Adding upstream version 1.21.8.upstream/1.21.8

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1431 files changed, 142743 insertions, 0 deletions

diff --git a/src/cmd/go/alldocs.go b/src/cmd/go/alldocs.go
new file mode 100644
index 0000000..bb28756
--- /dev/null
+++ b/src/cmd/go/alldocs.go
@@ -0,0 +1,3396 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Code generated by 'go test cmd/go -v -run=TestDocsUpToDate -fixdocs'; DO NOT EDIT.
+// Edit the documentation in other files and then execute 'go generate cmd/go' to generate this one.
+
+// Go is a tool for managing Go source code.
+//
+// Usage:
+//
+//	go <command> [arguments]
+//
+// The commands are:
+//
+//	bug         start a bug report
+//	build       compile packages and dependencies
+//	clean       remove object files and cached files
+//	doc         show documentation for package or symbol
+//	env         print Go environment information
+//	fix         update packages to use new APIs
+//	fmt         gofmt (reformat) package sources
+//	generate    generate Go files by processing source
+//	get         add dependencies to current module and install them
+//	install     compile and install packages and dependencies
+//	list        list packages or modules
+//	mod         module maintenance
+//	work        workspace maintenance
+//	run         compile and run Go program
+//	test        test packages
+//	tool        run specified go tool
+//	version     print Go version
+//	vet         report likely mistakes in packages
+//
+// Use "go help <command>" for more information about a command.
+//
+// Additional help topics:
+//
+//	buildconstraint build constraints
+//	buildmode       build modes
+//	c               calling between Go and C
+//	cache           build and test caching
+//	environment     environment variables
+//	filetype        file types
+//	go.mod          the go.mod file
+//	gopath          GOPATH environment variable
+//	gopath-get      legacy GOPATH go get
+//	goproxy         module proxy protocol
+//	importpath      import path syntax
+//	modules         modules, module versions, and more
+//	module-get      module-aware go get
+//	module-auth     module authentication using go.sum
+//	packages        package lists and patterns
+//	private         configuration for downloading non-public code
+//	testflag        testing flags
+//	testfunc        testing functions
+//	vcs             controlling version control with GOVCS
+//
+// Use "go help <topic>" for more information about that topic.
+//
+// # Start a bug report
+//
+// Usage:
+//
+//	go bug
+//
+// Bug opens the default browser and starts a new bug report.
+// The report includes useful system information.
+//
+// # Compile packages and dependencies
+//
+// Usage:
+//
+//	go build [-o output] [build flags] [packages]
+//
+// Build compiles the packages named by the import paths,
+// along with their dependencies, but it does not install the results.
+//
+// If the arguments to build are a list of .go files from a single directory,
+// build treats them as a list of source files specifying a single package.
+//
+// When compiling packages, build ignores files that end in '_test.go'.
+//
+// When compiling a single main package, build writes
+// the resulting executable to an output file named after
+// the first source file ('go build ed.go rx.go' writes 'ed' or 'ed.exe')
+// or the source code directory ('go build unix/sam' writes 'sam' or 'sam.exe').
+// The '.exe' suffix is added when writing a Windows executable.
+//
+// When compiling multiple packages or a single non-main package,
+// build compiles the packages but discards the resulting object,
+// serving only as a check that the packages can be built.
+//
+// The -o flag forces build to write the resulting executable or object
+// to the named output file or directory, instead of the default behavior described
+// in the last two paragraphs. If the named output is an existing directory or
+// ends with a slash or backslash, then any resulting executables
+// will be written to that directory.
+//
+// The build flags are shared by the build, clean, get, install, list, run,
+// and test commands:
+//
+//	-C dir
+//		Change to dir before running the command.
+//		Any files named on the command line are interpreted after
+//		changing directories.
+//		If used, this flag must be the first one in the command line.
+//	-a
+//		force rebuilding of packages that are already up-to-date.
+//	-n
+//		print the commands but do not run them.
+//	-p n
+//		the number of programs, such as build commands or
+//		test binaries, that can be run in parallel.
+//		The default is GOMAXPROCS, normally the number of CPUs available.
+//	-race
+//		enable data race detection.
+//		Supported only on linux/amd64, freebsd/amd64, darwin/amd64, darwin/arm64, windows/amd64,
+//		linux/ppc64le and linux/arm64 (only for 48-bit VMA).
+//	-msan
+//		enable interoperation with memory sanitizer.
+//		Supported only on linux/amd64, linux/arm64, freebsd/amd64
+//		and only with Clang/LLVM as the host C compiler.
+//		PIE build mode will be used on all platforms except linux/amd64.
+//	-asan
+//		enable interoperation with address sanitizer.
+//		Supported only on linux/arm64, linux/amd64.
+//		Supported only on linux/amd64 or linux/arm64 and only with GCC 7 and higher
+//		or Clang/LLVM 9 and higher.
+//	-cover
+//		enable code coverage instrumentation.
+//	-covermode set,count,atomic
+//		set the mode for coverage analysis.
+//		The default is "set" unless -race is enabled,
+//		in which case it is "atomic".
+//		The values:
+//		set: bool: does this statement run?
+//		count: int: how many times does this statement run?
+//		atomic: int: count, but correct in multithreaded tests;
+//			significantly more expensive.
+//		Sets -cover.
+//	-coverpkg pattern1,pattern2,pattern3
+//		For a build that targets package 'main' (e.g. building a Go
+//		executable), apply coverage analysis to each package matching
+//		the patterns. The default is to apply coverage analysis to
+//		packages in the main Go module. See 'go help packages' for a
+//		description of package patterns.  Sets -cover.
+//	-v
+//		print the names of packages as they are compiled.
+//	-work
+//		print the name of the temporary work directory and
+//		do not delete it when exiting.
+//	-x
+//		print the commands.
+//	-asmflags '[pattern=]arg list'
+//		arguments to pass on each go tool asm invocation.
+//	-buildmode mode
+//		build mode to use. See 'go help buildmode' for more.
+//	-buildvcs
+//		Whether to stamp binaries with version control information
+//		("true", "false", or "auto"). By default ("auto"), version control
+//		information is stamped into a binary if the main package, the main module
+//		containing it, and the current directory are all in the same repository.
+//		Use -buildvcs=false to always omit version control information, or
+//		-buildvcs=true to error out if version control information is available but
+//		cannot be included due to a missing tool or ambiguous directory structure.
+//	-compiler name
+//		name of compiler to use, as in runtime.Compiler (gccgo or gc).
+//	-gccgoflags '[pattern=]arg list'
+//		arguments to pass on each gccgo compiler/linker invocation.
+//	-gcflags '[pattern=]arg list'
+//		arguments to pass on each go tool compile invocation.
+//	-installsuffix suffix
+//		a suffix to use in the name of the package installation directory,
+//		in order to keep output separate from default builds.
+//		If using the -race flag, the install suffix is automatically set to race
+//		or, if set explicitly, has _race appended to it. Likewise for the -msan
+//		and -asan flags. Using a -buildmode option that requires non-default compile
+//		flags has a similar effect.
+//	-ldflags '[pattern=]arg list'
+//		arguments to pass on each go tool link invocation.
+//	-linkshared
+//		build code that will be linked against shared libraries previously
+//		created with -buildmode=shared.
+//	-mod mode
+//		module download mode to use: readonly, vendor, or mod.
+//		By default, if a vendor directory is present and the go version in go.mod
+//		is 1.14 or higher, the go command acts as if -mod=vendor were set.
+//		Otherwise, the go command acts as if -mod=readonly were set.
+//		See https://golang.org/ref/mod#build-commands for details.
+//	-modcacherw
+//		leave newly-created directories in the module cache read-write
+//		instead of making them read-only.
+//	-modfile file
+//		in module aware mode, read (and possibly write) an alternate go.mod
+//		file instead of the one in the module root directory. A file named
+//		"go.mod" must still be present in order to determine the module root
+//		directory, but it is not accessed. When -modfile is specified, an
+//		alternate go.sum file is also used: its path is derived from the
+//		-modfile flag by trimming the ".mod" extension and appending ".sum".
+//	-overlay file
+//		read a JSON config file that provides an overlay for build operations.
+//		The file is a JSON struct with a single field, named 'Replace', that
+//		maps each disk file path (a string) to its backing file path, so that
+//		a build will run as if the disk file path exists with the contents
+//		given by the backing file paths, or as if the disk file path does not
+//		exist if its backing file path is empty. Support for the -overlay flag
+//		has some limitations: importantly, cgo files included from outside the
+//		include path must be in the same directory as the Go package they are
+//		included from, and overlays will not appear when binaries and tests are
+//		run through go run and go test respectively.
+//	-pgo file
+//		specify the file path of a profile for profile-guided optimization (PGO).
+//		When the special name "auto" is specified, for each main package in the
+//		build, the go command selects a file named "default.pgo" in the package's
+//		directory if that file exists, and applies it to the (transitive)
+//		dependencies of the main package (other packages are not affected).
+//		Special name "off" turns off PGO. The default is "auto".
+//	-pkgdir dir
+//		install and load all packages from dir instead of the usual locations.
+//		For example, when building with a non-standard configuration,
+//		use -pkgdir to keep generated packages in a separate location.
+//	-tags tag,list
+//		a comma-separated list of additional build tags to consider satisfied
+//		during the build. For more information about build tags, see
+//		'go help buildconstraint'. (Earlier versions of Go used a
+//		space-separated list, and that form is deprecated but still recognized.)
+//	-trimpath
+//		remove all file system paths from the resulting executable.
+//		Instead of absolute file system paths, the recorded file names
+//		will begin either a module path@version (when using modules),
+//		or a plain import path (when using the standard library, or GOPATH).
+//	-toolexec 'cmd args'
+//		a program to use to invoke toolchain programs like vet and asm.
+//		For example, instead of running asm, the go command will run
+//		'cmd args /path/to/asm <arguments for asm>'.
+//		The TOOLEXEC_IMPORTPATH environment variable will be set,
+//		matching 'go list -f {{.ImportPath}}' for the package being built.
+//
+// The -asmflags, -gccgoflags, -gcflags, and -ldflags flags accept a
+// space-separated list of arguments to pass to an underlying tool
+// during the build. To embed spaces in an element in the list, surround
+// it with either single or double quotes. The argument list may be
+// preceded by a package pattern and an equal sign, which restricts
+// the use of that argument list to the building of packages matching
+// that pattern (see 'go help packages' for a description of package
+// patterns). Without a pattern, the argument list applies only to the
+// packages named on the command line. The flags may be repeated
+// with different patterns in order to specify different arguments for
+// different sets of packages. If a package matches patterns given in
+// multiple flags, the latest match on the command line wins.
+// For example, 'go build -gcflags=-S fmt' prints the disassembly
+// only for package fmt, while 'go build -gcflags=all=-S fmt'
+// prints the disassembly for fmt and all its dependencies.
+//
+// For more about specifying packages, see 'go help packages'.
+// For more about where packages and binaries are installed,
+// run 'go help gopath'.
+// For more about calling between Go and C/C++, run 'go help c'.
+//
+// Note: Build adheres to certain conventions such as those described
+// by 'go help gopath'. Not all projects can follow these conventions,
+// however. Installations that have their own conventions or that use
+// a separate software build system may choose to use lower-level
+// invocations such as 'go tool compile' and 'go tool link' to avoid
+// some of the overheads and design decisions of the build tool.
+//
+// See also: go install, go get, go clean.
+//
+// # Remove object files and cached files
+//
+// Usage:
+//
+//	go clean [clean flags] [build flags] [packages]
+//
+// Clean removes object files from package source directories.
+// The go command builds most objects in a temporary directory,
+// so go clean is mainly concerned with object files left by other
+// tools or by manual invocations of go build.
+//
+// If a package argument is given or the -i or -r flag is set,
+// clean removes the following files from each of the
+// source directories corresponding to the import paths:
+//
+//	_obj/            old object directory, left from Makefiles
+//	_test/           old test directory, left from Makefiles
+//	_testmain.go     old gotest file, left from Makefiles
+//	test.out         old test log, left from Makefiles
+//	build.out        old test log, left from Makefiles
+//	*.[568ao]        object files, left from Makefiles
+//
+//	DIR(.exe)        from go build
+//	DIR.test(.exe)   from go test -c
+//	MAINFILE(.exe)   from go build MAINFILE.go
+//	*.so             from SWIG
+//
+// In the list, DIR represents the final path element of the
+// directory, and MAINFILE is the base name of any Go source
+// file in the directory that is not included when building
+// the package.
+//
+// The -i flag causes clean to remove the corresponding installed
+// archive or binary (what 'go install' would create).
+//
+// The -n flag causes clean to print the remove commands it would execute,
+// but not run them.
+//
+// The -r flag causes clean to be applied recursively to all the
+// dependencies of the packages named by the import paths.
+//
+// The -x flag causes clean to print remove commands as it executes them.
+//
+// The -cache flag causes clean to remove the entire go build cache.
+//
+// The -testcache flag causes clean to expire all test results in the
+// go build cache.
+//
+// The -modcache flag causes clean to remove the entire module
+// download cache, including unpacked source code of versioned
+// dependencies.
+//
+// The -fuzzcache flag causes clean to remove files stored in the Go build
+// cache for fuzz testing. The fuzzing engine caches files that expand
+// code coverage, so removing them may make fuzzing less effective until
+// new inputs are found that provide the same coverage. These files are
+// distinct from those stored in testdata directory; clean does not remove
+// those files.
+//
+// For more about build flags, see 'go help build'.
+//
+// For more about specifying packages, see 'go help packages'.
+//
+// # Show documentation for package or symbol
+//
+// Usage:
+//
+//	go doc [doc flags] [package|[package.]symbol[.methodOrField]]
+//
+// Doc prints the documentation comments associated with the item identified by its
+// arguments (a package, const, func, type, var, method, or struct field)
+// followed by a one-line summary of each of the first-level items "under"
+// that item (package-level declarations for a package, methods for a type,
+// etc.).
+//
+// Doc accepts zero, one, or two arguments.
+//
+// Given no arguments, that is, when run as
+//
+//	go doc
+//
+// it prints the package documentation for the package in the current directory.
+// If the package is a command (package main), the exported symbols of the package
+// are elided from the presentation unless the -cmd flag is provided.
+//
+// When run with one argument, the argument is treated as a Go-syntax-like
+// representation of the item to be documented. What the argument selects depends
+// on what is installed in GOROOT and GOPATH, as well as the form of the argument,
+// which is schematically one of these:
+//
+//	go doc <pkg>
+//	go doc <sym>[.<methodOrField>]
+//	go doc [<pkg>.]<sym>[.<methodOrField>]
+//	go doc [<pkg>.][<sym>.]<methodOrField>
+//
+// The first item in this list matched by the argument is the one whose documentation
+// is printed. (See the examples below.) However, if the argument starts with a capital
+// letter it is assumed to identify a symbol or method in the current directory.
+//
+// For packages, the order of scanning is determined lexically in breadth-first order.
+// That is, the package presented is the one that matches the search and is nearest
+// the root and lexically first at its level of the hierarchy. The GOROOT tree is
+// always scanned in its entirety before GOPATH.
+//
+// If there is no package specified or matched, the package in the current
+// directory is selected, so "go doc Foo" shows the documentation for symbol Foo in
+// the current package.
+//
+// The package path must be either a qualified path or a proper suffix of a
+// path. The go tool's usual package mechanism does not apply: package path
+// elements like . and ... are not implemented by go doc.
+//
+// When run with two arguments, the first is a package path (full path or suffix),
+// and the second is a symbol, or symbol with method or struct field:
+//
+//	go doc <pkg> <sym>[.<methodOrField>]
+//
+// In all forms, when matching symbols, lower-case letters in the argument match
+// either case but upper-case letters match exactly. This means that there may be
+// multiple matches of a lower-case argument in a package if different symbols have
+// different cases. If this occurs, documentation for all matches is printed.
+//
+// Examples:
+//
+//	go doc
+//		Show documentation for current package.
+//	go doc Foo
+//		Show documentation for Foo in the current package.
+//		(Foo starts with a capital letter so it cannot match
+//		a package path.)
+//	go doc encoding/json
+//		Show documentation for the encoding/json package.
+//	go doc json
+//		Shorthand for encoding/json.
+//	go doc json.Number (or go doc json.number)
+//		Show documentation and method summary for json.Number.
+//	go doc json.Number.Int64 (or go doc json.number.int64)
+//		Show documentation for json.Number's Int64 method.
+//	go doc cmd/doc
+//		Show package docs for the doc command.
+//	go doc -cmd cmd/doc
+//		Show package docs and exported symbols within the doc command.
+//	go doc template.new
+//		Show documentation for html/template's New function.
+//		(html/template is lexically before text/template)
+//	go doc text/template.new # One argument
+//		Show documentation for text/template's New function.
+//	go doc text/template new # Two arguments
+//		Show documentation for text/template's New function.
+//
+//	At least in the current tree, these invocations all print the
+//	documentation for json.Decoder's Decode method:
+//
+//	go doc json.Decoder.Decode
+//	go doc json.decoder.decode
+//	go doc json.decode
+//	cd go/src/encoding/json; go doc decode
+//
+// Flags:
+//
+//	-all
+//		Show all the documentation for the package.
+//	-c
+//		Respect case when matching symbols.
+//	-cmd
+//		Treat a command (package main) like a regular package.
+//		Otherwise package main's exported symbols are hidden
+//		when showing the package's top-level documentation.
+//	-short
+//		One-line representation for each symbol.
+//	-src
+//		Show the full source code for the symbol. This will
+//		display the full Go source of its declaration and
+//		definition, such as a function definition (including
+//		the body), type declaration or enclosing const
+//		block. The output may therefore include unexported
+//		details.
+//	-u
+//		Show documentation for unexported as well as exported
+//		symbols, methods, and fields.
+//
+// # Print Go environment information
+//
+// Usage:
+//
+//	go env [-json] [-u] [-w] [var ...]
+//
+// Env prints Go environment information.
+//
+// By default env prints information as a shell script
+// (on Windows, a batch file). If one or more variable
+// names is given as arguments, env prints the value of
+// each named variable on its own line.
+//
+// The -json flag prints the environment in JSON format
+// instead of as a shell script.
+//
+// The -u flag requires one or more arguments and unsets
+// the default setting for the named environment variables,
+// if one has been set with 'go env -w'.
+//
+// The -w flag requires one or more arguments of the
+// form NAME=VALUE and changes the default settings
+// of the named environment variables to the given values.
+//
+// For more about environment variables, see 'go help environment'.
+//
+// # Update packages to use new APIs
+//
+// Usage:
+//
+//	go fix [-fix list] [packages]
+//
+// Fix runs the Go fix command on the packages named by the import paths.
+//
+// The -fix flag sets a comma-separated list of fixes to run.
+// The default is all known fixes.
+// (Its value is passed to 'go tool fix -r'.)
+//
+// For more about fix, see 'go doc cmd/fix'.
+// For more about specifying packages, see 'go help packages'.
+//
+// To run fix with other options, run 'go tool fix'.
+//
+// See also: go fmt, go vet.
+//
+// # Gofmt (reformat) package sources
+//
+// Usage:
+//
+//	go fmt [-n] [-x] [packages]
+//
+// Fmt runs the command 'gofmt -l -w' on the packages named
+// by the import paths. It prints the names of the files that are modified.
+//
+// For more about gofmt, see 'go doc cmd/gofmt'.
+// For more about specifying packages, see 'go help packages'.
+//
+// The -n flag prints commands that would be executed.
+// The -x flag prints commands as they are executed.
+//
+// The -mod flag's value sets which module download mode
+// to use: readonly or vendor. See 'go help modules' for more.
+//
+// To run gofmt with specific options, run gofmt itself.
+//
+// See also: go fix, go vet.
+//
+// # Generate Go files by processing source
+//
+// Usage:
+//
+//	go generate [-run regexp] [-n] [-v] [-x] [build flags] [file.go... | packages]
+//
+// Generate runs commands described by directives within existing
+// files. Those commands can run any process but the intent is to
+// create or update Go source files.
+//
+// Go generate is never run automatically by go build, go test,
+// and so on. It must be run explicitly.
+//
+// Go generate scans the file for directives, which are lines of
+// the form,
+//
+//	//go:generate command argument...
+//
+// (note: no leading spaces and no space in "//go") where command
+// is the generator to be run, corresponding to an executable file
+// that can be run locally. It must either be in the shell path
+// (gofmt), a fully qualified path (/usr/you/bin/mytool), or a
+// command alias, described below.
+//
+// Note that go generate does not parse the file, so lines that look
+// like directives in comments or multiline strings will be treated
+// as directives.
+//
+// The arguments to the directive are space-separated tokens or
+// double-quoted strings passed to the generator as individual
+// arguments when it is run.
+//
+// Quoted strings use Go syntax and are evaluated before execution; a
+// quoted string appears as a single argument to the generator.
+//
+// To convey to humans and machine tools that code is generated,
+// generated source should have a line that matches the following
+// regular expression (in Go syntax):
+//
+//	^// Code generated .* DO NOT EDIT\.$
+//
+// This line must appear before the first non-comment, non-blank
+// text in the file.
+//
+// Go generate sets several variables when it runs the generator:
+//
+//	$GOARCH
+//		The execution architecture (arm, amd64, etc.)
+//	$GOOS
+//		The execution operating system (linux, windows, etc.)
+//	$GOFILE
+//		The base name of the file.
+//	$GOLINE
+//		The line number of the directive in the source file.
+//	$GOPACKAGE
+//		The name of the package of the file containing the directive.
+//	$GOROOT
+//		The GOROOT directory for the 'go' command that invoked the
+//		generator, containing the Go toolchain and standard library.
+//	$DOLLAR
+//		A dollar sign.
+//	$PATH
+//		The $PATH of the parent process, with $GOROOT/bin
+//		placed at the beginning. This causes generators
+//		that execute 'go' commands to use the same 'go'
+//		as the parent 'go generate' command.
+//
+// Other than variable substitution and quoted-string evaluation, no
+// special processing such as "globbing" is performed on the command
+// line.
+//
+// As a last step before running the command, any invocations of any
+// environment variables with alphanumeric names, such as $GOFILE or
+// $HOME, are expanded throughout the command line. The syntax for
+// variable expansion is $NAME on all operating systems. Due to the
+// order of evaluation, variables are expanded even inside quoted
+// strings. If the variable NAME is not set, $NAME expands to the
+// empty string.
+//
+// A directive of the form,
+//
+//	//go:generate -command xxx args...
+//
+// specifies, for the remainder of this source file only, that the
+// string xxx represents the command identified by the arguments. This
+// can be used to create aliases or to handle multiword generators.
+// For example,
+//
+//	//go:generate -command foo go tool foo
+//
+// specifies that the command "foo" represents the generator
+// "go tool foo".
+//
+// Generate processes packages in the order given on the command line,
+// one at a time. If the command line lists .go files from a single directory,
+// they are treated as a single package. Within a package, generate processes the
+// source files in a package in file name order, one at a time. Within
+// a source file, generate runs generators in the order they appear
+// in the file, one at a time. The go generate tool also sets the build
+// tag "generate" so that files may be examined by go generate but ignored
+// during build.
+//
+// For packages with invalid code, generate processes only source files with a
+// valid package clause.
+//
+// If any generator returns an error exit status, "go generate" skips
+// all further processing for that package.
+//
+// The generator is run in the package's source directory.
+//
+// Go generate accepts two specific flags:
+//
+//	-run=""
+//		if non-empty, specifies a regular expression to select
+//		directives whose full original source text (excluding
+//		any trailing spaces and final newline) matches the
+//		expression.
+//
+//	-skip=""
+//		if non-empty, specifies a regular expression to suppress
+//		directives whose full original source text (excluding
+//		any trailing spaces and final newline) matches the
+//		expression. If a directive matches both the -run and
+//		the -skip arguments, it is skipped.
+//
+// It also accepts the standard build flags including -v, -n, and -x.
+// The -v flag prints the names of packages and files as they are
+// processed.
+// The -n flag prints commands that would be executed.
+// The -x flag prints commands as they are executed.
+//
+// For more about build flags, see 'go help build'.
+//
+// For more about specifying packages, see 'go help packages'.
+//
+// # Add dependencies to current module and install them
+//
+// Usage:
+//
+//	go get [-t] [-u] [-v] [build flags] [packages]
+//
+// Get resolves its command-line arguments to packages at specific module versions,
+// updates go.mod to require those versions, and downloads source code into the
+// module cache.
+//
+// To add a dependency for a package or upgrade it to its latest version:
+//
+//	go get example.com/pkg
+//
+// To upgrade or downgrade a package to a specific version:
+//
+//	go get example.com/pkg@v1.2.3
+//
+// To remove a dependency on a module and downgrade modules that require it:
+//
+//	go get example.com/mod@none
+//
+// To upgrade the minimum required Go version to the latest released Go version:
+//
+//	go get go@latest
+//
+// To upgrade the Go toolchain to the latest patch release of the current Go toolchain:
+//
+//	go get toolchain@patch
+//
+// See https://golang.org/ref/mod#go-get for details.
+//
+// In earlier versions of Go, 'go get' was used to build and install packages.
+// Now, 'go get' is dedicated to adjusting dependencies in go.mod. 'go install'
+// may be used to build and install commands instead. When a version is specified,
+// 'go install' runs in module-aware mode and ignores the go.mod file in the
+// current directory. For example:
+//
+//	go install example.com/pkg@v1.2.3
+//	go install example.com/pkg@latest
+//
+// See 'go help install' or https://golang.org/ref/mod#go-install for details.
+//
+// 'go get' accepts the following flags.
+//
+// The -t flag instructs get to consider modules needed to build tests of
+// packages specified on the command line.
+//
+// The -u flag instructs get to update modules providing dependencies
+// of packages named on the command line to use newer minor or patch
+// releases when available.
+//
+// The -u=patch flag (not -u patch) also instructs get to update dependencies,
+// but changes the default to select patch releases.
+//
+// When the -t and -u flags are used together, get will update
+// test dependencies as well.
+//
+// The -x flag prints commands as they are executed. This is useful for
+// debugging version control commands when a module is downloaded directly
+// from a repository.
+//
+// For more about modules, see https://golang.org/ref/mod.
+//
+// For more about using 'go get' to update the minimum Go version and
+// suggested Go toolchain, see https://go.dev/doc/toolchain.
+//
+// For more about specifying packages, see 'go help packages'.
+//
+// This text describes the behavior of get using modules to manage source
+// code and dependencies. If instead the go command is running in GOPATH
+// mode, the details of get's flags and effects change, as does 'go help get'.
+// See 'go help gopath-get'.
+//
+// See also: go build, go install, go clean, go mod.
+//
+// # Compile and install packages and dependencies
+//
+// Usage:
+//
+//	go install [build flags] [packages]
+//
+// Install compiles and installs the packages named by the import paths.
+//
+// Executables are installed in the directory named by the GOBIN environment
+// variable, which defaults to $GOPATH/bin or $HOME/go/bin if the GOPATH
+// environment variable is not set. Executables in $GOROOT
+// are installed in $GOROOT/bin or $GOTOOLDIR instead of $GOBIN.
+//
+// If the arguments have version suffixes (like @latest or @v1.0.0), "go install"
+// builds packages in module-aware mode, ignoring the go.mod file in the current
+// directory or any parent directory, if there is one. This is useful for
+// installing executables without affecting the dependencies of the main module.
+// To eliminate ambiguity about which module versions are used in the build, the
+// arguments must satisfy the following constraints:
+//
+// - Arguments must be package paths or package patterns (with "..." wildcards).
+// They must not be standard packages (like fmt), meta-patterns (std, cmd,
+// all), or relative or absolute file paths.
+//
+// - All arguments must have the same version suffix. Different queries are not
+// allowed, even if they refer to the same version.
+//
+// - All arguments must refer to packages in the same module at the same version.
+//
+// - Package path arguments must refer to main packages. Pattern arguments
+// will only match main packages.
+//
+// - No module is considered the "main" module. If the module containing
+// packages named on the command line has a go.mod file, it must not contain
+// directives (replace and exclude) that would cause it to be interpreted
+// differently than if it were the main module. The module must not require
+// a higher version of itself.
+//
+// - Vendor directories are not used in any module. (Vendor directories are not
+// included in the module zip files downloaded by 'go install'.)
+//
+// If the arguments don't have version suffixes, "go install" may run in
+// module-aware mode or GOPATH mode, depending on the GO111MODULE environment
+// variable and the presence of a go.mod file. See 'go help modules' for details.
+// If module-aware mode is enabled, "go install" runs in the context of the main
+// module.
+//
+// When module-aware mode is disabled, non-main packages are installed in the
+// directory $GOPATH/pkg/$GOOS_$GOARCH. When module-aware mode is enabled,
+// non-main packages are built and cached but not installed.
+//
+// Before Go 1.20, the standard library was installed to
+// $GOROOT/pkg/$GOOS_$GOARCH.
+// Starting in Go 1.20, the standard library is built and cached but not installed.
+// Setting GODEBUG=installgoroot=all restores the use of
+// $GOROOT/pkg/$GOOS_$GOARCH.
+//
+// For more about build flags, see 'go help build'.
+//
+// For more about specifying packages, see 'go help packages'.
+//
+// See also: go build, go get, go clean.
+//
+// # List packages or modules
+//
+// Usage:
+//
+//	go list [-f format] [-json] [-m] [list flags] [build flags] [packages]
+//
+// List lists the named packages, one per line.
+// The most commonly-used flags are -f and -json, which control the form
+// of the output printed for each package. Other list flags, documented below,
+// control more specific details.
+//
+// The default output shows the package import path:
+//
+//	bytes
+//	encoding/json
+//	github.com/gorilla/mux
+//	golang.org/x/net/html
+//
+// The -f flag specifies an alternate format for the list, using the
+// syntax of package template. The default output is equivalent
+// to -f '{{.ImportPath}}'. The struct being passed to the template is:
+//
+//	type Package struct {
+//	    Dir            string   // directory containing package sources
+//	    ImportPath     string   // import path of package in dir
+//	    ImportComment  string   // path in import comment on package statement
+//	    Name           string   // package name
+//	    Doc            string   // package documentation string
+//	    Target         string   // install path
+//	    Shlib          string   // the shared library that contains this package (only set when -linkshared)
+//	    Goroot         bool     // is this package in the Go root?
+//	    Standard       bool     // is this package part of the standard Go library?
+//	    Stale          bool     // would 'go install' do anything for this package?
+//	    StaleReason    string   // explanation for Stale==true
+//	    Root           string   // Go root or Go path dir containing this package
+//	    ConflictDir    string   // this directory shadows Dir in $GOPATH
+//	    BinaryOnly     bool     // binary-only package (no longer supported)
+//	    ForTest        string   // package is only for use in named test
+//	    Export         string   // file containing export data (when using -export)
+//	    BuildID        string   // build ID of the compiled package (when using -export)
+//	    Module         *Module  // info about package's containing module, if any (can be nil)
+//	    Match          []string // command-line patterns matching this package
+//	    DepOnly        bool     // package is only a dependency, not explicitly listed
+//	    DefaultGODEBUG string  // default GODEBUG setting, for main packages
+//
+//	    // Source files
+//	    GoFiles           []string   // .go source files (excluding CgoFiles, TestGoFiles, XTestGoFiles)
+//	    CgoFiles          []string   // .go source files that import "C"
+//	    CompiledGoFiles   []string   // .go files presented to compiler (when using -compiled)
+//	    IgnoredGoFiles    []string   // .go source files ignored due to build constraints
+//	    IgnoredOtherFiles []string // non-.go source files ignored due to build constraints
+//	    CFiles            []string   // .c source files
+//	    CXXFiles          []string   // .cc, .cxx and .cpp source files
+//	    MFiles            []string   // .m source files
+//	    HFiles            []string   // .h, .hh, .hpp and .hxx source files
+//	    FFiles            []string   // .f, .F, .for and .f90 Fortran source files
+//	    SFiles            []string   // .s source files
+//	    SwigFiles         []string   // .swig files
+//	    SwigCXXFiles      []string   // .swigcxx files
+//	    SysoFiles         []string   // .syso object files to add to archive
+//	    TestGoFiles       []string   // _test.go files in package
+//	    XTestGoFiles      []string   // _test.go files outside package
+//
+//	    // Embedded files
+//	    EmbedPatterns      []string // //go:embed patterns
+//	    EmbedFiles         []string // files matched by EmbedPatterns
+//	    TestEmbedPatterns  []string // //go:embed patterns in TestGoFiles
+//	    TestEmbedFiles     []string // files matched by TestEmbedPatterns
+//	    XTestEmbedPatterns []string // //go:embed patterns in XTestGoFiles
+//	    XTestEmbedFiles    []string // files matched by XTestEmbedPatterns
+//
+//	    // Cgo directives
+//	    CgoCFLAGS    []string // cgo: flags for C compiler
+//	    CgoCPPFLAGS  []string // cgo: flags for C preprocessor
+//	    CgoCXXFLAGS  []string // cgo: flags for C++ compiler
+//	    CgoFFLAGS    []string // cgo: flags for Fortran compiler
+//	    CgoLDFLAGS   []string // cgo: flags for linker
+//	    CgoPkgConfig []string // cgo: pkg-config names
+//
+//	    // Dependency information
+//	    Imports      []string          // import paths used by this package
+//	    ImportMap    map[string]string // map from source import to ImportPath (identity entries omitted)
+//	    Deps         []string          // all (recursively) imported dependencies
+//	    TestImports  []string          // imports from TestGoFiles
+//	    XTestImports []string          // imports from XTestGoFiles
+//
+//	    // Error information
+//	    Incomplete bool            // this package or a dependency has an error
+//	    Error      *PackageError   // error loading package
+//	    DepsErrors []*PackageError // errors loading dependencies
+//	}
+//
+// Packages stored in vendor directories report an ImportPath that includes the
+// path to the vendor directory (for example, "d/vendor/p" instead of "p"),
+// so that the ImportPath uniquely identifies a given copy of a package.
+// The Imports, Deps, TestImports, and XTestImports lists also contain these
+// expanded import paths. See golang.org/s/go15vendor for more about vendoring.
+//
+// The error information, if any, is
+//
+//	type PackageError struct {
+//	    ImportStack   []string // shortest path from package named on command line to this one
+//	    Pos           string   // position of error (if present, file:line:col)
+//	    Err           string   // the error itself
+//	}
+//
+// The module information is a Module struct, defined in the discussion
+// of list -m below.
+//
+// The template function "join" calls strings.Join.
+//
+// The template function "context" returns the build context, defined as:
+//
+//	type Context struct {
+//	    GOARCH        string   // target architecture
+//	    GOOS          string   // target operating system
+//	    GOROOT        string   // Go root
+//	    GOPATH        string   // Go path
+//	    CgoEnabled    bool     // whether cgo can be used
+//	    UseAllFiles   bool     // use files regardless of //go:build lines, file names
+//	    Compiler      string   // compiler to assume when computing target paths
+//	    BuildTags     []string // build constraints to match in //go:build lines
+//	    ToolTags      []string // toolchain-specific build constraints
+//	    ReleaseTags   []string // releases the current release is compatible with
+//	    InstallSuffix string   // suffix to use in the name of the install dir
+//	}
+//
+// For more information about the meaning of these fields see the documentation
+// for the go/build package's Context type.
+//
+// The -json flag causes the package data to be printed in JSON format
+// instead of using the template format. The JSON flag can optionally be
+// provided with a set of comma-separated required field names to be output.
+// If so, those required fields will always appear in JSON output, but
+// others may be omitted to save work in computing the JSON struct.
+//
+// The -compiled flag causes list to set CompiledGoFiles to the Go source
+// files presented to the compiler. Typically this means that it repeats
+// the files listed in GoFiles and then also adds the Go code generated
+// by processing CgoFiles and SwigFiles. The Imports list contains the
+// union of all imports from both GoFiles and CompiledGoFiles.
+//
+// The -deps flag causes list to iterate over not just the named packages
+// but also all their dependencies. It visits them in a depth-first post-order
+// traversal, so that a package is listed only after all its dependencies.
+// Packages not explicitly listed on the command line will have the DepOnly
+// field set to true.
+//
+// The -e flag changes the handling of erroneous packages, those that
+// cannot be found or are malformed. By default, the list command
+// prints an error to standard error for each erroneous package and
+// omits the packages from consideration during the usual printing.
+// With the -e flag, the list command never prints errors to standard
+// error and instead processes the erroneous packages with the usual
+// printing. Erroneous packages will have a non-empty ImportPath and
+// a non-nil Error field; other information may or may not be missing
+// (zeroed).
+//
+// The -export flag causes list to set the Export field to the name of a
+// file containing up-to-date export information for the given package,
+// and the BuildID field to the build ID of the compiled package.
+//
+// The -find flag causes list to identify the named packages but not
+// resolve their dependencies: the Imports and Deps lists will be empty.
+// With the -find flag, the -deps, -test and -export commands cannot be
+// used.
+//
+// The -test flag causes list to report not only the named packages
+// but also their test binaries (for packages with tests), to convey to
+// source code analysis tools exactly how test binaries are constructed.
+// The reported import path for a test binary is the import path of
+// the package followed by a ".test" suffix, as in "math/rand.test".
+// When building a test, it is sometimes necessary to rebuild certain
+// dependencies specially for that test (most commonly the tested
+// package itself). The reported import path of a package recompiled
+// for a particular test binary is followed by a space and the name of
+// the test binary in brackets, as in "math/rand [math/rand.test]"
+// or "regexp [sort.test]". The ForTest field is also set to the name
+// of the package being tested ("math/rand" or "sort" in the previous
+// examples).
+//
+// The Dir, Target, Shlib, Root, ConflictDir, and Export file paths
+// are all absolute paths.
+//
+// By default, the lists GoFiles, CgoFiles, and so on hold names of files in Dir
+// (that is, paths relative to Dir, not absolute paths).
+// The generated files added when using the -compiled and -test flags
+// are absolute paths referring to cached copies of generated Go source files.
+// Although they are Go source files, the paths may not end in ".go".
+//
+// The -m flag causes list to list modules instead of packages.
+//
+// When listing modules, the -f flag still specifies a format template
+// applied to a Go struct, but now a Module struct:
+//
+//	type Module struct {
+//	    Path       string        // module path
+//	    Query      string        // version query corresponding to this version
+//	    Version    string        // module version
+//	    Versions   []string      // available module versions
+//	    Replace    *Module       // replaced by this module
+//	    Time       *time.Time    // time version was created
+//	    Update     *Module       // available update (with -u)
+//	    Main       bool          // is this the main module?
+//	    Indirect   bool          // module is only indirectly needed by main module
+//	    Dir        string        // directory holding local copy of files, if any
+//	    GoMod      string        // path to go.mod file describing module, if any
+//	    GoVersion  string        // go version used in module
+//	    Retracted  []string      // retraction information, if any (with -retracted or -u)
+//	    Deprecated string        // deprecation message, if any (with -u)
+//	    Error      *ModuleError  // error loading module
+//	    Origin     any           // provenance of module
+//	    Reuse      bool          // reuse of old module info is safe
+//	}
+//
+//	type ModuleError struct {
+//	    Err string // the error itself
+//	}
+//
+// The file GoMod refers to may be outside the module directory if the
+// module is in the module cache or if the -modfile flag is used.
+//
+// The default output is to print the module path and then
+// information about the version and replacement if any.
+// For example, 'go list -m all' might print:
+//
+//	my/main/module
+//	golang.org/x/text v0.3.0 => /tmp/text
+//	rsc.io/pdf v0.1.1
+//
+// The Module struct has a String method that formats this
+// line of output, so that the default format is equivalent
+// to -f '{{.String}}'.
+//
+// Note that when a module has been replaced, its Replace field
+// describes the replacement module, and its Dir field is set to
+// the replacement's source code, if present. (That is, if Replace
+// is non-nil, then Dir is set to Replace.Dir, with no access to
+// the replaced source code.)
+//
+// The -u flag adds information about available upgrades.
+// When the latest version of a given module is newer than
+// the current one, list -u sets the Module's Update field
+// to information about the newer module. list -u will also set
+// the module's Retracted field if the current version is retracted.
+// The Module's String method indicates an available upgrade by
+// formatting the newer version in brackets after the current version.
+// If a version is retracted, the string "(retracted)" will follow it.
+// For example, 'go list -m -u all' might print:
+//
+//	my/main/module
+//	golang.org/x/text v0.3.0 [v0.4.0] => /tmp/text
+//	rsc.io/pdf v0.1.1 (retracted) [v0.1.2]
+//
+// (For tools, 'go list -m -u -json all' may be more convenient to parse.)
+//
+// The -versions flag causes list to set the Module's Versions field
+// to a list of all known versions of that module, ordered according
+// to semantic versioning, earliest to latest. The flag also changes
+// the default output format to display the module path followed by the
+// space-separated version list.
+//
+// The -retracted flag causes list to report information about retracted
+// module versions. When -retracted is used with -f or -json, the Retracted
+// field will be set to a string explaining why the version was retracted.
+// The string is taken from comments on the retract directive in the
+// module's go.mod file. When -retracted is used with -versions, retracted
+// versions are listed together with unretracted versions. The -retracted
+// flag may be used with or without -m.
+//
+// The arguments to list -m are interpreted as a list of modules, not packages.
+// The main module is the module containing the current directory.
+// The active modules are the main module and its dependencies.
+// With no arguments, list -m shows the main module.
+// With arguments, list -m shows the modules specified by the arguments.
+// Any of the active modules can be specified by its module path.
+// The special pattern "all" specifies all the active modules, first the main
+// module and then dependencies sorted by module path.
+// A pattern containing "..." specifies the active modules whose
+// module paths match the pattern.
+// A query of the form path@version specifies the result of that query,
+// which is not limited to active modules.
+// See 'go help modules' for more about module queries.
+//
+// The template function "module" takes a single string argument
+// that must be a module path or query and returns the specified
+// module as a Module struct. If an error occurs, the result will
+// be a Module struct with a non-nil Error field.
+//
+// When using -m, the -reuse=old.json flag accepts the name of file containing
+// the JSON output of a previous 'go list -m -json' invocation with the
+// same set of modifier flags (such as -u, -retracted, and -versions).
+// The go command may use this file to determine that a module is unchanged
+// since the previous invocation and avoid redownloading information about it.
+// Modules that are not redownloaded will be marked in the new output by
+// setting the Reuse field to true. Normally the module cache provides this
+// kind of reuse automatically; the -reuse flag can be useful on systems that
+// do not preserve the module cache.
+//
+// For more about build flags, see 'go help build'.
+//
+// For more about specifying packages, see 'go help packages'.
+//
+// For more about modules, see https://golang.org/ref/mod.
+//
+// # Module maintenance
+//
+// Go mod provides access to operations on modules.
+//
+// Note that support for modules is built into all the go commands,
+// not just 'go mod'. For example, day-to-day adding, removing, upgrading,
+// and downgrading of dependencies should be done using 'go get'.
+// See 'go help modules' for an overview of module functionality.
+//
+// Usage:
+//
+//	go mod <command> [arguments]
+//
+// The commands are:
+//
+//	download    download modules to local cache
+//	edit        edit go.mod from tools or scripts
+//	graph       print module requirement graph
+//	init        initialize new module in current directory
+//	tidy        add missing and remove unused modules
+//	vendor      make vendored copy of dependencies
+//	verify      verify dependencies have expected content
+//	why         explain why packages or modules are needed
+//
+// Use "go help mod <command>" for more information about a command.
+//
+// # Download modules to local cache
+//
+// Usage:
+//
+//	go mod download [-x] [-json] [-reuse=old.json] [modules]
+//
+// Download downloads the named modules, which can be module patterns selecting
+// dependencies of the main module or module queries of the form path@version.
+//
+// With no arguments, download applies to the modules needed to build and test
+// the packages in the main module: the modules explicitly required by the main
+// module if it is at 'go 1.17' or higher, or all transitively-required modules
+// if at 'go 1.16' or lower.
+//
+// The go command will automatically download modules as needed during ordinary
+// execution. The "go mod download" command is useful mainly for pre-filling
+// the local cache or to compute the answers for a Go module proxy.
+//
+// By default, download writes nothing to standard output. It may print progress
+// messages and errors to standard error.
+//
+// The -json flag causes download to print a sequence of JSON objects
+// to standard output, describing each downloaded module (or failure),
+// corresponding to this Go struct:
+//
+//	type Module struct {
+//	    Path     string // module path
+//	    Query    string // version query corresponding to this version
+//	    Version  string // module version
+//	    Error    string // error loading module
+//	    Info     string // absolute path to cached .info file
+//	    GoMod    string // absolute path to cached .mod file
+//	    Zip      string // absolute path to cached .zip file
+//	    Dir      string // absolute path to cached source root directory
+//	    Sum      string // checksum for path, version (as in go.sum)
+//	    GoModSum string // checksum for go.mod (as in go.sum)
+//	    Origin   any    // provenance of module
+//	    Reuse    bool   // reuse of old module info is safe
+//	}
+//
+// The -reuse flag accepts the name of file containing the JSON output of a
+// previous 'go mod download -json' invocation. The go command may use this
+// file to determine that a module is unchanged since the previous invocation
+// and avoid redownloading it. Modules that are not redownloaded will be marked
+// in the new output by setting the Reuse field to true. Normally the module
+// cache provides this kind of reuse automatically; the -reuse flag can be
+// useful on systems that do not preserve the module cache.
+//
+// The -x flag causes download to print the commands download executes.
+//
+// See https://golang.org/ref/mod#go-mod-download for more about 'go mod download'.
+//
+// See https://golang.org/ref/mod#version-queries for more about version queries.
+//
+// # Edit go.mod from tools or scripts
+//
+// Usage:
+//
+//	go mod edit [editing flags] [-fmt|-print|-json] [go.mod]
+//
+// Edit provides a command-line interface for editing go.mod,
+// for use primarily by tools or scripts. It reads only go.mod;
+// it does not look up information about the modules involved.
+// By default, edit reads and writes the go.mod file of the main module,
+// but a different target file can be specified after the editing flags.
+//
+// The editing flags specify a sequence of editing operations.
+//
+// The -fmt flag reformats the go.mod file without making other changes.
+// This reformatting is also implied by any other modifications that use or
+// rewrite the go.mod file. The only time this flag is needed is if no other
+// flags are specified, as in 'go mod edit -fmt'.
+//
+// The -module flag changes the module's path (the go.mod file's module line).
+//
+// The -require=path@version and -droprequire=path flags
+// add and drop a requirement on the given module path and version.
+// Note that -require overrides any existing requirements on path.
+// These flags are mainly for tools that understand the module graph.
+// Users should prefer 'go get path@version' or 'go get path@none',
+// which make other go.mod adjustments as needed to satisfy
+// constraints imposed by other modules.
+//
+// The -exclude=path@version and -dropexclude=path@version flags
+// add and drop an exclusion for the given module path and version.
+// Note that -exclude=path@version is a no-op if that exclusion already exists.
+//
+// The -replace=old[@v]=new[@v] flag adds a replacement of the given
+// module path and version pair. If the @v in old@v is omitted, a
+// replacement without a version on the left side is added, which applies
+// to all versions of the old module path. If the @v in new@v is omitted,
+// the new path should be a local module root directory, not a module
+// path. Note that -replace overrides any redundant replacements for old[@v],
+// so omitting @v will drop existing replacements for specific versions.
+//
+// The -dropreplace=old[@v] flag drops a replacement of the given
+// module path and version pair. If the @v is omitted, a replacement without
+// a version on the left side is dropped.
+//
+// The -retract=version and -dropretract=version flags add and drop a
+// retraction on the given version. The version may be a single version
+// like "v1.2.3" or a closed interval like "[v1.1.0,v1.1.9]". Note that
+// -retract=version is a no-op if that retraction already exists.
+//
+// The -require, -droprequire, -exclude, -dropexclude, -replace,
+// -dropreplace, -retract, and -dropretract editing flags may be repeated,
+// and the changes are applied in the order given.
+//
+// The -go=version flag sets the expected Go language version.
+//
+// The -toolchain=name flag sets the Go toolchain to use.
+//
+// The -print flag prints the final go.mod in its text format instead of
+// writing it back to go.mod.
+//
+// The -json flag prints the final go.mod file in JSON format instead of
+// writing it back to go.mod. The JSON output corresponds to these Go types:
+//
+//	type Module struct {
+//		Path    string
+//		Version string
+//	}
+//
+//	type GoMod struct {
+//		Module    ModPath
+//		Go        string
+//		Toolchain string
+//		Require   []Require
+//		Exclude   []Module
+//		Replace   []Replace
+//		Retract   []Retract
+//	}
+//
+//	type ModPath struct {
+//		Path       string
+//		Deprecated string
+//	}
+//
+//	type Require struct {
+//		Path string
+//		Version string
+//		Indirect bool
+//	}
+//
+//	type Replace struct {
+//		Old Module
+//		New Module
+//	}
+//
+//	type Retract struct {
+//		Low       string
+//		High      string
+//		Rationale string
+//	}
+//
+// Retract entries representing a single version (not an interval) will have
+// the "Low" and "High" fields set to the same value.
+//
+// Note that this only describes the go.mod file itself, not other modules
+// referred to indirectly. For the full set of modules available to a build,
+// use 'go list -m -json all'.
+//
+// Edit also provides the -C, -n, and -x build flags.
+//
+// See https://golang.org/ref/mod#go-mod-edit for more about 'go mod edit'.
+//
+// # Print module requirement graph
+//
+// Usage:
+//
+//	go mod graph [-go=version] [-x]
+//
+// Graph prints the module requirement graph (with replacements applied)
+// in text form. Each line in the output has two space-separated fields: a module
+// and one of its requirements. Each module is identified as a string of the form
+// path@version, except for the main module, which has no @version suffix.
+//
+// The -go flag causes graph to report the module graph as loaded by the
+// given Go version, instead of the version indicated by the 'go' directive
+// in the go.mod file.
+//
+// The -x flag causes graph to print the commands graph executes.
+//
+// See https://golang.org/ref/mod#go-mod-graph for more about 'go mod graph'.
+//
+// # Initialize new module in current directory
+//
+// Usage:
+//
+//	go mod init [module-path]
+//
+// Init initializes and writes a new go.mod file in the current directory, in
+// effect creating a new module rooted at the current directory. The go.mod file
+// must not already exist.
+//
+// Init accepts one optional argument, the module path for the new module. If the
+// module path argument is omitted, init will attempt to infer the module path
+// using import comments in .go files, vendoring tool configuration files (like
+// Gopkg.lock), and the current directory (if in GOPATH).
+//
+// If a configuration file for a vendoring tool is present, init will attempt to
+// import module requirements from it.
+//
+// See https://golang.org/ref/mod#go-mod-init for more about 'go mod init'.
+//
+// # Add missing and remove unused modules
+//
+// Usage:
+//
+//	go mod tidy [-e] [-v] [-x] [-go=version] [-compat=version]
+//
+// Tidy makes sure go.mod matches the source code in the module.
+// It adds any missing modules necessary to build the current module's
+// packages and dependencies, and it removes unused modules that
+// don't provide any relevant packages. It also adds any missing entries
+// to go.sum and removes any unnecessary ones.
+//
+// The -v flag causes tidy to print information about removed modules
+// to standard error.
+//
+// The -e flag causes tidy to attempt to proceed despite errors
+// encountered while loading packages.
+//
+// The -go flag causes tidy to update the 'go' directive in the go.mod
+// file to the given version, which may change which module dependencies
+// are retained as explicit requirements in the go.mod file.
+// (Go versions 1.17 and higher retain more requirements in order to
+// support lazy module loading.)
+//
+// The -compat flag preserves any additional checksums needed for the
+// 'go' command from the indicated major Go release to successfully load
+// the module graph, and causes tidy to error out if that version of the
+// 'go' command would load any imported package from a different module
+// version. By default, tidy acts as if the -compat flag were set to the
+// version prior to the one indicated by the 'go' directive in the go.mod
+// file.
+//
+// The -x flag causes tidy to print the commands download executes.
+//
+// See https://golang.org/ref/mod#go-mod-tidy for more about 'go mod tidy'.
+//
+// # Make vendored copy of dependencies
+//
+// Usage:
+//
+//	go mod vendor [-e] [-v] [-o outdir]
+//
+// Vendor resets the main module's vendor directory to include all packages
+// needed to build and test all the main module's packages.
+// It does not include test code for vendored packages.
+//
+// The -v flag causes vendor to print the names of vendored
+// modules and packages to standard error.
+//
+// The -e flag causes vendor to attempt to proceed despite errors
+// encountered while loading packages.
+//
+// The -o flag causes vendor to create the vendor directory at the given
+// path instead of "vendor". The go command can only use a vendor directory
+// named "vendor" within the module root directory, so this flag is
+// primarily useful for other tools.
+//
+// See https://golang.org/ref/mod#go-mod-vendor for more about 'go mod vendor'.
+//
+// # Verify dependencies have expected content
+//
+// Usage:
+//
+//	go mod verify
+//
+// Verify checks that the dependencies of the current module,
+// which are stored in a local downloaded source cache, have not been
+// modified since being downloaded. If all the modules are unmodified,
+// verify prints "all modules verified." Otherwise it reports which
+// modules have been changed and causes 'go mod' to exit with a
+// non-zero status.
+//
+// See https://golang.org/ref/mod#go-mod-verify for more about 'go mod verify'.
+//
+// # Explain why packages or modules are needed
+//
+// Usage:
+//
+//	go mod why [-m] [-vendor] packages...
+//
+// Why shows a shortest path in the import graph from the main module to
+// each of the listed packages. If the -m flag is given, why treats the
+// arguments as a list of modules and finds a path to any package in each
+// of the modules.
+//
+// By default, why queries the graph of packages matched by "go list all",
+// which includes tests for reachable packages. The -vendor flag causes why
+// to exclude tests of dependencies.
+//
+// The output is a sequence of stanzas, one for each package or module
+// name on the command line, separated by blank lines. Each stanza begins
+// with a comment line "# package" or "# module" giving the target
+// package or module. Subsequent lines give a path through the import
+// graph, one package per line. If the package or module is not
+// referenced from the main module, the stanza will display a single
+// parenthesized note indicating that fact.
+//
+// For example:
+//
+//	$ go mod why golang.org/x/text/language golang.org/x/text/encoding
+//	# golang.org/x/text/language
+//	rsc.io/quote
+//	rsc.io/sampler
+//	golang.org/x/text/language
+//
+//	# golang.org/x/text/encoding
+//	(main module does not need package golang.org/x/text/encoding)
+//	$
+//
+// See https://golang.org/ref/mod#go-mod-why for more about 'go mod why'.
+//
+// # Workspace maintenance
+//
+// Work provides access to operations on workspaces.
+//
+// Note that support for workspaces is built into many other commands, not
+// just 'go work'.
+//
+// See 'go help modules' for information about Go's module system of which
+// workspaces are a part.
+//
+// See https://go.dev/ref/mod#workspaces for an in-depth reference on
+// workspaces.
+//
+// See https://go.dev/doc/tutorial/workspaces for an introductory
+// tutorial on workspaces.
+//
+// A workspace is specified by a go.work file that specifies a set of
+// module directories with the "use" directive. These modules are used as
+// root modules by the go command for builds and related operations.  A
+// workspace that does not specify modules to be used cannot be used to do
+// builds from local modules.
+//
+// go.work files are line-oriented. Each line holds a single directive,
+// made up of a keyword followed by arguments. For example:
+//
+//	go 1.18
+//
+//	use ../foo/bar
+//	use ./baz
+//
+//	replace example.com/foo v1.2.3 => example.com/bar v1.4.5
+//
+// The leading keyword can be factored out of adjacent lines to create a block,
+// like in Go imports.
+//
+//	use (
+//	  ../foo/bar
+//	  ./baz
+//	)
+//
+// The use directive specifies a module to be included in the workspace's
+// set of main modules. The argument to the use directive is the directory
+// containing the module's go.mod file.
+//
+// The go directive specifies the version of Go the file was written at. It
+// is possible there may be future changes in the semantics of workspaces
+// that could be controlled by this version, but for now the version
+// specified has no effect.
+//
+// The replace directive has the same syntax as the replace directive in a
+// go.mod file and takes precedence over replaces in go.mod files.  It is
+// primarily intended to override conflicting replaces in different workspace
+// modules.
+//
+// To determine whether the go command is operating in workspace mode, use
+// the "go env GOWORK" command. This will specify the workspace file being
+// used.
+//
+// Usage:
+//
+//	go work <command> [arguments]
+//
+// The commands are:
+//
+//	edit        edit go.work from tools or scripts
+//	init        initialize workspace file
+//	sync        sync workspace build list to modules
+//	use         add modules to workspace file
+//
+// Use "go help work <command>" for more information about a command.
+//
+// # Edit go.work from tools or scripts
+//
+// Usage:
+//
+//	go work edit [editing flags] [go.work]
+//
+// Edit provides a command-line interface for editing go.work,
+// for use primarily by tools or scripts. It only reads go.work;
+// it does not look up information about the modules involved.
+// If no file is specified, Edit looks for a go.work file in the current
+// directory and its parent directories
+//
+// The editing flags specify a sequence of editing operations.
+//
+// The -fmt flag reformats the go.work file without making other changes.
+// This reformatting is also implied by any other modifications that use or
+// rewrite the go.mod file. The only time this flag is needed is if no other
+// flags are specified, as in 'go work edit -fmt'.
+//
+// The -use=path and -dropuse=path flags
+// add and drop a use directive from the go.work file's set of module directories.
+//
+// The -replace=old[@v]=new[@v] flag adds a replacement of the given
+// module path and version pair. If the @v in old@v is omitted, a
+// replacement without a version on the left side is added, which applies
+// to all versions of the old module path. If the @v in new@v is omitted,
+// the new path should be a local module root directory, not a module
+// path. Note that -replace overrides any redundant replacements for old[@v],
+// so omitting @v will drop existing replacements for specific versions.
+//
+// The -dropreplace=old[@v] flag drops a replacement of the given
+// module path and version pair. If the @v is omitted, a replacement without
+// a version on the left side is dropped.
+//
+// The -use, -dropuse, -replace, and -dropreplace,
+// editing flags may be repeated, and the changes are applied in the order given.
+//
+// The -go=version flag sets the expected Go language version.
+//
+// The -toolchain=name flag sets the Go toolchain to use.
+//
+// The -print flag prints the final go.work in its text format instead of
+// writing it back to go.mod.
+//
+// The -json flag prints the final go.work file in JSON format instead of
+// writing it back to go.mod. The JSON output corresponds to these Go types:
+//
+//	type GoWork struct {
+//		Go        string
+//		Toolchain string
+//		Use       []Use
+//		Replace   []Replace
+//	}
+//
+//	type Use struct {
+//		DiskPath   string
+//		ModulePath string
+//	}
+//
+//	type Replace struct {
+//		Old Module
+//		New Module
+//	}
+//
+//	type Module struct {
+//		Path    string
+//		Version string
+//	}
+//
+// See the workspaces reference at https://go.dev/ref/mod#workspaces
+// for more information.
+//
+// # Initialize workspace file
+//
+// Usage:
+//
+//	go work init [moddirs]
+//
+// Init initializes and writes a new go.work file in the
+// current directory, in effect creating a new workspace at the current
+// directory.
+//
+// go work init optionally accepts paths to the workspace modules as
+// arguments. If the argument is omitted, an empty workspace with no
+// modules will be created.
+//
+// Each argument path is added to a use directive in the go.work file. The
+// current go version will also be listed in the go.work file.
+//
+// See the workspaces reference at https://go.dev/ref/mod#workspaces
+// for more information.
+//
+// # Sync workspace build list to modules
+//
+// Usage:
+//
+//	go work sync
+//
+// Sync syncs the workspace's build list back to the
+// workspace's modules
+//
+// The workspace's build list is the set of versions of all the
+// (transitive) dependency modules used to do builds in the workspace. go
+// work sync generates that build list using the Minimal Version Selection
+// algorithm, and then syncs those versions back to each of modules
+// specified in the workspace (with use directives).
+//
+// The syncing is done by sequentially upgrading each of the dependency
+// modules specified in a workspace module to the version in the build list
+// if the dependency module's version is not already the same as the build
+// list's version. Note that Minimal Version Selection guarantees that the
+// build list's version of each module is always the same or higher than
+// that in each workspace module.
+//
+// See the workspaces reference at https://go.dev/ref/mod#workspaces
+// for more information.
+//
+// # Add modules to workspace file
+//
+// Usage:
+//
+//	go work use [-r] [moddirs]
+//
+// Use provides a command-line interface for adding
+// directories, optionally recursively, to a go.work file.
+//
+// A use directive will be added to the go.work file for each argument
+// directory listed on the command line go.work file, if it exists,
+// or removed from the go.work file if it does not exist.
+// Use fails if any remaining use directives refer to modules that
+// do not exist.
+//
+// Use updates the go line in go.work to specify a version at least as
+// new as all the go lines in the used modules, both preexisting ones
+// and newly added ones. With no arguments, this update is the only
+// thing that go work use does.
+//
+// The -r flag searches recursively for modules in the argument
+// directories, and the use command operates as if each of the directories
+// were specified as arguments: namely, use directives will be added for
+// directories that exist, and removed for directories that do not exist.
+//
+// See the workspaces reference at https://go.dev/ref/mod#workspaces
+// for more information.
+//
+// # Compile and run Go program
+//
+// Usage:
+//
+//	go run [build flags] [-exec xprog] package [arguments...]
+//
+// Run compiles and runs the named main Go package.
+// Typically the package is specified as a list of .go source files from a single
+// directory, but it may also be an import path, file system path, or pattern
+// matching a single known package, as in 'go run .' or 'go run my/cmd'.
+//
+// If the package argument has a version suffix (like @latest or @v1.0.0),
+// "go run" builds the program in module-aware mode, ignoring the go.mod file in
+// the current directory or any parent directory, if there is one. This is useful
+// for running programs without affecting the dependencies of the main module.
+//
+// If the package argument doesn't have a version suffix, "go run" may run in
+// module-aware mode or GOPATH mode, depending on the GO111MODULE environment
+// variable and the presence of a go.mod file. See 'go help modules' for details.
+// If module-aware mode is enabled, "go run" runs in the context of the main
+// module.
+//
+// By default, 'go run' runs the compiled binary directly: 'a.out arguments...'.
+// If the -exec flag is given, 'go run' invokes the binary using xprog:
+//
+//	'xprog a.out arguments...'.
+//
+// If the -exec flag is not given, GOOS or GOARCH is different from the system
+// default, and a program named go_$GOOS_$GOARCH_exec can be found
+// on the current search path, 'go run' invokes the binary using that program,
+// for example 'go_js_wasm_exec a.out arguments...'. This allows execution of
+// cross-compiled programs when a simulator or other execution method is
+// available.
+//
+// By default, 'go run' compiles the binary without generating the information
+// used by debuggers, to reduce build time. To include debugger information in
+// the binary, use 'go build'.
+//
+// The exit status of Run is not the exit status of the compiled binary.
+//
+// For more about build flags, see 'go help build'.
+// For more about specifying packages, see 'go help packages'.
+//
+// See also: go build.
+//
+// # Test packages
+//
+// Usage:
+//
+//	go test [build/test flags] [packages] [build/test flags & test binary flags]
+//
+// 'Go test' automates testing the packages named by the import paths.
+// It prints a summary of the test results in the format:
+//
+//	ok   archive/tar   0.011s
+//	FAIL archive/zip   0.022s
+//	ok   compress/gzip 0.033s
+//	...
+//
+// followed by detailed output for each failed package.
+//
+// 'Go test' recompiles each package along with any files with names matching
+// the file pattern "*_test.go".
+// These additional files can contain test functions, benchmark functions, fuzz
+// tests and example functions. See 'go help testfunc' for more.
+// Each listed package causes the execution of a separate test binary.
+// Files whose names begin with "_" (including "_test.go") or "." are ignored.
+//
+// Test files that declare a package with the suffix "_test" will be compiled as a
+// separate package, and then linked and run with the main test binary.
+//
+// The go tool will ignore a directory named "testdata", making it available
+// to hold ancillary data needed by the tests.
+//
+// As part of building a test binary, go test runs go vet on the package
+// and its test source files to identify significant problems. If go vet
+// finds any problems, go test reports those and does not run the test
+// binary. Only a high-confidence subset of the default go vet checks are
+// used. That subset is: atomic, bool, buildtags, directive, errorsas,
+// ifaceassert, nilfunc, printf, and stringintconv. You can see
+// the documentation for these and other vet tests via "go doc cmd/vet".
+// To disable the running of go vet, use the -vet=off flag. To run all
+// checks, use the -vet=all flag.
+//
+// All test output and summary lines are printed to the go command's
+// standard output, even if the test printed them to its own standard
+// error. (The go command's standard error is reserved for printing
+// errors building the tests.)
+//
+// The go command places $GOROOT/bin at the beginning of $PATH
+// in the test's environment, so that tests that execute
+// 'go' commands use the same 'go' as the parent 'go test' command.
+//
+// Go test runs in two different modes:
+//
+// The first, called local directory mode, occurs when go test is
+// invoked with no package arguments (for example, 'go test' or 'go
+// test -v'). In this mode, go test compiles the package sources and
+// tests found in the current directory and then runs the resulting
+// test binary. In this mode, caching (discussed below) is disabled.
+// After the package test finishes, go test prints a summary line
+// showing the test status ('ok' or 'FAIL'), package name, and elapsed
+// time.
+//
+// The second, called package list mode, occurs when go test is invoked
+// with explicit package arguments (for example 'go test math', 'go
+// test ./...', and even 'go test .'). In this mode, go test compiles
+// and tests each of the packages listed on the command line. If a
+// package test passes, go test prints only the final 'ok' summary
+// line. If a package test fails, go test prints the full test output.
+// If invoked with the -bench or -v flag, go test prints the full
+// output even for passing package tests, in order to display the
+// requested benchmark results or verbose logging. After the package
+// tests for all of the listed packages finish, and their output is
+// printed, go test prints a final 'FAIL' status if any package test
+// has failed.
+//
+// In package list mode only, go test caches successful package test
+// results to avoid unnecessary repeated running of tests. When the
+// result of a test can be recovered from the cache, go test will
+// redisplay the previous output instead of running the test binary
+// again. When this happens, go test prints '(cached)' in place of the
+// elapsed time in the summary line.
+//
+// The rule for a match in the cache is that the run involves the same
+// test binary and the flags on the command line come entirely from a
+// restricted set of 'cacheable' test flags, defined as -benchtime, -cpu,
+// -list, -parallel, -run, -short, -timeout, -failfast, and -v.
+// If a run of go test has any test or non-test flags outside this set,
+// the result is not cached. To disable test caching, use any test flag
+// or argument other than the cacheable flags. The idiomatic way to disable
+// test caching explicitly is to use -count=1. Tests that open files within
+// the package's source root (usually $GOPATH) or that consult environment
+// variables only match future runs in which the files and environment
+// variables are unchanged. A cached test result is treated as executing
+// in no time at all, so a successful package test result will be cached and
+// reused regardless of -timeout setting.
+//
+// In addition to the build flags, the flags handled by 'go test' itself are:
+//
+//	-args
+//	    Pass the remainder of the command line (everything after -args)
+//	    to the test binary, uninterpreted and unchanged.
+//	    Because this flag consumes the remainder of the command line,
+//	    the package list (if present) must appear before this flag.
+//
+//	-c
+//	    Compile the test binary to pkg.test in the current directory but do not run it
+//	    (where pkg is the last element of the package's import path).
+//	    The file name or target directory can be changed with the -o flag.
+//
+//	-exec xprog
+//	    Run the test binary using xprog. The behavior is the same as
+//	    in 'go run'. See 'go help run' for details.
+//
+//	-json
+//	    Convert test output to JSON suitable for automated processing.
+//	    See 'go doc test2json' for the encoding details.
+//
+//	-o file
+//	    Compile the test binary to the named file.
+//	    The test still runs (unless -c or -i is specified).
+//	    If file ends in a slash or names an existing directory,
+//	    the test is written to pkg.test in that directory.
+//
+// The test binary also accepts flags that control execution of the test; these
+// flags are also accessible by 'go test'. See 'go help testflag' for details.
+//
+// For more about build flags, see 'go help build'.
+// For more about specifying packages, see 'go help packages'.
+//
+// See also: go build, go vet.
+//
+// # Run specified go tool
+//
+// Usage:
+//
+//	go tool [-n] command [args...]
+//
+// Tool runs the go tool command identified by the arguments.
+// With no arguments it prints the list of known tools.
+//
+// The -n flag causes tool to print the command that would be
+// executed but not execute it.
+//
+// For more about each tool command, see 'go doc cmd/<command>'.
+//
+// # Print Go version
+//
+// Usage:
+//
+//	go version [-m] [-v] [file ...]
+//
+// Version prints the build information for Go binary files.
+//
+// Go version reports the Go version used to build each of the named files.
+//
+// If no files are named on the command line, go version prints its own
+// version information.
+//
+// If a directory is named, go version walks that directory, recursively,
+// looking for recognized Go binaries and reporting their versions.
+// By default, go version does not report unrecognized files found
+// during a directory scan. The -v flag causes it to report unrecognized files.
+//
+// The -m flag causes go version to print each file's embedded
+// module version information, when available. In the output, the module
+// information consists of multiple lines following the version line, each
+// indented by a leading tab character.
+//
+// See also: go doc runtime/debug.BuildInfo.
+//
+// # Report likely mistakes in packages
+//
+// Usage:
+//
+//	go vet [build flags] [-vettool prog] [vet flags] [packages]
+//
+// Vet runs the Go vet command on the packages named by the import paths.
+//
+// For more about vet and its flags, see 'go doc cmd/vet'.
+// For more about specifying packages, see 'go help packages'.
+// For a list of checkers and their flags, see 'go tool vet help'.
+// For details of a specific checker such as 'printf', see 'go tool vet help printf'.
+//
+// The -vettool=prog flag selects a different analysis tool with alternative
+// or additional checks.
+// For example, the 'shadow' analyzer can be built and run using these commands:
+//
+//	go install golang.org/x/tools/go/analysis/passes/shadow/cmd/shadow@latest
+//	go vet -vettool=$(which shadow)
+//
+// The build flags supported by go vet are those that control package resolution
+// and execution, such as -C, -n, -x, -v, -tags, and -toolexec.
+// For more about these flags, see 'go help build'.
+//
+// See also: go fmt, go fix.
+//
+// # Build constraints
+//
+// A build constraint, also known as a build tag, is a condition under which a
+// file should be included in the package. Build constraints are given by a
+// line comment that begins
+//
+//	//go:build
+//
+// Constraints may appear in any kind of source file (not just Go), but
+// they must appear near the top of the file, preceded
+// only by blank lines and other line comments. These rules mean that in Go
+// files a build constraint must appear before the package clause.
+//
+// To distinguish build constraints from package documentation,
+// a build constraint should be followed by a blank line.
+//
+// A build constraint comment is evaluated as an expression containing
+// build tags combined by ||, &&, and ! operators and parentheses.
+// Operators have the same meaning as in Go.
+//
+// For example, the following build constraint constrains a file to
+// build when the "linux" and "386" constraints are satisfied, or when
+// "darwin" is satisfied and "cgo" is not:
+//
+//	//go:build (linux && 386) || (darwin && !cgo)
+//
+// It is an error for a file to have more than one //go:build line.
+//
+// During a particular build, the following build tags are satisfied:
+//
+//   - the target operating system, as spelled by runtime.GOOS, set with the
+//     GOOS environment variable.
+//   - the target architecture, as spelled by runtime.GOARCH, set with the
+//     GOARCH environment variable.
+//   - any architecture features, in the form GOARCH.feature
+//     (for example, "amd64.v2"), as detailed below.
+//   - "unix", if GOOS is a Unix or Unix-like system.
+//   - the compiler being used, either "gc" or "gccgo"
+//   - "cgo", if the cgo command is supported (see CGO_ENABLED in
+//     'go help environment').
+//   - a term for each Go major release, through the current version:
+//     "go1.1" from Go version 1.1 onward, "go1.12" from Go 1.12, and so on.
+//   - any additional tags given by the -tags flag (see 'go help build').
+//
+// There are no separate build tags for beta or minor releases.
+//
+// If a file's name, after stripping the extension and a possible _test suffix,
+// matches any of the following patterns:
+//
+//	*_GOOS
+//	*_GOARCH
+//	*_GOOS_GOARCH
+//
+// (example: source_windows_amd64.go) where GOOS and GOARCH represent
+// any known operating system and architecture values respectively, then
+// the file is considered to have an implicit build constraint requiring
+// those terms (in addition to any explicit constraints in the file).
+//
+// Using GOOS=android matches build tags and files as for GOOS=linux
+// in addition to android tags and files.
+//
+// Using GOOS=illumos matches build tags and files as for GOOS=solaris
+// in addition to illumos tags and files.
+//
+// Using GOOS=ios matches build tags and files as for GOOS=darwin
+// in addition to ios tags and files.
+//
+// The defined architecture feature build tags are:
+//
+//   - For GOARCH=386, GO386=387 and GO386=sse2
+//     set the 386.387 and 386.sse2 build tags, respectively.
+//   - For GOARCH=amd64, GOAMD64=v1, v2, and v3
+//     correspond to the amd64.v1, amd64.v2, and amd64.v3 feature build tags.
+//   - For GOARCH=arm, GOARM=5, 6, and 7
+//     correspond to the arm.5, arm.6, and arm.7 feature build tags.
+//   - For GOARCH=mips or mipsle,
+//     GOMIPS=hardfloat and softfloat
+//     correspond to the mips.hardfloat and mips.softfloat
+//     (or mipsle.hardfloat and mipsle.softfloat) feature build tags.
+//   - For GOARCH=mips64 or mips64le,
+//     GOMIPS64=hardfloat and softfloat
+//     correspond to the mips64.hardfloat and mips64.softfloat
+//     (or mips64le.hardfloat and mips64le.softfloat) feature build tags.
+//   - For GOARCH=ppc64 or ppc64le,
+//     GOPPC64=power8, power9, and power10 correspond to the
+//     ppc64.power8, ppc64.power9, and ppc64.power10
+//     (or ppc64le.power8, ppc64le.power9, and ppc64le.power10)
+//     feature build tags.
+//   - For GOARCH=wasm, GOWASM=satconv and signext
+//     correspond to the wasm.satconv and wasm.signext feature build tags.
+//
+// For GOARCH=amd64, arm, ppc64, and ppc64le, a particular feature level
+// sets the feature build tags for all previous levels as well.
+// For example, GOAMD64=v2 sets the amd64.v1 and amd64.v2 feature flags.
+// This ensures that code making use of v2 features continues to compile
+// when, say, GOAMD64=v4 is introduced.
+// Code handling the absence of a particular feature level
+// should use a negation:
+//
+//	//go:build !amd64.v2
+//
+// To keep a file from being considered for any build:
+//
+//	//go:build ignore
+//
+// (Any other unsatisfied word will work as well, but "ignore" is conventional.)
+//
+// To build a file only when using cgo, and only on Linux and OS X:
+//
+//	//go:build cgo && (linux || darwin)
+//
+// Such a file is usually paired with another file implementing the
+// default functionality for other systems, which in this case would
+// carry the constraint:
+//
+//	//go:build !(cgo && (linux || darwin))
+//
+// Naming a file dns_windows.go will cause it to be included only when
+// building the package for Windows; similarly, math_386.s will be included
+// only when building the package for 32-bit x86.
+//
+// Go versions 1.16 and earlier used a different syntax for build constraints,
+// with a "// +build" prefix. The gofmt command will add an equivalent //go:build
+// constraint when encountering the older syntax.
+//
+// # Build modes
+//
+// The 'go build' and 'go install' commands take a -buildmode argument which
+// indicates which kind of object file is to be built. Currently supported values
+// are:
+//
+//	-buildmode=archive
+//		Build the listed non-main packages into .a files. Packages named
+//		main are ignored.
+//
+//	-buildmode=c-archive
+//		Build the listed main package, plus all packages it imports,
+//		into a C archive file. The only callable symbols will be those
+//		functions exported using a cgo //export comment. Requires
+//		exactly one main package to be listed.
+//
+//	-buildmode=c-shared
+//		Build the listed main package, plus all packages it imports,
+//		into a C shared library. The only callable symbols will
+//		be those functions exported using a cgo //export comment.
+//		Requires exactly one main package to be listed.
+//
+//	-buildmode=default
+//		Listed main packages are built into executables and listed
+//		non-main packages are built into .a files (the default
+//		behavior).
+//
+//	-buildmode=shared
+//		Combine all the listed non-main packages into a single shared
+//		library that will be used when building with the -linkshared
+//		option. Packages named main are ignored.
+//
+//	-buildmode=exe
+//		Build the listed main packages and everything they import into
+//		executables. Packages not named main are ignored.
+//
+//	-buildmode=pie
+//		Build the listed main packages and everything they import into
+//		position independent executables (PIE). Packages not named
+//		main are ignored.
+//
+//	-buildmode=plugin
+//		Build the listed main packages, plus all packages that they
+//		import, into a Go plugin. Packages not named main are ignored.
+//
+// On AIX, when linking a C program that uses a Go archive built with
+// -buildmode=c-archive, you must pass -Wl,-bnoobjreorder to the C compiler.
+//
+// # Calling between Go and C
+//
+// There are two different ways to call between Go and C/C++ code.
+//
+// The first is the cgo tool, which is part of the Go distribution. For
+// information on how to use it see the cgo documentation (go doc cmd/cgo).
+//
+// The second is the SWIG program, which is a general tool for
+// interfacing between languages. For information on SWIG see
+// http://swig.org/. When running go build, any file with a .swig
+// extension will be passed to SWIG. Any file with a .swigcxx extension
+// will be passed to SWIG with the -c++ option.
+//
+// When either cgo or SWIG is used, go build will pass any .c, .m, .s, .S
+// or .sx files to the C compiler, and any .cc, .cpp, .cxx files to the C++
+// compiler. The CC or CXX environment variables may be set to determine
+// the C or C++ compiler, respectively, to use.
+//
+// # Build and test caching
+//
+// The go command caches build outputs for reuse in future builds.
+// The default location for cache data is a subdirectory named go-build
+// in the standard user cache directory for the current operating system.
+// Setting the GOCACHE environment variable overrides this default,
+// and running 'go env GOCACHE' prints the current cache directory.
+//
+// The go command periodically deletes cached data that has not been
+// used recently. Running 'go clean -cache' deletes all cached data.
+//
+// The build cache correctly accounts for changes to Go source files,
+// compilers, compiler options, and so on: cleaning the cache explicitly
+// should not be necessary in typical use. However, the build cache
+// does not detect changes to C libraries imported with cgo.
+// If you have made changes to the C libraries on your system, you
+// will need to clean the cache explicitly or else use the -a build flag
+// (see 'go help build') to force rebuilding of packages that
+// depend on the updated C libraries.
+//
+// The go command also caches successful package test results.
+// See 'go help test' for details. Running 'go clean -testcache' removes
+// all cached test results (but not cached build results).
+//
+// The go command also caches values used in fuzzing with 'go test -fuzz',
+// specifically, values that expanded code coverage when passed to a
+// fuzz function. These values are not used for regular building and
+// testing, but they're stored in a subdirectory of the build cache.
+// Running 'go clean -fuzzcache' removes all cached fuzzing values.
+// This may make fuzzing less effective, temporarily.
+//
+// The GODEBUG environment variable can enable printing of debugging
+// information about the state of the cache:
+//
+// GODEBUG=gocacheverify=1 causes the go command to bypass the
+// use of any cache entries and instead rebuild everything and check
+// that the results match existing cache entries.
+//
+// GODEBUG=gocachehash=1 causes the go command to print the inputs
+// for all of the content hashes it uses to construct cache lookup keys.
+// The output is voluminous but can be useful for debugging the cache.
+//
+// GODEBUG=gocachetest=1 causes the go command to print details of its
+// decisions about whether to reuse a cached test result.
+//
+// # Environment variables
+//
+// The go command and the tools it invokes consult environment variables
+// for configuration. If an environment variable is unset or empty, the go
+// command uses a sensible default setting. To see the effective setting of
+// the variable <NAME>, run 'go env <NAME>'. To change the default setting,
+// run 'go env -w <NAME>=<VALUE>'. Defaults changed using 'go env -w'
+// are recorded in a Go environment configuration file stored in the
+// per-user configuration directory, as reported by os.UserConfigDir.
+// The location of the configuration file can be changed by setting
+// the environment variable GOENV, and 'go env GOENV' prints the
+// effective location, but 'go env -w' cannot change the default location.
+// See 'go help env' for details.
+//
+// General-purpose environment variables:
+//
+//	GO111MODULE
+//		Controls whether the go command runs in module-aware mode or GOPATH mode.
+//		May be "off", "on", or "auto".
+//		See https://golang.org/ref/mod#mod-commands.
+//	GCCGO
+//		The gccgo command to run for 'go build -compiler=gccgo'.
+//	GOARCH
+//		The architecture, or processor, for which to compile code.
+//		Examples are amd64, 386, arm, ppc64.
+//	GOBIN
+//		The directory where 'go install' will install a command.
+//	GOCACHE
+//		The directory where the go command will store cached
+//		information for reuse in future builds.
+//	GOMODCACHE
+//		The directory where the go command will store downloaded modules.
+//	GODEBUG
+//		Enable various debugging facilities. See https://go.dev/doc/godebug
+//		for details.
+//	GOENV
+//		The location of the Go environment configuration file.
+//		Cannot be set using 'go env -w'.
+//		Setting GOENV=off in the environment disables the use of the
+//		default configuration file.
+//	GOFLAGS
+//		A space-separated list of -flag=value settings to apply
+//		to go commands by default, when the given flag is known by
+//		the current command. Each entry must be a standalone flag.
+//		Because the entries are space-separated, flag values must
+//		not contain spaces. Flags listed on the command line
+//		are applied after this list and therefore override it.
+//	GOINSECURE
+//		Comma-separated list of glob patterns (in the syntax of Go's path.Match)
+//		of module path prefixes that should always be fetched in an insecure
+//		manner. Only applies to dependencies that are being fetched directly.
+//		GOINSECURE does not disable checksum database validation. GOPRIVATE or
+//		GONOSUMDB may be used to achieve that.
+//	GOOS
+//		The operating system for which to compile code.
+//		Examples are linux, darwin, windows, netbsd.
+//	GOPATH
+//		Controls where various files are stored. See: 'go help gopath'.
+//	GOPROXY
+//		URL of Go module proxy. See https://golang.org/ref/mod#environment-variables
+//		and https://golang.org/ref/mod#module-proxy for details.
+//	GOPRIVATE, GONOPROXY, GONOSUMDB
+//		Comma-separated list of glob patterns (in the syntax of Go's path.Match)
+//		of module path prefixes that should always be fetched directly
+//		or that should not be compared against the checksum database.
+//		See https://golang.org/ref/mod#private-modules.
+//	GOROOT
+//		The root of the go tree.
+//	GOSUMDB
+//		The name of checksum database to use and optionally its public key and
+//		URL. See https://golang.org/ref/mod#authenticating.
+//	GOTOOLCHAIN
+//		Controls which Go toolchain is used. See https://go.dev/doc/toolchain.
+//	GOTMPDIR
+//		The directory where the go command will write
+//		temporary source files, packages, and binaries.
+//	GOVCS
+//		Lists version control commands that may be used with matching servers.
+//		See 'go help vcs'.
+//	GOWORK
+//		In module aware mode, use the given go.work file as a workspace file.
+//		By default or when GOWORK is "auto", the go command searches for a
+//		file named go.work in the current directory and then containing directories
+//		until one is found. If a valid go.work file is found, the modules
+//		specified will collectively be used as the main modules. If GOWORK
+//		is "off", or a go.work file is not found in "auto" mode, workspace
+//		mode is disabled.
+//
+// Environment variables for use with cgo:
+//
+//	AR
+//		The command to use to manipulate library archives when
+//		building with the gccgo compiler.
+//		The default is 'ar'.
+//	CC
+//		The command to use to compile C code.
+//	CGO_ENABLED
+//		Whether the cgo command is supported. Either 0 or 1.
+//	CGO_CFLAGS
+//		Flags that cgo will pass to the compiler when compiling
+//		C code.
+//	CGO_CFLAGS_ALLOW
+//		A regular expression specifying additional flags to allow
+//		to appear in #cgo CFLAGS source code directives.
+//		Does not apply to the CGO_CFLAGS environment variable.
+//	CGO_CFLAGS_DISALLOW
+//		A regular expression specifying flags that must be disallowed
+//		from appearing in #cgo CFLAGS source code directives.
+//		Does not apply to the CGO_CFLAGS environment variable.
+//	CGO_CPPFLAGS, CGO_CPPFLAGS_ALLOW, CGO_CPPFLAGS_DISALLOW
+//		Like CGO_CFLAGS, CGO_CFLAGS_ALLOW, and CGO_CFLAGS_DISALLOW,
+//		but for the C preprocessor.
+//	CGO_CXXFLAGS, CGO_CXXFLAGS_ALLOW, CGO_CXXFLAGS_DISALLOW
+//		Like CGO_CFLAGS, CGO_CFLAGS_ALLOW, and CGO_CFLAGS_DISALLOW,
+//		but for the C++ compiler.
+//	CGO_FFLAGS, CGO_FFLAGS_ALLOW, CGO_FFLAGS_DISALLOW
+//		Like CGO_CFLAGS, CGO_CFLAGS_ALLOW, and CGO_CFLAGS_DISALLOW,
+//		but for the Fortran compiler.
+//	CGO_LDFLAGS, CGO_LDFLAGS_ALLOW, CGO_LDFLAGS_DISALLOW
+//		Like CGO_CFLAGS, CGO_CFLAGS_ALLOW, and CGO_CFLAGS_DISALLOW,
+//		but for the linker.
+//	CXX
+//		The command to use to compile C++ code.
+//	FC
+//		The command to use to compile Fortran code.
+//	PKG_CONFIG
+//		Path to pkg-config tool.
+//
+// Architecture-specific environment variables:
+//
+//	GOARM
+//		For GOARCH=arm, the ARM architecture for which to compile.
+//		Valid values are 5, 6, 7.
+//	GO386
+//		For GOARCH=386, how to implement floating point instructions.
+//		Valid values are sse2 (default), softfloat.
+//	GOAMD64
+//		For GOARCH=amd64, the microarchitecture level for which to compile.
+//		Valid values are v1 (default), v2, v3, v4.
+//		See https://golang.org/wiki/MinimumRequirements#amd64
+//	GOMIPS
+//		For GOARCH=mips{,le}, whether to use floating point instructions.
+//		Valid values are hardfloat (default), softfloat.
+//	GOMIPS64
+//		For GOARCH=mips64{,le}, whether to use floating point instructions.
+//		Valid values are hardfloat (default), softfloat.
+//	GOPPC64
+//		For GOARCH=ppc64{,le}, the target ISA (Instruction Set Architecture).
+//		Valid values are power8 (default), power9, power10.
+//	GOWASM
+//		For GOARCH=wasm, comma-separated list of experimental WebAssembly features to use.
+//		Valid values are satconv, signext.
+//
+// Environment variables for use with code coverage:
+//
+//	GOCOVERDIR
+//		Directory into which to write code coverage data files
+//		generated by running a "go build -cover" binary.
+//		Requires that GOEXPERIMENT=coverageredesign is enabled.
+//
+// Special-purpose environment variables:
+//
+//	GCCGOTOOLDIR
+//		If set, where to find gccgo tools, such as cgo.
+//		The default is based on how gccgo was configured.
+//	GOEXPERIMENT
+//		Comma-separated list of toolchain experiments to enable or disable.
+//		The list of available experiments may change arbitrarily over time.
+//		See src/internal/goexperiment/flags.go for currently valid values.
+//		Warning: This variable is provided for the development and testing
+//		of the Go toolchain itself. Use beyond that purpose is unsupported.
+//	GOROOT_FINAL
+//		The root of the installed Go tree, when it is
+//		installed in a location other than where it is built.
+//		File names in stack traces are rewritten from GOROOT to
+//		GOROOT_FINAL.
+//	GO_EXTLINK_ENABLED
+//		Whether the linker should use external linking mode
+//		when using -linkmode=auto with code that uses cgo.
+//		Set to 0 to disable external linking mode, 1 to enable it.
+//	GIT_ALLOW_PROTOCOL
+//		Defined by Git. A colon-separated list of schemes that are allowed
+//		to be used with git fetch/clone. If set, any scheme not explicitly
+//		mentioned will be considered insecure by 'go get'.
+//		Because the variable is defined by Git, the default value cannot
+//		be set using 'go env -w'.
+//
+// Additional information available from 'go env' but not read from the environment:
+//
+//	GOEXE
+//		The executable file name suffix (".exe" on Windows, "" on other systems).
+//	GOGCCFLAGS
+//		A space-separated list of arguments supplied to the CC command.
+//	GOHOSTARCH
+//		The architecture (GOARCH) of the Go toolchain binaries.
+//	GOHOSTOS
+//		The operating system (GOOS) of the Go toolchain binaries.
+//	GOMOD
+//		The absolute path to the go.mod of the main module.
+//		If module-aware mode is enabled, but there is no go.mod, GOMOD will be
+//		os.DevNull ("/dev/null" on Unix-like systems, "NUL" on Windows).
+//		If module-aware mode is disabled, GOMOD will be the empty string.
+//	GOTOOLDIR
+//		The directory where the go tools (compile, cover, doc, etc...) are installed.
+//	GOVERSION
+//		The version of the installed Go tree, as reported by runtime.Version.
+//
+// # File types
+//
+// The go command examines the contents of a restricted set of files
+// in each directory. It identifies which files to examine based on
+// the extension of the file name. These extensions are:
+//
+//	.go
+//		Go source files.
+//	.c, .h
+//		C source files.
+//		If the package uses cgo or SWIG, these will be compiled with the
+//		OS-native compiler (typically gcc); otherwise they will
+//		trigger an error.
+//	.cc, .cpp, .cxx, .hh, .hpp, .hxx
+//		C++ source files. Only useful with cgo or SWIG, and always
+//		compiled with the OS-native compiler.
+//	.m
+//		Objective-C source files. Only useful with cgo, and always
+//		compiled with the OS-native compiler.
+//	.s, .S, .sx
+//		Assembler source files.
+//		If the package uses cgo or SWIG, these will be assembled with the
+//		OS-native assembler (typically gcc (sic)); otherwise they
+//		will be assembled with the Go assembler.
+//	.swig, .swigcxx
+//		SWIG definition files.
+//	.syso
+//		System object files.
+//
+// Files of each of these types except .syso may contain build
+// constraints, but the go command stops scanning for build constraints
+// at the first item in the file that is not a blank line or //-style
+// line comment. See the go/build package documentation for
+// more details.
+//
+// # The go.mod file
+//
+// A module version is defined by a tree of source files, with a go.mod
+// file in its root. When the go command is run, it looks in the current
+// directory and then successive parent directories to find the go.mod
+// marking the root of the main (current) module.
+//
+// The go.mod file format is described in detail at
+// https://golang.org/ref/mod#go-mod-file.
+//
+// To create a new go.mod file, use 'go mod init'. For details see
+// 'go help mod init' or https://golang.org/ref/mod#go-mod-init.
+//
+// To add missing module requirements or remove unneeded requirements,
+// use 'go mod tidy'. For details, see 'go help mod tidy' or
+// https://golang.org/ref/mod#go-mod-tidy.
+//
+// To add, upgrade, downgrade, or remove a specific module requirement, use
+// 'go get'. For details, see 'go help module-get' or
+// https://golang.org/ref/mod#go-get.
+//
+// To make other changes or to parse go.mod as JSON for use by other tools,
+// use 'go mod edit'. See 'go help mod edit' or
+// https://golang.org/ref/mod#go-mod-edit.
+//
+// # GOPATH environment variable
+//
+// The Go path is used to resolve import statements.
+// It is implemented by and documented in the go/build package.
+//
+// The GOPATH environment variable lists places to look for Go code.
+// On Unix, the value is a colon-separated string.
+// On Windows, the value is a semicolon-separated string.
+// On Plan 9, the value is a list.
+//
+// If the environment variable is unset, GOPATH defaults
+// to a subdirectory named "go" in the user's home directory
+// ($HOME/go on Unix, %USERPROFILE%\go on Windows),
+// unless that directory holds a Go distribution.
+// Run "go env GOPATH" to see the current GOPATH.
+//
+// See https://golang.org/wiki/SettingGOPATH to set a custom GOPATH.
+//
+// Each directory listed in GOPATH must have a prescribed structure:
+//
+// The src directory holds source code. The path below src
+// determines the import path or executable name.
+//
+// The pkg directory holds installed package objects.
+// As in the Go tree, each target operating system and
+// architecture pair has its own subdirectory of pkg
+// (pkg/GOOS_GOARCH).
+//
+// If DIR is a directory listed in the GOPATH, a package with
+// source in DIR/src/foo/bar can be imported as "foo/bar" and
+// has its compiled form installed to "DIR/pkg/GOOS_GOARCH/foo/bar.a".
+//
+// The bin directory holds compiled commands.
+// Each command is named for its source directory, but only
+// the final element, not the entire path. That is, the
+// command with source in DIR/src/foo/quux is installed into
+// DIR/bin/quux, not DIR/bin/foo/quux. The "foo/" prefix is stripped
+// so that you can add DIR/bin to your PATH to get at the
+// installed commands. If the GOBIN environment variable is
+// set, commands are installed to the directory it names instead
+// of DIR/bin. GOBIN must be an absolute path.
+//
+// Here's an example directory layout:
+//
+//	GOPATH=/home/user/go
+//
+//	/home/user/go/
+//	    src/
+//	        foo/
+//	            bar/               (go code in package bar)
+//	                x.go
+//	            quux/              (go code in package main)
+//	                y.go
+//	    bin/
+//	        quux                   (installed command)
+//	    pkg/
+//	        linux_amd64/
+//	            foo/
+//	                bar.a          (installed package object)
+//
+// Go searches each directory listed in GOPATH to find source code,
+// but new packages are always downloaded into the first directory
+// in the list.
+//
+// See https://golang.org/doc/code.html for an example.
+//
+// # GOPATH and Modules
+//
+// When using modules, GOPATH is no longer used for resolving imports.
+// However, it is still used to store downloaded source code (in GOPATH/pkg/mod)
+// and compiled commands (in GOPATH/bin).
+//
+// # Internal Directories
+//
+// Code in or below a directory named "internal" is importable only
+// by code in the directory tree rooted at the parent of "internal".
+// Here's an extended version of the directory layout above:
+//
+//	/home/user/go/
+//	    src/
+//	        crash/
+//	            bang/              (go code in package bang)
+//	                b.go
+//	        foo/                   (go code in package foo)
+//	            f.go
+//	            bar/               (go code in package bar)
+//	                x.go
+//	            internal/
+//	                baz/           (go code in package baz)
+//	                    z.go
+//	            quux/              (go code in package main)
+//	                y.go
+//
+// The code in z.go is imported as "foo/internal/baz", but that
+// import statement can only appear in source files in the subtree
+// rooted at foo. The source files foo/f.go, foo/bar/x.go, and
+// foo/quux/y.go can all import "foo/internal/baz", but the source file
+// crash/bang/b.go cannot.
+//
+// See https://golang.org/s/go14internal for details.
+//
+// # Vendor Directories
+//
+// Go 1.6 includes support for using local copies of external dependencies
+// to satisfy imports of those dependencies, often referred to as vendoring.
+//
+// Code below a directory named "vendor" is importable only
+// by code in the directory tree rooted at the parent of "vendor",
+// and only using an import path that omits the prefix up to and
+// including the vendor element.
+//
+// Here's the example from the previous section,
+// but with the "internal" directory renamed to "vendor"
+// and a new foo/vendor/crash/bang directory added:
+//
+//	/home/user/go/
+//	    src/
+//	        crash/
+//	            bang/              (go code in package bang)
+//	                b.go
+//	        foo/                   (go code in package foo)
+//	            f.go
+//	            bar/               (go code in package bar)
+//	                x.go
+//	            vendor/
+//	                crash/
+//	                    bang/      (go code in package bang)
+//	                        b.go
+//	                baz/           (go code in package baz)
+//	                    z.go
+//	            quux/              (go code in package main)
+//	                y.go
+//
+// The same visibility rules apply as for internal, but the code
+// in z.go is imported as "baz", not as "foo/vendor/baz".
+//
+// Code in vendor directories deeper in the source tree shadows
+// code in higher directories. Within the subtree rooted at foo, an import
+// of "crash/bang" resolves to "foo/vendor/crash/bang", not the
+// top-level "crash/bang".
+//
+// Code in vendor directories is not subject to import path
+// checking (see 'go help importpath').
+//
+// When 'go get' checks out or updates a git repository, it now also
+// updates submodules.
+//
+// Vendor directories do not affect the placement of new repositories
+// being checked out for the first time by 'go get': those are always
+// placed in the main GOPATH, never in a vendor subtree.
+//
+// See https://golang.org/s/go15vendor for details.
+//
+// # Legacy GOPATH go get
+//
+// The 'go get' command changes behavior depending on whether the
+// go command is running in module-aware mode or legacy GOPATH mode.
+// This help text, accessible as 'go help gopath-get' even in module-aware mode,
+// describes 'go get' as it operates in legacy GOPATH mode.
+//
+// Usage: go get [-d] [-f] [-t] [-u] [-v] [-fix] [build flags] [packages]
+//
+// Get downloads the packages named by the import paths, along with their
+// dependencies. It then installs the named packages, like 'go install'.
+//
+// The -d flag instructs get to stop after downloading the packages; that is,
+// it instructs get not to install the packages.
+//
+// The -f flag, valid only when -u is set, forces get -u not to verify that
+// each package has been checked out from the source control repository
+// implied by its import path. This can be useful if the source is a local fork
+// of the original.
+//
+// The -fix flag instructs get to run the fix tool on the downloaded packages
+// before resolving dependencies or building the code.
+//
+// The -t flag instructs get to also download the packages required to build
+// the tests for the specified packages.
+//
+// The -u flag instructs get to use the network to update the named packages
+// and their dependencies. By default, get uses the network to check out
+// missing packages but does not use it to look for updates to existing packages.
+//
+// The -v flag enables verbose progress and debug output.
+//
+// Get also accepts build flags to control the installation. See 'go help build'.
+//
+// When checking out a new package, get creates the target directory
+// GOPATH/src/<import-path>. If the GOPATH contains multiple entries,
+// get uses the first one. For more details see: 'go help gopath'.
+//
+// When checking out or updating a package, get looks for a branch or tag
+// that matches the locally installed version of Go. The most important
+// rule is that if the local installation is running version "go1", get
+// searches for a branch or tag named "go1". If no such version exists
+// it retrieves the default branch of the package.
+//
+// When go get checks out or updates a Git repository,
+// it also updates any git submodules referenced by the repository.
+//
+// Get never checks out or updates code stored in vendor directories.
+//
+// For more about build flags, see 'go help build'.
+//
+// For more about specifying packages, see 'go help packages'.
+//
+// For more about how 'go get' finds source code to
+// download, see 'go help importpath'.
+//
+// This text describes the behavior of get when using GOPATH
+// to manage source code and dependencies.
+// If instead the go command is running in module-aware mode,
+// the details of get's flags and effects change, as does 'go help get'.
+// See 'go help modules' and 'go help module-get'.
+//
+// See also: go build, go install, go clean.
+//
+// # Module proxy protocol
+//
+// A Go module proxy is any web server that can respond to GET requests for
+// URLs of a specified form. The requests have no query parameters, so even
+// a site serving from a fixed file system (including a file:/// URL)
+// can be a module proxy.
+//
+// For details on the GOPROXY protocol, see
+// https://golang.org/ref/mod#goproxy-protocol.
+//
+// # Import path syntax
+//
+// An import path (see 'go help packages') denotes a package stored in the local
+// file system. In general, an import path denotes either a standard package (such
+// as "unicode/utf8") or a package found in one of the work spaces (For more
+// details see: 'go help gopath').
+//
+// # Relative import paths
+//
+// An import path beginning with ./ or ../ is called a relative path.
+// The toolchain supports relative import paths as a shortcut in two ways.
+//
+// First, a relative path can be used as a shorthand on the command line.
+// If you are working in the directory containing the code imported as
+// "unicode" and want to run the tests for "unicode/utf8", you can type
+// "go test ./utf8" instead of needing to specify the full path.
+// Similarly, in the reverse situation, "go test .." will test "unicode" from
+// the "unicode/utf8" directory. Relative patterns are also allowed, like
+// "go test ./..." to test all subdirectories. See 'go help packages' for details
+// on the pattern syntax.
+//
+// Second, if you are compiling a Go program not in a work space,
+// you can use a relative path in an import statement in that program
+// to refer to nearby code also not in a work space.
+// This makes it easy to experiment with small multipackage programs
+// outside of the usual work spaces, but such programs cannot be
+// installed with "go install" (there is no work space in which to install them),
+// so they are rebuilt from scratch each time they are built.
+// To avoid ambiguity, Go programs cannot use relative import paths
+// within a work space.
+//
+// # Remote import paths
+//
+// Certain import paths also
+// describe how to obtain the source code for the package using
+// a revision control system.
+//
+// A few common code hosting sites have special syntax:
+//
+//	Bitbucket (Git, Mercurial)
+//
+//		import "bitbucket.org/user/project"
+//		import "bitbucket.org/user/project/sub/directory"
+//
+//	GitHub (Git)
+//
+//		import "github.com/user/project"
+//		import "github.com/user/project/sub/directory"
+//
+//	Launchpad (Bazaar)
+//
+//		import "launchpad.net/project"
+//		import "launchpad.net/project/series"
+//		import "launchpad.net/project/series/sub/directory"
+//
+//		import "launchpad.net/~user/project/branch"
+//		import "launchpad.net/~user/project/branch/sub/directory"
+//
+//	IBM DevOps Services (Git)
+//
+//		import "hub.jazz.net/git/user/project"
+//		import "hub.jazz.net/git/user/project/sub/directory"
+//
+// For code hosted on other servers, import paths may either be qualified
+// with the version control type, or the go tool can dynamically fetch
+// the import path over https/http and discover where the code resides
+// from a <meta> tag in the HTML.
+//
+// To declare the code location, an import path of the form
+//
+//	repository.vcs/path
+//
+// specifies the given repository, with or without the .vcs suffix,
+// using the named version control system, and then the path inside
+// that repository. The supported version control systems are:
+//
+//	Bazaar      .bzr
+//	Fossil      .fossil
+//	Git         .git
+//	Mercurial   .hg
+//	Subversion  .svn
+//
+// For example,
+//
+//	import "example.org/user/foo.hg"
+//
+// denotes the root directory of the Mercurial repository at
+// example.org/user/foo or foo.hg, and
+//
+//	import "example.org/repo.git/foo/bar"
+//
+// denotes the foo/bar directory of the Git repository at
+// example.org/repo or repo.git.
+//
+// When a version control system supports multiple protocols,
+// each is tried in turn when downloading. For example, a Git
+// download tries https://, then git+ssh://.
+//
+// By default, downloads are restricted to known secure protocols
+// (e.g. https, ssh). To override this setting for Git downloads, the
+// GIT_ALLOW_PROTOCOL environment variable can be set (For more details see:
+// 'go help environment').
+//
+// If the import path is not a known code hosting site and also lacks a
+// version control qualifier, the go tool attempts to fetch the import
+// over https/http and looks for a <meta> tag in the document's HTML
+// <head>.
+//
+// The meta tag has the form:
+//
+//	<meta name="go-import" content="import-prefix vcs repo-root">
+//
+// The import-prefix is the import path corresponding to the repository
+// root. It must be a prefix or an exact match of the package being
+// fetched with "go get". If it's not an exact match, another http
+// request is made at the prefix to verify the <meta> tags match.
+//
+// The meta tag should appear as early in the file as possible.
+// In particular, it should appear before any raw JavaScript or CSS,
+// to avoid confusing the go command's restricted parser.
+//
+// The vcs is one of "bzr", "fossil", "git", "hg", "svn".
+//
+// The repo-root is the root of the version control system
+// containing a scheme and not containing a .vcs qualifier.
+//
+// For example,
+//
+//	import "example.org/pkg/foo"
+//
+// will result in the following requests:
+//
+//	https://example.org/pkg/foo?go-get=1 (preferred)
+//	http://example.org/pkg/foo?go-get=1  (fallback, only with use of correctly set GOINSECURE)
+//
+// If that page contains the meta tag
+//
+//	<meta name="go-import" content="example.org git https://code.org/r/p/exproj">
+//
+// the go tool will verify that https://example.org/?go-get=1 contains the
+// same meta tag and then git clone https://code.org/r/p/exproj into
+// GOPATH/src/example.org.
+//
+// When using GOPATH, downloaded packages are written to the first directory
+// listed in the GOPATH environment variable.
+// (See 'go help gopath-get' and 'go help gopath'.)
+//
+// When using modules, downloaded packages are stored in the module cache.
+// See https://golang.org/ref/mod#module-cache.
+//
+// When using modules, an additional variant of the go-import meta tag is
+// recognized and is preferred over those listing version control systems.
+// That variant uses "mod" as the vcs in the content value, as in:
+//
+//	<meta name="go-import" content="example.org mod https://code.org/moduleproxy">
+//
+// This tag means to fetch modules with paths beginning with example.org
+// from the module proxy available at the URL https://code.org/moduleproxy.
+// See https://golang.org/ref/mod#goproxy-protocol for details about the
+// proxy protocol.
+//
+// # Import path checking
+//
+// When the custom import path feature described above redirects to a
+// known code hosting site, each of the resulting packages has two possible
+// import paths, using the custom domain or the known hosting site.
+//
+// A package statement is said to have an "import comment" if it is immediately
+// followed (before the next newline) by a comment of one of these two forms:
+//
+//	package math // import "path"
+//	package math /* import "path" */
+//
+// The go command will refuse to install a package with an import comment
+// unless it is being referred to by that import path. In this way, import comments
+// let package authors make sure the custom import path is used and not a
+// direct path to the underlying code hosting site.
+//
+// Import path checking is disabled for code found within vendor trees.
+// This makes it possible to copy code into alternate locations in vendor trees
+// without needing to update import comments.
+//
+// Import path checking is also disabled when using modules.
+// Import path comments are obsoleted by the go.mod file's module statement.
+//
+// See https://golang.org/s/go14customimport for details.
+//
+// # Modules, module versions, and more
+//
+// Modules are how Go manages dependencies.
+//
+// A module is a collection of packages that are released, versioned, and
+// distributed together. Modules may be downloaded directly from version control
+// repositories or from module proxy servers.
+//
+// For a series of tutorials on modules, see
+// https://golang.org/doc/tutorial/create-module.
+//
+// For a detailed reference on modules, see https://golang.org/ref/mod.
+//
+// By default, the go command may download modules from https://proxy.golang.org.
+// It may authenticate modules using the checksum database at
+// https://sum.golang.org. Both services are operated by the Go team at Google.
+// The privacy policies for these services are available at
+// https://proxy.golang.org/privacy and https://sum.golang.org/privacy,
+// respectively.
+//
+// The go command's download behavior may be configured using GOPROXY, GOSUMDB,
+// GOPRIVATE, and other environment variables. See 'go help environment'
+// and https://golang.org/ref/mod#private-module-privacy for more information.
+//
+// # Module authentication using go.sum
+//
+// When the go command downloads a module zip file or go.mod file into the
+// module cache, it computes a cryptographic hash and compares it with a known
+// value to verify the file hasn't changed since it was first downloaded. Known
+// hashes are stored in a file in the module root directory named go.sum. Hashes
+// may also be downloaded from the checksum database depending on the values of
+// GOSUMDB, GOPRIVATE, and GONOSUMDB.
+//
+// For details, see https://golang.org/ref/mod#authenticating.
+//
+// # Package lists and patterns
+//
+// Many commands apply to a set of packages:
+//
+//	go <action> [packages]
+//
+// Usually, [packages] is a list of import paths.
+//
+// An import path that is a rooted path or that begins with
+// a . or .. element is interpreted as a file system path and
+// denotes the package in that directory.
+//
+// Otherwise, the import path P denotes the package found in
+// the directory DIR/src/P for some DIR listed in the GOPATH
+// environment variable (For more details see: 'go help gopath').
+//
+// If no import paths are given, the action applies to the
+// package in the current directory.
+//
+// There are four reserved names for paths that should not be used
+// for packages to be built with the go tool:
+//
+// - "main" denotes the top-level package in a stand-alone executable.
+//
+// - "all" expands to all packages found in all the GOPATH
+// trees. For example, 'go list all' lists all the packages on the local
+// system. When using modules, "all" expands to all packages in
+// the main module and their dependencies, including dependencies
+// needed by tests of any of those.
+//
+// - "std" is like all but expands to just the packages in the standard
+// Go library.
+//
+// - "cmd" expands to the Go repository's commands and their
+// internal libraries.
+//
+// Import paths beginning with "cmd/" only match source code in
+// the Go repository.
+//
+// An import path is a pattern if it includes one or more "..." wildcards,
+// each of which can match any string, including the empty string and
+// strings containing slashes. Such a pattern expands to all package
+// directories found in the GOPATH trees with names matching the
+// patterns.
+//
+// To make common patterns more convenient, there are two special cases.
+// First, /... at the end of the pattern can match an empty string,
+// so that net/... matches both net and packages in its subdirectories, like net/http.
+// Second, any slash-separated pattern element containing a wildcard never
+// participates in a match of the "vendor" element in the path of a vendored
+// package, so that ./... does not match packages in subdirectories of
+// ./vendor or ./mycode/vendor, but ./vendor/... and ./mycode/vendor/... do.
+// Note, however, that a directory named vendor that itself contains code
+// is not a vendored package: cmd/vendor would be a command named vendor,
+// and the pattern cmd/... matches it.
+// See golang.org/s/go15vendor for more about vendoring.
+//
+// An import path can also name a package to be downloaded from
+// a remote repository. Run 'go help importpath' for details.
+//
+// Every package in a program must have a unique import path.
+// By convention, this is arranged by starting each path with a
+// unique prefix that belongs to you. For example, paths used
+// internally at Google all begin with 'google', and paths
+// denoting remote repositories begin with the path to the code,
+// such as 'github.com/user/repo'.
+//
+// Packages in a program need not have unique package names,
+// but there are two reserved package names with special meaning.
+// The name main indicates a command, not a library.
+// Commands are built into binaries and cannot be imported.
+// The name documentation indicates documentation for
+// a non-Go program in the directory. Files in package documentation
+// are ignored by the go command.
+//
+// As a special case, if the package list is a list of .go files from a
+// single directory, the command is applied to a single synthesized
+// package made up of exactly those files, ignoring any build constraints
+// in those files and ignoring any other files in the directory.
+//
+// Directory and file names that begin with "." or "_" are ignored
+// by the go tool, as are directories named "testdata".
+//
+// # Configuration for downloading non-public code
+//
+// The go command defaults to downloading modules from the public Go module
+// mirror at proxy.golang.org. It also defaults to validating downloaded modules,
+// regardless of source, against the public Go checksum database at sum.golang.org.
+// These defaults work well for publicly available source code.
+//
+// The GOPRIVATE environment variable controls which modules the go command
+// considers to be private (not available publicly) and should therefore not use
+// the proxy or checksum database. The variable is a comma-separated list of
+// glob patterns (in the syntax of Go's path.Match) of module path prefixes.
+// For example,
+//
+//	GOPRIVATE=*.corp.example.com,rsc.io/private
+//
+// causes the go command to treat as private any module with a path prefix
+// matching either pattern, including git.corp.example.com/xyzzy, rsc.io/private,
+// and rsc.io/private/quux.
+//
+// For fine-grained control over module download and validation, the GONOPROXY
+// and GONOSUMDB environment variables accept the same kind of glob list
+// and override GOPRIVATE for the specific decision of whether to use the proxy
+// and checksum database, respectively.
+//
+// For example, if a company ran a module proxy serving private modules,
+// users would configure go using:
+//
+//	GOPRIVATE=*.corp.example.com
+//	GOPROXY=proxy.example.com
+//	GONOPROXY=none
+//
+// The GOPRIVATE variable is also used to define the "public" and "private"
+// patterns for the GOVCS variable; see 'go help vcs'. For that usage,
+// GOPRIVATE applies even in GOPATH mode. In that case, it matches import paths
+// instead of module paths.
+//
+// The 'go env -w' command (see 'go help env') can be used to set these variables
+// for future go command invocations.
+//
+// For more details, see https://golang.org/ref/mod#private-modules.
+//
+// # Testing flags
+//
+// The 'go test' command takes both flags that apply to 'go test' itself
+// and flags that apply to the resulting test binary.
+//
+// Several of the flags control profiling and write an execution profile
+// suitable for "go tool pprof"; run "go tool pprof -h" for more
+// information. The --alloc_space, --alloc_objects, and --show_bytes
+// options of pprof control how the information is presented.
+//
+// The following flags are recognized by the 'go test' command and
+// control the execution of any test:
+//
+//	-bench regexp
+//	    Run only those benchmarks matching a regular expression.
+//	    By default, no benchmarks are run.
+//	    To run all benchmarks, use '-bench .' or '-bench=.'.
+//	    The regular expression is split by unbracketed slash (/)
+//	    characters into a sequence of regular expressions, and each
+//	    part of a benchmark's identifier must match the corresponding
+//	    element in the sequence, if any. Possible parents of matches
+//	    are run with b.N=1 to identify sub-benchmarks. For example,
+//	    given -bench=X/Y, top-level benchmarks matching X are run
+//	    with b.N=1 to find any sub-benchmarks matching Y, which are
+//	    then run in full.
+//
+//	-benchtime t
+//	    Run enough iterations of each benchmark to take t, specified
+//	    as a time.Duration (for example, -benchtime 1h30s).
+//	    The default is 1 second (1s).
+//	    The special syntax Nx means to run the benchmark N times
+//	    (for example, -benchtime 100x).
+//
+//	-count n
+//	    Run each test, benchmark, and fuzz seed n times (default 1).
+//	    If -cpu is set, run n times for each GOMAXPROCS value.
+//	    Examples are always run once. -count does not apply to
+//	    fuzz tests matched by -fuzz.
+//
+//	-cover
+//	    Enable coverage analysis.
+//	    Note that because coverage works by annotating the source
+//	    code before compilation, compilation and test failures with
+//	    coverage enabled may report line numbers that don't correspond
+//	    to the original sources.
+//
+//	-covermode set,count,atomic
+//	    Set the mode for coverage analysis for the package[s]
+//	    being tested. The default is "set" unless -race is enabled,
+//	    in which case it is "atomic".
+//	    The values:
+//		set: bool: does this statement run?
+//		count: int: how many times does this statement run?
+//		atomic: int: count, but correct in multithreaded tests;
+//			significantly more expensive.
+//	    Sets -cover.
+//
+//	-coverpkg pattern1,pattern2,pattern3
+//	    Apply coverage analysis in each test to packages matching the patterns.
+//	    The default is for each test to analyze only the package being tested.
+//	    See 'go help packages' for a description of package patterns.
+//	    Sets -cover.
+//
+//	-cpu 1,2,4
+//	    Specify a list of GOMAXPROCS values for which the tests, benchmarks or
+//	    fuzz tests should be executed. The default is the current value
+//	    of GOMAXPROCS. -cpu does not apply to fuzz tests matched by -fuzz.
+//
+//	-failfast
+//	    Do not start new tests after the first test failure.
+//
+//	-fullpath
+//	    Show full file names in the error messages.
+//
+//	-fuzz regexp
+//	    Run the fuzz test matching the regular expression. When specified,
+//	    the command line argument must match exactly one package within the
+//	    main module, and regexp must match exactly one fuzz test within
+//	    that package. Fuzzing will occur after tests, benchmarks, seed corpora
+//	    of other fuzz tests, and examples have completed. See the Fuzzing
+//	    section of the testing package documentation for details.
+//
+//	-fuzztime t
+//	    Run enough iterations of the fuzz target during fuzzing to take t,
+//	    specified as a time.Duration (for example, -fuzztime 1h30s).
+//		The default is to run forever.
+//	    The special syntax Nx means to run the fuzz target N times
+//	    (for example, -fuzztime 1000x).
+//
+//	-fuzzminimizetime t
+//	    Run enough iterations of the fuzz target during each minimization
+//	    attempt to take t, as specified as a time.Duration (for example,
+//	    -fuzzminimizetime 30s).
+//		The default is 60s.
+//	    The special syntax Nx means to run the fuzz target N times
+//	    (for example, -fuzzminimizetime 100x).
+//
+//	-json
+//	    Log verbose output and test results in JSON. This presents the
+//	    same information as the -v flag in a machine-readable format.
+//
+//	-list regexp
+//	    List tests, benchmarks, fuzz tests, or examples matching the regular
+//	    expression. No tests, benchmarks, fuzz tests, or examples will be run.
+//	    This will only list top-level tests. No subtest or subbenchmarks will be
+//	    shown.
+//
+//	-parallel n
+//	    Allow parallel execution of test functions that call t.Parallel, and
+//	    fuzz targets that call t.Parallel when running the seed corpus.
+//	    The value of this flag is the maximum number of tests to run
+//	    simultaneously.
+//	    While fuzzing, the value of this flag is the maximum number of
+//	    subprocesses that may call the fuzz function simultaneously, regardless of
+//	    whether T.Parallel is called.
+//	    By default, -parallel is set to the value of GOMAXPROCS.
+//	    Setting -parallel to values higher than GOMAXPROCS may cause degraded
+//	    performance due to CPU contention, especially when fuzzing.
+//	    Note that -parallel only applies within a single test binary.
+//	    The 'go test' command may run tests for different packages
+//	    in parallel as well, according to the setting of the -p flag
+//	    (see 'go help build').
+//
+//	-run regexp
+//	    Run only those tests, examples, and fuzz tests matching the regular
+//	    expression. For tests, the regular expression is split by unbracketed
+//	    slash (/) characters into a sequence of regular expressions, and each
+//	    part of a test's identifier must match the corresponding element in
+//	    the sequence, if any. Note that possible parents of matches are
+//	    run too, so that -run=X/Y matches and runs and reports the result
+//	    of all tests matching X, even those without sub-tests matching Y,
+//	    because it must run them to look for those sub-tests.
+//	    See also -skip.
+//
+//	-short
+//	    Tell long-running tests to shorten their run time.
+//	    It is off by default but set during all.bash so that installing
+//	    the Go tree can run a sanity check but not spend time running
+//	    exhaustive tests.
+//
+//	-shuffle off,on,N
+//	    Randomize the execution order of tests and benchmarks.
+//	    It is off by default. If -shuffle is set to on, then it will seed
+//	    the randomizer using the system clock. If -shuffle is set to an
+//	    integer N, then N will be used as the seed value. In both cases,
+//	    the seed will be reported for reproducibility.
+//
+//	-skip regexp
+//	    Run only those tests, examples, fuzz tests, and benchmarks that
+//	    do not match the regular expression. Like for -run and -bench,
+//	    for tests and benchmarks, the regular expression is split by unbracketed
+//	    slash (/) characters into a sequence of regular expressions, and each
+//	    part of a test's identifier must match the corresponding element in
+//	    the sequence, if any.
+//
+//	-timeout d
+//	    If a test binary runs longer than duration d, panic.
+//	    If d is 0, the timeout is disabled.
+//	    The default is 10 minutes (10m).
+//
+//	-v
+//	    Verbose output: log all tests as they are run. Also print all
+//	    text from Log and Logf calls even if the test succeeds.
+//
+//	-vet list
+//	    Configure the invocation of "go vet" during "go test"
+//	    to use the comma-separated list of vet checks.
+//	    If list is empty, "go test" runs "go vet" with a curated list of
+//	    checks believed to be always worth addressing.
+//	    If list is "off", "go test" does not run "go vet" at all.
+//
+// The following flags are also recognized by 'go test' and can be used to
+// profile the tests during execution:
+//
+//	-benchmem
+//	    Print memory allocation statistics for benchmarks.
+//
+//	-blockprofile block.out
+//	    Write a goroutine blocking profile to the specified file
+//	    when all tests are complete.
+//	    Writes test binary as -c would.
+//
+//	-blockprofilerate n
+//	    Control the detail provided in goroutine blocking profiles by
+//	    calling runtime.SetBlockProfileRate with n.
+//	    See 'go doc runtime.SetBlockProfileRate'.
+//	    The profiler aims to sample, on average, one blocking event every
+//	    n nanoseconds the program spends blocked. By default,
+//	    if -test.blockprofile is set without this flag, all blocking events
+//	    are recorded, equivalent to -test.blockprofilerate=1.
+//
+//	-coverprofile cover.out
+//	    Write a coverage profile to the file after all tests have passed.
+//	    Sets -cover.
+//
+//	-cpuprofile cpu.out
+//	    Write a CPU profile to the specified file before exiting.
+//	    Writes test binary as -c would.
+//
+//	-memprofile mem.out
+//	    Write an allocation profile to the file after all tests have passed.
+//	    Writes test binary as -c would.
+//
+//	-memprofilerate n
+//	    Enable more precise (and expensive) memory allocation profiles by
+//	    setting runtime.MemProfileRate. See 'go doc runtime.MemProfileRate'.
+//	    To profile all memory allocations, use -test.memprofilerate=1.
+//
+//	-mutexprofile mutex.out
+//	    Write a mutex contention profile to the specified file
+//	    when all tests are complete.
+//	    Writes test binary as -c would.
+//
+//	-mutexprofilefraction n
+//	    Sample 1 in n stack traces of goroutines holding a
+//	    contended mutex.
+//
+//	-outputdir directory
+//	    Place output files from profiling in the specified directory,
+//	    by default the directory in which "go test" is running.
+//
+//	-trace trace.out
+//	    Write an execution trace to the specified file before exiting.
+//
+// Each of these flags is also recognized with an optional 'test.' prefix,
+// as in -test.v. When invoking the generated test binary (the result of
+// 'go test -c') directly, however, the prefix is mandatory.
+//
+// The 'go test' command rewrites or removes recognized flags,
+// as appropriate, both before and after the optional package list,
+// before invoking the test binary.
+//
+// For instance, the command
+//
+//	go test -v -myflag testdata -cpuprofile=prof.out -x
+//
+// will compile the test binary and then run it as
+//
+//	pkg.test -test.v -myflag testdata -test.cpuprofile=prof.out
+//
+// (The -x flag is removed because it applies only to the go command's
+// execution, not to the test itself.)
+//
+// The test flags that generate profiles (other than for coverage) also
+// leave the test binary in pkg.test for use when analyzing the profiles.
+//
+// When 'go test' runs a test binary, it does so from within the
+// corresponding package's source code directory. Depending on the test,
+// it may be necessary to do the same when invoking a generated test
+// binary directly. Because that directory may be located within the
+// module cache, which may be read-only and is verified by checksums, the
+// test must not write to it or any other directory within the module
+// unless explicitly requested by the user (such as with the -fuzz flag,
+// which writes failures to testdata/fuzz).
+//
+// The command-line package list, if present, must appear before any
+// flag not known to the go test command. Continuing the example above,
+// the package list would have to appear before -myflag, but could appear
+// on either side of -v.
+//
+// When 'go test' runs in package list mode, 'go test' caches successful
+// package test results to avoid unnecessary repeated running of tests. To
+// disable test caching, use any test flag or argument other than the
+// cacheable flags. The idiomatic way to disable test caching explicitly
+// is to use -count=1.
+//
+// To keep an argument for a test binary from being interpreted as a
+// known flag or a package name, use -args (see 'go help test') which
+// passes the remainder of the command line through to the test binary
+// uninterpreted and unaltered.
+//
+// For instance, the command
+//
+//	go test -v -args -x -v
+//
+// will compile the test binary and then run it as
+//
+//	pkg.test -test.v -x -v
+//
+// Similarly,
+//
+//	go test -args math
+//
+// will compile the test binary and then run it as
+//
+//	pkg.test math
+//
+// In the first example, the -x and the second -v are passed through to the
+// test binary unchanged and with no effect on the go command itself.
+// In the second example, the argument math is passed through to the test
+// binary, instead of being interpreted as the package list.
+//
+// # Testing functions
+//
+// The 'go test' command expects to find test, benchmark, and example functions
+// in the "*_test.go" files corresponding to the package under test.
+//
+// A test function is one named TestXxx (where Xxx does not start with a
+// lower case letter) and should have the signature,
+//
+//	func TestXxx(t *testing.T) { ... }
+//
+// A benchmark function is one named BenchmarkXxx and should have the signature,
+//
+//	func BenchmarkXxx(b *testing.B) { ... }
+//
+// A fuzz test is one named FuzzXxx and should have the signature,
+//
+//	func FuzzXxx(f *testing.F) { ... }
+//
+// An example function is similar to a test function but, instead of using
+// *testing.T to report success or failure, prints output to os.Stdout.
+// If the last comment in the function starts with "Output:" then the output
+// is compared exactly against the comment (see examples below). If the last
+// comment begins with "Unordered output:" then the output is compared to the
+// comment, however the order of the lines is ignored. An example with no such
+// comment is compiled but not executed. An example with no text after
+// "Output:" is compiled, executed, and expected to produce no output.
+//
+// Godoc displays the body of ExampleXxx to demonstrate the use
+// of the function, constant, or variable Xxx. An example of a method M with
+// receiver type T or *T is named ExampleT_M. There may be multiple examples
+// for a given function, constant, or variable, distinguished by a trailing _xxx,
+// where xxx is a suffix not beginning with an upper case letter.
+//
+// Here is an example of an example:
+//
+//	func ExamplePrintln() {
+//		Println("The output of\nthis example.")
+//		// Output: The output of
+//		// this example.
+//	}
+//
+// Here is another example where the ordering of the output is ignored:
+//
+//	func ExamplePerm() {
+//		for _, value := range Perm(4) {
+//			fmt.Println(value)
+//		}
+//
+//		// Unordered output: 4
+//		// 2
+//		// 1
+//		// 3
+//		// 0
+//	}
+//
+// The entire test file is presented as the example when it contains a single
+// example function, at least one other function, type, variable, or constant
+// declaration, and no tests, benchmarks, or fuzz tests.
+//
+// See the documentation of the testing package for more information.
+//
+// # Controlling version control with GOVCS
+//
+// The 'go get' command can run version control commands like git
+// to download imported code. This functionality is critical to the decentralized
+// Go package ecosystem, in which code can be imported from any server,
+// but it is also a potential security problem, if a malicious server finds a
+// way to cause the invoked version control command to run unintended code.
+//
+// To balance the functionality and security concerns, the 'go get' command
+// by default will only use git and hg to download code from public servers.
+// But it will use any known version control system (bzr, fossil, git, hg, svn)
+// to download code from private servers, defined as those hosting packages
+// matching the GOPRIVATE variable (see 'go help private'). The rationale behind
+// allowing only Git and Mercurial is that these two systems have had the most
+// attention to issues of being run as clients of untrusted servers. In contrast,
+// Bazaar, Fossil, and Subversion have primarily been used in trusted,
+// authenticated environments and are not as well scrutinized as attack surfaces.
+//
+// The version control command restrictions only apply when using direct version
+// control access to download code. When downloading modules from a proxy,
+// 'go get' uses the proxy protocol instead, which is always permitted.
+// By default, the 'go get' command uses the Go module mirror (proxy.golang.org)
+// for public packages and only falls back to version control for private
+// packages or when the mirror refuses to serve a public package (typically for
+// legal reasons). Therefore, clients can still access public code served from
+// Bazaar, Fossil, or Subversion repositories by default, because those downloads
+// use the Go module mirror, which takes on the security risk of running the
+// version control commands using a custom sandbox.
+//
+// The GOVCS variable can be used to change the allowed version control systems
+// for specific packages (identified by a module or import path).
+// The GOVCS variable applies when building package in both module-aware mode
+// and GOPATH mode. When using modules, the patterns match against the module path.
+// When using GOPATH, the patterns match against the import path corresponding to
+// the root of the version control repository.
+//
+// The general form of the GOVCS setting is a comma-separated list of
+// pattern:vcslist rules. The pattern is a glob pattern that must match
+// one or more leading elements of the module or import path. The vcslist
+// is a pipe-separated list of allowed version control commands, or "all"
+// to allow use of any known command, or "off" to disallow all commands.
+// Note that if a module matches a pattern with vcslist "off", it may still be
+// downloaded if the origin server uses the "mod" scheme, which instructs the
+// go command to download the module using the GOPROXY protocol.
+// The earliest matching pattern in the list applies, even if later patterns
+// might also match.
+//
+// For example, consider:
+//
+//	GOVCS=github.com:git,evil.com:off,*:git|hg
+//
+// With this setting, code with a module or import path beginning with
+// github.com/ can only use git; paths on evil.com cannot use any version
+// control command, and all other paths (* matches everything) can use
+// only git or hg.
+//
+// The special patterns "public" and "private" match public and private
+// module or import paths. A path is private if it matches the GOPRIVATE
+// variable; otherwise it is public.
+//
+// If no rules in the GOVCS variable match a particular module or import path,
+// the 'go get' command applies its default rule, which can now be summarized
+// in GOVCS notation as 'public:git|hg,private:all'.
+//
+// To allow unfettered use of any version control system for any package, use:
+//
+//	GOVCS=*:all
+//
+// To disable all use of version control, use:
+//
+//	GOVCS=*:off
+//
+// The 'go env -w' command (see 'go help env') can be used to set the GOVCS
+// variable for future go command invocations.
+package main
diff --git a/src/cmd/go/chdir_test.go b/src/cmd/go/chdir_test.go
new file mode 100644
index 0000000..44cbb9c
--- /dev/null
+++ b/src/cmd/go/chdir_test.go
@@ -0,0 +1,49 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"cmd/go/internal/base"
+	"os"
+	"strings"
+	"testing"
+)
+
+func TestChdir(t *testing.T) {
+	// We want -C to apply to every go subcommand.
+	// Test that every command either has a -C flag registered
+	// or has CustomFlags set. In the latter case, the command
+	// must be explicitly tested in TestScript/chdir.
+	script, err := os.ReadFile("testdata/script/chdir.txt")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var walk func(string, *base.Command)
+	walk = func(name string, cmd *base.Command) {
+		if len(cmd.Commands) > 0 {
+			for _, sub := range cmd.Commands {
+				walk(name+" "+sub.Name(), sub)
+			}
+			return
+		}
+		if !cmd.Runnable() {
+			return
+		}
+		if cmd.CustomFlags {
+			if !strings.Contains(string(script), "# "+name+"\n") {
+				t.Errorf("%s has custom flags, not tested in testdata/script/chdir.txt", name)
+			}
+			return
+		}
+		f := cmd.Flag.Lookup("C")
+		if f == nil {
+			t.Errorf("%s has no -C flag", name)
+		} else if f.Usage != "AddChdirFlag" {
+			t.Errorf("%s has -C flag but not from AddChdirFlag", name)
+		}
+	}
+	walk("go", base.Go)
+}
diff --git a/src/cmd/go/export_test.go b/src/cmd/go/export_test.go
new file mode 100644
index 0000000..155ab8c
--- /dev/null
+++ b/src/cmd/go/export_test.go
@@ -0,0 +1,7 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+func Main() { main() }
diff --git a/src/cmd/go/go11.go b/src/cmd/go/go11.go
new file mode 100644
index 0000000..9faa7cb
--- /dev/null
+++ b/src/cmd/go/go11.go
@@ -0,0 +1,10 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.1
+
+package main
+
+// Test that go1.1 tag above is included in builds. main.go refers to this definition.
+const go11tag = true
diff --git a/src/cmd/go/go_boring_test.go b/src/cmd/go/go_boring_test.go
new file mode 100644
index 0000000..ed0fbf3
--- /dev/null
+++ b/src/cmd/go/go_boring_test.go
@@ -0,0 +1,22 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build boringcrypto
+
+package main_test
+
+import "testing"
+
+func TestBoringInternalLink(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempFile("main.go", `package main
+		import "crypto/sha1"
+		func main() {
+			sha1.New()
+		}`)
+	tg.run("build", "-ldflags=-w -extld=false", tg.path("main.go"))
+	tg.run("build", "-ldflags=-extld=false", tg.path("main.go"))
+}
diff --git a/src/cmd/go/go_test.go b/src/cmd/go/go_test.go
new file mode 100644
index 0000000..54249f6
--- /dev/null
+++ b/src/cmd/go/go_test.go
@@ -0,0 +1,2978 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main_test
+
+import (
+	"bytes"
+	"debug/elf"
+	"debug/macho"
+	"debug/pe"
+	"encoding/binary"
+	"flag"
+	"fmt"
+	"go/format"
+	"internal/godebug"
+	"internal/platform"
+	"internal/testenv"
+	"io"
+	"io/fs"
+	"log"
+	"math"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
+	"runtime"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cache"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/robustio"
+	"cmd/go/internal/search"
+	"cmd/go/internal/toolchain"
+	"cmd/go/internal/vcs"
+	"cmd/go/internal/vcweb/vcstest"
+	"cmd/go/internal/web"
+	"cmd/go/internal/work"
+	"cmd/internal/sys"
+
+	cmdgo "cmd/go"
+)
+
+func init() {
+	// GOVCS defaults to public:git|hg,private:all,
+	// which breaks many tests here - they can't use non-git, non-hg VCS at all!
+	// Change to fully permissive.
+	// The tests of the GOVCS setting itself are in ../../testdata/script/govcs.txt.
+	os.Setenv("GOVCS", "*:all")
+}
+
+var (
+	canRace = false // whether we can run the race detector
+	canMSan = false // whether we can run the memory sanitizer
+	canASan = false // whether we can run the address sanitizer
+)
+
+var (
+	goHostOS, goHostArch string
+	cgoEnabled           string // raw value from 'go env CGO_ENABLED'
+)
+
+// netTestSem is a semaphore limiting the number of tests that may use the
+// external network in parallel. If non-nil, it contains one buffer slot per
+// test (send to acquire), with a low enough limit that the overall number of
+// connections (summed across subprocesses) stays at or below base.NetLimit.
+var netTestSem chan struct{}
+
+var exeSuffix string = func() string {
+	if runtime.GOOS == "windows" {
+		return ".exe"
+	}
+	return ""
+}()
+
+func tooSlow(t *testing.T, reason string) {
+	if testing.Short() {
+		t.Helper()
+		t.Skipf("skipping test in -short mode: %s", reason)
+	}
+}
+
+// testGOROOT is the GOROOT to use when running testgo, a cmd/go binary
+// build from this process's current GOROOT, but run from a different
+// (temp) directory.
+var testGOROOT string
+
+// testGOROOT_FINAL is the GOROOT_FINAL with which the test binary is assumed to
+// have been built.
+var testGOROOT_FINAL = os.Getenv("GOROOT_FINAL")
+
+var testGOCACHE string
+
+var testGo string
+var testTmpDir string
+var testBin string
+
+// The TestMain function creates a go command for testing purposes and
+// deletes it after the tests have been run.
+func TestMain(m *testing.M) {
+	// When CMDGO_TEST_RUN_MAIN is set, we're reusing the test binary as cmd/go.
+	// Enable the special behavior needed in cmd/go/internal/work,
+	// run the main func exported via export_test.go, and exit.
+	// We set CMDGO_TEST_RUN_MAIN via os.Setenv and testScript.setup.
+	if os.Getenv("CMDGO_TEST_RUN_MAIN") != "" {
+		cfg.SetGOROOT(cfg.GOROOT, true)
+		gover.TestVersion = os.Getenv("TESTGO_VERSION")
+		toolchain.TestVersionSwitch = os.Getenv("TESTGO_VERSION_SWITCH")
+		if v := os.Getenv("TESTGO_TOOLCHAIN_VERSION"); v != "" {
+			work.ToolchainVersion = v
+		}
+
+		if testGOROOT := os.Getenv("TESTGO_GOROOT"); testGOROOT != "" {
+			// Disallow installs to the GOROOT from which testgo was built.
+			// Installs to other GOROOTs — such as one set explicitly within a test — are ok.
+			work.AllowInstall = func(a *work.Action) error {
+				if cfg.BuildN {
+					return nil
+				}
+
+				rel := search.InDir(a.Target, testGOROOT)
+				if rel == "" {
+					return nil
+				}
+
+				callerPos := ""
+				if _, file, line, ok := runtime.Caller(1); ok {
+					if shortFile := search.InDir(file, filepath.Join(testGOROOT, "src")); shortFile != "" {
+						file = shortFile
+					}
+					callerPos = fmt.Sprintf("%s:%d: ", file, line)
+				}
+				notice := "This error error can occur if GOROOT is stale, in which case rerunning make.bash will fix it."
+				return fmt.Errorf("%stestgo must not write to GOROOT (installing to %s) (%v)", callerPos, filepath.Join("GOROOT", rel), notice)
+			}
+		}
+
+		if vcsTestHost := os.Getenv("TESTGO_VCSTEST_HOST"); vcsTestHost != "" {
+			vcs.VCSTestRepoURL = "http://" + vcsTestHost
+			vcs.VCSTestHosts = vcstest.Hosts
+			vcsTestTLSHost := os.Getenv("TESTGO_VCSTEST_TLS_HOST")
+			vcsTestClient, err := vcstest.TLSClient(os.Getenv("TESTGO_VCSTEST_CERT"))
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "loading certificates from $TESTGO_VCSTEST_CERT: %v", err)
+			}
+			var interceptors []web.Interceptor
+			for _, host := range vcstest.Hosts {
+				interceptors = append(interceptors,
+					web.Interceptor{Scheme: "http", FromHost: host, ToHost: vcsTestHost},
+					web.Interceptor{Scheme: "https", FromHost: host, ToHost: vcsTestTLSHost, Client: vcsTestClient})
+			}
+			web.EnableTestHooks(interceptors)
+		}
+
+		cmdgo.Main()
+		os.Exit(0)
+	}
+	os.Setenv("CMDGO_TEST_RUN_MAIN", "true")
+
+	// $GO_GCFLAGS a compiler debug flag known to cmd/dist, make.bash, etc.
+	// It is not a standard go command flag; use os.Getenv, not cfg.Getenv.
+	if os.Getenv("GO_GCFLAGS") != "" {
+		fmt.Fprintf(os.Stderr, "testing: warning: no tests to run\n") // magic string for cmd/go
+		fmt.Printf("cmd/go test is not compatible with $GO_GCFLAGS being set\n")
+		fmt.Printf("SKIP\n")
+		return
+	}
+
+	flag.Parse()
+
+	if *proxyAddr != "" {
+		StartProxy()
+		select {}
+	}
+
+	// Run with a temporary TMPDIR to check that the tests don't
+	// leave anything behind.
+	topTmpdir, err := os.MkdirTemp("", "cmd-go-test-")
+	if err != nil {
+		log.Fatal(err)
+	}
+	if !*testWork {
+		defer removeAll(topTmpdir)
+	} else {
+		fmt.Fprintf(os.Stderr, "TESTWORK: preserving top level tempdir %s\n", topTmpdir)
+	}
+	os.Setenv(tempEnvName(), topTmpdir)
+
+	dir, err := os.MkdirTemp(topTmpdir, "tmpdir")
+	if err != nil {
+		log.Fatal(err)
+	}
+	testTmpDir = dir
+	if !*testWork {
+		defer removeAll(testTmpDir)
+	}
+
+	testGOCACHE = cache.DefaultDir()
+	if testenv.HasGoBuild() {
+		testBin = filepath.Join(testTmpDir, "testbin")
+		if err := os.Mkdir(testBin, 0777); err != nil {
+			log.Fatal(err)
+		}
+		testGo = filepath.Join(testBin, "go"+exeSuffix)
+		gotool, err := testenv.GoTool()
+		if err != nil {
+			fmt.Fprintln(os.Stderr, "locating go tool: ", err)
+			os.Exit(2)
+		}
+
+		goEnv := func(name string) string {
+			out, err := exec.Command(gotool, "env", name).CombinedOutput()
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "go env %s: %v\n%s", name, err, out)
+				os.Exit(2)
+			}
+			return strings.TrimSpace(string(out))
+		}
+		testGOROOT = goEnv("GOROOT")
+		os.Setenv("TESTGO_GOROOT", testGOROOT)
+		// Ensure that GOROOT is set explicitly.
+		// Otherwise, if the toolchain was built with GOROOT_FINAL set but has not
+		// yet been moved to its final location, programs that invoke runtime.GOROOT
+		// may accidentally use the wrong path.
+		os.Setenv("GOROOT", testGOROOT)
+
+		// The whole GOROOT/pkg tree was installed using the GOHOSTOS/GOHOSTARCH
+		// toolchain (installed in GOROOT/pkg/tool/GOHOSTOS_GOHOSTARCH).
+		// The testgo.exe we are about to create will be built for GOOS/GOARCH,
+		// which means it will use the GOOS/GOARCH toolchain
+		// (installed in GOROOT/pkg/tool/GOOS_GOARCH).
+		// If these are not the same toolchain, then the entire standard library
+		// will look out of date (the compilers in those two different tool directories
+		// are built for different architectures and have different build IDs),
+		// which will cause many tests to do unnecessary rebuilds and some
+		// tests to attempt to overwrite the installed standard library.
+		// Bail out entirely in this case.
+		goHostOS = goEnv("GOHOSTOS")
+		os.Setenv("TESTGO_GOHOSTOS", goHostOS)
+		goHostArch = goEnv("GOHOSTARCH")
+		os.Setenv("TESTGO_GOHOSTARCH", goHostArch)
+
+		cgoEnabled = goEnv("CGO_ENABLED")
+
+		// Duplicate the test executable into the path at testGo, for $PATH.
+		// If the OS supports symlinks, use them instead of copying bytes.
+		testExe, err := os.Executable()
+		if err != nil {
+			log.Fatal(err)
+		}
+		if err := os.Symlink(testExe, testGo); err != nil {
+			// Otherwise, copy the bytes.
+			src, err := os.Open(testExe)
+			if err != nil {
+				log.Fatal(err)
+			}
+			defer src.Close()
+
+			dst, err := os.OpenFile(testGo, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o777)
+			if err != nil {
+				log.Fatal(err)
+			}
+
+			_, err = io.Copy(dst, src)
+			if closeErr := dst.Close(); err == nil {
+				err = closeErr
+			}
+			if err != nil {
+				log.Fatal(err)
+			}
+		}
+
+		out, err := exec.Command(gotool, "env", "GOCACHE").CombinedOutput()
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "could not find testing GOCACHE: %v\n%s", err, out)
+			os.Exit(2)
+		}
+		testGOCACHE = strings.TrimSpace(string(out))
+
+		canMSan = testenv.HasCGO() && platform.MSanSupported(runtime.GOOS, runtime.GOARCH)
+		canASan = testenv.HasCGO() && platform.ASanSupported(runtime.GOOS, runtime.GOARCH)
+		canRace = testenv.HasCGO() && platform.RaceDetectorSupported(runtime.GOOS, runtime.GOARCH)
+		// The race detector doesn't work on Alpine Linux:
+		// golang.org/issue/14481
+		// gccgo does not support the race detector.
+		if isAlpineLinux() || runtime.Compiler == "gccgo" {
+			canRace = false
+		}
+	}
+
+	if n, limited := base.NetLimit(); limited && n > 0 {
+		// Split the network limit into chunks, so that each parallel script can
+		// have one chunk. We want to run as many parallel scripts as possible, but
+		// also want to give each script as high a limit as possible.
+		// We arbitrarily split by sqrt(n) to try to balance those two goals.
+		netTestLimit := int(math.Sqrt(float64(n)))
+		netTestSem = make(chan struct{}, netTestLimit)
+		reducedLimit := fmt.Sprintf(",%s=%d", base.NetLimitGodebug.Name(), n/netTestLimit)
+		os.Setenv("GODEBUG", os.Getenv("GODEBUG")+reducedLimit)
+	}
+
+	// Don't let these environment variables confuse the test.
+	os.Setenv("GOENV", "off")
+	os.Unsetenv("GOFLAGS")
+	os.Unsetenv("GOBIN")
+	os.Unsetenv("GOPATH")
+	os.Unsetenv("GIT_ALLOW_PROTOCOL")
+	os.Setenv("HOME", "/test-go-home-does-not-exist")
+	// On some systems the default C compiler is ccache.
+	// Setting HOME to a non-existent directory will break
+	// those systems. Disable ccache and use real compiler. Issue 17668.
+	os.Setenv("CCACHE_DISABLE", "1")
+	if cfg.Getenv("GOCACHE") == "" {
+		os.Setenv("GOCACHE", testGOCACHE) // because $HOME is gone
+	}
+
+	if testenv.Builder() != "" || os.Getenv("GIT_TRACE_CURL") == "1" {
+		// To help diagnose https://go.dev/issue/52545,
+		// enable tracing for Git HTTPS requests.
+		os.Setenv("GIT_TRACE_CURL", "1")
+		os.Setenv("GIT_TRACE_CURL_NO_DATA", "1")
+		os.Setenv("GIT_REDACT_COOKIES", "o,SSO,GSSO_Uberproxy")
+	}
+
+	r := m.Run()
+	if !*testWork {
+		removeAll(testTmpDir) // os.Exit won't run defer
+	}
+
+	if !*testWork {
+		// There shouldn't be anything left in topTmpdir.
+		var extraFiles, extraDirs []string
+		err := filepath.WalkDir(topTmpdir, func(path string, d fs.DirEntry, err error) error {
+			if err != nil {
+				return err
+			}
+			if path == topTmpdir {
+				return nil
+			}
+
+			if rel, err := filepath.Rel(topTmpdir, path); err == nil {
+				path = rel
+			}
+			if d.IsDir() {
+				extraDirs = append(extraDirs, path)
+			} else {
+				extraFiles = append(extraFiles, path)
+			}
+			return nil
+		})
+		if err != nil {
+			log.Fatal(err)
+		}
+
+		if len(extraFiles) > 0 {
+			log.Fatalf("unexpected files left in tmpdir: %q", extraFiles)
+		} else if len(extraDirs) > 0 {
+			log.Fatalf("unexpected subdirectories left in tmpdir: %q", extraDirs)
+		}
+
+		removeAll(topTmpdir)
+	}
+
+	os.Exit(r)
+}
+
+func isAlpineLinux() bool {
+	if runtime.GOOS != "linux" {
+		return false
+	}
+	fi, err := os.Lstat("/etc/alpine-release")
+	return err == nil && fi.Mode().IsRegular()
+}
+
+// The length of an mtime tick on this system. This is an estimate of
+// how long we need to sleep to ensure that the mtime of two files is
+// different.
+// We used to try to be clever but that didn't always work (see golang.org/issue/12205).
+var mtimeTick time.Duration = 1 * time.Second
+
+// Manage a single run of the testgo binary.
+type testgoData struct {
+	t              *testing.T
+	temps          []string
+	env            []string
+	tempdir        string
+	ran            bool
+	inParallel     bool
+	hasNet         bool
+	stdout, stderr bytes.Buffer
+	execDir        string // dir for tg.run
+}
+
+// skipIfGccgo skips the test if using gccgo.
+func skipIfGccgo(t *testing.T, msg string) {
+	if runtime.Compiler == "gccgo" {
+		t.Skipf("skipping test not supported on gccgo: %s", msg)
+	}
+}
+
+// testgo sets up for a test that runs testgo.
+func testgo(t *testing.T) *testgoData {
+	t.Helper()
+	testenv.MustHaveGoBuild(t)
+	testenv.SkipIfShortAndSlow(t)
+
+	return &testgoData{t: t}
+}
+
+// must gives a fatal error if err is not nil.
+func (tg *testgoData) must(err error) {
+	tg.t.Helper()
+	if err != nil {
+		tg.t.Fatal(err)
+	}
+}
+
+// check gives a test non-fatal error if err is not nil.
+func (tg *testgoData) check(err error) {
+	tg.t.Helper()
+	if err != nil {
+		tg.t.Error(err)
+	}
+}
+
+// parallel runs the test in parallel by calling t.Parallel.
+func (tg *testgoData) parallel() {
+	tg.t.Helper()
+	if tg.ran {
+		tg.t.Fatal("internal testsuite error: call to parallel after run")
+	}
+	if tg.hasNet {
+		tg.t.Fatal("internal testsuite error: call to parallel after acquireNet")
+	}
+	for _, e := range tg.env {
+		if strings.HasPrefix(e, "GOROOT=") || strings.HasPrefix(e, "GOPATH=") || strings.HasPrefix(e, "GOBIN=") {
+			val := e[strings.Index(e, "=")+1:]
+			if strings.HasPrefix(val, "testdata") || strings.HasPrefix(val, "./testdata") {
+				tg.t.Fatalf("internal testsuite error: call to parallel with testdata in environment (%s)", e)
+			}
+		}
+	}
+	tg.inParallel = true
+	tg.t.Parallel()
+}
+
+// acquireNet skips t if the network is unavailable, and otherwise acquires a
+// netTestSem token for t to be released at the end of the test.
+//
+// t.Parallel must not be called after acquireNet.
+func (tg *testgoData) acquireNet() {
+	tg.t.Helper()
+	if tg.hasNet {
+		return
+	}
+
+	testenv.MustHaveExternalNetwork(tg.t)
+	if netTestSem != nil {
+		netTestSem <- struct{}{}
+		tg.t.Cleanup(func() { <-netTestSem })
+	}
+	tg.setenv("TESTGONETWORK", "")
+	tg.hasNet = true
+}
+
+// pwd returns the current directory.
+func (tg *testgoData) pwd() string {
+	tg.t.Helper()
+	wd, err := os.Getwd()
+	if err != nil {
+		tg.t.Fatalf("could not get working directory: %v", err)
+	}
+	return wd
+}
+
+// sleep sleeps for one tick, where a tick is a conservative estimate
+// of how long it takes for a file modification to get a different
+// mtime.
+func (tg *testgoData) sleep() {
+	time.Sleep(mtimeTick)
+}
+
+// setenv sets an environment variable to use when running the test go
+// command.
+func (tg *testgoData) setenv(name, val string) {
+	tg.t.Helper()
+	tg.unsetenv(name)
+	tg.env = append(tg.env, name+"="+val)
+}
+
+// unsetenv removes an environment variable.
+func (tg *testgoData) unsetenv(name string) {
+	if tg.env == nil {
+		tg.env = append([]string(nil), os.Environ()...)
+		tg.env = append(tg.env, "GO111MODULE=off", "TESTGONETWORK=panic")
+		if testing.Short() {
+			tg.env = append(tg.env, "TESTGOVCS=panic")
+		}
+	}
+	for i, v := range tg.env {
+		if strings.HasPrefix(v, name+"=") {
+			tg.env = append(tg.env[:i], tg.env[i+1:]...)
+			break
+		}
+	}
+}
+
+func (tg *testgoData) goTool() string {
+	return testGo
+}
+
+// doRun runs the test go command, recording stdout and stderr and
+// returning exit status.
+func (tg *testgoData) doRun(args []string) error {
+	tg.t.Helper()
+	if tg.inParallel {
+		for _, arg := range args {
+			if strings.HasPrefix(arg, "testdata") || strings.HasPrefix(arg, "./testdata") {
+				tg.t.Fatal("internal testsuite error: parallel run using testdata")
+			}
+		}
+	}
+
+	hasGoroot := false
+	for _, v := range tg.env {
+		if strings.HasPrefix(v, "GOROOT=") {
+			hasGoroot = true
+			break
+		}
+	}
+	prog := tg.goTool()
+	if !hasGoroot {
+		tg.setenv("GOROOT", testGOROOT)
+	}
+
+	tg.t.Logf("running testgo %v", args)
+	cmd := testenv.Command(tg.t, prog, args...)
+	tg.stdout.Reset()
+	tg.stderr.Reset()
+	cmd.Dir = tg.execDir
+	cmd.Stdout = &tg.stdout
+	cmd.Stderr = &tg.stderr
+	cmd.Env = tg.env
+	status := cmd.Run()
+	if tg.stdout.Len() > 0 {
+		tg.t.Log("standard output:")
+		tg.t.Log(tg.stdout.String())
+	}
+	if tg.stderr.Len() > 0 {
+		tg.t.Log("standard error:")
+		tg.t.Log(tg.stderr.String())
+	}
+	tg.ran = true
+	return status
+}
+
+// run runs the test go command, and expects it to succeed.
+func (tg *testgoData) run(args ...string) {
+	tg.t.Helper()
+	if status := tg.doRun(args); status != nil {
+		wd, _ := os.Getwd()
+		tg.t.Logf("go %v failed unexpectedly in %s: %v", args, wd, status)
+		tg.t.FailNow()
+	}
+}
+
+// runFail runs the test go command, and expects it to fail.
+func (tg *testgoData) runFail(args ...string) {
+	tg.t.Helper()
+	if status := tg.doRun(args); status == nil {
+		tg.t.Fatal("testgo succeeded unexpectedly")
+	} else {
+		tg.t.Log("testgo failed as expected:", status)
+	}
+}
+
+// runGit runs a git command, and expects it to succeed.
+func (tg *testgoData) runGit(dir string, args ...string) {
+	tg.t.Helper()
+	cmd := testenv.Command(tg.t, "git", args...)
+	tg.stdout.Reset()
+	tg.stderr.Reset()
+	cmd.Stdout = &tg.stdout
+	cmd.Stderr = &tg.stderr
+	cmd.Dir = dir
+	cmd.Env = tg.env
+	status := cmd.Run()
+	if tg.stdout.Len() > 0 {
+		tg.t.Log("git standard output:")
+		tg.t.Log(tg.stdout.String())
+	}
+	if tg.stderr.Len() > 0 {
+		tg.t.Log("git standard error:")
+		tg.t.Log(tg.stderr.String())
+	}
+	if status != nil {
+		tg.t.Logf("git %v failed unexpectedly: %v", args, status)
+		tg.t.FailNow()
+	}
+}
+
+// getStdout returns standard output of the testgo run as a string.
+func (tg *testgoData) getStdout() string {
+	tg.t.Helper()
+	if !tg.ran {
+		tg.t.Fatal("internal testsuite error: stdout called before run")
+	}
+	return tg.stdout.String()
+}
+
+// getStderr returns standard error of the testgo run as a string.
+func (tg *testgoData) getStderr() string {
+	tg.t.Helper()
+	if !tg.ran {
+		tg.t.Fatal("internal testsuite error: stdout called before run")
+	}
+	return tg.stderr.String()
+}
+
+// doGrepMatch looks for a regular expression in a buffer, and returns
+// whether it is found. The regular expression is matched against
+// each line separately, as with the grep command.
+func (tg *testgoData) doGrepMatch(match string, b *bytes.Buffer) bool {
+	tg.t.Helper()
+	if !tg.ran {
+		tg.t.Fatal("internal testsuite error: grep called before run")
+	}
+	re := regexp.MustCompile(match)
+	for _, ln := range bytes.Split(b.Bytes(), []byte{'\n'}) {
+		if re.Match(ln) {
+			return true
+		}
+	}
+	return false
+}
+
+// doGrep looks for a regular expression in a buffer and fails if it
+// is not found. The name argument is the name of the output we are
+// searching, "output" or "error". The msg argument is logged on
+// failure.
+func (tg *testgoData) doGrep(match string, b *bytes.Buffer, name, msg string) {
+	tg.t.Helper()
+	if !tg.doGrepMatch(match, b) {
+		tg.t.Log(msg)
+		tg.t.Logf("pattern %v not found in standard %s", match, name)
+		tg.t.FailNow()
+	}
+}
+
+// grepStdout looks for a regular expression in the test run's
+// standard output and fails, logging msg, if it is not found.
+func (tg *testgoData) grepStdout(match, msg string) {
+	tg.t.Helper()
+	tg.doGrep(match, &tg.stdout, "output", msg)
+}
+
+// grepStderr looks for a regular expression in the test run's
+// standard error and fails, logging msg, if it is not found.
+func (tg *testgoData) grepStderr(match, msg string) {
+	tg.t.Helper()
+	tg.doGrep(match, &tg.stderr, "error", msg)
+}
+
+// grepBoth looks for a regular expression in the test run's standard
+// output or stand error and fails, logging msg, if it is not found.
+func (tg *testgoData) grepBoth(match, msg string) {
+	tg.t.Helper()
+	if !tg.doGrepMatch(match, &tg.stdout) && !tg.doGrepMatch(match, &tg.stderr) {
+		tg.t.Log(msg)
+		tg.t.Logf("pattern %v not found in standard output or standard error", match)
+		tg.t.FailNow()
+	}
+}
+
+// doGrepNot looks for a regular expression in a buffer and fails if
+// it is found. The name and msg arguments are as for doGrep.
+func (tg *testgoData) doGrepNot(match string, b *bytes.Buffer, name, msg string) {
+	tg.t.Helper()
+	if tg.doGrepMatch(match, b) {
+		tg.t.Log(msg)
+		tg.t.Logf("pattern %v found unexpectedly in standard %s", match, name)
+		tg.t.FailNow()
+	}
+}
+
+// grepStdoutNot looks for a regular expression in the test run's
+// standard output and fails, logging msg, if it is found.
+func (tg *testgoData) grepStdoutNot(match, msg string) {
+	tg.t.Helper()
+	tg.doGrepNot(match, &tg.stdout, "output", msg)
+}
+
+// grepStderrNot looks for a regular expression in the test run's
+// standard error and fails, logging msg, if it is found.
+func (tg *testgoData) grepStderrNot(match, msg string) {
+	tg.t.Helper()
+	tg.doGrepNot(match, &tg.stderr, "error", msg)
+}
+
+// grepBothNot looks for a regular expression in the test run's
+// standard output or standard error and fails, logging msg, if it is
+// found.
+func (tg *testgoData) grepBothNot(match, msg string) {
+	tg.t.Helper()
+	if tg.doGrepMatch(match, &tg.stdout) || tg.doGrepMatch(match, &tg.stderr) {
+		tg.t.Log(msg)
+		tg.t.Fatalf("pattern %v found unexpectedly in standard output or standard error", match)
+	}
+}
+
+// doGrepCount counts the number of times a regexp is seen in a buffer.
+func (tg *testgoData) doGrepCount(match string, b *bytes.Buffer) int {
+	tg.t.Helper()
+	if !tg.ran {
+		tg.t.Fatal("internal testsuite error: doGrepCount called before run")
+	}
+	re := regexp.MustCompile(match)
+	c := 0
+	for _, ln := range bytes.Split(b.Bytes(), []byte{'\n'}) {
+		if re.Match(ln) {
+			c++
+		}
+	}
+	return c
+}
+
+// grepCountBoth returns the number of times a regexp is seen in both
+// standard output and standard error.
+func (tg *testgoData) grepCountBoth(match string) int {
+	tg.t.Helper()
+	return tg.doGrepCount(match, &tg.stdout) + tg.doGrepCount(match, &tg.stderr)
+}
+
+// creatingTemp records that the test plans to create a temporary file
+// or directory. If the file or directory exists already, it will be
+// removed. When the test completes, the file or directory will be
+// removed if it exists.
+func (tg *testgoData) creatingTemp(path string) {
+	tg.t.Helper()
+	if filepath.IsAbs(path) && !strings.HasPrefix(path, tg.tempdir) {
+		tg.t.Fatalf("internal testsuite error: creatingTemp(%q) with absolute path not in temporary directory", path)
+	}
+	tg.must(robustio.RemoveAll(path))
+	tg.temps = append(tg.temps, path)
+}
+
+// makeTempdir makes a temporary directory for a run of testgo. If
+// the temporary directory was already created, this does nothing.
+func (tg *testgoData) makeTempdir() {
+	tg.t.Helper()
+	if tg.tempdir == "" {
+		var err error
+		tg.tempdir, err = os.MkdirTemp("", "gotest")
+		tg.must(err)
+	}
+}
+
+// tempFile adds a temporary file for a run of testgo.
+func (tg *testgoData) tempFile(path, contents string) {
+	tg.t.Helper()
+	tg.makeTempdir()
+	tg.must(os.MkdirAll(filepath.Join(tg.tempdir, filepath.Dir(path)), 0755))
+	bytes := []byte(contents)
+	if strings.HasSuffix(path, ".go") {
+		formatted, err := format.Source(bytes)
+		if err == nil {
+			bytes = formatted
+		}
+	}
+	tg.must(os.WriteFile(filepath.Join(tg.tempdir, path), bytes, 0644))
+}
+
+// tempDir adds a temporary directory for a run of testgo.
+func (tg *testgoData) tempDir(path string) {
+	tg.t.Helper()
+	tg.makeTempdir()
+	if err := os.MkdirAll(filepath.Join(tg.tempdir, path), 0755); err != nil && !os.IsExist(err) {
+		tg.t.Fatal(err)
+	}
+}
+
+// path returns the absolute pathname to file with the temporary
+// directory.
+func (tg *testgoData) path(name string) string {
+	tg.t.Helper()
+	if tg.tempdir == "" {
+		tg.t.Fatalf("internal testsuite error: path(%q) with no tempdir", name)
+	}
+	if name == "." {
+		return tg.tempdir
+	}
+	return filepath.Join(tg.tempdir, name)
+}
+
+// mustExist fails if path does not exist.
+func (tg *testgoData) mustExist(path string) {
+	tg.t.Helper()
+	if _, err := os.Stat(path); err != nil {
+		if os.IsNotExist(err) {
+			tg.t.Fatalf("%s does not exist but should", path)
+		}
+		tg.t.Fatalf("%s stat failed: %v", path, err)
+	}
+}
+
+// mustNotExist fails if path exists.
+func (tg *testgoData) mustNotExist(path string) {
+	tg.t.Helper()
+	if _, err := os.Stat(path); err == nil || !os.IsNotExist(err) {
+		tg.t.Fatalf("%s exists but should not (%v)", path, err)
+	}
+}
+
+// mustHaveContent succeeds if filePath is a path to a file,
+// and that file is readable and not empty.
+func (tg *testgoData) mustHaveContent(filePath string) {
+	tg.mustExist(filePath)
+	f, err := os.Stat(filePath)
+	if err != nil {
+		tg.t.Fatal(err)
+	}
+	if f.Size() == 0 {
+		tg.t.Fatalf("expected %s to have data, but is empty", filePath)
+	}
+}
+
+// wantExecutable fails with msg if path is not executable.
+func (tg *testgoData) wantExecutable(path, msg string) {
+	tg.t.Helper()
+	if st, err := os.Stat(path); err != nil {
+		if !os.IsNotExist(err) {
+			tg.t.Log(err)
+		}
+		tg.t.Fatal(msg)
+	} else {
+		if runtime.GOOS != "windows" && st.Mode()&0111 == 0 {
+			tg.t.Fatalf("binary %s exists but is not executable", path)
+		}
+	}
+}
+
+// isStale reports whether pkg is stale, and why
+func (tg *testgoData) isStale(pkg string) (bool, string) {
+	tg.t.Helper()
+	tg.run("list", "-f", "{{.Stale}}:{{.StaleReason}}", pkg)
+	v := strings.TrimSpace(tg.getStdout())
+	f := strings.SplitN(v, ":", 2)
+	if len(f) == 2 {
+		switch f[0] {
+		case "true":
+			return true, f[1]
+		case "false":
+			return false, f[1]
+		}
+	}
+	tg.t.Fatalf("unexpected output checking staleness of package %v: %v", pkg, v)
+	panic("unreachable")
+}
+
+// wantStale fails with msg if pkg is not stale.
+func (tg *testgoData) wantStale(pkg, reason, msg string) {
+	tg.t.Helper()
+	stale, why := tg.isStale(pkg)
+	if !stale {
+		tg.t.Fatal(msg)
+	}
+	// We always accept the reason as being "not installed but
+	// available in build cache", because when that is the case go
+	// list doesn't try to sort out the underlying reason why the
+	// package is not installed.
+	if reason == "" && why != "" || !strings.Contains(why, reason) && !strings.Contains(why, "not installed but available in build cache") {
+		tg.t.Errorf("wrong reason for Stale=true: %q, want %q", why, reason)
+	}
+}
+
+// wantNotStale fails with msg if pkg is stale.
+func (tg *testgoData) wantNotStale(pkg, reason, msg string) {
+	tg.t.Helper()
+	stale, why := tg.isStale(pkg)
+	if stale {
+		tg.t.Fatal(msg)
+	}
+	if reason == "" && why != "" || !strings.Contains(why, reason) {
+		tg.t.Errorf("wrong reason for Stale=false: %q, want %q", why, reason)
+	}
+}
+
+// If -testwork is specified, the test prints the name of the temp directory
+// and does not remove it when done, so that a programmer can
+// poke at the test file tree afterward.
+var testWork = flag.Bool("testwork", false, "")
+
+// cleanup cleans up a test that runs testgo.
+func (tg *testgoData) cleanup() {
+	tg.t.Helper()
+	if *testWork {
+		if tg.tempdir != "" {
+			tg.t.Logf("TESTWORK=%s\n", tg.path("."))
+		}
+		return
+	}
+	for _, path := range tg.temps {
+		tg.check(removeAll(path))
+	}
+	if tg.tempdir != "" {
+		tg.check(removeAll(tg.tempdir))
+	}
+}
+
+func removeAll(dir string) error {
+	// module cache has 0444 directories;
+	// make them writable in order to remove content.
+	filepath.WalkDir(dir, func(path string, info fs.DirEntry, err error) error {
+		// chmod not only directories, but also things that we couldn't even stat
+		// due to permission errors: they may also be unreadable directories.
+		if err != nil || info.IsDir() {
+			os.Chmod(path, 0777)
+		}
+		return nil
+	})
+	return robustio.RemoveAll(dir)
+}
+
+// failSSH puts an ssh executable in the PATH that always fails.
+// This is to stub out uses of ssh by go get.
+func (tg *testgoData) failSSH() {
+	tg.t.Helper()
+	wd, err := os.Getwd()
+	if err != nil {
+		tg.t.Fatal(err)
+	}
+	fail := filepath.Join(wd, "testdata/failssh")
+	tg.setenv("PATH", fmt.Sprintf("%v%c%v", fail, filepath.ListSeparator, os.Getenv("PATH")))
+}
+
+func TestNewReleaseRebuildsStalePackagesInGOPATH(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping lengthy test in short mode")
+	}
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+
+	// Set GOCACHE to an empty directory so that a previous run of
+	// this test does not affect the staleness of the packages it builds.
+	tg.tempDir("gocache")
+	tg.setenv("GOCACHE", tg.path("gocache"))
+
+	// Copy the runtime packages into a temporary GOROOT
+	// so that we can change files.
+	for _, copydir := range []string{
+		"src/runtime",
+		"src/internal/abi",
+		"src/internal/bytealg",
+		"src/internal/coverage/rtcov",
+		"src/internal/cpu",
+		"src/internal/goarch",
+		"src/internal/godebugs",
+		"src/internal/goexperiment",
+		"src/internal/goos",
+		"src/internal/coverage/rtcov",
+		"src/math/bits",
+		"src/unsafe",
+		filepath.Join("pkg/tool", goHostOS+"_"+goHostArch),
+		"pkg/include",
+	} {
+		srcdir := filepath.Join(testGOROOT, copydir)
+		tg.tempDir(filepath.Join("goroot", copydir))
+		err := filepath.WalkDir(srcdir,
+			func(path string, info fs.DirEntry, err error) error {
+				if err != nil {
+					return err
+				}
+				if info.IsDir() {
+					return nil
+				}
+				srcrel, err := filepath.Rel(srcdir, path)
+				if err != nil {
+					return err
+				}
+				dest := filepath.Join("goroot", copydir, srcrel)
+				data, err := os.ReadFile(path)
+				if err != nil {
+					return err
+				}
+				tg.tempFile(dest, string(data))
+				if strings.Contains(copydir, filepath.Join("pkg", "tool")) {
+					os.Chmod(tg.path(dest), 0777)
+				}
+				return nil
+			})
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+	tg.setenv("GOROOT", tg.path("goroot"))
+
+	addVar := func(name string, idx int) (restore func()) {
+		data, err := os.ReadFile(name)
+		if err != nil {
+			t.Fatal(err)
+		}
+		old := data
+		data = append(data, fmt.Sprintf("var DummyUnusedVar%d bool\n", idx)...)
+		if err := os.WriteFile(name, append(data, '\n'), 0666); err != nil {
+			t.Fatal(err)
+		}
+		tg.sleep()
+		return func() {
+			if err := os.WriteFile(name, old, 0666); err != nil {
+				t.Fatal(err)
+			}
+		}
+	}
+
+	// Every main package depends on the "runtime".
+	tg.tempFile("d1/src/p1/p1.go", `package main; func main(){}`)
+	tg.setenv("GOPATH", tg.path("d1"))
+	// Pass -i flag to rebuild everything outdated.
+	tg.run("install", "p1")
+	tg.wantNotStale("p1", "", "./testgo list claims p1 is stale, incorrectly, before any changes")
+
+	// Changing mtime of runtime/internal/sys/sys.go
+	// should have no effect: only the content matters.
+	// In fact this should be true even outside a release branch.
+	sys := tg.path("goroot/src/runtime/internal/sys/sys.go")
+	tg.sleep()
+	restore := addVar(sys, 0)
+	restore()
+	tg.wantNotStale("p1", "", "./testgo list claims p1 is stale, incorrectly, after updating mtime of runtime/internal/sys/sys.go")
+
+	// But changing content of any file should have an effect.
+	// Previously zversion.go was the only one that mattered;
+	// now they all matter, so keep using sys.go.
+	restore = addVar(sys, 1)
+	defer restore()
+	tg.wantStale("p1", "stale dependency: runtime/internal", "./testgo list claims p1 is NOT stale, incorrectly, after changing sys.go")
+	restore()
+	tg.wantNotStale("p1", "", "./testgo list claims p1 is stale, incorrectly, after changing back to old release")
+	addVar(sys, 2)
+	tg.wantStale("p1", "stale dependency: runtime", "./testgo list claims p1 is NOT stale, incorrectly, after changing sys.go again")
+	tg.run("install", "p1")
+	tg.wantNotStale("p1", "", "./testgo list claims p1 is stale after building with new release")
+
+	// Restore to "old" release.
+	restore()
+	tg.wantStale("p1", "stale dependency: runtime/internal", "./testgo list claims p1 is NOT stale, incorrectly, after restoring sys.go")
+	tg.run("install", "p1")
+	tg.wantNotStale("p1", "", "./testgo list claims p1 is stale after building with old release")
+}
+
+// cmd/go: custom import path checking should not apply to Go packages without import comment.
+func TestIssue10952(t *testing.T) {
+	testenv.MustHaveExecPath(t, "git")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.acquireNet()
+
+	tg.tempDir("src")
+	tg.setenv("GOPATH", tg.path("."))
+	const importPath = "github.com/zombiezen/go-get-issue-10952"
+	tg.run("get", "-d", "-u", importPath)
+	repoDir := tg.path("src/" + importPath)
+	tg.runGit(repoDir, "remote", "set-url", "origin", "https://"+importPath+".git")
+	tg.run("get", "-d", "-u", importPath)
+}
+
+// Test git clone URL that uses SCP-like syntax and custom import path checking.
+func TestIssue11457(t *testing.T) {
+	testenv.MustHaveExecPath(t, "git")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.acquireNet()
+
+	tg.tempDir("src")
+	tg.setenv("GOPATH", tg.path("."))
+	const importPath = "rsc.io/go-get-issue-11457"
+	tg.run("get", "-d", "-u", importPath)
+	repoDir := tg.path("src/" + importPath)
+	tg.runGit(repoDir, "remote", "set-url", "origin", "git@github.com:rsc/go-get-issue-11457")
+
+	// At this time, custom import path checking compares remotes verbatim (rather than
+	// just the host and path, skipping scheme and user), so we expect go get -u to fail.
+	// However, the goal of this test is to verify that gitRemoteRepo correctly parsed
+	// the SCP-like syntax, and we expect it to appear in the error message.
+	tg.runFail("get", "-d", "-u", importPath)
+	want := " is checked out from ssh://git@github.com/rsc/go-get-issue-11457"
+	if !strings.HasSuffix(strings.TrimSpace(tg.getStderr()), want) {
+		t.Error("expected clone URL to appear in stderr")
+	}
+}
+
+func TestGetGitDefaultBranch(t *testing.T) {
+	testenv.MustHaveExecPath(t, "git")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.acquireNet()
+
+	tg.tempDir("src")
+	tg.setenv("GOPATH", tg.path("."))
+
+	// This repo has two branches, master and another-branch.
+	// The another-branch is the default that you get from 'git clone'.
+	// The go get command variants should not override this.
+	const importPath = "github.com/rsc/go-get-default-branch"
+
+	tg.run("get", "-d", importPath)
+	repoDir := tg.path("src/" + importPath)
+	tg.runGit(repoDir, "branch", "--contains", "HEAD")
+	tg.grepStdout(`\* another-branch`, "not on correct default branch")
+
+	tg.run("get", "-d", "-u", importPath)
+	tg.runGit(repoDir, "branch", "--contains", "HEAD")
+	tg.grepStdout(`\* another-branch`, "not on correct default branch")
+}
+
+func TestPackageMainTestCompilerFlags(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.makeTempdir()
+	tg.setenv("GOPATH", tg.path("."))
+	tg.tempFile("src/p1/p1.go", "package main\n")
+	tg.tempFile("src/p1/p1_test.go", "package main\nimport \"testing\"\nfunc Test(t *testing.T){}\n")
+	tg.run("test", "-c", "-n", "p1")
+	tg.grepBothNot(`([\\/]compile|gccgo).* (-p main|-fgo-pkgpath=main).*p1\.go`, "should not have run compile -p main p1.go")
+	tg.grepStderr(`([\\/]compile|gccgo).* (-p p1|-fgo-pkgpath=p1).*p1\.go`, "should have run compile -p p1 p1.go")
+}
+
+// Issue 4104.
+func TestGoTestWithPackageListedMultipleTimes(t *testing.T) {
+	tooSlow(t, "links and runs a test")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.run("test", "errors", "errors", "errors", "errors", "errors")
+	if strings.Contains(strings.TrimSpace(tg.getStdout()), "\n") {
+		t.Error("go test errors errors errors errors errors tested the same package multiple times")
+	}
+}
+
+func TestGoListHasAConsistentOrder(t *testing.T) {
+	tooSlow(t, "walks all of GOROOT/src twice")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.run("list", "std")
+	first := tg.getStdout()
+	tg.run("list", "std")
+	if first != tg.getStdout() {
+		t.Error("go list std ordering is inconsistent")
+	}
+}
+
+func TestGoListStdDoesNotIncludeCommands(t *testing.T) {
+	tooSlow(t, "walks all of GOROOT/src")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.run("list", "std")
+	tg.grepStdoutNot("cmd/", "go list std shows commands")
+}
+
+func TestGoListCmdOnlyShowsCommands(t *testing.T) {
+	skipIfGccgo(t, "gccgo does not have GOROOT")
+	tooSlow(t, "walks all of GOROOT/src/cmd")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.run("list", "cmd")
+	out := strings.TrimSpace(tg.getStdout())
+	for _, line := range strings.Split(out, "\n") {
+		if !strings.Contains(line, "cmd/") {
+			t.Error("go list cmd shows non-commands")
+			break
+		}
+	}
+}
+
+func TestGoListDeps(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempDir("src/p1/p2/p3/p4")
+	tg.setenv("GOPATH", tg.path("."))
+	tg.tempFile("src/p1/p.go", "package p1\nimport _ \"p1/p2\"\n")
+	tg.tempFile("src/p1/p2/p.go", "package p2\nimport _ \"p1/p2/p3\"\n")
+	tg.tempFile("src/p1/p2/p3/p.go", "package p3\nimport _ \"p1/p2/p3/p4\"\n")
+	tg.tempFile("src/p1/p2/p3/p4/p.go", "package p4\n")
+	tg.run("list", "-f", "{{.Deps}}", "p1")
+	tg.grepStdout("p1/p2/p3/p4", "Deps(p1) does not mention p4")
+
+	tg.run("list", "-deps", "p1")
+	tg.grepStdout("p1/p2/p3/p4", "-deps p1 does not mention p4")
+
+	if runtime.Compiler != "gccgo" {
+		// Check the list is in dependency order.
+		tg.run("list", "-deps", "math")
+		want := "internal/cpu\nunsafe\nmath/bits\nmath\n"
+		out := tg.stdout.String()
+		if !strings.Contains(out, "internal/cpu") {
+			// Some systems don't use internal/cpu.
+			want = "unsafe\nmath/bits\nmath\n"
+		}
+		if tg.stdout.String() != want {
+			t.Fatalf("list -deps math: wrong order\nhave %q\nwant %q", tg.stdout.String(), want)
+		}
+	}
+}
+
+func TestGoListTest(t *testing.T) {
+	skipIfGccgo(t, "gccgo does not have standard packages")
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.makeTempdir()
+	tg.setenv("GOCACHE", tg.tempdir)
+
+	tg.run("list", "-test", "-deps", "sort")
+	tg.grepStdout(`^sort.test$`, "missing test main")
+	tg.grepStdout(`^sort$`, "missing real sort")
+	tg.grepStdout(`^sort \[sort.test\]$`, "missing test copy of sort")
+	tg.grepStdout(`^testing \[sort.test\]$`, "missing test copy of testing")
+	tg.grepStdoutNot(`^testing$`, "unexpected real copy of testing")
+
+	tg.run("list", "-test", "sort")
+	tg.grepStdout(`^sort.test$`, "missing test main")
+	tg.grepStdout(`^sort$`, "missing real sort")
+	tg.grepStdout(`^sort \[sort.test\]$`, "unexpected test copy of sort")
+	tg.grepStdoutNot(`^testing \[sort.test\]$`, "unexpected test copy of testing")
+	tg.grepStdoutNot(`^testing$`, "unexpected real copy of testing")
+
+	tg.run("list", "-test", "cmd/buildid", "cmd/doc")
+	tg.grepStdout(`^cmd/buildid$`, "missing cmd/buildid")
+	tg.grepStdout(`^cmd/doc$`, "missing cmd/doc")
+	tg.grepStdout(`^cmd/doc\.test$`, "missing cmd/doc test")
+	tg.grepStdoutNot(`^cmd/buildid\.test$`, "unexpected cmd/buildid test")
+	tg.grepStdoutNot(`^testing`, "unexpected testing")
+
+	tg.run("list", "-test", "runtime/cgo")
+	tg.grepStdout(`^runtime/cgo$`, "missing runtime/cgo")
+
+	tg.run("list", "-deps", "-f", "{{if .DepOnly}}{{.ImportPath}}{{end}}", "sort")
+	tg.grepStdout(`^internal/reflectlite$`, "missing internal/reflectlite")
+	tg.grepStdoutNot(`^sort`, "unexpected sort")
+}
+
+func TestGoListCompiledCgo(t *testing.T) {
+	tooSlow(t, "compiles cgo files")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.makeTempdir()
+	tg.setenv("GOCACHE", tg.tempdir)
+
+	tg.run("list", "-f", `{{join .CgoFiles "\n"}}`, "net")
+	if tg.stdout.String() == "" {
+		t.Skip("net does not use cgo")
+	}
+	if strings.Contains(tg.stdout.String(), tg.tempdir) {
+		t.Fatalf(".CgoFiles unexpectedly mentioned cache %s", tg.tempdir)
+	}
+	tg.run("list", "-compiled", "-f", `{{.Dir}}{{"\n"}}{{join .CompiledGoFiles "\n"}}`, "net")
+	if !strings.Contains(tg.stdout.String(), tg.tempdir) {
+		t.Fatalf(".CompiledGoFiles with -compiled did not mention cache %s", tg.tempdir)
+	}
+	dir := ""
+	for _, file := range strings.Split(tg.stdout.String(), "\n") {
+		if file == "" {
+			continue
+		}
+		if dir == "" {
+			dir = file
+			continue
+		}
+		if !strings.Contains(file, "/") && !strings.Contains(file, `\`) {
+			file = filepath.Join(dir, file)
+		}
+		if _, err := os.Stat(file); err != nil {
+			t.Fatalf("cannot find .CompiledGoFiles result %s: %v", file, err)
+		}
+	}
+}
+
+func TestGoListExport(t *testing.T) {
+	skipIfGccgo(t, "gccgo does not have standard packages")
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.makeTempdir()
+	tg.setenv("GOCACHE", tg.tempdir)
+
+	tg.run("list", "-f", "{{.Export}}", "strings")
+	if tg.stdout.String() != "" {
+		t.Fatalf(".Export without -export unexpectedly set")
+	}
+	tg.run("list", "-export", "-f", "{{.Export}}", "strings")
+	file := strings.TrimSpace(tg.stdout.String())
+	if file == "" {
+		t.Fatalf(".Export with -export was empty")
+	}
+	if _, err := os.Stat(file); err != nil {
+		t.Fatalf("cannot find .Export result %s: %v", file, err)
+	}
+
+	tg.run("list", "-export", "-f", "{{.BuildID}}", "strings")
+	buildID := strings.TrimSpace(tg.stdout.String())
+	if buildID == "" {
+		t.Fatalf(".BuildID with -export was empty")
+	}
+
+	tg.run("tool", "buildid", file)
+	toolBuildID := strings.TrimSpace(tg.stdout.String())
+	if buildID != toolBuildID {
+		t.Fatalf(".BuildID with -export %q disagrees with 'go tool buildid' %q", buildID, toolBuildID)
+	}
+}
+
+// Issue 4096. Validate the output of unsuccessful go install foo/quxx.
+func TestUnsuccessfulGoInstallShouldMentionMissingPackage(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.runFail("install", "foo/quxx")
+	if tg.grepCountBoth(`cannot find package "foo/quxx" in any of`) != 1 {
+		t.Error(`go install foo/quxx expected error: .*cannot find package "foo/quxx" in any of`)
+	}
+}
+
+func TestGOROOTSearchFailureReporting(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.runFail("install", "foo/quxx")
+	if tg.grepCountBoth(regexp.QuoteMeta(filepath.Join("foo", "quxx"))+` \(from \$GOROOT\)$`) != 1 {
+		t.Error(`go install foo/quxx expected error: .*foo/quxx (from $GOROOT)`)
+	}
+}
+
+func TestMultipleGOPATHEntriesReportedSeparately(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	sep := string(filepath.ListSeparator)
+	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata", "a")+sep+filepath.Join(tg.pwd(), "testdata", "b"))
+	tg.runFail("install", "foo/quxx")
+	if tg.grepCountBoth(`testdata[/\\].[/\\]src[/\\]foo[/\\]quxx`) != 2 {
+		t.Error(`go install foo/quxx expected error: .*testdata/a/src/foo/quxx (from $GOPATH)\n.*testdata/b/src/foo/quxx`)
+	}
+}
+
+// Test (from $GOPATH) annotation is reported for the first GOPATH entry,
+func TestMentionGOPATHInFirstGOPATHEntry(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	sep := string(filepath.ListSeparator)
+	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata", "a")+sep+filepath.Join(tg.pwd(), "testdata", "b"))
+	tg.runFail("install", "foo/quxx")
+	if tg.grepCountBoth(regexp.QuoteMeta(filepath.Join("testdata", "a", "src", "foo", "quxx"))+` \(from \$GOPATH\)$`) != 1 {
+		t.Error(`go install foo/quxx expected error: .*testdata/a/src/foo/quxx (from $GOPATH)`)
+	}
+}
+
+// but not on the second.
+func TestMentionGOPATHNotOnSecondEntry(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	sep := string(filepath.ListSeparator)
+	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata", "a")+sep+filepath.Join(tg.pwd(), "testdata", "b"))
+	tg.runFail("install", "foo/quxx")
+	if tg.grepCountBoth(regexp.QuoteMeta(filepath.Join("testdata", "b", "src", "foo", "quxx"))+`$`) != 1 {
+		t.Error(`go install foo/quxx expected error: .*testdata/b/src/foo/quxx`)
+	}
+}
+
+func homeEnvName() string {
+	switch runtime.GOOS {
+	case "windows":
+		return "USERPROFILE"
+	case "plan9":
+		return "home"
+	default:
+		return "HOME"
+	}
+}
+
+func tempEnvName() string {
+	switch runtime.GOOS {
+	case "windows":
+		return "TMP"
+	case "plan9":
+		return "TMPDIR" // actually plan 9 doesn't have one at all but this is fine
+	default:
+		return "TMPDIR"
+	}
+}
+
+func pathEnvName() string {
+	switch runtime.GOOS {
+	case "plan9":
+		return "path"
+	default:
+		return "PATH"
+	}
+}
+
+func TestDefaultGOPATH(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempDir("home/go")
+	tg.setenv(homeEnvName(), tg.path("home"))
+
+	tg.run("env", "GOPATH")
+	tg.grepStdout(regexp.QuoteMeta(tg.path("home/go")), "want GOPATH=$HOME/go")
+
+	tg.setenv("GOROOT", tg.path("home/go"))
+	tg.run("env", "GOPATH")
+	tg.grepStdoutNot(".", "want unset GOPATH because GOROOT=$HOME/go")
+
+	tg.setenv("GOROOT", tg.path("home/go")+"/")
+	tg.run("env", "GOPATH")
+	tg.grepStdoutNot(".", "want unset GOPATH because GOROOT=$HOME/go/")
+}
+
+func TestDefaultGOPATHGet(t *testing.T) {
+	testenv.MustHaveExecPath(t, "git")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.acquireNet()
+
+	tg.setenv("GOPATH", "")
+	tg.tempDir("home")
+	tg.setenv(homeEnvName(), tg.path("home"))
+
+	// warn for creating directory
+	tg.run("get", "-v", "github.com/golang/example/hello")
+	tg.grepStderr("created GOPATH="+regexp.QuoteMeta(tg.path("home/go"))+"; see 'go help gopath'", "did not create GOPATH")
+
+	// no warning if directory already exists
+	tg.must(robustio.RemoveAll(tg.path("home/go")))
+	tg.tempDir("home/go")
+	tg.run("get", "github.com/golang/example/hello")
+	tg.grepStderrNot(".", "expected no output on standard error")
+
+	// error if $HOME/go is a file
+	tg.must(robustio.RemoveAll(tg.path("home/go")))
+	tg.tempFile("home/go", "")
+	tg.runFail("get", "github.com/golang/example/hello")
+	tg.grepStderr(`mkdir .*[/\\]go: .*(not a directory|cannot find the path)`, "expected error because $HOME/go is a file")
+}
+
+func TestDefaultGOPATHPrintedSearchList(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.setenv("GOPATH", "")
+	tg.tempDir("home")
+	tg.setenv(homeEnvName(), tg.path("home"))
+
+	tg.runFail("install", "github.com/golang/example/hello")
+	tg.grepStderr(regexp.QuoteMeta(tg.path("home/go/src/github.com/golang/example/hello"))+`.*from \$GOPATH`, "expected default GOPATH")
+}
+
+func TestLdflagsArgumentsWithSpacesIssue3941(t *testing.T) {
+	skipIfGccgo(t, "gccgo does not support -ldflags -X")
+	tooSlow(t, "compiles and links a binary")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempFile("main.go", `package main
+		var extern string
+		func main() {
+			println(extern)
+		}`)
+	tg.run("run", "-ldflags", `-X "main.extern=hello world"`, tg.path("main.go"))
+	tg.grepStderr("^hello world", `ldflags -X "main.extern=hello world"' failed`)
+}
+
+func TestLdFlagsLongArgumentsIssue42295(t *testing.T) {
+	// Test the extremely long command line arguments that contain '\n' characters
+	// get encoded and passed correctly.
+	skipIfGccgo(t, "gccgo does not support -ldflags -X")
+	tooSlow(t, "compiles and links a binary")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempFile("main.go", `package main
+		var extern string
+		func main() {
+			print(extern)
+		}`)
+	testStr := "test test test test test \n\\ "
+	var buf strings.Builder
+	for buf.Len() < sys.ExecArgLengthLimit+1 {
+		buf.WriteString(testStr)
+	}
+	tg.run("run", "-ldflags", fmt.Sprintf(`-X "main.extern=%s"`, buf.String()), tg.path("main.go"))
+	if tg.stderr.String() != buf.String() {
+		t.Errorf("strings differ")
+	}
+}
+
+func TestGoTestDashCDashOControlsBinaryLocation(t *testing.T) {
+	skipIfGccgo(t, "gccgo has no standard packages")
+	tooSlow(t, "compiles and links a test binary")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.makeTempdir()
+	tg.run("test", "-c", "-o", tg.path("myerrors.test"+exeSuffix), "errors")
+	tg.wantExecutable(tg.path("myerrors.test"+exeSuffix), "go test -c -o myerrors.test did not create myerrors.test")
+}
+
+func TestGoTestDashOWritesBinary(t *testing.T) {
+	skipIfGccgo(t, "gccgo has no standard packages")
+	tooSlow(t, "compiles and runs a test binary")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.makeTempdir()
+	tg.run("test", "-o", tg.path("myerrors.test"+exeSuffix), "errors")
+	tg.wantExecutable(tg.path("myerrors.test"+exeSuffix), "go test -o myerrors.test did not create myerrors.test")
+}
+
+// Issue 4515.
+func TestInstallWithTags(t *testing.T) {
+	tooSlow(t, "compiles and links binaries")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempDir("bin")
+	tg.tempFile("src/example/a/main.go", `package main
+		func main() {}`)
+	tg.tempFile("src/example/b/main.go", `// +build mytag
+
+		package main
+		func main() {}`)
+	tg.setenv("GOPATH", tg.path("."))
+	tg.run("install", "-tags", "mytag", "example/a", "example/b")
+	tg.wantExecutable(tg.path("bin/a"+exeSuffix), "go install example/a example/b did not install binaries")
+	tg.wantExecutable(tg.path("bin/b"+exeSuffix), "go install example/a example/b did not install binaries")
+	tg.must(os.Remove(tg.path("bin/a" + exeSuffix)))
+	tg.must(os.Remove(tg.path("bin/b" + exeSuffix)))
+	tg.run("install", "-tags", "mytag", "example/...")
+	tg.wantExecutable(tg.path("bin/a"+exeSuffix), "go install example/... did not install binaries")
+	tg.wantExecutable(tg.path("bin/b"+exeSuffix), "go install example/... did not install binaries")
+	tg.run("list", "-tags", "mytag", "example/b...")
+	if strings.TrimSpace(tg.getStdout()) != "example/b" {
+		t.Error("go list example/b did not find example/b")
+	}
+}
+
+// Issue 17451, 17662.
+func TestSymlinkWarning(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.makeTempdir()
+	tg.setenv("GOPATH", tg.path("."))
+
+	tg.tempDir("src/example/xx")
+	tg.tempDir("yy/zz")
+	tg.tempFile("yy/zz/zz.go", "package zz\n")
+	if err := os.Symlink(tg.path("yy"), tg.path("src/example/xx/yy")); err != nil {
+		t.Skipf("symlink failed: %v", err)
+	}
+	tg.run("list", "example/xx/z...")
+	tg.grepStdoutNot(".", "list should not have matched anything")
+	tg.grepStderr("matched no packages", "list should have reported that pattern matched no packages")
+	tg.grepStderrNot("symlink", "list should not have reported symlink")
+
+	tg.run("list", "example/xx/...")
+	tg.grepStdoutNot(".", "list should not have matched anything")
+	tg.grepStderr("matched no packages", "list should have reported that pattern matched no packages")
+	tg.grepStderr("ignoring symlink", "list should have reported symlink")
+}
+
+func TestCgoShowsFullPathNames(t *testing.T) {
+	testenv.MustHaveCGO(t)
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempFile("src/x/y/dirname/foo.go", `
+		package foo
+		import "C"
+		func f() {`)
+	tg.setenv("GOPATH", tg.path("."))
+	tg.runFail("build", "x/y/dirname")
+	tg.grepBoth("x/y/dirname", "error did not use full path")
+}
+
+func TestCgoHandlesWlORIGIN(t *testing.T) {
+	tooSlow(t, "compiles cgo files")
+	testenv.MustHaveCGO(t)
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempFile("src/origin/origin.go", `package origin
+		// #cgo !darwin,!windows LDFLAGS: -Wl,-rpath,$ORIGIN
+		// void f(void) {}
+		import "C"
+		func f() { C.f() }`)
+	tg.setenv("GOPATH", tg.path("."))
+	tg.run("build", "origin")
+}
+
+func TestCgoPkgConfig(t *testing.T) {
+	tooSlow(t, "compiles cgo files")
+	testenv.MustHaveCGO(t)
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+
+	tg.run("env", "PKG_CONFIG")
+	pkgConfig := strings.TrimSpace(tg.getStdout())
+	testenv.MustHaveExecPath(t, pkgConfig)
+	if out, err := testenv.Command(t, pkgConfig, "--atleast-pkgconfig-version", "0.24").CombinedOutput(); err != nil {
+		t.Skipf("%s --atleast-pkgconfig-version 0.24: %v\n%s", pkgConfig, err, out)
+	}
+
+	// OpenBSD's pkg-config is strict about whitespace and only
+	// supports backslash-escaped whitespace. It does not support
+	// quotes, which the normal freedesktop.org pkg-config does
+	// support. See https://man.openbsd.org/pkg-config.1
+	tg.tempFile("foo.pc", `
+Name: foo
+Description: The foo library
+Version: 1.0.0
+Cflags: -Dhello=10 -Dworld=+32 -DDEFINED_FROM_PKG_CONFIG=hello\ world
+`)
+	tg.tempFile("foo.go", `package main
+
+/*
+#cgo pkg-config: foo
+int value() {
+	return DEFINED_FROM_PKG_CONFIG;
+}
+*/
+import "C"
+import "os"
+
+func main() {
+	if C.value() != 42 {
+		println("value() =", C.value(), "wanted 42")
+		os.Exit(1)
+	}
+}
+`)
+	tg.setenv("PKG_CONFIG_PATH", tg.path("."))
+	tg.run("run", tg.path("foo.go"))
+
+	// test for ldflags
+	tg.tempFile("bar.pc", `
+Name: bar
+Description: The bar library
+Version: 1.0.0
+Libs: -Wl,-rpath=/path\ with\ spaces/bin
+`)
+	tg.tempFile("bar.go", `package main
+/*
+#cgo pkg-config: bar
+*/
+import "C"
+func main() {}
+`)
+	tg.run("run", tg.path("bar.go"))
+}
+
+func TestListTemplateContextFunction(t *testing.T) {
+	t.Parallel()
+	for _, tt := range []struct {
+		v    string
+		want string
+	}{
+		{"GOARCH", runtime.GOARCH},
+		{"GOOS", runtime.GOOS},
+		{"GOROOT", testGOROOT},
+		{"GOPATH", os.Getenv("GOPATH")},
+		{"CgoEnabled", ""},
+		{"UseAllFiles", ""},
+		{"Compiler", ""},
+		{"BuildTags", ""},
+		{"ReleaseTags", ""},
+		{"InstallSuffix", ""},
+	} {
+		tt := tt
+		t.Run(tt.v, func(t *testing.T) {
+			tg := testgo(t)
+			tg.parallel()
+			defer tg.cleanup()
+			tmpl := "{{context." + tt.v + "}}"
+			tg.run("list", "-f", tmpl)
+			if tt.want == "" {
+				return
+			}
+			if got := strings.TrimSpace(tg.getStdout()); got != tt.want {
+				t.Errorf("go list -f %q: got %q; want %q", tmpl, got, tt.want)
+			}
+		})
+	}
+}
+
+// Test that you cannot use a local import in a package
+// accessed by a non-local import (found in a GOPATH/GOROOT).
+// See golang.org/issue/17475.
+func TestImportLocal(t *testing.T) {
+	tooSlow(t, "builds a lot of sequential packages")
+
+	tg := testgo(t)
+	tg.parallel()
+	defer tg.cleanup()
+
+	tg.tempFile("src/dir/x/x.go", `package x
+		var X int
+	`)
+	tg.setenv("GOPATH", tg.path("."))
+	tg.run("build", "dir/x")
+
+	// Ordinary import should work.
+	tg.tempFile("src/dir/p0/p.go", `package p0
+		import "dir/x"
+		var _ = x.X
+	`)
+	tg.run("build", "dir/p0")
+
+	// Relative import should not.
+	tg.tempFile("src/dir/p1/p.go", `package p1
+		import "../x"
+		var _ = x.X
+	`)
+	tg.runFail("build", "dir/p1")
+	tg.grepStderr("local import.*in non-local package", "did not diagnose local import")
+
+	// ... even in a test.
+	tg.tempFile("src/dir/p2/p.go", `package p2
+	`)
+	tg.tempFile("src/dir/p2/p_test.go", `package p2
+		import "../x"
+		import "testing"
+		var _ = x.X
+		func TestFoo(t *testing.T) {}
+	`)
+	tg.run("build", "dir/p2")
+	tg.runFail("test", "dir/p2")
+	tg.grepStderr("local import.*in non-local package", "did not diagnose local import")
+
+	// ... even in an xtest.
+	tg.tempFile("src/dir/p2/p_test.go", `package p2_test
+		import "../x"
+		import "testing"
+		var _ = x.X
+		func TestFoo(t *testing.T) {}
+	`)
+	tg.run("build", "dir/p2")
+	tg.runFail("test", "dir/p2")
+	tg.grepStderr("local import.*in non-local package", "did not diagnose local import")
+
+	// Relative import starting with ./ should not work either.
+	tg.tempFile("src/dir/d.go", `package dir
+		import "./x"
+		var _ = x.X
+	`)
+	tg.runFail("build", "dir")
+	tg.grepStderr("local import.*in non-local package", "did not diagnose local import")
+
+	// ... even in a test.
+	tg.tempFile("src/dir/d.go", `package dir
+	`)
+	tg.tempFile("src/dir/d_test.go", `package dir
+		import "./x"
+		import "testing"
+		var _ = x.X
+		func TestFoo(t *testing.T) {}
+	`)
+	tg.run("build", "dir")
+	tg.runFail("test", "dir")
+	tg.grepStderr("local import.*in non-local package", "did not diagnose local import")
+
+	// ... even in an xtest.
+	tg.tempFile("src/dir/d_test.go", `package dir_test
+		import "./x"
+		import "testing"
+		var _ = x.X
+		func TestFoo(t *testing.T) {}
+	`)
+	tg.run("build", "dir")
+	tg.runFail("test", "dir")
+	tg.grepStderr("local import.*in non-local package", "did not diagnose local import")
+
+	// Relative import plain ".." should not work.
+	tg.tempFile("src/dir/x/y/y.go", `package dir
+		import ".."
+		var _ = x.X
+	`)
+	tg.runFail("build", "dir/x/y")
+	tg.grepStderr("local import.*in non-local package", "did not diagnose local import")
+
+	// ... even in a test.
+	tg.tempFile("src/dir/x/y/y.go", `package y
+	`)
+	tg.tempFile("src/dir/x/y/y_test.go", `package y
+		import ".."
+		import "testing"
+		var _ = x.X
+		func TestFoo(t *testing.T) {}
+	`)
+	tg.run("build", "dir/x/y")
+	tg.runFail("test", "dir/x/y")
+	tg.grepStderr("local import.*in non-local package", "did not diagnose local import")
+
+	// ... even in an x test.
+	tg.tempFile("src/dir/x/y/y_test.go", `package y_test
+		import ".."
+		import "testing"
+		var _ = x.X
+		func TestFoo(t *testing.T) {}
+	`)
+	tg.run("build", "dir/x/y")
+	tg.runFail("test", "dir/x/y")
+	tg.grepStderr("local import.*in non-local package", "did not diagnose local import")
+
+	// Relative import "." should not work.
+	tg.tempFile("src/dir/x/xx.go", `package x
+		import "."
+		var _ = x.X
+	`)
+	tg.runFail("build", "dir/x")
+	tg.grepStderr("cannot import current directory", "did not diagnose import current directory")
+
+	// ... even in a test.
+	tg.tempFile("src/dir/x/xx.go", `package x
+	`)
+	tg.tempFile("src/dir/x/xx_test.go", `package x
+		import "."
+		import "testing"
+		var _ = x.X
+		func TestFoo(t *testing.T) {}
+	`)
+	tg.run("build", "dir/x")
+	tg.runFail("test", "dir/x")
+	tg.grepStderr("cannot import current directory", "did not diagnose import current directory")
+
+	// ... even in an xtest.
+	tg.tempFile("src/dir/x/xx.go", `package x
+	`)
+	tg.tempFile("src/dir/x/xx_test.go", `package x_test
+		import "."
+		import "testing"
+		var _ = x.X
+		func TestFoo(t *testing.T) {}
+	`)
+	tg.run("build", "dir/x")
+	tg.runFail("test", "dir/x")
+	tg.grepStderr("cannot import current directory", "did not diagnose import current directory")
+}
+
+func TestGoInstallPkgdir(t *testing.T) {
+	skipIfGccgo(t, "gccgo has no standard packages")
+	tooSlow(t, "builds a package with cgo dependencies")
+	// Only the stdlib packages that use cgo have install
+	// targets, (we're using net below) so cgo is required
+	// for the install.
+	testenv.MustHaveCGO(t)
+
+	tg := testgo(t)
+	tg.parallel()
+	tg.setenv("GODEBUG", "installgoroot=all")
+	defer tg.cleanup()
+	tg.makeTempdir()
+	pkg := tg.path(".")
+	tg.run("install", "-pkgdir", pkg, "net")
+	tg.mustExist(filepath.Join(pkg, "net.a"))
+	tg.mustNotExist(filepath.Join(pkg, "runtime/cgo.a"))
+}
+
+// For issue 14337.
+func TestParallelTest(t *testing.T) {
+	tooSlow(t, "links and runs test binaries")
+
+	tg := testgo(t)
+	tg.parallel()
+	defer tg.cleanup()
+	tg.makeTempdir()
+	const testSrc = `package package_test
+		import (
+			"testing"
+		)
+		func TestTest(t *testing.T) {
+		}`
+	tg.tempFile("src/p1/p1_test.go", strings.Replace(testSrc, "package_test", "p1_test", 1))
+	tg.tempFile("src/p2/p2_test.go", strings.Replace(testSrc, "package_test", "p2_test", 1))
+	tg.tempFile("src/p3/p3_test.go", strings.Replace(testSrc, "package_test", "p3_test", 1))
+	tg.tempFile("src/p4/p4_test.go", strings.Replace(testSrc, "package_test", "p4_test", 1))
+	tg.setenv("GOPATH", tg.path("."))
+	tg.run("test", "-p=4", "p1", "p2", "p3", "p4")
+}
+
+func TestBinaryOnlyPackages(t *testing.T) {
+	tooSlow(t, "compiles several packages sequentially")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.makeTempdir()
+	tg.setenv("GOPATH", tg.path("."))
+
+	tg.tempFile("src/p1/p1.go", `//go:binary-only-package
+
+		package p1
+	`)
+	tg.wantStale("p1", "binary-only packages are no longer supported", "p1 is binary-only, and this message should always be printed")
+	tg.runFail("install", "p1")
+	tg.grepStderr("binary-only packages are no longer supported", "did not report attempt to compile binary-only package")
+
+	tg.tempFile("src/p1/p1.go", `
+		package p1
+		import "fmt"
+		func F(b bool) { fmt.Printf("hello from p1\n"); if b { F(false) } }
+	`)
+	tg.run("install", "p1")
+	os.Remove(tg.path("src/p1/p1.go"))
+	tg.mustNotExist(tg.path("src/p1/p1.go"))
+
+	tg.tempFile("src/p2/p2.go", `//go:binary-only-packages-are-not-great
+
+		package p2
+		import "p1"
+		func F() { p1.F(true) }
+	`)
+	tg.runFail("install", "p2")
+	tg.grepStderr("no Go files", "did not complain about missing sources")
+
+	tg.tempFile("src/p1/missing.go", `//go:binary-only-package
+
+		package p1
+		import _ "fmt"
+		func G()
+	`)
+	tg.wantStale("p1", "binary-only package", "should NOT want to rebuild p1 (first)")
+	tg.runFail("install", "p2")
+	tg.grepStderr("p1: binary-only packages are no longer supported", "did not report error for binary-only p1")
+
+	tg.run("list", "-deps", "-f", "{{.ImportPath}}: {{.BinaryOnly}}", "p2")
+	tg.grepStdout("p1: true", "p1 not listed as BinaryOnly")
+	tg.grepStdout("p2: false", "p2 listed as BinaryOnly")
+}
+
+// Issue 16050 and 21884.
+func TestLinkSysoFiles(t *testing.T) {
+	if runtime.GOOS != "linux" || runtime.GOARCH != "amd64" {
+		t.Skip("not linux/amd64")
+	}
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempDir("src/syso")
+	tg.tempFile("src/syso/a.syso", ``)
+	tg.tempFile("src/syso/b.go", `package syso`)
+	tg.setenv("GOPATH", tg.path("."))
+
+	// We should see the .syso file regardless of the setting of
+	// CGO_ENABLED.
+
+	tg.setenv("CGO_ENABLED", "1")
+	tg.run("list", "-f", "{{.SysoFiles}}", "syso")
+	tg.grepStdout("a.syso", "missing syso file with CGO_ENABLED=1")
+
+	tg.setenv("CGO_ENABLED", "0")
+	tg.run("list", "-f", "{{.SysoFiles}}", "syso")
+	tg.grepStdout("a.syso", "missing syso file with CGO_ENABLED=0")
+
+	tg.setenv("CGO_ENABLED", "1")
+	tg.run("list", "-msan", "-f", "{{.SysoFiles}}", "syso")
+	tg.grepStdoutNot("a.syso", "unexpected syso file with -msan")
+}
+
+// Issue 16120.
+func TestGenerateUsesBuildContext(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("this test won't run under Windows")
+	}
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempDir("src/gen")
+	tg.tempFile("src/gen/gen.go", "package gen\n//go:generate echo $GOOS $GOARCH\n")
+	tg.setenv("GOPATH", tg.path("."))
+
+	tg.setenv("GOOS", "linux")
+	tg.setenv("GOARCH", "amd64")
+	tg.run("generate", "gen")
+	tg.grepStdout("linux amd64", "unexpected GOOS/GOARCH combination")
+
+	tg.setenv("GOOS", "darwin")
+	tg.setenv("GOARCH", "arm64")
+	tg.run("generate", "gen")
+	tg.grepStdout("darwin arm64", "unexpected GOOS/GOARCH combination")
+}
+
+func TestGoEnv(t *testing.T) {
+	tg := testgo(t)
+	tg.parallel()
+	defer tg.cleanup()
+	tg.setenv("GOOS", "freebsd") // to avoid invalid pair errors
+	tg.setenv("GOARCH", "arm")
+	tg.run("env", "GOARCH")
+	tg.grepStdout("^arm$", "GOARCH not honored")
+
+	tg.run("env", "GCCGO")
+	tg.grepStdout(".", "GCCGO unexpectedly empty")
+
+	tg.run("env", "CGO_CFLAGS")
+	tg.grepStdout(".", "default CGO_CFLAGS unexpectedly empty")
+
+	tg.setenv("CGO_CFLAGS", "-foobar")
+	tg.run("env", "CGO_CFLAGS")
+	tg.grepStdout("^-foobar$", "CGO_CFLAGS not honored")
+
+	tg.setenv("CC", "gcc -fmust -fgo -ffaster")
+	tg.run("env", "CC")
+	tg.grepStdout("gcc", "CC not found")
+	tg.run("env", "GOGCCFLAGS")
+	tg.grepStdout("-ffaster", "CC arguments not found")
+
+	tg.run("env", "GOVERSION")
+	envVersion := strings.TrimSpace(tg.stdout.String())
+
+	tg.run("version")
+	cmdVersion := strings.TrimSpace(tg.stdout.String())
+
+	// If 'go version' is "go version <version> <goos>/<goarch>", then
+	// 'go env GOVERSION' is just "<version>".
+	if cmdVersion == envVersion || !strings.Contains(cmdVersion, envVersion) {
+		t.Fatalf("'go env GOVERSION' %q should be a shorter substring of 'go version' %q", envVersion, cmdVersion)
+	}
+}
+
+const (
+	noMatchesPattern = `(?m)^ok.*\[no tests to run\]`
+	okPattern        = `(?m)^ok`
+)
+
+// Issue 18044.
+func TestLdBindNow(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.setenv("LD_BIND_NOW", "1")
+	tg.run("help")
+}
+
+// Issue 18225.
+// This is really a cmd/asm issue but this is a convenient place to test it.
+func TestConcurrentAsm(t *testing.T) {
+	skipIfGccgo(t, "gccgo does not use cmd/asm")
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	asm := `DATA ·constants<>+0x0(SB)/8,$0
+GLOBL ·constants<>(SB),8,$8
+`
+	tg.tempFile("go/src/p/a.s", asm)
+	tg.tempFile("go/src/p/b.s", asm)
+	tg.tempFile("go/src/p/p.go", `package p`)
+	tg.setenv("GOPATH", tg.path("go"))
+	tg.run("build", "p")
+}
+
+// Issue 18975.
+func TestFFLAGS(t *testing.T) {
+	testenv.MustHaveCGO(t)
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+
+	tg.tempFile("p/src/p/main.go", `package main
+		// #cgo FFLAGS: -no-such-fortran-flag
+		import "C"
+		func main() {}
+	`)
+	tg.tempFile("p/src/p/a.f", `! comment`)
+	tg.setenv("GOPATH", tg.path("p"))
+
+	// This should normally fail because we are passing an unknown flag,
+	// but issue #19080 points to Fortran compilers that succeed anyhow.
+	// To work either way we call doRun directly rather than run or runFail.
+	tg.doRun([]string{"build", "-x", "p"})
+
+	tg.grepStderr("no-such-fortran-flag", `missing expected "-no-such-fortran-flag"`)
+}
+
+// Issue 19198.
+// This is really a cmd/link issue but this is a convenient place to test it.
+func TestDuplicateGlobalAsmSymbols(t *testing.T) {
+	skipIfGccgo(t, "gccgo does not use cmd/asm")
+	tooSlow(t, "links a binary with cgo dependencies")
+	if runtime.GOARCH != "386" && runtime.GOARCH != "amd64" {
+		t.Skipf("skipping test on %s", runtime.GOARCH)
+	}
+	testenv.MustHaveCGO(t)
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+
+	asm := `
+#include "textflag.h"
+
+DATA sym<>+0x0(SB)/8,$0
+GLOBL sym<>(SB),(NOPTR+RODATA),$8
+
+TEXT ·Data(SB),NOSPLIT,$0
+	MOVB sym<>(SB), AX
+	MOVB AX, ret+0(FP)
+	RET
+`
+	tg.tempFile("go/src/a/a.s", asm)
+	tg.tempFile("go/src/a/a.go", `package a; func Data() uint8`)
+	tg.tempFile("go/src/b/b.s", asm)
+	tg.tempFile("go/src/b/b.go", `package b; func Data() uint8`)
+	tg.tempFile("go/src/p/p.go", `
+package main
+import "a"
+import "b"
+import "C"
+func main() {
+	_ = a.Data() + b.Data()
+}
+`)
+	tg.setenv("GOPATH", tg.path("go"))
+	exe := tg.path("p.exe")
+	tg.creatingTemp(exe)
+	tg.run("build", "-o", exe, "p")
+}
+
+func copyFile(src, dst string, perm fs.FileMode) error {
+	sf, err := os.Open(src)
+	if err != nil {
+		return err
+	}
+	defer sf.Close()
+
+	df, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
+	if err != nil {
+		return err
+	}
+
+	_, err = io.Copy(df, sf)
+	err2 := df.Close()
+	if err != nil {
+		return err
+	}
+	return err2
+}
+
+func TestNeedVersion(t *testing.T) {
+	skipIfGccgo(t, "gccgo does not use cmd/compile")
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempFile("goversion.go", `package main; func main() {}`)
+	path := tg.path("goversion.go")
+	tg.setenv("TESTGO_TOOLCHAIN_VERSION", "go1.testgo")
+	tg.runFail("run", path)
+	tg.grepStderr("compile", "does not match go tool version")
+}
+
+func TestBuildmodePIE(t *testing.T) {
+	tooSlow(t, "links binaries")
+
+	if !platform.BuildModeSupported(runtime.Compiler, "pie", runtime.GOOS, runtime.GOARCH) {
+		t.Skipf("skipping test because buildmode=pie is not supported on %s/%s", runtime.GOOS, runtime.GOARCH)
+	}
+	// Skip on alpine until https://go.dev/issues/54354 resolved.
+	if strings.HasSuffix(testenv.Builder(), "-alpine") {
+		t.Skip("skipping PIE tests on alpine; see https://go.dev/issues/54354")
+	}
+	t.Run("non-cgo", func(t *testing.T) {
+		testBuildmodePIE(t, false, true)
+	})
+	t.Run("cgo", func(t *testing.T) {
+		testenv.MustHaveCGO(t)
+		testBuildmodePIE(t, true, true)
+	})
+}
+
+func TestWindowsDefaultBuildmodIsPIE(t *testing.T) {
+	if runtime.GOOS != "windows" {
+		t.Skip("skipping windows only test")
+	}
+	tooSlow(t, "links binaries")
+
+	t.Run("non-cgo", func(t *testing.T) {
+		testBuildmodePIE(t, false, false)
+	})
+	t.Run("cgo", func(t *testing.T) {
+		testenv.MustHaveCGO(t)
+		testBuildmodePIE(t, true, false)
+	})
+}
+
+func testBuildmodePIE(t *testing.T, useCgo, setBuildmodeToPIE bool) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+
+	var s string
+	if useCgo {
+		s = `import "C";`
+	}
+	tg.tempFile("main.go", fmt.Sprintf(`package main;%s func main() { print("hello") }`, s))
+	src := tg.path("main.go")
+	obj := tg.path("main.exe")
+	args := []string{"build"}
+	if setBuildmodeToPIE {
+		args = append(args, "-buildmode=pie")
+	}
+	args = append(args, "-o", obj, src)
+	tg.run(args...)
+
+	switch runtime.GOOS {
+	case "linux", "android", "freebsd":
+		f, err := elf.Open(obj)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer f.Close()
+		if f.Type != elf.ET_DYN {
+			t.Errorf("PIE type must be ET_DYN, but %s", f.Type)
+		}
+	case "darwin", "ios":
+		f, err := macho.Open(obj)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer f.Close()
+		if f.Flags&macho.FlagDyldLink == 0 {
+			t.Error("PIE must have DyldLink flag, but not")
+		}
+		if f.Flags&macho.FlagPIE == 0 {
+			t.Error("PIE must have PIE flag, but not")
+		}
+	case "windows":
+		f, err := pe.Open(obj)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer f.Close()
+		if f.Section(".reloc") == nil {
+			t.Error(".reloc section is not present")
+		}
+		if (f.FileHeader.Characteristics & pe.IMAGE_FILE_RELOCS_STRIPPED) != 0 {
+			t.Error("IMAGE_FILE_RELOCS_STRIPPED flag is set")
+		}
+		var dc uint16
+		switch oh := f.OptionalHeader.(type) {
+		case *pe.OptionalHeader32:
+			dc = oh.DllCharacteristics
+		case *pe.OptionalHeader64:
+			dc = oh.DllCharacteristics
+			if (dc & pe.IMAGE_DLLCHARACTERISTICS_HIGH_ENTROPY_VA) == 0 {
+				t.Error("IMAGE_DLLCHARACTERISTICS_HIGH_ENTROPY_VA flag is not set")
+			}
+		default:
+			t.Fatalf("unexpected optional header type of %T", f.OptionalHeader)
+		}
+		if (dc & pe.IMAGE_DLLCHARACTERISTICS_DYNAMIC_BASE) == 0 {
+			t.Error("IMAGE_DLLCHARACTERISTICS_DYNAMIC_BASE flag is not set")
+		}
+		if useCgo {
+			// Test that only one symbol is exported (#40795).
+			// PIE binaries don´t require .edata section but unfortunately
+			// binutils doesn´t generate a .reloc section unless there is
+			// at least one symbol exported.
+			// See https://sourceware.org/bugzilla/show_bug.cgi?id=19011
+			section := f.Section(".edata")
+			if section == nil {
+				t.Skip(".edata section is not present")
+			}
+			// TODO: deduplicate this struct from cmd/link/internal/ld/pe.go
+			type IMAGE_EXPORT_DIRECTORY struct {
+				_                 [2]uint32
+				_                 [2]uint16
+				_                 [2]uint32
+				NumberOfFunctions uint32
+				NumberOfNames     uint32
+				_                 [3]uint32
+			}
+			var e IMAGE_EXPORT_DIRECTORY
+			if err := binary.Read(section.Open(), binary.LittleEndian, &e); err != nil {
+				t.Fatalf("binary.Read failed: %v", err)
+			}
+
+			// Only _cgo_dummy_export should be exported
+			if e.NumberOfFunctions != 1 {
+				t.Fatalf("got %d exported functions; want 1", e.NumberOfFunctions)
+			}
+			if e.NumberOfNames != 1 {
+				t.Fatalf("got %d exported names; want 1", e.NumberOfNames)
+			}
+		}
+	default:
+		// testBuildmodePIE opens object files, so it needs to understand the object
+		// file format.
+		t.Skipf("skipping test: test helper does not support %s", runtime.GOOS)
+	}
+
+	out, err := testenv.Command(t, obj).CombinedOutput()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if string(out) != "hello" {
+		t.Errorf("got %q; want %q", out, "hello")
+	}
+}
+
+func TestUpxCompression(t *testing.T) {
+	if runtime.GOOS != "linux" ||
+		(runtime.GOARCH != "amd64" && runtime.GOARCH != "386") {
+		t.Skipf("skipping upx test on %s/%s", runtime.GOOS, runtime.GOARCH)
+	}
+
+	testenv.MustHaveExecPath(t, "upx")
+	out, err := testenv.Command(t, "upx", "--version").CombinedOutput()
+	if err != nil {
+		t.Fatalf("upx --version failed: %v", err)
+	}
+
+	// upx --version prints `upx <version>` in the first line of output:
+	//   upx 3.94
+	//   [...]
+	re := regexp.MustCompile(`([[:digit:]]+)\.([[:digit:]]+)`)
+	upxVersion := re.FindStringSubmatch(string(out))
+	if len(upxVersion) != 3 {
+		t.Fatalf("bad upx version string: %s", upxVersion)
+	}
+
+	major, err1 := strconv.Atoi(upxVersion[1])
+	minor, err2 := strconv.Atoi(upxVersion[2])
+	if err1 != nil || err2 != nil {
+		t.Fatalf("bad upx version string: %s", upxVersion[0])
+	}
+
+	// Anything below 3.94 is known not to work with go binaries
+	if (major < 3) || (major == 3 && minor < 94) {
+		t.Skipf("skipping because upx version %v.%v is too old", major, minor)
+	}
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+
+	tg.tempFile("main.go", `package main; import "fmt"; func main() { fmt.Print("hello upx") }`)
+	src := tg.path("main.go")
+	obj := tg.path("main")
+	tg.run("build", "-o", obj, src)
+
+	out, err = testenv.Command(t, "upx", obj).CombinedOutput()
+	if err != nil {
+		t.Logf("executing upx\n%s\n", out)
+		t.Fatalf("upx failed with %v", err)
+	}
+
+	out, err = testenv.Command(t, obj).CombinedOutput()
+	if err != nil {
+		t.Logf("%s", out)
+		t.Fatalf("running compressed go binary failed with error %s", err)
+	}
+	if string(out) != "hello upx" {
+		t.Fatalf("bad output from compressed go binary:\ngot %q; want %q", out, "hello upx")
+	}
+}
+
+var gocacheverify = godebug.New("#gocacheverify")
+
+func TestCacheListStale(t *testing.T) {
+	tooSlow(t, "links a binary")
+	if gocacheverify.Value() == "1" {
+		t.Skip("GODEBUG gocacheverify")
+	}
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.makeTempdir()
+	tg.setenv("GOCACHE", tg.path("cache"))
+	tg.tempFile("gopath/src/p/p.go", "package p; import _ \"q\"; func F(){}\n")
+	tg.tempFile("gopath/src/q/q.go", "package q; func F(){}\n")
+	tg.tempFile("gopath/src/m/m.go", "package main; import _ \"q\"; func main(){}\n")
+
+	tg.setenv("GOPATH", tg.path("gopath"))
+	tg.run("install", "p", "m")
+	tg.run("list", "-f={{.ImportPath}} {{.Stale}}", "m", "q", "p")
+	tg.grepStdout("^m false", "m should not be stale")
+	tg.grepStdout("^q true", "q should be stale")
+	tg.grepStdout("^p false", "p should not be stale")
+}
+
+func TestCacheCoverage(t *testing.T) {
+	tooSlow(t, "links and runs a test binary with coverage enabled")
+	if gocacheverify.Value() == "1" {
+		t.Skip("GODEBUG gocacheverify")
+	}
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata"))
+	tg.makeTempdir()
+
+	tg.setenv("GOCACHE", tg.path("c1"))
+	tg.run("test", "-cover", "-short", "strings")
+	tg.run("test", "-cover", "-short", "math", "strings")
+}
+
+func TestIssue22588(t *testing.T) {
+	// Don't get confused by stderr coming from tools.
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+
+	tg.wantNotStale("runtime", "", "must be non-stale to compare staleness under -toolexec")
+
+	if _, err := os.Stat("/usr/bin/time"); err != nil {
+		t.Skip(err)
+	}
+
+	tg.run("list", "-f={{.Stale}}", "runtime")
+	tg.run("list", "-toolexec=/usr/bin/time", "-f={{.Stale}}", "runtime")
+	tg.grepStdout("false", "incorrectly reported runtime as stale")
+}
+
+func TestIssue22531(t *testing.T) {
+	tooSlow(t, "links binaries")
+	if gocacheverify.Value() == "1" {
+		t.Skip("GODEBUG gocacheverify")
+	}
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.makeTempdir()
+	tg.setenv("GOPATH", tg.tempdir)
+	tg.setenv("GOCACHE", tg.path("cache"))
+	tg.tempFile("src/m/main.go", "package main /* c1 */; func main() {}\n")
+	tg.run("install", "-x", "m")
+	tg.run("list", "-f", "{{.Stale}}", "m")
+	tg.grepStdout("false", "reported m as stale after install")
+	tg.run("tool", "buildid", tg.path("bin/m"+exeSuffix))
+
+	// The link action ID did not include the full main build ID,
+	// even though the full main build ID is written into the
+	// eventual binary. That caused the following install to
+	// be a no-op, thinking the gofmt binary was up-to-date,
+	// even though .Stale could see it was not.
+	tg.tempFile("src/m/main.go", "package main /* c2 */; func main() {}\n")
+	tg.run("install", "-x", "m")
+	tg.run("list", "-f", "{{.Stale}}", "m")
+	tg.grepStdout("false", "reported m as stale after reinstall")
+	tg.run("tool", "buildid", tg.path("bin/m"+exeSuffix))
+}
+
+func TestIssue22596(t *testing.T) {
+	tooSlow(t, "links binaries")
+	if gocacheverify.Value() == "1" {
+		t.Skip("GODEBUG gocacheverify")
+	}
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.makeTempdir()
+	tg.setenv("GOCACHE", tg.path("cache"))
+	tg.tempFile("gopath1/src/p/p.go", "package p; func F(){}\n")
+	tg.tempFile("gopath2/src/p/p.go", "package p; func F(){}\n")
+
+	tg.setenv("GOPATH", tg.path("gopath1"))
+	tg.run("list", "-f={{.Target}}", "p")
+	target1 := strings.TrimSpace(tg.getStdout())
+	tg.run("install", "p")
+	tg.wantNotStale("p", "", "p stale after install")
+
+	tg.setenv("GOPATH", tg.path("gopath2"))
+	tg.run("list", "-f={{.Target}}", "p")
+	target2 := strings.TrimSpace(tg.getStdout())
+	tg.must(os.MkdirAll(filepath.Dir(target2), 0777))
+	tg.must(copyFile(target1, target2, 0666))
+	tg.wantStale("p", "build ID mismatch", "p not stale after copy from gopath1")
+	tg.run("install", "p")
+	tg.wantNotStale("p", "", "p stale after install2")
+}
+
+func TestTestCache(t *testing.T) {
+	tooSlow(t, "links and runs test binaries")
+	if gocacheverify.Value() == "1" {
+		t.Skip("GODEBUG gocacheverify")
+	}
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.makeTempdir()
+	tg.setenv("GOPATH", tg.tempdir)
+	tg.setenv("GOCACHE", tg.path("cache"))
+
+	// The -p=1 in the commands below just makes the -x output easier to read.
+
+	t.Log("\n\nINITIAL\n\n")
+
+	tg.tempFile("src/p1/p1.go", "package p1\nvar X =  1\n")
+	tg.tempFile("src/p2/p2.go", "package p2\nimport _ \"p1\"\nvar X = 1\n")
+	tg.tempFile("src/t/t1/t1_test.go", "package t\nimport \"testing\"\nfunc Test1(*testing.T) {}\n")
+	tg.tempFile("src/t/t2/t2_test.go", "package t\nimport _ \"p1\"\nimport \"testing\"\nfunc Test2(*testing.T) {}\n")
+	tg.tempFile("src/t/t3/t3_test.go", "package t\nimport \"p1\"\nimport \"testing\"\nfunc Test3(t *testing.T) {t.Log(p1.X)}\n")
+	tg.tempFile("src/t/t4/t4_test.go", "package t\nimport \"p2\"\nimport \"testing\"\nfunc Test4(t *testing.T) {t.Log(p2.X)}")
+	tg.run("test", "-x", "-v", "-short", "t/...")
+
+	t.Log("\n\nREPEAT\n\n")
+
+	tg.run("test", "-x", "-v", "-short", "t/...")
+	tg.grepStdout(`ok  \tt/t1\t\(cached\)`, "did not cache t1")
+	tg.grepStdout(`ok  \tt/t2\t\(cached\)`, "did not cache t2")
+	tg.grepStdout(`ok  \tt/t3\t\(cached\)`, "did not cache t3")
+	tg.grepStdout(`ok  \tt/t4\t\(cached\)`, "did not cache t4")
+	tg.grepStderrNot(`[\\/](compile|gccgo) `, "incorrectly ran compiler")
+	tg.grepStderrNot(`[\\/](link|gccgo) `, "incorrectly ran linker")
+	tg.grepStderrNot(`p[0-9]\.test`, "incorrectly ran test")
+
+	t.Log("\n\nCOMMENT\n\n")
+
+	// Changing the program text without affecting the compiled package
+	// should result in the package being rebuilt but nothing more.
+	tg.tempFile("src/p1/p1.go", "package p1\nvar X = 01\n")
+	tg.run("test", "-p=1", "-x", "-v", "-short", "t/...")
+	tg.grepStdout(`ok  \tt/t1\t\(cached\)`, "did not cache t1")
+	tg.grepStdout(`ok  \tt/t2\t\(cached\)`, "did not cache t2")
+	tg.grepStdout(`ok  \tt/t3\t\(cached\)`, "did not cache t3")
+	tg.grepStdout(`ok  \tt/t4\t\(cached\)`, "did not cache t4")
+	tg.grepStderrNot(`([\\/](compile|gccgo) ).*t[0-9]_test\.go`, "incorrectly ran compiler")
+	tg.grepStderrNot(`[\\/](link|gccgo) `, "incorrectly ran linker")
+	tg.grepStderrNot(`t[0-9]\.test.*test\.short`, "incorrectly ran test")
+
+	t.Log("\n\nCHANGE\n\n")
+
+	// Changing the actual package should have limited effects.
+	tg.tempFile("src/p1/p1.go", "package p1\nvar X = 02\n")
+	tg.run("test", "-p=1", "-x", "-v", "-short", "t/...")
+
+	// p2 should have been rebuilt.
+	tg.grepStderr(`([\\/]compile|gccgo).*p2.go`, "did not recompile p2")
+
+	// t1 does not import anything, should not have been rebuilt.
+	tg.grepStderrNot(`([\\/]compile|gccgo).*t1_test.go`, "incorrectly recompiled t1")
+	tg.grepStderrNot(`([\\/]link|gccgo).*t1_test`, "incorrectly relinked t1_test")
+	tg.grepStdout(`ok  \tt/t1\t\(cached\)`, "did not cache t/t1")
+
+	// t2 imports p1 and must be rebuilt and relinked,
+	// but the change should not have any effect on the test binary,
+	// so the test should not have been rerun.
+	tg.grepStderr(`([\\/]compile|gccgo).*t2_test.go`, "did not recompile t2")
+	tg.grepStderr(`([\\/]link|gccgo).*t2\.test`, "did not relink t2_test")
+	// This check does not currently work with gccgo, as garbage
+	// collection of unused variables is not turned on by default.
+	if runtime.Compiler != "gccgo" {
+		tg.grepStdout(`ok  \tt/t2\t\(cached\)`, "did not cache t/t2")
+	}
+
+	// t3 imports p1, and changing X changes t3's test binary.
+	tg.grepStderr(`([\\/]compile|gccgo).*t3_test.go`, "did not recompile t3")
+	tg.grepStderr(`([\\/]link|gccgo).*t3\.test`, "did not relink t3_test")
+	tg.grepStderr(`t3\.test.*-test.short`, "did not rerun t3_test")
+	tg.grepStdoutNot(`ok  \tt/t3\t\(cached\)`, "reported cached t3_test result")
+
+	// t4 imports p2, but p2 did not change, so t4 should be relinked, not recompiled,
+	// and not rerun.
+	tg.grepStderrNot(`([\\/]compile|gccgo).*t4_test.go`, "incorrectly recompiled t4")
+	tg.grepStderr(`([\\/]link|gccgo).*t4\.test`, "did not relink t4_test")
+	// This check does not currently work with gccgo, as garbage
+	// collection of unused variables is not turned on by default.
+	if runtime.Compiler != "gccgo" {
+		tg.grepStdout(`ok  \tt/t4\t\(cached\)`, "did not cache t/t4")
+	}
+}
+
+func TestTestSkipVetAfterFailedBuild(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+
+	tg.tempFile("x_test.go", `package x
+		func f() {
+			return 1
+		}
+	`)
+
+	tg.runFail("test", tg.path("x_test.go"))
+	tg.grepStderrNot(`vet`, "vet should be skipped after the failed build")
+}
+
+func TestTestVetRebuild(t *testing.T) {
+	tooSlow(t, "links and runs test binaries")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+
+	// golang.org/issue/23701.
+	// b_test imports b with augmented method from export_test.go.
+	// b_test also imports a, which imports b.
+	// Must not accidentally see un-augmented b propagate through a to b_test.
+	tg.tempFile("src/a/a.go", `package a
+		import "b"
+		type Type struct{}
+		func (*Type) M() b.T {return 0}
+	`)
+	tg.tempFile("src/b/b.go", `package b
+		type T int
+		type I interface {M() T}
+	`)
+	tg.tempFile("src/b/export_test.go", `package b
+		func (*T) Method() *T { return nil }
+	`)
+	tg.tempFile("src/b/b_test.go", `package b_test
+		import (
+			"testing"
+			"a"
+			. "b"
+		)
+		func TestBroken(t *testing.T) {
+			x := new(T)
+			x.Method()
+			_ = new(a.Type)
+		}
+	`)
+
+	tg.setenv("GOPATH", tg.path("."))
+	tg.run("test", "b")
+	tg.run("vet", "b")
+}
+
+func TestInstallDeps(t *testing.T) {
+	tooSlow(t, "links a binary")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.makeTempdir()
+	tg.setenv("GOPATH", tg.tempdir)
+
+	tg.tempFile("src/p1/p1.go", "package p1\nvar X =  1\n")
+	tg.tempFile("src/p2/p2.go", "package p2\nimport _ \"p1\"\n")
+	tg.tempFile("src/main1/main.go", "package main\nimport _ \"p2\"\nfunc main() {}\n")
+
+	tg.run("list", "-f={{.Target}}", "p1")
+	p1 := strings.TrimSpace(tg.getStdout())
+	tg.run("list", "-f={{.Target}}", "p2")
+	p2 := strings.TrimSpace(tg.getStdout())
+	tg.run("list", "-f={{.Target}}", "main1")
+	main1 := strings.TrimSpace(tg.getStdout())
+
+	tg.run("install", "main1")
+
+	tg.mustExist(main1)
+	tg.mustNotExist(p2)
+	tg.mustNotExist(p1)
+
+	tg.run("install", "p2")
+	tg.mustExist(p2)
+	tg.mustNotExist(p1)
+}
+
+// Issue 22986.
+func TestImportPath(t *testing.T) {
+	tooSlow(t, "links and runs a test binary")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+
+	tg.tempFile("src/a/a.go", `
+package main
+
+import (
+	"log"
+	p "a/p-1.0"
+)
+
+func main() {
+	if !p.V {
+		log.Fatal("false")
+	}
+}`)
+
+	tg.tempFile("src/a/a_test.go", `
+package main_test
+
+import (
+	p "a/p-1.0"
+	"testing"
+)
+
+func TestV(t *testing.T) {
+	if !p.V {
+		t.Fatal("false")
+	}
+}`)
+
+	tg.tempFile("src/a/p-1.0/p.go", `
+package p
+
+var V = true
+
+func init() {}
+`)
+
+	tg.setenv("GOPATH", tg.path("."))
+	tg.run("build", "-o", tg.path("a.exe"), "a")
+	tg.run("test", "a")
+}
+
+func TestBadCommandLines(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+
+	tg.tempFile("src/x/x.go", "package x\n")
+	tg.setenv("GOPATH", tg.path("."))
+
+	tg.run("build", "x")
+
+	tg.tempFile("src/x/@y.go", "package x\n")
+	tg.runFail("build", "x")
+	tg.grepStderr("invalid input file name \"@y.go\"", "did not reject @y.go")
+	tg.must(os.Remove(tg.path("src/x/@y.go")))
+
+	tg.tempFile("src/x/-y.go", "package x\n")
+	tg.runFail("build", "x")
+	tg.grepStderr("invalid input file name \"-y.go\"", "did not reject -y.go")
+	tg.must(os.Remove(tg.path("src/x/-y.go")))
+
+	if runtime.Compiler == "gccgo" {
+		tg.runFail("build", "-gccgoflags=all=@x", "x")
+	} else {
+		tg.runFail("build", "-gcflags=all=@x", "x")
+	}
+	tg.grepStderr("invalid command-line argument @x in command", "did not reject @x during exec")
+
+	tg.tempFile("src/@x/x.go", "package x\n")
+	tg.setenv("GOPATH", tg.path("."))
+	tg.runFail("build", "@x")
+	tg.grepStderr("invalid input directory name \"@x\"|can only use path@version syntax with 'go get' and 'go install' in module-aware mode", "did not reject @x directory")
+
+	tg.tempFile("src/@x/y/y.go", "package y\n")
+	tg.setenv("GOPATH", tg.path("."))
+	tg.runFail("build", "@x/y")
+	tg.grepStderr("invalid import path \"@x/y\"|can only use path@version syntax with 'go get' and 'go install' in module-aware mode", "did not reject @x/y import path")
+
+	tg.tempFile("src/-x/x.go", "package x\n")
+	tg.setenv("GOPATH", tg.path("."))
+	tg.runFail("build", "--", "-x")
+	tg.grepStderr("invalid import path \"-x\"", "did not reject -x import path")
+
+	tg.tempFile("src/-x/y/y.go", "package y\n")
+	tg.setenv("GOPATH", tg.path("."))
+	tg.runFail("build", "--", "-x/y")
+	tg.grepStderr("invalid import path \"-x/y\"", "did not reject -x/y import path")
+}
+
+func TestTwoPkgConfigs(t *testing.T) {
+	testenv.MustHaveCGO(t)
+	if runtime.GOOS == "windows" || runtime.GOOS == "plan9" {
+		t.Skipf("no shell scripts on %s", runtime.GOOS)
+	}
+	tooSlow(t, "builds a package with cgo dependencies")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempFile("src/x/a.go", `package x
+		// #cgo pkg-config: --static a
+		import "C"
+	`)
+	tg.tempFile("src/x/b.go", `package x
+		// #cgo pkg-config: --static a
+		import "C"
+	`)
+	tg.tempFile("pkg-config.sh", `#!/bin/sh
+echo $* >>`+tg.path("pkg-config.out"))
+	tg.must(os.Chmod(tg.path("pkg-config.sh"), 0755))
+	tg.setenv("GOPATH", tg.path("."))
+	tg.setenv("PKG_CONFIG", tg.path("pkg-config.sh"))
+	tg.run("build", "x")
+	out, err := os.ReadFile(tg.path("pkg-config.out"))
+	tg.must(err)
+	out = bytes.TrimSpace(out)
+	want := "--cflags --static --static -- a a\n--libs --static --static -- a a"
+	if !bytes.Equal(out, []byte(want)) {
+		t.Errorf("got %q want %q", out, want)
+	}
+}
+
+func TestCgoCache(t *testing.T) {
+	testenv.MustHaveCGO(t)
+	tooSlow(t, "builds a package with cgo dependencies")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempFile("src/x/a.go", `package main
+		// #ifndef VAL
+		// #define VAL 0
+		// #endif
+		// int val = VAL;
+		import "C"
+		import "fmt"
+		func main() { fmt.Println(C.val) }
+	`)
+	tg.setenv("GOPATH", tg.path("."))
+	exe := tg.path("x.exe")
+	tg.run("build", "-o", exe, "x")
+	tg.setenv("CGO_LDFLAGS", "-lnosuchlibraryexists")
+	tg.runFail("build", "-o", exe, "x")
+	tg.grepStderr(`nosuchlibraryexists`, "did not run linker with changed CGO_LDFLAGS")
+}
+
+// Issue 23982
+func TestFilepathUnderCwdFormat(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.run("test", "-x", "-cover", "log")
+	tg.grepStderrNot(`\.log\.cover\.go`, "-x output should contain correctly formatted filepath under cwd")
+}
+
+// Issue 24396.
+func TestDontReportRemoveOfEmptyDir(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempFile("src/a/a.go", `package a`)
+	tg.setenv("GOPATH", tg.path("."))
+	tg.run("install", "-x", "a")
+	tg.run("install", "-x", "a")
+	// The second install should have printed only a WORK= line,
+	// nothing else.
+	if bytes.Count(tg.stdout.Bytes(), []byte{'\n'})+bytes.Count(tg.stderr.Bytes(), []byte{'\n'}) > 1 {
+		t.Error("unnecessary output when installing installed package")
+	}
+}
+
+// Issue 24704.
+func TestLinkerTmpDirIsDeleted(t *testing.T) {
+	skipIfGccgo(t, "gccgo does not use cmd/link")
+	testenv.MustHaveCGO(t)
+	tooSlow(t, "builds a package with cgo dependencies")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempFile("a.go", `package main; import "C"; func main() {}`)
+	tg.run("build", "-ldflags", "-v", "-o", os.DevNull, tg.path("a.go"))
+	// Find line that has "host link:" in linker output.
+	stderr := tg.getStderr()
+	var hostLinkLine string
+	for _, line := range strings.Split(stderr, "\n") {
+		if !strings.Contains(line, "host link:") {
+			continue
+		}
+		hostLinkLine = line
+		break
+	}
+	if hostLinkLine == "" {
+		t.Fatal(`fail to find with "host link:" string in linker output`)
+	}
+	// Find parameter, like "/tmp/go-link-408556474/go.o" inside of
+	// "host link:" line, and extract temp directory /tmp/go-link-408556474
+	// out of it.
+	tmpdir := hostLinkLine
+	i := strings.Index(tmpdir, `go.o"`)
+	if i == -1 {
+		t.Fatalf(`fail to find "go.o" in "host link:" line %q`, hostLinkLine)
+	}
+	tmpdir = tmpdir[:i-1]
+	i = strings.LastIndex(tmpdir, `"`)
+	if i == -1 {
+		t.Fatalf(`fail to find " in "host link:" line %q`, hostLinkLine)
+	}
+	tmpdir = tmpdir[i+1:]
+	// Verify that temp directory has been removed.
+	_, err := os.Stat(tmpdir)
+	if err == nil {
+		t.Fatalf("temp directory %q has not been removed", tmpdir)
+	}
+	if !os.IsNotExist(err) {
+		t.Fatalf("Stat(%q) returns unexpected error: %v", tmpdir, err)
+	}
+}
+
+// Issue 25093.
+func TestCoverpkgTestOnly(t *testing.T) {
+	skipIfGccgo(t, "gccgo has no cover tool")
+	tooSlow(t, "links and runs a test binary with coverage enabled")
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.tempFile("src/a/a.go", `package a
+		func F(i int) int {
+			return i*i
+		}`)
+	tg.tempFile("src/atest/a_test.go", `
+		package a_test
+		import ( "a"; "testing" )
+		func TestF(t *testing.T) { a.F(2) }
+	`)
+	tg.setenv("GOPATH", tg.path("."))
+	tg.run("test", "-coverpkg=a", "atest")
+	tg.grepStderrNot("no packages being tested depend on matches", "bad match message")
+	tg.grepStdout("coverage: 100", "no coverage")
+}
+
+// Regression test for golang.org/issue/34499: version command should not crash
+// when executed in a deleted directory on Linux.
+func TestExecInDeletedDir(t *testing.T) {
+	switch runtime.GOOS {
+	case "windows", "plan9",
+		"aix",                // Fails with "device busy".
+		"solaris", "illumos": // Fails with "invalid argument".
+		t.Skipf("%v does not support removing the current working directory", runtime.GOOS)
+	}
+	tg := testgo(t)
+	defer tg.cleanup()
+
+	wd, err := os.Getwd()
+	tg.check(err)
+	tg.makeTempdir()
+	tg.check(os.Chdir(tg.tempdir))
+	defer func() { tg.check(os.Chdir(wd)) }()
+
+	tg.check(os.Remove(tg.tempdir))
+
+	// `go version` should not fail
+	tg.run("version")
+}
diff --git a/src/cmd/go/go_unix_test.go b/src/cmd/go/go_unix_test.go
new file mode 100644
index 0000000..d04e496
--- /dev/null
+++ b/src/cmd/go/go_unix_test.go
@@ -0,0 +1,119 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix
+
+package main_test
+
+import (
+	"bufio"
+	"context"
+	"internal/testenv"
+	"io"
+	"os"
+	"os/exec"
+	"slices"
+	"strings"
+	"syscall"
+	"testing"
+)
+
+func TestGoBuildUmask(t *testing.T) {
+	// Do not use tg.parallel; avoid other tests seeing umask manipulation.
+	mask := syscall.Umask(0077) // prohibit low bits
+	defer syscall.Umask(mask)
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.tempFile("x.go", `package main; func main() {}`)
+	// Make sure artifact will be output to /tmp/... in case the user
+	// has POSIX acl's on their go source tree.
+	// See issue 17909.
+	exe := tg.path("x")
+	tg.creatingTemp(exe)
+	tg.run("build", "-o", exe, tg.path("x.go"))
+	fi, err := os.Stat(exe)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if mode := fi.Mode(); mode&0077 != 0 {
+		t.Fatalf("wrote x with mode=%v, wanted no 0077 bits", mode)
+	}
+}
+
+// TestTestInterrupt verifies the fix for issue #60203.
+//
+// If the whole process group for a 'go test' invocation receives
+// SIGINT (as would be sent by pressing ^C on a console),
+// it should return quickly, not deadlock.
+func TestTestInterrupt(t *testing.T) {
+	if testing.Short() {
+		t.Skipf("skipping in short mode: test executes many subprocesses")
+	}
+	// Don't run this test in parallel, for the same reason.
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.setenv("GOROOT", testGOROOT)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	cmd := testenv.CommandContext(t, ctx, tg.goTool(), "test", "std", "-short", "-count=1")
+	cmd.Dir = tg.execDir
+
+	// Override $TMPDIR when running the tests: since we're terminating the tests
+	// with a signal they might fail to clean up some temp files, and we don't
+	// want that to cause an "unexpected files" failure at the end of the run.
+	cmd.Env = append(slices.Clip(tg.env), tempEnvName()+"="+t.TempDir())
+
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		Setpgid: true,
+	}
+	cmd.Cancel = func() error {
+		pgid := cmd.Process.Pid
+		return syscall.Kill(-pgid, syscall.SIGINT)
+	}
+
+	pipe, err := cmd.StdoutPipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	t.Logf("running %v", cmd)
+	if err := cmd.Start(); err != nil {
+		t.Fatal(err)
+	}
+
+	stdout := new(strings.Builder)
+	r := bufio.NewReader(pipe)
+	line, err := r.ReadString('\n')
+	if err != nil {
+		t.Fatal(err)
+	}
+	stdout.WriteString(line)
+
+	// The output line for some test was written, so we know things are in progress.
+	//
+	// Cancel the rest of the run by sending SIGINT to the process group:
+	// it should finish up and exit with a nonzero status,
+	// not have to be killed with SIGKILL.
+	cancel()
+
+	io.Copy(stdout, r)
+	if stdout.Len() > 0 {
+		t.Logf("stdout:\n%s", stdout)
+	}
+	err = cmd.Wait()
+
+	ee, _ := err.(*exec.ExitError)
+	if ee == nil {
+		t.Fatalf("unexpectedly finished with nonzero status")
+	}
+	if len(ee.Stderr) > 0 {
+		t.Logf("stderr:\n%s", ee.Stderr)
+	}
+	if !ee.Exited() {
+		t.Fatalf("'go test' did not exit after interrupt: %v", err)
+	}
+
+	t.Logf("interrupted tests without deadlocking")
+}
diff --git a/src/cmd/go/go_windows_test.go b/src/cmd/go/go_windows_test.go
new file mode 100644
index 0000000..0c443eb
--- /dev/null
+++ b/src/cmd/go/go_windows_test.go
@@ -0,0 +1,50 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main_test
+
+import (
+	"internal/testenv"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"cmd/go/internal/robustio"
+)
+
+func TestAbsolutePath(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+
+	tmp, err := os.MkdirTemp("", "TestAbsolutePath")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer robustio.RemoveAll(tmp)
+
+	file := filepath.Join(tmp, "a.go")
+	err = os.WriteFile(file, []byte{}, 0644)
+	if err != nil {
+		t.Fatal(err)
+	}
+	dir := filepath.Join(tmp, "dir")
+	err = os.Mkdir(dir, 0777)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	noVolume := file[len(filepath.VolumeName(file)):]
+	wrongPath := filepath.Join(dir, noVolume)
+	cmd := testenv.Command(t, tg.goTool(), "build", noVolume)
+	cmd.Dir = dir
+	output, err := cmd.CombinedOutput()
+	if err == nil {
+		t.Fatal("build should fail")
+	}
+	if strings.Contains(string(output), wrongPath) {
+		t.Fatalf("wrong output found: %v %v", err, string(output))
+	}
+}
diff --git a/src/cmd/go/help_test.go b/src/cmd/go/help_test.go
new file mode 100644
index 0000000..de3b966
--- /dev/null
+++ b/src/cmd/go/help_test.go
@@ -0,0 +1,63 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main_test
+
+import (
+	"flag"
+	"go/format"
+	"internal/diff"
+	"internal/testenv"
+	"os"
+	"strings"
+	"testing"
+)
+
+var fixDocs = flag.Bool("fixdocs", false, "if true, update alldocs.go")
+
+func TestDocsUpToDate(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+	if !*fixDocs {
+		t.Parallel()
+	}
+
+	// We run 'go help documentation' as a subprocess instead of
+	// calling help.Help directly because it may be sensitive to
+	// init-time configuration
+	cmd := testenv.Command(t, testGo, "help", "documentation")
+	// Unset GO111MODULE so that the 'go get' section matches
+	// the default 'go get' implementation.
+	cmd.Env = append(cmd.Environ(), "GO111MODULE=")
+	cmd.Stderr = new(strings.Builder)
+	out, err := cmd.Output()
+	if err != nil {
+		t.Fatalf("%v: %v\n%s", cmd, err, cmd.Stderr)
+	}
+
+	alldocs, err := format.Source(out)
+	if err != nil {
+		t.Fatalf("format.Source($(%v)): %v", cmd, err)
+	}
+
+	const srcPath = `alldocs.go`
+	old, err := os.ReadFile(srcPath)
+	if err != nil {
+		t.Fatalf("error reading %s: %v", srcPath, err)
+	}
+	diff := diff.Diff(srcPath, old, "go help documentation | gofmt", alldocs)
+	if diff == nil {
+		t.Logf("%s is up to date.", srcPath)
+		return
+	}
+
+	if *fixDocs {
+		if err := os.WriteFile(srcPath, alldocs, 0666); err != nil {
+			t.Fatal(err)
+		}
+		t.Logf("wrote %d bytes to %s", len(alldocs), srcPath)
+	} else {
+		t.Logf("\n%s", diff)
+		t.Errorf("%s is stale. To update, run 'go generate cmd/go'.", srcPath)
+	}
+}
diff --git a/src/cmd/go/init_test.go b/src/cmd/go/init_test.go
new file mode 100644
index 0000000..f76425d
--- /dev/null
+++ b/src/cmd/go/init_test.go
@@ -0,0 +1,41 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main_test
+
+import (
+	"internal/testenv"
+	"sync/atomic"
+	"testing"
+)
+
+// BenchmarkExecGoEnv measures how long it takes for 'go env GOARCH' to run.
+// Since 'go' is executed, remember to run 'go install cmd/go' before running
+// the benchmark if any changes were done.
+func BenchmarkExecGoEnv(b *testing.B) {
+	testenv.MustHaveExec(b)
+	gotool, err := testenv.GoTool()
+	if err != nil {
+		b.Fatal(err)
+	}
+
+	// We collect extra metrics.
+	var n, userTime, systemTime int64
+
+	b.ResetTimer()
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			cmd := testenv.Command(b, gotool, "env", "GOARCH")
+
+			if err := cmd.Run(); err != nil {
+				b.Fatal(err)
+			}
+			atomic.AddInt64(&n, 1)
+			atomic.AddInt64(&userTime, int64(cmd.ProcessState.UserTime()))
+			atomic.AddInt64(&systemTime, int64(cmd.ProcessState.SystemTime()))
+		}
+	})
+	b.ReportMetric(float64(userTime)/float64(n), "user-ns/op")
+	b.ReportMetric(float64(systemTime)/float64(n), "sys-ns/op")
+}
diff --git a/src/cmd/go/internal/auth/auth.go b/src/cmd/go/internal/auth/auth.go
new file mode 100644
index 0000000..77edeb8
--- /dev/null
+++ b/src/cmd/go/internal/auth/auth.go
@@ -0,0 +1,28 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package auth provides access to user-provided authentication credentials.
+package auth
+
+import "net/http"
+
+// AddCredentials fills in the user's credentials for req, if any.
+// The return value reports whether any matching credentials were found.
+func AddCredentials(req *http.Request) (added bool) {
+	host := req.Host
+	if host == "" {
+		host = req.URL.Hostname()
+	}
+
+	// TODO(golang.org/issue/26232): Support arbitrary user-provided credentials.
+	netrcOnce.Do(readNetrc)
+	for _, l := range netrc {
+		if l.machine == host {
+			req.SetBasicAuth(l.login, l.password)
+			return true
+		}
+	}
+
+	return false
+}
diff --git a/src/cmd/go/internal/auth/netrc.go b/src/cmd/go/internal/auth/netrc.go
new file mode 100644
index 0000000..0107f20
--- /dev/null
+++ b/src/cmd/go/internal/auth/netrc.go
@@ -0,0 +1,110 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package auth
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"sync"
+)
+
+type netrcLine struct {
+	machine  string
+	login    string
+	password string
+}
+
+var (
+	netrcOnce sync.Once
+	netrc     []netrcLine
+	netrcErr  error
+)
+
+func parseNetrc(data string) []netrcLine {
+	// See https://www.gnu.org/software/inetutils/manual/html_node/The-_002enetrc-file.html
+	// for documentation on the .netrc format.
+	var nrc []netrcLine
+	var l netrcLine
+	inMacro := false
+	for _, line := range strings.Split(data, "\n") {
+		if inMacro {
+			if line == "" {
+				inMacro = false
+			}
+			continue
+		}
+
+		f := strings.Fields(line)
+		i := 0
+		for ; i < len(f)-1; i += 2 {
+			// Reset at each "machine" token.
+			// “The auto-login process searches the .netrc file for a machine token
+			// that matches […]. Once a match is made, the subsequent .netrc tokens
+			// are processed, stopping when the end of file is reached or another
+			// machine or a default token is encountered.”
+			switch f[i] {
+			case "machine":
+				l = netrcLine{machine: f[i+1]}
+			case "default":
+				break
+			case "login":
+				l.login = f[i+1]
+			case "password":
+				l.password = f[i+1]
+			case "macdef":
+				// “A macro is defined with the specified name; its contents begin with
+				// the next .netrc line and continue until a null line (consecutive
+				// new-line characters) is encountered.”
+				inMacro = true
+			}
+			if l.machine != "" && l.login != "" && l.password != "" {
+				nrc = append(nrc, l)
+				l = netrcLine{}
+			}
+		}
+
+		if i < len(f) && f[i] == "default" {
+			// “There can be only one default token, and it must be after all machine tokens.”
+			break
+		}
+	}
+
+	return nrc
+}
+
+func netrcPath() (string, error) {
+	if env := os.Getenv("NETRC"); env != "" {
+		return env, nil
+	}
+	dir, err := os.UserHomeDir()
+	if err != nil {
+		return "", err
+	}
+	base := ".netrc"
+	if runtime.GOOS == "windows" {
+		base = "_netrc"
+	}
+	return filepath.Join(dir, base), nil
+}
+
+func readNetrc() {
+	path, err := netrcPath()
+	if err != nil {
+		netrcErr = err
+		return
+	}
+
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if !os.IsNotExist(err) {
+			netrcErr = err
+		}
+		return
+	}
+
+	netrc = parseNetrc(string(data))
+}
diff --git a/src/cmd/go/internal/auth/netrc_test.go b/src/cmd/go/internal/auth/netrc_test.go
new file mode 100644
index 0000000..e06c545
--- /dev/null
+++ b/src/cmd/go/internal/auth/netrc_test.go
@@ -0,0 +1,58 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package auth
+
+import (
+	"reflect"
+	"testing"
+)
+
+var testNetrc = `
+machine incomplete
+password none
+
+machine api.github.com
+  login user
+  password pwd
+
+machine incomlete.host
+  login justlogin
+
+machine test.host
+login user2
+password pwd2
+
+machine oneline login user3 password pwd3
+
+machine ignore.host macdef ignore
+  login nobody
+  password nothing
+
+machine hasmacro.too macdef ignore-next-lines login user4 password pwd4
+  login nobody
+  password nothing
+
+default
+login anonymous
+password gopher@golang.org
+
+machine after.default
+login oops
+password too-late-in-file
+`
+
+func TestParseNetrc(t *testing.T) {
+	lines := parseNetrc(testNetrc)
+	want := []netrcLine{
+		{"api.github.com", "user", "pwd"},
+		{"test.host", "user2", "pwd2"},
+		{"oneline", "user3", "pwd3"},
+		{"hasmacro.too", "user4", "pwd4"},
+	}
+
+	if !reflect.DeepEqual(lines, want) {
+		t.Errorf("parseNetrc:\nhave %q\nwant %q", lines, want)
+	}
+}
diff --git a/src/cmd/go/internal/base/base.go b/src/cmd/go/internal/base/base.go
new file mode 100644
index 0000000..2171d13
--- /dev/null
+++ b/src/cmd/go/internal/base/base.go
@@ -0,0 +1,223 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package base defines shared basic pieces of the go command,
+// in particular logging and the Command structure.
+package base
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"log"
+	"os"
+	"os/exec"
+	"reflect"
+	"strings"
+	"sync"
+
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/str"
+)
+
+// A Command is an implementation of a go command
+// like go build or go fix.
+type Command struct {
+	// Run runs the command.
+	// The args are the arguments after the command name.
+	Run func(ctx context.Context, cmd *Command, args []string)
+
+	// UsageLine is the one-line usage message.
+	// The words between "go" and the first flag or argument in the line are taken to be the command name.
+	UsageLine string
+
+	// Short is the short description shown in the 'go help' output.
+	Short string
+
+	// Long is the long message shown in the 'go help <this-command>' output.
+	Long string
+
+	// Flag is a set of flags specific to this command.
+	Flag flag.FlagSet
+
+	// CustomFlags indicates that the command will do its own
+	// flag parsing.
+	CustomFlags bool
+
+	// Commands lists the available commands and help topics.
+	// The order here is the order in which they are printed by 'go help'.
+	// Note that subcommands are in general best avoided.
+	Commands []*Command
+}
+
+var Go = &Command{
+	UsageLine: "go",
+	Long:      `Go is a tool for managing Go source code.`,
+	// Commands initialized in package main
+}
+
+// Lookup returns the subcommand with the given name, if any.
+// Otherwise it returns nil.
+//
+// Lookup ignores subcommands that have len(c.Commands) == 0 and c.Run == nil.
+// Such subcommands are only for use as arguments to "help".
+func (c *Command) Lookup(name string) *Command {
+	for _, sub := range c.Commands {
+		if sub.Name() == name && (len(c.Commands) > 0 || c.Runnable()) {
+			return sub
+		}
+	}
+	return nil
+}
+
+// hasFlag reports whether a command or any of its subcommands contain the given
+// flag.
+func hasFlag(c *Command, name string) bool {
+	if f := c.Flag.Lookup(name); f != nil {
+		return true
+	}
+	for _, sub := range c.Commands {
+		if hasFlag(sub, name) {
+			return true
+		}
+	}
+	return false
+}
+
+// LongName returns the command's long name: all the words in the usage line between "go" and a flag or argument,
+func (c *Command) LongName() string {
+	name := c.UsageLine
+	if i := strings.Index(name, " ["); i >= 0 {
+		name = name[:i]
+	}
+	if name == "go" {
+		return ""
+	}
+	return strings.TrimPrefix(name, "go ")
+}
+
+// Name returns the command's short name: the last word in the usage line before a flag or argument.
+func (c *Command) Name() string {
+	name := c.LongName()
+	if i := strings.LastIndex(name, " "); i >= 0 {
+		name = name[i+1:]
+	}
+	return name
+}
+
+func (c *Command) Usage() {
+	fmt.Fprintf(os.Stderr, "usage: %s\n", c.UsageLine)
+	fmt.Fprintf(os.Stderr, "Run 'go help %s' for details.\n", c.LongName())
+	SetExitStatus(2)
+	Exit()
+}
+
+// Runnable reports whether the command can be run; otherwise
+// it is a documentation pseudo-command such as importpath.
+func (c *Command) Runnable() bool {
+	return c.Run != nil
+}
+
+var atExitFuncs []func()
+
+func AtExit(f func()) {
+	atExitFuncs = append(atExitFuncs, f)
+}
+
+func Exit() {
+	for _, f := range atExitFuncs {
+		f()
+	}
+	os.Exit(exitStatus)
+}
+
+func Fatalf(format string, args ...any) {
+	Errorf(format, args...)
+	Exit()
+}
+
+func Errorf(format string, args ...any) {
+	log.Printf(format, args...)
+	SetExitStatus(1)
+}
+
+func ExitIfErrors() {
+	if exitStatus != 0 {
+		Exit()
+	}
+}
+
+func Error(err error) {
+	// We use errors.Join to return multiple errors from various routines.
+	// If we receive multiple errors joined with a basic errors.Join,
+	// handle each one separately so that they all have the leading "go: " prefix.
+	// A plain interface check is not good enough because there might be
+	// other kinds of structured errors that are logically one unit and that
+	// add other context: only handling the wrapped errors would lose
+	// that context.
+	if err != nil && reflect.TypeOf(err).String() == "*errors.joinError" {
+		for _, e := range err.(interface{ Unwrap() []error }).Unwrap() {
+			Error(e)
+		}
+		return
+	}
+	Errorf("go: %v", err)
+}
+
+func Fatal(err error) {
+	Error(err)
+	Exit()
+}
+
+var exitStatus = 0
+var exitMu sync.Mutex
+
+func SetExitStatus(n int) {
+	exitMu.Lock()
+	if exitStatus < n {
+		exitStatus = n
+	}
+	exitMu.Unlock()
+}
+
+func GetExitStatus() int {
+	return exitStatus
+}
+
+// Run runs the command, with stdout and stderr
+// connected to the go command's own stdout and stderr.
+// If the command fails, Run reports the error using Errorf.
+func Run(cmdargs ...any) {
+	cmdline := str.StringList(cmdargs...)
+	if cfg.BuildN || cfg.BuildX {
+		fmt.Printf("%s\n", strings.Join(cmdline, " "))
+		if cfg.BuildN {
+			return
+		}
+	}
+
+	cmd := exec.Command(cmdline[0], cmdline[1:]...)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	if err := cmd.Run(); err != nil {
+		Errorf("%v", err)
+	}
+}
+
+// RunStdin is like run but connects Stdin.
+func RunStdin(cmdline []string) {
+	cmd := exec.Command(cmdline[0], cmdline[1:]...)
+	cmd.Stdin = os.Stdin
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	cmd.Env = cfg.OrigEnv
+	StartSigHandlers()
+	if err := cmd.Run(); err != nil {
+		Errorf("%v", err)
+	}
+}
+
+// Usage is the usage-reporting function, filled in by package main
+// but here for reference by other packages.
+var Usage func()
diff --git a/src/cmd/go/internal/base/env.go b/src/cmd/go/internal/base/env.go
new file mode 100644
index 0000000..20ae06d
--- /dev/null
+++ b/src/cmd/go/internal/base/env.go
@@ -0,0 +1,46 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package base
+
+import (
+	"cmd/go/internal/cfg"
+	"fmt"
+	"os"
+	"path/filepath"
+	"runtime"
+)
+
+// AppendPWD returns the result of appending PWD=dir to the environment base.
+//
+// The resulting environment makes os.Getwd more efficient for a subprocess
+// running in dir, and also improves the accuracy of paths relative to dir
+// if one or more elements of dir is a symlink.
+func AppendPWD(base []string, dir string) []string {
+	// POSIX requires PWD to be absolute.
+	// Internally we only use absolute paths, so dir should already be absolute.
+	if !filepath.IsAbs(dir) {
+		panic(fmt.Sprintf("AppendPWD with relative path %q", dir))
+	}
+	return append(base, "PWD="+dir)
+}
+
+// AppendPATH returns the result of appending PATH=$GOROOT/bin:$PATH
+// (or the platform equivalent) to the environment base.
+func AppendPATH(base []string) []string {
+	if cfg.GOROOTbin == "" {
+		return base
+	}
+
+	pathVar := "PATH"
+	if runtime.GOOS == "plan9" {
+		pathVar = "path"
+	}
+
+	path := os.Getenv(pathVar)
+	if path == "" {
+		return append(base, pathVar+"="+cfg.GOROOTbin)
+	}
+	return append(base, pathVar+"="+cfg.GOROOTbin+string(os.PathListSeparator)+path)
+}
diff --git a/src/cmd/go/internal/base/flag.go b/src/cmd/go/internal/base/flag.go
new file mode 100644
index 0000000..74e1275
--- /dev/null
+++ b/src/cmd/go/internal/base/flag.go
@@ -0,0 +1,85 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package base
+
+import (
+	"flag"
+	"fmt"
+
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/internal/quoted"
+)
+
+// A StringsFlag is a command-line flag that interprets its argument
+// as a space-separated list of possibly-quoted strings.
+type StringsFlag []string
+
+func (v *StringsFlag) Set(s string) error {
+	var err error
+	*v, err = quoted.Split(s)
+	if *v == nil {
+		*v = []string{}
+	}
+	return err
+}
+
+func (v *StringsFlag) String() string {
+	return "<StringsFlag>"
+}
+
+// explicitStringFlag is like a regular string flag, but it also tracks whether
+// the string was set explicitly to a non-empty value.
+type explicitStringFlag struct {
+	value    *string
+	explicit *bool
+}
+
+func (f explicitStringFlag) String() string {
+	if f.value == nil {
+		return ""
+	}
+	return *f.value
+}
+
+func (f explicitStringFlag) Set(v string) error {
+	*f.value = v
+	if v != "" {
+		*f.explicit = true
+	}
+	return nil
+}
+
+// AddBuildFlagsNX adds the -n and -x build flags to the flag set.
+func AddBuildFlagsNX(flags *flag.FlagSet) {
+	flags.BoolVar(&cfg.BuildN, "n", false, "")
+	flags.BoolVar(&cfg.BuildX, "x", false, "")
+}
+
+// AddChdirFlag adds the -C flag to the flag set.
+func AddChdirFlag(flags *flag.FlagSet) {
+	// The usage message is never printed, but it's used in chdir_test.go
+	// to identify that the -C flag is from AddChdirFlag.
+	flags.Func("C", "AddChdirFlag", ChdirFlag)
+}
+
+// AddModFlag adds the -mod build flag to the flag set.
+func AddModFlag(flags *flag.FlagSet) {
+	flags.Var(explicitStringFlag{value: &cfg.BuildMod, explicit: &cfg.BuildModExplicit}, "mod", "")
+}
+
+// AddModCommonFlags adds the module-related flags common to build commands
+// and 'go mod' subcommands.
+func AddModCommonFlags(flags *flag.FlagSet) {
+	flags.BoolVar(&cfg.ModCacheRW, "modcacherw", false, "")
+	flags.StringVar(&cfg.ModFile, "modfile", "", "")
+	flags.StringVar(&fsys.OverlayFile, "overlay", "", "")
+}
+
+func ChdirFlag(s string) error {
+	// main handles -C by removing it from the command line.
+	// If we see one during flag parsing, that's an error.
+	return fmt.Errorf("-C flag must be first flag on command line")
+}
diff --git a/src/cmd/go/internal/base/goflags.go b/src/cmd/go/internal/base/goflags.go
new file mode 100644
index 0000000..eced2c5
--- /dev/null
+++ b/src/cmd/go/internal/base/goflags.go
@@ -0,0 +1,162 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package base
+
+import (
+	"flag"
+	"fmt"
+	"runtime"
+	"strings"
+
+	"cmd/go/internal/cfg"
+	"cmd/internal/quoted"
+)
+
+var goflags []string // cached $GOFLAGS list; can be -x or --x form
+
+// GOFLAGS returns the flags from $GOFLAGS.
+// The list can be assumed to contain one string per flag,
+// with each string either beginning with -name or --name.
+func GOFLAGS() []string {
+	InitGOFLAGS()
+	return goflags
+}
+
+// InitGOFLAGS initializes the goflags list from $GOFLAGS.
+// If goflags is already initialized, it does nothing.
+func InitGOFLAGS() {
+	if goflags != nil { // already initialized
+		return
+	}
+
+	// Ignore bad flag in go env and go bug, because
+	// they are what people reach for when debugging
+	// a problem, and maybe they're debugging GOFLAGS.
+	// (Both will show the GOFLAGS setting if let succeed.)
+	hideErrors := cfg.CmdName == "env" || cfg.CmdName == "bug"
+
+	var err error
+	goflags, err = quoted.Split(cfg.Getenv("GOFLAGS"))
+	if err != nil {
+		if hideErrors {
+			return
+		}
+		Fatalf("go: parsing $GOFLAGS: %v", err)
+	}
+
+	if len(goflags) == 0 {
+		// nothing to do; avoid work on later InitGOFLAGS call
+		goflags = []string{}
+		return
+	}
+
+	// Each of the words returned by strings.Fields must be its own flag.
+	// To set flag arguments use -x=value instead of -x value.
+	// For boolean flags, -x is fine instead of -x=true.
+	for _, f := range goflags {
+		// Check that every flag looks like -x --x -x=value or --x=value.
+		if !strings.HasPrefix(f, "-") || f == "-" || f == "--" || strings.HasPrefix(f, "---") || strings.HasPrefix(f, "-=") || strings.HasPrefix(f, "--=") {
+			if hideErrors {
+				continue
+			}
+			Fatalf("go: parsing $GOFLAGS: non-flag %q", f)
+		}
+
+		name := f[1:]
+		if name[0] == '-' {
+			name = name[1:]
+		}
+		if i := strings.Index(name, "="); i >= 0 {
+			name = name[:i]
+		}
+		if !hasFlag(Go, name) {
+			if hideErrors {
+				continue
+			}
+			Fatalf("go: parsing $GOFLAGS: unknown flag -%s", name)
+		}
+	}
+}
+
+// boolFlag is the optional interface for flag.Value known to the flag package.
+// (It is not clear why package flag does not export this interface.)
+type boolFlag interface {
+	flag.Value
+	IsBoolFlag() bool
+}
+
+// SetFromGOFLAGS sets the flags in the given flag set using settings in $GOFLAGS.
+func SetFromGOFLAGS(flags *flag.FlagSet) {
+	InitGOFLAGS()
+
+	// This loop is similar to flag.Parse except that it ignores
+	// unknown flags found in goflags, so that setting, say, GOFLAGS=-ldflags=-w
+	// does not break commands that don't have a -ldflags.
+	// It also adjusts the output to be clear that the reported problem is from $GOFLAGS.
+	where := "$GOFLAGS"
+	if runtime.GOOS == "windows" {
+		where = "%GOFLAGS%"
+	}
+	for _, goflag := range goflags {
+		name, value, hasValue := goflag, "", false
+		// Ignore invalid flags like '=' or '=value'.
+		// If it is not reported in InitGOFlags it means we don't want to report it.
+		if i := strings.Index(goflag, "="); i == 0 {
+			continue
+		} else if i > 0 {
+			name, value, hasValue = goflag[:i], goflag[i+1:], true
+		}
+		if strings.HasPrefix(name, "--") {
+			name = name[1:]
+		}
+		f := flags.Lookup(name[1:])
+		if f == nil {
+			continue
+		}
+
+		// Use flags.Set consistently (instead of f.Value.Set) so that a subsequent
+		// call to flags.Visit will correctly visit the flags that have been set.
+
+		if fb, ok := f.Value.(boolFlag); ok && fb.IsBoolFlag() {
+			if hasValue {
+				if err := flags.Set(f.Name, value); err != nil {
+					fmt.Fprintf(flags.Output(), "go: invalid boolean value %q for flag %s (from %s): %v\n", value, name, where, err)
+					flags.Usage()
+				}
+			} else {
+				if err := flags.Set(f.Name, "true"); err != nil {
+					fmt.Fprintf(flags.Output(), "go: invalid boolean flag %s (from %s): %v\n", name, where, err)
+					flags.Usage()
+				}
+			}
+		} else {
+			if !hasValue {
+				fmt.Fprintf(flags.Output(), "go: flag needs an argument: %s (from %s)\n", name, where)
+				flags.Usage()
+			}
+			if err := flags.Set(f.Name, value); err != nil {
+				fmt.Fprintf(flags.Output(), "go: invalid value %q for flag %s (from %s): %v\n", value, name, where, err)
+				flags.Usage()
+			}
+		}
+	}
+}
+
+// InGOFLAGS returns whether GOFLAGS contains the given flag, such as "-mod".
+func InGOFLAGS(flag string) bool {
+	for _, goflag := range GOFLAGS() {
+		name := goflag
+		if strings.HasPrefix(name, "--") {
+			name = name[1:]
+		}
+		if i := strings.Index(name, "="); i >= 0 {
+			name = name[:i]
+		}
+		if name == flag {
+			return true
+		}
+	}
+	return false
+}
diff --git a/src/cmd/go/internal/base/limit.go b/src/cmd/go/internal/base/limit.go
new file mode 100644
index 0000000..b4160bd
--- /dev/null
+++ b/src/cmd/go/internal/base/limit.go
@@ -0,0 +1,84 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package base
+
+import (
+	"fmt"
+	"internal/godebug"
+	"runtime"
+	"strconv"
+	"sync"
+)
+
+var NetLimitGodebug = godebug.New("#cmdgonetlimit")
+
+// NetLimit returns the limit on concurrent network operations
+// configured by GODEBUG=cmdgonetlimit, if any.
+//
+// A limit of 0 (indicated by 0, true) means that network operations should not
+// be allowed.
+func NetLimit() (int, bool) {
+	netLimitOnce.Do(func() {
+		s := NetLimitGodebug.Value()
+		if s == "" {
+			return
+		}
+
+		n, err := strconv.Atoi(s)
+		if err != nil {
+			Fatalf("invalid %s: %v", NetLimitGodebug.Name(), err)
+		}
+		if n < 0 {
+			// Treat negative values as unlimited.
+			return
+		}
+		netLimitSem = make(chan struct{}, n)
+	})
+
+	return cap(netLimitSem), netLimitSem != nil
+}
+
+// AcquireNet acquires a semaphore token for a network operation.
+func AcquireNet() (release func(), err error) {
+	hasToken := false
+	if n, ok := NetLimit(); ok {
+		if n == 0 {
+			return nil, fmt.Errorf("network disabled by %v=%v", NetLimitGodebug.Name(), NetLimitGodebug.Value())
+		}
+		netLimitSem <- struct{}{}
+		hasToken = true
+	}
+
+	checker := new(netTokenChecker)
+	runtime.SetFinalizer(checker, (*netTokenChecker).panicUnreleased)
+
+	return func() {
+		if checker.released {
+			panic("internal error: net token released twice")
+		}
+		checker.released = true
+		if hasToken {
+			<-netLimitSem
+		}
+		runtime.SetFinalizer(checker, nil)
+	}, nil
+}
+
+var (
+	netLimitOnce sync.Once
+	netLimitSem  chan struct{}
+)
+
+type netTokenChecker struct {
+	released bool
+	// We want to use a finalizer to check that all acquired tokens are returned,
+	// so we arbitrarily pad the tokens with a string to defeat the runtime's
+	// “tiny allocator”.
+	unusedAvoidTinyAllocator string
+}
+
+func (c *netTokenChecker) panicUnreleased() {
+	panic("internal error: net token acquired but not released")
+}
diff --git a/src/cmd/go/internal/base/path.go b/src/cmd/go/internal/base/path.go
new file mode 100644
index 0000000..64f213b
--- /dev/null
+++ b/src/cmd/go/internal/base/path.go
@@ -0,0 +1,79 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package base
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"sync"
+)
+
+var cwd string
+var cwdOnce sync.Once
+
+// UncachedCwd returns the current working directory.
+// Most callers should use Cwd, which caches the result for future use.
+// UncachedCwd is appropriate to call early in program startup before flag parsing,
+// because the -C flag may change the current directory.
+func UncachedCwd() string {
+	wd, err := os.Getwd()
+	if err != nil {
+		Fatalf("cannot determine current directory: %v", err)
+	}
+	return wd
+}
+
+// Cwd returns the current working directory at the time of the first call.
+func Cwd() string {
+	cwdOnce.Do(func() {
+		cwd = UncachedCwd()
+	})
+	return cwd
+}
+
+// ShortPath returns an absolute or relative name for path, whatever is shorter.
+func ShortPath(path string) string {
+	if rel, err := filepath.Rel(Cwd(), path); err == nil && len(rel) < len(path) {
+		return rel
+	}
+	return path
+}
+
+// RelPaths returns a copy of paths with absolute paths
+// made relative to the current directory if they would be shorter.
+func RelPaths(paths []string) []string {
+	var out []string
+	for _, p := range paths {
+		rel, err := filepath.Rel(Cwd(), p)
+		if err == nil && len(rel) < len(p) {
+			p = rel
+		}
+		out = append(out, p)
+	}
+	return out
+}
+
+// IsTestFile reports whether the source file is a set of tests and should therefore
+// be excluded from coverage analysis.
+func IsTestFile(file string) bool {
+	// We don't cover tests, only the code they test.
+	return strings.HasSuffix(file, "_test.go")
+}
+
+// IsNull reports whether the path is a common name for the null device.
+// It returns true for /dev/null on Unix, or NUL (case-insensitive) on Windows.
+func IsNull(path string) bool {
+	if path == os.DevNull {
+		return true
+	}
+	if runtime.GOOS == "windows" {
+		if strings.EqualFold(path, "NUL") {
+			return true
+		}
+	}
+	return false
+}
diff --git a/src/cmd/go/internal/base/signal.go b/src/cmd/go/internal/base/signal.go
new file mode 100644
index 0000000..05befcf
--- /dev/null
+++ b/src/cmd/go/internal/base/signal.go
@@ -0,0 +1,31 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package base
+
+import (
+	"os"
+	"os/signal"
+	"sync"
+)
+
+// Interrupted is closed when the go command receives an interrupt signal.
+var Interrupted = make(chan struct{})
+
+// processSignals setups signal handler.
+func processSignals() {
+	sig := make(chan os.Signal, 1)
+	signal.Notify(sig, signalsToIgnore...)
+	go func() {
+		<-sig
+		close(Interrupted)
+	}()
+}
+
+var onceProcessSignals sync.Once
+
+// StartSigHandlers starts the signal handlers.
+func StartSigHandlers() {
+	onceProcessSignals.Do(processSignals)
+}
diff --git a/src/cmd/go/internal/base/signal_notunix.go b/src/cmd/go/internal/base/signal_notunix.go
new file mode 100644
index 0000000..682705f
--- /dev/null
+++ b/src/cmd/go/internal/base/signal_notunix.go
@@ -0,0 +1,17 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build plan9 || windows
+
+package base
+
+import (
+	"os"
+)
+
+var signalsToIgnore = []os.Signal{os.Interrupt}
+
+// SignalTrace is the signal to send to make a Go program
+// crash with a stack trace (no such signal in this case).
+var SignalTrace os.Signal = nil
diff --git a/src/cmd/go/internal/base/signal_unix.go b/src/cmd/go/internal/base/signal_unix.go
new file mode 100644
index 0000000..0905971
--- /dev/null
+++ b/src/cmd/go/internal/base/signal_unix.go
@@ -0,0 +1,18 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix || js || wasip1
+
+package base
+
+import (
+	"os"
+	"syscall"
+)
+
+var signalsToIgnore = []os.Signal{os.Interrupt, syscall.SIGQUIT}
+
+// SignalTrace is the signal to send to make a Go program
+// crash with a stack trace.
+var SignalTrace os.Signal = syscall.SIGQUIT
diff --git a/src/cmd/go/internal/base/tool.go b/src/cmd/go/internal/base/tool.go
new file mode 100644
index 0000000..ab623da
--- /dev/null
+++ b/src/cmd/go/internal/base/tool.go
@@ -0,0 +1,41 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package base
+
+import (
+	"fmt"
+	"go/build"
+	"os"
+	"path/filepath"
+
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/par"
+)
+
+// Tool returns the path to the named tool (for example, "vet").
+// If the tool cannot be found, Tool exits the process.
+func Tool(toolName string) string {
+	toolPath, err := ToolPath(toolName)
+	if err != nil && len(cfg.BuildToolexec) == 0 {
+		// Give a nice message if there is no tool with that name.
+		fmt.Fprintf(os.Stderr, "go: no such tool %q\n", toolName)
+		SetExitStatus(2)
+		Exit()
+	}
+	return toolPath
+}
+
+// Tool returns the path at which we expect to find the named tool
+// (for example, "vet"), and the error (if any) from statting that path.
+func ToolPath(toolName string) (string, error) {
+	toolPath := filepath.Join(build.ToolDir, toolName) + cfg.ToolExeSuffix()
+	err := toolStatCache.Do(toolPath, func() error {
+		_, err := os.Stat(toolPath)
+		return err
+	})
+	return toolPath, err
+}
+
+var toolStatCache par.Cache[string, error]
diff --git a/src/cmd/go/internal/bug/bug.go b/src/cmd/go/internal/bug/bug.go
new file mode 100644
index 0000000..ed18136
--- /dev/null
+++ b/src/cmd/go/internal/bug/bug.go
@@ -0,0 +1,224 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package bug implements the “go bug” command.
+package bug
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"io"
+	urlpkg "net/url"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
+	"runtime"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/envcmd"
+	"cmd/go/internal/web"
+	"cmd/go/internal/work"
+)
+
+var CmdBug = &base.Command{
+	Run:       runBug,
+	UsageLine: "go bug",
+	Short:     "start a bug report",
+	Long: `
+Bug opens the default browser and starts a new bug report.
+The report includes useful system information.
+	`,
+}
+
+func init() {
+	CmdBug.Flag.BoolVar(&cfg.BuildV, "v", false, "")
+	base.AddChdirFlag(&CmdBug.Flag)
+}
+
+func runBug(ctx context.Context, cmd *base.Command, args []string) {
+	if len(args) > 0 {
+		base.Fatalf("go: bug takes no arguments")
+	}
+	work.BuildInit()
+
+	var buf strings.Builder
+	buf.WriteString(bugHeader)
+	printGoVersion(&buf)
+	buf.WriteString("### Does this issue reproduce with the latest release?\n\n\n")
+	printEnvDetails(&buf)
+	buf.WriteString(bugFooter)
+
+	body := buf.String()
+	url := "https://github.com/golang/go/issues/new?body=" + urlpkg.QueryEscape(body)
+	if !web.OpenBrowser(url) {
+		fmt.Print("Please file a new issue at golang.org/issue/new using this template:\n\n")
+		fmt.Print(body)
+	}
+}
+
+const bugHeader = `<!-- Please answer these questions before submitting your issue. Thanks! -->
+
+`
+const bugFooter = `### What did you do?
+
+<!--
+If possible, provide a recipe for reproducing the error.
+A complete runnable program is good.
+A link on play.golang.org is best.
+-->
+
+
+
+### What did you expect to see?
+
+
+
+### What did you see instead?
+
+`
+
+func printGoVersion(w io.Writer) {
+	fmt.Fprintf(w, "### What version of Go are you using (`go version`)?\n\n")
+	fmt.Fprintf(w, "<pre>\n")
+	fmt.Fprintf(w, "$ go version\n")
+	fmt.Fprintf(w, "go version %s %s/%s\n", runtime.Version(), runtime.GOOS, runtime.GOARCH)
+	fmt.Fprintf(w, "</pre>\n")
+	fmt.Fprintf(w, "\n")
+}
+
+func printEnvDetails(w io.Writer) {
+	fmt.Fprintf(w, "### What operating system and processor architecture are you using (`go env`)?\n\n")
+	fmt.Fprintf(w, "<details><summary><code>go env</code> Output</summary><br><pre>\n")
+	fmt.Fprintf(w, "$ go env\n")
+	printGoEnv(w)
+	printGoDetails(w)
+	printOSDetails(w)
+	printCDetails(w)
+	fmt.Fprintf(w, "</pre></details>\n\n")
+}
+
+func printGoEnv(w io.Writer) {
+	env := envcmd.MkEnv()
+	env = append(env, envcmd.ExtraEnvVars()...)
+	env = append(env, envcmd.ExtraEnvVarsCostly()...)
+	envcmd.PrintEnv(w, env)
+}
+
+func printGoDetails(w io.Writer) {
+	gocmd := filepath.Join(runtime.GOROOT(), "bin/go")
+	printCmdOut(w, "GOROOT/bin/go version: ", gocmd, "version")
+	printCmdOut(w, "GOROOT/bin/go tool compile -V: ", gocmd, "tool", "compile", "-V")
+}
+
+func printOSDetails(w io.Writer) {
+	switch runtime.GOOS {
+	case "darwin", "ios":
+		printCmdOut(w, "uname -v: ", "uname", "-v")
+		printCmdOut(w, "", "sw_vers")
+	case "linux":
+		printCmdOut(w, "uname -sr: ", "uname", "-sr")
+		printCmdOut(w, "", "lsb_release", "-a")
+		printGlibcVersion(w)
+	case "openbsd", "netbsd", "freebsd", "dragonfly":
+		printCmdOut(w, "uname -v: ", "uname", "-v")
+	case "illumos", "solaris":
+		// Be sure to use the OS-supplied uname, in "/usr/bin":
+		printCmdOut(w, "uname -srv: ", "/usr/bin/uname", "-srv")
+		out, err := os.ReadFile("/etc/release")
+		if err == nil {
+			fmt.Fprintf(w, "/etc/release: %s\n", out)
+		} else {
+			if cfg.BuildV {
+				fmt.Printf("failed to read /etc/release: %v\n", err)
+			}
+		}
+	}
+}
+
+func printCDetails(w io.Writer) {
+	printCmdOut(w, "lldb --version: ", "lldb", "--version")
+	cmd := exec.Command("gdb", "--version")
+	out, err := cmd.Output()
+	if err == nil {
+		// There's apparently no combination of command line flags
+		// to get gdb to spit out its version without the license and warranty.
+		// Print up to the first newline.
+		fmt.Fprintf(w, "gdb --version: %s\n", firstLine(out))
+	} else {
+		if cfg.BuildV {
+			fmt.Printf("failed to run gdb --version: %v\n", err)
+		}
+	}
+}
+
+// printCmdOut prints the output of running the given command.
+// It ignores failures; 'go bug' is best effort.
+func printCmdOut(w io.Writer, prefix, path string, args ...string) {
+	cmd := exec.Command(path, args...)
+	out, err := cmd.Output()
+	if err != nil {
+		if cfg.BuildV {
+			fmt.Printf("%s %s: %v\n", path, strings.Join(args, " "), err)
+		}
+		return
+	}
+	fmt.Fprintf(w, "%s%s\n", prefix, bytes.TrimSpace(out))
+}
+
+// firstLine returns the first line of a given byte slice.
+func firstLine(buf []byte) []byte {
+	idx := bytes.IndexByte(buf, '\n')
+	if idx > 0 {
+		buf = buf[:idx]
+	}
+	return bytes.TrimSpace(buf)
+}
+
+// printGlibcVersion prints information about the glibc version.
+// It ignores failures.
+func printGlibcVersion(w io.Writer) {
+	tempdir := os.TempDir()
+	if tempdir == "" {
+		return
+	}
+	src := []byte(`int main() {}`)
+	srcfile := filepath.Join(tempdir, "go-bug.c")
+	outfile := filepath.Join(tempdir, "go-bug")
+	err := os.WriteFile(srcfile, src, 0644)
+	if err != nil {
+		return
+	}
+	defer os.Remove(srcfile)
+	cmd := exec.Command("gcc", "-o", outfile, srcfile)
+	if _, err = cmd.CombinedOutput(); err != nil {
+		return
+	}
+	defer os.Remove(outfile)
+
+	cmd = exec.Command("ldd", outfile)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return
+	}
+	re := regexp.MustCompile(`libc\.so[^ ]* => ([^ ]+)`)
+	m := re.FindStringSubmatch(string(out))
+	if m == nil {
+		return
+	}
+	cmd = exec.Command(m[1])
+	out, err = cmd.Output()
+	if err != nil {
+		return
+	}
+	fmt.Fprintf(w, "%s: %s\n", m[1], firstLine(out))
+
+	// print another line (the one containing version string) in case of musl libc
+	if idx := bytes.IndexByte(out, '\n'); bytes.Contains(out, []byte("musl")) && idx > -1 {
+		fmt.Fprintf(w, "%s\n", firstLine(out[idx+1:]))
+	}
+}
diff --git a/src/cmd/go/internal/cache/cache.go b/src/cmd/go/internal/cache/cache.go
new file mode 100644
index 0000000..4a82d27
--- /dev/null
+++ b/src/cmd/go/internal/cache/cache.go
@@ -0,0 +1,627 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package cache implements a build artifact cache.
+package cache
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"internal/godebug"
+	"io"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"time"
+
+	"cmd/go/internal/lockedfile"
+	"cmd/go/internal/mmap"
+)
+
+// An ActionID is a cache action key, the hash of a complete description of a
+// repeatable computation (command line, environment variables,
+// input file contents, executable contents).
+type ActionID [HashSize]byte
+
+// An OutputID is a cache output key, the hash of an output of a computation.
+type OutputID [HashSize]byte
+
+// Cache is the interface as used by the cmd/go.
+type Cache interface {
+	// Get returns the cache entry for the provided ActionID.
+	// On miss, the error type should be of type *entryNotFoundError.
+	//
+	// After a success call to Get, OutputFile(Entry.OutputID) must
+	// exist on disk for until Close is called (at the end of the process).
+	Get(ActionID) (Entry, error)
+
+	// Put adds an item to the cache.
+	//
+	// The seeker is only used to seek to the beginning. After a call to Put,
+	// the seek position is not guaranteed to be in any particular state.
+	//
+	// As a special case, if the ReadSeeker is of type noVerifyReadSeeker,
+	// the verification from GODEBUG=goverifycache=1 is skipped.
+	//
+	// After a success call to Get, OutputFile(Entry.OutputID) must
+	// exist on disk for until Close is called (at the end of the process).
+	Put(ActionID, io.ReadSeeker) (_ OutputID, size int64, _ error)
+
+	// Close is called at the end of the go process. Implementations can do
+	// cache cleanup work at this phase, or wait for and report any errors from
+	// background cleanup work started earlier. Any cache trimming should in one
+	// process should not violate cause the invariants of this interface to be
+	// violated in another process. Namely, a cache trim from one process should
+	// not delete an ObjectID from disk that was recently Get or Put from
+	// another process. As a rule of thumb, don't trim things used in the last
+	// day.
+	Close() error
+
+	// OutputFile returns the path on disk where OutputID is stored.
+	//
+	// It's only called after a successful get or put call so it doesn't need
+	// to return an error; it's assumed that if the previous get or put succeeded,
+	// it's already on disk.
+	OutputFile(OutputID) string
+
+	// FuzzDir returns where fuzz files are stored.
+	FuzzDir() string
+}
+
+// A Cache is a package cache, backed by a file system directory tree.
+type DiskCache struct {
+	dir string
+	now func() time.Time
+}
+
+// Open opens and returns the cache in the given directory.
+//
+// It is safe for multiple processes on a single machine to use the
+// same cache directory in a local file system simultaneously.
+// They will coordinate using operating system file locks and may
+// duplicate effort but will not corrupt the cache.
+//
+// However, it is NOT safe for multiple processes on different machines
+// to share a cache directory (for example, if the directory were stored
+// in a network file system). File locking is notoriously unreliable in
+// network file systems and may not suffice to protect the cache.
+func Open(dir string) (*DiskCache, error) {
+	info, err := os.Stat(dir)
+	if err != nil {
+		return nil, err
+	}
+	if !info.IsDir() {
+		return nil, &fs.PathError{Op: "open", Path: dir, Err: fmt.Errorf("not a directory")}
+	}
+	for i := 0; i < 256; i++ {
+		name := filepath.Join(dir, fmt.Sprintf("%02x", i))
+		if err := os.MkdirAll(name, 0777); err != nil {
+			return nil, err
+		}
+	}
+	c := &DiskCache{
+		dir: dir,
+		now: time.Now,
+	}
+	return c, nil
+}
+
+// fileName returns the name of the file corresponding to the given id.
+func (c *DiskCache) fileName(id [HashSize]byte, key string) string {
+	return filepath.Join(c.dir, fmt.Sprintf("%02x", id[0]), fmt.Sprintf("%x", id)+"-"+key)
+}
+
+// An entryNotFoundError indicates that a cache entry was not found, with an
+// optional underlying reason.
+type entryNotFoundError struct {
+	Err error
+}
+
+func (e *entryNotFoundError) Error() string {
+	if e.Err == nil {
+		return "cache entry not found"
+	}
+	return fmt.Sprintf("cache entry not found: %v", e.Err)
+}
+
+func (e *entryNotFoundError) Unwrap() error {
+	return e.Err
+}
+
+const (
+	// action entry file is "v1 <hex id> <hex out> <decimal size space-padded to 20 bytes> <unixnano space-padded to 20 bytes>\n"
+	hexSize   = HashSize * 2
+	entrySize = 2 + 1 + hexSize + 1 + hexSize + 1 + 20 + 1 + 20 + 1
+)
+
+// verify controls whether to run the cache in verify mode.
+// In verify mode, the cache always returns errMissing from Get
+// but then double-checks in Put that the data being written
+// exactly matches any existing entry. This provides an easy
+// way to detect program behavior that would have been different
+// had the cache entry been returned from Get.
+//
+// verify is enabled by setting the environment variable
+// GODEBUG=gocacheverify=1.
+var verify = false
+
+var errVerifyMode = errors.New("gocacheverify=1")
+
+// DebugTest is set when GODEBUG=gocachetest=1 is in the environment.
+var DebugTest = false
+
+func init() { initEnv() }
+
+var (
+	goCacheVerify = godebug.New("gocacheverify")
+	goDebugHash   = godebug.New("gocachehash")
+	goCacheTest   = godebug.New("gocachetest")
+)
+
+func initEnv() {
+	if goCacheVerify.Value() == "1" {
+		goCacheVerify.IncNonDefault()
+		verify = true
+	}
+	if goDebugHash.Value() == "1" {
+		goDebugHash.IncNonDefault()
+		debugHash = true
+	}
+	if goCacheTest.Value() == "1" {
+		goCacheTest.IncNonDefault()
+		DebugTest = true
+	}
+}
+
+// Get looks up the action ID in the cache,
+// returning the corresponding output ID and file size, if any.
+// Note that finding an output ID does not guarantee that the
+// saved file for that output ID is still available.
+func (c *DiskCache) Get(id ActionID) (Entry, error) {
+	if verify {
+		return Entry{}, &entryNotFoundError{Err: errVerifyMode}
+	}
+	return c.get(id)
+}
+
+type Entry struct {
+	OutputID OutputID
+	Size     int64
+	Time     time.Time // when added to cache
+}
+
+// get is Get but does not respect verify mode, so that Put can use it.
+func (c *DiskCache) get(id ActionID) (Entry, error) {
+	missing := func(reason error) (Entry, error) {
+		return Entry{}, &entryNotFoundError{Err: reason}
+	}
+	f, err := os.Open(c.fileName(id, "a"))
+	if err != nil {
+		return missing(err)
+	}
+	defer f.Close()
+	entry := make([]byte, entrySize+1) // +1 to detect whether f is too long
+	if n, err := io.ReadFull(f, entry); n > entrySize {
+		return missing(errors.New("too long"))
+	} else if err != io.ErrUnexpectedEOF {
+		if err == io.EOF {
+			return missing(errors.New("file is empty"))
+		}
+		return missing(err)
+	} else if n < entrySize {
+		return missing(errors.New("entry file incomplete"))
+	}
+	if entry[0] != 'v' || entry[1] != '1' || entry[2] != ' ' || entry[3+hexSize] != ' ' || entry[3+hexSize+1+hexSize] != ' ' || entry[3+hexSize+1+hexSize+1+20] != ' ' || entry[entrySize-1] != '\n' {
+		return missing(errors.New("invalid header"))
+	}
+	eid, entry := entry[3:3+hexSize], entry[3+hexSize:]
+	eout, entry := entry[1:1+hexSize], entry[1+hexSize:]
+	esize, entry := entry[1:1+20], entry[1+20:]
+	etime, entry := entry[1:1+20], entry[1+20:]
+	var buf [HashSize]byte
+	if _, err := hex.Decode(buf[:], eid); err != nil {
+		return missing(fmt.Errorf("decoding ID: %v", err))
+	} else if buf != id {
+		return missing(errors.New("mismatched ID"))
+	}
+	if _, err := hex.Decode(buf[:], eout); err != nil {
+		return missing(fmt.Errorf("decoding output ID: %v", err))
+	}
+	i := 0
+	for i < len(esize) && esize[i] == ' ' {
+		i++
+	}
+	size, err := strconv.ParseInt(string(esize[i:]), 10, 64)
+	if err != nil {
+		return missing(fmt.Errorf("parsing size: %v", err))
+	} else if size < 0 {
+		return missing(errors.New("negative size"))
+	}
+	i = 0
+	for i < len(etime) && etime[i] == ' ' {
+		i++
+	}
+	tm, err := strconv.ParseInt(string(etime[i:]), 10, 64)
+	if err != nil {
+		return missing(fmt.Errorf("parsing timestamp: %v", err))
+	} else if tm < 0 {
+		return missing(errors.New("negative timestamp"))
+	}
+
+	c.used(c.fileName(id, "a"))
+
+	return Entry{buf, size, time.Unix(0, tm)}, nil
+}
+
+// GetFile looks up the action ID in the cache and returns
+// the name of the corresponding data file.
+func GetFile(c Cache, id ActionID) (file string, entry Entry, err error) {
+	entry, err = c.Get(id)
+	if err != nil {
+		return "", Entry{}, err
+	}
+	file = c.OutputFile(entry.OutputID)
+	info, err := os.Stat(file)
+	if err != nil {
+		return "", Entry{}, &entryNotFoundError{Err: err}
+	}
+	if info.Size() != entry.Size {
+		return "", Entry{}, &entryNotFoundError{Err: errors.New("file incomplete")}
+	}
+	return file, entry, nil
+}
+
+// GetBytes looks up the action ID in the cache and returns
+// the corresponding output bytes.
+// GetBytes should only be used for data that can be expected to fit in memory.
+func GetBytes(c Cache, id ActionID) ([]byte, Entry, error) {
+	entry, err := c.Get(id)
+	if err != nil {
+		return nil, entry, err
+	}
+	data, _ := os.ReadFile(c.OutputFile(entry.OutputID))
+	if sha256.Sum256(data) != entry.OutputID {
+		return nil, entry, &entryNotFoundError{Err: errors.New("bad checksum")}
+	}
+	return data, entry, nil
+}
+
+// GetMmap looks up the action ID in the cache and returns
+// the corresponding output bytes.
+// GetMmap should only be used for data that can be expected to fit in memory.
+func GetMmap(c Cache, id ActionID) ([]byte, Entry, error) {
+	entry, err := c.Get(id)
+	if err != nil {
+		return nil, entry, err
+	}
+	md, err := mmap.Mmap(c.OutputFile(entry.OutputID))
+	if err != nil {
+		return nil, Entry{}, err
+	}
+	if int64(len(md.Data)) != entry.Size {
+		return nil, Entry{}, &entryNotFoundError{Err: errors.New("file incomplete")}
+	}
+	return md.Data, entry, nil
+}
+
+// OutputFile returns the name of the cache file storing output with the given OutputID.
+func (c *DiskCache) OutputFile(out OutputID) string {
+	file := c.fileName(out, "d")
+	c.used(file)
+	return file
+}
+
+// Time constants for cache expiration.
+//
+// We set the mtime on a cache file on each use, but at most one per mtimeInterval (1 hour),
+// to avoid causing many unnecessary inode updates. The mtimes therefore
+// roughly reflect "time of last use" but may in fact be older by at most an hour.
+//
+// We scan the cache for entries to delete at most once per trimInterval (1 day).
+//
+// When we do scan the cache, we delete entries that have not been used for
+// at least trimLimit (5 days). Statistics gathered from a month of usage by
+// Go developers found that essentially all reuse of cached entries happened
+// within 5 days of the previous reuse. See golang.org/issue/22990.
+const (
+	mtimeInterval = 1 * time.Hour
+	trimInterval  = 24 * time.Hour
+	trimLimit     = 5 * 24 * time.Hour
+)
+
+// used makes a best-effort attempt to update mtime on file,
+// so that mtime reflects cache access time.
+//
+// Because the reflection only needs to be approximate,
+// and to reduce the amount of disk activity caused by using
+// cache entries, used only updates the mtime if the current
+// mtime is more than an hour old. This heuristic eliminates
+// nearly all of the mtime updates that would otherwise happen,
+// while still keeping the mtimes useful for cache trimming.
+func (c *DiskCache) used(file string) {
+	info, err := os.Stat(file)
+	if err == nil && c.now().Sub(info.ModTime()) < mtimeInterval {
+		return
+	}
+	os.Chtimes(file, c.now(), c.now())
+}
+
+func (c *DiskCache) Close() error { return c.Trim() }
+
+// Trim removes old cache entries that are likely not to be reused.
+func (c *DiskCache) Trim() error {
+	now := c.now()
+
+	// We maintain in dir/trim.txt the time of the last completed cache trim.
+	// If the cache has been trimmed recently enough, do nothing.
+	// This is the common case.
+	// If the trim file is corrupt, detected if the file can't be parsed, or the
+	// trim time is too far in the future, attempt the trim anyway. It's possible that
+	// the cache was full when the corruption happened. Attempting a trim on
+	// an empty cache is cheap, so there wouldn't be a big performance hit in that case.
+	if data, err := lockedfile.Read(filepath.Join(c.dir, "trim.txt")); err == nil {
+		if t, err := strconv.ParseInt(strings.TrimSpace(string(data)), 10, 64); err == nil {
+			lastTrim := time.Unix(t, 0)
+			if d := now.Sub(lastTrim); d < trimInterval && d > -mtimeInterval {
+				return nil
+			}
+		}
+	}
+
+	// Trim each of the 256 subdirectories.
+	// We subtract an additional mtimeInterval
+	// to account for the imprecision of our "last used" mtimes.
+	cutoff := now.Add(-trimLimit - mtimeInterval)
+	for i := 0; i < 256; i++ {
+		subdir := filepath.Join(c.dir, fmt.Sprintf("%02x", i))
+		c.trimSubdir(subdir, cutoff)
+	}
+
+	// Ignore errors from here: if we don't write the complete timestamp, the
+	// cache will appear older than it is, and we'll trim it again next time.
+	var b bytes.Buffer
+	fmt.Fprintf(&b, "%d", now.Unix())
+	if err := lockedfile.Write(filepath.Join(c.dir, "trim.txt"), &b, 0666); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// trimSubdir trims a single cache subdirectory.
+func (c *DiskCache) trimSubdir(subdir string, cutoff time.Time) {
+	// Read all directory entries from subdir before removing
+	// any files, in case removing files invalidates the file offset
+	// in the directory scan. Also, ignore error from f.Readdirnames,
+	// because we don't care about reporting the error and we still
+	// want to process any entries found before the error.
+	f, err := os.Open(subdir)
+	if err != nil {
+		return
+	}
+	names, _ := f.Readdirnames(-1)
+	f.Close()
+
+	for _, name := range names {
+		// Remove only cache entries (xxxx-a and xxxx-d).
+		if !strings.HasSuffix(name, "-a") && !strings.HasSuffix(name, "-d") {
+			continue
+		}
+		entry := filepath.Join(subdir, name)
+		info, err := os.Stat(entry)
+		if err == nil && info.ModTime().Before(cutoff) {
+			os.Remove(entry)
+		}
+	}
+}
+
+// putIndexEntry adds an entry to the cache recording that executing the action
+// with the given id produces an output with the given output id (hash) and size.
+func (c *DiskCache) putIndexEntry(id ActionID, out OutputID, size int64, allowVerify bool) error {
+	// Note: We expect that for one reason or another it may happen
+	// that repeating an action produces a different output hash
+	// (for example, if the output contains a time stamp or temp dir name).
+	// While not ideal, this is also not a correctness problem, so we
+	// don't make a big deal about it. In particular, we leave the action
+	// cache entries writable specifically so that they can be overwritten.
+	//
+	// Setting GODEBUG=gocacheverify=1 does make a big deal:
+	// in verify mode we are double-checking that the cache entries
+	// are entirely reproducible. As just noted, this may be unrealistic
+	// in some cases but the check is also useful for shaking out real bugs.
+	entry := fmt.Sprintf("v1 %x %x %20d %20d\n", id, out, size, time.Now().UnixNano())
+	if verify && allowVerify {
+		old, err := c.get(id)
+		if err == nil && (old.OutputID != out || old.Size != size) {
+			// panic to show stack trace, so we can see what code is generating this cache entry.
+			msg := fmt.Sprintf("go: internal cache error: cache verify failed: id=%x changed:<<<\n%s\n>>>\nold: %x %d\nnew: %x %d", id, reverseHash(id), out, size, old.OutputID, old.Size)
+			panic(msg)
+		}
+	}
+	file := c.fileName(id, "a")
+
+	// Copy file to cache directory.
+	mode := os.O_WRONLY | os.O_CREATE
+	f, err := os.OpenFile(file, mode, 0666)
+	if err != nil {
+		return err
+	}
+	_, err = f.WriteString(entry)
+	if err == nil {
+		// Truncate the file only *after* writing it.
+		// (This should be a no-op, but truncate just in case of previous corruption.)
+		//
+		// This differs from os.WriteFile, which truncates to 0 *before* writing
+		// via os.O_TRUNC. Truncating only after writing ensures that a second write
+		// of the same content to the same file is idempotent, and does not — even
+		// temporarily! — undo the effect of the first write.
+		err = f.Truncate(int64(len(entry)))
+	}
+	if closeErr := f.Close(); err == nil {
+		err = closeErr
+	}
+	if err != nil {
+		// TODO(bcmills): This Remove potentially races with another go command writing to file.
+		// Can we eliminate it?
+		os.Remove(file)
+		return err
+	}
+	os.Chtimes(file, c.now(), c.now()) // mainly for tests
+
+	return nil
+}
+
+// noVerifyReadSeeker is a io.ReadSeeker wrapper sentinel type
+// that says that Cache.Put should skip the verify check
+// (from GODEBUG=goverifycache=1).
+type noVerifyReadSeeker struct {
+	io.ReadSeeker
+}
+
+// Put stores the given output in the cache as the output for the action ID.
+// It may read file twice. The content of file must not change between the two passes.
+func (c *DiskCache) Put(id ActionID, file io.ReadSeeker) (OutputID, int64, error) {
+	wrapper, isNoVerify := file.(noVerifyReadSeeker)
+	if isNoVerify {
+		file = wrapper.ReadSeeker
+	}
+	return c.put(id, file, !isNoVerify)
+}
+
+// PutNoVerify is like Put but disables the verify check
+// when GODEBUG=goverifycache=1 is set.
+// It is meant for data that is OK to cache but that we expect to vary slightly from run to run,
+// like test output containing times and the like.
+func PutNoVerify(c Cache, id ActionID, file io.ReadSeeker) (OutputID, int64, error) {
+	return c.Put(id, noVerifyReadSeeker{file})
+}
+
+func (c *DiskCache) put(id ActionID, file io.ReadSeeker, allowVerify bool) (OutputID, int64, error) {
+	// Compute output ID.
+	h := sha256.New()
+	if _, err := file.Seek(0, 0); err != nil {
+		return OutputID{}, 0, err
+	}
+	size, err := io.Copy(h, file)
+	if err != nil {
+		return OutputID{}, 0, err
+	}
+	var out OutputID
+	h.Sum(out[:0])
+
+	// Copy to cached output file (if not already present).
+	if err := c.copyFile(file, out, size); err != nil {
+		return out, size, err
+	}
+
+	// Add to cache index.
+	return out, size, c.putIndexEntry(id, out, size, allowVerify)
+}
+
+// PutBytes stores the given bytes in the cache as the output for the action ID.
+func PutBytes(c Cache, id ActionID, data []byte) error {
+	_, _, err := c.Put(id, bytes.NewReader(data))
+	return err
+}
+
+// copyFile copies file into the cache, expecting it to have the given
+// output ID and size, if that file is not present already.
+func (c *DiskCache) copyFile(file io.ReadSeeker, out OutputID, size int64) error {
+	name := c.fileName(out, "d")
+	info, err := os.Stat(name)
+	if err == nil && info.Size() == size {
+		// Check hash.
+		if f, err := os.Open(name); err == nil {
+			h := sha256.New()
+			io.Copy(h, f)
+			f.Close()
+			var out2 OutputID
+			h.Sum(out2[:0])
+			if out == out2 {
+				return nil
+			}
+		}
+		// Hash did not match. Fall through and rewrite file.
+	}
+
+	// Copy file to cache directory.
+	mode := os.O_RDWR | os.O_CREATE
+	if err == nil && info.Size() > size { // shouldn't happen but fix in case
+		mode |= os.O_TRUNC
+	}
+	f, err := os.OpenFile(name, mode, 0666)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+	if size == 0 {
+		// File now exists with correct size.
+		// Only one possible zero-length file, so contents are OK too.
+		// Early return here makes sure there's a "last byte" for code below.
+		return nil
+	}
+
+	// From here on, if any of the I/O writing the file fails,
+	// we make a best-effort attempt to truncate the file f
+	// before returning, to avoid leaving bad bytes in the file.
+
+	// Copy file to f, but also into h to double-check hash.
+	if _, err := file.Seek(0, 0); err != nil {
+		f.Truncate(0)
+		return err
+	}
+	h := sha256.New()
+	w := io.MultiWriter(f, h)
+	if _, err := io.CopyN(w, file, size-1); err != nil {
+		f.Truncate(0)
+		return err
+	}
+	// Check last byte before writing it; writing it will make the size match
+	// what other processes expect to find and might cause them to start
+	// using the file.
+	buf := make([]byte, 1)
+	if _, err := file.Read(buf); err != nil {
+		f.Truncate(0)
+		return err
+	}
+	h.Write(buf)
+	sum := h.Sum(nil)
+	if !bytes.Equal(sum, out[:]) {
+		f.Truncate(0)
+		return fmt.Errorf("file content changed underfoot")
+	}
+
+	// Commit cache file entry.
+	if _, err := f.Write(buf); err != nil {
+		f.Truncate(0)
+		return err
+	}
+	if err := f.Close(); err != nil {
+		// Data might not have been written,
+		// but file may look like it is the right size.
+		// To be extra careful, remove cached file.
+		os.Remove(name)
+		return err
+	}
+	os.Chtimes(name, c.now(), c.now()) // mainly for tests
+
+	return nil
+}
+
+// FuzzDir returns a subdirectory within the cache for storing fuzzing data.
+// The subdirectory may not exist.
+//
+// This directory is managed by the internal/fuzz package. Files in this
+// directory aren't removed by the 'go clean -cache' command or by Trim.
+// They may be removed with 'go clean -fuzzcache'.
+//
+// TODO(#48526): make Trim remove unused files from this directory.
+func (c *DiskCache) FuzzDir() string {
+	return filepath.Join(c.dir, "fuzz")
+}
diff --git a/src/cmd/go/internal/cache/cache_test.go b/src/cmd/go/internal/cache/cache_test.go
new file mode 100644
index 0000000..a12f1d2
--- /dev/null
+++ b/src/cmd/go/internal/cache/cache_test.go
@@ -0,0 +1,285 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cache
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"internal/testenv"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+func init() {
+	verify = false // even if GODEBUG is set
+}
+
+func TestBasic(t *testing.T) {
+	dir, err := os.MkdirTemp("", "cachetest-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+	_, err = Open(filepath.Join(dir, "notexist"))
+	if err == nil {
+		t.Fatal(`Open("tmp/notexist") succeeded, want failure`)
+	}
+
+	cdir := filepath.Join(dir, "c1")
+	if err := os.Mkdir(cdir, 0777); err != nil {
+		t.Fatal(err)
+	}
+
+	c1, err := Open(cdir)
+	if err != nil {
+		t.Fatalf("Open(c1) (create): %v", err)
+	}
+	if err := c1.putIndexEntry(dummyID(1), dummyID(12), 13, true); err != nil {
+		t.Fatalf("addIndexEntry: %v", err)
+	}
+	if err := c1.putIndexEntry(dummyID(1), dummyID(2), 3, true); err != nil { // overwrite entry
+		t.Fatalf("addIndexEntry: %v", err)
+	}
+	if entry, err := c1.Get(dummyID(1)); err != nil || entry.OutputID != dummyID(2) || entry.Size != 3 {
+		t.Fatalf("c1.Get(1) = %x, %v, %v, want %x, %v, nil", entry.OutputID, entry.Size, err, dummyID(2), 3)
+	}
+
+	c2, err := Open(cdir)
+	if err != nil {
+		t.Fatalf("Open(c2) (reuse): %v", err)
+	}
+	if entry, err := c2.Get(dummyID(1)); err != nil || entry.OutputID != dummyID(2) || entry.Size != 3 {
+		t.Fatalf("c2.Get(1) = %x, %v, %v, want %x, %v, nil", entry.OutputID, entry.Size, err, dummyID(2), 3)
+	}
+	if err := c2.putIndexEntry(dummyID(2), dummyID(3), 4, true); err != nil {
+		t.Fatalf("addIndexEntry: %v", err)
+	}
+	if entry, err := c1.Get(dummyID(2)); err != nil || entry.OutputID != dummyID(3) || entry.Size != 4 {
+		t.Fatalf("c1.Get(2) = %x, %v, %v, want %x, %v, nil", entry.OutputID, entry.Size, err, dummyID(3), 4)
+	}
+}
+
+func TestGrowth(t *testing.T) {
+	dir, err := os.MkdirTemp("", "cachetest-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+
+	c, err := Open(dir)
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+
+	n := 10000
+	if testing.Short() {
+		n = 10
+	}
+
+	for i := 0; i < n; i++ {
+		if err := c.putIndexEntry(dummyID(i), dummyID(i*99), int64(i)*101, true); err != nil {
+			t.Fatalf("addIndexEntry: %v", err)
+		}
+		id := ActionID(dummyID(i))
+		entry, err := c.Get(id)
+		if err != nil {
+			t.Fatalf("Get(%x): %v", id, err)
+		}
+		if entry.OutputID != dummyID(i*99) || entry.Size != int64(i)*101 {
+			t.Errorf("Get(%x) = %x, %d, want %x, %d", id, entry.OutputID, entry.Size, dummyID(i*99), int64(i)*101)
+		}
+	}
+	for i := 0; i < n; i++ {
+		id := ActionID(dummyID(i))
+		entry, err := c.Get(id)
+		if err != nil {
+			t.Fatalf("Get2(%x): %v", id, err)
+		}
+		if entry.OutputID != dummyID(i*99) || entry.Size != int64(i)*101 {
+			t.Errorf("Get2(%x) = %x, %d, want %x, %d", id, entry.OutputID, entry.Size, dummyID(i*99), int64(i)*101)
+		}
+	}
+}
+
+func TestVerifyPanic(t *testing.T) {
+	os.Setenv("GODEBUG", "gocacheverify=1")
+	initEnv()
+	defer func() {
+		os.Unsetenv("GODEBUG")
+		verify = false
+	}()
+
+	if !verify {
+		t.Fatal("initEnv did not set verify")
+	}
+
+	dir, err := os.MkdirTemp("", "cachetest-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+
+	c, err := Open(dir)
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+
+	id := ActionID(dummyID(1))
+	if err := PutBytes(c, id, []byte("abc")); err != nil {
+		t.Fatal(err)
+	}
+
+	defer func() {
+		if err := recover(); err != nil {
+			t.Log(err)
+			return
+		}
+	}()
+	PutBytes(c, id, []byte("def"))
+	t.Fatal("mismatched Put did not panic in verify mode")
+}
+
+func dummyID(x int) [HashSize]byte {
+	var out [HashSize]byte
+	binary.LittleEndian.PutUint64(out[:], uint64(x))
+	return out
+}
+
+func TestCacheTrim(t *testing.T) {
+	dir, err := os.MkdirTemp("", "cachetest-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+
+	c, err := Open(dir)
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+	const start = 1000000000
+	now := int64(start)
+	c.now = func() time.Time { return time.Unix(now, 0) }
+
+	checkTime := func(name string, mtime int64) {
+		t.Helper()
+		file := filepath.Join(c.dir, name[:2], name)
+		info, err := os.Stat(file)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if info.ModTime().Unix() != mtime {
+			t.Fatalf("%s mtime = %d, want %d", name, info.ModTime().Unix(), mtime)
+		}
+	}
+
+	id := ActionID(dummyID(1))
+	PutBytes(c, id, []byte("abc"))
+	entry, _ := c.Get(id)
+	PutBytes(c, ActionID(dummyID(2)), []byte("def"))
+	mtime := now
+	checkTime(fmt.Sprintf("%x-a", id), mtime)
+	checkTime(fmt.Sprintf("%x-d", entry.OutputID), mtime)
+
+	// Get should not change recent mtimes.
+	now = start + 10
+	c.Get(id)
+	checkTime(fmt.Sprintf("%x-a", id), mtime)
+	checkTime(fmt.Sprintf("%x-d", entry.OutputID), mtime)
+
+	// Get should change distant mtimes.
+	now = start + 5000
+	mtime2 := now
+	if _, err := c.Get(id); err != nil {
+		t.Fatal(err)
+	}
+	c.OutputFile(entry.OutputID)
+	checkTime(fmt.Sprintf("%x-a", id), mtime2)
+	checkTime(fmt.Sprintf("%x-d", entry.OutputID), mtime2)
+
+	// Trim should leave everything alone: it's all too new.
+	if err := c.Trim(); err != nil {
+		if testenv.SyscallIsNotSupported(err) {
+			t.Skipf("skipping: Trim is unsupported (%v)", err)
+		}
+		t.Fatal(err)
+	}
+	if _, err := c.Get(id); err != nil {
+		t.Fatal(err)
+	}
+	c.OutputFile(entry.OutputID)
+	data, err := os.ReadFile(filepath.Join(dir, "trim.txt"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	checkTime(fmt.Sprintf("%x-a", dummyID(2)), start)
+
+	// Trim less than a day later should not do any work at all.
+	now = start + 80000
+	if err := c.Trim(); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := c.Get(id); err != nil {
+		t.Fatal(err)
+	}
+	c.OutputFile(entry.OutputID)
+	data2, err := os.ReadFile(filepath.Join(dir, "trim.txt"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(data, data2) {
+		t.Fatalf("second trim did work: %q -> %q", data, data2)
+	}
+
+	// Fast forward and do another trim just before the 5 day cutoff.
+	// Note that because of usedQuantum the cutoff is actually 5 days + 1 hour.
+	// We used c.Get(id) just now, so 5 days later it should still be kept.
+	// On the other hand almost a full day has gone by since we wrote dummyID(2)
+	// and we haven't looked at it since, so 5 days later it should be gone.
+	now += 5 * 86400
+	checkTime(fmt.Sprintf("%x-a", dummyID(2)), start)
+	if err := c.Trim(); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := c.Get(id); err != nil {
+		t.Fatal(err)
+	}
+	c.OutputFile(entry.OutputID)
+	mtime3 := now
+	if _, err := c.Get(dummyID(2)); err == nil { // haven't done a Get for this since original write above
+		t.Fatalf("Trim did not remove dummyID(2)")
+	}
+
+	// The c.Get(id) refreshed id's mtime again.
+	// Check that another 5 days later it is still not gone,
+	// but check by using checkTime, which doesn't bring mtime forward.
+	now += 5 * 86400
+	if err := c.Trim(); err != nil {
+		t.Fatal(err)
+	}
+	checkTime(fmt.Sprintf("%x-a", id), mtime3)
+	checkTime(fmt.Sprintf("%x-d", entry.OutputID), mtime3)
+
+	// Half a day later Trim should still be a no-op, because there was a Trim recently.
+	// Even though the entry for id is now old enough to be trimmed,
+	// it gets a reprieve until the time comes for a new Trim scan.
+	now += 86400 / 2
+	if err := c.Trim(); err != nil {
+		t.Fatal(err)
+	}
+	checkTime(fmt.Sprintf("%x-a", id), mtime3)
+	checkTime(fmt.Sprintf("%x-d", entry.OutputID), mtime3)
+
+	// Another half a day later, Trim should actually run, and it should remove id.
+	now += 86400/2 + 1
+	if err := c.Trim(); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := c.Get(dummyID(1)); err == nil {
+		t.Fatal("Trim did not remove dummyID(1)")
+	}
+}
diff --git a/src/cmd/go/internal/cache/default.go b/src/cmd/go/internal/cache/default.go
new file mode 100644
index 0000000..b5650ea
--- /dev/null
+++ b/src/cmd/go/internal/cache/default.go
@@ -0,0 +1,105 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cache
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"sync"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"internal/goexperiment"
+)
+
+// Default returns the default cache to use.
+// It never returns nil.
+func Default() Cache {
+	defaultOnce.Do(initDefaultCache)
+	return defaultCache
+}
+
+var (
+	defaultOnce  sync.Once
+	defaultCache Cache
+)
+
+// cacheREADME is a message stored in a README in the cache directory.
+// Because the cache lives outside the normal Go trees, we leave the
+// README as a courtesy to explain where it came from.
+const cacheREADME = `This directory holds cached build artifacts from the Go build system.
+Run "go clean -cache" if the directory is getting too large.
+Run "go clean -fuzzcache" to delete the fuzz cache.
+See golang.org to learn more about Go.
+`
+
+// initDefaultCache does the work of finding the default cache
+// the first time Default is called.
+func initDefaultCache() {
+	dir := DefaultDir()
+	if dir == "off" {
+		if defaultDirErr != nil {
+			base.Fatalf("build cache is required, but could not be located: %v", defaultDirErr)
+		}
+		base.Fatalf("build cache is disabled by GOCACHE=off, but required as of Go 1.12")
+	}
+	if err := os.MkdirAll(dir, 0777); err != nil {
+		base.Fatalf("failed to initialize build cache at %s: %s\n", dir, err)
+	}
+	if _, err := os.Stat(filepath.Join(dir, "README")); err != nil {
+		// Best effort.
+		os.WriteFile(filepath.Join(dir, "README"), []byte(cacheREADME), 0666)
+	}
+
+	diskCache, err := Open(dir)
+	if err != nil {
+		base.Fatalf("failed to initialize build cache at %s: %s\n", dir, err)
+	}
+
+	if v := cfg.Getenv("GOCACHEPROG"); v != "" && goexperiment.CacheProg {
+		defaultCache = startCacheProg(v, diskCache)
+	} else {
+		defaultCache = diskCache
+	}
+}
+
+var (
+	defaultDirOnce sync.Once
+	defaultDir     string
+	defaultDirErr  error
+)
+
+// DefaultDir returns the effective GOCACHE setting.
+// It returns "off" if the cache is disabled.
+func DefaultDir() string {
+	// Save the result of the first call to DefaultDir for later use in
+	// initDefaultCache. cmd/go/main.go explicitly sets GOCACHE so that
+	// subprocesses will inherit it, but that means initDefaultCache can't
+	// otherwise distinguish between an explicit "off" and a UserCacheDir error.
+
+	defaultDirOnce.Do(func() {
+		defaultDir = cfg.Getenv("GOCACHE")
+		if filepath.IsAbs(defaultDir) || defaultDir == "off" {
+			return
+		}
+		if defaultDir != "" {
+			defaultDir = "off"
+			defaultDirErr = fmt.Errorf("GOCACHE is not an absolute path")
+			return
+		}
+
+		// Compute default location.
+		dir, err := os.UserCacheDir()
+		if err != nil {
+			defaultDir = "off"
+			defaultDirErr = fmt.Errorf("GOCACHE is not defined and %v", err)
+			return
+		}
+		defaultDir = filepath.Join(dir, "go-build")
+	})
+
+	return defaultDir
+}
diff --git a/src/cmd/go/internal/cache/hash.go b/src/cmd/go/internal/cache/hash.go
new file mode 100644
index 0000000..4f79c31
--- /dev/null
+++ b/src/cmd/go/internal/cache/hash.go
@@ -0,0 +1,190 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cache
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"fmt"
+	"hash"
+	"io"
+	"os"
+	"runtime"
+	"strings"
+	"sync"
+)
+
+var debugHash = false // set when GODEBUG=gocachehash=1
+
+// HashSize is the number of bytes in a hash.
+const HashSize = 32
+
+// A Hash provides access to the canonical hash function used to index the cache.
+// The current implementation uses salted SHA256, but clients must not assume this.
+type Hash struct {
+	h    hash.Hash
+	name string        // for debugging
+	buf  *bytes.Buffer // for verify
+}
+
+// hashSalt is a salt string added to the beginning of every hash
+// created by NewHash. Using the Go version makes sure that different
+// versions of the go command (or even different Git commits during
+// work on the development branch) do not address the same cache
+// entries, so that a bug in one version does not affect the execution
+// of other versions. This salt will result in additional ActionID files
+// in the cache, but not additional copies of the large output files,
+// which are still addressed by unsalted SHA256.
+//
+// We strip any GOEXPERIMENTs the go tool was built with from this
+// version string on the assumption that they shouldn't affect go tool
+// execution. This allows bootstrapping to converge faster: dist builds
+// go_bootstrap without any experiments, so by stripping experiments
+// go_bootstrap and the final go binary will use the same salt.
+var hashSalt = []byte(stripExperiment(runtime.Version()))
+
+// stripExperiment strips any GOEXPERIMENT configuration from the Go
+// version string.
+func stripExperiment(version string) string {
+	if i := strings.Index(version, " X:"); i >= 0 {
+		return version[:i]
+	}
+	return version
+}
+
+// Subkey returns an action ID corresponding to mixing a parent
+// action ID with a string description of the subkey.
+func Subkey(parent ActionID, desc string) ActionID {
+	h := sha256.New()
+	h.Write([]byte("subkey:"))
+	h.Write(parent[:])
+	h.Write([]byte(desc))
+	var out ActionID
+	h.Sum(out[:0])
+	if debugHash {
+		fmt.Fprintf(os.Stderr, "HASH subkey %x %q = %x\n", parent, desc, out)
+	}
+	if verify {
+		hashDebug.Lock()
+		hashDebug.m[out] = fmt.Sprintf("subkey %x %q", parent, desc)
+		hashDebug.Unlock()
+	}
+	return out
+}
+
+// NewHash returns a new Hash.
+// The caller is expected to Write data to it and then call Sum.
+func NewHash(name string) *Hash {
+	h := &Hash{h: sha256.New(), name: name}
+	if debugHash {
+		fmt.Fprintf(os.Stderr, "HASH[%s]\n", h.name)
+	}
+	h.Write(hashSalt)
+	if verify {
+		h.buf = new(bytes.Buffer)
+	}
+	return h
+}
+
+// Write writes data to the running hash.
+func (h *Hash) Write(b []byte) (int, error) {
+	if debugHash {
+		fmt.Fprintf(os.Stderr, "HASH[%s]: %q\n", h.name, b)
+	}
+	if h.buf != nil {
+		h.buf.Write(b)
+	}
+	return h.h.Write(b)
+}
+
+// Sum returns the hash of the data written previously.
+func (h *Hash) Sum() [HashSize]byte {
+	var out [HashSize]byte
+	h.h.Sum(out[:0])
+	if debugHash {
+		fmt.Fprintf(os.Stderr, "HASH[%s]: %x\n", h.name, out)
+	}
+	if h.buf != nil {
+		hashDebug.Lock()
+		if hashDebug.m == nil {
+			hashDebug.m = make(map[[HashSize]byte]string)
+		}
+		hashDebug.m[out] = h.buf.String()
+		hashDebug.Unlock()
+	}
+	return out
+}
+
+// In GODEBUG=gocacheverify=1 mode,
+// hashDebug holds the input to every computed hash ID,
+// so that we can work backward from the ID involved in a
+// cache entry mismatch to a description of what should be there.
+var hashDebug struct {
+	sync.Mutex
+	m map[[HashSize]byte]string
+}
+
+// reverseHash returns the input used to compute the hash id.
+func reverseHash(id [HashSize]byte) string {
+	hashDebug.Lock()
+	s := hashDebug.m[id]
+	hashDebug.Unlock()
+	return s
+}
+
+var hashFileCache struct {
+	sync.Mutex
+	m map[string][HashSize]byte
+}
+
+// FileHash returns the hash of the named file.
+// It caches repeated lookups for a given file,
+// and the cache entry for a file can be initialized
+// using SetFileHash.
+// The hash used by FileHash is not the same as
+// the hash used by NewHash.
+func FileHash(file string) ([HashSize]byte, error) {
+	hashFileCache.Lock()
+	out, ok := hashFileCache.m[file]
+	hashFileCache.Unlock()
+
+	if ok {
+		return out, nil
+	}
+
+	h := sha256.New()
+	f, err := os.Open(file)
+	if err != nil {
+		if debugHash {
+			fmt.Fprintf(os.Stderr, "HASH %s: %v\n", file, err)
+		}
+		return [HashSize]byte{}, err
+	}
+	_, err = io.Copy(h, f)
+	f.Close()
+	if err != nil {
+		if debugHash {
+			fmt.Fprintf(os.Stderr, "HASH %s: %v\n", file, err)
+		}
+		return [HashSize]byte{}, err
+	}
+	h.Sum(out[:0])
+	if debugHash {
+		fmt.Fprintf(os.Stderr, "HASH %s: %x\n", file, out)
+	}
+
+	SetFileHash(file, out)
+	return out, nil
+}
+
+// SetFileHash sets the hash returned by FileHash for file.
+func SetFileHash(file string, sum [HashSize]byte) {
+	hashFileCache.Lock()
+	if hashFileCache.m == nil {
+		hashFileCache.m = make(map[string][HashSize]byte)
+	}
+	hashFileCache.m[file] = sum
+	hashFileCache.Unlock()
+}
diff --git a/src/cmd/go/internal/cache/hash_test.go b/src/cmd/go/internal/cache/hash_test.go
new file mode 100644
index 0000000..a035677
--- /dev/null
+++ b/src/cmd/go/internal/cache/hash_test.go
@@ -0,0 +1,51 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cache
+
+import (
+	"fmt"
+	"os"
+	"testing"
+)
+
+func TestHash(t *testing.T) {
+	oldSalt := hashSalt
+	hashSalt = nil
+	defer func() {
+		hashSalt = oldSalt
+	}()
+
+	h := NewHash("alice")
+	h.Write([]byte("hello world"))
+	sum := fmt.Sprintf("%x", h.Sum())
+	want := "b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9"
+	if sum != want {
+		t.Errorf("hash(hello world) = %v, want %v", sum, want)
+	}
+}
+
+func TestHashFile(t *testing.T) {
+	f, err := os.CreateTemp("", "cmd-go-test-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	name := f.Name()
+	fmt.Fprintf(f, "hello world")
+	defer os.Remove(name)
+	if err := f.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	var h ActionID // make sure hash result is assignable to ActionID
+	h, err = FileHash(name)
+	if err != nil {
+		t.Fatal(err)
+	}
+	sum := fmt.Sprintf("%x", h)
+	want := "b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9"
+	if sum != want {
+		t.Errorf("hash(hello world) = %v, want %v", sum, want)
+	}
+}
diff --git a/src/cmd/go/internal/cache/prog.go b/src/cmd/go/internal/cache/prog.go
new file mode 100644
index 0000000..30f69b3
--- /dev/null
+++ b/src/cmd/go/internal/cache/prog.go
@@ -0,0 +1,427 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cache
+
+import (
+	"bufio"
+	"cmd/go/internal/base"
+	"cmd/internal/quoted"
+	"context"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"log"
+	"os"
+	"os/exec"
+	"sync"
+	"sync/atomic"
+	"time"
+)
+
+// ProgCache implements Cache via JSON messages over stdin/stdout to a child
+// helper process which can then implement whatever caching policy/mechanism it
+// wants.
+//
+// See https://github.com/golang/go/issues/59719
+type ProgCache struct {
+	cmd    *exec.Cmd
+	stdout io.ReadCloser  // from the child process
+	stdin  io.WriteCloser // to the child process
+	bw     *bufio.Writer  // to stdin
+	jenc   *json.Encoder  // to bw
+
+	// can are the commands that the child process declared that it supports.
+	// This is effectively the versioning mechanism.
+	can map[ProgCmd]bool
+
+	// fuzzDirCache is another Cache implementation to use for the FuzzDir
+	// method. In practice this is the default GOCACHE disk-based
+	// implementation.
+	//
+	// TODO(bradfitz): maybe this isn't ideal. But we'd need to extend the Cache
+	// interface and the fuzzing callers to be less disk-y to do more here.
+	fuzzDirCache Cache
+
+	closing      atomic.Bool
+	ctx          context.Context    // valid until Close via ctxClose
+	ctxCancel    context.CancelFunc // called on Close
+	readLoopDone chan struct{}      // closed when readLoop returns
+
+	mu         sync.Mutex // guards following fields
+	nextID     int64
+	inFlight   map[int64]chan<- *ProgResponse
+	outputFile map[OutputID]string // object => abs path on disk
+
+	// writeMu serializes writing to the child process.
+	// It must never be held at the same time as mu.
+	writeMu sync.Mutex
+}
+
+// ProgCmd is a command that can be issued to a child process.
+//
+// If the interface needs to grow, we can add new commands or new versioned
+// commands like "get2".
+type ProgCmd string
+
+const (
+	cmdGet   = ProgCmd("get")
+	cmdPut   = ProgCmd("put")
+	cmdClose = ProgCmd("close")
+)
+
+// ProgRequest is the JSON-encoded message that's sent from cmd/go to
+// the GOCACHEPROG child process over stdin. Each JSON object is on its
+// own line. A ProgRequest of Type "put" with BodySize > 0 will be followed
+// by a line containing a base64-encoded JSON string literal of the body.
+type ProgRequest struct {
+	// ID is a unique number per process across all requests.
+	// It must be echoed in the ProgResponse from the child.
+	ID int64
+
+	// Command is the type of request.
+	// The cmd/go tool will only send commands that were declared
+	// as supported by the child.
+	Command ProgCmd
+
+	// ActionID is non-nil for get and puts.
+	ActionID []byte `json:",omitempty"` // or nil if not used
+
+	// ObjectID is set for Type "put" and "output-file".
+	ObjectID []byte `json:",omitempty"` // or nil if not used
+
+	// Body is the body for "put" requests. It's sent after the JSON object
+	// as a base64-encoded JSON string when BodySize is non-zero.
+	// It's sent as a separate JSON value instead of being a struct field
+	// send in this JSON object so large values can be streamed in both directions.
+	// The base64 string body of a ProgRequest will always be written
+	// immediately after the JSON object and a newline.
+	Body io.Reader `json:"-"`
+
+	// BodySize is the number of bytes of Body. If zero, the body isn't written.
+	BodySize int64 `json:",omitempty"`
+}
+
+// ProgResponse is the JSON response from the child process to cmd/go.
+//
+// With the exception of the first protocol message that the child writes to its
+// stdout with ID==0 and KnownCommands populated, these are only sent in
+// response to a ProgRequest from cmd/go.
+//
+// ProgResponses can be sent in any order. The ID must match the request they're
+// replying to.
+type ProgResponse struct {
+	ID  int64  // that corresponds to ProgRequest; they can be answered out of order
+	Err string `json:",omitempty"` // if non-empty, the error
+
+	// KnownCommands is included in the first message that cache helper program
+	// writes to stdout on startup (with ID==0). It includes the
+	// ProgRequest.Command types that are supported by the program.
+	//
+	// This lets us extend the protocol gracefully over time (adding "get2",
+	// etc), or fail gracefully when needed. It also lets us verify the program
+	// wants to be a cache helper.
+	KnownCommands []ProgCmd `json:",omitempty"`
+
+	// For Get requests.
+
+	Miss     bool       `json:",omitempty"` // cache miss
+	OutputID []byte     `json:",omitempty"`
+	Size     int64      `json:",omitempty"` // in bytes
+	Time     *time.Time `json:",omitempty"` // an Entry.Time; when the object was added to the docs
+
+	// DiskPath is the absolute path on disk of the ObjectID corresponding
+	// a "get" request's ActionID (on cache hit) or a "put" request's
+	// provided ObjectID.
+	DiskPath string `json:",omitempty"`
+}
+
+// startCacheProg starts the prog binary (with optional space-separated flags)
+// and returns a Cache implementation that talks to it.
+//
+// It blocks a few seconds to wait for the child process to successfully start
+// and advertise its capabilities.
+func startCacheProg(progAndArgs string, fuzzDirCache Cache) Cache {
+	if fuzzDirCache == nil {
+		panic("missing fuzzDirCache")
+	}
+	args, err := quoted.Split(progAndArgs)
+	if err != nil {
+		base.Fatalf("GOCACHEPROG args: %v", err)
+	}
+	var prog string
+	if len(args) > 0 {
+		prog = args[0]
+		args = args[1:]
+	}
+
+	ctx, ctxCancel := context.WithCancel(context.Background())
+
+	cmd := exec.CommandContext(ctx, prog, args...)
+	out, err := cmd.StdoutPipe()
+	if err != nil {
+		base.Fatalf("StdoutPipe to GOCACHEPROG: %v", err)
+	}
+	in, err := cmd.StdinPipe()
+	if err != nil {
+		base.Fatalf("StdinPipe to GOCACHEPROG: %v", err)
+	}
+	cmd.Stderr = os.Stderr
+	cmd.Cancel = in.Close
+
+	if err := cmd.Start(); err != nil {
+		base.Fatalf("error starting GOCACHEPROG program %q: %v", prog, err)
+	}
+
+	pc := &ProgCache{
+		ctx:          ctx,
+		ctxCancel:    ctxCancel,
+		fuzzDirCache: fuzzDirCache,
+		cmd:          cmd,
+		stdout:       out,
+		stdin:        in,
+		bw:           bufio.NewWriter(in),
+		inFlight:     make(map[int64]chan<- *ProgResponse),
+		outputFile:   make(map[OutputID]string),
+		readLoopDone: make(chan struct{}),
+	}
+
+	// Register our interest in the initial protocol message from the child to
+	// us, saying what it can do.
+	capResc := make(chan *ProgResponse, 1)
+	pc.inFlight[0] = capResc
+
+	pc.jenc = json.NewEncoder(pc.bw)
+	go pc.readLoop(pc.readLoopDone)
+
+	// Give the child process a few seconds to report its capabilities. This
+	// should be instant and not require any slow work by the program.
+	timer := time.NewTicker(5 * time.Second)
+	defer timer.Stop()
+	for {
+		select {
+		case <-timer.C:
+			log.Printf("# still waiting for GOCACHEPROG %v ...", prog)
+		case capRes := <-capResc:
+			can := map[ProgCmd]bool{}
+			for _, cmd := range capRes.KnownCommands {
+				can[cmd] = true
+			}
+			if len(can) == 0 {
+				base.Fatalf("GOCACHEPROG %v declared no supported commands", prog)
+			}
+			pc.can = can
+			return pc
+		}
+	}
+}
+
+func (c *ProgCache) readLoop(readLoopDone chan<- struct{}) {
+	defer close(readLoopDone)
+	jd := json.NewDecoder(c.stdout)
+	for {
+		res := new(ProgResponse)
+		if err := jd.Decode(res); err != nil {
+			if c.closing.Load() {
+				return // quietly
+			}
+			if errors.Is(err, io.EOF) {
+				c.mu.Lock()
+				inFlight := len(c.inFlight)
+				c.mu.Unlock()
+				base.Fatalf("GOCACHEPROG exited pre-Close with %v pending requests", inFlight)
+			}
+			base.Fatalf("error reading JSON from GOCACHEPROG: %v", err)
+		}
+		c.mu.Lock()
+		ch, ok := c.inFlight[res.ID]
+		delete(c.inFlight, res.ID)
+		c.mu.Unlock()
+		if ok {
+			ch <- res
+		} else {
+			base.Fatalf("GOCACHEPROG sent response for unknown request ID %v", res.ID)
+		}
+	}
+}
+
+func (c *ProgCache) send(ctx context.Context, req *ProgRequest) (*ProgResponse, error) {
+	resc := make(chan *ProgResponse, 1)
+	if err := c.writeToChild(req, resc); err != nil {
+		return nil, err
+	}
+	select {
+	case res := <-resc:
+		if res.Err != "" {
+			return nil, errors.New(res.Err)
+		}
+		return res, nil
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	}
+}
+
+func (c *ProgCache) writeToChild(req *ProgRequest, resc chan<- *ProgResponse) (err error) {
+	c.mu.Lock()
+	c.nextID++
+	req.ID = c.nextID
+	c.inFlight[req.ID] = resc
+	c.mu.Unlock()
+
+	defer func() {
+		if err != nil {
+			c.mu.Lock()
+			delete(c.inFlight, req.ID)
+			c.mu.Unlock()
+		}
+	}()
+
+	c.writeMu.Lock()
+	defer c.writeMu.Unlock()
+
+	if err := c.jenc.Encode(req); err != nil {
+		return err
+	}
+	if err := c.bw.WriteByte('\n'); err != nil {
+		return err
+	}
+	if req.Body != nil && req.BodySize > 0 {
+		if err := c.bw.WriteByte('"'); err != nil {
+			return err
+		}
+		e := base64.NewEncoder(base64.StdEncoding, c.bw)
+		wrote, err := io.Copy(e, req.Body)
+		if err != nil {
+			return err
+		}
+		if err := e.Close(); err != nil {
+			return nil
+		}
+		if wrote != req.BodySize {
+			return fmt.Errorf("short write writing body to GOCACHEPROG for action %x, object %x: wrote %v; expected %v",
+				req.ActionID, req.ObjectID, wrote, req.BodySize)
+		}
+		if _, err := c.bw.WriteString("\"\n"); err != nil {
+			return err
+		}
+	}
+	if err := c.bw.Flush(); err != nil {
+		return err
+	}
+	return nil
+}
+
+func (c *ProgCache) Get(a ActionID) (Entry, error) {
+	if !c.can[cmdGet] {
+		// They can't do a "get". Maybe they're a write-only cache.
+		//
+		// TODO(bradfitz,bcmills): figure out the proper error type here. Maybe
+		// errors.ErrUnsupported? Is entryNotFoundError even appropriate? There
+		// might be places where we rely on the fact that a recent Put can be
+		// read through a corresponding Get. Audit callers and check, and document
+		// error types on the Cache interface.
+		return Entry{}, &entryNotFoundError{}
+	}
+	res, err := c.send(c.ctx, &ProgRequest{
+		Command:  cmdGet,
+		ActionID: a[:],
+	})
+	if err != nil {
+		return Entry{}, err // TODO(bradfitz): or entryNotFoundError? Audit callers.
+	}
+	if res.Miss {
+		return Entry{}, &entryNotFoundError{}
+	}
+	e := Entry{
+		Size: res.Size,
+	}
+	if res.Time != nil {
+		e.Time = *res.Time
+	} else {
+		e.Time = time.Now()
+	}
+	if res.DiskPath == "" {
+		return Entry{}, &entryNotFoundError{errors.New("GOCACHEPROG didn't populate DiskPath on get hit")}
+	}
+	if copy(e.OutputID[:], res.OutputID) != len(res.OutputID) {
+		return Entry{}, &entryNotFoundError{errors.New("incomplete ProgResponse OutputID")}
+	}
+	c.noteOutputFile(e.OutputID, res.DiskPath)
+	return e, nil
+}
+
+func (c *ProgCache) noteOutputFile(o OutputID, diskPath string) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.outputFile[o] = diskPath
+}
+
+func (c *ProgCache) OutputFile(o OutputID) string {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.outputFile[o]
+}
+
+func (c *ProgCache) Put(a ActionID, file io.ReadSeeker) (_ OutputID, size int64, _ error) {
+	// Compute output ID.
+	h := sha256.New()
+	if _, err := file.Seek(0, 0); err != nil {
+		return OutputID{}, 0, err
+	}
+	size, err := io.Copy(h, file)
+	if err != nil {
+		return OutputID{}, 0, err
+	}
+	var out OutputID
+	h.Sum(out[:0])
+
+	if _, err := file.Seek(0, 0); err != nil {
+		return OutputID{}, 0, err
+	}
+
+	if !c.can[cmdPut] {
+		// Child is a read-only cache. Do nothing.
+		return out, size, nil
+	}
+
+	res, err := c.send(c.ctx, &ProgRequest{
+		Command:  cmdPut,
+		ActionID: a[:],
+		ObjectID: out[:],
+		Body:     file,
+		BodySize: size,
+	})
+	if err != nil {
+		return OutputID{}, 0, err
+	}
+	if res.DiskPath == "" {
+		return OutputID{}, 0, errors.New("GOCACHEPROG didn't return DiskPath in put response")
+	}
+	c.noteOutputFile(out, res.DiskPath)
+	return out, size, err
+}
+
+func (c *ProgCache) Close() error {
+	c.closing.Store(true)
+	var err error
+
+	// First write a "close" message to the child so it can exit nicely
+	// and clean up if it wants. Only after that exchange do we cancel
+	// the context that kills the process.
+	if c.can[cmdClose] {
+		_, err = c.send(c.ctx, &ProgRequest{Command: cmdClose})
+	}
+	c.ctxCancel()
+	<-c.readLoopDone
+	return err
+}
+
+func (c *ProgCache) FuzzDir() string {
+	// TODO(bradfitz): figure out what to do here. For now just use the
+	// disk-based default.
+	return c.fuzzDirCache.FuzzDir()
+}
diff --git a/src/cmd/go/internal/cfg/cfg.go b/src/cmd/go/internal/cfg/cfg.go
new file mode 100644
index 0000000..8caa22a
--- /dev/null
+++ b/src/cmd/go/internal/cfg/cfg.go
@@ -0,0 +1,619 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package cfg holds configuration shared by multiple parts
+// of the go command.
+package cfg
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"go/build"
+	"internal/buildcfg"
+	"internal/cfg"
+	"io"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"sync"
+
+	"cmd/go/internal/fsys"
+)
+
+// Global build parameters (used during package load)
+var (
+	Goos   = envOr("GOOS", build.Default.GOOS)
+	Goarch = envOr("GOARCH", build.Default.GOARCH)
+
+	ExeSuffix = exeSuffix()
+
+	// ModulesEnabled specifies whether the go command is running
+	// in module-aware mode (as opposed to GOPATH mode).
+	// It is equal to modload.Enabled, but not all packages can import modload.
+	ModulesEnabled bool
+)
+
+func exeSuffix() string {
+	if Goos == "windows" {
+		return ".exe"
+	}
+	return ""
+}
+
+// Configuration for tools installed to GOROOT/bin.
+// Normally these match runtime.GOOS and runtime.GOARCH,
+// but when testing a cross-compiled cmd/go they will
+// indicate the GOOS and GOARCH of the installed cmd/go
+// rather than the test binary.
+var (
+	installedGOOS   string
+	installedGOARCH string
+)
+
+// ToolExeSuffix returns the suffix for executables installed
+// in build.ToolDir.
+func ToolExeSuffix() string {
+	if installedGOOS == "windows" {
+		return ".exe"
+	}
+	return ""
+}
+
+// These are general "build flags" used by build and other commands.
+var (
+	BuildA                 bool     // -a flag
+	BuildBuildmode         string   // -buildmode flag
+	BuildBuildvcs          = "auto" // -buildvcs flag: "true", "false", or "auto"
+	BuildContext           = defaultContext()
+	BuildMod               string                  // -mod flag
+	BuildModExplicit       bool                    // whether -mod was set explicitly
+	BuildModReason         string                  // reason -mod was set, if set by default
+	BuildLinkshared        bool                    // -linkshared flag
+	BuildMSan              bool                    // -msan flag
+	BuildASan              bool                    // -asan flag
+	BuildCover             bool                    // -cover flag
+	BuildCoverMode         string                  // -covermode flag
+	BuildCoverPkg          []string                // -coverpkg flag
+	BuildN                 bool                    // -n flag
+	BuildO                 string                  // -o flag
+	BuildP                 = runtime.GOMAXPROCS(0) // -p flag
+	BuildPGO               string                  // -pgo flag
+	BuildPkgdir            string                  // -pkgdir flag
+	BuildRace              bool                    // -race flag
+	BuildToolexec          []string                // -toolexec flag
+	BuildToolchainName     string
+	BuildToolchainCompiler func() string
+	BuildToolchainLinker   func() string
+	BuildTrimpath          bool // -trimpath flag
+	BuildV                 bool // -v flag
+	BuildWork              bool // -work flag
+	BuildX                 bool // -x flag
+
+	ModCacheRW bool   // -modcacherw flag
+	ModFile    string // -modfile flag
+
+	CmdName string // "build", "install", "list", "mod tidy", etc.
+
+	DebugActiongraph  string // -debug-actiongraph flag (undocumented, unstable)
+	DebugTrace        string // -debug-trace flag
+	DebugRuntimeTrace string // -debug-runtime-trace flag (undocumented, unstable)
+
+	// GoPathError is set when GOPATH is not set. it contains an
+	// explanation why GOPATH is unset.
+	GoPathError string
+)
+
+func defaultContext() build.Context {
+	ctxt := build.Default
+
+	ctxt.JoinPath = filepath.Join // back door to say "do not use go command"
+
+	// Override defaults computed in go/build with defaults
+	// from go environment configuration file, if known.
+	ctxt.GOPATH = envOr("GOPATH", gopath(ctxt))
+	ctxt.GOOS = Goos
+	ctxt.GOARCH = Goarch
+
+	// Clear the GOEXPERIMENT-based tool tags, which we will recompute later.
+	var save []string
+	for _, tag := range ctxt.ToolTags {
+		if !strings.HasPrefix(tag, "goexperiment.") {
+			save = append(save, tag)
+		}
+	}
+	ctxt.ToolTags = save
+
+	// The go/build rule for whether cgo is enabled is:
+	//	1. If $CGO_ENABLED is set, respect it.
+	//	2. Otherwise, if this is a cross-compile, disable cgo.
+	//	3. Otherwise, use built-in default for GOOS/GOARCH.
+	// Recreate that logic here with the new GOOS/GOARCH setting.
+	if v := Getenv("CGO_ENABLED"); v == "0" || v == "1" {
+		ctxt.CgoEnabled = v[0] == '1'
+	} else if ctxt.GOOS != runtime.GOOS || ctxt.GOARCH != runtime.GOARCH {
+		ctxt.CgoEnabled = false
+	} else {
+		// Use built-in default cgo setting for GOOS/GOARCH.
+		// Note that ctxt.GOOS/GOARCH are derived from the preference list
+		// (1) environment, (2) go/env file, (3) runtime constants,
+		// while go/build.Default.GOOS/GOARCH are derived from the preference list
+		// (1) environment, (2) runtime constants.
+		//
+		// We know ctxt.GOOS/GOARCH == runtime.GOOS/GOARCH;
+		// no matter how that happened, go/build.Default will make the
+		// same decision (either the environment variables are set explicitly
+		// to match the runtime constants, or else they are unset, in which
+		// case go/build falls back to the runtime constants), so
+		// go/build.Default.GOOS/GOARCH == runtime.GOOS/GOARCH.
+		// So ctxt.CgoEnabled (== go/build.Default.CgoEnabled) is correct
+		// as is and can be left unmodified.
+		//
+		// All that said, starting in Go 1.20 we layer one more rule
+		// on top of the go/build decision: if CC is unset and
+		// the default C compiler we'd look for is not in the PATH,
+		// we automatically default cgo to off.
+		// This makes go builds work automatically on systems
+		// without a C compiler installed.
+		if ctxt.CgoEnabled {
+			if os.Getenv("CC") == "" {
+				cc := DefaultCC(ctxt.GOOS, ctxt.GOARCH)
+				if _, err := exec.LookPath(cc); err != nil {
+					ctxt.CgoEnabled = false
+				}
+			}
+		}
+	}
+
+	ctxt.OpenFile = func(path string) (io.ReadCloser, error) {
+		return fsys.Open(path)
+	}
+	ctxt.ReadDir = fsys.ReadDir
+	ctxt.IsDir = func(path string) bool {
+		isDir, err := fsys.IsDir(path)
+		return err == nil && isDir
+	}
+
+	return ctxt
+}
+
+func init() {
+	SetGOROOT(Getenv("GOROOT"), false)
+	BuildToolchainCompiler = func() string { return "missing-compiler" }
+	BuildToolchainLinker = func() string { return "missing-linker" }
+}
+
+// SetGOROOT sets GOROOT and associated variables to the given values.
+//
+// If isTestGo is true, build.ToolDir is set based on the TESTGO_GOHOSTOS and
+// TESTGO_GOHOSTARCH environment variables instead of runtime.GOOS and
+// runtime.GOARCH.
+func SetGOROOT(goroot string, isTestGo bool) {
+	BuildContext.GOROOT = goroot
+
+	GOROOT = goroot
+	if goroot == "" {
+		GOROOTbin = ""
+		GOROOTpkg = ""
+		GOROOTsrc = ""
+	} else {
+		GOROOTbin = filepath.Join(goroot, "bin")
+		GOROOTpkg = filepath.Join(goroot, "pkg")
+		GOROOTsrc = filepath.Join(goroot, "src")
+	}
+	GOROOT_FINAL = findGOROOT_FINAL(goroot)
+
+	installedGOOS = runtime.GOOS
+	installedGOARCH = runtime.GOARCH
+	if isTestGo {
+		if testOS := os.Getenv("TESTGO_GOHOSTOS"); testOS != "" {
+			installedGOOS = testOS
+		}
+		if testArch := os.Getenv("TESTGO_GOHOSTARCH"); testArch != "" {
+			installedGOARCH = testArch
+		}
+	}
+
+	if runtime.Compiler != "gccgo" {
+		if goroot == "" {
+			build.ToolDir = ""
+		} else {
+			// Note that we must use the installed OS and arch here: the tool
+			// directory does not move based on environment variables, and even if we
+			// are testing a cross-compiled cmd/go all of the installed packages and
+			// tools would have been built using the native compiler and linker (and
+			// would spuriously appear stale if we used a cross-compiled compiler and
+			// linker).
+			//
+			// This matches the initialization of ToolDir in go/build, except for
+			// using ctxt.GOROOT and the installed GOOS and GOARCH rather than the
+			// GOROOT, GOOS, and GOARCH reported by the runtime package.
+			build.ToolDir = filepath.Join(GOROOTpkg, "tool", installedGOOS+"_"+installedGOARCH)
+		}
+	}
+}
+
+// Experiment configuration.
+var (
+	// RawGOEXPERIMENT is the GOEXPERIMENT value set by the user.
+	RawGOEXPERIMENT = envOr("GOEXPERIMENT", buildcfg.DefaultGOEXPERIMENT)
+	// CleanGOEXPERIMENT is the minimal GOEXPERIMENT value needed to reproduce the
+	// experiments enabled by RawGOEXPERIMENT.
+	CleanGOEXPERIMENT = RawGOEXPERIMENT
+
+	Experiment    *buildcfg.ExperimentFlags
+	ExperimentErr error
+)
+
+func init() {
+	Experiment, ExperimentErr = buildcfg.ParseGOEXPERIMENT(Goos, Goarch, RawGOEXPERIMENT)
+	if ExperimentErr != nil {
+		return
+	}
+
+	// GOEXPERIMENT is valid, so convert it to canonical form.
+	CleanGOEXPERIMENT = Experiment.String()
+
+	// Add build tags based on the experiments in effect.
+	exps := Experiment.Enabled()
+	expTags := make([]string, 0, len(exps)+len(BuildContext.ToolTags))
+	for _, exp := range exps {
+		expTags = append(expTags, "goexperiment."+exp)
+	}
+	BuildContext.ToolTags = append(expTags, BuildContext.ToolTags...)
+}
+
+// An EnvVar is an environment variable Name=Value.
+type EnvVar struct {
+	Name  string
+	Value string
+}
+
+// OrigEnv is the original environment of the program at startup.
+var OrigEnv []string
+
+// CmdEnv is the new environment for running go tool commands.
+// User binaries (during go test or go run) are run with OrigEnv,
+// not CmdEnv.
+var CmdEnv []EnvVar
+
+var envCache struct {
+	once sync.Once
+	m    map[string]string
+}
+
+// EnvFile returns the name of the Go environment configuration file.
+func EnvFile() (string, error) {
+	if file := os.Getenv("GOENV"); file != "" {
+		if file == "off" {
+			return "", fmt.Errorf("GOENV=off")
+		}
+		return file, nil
+	}
+	dir, err := os.UserConfigDir()
+	if err != nil {
+		return "", err
+	}
+	if dir == "" {
+		return "", fmt.Errorf("missing user-config dir")
+	}
+	return filepath.Join(dir, "go/env"), nil
+}
+
+func initEnvCache() {
+	envCache.m = make(map[string]string)
+	if file, _ := EnvFile(); file != "" {
+		readEnvFile(file, "user")
+	}
+	goroot := findGOROOT(envCache.m["GOROOT"])
+	if goroot != "" {
+		readEnvFile(filepath.Join(goroot, "go.env"), "GOROOT")
+	}
+
+	// Save the goroot for func init calling SetGOROOT,
+	// and also overwrite anything that might have been in go.env.
+	// It makes no sense for GOROOT/go.env to specify
+	// a different GOROOT.
+	envCache.m["GOROOT"] = goroot
+}
+
+func readEnvFile(file string, source string) {
+	if file == "" {
+		return
+	}
+	data, err := os.ReadFile(file)
+	if err != nil {
+		return
+	}
+
+	for len(data) > 0 {
+		// Get next line.
+		line := data
+		i := bytes.IndexByte(data, '\n')
+		if i >= 0 {
+			line, data = line[:i], data[i+1:]
+		} else {
+			data = nil
+		}
+
+		i = bytes.IndexByte(line, '=')
+		if i < 0 || line[0] < 'A' || 'Z' < line[0] {
+			// Line is missing = (or empty) or a comment or not a valid env name. Ignore.
+			// This should not happen in the user file, since the file should be maintained almost
+			// exclusively by "go env -w", but better to silently ignore than to make
+			// the go command unusable just because somehow the env file has
+			// gotten corrupted.
+			// In the GOROOT/go.env file, we expect comments.
+			continue
+		}
+		key, val := line[:i], line[i+1:]
+
+		if source == "GOROOT" {
+			// In the GOROOT/go.env file, do not overwrite fields loaded from the user's go/env file.
+			if _, ok := envCache.m[string(key)]; ok {
+				continue
+			}
+		}
+		envCache.m[string(key)] = string(val)
+	}
+}
+
+// Getenv gets the value for the configuration key.
+// It consults the operating system environment
+// and then the go/env file.
+// If Getenv is called for a key that cannot be set
+// in the go/env file (for example GODEBUG), it panics.
+// This ensures that CanGetenv is accurate, so that
+// 'go env -w' stays in sync with what Getenv can retrieve.
+func Getenv(key string) string {
+	if !CanGetenv(key) {
+		switch key {
+		case "CGO_TEST_ALLOW", "CGO_TEST_DISALLOW", "CGO_test_ALLOW", "CGO_test_DISALLOW":
+			// used by internal/work/security_test.go; allow
+		default:
+			panic("internal error: invalid Getenv " + key)
+		}
+	}
+	val := os.Getenv(key)
+	if val != "" {
+		return val
+	}
+	envCache.once.Do(initEnvCache)
+	return envCache.m[key]
+}
+
+// CanGetenv reports whether key is a valid go/env configuration key.
+func CanGetenv(key string) bool {
+	envCache.once.Do(initEnvCache)
+	if _, ok := envCache.m[key]; ok {
+		// Assume anything in the user file or go.env file is valid.
+		return true
+	}
+	return strings.Contains(cfg.KnownEnv, "\t"+key+"\n")
+}
+
+var (
+	GOROOT string
+
+	// Either empty or produced by filepath.Join(GOROOT, …).
+	GOROOTbin string
+	GOROOTpkg string
+	GOROOTsrc string
+
+	GOROOT_FINAL string
+
+	GOBIN      = Getenv("GOBIN")
+	GOMODCACHE = envOr("GOMODCACHE", gopathDir("pkg/mod"))
+
+	// Used in envcmd.MkEnv and build ID computations.
+	GOARM    = envOr("GOARM", fmt.Sprint(buildcfg.GOARM))
+	GO386    = envOr("GO386", buildcfg.GO386)
+	GOAMD64  = envOr("GOAMD64", fmt.Sprintf("%s%d", "v", buildcfg.GOAMD64))
+	GOMIPS   = envOr("GOMIPS", buildcfg.GOMIPS)
+	GOMIPS64 = envOr("GOMIPS64", buildcfg.GOMIPS64)
+	GOPPC64  = envOr("GOPPC64", fmt.Sprintf("%s%d", "power", buildcfg.GOPPC64))
+	GOWASM   = envOr("GOWASM", fmt.Sprint(buildcfg.GOWASM))
+
+	GOPROXY    = envOr("GOPROXY", "")
+	GOSUMDB    = envOr("GOSUMDB", "")
+	GOPRIVATE  = Getenv("GOPRIVATE")
+	GONOPROXY  = envOr("GONOPROXY", GOPRIVATE)
+	GONOSUMDB  = envOr("GONOSUMDB", GOPRIVATE)
+	GOINSECURE = Getenv("GOINSECURE")
+	GOVCS      = Getenv("GOVCS")
+)
+
+var SumdbDir = gopathDir("pkg/sumdb")
+
+// GetArchEnv returns the name and setting of the
+// GOARCH-specific architecture environment variable.
+// If the current architecture has no GOARCH-specific variable,
+// GetArchEnv returns empty key and value.
+func GetArchEnv() (key, val string) {
+	switch Goarch {
+	case "arm":
+		return "GOARM", GOARM
+	case "386":
+		return "GO386", GO386
+	case "amd64":
+		return "GOAMD64", GOAMD64
+	case "mips", "mipsle":
+		return "GOMIPS", GOMIPS
+	case "mips64", "mips64le":
+		return "GOMIPS64", GOMIPS64
+	case "ppc64", "ppc64le":
+		return "GOPPC64", GOPPC64
+	case "wasm":
+		return "GOWASM", GOWASM
+	}
+	return "", ""
+}
+
+// envOr returns Getenv(key) if set, or else def.
+func envOr(key, def string) string {
+	val := Getenv(key)
+	if val == "" {
+		val = def
+	}
+	return val
+}
+
+// There is a copy of findGOROOT, isSameDir, and isGOROOT in
+// x/tools/cmd/godoc/goroot.go.
+// Try to keep them in sync for now.
+
+// findGOROOT returns the GOROOT value, using either an explicitly
+// provided environment variable, a GOROOT that contains the current
+// os.Executable value, or else the GOROOT that the binary was built
+// with from runtime.GOROOT().
+//
+// There is a copy of this code in x/tools/cmd/godoc/goroot.go.
+func findGOROOT(env string) string {
+	if env == "" {
+		// Not using Getenv because findGOROOT is called
+		// to find the GOROOT/go.env file. initEnvCache
+		// has passed in the setting from the user go/env file.
+		env = os.Getenv("GOROOT")
+	}
+	if env != "" {
+		return filepath.Clean(env)
+	}
+	def := ""
+	if r := runtime.GOROOT(); r != "" {
+		def = filepath.Clean(r)
+	}
+	if runtime.Compiler == "gccgo" {
+		// gccgo has no real GOROOT, and it certainly doesn't
+		// depend on the executable's location.
+		return def
+	}
+
+	// canonical returns a directory path that represents
+	// the same directory as dir,
+	// preferring the spelling in def if the two are the same.
+	canonical := func(dir string) string {
+		if isSameDir(def, dir) {
+			return def
+		}
+		return dir
+	}
+
+	exe, err := os.Executable()
+	if err == nil {
+		exe, err = filepath.Abs(exe)
+		if err == nil {
+			// cmd/go may be installed in GOROOT/bin or GOROOT/bin/GOOS_GOARCH,
+			// depending on whether it was cross-compiled with a different
+			// GOHOSTOS (see https://go.dev/issue/62119). Try both.
+			if dir := filepath.Join(exe, "../.."); isGOROOT(dir) {
+				return canonical(dir)
+			}
+			if dir := filepath.Join(exe, "../../.."); isGOROOT(dir) {
+				return canonical(dir)
+			}
+
+			// Depending on what was passed on the command line, it is possible
+			// that os.Executable is a symlink (like /usr/local/bin/go) referring
+			// to a binary installed in a real GOROOT elsewhere
+			// (like /usr/lib/go/bin/go).
+			// Try to find that GOROOT by resolving the symlinks.
+			exe, err = filepath.EvalSymlinks(exe)
+			if err == nil {
+				if dir := filepath.Join(exe, "../.."); isGOROOT(dir) {
+					return canonical(dir)
+				}
+				if dir := filepath.Join(exe, "../../.."); isGOROOT(dir) {
+					return canonical(dir)
+				}
+			}
+		}
+	}
+	return def
+}
+
+func findGOROOT_FINAL(goroot string) string {
+	// $GOROOT_FINAL is only for use during make.bash
+	// so it is not settable using go/env, so we use os.Getenv here.
+	def := goroot
+	if env := os.Getenv("GOROOT_FINAL"); env != "" {
+		def = filepath.Clean(env)
+	}
+	return def
+}
+
+// isSameDir reports whether dir1 and dir2 are the same directory.
+func isSameDir(dir1, dir2 string) bool {
+	if dir1 == dir2 {
+		return true
+	}
+	info1, err1 := os.Stat(dir1)
+	info2, err2 := os.Stat(dir2)
+	return err1 == nil && err2 == nil && os.SameFile(info1, info2)
+}
+
+// isGOROOT reports whether path looks like a GOROOT.
+//
+// It does this by looking for the path/pkg/tool directory,
+// which is necessary for useful operation of the cmd/go tool,
+// and is not typically present in a GOPATH.
+//
+// There is a copy of this code in x/tools/cmd/godoc/goroot.go.
+func isGOROOT(path string) bool {
+	stat, err := os.Stat(filepath.Join(path, "pkg", "tool"))
+	if err != nil {
+		return false
+	}
+	return stat.IsDir()
+}
+
+func gopathDir(rel string) string {
+	list := filepath.SplitList(BuildContext.GOPATH)
+	if len(list) == 0 || list[0] == "" {
+		return ""
+	}
+	return filepath.Join(list[0], rel)
+}
+
+func gopath(ctxt build.Context) string {
+	if len(ctxt.GOPATH) > 0 {
+		return ctxt.GOPATH
+	}
+	env := "HOME"
+	if runtime.GOOS == "windows" {
+		env = "USERPROFILE"
+	} else if runtime.GOOS == "plan9" {
+		env = "home"
+	}
+	if home := os.Getenv(env); home != "" {
+		def := filepath.Join(home, "go")
+		if filepath.Clean(def) == filepath.Clean(runtime.GOROOT()) {
+			GoPathError = "cannot set GOROOT as GOPATH"
+		}
+		return ""
+	}
+	GoPathError = fmt.Sprintf("%s is not set", env)
+	return ""
+}
+
+// WithBuildXWriter returns a Context in which BuildX output is written
+// to given io.Writer.
+func WithBuildXWriter(ctx context.Context, xLog io.Writer) context.Context {
+	return context.WithValue(ctx, buildXContextKey{}, xLog)
+}
+
+type buildXContextKey struct{}
+
+// BuildXWriter returns nil if BuildX is false, or
+// the writer to which BuildX output should be written otherwise.
+func BuildXWriter(ctx context.Context) (io.Writer, bool) {
+	if !BuildX {
+		return nil, false
+	}
+	if v := ctx.Value(buildXContextKey{}); v != nil {
+		return v.(io.Writer), true
+	}
+	return os.Stderr, true
+}
diff --git a/src/cmd/go/internal/clean/clean.go b/src/cmd/go/internal/clean/clean.go
new file mode 100644
index 0000000..e011237
--- /dev/null
+++ b/src/cmd/go/internal/clean/clean.go
@@ -0,0 +1,428 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package clean implements the “go clean” command.
+package clean
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strconv"
+	"strings"
+	"time"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cache"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/load"
+	"cmd/go/internal/lockedfile"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/str"
+	"cmd/go/internal/work"
+)
+
+var CmdClean = &base.Command{
+	UsageLine: "go clean [clean flags] [build flags] [packages]",
+	Short:     "remove object files and cached files",
+	Long: `
+Clean removes object files from package source directories.
+The go command builds most objects in a temporary directory,
+so go clean is mainly concerned with object files left by other
+tools or by manual invocations of go build.
+
+If a package argument is given or the -i or -r flag is set,
+clean removes the following files from each of the
+source directories corresponding to the import paths:
+
+	_obj/            old object directory, left from Makefiles
+	_test/           old test directory, left from Makefiles
+	_testmain.go     old gotest file, left from Makefiles
+	test.out         old test log, left from Makefiles
+	build.out        old test log, left from Makefiles
+	*.[568ao]        object files, left from Makefiles
+
+	DIR(.exe)        from go build
+	DIR.test(.exe)   from go test -c
+	MAINFILE(.exe)   from go build MAINFILE.go
+	*.so             from SWIG
+
+In the list, DIR represents the final path element of the
+directory, and MAINFILE is the base name of any Go source
+file in the directory that is not included when building
+the package.
+
+The -i flag causes clean to remove the corresponding installed
+archive or binary (what 'go install' would create).
+
+The -n flag causes clean to print the remove commands it would execute,
+but not run them.
+
+The -r flag causes clean to be applied recursively to all the
+dependencies of the packages named by the import paths.
+
+The -x flag causes clean to print remove commands as it executes them.
+
+The -cache flag causes clean to remove the entire go build cache.
+
+The -testcache flag causes clean to expire all test results in the
+go build cache.
+
+The -modcache flag causes clean to remove the entire module
+download cache, including unpacked source code of versioned
+dependencies.
+
+The -fuzzcache flag causes clean to remove files stored in the Go build
+cache for fuzz testing. The fuzzing engine caches files that expand
+code coverage, so removing them may make fuzzing less effective until
+new inputs are found that provide the same coverage. These files are
+distinct from those stored in testdata directory; clean does not remove
+those files.
+
+For more about build flags, see 'go help build'.
+
+For more about specifying packages, see 'go help packages'.
+	`,
+}
+
+var (
+	cleanI         bool // clean -i flag
+	cleanR         bool // clean -r flag
+	cleanCache     bool // clean -cache flag
+	cleanFuzzcache bool // clean -fuzzcache flag
+	cleanModcache  bool // clean -modcache flag
+	cleanTestcache bool // clean -testcache flag
+)
+
+func init() {
+	// break init cycle
+	CmdClean.Run = runClean
+
+	CmdClean.Flag.BoolVar(&cleanI, "i", false, "")
+	CmdClean.Flag.BoolVar(&cleanR, "r", false, "")
+	CmdClean.Flag.BoolVar(&cleanCache, "cache", false, "")
+	CmdClean.Flag.BoolVar(&cleanFuzzcache, "fuzzcache", false, "")
+	CmdClean.Flag.BoolVar(&cleanModcache, "modcache", false, "")
+	CmdClean.Flag.BoolVar(&cleanTestcache, "testcache", false, "")
+
+	// -n and -x are important enough to be
+	// mentioned explicitly in the docs but they
+	// are part of the build flags.
+
+	work.AddBuildFlags(CmdClean, work.DefaultBuildFlags)
+}
+
+func runClean(ctx context.Context, cmd *base.Command, args []string) {
+	if len(args) > 0 {
+		cacheFlag := ""
+		switch {
+		case cleanCache:
+			cacheFlag = "-cache"
+		case cleanTestcache:
+			cacheFlag = "-testcache"
+		case cleanFuzzcache:
+			cacheFlag = "-fuzzcache"
+		case cleanModcache:
+			cacheFlag = "-modcache"
+		}
+		if cacheFlag != "" {
+			base.Fatalf("go: clean %s cannot be used with package arguments", cacheFlag)
+		}
+	}
+
+	// golang.org/issue/29925: only load packages before cleaning if
+	// either the flags and arguments explicitly imply a package,
+	// or no other target (such as a cache) was requested to be cleaned.
+	cleanPkg := len(args) > 0 || cleanI || cleanR
+	if (!modload.Enabled() || modload.HasModRoot()) &&
+		!cleanCache && !cleanModcache && !cleanTestcache && !cleanFuzzcache {
+		cleanPkg = true
+	}
+
+	if cleanPkg {
+		for _, pkg := range load.PackagesAndErrors(ctx, load.PackageOpts{}, args) {
+			clean(pkg)
+		}
+	}
+
+	var b work.Builder
+	b.Print = fmt.Print
+
+	if cleanCache {
+		dir := cache.DefaultDir()
+		if dir != "off" {
+			// Remove the cache subdirectories but not the top cache directory.
+			// The top cache directory may have been created with special permissions
+			// and not something that we want to remove. Also, we'd like to preserve
+			// the access log for future analysis, even if the cache is cleared.
+			subdirs, _ := filepath.Glob(filepath.Join(str.QuoteGlob(dir), "[0-9a-f][0-9a-f]"))
+			printedErrors := false
+			if len(subdirs) > 0 {
+				if cfg.BuildN || cfg.BuildX {
+					b.Showcmd("", "rm -r %s", strings.Join(subdirs, " "))
+				}
+				if !cfg.BuildN {
+					for _, d := range subdirs {
+						// Only print the first error - there may be many.
+						// This also mimics what os.RemoveAll(dir) would do.
+						if err := os.RemoveAll(d); err != nil && !printedErrors {
+							printedErrors = true
+							base.Error(err)
+						}
+					}
+				}
+			}
+
+			logFile := filepath.Join(dir, "log.txt")
+			if cfg.BuildN || cfg.BuildX {
+				b.Showcmd("", "rm -f %s", logFile)
+			}
+			if !cfg.BuildN {
+				if err := os.RemoveAll(logFile); err != nil && !printedErrors {
+					printedErrors = true
+					base.Error(err)
+				}
+			}
+		}
+	}
+
+	if cleanTestcache && !cleanCache {
+		// Instead of walking through the entire cache looking for test results,
+		// we write a file to the cache indicating that all test results from before
+		// right now are to be ignored.
+		dir := cache.DefaultDir()
+		if dir != "off" {
+			f, err := lockedfile.Edit(filepath.Join(dir, "testexpire.txt"))
+			if err == nil {
+				now := time.Now().UnixNano()
+				buf, _ := io.ReadAll(f)
+				prev, _ := strconv.ParseInt(strings.TrimSpace(string(buf)), 10, 64)
+				if now > prev {
+					if err = f.Truncate(0); err == nil {
+						if _, err = f.Seek(0, 0); err == nil {
+							_, err = fmt.Fprintf(f, "%d\n", now)
+						}
+					}
+				}
+				if closeErr := f.Close(); err == nil {
+					err = closeErr
+				}
+			}
+			if err != nil {
+				if _, statErr := os.Stat(dir); !os.IsNotExist(statErr) {
+					base.Error(err)
+				}
+			}
+		}
+	}
+
+	if cleanModcache {
+		if cfg.GOMODCACHE == "" {
+			base.Fatalf("go: cannot clean -modcache without a module cache")
+		}
+		if cfg.BuildN || cfg.BuildX {
+			b.Showcmd("", "rm -rf %s", cfg.GOMODCACHE)
+		}
+		if !cfg.BuildN {
+			if err := modfetch.RemoveAll(cfg.GOMODCACHE); err != nil {
+				base.Error(err)
+			}
+		}
+	}
+
+	if cleanFuzzcache {
+		fuzzDir := cache.Default().FuzzDir()
+		if cfg.BuildN || cfg.BuildX {
+			b.Showcmd("", "rm -rf %s", fuzzDir)
+		}
+		if !cfg.BuildN {
+			if err := os.RemoveAll(fuzzDir); err != nil {
+				base.Error(err)
+			}
+		}
+	}
+}
+
+var cleaned = map[*load.Package]bool{}
+
+// TODO: These are dregs left by Makefile-based builds.
+// Eventually, can stop deleting these.
+var cleanDir = map[string]bool{
+	"_test": true,
+	"_obj":  true,
+}
+
+var cleanFile = map[string]bool{
+	"_testmain.go": true,
+	"test.out":     true,
+	"build.out":    true,
+	"a.out":        true,
+}
+
+var cleanExt = map[string]bool{
+	".5":  true,
+	".6":  true,
+	".8":  true,
+	".a":  true,
+	".o":  true,
+	".so": true,
+}
+
+func clean(p *load.Package) {
+	if cleaned[p] {
+		return
+	}
+	cleaned[p] = true
+
+	if p.Dir == "" {
+		base.Errorf("%v", p.Error)
+		return
+	}
+	dirs, err := os.ReadDir(p.Dir)
+	if err != nil {
+		base.Errorf("go: %s: %v", p.Dir, err)
+		return
+	}
+
+	var b work.Builder
+	b.Print = fmt.Print
+
+	packageFile := map[string]bool{}
+	if p.Name != "main" {
+		// Record which files are not in package main.
+		// The others are.
+		keep := func(list []string) {
+			for _, f := range list {
+				packageFile[f] = true
+			}
+		}
+		keep(p.GoFiles)
+		keep(p.CgoFiles)
+		keep(p.TestGoFiles)
+		keep(p.XTestGoFiles)
+	}
+
+	_, elem := filepath.Split(p.Dir)
+	var allRemove []string
+
+	// Remove dir-named executable only if this is package main.
+	if p.Name == "main" {
+		allRemove = append(allRemove,
+			elem,
+			elem+".exe",
+			p.DefaultExecName(),
+			p.DefaultExecName()+".exe",
+		)
+	}
+
+	// Remove package test executables.
+	allRemove = append(allRemove,
+		elem+".test",
+		elem+".test.exe",
+		p.DefaultExecName()+".test",
+		p.DefaultExecName()+".test.exe",
+	)
+
+	// Remove a potential executable, test executable for each .go file in the directory that
+	// is not part of the directory's package.
+	for _, dir := range dirs {
+		name := dir.Name()
+		if packageFile[name] {
+			continue
+		}
+
+		if dir.IsDir() {
+			continue
+		}
+
+		if base, found := strings.CutSuffix(name, "_test.go"); found {
+			allRemove = append(allRemove, base+".test", base+".test.exe")
+		}
+
+		if base, found := strings.CutSuffix(name, ".go"); found {
+			// TODO(adg,rsc): check that this .go file is actually
+			// in "package main", and therefore capable of building
+			// to an executable file.
+			allRemove = append(allRemove, base, base+".exe")
+		}
+	}
+
+	if cfg.BuildN || cfg.BuildX {
+		b.Showcmd(p.Dir, "rm -f %s", strings.Join(allRemove, " "))
+	}
+
+	toRemove := map[string]bool{}
+	for _, name := range allRemove {
+		toRemove[name] = true
+	}
+	for _, dir := range dirs {
+		name := dir.Name()
+		if dir.IsDir() {
+			// TODO: Remove once Makefiles are forgotten.
+			if cleanDir[name] {
+				if cfg.BuildN || cfg.BuildX {
+					b.Showcmd(p.Dir, "rm -r %s", name)
+					if cfg.BuildN {
+						continue
+					}
+				}
+				if err := os.RemoveAll(filepath.Join(p.Dir, name)); err != nil {
+					base.Error(err)
+				}
+			}
+			continue
+		}
+
+		if cfg.BuildN {
+			continue
+		}
+
+		if cleanFile[name] || cleanExt[filepath.Ext(name)] || toRemove[name] {
+			removeFile(filepath.Join(p.Dir, name))
+		}
+	}
+
+	if cleanI && p.Target != "" {
+		if cfg.BuildN || cfg.BuildX {
+			b.Showcmd("", "rm -f %s", p.Target)
+		}
+		if !cfg.BuildN {
+			removeFile(p.Target)
+		}
+	}
+
+	if cleanR {
+		for _, p1 := range p.Internal.Imports {
+			clean(p1)
+		}
+	}
+}
+
+// removeFile tries to remove file f, if error other than file doesn't exist
+// occurs, it will report the error.
+func removeFile(f string) {
+	err := os.Remove(f)
+	if err == nil || os.IsNotExist(err) {
+		return
+	}
+	// Windows does not allow deletion of a binary file while it is executing.
+	if runtime.GOOS == "windows" {
+		// Remove lingering ~ file from last attempt.
+		if _, err2 := os.Stat(f + "~"); err2 == nil {
+			os.Remove(f + "~")
+		}
+		// Try to move it out of the way. If the move fails,
+		// which is likely, we'll try again the
+		// next time we do an install of this binary.
+		if err2 := os.Rename(f, f+"~"); err2 == nil {
+			os.Remove(f + "~")
+			return
+		}
+	}
+	base.Error(err)
+}
diff --git a/src/cmd/go/internal/cmdflag/flag.go b/src/cmd/go/internal/cmdflag/flag.go
new file mode 100644
index 0000000..86e33ea
--- /dev/null
+++ b/src/cmd/go/internal/cmdflag/flag.go
@@ -0,0 +1,122 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package cmdflag handles flag processing common to several go tools.
+package cmdflag
+
+import (
+	"errors"
+	"flag"
+	"fmt"
+	"strings"
+)
+
+// The flag handling part of go commands such as test is large and distracting.
+// We can't use the standard flag package because some of the flags from
+// our command line are for us, and some are for the binary we're running,
+// and some are for both.
+
+// ErrFlagTerminator indicates the distinguished token "--", which causes the
+// flag package to treat all subsequent arguments as non-flags.
+var ErrFlagTerminator = errors.New("flag terminator")
+
+// A FlagNotDefinedError indicates a flag-like argument that does not correspond
+// to any registered flag in a FlagSet.
+type FlagNotDefinedError struct {
+	RawArg   string // the original argument, like --foo or -foo=value
+	Name     string
+	HasValue bool   // is this the -foo=value or --foo=value form?
+	Value    string // only provided if HasValue is true
+}
+
+func (e FlagNotDefinedError) Error() string {
+	return fmt.Sprintf("flag provided but not defined: -%s", e.Name)
+}
+
+// A NonFlagError indicates an argument that is not a syntactically-valid flag.
+type NonFlagError struct {
+	RawArg string
+}
+
+func (e NonFlagError) Error() string {
+	return fmt.Sprintf("not a flag: %q", e.RawArg)
+}
+
+// ParseOne sees if args[0] is present in the given flag set and if so,
+// sets its value and returns the flag along with the remaining (unused) arguments.
+//
+// ParseOne always returns either a non-nil Flag or a non-nil error,
+// and always consumes at least one argument (even on error).
+//
+// Unlike (*flag.FlagSet).Parse, ParseOne does not log its own errors.
+func ParseOne(fs *flag.FlagSet, args []string) (f *flag.Flag, remainingArgs []string, err error) {
+	// This function is loosely derived from (*flag.FlagSet).parseOne.
+
+	raw, args := args[0], args[1:]
+	arg := raw
+	if strings.HasPrefix(arg, "--") {
+		if arg == "--" {
+			return nil, args, ErrFlagTerminator
+		}
+		arg = arg[1:] // reduce two minuses to one
+	}
+
+	switch arg {
+	case "-?", "-h", "-help":
+		return nil, args, flag.ErrHelp
+	}
+	if len(arg) < 2 || arg[0] != '-' || arg[1] == '-' || arg[1] == '=' {
+		return nil, args, NonFlagError{RawArg: raw}
+	}
+
+	name, value, hasValue := strings.Cut(arg[1:], "=")
+
+	f = fs.Lookup(name)
+	if f == nil {
+		return nil, args, FlagNotDefinedError{
+			RawArg:   raw,
+			Name:     name,
+			HasValue: hasValue,
+			Value:    value,
+		}
+	}
+
+	// Use fs.Set instead of f.Value.Set below so that any subsequent call to
+	// fs.Visit will correctly visit the flags that have been set.
+
+	failf := func(format string, a ...any) (*flag.Flag, []string, error) {
+		return f, args, fmt.Errorf(format, a...)
+	}
+
+	if fv, ok := f.Value.(boolFlag); ok && fv.IsBoolFlag() { // special case: doesn't need an arg
+		if hasValue {
+			if err := fs.Set(name, value); err != nil {
+				return failf("invalid boolean value %q for -%s: %v", value, name, err)
+			}
+		} else {
+			if err := fs.Set(name, "true"); err != nil {
+				return failf("invalid boolean flag %s: %v", name, err)
+			}
+		}
+	} else {
+		// It must have a value, which might be the next argument.
+		if !hasValue && len(args) > 0 {
+			// value is the next arg
+			hasValue = true
+			value, args = args[0], args[1:]
+		}
+		if !hasValue {
+			return failf("flag needs an argument: -%s", name)
+		}
+		if err := fs.Set(name, value); err != nil {
+			return failf("invalid value %q for flag -%s: %v", value, name, err)
+		}
+	}
+
+	return f, args, nil
+}
+
+type boolFlag interface {
+	IsBoolFlag() bool
+}
diff --git a/src/cmd/go/internal/doc/doc.go b/src/cmd/go/internal/doc/doc.go
new file mode 100644
index 0000000..3b6cd94
--- /dev/null
+++ b/src/cmd/go/internal/doc/doc.go
@@ -0,0 +1,134 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package doc implements the “go doc” command.
+package doc
+
+import (
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"context"
+)
+
+var CmdDoc = &base.Command{
+	Run:         runDoc,
+	UsageLine:   "go doc [doc flags] [package|[package.]symbol[.methodOrField]]",
+	CustomFlags: true,
+	Short:       "show documentation for package or symbol",
+	Long: `
+Doc prints the documentation comments associated with the item identified by its
+arguments (a package, const, func, type, var, method, or struct field)
+followed by a one-line summary of each of the first-level items "under"
+that item (package-level declarations for a package, methods for a type,
+etc.).
+
+Doc accepts zero, one, or two arguments.
+
+Given no arguments, that is, when run as
+
+	go doc
+
+it prints the package documentation for the package in the current directory.
+If the package is a command (package main), the exported symbols of the package
+are elided from the presentation unless the -cmd flag is provided.
+
+When run with one argument, the argument is treated as a Go-syntax-like
+representation of the item to be documented. What the argument selects depends
+on what is installed in GOROOT and GOPATH, as well as the form of the argument,
+which is schematically one of these:
+
+	go doc <pkg>
+	go doc <sym>[.<methodOrField>]
+	go doc [<pkg>.]<sym>[.<methodOrField>]
+	go doc [<pkg>.][<sym>.]<methodOrField>
+
+The first item in this list matched by the argument is the one whose documentation
+is printed. (See the examples below.) However, if the argument starts with a capital
+letter it is assumed to identify a symbol or method in the current directory.
+
+For packages, the order of scanning is determined lexically in breadth-first order.
+That is, the package presented is the one that matches the search and is nearest
+the root and lexically first at its level of the hierarchy. The GOROOT tree is
+always scanned in its entirety before GOPATH.
+
+If there is no package specified or matched, the package in the current
+directory is selected, so "go doc Foo" shows the documentation for symbol Foo in
+the current package.
+
+The package path must be either a qualified path or a proper suffix of a
+path. The go tool's usual package mechanism does not apply: package path
+elements like . and ... are not implemented by go doc.
+
+When run with two arguments, the first is a package path (full path or suffix),
+and the second is a symbol, or symbol with method or struct field:
+
+	go doc <pkg> <sym>[.<methodOrField>]
+
+In all forms, when matching symbols, lower-case letters in the argument match
+either case but upper-case letters match exactly. This means that there may be
+multiple matches of a lower-case argument in a package if different symbols have
+different cases. If this occurs, documentation for all matches is printed.
+
+Examples:
+	go doc
+		Show documentation for current package.
+	go doc Foo
+		Show documentation for Foo in the current package.
+		(Foo starts with a capital letter so it cannot match
+		a package path.)
+	go doc encoding/json
+		Show documentation for the encoding/json package.
+	go doc json
+		Shorthand for encoding/json.
+	go doc json.Number (or go doc json.number)
+		Show documentation and method summary for json.Number.
+	go doc json.Number.Int64 (or go doc json.number.int64)
+		Show documentation for json.Number's Int64 method.
+	go doc cmd/doc
+		Show package docs for the doc command.
+	go doc -cmd cmd/doc
+		Show package docs and exported symbols within the doc command.
+	go doc template.new
+		Show documentation for html/template's New function.
+		(html/template is lexically before text/template)
+	go doc text/template.new # One argument
+		Show documentation for text/template's New function.
+	go doc text/template new # Two arguments
+		Show documentation for text/template's New function.
+
+	At least in the current tree, these invocations all print the
+	documentation for json.Decoder's Decode method:
+
+	go doc json.Decoder.Decode
+	go doc json.decoder.decode
+	go doc json.decode
+	cd go/src/encoding/json; go doc decode
+
+Flags:
+	-all
+		Show all the documentation for the package.
+	-c
+		Respect case when matching symbols.
+	-cmd
+		Treat a command (package main) like a regular package.
+		Otherwise package main's exported symbols are hidden
+		when showing the package's top-level documentation.
+	-short
+		One-line representation for each symbol.
+	-src
+		Show the full source code for the symbol. This will
+		display the full Go source of its declaration and
+		definition, such as a function definition (including
+		the body), type declaration or enclosing const
+		block. The output may therefore include unexported
+		details.
+	-u
+		Show documentation for unexported as well as exported
+		symbols, methods, and fields.
+`,
+}
+
+func runDoc(ctx context.Context, cmd *base.Command, args []string) {
+	base.Run(cfg.BuildToolexec, base.Tool("doc"), args)
+}
diff --git a/src/cmd/go/internal/envcmd/env.go b/src/cmd/go/internal/envcmd/env.go
new file mode 100644
index 0000000..c7c2e83
--- /dev/null
+++ b/src/cmd/go/internal/envcmd/env.go
@@ -0,0 +1,691 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package envcmd implements the “go env” command.
+package envcmd
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"go/build"
+	"internal/buildcfg"
+	"io"
+	"os"
+	"path/filepath"
+	"runtime"
+	"sort"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cache"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/load"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/work"
+	"cmd/internal/quoted"
+)
+
+var CmdEnv = &base.Command{
+	UsageLine: "go env [-json] [-u] [-w] [var ...]",
+	Short:     "print Go environment information",
+	Long: `
+Env prints Go environment information.
+
+By default env prints information as a shell script
+(on Windows, a batch file). If one or more variable
+names is given as arguments, env prints the value of
+each named variable on its own line.
+
+The -json flag prints the environment in JSON format
+instead of as a shell script.
+
+The -u flag requires one or more arguments and unsets
+the default setting for the named environment variables,
+if one has been set with 'go env -w'.
+
+The -w flag requires one or more arguments of the
+form NAME=VALUE and changes the default settings
+of the named environment variables to the given values.
+
+For more about environment variables, see 'go help environment'.
+	`,
+}
+
+func init() {
+	CmdEnv.Run = runEnv // break init cycle
+	base.AddChdirFlag(&CmdEnv.Flag)
+	base.AddBuildFlagsNX(&CmdEnv.Flag)
+}
+
+var (
+	envJson = CmdEnv.Flag.Bool("json", false, "")
+	envU    = CmdEnv.Flag.Bool("u", false, "")
+	envW    = CmdEnv.Flag.Bool("w", false, "")
+)
+
+func MkEnv() []cfg.EnvVar {
+	envFile, _ := cfg.EnvFile()
+	env := []cfg.EnvVar{
+		{Name: "GO111MODULE", Value: cfg.Getenv("GO111MODULE")},
+		{Name: "GOARCH", Value: cfg.Goarch},
+		{Name: "GOBIN", Value: cfg.GOBIN},
+		{Name: "GOCACHE", Value: cache.DefaultDir()},
+		{Name: "GOENV", Value: envFile},
+		{Name: "GOEXE", Value: cfg.ExeSuffix},
+
+		// List the raw value of GOEXPERIMENT, not the cleaned one.
+		// The set of default experiments may change from one release
+		// to the next, so a GOEXPERIMENT setting that is redundant
+		// with the current toolchain might actually be relevant with
+		// a different version (for example, when bisecting a regression).
+		{Name: "GOEXPERIMENT", Value: cfg.RawGOEXPERIMENT},
+
+		{Name: "GOFLAGS", Value: cfg.Getenv("GOFLAGS")},
+		{Name: "GOHOSTARCH", Value: runtime.GOARCH},
+		{Name: "GOHOSTOS", Value: runtime.GOOS},
+		{Name: "GOINSECURE", Value: cfg.GOINSECURE},
+		{Name: "GOMODCACHE", Value: cfg.GOMODCACHE},
+		{Name: "GONOPROXY", Value: cfg.GONOPROXY},
+		{Name: "GONOSUMDB", Value: cfg.GONOSUMDB},
+		{Name: "GOOS", Value: cfg.Goos},
+		{Name: "GOPATH", Value: cfg.BuildContext.GOPATH},
+		{Name: "GOPRIVATE", Value: cfg.GOPRIVATE},
+		{Name: "GOPROXY", Value: cfg.GOPROXY},
+		{Name: "GOROOT", Value: cfg.GOROOT},
+		{Name: "GOSUMDB", Value: cfg.GOSUMDB},
+		{Name: "GOTMPDIR", Value: cfg.Getenv("GOTMPDIR")},
+		{Name: "GOTOOLCHAIN", Value: cfg.Getenv("GOTOOLCHAIN")},
+		{Name: "GOTOOLDIR", Value: build.ToolDir},
+		{Name: "GOVCS", Value: cfg.GOVCS},
+		{Name: "GOVERSION", Value: runtime.Version()},
+	}
+
+	if work.GccgoBin != "" {
+		env = append(env, cfg.EnvVar{Name: "GCCGO", Value: work.GccgoBin})
+	} else {
+		env = append(env, cfg.EnvVar{Name: "GCCGO", Value: work.GccgoName})
+	}
+
+	key, val := cfg.GetArchEnv()
+	if key != "" {
+		env = append(env, cfg.EnvVar{Name: key, Value: val})
+	}
+
+	cc := cfg.Getenv("CC")
+	if cc == "" {
+		cc = cfg.DefaultCC(cfg.Goos, cfg.Goarch)
+	}
+	cxx := cfg.Getenv("CXX")
+	if cxx == "" {
+		cxx = cfg.DefaultCXX(cfg.Goos, cfg.Goarch)
+	}
+	env = append(env, cfg.EnvVar{Name: "AR", Value: envOr("AR", "ar")})
+	env = append(env, cfg.EnvVar{Name: "CC", Value: cc})
+	env = append(env, cfg.EnvVar{Name: "CXX", Value: cxx})
+
+	if cfg.BuildContext.CgoEnabled {
+		env = append(env, cfg.EnvVar{Name: "CGO_ENABLED", Value: "1"})
+	} else {
+		env = append(env, cfg.EnvVar{Name: "CGO_ENABLED", Value: "0"})
+	}
+
+	return env
+}
+
+func envOr(name, def string) string {
+	val := cfg.Getenv(name)
+	if val != "" {
+		return val
+	}
+	return def
+}
+
+func findEnv(env []cfg.EnvVar, name string) string {
+	for _, e := range env {
+		if e.Name == name {
+			return e.Value
+		}
+	}
+	if cfg.CanGetenv(name) {
+		return cfg.Getenv(name)
+	}
+	return ""
+}
+
+// ExtraEnvVars returns environment variables that should not leak into child processes.
+func ExtraEnvVars() []cfg.EnvVar {
+	gomod := ""
+	modload.Init()
+	if modload.HasModRoot() {
+		gomod = modload.ModFilePath()
+	} else if modload.Enabled() {
+		gomod = os.DevNull
+	}
+	modload.InitWorkfile()
+	gowork := modload.WorkFilePath()
+	// As a special case, if a user set off explicitly, report that in GOWORK.
+	if cfg.Getenv("GOWORK") == "off" {
+		gowork = "off"
+	}
+	return []cfg.EnvVar{
+		{Name: "GOMOD", Value: gomod},
+		{Name: "GOWORK", Value: gowork},
+	}
+}
+
+// ExtraEnvVarsCostly returns environment variables that should not leak into child processes
+// but are costly to evaluate.
+func ExtraEnvVarsCostly() []cfg.EnvVar {
+	b := work.NewBuilder("")
+	defer func() {
+		if err := b.Close(); err != nil {
+			base.Fatal(err)
+		}
+	}()
+
+	cppflags, cflags, cxxflags, fflags, ldflags, err := b.CFlags(&load.Package{})
+	if err != nil {
+		// Should not happen - b.CFlags was given an empty package.
+		fmt.Fprintf(os.Stderr, "go: invalid cflags: %v\n", err)
+		return nil
+	}
+	cmd := b.GccCmd(".", "")
+
+	join := func(s []string) string {
+		q, err := quoted.Join(s)
+		if err != nil {
+			return strings.Join(s, " ")
+		}
+		return q
+	}
+
+	return []cfg.EnvVar{
+		// Note: Update the switch in runEnv below when adding to this list.
+		{Name: "CGO_CFLAGS", Value: join(cflags)},
+		{Name: "CGO_CPPFLAGS", Value: join(cppflags)},
+		{Name: "CGO_CXXFLAGS", Value: join(cxxflags)},
+		{Name: "CGO_FFLAGS", Value: join(fflags)},
+		{Name: "CGO_LDFLAGS", Value: join(ldflags)},
+		{Name: "PKG_CONFIG", Value: b.PkgconfigCmd()},
+		{Name: "GOGCCFLAGS", Value: join(cmd[3:])},
+	}
+}
+
+// argKey returns the KEY part of the arg KEY=VAL, or else arg itself.
+func argKey(arg string) string {
+	i := strings.Index(arg, "=")
+	if i < 0 {
+		return arg
+	}
+	return arg[:i]
+}
+
+func runEnv(ctx context.Context, cmd *base.Command, args []string) {
+	if *envJson && *envU {
+		base.Fatalf("go: cannot use -json with -u")
+	}
+	if *envJson && *envW {
+		base.Fatalf("go: cannot use -json with -w")
+	}
+	if *envU && *envW {
+		base.Fatalf("go: cannot use -u with -w")
+	}
+
+	// Handle 'go env -w' and 'go env -u' before calling buildcfg.Check,
+	// so they can be used to recover from an invalid configuration.
+	if *envW {
+		runEnvW(args)
+		return
+	}
+
+	if *envU {
+		runEnvU(args)
+		return
+	}
+
+	buildcfg.Check()
+	if cfg.ExperimentErr != nil {
+		base.Fatal(cfg.ExperimentErr)
+	}
+
+	for _, arg := range args {
+		if strings.Contains(arg, "=") {
+			base.Fatalf("go: invalid variable name %q (use -w to set variable)", arg)
+		}
+	}
+
+	env := cfg.CmdEnv
+	env = append(env, ExtraEnvVars()...)
+
+	if err := fsys.Init(base.Cwd()); err != nil {
+		base.Fatal(err)
+	}
+
+	// Do we need to call ExtraEnvVarsCostly, which is a bit expensive?
+	needCostly := false
+	if len(args) == 0 {
+		// We're listing all environment variables ("go env"),
+		// including the expensive ones.
+		needCostly = true
+	} else {
+		needCostly = false
+	checkCostly:
+		for _, arg := range args {
+			switch argKey(arg) {
+			case "CGO_CFLAGS",
+				"CGO_CPPFLAGS",
+				"CGO_CXXFLAGS",
+				"CGO_FFLAGS",
+				"CGO_LDFLAGS",
+				"PKG_CONFIG",
+				"GOGCCFLAGS":
+				needCostly = true
+				break checkCostly
+			}
+		}
+	}
+	if needCostly {
+		work.BuildInit()
+		env = append(env, ExtraEnvVarsCostly()...)
+	}
+
+	if len(args) > 0 {
+		if *envJson {
+			var es []cfg.EnvVar
+			for _, name := range args {
+				e := cfg.EnvVar{Name: name, Value: findEnv(env, name)}
+				es = append(es, e)
+			}
+			printEnvAsJSON(es)
+		} else {
+			for _, name := range args {
+				fmt.Printf("%s\n", findEnv(env, name))
+			}
+		}
+		return
+	}
+
+	if *envJson {
+		printEnvAsJSON(env)
+		return
+	}
+
+	PrintEnv(os.Stdout, env)
+}
+
+func runEnvW(args []string) {
+	// Process and sanity-check command line.
+	if len(args) == 0 {
+		base.Fatalf("go: no KEY=VALUE arguments given")
+	}
+	osEnv := make(map[string]string)
+	for _, e := range cfg.OrigEnv {
+		if i := strings.Index(e, "="); i >= 0 {
+			osEnv[e[:i]] = e[i+1:]
+		}
+	}
+	add := make(map[string]string)
+	for _, arg := range args {
+		key, val, found := strings.Cut(arg, "=")
+		if !found {
+			base.Fatalf("go: arguments must be KEY=VALUE: invalid argument: %s", arg)
+		}
+		if err := checkEnvWrite(key, val); err != nil {
+			base.Fatal(err)
+		}
+		if _, ok := add[key]; ok {
+			base.Fatalf("go: multiple values for key: %s", key)
+		}
+		add[key] = val
+		if osVal := osEnv[key]; osVal != "" && osVal != val {
+			fmt.Fprintf(os.Stderr, "warning: go env -w %s=... does not override conflicting OS environment variable\n", key)
+		}
+	}
+
+	if err := checkBuildConfig(add, nil); err != nil {
+		base.Fatal(err)
+	}
+
+	gotmp, okGOTMP := add["GOTMPDIR"]
+	if okGOTMP {
+		if !filepath.IsAbs(gotmp) && gotmp != "" {
+			base.Fatalf("go: GOTMPDIR must be an absolute path")
+		}
+	}
+
+	updateEnvFile(add, nil)
+}
+
+func runEnvU(args []string) {
+	// Process and sanity-check command line.
+	if len(args) == 0 {
+		base.Fatalf("go: 'go env -u' requires an argument")
+	}
+	del := make(map[string]bool)
+	for _, arg := range args {
+		if err := checkEnvWrite(arg, ""); err != nil {
+			base.Fatal(err)
+		}
+		del[arg] = true
+	}
+
+	if err := checkBuildConfig(nil, del); err != nil {
+		base.Fatal(err)
+	}
+
+	updateEnvFile(nil, del)
+}
+
+// checkBuildConfig checks whether the build configuration is valid
+// after the specified configuration environment changes are applied.
+func checkBuildConfig(add map[string]string, del map[string]bool) error {
+	// get returns the value for key after applying add and del and
+	// reports whether it changed. cur should be the current value
+	// (i.e., before applying changes) and def should be the default
+	// value (i.e., when no environment variables are provided at all).
+	get := func(key, cur, def string) (string, bool) {
+		if val, ok := add[key]; ok {
+			return val, true
+		}
+		if del[key] {
+			val := getOrigEnv(key)
+			if val == "" {
+				val = def
+			}
+			return val, true
+		}
+		return cur, false
+	}
+
+	goos, okGOOS := get("GOOS", cfg.Goos, build.Default.GOOS)
+	goarch, okGOARCH := get("GOARCH", cfg.Goarch, build.Default.GOARCH)
+	if okGOOS || okGOARCH {
+		if err := work.CheckGOOSARCHPair(goos, goarch); err != nil {
+			return err
+		}
+	}
+
+	goexperiment, okGOEXPERIMENT := get("GOEXPERIMENT", cfg.RawGOEXPERIMENT, buildcfg.DefaultGOEXPERIMENT)
+	if okGOEXPERIMENT {
+		if _, err := buildcfg.ParseGOEXPERIMENT(goos, goarch, goexperiment); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// PrintEnv prints the environment variables to w.
+func PrintEnv(w io.Writer, env []cfg.EnvVar) {
+	for _, e := range env {
+		if e.Name != "TERM" {
+			if runtime.GOOS != "plan9" && bytes.Contains([]byte(e.Value), []byte{0}) {
+				base.Fatalf("go: internal error: encountered null byte in environment variable %s on non-plan9 platform", e.Name)
+			}
+			switch runtime.GOOS {
+			default:
+				fmt.Fprintf(w, "%s=%s\n", e.Name, shellQuote(e.Value))
+			case "plan9":
+				if strings.IndexByte(e.Value, '\x00') < 0 {
+					fmt.Fprintf(w, "%s='%s'\n", e.Name, strings.ReplaceAll(e.Value, "'", "''"))
+				} else {
+					v := strings.Split(e.Value, "\x00")
+					fmt.Fprintf(w, "%s=(", e.Name)
+					for x, s := range v {
+						if x > 0 {
+							fmt.Fprintf(w, " ")
+						}
+						fmt.Fprintf(w, "'%s'", strings.ReplaceAll(s, "'", "''"))
+					}
+					fmt.Fprintf(w, ")\n")
+				}
+			case "windows":
+				if hasNonGraphic(e.Value) {
+					base.Errorf("go: stripping unprintable or unescapable characters from %%%q%%", e.Name)
+				}
+				fmt.Fprintf(w, "set %s=%s\n", e.Name, batchEscape(e.Value))
+			}
+		}
+	}
+}
+
+func hasNonGraphic(s string) bool {
+	for _, c := range []byte(s) {
+		if c == '\r' || c == '\n' || (!unicode.IsGraphic(rune(c)) && !unicode.IsSpace(rune(c))) {
+			return true
+		}
+	}
+	return false
+}
+
+func shellQuote(s string) string {
+	var b bytes.Buffer
+	b.WriteByte('\'')
+	for _, x := range []byte(s) {
+		if x == '\'' {
+			// Close the single quoted string, add an escaped single quote,
+			// and start another single quoted string.
+			b.WriteString(`'\''`)
+		} else {
+			b.WriteByte(x)
+		}
+	}
+	b.WriteByte('\'')
+	return b.String()
+}
+
+func batchEscape(s string) string {
+	var b bytes.Buffer
+	for _, x := range []byte(s) {
+		if x == '\r' || x == '\n' || (!unicode.IsGraphic(rune(x)) && !unicode.IsSpace(rune(x))) {
+			b.WriteRune(unicode.ReplacementChar)
+			continue
+		}
+		switch x {
+		case '%':
+			b.WriteString("%%")
+		case '<', '>', '|', '&', '^':
+			// These are special characters that need to be escaped with ^. See
+			// https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/set_1.
+			b.WriteByte('^')
+			b.WriteByte(x)
+		default:
+			b.WriteByte(x)
+		}
+	}
+	return b.String()
+}
+
+func printEnvAsJSON(env []cfg.EnvVar) {
+	m := make(map[string]string)
+	for _, e := range env {
+		if e.Name == "TERM" {
+			continue
+		}
+		m[e.Name] = e.Value
+	}
+	enc := json.NewEncoder(os.Stdout)
+	enc.SetIndent("", "\t")
+	if err := enc.Encode(m); err != nil {
+		base.Fatalf("go: %s", err)
+	}
+}
+
+func getOrigEnv(key string) string {
+	for _, v := range cfg.OrigEnv {
+		if v, found := strings.CutPrefix(v, key+"="); found {
+			return v
+		}
+	}
+	return ""
+}
+
+func checkEnvWrite(key, val string) error {
+	switch key {
+	case "GOEXE", "GOGCCFLAGS", "GOHOSTARCH", "GOHOSTOS", "GOMOD", "GOWORK", "GOTOOLDIR", "GOVERSION":
+		return fmt.Errorf("%s cannot be modified", key)
+	case "GOENV":
+		return fmt.Errorf("%s can only be set using the OS environment", key)
+	}
+
+	// To catch typos and the like, check that we know the variable.
+	// If it's already in the env file, we assume it's known.
+	if !cfg.CanGetenv(key) {
+		return fmt.Errorf("unknown go command variable %s", key)
+	}
+
+	// Some variables can only have one of a few valid values. If set to an
+	// invalid value, the next cmd/go invocation might fail immediately,
+	// even 'go env -w' itself.
+	switch key {
+	case "GO111MODULE":
+		switch val {
+		case "", "auto", "on", "off":
+		default:
+			return fmt.Errorf("invalid %s value %q", key, val)
+		}
+	case "GOPATH":
+		if strings.HasPrefix(val, "~") {
+			return fmt.Errorf("GOPATH entry cannot start with shell metacharacter '~': %q", val)
+		}
+		if !filepath.IsAbs(val) && val != "" {
+			return fmt.Errorf("GOPATH entry is relative; must be absolute path: %q", val)
+		}
+	case "GOMODCACHE":
+		if !filepath.IsAbs(val) && val != "" {
+			return fmt.Errorf("GOMODCACHE entry is relative; must be absolute path: %q", val)
+		}
+	case "CC", "CXX":
+		if val == "" {
+			break
+		}
+		args, err := quoted.Split(val)
+		if err != nil {
+			return fmt.Errorf("invalid %s: %v", key, err)
+		}
+		if len(args) == 0 {
+			return fmt.Errorf("%s entry cannot contain only space", key)
+		}
+		if !filepath.IsAbs(args[0]) && args[0] != filepath.Base(args[0]) {
+			return fmt.Errorf("%s entry is relative; must be absolute path: %q", key, args[0])
+		}
+	}
+
+	if !utf8.ValidString(val) {
+		return fmt.Errorf("invalid UTF-8 in %s=... value", key)
+	}
+	if strings.Contains(val, "\x00") {
+		return fmt.Errorf("invalid NUL in %s=... value", key)
+	}
+	if strings.ContainsAny(val, "\v\r\n") {
+		return fmt.Errorf("invalid newline in %s=... value", key)
+	}
+	return nil
+}
+
+func readEnvFileLines(mustExist bool) []string {
+	file, err := cfg.EnvFile()
+	if file == "" {
+		if mustExist {
+			base.Fatalf("go: cannot find go env config: %v", err)
+		}
+		return nil
+	}
+	data, err := os.ReadFile(file)
+	if err != nil && (!os.IsNotExist(err) || mustExist) {
+		base.Fatalf("go: reading go env config: %v", err)
+	}
+	lines := strings.SplitAfter(string(data), "\n")
+	if lines[len(lines)-1] == "" {
+		lines = lines[:len(lines)-1]
+	} else {
+		lines[len(lines)-1] += "\n"
+	}
+	return lines
+}
+
+func updateEnvFile(add map[string]string, del map[string]bool) {
+	lines := readEnvFileLines(len(add) == 0)
+
+	// Delete all but last copy of any duplicated variables,
+	// since the last copy is the one that takes effect.
+	prev := make(map[string]int)
+	for l, line := range lines {
+		if key := lineToKey(line); key != "" {
+			if p, ok := prev[key]; ok {
+				lines[p] = ""
+			}
+			prev[key] = l
+		}
+	}
+
+	// Add variables (go env -w). Update existing lines in file if present, add to end otherwise.
+	for key, val := range add {
+		if p, ok := prev[key]; ok {
+			lines[p] = key + "=" + val + "\n"
+			delete(add, key)
+		}
+	}
+	for key, val := range add {
+		lines = append(lines, key+"="+val+"\n")
+	}
+
+	// Delete requested variables (go env -u).
+	for key := range del {
+		if p, ok := prev[key]; ok {
+			lines[p] = ""
+		}
+	}
+
+	// Sort runs of KEY=VALUE lines
+	// (that is, blocks of lines where blocks are separated
+	// by comments, blank lines, or invalid lines).
+	start := 0
+	for i := 0; i <= len(lines); i++ {
+		if i == len(lines) || lineToKey(lines[i]) == "" {
+			sortKeyValues(lines[start:i])
+			start = i + 1
+		}
+	}
+
+	file, err := cfg.EnvFile()
+	if file == "" {
+		base.Fatalf("go: cannot find go env config: %v", err)
+	}
+	data := []byte(strings.Join(lines, ""))
+	err = os.WriteFile(file, data, 0666)
+	if err != nil {
+		// Try creating directory.
+		os.MkdirAll(filepath.Dir(file), 0777)
+		err = os.WriteFile(file, data, 0666)
+		if err != nil {
+			base.Fatalf("go: writing go env config: %v", err)
+		}
+	}
+}
+
+// lineToKey returns the KEY part of the line KEY=VALUE or else an empty string.
+func lineToKey(line string) string {
+	i := strings.Index(line, "=")
+	if i < 0 || strings.Contains(line[:i], "#") {
+		return ""
+	}
+	return line[:i]
+}
+
+// sortKeyValues sorts a sequence of lines by key.
+// It differs from sort.Strings in that keys which are GOx where x is an ASCII
+// character smaller than = sort after GO=.
+// (There are no such keys currently. It used to matter for GO386 which was
+// removed in Go 1.16.)
+func sortKeyValues(lines []string) {
+	sort.Slice(lines, func(i, j int) bool {
+		return lineToKey(lines[i]) < lineToKey(lines[j])
+	})
+}
diff --git a/src/cmd/go/internal/envcmd/env_test.go b/src/cmd/go/internal/envcmd/env_test.go
new file mode 100644
index 0000000..7419cf3
--- /dev/null
+++ b/src/cmd/go/internal/envcmd/env_test.go
@@ -0,0 +1,93 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix || windows
+
+package envcmd
+
+import (
+	"bytes"
+	"cmd/go/internal/cfg"
+	"fmt"
+	"internal/testenv"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"testing"
+	"unicode"
+)
+
+func FuzzPrintEnvEscape(f *testing.F) {
+	f.Add(`$(echo 'cc"'; echo 'OOPS="oops')`)
+	f.Add("$(echo shell expansion 1>&2)")
+	f.Add("''")
+	f.Add(`C:\"Program Files"\`)
+	f.Add(`\\"Quoted Host"\\share`)
+	f.Add("\xfb")
+	f.Add("0")
+	f.Add("")
+	f.Add("''''''''")
+	f.Add("\r")
+	f.Add("\n")
+	f.Add("E,%")
+	f.Fuzz(func(t *testing.T, s string) {
+		t.Parallel()
+
+		for _, c := range []byte(s) {
+			if c == 0 {
+				t.Skipf("skipping %q: contains a null byte. Null bytes can't occur in the environment"+
+					" outside of Plan 9, which has different code path than Windows and Unix that this test"+
+					" isn't testing.", s)
+			}
+			if c > unicode.MaxASCII {
+				t.Skipf("skipping %#q: contains a non-ASCII character %q", s, c)
+			}
+			if !unicode.IsGraphic(rune(c)) && !unicode.IsSpace(rune(c)) {
+				t.Skipf("skipping %#q: contains non-graphic character %q", s, c)
+			}
+			if runtime.GOOS == "windows" && c == '\r' || c == '\n' {
+				t.Skipf("skipping %#q on Windows: contains unescapable character %q", s, c)
+			}
+		}
+
+		var b bytes.Buffer
+		if runtime.GOOS == "windows" {
+			b.WriteString("@echo off\n")
+		}
+		PrintEnv(&b, []cfg.EnvVar{{Name: "var", Value: s}})
+		var want string
+		if runtime.GOOS == "windows" {
+			fmt.Fprintf(&b, "echo \"%%var%%\"\n")
+			want += "\"" + s + "\"\r\n"
+		} else {
+			fmt.Fprintf(&b, "printf '%%s\\n' \"$var\"\n")
+			want += s + "\n"
+		}
+		scriptfilename := "script.sh"
+		if runtime.GOOS == "windows" {
+			scriptfilename = "script.bat"
+		}
+		var cmd *exec.Cmd
+		if runtime.GOOS == "windows" {
+			scriptfile := filepath.Join(t.TempDir(), scriptfilename)
+			if err := os.WriteFile(scriptfile, b.Bytes(), 0777); err != nil {
+				t.Fatal(err)
+			}
+			cmd = testenv.Command(t, "cmd.exe", "/C", scriptfile)
+		} else {
+			cmd = testenv.Command(t, "sh", "-c", b.String())
+		}
+		out, err := cmd.Output()
+		t.Log(string(out))
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if string(out) != want {
+			t.Fatalf("output of running PrintEnv script and echoing variable: got: %q, want: %q",
+				string(out), want)
+		}
+	})
+}
diff --git a/src/cmd/go/internal/fix/fix.go b/src/cmd/go/internal/fix/fix.go
new file mode 100644
index 0000000..3705b30
--- /dev/null
+++ b/src/cmd/go/internal/fix/fix.go
@@ -0,0 +1,85 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package fix implements the “go fix” command.
+package fix
+
+import (
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/load"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/str"
+	"cmd/go/internal/work"
+	"context"
+	"fmt"
+	"go/build"
+	"os"
+)
+
+var CmdFix = &base.Command{
+	UsageLine: "go fix [-fix list] [packages]",
+	Short:     "update packages to use new APIs",
+	Long: `
+Fix runs the Go fix command on the packages named by the import paths.
+
+The -fix flag sets a comma-separated list of fixes to run.
+The default is all known fixes.
+(Its value is passed to 'go tool fix -r'.)
+
+For more about fix, see 'go doc cmd/fix'.
+For more about specifying packages, see 'go help packages'.
+
+To run fix with other options, run 'go tool fix'.
+
+See also: go fmt, go vet.
+	`,
+}
+
+var fixes = CmdFix.Flag.String("fix", "", "comma-separated list of fixes to apply")
+
+func init() {
+	work.AddBuildFlags(CmdFix, work.DefaultBuildFlags)
+	CmdFix.Run = runFix // fix cycle
+}
+
+func runFix(ctx context.Context, cmd *base.Command, args []string) {
+	pkgs := load.PackagesAndErrors(ctx, load.PackageOpts{}, args)
+	w := 0
+	for _, pkg := range pkgs {
+		if pkg.Error != nil {
+			base.Errorf("%v", pkg.Error)
+			continue
+		}
+		pkgs[w] = pkg
+		w++
+	}
+	pkgs = pkgs[:w]
+
+	printed := false
+	for _, pkg := range pkgs {
+		if modload.Enabled() && pkg.Module != nil && !pkg.Module.Main {
+			if !printed {
+				fmt.Fprintf(os.Stderr, "go: not fixing packages in dependency modules\n")
+				printed = true
+			}
+			continue
+		}
+		// Use pkg.gofiles instead of pkg.Dir so that
+		// the command only applies to this package,
+		// not to packages in subdirectories.
+		files := base.RelPaths(pkg.InternalAllGoFiles())
+		goVersion := ""
+		if pkg.Module != nil {
+			goVersion = "go" + pkg.Module.GoVersion
+		} else if pkg.Standard {
+			goVersion = build.Default.ReleaseTags[len(build.Default.ReleaseTags)-1]
+		}
+		var fixArg []string
+		if *fixes != "" {
+			fixArg = []string{"-r=" + *fixes}
+		}
+		base.Run(str.StringList(cfg.BuildToolexec, base.Tool("fix"), "-go="+goVersion, fixArg, files))
+	}
+}
diff --git a/src/cmd/go/internal/fmtcmd/fmt.go b/src/cmd/go/internal/fmtcmd/fmt.go
new file mode 100644
index 0000000..62b22f6
--- /dev/null
+++ b/src/cmd/go/internal/fmtcmd/fmt.go
@@ -0,0 +1,115 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package fmtcmd implements the “go fmt” command.
+package fmtcmd
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/load"
+	"cmd/go/internal/modload"
+	"cmd/internal/sys"
+)
+
+func init() {
+	base.AddBuildFlagsNX(&CmdFmt.Flag)
+	base.AddChdirFlag(&CmdFmt.Flag)
+	base.AddModFlag(&CmdFmt.Flag)
+	base.AddModCommonFlags(&CmdFmt.Flag)
+}
+
+var CmdFmt = &base.Command{
+	Run:       runFmt,
+	UsageLine: "go fmt [-n] [-x] [packages]",
+	Short:     "gofmt (reformat) package sources",
+	Long: `
+Fmt runs the command 'gofmt -l -w' on the packages named
+by the import paths. It prints the names of the files that are modified.
+
+For more about gofmt, see 'go doc cmd/gofmt'.
+For more about specifying packages, see 'go help packages'.
+
+The -n flag prints commands that would be executed.
+The -x flag prints commands as they are executed.
+
+The -mod flag's value sets which module download mode
+to use: readonly or vendor. See 'go help modules' for more.
+
+To run gofmt with specific options, run gofmt itself.
+
+See also: go fix, go vet.
+	`,
+}
+
+func runFmt(ctx context.Context, cmd *base.Command, args []string) {
+	printed := false
+	gofmt := gofmtPath()
+
+	gofmtArgs := []string{gofmt, "-l", "-w"}
+	gofmtArgLen := len(gofmt) + len(" -l -w")
+
+	baseGofmtArgs := len(gofmtArgs)
+	baseGofmtArgLen := gofmtArgLen
+
+	for _, pkg := range load.PackagesAndErrors(ctx, load.PackageOpts{}, args) {
+		if modload.Enabled() && pkg.Module != nil && !pkg.Module.Main {
+			if !printed {
+				fmt.Fprintf(os.Stderr, "go: not formatting packages in dependency modules\n")
+				printed = true
+			}
+			continue
+		}
+		if pkg.Error != nil {
+			var nogo *load.NoGoError
+			var embed *load.EmbedError
+			if (errors.As(pkg.Error, &nogo) || errors.As(pkg.Error, &embed)) && len(pkg.InternalAllGoFiles()) > 0 {
+				// Skip this error, as we will format
+				// all files regardless.
+			} else {
+				base.Errorf("%v", pkg.Error)
+				continue
+			}
+		}
+		// Use pkg.gofiles instead of pkg.Dir so that
+		// the command only applies to this package,
+		// not to packages in subdirectories.
+		files := base.RelPaths(pkg.InternalAllGoFiles())
+		for _, file := range files {
+			gofmtArgs = append(gofmtArgs, file)
+			gofmtArgLen += 1 + len(file) // plus separator
+			if gofmtArgLen >= sys.ExecArgLengthLimit {
+				base.Run(gofmtArgs)
+				gofmtArgs = gofmtArgs[:baseGofmtArgs]
+				gofmtArgLen = baseGofmtArgLen
+			}
+		}
+	}
+	if len(gofmtArgs) > baseGofmtArgs {
+		base.Run(gofmtArgs)
+	}
+}
+
+func gofmtPath() string {
+	gofmt := "gofmt" + cfg.ToolExeSuffix()
+
+	gofmtPath := filepath.Join(cfg.GOBIN, gofmt)
+	if _, err := os.Stat(gofmtPath); err == nil {
+		return gofmtPath
+	}
+
+	gofmtPath = filepath.Join(cfg.GOROOT, "bin", gofmt)
+	if _, err := os.Stat(gofmtPath); err == nil {
+		return gofmtPath
+	}
+
+	// fallback to looking for gofmt in $PATH
+	return "gofmt"
+}
diff --git a/src/cmd/go/internal/fsys/fsys.go b/src/cmd/go/internal/fsys/fsys.go
new file mode 100644
index 0000000..b83c5a3
--- /dev/null
+++ b/src/cmd/go/internal/fsys/fsys.go
@@ -0,0 +1,784 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package fsys is an abstraction for reading files that
+// allows for virtual overlays on top of the files on disk.
+package fsys
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"internal/godebug"
+	"io/fs"
+	"log"
+	"os"
+	pathpkg "path"
+	"path/filepath"
+	"runtime"
+	"runtime/debug"
+	"sort"
+	"strings"
+	"sync"
+	"time"
+)
+
+// Trace emits a trace event for the operation and file path to the trace log,
+// but only when $GODEBUG contains gofsystrace=1.
+// The traces are appended to the file named by the $GODEBUG setting gofsystracelog, or else standard error.
+// For debugging, if the $GODEBUG setting gofsystracestack is non-empty, then trace events for paths
+// matching that glob pattern (using path.Match) will be followed by a full stack trace.
+func Trace(op, path string) {
+	if !doTrace {
+		return
+	}
+	traceMu.Lock()
+	defer traceMu.Unlock()
+	fmt.Fprintf(traceFile, "%d gofsystrace %s %s\n", os.Getpid(), op, path)
+	if pattern := gofsystracestack.Value(); pattern != "" {
+		if match, _ := pathpkg.Match(pattern, path); match {
+			traceFile.Write(debug.Stack())
+		}
+	}
+}
+
+var (
+	doTrace   bool
+	traceFile *os.File
+	traceMu   sync.Mutex
+
+	gofsystrace      = godebug.New("#gofsystrace")
+	gofsystracelog   = godebug.New("#gofsystracelog")
+	gofsystracestack = godebug.New("#gofsystracestack")
+)
+
+func init() {
+	if gofsystrace.Value() != "1" {
+		return
+	}
+	doTrace = true
+	if f := gofsystracelog.Value(); f != "" {
+		// Note: No buffering on writes to this file, so no need to worry about closing it at exit.
+		var err error
+		traceFile, err = os.OpenFile(f, os.O_WRONLY|os.O_APPEND|os.O_CREATE, 0666)
+		if err != nil {
+			log.Fatal(err)
+		}
+	} else {
+		traceFile = os.Stderr
+	}
+}
+
+// OverlayFile is the path to a text file in the OverlayJSON format.
+// It is the value of the -overlay flag.
+var OverlayFile string
+
+// OverlayJSON is the format overlay files are expected to be in.
+// The Replace map maps from overlaid paths to replacement paths:
+// the Go command will forward all reads trying to open
+// each overlaid path to its replacement path, or consider the overlaid
+// path not to exist if the replacement path is empty.
+type OverlayJSON struct {
+	Replace map[string]string
+}
+
+type node struct {
+	actualFilePath string           // empty if a directory
+	children       map[string]*node // path element → file or directory
+}
+
+func (n *node) isDir() bool {
+	return n.actualFilePath == "" && n.children != nil
+}
+
+func (n *node) isDeleted() bool {
+	return n.actualFilePath == "" && n.children == nil
+}
+
+// TODO(matloob): encapsulate these in an io/fs-like interface
+var overlay map[string]*node // path -> file or directory node
+var cwd string               // copy of base.Cwd() to avoid dependency
+
+// canonicalize a path for looking it up in the overlay.
+// Important: filepath.Join(cwd, path) doesn't always produce
+// the correct absolute path if path is relative, because on
+// Windows producing the correct absolute path requires making
+// a syscall. So this should only be used when looking up paths
+// in the overlay, or canonicalizing the paths in the overlay.
+func canonicalize(path string) string {
+	if path == "" {
+		return ""
+	}
+	if filepath.IsAbs(path) {
+		return filepath.Clean(path)
+	}
+
+	if v := filepath.VolumeName(cwd); v != "" && path[0] == filepath.Separator {
+		// On Windows filepath.Join(cwd, path) doesn't always work. In general
+		// filepath.Abs needs to make a syscall on Windows. Elsewhere in cmd/go
+		// use filepath.Join(cwd, path), but cmd/go specifically supports Windows
+		// paths that start with "\" which implies the path is relative to the
+		// volume of the working directory. See golang.org/issue/8130.
+		return filepath.Join(v, path)
+	}
+
+	// Make the path absolute.
+	return filepath.Join(cwd, path)
+}
+
+// Init initializes the overlay, if one is being used.
+func Init(wd string) error {
+	if overlay != nil {
+		// already initialized
+		return nil
+	}
+
+	cwd = wd
+
+	if OverlayFile == "" {
+		return nil
+	}
+
+	Trace("ReadFile", OverlayFile)
+	b, err := os.ReadFile(OverlayFile)
+	if err != nil {
+		return fmt.Errorf("reading overlay file: %v", err)
+	}
+
+	var overlayJSON OverlayJSON
+	if err := json.Unmarshal(b, &overlayJSON); err != nil {
+		return fmt.Errorf("parsing overlay JSON: %v", err)
+	}
+
+	return initFromJSON(overlayJSON)
+}
+
+func initFromJSON(overlayJSON OverlayJSON) error {
+	// Canonicalize the paths in the overlay map.
+	// Use reverseCanonicalized to check for collisions:
+	// no two 'from' paths should canonicalize to the same path.
+	overlay = make(map[string]*node)
+	reverseCanonicalized := make(map[string]string) // inverse of canonicalize operation, to check for duplicates
+	// Build a table of file and directory nodes from the replacement map.
+
+	// Remove any potential non-determinism from iterating over map by sorting it.
+	replaceFrom := make([]string, 0, len(overlayJSON.Replace))
+	for k := range overlayJSON.Replace {
+		replaceFrom = append(replaceFrom, k)
+	}
+	sort.Strings(replaceFrom)
+
+	for _, from := range replaceFrom {
+		to := overlayJSON.Replace[from]
+		// Canonicalize paths and check for a collision.
+		if from == "" {
+			return fmt.Errorf("empty string key in overlay file Replace map")
+		}
+		cfrom := canonicalize(from)
+		if to != "" {
+			// Don't canonicalize "", meaning to delete a file, because then it will turn into ".".
+			to = canonicalize(to)
+		}
+		if otherFrom, seen := reverseCanonicalized[cfrom]; seen {
+			return fmt.Errorf(
+				"paths %q and %q both canonicalize to %q in overlay file Replace map", otherFrom, from, cfrom)
+		}
+		reverseCanonicalized[cfrom] = from
+		from = cfrom
+
+		// Create node for overlaid file.
+		dir, base := filepath.Dir(from), filepath.Base(from)
+		if n, ok := overlay[from]; ok {
+			// All 'from' paths in the overlay are file paths. Since the from paths
+			// are in a map, they are unique, so if the node already exists we added
+			// it below when we create parent directory nodes. That is, that
+			// both a file and a path to one of its parent directories exist as keys
+			// in the Replace map.
+			//
+			// This only applies if the overlay directory has any files or directories
+			// in it: placeholder directories that only contain deleted files don't
+			// count. They are safe to be overwritten with actual files.
+			for _, f := range n.children {
+				if !f.isDeleted() {
+					return fmt.Errorf("invalid overlay: path %v is used as both file and directory", from)
+				}
+			}
+		}
+		overlay[from] = &node{actualFilePath: to}
+
+		// Add parent directory nodes to overlay structure.
+		childNode := overlay[from]
+		for {
+			dirNode := overlay[dir]
+			if dirNode == nil || dirNode.isDeleted() {
+				dirNode = &node{children: make(map[string]*node)}
+				overlay[dir] = dirNode
+			}
+			if childNode.isDeleted() {
+				// Only create one parent for a deleted file:
+				// the directory only conditionally exists if
+				// there are any non-deleted children, so
+				// we don't create their parents.
+				if dirNode.isDir() {
+					dirNode.children[base] = childNode
+				}
+				break
+			}
+			if !dirNode.isDir() {
+				// This path already exists as a file, so it can't be a parent
+				// directory. See comment at error above.
+				return fmt.Errorf("invalid overlay: path %v is used as both file and directory", dir)
+			}
+			dirNode.children[base] = childNode
+			parent := filepath.Dir(dir)
+			if parent == dir {
+				break // reached the top; there is no parent
+			}
+			dir, base = parent, filepath.Base(dir)
+			childNode = dirNode
+		}
+	}
+
+	return nil
+}
+
+// IsDir returns true if path is a directory on disk or in the
+// overlay.
+func IsDir(path string) (bool, error) {
+	Trace("IsDir", path)
+	path = canonicalize(path)
+
+	if _, ok := parentIsOverlayFile(path); ok {
+		return false, nil
+	}
+
+	if n, ok := overlay[path]; ok {
+		return n.isDir(), nil
+	}
+
+	fi, err := os.Stat(path)
+	if err != nil {
+		return false, err
+	}
+
+	return fi.IsDir(), nil
+}
+
+// parentIsOverlayFile returns whether name or any of
+// its parents are files in the overlay, and the first parent found,
+// including name itself, that's a file in the overlay.
+func parentIsOverlayFile(name string) (string, bool) {
+	if overlay != nil {
+		// Check if name can't possibly be a directory because
+		// it or one of its parents is overlaid with a file.
+		// TODO(matloob): Maybe save this to avoid doing it every time?
+		prefix := name
+		for {
+			node := overlay[prefix]
+			if node != nil && !node.isDir() {
+				return prefix, true
+			}
+			parent := filepath.Dir(prefix)
+			if parent == prefix {
+				break
+			}
+			prefix = parent
+		}
+	}
+
+	return "", false
+}
+
+// errNotDir is used to communicate from ReadDir to IsDirWithGoFiles
+// that the argument is not a directory, so that IsDirWithGoFiles doesn't
+// return an error.
+var errNotDir = errors.New("not a directory")
+
+func nonFileInOverlayError(overlayPath string) error {
+	return fmt.Errorf("replacement path %q is a directory, not a file", overlayPath)
+}
+
+// readDir reads a dir on disk, returning an error that is errNotDir if the dir is not a directory.
+// Unfortunately, the error returned by os.ReadDir if dir is not a directory
+// can vary depending on the OS (Linux, Mac, Windows return ENOTDIR; BSD returns EINVAL).
+func readDir(dir string) ([]fs.FileInfo, error) {
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, err
+		}
+		if dirfi, staterr := os.Stat(dir); staterr == nil && !dirfi.IsDir() {
+			return nil, &fs.PathError{Op: "ReadDir", Path: dir, Err: errNotDir}
+		}
+		return nil, err
+	}
+
+	fis := make([]fs.FileInfo, 0, len(entries))
+	for _, entry := range entries {
+		info, err := entry.Info()
+		if err != nil {
+			continue
+		}
+		fis = append(fis, info)
+	}
+	return fis, nil
+}
+
+// ReadDir provides a slice of fs.FileInfo entries corresponding
+// to the overlaid files in the directory.
+func ReadDir(dir string) ([]fs.FileInfo, error) {
+	Trace("ReadDir", dir)
+	dir = canonicalize(dir)
+	if _, ok := parentIsOverlayFile(dir); ok {
+		return nil, &fs.PathError{Op: "ReadDir", Path: dir, Err: errNotDir}
+	}
+
+	dirNode := overlay[dir]
+	if dirNode == nil {
+		return readDir(dir)
+	}
+	if dirNode.isDeleted() {
+		return nil, &fs.PathError{Op: "ReadDir", Path: dir, Err: fs.ErrNotExist}
+	}
+	diskfis, err := readDir(dir)
+	if err != nil && !os.IsNotExist(err) && !errors.Is(err, errNotDir) {
+		return nil, err
+	}
+
+	// Stat files in overlay to make composite list of fileinfos
+	files := make(map[string]fs.FileInfo)
+	for _, f := range diskfis {
+		files[f.Name()] = f
+	}
+	for name, to := range dirNode.children {
+		switch {
+		case to.isDir():
+			files[name] = fakeDir(name)
+		case to.isDeleted():
+			delete(files, name)
+		default:
+			// To keep the data model simple, if the overlay contains a symlink we
+			// always stat through it (using Stat, not Lstat). That way we don't need
+			// to worry about the interaction between Lstat and directories: if a
+			// symlink in the overlay points to a directory, we reject it like an
+			// ordinary directory.
+			fi, err := os.Stat(to.actualFilePath)
+			if err != nil {
+				files[name] = missingFile(name)
+				continue
+			} else if fi.IsDir() {
+				return nil, &fs.PathError{Op: "Stat", Path: filepath.Join(dir, name), Err: nonFileInOverlayError(to.actualFilePath)}
+			}
+			// Add a fileinfo for the overlaid file, so that it has
+			// the original file's name, but the overlaid file's metadata.
+			files[name] = fakeFile{name, fi}
+		}
+	}
+	sortedFiles := diskfis[:0]
+	for _, f := range files {
+		sortedFiles = append(sortedFiles, f)
+	}
+	sort.Slice(sortedFiles, func(i, j int) bool { return sortedFiles[i].Name() < sortedFiles[j].Name() })
+	return sortedFiles, nil
+}
+
+// OverlayPath returns the path to the overlaid contents of the
+// file, the empty string if the overlay deletes the file, or path
+// itself if the file is not in the overlay, the file is a directory
+// in the overlay, or there is no overlay.
+// It returns true if the path is overlaid with a regular file
+// or deleted, and false otherwise.
+func OverlayPath(path string) (string, bool) {
+	if p, ok := overlay[canonicalize(path)]; ok && !p.isDir() {
+		return p.actualFilePath, ok
+	}
+
+	return path, false
+}
+
+// Open opens the file at or overlaid on the given path.
+func Open(path string) (*os.File, error) {
+	Trace("Open", path)
+	return openFile(path, os.O_RDONLY, 0)
+}
+
+// OpenFile opens the file at or overlaid on the given path with the flag and perm.
+func OpenFile(path string, flag int, perm os.FileMode) (*os.File, error) {
+	Trace("OpenFile", path)
+	return openFile(path, flag, perm)
+}
+
+func openFile(path string, flag int, perm os.FileMode) (*os.File, error) {
+	cpath := canonicalize(path)
+	if node, ok := overlay[cpath]; ok {
+		// Opening a file in the overlay.
+		if node.isDir() {
+			return nil, &fs.PathError{Op: "OpenFile", Path: path, Err: errors.New("fsys.OpenFile doesn't support opening directories yet")}
+		}
+		// We can't open overlaid paths for write.
+		if perm != os.FileMode(os.O_RDONLY) {
+			return nil, &fs.PathError{Op: "OpenFile", Path: path, Err: errors.New("overlaid files can't be opened for write")}
+		}
+		return os.OpenFile(node.actualFilePath, flag, perm)
+	}
+	if parent, ok := parentIsOverlayFile(filepath.Dir(cpath)); ok {
+		// The file is deleted explicitly in the Replace map,
+		// or implicitly because one of its parent directories was
+		// replaced by a file.
+		return nil, &fs.PathError{
+			Op:   "Open",
+			Path: path,
+			Err:  fmt.Errorf("file %s does not exist: parent directory %s is replaced by a file in overlay", path, parent),
+		}
+	}
+	return os.OpenFile(cpath, flag, perm)
+}
+
+// IsDirWithGoFiles reports whether dir is a directory containing Go files
+// either on disk or in the overlay.
+func IsDirWithGoFiles(dir string) (bool, error) {
+	Trace("IsDirWithGoFiles", dir)
+	fis, err := ReadDir(dir)
+	if os.IsNotExist(err) || errors.Is(err, errNotDir) {
+		return false, nil
+	}
+	if err != nil {
+		return false, err
+	}
+
+	var firstErr error
+	for _, fi := range fis {
+		if fi.IsDir() {
+			continue
+		}
+
+		// TODO(matloob): this enforces that the "from" in the map
+		// has a .go suffix, but the actual destination file
+		// doesn't need to have a .go suffix. Is this okay with the
+		// compiler?
+		if !strings.HasSuffix(fi.Name(), ".go") {
+			continue
+		}
+		if fi.Mode().IsRegular() {
+			return true, nil
+		}
+
+		// fi is the result of an Lstat, so it doesn't follow symlinks.
+		// But it's okay if the file is a symlink pointing to a regular
+		// file, so use os.Stat to follow symlinks and check that.
+		actualFilePath, _ := OverlayPath(filepath.Join(dir, fi.Name()))
+		fi, err := os.Stat(actualFilePath)
+		if err == nil && fi.Mode().IsRegular() {
+			return true, nil
+		}
+		if err != nil && firstErr == nil {
+			firstErr = err
+		}
+	}
+
+	// No go files found in directory.
+	return false, firstErr
+}
+
+// walk recursively descends path, calling walkFn. Copied, with some
+// modifications from path/filepath.walk.
+func walk(path string, info fs.FileInfo, walkFn filepath.WalkFunc) error {
+	if err := walkFn(path, info, nil); err != nil || !info.IsDir() {
+		return err
+	}
+
+	fis, err := ReadDir(path)
+	if err != nil {
+		return walkFn(path, info, err)
+	}
+
+	for _, fi := range fis {
+		filename := filepath.Join(path, fi.Name())
+		if err := walk(filename, fi, walkFn); err != nil {
+			if !fi.IsDir() || err != filepath.SkipDir {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+// Walk walks the file tree rooted at root, calling walkFn for each file or
+// directory in the tree, including root.
+func Walk(root string, walkFn filepath.WalkFunc) error {
+	Trace("Walk", root)
+	info, err := Lstat(root)
+	if err != nil {
+		err = walkFn(root, nil, err)
+	} else {
+		err = walk(root, info, walkFn)
+	}
+	if err == filepath.SkipDir {
+		return nil
+	}
+	return err
+}
+
+// Lstat implements a version of os.Lstat that operates on the overlay filesystem.
+func Lstat(path string) (fs.FileInfo, error) {
+	Trace("Lstat", path)
+	return overlayStat(path, os.Lstat, "lstat")
+}
+
+// Stat implements a version of os.Stat that operates on the overlay filesystem.
+func Stat(path string) (fs.FileInfo, error) {
+	Trace("Stat", path)
+	return overlayStat(path, os.Stat, "stat")
+}
+
+// overlayStat implements lstat or Stat (depending on whether os.Lstat or os.Stat is passed in).
+func overlayStat(path string, osStat func(string) (fs.FileInfo, error), opName string) (fs.FileInfo, error) {
+	cpath := canonicalize(path)
+
+	if _, ok := parentIsOverlayFile(filepath.Dir(cpath)); ok {
+		return nil, &fs.PathError{Op: opName, Path: cpath, Err: fs.ErrNotExist}
+	}
+
+	node, ok := overlay[cpath]
+	if !ok {
+		// The file or directory is not overlaid.
+		return osStat(path)
+	}
+
+	switch {
+	case node.isDeleted():
+		return nil, &fs.PathError{Op: opName, Path: cpath, Err: fs.ErrNotExist}
+	case node.isDir():
+		return fakeDir(filepath.Base(path)), nil
+	default:
+		// To keep the data model simple, if the overlay contains a symlink we
+		// always stat through it (using Stat, not Lstat). That way we don't need to
+		// worry about the interaction between Lstat and directories: if a symlink
+		// in the overlay points to a directory, we reject it like an ordinary
+		// directory.
+		fi, err := os.Stat(node.actualFilePath)
+		if err != nil {
+			return nil, err
+		}
+		if fi.IsDir() {
+			return nil, &fs.PathError{Op: opName, Path: cpath, Err: nonFileInOverlayError(node.actualFilePath)}
+		}
+		return fakeFile{name: filepath.Base(path), real: fi}, nil
+	}
+}
+
+// fakeFile provides an fs.FileInfo implementation for an overlaid file,
+// so that the file has the name of the overlaid file, but takes all
+// other characteristics of the replacement file.
+type fakeFile struct {
+	name string
+	real fs.FileInfo
+}
+
+func (f fakeFile) Name() string       { return f.name }
+func (f fakeFile) Size() int64        { return f.real.Size() }
+func (f fakeFile) Mode() fs.FileMode  { return f.real.Mode() }
+func (f fakeFile) ModTime() time.Time { return f.real.ModTime() }
+func (f fakeFile) IsDir() bool        { return f.real.IsDir() }
+func (f fakeFile) Sys() any           { return f.real.Sys() }
+
+func (f fakeFile) String() string {
+	return fs.FormatFileInfo(f)
+}
+
+// missingFile provides an fs.FileInfo for an overlaid file where the
+// destination file in the overlay doesn't exist. It returns zero values
+// for the fileInfo methods other than Name, set to the file's name, and Mode
+// set to ModeIrregular.
+type missingFile string
+
+func (f missingFile) Name() string       { return string(f) }
+func (f missingFile) Size() int64        { return 0 }
+func (f missingFile) Mode() fs.FileMode  { return fs.ModeIrregular }
+func (f missingFile) ModTime() time.Time { return time.Unix(0, 0) }
+func (f missingFile) IsDir() bool        { return false }
+func (f missingFile) Sys() any           { return nil }
+
+func (f missingFile) String() string {
+	return fs.FormatFileInfo(f)
+}
+
+// fakeDir provides an fs.FileInfo implementation for directories that are
+// implicitly created by overlaid files. Each directory in the
+// path of an overlaid file is considered to exist in the overlay filesystem.
+type fakeDir string
+
+func (f fakeDir) Name() string       { return string(f) }
+func (f fakeDir) Size() int64        { return 0 }
+func (f fakeDir) Mode() fs.FileMode  { return fs.ModeDir | 0500 }
+func (f fakeDir) ModTime() time.Time { return time.Unix(0, 0) }
+func (f fakeDir) IsDir() bool        { return true }
+func (f fakeDir) Sys() any           { return nil }
+
+func (f fakeDir) String() string {
+	return fs.FormatFileInfo(f)
+}
+
+// Glob is like filepath.Glob but uses the overlay file system.
+func Glob(pattern string) (matches []string, err error) {
+	Trace("Glob", pattern)
+	// Check pattern is well-formed.
+	if _, err := filepath.Match(pattern, ""); err != nil {
+		return nil, err
+	}
+	if !hasMeta(pattern) {
+		if _, err = Lstat(pattern); err != nil {
+			return nil, nil
+		}
+		return []string{pattern}, nil
+	}
+
+	dir, file := filepath.Split(pattern)
+	volumeLen := 0
+	if runtime.GOOS == "windows" {
+		volumeLen, dir = cleanGlobPathWindows(dir)
+	} else {
+		dir = cleanGlobPath(dir)
+	}
+
+	if !hasMeta(dir[volumeLen:]) {
+		return glob(dir, file, nil)
+	}
+
+	// Prevent infinite recursion. See issue 15879.
+	if dir == pattern {
+		return nil, filepath.ErrBadPattern
+	}
+
+	var m []string
+	m, err = Glob(dir)
+	if err != nil {
+		return
+	}
+	for _, d := range m {
+		matches, err = glob(d, file, matches)
+		if err != nil {
+			return
+		}
+	}
+	return
+}
+
+// cleanGlobPath prepares path for glob matching.
+func cleanGlobPath(path string) string {
+	switch path {
+	case "":
+		return "."
+	case string(filepath.Separator):
+		// do nothing to the path
+		return path
+	default:
+		return path[0 : len(path)-1] // chop off trailing separator
+	}
+}
+
+func volumeNameLen(path string) int {
+	isSlash := func(c uint8) bool {
+		return c == '\\' || c == '/'
+	}
+	if len(path) < 2 {
+		return 0
+	}
+	// with drive letter
+	c := path[0]
+	if path[1] == ':' && ('a' <= c && c <= 'z' || 'A' <= c && c <= 'Z') {
+		return 2
+	}
+	// is it UNC? https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx
+	if l := len(path); l >= 5 && isSlash(path[0]) && isSlash(path[1]) &&
+		!isSlash(path[2]) && path[2] != '.' {
+		// first, leading `\\` and next shouldn't be `\`. its server name.
+		for n := 3; n < l-1; n++ {
+			// second, next '\' shouldn't be repeated.
+			if isSlash(path[n]) {
+				n++
+				// third, following something characters. its share name.
+				if !isSlash(path[n]) {
+					if path[n] == '.' {
+						break
+					}
+					for ; n < l; n++ {
+						if isSlash(path[n]) {
+							break
+						}
+					}
+					return n
+				}
+				break
+			}
+		}
+	}
+	return 0
+}
+
+// cleanGlobPathWindows is windows version of cleanGlobPath.
+func cleanGlobPathWindows(path string) (prefixLen int, cleaned string) {
+	vollen := volumeNameLen(path)
+	switch {
+	case path == "":
+		return 0, "."
+	case vollen+1 == len(path) && os.IsPathSeparator(path[len(path)-1]): // /, \, C:\ and C:/
+		// do nothing to the path
+		return vollen + 1, path
+	case vollen == len(path) && len(path) == 2: // C:
+		return vollen, path + "." // convert C: into C:.
+	default:
+		if vollen >= len(path) {
+			vollen = len(path) - 1
+		}
+		return vollen, path[0 : len(path)-1] // chop off trailing separator
+	}
+}
+
+// glob searches for files matching pattern in the directory dir
+// and appends them to matches. If the directory cannot be
+// opened, it returns the existing matches. New matches are
+// added in lexicographical order.
+func glob(dir, pattern string, matches []string) (m []string, e error) {
+	m = matches
+	fi, err := Stat(dir)
+	if err != nil {
+		return // ignore I/O error
+	}
+	if !fi.IsDir() {
+		return // ignore I/O error
+	}
+
+	list, err := ReadDir(dir)
+	if err != nil {
+		return // ignore I/O error
+	}
+
+	var names []string
+	for _, info := range list {
+		names = append(names, info.Name())
+	}
+	sort.Strings(names)
+
+	for _, n := range names {
+		matched, err := filepath.Match(pattern, n)
+		if err != nil {
+			return m, err
+		}
+		if matched {
+			m = append(m, filepath.Join(dir, n))
+		}
+	}
+	return
+}
+
+// hasMeta reports whether path contains any of the magic characters
+// recognized by filepath.Match.
+func hasMeta(path string) bool {
+	magicChars := `*?[`
+	if runtime.GOOS != "windows" {
+		magicChars = `*?[\`
+	}
+	return strings.ContainsAny(path, magicChars)
+}
diff --git a/src/cmd/go/internal/fsys/fsys_test.go b/src/cmd/go/internal/fsys/fsys_test.go
new file mode 100644
index 0000000..2ab2bb2
--- /dev/null
+++ b/src/cmd/go/internal/fsys/fsys_test.go
@@ -0,0 +1,1139 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fsys
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"internal/testenv"
+	"internal/txtar"
+	"io"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"reflect"
+	"testing"
+)
+
+// initOverlay resets the overlay state to reflect the config.
+// config should be a text archive string. The comment is the overlay config
+// json, and the files, in the archive are laid out in a temp directory
+// that cwd is set to.
+func initOverlay(t *testing.T, config string) {
+	t.Helper()
+
+	// Create a temporary directory and chdir to it.
+	prevwd, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	cwd = filepath.Join(t.TempDir(), "root")
+	if err := os.Mkdir(cwd, 0777); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Chdir(cwd); err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() {
+		overlay = nil
+		if err := os.Chdir(prevwd); err != nil {
+			t.Fatal(err)
+		}
+	})
+
+	a := txtar.Parse([]byte(config))
+	for _, f := range a.Files {
+		name := filepath.Join(cwd, f.Name)
+		if err := os.MkdirAll(filepath.Dir(name), 0777); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(name, f.Data, 0666); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	var overlayJSON OverlayJSON
+	if err := json.Unmarshal(a.Comment, &overlayJSON); err != nil {
+		t.Fatal(fmt.Errorf("parsing overlay JSON: %v", err))
+	}
+
+	initFromJSON(overlayJSON)
+}
+
+func TestIsDir(t *testing.T) {
+	initOverlay(t, `
+{
+	"Replace": {
+		"subdir2/file2.txt":  "overlayfiles/subdir2_file2.txt",
+		"subdir4":            "overlayfiles/subdir4",
+		"subdir3/file3b.txt": "overlayfiles/subdir3_file3b.txt",
+		"subdir5":            "",
+		"subdir6":            ""
+	}
+}
+-- subdir1/file1.txt --
+
+-- subdir3/file3a.txt --
+33
+-- subdir4/file4.txt --
+444
+-- overlayfiles/subdir2_file2.txt --
+2
+-- overlayfiles/subdir3_file3b.txt --
+66666
+-- overlayfiles/subdir4 --
+x
+-- subdir6/file6.txt --
+six
+`)
+
+	testCases := []struct {
+		path          string
+		want, wantErr bool
+	}{
+		{"", true, true},
+		{".", true, false},
+		{cwd, true, false},
+		{cwd + string(filepath.Separator), true, false},
+		// subdir1 is only on disk
+		{filepath.Join(cwd, "subdir1"), true, false},
+		{"subdir1", true, false},
+		{"subdir1" + string(filepath.Separator), true, false},
+		{"subdir1/file1.txt", false, false},
+		{"subdir1/doesntexist.txt", false, true},
+		{"doesntexist", false, true},
+		// subdir2 is only in overlay
+		{filepath.Join(cwd, "subdir2"), true, false},
+		{"subdir2", true, false},
+		{"subdir2" + string(filepath.Separator), true, false},
+		{"subdir2/file2.txt", false, false},
+		{"subdir2/doesntexist.txt", false, true},
+		// subdir3 has files on disk and in overlay
+		{filepath.Join(cwd, "subdir3"), true, false},
+		{"subdir3", true, false},
+		{"subdir3" + string(filepath.Separator), true, false},
+		{"subdir3/file3a.txt", false, false},
+		{"subdir3/file3b.txt", false, false},
+		{"subdir3/doesntexist.txt", false, true},
+		// subdir4 is overlaid with a file
+		{filepath.Join(cwd, "subdir4"), false, false},
+		{"subdir4", false, false},
+		{"subdir4" + string(filepath.Separator), false, false},
+		{"subdir4/file4.txt", false, false},
+		{"subdir4/doesntexist.txt", false, false},
+		// subdir5 doesn't exist, and is overlaid with a "delete" entry
+		{filepath.Join(cwd, "subdir5"), false, false},
+		{"subdir5", false, false},
+		{"subdir5" + string(filepath.Separator), false, false},
+		{"subdir5/file5.txt", false, false},
+		{"subdir5/doesntexist.txt", false, false},
+		// subdir6 does exist, and is overlaid with a "delete" entry
+		{filepath.Join(cwd, "subdir6"), false, false},
+		{"subdir6", false, false},
+		{"subdir6" + string(filepath.Separator), false, false},
+		{"subdir6/file6.txt", false, false},
+		{"subdir6/doesntexist.txt", false, false},
+	}
+
+	for _, tc := range testCases {
+		got, err := IsDir(tc.path)
+		if err != nil {
+			if !tc.wantErr {
+				t.Errorf("IsDir(%q): got error with string %q, want no error", tc.path, err.Error())
+			}
+			continue
+		}
+		if tc.wantErr {
+			t.Errorf("IsDir(%q): got no error, want error", tc.path)
+		}
+		if tc.want != got {
+			t.Errorf("IsDir(%q) = %v, want %v", tc.path, got, tc.want)
+		}
+	}
+}
+
+const readDirOverlay = `
+{
+	"Replace": {
+		"subdir2/file2.txt":                 "overlayfiles/subdir2_file2.txt",
+		"subdir4":                           "overlayfiles/subdir4",
+		"subdir3/file3b.txt":                "overlayfiles/subdir3_file3b.txt",
+		"subdir5":                           "",
+		"subdir6/asubsubdir/afile.txt":      "overlayfiles/subdir6_asubsubdir_afile.txt",
+		"subdir6/asubsubdir/zfile.txt":      "overlayfiles/subdir6_asubsubdir_zfile.txt",
+		"subdir6/zsubsubdir/file.txt":       "overlayfiles/subdir6_zsubsubdir_file.txt",
+		"subdir7/asubsubdir/file.txt":       "overlayfiles/subdir7_asubsubdir_file.txt",
+		"subdir7/zsubsubdir/file.txt":       "overlayfiles/subdir7_zsubsubdir_file.txt",
+		"subdir8/doesntexist":               "this_file_doesnt_exist_anywhere",
+		"other/pointstodir":                 "overlayfiles/this_is_a_directory",
+		"parentoverwritten/subdir1":         "overlayfiles/parentoverwritten_subdir1",
+		"subdir9/this_file_is_overlaid.txt": "overlayfiles/subdir9_this_file_is_overlaid.txt",
+		"subdir10/only_deleted_file.txt":    "",
+		"subdir11/deleted.txt":              "",
+		"subdir11":                          "overlayfiles/subdir11",
+		"textfile.txt/file.go":              "overlayfiles/textfile_txt_file.go"
+	}
+}
+-- subdir1/file1.txt --
+
+-- subdir3/file3a.txt --
+33
+-- subdir4/file4.txt --
+444
+-- subdir6/file.txt --
+-- subdir6/asubsubdir/file.txt --
+-- subdir6/anothersubsubdir/file.txt --
+-- subdir9/this_file_is_overlaid.txt --
+-- subdir10/only_deleted_file.txt --
+this will be deleted in overlay
+-- subdir11/deleted.txt --
+-- parentoverwritten/subdir1/subdir2/subdir3/file.txt --
+-- textfile.txt --
+this will be overridden by textfile.txt/file.go
+-- overlayfiles/subdir2_file2.txt --
+2
+-- overlayfiles/subdir3_file3b.txt --
+66666
+-- overlayfiles/subdir4 --
+x
+-- overlayfiles/subdir6_asubsubdir_afile.txt --
+-- overlayfiles/subdir6_asubsubdir_zfile.txt --
+-- overlayfiles/subdir6_zsubsubdir_file.txt --
+-- overlayfiles/subdir7_asubsubdir_file.txt --
+-- overlayfiles/subdir7_zsubsubdir_file.txt --
+-- overlayfiles/parentoverwritten_subdir1 --
+x
+-- overlayfiles/subdir9_this_file_is_overlaid.txt --
+99999999
+-- overlayfiles/subdir11 --
+-- overlayfiles/this_is_a_directory/file.txt --
+-- overlayfiles/textfile_txt_file.go --
+x
+`
+
+func TestReadDir(t *testing.T) {
+	initOverlay(t, readDirOverlay)
+
+	type entry struct {
+		name  string
+		size  int64
+		isDir bool
+	}
+
+	testCases := []struct {
+		dir  string
+		want []entry
+	}{
+		{
+			".", []entry{
+				{"other", 0, true},
+				{"overlayfiles", 0, true},
+				{"parentoverwritten", 0, true},
+				{"subdir1", 0, true},
+				{"subdir10", 0, true},
+				{"subdir11", 0, false},
+				{"subdir2", 0, true},
+				{"subdir3", 0, true},
+				{"subdir4", 2, false},
+				// no subdir5.
+				{"subdir6", 0, true},
+				{"subdir7", 0, true},
+				{"subdir8", 0, true},
+				{"subdir9", 0, true},
+				{"textfile.txt", 0, true},
+			},
+		},
+		{
+			"subdir1", []entry{
+				{"file1.txt", 1, false},
+			},
+		},
+		{
+			"subdir2", []entry{
+				{"file2.txt", 2, false},
+			},
+		},
+		{
+			"subdir3", []entry{
+				{"file3a.txt", 3, false},
+				{"file3b.txt", 6, false},
+			},
+		},
+		{
+			"subdir6", []entry{
+				{"anothersubsubdir", 0, true},
+				{"asubsubdir", 0, true},
+				{"file.txt", 0, false},
+				{"zsubsubdir", 0, true},
+			},
+		},
+		{
+			"subdir6/asubsubdir", []entry{
+				{"afile.txt", 0, false},
+				{"file.txt", 0, false},
+				{"zfile.txt", 0, false},
+			},
+		},
+		{
+			"subdir8", []entry{
+				{"doesntexist", 0, false}, // entry is returned even if destination file doesn't exist
+			},
+		},
+		{
+			// check that read dir actually redirects files that already exist
+			// the original this_file_is_overlaid.txt is empty
+			"subdir9", []entry{
+				{"this_file_is_overlaid.txt", 9, false},
+			},
+		},
+		{
+			"subdir10", []entry{},
+		},
+		{
+			"parentoverwritten", []entry{
+				{"subdir1", 2, false},
+			},
+		},
+		{
+			"textfile.txt", []entry{
+				{"file.go", 2, false},
+			},
+		},
+	}
+
+	for _, tc := range testCases {
+		dir, want := tc.dir, tc.want
+		infos, err := ReadDir(dir)
+		if err != nil {
+			t.Errorf("ReadDir(%q): %v", dir, err)
+			continue
+		}
+		// Sorted diff of want and infos.
+		for len(infos) > 0 || len(want) > 0 {
+			switch {
+			case len(want) == 0 || len(infos) > 0 && infos[0].Name() < want[0].name:
+				t.Errorf("ReadDir(%q): unexpected entry: %s IsDir=%v Size=%v", dir, infos[0].Name(), infos[0].IsDir(), infos[0].Size())
+				infos = infos[1:]
+			case len(infos) == 0 || len(want) > 0 && want[0].name < infos[0].Name():
+				t.Errorf("ReadDir(%q): missing entry: %s IsDir=%v Size=%v", dir, want[0].name, want[0].isDir, want[0].size)
+				want = want[1:]
+			default:
+				infoSize := infos[0].Size()
+				if want[0].isDir {
+					infoSize = 0
+				}
+				if infos[0].IsDir() != want[0].isDir || want[0].isDir && infoSize != want[0].size {
+					t.Errorf("ReadDir(%q): %s: IsDir=%v Size=%v, want IsDir=%v Size=%v", dir, want[0].name, infos[0].IsDir(), infoSize, want[0].isDir, want[0].size)
+				}
+				infos = infos[1:]
+				want = want[1:]
+			}
+		}
+	}
+
+	errCases := []string{
+		"subdir1/file1.txt", // regular file on disk
+		"subdir2/file2.txt", // regular file in overlay
+		"subdir4",           // directory overlaid with regular file
+		"subdir5",           // directory deleted in overlay
+		"parentoverwritten/subdir1/subdir2/subdir3", // parentoverwritten/subdir1 overlaid with regular file
+		"parentoverwritten/subdir1/subdir2",         // parentoverwritten/subdir1 overlaid with regular file
+		"subdir11",                                  // directory with deleted child, overlaid with regular file
+		"other/pointstodir",
+	}
+
+	for _, dir := range errCases {
+		_, err := ReadDir(dir)
+		if _, ok := err.(*fs.PathError); !ok {
+			t.Errorf("ReadDir(%q): err = %T (%v), want fs.PathError", dir, err, err)
+		}
+	}
+}
+
+func TestGlob(t *testing.T) {
+	initOverlay(t, readDirOverlay)
+
+	testCases := []struct {
+		pattern string
+		match   []string
+	}{
+		{
+			"*o*",
+			[]string{
+				"other",
+				"overlayfiles",
+				"parentoverwritten",
+			},
+		},
+		{
+			"subdir2/file2.txt",
+			[]string{
+				"subdir2/file2.txt",
+			},
+		},
+		{
+			"*/*.txt",
+			[]string{
+				"overlayfiles/subdir2_file2.txt",
+				"overlayfiles/subdir3_file3b.txt",
+				"overlayfiles/subdir6_asubsubdir_afile.txt",
+				"overlayfiles/subdir6_asubsubdir_zfile.txt",
+				"overlayfiles/subdir6_zsubsubdir_file.txt",
+				"overlayfiles/subdir7_asubsubdir_file.txt",
+				"overlayfiles/subdir7_zsubsubdir_file.txt",
+				"overlayfiles/subdir9_this_file_is_overlaid.txt",
+				"subdir1/file1.txt",
+				"subdir2/file2.txt",
+				"subdir3/file3a.txt",
+				"subdir3/file3b.txt",
+				"subdir6/file.txt",
+				"subdir9/this_file_is_overlaid.txt",
+			},
+		},
+	}
+
+	for _, tc := range testCases {
+		pattern := tc.pattern
+		match, err := Glob(pattern)
+		if err != nil {
+			t.Errorf("Glob(%q): %v", pattern, err)
+			continue
+		}
+		want := tc.match
+		for i, name := range want {
+			if name != tc.pattern {
+				want[i] = filepath.FromSlash(name)
+			}
+		}
+		for len(match) > 0 || len(want) > 0 {
+			switch {
+			case len(match) == 0 || len(want) > 0 && want[0] < match[0]:
+				t.Errorf("Glob(%q): missing match: %s", pattern, want[0])
+				want = want[1:]
+			case len(want) == 0 || len(match) > 0 && match[0] < want[0]:
+				t.Errorf("Glob(%q): extra match: %s", pattern, match[0])
+				match = match[1:]
+			default:
+				want = want[1:]
+				match = match[1:]
+			}
+		}
+	}
+}
+
+func TestOverlayPath(t *testing.T) {
+	initOverlay(t, `
+{
+	"Replace": {
+		"subdir2/file2.txt":                 "overlayfiles/subdir2_file2.txt",
+		"subdir3/doesntexist":               "this_file_doesnt_exist_anywhere",
+		"subdir4/this_file_is_overlaid.txt": "overlayfiles/subdir4_this_file_is_overlaid.txt",
+		"subdir5/deleted.txt":               "",
+		"parentoverwritten/subdir1":         ""
+	}
+}
+-- subdir1/file1.txt --
+file 1
+-- subdir4/this_file_is_overlaid.txt --
+these contents are replaced by the overlay
+-- parentoverwritten/subdir1/subdir2/subdir3/file.txt --
+-- subdir5/deleted.txt --
+deleted
+-- overlayfiles/subdir2_file2.txt --
+file 2
+-- overlayfiles/subdir4_this_file_is_overlaid.txt --
+99999999
+`)
+
+	testCases := []struct {
+		path     string
+		wantPath string
+		wantOK   bool
+	}{
+		{"subdir1/file1.txt", "subdir1/file1.txt", false},
+		// OverlayPath returns false for directories
+		{"subdir2", "subdir2", false},
+		{"subdir2/file2.txt", filepath.Join(cwd, "overlayfiles/subdir2_file2.txt"), true},
+		// OverlayPath doesn't stat a file to see if it exists, so it happily returns
+		// the 'to' path and true even if the 'to' path doesn't exist on disk.
+		{"subdir3/doesntexist", filepath.Join(cwd, "this_file_doesnt_exist_anywhere"), true},
+		// Like the subdir2/file2.txt case above, but subdir4 exists on disk, but subdir2 does not.
+		{"subdir4/this_file_is_overlaid.txt", filepath.Join(cwd, "overlayfiles/subdir4_this_file_is_overlaid.txt"), true},
+		{"subdir5", "subdir5", false},
+		{"subdir5/deleted.txt", "", true},
+	}
+
+	for _, tc := range testCases {
+		gotPath, gotOK := OverlayPath(tc.path)
+		if gotPath != tc.wantPath || gotOK != tc.wantOK {
+			t.Errorf("OverlayPath(%q): got %v, %v; want %v, %v",
+				tc.path, gotPath, gotOK, tc.wantPath, tc.wantOK)
+		}
+	}
+}
+
+func TestOpen(t *testing.T) {
+	initOverlay(t, `
+{
+    "Replace": {
+		"subdir2/file2.txt":                  "overlayfiles/subdir2_file2.txt",
+		"subdir3/doesntexist":                "this_file_doesnt_exist_anywhere",
+		"subdir4/this_file_is_overlaid.txt":  "overlayfiles/subdir4_this_file_is_overlaid.txt",
+		"subdir5/deleted.txt":                "",
+		"parentoverwritten/subdir1":          "",
+		"childoverlay/subdir1.txt/child.txt": "overlayfiles/child.txt",
+		"subdir11/deleted.txt":               "",
+		"subdir11":                           "overlayfiles/subdir11",
+		"parentdeleted":                      "",
+		"parentdeleted/file.txt":             "overlayfiles/parentdeleted_file.txt"
+	}
+}
+-- subdir11/deleted.txt --
+-- subdir1/file1.txt --
+file 1
+-- subdir4/this_file_is_overlaid.txt --
+these contents are replaced by the overlay
+-- parentoverwritten/subdir1/subdir2/subdir3/file.txt --
+-- childoverlay/subdir1.txt --
+this file doesn't exist because the path
+childoverlay/subdir1.txt/child.txt is in the overlay
+-- subdir5/deleted.txt --
+deleted
+-- parentdeleted --
+this will be deleted so that parentdeleted/file.txt can exist
+-- overlayfiles/subdir2_file2.txt --
+file 2
+-- overlayfiles/subdir4_this_file_is_overlaid.txt --
+99999999
+-- overlayfiles/child.txt --
+-- overlayfiles/subdir11 --
+11
+-- overlayfiles/parentdeleted_file.txt --
+this can exist because the parent directory is deleted
+`)
+
+	testCases := []struct {
+		path         string
+		wantContents string
+		isErr        bool
+	}{
+		{"subdir1/file1.txt", "file 1\n", false},
+		{"subdir2/file2.txt", "file 2\n", false},
+		{"subdir3/doesntexist", "", true},
+		{"subdir4/this_file_is_overlaid.txt", "99999999\n", false},
+		{"subdir5/deleted.txt", "", true},
+		{"parentoverwritten/subdir1/subdir2/subdir3/file.txt", "", true},
+		{"childoverlay/subdir1.txt", "", true},
+		{"subdir11", "11\n", false},
+		{"parentdeleted/file.txt", "this can exist because the parent directory is deleted\n", false},
+	}
+
+	for _, tc := range testCases {
+		f, err := Open(tc.path)
+		if tc.isErr {
+			if err == nil {
+				f.Close()
+				t.Errorf("Open(%q): got no error, but want error", tc.path)
+			}
+			continue
+		}
+		if err != nil {
+			t.Errorf("Open(%q): got error %v, want nil", tc.path, err)
+			continue
+		}
+		contents, err := io.ReadAll(f)
+		if err != nil {
+			t.Errorf("unexpected error reading contents of file: %v", err)
+		}
+		if string(contents) != tc.wantContents {
+			t.Errorf("contents of file opened with Open(%q): got %q, want %q",
+				tc.path, contents, tc.wantContents)
+		}
+		f.Close()
+	}
+}
+
+func TestIsDirWithGoFiles(t *testing.T) {
+	initOverlay(t, `
+{
+	"Replace": {
+		"goinoverlay/file.go":       "dummy",
+		"directory/removed/by/file": "dummy",
+		"directory_with_go_dir/dir.go/file.txt": "dummy",
+		"otherdirectory/deleted.go": "",
+		"nonexistentdirectory/deleted.go": "",
+		"textfile.txt/file.go": "dummy"
+	}
+}
+-- dummy --
+a destination file for the overlay entries to point to
+contents don't matter for this test
+-- nogo/file.txt --
+-- goondisk/file.go --
+-- goinoverlay/file.txt --
+-- directory/removed/by/file/in/overlay/file.go --
+-- otherdirectory/deleted.go --
+-- textfile.txt --
+`)
+
+	testCases := []struct {
+		dir     string
+		want    bool
+		wantErr bool
+	}{
+		{"nogo", false, false},
+		{"goondisk", true, false},
+		{"goinoverlay", true, false},
+		{"directory/removed/by/file/in/overlay", false, false},
+		{"directory_with_go_dir", false, false},
+		{"otherdirectory", false, false},
+		{"nonexistentdirectory", false, false},
+		{"textfile.txt", true, false},
+	}
+
+	for _, tc := range testCases {
+		got, gotErr := IsDirWithGoFiles(tc.dir)
+		if tc.wantErr {
+			if gotErr == nil {
+				t.Errorf("IsDirWithGoFiles(%q): got %v, %v; want non-nil error", tc.dir, got, gotErr)
+			}
+			continue
+		}
+		if gotErr != nil {
+			t.Errorf("IsDirWithGoFiles(%q): got %v, %v; want nil error", tc.dir, got, gotErr)
+		}
+		if got != tc.want {
+			t.Errorf("IsDirWithGoFiles(%q) = %v; want %v", tc.dir, got, tc.want)
+		}
+	}
+}
+
+func TestWalk(t *testing.T) {
+	// The root of the walk must be a name with an actual basename, not just ".".
+	// Walk uses Lstat to obtain the name of the root, and Lstat on platforms
+	// other than Plan 9 reports the name "." instead of the actual base name of
+	// the directory. (See https://golang.org/issue/42115.)
+
+	type file struct {
+		path  string
+		name  string
+		size  int64
+		mode  fs.FileMode
+		isDir bool
+	}
+	testCases := []struct {
+		name      string
+		overlay   string
+		root      string
+		wantFiles []file
+	}{
+		{"no overlay", `
+{}
+-- dir/file.txt --
+`,
+			"dir",
+			[]file{
+				{"dir", "dir", 0, fs.ModeDir | 0700, true},
+				{"dir/file.txt", "file.txt", 0, 0600, false},
+			},
+		},
+		{"overlay with different file", `
+{
+	"Replace": {
+		"dir/file.txt": "dir/other.txt"
+	}
+}
+-- dir/file.txt --
+-- dir/other.txt --
+contents of other file
+`,
+			"dir",
+			[]file{
+				{"dir", "dir", 0, fs.ModeDir | 0500, true},
+				{"dir/file.txt", "file.txt", 23, 0600, false},
+				{"dir/other.txt", "other.txt", 23, 0600, false},
+			},
+		},
+		{"overlay with new file", `
+{
+	"Replace": {
+		"dir/file.txt": "dir/other.txt"
+	}
+}
+-- dir/other.txt --
+contents of other file
+`,
+			"dir",
+			[]file{
+				{"dir", "dir", 0, fs.ModeDir | 0500, true},
+				{"dir/file.txt", "file.txt", 23, 0600, false},
+				{"dir/other.txt", "other.txt", 23, 0600, false},
+			},
+		},
+		{"overlay with new directory", `
+{
+	"Replace": {
+		"dir/subdir/file.txt": "dir/other.txt"
+	}
+}
+-- dir/other.txt --
+contents of other file
+`,
+			"dir",
+			[]file{
+				{"dir", "dir", 0, fs.ModeDir | 0500, true},
+				{"dir/other.txt", "other.txt", 23, 0600, false},
+				{"dir/subdir", "subdir", 0, fs.ModeDir | 0500, true},
+				{"dir/subdir/file.txt", "file.txt", 23, 0600, false},
+			},
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			initOverlay(t, tc.overlay)
+
+			var got []file
+			Walk(tc.root, func(path string, info fs.FileInfo, err error) error {
+				got = append(got, file{path, info.Name(), info.Size(), info.Mode(), info.IsDir()})
+				return nil
+			})
+
+			if len(got) != len(tc.wantFiles) {
+				t.Errorf("Walk: saw %#v in walk; want %#v", got, tc.wantFiles)
+			}
+			for i := 0; i < len(got) && i < len(tc.wantFiles); i++ {
+				wantPath := filepath.FromSlash(tc.wantFiles[i].path)
+				if got[i].path != wantPath {
+					t.Errorf("path of file #%v in walk, got %q, want %q", i, got[i].path, wantPath)
+				}
+				if got[i].name != tc.wantFiles[i].name {
+					t.Errorf("name of file #%v in walk, got %q, want %q", i, got[i].name, tc.wantFiles[i].name)
+				}
+				if got[i].mode&(fs.ModeDir|0700) != tc.wantFiles[i].mode {
+					t.Errorf("mode&(fs.ModeDir|0700) for mode of file #%v in walk, got %v, want %v", i, got[i].mode&(fs.ModeDir|0700), tc.wantFiles[i].mode)
+				}
+				if got[i].isDir != tc.wantFiles[i].isDir {
+					t.Errorf("isDir for file #%v in walk, got %v, want %v", i, got[i].isDir, tc.wantFiles[i].isDir)
+				}
+				if tc.wantFiles[i].isDir {
+					continue // don't check size for directories
+				}
+				if got[i].size != tc.wantFiles[i].size {
+					t.Errorf("size of file #%v in walk, got %v, want %v", i, got[i].size, tc.wantFiles[i].size)
+				}
+			}
+		})
+	}
+}
+
+func TestWalkSkipDir(t *testing.T) {
+	initOverlay(t, `
+{
+	"Replace": {
+		"dir/skip/file.go": "dummy.txt",
+		"dir/dontskip/file.go": "dummy.txt",
+		"dir/dontskip/skip/file.go": "dummy.txt"
+	}
+}
+-- dummy.txt --
+`)
+
+	var seen []string
+	Walk("dir", func(path string, info fs.FileInfo, err error) error {
+		seen = append(seen, filepath.ToSlash(path))
+		if info.Name() == "skip" {
+			return filepath.SkipDir
+		}
+		return nil
+	})
+
+	wantSeen := []string{"dir", "dir/dontskip", "dir/dontskip/file.go", "dir/dontskip/skip", "dir/skip"}
+
+	if len(seen) != len(wantSeen) {
+		t.Errorf("paths seen in walk: got %v entries; want %v entries", len(seen), len(wantSeen))
+	}
+
+	for i := 0; i < len(seen) && i < len(wantSeen); i++ {
+		if seen[i] != wantSeen[i] {
+			t.Errorf("path #%v seen walking tree: want %q, got %q", i, seen[i], wantSeen[i])
+		}
+	}
+}
+
+func TestWalkSkipAll(t *testing.T) {
+	initOverlay(t, `
+{
+	"Replace": {
+		"dir/subdir1/foo1": "dummy.txt",
+		"dir/subdir1/foo2": "dummy.txt",
+		"dir/subdir1/foo3": "dummy.txt",
+		"dir/subdir2/foo4": "dummy.txt",
+		"dir/zzlast": "dummy.txt"
+	}
+}
+-- dummy.txt --
+`)
+
+	var seen []string
+	Walk("dir", func(path string, info fs.FileInfo, err error) error {
+		seen = append(seen, filepath.ToSlash(path))
+		if info.Name() == "foo2" {
+			return filepath.SkipAll
+		}
+		return nil
+	})
+
+	wantSeen := []string{"dir", "dir/subdir1", "dir/subdir1/foo1", "dir/subdir1/foo2"}
+
+	if len(seen) != len(wantSeen) {
+		t.Errorf("paths seen in walk: got %v entries; want %v entries", len(seen), len(wantSeen))
+	}
+
+	for i := 0; i < len(seen) && i < len(wantSeen); i++ {
+		if seen[i] != wantSeen[i] {
+			t.Errorf("path %#v seen walking tree: got %q, want %q", i, seen[i], wantSeen[i])
+		}
+	}
+}
+
+func TestWalkError(t *testing.T) {
+	initOverlay(t, "{}")
+
+	alreadyCalled := false
+	err := Walk("foo", func(path string, info fs.FileInfo, err error) error {
+		if alreadyCalled {
+			t.Fatal("expected walk function to be called exactly once, but it was called more than once")
+		}
+		alreadyCalled = true
+		return errors.New("returned from function")
+	})
+	if !alreadyCalled {
+		t.Fatal("expected walk function to be called exactly once, but it was never called")
+
+	}
+	if err == nil {
+		t.Fatalf("Walk: got no error, want error")
+	}
+	if err.Error() != "returned from function" {
+		t.Fatalf("Walk: got error %v, want \"returned from function\" error", err)
+	}
+}
+
+func TestWalkSymlink(t *testing.T) {
+	testenv.MustHaveSymlink(t)
+
+	initOverlay(t, `{
+	"Replace": {"overlay_symlink/file": "symlink/file"}
+}
+-- dir/file --`)
+
+	// Create symlink
+	if err := os.Symlink("dir", "symlink"); err != nil {
+		t.Error(err)
+	}
+
+	testCases := []struct {
+		name      string
+		dir       string
+		wantFiles []string
+	}{
+		{"control", "dir", []string{"dir", filepath.Join("dir", "file")}},
+		// ensure Walk doesn't walk into the directory pointed to by the symlink
+		// (because it's supposed to use Lstat instead of Stat).
+		{"symlink_to_dir", "symlink", []string{"symlink"}},
+		{"overlay_to_symlink_to_dir", "overlay_symlink", []string{"overlay_symlink", filepath.Join("overlay_symlink", "file")}},
+
+		// However, adding filepath.Separator should cause the link to be resolved.
+		{"symlink_with_slash", "symlink" + string(filepath.Separator), []string{"symlink" + string(filepath.Separator), filepath.Join("symlink", "file")}},
+		{"overlay_to_symlink_to_dir", "overlay_symlink" + string(filepath.Separator), []string{"overlay_symlink" + string(filepath.Separator), filepath.Join("overlay_symlink", "file")}},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			var got []string
+
+			err := Walk(tc.dir, func(path string, info fs.FileInfo, err error) error {
+				t.Logf("walk %q", path)
+				got = append(got, path)
+				if err != nil {
+					t.Errorf("walkfn: got non nil err argument: %v, want nil err argument", err)
+				}
+				return nil
+			})
+			if err != nil {
+				t.Errorf("Walk: got error %q, want nil", err)
+			}
+
+			if !reflect.DeepEqual(got, tc.wantFiles) {
+				t.Errorf("files examined by walk: got %v, want %v", got, tc.wantFiles)
+			}
+		})
+	}
+
+}
+
+func TestLstat(t *testing.T) {
+	type file struct {
+		name  string
+		size  int64
+		mode  fs.FileMode // mode & (fs.ModeDir|0x700): only check 'user' permissions
+		isDir bool
+	}
+
+	testCases := []struct {
+		name    string
+		overlay string
+		path    string
+
+		want    file
+		wantErr bool
+	}{
+		{
+			"regular_file",
+			`{}
+-- file.txt --
+contents`,
+			"file.txt",
+			file{"file.txt", 9, 0600, false},
+			false,
+		},
+		{
+			"new_file_in_overlay",
+			`{"Replace": {"file.txt": "dummy.txt"}}
+-- dummy.txt --
+contents`,
+			"file.txt",
+			file{"file.txt", 9, 0600, false},
+			false,
+		},
+		{
+			"file_replaced_in_overlay",
+			`{"Replace": {"file.txt": "dummy.txt"}}
+-- file.txt --
+-- dummy.txt --
+contents`,
+			"file.txt",
+			file{"file.txt", 9, 0600, false},
+			false,
+		},
+		{
+			"file_cant_exist",
+			`{"Replace": {"deleted": "dummy.txt"}}
+-- deleted/file.txt --
+-- dummy.txt --
+`,
+			"deleted/file.txt",
+			file{},
+			true,
+		},
+		{
+			"deleted",
+			`{"Replace": {"deleted": ""}}
+-- deleted --
+`,
+			"deleted",
+			file{},
+			true,
+		},
+		{
+			"dir_on_disk",
+			`{}
+-- dir/foo.txt --
+`,
+			"dir",
+			file{"dir", 0, 0700 | fs.ModeDir, true},
+			false,
+		},
+		{
+			"dir_in_overlay",
+			`{"Replace": {"dir/file.txt": "dummy.txt"}}
+-- dummy.txt --
+`,
+			"dir",
+			file{"dir", 0, 0500 | fs.ModeDir, true},
+			false,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			initOverlay(t, tc.overlay)
+			got, err := Lstat(tc.path)
+			if tc.wantErr {
+				if err == nil {
+					t.Errorf("lstat(%q): got no error, want error", tc.path)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("lstat(%q): got error %v, want no error", tc.path, err)
+			}
+			if got.Name() != tc.want.name {
+				t.Errorf("lstat(%q).Name(): got %q, want %q", tc.path, got.Name(), tc.want.name)
+			}
+			if got.Mode()&(fs.ModeDir|0700) != tc.want.mode {
+				t.Errorf("lstat(%q).Mode()&(fs.ModeDir|0700): got %v, want %v", tc.path, got.Mode()&(fs.ModeDir|0700), tc.want.mode)
+			}
+			if got.IsDir() != tc.want.isDir {
+				t.Errorf("lstat(%q).IsDir(): got %v, want %v", tc.path, got.IsDir(), tc.want.isDir)
+			}
+			if tc.want.isDir {
+				return // don't check size for directories
+			}
+			if got.Size() != tc.want.size {
+				t.Errorf("lstat(%q).Size(): got %v, want %v", tc.path, got.Size(), tc.want.size)
+			}
+		})
+	}
+}
+
+func TestStat(t *testing.T) {
+	testenv.MustHaveSymlink(t)
+
+	type file struct {
+		name  string
+		size  int64
+		mode  os.FileMode // mode & (os.ModeDir|0x700): only check 'user' permissions
+		isDir bool
+	}
+
+	testCases := []struct {
+		name    string
+		overlay string
+		path    string
+
+		want    file
+		wantErr bool
+	}{
+		{
+			"regular_file",
+			`{}
+-- file.txt --
+contents`,
+			"file.txt",
+			file{"file.txt", 9, 0600, false},
+			false,
+		},
+		{
+			"new_file_in_overlay",
+			`{"Replace": {"file.txt": "dummy.txt"}}
+-- dummy.txt --
+contents`,
+			"file.txt",
+			file{"file.txt", 9, 0600, false},
+			false,
+		},
+		{
+			"file_replaced_in_overlay",
+			`{"Replace": {"file.txt": "dummy.txt"}}
+-- file.txt --
+-- dummy.txt --
+contents`,
+			"file.txt",
+			file{"file.txt", 9, 0600, false},
+			false,
+		},
+		{
+			"file_cant_exist",
+			`{"Replace": {"deleted": "dummy.txt"}}
+-- deleted/file.txt --
+-- dummy.txt --
+`,
+			"deleted/file.txt",
+			file{},
+			true,
+		},
+		{
+			"deleted",
+			`{"Replace": {"deleted": ""}}
+-- deleted --
+`,
+			"deleted",
+			file{},
+			true,
+		},
+		{
+			"dir_on_disk",
+			`{}
+-- dir/foo.txt --
+`,
+			"dir",
+			file{"dir", 0, 0700 | os.ModeDir, true},
+			false,
+		},
+		{
+			"dir_in_overlay",
+			`{"Replace": {"dir/file.txt": "dummy.txt"}}
+-- dummy.txt --
+`,
+			"dir",
+			file{"dir", 0, 0500 | os.ModeDir, true},
+			false,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			initOverlay(t, tc.overlay)
+			got, err := Stat(tc.path)
+			if tc.wantErr {
+				if err == nil {
+					t.Errorf("Stat(%q): got no error, want error", tc.path)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("Stat(%q): got error %v, want no error", tc.path, err)
+			}
+			if got.Name() != tc.want.name {
+				t.Errorf("Stat(%q).Name(): got %q, want %q", tc.path, got.Name(), tc.want.name)
+			}
+			if got.Mode()&(os.ModeDir|0700) != tc.want.mode {
+				t.Errorf("Stat(%q).Mode()&(os.ModeDir|0700): got %v, want %v", tc.path, got.Mode()&(os.ModeDir|0700), tc.want.mode)
+			}
+			if got.IsDir() != tc.want.isDir {
+				t.Errorf("Stat(%q).IsDir(): got %v, want %v", tc.path, got.IsDir(), tc.want.isDir)
+			}
+			if tc.want.isDir {
+				return // don't check size for directories
+			}
+			if got.Size() != tc.want.size {
+				t.Errorf("Stat(%q).Size(): got %v, want %v", tc.path, got.Size(), tc.want.size)
+			}
+		})
+	}
+}
+
+func TestStatSymlink(t *testing.T) {
+	testenv.MustHaveSymlink(t)
+
+	initOverlay(t, `{
+	"Replace": {"file.go": "symlink"}
+}
+-- to.go --
+0123456789
+`)
+
+	// Create symlink
+	if err := os.Symlink("to.go", "symlink"); err != nil {
+		t.Error(err)
+	}
+
+	f := "file.go"
+	fi, err := Stat(f)
+	if err != nil {
+		t.Errorf("Stat(%q): got error %q, want nil error", f, err)
+	}
+
+	if !fi.Mode().IsRegular() {
+		t.Errorf("Stat(%q).Mode(): got %v, want regular mode", f, fi.Mode())
+	}
+
+	if fi.Size() != 11 {
+		t.Errorf("Stat(%q).Size(): got %v, want 11", f, fi.Size())
+	}
+}
diff --git a/src/cmd/go/internal/generate/generate.go b/src/cmd/go/internal/generate/generate.go
new file mode 100644
index 0000000..353506f
--- /dev/null
+++ b/src/cmd/go/internal/generate/generate.go
@@ -0,0 +1,502 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package generate implements the “go generate” command.
+package generate
+
+import (
+	"bufio"
+	"bytes"
+	"context"
+	"fmt"
+	"go/parser"
+	"go/token"
+	"io"
+	"log"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
+	"slices"
+	"strconv"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/load"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/str"
+	"cmd/go/internal/work"
+)
+
+var CmdGenerate = &base.Command{
+	Run:       runGenerate,
+	UsageLine: "go generate [-run regexp] [-n] [-v] [-x] [build flags] [file.go... | packages]",
+	Short:     "generate Go files by processing source",
+	Long: `
+Generate runs commands described by directives within existing
+files. Those commands can run any process but the intent is to
+create or update Go source files.
+
+Go generate is never run automatically by go build, go test,
+and so on. It must be run explicitly.
+
+Go generate scans the file for directives, which are lines of
+the form,
+
+	//go:generate command argument...
+
+(note: no leading spaces and no space in "//go") where command
+is the generator to be run, corresponding to an executable file
+that can be run locally. It must either be in the shell path
+(gofmt), a fully qualified path (/usr/you/bin/mytool), or a
+command alias, described below.
+
+Note that go generate does not parse the file, so lines that look
+like directives in comments or multiline strings will be treated
+as directives.
+
+The arguments to the directive are space-separated tokens or
+double-quoted strings passed to the generator as individual
+arguments when it is run.
+
+Quoted strings use Go syntax and are evaluated before execution; a
+quoted string appears as a single argument to the generator.
+
+To convey to humans and machine tools that code is generated,
+generated source should have a line that matches the following
+regular expression (in Go syntax):
+
+	^// Code generated .* DO NOT EDIT\.$
+
+This line must appear before the first non-comment, non-blank
+text in the file.
+
+Go generate sets several variables when it runs the generator:
+
+	$GOARCH
+		The execution architecture (arm, amd64, etc.)
+	$GOOS
+		The execution operating system (linux, windows, etc.)
+	$GOFILE
+		The base name of the file.
+	$GOLINE
+		The line number of the directive in the source file.
+	$GOPACKAGE
+		The name of the package of the file containing the directive.
+	$GOROOT
+		The GOROOT directory for the 'go' command that invoked the
+		generator, containing the Go toolchain and standard library.
+	$DOLLAR
+		A dollar sign.
+	$PATH
+		The $PATH of the parent process, with $GOROOT/bin
+		placed at the beginning. This causes generators
+		that execute 'go' commands to use the same 'go'
+		as the parent 'go generate' command.
+
+Other than variable substitution and quoted-string evaluation, no
+special processing such as "globbing" is performed on the command
+line.
+
+As a last step before running the command, any invocations of any
+environment variables with alphanumeric names, such as $GOFILE or
+$HOME, are expanded throughout the command line. The syntax for
+variable expansion is $NAME on all operating systems. Due to the
+order of evaluation, variables are expanded even inside quoted
+strings. If the variable NAME is not set, $NAME expands to the
+empty string.
+
+A directive of the form,
+
+	//go:generate -command xxx args...
+
+specifies, for the remainder of this source file only, that the
+string xxx represents the command identified by the arguments. This
+can be used to create aliases or to handle multiword generators.
+For example,
+
+	//go:generate -command foo go tool foo
+
+specifies that the command "foo" represents the generator
+"go tool foo".
+
+Generate processes packages in the order given on the command line,
+one at a time. If the command line lists .go files from a single directory,
+they are treated as a single package. Within a package, generate processes the
+source files in a package in file name order, one at a time. Within
+a source file, generate runs generators in the order they appear
+in the file, one at a time. The go generate tool also sets the build
+tag "generate" so that files may be examined by go generate but ignored
+during build.
+
+For packages with invalid code, generate processes only source files with a
+valid package clause.
+
+If any generator returns an error exit status, "go generate" skips
+all further processing for that package.
+
+The generator is run in the package's source directory.
+
+Go generate accepts two specific flags:
+
+	-run=""
+		if non-empty, specifies a regular expression to select
+		directives whose full original source text (excluding
+		any trailing spaces and final newline) matches the
+		expression.
+
+	-skip=""
+		if non-empty, specifies a regular expression to suppress
+		directives whose full original source text (excluding
+		any trailing spaces and final newline) matches the
+		expression. If a directive matches both the -run and
+		the -skip arguments, it is skipped.
+
+It also accepts the standard build flags including -v, -n, and -x.
+The -v flag prints the names of packages and files as they are
+processed.
+The -n flag prints commands that would be executed.
+The -x flag prints commands as they are executed.
+
+For more about build flags, see 'go help build'.
+
+For more about specifying packages, see 'go help packages'.
+	`,
+}
+
+var (
+	generateRunFlag string         // generate -run flag
+	generateRunRE   *regexp.Regexp // compiled expression for -run
+
+	generateSkipFlag string         // generate -skip flag
+	generateSkipRE   *regexp.Regexp // compiled expression for -skip
+)
+
+func init() {
+	work.AddBuildFlags(CmdGenerate, work.DefaultBuildFlags)
+	CmdGenerate.Flag.StringVar(&generateRunFlag, "run", "", "")
+	CmdGenerate.Flag.StringVar(&generateSkipFlag, "skip", "", "")
+}
+
+func runGenerate(ctx context.Context, cmd *base.Command, args []string) {
+	modload.InitWorkfile()
+
+	if generateRunFlag != "" {
+		var err error
+		generateRunRE, err = regexp.Compile(generateRunFlag)
+		if err != nil {
+			log.Fatalf("generate: %s", err)
+		}
+	}
+	if generateSkipFlag != "" {
+		var err error
+		generateSkipRE, err = regexp.Compile(generateSkipFlag)
+		if err != nil {
+			log.Fatalf("generate: %s", err)
+		}
+	}
+
+	cfg.BuildContext.BuildTags = append(cfg.BuildContext.BuildTags, "generate")
+
+	// Even if the arguments are .go files, this loop suffices.
+	printed := false
+	pkgOpts := load.PackageOpts{IgnoreImports: true}
+	for _, pkg := range load.PackagesAndErrors(ctx, pkgOpts, args) {
+		if modload.Enabled() && pkg.Module != nil && !pkg.Module.Main {
+			if !printed {
+				fmt.Fprintf(os.Stderr, "go: not generating in packages in dependency modules\n")
+				printed = true
+			}
+			continue
+		}
+
+		for _, file := range pkg.InternalGoFiles() {
+			if !generate(file) {
+				break
+			}
+		}
+
+		for _, file := range pkg.InternalXGoFiles() {
+			if !generate(file) {
+				break
+			}
+		}
+	}
+}
+
+// generate runs the generation directives for a single file.
+func generate(absFile string) bool {
+	src, err := os.ReadFile(absFile)
+	if err != nil {
+		log.Fatalf("generate: %s", err)
+	}
+
+	// Parse package clause
+	filePkg, err := parser.ParseFile(token.NewFileSet(), "", src, parser.PackageClauseOnly)
+	if err != nil {
+		// Invalid package clause - ignore file.
+		return true
+	}
+
+	g := &Generator{
+		r:        bytes.NewReader(src),
+		path:     absFile,
+		pkg:      filePkg.Name.String(),
+		commands: make(map[string][]string),
+	}
+	return g.run()
+}
+
+// A Generator represents the state of a single Go source file
+// being scanned for generator commands.
+type Generator struct {
+	r        io.Reader
+	path     string // full rooted path name.
+	dir      string // full rooted directory of file.
+	file     string // base name of file.
+	pkg      string
+	commands map[string][]string
+	lineNum  int // current line number.
+	env      []string
+}
+
+// run runs the generators in the current file.
+func (g *Generator) run() (ok bool) {
+	// Processing below here calls g.errorf on failure, which does panic(stop).
+	// If we encounter an error, we abort the package.
+	defer func() {
+		e := recover()
+		if e != nil {
+			ok = false
+			if e != stop {
+				panic(e)
+			}
+			base.SetExitStatus(1)
+		}
+	}()
+	g.dir, g.file = filepath.Split(g.path)
+	g.dir = filepath.Clean(g.dir) // No final separator please.
+	if cfg.BuildV {
+		fmt.Fprintf(os.Stderr, "%s\n", base.ShortPath(g.path))
+	}
+
+	// Scan for lines that start "//go:generate".
+	// Can't use bufio.Scanner because it can't handle long lines,
+	// which are likely to appear when using generate.
+	input := bufio.NewReader(g.r)
+	var err error
+	// One line per loop.
+	for {
+		g.lineNum++ // 1-indexed.
+		var buf []byte
+		buf, err = input.ReadSlice('\n')
+		if err == bufio.ErrBufferFull {
+			// Line too long - consume and ignore.
+			if isGoGenerate(buf) {
+				g.errorf("directive too long")
+			}
+			for err == bufio.ErrBufferFull {
+				_, err = input.ReadSlice('\n')
+			}
+			if err != nil {
+				break
+			}
+			continue
+		}
+
+		if err != nil {
+			// Check for marker at EOF without final \n.
+			if err == io.EOF && isGoGenerate(buf) {
+				err = io.ErrUnexpectedEOF
+			}
+			break
+		}
+
+		if !isGoGenerate(buf) {
+			continue
+		}
+		if generateRunFlag != "" && !generateRunRE.Match(bytes.TrimSpace(buf)) {
+			continue
+		}
+		if generateSkipFlag != "" && generateSkipRE.Match(bytes.TrimSpace(buf)) {
+			continue
+		}
+
+		g.setEnv()
+		words := g.split(string(buf))
+		if len(words) == 0 {
+			g.errorf("no arguments to directive")
+		}
+		if words[0] == "-command" {
+			g.setShorthand(words)
+			continue
+		}
+		// Run the command line.
+		if cfg.BuildN || cfg.BuildX {
+			fmt.Fprintf(os.Stderr, "%s\n", strings.Join(words, " "))
+		}
+		if cfg.BuildN {
+			continue
+		}
+		g.exec(words)
+	}
+	if err != nil && err != io.EOF {
+		g.errorf("error reading %s: %s", base.ShortPath(g.path), err)
+	}
+	return true
+}
+
+func isGoGenerate(buf []byte) bool {
+	return bytes.HasPrefix(buf, []byte("//go:generate ")) || bytes.HasPrefix(buf, []byte("//go:generate\t"))
+}
+
+// setEnv sets the extra environment variables used when executing a
+// single go:generate command.
+func (g *Generator) setEnv() {
+	env := []string{
+		"GOROOT=" + cfg.GOROOT,
+		"GOARCH=" + cfg.BuildContext.GOARCH,
+		"GOOS=" + cfg.BuildContext.GOOS,
+		"GOFILE=" + g.file,
+		"GOLINE=" + strconv.Itoa(g.lineNum),
+		"GOPACKAGE=" + g.pkg,
+		"DOLLAR=" + "$",
+	}
+	env = base.AppendPATH(env)
+	env = base.AppendPWD(env, g.dir)
+	g.env = env
+}
+
+// split breaks the line into words, evaluating quoted
+// strings and evaluating environment variables.
+// The initial //go:generate element is present in line.
+func (g *Generator) split(line string) []string {
+	// Parse line, obeying quoted strings.
+	var words []string
+	line = line[len("//go:generate ") : len(line)-1] // Drop preamble and final newline.
+	// There may still be a carriage return.
+	if len(line) > 0 && line[len(line)-1] == '\r' {
+		line = line[:len(line)-1]
+	}
+	// One (possibly quoted) word per iteration.
+Words:
+	for {
+		line = strings.TrimLeft(line, " \t")
+		if len(line) == 0 {
+			break
+		}
+		if line[0] == '"' {
+			for i := 1; i < len(line); i++ {
+				c := line[i] // Only looking for ASCII so this is OK.
+				switch c {
+				case '\\':
+					if i+1 == len(line) {
+						g.errorf("bad backslash")
+					}
+					i++ // Absorb next byte (If it's a multibyte we'll get an error in Unquote).
+				case '"':
+					word, err := strconv.Unquote(line[0 : i+1])
+					if err != nil {
+						g.errorf("bad quoted string")
+					}
+					words = append(words, word)
+					line = line[i+1:]
+					// Check the next character is space or end of line.
+					if len(line) > 0 && line[0] != ' ' && line[0] != '\t' {
+						g.errorf("expect space after quoted argument")
+					}
+					continue Words
+				}
+			}
+			g.errorf("mismatched quoted string")
+		}
+		i := strings.IndexAny(line, " \t")
+		if i < 0 {
+			i = len(line)
+		}
+		words = append(words, line[0:i])
+		line = line[i:]
+	}
+	// Substitute command if required.
+	if len(words) > 0 && g.commands[words[0]] != nil {
+		// Replace 0th word by command substitution.
+		//
+		// Force a copy of the command definition to
+		// ensure words doesn't end up as a reference
+		// to the g.commands content.
+		tmpCmdWords := append([]string(nil), (g.commands[words[0]])...)
+		words = append(tmpCmdWords, words[1:]...)
+	}
+	// Substitute environment variables.
+	for i, word := range words {
+		words[i] = os.Expand(word, g.expandVar)
+	}
+	return words
+}
+
+var stop = fmt.Errorf("error in generation")
+
+// errorf logs an error message prefixed with the file and line number.
+// It then exits the program (with exit status 1) because generation stops
+// at the first error.
+func (g *Generator) errorf(format string, args ...any) {
+	fmt.Fprintf(os.Stderr, "%s:%d: %s\n", base.ShortPath(g.path), g.lineNum,
+		fmt.Sprintf(format, args...))
+	panic(stop)
+}
+
+// expandVar expands the $XXX invocation in word. It is called
+// by os.Expand.
+func (g *Generator) expandVar(word string) string {
+	w := word + "="
+	for _, e := range g.env {
+		if strings.HasPrefix(e, w) {
+			return e[len(w):]
+		}
+	}
+	return os.Getenv(word)
+}
+
+// setShorthand installs a new shorthand as defined by a -command directive.
+func (g *Generator) setShorthand(words []string) {
+	// Create command shorthand.
+	if len(words) == 1 {
+		g.errorf("no command specified for -command")
+	}
+	command := words[1]
+	if g.commands[command] != nil {
+		g.errorf("command %q multiply defined", command)
+	}
+	g.commands[command] = slices.Clip(words[2:])
+}
+
+// exec runs the command specified by the argument. The first word is
+// the command name itself.
+func (g *Generator) exec(words []string) {
+	path := words[0]
+	if path != "" && !strings.Contains(path, string(os.PathSeparator)) {
+		// If a generator says '//go:generate go run <blah>' it almost certainly
+		// intends to use the same 'go' as 'go generate' itself.
+		// Prefer to resolve the binary from GOROOT/bin, and for consistency
+		// prefer to resolve any other commands there too.
+		gorootBinPath, err := exec.LookPath(filepath.Join(cfg.GOROOTbin, path))
+		if err == nil {
+			path = gorootBinPath
+		}
+	}
+	cmd := exec.Command(path, words[1:]...)
+	cmd.Args[0] = words[0] // Overwrite with the original in case it was rewritten above.
+
+	// Standard in and out of generator should be the usual.
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	// Run the command in the package directory.
+	cmd.Dir = g.dir
+	cmd.Env = str.StringList(cfg.OrigEnv, g.env)
+	err := cmd.Run()
+	if err != nil {
+		g.errorf("running %q: %s", words[0], err)
+	}
+}
diff --git a/src/cmd/go/internal/generate/generate_test.go b/src/cmd/go/internal/generate/generate_test.go
new file mode 100644
index 0000000..d61ecf1
--- /dev/null
+++ b/src/cmd/go/internal/generate/generate_test.go
@@ -0,0 +1,259 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package generate
+
+import (
+	"internal/testenv"
+	"os"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"testing"
+)
+
+type splitTest struct {
+	in  string
+	out []string
+}
+
+// Same as above, except including source line number to set
+type splitTestWithLine struct {
+	in         string
+	out        []string
+	lineNumber int
+}
+
+const anyLineNo = 0
+
+var splitTests = []splitTest{
+	{"", nil},
+	{"x", []string{"x"}},
+	{" a b\tc ", []string{"a", "b", "c"}},
+	{` " a " `, []string{" a "}},
+	{"$GOARCH", []string{runtime.GOARCH}},
+	{"$GOOS", []string{runtime.GOOS}},
+	{"$GOFILE", []string{"proc.go"}},
+	{"$GOPACKAGE", []string{"sys"}},
+	{"a $XXNOTDEFINEDXX b", []string{"a", "", "b"}},
+	{"/$XXNOTDEFINED/", []string{"//"}},
+	{"/$DOLLAR/", []string{"/$/"}},
+	{"yacc -o $GOARCH/yacc_$GOFILE", []string{"go", "tool", "yacc", "-o", runtime.GOARCH + "/yacc_proc.go"}},
+}
+
+func TestGenerateCommandParse(t *testing.T) {
+	dir := filepath.Join(testenv.GOROOT(t), "src", "sys")
+	g := &Generator{
+		r:        nil, // Unused here.
+		path:     filepath.Join(dir, "proc.go"),
+		dir:      dir,
+		file:     "proc.go",
+		pkg:      "sys",
+		commands: make(map[string][]string),
+	}
+	g.setEnv()
+	g.setShorthand([]string{"-command", "yacc", "go", "tool", "yacc"})
+	for _, test := range splitTests {
+		// First with newlines.
+		got := g.split("//go:generate " + test.in + "\n")
+		if !reflect.DeepEqual(got, test.out) {
+			t.Errorf("split(%q): got %q expected %q", test.in, got, test.out)
+		}
+		// Then with CRLFs, thank you Windows.
+		got = g.split("//go:generate " + test.in + "\r\n")
+		if !reflect.DeepEqual(got, test.out) {
+			t.Errorf("split(%q): got %q expected %q", test.in, got, test.out)
+		}
+	}
+}
+
+// These environment variables will be undefined before the splitTestWithLine tests
+var undefEnvList = []string{
+	"_XYZZY_",
+}
+
+// These environment variables will be defined before the splitTestWithLine tests
+var defEnvMap = map[string]string{
+	"_PLUGH_": "SomeVal",
+	"_X":      "Y",
+}
+
+// TestGenerateCommandShortHand - similar to TestGenerateCommandParse,
+// except:
+//  1. if the result starts with -command, record that shorthand
+//     before moving on to the next test.
+//  2. If a source line number is specified, set that in the parser
+//     before executing the test.  i.e., execute the split as if it
+//     processing that source line.
+func TestGenerateCommandShorthand(t *testing.T) {
+	dir := filepath.Join(testenv.GOROOT(t), "src", "sys")
+	g := &Generator{
+		r:        nil, // Unused here.
+		path:     filepath.Join(dir, "proc.go"),
+		dir:      dir,
+		file:     "proc.go",
+		pkg:      "sys",
+		commands: make(map[string][]string),
+	}
+
+	var inLine string
+	var expected, got []string
+
+	g.setEnv()
+
+	// Set up the system environment variables
+	for i := range undefEnvList {
+		os.Unsetenv(undefEnvList[i])
+	}
+	for k := range defEnvMap {
+		os.Setenv(k, defEnvMap[k])
+	}
+
+	// simple command from environment variable
+	inLine = "//go:generate -command CMD0 \"ab${_X}cd\""
+	expected = []string{"-command", "CMD0", "abYcd"}
+	got = g.split(inLine + "\n")
+
+	if !reflect.DeepEqual(got, expected) {
+		t.Errorf("split(%q): got %q expected %q", inLine, got, expected)
+	}
+
+	// try again, with an extra level of indirection (should leave variable in command)
+	inLine = "//go:generate -command CMD0 \"ab${DOLLAR}{_X}cd\""
+	expected = []string{"-command", "CMD0", "ab${_X}cd"}
+	got = g.split(inLine + "\n")
+
+	if !reflect.DeepEqual(got, expected) {
+		t.Errorf("split(%q): got %q expected %q", inLine, got, expected)
+	}
+
+	// Now the interesting part, record that output as a command
+	g.setShorthand(got)
+
+	// see that the command still substitutes correctly from env. variable
+	inLine = "//go:generate CMD0"
+	expected = []string{"abYcd"}
+	got = g.split(inLine + "\n")
+
+	if !reflect.DeepEqual(got, expected) {
+		t.Errorf("split(%q): got %q expected %q", inLine, got, expected)
+	}
+
+	// Now change the value of $X and see if the recorded definition is
+	// still intact (vs. having the $_X already substituted out)
+
+	os.Setenv("_X", "Z")
+	inLine = "//go:generate CMD0"
+	expected = []string{"abZcd"}
+	got = g.split(inLine + "\n")
+
+	if !reflect.DeepEqual(got, expected) {
+		t.Errorf("split(%q): got %q expected %q", inLine, got, expected)
+	}
+
+	// What if the variable is now undefined?  Should be empty substitution.
+
+	os.Unsetenv("_X")
+	inLine = "//go:generate CMD0"
+	expected = []string{"abcd"}
+	got = g.split(inLine + "\n")
+
+	if !reflect.DeepEqual(got, expected) {
+		t.Errorf("split(%q): got %q expected %q", inLine, got, expected)
+	}
+
+	// Try another undefined variable as an extra check
+	os.Unsetenv("_Z")
+	inLine = "//go:generate -command CMD1 \"ab${_Z}cd\""
+	expected = []string{"-command", "CMD1", "abcd"}
+	got = g.split(inLine + "\n")
+
+	if !reflect.DeepEqual(got, expected) {
+		t.Errorf("split(%q): got %q expected %q", inLine, got, expected)
+	}
+
+	g.setShorthand(got)
+
+	inLine = "//go:generate CMD1"
+	expected = []string{"abcd"}
+	got = g.split(inLine + "\n")
+
+	if !reflect.DeepEqual(got, expected) {
+		t.Errorf("split(%q): got %q expected %q", inLine, got, expected)
+	}
+
+	const val = "someNewValue"
+	os.Setenv("_Z", val)
+
+	// try again with the properly-escaped variable.
+
+	inLine = "//go:generate -command CMD2 \"ab${DOLLAR}{_Z}cd\""
+	expected = []string{"-command", "CMD2", "ab${_Z}cd"}
+	got = g.split(inLine + "\n")
+
+	if !reflect.DeepEqual(got, expected) {
+		t.Errorf("split(%q): got %q expected %q", inLine, got, expected)
+	}
+
+	g.setShorthand(got)
+
+	inLine = "//go:generate CMD2"
+	expected = []string{"ab" + val + "cd"}
+	got = g.split(inLine + "\n")
+
+	if !reflect.DeepEqual(got, expected) {
+		t.Errorf("split(%q): got %q expected %q", inLine, got, expected)
+	}
+}
+
+// Command-related tests for TestGenerateCommandShortHand2
+// -- Note line numbers included to check substitutions from "build-in" variable - $GOLINE
+var splitTestsLines = []splitTestWithLine{
+	{"-command TEST1 $GOLINE", []string{"-command", "TEST1", "22"}, 22},
+	{"-command TEST2 ${DOLLAR}GOLINE", []string{"-command", "TEST2", "$GOLINE"}, 26},
+	{"TEST1", []string{"22"}, 33},
+	{"TEST2", []string{"66"}, 66},
+	{"TEST1 ''", []string{"22", "''"}, 99},
+	{"TEST2 ''", []string{"44", "''"}, 44},
+}
+
+// TestGenerateCommandShortHand - similar to TestGenerateCommandParse,
+// except:
+//  1. if the result starts with -command, record that shorthand
+//     before moving on to the next test.
+//  2. If a source line number is specified, set that in the parser
+//     before executing the test.  i.e., execute the split as if it
+//     processing that source line.
+func TestGenerateCommandShortHand2(t *testing.T) {
+	dir := filepath.Join(testenv.GOROOT(t), "src", "sys")
+	g := &Generator{
+		r:        nil, // Unused here.
+		path:     filepath.Join(dir, "proc.go"),
+		dir:      dir,
+		file:     "proc.go",
+		pkg:      "sys",
+		commands: make(map[string][]string),
+	}
+	g.setEnv()
+	for _, test := range splitTestsLines {
+		// if the test specified a line number, reflect that
+		if test.lineNumber != anyLineNo {
+			g.lineNum = test.lineNumber
+			g.setEnv()
+		}
+		// First with newlines.
+		got := g.split("//go:generate " + test.in + "\n")
+		if !reflect.DeepEqual(got, test.out) {
+			t.Errorf("split(%q): got %q expected %q", test.in, got, test.out)
+		}
+		// Then with CRLFs, thank you Windows.
+		got = g.split("//go:generate " + test.in + "\r\n")
+		if !reflect.DeepEqual(got, test.out) {
+			t.Errorf("split(%q): got %q expected %q", test.in, got, test.out)
+		}
+		if got[0] == "-command" { // record commands
+			g.setShorthand(got)
+		}
+	}
+}
diff --git a/src/cmd/go/internal/get/get.go b/src/cmd/go/internal/get/get.go
new file mode 100644
index 0000000..06b567a
--- /dev/null
+++ b/src/cmd/go/internal/get/get.go
@@ -0,0 +1,640 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package get implements the “go get” command.
+package get
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/load"
+	"cmd/go/internal/search"
+	"cmd/go/internal/str"
+	"cmd/go/internal/vcs"
+	"cmd/go/internal/web"
+	"cmd/go/internal/work"
+
+	"golang.org/x/mod/module"
+)
+
+var CmdGet = &base.Command{
+	UsageLine: "go get [-d] [-f] [-t] [-u] [-v] [-fix] [build flags] [packages]",
+	Short:     "download and install packages and dependencies",
+	Long: `
+Get downloads the packages named by the import paths, along with their
+dependencies. It then installs the named packages, like 'go install'.
+
+The -d flag instructs get to stop after downloading the packages; that is,
+it instructs get not to install the packages.
+
+The -f flag, valid only when -u is set, forces get -u not to verify that
+each package has been checked out from the source control repository
+implied by its import path. This can be useful if the source is a local fork
+of the original.
+
+The -fix flag instructs get to run the fix tool on the downloaded packages
+before resolving dependencies or building the code.
+
+The -t flag instructs get to also download the packages required to build
+the tests for the specified packages.
+
+The -u flag instructs get to use the network to update the named packages
+and their dependencies. By default, get uses the network to check out
+missing packages but does not use it to look for updates to existing packages.
+
+The -v flag enables verbose progress and debug output.
+
+Get also accepts build flags to control the installation. See 'go help build'.
+
+When checking out a new package, get creates the target directory
+GOPATH/src/<import-path>. If the GOPATH contains multiple entries,
+get uses the first one. For more details see: 'go help gopath'.
+
+When checking out or updating a package, get looks for a branch or tag
+that matches the locally installed version of Go. The most important
+rule is that if the local installation is running version "go1", get
+searches for a branch or tag named "go1". If no such version exists
+it retrieves the default branch of the package.
+
+When go get checks out or updates a Git repository,
+it also updates any git submodules referenced by the repository.
+
+Get never checks out or updates code stored in vendor directories.
+
+For more about build flags, see 'go help build'.
+
+For more about specifying packages, see 'go help packages'.
+
+For more about how 'go get' finds source code to
+download, see 'go help importpath'.
+
+This text describes the behavior of get when using GOPATH
+to manage source code and dependencies.
+If instead the go command is running in module-aware mode,
+the details of get's flags and effects change, as does 'go help get'.
+See 'go help modules' and 'go help module-get'.
+
+See also: go build, go install, go clean.
+	`,
+}
+
+var HelpGopathGet = &base.Command{
+	UsageLine: "gopath-get",
+	Short:     "legacy GOPATH go get",
+	Long: `
+The 'go get' command changes behavior depending on whether the
+go command is running in module-aware mode or legacy GOPATH mode.
+This help text, accessible as 'go help gopath-get' even in module-aware mode,
+describes 'go get' as it operates in legacy GOPATH mode.
+
+Usage: ` + CmdGet.UsageLine + `
+` + CmdGet.Long,
+}
+
+var (
+	getD        = CmdGet.Flag.Bool("d", false, "")
+	getF        = CmdGet.Flag.Bool("f", false, "")
+	getT        = CmdGet.Flag.Bool("t", false, "")
+	getU        = CmdGet.Flag.Bool("u", false, "")
+	getFix      = CmdGet.Flag.Bool("fix", false, "")
+	getInsecure = CmdGet.Flag.Bool("insecure", false, "")
+)
+
+func init() {
+	work.AddBuildFlags(CmdGet, work.OmitModFlag|work.OmitModCommonFlags)
+	CmdGet.Run = runGet // break init loop
+}
+
+func runGet(ctx context.Context, cmd *base.Command, args []string) {
+	if cfg.ModulesEnabled {
+		// Should not happen: main.go should install the separate module-enabled get code.
+		base.Fatalf("go: modules not implemented")
+	}
+
+	work.BuildInit()
+
+	if *getF && !*getU {
+		base.Fatalf("go: cannot use -f flag without -u")
+	}
+	if *getInsecure {
+		base.Fatalf("go: -insecure flag is no longer supported; use GOINSECURE instead")
+	}
+
+	// Disable any prompting for passwords by Git itself.
+	// Only has an effect for 2.3.0 or later, but avoiding
+	// the prompt in earlier versions is just too hard.
+	// If user has explicitly set GIT_TERMINAL_PROMPT=1, keep
+	// prompting.
+	// See golang.org/issue/9341 and golang.org/issue/12706.
+	if os.Getenv("GIT_TERMINAL_PROMPT") == "" {
+		os.Setenv("GIT_TERMINAL_PROMPT", "0")
+	}
+
+	// Also disable prompting for passwords by the 'ssh' subprocess spawned by
+	// Git, because apparently GIT_TERMINAL_PROMPT isn't sufficient to do that.
+	// Adding '-o BatchMode=yes' should do the trick.
+	//
+	// If a Git subprocess forks a child into the background to cache a new connection,
+	// that child keeps stdout/stderr open. After the Git subprocess exits,
+	// os /exec expects to be able to read from the stdout/stderr pipe
+	// until EOF to get all the data that the Git subprocess wrote before exiting.
+	// The EOF doesn't come until the child exits too, because the child
+	// is holding the write end of the pipe.
+	// This is unfortunate, but it has come up at least twice
+	// (see golang.org/issue/13453 and golang.org/issue/16104)
+	// and confuses users when it does.
+	// If the user has explicitly set GIT_SSH or GIT_SSH_COMMAND,
+	// assume they know what they are doing and don't step on it.
+	// But default to turning off ControlMaster.
+	if os.Getenv("GIT_SSH") == "" && os.Getenv("GIT_SSH_COMMAND") == "" {
+		os.Setenv("GIT_SSH_COMMAND", "ssh -o ControlMaster=no -o BatchMode=yes")
+	}
+
+	// And one more source of Git prompts: the Git Credential Manager Core for Windows.
+	//
+	// See https://github.com/microsoft/Git-Credential-Manager-Core/blob/master/docs/environment.md#gcm_interactive.
+	if os.Getenv("GCM_INTERACTIVE") == "" {
+		os.Setenv("GCM_INTERACTIVE", "never")
+	}
+
+	// Phase 1. Download/update.
+	var stk load.ImportStack
+	mode := 0
+	if *getT {
+		mode |= load.GetTestDeps
+	}
+	for _, pkg := range downloadPaths(args) {
+		download(ctx, pkg, nil, &stk, mode)
+	}
+	base.ExitIfErrors()
+
+	// Phase 2. Rescan packages and re-evaluate args list.
+
+	// Code we downloaded and all code that depends on it
+	// needs to be evicted from the package cache so that
+	// the information will be recomputed. Instead of keeping
+	// track of the reverse dependency information, evict
+	// everything.
+	load.ClearPackageCache()
+
+	pkgs := load.PackagesAndErrors(ctx, load.PackageOpts{}, args)
+	load.CheckPackageErrors(pkgs)
+
+	// Phase 3. Install.
+	if *getD {
+		// Download only.
+		// Check delayed until now so that downloadPaths
+		// and CheckPackageErrors have a chance to print errors.
+		return
+	}
+
+	work.InstallPackages(ctx, args, pkgs)
+}
+
+// downloadPaths prepares the list of paths to pass to download.
+// It expands ... patterns that can be expanded. If there is no match
+// for a particular pattern, downloadPaths leaves it in the result list,
+// in the hope that we can figure out the repository from the
+// initial ...-free prefix.
+func downloadPaths(patterns []string) []string {
+	for _, arg := range patterns {
+		if strings.Contains(arg, "@") {
+			base.Fatalf("go: can only use path@version syntax with 'go get' and 'go install' in module-aware mode")
+		}
+
+		// Guard against 'go get x.go', a common mistake.
+		// Note that package and module paths may end with '.go', so only print an error
+		// if the argument has no slash or refers to an existing file.
+		if strings.HasSuffix(arg, ".go") {
+			if !strings.Contains(arg, "/") {
+				base.Errorf("go: %s: arguments must be package or module paths", arg)
+				continue
+			}
+			if fi, err := os.Stat(arg); err == nil && !fi.IsDir() {
+				base.Errorf("go: %s exists as a file, but 'go get' requires package arguments", arg)
+			}
+		}
+	}
+	base.ExitIfErrors()
+
+	var pkgs []string
+	noModRoots := []string{}
+	for _, m := range search.ImportPathsQuiet(patterns, noModRoots) {
+		if len(m.Pkgs) == 0 && strings.Contains(m.Pattern(), "...") {
+			pkgs = append(pkgs, m.Pattern())
+		} else {
+			pkgs = append(pkgs, m.Pkgs...)
+		}
+	}
+	return pkgs
+}
+
+// downloadCache records the import paths we have already
+// considered during the download, to avoid duplicate work when
+// there is more than one dependency sequence leading to
+// a particular package.
+var downloadCache = map[string]bool{}
+
+// downloadRootCache records the version control repository
+// root directories we have already considered during the download.
+// For example, all the packages in the github.com/google/codesearch repo
+// share the same root (the directory for that path), and we only need
+// to run the hg commands to consider each repository once.
+var downloadRootCache = map[string]bool{}
+
+// download runs the download half of the get command
+// for the package or pattern named by the argument.
+func download(ctx context.Context, arg string, parent *load.Package, stk *load.ImportStack, mode int) {
+	if mode&load.ResolveImport != 0 {
+		// Caller is responsible for expanding vendor paths.
+		panic("internal error: download mode has useVendor set")
+	}
+	load1 := func(path string, mode int) *load.Package {
+		if parent == nil {
+			mode := 0 // don't do module or vendor resolution
+			return load.LoadPackage(ctx, load.PackageOpts{}, path, base.Cwd(), stk, nil, mode)
+		}
+		p, err := load.LoadImport(ctx, load.PackageOpts{}, path, parent.Dir, parent, stk, nil, mode|load.ResolveModule)
+		if err != nil {
+			base.Errorf("%s", err)
+		}
+		return p
+	}
+
+	p := load1(arg, mode)
+	if p.Error != nil && p.Error.Hard {
+		base.Errorf("%s", p.Error)
+		return
+	}
+
+	// loadPackage inferred the canonical ImportPath from arg.
+	// Use that in the following to prevent hysteresis effects
+	// in e.g. downloadCache and packageCache.
+	// This allows invocations such as:
+	//   mkdir -p $GOPATH/src/github.com/user
+	//   cd $GOPATH/src/github.com/user
+	//   go get ./foo
+	// see: golang.org/issue/9767
+	arg = p.ImportPath
+
+	// There's nothing to do if this is a package in the standard library.
+	if p.Standard {
+		return
+	}
+
+	// Only process each package once.
+	// (Unless we're fetching test dependencies for this package,
+	// in which case we want to process it again.)
+	if downloadCache[arg] && mode&load.GetTestDeps == 0 {
+		return
+	}
+	downloadCache[arg] = true
+
+	pkgs := []*load.Package{p}
+	wildcardOkay := len(*stk) == 0
+	isWildcard := false
+
+	// Download if the package is missing, or update if we're using -u.
+	if p.Dir == "" || *getU {
+		// The actual download.
+		stk.Push(arg)
+		err := downloadPackage(p)
+		if err != nil {
+			base.Errorf("%s", &load.PackageError{ImportStack: stk.Copy(), Err: err})
+			stk.Pop()
+			return
+		}
+		stk.Pop()
+
+		args := []string{arg}
+		// If the argument has a wildcard in it, re-evaluate the wildcard.
+		// We delay this until after reloadPackage so that the old entry
+		// for p has been replaced in the package cache.
+		if wildcardOkay && strings.Contains(arg, "...") {
+			match := search.NewMatch(arg)
+			if match.IsLocal() {
+				noModRoots := []string{} // We're in gopath mode, so there are no modroots.
+				match.MatchDirs(noModRoots)
+				args = match.Dirs
+			} else {
+				match.MatchPackages()
+				args = match.Pkgs
+			}
+			for _, err := range match.Errs {
+				base.Errorf("%s", err)
+			}
+			isWildcard = true
+		}
+
+		// Clear all relevant package cache entries before
+		// doing any new loads.
+		load.ClearPackageCachePartial(args)
+
+		pkgs = pkgs[:0]
+		for _, arg := range args {
+			// Note: load calls loadPackage or loadImport,
+			// which push arg onto stk already.
+			// Do not push here too, or else stk will say arg imports arg.
+			p := load1(arg, mode)
+			if p.Error != nil {
+				base.Errorf("%s", p.Error)
+				continue
+			}
+			pkgs = append(pkgs, p)
+		}
+	}
+
+	// Process package, which might now be multiple packages
+	// due to wildcard expansion.
+	for _, p := range pkgs {
+		if *getFix {
+			files := base.RelPaths(p.InternalAllGoFiles())
+			base.Run(cfg.BuildToolexec, str.StringList(base.Tool("fix"), files))
+
+			// The imports might have changed, so reload again.
+			p = load.ReloadPackageNoFlags(arg, stk)
+			if p.Error != nil {
+				base.Errorf("%s", p.Error)
+				return
+			}
+		}
+
+		if isWildcard {
+			// Report both the real package and the
+			// wildcard in any error message.
+			stk.Push(p.ImportPath)
+		}
+
+		// Process dependencies, now that we know what they are.
+		imports := p.Imports
+		if mode&load.GetTestDeps != 0 {
+			// Process test dependencies when -t is specified.
+			// (But don't get test dependencies for test dependencies:
+			// we always pass mode 0 to the recursive calls below.)
+			imports = str.StringList(imports, p.TestImports, p.XTestImports)
+		}
+		for i, path := range imports {
+			if path == "C" {
+				continue
+			}
+			// Fail fast on import naming full vendor path.
+			// Otherwise expand path as needed for test imports.
+			// Note that p.Imports can have additional entries beyond p.Internal.Build.Imports.
+			orig := path
+			if i < len(p.Internal.Build.Imports) {
+				orig = p.Internal.Build.Imports[i]
+			}
+			if j, ok := load.FindVendor(orig); ok {
+				stk.Push(path)
+				err := &load.PackageError{
+					ImportStack: stk.Copy(),
+					Err:         load.ImportErrorf(path, "%s must be imported as %s", path, path[j+len("vendor/"):]),
+				}
+				stk.Pop()
+				base.Errorf("%s", err)
+				continue
+			}
+			// If this is a test import, apply module and vendor lookup now.
+			// We cannot pass ResolveImport to download, because
+			// download does caching based on the value of path,
+			// so it must be the fully qualified path already.
+			if i >= len(p.Imports) {
+				path = load.ResolveImportPath(p, path)
+			}
+			download(ctx, path, p, stk, 0)
+		}
+
+		if isWildcard {
+			stk.Pop()
+		}
+	}
+}
+
+// downloadPackage runs the create or download command
+// to make the first copy of or update a copy of the given package.
+func downloadPackage(p *load.Package) error {
+	var (
+		vcsCmd                  *vcs.Cmd
+		repo, rootPath, repoDir string
+		err                     error
+		blindRepo               bool // set if the repo has unusual configuration
+	)
+
+	// p can be either a real package, or a pseudo-package whose “import path” is
+	// actually a wildcard pattern.
+	// Trim the path at the element containing the first wildcard,
+	// and hope that it applies to the wildcarded parts too.
+	// This makes 'go get rsc.io/pdf/...' work in a fresh GOPATH.
+	importPrefix := p.ImportPath
+	if i := strings.Index(importPrefix, "..."); i >= 0 {
+		slash := strings.LastIndexByte(importPrefix[:i], '/')
+		if slash < 0 {
+			return fmt.Errorf("cannot expand ... in %q", p.ImportPath)
+		}
+		importPrefix = importPrefix[:slash]
+	}
+	if err := checkImportPath(importPrefix); err != nil {
+		return fmt.Errorf("%s: invalid import path: %v", p.ImportPath, err)
+	}
+	security := web.SecureOnly
+	if module.MatchPrefixPatterns(cfg.GOINSECURE, importPrefix) {
+		security = web.Insecure
+	}
+
+	if p.Internal.Build.SrcRoot != "" {
+		// Directory exists. Look for checkout along path to src.
+		const allowNesting = false
+		repoDir, vcsCmd, err = vcs.FromDir(p.Dir, p.Internal.Build.SrcRoot, allowNesting)
+		if err != nil {
+			return err
+		}
+		if !str.HasFilePathPrefix(repoDir, p.Internal.Build.SrcRoot) {
+			panic(fmt.Sprintf("repository %q not in source root %q", repo, p.Internal.Build.SrcRoot))
+		}
+		rootPath = str.TrimFilePathPrefix(repoDir, p.Internal.Build.SrcRoot)
+		if err := vcs.CheckGOVCS(vcsCmd, rootPath); err != nil {
+			return err
+		}
+
+		repo = "<local>" // should be unused; make distinctive
+
+		// Double-check where it came from.
+		if *getU && vcsCmd.RemoteRepo != nil {
+			dir := filepath.Join(p.Internal.Build.SrcRoot, filepath.FromSlash(rootPath))
+			remote, err := vcsCmd.RemoteRepo(vcsCmd, dir)
+			if err != nil {
+				// Proceed anyway. The package is present; we likely just don't understand
+				// the repo configuration (e.g. unusual remote protocol).
+				blindRepo = true
+			}
+			repo = remote
+			if !*getF && err == nil {
+				if rr, err := vcs.RepoRootForImportPath(importPrefix, vcs.IgnoreMod, security); err == nil {
+					repo := rr.Repo
+					if rr.VCS.ResolveRepo != nil {
+						resolved, err := rr.VCS.ResolveRepo(rr.VCS, dir, repo)
+						if err == nil {
+							repo = resolved
+						}
+					}
+					if remote != repo && rr.IsCustom {
+						return fmt.Errorf("%s is a custom import path for %s, but %s is checked out from %s", rr.Root, repo, dir, remote)
+					}
+				}
+			}
+		}
+	} else {
+		// Analyze the import path to determine the version control system,
+		// repository, and the import path for the root of the repository.
+		rr, err := vcs.RepoRootForImportPath(importPrefix, vcs.IgnoreMod, security)
+		if err != nil {
+			return err
+		}
+		vcsCmd, repo, rootPath = rr.VCS, rr.Repo, rr.Root
+	}
+	if !blindRepo && !vcsCmd.IsSecure(repo) && security != web.Insecure {
+		return fmt.Errorf("cannot download: %v uses insecure protocol", repo)
+	}
+
+	if p.Internal.Build.SrcRoot == "" {
+		// Package not found. Put in first directory of $GOPATH.
+		list := filepath.SplitList(cfg.BuildContext.GOPATH)
+		if len(list) == 0 {
+			return fmt.Errorf("cannot download: $GOPATH not set. For more details see: 'go help gopath'")
+		}
+		// Guard against people setting GOPATH=$GOROOT.
+		if filepath.Clean(list[0]) == filepath.Clean(cfg.GOROOT) {
+			return fmt.Errorf("cannot download: $GOPATH must not be set to $GOROOT. For more details see: 'go help gopath'")
+		}
+		if _, err := os.Stat(filepath.Join(list[0], "src/cmd/go/alldocs.go")); err == nil {
+			return fmt.Errorf("cannot download: %s is a GOROOT, not a GOPATH. For more details see: 'go help gopath'", list[0])
+		}
+		p.Internal.Build.Root = list[0]
+		p.Internal.Build.SrcRoot = filepath.Join(list[0], "src")
+		p.Internal.Build.PkgRoot = filepath.Join(list[0], "pkg")
+	}
+	root := filepath.Join(p.Internal.Build.SrcRoot, filepath.FromSlash(rootPath))
+
+	if err := vcs.CheckNested(vcsCmd, root, p.Internal.Build.SrcRoot); err != nil {
+		return err
+	}
+
+	// If we've considered this repository already, don't do it again.
+	if downloadRootCache[root] {
+		return nil
+	}
+	downloadRootCache[root] = true
+
+	if cfg.BuildV {
+		fmt.Fprintf(os.Stderr, "%s (download)\n", rootPath)
+	}
+
+	// Check that this is an appropriate place for the repo to be checked out.
+	// The target directory must either not exist or have a repo checked out already.
+	meta := filepath.Join(root, "."+vcsCmd.Cmd)
+	if _, err := os.Stat(meta); err != nil {
+		// Metadata file or directory does not exist. Prepare to checkout new copy.
+		// Some version control tools require the target directory not to exist.
+		// We require that too, just to avoid stepping on existing work.
+		if _, err := os.Stat(root); err == nil {
+			return fmt.Errorf("%s exists but %s does not - stale checkout?", root, meta)
+		}
+
+		_, err := os.Stat(p.Internal.Build.Root)
+		gopathExisted := err == nil
+
+		// Some version control tools require the parent of the target to exist.
+		parent, _ := filepath.Split(root)
+		if err = os.MkdirAll(parent, 0777); err != nil {
+			return err
+		}
+		if cfg.BuildV && !gopathExisted && p.Internal.Build.Root == cfg.BuildContext.GOPATH {
+			fmt.Fprintf(os.Stderr, "created GOPATH=%s; see 'go help gopath'\n", p.Internal.Build.Root)
+		}
+
+		if err = vcsCmd.Create(root, repo); err != nil {
+			return err
+		}
+	} else {
+		// Metadata directory does exist; download incremental updates.
+		if err = vcsCmd.Download(root); err != nil {
+			return err
+		}
+	}
+
+	if cfg.BuildN {
+		// Do not show tag sync in -n; it's noise more than anything,
+		// and since we're not running commands, no tag will be found.
+		// But avoid printing nothing.
+		fmt.Fprintf(os.Stderr, "# cd %s; %s sync/update\n", root, vcsCmd.Cmd)
+		return nil
+	}
+
+	// Select and sync to appropriate version of the repository.
+	tags, err := vcsCmd.Tags(root)
+	if err != nil {
+		return err
+	}
+	vers := runtime.Version()
+	if i := strings.Index(vers, " "); i >= 0 {
+		vers = vers[:i]
+	}
+	if err := vcsCmd.TagSync(root, selectTag(vers, tags)); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// selectTag returns the closest matching tag for a given version.
+// Closest means the latest one that is not after the current release.
+// Version "goX" (or "goX.Y" or "goX.Y.Z") matches tags of the same form.
+// Version "release.rN" matches tags of the form "go.rN" (N being a floating-point number).
+// Version "weekly.YYYY-MM-DD" matches tags like "go.weekly.YYYY-MM-DD".
+//
+// NOTE(rsc): Eventually we will need to decide on some logic here.
+// For now, there is only "go1". This matches the docs in go help get.
+func selectTag(goVersion string, tags []string) (match string) {
+	for _, t := range tags {
+		if t == "go1" {
+			return "go1"
+		}
+	}
+	return ""
+}
+
+// checkImportPath is like module.CheckImportPath, but it forbids leading dots
+// in path elements. This can lead to 'go get' creating .git and other VCS
+// directories in places we might run VCS tools later.
+func checkImportPath(path string) error {
+	if err := module.CheckImportPath(path); err != nil {
+		return err
+	}
+	checkElem := func(elem string) error {
+		if elem[0] == '.' {
+			return fmt.Errorf("malformed import path %q: leading dot in path element", path)
+		}
+		return nil
+	}
+	elemStart := 0
+	for i, r := range path {
+		if r == '/' {
+			if err := checkElem(path[elemStart:]); err != nil {
+				return err
+			}
+			elemStart = i + 1
+		}
+	}
+	if err := checkElem(path[elemStart:]); err != nil {
+		return err
+	}
+	return nil
+}
diff --git a/src/cmd/go/internal/get/tag_test.go b/src/cmd/go/internal/get/tag_test.go
new file mode 100644
index 0000000..9a25dfa
--- /dev/null
+++ b/src/cmd/go/internal/get/tag_test.go
@@ -0,0 +1,100 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package get
+
+import "testing"
+
+var selectTagTestTags = []string{
+	"go.r58",
+	"go.r58.1",
+	"go.r59",
+	"go.r59.1",
+	"go.r61",
+	"go.r61.1",
+	"go.weekly.2010-01-02",
+	"go.weekly.2011-10-12",
+	"go.weekly.2011-10-12.1",
+	"go.weekly.2011-10-14",
+	"go.weekly.2011-11-01",
+	"go1",
+	"go1.0.1",
+	"go1.999",
+	"go1.9.2",
+	"go5",
+
+	// these should be ignored:
+	"release.r59",
+	"release.r59.1",
+	"release",
+	"weekly.2011-10-12",
+	"weekly.2011-10-12.1",
+	"weekly",
+	"foo",
+	"bar",
+	"go.f00",
+	"go!r60",
+	"go.1999-01-01",
+	"go.2x",
+	"go.20000000000000",
+	"go.2.",
+	"go.2.0",
+	"go2x",
+	"go20000000000000",
+	"go2.",
+	"go2.0",
+}
+
+var selectTagTests = []struct {
+	version  string
+	selected string
+}{
+	/*
+		{"release.r57", ""},
+		{"release.r58.2", "go.r58.1"},
+		{"release.r59", "go.r59"},
+		{"release.r59.1", "go.r59.1"},
+		{"release.r60", "go.r59.1"},
+		{"release.r60.1", "go.r59.1"},
+		{"release.r61", "go.r61"},
+		{"release.r66", "go.r61.1"},
+		{"weekly.2010-01-01", ""},
+		{"weekly.2010-01-02", "go.weekly.2010-01-02"},
+		{"weekly.2010-01-02.1", "go.weekly.2010-01-02"},
+		{"weekly.2010-01-03", "go.weekly.2010-01-02"},
+		{"weekly.2011-10-12", "go.weekly.2011-10-12"},
+		{"weekly.2011-10-12.1", "go.weekly.2011-10-12.1"},
+		{"weekly.2011-10-13", "go.weekly.2011-10-12.1"},
+		{"weekly.2011-10-14", "go.weekly.2011-10-14"},
+		{"weekly.2011-10-14.1", "go.weekly.2011-10-14"},
+		{"weekly.2011-11-01", "go.weekly.2011-11-01"},
+		{"weekly.2014-01-01", "go.weekly.2011-11-01"},
+		{"weekly.3000-01-01", "go.weekly.2011-11-01"},
+		{"go1", "go1"},
+		{"go1.1", "go1.0.1"},
+		{"go1.998", "go1.9.2"},
+		{"go1.1000", "go1.999"},
+		{"go6", "go5"},
+
+		// faulty versions:
+		{"release.f00", ""},
+		{"weekly.1999-01-01", ""},
+		{"junk", ""},
+		{"", ""},
+		{"go2x", ""},
+		{"go200000000000", ""},
+		{"go2.", ""},
+		{"go2.0", ""},
+	*/
+	{"anything", "go1"},
+}
+
+func TestSelectTag(t *testing.T) {
+	for _, c := range selectTagTests {
+		selected := selectTag(c.version, selectTagTestTags)
+		if selected != c.selected {
+			t.Errorf("selectTag(%q) = %q, want %q", c.version, selected, c.selected)
+		}
+	}
+}
diff --git a/src/cmd/go/internal/gover/gomod.go b/src/cmd/go/internal/gover/gomod.go
new file mode 100644
index 0000000..4a4ae53
--- /dev/null
+++ b/src/cmd/go/internal/gover/gomod.go
@@ -0,0 +1,43 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gover
+
+import (
+	"bytes"
+	"strings"
+)
+
+var nl = []byte("\n")
+
+// GoModLookup takes go.mod or go.work content,
+// finds the first line in the file starting with the given key,
+// and returns the value associated with that key.
+//
+// Lookup should only be used with non-factored verbs
+// such as "go" and "toolchain", usually to find versions
+// or version-like strings.
+func GoModLookup(gomod []byte, key string) string {
+	for len(gomod) > 0 {
+		var line []byte
+		line, gomod, _ = bytes.Cut(gomod, nl)
+		line = bytes.TrimSpace(line)
+		if v, ok := parseKey(line, key); ok {
+			return v
+		}
+	}
+	return ""
+}
+
+func parseKey(line []byte, key string) (string, bool) {
+	if !strings.HasPrefix(string(line), key) {
+		return "", false
+	}
+	s := strings.TrimPrefix(string(line), key)
+	if len(s) == 0 || (s[0] != ' ' && s[0] != '\t') {
+		return "", false
+	}
+	s, _, _ = strings.Cut(s, "//") // strip comments
+	return strings.TrimSpace(s), true
+}
diff --git a/src/cmd/go/internal/gover/gover.go b/src/cmd/go/internal/gover/gover.go
new file mode 100644
index 0000000..b2a8261
--- /dev/null
+++ b/src/cmd/go/internal/gover/gover.go
@@ -0,0 +1,254 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package gover implements support for Go toolchain versions like 1.21.0 and 1.21rc1.
+// (For historical reasons, Go does not use semver for its toolchains.)
+// This package provides the same basic analysis that golang.org/x/mod/semver does for semver.
+// It also provides some helpers for extracting versions from go.mod files
+// and for dealing with module.Versions that may use Go versions or semver
+// depending on the module path.
+package gover
+
+import (
+	"cmp"
+)
+
+// A version is a parsed Go version: major[.minor[.patch]][kind[pre]]
+// The numbers are the original decimal strings to avoid integer overflows
+// and since there is very little actual math. (Probably overflow doesn't matter in practice,
+// but at the time this code was written, there was an existing test that used
+// go1.99999999999, which does not fit in an int on 32-bit platforms.
+// The "big decimal" representation avoids the problem entirely.)
+type version struct {
+	major string // decimal
+	minor string // decimal or ""
+	patch string // decimal or ""
+	kind  string // "", "alpha", "beta", "rc"
+	pre   string // decimal or ""
+}
+
+// Compare returns -1, 0, or +1 depending on whether
+// x < y, x == y, or x > y, interpreted as toolchain versions.
+// The versions x and y must not begin with a "go" prefix: just "1.21" not "go1.21".
+// Malformed versions compare less than well-formed versions and equal to each other.
+// The language version "1.21" compares less than the release candidate and eventual releases "1.21rc1" and "1.21.0".
+func Compare(x, y string) int {
+	vx := parse(x)
+	vy := parse(y)
+
+	if c := cmpInt(vx.major, vy.major); c != 0 {
+		return c
+	}
+	if c := cmpInt(vx.minor, vy.minor); c != 0 {
+		return c
+	}
+	if c := cmpInt(vx.patch, vy.patch); c != 0 {
+		return c
+	}
+	if c := cmp.Compare(vx.kind, vy.kind); c != 0 { // "" < alpha < beta < rc
+		return c
+	}
+	if c := cmpInt(vx.pre, vy.pre); c != 0 {
+		return c
+	}
+	return 0
+}
+
+// Max returns the maximum of x and y interpreted as toolchain versions,
+// compared using Compare.
+// If x and y compare equal, Max returns x.
+func Max(x, y string) string {
+	if Compare(x, y) < 0 {
+		return y
+	}
+	return x
+}
+
+// Toolchain returns the maximum of x and y interpreted as toolchain names,
+// compared using Compare(FromToolchain(x), FromToolchain(y)).
+// If x and y compare equal, Max returns x.
+func ToolchainMax(x, y string) string {
+	if Compare(FromToolchain(x), FromToolchain(y)) < 0 {
+		return y
+	}
+	return x
+}
+
+// IsLang reports whether v denotes the overall Go language version
+// and not a specific release. Starting with the Go 1.21 release, "1.x" denotes
+// the overall language version; the first release is "1.x.0".
+// The distinction is important because the relative ordering is
+//
+//	1.21 < 1.21rc1 < 1.21.0
+//
+// meaning that Go 1.21rc1 and Go 1.21.0 will both handle go.mod files that
+// say "go 1.21", but Go 1.21rc1 will not handle files that say "go 1.21.0".
+func IsLang(x string) bool {
+	v := parse(x)
+	return v != version{} && v.patch == "" && v.kind == "" && v.pre == ""
+}
+
+// Lang returns the Go language version. For example, Lang("1.2.3") == "1.2".
+func Lang(x string) string {
+	v := parse(x)
+	if v.minor == "" {
+		return v.major
+	}
+	return v.major + "." + v.minor
+}
+
+// IsPrerelease reports whether v denotes a Go prerelease version.
+func IsPrerelease(x string) bool {
+	return parse(x).kind != ""
+}
+
+// Prev returns the Go major release immediately preceding v,
+// or v itself if v is the first Go major release (1.0) or not a supported
+// Go version.
+//
+// Examples:
+//
+//	Prev("1.2") = "1.1"
+//	Prev("1.3rc4") = "1.2"
+func Prev(x string) string {
+	v := parse(x)
+	if cmpInt(v.minor, "1") <= 0 {
+		return v.major
+	}
+	return v.major + "." + decInt(v.minor)
+}
+
+// IsValid reports whether the version x is valid.
+func IsValid(x string) bool {
+	return parse(x) != version{}
+}
+
+// parse parses the Go version string x into a version.
+// It returns the zero version if x is malformed.
+func parse(x string) version {
+	var v version
+
+	// Parse major version.
+	var ok bool
+	v.major, x, ok = cutInt(x)
+	if !ok {
+		return version{}
+	}
+	if x == "" {
+		// Interpret "1" as "1.0.0".
+		v.minor = "0"
+		v.patch = "0"
+		return v
+	}
+
+	// Parse . before minor version.
+	if x[0] != '.' {
+		return version{}
+	}
+
+	// Parse minor version.
+	v.minor, x, ok = cutInt(x[1:])
+	if !ok {
+		return version{}
+	}
+	if x == "" {
+		// Patch missing is same as "0" for older versions.
+		// Starting in Go 1.21, patch missing is different from explicit .0.
+		if cmpInt(v.minor, "21") < 0 {
+			v.patch = "0"
+		}
+		return v
+	}
+
+	// Parse patch if present.
+	if x[0] == '.' {
+		v.patch, x, ok = cutInt(x[1:])
+		if !ok || x != "" {
+			// Note that we are disallowing prereleases (alpha, beta, rc) for patch releases here (x != "").
+			// Allowing them would be a bit confusing because we already have:
+			//	1.21 < 1.21rc1
+			// But a prerelease of a patch would have the opposite effect:
+			//	1.21.3rc1 < 1.21.3
+			// We've never needed them before, so let's not start now.
+			return version{}
+		}
+		return v
+	}
+
+	// Parse prerelease.
+	i := 0
+	for i < len(x) && (x[i] < '0' || '9' < x[i]) {
+		if x[i] < 'a' || 'z' < x[i] {
+			return version{}
+		}
+		i++
+	}
+	if i == 0 {
+		return version{}
+	}
+	v.kind, x = x[:i], x[i:]
+	if x == "" {
+		return v
+	}
+	v.pre, x, ok = cutInt(x)
+	if !ok || x != "" {
+		return version{}
+	}
+
+	return v
+}
+
+// cutInt scans the leading decimal number at the start of x to an integer
+// and returns that value and the rest of the string.
+func cutInt(x string) (n, rest string, ok bool) {
+	i := 0
+	for i < len(x) && '0' <= x[i] && x[i] <= '9' {
+		i++
+	}
+	if i == 0 || x[0] == '0' && i != 1 {
+		return "", "", false
+	}
+	return x[:i], x[i:], true
+}
+
+// cmpInt returns cmp.Compare(x, y) interpreting x and y as decimal numbers.
+// (Copied from golang.org/x/mod/semver's compareInt.)
+func cmpInt(x, y string) int {
+	if x == y {
+		return 0
+	}
+	if len(x) < len(y) {
+		return -1
+	}
+	if len(x) > len(y) {
+		return +1
+	}
+	if x < y {
+		return -1
+	} else {
+		return +1
+	}
+}
+
+// decInt returns the decimal string decremented by 1, or the empty string
+// if the decimal is all zeroes.
+// (Copied from golang.org/x/mod/module's decDecimal.)
+func decInt(decimal string) string {
+	// Scan right to left turning 0s to 9s until you find a digit to decrement.
+	digits := []byte(decimal)
+	i := len(digits) - 1
+	for ; i >= 0 && digits[i] == '0'; i-- {
+		digits[i] = '9'
+	}
+	if i < 0 {
+		// decimal is all zeros
+		return ""
+	}
+	if i == 0 && digits[i] == '1' && len(digits) > 1 {
+		digits = digits[1:]
+	} else {
+		digits[i]--
+	}
+	return string(digits)
+}
diff --git a/src/cmd/go/internal/gover/gover_test.go b/src/cmd/go/internal/gover/gover_test.go
new file mode 100644
index 0000000..3a0bf10
--- /dev/null
+++ b/src/cmd/go/internal/gover/gover_test.go
@@ -0,0 +1,160 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gover
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestCompare(t *testing.T) { test2(t, compareTests, "Compare", Compare) }
+
+var compareTests = []testCase2[string, string, int]{
+	{"", "", 0},
+	{"x", "x", 0},
+	{"", "x", 0},
+	{"1", "1.1", -1},
+	{"1.5", "1.6", -1},
+	{"1.5", "1.10", -1},
+	{"1.6", "1.6.1", -1},
+	{"1.19", "1.19.0", 0},
+	{"1.19rc1", "1.19", -1},
+	{"1.20", "1.20.0", 0},
+	{"1.20rc1", "1.20", -1},
+	{"1.21", "1.21.0", -1},
+	{"1.21", "1.21rc1", -1},
+	{"1.21rc1", "1.21.0", -1},
+	{"1.6", "1.19", -1},
+	{"1.19", "1.19.1", -1},
+	{"1.19rc1", "1.19", -1},
+	{"1.19rc1", "1.19.1", -1},
+	{"1.19rc1", "1.19rc2", -1},
+	{"1.19.0", "1.19.1", -1},
+	{"1.19rc1", "1.19.0", -1},
+	{"1.19alpha3", "1.19beta2", -1},
+	{"1.19beta2", "1.19rc1", -1},
+	{"1.1", "1.99999999999999998", -1},
+	{"1.99999999999999998", "1.99999999999999999", -1},
+}
+
+func TestParse(t *testing.T) { test1(t, parseTests, "parse", parse) }
+
+var parseTests = []testCase1[string, version]{
+	{"1", version{"1", "0", "0", "", ""}},
+	{"1.2", version{"1", "2", "0", "", ""}},
+	{"1.2.3", version{"1", "2", "3", "", ""}},
+	{"1.2rc3", version{"1", "2", "", "rc", "3"}},
+	{"1.20", version{"1", "20", "0", "", ""}},
+	{"1.21", version{"1", "21", "", "", ""}},
+	{"1.21rc3", version{"1", "21", "", "rc", "3"}},
+	{"1.21.0", version{"1", "21", "0", "", ""}},
+	{"1.24", version{"1", "24", "", "", ""}},
+	{"1.24rc3", version{"1", "24", "", "rc", "3"}},
+	{"1.24.0", version{"1", "24", "0", "", ""}},
+	{"1.999testmod", version{"1", "999", "", "testmod", ""}},
+	{"1.99999999999999999", version{"1", "99999999999999999", "", "", ""}},
+}
+
+func TestLang(t *testing.T) { test1(t, langTests, "Lang", Lang) }
+
+var langTests = []testCase1[string, string]{
+	{"1.2rc3", "1.2"},
+	{"1.2.3", "1.2"},
+	{"1.2", "1.2"},
+	{"1", "1.0"},
+	{"1.999testmod", "1.999"},
+}
+
+func TestIsLang(t *testing.T) { test1(t, isLangTests, "IsLang", IsLang) }
+
+var isLangTests = []testCase1[string, bool]{
+	{"1.2rc3", false},
+	{"1.2.3", false},
+	{"1.999testmod", false},
+	{"1.22", true},
+	{"1.21", true},
+	{"1.20", false}, // == 1.20.0
+	{"1.19", false}, // == 1.20.0
+	{"1.3", false},  // == 1.3.0
+	{"1.2", false},  // == 1.2.0
+	{"1", false},    // == 1.0.0
+}
+
+func TestPrev(t *testing.T) { test1(t, prevTests, "Prev", Prev) }
+
+var prevTests = []testCase1[string, string]{
+	{"", ""},
+	{"0", "0"},
+	{"1.3rc4", "1.2"},
+	{"1.3.5", "1.2"},
+	{"1.3", "1.2"},
+	{"1", "1"},
+	{"1.99999999999999999", "1.99999999999999998"},
+	{"1.40000000000000000", "1.39999999999999999"},
+}
+
+func TestIsValid(t *testing.T) { test1(t, isValidTests, "IsValid", IsValid) }
+
+var isValidTests = []testCase1[string, bool]{
+	{"1.2rc3", true},
+	{"1.2.3", true},
+	{"1.999testmod", true},
+	{"1.600+auto", false},
+	{"1.22", true},
+	{"1.21.0", true},
+	{"1.21rc2", true},
+	{"1.21", true},
+	{"1.20.0", true},
+	{"1.20", true},
+	{"1.19", true},
+	{"1.3", true},
+	{"1.2", true},
+	{"1", true},
+}
+
+type testCase1[In, Out any] struct {
+	in  In
+	out Out
+}
+
+type testCase2[In1, In2, Out any] struct {
+	in1 In1
+	in2 In2
+	out Out
+}
+
+type testCase3[In1, In2, In3, Out any] struct {
+	in1 In1
+	in2 In2
+	in3 In3
+	out Out
+}
+
+func test1[In, Out any](t *testing.T, tests []testCase1[In, Out], name string, f func(In) Out) {
+	t.Helper()
+	for _, tt := range tests {
+		if out := f(tt.in); !reflect.DeepEqual(out, tt.out) {
+			t.Errorf("%s(%v) = %v, want %v", name, tt.in, out, tt.out)
+		}
+	}
+}
+
+func test2[In1, In2, Out any](t *testing.T, tests []testCase2[In1, In2, Out], name string, f func(In1, In2) Out) {
+	t.Helper()
+	for _, tt := range tests {
+		if out := f(tt.in1, tt.in2); !reflect.DeepEqual(out, tt.out) {
+			t.Errorf("%s(%+v, %+v) = %+v, want %+v", name, tt.in1, tt.in2, out, tt.out)
+		}
+	}
+}
+
+func test3[In1, In2, In3, Out any](t *testing.T, tests []testCase3[In1, In2, In3, Out], name string, f func(In1, In2, In3) Out) {
+	t.Helper()
+	for _, tt := range tests {
+		if out := f(tt.in1, tt.in2, tt.in3); !reflect.DeepEqual(out, tt.out) {
+			t.Errorf("%s(%+v, %+v, %+v) = %+v, want %+v", name, tt.in1, tt.in2, tt.in3, out, tt.out)
+		}
+	}
+}
diff --git a/src/cmd/go/internal/gover/local.go b/src/cmd/go/internal/gover/local.go
new file mode 100644
index 0000000..8183a5c
--- /dev/null
+++ b/src/cmd/go/internal/gover/local.go
@@ -0,0 +1,42 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gover
+
+import (
+	"internal/goversion"
+	"runtime"
+	"strconv"
+)
+
+// TestVersion is initialized in the go command test binary
+// to be $TESTGO_VERSION, to allow tests to override the
+// go command's idea of its own version as returned by Local.
+var TestVersion string
+
+// Local returns the local Go version, the one implemented by this go command.
+func Local() string {
+	v, _ := local()
+	return v
+}
+
+// LocalToolchain returns the local toolchain name, the one implemented by this go command.
+func LocalToolchain() string {
+	_, t := local()
+	return t
+}
+
+func local() (goVers, toolVers string) {
+	toolVers = runtime.Version()
+	if TestVersion != "" {
+		toolVers = TestVersion
+	}
+	goVers = FromToolchain(toolVers)
+	if goVers == "" {
+		// Development branch. Use "Dev" version with just 1.N, no rc1 or .0 suffix.
+		goVers = "1." + strconv.Itoa(goversion.Version)
+		toolVers = "go" + goVers
+	}
+	return goVers, toolVers
+}
diff --git a/src/cmd/go/internal/gover/mod.go b/src/cmd/go/internal/gover/mod.go
new file mode 100644
index 0000000..d3cc170
--- /dev/null
+++ b/src/cmd/go/internal/gover/mod.go
@@ -0,0 +1,127 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gover
+
+import (
+	"sort"
+	"strings"
+
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
+)
+
+// IsToolchain reports whether the module path corresponds to the
+// virtual, non-downloadable module tracking go or toolchain directives in the go.mod file.
+//
+// Note that IsToolchain only matches "go" and "toolchain", not the
+// real, downloadable module "golang.org/toolchain" containing toolchain files.
+//
+//	IsToolchain("go") = true
+//	IsToolchain("toolchain") = true
+//	IsToolchain("golang.org/x/tools") = false
+//	IsToolchain("golang.org/toolchain") = false
+func IsToolchain(path string) bool {
+	return path == "go" || path == "toolchain"
+}
+
+// ModCompare returns the result of comparing the versions x and y
+// for the module with the given path.
+// The path is necessary because the "go" and "toolchain" modules
+// use a different version syntax and semantics (gover, this package)
+// than most modules (semver).
+func ModCompare(path string, x, y string) int {
+	if path == "go" {
+		return Compare(x, y)
+	}
+	if path == "toolchain" {
+		return Compare(maybeToolchainVersion(x), maybeToolchainVersion(y))
+	}
+	return semver.Compare(x, y)
+}
+
+// ModSort is like module.Sort but understands the "go" and "toolchain"
+// modules and their version ordering.
+func ModSort(list []module.Version) {
+	sort.Slice(list, func(i, j int) bool {
+		mi := list[i]
+		mj := list[j]
+		if mi.Path != mj.Path {
+			return mi.Path < mj.Path
+		}
+		// To help go.sum formatting, allow version/file.
+		// Compare semver prefix by semver rules,
+		// file by string order.
+		vi := mi.Version
+		vj := mj.Version
+		var fi, fj string
+		if k := strings.Index(vi, "/"); k >= 0 {
+			vi, fi = vi[:k], vi[k:]
+		}
+		if k := strings.Index(vj, "/"); k >= 0 {
+			vj, fj = vj[:k], vj[k:]
+		}
+		if vi != vj {
+			return ModCompare(mi.Path, vi, vj) < 0
+		}
+		return fi < fj
+	})
+}
+
+// ModIsValid reports whether vers is a valid version syntax for the module with the given path.
+func ModIsValid(path, vers string) bool {
+	if IsToolchain(path) {
+		if path == "toolchain" {
+			return IsValid(FromToolchain(vers))
+		}
+		return IsValid(vers)
+	}
+	return semver.IsValid(vers)
+}
+
+// ModIsPrefix reports whether v is a valid version syntax prefix for the module with the given path.
+// The caller is assumed to have checked that ModIsValid(path, vers) is true.
+func ModIsPrefix(path, vers string) bool {
+	if IsToolchain(path) {
+		if path == "toolchain" {
+			return IsLang(FromToolchain(vers))
+		}
+		return IsLang(vers)
+	}
+	// Semver
+	dots := 0
+	for i := 0; i < len(vers); i++ {
+		switch vers[i] {
+		case '-', '+':
+			return false
+		case '.':
+			dots++
+			if dots >= 2 {
+				return false
+			}
+		}
+	}
+	return true
+}
+
+// ModIsPrerelease reports whether v is a prerelease version for the module with the given path.
+// The caller is assumed to have checked that ModIsValid(path, vers) is true.
+func ModIsPrerelease(path, vers string) bool {
+	if IsToolchain(path) {
+		return IsPrerelease(vers)
+	}
+	return semver.Prerelease(vers) != ""
+}
+
+// ModMajorMinor returns the "major.minor" truncation of the version v,
+// for use as a prefix in "@patch" queries.
+func ModMajorMinor(path, vers string) string {
+	if IsToolchain(path) {
+		if path == "toolchain" {
+			return "go" + Lang(FromToolchain(vers))
+		}
+		return Lang(vers)
+	}
+	return semver.MajorMinor(vers)
+}
diff --git a/src/cmd/go/internal/gover/mod_test.go b/src/cmd/go/internal/gover/mod_test.go
new file mode 100644
index 0000000..c92169c
--- /dev/null
+++ b/src/cmd/go/internal/gover/mod_test.go
@@ -0,0 +1,72 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gover
+
+import (
+	"slices"
+	"strings"
+	"testing"
+
+	"golang.org/x/mod/module"
+)
+
+func TestIsToolchain(t *testing.T) { test1(t, isToolchainTests, "IsToolchain", IsToolchain) }
+
+var isToolchainTests = []testCase1[string, bool]{
+	{"go", true},
+	{"toolchain", true},
+	{"anything", false},
+	{"golang.org/toolchain", false},
+}
+
+func TestModCompare(t *testing.T) { test3(t, modCompareTests, "ModCompare", ModCompare) }
+
+var modCompareTests = []testCase3[string, string, string, int]{
+	{"go", "1.2", "1.3", -1},
+	{"go", "v1.2", "v1.3", 0}, // equal because invalid
+	{"go", "1.2", "1.2", 0},
+	{"toolchain", "go1.2", "go1.3", -1},
+	{"toolchain", "go1.2", "go1.2", 0},
+	{"toolchain", "1.2", "1.3", -1},  // accepted but non-standard
+	{"toolchain", "v1.2", "v1.3", 0}, // equal because invalid
+	{"rsc.io/quote", "v1.2", "v1.3", -1},
+	{"rsc.io/quote", "1.2", "1.3", 0}, // equal because invalid
+}
+
+func TestModIsValid(t *testing.T) { test2(t, modIsValidTests, "ModIsValid", ModIsValid) }
+
+var modIsValidTests = []testCase2[string, string, bool]{
+	{"go", "1.2", true},
+	{"go", "v1.2", false},
+	{"toolchain", "go1.2", true},
+	{"toolchain", "v1.2", false},
+	{"rsc.io/quote", "v1.2", true},
+	{"rsc.io/quote", "1.2", false},
+}
+
+func TestModSort(t *testing.T) {
+	test1(t, modSortTests, "ModSort", func(list []module.Version) []module.Version {
+		out := slices.Clone(list)
+		ModSort(out)
+		return out
+	})
+}
+
+var modSortTests = []testCase1[[]module.Version, []module.Version]{
+	{
+		mvl(`z v1.1; a v1.2; a v1.1; go 1.3; toolchain 1.3; toolchain 1.2; go 1.2`),
+		mvl(`a v1.1; a v1.2; go 1.2; go 1.3; toolchain 1.2; toolchain 1.3; z v1.1`),
+	},
+}
+
+func mvl(s string) []module.Version {
+	var list []module.Version
+	for _, f := range strings.Split(s, ";") {
+		f = strings.TrimSpace(f)
+		path, vers, _ := strings.Cut(f, " ")
+		list = append(list, module.Version{Path: path, Version: vers})
+	}
+	return list
+}
diff --git a/src/cmd/go/internal/gover/toolchain.go b/src/cmd/go/internal/gover/toolchain.go
new file mode 100644
index 0000000..a24df98
--- /dev/null
+++ b/src/cmd/go/internal/gover/toolchain.go
@@ -0,0 +1,98 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gover
+
+import (
+	"cmd/go/internal/base"
+	"context"
+	"errors"
+	"fmt"
+	"strings"
+)
+
+// FromToolchain returns the Go version for the named toolchain,
+// derived from the name itself (not by running the toolchain).
+// A toolchain is named "goVERSION".
+// A suffix after the VERSION introduced by a -, space, or tab is removed.
+// Examples:
+//
+//	FromToolchain("go1.2.3") == "1.2.3"
+//	FromToolchain("go1.2.3-bigcorp") == "1.2.3"
+//	FromToolchain("invalid") == ""
+func FromToolchain(name string) string {
+	if strings.ContainsAny(name, "\\/") {
+		// The suffix must not include a path separator, since that would cause
+		// exec.LookPath to resolve it from a relative directory instead of from
+		// $PATH.
+		return ""
+	}
+
+	var v string
+	if strings.HasPrefix(name, "go") {
+		v = name[2:]
+	} else {
+		return ""
+	}
+	// Some builds use custom suffixes; strip them.
+	if i := strings.IndexAny(v, " \t-"); i >= 0 {
+		v = v[:i]
+	}
+	if !IsValid(v) {
+		return ""
+	}
+	return v
+}
+
+func maybeToolchainVersion(name string) string {
+	if IsValid(name) {
+		return name
+	}
+	return FromToolchain(name)
+}
+
+// Startup records the information that went into the startup-time version switch.
+// It is initialized by switchGoToolchain.
+var Startup struct {
+	GOTOOLCHAIN   string // $GOTOOLCHAIN setting
+	AutoFile      string // go.mod or go.work file consulted
+	AutoGoVersion string // go line found in file
+	AutoToolchain string // toolchain line found in file
+}
+
+// A TooNewError explains that a module is too new for this version of Go.
+type TooNewError struct {
+	What      string
+	GoVersion string
+	Toolchain string // for callers if they want to use it, but not printed
+}
+
+func (e *TooNewError) Error() string {
+	var explain string
+	if Startup.GOTOOLCHAIN != "" && Startup.GOTOOLCHAIN != "auto" {
+		explain = "; GOTOOLCHAIN=" + Startup.GOTOOLCHAIN
+	}
+	if Startup.AutoFile != "" && (Startup.AutoGoVersion != "" || Startup.AutoToolchain != "") {
+		explain += fmt.Sprintf("; %s sets ", base.ShortPath(Startup.AutoFile))
+		if Startup.AutoToolchain != "" {
+			explain += "toolchain " + Startup.AutoToolchain
+		} else {
+			explain += "go " + Startup.AutoGoVersion
+		}
+	}
+	return fmt.Sprintf("%v requires go >= %v (running go %v%v)", e.What, e.GoVersion, Local(), explain)
+}
+
+var ErrTooNew = errors.New("module too new")
+
+func (e *TooNewError) Is(err error) bool {
+	return err == ErrTooNew
+}
+
+// A Switcher provides the ability to switch to a new toolchain in response to TooNewErrors.
+// See [cmd/go/internal/toolchain.Switcher] for documentation.
+type Switcher interface {
+	Error(err error)
+	Switch(ctx context.Context)
+}
diff --git a/src/cmd/go/internal/gover/toolchain_test.go b/src/cmd/go/internal/gover/toolchain_test.go
new file mode 100644
index 0000000..d1c22fb
--- /dev/null
+++ b/src/cmd/go/internal/gover/toolchain_test.go
@@ -0,0 +1,19 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gover
+
+import "testing"
+
+func TestFromToolchain(t *testing.T) { test1(t, fromToolchainTests, "FromToolchain", FromToolchain) }
+
+var fromToolchainTests = []testCase1[string, string]{
+	{"go1.2.3", "1.2.3"},
+	{"1.2.3", ""},
+	{"go1.2.3+bigcorp", ""},
+	{"go1.2.3-bigcorp", "1.2.3"},
+	{"go1.2.3-bigcorp more text", "1.2.3"},
+	{"gccgo-go1.23rc4", ""},
+	{"gccgo-go1.23rc4-bigdwarf", ""},
+}
diff --git a/src/cmd/go/internal/gover/version.go b/src/cmd/go/internal/gover/version.go
new file mode 100644
index 0000000..2681013
--- /dev/null
+++ b/src/cmd/go/internal/gover/version.go
@@ -0,0 +1,74 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gover
+
+import "golang.org/x/mod/modfile"
+
+const (
+	// narrowAllVersion is the Go version at which the
+	// module-module "all" pattern no longer closes over the dependencies of
+	// tests outside of the main module.
+	NarrowAllVersion = "1.16"
+
+	// DefaultGoModVersion is the Go version to assume for go.mod files
+	// that do not declare a Go version. The go command has been
+	// writing go versions to modules since Go 1.12, so a go.mod
+	// without a version is either very old or recently hand-written.
+	// Since we can't tell which, we have to assume it's very old.
+	// The semantics of the go.mod changed at Go 1.17 to support
+	// graph pruning. If see a go.mod without a go line, we have to
+	// assume Go 1.16 so that we interpret the requirements correctly.
+	// Note that this default must stay at Go 1.16; it cannot be moved forward.
+	DefaultGoModVersion = "1.16"
+
+	// DefaultGoWorkVersion is the Go version to assume for go.work files
+	// that do not declare a Go version. Workspaces were added in Go 1.18,
+	// so use that.
+	DefaultGoWorkVersion = "1.18"
+
+	// ExplicitIndirectVersion is the Go version at which a
+	// module's go.mod file is expected to list explicit requirements on every
+	// module that provides any package transitively imported by that module.
+	//
+	// Other indirect dependencies of such a module can be safely pruned out of
+	// the module graph; see https://golang.org/ref/mod#graph-pruning.
+	ExplicitIndirectVersion = "1.17"
+
+	// separateIndirectVersion is the Go version at which
+	// "// indirect" dependencies are added in a block separate from the direct
+	// ones. See https://golang.org/issue/45965.
+	SeparateIndirectVersion = "1.17"
+
+	// tidyGoModSumVersion is the Go version at which
+	// 'go mod tidy' preserves go.mod checksums needed to build test dependencies
+	// of packages in "all", so that 'go test all' can be run without checksum
+	// errors.
+	// See https://go.dev/issue/56222.
+	TidyGoModSumVersion = "1.21"
+
+	// goStrictVersion is the Go version at which the Go versions
+	// became "strict" in the sense that, restricted to modules at this version
+	// or later, every module must have a go version line ≥ all its dependencies.
+	// It is also the version after which "too new" a version is considered a fatal error.
+	GoStrictVersion = "1.21"
+)
+
+// FromGoMod returns the go version from the go.mod file.
+// It returns DefaultGoModVersion if the go.mod file does not contain a go line or if mf is nil.
+func FromGoMod(mf *modfile.File) string {
+	if mf == nil || mf.Go == nil {
+		return DefaultGoModVersion
+	}
+	return mf.Go.Version
+}
+
+// FromGoWork returns the go version from the go.mod file.
+// It returns DefaultGoWorkVersion if the go.mod file does not contain a go line or if wf is nil.
+func FromGoWork(wf *modfile.WorkFile) string {
+	if wf == nil || wf.Go == nil {
+		return DefaultGoWorkVersion
+	}
+	return wf.Go.Version
+}
diff --git a/src/cmd/go/internal/help/help.go b/src/cmd/go/internal/help/help.go
new file mode 100644
index 0000000..aeaba78
--- /dev/null
+++ b/src/cmd/go/internal/help/help.go
@@ -0,0 +1,195 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package help implements the “go help” command.
+package help
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+	"text/template"
+	"unicode"
+	"unicode/utf8"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/modload"
+)
+
+// Help implements the 'help' command.
+func Help(w io.Writer, args []string) {
+	// 'go help documentation' generates doc.go.
+	if len(args) == 1 && args[0] == "documentation" {
+		fmt.Fprintln(w, "// Copyright 2011 The Go Authors. All rights reserved.")
+		fmt.Fprintln(w, "// Use of this source code is governed by a BSD-style")
+		fmt.Fprintln(w, "// license that can be found in the LICENSE file.")
+		fmt.Fprintln(w)
+		fmt.Fprintln(w, "// Code generated by 'go test cmd/go -v -run=TestDocsUpToDate -fixdocs'; DO NOT EDIT.")
+		fmt.Fprintln(w, "// Edit the documentation in other files and then execute 'go generate cmd/go' to generate this one.")
+		fmt.Fprintln(w)
+		buf := new(strings.Builder)
+		PrintUsage(buf, base.Go)
+		usage := &base.Command{Long: buf.String()}
+		cmds := []*base.Command{usage}
+		for _, cmd := range base.Go.Commands {
+			// Avoid duplication of the "get" documentation.
+			if cmd.UsageLine == "module-get" && modload.Enabled() {
+				continue
+			} else if cmd.UsageLine == "gopath-get" && !modload.Enabled() {
+				continue
+			}
+			cmds = append(cmds, cmd)
+			cmds = append(cmds, cmd.Commands...)
+		}
+		tmpl(&commentWriter{W: w}, documentationTemplate, cmds)
+		fmt.Fprintln(w, "package main")
+		return
+	}
+
+	cmd := base.Go
+Args:
+	for i, arg := range args {
+		for _, sub := range cmd.Commands {
+			if sub.Name() == arg {
+				cmd = sub
+				continue Args
+			}
+		}
+
+		// helpSuccess is the help command using as many args as possible that would succeed.
+		helpSuccess := "go help"
+		if i > 0 {
+			helpSuccess += " " + strings.Join(args[:i], " ")
+		}
+		fmt.Fprintf(os.Stderr, "go help %s: unknown help topic. Run '%s'.\n", strings.Join(args, " "), helpSuccess)
+		base.SetExitStatus(2) // failed at 'go help cmd'
+		base.Exit()
+	}
+
+	if len(cmd.Commands) > 0 {
+		PrintUsage(os.Stdout, cmd)
+	} else {
+		tmpl(os.Stdout, helpTemplate, cmd)
+	}
+	// not exit 2: succeeded at 'go help cmd'.
+	return
+}
+
+var usageTemplate = `{{.Long | trim}}
+
+Usage:
+
+	{{.UsageLine}} <command> [arguments]
+
+The commands are:
+{{range .Commands}}{{if or (.Runnable) .Commands}}
+	{{.Name | printf "%-11s"}} {{.Short}}{{end}}{{end}}
+
+Use "go help{{with .LongName}} {{.}}{{end}} <command>" for more information about a command.
+{{if eq (.UsageLine) "go"}}
+Additional help topics:
+{{range .Commands}}{{if and (not .Runnable) (not .Commands)}}
+	{{.Name | printf "%-15s"}} {{.Short}}{{end}}{{end}}
+
+Use "go help{{with .LongName}} {{.}}{{end}} <topic>" for more information about that topic.
+{{end}}
+`
+
+var helpTemplate = `{{if .Runnable}}usage: {{.UsageLine}}
+
+{{end}}{{.Long | trim}}
+`
+
+var documentationTemplate = `{{range .}}{{if .Short}}{{.Short | capitalize}}
+
+{{end}}{{if .Commands}}` + usageTemplate + `{{else}}{{if .Runnable}}Usage:
+
+	{{.UsageLine}}
+
+{{end}}{{.Long | trim}}
+
+
+{{end}}{{end}}`
+
+// commentWriter writes a Go comment to the underlying io.Writer,
+// using line comment form (//).
+type commentWriter struct {
+	W            io.Writer
+	wroteSlashes bool // Wrote "//" at the beginning of the current line.
+}
+
+func (c *commentWriter) Write(p []byte) (int, error) {
+	var n int
+	for i, b := range p {
+		if !c.wroteSlashes {
+			s := "//"
+			if b != '\n' {
+				s = "// "
+			}
+			if _, err := io.WriteString(c.W, s); err != nil {
+				return n, err
+			}
+			c.wroteSlashes = true
+		}
+		n0, err := c.W.Write(p[i : i+1])
+		n += n0
+		if err != nil {
+			return n, err
+		}
+		if b == '\n' {
+			c.wroteSlashes = false
+		}
+	}
+	return len(p), nil
+}
+
+// An errWriter wraps a writer, recording whether a write error occurred.
+type errWriter struct {
+	w   io.Writer
+	err error
+}
+
+func (w *errWriter) Write(b []byte) (int, error) {
+	n, err := w.w.Write(b)
+	if err != nil {
+		w.err = err
+	}
+	return n, err
+}
+
+// tmpl executes the given template text on data, writing the result to w.
+func tmpl(w io.Writer, text string, data any) {
+	t := template.New("top")
+	t.Funcs(template.FuncMap{"trim": strings.TrimSpace, "capitalize": capitalize})
+	template.Must(t.Parse(text))
+	ew := &errWriter{w: w}
+	err := t.Execute(ew, data)
+	if ew.err != nil {
+		// I/O error writing. Ignore write on closed pipe.
+		if strings.Contains(ew.err.Error(), "pipe") {
+			base.SetExitStatus(1)
+			base.Exit()
+		}
+		base.Fatalf("writing output: %v", ew.err)
+	}
+	if err != nil {
+		panic(err)
+	}
+}
+
+func capitalize(s string) string {
+	if s == "" {
+		return s
+	}
+	r, n := utf8.DecodeRuneInString(s)
+	return string(unicode.ToTitle(r)) + s[n:]
+}
+
+func PrintUsage(w io.Writer, cmd *base.Command) {
+	bw := bufio.NewWriter(w)
+	tmpl(bw, usageTemplate, cmd)
+	bw.Flush()
+}
diff --git a/src/cmd/go/internal/help/helpdoc.go b/src/cmd/go/internal/help/helpdoc.go
new file mode 100644
index 0000000..68ac4d2
--- /dev/null
+++ b/src/cmd/go/internal/help/helpdoc.go
@@ -0,0 +1,945 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package help
+
+import "cmd/go/internal/base"
+
+var HelpC = &base.Command{
+	UsageLine: "c",
+	Short:     "calling between Go and C",
+	Long: `
+There are two different ways to call between Go and C/C++ code.
+
+The first is the cgo tool, which is part of the Go distribution. For
+information on how to use it see the cgo documentation (go doc cmd/cgo).
+
+The second is the SWIG program, which is a general tool for
+interfacing between languages. For information on SWIG see
+http://swig.org/. When running go build, any file with a .swig
+extension will be passed to SWIG. Any file with a .swigcxx extension
+will be passed to SWIG with the -c++ option.
+
+When either cgo or SWIG is used, go build will pass any .c, .m, .s, .S
+or .sx files to the C compiler, and any .cc, .cpp, .cxx files to the C++
+compiler. The CC or CXX environment variables may be set to determine
+the C or C++ compiler, respectively, to use.
+	`,
+}
+
+var HelpPackages = &base.Command{
+	UsageLine: "packages",
+	Short:     "package lists and patterns",
+	Long: `
+Many commands apply to a set of packages:
+
+	go <action> [packages]
+
+Usually, [packages] is a list of import paths.
+
+An import path that is a rooted path or that begins with
+a . or .. element is interpreted as a file system path and
+denotes the package in that directory.
+
+Otherwise, the import path P denotes the package found in
+the directory DIR/src/P for some DIR listed in the GOPATH
+environment variable (For more details see: 'go help gopath').
+
+If no import paths are given, the action applies to the
+package in the current directory.
+
+There are four reserved names for paths that should not be used
+for packages to be built with the go tool:
+
+- "main" denotes the top-level package in a stand-alone executable.
+
+- "all" expands to all packages found in all the GOPATH
+trees. For example, 'go list all' lists all the packages on the local
+system. When using modules, "all" expands to all packages in
+the main module and their dependencies, including dependencies
+needed by tests of any of those.
+
+- "std" is like all but expands to just the packages in the standard
+Go library.
+
+- "cmd" expands to the Go repository's commands and their
+internal libraries.
+
+Import paths beginning with "cmd/" only match source code in
+the Go repository.
+
+An import path is a pattern if it includes one or more "..." wildcards,
+each of which can match any string, including the empty string and
+strings containing slashes. Such a pattern expands to all package
+directories found in the GOPATH trees with names matching the
+patterns.
+
+To make common patterns more convenient, there are two special cases.
+First, /... at the end of the pattern can match an empty string,
+so that net/... matches both net and packages in its subdirectories, like net/http.
+Second, any slash-separated pattern element containing a wildcard never
+participates in a match of the "vendor" element in the path of a vendored
+package, so that ./... does not match packages in subdirectories of
+./vendor or ./mycode/vendor, but ./vendor/... and ./mycode/vendor/... do.
+Note, however, that a directory named vendor that itself contains code
+is not a vendored package: cmd/vendor would be a command named vendor,
+and the pattern cmd/... matches it.
+See golang.org/s/go15vendor for more about vendoring.
+
+An import path can also name a package to be downloaded from
+a remote repository. Run 'go help importpath' for details.
+
+Every package in a program must have a unique import path.
+By convention, this is arranged by starting each path with a
+unique prefix that belongs to you. For example, paths used
+internally at Google all begin with 'google', and paths
+denoting remote repositories begin with the path to the code,
+such as 'github.com/user/repo'.
+
+Packages in a program need not have unique package names,
+but there are two reserved package names with special meaning.
+The name main indicates a command, not a library.
+Commands are built into binaries and cannot be imported.
+The name documentation indicates documentation for
+a non-Go program in the directory. Files in package documentation
+are ignored by the go command.
+
+As a special case, if the package list is a list of .go files from a
+single directory, the command is applied to a single synthesized
+package made up of exactly those files, ignoring any build constraints
+in those files and ignoring any other files in the directory.
+
+Directory and file names that begin with "." or "_" are ignored
+by the go tool, as are directories named "testdata".
+	`,
+}
+
+var HelpImportPath = &base.Command{
+	UsageLine: "importpath",
+	Short:     "import path syntax",
+	Long: `
+
+An import path (see 'go help packages') denotes a package stored in the local
+file system. In general, an import path denotes either a standard package (such
+as "unicode/utf8") or a package found in one of the work spaces (For more
+details see: 'go help gopath').
+
+Relative import paths
+
+An import path beginning with ./ or ../ is called a relative path.
+The toolchain supports relative import paths as a shortcut in two ways.
+
+First, a relative path can be used as a shorthand on the command line.
+If you are working in the directory containing the code imported as
+"unicode" and want to run the tests for "unicode/utf8", you can type
+"go test ./utf8" instead of needing to specify the full path.
+Similarly, in the reverse situation, "go test .." will test "unicode" from
+the "unicode/utf8" directory. Relative patterns are also allowed, like
+"go test ./..." to test all subdirectories. See 'go help packages' for details
+on the pattern syntax.
+
+Second, if you are compiling a Go program not in a work space,
+you can use a relative path in an import statement in that program
+to refer to nearby code also not in a work space.
+This makes it easy to experiment with small multipackage programs
+outside of the usual work spaces, but such programs cannot be
+installed with "go install" (there is no work space in which to install them),
+so they are rebuilt from scratch each time they are built.
+To avoid ambiguity, Go programs cannot use relative import paths
+within a work space.
+
+Remote import paths
+
+Certain import paths also
+describe how to obtain the source code for the package using
+a revision control system.
+
+A few common code hosting sites have special syntax:
+
+	Bitbucket (Git, Mercurial)
+
+		import "bitbucket.org/user/project"
+		import "bitbucket.org/user/project/sub/directory"
+
+	GitHub (Git)
+
+		import "github.com/user/project"
+		import "github.com/user/project/sub/directory"
+
+	Launchpad (Bazaar)
+
+		import "launchpad.net/project"
+		import "launchpad.net/project/series"
+		import "launchpad.net/project/series/sub/directory"
+
+		import "launchpad.net/~user/project/branch"
+		import "launchpad.net/~user/project/branch/sub/directory"
+
+	IBM DevOps Services (Git)
+
+		import "hub.jazz.net/git/user/project"
+		import "hub.jazz.net/git/user/project/sub/directory"
+
+For code hosted on other servers, import paths may either be qualified
+with the version control type, or the go tool can dynamically fetch
+the import path over https/http and discover where the code resides
+from a <meta> tag in the HTML.
+
+To declare the code location, an import path of the form
+
+	repository.vcs/path
+
+specifies the given repository, with or without the .vcs suffix,
+using the named version control system, and then the path inside
+that repository. The supported version control systems are:
+
+	Bazaar      .bzr
+	Fossil      .fossil
+	Git         .git
+	Mercurial   .hg
+	Subversion  .svn
+
+For example,
+
+	import "example.org/user/foo.hg"
+
+denotes the root directory of the Mercurial repository at
+example.org/user/foo or foo.hg, and
+
+	import "example.org/repo.git/foo/bar"
+
+denotes the foo/bar directory of the Git repository at
+example.org/repo or repo.git.
+
+When a version control system supports multiple protocols,
+each is tried in turn when downloading. For example, a Git
+download tries https://, then git+ssh://.
+
+By default, downloads are restricted to known secure protocols
+(e.g. https, ssh). To override this setting for Git downloads, the
+GIT_ALLOW_PROTOCOL environment variable can be set (For more details see:
+'go help environment').
+
+If the import path is not a known code hosting site and also lacks a
+version control qualifier, the go tool attempts to fetch the import
+over https/http and looks for a <meta> tag in the document's HTML
+<head>.
+
+The meta tag has the form:
+
+	<meta name="go-import" content="import-prefix vcs repo-root">
+
+The import-prefix is the import path corresponding to the repository
+root. It must be a prefix or an exact match of the package being
+fetched with "go get". If it's not an exact match, another http
+request is made at the prefix to verify the <meta> tags match.
+
+The meta tag should appear as early in the file as possible.
+In particular, it should appear before any raw JavaScript or CSS,
+to avoid confusing the go command's restricted parser.
+
+The vcs is one of "bzr", "fossil", "git", "hg", "svn".
+
+The repo-root is the root of the version control system
+containing a scheme and not containing a .vcs qualifier.
+
+For example,
+
+	import "example.org/pkg/foo"
+
+will result in the following requests:
+
+	https://example.org/pkg/foo?go-get=1 (preferred)
+	http://example.org/pkg/foo?go-get=1  (fallback, only with use of correctly set GOINSECURE)
+
+If that page contains the meta tag
+
+	<meta name="go-import" content="example.org git https://code.org/r/p/exproj">
+
+the go tool will verify that https://example.org/?go-get=1 contains the
+same meta tag and then git clone https://code.org/r/p/exproj into
+GOPATH/src/example.org.
+
+When using GOPATH, downloaded packages are written to the first directory
+listed in the GOPATH environment variable.
+(See 'go help gopath-get' and 'go help gopath'.)
+
+When using modules, downloaded packages are stored in the module cache.
+See https://golang.org/ref/mod#module-cache.
+
+When using modules, an additional variant of the go-import meta tag is
+recognized and is preferred over those listing version control systems.
+That variant uses "mod" as the vcs in the content value, as in:
+
+	<meta name="go-import" content="example.org mod https://code.org/moduleproxy">
+
+This tag means to fetch modules with paths beginning with example.org
+from the module proxy available at the URL https://code.org/moduleproxy.
+See https://golang.org/ref/mod#goproxy-protocol for details about the
+proxy protocol.
+
+Import path checking
+
+When the custom import path feature described above redirects to a
+known code hosting site, each of the resulting packages has two possible
+import paths, using the custom domain or the known hosting site.
+
+A package statement is said to have an "import comment" if it is immediately
+followed (before the next newline) by a comment of one of these two forms:
+
+	package math // import "path"
+	package math /* import "path" */
+
+The go command will refuse to install a package with an import comment
+unless it is being referred to by that import path. In this way, import comments
+let package authors make sure the custom import path is used and not a
+direct path to the underlying code hosting site.
+
+Import path checking is disabled for code found within vendor trees.
+This makes it possible to copy code into alternate locations in vendor trees
+without needing to update import comments.
+
+Import path checking is also disabled when using modules.
+Import path comments are obsoleted by the go.mod file's module statement.
+
+See https://golang.org/s/go14customimport for details.
+	`,
+}
+
+var HelpGopath = &base.Command{
+	UsageLine: "gopath",
+	Short:     "GOPATH environment variable",
+	Long: `
+The Go path is used to resolve import statements.
+It is implemented by and documented in the go/build package.
+
+The GOPATH environment variable lists places to look for Go code.
+On Unix, the value is a colon-separated string.
+On Windows, the value is a semicolon-separated string.
+On Plan 9, the value is a list.
+
+If the environment variable is unset, GOPATH defaults
+to a subdirectory named "go" in the user's home directory
+($HOME/go on Unix, %USERPROFILE%\go on Windows),
+unless that directory holds a Go distribution.
+Run "go env GOPATH" to see the current GOPATH.
+
+See https://golang.org/wiki/SettingGOPATH to set a custom GOPATH.
+
+Each directory listed in GOPATH must have a prescribed structure:
+
+The src directory holds source code. The path below src
+determines the import path or executable name.
+
+The pkg directory holds installed package objects.
+As in the Go tree, each target operating system and
+architecture pair has its own subdirectory of pkg
+(pkg/GOOS_GOARCH).
+
+If DIR is a directory listed in the GOPATH, a package with
+source in DIR/src/foo/bar can be imported as "foo/bar" and
+has its compiled form installed to "DIR/pkg/GOOS_GOARCH/foo/bar.a".
+
+The bin directory holds compiled commands.
+Each command is named for its source directory, but only
+the final element, not the entire path. That is, the
+command with source in DIR/src/foo/quux is installed into
+DIR/bin/quux, not DIR/bin/foo/quux. The "foo/" prefix is stripped
+so that you can add DIR/bin to your PATH to get at the
+installed commands. If the GOBIN environment variable is
+set, commands are installed to the directory it names instead
+of DIR/bin. GOBIN must be an absolute path.
+
+Here's an example directory layout:
+
+    GOPATH=/home/user/go
+
+    /home/user/go/
+        src/
+            foo/
+                bar/               (go code in package bar)
+                    x.go
+                quux/              (go code in package main)
+                    y.go
+        bin/
+            quux                   (installed command)
+        pkg/
+            linux_amd64/
+                foo/
+                    bar.a          (installed package object)
+
+Go searches each directory listed in GOPATH to find source code,
+but new packages are always downloaded into the first directory
+in the list.
+
+See https://golang.org/doc/code.html for an example.
+
+GOPATH and Modules
+
+When using modules, GOPATH is no longer used for resolving imports.
+However, it is still used to store downloaded source code (in GOPATH/pkg/mod)
+and compiled commands (in GOPATH/bin).
+
+Internal Directories
+
+Code in or below a directory named "internal" is importable only
+by code in the directory tree rooted at the parent of "internal".
+Here's an extended version of the directory layout above:
+
+    /home/user/go/
+        src/
+            crash/
+                bang/              (go code in package bang)
+                    b.go
+            foo/                   (go code in package foo)
+                f.go
+                bar/               (go code in package bar)
+                    x.go
+                internal/
+                    baz/           (go code in package baz)
+                        z.go
+                quux/              (go code in package main)
+                    y.go
+
+
+The code in z.go is imported as "foo/internal/baz", but that
+import statement can only appear in source files in the subtree
+rooted at foo. The source files foo/f.go, foo/bar/x.go, and
+foo/quux/y.go can all import "foo/internal/baz", but the source file
+crash/bang/b.go cannot.
+
+See https://golang.org/s/go14internal for details.
+
+Vendor Directories
+
+Go 1.6 includes support for using local copies of external dependencies
+to satisfy imports of those dependencies, often referred to as vendoring.
+
+Code below a directory named "vendor" is importable only
+by code in the directory tree rooted at the parent of "vendor",
+and only using an import path that omits the prefix up to and
+including the vendor element.
+
+Here's the example from the previous section,
+but with the "internal" directory renamed to "vendor"
+and a new foo/vendor/crash/bang directory added:
+
+    /home/user/go/
+        src/
+            crash/
+                bang/              (go code in package bang)
+                    b.go
+            foo/                   (go code in package foo)
+                f.go
+                bar/               (go code in package bar)
+                    x.go
+                vendor/
+                    crash/
+                        bang/      (go code in package bang)
+                            b.go
+                    baz/           (go code in package baz)
+                        z.go
+                quux/              (go code in package main)
+                    y.go
+
+The same visibility rules apply as for internal, but the code
+in z.go is imported as "baz", not as "foo/vendor/baz".
+
+Code in vendor directories deeper in the source tree shadows
+code in higher directories. Within the subtree rooted at foo, an import
+of "crash/bang" resolves to "foo/vendor/crash/bang", not the
+top-level "crash/bang".
+
+Code in vendor directories is not subject to import path
+checking (see 'go help importpath').
+
+When 'go get' checks out or updates a git repository, it now also
+updates submodules.
+
+Vendor directories do not affect the placement of new repositories
+being checked out for the first time by 'go get': those are always
+placed in the main GOPATH, never in a vendor subtree.
+
+See https://golang.org/s/go15vendor for details.
+	`,
+}
+
+var HelpEnvironment = &base.Command{
+	UsageLine: "environment",
+	Short:     "environment variables",
+	Long: `
+
+The go command and the tools it invokes consult environment variables
+for configuration. If an environment variable is unset or empty, the go
+command uses a sensible default setting. To see the effective setting of
+the variable <NAME>, run 'go env <NAME>'. To change the default setting,
+run 'go env -w <NAME>=<VALUE>'. Defaults changed using 'go env -w'
+are recorded in a Go environment configuration file stored in the
+per-user configuration directory, as reported by os.UserConfigDir.
+The location of the configuration file can be changed by setting
+the environment variable GOENV, and 'go env GOENV' prints the
+effective location, but 'go env -w' cannot change the default location.
+See 'go help env' for details.
+
+General-purpose environment variables:
+
+	GO111MODULE
+		Controls whether the go command runs in module-aware mode or GOPATH mode.
+		May be "off", "on", or "auto".
+		See https://golang.org/ref/mod#mod-commands.
+	GCCGO
+		The gccgo command to run for 'go build -compiler=gccgo'.
+	GOARCH
+		The architecture, or processor, for which to compile code.
+		Examples are amd64, 386, arm, ppc64.
+	GOBIN
+		The directory where 'go install' will install a command.
+	GOCACHE
+		The directory where the go command will store cached
+		information for reuse in future builds.
+	GOMODCACHE
+		The directory where the go command will store downloaded modules.
+	GODEBUG
+		Enable various debugging facilities. See https://go.dev/doc/godebug
+		for details.
+	GOENV
+		The location of the Go environment configuration file.
+		Cannot be set using 'go env -w'.
+		Setting GOENV=off in the environment disables the use of the
+		default configuration file.
+	GOFLAGS
+		A space-separated list of -flag=value settings to apply
+		to go commands by default, when the given flag is known by
+		the current command. Each entry must be a standalone flag.
+		Because the entries are space-separated, flag values must
+		not contain spaces. Flags listed on the command line
+		are applied after this list and therefore override it.
+	GOINSECURE
+		Comma-separated list of glob patterns (in the syntax of Go's path.Match)
+		of module path prefixes that should always be fetched in an insecure
+		manner. Only applies to dependencies that are being fetched directly.
+		GOINSECURE does not disable checksum database validation. GOPRIVATE or
+		GONOSUMDB may be used to achieve that.
+	GOOS
+		The operating system for which to compile code.
+		Examples are linux, darwin, windows, netbsd.
+	GOPATH
+		Controls where various files are stored. See: 'go help gopath'.
+	GOPROXY
+		URL of Go module proxy. See https://golang.org/ref/mod#environment-variables
+		and https://golang.org/ref/mod#module-proxy for details.
+	GOPRIVATE, GONOPROXY, GONOSUMDB
+		Comma-separated list of glob patterns (in the syntax of Go's path.Match)
+		of module path prefixes that should always be fetched directly
+		or that should not be compared against the checksum database.
+		See https://golang.org/ref/mod#private-modules.
+	GOROOT
+		The root of the go tree.
+	GOSUMDB
+		The name of checksum database to use and optionally its public key and
+		URL. See https://golang.org/ref/mod#authenticating.
+	GOTOOLCHAIN
+		Controls which Go toolchain is used. See https://go.dev/doc/toolchain.
+	GOTMPDIR
+		The directory where the go command will write
+		temporary source files, packages, and binaries.
+	GOVCS
+		Lists version control commands that may be used with matching servers.
+		See 'go help vcs'.
+	GOWORK
+		In module aware mode, use the given go.work file as a workspace file.
+		By default or when GOWORK is "auto", the go command searches for a
+		file named go.work in the current directory and then containing directories
+		until one is found. If a valid go.work file is found, the modules
+		specified will collectively be used as the main modules. If GOWORK
+		is "off", or a go.work file is not found in "auto" mode, workspace
+		mode is disabled.
+
+Environment variables for use with cgo:
+
+	AR
+		The command to use to manipulate library archives when
+		building with the gccgo compiler.
+		The default is 'ar'.
+	CC
+		The command to use to compile C code.
+	CGO_ENABLED
+		Whether the cgo command is supported. Either 0 or 1.
+	CGO_CFLAGS
+		Flags that cgo will pass to the compiler when compiling
+		C code.
+	CGO_CFLAGS_ALLOW
+		A regular expression specifying additional flags to allow
+		to appear in #cgo CFLAGS source code directives.
+		Does not apply to the CGO_CFLAGS environment variable.
+	CGO_CFLAGS_DISALLOW
+		A regular expression specifying flags that must be disallowed
+		from appearing in #cgo CFLAGS source code directives.
+		Does not apply to the CGO_CFLAGS environment variable.
+	CGO_CPPFLAGS, CGO_CPPFLAGS_ALLOW, CGO_CPPFLAGS_DISALLOW
+		Like CGO_CFLAGS, CGO_CFLAGS_ALLOW, and CGO_CFLAGS_DISALLOW,
+		but for the C preprocessor.
+	CGO_CXXFLAGS, CGO_CXXFLAGS_ALLOW, CGO_CXXFLAGS_DISALLOW
+		Like CGO_CFLAGS, CGO_CFLAGS_ALLOW, and CGO_CFLAGS_DISALLOW,
+		but for the C++ compiler.
+	CGO_FFLAGS, CGO_FFLAGS_ALLOW, CGO_FFLAGS_DISALLOW
+		Like CGO_CFLAGS, CGO_CFLAGS_ALLOW, and CGO_CFLAGS_DISALLOW,
+		but for the Fortran compiler.
+	CGO_LDFLAGS, CGO_LDFLAGS_ALLOW, CGO_LDFLAGS_DISALLOW
+		Like CGO_CFLAGS, CGO_CFLAGS_ALLOW, and CGO_CFLAGS_DISALLOW,
+		but for the linker.
+	CXX
+		The command to use to compile C++ code.
+	FC
+		The command to use to compile Fortran code.
+	PKG_CONFIG
+		Path to pkg-config tool.
+
+Architecture-specific environment variables:
+
+	GOARM
+		For GOARCH=arm, the ARM architecture for which to compile.
+		Valid values are 5, 6, 7.
+	GO386
+		For GOARCH=386, how to implement floating point instructions.
+		Valid values are sse2 (default), softfloat.
+	GOAMD64
+		For GOARCH=amd64, the microarchitecture level for which to compile.
+		Valid values are v1 (default), v2, v3, v4.
+		See https://golang.org/wiki/MinimumRequirements#amd64
+	GOMIPS
+		For GOARCH=mips{,le}, whether to use floating point instructions.
+		Valid values are hardfloat (default), softfloat.
+	GOMIPS64
+		For GOARCH=mips64{,le}, whether to use floating point instructions.
+		Valid values are hardfloat (default), softfloat.
+	GOPPC64
+		For GOARCH=ppc64{,le}, the target ISA (Instruction Set Architecture).
+		Valid values are power8 (default), power9, power10.
+	GOWASM
+		For GOARCH=wasm, comma-separated list of experimental WebAssembly features to use.
+		Valid values are satconv, signext.
+
+Environment variables for use with code coverage:
+
+	GOCOVERDIR
+		Directory into which to write code coverage data files
+		generated by running a "go build -cover" binary.
+		Requires that GOEXPERIMENT=coverageredesign is enabled.
+
+Special-purpose environment variables:
+
+	GCCGOTOOLDIR
+		If set, where to find gccgo tools, such as cgo.
+		The default is based on how gccgo was configured.
+	GOEXPERIMENT
+		Comma-separated list of toolchain experiments to enable or disable.
+		The list of available experiments may change arbitrarily over time.
+		See src/internal/goexperiment/flags.go for currently valid values.
+		Warning: This variable is provided for the development and testing
+		of the Go toolchain itself. Use beyond that purpose is unsupported.
+	GOROOT_FINAL
+		The root of the installed Go tree, when it is
+		installed in a location other than where it is built.
+		File names in stack traces are rewritten from GOROOT to
+		GOROOT_FINAL.
+	GO_EXTLINK_ENABLED
+		Whether the linker should use external linking mode
+		when using -linkmode=auto with code that uses cgo.
+		Set to 0 to disable external linking mode, 1 to enable it.
+	GIT_ALLOW_PROTOCOL
+		Defined by Git. A colon-separated list of schemes that are allowed
+		to be used with git fetch/clone. If set, any scheme not explicitly
+		mentioned will be considered insecure by 'go get'.
+		Because the variable is defined by Git, the default value cannot
+		be set using 'go env -w'.
+
+Additional information available from 'go env' but not read from the environment:
+
+	GOEXE
+		The executable file name suffix (".exe" on Windows, "" on other systems).
+	GOGCCFLAGS
+		A space-separated list of arguments supplied to the CC command.
+	GOHOSTARCH
+		The architecture (GOARCH) of the Go toolchain binaries.
+	GOHOSTOS
+		The operating system (GOOS) of the Go toolchain binaries.
+	GOMOD
+		The absolute path to the go.mod of the main module.
+		If module-aware mode is enabled, but there is no go.mod, GOMOD will be
+		os.DevNull ("/dev/null" on Unix-like systems, "NUL" on Windows).
+		If module-aware mode is disabled, GOMOD will be the empty string.
+	GOTOOLDIR
+		The directory where the go tools (compile, cover, doc, etc...) are installed.
+	GOVERSION
+		The version of the installed Go tree, as reported by runtime.Version.
+	`,
+}
+
+var HelpFileType = &base.Command{
+	UsageLine: "filetype",
+	Short:     "file types",
+	Long: `
+The go command examines the contents of a restricted set of files
+in each directory. It identifies which files to examine based on
+the extension of the file name. These extensions are:
+
+	.go
+		Go source files.
+	.c, .h
+		C source files.
+		If the package uses cgo or SWIG, these will be compiled with the
+		OS-native compiler (typically gcc); otherwise they will
+		trigger an error.
+	.cc, .cpp, .cxx, .hh, .hpp, .hxx
+		C++ source files. Only useful with cgo or SWIG, and always
+		compiled with the OS-native compiler.
+	.m
+		Objective-C source files. Only useful with cgo, and always
+		compiled with the OS-native compiler.
+	.s, .S, .sx
+		Assembler source files.
+		If the package uses cgo or SWIG, these will be assembled with the
+		OS-native assembler (typically gcc (sic)); otherwise they
+		will be assembled with the Go assembler.
+	.swig, .swigcxx
+		SWIG definition files.
+	.syso
+		System object files.
+
+Files of each of these types except .syso may contain build
+constraints, but the go command stops scanning for build constraints
+at the first item in the file that is not a blank line or //-style
+line comment. See the go/build package documentation for
+more details.
+	`,
+}
+
+var HelpBuildmode = &base.Command{
+	UsageLine: "buildmode",
+	Short:     "build modes",
+	Long: `
+The 'go build' and 'go install' commands take a -buildmode argument which
+indicates which kind of object file is to be built. Currently supported values
+are:
+
+	-buildmode=archive
+		Build the listed non-main packages into .a files. Packages named
+		main are ignored.
+
+	-buildmode=c-archive
+		Build the listed main package, plus all packages it imports,
+		into a C archive file. The only callable symbols will be those
+		functions exported using a cgo //export comment. Requires
+		exactly one main package to be listed.
+
+	-buildmode=c-shared
+		Build the listed main package, plus all packages it imports,
+		into a C shared library. The only callable symbols will
+		be those functions exported using a cgo //export comment.
+		Requires exactly one main package to be listed.
+
+	-buildmode=default
+		Listed main packages are built into executables and listed
+		non-main packages are built into .a files (the default
+		behavior).
+
+	-buildmode=shared
+		Combine all the listed non-main packages into a single shared
+		library that will be used when building with the -linkshared
+		option. Packages named main are ignored.
+
+	-buildmode=exe
+		Build the listed main packages and everything they import into
+		executables. Packages not named main are ignored.
+
+	-buildmode=pie
+		Build the listed main packages and everything they import into
+		position independent executables (PIE). Packages not named
+		main are ignored.
+
+	-buildmode=plugin
+		Build the listed main packages, plus all packages that they
+		import, into a Go plugin. Packages not named main are ignored.
+
+On AIX, when linking a C program that uses a Go archive built with
+-buildmode=c-archive, you must pass -Wl,-bnoobjreorder to the C compiler.
+`,
+}
+
+var HelpCache = &base.Command{
+	UsageLine: "cache",
+	Short:     "build and test caching",
+	Long: `
+The go command caches build outputs for reuse in future builds.
+The default location for cache data is a subdirectory named go-build
+in the standard user cache directory for the current operating system.
+Setting the GOCACHE environment variable overrides this default,
+and running 'go env GOCACHE' prints the current cache directory.
+
+The go command periodically deletes cached data that has not been
+used recently. Running 'go clean -cache' deletes all cached data.
+
+The build cache correctly accounts for changes to Go source files,
+compilers, compiler options, and so on: cleaning the cache explicitly
+should not be necessary in typical use. However, the build cache
+does not detect changes to C libraries imported with cgo.
+If you have made changes to the C libraries on your system, you
+will need to clean the cache explicitly or else use the -a build flag
+(see 'go help build') to force rebuilding of packages that
+depend on the updated C libraries.
+
+The go command also caches successful package test results.
+See 'go help test' for details. Running 'go clean -testcache' removes
+all cached test results (but not cached build results).
+
+The go command also caches values used in fuzzing with 'go test -fuzz',
+specifically, values that expanded code coverage when passed to a
+fuzz function. These values are not used for regular building and
+testing, but they're stored in a subdirectory of the build cache.
+Running 'go clean -fuzzcache' removes all cached fuzzing values.
+This may make fuzzing less effective, temporarily.
+
+The GODEBUG environment variable can enable printing of debugging
+information about the state of the cache:
+
+GODEBUG=gocacheverify=1 causes the go command to bypass the
+use of any cache entries and instead rebuild everything and check
+that the results match existing cache entries.
+
+GODEBUG=gocachehash=1 causes the go command to print the inputs
+for all of the content hashes it uses to construct cache lookup keys.
+The output is voluminous but can be useful for debugging the cache.
+
+GODEBUG=gocachetest=1 causes the go command to print details of its
+decisions about whether to reuse a cached test result.
+`,
+}
+
+var HelpBuildConstraint = &base.Command{
+	UsageLine: "buildconstraint",
+	Short:     "build constraints",
+	Long: `
+A build constraint, also known as a build tag, is a condition under which a
+file should be included in the package. Build constraints are given by a
+line comment that begins
+
+	//go:build
+
+Constraints may appear in any kind of source file (not just Go), but
+they must appear near the top of the file, preceded
+only by blank lines and other line comments. These rules mean that in Go
+files a build constraint must appear before the package clause.
+
+To distinguish build constraints from package documentation,
+a build constraint should be followed by a blank line.
+
+A build constraint comment is evaluated as an expression containing
+build tags combined by ||, &&, and ! operators and parentheses.
+Operators have the same meaning as in Go.
+
+For example, the following build constraint constrains a file to
+build when the "linux" and "386" constraints are satisfied, or when
+"darwin" is satisfied and "cgo" is not:
+
+	//go:build (linux && 386) || (darwin && !cgo)
+
+It is an error for a file to have more than one //go:build line.
+
+During a particular build, the following build tags are satisfied:
+
+	- the target operating system, as spelled by runtime.GOOS, set with the
+	  GOOS environment variable.
+	- the target architecture, as spelled by runtime.GOARCH, set with the
+	  GOARCH environment variable.
+	- any architecture features, in the form GOARCH.feature
+	  (for example, "amd64.v2"), as detailed below.
+	- "unix", if GOOS is a Unix or Unix-like system.
+	- the compiler being used, either "gc" or "gccgo"
+	- "cgo", if the cgo command is supported (see CGO_ENABLED in
+	  'go help environment').
+	- a term for each Go major release, through the current version:
+	  "go1.1" from Go version 1.1 onward, "go1.12" from Go 1.12, and so on.
+	- any additional tags given by the -tags flag (see 'go help build').
+
+There are no separate build tags for beta or minor releases.
+
+If a file's name, after stripping the extension and a possible _test suffix,
+matches any of the following patterns:
+	*_GOOS
+	*_GOARCH
+	*_GOOS_GOARCH
+(example: source_windows_amd64.go) where GOOS and GOARCH represent
+any known operating system and architecture values respectively, then
+the file is considered to have an implicit build constraint requiring
+those terms (in addition to any explicit constraints in the file).
+
+Using GOOS=android matches build tags and files as for GOOS=linux
+in addition to android tags and files.
+
+Using GOOS=illumos matches build tags and files as for GOOS=solaris
+in addition to illumos tags and files.
+
+Using GOOS=ios matches build tags and files as for GOOS=darwin
+in addition to ios tags and files.
+
+The defined architecture feature build tags are:
+
+	- For GOARCH=386, GO386=387 and GO386=sse2
+	  set the 386.387 and 386.sse2 build tags, respectively.
+	- For GOARCH=amd64, GOAMD64=v1, v2, and v3
+	  correspond to the amd64.v1, amd64.v2, and amd64.v3 feature build tags.
+	- For GOARCH=arm, GOARM=5, 6, and 7
+	  correspond to the arm.5, arm.6, and arm.7 feature build tags.
+	- For GOARCH=mips or mipsle,
+	  GOMIPS=hardfloat and softfloat
+	  correspond to the mips.hardfloat and mips.softfloat
+	  (or mipsle.hardfloat and mipsle.softfloat) feature build tags.
+	- For GOARCH=mips64 or mips64le,
+	  GOMIPS64=hardfloat and softfloat
+	  correspond to the mips64.hardfloat and mips64.softfloat
+	  (or mips64le.hardfloat and mips64le.softfloat) feature build tags.
+	- For GOARCH=ppc64 or ppc64le,
+	  GOPPC64=power8, power9, and power10 correspond to the
+	  ppc64.power8, ppc64.power9, and ppc64.power10
+	  (or ppc64le.power8, ppc64le.power9, and ppc64le.power10)
+	  feature build tags.
+	- For GOARCH=wasm, GOWASM=satconv and signext
+	  correspond to the wasm.satconv and wasm.signext feature build tags.
+
+For GOARCH=amd64, arm, ppc64, and ppc64le, a particular feature level
+sets the feature build tags for all previous levels as well.
+For example, GOAMD64=v2 sets the amd64.v1 and amd64.v2 feature flags.
+This ensures that code making use of v2 features continues to compile
+when, say, GOAMD64=v4 is introduced.
+Code handling the absence of a particular feature level
+should use a negation:
+
+	//go:build !amd64.v2
+
+To keep a file from being considered for any build:
+
+	//go:build ignore
+
+(Any other unsatisfied word will work as well, but "ignore" is conventional.)
+
+To build a file only when using cgo, and only on Linux and OS X:
+
+	//go:build cgo && (linux || darwin)
+
+Such a file is usually paired with another file implementing the
+default functionality for other systems, which in this case would
+carry the constraint:
+
+	//go:build !(cgo && (linux || darwin))
+
+Naming a file dns_windows.go will cause it to be included only when
+building the package for Windows; similarly, math_386.s will be included
+only when building the package for 32-bit x86.
+
+Go versions 1.16 and earlier used a different syntax for build constraints,
+with a "// +build" prefix. The gofmt command will add an equivalent //go:build
+constraint when encountering the older syntax.
+`,
+}
diff --git a/src/cmd/go/internal/imports/build.go b/src/cmd/go/internal/imports/build.go
new file mode 100644
index 0000000..3a4a66b
--- /dev/null
+++ b/src/cmd/go/internal/imports/build.go
@@ -0,0 +1,374 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Copied from Go distribution src/go/build/build.go, syslist.go.
+// That package does not export the ability to process raw file data,
+// although we could fake it with an appropriate build.Context
+// and a lot of unwrapping.
+// More importantly, that package does not implement the tags["*"]
+// special case, in which both tag and !tag are considered to be true
+// for essentially all tags (except "ignore").
+//
+// If we added this API to go/build directly, we wouldn't need this
+// file anymore, but this API is not terribly general-purpose and we
+// don't really want to commit to any public form of it, nor do we
+// want to move the core parts of go/build into a top-level internal package.
+// These details change very infrequently, so the copy is fine.
+
+package imports
+
+import (
+	"bytes"
+	"cmd/go/internal/cfg"
+	"errors"
+	"fmt"
+	"go/build/constraint"
+	"strings"
+	"unicode"
+)
+
+var (
+	bSlashSlash = []byte("//")
+	bStarSlash  = []byte("*/")
+	bSlashStar  = []byte("/*")
+	bPlusBuild  = []byte("+build")
+
+	goBuildComment = []byte("//go:build")
+
+	errMultipleGoBuild = errors.New("multiple //go:build comments")
+)
+
+func isGoBuildComment(line []byte) bool {
+	if !bytes.HasPrefix(line, goBuildComment) {
+		return false
+	}
+	line = bytes.TrimSpace(line)
+	rest := line[len(goBuildComment):]
+	return len(rest) == 0 || len(bytes.TrimSpace(rest)) < len(rest)
+}
+
+// ShouldBuild reports whether it is okay to use this file,
+// The rule is that in the file's leading run of // comments
+// and blank lines, which must be followed by a blank line
+// (to avoid including a Go package clause doc comment),
+// lines beginning with '// +build' are taken as build directives.
+//
+// The file is accepted only if each such line lists something
+// matching the file. For example:
+//
+//	// +build windows linux
+//
+// marks the file as applicable only on Windows and Linux.
+//
+// If tags["*"] is true, then ShouldBuild will consider every
+// build tag except "ignore" to be both true and false for
+// the purpose of satisfying build tags, in order to estimate
+// (conservatively) whether a file could ever possibly be used
+// in any build.
+func ShouldBuild(content []byte, tags map[string]bool) bool {
+	// Identify leading run of // comments and blank lines,
+	// which must be followed by a blank line.
+	// Also identify any //go:build comments.
+	content, goBuild, _, err := parseFileHeader(content)
+	if err != nil {
+		return false
+	}
+
+	// If //go:build line is present, it controls.
+	// Otherwise fall back to +build processing.
+	var shouldBuild bool
+	switch {
+	case goBuild != nil:
+		x, err := constraint.Parse(string(goBuild))
+		if err != nil {
+			return false
+		}
+		shouldBuild = eval(x, tags, true)
+
+	default:
+		shouldBuild = true
+		p := content
+		for len(p) > 0 {
+			line := p
+			if i := bytes.IndexByte(line, '\n'); i >= 0 {
+				line, p = line[:i], p[i+1:]
+			} else {
+				p = p[len(p):]
+			}
+			line = bytes.TrimSpace(line)
+			if !bytes.HasPrefix(line, bSlashSlash) || !bytes.Contains(line, bPlusBuild) {
+				continue
+			}
+			text := string(line)
+			if !constraint.IsPlusBuild(text) {
+				continue
+			}
+			if x, err := constraint.Parse(text); err == nil {
+				if !eval(x, tags, true) {
+					shouldBuild = false
+				}
+			}
+		}
+	}
+
+	return shouldBuild
+}
+
+func parseFileHeader(content []byte) (trimmed, goBuild []byte, sawBinaryOnly bool, err error) {
+	end := 0
+	p := content
+	ended := false       // found non-blank, non-// line, so stopped accepting // +build lines
+	inSlashStar := false // in /* */ comment
+
+Lines:
+	for len(p) > 0 {
+		line := p
+		if i := bytes.IndexByte(line, '\n'); i >= 0 {
+			line, p = line[:i], p[i+1:]
+		} else {
+			p = p[len(p):]
+		}
+		line = bytes.TrimSpace(line)
+		if len(line) == 0 && !ended { // Blank line
+			// Remember position of most recent blank line.
+			// When we find the first non-blank, non-// line,
+			// this "end" position marks the latest file position
+			// where a // +build line can appear.
+			// (It must appear _before_ a blank line before the non-blank, non-// line.
+			// Yes, that's confusing, which is part of why we moved to //go:build lines.)
+			// Note that ended==false here means that inSlashStar==false,
+			// since seeing a /* would have set ended==true.
+			end = len(content) - len(p)
+			continue Lines
+		}
+		if !bytes.HasPrefix(line, bSlashSlash) { // Not comment line
+			ended = true
+		}
+
+		if !inSlashStar && isGoBuildComment(line) {
+			if goBuild != nil {
+				return nil, nil, false, errMultipleGoBuild
+			}
+			goBuild = line
+		}
+
+	Comments:
+		for len(line) > 0 {
+			if inSlashStar {
+				if i := bytes.Index(line, bStarSlash); i >= 0 {
+					inSlashStar = false
+					line = bytes.TrimSpace(line[i+len(bStarSlash):])
+					continue Comments
+				}
+				continue Lines
+			}
+			if bytes.HasPrefix(line, bSlashSlash) {
+				continue Lines
+			}
+			if bytes.HasPrefix(line, bSlashStar) {
+				inSlashStar = true
+				line = bytes.TrimSpace(line[len(bSlashStar):])
+				continue Comments
+			}
+			// Found non-comment text.
+			break Lines
+		}
+	}
+
+	return content[:end], goBuild, sawBinaryOnly, nil
+}
+
+// matchTag reports whether the tag name is valid and tags[name] is true.
+// As a special case, if tags["*"] is true and name is not empty or ignore,
+// then matchTag will return prefer instead of the actual answer,
+// which allows the caller to pretend in that case that most tags are
+// both true and false.
+func matchTag(name string, tags map[string]bool, prefer bool) bool {
+	// Tags must be letters, digits, underscores or dots.
+	// Unlike in Go identifiers, all digits are fine (e.g., "386").
+	for _, c := range name {
+		if !unicode.IsLetter(c) && !unicode.IsDigit(c) && c != '_' && c != '.' {
+			return false
+		}
+	}
+
+	if tags["*"] && name != "" && name != "ignore" {
+		// Special case for gathering all possible imports:
+		// if we put * in the tags map then all tags
+		// except "ignore" are considered both present and not
+		// (so we return true no matter how 'want' is set).
+		return prefer
+	}
+
+	if tags[name] {
+		return true
+	}
+
+	switch name {
+	case "linux":
+		return tags["android"]
+	case "solaris":
+		return tags["illumos"]
+	case "darwin":
+		return tags["ios"]
+	case "unix":
+		return unixOS[cfg.BuildContext.GOOS]
+	default:
+		return false
+	}
+}
+
+// eval is like
+//
+//	x.Eval(func(tag string) bool { return matchTag(tag, tags) })
+//
+// except that it implements the special case for tags["*"] meaning
+// all tags are both true and false at the same time.
+func eval(x constraint.Expr, tags map[string]bool, prefer bool) bool {
+	switch x := x.(type) {
+	case *constraint.TagExpr:
+		return matchTag(x.Tag, tags, prefer)
+	case *constraint.NotExpr:
+		return !eval(x.X, tags, !prefer)
+	case *constraint.AndExpr:
+		return eval(x.X, tags, prefer) && eval(x.Y, tags, prefer)
+	case *constraint.OrExpr:
+		return eval(x.X, tags, prefer) || eval(x.Y, tags, prefer)
+	}
+	panic(fmt.Sprintf("unexpected constraint expression %T", x))
+}
+
+// Eval is like
+//
+//	x.Eval(func(tag string) bool { return matchTag(tag, tags) })
+//
+// except that it implements the special case for tags["*"] meaning
+// all tags are both true and false at the same time.
+func Eval(x constraint.Expr, tags map[string]bool, prefer bool) bool {
+	return eval(x, tags, prefer)
+}
+
+// MatchFile returns false if the name contains a $GOOS or $GOARCH
+// suffix which does not match the current system.
+// The recognized name formats are:
+//
+//	name_$(GOOS).*
+//	name_$(GOARCH).*
+//	name_$(GOOS)_$(GOARCH).*
+//	name_$(GOOS)_test.*
+//	name_$(GOARCH)_test.*
+//	name_$(GOOS)_$(GOARCH)_test.*
+//
+// Exceptions:
+//
+//	if GOOS=android, then files with GOOS=linux are also matched.
+//	if GOOS=illumos, then files with GOOS=solaris are also matched.
+//	if GOOS=ios, then files with GOOS=darwin are also matched.
+//
+// If tags["*"] is true, then MatchFile will consider all possible
+// GOOS and GOARCH to be available and will consequently
+// always return true.
+func MatchFile(name string, tags map[string]bool) bool {
+	if tags["*"] {
+		return true
+	}
+	if dot := strings.Index(name, "."); dot != -1 {
+		name = name[:dot]
+	}
+
+	// Before Go 1.4, a file called "linux.go" would be equivalent to having a
+	// build tag "linux" in that file. For Go 1.4 and beyond, we require this
+	// auto-tagging to apply only to files with a non-empty prefix, so
+	// "foo_linux.go" is tagged but "linux.go" is not. This allows new operating
+	// systems, such as android, to arrive without breaking existing code with
+	// innocuous source code in "android.go". The easiest fix: cut everything
+	// in the name before the initial _.
+	i := strings.Index(name, "_")
+	if i < 0 {
+		return true
+	}
+	name = name[i:] // ignore everything before first _
+
+	l := strings.Split(name, "_")
+	if n := len(l); n > 0 && l[n-1] == "test" {
+		l = l[:n-1]
+	}
+	n := len(l)
+	if n >= 2 && KnownOS[l[n-2]] && KnownArch[l[n-1]] {
+		return matchTag(l[n-2], tags, true) && matchTag(l[n-1], tags, true)
+	}
+	if n >= 1 && KnownOS[l[n-1]] {
+		return matchTag(l[n-1], tags, true)
+	}
+	if n >= 1 && KnownArch[l[n-1]] {
+		return matchTag(l[n-1], tags, true)
+	}
+	return true
+}
+
+var KnownOS = map[string]bool{
+	"aix":       true,
+	"android":   true,
+	"darwin":    true,
+	"dragonfly": true,
+	"freebsd":   true,
+	"hurd":      true,
+	"illumos":   true,
+	"ios":       true,
+	"js":        true,
+	"linux":     true,
+	"nacl":      true, // legacy; don't remove
+	"netbsd":    true,
+	"openbsd":   true,
+	"plan9":     true,
+	"solaris":   true,
+	"wasip1":    true,
+	"windows":   true,
+	"zos":       true,
+}
+
+// unixOS is the set of GOOS values matched by the "unix" build tag.
+// This is not used for filename matching.
+// This is the same list as in go/build/syslist.go and cmd/dist/build.go.
+var unixOS = map[string]bool{
+	"aix":       true,
+	"android":   true,
+	"darwin":    true,
+	"dragonfly": true,
+	"freebsd":   true,
+	"hurd":      true,
+	"illumos":   true,
+	"ios":       true,
+	"linux":     true,
+	"netbsd":    true,
+	"openbsd":   true,
+	"solaris":   true,
+}
+
+var KnownArch = map[string]bool{
+	"386":         true,
+	"amd64":       true,
+	"amd64p32":    true, // legacy; don't remove
+	"arm":         true,
+	"armbe":       true,
+	"arm64":       true,
+	"arm64be":     true,
+	"ppc64":       true,
+	"ppc64le":     true,
+	"mips":        true,
+	"mipsle":      true,
+	"mips64":      true,
+	"mips64le":    true,
+	"mips64p32":   true,
+	"mips64p32le": true,
+	"loong64":     true,
+	"ppc":         true,
+	"riscv":       true,
+	"riscv64":     true,
+	"s390":        true,
+	"s390x":       true,
+	"sparc":       true,
+	"sparc64":     true,
+	"wasm":        true,
+}
diff --git a/src/cmd/go/internal/imports/read.go b/src/cmd/go/internal/imports/read.go
new file mode 100644
index 0000000..70d5190
--- /dev/null
+++ b/src/cmd/go/internal/imports/read.go
@@ -0,0 +1,263 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Copied from Go distribution src/go/build/read.go.
+
+package imports
+
+import (
+	"bufio"
+	"bytes"
+	"errors"
+	"io"
+	"unicode/utf8"
+)
+
+type importReader struct {
+	b    *bufio.Reader
+	buf  []byte
+	peek byte
+	err  error
+	eof  bool
+	nerr int
+}
+
+var bom = []byte{0xef, 0xbb, 0xbf}
+
+func newImportReader(b *bufio.Reader) *importReader {
+	// Remove leading UTF-8 BOM.
+	// Per https://golang.org/ref/spec#Source_code_representation:
+	// a compiler may ignore a UTF-8-encoded byte order mark (U+FEFF)
+	// if it is the first Unicode code point in the source text.
+	if leadingBytes, err := b.Peek(3); err == nil && bytes.Equal(leadingBytes, bom) {
+		b.Discard(3)
+	}
+	return &importReader{b: b}
+}
+
+func isIdent(c byte) bool {
+	return 'A' <= c && c <= 'Z' || 'a' <= c && c <= 'z' || '0' <= c && c <= '9' || c == '_' || c >= utf8.RuneSelf
+}
+
+var (
+	errSyntax = errors.New("syntax error")
+	errNUL    = errors.New("unexpected NUL in input")
+)
+
+// syntaxError records a syntax error, but only if an I/O error has not already been recorded.
+func (r *importReader) syntaxError() {
+	if r.err == nil {
+		r.err = errSyntax
+	}
+}
+
+// readByte reads the next byte from the input, saves it in buf, and returns it.
+// If an error occurs, readByte records the error in r.err and returns 0.
+func (r *importReader) readByte() byte {
+	c, err := r.b.ReadByte()
+	if err == nil {
+		r.buf = append(r.buf, c)
+		if c == 0 {
+			err = errNUL
+		}
+	}
+	if err != nil {
+		if err == io.EOF {
+			r.eof = true
+		} else if r.err == nil {
+			r.err = err
+		}
+		c = 0
+	}
+	return c
+}
+
+// peekByte returns the next byte from the input reader but does not advance beyond it.
+// If skipSpace is set, peekByte skips leading spaces and comments.
+func (r *importReader) peekByte(skipSpace bool) byte {
+	if r.err != nil {
+		if r.nerr++; r.nerr > 10000 {
+			panic("go/build: import reader looping")
+		}
+		return 0
+	}
+
+	// Use r.peek as first input byte.
+	// Don't just return r.peek here: it might have been left by peekByte(false)
+	// and this might be peekByte(true).
+	c := r.peek
+	if c == 0 {
+		c = r.readByte()
+	}
+	for r.err == nil && !r.eof {
+		if skipSpace {
+			// For the purposes of this reader, semicolons are never necessary to
+			// understand the input and are treated as spaces.
+			switch c {
+			case ' ', '\f', '\t', '\r', '\n', ';':
+				c = r.readByte()
+				continue
+
+			case '/':
+				c = r.readByte()
+				if c == '/' {
+					for c != '\n' && r.err == nil && !r.eof {
+						c = r.readByte()
+					}
+				} else if c == '*' {
+					var c1 byte
+					for (c != '*' || c1 != '/') && r.err == nil {
+						if r.eof {
+							r.syntaxError()
+						}
+						c, c1 = c1, r.readByte()
+					}
+				} else {
+					r.syntaxError()
+				}
+				c = r.readByte()
+				continue
+			}
+		}
+		break
+	}
+	r.peek = c
+	return r.peek
+}
+
+// nextByte is like peekByte but advances beyond the returned byte.
+func (r *importReader) nextByte(skipSpace bool) byte {
+	c := r.peekByte(skipSpace)
+	r.peek = 0
+	return c
+}
+
+// readKeyword reads the given keyword from the input.
+// If the keyword is not present, readKeyword records a syntax error.
+func (r *importReader) readKeyword(kw string) {
+	r.peekByte(true)
+	for i := 0; i < len(kw); i++ {
+		if r.nextByte(false) != kw[i] {
+			r.syntaxError()
+			return
+		}
+	}
+	if isIdent(r.peekByte(false)) {
+		r.syntaxError()
+	}
+}
+
+// readIdent reads an identifier from the input.
+// If an identifier is not present, readIdent records a syntax error.
+func (r *importReader) readIdent() {
+	c := r.peekByte(true)
+	if !isIdent(c) {
+		r.syntaxError()
+		return
+	}
+	for isIdent(r.peekByte(false)) {
+		r.peek = 0
+	}
+}
+
+// readString reads a quoted string literal from the input.
+// If an identifier is not present, readString records a syntax error.
+func (r *importReader) readString(save *[]string) {
+	switch r.nextByte(true) {
+	case '`':
+		start := len(r.buf) - 1
+		for r.err == nil {
+			if r.nextByte(false) == '`' {
+				if save != nil {
+					*save = append(*save, string(r.buf[start:]))
+				}
+				break
+			}
+			if r.eof {
+				r.syntaxError()
+			}
+		}
+	case '"':
+		start := len(r.buf) - 1
+		for r.err == nil {
+			c := r.nextByte(false)
+			if c == '"' {
+				if save != nil {
+					*save = append(*save, string(r.buf[start:]))
+				}
+				break
+			}
+			if r.eof || c == '\n' {
+				r.syntaxError()
+			}
+			if c == '\\' {
+				r.nextByte(false)
+			}
+		}
+	default:
+		r.syntaxError()
+	}
+}
+
+// readImport reads an import clause - optional identifier followed by quoted string -
+// from the input.
+func (r *importReader) readImport(imports *[]string) {
+	c := r.peekByte(true)
+	if c == '.' {
+		r.peek = 0
+	} else if isIdent(c) {
+		r.readIdent()
+	}
+	r.readString(imports)
+}
+
+// ReadComments is like io.ReadAll, except that it only reads the leading
+// block of comments in the file.
+func ReadComments(f io.Reader) ([]byte, error) {
+	r := newImportReader(bufio.NewReader(f))
+	r.peekByte(true)
+	if r.err == nil && !r.eof {
+		// Didn't reach EOF, so must have found a non-space byte. Remove it.
+		r.buf = r.buf[:len(r.buf)-1]
+	}
+	return r.buf, r.err
+}
+
+// ReadImports is like io.ReadAll, except that it expects a Go file as input
+// and stops reading the input once the imports have completed.
+func ReadImports(f io.Reader, reportSyntaxError bool, imports *[]string) ([]byte, error) {
+	r := newImportReader(bufio.NewReader(f))
+
+	r.readKeyword("package")
+	r.readIdent()
+	for r.peekByte(true) == 'i' {
+		r.readKeyword("import")
+		if r.peekByte(true) == '(' {
+			r.nextByte(false)
+			for r.peekByte(true) != ')' && r.err == nil {
+				r.readImport(imports)
+			}
+			r.nextByte(false)
+		} else {
+			r.readImport(imports)
+		}
+	}
+
+	// If we stopped successfully before EOF, we read a byte that told us we were done.
+	// Return all but that last byte, which would cause a syntax error if we let it through.
+	if r.err == nil && !r.eof {
+		return r.buf[:len(r.buf)-1], nil
+	}
+
+	// If we stopped for a syntax error, consume the whole file so that
+	// we are sure we don't change the errors that go/parser returns.
+	if r.err == errSyntax && !reportSyntaxError {
+		r.err = nil
+		for r.err == nil && !r.eof {
+			r.readByte()
+		}
+	}
+
+	return r.buf, r.err
+}
diff --git a/src/cmd/go/internal/imports/read_test.go b/src/cmd/go/internal/imports/read_test.go
new file mode 100644
index 0000000..6a1a652
--- /dev/null
+++ b/src/cmd/go/internal/imports/read_test.go
@@ -0,0 +1,254 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Copied from Go distribution src/go/build/read.go.
+
+package imports
+
+import (
+	"io"
+	"strings"
+	"testing"
+)
+
+const quote = "`"
+
+type readTest struct {
+	// Test input contains ℙ where readImports should stop.
+	in  string
+	err string
+}
+
+var readImportsTests = []readTest{
+	{
+		`package p`,
+		"",
+	},
+	{
+		`package p; import "x"`,
+		"",
+	},
+	{
+		`package p; import . "x"`,
+		"",
+	},
+	{
+		`package p; import "x";ℙvar x = 1`,
+		"",
+	},
+	{
+		`package p
+		
+		// comment
+		
+		import "x"
+		import _ "x"
+		import a "x"
+		
+		/* comment */
+		
+		import (
+			"x" /* comment */
+			_ "x"
+			a "x" // comment
+			` + quote + `x` + quote + `
+			_ /*comment*/ ` + quote + `x` + quote + `
+			a ` + quote + `x` + quote + `
+		)
+		import (
+		)
+		import ()
+		import()import()import()
+		import();import();import()
+		
+		ℙvar x = 1
+		`,
+		"",
+	},
+	{
+		"\ufeff𝔻" + `package p; import "x";ℙvar x = 1`,
+		"",
+	},
+}
+
+var readCommentsTests = []readTest{
+	{
+		`ℙpackage p`,
+		"",
+	},
+	{
+		`ℙpackage p; import "x"`,
+		"",
+	},
+	{
+		`ℙpackage p; import . "x"`,
+		"",
+	},
+	{
+		"\ufeff𝔻" + `ℙpackage p; import . "x"`,
+		"",
+	},
+	{
+		`// foo
+
+		/* bar */
+
+		/* quux */ // baz
+		
+		/*/ zot */
+
+		// asdf
+		ℙHello, world`,
+		"",
+	},
+	{
+		"\ufeff𝔻" + `// foo
+
+		/* bar */
+
+		/* quux */ // baz
+
+		/*/ zot */
+
+		// asdf
+		ℙHello, world`,
+		"",
+	},
+}
+
+func testRead(t *testing.T, tests []readTest, read func(io.Reader) ([]byte, error)) {
+	for i, tt := range tests {
+		var in, testOut string
+		j := strings.Index(tt.in, "ℙ")
+		if j < 0 {
+			in = tt.in
+			testOut = tt.in
+		} else {
+			in = tt.in[:j] + tt.in[j+len("ℙ"):]
+			testOut = tt.in[:j]
+		}
+		d := strings.Index(tt.in, "𝔻")
+		if d >= 0 {
+			in = in[:d] + in[d+len("𝔻"):]
+			testOut = testOut[d+len("𝔻"):]
+		}
+		r := strings.NewReader(in)
+		buf, err := read(r)
+		if err != nil {
+			if tt.err == "" {
+				t.Errorf("#%d: err=%q, expected success (%q)", i, err, string(buf))
+				continue
+			}
+			if !strings.Contains(err.Error(), tt.err) {
+				t.Errorf("#%d: err=%q, expected %q", i, err, tt.err)
+				continue
+			}
+			continue
+		}
+		if err == nil && tt.err != "" {
+			t.Errorf("#%d: success, expected %q", i, tt.err)
+			continue
+		}
+
+		out := string(buf)
+		if out != testOut {
+			t.Errorf("#%d: wrong output:\nhave %q\nwant %q\n", i, out, testOut)
+		}
+	}
+}
+
+func TestReadImports(t *testing.T) {
+	testRead(t, readImportsTests, func(r io.Reader) ([]byte, error) { return ReadImports(r, true, nil) })
+}
+
+func TestReadComments(t *testing.T) {
+	testRead(t, readCommentsTests, ReadComments)
+}
+
+var readFailuresTests = []readTest{
+	{
+		`package`,
+		"syntax error",
+	},
+	{
+		"package p\n\x00\nimport `math`\n",
+		"unexpected NUL in input",
+	},
+	{
+		`package p; import`,
+		"syntax error",
+	},
+	{
+		`package p; import "`,
+		"syntax error",
+	},
+	{
+		"package p; import ` \n\n",
+		"syntax error",
+	},
+	{
+		`package p; import "x`,
+		"syntax error",
+	},
+	{
+		`package p; import _`,
+		"syntax error",
+	},
+	{
+		`package p; import _ "`,
+		"syntax error",
+	},
+	{
+		`package p; import _ "x`,
+		"syntax error",
+	},
+	{
+		`package p; import .`,
+		"syntax error",
+	},
+	{
+		`package p; import . "`,
+		"syntax error",
+	},
+	{
+		`package p; import . "x`,
+		"syntax error",
+	},
+	{
+		`package p; import (`,
+		"syntax error",
+	},
+	{
+		`package p; import ("`,
+		"syntax error",
+	},
+	{
+		`package p; import ("x`,
+		"syntax error",
+	},
+	{
+		`package p; import ("x"`,
+		"syntax error",
+	},
+}
+
+func TestReadFailures(t *testing.T) {
+	// Errors should be reported (true arg to readImports).
+	testRead(t, readFailuresTests, func(r io.Reader) ([]byte, error) { return ReadImports(r, true, nil) })
+}
+
+func TestReadFailuresIgnored(t *testing.T) {
+	// Syntax errors should not be reported (false arg to readImports).
+	// Instead, entire file should be the output and no error.
+	// Convert tests not to return syntax errors.
+	tests := make([]readTest, len(readFailuresTests))
+	copy(tests, readFailuresTests)
+	for i := range tests {
+		tt := &tests[i]
+		if !strings.Contains(tt.err, "NUL") {
+			tt.err = ""
+		}
+	}
+	testRead(t, tests, func(r io.Reader) ([]byte, error) { return ReadImports(r, false, nil) })
+}
diff --git a/src/cmd/go/internal/imports/scan.go b/src/cmd/go/internal/imports/scan.go
new file mode 100644
index 0000000..ee11a87
--- /dev/null
+++ b/src/cmd/go/internal/imports/scan.go
@@ -0,0 +1,107 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package imports
+
+import (
+	"fmt"
+	"io/fs"
+	"path/filepath"
+	"sort"
+	"strconv"
+	"strings"
+
+	"cmd/go/internal/fsys"
+)
+
+func ScanDir(dir string, tags map[string]bool) ([]string, []string, error) {
+	infos, err := fsys.ReadDir(dir)
+	if err != nil {
+		return nil, nil, err
+	}
+	var files []string
+	for _, info := range infos {
+		name := info.Name()
+
+		// If the directory entry is a symlink, stat it to obtain the info for the
+		// link target instead of the link itself.
+		if info.Mode()&fs.ModeSymlink != 0 {
+			info, err = fsys.Stat(filepath.Join(dir, name))
+			if err != nil {
+				continue // Ignore broken symlinks.
+			}
+		}
+
+		if info.Mode().IsRegular() && !strings.HasPrefix(name, "_") && !strings.HasPrefix(name, ".") && strings.HasSuffix(name, ".go") && MatchFile(name, tags) {
+			files = append(files, filepath.Join(dir, name))
+		}
+	}
+	return scanFiles(files, tags, false)
+}
+
+func ScanFiles(files []string, tags map[string]bool) ([]string, []string, error) {
+	return scanFiles(files, tags, true)
+}
+
+func scanFiles(files []string, tags map[string]bool, explicitFiles bool) ([]string, []string, error) {
+	imports := make(map[string]bool)
+	testImports := make(map[string]bool)
+	numFiles := 0
+Files:
+	for _, name := range files {
+		r, err := fsys.Open(name)
+		if err != nil {
+			return nil, nil, err
+		}
+		var list []string
+		data, err := ReadImports(r, false, &list)
+		r.Close()
+		if err != nil {
+			return nil, nil, fmt.Errorf("reading %s: %v", name, err)
+		}
+
+		// import "C" is implicit requirement of cgo tag.
+		// When listing files on the command line (explicitFiles=true)
+		// we do not apply build tag filtering but we still do apply
+		// cgo filtering, so no explicitFiles check here.
+		// Why? Because we always have, and it's not worth breaking
+		// that behavior now.
+		for _, path := range list {
+			if path == `"C"` && !tags["cgo"] && !tags["*"] {
+				continue Files
+			}
+		}
+
+		if !explicitFiles && !ShouldBuild(data, tags) {
+			continue
+		}
+		numFiles++
+		m := imports
+		if strings.HasSuffix(name, "_test.go") {
+			m = testImports
+		}
+		for _, p := range list {
+			q, err := strconv.Unquote(p)
+			if err != nil {
+				continue
+			}
+			m[q] = true
+		}
+	}
+	if numFiles == 0 {
+		return nil, nil, ErrNoGo
+	}
+	return keys(imports), keys(testImports), nil
+}
+
+var ErrNoGo = fmt.Errorf("no Go source files")
+
+func keys(m map[string]bool) []string {
+	var list []string
+	for k := range m {
+		list = append(list, k)
+	}
+	sort.Strings(list)
+	return list
+}
diff --git a/src/cmd/go/internal/imports/scan_test.go b/src/cmd/go/internal/imports/scan_test.go
new file mode 100644
index 0000000..56efa90
--- /dev/null
+++ b/src/cmd/go/internal/imports/scan_test.go
@@ -0,0 +1,93 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package imports
+
+import (
+	"bytes"
+	"internal/testenv"
+	"os"
+	"path"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestScan(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	imports, testImports, err := ScanDir(filepath.Join(testenv.GOROOT(t), "src/encoding/json"), Tags())
+	if err != nil {
+		t.Fatal(err)
+	}
+	foundBase64 := false
+	for _, p := range imports {
+		if p == "encoding/base64" {
+			foundBase64 = true
+		}
+		if p == "encoding/binary" {
+			// A dependency but not an import
+			t.Errorf("json reported as importing encoding/binary but does not")
+		}
+		if p == "net/http" {
+			// A test import but not an import
+			t.Errorf("json reported as importing net/http but does not")
+		}
+	}
+	if !foundBase64 {
+		t.Errorf("json missing import encoding/base64 (%q)", imports)
+	}
+
+	foundHTTP := false
+	for _, p := range testImports {
+		if p == "net/http" {
+			foundHTTP = true
+		}
+		if p == "unicode/utf16" {
+			// A package import but not a test import
+			t.Errorf("json reported as test-importing unicode/utf16  but does not")
+		}
+	}
+	if !foundHTTP {
+		t.Errorf("json missing test import net/http (%q)", testImports)
+	}
+}
+func TestScanDir(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	dirs, err := os.ReadDir("testdata")
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, dir := range dirs {
+		if !dir.IsDir() || strings.HasPrefix(dir.Name(), ".") {
+			continue
+		}
+		t.Run(dir.Name(), func(t *testing.T) {
+			tagsData, err := os.ReadFile(filepath.Join("testdata", dir.Name(), "tags.txt"))
+			if err != nil {
+				t.Fatalf("error reading tags: %v", err)
+			}
+			tags := make(map[string]bool)
+			for _, t := range strings.Fields(string(tagsData)) {
+				tags[t] = true
+			}
+
+			wantData, err := os.ReadFile(filepath.Join("testdata", dir.Name(), "want.txt"))
+			if err != nil {
+				t.Fatalf("error reading want: %v", err)
+			}
+			want := string(bytes.TrimSpace(wantData))
+
+			imports, _, err := ScanDir(path.Join("testdata", dir.Name()), tags)
+			if err != nil {
+				t.Fatal(err)
+			}
+			got := strings.Join(imports, "\n")
+			if got != want {
+				t.Errorf("ScanDir: got imports:\n%s\n\nwant:\n%s", got, want)
+			}
+		})
+	}
+}
diff --git a/src/cmd/go/internal/imports/tags.go b/src/cmd/go/internal/imports/tags.go
new file mode 100644
index 0000000..d1467b8
--- /dev/null
+++ b/src/cmd/go/internal/imports/tags.go
@@ -0,0 +1,61 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package imports
+
+import (
+	"cmd/go/internal/cfg"
+	"sync"
+)
+
+var (
+	tags     map[string]bool
+	tagsOnce sync.Once
+)
+
+// Tags returns a set of build tags that are true for the target platform.
+// It includes GOOS, GOARCH, the compiler, possibly "cgo",
+// release tags like "go1.13", and user-specified build tags.
+func Tags() map[string]bool {
+	tagsOnce.Do(func() {
+		tags = loadTags()
+	})
+	return tags
+}
+
+func loadTags() map[string]bool {
+	tags := map[string]bool{
+		cfg.BuildContext.GOOS:     true,
+		cfg.BuildContext.GOARCH:   true,
+		cfg.BuildContext.Compiler: true,
+	}
+	if cfg.BuildContext.CgoEnabled {
+		tags["cgo"] = true
+	}
+	for _, tag := range cfg.BuildContext.BuildTags {
+		tags[tag] = true
+	}
+	for _, tag := range cfg.BuildContext.ToolTags {
+		tags[tag] = true
+	}
+	for _, tag := range cfg.BuildContext.ReleaseTags {
+		tags[tag] = true
+	}
+	return tags
+}
+
+var (
+	anyTags     map[string]bool
+	anyTagsOnce sync.Once
+)
+
+// AnyTags returns a special set of build tags that satisfy nearly all
+// build tag expressions. Only "ignore" and malformed build tag requirements
+// are considered false.
+func AnyTags() map[string]bool {
+	anyTagsOnce.Do(func() {
+		anyTags = map[string]bool{"*": true}
+	})
+	return anyTags
+}
diff --git a/src/cmd/go/internal/imports/testdata/android/.h.go b/src/cmd/go/internal/imports/testdata/android/.h.go
new file mode 100644
index 0000000..53c529e
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/android/.h.go
@@ -0,0 +1,3 @@
+package android
+
+import _ "h"
diff --git a/src/cmd/go/internal/imports/testdata/android/a_android.go b/src/cmd/go/internal/imports/testdata/android/a_android.go
new file mode 100644
index 0000000..2ed972e
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/android/a_android.go
@@ -0,0 +1,3 @@
+package android
+
+import _ "a"
diff --git a/src/cmd/go/internal/imports/testdata/android/b_android_arm64.go b/src/cmd/go/internal/imports/testdata/android/b_android_arm64.go
new file mode 100644
index 0000000..ee9c312
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/android/b_android_arm64.go
@@ -0,0 +1,3 @@
+package android
+
+import _ "b"
diff --git a/src/cmd/go/internal/imports/testdata/android/c_linux.go b/src/cmd/go/internal/imports/testdata/android/c_linux.go
new file mode 100644
index 0000000..91624ce
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/android/c_linux.go
@@ -0,0 +1,3 @@
+package android
+
+import _ "c"
diff --git a/src/cmd/go/internal/imports/testdata/android/d_linux_arm64.go b/src/cmd/go/internal/imports/testdata/android/d_linux_arm64.go
new file mode 100644
index 0000000..34e07df
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/android/d_linux_arm64.go
@@ -0,0 +1,3 @@
+package android
+
+import _ "d"
diff --git a/src/cmd/go/internal/imports/testdata/android/e.go b/src/cmd/go/internal/imports/testdata/android/e.go
new file mode 100644
index 0000000..f1b9c88
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/android/e.go
@@ -0,0 +1,6 @@
+//go:build android
+// +build android
+
+package android
+
+import _ "e"
diff --git a/src/cmd/go/internal/imports/testdata/android/f.go b/src/cmd/go/internal/imports/testdata/android/f.go
new file mode 100644
index 0000000..bb0ff7b
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/android/f.go
@@ -0,0 +1,6 @@
+//go:build linux
+// +build linux
+
+package android
+
+import _ "f"
diff --git a/src/cmd/go/internal/imports/testdata/android/g.go b/src/cmd/go/internal/imports/testdata/android/g.go
new file mode 100644
index 0000000..ee19424
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/android/g.go
@@ -0,0 +1,6 @@
+//go:build !android
+// +build !android
+
+package android
+
+import _ "g"
diff --git a/src/cmd/go/internal/imports/testdata/android/tags.txt b/src/cmd/go/internal/imports/testdata/android/tags.txt
new file mode 100644
index 0000000..aaf5a6b
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/android/tags.txt
@@ -0,0 +1 @@
+android arm64
\ No newline at end of file
diff --git a/src/cmd/go/internal/imports/testdata/android/want.txt b/src/cmd/go/internal/imports/testdata/android/want.txt
new file mode 100644
index 0000000..0fdf397
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/android/want.txt
@@ -0,0 +1,6 @@
+a
+b
+c
+d
+e
+f
diff --git a/src/cmd/go/internal/imports/testdata/illumos/.h.go b/src/cmd/go/internal/imports/testdata/illumos/.h.go
new file mode 100644
index 0000000..53c529e
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/illumos/.h.go
@@ -0,0 +1,3 @@
+package android
+
+import _ "h"
diff --git a/src/cmd/go/internal/imports/testdata/illumos/a_illumos.go b/src/cmd/go/internal/imports/testdata/illumos/a_illumos.go
new file mode 100644
index 0000000..2e6cb50
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/illumos/a_illumos.go
@@ -0,0 +1,3 @@
+package illumos
+
+import _ "a"
diff --git a/src/cmd/go/internal/imports/testdata/illumos/b_illumos_amd64.go b/src/cmd/go/internal/imports/testdata/illumos/b_illumos_amd64.go
new file mode 100644
index 0000000..2834d80
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/illumos/b_illumos_amd64.go
@@ -0,0 +1,3 @@
+package illumos
+
+import _ "b"
diff --git a/src/cmd/go/internal/imports/testdata/illumos/c_solaris.go b/src/cmd/go/internal/imports/testdata/illumos/c_solaris.go
new file mode 100644
index 0000000..d7f9462
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/illumos/c_solaris.go
@@ -0,0 +1,3 @@
+package illumos
+
+import _ "c"
diff --git a/src/cmd/go/internal/imports/testdata/illumos/d_solaris_amd64.go b/src/cmd/go/internal/imports/testdata/illumos/d_solaris_amd64.go
new file mode 100644
index 0000000..0f52c2b
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/illumos/d_solaris_amd64.go
@@ -0,0 +1,3 @@
+package illumos
+
+import _ "d"
diff --git a/src/cmd/go/internal/imports/testdata/illumos/e.go b/src/cmd/go/internal/imports/testdata/illumos/e.go
new file mode 100644
index 0000000..fddf2c4
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/illumos/e.go
@@ -0,0 +1,6 @@
+//go:build illumos
+// +build illumos
+
+package illumos
+
+import _ "e"
diff --git a/src/cmd/go/internal/imports/testdata/illumos/f.go b/src/cmd/go/internal/imports/testdata/illumos/f.go
new file mode 100644
index 0000000..4b6d528
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/illumos/f.go
@@ -0,0 +1,6 @@
+//go:build solaris
+// +build solaris
+
+package illumos
+
+import _ "f"
diff --git a/src/cmd/go/internal/imports/testdata/illumos/g.go b/src/cmd/go/internal/imports/testdata/illumos/g.go
new file mode 100644
index 0000000..1bf826b
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/illumos/g.go
@@ -0,0 +1,6 @@
+//go:build !illumos
+// +build !illumos
+
+package illumos
+
+import _ "g"
diff --git a/src/cmd/go/internal/imports/testdata/illumos/tags.txt b/src/cmd/go/internal/imports/testdata/illumos/tags.txt
new file mode 100644
index 0000000..b6386a3
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/illumos/tags.txt
@@ -0,0 +1 @@
+illumos amd64
diff --git a/src/cmd/go/internal/imports/testdata/illumos/want.txt b/src/cmd/go/internal/imports/testdata/illumos/want.txt
new file mode 100644
index 0000000..0fdf397
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/illumos/want.txt
@@ -0,0 +1,6 @@
+a
+b
+c
+d
+e
+f
diff --git a/src/cmd/go/internal/imports/testdata/star/tags.txt b/src/cmd/go/internal/imports/testdata/star/tags.txt
new file mode 100644
index 0000000..f59ec20
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/star/tags.txt
@@ -0,0 +1 @@
+*
\ No newline at end of file
diff --git a/src/cmd/go/internal/imports/testdata/star/want.txt b/src/cmd/go/internal/imports/testdata/star/want.txt
new file mode 100644
index 0000000..139f5f4
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/star/want.txt
@@ -0,0 +1,4 @@
+import1
+import2
+import3
+import4
diff --git a/src/cmd/go/internal/imports/testdata/star/x.go b/src/cmd/go/internal/imports/testdata/star/x.go
new file mode 100644
index 0000000..98f9191
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/star/x.go
@@ -0,0 +1,3 @@
+package x
+
+import "import1"
diff --git a/src/cmd/go/internal/imports/testdata/star/x1.go b/src/cmd/go/internal/imports/testdata/star/x1.go
new file mode 100644
index 0000000..eaaea97
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/star/x1.go
@@ -0,0 +1,6 @@
+//go:build blahblh && linux && !linux && windows && darwin
+// +build blahblh,linux,!linux,windows,darwin
+
+package x
+
+import "import4"
diff --git a/src/cmd/go/internal/imports/testdata/star/x_darwin.go b/src/cmd/go/internal/imports/testdata/star/x_darwin.go
new file mode 100644
index 0000000..a0c3fdd
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/star/x_darwin.go
@@ -0,0 +1,3 @@
+package xxxx
+
+import "import3"
diff --git a/src/cmd/go/internal/imports/testdata/star/x_windows.go b/src/cmd/go/internal/imports/testdata/star/x_windows.go
new file mode 100644
index 0000000..63c5082
--- /dev/null
+++ b/src/cmd/go/internal/imports/testdata/star/x_windows.go
@@ -0,0 +1,3 @@
+package x
+
+import "import2"
diff --git a/src/cmd/go/internal/list/context.go b/src/cmd/go/internal/list/context.go
new file mode 100644
index 0000000..9d6494c
--- /dev/null
+++ b/src/cmd/go/internal/list/context.go
@@ -0,0 +1,39 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package list
+
+import (
+	"go/build"
+)
+
+type Context struct {
+	GOARCH        string   `json:",omitempty"` // target architecture
+	GOOS          string   `json:",omitempty"` // target operating system
+	GOROOT        string   `json:",omitempty"` // Go root
+	GOPATH        string   `json:",omitempty"` // Go path
+	CgoEnabled    bool     `json:",omitempty"` // whether cgo can be used
+	UseAllFiles   bool     `json:",omitempty"` // use files regardless of //go:build lines, file names
+	Compiler      string   `json:",omitempty"` // compiler to assume when computing target paths
+	BuildTags     []string `json:",omitempty"` // build constraints to match in +build lines
+	ToolTags      []string `json:",omitempty"` // toolchain-specific build constraints
+	ReleaseTags   []string `json:",omitempty"` // releases the current release is compatible with
+	InstallSuffix string   `json:",omitempty"` // suffix to use in the name of the install dir
+}
+
+func newContext(c *build.Context) *Context {
+	return &Context{
+		GOARCH:        c.GOARCH,
+		GOOS:          c.GOOS,
+		GOROOT:        c.GOROOT,
+		GOPATH:        c.GOPATH,
+		CgoEnabled:    c.CgoEnabled,
+		UseAllFiles:   c.UseAllFiles,
+		Compiler:      c.Compiler,
+		BuildTags:     c.BuildTags,
+		ToolTags:      c.ToolTags,
+		ReleaseTags:   c.ReleaseTags,
+		InstallSuffix: c.InstallSuffix,
+	}
+}
diff --git a/src/cmd/go/internal/list/list.go b/src/cmd/go/internal/list/list.go
new file mode 100644
index 0000000..92020da
--- /dev/null
+++ b/src/cmd/go/internal/list/list.go
@@ -0,0 +1,1001 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package list implements the “go list” command.
+package list
+
+import (
+	"bufio"
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"reflect"
+	"runtime"
+	"sort"
+	"strconv"
+	"strings"
+	"sync"
+	"text/template"
+
+	"golang.org/x/sync/semaphore"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cache"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/load"
+	"cmd/go/internal/modinfo"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/str"
+	"cmd/go/internal/work"
+)
+
+var CmdList = &base.Command{
+	// Note: -f -json -m are listed explicitly because they are the most common list flags.
+	// Do not send CLs removing them because they're covered by [list flags].
+	UsageLine: "go list [-f format] [-json] [-m] [list flags] [build flags] [packages]",
+	Short:     "list packages or modules",
+	Long: `
+List lists the named packages, one per line.
+The most commonly-used flags are -f and -json, which control the form
+of the output printed for each package. Other list flags, documented below,
+control more specific details.
+
+The default output shows the package import path:
+
+    bytes
+    encoding/json
+    github.com/gorilla/mux
+    golang.org/x/net/html
+
+The -f flag specifies an alternate format for the list, using the
+syntax of package template. The default output is equivalent
+to -f '{{.ImportPath}}'. The struct being passed to the template is:
+
+    type Package struct {
+        Dir            string   // directory containing package sources
+        ImportPath     string   // import path of package in dir
+        ImportComment  string   // path in import comment on package statement
+        Name           string   // package name
+        Doc            string   // package documentation string
+        Target         string   // install path
+        Shlib          string   // the shared library that contains this package (only set when -linkshared)
+        Goroot         bool     // is this package in the Go root?
+        Standard       bool     // is this package part of the standard Go library?
+        Stale          bool     // would 'go install' do anything for this package?
+        StaleReason    string   // explanation for Stale==true
+        Root           string   // Go root or Go path dir containing this package
+        ConflictDir    string   // this directory shadows Dir in $GOPATH
+        BinaryOnly     bool     // binary-only package (no longer supported)
+        ForTest        string   // package is only for use in named test
+        Export         string   // file containing export data (when using -export)
+        BuildID        string   // build ID of the compiled package (when using -export)
+        Module         *Module  // info about package's containing module, if any (can be nil)
+        Match          []string // command-line patterns matching this package
+        DepOnly        bool     // package is only a dependency, not explicitly listed
+        DefaultGODEBUG string  // default GODEBUG setting, for main packages
+
+        // Source files
+        GoFiles           []string   // .go source files (excluding CgoFiles, TestGoFiles, XTestGoFiles)
+        CgoFiles          []string   // .go source files that import "C"
+        CompiledGoFiles   []string   // .go files presented to compiler (when using -compiled)
+        IgnoredGoFiles    []string   // .go source files ignored due to build constraints
+        IgnoredOtherFiles []string // non-.go source files ignored due to build constraints
+        CFiles            []string   // .c source files
+        CXXFiles          []string   // .cc, .cxx and .cpp source files
+        MFiles            []string   // .m source files
+        HFiles            []string   // .h, .hh, .hpp and .hxx source files
+        FFiles            []string   // .f, .F, .for and .f90 Fortran source files
+        SFiles            []string   // .s source files
+        SwigFiles         []string   // .swig files
+        SwigCXXFiles      []string   // .swigcxx files
+        SysoFiles         []string   // .syso object files to add to archive
+        TestGoFiles       []string   // _test.go files in package
+        XTestGoFiles      []string   // _test.go files outside package
+
+        // Embedded files
+        EmbedPatterns      []string // //go:embed patterns
+        EmbedFiles         []string // files matched by EmbedPatterns
+        TestEmbedPatterns  []string // //go:embed patterns in TestGoFiles
+        TestEmbedFiles     []string // files matched by TestEmbedPatterns
+        XTestEmbedPatterns []string // //go:embed patterns in XTestGoFiles
+        XTestEmbedFiles    []string // files matched by XTestEmbedPatterns
+
+        // Cgo directives
+        CgoCFLAGS    []string // cgo: flags for C compiler
+        CgoCPPFLAGS  []string // cgo: flags for C preprocessor
+        CgoCXXFLAGS  []string // cgo: flags for C++ compiler
+        CgoFFLAGS    []string // cgo: flags for Fortran compiler
+        CgoLDFLAGS   []string // cgo: flags for linker
+        CgoPkgConfig []string // cgo: pkg-config names
+
+        // Dependency information
+        Imports      []string          // import paths used by this package
+        ImportMap    map[string]string // map from source import to ImportPath (identity entries omitted)
+        Deps         []string          // all (recursively) imported dependencies
+        TestImports  []string          // imports from TestGoFiles
+        XTestImports []string          // imports from XTestGoFiles
+
+        // Error information
+        Incomplete bool            // this package or a dependency has an error
+        Error      *PackageError   // error loading package
+        DepsErrors []*PackageError // errors loading dependencies
+    }
+
+Packages stored in vendor directories report an ImportPath that includes the
+path to the vendor directory (for example, "d/vendor/p" instead of "p"),
+so that the ImportPath uniquely identifies a given copy of a package.
+The Imports, Deps, TestImports, and XTestImports lists also contain these
+expanded import paths. See golang.org/s/go15vendor for more about vendoring.
+
+The error information, if any, is
+
+    type PackageError struct {
+        ImportStack   []string // shortest path from package named on command line to this one
+        Pos           string   // position of error (if present, file:line:col)
+        Err           string   // the error itself
+    }
+
+The module information is a Module struct, defined in the discussion
+of list -m below.
+
+The template function "join" calls strings.Join.
+
+The template function "context" returns the build context, defined as:
+
+    type Context struct {
+        GOARCH        string   // target architecture
+        GOOS          string   // target operating system
+        GOROOT        string   // Go root
+        GOPATH        string   // Go path
+        CgoEnabled    bool     // whether cgo can be used
+        UseAllFiles   bool     // use files regardless of //go:build lines, file names
+        Compiler      string   // compiler to assume when computing target paths
+        BuildTags     []string // build constraints to match in //go:build lines
+        ToolTags      []string // toolchain-specific build constraints
+        ReleaseTags   []string // releases the current release is compatible with
+        InstallSuffix string   // suffix to use in the name of the install dir
+    }
+
+For more information about the meaning of these fields see the documentation
+for the go/build package's Context type.
+
+The -json flag causes the package data to be printed in JSON format
+instead of using the template format. The JSON flag can optionally be
+provided with a set of comma-separated required field names to be output.
+If so, those required fields will always appear in JSON output, but
+others may be omitted to save work in computing the JSON struct.
+
+The -compiled flag causes list to set CompiledGoFiles to the Go source
+files presented to the compiler. Typically this means that it repeats
+the files listed in GoFiles and then also adds the Go code generated
+by processing CgoFiles and SwigFiles. The Imports list contains the
+union of all imports from both GoFiles and CompiledGoFiles.
+
+The -deps flag causes list to iterate over not just the named packages
+but also all their dependencies. It visits them in a depth-first post-order
+traversal, so that a package is listed only after all its dependencies.
+Packages not explicitly listed on the command line will have the DepOnly
+field set to true.
+
+The -e flag changes the handling of erroneous packages, those that
+cannot be found or are malformed. By default, the list command
+prints an error to standard error for each erroneous package and
+omits the packages from consideration during the usual printing.
+With the -e flag, the list command never prints errors to standard
+error and instead processes the erroneous packages with the usual
+printing. Erroneous packages will have a non-empty ImportPath and
+a non-nil Error field; other information may or may not be missing
+(zeroed).
+
+The -export flag causes list to set the Export field to the name of a
+file containing up-to-date export information for the given package,
+and the BuildID field to the build ID of the compiled package.
+
+The -find flag causes list to identify the named packages but not
+resolve their dependencies: the Imports and Deps lists will be empty.
+With the -find flag, the -deps, -test and -export commands cannot be
+used.
+
+The -test flag causes list to report not only the named packages
+but also their test binaries (for packages with tests), to convey to
+source code analysis tools exactly how test binaries are constructed.
+The reported import path for a test binary is the import path of
+the package followed by a ".test" suffix, as in "math/rand.test".
+When building a test, it is sometimes necessary to rebuild certain
+dependencies specially for that test (most commonly the tested
+package itself). The reported import path of a package recompiled
+for a particular test binary is followed by a space and the name of
+the test binary in brackets, as in "math/rand [math/rand.test]"
+or "regexp [sort.test]". The ForTest field is also set to the name
+of the package being tested ("math/rand" or "sort" in the previous
+examples).
+
+The Dir, Target, Shlib, Root, ConflictDir, and Export file paths
+are all absolute paths.
+
+By default, the lists GoFiles, CgoFiles, and so on hold names of files in Dir
+(that is, paths relative to Dir, not absolute paths).
+The generated files added when using the -compiled and -test flags
+are absolute paths referring to cached copies of generated Go source files.
+Although they are Go source files, the paths may not end in ".go".
+
+The -m flag causes list to list modules instead of packages.
+
+When listing modules, the -f flag still specifies a format template
+applied to a Go struct, but now a Module struct:
+
+    type Module struct {
+        Path       string        // module path
+        Query      string        // version query corresponding to this version
+        Version    string        // module version
+        Versions   []string      // available module versions
+        Replace    *Module       // replaced by this module
+        Time       *time.Time    // time version was created
+        Update     *Module       // available update (with -u)
+        Main       bool          // is this the main module?
+        Indirect   bool          // module is only indirectly needed by main module
+        Dir        string        // directory holding local copy of files, if any
+        GoMod      string        // path to go.mod file describing module, if any
+        GoVersion  string        // go version used in module
+        Retracted  []string      // retraction information, if any (with -retracted or -u)
+        Deprecated string        // deprecation message, if any (with -u)
+        Error      *ModuleError  // error loading module
+        Origin     any           // provenance of module
+        Reuse      bool          // reuse of old module info is safe
+    }
+
+    type ModuleError struct {
+        Err string // the error itself
+    }
+
+The file GoMod refers to may be outside the module directory if the
+module is in the module cache or if the -modfile flag is used.
+
+The default output is to print the module path and then
+information about the version and replacement if any.
+For example, 'go list -m all' might print:
+
+    my/main/module
+    golang.org/x/text v0.3.0 => /tmp/text
+    rsc.io/pdf v0.1.1
+
+The Module struct has a String method that formats this
+line of output, so that the default format is equivalent
+to -f '{{.String}}'.
+
+Note that when a module has been replaced, its Replace field
+describes the replacement module, and its Dir field is set to
+the replacement's source code, if present. (That is, if Replace
+is non-nil, then Dir is set to Replace.Dir, with no access to
+the replaced source code.)
+
+The -u flag adds information about available upgrades.
+When the latest version of a given module is newer than
+the current one, list -u sets the Module's Update field
+to information about the newer module. list -u will also set
+the module's Retracted field if the current version is retracted.
+The Module's String method indicates an available upgrade by
+formatting the newer version in brackets after the current version.
+If a version is retracted, the string "(retracted)" will follow it.
+For example, 'go list -m -u all' might print:
+
+    my/main/module
+    golang.org/x/text v0.3.0 [v0.4.0] => /tmp/text
+    rsc.io/pdf v0.1.1 (retracted) [v0.1.2]
+
+(For tools, 'go list -m -u -json all' may be more convenient to parse.)
+
+The -versions flag causes list to set the Module's Versions field
+to a list of all known versions of that module, ordered according
+to semantic versioning, earliest to latest. The flag also changes
+the default output format to display the module path followed by the
+space-separated version list.
+
+The -retracted flag causes list to report information about retracted
+module versions. When -retracted is used with -f or -json, the Retracted
+field will be set to a string explaining why the version was retracted.
+The string is taken from comments on the retract directive in the
+module's go.mod file. When -retracted is used with -versions, retracted
+versions are listed together with unretracted versions. The -retracted
+flag may be used with or without -m.
+
+The arguments to list -m are interpreted as a list of modules, not packages.
+The main module is the module containing the current directory.
+The active modules are the main module and its dependencies.
+With no arguments, list -m shows the main module.
+With arguments, list -m shows the modules specified by the arguments.
+Any of the active modules can be specified by its module path.
+The special pattern "all" specifies all the active modules, first the main
+module and then dependencies sorted by module path.
+A pattern containing "..." specifies the active modules whose
+module paths match the pattern.
+A query of the form path@version specifies the result of that query,
+which is not limited to active modules.
+See 'go help modules' for more about module queries.
+
+The template function "module" takes a single string argument
+that must be a module path or query and returns the specified
+module as a Module struct. If an error occurs, the result will
+be a Module struct with a non-nil Error field.
+
+When using -m, the -reuse=old.json flag accepts the name of file containing
+the JSON output of a previous 'go list -m -json' invocation with the
+same set of modifier flags (such as -u, -retracted, and -versions).
+The go command may use this file to determine that a module is unchanged
+since the previous invocation and avoid redownloading information about it.
+Modules that are not redownloaded will be marked in the new output by
+setting the Reuse field to true. Normally the module cache provides this
+kind of reuse automatically; the -reuse flag can be useful on systems that
+do not preserve the module cache.
+
+For more about build flags, see 'go help build'.
+
+For more about specifying packages, see 'go help packages'.
+
+For more about modules, see https://golang.org/ref/mod.
+	`,
+}
+
+func init() {
+	CmdList.Run = runList // break init cycle
+	work.AddBuildFlags(CmdList, work.DefaultBuildFlags)
+	if cfg.Experiment != nil && cfg.Experiment.CoverageRedesign {
+		work.AddCoverFlags(CmdList, nil)
+	}
+	CmdList.Flag.Var(&listJsonFields, "json", "")
+}
+
+var (
+	listCompiled   = CmdList.Flag.Bool("compiled", false, "")
+	listDeps       = CmdList.Flag.Bool("deps", false, "")
+	listE          = CmdList.Flag.Bool("e", false, "")
+	listExport     = CmdList.Flag.Bool("export", false, "")
+	listFmt        = CmdList.Flag.String("f", "", "")
+	listFind       = CmdList.Flag.Bool("find", false, "")
+	listJson       bool
+	listJsonFields jsonFlag // If not empty, only output these fields.
+	listM          = CmdList.Flag.Bool("m", false, "")
+	listRetracted  = CmdList.Flag.Bool("retracted", false, "")
+	listReuse      = CmdList.Flag.String("reuse", "", "")
+	listTest       = CmdList.Flag.Bool("test", false, "")
+	listU          = CmdList.Flag.Bool("u", false, "")
+	listVersions   = CmdList.Flag.Bool("versions", false, "")
+)
+
+// A StringsFlag is a command-line flag that interprets its argument
+// as a space-separated list of possibly-quoted strings.
+type jsonFlag map[string]bool
+
+func (v *jsonFlag) Set(s string) error {
+	if v, err := strconv.ParseBool(s); err == nil {
+		listJson = v
+		return nil
+	}
+	listJson = true
+	if *v == nil {
+		*v = make(map[string]bool)
+	}
+	for _, f := range strings.Split(s, ",") {
+		(*v)[f] = true
+	}
+	return nil
+}
+
+func (v *jsonFlag) String() string {
+	var fields []string
+	for f := range *v {
+		fields = append(fields, f)
+	}
+	sort.Strings(fields)
+	return strings.Join(fields, ",")
+}
+
+func (v *jsonFlag) IsBoolFlag() bool {
+	return true
+}
+
+func (v *jsonFlag) needAll() bool {
+	return len(*v) == 0
+}
+
+func (v *jsonFlag) needAny(fields ...string) bool {
+	if v.needAll() {
+		return true
+	}
+	for _, f := range fields {
+		if (*v)[f] {
+			return true
+		}
+	}
+	return false
+}
+
+var nl = []byte{'\n'}
+
+func runList(ctx context.Context, cmd *base.Command, args []string) {
+	modload.InitWorkfile()
+
+	if *listFmt != "" && listJson {
+		base.Fatalf("go list -f cannot be used with -json")
+	}
+	if *listReuse != "" && !*listM {
+		base.Fatalf("go list -reuse cannot be used without -m")
+	}
+	if *listReuse != "" && modload.HasModRoot() {
+		base.Fatalf("go list -reuse cannot be used inside a module")
+	}
+
+	work.BuildInit()
+	out := newTrackingWriter(os.Stdout)
+	defer out.w.Flush()
+
+	if *listFmt == "" {
+		if *listM {
+			*listFmt = "{{.String}}"
+			if *listVersions {
+				*listFmt = `{{.Path}}{{range .Versions}} {{.}}{{end}}{{if .Deprecated}} (deprecated){{end}}`
+			}
+		} else {
+			*listFmt = "{{.ImportPath}}"
+		}
+	}
+
+	var do func(x any)
+	if listJson {
+		do = func(x any) {
+			if !listJsonFields.needAll() {
+				v := reflect.ValueOf(x).Elem() // do is always called with a non-nil pointer.
+				// Clear all non-requested fields.
+				for i := 0; i < v.NumField(); i++ {
+					if !listJsonFields.needAny(v.Type().Field(i).Name) {
+						v.Field(i).SetZero()
+					}
+				}
+			}
+			b, err := json.MarshalIndent(x, "", "\t")
+			if err != nil {
+				out.Flush()
+				base.Fatalf("%s", err)
+			}
+			out.Write(b)
+			out.Write(nl)
+		}
+	} else {
+		var cachedCtxt *Context
+		context := func() *Context {
+			if cachedCtxt == nil {
+				cachedCtxt = newContext(&cfg.BuildContext)
+			}
+			return cachedCtxt
+		}
+		fm := template.FuncMap{
+			"join":    strings.Join,
+			"context": context,
+			"module":  func(path string) *modinfo.ModulePublic { return modload.ModuleInfo(ctx, path) },
+		}
+		tmpl, err := template.New("main").Funcs(fm).Parse(*listFmt)
+		if err != nil {
+			base.Fatalf("%s", err)
+		}
+		do = func(x any) {
+			if err := tmpl.Execute(out, x); err != nil {
+				out.Flush()
+				base.Fatalf("%s", err)
+			}
+			if out.NeedNL() {
+				out.Write(nl)
+			}
+		}
+	}
+
+	modload.Init()
+	if *listRetracted {
+		if cfg.BuildMod == "vendor" {
+			base.Fatalf("go list -retracted cannot be used when vendoring is enabled")
+		}
+		if !modload.Enabled() {
+			base.Fatalf("go list -retracted can only be used in module-aware mode")
+		}
+	}
+
+	if *listM {
+		// Module mode.
+		if *listCompiled {
+			base.Fatalf("go list -compiled cannot be used with -m")
+		}
+		if *listDeps {
+			// TODO(rsc): Could make this mean something with -m.
+			base.Fatalf("go list -deps cannot be used with -m")
+		}
+		if *listExport {
+			base.Fatalf("go list -export cannot be used with -m")
+		}
+		if *listFind {
+			base.Fatalf("go list -find cannot be used with -m")
+		}
+		if *listTest {
+			base.Fatalf("go list -test cannot be used with -m")
+		}
+
+		if modload.Init(); !modload.Enabled() {
+			base.Fatalf("go: list -m cannot be used with GO111MODULE=off")
+		}
+
+		modload.LoadModFile(ctx) // Sets cfg.BuildMod as a side-effect.
+		if cfg.BuildMod == "vendor" {
+			const actionDisabledFormat = "go: can't %s using the vendor directory\n\t(Use -mod=mod or -mod=readonly to bypass.)"
+
+			if *listVersions {
+				base.Fatalf(actionDisabledFormat, "determine available versions")
+			}
+			if *listU {
+				base.Fatalf(actionDisabledFormat, "determine available upgrades")
+			}
+
+			for _, arg := range args {
+				// In vendor mode, the module graph is incomplete: it contains only the
+				// explicit module dependencies and the modules that supply packages in
+				// the import graph. Reject queries that imply more information than that.
+				if arg == "all" {
+					base.Fatalf(actionDisabledFormat, "compute 'all'")
+				}
+				if strings.Contains(arg, "...") {
+					base.Fatalf(actionDisabledFormat, "match module patterns")
+				}
+			}
+		}
+
+		var mode modload.ListMode
+		if *listU {
+			mode |= modload.ListU | modload.ListRetracted | modload.ListDeprecated
+		}
+		if *listRetracted {
+			mode |= modload.ListRetracted
+		}
+		if *listVersions {
+			mode |= modload.ListVersions
+			if *listRetracted {
+				mode |= modload.ListRetractedVersions
+			}
+		}
+		if *listReuse != "" && len(args) == 0 {
+			base.Fatalf("go: list -m -reuse only has an effect with module@version arguments")
+		}
+		mods, err := modload.ListModules(ctx, args, mode, *listReuse)
+		if !*listE {
+			for _, m := range mods {
+				if m.Error != nil {
+					base.Error(errors.New(m.Error.Err))
+				}
+			}
+			if err != nil {
+				base.Error(err)
+			}
+			base.ExitIfErrors()
+		}
+		for _, m := range mods {
+			do(m)
+		}
+		return
+	}
+
+	// Package mode (not -m).
+	if *listU {
+		base.Fatalf("go list -u can only be used with -m")
+	}
+	if *listVersions {
+		base.Fatalf("go list -versions can only be used with -m")
+	}
+
+	// These pairings make no sense.
+	if *listFind && *listDeps {
+		base.Fatalf("go list -deps cannot be used with -find")
+	}
+	if *listFind && *listTest {
+		base.Fatalf("go list -test cannot be used with -find")
+	}
+	if *listFind && *listExport {
+		base.Fatalf("go list -export cannot be used with -find")
+	}
+
+	pkgOpts := load.PackageOpts{
+		IgnoreImports:      *listFind,
+		ModResolveTests:    *listTest,
+		AutoVCS:            true,
+		SuppressBuildInfo:  !*listExport && !listJsonFields.needAny("Stale", "StaleReason"),
+		SuppressEmbedFiles: !*listExport && !listJsonFields.needAny("EmbedFiles", "TestEmbedFiles", "XTestEmbedFiles"),
+	}
+	pkgs := load.PackagesAndErrors(ctx, pkgOpts, args)
+	if !*listE {
+		w := 0
+		for _, pkg := range pkgs {
+			if pkg.Error != nil {
+				base.Errorf("%v", pkg.Error)
+				continue
+			}
+			pkgs[w] = pkg
+			w++
+		}
+		pkgs = pkgs[:w]
+		base.ExitIfErrors()
+	}
+
+	if *listTest {
+		c := cache.Default()
+		// Add test binaries to packages to be listed.
+
+		var wg sync.WaitGroup
+		sema := semaphore.NewWeighted(int64(runtime.GOMAXPROCS(0)))
+		type testPackageSet struct {
+			p, pmain, ptest, pxtest *load.Package
+		}
+		var testPackages []testPackageSet
+		for _, p := range pkgs {
+			if len(p.TestGoFiles)+len(p.XTestGoFiles) > 0 {
+				var pmain, ptest, pxtest *load.Package
+				var err error
+				if *listE {
+					sema.Acquire(ctx, 1)
+					wg.Add(1)
+					done := func() {
+						sema.Release(1)
+						wg.Done()
+					}
+					pmain, ptest, pxtest = load.TestPackagesAndErrors(ctx, done, pkgOpts, p, nil)
+				} else {
+					pmain, ptest, pxtest, err = load.TestPackagesFor(ctx, pkgOpts, p, nil)
+					if err != nil {
+						base.Fatalf("go: can't load test package: %s", err)
+					}
+				}
+				testPackages = append(testPackages, testPackageSet{p, pmain, ptest, pxtest})
+			}
+		}
+		wg.Wait()
+		for _, pkgset := range testPackages {
+			p, pmain, ptest, pxtest := pkgset.p, pkgset.pmain, pkgset.ptest, pkgset.pxtest
+			if pmain != nil {
+				pkgs = append(pkgs, pmain)
+				data := *pmain.Internal.TestmainGo
+				sema.Acquire(ctx, 1)
+				wg.Add(1)
+				go func() {
+					h := cache.NewHash("testmain")
+					h.Write([]byte("testmain\n"))
+					h.Write(data)
+					out, _, err := c.Put(h.Sum(), bytes.NewReader(data))
+					if err != nil {
+						base.Fatalf("%s", err)
+					}
+					pmain.GoFiles[0] = c.OutputFile(out)
+					sema.Release(1)
+					wg.Done()
+				}()
+
+			}
+			if ptest != nil && ptest != p {
+				pkgs = append(pkgs, ptest)
+			}
+			if pxtest != nil {
+				pkgs = append(pkgs, pxtest)
+			}
+		}
+
+		wg.Wait()
+	}
+
+	// Remember which packages are named on the command line.
+	cmdline := make(map[*load.Package]bool)
+	for _, p := range pkgs {
+		cmdline[p] = true
+	}
+
+	if *listDeps {
+		// Note: This changes the order of the listed packages
+		// from "as written on the command line" to
+		// "a depth-first post-order traversal".
+		// (The dependency exploration order for a given node
+		// is alphabetical, same as listed in .Deps.)
+		// Note that -deps is applied after -test,
+		// so that you only get descriptions of tests for the things named
+		// explicitly on the command line, not for all dependencies.
+		pkgs = loadPackageList(pkgs)
+	}
+
+	// Do we need to run a build to gather information?
+	needStale := (listJson && listJsonFields.needAny("Stale", "StaleReason")) || strings.Contains(*listFmt, ".Stale")
+	if needStale || *listExport || *listCompiled {
+		b := work.NewBuilder("")
+		if *listE {
+			b.AllowErrors = true
+		}
+		defer func() {
+			if err := b.Close(); err != nil {
+				base.Fatal(err)
+			}
+		}()
+
+		b.IsCmdList = true
+		b.NeedExport = *listExport
+		b.NeedCompiledGoFiles = *listCompiled
+		a := &work.Action{}
+		// TODO: Use pkgsFilter?
+		for _, p := range pkgs {
+			if len(p.GoFiles)+len(p.CgoFiles) > 0 {
+				a.Deps = append(a.Deps, b.AutoAction(work.ModeInstall, work.ModeInstall, p))
+			}
+		}
+		if cfg.Experiment.CoverageRedesign && cfg.BuildCover {
+			load.PrepareForCoverageBuild(pkgs)
+		}
+		b.Do(ctx, a)
+	}
+
+	for _, p := range pkgs {
+		// Show vendor-expanded paths in listing
+		p.TestImports = p.Resolve(p.TestImports)
+		p.XTestImports = p.Resolve(p.XTestImports)
+		p.DepOnly = !cmdline[p]
+
+		if *listCompiled {
+			p.Imports = str.StringList(p.Imports, p.Internal.CompiledImports)
+		}
+	}
+
+	if *listTest || (cfg.BuildPGO == "auto" && len(cmdline) > 1) {
+		all := pkgs
+		if !*listDeps {
+			all = loadPackageList(pkgs)
+		}
+		// Update import paths to distinguish the real package p
+		// from p recompiled for q.test, or to distinguish between
+		// p compiled with different PGO profiles.
+		// This must happen only once the build code is done
+		// looking at import paths, because it will get very confused
+		// if it sees these.
+		old := make(map[string]string)
+		for _, p := range all {
+			if p.ForTest != "" || p.Internal.ForMain != "" {
+				new := p.Desc()
+				old[new] = p.ImportPath
+				p.ImportPath = new
+			}
+			p.DepOnly = !cmdline[p]
+		}
+		// Update import path lists to use new strings.
+		m := make(map[string]string)
+		for _, p := range all {
+			for _, p1 := range p.Internal.Imports {
+				if p1.ForTest != "" || p1.Internal.ForMain != "" {
+					m[old[p1.ImportPath]] = p1.ImportPath
+				}
+			}
+			for i, old := range p.Imports {
+				if new := m[old]; new != "" {
+					p.Imports[i] = new
+				}
+			}
+			for old := range m {
+				delete(m, old)
+			}
+		}
+	}
+
+	if listJsonFields.needAny("Deps", "DepsErrors") {
+		all := pkgs
+		// Make sure we iterate through packages in a postorder traversal,
+		// which load.PackageList guarantees. If *listDeps, then all is
+		// already in PackageList order. Otherwise, calling load.PackageList
+		// provides the guarantee. In the case of an import cycle, the last package
+		// visited in the cycle, importing the first encountered package in the cycle,
+		// is visited first. The cycle import error will be bubbled up in the traversal
+		// order up to the first package in the cycle, covering all the packages
+		// in the cycle.
+		if !*listDeps {
+			all = load.PackageList(pkgs)
+		}
+		if listJsonFields.needAny("Deps") {
+			for _, p := range all {
+				collectDeps(p)
+			}
+		}
+		if listJsonFields.needAny("DepsErrors") {
+			for _, p := range all {
+				collectDepsErrors(p)
+			}
+		}
+	}
+
+	// TODO(golang.org/issue/40676): This mechanism could be extended to support
+	// -u without -m.
+	if *listRetracted {
+		// Load retractions for modules that provide packages that will be printed.
+		// TODO(golang.org/issue/40775): Packages from the same module refer to
+		// distinct ModulePublic instance. It would be nice if they could all point
+		// to the same instance. This would require additional global state in
+		// modload.loaded, so that should be refactored first. For now, we update
+		// all instances.
+		modToArg := make(map[*modinfo.ModulePublic]string)
+		argToMods := make(map[string][]*modinfo.ModulePublic)
+		var args []string
+		addModule := func(mod *modinfo.ModulePublic) {
+			if mod.Version == "" {
+				return
+			}
+			arg := fmt.Sprintf("%s@%s", mod.Path, mod.Version)
+			if argToMods[arg] == nil {
+				args = append(args, arg)
+			}
+			argToMods[arg] = append(argToMods[arg], mod)
+			modToArg[mod] = arg
+		}
+		for _, p := range pkgs {
+			if p.Module == nil {
+				continue
+			}
+			addModule(p.Module)
+			if p.Module.Replace != nil {
+				addModule(p.Module.Replace)
+			}
+		}
+
+		if len(args) > 0 {
+			var mode modload.ListMode
+			if *listRetracted {
+				mode |= modload.ListRetracted
+			}
+			rmods, err := modload.ListModules(ctx, args, mode, *listReuse)
+			if err != nil && !*listE {
+				base.Error(err)
+			}
+			for i, arg := range args {
+				rmod := rmods[i]
+				for _, mod := range argToMods[arg] {
+					mod.Retracted = rmod.Retracted
+					if rmod.Error != nil && mod.Error == nil {
+						mod.Error = rmod.Error
+					}
+				}
+			}
+		}
+	}
+
+	// Record non-identity import mappings in p.ImportMap.
+	for _, p := range pkgs {
+		nRaw := len(p.Internal.RawImports)
+		for i, path := range p.Imports {
+			var srcPath string
+			if i < nRaw {
+				srcPath = p.Internal.RawImports[i]
+			} else {
+				// This path is not within the raw imports, so it must be an import
+				// found only within CompiledGoFiles. Those paths are found in
+				// CompiledImports.
+				srcPath = p.Internal.CompiledImports[i-nRaw]
+			}
+
+			if path != srcPath {
+				if p.ImportMap == nil {
+					p.ImportMap = make(map[string]string)
+				}
+				p.ImportMap[srcPath] = path
+			}
+		}
+	}
+
+	for _, p := range pkgs {
+		do(&p.PackagePublic)
+	}
+}
+
+// loadPackageList is like load.PackageList, but prints error messages and exits
+// with nonzero status if listE is not set and any package in the expanded list
+// has errors.
+func loadPackageList(roots []*load.Package) []*load.Package {
+	pkgs := load.PackageList(roots)
+
+	if !*listE {
+		for _, pkg := range pkgs {
+			if pkg.Error != nil {
+				base.Errorf("%v", pkg.Error)
+			}
+		}
+	}
+
+	return pkgs
+}
+
+// collectDeps populates p.Deps by iterating over p.Internal.Imports.
+// collectDeps must be called on all of p's Imports before being called on p.
+func collectDeps(p *load.Package) {
+	deps := make(map[string]bool)
+
+	for _, p := range p.Internal.Imports {
+		deps[p.ImportPath] = true
+		for _, q := range p.Deps {
+			deps[q] = true
+		}
+	}
+
+	p.Deps = make([]string, 0, len(deps))
+	for dep := range deps {
+		p.Deps = append(p.Deps, dep)
+	}
+	sort.Strings(p.Deps)
+}
+
+// collectDeps populates p.DepsErrors by iterating over p.Internal.Imports.
+// collectDepsErrors must be called on all of p's Imports before being called on p.
+func collectDepsErrors(p *load.Package) {
+	depsErrors := make(map[*load.PackageError]bool)
+
+	for _, p := range p.Internal.Imports {
+		if p.Error != nil {
+			depsErrors[p.Error] = true
+		}
+		for _, q := range p.DepsErrors {
+			depsErrors[q] = true
+		}
+	}
+
+	p.DepsErrors = make([]*load.PackageError, 0, len(depsErrors))
+	for deperr := range depsErrors {
+		p.DepsErrors = append(p.DepsErrors, deperr)
+	}
+	// Sort packages by the package on the top of the stack, which should be
+	// the package the error was produced for. Each package can have at most
+	// one error set on it.
+	sort.Slice(p.DepsErrors, func(i, j int) bool {
+		stki, stkj := p.DepsErrors[i].ImportStack, p.DepsErrors[j].ImportStack
+		// Some packages are missing import stacks. To ensure deterministic
+		// sort order compare two errors that are missing import stacks by
+		// their errors' error texts.
+		if len(stki) == 0 {
+			if len(stkj) != 0 {
+				return true
+			}
+
+			return p.DepsErrors[i].Err.Error() < p.DepsErrors[j].Err.Error()
+		} else if len(stkj) == 0 {
+			return false
+		}
+		pathi, pathj := stki[len(stki)-1], stkj[len(stkj)-1]
+		return pathi < pathj
+	})
+}
+
+// TrackingWriter tracks the last byte written on every write so
+// we can avoid printing a newline if one was already written or
+// if there is no output at all.
+type TrackingWriter struct {
+	w    *bufio.Writer
+	last byte
+}
+
+func newTrackingWriter(w io.Writer) *TrackingWriter {
+	return &TrackingWriter{
+		w:    bufio.NewWriter(w),
+		last: '\n',
+	}
+}
+
+func (t *TrackingWriter) Write(p []byte) (n int, err error) {
+	n, err = t.w.Write(p)
+	if n > 0 {
+		t.last = p[n-1]
+	}
+	return
+}
+
+func (t *TrackingWriter) Flush() {
+	t.w.Flush()
+}
+
+func (t *TrackingWriter) NeedNL() bool {
+	return t.last != '\n'
+}
diff --git a/src/cmd/go/internal/load/flag.go b/src/cmd/go/internal/load/flag.go
new file mode 100644
index 0000000..55bdab0
--- /dev/null
+++ b/src/cmd/go/internal/load/flag.go
@@ -0,0 +1,96 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package load
+
+import (
+	"cmd/go/internal/base"
+	"cmd/internal/quoted"
+	"fmt"
+	"strings"
+)
+
+var (
+	BuildAsmflags   PerPackageFlag // -asmflags
+	BuildGcflags    PerPackageFlag // -gcflags
+	BuildLdflags    PerPackageFlag // -ldflags
+	BuildGccgoflags PerPackageFlag // -gccgoflags
+)
+
+// A PerPackageFlag is a command-line flag implementation (a flag.Value)
+// that allows specifying different effective flags for different packages.
+// See 'go help build' for more details about per-package flags.
+type PerPackageFlag struct {
+	raw     string
+	present bool
+	values  []ppfValue
+}
+
+// A ppfValue is a single <pattern>=<flags> per-package flag value.
+type ppfValue struct {
+	match func(*Package) bool // compiled pattern
+	flags []string
+}
+
+// Set is called each time the flag is encountered on the command line.
+func (f *PerPackageFlag) Set(v string) error {
+	return f.set(v, base.Cwd())
+}
+
+// set is the implementation of Set, taking a cwd (current working directory) for easier testing.
+func (f *PerPackageFlag) set(v, cwd string) error {
+	f.raw = v
+	f.present = true
+	match := func(p *Package) bool { return p.Internal.CmdlinePkg || p.Internal.CmdlineFiles } // default predicate with no pattern
+	// For backwards compatibility with earlier flag splitting, ignore spaces around flags.
+	v = strings.TrimSpace(v)
+	if v == "" {
+		// Special case: -gcflags="" means no flags for command-line arguments
+		// (overrides previous -gcflags="-whatever").
+		f.values = append(f.values, ppfValue{match, []string{}})
+		return nil
+	}
+	if !strings.HasPrefix(v, "-") {
+		i := strings.Index(v, "=")
+		if i < 0 {
+			return fmt.Errorf("missing =<value> in <pattern>=<value>")
+		}
+		if i == 0 {
+			return fmt.Errorf("missing <pattern> in <pattern>=<value>")
+		}
+		if v[0] == '\'' || v[0] == '"' {
+			return fmt.Errorf("parameter may not start with quote character %c", v[0])
+		}
+		pattern := strings.TrimSpace(v[:i])
+		match = MatchPackage(pattern, cwd)
+		v = v[i+1:]
+	}
+	flags, err := quoted.Split(v)
+	if err != nil {
+		return err
+	}
+	if flags == nil {
+		flags = []string{}
+	}
+	f.values = append(f.values, ppfValue{match, flags})
+	return nil
+}
+
+func (f *PerPackageFlag) String() string { return f.raw }
+
+// Present reports whether the flag appeared on the command line.
+func (f *PerPackageFlag) Present() bool {
+	return f.present
+}
+
+// For returns the flags to use for the given package.
+func (f *PerPackageFlag) For(p *Package) []string {
+	flags := []string{}
+	for _, v := range f.values {
+		if v.match(p) {
+			flags = v.flags
+		}
+	}
+	return flags
+}
diff --git a/src/cmd/go/internal/load/flag_test.go b/src/cmd/go/internal/load/flag_test.go
new file mode 100644
index 0000000..d3223e1
--- /dev/null
+++ b/src/cmd/go/internal/load/flag_test.go
@@ -0,0 +1,135 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package load
+
+import (
+	"fmt"
+	"path/filepath"
+	"reflect"
+	"testing"
+)
+
+type ppfTestPackage struct {
+	path    string
+	dir     string
+	cmdline bool
+	flags   []string
+}
+
+type ppfTest struct {
+	args []string
+	pkgs []ppfTestPackage
+}
+
+var ppfTests = []ppfTest{
+	// -gcflags=-S applies only to packages on command line.
+	{
+		args: []string{"-S"},
+		pkgs: []ppfTestPackage{
+			{cmdline: true, flags: []string{"-S"}},
+			{cmdline: false, flags: []string{}},
+		},
+	},
+
+	// -gcflags=-S -gcflags= overrides the earlier -S.
+	{
+		args: []string{"-S", ""},
+		pkgs: []ppfTestPackage{
+			{cmdline: true, flags: []string{}},
+		},
+	},
+
+	// -gcflags=net=-S applies only to package net
+	{
+		args: []string{"net=-S"},
+		pkgs: []ppfTestPackage{
+			{path: "math", cmdline: true, flags: []string{}},
+			{path: "net", flags: []string{"-S"}},
+		},
+	},
+
+	// -gcflags=net=-S -gcflags=net= also overrides the earlier -S
+	{
+		args: []string{"net=-S", "net="},
+		pkgs: []ppfTestPackage{
+			{path: "net", flags: []string{}},
+		},
+	},
+
+	// -gcflags=net/...=-S net math
+	// applies -S to net and net/http but not math
+	{
+		args: []string{"net/...=-S"},
+		pkgs: []ppfTestPackage{
+			{path: "net", flags: []string{"-S"}},
+			{path: "net/http", flags: []string{"-S"}},
+			{path: "math", flags: []string{}},
+		},
+	},
+
+	// -gcflags=net/...=-S -gcflags=-m net math
+	// applies -m to net and math and -S to other packages matching net/...
+	// (net matches too, but it was grabbed by the later -gcflags).
+	{
+		args: []string{"net/...=-S", "-m"},
+		pkgs: []ppfTestPackage{
+			{path: "net", cmdline: true, flags: []string{"-m"}},
+			{path: "math", cmdline: true, flags: []string{"-m"}},
+			{path: "net", cmdline: false, flags: []string{"-S"}},
+			{path: "net/http", flags: []string{"-S"}},
+			{path: "math", flags: []string{}},
+		},
+	},
+
+	// relative path patterns
+	// ppfDirTest(pattern, n, dirs...) says the first n dirs should match and the others should not.
+	ppfDirTest(".", 1, "/my/test/dir", "/my/test", "/my/test/other", "/my/test/dir/sub"),
+	ppfDirTest("..", 1, "/my/test", "/my/test/dir", "/my/test/other", "/my/test/dir/sub"),
+	ppfDirTest("./sub", 1, "/my/test/dir/sub", "/my/test", "/my/test/dir", "/my/test/other", "/my/test/dir/sub/sub"),
+	ppfDirTest("../other", 1, "/my/test/other", "/my/test", "/my/test/dir", "/my/test/other/sub", "/my/test/dir/other", "/my/test/dir/sub"),
+	ppfDirTest("./...", 3, "/my/test/dir", "/my/test/dir/sub", "/my/test/dir/sub/sub", "/my/test/other", "/my/test/other/sub"),
+	ppfDirTest("../...", 4, "/my/test/dir", "/my/test/other", "/my/test/dir/sub", "/my/test/other/sub", "/my/other/test"),
+	ppfDirTest("../...sub...", 3, "/my/test/dir/sub", "/my/test/othersub", "/my/test/yellowsubmarine", "/my/other/test"),
+}
+
+func ppfDirTest(pattern string, nmatch int, dirs ...string) ppfTest {
+	var pkgs []ppfTestPackage
+	for i, d := range dirs {
+		flags := []string{}
+		if i < nmatch {
+			flags = []string{"-S"}
+		}
+		pkgs = append(pkgs, ppfTestPackage{path: "p", dir: d, flags: flags})
+	}
+	return ppfTest{args: []string{pattern + "=-S"}, pkgs: pkgs}
+}
+
+func TestPerPackageFlag(t *testing.T) {
+	nativeDir := func(d string) string {
+		if filepath.Separator == '\\' {
+			return `C:` + filepath.FromSlash(d)
+		}
+		return d
+	}
+
+	for i, tt := range ppfTests {
+		t.Run(fmt.Sprintf("#%d", i), func(t *testing.T) {
+			ppFlags := new(PerPackageFlag)
+			for _, arg := range tt.args {
+				t.Logf("set(%s)", arg)
+				if err := ppFlags.set(arg, nativeDir("/my/test/dir")); err != nil {
+					t.Fatal(err)
+				}
+			}
+			for _, p := range tt.pkgs {
+				dir := nativeDir(p.dir)
+				flags := ppFlags.For(&Package{PackagePublic: PackagePublic{ImportPath: p.path, Dir: dir}, Internal: PackageInternal{CmdlinePkg: p.cmdline}})
+				if !reflect.DeepEqual(flags, p.flags) {
+					t.Errorf("For(%v, %v, %v) = %v, want %v", p.path, dir, p.cmdline, flags, p.flags)
+				}
+			}
+		})
+	}
+}
diff --git a/src/cmd/go/internal/load/godebug.go b/src/cmd/go/internal/load/godebug.go
new file mode 100644
index 0000000..c79245e
--- /dev/null
+++ b/src/cmd/go/internal/load/godebug.go
@@ -0,0 +1,126 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package load
+
+import (
+	"cmd/go/internal/modload"
+	"errors"
+	"fmt"
+	"go/build"
+	"internal/godebugs"
+	"sort"
+	"strconv"
+	"strings"
+)
+
+var ErrNotGoDebug = errors.New("not //go:debug line")
+
+func ParseGoDebug(text string) (key, value string, err error) {
+	if !strings.HasPrefix(text, "//go:debug") {
+		return "", "", ErrNotGoDebug
+	}
+	i := strings.IndexAny(text, " \t")
+	if i < 0 {
+		if strings.TrimSpace(text) == "//go:debug" {
+			return "", "", fmt.Errorf("missing key=value")
+		}
+		return "", "", ErrNotGoDebug
+	}
+	k, v, ok := strings.Cut(strings.TrimSpace(text[i:]), "=")
+	if !ok {
+		return "", "", fmt.Errorf("missing key=value")
+	}
+	if strings.ContainsAny(k, " \t") {
+		return "", "", fmt.Errorf("key contains space")
+	}
+	if strings.ContainsAny(v, " \t") {
+		return "", "", fmt.Errorf("value contains space")
+	}
+	if strings.ContainsAny(k, ",") {
+		return "", "", fmt.Errorf("key contains comma")
+	}
+	if strings.ContainsAny(v, ",") {
+		return "", "", fmt.Errorf("value contains comma")
+	}
+
+	for _, info := range godebugs.All {
+		if k == info.Name {
+			return k, v, nil
+		}
+	}
+	return "", "", fmt.Errorf("unknown //go:debug setting %q", k)
+}
+
+// defaultGODEBUG returns the default GODEBUG setting for the main package p.
+// When building a test binary, directives, testDirectives, and xtestDirectives
+// list additional directives from the package under test.
+func defaultGODEBUG(p *Package, directives, testDirectives, xtestDirectives []build.Directive) string {
+	if p.Name != "main" {
+		return ""
+	}
+	goVersion := modload.MainModules.GoVersion()
+	if modload.RootMode == modload.NoRoot && p.Module != nil {
+		// This is go install pkg@version or go run pkg@version.
+		// Use the Go version from the package.
+		// If there isn't one, then
+		goVersion = p.Module.GoVersion
+		if goVersion == "" {
+			goVersion = "1.20"
+		}
+	}
+
+	m := godebugForGoVersion(goVersion)
+	for _, list := range [][]build.Directive{p.Internal.Build.Directives, directives, testDirectives, xtestDirectives} {
+		for _, d := range list {
+			k, v, err := ParseGoDebug(d.Text)
+			if err != nil {
+				continue
+			}
+			if m == nil {
+				m = make(map[string]string)
+			}
+			m[k] = v
+		}
+	}
+	var keys []string
+	for k := range m {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+	var b strings.Builder
+	for _, k := range keys {
+		if b.Len() > 0 {
+			b.WriteString(",")
+		}
+		b.WriteString(k)
+		b.WriteString("=")
+		b.WriteString(m[k])
+	}
+	return b.String()
+}
+
+func godebugForGoVersion(v string) map[string]string {
+	if strings.Count(v, ".") >= 2 {
+		i := strings.Index(v, ".")
+		j := i + 1 + strings.Index(v[i+1:], ".")
+		v = v[:j]
+	}
+
+	if !strings.HasPrefix(v, "1.") {
+		return nil
+	}
+	n, err := strconv.Atoi(v[len("1."):])
+	if err != nil {
+		return nil
+	}
+
+	def := make(map[string]string)
+	for _, info := range godebugs.All {
+		if n < info.Changed {
+			def[info.Name] = info.Old
+		}
+	}
+	return def
+}
diff --git a/src/cmd/go/internal/load/path.go b/src/cmd/go/internal/load/path.go
new file mode 100644
index 0000000..584cdff
--- /dev/null
+++ b/src/cmd/go/internal/load/path.go
@@ -0,0 +1,18 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package load
+
+import (
+	"path/filepath"
+)
+
+// expandPath returns the symlink-expanded form of path.
+func expandPath(p string) string {
+	x, err := filepath.EvalSymlinks(p)
+	if err == nil {
+		return x
+	}
+	return p
+}
diff --git a/src/cmd/go/internal/load/pkg.go b/src/cmd/go/internal/load/pkg.go
new file mode 100644
index 0000000..c0e6265
--- /dev/null
+++ b/src/cmd/go/internal/load/pkg.go
@@ -0,0 +1,3554 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package load loads packages.
+package load
+
+import (
+	"bytes"
+	"context"
+	"crypto/sha256"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"go/build"
+	"go/scanner"
+	"go/token"
+	"internal/platform"
+	"io/fs"
+	"os"
+	"os/exec"
+	pathpkg "path"
+	"path/filepath"
+	"runtime"
+	"runtime/debug"
+	"slices"
+	"sort"
+	"strconv"
+	"strings"
+	"time"
+	"unicode"
+	"unicode/utf8"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/imports"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/modindex"
+	"cmd/go/internal/modinfo"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/par"
+	"cmd/go/internal/search"
+	"cmd/go/internal/str"
+	"cmd/go/internal/trace"
+	"cmd/go/internal/vcs"
+	"cmd/internal/pkgpattern"
+
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/module"
+)
+
+// A Package describes a single package found in a directory.
+type Package struct {
+	PackagePublic                 // visible in 'go list'
+	Internal      PackageInternal // for use inside go command only
+}
+
+type PackagePublic struct {
+	// Note: These fields are part of the go command's public API.
+	// See list.go. It is okay to add fields, but not to change or
+	// remove existing ones. Keep in sync with ../list/list.go
+	Dir           string                `json:",omitempty"` // directory containing package sources
+	ImportPath    string                `json:",omitempty"` // import path of package in dir
+	ImportComment string                `json:",omitempty"` // path in import comment on package statement
+	Name          string                `json:",omitempty"` // package name
+	Doc           string                `json:",omitempty"` // package documentation string
+	Target        string                `json:",omitempty"` // installed target for this package (may be executable)
+	Shlib         string                `json:",omitempty"` // the shared library that contains this package (only set when -linkshared)
+	Root          string                `json:",omitempty"` // Go root, Go path dir, or module root dir containing this package
+	ConflictDir   string                `json:",omitempty"` // Dir is hidden by this other directory
+	ForTest       string                `json:",omitempty"` // package is only for use in named test
+	Export        string                `json:",omitempty"` // file containing export data (set by go list -export)
+	BuildID       string                `json:",omitempty"` // build ID of the compiled package (set by go list -export)
+	Module        *modinfo.ModulePublic `json:",omitempty"` // info about package's module, if any
+	Match         []string              `json:",omitempty"` // command-line patterns matching this package
+	Goroot        bool                  `json:",omitempty"` // is this package found in the Go root?
+	Standard      bool                  `json:",omitempty"` // is this package part of the standard Go library?
+	DepOnly       bool                  `json:",omitempty"` // package is only as a dependency, not explicitly listed
+	BinaryOnly    bool                  `json:",omitempty"` // package cannot be recompiled
+	Incomplete    bool                  `json:",omitempty"` // was there an error loading this package or dependencies?
+
+	DefaultGODEBUG string `json:",omitempty"` // default GODEBUG setting (only for Name=="main")
+
+	// Stale and StaleReason remain here *only* for the list command.
+	// They are only initialized in preparation for list execution.
+	// The regular build determines staleness on the fly during action execution.
+	Stale       bool   `json:",omitempty"` // would 'go install' do anything for this package?
+	StaleReason string `json:",omitempty"` // why is Stale true?
+
+	// Source files
+	// If you add to this list you MUST add to p.AllFiles (below) too.
+	// Otherwise file name security lists will not apply to any new additions.
+	GoFiles           []string `json:",omitempty"` // .go source files (excluding CgoFiles, TestGoFiles, XTestGoFiles)
+	CgoFiles          []string `json:",omitempty"` // .go source files that import "C"
+	CompiledGoFiles   []string `json:",omitempty"` // .go output from running cgo on CgoFiles
+	IgnoredGoFiles    []string `json:",omitempty"` // .go source files ignored due to build constraints
+	InvalidGoFiles    []string `json:",omitempty"` // .go source files with detected problems (parse error, wrong package name, and so on)
+	IgnoredOtherFiles []string `json:",omitempty"` // non-.go source files ignored due to build constraints
+	CFiles            []string `json:",omitempty"` // .c source files
+	CXXFiles          []string `json:",omitempty"` // .cc, .cpp and .cxx source files
+	MFiles            []string `json:",omitempty"` // .m source files
+	HFiles            []string `json:",omitempty"` // .h, .hh, .hpp and .hxx source files
+	FFiles            []string `json:",omitempty"` // .f, .F, .for and .f90 Fortran source files
+	SFiles            []string `json:",omitempty"` // .s source files
+	SwigFiles         []string `json:",omitempty"` // .swig files
+	SwigCXXFiles      []string `json:",omitempty"` // .swigcxx files
+	SysoFiles         []string `json:",omitempty"` // .syso system object files added to package
+
+	// Embedded files
+	EmbedPatterns []string `json:",omitempty"` // //go:embed patterns
+	EmbedFiles    []string `json:",omitempty"` // files matched by EmbedPatterns
+
+	// Cgo directives
+	CgoCFLAGS    []string `json:",omitempty"` // cgo: flags for C compiler
+	CgoCPPFLAGS  []string `json:",omitempty"` // cgo: flags for C preprocessor
+	CgoCXXFLAGS  []string `json:",omitempty"` // cgo: flags for C++ compiler
+	CgoFFLAGS    []string `json:",omitempty"` // cgo: flags for Fortran compiler
+	CgoLDFLAGS   []string `json:",omitempty"` // cgo: flags for linker
+	CgoPkgConfig []string `json:",omitempty"` // cgo: pkg-config names
+
+	// Dependency information
+	Imports   []string          `json:",omitempty"` // import paths used by this package
+	ImportMap map[string]string `json:",omitempty"` // map from source import to ImportPath (identity entries omitted)
+	Deps      []string          `json:",omitempty"` // all (recursively) imported dependencies
+
+	// Error information
+	// Incomplete is above, packed into the other bools
+	Error      *PackageError   `json:",omitempty"` // error loading this package (not dependencies)
+	DepsErrors []*PackageError `json:",omitempty"` // errors loading dependencies, collected by go list before output
+
+	// Test information
+	// If you add to this list you MUST add to p.AllFiles (below) too.
+	// Otherwise file name security lists will not apply to any new additions.
+	TestGoFiles        []string `json:",omitempty"` // _test.go files in package
+	TestImports        []string `json:",omitempty"` // imports from TestGoFiles
+	TestEmbedPatterns  []string `json:",omitempty"` // //go:embed patterns
+	TestEmbedFiles     []string `json:",omitempty"` // files matched by TestEmbedPatterns
+	XTestGoFiles       []string `json:",omitempty"` // _test.go files outside package
+	XTestImports       []string `json:",omitempty"` // imports from XTestGoFiles
+	XTestEmbedPatterns []string `json:",omitempty"` // //go:embed patterns
+	XTestEmbedFiles    []string `json:",omitempty"` // files matched by XTestEmbedPatterns
+}
+
+// AllFiles returns the names of all the files considered for the package.
+// This is used for sanity and security checks, so we include all files,
+// even IgnoredGoFiles, because some subcommands consider them.
+// The go/build package filtered others out (like foo_wrongGOARCH.s)
+// and that's OK.
+func (p *Package) AllFiles() []string {
+	files := str.StringList(
+		p.GoFiles,
+		p.CgoFiles,
+		// no p.CompiledGoFiles, because they are from GoFiles or generated by us
+		p.IgnoredGoFiles,
+		// no p.InvalidGoFiles, because they are from GoFiles
+		p.IgnoredOtherFiles,
+		p.CFiles,
+		p.CXXFiles,
+		p.MFiles,
+		p.HFiles,
+		p.FFiles,
+		p.SFiles,
+		p.SwigFiles,
+		p.SwigCXXFiles,
+		p.SysoFiles,
+		p.TestGoFiles,
+		p.XTestGoFiles,
+	)
+
+	// EmbedFiles may overlap with the other files.
+	// Dedup, but delay building the map as long as possible.
+	// Only files in the current directory (no slash in name)
+	// need to be checked against the files variable above.
+	var have map[string]bool
+	for _, file := range p.EmbedFiles {
+		if !strings.Contains(file, "/") {
+			if have == nil {
+				have = make(map[string]bool)
+				for _, file := range files {
+					have[file] = true
+				}
+			}
+			if have[file] {
+				continue
+			}
+		}
+		files = append(files, file)
+	}
+	return files
+}
+
+// Desc returns the package "description", for use in b.showOutput.
+func (p *Package) Desc() string {
+	if p.ForTest != "" {
+		return p.ImportPath + " [" + p.ForTest + ".test]"
+	}
+	if p.Internal.ForMain != "" {
+		return p.ImportPath + " [" + p.Internal.ForMain + "]"
+	}
+	return p.ImportPath
+}
+
+// IsTestOnly reports whether p is a test-only package.
+//
+// A “test-only” package is one that:
+//   - is a test-only variant of an ordinary package, or
+//   - is a synthesized "main" package for a test binary, or
+//   - contains only _test.go files.
+func (p *Package) IsTestOnly() bool {
+	return p.ForTest != "" ||
+		p.Internal.TestmainGo != nil ||
+		len(p.TestGoFiles)+len(p.XTestGoFiles) > 0 && len(p.GoFiles)+len(p.CgoFiles) == 0
+}
+
+type PackageInternal struct {
+	// Unexported fields are not part of the public API.
+	Build             *build.Package
+	Imports           []*Package           // this package's direct imports
+	CompiledImports   []string             // additional Imports necessary when using CompiledGoFiles (all from standard library); 1:1 with the end of PackagePublic.Imports
+	RawImports        []string             // this package's original imports as they appear in the text of the program; 1:1 with the end of PackagePublic.Imports
+	ForceLibrary      bool                 // this package is a library (even if named "main")
+	CmdlineFiles      bool                 // package built from files listed on command line
+	CmdlinePkg        bool                 // package listed on command line
+	CmdlinePkgLiteral bool                 // package listed as literal on command line (not via wildcard)
+	Local             bool                 // imported via local path (./ or ../)
+	LocalPrefix       string               // interpret ./ and ../ imports relative to this prefix
+	ExeName           string               // desired name for temporary executable
+	FuzzInstrument    bool                 // package should be instrumented for fuzzing
+	CoverMode         string               // preprocess Go source files with the coverage tool in this mode
+	CoverVars         map[string]*CoverVar // variables created by coverage analysis
+	CoverageCfg       string               // coverage info config file path (passed to compiler)
+	OmitDebug         bool                 // tell linker not to write debug information
+	GobinSubdir       bool                 // install target would be subdir of GOBIN
+	BuildInfo         *debug.BuildInfo     // add this info to package main
+	TestmainGo        *[]byte              // content for _testmain.go
+	Embed             map[string][]string  // //go:embed comment mapping
+	OrigImportPath    string               // original import path before adding '_test' suffix
+	PGOProfile        string               // path to PGO profile
+	ForMain           string               // the main package if this package is built specifically for it
+
+	Asmflags   []string // -asmflags for this package
+	Gcflags    []string // -gcflags for this package
+	Ldflags    []string // -ldflags for this package
+	Gccgoflags []string // -gccgoflags for this package
+}
+
+// A NoGoError indicates that no Go files for the package were applicable to the
+// build for that package.
+//
+// That may be because there were no files whatsoever, or because all files were
+// excluded, or because all non-excluded files were test sources.
+type NoGoError struct {
+	Package *Package
+}
+
+func (e *NoGoError) Error() string {
+	if len(e.Package.IgnoredGoFiles) > 0 {
+		// Go files exist, but they were ignored due to build constraints.
+		return "build constraints exclude all Go files in " + e.Package.Dir
+	}
+	if len(e.Package.TestGoFiles)+len(e.Package.XTestGoFiles) > 0 {
+		// Test Go files exist, but we're not interested in them.
+		// The double-negative is unfortunate but we want e.Package.Dir
+		// to appear at the end of error message.
+		return "no non-test Go files in " + e.Package.Dir
+	}
+	return "no Go files in " + e.Package.Dir
+}
+
+// setLoadPackageDataError presents an error found when loading package data
+// as a *PackageError. It has special cases for some common errors to improve
+// messages shown to users and reduce redundancy.
+//
+// setLoadPackageDataError returns true if it's safe to load information about
+// imported packages, for example, if there was a parse error loading imports
+// in one file, but other files are okay.
+func (p *Package) setLoadPackageDataError(err error, path string, stk *ImportStack, importPos []token.Position) {
+	matchErr, isMatchErr := err.(*search.MatchError)
+	if isMatchErr && matchErr.Match.Pattern() == path {
+		if matchErr.Match.IsLiteral() {
+			// The error has a pattern has a pattern similar to the import path.
+			// It may be slightly different (./foo matching example.com/foo),
+			// but close enough to seem redundant.
+			// Unwrap the error so we don't show the pattern.
+			err = matchErr.Err
+		}
+	}
+
+	// Replace (possibly wrapped) *build.NoGoError with *load.NoGoError.
+	// The latter is more specific about the cause.
+	var nogoErr *build.NoGoError
+	if errors.As(err, &nogoErr) {
+		if p.Dir == "" && nogoErr.Dir != "" {
+			p.Dir = nogoErr.Dir
+		}
+		err = &NoGoError{Package: p}
+	}
+
+	// Take only the first error from a scanner.ErrorList. PackageError only
+	// has room for one position, so we report the first error with a position
+	// instead of all of the errors without a position.
+	var pos string
+	var isScanErr bool
+	if scanErr, ok := err.(scanner.ErrorList); ok && len(scanErr) > 0 {
+		isScanErr = true // For stack push/pop below.
+
+		scanPos := scanErr[0].Pos
+		scanPos.Filename = base.ShortPath(scanPos.Filename)
+		pos = scanPos.String()
+		err = errors.New(scanErr[0].Msg)
+	}
+
+	// Report the error on the importing package if the problem is with the import declaration
+	// for example, if the package doesn't exist or if the import path is malformed.
+	// On the other hand, don't include a position if the problem is with the imported package,
+	// for example there are no Go files (NoGoError), or there's a problem in the imported
+	// package's source files themselves (scanner errors).
+	//
+	// TODO(matloob): Perhaps make each of those the errors in the first group
+	// (including modload.ImportMissingError, ImportMissingSumError, and the
+	// corresponding "cannot find package %q in any of" GOPATH-mode error
+	// produced in build.(*Context).Import; modload.AmbiguousImportError,
+	// and modload.PackageNotInModuleError; and the malformed module path errors
+	// produced in golang.org/x/mod/module.CheckMod) implement an interface
+	// to make it easier to check for them? That would save us from having to
+	// move the modload errors into this package to avoid a package import cycle,
+	// and from having to export an error type for the errors produced in build.
+	if !isMatchErr && (nogoErr != nil || isScanErr) {
+		stk.Push(path)
+		defer stk.Pop()
+	}
+
+	p.Error = &PackageError{
+		ImportStack: stk.Copy(),
+		Pos:         pos,
+		Err:         err,
+	}
+	p.Incomplete = true
+
+	if path != stk.Top() {
+		p.Error.setPos(importPos)
+	}
+}
+
+// Resolve returns the resolved version of imports,
+// which should be p.TestImports or p.XTestImports, NOT p.Imports.
+// The imports in p.TestImports and p.XTestImports are not recursively
+// loaded during the initial load of p, so they list the imports found in
+// the source file, but most processing should be over the vendor-resolved
+// import paths. We do this resolution lazily both to avoid file system work
+// and because the eventual real load of the test imports (during 'go test')
+// can produce better error messages if it starts with the original paths.
+// The initial load of p loads all the non-test imports and rewrites
+// the vendored paths, so nothing should ever call p.vendored(p.Imports).
+func (p *Package) Resolve(imports []string) []string {
+	if len(imports) > 0 && len(p.Imports) > 0 && &imports[0] == &p.Imports[0] {
+		panic("internal error: p.Resolve(p.Imports) called")
+	}
+	seen := make(map[string]bool)
+	var all []string
+	for _, path := range imports {
+		path = ResolveImportPath(p, path)
+		if !seen[path] {
+			seen[path] = true
+			all = append(all, path)
+		}
+	}
+	sort.Strings(all)
+	return all
+}
+
+// CoverVar holds the name of the generated coverage variables targeting the named file.
+type CoverVar struct {
+	File string // local file name
+	Var  string // name of count struct
+}
+
+func (p *Package) copyBuild(opts PackageOpts, pp *build.Package) {
+	p.Internal.Build = pp
+
+	if pp.PkgTargetRoot != "" && cfg.BuildPkgdir != "" {
+		old := pp.PkgTargetRoot
+		pp.PkgRoot = cfg.BuildPkgdir
+		pp.PkgTargetRoot = cfg.BuildPkgdir
+		if pp.PkgObj != "" {
+			pp.PkgObj = filepath.Join(cfg.BuildPkgdir, strings.TrimPrefix(pp.PkgObj, old))
+		}
+	}
+
+	p.Dir = pp.Dir
+	p.ImportPath = pp.ImportPath
+	p.ImportComment = pp.ImportComment
+	p.Name = pp.Name
+	p.Doc = pp.Doc
+	p.Root = pp.Root
+	p.ConflictDir = pp.ConflictDir
+	p.BinaryOnly = pp.BinaryOnly
+
+	// TODO? Target
+	p.Goroot = pp.Goroot
+	p.Standard = p.Goroot && p.ImportPath != "" && search.IsStandardImportPath(p.ImportPath)
+	p.GoFiles = pp.GoFiles
+	p.CgoFiles = pp.CgoFiles
+	p.IgnoredGoFiles = pp.IgnoredGoFiles
+	p.InvalidGoFiles = pp.InvalidGoFiles
+	p.IgnoredOtherFiles = pp.IgnoredOtherFiles
+	p.CFiles = pp.CFiles
+	p.CXXFiles = pp.CXXFiles
+	p.MFiles = pp.MFiles
+	p.HFiles = pp.HFiles
+	p.FFiles = pp.FFiles
+	p.SFiles = pp.SFiles
+	p.SwigFiles = pp.SwigFiles
+	p.SwigCXXFiles = pp.SwigCXXFiles
+	p.SysoFiles = pp.SysoFiles
+	if cfg.BuildMSan {
+		// There's no way for .syso files to be built both with and without
+		// support for memory sanitizer. Assume they are built without,
+		// and drop them.
+		p.SysoFiles = nil
+	}
+	p.CgoCFLAGS = pp.CgoCFLAGS
+	p.CgoCPPFLAGS = pp.CgoCPPFLAGS
+	p.CgoCXXFLAGS = pp.CgoCXXFLAGS
+	p.CgoFFLAGS = pp.CgoFFLAGS
+	p.CgoLDFLAGS = pp.CgoLDFLAGS
+	p.CgoPkgConfig = pp.CgoPkgConfig
+	// We modify p.Imports in place, so make copy now.
+	p.Imports = make([]string, len(pp.Imports))
+	copy(p.Imports, pp.Imports)
+	p.Internal.RawImports = pp.Imports
+	p.TestGoFiles = pp.TestGoFiles
+	p.TestImports = pp.TestImports
+	p.XTestGoFiles = pp.XTestGoFiles
+	p.XTestImports = pp.XTestImports
+	if opts.IgnoreImports {
+		p.Imports = nil
+		p.Internal.RawImports = nil
+		p.TestImports = nil
+		p.XTestImports = nil
+	}
+	p.EmbedPatterns = pp.EmbedPatterns
+	p.TestEmbedPatterns = pp.TestEmbedPatterns
+	p.XTestEmbedPatterns = pp.XTestEmbedPatterns
+	p.Internal.OrigImportPath = pp.ImportPath
+}
+
+// A PackageError describes an error loading information about a package.
+type PackageError struct {
+	ImportStack      []string // shortest path from package named on command line to this one
+	Pos              string   // position of error
+	Err              error    // the error itself
+	IsImportCycle    bool     // the error is an import cycle
+	Hard             bool     // whether the error is soft or hard; soft errors are ignored in some places
+	alwaysPrintStack bool     // whether to always print the ImportStack
+}
+
+func (p *PackageError) Error() string {
+	// TODO(#43696): decide when to print the stack or the position based on
+	// the error type and whether the package is in the main module.
+	// Document the rationale.
+	if p.Pos != "" && (len(p.ImportStack) == 0 || !p.alwaysPrintStack) {
+		// Omit import stack. The full path to the file where the error
+		// is the most important thing.
+		return p.Pos + ": " + p.Err.Error()
+	}
+
+	// If the error is an ImportPathError, and the last path on the stack appears
+	// in the error message, omit that path from the stack to avoid repetition.
+	// If an ImportPathError wraps another ImportPathError that matches the
+	// last path on the stack, we don't omit the path. An error like
+	// "package A imports B: error loading C caused by B" would not be clearer
+	// if "imports B" were omitted.
+	if len(p.ImportStack) == 0 {
+		return p.Err.Error()
+	}
+	var optpos string
+	if p.Pos != "" {
+		optpos = "\n\t" + p.Pos
+	}
+	return "package " + strings.Join(p.ImportStack, "\n\timports ") + optpos + ": " + p.Err.Error()
+}
+
+func (p *PackageError) Unwrap() error { return p.Err }
+
+// PackageError implements MarshalJSON so that Err is marshaled as a string
+// and non-essential fields are omitted.
+func (p *PackageError) MarshalJSON() ([]byte, error) {
+	perr := struct {
+		ImportStack []string
+		Pos         string
+		Err         string
+	}{p.ImportStack, p.Pos, p.Err.Error()}
+	return json.Marshal(perr)
+}
+
+func (p *PackageError) setPos(posList []token.Position) {
+	if len(posList) == 0 {
+		return
+	}
+	pos := posList[0]
+	pos.Filename = base.ShortPath(pos.Filename)
+	p.Pos = pos.String()
+}
+
+// ImportPathError is a type of error that prevents a package from being loaded
+// for a given import path. When such a package is loaded, a *Package is
+// returned with Err wrapping an ImportPathError: the error is attached to
+// the imported package, not the importing package.
+//
+// The string returned by ImportPath must appear in the string returned by
+// Error. Errors that wrap ImportPathError (such as PackageError) may omit
+// the import path.
+type ImportPathError interface {
+	error
+	ImportPath() string
+}
+
+var (
+	_ ImportPathError = (*importError)(nil)
+	_ ImportPathError = (*mainPackageError)(nil)
+	_ ImportPathError = (*modload.ImportMissingError)(nil)
+	_ ImportPathError = (*modload.ImportMissingSumError)(nil)
+	_ ImportPathError = (*modload.DirectImportFromImplicitDependencyError)(nil)
+)
+
+type importError struct {
+	importPath string
+	err        error // created with fmt.Errorf
+}
+
+func ImportErrorf(path, format string, args ...any) ImportPathError {
+	err := &importError{importPath: path, err: fmt.Errorf(format, args...)}
+	if errStr := err.Error(); !strings.Contains(errStr, path) {
+		panic(fmt.Sprintf("path %q not in error %q", path, errStr))
+	}
+	return err
+}
+
+func (e *importError) Error() string {
+	return e.err.Error()
+}
+
+func (e *importError) Unwrap() error {
+	// Don't return e.err directly, since we're only wrapping an error if %w
+	// was passed to ImportErrorf.
+	return errors.Unwrap(e.err)
+}
+
+func (e *importError) ImportPath() string {
+	return e.importPath
+}
+
+// An ImportStack is a stack of import paths, possibly with the suffix " (test)" appended.
+// The import path of a test package is the import path of the corresponding
+// non-test package with the suffix "_test" added.
+type ImportStack []string
+
+func (s *ImportStack) Push(p string) {
+	*s = append(*s, p)
+}
+
+func (s *ImportStack) Pop() {
+	*s = (*s)[0 : len(*s)-1]
+}
+
+func (s *ImportStack) Copy() []string {
+	return append([]string{}, *s...)
+}
+
+func (s *ImportStack) Top() string {
+	if len(*s) == 0 {
+		return ""
+	}
+	return (*s)[len(*s)-1]
+}
+
+// shorterThan reports whether sp is shorter than t.
+// We use this to record the shortest import sequence
+// that leads to a particular package.
+func (sp *ImportStack) shorterThan(t []string) bool {
+	s := *sp
+	if len(s) != len(t) {
+		return len(s) < len(t)
+	}
+	// If they are the same length, settle ties using string ordering.
+	for i := range s {
+		if s[i] != t[i] {
+			return s[i] < t[i]
+		}
+	}
+	return false // they are equal
+}
+
+// packageCache is a lookup cache for LoadImport,
+// so that if we look up a package multiple times
+// we return the same pointer each time.
+var packageCache = map[string]*Package{}
+
+// ClearPackageCache clears the in-memory package cache and the preload caches.
+// It is only for use by GOPATH-based "go get".
+// TODO(jayconrod): When GOPATH-based "go get" is removed, delete this function.
+func ClearPackageCache() {
+	for name := range packageCache {
+		delete(packageCache, name)
+	}
+	resolvedImportCache.Clear()
+	packageDataCache.Clear()
+}
+
+// ClearPackageCachePartial clears packages with the given import paths from the
+// in-memory package cache and the preload caches. It is only for use by
+// GOPATH-based "go get".
+// TODO(jayconrod): When GOPATH-based "go get" is removed, delete this function.
+func ClearPackageCachePartial(args []string) {
+	shouldDelete := make(map[string]bool)
+	for _, arg := range args {
+		shouldDelete[arg] = true
+		if p := packageCache[arg]; p != nil {
+			delete(packageCache, arg)
+		}
+	}
+	resolvedImportCache.DeleteIf(func(key importSpec) bool {
+		return shouldDelete[key.path]
+	})
+	packageDataCache.DeleteIf(func(key string) bool {
+		return shouldDelete[key]
+	})
+}
+
+// ReloadPackageNoFlags is like LoadImport but makes sure
+// not to use the package cache.
+// It is only for use by GOPATH-based "go get".
+// TODO(rsc): When GOPATH-based "go get" is removed, delete this function.
+func ReloadPackageNoFlags(arg string, stk *ImportStack) *Package {
+	p := packageCache[arg]
+	if p != nil {
+		delete(packageCache, arg)
+		resolvedImportCache.DeleteIf(func(key importSpec) bool {
+			return key.path == p.ImportPath
+		})
+		packageDataCache.Delete(p.ImportPath)
+	}
+	return LoadPackage(context.TODO(), PackageOpts{}, arg, base.Cwd(), stk, nil, 0)
+}
+
+// dirToImportPath returns the pseudo-import path we use for a package
+// outside the Go path. It begins with _/ and then contains the full path
+// to the directory. If the package lives in c:\home\gopher\my\pkg then
+// the pseudo-import path is _/c_/home/gopher/my/pkg.
+// Using a pseudo-import path like this makes the ./ imports no longer
+// a special case, so that all the code to deal with ordinary imports works
+// automatically.
+func dirToImportPath(dir string) string {
+	return pathpkg.Join("_", strings.Map(makeImportValid, filepath.ToSlash(dir)))
+}
+
+func makeImportValid(r rune) rune {
+	// Should match Go spec, compilers, and ../../go/parser/parser.go:/isValidImport.
+	const illegalChars = `!"#$%&'()*,:;<=>?[\]^{|}` + "`\uFFFD"
+	if !unicode.IsGraphic(r) || unicode.IsSpace(r) || strings.ContainsRune(illegalChars, r) {
+		return '_'
+	}
+	return r
+}
+
+// Mode flags for loadImport and download (in get.go).
+const (
+	// ResolveImport means that loadImport should do import path expansion.
+	// That is, ResolveImport means that the import path came from
+	// a source file and has not been expanded yet to account for
+	// vendoring or possible module adjustment.
+	// Every import path should be loaded initially with ResolveImport,
+	// and then the expanded version (for example with the /vendor/ in it)
+	// gets recorded as the canonical import path. At that point, future loads
+	// of that package must not pass ResolveImport, because
+	// disallowVendor will reject direct use of paths containing /vendor/.
+	ResolveImport = 1 << iota
+
+	// ResolveModule is for download (part of "go get") and indicates
+	// that the module adjustment should be done, but not vendor adjustment.
+	ResolveModule
+
+	// GetTestDeps is for download (part of "go get") and indicates
+	// that test dependencies should be fetched too.
+	GetTestDeps
+
+	// The remainder are internal modes for calls to loadImport.
+
+	// cmdlinePkg is for a package mentioned on the command line.
+	cmdlinePkg
+
+	// cmdlinePkgLiteral is for a package mentioned on the command line
+	// without using any wildcards or meta-patterns.
+	cmdlinePkgLiteral
+)
+
+// LoadImport scans the directory named by path, which must be an import path,
+// but possibly a local import path (an absolute file system path or one beginning
+// with ./ or ../). A local relative path is interpreted relative to srcDir.
+// It returns a *Package describing the package found in that directory.
+// LoadImport does not set tool flags and should only be used by
+// this package, as part of a bigger load operation, and by GOPATH-based "go get".
+// TODO(rsc): When GOPATH-based "go get" is removed, unexport this function.
+// The returned PackageError, if any, describes why parent is not allowed
+// to import the named package, with the error referring to importPos.
+// The PackageError can only be non-nil when parent is not nil.
+func LoadImport(ctx context.Context, opts PackageOpts, path, srcDir string, parent *Package, stk *ImportStack, importPos []token.Position, mode int) (*Package, *PackageError) {
+	return loadImport(ctx, opts, nil, path, srcDir, parent, stk, importPos, mode)
+}
+
+// LoadPackage does Load import, but without a parent package load contezt
+func LoadPackage(ctx context.Context, opts PackageOpts, path, srcDir string, stk *ImportStack, importPos []token.Position, mode int) *Package {
+	p, err := loadImport(ctx, opts, nil, path, srcDir, nil, stk, importPos, mode)
+	if err != nil {
+		base.Fatalf("internal error: loadImport of %q with nil parent returned an error", path)
+	}
+	return p
+}
+
+func loadImport(ctx context.Context, opts PackageOpts, pre *preload, path, srcDir string, parent *Package, stk *ImportStack, importPos []token.Position, mode int) (*Package, *PackageError) {
+	ctx, span := trace.StartSpan(ctx, "modload.loadImport "+path)
+	defer span.Done()
+
+	if path == "" {
+		panic("LoadImport called with empty package path")
+	}
+
+	var parentPath, parentRoot string
+	parentIsStd := false
+	if parent != nil {
+		parentPath = parent.ImportPath
+		parentRoot = parent.Root
+		parentIsStd = parent.Standard
+	}
+	bp, loaded, err := loadPackageData(ctx, path, parentPath, srcDir, parentRoot, parentIsStd, mode)
+	if loaded && pre != nil && !opts.IgnoreImports {
+		pre.preloadImports(ctx, opts, bp.Imports, bp)
+	}
+	if bp == nil {
+		p := &Package{
+			PackagePublic: PackagePublic{
+				ImportPath: path,
+				Incomplete: true,
+			},
+		}
+		if importErr, ok := err.(ImportPathError); !ok || importErr.ImportPath() != path {
+			// Only add path to the error's import stack if it's not already present
+			// in the error.
+			//
+			// TODO(bcmills): setLoadPackageDataError itself has a similar Push / Pop
+			// sequence that empirically doesn't trigger for these errors, guarded by
+			// a somewhat complex condition. Figure out how to generalize that
+			// condition and eliminate the explicit calls here.
+			stk.Push(path)
+			defer stk.Pop()
+		}
+		p.setLoadPackageDataError(err, path, stk, nil)
+		return p, nil
+	}
+
+	setCmdline := func(p *Package) {
+		if mode&cmdlinePkg != 0 {
+			p.Internal.CmdlinePkg = true
+		}
+		if mode&cmdlinePkgLiteral != 0 {
+			p.Internal.CmdlinePkgLiteral = true
+		}
+	}
+
+	importPath := bp.ImportPath
+	p := packageCache[importPath]
+	if p != nil {
+		stk.Push(path)
+		p = reusePackage(p, stk)
+		stk.Pop()
+		setCmdline(p)
+	} else {
+		p = new(Package)
+		p.Internal.Local = build.IsLocalImport(path)
+		p.ImportPath = importPath
+		packageCache[importPath] = p
+
+		setCmdline(p)
+
+		// Load package.
+		// loadPackageData may return bp != nil even if an error occurs,
+		// in order to return partial information.
+		p.load(ctx, opts, path, stk, importPos, bp, err)
+
+		if !cfg.ModulesEnabled && path != cleanImport(path) {
+			p.Error = &PackageError{
+				ImportStack: stk.Copy(),
+				Err:         ImportErrorf(path, "non-canonical import path %q: should be %q", path, pathpkg.Clean(path)),
+			}
+			p.Incomplete = true
+			p.Error.setPos(importPos)
+		}
+	}
+
+	// Checked on every import because the rules depend on the code doing the importing.
+	if perr := disallowInternal(ctx, srcDir, parent, parentPath, p, stk); perr != nil {
+		perr.setPos(importPos)
+		return p, perr
+	}
+	if mode&ResolveImport != 0 {
+		if perr := disallowVendor(srcDir, path, parentPath, p, stk); perr != nil {
+			perr.setPos(importPos)
+			return p, perr
+		}
+	}
+
+	if p.Name == "main" && parent != nil && parent.Dir != p.Dir {
+		perr := &PackageError{
+			ImportStack: stk.Copy(),
+			Err:         ImportErrorf(path, "import %q is a program, not an importable package", path),
+		}
+		perr.setPos(importPos)
+		return p, perr
+	}
+
+	if p.Internal.Local && parent != nil && !parent.Internal.Local {
+		var err error
+		if path == "." {
+			err = ImportErrorf(path, "%s: cannot import current directory", path)
+		} else {
+			err = ImportErrorf(path, "local import %q in non-local package", path)
+		}
+		perr := &PackageError{
+			ImportStack: stk.Copy(),
+			Err:         err,
+		}
+		perr.setPos(importPos)
+		return p, perr
+	}
+
+	return p, nil
+}
+
+// loadPackageData loads information needed to construct a *Package. The result
+// is cached, and later calls to loadPackageData for the same package will return
+// the same data.
+//
+// loadPackageData returns a non-nil package even if err is non-nil unless
+// the package path is malformed (for example, the path contains "mod/" or "@").
+//
+// loadPackageData returns a boolean, loaded, which is true if this is the
+// first time the package was loaded. Callers may preload imports in this case.
+func loadPackageData(ctx context.Context, path, parentPath, parentDir, parentRoot string, parentIsStd bool, mode int) (bp *build.Package, loaded bool, err error) {
+	ctx, span := trace.StartSpan(ctx, "load.loadPackageData "+path)
+	defer span.Done()
+
+	if path == "" {
+		panic("loadPackageData called with empty package path")
+	}
+
+	if strings.HasPrefix(path, "mod/") {
+		// Paths beginning with "mod/" might accidentally
+		// look in the module cache directory tree in $GOPATH/pkg/mod/.
+		// This prefix is owned by the Go core for possible use in the
+		// standard library (since it does not begin with a domain name),
+		// so it's OK to disallow entirely.
+		return nil, false, fmt.Errorf("disallowed import path %q", path)
+	}
+
+	if strings.Contains(path, "@") {
+		return nil, false, errors.New("can only use path@version syntax with 'go get' and 'go install' in module-aware mode")
+	}
+
+	// Determine canonical package path and directory.
+	// For a local import the identifier is the pseudo-import path
+	// we create from the full directory to the package.
+	// Otherwise it is the usual import path.
+	// For vendored imports, it is the expanded form.
+	//
+	// Note that when modules are enabled, local import paths are normally
+	// canonicalized by modload.LoadPackages before now. However, if there's an
+	// error resolving a local path, it will be returned untransformed
+	// so that 'go list -e' reports something useful.
+	importKey := importSpec{
+		path:        path,
+		parentPath:  parentPath,
+		parentDir:   parentDir,
+		parentRoot:  parentRoot,
+		parentIsStd: parentIsStd,
+		mode:        mode,
+	}
+	r := resolvedImportCache.Do(importKey, func() resolvedImport {
+		var r resolvedImport
+		if cfg.ModulesEnabled {
+			r.dir, r.path, r.err = modload.Lookup(parentPath, parentIsStd, path)
+		} else if build.IsLocalImport(path) {
+			r.dir = filepath.Join(parentDir, path)
+			r.path = dirToImportPath(r.dir)
+		} else if mode&ResolveImport != 0 {
+			// We do our own path resolution, because we want to
+			// find out the key to use in packageCache without the
+			// overhead of repeated calls to buildContext.Import.
+			// The code is also needed in a few other places anyway.
+			r.path = resolveImportPath(path, parentPath, parentDir, parentRoot, parentIsStd)
+		} else if mode&ResolveModule != 0 {
+			r.path = moduleImportPath(path, parentPath, parentDir, parentRoot)
+		}
+		if r.path == "" {
+			r.path = path
+		}
+		return r
+	})
+	// Invariant: r.path is set to the resolved import path. If the path cannot
+	// be resolved, r.path is set to path, the source import path.
+	// r.path is never empty.
+
+	// Load the package from its directory. If we already found the package's
+	// directory when resolving its import path, use that.
+	p, err := packageDataCache.Do(r.path, func() (*build.Package, error) {
+		loaded = true
+		var data struct {
+			p   *build.Package
+			err error
+		}
+		if r.dir != "" {
+			var buildMode build.ImportMode
+			buildContext := cfg.BuildContext
+			if !cfg.ModulesEnabled {
+				buildMode = build.ImportComment
+			} else {
+				buildContext.GOPATH = "" // Clear GOPATH so packages are imported as pure module packages
+			}
+			modroot := modload.PackageModRoot(ctx, r.path)
+			if modroot == "" && str.HasPathPrefix(r.dir, cfg.GOROOTsrc) {
+				modroot = cfg.GOROOTsrc
+				gorootSrcCmd := filepath.Join(cfg.GOROOTsrc, "cmd")
+				if str.HasPathPrefix(r.dir, gorootSrcCmd) {
+					modroot = gorootSrcCmd
+				}
+			}
+			if modroot != "" {
+				if rp, err := modindex.GetPackage(modroot, r.dir); err == nil {
+					data.p, data.err = rp.Import(cfg.BuildContext, buildMode)
+					goto Happy
+				} else if !errors.Is(err, modindex.ErrNotIndexed) {
+					base.Fatal(err)
+				}
+			}
+			data.p, data.err = buildContext.ImportDir(r.dir, buildMode)
+		Happy:
+			if cfg.ModulesEnabled {
+				// Override data.p.Root, since ImportDir sets it to $GOPATH, if
+				// the module is inside $GOPATH/src.
+				if info := modload.PackageModuleInfo(ctx, path); info != nil {
+					data.p.Root = info.Dir
+				}
+			}
+			if r.err != nil {
+				if data.err != nil {
+					// ImportDir gave us one error, and the module loader gave us another.
+					// We arbitrarily choose to keep the error from ImportDir because
+					// that's what our tests already expect, and it seems to provide a bit
+					// more detail in most cases.
+				} else if errors.Is(r.err, imports.ErrNoGo) {
+					// ImportDir said there were files in the package, but the module
+					// loader said there weren't. Which one is right?
+					// Without this special-case hack, the TestScript/test_vet case fails
+					// on the vetfail/p1 package (added in CL 83955).
+					// Apparently, imports.ShouldBuild biases toward rejecting files
+					// with invalid build constraints, whereas ImportDir biases toward
+					// accepting them.
+					//
+					// TODO(#41410: Figure out how this actually ought to work and fix
+					// this mess.
+				} else {
+					data.err = r.err
+				}
+			}
+		} else if r.err != nil {
+			data.p = new(build.Package)
+			data.err = r.err
+		} else if cfg.ModulesEnabled && path != "unsafe" {
+			data.p = new(build.Package)
+			data.err = fmt.Errorf("unknown import path %q: internal error: module loader did not resolve import", r.path)
+		} else {
+			buildMode := build.ImportComment
+			if mode&ResolveImport == 0 || r.path != path {
+				// Not vendoring, or we already found the vendored path.
+				buildMode |= build.IgnoreVendor
+			}
+			data.p, data.err = cfg.BuildContext.Import(r.path, parentDir, buildMode)
+		}
+		data.p.ImportPath = r.path
+
+		// Set data.p.BinDir in cases where go/build.Context.Import
+		// may give us a path we don't want.
+		if !data.p.Goroot {
+			if cfg.GOBIN != "" {
+				data.p.BinDir = cfg.GOBIN
+			} else if cfg.ModulesEnabled {
+				data.p.BinDir = modload.BinDir()
+			}
+		}
+
+		if !cfg.ModulesEnabled && data.err == nil &&
+			data.p.ImportComment != "" && data.p.ImportComment != path &&
+			!strings.Contains(path, "/vendor/") && !strings.HasPrefix(path, "vendor/") {
+			data.err = fmt.Errorf("code in directory %s expects import %q", data.p.Dir, data.p.ImportComment)
+		}
+		return data.p, data.err
+	})
+
+	return p, loaded, err
+}
+
+// importSpec describes an import declaration in source code. It is used as a
+// cache key for resolvedImportCache.
+type importSpec struct {
+	path                              string
+	parentPath, parentDir, parentRoot string
+	parentIsStd                       bool
+	mode                              int
+}
+
+// resolvedImport holds a canonical identifier for a package. It may also contain
+// a path to the package's directory and an error if one occurred. resolvedImport
+// is the value type in resolvedImportCache.
+type resolvedImport struct {
+	path, dir string
+	err       error
+}
+
+// resolvedImportCache maps import strings to canonical package names.
+var resolvedImportCache par.Cache[importSpec, resolvedImport]
+
+// packageDataCache maps canonical package names (string) to package metadata.
+var packageDataCache par.ErrCache[string, *build.Package]
+
+// preloadWorkerCount is the number of concurrent goroutines that can load
+// packages. Experimentally, there are diminishing returns with more than
+// 4 workers. This was measured on the following machines.
+//
+// * MacBookPro with a 4-core Intel Core i7 CPU
+// * Linux workstation with 6-core Intel Xeon CPU
+// * Linux workstation with 24-core Intel Xeon CPU
+//
+// It is very likely (though not confirmed) that this workload is limited
+// by memory bandwidth. We don't have a good way to determine the number of
+// workers that would saturate the bus though, so runtime.GOMAXPROCS
+// seems like a reasonable default.
+var preloadWorkerCount = runtime.GOMAXPROCS(0)
+
+// preload holds state for managing concurrent preloading of package data.
+//
+// A preload should be created with newPreload before loading a large
+// package graph. flush must be called when package loading is complete
+// to ensure preload goroutines are no longer active. This is necessary
+// because of global mutable state that cannot safely be read and written
+// concurrently. In particular, packageDataCache may be cleared by "go get"
+// in GOPATH mode, and modload.loaded (accessed via modload.Lookup) may be
+// modified by modload.LoadPackages.
+type preload struct {
+	cancel chan struct{}
+	sema   chan struct{}
+}
+
+// newPreload creates a new preloader. flush must be called later to avoid
+// accessing global state while it is being modified.
+func newPreload() *preload {
+	pre := &preload{
+		cancel: make(chan struct{}),
+		sema:   make(chan struct{}, preloadWorkerCount),
+	}
+	return pre
+}
+
+// preloadMatches loads data for package paths matched by patterns.
+// When preloadMatches returns, some packages may not be loaded yet, but
+// loadPackageData and loadImport are always safe to call.
+func (pre *preload) preloadMatches(ctx context.Context, opts PackageOpts, matches []*search.Match) {
+	for _, m := range matches {
+		for _, pkg := range m.Pkgs {
+			select {
+			case <-pre.cancel:
+				return
+			case pre.sema <- struct{}{}:
+				go func(pkg string) {
+					mode := 0 // don't use vendoring or module import resolution
+					bp, loaded, err := loadPackageData(ctx, pkg, "", base.Cwd(), "", false, mode)
+					<-pre.sema
+					if bp != nil && loaded && err == nil && !opts.IgnoreImports {
+						pre.preloadImports(ctx, opts, bp.Imports, bp)
+					}
+				}(pkg)
+			}
+		}
+	}
+}
+
+// preloadImports queues a list of imports for preloading.
+// When preloadImports returns, some packages may not be loaded yet,
+// but loadPackageData and loadImport are always safe to call.
+func (pre *preload) preloadImports(ctx context.Context, opts PackageOpts, imports []string, parent *build.Package) {
+	parentIsStd := parent.Goroot && parent.ImportPath != "" && search.IsStandardImportPath(parent.ImportPath)
+	for _, path := range imports {
+		if path == "C" || path == "unsafe" {
+			continue
+		}
+		select {
+		case <-pre.cancel:
+			return
+		case pre.sema <- struct{}{}:
+			go func(path string) {
+				bp, loaded, err := loadPackageData(ctx, path, parent.ImportPath, parent.Dir, parent.Root, parentIsStd, ResolveImport)
+				<-pre.sema
+				if bp != nil && loaded && err == nil && !opts.IgnoreImports {
+					pre.preloadImports(ctx, opts, bp.Imports, bp)
+				}
+			}(path)
+		}
+	}
+}
+
+// flush stops pending preload operations. flush blocks until preload calls to
+// loadPackageData have completed. The preloader will not make any new calls
+// to loadPackageData.
+func (pre *preload) flush() {
+	// flush is usually deferred.
+	// Don't hang program waiting for workers on panic.
+	if v := recover(); v != nil {
+		panic(v)
+	}
+
+	close(pre.cancel)
+	for i := 0; i < preloadWorkerCount; i++ {
+		pre.sema <- struct{}{}
+	}
+}
+
+func cleanImport(path string) string {
+	orig := path
+	path = pathpkg.Clean(path)
+	if strings.HasPrefix(orig, "./") && path != ".." && !strings.HasPrefix(path, "../") {
+		path = "./" + path
+	}
+	return path
+}
+
+var isDirCache par.Cache[string, bool]
+
+func isDir(path string) bool {
+	return isDirCache.Do(path, func() bool {
+		fi, err := fsys.Stat(path)
+		return err == nil && fi.IsDir()
+	})
+}
+
+// ResolveImportPath returns the true meaning of path when it appears in parent.
+// There are two different resolutions applied.
+// First, there is Go 1.5 vendoring (golang.org/s/go15vendor).
+// If vendor expansion doesn't trigger, then the path is also subject to
+// Go 1.11 module legacy conversion (golang.org/issue/25069).
+func ResolveImportPath(parent *Package, path string) (found string) {
+	var parentPath, parentDir, parentRoot string
+	parentIsStd := false
+	if parent != nil {
+		parentPath = parent.ImportPath
+		parentDir = parent.Dir
+		parentRoot = parent.Root
+		parentIsStd = parent.Standard
+	}
+	return resolveImportPath(path, parentPath, parentDir, parentRoot, parentIsStd)
+}
+
+func resolveImportPath(path, parentPath, parentDir, parentRoot string, parentIsStd bool) (found string) {
+	if cfg.ModulesEnabled {
+		if _, p, e := modload.Lookup(parentPath, parentIsStd, path); e == nil {
+			return p
+		}
+		return path
+	}
+	found = vendoredImportPath(path, parentPath, parentDir, parentRoot)
+	if found != path {
+		return found
+	}
+	return moduleImportPath(path, parentPath, parentDir, parentRoot)
+}
+
+// dirAndRoot returns the source directory and workspace root
+// for the package p, guaranteeing that root is a path prefix of dir.
+func dirAndRoot(path string, dir, root string) (string, string) {
+	origDir, origRoot := dir, root
+	dir = filepath.Clean(dir)
+	root = filepath.Join(root, "src")
+	if !str.HasFilePathPrefix(dir, root) || path != "command-line-arguments" && filepath.Join(root, path) != dir {
+		// Look for symlinks before reporting error.
+		dir = expandPath(dir)
+		root = expandPath(root)
+	}
+
+	if !str.HasFilePathPrefix(dir, root) || len(dir) <= len(root) || dir[len(root)] != filepath.Separator || path != "command-line-arguments" && !build.IsLocalImport(path) && filepath.Join(root, path) != dir {
+		debug.PrintStack()
+		base.Fatalf("unexpected directory layout:\n"+
+			"	import path: %s\n"+
+			"	root: %s\n"+
+			"	dir: %s\n"+
+			"	expand root: %s\n"+
+			"	expand dir: %s\n"+
+			"	separator: %s",
+			path,
+			filepath.Join(origRoot, "src"),
+			filepath.Clean(origDir),
+			origRoot,
+			origDir,
+			string(filepath.Separator))
+	}
+
+	return dir, root
+}
+
+// vendoredImportPath returns the vendor-expansion of path when it appears in parent.
+// If parent is x/y/z, then path might expand to x/y/z/vendor/path, x/y/vendor/path,
+// x/vendor/path, vendor/path, or else stay path if none of those exist.
+// vendoredImportPath returns the expanded path or, if no expansion is found, the original.
+func vendoredImportPath(path, parentPath, parentDir, parentRoot string) (found string) {
+	if parentRoot == "" {
+		return path
+	}
+
+	dir, root := dirAndRoot(parentPath, parentDir, parentRoot)
+
+	vpath := "vendor/" + path
+	for i := len(dir); i >= len(root); i-- {
+		if i < len(dir) && dir[i] != filepath.Separator {
+			continue
+		}
+		// Note: checking for the vendor directory before checking
+		// for the vendor/path directory helps us hit the
+		// isDir cache more often. It also helps us prepare a more useful
+		// list of places we looked, to report when an import is not found.
+		if !isDir(filepath.Join(dir[:i], "vendor")) {
+			continue
+		}
+		targ := filepath.Join(dir[:i], vpath)
+		if isDir(targ) && hasGoFiles(targ) {
+			importPath := parentPath
+			if importPath == "command-line-arguments" {
+				// If parent.ImportPath is 'command-line-arguments'.
+				// set to relative directory to root (also chopped root directory)
+				importPath = dir[len(root)+1:]
+			}
+			// We started with parent's dir c:\gopath\src\foo\bar\baz\quux\xyzzy.
+			// We know the import path for parent's dir.
+			// We chopped off some number of path elements and
+			// added vendor\path to produce c:\gopath\src\foo\bar\baz\vendor\path.
+			// Now we want to know the import path for that directory.
+			// Construct it by chopping the same number of path elements
+			// (actually the same number of bytes) from parent's import path
+			// and then append /vendor/path.
+			chopped := len(dir) - i
+			if chopped == len(importPath)+1 {
+				// We walked up from c:\gopath\src\foo\bar
+				// and found c:\gopath\src\vendor\path.
+				// We chopped \foo\bar (length 8) but the import path is "foo/bar" (length 7).
+				// Use "vendor/path" without any prefix.
+				return vpath
+			}
+			return importPath[:len(importPath)-chopped] + "/" + vpath
+		}
+	}
+	return path
+}
+
+var (
+	modulePrefix   = []byte("\nmodule ")
+	goModPathCache par.Cache[string, string]
+)
+
+// goModPath returns the module path in the go.mod in dir, if any.
+func goModPath(dir string) (path string) {
+	return goModPathCache.Do(dir, func() string {
+		data, err := os.ReadFile(filepath.Join(dir, "go.mod"))
+		if err != nil {
+			return ""
+		}
+		var i int
+		if bytes.HasPrefix(data, modulePrefix[1:]) {
+			i = 0
+		} else {
+			i = bytes.Index(data, modulePrefix)
+			if i < 0 {
+				return ""
+			}
+			i++
+		}
+		line := data[i:]
+
+		// Cut line at \n, drop trailing \r if present.
+		if j := bytes.IndexByte(line, '\n'); j >= 0 {
+			line = line[:j]
+		}
+		if line[len(line)-1] == '\r' {
+			line = line[:len(line)-1]
+		}
+		line = line[len("module "):]
+
+		// If quoted, unquote.
+		path = strings.TrimSpace(string(line))
+		if path != "" && path[0] == '"' {
+			s, err := strconv.Unquote(path)
+			if err != nil {
+				return ""
+			}
+			path = s
+		}
+		return path
+	})
+}
+
+// findVersionElement returns the slice indices of the final version element /vN in path.
+// If there is no such element, it returns -1, -1.
+func findVersionElement(path string) (i, j int) {
+	j = len(path)
+	for i = len(path) - 1; i >= 0; i-- {
+		if path[i] == '/' {
+			if isVersionElement(path[i+1 : j]) {
+				return i, j
+			}
+			j = i
+		}
+	}
+	return -1, -1
+}
+
+// isVersionElement reports whether s is a well-formed path version element:
+// v2, v3, v10, etc, but not v0, v05, v1.
+func isVersionElement(s string) bool {
+	if len(s) < 2 || s[0] != 'v' || s[1] == '0' || s[1] == '1' && len(s) == 2 {
+		return false
+	}
+	for i := 1; i < len(s); i++ {
+		if s[i] < '0' || '9' < s[i] {
+			return false
+		}
+	}
+	return true
+}
+
+// moduleImportPath translates import paths found in go modules
+// back down to paths that can be resolved in ordinary builds.
+//
+// Define “new” code as code with a go.mod file in the same directory
+// or a parent directory. If an import in new code says x/y/v2/z but
+// x/y/v2/z does not exist and x/y/go.mod says “module x/y/v2”,
+// then go build will read the import as x/y/z instead.
+// See golang.org/issue/25069.
+func moduleImportPath(path, parentPath, parentDir, parentRoot string) (found string) {
+	if parentRoot == "" {
+		return path
+	}
+
+	// If there are no vN elements in path, leave it alone.
+	// (The code below would do the same, but only after
+	// some other file system accesses that we can avoid
+	// here by returning early.)
+	if i, _ := findVersionElement(path); i < 0 {
+		return path
+	}
+
+	dir, root := dirAndRoot(parentPath, parentDir, parentRoot)
+
+	// Consider dir and parents, up to and including root.
+	for i := len(dir); i >= len(root); i-- {
+		if i < len(dir) && dir[i] != filepath.Separator {
+			continue
+		}
+		if goModPath(dir[:i]) != "" {
+			goto HaveGoMod
+		}
+	}
+	// This code is not in a tree with a go.mod,
+	// so apply no changes to the path.
+	return path
+
+HaveGoMod:
+	// This import is in a tree with a go.mod.
+	// Allow it to refer to code in GOPATH/src/x/y/z as x/y/v2/z
+	// if GOPATH/src/x/y/go.mod says module "x/y/v2",
+
+	// If x/y/v2/z exists, use it unmodified.
+	if bp, _ := cfg.BuildContext.Import(path, "", build.IgnoreVendor); bp.Dir != "" {
+		return path
+	}
+
+	// Otherwise look for a go.mod supplying a version element.
+	// Some version-like elements may appear in paths but not
+	// be module versions; we skip over those to look for module
+	// versions. For example the module m/v2 might have a
+	// package m/v2/api/v1/foo.
+	limit := len(path)
+	for limit > 0 {
+		i, j := findVersionElement(path[:limit])
+		if i < 0 {
+			return path
+		}
+		if bp, _ := cfg.BuildContext.Import(path[:i], "", build.IgnoreVendor); bp.Dir != "" {
+			if mpath := goModPath(bp.Dir); mpath != "" {
+				// Found a valid go.mod file, so we're stopping the search.
+				// If the path is m/v2/p and we found m/go.mod that says
+				// "module m/v2", then we return "m/p".
+				if mpath == path[:j] {
+					return path[:i] + path[j:]
+				}
+				// Otherwise just return the original path.
+				// We didn't find anything worth rewriting,
+				// and the go.mod indicates that we should
+				// not consider parent directories.
+				return path
+			}
+		}
+		limit = i
+	}
+	return path
+}
+
+// hasGoFiles reports whether dir contains any files with names ending in .go.
+// For a vendor check we must exclude directories that contain no .go files.
+// Otherwise it is not possible to vendor just a/b/c and still import the
+// non-vendored a/b. See golang.org/issue/13832.
+func hasGoFiles(dir string) bool {
+	files, _ := os.ReadDir(dir)
+	for _, f := range files {
+		if !f.IsDir() && strings.HasSuffix(f.Name(), ".go") {
+			return true
+		}
+	}
+	return false
+}
+
+// reusePackage reuses package p to satisfy the import at the top
+// of the import stack stk. If this use causes an import loop,
+// reusePackage updates p's error information to record the loop.
+func reusePackage(p *Package, stk *ImportStack) *Package {
+	// We use p.Internal.Imports==nil to detect a package that
+	// is in the midst of its own loadPackage call
+	// (all the recursion below happens before p.Internal.Imports gets set).
+	if p.Internal.Imports == nil {
+		if p.Error == nil {
+			p.Error = &PackageError{
+				ImportStack:   stk.Copy(),
+				Err:           errors.New("import cycle not allowed"),
+				IsImportCycle: true,
+			}
+		} else if !p.Error.IsImportCycle {
+			// If the error is already set, but it does not indicate that
+			// we are in an import cycle, set IsImportCycle so that we don't
+			// end up stuck in a loop down the road.
+			p.Error.IsImportCycle = true
+		}
+		p.Incomplete = true
+	}
+	// Don't rewrite the import stack in the error if we have an import cycle.
+	// If we do, we'll lose the path that describes the cycle.
+	if p.Error != nil && !p.Error.IsImportCycle && stk.shorterThan(p.Error.ImportStack) {
+		p.Error.ImportStack = stk.Copy()
+	}
+	return p
+}
+
+// disallowInternal checks that srcDir (containing package importerPath, if non-empty)
+// is allowed to import p.
+// If the import is allowed, disallowInternal returns the original package p.
+// If not, it returns a new package containing just an appropriate error.
+func disallowInternal(ctx context.Context, srcDir string, importer *Package, importerPath string, p *Package, stk *ImportStack) *PackageError {
+	// golang.org/s/go14internal:
+	// An import of a path containing the element “internal”
+	// is disallowed if the importing code is outside the tree
+	// rooted at the parent of the “internal” directory.
+
+	// There was an error loading the package; stop here.
+	if p.Error != nil {
+		return nil
+	}
+
+	// The generated 'testmain' package is allowed to access testing/internal/...,
+	// as if it were generated into the testing directory tree
+	// (it's actually in a temporary directory outside any Go tree).
+	// This cleans up a former kludge in passing functionality to the testing package.
+	if str.HasPathPrefix(p.ImportPath, "testing/internal") && importerPath == "testmain" {
+		return nil
+	}
+
+	// We can't check standard packages with gccgo.
+	if cfg.BuildContext.Compiler == "gccgo" && p.Standard {
+		return nil
+	}
+
+	// The sort package depends on internal/reflectlite, but during bootstrap
+	// the path rewriting causes the normal internal checks to fail.
+	// Instead, just ignore the internal rules during bootstrap.
+	if p.Standard && strings.HasPrefix(importerPath, "bootstrap/") {
+		return nil
+	}
+
+	// importerPath is empty: we started
+	// with a name given on the command line, not an
+	// import. Anything listed on the command line is fine.
+	if importerPath == "" {
+		return nil
+	}
+
+	// Check for "internal" element: three cases depending on begin of string and/or end of string.
+	i, ok := findInternal(p.ImportPath)
+	if !ok {
+		return nil
+	}
+
+	// Internal is present.
+	// Map import path back to directory corresponding to parent of internal.
+	if i > 0 {
+		i-- // rewind over slash in ".../internal"
+	}
+
+	if p.Module == nil {
+		parent := p.Dir[:i+len(p.Dir)-len(p.ImportPath)]
+
+		if str.HasFilePathPrefix(filepath.Clean(srcDir), filepath.Clean(parent)) {
+			return nil
+		}
+
+		// Look for symlinks before reporting error.
+		srcDir = expandPath(srcDir)
+		parent = expandPath(parent)
+		if str.HasFilePathPrefix(filepath.Clean(srcDir), filepath.Clean(parent)) {
+			return nil
+		}
+	} else {
+		// p is in a module, so make it available based on the importer's import path instead
+		// of the file path (https://golang.org/issue/23970).
+		if importer.Internal.CmdlineFiles {
+			// The importer is a list of command-line files.
+			// Pretend that the import path is the import path of the
+			// directory containing them.
+			// If the directory is outside the main modules, this will resolve to ".",
+			// which is not a prefix of any valid module.
+			importerPath, _ = modload.MainModules.DirImportPath(ctx, importer.Dir)
+		}
+		parentOfInternal := p.ImportPath[:i]
+		if str.HasPathPrefix(importerPath, parentOfInternal) {
+			return nil
+		}
+	}
+
+	// Internal is present, and srcDir is outside parent's tree. Not allowed.
+	perr := &PackageError{
+		alwaysPrintStack: true,
+		ImportStack:      stk.Copy(),
+		Err:              ImportErrorf(p.ImportPath, "use of internal package "+p.ImportPath+" not allowed"),
+	}
+	return perr
+}
+
+// findInternal looks for the final "internal" path element in the given import path.
+// If there isn't one, findInternal returns ok=false.
+// Otherwise, findInternal returns ok=true and the index of the "internal".
+func findInternal(path string) (index int, ok bool) {
+	// Three cases, depending on internal at start/end of string or not.
+	// The order matters: we must return the index of the final element,
+	// because the final one produces the most restrictive requirement
+	// on the importer.
+	switch {
+	case strings.HasSuffix(path, "/internal"):
+		return len(path) - len("internal"), true
+	case strings.Contains(path, "/internal/"):
+		return strings.LastIndex(path, "/internal/") + 1, true
+	case path == "internal", strings.HasPrefix(path, "internal/"):
+		return 0, true
+	}
+	return 0, false
+}
+
+// disallowVendor checks that srcDir is allowed to import p as path.
+// If the import is allowed, disallowVendor returns the original package p.
+// If not, it returns a PackageError.
+func disallowVendor(srcDir string, path string, importerPath string, p *Package, stk *ImportStack) *PackageError {
+	// If the importerPath is empty, we started
+	// with a name given on the command line, not an
+	// import. Anything listed on the command line is fine.
+	if importerPath == "" {
+		return nil
+	}
+
+	if perr := disallowVendorVisibility(srcDir, p, importerPath, stk); perr != nil {
+		return perr
+	}
+
+	// Paths like x/vendor/y must be imported as y, never as x/vendor/y.
+	if i, ok := FindVendor(path); ok {
+		perr := &PackageError{
+			ImportStack: stk.Copy(),
+			Err:         ImportErrorf(path, "%s must be imported as %s", path, path[i+len("vendor/"):]),
+		}
+		return perr
+	}
+
+	return nil
+}
+
+// disallowVendorVisibility checks that srcDir is allowed to import p.
+// The rules are the same as for /internal/ except that a path ending in /vendor
+// is not subject to the rules, only subdirectories of vendor.
+// This allows people to have packages and commands named vendor,
+// for maximal compatibility with existing source trees.
+func disallowVendorVisibility(srcDir string, p *Package, importerPath string, stk *ImportStack) *PackageError {
+	// The stack does not include p.ImportPath.
+	// If there's nothing on the stack, we started
+	// with a name given on the command line, not an
+	// import. Anything listed on the command line is fine.
+	if importerPath == "" {
+		return nil
+	}
+
+	// Check for "vendor" element.
+	i, ok := FindVendor(p.ImportPath)
+	if !ok {
+		return nil
+	}
+
+	// Vendor is present.
+	// Map import path back to directory corresponding to parent of vendor.
+	if i > 0 {
+		i-- // rewind over slash in ".../vendor"
+	}
+	truncateTo := i + len(p.Dir) - len(p.ImportPath)
+	if truncateTo < 0 || len(p.Dir) < truncateTo {
+		return nil
+	}
+	parent := p.Dir[:truncateTo]
+	if str.HasFilePathPrefix(filepath.Clean(srcDir), filepath.Clean(parent)) {
+		return nil
+	}
+
+	// Look for symlinks before reporting error.
+	srcDir = expandPath(srcDir)
+	parent = expandPath(parent)
+	if str.HasFilePathPrefix(filepath.Clean(srcDir), filepath.Clean(parent)) {
+		return nil
+	}
+
+	// Vendor is present, and srcDir is outside parent's tree. Not allowed.
+
+	perr := &PackageError{
+		ImportStack: stk.Copy(),
+		Err:         errors.New("use of vendored package not allowed"),
+	}
+	return perr
+}
+
+// FindVendor looks for the last non-terminating "vendor" path element in the given import path.
+// If there isn't one, FindVendor returns ok=false.
+// Otherwise, FindVendor returns ok=true and the index of the "vendor".
+//
+// Note that terminating "vendor" elements don't count: "x/vendor" is its own package,
+// not the vendored copy of an import "" (the empty import path).
+// This will allow people to have packages or commands named vendor.
+// This may help reduce breakage, or it may just be confusing. We'll see.
+func FindVendor(path string) (index int, ok bool) {
+	// Two cases, depending on internal at start of string or not.
+	// The order matters: we must return the index of the final element,
+	// because the final one is where the effective import path starts.
+	switch {
+	case strings.Contains(path, "/vendor/"):
+		return strings.LastIndex(path, "/vendor/") + 1, true
+	case strings.HasPrefix(path, "vendor/"):
+		return 0, true
+	}
+	return 0, false
+}
+
+type TargetDir int
+
+const (
+	ToTool    TargetDir = iota // to GOROOT/pkg/tool (default for cmd/*)
+	ToBin                      // to bin dir inside package root (default for non-cmd/*)
+	StalePath                  // an old import path; fail to build
+)
+
+// InstallTargetDir reports the target directory for installing the command p.
+func InstallTargetDir(p *Package) TargetDir {
+	if strings.HasPrefix(p.ImportPath, "code.google.com/p/go.tools/cmd/") {
+		return StalePath
+	}
+	if p.Goroot && strings.HasPrefix(p.ImportPath, "cmd/") && p.Name == "main" {
+		switch p.ImportPath {
+		case "cmd/go", "cmd/gofmt":
+			return ToBin
+		}
+		return ToTool
+	}
+	return ToBin
+}
+
+var cgoExclude = map[string]bool{
+	"runtime/cgo": true,
+}
+
+var cgoSyscallExclude = map[string]bool{
+	"runtime/cgo":  true,
+	"runtime/race": true,
+	"runtime/msan": true,
+	"runtime/asan": true,
+}
+
+var foldPath = make(map[string]string)
+
+// exeFromImportPath returns an executable name
+// for a package using the import path.
+//
+// The executable name is the last element of the import path.
+// In module-aware mode, an additional rule is used on import paths
+// consisting of two or more path elements. If the last element is
+// a vN path element specifying the major version, then the
+// second last element of the import path is used instead.
+func (p *Package) exeFromImportPath() string {
+	_, elem := pathpkg.Split(p.ImportPath)
+	if cfg.ModulesEnabled {
+		// If this is example.com/mycmd/v2, it's more useful to
+		// install it as mycmd than as v2. See golang.org/issue/24667.
+		if elem != p.ImportPath && isVersionElement(elem) {
+			_, elem = pathpkg.Split(pathpkg.Dir(p.ImportPath))
+		}
+	}
+	return elem
+}
+
+// exeFromFiles returns an executable name for a package
+// using the first element in GoFiles or CgoFiles collections without the prefix.
+//
+// Returns empty string in case of empty collection.
+func (p *Package) exeFromFiles() string {
+	var src string
+	if len(p.GoFiles) > 0 {
+		src = p.GoFiles[0]
+	} else if len(p.CgoFiles) > 0 {
+		src = p.CgoFiles[0]
+	} else {
+		return ""
+	}
+	_, elem := filepath.Split(src)
+	return elem[:len(elem)-len(".go")]
+}
+
+// DefaultExecName returns the default executable name for a package
+func (p *Package) DefaultExecName() string {
+	if p.Internal.CmdlineFiles {
+		return p.exeFromFiles()
+	}
+	return p.exeFromImportPath()
+}
+
+// load populates p using information from bp, err, which should
+// be the result of calling build.Context.Import.
+// stk contains the import stack, not including path itself.
+func (p *Package) load(ctx context.Context, opts PackageOpts, path string, stk *ImportStack, importPos []token.Position, bp *build.Package, err error) {
+	p.copyBuild(opts, bp)
+
+	// The localPrefix is the path we interpret ./ imports relative to,
+	// if we support them at all (not in module mode!).
+	// Synthesized main packages sometimes override this.
+	if p.Internal.Local && !cfg.ModulesEnabled {
+		p.Internal.LocalPrefix = dirToImportPath(p.Dir)
+	}
+
+	// setError sets p.Error if it hasn't already been set. We may proceed
+	// after encountering some errors so that 'go list -e' has more complete
+	// output. If there's more than one error, we should report the first.
+	setError := func(err error) {
+		if p.Error == nil {
+			p.Error = &PackageError{
+				ImportStack: stk.Copy(),
+				Err:         err,
+			}
+			p.Incomplete = true
+
+			// Add the importer's position information if the import position exists, and
+			// the current package being examined is the importer.
+			// If we have not yet accepted package p onto the import stack,
+			// then the cause of the error is not within p itself: the error
+			// must be either in an explicit command-line argument,
+			// or on the importer side (indicated by a non-empty importPos).
+			if path != stk.Top() && len(importPos) > 0 {
+				p.Error.setPos(importPos)
+			}
+		}
+	}
+
+	if err != nil {
+		p.Incomplete = true
+		p.setLoadPackageDataError(err, path, stk, importPos)
+	}
+
+	useBindir := p.Name == "main"
+	if !p.Standard {
+		switch cfg.BuildBuildmode {
+		case "c-archive", "c-shared", "plugin":
+			useBindir = false
+		}
+	}
+
+	if useBindir {
+		// Report an error when the old code.google.com/p/go.tools paths are used.
+		if InstallTargetDir(p) == StalePath {
+			// TODO(matloob): remove this branch, and StalePath itself. code.google.com/p/go is so
+			// old, even this code checking for it is stale now!
+			newPath := strings.Replace(p.ImportPath, "code.google.com/p/go.", "golang.org/x/", 1)
+			e := ImportErrorf(p.ImportPath, "the %v command has moved; use %v instead.", p.ImportPath, newPath)
+			setError(e)
+			return
+		}
+		elem := p.DefaultExecName() + cfg.ExeSuffix
+		full := filepath.Join(cfg.BuildContext.GOOS+"_"+cfg.BuildContext.GOARCH, elem)
+		if cfg.BuildContext.GOOS != runtime.GOOS || cfg.BuildContext.GOARCH != runtime.GOARCH {
+			// Install cross-compiled binaries to subdirectories of bin.
+			elem = full
+		}
+		if p.Internal.Build.BinDir == "" && cfg.ModulesEnabled {
+			p.Internal.Build.BinDir = modload.BinDir()
+		}
+		if p.Internal.Build.BinDir != "" {
+			// Install to GOBIN or bin of GOPATH entry.
+			p.Target = filepath.Join(p.Internal.Build.BinDir, elem)
+			if !p.Goroot && strings.Contains(elem, string(filepath.Separator)) && cfg.GOBIN != "" {
+				// Do not create $GOBIN/goos_goarch/elem.
+				p.Target = ""
+				p.Internal.GobinSubdir = true
+			}
+		}
+		if InstallTargetDir(p) == ToTool {
+			// This is for 'go tool'.
+			// Override all the usual logic and force it into the tool directory.
+			if cfg.BuildToolchainName == "gccgo" {
+				p.Target = filepath.Join(build.ToolDir, elem)
+			} else {
+				p.Target = filepath.Join(cfg.GOROOTpkg, "tool", full)
+			}
+		}
+	} else if p.Internal.Local {
+		// Local import turned into absolute path.
+		// No permanent install target.
+		p.Target = ""
+	} else if p.Standard && cfg.BuildContext.Compiler == "gccgo" {
+		// gccgo has a preinstalled standard library that cmd/go cannot rebuild.
+		p.Target = ""
+	} else {
+		p.Target = p.Internal.Build.PkgObj
+		if cfg.BuildBuildmode == "shared" && p.Internal.Build.PkgTargetRoot != "" {
+			// TODO(matloob): This shouldn't be necessary, but the cmd/cgo/internal/testshared
+			// test fails without Target set for this condition. Figure out why and
+			// fix it.
+			p.Target = filepath.Join(p.Internal.Build.PkgTargetRoot, p.ImportPath+".a")
+		}
+		if cfg.BuildLinkshared && p.Internal.Build.PkgTargetRoot != "" {
+			// TODO(bcmills): The reliance on PkgTargetRoot implies that -linkshared does
+			// not work for any package that lacks a PkgTargetRoot — such as a non-main
+			// package in module mode. We should probably fix that.
+			targetPrefix := filepath.Join(p.Internal.Build.PkgTargetRoot, p.ImportPath)
+			p.Target = targetPrefix + ".a"
+			shlibnamefile := targetPrefix + ".shlibname"
+			shlib, err := os.ReadFile(shlibnamefile)
+			if err != nil && !os.IsNotExist(err) {
+				base.Fatalf("reading shlibname: %v", err)
+			}
+			if err == nil {
+				libname := strings.TrimSpace(string(shlib))
+				if cfg.BuildContext.Compiler == "gccgo" {
+					p.Shlib = filepath.Join(p.Internal.Build.PkgTargetRoot, "shlibs", libname)
+				} else {
+					p.Shlib = filepath.Join(p.Internal.Build.PkgTargetRoot, libname)
+				}
+			}
+		}
+	}
+
+	// Build augmented import list to add implicit dependencies.
+	// Be careful not to add imports twice, just to avoid confusion.
+	importPaths := p.Imports
+	addImport := func(path string, forCompiler bool) {
+		for _, p := range importPaths {
+			if path == p {
+				return
+			}
+		}
+		importPaths = append(importPaths, path)
+		if forCompiler {
+			p.Internal.CompiledImports = append(p.Internal.CompiledImports, path)
+		}
+	}
+
+	if !opts.IgnoreImports {
+		// Cgo translation adds imports of "unsafe", "runtime/cgo" and "syscall",
+		// except for certain packages, to avoid circular dependencies.
+		if p.UsesCgo() {
+			addImport("unsafe", true)
+		}
+		if p.UsesCgo() && (!p.Standard || !cgoExclude[p.ImportPath]) && cfg.BuildContext.Compiler != "gccgo" {
+			addImport("runtime/cgo", true)
+		}
+		if p.UsesCgo() && (!p.Standard || !cgoSyscallExclude[p.ImportPath]) {
+			addImport("syscall", true)
+		}
+
+		// SWIG adds imports of some standard packages.
+		if p.UsesSwig() {
+			addImport("unsafe", true)
+			if cfg.BuildContext.Compiler != "gccgo" {
+				addImport("runtime/cgo", true)
+			}
+			addImport("syscall", true)
+			addImport("sync", true)
+
+			// TODO: The .swig and .swigcxx files can use
+			// %go_import directives to import other packages.
+		}
+
+		// The linker loads implicit dependencies.
+		if p.Name == "main" && !p.Internal.ForceLibrary {
+			for _, dep := range LinkerDeps(p) {
+				addImport(dep, false)
+			}
+		}
+	}
+
+	// Check for case-insensitive collisions of import paths.
+	fold := str.ToFold(p.ImportPath)
+	if other := foldPath[fold]; other == "" {
+		foldPath[fold] = p.ImportPath
+	} else if other != p.ImportPath {
+		setError(ImportErrorf(p.ImportPath, "case-insensitive import collision: %q and %q", p.ImportPath, other))
+		return
+	}
+
+	if !SafeArg(p.ImportPath) {
+		setError(ImportErrorf(p.ImportPath, "invalid import path %q", p.ImportPath))
+		return
+	}
+
+	// Errors after this point are caused by this package, not the importing
+	// package. Pushing the path here prevents us from reporting the error
+	// with the position of the import declaration.
+	stk.Push(path)
+	defer stk.Pop()
+
+	pkgPath := p.ImportPath
+	if p.Internal.CmdlineFiles {
+		pkgPath = "command-line-arguments"
+	}
+	if cfg.ModulesEnabled {
+		p.Module = modload.PackageModuleInfo(ctx, pkgPath)
+	}
+	p.DefaultGODEBUG = defaultGODEBUG(p, nil, nil, nil)
+
+	if !opts.SuppressEmbedFiles {
+		p.EmbedFiles, p.Internal.Embed, err = resolveEmbed(p.Dir, p.EmbedPatterns)
+		if err != nil {
+			p.Incomplete = true
+			setError(err)
+			embedErr := err.(*EmbedError)
+			p.Error.setPos(p.Internal.Build.EmbedPatternPos[embedErr.Pattern])
+		}
+	}
+
+	// Check for case-insensitive collision of input files.
+	// To avoid problems on case-insensitive files, we reject any package
+	// where two different input files have equal names under a case-insensitive
+	// comparison.
+	inputs := p.AllFiles()
+	f1, f2 := str.FoldDup(inputs)
+	if f1 != "" {
+		setError(fmt.Errorf("case-insensitive file name collision: %q and %q", f1, f2))
+		return
+	}
+
+	// If first letter of input file is ASCII, it must be alphanumeric.
+	// This avoids files turning into flags when invoking commands,
+	// and other problems we haven't thought of yet.
+	// Also, _cgo_ files must be generated by us, not supplied.
+	// They are allowed to have //go:cgo_ldflag directives.
+	// The directory scan ignores files beginning with _,
+	// so we shouldn't see any _cgo_ files anyway, but just be safe.
+	for _, file := range inputs {
+		if !SafeArg(file) || strings.HasPrefix(file, "_cgo_") {
+			setError(fmt.Errorf("invalid input file name %q", file))
+			return
+		}
+	}
+	if name := pathpkg.Base(p.ImportPath); !SafeArg(name) {
+		setError(fmt.Errorf("invalid input directory name %q", name))
+		return
+	}
+	if strings.ContainsAny(p.Dir, "\r\n") {
+		setError(fmt.Errorf("invalid package directory %q", p.Dir))
+		return
+	}
+
+	// Build list of imported packages and full dependency list.
+	imports := make([]*Package, 0, len(p.Imports))
+	for i, path := range importPaths {
+		if path == "C" {
+			continue
+		}
+		p1, err := LoadImport(ctx, opts, path, p.Dir, p, stk, p.Internal.Build.ImportPos[path], ResolveImport)
+		if err != nil && p.Error == nil {
+			p.Error = err
+			p.Incomplete = true
+		}
+
+		path = p1.ImportPath
+		importPaths[i] = path
+		if i < len(p.Imports) {
+			p.Imports[i] = path
+		}
+
+		imports = append(imports, p1)
+		if p1.Incomplete {
+			p.Incomplete = true
+		}
+	}
+	p.Internal.Imports = imports
+	if p.Error == nil && p.Name == "main" && !p.Internal.ForceLibrary && !p.Incomplete && !opts.SuppressBuildInfo {
+		// TODO(bcmills): loading VCS metadata can be fairly slow.
+		// Consider starting this as a background goroutine and retrieving the result
+		// asynchronously when we're actually ready to build the package, or when we
+		// actually need to evaluate whether the package's metadata is stale.
+		p.setBuildInfo(ctx, opts.AutoVCS)
+	}
+
+	// If cgo is not enabled, ignore cgo supporting sources
+	// just as we ignore go files containing import "C".
+	if !cfg.BuildContext.CgoEnabled {
+		p.CFiles = nil
+		p.CXXFiles = nil
+		p.MFiles = nil
+		p.SwigFiles = nil
+		p.SwigCXXFiles = nil
+		// Note that SFiles are okay (they go to the Go assembler)
+		// and HFiles are okay (they might be used by the SFiles).
+		// Also Sysofiles are okay (they might not contain object
+		// code; see issue #16050).
+	}
+
+	// The gc toolchain only permits C source files with cgo or SWIG.
+	if len(p.CFiles) > 0 && !p.UsesCgo() && !p.UsesSwig() && cfg.BuildContext.Compiler == "gc" {
+		setError(fmt.Errorf("C source files not allowed when not using cgo or SWIG: %s", strings.Join(p.CFiles, " ")))
+		return
+	}
+
+	// C++, Objective-C, and Fortran source files are permitted only with cgo or SWIG,
+	// regardless of toolchain.
+	if len(p.CXXFiles) > 0 && !p.UsesCgo() && !p.UsesSwig() {
+		setError(fmt.Errorf("C++ source files not allowed when not using cgo or SWIG: %s", strings.Join(p.CXXFiles, " ")))
+		return
+	}
+	if len(p.MFiles) > 0 && !p.UsesCgo() && !p.UsesSwig() {
+		setError(fmt.Errorf("Objective-C source files not allowed when not using cgo or SWIG: %s", strings.Join(p.MFiles, " ")))
+		return
+	}
+	if len(p.FFiles) > 0 && !p.UsesCgo() && !p.UsesSwig() {
+		setError(fmt.Errorf("Fortran source files not allowed when not using cgo or SWIG: %s", strings.Join(p.FFiles, " ")))
+		return
+	}
+}
+
+// An EmbedError indicates a problem with a go:embed directive.
+type EmbedError struct {
+	Pattern string
+	Err     error
+}
+
+func (e *EmbedError) Error() string {
+	return fmt.Sprintf("pattern %s: %v", e.Pattern, e.Err)
+}
+
+func (e *EmbedError) Unwrap() error {
+	return e.Err
+}
+
+// ResolveEmbed resolves //go:embed patterns and returns only the file list.
+// For use by go mod vendor to find embedded files it should copy into the
+// vendor directory.
+// TODO(#42504): Once go mod vendor uses load.PackagesAndErrors, just
+// call (*Package).ResolveEmbed
+func ResolveEmbed(dir string, patterns []string) ([]string, error) {
+	files, _, err := resolveEmbed(dir, patterns)
+	return files, err
+}
+
+// resolveEmbed resolves //go:embed patterns to precise file lists.
+// It sets files to the list of unique files matched (for go list),
+// and it sets pmap to the more precise mapping from
+// patterns to files.
+func resolveEmbed(pkgdir string, patterns []string) (files []string, pmap map[string][]string, err error) {
+	var pattern string
+	defer func() {
+		if err != nil {
+			err = &EmbedError{
+				Pattern: pattern,
+				Err:     err,
+			}
+		}
+	}()
+
+	// TODO(rsc): All these messages need position information for better error reports.
+	pmap = make(map[string][]string)
+	have := make(map[string]int)
+	dirOK := make(map[string]bool)
+	pid := 0 // pattern ID, to allow reuse of have map
+	for _, pattern = range patterns {
+		pid++
+
+		glob := pattern
+		all := strings.HasPrefix(pattern, "all:")
+		if all {
+			glob = pattern[len("all:"):]
+		}
+		// Check pattern is valid for //go:embed.
+		if _, err := pathpkg.Match(glob, ""); err != nil || !validEmbedPattern(glob) {
+			return nil, nil, fmt.Errorf("invalid pattern syntax")
+		}
+
+		// Glob to find matches.
+		match, err := fsys.Glob(str.QuoteGlob(str.WithFilePathSeparator(pkgdir)) + filepath.FromSlash(glob))
+		if err != nil {
+			return nil, nil, err
+		}
+
+		// Filter list of matches down to the ones that will still exist when
+		// the directory is packaged up as a module. (If p.Dir is in the module cache,
+		// only those files exist already, but if p.Dir is in the current module,
+		// then there may be other things lying around, like symbolic links or .git directories.)
+		var list []string
+		for _, file := range match {
+			// relative path to p.Dir which begins without prefix slash
+			rel := filepath.ToSlash(str.TrimFilePathPrefix(file, pkgdir))
+
+			what := "file"
+			info, err := fsys.Lstat(file)
+			if err != nil {
+				return nil, nil, err
+			}
+			if info.IsDir() {
+				what = "directory"
+			}
+
+			// Check that directories along path do not begin a new module
+			// (do not contain a go.mod).
+			for dir := file; len(dir) > len(pkgdir)+1 && !dirOK[dir]; dir = filepath.Dir(dir) {
+				if _, err := fsys.Stat(filepath.Join(dir, "go.mod")); err == nil {
+					return nil, nil, fmt.Errorf("cannot embed %s %s: in different module", what, rel)
+				}
+				if dir != file {
+					if info, err := fsys.Lstat(dir); err == nil && !info.IsDir() {
+						return nil, nil, fmt.Errorf("cannot embed %s %s: in non-directory %s", what, rel, dir[len(pkgdir)+1:])
+					}
+				}
+				dirOK[dir] = true
+				if elem := filepath.Base(dir); isBadEmbedName(elem) {
+					if dir == file {
+						return nil, nil, fmt.Errorf("cannot embed %s %s: invalid name %s", what, rel, elem)
+					} else {
+						return nil, nil, fmt.Errorf("cannot embed %s %s: in invalid directory %s", what, rel, elem)
+					}
+				}
+			}
+
+			switch {
+			default:
+				return nil, nil, fmt.Errorf("cannot embed irregular file %s", rel)
+
+			case info.Mode().IsRegular():
+				if have[rel] != pid {
+					have[rel] = pid
+					list = append(list, rel)
+				}
+
+			case info.IsDir():
+				// Gather all files in the named directory, stopping at module boundaries
+				// and ignoring files that wouldn't be packaged into a module.
+				count := 0
+				err := fsys.Walk(file, func(path string, info os.FileInfo, err error) error {
+					if err != nil {
+						return err
+					}
+					rel := filepath.ToSlash(str.TrimFilePathPrefix(path, pkgdir))
+					name := info.Name()
+					if path != file && (isBadEmbedName(name) || ((name[0] == '.' || name[0] == '_') && !all)) {
+						// Ignore bad names, assuming they won't go into modules.
+						// Also avoid hidden files that user may not know about.
+						// See golang.org/issue/42328.
+						if info.IsDir() {
+							return fs.SkipDir
+						}
+						return nil
+					}
+					if info.IsDir() {
+						if _, err := fsys.Stat(filepath.Join(path, "go.mod")); err == nil {
+							return filepath.SkipDir
+						}
+						return nil
+					}
+					if !info.Mode().IsRegular() {
+						return nil
+					}
+					count++
+					if have[rel] != pid {
+						have[rel] = pid
+						list = append(list, rel)
+					}
+					return nil
+				})
+				if err != nil {
+					return nil, nil, err
+				}
+				if count == 0 {
+					return nil, nil, fmt.Errorf("cannot embed directory %s: contains no embeddable files", rel)
+				}
+			}
+		}
+
+		if len(list) == 0 {
+			return nil, nil, fmt.Errorf("no matching files found")
+		}
+		sort.Strings(list)
+		pmap[pattern] = list
+	}
+
+	for file := range have {
+		files = append(files, file)
+	}
+	sort.Strings(files)
+	return files, pmap, nil
+}
+
+func validEmbedPattern(pattern string) bool {
+	return pattern != "." && fs.ValidPath(pattern)
+}
+
+// isBadEmbedName reports whether name is the base name of a file that
+// can't or won't be included in modules and therefore shouldn't be treated
+// as existing for embedding.
+func isBadEmbedName(name string) bool {
+	if err := module.CheckFilePath(name); err != nil {
+		return true
+	}
+	switch name {
+	// Empty string should be impossible but make it bad.
+	case "":
+		return true
+	// Version control directories won't be present in module.
+	case ".bzr", ".hg", ".git", ".svn":
+		return true
+	}
+	return false
+}
+
+// vcsStatusCache maps repository directories (string)
+// to their VCS information.
+var vcsStatusCache par.ErrCache[string, vcs.Status]
+
+func appendBuildSetting(info *debug.BuildInfo, key, value string) {
+	value = strings.ReplaceAll(value, "\n", " ") // make value safe
+	info.Settings = append(info.Settings, debug.BuildSetting{Key: key, Value: value})
+}
+
+// setBuildInfo gathers build information and sets it into
+// p.Internal.BuildInfo, which will later be formatted as a string and embedded
+// in the binary. setBuildInfo should only be called on a main package with no
+// errors.
+//
+// This information can be retrieved using debug.ReadBuildInfo.
+//
+// Note that the GoVersion field is not set here to avoid encoding it twice.
+// It is stored separately in the binary, mostly for historical reasons.
+func (p *Package) setBuildInfo(ctx context.Context, autoVCS bool) {
+	setPkgErrorf := func(format string, args ...any) {
+		if p.Error == nil {
+			p.Error = &PackageError{Err: fmt.Errorf(format, args...)}
+			p.Incomplete = true
+		}
+	}
+
+	var debugModFromModinfo func(*modinfo.ModulePublic) *debug.Module
+	debugModFromModinfo = func(mi *modinfo.ModulePublic) *debug.Module {
+		version := mi.Version
+		if version == "" {
+			version = "(devel)"
+		}
+		dm := &debug.Module{
+			Path:    mi.Path,
+			Version: version,
+		}
+		if mi.Replace != nil {
+			dm.Replace = debugModFromModinfo(mi.Replace)
+		} else if mi.Version != "" {
+			dm.Sum = modfetch.Sum(ctx, module.Version{Path: mi.Path, Version: mi.Version})
+		}
+		return dm
+	}
+
+	var main debug.Module
+	if p.Module != nil {
+		main = *debugModFromModinfo(p.Module)
+	}
+
+	visited := make(map[*Package]bool)
+	mdeps := make(map[module.Version]*debug.Module)
+	var q []*Package
+	q = append(q, p.Internal.Imports...)
+	for len(q) > 0 {
+		p1 := q[0]
+		q = q[1:]
+		if visited[p1] {
+			continue
+		}
+		visited[p1] = true
+		if p1.Module != nil {
+			m := module.Version{Path: p1.Module.Path, Version: p1.Module.Version}
+			if p1.Module.Path != main.Path && mdeps[m] == nil {
+				mdeps[m] = debugModFromModinfo(p1.Module)
+			}
+		}
+		q = append(q, p1.Internal.Imports...)
+	}
+	sortedMods := make([]module.Version, 0, len(mdeps))
+	for mod := range mdeps {
+		sortedMods = append(sortedMods, mod)
+	}
+	gover.ModSort(sortedMods)
+	deps := make([]*debug.Module, len(sortedMods))
+	for i, mod := range sortedMods {
+		deps[i] = mdeps[mod]
+	}
+
+	pkgPath := p.ImportPath
+	if p.Internal.CmdlineFiles {
+		pkgPath = "command-line-arguments"
+	}
+	info := &debug.BuildInfo{
+		Path: pkgPath,
+		Main: main,
+		Deps: deps,
+	}
+	appendSetting := func(key, value string) {
+		appendBuildSetting(info, key, value)
+	}
+
+	// Add command-line flags relevant to the build.
+	// This is informational, not an exhaustive list.
+	// Please keep the list sorted.
+	if cfg.BuildASan {
+		appendSetting("-asan", "true")
+	}
+	if BuildAsmflags.present {
+		appendSetting("-asmflags", BuildAsmflags.String())
+	}
+	buildmode := cfg.BuildBuildmode
+	if buildmode == "default" {
+		if p.Name == "main" {
+			buildmode = "exe"
+		} else {
+			buildmode = "archive"
+		}
+	}
+	appendSetting("-buildmode", buildmode)
+	appendSetting("-compiler", cfg.BuildContext.Compiler)
+	if gccgoflags := BuildGccgoflags.String(); gccgoflags != "" && cfg.BuildContext.Compiler == "gccgo" {
+		appendSetting("-gccgoflags", gccgoflags)
+	}
+	if gcflags := BuildGcflags.String(); gcflags != "" && cfg.BuildContext.Compiler == "gc" {
+		appendSetting("-gcflags", gcflags)
+	}
+	if ldflags := BuildLdflags.String(); ldflags != "" {
+		// https://go.dev/issue/52372: only include ldflags if -trimpath is not set,
+		// since it can include system paths through various linker flags (notably
+		// -extar, -extld, and -extldflags).
+		//
+		// TODO: since we control cmd/link, in theory we can parse ldflags to
+		// determine whether they may refer to system paths. If we do that, we can
+		// redact only those paths from the recorded -ldflags setting and still
+		// record the system-independent parts of the flags.
+		if !cfg.BuildTrimpath {
+			appendSetting("-ldflags", ldflags)
+		}
+	}
+	// N.B. -pgo added later by setPGOProfilePath.
+	if cfg.BuildMSan {
+		appendSetting("-msan", "true")
+	}
+	if cfg.BuildRace {
+		appendSetting("-race", "true")
+	}
+	if tags := cfg.BuildContext.BuildTags; len(tags) > 0 {
+		appendSetting("-tags", strings.Join(tags, ","))
+	}
+	if cfg.BuildTrimpath {
+		appendSetting("-trimpath", "true")
+	}
+	if p.DefaultGODEBUG != "" {
+		appendSetting("DefaultGODEBUG", p.DefaultGODEBUG)
+	}
+	cgo := "0"
+	if cfg.BuildContext.CgoEnabled {
+		cgo = "1"
+	}
+	appendSetting("CGO_ENABLED", cgo)
+	// https://go.dev/issue/52372: only include CGO flags if -trimpath is not set.
+	// (If -trimpath is set, it is possible that these flags include system paths.)
+	// If cgo is involved, reproducibility is already pretty well ruined anyway,
+	// given that we aren't stamping header or library versions.
+	//
+	// TODO(bcmills): perhaps we could at least parse the flags and stamp the
+	// subset of flags that are known not to be paths?
+	if cfg.BuildContext.CgoEnabled && !cfg.BuildTrimpath {
+		for _, name := range []string{"CGO_CFLAGS", "CGO_CPPFLAGS", "CGO_CXXFLAGS", "CGO_LDFLAGS"} {
+			appendSetting(name, cfg.Getenv(name))
+		}
+	}
+	appendSetting("GOARCH", cfg.BuildContext.GOARCH)
+	if cfg.RawGOEXPERIMENT != "" {
+		appendSetting("GOEXPERIMENT", cfg.RawGOEXPERIMENT)
+	}
+	appendSetting("GOOS", cfg.BuildContext.GOOS)
+	if key, val := cfg.GetArchEnv(); key != "" && val != "" {
+		appendSetting(key, val)
+	}
+
+	// Add VCS status if all conditions are true:
+	//
+	// - -buildvcs is enabled.
+	// - p is a non-test contained within a main module (there may be multiple
+	//   main modules in a workspace, but local replacements don't count).
+	// - Both the current directory and p's module's root directory are contained
+	//   in the same local repository.
+	// - We know the VCS commands needed to get the status.
+	setVCSError := func(err error) {
+		setPkgErrorf("error obtaining VCS status: %v\n\tUse -buildvcs=false to disable VCS stamping.", err)
+	}
+
+	var repoDir string
+	var vcsCmd *vcs.Cmd
+	var err error
+	const allowNesting = true
+
+	wantVCS := false
+	switch cfg.BuildBuildvcs {
+	case "true":
+		wantVCS = true // Include VCS metadata even for tests if requested explicitly; see https://go.dev/issue/52648.
+	case "auto":
+		wantVCS = autoVCS && !p.IsTestOnly()
+	case "false":
+	default:
+		panic(fmt.Sprintf("unexpected value for cfg.BuildBuildvcs: %q", cfg.BuildBuildvcs))
+	}
+
+	if wantVCS && p.Module != nil && p.Module.Version == "" && !p.Standard {
+		if p.Module.Path == "bootstrap" && cfg.GOROOT == os.Getenv("GOROOT_BOOTSTRAP") {
+			// During bootstrapping, the bootstrap toolchain is built in module
+			// "bootstrap" (instead of "std"), with GOROOT set to GOROOT_BOOTSTRAP
+			// (so the bootstrap toolchain packages don't even appear to be in GOROOT).
+			goto omitVCS
+		}
+		repoDir, vcsCmd, err = vcs.FromDir(base.Cwd(), "", allowNesting)
+		if err != nil && !errors.Is(err, os.ErrNotExist) {
+			setVCSError(err)
+			return
+		}
+		if !str.HasFilePathPrefix(p.Module.Dir, repoDir) &&
+			!str.HasFilePathPrefix(repoDir, p.Module.Dir) {
+			// The module containing the main package does not overlap with the
+			// repository containing the working directory. Don't include VCS info.
+			// If the repo contains the module or vice versa, but they are not
+			// the same directory, it's likely an error (see below).
+			goto omitVCS
+		}
+		if cfg.BuildBuildvcs == "auto" && vcsCmd != nil && vcsCmd.Cmd != "" {
+			if _, err := exec.LookPath(vcsCmd.Cmd); err != nil {
+				// We fould a repository, but the required VCS tool is not present.
+				// "-buildvcs=auto" means that we should silently drop the VCS metadata.
+				goto omitVCS
+			}
+		}
+	}
+	if repoDir != "" && vcsCmd.Status != nil {
+		// Check that the current directory, package, and module are in the same
+		// repository. vcs.FromDir allows nested Git repositories, but nesting
+		// is not allowed for other VCS tools. The current directory may be outside
+		// p.Module.Dir when a workspace is used.
+		pkgRepoDir, _, err := vcs.FromDir(p.Dir, "", allowNesting)
+		if err != nil {
+			setVCSError(err)
+			return
+		}
+		if pkgRepoDir != repoDir {
+			if cfg.BuildBuildvcs != "auto" {
+				setVCSError(fmt.Errorf("main package is in repository %q but current directory is in repository %q", pkgRepoDir, repoDir))
+				return
+			}
+			goto omitVCS
+		}
+		modRepoDir, _, err := vcs.FromDir(p.Module.Dir, "", allowNesting)
+		if err != nil {
+			setVCSError(err)
+			return
+		}
+		if modRepoDir != repoDir {
+			if cfg.BuildBuildvcs != "auto" {
+				setVCSError(fmt.Errorf("main module is in repository %q but current directory is in repository %q", modRepoDir, repoDir))
+				return
+			}
+			goto omitVCS
+		}
+
+		st, err := vcsStatusCache.Do(repoDir, func() (vcs.Status, error) {
+			return vcsCmd.Status(vcsCmd, repoDir)
+		})
+		if err != nil {
+			setVCSError(err)
+			return
+		}
+
+		appendSetting("vcs", vcsCmd.Cmd)
+		if st.Revision != "" {
+			appendSetting("vcs.revision", st.Revision)
+		}
+		if !st.CommitTime.IsZero() {
+			stamp := st.CommitTime.UTC().Format(time.RFC3339Nano)
+			appendSetting("vcs.time", stamp)
+		}
+		appendSetting("vcs.modified", strconv.FormatBool(st.Uncommitted))
+	}
+omitVCS:
+
+	p.Internal.BuildInfo = info
+}
+
+// SafeArg reports whether arg is a "safe" command-line argument,
+// meaning that when it appears in a command-line, it probably
+// doesn't have some special meaning other than its own name.
+// Obviously args beginning with - are not safe (they look like flags).
+// Less obviously, args beginning with @ are not safe (they look like
+// GNU binutils flagfile specifiers, sometimes called "response files").
+// To be conservative, we reject almost any arg beginning with non-alphanumeric ASCII.
+// We accept leading . _ and / as likely in file system paths.
+// There is a copy of this function in cmd/compile/internal/gc/noder.go.
+func SafeArg(name string) bool {
+	if name == "" {
+		return false
+	}
+	c := name[0]
+	return '0' <= c && c <= '9' || 'A' <= c && c <= 'Z' || 'a' <= c && c <= 'z' || c == '.' || c == '_' || c == '/' || c >= utf8.RuneSelf
+}
+
+// LinkerDeps returns the list of linker-induced dependencies for main package p.
+func LinkerDeps(p *Package) []string {
+	// Everything links runtime.
+	deps := []string{"runtime"}
+
+	// External linking mode forces an import of runtime/cgo.
+	if externalLinkingForced(p) && cfg.BuildContext.Compiler != "gccgo" {
+		deps = append(deps, "runtime/cgo")
+	}
+	// On ARM with GOARM=5, it forces an import of math, for soft floating point.
+	if cfg.Goarch == "arm" {
+		deps = append(deps, "math")
+	}
+	// Using the race detector forces an import of runtime/race.
+	if cfg.BuildRace {
+		deps = append(deps, "runtime/race")
+	}
+	// Using memory sanitizer forces an import of runtime/msan.
+	if cfg.BuildMSan {
+		deps = append(deps, "runtime/msan")
+	}
+	// Using address sanitizer forces an import of runtime/asan.
+	if cfg.BuildASan {
+		deps = append(deps, "runtime/asan")
+	}
+	// Building for coverage forces an import of runtime/coverage.
+	if cfg.BuildCover && cfg.Experiment.CoverageRedesign {
+		deps = append(deps, "runtime/coverage")
+	}
+
+	return deps
+}
+
+// externalLinkingForced reports whether external linking is being
+// forced even for programs that do not use cgo.
+func externalLinkingForced(p *Package) bool {
+	if !cfg.BuildContext.CgoEnabled {
+		return false
+	}
+
+	// Some targets must use external linking even inside GOROOT.
+	if platform.MustLinkExternal(cfg.BuildContext.GOOS, cfg.BuildContext.GOARCH, false) {
+		return true
+	}
+
+	// Some build modes always require external linking.
+	switch cfg.BuildBuildmode {
+	case "c-shared", "plugin":
+		return true
+	}
+
+	// Using -linkshared always requires external linking.
+	if cfg.BuildLinkshared {
+		return true
+	}
+
+	// Decide whether we are building a PIE,
+	// bearing in mind that some systems default to PIE.
+	isPIE := false
+	if cfg.BuildBuildmode == "pie" {
+		isPIE = true
+	} else if cfg.BuildBuildmode == "default" && platform.DefaultPIE(cfg.BuildContext.GOOS, cfg.BuildContext.GOARCH, cfg.BuildRace) {
+		isPIE = true
+	}
+	// If we are building a PIE, and we are on a system
+	// that does not support PIE with internal linking mode,
+	// then we must use external linking.
+	if isPIE && !platform.InternalLinkPIESupported(cfg.BuildContext.GOOS, cfg.BuildContext.GOARCH) {
+		return true
+	}
+
+	// Using -ldflags=-linkmode=external forces external linking.
+	// If there are multiple -linkmode options, the last one wins.
+	linkmodeExternal := false
+	if p != nil {
+		ldflags := BuildLdflags.For(p)
+		for i := len(ldflags) - 1; i >= 0; i-- {
+			a := ldflags[i]
+			if a == "-linkmode=external" ||
+				a == "-linkmode" && i+1 < len(ldflags) && ldflags[i+1] == "external" {
+				linkmodeExternal = true
+				break
+			} else if a == "-linkmode=internal" ||
+				a == "-linkmode" && i+1 < len(ldflags) && ldflags[i+1] == "internal" {
+				break
+			}
+		}
+	}
+	return linkmodeExternal
+}
+
+// mkAbs rewrites list, which must be paths relative to p.Dir,
+// into a sorted list of absolute paths. It edits list in place but for
+// convenience also returns list back to its caller.
+func (p *Package) mkAbs(list []string) []string {
+	for i, f := range list {
+		list[i] = filepath.Join(p.Dir, f)
+	}
+	sort.Strings(list)
+	return list
+}
+
+// InternalGoFiles returns the list of Go files being built for the package,
+// using absolute paths.
+func (p *Package) InternalGoFiles() []string {
+	return p.mkAbs(str.StringList(p.GoFiles, p.CgoFiles, p.TestGoFiles))
+}
+
+// InternalXGoFiles returns the list of Go files being built for the XTest package,
+// using absolute paths.
+func (p *Package) InternalXGoFiles() []string {
+	return p.mkAbs(p.XTestGoFiles)
+}
+
+// InternalAllGoFiles returns the list of all Go files possibly relevant for the package,
+// using absolute paths. "Possibly relevant" means that files are not excluded
+// due to build tags, but files with names beginning with . or _ are still excluded.
+func (p *Package) InternalAllGoFiles() []string {
+	return p.mkAbs(str.StringList(p.IgnoredGoFiles, p.GoFiles, p.CgoFiles, p.TestGoFiles, p.XTestGoFiles))
+}
+
+// UsesSwig reports whether the package needs to run SWIG.
+func (p *Package) UsesSwig() bool {
+	return len(p.SwigFiles) > 0 || len(p.SwigCXXFiles) > 0
+}
+
+// UsesCgo reports whether the package needs to run cgo
+func (p *Package) UsesCgo() bool {
+	return len(p.CgoFiles) > 0
+}
+
+// PackageList returns the list of packages in the dag rooted at roots
+// as visited in a depth-first post-order traversal.
+func PackageList(roots []*Package) []*Package {
+	seen := map[*Package]bool{}
+	all := []*Package{}
+	var walk func(*Package)
+	walk = func(p *Package) {
+		if seen[p] {
+			return
+		}
+		seen[p] = true
+		for _, p1 := range p.Internal.Imports {
+			walk(p1)
+		}
+		all = append(all, p)
+	}
+	for _, root := range roots {
+		walk(root)
+	}
+	return all
+}
+
+// TestPackageList returns the list of packages in the dag rooted at roots
+// as visited in a depth-first post-order traversal, including the test
+// imports of the roots. This ignores errors in test packages.
+func TestPackageList(ctx context.Context, opts PackageOpts, roots []*Package) []*Package {
+	seen := map[*Package]bool{}
+	all := []*Package{}
+	var walk func(*Package)
+	walk = func(p *Package) {
+		if seen[p] {
+			return
+		}
+		seen[p] = true
+		for _, p1 := range p.Internal.Imports {
+			walk(p1)
+		}
+		all = append(all, p)
+	}
+	walkTest := func(root *Package, path string) {
+		var stk ImportStack
+		p1, err := LoadImport(ctx, opts, path, root.Dir, root, &stk, root.Internal.Build.TestImportPos[path], ResolveImport)
+		if err != nil && root.Error == nil {
+			// Assign error importing the package to the importer.
+			root.Error = err
+			root.Incomplete = true
+		}
+		if p1.Error == nil {
+			walk(p1)
+		}
+	}
+	for _, root := range roots {
+		walk(root)
+		for _, path := range root.TestImports {
+			walkTest(root, path)
+		}
+		for _, path := range root.XTestImports {
+			walkTest(root, path)
+		}
+	}
+	return all
+}
+
+// LoadImportWithFlags loads the package with the given import path and
+// sets tool flags on that package. This function is useful loading implicit
+// dependencies (like sync/atomic for coverage).
+// TODO(jayconrod): delete this function and set flags automatically
+// in LoadImport instead.
+func LoadImportWithFlags(path, srcDir string, parent *Package, stk *ImportStack, importPos []token.Position, mode int) (*Package, *PackageError) {
+	p, err := LoadImport(context.TODO(), PackageOpts{}, path, srcDir, parent, stk, importPos, mode)
+	setToolFlags(p)
+	return p, err
+}
+
+// LoadPackageWithFlags is the same as LoadImportWithFlags but without a parent.
+// It's then guaranteed to not return an error
+func LoadPackageWithFlags(path, srcDir string, stk *ImportStack, importPos []token.Position, mode int) *Package {
+	p := LoadPackage(context.TODO(), PackageOpts{}, path, srcDir, stk, importPos, mode)
+	setToolFlags(p)
+	return p
+}
+
+// PackageOpts control the behavior of PackagesAndErrors and other package
+// loading functions.
+type PackageOpts struct {
+	// IgnoreImports controls whether we ignore explicit and implicit imports
+	// when loading packages.  Implicit imports are added when supporting Cgo
+	// or SWIG and when linking main packages.
+	IgnoreImports bool
+
+	// ModResolveTests indicates whether calls to the module loader should also
+	// resolve test dependencies of the requested packages.
+	//
+	// If ModResolveTests is true, then the module loader needs to resolve test
+	// dependencies at the same time as packages; otherwise, the test dependencies
+	// of those packages could be missing, and resolving those missing dependencies
+	// could change the selected versions of modules that provide other packages.
+	ModResolveTests bool
+
+	// MainOnly is true if the caller only wants to load main packages.
+	// For a literal argument matching a non-main package, a stub may be returned
+	// with an error. For a non-literal argument (with "..."), non-main packages
+	// are not be matched, and their dependencies may not be loaded. A warning
+	// may be printed for non-literal arguments that match no main packages.
+	MainOnly bool
+
+	// AutoVCS controls whether we also load version-control metadata for main packages
+	// when -buildvcs=auto (the default).
+	AutoVCS bool
+
+	// SuppressBuildInfo is true if the caller does not need p.Stale, p.StaleReason, or p.Internal.BuildInfo
+	// to be populated on the package.
+	SuppressBuildInfo bool
+
+	// SuppressEmbedFiles is true if the caller does not need any embed files to be populated on the
+	// package.
+	SuppressEmbedFiles bool
+}
+
+// PackagesAndErrors returns the packages named by the command line arguments
+// 'patterns'. If a named package cannot be loaded, PackagesAndErrors returns
+// a *Package with the Error field describing the failure. If errors are found
+// loading imported packages, the DepsErrors field is set. The Incomplete field
+// may be set as well.
+//
+// To obtain a flat list of packages, use PackageList.
+// To report errors loading packages, use ReportPackageErrors.
+func PackagesAndErrors(ctx context.Context, opts PackageOpts, patterns []string) []*Package {
+	ctx, span := trace.StartSpan(ctx, "load.PackagesAndErrors")
+	defer span.Done()
+
+	for _, p := range patterns {
+		// Listing is only supported with all patterns referring to either:
+		// - Files that are part of the same directory.
+		// - Explicit package paths or patterns.
+		if strings.HasSuffix(p, ".go") {
+			// We need to test whether the path is an actual Go file and not a
+			// package path or pattern ending in '.go' (see golang.org/issue/34653).
+			if fi, err := fsys.Stat(p); err == nil && !fi.IsDir() {
+				pkgs := []*Package{GoFilesPackage(ctx, opts, patterns)}
+				setPGOProfilePath(pkgs)
+				return pkgs
+			}
+		}
+	}
+
+	var matches []*search.Match
+	if modload.Init(); cfg.ModulesEnabled {
+		modOpts := modload.PackageOpts{
+			ResolveMissingImports: true,
+			LoadTests:             opts.ModResolveTests,
+			SilencePackageErrors:  true,
+		}
+		matches, _ = modload.LoadPackages(ctx, modOpts, patterns...)
+	} else {
+		noModRoots := []string{}
+		matches = search.ImportPaths(patterns, noModRoots)
+	}
+
+	var (
+		pkgs    []*Package
+		stk     ImportStack
+		seenPkg = make(map[*Package]bool)
+	)
+
+	pre := newPreload()
+	defer pre.flush()
+	pre.preloadMatches(ctx, opts, matches)
+
+	for _, m := range matches {
+		for _, pkg := range m.Pkgs {
+			if pkg == "" {
+				panic(fmt.Sprintf("ImportPaths returned empty package for pattern %s", m.Pattern()))
+			}
+			mode := cmdlinePkg
+			if m.IsLiteral() {
+				// Note: do not set = m.IsLiteral unconditionally
+				// because maybe we'll see p matching both
+				// a literal and also a non-literal pattern.
+				mode |= cmdlinePkgLiteral
+			}
+			p, perr := loadImport(ctx, opts, pre, pkg, base.Cwd(), nil, &stk, nil, mode)
+			if perr != nil {
+				base.Fatalf("internal error: loadImport of %q with nil parent returned an error", pkg)
+			}
+			p.Match = append(p.Match, m.Pattern())
+			if seenPkg[p] {
+				continue
+			}
+			seenPkg[p] = true
+			pkgs = append(pkgs, p)
+		}
+
+		if len(m.Errs) > 0 {
+			// In addition to any packages that were actually resolved from the
+			// pattern, there was some error in resolving the pattern itself.
+			// Report it as a synthetic package.
+			p := new(Package)
+			p.ImportPath = m.Pattern()
+			// Pass an empty ImportStack and nil importPos: the error arose from a pattern, not an import.
+			var stk ImportStack
+			var importPos []token.Position
+			p.setLoadPackageDataError(m.Errs[0], m.Pattern(), &stk, importPos)
+			p.Incomplete = true
+			p.Match = append(p.Match, m.Pattern())
+			p.Internal.CmdlinePkg = true
+			if m.IsLiteral() {
+				p.Internal.CmdlinePkgLiteral = true
+			}
+			pkgs = append(pkgs, p)
+		}
+	}
+
+	if opts.MainOnly {
+		pkgs = mainPackagesOnly(pkgs, matches)
+	}
+
+	// Now that CmdlinePkg is set correctly,
+	// compute the effective flags for all loaded packages
+	// (not just the ones matching the patterns but also
+	// their dependencies).
+	setToolFlags(pkgs...)
+
+	setPGOProfilePath(pkgs)
+
+	return pkgs
+}
+
+// setPGOProfilePath sets the PGO profile path for pkgs.
+// In -pgo=auto mode, it finds the default PGO profile.
+func setPGOProfilePath(pkgs []*Package) {
+	updateBuildInfo := func(p *Package, file string) {
+		// Don't create BuildInfo for packages that didn't already have it.
+		if p.Internal.BuildInfo == nil {
+			return
+		}
+
+		if cfg.BuildTrimpath {
+			appendBuildSetting(p.Internal.BuildInfo, "-pgo", filepath.Base(file))
+		} else {
+			appendBuildSetting(p.Internal.BuildInfo, "-pgo", file)
+		}
+	}
+
+	switch cfg.BuildPGO {
+	case "off":
+		return
+
+	case "auto":
+		// Locate PGO profiles from the main packages, and
+		// attach the profile to the main package and its
+		// dependencies.
+		// If we're building multiple main packages, they may
+		// have different profiles. We may need to split (unshare)
+		// the dependency graph so they can attach different
+		// profiles.
+		for _, p := range pkgs {
+			if p.Name != "main" {
+				continue
+			}
+			pmain := p
+			file := filepath.Join(pmain.Dir, "default.pgo")
+			if _, err := os.Stat(file); err != nil {
+				continue // no profile
+			}
+
+			// Packages already visited. The value should replace
+			// the key, as it may be a forked copy of the original
+			// Package.
+			visited := make(map[*Package]*Package)
+			var split func(p *Package) *Package
+			split = func(p *Package) *Package {
+				if p1 := visited[p]; p1 != nil {
+					return p1
+				}
+
+				if len(pkgs) > 1 && p != pmain {
+					// Make a copy, then attach profile.
+					// No need to copy if there is only one root package (we can
+					// attach profile directly in-place).
+					// Also no need to copy the main package.
+					if p.Internal.PGOProfile != "" {
+						panic("setPGOProfilePath: already have profile")
+					}
+					p1 := new(Package)
+					*p1 = *p
+					// Unalias the Internal.Imports slice, which is we're going to
+					// modify. We don't copy other slices as we don't change them.
+					p1.Internal.Imports = slices.Clone(p.Internal.Imports)
+					p1.Internal.ForMain = pmain.ImportPath
+					visited[p] = p1
+					p = p1
+				} else {
+					visited[p] = p
+				}
+				p.Internal.PGOProfile = file
+				updateBuildInfo(p, file)
+				// Recurse to dependencies.
+				for i, pp := range p.Internal.Imports {
+					p.Internal.Imports[i] = split(pp)
+				}
+				return p
+			}
+
+			// Replace the package and imports with the PGO version.
+			split(pmain)
+		}
+
+	default:
+		// Profile specified from the command line.
+		// Make it absolute path, as the compiler runs on various directories.
+		file, err := filepath.Abs(cfg.BuildPGO)
+		if err != nil {
+			base.Fatalf("fail to get absolute path of PGO file %s: %v", cfg.BuildPGO, err)
+		}
+
+		for _, p := range PackageList(pkgs) {
+			p.Internal.PGOProfile = file
+			updateBuildInfo(p, file)
+		}
+	}
+}
+
+// CheckPackageErrors prints errors encountered loading pkgs and their
+// dependencies, then exits with a non-zero status if any errors were found.
+func CheckPackageErrors(pkgs []*Package) {
+	var anyIncomplete bool
+	for _, pkg := range pkgs {
+		if pkg.Incomplete {
+			anyIncomplete = true
+		}
+	}
+	if anyIncomplete {
+		all := PackageList(pkgs)
+		for _, p := range all {
+			if p.Error != nil {
+				base.Errorf("%v", p.Error)
+			}
+		}
+	}
+	base.ExitIfErrors()
+
+	// Check for duplicate loads of the same package.
+	// That should be impossible, but if it does happen then
+	// we end up trying to build the same package twice,
+	// usually in parallel overwriting the same files,
+	// which doesn't work very well.
+	seen := map[string]bool{}
+	reported := map[string]bool{}
+	for _, pkg := range PackageList(pkgs) {
+		// -pgo=auto with multiple main packages can cause a package being
+		// built multiple times (with different profiles).
+		// We check that package import path + profile path is unique.
+		key := pkg.ImportPath
+		if pkg.Internal.PGOProfile != "" {
+			key += " pgo:" + pkg.Internal.PGOProfile
+		}
+		if seen[key] && !reported[key] {
+			reported[key] = true
+			base.Errorf("internal error: duplicate loads of %s", pkg.ImportPath)
+		}
+		seen[key] = true
+	}
+	base.ExitIfErrors()
+}
+
+// mainPackagesOnly filters out non-main packages matched only by arguments
+// containing "..." and returns the remaining main packages.
+//
+// Packages with missing, invalid, or ambiguous names may be treated as
+// possibly-main packages.
+//
+// mainPackagesOnly sets a non-main package's Error field and returns it if it
+// is named by a literal argument.
+//
+// mainPackagesOnly prints warnings for non-literal arguments that only match
+// non-main packages.
+func mainPackagesOnly(pkgs []*Package, matches []*search.Match) []*Package {
+	treatAsMain := map[string]bool{}
+	for _, m := range matches {
+		if m.IsLiteral() {
+			for _, path := range m.Pkgs {
+				treatAsMain[path] = true
+			}
+		}
+	}
+
+	var mains []*Package
+	for _, pkg := range pkgs {
+		if pkg.Name == "main" || (pkg.Name == "" && pkg.Error != nil) {
+			treatAsMain[pkg.ImportPath] = true
+			mains = append(mains, pkg)
+			continue
+		}
+
+		if len(pkg.InvalidGoFiles) > 0 { // TODO(#45999): && pkg.Name == "", but currently go/build sets pkg.Name arbitrarily if it is ambiguous.
+			// The package has (or may have) conflicting names, and we can't easily
+			// tell whether one of them is "main". So assume that it could be, and
+			// report an error for the package.
+			treatAsMain[pkg.ImportPath] = true
+		}
+		if treatAsMain[pkg.ImportPath] {
+			if pkg.Error == nil {
+				pkg.Error = &PackageError{Err: &mainPackageError{importPath: pkg.ImportPath}}
+				pkg.Incomplete = true
+			}
+			mains = append(mains, pkg)
+		}
+	}
+
+	for _, m := range matches {
+		if m.IsLiteral() || len(m.Pkgs) == 0 {
+			continue
+		}
+		foundMain := false
+		for _, path := range m.Pkgs {
+			if treatAsMain[path] {
+				foundMain = true
+				break
+			}
+		}
+		if !foundMain {
+			fmt.Fprintf(os.Stderr, "go: warning: %q matched only non-main packages\n", m.Pattern())
+		}
+	}
+
+	return mains
+}
+
+type mainPackageError struct {
+	importPath string
+}
+
+func (e *mainPackageError) Error() string {
+	return fmt.Sprintf("package %s is not a main package", e.importPath)
+}
+
+func (e *mainPackageError) ImportPath() string {
+	return e.importPath
+}
+
+func setToolFlags(pkgs ...*Package) {
+	for _, p := range PackageList(pkgs) {
+		p.Internal.Asmflags = BuildAsmflags.For(p)
+		p.Internal.Gcflags = BuildGcflags.For(p)
+		p.Internal.Ldflags = BuildLdflags.For(p)
+		p.Internal.Gccgoflags = BuildGccgoflags.For(p)
+	}
+}
+
+// GoFilesPackage creates a package for building a collection of Go files
+// (typically named on the command line). The target is named p.a for
+// package p or named after the first Go file for package main.
+func GoFilesPackage(ctx context.Context, opts PackageOpts, gofiles []string) *Package {
+	modload.Init()
+
+	for _, f := range gofiles {
+		if !strings.HasSuffix(f, ".go") {
+			pkg := new(Package)
+			pkg.Internal.Local = true
+			pkg.Internal.CmdlineFiles = true
+			pkg.Name = f
+			pkg.Error = &PackageError{
+				Err: fmt.Errorf("named files must be .go files: %s", pkg.Name),
+			}
+			pkg.Incomplete = true
+			return pkg
+		}
+	}
+
+	var stk ImportStack
+	ctxt := cfg.BuildContext
+	ctxt.UseAllFiles = true
+
+	// Synthesize fake "directory" that only shows the named files,
+	// to make it look like this is a standard package or
+	// command directory. So that local imports resolve
+	// consistently, the files must all be in the same directory.
+	var dirent []fs.FileInfo
+	var dir string
+	for _, file := range gofiles {
+		fi, err := fsys.Stat(file)
+		if err != nil {
+			base.Fatalf("%s", err)
+		}
+		if fi.IsDir() {
+			base.Fatalf("%s is a directory, should be a Go file", file)
+		}
+		dir1 := filepath.Dir(file)
+		if dir == "" {
+			dir = dir1
+		} else if dir != dir1 {
+			base.Fatalf("named files must all be in one directory; have %s and %s", dir, dir1)
+		}
+		dirent = append(dirent, fi)
+	}
+	ctxt.ReadDir = func(string) ([]fs.FileInfo, error) { return dirent, nil }
+
+	if cfg.ModulesEnabled {
+		modload.ImportFromFiles(ctx, gofiles)
+	}
+
+	var err error
+	if dir == "" {
+		dir = base.Cwd()
+	}
+	dir, err = filepath.Abs(dir)
+	if err != nil {
+		base.Fatalf("%s", err)
+	}
+
+	bp, err := ctxt.ImportDir(dir, 0)
+	pkg := new(Package)
+	pkg.Internal.Local = true
+	pkg.Internal.CmdlineFiles = true
+	pkg.load(ctx, opts, "command-line-arguments", &stk, nil, bp, err)
+	if !cfg.ModulesEnabled {
+		pkg.Internal.LocalPrefix = dirToImportPath(dir)
+	}
+	pkg.ImportPath = "command-line-arguments"
+	pkg.Target = ""
+	pkg.Match = gofiles
+
+	if pkg.Name == "main" {
+		exe := pkg.DefaultExecName() + cfg.ExeSuffix
+
+		if cfg.GOBIN != "" {
+			pkg.Target = filepath.Join(cfg.GOBIN, exe)
+		} else if cfg.ModulesEnabled {
+			pkg.Target = filepath.Join(modload.BinDir(), exe)
+		}
+	}
+
+	if opts.MainOnly && pkg.Name != "main" && pkg.Error == nil {
+		pkg.Error = &PackageError{Err: &mainPackageError{importPath: pkg.ImportPath}}
+		pkg.Incomplete = true
+	}
+	setToolFlags(pkg)
+
+	return pkg
+}
+
+// PackagesAndErrorsOutsideModule is like PackagesAndErrors but runs in
+// module-aware mode and ignores the go.mod file in the current directory or any
+// parent directory, if there is one. This is used in the implementation of 'go
+// install pkg@version' and other commands that support similar forms.
+//
+// modload.ForceUseModules must be true, and modload.RootMode must be NoRoot
+// before calling this function.
+//
+// PackagesAndErrorsOutsideModule imposes several constraints to avoid
+// ambiguity. All arguments must have the same version suffix (not just a suffix
+// that resolves to the same version). They must refer to packages in the same
+// module, which must not be std or cmd. That module is not considered the main
+// module, but its go.mod file (if it has one) must not contain directives that
+// would cause it to be interpreted differently if it were the main module
+// (replace, exclude).
+func PackagesAndErrorsOutsideModule(ctx context.Context, opts PackageOpts, args []string) ([]*Package, error) {
+	if !modload.ForceUseModules {
+		panic("modload.ForceUseModules must be true")
+	}
+	if modload.RootMode != modload.NoRoot {
+		panic("modload.RootMode must be NoRoot")
+	}
+
+	// Check that the arguments satisfy syntactic constraints.
+	var version string
+	var firstPath string
+	for _, arg := range args {
+		if i := strings.Index(arg, "@"); i >= 0 {
+			firstPath, version = arg[:i], arg[i+1:]
+			if version == "" {
+				return nil, fmt.Errorf("%s: version must not be empty", arg)
+			}
+			break
+		}
+	}
+	patterns := make([]string, len(args))
+	for i, arg := range args {
+		p, found := strings.CutSuffix(arg, "@"+version)
+		if !found {
+			return nil, fmt.Errorf("%s: all arguments must refer to packages in the same module at the same version (@%s)", arg, version)
+		}
+		switch {
+		case build.IsLocalImport(p):
+			return nil, fmt.Errorf("%s: argument must be a package path, not a relative path", arg)
+		case filepath.IsAbs(p):
+			return nil, fmt.Errorf("%s: argument must be a package path, not an absolute path", arg)
+		case search.IsMetaPackage(p):
+			return nil, fmt.Errorf("%s: argument must be a package path, not a meta-package", arg)
+		case pathpkg.Clean(p) != p:
+			return nil, fmt.Errorf("%s: argument must be a clean package path", arg)
+		case !strings.Contains(p, "...") && search.IsStandardImportPath(p) && modindex.IsStandardPackage(cfg.GOROOT, cfg.BuildContext.Compiler, p):
+			return nil, fmt.Errorf("%s: argument must not be a package in the standard library", arg)
+		default:
+			patterns[i] = p
+		}
+	}
+
+	// Query the module providing the first argument, load its go.mod file, and
+	// check that it doesn't contain directives that would cause it to be
+	// interpreted differently if it were the main module.
+	//
+	// If multiple modules match the first argument, accept the longest match
+	// (first result). It's possible this module won't provide packages named by
+	// later arguments, and other modules would. Let's not try to be too
+	// magical though.
+	allowed := modload.CheckAllowed
+	if modload.IsRevisionQuery(firstPath, version) {
+		// Don't check for retractions if a specific revision is requested.
+		allowed = nil
+	}
+	noneSelected := func(path string) (version string) { return "none" }
+	qrs, err := modload.QueryPackages(ctx, patterns[0], version, noneSelected, allowed)
+	if err != nil {
+		return nil, fmt.Errorf("%s: %w", args[0], err)
+	}
+	rootMod := qrs[0].Mod
+	data, err := modfetch.GoMod(ctx, rootMod.Path, rootMod.Version)
+	if err != nil {
+		return nil, fmt.Errorf("%s: %w", args[0], err)
+	}
+	f, err := modfile.Parse("go.mod", data, nil)
+	if err != nil {
+		return nil, fmt.Errorf("%s (in %s): %w", args[0], rootMod, err)
+	}
+	directiveFmt := "%s (in %s):\n" +
+		"\tThe go.mod file for the module providing named packages contains one or\n" +
+		"\tmore %s directives. It must not contain directives that would cause\n" +
+		"\tit to be interpreted differently than if it were the main module."
+	if len(f.Replace) > 0 {
+		return nil, fmt.Errorf(directiveFmt, args[0], rootMod, "replace")
+	}
+	if len(f.Exclude) > 0 {
+		return nil, fmt.Errorf(directiveFmt, args[0], rootMod, "exclude")
+	}
+
+	// Since we are in NoRoot mode, the build list initially contains only
+	// the dummy command-line-arguments module. Add a requirement on the
+	// module that provides the packages named on the command line.
+	if _, err := modload.EditBuildList(ctx, nil, []module.Version{rootMod}); err != nil {
+		return nil, fmt.Errorf("%s: %w", args[0], err)
+	}
+
+	// Load packages for all arguments.
+	pkgs := PackagesAndErrors(ctx, opts, patterns)
+
+	// Check that named packages are all provided by the same module.
+	for _, pkg := range pkgs {
+		var pkgErr error
+		if pkg.Module == nil {
+			// Packages in std, cmd, and their vendored dependencies
+			// don't have this field set.
+			pkgErr = fmt.Errorf("package %s not provided by module %s", pkg.ImportPath, rootMod)
+		} else if pkg.Module.Path != rootMod.Path || pkg.Module.Version != rootMod.Version {
+			pkgErr = fmt.Errorf("package %s provided by module %s@%s\n\tAll packages must be provided by the same module (%s).", pkg.ImportPath, pkg.Module.Path, pkg.Module.Version, rootMod)
+		}
+		if pkgErr != nil && pkg.Error == nil {
+			pkg.Error = &PackageError{Err: pkgErr}
+			pkg.Incomplete = true
+		}
+	}
+
+	matchers := make([]func(string) bool, len(patterns))
+	for i, p := range patterns {
+		if strings.Contains(p, "...") {
+			matchers[i] = pkgpattern.MatchPattern(p)
+		}
+	}
+	return pkgs, nil
+}
+
+// EnsureImport ensures that package p imports the named package.
+func EnsureImport(p *Package, pkg string) {
+	for _, d := range p.Internal.Imports {
+		if d.Name == pkg {
+			return
+		}
+	}
+
+	p1, err := LoadImportWithFlags(pkg, p.Dir, p, &ImportStack{}, nil, 0)
+	if err != nil {
+		base.Fatalf("load %s: %v", pkg, err)
+	}
+	if p1.Error != nil {
+		base.Fatalf("load %s: %v", pkg, p1.Error)
+	}
+
+	p.Internal.Imports = append(p.Internal.Imports, p1)
+}
+
+// PrepareForCoverageBuild is a helper invoked for "go install
+// -cover", "go run -cover", and "go build -cover" (but not used by
+// "go test -cover"). It walks through the packages being built (and
+// dependencies) and marks them for coverage instrumentation when
+// appropriate, and possibly adding additional deps where needed.
+func PrepareForCoverageBuild(pkgs []*Package) {
+	var match []func(*Package) bool
+
+	matchMainModAndCommandLine := func(p *Package) bool {
+		// note that p.Standard implies p.Module == nil below.
+		return p.Internal.CmdlineFiles || p.Internal.CmdlinePkg || (p.Module != nil && p.Module.Main)
+	}
+
+	if len(cfg.BuildCoverPkg) != 0 {
+		// If -coverpkg has been specified, then we instrument only
+		// the specific packages selected by the user-specified pattern(s).
+		match = make([]func(*Package) bool, len(cfg.BuildCoverPkg))
+		for i := range cfg.BuildCoverPkg {
+			match[i] = MatchPackage(cfg.BuildCoverPkg[i], base.Cwd())
+		}
+	} else {
+		// Without -coverpkg, instrument only packages in the main module
+		// (if any), as well as packages/files specifically named on the
+		// command line.
+		match = []func(*Package) bool{matchMainModAndCommandLine}
+	}
+
+	// Visit the packages being built or installed, along with all of
+	// their dependencies, and mark them to be instrumented, taking
+	// into account the matchers we've set up in the sequence above.
+	SelectCoverPackages(PackageList(pkgs), match, "build")
+}
+
+func SelectCoverPackages(roots []*Package, match []func(*Package) bool, op string) []*Package {
+	var warntag string
+	var includeMain bool
+	switch op {
+	case "build":
+		warntag = "built"
+		includeMain = true
+	case "test":
+		warntag = "tested"
+	default:
+		panic("internal error, bad mode passed to SelectCoverPackages")
+	}
+
+	covered := []*Package{}
+	matched := make([]bool, len(match))
+	for _, p := range roots {
+		haveMatch := false
+		for i := range match {
+			if match[i](p) {
+				matched[i] = true
+				haveMatch = true
+			}
+		}
+		if !haveMatch {
+			continue
+		}
+
+		// There is nothing to cover in package unsafe; it comes from
+		// the compiler.
+		if p.ImportPath == "unsafe" {
+			continue
+		}
+
+		// A package which only has test files can't be imported as a
+		// dependency, and at the moment we don't try to instrument it
+		// for coverage. There isn't any technical reason why
+		// *_test.go files couldn't be instrumented, but it probably
+		// doesn't make much sense to lump together coverage metrics
+		// (ex: percent stmts covered) of *_test.go files with
+		// non-test Go code.
+		if len(p.GoFiles)+len(p.CgoFiles) == 0 {
+			continue
+		}
+
+		// Silently ignore attempts to run coverage on sync/atomic
+		// and/or runtime/internal/atomic when using atomic coverage
+		// mode. Atomic coverage mode uses sync/atomic, so we can't
+		// also do coverage on it.
+		if cfg.BuildCoverMode == "atomic" && p.Standard &&
+			(p.ImportPath == "sync/atomic" || p.ImportPath == "runtime/internal/atomic") {
+			continue
+		}
+
+		// If using the race detector, silently ignore attempts to run
+		// coverage on the runtime packages. It will cause the race
+		// detector to be invoked before it has been initialized. Note
+		// the use of "regonly" instead of just ignoring the package
+		// completely-- we do this due to the requirements of the
+		// package ID numbering scheme. See the comment in
+		// $GOROOT/src/internal/coverage/pkid.go dealing with
+		// hard-coding of runtime package IDs.
+		cmode := cfg.BuildCoverMode
+		if cfg.BuildRace && p.Standard && (p.ImportPath == "runtime" || strings.HasPrefix(p.ImportPath, "runtime/internal")) {
+			cmode = "regonly"
+		}
+
+		// If -coverpkg is in effect and for some reason we don't want
+		// coverage data for the main package, make sure that we at
+		// least process it for registration hooks.
+		if includeMain && p.Name == "main" && !haveMatch {
+			haveMatch = true
+			cmode = "regonly"
+		}
+
+		// Mark package for instrumentation.
+		p.Internal.CoverMode = cmode
+		covered = append(covered, p)
+
+		// Force import of sync/atomic into package if atomic mode.
+		if cfg.BuildCoverMode == "atomic" {
+			EnsureImport(p, "sync/atomic")
+		}
+
+		// Generate covervars if using legacy coverage design.
+		if !cfg.Experiment.CoverageRedesign {
+			var coverFiles []string
+			coverFiles = append(coverFiles, p.GoFiles...)
+			coverFiles = append(coverFiles, p.CgoFiles...)
+			p.Internal.CoverVars = DeclareCoverVars(p, coverFiles...)
+		}
+	}
+
+	// Warn about -coverpkg arguments that are not actually used.
+	for i := range cfg.BuildCoverPkg {
+		if !matched[i] {
+			fmt.Fprintf(os.Stderr, "warning: no packages being %s depend on matches for pattern %s\n", warntag, cfg.BuildCoverPkg[i])
+		}
+	}
+
+	return covered
+}
+
+// DeclareCoverVars attaches the required cover variables names
+// to the files, to be used when annotating the files. This
+// function only called when using legacy coverage test/build
+// (e.g. GOEXPERIMENT=coverageredesign is off).
+func DeclareCoverVars(p *Package, files ...string) map[string]*CoverVar {
+	coverVars := make(map[string]*CoverVar)
+	coverIndex := 0
+	// We create the cover counters as new top-level variables in the package.
+	// We need to avoid collisions with user variables (GoCover_0 is unlikely but still)
+	// and more importantly with dot imports of other covered packages,
+	// so we append 12 hex digits from the SHA-256 of the import path.
+	// The point is only to avoid accidents, not to defeat users determined to
+	// break things.
+	sum := sha256.Sum256([]byte(p.ImportPath))
+	h := fmt.Sprintf("%x", sum[:6])
+	for _, file := range files {
+		if base.IsTestFile(file) {
+			continue
+		}
+		// For a package that is "local" (imported via ./ import or command line, outside GOPATH),
+		// we record the full path to the file name.
+		// Otherwise we record the import path, then a forward slash, then the file name.
+		// This makes profiles within GOPATH file system-independent.
+		// These names appear in the cmd/cover HTML interface.
+		var longFile string
+		if p.Internal.Local {
+			longFile = filepath.Join(p.Dir, file)
+		} else {
+			longFile = pathpkg.Join(p.ImportPath, file)
+		}
+		coverVars[file] = &CoverVar{
+			File: longFile,
+			Var:  fmt.Sprintf("GoCover_%d_%x", coverIndex, h),
+		}
+		coverIndex++
+	}
+	return coverVars
+}
diff --git a/src/cmd/go/internal/load/pkg_test.go b/src/cmd/go/internal/load/pkg_test.go
new file mode 100644
index 0000000..3bcddee
--- /dev/null
+++ b/src/cmd/go/internal/load/pkg_test.go
@@ -0,0 +1,82 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package load
+
+import (
+	"cmd/go/internal/cfg"
+	"testing"
+)
+
+func TestPkgDefaultExecName(t *testing.T) {
+	oldModulesEnabled := cfg.ModulesEnabled
+	defer func() { cfg.ModulesEnabled = oldModulesEnabled }()
+	for _, tt := range []struct {
+		in         string
+		files      []string
+		wantMod    string
+		wantGopath string
+	}{
+		{"example.com/mycmd", []string{}, "mycmd", "mycmd"},
+		{"example.com/mycmd/v0", []string{}, "v0", "v0"},
+		{"example.com/mycmd/v1", []string{}, "v1", "v1"},
+		{"example.com/mycmd/v2", []string{}, "mycmd", "v2"}, // Semantic import versioning, use second last element in module mode.
+		{"example.com/mycmd/v3", []string{}, "mycmd", "v3"}, // Semantic import versioning, use second last element in module mode.
+		{"mycmd", []string{}, "mycmd", "mycmd"},
+		{"mycmd/v0", []string{}, "v0", "v0"},
+		{"mycmd/v1", []string{}, "v1", "v1"},
+		{"mycmd/v2", []string{}, "mycmd", "v2"}, // Semantic import versioning, use second last element in module mode.
+		{"v0", []string{}, "v0", "v0"},
+		{"v1", []string{}, "v1", "v1"},
+		{"v2", []string{}, "v2", "v2"},
+		{"command-line-arguments", []string{"output.go", "foo.go"}, "output", "output"},
+	} {
+		{
+			cfg.ModulesEnabled = true
+			pkg := new(Package)
+			pkg.ImportPath = tt.in
+			pkg.GoFiles = tt.files
+			pkg.Internal.CmdlineFiles = len(tt.files) > 0
+			gotMod := pkg.DefaultExecName()
+			if gotMod != tt.wantMod {
+				t.Errorf("pkg.DefaultExecName with ImportPath = %q in module mode = %v; want %v", tt.in, gotMod, tt.wantMod)
+			}
+		}
+		{
+			cfg.ModulesEnabled = false
+			pkg := new(Package)
+			pkg.ImportPath = tt.in
+			pkg.GoFiles = tt.files
+			pkg.Internal.CmdlineFiles = len(tt.files) > 0
+			gotGopath := pkg.DefaultExecName()
+			if gotGopath != tt.wantGopath {
+				t.Errorf("pkg.DefaultExecName with ImportPath = %q in gopath mode = %v; want %v", tt.in, gotGopath, tt.wantGopath)
+			}
+		}
+	}
+}
+
+func TestIsVersionElement(t *testing.T) {
+	t.Parallel()
+	for _, tt := range []struct {
+		in   string
+		want bool
+	}{
+		{"v0", false},
+		{"v05", false},
+		{"v1", false},
+		{"v2", true},
+		{"v3", true},
+		{"v9", true},
+		{"v10", true},
+		{"v11", true},
+		{"v", false},
+		{"vx", false},
+	} {
+		got := isVersionElement(tt.in)
+		if got != tt.want {
+			t.Errorf("isVersionElement(%q) = %v; want %v", tt.in, got, tt.want)
+		}
+	}
+}
diff --git a/src/cmd/go/internal/load/search.go b/src/cmd/go/internal/load/search.go
new file mode 100644
index 0000000..565996a
--- /dev/null
+++ b/src/cmd/go/internal/load/search.go
@@ -0,0 +1,57 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package load
+
+import (
+	"path/filepath"
+	"strings"
+
+	"cmd/go/internal/search"
+	"cmd/internal/pkgpattern"
+)
+
+// MatchPackage(pattern, cwd)(p) reports whether package p matches pattern in the working directory cwd.
+func MatchPackage(pattern, cwd string) func(*Package) bool {
+	switch {
+	case search.IsRelativePath(pattern):
+		// Split pattern into leading pattern-free directory path
+		// (including all . and .. elements) and the final pattern.
+		var dir string
+		i := strings.Index(pattern, "...")
+		if i < 0 {
+			dir, pattern = pattern, ""
+		} else {
+			j := strings.LastIndex(pattern[:i], "/")
+			dir, pattern = pattern[:j], pattern[j+1:]
+		}
+		dir = filepath.Join(cwd, dir)
+		if pattern == "" {
+			return func(p *Package) bool { return p.Dir == dir }
+		}
+		matchPath := pkgpattern.MatchPattern(pattern)
+		return func(p *Package) bool {
+			// Compute relative path to dir and see if it matches the pattern.
+			rel, err := filepath.Rel(dir, p.Dir)
+			if err != nil {
+				// Cannot make relative - e.g. different drive letters on Windows.
+				return false
+			}
+			rel = filepath.ToSlash(rel)
+			if rel == ".." || strings.HasPrefix(rel, "../") {
+				return false
+			}
+			return matchPath(rel)
+		}
+	case pattern == "all":
+		return func(p *Package) bool { return true }
+	case pattern == "std":
+		return func(p *Package) bool { return p.Standard }
+	case pattern == "cmd":
+		return func(p *Package) bool { return p.Standard && strings.HasPrefix(p.ImportPath, "cmd/") }
+	default:
+		matchPath := pkgpattern.MatchPattern(pattern)
+		return func(p *Package) bool { return matchPath(p.ImportPath) }
+	}
+}
diff --git a/src/cmd/go/internal/load/test.go b/src/cmd/go/internal/load/test.go
new file mode 100644
index 0000000..e9ed0d3
--- /dev/null
+++ b/src/cmd/go/internal/load/test.go
@@ -0,0 +1,991 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package load
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"go/ast"
+	"go/build"
+	"go/doc"
+	"go/parser"
+	"go/token"
+	"internal/lazytemplate"
+	"path/filepath"
+	"slices"
+	"sort"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/str"
+	"cmd/go/internal/trace"
+)
+
+var TestMainDeps = []string{
+	// Dependencies for testmain.
+	"os",
+	"reflect",
+	"testing",
+	"testing/internal/testdeps",
+}
+
+type TestCover struct {
+	Mode  string
+	Local bool
+	Pkgs  []*Package
+	Paths []string
+	Vars  []coverInfo
+}
+
+// TestPackagesFor is like TestPackagesAndErrors but it returns
+// an error if the test packages or their dependencies have errors.
+// Only test packages without errors are returned.
+func TestPackagesFor(ctx context.Context, opts PackageOpts, p *Package, cover *TestCover) (pmain, ptest, pxtest *Package, err error) {
+	pmain, ptest, pxtest = TestPackagesAndErrors(ctx, nil, opts, p, cover)
+	for _, p1 := range []*Package{ptest, pxtest, pmain} {
+		if p1 == nil {
+			// pxtest may be nil
+			continue
+		}
+		if p1.Error != nil {
+			err = p1.Error
+			break
+		}
+		if p1.Incomplete {
+			ps := PackageList([]*Package{p1})
+			for _, p := range ps {
+				if p.Error != nil {
+					err = p.Error
+					break
+				}
+			}
+			break
+		}
+	}
+	if pmain.Error != nil || pmain.Incomplete {
+		pmain = nil
+	}
+	if ptest.Error != nil || ptest.Incomplete {
+		ptest = nil
+	}
+	if pxtest != nil && (pxtest.Error != nil || pxtest.Incomplete) {
+		pxtest = nil
+	}
+	return pmain, ptest, pxtest, err
+}
+
+// TestPackagesAndErrors returns three packages:
+//   - pmain, the package main corresponding to the test binary (running tests in ptest and pxtest).
+//   - ptest, the package p compiled with added "package p" test files.
+//   - pxtest, the result of compiling any "package p_test" (external) test files.
+//
+// If the package has no "package p_test" test files, pxtest will be nil.
+// If the non-test compilation of package p can be reused
+// (for example, if there are no "package p" test files and
+// package p need not be instrumented for coverage or any other reason),
+// then the returned ptest == p.
+//
+// If done is non-nil, TestPackagesAndErrors will finish filling out the returned
+// package structs in a goroutine and call done once finished. The members of the
+// returned packages should not be accessed until done is called.
+//
+// The caller is expected to have checked that len(p.TestGoFiles)+len(p.XTestGoFiles) > 0,
+// or else there's no point in any of this.
+func TestPackagesAndErrors(ctx context.Context, done func(), opts PackageOpts, p *Package, cover *TestCover) (pmain, ptest, pxtest *Package) {
+	ctx, span := trace.StartSpan(ctx, "load.TestPackagesAndErrors")
+	defer span.Done()
+
+	pre := newPreload()
+	defer pre.flush()
+	allImports := append([]string{}, p.TestImports...)
+	allImports = append(allImports, p.XTestImports...)
+	pre.preloadImports(ctx, opts, allImports, p.Internal.Build)
+
+	var ptestErr, pxtestErr *PackageError
+	var imports, ximports []*Package
+	var stk ImportStack
+	var testEmbed, xtestEmbed map[string][]string
+	var incomplete bool
+	stk.Push(p.ImportPath + " (test)")
+	rawTestImports := str.StringList(p.TestImports)
+	for i, path := range p.TestImports {
+		p1, err := loadImport(ctx, opts, pre, path, p.Dir, p, &stk, p.Internal.Build.TestImportPos[path], ResolveImport)
+		if err != nil && ptestErr == nil {
+			ptestErr = err
+			incomplete = true
+		}
+		if p1.Incomplete {
+			incomplete = true
+		}
+		p.TestImports[i] = p1.ImportPath
+		imports = append(imports, p1)
+	}
+	var err error
+	p.TestEmbedFiles, testEmbed, err = resolveEmbed(p.Dir, p.TestEmbedPatterns)
+	if err != nil {
+		ptestErr = &PackageError{
+			ImportStack: stk.Copy(),
+			Err:         err,
+		}
+		incomplete = true
+		embedErr := err.(*EmbedError)
+		ptestErr.setPos(p.Internal.Build.TestEmbedPatternPos[embedErr.Pattern])
+	}
+	stk.Pop()
+
+	stk.Push(p.ImportPath + "_test")
+	pxtestNeedsPtest := false
+	var pxtestIncomplete bool
+	rawXTestImports := str.StringList(p.XTestImports)
+	for i, path := range p.XTestImports {
+		p1, err := loadImport(ctx, opts, pre, path, p.Dir, p, &stk, p.Internal.Build.XTestImportPos[path], ResolveImport)
+		if err != nil && pxtestErr == nil {
+			pxtestErr = err
+		}
+		if p1.Incomplete {
+			pxtestIncomplete = true
+		}
+		if p1.ImportPath == p.ImportPath {
+			pxtestNeedsPtest = true
+		} else {
+			ximports = append(ximports, p1)
+		}
+		p.XTestImports[i] = p1.ImportPath
+	}
+	p.XTestEmbedFiles, xtestEmbed, err = resolveEmbed(p.Dir, p.XTestEmbedPatterns)
+	if err != nil && pxtestErr == nil {
+		pxtestErr = &PackageError{
+			ImportStack: stk.Copy(),
+			Err:         err,
+		}
+		embedErr := err.(*EmbedError)
+		pxtestErr.setPos(p.Internal.Build.XTestEmbedPatternPos[embedErr.Pattern])
+	}
+	pxtestIncomplete = pxtestIncomplete || pxtestErr != nil
+	stk.Pop()
+
+	// Test package.
+	if len(p.TestGoFiles) > 0 || p.Name == "main" || cover != nil && cover.Local {
+		ptest = new(Package)
+		*ptest = *p
+		ptest.Error = ptestErr
+		ptest.Incomplete = incomplete
+		ptest.ForTest = p.ImportPath
+		ptest.GoFiles = nil
+		ptest.GoFiles = append(ptest.GoFiles, p.GoFiles...)
+		ptest.GoFiles = append(ptest.GoFiles, p.TestGoFiles...)
+		ptest.Target = ""
+		// Note: The preparation of the vet config requires that common
+		// indexes in ptest.Imports and ptest.Internal.RawImports
+		// all line up (but RawImports can be shorter than the others).
+		// That is, for 0 ≤ i < len(RawImports),
+		// RawImports[i] is the import string in the program text, and
+		// Imports[i] is the expanded import string (vendoring applied or relative path expanded away).
+		// Any implicitly added imports appear in Imports and Internal.Imports
+		// but not RawImports (because they were not in the source code).
+		// We insert TestImports, imports, and rawTestImports at the start of
+		// these lists to preserve the alignment.
+		// Note that p.Internal.Imports may not be aligned with p.Imports/p.Internal.RawImports,
+		// but we insert at the beginning there too just for consistency.
+		ptest.Imports = str.StringList(p.TestImports, p.Imports)
+		ptest.Internal.Imports = append(imports, p.Internal.Imports...)
+		ptest.Internal.RawImports = str.StringList(rawTestImports, p.Internal.RawImports)
+		ptest.Internal.ForceLibrary = true
+		ptest.Internal.BuildInfo = nil
+		ptest.Internal.Build = new(build.Package)
+		*ptest.Internal.Build = *p.Internal.Build
+		m := map[string][]token.Position{}
+		for k, v := range p.Internal.Build.ImportPos {
+			m[k] = append(m[k], v...)
+		}
+		for k, v := range p.Internal.Build.TestImportPos {
+			m[k] = append(m[k], v...)
+		}
+		ptest.Internal.Build.ImportPos = m
+		if testEmbed == nil && len(p.Internal.Embed) > 0 {
+			testEmbed = map[string][]string{}
+		}
+		for k, v := range p.Internal.Embed {
+			testEmbed[k] = v
+		}
+		ptest.Internal.Embed = testEmbed
+		ptest.EmbedFiles = str.StringList(p.EmbedFiles, p.TestEmbedFiles)
+		ptest.Internal.OrigImportPath = p.Internal.OrigImportPath
+		ptest.Internal.PGOProfile = p.Internal.PGOProfile
+		ptest.Internal.Build.Directives = append(slices.Clip(p.Internal.Build.Directives), p.Internal.Build.TestDirectives...)
+	} else {
+		ptest = p
+	}
+
+	// External test package.
+	if len(p.XTestGoFiles) > 0 {
+		pxtest = &Package{
+			PackagePublic: PackagePublic{
+				Name:       p.Name + "_test",
+				ImportPath: p.ImportPath + "_test",
+				Root:       p.Root,
+				Dir:        p.Dir,
+				Goroot:     p.Goroot,
+				GoFiles:    p.XTestGoFiles,
+				Imports:    p.XTestImports,
+				ForTest:    p.ImportPath,
+				Module:     p.Module,
+				Error:      pxtestErr,
+				Incomplete: pxtestIncomplete,
+				EmbedFiles: p.XTestEmbedFiles,
+			},
+			Internal: PackageInternal{
+				LocalPrefix: p.Internal.LocalPrefix,
+				Build: &build.Package{
+					ImportPos:  p.Internal.Build.XTestImportPos,
+					Directives: p.Internal.Build.XTestDirectives,
+				},
+				Imports:    ximports,
+				RawImports: rawXTestImports,
+
+				Asmflags:       p.Internal.Asmflags,
+				Gcflags:        p.Internal.Gcflags,
+				Ldflags:        p.Internal.Ldflags,
+				Gccgoflags:     p.Internal.Gccgoflags,
+				Embed:          xtestEmbed,
+				OrigImportPath: p.Internal.OrigImportPath,
+				PGOProfile:     p.Internal.PGOProfile,
+			},
+		}
+		if pxtestNeedsPtest {
+			pxtest.Internal.Imports = append(pxtest.Internal.Imports, ptest)
+		}
+	}
+
+	// Arrange for testing.Testing to report true.
+	ldflags := append(p.Internal.Ldflags, "-X", "testing.testBinary=1")
+	gccgoflags := append(p.Internal.Gccgoflags, "-Wl,--defsym,testing.gccgoTestBinary=1")
+
+	// Build main package.
+	pmain = &Package{
+		PackagePublic: PackagePublic{
+			Name:       "main",
+			Dir:        p.Dir,
+			GoFiles:    []string{"_testmain.go"},
+			ImportPath: p.ImportPath + ".test",
+			Root:       p.Root,
+			Imports:    str.StringList(TestMainDeps),
+			Module:     p.Module,
+		},
+		Internal: PackageInternal{
+			Build:          &build.Package{Name: "main"},
+			BuildInfo:      p.Internal.BuildInfo,
+			Asmflags:       p.Internal.Asmflags,
+			Gcflags:        p.Internal.Gcflags,
+			Ldflags:        ldflags,
+			Gccgoflags:     gccgoflags,
+			OrigImportPath: p.Internal.OrigImportPath,
+			PGOProfile:     p.Internal.PGOProfile,
+		},
+	}
+
+	pb := p.Internal.Build
+	pmain.DefaultGODEBUG = defaultGODEBUG(pmain, pb.Directives, pb.TestDirectives, pb.XTestDirectives)
+
+	// The generated main also imports testing, regexp, and os.
+	// Also the linker introduces implicit dependencies reported by LinkerDeps.
+	stk.Push("testmain")
+	deps := TestMainDeps // cap==len, so safe for append
+	for _, d := range LinkerDeps(p) {
+		deps = append(deps, d)
+	}
+	for _, dep := range deps {
+		if dep == ptest.ImportPath {
+			pmain.Internal.Imports = append(pmain.Internal.Imports, ptest)
+		} else {
+			p1, err := loadImport(ctx, opts, pre, dep, "", nil, &stk, nil, 0)
+			if err != nil && pmain.Error == nil {
+				pmain.Error = err
+				pmain.Incomplete = true
+			}
+			pmain.Internal.Imports = append(pmain.Internal.Imports, p1)
+		}
+	}
+	stk.Pop()
+
+	parallelizablePart := func() {
+		if cover != nil && cover.Pkgs != nil && !cfg.Experiment.CoverageRedesign {
+			// Add imports, but avoid duplicates.
+			seen := map[*Package]bool{p: true, ptest: true}
+			for _, p1 := range pmain.Internal.Imports {
+				seen[p1] = true
+			}
+			for _, p1 := range cover.Pkgs {
+				if seen[p1] {
+					// Don't add duplicate imports.
+					continue
+				}
+				seen[p1] = true
+				pmain.Internal.Imports = append(pmain.Internal.Imports, p1)
+			}
+		}
+
+		allTestImports := make([]*Package, 0, len(pmain.Internal.Imports)+len(imports)+len(ximports))
+		allTestImports = append(allTestImports, pmain.Internal.Imports...)
+		allTestImports = append(allTestImports, imports...)
+		allTestImports = append(allTestImports, ximports...)
+		setToolFlags(allTestImports...)
+
+		// Do initial scan for metadata needed for writing _testmain.go
+		// Use that metadata to update the list of imports for package main.
+		// The list of imports is used by recompileForTest and by the loop
+		// afterward that gathers t.Cover information.
+		t, err := loadTestFuncs(p)
+		if err != nil && pmain.Error == nil {
+			pmain.setLoadPackageDataError(err, p.ImportPath, &stk, nil)
+		}
+		t.Cover = cover
+		if len(ptest.GoFiles)+len(ptest.CgoFiles) > 0 {
+			pmain.Internal.Imports = append(pmain.Internal.Imports, ptest)
+			pmain.Imports = append(pmain.Imports, ptest.ImportPath)
+			t.ImportTest = true
+		}
+		if pxtest != nil {
+			pmain.Internal.Imports = append(pmain.Internal.Imports, pxtest)
+			pmain.Imports = append(pmain.Imports, pxtest.ImportPath)
+			t.ImportXtest = true
+		}
+
+		// Sort and dedup pmain.Imports.
+		// Only matters for go list -test output.
+		sort.Strings(pmain.Imports)
+		w := 0
+		for _, path := range pmain.Imports {
+			if w == 0 || path != pmain.Imports[w-1] {
+				pmain.Imports[w] = path
+				w++
+			}
+		}
+		pmain.Imports = pmain.Imports[:w]
+		pmain.Internal.RawImports = str.StringList(pmain.Imports)
+
+		// Replace pmain's transitive dependencies with test copies, as necessary.
+		cycleErr := recompileForTest(pmain, p, ptest, pxtest)
+		if cycleErr != nil {
+			ptest.Error = cycleErr
+			ptest.Incomplete = true
+		}
+
+		if cover != nil {
+			if cfg.Experiment.CoverageRedesign {
+				// Here ptest needs to inherit the proper coverage mode (since
+				// it contains p's Go files), whereas pmain contains only
+				// test harness code (don't want to instrument it, and
+				// we don't want coverage hooks in the pkg init).
+				ptest.Internal.CoverMode = p.Internal.CoverMode
+				pmain.Internal.CoverMode = "testmain"
+			}
+			// Should we apply coverage analysis locally, only for this
+			// package and only for this test? Yes, if -cover is on but
+			// -coverpkg has not specified a list of packages for global
+			// coverage.
+			if cover.Local {
+				ptest.Internal.CoverMode = cover.Mode
+
+				if !cfg.Experiment.CoverageRedesign {
+					var coverFiles []string
+					coverFiles = append(coverFiles, ptest.GoFiles...)
+					coverFiles = append(coverFiles, ptest.CgoFiles...)
+					ptest.Internal.CoverVars = DeclareCoverVars(ptest, coverFiles...)
+				}
+			}
+
+			if !cfg.Experiment.CoverageRedesign {
+				for _, cp := range pmain.Internal.Imports {
+					if len(cp.Internal.CoverVars) > 0 {
+						t.Cover.Vars = append(t.Cover.Vars, coverInfo{cp, cp.Internal.CoverVars})
+					}
+				}
+			}
+		}
+
+		data, err := formatTestmain(t)
+		if err != nil && pmain.Error == nil {
+			pmain.Error = &PackageError{Err: err}
+			pmain.Incomplete = true
+		}
+		// Set TestmainGo even if it is empty: the presence of a TestmainGo
+		// indicates that this package is, in fact, a test main.
+		pmain.Internal.TestmainGo = &data
+	}
+
+	if done != nil {
+		go func() {
+			parallelizablePart()
+			done()
+		}()
+	} else {
+		parallelizablePart()
+	}
+
+	return pmain, ptest, pxtest
+}
+
+// recompileForTest copies and replaces certain packages in pmain's dependency
+// graph. This is necessary for two reasons. First, if ptest is different than
+// preal, packages that import the package under test should get ptest instead
+// of preal. This is particularly important if pxtest depends on functionality
+// exposed in test sources in ptest. Second, if there is a main package
+// (other than pmain) anywhere, we need to set p.Internal.ForceLibrary and
+// clear p.Internal.BuildInfo in the test copy to prevent link conflicts.
+// This may happen if both -coverpkg and the command line patterns include
+// multiple main packages.
+func recompileForTest(pmain, preal, ptest, pxtest *Package) *PackageError {
+	// The "test copy" of preal is ptest.
+	// For each package that depends on preal, make a "test copy"
+	// that depends on ptest. And so on, up the dependency tree.
+	testCopy := map[*Package]*Package{preal: ptest}
+	for _, p := range PackageList([]*Package{pmain}) {
+		if p == preal {
+			continue
+		}
+		// Copy on write.
+		didSplit := p == pmain || p == pxtest || p == ptest
+		split := func() {
+			if didSplit {
+				return
+			}
+			didSplit = true
+			if testCopy[p] != nil {
+				panic("recompileForTest loop")
+			}
+			p1 := new(Package)
+			testCopy[p] = p1
+			*p1 = *p
+			p1.ForTest = preal.ImportPath
+			p1.Internal.Imports = make([]*Package, len(p.Internal.Imports))
+			copy(p1.Internal.Imports, p.Internal.Imports)
+			p1.Imports = make([]string, len(p.Imports))
+			copy(p1.Imports, p.Imports)
+			p = p1
+			p.Target = ""
+			p.Internal.BuildInfo = nil
+			p.Internal.ForceLibrary = true
+			p.Internal.PGOProfile = preal.Internal.PGOProfile
+		}
+
+		// Update p.Internal.Imports to use test copies.
+		for i, imp := range p.Internal.Imports {
+			if p1 := testCopy[imp]; p1 != nil && p1 != imp {
+				split()
+
+				// If the test dependencies cause a cycle with pmain, this is
+				// where it is introduced.
+				// (There are no cycles in the graph until this assignment occurs.)
+				p.Internal.Imports[i] = p1
+			}
+		}
+
+		// Force main packages the test imports to be built as libraries.
+		// Normal imports of main packages are forbidden by the package loader,
+		// but this can still happen if -coverpkg patterns include main packages:
+		// covered packages are imported by pmain. Linking multiple packages
+		// compiled with '-p main' causes duplicate symbol errors.
+		// See golang.org/issue/30907, golang.org/issue/34114.
+		if p.Name == "main" && p != pmain && p != ptest {
+			split()
+		}
+		// Split and attach PGO information to test dependencies if preal
+		// is built with PGO.
+		if preal.Internal.PGOProfile != "" && p.Internal.PGOProfile == "" {
+			split()
+		}
+	}
+
+	// Do search to find cycle.
+	// importerOf maps each import path to its importer nearest to p.
+	importerOf := map[*Package]*Package{}
+	for _, p := range ptest.Internal.Imports {
+		importerOf[p] = nil
+	}
+
+	// q is a breadth-first queue of packages to search for target.
+	// Every package added to q has a corresponding entry in pathTo.
+	//
+	// We search breadth-first for two reasons:
+	//
+	// 	1. We want to report the shortest cycle.
+	//
+	// 	2. If p contains multiple cycles, the first cycle we encounter might not
+	// 	   contain target. To ensure termination, we have to break all cycles
+	// 	   other than the first.
+	q := slices.Clip(ptest.Internal.Imports)
+	for len(q) > 0 {
+		p := q[0]
+		q = q[1:]
+		if p == ptest {
+			// The stack is supposed to be in the order x imports y imports z.
+			// We collect in the reverse order: z is imported by y is imported
+			// by x, and then we reverse it.
+			var stk []string
+			for p != nil {
+				stk = append(stk, p.ImportPath)
+				p = importerOf[p]
+			}
+			// complete the cycle: we set importer[p] = nil to break the cycle
+			// in importerOf, it's an implicit importerOf[p] == pTest. Add it
+			// back here since we reached nil in the loop above to demonstrate
+			// the cycle as (for example) package p imports package q imports package r
+			// imports package p.
+			stk = append(stk, ptest.ImportPath)
+			slices.Reverse(stk)
+
+			return &PackageError{
+				ImportStack:   stk,
+				Err:           errors.New("import cycle not allowed in test"),
+				IsImportCycle: true,
+			}
+		}
+		for _, dep := range p.Internal.Imports {
+			if _, ok := importerOf[dep]; !ok {
+				importerOf[dep] = p
+				q = append(q, dep)
+			}
+		}
+	}
+
+	return nil
+}
+
+// isTestFunc tells whether fn has the type of a testing function. arg
+// specifies the parameter type we look for: B, M or T.
+func isTestFunc(fn *ast.FuncDecl, arg string) bool {
+	if fn.Type.Results != nil && len(fn.Type.Results.List) > 0 ||
+		fn.Type.Params.List == nil ||
+		len(fn.Type.Params.List) != 1 ||
+		len(fn.Type.Params.List[0].Names) > 1 {
+		return false
+	}
+	ptr, ok := fn.Type.Params.List[0].Type.(*ast.StarExpr)
+	if !ok {
+		return false
+	}
+	// We can't easily check that the type is *testing.M
+	// because we don't know how testing has been imported,
+	// but at least check that it's *M or *something.M.
+	// Same applies for B and T.
+	if name, ok := ptr.X.(*ast.Ident); ok && name.Name == arg {
+		return true
+	}
+	if sel, ok := ptr.X.(*ast.SelectorExpr); ok && sel.Sel.Name == arg {
+		return true
+	}
+	return false
+}
+
+// isTest tells whether name looks like a test (or benchmark, according to prefix).
+// It is a Test (say) if there is a character after Test that is not a lower-case letter.
+// We don't want TesticularCancer.
+func isTest(name, prefix string) bool {
+	if !strings.HasPrefix(name, prefix) {
+		return false
+	}
+	if len(name) == len(prefix) { // "Test" is ok
+		return true
+	}
+	rune, _ := utf8.DecodeRuneInString(name[len(prefix):])
+	return !unicode.IsLower(rune)
+}
+
+type coverInfo struct {
+	Package *Package
+	Vars    map[string]*CoverVar
+}
+
+// loadTestFuncs returns the testFuncs describing the tests that will be run.
+// The returned testFuncs is always non-nil, even if an error occurred while
+// processing test files.
+func loadTestFuncs(ptest *Package) (*testFuncs, error) {
+	t := &testFuncs{
+		Package: ptest,
+	}
+	var err error
+	for _, file := range ptest.TestGoFiles {
+		if lerr := t.load(filepath.Join(ptest.Dir, file), "_test", &t.ImportTest, &t.NeedTest); lerr != nil && err == nil {
+			err = lerr
+		}
+	}
+	for _, file := range ptest.XTestGoFiles {
+		if lerr := t.load(filepath.Join(ptest.Dir, file), "_xtest", &t.ImportXtest, &t.NeedXtest); lerr != nil && err == nil {
+			err = lerr
+		}
+	}
+	return t, err
+}
+
+// formatTestmain returns the content of the _testmain.go file for t.
+func formatTestmain(t *testFuncs) ([]byte, error) {
+	var buf bytes.Buffer
+	tmpl := testmainTmpl
+	if cfg.Experiment.CoverageRedesign {
+		tmpl = testmainTmplNewCoverage
+	}
+	if err := tmpl.Execute(&buf, t); err != nil {
+		return nil, err
+	}
+	return buf.Bytes(), nil
+}
+
+type testFuncs struct {
+	Tests       []testFunc
+	Benchmarks  []testFunc
+	FuzzTargets []testFunc
+	Examples    []testFunc
+	TestMain    *testFunc
+	Package     *Package
+	ImportTest  bool
+	NeedTest    bool
+	ImportXtest bool
+	NeedXtest   bool
+	Cover       *TestCover
+}
+
+// ImportPath returns the import path of the package being tested, if it is within GOPATH.
+// This is printed by the testing package when running benchmarks.
+func (t *testFuncs) ImportPath() string {
+	pkg := t.Package.ImportPath
+	if strings.HasPrefix(pkg, "_/") {
+		return ""
+	}
+	if pkg == "command-line-arguments" {
+		return ""
+	}
+	return pkg
+}
+
+// Covered returns a string describing which packages are being tested for coverage.
+// If the covered package is the same as the tested package, it returns the empty string.
+// Otherwise it is a comma-separated human-readable list of packages beginning with
+// " in", ready for use in the coverage message.
+func (t *testFuncs) Covered() string {
+	if t.Cover == nil || t.Cover.Paths == nil {
+		return ""
+	}
+	return " in " + strings.Join(t.Cover.Paths, ", ")
+}
+
+// Tested returns the name of the package being tested.
+func (t *testFuncs) Tested() string {
+	return t.Package.Name
+}
+
+type testFunc struct {
+	Package   string // imported package name (_test or _xtest)
+	Name      string // function name
+	Output    string // output, for examples
+	Unordered bool   // output is allowed to be unordered.
+}
+
+var testFileSet = token.NewFileSet()
+
+func (t *testFuncs) load(filename, pkg string, doImport, seen *bool) error {
+	// Pass in the overlaid source if we have an overlay for this file.
+	src, err := fsys.Open(filename)
+	if err != nil {
+		return err
+	}
+	defer src.Close()
+	f, err := parser.ParseFile(testFileSet, filename, src, parser.ParseComments|parser.SkipObjectResolution)
+	if err != nil {
+		return err
+	}
+	for _, d := range f.Decls {
+		n, ok := d.(*ast.FuncDecl)
+		if !ok {
+			continue
+		}
+		if n.Recv != nil {
+			continue
+		}
+		name := n.Name.String()
+		switch {
+		case name == "TestMain":
+			if isTestFunc(n, "T") {
+				t.Tests = append(t.Tests, testFunc{pkg, name, "", false})
+				*doImport, *seen = true, true
+				continue
+			}
+			err := checkTestFunc(n, "M")
+			if err != nil {
+				return err
+			}
+			if t.TestMain != nil {
+				return errors.New("multiple definitions of TestMain")
+			}
+			t.TestMain = &testFunc{pkg, name, "", false}
+			*doImport, *seen = true, true
+		case isTest(name, "Test"):
+			err := checkTestFunc(n, "T")
+			if err != nil {
+				return err
+			}
+			t.Tests = append(t.Tests, testFunc{pkg, name, "", false})
+			*doImport, *seen = true, true
+		case isTest(name, "Benchmark"):
+			err := checkTestFunc(n, "B")
+			if err != nil {
+				return err
+			}
+			t.Benchmarks = append(t.Benchmarks, testFunc{pkg, name, "", false})
+			*doImport, *seen = true, true
+		case isTest(name, "Fuzz"):
+			err := checkTestFunc(n, "F")
+			if err != nil {
+				return err
+			}
+			t.FuzzTargets = append(t.FuzzTargets, testFunc{pkg, name, "", false})
+			*doImport, *seen = true, true
+		}
+	}
+	ex := doc.Examples(f)
+	sort.Slice(ex, func(i, j int) bool { return ex[i].Order < ex[j].Order })
+	for _, e := range ex {
+		*doImport = true // import test file whether executed or not
+		if e.Output == "" && !e.EmptyOutput {
+			// Don't run examples with no output.
+			continue
+		}
+		t.Examples = append(t.Examples, testFunc{pkg, "Example" + e.Name, e.Output, e.Unordered})
+		*seen = true
+	}
+	return nil
+}
+
+func checkTestFunc(fn *ast.FuncDecl, arg string) error {
+	var why string
+	if !isTestFunc(fn, arg) {
+		why = fmt.Sprintf("must be: func %s(%s *testing.%s)", fn.Name.String(), strings.ToLower(arg), arg)
+	}
+	if fn.Type.TypeParams.NumFields() > 0 {
+		why = "test functions cannot have type parameters"
+	}
+	if why != "" {
+		pos := testFileSet.Position(fn.Pos())
+		return fmt.Errorf("%s: wrong signature for %s, %s", pos, fn.Name.String(), why)
+	}
+	return nil
+}
+
+var testmainTmpl = lazytemplate.New("main", `
+// Code generated by 'go test'. DO NOT EDIT.
+
+package main
+
+import (
+	"os"
+{{if .TestMain}}
+	"reflect"
+{{end}}
+	"testing"
+	"testing/internal/testdeps"
+
+{{if .ImportTest}}
+	{{if .NeedTest}}_test{{else}}_{{end}} {{.Package.ImportPath | printf "%q"}}
+{{end}}
+{{if .ImportXtest}}
+	{{if .NeedXtest}}_xtest{{else}}_{{end}} {{.Package.ImportPath | printf "%s_test" | printf "%q"}}
+{{end}}
+{{if .Cover}}
+{{range $i, $p := .Cover.Vars}}
+	_cover{{$i}} {{$p.Package.ImportPath | printf "%q"}}
+{{end}}
+{{end}}
+)
+
+var tests = []testing.InternalTest{
+{{range .Tests}}
+	{"{{.Name}}", {{.Package}}.{{.Name}}},
+{{end}}
+}
+
+var benchmarks = []testing.InternalBenchmark{
+{{range .Benchmarks}}
+	{"{{.Name}}", {{.Package}}.{{.Name}}},
+{{end}}
+}
+
+var fuzzTargets = []testing.InternalFuzzTarget{
+{{range .FuzzTargets}}
+	{"{{.Name}}", {{.Package}}.{{.Name}}},
+{{end}}
+}
+
+var examples = []testing.InternalExample{
+{{range .Examples}}
+	{"{{.Name}}", {{.Package}}.{{.Name}}, {{.Output | printf "%q"}}, {{.Unordered}}},
+{{end}}
+}
+
+func init() {
+	testdeps.ImportPath = {{.ImportPath | printf "%q"}}
+}
+
+{{if .Cover}}
+
+// Only updated by init functions, so no need for atomicity.
+var (
+	coverCounters = make(map[string][]uint32)
+	coverBlocks = make(map[string][]testing.CoverBlock)
+)
+
+func init() {
+	{{range $i, $p := .Cover.Vars}}
+	{{range $file, $cover := $p.Vars}}
+	coverRegisterFile({{printf "%q" $cover.File}}, _cover{{$i}}.{{$cover.Var}}.Count[:], _cover{{$i}}.{{$cover.Var}}.Pos[:], _cover{{$i}}.{{$cover.Var}}.NumStmt[:])
+	{{end}}
+	{{end}}
+}
+
+func coverRegisterFile(fileName string, counter []uint32, pos []uint32, numStmts []uint16) {
+	if 3*len(counter) != len(pos) || len(counter) != len(numStmts) {
+		panic("coverage: mismatched sizes")
+	}
+	if coverCounters[fileName] != nil {
+		// Already registered.
+		return
+	}
+	coverCounters[fileName] = counter
+	block := make([]testing.CoverBlock, len(counter))
+	for i := range counter {
+		block[i] = testing.CoverBlock{
+			Line0: pos[3*i+0],
+			Col0: uint16(pos[3*i+2]),
+			Line1: pos[3*i+1],
+			Col1: uint16(pos[3*i+2]>>16),
+			Stmts: numStmts[i],
+		}
+	}
+	coverBlocks[fileName] = block
+}
+{{end}}
+
+func main() {
+{{if .Cover}}
+	testing.RegisterCover(testing.Cover{
+		Mode: {{printf "%q" .Cover.Mode}},
+		Counters: coverCounters,
+		Blocks: coverBlocks,
+		CoveredPackages: {{printf "%q" .Covered}},
+	})
+{{end}}
+	m := testing.MainStart(testdeps.TestDeps{}, tests, benchmarks, fuzzTargets, examples)
+{{with .TestMain}}
+	{{.Package}}.{{.Name}}(m)
+	os.Exit(int(reflect.ValueOf(m).Elem().FieldByName("exitCode").Int()))
+{{else}}
+	os.Exit(m.Run())
+{{end}}
+}
+
+`)
+
+var testmainTmplNewCoverage = lazytemplate.New("main", `
+// Code generated by 'go test'. DO NOT EDIT.
+
+package main
+
+import (
+	"os"
+{{if .Cover}}
+	_ "unsafe"
+{{end}}
+{{if .TestMain}}
+	"reflect"
+{{end}}
+	"testing"
+	"testing/internal/testdeps"
+
+{{if .ImportTest}}
+	{{if .NeedTest}}_test{{else}}_{{end}} {{.Package.ImportPath | printf "%q"}}
+{{end}}
+{{if .ImportXtest}}
+	{{if .NeedXtest}}_xtest{{else}}_{{end}} {{.Package.ImportPath | printf "%s_test" | printf "%q"}}
+{{end}}
+)
+
+var tests = []testing.InternalTest{
+{{range .Tests}}
+	{"{{.Name}}", {{.Package}}.{{.Name}}},
+{{end}}
+}
+
+var benchmarks = []testing.InternalBenchmark{
+{{range .Benchmarks}}
+	{"{{.Name}}", {{.Package}}.{{.Name}}},
+{{end}}
+}
+
+var fuzzTargets = []testing.InternalFuzzTarget{
+{{range .FuzzTargets}}
+	{"{{.Name}}", {{.Package}}.{{.Name}}},
+{{end}}
+}
+
+var examples = []testing.InternalExample{
+{{range .Examples}}
+	{"{{.Name}}", {{.Package}}.{{.Name}}, {{.Output | printf "%q"}}, {{.Unordered}}},
+{{end}}
+}
+
+func init() {
+	testdeps.ImportPath = {{.ImportPath | printf "%q"}}
+}
+
+{{if .Cover}}
+
+//go:linkname runtime_coverage_processCoverTestDir runtime/coverage.processCoverTestDir
+func runtime_coverage_processCoverTestDir(dir string, cfile string, cmode string, cpkgs string) error
+
+//go:linkname testing_registerCover2 testing.registerCover2
+func testing_registerCover2(mode string, tearDown func(coverprofile string, gocoverdir string) (string, error), snapcov func() float64)
+
+//go:linkname runtime_coverage_markProfileEmitted runtime/coverage.markProfileEmitted
+func runtime_coverage_markProfileEmitted(val bool)
+
+//go:linkname runtime_coverage_snapshot runtime/coverage.snapshot
+func runtime_coverage_snapshot() float64
+
+func coverTearDown(coverprofile string, gocoverdir string) (string, error) {
+	var err error
+	if gocoverdir == "" {
+		gocoverdir, err = os.MkdirTemp("", "gocoverdir")
+		if err != nil {
+			return "error setting GOCOVERDIR: bad os.MkdirTemp return", err
+		}
+		defer os.RemoveAll(gocoverdir)
+	}
+	runtime_coverage_markProfileEmitted(true)
+	cmode := {{printf "%q" .Cover.Mode}}
+	if err := runtime_coverage_processCoverTestDir(gocoverdir, coverprofile, cmode, {{printf "%q" .Covered}}); err != nil {
+		return "error generating coverage report", err
+	}
+	return "", nil
+}
+{{end}}
+
+func main() {
+{{if .Cover}}
+	testing_registerCover2({{printf "%q" .Cover.Mode}}, coverTearDown, runtime_coverage_snapshot)
+{{end}}
+	m := testing.MainStart(testdeps.TestDeps{}, tests, benchmarks, fuzzTargets, examples)
+{{with .TestMain}}
+	{{.Package}}.{{.Name}}(m)
+	os.Exit(int(reflect.ValueOf(m).Elem().FieldByName("exitCode").Int()))
+{{else}}
+	os.Exit(m.Run())
+{{end}}
+}
+
+`)
diff --git a/src/cmd/go/internal/lockedfile/internal/filelock/filelock.go b/src/cmd/go/internal/lockedfile/internal/filelock/filelock.go
new file mode 100644
index 0000000..d373318
--- /dev/null
+++ b/src/cmd/go/internal/lockedfile/internal/filelock/filelock.go
@@ -0,0 +1,83 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package filelock provides a platform-independent API for advisory file
+// locking. Calls to functions in this package on platforms that do not support
+// advisory locks will return errors for which IsNotSupported returns true.
+package filelock
+
+import (
+	"errors"
+	"io/fs"
+)
+
+// A File provides the minimal set of methods required to lock an open file.
+// File implementations must be usable as map keys.
+// The usual implementation is *os.File.
+type File interface {
+	// Name returns the name of the file.
+	Name() string
+
+	// Fd returns a valid file descriptor.
+	// (If the File is an *os.File, it must not be closed.)
+	Fd() uintptr
+
+	// Stat returns the FileInfo structure describing file.
+	Stat() (fs.FileInfo, error)
+}
+
+// Lock places an advisory write lock on the file, blocking until it can be
+// locked.
+//
+// If Lock returns nil, no other process will be able to place a read or write
+// lock on the file until this process exits, closes f, or calls Unlock on it.
+//
+// If f's descriptor is already read- or write-locked, the behavior of Lock is
+// unspecified.
+//
+// Closing the file may or may not release the lock promptly. Callers should
+// ensure that Unlock is always called when Lock succeeds.
+func Lock(f File) error {
+	return lock(f, writeLock)
+}
+
+// RLock places an advisory read lock on the file, blocking until it can be locked.
+//
+// If RLock returns nil, no other process will be able to place a write lock on
+// the file until this process exits, closes f, or calls Unlock on it.
+//
+// If f is already read- or write-locked, the behavior of RLock is unspecified.
+//
+// Closing the file may or may not release the lock promptly. Callers should
+// ensure that Unlock is always called if RLock succeeds.
+func RLock(f File) error {
+	return lock(f, readLock)
+}
+
+// Unlock removes an advisory lock placed on f by this process.
+//
+// The caller must not attempt to unlock a file that is not locked.
+func Unlock(f File) error {
+	return unlock(f)
+}
+
+// String returns the name of the function corresponding to lt
+// (Lock, RLock, or Unlock).
+func (lt lockType) String() string {
+	switch lt {
+	case readLock:
+		return "RLock"
+	case writeLock:
+		return "Lock"
+	default:
+		return "Unlock"
+	}
+}
+
+// IsNotSupported returns a boolean indicating whether the error is known to
+// report that a function is not supported (possibly for a specific input).
+// It is satisfied by errors.ErrUnsupported as well as some syscall errors.
+func IsNotSupported(err error) bool {
+	return errors.Is(err, errors.ErrUnsupported)
+}
diff --git a/src/cmd/go/internal/lockedfile/internal/filelock/filelock_fcntl.go b/src/cmd/go/internal/lockedfile/internal/filelock/filelock_fcntl.go
new file mode 100644
index 0000000..8a62839
--- /dev/null
+++ b/src/cmd/go/internal/lockedfile/internal/filelock/filelock_fcntl.go
@@ -0,0 +1,210 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build aix || (solaris && !illumos)
+
+// This code implements the filelock API using POSIX 'fcntl' locks, which attach
+// to an (inode, process) pair rather than a file descriptor. To avoid unlocking
+// files prematurely when the same file is opened through different descriptors,
+// we allow only one read-lock at a time.
+//
+// Most platforms provide some alternative API, such as an 'flock' system call
+// or an F_OFD_SETLK command for 'fcntl', that allows for better concurrency and
+// does not require per-inode bookkeeping in the application.
+
+package filelock
+
+import (
+	"errors"
+	"io"
+	"io/fs"
+	"math/rand"
+	"sync"
+	"syscall"
+	"time"
+)
+
+type lockType int16
+
+const (
+	readLock  lockType = syscall.F_RDLCK
+	writeLock lockType = syscall.F_WRLCK
+)
+
+type inode = uint64 // type of syscall.Stat_t.Ino
+
+type inodeLock struct {
+	owner File
+	queue []<-chan File
+}
+
+var (
+	mu     sync.Mutex
+	inodes = map[File]inode{}
+	locks  = map[inode]inodeLock{}
+)
+
+func lock(f File, lt lockType) (err error) {
+	// POSIX locks apply per inode and process, and the lock for an inode is
+	// released when *any* descriptor for that inode is closed. So we need to
+	// synchronize access to each inode internally, and must serialize lock and
+	// unlock calls that refer to the same inode through different descriptors.
+	fi, err := f.Stat()
+	if err != nil {
+		return err
+	}
+	ino := fi.Sys().(*syscall.Stat_t).Ino
+
+	mu.Lock()
+	if i, dup := inodes[f]; dup && i != ino {
+		mu.Unlock()
+		return &fs.PathError{
+			Op:   lt.String(),
+			Path: f.Name(),
+			Err:  errors.New("inode for file changed since last Lock or RLock"),
+		}
+	}
+	inodes[f] = ino
+
+	var wait chan File
+	l := locks[ino]
+	if l.owner == f {
+		// This file already owns the lock, but the call may change its lock type.
+	} else if l.owner == nil {
+		// No owner: it's ours now.
+		l.owner = f
+	} else {
+		// Already owned: add a channel to wait on.
+		wait = make(chan File)
+		l.queue = append(l.queue, wait)
+	}
+	locks[ino] = l
+	mu.Unlock()
+
+	if wait != nil {
+		wait <- f
+	}
+
+	// Spurious EDEADLK errors arise on platforms that compute deadlock graphs at
+	// the process, rather than thread, level. Consider processes P and Q, with
+	// threads P.1, P.2, and Q.3. The following trace is NOT a deadlock, but will be
+	// reported as a deadlock on systems that consider only process granularity:
+	//
+	// 	P.1 locks file A.
+	// 	Q.3 locks file B.
+	// 	Q.3 blocks on file A.
+	// 	P.2 blocks on file B. (This is erroneously reported as a deadlock.)
+	// 	P.1 unlocks file A.
+	// 	Q.3 unblocks and locks file A.
+	// 	Q.3 unlocks files A and B.
+	// 	P.2 unblocks and locks file B.
+	// 	P.2 unlocks file B.
+	//
+	// These spurious errors were observed in practice on AIX and Solaris in
+	// cmd/go: see https://golang.org/issue/32817.
+	//
+	// We work around this bug by treating EDEADLK as always spurious. If there
+	// really is a lock-ordering bug between the interacting processes, it will
+	// become a livelock instead, but that's not appreciably worse than if we had
+	// a proper flock implementation (which generally does not even attempt to
+	// diagnose deadlocks).
+	//
+	// In the above example, that changes the trace to:
+	//
+	// 	P.1 locks file A.
+	// 	Q.3 locks file B.
+	// 	Q.3 blocks on file A.
+	// 	P.2 spuriously fails to lock file B and goes to sleep.
+	// 	P.1 unlocks file A.
+	// 	Q.3 unblocks and locks file A.
+	// 	Q.3 unlocks files A and B.
+	// 	P.2 wakes up and locks file B.
+	// 	P.2 unlocks file B.
+	//
+	// We know that the retry loop will not introduce a *spurious* livelock
+	// because, according to the POSIX specification, EDEADLK is only to be
+	// returned when “the lock is blocked by a lock from another process”.
+	// If that process is blocked on some lock that we are holding, then the
+	// resulting livelock is due to a real deadlock (and would manifest as such
+	// when using, for example, the flock implementation of this package).
+	// If the other process is *not* blocked on some other lock that we are
+	// holding, then it will eventually release the requested lock.
+
+	nextSleep := 1 * time.Millisecond
+	const maxSleep = 500 * time.Millisecond
+	for {
+		err = setlkw(f.Fd(), lt)
+		if err != syscall.EDEADLK {
+			break
+		}
+		time.Sleep(nextSleep)
+
+		nextSleep += nextSleep
+		if nextSleep > maxSleep {
+			nextSleep = maxSleep
+		}
+		// Apply 10% jitter to avoid synchronizing collisions when we finally unblock.
+		nextSleep += time.Duration((0.1*rand.Float64() - 0.05) * float64(nextSleep))
+	}
+
+	if err != nil {
+		unlock(f)
+		return &fs.PathError{
+			Op:   lt.String(),
+			Path: f.Name(),
+			Err:  err,
+		}
+	}
+
+	return nil
+}
+
+func unlock(f File) error {
+	var owner File
+
+	mu.Lock()
+	ino, ok := inodes[f]
+	if ok {
+		owner = locks[ino].owner
+	}
+	mu.Unlock()
+
+	if owner != f {
+		panic("unlock called on a file that is not locked")
+	}
+
+	err := setlkw(f.Fd(), syscall.F_UNLCK)
+
+	mu.Lock()
+	l := locks[ino]
+	if len(l.queue) == 0 {
+		// No waiters: remove the map entry.
+		delete(locks, ino)
+	} else {
+		// The first waiter is sending us their file now.
+		// Receive it and update the queue.
+		l.owner = <-l.queue[0]
+		l.queue = l.queue[1:]
+		locks[ino] = l
+	}
+	delete(inodes, f)
+	mu.Unlock()
+
+	return err
+}
+
+// setlkw calls FcntlFlock with F_SETLKW for the entire file indicated by fd.
+func setlkw(fd uintptr, lt lockType) error {
+	for {
+		err := syscall.FcntlFlock(fd, syscall.F_SETLKW, &syscall.Flock_t{
+			Type:   int16(lt),
+			Whence: io.SeekStart,
+			Start:  0,
+			Len:    0, // All bytes.
+		})
+		if err != syscall.EINTR {
+			return err
+		}
+	}
+}
diff --git a/src/cmd/go/internal/lockedfile/internal/filelock/filelock_other.go b/src/cmd/go/internal/lockedfile/internal/filelock/filelock_other.go
new file mode 100644
index 0000000..b16709e
--- /dev/null
+++ b/src/cmd/go/internal/lockedfile/internal/filelock/filelock_other.go
@@ -0,0 +1,35 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !unix && !windows
+
+package filelock
+
+import (
+	"errors"
+	"io/fs"
+)
+
+type lockType int8
+
+const (
+	readLock = iota + 1
+	writeLock
+)
+
+func lock(f File, lt lockType) error {
+	return &fs.PathError{
+		Op:   lt.String(),
+		Path: f.Name(),
+		Err:  errors.ErrUnsupported,
+	}
+}
+
+func unlock(f File) error {
+	return &fs.PathError{
+		Op:   "Unlock",
+		Path: f.Name(),
+		Err:  errors.ErrUnsupported,
+	}
+}
diff --git a/src/cmd/go/internal/lockedfile/internal/filelock/filelock_test.go b/src/cmd/go/internal/lockedfile/internal/filelock/filelock_test.go
new file mode 100644
index 0000000..d32bf06
--- /dev/null
+++ b/src/cmd/go/internal/lockedfile/internal/filelock/filelock_test.go
@@ -0,0 +1,210 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !js && !plan9 && !wasip1
+
+package filelock_test
+
+import (
+	"fmt"
+	"internal/testenv"
+	"os"
+	"path/filepath"
+	"runtime"
+	"testing"
+	"time"
+
+	"cmd/go/internal/lockedfile/internal/filelock"
+)
+
+func lock(t *testing.T, f *os.File) {
+	t.Helper()
+	err := filelock.Lock(f)
+	t.Logf("Lock(fd %d) = %v", f.Fd(), err)
+	if err != nil {
+		t.Fail()
+	}
+}
+
+func rLock(t *testing.T, f *os.File) {
+	t.Helper()
+	err := filelock.RLock(f)
+	t.Logf("RLock(fd %d) = %v", f.Fd(), err)
+	if err != nil {
+		t.Fail()
+	}
+}
+
+func unlock(t *testing.T, f *os.File) {
+	t.Helper()
+	err := filelock.Unlock(f)
+	t.Logf("Unlock(fd %d) = %v", f.Fd(), err)
+	if err != nil {
+		t.Fail()
+	}
+}
+
+func mustTempFile(t *testing.T) (f *os.File, remove func()) {
+	t.Helper()
+
+	base := filepath.Base(t.Name())
+	f, err := os.CreateTemp("", base)
+	if err != nil {
+		t.Fatalf(`os.CreateTemp("", %q) = %v`, base, err)
+	}
+	t.Logf("fd %d = %s", f.Fd(), f.Name())
+
+	return f, func() {
+		f.Close()
+		os.Remove(f.Name())
+	}
+}
+
+func mustOpen(t *testing.T, name string) *os.File {
+	t.Helper()
+
+	f, err := os.OpenFile(name, os.O_RDWR, 0)
+	if err != nil {
+		t.Fatalf("os.Open(%q) = %v", name, err)
+	}
+
+	t.Logf("fd %d = os.Open(%q)", f.Fd(), name)
+	return f
+}
+
+const (
+	quiescent            = 10 * time.Millisecond
+	probablyStillBlocked = 10 * time.Second
+)
+
+func mustBlock(t *testing.T, op string, f *os.File) (wait func(*testing.T)) {
+	t.Helper()
+
+	desc := fmt.Sprintf("%s(fd %d)", op, f.Fd())
+
+	done := make(chan struct{})
+	go func() {
+		t.Helper()
+		switch op {
+		case "Lock":
+			lock(t, f)
+		case "RLock":
+			rLock(t, f)
+		default:
+			panic("invalid op: " + op)
+		}
+		close(done)
+	}()
+
+	select {
+	case <-done:
+		t.Fatalf("%s unexpectedly did not block", desc)
+		return nil
+
+	case <-time.After(quiescent):
+		t.Logf("%s is blocked (as expected)", desc)
+		return func(t *testing.T) {
+			t.Helper()
+			select {
+			case <-time.After(probablyStillBlocked):
+				t.Fatalf("%s is unexpectedly still blocked", desc)
+			case <-done:
+			}
+		}
+	}
+}
+
+func TestLockExcludesLock(t *testing.T) {
+	t.Parallel()
+
+	f, remove := mustTempFile(t)
+	defer remove()
+
+	other := mustOpen(t, f.Name())
+	defer other.Close()
+
+	lock(t, f)
+	lockOther := mustBlock(t, "Lock", other)
+	unlock(t, f)
+	lockOther(t)
+	unlock(t, other)
+}
+
+func TestLockExcludesRLock(t *testing.T) {
+	t.Parallel()
+
+	f, remove := mustTempFile(t)
+	defer remove()
+
+	other := mustOpen(t, f.Name())
+	defer other.Close()
+
+	lock(t, f)
+	rLockOther := mustBlock(t, "RLock", other)
+	unlock(t, f)
+	rLockOther(t)
+	unlock(t, other)
+}
+
+func TestRLockExcludesOnlyLock(t *testing.T) {
+	t.Parallel()
+
+	f, remove := mustTempFile(t)
+	defer remove()
+	rLock(t, f)
+
+	f2 := mustOpen(t, f.Name())
+	defer f2.Close()
+
+	doUnlockTF := false
+	switch runtime.GOOS {
+	case "aix", "solaris":
+		// When using POSIX locks (as on Solaris), we can't safely read-lock the
+		// same inode through two different descriptors at the same time: when the
+		// first descriptor is closed, the second descriptor would still be open but
+		// silently unlocked. So a second RLock must block instead of proceeding.
+		lockF2 := mustBlock(t, "RLock", f2)
+		unlock(t, f)
+		lockF2(t)
+	default:
+		rLock(t, f2)
+		doUnlockTF = true
+	}
+
+	other := mustOpen(t, f.Name())
+	defer other.Close()
+	lockOther := mustBlock(t, "Lock", other)
+
+	unlock(t, f2)
+	if doUnlockTF {
+		unlock(t, f)
+	}
+	lockOther(t)
+	unlock(t, other)
+}
+
+func TestLockNotDroppedByExecCommand(t *testing.T) {
+	testenv.MustHaveExec(t)
+
+	f, remove := mustTempFile(t)
+	defer remove()
+
+	lock(t, f)
+
+	other := mustOpen(t, f.Name())
+	defer other.Close()
+
+	// Some kinds of file locks are dropped when a duplicated or forked file
+	// descriptor is unlocked. Double-check that the approach used by os/exec does
+	// not accidentally drop locks.
+	cmd := testenv.Command(t, os.Args[0], "-test.run=^$")
+	if err := cmd.Run(); err != nil {
+		t.Fatalf("exec failed: %v", err)
+	}
+
+	lockOther := mustBlock(t, "Lock", other)
+	unlock(t, f)
+	lockOther(t)
+	unlock(t, other)
+}
diff --git a/src/cmd/go/internal/lockedfile/internal/filelock/filelock_unix.go b/src/cmd/go/internal/lockedfile/internal/filelock/filelock_unix.go
new file mode 100644
index 0000000..6f73b1b
--- /dev/null
+++ b/src/cmd/go/internal/lockedfile/internal/filelock/filelock_unix.go
@@ -0,0 +1,40 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build darwin || dragonfly || freebsd || illumos || linux || netbsd || openbsd
+
+package filelock
+
+import (
+	"io/fs"
+	"syscall"
+)
+
+type lockType int16
+
+const (
+	readLock  lockType = syscall.LOCK_SH
+	writeLock lockType = syscall.LOCK_EX
+)
+
+func lock(f File, lt lockType) (err error) {
+	for {
+		err = syscall.Flock(int(f.Fd()), int(lt))
+		if err != syscall.EINTR {
+			break
+		}
+	}
+	if err != nil {
+		return &fs.PathError{
+			Op:   lt.String(),
+			Path: f.Name(),
+			Err:  err,
+		}
+	}
+	return nil
+}
+
+func unlock(f File) error {
+	return lock(f, syscall.LOCK_UN)
+}
diff --git a/src/cmd/go/internal/lockedfile/internal/filelock/filelock_windows.go b/src/cmd/go/internal/lockedfile/internal/filelock/filelock_windows.go
new file mode 100644
index 0000000..647ee99
--- /dev/null
+++ b/src/cmd/go/internal/lockedfile/internal/filelock/filelock_windows.go
@@ -0,0 +1,57 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build windows
+
+package filelock
+
+import (
+	"internal/syscall/windows"
+	"io/fs"
+	"syscall"
+)
+
+type lockType uint32
+
+const (
+	readLock  lockType = 0
+	writeLock lockType = windows.LOCKFILE_EXCLUSIVE_LOCK
+)
+
+const (
+	reserved = 0
+	allBytes = ^uint32(0)
+)
+
+func lock(f File, lt lockType) error {
+	// Per https://golang.org/issue/19098, “Programs currently expect the Fd
+	// method to return a handle that uses ordinary synchronous I/O.”
+	// However, LockFileEx still requires an OVERLAPPED structure,
+	// which contains the file offset of the beginning of the lock range.
+	// We want to lock the entire file, so we leave the offset as zero.
+	ol := new(syscall.Overlapped)
+
+	err := windows.LockFileEx(syscall.Handle(f.Fd()), uint32(lt), reserved, allBytes, allBytes, ol)
+	if err != nil {
+		return &fs.PathError{
+			Op:   lt.String(),
+			Path: f.Name(),
+			Err:  err,
+		}
+	}
+	return nil
+}
+
+func unlock(f File) error {
+	ol := new(syscall.Overlapped)
+	err := windows.UnlockFileEx(syscall.Handle(f.Fd()), reserved, allBytes, allBytes, ol)
+	if err != nil {
+		return &fs.PathError{
+			Op:   "Unlock",
+			Path: f.Name(),
+			Err:  err,
+		}
+	}
+	return nil
+}
diff --git a/src/cmd/go/internal/lockedfile/lockedfile.go b/src/cmd/go/internal/lockedfile/lockedfile.go
new file mode 100644
index 0000000..82e1a89
--- /dev/null
+++ b/src/cmd/go/internal/lockedfile/lockedfile.go
@@ -0,0 +1,187 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package lockedfile creates and manipulates files whose contents should only
+// change atomically.
+package lockedfile
+
+import (
+	"fmt"
+	"io"
+	"io/fs"
+	"os"
+	"runtime"
+)
+
+// A File is a locked *os.File.
+//
+// Closing the file releases the lock.
+//
+// If the program exits while a file is locked, the operating system releases
+// the lock but may not do so promptly: callers must ensure that all locked
+// files are closed before exiting.
+type File struct {
+	osFile
+	closed bool
+}
+
+// osFile embeds a *os.File while keeping the pointer itself unexported.
+// (When we close a File, it must be the same file descriptor that we opened!)
+type osFile struct {
+	*os.File
+}
+
+// OpenFile is like os.OpenFile, but returns a locked file.
+// If flag includes os.O_WRONLY or os.O_RDWR, the file is write-locked;
+// otherwise, it is read-locked.
+func OpenFile(name string, flag int, perm fs.FileMode) (*File, error) {
+	var (
+		f   = new(File)
+		err error
+	)
+	f.osFile.File, err = openFile(name, flag, perm)
+	if err != nil {
+		return nil, err
+	}
+
+	// Although the operating system will drop locks for open files when the go
+	// command exits, we want to hold locks for as little time as possible, and we
+	// especially don't want to leave a file locked after we're done with it. Our
+	// Close method is what releases the locks, so use a finalizer to report
+	// missing Close calls on a best-effort basis.
+	runtime.SetFinalizer(f, func(f *File) {
+		panic(fmt.Sprintf("lockedfile.File %s became unreachable without a call to Close", f.Name()))
+	})
+
+	return f, nil
+}
+
+// Open is like os.Open, but returns a read-locked file.
+func Open(name string) (*File, error) {
+	return OpenFile(name, os.O_RDONLY, 0)
+}
+
+// Create is like os.Create, but returns a write-locked file.
+func Create(name string) (*File, error) {
+	return OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0666)
+}
+
+// Edit creates the named file with mode 0666 (before umask),
+// but does not truncate existing contents.
+//
+// If Edit succeeds, methods on the returned File can be used for I/O.
+// The associated file descriptor has mode O_RDWR and the file is write-locked.
+func Edit(name string) (*File, error) {
+	return OpenFile(name, os.O_RDWR|os.O_CREATE, 0666)
+}
+
+// Close unlocks and closes the underlying file.
+//
+// Close may be called multiple times; all calls after the first will return a
+// non-nil error.
+func (f *File) Close() error {
+	if f.closed {
+		return &fs.PathError{
+			Op:   "close",
+			Path: f.Name(),
+			Err:  fs.ErrClosed,
+		}
+	}
+	f.closed = true
+
+	err := closeFile(f.osFile.File)
+	runtime.SetFinalizer(f, nil)
+	return err
+}
+
+// Read opens the named file with a read-lock and returns its contents.
+func Read(name string) ([]byte, error) {
+	f, err := Open(name)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	return io.ReadAll(f)
+}
+
+// Write opens the named file (creating it with the given permissions if needed),
+// then write-locks it and overwrites it with the given content.
+func Write(name string, content io.Reader, perm fs.FileMode) (err error) {
+	f, err := OpenFile(name, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
+	if err != nil {
+		return err
+	}
+
+	_, err = io.Copy(f, content)
+	if closeErr := f.Close(); err == nil {
+		err = closeErr
+	}
+	return err
+}
+
+// Transform invokes t with the result of reading the named file, with its lock
+// still held.
+//
+// If t returns a nil error, Transform then writes the returned contents back to
+// the file, making a best effort to preserve existing contents on error.
+//
+// t must not modify the slice passed to it.
+func Transform(name string, t func([]byte) ([]byte, error)) (err error) {
+	f, err := Edit(name)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	old, err := io.ReadAll(f)
+	if err != nil {
+		return err
+	}
+
+	new, err := t(old)
+	if err != nil {
+		return err
+	}
+
+	if len(new) > len(old) {
+		// The overall file size is increasing, so write the tail first: if we're
+		// about to run out of space on the disk, we would rather detect that
+		// failure before we have overwritten the original contents.
+		if _, err := f.WriteAt(new[len(old):], int64(len(old))); err != nil {
+			// Make a best effort to remove the incomplete tail.
+			f.Truncate(int64(len(old)))
+			return err
+		}
+	}
+
+	// We're about to overwrite the old contents. In case of failure, make a best
+	// effort to roll back before we close the file.
+	defer func() {
+		if err != nil {
+			if _, err := f.WriteAt(old, 0); err == nil {
+				f.Truncate(int64(len(old)))
+			}
+		}
+	}()
+
+	if len(new) >= len(old) {
+		if _, err := f.WriteAt(new[:len(old)], 0); err != nil {
+			return err
+		}
+	} else {
+		if _, err := f.WriteAt(new, 0); err != nil {
+			return err
+		}
+		// The overall file size is decreasing, so shrink the file to its final size
+		// after writing. We do this after writing (instead of before) so that if
+		// the write fails, enough filesystem space will likely still be reserved
+		// to contain the previous contents.
+		if err := f.Truncate(int64(len(new))); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
diff --git a/src/cmd/go/internal/lockedfile/lockedfile_filelock.go b/src/cmd/go/internal/lockedfile/lockedfile_filelock.go
new file mode 100644
index 0000000..1a677a7
--- /dev/null
+++ b/src/cmd/go/internal/lockedfile/lockedfile_filelock.go
@@ -0,0 +1,65 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !plan9
+
+package lockedfile
+
+import (
+	"io/fs"
+	"os"
+
+	"cmd/go/internal/lockedfile/internal/filelock"
+)
+
+func openFile(name string, flag int, perm fs.FileMode) (*os.File, error) {
+	// On BSD systems, we could add the O_SHLOCK or O_EXLOCK flag to the OpenFile
+	// call instead of locking separately, but we have to support separate locking
+	// calls for Linux and Windows anyway, so it's simpler to use that approach
+	// consistently.
+
+	f, err := os.OpenFile(name, flag&^os.O_TRUNC, perm)
+	if err != nil {
+		return nil, err
+	}
+
+	switch flag & (os.O_RDONLY | os.O_WRONLY | os.O_RDWR) {
+	case os.O_WRONLY, os.O_RDWR:
+		err = filelock.Lock(f)
+	default:
+		err = filelock.RLock(f)
+	}
+	if err != nil {
+		f.Close()
+		return nil, err
+	}
+
+	if flag&os.O_TRUNC == os.O_TRUNC {
+		if err := f.Truncate(0); err != nil {
+			// The documentation for os.O_TRUNC says “if possible, truncate file when
+			// opened”, but doesn't define “possible” (golang.org/issue/28699).
+			// We'll treat regular files (and symlinks to regular files) as “possible”
+			// and ignore errors for the rest.
+			if fi, statErr := f.Stat(); statErr != nil || fi.Mode().IsRegular() {
+				filelock.Unlock(f)
+				f.Close()
+				return nil, err
+			}
+		}
+	}
+
+	return f, nil
+}
+
+func closeFile(f *os.File) error {
+	// Since locking syscalls operate on file descriptors, we must unlock the file
+	// while the descriptor is still valid — that is, before the file is closed —
+	// and avoid unlocking files that are already closed.
+	err := filelock.Unlock(f)
+
+	if closeErr := f.Close(); err == nil {
+		err = closeErr
+	}
+	return err
+}
diff --git a/src/cmd/go/internal/lockedfile/lockedfile_plan9.go b/src/cmd/go/internal/lockedfile/lockedfile_plan9.go
new file mode 100644
index 0000000..a2ce794
--- /dev/null
+++ b/src/cmd/go/internal/lockedfile/lockedfile_plan9.go
@@ -0,0 +1,94 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build plan9
+
+package lockedfile
+
+import (
+	"io/fs"
+	"math/rand"
+	"os"
+	"strings"
+	"time"
+)
+
+// Opening an exclusive-use file returns an error.
+// The expected error strings are:
+//
+//   - "open/create -- file is locked" (cwfs, kfs)
+//   - "exclusive lock" (fossil)
+//   - "exclusive use file already open" (ramfs)
+var lockedErrStrings = [...]string{
+	"file is locked",
+	"exclusive lock",
+	"exclusive use file already open",
+}
+
+// Even though plan9 doesn't support the Lock/RLock/Unlock functions to
+// manipulate already-open files, IsLocked is still meaningful: os.OpenFile
+// itself may return errors that indicate that a file with the ModeExclusive bit
+// set is already open.
+func isLocked(err error) bool {
+	s := err.Error()
+
+	for _, frag := range lockedErrStrings {
+		if strings.Contains(s, frag) {
+			return true
+		}
+	}
+
+	return false
+}
+
+func openFile(name string, flag int, perm fs.FileMode) (*os.File, error) {
+	// Plan 9 uses a mode bit instead of explicit lock/unlock syscalls.
+	//
+	// Per http://man.cat-v.org/plan_9/5/stat: “Exclusive use files may be open
+	// for I/O by only one fid at a time across all clients of the server. If a
+	// second open is attempted, it draws an error.”
+	//
+	// So we can try to open a locked file, but if it fails we're on our own to
+	// figure out when it becomes available. We'll use exponential backoff with
+	// some jitter and an arbitrary limit of 500ms.
+
+	// If the file was unpacked or created by some other program, it might not
+	// have the ModeExclusive bit set. Set it before we call OpenFile, so that we
+	// can be confident that a successful OpenFile implies exclusive use.
+	if fi, err := os.Stat(name); err == nil {
+		if fi.Mode()&fs.ModeExclusive == 0 {
+			if err := os.Chmod(name, fi.Mode()|fs.ModeExclusive); err != nil {
+				return nil, err
+			}
+		}
+	} else if !os.IsNotExist(err) {
+		return nil, err
+	}
+
+	nextSleep := 1 * time.Millisecond
+	const maxSleep = 500 * time.Millisecond
+	for {
+		f, err := os.OpenFile(name, flag, perm|fs.ModeExclusive)
+		if err == nil {
+			return f, nil
+		}
+
+		if !isLocked(err) {
+			return nil, err
+		}
+
+		time.Sleep(nextSleep)
+
+		nextSleep += nextSleep
+		if nextSleep > maxSleep {
+			nextSleep = maxSleep
+		}
+		// Apply 10% jitter to avoid synchronizing collisions.
+		nextSleep += time.Duration((0.1*rand.Float64() - 0.05) * float64(nextSleep))
+	}
+}
+
+func closeFile(f *os.File) error {
+	return f.Close()
+}
diff --git a/src/cmd/go/internal/lockedfile/lockedfile_test.go b/src/cmd/go/internal/lockedfile/lockedfile_test.go
new file mode 100644
index 0000000..8dea8f7
--- /dev/null
+++ b/src/cmd/go/internal/lockedfile/lockedfile_test.go
@@ -0,0 +1,286 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// js and wasip1 do not support inter-process file locking.
+//
+//go:build !js && !wasip1
+
+package lockedfile_test
+
+import (
+	"fmt"
+	"internal/testenv"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"cmd/go/internal/lockedfile"
+)
+
+func mustTempDir(t *testing.T) (dir string, remove func()) {
+	t.Helper()
+
+	dir, err := os.MkdirTemp("", filepath.Base(t.Name()))
+	if err != nil {
+		t.Fatal(err)
+	}
+	return dir, func() { os.RemoveAll(dir) }
+}
+
+const (
+	quiescent            = 10 * time.Millisecond
+	probablyStillBlocked = 10 * time.Second
+)
+
+func mustBlock(t *testing.T, desc string, f func()) (wait func(*testing.T)) {
+	t.Helper()
+
+	done := make(chan struct{})
+	go func() {
+		f()
+		close(done)
+	}()
+
+	timer := time.NewTimer(quiescent)
+	defer timer.Stop()
+	select {
+	case <-done:
+		t.Fatalf("%s unexpectedly did not block", desc)
+	case <-timer.C:
+	}
+
+	return func(t *testing.T) {
+		logTimer := time.NewTimer(quiescent)
+		defer logTimer.Stop()
+
+		select {
+		case <-logTimer.C:
+			// We expect the operation to have unblocked by now,
+			// but maybe it's just slow. Write to the test log
+			// in case the test times out, but don't fail it.
+			t.Helper()
+			t.Logf("%s is unexpectedly still blocked after %v", desc, quiescent)
+
+			// Wait for the operation to actually complete, no matter how long it
+			// takes. If the test has deadlocked, this will cause the test to time out
+			// and dump goroutines.
+			<-done
+
+		case <-done:
+		}
+	}
+}
+
+func TestMutexExcludes(t *testing.T) {
+	t.Parallel()
+
+	dir, remove := mustTempDir(t)
+	defer remove()
+
+	path := filepath.Join(dir, "lock")
+
+	mu := lockedfile.MutexAt(path)
+	t.Logf("mu := MutexAt(_)")
+
+	unlock, err := mu.Lock()
+	if err != nil {
+		t.Fatalf("mu.Lock: %v", err)
+	}
+	t.Logf("unlock, _  := mu.Lock()")
+
+	mu2 := lockedfile.MutexAt(mu.Path)
+	t.Logf("mu2 := MutexAt(mu.Path)")
+
+	wait := mustBlock(t, "mu2.Lock()", func() {
+		unlock2, err := mu2.Lock()
+		if err != nil {
+			t.Errorf("mu2.Lock: %v", err)
+			return
+		}
+		t.Logf("unlock2, _ := mu2.Lock()")
+		t.Logf("unlock2()")
+		unlock2()
+	})
+
+	t.Logf("unlock()")
+	unlock()
+	wait(t)
+}
+
+func TestReadWaitsForLock(t *testing.T) {
+	t.Parallel()
+
+	dir, remove := mustTempDir(t)
+	defer remove()
+
+	path := filepath.Join(dir, "timestamp.txt")
+
+	f, err := lockedfile.Create(path)
+	if err != nil {
+		t.Fatalf("Create: %v", err)
+	}
+	defer f.Close()
+
+	const (
+		part1 = "part 1\n"
+		part2 = "part 2\n"
+	)
+	_, err = f.WriteString(part1)
+	if err != nil {
+		t.Fatalf("WriteString: %v", err)
+	}
+	t.Logf("WriteString(%q) = <nil>", part1)
+
+	wait := mustBlock(t, "Read", func() {
+		b, err := lockedfile.Read(path)
+		if err != nil {
+			t.Errorf("Read: %v", err)
+			return
+		}
+
+		const want = part1 + part2
+		got := string(b)
+		if got == want {
+			t.Logf("Read(_) = %q", got)
+		} else {
+			t.Errorf("Read(_) = %q, _; want %q", got, want)
+		}
+	})
+
+	_, err = f.WriteString(part2)
+	if err != nil {
+		t.Errorf("WriteString: %v", err)
+	} else {
+		t.Logf("WriteString(%q) = <nil>", part2)
+	}
+	f.Close()
+
+	wait(t)
+}
+
+func TestCanLockExistingFile(t *testing.T) {
+	t.Parallel()
+
+	dir, remove := mustTempDir(t)
+	defer remove()
+	path := filepath.Join(dir, "existing.txt")
+
+	if err := os.WriteFile(path, []byte("ok"), 0777); err != nil {
+		t.Fatalf("os.WriteFile: %v", err)
+	}
+
+	f, err := lockedfile.Edit(path)
+	if err != nil {
+		t.Fatalf("first Edit: %v", err)
+	}
+
+	wait := mustBlock(t, "Edit", func() {
+		other, err := lockedfile.Edit(path)
+		if err != nil {
+			t.Errorf("second Edit: %v", err)
+		}
+		other.Close()
+	})
+
+	f.Close()
+	wait(t)
+}
+
+// TestSpuriousEDEADLK verifies that the spurious EDEADLK reported in
+// https://golang.org/issue/32817 no longer occurs.
+func TestSpuriousEDEADLK(t *testing.T) {
+	// 	P.1 locks file A.
+	// 	Q.3 locks file B.
+	// 	Q.3 blocks on file A.
+	// 	P.2 blocks on file B. (Spurious EDEADLK occurs here.)
+	// 	P.1 unlocks file A.
+	// 	Q.3 unblocks and locks file A.
+	// 	Q.3 unlocks files A and B.
+	// 	P.2 unblocks and locks file B.
+	// 	P.2 unlocks file B.
+
+	testenv.MustHaveExec(t)
+
+	dirVar := t.Name() + "DIR"
+
+	if dir := os.Getenv(dirVar); dir != "" {
+		// Q.3 locks file B.
+		b, err := lockedfile.Edit(filepath.Join(dir, "B"))
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer b.Close()
+
+		if err := os.WriteFile(filepath.Join(dir, "locked"), []byte("ok"), 0666); err != nil {
+			t.Fatal(err)
+		}
+
+		// Q.3 blocks on file A.
+		a, err := lockedfile.Edit(filepath.Join(dir, "A"))
+		// Q.3 unblocks and locks file A.
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer a.Close()
+
+		// Q.3 unlocks files A and B.
+		return
+	}
+
+	dir, remove := mustTempDir(t)
+	defer remove()
+
+	// P.1 locks file A.
+	a, err := lockedfile.Edit(filepath.Join(dir, "A"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cmd := testenv.Command(t, os.Args[0], "-test.run="+t.Name())
+	cmd.Env = append(os.Environ(), fmt.Sprintf("%s=%s", dirVar, dir))
+
+	qDone := make(chan struct{})
+	waitQ := mustBlock(t, "Edit A and B in subprocess", func() {
+		out, err := cmd.CombinedOutput()
+		if err != nil {
+			t.Errorf("%v:\n%s", err, out)
+		}
+		close(qDone)
+	})
+
+	// Wait until process Q has either failed or locked file B.
+	// Otherwise, P.2 might not block on file B as intended.
+locked:
+	for {
+		if _, err := os.Stat(filepath.Join(dir, "locked")); !os.IsNotExist(err) {
+			break locked
+		}
+		timer := time.NewTimer(1 * time.Millisecond)
+		select {
+		case <-qDone:
+			timer.Stop()
+			break locked
+		case <-timer.C:
+		}
+	}
+
+	waitP2 := mustBlock(t, "Edit B", func() {
+		// P.2 blocks on file B. (Spurious EDEADLK occurs here.)
+		b, err := lockedfile.Edit(filepath.Join(dir, "B"))
+		// P.2 unblocks and locks file B.
+		if err != nil {
+			t.Error(err)
+			return
+		}
+		// P.2 unlocks file B.
+		b.Close()
+	})
+
+	// P.1 unlocks file A.
+	a.Close()
+
+	waitQ(t)
+	waitP2(t)
+}
diff --git a/src/cmd/go/internal/lockedfile/mutex.go b/src/cmd/go/internal/lockedfile/mutex.go
new file mode 100644
index 0000000..180a36c
--- /dev/null
+++ b/src/cmd/go/internal/lockedfile/mutex.go
@@ -0,0 +1,67 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package lockedfile
+
+import (
+	"fmt"
+	"os"
+	"sync"
+)
+
+// A Mutex provides mutual exclusion within and across processes by locking a
+// well-known file. Such a file generally guards some other part of the
+// filesystem: for example, a Mutex file in a directory might guard access to
+// the entire tree rooted in that directory.
+//
+// Mutex does not implement sync.Locker: unlike a sync.Mutex, a lockedfile.Mutex
+// can fail to lock (e.g. if there is a permission error in the filesystem).
+//
+// Like a sync.Mutex, a Mutex may be included as a field of a larger struct but
+// must not be copied after first use. The Path field must be set before first
+// use and must not be change thereafter.
+type Mutex struct {
+	Path string     // The path to the well-known lock file. Must be non-empty.
+	mu   sync.Mutex // A redundant mutex. The race detector doesn't know about file locking, so in tests we may need to lock something that it understands.
+}
+
+// MutexAt returns a new Mutex with Path set to the given non-empty path.
+func MutexAt(path string) *Mutex {
+	if path == "" {
+		panic("lockedfile.MutexAt: path must be non-empty")
+	}
+	return &Mutex{Path: path}
+}
+
+func (mu *Mutex) String() string {
+	return fmt.Sprintf("lockedfile.Mutex(%s)", mu.Path)
+}
+
+// Lock attempts to lock the Mutex.
+//
+// If successful, Lock returns a non-nil unlock function: it is provided as a
+// return-value instead of a separate method to remind the caller to check the
+// accompanying error. (See https://golang.org/issue/20803.)
+func (mu *Mutex) Lock() (unlock func(), err error) {
+	if mu.Path == "" {
+		panic("lockedfile.Mutex: missing Path during Lock")
+	}
+
+	// We could use either O_RDWR or O_WRONLY here. If we choose O_RDWR and the
+	// file at mu.Path is write-only, the call to OpenFile will fail with a
+	// permission error. That's actually what we want: if we add an RLock method
+	// in the future, it should call OpenFile with O_RDONLY and will require the
+	// files must be readable, so we should not let the caller make any
+	// assumptions about Mutex working with write-only files.
+	f, err := OpenFile(mu.Path, os.O_RDWR|os.O_CREATE, 0666)
+	if err != nil {
+		return nil, err
+	}
+	mu.mu.Lock()
+
+	return func() {
+		mu.mu.Unlock()
+		f.Close()
+	}, nil
+}
diff --git a/src/cmd/go/internal/lockedfile/transform_test.go b/src/cmd/go/internal/lockedfile/transform_test.go
new file mode 100644
index 0000000..f8b2802
--- /dev/null
+++ b/src/cmd/go/internal/lockedfile/transform_test.go
@@ -0,0 +1,105 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// js and wasip1 do not support inter-process file locking.
+//
+//go:build !js && !wasip1
+
+package lockedfile_test
+
+import (
+	"bytes"
+	"encoding/binary"
+	"math/rand"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"cmd/go/internal/lockedfile"
+)
+
+func isPowerOf2(x int) bool {
+	return x > 0 && x&(x-1) == 0
+}
+
+func roundDownToPowerOf2(x int) int {
+	if x <= 0 {
+		panic("nonpositive x")
+	}
+	bit := 1
+	for x != bit {
+		x = x &^ bit
+		bit <<= 1
+	}
+	return x
+}
+
+func TestTransform(t *testing.T) {
+	dir, remove := mustTempDir(t)
+	defer remove()
+	path := filepath.Join(dir, "blob.bin")
+
+	const maxChunkWords = 8 << 10
+	buf := make([]byte, 2*maxChunkWords*8)
+	for i := uint64(0); i < 2*maxChunkWords; i++ {
+		binary.LittleEndian.PutUint64(buf[i*8:], i)
+	}
+	if err := lockedfile.Write(path, bytes.NewReader(buf[:8]), 0666); err != nil {
+		t.Fatal(err)
+	}
+
+	var attempts int64 = 128
+	if !testing.Short() {
+		attempts *= 16
+	}
+	const parallel = 32
+
+	var sem = make(chan bool, parallel)
+
+	for n := attempts; n > 0; n-- {
+		sem <- true
+		go func() {
+			defer func() { <-sem }()
+
+			time.Sleep(time.Duration(rand.Intn(100)) * time.Microsecond)
+			chunkWords := roundDownToPowerOf2(rand.Intn(maxChunkWords) + 1)
+			offset := rand.Intn(chunkWords)
+
+			err := lockedfile.Transform(path, func(data []byte) (chunk []byte, err error) {
+				chunk = buf[offset*8 : (offset+chunkWords)*8]
+
+				if len(data)&^7 != len(data) {
+					t.Errorf("read %d bytes, but each write is an integer multiple of 8 bytes", len(data))
+					return chunk, nil
+				}
+
+				words := len(data) / 8
+				if !isPowerOf2(words) {
+					t.Errorf("read %d 8-byte words, but each write is a power-of-2 number of words", words)
+					return chunk, nil
+				}
+
+				u := binary.LittleEndian.Uint64(data)
+				for i := 1; i < words; i++ {
+					next := binary.LittleEndian.Uint64(data[i*8:])
+					if next != u+1 {
+						t.Errorf("wrote sequential integers, but read integer out of sequence at offset %d", i)
+						return chunk, nil
+					}
+					u = next
+				}
+
+				return chunk, nil
+			})
+
+			if err != nil {
+				t.Errorf("unexpected error from Transform: %v", err)
+			}
+		}()
+	}
+
+	for n := parallel; n > 0; n-- {
+		sem <- true
+	}
+}
diff --git a/src/cmd/go/internal/mmap/mmap.go b/src/cmd/go/internal/mmap/mmap.go
new file mode 100644
index 0000000..0cad9ca
--- /dev/null
+++ b/src/cmd/go/internal/mmap/mmap.go
@@ -0,0 +1,31 @@
+// Copyright 2011 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This package is a lightly modified version of the mmap code
+// in github.com/google/codesearch/index.
+
+// The mmap package provides an abstraction for memory mapping files
+// on different platforms.
+package mmap
+
+import (
+	"os"
+)
+
+// Data is mmap'ed read-only data from a file.
+// The backing file is never closed, so Data
+// remains valid for the lifetime of the process.
+type Data struct {
+	f    *os.File
+	Data []byte
+}
+
+// Mmap maps the given file into memory.
+func Mmap(file string) (Data, error) {
+	f, err := os.Open(file)
+	if err != nil {
+		return Data{}, err
+	}
+	return mmapFile(f)
+}
diff --git a/src/cmd/go/internal/mmap/mmap_other.go b/src/cmd/go/internal/mmap/mmap_other.go
new file mode 100644
index 0000000..22e9395
--- /dev/null
+++ b/src/cmd/go/internal/mmap/mmap_other.go
@@ -0,0 +1,21 @@
+// Copyright 2022 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build (js && wasm) || wasip1 || plan9
+
+package mmap
+
+import (
+	"io"
+	"os"
+)
+
+// mmapFile on other systems doesn't mmap the file. It just reads everything.
+func mmapFile(f *os.File) (Data, error) {
+	b, err := io.ReadAll(f)
+	if err != nil {
+		return Data{}, err
+	}
+	return Data{f, b}, nil
+}
diff --git a/src/cmd/go/internal/mmap/mmap_unix.go b/src/cmd/go/internal/mmap/mmap_unix.go
new file mode 100644
index 0000000..53bcbb9
--- /dev/null
+++ b/src/cmd/go/internal/mmap/mmap_unix.go
@@ -0,0 +1,36 @@
+// Copyright 2011 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix
+
+package mmap
+
+import (
+	"fmt"
+	"io/fs"
+	"os"
+	"syscall"
+)
+
+func mmapFile(f *os.File) (Data, error) {
+	st, err := f.Stat()
+	if err != nil {
+		return Data{}, err
+	}
+	size := st.Size()
+	pagesize := int64(os.Getpagesize())
+	if int64(int(size+(pagesize-1))) != size+(pagesize-1) {
+		return Data{}, fmt.Errorf("%s: too large for mmap", f.Name())
+	}
+	n := int(size)
+	if n == 0 {
+		return Data{f, nil}, nil
+	}
+	mmapLength := int(((size + pagesize - 1) / pagesize) * pagesize) // round up to page size
+	data, err := syscall.Mmap(int(f.Fd()), 0, mmapLength, syscall.PROT_READ, syscall.MAP_SHARED)
+	if err != nil {
+		return Data{}, &fs.PathError{Op: "mmap", Path: f.Name(), Err: err}
+	}
+	return Data{f, data[:n]}, nil
+}
diff --git a/src/cmd/go/internal/mmap/mmap_windows.go b/src/cmd/go/internal/mmap/mmap_windows.go
new file mode 100644
index 0000000..1cf62fe
--- /dev/null
+++ b/src/cmd/go/internal/mmap/mmap_windows.go
@@ -0,0 +1,41 @@
+// Copyright 2011 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package mmap
+
+import (
+	"fmt"
+	"os"
+	"syscall"
+	"unsafe"
+
+	"internal/syscall/windows"
+)
+
+func mmapFile(f *os.File) (Data, error) {
+	st, err := f.Stat()
+	if err != nil {
+		return Data{}, err
+	}
+	size := st.Size()
+	if size == 0 {
+		return Data{f, nil}, nil
+	}
+	h, err := syscall.CreateFileMapping(syscall.Handle(f.Fd()), nil, syscall.PAGE_READONLY, 0, 0, nil)
+	if err != nil {
+		return Data{}, fmt.Errorf("CreateFileMapping %s: %w", f.Name(), err)
+	}
+
+	addr, err := syscall.MapViewOfFile(h, syscall.FILE_MAP_READ, 0, 0, 0)
+	if err != nil {
+		return Data{}, fmt.Errorf("MapViewOfFile %s: %w", f.Name(), err)
+	}
+	var info windows.MemoryBasicInformation
+	err = windows.VirtualQuery(addr, &info, unsafe.Sizeof(info))
+	if err != nil {
+		return Data{}, fmt.Errorf("VirtualQuery %s: %w", f.Name(), err)
+	}
+	data := unsafe.Slice((*byte)(unsafe.Pointer(addr)), int(info.RegionSize))
+	return Data{f, data}, nil
+}
diff --git a/src/cmd/go/internal/modcmd/download.go b/src/cmd/go/internal/modcmd/download.go
new file mode 100644
index 0000000..373acce
--- /dev/null
+++ b/src/cmd/go/internal/modcmd/download.go
@@ -0,0 +1,389 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modcmd
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"os"
+	"runtime"
+	"sync"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/modfetch/codehost"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/toolchain"
+
+	"golang.org/x/mod/module"
+)
+
+var cmdDownload = &base.Command{
+	UsageLine: "go mod download [-x] [-json] [-reuse=old.json] [modules]",
+	Short:     "download modules to local cache",
+	Long: `
+Download downloads the named modules, which can be module patterns selecting
+dependencies of the main module or module queries of the form path@version.
+
+With no arguments, download applies to the modules needed to build and test
+the packages in the main module: the modules explicitly required by the main
+module if it is at 'go 1.17' or higher, or all transitively-required modules
+if at 'go 1.16' or lower.
+
+The go command will automatically download modules as needed during ordinary
+execution. The "go mod download" command is useful mainly for pre-filling
+the local cache or to compute the answers for a Go module proxy.
+
+By default, download writes nothing to standard output. It may print progress
+messages and errors to standard error.
+
+The -json flag causes download to print a sequence of JSON objects
+to standard output, describing each downloaded module (or failure),
+corresponding to this Go struct:
+
+    type Module struct {
+        Path     string // module path
+        Query    string // version query corresponding to this version
+        Version  string // module version
+        Error    string // error loading module
+        Info     string // absolute path to cached .info file
+        GoMod    string // absolute path to cached .mod file
+        Zip      string // absolute path to cached .zip file
+        Dir      string // absolute path to cached source root directory
+        Sum      string // checksum for path, version (as in go.sum)
+        GoModSum string // checksum for go.mod (as in go.sum)
+        Origin   any    // provenance of module
+        Reuse    bool   // reuse of old module info is safe
+    }
+
+The -reuse flag accepts the name of file containing the JSON output of a
+previous 'go mod download -json' invocation. The go command may use this
+file to determine that a module is unchanged since the previous invocation
+and avoid redownloading it. Modules that are not redownloaded will be marked
+in the new output by setting the Reuse field to true. Normally the module
+cache provides this kind of reuse automatically; the -reuse flag can be
+useful on systems that do not preserve the module cache.
+
+The -x flag causes download to print the commands download executes.
+
+See https://golang.org/ref/mod#go-mod-download for more about 'go mod download'.
+
+See https://golang.org/ref/mod#version-queries for more about version queries.
+	`,
+}
+
+var (
+	downloadJSON  = cmdDownload.Flag.Bool("json", false, "")
+	downloadReuse = cmdDownload.Flag.String("reuse", "", "")
+)
+
+func init() {
+	cmdDownload.Run = runDownload // break init cycle
+
+	// TODO(jayconrod): https://golang.org/issue/35849 Apply -x to other 'go mod' commands.
+	cmdDownload.Flag.BoolVar(&cfg.BuildX, "x", false, "")
+	base.AddChdirFlag(&cmdDownload.Flag)
+	base.AddModCommonFlags(&cmdDownload.Flag)
+}
+
+// A ModuleJSON describes the result of go mod download.
+type ModuleJSON struct {
+	Path     string `json:",omitempty"`
+	Version  string `json:",omitempty"`
+	Query    string `json:",omitempty"`
+	Error    string `json:",omitempty"`
+	Info     string `json:",omitempty"`
+	GoMod    string `json:",omitempty"`
+	Zip      string `json:",omitempty"`
+	Dir      string `json:",omitempty"`
+	Sum      string `json:",omitempty"`
+	GoModSum string `json:",omitempty"`
+
+	Origin *codehost.Origin `json:",omitempty"`
+	Reuse  bool             `json:",omitempty"`
+}
+
+func runDownload(ctx context.Context, cmd *base.Command, args []string) {
+	modload.InitWorkfile()
+
+	// Check whether modules are enabled and whether we're in a module.
+	modload.ForceUseModules = true
+	modload.ExplicitWriteGoMod = true
+	haveExplicitArgs := len(args) > 0
+
+	if modload.HasModRoot() || modload.WorkFilePath() != "" {
+		modload.LoadModFile(ctx) // to fill MainModules
+
+		if haveExplicitArgs {
+			for _, mainModule := range modload.MainModules.Versions() {
+				targetAtUpgrade := mainModule.Path + "@upgrade"
+				targetAtPatch := mainModule.Path + "@patch"
+				for _, arg := range args {
+					switch arg {
+					case mainModule.Path, targetAtUpgrade, targetAtPatch:
+						os.Stderr.WriteString("go: skipping download of " + arg + " that resolves to the main module\n")
+					}
+				}
+			}
+		} else if modload.WorkFilePath() != "" {
+			// TODO(#44435): Think about what the correct query is to download the
+			// right set of modules. Also see code review comment at
+			// https://go-review.googlesource.com/c/go/+/359794/comments/ce946a80_6cf53992.
+			args = []string{"all"}
+		} else {
+			mainModule := modload.MainModules.Versions()[0]
+			modFile := modload.MainModules.ModFile(mainModule)
+			if modFile.Go == nil || gover.Compare(modFile.Go.Version, gover.ExplicitIndirectVersion) < 0 {
+				if len(modFile.Require) > 0 {
+					args = []string{"all"}
+				}
+			} else {
+				// As of Go 1.17, the go.mod file explicitly requires every module
+				// that provides any package imported by the main module.
+				// 'go mod download' is typically run before testing packages in the
+				// main module, so by default we shouldn't download the others
+				// (which are presumed irrelevant to the packages in the main module).
+				// See https://golang.org/issue/44435.
+				//
+				// However, we also need to load the full module graph, to ensure that
+				// we have downloaded enough of the module graph to run 'go list all',
+				// 'go mod graph', and similar commands.
+				_, err := modload.LoadModGraph(ctx, "")
+				if err != nil {
+					// TODO(#64008): call base.Fatalf instead of toolchain.SwitchOrFatal
+					// here, since we can only reach this point with an outdated toolchain
+					// if the go.mod file is inconsistent.
+					toolchain.SwitchOrFatal(ctx, err)
+				}
+
+				for _, m := range modFile.Require {
+					args = append(args, m.Mod.Path)
+				}
+			}
+		}
+	}
+
+	if len(args) == 0 {
+		if modload.HasModRoot() {
+			os.Stderr.WriteString("go: no module dependencies to download\n")
+		} else {
+			base.Errorf("go: no modules specified (see 'go help mod download')")
+		}
+		base.Exit()
+	}
+
+	if *downloadReuse != "" && modload.HasModRoot() {
+		base.Fatalf("go mod download -reuse cannot be used inside a module")
+	}
+
+	var mods []*ModuleJSON
+	type token struct{}
+	sem := make(chan token, runtime.GOMAXPROCS(0))
+	infos, infosErr := modload.ListModules(ctx, args, 0, *downloadReuse)
+
+	// There is a bit of a chicken-and-egg problem here: ideally we need to know
+	// which Go version to switch to to download the requested modules, but if we
+	// haven't downloaded the module's go.mod file yet the GoVersion field of its
+	// info struct is not yet populated.
+	//
+	// We also need to be careful to only print the info for each module once
+	// if the -json flag is set.
+	//
+	// In theory we could go through each module in the list, attempt to download
+	// its go.mod file, and record the maximum version (either from the file or
+	// from the resulting TooNewError), all before we try the actual full download
+	// of each module.
+	//
+	// For now, we go ahead and try all the downloads and collect the errors, and
+	// if any download failed due to a TooNewError, we switch toolchains and try
+	// again. Any downloads that already succeeded will still be in cache.
+	// That won't give optimal concurrency (we'll do two batches of concurrent
+	// downloads instead of all in one batch), and it might add a little overhead
+	// to look up the downloads from the first batch in the module cache when
+	// we see them again in the second batch. On the other hand, it's way simpler
+	// to implement, and not really any more expensive if the user is requesting
+	// no explicit arguments (their go.mod file should already list an appropriate
+	// toolchain version) or only one module (as is used by the Go Module Proxy).
+
+	if infosErr != nil {
+		var sw toolchain.Switcher
+		sw.Error(infosErr)
+		if sw.NeedSwitch() {
+			sw.Switch(ctx)
+		}
+		// Otherwise, wait to report infosErr after we have downloaded
+		// when we can.
+	}
+
+	if !haveExplicitArgs && modload.WorkFilePath() == "" {
+		// 'go mod download' is sometimes run without arguments to pre-populate the
+		// module cache. In modules that aren't at go 1.17 or higher, it may fetch
+		// modules that aren't needed to build packages in the main module. This is
+		// usually not intended, so don't save sums for downloaded modules
+		// (golang.org/issue/45332). We do still fix inconsistencies in go.mod
+		// though.
+		//
+		// TODO(#64008): In the future, report an error if go.mod or go.sum need to
+		// be updated after loading the build list. This may require setting
+		// the mode to "mod" or "readonly" depending on haveExplicitArgs.
+		if err := modload.WriteGoMod(ctx, modload.WriteOpts{}); err != nil {
+			base.Fatal(err)
+		}
+	}
+
+	var downloadErrs sync.Map
+	for _, info := range infos {
+		if info.Replace != nil {
+			info = info.Replace
+		}
+		if info.Version == "" && info.Error == nil {
+			// main module or module replaced with file path.
+			// Nothing to download.
+			continue
+		}
+		m := &ModuleJSON{
+			Path:    info.Path,
+			Version: info.Version,
+			Query:   info.Query,
+			Reuse:   info.Reuse,
+			Origin:  info.Origin,
+		}
+		mods = append(mods, m)
+		if info.Error != nil {
+			m.Error = info.Error.Err
+			continue
+		}
+		if m.Reuse {
+			continue
+		}
+		sem <- token{}
+		go func() {
+			err := DownloadModule(ctx, m)
+			if err != nil {
+				downloadErrs.Store(m, err)
+				m.Error = err.Error()
+			}
+			<-sem
+		}()
+	}
+
+	// Fill semaphore channel to wait for goroutines to finish.
+	for n := cap(sem); n > 0; n-- {
+		sem <- token{}
+	}
+
+	// If there were explicit arguments
+	// (like 'go mod download golang.org/x/tools@latest'),
+	// check whether we need to upgrade the toolchain in order to download them.
+	//
+	// (If invoked without arguments, we expect the module graph to already
+	// be tidy and the go.mod file to declare a 'go' version that satisfies
+	// transitive requirements. If that invariant holds, then we should have
+	// already upgraded when we loaded the module graph, and should not need
+	// an additional check here. See https://go.dev/issue/45551.)
+	//
+	// We also allow upgrades if in a workspace because in workspace mode
+	// with no arguments we download the module pattern "all",
+	// which may include dependencies that are normally pruned out
+	// of the individual modules in the workspace.
+	if haveExplicitArgs || modload.WorkFilePath() != "" {
+		var sw toolchain.Switcher
+		// Add errors to the Switcher in deterministic order so that they will be
+		// logged deterministically.
+		for _, m := range mods {
+			if erri, ok := downloadErrs.Load(m); ok {
+				sw.Error(erri.(error))
+			}
+		}
+		// Only call sw.Switch if it will actually switch.
+		// Otherwise, we may want to write the errors as JSON
+		// (instead of using base.Error as sw.Switch would),
+		// and we may also have other errors to report from the
+		// initial infos returned by ListModules.
+		if sw.NeedSwitch() {
+			sw.Switch(ctx)
+		}
+	}
+
+	if *downloadJSON {
+		for _, m := range mods {
+			b, err := json.MarshalIndent(m, "", "\t")
+			if err != nil {
+				base.Fatal(err)
+			}
+			os.Stdout.Write(append(b, '\n'))
+			if m.Error != "" {
+				base.SetExitStatus(1)
+			}
+		}
+	} else {
+		for _, m := range mods {
+			if m.Error != "" {
+				base.Error(errors.New(m.Error))
+			}
+		}
+		base.ExitIfErrors()
+	}
+
+	// If there were explicit arguments, update go.mod and especially go.sum.
+	// 'go mod download mod@version' is a useful way to add a sum without using
+	// 'go get mod@version', which may have other side effects. We print this in
+	// some error message hints.
+	//
+	// If we're in workspace mode, update go.work.sum with checksums for all of
+	// the modules we downloaded that aren't already recorded. Since a requirement
+	// in one module may upgrade a dependency of another, we can't be sure that
+	// the import graph matches the import graph of any given module in isolation,
+	// so we may end up needing to load packages from modules that wouldn't
+	// otherwise be relevant.
+	//
+	// TODO(#44435): If we adjust the set of modules downloaded in workspace mode,
+	// we may also need to adjust the logic for saving checksums here.
+	//
+	// Don't save sums for 'go mod download' without arguments unless we're in
+	// workspace mode; see comment above.
+	if haveExplicitArgs || modload.WorkFilePath() != "" {
+		if err := modload.WriteGoMod(ctx, modload.WriteOpts{}); err != nil {
+			base.Error(err)
+		}
+	}
+
+	// If there was an error matching some of the requested packages, emit it now
+	// (after we've written the checksums for the modules that were downloaded
+	// successfully).
+	if infosErr != nil {
+		base.Error(infosErr)
+	}
+}
+
+// DownloadModule runs 'go mod download' for m.Path@m.Version,
+// leaving the results (including any error) in m itself.
+func DownloadModule(ctx context.Context, m *ModuleJSON) error {
+	var err error
+	_, file, err := modfetch.InfoFile(ctx, m.Path, m.Version)
+	if err != nil {
+		return err
+	}
+	m.Info = file
+	m.GoMod, err = modfetch.GoModFile(ctx, m.Path, m.Version)
+	if err != nil {
+		return err
+	}
+	m.GoModSum, err = modfetch.GoModSum(ctx, m.Path, m.Version)
+	if err != nil {
+		return err
+	}
+	mod := module.Version{Path: m.Path, Version: m.Version}
+	m.Zip, err = modfetch.DownloadZip(ctx, mod)
+	if err != nil {
+		return err
+	}
+	m.Sum = modfetch.Sum(ctx, mod)
+	m.Dir, err = modfetch.Download(ctx, mod)
+	return err
+}
diff --git a/src/cmd/go/internal/modcmd/edit.go b/src/cmd/go/internal/modcmd/edit.go
new file mode 100644
index 0000000..96bd608
--- /dev/null
+++ b/src/cmd/go/internal/modcmd/edit.go
@@ -0,0 +1,545 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// go mod edit
+
+package modcmd
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"os"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/lockedfile"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/modload"
+
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/module"
+)
+
+var cmdEdit = &base.Command{
+	UsageLine: "go mod edit [editing flags] [-fmt|-print|-json] [go.mod]",
+	Short:     "edit go.mod from tools or scripts",
+	Long: `
+Edit provides a command-line interface for editing go.mod,
+for use primarily by tools or scripts. It reads only go.mod;
+it does not look up information about the modules involved.
+By default, edit reads and writes the go.mod file of the main module,
+but a different target file can be specified after the editing flags.
+
+The editing flags specify a sequence of editing operations.
+
+The -fmt flag reformats the go.mod file without making other changes.
+This reformatting is also implied by any other modifications that use or
+rewrite the go.mod file. The only time this flag is needed is if no other
+flags are specified, as in 'go mod edit -fmt'.
+
+The -module flag changes the module's path (the go.mod file's module line).
+
+The -require=path@version and -droprequire=path flags
+add and drop a requirement on the given module path and version.
+Note that -require overrides any existing requirements on path.
+These flags are mainly for tools that understand the module graph.
+Users should prefer 'go get path@version' or 'go get path@none',
+which make other go.mod adjustments as needed to satisfy
+constraints imposed by other modules.
+
+The -exclude=path@version and -dropexclude=path@version flags
+add and drop an exclusion for the given module path and version.
+Note that -exclude=path@version is a no-op if that exclusion already exists.
+
+The -replace=old[@v]=new[@v] flag adds a replacement of the given
+module path and version pair. If the @v in old@v is omitted, a
+replacement without a version on the left side is added, which applies
+to all versions of the old module path. If the @v in new@v is omitted,
+the new path should be a local module root directory, not a module
+path. Note that -replace overrides any redundant replacements for old[@v],
+so omitting @v will drop existing replacements for specific versions.
+
+The -dropreplace=old[@v] flag drops a replacement of the given
+module path and version pair. If the @v is omitted, a replacement without
+a version on the left side is dropped.
+
+The -retract=version and -dropretract=version flags add and drop a
+retraction on the given version. The version may be a single version
+like "v1.2.3" or a closed interval like "[v1.1.0,v1.1.9]". Note that
+-retract=version is a no-op if that retraction already exists.
+
+The -require, -droprequire, -exclude, -dropexclude, -replace,
+-dropreplace, -retract, and -dropretract editing flags may be repeated,
+and the changes are applied in the order given.
+
+The -go=version flag sets the expected Go language version.
+
+The -toolchain=name flag sets the Go toolchain to use.
+
+The -print flag prints the final go.mod in its text format instead of
+writing it back to go.mod.
+
+The -json flag prints the final go.mod file in JSON format instead of
+writing it back to go.mod. The JSON output corresponds to these Go types:
+
+	type Module struct {
+		Path    string
+		Version string
+	}
+
+	type GoMod struct {
+		Module    ModPath
+		Go        string
+		Toolchain string
+		Require   []Require
+		Exclude   []Module
+		Replace   []Replace
+		Retract   []Retract
+	}
+
+	type ModPath struct {
+		Path       string
+		Deprecated string
+	}
+
+	type Require struct {
+		Path string
+		Version string
+		Indirect bool
+	}
+
+	type Replace struct {
+		Old Module
+		New Module
+	}
+
+	type Retract struct {
+		Low       string
+		High      string
+		Rationale string
+	}
+
+Retract entries representing a single version (not an interval) will have
+the "Low" and "High" fields set to the same value.
+
+Note that this only describes the go.mod file itself, not other modules
+referred to indirectly. For the full set of modules available to a build,
+use 'go list -m -json all'.
+
+Edit also provides the -C, -n, and -x build flags.
+
+See https://golang.org/ref/mod#go-mod-edit for more about 'go mod edit'.
+	`,
+}
+
+var (
+	editFmt       = cmdEdit.Flag.Bool("fmt", false, "")
+	editGo        = cmdEdit.Flag.String("go", "", "")
+	editToolchain = cmdEdit.Flag.String("toolchain", "", "")
+	editJSON      = cmdEdit.Flag.Bool("json", false, "")
+	editPrint     = cmdEdit.Flag.Bool("print", false, "")
+	editModule    = cmdEdit.Flag.String("module", "", "")
+	edits         []func(*modfile.File) // edits specified in flags
+)
+
+type flagFunc func(string)
+
+func (f flagFunc) String() string     { return "" }
+func (f flagFunc) Set(s string) error { f(s); return nil }
+
+func init() {
+	cmdEdit.Run = runEdit // break init cycle
+
+	cmdEdit.Flag.Var(flagFunc(flagRequire), "require", "")
+	cmdEdit.Flag.Var(flagFunc(flagDropRequire), "droprequire", "")
+	cmdEdit.Flag.Var(flagFunc(flagExclude), "exclude", "")
+	cmdEdit.Flag.Var(flagFunc(flagDropReplace), "dropreplace", "")
+	cmdEdit.Flag.Var(flagFunc(flagReplace), "replace", "")
+	cmdEdit.Flag.Var(flagFunc(flagDropExclude), "dropexclude", "")
+	cmdEdit.Flag.Var(flagFunc(flagRetract), "retract", "")
+	cmdEdit.Flag.Var(flagFunc(flagDropRetract), "dropretract", "")
+
+	base.AddBuildFlagsNX(&cmdEdit.Flag)
+	base.AddChdirFlag(&cmdEdit.Flag)
+	base.AddModCommonFlags(&cmdEdit.Flag)
+}
+
+func runEdit(ctx context.Context, cmd *base.Command, args []string) {
+	anyFlags := *editModule != "" ||
+		*editGo != "" ||
+		*editToolchain != "" ||
+		*editJSON ||
+		*editPrint ||
+		*editFmt ||
+		len(edits) > 0
+
+	if !anyFlags {
+		base.Fatalf("go: no flags specified (see 'go help mod edit').")
+	}
+
+	if *editJSON && *editPrint {
+		base.Fatalf("go: cannot use both -json and -print")
+	}
+
+	if len(args) > 1 {
+		base.Fatalf("go: too many arguments")
+	}
+	var gomod string
+	if len(args) == 1 {
+		gomod = args[0]
+	} else {
+		gomod = modload.ModFilePath()
+	}
+
+	if *editModule != "" {
+		if err := module.CheckImportPath(*editModule); err != nil {
+			base.Fatalf("go: invalid -module: %v", err)
+		}
+	}
+
+	if *editGo != "" && *editGo != "none" {
+		if !modfile.GoVersionRE.MatchString(*editGo) {
+			base.Fatalf(`go mod: invalid -go option; expecting something like "-go %s"`, gover.Local())
+		}
+	}
+	if *editToolchain != "" && *editToolchain != "none" {
+		if !modfile.ToolchainRE.MatchString(*editToolchain) {
+			base.Fatalf(`go mod: invalid -toolchain option; expecting something like "-toolchain go%s"`, gover.Local())
+		}
+	}
+
+	data, err := lockedfile.Read(gomod)
+	if err != nil {
+		base.Fatal(err)
+	}
+
+	modFile, err := modfile.Parse(gomod, data, nil)
+	if err != nil {
+		base.Fatalf("go: errors parsing %s:\n%s", base.ShortPath(gomod), err)
+	}
+
+	if *editModule != "" {
+		modFile.AddModuleStmt(*editModule)
+	}
+
+	if *editGo == "none" {
+		modFile.DropGoStmt()
+	} else if *editGo != "" {
+		if err := modFile.AddGoStmt(*editGo); err != nil {
+			base.Fatalf("go: internal error: %v", err)
+		}
+	}
+	if *editToolchain == "none" {
+		modFile.DropToolchainStmt()
+	} else if *editToolchain != "" {
+		if err := modFile.AddToolchainStmt(*editToolchain); err != nil {
+			base.Fatalf("go: internal error: %v", err)
+		}
+	}
+
+	if len(edits) > 0 {
+		for _, edit := range edits {
+			edit(modFile)
+		}
+	}
+	modFile.SortBlocks()
+	modFile.Cleanup() // clean file after edits
+
+	if *editJSON {
+		editPrintJSON(modFile)
+		return
+	}
+
+	out, err := modFile.Format()
+	if err != nil {
+		base.Fatal(err)
+	}
+
+	if *editPrint {
+		os.Stdout.Write(out)
+		return
+	}
+
+	// Make a best-effort attempt to acquire the side lock, only to exclude
+	// previous versions of the 'go' command from making simultaneous edits.
+	if unlock, err := modfetch.SideLock(ctx); err == nil {
+		defer unlock()
+	}
+
+	err = lockedfile.Transform(gomod, func(lockedData []byte) ([]byte, error) {
+		if !bytes.Equal(lockedData, data) {
+			return nil, errors.New("go.mod changed during editing; not overwriting")
+		}
+		return out, nil
+	})
+	if err != nil {
+		base.Fatal(err)
+	}
+}
+
+// parsePathVersion parses -flag=arg expecting arg to be path@version.
+func parsePathVersion(flag, arg string) (path, version string) {
+	before, after, found := strings.Cut(arg, "@")
+	if !found {
+		base.Fatalf("go: -%s=%s: need path@version", flag, arg)
+	}
+	path, version = strings.TrimSpace(before), strings.TrimSpace(after)
+	if err := module.CheckImportPath(path); err != nil {
+		base.Fatalf("go: -%s=%s: invalid path: %v", flag, arg, err)
+	}
+
+	if !allowedVersionArg(version) {
+		base.Fatalf("go: -%s=%s: invalid version %q", flag, arg, version)
+	}
+
+	return path, version
+}
+
+// parsePath parses -flag=arg expecting arg to be path (not path@version).
+func parsePath(flag, arg string) (path string) {
+	if strings.Contains(arg, "@") {
+		base.Fatalf("go: -%s=%s: need just path, not path@version", flag, arg)
+	}
+	path = arg
+	if err := module.CheckImportPath(path); err != nil {
+		base.Fatalf("go: -%s=%s: invalid path: %v", flag, arg, err)
+	}
+	return path
+}
+
+// parsePathVersionOptional parses path[@version], using adj to
+// describe any errors.
+func parsePathVersionOptional(adj, arg string, allowDirPath bool) (path, version string, err error) {
+	before, after, found := strings.Cut(arg, "@")
+	if !found {
+		path = arg
+	} else {
+		path, version = strings.TrimSpace(before), strings.TrimSpace(after)
+	}
+	if err := module.CheckImportPath(path); err != nil {
+		if !allowDirPath || !modfile.IsDirectoryPath(path) {
+			return path, version, fmt.Errorf("invalid %s path: %v", adj, err)
+		}
+	}
+	if path != arg && !allowedVersionArg(version) {
+		return path, version, fmt.Errorf("invalid %s version: %q", adj, version)
+	}
+	return path, version, nil
+}
+
+// parseVersionInterval parses a single version like "v1.2.3" or a closed
+// interval like "[v1.2.3,v1.4.5]". Note that a single version has the same
+// representation as an interval with equal upper and lower bounds: both
+// Low and High are set.
+func parseVersionInterval(arg string) (modfile.VersionInterval, error) {
+	if !strings.HasPrefix(arg, "[") {
+		if !allowedVersionArg(arg) {
+			return modfile.VersionInterval{}, fmt.Errorf("invalid version: %q", arg)
+		}
+		return modfile.VersionInterval{Low: arg, High: arg}, nil
+	}
+	if !strings.HasSuffix(arg, "]") {
+		return modfile.VersionInterval{}, fmt.Errorf("invalid version interval: %q", arg)
+	}
+	s := arg[1 : len(arg)-1]
+	before, after, found := strings.Cut(s, ",")
+	if !found {
+		return modfile.VersionInterval{}, fmt.Errorf("invalid version interval: %q", arg)
+	}
+	low := strings.TrimSpace(before)
+	high := strings.TrimSpace(after)
+	if !allowedVersionArg(low) || !allowedVersionArg(high) {
+		return modfile.VersionInterval{}, fmt.Errorf("invalid version interval: %q", arg)
+	}
+	return modfile.VersionInterval{Low: low, High: high}, nil
+}
+
+// allowedVersionArg returns whether a token may be used as a version in go.mod.
+// We don't call modfile.CheckPathVersion, because that insists on versions
+// being in semver form, but here we want to allow versions like "master" or
+// "1234abcdef", which the go command will resolve the next time it runs (or
+// during -fix).  Even so, we need to make sure the version is a valid token.
+func allowedVersionArg(arg string) bool {
+	return !modfile.MustQuote(arg)
+}
+
+// flagRequire implements the -require flag.
+func flagRequire(arg string) {
+	path, version := parsePathVersion("require", arg)
+	edits = append(edits, func(f *modfile.File) {
+		if err := f.AddRequire(path, version); err != nil {
+			base.Fatalf("go: -require=%s: %v", arg, err)
+		}
+	})
+}
+
+// flagDropRequire implements the -droprequire flag.
+func flagDropRequire(arg string) {
+	path := parsePath("droprequire", arg)
+	edits = append(edits, func(f *modfile.File) {
+		if err := f.DropRequire(path); err != nil {
+			base.Fatalf("go: -droprequire=%s: %v", arg, err)
+		}
+	})
+}
+
+// flagExclude implements the -exclude flag.
+func flagExclude(arg string) {
+	path, version := parsePathVersion("exclude", arg)
+	edits = append(edits, func(f *modfile.File) {
+		if err := f.AddExclude(path, version); err != nil {
+			base.Fatalf("go: -exclude=%s: %v", arg, err)
+		}
+	})
+}
+
+// flagDropExclude implements the -dropexclude flag.
+func flagDropExclude(arg string) {
+	path, version := parsePathVersion("dropexclude", arg)
+	edits = append(edits, func(f *modfile.File) {
+		if err := f.DropExclude(path, version); err != nil {
+			base.Fatalf("go: -dropexclude=%s: %v", arg, err)
+		}
+	})
+}
+
+// flagReplace implements the -replace flag.
+func flagReplace(arg string) {
+	before, after, found := strings.Cut(arg, "=")
+	if !found {
+		base.Fatalf("go: -replace=%s: need old[@v]=new[@w] (missing =)", arg)
+	}
+	old, new := strings.TrimSpace(before), strings.TrimSpace(after)
+	if strings.HasPrefix(new, ">") {
+		base.Fatalf("go: -replace=%s: separator between old and new is =, not =>", arg)
+	}
+	oldPath, oldVersion, err := parsePathVersionOptional("old", old, false)
+	if err != nil {
+		base.Fatalf("go: -replace=%s: %v", arg, err)
+	}
+	newPath, newVersion, err := parsePathVersionOptional("new", new, true)
+	if err != nil {
+		base.Fatalf("go: -replace=%s: %v", arg, err)
+	}
+	if newPath == new && !modfile.IsDirectoryPath(new) {
+		base.Fatalf("go: -replace=%s: unversioned new path must be local directory", arg)
+	}
+
+	edits = append(edits, func(f *modfile.File) {
+		if err := f.AddReplace(oldPath, oldVersion, newPath, newVersion); err != nil {
+			base.Fatalf("go: -replace=%s: %v", arg, err)
+		}
+	})
+}
+
+// flagDropReplace implements the -dropreplace flag.
+func flagDropReplace(arg string) {
+	path, version, err := parsePathVersionOptional("old", arg, true)
+	if err != nil {
+		base.Fatalf("go: -dropreplace=%s: %v", arg, err)
+	}
+	edits = append(edits, func(f *modfile.File) {
+		if err := f.DropReplace(path, version); err != nil {
+			base.Fatalf("go: -dropreplace=%s: %v", arg, err)
+		}
+	})
+}
+
+// flagRetract implements the -retract flag.
+func flagRetract(arg string) {
+	vi, err := parseVersionInterval(arg)
+	if err != nil {
+		base.Fatalf("go: -retract=%s: %v", arg, err)
+	}
+	edits = append(edits, func(f *modfile.File) {
+		if err := f.AddRetract(vi, ""); err != nil {
+			base.Fatalf("go: -retract=%s: %v", arg, err)
+		}
+	})
+}
+
+// flagDropRetract implements the -dropretract flag.
+func flagDropRetract(arg string) {
+	vi, err := parseVersionInterval(arg)
+	if err != nil {
+		base.Fatalf("go: -dropretract=%s: %v", arg, err)
+	}
+	edits = append(edits, func(f *modfile.File) {
+		if err := f.DropRetract(vi); err != nil {
+			base.Fatalf("go: -dropretract=%s: %v", arg, err)
+		}
+	})
+}
+
+// fileJSON is the -json output data structure.
+type fileJSON struct {
+	Module    editModuleJSON
+	Go        string `json:",omitempty"`
+	Toolchain string `json:",omitempty"`
+	Require   []requireJSON
+	Exclude   []module.Version
+	Replace   []replaceJSON
+	Retract   []retractJSON
+}
+
+type editModuleJSON struct {
+	Path       string
+	Deprecated string `json:",omitempty"`
+}
+
+type requireJSON struct {
+	Path     string
+	Version  string `json:",omitempty"`
+	Indirect bool   `json:",omitempty"`
+}
+
+type replaceJSON struct {
+	Old module.Version
+	New module.Version
+}
+
+type retractJSON struct {
+	Low       string `json:",omitempty"`
+	High      string `json:",omitempty"`
+	Rationale string `json:",omitempty"`
+}
+
+// editPrintJSON prints the -json output.
+func editPrintJSON(modFile *modfile.File) {
+	var f fileJSON
+	if modFile.Module != nil {
+		f.Module = editModuleJSON{
+			Path:       modFile.Module.Mod.Path,
+			Deprecated: modFile.Module.Deprecated,
+		}
+	}
+	if modFile.Go != nil {
+		f.Go = modFile.Go.Version
+	}
+	if modFile.Toolchain != nil {
+		f.Toolchain = modFile.Toolchain.Name
+	}
+	for _, r := range modFile.Require {
+		f.Require = append(f.Require, requireJSON{Path: r.Mod.Path, Version: r.Mod.Version, Indirect: r.Indirect})
+	}
+	for _, x := range modFile.Exclude {
+		f.Exclude = append(f.Exclude, x.Mod)
+	}
+	for _, r := range modFile.Replace {
+		f.Replace = append(f.Replace, replaceJSON{r.Old, r.New})
+	}
+	for _, r := range modFile.Retract {
+		f.Retract = append(f.Retract, retractJSON{r.Low, r.High, r.Rationale})
+	}
+	data, err := json.MarshalIndent(&f, "", "\t")
+	if err != nil {
+		base.Fatalf("go: internal error: %v", err)
+	}
+	data = append(data, '\n')
+	os.Stdout.Write(data)
+}
diff --git a/src/cmd/go/internal/modcmd/graph.go b/src/cmd/go/internal/modcmd/graph.go
new file mode 100644
index 0000000..172c1dd
--- /dev/null
+++ b/src/cmd/go/internal/modcmd/graph.go
@@ -0,0 +1,96 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// go mod graph
+
+package modcmd
+
+import (
+	"bufio"
+	"context"
+	"os"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/toolchain"
+
+	"golang.org/x/mod/module"
+)
+
+var cmdGraph = &base.Command{
+	UsageLine: "go mod graph [-go=version] [-x]",
+	Short:     "print module requirement graph",
+	Long: `
+Graph prints the module requirement graph (with replacements applied)
+in text form. Each line in the output has two space-separated fields: a module
+and one of its requirements. Each module is identified as a string of the form
+path@version, except for the main module, which has no @version suffix.
+
+The -go flag causes graph to report the module graph as loaded by the
+given Go version, instead of the version indicated by the 'go' directive
+in the go.mod file.
+
+The -x flag causes graph to print the commands graph executes.
+
+See https://golang.org/ref/mod#go-mod-graph for more about 'go mod graph'.
+	`,
+	Run: runGraph,
+}
+
+var (
+	graphGo goVersionFlag
+)
+
+func init() {
+	cmdGraph.Flag.Var(&graphGo, "go", "")
+	cmdGraph.Flag.BoolVar(&cfg.BuildX, "x", false, "")
+	base.AddChdirFlag(&cmdGraph.Flag)
+	base.AddModCommonFlags(&cmdGraph.Flag)
+}
+
+func runGraph(ctx context.Context, cmd *base.Command, args []string) {
+	modload.InitWorkfile()
+
+	if len(args) > 0 {
+		base.Fatalf("go: 'go mod graph' accepts no arguments")
+	}
+	modload.ForceUseModules = true
+	modload.RootMode = modload.NeedRoot
+
+	goVersion := graphGo.String()
+	if goVersion != "" && gover.Compare(gover.Local(), goVersion) < 0 {
+		toolchain.SwitchOrFatal(ctx, &gover.TooNewError{
+			What:      "-go flag",
+			GoVersion: goVersion,
+		})
+	}
+
+	mg, err := modload.LoadModGraph(ctx, goVersion)
+	if err != nil {
+		base.Fatal(err)
+	}
+
+	w := bufio.NewWriter(os.Stdout)
+	defer w.Flush()
+
+	format := func(m module.Version) {
+		w.WriteString(m.Path)
+		if m.Version != "" {
+			w.WriteString("@")
+			w.WriteString(m.Version)
+		}
+	}
+
+	mg.WalkBreadthFirst(func(m module.Version) {
+		reqs, _ := mg.RequiredBy(m)
+		for _, r := range reqs {
+			format(m)
+			w.WriteByte(' ')
+			format(r)
+			w.WriteByte('\n')
+		}
+	})
+}
diff --git a/src/cmd/go/internal/modcmd/init.go b/src/cmd/go/internal/modcmd/init.go
new file mode 100644
index 0000000..e4be73f
--- /dev/null
+++ b/src/cmd/go/internal/modcmd/init.go
@@ -0,0 +1,52 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// go mod init
+
+package modcmd
+
+import (
+	"cmd/go/internal/base"
+	"cmd/go/internal/modload"
+	"context"
+)
+
+var cmdInit = &base.Command{
+	UsageLine: "go mod init [module-path]",
+	Short:     "initialize new module in current directory",
+	Long: `
+Init initializes and writes a new go.mod file in the current directory, in
+effect creating a new module rooted at the current directory. The go.mod file
+must not already exist.
+
+Init accepts one optional argument, the module path for the new module. If the
+module path argument is omitted, init will attempt to infer the module path
+using import comments in .go files, vendoring tool configuration files (like
+Gopkg.lock), and the current directory (if in GOPATH).
+
+If a configuration file for a vendoring tool is present, init will attempt to
+import module requirements from it.
+
+See https://golang.org/ref/mod#go-mod-init for more about 'go mod init'.
+`,
+	Run: runInit,
+}
+
+func init() {
+	base.AddChdirFlag(&cmdInit.Flag)
+	base.AddModCommonFlags(&cmdInit.Flag)
+}
+
+func runInit(ctx context.Context, cmd *base.Command, args []string) {
+	if len(args) > 1 {
+		base.Fatalf("go: 'go mod init' accepts at most one argument")
+	}
+	var modPath string
+	if len(args) == 1 {
+		modPath = args[0]
+	}
+
+	modload.ForceUseModules = true
+	modload.CreateModFile(ctx, modPath) // does all the hard work
+}
diff --git a/src/cmd/go/internal/modcmd/mod.go b/src/cmd/go/internal/modcmd/mod.go
new file mode 100644
index 0000000..125ba33
--- /dev/null
+++ b/src/cmd/go/internal/modcmd/mod.go
@@ -0,0 +1,33 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package modcmd implements the “go mod” command.
+package modcmd
+
+import (
+	"cmd/go/internal/base"
+)
+
+var CmdMod = &base.Command{
+	UsageLine: "go mod",
+	Short:     "module maintenance",
+	Long: `Go mod provides access to operations on modules.
+
+Note that support for modules is built into all the go commands,
+not just 'go mod'. For example, day-to-day adding, removing, upgrading,
+and downgrading of dependencies should be done using 'go get'.
+See 'go help modules' for an overview of module functionality.
+	`,
+
+	Commands: []*base.Command{
+		cmdDownload,
+		cmdEdit,
+		cmdGraph,
+		cmdInit,
+		cmdTidy,
+		cmdVendor,
+		cmdVerify,
+		cmdWhy,
+	},
+}
diff --git a/src/cmd/go/internal/modcmd/tidy.go b/src/cmd/go/internal/modcmd/tidy.go
new file mode 100644
index 0000000..36be926
--- /dev/null
+++ b/src/cmd/go/internal/modcmd/tidy.go
@@ -0,0 +1,139 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// go mod tidy
+
+package modcmd
+
+import (
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/imports"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/toolchain"
+	"context"
+	"fmt"
+
+	"golang.org/x/mod/modfile"
+)
+
+var cmdTidy = &base.Command{
+	UsageLine: "go mod tidy [-e] [-v] [-x] [-go=version] [-compat=version]",
+	Short:     "add missing and remove unused modules",
+	Long: `
+Tidy makes sure go.mod matches the source code in the module.
+It adds any missing modules necessary to build the current module's
+packages and dependencies, and it removes unused modules that
+don't provide any relevant packages. It also adds any missing entries
+to go.sum and removes any unnecessary ones.
+
+The -v flag causes tidy to print information about removed modules
+to standard error.
+
+The -e flag causes tidy to attempt to proceed despite errors
+encountered while loading packages.
+
+The -go flag causes tidy to update the 'go' directive in the go.mod
+file to the given version, which may change which module dependencies
+are retained as explicit requirements in the go.mod file.
+(Go versions 1.17 and higher retain more requirements in order to
+support lazy module loading.)
+
+The -compat flag preserves any additional checksums needed for the
+'go' command from the indicated major Go release to successfully load
+the module graph, and causes tidy to error out if that version of the
+'go' command would load any imported package from a different module
+version. By default, tidy acts as if the -compat flag were set to the
+version prior to the one indicated by the 'go' directive in the go.mod
+file.
+
+The -x flag causes tidy to print the commands download executes.
+
+See https://golang.org/ref/mod#go-mod-tidy for more about 'go mod tidy'.
+	`,
+	Run: runTidy,
+}
+
+var (
+	tidyE      bool          // if true, report errors but proceed anyway.
+	tidyGo     goVersionFlag // go version to write to the tidied go.mod file (toggles lazy loading)
+	tidyCompat goVersionFlag // go version for which the tidied go.mod and go.sum files should be “compatible”
+)
+
+func init() {
+	cmdTidy.Flag.BoolVar(&cfg.BuildV, "v", false, "")
+	cmdTidy.Flag.BoolVar(&cfg.BuildX, "x", false, "")
+	cmdTidy.Flag.BoolVar(&tidyE, "e", false, "")
+	cmdTidy.Flag.Var(&tidyGo, "go", "")
+	cmdTidy.Flag.Var(&tidyCompat, "compat", "")
+	base.AddChdirFlag(&cmdTidy.Flag)
+	base.AddModCommonFlags(&cmdTidy.Flag)
+}
+
+// A goVersionFlag is a flag.Value representing a supported Go version.
+//
+// (Note that the -go argument to 'go mod edit' is *not* a goVersionFlag.
+// It intentionally allows newer-than-supported versions as arguments.)
+type goVersionFlag struct {
+	v string
+}
+
+func (f *goVersionFlag) String() string { return f.v }
+func (f *goVersionFlag) Get() any       { return f.v }
+
+func (f *goVersionFlag) Set(s string) error {
+	if s != "" {
+		latest := gover.Local()
+		if !modfile.GoVersionRE.MatchString(s) {
+			return fmt.Errorf("expecting a Go version like %q", latest)
+		}
+		if gover.Compare(s, latest) > 0 {
+			return fmt.Errorf("maximum supported Go version is %s", latest)
+		}
+	}
+
+	f.v = s
+	return nil
+}
+
+func runTidy(ctx context.Context, cmd *base.Command, args []string) {
+	if len(args) > 0 {
+		base.Fatalf("go: 'go mod tidy' accepts no arguments")
+	}
+
+	// Tidy aims to make 'go test' reproducible for any package in 'all', so we
+	// need to include test dependencies. For modules that specify go 1.15 or
+	// earlier this is a no-op (because 'all' saturates transitive test
+	// dependencies).
+	//
+	// However, with lazy loading (go 1.16+) 'all' includes only the packages that
+	// are transitively imported by the main module, not the test dependencies of
+	// those packages. In order to make 'go test' reproducible for the packages
+	// that are in 'all' but outside of the main module, we must explicitly
+	// request that their test dependencies be included.
+	modload.ForceUseModules = true
+	modload.RootMode = modload.NeedRoot
+
+	goVersion := tidyGo.String()
+	if goVersion != "" && gover.Compare(gover.Local(), goVersion) < 0 {
+		toolchain.SwitchOrFatal(ctx, &gover.TooNewError{
+			What:      "-go flag",
+			GoVersion: goVersion,
+		})
+	}
+
+	modload.LoadPackages(ctx, modload.PackageOpts{
+		TidyGoVersion:            tidyGo.String(),
+		Tags:                     imports.AnyTags(),
+		Tidy:                     true,
+		TidyCompatibleVersion:    tidyCompat.String(),
+		VendorModulesInGOROOTSrc: true,
+		ResolveMissingImports:    true,
+		LoadTests:                true,
+		AllowErrors:              tidyE,
+		SilenceMissingStdImports: true,
+		Switcher:                 new(toolchain.Switcher),
+	}, "all")
+}
diff --git a/src/cmd/go/internal/modcmd/vendor.go b/src/cmd/go/internal/modcmd/vendor.go
new file mode 100644
index 0000000..1a0d69e
--- /dev/null
+++ b/src/cmd/go/internal/modcmd/vendor.go
@@ -0,0 +1,431 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modcmd
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"go/build"
+	"io"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"sort"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/imports"
+	"cmd/go/internal/load"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/str"
+
+	"golang.org/x/mod/module"
+)
+
+var cmdVendor = &base.Command{
+	UsageLine: "go mod vendor [-e] [-v] [-o outdir]",
+	Short:     "make vendored copy of dependencies",
+	Long: `
+Vendor resets the main module's vendor directory to include all packages
+needed to build and test all the main module's packages.
+It does not include test code for vendored packages.
+
+The -v flag causes vendor to print the names of vendored
+modules and packages to standard error.
+
+The -e flag causes vendor to attempt to proceed despite errors
+encountered while loading packages.
+
+The -o flag causes vendor to create the vendor directory at the given
+path instead of "vendor". The go command can only use a vendor directory
+named "vendor" within the module root directory, so this flag is
+primarily useful for other tools.
+
+See https://golang.org/ref/mod#go-mod-vendor for more about 'go mod vendor'.
+	`,
+	Run: runVendor,
+}
+
+var vendorE bool   // if true, report errors but proceed anyway
+var vendorO string // if set, overrides the default output directory
+
+func init() {
+	cmdVendor.Flag.BoolVar(&cfg.BuildV, "v", false, "")
+	cmdVendor.Flag.BoolVar(&vendorE, "e", false, "")
+	cmdVendor.Flag.StringVar(&vendorO, "o", "", "")
+	base.AddChdirFlag(&cmdVendor.Flag)
+	base.AddModCommonFlags(&cmdVendor.Flag)
+}
+
+func runVendor(ctx context.Context, cmd *base.Command, args []string) {
+	if len(args) != 0 {
+		base.Fatalf("go: 'go mod vendor' accepts no arguments")
+	}
+	modload.ForceUseModules = true
+	modload.RootMode = modload.NeedRoot
+
+	loadOpts := modload.PackageOpts{
+		Tags:                     imports.AnyTags(),
+		VendorModulesInGOROOTSrc: true,
+		ResolveMissingImports:    true,
+		UseVendorAll:             true,
+		AllowErrors:              vendorE,
+		SilenceMissingStdImports: true,
+	}
+	_, pkgs := modload.LoadPackages(ctx, loadOpts, "all")
+
+	var vdir string
+	switch {
+	case filepath.IsAbs(vendorO):
+		vdir = vendorO
+	case vendorO != "":
+		vdir = filepath.Join(base.Cwd(), vendorO)
+	default:
+		vdir = filepath.Join(modload.VendorDir())
+	}
+	if err := os.RemoveAll(vdir); err != nil {
+		base.Fatal(err)
+	}
+
+	modpkgs := make(map[module.Version][]string)
+	for _, pkg := range pkgs {
+		m := modload.PackageModule(pkg)
+		if m.Path == "" || m.Version == "" && modload.MainModules.Contains(m.Path) {
+			continue
+		}
+		modpkgs[m] = append(modpkgs[m], pkg)
+	}
+
+	includeAllReplacements := false
+	includeGoVersions := false
+	isExplicit := map[module.Version]bool{}
+	if gv := modload.ModFile().Go; gv != nil {
+		if gover.Compare(gv.Version, "1.14") >= 0 {
+			// If the Go version is at least 1.14, annotate all explicit 'require' and
+			// 'replace' targets found in the go.mod file so that we can perform a
+			// stronger consistency check when -mod=vendor is set.
+			for _, r := range modload.ModFile().Require {
+				isExplicit[r.Mod] = true
+			}
+			includeAllReplacements = true
+		}
+		if gover.Compare(gv.Version, "1.17") >= 0 {
+			// If the Go version is at least 1.17, annotate all modules with their
+			// 'go' version directives.
+			includeGoVersions = true
+		}
+	}
+
+	var vendorMods []module.Version
+	for m := range isExplicit {
+		vendorMods = append(vendorMods, m)
+	}
+	for m := range modpkgs {
+		if !isExplicit[m] {
+			vendorMods = append(vendorMods, m)
+		}
+	}
+	gover.ModSort(vendorMods)
+
+	var (
+		buf bytes.Buffer
+		w   io.Writer = &buf
+	)
+	if cfg.BuildV {
+		w = io.MultiWriter(&buf, os.Stderr)
+	}
+
+	for _, m := range vendorMods {
+		replacement := modload.Replacement(m)
+		line := moduleLine(m, replacement)
+		io.WriteString(w, line)
+
+		goVersion := ""
+		if includeGoVersions {
+			goVersion = modload.ModuleInfo(ctx, m.Path).GoVersion
+		}
+		switch {
+		case isExplicit[m] && goVersion != "":
+			fmt.Fprintf(w, "## explicit; go %s\n", goVersion)
+		case isExplicit[m]:
+			io.WriteString(w, "## explicit\n")
+		case goVersion != "":
+			fmt.Fprintf(w, "## go %s\n", goVersion)
+		}
+
+		pkgs := modpkgs[m]
+		sort.Strings(pkgs)
+		for _, pkg := range pkgs {
+			fmt.Fprintf(w, "%s\n", pkg)
+			vendorPkg(vdir, pkg)
+		}
+	}
+
+	if includeAllReplacements {
+		// Record unused and wildcard replacements at the end of the modules.txt file:
+		// without access to the complete build list, the consumer of the vendor
+		// directory can't otherwise determine that those replacements had no effect.
+		for _, r := range modload.ModFile().Replace {
+			if len(modpkgs[r.Old]) > 0 {
+				// We we already recorded this replacement in the entry for the replaced
+				// module with the packages it provides.
+				continue
+			}
+
+			line := moduleLine(r.Old, r.New)
+			buf.WriteString(line)
+			if cfg.BuildV {
+				os.Stderr.WriteString(line)
+			}
+		}
+	}
+
+	if buf.Len() == 0 {
+		fmt.Fprintf(os.Stderr, "go: no dependencies to vendor\n")
+		return
+	}
+
+	if err := os.MkdirAll(vdir, 0777); err != nil {
+		base.Fatal(err)
+	}
+
+	if err := os.WriteFile(filepath.Join(vdir, "modules.txt"), buf.Bytes(), 0666); err != nil {
+		base.Fatal(err)
+	}
+}
+
+func moduleLine(m, r module.Version) string {
+	b := new(strings.Builder)
+	b.WriteString("# ")
+	b.WriteString(m.Path)
+	if m.Version != "" {
+		b.WriteString(" ")
+		b.WriteString(m.Version)
+	}
+	if r.Path != "" {
+		if str.HasFilePathPrefix(filepath.Clean(r.Path), "vendor") {
+			base.Fatalf("go: replacement path %s inside vendor directory", r.Path)
+		}
+		b.WriteString(" => ")
+		b.WriteString(r.Path)
+		if r.Version != "" {
+			b.WriteString(" ")
+			b.WriteString(r.Version)
+		}
+	}
+	b.WriteString("\n")
+	return b.String()
+}
+
+func vendorPkg(vdir, pkg string) {
+	src, realPath, _ := modload.Lookup("", false, pkg)
+	if src == "" {
+		base.Errorf("internal error: no pkg for %s\n", pkg)
+		return
+	}
+	if realPath != pkg {
+		// TODO(#26904): Revisit whether this behavior still makes sense.
+		// This should actually be impossible today, because the import map is the
+		// identity function for packages outside of the standard library.
+		//
+		// Part of the purpose of the vendor directory is to allow the packages in
+		// the module to continue to build in GOPATH mode, and GOPATH-mode users
+		// won't know about replacement aliasing. How important is it to maintain
+		// compatibility?
+		fmt.Fprintf(os.Stderr, "warning: %s imported as both %s and %s; making two copies.\n", realPath, realPath, pkg)
+	}
+
+	copiedFiles := make(map[string]bool)
+	dst := filepath.Join(vdir, pkg)
+	copyDir(dst, src, matchPotentialSourceFile, copiedFiles)
+	if m := modload.PackageModule(realPath); m.Path != "" {
+		copyMetadata(m.Path, realPath, dst, src, copiedFiles)
+	}
+
+	ctx := build.Default
+	ctx.UseAllFiles = true
+	bp, err := ctx.ImportDir(src, build.IgnoreVendor)
+	// Because UseAllFiles is set on the build.Context, it's possible ta get
+	// a MultiplePackageError on an otherwise valid package: the package could
+	// have different names for GOOS=windows and GOOS=mac for example. On the
+	// other hand if there's a NoGoError, the package might have source files
+	// specifying "// +build ignore" those packages should be skipped because
+	// embeds from ignored files can't be used.
+	// TODO(#42504): Find a better way to avoid errors from ImportDir. We'll
+	// need to figure this out when we switch to PackagesAndErrors as per the
+	// TODO above.
+	var multiplePackageError *build.MultiplePackageError
+	var noGoError *build.NoGoError
+	if err != nil {
+		if errors.As(err, &noGoError) {
+			return // No source files in this package are built. Skip embeds in ignored files.
+		} else if !errors.As(err, &multiplePackageError) { // multiplePackageErrors are OK, but others are not.
+			base.Fatalf("internal error: failed to find embedded files of %s: %v\n", pkg, err)
+		}
+	}
+	embedPatterns := str.StringList(bp.EmbedPatterns, bp.TestEmbedPatterns, bp.XTestEmbedPatterns)
+	embeds, err := load.ResolveEmbed(bp.Dir, embedPatterns)
+	if err != nil {
+		base.Fatal(err)
+	}
+	for _, embed := range embeds {
+		embedDst := filepath.Join(dst, embed)
+		if copiedFiles[embedDst] {
+			continue
+		}
+
+		// Copy the file as is done by copyDir below.
+		r, err := os.Open(filepath.Join(src, embed))
+		if err != nil {
+			base.Fatal(err)
+		}
+		if err := os.MkdirAll(filepath.Dir(embedDst), 0777); err != nil {
+			base.Fatal(err)
+		}
+		w, err := os.Create(embedDst)
+		if err != nil {
+			base.Fatal(err)
+		}
+		if _, err := io.Copy(w, r); err != nil {
+			base.Fatal(err)
+		}
+		r.Close()
+		if err := w.Close(); err != nil {
+			base.Fatal(err)
+		}
+	}
+}
+
+type metakey struct {
+	modPath string
+	dst     string
+}
+
+var copiedMetadata = make(map[metakey]bool)
+
+// copyMetadata copies metadata files from parents of src to parents of dst,
+// stopping after processing the src parent for modPath.
+func copyMetadata(modPath, pkg, dst, src string, copiedFiles map[string]bool) {
+	for parent := 0; ; parent++ {
+		if copiedMetadata[metakey{modPath, dst}] {
+			break
+		}
+		copiedMetadata[metakey{modPath, dst}] = true
+		if parent > 0 {
+			copyDir(dst, src, matchMetadata, copiedFiles)
+		}
+		if modPath == pkg {
+			break
+		}
+		pkg = path.Dir(pkg)
+		dst = filepath.Dir(dst)
+		src = filepath.Dir(src)
+	}
+}
+
+// metaPrefixes is the list of metadata file prefixes.
+// Vendoring copies metadata files from parents of copied directories.
+// Note that this list could be arbitrarily extended, and it is longer
+// in other tools (such as godep or dep). By using this limited set of
+// prefixes and also insisting on capitalized file names, we are trying
+// to nudge people toward more agreement on the naming
+// and also trying to avoid false positives.
+var metaPrefixes = []string{
+	"AUTHORS",
+	"CONTRIBUTORS",
+	"COPYLEFT",
+	"COPYING",
+	"COPYRIGHT",
+	"LEGAL",
+	"LICENSE",
+	"NOTICE",
+	"PATENTS",
+}
+
+// matchMetadata reports whether info is a metadata file.
+func matchMetadata(dir string, info fs.DirEntry) bool {
+	name := info.Name()
+	for _, p := range metaPrefixes {
+		if strings.HasPrefix(name, p) {
+			return true
+		}
+	}
+	return false
+}
+
+// matchPotentialSourceFile reports whether info may be relevant to a build operation.
+func matchPotentialSourceFile(dir string, info fs.DirEntry) bool {
+	if strings.HasSuffix(info.Name(), "_test.go") {
+		return false
+	}
+	if info.Name() == "go.mod" || info.Name() == "go.sum" {
+		if gv := modload.ModFile().Go; gv != nil && gover.Compare(gv.Version, "1.17") >= 0 {
+			// As of Go 1.17, we strip go.mod and go.sum files from dependency modules.
+			// Otherwise, 'go' commands invoked within the vendor subtree may misidentify
+			// an arbitrary directory within the vendor tree as a module root.
+			// (See https://golang.org/issue/42970.)
+			return false
+		}
+	}
+	if strings.HasSuffix(info.Name(), ".go") {
+		f, err := fsys.Open(filepath.Join(dir, info.Name()))
+		if err != nil {
+			base.Fatal(err)
+		}
+		defer f.Close()
+
+		content, err := imports.ReadImports(f, false, nil)
+		if err == nil && !imports.ShouldBuild(content, imports.AnyTags()) {
+			// The file is explicitly tagged "ignore", so it can't affect the build.
+			// Leave it out.
+			return false
+		}
+		return true
+	}
+
+	// We don't know anything about this file, so optimistically assume that it is
+	// needed.
+	return true
+}
+
+// copyDir copies all regular files satisfying match(info) from src to dst.
+func copyDir(dst, src string, match func(dir string, info fs.DirEntry) bool, copiedFiles map[string]bool) {
+	files, err := os.ReadDir(src)
+	if err != nil {
+		base.Fatal(err)
+	}
+	if err := os.MkdirAll(dst, 0777); err != nil {
+		base.Fatal(err)
+	}
+	for _, file := range files {
+		if file.IsDir() || !file.Type().IsRegular() || !match(src, file) {
+			continue
+		}
+		copiedFiles[file.Name()] = true
+		r, err := os.Open(filepath.Join(src, file.Name()))
+		if err != nil {
+			base.Fatal(err)
+		}
+		dstPath := filepath.Join(dst, file.Name())
+		copiedFiles[dstPath] = true
+		w, err := os.Create(dstPath)
+		if err != nil {
+			base.Fatal(err)
+		}
+		if _, err := io.Copy(w, r); err != nil {
+			base.Fatal(err)
+		}
+		r.Close()
+		if err := w.Close(); err != nil {
+			base.Fatal(err)
+		}
+	}
+}
diff --git a/src/cmd/go/internal/modcmd/verify.go b/src/cmd/go/internal/modcmd/verify.go
new file mode 100644
index 0000000..d07f730
--- /dev/null
+++ b/src/cmd/go/internal/modcmd/verify.go
@@ -0,0 +1,143 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modcmd
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"runtime"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/modload"
+
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/sumdb/dirhash"
+)
+
+var cmdVerify = &base.Command{
+	UsageLine: "go mod verify",
+	Short:     "verify dependencies have expected content",
+	Long: `
+Verify checks that the dependencies of the current module,
+which are stored in a local downloaded source cache, have not been
+modified since being downloaded. If all the modules are unmodified,
+verify prints "all modules verified." Otherwise it reports which
+modules have been changed and causes 'go mod' to exit with a
+non-zero status.
+
+See https://golang.org/ref/mod#go-mod-verify for more about 'go mod verify'.
+	`,
+	Run: runVerify,
+}
+
+func init() {
+	base.AddChdirFlag(&cmdVerify.Flag)
+	base.AddModCommonFlags(&cmdVerify.Flag)
+}
+
+func runVerify(ctx context.Context, cmd *base.Command, args []string) {
+	modload.InitWorkfile()
+
+	if len(args) != 0 {
+		// NOTE(rsc): Could take a module pattern.
+		base.Fatalf("go: verify takes no arguments")
+	}
+	modload.ForceUseModules = true
+	modload.RootMode = modload.NeedRoot
+
+	// Only verify up to GOMAXPROCS zips at once.
+	type token struct{}
+	sem := make(chan token, runtime.GOMAXPROCS(0))
+
+	mg, err := modload.LoadModGraph(ctx, "")
+	if err != nil {
+		base.Fatal(err)
+	}
+	mods := mg.BuildList()
+	// Use a slice of result channels, so that the output is deterministic.
+	errsChans := make([]<-chan []error, len(mods))
+
+	for i, mod := range mods {
+		sem <- token{}
+		errsc := make(chan []error, 1)
+		errsChans[i] = errsc
+		mod := mod // use a copy to avoid data races
+		go func() {
+			errsc <- verifyMod(ctx, mod)
+			<-sem
+		}()
+	}
+
+	ok := true
+	for _, errsc := range errsChans {
+		errs := <-errsc
+		for _, err := range errs {
+			base.Errorf("%s", err)
+			ok = false
+		}
+	}
+	if ok {
+		fmt.Printf("all modules verified\n")
+	}
+}
+
+func verifyMod(ctx context.Context, mod module.Version) []error {
+	if gover.IsToolchain(mod.Path) {
+		// "go" and "toolchain" have no disk footprint; nothing to verify.
+		return nil
+	}
+	if modload.MainModules.Contains(mod.Path) {
+		return nil
+	}
+	var errs []error
+	zip, zipErr := modfetch.CachePath(ctx, mod, "zip")
+	if zipErr == nil {
+		_, zipErr = os.Stat(zip)
+	}
+	dir, dirErr := modfetch.DownloadDir(ctx, mod)
+	data, err := os.ReadFile(zip + "hash")
+	if err != nil {
+		if zipErr != nil && errors.Is(zipErr, fs.ErrNotExist) &&
+			dirErr != nil && errors.Is(dirErr, fs.ErrNotExist) {
+			// Nothing downloaded yet. Nothing to verify.
+			return nil
+		}
+		errs = append(errs, fmt.Errorf("%s %s: missing ziphash: %v", mod.Path, mod.Version, err))
+		return errs
+	}
+	h := string(bytes.TrimSpace(data))
+
+	if zipErr != nil && errors.Is(zipErr, fs.ErrNotExist) {
+		// ok
+	} else {
+		hZ, err := dirhash.HashZip(zip, dirhash.DefaultHash)
+		if err != nil {
+			errs = append(errs, fmt.Errorf("%s %s: %v", mod.Path, mod.Version, err))
+			return errs
+		} else if hZ != h {
+			errs = append(errs, fmt.Errorf("%s %s: zip has been modified (%v)", mod.Path, mod.Version, zip))
+		}
+	}
+	if dirErr != nil && errors.Is(dirErr, fs.ErrNotExist) {
+		// ok
+	} else {
+		hD, err := dirhash.HashDir(dir, mod.Path+"@"+mod.Version, dirhash.DefaultHash)
+		if err != nil {
+
+			errs = append(errs, fmt.Errorf("%s %s: %v", mod.Path, mod.Version, err))
+			return errs
+		}
+		if hD != h {
+			errs = append(errs, fmt.Errorf("%s %s: dir has been modified (%v)", mod.Path, mod.Version, dir))
+		}
+	}
+	return errs
+}
diff --git a/src/cmd/go/internal/modcmd/why.go b/src/cmd/go/internal/modcmd/why.go
new file mode 100644
index 0000000..198672d
--- /dev/null
+++ b/src/cmd/go/internal/modcmd/why.go
@@ -0,0 +1,143 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modcmd
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/imports"
+	"cmd/go/internal/modload"
+)
+
+var cmdWhy = &base.Command{
+	UsageLine: "go mod why [-m] [-vendor] packages...",
+	Short:     "explain why packages or modules are needed",
+	Long: `
+Why shows a shortest path in the import graph from the main module to
+each of the listed packages. If the -m flag is given, why treats the
+arguments as a list of modules and finds a path to any package in each
+of the modules.
+
+By default, why queries the graph of packages matched by "go list all",
+which includes tests for reachable packages. The -vendor flag causes why
+to exclude tests of dependencies.
+
+The output is a sequence of stanzas, one for each package or module
+name on the command line, separated by blank lines. Each stanza begins
+with a comment line "# package" or "# module" giving the target
+package or module. Subsequent lines give a path through the import
+graph, one package per line. If the package or module is not
+referenced from the main module, the stanza will display a single
+parenthesized note indicating that fact.
+
+For example:
+
+	$ go mod why golang.org/x/text/language golang.org/x/text/encoding
+	# golang.org/x/text/language
+	rsc.io/quote
+	rsc.io/sampler
+	golang.org/x/text/language
+
+	# golang.org/x/text/encoding
+	(main module does not need package golang.org/x/text/encoding)
+	$
+
+See https://golang.org/ref/mod#go-mod-why for more about 'go mod why'.
+	`,
+}
+
+var (
+	whyM      = cmdWhy.Flag.Bool("m", false, "")
+	whyVendor = cmdWhy.Flag.Bool("vendor", false, "")
+)
+
+func init() {
+	cmdWhy.Run = runWhy // break init cycle
+	base.AddChdirFlag(&cmdWhy.Flag)
+	base.AddModCommonFlags(&cmdWhy.Flag)
+}
+
+func runWhy(ctx context.Context, cmd *base.Command, args []string) {
+	modload.InitWorkfile()
+	modload.ForceUseModules = true
+	modload.RootMode = modload.NeedRoot
+	modload.ExplicitWriteGoMod = true // don't write go.mod in ListModules
+
+	loadOpts := modload.PackageOpts{
+		Tags:                     imports.AnyTags(),
+		VendorModulesInGOROOTSrc: true,
+		LoadTests:                !*whyVendor,
+		SilencePackageErrors:     true,
+		UseVendorAll:             *whyVendor,
+	}
+
+	if *whyM {
+		for _, arg := range args {
+			if strings.Contains(arg, "@") {
+				base.Fatalf("go: %s: 'go mod why' requires a module path, not a version query", arg)
+			}
+		}
+
+		mods, err := modload.ListModules(ctx, args, 0, "")
+		if err != nil {
+			base.Fatal(err)
+		}
+
+		byModule := make(map[string][]string)
+		_, pkgs := modload.LoadPackages(ctx, loadOpts, "all")
+		for _, path := range pkgs {
+			m := modload.PackageModule(path)
+			if m.Path != "" {
+				byModule[m.Path] = append(byModule[m.Path], path)
+			}
+		}
+		sep := ""
+		for _, m := range mods {
+			best := ""
+			bestDepth := 1000000000
+			for _, path := range byModule[m.Path] {
+				d := modload.WhyDepth(path)
+				if d > 0 && d < bestDepth {
+					best = path
+					bestDepth = d
+				}
+			}
+			why := modload.Why(best)
+			if why == "" {
+				vendoring := ""
+				if *whyVendor {
+					vendoring = " to vendor"
+				}
+				why = "(main module does not need" + vendoring + " module " + m.Path + ")\n"
+			}
+			fmt.Printf("%s# %s\n%s", sep, m.Path, why)
+			sep = "\n"
+		}
+	} else {
+		// Resolve to packages.
+		matches, _ := modload.LoadPackages(ctx, loadOpts, args...)
+
+		modload.LoadPackages(ctx, loadOpts, "all") // rebuild graph, from main module (not from named packages)
+
+		sep := ""
+		for _, m := range matches {
+			for _, path := range m.Pkgs {
+				why := modload.Why(path)
+				if why == "" {
+					vendoring := ""
+					if *whyVendor {
+						vendoring = " to vendor"
+					}
+					why = "(main module does not need" + vendoring + " package " + path + ")\n"
+				}
+				fmt.Printf("%s# %s\n%s", sep, path, why)
+				sep = "\n"
+			}
+		}
+	}
+}
diff --git a/src/cmd/go/internal/modconv/convert.go b/src/cmd/go/internal/modconv/convert.go
new file mode 100644
index 0000000..9c861f8
--- /dev/null
+++ b/src/cmd/go/internal/modconv/convert.go
@@ -0,0 +1,105 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modconv
+
+import (
+	"fmt"
+	"os"
+	"runtime"
+	"sort"
+	"strings"
+
+	"cmd/go/internal/base"
+
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
+)
+
+// ConvertLegacyConfig converts legacy config to modfile.
+// The file argument is slash-delimited.
+func ConvertLegacyConfig(f *modfile.File, file string, data []byte, queryPackage func(path, rev string) (module.Version, error)) error {
+	i := strings.LastIndex(file, "/")
+	j := -2
+	if i >= 0 {
+		j = strings.LastIndex(file[:i], "/")
+	}
+	convert := Converters[file[i+1:]]
+	if convert == nil && j != -2 {
+		convert = Converters[file[j+1:]]
+	}
+	if convert == nil {
+		return fmt.Errorf("unknown legacy config file %s", file)
+	}
+	mf, err := convert(file, data)
+	if err != nil {
+		return fmt.Errorf("parsing %s: %v", file, err)
+	}
+
+	// Convert requirements block, which may use raw SHA1 hashes as versions,
+	// to valid semver requirement list, respecting major versions.
+	versions := make([]module.Version, len(mf.Require))
+	replace := make(map[string]*modfile.Replace)
+
+	for _, r := range mf.Replace {
+		replace[r.New.Path] = r
+		replace[r.Old.Path] = r
+	}
+
+	type token struct{}
+	sem := make(chan token, runtime.GOMAXPROCS(0))
+	for i, r := range mf.Require {
+		m := r.Mod
+		if m.Path == "" {
+			continue
+		}
+		if re, ok := replace[m.Path]; ok {
+			m = re.New
+		}
+		sem <- token{}
+		go func(i int, m module.Version) {
+			defer func() { <-sem }()
+			version, err := queryPackage(m.Path, m.Version)
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "go: converting %s: stat %s@%s: %v\n", base.ShortPath(file), m.Path, m.Version, err)
+				return
+			}
+
+			versions[i] = version
+		}(i, m)
+	}
+	// Fill semaphore channel to wait for all tasks to finish.
+	for n := cap(sem); n > 0; n-- {
+		sem <- token{}
+	}
+
+	need := map[string]string{}
+	for _, v := range versions {
+		if v.Path == "" {
+			continue
+		}
+		// Don't use semver.Max here; need to preserve +incompatible suffix.
+		if needv, ok := need[v.Path]; !ok || semver.Compare(needv, v.Version) < 0 {
+			need[v.Path] = v.Version
+		}
+	}
+	paths := make([]string, 0, len(need))
+	for path := range need {
+		paths = append(paths, path)
+	}
+	sort.Strings(paths)
+	for _, path := range paths {
+		if re, ok := replace[path]; ok {
+			err := f.AddReplace(re.Old.Path, re.Old.Version, path, need[path])
+			if err != nil {
+				return fmt.Errorf("add replace: %v", err)
+			}
+		}
+		f.AddNewRequire(path, need[path], false)
+	}
+
+	f.Cleanup()
+	return nil
+}
diff --git a/src/cmd/go/internal/modconv/dep.go b/src/cmd/go/internal/modconv/dep.go
new file mode 100644
index 0000000..9bea761
--- /dev/null
+++ b/src/cmd/go/internal/modconv/dep.go
@@ -0,0 +1,132 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modconv
+
+import (
+	"fmt"
+	"internal/lazyregexp"
+	"net/url"
+	"path"
+	"strconv"
+	"strings"
+
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
+)
+
+func ParseGopkgLock(file string, data []byte) (*modfile.File, error) {
+	type pkg struct {
+		Path    string
+		Version string
+		Source  string
+	}
+	mf := new(modfile.File)
+	var list []pkg
+	var r *pkg
+	for lineno, line := range strings.Split(string(data), "\n") {
+		lineno++
+		if i := strings.Index(line, "#"); i >= 0 {
+			line = line[:i]
+		}
+		line = strings.TrimSpace(line)
+		if line == "[[projects]]" {
+			list = append(list, pkg{})
+			r = &list[len(list)-1]
+			continue
+		}
+		if strings.HasPrefix(line, "[") {
+			r = nil
+			continue
+		}
+		if r == nil {
+			continue
+		}
+		before, after, found := strings.Cut(line, "=")
+		if !found {
+			continue
+		}
+		key := strings.TrimSpace(before)
+		val := strings.TrimSpace(after)
+		if len(val) >= 2 && val[0] == '"' && val[len(val)-1] == '"' {
+			q, err := strconv.Unquote(val) // Go unquoting, but close enough for now
+			if err != nil {
+				return nil, fmt.Errorf("%s:%d: invalid quoted string: %v", file, lineno, err)
+			}
+			val = q
+		}
+		switch key {
+		case "name":
+			r.Path = val
+		case "source":
+			r.Source = val
+		case "revision", "version":
+			// Note: key "version" should take priority over "revision",
+			// and it does, because dep writes toml keys in alphabetical order,
+			// so we see version (if present) second.
+			if key == "version" {
+				if !semver.IsValid(val) || semver.Canonical(val) != val {
+					break
+				}
+			}
+			r.Version = val
+		}
+	}
+	for _, r := range list {
+		if r.Path == "" || r.Version == "" {
+			return nil, fmt.Errorf("%s: empty [[projects]] stanza (%s)", file, r.Path)
+		}
+		mf.Require = append(mf.Require, &modfile.Require{Mod: module.Version{Path: r.Path, Version: r.Version}})
+
+		if r.Source != "" {
+			// Convert "source" to import path, such as
+			// git@test.com:x/y.git and https://test.com/x/y.git.
+			// We get "test.com/x/y" at last.
+			source, err := decodeSource(r.Source)
+			if err != nil {
+				return nil, err
+			}
+			old := module.Version{Path: r.Path, Version: r.Version}
+			new := module.Version{Path: source, Version: r.Version}
+			mf.Replace = append(mf.Replace, &modfile.Replace{Old: old, New: new})
+		}
+	}
+	return mf, nil
+}
+
+var scpSyntaxReg = lazyregexp.New(`^([a-zA-Z0-9_]+)@([a-zA-Z0-9._-]+):(.*)$`)
+
+func decodeSource(source string) (string, error) {
+	var u *url.URL
+	var p string
+	if m := scpSyntaxReg.FindStringSubmatch(source); m != nil {
+		// Match SCP-like syntax and convert it to a URL.
+		// Eg, "git@github.com:user/repo" becomes
+		// "ssh://git@github.com/user/repo".
+		u = &url.URL{
+			Scheme: "ssh",
+			User:   url.User(m[1]),
+			Host:   m[2],
+			Path:   "/" + m[3],
+		}
+	} else {
+		var err error
+		u, err = url.Parse(source)
+		if err != nil {
+			return "", fmt.Errorf("%q is not a valid URI", source)
+		}
+	}
+
+	// If no scheme was passed, then the entire path will have been put into
+	// u.Path. Either way, construct the normalized path correctly.
+	if u.Host == "" {
+		p = source
+	} else {
+		p = path.Join(u.Host, u.Path)
+	}
+	p = strings.TrimSuffix(p, ".git")
+	p = strings.TrimSuffix(p, ".hg")
+	return p, nil
+}
diff --git a/src/cmd/go/internal/modconv/glide.go b/src/cmd/go/internal/modconv/glide.go
new file mode 100644
index 0000000..d1de3f7
--- /dev/null
+++ b/src/cmd/go/internal/modconv/glide.go
@@ -0,0 +1,41 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modconv
+
+import (
+	"strings"
+
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/module"
+)
+
+func ParseGlideLock(file string, data []byte) (*modfile.File, error) {
+	mf := new(modfile.File)
+	imports := false
+	name := ""
+	for _, line := range strings.Split(string(data), "\n") {
+		if line == "" {
+			continue
+		}
+		if strings.HasPrefix(line, "imports:") {
+			imports = true
+		} else if line[0] != '-' && line[0] != ' ' && line[0] != '\t' {
+			imports = false
+		}
+		if !imports {
+			continue
+		}
+		if strings.HasPrefix(line, "- name:") {
+			name = strings.TrimSpace(line[len("- name:"):])
+		}
+		if strings.HasPrefix(line, "  version:") {
+			version := strings.TrimSpace(line[len("  version:"):])
+			if name != "" && version != "" {
+				mf.Require = append(mf.Require, &modfile.Require{Mod: module.Version{Path: name, Version: version}})
+			}
+		}
+	}
+	return mf, nil
+}
diff --git a/src/cmd/go/internal/modconv/glock.go b/src/cmd/go/internal/modconv/glock.go
new file mode 100644
index 0000000..b8dc204
--- /dev/null
+++ b/src/cmd/go/internal/modconv/glock.go
@@ -0,0 +1,23 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modconv
+
+import (
+	"strings"
+
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/module"
+)
+
+func ParseGLOCKFILE(file string, data []byte) (*modfile.File, error) {
+	mf := new(modfile.File)
+	for _, line := range strings.Split(string(data), "\n") {
+		f := strings.Fields(line)
+		if len(f) >= 2 && f[0] != "cmd" {
+			mf.Require = append(mf.Require, &modfile.Require{Mod: module.Version{Path: f[0], Version: f[1]}})
+		}
+	}
+	return mf, nil
+}
diff --git a/src/cmd/go/internal/modconv/godeps.go b/src/cmd/go/internal/modconv/godeps.go
new file mode 100644
index 0000000..09c0fa3
--- /dev/null
+++ b/src/cmd/go/internal/modconv/godeps.go
@@ -0,0 +1,30 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modconv
+
+import (
+	"encoding/json"
+
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/module"
+)
+
+func ParseGodepsJSON(file string, data []byte) (*modfile.File, error) {
+	var cfg struct {
+		ImportPath string
+		Deps       []struct {
+			ImportPath string
+			Rev        string
+		}
+	}
+	if err := json.Unmarshal(data, &cfg); err != nil {
+		return nil, err
+	}
+	mf := new(modfile.File)
+	for _, d := range cfg.Deps {
+		mf.Require = append(mf.Require, &modfile.Require{Mod: module.Version{Path: d.ImportPath, Version: d.Rev}})
+	}
+	return mf, nil
+}
diff --git a/src/cmd/go/internal/modconv/modconv.go b/src/cmd/go/internal/modconv/modconv.go
new file mode 100644
index 0000000..dc06072
--- /dev/null
+++ b/src/cmd/go/internal/modconv/modconv.go
@@ -0,0 +1,19 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modconv
+
+import "golang.org/x/mod/modfile"
+
+var Converters = map[string]func(string, []byte) (*modfile.File, error){
+	"GLOCKFILE":          ParseGLOCKFILE,
+	"Godeps/Godeps.json": ParseGodepsJSON,
+	"Gopkg.lock":         ParseGopkgLock,
+	"dependencies.tsv":   ParseDependenciesTSV,
+	"glide.lock":         ParseGlideLock,
+	"vendor.conf":        ParseVendorConf,
+	"vendor.yml":         ParseVendorYML,
+	"vendor/manifest":    ParseVendorManifest,
+	"vendor/vendor.json": ParseVendorJSON,
+}
diff --git a/src/cmd/go/internal/modconv/modconv_test.go b/src/cmd/go/internal/modconv/modconv_test.go
new file mode 100644
index 0000000..750525d
--- /dev/null
+++ b/src/cmd/go/internal/modconv/modconv_test.go
@@ -0,0 +1,69 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modconv
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+var extMap = map[string]string{
+	".dep":       "Gopkg.lock",
+	".glide":     "glide.lock",
+	".glock":     "GLOCKFILE",
+	".godeps":    "Godeps/Godeps.json",
+	".tsv":       "dependencies.tsv",
+	".vconf":     "vendor.conf",
+	".vjson":     "vendor/vendor.json",
+	".vyml":      "vendor.yml",
+	".vmanifest": "vendor/manifest",
+}
+
+func Test(t *testing.T) {
+	tests, _ := filepath.Glob("testdata/*")
+	if len(tests) == 0 {
+		t.Fatalf("no tests found")
+	}
+	for _, test := range tests {
+		file := filepath.Base(test)
+		ext := filepath.Ext(file)
+		if ext == ".out" {
+			continue
+		}
+		t.Run(file, func(t *testing.T) {
+			if extMap[ext] == "" {
+				t.Fatal("unknown extension")
+			}
+			if Converters[extMap[ext]] == nil {
+				t.Fatalf("Converters[%q] == nil", extMap[ext])
+			}
+			data, err := os.ReadFile(test)
+			if err != nil {
+				t.Fatal(err)
+			}
+			out, err := Converters[extMap[ext]](test, data)
+			if err != nil {
+				t.Fatal(err)
+			}
+			want, err := os.ReadFile(test[:len(test)-len(ext)] + ".out")
+			if err != nil {
+				t.Error(err)
+			}
+			var buf bytes.Buffer
+			for _, r := range out.Require {
+				fmt.Fprintf(&buf, "%s %s\n", r.Mod.Path, r.Mod.Version)
+			}
+			for _, r := range out.Replace {
+				fmt.Fprintf(&buf, "replace: %s %s %s %s\n", r.Old.Path, r.Old.Version, r.New.Path, r.New.Version)
+			}
+			if !bytes.Equal(buf.Bytes(), want) {
+				t.Errorf("have:\n%s\nwant:\n%s", buf.Bytes(), want)
+			}
+		})
+	}
+}
diff --git a/src/cmd/go/internal/modconv/testdata/cockroach.glock b/src/cmd/go/internal/modconv/testdata/cockroach.glock
new file mode 100644
index 0000000..221c8ac
--- /dev/null
+++ b/src/cmd/go/internal/modconv/testdata/cockroach.glock
@@ -0,0 +1,41 @@
+cmd github.com/cockroachdb/c-protobuf/cmd/protoc
+cmd github.com/cockroachdb/yacc
+cmd github.com/gogo/protobuf/protoc-gen-gogo
+cmd github.com/golang/lint/golint
+cmd github.com/jteeuwen/go-bindata/go-bindata
+cmd github.com/kisielk/errcheck
+cmd github.com/robfig/glock
+cmd github.com/tebeka/go2xunit
+cmd golang.org/x/tools/cmd/goimports
+cmd golang.org/x/tools/cmd/stringer
+github.com/agtorre/gocolorize f42b554bf7f006936130c9bb4f971afd2d87f671
+github.com/biogo/store e1f74b3c58befe661feed7fa4cf52436de753128
+github.com/cockroachdb/c-lz4 6e71f140a365017bbe0904710007f8725fd3f809
+github.com/cockroachdb/c-protobuf 0f9ab7b988ca7474cf76b9a961ab03c0552abcb3
+github.com/cockroachdb/c-rocksdb 7fc876fe79b96de0e25069c9ae27e6444637bd54
+github.com/cockroachdb/c-snappy 618733f9e5bab8463b9049117a335a7a1bfc9fd5
+github.com/cockroachdb/yacc 572e006f8e6b0061ebda949d13744f5108389514
+github.com/coreos/etcd 18ecc297bc913bed6fc093d66b1fa22020dba7dc
+github.com/docker/docker 7374852be9def787921aea2ca831771982badecf
+github.com/elazarl/go-bindata-assetfs 3dcc96556217539f50599357fb481ac0dc7439b9
+github.com/gogo/protobuf 98e73e511a62a9c232152f94999112c80142a813
+github.com/golang/lint 7b7f4364ff76043e6c3610281525fabc0d90f0e4
+github.com/google/btree cc6329d4279e3f025a53a83c397d2339b5705c45
+github.com/inconshreveable/mousetrap 76626ae9c91c4f2a10f34cad8ce83ea42c93bb75
+github.com/jteeuwen/go-bindata dce55d09e24ac40a6e725c8420902b86554f8046
+github.com/julienschmidt/httprouter 6aacfd5ab513e34f7e64ea9627ab9670371b34e7
+github.com/kisielk/errcheck 50b84cf7fa18ee2985b8c63ba3de5edd604b9259
+github.com/kisielk/gotool d678387370a2eb9b5b0a33218bc8c9d8de15b6be
+github.com/lib/pq a8d8d01c4f91602f876bf5aa210274e8203a6b45
+github.com/montanaflynn/stats 44fb56da2a2a67d394dec0e18a82dd316f192529
+github.com/peterh/liner 1bb0d1c1a25ed393d8feb09bab039b2b1b1fbced
+github.com/robfig/glock cb3c3ec56de988289cab7bbd284eddc04dfee6c9
+github.com/samalba/dockerclient 12570e600d71374233e5056ba315f657ced496c7
+github.com/spf13/cobra 66816bcd0378e248c613e3c443c020f544c28804
+github.com/spf13/pflag 67cbc198fd11dab704b214c1e629a97af392c085
+github.com/tebeka/go2xunit d45000af2242dd0e7b8c7b07d82a1068adc5fd40
+golang.org/x/crypto cc04154d65fb9296747569b107cfd05380b1ea3e
+golang.org/x/net 8bfde94a845cb31000de3266ac83edbda58dab09
+golang.org/x/text d4cc1b1e16b49d6dafc4982403b40fe89c512cd5
+golang.org/x/tools d02228d1857b9f49cd0252788516ff5584266eb6
+gopkg.in/yaml.v1 9f9df34309c04878acc86042b16630b0f696e1de
diff --git a/src/cmd/go/internal/modconv/testdata/cockroach.out b/src/cmd/go/internal/modconv/testdata/cockroach.out
new file mode 100644
index 0000000..30cdbb7
--- /dev/null
+++ b/src/cmd/go/internal/modconv/testdata/cockroach.out
@@ -0,0 +1,31 @@
+github.com/agtorre/gocolorize f42b554bf7f006936130c9bb4f971afd2d87f671
+github.com/biogo/store e1f74b3c58befe661feed7fa4cf52436de753128
+github.com/cockroachdb/c-lz4 6e71f140a365017bbe0904710007f8725fd3f809
+github.com/cockroachdb/c-protobuf 0f9ab7b988ca7474cf76b9a961ab03c0552abcb3
+github.com/cockroachdb/c-rocksdb 7fc876fe79b96de0e25069c9ae27e6444637bd54
+github.com/cockroachdb/c-snappy 618733f9e5bab8463b9049117a335a7a1bfc9fd5
+github.com/cockroachdb/yacc 572e006f8e6b0061ebda949d13744f5108389514
+github.com/coreos/etcd 18ecc297bc913bed6fc093d66b1fa22020dba7dc
+github.com/docker/docker 7374852be9def787921aea2ca831771982badecf
+github.com/elazarl/go-bindata-assetfs 3dcc96556217539f50599357fb481ac0dc7439b9
+github.com/gogo/protobuf 98e73e511a62a9c232152f94999112c80142a813
+github.com/golang/lint 7b7f4364ff76043e6c3610281525fabc0d90f0e4
+github.com/google/btree cc6329d4279e3f025a53a83c397d2339b5705c45
+github.com/inconshreveable/mousetrap 76626ae9c91c4f2a10f34cad8ce83ea42c93bb75
+github.com/jteeuwen/go-bindata dce55d09e24ac40a6e725c8420902b86554f8046
+github.com/julienschmidt/httprouter 6aacfd5ab513e34f7e64ea9627ab9670371b34e7
+github.com/kisielk/errcheck 50b84cf7fa18ee2985b8c63ba3de5edd604b9259
+github.com/kisielk/gotool d678387370a2eb9b5b0a33218bc8c9d8de15b6be
+github.com/lib/pq a8d8d01c4f91602f876bf5aa210274e8203a6b45
+github.com/montanaflynn/stats 44fb56da2a2a67d394dec0e18a82dd316f192529
+github.com/peterh/liner 1bb0d1c1a25ed393d8feb09bab039b2b1b1fbced
+github.com/robfig/glock cb3c3ec56de988289cab7bbd284eddc04dfee6c9
+github.com/samalba/dockerclient 12570e600d71374233e5056ba315f657ced496c7
+github.com/spf13/cobra 66816bcd0378e248c613e3c443c020f544c28804
+github.com/spf13/pflag 67cbc198fd11dab704b214c1e629a97af392c085
+github.com/tebeka/go2xunit d45000af2242dd0e7b8c7b07d82a1068adc5fd40
+golang.org/x/crypto cc04154d65fb9296747569b107cfd05380b1ea3e
+golang.org/x/net 8bfde94a845cb31000de3266ac83edbda58dab09
+golang.org/x/text d4cc1b1e16b49d6dafc4982403b40fe89c512cd5
+golang.org/x/tools d02228d1857b9f49cd0252788516ff5584266eb6
+gopkg.in/yaml.v1 9f9df34309c04878acc86042b16630b0f696e1de
diff --git a/src/cmd/go/internal/modconv/testdata/dockermachine.godeps b/src/cmd/go/internal/modconv/testdata/dockermachine.godeps
new file mode 100644
index 0000000..a551002
--- /dev/null
+++ b/src/cmd/go/internal/modconv/testdata/dockermachine.godeps
@@ -0,0 +1,159 @@
+{
+	"ImportPath": "github.com/docker/machine",
+	"GoVersion": "go1.4.2",
+	"Deps": [
+		{
+			"ImportPath": "code.google.com/p/goauth2/oauth",
+			"Comment": "weekly-56",
+			"Rev": "afe77d958c701557ec5dc56f6936fcc194d15520"
+		},
+		{
+			"ImportPath": "github.com/MSOpenTech/azure-sdk-for-go",
+			"Comment": "v1.1-17-g515f3ec",
+			"Rev": "515f3ec74ce6a5b31e934cefae997c97bd0a1b1e"
+		},
+		{
+			"ImportPath": "github.com/cenkalti/backoff",
+			"Rev": "9831e1e25c874e0a0601b6dc43641071414eec7a"
+		},
+		{
+			"ImportPath": "github.com/codegangsta/cli",
+			"Comment": "1.2.0-64-ge1712f3",
+			"Rev": "e1712f381785e32046927f64a7c86fe569203196"
+		},
+		{
+			"ImportPath": "github.com/digitalocean/godo",
+			"Comment": "v0.5.0",
+			"Rev": "5478aae80694de1d2d0e02c386bbedd201266234"
+		},
+		{
+			"ImportPath": "github.com/docker/docker/dockerversion",
+			"Comment": "v1.5.0",
+			"Rev": "a8a31eff10544860d2188dddabdee4d727545796"
+		},
+		{
+			"ImportPath": "github.com/docker/docker/engine",
+			"Comment": "v1.5.0",
+			"Rev": "a8a31eff10544860d2188dddabdee4d727545796"
+		},
+		{
+			"ImportPath": "github.com/docker/docker/pkg/archive",
+			"Comment": "v1.5.0",
+			"Rev": "a8a31eff10544860d2188dddabdee4d727545796"
+		},
+		{
+			"ImportPath": "github.com/docker/docker/pkg/fileutils",
+			"Comment": "v1.5.0",
+			"Rev": "a8a31eff10544860d2188dddabdee4d727545796"
+		},
+		{
+			"ImportPath": "github.com/docker/docker/pkg/ioutils",
+			"Comment": "v1.5.0",
+			"Rev": "a8a31eff10544860d2188dddabdee4d727545796"
+		},
+		{
+			"ImportPath": "github.com/docker/docker/pkg/mflag",
+			"Comment": "v1.5.0",
+			"Rev": "a8a31eff10544860d2188dddabdee4d727545796"
+		},
+		{
+			"ImportPath": "github.com/docker/docker/pkg/parsers",
+			"Comment": "v1.5.0",
+			"Rev": "a8a31eff10544860d2188dddabdee4d727545796"
+		},
+		{
+			"ImportPath": "github.com/docker/docker/pkg/pools",
+			"Comment": "v1.5.0",
+			"Rev": "a8a31eff10544860d2188dddabdee4d727545796"
+		},
+		{
+			"ImportPath": "github.com/docker/docker/pkg/promise",
+			"Comment": "v1.5.0",
+			"Rev": "a8a31eff10544860d2188dddabdee4d727545796"
+		},
+		{
+			"ImportPath": "github.com/docker/docker/pkg/system",
+			"Comment": "v1.5.0",
+			"Rev": "a8a31eff10544860d2188dddabdee4d727545796"
+		},
+		{
+			"ImportPath": "github.com/docker/docker/pkg/term",
+			"Comment": "v1.5.0",
+			"Rev": "a8a31eff10544860d2188dddabdee4d727545796"
+		},
+		{
+			"ImportPath": "github.com/docker/docker/pkg/timeutils",
+			"Comment": "v1.5.0",
+			"Rev": "a8a31eff10544860d2188dddabdee4d727545796"
+		},
+		{
+			"ImportPath": "github.com/docker/docker/pkg/units",
+			"Comment": "v1.5.0",
+			"Rev": "a8a31eff10544860d2188dddabdee4d727545796"
+		},
+		{
+			"ImportPath": "github.com/docker/docker/pkg/version",
+			"Comment": "v1.5.0",
+			"Rev": "a8a31eff10544860d2188dddabdee4d727545796"
+		},
+		{
+			"ImportPath": "github.com/docker/docker/vendor/src/code.google.com/p/go/src/pkg/archive/tar",
+			"Comment": "v1.5.0",
+			"Rev": "a8a31eff10544860d2188dddabdee4d727545796"
+		},
+		{
+			"ImportPath": "github.com/docker/libtrust",
+			"Rev": "c54fbb67c1f1e68d7d6f8d2ad7c9360404616a41"
+		},
+		{
+			"ImportPath": "github.com/google/go-querystring/query",
+			"Rev": "30f7a39f4a218feb5325f3aebc60c32a572a8274"
+		},
+		{
+			"ImportPath": "github.com/mitchellh/mapstructure",
+			"Rev": "740c764bc6149d3f1806231418adb9f52c11bcbf"
+		},
+		{
+			"ImportPath": "github.com/rackspace/gophercloud",
+			"Comment": "v1.0.0-558-ce0f487",
+			"Rev": "ce0f487f6747ab43c4e4404722df25349385bebd"
+		},
+		{
+			"ImportPath": "github.com/skarademir/naturalsort",
+			"Rev": "983d4d86054d80f91fd04dd62ec52c1d078ce403"
+		},
+		{
+			"ImportPath": "github.com/smartystreets/go-aws-auth",
+			"Rev": "1f0db8c0ee6362470abe06a94e3385927ed72a4b"
+		},
+		{
+			"ImportPath": "github.com/stretchr/testify/assert",
+			"Rev": "e4ec8152c15fc46bd5056ce65997a07c7d415325"
+		},
+		{
+			"ImportPath": "github.com/pyr/egoscale/src/egoscale",
+			"Rev": "bbaa67324aeeacc90430c1fe0a9c620d3929512e"
+		},
+		{
+			"ImportPath": "github.com/tent/http-link-go",
+			"Rev": "ac974c61c2f990f4115b119354b5e0b47550e888"
+		},
+		{
+			"ImportPath": "github.com/vmware/govcloudair",
+			"Comment": "v0.0.2",
+			"Rev": "66a23eaabc61518f91769939ff541886fe1dceef"
+		},
+		{
+			"ImportPath": "golang.org/x/crypto/ssh",
+			"Rev": "1fbbd62cfec66bd39d91e97749579579d4d3037e"
+		},
+		{
+			"ImportPath": "google.golang.org/api/compute/v1",
+			"Rev": "aa91ac681e18e52b1a0dfe29b9d8354e88c0dcf5"
+		},
+		{
+			"ImportPath": "google.golang.org/api/googleapi",
+			"Rev": "aa91ac681e18e52b1a0dfe29b9d8354e88c0dcf5"
+		}
+	]
+}
diff --git a/src/cmd/go/internal/modconv/testdata/dockermachine.out b/src/cmd/go/internal/modconv/testdata/dockermachine.out
new file mode 100644
index 0000000..0b39cea
--- /dev/null
+++ b/src/cmd/go/internal/modconv/testdata/dockermachine.out
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diff --git a/src/cmd/go/internal/modconv/testdata/dockerman.glide b/src/cmd/go/internal/modconv/testdata/dockerman.glide
new file mode 100644
index 0000000..5ec765a
--- /dev/null
+++ b/src/cmd/go/internal/modconv/testdata/dockerman.glide
@@ -0,0 +1,52 @@
+hash: ead3ea293a6143fe41069ebec814bf197d8c43a92cc7666b1f7e21a419b46feb
+updated: 2016-06-20T21:53:35.420817456Z
+imports:
+- name: github.com/BurntSushi/toml
+  version: f0aeabca5a127c4078abb8c8d64298b147264b55
+- name: github.com/cpuguy83/go-md2man
+  version: a65d4d2de4d5f7c74868dfa9b202a3c8be315aaa
+  subpackages:
+  - md2man
+- name: github.com/fsnotify/fsnotify
+  version: 30411dbcefb7a1da7e84f75530ad3abe4011b4f8
+- name: github.com/hashicorp/hcl
+  version: da486364306ed66c218be9b7953e19173447c18b
+  subpackages:
+  - hcl/ast
+  - hcl/parser
+  - hcl/token
+  - json/parser
+  - hcl/scanner
+  - hcl/strconv
+  - json/scanner
+  - json/token
+- name: github.com/inconshreveable/mousetrap
+  version: 76626ae9c91c4f2a10f34cad8ce83ea42c93bb75
+- name: github.com/magiconair/properties
+  version: c265cfa48dda6474e208715ca93e987829f572f8
+- name: github.com/mitchellh/mapstructure
+  version: d2dd0262208475919e1a362f675cfc0e7c10e905
+- name: github.com/russross/blackfriday
+  version: 1d6b8e9301e720b08a8938b8c25c018285885438
+- name: github.com/shurcooL/sanitized_anchor_name
+  version: 10ef21a441db47d8b13ebcc5fd2310f636973c77
+- name: github.com/spf13/cast
+  version: 27b586b42e29bec072fe7379259cc719e1289da6
+- name: github.com/spf13/jwalterweatherman
+  version: 33c24e77fb80341fe7130ee7c594256ff08ccc46
+- name: github.com/spf13/pflag
+  version: dabebe21bf790f782ea4c7bbd2efc430de182afd
+- name: github.com/spf13/viper
+  version: c1ccc378a054ea8d4e38d8c67f6938d4760b53dd
+- name: golang.org/x/sys
+  version: 62bee037599929a6e9146f29d10dd5208c43507d
+  subpackages:
+  - unix
+- name: gopkg.in/yaml.v2
+  version: a83829b6f1293c91addabc89d0571c246397bbf4
+- name: github.com/spf13/cobra
+  repo: https://github.com/dnephin/cobra
+  subpackages:
+  - doc
+  version: v1.3
+devImports: []
diff --git a/src/cmd/go/internal/modconv/testdata/dockerman.out b/src/cmd/go/internal/modconv/testdata/dockerman.out
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diff --git a/src/cmd/go/internal/modconv/testdata/govmomi.out b/src/cmd/go/internal/modconv/testdata/govmomi.out
new file mode 100644
index 0000000..188c458
--- /dev/null
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diff --git a/src/cmd/go/internal/modconv/testdata/govmomi.vmanifest b/src/cmd/go/internal/modconv/testdata/govmomi.vmanifest
new file mode 100644
index 0000000..b89e4ab
--- /dev/null
+++ b/src/cmd/go/internal/modconv/testdata/govmomi.vmanifest
@@ -0,0 +1,46 @@
+{
+	"version": 0,
+	"dependencies": [
+		{
+			"importpath": "github.com/davecgh/go-xdr/xdr2",
+			"repository": "https://github.com/rasky/go-xdr",
+			"vcs": "git",
+			"revision": "4930550ba2e22f87187498acfd78348b15f4e7a8",
+			"branch": "improvements",
+			"path": "/xdr2",
+			"notests": true
+		},
+		{
+			"importpath": "github.com/google/uuid",
+			"repository": "https://github.com/google/uuid",
+			"vcs": "git",
+			"revision": "6a5e28554805e78ea6141142aba763936c4761c0",
+			"branch": "master",
+			"notests": true
+		},
+		{
+			"importpath": "github.com/kr/pretty",
+			"repository": "https://github.com/dougm/pretty",
+			"vcs": "git",
+			"revision": "2ee9d7453c02ef7fa518a83ae23644eb8872186a",
+			"branch": "govmomi",
+			"notests": true
+		},
+		{
+			"importpath": "github.com/kr/pty",
+			"repository": "https://github.com/kr/pty",
+			"vcs": "git",
+			"revision": "95d05c1eef33a45bd58676b6ce28d105839b8d0b",
+			"branch": "master",
+			"notests": true
+		},
+		{
+			"importpath": "github.com/vmware/vmw-guestinfo",
+			"repository": "https://github.com/vmware/vmw-guestinfo",
+			"vcs": "git",
+			"revision": "25eff159a728be87e103a0b8045e08273f4dbec4",
+			"branch": "master",
+			"notests": true
+		}
+	]
+}
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diff --git a/src/cmd/go/internal/modconv/testdata/juju.tsv b/src/cmd/go/internal/modconv/testdata/juju.tsv
new file mode 100644
index 0000000..0bddcef
--- /dev/null
+++ b/src/cmd/go/internal/modconv/testdata/juju.tsv
@@ -0,0 +1,106 @@
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diff --git a/src/cmd/go/internal/modconv/testdata/moby.out b/src/cmd/go/internal/modconv/testdata/moby.out
new file mode 100644
index 0000000..2cb2e05
--- /dev/null
+++ b/src/cmd/go/internal/modconv/testdata/moby.out
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+github.com/coreos/etcd v3.2.1
+github.com/coreos/go-semver v0.2.0
+github.com/ugorji/go f1f1a805ed361a0e078bb537e4ea78cd37dcf065
+github.com/hashicorp/consul v0.5.2
+github.com/boltdb/bolt fff57c100f4dea1905678da7e90d92429dff2904
+github.com/miekg/dns 75e6e86cc601825c5dbcd4e0c209eab180997cd7
+github.com/docker/distribution edc3ab29cdff8694dd6feb85cfeb4b5f1b38ed9c
+github.com/vbatts/tar-split v0.10.1
+github.com/opencontainers/go-digest a6d0ee40d4207ea02364bd3b9e8e77b9159ba1eb
+github.com/mistifyio/go-zfs 22c9b32c84eb0d0c6f4043b6e90fc94073de92fa
+github.com/pborman/uuid v1.0
+google.golang.org/grpc v1.3.0
+github.com/opencontainers/runc 0351df1c5a66838d0c392b4ac4cf9450de844e2d
+github.com/opencontainers/image-spec 372ad780f63454fbbbbcc7cf80e5b90245c13e13
+github.com/opencontainers/runtime-spec v1.0.0
+github.com/seccomp/libseccomp-golang 32f571b70023028bd57d9288c20efbcb237f3ce0
+github.com/coreos/go-systemd v4
+github.com/godbus/dbus v4.0.0
+github.com/syndtr/gocapability 2c00daeb6c3b45114c80ac44119e7b8801fdd852
+github.com/golang/protobuf 7a211bcf3bce0e3f1d74f9894916e6f116ae83b4
+github.com/Graylog2/go-gelf v2
+github.com/fluent/fluent-logger-golang v1.2.1
+github.com/philhofer/fwd 98c11a7a6ec829d672b03833c3d69a7fae1ca972
+github.com/tinylib/msgp 75ee40d2601edf122ef667e2a07d600d4c44490c
+github.com/fsnotify/fsnotify v1.4.2
+github.com/aws/aws-sdk-go v1.4.22
+github.com/go-ini/ini 060d7da055ba6ec5ea7a31f116332fe5efa04ce0
+github.com/jmespath/go-jmespath 0b12d6b521d83fc7f755e7cfc1b1fbdd35a01a74
+github.com/bsphere/le_go 7a984a84b5492ae539b79b62fb4a10afc63c7bcf
+golang.org/x/oauth2 96382aa079b72d8c014eb0c50f6c223d1e6a2de0
+google.golang.org/api 3cc2e591b550923a2c5f0ab5a803feda924d5823
+cloud.google.com/go 9d965e63e8cceb1b5d7977a202f0fcb8866d6525
+github.com/googleapis/gax-go da06d194a00e19ce00d9011a13931c3f6f6887c7
+google.golang.org/genproto d80a6e20e776b0b17a324d0ba1ab50a39c8e8944
+github.com/containerd/containerd 06b9cb35161009dcb7123345749fef02f7cea8e0
+github.com/tonistiigi/fifo 1405643975692217d6720f8b54aeee1bf2cd5cf4
+github.com/docker/swarmkit 872861d2ae46958af7ead1d5fffb092c73afbaf0
+github.com/gogo/protobuf v0.4
+github.com/cloudflare/cfssl 7fb22c8cba7ecaf98e4082d22d65800cf45e042a
+github.com/google/certificate-transparency d90e65c3a07988180c5b1ece71791c0b6506826e
+golang.org/x/crypto 558b6879de74bc843225cde5686419267ff707ca
+golang.org/x/time a4bde12657593d5e90d0533a3e4fd95e635124cb
+github.com/hashicorp/go-memdb cb9a474f84cc5e41b273b20c6927680b2a8776ad
+github.com/hashicorp/go-immutable-radix 8e8ed81f8f0bf1bdd829593fdd5c29922c1ea990
+github.com/hashicorp/golang-lru a0d98a5f288019575c6d1f4bb1573fef2d1fcdc4
+github.com/coreos/pkg fa29b1d70f0beaddd4c7021607cc3c3be8ce94b8
+github.com/pivotal-golang/clock 3fd3c1944c59d9742e1cd333672181cd1a6f9fa0
+github.com/prometheus/client_golang 52437c81da6b127a9925d17eb3a382a2e5fd395e
+github.com/beorn7/perks 4c0e84591b9aa9e6dcfdf3e020114cd81f89d5f9
+github.com/prometheus/client_model fa8ad6fec33561be4280a8f0514318c79d7f6cb6
+github.com/prometheus/common ebdfc6da46522d58825777cf1f90490a5b1ef1d8
+github.com/prometheus/procfs abf152e5f3e97f2fafac028d2cc06c1feb87ffa5
+github.com/matttproud/golang_protobuf_extensions v1.0.0
+github.com/pkg/errors 839d9e913e063e28dfd0e6c7b7512793e0a48be9
+github.com/grpc-ecosystem/go-grpc-prometheus 6b7015e65d366bf3f19b2b2a000a831940f0f7e0
+github.com/spf13/cobra v1.5.1
+github.com/spf13/pflag 9ff6c6923cfffbcd502984b8e0c80539a94968b7
+github.com/inconshreveable/mousetrap 76626ae9c91c4f2a10f34cad8ce83ea42c93bb75
+github.com/Nvveen/Gotty a8b993ba6abdb0e0c12b0125c603323a71c7790c
+github.com/docker/go-metrics d466d4f6fd960e01820085bd7e1a24426ee7ef18
+github.com/opencontainers/selinux v1.0.0-rc1
diff --git a/src/cmd/go/internal/modconv/testdata/moby.vconf b/src/cmd/go/internal/modconv/testdata/moby.vconf
new file mode 100644
index 0000000..53b90d1
--- /dev/null
+++ b/src/cmd/go/internal/modconv/testdata/moby.vconf
@@ -0,0 +1,149 @@
+# the following lines are in sorted order, FYI
+github.com/Azure/go-ansiterm d6e3b3328b783f23731bc4d058875b0371ff8109
+github.com/Microsoft/hcsshim v0.6.5
+github.com/Microsoft/go-winio v0.4.5
+github.com/davecgh/go-spew 346938d642f2ec3594ed81d874461961cd0faa76
+github.com/docker/libtrust 9cbd2a1374f46905c68a4eb3694a130610adc62a
+github.com/go-check/check 4ed411733c5785b40214c70bce814c3a3a689609 https://github.com/cpuguy83/check.git
+github.com/gorilla/context v1.1
+github.com/gorilla/mux v1.1
+github.com/Microsoft/opengcs v0.3.4
+github.com/kr/pty 5cf931ef8f
+github.com/mattn/go-shellwords v1.0.3
+github.com/sirupsen/logrus v1.0.3
+github.com/tchap/go-patricia v2.2.6
+github.com/vdemeester/shakers 24d7f1d6a71aa5d9cbe7390e4afb66b7eef9e1b3
+golang.org/x/net 7dcfb8076726a3fdd9353b6b8a1f1b6be6811bd6
+golang.org/x/sys 07c182904dbd53199946ba614a412c61d3c548f5
+github.com/docker/go-units 9e638d38cf6977a37a8ea0078f3ee75a7cdb2dd1
+github.com/docker/go-connections 3ede32e2033de7505e6500d6c868c2b9ed9f169d
+golang.org/x/text f72d8390a633d5dfb0cc84043294db9f6c935756
+github.com/stretchr/testify 4d4bfba8f1d1027c4fdbe371823030df51419987
+github.com/pmezard/go-difflib v1.0.0
+github.com/gotestyourself/gotestyourself v1.1.0
+
+github.com/RackSec/srslog 456df3a81436d29ba874f3590eeeee25d666f8a5
+github.com/imdario/mergo 0.2.1
+golang.org/x/sync de49d9dcd27d4f764488181bea099dfe6179bcf0
+
+github.com/containerd/continuity 22694c680ee48fb8f50015b44618517e2bde77e8
+github.com/moby/buildkit aaff9d591ef128560018433fe61beb802e149de8
+github.com/tonistiigi/fsutil dea3a0da73aee887fc02142d995be764106ac5e2
+
+#get libnetwork packages
+github.com/docker/libnetwork 68f1039f172434709a4550fe92e3e058406c74ce 
+github.com/docker/go-events 9461782956ad83b30282bf90e31fa6a70c255ba9
+github.com/armon/go-radix e39d623f12e8e41c7b5529e9a9dd67a1e2261f80
+github.com/armon/go-metrics eb0af217e5e9747e41dd5303755356b62d28e3ec
+github.com/hashicorp/go-msgpack 71c2886f5a673a35f909803f38ece5810165097b
+github.com/hashicorp/memberlist v0.1.0
+github.com/sean-/seed e2103e2c35297fb7e17febb81e49b312087a2372
+github.com/hashicorp/go-sockaddr acd314c5781ea706c710d9ea70069fd2e110d61d
+github.com/hashicorp/go-multierror fcdddc395df1ddf4247c69bd436e84cfa0733f7e
+github.com/hashicorp/serf 598c54895cc5a7b1a24a398d635e8c0ea0959870
+github.com/docker/libkv 1d8431073ae03cdaedb198a89722f3aab6d418ef
+github.com/vishvananda/netns 604eaf189ee867d8c147fafc28def2394e878d25
+github.com/vishvananda/netlink bd6d5de5ccef2d66b0a26177928d0d8895d7f969
+github.com/BurntSushi/toml f706d00e3de6abe700c994cdd545a1a4915af060
+github.com/samuel/go-zookeeper d0e0d8e11f318e000a8cc434616d69e329edc374
+github.com/deckarep/golang-set ef32fa3046d9f249d399f98ebaf9be944430fd1d
+github.com/coreos/etcd v3.2.1
+github.com/coreos/go-semver v0.2.0
+github.com/ugorji/go f1f1a805ed361a0e078bb537e4ea78cd37dcf065
+github.com/hashicorp/consul v0.5.2
+github.com/boltdb/bolt fff57c100f4dea1905678da7e90d92429dff2904
+github.com/miekg/dns 75e6e86cc601825c5dbcd4e0c209eab180997cd7
+
+# get graph and distribution packages
+github.com/docker/distribution edc3ab29cdff8694dd6feb85cfeb4b5f1b38ed9c
+github.com/vbatts/tar-split v0.10.1
+github.com/opencontainers/go-digest a6d0ee40d4207ea02364bd3b9e8e77b9159ba1eb
+
+# get go-zfs packages
+github.com/mistifyio/go-zfs 22c9b32c84eb0d0c6f4043b6e90fc94073de92fa
+github.com/pborman/uuid v1.0
+
+google.golang.org/grpc v1.3.0
+
+# When updating, also update RUNC_COMMIT in hack/dockerfile/binaries-commits accordingly
+github.com/opencontainers/runc 0351df1c5a66838d0c392b4ac4cf9450de844e2d
+github.com/opencontainers/image-spec 372ad780f63454fbbbbcc7cf80e5b90245c13e13
+github.com/opencontainers/runtime-spec v1.0.0
+
+github.com/seccomp/libseccomp-golang 32f571b70023028bd57d9288c20efbcb237f3ce0
+
+# libcontainer deps (see src/github.com/opencontainers/runc/Godeps/Godeps.json)
+github.com/coreos/go-systemd v4
+github.com/godbus/dbus v4.0.0
+github.com/syndtr/gocapability 2c00daeb6c3b45114c80ac44119e7b8801fdd852
+github.com/golang/protobuf 7a211bcf3bce0e3f1d74f9894916e6f116ae83b4
+
+# gelf logging driver deps
+github.com/Graylog2/go-gelf v2
+
+github.com/fluent/fluent-logger-golang v1.2.1
+# fluent-logger-golang deps
+github.com/philhofer/fwd 98c11a7a6ec829d672b03833c3d69a7fae1ca972
+github.com/tinylib/msgp 75ee40d2601edf122ef667e2a07d600d4c44490c
+
+# fsnotify
+github.com/fsnotify/fsnotify v1.4.2
+
+# awslogs deps
+github.com/aws/aws-sdk-go v1.4.22
+github.com/go-ini/ini 060d7da055ba6ec5ea7a31f116332fe5efa04ce0
+github.com/jmespath/go-jmespath 0b12d6b521d83fc7f755e7cfc1b1fbdd35a01a74
+
+# logentries
+github.com/bsphere/le_go 7a984a84b5492ae539b79b62fb4a10afc63c7bcf
+
+# gcplogs deps
+golang.org/x/oauth2 96382aa079b72d8c014eb0c50f6c223d1e6a2de0
+google.golang.org/api 3cc2e591b550923a2c5f0ab5a803feda924d5823
+cloud.google.com/go 9d965e63e8cceb1b5d7977a202f0fcb8866d6525
+github.com/googleapis/gax-go da06d194a00e19ce00d9011a13931c3f6f6887c7
+google.golang.org/genproto d80a6e20e776b0b17a324d0ba1ab50a39c8e8944
+
+# containerd
+github.com/containerd/containerd 06b9cb35161009dcb7123345749fef02f7cea8e0
+github.com/tonistiigi/fifo 1405643975692217d6720f8b54aeee1bf2cd5cf4
+
+# cluster
+github.com/docker/swarmkit 872861d2ae46958af7ead1d5fffb092c73afbaf0
+github.com/gogo/protobuf v0.4
+github.com/cloudflare/cfssl 7fb22c8cba7ecaf98e4082d22d65800cf45e042a
+github.com/google/certificate-transparency d90e65c3a07988180c5b1ece71791c0b6506826e
+golang.org/x/crypto 558b6879de74bc843225cde5686419267ff707ca
+golang.org/x/time a4bde12657593d5e90d0533a3e4fd95e635124cb
+github.com/hashicorp/go-memdb cb9a474f84cc5e41b273b20c6927680b2a8776ad
+github.com/hashicorp/go-immutable-radix 8e8ed81f8f0bf1bdd829593fdd5c29922c1ea990
+github.com/hashicorp/golang-lru a0d98a5f288019575c6d1f4bb1573fef2d1fcdc4
+github.com/coreos/pkg fa29b1d70f0beaddd4c7021607cc3c3be8ce94b8
+github.com/pivotal-golang/clock 3fd3c1944c59d9742e1cd333672181cd1a6f9fa0
+github.com/prometheus/client_golang 52437c81da6b127a9925d17eb3a382a2e5fd395e
+github.com/beorn7/perks 4c0e84591b9aa9e6dcfdf3e020114cd81f89d5f9
+github.com/prometheus/client_model fa8ad6fec33561be4280a8f0514318c79d7f6cb6
+github.com/prometheus/common ebdfc6da46522d58825777cf1f90490a5b1ef1d8
+github.com/prometheus/procfs abf152e5f3e97f2fafac028d2cc06c1feb87ffa5
+github.com/matttproud/golang_protobuf_extensions v1.0.0
+github.com/pkg/errors 839d9e913e063e28dfd0e6c7b7512793e0a48be9
+github.com/grpc-ecosystem/go-grpc-prometheus 6b7015e65d366bf3f19b2b2a000a831940f0f7e0
+
+# cli
+github.com/spf13/cobra v1.5.1 https://github.com/dnephin/cobra.git
+github.com/spf13/pflag 9ff6c6923cfffbcd502984b8e0c80539a94968b7
+github.com/inconshreveable/mousetrap 76626ae9c91c4f2a10f34cad8ce83ea42c93bb75
+github.com/Nvveen/Gotty a8b993ba6abdb0e0c12b0125c603323a71c7790c https://github.com/ijc25/Gotty
+
+# metrics
+github.com/docker/go-metrics d466d4f6fd960e01820085bd7e1a24426ee7ef18
+
+github.com/opencontainers/selinux v1.0.0-rc1
+
+# archive/tar
+# mkdir -p ./vendor/archive
+# git clone git://github.com/tonistiigi/go-1.git ./go
+# git --git-dir ./go/.git --work-tree ./go checkout revert-prefix-ignore
+# cp -a go/src/archive/tar ./vendor/archive/tar
+# rm -rf ./go
+# vndr
diff --git a/src/cmd/go/internal/modconv/testdata/panicparse.out b/src/cmd/go/internal/modconv/testdata/panicparse.out
new file mode 100644
index 0000000..8830033
--- /dev/null
+++ b/src/cmd/go/internal/modconv/testdata/panicparse.out
@@ -0,0 +1,8 @@
+github.com/kr/pretty 737b74a46c4bf788349f72cb256fed10aea4d0ac
+github.com/kr/text 7cafcd837844e784b526369c9bce262804aebc60
+github.com/maruel/ut a9c9f15ccfa6f8b90182a53df32f4745586fbae3
+github.com/mattn/go-colorable 9056b7a9f2d1f2d96498d6d146acd1f9d5ed3d59
+github.com/mattn/go-isatty 56b76bdf51f7708750eac80fa38b952bb9f32639
+github.com/mgutz/ansi c286dcecd19ff979eeb73ea444e479b903f2cfcb
+github.com/pmezard/go-difflib 792786c7400a136282c1664665ae0a8db921c6c2
+golang.org/x/sys a646d33e2ee3172a661fc09bca23bb4889a41bc8
diff --git a/src/cmd/go/internal/modconv/testdata/panicparse.vyml b/src/cmd/go/internal/modconv/testdata/panicparse.vyml
new file mode 100644
index 0000000..ff3d43f
--- /dev/null
+++ b/src/cmd/go/internal/modconv/testdata/panicparse.vyml
@@ -0,0 +1,17 @@
+vendors:
+- path: github.com/kr/pretty
+  rev: 737b74a46c4bf788349f72cb256fed10aea4d0ac
+- path: github.com/kr/text
+  rev: 7cafcd837844e784b526369c9bce262804aebc60
+- path: github.com/maruel/ut
+  rev: a9c9f15ccfa6f8b90182a53df32f4745586fbae3
+- path: github.com/mattn/go-colorable
+  rev: 9056b7a9f2d1f2d96498d6d146acd1f9d5ed3d59
+- path: github.com/mattn/go-isatty
+  rev: 56b76bdf51f7708750eac80fa38b952bb9f32639
+- path: github.com/mgutz/ansi
+  rev: c286dcecd19ff979eeb73ea444e479b903f2cfcb
+- path: github.com/pmezard/go-difflib
+  rev: 792786c7400a136282c1664665ae0a8db921c6c2
+- path: golang.org/x/sys
+  rev: a646d33e2ee3172a661fc09bca23bb4889a41bc8
diff --git a/src/cmd/go/internal/modconv/testdata/prometheus.out b/src/cmd/go/internal/modconv/testdata/prometheus.out
new file mode 100644
index 0000000..d11b8ec
--- /dev/null
+++ b/src/cmd/go/internal/modconv/testdata/prometheus.out
@@ -0,0 +1,258 @@
+cloud.google.com/go/compute/metadata c589d0c9f0d81640c518354c7bcae77d99820aa3
+cloud.google.com/go/internal c589d0c9f0d81640c518354c7bcae77d99820aa3
+github.com/Azure/azure-sdk-for-go/arm/compute bd73d950fa4440dae889bd9917bff7cef539f86e
+github.com/Azure/azure-sdk-for-go/arm/network bd73d950fa4440dae889bd9917bff7cef539f86e
+github.com/Azure/go-autorest/autorest 8a25372bbfec739b8719a9e3987400d15ef9e179
+github.com/Azure/go-autorest/autorest/azure 8a25372bbfec739b8719a9e3987400d15ef9e179
+github.com/Azure/go-autorest/autorest/date 8a25372bbfec739b8719a9e3987400d15ef9e179
+github.com/Azure/go-autorest/autorest/to 8a25372bbfec739b8719a9e3987400d15ef9e179
+github.com/Azure/go-autorest/autorest/validation 8a25372bbfec739b8719a9e3987400d15ef9e179
+github.com/PuerkitoBio/purell c589d0c9f0d81640c518354c7bcae77d99820aa3
+github.com/PuerkitoBio/urlesc c589d0c9f0d81640c518354c7bcae77d99820aa3
+github.com/asaskevich/govalidator 7b3beb6df3c42abd3509abfc3bcacc0fbfb7c877
+github.com/aws/aws-sdk-go/aws 707203bc55114ed114446bf57949c5c211d8b7c0
+github.com/aws/aws-sdk-go/aws/awserr 707203bc55114ed114446bf57949c5c211d8b7c0
+github.com/aws/aws-sdk-go/aws/awsutil 707203bc55114ed114446bf57949c5c211d8b7c0
+github.com/aws/aws-sdk-go/aws/client 707203bc55114ed114446bf57949c5c211d8b7c0
+github.com/aws/aws-sdk-go/aws/client/metadata 707203bc55114ed114446bf57949c5c211d8b7c0
+github.com/aws/aws-sdk-go/aws/corehandlers 707203bc55114ed114446bf57949c5c211d8b7c0
+github.com/aws/aws-sdk-go/aws/credentials 707203bc55114ed114446bf57949c5c211d8b7c0
+github.com/aws/aws-sdk-go/aws/credentials/ec2rolecreds 707203bc55114ed114446bf57949c5c211d8b7c0
+github.com/aws/aws-sdk-go/aws/credentials/endpointcreds 707203bc55114ed114446bf57949c5c211d8b7c0
+github.com/aws/aws-sdk-go/aws/credentials/stscreds 707203bc55114ed114446bf57949c5c211d8b7c0
+github.com/aws/aws-sdk-go/aws/defaults 707203bc55114ed114446bf57949c5c211d8b7c0
+github.com/aws/aws-sdk-go/aws/ec2metadata 707203bc55114ed114446bf57949c5c211d8b7c0
+github.com/aws/aws-sdk-go/aws/request 707203bc55114ed114446bf57949c5c211d8b7c0
+github.com/aws/aws-sdk-go/aws/session 707203bc55114ed114446bf57949c5c211d8b7c0
+github.com/aws/aws-sdk-go/aws/signer/v4 707203bc55114ed114446bf57949c5c211d8b7c0
+github.com/aws/aws-sdk-go/private/endpoints 707203bc55114ed114446bf57949c5c211d8b7c0
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diff --git a/src/cmd/go/internal/modconv/testdata/prometheus.vjson b/src/cmd/go/internal/modconv/testdata/prometheus.vjson
new file mode 100644
index 0000000..648bec4
--- /dev/null
+++ b/src/cmd/go/internal/modconv/testdata/prometheus.vjson
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+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "F7h+8zZ0JPLYkac4KgSVljguBE4=",
+			"path": "k8s.io/client-go/1.5/pkg/runtime/serializer/protobuf",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "CvySOL8C85e3y7EWQ+Au4cwUZJM=",
+			"path": "k8s.io/client-go/1.5/pkg/runtime/serializer/recognizer",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "eCitoKeIun+lJzYFhAfdSIIicSM=",
+			"path": "k8s.io/client-go/1.5/pkg/runtime/serializer/streaming",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "kVWvZuLGltJ4YqQsiaCLRRLDDK0=",
+			"path": "k8s.io/client-go/1.5/pkg/runtime/serializer/versioning",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "m51+LAeQ9RK1KHX+l2iGcwbVCKs=",
+			"path": "k8s.io/client-go/1.5/pkg/selection",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "dp4IWcC3U6a0HeOdVCDQWODWCbw=",
+			"path": "k8s.io/client-go/1.5/pkg/third_party/forked/golang/reflect",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "ER898XJD1ox4d71gKZD8TLtTSpM=",
+			"path": "k8s.io/client-go/1.5/pkg/types",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "BVdXtnLDlmBQksRPfHOIG+qdeVg=",
+			"path": "k8s.io/client-go/1.5/pkg/util",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "nnh8Sa4dCupxRI4bbKaozGp1d/A=",
+			"path": "k8s.io/client-go/1.5/pkg/util/cert",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "S32d5uduNlwouM8+mIz+ALpliUQ=",
+			"path": "k8s.io/client-go/1.5/pkg/util/clock",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "Y6rWC0TUw2/uUeUjJ7kazyEUzBQ=",
+			"path": "k8s.io/client-go/1.5/pkg/util/errors",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "C7IfEAdCOePw3/IraaZCNXuYXLw=",
+			"path": "k8s.io/client-go/1.5/pkg/util/flowcontrol",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "EuslQHnhBSRXaWimYqLEqhMPV48=",
+			"path": "k8s.io/client-go/1.5/pkg/util/framer",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "ByO18NbZwiifFr8qtLyfJAHXguA=",
+			"path": "k8s.io/client-go/1.5/pkg/util/integer",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "ww+RfsoIlUBDwThg2oqC5QVz33Y=",
+			"path": "k8s.io/client-go/1.5/pkg/util/intstr",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "7E8f8dLlXW7u6r9sggMjvB4HEiw=",
+			"path": "k8s.io/client-go/1.5/pkg/util/json",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "d0pFZxMJG9j95acNmaIM1l+X+QU=",
+			"path": "k8s.io/client-go/1.5/pkg/util/labels",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "wCN7u1lE+25neM9jXeI7aE8EAfk=",
+			"path": "k8s.io/client-go/1.5/pkg/util/net",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "g+kBkxcb+tYmFtRRly+VE+JAIfw=",
+			"path": "k8s.io/client-go/1.5/pkg/util/parsers",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "S4wUnE5VkaWWrkLbgPL/1oNLJ4g=",
+			"path": "k8s.io/client-go/1.5/pkg/util/rand",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "8j9c2PqTKybtnymXbStNYRexRj8=",
+			"path": "k8s.io/client-go/1.5/pkg/util/runtime",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "aAz4e8hLGs0+ZAz1TdA5tY/9e1A=",
+			"path": "k8s.io/client-go/1.5/pkg/util/sets",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "P/fwh6QZ5tsjVyHTaASDWL3WaGs=",
+			"path": "k8s.io/client-go/1.5/pkg/util/uuid",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "P9Bq/1qbF4SvnN9HyCTRpbUz7sQ=",
+			"path": "k8s.io/client-go/1.5/pkg/util/validation",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "D0JIEjlP69cuPOZEdsSKeFgsnI8=",
+			"path": "k8s.io/client-go/1.5/pkg/util/validation/field",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "T7ba8t8i+BtgClMgL+aMZM94fcI=",
+			"path": "k8s.io/client-go/1.5/pkg/util/wait",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "6RCTv/KDiw7as4KeyrgU3XrUSQI=",
+			"path": "k8s.io/client-go/1.5/pkg/util/yaml",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "OwKlsSeKtz1FBVC9cQ5gWRL5pKc=",
+			"path": "k8s.io/client-go/1.5/pkg/version",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "Oil9WGw/dODbpBopn6LWQGS3DYg=",
+			"path": "k8s.io/client-go/1.5/pkg/watch",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "r5alnRCbLaPsbTeJjjTVn/bt6uw=",
+			"path": "k8s.io/client-go/1.5/pkg/watch/versioned",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "X1+ltyfHui/XCwDupXIf39+9gWQ=",
+			"path": "k8s.io/client-go/1.5/plugin/pkg/client/auth",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "KYy+js37AS0ZT08g5uBr1ZoMPmE=",
+			"path": "k8s.io/client-go/1.5/plugin/pkg/client/auth/gcp",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "wQ9G5++lbQpejqCzGHo037N3YcY=",
+			"path": "k8s.io/client-go/1.5/plugin/pkg/client/auth/oidc",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "ABe8YfZVEDoRpAUqp2BKP8o1VIA=",
+			"path": "k8s.io/client-go/1.5/rest",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "Gbe0Vs9hkI7X5hhbXUuWdRFffSI=",
+			"path": "k8s.io/client-go/1.5/tools/cache",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "K/oOznXABjqSS1c2Fs407c5F8KA=",
+			"path": "k8s.io/client-go/1.5/tools/clientcmd/api",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "c1PQ4WJRfpA9BYcFHW2+46hu5IE=",
+			"path": "k8s.io/client-go/1.5/tools/metrics",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		},
+		{
+			"checksumSHA1": "e4W2q+6wvjejv3V0UCI1mewTTro=",
+			"path": "k8s.io/client-go/1.5/transport",
+			"revision": "c589d0c9f0d81640c518354c7bcae77d99820aa3",
+			"revisionTime": "2016-09-30T00:14:02Z"
+		}
+	],
+	"rootPath": "github.com/prometheus/prometheus"
+}
diff --git a/src/cmd/go/internal/modconv/testdata/traefik.dep b/src/cmd/go/internal/modconv/testdata/traefik.dep
new file mode 100644
index 0000000..8510f0f
--- /dev/null
+++ b/src/cmd/go/internal/modconv/testdata/traefik.dep
@@ -0,0 +1,79 @@
+# This file is autogenerated, do not edit; changes may be undone by the next 'dep ensure'.
+
+[[projects]]
+  name = "github.com/Nvveen/Gotty"
+  packages = ["."]
+  revision = "a8b993ba6abdb0e0c12b0125c603323a71c7790c"
+  source = "github.com/ijc25/Gotty"
+
+[[projects]]
+  branch = "master"
+  name = "github.com/OpenDNS/vegadns2client"
+  packages = ["."]
+  revision = "a3fa4a771d87bda2514a90a157e1fed1b6897d2e"
+
+[[projects]]
+  name = "github.com/PuerkitoBio/purell"
+  packages = ["."]
+  revision = "8a290539e2e8629dbc4e6bad948158f790ec31f4"
+  version = "v1.0.0"
+
+[[projects]]
+  name = "github.com/PuerkitoBio/urlesc"
+  packages = ["."]
+  revision = "5bd2802263f21d8788851d5305584c82a5c75d7e"
+
+[[projects]]
+  name = "github.com/Shopify/sarama"
+  packages = ["."]
+  revision = "70f6a705d4a17af059acbc6946fb2bd30762acd7"
+
+[[projects]]
+  name = "github.com/VividCortex/gohistogram"
+  packages = ["."]
+  revision = "51564d9861991fb0ad0f531c99ef602d0f9866e6"
+  version = "v1.0.0"
+
+[[projects]]
+  branch = "containous-fork"
+  name = "github.com/abbot/go-http-auth"
+  packages = ["."]
+  revision = "65b0cdae8d7fe5c05c7430e055938ef6d24a66c9"
+  source = "github.com/containous/go-http-auth"
+
+[[projects]]
+  branch = "master"
+  name = "github.com/abronan/valkeyrie"
+  packages = [
+    ".",
+    "store",
+    "store/boltdb",
+    "store/consul",
+    "store/etcd/v2",
+    "store/etcd/v3",
+    "store/zookeeper"
+  ]
+  revision = "063d875e3c5fd734fa2aa12fac83829f62acfc70"
+  
+[[projects]]
+  branch = "master"
+  name = "github.com/mesosphere/mesos-dns"
+  packages = [
+    "detect",
+    "errorutil",
+    "logging",
+    "models",
+    "records",
+    "records/labels",
+    "records/state",
+    "util"
+  ]
+  revision = "b47dc4c19f215e98da687b15b4c64e70f629bea5"
+  source = "git@github.com:containous/mesos-dns.git"
+
+  [[projects]]
+  name = "gopkg.in/fsnotify.v1"
+  packages = ["."]
+  revision = "629574ca2a5df945712d3079857300b5e4da0236"
+  source = "github.com/fsnotify/fsnotify"
+  version = "v1.4.2"
\ No newline at end of file
diff --git a/src/cmd/go/internal/modconv/testdata/traefik.out b/src/cmd/go/internal/modconv/testdata/traefik.out
new file mode 100644
index 0000000..5054295
--- /dev/null
+++ b/src/cmd/go/internal/modconv/testdata/traefik.out
@@ -0,0 +1,14 @@
+github.com/Nvveen/Gotty a8b993ba6abdb0e0c12b0125c603323a71c7790c
+github.com/OpenDNS/vegadns2client a3fa4a771d87bda2514a90a157e1fed1b6897d2e
+github.com/PuerkitoBio/purell v1.0.0
+github.com/PuerkitoBio/urlesc 5bd2802263f21d8788851d5305584c82a5c75d7e
+github.com/Shopify/sarama 70f6a705d4a17af059acbc6946fb2bd30762acd7
+github.com/VividCortex/gohistogram v1.0.0
+github.com/abbot/go-http-auth 65b0cdae8d7fe5c05c7430e055938ef6d24a66c9
+github.com/abronan/valkeyrie 063d875e3c5fd734fa2aa12fac83829f62acfc70
+github.com/mesosphere/mesos-dns b47dc4c19f215e98da687b15b4c64e70f629bea5
+gopkg.in/fsnotify.v1 v1.4.2
+replace: github.com/Nvveen/Gotty a8b993ba6abdb0e0c12b0125c603323a71c7790c github.com/ijc25/Gotty a8b993ba6abdb0e0c12b0125c603323a71c7790c
+replace: github.com/abbot/go-http-auth 65b0cdae8d7fe5c05c7430e055938ef6d24a66c9 github.com/containous/go-http-auth 65b0cdae8d7fe5c05c7430e055938ef6d24a66c9
+replace: github.com/mesosphere/mesos-dns b47dc4c19f215e98da687b15b4c64e70f629bea5 github.com/containous/mesos-dns b47dc4c19f215e98da687b15b4c64e70f629bea5
+replace: gopkg.in/fsnotify.v1 v1.4.2 github.com/fsnotify/fsnotify v1.4.2
diff --git a/src/cmd/go/internal/modconv/testdata/upspin.dep b/src/cmd/go/internal/modconv/testdata/upspin.dep
new file mode 100644
index 0000000..be77bcb
--- /dev/null
+++ b/src/cmd/go/internal/modconv/testdata/upspin.dep
@@ -0,0 +1,57 @@
+# This file is autogenerated, do not edit; changes may be undone by the next 'dep ensure'.
+
+
+[[projects]]
+  branch = "master"
+  name = "bazil.org/fuse"
+  packages = [".","fs","fuseutil"]
+  revision = "371fbbdaa8987b715bdd21d6adc4c9b20155f748"
+
+[[projects]]
+  branch = "master"
+  name = "github.com/NYTimes/gziphandler"
+  packages = ["."]
+  revision = "97ae7fbaf81620fe97840685304a78a306a39c64"
+
+[[projects]]
+  branch = "master"
+  name = "github.com/golang/protobuf"
+  packages = ["proto"]
+  revision = "1643683e1b54a9e88ad26d98f81400c8c9d9f4f9"
+
+[[projects]]
+  branch = "master"
+  name = "github.com/russross/blackfriday"
+  packages = ["."]
+  revision = "6d1ef893fcb01b4f50cb6e57ed7df3e2e627b6b2"
+
+[[projects]]
+  branch = "master"
+  name = "golang.org/x/crypto"
+  packages = ["acme","acme/autocert","hkdf"]
+  revision = "13931e22f9e72ea58bb73048bc752b48c6d4d4ac"
+
+[[projects]]
+  branch = "master"
+  name = "golang.org/x/net"
+  packages = ["context"]
+  revision = "4b14673ba32bee7f5ac0f990a48f033919fd418b"
+
+[[projects]]
+  branch = "master"
+  name = "golang.org/x/text"
+  packages = ["cases","internal","internal/gen","internal/tag","internal/triegen","internal/ucd","language","runes","secure/bidirule","secure/precis","transform","unicode/bidi","unicode/cldr","unicode/norm","unicode/rangetable","width"]
+  revision = "6eab0e8f74e86c598ec3b6fad4888e0c11482d48"
+
+[[projects]]
+  branch = "v2"
+  name = "gopkg.in/yaml.v2"
+  packages = ["."]
+  revision = "eb3733d160e74a9c7e442f435eb3bea458e1d19f"
+
+[solve-meta]
+  analyzer-name = "dep"
+  analyzer-version = 1
+  inputs-digest = "2246e647ba1c78b0b9f948f9fb072fff1467284fb138709c063e99736f646b90"
+  solver-name = "gps-cdcl"
+  solver-version = 1
diff --git a/src/cmd/go/internal/modconv/testdata/upspin.out b/src/cmd/go/internal/modconv/testdata/upspin.out
new file mode 100644
index 0000000..00597db
--- /dev/null
+++ b/src/cmd/go/internal/modconv/testdata/upspin.out
@@ -0,0 +1,8 @@
+bazil.org/fuse 371fbbdaa8987b715bdd21d6adc4c9b20155f748
+github.com/NYTimes/gziphandler 97ae7fbaf81620fe97840685304a78a306a39c64
+github.com/golang/protobuf 1643683e1b54a9e88ad26d98f81400c8c9d9f4f9
+github.com/russross/blackfriday 6d1ef893fcb01b4f50cb6e57ed7df3e2e627b6b2
+golang.org/x/crypto 13931e22f9e72ea58bb73048bc752b48c6d4d4ac
+golang.org/x/net 4b14673ba32bee7f5ac0f990a48f033919fd418b
+golang.org/x/text 6eab0e8f74e86c598ec3b6fad4888e0c11482d48
+gopkg.in/yaml.v2 eb3733d160e74a9c7e442f435eb3bea458e1d19f
diff --git a/src/cmd/go/internal/modconv/tsv.go b/src/cmd/go/internal/modconv/tsv.go
new file mode 100644
index 0000000..4649579
--- /dev/null
+++ b/src/cmd/go/internal/modconv/tsv.go
@@ -0,0 +1,23 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modconv
+
+import (
+	"strings"
+
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/module"
+)
+
+func ParseDependenciesTSV(file string, data []byte) (*modfile.File, error) {
+	mf := new(modfile.File)
+	for _, line := range strings.Split(string(data), "\n") {
+		f := strings.Split(line, "\t")
+		if len(f) >= 3 {
+			mf.Require = append(mf.Require, &modfile.Require{Mod: module.Version{Path: f[0], Version: f[2]}})
+		}
+	}
+	return mf, nil
+}
diff --git a/src/cmd/go/internal/modconv/vconf.go b/src/cmd/go/internal/modconv/vconf.go
new file mode 100644
index 0000000..9bad2ba
--- /dev/null
+++ b/src/cmd/go/internal/modconv/vconf.go
@@ -0,0 +1,26 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modconv
+
+import (
+	"strings"
+
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/module"
+)
+
+func ParseVendorConf(file string, data []byte) (*modfile.File, error) {
+	mf := new(modfile.File)
+	for _, line := range strings.Split(string(data), "\n") {
+		if i := strings.Index(line, "#"); i >= 0 {
+			line = line[:i]
+		}
+		f := strings.Fields(line)
+		if len(f) >= 2 {
+			mf.Require = append(mf.Require, &modfile.Require{Mod: module.Version{Path: f[0], Version: f[1]}})
+		}
+	}
+	return mf, nil
+}
diff --git a/src/cmd/go/internal/modconv/vjson.go b/src/cmd/go/internal/modconv/vjson.go
new file mode 100644
index 0000000..1bd025c
--- /dev/null
+++ b/src/cmd/go/internal/modconv/vjson.go
@@ -0,0 +1,29 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modconv
+
+import (
+	"encoding/json"
+
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/module"
+)
+
+func ParseVendorJSON(file string, data []byte) (*modfile.File, error) {
+	var cfg struct {
+		Package []struct {
+			Path     string
+			Revision string
+		}
+	}
+	if err := json.Unmarshal(data, &cfg); err != nil {
+		return nil, err
+	}
+	mf := new(modfile.File)
+	for _, d := range cfg.Package {
+		mf.Require = append(mf.Require, &modfile.Require{Mod: module.Version{Path: d.Path, Version: d.Revision}})
+	}
+	return mf, nil
+}
diff --git a/src/cmd/go/internal/modconv/vmanifest.go b/src/cmd/go/internal/modconv/vmanifest.go
new file mode 100644
index 0000000..bcf0008
--- /dev/null
+++ b/src/cmd/go/internal/modconv/vmanifest.go
@@ -0,0 +1,29 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modconv
+
+import (
+	"encoding/json"
+
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/module"
+)
+
+func ParseVendorManifest(file string, data []byte) (*modfile.File, error) {
+	var cfg struct {
+		Dependencies []struct {
+			ImportPath string
+			Revision   string
+		}
+	}
+	if err := json.Unmarshal(data, &cfg); err != nil {
+		return nil, err
+	}
+	mf := new(modfile.File)
+	for _, d := range cfg.Dependencies {
+		mf.Require = append(mf.Require, &modfile.Require{Mod: module.Version{Path: d.ImportPath, Version: d.Revision}})
+	}
+	return mf, nil
+}
diff --git a/src/cmd/go/internal/modconv/vyml.go b/src/cmd/go/internal/modconv/vyml.go
new file mode 100644
index 0000000..cfa4194
--- /dev/null
+++ b/src/cmd/go/internal/modconv/vyml.go
@@ -0,0 +1,41 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modconv
+
+import (
+	"strings"
+
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/module"
+)
+
+func ParseVendorYML(file string, data []byte) (*modfile.File, error) {
+	mf := new(modfile.File)
+	vendors := false
+	path := ""
+	for _, line := range strings.Split(string(data), "\n") {
+		if line == "" {
+			continue
+		}
+		if strings.HasPrefix(line, "vendors:") {
+			vendors = true
+		} else if line[0] != '-' && line[0] != ' ' && line[0] != '\t' {
+			vendors = false
+		}
+		if !vendors {
+			continue
+		}
+		if strings.HasPrefix(line, "- path:") {
+			path = strings.TrimSpace(line[len("- path:"):])
+		}
+		if strings.HasPrefix(line, "  rev:") {
+			rev := strings.TrimSpace(line[len("  rev:"):])
+			if path != "" && rev != "" {
+				mf.Require = append(mf.Require, &modfile.Require{Mod: module.Version{Path: path, Version: rev}})
+			}
+		}
+	}
+	return mf, nil
+}
diff --git a/src/cmd/go/internal/modfetch/bootstrap.go b/src/cmd/go/internal/modfetch/bootstrap.go
new file mode 100644
index 0000000..e23669f
--- /dev/null
+++ b/src/cmd/go/internal/modfetch/bootstrap.go
@@ -0,0 +1,17 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build cmd_go_bootstrap
+
+package modfetch
+
+import "golang.org/x/mod/module"
+
+func useSumDB(mod module.Version) bool {
+	return false
+}
+
+func lookupSumDB(mod module.Version) (string, []string, error) {
+	panic("bootstrap")
+}
diff --git a/src/cmd/go/internal/modfetch/cache.go b/src/cmd/go/internal/modfetch/cache.go
new file mode 100644
index 0000000..5a727c6
--- /dev/null
+++ b/src/cmd/go/internal/modfetch/cache.go
@@ -0,0 +1,815 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modfetch
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"io/fs"
+	"math/rand"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"sync"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/lockedfile"
+	"cmd/go/internal/modfetch/codehost"
+	"cmd/go/internal/par"
+	"cmd/go/internal/robustio"
+
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
+)
+
+func cacheDir(ctx context.Context, path string) (string, error) {
+	if err := checkCacheDir(ctx); err != nil {
+		return "", err
+	}
+	enc, err := module.EscapePath(path)
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(cfg.GOMODCACHE, "cache/download", enc, "/@v"), nil
+}
+
+func CachePath(ctx context.Context, m module.Version, suffix string) (string, error) {
+	if gover.IsToolchain(m.Path) {
+		return "", ErrToolchain
+	}
+	dir, err := cacheDir(ctx, m.Path)
+	if err != nil {
+		return "", err
+	}
+	if !gover.ModIsValid(m.Path, m.Version) {
+		return "", fmt.Errorf("non-semver module version %q", m.Version)
+	}
+	if module.CanonicalVersion(m.Version) != m.Version {
+		return "", fmt.Errorf("non-canonical module version %q", m.Version)
+	}
+	encVer, err := module.EscapeVersion(m.Version)
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(dir, encVer+"."+suffix), nil
+}
+
+// DownloadDir returns the directory to which m should have been downloaded.
+// An error will be returned if the module path or version cannot be escaped.
+// An error satisfying errors.Is(err, fs.ErrNotExist) will be returned
+// along with the directory if the directory does not exist or if the directory
+// is not completely populated.
+func DownloadDir(ctx context.Context, m module.Version) (string, error) {
+	if gover.IsToolchain(m.Path) {
+		return "", ErrToolchain
+	}
+	if err := checkCacheDir(ctx); err != nil {
+		return "", err
+	}
+	enc, err := module.EscapePath(m.Path)
+	if err != nil {
+		return "", err
+	}
+	if !gover.ModIsValid(m.Path, m.Version) {
+		return "", fmt.Errorf("non-semver module version %q", m.Version)
+	}
+	if module.CanonicalVersion(m.Version) != m.Version {
+		return "", fmt.Errorf("non-canonical module version %q", m.Version)
+	}
+	encVer, err := module.EscapeVersion(m.Version)
+	if err != nil {
+		return "", err
+	}
+
+	// Check whether the directory itself exists.
+	dir := filepath.Join(cfg.GOMODCACHE, enc+"@"+encVer)
+	if fi, err := os.Stat(dir); os.IsNotExist(err) {
+		return dir, err
+	} else if err != nil {
+		return dir, &DownloadDirPartialError{dir, err}
+	} else if !fi.IsDir() {
+		return dir, &DownloadDirPartialError{dir, errors.New("not a directory")}
+	}
+
+	// Check if a .partial file exists. This is created at the beginning of
+	// a download and removed after the zip is extracted.
+	partialPath, err := CachePath(ctx, m, "partial")
+	if err != nil {
+		return dir, err
+	}
+	if _, err := os.Stat(partialPath); err == nil {
+		return dir, &DownloadDirPartialError{dir, errors.New("not completely extracted")}
+	} else if !os.IsNotExist(err) {
+		return dir, err
+	}
+
+	// Check if a .ziphash file exists. It should be created before the
+	// zip is extracted, but if it was deleted (by another program?), we need
+	// to re-calculate it. Note that checkMod will repopulate the ziphash
+	// file if it doesn't exist, but if the module is excluded by checks
+	// through GONOSUMDB or GOPRIVATE, that check and repopulation won't happen.
+	ziphashPath, err := CachePath(ctx, m, "ziphash")
+	if err != nil {
+		return dir, err
+	}
+	if _, err := os.Stat(ziphashPath); os.IsNotExist(err) {
+		return dir, &DownloadDirPartialError{dir, errors.New("ziphash file is missing")}
+	} else if err != nil {
+		return dir, err
+	}
+	return dir, nil
+}
+
+// DownloadDirPartialError is returned by DownloadDir if a module directory
+// exists but was not completely populated.
+//
+// DownloadDirPartialError is equivalent to fs.ErrNotExist.
+type DownloadDirPartialError struct {
+	Dir string
+	Err error
+}
+
+func (e *DownloadDirPartialError) Error() string     { return fmt.Sprintf("%s: %v", e.Dir, e.Err) }
+func (e *DownloadDirPartialError) Is(err error) bool { return err == fs.ErrNotExist }
+
+// lockVersion locks a file within the module cache that guards the downloading
+// and extraction of the zipfile for the given module version.
+func lockVersion(ctx context.Context, mod module.Version) (unlock func(), err error) {
+	path, err := CachePath(ctx, mod, "lock")
+	if err != nil {
+		return nil, err
+	}
+	if err := os.MkdirAll(filepath.Dir(path), 0777); err != nil {
+		return nil, err
+	}
+	return lockedfile.MutexAt(path).Lock()
+}
+
+// SideLock locks a file within the module cache that previously guarded
+// edits to files outside the cache, such as go.sum and go.mod files in the
+// user's working directory.
+// If err is nil, the caller MUST eventually call the unlock function.
+func SideLock(ctx context.Context) (unlock func(), err error) {
+	if err := checkCacheDir(ctx); err != nil {
+		return nil, err
+	}
+
+	path := filepath.Join(cfg.GOMODCACHE, "cache", "lock")
+	if err := os.MkdirAll(filepath.Dir(path), 0777); err != nil {
+		return nil, fmt.Errorf("failed to create cache directory: %w", err)
+	}
+
+	return lockedfile.MutexAt(path).Lock()
+}
+
+// A cachingRepo is a cache around an underlying Repo,
+// avoiding redundant calls to ModulePath, Versions, Stat, Latest, and GoMod (but not CheckReuse or Zip).
+// It is also safe for simultaneous use by multiple goroutines
+// (so that it can be returned from Lookup multiple times).
+// It serializes calls to the underlying Repo.
+type cachingRepo struct {
+	path          string
+	versionsCache par.ErrCache[string, *Versions]
+	statCache     par.ErrCache[string, *RevInfo]
+	latestCache   par.ErrCache[struct{}, *RevInfo]
+	gomodCache    par.ErrCache[string, []byte]
+
+	once     sync.Once
+	initRepo func(context.Context) (Repo, error)
+	r        Repo
+}
+
+func newCachingRepo(ctx context.Context, path string, initRepo func(context.Context) (Repo, error)) *cachingRepo {
+	return &cachingRepo{
+		path:     path,
+		initRepo: initRepo,
+	}
+}
+
+func (r *cachingRepo) repo(ctx context.Context) Repo {
+	r.once.Do(func() {
+		var err error
+		r.r, err = r.initRepo(ctx)
+		if err != nil {
+			r.r = errRepo{r.path, err}
+		}
+	})
+	return r.r
+}
+
+func (r *cachingRepo) CheckReuse(ctx context.Context, old *codehost.Origin) error {
+	return r.repo(ctx).CheckReuse(ctx, old)
+}
+
+func (r *cachingRepo) ModulePath() string {
+	return r.path
+}
+
+func (r *cachingRepo) Versions(ctx context.Context, prefix string) (*Versions, error) {
+	v, err := r.versionsCache.Do(prefix, func() (*Versions, error) {
+		return r.repo(ctx).Versions(ctx, prefix)
+	})
+
+	if err != nil {
+		return nil, err
+	}
+	return &Versions{
+		Origin: v.Origin,
+		List:   append([]string(nil), v.List...),
+	}, nil
+}
+
+type cachedInfo struct {
+	info *RevInfo
+	err  error
+}
+
+func (r *cachingRepo) Stat(ctx context.Context, rev string) (*RevInfo, error) {
+	if gover.IsToolchain(r.path) {
+		// Skip disk cache; the underlying golang.org/toolchain repo is cached instead.
+		return r.repo(ctx).Stat(ctx, rev)
+	}
+	info, err := r.statCache.Do(rev, func() (*RevInfo, error) {
+		file, info, err := readDiskStat(ctx, r.path, rev)
+		if err == nil {
+			return info, err
+		}
+
+		info, err = r.repo(ctx).Stat(ctx, rev)
+		if err == nil {
+			// If we resolved, say, 1234abcde to v0.0.0-20180604122334-1234abcdef78,
+			// then save the information under the proper version, for future use.
+			if info.Version != rev {
+				file, _ = CachePath(ctx, module.Version{Path: r.path, Version: info.Version}, "info")
+				r.statCache.Do(info.Version, func() (*RevInfo, error) {
+					return info, nil
+				})
+			}
+
+			if err := writeDiskStat(ctx, file, info); err != nil {
+				fmt.Fprintf(os.Stderr, "go: writing stat cache: %v\n", err)
+			}
+		}
+		return info, err
+	})
+	if info != nil {
+		copy := *info
+		info = &copy
+	}
+	return info, err
+}
+
+func (r *cachingRepo) Latest(ctx context.Context) (*RevInfo, error) {
+	if gover.IsToolchain(r.path) {
+		// Skip disk cache; the underlying golang.org/toolchain repo is cached instead.
+		return r.repo(ctx).Latest(ctx)
+	}
+	info, err := r.latestCache.Do(struct{}{}, func() (*RevInfo, error) {
+		info, err := r.repo(ctx).Latest(ctx)
+
+		// Save info for likely future Stat call.
+		if err == nil {
+			r.statCache.Do(info.Version, func() (*RevInfo, error) {
+				return info, nil
+			})
+			if file, _, err := readDiskStat(ctx, r.path, info.Version); err != nil {
+				writeDiskStat(ctx, file, info)
+			}
+		}
+
+		return info, err
+	})
+	if info != nil {
+		copy := *info
+		info = &copy
+	}
+	return info, err
+}
+
+func (r *cachingRepo) GoMod(ctx context.Context, version string) ([]byte, error) {
+	if gover.IsToolchain(r.path) {
+		// Skip disk cache; the underlying golang.org/toolchain repo is cached instead.
+		return r.repo(ctx).GoMod(ctx, version)
+	}
+	text, err := r.gomodCache.Do(version, func() ([]byte, error) {
+		file, text, err := readDiskGoMod(ctx, r.path, version)
+		if err == nil {
+			// Note: readDiskGoMod already called checkGoMod.
+			return text, nil
+		}
+
+		text, err = r.repo(ctx).GoMod(ctx, version)
+		if err == nil {
+			if err := checkGoMod(r.path, version, text); err != nil {
+				return text, err
+			}
+			if err := writeDiskGoMod(ctx, file, text); err != nil {
+				fmt.Fprintf(os.Stderr, "go: writing go.mod cache: %v\n", err)
+			}
+		}
+		return text, err
+	})
+	if err != nil {
+		return nil, err
+	}
+	return append([]byte(nil), text...), nil
+}
+
+func (r *cachingRepo) Zip(ctx context.Context, dst io.Writer, version string) error {
+	if gover.IsToolchain(r.path) {
+		return ErrToolchain
+	}
+	return r.repo(ctx).Zip(ctx, dst, version)
+}
+
+// InfoFile is like Lookup(ctx, path).Stat(version) but also returns the name of the file
+// containing the cached information.
+func InfoFile(ctx context.Context, path, version string) (*RevInfo, string, error) {
+	if !gover.ModIsValid(path, version) {
+		return nil, "", fmt.Errorf("invalid version %q", version)
+	}
+
+	if file, info, err := readDiskStat(ctx, path, version); err == nil {
+		return info, file, nil
+	}
+
+	var info *RevInfo
+	var err2info map[error]*RevInfo
+	err := TryProxies(func(proxy string) error {
+		i, err := Lookup(ctx, proxy, path).Stat(ctx, version)
+		if err == nil {
+			info = i
+		} else {
+			if err2info == nil {
+				err2info = make(map[error]*RevInfo)
+			}
+			err2info[err] = info
+		}
+		return err
+	})
+	if err != nil {
+		return err2info[err], "", err
+	}
+
+	// Stat should have populated the disk cache for us.
+	file, err := CachePath(ctx, module.Version{Path: path, Version: version}, "info")
+	if err != nil {
+		return nil, "", err
+	}
+	return info, file, nil
+}
+
+// GoMod is like Lookup(ctx, path).GoMod(rev) but avoids the
+// repository path resolution in Lookup if the result is
+// already cached on local disk.
+func GoMod(ctx context.Context, path, rev string) ([]byte, error) {
+	// Convert commit hash to pseudo-version
+	// to increase cache hit rate.
+	if !gover.ModIsValid(path, rev) {
+		if _, info, err := readDiskStat(ctx, path, rev); err == nil {
+			rev = info.Version
+		} else {
+			if errors.Is(err, statCacheErr) {
+				return nil, err
+			}
+			err := TryProxies(func(proxy string) error {
+				info, err := Lookup(ctx, proxy, path).Stat(ctx, rev)
+				if err == nil {
+					rev = info.Version
+				}
+				return err
+			})
+			if err != nil {
+				return nil, err
+			}
+		}
+	}
+
+	_, data, err := readDiskGoMod(ctx, path, rev)
+	if err == nil {
+		return data, nil
+	}
+
+	err = TryProxies(func(proxy string) (err error) {
+		data, err = Lookup(ctx, proxy, path).GoMod(ctx, rev)
+		return err
+	})
+	return data, err
+}
+
+// GoModFile is like GoMod but returns the name of the file containing
+// the cached information.
+func GoModFile(ctx context.Context, path, version string) (string, error) {
+	if !gover.ModIsValid(path, version) {
+		return "", fmt.Errorf("invalid version %q", version)
+	}
+	if _, err := GoMod(ctx, path, version); err != nil {
+		return "", err
+	}
+	// GoMod should have populated the disk cache for us.
+	file, err := CachePath(ctx, module.Version{Path: path, Version: version}, "mod")
+	if err != nil {
+		return "", err
+	}
+	return file, nil
+}
+
+// GoModSum returns the go.sum entry for the module version's go.mod file.
+// (That is, it returns the entry listed in go.sum as "path version/go.mod".)
+func GoModSum(ctx context.Context, path, version string) (string, error) {
+	if !gover.ModIsValid(path, version) {
+		return "", fmt.Errorf("invalid version %q", version)
+	}
+	data, err := GoMod(ctx, path, version)
+	if err != nil {
+		return "", err
+	}
+	sum, err := goModSum(data)
+	if err != nil {
+		return "", err
+	}
+	return sum, nil
+}
+
+var errNotCached = fmt.Errorf("not in cache")
+
+// readDiskStat reads a cached stat result from disk,
+// returning the name of the cache file and the result.
+// If the read fails, the caller can use
+// writeDiskStat(file, info) to write a new cache entry.
+func readDiskStat(ctx context.Context, path, rev string) (file string, info *RevInfo, err error) {
+	if gover.IsToolchain(path) {
+		return "", nil, errNotCached
+	}
+	file, data, err := readDiskCache(ctx, path, rev, "info")
+	if err != nil {
+		// If the cache already contains a pseudo-version with the given hash, we
+		// would previously return that pseudo-version without checking upstream.
+		// However, that produced an unfortunate side-effect: if the author added a
+		// tag to the repository, 'go get' would not pick up the effect of that new
+		// tag on the existing commits, and 'go' commands that referred to those
+		// commits would use the previous name instead of the new one.
+		//
+		// That's especially problematic if the original pseudo-version starts with
+		// v0.0.0-, as was the case for all pseudo-versions during vgo development,
+		// since a v0.0.0- pseudo-version has lower precedence than pretty much any
+		// tagged version.
+		//
+		// In practice, we're only looking up by hash during initial conversion of a
+		// legacy config and during an explicit 'go get', and a little extra latency
+		// for those operations seems worth the benefit of picking up more accurate
+		// versions.
+		//
+		// Fall back to this resolution scheme only if the GOPROXY setting prohibits
+		// us from resolving upstream tags.
+		if cfg.GOPROXY == "off" {
+			if file, info, err := readDiskStatByHash(ctx, path, rev); err == nil {
+				return file, info, nil
+			}
+		}
+		return file, nil, err
+	}
+	info = new(RevInfo)
+	if err := json.Unmarshal(data, info); err != nil {
+		return file, nil, errNotCached
+	}
+	// The disk might have stale .info files that have Name and Short fields set.
+	// We want to canonicalize to .info files with those fields omitted.
+	// Remarshal and update the cache file if needed.
+	data2, err := json.Marshal(info)
+	if err == nil && !bytes.Equal(data2, data) {
+		writeDiskCache(ctx, file, data)
+	}
+	return file, info, nil
+}
+
+// readDiskStatByHash is a fallback for readDiskStat for the case
+// where rev is a commit hash instead of a proper semantic version.
+// In that case, we look for a cached pseudo-version that matches
+// the commit hash. If we find one, we use it.
+// This matters most for converting legacy package management
+// configs, when we are often looking up commits by full hash.
+// Without this check we'd be doing network I/O to the remote repo
+// just to find out about a commit we already know about
+// (and have cached under its pseudo-version).
+func readDiskStatByHash(ctx context.Context, path, rev string) (file string, info *RevInfo, err error) {
+	if gover.IsToolchain(path) {
+		return "", nil, errNotCached
+	}
+	if cfg.GOMODCACHE == "" {
+		// Do not download to current directory.
+		return "", nil, errNotCached
+	}
+
+	if !codehost.AllHex(rev) || len(rev) < 12 {
+		return "", nil, errNotCached
+	}
+	rev = rev[:12]
+	cdir, err := cacheDir(ctx, path)
+	if err != nil {
+		return "", nil, errNotCached
+	}
+	dir, err := os.Open(cdir)
+	if err != nil {
+		return "", nil, errNotCached
+	}
+	names, err := dir.Readdirnames(-1)
+	dir.Close()
+	if err != nil {
+		return "", nil, errNotCached
+	}
+
+	// A given commit hash may map to more than one pseudo-version,
+	// depending on which tags are present on the repository.
+	// Take the highest such version.
+	var maxVersion string
+	suffix := "-" + rev + ".info"
+	err = errNotCached
+	for _, name := range names {
+		if strings.HasSuffix(name, suffix) {
+			v := strings.TrimSuffix(name, ".info")
+			if module.IsPseudoVersion(v) && semver.Compare(v, maxVersion) > 0 {
+				maxVersion = v
+				file, info, err = readDiskStat(ctx, path, strings.TrimSuffix(name, ".info"))
+			}
+		}
+	}
+	return file, info, err
+}
+
+// oldVgoPrefix is the prefix in the old auto-generated cached go.mod files.
+// We stopped trying to auto-generate the go.mod files. Now we use a trivial
+// go.mod with only a module line, and we've dropped the version prefix
+// entirely. If we see a version prefix, that means we're looking at an old copy
+// and should ignore it.
+var oldVgoPrefix = []byte("//vgo 0.0.")
+
+// readDiskGoMod reads a cached go.mod file from disk,
+// returning the name of the cache file and the result.
+// If the read fails, the caller can use
+// writeDiskGoMod(file, data) to write a new cache entry.
+func readDiskGoMod(ctx context.Context, path, rev string) (file string, data []byte, err error) {
+	if gover.IsToolchain(path) {
+		return "", nil, errNotCached
+	}
+	file, data, err = readDiskCache(ctx, path, rev, "mod")
+
+	// If the file has an old auto-conversion prefix, pretend it's not there.
+	if bytes.HasPrefix(data, oldVgoPrefix) {
+		err = errNotCached
+		data = nil
+	}
+
+	if err == nil {
+		if err := checkGoMod(path, rev, data); err != nil {
+			return "", nil, err
+		}
+	}
+
+	return file, data, err
+}
+
+// readDiskCache is the generic "read from a cache file" implementation.
+// It takes the revision and an identifying suffix for the kind of data being cached.
+// It returns the name of the cache file and the content of the file.
+// If the read fails, the caller can use
+// writeDiskCache(file, data) to write a new cache entry.
+func readDiskCache(ctx context.Context, path, rev, suffix string) (file string, data []byte, err error) {
+	if gover.IsToolchain(path) {
+		return "", nil, errNotCached
+	}
+	file, err = CachePath(ctx, module.Version{Path: path, Version: rev}, suffix)
+	if err != nil {
+		return "", nil, errNotCached
+	}
+	data, err = robustio.ReadFile(file)
+	if err != nil {
+		return file, nil, errNotCached
+	}
+	return file, data, nil
+}
+
+// writeDiskStat writes a stat result cache entry.
+// The file name must have been returned by a previous call to readDiskStat.
+func writeDiskStat(ctx context.Context, file string, info *RevInfo) error {
+	if file == "" {
+		return nil
+	}
+
+	if info.Origin != nil {
+		// Clean the origin information, which might have too many
+		// validation criteria, for example if we are saving the result of
+		// m@master as m@pseudo-version.
+		clean := *info
+		info = &clean
+		o := *info.Origin
+		info.Origin = &o
+
+		// Tags never matter if you are starting with a semver version,
+		// as we would be when finding this cache entry.
+		o.TagSum = ""
+		o.TagPrefix = ""
+		// Ref doesn't matter if you have a pseudoversion.
+		if module.IsPseudoVersion(info.Version) {
+			o.Ref = ""
+		}
+	}
+
+	js, err := json.Marshal(info)
+	if err != nil {
+		return err
+	}
+	return writeDiskCache(ctx, file, js)
+}
+
+// writeDiskGoMod writes a go.mod cache entry.
+// The file name must have been returned by a previous call to readDiskGoMod.
+func writeDiskGoMod(ctx context.Context, file string, text []byte) error {
+	return writeDiskCache(ctx, file, text)
+}
+
+// writeDiskCache is the generic "write to a cache file" implementation.
+// The file must have been returned by a previous call to readDiskCache.
+func writeDiskCache(ctx context.Context, file string, data []byte) error {
+	if file == "" {
+		return nil
+	}
+	// Make sure directory for file exists.
+	if err := os.MkdirAll(filepath.Dir(file), 0777); err != nil {
+		return err
+	}
+
+	// Write the file to a temporary location, and then rename it to its final
+	// path to reduce the likelihood of a corrupt file existing at that final path.
+	f, err := tempFile(ctx, filepath.Dir(file), filepath.Base(file), 0666)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		// Only call os.Remove on f.Name() if we failed to rename it: otherwise,
+		// some other process may have created a new file with the same name after
+		// the rename completed.
+		if err != nil {
+			f.Close()
+			os.Remove(f.Name())
+		}
+	}()
+
+	if _, err := f.Write(data); err != nil {
+		return err
+	}
+	if err := f.Close(); err != nil {
+		return err
+	}
+	if err := robustio.Rename(f.Name(), file); err != nil {
+		return err
+	}
+
+	if strings.HasSuffix(file, ".mod") {
+		rewriteVersionList(ctx, filepath.Dir(file))
+	}
+	return nil
+}
+
+// tempFile creates a new temporary file with given permission bits.
+func tempFile(ctx context.Context, dir, prefix string, perm fs.FileMode) (f *os.File, err error) {
+	for i := 0; i < 10000; i++ {
+		name := filepath.Join(dir, prefix+strconv.Itoa(rand.Intn(1000000000))+".tmp")
+		f, err = os.OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_EXCL, perm)
+		if os.IsExist(err) {
+			if ctx.Err() != nil {
+				return nil, ctx.Err()
+			}
+			continue
+		}
+		break
+	}
+	return
+}
+
+// rewriteVersionList rewrites the version list in dir
+// after a new *.mod file has been written.
+func rewriteVersionList(ctx context.Context, dir string) (err error) {
+	if filepath.Base(dir) != "@v" {
+		base.Fatalf("go: internal error: misuse of rewriteVersionList")
+	}
+
+	listFile := filepath.Join(dir, "list")
+
+	// Lock listfile when writing to it to try to avoid corruption to the file.
+	// Under rare circumstances, for instance, if the system loses power in the
+	// middle of a write it is possible for corrupt data to be written. This is
+	// not a problem for the go command itself, but may be an issue if the
+	// cache is being served by a GOPROXY HTTP server. This will be corrected
+	// the next time a new version of the module is fetched and the file is rewritten.
+	// TODO(matloob): golang.org/issue/43313 covers adding a go mod verify
+	// command that removes module versions that fail checksums. It should also
+	// remove list files that are detected to be corrupt.
+	f, err := lockedfile.Edit(listFile)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if cerr := f.Close(); cerr != nil && err == nil {
+			err = cerr
+		}
+	}()
+	infos, err := os.ReadDir(dir)
+	if err != nil {
+		return err
+	}
+	var list []string
+	for _, info := range infos {
+		// We look for *.mod files on the theory that if we can't supply
+		// the .mod file then there's no point in listing that version,
+		// since it's unusable. (We can have *.info without *.mod.)
+		// We don't require *.zip files on the theory that for code only
+		// involved in module graph construction, many *.zip files
+		// will never be requested.
+		name := info.Name()
+		if v, found := strings.CutSuffix(name, ".mod"); found {
+			if v != "" && module.CanonicalVersion(v) == v {
+				list = append(list, v)
+			}
+		}
+	}
+	semver.Sort(list)
+
+	var buf bytes.Buffer
+	for _, v := range list {
+		buf.WriteString(v)
+		buf.WriteString("\n")
+	}
+	if fi, err := f.Stat(); err == nil && int(fi.Size()) == buf.Len() {
+		old := make([]byte, buf.Len()+1)
+		if n, err := f.ReadAt(old, 0); err == io.EOF && n == buf.Len() && bytes.Equal(buf.Bytes(), old) {
+			return nil // No edit needed.
+		}
+	}
+	// Remove existing contents, so that when we truncate to the actual size it will zero-fill,
+	// and we will be able to detect (some) incomplete writes as files containing trailing NUL bytes.
+	if err := f.Truncate(0); err != nil {
+		return err
+	}
+	// Reserve the final size and zero-fill.
+	if err := f.Truncate(int64(buf.Len())); err != nil {
+		return err
+	}
+	// Write the actual contents. If this fails partway through,
+	// the remainder of the file should remain as zeroes.
+	if _, err := f.Write(buf.Bytes()); err != nil {
+		f.Truncate(0)
+		return err
+	}
+
+	return nil
+}
+
+var (
+	statCacheOnce sync.Once
+	statCacheErr  error
+)
+
+// checkCacheDir checks if the directory specified by GOMODCACHE exists. An
+// error is returned if it does not.
+func checkCacheDir(ctx context.Context) error {
+	if cfg.GOMODCACHE == "" {
+		// modload.Init exits if GOPATH[0] is empty, and cfg.GOMODCACHE
+		// is set to GOPATH[0]/pkg/mod if GOMODCACHE is empty, so this should never happen.
+		return fmt.Errorf("module cache not found: neither GOMODCACHE nor GOPATH is set")
+	}
+	if !filepath.IsAbs(cfg.GOMODCACHE) {
+		return fmt.Errorf("GOMODCACHE entry is relative; must be absolute path: %q.\n", cfg.GOMODCACHE)
+	}
+
+	// os.Stat is slow on Windows, so we only call it once to prevent unnecessary
+	// I/O every time this function is called.
+	statCacheOnce.Do(func() {
+		fi, err := os.Stat(cfg.GOMODCACHE)
+		if err != nil {
+			if !os.IsNotExist(err) {
+				statCacheErr = fmt.Errorf("could not create module cache: %w", err)
+				return
+			}
+			if err := os.MkdirAll(cfg.GOMODCACHE, 0777); err != nil {
+				statCacheErr = fmt.Errorf("could not create module cache: %w", err)
+				return
+			}
+			return
+		}
+		if !fi.IsDir() {
+			statCacheErr = fmt.Errorf("could not create module cache: %q is not a directory", cfg.GOMODCACHE)
+			return
+		}
+	})
+	return statCacheErr
+}
diff --git a/src/cmd/go/internal/modfetch/cache_test.go b/src/cmd/go/internal/modfetch/cache_test.go
new file mode 100644
index 0000000..6aada66
--- /dev/null
+++ b/src/cmd/go/internal/modfetch/cache_test.go
@@ -0,0 +1,27 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modfetch
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestWriteDiskCache(t *testing.T) {
+	ctx := context.Background()
+
+	tmpdir, err := os.MkdirTemp("", "go-writeCache-test-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpdir)
+
+	err = writeDiskCache(ctx, filepath.Join(tmpdir, "file"), []byte("data"))
+	if err != nil {
+		t.Fatal(err)
+	}
+}
diff --git a/src/cmd/go/internal/modfetch/codehost/codehost.go b/src/cmd/go/internal/modfetch/codehost/codehost.go
new file mode 100644
index 0000000..ca57762
--- /dev/null
+++ b/src/cmd/go/internal/modfetch/codehost/codehost.go
@@ -0,0 +1,390 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package codehost defines the interface implemented by a code hosting source,
+// along with support code for use by implementations.
+package codehost
+
+import (
+	"bytes"
+	"context"
+	"crypto/sha256"
+	"fmt"
+	"io"
+	"io/fs"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"sync"
+	"time"
+
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/lockedfile"
+	"cmd/go/internal/str"
+
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
+)
+
+// Downloaded size limits.
+const (
+	MaxGoMod   = 16 << 20  // maximum size of go.mod file
+	MaxLICENSE = 16 << 20  // maximum size of LICENSE file
+	MaxZipFile = 500 << 20 // maximum size of downloaded zip file
+)
+
+// A Repo represents a code hosting source.
+// Typical implementations include local version control repositories,
+// remote version control servers, and code hosting sites.
+//
+// A Repo must be safe for simultaneous use by multiple goroutines,
+// and callers must not modify returned values, which may be cached and shared.
+type Repo interface {
+	// CheckReuse checks whether the old origin information
+	// remains up to date. If so, whatever cached object it was
+	// taken from can be reused.
+	// The subdir gives subdirectory name where the module root is expected to be found,
+	// "" for the root or "sub/dir" for a subdirectory (no trailing slash).
+	CheckReuse(ctx context.Context, old *Origin, subdir string) error
+
+	// List lists all tags with the given prefix.
+	Tags(ctx context.Context, prefix string) (*Tags, error)
+
+	// Stat returns information about the revision rev.
+	// A revision can be any identifier known to the underlying service:
+	// commit hash, branch, tag, and so on.
+	Stat(ctx context.Context, rev string) (*RevInfo, error)
+
+	// Latest returns the latest revision on the default branch,
+	// whatever that means in the underlying implementation.
+	Latest(ctx context.Context) (*RevInfo, error)
+
+	// ReadFile reads the given file in the file tree corresponding to revision rev.
+	// It should refuse to read more than maxSize bytes.
+	//
+	// If the requested file does not exist it should return an error for which
+	// os.IsNotExist(err) returns true.
+	ReadFile(ctx context.Context, rev, file string, maxSize int64) (data []byte, err error)
+
+	// ReadZip downloads a zip file for the subdir subdirectory
+	// of the given revision to a new file in a given temporary directory.
+	// It should refuse to read more than maxSize bytes.
+	// It returns a ReadCloser for a streamed copy of the zip file.
+	// All files in the zip file are expected to be
+	// nested in a single top-level directory, whose name is not specified.
+	ReadZip(ctx context.Context, rev, subdir string, maxSize int64) (zip io.ReadCloser, err error)
+
+	// RecentTag returns the most recent tag on rev or one of its predecessors
+	// with the given prefix. allowed may be used to filter out unwanted versions.
+	RecentTag(ctx context.Context, rev, prefix string, allowed func(tag string) bool) (tag string, err error)
+
+	// DescendsFrom reports whether rev or any of its ancestors has the given tag.
+	//
+	// DescendsFrom must return true for any tag returned by RecentTag for the
+	// same revision.
+	DescendsFrom(ctx context.Context, rev, tag string) (bool, error)
+}
+
+// An Origin describes the provenance of a given repo method result.
+// It can be passed to CheckReuse (usually in a different go command invocation)
+// to see whether the result remains up-to-date.
+type Origin struct {
+	VCS    string `json:",omitempty"` // "git" etc
+	URL    string `json:",omitempty"` // URL of repository
+	Subdir string `json:",omitempty"` // subdirectory in repo
+
+	// If TagSum is non-empty, then the resolution of this module version
+	// depends on the set of tags present in the repo, specifically the tags
+	// of the form TagPrefix + a valid semver version.
+	// If the matching repo tags and their commit hashes still hash to TagSum,
+	// the Origin is still valid (at least as far as the tags are concerned).
+	// The exact checksum is up to the Repo implementation; see (*gitRepo).Tags.
+	TagPrefix string `json:",omitempty"`
+	TagSum    string `json:",omitempty"`
+
+	// If Ref is non-empty, then the resolution of this module version
+	// depends on Ref resolving to the revision identified by Hash.
+	// If Ref still resolves to Hash, the Origin is still valid (at least as far as Ref is concerned).
+	// For Git, the Ref is a full ref like "refs/heads/main" or "refs/tags/v1.2.3",
+	// and the Hash is the Git object hash the ref maps to.
+	// Other VCS might choose differently, but the idea is that Ref is the name
+	// with a mutable meaning while Hash is a name with an immutable meaning.
+	Ref  string `json:",omitempty"`
+	Hash string `json:",omitempty"`
+
+	// If RepoSum is non-empty, then the resolution of this module version
+	// failed due to the repo being available but the version not being present.
+	// This depends on the entire state of the repo, which RepoSum summarizes.
+	// For Git, this is a hash of all the refs and their hashes.
+	RepoSum string `json:",omitempty"`
+}
+
+// Checkable reports whether the Origin contains anything that can be checked.
+// If not, the Origin is purely informational and should fail a CheckReuse call.
+func (o *Origin) Checkable() bool {
+	return o.TagSum != "" || o.Ref != "" || o.Hash != "" || o.RepoSum != ""
+}
+
+// ClearCheckable clears the Origin enough to make Checkable return false.
+func (o *Origin) ClearCheckable() {
+	o.TagSum = ""
+	o.TagPrefix = ""
+	o.Ref = ""
+	o.Hash = ""
+	o.RepoSum = ""
+}
+
+// A Tags describes the available tags in a code repository.
+type Tags struct {
+	Origin *Origin
+	List   []Tag
+}
+
+// A Tag describes a single tag in a code repository.
+type Tag struct {
+	Name string
+	Hash string // content hash identifying tag's content, if available
+}
+
+// isOriginTag reports whether tag should be preserved
+// in the Tags method's Origin calculation.
+// We can safely ignore tags that are not look like pseudo-versions,
+// because ../coderepo.go's (*codeRepo).Versions ignores them too.
+// We can also ignore non-semver tags, but we have to include semver
+// tags with extra suffixes, because the pseudo-version base finder uses them.
+func isOriginTag(tag string) bool {
+	// modfetch.(*codeRepo).Versions uses Canonical == tag,
+	// but pseudo-version calculation has a weaker condition that
+	// the canonical is a prefix of the tag.
+	// Include those too, so that if any new one appears, we'll invalidate the cache entry.
+	// This will lead to spurious invalidation of version list results,
+	// but tags of this form being created should be fairly rare
+	// (and invalidate pseudo-version results anyway).
+	c := semver.Canonical(tag)
+	return c != "" && strings.HasPrefix(tag, c) && !module.IsPseudoVersion(tag)
+}
+
+// A RevInfo describes a single revision in a source code repository.
+type RevInfo struct {
+	Origin  *Origin
+	Name    string    // complete ID in underlying repository
+	Short   string    // shortened ID, for use in pseudo-version
+	Version string    // version used in lookup
+	Time    time.Time // commit time
+	Tags    []string  // known tags for commit
+}
+
+// UnknownRevisionError is an error equivalent to fs.ErrNotExist, but for a
+// revision rather than a file.
+type UnknownRevisionError struct {
+	Rev string
+}
+
+func (e *UnknownRevisionError) Error() string {
+	return "unknown revision " + e.Rev
+}
+func (UnknownRevisionError) Is(err error) bool {
+	return err == fs.ErrNotExist
+}
+
+// ErrNoCommits is an error equivalent to fs.ErrNotExist indicating that a given
+// repository or module contains no commits.
+var ErrNoCommits error = noCommitsError{}
+
+type noCommitsError struct{}
+
+func (noCommitsError) Error() string {
+	return "no commits"
+}
+func (noCommitsError) Is(err error) bool {
+	return err == fs.ErrNotExist
+}
+
+// AllHex reports whether the revision rev is entirely lower-case hexadecimal digits.
+func AllHex(rev string) bool {
+	for i := 0; i < len(rev); i++ {
+		c := rev[i]
+		if '0' <= c && c <= '9' || 'a' <= c && c <= 'f' {
+			continue
+		}
+		return false
+	}
+	return true
+}
+
+// ShortenSHA1 shortens a SHA1 hash (40 hex digits) to the canonical length
+// used in pseudo-versions (12 hex digits).
+func ShortenSHA1(rev string) string {
+	if AllHex(rev) && len(rev) == 40 {
+		return rev[:12]
+	}
+	return rev
+}
+
+// WorkDir returns the name of the cached work directory to use for the
+// given repository type and name.
+func WorkDir(ctx context.Context, typ, name string) (dir, lockfile string, err error) {
+	if cfg.GOMODCACHE == "" {
+		return "", "", fmt.Errorf("neither GOPATH nor GOMODCACHE are set")
+	}
+
+	// We name the work directory for the SHA256 hash of the type and name.
+	// We intentionally avoid the actual name both because of possible
+	// conflicts with valid file system paths and because we want to ensure
+	// that one checkout is never nested inside another. That nesting has
+	// led to security problems in the past.
+	if strings.Contains(typ, ":") {
+		return "", "", fmt.Errorf("codehost.WorkDir: type cannot contain colon")
+	}
+	key := typ + ":" + name
+	dir = filepath.Join(cfg.GOMODCACHE, "cache/vcs", fmt.Sprintf("%x", sha256.Sum256([]byte(key))))
+
+	xLog, buildX := cfg.BuildXWriter(ctx)
+	if buildX {
+		fmt.Fprintf(xLog, "mkdir -p %s # %s %s\n", filepath.Dir(dir), typ, name)
+	}
+	if err := os.MkdirAll(filepath.Dir(dir), 0777); err != nil {
+		return "", "", err
+	}
+
+	lockfile = dir + ".lock"
+	if buildX {
+		fmt.Fprintf(xLog, "# lock %s\n", lockfile)
+	}
+
+	unlock, err := lockedfile.MutexAt(lockfile).Lock()
+	if err != nil {
+		return "", "", fmt.Errorf("codehost.WorkDir: can't find or create lock file: %v", err)
+	}
+	defer unlock()
+
+	data, err := os.ReadFile(dir + ".info")
+	info, err2 := os.Stat(dir)
+	if err == nil && err2 == nil && info.IsDir() {
+		// Info file and directory both already exist: reuse.
+		have := strings.TrimSuffix(string(data), "\n")
+		if have != key {
+			return "", "", fmt.Errorf("%s exists with wrong content (have %q want %q)", dir+".info", have, key)
+		}
+		if buildX {
+			fmt.Fprintf(xLog, "# %s for %s %s\n", dir, typ, name)
+		}
+		return dir, lockfile, nil
+	}
+
+	// Info file or directory missing. Start from scratch.
+	if xLog != nil {
+		fmt.Fprintf(xLog, "mkdir -p %s # %s %s\n", dir, typ, name)
+	}
+	os.RemoveAll(dir)
+	if err := os.MkdirAll(dir, 0777); err != nil {
+		return "", "", err
+	}
+	if err := os.WriteFile(dir+".info", []byte(key), 0666); err != nil {
+		os.RemoveAll(dir)
+		return "", "", err
+	}
+	return dir, lockfile, nil
+}
+
+type RunError struct {
+	Cmd      string
+	Err      error
+	Stderr   []byte
+	HelpText string
+}
+
+func (e *RunError) Error() string {
+	text := e.Cmd + ": " + e.Err.Error()
+	stderr := bytes.TrimRight(e.Stderr, "\n")
+	if len(stderr) > 0 {
+		text += ":\n\t" + strings.ReplaceAll(string(stderr), "\n", "\n\t")
+	}
+	if len(e.HelpText) > 0 {
+		text += "\n" + e.HelpText
+	}
+	return text
+}
+
+var dirLock sync.Map
+
+// Run runs the command line in the given directory
+// (an empty dir means the current directory).
+// It returns the standard output and, for a non-zero exit,
+// a *RunError indicating the command, exit status, and standard error.
+// Standard error is unavailable for commands that exit successfully.
+func Run(ctx context.Context, dir string, cmdline ...any) ([]byte, error) {
+	return RunWithStdin(ctx, dir, nil, cmdline...)
+}
+
+// bashQuoter escapes characters that have special meaning in double-quoted strings in the bash shell.
+// See https://www.gnu.org/software/bash/manual/html_node/Double-Quotes.html.
+var bashQuoter = strings.NewReplacer(`"`, `\"`, `$`, `\$`, "`", "\\`", `\`, `\\`)
+
+func RunWithStdin(ctx context.Context, dir string, stdin io.Reader, cmdline ...any) ([]byte, error) {
+	if dir != "" {
+		muIface, ok := dirLock.Load(dir)
+		if !ok {
+			muIface, _ = dirLock.LoadOrStore(dir, new(sync.Mutex))
+		}
+		mu := muIface.(*sync.Mutex)
+		mu.Lock()
+		defer mu.Unlock()
+	}
+
+	cmd := str.StringList(cmdline...)
+	if os.Getenv("TESTGOVCS") == "panic" {
+		panic(fmt.Sprintf("use of vcs: %v", cmd))
+	}
+	if xLog, ok := cfg.BuildXWriter(ctx); ok {
+		text := new(strings.Builder)
+		if dir != "" {
+			text.WriteString("cd ")
+			text.WriteString(dir)
+			text.WriteString("; ")
+		}
+		for i, arg := range cmd {
+			if i > 0 {
+				text.WriteByte(' ')
+			}
+			switch {
+			case strings.ContainsAny(arg, "'"):
+				// Quote args that could be mistaken for quoted args.
+				text.WriteByte('"')
+				text.WriteString(bashQuoter.Replace(arg))
+				text.WriteByte('"')
+			case strings.ContainsAny(arg, "$`\\*?[\"\t\n\v\f\r \u0085\u00a0"):
+				// Quote args that contain special characters, glob patterns, or spaces.
+				text.WriteByte('\'')
+				text.WriteString(arg)
+				text.WriteByte('\'')
+			default:
+				text.WriteString(arg)
+			}
+		}
+		fmt.Fprintf(xLog, "%s\n", text)
+		start := time.Now()
+		defer func() {
+			fmt.Fprintf(xLog, "%.3fs # %s\n", time.Since(start).Seconds(), text)
+		}()
+	}
+	// TODO: Impose limits on command output size.
+	// TODO: Set environment to get English error messages.
+	var stderr bytes.Buffer
+	var stdout bytes.Buffer
+	c := exec.CommandContext(ctx, cmd[0], cmd[1:]...)
+	c.Cancel = func() error { return c.Process.Signal(os.Interrupt) }
+	c.Dir = dir
+	c.Stdin = stdin
+	c.Stderr = &stderr
+	c.Stdout = &stdout
+	// For Git commands, manually supply GIT_DIR so Git works with safe.bareRepository=explicit set. Noop for other commands.
+	c.Env = append(c.Environ(), "GIT_DIR="+dir)
+	err := c.Run()
+	if err != nil {
+		err = &RunError{Cmd: strings.Join(cmd, " ") + " in " + dir, Stderr: stderr.Bytes(), Err: err}
+	}
+	return stdout.Bytes(), err
+}
diff --git a/src/cmd/go/internal/modfetch/codehost/git.go b/src/cmd/go/internal/modfetch/codehost/git.go
new file mode 100644
index 0000000..d1a18a8
--- /dev/null
+++ b/src/cmd/go/internal/modfetch/codehost/git.go
@@ -0,0 +1,915 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package codehost
+
+import (
+	"bytes"
+	"context"
+	"crypto/sha256"
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"io"
+	"io/fs"
+	"net/url"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"sort"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/lockedfile"
+	"cmd/go/internal/par"
+	"cmd/go/internal/web"
+
+	"golang.org/x/mod/semver"
+)
+
+// LocalGitRepo is like Repo but accepts both Git remote references
+// and paths to repositories on the local file system.
+func LocalGitRepo(ctx context.Context, remote string) (Repo, error) {
+	return newGitRepoCached(ctx, remote, true)
+}
+
+// A notExistError wraps another error to retain its original text
+// but makes it opaquely equivalent to fs.ErrNotExist.
+type notExistError struct {
+	err error
+}
+
+func (e notExistError) Error() string   { return e.err.Error() }
+func (notExistError) Is(err error) bool { return err == fs.ErrNotExist }
+
+const gitWorkDirType = "git3"
+
+var gitRepoCache par.ErrCache[gitCacheKey, Repo]
+
+type gitCacheKey struct {
+	remote  string
+	localOK bool
+}
+
+func newGitRepoCached(ctx context.Context, remote string, localOK bool) (Repo, error) {
+	return gitRepoCache.Do(gitCacheKey{remote, localOK}, func() (Repo, error) {
+		return newGitRepo(ctx, remote, localOK)
+	})
+}
+
+func newGitRepo(ctx context.Context, remote string, localOK bool) (Repo, error) {
+	r := &gitRepo{remote: remote}
+	if strings.Contains(remote, "://") {
+		// This is a remote path.
+		var err error
+		r.dir, r.mu.Path, err = WorkDir(ctx, gitWorkDirType, r.remote)
+		if err != nil {
+			return nil, err
+		}
+
+		unlock, err := r.mu.Lock()
+		if err != nil {
+			return nil, err
+		}
+		defer unlock()
+
+		if _, err := os.Stat(filepath.Join(r.dir, "objects")); err != nil {
+			if _, err := Run(ctx, r.dir, "git", "init", "--bare"); err != nil {
+				os.RemoveAll(r.dir)
+				return nil, err
+			}
+			// We could just say git fetch https://whatever later,
+			// but this lets us say git fetch origin instead, which
+			// is a little nicer. More importantly, using a named remote
+			// avoids a problem with Git LFS. See golang.org/issue/25605.
+			if _, err := Run(ctx, r.dir, "git", "remote", "add", "origin", "--", r.remote); err != nil {
+				os.RemoveAll(r.dir)
+				return nil, err
+			}
+			if runtime.GOOS == "windows" {
+				// Git for Windows by default does not support paths longer than
+				// MAX_PATH (260 characters) because that may interfere with navigation
+				// in some Windows programs. However, cmd/go should be able to handle
+				// long paths just fine, and we expect people to use 'go clean' to
+				// manipulate the module cache, so it should be harmless to set here,
+				// and in some cases may be necessary in order to download modules with
+				// long branch names.
+				//
+				// See https://github.com/git-for-windows/git/wiki/Git-cannot-create-a-file-or-directory-with-a-long-path.
+				if _, err := Run(ctx, r.dir, "git", "config", "core.longpaths", "true"); err != nil {
+					os.RemoveAll(r.dir)
+					return nil, err
+				}
+			}
+		}
+		r.remoteURL = r.remote
+		r.remote = "origin"
+	} else {
+		// Local path.
+		// Disallow colon (not in ://) because sometimes
+		// that's rcp-style host:path syntax and sometimes it's not (c:\work).
+		// The go command has always insisted on URL syntax for ssh.
+		if strings.Contains(remote, ":") {
+			return nil, fmt.Errorf("git remote cannot use host:path syntax")
+		}
+		if !localOK {
+			return nil, fmt.Errorf("git remote must not be local directory")
+		}
+		r.local = true
+		info, err := os.Stat(remote)
+		if err != nil {
+			return nil, err
+		}
+		if !info.IsDir() {
+			return nil, fmt.Errorf("%s exists but is not a directory", remote)
+		}
+		r.dir = remote
+		r.mu.Path = r.dir + ".lock"
+	}
+	return r, nil
+}
+
+type gitRepo struct {
+	ctx context.Context
+
+	remote, remoteURL string
+	local             bool
+	dir               string
+
+	mu lockedfile.Mutex // protects fetchLevel and git repo state
+
+	fetchLevel int
+
+	statCache par.ErrCache[string, *RevInfo]
+
+	refsOnce sync.Once
+	// refs maps branch and tag refs (e.g., "HEAD", "refs/heads/master")
+	// to commits (e.g., "37ffd2e798afde829a34e8955b716ab730b2a6d6")
+	refs    map[string]string
+	refsErr error
+
+	localTagsOnce sync.Once
+	localTags     map[string]bool
+}
+
+const (
+	// How much have we fetched into the git repo (in this process)?
+	fetchNone = iota // nothing yet
+	fetchSome        // shallow fetches of individual hashes
+	fetchAll         // "fetch -t origin": get all remote branches and tags
+)
+
+// loadLocalTags loads tag references from the local git cache
+// into the map r.localTags.
+// Should only be called as r.localTagsOnce.Do(r.loadLocalTags).
+func (r *gitRepo) loadLocalTags(ctx context.Context) {
+	// The git protocol sends all known refs and ls-remote filters them on the client side,
+	// so we might as well record both heads and tags in one shot.
+	// Most of the time we only care about tags but sometimes we care about heads too.
+	out, err := Run(ctx, r.dir, "git", "tag", "-l")
+	if err != nil {
+		return
+	}
+
+	r.localTags = make(map[string]bool)
+	for _, line := range strings.Split(string(out), "\n") {
+		if line != "" {
+			r.localTags[line] = true
+		}
+	}
+}
+
+func (r *gitRepo) CheckReuse(ctx context.Context, old *Origin, subdir string) error {
+	if old == nil {
+		return fmt.Errorf("missing origin")
+	}
+	if old.VCS != "git" || old.URL != r.remoteURL {
+		return fmt.Errorf("origin moved from %v %q to %v %q", old.VCS, old.URL, "git", r.remoteURL)
+	}
+	if old.Subdir != subdir {
+		return fmt.Errorf("origin moved from %v %q %q to %v %q %q", old.VCS, old.URL, old.Subdir, "git", r.remoteURL, subdir)
+	}
+
+	// Note: Can have Hash with no Ref and no TagSum and no RepoSum,
+	// meaning the Hash simply has to remain in the repo.
+	// In that case we assume it does in the absence of any real way to check.
+	// But if neither Hash nor TagSum is present, we have nothing to check,
+	// which we take to mean we didn't record enough information to be sure.
+	if old.Hash == "" && old.TagSum == "" && old.RepoSum == "" {
+		return fmt.Errorf("non-specific origin")
+	}
+
+	r.loadRefs(ctx)
+	if r.refsErr != nil {
+		return r.refsErr
+	}
+
+	if old.Ref != "" {
+		hash, ok := r.refs[old.Ref]
+		if !ok {
+			return fmt.Errorf("ref %q deleted", old.Ref)
+		}
+		if hash != old.Hash {
+			return fmt.Errorf("ref %q moved from %s to %s", old.Ref, old.Hash, hash)
+		}
+	}
+	if old.TagSum != "" {
+		tags, err := r.Tags(ctx, old.TagPrefix)
+		if err != nil {
+			return err
+		}
+		if tags.Origin.TagSum != old.TagSum {
+			return fmt.Errorf("tags changed")
+		}
+	}
+	if old.RepoSum != "" {
+		if r.repoSum(r.refs) != old.RepoSum {
+			return fmt.Errorf("refs changed")
+		}
+	}
+	return nil
+}
+
+// loadRefs loads heads and tags references from the remote into the map r.refs.
+// The result is cached in memory.
+func (r *gitRepo) loadRefs(ctx context.Context) (map[string]string, error) {
+	r.refsOnce.Do(func() {
+		// The git protocol sends all known refs and ls-remote filters them on the client side,
+		// so we might as well record both heads and tags in one shot.
+		// Most of the time we only care about tags but sometimes we care about heads too.
+		release, err := base.AcquireNet()
+		if err != nil {
+			r.refsErr = err
+			return
+		}
+		out, gitErr := Run(ctx, r.dir, "git", "ls-remote", "-q", r.remote)
+		release()
+
+		if gitErr != nil {
+			if rerr, ok := gitErr.(*RunError); ok {
+				if bytes.Contains(rerr.Stderr, []byte("fatal: could not read Username")) {
+					rerr.HelpText = "Confirm the import path was entered correctly.\nIf this is a private repository, see https://golang.org/doc/faq#git_https for additional information."
+				}
+			}
+
+			// If the remote URL doesn't exist at all, ideally we should treat the whole
+			// repository as nonexistent by wrapping the error in a notExistError.
+			// For HTTP and HTTPS, that's easy to detect: we'll try to fetch the URL
+			// ourselves and see what code it serves.
+			if u, err := url.Parse(r.remoteURL); err == nil && (u.Scheme == "http" || u.Scheme == "https") {
+				if _, err := web.GetBytes(u); errors.Is(err, fs.ErrNotExist) {
+					gitErr = notExistError{gitErr}
+				}
+			}
+
+			r.refsErr = gitErr
+			return
+		}
+
+		refs := make(map[string]string)
+		for _, line := range strings.Split(string(out), "\n") {
+			f := strings.Fields(line)
+			if len(f) != 2 {
+				continue
+			}
+			if f[1] == "HEAD" || strings.HasPrefix(f[1], "refs/heads/") || strings.HasPrefix(f[1], "refs/tags/") {
+				refs[f[1]] = f[0]
+			}
+		}
+		for ref, hash := range refs {
+			if k, found := strings.CutSuffix(ref, "^{}"); found { // record unwrapped annotated tag as value of tag
+				refs[k] = hash
+				delete(refs, ref)
+			}
+		}
+		r.refs = refs
+	})
+	return r.refs, r.refsErr
+}
+
+func (r *gitRepo) Tags(ctx context.Context, prefix string) (*Tags, error) {
+	refs, err := r.loadRefs(ctx)
+	if err != nil {
+		return nil, err
+	}
+
+	tags := &Tags{
+		Origin: &Origin{
+			VCS:       "git",
+			URL:       r.remoteURL,
+			TagPrefix: prefix,
+		},
+		List: []Tag{},
+	}
+	for ref, hash := range refs {
+		if !strings.HasPrefix(ref, "refs/tags/") {
+			continue
+		}
+		tag := ref[len("refs/tags/"):]
+		if !strings.HasPrefix(tag, prefix) {
+			continue
+		}
+		tags.List = append(tags.List, Tag{tag, hash})
+	}
+	sort.Slice(tags.List, func(i, j int) bool {
+		return tags.List[i].Name < tags.List[j].Name
+	})
+
+	dir := prefix[:strings.LastIndex(prefix, "/")+1]
+	h := sha256.New()
+	for _, tag := range tags.List {
+		if isOriginTag(strings.TrimPrefix(tag.Name, dir)) {
+			fmt.Fprintf(h, "%q %s\n", tag.Name, tag.Hash)
+		}
+	}
+	tags.Origin.TagSum = "t1:" + base64.StdEncoding.EncodeToString(h.Sum(nil))
+	return tags, nil
+}
+
+// repoSum returns a checksum of the entire repo state,
+// which can be checked (as Origin.RepoSum) to cache
+// the absence of a specific module version.
+// The caller must supply refs, the result of a successful r.loadRefs.
+func (r *gitRepo) repoSum(refs map[string]string) string {
+	var list []string
+	for ref := range refs {
+		list = append(list, ref)
+	}
+	sort.Strings(list)
+	h := sha256.New()
+	for _, ref := range list {
+		fmt.Fprintf(h, "%q %s\n", ref, refs[ref])
+	}
+	return "r1:" + base64.StdEncoding.EncodeToString(h.Sum(nil))
+}
+
+// unknownRevisionInfo returns a RevInfo containing an Origin containing a RepoSum of refs,
+// for use when returning an UnknownRevisionError.
+func (r *gitRepo) unknownRevisionInfo(refs map[string]string) *RevInfo {
+	return &RevInfo{
+		Origin: &Origin{
+			VCS:     "git",
+			URL:     r.remoteURL,
+			RepoSum: r.repoSum(refs),
+		},
+	}
+}
+
+func (r *gitRepo) Latest(ctx context.Context) (*RevInfo, error) {
+	refs, err := r.loadRefs(ctx)
+	if err != nil {
+		return nil, err
+	}
+	if refs["HEAD"] == "" {
+		return nil, ErrNoCommits
+	}
+	statInfo, err := r.Stat(ctx, refs["HEAD"])
+	if err != nil {
+		return nil, err
+	}
+
+	// Stat may return cached info, so make a copy to modify here.
+	info := new(RevInfo)
+	*info = *statInfo
+	info.Origin = new(Origin)
+	if statInfo.Origin != nil {
+		*info.Origin = *statInfo.Origin
+	}
+	info.Origin.Ref = "HEAD"
+	info.Origin.Hash = refs["HEAD"]
+
+	return info, nil
+}
+
+// findRef finds some ref name for the given hash,
+// for use when the server requires giving a ref instead of a hash.
+// There may be multiple ref names for a given hash,
+// in which case this returns some name - it doesn't matter which.
+func (r *gitRepo) findRef(ctx context.Context, hash string) (ref string, ok bool) {
+	refs, err := r.loadRefs(ctx)
+	if err != nil {
+		return "", false
+	}
+	for ref, h := range refs {
+		if h == hash {
+			return ref, true
+		}
+	}
+	return "", false
+}
+
+// minHashDigits is the minimum number of digits to require
+// before accepting a hex digit sequence as potentially identifying
+// a specific commit in a git repo. (Of course, users can always
+// specify more digits, and many will paste in all 40 digits,
+// but many of git's commands default to printing short hashes
+// as 7 digits.)
+const minHashDigits = 7
+
+// stat stats the given rev in the local repository,
+// or else it fetches more info from the remote repository and tries again.
+func (r *gitRepo) stat(ctx context.Context, rev string) (info *RevInfo, err error) {
+	if r.local {
+		return r.statLocal(ctx, rev, rev)
+	}
+
+	// Fast path: maybe rev is a hash we already have locally.
+	didStatLocal := false
+	if len(rev) >= minHashDigits && len(rev) <= 40 && AllHex(rev) {
+		if info, err := r.statLocal(ctx, rev, rev); err == nil {
+			return info, nil
+		}
+		didStatLocal = true
+	}
+
+	// Maybe rev is a tag we already have locally.
+	// (Note that we're excluding branches, which can be stale.)
+	r.localTagsOnce.Do(func() { r.loadLocalTags(ctx) })
+	if r.localTags[rev] {
+		return r.statLocal(ctx, rev, "refs/tags/"+rev)
+	}
+
+	// Maybe rev is the name of a tag or branch on the remote server.
+	// Or maybe it's the prefix of a hash of a named ref.
+	// Try to resolve to both a ref (git name) and full (40-hex-digit) commit hash.
+	refs, err := r.loadRefs(ctx)
+	if err != nil {
+		return nil, err
+	}
+	// loadRefs may return an error if git fails, for example segfaults, or
+	// could not load a private repo, but defer checking to the else block
+	// below, in case we already have the rev in question in the local cache.
+	var ref, hash string
+	if refs["refs/tags/"+rev] != "" {
+		ref = "refs/tags/" + rev
+		hash = refs[ref]
+		// Keep rev as is: tags are assumed not to change meaning.
+	} else if refs["refs/heads/"+rev] != "" {
+		ref = "refs/heads/" + rev
+		hash = refs[ref]
+		rev = hash // Replace rev, because meaning of refs/heads/foo can change.
+	} else if rev == "HEAD" && refs["HEAD"] != "" {
+		ref = "HEAD"
+		hash = refs[ref]
+		rev = hash // Replace rev, because meaning of HEAD can change.
+	} else if len(rev) >= minHashDigits && len(rev) <= 40 && AllHex(rev) {
+		// At the least, we have a hash prefix we can look up after the fetch below.
+		// Maybe we can map it to a full hash using the known refs.
+		prefix := rev
+		// Check whether rev is prefix of known ref hash.
+		for k, h := range refs {
+			if strings.HasPrefix(h, prefix) {
+				if hash != "" && hash != h {
+					// Hash is an ambiguous hash prefix.
+					// More information will not change that.
+					return nil, fmt.Errorf("ambiguous revision %s", rev)
+				}
+				if ref == "" || ref > k { // Break ties deterministically when multiple refs point at same hash.
+					ref = k
+				}
+				rev = h
+				hash = h
+			}
+		}
+		if hash == "" && len(rev) == 40 { // Didn't find a ref, but rev is a full hash.
+			hash = rev
+		}
+	} else {
+		return r.unknownRevisionInfo(refs), &UnknownRevisionError{Rev: rev}
+	}
+
+	defer func() {
+		if info != nil {
+			info.Origin.Hash = info.Name
+			// There's a ref = hash below; don't write that hash down as Origin.Ref.
+			if ref != info.Origin.Hash {
+				info.Origin.Ref = ref
+			}
+		}
+	}()
+
+	// Protect r.fetchLevel and the "fetch more and more" sequence.
+	unlock, err := r.mu.Lock()
+	if err != nil {
+		return nil, err
+	}
+	defer unlock()
+
+	// Perhaps r.localTags did not have the ref when we loaded local tags,
+	// but we've since done fetches that pulled down the hash we need
+	// (or already have the hash we need, just without its tag).
+	// Either way, try a local stat before falling back to network I/O.
+	if !didStatLocal {
+		if info, err := r.statLocal(ctx, rev, hash); err == nil {
+			if after, found := strings.CutPrefix(ref, "refs/tags/"); found {
+				// Make sure tag exists, so it will be in localTags next time the go command is run.
+				Run(ctx, r.dir, "git", "tag", after, hash)
+			}
+			return info, nil
+		}
+	}
+
+	// If we know a specific commit we need and its ref, fetch it.
+	// We do NOT fetch arbitrary hashes (when we don't know the ref)
+	// because we want to avoid ever importing a commit that isn't
+	// reachable from refs/tags/* or refs/heads/* or HEAD.
+	// Both Gerrit and GitHub expose every CL/PR as a named ref,
+	// and we don't want those commits masquerading as being real
+	// pseudo-versions in the main repo.
+	if r.fetchLevel <= fetchSome && ref != "" && hash != "" && !r.local {
+		r.fetchLevel = fetchSome
+		var refspec string
+		if ref != "" && ref != "HEAD" {
+			// If we do know the ref name, save the mapping locally
+			// so that (if it is a tag) it can show up in localTags
+			// on a future call. Also, some servers refuse to allow
+			// full hashes in ref specs, so prefer a ref name if known.
+			refspec = ref + ":" + ref
+		} else {
+			// Fetch the hash but give it a local name (refs/dummy),
+			// because that triggers the fetch behavior of creating any
+			// other known remote tags for the hash. We never use
+			// refs/dummy (it's not refs/tags/dummy) and it will be
+			// overwritten in the next command, and that's fine.
+			ref = hash
+			refspec = hash + ":refs/dummy"
+		}
+
+		release, err := base.AcquireNet()
+		if err != nil {
+			return nil, err
+		}
+		_, err = Run(ctx, r.dir, "git", "fetch", "-f", "--depth=1", r.remote, refspec)
+		release()
+
+		if err == nil {
+			return r.statLocal(ctx, rev, ref)
+		}
+		// Don't try to be smart about parsing the error.
+		// It's too complex and varies too much by git version.
+		// No matter what went wrong, fall back to a complete fetch.
+	}
+
+	// Last resort.
+	// Fetch all heads and tags and hope the hash we want is in the history.
+	if err := r.fetchRefsLocked(ctx); err != nil {
+		return nil, err
+	}
+
+	return r.statLocal(ctx, rev, rev)
+}
+
+// fetchRefsLocked fetches all heads and tags from the origin, along with the
+// ancestors of those commits.
+//
+// We only fetch heads and tags, not arbitrary other commits: we don't want to
+// pull in off-branch commits (such as rejected GitHub pull requests) that the
+// server may be willing to provide. (See the comments within the stat method
+// for more detail.)
+//
+// fetchRefsLocked requires that r.mu remain locked for the duration of the call.
+func (r *gitRepo) fetchRefsLocked(ctx context.Context) error {
+	if r.fetchLevel < fetchAll {
+		// NOTE: To work around a bug affecting Git clients up to at least 2.23.0
+		// (2019-08-16), we must first expand the set of local refs, and only then
+		// unshallow the repository as a separate fetch operation. (See
+		// golang.org/issue/34266 and
+		// https://github.com/git/git/blob/4c86140027f4a0d2caaa3ab4bd8bfc5ce3c11c8a/transport.c#L1303-L1309.)
+
+		release, err := base.AcquireNet()
+		if err != nil {
+			return err
+		}
+		defer release()
+
+		if _, err := Run(ctx, r.dir, "git", "fetch", "-f", r.remote, "refs/heads/*:refs/heads/*", "refs/tags/*:refs/tags/*"); err != nil {
+			return err
+		}
+
+		if _, err := os.Stat(filepath.Join(r.dir, "shallow")); err == nil {
+			if _, err := Run(ctx, r.dir, "git", "fetch", "--unshallow", "-f", r.remote); err != nil {
+				return err
+			}
+		}
+
+		r.fetchLevel = fetchAll
+	}
+	return nil
+}
+
+// statLocal returns a new RevInfo describing rev in the local git repository.
+// It uses version as info.Version.
+func (r *gitRepo) statLocal(ctx context.Context, version, rev string) (*RevInfo, error) {
+	out, err := Run(ctx, r.dir, "git", "-c", "log.showsignature=false", "log", "--no-decorate", "-n1", "--format=format:%H %ct %D", rev, "--")
+	if err != nil {
+		// Return info with Origin.RepoSum if possible to allow caching of negative lookup.
+		var info *RevInfo
+		if refs, err := r.loadRefs(ctx); err == nil {
+			info = r.unknownRevisionInfo(refs)
+		}
+		return info, &UnknownRevisionError{Rev: rev}
+	}
+	f := strings.Fields(string(out))
+	if len(f) < 2 {
+		return nil, fmt.Errorf("unexpected response from git log: %q", out)
+	}
+	hash := f[0]
+	if strings.HasPrefix(hash, version) {
+		version = hash // extend to full hash
+	}
+	t, err := strconv.ParseInt(f[1], 10, 64)
+	if err != nil {
+		return nil, fmt.Errorf("invalid time from git log: %q", out)
+	}
+
+	info := &RevInfo{
+		Origin: &Origin{
+			VCS:  "git",
+			URL:  r.remoteURL,
+			Hash: hash,
+		},
+		Name:    hash,
+		Short:   ShortenSHA1(hash),
+		Time:    time.Unix(t, 0).UTC(),
+		Version: hash,
+	}
+	if !strings.HasPrefix(hash, rev) {
+		info.Origin.Ref = rev
+	}
+
+	// Add tags. Output looks like:
+	//	ede458df7cd0fdca520df19a33158086a8a68e81 1523994202 HEAD -> master, tag: v1.2.4-annotated, tag: v1.2.3, origin/master, origin/HEAD
+	for i := 2; i < len(f); i++ {
+		if f[i] == "tag:" {
+			i++
+			if i < len(f) {
+				info.Tags = append(info.Tags, strings.TrimSuffix(f[i], ","))
+			}
+		}
+	}
+	sort.Strings(info.Tags)
+
+	// Used hash as info.Version above.
+	// Use caller's suggested version if it appears in the tag list
+	// (filters out branch names, HEAD).
+	for _, tag := range info.Tags {
+		if version == tag {
+			info.Version = version
+		}
+	}
+
+	return info, nil
+}
+
+func (r *gitRepo) Stat(ctx context.Context, rev string) (*RevInfo, error) {
+	if rev == "latest" {
+		return r.Latest(ctx)
+	}
+	return r.statCache.Do(rev, func() (*RevInfo, error) {
+		return r.stat(ctx, rev)
+	})
+}
+
+func (r *gitRepo) ReadFile(ctx context.Context, rev, file string, maxSize int64) ([]byte, error) {
+	// TODO: Could use git cat-file --batch.
+	info, err := r.Stat(ctx, rev) // download rev into local git repo
+	if err != nil {
+		return nil, err
+	}
+	out, err := Run(ctx, r.dir, "git", "cat-file", "blob", info.Name+":"+file)
+	if err != nil {
+		return nil, fs.ErrNotExist
+	}
+	return out, nil
+}
+
+func (r *gitRepo) RecentTag(ctx context.Context, rev, prefix string, allowed func(tag string) bool) (tag string, err error) {
+	info, err := r.Stat(ctx, rev)
+	if err != nil {
+		return "", err
+	}
+	rev = info.Name // expand hash prefixes
+
+	// describe sets tag and err using 'git for-each-ref' and reports whether the
+	// result is definitive.
+	describe := func() (definitive bool) {
+		var out []byte
+		out, err = Run(ctx, r.dir, "git", "for-each-ref", "--format", "%(refname)", "refs/tags", "--merged", rev)
+		if err != nil {
+			return true
+		}
+
+		// prefixed tags aren't valid semver tags so compare without prefix, but only tags with correct prefix
+		var highest string
+		for _, line := range strings.Split(string(out), "\n") {
+			line = strings.TrimSpace(line)
+			// git do support lstrip in for-each-ref format, but it was added in v2.13.0. Stripping here
+			// instead gives support for git v2.7.0.
+			if !strings.HasPrefix(line, "refs/tags/") {
+				continue
+			}
+			line = line[len("refs/tags/"):]
+
+			if !strings.HasPrefix(line, prefix) {
+				continue
+			}
+			if !allowed(line) {
+				continue
+			}
+
+			semtag := line[len(prefix):]
+			if semver.Compare(semtag, highest) > 0 {
+				highest = semtag
+			}
+		}
+
+		if highest != "" {
+			tag = prefix + highest
+		}
+
+		return tag != "" && !AllHex(tag)
+	}
+
+	if describe() {
+		return tag, err
+	}
+
+	// Git didn't find a version tag preceding the requested rev.
+	// See whether any plausible tag exists.
+	tags, err := r.Tags(ctx, prefix+"v")
+	if err != nil {
+		return "", err
+	}
+	if len(tags.List) == 0 {
+		return "", nil
+	}
+
+	// There are plausible tags, but we don't know if rev is a descendent of any of them.
+	// Fetch the history to find out.
+
+	unlock, err := r.mu.Lock()
+	if err != nil {
+		return "", err
+	}
+	defer unlock()
+
+	if err := r.fetchRefsLocked(ctx); err != nil {
+		return "", err
+	}
+
+	// If we've reached this point, we have all of the commits that are reachable
+	// from all heads and tags.
+	//
+	// The only refs we should be missing are those that are no longer reachable
+	// (or never were reachable) from any branch or tag, including the master
+	// branch, and we don't want to resolve them anyway (they're probably
+	// unreachable for a reason).
+	//
+	// Try one last time in case some other goroutine fetched rev while we were
+	// waiting on the lock.
+	describe()
+	return tag, err
+}
+
+func (r *gitRepo) DescendsFrom(ctx context.Context, rev, tag string) (bool, error) {
+	// The "--is-ancestor" flag was added to "git merge-base" in version 1.8.0, so
+	// this won't work with Git 1.7.1. According to golang.org/issue/28550, cmd/go
+	// already doesn't work with Git 1.7.1, so at least it's not a regression.
+	//
+	// git merge-base --is-ancestor exits with status 0 if rev is an ancestor, or
+	// 1 if not.
+	_, err := Run(ctx, r.dir, "git", "merge-base", "--is-ancestor", "--", tag, rev)
+
+	// Git reports "is an ancestor" with exit code 0 and "not an ancestor" with
+	// exit code 1.
+	// Unfortunately, if we've already fetched rev with a shallow history, git
+	// merge-base has been observed to report a false-negative, so don't stop yet
+	// even if the exit code is 1!
+	if err == nil {
+		return true, nil
+	}
+
+	// See whether the tag and rev even exist.
+	tags, err := r.Tags(ctx, tag)
+	if err != nil {
+		return false, err
+	}
+	if len(tags.List) == 0 {
+		return false, nil
+	}
+
+	// NOTE: r.stat is very careful not to fetch commits that we shouldn't know
+	// about, like rejected GitHub pull requests, so don't try to short-circuit
+	// that here.
+	if _, err = r.stat(ctx, rev); err != nil {
+		return false, err
+	}
+
+	// Now fetch history so that git can search for a path.
+	unlock, err := r.mu.Lock()
+	if err != nil {
+		return false, err
+	}
+	defer unlock()
+
+	if r.fetchLevel < fetchAll {
+		// Fetch the complete history for all refs and heads. It would be more
+		// efficient to only fetch the history from rev to tag, but that's much more
+		// complicated, and any kind of shallow fetch is fairly likely to trigger
+		// bugs in JGit servers and/or the go command anyway.
+		if err := r.fetchRefsLocked(ctx); err != nil {
+			return false, err
+		}
+	}
+
+	_, err = Run(ctx, r.dir, "git", "merge-base", "--is-ancestor", "--", tag, rev)
+	if err == nil {
+		return true, nil
+	}
+	if ee, ok := err.(*RunError).Err.(*exec.ExitError); ok && ee.ExitCode() == 1 {
+		return false, nil
+	}
+	return false, err
+}
+
+func (r *gitRepo) ReadZip(ctx context.Context, rev, subdir string, maxSize int64) (zip io.ReadCloser, err error) {
+	// TODO: Use maxSize or drop it.
+	args := []string{}
+	if subdir != "" {
+		args = append(args, "--", subdir)
+	}
+	info, err := r.Stat(ctx, rev) // download rev into local git repo
+	if err != nil {
+		return nil, err
+	}
+
+	unlock, err := r.mu.Lock()
+	if err != nil {
+		return nil, err
+	}
+	defer unlock()
+
+	if err := ensureGitAttributes(r.dir); err != nil {
+		return nil, err
+	}
+
+	// Incredibly, git produces different archives depending on whether
+	// it is running on a Windows system or not, in an attempt to normalize
+	// text file line endings. Setting -c core.autocrlf=input means only
+	// translate files on the way into the repo, not on the way out (archive).
+	// The -c core.eol=lf should be unnecessary but set it anyway.
+	archive, err := Run(ctx, r.dir, "git", "-c", "core.autocrlf=input", "-c", "core.eol=lf", "archive", "--format=zip", "--prefix=prefix/", info.Name, args)
+	if err != nil {
+		if bytes.Contains(err.(*RunError).Stderr, []byte("did not match any files")) {
+			return nil, fs.ErrNotExist
+		}
+		return nil, err
+	}
+
+	return io.NopCloser(bytes.NewReader(archive)), nil
+}
+
+// ensureGitAttributes makes sure export-subst and export-ignore features are
+// disabled for this repo. This is intended to be run prior to running git
+// archive so that zip files are generated that produce consistent ziphashes
+// for a given revision, independent of variables such as git version and the
+// size of the repo.
+//
+// See: https://github.com/golang/go/issues/27153
+func ensureGitAttributes(repoDir string) (err error) {
+	const attr = "\n* -export-subst -export-ignore\n"
+
+	d := repoDir + "/info"
+	p := d + "/attributes"
+
+	if err := os.MkdirAll(d, 0755); err != nil {
+		return err
+	}
+
+	f, err := os.OpenFile(p, os.O_CREATE|os.O_APPEND|os.O_RDWR, 0666)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		closeErr := f.Close()
+		if closeErr != nil {
+			err = closeErr
+		}
+	}()
+
+	b, err := io.ReadAll(f)
+	if err != nil {
+		return err
+	}
+	if !bytes.HasSuffix(b, []byte(attr)) {
+		_, err := f.WriteString(attr)
+		return err
+	}
+
+	return nil
+}
diff --git a/src/cmd/go/internal/modfetch/codehost/git_test.go b/src/cmd/go/internal/modfetch/codehost/git_test.go
new file mode 100644
index 0000000..328ab5b
--- /dev/null
+++ b/src/cmd/go/internal/modfetch/codehost/git_test.go
@@ -0,0 +1,801 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package codehost
+
+import (
+	"archive/zip"
+	"bytes"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/vcweb/vcstest"
+	"context"
+	"flag"
+	"internal/testenv"
+	"io"
+	"io/fs"
+	"log"
+	"os"
+	"path"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+)
+
+func TestMain(m *testing.M) {
+	flag.Parse()
+	if err := testMain(m); err != nil {
+		log.Fatal(err)
+	}
+}
+
+var gitrepo1, hgrepo1, vgotest1 string
+
+var altRepos = func() []string {
+	return []string{
+		"localGitRepo",
+		hgrepo1,
+	}
+}
+
+// TODO: Convert gitrepo1 to svn, bzr, fossil and add tests.
+// For now, at least the hgrepo1 tests check the general vcs.go logic.
+
+// localGitRepo is like gitrepo1 but allows archive access
+// (although that doesn't really matter after CL 120041),
+// and has a file:// URL instead of http:// or https://
+// (which might still matter).
+var localGitRepo string
+
+// localGitURL initializes the repo in localGitRepo and returns its URL.
+func localGitURL(t testing.TB) string {
+	testenv.MustHaveExecPath(t, "git")
+	if runtime.GOOS == "android" && strings.HasSuffix(testenv.Builder(), "-corellium") {
+		testenv.SkipFlaky(t, 59940)
+	}
+
+	localGitURLOnce.Do(func() {
+		// Clone gitrepo1 into a local directory.
+		// If we use a file:// URL to access the local directory,
+		// then git starts up all the usual protocol machinery,
+		// which will let us test remote git archive invocations.
+		_, localGitURLErr = Run(context.Background(), "", "git", "clone", "--mirror", gitrepo1, localGitRepo)
+		if localGitURLErr != nil {
+			return
+		}
+		_, localGitURLErr = Run(context.Background(), localGitRepo, "git", "config", "daemon.uploadarch", "true")
+	})
+
+	if localGitURLErr != nil {
+		t.Fatal(localGitURLErr)
+	}
+	// Convert absolute path to file URL. LocalGitRepo will not accept
+	// Windows absolute paths because they look like a host:path remote.
+	// TODO(golang.org/issue/32456): use url.FromFilePath when implemented.
+	if strings.HasPrefix(localGitRepo, "/") {
+		return "file://" + localGitRepo
+	} else {
+		return "file:///" + filepath.ToSlash(localGitRepo)
+	}
+}
+
+var (
+	localGitURLOnce sync.Once
+	localGitURLErr  error
+)
+
+func testMain(m *testing.M) (err error) {
+	cfg.BuildX = testing.Verbose()
+
+	srv, err := vcstest.NewServer()
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if closeErr := srv.Close(); err == nil {
+			err = closeErr
+		}
+	}()
+
+	gitrepo1 = srv.HTTP.URL + "/git/gitrepo1"
+	hgrepo1 = srv.HTTP.URL + "/hg/hgrepo1"
+	vgotest1 = srv.HTTP.URL + "/git/vgotest1"
+
+	dir, err := os.MkdirTemp("", "gitrepo-test-")
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if rmErr := os.RemoveAll(dir); err == nil {
+			err = rmErr
+		}
+	}()
+
+	localGitRepo = filepath.Join(dir, "gitrepo2")
+
+	// Redirect the module cache to a fresh directory to avoid crosstalk, and make
+	// it read/write so that the test can still clean it up easily when done.
+	cfg.GOMODCACHE = filepath.Join(dir, "modcache")
+	cfg.ModCacheRW = true
+
+	m.Run()
+	return nil
+}
+
+func testContext(t testing.TB) context.Context {
+	w := newTestWriter(t)
+	return cfg.WithBuildXWriter(context.Background(), w)
+}
+
+// A testWriter is an io.Writer that writes to a test's log.
+//
+// The writer batches written data until the last byte of a write is a newline
+// character, then flushes the batched data as a single call to Logf.
+// Any remaining unflushed data is logged during Cleanup.
+type testWriter struct {
+	t testing.TB
+
+	mu  sync.Mutex
+	buf bytes.Buffer
+}
+
+func newTestWriter(t testing.TB) *testWriter {
+	w := &testWriter{t: t}
+
+	t.Cleanup(func() {
+		w.mu.Lock()
+		defer w.mu.Unlock()
+		if b := w.buf.Bytes(); len(b) > 0 {
+			w.t.Logf("%s", b)
+			w.buf.Reset()
+		}
+	})
+
+	return w
+}
+
+func (w *testWriter) Write(p []byte) (int, error) {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+	n, err := w.buf.Write(p)
+	if b := w.buf.Bytes(); len(b) > 0 && b[len(b)-1] == '\n' {
+		w.t.Logf("%s", b)
+		w.buf.Reset()
+	}
+	return n, err
+}
+
+func testRepo(ctx context.Context, t *testing.T, remote string) (Repo, error) {
+	if remote == "localGitRepo" {
+		return LocalGitRepo(ctx, localGitURL(t))
+	}
+	vcsName := "git"
+	for _, k := range []string{"hg"} {
+		if strings.Contains(remote, "/"+k+"/") {
+			vcsName = k
+		}
+	}
+	if testing.Short() && vcsName == "hg" {
+		t.Skipf("skipping hg test in short mode: hg is slow")
+	}
+	testenv.MustHaveExecPath(t, vcsName)
+	if runtime.GOOS == "android" && strings.HasSuffix(testenv.Builder(), "-corellium") {
+		testenv.SkipFlaky(t, 59940)
+	}
+	return NewRepo(ctx, vcsName, remote)
+}
+
+func TestTags(t *testing.T) {
+	t.Parallel()
+
+	type tagsTest struct {
+		repo   string
+		prefix string
+		tags   []Tag
+	}
+
+	runTest := func(tt tagsTest) func(*testing.T) {
+		return func(t *testing.T) {
+			t.Parallel()
+			ctx := testContext(t)
+
+			r, err := testRepo(ctx, t, tt.repo)
+			if err != nil {
+				t.Fatal(err)
+			}
+			tags, err := r.Tags(ctx, tt.prefix)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if tags == nil || !reflect.DeepEqual(tags.List, tt.tags) {
+				t.Errorf("Tags(%q): incorrect tags\nhave %v\nwant %v", tt.prefix, tags, tt.tags)
+			}
+		}
+	}
+
+	for _, tt := range []tagsTest{
+		{gitrepo1, "xxx", []Tag{}},
+		{gitrepo1, "", []Tag{
+			{"v1.2.3", "ede458df7cd0fdca520df19a33158086a8a68e81"},
+			{"v1.2.4-annotated", "ede458df7cd0fdca520df19a33158086a8a68e81"},
+			{"v2.0.1", "76a00fb249b7f93091bc2c89a789dab1fc1bc26f"},
+			{"v2.0.2", "9d02800338b8a55be062c838d1f02e0c5780b9eb"},
+			{"v2.3", "76a00fb249b7f93091bc2c89a789dab1fc1bc26f"},
+		}},
+		{gitrepo1, "v", []Tag{
+			{"v1.2.3", "ede458df7cd0fdca520df19a33158086a8a68e81"},
+			{"v1.2.4-annotated", "ede458df7cd0fdca520df19a33158086a8a68e81"},
+			{"v2.0.1", "76a00fb249b7f93091bc2c89a789dab1fc1bc26f"},
+			{"v2.0.2", "9d02800338b8a55be062c838d1f02e0c5780b9eb"},
+			{"v2.3", "76a00fb249b7f93091bc2c89a789dab1fc1bc26f"},
+		}},
+		{gitrepo1, "v1", []Tag{
+			{"v1.2.3", "ede458df7cd0fdca520df19a33158086a8a68e81"},
+			{"v1.2.4-annotated", "ede458df7cd0fdca520df19a33158086a8a68e81"},
+		}},
+		{gitrepo1, "2", []Tag{}},
+	} {
+		t.Run(path.Base(tt.repo)+"/"+tt.prefix, runTest(tt))
+		if tt.repo == gitrepo1 {
+			// Clear hashes.
+			clearTags := []Tag{}
+			for _, tag := range tt.tags {
+				clearTags = append(clearTags, Tag{tag.Name, ""})
+			}
+			tags := tt.tags
+			for _, tt.repo = range altRepos() {
+				if strings.Contains(tt.repo, "Git") {
+					tt.tags = tags
+				} else {
+					tt.tags = clearTags
+				}
+				t.Run(path.Base(tt.repo)+"/"+tt.prefix, runTest(tt))
+			}
+		}
+	}
+}
+
+func TestLatest(t *testing.T) {
+	t.Parallel()
+
+	type latestTest struct {
+		repo string
+		info *RevInfo
+	}
+	runTest := func(tt latestTest) func(*testing.T) {
+		return func(t *testing.T) {
+			t.Parallel()
+			ctx := testContext(t)
+
+			r, err := testRepo(ctx, t, tt.repo)
+			if err != nil {
+				t.Fatal(err)
+			}
+			info, err := r.Latest(ctx)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if !reflect.DeepEqual(info, tt.info) {
+				if !reflect.DeepEqual(info.Tags, tt.info.Tags) {
+					testenv.SkipFlaky(t, 56881)
+				}
+				t.Errorf("Latest: incorrect info\nhave %+v (origin %+v)\nwant %+v (origin %+v)", info, info.Origin, tt.info, tt.info.Origin)
+			}
+		}
+	}
+
+	for _, tt := range []latestTest{
+		{
+			gitrepo1,
+			&RevInfo{
+				Origin: &Origin{
+					VCS:  "git",
+					URL:  gitrepo1,
+					Ref:  "HEAD",
+					Hash: "ede458df7cd0fdca520df19a33158086a8a68e81",
+				},
+				Name:    "ede458df7cd0fdca520df19a33158086a8a68e81",
+				Short:   "ede458df7cd0",
+				Version: "ede458df7cd0fdca520df19a33158086a8a68e81",
+				Time:    time.Date(2018, 4, 17, 19, 43, 22, 0, time.UTC),
+				Tags:    []string{"v1.2.3", "v1.2.4-annotated"},
+			},
+		},
+		{
+			hgrepo1,
+			&RevInfo{
+				Origin: &Origin{
+					VCS:  "hg",
+					URL:  hgrepo1,
+					Hash: "18518c07eb8ed5c80221e997e518cccaa8c0c287",
+				},
+				Name:    "18518c07eb8ed5c80221e997e518cccaa8c0c287",
+				Short:   "18518c07eb8e",
+				Version: "18518c07eb8ed5c80221e997e518cccaa8c0c287",
+				Time:    time.Date(2018, 6, 27, 16, 16, 30, 0, time.UTC),
+			},
+		},
+	} {
+		t.Run(path.Base(tt.repo), runTest(tt))
+		if tt.repo == gitrepo1 {
+			tt.repo = "localGitRepo"
+			info := *tt.info
+			tt.info = &info
+			o := *info.Origin
+			info.Origin = &o
+			o.URL = localGitURL(t)
+			t.Run(path.Base(tt.repo), runTest(tt))
+		}
+	}
+}
+
+func TestReadFile(t *testing.T) {
+	t.Parallel()
+
+	type readFileTest struct {
+		repo string
+		rev  string
+		file string
+		err  string
+		data string
+	}
+	runTest := func(tt readFileTest) func(*testing.T) {
+		return func(t *testing.T) {
+			t.Parallel()
+			ctx := testContext(t)
+
+			r, err := testRepo(ctx, t, tt.repo)
+			if err != nil {
+				t.Fatal(err)
+			}
+			data, err := r.ReadFile(ctx, tt.rev, tt.file, 100)
+			if err != nil {
+				if tt.err == "" {
+					t.Fatalf("ReadFile: unexpected error %v", err)
+				}
+				if !strings.Contains(err.Error(), tt.err) {
+					t.Fatalf("ReadFile: wrong error %q, want %q", err, tt.err)
+				}
+				if len(data) != 0 {
+					t.Errorf("ReadFile: non-empty data %q with error %v", data, err)
+				}
+				return
+			}
+			if tt.err != "" {
+				t.Fatalf("ReadFile: no error, wanted %v", tt.err)
+			}
+			if string(data) != tt.data {
+				t.Errorf("ReadFile: incorrect data\nhave %q\nwant %q", data, tt.data)
+			}
+		}
+	}
+
+	for _, tt := range []readFileTest{
+		{
+			repo: gitrepo1,
+			rev:  "latest",
+			file: "README",
+			data: "",
+		},
+		{
+			repo: gitrepo1,
+			rev:  "v2",
+			file: "another.txt",
+			data: "another\n",
+		},
+		{
+			repo: gitrepo1,
+			rev:  "v2.3.4",
+			file: "another.txt",
+			err:  fs.ErrNotExist.Error(),
+		},
+	} {
+		t.Run(path.Base(tt.repo)+"/"+tt.rev+"/"+tt.file, runTest(tt))
+		if tt.repo == gitrepo1 {
+			for _, tt.repo = range altRepos() {
+				t.Run(path.Base(tt.repo)+"/"+tt.rev+"/"+tt.file, runTest(tt))
+			}
+		}
+	}
+}
+
+type zipFile struct {
+	name string
+	size int64
+}
+
+func TestReadZip(t *testing.T) {
+	t.Parallel()
+
+	type readZipTest struct {
+		repo   string
+		rev    string
+		subdir string
+		err    string
+		files  map[string]uint64
+	}
+	runTest := func(tt readZipTest) func(*testing.T) {
+		return func(t *testing.T) {
+			t.Parallel()
+			ctx := testContext(t)
+
+			r, err := testRepo(ctx, t, tt.repo)
+			if err != nil {
+				t.Fatal(err)
+			}
+			rc, err := r.ReadZip(ctx, tt.rev, tt.subdir, 100000)
+			if err != nil {
+				if tt.err == "" {
+					t.Fatalf("ReadZip: unexpected error %v", err)
+				}
+				if !strings.Contains(err.Error(), tt.err) {
+					t.Fatalf("ReadZip: wrong error %q, want %q", err, tt.err)
+				}
+				if rc != nil {
+					t.Errorf("ReadZip: non-nil io.ReadCloser with error %v", err)
+				}
+				return
+			}
+			defer rc.Close()
+			if tt.err != "" {
+				t.Fatalf("ReadZip: no error, wanted %v", tt.err)
+			}
+			zipdata, err := io.ReadAll(rc)
+			if err != nil {
+				t.Fatal(err)
+			}
+			z, err := zip.NewReader(bytes.NewReader(zipdata), int64(len(zipdata)))
+			if err != nil {
+				t.Fatalf("ReadZip: cannot read zip file: %v", err)
+			}
+			have := make(map[string]bool)
+			for _, f := range z.File {
+				size, ok := tt.files[f.Name]
+				if !ok {
+					t.Errorf("ReadZip: unexpected file %s", f.Name)
+					continue
+				}
+				have[f.Name] = true
+				if size != ^uint64(0) && f.UncompressedSize64 != size {
+					t.Errorf("ReadZip: file %s has unexpected size %d != %d", f.Name, f.UncompressedSize64, size)
+				}
+			}
+			for name := range tt.files {
+				if !have[name] {
+					t.Errorf("ReadZip: missing file %s", name)
+				}
+			}
+		}
+	}
+
+	for _, tt := range []readZipTest{
+		{
+			repo:   gitrepo1,
+			rev:    "v2.3.4",
+			subdir: "",
+			files: map[string]uint64{
+				"prefix/":       0,
+				"prefix/README": 0,
+				"prefix/v2":     3,
+			},
+		},
+		{
+			repo:   hgrepo1,
+			rev:    "v2.3.4",
+			subdir: "",
+			files: map[string]uint64{
+				"prefix/.hg_archival.txt": ^uint64(0),
+				"prefix/README":           0,
+				"prefix/v2":               3,
+			},
+		},
+
+		{
+			repo:   gitrepo1,
+			rev:    "v2",
+			subdir: "",
+			files: map[string]uint64{
+				"prefix/":            0,
+				"prefix/README":      0,
+				"prefix/v2":          3,
+				"prefix/another.txt": 8,
+				"prefix/foo.txt":     13,
+			},
+		},
+		{
+			repo:   hgrepo1,
+			rev:    "v2",
+			subdir: "",
+			files: map[string]uint64{
+				"prefix/.hg_archival.txt": ^uint64(0),
+				"prefix/README":           0,
+				"prefix/v2":               3,
+				"prefix/another.txt":      8,
+				"prefix/foo.txt":          13,
+			},
+		},
+
+		{
+			repo:   gitrepo1,
+			rev:    "v3",
+			subdir: "",
+			files: map[string]uint64{
+				"prefix/":                    0,
+				"prefix/v3/":                 0,
+				"prefix/v3/sub/":             0,
+				"prefix/v3/sub/dir/":         0,
+				"prefix/v3/sub/dir/file.txt": 16,
+				"prefix/README":              0,
+			},
+		},
+		{
+			repo:   hgrepo1,
+			rev:    "v3",
+			subdir: "",
+			files: map[string]uint64{
+				"prefix/.hg_archival.txt":    ^uint64(0),
+				"prefix/.hgtags":             405,
+				"prefix/v3/sub/dir/file.txt": 16,
+				"prefix/README":              0,
+			},
+		},
+
+		{
+			repo:   gitrepo1,
+			rev:    "v3",
+			subdir: "v3/sub/dir",
+			files: map[string]uint64{
+				"prefix/":                    0,
+				"prefix/v3/":                 0,
+				"prefix/v3/sub/":             0,
+				"prefix/v3/sub/dir/":         0,
+				"prefix/v3/sub/dir/file.txt": 16,
+			},
+		},
+		{
+			repo:   hgrepo1,
+			rev:    "v3",
+			subdir: "v3/sub/dir",
+			files: map[string]uint64{
+				"prefix/v3/sub/dir/file.txt": 16,
+			},
+		},
+
+		{
+			repo:   gitrepo1,
+			rev:    "v3",
+			subdir: "v3/sub",
+			files: map[string]uint64{
+				"prefix/":                    0,
+				"prefix/v3/":                 0,
+				"prefix/v3/sub/":             0,
+				"prefix/v3/sub/dir/":         0,
+				"prefix/v3/sub/dir/file.txt": 16,
+			},
+		},
+		{
+			repo:   hgrepo1,
+			rev:    "v3",
+			subdir: "v3/sub",
+			files: map[string]uint64{
+				"prefix/v3/sub/dir/file.txt": 16,
+			},
+		},
+
+		{
+			repo:   gitrepo1,
+			rev:    "aaaaaaaaab",
+			subdir: "",
+			err:    "unknown revision",
+		},
+		{
+			repo:   hgrepo1,
+			rev:    "aaaaaaaaab",
+			subdir: "",
+			err:    "unknown revision",
+		},
+
+		{
+			repo:   vgotest1,
+			rev:    "submod/v1.0.4",
+			subdir: "submod",
+			files: map[string]uint64{
+				"prefix/":                0,
+				"prefix/submod/":         0,
+				"prefix/submod/go.mod":   53,
+				"prefix/submod/pkg/":     0,
+				"prefix/submod/pkg/p.go": 31,
+			},
+		},
+	} {
+		t.Run(path.Base(tt.repo)+"/"+tt.rev+"/"+tt.subdir, runTest(tt))
+		if tt.repo == gitrepo1 {
+			tt.repo = "localGitRepo"
+			t.Run(path.Base(tt.repo)+"/"+tt.rev+"/"+tt.subdir, runTest(tt))
+		}
+	}
+}
+
+var hgmap = map[string]string{
+	"HEAD": "41964ddce1180313bdc01d0a39a2813344d6261d", // not tip due to bad hgrepo1 conversion
+	"9d02800338b8a55be062c838d1f02e0c5780b9eb": "8f49ee7a6ddcdec6f0112d9dca48d4a2e4c3c09e",
+	"76a00fb249b7f93091bc2c89a789dab1fc1bc26f": "88fde824ec8b41a76baa16b7e84212cee9f3edd0",
+	"ede458df7cd0fdca520df19a33158086a8a68e81": "41964ddce1180313bdc01d0a39a2813344d6261d",
+	"97f6aa59c81c623494825b43d39e445566e429a4": "c0cbbfb24c7c3c50c35c7b88e7db777da4ff625d",
+}
+
+func TestStat(t *testing.T) {
+	t.Parallel()
+
+	type statTest struct {
+		repo string
+		rev  string
+		err  string
+		info *RevInfo
+	}
+	runTest := func(tt statTest) func(*testing.T) {
+		return func(t *testing.T) {
+			t.Parallel()
+			ctx := testContext(t)
+
+			r, err := testRepo(ctx, t, tt.repo)
+			if err != nil {
+				t.Fatal(err)
+			}
+			info, err := r.Stat(ctx, tt.rev)
+			if err != nil {
+				if tt.err == "" {
+					t.Fatalf("Stat: unexpected error %v", err)
+				}
+				if !strings.Contains(err.Error(), tt.err) {
+					t.Fatalf("Stat: wrong error %q, want %q", err, tt.err)
+				}
+				if info != nil && info.Origin == nil {
+					t.Errorf("Stat: non-nil info with nil Origin with error %q", err)
+				}
+				return
+			}
+			info.Origin = nil // TestLatest and ../../../testdata/script/reuse_git.txt test Origin well enough
+			if !reflect.DeepEqual(info, tt.info) {
+				if !reflect.DeepEqual(info.Tags, tt.info.Tags) {
+					testenv.SkipFlaky(t, 56881)
+				}
+				t.Errorf("Stat: incorrect info\nhave %+v\nwant %+v", *info, *tt.info)
+			}
+		}
+	}
+
+	for _, tt := range []statTest{
+		{
+			repo: gitrepo1,
+			rev:  "HEAD",
+			info: &RevInfo{
+				Name:    "ede458df7cd0fdca520df19a33158086a8a68e81",
+				Short:   "ede458df7cd0",
+				Version: "ede458df7cd0fdca520df19a33158086a8a68e81",
+				Time:    time.Date(2018, 4, 17, 19, 43, 22, 0, time.UTC),
+				Tags:    []string{"v1.2.3", "v1.2.4-annotated"},
+			},
+		},
+		{
+			repo: gitrepo1,
+			rev:  "v2", // branch
+			info: &RevInfo{
+				Name:    "9d02800338b8a55be062c838d1f02e0c5780b9eb",
+				Short:   "9d02800338b8",
+				Version: "9d02800338b8a55be062c838d1f02e0c5780b9eb",
+				Time:    time.Date(2018, 4, 17, 20, 00, 32, 0, time.UTC),
+				Tags:    []string{"v2.0.2"},
+			},
+		},
+		{
+			repo: gitrepo1,
+			rev:  "v2.3.4", // badly-named branch (semver should be a tag)
+			info: &RevInfo{
+				Name:    "76a00fb249b7f93091bc2c89a789dab1fc1bc26f",
+				Short:   "76a00fb249b7",
+				Version: "76a00fb249b7f93091bc2c89a789dab1fc1bc26f",
+				Time:    time.Date(2018, 4, 17, 19, 45, 48, 0, time.UTC),
+				Tags:    []string{"v2.0.1", "v2.3"},
+			},
+		},
+		{
+			repo: gitrepo1,
+			rev:  "v2.3", // badly-named tag (we only respect full semver v2.3.0)
+			info: &RevInfo{
+				Name:    "76a00fb249b7f93091bc2c89a789dab1fc1bc26f",
+				Short:   "76a00fb249b7",
+				Version: "v2.3",
+				Time:    time.Date(2018, 4, 17, 19, 45, 48, 0, time.UTC),
+				Tags:    []string{"v2.0.1", "v2.3"},
+			},
+		},
+		{
+			repo: gitrepo1,
+			rev:  "v1.2.3", // tag
+			info: &RevInfo{
+				Name:    "ede458df7cd0fdca520df19a33158086a8a68e81",
+				Short:   "ede458df7cd0",
+				Version: "v1.2.3",
+				Time:    time.Date(2018, 4, 17, 19, 43, 22, 0, time.UTC),
+				Tags:    []string{"v1.2.3", "v1.2.4-annotated"},
+			},
+		},
+		{
+			repo: gitrepo1,
+			rev:  "ede458df", // hash prefix in refs
+			info: &RevInfo{
+				Name:    "ede458df7cd0fdca520df19a33158086a8a68e81",
+				Short:   "ede458df7cd0",
+				Version: "ede458df7cd0fdca520df19a33158086a8a68e81",
+				Time:    time.Date(2018, 4, 17, 19, 43, 22, 0, time.UTC),
+				Tags:    []string{"v1.2.3", "v1.2.4-annotated"},
+			},
+		},
+		{
+			repo: gitrepo1,
+			rev:  "97f6aa59", // hash prefix not in refs
+			info: &RevInfo{
+				Name:    "97f6aa59c81c623494825b43d39e445566e429a4",
+				Short:   "97f6aa59c81c",
+				Version: "97f6aa59c81c623494825b43d39e445566e429a4",
+				Time:    time.Date(2018, 4, 17, 20, 0, 19, 0, time.UTC),
+			},
+		},
+		{
+			repo: gitrepo1,
+			rev:  "v1.2.4-annotated", // annotated tag uses unwrapped commit hash
+			info: &RevInfo{
+				Name:    "ede458df7cd0fdca520df19a33158086a8a68e81",
+				Short:   "ede458df7cd0",
+				Version: "v1.2.4-annotated",
+				Time:    time.Date(2018, 4, 17, 19, 43, 22, 0, time.UTC),
+				Tags:    []string{"v1.2.3", "v1.2.4-annotated"},
+			},
+		},
+		{
+			repo: gitrepo1,
+			rev:  "aaaaaaaaab",
+			err:  "unknown revision",
+		},
+	} {
+		t.Run(path.Base(tt.repo)+"/"+tt.rev, runTest(tt))
+		if tt.repo == gitrepo1 {
+			for _, tt.repo = range altRepos() {
+				old := tt
+				var m map[string]string
+				if tt.repo == hgrepo1 {
+					m = hgmap
+				}
+				if tt.info != nil {
+					info := *tt.info
+					tt.info = &info
+					tt.info.Name = remap(tt.info.Name, m)
+					tt.info.Version = remap(tt.info.Version, m)
+					tt.info.Short = remap(tt.info.Short, m)
+				}
+				tt.rev = remap(tt.rev, m)
+				t.Run(path.Base(tt.repo)+"/"+tt.rev, runTest(tt))
+				tt = old
+			}
+		}
+	}
+}
+
+func remap(name string, m map[string]string) string {
+	if m[name] != "" {
+		return m[name]
+	}
+	if AllHex(name) {
+		for k, v := range m {
+			if strings.HasPrefix(k, name) {
+				return v[:len(name)]
+			}
+		}
+	}
+	return name
+}
diff --git a/src/cmd/go/internal/modfetch/codehost/shell.go b/src/cmd/go/internal/modfetch/codehost/shell.go
new file mode 100644
index 0000000..eaa0195
--- /dev/null
+++ b/src/cmd/go/internal/modfetch/codehost/shell.go
@@ -0,0 +1,141 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ignore
+
+// Interactive debugging shell for codehost.Repo implementations.
+
+package main
+
+import (
+	"archive/zip"
+	"bufio"
+	"bytes"
+	"flag"
+	"fmt"
+	"io"
+	"log"
+	"os"
+	"strings"
+	"time"
+
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/modfetch/codehost"
+)
+
+func usage() {
+	fmt.Fprintf(os.Stderr, "usage: go run shell.go vcs remote\n")
+	os.Exit(2)
+}
+
+func main() {
+	cfg.GOMODCACHE = "/tmp/vcswork"
+	log.SetFlags(0)
+	log.SetPrefix("shell: ")
+	flag.Usage = usage
+	flag.Parse()
+	if flag.NArg() != 2 {
+		usage()
+	}
+
+	repo, err := codehost.NewRepo(flag.Arg(0), flag.Arg(1))
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	b := bufio.NewReader(os.Stdin)
+	for {
+		fmt.Fprintf(os.Stderr, ">>> ")
+		line, err := b.ReadString('\n')
+		if err != nil {
+			log.Fatal(err)
+		}
+		f := strings.Fields(line)
+		if len(f) == 0 {
+			continue
+		}
+		switch f[0] {
+		default:
+			fmt.Fprintf(os.Stderr, "?unknown command\n")
+			continue
+		case "tags":
+			prefix := ""
+			if len(f) == 2 {
+				prefix = f[1]
+			}
+			if len(f) > 2 {
+				fmt.Fprintf(os.Stderr, "?usage: tags [prefix]\n")
+				continue
+			}
+			tags, err := repo.Tags(prefix)
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "?%s\n", err)
+				continue
+			}
+			for _, tag := range tags {
+				fmt.Printf("%s\n", tag)
+			}
+
+		case "stat":
+			if len(f) != 2 {
+				fmt.Fprintf(os.Stderr, "?usage: stat rev\n")
+				continue
+			}
+			info, err := repo.Stat(f[1])
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "?%s\n", err)
+				continue
+			}
+			fmt.Printf("name=%s short=%s version=%s time=%s\n", info.Name, info.Short, info.Version, info.Time.UTC().Format(time.RFC3339))
+
+		case "read":
+			if len(f) != 3 {
+				fmt.Fprintf(os.Stderr, "?usage: read rev file\n")
+				continue
+			}
+			data, err := repo.ReadFile(f[1], f[2], 10<<20)
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "?%s\n", err)
+				continue
+			}
+			os.Stdout.Write(data)
+
+		case "zip":
+			if len(f) != 4 {
+				fmt.Fprintf(os.Stderr, "?usage: zip rev subdir output\n")
+				continue
+			}
+			subdir := f[2]
+			if subdir == "-" {
+				subdir = ""
+			}
+			rc, err := repo.ReadZip(f[1], subdir, 10<<20)
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "?%s\n", err)
+				continue
+			}
+			data, err := io.ReadAll(rc)
+			rc.Close()
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "?%s\n", err)
+				continue
+			}
+
+			if f[3] != "-" {
+				if err := os.WriteFile(f[3], data, 0666); err != nil {
+					fmt.Fprintf(os.Stderr, "?%s\n", err)
+					continue
+				}
+			}
+			z, err := zip.NewReader(bytes.NewReader(data), int64(len(data)))
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "?%s\n", err)
+				continue
+			}
+			for _, f := range z.File {
+				fmt.Printf("%s %d\n", f.Name, f.UncompressedSize64)
+			}
+		}
+	}
+}
diff --git a/src/cmd/go/internal/modfetch/codehost/svn.go b/src/cmd/go/internal/modfetch/codehost/svn.go
new file mode 100644
index 0000000..9c1c100
--- /dev/null
+++ b/src/cmd/go/internal/modfetch/codehost/svn.go
@@ -0,0 +1,168 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package codehost
+
+import (
+	"archive/zip"
+	"context"
+	"encoding/xml"
+	"fmt"
+	"io"
+	"os"
+	"path"
+	"path/filepath"
+	"strconv"
+	"time"
+
+	"cmd/go/internal/base"
+)
+
+func svnParseStat(rev, out string) (*RevInfo, error) {
+	var log struct {
+		Logentry struct {
+			Revision int64  `xml:"revision,attr"`
+			Date     string `xml:"date"`
+		} `xml:"logentry"`
+	}
+	if err := xml.Unmarshal([]byte(out), &log); err != nil {
+		return nil, vcsErrorf("unexpected response from svn log --xml: %v\n%s", err, out)
+	}
+
+	t, err := time.Parse(time.RFC3339, log.Logentry.Date)
+	if err != nil {
+		return nil, vcsErrorf("unexpected response from svn log --xml: %v\n%s", err, out)
+	}
+
+	info := &RevInfo{
+		Name:    strconv.FormatInt(log.Logentry.Revision, 10),
+		Short:   fmt.Sprintf("%012d", log.Logentry.Revision),
+		Time:    t.UTC(),
+		Version: rev,
+	}
+	return info, nil
+}
+
+func svnReadZip(ctx context.Context, dst io.Writer, workDir, rev, subdir, remote string) (err error) {
+	// The subversion CLI doesn't provide a command to write the repository
+	// directly to an archive, so we need to export it to the local filesystem
+	// instead. Unfortunately, the local filesystem might apply arbitrary
+	// normalization to the filenames, so we need to obtain those directly.
+	//
+	// 'svn export' prints the filenames as they are written, but from reading the
+	// svn source code (as of revision 1868933), those filenames are encoded using
+	// the system locale rather than preserved byte-for-byte from the origin. For
+	// our purposes, that won't do, but we don't want to go mucking around with
+	// the user's locale settings either — that could impact error messages, and
+	// we don't know what locales the user has available or what LC_* variables
+	// their platform supports.
+	//
+	// Instead, we'll do a two-pass export: first we'll run 'svn list' to get the
+	// canonical filenames, then we'll 'svn export' and look for those filenames
+	// in the local filesystem. (If there is an encoding problem at that point, we
+	// would probably reject the resulting module anyway.)
+
+	remotePath := remote
+	if subdir != "" {
+		remotePath += "/" + subdir
+	}
+
+	release, err := base.AcquireNet()
+	if err != nil {
+		return err
+	}
+	out, err := Run(ctx, workDir, []string{
+		"svn", "list",
+		"--non-interactive",
+		"--xml",
+		"--incremental",
+		"--recursive",
+		"--revision", rev,
+		"--", remotePath,
+	})
+	release()
+	if err != nil {
+		return err
+	}
+
+	type listEntry struct {
+		Kind string `xml:"kind,attr"`
+		Name string `xml:"name"`
+		Size int64  `xml:"size"`
+	}
+	var list struct {
+		Entries []listEntry `xml:"entry"`
+	}
+	if err := xml.Unmarshal(out, &list); err != nil {
+		return vcsErrorf("unexpected response from svn list --xml: %v\n%s", err, out)
+	}
+
+	exportDir := filepath.Join(workDir, "export")
+	// Remove any existing contents from a previous (failed) run.
+	if err := os.RemoveAll(exportDir); err != nil {
+		return err
+	}
+	defer os.RemoveAll(exportDir) // best-effort
+
+	release, err = base.AcquireNet()
+	if err != nil {
+		return err
+	}
+	_, err = Run(ctx, workDir, []string{
+		"svn", "export",
+		"--non-interactive",
+		"--quiet",
+
+		// Suppress any platform- or host-dependent transformations.
+		"--native-eol", "LF",
+		"--ignore-externals",
+		"--ignore-keywords",
+
+		"--revision", rev,
+		"--", remotePath,
+		exportDir,
+	})
+	release()
+	if err != nil {
+		return err
+	}
+
+	// Scrape the exported files out of the filesystem and encode them in the zipfile.
+
+	// “All files in the zip file are expected to be
+	// nested in a single top-level directory, whose name is not specified.”
+	// We'll (arbitrarily) choose the base of the remote path.
+	basePath := path.Join(path.Base(remote), subdir)
+
+	zw := zip.NewWriter(dst)
+	for _, e := range list.Entries {
+		if e.Kind != "file" {
+			continue
+		}
+
+		zf, err := zw.Create(path.Join(basePath, e.Name))
+		if err != nil {
+			return err
+		}
+
+		f, err := os.Open(filepath.Join(exportDir, e.Name))
+		if err != nil {
+			if os.IsNotExist(err) {
+				return vcsErrorf("file reported by 'svn list', but not written by 'svn export': %s", e.Name)
+			}
+			return fmt.Errorf("error opening file created by 'svn export': %v", err)
+		}
+
+		n, err := io.Copy(zf, f)
+		f.Close()
+		if err != nil {
+			return err
+		}
+		if n != e.Size {
+			return vcsErrorf("file size differs between 'svn list' and 'svn export': file %s listed as %v bytes, but exported as %v bytes", e.Name, e.Size, n)
+		}
+	}
+
+	return zw.Close()
+}
diff --git a/src/cmd/go/internal/modfetch/codehost/vcs.go b/src/cmd/go/internal/modfetch/codehost/vcs.go
new file mode 100644
index 0000000..5bd1005
--- /dev/null
+++ b/src/cmd/go/internal/modfetch/codehost/vcs.go
@@ -0,0 +1,644 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package codehost
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"internal/lazyregexp"
+	"io"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"sort"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/lockedfile"
+	"cmd/go/internal/par"
+	"cmd/go/internal/str"
+)
+
+// A VCSError indicates an error using a version control system.
+// The implication of a VCSError is that we know definitively where
+// to get the code, but we can't access it due to the error.
+// The caller should report this error instead of continuing to probe
+// other possible module paths.
+//
+// TODO(golang.org/issue/31730): See if we can invert this. (Return a
+// distinguished error for “repo not found” and treat everything else
+// as terminal.)
+type VCSError struct {
+	Err error
+}
+
+func (e *VCSError) Error() string { return e.Err.Error() }
+
+func (e *VCSError) Unwrap() error { return e.Err }
+
+func vcsErrorf(format string, a ...any) error {
+	return &VCSError{Err: fmt.Errorf(format, a...)}
+}
+
+type vcsCacheKey struct {
+	vcs    string
+	remote string
+}
+
+func NewRepo(ctx context.Context, vcs, remote string) (Repo, error) {
+	return vcsRepoCache.Do(vcsCacheKey{vcs, remote}, func() (Repo, error) {
+		repo, err := newVCSRepo(ctx, vcs, remote)
+		if err != nil {
+			return nil, &VCSError{err}
+		}
+		return repo, nil
+	})
+}
+
+var vcsRepoCache par.ErrCache[vcsCacheKey, Repo]
+
+type vcsRepo struct {
+	mu lockedfile.Mutex // protects all commands, so we don't have to decide which are safe on a per-VCS basis
+
+	remote string
+	cmd    *vcsCmd
+	dir    string
+
+	tagsOnce sync.Once
+	tags     map[string]bool
+
+	branchesOnce sync.Once
+	branches     map[string]bool
+
+	fetchOnce sync.Once
+	fetchErr  error
+}
+
+func newVCSRepo(ctx context.Context, vcs, remote string) (Repo, error) {
+	if vcs == "git" {
+		return newGitRepo(ctx, remote, false)
+	}
+	cmd := vcsCmds[vcs]
+	if cmd == nil {
+		return nil, fmt.Errorf("unknown vcs: %s %s", vcs, remote)
+	}
+	if !strings.Contains(remote, "://") {
+		return nil, fmt.Errorf("invalid vcs remote: %s %s", vcs, remote)
+	}
+
+	r := &vcsRepo{remote: remote, cmd: cmd}
+	var err error
+	r.dir, r.mu.Path, err = WorkDir(ctx, vcsWorkDirType+vcs, r.remote)
+	if err != nil {
+		return nil, err
+	}
+
+	if cmd.init == nil {
+		return r, nil
+	}
+
+	unlock, err := r.mu.Lock()
+	if err != nil {
+		return nil, err
+	}
+	defer unlock()
+
+	if _, err := os.Stat(filepath.Join(r.dir, "."+vcs)); err != nil {
+		release, err := base.AcquireNet()
+		if err != nil {
+			return nil, err
+		}
+		_, err = Run(ctx, r.dir, cmd.init(r.remote))
+		release()
+
+		if err != nil {
+			os.RemoveAll(r.dir)
+			return nil, err
+		}
+	}
+	return r, nil
+}
+
+const vcsWorkDirType = "vcs1."
+
+type vcsCmd struct {
+	vcs           string                                                                              // vcs name "hg"
+	init          func(remote string) []string                                                        // cmd to init repo to track remote
+	tags          func(remote string) []string                                                        // cmd to list local tags
+	tagRE         *lazyregexp.Regexp                                                                  // regexp to extract tag names from output of tags cmd
+	branches      func(remote string) []string                                                        // cmd to list local branches
+	branchRE      *lazyregexp.Regexp                                                                  // regexp to extract branch names from output of tags cmd
+	badLocalRevRE *lazyregexp.Regexp                                                                  // regexp of names that must not be served out of local cache without doing fetch first
+	statLocal     func(rev, remote string) []string                                                   // cmd to stat local rev
+	parseStat     func(rev, out string) (*RevInfo, error)                                             // cmd to parse output of statLocal
+	fetch         []string                                                                            // cmd to fetch everything from remote
+	latest        string                                                                              // name of latest commit on remote (tip, HEAD, etc)
+	readFile      func(rev, file, remote string) []string                                             // cmd to read rev's file
+	readZip       func(rev, subdir, remote, target string) []string                                   // cmd to read rev's subdir as zip file
+	doReadZip     func(ctx context.Context, dst io.Writer, workDir, rev, subdir, remote string) error // arbitrary function to read rev's subdir as zip file
+}
+
+var re = lazyregexp.New
+
+var vcsCmds = map[string]*vcsCmd{
+	"hg": {
+		vcs: "hg",
+		init: func(remote string) []string {
+			return []string{"hg", "clone", "-U", "--", remote, "."}
+		},
+		tags: func(remote string) []string {
+			return []string{"hg", "tags", "-q"}
+		},
+		tagRE: re(`(?m)^[^\n]+$`),
+		branches: func(remote string) []string {
+			return []string{"hg", "branches", "-c", "-q"}
+		},
+		branchRE:      re(`(?m)^[^\n]+$`),
+		badLocalRevRE: re(`(?m)^(tip)$`),
+		statLocal: func(rev, remote string) []string {
+			return []string{"hg", "log", "-l1", "-r", rev, "--template", "{node} {date|hgdate} {tags}"}
+		},
+		parseStat: hgParseStat,
+		fetch:     []string{"hg", "pull", "-f"},
+		latest:    "tip",
+		readFile: func(rev, file, remote string) []string {
+			return []string{"hg", "cat", "-r", rev, file}
+		},
+		readZip: func(rev, subdir, remote, target string) []string {
+			pattern := []string{}
+			if subdir != "" {
+				pattern = []string{"-I", subdir + "/**"}
+			}
+			return str.StringList("hg", "archive", "-t", "zip", "--no-decode", "-r", rev, "--prefix=prefix/", pattern, "--", target)
+		},
+	},
+
+	"svn": {
+		vcs:  "svn",
+		init: nil, // no local checkout
+		tags: func(remote string) []string {
+			return []string{"svn", "list", "--", strings.TrimSuffix(remote, "/trunk") + "/tags"}
+		},
+		tagRE: re(`(?m)^(.*?)/?$`),
+		statLocal: func(rev, remote string) []string {
+			suffix := "@" + rev
+			if rev == "latest" {
+				suffix = ""
+			}
+			return []string{"svn", "log", "-l1", "--xml", "--", remote + suffix}
+		},
+		parseStat: svnParseStat,
+		latest:    "latest",
+		readFile: func(rev, file, remote string) []string {
+			return []string{"svn", "cat", "--", remote + "/" + file + "@" + rev}
+		},
+		doReadZip: svnReadZip,
+	},
+
+	"bzr": {
+		vcs: "bzr",
+		init: func(remote string) []string {
+			return []string{"bzr", "branch", "--use-existing-dir", "--", remote, "."}
+		},
+		fetch: []string{
+			"bzr", "pull", "--overwrite-tags",
+		},
+		tags: func(remote string) []string {
+			return []string{"bzr", "tags"}
+		},
+		tagRE:         re(`(?m)^\S+`),
+		badLocalRevRE: re(`^revno:-`),
+		statLocal: func(rev, remote string) []string {
+			return []string{"bzr", "log", "-l1", "--long", "--show-ids", "-r", rev}
+		},
+		parseStat: bzrParseStat,
+		latest:    "revno:-1",
+		readFile: func(rev, file, remote string) []string {
+			return []string{"bzr", "cat", "-r", rev, file}
+		},
+		readZip: func(rev, subdir, remote, target string) []string {
+			extra := []string{}
+			if subdir != "" {
+				extra = []string{"./" + subdir}
+			}
+			return str.StringList("bzr", "export", "--format=zip", "-r", rev, "--root=prefix/", "--", target, extra)
+		},
+	},
+
+	"fossil": {
+		vcs: "fossil",
+		init: func(remote string) []string {
+			return []string{"fossil", "clone", "--", remote, ".fossil"}
+		},
+		fetch: []string{"fossil", "pull", "-R", ".fossil"},
+		tags: func(remote string) []string {
+			return []string{"fossil", "tag", "-R", ".fossil", "list"}
+		},
+		tagRE: re(`XXXTODO`),
+		statLocal: func(rev, remote string) []string {
+			return []string{"fossil", "info", "-R", ".fossil", rev}
+		},
+		parseStat: fossilParseStat,
+		latest:    "trunk",
+		readFile: func(rev, file, remote string) []string {
+			return []string{"fossil", "cat", "-R", ".fossil", "-r", rev, file}
+		},
+		readZip: func(rev, subdir, remote, target string) []string {
+			extra := []string{}
+			if subdir != "" && !strings.ContainsAny(subdir, "*?[],") {
+				extra = []string{"--include", subdir}
+			}
+			// Note that vcsRepo.ReadZip below rewrites this command
+			// to run in a different directory, to work around a fossil bug.
+			return str.StringList("fossil", "zip", "-R", ".fossil", "--name", "prefix", extra, "--", rev, target)
+		},
+	},
+}
+
+func (r *vcsRepo) loadTags(ctx context.Context) {
+	out, err := Run(ctx, r.dir, r.cmd.tags(r.remote))
+	if err != nil {
+		return
+	}
+
+	// Run tag-listing command and extract tags.
+	r.tags = make(map[string]bool)
+	for _, tag := range r.cmd.tagRE.FindAllString(string(out), -1) {
+		if r.cmd.badLocalRevRE != nil && r.cmd.badLocalRevRE.MatchString(tag) {
+			continue
+		}
+		r.tags[tag] = true
+	}
+}
+
+func (r *vcsRepo) loadBranches(ctx context.Context) {
+	if r.cmd.branches == nil {
+		return
+	}
+
+	out, err := Run(ctx, r.dir, r.cmd.branches(r.remote))
+	if err != nil {
+		return
+	}
+
+	r.branches = make(map[string]bool)
+	for _, branch := range r.cmd.branchRE.FindAllString(string(out), -1) {
+		if r.cmd.badLocalRevRE != nil && r.cmd.badLocalRevRE.MatchString(branch) {
+			continue
+		}
+		r.branches[branch] = true
+	}
+}
+
+func (r *vcsRepo) CheckReuse(ctx context.Context, old *Origin, subdir string) error {
+	return fmt.Errorf("vcs %s: CheckReuse: %w", r.cmd.vcs, errors.ErrUnsupported)
+}
+
+func (r *vcsRepo) Tags(ctx context.Context, prefix string) (*Tags, error) {
+	unlock, err := r.mu.Lock()
+	if err != nil {
+		return nil, err
+	}
+	defer unlock()
+
+	r.tagsOnce.Do(func() { r.loadTags(ctx) })
+	tags := &Tags{
+		// None of the other VCS provide a reasonable way to compute TagSum
+		// without downloading the whole repo, so we only include VCS and URL
+		// in the Origin.
+		Origin: &Origin{
+			VCS: r.cmd.vcs,
+			URL: r.remote,
+		},
+		List: []Tag{},
+	}
+	for tag := range r.tags {
+		if strings.HasPrefix(tag, prefix) {
+			tags.List = append(tags.List, Tag{tag, ""})
+		}
+	}
+	sort.Slice(tags.List, func(i, j int) bool {
+		return tags.List[i].Name < tags.List[j].Name
+	})
+	return tags, nil
+}
+
+func (r *vcsRepo) Stat(ctx context.Context, rev string) (*RevInfo, error) {
+	unlock, err := r.mu.Lock()
+	if err != nil {
+		return nil, err
+	}
+	defer unlock()
+
+	if rev == "latest" {
+		rev = r.cmd.latest
+	}
+	r.branchesOnce.Do(func() { r.loadBranches(ctx) })
+	revOK := (r.cmd.badLocalRevRE == nil || !r.cmd.badLocalRevRE.MatchString(rev)) && !r.branches[rev]
+	if revOK {
+		if info, err := r.statLocal(ctx, rev); err == nil {
+			return info, nil
+		}
+	}
+
+	r.fetchOnce.Do(func() { r.fetch(ctx) })
+	if r.fetchErr != nil {
+		return nil, r.fetchErr
+	}
+	info, err := r.statLocal(ctx, rev)
+	if err != nil {
+		return nil, err
+	}
+	if !revOK {
+		info.Version = info.Name
+	}
+	return info, nil
+}
+
+func (r *vcsRepo) fetch(ctx context.Context) {
+	if len(r.cmd.fetch) > 0 {
+		release, err := base.AcquireNet()
+		if err != nil {
+			r.fetchErr = err
+			return
+		}
+		_, r.fetchErr = Run(ctx, r.dir, r.cmd.fetch)
+		release()
+	}
+}
+
+func (r *vcsRepo) statLocal(ctx context.Context, rev string) (*RevInfo, error) {
+	out, err := Run(ctx, r.dir, r.cmd.statLocal(rev, r.remote))
+	if err != nil {
+		return nil, &UnknownRevisionError{Rev: rev}
+	}
+	info, err := r.cmd.parseStat(rev, string(out))
+	if err != nil {
+		return nil, err
+	}
+	if info.Origin == nil {
+		info.Origin = new(Origin)
+	}
+	info.Origin.VCS = r.cmd.vcs
+	info.Origin.URL = r.remote
+	return info, nil
+}
+
+func (r *vcsRepo) Latest(ctx context.Context) (*RevInfo, error) {
+	return r.Stat(ctx, "latest")
+}
+
+func (r *vcsRepo) ReadFile(ctx context.Context, rev, file string, maxSize int64) ([]byte, error) {
+	if rev == "latest" {
+		rev = r.cmd.latest
+	}
+	_, err := r.Stat(ctx, rev) // download rev into local repo
+	if err != nil {
+		return nil, err
+	}
+
+	// r.Stat acquires r.mu, so lock after that.
+	unlock, err := r.mu.Lock()
+	if err != nil {
+		return nil, err
+	}
+	defer unlock()
+
+	out, err := Run(ctx, r.dir, r.cmd.readFile(rev, file, r.remote))
+	if err != nil {
+		return nil, fs.ErrNotExist
+	}
+	return out, nil
+}
+
+func (r *vcsRepo) RecentTag(ctx context.Context, rev, prefix string, allowed func(string) bool) (tag string, err error) {
+	// We don't technically need to lock here since we're returning an error
+	// uncondititonally, but doing so anyway will help to avoid baking in
+	// lock-inversion bugs.
+	unlock, err := r.mu.Lock()
+	if err != nil {
+		return "", err
+	}
+	defer unlock()
+
+	return "", vcsErrorf("vcs %s: RecentTag: %w", r.cmd.vcs, errors.ErrUnsupported)
+}
+
+func (r *vcsRepo) DescendsFrom(ctx context.Context, rev, tag string) (bool, error) {
+	unlock, err := r.mu.Lock()
+	if err != nil {
+		return false, err
+	}
+	defer unlock()
+
+	return false, vcsErrorf("vcs %s: DescendsFrom: %w", r.cmd.vcs, errors.ErrUnsupported)
+}
+
+func (r *vcsRepo) ReadZip(ctx context.Context, rev, subdir string, maxSize int64) (zip io.ReadCloser, err error) {
+	if r.cmd.readZip == nil && r.cmd.doReadZip == nil {
+		return nil, vcsErrorf("vcs %s: ReadZip: %w", r.cmd.vcs, errors.ErrUnsupported)
+	}
+
+	unlock, err := r.mu.Lock()
+	if err != nil {
+		return nil, err
+	}
+	defer unlock()
+
+	if rev == "latest" {
+		rev = r.cmd.latest
+	}
+	f, err := os.CreateTemp("", "go-readzip-*.zip")
+	if err != nil {
+		return nil, err
+	}
+	if r.cmd.doReadZip != nil {
+		lw := &limitedWriter{
+			W:               f,
+			N:               maxSize,
+			ErrLimitReached: errors.New("ReadZip: encoded file exceeds allowed size"),
+		}
+		err = r.cmd.doReadZip(ctx, lw, r.dir, rev, subdir, r.remote)
+		if err == nil {
+			_, err = f.Seek(0, io.SeekStart)
+		}
+	} else if r.cmd.vcs == "fossil" {
+		// If you run
+		//	fossil zip -R .fossil --name prefix trunk /tmp/x.zip
+		// fossil fails with "unable to create directory /tmp" [sic].
+		// Change the command to run in /tmp instead,
+		// replacing the -R argument with an absolute path.
+		args := r.cmd.readZip(rev, subdir, r.remote, filepath.Base(f.Name()))
+		for i := range args {
+			if args[i] == ".fossil" {
+				args[i] = filepath.Join(r.dir, ".fossil")
+			}
+		}
+		_, err = Run(ctx, filepath.Dir(f.Name()), args)
+	} else {
+		_, err = Run(ctx, r.dir, r.cmd.readZip(rev, subdir, r.remote, f.Name()))
+	}
+	if err != nil {
+		f.Close()
+		os.Remove(f.Name())
+		return nil, err
+	}
+	return &deleteCloser{f}, nil
+}
+
+// deleteCloser is a file that gets deleted on Close.
+type deleteCloser struct {
+	*os.File
+}
+
+func (d *deleteCloser) Close() error {
+	defer os.Remove(d.File.Name())
+	return d.File.Close()
+}
+
+func hgParseStat(rev, out string) (*RevInfo, error) {
+	f := strings.Fields(out)
+	if len(f) < 3 {
+		return nil, vcsErrorf("unexpected response from hg log: %q", out)
+	}
+	hash := f[0]
+	version := rev
+	if strings.HasPrefix(hash, version) {
+		version = hash // extend to full hash
+	}
+	t, err := strconv.ParseInt(f[1], 10, 64)
+	if err != nil {
+		return nil, vcsErrorf("invalid time from hg log: %q", out)
+	}
+
+	var tags []string
+	for _, tag := range f[3:] {
+		if tag != "tip" {
+			tags = append(tags, tag)
+		}
+	}
+	sort.Strings(tags)
+
+	info := &RevInfo{
+		Origin: &Origin{
+			Hash: hash,
+		},
+		Name:    hash,
+		Short:   ShortenSHA1(hash),
+		Time:    time.Unix(t, 0).UTC(),
+		Version: version,
+		Tags:    tags,
+	}
+	return info, nil
+}
+
+func bzrParseStat(rev, out string) (*RevInfo, error) {
+	var revno int64
+	var tm time.Time
+	for _, line := range strings.Split(out, "\n") {
+		if line == "" || line[0] == ' ' || line[0] == '\t' {
+			// End of header, start of commit message.
+			break
+		}
+		if line[0] == '-' {
+			continue
+		}
+		before, after, found := strings.Cut(line, ":")
+		if !found {
+			// End of header, start of commit message.
+			break
+		}
+		key, val := before, strings.TrimSpace(after)
+		switch key {
+		case "revno":
+			if j := strings.Index(val, " "); j >= 0 {
+				val = val[:j]
+			}
+			i, err := strconv.ParseInt(val, 10, 64)
+			if err != nil {
+				return nil, vcsErrorf("unexpected revno from bzr log: %q", line)
+			}
+			revno = i
+		case "timestamp":
+			j := strings.Index(val, " ")
+			if j < 0 {
+				return nil, vcsErrorf("unexpected timestamp from bzr log: %q", line)
+			}
+			t, err := time.Parse("2006-01-02 15:04:05 -0700", val[j+1:])
+			if err != nil {
+				return nil, vcsErrorf("unexpected timestamp from bzr log: %q", line)
+			}
+			tm = t.UTC()
+		}
+	}
+	if revno == 0 || tm.IsZero() {
+		return nil, vcsErrorf("unexpected response from bzr log: %q", out)
+	}
+
+	info := &RevInfo{
+		Name:    strconv.FormatInt(revno, 10),
+		Short:   fmt.Sprintf("%012d", revno),
+		Time:    tm,
+		Version: rev,
+	}
+	return info, nil
+}
+
+func fossilParseStat(rev, out string) (*RevInfo, error) {
+	for _, line := range strings.Split(out, "\n") {
+		if strings.HasPrefix(line, "uuid:") || strings.HasPrefix(line, "hash:") {
+			f := strings.Fields(line)
+			if len(f) != 5 || len(f[1]) != 40 || f[4] != "UTC" {
+				return nil, vcsErrorf("unexpected response from fossil info: %q", line)
+			}
+			t, err := time.Parse(time.DateTime, f[2]+" "+f[3])
+			if err != nil {
+				return nil, vcsErrorf("unexpected response from fossil info: %q", line)
+			}
+			hash := f[1]
+			version := rev
+			if strings.HasPrefix(hash, version) {
+				version = hash // extend to full hash
+			}
+			info := &RevInfo{
+				Origin: &Origin{
+					Hash: hash,
+				},
+				Name:    hash,
+				Short:   ShortenSHA1(hash),
+				Time:    t,
+				Version: version,
+			}
+			return info, nil
+		}
+	}
+	return nil, vcsErrorf("unexpected response from fossil info: %q", out)
+}
+
+type limitedWriter struct {
+	W               io.Writer
+	N               int64
+	ErrLimitReached error
+}
+
+func (l *limitedWriter) Write(p []byte) (n int, err error) {
+	if l.N > 0 {
+		max := len(p)
+		if l.N < int64(max) {
+			max = int(l.N)
+		}
+		n, err = l.W.Write(p[:max])
+		l.N -= int64(n)
+		if err != nil || n >= len(p) {
+			return n, err
+		}
+	}
+
+	return n, l.ErrLimitReached
+}
diff --git a/src/cmd/go/internal/modfetch/coderepo.go b/src/cmd/go/internal/modfetch/coderepo.go
new file mode 100644
index 0000000..8fe432a
--- /dev/null
+++ b/src/cmd/go/internal/modfetch/coderepo.go
@@ -0,0 +1,1189 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modfetch
+
+import (
+	"archive/zip"
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"sort"
+	"strings"
+	"time"
+
+	"cmd/go/internal/gover"
+	"cmd/go/internal/modfetch/codehost"
+
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
+	modzip "golang.org/x/mod/zip"
+)
+
+// A codeRepo implements modfetch.Repo using an underlying codehost.Repo.
+type codeRepo struct {
+	modPath string
+
+	// code is the repository containing this module.
+	code codehost.Repo
+	// codeRoot is the import path at the root of code.
+	codeRoot string
+	// codeDir is the directory (relative to root) at which we expect to find the module.
+	// If pathMajor is non-empty and codeRoot is not the full modPath,
+	// then we look in both codeDir and codeDir/pathMajor[1:].
+	codeDir string
+
+	// pathMajor is the suffix of modPath that indicates its major version,
+	// or the empty string if modPath is at major version 0 or 1.
+	//
+	// pathMajor is typically of the form "/vN", but possibly ".vN", or
+	// ".vN-unstable" for modules resolved using gopkg.in.
+	pathMajor string
+	// pathPrefix is the prefix of modPath that excludes pathMajor.
+	// It is used only for logging.
+	pathPrefix string
+
+	// pseudoMajor is the major version prefix to require when generating
+	// pseudo-versions for this module, derived from the module path. pseudoMajor
+	// is empty if the module path does not include a version suffix (that is,
+	// accepts either v0 or v1).
+	pseudoMajor string
+}
+
+// newCodeRepo returns a Repo that reads the source code for the module with the
+// given path, from the repo stored in code, with the root of the repo
+// containing the path given by codeRoot.
+func newCodeRepo(code codehost.Repo, codeRoot, path string) (Repo, error) {
+	if !hasPathPrefix(path, codeRoot) {
+		return nil, fmt.Errorf("mismatched repo: found %s for %s", codeRoot, path)
+	}
+	pathPrefix, pathMajor, ok := module.SplitPathVersion(path)
+	if !ok {
+		return nil, fmt.Errorf("invalid module path %q", path)
+	}
+	if codeRoot == path {
+		pathPrefix = path
+	}
+	pseudoMajor := module.PathMajorPrefix(pathMajor)
+
+	// Compute codeDir = bar, the subdirectory within the repo
+	// corresponding to the module root.
+	//
+	// At this point we might have:
+	//	path = github.com/rsc/foo/bar/v2
+	//	codeRoot = github.com/rsc/foo
+	//	pathPrefix = github.com/rsc/foo/bar
+	//	pathMajor = /v2
+	//	pseudoMajor = v2
+	//
+	// which gives
+	//	codeDir = bar
+	//
+	// We know that pathPrefix is a prefix of path, and codeRoot is a prefix of
+	// path, but codeRoot may or may not be a prefix of pathPrefix, because
+	// codeRoot may be the entire path (in which case codeDir should be empty).
+	// That occurs in two situations.
+	//
+	// One is when a go-import meta tag resolves the complete module path,
+	// including the pathMajor suffix:
+	//	path = nanomsg.org/go/mangos/v2
+	//	codeRoot = nanomsg.org/go/mangos/v2
+	//	pathPrefix = nanomsg.org/go/mangos
+	//	pathMajor = /v2
+	//	pseudoMajor = v2
+	//
+	// The other is similar: for gopkg.in only, the major version is encoded
+	// with a dot rather than a slash, and thus can't be in a subdirectory.
+	//	path = gopkg.in/yaml.v2
+	//	codeRoot = gopkg.in/yaml.v2
+	//	pathPrefix = gopkg.in/yaml
+	//	pathMajor = .v2
+	//	pseudoMajor = v2
+	//
+	codeDir := ""
+	if codeRoot != path {
+		if !hasPathPrefix(pathPrefix, codeRoot) {
+			return nil, fmt.Errorf("repository rooted at %s cannot contain module %s", codeRoot, path)
+		}
+		codeDir = strings.Trim(pathPrefix[len(codeRoot):], "/")
+	}
+
+	r := &codeRepo{
+		modPath:     path,
+		code:        code,
+		codeRoot:    codeRoot,
+		codeDir:     codeDir,
+		pathPrefix:  pathPrefix,
+		pathMajor:   pathMajor,
+		pseudoMajor: pseudoMajor,
+	}
+
+	return r, nil
+}
+
+func (r *codeRepo) ModulePath() string {
+	return r.modPath
+}
+
+func (r *codeRepo) CheckReuse(ctx context.Context, old *codehost.Origin) error {
+	return r.code.CheckReuse(ctx, old, r.codeDir)
+}
+
+func (r *codeRepo) Versions(ctx context.Context, prefix string) (*Versions, error) {
+	// Special case: gopkg.in/macaroon-bakery.v2-unstable
+	// does not use the v2 tags (those are for macaroon-bakery.v2).
+	// It has no possible tags at all.
+	if strings.HasPrefix(r.modPath, "gopkg.in/") && strings.HasSuffix(r.modPath, "-unstable") {
+		return &Versions{}, nil
+	}
+
+	p := prefix
+	if r.codeDir != "" {
+		p = r.codeDir + "/" + p
+	}
+	tags, err := r.code.Tags(ctx, p)
+	if err != nil {
+		return nil, &module.ModuleError{
+			Path: r.modPath,
+			Err:  err,
+		}
+	}
+	if tags.Origin != nil {
+		tags.Origin.Subdir = r.codeDir
+	}
+
+	var list, incompatible []string
+	for _, tag := range tags.List {
+		if !strings.HasPrefix(tag.Name, p) {
+			continue
+		}
+		v := tag.Name
+		if r.codeDir != "" {
+			v = v[len(r.codeDir)+1:]
+		}
+		// Note: ./codehost/codehost.go's isOriginTag knows about these conditions too.
+		// If these are relaxed, isOriginTag will need to be relaxed as well.
+		if v == "" || v != semver.Canonical(v) {
+			// Ignore non-canonical tags: Stat rewrites those to canonical
+			// pseudo-versions. Note that we compare against semver.Canonical here
+			// instead of module.CanonicalVersion: revToRev strips "+incompatible"
+			// suffixes before looking up tags, so a tag like "v2.0.0+incompatible"
+			// would not resolve at all. (The Go version string "v2.0.0+incompatible"
+			// refers to the "v2.0.0" version tag, which we handle below.)
+			continue
+		}
+		if module.IsPseudoVersion(v) {
+			// Ignore tags that look like pseudo-versions: Stat rewrites those
+			// unambiguously to the underlying commit, and tagToVersion drops them.
+			continue
+		}
+
+		if err := module.CheckPathMajor(v, r.pathMajor); err != nil {
+			if r.codeDir == "" && r.pathMajor == "" && semver.Major(v) > "v1" {
+				incompatible = append(incompatible, v)
+			}
+			continue
+		}
+
+		list = append(list, v)
+	}
+	semver.Sort(list)
+	semver.Sort(incompatible)
+
+	return r.appendIncompatibleVersions(ctx, tags.Origin, list, incompatible)
+}
+
+// appendIncompatibleVersions appends "+incompatible" versions to list if
+// appropriate, returning the final list.
+//
+// The incompatible list contains candidate versions without the '+incompatible'
+// prefix.
+//
+// Both list and incompatible must be sorted in semantic order.
+func (r *codeRepo) appendIncompatibleVersions(ctx context.Context, origin *codehost.Origin, list, incompatible []string) (*Versions, error) {
+	versions := &Versions{
+		Origin: origin,
+		List:   list,
+	}
+	if len(incompatible) == 0 || r.pathMajor != "" {
+		// No +incompatible versions are possible, so no need to check them.
+		return versions, nil
+	}
+
+	versionHasGoMod := func(v string) (bool, error) {
+		_, err := r.code.ReadFile(ctx, v, "go.mod", codehost.MaxGoMod)
+		if err == nil {
+			return true, nil
+		}
+		if !os.IsNotExist(err) {
+			return false, &module.ModuleError{
+				Path: r.modPath,
+				Err:  err,
+			}
+		}
+		return false, nil
+	}
+
+	if len(list) > 0 {
+		ok, err := versionHasGoMod(list[len(list)-1])
+		if err != nil {
+			return nil, err
+		}
+		if ok {
+			// The latest compatible version has a go.mod file, so assume that all
+			// subsequent versions do as well, and do not include any +incompatible
+			// versions. Even if we are wrong, the author clearly intends module
+			// consumers to be on the v0/v1 line instead of a higher +incompatible
+			// version. (See https://golang.org/issue/34189.)
+			//
+			// We know of at least two examples where this behavior is desired
+			// (github.com/russross/blackfriday@v2.0.0 and
+			// github.com/libp2p/go-libp2p@v6.0.23), and (as of 2019-10-29) have no
+			// concrete examples for which it is undesired.
+			return versions, nil
+		}
+	}
+
+	var (
+		lastMajor         string
+		lastMajorHasGoMod bool
+	)
+	for i, v := range incompatible {
+		major := semver.Major(v)
+
+		if major != lastMajor {
+			rem := incompatible[i:]
+			j := sort.Search(len(rem), func(j int) bool {
+				return semver.Major(rem[j]) != major
+			})
+			latestAtMajor := rem[j-1]
+
+			var err error
+			lastMajor = major
+			lastMajorHasGoMod, err = versionHasGoMod(latestAtMajor)
+			if err != nil {
+				return nil, err
+			}
+		}
+
+		if lastMajorHasGoMod {
+			// The latest release of this major version has a go.mod file, so it is
+			// not allowed as +incompatible. It would be confusing to include some
+			// minor versions of this major version as +incompatible but require
+			// semantic import versioning for others, so drop all +incompatible
+			// versions for this major version.
+			//
+			// If we're wrong about a minor version in the middle, users will still be
+			// able to 'go get' specific tags for that version explicitly — they just
+			// won't appear in 'go list' or as the results for queries with inequality
+			// bounds.
+			continue
+		}
+		versions.List = append(versions.List, v+"+incompatible")
+	}
+
+	return versions, nil
+}
+
+func (r *codeRepo) Stat(ctx context.Context, rev string) (*RevInfo, error) {
+	if rev == "latest" {
+		return r.Latest(ctx)
+	}
+	codeRev := r.revToRev(rev)
+	info, err := r.code.Stat(ctx, codeRev)
+	if err != nil {
+		// Note: info may be non-nil to supply Origin for caching error.
+		var revInfo *RevInfo
+		if info != nil {
+			revInfo = &RevInfo{
+				Origin:  info.Origin,
+				Version: rev,
+			}
+		}
+		return revInfo, &module.ModuleError{
+			Path: r.modPath,
+			Err: &module.InvalidVersionError{
+				Version: rev,
+				Err:     err,
+			},
+		}
+	}
+	return r.convert(ctx, info, rev)
+}
+
+func (r *codeRepo) Latest(ctx context.Context) (*RevInfo, error) {
+	info, err := r.code.Latest(ctx)
+	if err != nil {
+		return nil, err
+	}
+	return r.convert(ctx, info, "")
+}
+
+// convert converts a version as reported by the code host to a version as
+// interpreted by the module system.
+//
+// If statVers is a valid module version, it is used for the Version field.
+// Otherwise, the Version is derived from the passed-in info and recent tags.
+func (r *codeRepo) convert(ctx context.Context, info *codehost.RevInfo, statVers string) (*RevInfo, error) {
+	// If this is a plain tag (no dir/ prefix)
+	// and the module path is unversioned,
+	// and if the underlying file tree has no go.mod,
+	// then allow using the tag with a +incompatible suffix.
+	//
+	// (If the version is +incompatible, then the go.mod file must not exist:
+	// +incompatible is not an ongoing opt-out from semantic import versioning.)
+	incompatibleOk := map[string]bool{}
+	canUseIncompatible := func(v string) bool {
+		if r.codeDir != "" || r.pathMajor != "" {
+			// A non-empty codeDir indicates a module within a subdirectory,
+			// which necessarily has a go.mod file indicating the module boundary.
+			// A non-empty pathMajor indicates a module path with a major-version
+			// suffix, which must match.
+			return false
+		}
+
+		ok, seen := incompatibleOk[""]
+		if !seen {
+			_, errGoMod := r.code.ReadFile(ctx, info.Name, "go.mod", codehost.MaxGoMod)
+			ok = (errGoMod != nil)
+			incompatibleOk[""] = ok
+		}
+		if !ok {
+			// A go.mod file exists at the repo root.
+			return false
+		}
+
+		// Per https://go.dev/issue/51324, previous versions of the 'go' command
+		// didn't always check for go.mod files in subdirectories, so if the user
+		// requests a +incompatible version explicitly, we should continue to allow
+		// it. Otherwise, if vN/go.mod exists, expect that release tags for that
+		// major version are intended for the vN module.
+		if v != "" && !strings.HasSuffix(statVers, "+incompatible") {
+			major := semver.Major(v)
+			ok, seen = incompatibleOk[major]
+			if !seen {
+				_, errGoModSub := r.code.ReadFile(ctx, info.Name, path.Join(major, "go.mod"), codehost.MaxGoMod)
+				ok = (errGoModSub != nil)
+				incompatibleOk[major] = ok
+			}
+			if !ok {
+				return false
+			}
+		}
+
+		return true
+	}
+
+	// checkCanonical verifies that the canonical version v is compatible with the
+	// module path represented by r, adding a "+incompatible" suffix if needed.
+	//
+	// If statVers is also canonical, checkCanonical also verifies that v is
+	// either statVers or statVers with the added "+incompatible" suffix.
+	checkCanonical := func(v string) (*RevInfo, error) {
+		// If r.codeDir is non-empty, then the go.mod file must exist: the module
+		// author — not the module consumer, — gets to decide how to carve up the repo
+		// into modules.
+		//
+		// Conversely, if the go.mod file exists, the module author — not the module
+		// consumer — gets to determine the module's path
+		//
+		// r.findDir verifies both of these conditions. Execute it now so that
+		// r.Stat will correctly return a notExistError if the go.mod location or
+		// declared module path doesn't match.
+		_, _, _, err := r.findDir(ctx, v)
+		if err != nil {
+			// TODO: It would be nice to return an error like "not a module".
+			// Right now we return "missing go.mod", which is a little confusing.
+			return nil, &module.ModuleError{
+				Path: r.modPath,
+				Err: &module.InvalidVersionError{
+					Version: v,
+					Err:     notExistError{err: err},
+				},
+			}
+		}
+
+		invalidf := func(format string, args ...any) error {
+			return &module.ModuleError{
+				Path: r.modPath,
+				Err: &module.InvalidVersionError{
+					Version: v,
+					Err:     fmt.Errorf(format, args...),
+				},
+			}
+		}
+
+		// Add the +incompatible suffix if needed or requested explicitly, and
+		// verify that its presence or absence is appropriate for this version
+		// (which depends on whether it has an explicit go.mod file).
+
+		if v == strings.TrimSuffix(statVers, "+incompatible") {
+			v = statVers
+		}
+		base := strings.TrimSuffix(v, "+incompatible")
+		var errIncompatible error
+		if !module.MatchPathMajor(base, r.pathMajor) {
+			if canUseIncompatible(base) {
+				v = base + "+incompatible"
+			} else {
+				if r.pathMajor != "" {
+					errIncompatible = invalidf("module path includes a major version suffix, so major version must match")
+				} else {
+					errIncompatible = invalidf("module contains a go.mod file, so module path must match major version (%q)", path.Join(r.pathPrefix, semver.Major(v)))
+				}
+			}
+		} else if strings.HasSuffix(v, "+incompatible") {
+			errIncompatible = invalidf("+incompatible suffix not allowed: major version %s is compatible", semver.Major(v))
+		}
+
+		if statVers != "" && statVers == module.CanonicalVersion(statVers) {
+			// Since the caller-requested version is canonical, it would be very
+			// confusing to resolve it to anything but itself, possibly with a
+			// "+incompatible" suffix. Error out explicitly.
+			if statBase := strings.TrimSuffix(statVers, "+incompatible"); statBase != base {
+				return nil, &module.ModuleError{
+					Path: r.modPath,
+					Err: &module.InvalidVersionError{
+						Version: statVers,
+						Err:     fmt.Errorf("resolves to version %v (%s is not a tag)", v, statBase),
+					},
+				}
+			}
+		}
+
+		if errIncompatible != nil {
+			return nil, errIncompatible
+		}
+
+		origin := info.Origin
+		if origin != nil {
+			o := *origin
+			origin = &o
+			origin.Subdir = r.codeDir
+			if module.IsPseudoVersion(v) && (v != statVers || !strings.HasPrefix(v, "v0.0.0-")) {
+				// Add tags that are relevant to pseudo-version calculation to origin.
+				prefix := r.codeDir
+				if prefix != "" {
+					prefix += "/"
+				}
+				if r.pathMajor != "" { // "/v2" or "/.v2"
+					prefix += r.pathMajor[1:] + "." // += "v2."
+				}
+				tags, err := r.code.Tags(ctx, prefix)
+				if err != nil {
+					return nil, err
+				}
+				origin.TagPrefix = tags.Origin.TagPrefix
+				origin.TagSum = tags.Origin.TagSum
+			}
+		}
+
+		return &RevInfo{
+			Origin:  origin,
+			Name:    info.Name,
+			Short:   info.Short,
+			Time:    info.Time,
+			Version: v,
+		}, nil
+	}
+
+	// Determine version.
+
+	if module.IsPseudoVersion(statVers) {
+		if err := r.validatePseudoVersion(ctx, info, statVers); err != nil {
+			return nil, err
+		}
+		return checkCanonical(statVers)
+	}
+
+	// statVers is not a pseudo-version, so we need to either resolve it to a
+	// canonical version or verify that it is already a canonical tag
+	// (not a branch).
+
+	// Derive or verify a version from a code repo tag.
+	// Tag must have a prefix matching codeDir.
+	tagPrefix := ""
+	if r.codeDir != "" {
+		tagPrefix = r.codeDir + "/"
+	}
+
+	isRetracted, err := r.retractedVersions(ctx)
+	if err != nil {
+		isRetracted = func(string) bool { return false }
+	}
+
+	// tagToVersion returns the version obtained by trimming tagPrefix from tag.
+	// If the tag is invalid, retracted, or a pseudo-version, tagToVersion returns
+	// an empty version.
+	tagToVersion := func(tag string) (v string, tagIsCanonical bool) {
+		if !strings.HasPrefix(tag, tagPrefix) {
+			return "", false
+		}
+		trimmed := tag[len(tagPrefix):]
+		// Tags that look like pseudo-versions would be confusing. Ignore them.
+		if module.IsPseudoVersion(tag) {
+			return "", false
+		}
+
+		v = semver.Canonical(trimmed) // Not module.Canonical: we don't want to pick up an explicit "+incompatible" suffix from the tag.
+		if v == "" || !strings.HasPrefix(trimmed, v) {
+			return "", false // Invalid or incomplete version (just vX or vX.Y).
+		}
+		if v == trimmed {
+			tagIsCanonical = true
+		}
+		return v, tagIsCanonical
+	}
+
+	// If the VCS gave us a valid version, use that.
+	if v, tagIsCanonical := tagToVersion(info.Version); tagIsCanonical {
+		if info, err := checkCanonical(v); err == nil {
+			return info, err
+		}
+	}
+
+	// Look through the tags on the revision for either a usable canonical version
+	// or an appropriate base for a pseudo-version.
+	var (
+		highestCanonical string
+		pseudoBase       string
+	)
+	for _, pathTag := range info.Tags {
+		v, tagIsCanonical := tagToVersion(pathTag)
+		if statVers != "" && semver.Compare(v, statVers) == 0 {
+			// The tag is equivalent to the version requested by the user.
+			if tagIsCanonical {
+				// This tag is the canonical form of the requested version,
+				// not some other form with extra build metadata.
+				// Use this tag so that the resolved version will match exactly.
+				// (If it isn't actually allowed, we'll error out in checkCanonical.)
+				return checkCanonical(v)
+			} else {
+				// The user explicitly requested something equivalent to this tag. We
+				// can't use the version from the tag directly: since the tag is not
+				// canonical, it could be ambiguous. For example, tags v0.0.1+a and
+				// v0.0.1+b might both exist and refer to different revisions.
+				//
+				// The tag is otherwise valid for the module, so we can at least use it as
+				// the base of an unambiguous pseudo-version.
+				//
+				// If multiple tags match, tagToVersion will canonicalize them to the same
+				// base version.
+				pseudoBase = v
+			}
+		}
+		// Save the highest non-retracted canonical tag for the revision.
+		// If we don't find a better match, we'll use it as the canonical version.
+		if tagIsCanonical && semver.Compare(highestCanonical, v) < 0 && !isRetracted(v) {
+			if module.MatchPathMajor(v, r.pathMajor) || canUseIncompatible(v) {
+				highestCanonical = v
+			}
+		}
+	}
+
+	// If we found a valid canonical tag for the revision, return it.
+	// Even if we found a good pseudo-version base, a canonical version is better.
+	if highestCanonical != "" {
+		return checkCanonical(highestCanonical)
+	}
+
+	// Find the highest tagged version in the revision's history, subject to
+	// major version and +incompatible constraints. Use that version as the
+	// pseudo-version base so that the pseudo-version sorts higher. Ignore
+	// retracted versions.
+	tagAllowed := func(tag string) bool {
+		v, _ := tagToVersion(tag)
+		if v == "" {
+			return false
+		}
+		if !module.MatchPathMajor(v, r.pathMajor) && !canUseIncompatible(v) {
+			return false
+		}
+		return !isRetracted(v)
+	}
+	if pseudoBase == "" {
+		tag, err := r.code.RecentTag(ctx, info.Name, tagPrefix, tagAllowed)
+		if err != nil && !errors.Is(err, errors.ErrUnsupported) {
+			return nil, err
+		}
+		if tag != "" {
+			pseudoBase, _ = tagToVersion(tag)
+		}
+	}
+
+	return checkCanonical(module.PseudoVersion(r.pseudoMajor, pseudoBase, info.Time, info.Short))
+}
+
+// validatePseudoVersion checks that version has a major version compatible with
+// r.modPath and encodes a base version and commit metadata that agrees with
+// info.
+//
+// Note that verifying a nontrivial base version in particular may be somewhat
+// expensive: in order to do so, r.code.DescendsFrom will need to fetch at least
+// enough of the commit history to find a path between version and its base.
+// Fortunately, many pseudo-versions — such as those for untagged repositories —
+// have trivial bases!
+func (r *codeRepo) validatePseudoVersion(ctx context.Context, info *codehost.RevInfo, version string) (err error) {
+	defer func() {
+		if err != nil {
+			if _, ok := err.(*module.ModuleError); !ok {
+				if _, ok := err.(*module.InvalidVersionError); !ok {
+					err = &module.InvalidVersionError{Version: version, Pseudo: true, Err: err}
+				}
+				err = &module.ModuleError{Path: r.modPath, Err: err}
+			}
+		}
+	}()
+
+	rev, err := module.PseudoVersionRev(version)
+	if err != nil {
+		return err
+	}
+	if rev != info.Short {
+		switch {
+		case strings.HasPrefix(rev, info.Short):
+			return fmt.Errorf("revision is longer than canonical (expected %s)", info.Short)
+		case strings.HasPrefix(info.Short, rev):
+			return fmt.Errorf("revision is shorter than canonical (expected %s)", info.Short)
+		default:
+			return fmt.Errorf("does not match short name of revision (expected %s)", info.Short)
+		}
+	}
+
+	t, err := module.PseudoVersionTime(version)
+	if err != nil {
+		return err
+	}
+	if !t.Equal(info.Time.Truncate(time.Second)) {
+		return fmt.Errorf("does not match version-control timestamp (expected %s)", info.Time.UTC().Format(module.PseudoVersionTimestampFormat))
+	}
+
+	tagPrefix := ""
+	if r.codeDir != "" {
+		tagPrefix = r.codeDir + "/"
+	}
+
+	// A pseudo-version should have a precedence just above its parent revisions,
+	// and no higher. Otherwise, it would be possible for library authors to "pin"
+	// dependency versions (and bypass the usual minimum version selection) by
+	// naming an extremely high pseudo-version rather than an accurate one.
+	//
+	// Moreover, if we allow a pseudo-version to use any arbitrary pre-release
+	// tag, we end up with infinitely many possible names for each commit. Each
+	// name consumes resources in the module cache and proxies, so we want to
+	// restrict them to a finite set under control of the module author.
+	//
+	// We address both of these issues by requiring the tag upon which the
+	// pseudo-version is based to refer to some ancestor of the revision. We
+	// prefer the highest such tag when constructing a new pseudo-version, but do
+	// not enforce that property when resolving existing pseudo-versions: we don't
+	// know when the parent tags were added, and the highest-tagged parent may not
+	// have existed when the pseudo-version was first resolved.
+	base, err := module.PseudoVersionBase(strings.TrimSuffix(version, "+incompatible"))
+	if err != nil {
+		return err
+	}
+	if base == "" {
+		if r.pseudoMajor == "" && semver.Major(version) == "v1" {
+			return fmt.Errorf("major version without preceding tag must be v0, not v1")
+		}
+		return nil
+	} else {
+		for _, tag := range info.Tags {
+			versionOnly := strings.TrimPrefix(tag, tagPrefix)
+			if versionOnly == base {
+				// The base version is canonical, so if the version from the tag is
+				// literally equal (not just equivalent), then the tag is canonical too.
+				//
+				// We allow pseudo-versions to be derived from non-canonical tags on the
+				// same commit, so that tags like "v1.1.0+some-metadata" resolve as
+				// close as possible to the canonical version ("v1.1.0") while still
+				// enforcing a total ordering ("v1.1.1-0.[…]" with a unique suffix).
+				//
+				// However, canonical tags already have a total ordering, so there is no
+				// reason not to use the canonical tag directly, and we know that the
+				// canonical tag must already exist because the pseudo-version is
+				// derived from it. In that case, referring to the revision by a
+				// pseudo-version derived from its own canonical tag is just confusing.
+				return fmt.Errorf("tag (%s) found on revision %s is already canonical, so should not be replaced with a pseudo-version derived from that tag", tag, rev)
+			}
+		}
+	}
+
+	tags, err := r.code.Tags(ctx, tagPrefix+base)
+	if err != nil {
+		return err
+	}
+
+	var lastTag string // Prefer to log some real tag rather than a canonically-equivalent base.
+	ancestorFound := false
+	for _, tag := range tags.List {
+		versionOnly := strings.TrimPrefix(tag.Name, tagPrefix)
+		if semver.Compare(versionOnly, base) == 0 {
+			lastTag = tag.Name
+			ancestorFound, err = r.code.DescendsFrom(ctx, info.Name, tag.Name)
+			if ancestorFound {
+				break
+			}
+		}
+	}
+
+	if lastTag == "" {
+		return fmt.Errorf("preceding tag (%s) not found", base)
+	}
+
+	if !ancestorFound {
+		if err != nil {
+			return err
+		}
+		rev, err := module.PseudoVersionRev(version)
+		if err != nil {
+			return fmt.Errorf("not a descendent of preceding tag (%s)", lastTag)
+		}
+		return fmt.Errorf("revision %s is not a descendent of preceding tag (%s)", rev, lastTag)
+	}
+	return nil
+}
+
+func (r *codeRepo) revToRev(rev string) string {
+	if semver.IsValid(rev) {
+		if module.IsPseudoVersion(rev) {
+			r, _ := module.PseudoVersionRev(rev)
+			return r
+		}
+		if semver.Build(rev) == "+incompatible" {
+			rev = rev[:len(rev)-len("+incompatible")]
+		}
+		if r.codeDir == "" {
+			return rev
+		}
+		return r.codeDir + "/" + rev
+	}
+	return rev
+}
+
+func (r *codeRepo) versionToRev(version string) (rev string, err error) {
+	if !semver.IsValid(version) {
+		return "", &module.ModuleError{
+			Path: r.modPath,
+			Err: &module.InvalidVersionError{
+				Version: version,
+				Err:     errors.New("syntax error"),
+			},
+		}
+	}
+	return r.revToRev(version), nil
+}
+
+// findDir locates the directory within the repo containing the module.
+//
+// If r.pathMajor is non-empty, this can be either r.codeDir or — if a go.mod
+// file exists — r.codeDir/r.pathMajor[1:].
+func (r *codeRepo) findDir(ctx context.Context, version string) (rev, dir string, gomod []byte, err error) {
+	rev, err = r.versionToRev(version)
+	if err != nil {
+		return "", "", nil, err
+	}
+
+	// Load info about go.mod but delay consideration
+	// (except I/O error) until we rule out v2/go.mod.
+	file1 := path.Join(r.codeDir, "go.mod")
+	gomod1, err1 := r.code.ReadFile(ctx, rev, file1, codehost.MaxGoMod)
+	if err1 != nil && !os.IsNotExist(err1) {
+		return "", "", nil, fmt.Errorf("reading %s/%s at revision %s: %v", r.codeRoot, file1, rev, err1)
+	}
+	mpath1 := modfile.ModulePath(gomod1)
+	found1 := err1 == nil && (isMajor(mpath1, r.pathMajor) || r.canReplaceMismatchedVersionDueToBug(mpath1))
+
+	var file2 string
+	if r.pathMajor != "" && r.codeRoot != r.modPath && !strings.HasPrefix(r.pathMajor, ".") {
+		// Suppose pathMajor is "/v2".
+		// Either go.mod should claim v2 and v2/go.mod should not exist,
+		// or v2/go.mod should exist and claim v2. Not both.
+		// Note that we don't check the full path, just the major suffix,
+		// because of replacement modules. This might be a fork of
+		// the real module, found at a different path, usable only in
+		// a replace directive.
+		dir2 := path.Join(r.codeDir, r.pathMajor[1:])
+		file2 = path.Join(dir2, "go.mod")
+		gomod2, err2 := r.code.ReadFile(ctx, rev, file2, codehost.MaxGoMod)
+		if err2 != nil && !os.IsNotExist(err2) {
+			return "", "", nil, fmt.Errorf("reading %s/%s at revision %s: %v", r.codeRoot, file2, rev, err2)
+		}
+		mpath2 := modfile.ModulePath(gomod2)
+		found2 := err2 == nil && isMajor(mpath2, r.pathMajor)
+
+		if found1 && found2 {
+			return "", "", nil, fmt.Errorf("%s/%s and ...%s/go.mod both have ...%s module paths at revision %s", r.pathPrefix, file1, r.pathMajor, r.pathMajor, rev)
+		}
+		if found2 {
+			return rev, dir2, gomod2, nil
+		}
+		if err2 == nil {
+			if mpath2 == "" {
+				return "", "", nil, fmt.Errorf("%s/%s is missing module path at revision %s", r.codeRoot, file2, rev)
+			}
+			return "", "", nil, fmt.Errorf("%s/%s has non-...%s module path %q at revision %s", r.codeRoot, file2, r.pathMajor, mpath2, rev)
+		}
+	}
+
+	// Not v2/go.mod, so it's either go.mod or nothing. Which is it?
+	if found1 {
+		// Explicit go.mod with matching major version ok.
+		return rev, r.codeDir, gomod1, nil
+	}
+	if err1 == nil {
+		// Explicit go.mod with non-matching major version disallowed.
+		suffix := ""
+		if file2 != "" {
+			suffix = fmt.Sprintf(" (and ...%s/go.mod does not exist)", r.pathMajor)
+		}
+		if mpath1 == "" {
+			return "", "", nil, fmt.Errorf("%s is missing module path%s at revision %s", file1, suffix, rev)
+		}
+		if r.pathMajor != "" { // ".v1", ".v2" for gopkg.in
+			return "", "", nil, fmt.Errorf("%s has non-...%s module path %q%s at revision %s", file1, r.pathMajor, mpath1, suffix, rev)
+		}
+		if _, _, ok := module.SplitPathVersion(mpath1); !ok {
+			return "", "", nil, fmt.Errorf("%s has malformed module path %q%s at revision %s", file1, mpath1, suffix, rev)
+		}
+		return "", "", nil, fmt.Errorf("%s has post-%s module path %q%s at revision %s", file1, semver.Major(version), mpath1, suffix, rev)
+	}
+
+	if r.codeDir == "" && (r.pathMajor == "" || strings.HasPrefix(r.pathMajor, ".")) {
+		// Implicit go.mod at root of repo OK for v0/v1 and for gopkg.in.
+		return rev, "", nil, nil
+	}
+
+	// Implicit go.mod below root of repo or at v2+ disallowed.
+	// Be clear about possibility of using either location for v2+.
+	if file2 != "" {
+		return "", "", nil, fmt.Errorf("missing %s/go.mod and ...%s/go.mod at revision %s", r.pathPrefix, r.pathMajor, rev)
+	}
+	return "", "", nil, fmt.Errorf("missing %s/go.mod at revision %s", r.pathPrefix, rev)
+}
+
+// isMajor reports whether the versions allowed for mpath are compatible with
+// the major version(s) implied by pathMajor, or false if mpath has an invalid
+// version suffix.
+func isMajor(mpath, pathMajor string) bool {
+	if mpath == "" {
+		// If we don't have a path, we don't know what version(s) it is compatible with.
+		return false
+	}
+	_, mpathMajor, ok := module.SplitPathVersion(mpath)
+	if !ok {
+		// An invalid module path is not compatible with any version.
+		return false
+	}
+	if pathMajor == "" {
+		// All of the valid versions for a gopkg.in module that requires major
+		// version v0 or v1 are compatible with the "v0 or v1" implied by an empty
+		// pathMajor.
+		switch module.PathMajorPrefix(mpathMajor) {
+		case "", "v0", "v1":
+			return true
+		default:
+			return false
+		}
+	}
+	if mpathMajor == "" {
+		// Even if pathMajor is ".v0" or ".v1", we can't be sure that a module
+		// without a suffix is tagged appropriately. Besides, we don't expect clones
+		// of non-gopkg.in modules to have gopkg.in paths, so a non-empty,
+		// non-gopkg.in mpath is probably the wrong module for any such pathMajor
+		// anyway.
+		return false
+	}
+	// If both pathMajor and mpathMajor are non-empty, then we only care that they
+	// have the same major-version validation rules. A clone fetched via a /v2
+	// path might replace a module with path gopkg.in/foo.v2-unstable, and that's
+	// ok.
+	return pathMajor[1:] == mpathMajor[1:]
+}
+
+// canReplaceMismatchedVersionDueToBug reports whether versions of r
+// could replace versions of mpath with otherwise-mismatched major versions
+// due to a historical bug in the Go command (golang.org/issue/34254).
+func (r *codeRepo) canReplaceMismatchedVersionDueToBug(mpath string) bool {
+	// The bug caused us to erroneously accept unversioned paths as replacements
+	// for versioned gopkg.in paths.
+	unversioned := r.pathMajor == ""
+	replacingGopkgIn := strings.HasPrefix(mpath, "gopkg.in/")
+	return unversioned && replacingGopkgIn
+}
+
+func (r *codeRepo) GoMod(ctx context.Context, version string) (data []byte, err error) {
+	if version != module.CanonicalVersion(version) {
+		return nil, fmt.Errorf("version %s is not canonical", version)
+	}
+
+	if module.IsPseudoVersion(version) {
+		// findDir ignores the metadata encoded in a pseudo-version,
+		// only using the revision at the end.
+		// Invoke Stat to verify the metadata explicitly so we don't return
+		// a bogus file for an invalid version.
+		_, err := r.Stat(ctx, version)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	rev, dir, gomod, err := r.findDir(ctx, version)
+	if err != nil {
+		return nil, err
+	}
+	if gomod != nil {
+		return gomod, nil
+	}
+	data, err = r.code.ReadFile(ctx, rev, path.Join(dir, "go.mod"), codehost.MaxGoMod)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return LegacyGoMod(r.modPath), nil
+		}
+		return nil, err
+	}
+	return data, nil
+}
+
+// LegacyGoMod generates a fake go.mod file for a module that doesn't have one.
+// The go.mod file contains a module directive and nothing else: no go version,
+// no requirements.
+//
+// We used to try to build a go.mod reflecting pre-existing
+// package management metadata files, but the conversion
+// was inherently imperfect (because those files don't have
+// exactly the same semantics as go.mod) and, when done
+// for dependencies in the middle of a build, impossible to
+// correct. So we stopped.
+func LegacyGoMod(modPath string) []byte {
+	return fmt.Appendf(nil, "module %s\n", modfile.AutoQuote(modPath))
+}
+
+func (r *codeRepo) modPrefix(rev string) string {
+	return r.modPath + "@" + rev
+}
+
+func (r *codeRepo) retractedVersions(ctx context.Context) (func(string) bool, error) {
+	vs, err := r.Versions(ctx, "")
+	if err != nil {
+		return nil, err
+	}
+	versions := vs.List
+
+	for i, v := range versions {
+		if strings.HasSuffix(v, "+incompatible") {
+			// We're looking for the latest release tag that may list retractions in a
+			// go.mod file. +incompatible versions necessarily do not, and they start
+			// at major version 2 — which is higher than any version that could
+			// validly contain a go.mod file.
+			versions = versions[:i]
+			break
+		}
+	}
+	if len(versions) == 0 {
+		return func(string) bool { return false }, nil
+	}
+
+	var highest string
+	for i := len(versions) - 1; i >= 0; i-- {
+		v := versions[i]
+		if semver.Prerelease(v) == "" {
+			highest = v
+			break
+		}
+	}
+	if highest == "" {
+		highest = versions[len(versions)-1]
+	}
+
+	data, err := r.GoMod(ctx, highest)
+	if err != nil {
+		return nil, err
+	}
+	f, err := modfile.ParseLax("go.mod", data, nil)
+	if err != nil {
+		return nil, err
+	}
+	retractions := make([]modfile.VersionInterval, 0, len(f.Retract))
+	for _, r := range f.Retract {
+		retractions = append(retractions, r.VersionInterval)
+	}
+
+	return func(v string) bool {
+		for _, r := range retractions {
+			if semver.Compare(r.Low, v) <= 0 && semver.Compare(v, r.High) <= 0 {
+				return true
+			}
+		}
+		return false
+	}, nil
+}
+
+func (r *codeRepo) Zip(ctx context.Context, dst io.Writer, version string) error {
+	if version != module.CanonicalVersion(version) {
+		return fmt.Errorf("version %s is not canonical", version)
+	}
+
+	if module.IsPseudoVersion(version) {
+		// findDir ignores the metadata encoded in a pseudo-version,
+		// only using the revision at the end.
+		// Invoke Stat to verify the metadata explicitly so we don't return
+		// a bogus file for an invalid version.
+		_, err := r.Stat(ctx, version)
+		if err != nil {
+			return err
+		}
+	}
+
+	rev, subdir, _, err := r.findDir(ctx, version)
+	if err != nil {
+		return err
+	}
+
+	if gomod, err := r.code.ReadFile(ctx, rev, filepath.Join(subdir, "go.mod"), codehost.MaxGoMod); err == nil {
+		goVers := gover.GoModLookup(gomod, "go")
+		if gover.Compare(goVers, gover.Local()) > 0 {
+			return &gover.TooNewError{What: r.ModulePath() + "@" + version, GoVersion: goVers}
+		}
+	} else if !errors.Is(err, fs.ErrNotExist) {
+		return err
+	}
+
+	dl, err := r.code.ReadZip(ctx, rev, subdir, codehost.MaxZipFile)
+	if err != nil {
+		return err
+	}
+	defer dl.Close()
+	subdir = strings.Trim(subdir, "/")
+
+	// Spool to local file.
+	f, err := os.CreateTemp("", "go-codehost-")
+	if err != nil {
+		dl.Close()
+		return err
+	}
+	defer os.Remove(f.Name())
+	defer f.Close()
+	maxSize := int64(codehost.MaxZipFile)
+	lr := &io.LimitedReader{R: dl, N: maxSize + 1}
+	if _, err := io.Copy(f, lr); err != nil {
+		dl.Close()
+		return err
+	}
+	dl.Close()
+	if lr.N <= 0 {
+		return fmt.Errorf("downloaded zip file too large")
+	}
+	size := (maxSize + 1) - lr.N
+	if _, err := f.Seek(0, 0); err != nil {
+		return err
+	}
+
+	// Translate from zip file we have to zip file we want.
+	zr, err := zip.NewReader(f, size)
+	if err != nil {
+		return err
+	}
+
+	var files []modzip.File
+	if subdir != "" {
+		subdir += "/"
+	}
+	haveLICENSE := false
+	topPrefix := ""
+	for _, zf := range zr.File {
+		if topPrefix == "" {
+			i := strings.Index(zf.Name, "/")
+			if i < 0 {
+				return fmt.Errorf("missing top-level directory prefix")
+			}
+			topPrefix = zf.Name[:i+1]
+		}
+		var name string
+		var found bool
+		if name, found = strings.CutPrefix(zf.Name, topPrefix); !found {
+			return fmt.Errorf("zip file contains more than one top-level directory")
+		}
+
+		if name, found = strings.CutPrefix(name, subdir); !found {
+			continue
+		}
+
+		if name == "" || strings.HasSuffix(name, "/") {
+			continue
+		}
+		files = append(files, zipFile{name: name, f: zf})
+		if name == "LICENSE" {
+			haveLICENSE = true
+		}
+	}
+
+	if !haveLICENSE && subdir != "" {
+		data, err := r.code.ReadFile(ctx, rev, "LICENSE", codehost.MaxLICENSE)
+		if err == nil {
+			files = append(files, dataFile{name: "LICENSE", data: data})
+		}
+	}
+
+	return modzip.Create(dst, module.Version{Path: r.modPath, Version: version}, files)
+}
+
+type zipFile struct {
+	name string
+	f    *zip.File
+}
+
+func (f zipFile) Path() string                 { return f.name }
+func (f zipFile) Lstat() (fs.FileInfo, error)  { return f.f.FileInfo(), nil }
+func (f zipFile) Open() (io.ReadCloser, error) { return f.f.Open() }
+
+type dataFile struct {
+	name string
+	data []byte
+}
+
+func (f dataFile) Path() string                { return f.name }
+func (f dataFile) Lstat() (fs.FileInfo, error) { return dataFileInfo{f}, nil }
+func (f dataFile) Open() (io.ReadCloser, error) {
+	return io.NopCloser(bytes.NewReader(f.data)), nil
+}
+
+type dataFileInfo struct {
+	f dataFile
+}
+
+func (fi dataFileInfo) Name() string       { return path.Base(fi.f.name) }
+func (fi dataFileInfo) Size() int64        { return int64(len(fi.f.data)) }
+func (fi dataFileInfo) Mode() fs.FileMode  { return 0644 }
+func (fi dataFileInfo) ModTime() time.Time { return time.Time{} }
+func (fi dataFileInfo) IsDir() bool        { return false }
+func (fi dataFileInfo) Sys() any           { return nil }
+
+func (fi dataFileInfo) String() string {
+	return fs.FormatFileInfo(fi)
+}
+
+// hasPathPrefix reports whether the path s begins with the
+// elements in prefix.
+func hasPathPrefix(s, prefix string) bool {
+	switch {
+	default:
+		return false
+	case len(s) == len(prefix):
+		return s == prefix
+	case len(s) > len(prefix):
+		if prefix != "" && prefix[len(prefix)-1] == '/' {
+			return strings.HasPrefix(s, prefix)
+		}
+		return s[len(prefix)] == '/' && s[:len(prefix)] == prefix
+	}
+}
diff --git a/src/cmd/go/internal/modfetch/coderepo_test.go b/src/cmd/go/internal/modfetch/coderepo_test.go
new file mode 100644
index 0000000..aad7872
--- /dev/null
+++ b/src/cmd/go/internal/modfetch/coderepo_test.go
@@ -0,0 +1,965 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modfetch
+
+import (
+	"archive/zip"
+	"context"
+	"crypto/sha256"
+	"encoding/hex"
+	"flag"
+	"hash"
+	"internal/testenv"
+	"io"
+	"log"
+	"os"
+	"path/filepath"
+	"reflect"
+	"strings"
+	"testing"
+	"time"
+
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/modfetch/codehost"
+	"cmd/go/internal/vcweb/vcstest"
+
+	"golang.org/x/mod/sumdb/dirhash"
+)
+
+func TestMain(m *testing.M) {
+	flag.Parse()
+	if err := testMain(m); err != nil {
+		log.Fatal(err)
+	}
+}
+
+func testMain(m *testing.M) (err error) {
+
+	cfg.GOPROXY = "direct"
+
+	// The sum database is populated using a released version of the go command,
+	// but this test may include fixes for additional modules that previously
+	// could not be fetched. Since this test isn't executing any of the resolved
+	// code, bypass the sum database.
+	cfg.GOSUMDB = "off"
+
+	dir, err := os.MkdirTemp("", "gitrepo-test-")
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if rmErr := os.RemoveAll(dir); err == nil {
+			err = rmErr
+		}
+	}()
+
+	cfg.GOMODCACHE = filepath.Join(dir, "modcache")
+	if err := os.Mkdir(cfg.GOMODCACHE, 0755); err != nil {
+		return err
+	}
+
+	srv, err := vcstest.NewServer()
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if closeErr := srv.Close(); err == nil {
+			err = closeErr
+		}
+	}()
+
+	m.Run()
+	return nil
+}
+
+const (
+	vgotest1git = "github.com/rsc/vgotest1"
+	vgotest1hg  = "vcs-test.golang.org/hg/vgotest1.hg"
+)
+
+var altVgotests = map[string]string{
+	"hg": vgotest1hg,
+}
+
+type codeRepoTest struct {
+	vcs         string
+	path        string
+	mpath       string
+	rev         string
+	err         string
+	version     string
+	name        string
+	short       string
+	time        time.Time
+	gomod       string
+	gomodErr    string
+	zip         []string
+	zipErr      string
+	zipSum      string
+	zipFileHash string
+}
+
+var codeRepoTests = []codeRepoTest{
+	{
+		vcs:     "git",
+		path:    "github.com/rsc/vgotest1",
+		rev:     "v0.0.0",
+		version: "v0.0.0",
+		name:    "80d85c5d4d17598a0e9055e7c175a32b415d6128",
+		short:   "80d85c5d4d17",
+		time:    time.Date(2018, 2, 19, 23, 10, 6, 0, time.UTC),
+		zip: []string{
+			"LICENSE",
+			"README.md",
+			"pkg/p.go",
+		},
+		zipSum:      "h1:zVEjciLdlk/TPWCOyZo7k24T+tOKRQC+u8MKq/xS80I=",
+		zipFileHash: "738a00ddbfe8c329dce6b48e1f23c8e22a92db50f3cfb2653caa0d62676bc09c",
+	},
+	{
+		vcs:     "git",
+		path:    "github.com/rsc/vgotest1",
+		rev:     "v0.0.0-20180219231006-80d85c5d4d17",
+		version: "v0.0.0-20180219231006-80d85c5d4d17",
+		name:    "80d85c5d4d17598a0e9055e7c175a32b415d6128",
+		short:   "80d85c5d4d17",
+		time:    time.Date(2018, 2, 19, 23, 10, 6, 0, time.UTC),
+		zip: []string{
+			"LICENSE",
+			"README.md",
+			"pkg/p.go",
+		},
+		zipSum:      "h1:nOznk2xKsLGkTnXe0q9t1Ewt9jxK+oadtafSUqHM3Ec=",
+		zipFileHash: "bacb08f391e29d2eaaef8281b5c129ee6d890e608ee65877e0003c0181a766c8",
+	},
+	{
+		vcs:  "git",
+		path: "github.com/rsc/vgotest1",
+		rev:  "v0.0.1-0.20180219231006-80d85c5d4d17",
+		err:  `github.com/rsc/vgotest1@v0.0.1-0.20180219231006-80d85c5d4d17: invalid pseudo-version: tag (v0.0.0) found on revision 80d85c5d4d17 is already canonical, so should not be replaced with a pseudo-version derived from that tag`,
+	},
+	{
+		vcs:     "git",
+		path:    "github.com/rsc/vgotest1",
+		rev:     "v1.0.0",
+		version: "v1.0.0",
+		name:    "80d85c5d4d17598a0e9055e7c175a32b415d6128",
+		short:   "80d85c5d4d17",
+		time:    time.Date(2018, 2, 19, 23, 10, 6, 0, time.UTC),
+		zip: []string{
+			"LICENSE",
+			"README.md",
+			"pkg/p.go",
+		},
+		zipSum:      "h1:e040hOoWGeuJLawDjK9DW6med+cz9FxMFYDMOVG8ctQ=",
+		zipFileHash: "74caab65cfbea427c341fa815f3bb0378681d8f0e3cf62a7f207014263ec7be3",
+	},
+	{
+		vcs:     "git",
+		path:    "github.com/rsc/vgotest1/v2",
+		rev:     "v2.0.0",
+		version: "v2.0.0",
+		name:    "45f53230a74ad275c7127e117ac46914c8126160",
+		short:   "45f53230a74a",
+		time:    time.Date(2018, 7, 19, 1, 21, 27, 0, time.UTC),
+		err:     "missing github.com/rsc/vgotest1/go.mod and .../v2/go.mod at revision v2.0.0",
+	},
+	{
+		vcs:     "git",
+		path:    "github.com/rsc/vgotest1",
+		rev:     "80d85c5",
+		version: "v1.0.0",
+		name:    "80d85c5d4d17598a0e9055e7c175a32b415d6128",
+		short:   "80d85c5d4d17",
+		time:    time.Date(2018, 2, 19, 23, 10, 6, 0, time.UTC),
+		zip: []string{
+			"LICENSE",
+			"README.md",
+			"pkg/p.go",
+		},
+		zipSum:      "h1:e040hOoWGeuJLawDjK9DW6med+cz9FxMFYDMOVG8ctQ=",
+		zipFileHash: "74caab65cfbea427c341fa815f3bb0378681d8f0e3cf62a7f207014263ec7be3",
+	},
+	{
+		vcs:     "git",
+		path:    "github.com/rsc/vgotest1",
+		rev:     "mytag",
+		version: "v1.0.0",
+		name:    "80d85c5d4d17598a0e9055e7c175a32b415d6128",
+		short:   "80d85c5d4d17",
+		time:    time.Date(2018, 2, 19, 23, 10, 6, 0, time.UTC),
+		zip: []string{
+			"LICENSE",
+			"README.md",
+			"pkg/p.go",
+		},
+	},
+	{
+		vcs:     "git",
+		path:    "github.com/rsc/vgotest1/v2",
+		rev:     "45f53230a",
+		version: "v2.0.0",
+		name:    "45f53230a74ad275c7127e117ac46914c8126160",
+		short:   "45f53230a74a",
+		time:    time.Date(2018, 7, 19, 1, 21, 27, 0, time.UTC),
+		err:     "missing github.com/rsc/vgotest1/go.mod and .../v2/go.mod at revision v2.0.0",
+	},
+	{
+		vcs:     "git",
+		path:    "github.com/rsc/vgotest1/v54321",
+		rev:     "80d85c5",
+		version: "v54321.0.0-20180219231006-80d85c5d4d17",
+		name:    "80d85c5d4d17598a0e9055e7c175a32b415d6128",
+		short:   "80d85c5d4d17",
+		time:    time.Date(2018, 2, 19, 23, 10, 6, 0, time.UTC),
+		err:     "missing github.com/rsc/vgotest1/go.mod and .../v54321/go.mod at revision 80d85c5d4d17",
+	},
+	{
+		vcs:  "git",
+		path: "github.com/rsc/vgotest1/submod",
+		rev:  "v1.0.0",
+		err:  "unknown revision submod/v1.0.0",
+	},
+	{
+		vcs:  "git",
+		path: "github.com/rsc/vgotest1/submod",
+		rev:  "v1.0.3",
+		err:  "unknown revision submod/v1.0.3",
+	},
+	{
+		vcs:     "git",
+		path:    "github.com/rsc/vgotest1/submod",
+		rev:     "v1.0.4",
+		version: "v1.0.4",
+		name:    "8afe2b2efed96e0880ecd2a69b98a53b8c2738b6",
+		short:   "8afe2b2efed9",
+		time:    time.Date(2018, 2, 19, 23, 12, 7, 0, time.UTC),
+		gomod:   "module \"github.com/vgotest1/submod\" // submod/go.mod\n",
+		zip: []string{
+			"go.mod",
+			"pkg/p.go",
+			"LICENSE",
+		},
+		zipSum:      "h1:iMsJ/9uQsk6MnZNnJK311f11QiSlmN92Q2aSjCywuJY=",
+		zipFileHash: "95801bfa69c5197ae809af512946d22f22850068527cd78100ae3f176bc8043b",
+	},
+	{
+		vcs:     "git",
+		path:    "github.com/rsc/vgotest1",
+		rev:     "v1.1.0",
+		version: "v1.1.0",
+		name:    "b769f2de407a4db81af9c5de0a06016d60d2ea09",
+		short:   "b769f2de407a",
+		time:    time.Date(2018, 2, 19, 23, 13, 36, 0, time.UTC),
+		gomod:   "module \"github.com/rsc/vgotest1\" // root go.mod\nrequire \"github.com/rsc/vgotest1/submod\" v1.0.5\n",
+		zip: []string{
+			"LICENSE",
+			"README.md",
+			"go.mod",
+			"pkg/p.go",
+		},
+		zipSum:      "h1:M69k7q+8bQ+QUpHov45Z/NoR8rj3DsQJUnXLWvf01+Q=",
+		zipFileHash: "58af45fb248d320ea471f568e006379e2b8d71d6d1663f9b19b2e00fd9ac9265",
+	},
+	{
+		vcs:         "git",
+		path:        "github.com/rsc/vgotest1/v2",
+		rev:         "v2.0.1",
+		version:     "v2.0.1",
+		name:        "ea65f87c8f52c15ea68f3bdd9925ef17e20d91e9",
+		short:       "ea65f87c8f52",
+		time:        time.Date(2018, 2, 19, 23, 14, 23, 0, time.UTC),
+		gomod:       "module \"github.com/rsc/vgotest1/v2\" // root go.mod\n",
+		zipSum:      "h1:QmgYy/zt+uoWhDpcsgrSVzYFvKtBEjl5zT/FRz9GTzA=",
+		zipFileHash: "1aedf1546d322a0121879ddfd6d0e8bfbd916d2cafbeb538ddb440e04b04b9ef",
+	},
+	{
+		vcs:     "git",
+		path:    "github.com/rsc/vgotest1/v2",
+		rev:     "v2.0.3",
+		version: "v2.0.3",
+		name:    "f18795870fb14388a21ef3ebc1d75911c8694f31",
+		short:   "f18795870fb1",
+		time:    time.Date(2018, 2, 19, 23, 16, 4, 0, time.UTC),
+		err:     "github.com/rsc/vgotest1/v2/go.mod has non-.../v2 module path \"github.com/rsc/vgotest\" at revision v2.0.3",
+	},
+	{
+		vcs:     "git",
+		path:    "github.com/rsc/vgotest1/v2",
+		rev:     "v2.0.4",
+		version: "v2.0.4",
+		name:    "1f863feb76bc7029b78b21c5375644838962f88d",
+		short:   "1f863feb76bc",
+		time:    time.Date(2018, 2, 20, 0, 3, 38, 0, time.UTC),
+		err:     "github.com/rsc/vgotest1/go.mod and .../v2/go.mod both have .../v2 module paths at revision v2.0.4",
+	},
+	{
+		vcs:         "git",
+		path:        "github.com/rsc/vgotest1/v2",
+		rev:         "v2.0.5",
+		version:     "v2.0.5",
+		name:        "2f615117ce481c8efef46e0cc0b4b4dccfac8fea",
+		short:       "2f615117ce48",
+		time:        time.Date(2018, 2, 20, 0, 3, 59, 0, time.UTC),
+		gomod:       "module \"github.com/rsc/vgotest1/v2\" // v2/go.mod\n",
+		zipSum:      "h1:RIEb9q1SUSEQOzMn0zfl/LQxGFWlhWEAdeEguf1MLGU=",
+		zipFileHash: "7d92c2c328c5e9b0694101353705d5843746ec1d93a1e986d0da54c8a14dfe6d",
+	},
+	{
+		// redirect to github
+		vcs:         "git",
+		path:        "rsc.io/quote",
+		rev:         "v1.0.0",
+		version:     "v1.0.0",
+		name:        "f488df80bcdbd3e5bafdc24ad7d1e79e83edd7e6",
+		short:       "f488df80bcdb",
+		time:        time.Date(2018, 2, 14, 0, 45, 20, 0, time.UTC),
+		gomod:       "module \"rsc.io/quote\"\n",
+		zipSum:      "h1:haUSojyo3j2M9g7CEUFG8Na09dtn7QKxvPGaPVQdGwM=",
+		zipFileHash: "5c08ba2c09a364f93704aaa780e7504346102c6ef4fe1333a11f09904a732078",
+	},
+	{
+		// redirect to static hosting proxy
+		vcs:     "mod",
+		path:    "swtch.com/testmod",
+		rev:     "v1.0.0",
+		version: "v1.0.0",
+		// NO name or short - we intentionally ignore those in the proxy protocol
+		time:  time.Date(1972, 7, 18, 12, 34, 56, 0, time.UTC),
+		gomod: "module \"swtch.com/testmod\"\n",
+	},
+	{
+		// redirect to googlesource
+		vcs:         "git",
+		path:        "golang.org/x/text",
+		rev:         "4e4a3210bb",
+		version:     "v0.3.1-0.20180208041248-4e4a3210bb54",
+		name:        "4e4a3210bb54bb31f6ab2cdca2edcc0b50c420c1",
+		short:       "4e4a3210bb54",
+		time:        time.Date(2018, 2, 8, 4, 12, 48, 0, time.UTC),
+		zipSum:      "h1:Yxu6pHX9X2RECiuw/Q5/4uvajuaowck8zOFKXgbfNBk=",
+		zipFileHash: "ac2c165a5c10aa5a7545dea60a08e019270b982fa6c8bdcb5943931de64922fe",
+	},
+	{
+		vcs:         "git",
+		path:        "github.com/pkg/errors",
+		rev:         "v0.8.0",
+		version:     "v0.8.0",
+		name:        "645ef00459ed84a119197bfb8d8205042c6df63d",
+		short:       "645ef00459ed",
+		time:        time.Date(2016, 9, 29, 1, 48, 1, 0, time.UTC),
+		zipSum:      "h1:WdK/asTD0HN+q6hsWO3/vpuAkAr+tw6aNJNDFFf0+qw=",
+		zipFileHash: "e4fa69ba057356614edbc1da881a7d3ebb688505be49f65965686bcb859e2fae",
+	},
+	{
+		// package in subdirectory - custom domain
+		// In general we can't reject these definitively in Lookup,
+		// but gopkg.in is special.
+		vcs:  "git",
+		path: "gopkg.in/yaml.v2/abc",
+		err:  "invalid module path \"gopkg.in/yaml.v2/abc\"",
+	},
+	{
+		// package in subdirectory - github
+		// Because it's a package, Stat should fail entirely.
+		vcs:  "git",
+		path: "github.com/rsc/quote/buggy",
+		rev:  "c4d4236f",
+		err:  "missing github.com/rsc/quote/buggy/go.mod at revision c4d4236f9242",
+	},
+	{
+		vcs:         "git",
+		path:        "gopkg.in/yaml.v2",
+		rev:         "d670f940",
+		version:     "v2.0.0",
+		name:        "d670f9405373e636a5a2765eea47fac0c9bc91a4",
+		short:       "d670f9405373",
+		time:        time.Date(2018, 1, 9, 11, 43, 31, 0, time.UTC),
+		gomod:       "module gopkg.in/yaml.v2\n",
+		zipSum:      "h1:uUkhRGrsEyx/laRdeS6YIQKIys8pg+lRSRdVMTYjivs=",
+		zipFileHash: "7b0a141b1b0b49772ab4eecfd11dfd6609a94a5e868cab04a3abb1861ffaa877",
+	},
+	{
+		vcs:         "git",
+		path:        "gopkg.in/check.v1",
+		rev:         "20d25e280405",
+		version:     "v1.0.0-20161208181325-20d25e280405",
+		name:        "20d25e2804050c1cd24a7eea1e7a6447dd0e74ec",
+		short:       "20d25e280405",
+		time:        time.Date(2016, 12, 8, 18, 13, 25, 0, time.UTC),
+		gomod:       "module gopkg.in/check.v1\n",
+		zipSum:      "h1:829vOVxxusYHC+IqBtkX5mbKtsY9fheQiQn0MZRVLfQ=",
+		zipFileHash: "9e7cb3f4f1e66d722306442b0dbe1f6f43d74d1736d54c510537bdfb1d6f432f",
+	},
+	{
+		vcs:         "git",
+		path:        "vcs-test.golang.org/go/mod/gitrepo1",
+		rev:         "master",
+		version:     "v1.2.4-annotated",
+		name:        "ede458df7cd0fdca520df19a33158086a8a68e81",
+		short:       "ede458df7cd0",
+		time:        time.Date(2018, 4, 17, 19, 43, 22, 0, time.UTC),
+		gomod:       "module vcs-test.golang.org/go/mod/gitrepo1\n",
+		zipSum:      "h1:YJYZRsM9BHFTlVr8YADjT0cJH8uFIDtoc5NLiVqZEx8=",
+		zipFileHash: "c15e49d58b7a4c37966cbe5bc01a0330cd5f2927e990e1839bda1d407766d9c5",
+	},
+	{
+		vcs:  "git",
+		path: "gopkg.in/natefinch/lumberjack.v2",
+		// This repo has a v2.1 tag.
+		// We only allow semver references to tags that are fully qualified, as in v2.1.0.
+		// Because we can't record v2.1.0 (the actual tag is v2.1), we record a pseudo-version
+		// instead, same as if the tag were any other non-version-looking string.
+		// We use a v2 pseudo-version here because of the .v2 in the path, not because
+		// of the v2 in the rev.
+		rev:     "v2.1", // non-canonical semantic version turns into pseudo-version
+		version: "v2.0.0-20170531160350-a96e63847dc3",
+		name:    "a96e63847dc3c67d17befa69c303767e2f84e54f",
+		short:   "a96e63847dc3",
+		time:    time.Date(2017, 5, 31, 16, 3, 50, 0, time.UTC),
+		gomod:   "module gopkg.in/natefinch/lumberjack.v2\n",
+	},
+	{
+		vcs:         "git",
+		path:        "vcs-test.golang.org/go/v2module/v2",
+		rev:         "v2.0.0",
+		version:     "v2.0.0",
+		name:        "203b91c896acd173aa719e4cdcb7d463c4b090fa",
+		short:       "203b91c896ac",
+		time:        time.Date(2019, 4, 3, 15, 52, 15, 0, time.UTC),
+		gomod:       "module vcs-test.golang.org/go/v2module/v2\n\ngo 1.12\n",
+		zipSum:      "h1:JItBZ+gwA5WvtZEGEbuDL4lUttGtLrs53lmdurq3bOg=",
+		zipFileHash: "9ea9ae1673cffcc44b7fdd3cc89953d68c102449b46c982dbf085e4f2e394da5",
+	},
+	{
+		// Git branch with a semver name, +incompatible version, and no go.mod file.
+		vcs:  "git",
+		path: "vcs-test.golang.org/go/mod/gitrepo1",
+		rev:  "v2.3.4+incompatible",
+		err:  `resolves to version v2.0.1+incompatible (v2.3.4 is not a tag)`,
+	},
+	{
+		// Git branch with a semver name, matching go.mod file, and compatible version.
+		vcs:  "git",
+		path: "vcs-test.golang.org/git/semver-branch.git",
+		rev:  "v1.0.0",
+		err:  `resolves to version v0.1.1-0.20220202191944-09c4d8f6938c (v1.0.0 is not a tag)`,
+	},
+	{
+		// Git branch with a semver name, matching go.mod file, and disallowed +incompatible version.
+		// The version/tag mismatch takes precedence over the +incompatible mismatched.
+		vcs:  "git",
+		path: "vcs-test.golang.org/git/semver-branch.git",
+		rev:  "v2.0.0+incompatible",
+		err:  `resolves to version v0.1.0 (v2.0.0 is not a tag)`,
+	},
+	{
+		// Git branch with a semver name, matching go.mod file, and mismatched version.
+		// The version/tag mismatch takes precedence over the +incompatible mismatched.
+		vcs:  "git",
+		path: "vcs-test.golang.org/git/semver-branch.git",
+		rev:  "v2.0.0",
+		err:  `resolves to version v0.1.0 (v2.0.0 is not a tag)`,
+	},
+	{
+		// v3.0.0-devel is the same as tag v4.0.0-beta.1, but v4.0.0-beta.1 would
+		// not be allowed because it is incompatible and a go.mod file exists.
+		// The error message should refer to a valid pseudo-version, not the
+		// unusable semver tag.
+		vcs:  "git",
+		path: "vcs-test.golang.org/git/semver-branch.git",
+		rev:  "v3.0.0-devel",
+		err:  `resolves to version v0.1.1-0.20220203155313-d59622f6e4d7 (v3.0.0-devel is not a tag)`,
+	},
+
+	// If v2/go.mod exists, then we should prefer to match the "v2"
+	// pseudo-versions to the nested module, and resolve the module in the parent
+	// directory to only compatible versions.
+	//
+	// However (https://go.dev/issue/51324), previous versions of the 'go' command
+	// didn't always do so, so if the user explicitly requests a +incompatible
+	// version (as would be present in an existing go.mod file), we should
+	// continue to allow it.
+	{
+		vcs:     "git",
+		path:    "vcs-test.golang.org/git/v2sub.git",
+		rev:     "80beb17a1603",
+		version: "v0.0.0-20220222205507-80beb17a1603",
+		name:    "80beb17a16036f17a5aedd1bb5bd6d407b3c6dc5",
+		short:   "80beb17a1603",
+		time:    time.Date(2022, 2, 22, 20, 55, 7, 0, time.UTC),
+	},
+	{
+		vcs:  "git",
+		path: "vcs-test.golang.org/git/v2sub.git",
+		rev:  "v2.0.0",
+		err:  `module contains a go.mod file, so module path must match major version ("vcs-test.golang.org/git/v2sub.git/v2")`,
+	},
+	{
+		vcs:  "git",
+		path: "vcs-test.golang.org/git/v2sub.git",
+		rev:  "v2.0.1-0.20220222205507-80beb17a1603",
+		err:  `module contains a go.mod file, so module path must match major version ("vcs-test.golang.org/git/v2sub.git/v2")`,
+	},
+	{
+		vcs:     "git",
+		path:    "vcs-test.golang.org/git/v2sub.git",
+		rev:     "v2.0.0+incompatible",
+		version: "v2.0.0+incompatible",
+		name:    "5fcd3eaeeb391d399f562fd45a50dac9fc34ae8b",
+		short:   "5fcd3eaeeb39",
+		time:    time.Date(2022, 2, 22, 20, 53, 33, 0, time.UTC),
+	},
+	{
+		vcs:     "git",
+		path:    "vcs-test.golang.org/git/v2sub.git",
+		rev:     "v2.0.1-0.20220222205507-80beb17a1603+incompatible",
+		version: "v2.0.1-0.20220222205507-80beb17a1603+incompatible",
+		name:    "80beb17a16036f17a5aedd1bb5bd6d407b3c6dc5",
+		short:   "80beb17a1603",
+		time:    time.Date(2022, 2, 22, 20, 55, 7, 0, time.UTC),
+	},
+
+	// A version tag with explicit build metadata is valid but not canonical.
+	// It should resolve to a pseudo-version based on the same tag.
+	{
+		vcs:     "git",
+		path:    "vcs-test.golang.org/git/odd-tags.git",
+		rev:     "v0.1.0+build-metadata",
+		version: "v0.1.1-0.20220223184835-9d863d525bbf",
+		name:    "9d863d525bbfcc8eda09364738c4032393711a56",
+		short:   "9d863d525bbf",
+		time:    time.Date(2022, 2, 23, 18, 48, 35, 0, time.UTC),
+	},
+	{
+		vcs:     "git",
+		path:    "vcs-test.golang.org/git/odd-tags.git",
+		rev:     "9d863d525bbf",
+		version: "v0.1.1-0.20220223184835-9d863d525bbf",
+		name:    "9d863d525bbfcc8eda09364738c4032393711a56",
+		short:   "9d863d525bbf",
+		time:    time.Date(2022, 2, 23, 18, 48, 35, 0, time.UTC),
+	},
+	{
+		vcs:     "git",
+		path:    "vcs-test.golang.org/git/odd-tags.git",
+		rev:     "latest",
+		version: "v0.1.1-0.20220223184835-9d863d525bbf",
+		name:    "9d863d525bbfcc8eda09364738c4032393711a56",
+		short:   "9d863d525bbf",
+		time:    time.Date(2022, 2, 23, 18, 48, 35, 0, time.UTC),
+	},
+
+	// A version tag with an erroneous "+incompatible" suffix should resolve using
+	// only the prefix before the "+incompatible" suffix, not the "+incompatible"
+	// tag itself. (Otherwise, we would potentially have two different commits
+	// both named "v2.0.0+incompatible".) However, the tag is still valid semver
+	// and can still be used as the base for an unambiguous pseudo-version.
+	{
+		vcs:  "git",
+		path: "vcs-test.golang.org/git/odd-tags.git",
+		rev:  "v2.0.0+incompatible",
+		err:  `unknown revision v2.0.0`,
+	},
+	{
+		vcs:     "git",
+		path:    "vcs-test.golang.org/git/odd-tags.git",
+		rev:     "12d19af20458",
+		version: "v2.0.1-0.20220223184802-12d19af20458+incompatible",
+		name:    "12d19af204585b0db3d2a876ceddf5b9323f5a4a",
+		short:   "12d19af20458",
+		time:    time.Date(2022, 2, 23, 18, 48, 2, 0, time.UTC),
+	},
+
+	// Similarly, a pseudo-version must resolve to the named commit, even if a tag
+	// matching that pseudo-version is present on a *different* commit.
+	{
+		vcs:     "git",
+		path:    "vcs-test.golang.org/git/odd-tags.git",
+		rev:     "v3.0.0-20220223184802-12d19af20458",
+		version: "v3.0.0-20220223184802-12d19af20458+incompatible",
+		name:    "12d19af204585b0db3d2a876ceddf5b9323f5a4a",
+		short:   "12d19af20458",
+		time:    time.Date(2022, 2, 23, 18, 48, 2, 0, time.UTC),
+	},
+}
+
+func TestCodeRepo(t *testing.T) {
+	testenv.MustHaveExternalNetwork(t)
+	tmpdir := t.TempDir()
+
+	for _, tt := range codeRepoTests {
+		f := func(tt codeRepoTest) func(t *testing.T) {
+			return func(t *testing.T) {
+				if strings.Contains(tt.path, "gopkg.in") {
+					testenv.SkipFlaky(t, 54503)
+				}
+
+				t.Parallel()
+				if tt.vcs != "mod" {
+					testenv.MustHaveExecPath(t, tt.vcs)
+				}
+				ctx := context.Background()
+
+				repo := Lookup(ctx, "direct", tt.path)
+
+				if tt.mpath == "" {
+					tt.mpath = tt.path
+				}
+				if mpath := repo.ModulePath(); mpath != tt.mpath {
+					t.Errorf("repo.ModulePath() = %q, want %q", mpath, tt.mpath)
+				}
+
+				info, err := repo.Stat(ctx, tt.rev)
+				if err != nil {
+					if tt.err != "" {
+						if !strings.Contains(err.Error(), tt.err) {
+							t.Fatalf("repoStat(%q): %v, wanted %q", tt.rev, err, tt.err)
+						}
+						return
+					}
+					t.Fatalf("repo.Stat(%q): %v", tt.rev, err)
+				}
+				if tt.err != "" {
+					t.Errorf("repo.Stat(%q): success, wanted error", tt.rev)
+				}
+				if info.Version != tt.version {
+					t.Errorf("info.Version = %q, want %q", info.Version, tt.version)
+				}
+				if info.Name != tt.name {
+					t.Errorf("info.Name = %q, want %q", info.Name, tt.name)
+				}
+				if info.Short != tt.short {
+					t.Errorf("info.Short = %q, want %q", info.Short, tt.short)
+				}
+				if !info.Time.Equal(tt.time) {
+					t.Errorf("info.Time = %v, want %v", info.Time, tt.time)
+				}
+
+				if tt.gomod != "" || tt.gomodErr != "" {
+					data, err := repo.GoMod(ctx, tt.version)
+					if err != nil && tt.gomodErr == "" {
+						t.Errorf("repo.GoMod(%q): %v", tt.version, err)
+					} else if err != nil && tt.gomodErr != "" {
+						if err.Error() != tt.gomodErr {
+							t.Errorf("repo.GoMod(%q): %v, want %q", tt.version, err, tt.gomodErr)
+						}
+					} else if tt.gomodErr != "" {
+						t.Errorf("repo.GoMod(%q) = %q, want error %q", tt.version, data, tt.gomodErr)
+					} else if string(data) != tt.gomod {
+						t.Errorf("repo.GoMod(%q) = %q, want %q", tt.version, data, tt.gomod)
+					}
+				}
+
+				needHash := !testing.Short() && (tt.zipFileHash != "" || tt.zipSum != "")
+				if tt.zip != nil || tt.zipErr != "" || needHash {
+					f, err := os.CreateTemp(tmpdir, tt.version+".zip.")
+					if err != nil {
+						t.Fatalf("os.CreateTemp: %v", err)
+					}
+					zipfile := f.Name()
+					defer func() {
+						f.Close()
+						os.Remove(zipfile)
+					}()
+
+					var w io.Writer
+					var h hash.Hash
+					if needHash {
+						h = sha256.New()
+						w = io.MultiWriter(f, h)
+					} else {
+						w = f
+					}
+					err = repo.Zip(ctx, w, tt.version)
+					f.Close()
+					if err != nil {
+						if tt.zipErr != "" {
+							if err.Error() == tt.zipErr {
+								return
+							}
+							t.Fatalf("repo.Zip(%q): %v, want error %q", tt.version, err, tt.zipErr)
+						}
+						t.Fatalf("repo.Zip(%q): %v", tt.version, err)
+					}
+					if tt.zipErr != "" {
+						t.Errorf("repo.Zip(%q): success, want error %q", tt.version, tt.zipErr)
+					}
+
+					if tt.zip != nil {
+						prefix := tt.path + "@" + tt.version + "/"
+						z, err := zip.OpenReader(zipfile)
+						if err != nil {
+							t.Fatalf("open zip %s: %v", zipfile, err)
+						}
+						var names []string
+						for _, file := range z.File {
+							if !strings.HasPrefix(file.Name, prefix) {
+								t.Errorf("zip entry %v does not start with prefix %v", file.Name, prefix)
+								continue
+							}
+							names = append(names, file.Name[len(prefix):])
+						}
+						z.Close()
+						if !reflect.DeepEqual(names, tt.zip) {
+							t.Fatalf("zip = %v\nwant %v\n", names, tt.zip)
+						}
+					}
+
+					if needHash {
+						sum, err := dirhash.HashZip(zipfile, dirhash.Hash1)
+						if err != nil {
+							t.Errorf("repo.Zip(%q): %v", tt.version, err)
+						} else if sum != tt.zipSum {
+							t.Errorf("repo.Zip(%q): got file with sum %q, want %q", tt.version, sum, tt.zipSum)
+						} else if zipFileHash := hex.EncodeToString(h.Sum(nil)); zipFileHash != tt.zipFileHash {
+							t.Errorf("repo.Zip(%q): got file with hash %q, want %q (but content has correct sum)", tt.version, zipFileHash, tt.zipFileHash)
+						}
+					}
+				}
+			}
+		}
+		t.Run(strings.ReplaceAll(tt.path, "/", "_")+"/"+tt.rev, f(tt))
+		if strings.HasPrefix(tt.path, vgotest1git) {
+			for vcs, alt := range altVgotests {
+				altTest := tt
+				altTest.vcs = vcs
+				altTest.path = alt + strings.TrimPrefix(altTest.path, vgotest1git)
+				if strings.HasPrefix(altTest.mpath, vgotest1git) {
+					altTest.mpath = alt + strings.TrimPrefix(altTest.mpath, vgotest1git)
+				}
+				var m map[string]string
+				if alt == vgotest1hg {
+					m = hgmap
+				}
+				altTest.version = remap(altTest.version, m)
+				altTest.name = remap(altTest.name, m)
+				altTest.short = remap(altTest.short, m)
+				altTest.rev = remap(altTest.rev, m)
+				altTest.err = remap(altTest.err, m)
+				altTest.gomodErr = remap(altTest.gomodErr, m)
+				altTest.zipErr = remap(altTest.zipErr, m)
+				altTest.zipSum = ""
+				altTest.zipFileHash = ""
+				t.Run(strings.ReplaceAll(altTest.path, "/", "_")+"/"+altTest.rev, f(altTest))
+			}
+		}
+	}
+}
+
+var hgmap = map[string]string{
+	"github.com/rsc/vgotest1":                  "vcs-test.golang.org/hg/vgotest1.hg",
+	"f18795870fb14388a21ef3ebc1d75911c8694f31": "a9ad6d1d14eb544f459f446210c7eb3b009807c6",
+	"ea65f87c8f52c15ea68f3bdd9925ef17e20d91e9": "f1fc0f22021b638d073d31c752847e7bf385def7",
+	"b769f2de407a4db81af9c5de0a06016d60d2ea09": "92c7eb888b4fac17f1c6bd2e1060a1b881a3b832",
+	"8afe2b2efed96e0880ecd2a69b98a53b8c2738b6": "4e58084d459ae7e79c8c2264d0e8e9a92eb5cd44",
+	"2f615117ce481c8efef46e0cc0b4b4dccfac8fea": "879ea98f7743c8eff54f59a918f3a24123d1cf46",
+	"80d85c5d4d17598a0e9055e7c175a32b415d6128": "e125018e286a4b09061079a81e7b537070b7ff71",
+	"1f863feb76bc7029b78b21c5375644838962f88d": "bf63880162304a9337477f3858f5b7e255c75459",
+	"45f53230a74ad275c7127e117ac46914c8126160": "814fce58e83abd5bf2a13892e0b0e1198abefcd4",
+}
+
+func remap(name string, m map[string]string) string {
+	if m[name] != "" {
+		return m[name]
+	}
+	if codehost.AllHex(name) {
+		for k, v := range m {
+			if strings.HasPrefix(k, name) {
+				return v[:len(name)]
+			}
+		}
+	}
+	for k, v := range m {
+		name = strings.ReplaceAll(name, k, v)
+		if codehost.AllHex(k) {
+			name = strings.ReplaceAll(name, k[:12], v[:12])
+		}
+	}
+	return name
+}
+
+var codeRepoVersionsTests = []struct {
+	vcs      string
+	path     string
+	prefix   string
+	versions []string
+}{
+	{
+		vcs:      "git",
+		path:     "github.com/rsc/vgotest1",
+		versions: []string{"v0.0.0", "v0.0.1", "v1.0.0", "v1.0.1", "v1.0.2", "v1.0.3", "v1.1.0"},
+	},
+	{
+		vcs:      "git",
+		path:     "github.com/rsc/vgotest1",
+		prefix:   "v1.0",
+		versions: []string{"v1.0.0", "v1.0.1", "v1.0.2", "v1.0.3"},
+	},
+	{
+		vcs:      "git",
+		path:     "github.com/rsc/vgotest1/v2",
+		versions: []string{"v2.0.0", "v2.0.1", "v2.0.2", "v2.0.3", "v2.0.4", "v2.0.5", "v2.0.6"},
+	},
+	{
+		vcs:      "mod",
+		path:     "swtch.com/testmod",
+		versions: []string{"v1.0.0", "v1.1.1"},
+	},
+	{
+		vcs:      "git",
+		path:     "vcs-test.golang.org/git/odd-tags.git",
+		versions: nil,
+	},
+}
+
+func TestCodeRepoVersions(t *testing.T) {
+	testenv.MustHaveExternalNetwork(t)
+
+	for _, tt := range codeRepoVersionsTests {
+		tt := tt
+		t.Run(strings.ReplaceAll(tt.path, "/", "_"), func(t *testing.T) {
+			if strings.Contains(tt.path, "gopkg.in") {
+				testenv.SkipFlaky(t, 54503)
+			}
+
+			t.Parallel()
+			if tt.vcs != "mod" {
+				testenv.MustHaveExecPath(t, tt.vcs)
+			}
+			ctx := context.Background()
+
+			repo := Lookup(ctx, "direct", tt.path)
+			list, err := repo.Versions(ctx, tt.prefix)
+			if err != nil {
+				t.Fatalf("Versions(%q): %v", tt.prefix, err)
+			}
+			if !reflect.DeepEqual(list.List, tt.versions) {
+				t.Fatalf("Versions(%q):\nhave %v\nwant %v", tt.prefix, list, tt.versions)
+			}
+		})
+	}
+}
+
+var latestTests = []struct {
+	vcs     string
+	path    string
+	version string
+	err     string
+}{
+	{
+		vcs:  "git",
+		path: "github.com/rsc/empty",
+		err:  "no commits",
+	},
+	{
+		vcs:  "git",
+		path: "github.com/rsc/vgotest1",
+		err:  `github.com/rsc/vgotest1@v0.0.0-20180219223237-a08abb797a67: invalid version: go.mod has post-v0 module path "github.com/vgotest1/v2" at revision a08abb797a67`,
+	},
+	{
+		vcs:  "git",
+		path: "github.com/rsc/vgotest1/v2",
+		err:  `github.com/rsc/vgotest1/v2@v2.0.0-20180219223237-a08abb797a67: invalid version: github.com/rsc/vgotest1/go.mod and .../v2/go.mod both have .../v2 module paths at revision a08abb797a67`,
+	},
+	{
+		vcs:  "git",
+		path: "github.com/rsc/vgotest1/subdir",
+		err:  "github.com/rsc/vgotest1/subdir@v0.0.0-20180219223237-a08abb797a67: invalid version: missing github.com/rsc/vgotest1/subdir/go.mod at revision a08abb797a67",
+	},
+	{
+		vcs:     "git",
+		path:    "vcs-test.golang.org/git/commit-after-tag.git",
+		version: "v1.0.1-0.20190715211727-b325d8217783",
+	},
+	{
+		vcs:     "git",
+		path:    "vcs-test.golang.org/git/no-tags.git",
+		version: "v0.0.0-20190715212047-e706ba1d9f6d",
+	},
+	{
+		vcs:     "mod",
+		path:    "swtch.com/testmod",
+		version: "v1.1.1",
+	},
+}
+
+func TestLatest(t *testing.T) {
+	testenv.MustHaveExternalNetwork(t)
+
+	for _, tt := range latestTests {
+		name := strings.ReplaceAll(tt.path, "/", "_")
+		t.Run(name, func(t *testing.T) {
+			tt := tt
+			t.Parallel()
+			if tt.vcs != "mod" {
+				testenv.MustHaveExecPath(t, tt.vcs)
+			}
+			ctx := context.Background()
+
+			repo := Lookup(ctx, "direct", tt.path)
+			info, err := repo.Latest(ctx)
+			if err != nil {
+				if tt.err != "" {
+					if err.Error() == tt.err {
+						return
+					}
+					t.Fatalf("Latest(): %v, want %q", err, tt.err)
+				}
+				t.Fatalf("Latest(): %v", err)
+			}
+			if tt.err != "" {
+				t.Fatalf("Latest() = %v, want error %q", info.Version, tt.err)
+			}
+			if info.Version != tt.version {
+				t.Fatalf("Latest() = %v, want %v", info.Version, tt.version)
+			}
+		})
+	}
+}
+
+// fixedTagsRepo is a fake codehost.Repo that returns a fixed list of tags
+type fixedTagsRepo struct {
+	tags []string
+	codehost.Repo
+}
+
+func (ch *fixedTagsRepo) Tags(ctx context.Context, prefix string) (*codehost.Tags, error) {
+	tags := &codehost.Tags{}
+	for _, t := range ch.tags {
+		tags.List = append(tags.List, codehost.Tag{Name: t})
+	}
+	return tags, nil
+}
+
+func TestNonCanonicalSemver(t *testing.T) {
+	t.Parallel()
+	ctx := context.Background()
+
+	root := "golang.org/x/issue24476"
+	ch := &fixedTagsRepo{
+		tags: []string{
+			"", "huh?", "1.0.1",
+			// what about "version 1 dot dogcow"?
+			"v1.🐕.🐄",
+			"v1", "v0.1",
+			// and one normal one that should pass through
+			"v1.0.1",
+		},
+	}
+
+	cr, err := newCodeRepo(ch, root, root)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	v, err := cr.Versions(ctx, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(v.List) != 1 || v.List[0] != "v1.0.1" {
+		t.Fatal("unexpected versions returned:", v)
+	}
+}
diff --git a/src/cmd/go/internal/modfetch/fetch.go b/src/cmd/go/internal/modfetch/fetch.go
new file mode 100644
index 0000000..4279686
--- /dev/null
+++ b/src/cmd/go/internal/modfetch/fetch.go
@@ -0,0 +1,998 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modfetch
+
+import (
+	"archive/zip"
+	"bytes"
+	"context"
+	"crypto/sha256"
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"io"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+	"sync"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/lockedfile"
+	"cmd/go/internal/par"
+	"cmd/go/internal/robustio"
+	"cmd/go/internal/str"
+	"cmd/go/internal/trace"
+
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/sumdb/dirhash"
+	modzip "golang.org/x/mod/zip"
+)
+
+var downloadCache par.ErrCache[module.Version, string] // version → directory
+
+var ErrToolchain = errors.New("internal error: invalid operation on toolchain module")
+
+// Download downloads the specific module version to the
+// local download cache and returns the name of the directory
+// corresponding to the root of the module's file tree.
+func Download(ctx context.Context, mod module.Version) (dir string, err error) {
+	if gover.IsToolchain(mod.Path) {
+		return "", ErrToolchain
+	}
+	if err := checkCacheDir(ctx); err != nil {
+		base.Fatal(err)
+	}
+
+	// The par.Cache here avoids duplicate work.
+	return downloadCache.Do(mod, func() (string, error) {
+		dir, err := download(ctx, mod)
+		if err != nil {
+			return "", err
+		}
+		checkMod(ctx, mod)
+
+		// If go.mod exists (not an old legacy module), check version is not too new.
+		if data, err := os.ReadFile(filepath.Join(dir, "go.mod")); err == nil {
+			goVersion := gover.GoModLookup(data, "go")
+			if gover.Compare(goVersion, gover.Local()) > 0 {
+				return "", &gover.TooNewError{What: mod.String(), GoVersion: goVersion}
+			}
+		} else if !errors.Is(err, fs.ErrNotExist) {
+			return "", err
+		}
+
+		return dir, nil
+	})
+}
+
+func download(ctx context.Context, mod module.Version) (dir string, err error) {
+	ctx, span := trace.StartSpan(ctx, "modfetch.download "+mod.String())
+	defer span.Done()
+
+	dir, err = DownloadDir(ctx, mod)
+	if err == nil {
+		// The directory has already been completely extracted (no .partial file exists).
+		return dir, nil
+	} else if dir == "" || !errors.Is(err, fs.ErrNotExist) {
+		return "", err
+	}
+
+	// To avoid cluttering the cache with extraneous files,
+	// DownloadZip uses the same lockfile as Download.
+	// Invoke DownloadZip before locking the file.
+	zipfile, err := DownloadZip(ctx, mod)
+	if err != nil {
+		return "", err
+	}
+
+	unlock, err := lockVersion(ctx, mod)
+	if err != nil {
+		return "", err
+	}
+	defer unlock()
+
+	ctx, span = trace.StartSpan(ctx, "unzip "+zipfile)
+	defer span.Done()
+
+	// Check whether the directory was populated while we were waiting on the lock.
+	_, dirErr := DownloadDir(ctx, mod)
+	if dirErr == nil {
+		return dir, nil
+	}
+	_, dirExists := dirErr.(*DownloadDirPartialError)
+
+	// Clean up any remaining temporary directories created by old versions
+	// (before 1.16), as well as partially extracted directories (indicated by
+	// DownloadDirPartialError, usually because of a .partial file). This is only
+	// safe to do because the lock file ensures that their writers are no longer
+	// active.
+	parentDir := filepath.Dir(dir)
+	tmpPrefix := filepath.Base(dir) + ".tmp-"
+	if old, err := filepath.Glob(filepath.Join(str.QuoteGlob(parentDir), str.QuoteGlob(tmpPrefix)+"*")); err == nil {
+		for _, path := range old {
+			RemoveAll(path) // best effort
+		}
+	}
+	if dirExists {
+		if err := RemoveAll(dir); err != nil {
+			return "", err
+		}
+	}
+
+	partialPath, err := CachePath(ctx, mod, "partial")
+	if err != nil {
+		return "", err
+	}
+
+	// Extract the module zip directory at its final location.
+	//
+	// To prevent other processes from reading the directory if we crash,
+	// create a .partial file before extracting the directory, and delete
+	// the .partial file afterward (all while holding the lock).
+	//
+	// Before Go 1.16, we extracted to a temporary directory with a random name
+	// then renamed it into place with os.Rename. On Windows, this failed with
+	// ERROR_ACCESS_DENIED when another process (usually an anti-virus scanner)
+	// opened files in the temporary directory.
+	//
+	// Go 1.14.2 and higher respect .partial files. Older versions may use
+	// partially extracted directories. 'go mod verify' can detect this,
+	// and 'go clean -modcache' can fix it.
+	if err := os.MkdirAll(parentDir, 0777); err != nil {
+		return "", err
+	}
+	if err := os.WriteFile(partialPath, nil, 0666); err != nil {
+		return "", err
+	}
+	if err := modzip.Unzip(dir, mod, zipfile); err != nil {
+		fmt.Fprintf(os.Stderr, "-> %s\n", err)
+		if rmErr := RemoveAll(dir); rmErr == nil {
+			os.Remove(partialPath)
+		}
+		return "", err
+	}
+	if err := os.Remove(partialPath); err != nil {
+		return "", err
+	}
+
+	if !cfg.ModCacheRW {
+		makeDirsReadOnly(dir)
+	}
+	return dir, nil
+}
+
+var downloadZipCache par.ErrCache[module.Version, string]
+
+// DownloadZip downloads the specific module version to the
+// local zip cache and returns the name of the zip file.
+func DownloadZip(ctx context.Context, mod module.Version) (zipfile string, err error) {
+	// The par.Cache here avoids duplicate work.
+	return downloadZipCache.Do(mod, func() (string, error) {
+		zipfile, err := CachePath(ctx, mod, "zip")
+		if err != nil {
+			return "", err
+		}
+		ziphashfile := zipfile + "hash"
+
+		// Return without locking if the zip and ziphash files exist.
+		if _, err := os.Stat(zipfile); err == nil {
+			if _, err := os.Stat(ziphashfile); err == nil {
+				return zipfile, nil
+			}
+		}
+
+		// The zip or ziphash file does not exist. Acquire the lock and create them.
+		if cfg.CmdName != "mod download" {
+			vers := mod.Version
+			if mod.Path == "golang.org/toolchain" {
+				// Shorten v0.0.1-go1.13.1.darwin-amd64 to go1.13.1.darwin-amd64
+				_, vers, _ = strings.Cut(vers, "-")
+				if i := strings.LastIndex(vers, "."); i >= 0 {
+					goos, goarch, _ := strings.Cut(vers[i+1:], "-")
+					vers = vers[:i] + " (" + goos + "/" + goarch + ")"
+				}
+				fmt.Fprintf(os.Stderr, "go: downloading %s\n", vers)
+			} else {
+				fmt.Fprintf(os.Stderr, "go: downloading %s %s\n", mod.Path, vers)
+			}
+		}
+		unlock, err := lockVersion(ctx, mod)
+		if err != nil {
+			return "", err
+		}
+		defer unlock()
+
+		if err := downloadZip(ctx, mod, zipfile); err != nil {
+			return "", err
+		}
+		return zipfile, nil
+	})
+}
+
+func downloadZip(ctx context.Context, mod module.Version, zipfile string) (err error) {
+	ctx, span := trace.StartSpan(ctx, "modfetch.downloadZip "+zipfile)
+	defer span.Done()
+
+	// Double-check that the zipfile was not created while we were waiting for
+	// the lock in DownloadZip.
+	ziphashfile := zipfile + "hash"
+	var zipExists, ziphashExists bool
+	if _, err := os.Stat(zipfile); err == nil {
+		zipExists = true
+	}
+	if _, err := os.Stat(ziphashfile); err == nil {
+		ziphashExists = true
+	}
+	if zipExists && ziphashExists {
+		return nil
+	}
+
+	// Create parent directories.
+	if err := os.MkdirAll(filepath.Dir(zipfile), 0777); err != nil {
+		return err
+	}
+
+	// Clean up any remaining tempfiles from previous runs.
+	// This is only safe to do because the lock file ensures that their
+	// writers are no longer active.
+	tmpPattern := filepath.Base(zipfile) + "*.tmp"
+	if old, err := filepath.Glob(filepath.Join(str.QuoteGlob(filepath.Dir(zipfile)), tmpPattern)); err == nil {
+		for _, path := range old {
+			os.Remove(path) // best effort
+		}
+	}
+
+	// If the zip file exists, the ziphash file must have been deleted
+	// or lost after a file system crash. Re-hash the zip without downloading.
+	if zipExists {
+		return hashZip(mod, zipfile, ziphashfile)
+	}
+
+	// From here to the os.Rename call below is functionally almost equivalent to
+	// renameio.WriteToFile, with one key difference: we want to validate the
+	// contents of the file (by hashing it) before we commit it. Because the file
+	// is zip-compressed, we need an actual file — or at least an io.ReaderAt — to
+	// validate it: we can't just tee the stream as we write it.
+	f, err := tempFile(ctx, filepath.Dir(zipfile), filepath.Base(zipfile), 0666)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if err != nil {
+			f.Close()
+			os.Remove(f.Name())
+		}
+	}()
+
+	var unrecoverableErr error
+	err = TryProxies(func(proxy string) error {
+		if unrecoverableErr != nil {
+			return unrecoverableErr
+		}
+		repo := Lookup(ctx, proxy, mod.Path)
+		err := repo.Zip(ctx, f, mod.Version)
+		if err != nil {
+			// Zip may have partially written to f before failing.
+			// (Perhaps the server crashed while sending the file?)
+			// Since we allow fallback on error in some cases, we need to fix up the
+			// file to be empty again for the next attempt.
+			if _, err := f.Seek(0, io.SeekStart); err != nil {
+				unrecoverableErr = err
+				return err
+			}
+			if err := f.Truncate(0); err != nil {
+				unrecoverableErr = err
+				return err
+			}
+		}
+		return err
+	})
+	if err != nil {
+		return err
+	}
+
+	// Double-check that the paths within the zip file are well-formed.
+	//
+	// TODO(bcmills): There is a similar check within the Unzip function. Can we eliminate one?
+	fi, err := f.Stat()
+	if err != nil {
+		return err
+	}
+	z, err := zip.NewReader(f, fi.Size())
+	if err != nil {
+		return err
+	}
+	prefix := mod.Path + "@" + mod.Version + "/"
+	for _, f := range z.File {
+		if !strings.HasPrefix(f.Name, prefix) {
+			return fmt.Errorf("zip for %s has unexpected file %s", prefix[:len(prefix)-1], f.Name)
+		}
+	}
+
+	if err := f.Close(); err != nil {
+		return err
+	}
+
+	// Hash the zip file and check the sum before renaming to the final location.
+	if err := hashZip(mod, f.Name(), ziphashfile); err != nil {
+		return err
+	}
+	if err := os.Rename(f.Name(), zipfile); err != nil {
+		return err
+	}
+
+	// TODO(bcmills): Should we make the .zip and .ziphash files read-only to discourage tampering?
+
+	return nil
+}
+
+// hashZip reads the zip file opened in f, then writes the hash to ziphashfile,
+// overwriting that file if it exists.
+//
+// If the hash does not match go.sum (or the sumdb if enabled), hashZip returns
+// an error and does not write ziphashfile.
+func hashZip(mod module.Version, zipfile, ziphashfile string) (err error) {
+	hash, err := dirhash.HashZip(zipfile, dirhash.DefaultHash)
+	if err != nil {
+		return err
+	}
+	if err := checkModSum(mod, hash); err != nil {
+		return err
+	}
+	hf, err := lockedfile.Create(ziphashfile)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if closeErr := hf.Close(); err == nil && closeErr != nil {
+			err = closeErr
+		}
+	}()
+	if err := hf.Truncate(int64(len(hash))); err != nil {
+		return err
+	}
+	if _, err := hf.WriteAt([]byte(hash), 0); err != nil {
+		return err
+	}
+	return nil
+}
+
+// makeDirsReadOnly makes a best-effort attempt to remove write permissions for dir
+// and its transitive contents.
+func makeDirsReadOnly(dir string) {
+	type pathMode struct {
+		path string
+		mode fs.FileMode
+	}
+	var dirs []pathMode // in lexical order
+	filepath.WalkDir(dir, func(path string, d fs.DirEntry, err error) error {
+		if err == nil && d.IsDir() {
+			info, err := d.Info()
+			if err == nil && info.Mode()&0222 != 0 {
+				dirs = append(dirs, pathMode{path, info.Mode()})
+			}
+		}
+		return nil
+	})
+
+	// Run over list backward to chmod children before parents.
+	for i := len(dirs) - 1; i >= 0; i-- {
+		os.Chmod(dirs[i].path, dirs[i].mode&^0222)
+	}
+}
+
+// RemoveAll removes a directory written by Download or Unzip, first applying
+// any permission changes needed to do so.
+func RemoveAll(dir string) error {
+	// Module cache has 0555 directories; make them writable in order to remove content.
+	filepath.WalkDir(dir, func(path string, info fs.DirEntry, err error) error {
+		if err != nil {
+			return nil // ignore errors walking in file system
+		}
+		if info.IsDir() {
+			os.Chmod(path, 0777)
+		}
+		return nil
+	})
+	return robustio.RemoveAll(dir)
+}
+
+var GoSumFile string             // path to go.sum; set by package modload
+var WorkspaceGoSumFiles []string // path to module go.sums in workspace; set by package modload
+
+type modSum struct {
+	mod module.Version
+	sum string
+}
+
+var goSum struct {
+	mu        sync.Mutex
+	m         map[module.Version][]string            // content of go.sum file
+	w         map[string]map[module.Version][]string // sum file in workspace -> content of that sum file
+	status    map[modSum]modSumStatus                // state of sums in m
+	overwrite bool                                   // if true, overwrite go.sum without incorporating its contents
+	enabled   bool                                   // whether to use go.sum at all
+}
+
+type modSumStatus struct {
+	used, dirty bool
+}
+
+// Reset resets globals in the modfetch package, so previous loads don't affect
+// contents of go.sum files.
+func Reset() {
+	GoSumFile = ""
+	WorkspaceGoSumFiles = nil
+
+	// Uses of lookupCache and downloadCache both can call checkModSum,
+	// which in turn sets the used bit on goSum.status for modules.
+	// Reset them so used can be computed properly.
+	lookupCache = par.Cache[lookupCacheKey, Repo]{}
+	downloadCache = par.ErrCache[module.Version, string]{}
+
+	// Clear all fields on goSum. It will be initialized later
+	goSum.mu.Lock()
+	goSum.m = nil
+	goSum.w = nil
+	goSum.status = nil
+	goSum.overwrite = false
+	goSum.enabled = false
+	goSum.mu.Unlock()
+}
+
+// initGoSum initializes the go.sum data.
+// The boolean it returns reports whether the
+// use of go.sum is now enabled.
+// The goSum lock must be held.
+func initGoSum() (bool, error) {
+	if GoSumFile == "" {
+		return false, nil
+	}
+	if goSum.m != nil {
+		return true, nil
+	}
+
+	goSum.m = make(map[module.Version][]string)
+	goSum.status = make(map[modSum]modSumStatus)
+	goSum.w = make(map[string]map[module.Version][]string)
+
+	for _, f := range WorkspaceGoSumFiles {
+		goSum.w[f] = make(map[module.Version][]string)
+		_, err := readGoSumFile(goSum.w[f], f)
+		if err != nil {
+			return false, err
+		}
+	}
+
+	enabled, err := readGoSumFile(goSum.m, GoSumFile)
+	goSum.enabled = enabled
+	return enabled, err
+}
+
+func readGoSumFile(dst map[module.Version][]string, file string) (bool, error) {
+	var (
+		data []byte
+		err  error
+	)
+	if actualSumFile, ok := fsys.OverlayPath(file); ok {
+		// Don't lock go.sum if it's part of the overlay.
+		// On Plan 9, locking requires chmod, and we don't want to modify any file
+		// in the overlay. See #44700.
+		data, err = os.ReadFile(actualSumFile)
+	} else {
+		data, err = lockedfile.Read(file)
+	}
+	if err != nil && !os.IsNotExist(err) {
+		return false, err
+	}
+	readGoSum(dst, file, data)
+
+	return true, nil
+}
+
+// emptyGoModHash is the hash of a 1-file tree containing a 0-length go.mod.
+// A bug caused us to write these into go.sum files for non-modules.
+// We detect and remove them.
+const emptyGoModHash = "h1:G7mAYYxgmS0lVkHyy2hEOLQCFB0DlQFTMLWggykrydY="
+
+// readGoSum parses data, which is the content of file,
+// and adds it to goSum.m. The goSum lock must be held.
+func readGoSum(dst map[module.Version][]string, file string, data []byte) error {
+	lineno := 0
+	for len(data) > 0 {
+		var line []byte
+		lineno++
+		i := bytes.IndexByte(data, '\n')
+		if i < 0 {
+			line, data = data, nil
+		} else {
+			line, data = data[:i], data[i+1:]
+		}
+		f := strings.Fields(string(line))
+		if len(f) == 0 {
+			// blank line; skip it
+			continue
+		}
+		if len(f) != 3 {
+			return fmt.Errorf("malformed go.sum:\n%s:%d: wrong number of fields %v", file, lineno, len(f))
+		}
+		if f[2] == emptyGoModHash {
+			// Old bug; drop it.
+			continue
+		}
+		mod := module.Version{Path: f[0], Version: f[1]}
+		dst[mod] = append(dst[mod], f[2])
+	}
+	return nil
+}
+
+// HaveSum returns true if the go.sum file contains an entry for mod.
+// The entry's hash must be generated with a known hash algorithm.
+// mod.Version may have a "/go.mod" suffix to distinguish sums for
+// .mod and .zip files.
+func HaveSum(mod module.Version) bool {
+	goSum.mu.Lock()
+	defer goSum.mu.Unlock()
+	inited, err := initGoSum()
+	if err != nil || !inited {
+		return false
+	}
+	for _, goSums := range goSum.w {
+		for _, h := range goSums[mod] {
+			if !strings.HasPrefix(h, "h1:") {
+				continue
+			}
+			if !goSum.status[modSum{mod, h}].dirty {
+				return true
+			}
+		}
+	}
+	for _, h := range goSum.m[mod] {
+		if !strings.HasPrefix(h, "h1:") {
+			continue
+		}
+		if !goSum.status[modSum{mod, h}].dirty {
+			return true
+		}
+	}
+	return false
+}
+
+// checkMod checks the given module's checksum and Go version.
+func checkMod(ctx context.Context, mod module.Version) {
+	// Do the file I/O before acquiring the go.sum lock.
+	ziphash, err := CachePath(ctx, mod, "ziphash")
+	if err != nil {
+		base.Fatalf("verifying %v", module.VersionError(mod, err))
+	}
+	data, err := lockedfile.Read(ziphash)
+	if err != nil {
+		base.Fatalf("verifying %v", module.VersionError(mod, err))
+	}
+	data = bytes.TrimSpace(data)
+	if !isValidSum(data) {
+		// Recreate ziphash file from zip file and use that to check the mod sum.
+		zip, err := CachePath(ctx, mod, "zip")
+		if err != nil {
+			base.Fatalf("verifying %v", module.VersionError(mod, err))
+		}
+		err = hashZip(mod, zip, ziphash)
+		if err != nil {
+			base.Fatalf("verifying %v", module.VersionError(mod, err))
+		}
+		return
+	}
+	h := string(data)
+	if !strings.HasPrefix(h, "h1:") {
+		base.Fatalf("verifying %v", module.VersionError(mod, fmt.Errorf("unexpected ziphash: %q", h)))
+	}
+
+	if err := checkModSum(mod, h); err != nil {
+		base.Fatalf("%s", err)
+	}
+}
+
+// goModSum returns the checksum for the go.mod contents.
+func goModSum(data []byte) (string, error) {
+	return dirhash.Hash1([]string{"go.mod"}, func(string) (io.ReadCloser, error) {
+		return io.NopCloser(bytes.NewReader(data)), nil
+	})
+}
+
+// checkGoMod checks the given module's go.mod checksum;
+// data is the go.mod content.
+func checkGoMod(path, version string, data []byte) error {
+	h, err := goModSum(data)
+	if err != nil {
+		return &module.ModuleError{Path: path, Version: version, Err: fmt.Errorf("verifying go.mod: %v", err)}
+	}
+
+	return checkModSum(module.Version{Path: path, Version: version + "/go.mod"}, h)
+}
+
+// checkModSum checks that the recorded checksum for mod is h.
+//
+// mod.Version may have the additional suffix "/go.mod" to request the checksum
+// for the module's go.mod file only.
+func checkModSum(mod module.Version, h string) error {
+	// We lock goSum when manipulating it,
+	// but we arrange to release the lock when calling checkSumDB,
+	// so that parallel calls to checkModHash can execute parallel calls
+	// to checkSumDB.
+
+	// Check whether mod+h is listed in go.sum already. If so, we're done.
+	goSum.mu.Lock()
+	inited, err := initGoSum()
+	if err != nil {
+		goSum.mu.Unlock()
+		return err
+	}
+	done := inited && haveModSumLocked(mod, h)
+	if inited {
+		st := goSum.status[modSum{mod, h}]
+		st.used = true
+		goSum.status[modSum{mod, h}] = st
+	}
+	goSum.mu.Unlock()
+
+	if done {
+		return nil
+	}
+
+	// Not listed, so we want to add them.
+	// Consult checksum database if appropriate.
+	if useSumDB(mod) {
+		// Calls base.Fatalf if mismatch detected.
+		if err := checkSumDB(mod, h); err != nil {
+			return err
+		}
+	}
+
+	// Add mod+h to go.sum, if it hasn't appeared already.
+	if inited {
+		goSum.mu.Lock()
+		addModSumLocked(mod, h)
+		st := goSum.status[modSum{mod, h}]
+		st.dirty = true
+		goSum.status[modSum{mod, h}] = st
+		goSum.mu.Unlock()
+	}
+	return nil
+}
+
+// haveModSumLocked reports whether the pair mod,h is already listed in go.sum.
+// If it finds a conflicting pair instead, it calls base.Fatalf.
+// goSum.mu must be locked.
+func haveModSumLocked(mod module.Version, h string) bool {
+	sumFileName := "go.sum"
+	if strings.HasSuffix(GoSumFile, "go.work.sum") {
+		sumFileName = "go.work.sum"
+	}
+	for _, vh := range goSum.m[mod] {
+		if h == vh {
+			return true
+		}
+		if strings.HasPrefix(vh, "h1:") {
+			base.Fatalf("verifying %s@%s: checksum mismatch\n\tdownloaded: %v\n\t%s:     %v"+goSumMismatch, mod.Path, mod.Version, h, sumFileName, vh)
+		}
+	}
+	// Also check workspace sums.
+	foundMatch := false
+	// Check sums from all files in case there are conflicts between
+	// the files.
+	for goSumFile, goSums := range goSum.w {
+		for _, vh := range goSums[mod] {
+			if h == vh {
+				foundMatch = true
+			} else if strings.HasPrefix(vh, "h1:") {
+				base.Fatalf("verifying %s@%s: checksum mismatch\n\tdownloaded: %v\n\t%s:     %v"+goSumMismatch, mod.Path, mod.Version, h, goSumFile, vh)
+			}
+		}
+	}
+	return foundMatch
+}
+
+// addModSumLocked adds the pair mod,h to go.sum.
+// goSum.mu must be locked.
+func addModSumLocked(mod module.Version, h string) {
+	if haveModSumLocked(mod, h) {
+		return
+	}
+	if len(goSum.m[mod]) > 0 {
+		fmt.Fprintf(os.Stderr, "warning: verifying %s@%s: unknown hashes in go.sum: %v; adding %v"+hashVersionMismatch, mod.Path, mod.Version, strings.Join(goSum.m[mod], ", "), h)
+	}
+	goSum.m[mod] = append(goSum.m[mod], h)
+}
+
+// checkSumDB checks the mod, h pair against the Go checksum database.
+// It calls base.Fatalf if the hash is to be rejected.
+func checkSumDB(mod module.Version, h string) error {
+	modWithoutSuffix := mod
+	noun := "module"
+	if before, found := strings.CutSuffix(mod.Version, "/go.mod"); found {
+		noun = "go.mod"
+		modWithoutSuffix.Version = before
+	}
+
+	db, lines, err := lookupSumDB(mod)
+	if err != nil {
+		return module.VersionError(modWithoutSuffix, fmt.Errorf("verifying %s: %v", noun, err))
+	}
+
+	have := mod.Path + " " + mod.Version + " " + h
+	prefix := mod.Path + " " + mod.Version + " h1:"
+	for _, line := range lines {
+		if line == have {
+			return nil
+		}
+		if strings.HasPrefix(line, prefix) {
+			return module.VersionError(modWithoutSuffix, fmt.Errorf("verifying %s: checksum mismatch\n\tdownloaded: %v\n\t%s: %v"+sumdbMismatch, noun, h, db, line[len(prefix)-len("h1:"):]))
+		}
+	}
+	return nil
+}
+
+// Sum returns the checksum for the downloaded copy of the given module,
+// if present in the download cache.
+func Sum(ctx context.Context, mod module.Version) string {
+	if cfg.GOMODCACHE == "" {
+		// Do not use current directory.
+		return ""
+	}
+
+	ziphash, err := CachePath(ctx, mod, "ziphash")
+	if err != nil {
+		return ""
+	}
+	data, err := lockedfile.Read(ziphash)
+	if err != nil {
+		return ""
+	}
+	data = bytes.TrimSpace(data)
+	if !isValidSum(data) {
+		return ""
+	}
+	return string(data)
+}
+
+// isValidSum returns true if data is the valid contents of a zip hash file.
+// Certain critical files are written to disk by first truncating
+// then writing the actual bytes, so that if the write fails
+// the corrupt file should contain at least one of the null
+// bytes written by the truncate operation.
+func isValidSum(data []byte) bool {
+	if bytes.IndexByte(data, '\000') >= 0 {
+		return false
+	}
+
+	if len(data) != len("h1:")+base64.StdEncoding.EncodedLen(sha256.Size) {
+		return false
+	}
+
+	return true
+}
+
+var ErrGoSumDirty = errors.New("updates to go.sum needed, disabled by -mod=readonly")
+
+// WriteGoSum writes the go.sum file if it needs to be updated.
+//
+// keep is used to check whether a newly added sum should be saved in go.sum.
+// It should have entries for both module content sums and go.mod sums
+// (version ends with "/go.mod"). Existing sums will be preserved unless they
+// have been marked for deletion with TrimGoSum.
+func WriteGoSum(ctx context.Context, keep map[module.Version]bool, readonly bool) error {
+	goSum.mu.Lock()
+	defer goSum.mu.Unlock()
+
+	// If we haven't read the go.sum file yet, don't bother writing it.
+	if !goSum.enabled {
+		return nil
+	}
+
+	// Check whether we need to add sums for which keep[m] is true or remove
+	// unused sums marked with TrimGoSum. If there are no changes to make,
+	// just return without opening go.sum.
+	dirty := false
+Outer:
+	for m, hs := range goSum.m {
+		for _, h := range hs {
+			st := goSum.status[modSum{m, h}]
+			if st.dirty && (!st.used || keep[m]) {
+				dirty = true
+				break Outer
+			}
+		}
+	}
+	if !dirty {
+		return nil
+	}
+	if readonly {
+		return ErrGoSumDirty
+	}
+	if _, ok := fsys.OverlayPath(GoSumFile); ok {
+		base.Fatalf("go: updates to go.sum needed, but go.sum is part of the overlay specified with -overlay")
+	}
+
+	// Make a best-effort attempt to acquire the side lock, only to exclude
+	// previous versions of the 'go' command from making simultaneous edits.
+	if unlock, err := SideLock(ctx); err == nil {
+		defer unlock()
+	}
+
+	err := lockedfile.Transform(GoSumFile, func(data []byte) ([]byte, error) {
+		if !goSum.overwrite {
+			// Incorporate any sums added by other processes in the meantime.
+			// Add only the sums that we actually checked: the user may have edited or
+			// truncated the file to remove erroneous hashes, and we shouldn't restore
+			// them without good reason.
+			goSum.m = make(map[module.Version][]string, len(goSum.m))
+			readGoSum(goSum.m, GoSumFile, data)
+			for ms, st := range goSum.status {
+				if st.used && !sumInWorkspaceModulesLocked(ms.mod) {
+					addModSumLocked(ms.mod, ms.sum)
+				}
+			}
+		}
+
+		var mods []module.Version
+		for m := range goSum.m {
+			mods = append(mods, m)
+		}
+		module.Sort(mods)
+
+		var buf bytes.Buffer
+		for _, m := range mods {
+			list := goSum.m[m]
+			sort.Strings(list)
+			str.Uniq(&list)
+			for _, h := range list {
+				st := goSum.status[modSum{m, h}]
+				if (!st.dirty || (st.used && keep[m])) && !sumInWorkspaceModulesLocked(m) {
+					fmt.Fprintf(&buf, "%s %s %s\n", m.Path, m.Version, h)
+				}
+			}
+		}
+		return buf.Bytes(), nil
+	})
+
+	if err != nil {
+		return fmt.Errorf("updating go.sum: %w", err)
+	}
+
+	goSum.status = make(map[modSum]modSumStatus)
+	goSum.overwrite = false
+	return nil
+}
+
+func sumInWorkspaceModulesLocked(m module.Version) bool {
+	for _, goSums := range goSum.w {
+		if _, ok := goSums[m]; ok {
+			return true
+		}
+	}
+	return false
+}
+
+// TrimGoSum trims go.sum to contain only the modules needed for reproducible
+// builds.
+//
+// keep is used to check whether a sum should be retained in go.mod. It should
+// have entries for both module content sums and go.mod sums (version ends
+// with "/go.mod").
+func TrimGoSum(keep map[module.Version]bool) {
+	goSum.mu.Lock()
+	defer goSum.mu.Unlock()
+	inited, err := initGoSum()
+	if err != nil {
+		base.Fatalf("%s", err)
+	}
+	if !inited {
+		return
+	}
+
+	for m, hs := range goSum.m {
+		if !keep[m] {
+			for _, h := range hs {
+				goSum.status[modSum{m, h}] = modSumStatus{used: false, dirty: true}
+			}
+			goSum.overwrite = true
+		}
+	}
+}
+
+const goSumMismatch = `
+
+SECURITY ERROR
+This download does NOT match an earlier download recorded in go.sum.
+The bits may have been replaced on the origin server, or an attacker may
+have intercepted the download attempt.
+
+For more information, see 'go help module-auth'.
+`
+
+const sumdbMismatch = `
+
+SECURITY ERROR
+This download does NOT match the one reported by the checksum server.
+The bits may have been replaced on the origin server, or an attacker may
+have intercepted the download attempt.
+
+For more information, see 'go help module-auth'.
+`
+
+const hashVersionMismatch = `
+
+SECURITY WARNING
+This download is listed in go.sum, but using an unknown hash algorithm.
+The download cannot be verified.
+
+For more information, see 'go help module-auth'.
+
+`
+
+var HelpModuleAuth = &base.Command{
+	UsageLine: "module-auth",
+	Short:     "module authentication using go.sum",
+	Long: `
+When the go command downloads a module zip file or go.mod file into the
+module cache, it computes a cryptographic hash and compares it with a known
+value to verify the file hasn't changed since it was first downloaded. Known
+hashes are stored in a file in the module root directory named go.sum. Hashes
+may also be downloaded from the checksum database depending on the values of
+GOSUMDB, GOPRIVATE, and GONOSUMDB.
+
+For details, see https://golang.org/ref/mod#authenticating.
+`,
+}
+
+var HelpPrivate = &base.Command{
+	UsageLine: "private",
+	Short:     "configuration for downloading non-public code",
+	Long: `
+The go command defaults to downloading modules from the public Go module
+mirror at proxy.golang.org. It also defaults to validating downloaded modules,
+regardless of source, against the public Go checksum database at sum.golang.org.
+These defaults work well for publicly available source code.
+
+The GOPRIVATE environment variable controls which modules the go command
+considers to be private (not available publicly) and should therefore not use
+the proxy or checksum database. The variable is a comma-separated list of
+glob patterns (in the syntax of Go's path.Match) of module path prefixes.
+For example,
+
+	GOPRIVATE=*.corp.example.com,rsc.io/private
+
+causes the go command to treat as private any module with a path prefix
+matching either pattern, including git.corp.example.com/xyzzy, rsc.io/private,
+and rsc.io/private/quux.
+
+For fine-grained control over module download and validation, the GONOPROXY
+and GONOSUMDB environment variables accept the same kind of glob list
+and override GOPRIVATE for the specific decision of whether to use the proxy
+and checksum database, respectively.
+
+For example, if a company ran a module proxy serving private modules,
+users would configure go using:
+
+	GOPRIVATE=*.corp.example.com
+	GOPROXY=proxy.example.com
+	GONOPROXY=none
+
+The GOPRIVATE variable is also used to define the "public" and "private"
+patterns for the GOVCS variable; see 'go help vcs'. For that usage,
+GOPRIVATE applies even in GOPATH mode. In that case, it matches import paths
+instead of module paths.
+
+The 'go env -w' command (see 'go help env') can be used to set these variables
+for future go command invocations.
+
+For more details, see https://golang.org/ref/mod#private-modules.
+`,
+}
diff --git a/src/cmd/go/internal/modfetch/key.go b/src/cmd/go/internal/modfetch/key.go
new file mode 100644
index 0000000..06f9989
--- /dev/null
+++ b/src/cmd/go/internal/modfetch/key.go
@@ -0,0 +1,9 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modfetch
+
+var knownGOSUMDB = map[string]string{
+	"sum.golang.org": "sum.golang.org+033de0ae+Ac4zctda0e5eza+HJyk9SxEdh+s3Ux18htTTAD8OuAn8",
+}
diff --git a/src/cmd/go/internal/modfetch/proxy.go b/src/cmd/go/internal/modfetch/proxy.go
new file mode 100644
index 0000000..dd37ba9
--- /dev/null
+++ b/src/cmd/go/internal/modfetch/proxy.go
@@ -0,0 +1,449 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modfetch
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"io/fs"
+	"net/url"
+	"path"
+	pathpkg "path"
+	"path/filepath"
+	"strings"
+	"sync"
+	"time"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/modfetch/codehost"
+	"cmd/go/internal/web"
+
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
+)
+
+var HelpGoproxy = &base.Command{
+	UsageLine: "goproxy",
+	Short:     "module proxy protocol",
+	Long: `
+A Go module proxy is any web server that can respond to GET requests for
+URLs of a specified form. The requests have no query parameters, so even
+a site serving from a fixed file system (including a file:/// URL)
+can be a module proxy.
+
+For details on the GOPROXY protocol, see
+https://golang.org/ref/mod#goproxy-protocol.
+`,
+}
+
+var proxyOnce struct {
+	sync.Once
+	list []proxySpec
+	err  error
+}
+
+type proxySpec struct {
+	// url is the proxy URL or one of "off", "direct", "noproxy".
+	url string
+
+	// fallBackOnError is true if a request should be attempted on the next proxy
+	// in the list after any error from this proxy. If fallBackOnError is false,
+	// the request will only be attempted on the next proxy if the error is
+	// equivalent to os.ErrNotFound, which is true for 404 and 410 responses.
+	fallBackOnError bool
+}
+
+func proxyList() ([]proxySpec, error) {
+	proxyOnce.Do(func() {
+		if cfg.GONOPROXY != "" && cfg.GOPROXY != "direct" {
+			proxyOnce.list = append(proxyOnce.list, proxySpec{url: "noproxy"})
+		}
+
+		goproxy := cfg.GOPROXY
+		for goproxy != "" {
+			var url string
+			fallBackOnError := false
+			if i := strings.IndexAny(goproxy, ",|"); i >= 0 {
+				url = goproxy[:i]
+				fallBackOnError = goproxy[i] == '|'
+				goproxy = goproxy[i+1:]
+			} else {
+				url = goproxy
+				goproxy = ""
+			}
+
+			url = strings.TrimSpace(url)
+			if url == "" {
+				continue
+			}
+			if url == "off" {
+				// "off" always fails hard, so can stop walking list.
+				proxyOnce.list = append(proxyOnce.list, proxySpec{url: "off"})
+				break
+			}
+			if url == "direct" {
+				proxyOnce.list = append(proxyOnce.list, proxySpec{url: "direct"})
+				// For now, "direct" is the end of the line. We may decide to add some
+				// sort of fallback behavior for them in the future, so ignore
+				// subsequent entries for forward-compatibility.
+				break
+			}
+
+			// Single-word tokens are reserved for built-in behaviors, and anything
+			// containing the string ":/" or matching an absolute file path must be a
+			// complete URL. For all other paths, implicitly add "https://".
+			if strings.ContainsAny(url, ".:/") && !strings.Contains(url, ":/") && !filepath.IsAbs(url) && !path.IsAbs(url) {
+				url = "https://" + url
+			}
+
+			// Check that newProxyRepo accepts the URL.
+			// It won't do anything with the path.
+			if _, err := newProxyRepo(url, "golang.org/x/text"); err != nil {
+				proxyOnce.err = err
+				return
+			}
+
+			proxyOnce.list = append(proxyOnce.list, proxySpec{
+				url:             url,
+				fallBackOnError: fallBackOnError,
+			})
+		}
+
+		if len(proxyOnce.list) == 0 ||
+			len(proxyOnce.list) == 1 && proxyOnce.list[0].url == "noproxy" {
+			// There were no proxies, other than the implicit "noproxy" added when
+			// GONOPROXY is set. This can happen if GOPROXY is a non-empty string
+			// like "," or " ".
+			proxyOnce.err = fmt.Errorf("GOPROXY list is not the empty string, but contains no entries")
+		}
+	})
+
+	return proxyOnce.list, proxyOnce.err
+}
+
+// TryProxies iterates f over each configured proxy (including "noproxy" and
+// "direct" if applicable) until f returns no error or until f returns an
+// error that is not equivalent to fs.ErrNotExist on a proxy configured
+// not to fall back on errors.
+//
+// TryProxies then returns that final error.
+//
+// If GOPROXY is set to "off", TryProxies invokes f once with the argument
+// "off".
+func TryProxies(f func(proxy string) error) error {
+	proxies, err := proxyList()
+	if err != nil {
+		return err
+	}
+	if len(proxies) == 0 {
+		panic("GOPROXY list is empty")
+	}
+
+	// We try to report the most helpful error to the user. "direct" and "noproxy"
+	// errors are best, followed by proxy errors other than ErrNotExist, followed
+	// by ErrNotExist.
+	//
+	// Note that errProxyOff, errNoproxy, and errUseProxy are equivalent to
+	// ErrNotExist. errUseProxy should only be returned if "noproxy" is the only
+	// proxy. errNoproxy should never be returned, since there should always be a
+	// more useful error from "noproxy" first.
+	const (
+		notExistRank = iota
+		proxyRank
+		directRank
+	)
+	var bestErr error
+	bestErrRank := notExistRank
+	for _, proxy := range proxies {
+		err := f(proxy.url)
+		if err == nil {
+			return nil
+		}
+		isNotExistErr := errors.Is(err, fs.ErrNotExist)
+
+		if proxy.url == "direct" || (proxy.url == "noproxy" && err != errUseProxy) {
+			bestErr = err
+			bestErrRank = directRank
+		} else if bestErrRank <= proxyRank && !isNotExistErr {
+			bestErr = err
+			bestErrRank = proxyRank
+		} else if bestErrRank == notExistRank {
+			bestErr = err
+		}
+
+		if !proxy.fallBackOnError && !isNotExistErr {
+			break
+		}
+	}
+	return bestErr
+}
+
+type proxyRepo struct {
+	url         *url.URL
+	path        string
+	redactedURL string
+
+	listLatestOnce sync.Once
+	listLatest     *RevInfo
+	listLatestErr  error
+}
+
+func newProxyRepo(baseURL, path string) (Repo, error) {
+	base, err := url.Parse(baseURL)
+	if err != nil {
+		return nil, err
+	}
+	switch base.Scheme {
+	case "http", "https":
+		// ok
+	case "file":
+		if *base != (url.URL{Scheme: base.Scheme, Path: base.Path, RawPath: base.RawPath}) {
+			return nil, fmt.Errorf("invalid file:// proxy URL with non-path elements: %s", base.Redacted())
+		}
+	case "":
+		return nil, fmt.Errorf("invalid proxy URL missing scheme: %s", base.Redacted())
+	default:
+		return nil, fmt.Errorf("invalid proxy URL scheme (must be https, http, file): %s", base.Redacted())
+	}
+
+	enc, err := module.EscapePath(path)
+	if err != nil {
+		return nil, err
+	}
+	redactedURL := base.Redacted()
+	base.Path = strings.TrimSuffix(base.Path, "/") + "/" + enc
+	base.RawPath = strings.TrimSuffix(base.RawPath, "/") + "/" + pathEscape(enc)
+	return &proxyRepo{base, path, redactedURL, sync.Once{}, nil, nil}, nil
+}
+
+func (p *proxyRepo) ModulePath() string {
+	return p.path
+}
+
+var errProxyReuse = fmt.Errorf("proxy does not support CheckReuse")
+
+func (p *proxyRepo) CheckReuse(ctx context.Context, old *codehost.Origin) error {
+	return errProxyReuse
+}
+
+// versionError returns err wrapped in a ModuleError for p.path.
+func (p *proxyRepo) versionError(version string, err error) error {
+	if version != "" && version != module.CanonicalVersion(version) {
+		return &module.ModuleError{
+			Path: p.path,
+			Err: &module.InvalidVersionError{
+				Version: version,
+				Pseudo:  module.IsPseudoVersion(version),
+				Err:     err,
+			},
+		}
+	}
+
+	return &module.ModuleError{
+		Path:    p.path,
+		Version: version,
+		Err:     err,
+	}
+}
+
+func (p *proxyRepo) getBytes(ctx context.Context, path string) ([]byte, error) {
+	body, err := p.getBody(ctx, path)
+	if err != nil {
+		return nil, err
+	}
+	defer body.Close()
+
+	b, err := io.ReadAll(body)
+	if err != nil {
+		// net/http doesn't add context to Body errors, so add it here.
+		// (See https://go.dev/issue/52727.)
+		return b, &url.Error{Op: "read", URL: strings.TrimSuffix(p.redactedURL, "/") + "/" + path, Err: err}
+	}
+	return b, nil
+}
+
+func (p *proxyRepo) getBody(ctx context.Context, path string) (r io.ReadCloser, err error) {
+	fullPath := pathpkg.Join(p.url.Path, path)
+
+	target := *p.url
+	target.Path = fullPath
+	target.RawPath = pathpkg.Join(target.RawPath, pathEscape(path))
+
+	resp, err := web.Get(web.DefaultSecurity, &target)
+	if err != nil {
+		return nil, err
+	}
+	if err := resp.Err(); err != nil {
+		resp.Body.Close()
+		return nil, err
+	}
+	return resp.Body, nil
+}
+
+func (p *proxyRepo) Versions(ctx context.Context, prefix string) (*Versions, error) {
+	data, err := p.getBytes(ctx, "@v/list")
+	if err != nil {
+		p.listLatestOnce.Do(func() {
+			p.listLatest, p.listLatestErr = nil, p.versionError("", err)
+		})
+		return nil, p.versionError("", err)
+	}
+	var list []string
+	allLine := strings.Split(string(data), "\n")
+	for _, line := range allLine {
+		f := strings.Fields(line)
+		if len(f) >= 1 && semver.IsValid(f[0]) && strings.HasPrefix(f[0], prefix) && !module.IsPseudoVersion(f[0]) {
+			list = append(list, f[0])
+		}
+	}
+	p.listLatestOnce.Do(func() {
+		p.listLatest, p.listLatestErr = p.latestFromList(ctx, allLine)
+	})
+	semver.Sort(list)
+	return &Versions{List: list}, nil
+}
+
+func (p *proxyRepo) latest(ctx context.Context) (*RevInfo, error) {
+	p.listLatestOnce.Do(func() {
+		data, err := p.getBytes(ctx, "@v/list")
+		if err != nil {
+			p.listLatestErr = p.versionError("", err)
+			return
+		}
+		list := strings.Split(string(data), "\n")
+		p.listLatest, p.listLatestErr = p.latestFromList(ctx, list)
+	})
+	return p.listLatest, p.listLatestErr
+}
+
+func (p *proxyRepo) latestFromList(ctx context.Context, allLine []string) (*RevInfo, error) {
+	var (
+		bestTime    time.Time
+		bestVersion string
+	)
+	for _, line := range allLine {
+		f := strings.Fields(line)
+		if len(f) >= 1 && semver.IsValid(f[0]) {
+			// If the proxy includes timestamps, prefer the timestamp it reports.
+			// Otherwise, derive the timestamp from the pseudo-version.
+			var (
+				ft time.Time
+			)
+			if len(f) >= 2 {
+				ft, _ = time.Parse(time.RFC3339, f[1])
+			} else if module.IsPseudoVersion(f[0]) {
+				ft, _ = module.PseudoVersionTime(f[0])
+			} else {
+				// Repo.Latest promises that this method is only called where there are
+				// no tagged versions. Ignore any tagged versions that were added in the
+				// meantime.
+				continue
+			}
+			if bestTime.Before(ft) {
+				bestTime = ft
+				bestVersion = f[0]
+			}
+		}
+	}
+	if bestVersion == "" {
+		return nil, p.versionError("", codehost.ErrNoCommits)
+	}
+
+	// Call Stat to get all the other fields, including Origin information.
+	return p.Stat(ctx, bestVersion)
+}
+
+func (p *proxyRepo) Stat(ctx context.Context, rev string) (*RevInfo, error) {
+	encRev, err := module.EscapeVersion(rev)
+	if err != nil {
+		return nil, p.versionError(rev, err)
+	}
+	data, err := p.getBytes(ctx, "@v/"+encRev+".info")
+	if err != nil {
+		return nil, p.versionError(rev, err)
+	}
+	info := new(RevInfo)
+	if err := json.Unmarshal(data, info); err != nil {
+		return nil, p.versionError(rev, fmt.Errorf("invalid response from proxy %q: %w", p.redactedURL, err))
+	}
+	if info.Version != rev && rev == module.CanonicalVersion(rev) && module.Check(p.path, rev) == nil {
+		// If we request a correct, appropriate version for the module path, the
+		// proxy must return either exactly that version or an error — not some
+		// arbitrary other version.
+		return nil, p.versionError(rev, fmt.Errorf("proxy returned info for version %s instead of requested version", info.Version))
+	}
+	return info, nil
+}
+
+func (p *proxyRepo) Latest(ctx context.Context) (*RevInfo, error) {
+	data, err := p.getBytes(ctx, "@latest")
+	if err != nil {
+		if !errors.Is(err, fs.ErrNotExist) {
+			return nil, p.versionError("", err)
+		}
+		return p.latest(ctx)
+	}
+	info := new(RevInfo)
+	if err := json.Unmarshal(data, info); err != nil {
+		return nil, p.versionError("", fmt.Errorf("invalid response from proxy %q: %w", p.redactedURL, err))
+	}
+	return info, nil
+}
+
+func (p *proxyRepo) GoMod(ctx context.Context, version string) ([]byte, error) {
+	if version != module.CanonicalVersion(version) {
+		return nil, p.versionError(version, fmt.Errorf("internal error: version passed to GoMod is not canonical"))
+	}
+
+	encVer, err := module.EscapeVersion(version)
+	if err != nil {
+		return nil, p.versionError(version, err)
+	}
+	data, err := p.getBytes(ctx, "@v/"+encVer+".mod")
+	if err != nil {
+		return nil, p.versionError(version, err)
+	}
+	return data, nil
+}
+
+func (p *proxyRepo) Zip(ctx context.Context, dst io.Writer, version string) error {
+	if version != module.CanonicalVersion(version) {
+		return p.versionError(version, fmt.Errorf("internal error: version passed to Zip is not canonical"))
+	}
+
+	encVer, err := module.EscapeVersion(version)
+	if err != nil {
+		return p.versionError(version, err)
+	}
+	path := "@v/" + encVer + ".zip"
+	body, err := p.getBody(ctx, path)
+	if err != nil {
+		return p.versionError(version, err)
+	}
+	defer body.Close()
+
+	lr := &io.LimitedReader{R: body, N: codehost.MaxZipFile + 1}
+	if _, err := io.Copy(dst, lr); err != nil {
+		// net/http doesn't add context to Body errors, so add it here.
+		// (See https://go.dev/issue/52727.)
+		err = &url.Error{Op: "read", URL: pathpkg.Join(p.redactedURL, path), Err: err}
+		return p.versionError(version, err)
+	}
+	if lr.N <= 0 {
+		return p.versionError(version, fmt.Errorf("downloaded zip file too large"))
+	}
+	return nil
+}
+
+// pathEscape escapes s so it can be used in a path.
+// That is, it escapes things like ? and # (which really shouldn't appear anyway).
+// It does not escape / to %2F: our REST API is designed so that / can be left as is.
+func pathEscape(s string) string {
+	return strings.ReplaceAll(url.PathEscape(s), "%2F", "/")
+}
diff --git a/src/cmd/go/internal/modfetch/repo.go b/src/cmd/go/internal/modfetch/repo.go
new file mode 100644
index 0000000..25fb02d
--- /dev/null
+++ b/src/cmd/go/internal/modfetch/repo.go
@@ -0,0 +1,411 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modfetch
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"io/fs"
+	"os"
+	"strconv"
+	"time"
+
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/modfetch/codehost"
+	"cmd/go/internal/par"
+	"cmd/go/internal/vcs"
+	web "cmd/go/internal/web"
+
+	"golang.org/x/mod/module"
+)
+
+const traceRepo = false // trace all repo actions, for debugging
+
+// A Repo represents a repository storing all versions of a single module.
+// It must be safe for simultaneous use by multiple goroutines.
+type Repo interface {
+	// ModulePath returns the module path.
+	ModulePath() string
+
+	// CheckReuse checks whether the validation criteria in the origin
+	// are still satisfied on the server corresponding to this module.
+	// If so, the caller can reuse any cached Versions or RevInfo containing
+	// this origin rather than redownloading those from the server.
+	CheckReuse(ctx context.Context, old *codehost.Origin) error
+
+	// Versions lists all known versions with the given prefix.
+	// Pseudo-versions are not included.
+	//
+	// Versions should be returned sorted in semver order
+	// (implementations can use semver.Sort).
+	//
+	// Versions returns a non-nil error only if there was a problem
+	// fetching the list of versions: it may return an empty list
+	// along with a nil error if the list of matching versions
+	// is known to be empty.
+	//
+	// If the underlying repository does not exist,
+	// Versions returns an error matching errors.Is(_, os.NotExist).
+	Versions(ctx context.Context, prefix string) (*Versions, error)
+
+	// Stat returns information about the revision rev.
+	// A revision can be any identifier known to the underlying service:
+	// commit hash, branch, tag, and so on.
+	Stat(ctx context.Context, rev string) (*RevInfo, error)
+
+	// Latest returns the latest revision on the default branch,
+	// whatever that means in the underlying source code repository.
+	// It is only used when there are no tagged versions.
+	Latest(ctx context.Context) (*RevInfo, error)
+
+	// GoMod returns the go.mod file for the given version.
+	GoMod(ctx context.Context, version string) (data []byte, err error)
+
+	// Zip writes a zip file for the given version to dst.
+	Zip(ctx context.Context, dst io.Writer, version string) error
+}
+
+// A Versions describes the available versions in a module repository.
+type Versions struct {
+	Origin *codehost.Origin `json:",omitempty"` // origin information for reuse
+
+	List []string // semver versions
+}
+
+// A RevInfo describes a single revision in a module repository.
+type RevInfo struct {
+	Version string    // suggested version string for this revision
+	Time    time.Time // commit time
+
+	// These fields are used for Stat of arbitrary rev,
+	// but they are not recorded when talking about module versions.
+	Name  string `json:"-"` // complete ID in underlying repository
+	Short string `json:"-"` // shortened ID, for use in pseudo-version
+
+	Origin *codehost.Origin `json:",omitempty"` // provenance for reuse
+}
+
+// Re: module paths, import paths, repository roots, and lookups
+//
+// A module is a collection of Go packages stored in a file tree
+// with a go.mod file at the root of the tree.
+// The go.mod defines the module path, which is the import path
+// corresponding to the root of the file tree.
+// The import path of a directory within that file tree is the module path
+// joined with the name of the subdirectory relative to the root.
+//
+// For example, the module with path rsc.io/qr corresponds to the
+// file tree in the repository https://github.com/rsc/qr.
+// That file tree has a go.mod that says "module rsc.io/qr".
+// The package in the root directory has import path "rsc.io/qr".
+// The package in the gf256 subdirectory has import path "rsc.io/qr/gf256".
+// In this example, "rsc.io/qr" is both a module path and an import path.
+// But "rsc.io/qr/gf256" is only an import path, not a module path:
+// it names an importable package, but not a module.
+//
+// As a special case to incorporate code written before modules were
+// introduced, if a path p resolves using the pre-module "go get" lookup
+// to the root of a source code repository without a go.mod file,
+// that repository is treated as if it had a go.mod in its root directory
+// declaring module path p. (The go.mod is further considered to
+// contain requirements corresponding to any legacy version
+// tracking format such as Gopkg.lock, vendor/vendor.conf, and so on.)
+//
+// The presentation so far ignores the fact that a source code repository
+// has many different versions of a file tree, and those versions may
+// differ in whether a particular go.mod exists and what it contains.
+// In fact there is a well-defined mapping only from a module path, version
+// pair - often written path@version - to a particular file tree.
+// For example rsc.io/qr@v0.1.0 depends on the "implicit go.mod at root of
+// repository" rule, while rsc.io/qr@v0.2.0 has an explicit go.mod.
+// Because the "go get" import paths rsc.io/qr and github.com/rsc/qr
+// both redirect to the Git repository https://github.com/rsc/qr,
+// github.com/rsc/qr@v0.1.0 is the same file tree as rsc.io/qr@v0.1.0
+// but a different module (a different name). In contrast, since v0.2.0
+// of that repository has an explicit go.mod that declares path rsc.io/qr,
+// github.com/rsc/qr@v0.2.0 is an invalid module path, version pair.
+// Before modules, import comments would have had the same effect.
+//
+// The set of import paths associated with a given module path is
+// clearly not fixed: at the least, new directories with new import paths
+// can always be added. But another potential operation is to split a
+// subtree out of a module into its own module. If done carefully,
+// this operation can be done while preserving compatibility for clients.
+// For example, suppose that we want to split rsc.io/qr/gf256 into its
+// own module, so that there would be two modules rsc.io/qr and rsc.io/qr/gf256.
+// Then we can simultaneously issue rsc.io/qr v0.3.0 (dropping the gf256 subdirectory)
+// and rsc.io/qr/gf256 v0.1.0, including in their respective go.mod
+// cyclic requirements pointing at each other: rsc.io/qr v0.3.0 requires
+// rsc.io/qr/gf256 v0.1.0 and vice versa. Then a build can be
+// using an older rsc.io/qr module that includes the gf256 package, but if
+// it adds a requirement on either the newer rsc.io/qr or the newer
+// rsc.io/qr/gf256 module, it will automatically add the requirement
+// on the complementary half, ensuring both that rsc.io/qr/gf256 is
+// available for importing by the build and also that it is only defined
+// by a single module. The gf256 package could move back into the
+// original by another simultaneous release of rsc.io/qr v0.4.0 including
+// the gf256 subdirectory and an rsc.io/qr/gf256 v0.2.0 with no code
+// in its root directory, along with a new requirement cycle.
+// The ability to shift module boundaries in this way is expected to be
+// important in large-scale program refactorings, similar to the ones
+// described in https://talks.golang.org/2016/refactor.article.
+//
+// The possibility of shifting module boundaries reemphasizes
+// that you must know both the module path and its version
+// to determine the set of packages provided directly by that module.
+//
+// On top of all this, it is possible for a single code repository
+// to contain multiple modules, either in branches or subdirectories,
+// as a limited kind of monorepo. For example rsc.io/qr/v2,
+// the v2.x.x continuation of rsc.io/qr, is expected to be found
+// in v2-tagged commits in https://github.com/rsc/qr, either
+// in the root or in a v2 subdirectory, disambiguated by go.mod.
+// Again the precise file tree corresponding to a module
+// depends on which version we are considering.
+//
+// It is also possible for the underlying repository to change over time,
+// without changing the module path. If I copy the github repo over
+// to https://bitbucket.org/rsc/qr and update https://rsc.io/qr?go-get=1,
+// then clients of all versions should start fetching from bitbucket
+// instead of github. That is, in contrast to the exact file tree,
+// the location of the source code repository associated with a module path
+// does not depend on the module version. (This is by design, as the whole
+// point of these redirects is to allow package authors to establish a stable
+// name that can be updated as code moves from one service to another.)
+//
+// All of this is important background for the lookup APIs defined in this
+// file.
+//
+// The Lookup function takes a module path and returns a Repo representing
+// that module path. Lookup can do only a little with the path alone.
+// It can check that the path is well-formed (see semver.CheckPath)
+// and it can check that the path can be resolved to a target repository.
+// To avoid version control access except when absolutely necessary,
+// Lookup does not attempt to connect to the repository itself.
+
+var lookupCache par.Cache[lookupCacheKey, Repo]
+
+type lookupCacheKey struct {
+	proxy, path string
+}
+
+// Lookup returns the module with the given module path,
+// fetched through the given proxy.
+//
+// The distinguished proxy "direct" indicates that the path should be fetched
+// from its origin, and "noproxy" indicates that the patch should be fetched
+// directly only if GONOPROXY matches the given path.
+//
+// For the distinguished proxy "off", Lookup always returns a Repo that returns
+// a non-nil error for every method call.
+//
+// A successful return does not guarantee that the module
+// has any defined versions.
+func Lookup(ctx context.Context, proxy, path string) Repo {
+	if traceRepo {
+		defer logCall("Lookup(%q, %q)", proxy, path)()
+	}
+
+	return lookupCache.Do(lookupCacheKey{proxy, path}, func() Repo {
+		return newCachingRepo(ctx, path, func(ctx context.Context) (Repo, error) {
+			r, err := lookup(ctx, proxy, path)
+			if err == nil && traceRepo {
+				r = newLoggingRepo(r)
+			}
+			return r, err
+		})
+	})
+}
+
+// lookup returns the module with the given module path.
+func lookup(ctx context.Context, proxy, path string) (r Repo, err error) {
+	if cfg.BuildMod == "vendor" {
+		return nil, errLookupDisabled
+	}
+
+	switch path {
+	case "go", "toolchain":
+		return &toolchainRepo{path, Lookup(ctx, proxy, "golang.org/toolchain")}, nil
+	}
+
+	if module.MatchPrefixPatterns(cfg.GONOPROXY, path) {
+		switch proxy {
+		case "noproxy", "direct":
+			return lookupDirect(ctx, path)
+		default:
+			return nil, errNoproxy
+		}
+	}
+
+	switch proxy {
+	case "off":
+		return errRepo{path, errProxyOff}, nil
+	case "direct":
+		return lookupDirect(ctx, path)
+	case "noproxy":
+		return nil, errUseProxy
+	default:
+		return newProxyRepo(proxy, path)
+	}
+}
+
+type lookupDisabledError struct{}
+
+func (lookupDisabledError) Error() string {
+	if cfg.BuildModReason == "" {
+		return fmt.Sprintf("module lookup disabled by -mod=%s", cfg.BuildMod)
+	}
+	return fmt.Sprintf("module lookup disabled by -mod=%s\n\t(%s)", cfg.BuildMod, cfg.BuildModReason)
+}
+
+var errLookupDisabled error = lookupDisabledError{}
+
+var (
+	errProxyOff       = notExistErrorf("module lookup disabled by GOPROXY=off")
+	errNoproxy  error = notExistErrorf("disabled by GOPRIVATE/GONOPROXY")
+	errUseProxy error = notExistErrorf("path does not match GOPRIVATE/GONOPROXY")
+)
+
+func lookupDirect(ctx context.Context, path string) (Repo, error) {
+	security := web.SecureOnly
+
+	if module.MatchPrefixPatterns(cfg.GOINSECURE, path) {
+		security = web.Insecure
+	}
+	rr, err := vcs.RepoRootForImportPath(path, vcs.PreferMod, security)
+	if err != nil {
+		// We don't know where to find code for a module with this path.
+		return nil, notExistError{err: err}
+	}
+
+	if rr.VCS.Name == "mod" {
+		// Fetch module from proxy with base URL rr.Repo.
+		return newProxyRepo(rr.Repo, path)
+	}
+
+	code, err := lookupCodeRepo(ctx, rr)
+	if err != nil {
+		return nil, err
+	}
+	return newCodeRepo(code, rr.Root, path)
+}
+
+func lookupCodeRepo(ctx context.Context, rr *vcs.RepoRoot) (codehost.Repo, error) {
+	code, err := codehost.NewRepo(ctx, rr.VCS.Cmd, rr.Repo)
+	if err != nil {
+		if _, ok := err.(*codehost.VCSError); ok {
+			return nil, err
+		}
+		return nil, fmt.Errorf("lookup %s: %v", rr.Root, err)
+	}
+	return code, nil
+}
+
+// A loggingRepo is a wrapper around an underlying Repo
+// that prints a log message at the start and end of each call.
+// It can be inserted when debugging.
+type loggingRepo struct {
+	r Repo
+}
+
+func newLoggingRepo(r Repo) *loggingRepo {
+	return &loggingRepo{r}
+}
+
+// logCall prints a log message using format and args and then
+// also returns a function that will print the same message again,
+// along with the elapsed time.
+// Typical usage is:
+//
+//	defer logCall("hello %s", arg)()
+//
+// Note the final ().
+func logCall(format string, args ...any) func() {
+	start := time.Now()
+	fmt.Fprintf(os.Stderr, "+++ %s\n", fmt.Sprintf(format, args...))
+	return func() {
+		fmt.Fprintf(os.Stderr, "%.3fs %s\n", time.Since(start).Seconds(), fmt.Sprintf(format, args...))
+	}
+}
+
+func (l *loggingRepo) ModulePath() string {
+	return l.r.ModulePath()
+}
+
+func (l *loggingRepo) CheckReuse(ctx context.Context, old *codehost.Origin) (err error) {
+	defer func() {
+		logCall("CheckReuse[%s]: %v", l.r.ModulePath(), err)
+	}()
+	return l.r.CheckReuse(ctx, old)
+}
+
+func (l *loggingRepo) Versions(ctx context.Context, prefix string) (*Versions, error) {
+	defer logCall("Repo[%s]: Versions(%q)", l.r.ModulePath(), prefix)()
+	return l.r.Versions(ctx, prefix)
+}
+
+func (l *loggingRepo) Stat(ctx context.Context, rev string) (*RevInfo, error) {
+	defer logCall("Repo[%s]: Stat(%q)", l.r.ModulePath(), rev)()
+	return l.r.Stat(ctx, rev)
+}
+
+func (l *loggingRepo) Latest(ctx context.Context) (*RevInfo, error) {
+	defer logCall("Repo[%s]: Latest()", l.r.ModulePath())()
+	return l.r.Latest(ctx)
+}
+
+func (l *loggingRepo) GoMod(ctx context.Context, version string) ([]byte, error) {
+	defer logCall("Repo[%s]: GoMod(%q)", l.r.ModulePath(), version)()
+	return l.r.GoMod(ctx, version)
+}
+
+func (l *loggingRepo) Zip(ctx context.Context, dst io.Writer, version string) error {
+	dstName := "_"
+	if dst, ok := dst.(interface{ Name() string }); ok {
+		dstName = strconv.Quote(dst.Name())
+	}
+	defer logCall("Repo[%s]: Zip(%s, %q)", l.r.ModulePath(), dstName, version)()
+	return l.r.Zip(ctx, dst, version)
+}
+
+// errRepo is a Repo that returns the same error for all operations.
+//
+// It is useful in conjunction with caching, since cache hits will not attempt
+// the prohibited operations.
+type errRepo struct {
+	modulePath string
+	err        error
+}
+
+func (r errRepo) ModulePath() string { return r.modulePath }
+
+func (r errRepo) CheckReuse(ctx context.Context, old *codehost.Origin) error     { return r.err }
+func (r errRepo) Versions(ctx context.Context, prefix string) (*Versions, error) { return nil, r.err }
+func (r errRepo) Stat(ctx context.Context, rev string) (*RevInfo, error)         { return nil, r.err }
+func (r errRepo) Latest(ctx context.Context) (*RevInfo, error)                   { return nil, r.err }
+func (r errRepo) GoMod(ctx context.Context, version string) ([]byte, error)      { return nil, r.err }
+func (r errRepo) Zip(ctx context.Context, dst io.Writer, version string) error   { return r.err }
+
+// A notExistError is like fs.ErrNotExist, but with a custom message
+type notExistError struct {
+	err error
+}
+
+func notExistErrorf(format string, args ...any) error {
+	return notExistError{fmt.Errorf(format, args...)}
+}
+
+func (e notExistError) Error() string {
+	return e.err.Error()
+}
+
+func (notExistError) Is(target error) bool {
+	return target == fs.ErrNotExist
+}
+
+func (e notExistError) Unwrap() error {
+	return e.err
+}
diff --git a/src/cmd/go/internal/modfetch/sumdb.go b/src/cmd/go/internal/modfetch/sumdb.go
new file mode 100644
index 0000000..ea7d561
--- /dev/null
+++ b/src/cmd/go/internal/modfetch/sumdb.go
@@ -0,0 +1,315 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Go checksum database lookup
+
+//go:build !cmd_go_bootstrap
+
+package modfetch
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"io"
+	"io/fs"
+	"net/url"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+	"time"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/lockedfile"
+	"cmd/go/internal/web"
+
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/sumdb"
+	"golang.org/x/mod/sumdb/note"
+)
+
+// useSumDB reports whether to use the Go checksum database for the given module.
+func useSumDB(mod module.Version) bool {
+	if mod.Path == "golang.org/toolchain" {
+		must := true
+		// Downloaded toolchains cannot be listed in go.sum,
+		// so we require checksum database lookups even if
+		// GOSUMDB=off or GONOSUMDB matches the pattern.
+		// If GOSUMDB=off, then the eventual lookup will fail
+		// with a good error message.
+
+		// Exception #1: using GOPROXY=file:// to test a distpack.
+		if strings.HasPrefix(cfg.GOPROXY, "file://") && !strings.ContainsAny(cfg.GOPROXY, ",|") {
+			must = false
+		}
+		// Exception #2: the Go proxy+checksum database cannot check itself
+		// while doing the initial download.
+		if strings.Contains(os.Getenv("GIT_HTTP_USER_AGENT"), "proxy.golang.org") {
+			must = false
+		}
+
+		// Another potential exception would be GOPROXY=direct,
+		// but that would make toolchain downloads only as secure
+		// as HTTPS, and in particular they'd be susceptible to MITM
+		// attacks on systems with less-than-trustworthy root certificates.
+		// The checksum database provides a stronger guarantee,
+		// so we don't make that exception.
+
+		// Otherwise, require the checksum database.
+		if must {
+			return true
+		}
+	}
+	return cfg.GOSUMDB != "off" && !module.MatchPrefixPatterns(cfg.GONOSUMDB, mod.Path)
+}
+
+// lookupSumDB returns the Go checksum database's go.sum lines for the given module,
+// along with the name of the database.
+func lookupSumDB(mod module.Version) (dbname string, lines []string, err error) {
+	dbOnce.Do(func() {
+		dbName, db, dbErr = dbDial()
+	})
+	if dbErr != nil {
+		return "", nil, dbErr
+	}
+	lines, err = db.Lookup(mod.Path, mod.Version)
+	return dbName, lines, err
+}
+
+var (
+	dbOnce sync.Once
+	dbName string
+	db     *sumdb.Client
+	dbErr  error
+)
+
+func dbDial() (dbName string, db *sumdb.Client, err error) {
+	// $GOSUMDB can be "key" or "key url",
+	// and the key can be a full verifier key
+	// or a host on our list of known keys.
+
+	// Special case: sum.golang.google.cn
+	// is an alias, reachable inside mainland China,
+	// for sum.golang.org. If there are more
+	// of these we should add a map like knownGOSUMDB.
+	gosumdb := cfg.GOSUMDB
+	if gosumdb == "sum.golang.google.cn" {
+		gosumdb = "sum.golang.org https://sum.golang.google.cn"
+	}
+
+	if gosumdb == "off" {
+		return "", nil, fmt.Errorf("checksum database disabled by GOSUMDB=off")
+	}
+
+	key := strings.Fields(gosumdb)
+	if len(key) >= 1 {
+		if k := knownGOSUMDB[key[0]]; k != "" {
+			key[0] = k
+		}
+	}
+	if len(key) == 0 {
+		return "", nil, fmt.Errorf("missing GOSUMDB")
+	}
+	if len(key) > 2 {
+		return "", nil, fmt.Errorf("invalid GOSUMDB: too many fields")
+	}
+	vkey, err := note.NewVerifier(key[0])
+	if err != nil {
+		return "", nil, fmt.Errorf("invalid GOSUMDB: %v", err)
+	}
+	name := vkey.Name()
+
+	// No funny business in the database name.
+	direct, err := url.Parse("https://" + name)
+	if err != nil || strings.HasSuffix(name, "/") || *direct != (url.URL{Scheme: "https", Host: direct.Host, Path: direct.Path, RawPath: direct.RawPath}) || direct.RawPath != "" || direct.Host == "" {
+		return "", nil, fmt.Errorf("invalid sumdb name (must be host[/path]): %s %+v", name, *direct)
+	}
+
+	// Determine how to get to database.
+	var base *url.URL
+	if len(key) >= 2 {
+		// Use explicit alternate URL listed in $GOSUMDB,
+		// bypassing both the default URL derivation and any proxies.
+		u, err := url.Parse(key[1])
+		if err != nil {
+			return "", nil, fmt.Errorf("invalid GOSUMDB URL: %v", err)
+		}
+		base = u
+	}
+
+	return name, sumdb.NewClient(&dbClient{key: key[0], name: name, direct: direct, base: base}), nil
+}
+
+type dbClient struct {
+	key    string
+	name   string
+	direct *url.URL
+
+	once    sync.Once
+	base    *url.URL
+	baseErr error
+}
+
+func (c *dbClient) ReadRemote(path string) ([]byte, error) {
+	c.once.Do(c.initBase)
+	if c.baseErr != nil {
+		return nil, c.baseErr
+	}
+
+	var data []byte
+	start := time.Now()
+	targ := web.Join(c.base, path)
+	data, err := web.GetBytes(targ)
+	if false {
+		fmt.Fprintf(os.Stderr, "%.3fs %s\n", time.Since(start).Seconds(), targ.Redacted())
+	}
+	return data, err
+}
+
+// initBase determines the base URL for connecting to the database.
+// Determining the URL requires sending network traffic to proxies,
+// so this work is delayed until we need to download something from
+// the database. If everything we need is in the local cache and
+// c.ReadRemote is never called, we will never do this work.
+func (c *dbClient) initBase() {
+	if c.base != nil {
+		return
+	}
+
+	// Try proxies in turn until we find out how to connect to this database.
+	//
+	// Before accessing any checksum database URL using a proxy, the proxy
+	// client should first fetch <proxyURL>/sumdb/<sumdb-name>/supported.
+	//
+	// If that request returns a successful (HTTP 200) response, then the proxy
+	// supports proxying checksum database requests. In that case, the client
+	// should use the proxied access method only, never falling back to a direct
+	// connection to the database.
+	//
+	// If the /sumdb/<sumdb-name>/supported check fails with a “not found” (HTTP
+	// 404) or “gone” (HTTP 410) response, or if the proxy is configured to fall
+	// back on errors, the client will try the next proxy. If there are no
+	// proxies left or if the proxy is "direct" or "off", the client should
+	// connect directly to that database.
+	//
+	// Any other response is treated as the database being unavailable.
+	//
+	// See https://golang.org/design/25530-sumdb#proxying-a-checksum-database.
+	err := TryProxies(func(proxy string) error {
+		switch proxy {
+		case "noproxy":
+			return errUseProxy
+		case "direct", "off":
+			return errProxyOff
+		default:
+			proxyURL, err := url.Parse(proxy)
+			if err != nil {
+				return err
+			}
+			if _, err := web.GetBytes(web.Join(proxyURL, "sumdb/"+c.name+"/supported")); err != nil {
+				return err
+			}
+			// Success! This proxy will help us.
+			c.base = web.Join(proxyURL, "sumdb/"+c.name)
+			return nil
+		}
+	})
+	if errors.Is(err, fs.ErrNotExist) {
+		// No proxies, or all proxies failed (with 404, 410, or were allowed
+		// to fall back), or we reached an explicit "direct" or "off".
+		c.base = c.direct
+	} else if err != nil {
+		c.baseErr = err
+	}
+}
+
+// ReadConfig reads the key from c.key
+// and otherwise reads the config (a latest tree head) from GOPATH/pkg/sumdb/<file>.
+func (c *dbClient) ReadConfig(file string) (data []byte, err error) {
+	if file == "key" {
+		return []byte(c.key), nil
+	}
+
+	if cfg.SumdbDir == "" {
+		return nil, fmt.Errorf("could not locate sumdb file: missing $GOPATH: %s",
+			cfg.GoPathError)
+	}
+	targ := filepath.Join(cfg.SumdbDir, file)
+	data, err = lockedfile.Read(targ)
+	if errors.Is(err, fs.ErrNotExist) {
+		// Treat non-existent as empty, to bootstrap the "latest" file
+		// the first time we connect to a given database.
+		return []byte{}, nil
+	}
+	return data, err
+}
+
+// WriteConfig rewrites the latest tree head.
+func (*dbClient) WriteConfig(file string, old, new []byte) error {
+	if file == "key" {
+		// Should not happen.
+		return fmt.Errorf("cannot write key")
+	}
+	if cfg.SumdbDir == "" {
+		return fmt.Errorf("could not locate sumdb file: missing $GOPATH: %s",
+			cfg.GoPathError)
+	}
+	targ := filepath.Join(cfg.SumdbDir, file)
+	os.MkdirAll(filepath.Dir(targ), 0777)
+	f, err := lockedfile.Edit(targ)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+	data, err := io.ReadAll(f)
+	if err != nil {
+		return err
+	}
+	if len(data) > 0 && !bytes.Equal(data, old) {
+		return sumdb.ErrWriteConflict
+	}
+	if _, err := f.Seek(0, 0); err != nil {
+		return err
+	}
+	if err := f.Truncate(0); err != nil {
+		return err
+	}
+	if _, err := f.Write(new); err != nil {
+		return err
+	}
+	return f.Close()
+}
+
+// ReadCache reads cached lookups or tiles from
+// GOPATH/pkg/mod/cache/download/sumdb,
+// which will be deleted by "go clean -modcache".
+func (*dbClient) ReadCache(file string) ([]byte, error) {
+	targ := filepath.Join(cfg.GOMODCACHE, "cache/download/sumdb", file)
+	data, err := lockedfile.Read(targ)
+	// lockedfile.Write does not atomically create the file with contents.
+	// There is a moment between file creation and locking the file for writing,
+	// during which the empty file can be locked for reading.
+	// Treat observing an empty file as file not found.
+	if err == nil && len(data) == 0 {
+		err = &fs.PathError{Op: "read", Path: targ, Err: fs.ErrNotExist}
+	}
+	return data, err
+}
+
+// WriteCache updates cached lookups or tiles.
+func (*dbClient) WriteCache(file string, data []byte) {
+	targ := filepath.Join(cfg.GOMODCACHE, "cache/download/sumdb", file)
+	os.MkdirAll(filepath.Dir(targ), 0777)
+	lockedfile.Write(targ, bytes.NewReader(data), 0666)
+}
+
+func (*dbClient) Log(msg string) {
+	// nothing for now
+}
+
+func (*dbClient) SecurityError(msg string) {
+	base.Fatalf("%s", msg)
+}
diff --git a/src/cmd/go/internal/modfetch/toolchain.go b/src/cmd/go/internal/modfetch/toolchain.go
new file mode 100644
index 0000000..0d7cfcf
--- /dev/null
+++ b/src/cmd/go/internal/modfetch/toolchain.go
@@ -0,0 +1,181 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modfetch
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"sort"
+	"strings"
+
+	"cmd/go/internal/gover"
+	"cmd/go/internal/modfetch/codehost"
+)
+
+// A toolchainRepo is a synthesized repository reporting Go toolchain versions.
+// It has path "go" or "toolchain". The "go" repo reports versions like "1.2".
+// The "toolchain" repo reports versions like "go1.2".
+//
+// Note that the repo ONLY reports versions. It does not actually support
+// downloading of the actual toolchains. Instead, that is done using
+// the regular repo code with "golang.org/toolchain".
+// The naming conflict is unfortunate: "golang.org/toolchain"
+// should perhaps have been "go.dev/dl", but it's too late.
+//
+// For clarity, this file refers to golang.org/toolchain as the "DL" repo,
+// the one you can actually download.
+type toolchainRepo struct {
+	path string // either "go" or "toolchain"
+	repo Repo   // underlying DL repo
+}
+
+func (r *toolchainRepo) ModulePath() string {
+	return r.path
+}
+
+func (r *toolchainRepo) Versions(ctx context.Context, prefix string) (*Versions, error) {
+	// Read DL repo list and convert to "go" or "toolchain" version list.
+	versions, err := r.repo.Versions(ctx, "")
+	if err != nil {
+		return nil, err
+	}
+	versions.Origin = nil
+	var list []string
+	have := make(map[string]bool)
+	goPrefix := ""
+	if r.path == "toolchain" {
+		goPrefix = "go"
+	}
+	for _, v := range versions.List {
+		v, ok := dlToGo(v)
+		if !ok {
+			continue
+		}
+		if !have[v] {
+			have[v] = true
+			list = append(list, goPrefix+v)
+		}
+	}
+
+	// Always include our own version.
+	// This means that the development branch of Go 1.21 (say) will allow 'go get go@1.21'
+	// even though there are no Go 1.21 releases yet.
+	// Once there is a release, 1.21 will be treated as a query matching the latest available release.
+	// Before then, 1.21 will be treated as a query that resolves to this entry we are adding (1.21).
+	if v := gover.Local(); !have[v] {
+		list = append(list, goPrefix+v)
+	}
+
+	if r.path == "go" {
+		sort.Slice(list, func(i, j int) bool {
+			return gover.Compare(list[i], list[j]) < 0
+		})
+	} else {
+		sort.Slice(list, func(i, j int) bool {
+			return gover.Compare(gover.FromToolchain(list[i]), gover.FromToolchain(list[j])) < 0
+		})
+	}
+	versions.List = list
+	return versions, nil
+}
+
+func (r *toolchainRepo) Stat(ctx context.Context, rev string) (*RevInfo, error) {
+	// Convert rev to DL version and stat that to make sure it exists.
+	// In theory the go@ versions should be like 1.21.0
+	// and the toolchain@ versions should be like go1.21.0
+	// but people will type the wrong one, and so we accept
+	// both and silently correct it to the standard form.
+	prefix := ""
+	v := rev
+	v = strings.TrimPrefix(v, "go")
+	if r.path == "toolchain" {
+		prefix = "go"
+	}
+
+	if !gover.IsValid(v) {
+		return nil, fmt.Errorf("invalid %s version %s", r.path, rev)
+	}
+
+	// If we're asking about "go" (not "toolchain"), pretend to have
+	// all earlier Go versions available without network access:
+	// we will provide those ourselves, at least in GOTOOLCHAIN=auto mode.
+	if r.path == "go" && gover.Compare(v, gover.Local()) <= 0 {
+		return &RevInfo{Version: prefix + v}, nil
+	}
+
+	// Similarly, if we're asking about *exactly* the current toolchain,
+	// we don't need to access the network to know that it exists.
+	if r.path == "toolchain" && v == gover.Local() {
+		return &RevInfo{Version: prefix + v}, nil
+	}
+
+	if gover.IsLang(v) {
+		// We can only use a language (development) version if the current toolchain
+		// implements that version, and the two checks above have ruled that out.
+		return nil, fmt.Errorf("go language version %s is not a toolchain version", rev)
+	}
+
+	// Check that the underlying toolchain exists.
+	// We always ask about linux-amd64 because that one
+	// has always existed and is likely to always exist in the future.
+	// This avoids different behavior validating go versions on different
+	// architectures. The eventual download uses the right GOOS-GOARCH.
+	info, err := r.repo.Stat(ctx, goToDL(v, "linux", "amd64"))
+	if err != nil {
+		return nil, err
+	}
+
+	// Return the info using the canonicalized rev
+	// (toolchain 1.2 => toolchain go1.2).
+	return &RevInfo{Version: prefix + v, Time: info.Time}, nil
+}
+
+func (r *toolchainRepo) Latest(ctx context.Context) (*RevInfo, error) {
+	versions, err := r.Versions(ctx, "")
+	if err != nil {
+		return nil, err
+	}
+	var max string
+	for _, v := range versions.List {
+		if max == "" || gover.ModCompare(r.path, v, max) > 0 {
+			max = v
+		}
+	}
+	return r.Stat(ctx, max)
+}
+
+func (r *toolchainRepo) GoMod(ctx context.Context, version string) (data []byte, err error) {
+	return []byte("module " + r.path + "\n"), nil
+}
+
+func (r *toolchainRepo) Zip(ctx context.Context, dst io.Writer, version string) error {
+	return fmt.Errorf("invalid use of toolchainRepo: Zip")
+}
+
+func (r *toolchainRepo) CheckReuse(ctx context.Context, old *codehost.Origin) error {
+	return fmt.Errorf("invalid use of toolchainRepo: CheckReuse")
+}
+
+// goToDL converts a Go version like "1.2" to a DL module version like "v0.0.1-go1.2.linux-amd64".
+func goToDL(v, goos, goarch string) string {
+	return "v0.0.1-go" + v + ".linux-amd64"
+}
+
+// dlToGo converts a DL module version like "v0.0.1-go1.2.linux-amd64" to a Go version like "1.2".
+func dlToGo(v string) (string, bool) {
+	// v0.0.1-go1.19.7.windows-amd64
+	// cut v0.0.1-
+	_, v, ok := strings.Cut(v, "-")
+	if !ok {
+		return "", false
+	}
+	// cut .windows-amd64
+	i := strings.LastIndex(v, ".")
+	if i < 0 || !strings.Contains(v[i+1:], "-") {
+		return "", false
+	}
+	return strings.TrimPrefix(v[:i], "go"), true
+}
diff --git a/src/cmd/go/internal/modfetch/zip_sum_test/testdata/zip_sums.csv b/src/cmd/go/internal/modfetch/zip_sum_test/testdata/zip_sums.csv
new file mode 100644
index 0000000..0906975
--- /dev/null
+++ b/src/cmd/go/internal/modfetch/zip_sum_test/testdata/zip_sums.csv
@@ -0,0 +1,2119 @@
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diff --git a/src/cmd/go/internal/modfetch/zip_sum_test/zip_sum_test.go b/src/cmd/go/internal/modfetch/zip_sum_test/zip_sum_test.go
new file mode 100644
index 0000000..16cc145
--- /dev/null
+++ b/src/cmd/go/internal/modfetch/zip_sum_test/zip_sum_test.go
@@ -0,0 +1,232 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package zip_sum_test tests that the module zip files produced by modfetch
+// have consistent content sums. Ideally the zip files themselves are also
+// stable over time, though this is not strictly necessary.
+//
+// This test loads a table from testdata/zip_sums.csv. The table has columns
+// for module path, version, content sum, and zip file hash. The table
+// includes a large number of real modules. The test downloads these modules
+// in direct mode and verifies the zip files.
+//
+// This test is very slow, and it depends on outside modules that change
+// frequently, so this is a manual test. To enable it, pass the -zipsum flag.
+package zip_sum_test
+
+import (
+	"context"
+	"crypto/sha256"
+	"encoding/csv"
+	"encoding/hex"
+	"flag"
+	"fmt"
+	"internal/testenv"
+	"io"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/modload"
+
+	"golang.org/x/mod/module"
+)
+
+var (
+	updateTestData = flag.Bool("u", false, "when set, tests may update files in testdata instead of failing")
+	enableZipSum   = flag.Bool("zipsum", false, "enable TestZipSums")
+	debugZipSum    = flag.Bool("testwork", false, "when set, TestZipSums will preserve its test directory")
+	modCacheDir    = flag.String("zipsumcache", "", "module cache to use instead of temp directory")
+	shardCount     = flag.Int("zipsumshardcount", 1, "number of shards to divide TestZipSums into")
+	shardIndex     = flag.Int("zipsumshard", 0, "index of TestZipSums shard to test (0 <= zipsumshard < zipsumshardcount)")
+)
+
+const zipSumsPath = "testdata/zip_sums.csv"
+
+type zipSumTest struct {
+	m                     module.Version
+	wantSum, wantFileHash string
+}
+
+func TestZipSums(t *testing.T) {
+	if !*enableZipSum {
+		// This test is very slow and heavily dependent on external repositories.
+		// Only run it explicitly.
+		t.Skip("TestZipSum not enabled with -zipsum")
+	}
+	if *shardCount < 1 {
+		t.Fatal("-zipsumshardcount must be a positive integer")
+	}
+	if *shardIndex < 0 || *shardCount <= *shardIndex {
+		t.Fatal("-zipsumshard must be between 0 and -zipsumshardcount")
+	}
+
+	testenv.MustHaveGoBuild(t)
+	testenv.MustHaveExternalNetwork(t)
+	testenv.MustHaveExecPath(t, "bzr")
+	testenv.MustHaveExecPath(t, "git")
+	// TODO(jayconrod): add hg, svn, and fossil modules to testdata.
+	// Could not find any for now.
+
+	tests, err := readZipSumTests()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if *modCacheDir != "" {
+		cfg.BuildContext.GOPATH = *modCacheDir
+	} else {
+		tmpDir, err := os.MkdirTemp("", "TestZipSums")
+		if err != nil {
+			t.Fatal(err)
+		}
+		if *debugZipSum {
+			fmt.Fprintf(os.Stderr, "TestZipSums: modCacheDir: %s\n", tmpDir)
+		} else {
+			defer os.RemoveAll(tmpDir)
+		}
+		cfg.BuildContext.GOPATH = tmpDir
+	}
+
+	cfg.GOPROXY = "direct"
+	cfg.GOSUMDB = "off"
+	modload.Init()
+
+	// Shard tests by downloading only every nth module when shard flags are set.
+	// This makes it easier to test small groups of modules quickly. We avoid
+	// testing similarly named modules together (the list is sorted by module
+	// path and version).
+	if *shardCount > 1 {
+		r := *shardIndex
+		w := 0
+		for r < len(tests) {
+			tests[w] = tests[r]
+			w++
+			r += *shardCount
+		}
+		tests = tests[:w]
+	}
+
+	// Download modules with a rate limit. We may run out of file descriptors
+	// or cause timeouts without a limit.
+	needUpdate := false
+	for i := range tests {
+		test := &tests[i]
+		name := fmt.Sprintf("%s@%s", strings.ReplaceAll(test.m.Path, "/", "_"), test.m.Version)
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+			ctx := context.Background()
+
+			zipPath, err := modfetch.DownloadZip(ctx, test.m)
+			if err != nil {
+				if *updateTestData {
+					t.Logf("%s: could not download module: %s (will remove from testdata)", test.m, err)
+					test.m.Path = "" // mark for deletion
+					needUpdate = true
+				} else {
+					t.Errorf("%s: could not download module: %s", test.m, err)
+				}
+				return
+			}
+
+			sum := modfetch.Sum(ctx, test.m)
+			if sum != test.wantSum {
+				if *updateTestData {
+					t.Logf("%s: updating content sum to %s", test.m, sum)
+					test.wantSum = sum
+					needUpdate = true
+				} else {
+					t.Errorf("%s: got content sum %s; want sum %s", test.m, sum, test.wantSum)
+					return
+				}
+			}
+
+			h := sha256.New()
+			f, err := os.Open(zipPath)
+			if err != nil {
+				t.Errorf("%s: %v", test.m, err)
+			}
+			defer f.Close()
+			if _, err := io.Copy(h, f); err != nil {
+				t.Errorf("%s: %v", test.m, err)
+			}
+			zipHash := hex.EncodeToString(h.Sum(nil))
+			if zipHash != test.wantFileHash {
+				if *updateTestData {
+					t.Logf("%s: updating zip file hash to %s", test.m, zipHash)
+					test.wantFileHash = zipHash
+					needUpdate = true
+				} else {
+					t.Errorf("%s: got zip file hash %s; want hash %s (but content sum matches)", test.m, zipHash, test.wantFileHash)
+				}
+			}
+		})
+	}
+
+	if needUpdate {
+		// Remove tests marked for deletion
+		r, w := 0, 0
+		for r < len(tests) {
+			if tests[r].m.Path != "" {
+				tests[w] = tests[r]
+				w++
+			}
+			r++
+		}
+		tests = tests[:w]
+
+		if err := writeZipSumTests(tests); err != nil {
+			t.Error(err)
+		}
+	}
+}
+
+func readZipSumTests() ([]zipSumTest, error) {
+	f, err := os.Open(filepath.FromSlash(zipSumsPath))
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	r := csv.NewReader(f)
+
+	var tests []zipSumTest
+	for {
+		line, err := r.Read()
+		if err == io.EOF {
+			break
+		} else if err != nil {
+			return nil, err
+		} else if len(line) != 4 {
+			return nil, fmt.Errorf("%s:%d: malformed line", f.Name(), len(tests)+1)
+		}
+		test := zipSumTest{m: module.Version{Path: line[0], Version: line[1]}, wantSum: line[2], wantFileHash: line[3]}
+		tests = append(tests, test)
+	}
+	return tests, nil
+}
+
+func writeZipSumTests(tests []zipSumTest) (err error) {
+	f, err := os.Create(filepath.FromSlash(zipSumsPath))
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if cerr := f.Close(); err == nil && cerr != nil {
+			err = cerr
+		}
+	}()
+	w := csv.NewWriter(f)
+	line := make([]string, 0, 4)
+	for _, test := range tests {
+		line = append(line[:0], test.m.Path, test.m.Version, test.wantSum, test.wantFileHash)
+		if err := w.Write(line); err != nil {
+			return err
+		}
+	}
+	w.Flush()
+	return nil
+}
diff --git a/src/cmd/go/internal/modget/get.go b/src/cmd/go/internal/modget/get.go
new file mode 100644
index 0000000..37befa3
--- /dev/null
+++ b/src/cmd/go/internal/modget/get.go
@@ -0,0 +1,1946 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package modget implements the module-aware “go get” command.
+package modget
+
+// The arguments to 'go get' are patterns with optional version queries, with
+// the version queries defaulting to "upgrade".
+//
+// The patterns are normally interpreted as package patterns. However, if a
+// pattern cannot match a package, it is instead interpreted as a *module*
+// pattern. For version queries such as "upgrade" and "patch" that depend on the
+// selected version of a module (or of the module containing a package),
+// whether a pattern denotes a package or module may change as updates are
+// applied (see the example in mod_get_patchmod.txt).
+//
+// There are a few other ambiguous cases to resolve, too. A package can exist in
+// two different modules at the same version: for example, the package
+// example.com/foo might be found in module example.com and also in module
+// example.com/foo, and those modules may have independent v0.1.0 tags — so the
+// input 'example.com/foo@v0.1.0' could syntactically refer to the variant of
+// the package loaded from either module! (See mod_get_ambiguous_pkg.txt.)
+// If the argument is ambiguous, the user can often disambiguate by specifying
+// explicit versions for *all* of the potential module paths involved.
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"runtime"
+	"sort"
+	"strings"
+	"sync"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/imports"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/par"
+	"cmd/go/internal/search"
+	"cmd/go/internal/toolchain"
+	"cmd/go/internal/work"
+
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/module"
+)
+
+var CmdGet = &base.Command{
+	// Note: flags below are listed explicitly because they're the most common.
+	// Do not send CLs removing them because they're covered by [get flags].
+	UsageLine: "go get [-t] [-u] [-v] [build flags] [packages]",
+	Short:     "add dependencies to current module and install them",
+	Long: `
+Get resolves its command-line arguments to packages at specific module versions,
+updates go.mod to require those versions, and downloads source code into the
+module cache.
+
+To add a dependency for a package or upgrade it to its latest version:
+
+	go get example.com/pkg
+
+To upgrade or downgrade a package to a specific version:
+
+	go get example.com/pkg@v1.2.3
+
+To remove a dependency on a module and downgrade modules that require it:
+
+	go get example.com/mod@none
+
+To upgrade the minimum required Go version to the latest released Go version:
+
+	go get go@latest
+
+To upgrade the Go toolchain to the latest patch release of the current Go toolchain:
+
+	go get toolchain@patch
+
+See https://golang.org/ref/mod#go-get for details.
+
+In earlier versions of Go, 'go get' was used to build and install packages.
+Now, 'go get' is dedicated to adjusting dependencies in go.mod. 'go install'
+may be used to build and install commands instead. When a version is specified,
+'go install' runs in module-aware mode and ignores the go.mod file in the
+current directory. For example:
+
+	go install example.com/pkg@v1.2.3
+	go install example.com/pkg@latest
+
+See 'go help install' or https://golang.org/ref/mod#go-install for details.
+
+'go get' accepts the following flags.
+
+The -t flag instructs get to consider modules needed to build tests of
+packages specified on the command line.
+
+The -u flag instructs get to update modules providing dependencies
+of packages named on the command line to use newer minor or patch
+releases when available.
+
+The -u=patch flag (not -u patch) also instructs get to update dependencies,
+but changes the default to select patch releases.
+
+When the -t and -u flags are used together, get will update
+test dependencies as well.
+
+The -x flag prints commands as they are executed. This is useful for
+debugging version control commands when a module is downloaded directly
+from a repository.
+
+For more about modules, see https://golang.org/ref/mod.
+
+For more about using 'go get' to update the minimum Go version and
+suggested Go toolchain, see https://go.dev/doc/toolchain.
+
+For more about specifying packages, see 'go help packages'.
+
+This text describes the behavior of get using modules to manage source
+code and dependencies. If instead the go command is running in GOPATH
+mode, the details of get's flags and effects change, as does 'go help get'.
+See 'go help gopath-get'.
+
+See also: go build, go install, go clean, go mod.
+	`,
+}
+
+// Note that this help text is a stopgap to make the module-aware get help text
+// available even in non-module settings. It should be deleted when the old get
+// is deleted. It should NOT be considered to set a precedent of having hierarchical
+// help names with dashes.
+var HelpModuleGet = &base.Command{
+	UsageLine: "module-get",
+	Short:     "module-aware go get",
+	Long: `
+The 'go get' command changes behavior depending on whether the
+go command is running in module-aware mode or legacy GOPATH mode.
+This help text, accessible as 'go help module-get' even in legacy GOPATH mode,
+describes 'go get' as it operates in module-aware mode.
+
+Usage: ` + CmdGet.UsageLine + `
+` + CmdGet.Long,
+}
+
+var HelpVCS = &base.Command{
+	UsageLine: "vcs",
+	Short:     "controlling version control with GOVCS",
+	Long: `
+The 'go get' command can run version control commands like git
+to download imported code. This functionality is critical to the decentralized
+Go package ecosystem, in which code can be imported from any server,
+but it is also a potential security problem, if a malicious server finds a
+way to cause the invoked version control command to run unintended code.
+
+To balance the functionality and security concerns, the 'go get' command
+by default will only use git and hg to download code from public servers.
+But it will use any known version control system (bzr, fossil, git, hg, svn)
+to download code from private servers, defined as those hosting packages
+matching the GOPRIVATE variable (see 'go help private'). The rationale behind
+allowing only Git and Mercurial is that these two systems have had the most
+attention to issues of being run as clients of untrusted servers. In contrast,
+Bazaar, Fossil, and Subversion have primarily been used in trusted,
+authenticated environments and are not as well scrutinized as attack surfaces.
+
+The version control command restrictions only apply when using direct version
+control access to download code. When downloading modules from a proxy,
+'go get' uses the proxy protocol instead, which is always permitted.
+By default, the 'go get' command uses the Go module mirror (proxy.golang.org)
+for public packages and only falls back to version control for private
+packages or when the mirror refuses to serve a public package (typically for
+legal reasons). Therefore, clients can still access public code served from
+Bazaar, Fossil, or Subversion repositories by default, because those downloads
+use the Go module mirror, which takes on the security risk of running the
+version control commands using a custom sandbox.
+
+The GOVCS variable can be used to change the allowed version control systems
+for specific packages (identified by a module or import path).
+The GOVCS variable applies when building package in both module-aware mode
+and GOPATH mode. When using modules, the patterns match against the module path.
+When using GOPATH, the patterns match against the import path corresponding to
+the root of the version control repository.
+
+The general form of the GOVCS setting is a comma-separated list of
+pattern:vcslist rules. The pattern is a glob pattern that must match
+one or more leading elements of the module or import path. The vcslist
+is a pipe-separated list of allowed version control commands, or "all"
+to allow use of any known command, or "off" to disallow all commands.
+Note that if a module matches a pattern with vcslist "off", it may still be
+downloaded if the origin server uses the "mod" scheme, which instructs the
+go command to download the module using the GOPROXY protocol.
+The earliest matching pattern in the list applies, even if later patterns
+might also match.
+
+For example, consider:
+
+	GOVCS=github.com:git,evil.com:off,*:git|hg
+
+With this setting, code with a module or import path beginning with
+github.com/ can only use git; paths on evil.com cannot use any version
+control command, and all other paths (* matches everything) can use
+only git or hg.
+
+The special patterns "public" and "private" match public and private
+module or import paths. A path is private if it matches the GOPRIVATE
+variable; otherwise it is public.
+
+If no rules in the GOVCS variable match a particular module or import path,
+the 'go get' command applies its default rule, which can now be summarized
+in GOVCS notation as 'public:git|hg,private:all'.
+
+To allow unfettered use of any version control system for any package, use:
+
+	GOVCS=*:all
+
+To disable all use of version control, use:
+
+	GOVCS=*:off
+
+The 'go env -w' command (see 'go help env') can be used to set the GOVCS
+variable for future go command invocations.
+`,
+}
+
+var (
+	getD        = CmdGet.Flag.Bool("d", true, "")
+	getF        = CmdGet.Flag.Bool("f", false, "")
+	getFix      = CmdGet.Flag.Bool("fix", false, "")
+	getM        = CmdGet.Flag.Bool("m", false, "")
+	getT        = CmdGet.Flag.Bool("t", false, "")
+	getU        upgradeFlag
+	getInsecure = CmdGet.Flag.Bool("insecure", false, "")
+	// -v is cfg.BuildV
+)
+
+// upgradeFlag is a custom flag.Value for -u.
+type upgradeFlag struct {
+	rawVersion string
+	version    string
+}
+
+func (*upgradeFlag) IsBoolFlag() bool { return true } // allow -u
+
+func (v *upgradeFlag) Set(s string) error {
+	if s == "false" {
+		v.version = ""
+		v.rawVersion = ""
+	} else if s == "true" {
+		v.version = "upgrade"
+		v.rawVersion = ""
+	} else {
+		v.version = s
+		v.rawVersion = s
+	}
+	return nil
+}
+
+func (v *upgradeFlag) String() string { return "" }
+
+func init() {
+	work.AddBuildFlags(CmdGet, work.OmitModFlag)
+	CmdGet.Run = runGet // break init loop
+	CmdGet.Flag.Var(&getU, "u", "")
+}
+
+func runGet(ctx context.Context, cmd *base.Command, args []string) {
+	switch getU.version {
+	case "", "upgrade", "patch":
+		// ok
+	default:
+		base.Fatalf("go: unknown upgrade flag -u=%s", getU.rawVersion)
+	}
+	// TODO(#43684): in the future (Go 1.20), warn that -d is a no-op.
+	if !*getD {
+		base.Fatalf("go: -d flag may not be disabled")
+	}
+	if *getF {
+		fmt.Fprintf(os.Stderr, "go: -f flag is a no-op when using modules\n")
+	}
+	if *getFix {
+		fmt.Fprintf(os.Stderr, "go: -fix flag is a no-op when using modules\n")
+	}
+	if *getM {
+		base.Fatalf("go: -m flag is no longer supported")
+	}
+	if *getInsecure {
+		base.Fatalf("go: -insecure flag is no longer supported; use GOINSECURE instead")
+	}
+
+	modload.ForceUseModules = true
+
+	// Do not allow any updating of go.mod until we've applied
+	// all the requested changes and checked that the result matches
+	// what was requested.
+	modload.ExplicitWriteGoMod = true
+
+	// Allow looking up modules for import paths when outside of a module.
+	// 'go get' is expected to do this, unlike other commands.
+	modload.AllowMissingModuleImports()
+
+	// 'go get' no longer builds or installs packages, so there's nothing to do
+	// if there's no go.mod file.
+	// TODO(#40775): make modload.Init return ErrNoModRoot instead of exiting.
+	// We could handle that here by printing a different message.
+	modload.Init()
+	if !modload.HasModRoot() {
+		base.Fatalf("go: go.mod file not found in current directory or any parent directory.\n" +
+			"\t'go get' is no longer supported outside a module.\n" +
+			"\tTo build and install a command, use 'go install' with a version,\n" +
+			"\tlike 'go install example.com/cmd@latest'\n" +
+			"\tFor more information, see https://golang.org/doc/go-get-install-deprecation\n" +
+			"\tor run 'go help get' or 'go help install'.")
+	}
+
+	dropToolchain, queries := parseArgs(ctx, args)
+	opts := modload.WriteOpts{
+		DropToolchain: dropToolchain,
+	}
+	for _, q := range queries {
+		if q.pattern == "toolchain" {
+			opts.ExplicitToolchain = true
+		}
+	}
+
+	r := newResolver(ctx, queries)
+	r.performLocalQueries(ctx)
+	r.performPathQueries(ctx)
+
+	for {
+		r.performWildcardQueries(ctx)
+		r.performPatternAllQueries(ctx)
+
+		if changed := r.resolveQueries(ctx, queries); changed {
+			// 'go get' arguments can be (and often are) package patterns rather than
+			// (just) modules. A package can be provided by any module with a prefix
+			// of its import path, and a wildcard can even match packages in modules
+			// with totally different paths. Because of these effects, and because any
+			// change to the selected version of a module can bring in entirely new
+			// module paths as dependencies, we need to reissue queries whenever we
+			// change the build list.
+			//
+			// The result of any version query for a given module — even "upgrade" or
+			// "patch" — is always relative to the build list at the start of
+			// the 'go get' command, not an intermediate state, and is therefore
+			// deterministic and therefore cachable, and the constraints on the
+			// selected version of each module can only narrow as we iterate.
+			//
+			// "all" is functionally very similar to a wildcard pattern. The set of
+			// packages imported by the main module does not change, and the query
+			// result for the module containing each such package also does not change
+			// (it is always relative to the initial build list, before applying
+			// queries). So the only way that the result of an "all" query can change
+			// is if some matching package moves from one module in the build list
+			// to another, which should not happen very often.
+			continue
+		}
+
+		// When we load imports, we detect the following conditions:
+		//
+		// - missing transitive dependencies that need to be resolved from outside the
+		//   current build list (note that these may add new matches for existing
+		//   pattern queries!)
+		//
+		// - transitive dependencies that didn't match any other query,
+		//   but need to be upgraded due to the -u flag
+		//
+		// - ambiguous import errors.
+		//   TODO(#27899): Try to resolve ambiguous import errors automatically.
+		upgrades := r.findAndUpgradeImports(ctx, queries)
+		if changed := r.applyUpgrades(ctx, upgrades); changed {
+			continue
+		}
+
+		r.findMissingWildcards(ctx)
+		if changed := r.resolveQueries(ctx, r.wildcardQueries); changed {
+			continue
+		}
+
+		break
+	}
+
+	r.checkWildcardVersions(ctx)
+
+	var pkgPatterns []string
+	for _, q := range queries {
+		if q.matchesPackages {
+			pkgPatterns = append(pkgPatterns, q.pattern)
+		}
+	}
+	r.checkPackageProblems(ctx, pkgPatterns)
+
+	// Everything succeeded. Update go.mod.
+	oldReqs := reqsFromGoMod(modload.ModFile())
+
+	if err := modload.WriteGoMod(ctx, opts); err != nil {
+		// A TooNewError can happen for 'go get go@newversion'
+		// when all the required modules are old enough
+		// but the command line is not.
+		// TODO(bcmills): modload.EditBuildList should catch this instead,
+		// and then this can be changed to base.Fatal(err).
+		toolchain.SwitchOrFatal(ctx, err)
+	}
+
+	newReqs := reqsFromGoMod(modload.ModFile())
+	r.reportChanges(oldReqs, newReqs)
+
+	if gowork := modload.FindGoWork(base.Cwd()); gowork != "" {
+		wf, err := modload.ReadWorkFile(gowork)
+		if err == nil && modload.UpdateWorkGoVersion(wf, modload.MainModules.GoVersion()) {
+			modload.WriteWorkFile(gowork, wf)
+		}
+	}
+}
+
+// parseArgs parses command-line arguments and reports errors.
+//
+// The command-line arguments are of the form path@version or simply path, with
+// implicit @upgrade. path@none is "downgrade away".
+func parseArgs(ctx context.Context, rawArgs []string) (dropToolchain bool, queries []*query) {
+	defer base.ExitIfErrors()
+
+	for _, arg := range search.CleanPatterns(rawArgs) {
+		q, err := newQuery(arg)
+		if err != nil {
+			base.Error(err)
+			continue
+		}
+
+		if q.version == "none" {
+			switch q.pattern {
+			case "go":
+				base.Errorf("go: cannot use go@none")
+				continue
+			case "toolchain":
+				dropToolchain = true
+				continue
+			}
+		}
+
+		// If there were no arguments, CleanPatterns returns ".". Set the raw
+		// string back to "" for better errors.
+		if len(rawArgs) == 0 {
+			q.raw = ""
+		}
+
+		// Guard against 'go get x.go', a common mistake.
+		// Note that package and module paths may end with '.go', so only print an error
+		// if the argument has no version and either has no slash or refers to an existing file.
+		if strings.HasSuffix(q.raw, ".go") && q.rawVersion == "" {
+			if !strings.Contains(q.raw, "/") {
+				base.Errorf("go: %s: arguments must be package or module paths", q.raw)
+				continue
+			}
+			if fi, err := os.Stat(q.raw); err == nil && !fi.IsDir() {
+				base.Errorf("go: %s exists as a file, but 'go get' requires package arguments", q.raw)
+				continue
+			}
+		}
+
+		queries = append(queries, q)
+	}
+
+	return dropToolchain, queries
+}
+
+type resolver struct {
+	localQueries      []*query // queries for absolute or relative paths
+	pathQueries       []*query // package path literal queries in original order
+	wildcardQueries   []*query // path wildcard queries in original order
+	patternAllQueries []*query // queries with the pattern "all"
+
+	// Indexed "none" queries. These are also included in the slices above;
+	// they are indexed here to speed up noneForPath.
+	nonesByPath   map[string]*query // path-literal "@none" queries indexed by path
+	wildcardNones []*query          // wildcard "@none" queries
+
+	// resolvedVersion maps each module path to the version of that module that
+	// must be selected in the final build list, along with the first query
+	// that resolved the module to that version (the “reason”).
+	resolvedVersion map[string]versionReason
+
+	buildList        []module.Version
+	buildListVersion map[string]string // index of buildList (module path → version)
+
+	initialVersion map[string]string // index of the initial build list at the start of 'go get'
+
+	missing []pathSet // candidates for missing transitive dependencies
+
+	work *par.Queue
+
+	matchInModuleCache par.ErrCache[matchInModuleKey, []string]
+}
+
+type versionReason struct {
+	version string
+	reason  *query
+}
+
+type matchInModuleKey struct {
+	pattern string
+	m       module.Version
+}
+
+func newResolver(ctx context.Context, queries []*query) *resolver {
+	// LoadModGraph also sets modload.Target, which is needed by various resolver
+	// methods.
+	mg, err := modload.LoadModGraph(ctx, "")
+	if err != nil {
+		toolchain.SwitchOrFatal(ctx, err)
+	}
+
+	buildList := mg.BuildList()
+	initialVersion := make(map[string]string, len(buildList))
+	for _, m := range buildList {
+		initialVersion[m.Path] = m.Version
+	}
+
+	r := &resolver{
+		work:             par.NewQueue(runtime.GOMAXPROCS(0)),
+		resolvedVersion:  map[string]versionReason{},
+		buildList:        buildList,
+		buildListVersion: initialVersion,
+		initialVersion:   initialVersion,
+		nonesByPath:      map[string]*query{},
+	}
+
+	for _, q := range queries {
+		if q.pattern == "all" {
+			r.patternAllQueries = append(r.patternAllQueries, q)
+		} else if q.patternIsLocal {
+			r.localQueries = append(r.localQueries, q)
+		} else if q.isWildcard() {
+			r.wildcardQueries = append(r.wildcardQueries, q)
+		} else {
+			r.pathQueries = append(r.pathQueries, q)
+		}
+
+		if q.version == "none" {
+			// Index "none" queries to make noneForPath more efficient.
+			if q.isWildcard() {
+				r.wildcardNones = append(r.wildcardNones, q)
+			} else {
+				// All "<path>@none" queries for the same path are identical; we only
+				// need to index one copy.
+				r.nonesByPath[q.pattern] = q
+			}
+		}
+	}
+
+	return r
+}
+
+// initialSelected returns the version of the module with the given path that
+// was selected at the start of this 'go get' invocation.
+func (r *resolver) initialSelected(mPath string) (version string) {
+	v, ok := r.initialVersion[mPath]
+	if !ok {
+		return "none"
+	}
+	return v
+}
+
+// selected returns the version of the module with the given path that is
+// selected in the resolver's current build list.
+func (r *resolver) selected(mPath string) (version string) {
+	v, ok := r.buildListVersion[mPath]
+	if !ok {
+		return "none"
+	}
+	return v
+}
+
+// noneForPath returns a "none" query matching the given module path,
+// or found == false if no such query exists.
+func (r *resolver) noneForPath(mPath string) (nq *query, found bool) {
+	if nq = r.nonesByPath[mPath]; nq != nil {
+		return nq, true
+	}
+	for _, nq := range r.wildcardNones {
+		if nq.matchesPath(mPath) {
+			return nq, true
+		}
+	}
+	return nil, false
+}
+
+// queryModule wraps modload.Query, substituting r.checkAllowedOr to decide
+// allowed versions.
+func (r *resolver) queryModule(ctx context.Context, mPath, query string, selected func(string) string) (module.Version, error) {
+	current := r.initialSelected(mPath)
+	rev, err := modload.Query(ctx, mPath, query, current, r.checkAllowedOr(query, selected))
+	if err != nil {
+		return module.Version{}, err
+	}
+	return module.Version{Path: mPath, Version: rev.Version}, nil
+}
+
+// queryPackages wraps modload.QueryPackage, substituting r.checkAllowedOr to
+// decide allowed versions.
+func (r *resolver) queryPackages(ctx context.Context, pattern, query string, selected func(string) string) (pkgMods []module.Version, err error) {
+	results, err := modload.QueryPackages(ctx, pattern, query, selected, r.checkAllowedOr(query, selected))
+	if len(results) > 0 {
+		pkgMods = make([]module.Version, 0, len(results))
+		for _, qr := range results {
+			pkgMods = append(pkgMods, qr.Mod)
+		}
+	}
+	return pkgMods, err
+}
+
+// queryPattern wraps modload.QueryPattern, substituting r.checkAllowedOr to
+// decide allowed versions.
+func (r *resolver) queryPattern(ctx context.Context, pattern, query string, selected func(string) string) (pkgMods []module.Version, mod module.Version, err error) {
+	results, modOnly, err := modload.QueryPattern(ctx, pattern, query, selected, r.checkAllowedOr(query, selected))
+	if len(results) > 0 {
+		pkgMods = make([]module.Version, 0, len(results))
+		for _, qr := range results {
+			pkgMods = append(pkgMods, qr.Mod)
+		}
+	}
+	if modOnly != nil {
+		mod = modOnly.Mod
+	}
+	return pkgMods, mod, err
+}
+
+// checkAllowedOr is like modload.CheckAllowed, but it always allows the requested
+// and current versions (even if they are retracted or otherwise excluded).
+func (r *resolver) checkAllowedOr(requested string, selected func(string) string) modload.AllowedFunc {
+	return func(ctx context.Context, m module.Version) error {
+		if m.Version == requested {
+			return modload.CheckExclusions(ctx, m)
+		}
+		if (requested == "upgrade" || requested == "patch") && m.Version == selected(m.Path) {
+			return nil
+		}
+		return modload.CheckAllowed(ctx, m)
+	}
+}
+
+// matchInModule is a caching wrapper around modload.MatchInModule.
+func (r *resolver) matchInModule(ctx context.Context, pattern string, m module.Version) (packages []string, err error) {
+	return r.matchInModuleCache.Do(matchInModuleKey{pattern, m}, func() ([]string, error) {
+		match := modload.MatchInModule(ctx, pattern, m, imports.AnyTags())
+		if len(match.Errs) > 0 {
+			return match.Pkgs, match.Errs[0]
+		}
+		return match.Pkgs, nil
+	})
+}
+
+// queryNone adds a candidate set to q for each module matching q.pattern.
+// Each candidate set has only one possible module version: the matched
+// module at version "none".
+//
+// We interpret arguments to 'go get' as packages first, and fall back to
+// modules second. However, no module exists at version "none", and therefore no
+// package exists at that version either: we know that the argument cannot match
+// any packages, and thus it must match modules instead.
+func (r *resolver) queryNone(ctx context.Context, q *query) {
+	if search.IsMetaPackage(q.pattern) {
+		panic(fmt.Sprintf("internal error: queryNone called with pattern %q", q.pattern))
+	}
+
+	if !q.isWildcard() {
+		q.pathOnce(q.pattern, func() pathSet {
+			hasModRoot := modload.HasModRoot()
+			if hasModRoot && modload.MainModules.Contains(q.pattern) {
+				v := module.Version{Path: q.pattern}
+				// The user has explicitly requested to downgrade their own module to
+				// version "none". This is not an entirely unreasonable request: it
+				// could plausibly mean “downgrade away everything that depends on any
+				// explicit version of the main module”, or “downgrade away the
+				// package with the same path as the main module, found in a module
+				// with a prefix of the main module's path”.
+				//
+				// However, neither of those behaviors would be consistent with the
+				// plain meaning of the query. To try to reduce confusion, reject the
+				// query explicitly.
+				return errSet(&modload.QueryMatchesMainModulesError{MainModules: []module.Version{v}, Pattern: q.pattern, Query: q.version})
+			}
+
+			return pathSet{mod: module.Version{Path: q.pattern, Version: "none"}}
+		})
+	}
+
+	for _, curM := range r.buildList {
+		if !q.matchesPath(curM.Path) {
+			continue
+		}
+		q.pathOnce(curM.Path, func() pathSet {
+			if modload.HasModRoot() && curM.Version == "" && modload.MainModules.Contains(curM.Path) {
+				return errSet(&modload.QueryMatchesMainModulesError{MainModules: []module.Version{curM}, Pattern: q.pattern, Query: q.version})
+			}
+			return pathSet{mod: module.Version{Path: curM.Path, Version: "none"}}
+		})
+	}
+}
+
+func (r *resolver) performLocalQueries(ctx context.Context) {
+	for _, q := range r.localQueries {
+		q.pathOnce(q.pattern, func() pathSet {
+			absDetail := ""
+			if !filepath.IsAbs(q.pattern) {
+				if absPath, err := filepath.Abs(q.pattern); err == nil {
+					absDetail = fmt.Sprintf(" (%s)", absPath)
+				}
+			}
+
+			// Absolute paths like C:\foo and relative paths like ../foo... are
+			// restricted to matching packages in the main module.
+			pkgPattern, mainModule := modload.MainModules.DirImportPath(ctx, q.pattern)
+			if pkgPattern == "." {
+				modload.MustHaveModRoot()
+				var modRoots []string
+				for _, m := range modload.MainModules.Versions() {
+					modRoots = append(modRoots, modload.MainModules.ModRoot(m))
+				}
+				var plural string
+				if len(modRoots) != 1 {
+					plural = "s"
+				}
+				return errSet(fmt.Errorf("%s%s is not within module%s rooted at %s", q.pattern, absDetail, plural, strings.Join(modRoots, ", ")))
+			}
+
+			match := modload.MatchInModule(ctx, pkgPattern, mainModule, imports.AnyTags())
+			if len(match.Errs) > 0 {
+				return pathSet{err: match.Errs[0]}
+			}
+
+			if len(match.Pkgs) == 0 {
+				if q.raw == "" || q.raw == "." {
+					return errSet(fmt.Errorf("no package to get in current directory"))
+				}
+				if !q.isWildcard() {
+					modload.MustHaveModRoot()
+					return errSet(fmt.Errorf("%s%s is not a package in module rooted at %s", q.pattern, absDetail, modload.MainModules.ModRoot(mainModule)))
+				}
+				search.WarnUnmatched([]*search.Match{match})
+				return pathSet{}
+			}
+
+			return pathSet{pkgMods: []module.Version{mainModule}}
+		})
+	}
+}
+
+// performWildcardQueries populates the candidates for each query whose pattern
+// is a wildcard.
+//
+// The candidates for a given module path matching (or containing a package
+// matching) a wildcard query depend only on the initial build list, but the set
+// of modules may be expanded by other queries, so wildcard queries need to be
+// re-evaluated whenever a potentially-matching module path is added to the
+// build list.
+func (r *resolver) performWildcardQueries(ctx context.Context) {
+	for _, q := range r.wildcardQueries {
+		q := q
+		r.work.Add(func() {
+			if q.version == "none" {
+				r.queryNone(ctx, q)
+			} else {
+				r.queryWildcard(ctx, q)
+			}
+		})
+	}
+	<-r.work.Idle()
+}
+
+// queryWildcard adds a candidate set to q for each module for which:
+//   - some version of the module is already in the build list, and
+//   - that module exists at some version matching q.version, and
+//   - either the module path itself matches q.pattern, or some package within
+//     the module at q.version matches q.pattern.
+func (r *resolver) queryWildcard(ctx context.Context, q *query) {
+	// For wildcard patterns, modload.QueryPattern only identifies modules
+	// matching the prefix of the path before the wildcard. However, the build
+	// list may already contain other modules with matching packages, and we
+	// should consider those modules to satisfy the query too.
+	// We want to match any packages in existing dependencies, but we only want to
+	// resolve new dependencies if nothing else turns up.
+	for _, curM := range r.buildList {
+		if !q.canMatchInModule(curM.Path) {
+			continue
+		}
+		q.pathOnce(curM.Path, func() pathSet {
+			if _, hit := r.noneForPath(curM.Path); hit {
+				// This module is being removed, so it will no longer be in the build list
+				// (and thus will no longer match the pattern).
+				return pathSet{}
+			}
+
+			if modload.MainModules.Contains(curM.Path) && !versionOkForMainModule(q.version) {
+				if q.matchesPath(curM.Path) {
+					return errSet(&modload.QueryMatchesMainModulesError{
+						MainModules: []module.Version{curM},
+						Pattern:     q.pattern,
+						Query:       q.version,
+					})
+				}
+
+				packages, err := r.matchInModule(ctx, q.pattern, curM)
+				if err != nil {
+					return errSet(err)
+				}
+				if len(packages) > 0 {
+					return errSet(&modload.QueryMatchesPackagesInMainModuleError{
+						Pattern:  q.pattern,
+						Query:    q.version,
+						Packages: packages,
+					})
+				}
+
+				return r.tryWildcard(ctx, q, curM)
+			}
+
+			m, err := r.queryModule(ctx, curM.Path, q.version, r.initialSelected)
+			if err != nil {
+				if !isNoSuchModuleVersion(err) {
+					// We can't tell whether a matching version exists.
+					return errSet(err)
+				}
+				// There is no version of curM.Path matching the query.
+
+				// We haven't checked whether curM contains any matching packages at its
+				// currently-selected version, or whether curM.Path itself matches q. If
+				// either of those conditions holds, *and* no other query changes the
+				// selected version of curM, then we will fail in checkWildcardVersions.
+				// (This could be an error, but it's too soon to tell.)
+				//
+				// However, even then the transitive requirements of some other query
+				// may downgrade this module out of the build list entirely, in which
+				// case the pattern will no longer include it and it won't be an error.
+				//
+				// Either way, punt on the query rather than erroring out just yet.
+				return pathSet{}
+			}
+
+			return r.tryWildcard(ctx, q, m)
+		})
+	}
+
+	// Even if no modules matched, we shouldn't query for a new module to provide
+	// the pattern yet: some other query may yet induce a new requirement that
+	// will match the wildcard. Instead, we'll check in findMissingWildcards.
+}
+
+// tryWildcard returns a pathSet for module m matching query q.
+// If m does not actually match q, tryWildcard returns an empty pathSet.
+func (r *resolver) tryWildcard(ctx context.Context, q *query, m module.Version) pathSet {
+	mMatches := q.matchesPath(m.Path)
+	packages, err := r.matchInModule(ctx, q.pattern, m)
+	if err != nil {
+		return errSet(err)
+	}
+	if len(packages) > 0 {
+		return pathSet{pkgMods: []module.Version{m}}
+	}
+	if mMatches {
+		return pathSet{mod: m}
+	}
+	return pathSet{}
+}
+
+// findMissingWildcards adds a candidate set for each query in r.wildcardQueries
+// that has not yet resolved to any version containing packages.
+func (r *resolver) findMissingWildcards(ctx context.Context) {
+	for _, q := range r.wildcardQueries {
+		if q.version == "none" || q.matchesPackages {
+			continue // q is not “missing”
+		}
+		r.work.Add(func() {
+			q.pathOnce(q.pattern, func() pathSet {
+				pkgMods, mod, err := r.queryPattern(ctx, q.pattern, q.version, r.initialSelected)
+				if err != nil {
+					if isNoSuchPackageVersion(err) && len(q.resolved) > 0 {
+						// q already resolved one or more modules but matches no packages.
+						// That's ok: this pattern is just a module pattern, and we don't
+						// need to add any more modules to satisfy it.
+						return pathSet{}
+					}
+					return errSet(err)
+				}
+
+				return pathSet{pkgMods: pkgMods, mod: mod}
+			})
+		})
+	}
+	<-r.work.Idle()
+}
+
+// checkWildcardVersions reports an error if any module in the build list has a
+// path (or contains a package) matching a query with a wildcard pattern, but
+// has a selected version that does *not* match the query.
+func (r *resolver) checkWildcardVersions(ctx context.Context) {
+	defer base.ExitIfErrors()
+
+	for _, q := range r.wildcardQueries {
+		for _, curM := range r.buildList {
+			if !q.canMatchInModule(curM.Path) {
+				continue
+			}
+			if !q.matchesPath(curM.Path) {
+				packages, err := r.matchInModule(ctx, q.pattern, curM)
+				if len(packages) == 0 {
+					if err != nil {
+						reportError(q, err)
+					}
+					continue // curM is not relevant to q.
+				}
+			}
+
+			rev, err := r.queryModule(ctx, curM.Path, q.version, r.initialSelected)
+			if err != nil {
+				reportError(q, err)
+				continue
+			}
+			if rev.Version == curM.Version {
+				continue // curM already matches q.
+			}
+
+			if !q.matchesPath(curM.Path) {
+				m := module.Version{Path: curM.Path, Version: rev.Version}
+				packages, err := r.matchInModule(ctx, q.pattern, m)
+				if err != nil {
+					reportError(q, err)
+					continue
+				}
+				if len(packages) == 0 {
+					// curM at its original version contains a path matching q.pattern,
+					// but at rev.Version it does not, so (somewhat paradoxically) if
+					// we changed the version of curM it would no longer match the query.
+					var version any = m
+					if rev.Version != q.version {
+						version = fmt.Sprintf("%s@%s (%s)", m.Path, q.version, m.Version)
+					}
+					reportError(q, fmt.Errorf("%v matches packages in %v but not %v: specify a different version for module %s", q, curM, version, m.Path))
+					continue
+				}
+			}
+
+			// Since queryModule succeeded and either curM or one of the packages it
+			// contains matches q.pattern, we should have either selected the version
+			// of curM matching q, or reported a conflict error (and exited).
+			// If we're still here and the version doesn't match,
+			// something has gone very wrong.
+			reportError(q, fmt.Errorf("internal error: selected %v instead of %v", curM, rev.Version))
+		}
+	}
+}
+
+// performPathQueries populates the candidates for each query whose pattern is
+// a path literal.
+//
+// The candidate packages and modules for path literals depend only on the
+// initial build list, not the current build list, so we only need to query path
+// literals once.
+func (r *resolver) performPathQueries(ctx context.Context) {
+	for _, q := range r.pathQueries {
+		q := q
+		r.work.Add(func() {
+			if q.version == "none" {
+				r.queryNone(ctx, q)
+			} else {
+				r.queryPath(ctx, q)
+			}
+		})
+	}
+	<-r.work.Idle()
+}
+
+// queryPath adds a candidate set to q for the package with path q.pattern.
+// The candidate set consists of all modules that could provide q.pattern
+// and have a version matching q, plus (if it exists) the module whose path
+// is itself q.pattern (at a matching version).
+func (r *resolver) queryPath(ctx context.Context, q *query) {
+	q.pathOnce(q.pattern, func() pathSet {
+		if search.IsMetaPackage(q.pattern) || q.isWildcard() {
+			panic(fmt.Sprintf("internal error: queryPath called with pattern %q", q.pattern))
+		}
+		if q.version == "none" {
+			panic(`internal error: queryPath called with version "none"`)
+		}
+
+		if search.IsStandardImportPath(q.pattern) {
+			stdOnly := module.Version{}
+			packages, _ := r.matchInModule(ctx, q.pattern, stdOnly)
+			if len(packages) > 0 {
+				if q.rawVersion != "" {
+					return errSet(fmt.Errorf("can't request explicit version %q of standard library package %s", q.version, q.pattern))
+				}
+
+				q.matchesPackages = true
+				return pathSet{} // No module needed for standard library.
+			}
+		}
+
+		pkgMods, mod, err := r.queryPattern(ctx, q.pattern, q.version, r.initialSelected)
+		if err != nil {
+			return errSet(err)
+		}
+		return pathSet{pkgMods: pkgMods, mod: mod}
+	})
+}
+
+// performPatternAllQueries populates the candidates for each query whose
+// pattern is "all".
+//
+// The candidate modules for a given package in "all" depend only on the initial
+// build list, but we cannot follow the dependencies of a given package until we
+// know which candidate is selected — and that selection may depend on the
+// results of other queries. We need to re-evaluate the "all" queries whenever
+// the module for one or more packages in "all" are resolved.
+func (r *resolver) performPatternAllQueries(ctx context.Context) {
+	if len(r.patternAllQueries) == 0 {
+		return
+	}
+
+	findPackage := func(ctx context.Context, path string, m module.Version) (versionOk bool) {
+		versionOk = true
+		for _, q := range r.patternAllQueries {
+			q.pathOnce(path, func() pathSet {
+				pkgMods, err := r.queryPackages(ctx, path, q.version, r.initialSelected)
+				if len(pkgMods) != 1 || pkgMods[0] != m {
+					// There are candidates other than m for the given path, so we can't
+					// be certain that m will actually be the module selected to provide
+					// the package. Don't load its dependencies just yet, because they
+					// might no longer be dependencies after we resolve the correct
+					// version.
+					versionOk = false
+				}
+				return pathSet{pkgMods: pkgMods, err: err}
+			})
+		}
+		return versionOk
+	}
+
+	r.loadPackages(ctx, []string{"all"}, findPackage)
+
+	// Since we built up the candidate lists concurrently, they may be in a
+	// nondeterministic order. We want 'go get' to be fully deterministic,
+	// including in which errors it chooses to report, so sort the candidates
+	// into a deterministic-but-arbitrary order.
+	for _, q := range r.patternAllQueries {
+		sort.Slice(q.candidates, func(i, j int) bool {
+			return q.candidates[i].path < q.candidates[j].path
+		})
+	}
+}
+
+// findAndUpgradeImports returns a pathSet for each package that is not yet
+// in the build list but is transitively imported by the packages matching the
+// given queries (which must already have been resolved).
+//
+// If the getU flag ("-u") is set, findAndUpgradeImports also returns a
+// pathSet for each module that is not constrained by any other
+// command-line argument and has an available matching upgrade.
+func (r *resolver) findAndUpgradeImports(ctx context.Context, queries []*query) (upgrades []pathSet) {
+	patterns := make([]string, 0, len(queries))
+	for _, q := range queries {
+		if q.matchesPackages {
+			patterns = append(patterns, q.pattern)
+		}
+	}
+	if len(patterns) == 0 {
+		return nil
+	}
+
+	// mu guards concurrent writes to upgrades, which will be sorted
+	// (to restore determinism) after loading.
+	var mu sync.Mutex
+
+	findPackage := func(ctx context.Context, path string, m module.Version) (versionOk bool) {
+		version := "latest"
+		if m.Path != "" {
+			if getU.version == "" {
+				// The user did not request that we upgrade transitive dependencies.
+				return true
+			}
+			if _, ok := r.resolvedVersion[m.Path]; ok {
+				// We cannot upgrade m implicitly because its version is determined by
+				// an explicit pattern argument.
+				return true
+			}
+			version = getU.version
+		}
+
+		// Unlike other queries, the "-u" flag upgrades relative to the build list
+		// after applying changes so far, not the initial build list.
+		// This is for two reasons:
+		//
+		// 	- The "-u" flag intentionally applies to transitive dependencies,
+		// 	  which may not be known or even resolved in advance of applying
+		// 	  other version changes.
+		//
+		// 	- The "-u" flag, unlike other arguments, does not cause version
+		// 	  conflicts with other queries. (The other query always wins.)
+
+		pkgMods, err := r.queryPackages(ctx, path, version, r.selected)
+		for _, u := range pkgMods {
+			if u == m {
+				// The selected package version is already upgraded appropriately; there
+				// is no need to change it.
+				return true
+			}
+		}
+
+		if err != nil {
+			if isNoSuchPackageVersion(err) || (m.Path == "" && module.CheckPath(path) != nil) {
+				// We can't find the package because it doesn't — or can't — even exist
+				// in any module at the latest version. (Note that invalid module paths
+				// could in general exist due to replacements, so we at least need to
+				// run the query to check those.)
+				//
+				// There is no version change we can make to fix the package, so leave
+				// it unresolved. Either some other query (perhaps a wildcard matching a
+				// newly-added dependency for some other missing package) will fill in
+				// the gaps, or we will report an error (with a better import stack) in
+				// the final LoadPackages call.
+				return true
+			}
+		}
+
+		mu.Lock()
+		upgrades = append(upgrades, pathSet{path: path, pkgMods: pkgMods, err: err})
+		mu.Unlock()
+		return false
+	}
+
+	r.loadPackages(ctx, patterns, findPackage)
+
+	// Since we built up the candidate lists concurrently, they may be in a
+	// nondeterministic order. We want 'go get' to be fully deterministic,
+	// including in which errors it chooses to report, so sort the candidates
+	// into a deterministic-but-arbitrary order.
+	sort.Slice(upgrades, func(i, j int) bool {
+		return upgrades[i].path < upgrades[j].path
+	})
+	return upgrades
+}
+
+// loadPackages loads the packages matching the given patterns, invoking the
+// findPackage function for each package that may require a change to the
+// build list.
+//
+// loadPackages invokes the findPackage function for each package loaded from a
+// module outside the main module. If the module or version that supplies that
+// package needs to be changed due to a query, findPackage may return false
+// and the imports of that package will not be loaded.
+//
+// loadPackages also invokes the findPackage function for each imported package
+// that is neither present in the standard library nor in any module in the
+// build list.
+func (r *resolver) loadPackages(ctx context.Context, patterns []string, findPackage func(ctx context.Context, path string, m module.Version) (versionOk bool)) {
+	opts := modload.PackageOpts{
+		Tags:                     imports.AnyTags(),
+		VendorModulesInGOROOTSrc: true,
+		LoadTests:                *getT,
+		AssumeRootsImported:      true, // After 'go get foo', imports of foo should build.
+		SilencePackageErrors:     true, // May be fixed by subsequent upgrades or downgrades.
+		Switcher:                 new(toolchain.Switcher),
+	}
+
+	opts.AllowPackage = func(ctx context.Context, path string, m module.Version) error {
+		if m.Path == "" || m.Version == "" {
+			// Packages in the standard library and main modules are already at their
+			// latest (and only) available versions.
+			return nil
+		}
+		if ok := findPackage(ctx, path, m); !ok {
+			return errVersionChange
+		}
+		return nil
+	}
+
+	_, pkgs := modload.LoadPackages(ctx, opts, patterns...)
+	for _, path := range pkgs {
+		const (
+			parentPath  = ""
+			parentIsStd = false
+		)
+		_, _, err := modload.Lookup(parentPath, parentIsStd, path)
+		if err == nil {
+			continue
+		}
+		if errors.Is(err, errVersionChange) {
+			// We already added candidates during loading.
+			continue
+		}
+
+		var (
+			importMissing *modload.ImportMissingError
+			ambiguous     *modload.AmbiguousImportError
+		)
+		if !errors.As(err, &importMissing) && !errors.As(err, &ambiguous) {
+			// The package, which is a dependency of something we care about, has some
+			// problem that we can't resolve with a version change.
+			// Leave the error for the final LoadPackages call.
+			continue
+		}
+
+		path := path
+		r.work.Add(func() {
+			findPackage(ctx, path, module.Version{})
+		})
+	}
+	<-r.work.Idle()
+}
+
+// errVersionChange is a sentinel error indicating that a module's version needs
+// to be updated before its dependencies can be loaded.
+var errVersionChange = errors.New("version change needed")
+
+// resolveQueries resolves candidate sets that are attached to the given
+// queries and/or needed to provide the given missing-package dependencies.
+//
+// resolveQueries starts by resolving one module version from each
+// unambiguous pathSet attached to the given queries.
+//
+// If no unambiguous query results in a change to the build list,
+// resolveQueries revisits the ambiguous query candidates and resolves them
+// arbitrarily in order to guarantee forward progress.
+//
+// If all pathSets are resolved without any changes to the build list,
+// resolveQueries returns with changed=false.
+func (r *resolver) resolveQueries(ctx context.Context, queries []*query) (changed bool) {
+	defer base.ExitIfErrors()
+
+	// Note: this is O(N²) with the number of pathSets in the worst case.
+	//
+	// We could perhaps get it down to O(N) if we were to index the pathSets
+	// by module path, so that we only revisit a given pathSet when the
+	// version of some module in its containingPackage list has been determined.
+	//
+	// However, N tends to be small, and most candidate sets will include only one
+	// candidate module (so they will be resolved in the first iteration), so for
+	// now we'll stick to the simple O(N²) approach.
+
+	resolved := 0
+	for {
+		prevResolved := resolved
+
+		// If we found modules that were too new, find the max of the required versions
+		// and then try to switch to a newer toolchain.
+		var sw toolchain.Switcher
+		for _, q := range queries {
+			for _, cs := range q.candidates {
+				sw.Error(cs.err)
+			}
+		}
+		// Only switch if we need a newer toolchain.
+		// Otherwise leave the cs.err for reporting later.
+		if sw.NeedSwitch() {
+			sw.Switch(ctx)
+			// If NeedSwitch is true and Switch returns, Switch has failed to locate a newer toolchain.
+			// It printed the errors along with one more about not finding a good toolchain.
+			base.Exit()
+		}
+
+		for _, q := range queries {
+			unresolved := q.candidates[:0]
+
+			for _, cs := range q.candidates {
+				if cs.err != nil {
+					reportError(q, cs.err)
+					resolved++
+					continue
+				}
+
+				filtered, isPackage, m, unique := r.disambiguate(cs)
+				if !unique {
+					unresolved = append(unresolved, filtered)
+					continue
+				}
+
+				if m.Path == "" {
+					// The query is not viable. Choose an arbitrary candidate from
+					// before filtering and “resolve” it to report a conflict.
+					isPackage, m = r.chooseArbitrarily(cs)
+				}
+				if isPackage {
+					q.matchesPackages = true
+				}
+				r.resolve(q, m)
+				resolved++
+			}
+
+			q.candidates = unresolved
+		}
+
+		base.ExitIfErrors()
+		if resolved == prevResolved {
+			break // No unambiguous candidate remains.
+		}
+	}
+
+	if resolved > 0 {
+		if changed = r.updateBuildList(ctx, nil); changed {
+			// The build list has changed, so disregard any remaining ambiguous queries:
+			// they might now be determined by requirements in the build list, which we
+			// would prefer to use instead of arbitrary versions.
+			return true
+		}
+	}
+
+	// The build list will be the same on the next iteration as it was on this
+	// iteration, so any ambiguous queries will remain so. In order to make
+	// progress, resolve them arbitrarily but deterministically.
+	//
+	// If that results in conflicting versions, the user can re-run 'go get'
+	// with additional explicit versions for the conflicting packages or
+	// modules.
+	resolvedArbitrarily := 0
+	for _, q := range queries {
+		for _, cs := range q.candidates {
+			isPackage, m := r.chooseArbitrarily(cs)
+			if isPackage {
+				q.matchesPackages = true
+			}
+			r.resolve(q, m)
+			resolvedArbitrarily++
+		}
+	}
+	if resolvedArbitrarily > 0 {
+		changed = r.updateBuildList(ctx, nil)
+	}
+	return changed
+}
+
+// applyUpgrades disambiguates candidate sets that are needed to upgrade (or
+// provide) transitive dependencies imported by previously-resolved packages.
+//
+// applyUpgrades modifies the build list by adding one module version from each
+// pathSet in upgrades, then downgrading (or further upgrading) those modules as
+// needed to maintain any already-resolved versions of other modules.
+// applyUpgrades does not mark the new versions as resolved, so they can still
+// be further modified by other queries (such as wildcards).
+//
+// If all pathSets are resolved without any changes to the build list,
+// applyUpgrades returns with changed=false.
+func (r *resolver) applyUpgrades(ctx context.Context, upgrades []pathSet) (changed bool) {
+	defer base.ExitIfErrors()
+
+	// Arbitrarily add a "latest" version that provides each missing package, but
+	// do not mark the version as resolved: we still want to allow the explicit
+	// queries to modify the resulting versions.
+	var tentative []module.Version
+	for _, cs := range upgrades {
+		if cs.err != nil {
+			base.Error(cs.err)
+			continue
+		}
+
+		filtered, _, m, unique := r.disambiguate(cs)
+		if !unique {
+			_, m = r.chooseArbitrarily(filtered)
+		}
+		if m.Path == "" {
+			// There is no viable candidate for the missing package.
+			// Leave it unresolved.
+			continue
+		}
+		tentative = append(tentative, m)
+	}
+	base.ExitIfErrors()
+
+	changed = r.updateBuildList(ctx, tentative)
+	return changed
+}
+
+// disambiguate eliminates candidates from cs that conflict with other module
+// versions that have already been resolved. If there is only one (unique)
+// remaining candidate, disambiguate returns that candidate, along with
+// an indication of whether that result interprets cs.path as a package
+//
+// Note: we're only doing very simple disambiguation here. The goal is to
+// reproduce the user's intent, not to find a solution that a human couldn't.
+// In the vast majority of cases, we expect only one module per pathSet,
+// but we want to give some minimal additional tools so that users can add an
+// extra argument or two on the command line to resolve simple ambiguities.
+func (r *resolver) disambiguate(cs pathSet) (filtered pathSet, isPackage bool, m module.Version, unique bool) {
+	if len(cs.pkgMods) == 0 && cs.mod.Path == "" {
+		panic("internal error: resolveIfUnambiguous called with empty pathSet")
+	}
+
+	for _, m := range cs.pkgMods {
+		if _, ok := r.noneForPath(m.Path); ok {
+			// A query with version "none" forces the candidate module to version
+			// "none", so we cannot use any other version for that module.
+			continue
+		}
+
+		if modload.MainModules.Contains(m.Path) {
+			if m.Version == "" {
+				return pathSet{}, true, m, true
+			}
+			// A main module can only be set to its own version.
+			continue
+		}
+
+		vr, ok := r.resolvedVersion[m.Path]
+		if !ok {
+			// m is a viable answer to the query, but other answers may also
+			// still be viable.
+			filtered.pkgMods = append(filtered.pkgMods, m)
+			continue
+		}
+
+		if vr.version != m.Version {
+			// Some query forces the candidate module to a version other than this
+			// one.
+			//
+			// The command could be something like
+			//
+			// 	go get example.com/foo/bar@none example.com/foo/bar/baz@latest
+			//
+			// in which case we *cannot* resolve the package from
+			// example.com/foo/bar (because it is constrained to version
+			// "none") and must fall through to module example.com/foo@latest.
+			continue
+		}
+
+		// Some query forces the candidate module *to* the candidate version.
+		// As a result, this candidate is the only viable choice to provide
+		// its package(s): any other choice would result in an ambiguous import
+		// for this path.
+		//
+		// For example, consider the command
+		//
+		// 	go get example.com/foo@latest example.com/foo/bar/baz@latest
+		//
+		// If modules example.com/foo and example.com/foo/bar both provide
+		// package example.com/foo/bar/baz, then we *must* resolve the package
+		// from example.com/foo: if we instead resolved it from
+		// example.com/foo/bar, we would have two copies of the package.
+		return pathSet{}, true, m, true
+	}
+
+	if cs.mod.Path != "" {
+		vr, ok := r.resolvedVersion[cs.mod.Path]
+		if !ok || vr.version == cs.mod.Version {
+			filtered.mod = cs.mod
+		}
+	}
+
+	if len(filtered.pkgMods) == 1 &&
+		(filtered.mod.Path == "" || filtered.mod == filtered.pkgMods[0]) {
+		// Exactly one viable module contains the package with the given path
+		// (by far the common case), so we can resolve it unambiguously.
+		return pathSet{}, true, filtered.pkgMods[0], true
+	}
+
+	if len(filtered.pkgMods) == 0 {
+		// All modules that could provide the path as a package conflict with other
+		// resolved arguments. If it can refer to a module instead, return that;
+		// otherwise, this pathSet cannot be resolved (and we will return the
+		// zero module.Version).
+		return pathSet{}, false, filtered.mod, true
+	}
+
+	// The query remains ambiguous: there are at least two different modules
+	// to which cs.path could refer.
+	return filtered, false, module.Version{}, false
+}
+
+// chooseArbitrarily returns an arbitrary (but deterministic) module version
+// from among those in the given set.
+//
+// chooseArbitrarily prefers module paths that were already in the build list at
+// the start of 'go get', prefers modules that provide packages over those that
+// do not, and chooses the first module meeting those criteria (so biases toward
+// longer paths).
+func (r *resolver) chooseArbitrarily(cs pathSet) (isPackage bool, m module.Version) {
+	// Prefer to upgrade some module that was already in the build list.
+	for _, m := range cs.pkgMods {
+		if r.initialSelected(m.Path) != "none" {
+			return true, m
+		}
+	}
+
+	// Otherwise, arbitrarily choose the first module that provides the package.
+	if len(cs.pkgMods) > 0 {
+		return true, cs.pkgMods[0]
+	}
+
+	return false, cs.mod
+}
+
+// checkPackageProblems reloads packages for the given patterns and reports
+// missing and ambiguous package errors. It also reports retractions and
+// deprecations for resolved modules and modules needed to build named packages.
+// It also adds a sum for each updated module in the build list if we had one
+// before and didn't get one while loading packages.
+//
+// We skip missing-package errors earlier in the process, since we want to
+// resolve pathSets ourselves, but at that point, we don't have enough context
+// to log the package-import chains leading to each error.
+func (r *resolver) checkPackageProblems(ctx context.Context, pkgPatterns []string) {
+	defer base.ExitIfErrors()
+
+	// Gather information about modules we might want to load retractions and
+	// deprecations for. Loading this metadata requires at least one version
+	// lookup per module, and we don't want to load information that's neither
+	// relevant nor actionable.
+	type modFlags int
+	const (
+		resolved modFlags = 1 << iota // version resolved by 'go get'
+		named                         // explicitly named on command line or provides a named package
+		hasPkg                        // needed to build named packages
+		direct                        // provides a direct dependency of the main module
+	)
+	relevantMods := make(map[module.Version]modFlags)
+	for path, reason := range r.resolvedVersion {
+		m := module.Version{Path: path, Version: reason.version}
+		relevantMods[m] |= resolved
+	}
+
+	// Reload packages, reporting errors for missing and ambiguous imports.
+	if len(pkgPatterns) > 0 {
+		// LoadPackages will print errors (since it has more context) but will not
+		// exit, since we need to load retractions later.
+		pkgOpts := modload.PackageOpts{
+			VendorModulesInGOROOTSrc: true,
+			LoadTests:                *getT,
+			ResolveMissingImports:    false,
+			AllowErrors:              true,
+			SilenceNoGoErrors:        true,
+		}
+		matches, pkgs := modload.LoadPackages(ctx, pkgOpts, pkgPatterns...)
+		for _, m := range matches {
+			if len(m.Errs) > 0 {
+				base.SetExitStatus(1)
+				break
+			}
+		}
+		for _, pkg := range pkgs {
+			if dir, _, err := modload.Lookup("", false, pkg); err != nil {
+				if dir != "" && errors.Is(err, imports.ErrNoGo) {
+					// Since dir is non-empty, we must have located source files
+					// associated with either the package or its test — ErrNoGo must
+					// indicate that none of those source files happen to apply in this
+					// configuration. If we are actually building the package (no -d
+					// flag), we will report the problem then; otherwise, assume that the
+					// user is going to build or test this package in some other
+					// configuration and suppress the error.
+					continue
+				}
+
+				base.SetExitStatus(1)
+				if ambiguousErr := (*modload.AmbiguousImportError)(nil); errors.As(err, &ambiguousErr) {
+					for _, m := range ambiguousErr.Modules {
+						relevantMods[m] |= hasPkg
+					}
+				}
+			}
+			if m := modload.PackageModule(pkg); m.Path != "" {
+				relevantMods[m] |= hasPkg
+			}
+		}
+		for _, match := range matches {
+			for _, pkg := range match.Pkgs {
+				m := modload.PackageModule(pkg)
+				relevantMods[m] |= named
+			}
+		}
+	}
+
+	reqs := modload.LoadModFile(ctx)
+	for m := range relevantMods {
+		if reqs.IsDirect(m.Path) {
+			relevantMods[m] |= direct
+		}
+	}
+
+	// Load retractions for modules mentioned on the command line and modules
+	// needed to build named packages. We care about retractions of indirect
+	// dependencies, since we might be able to upgrade away from them.
+	type modMessage struct {
+		m       module.Version
+		message string
+	}
+	retractions := make([]modMessage, 0, len(relevantMods))
+	for m, flags := range relevantMods {
+		if flags&(resolved|named|hasPkg) != 0 {
+			retractions = append(retractions, modMessage{m: m})
+		}
+	}
+	sort.Slice(retractions, func(i, j int) bool { return retractions[i].m.Path < retractions[j].m.Path })
+	for i := range retractions {
+		i := i
+		r.work.Add(func() {
+			err := modload.CheckRetractions(ctx, retractions[i].m)
+			if retractErr := (*modload.ModuleRetractedError)(nil); errors.As(err, &retractErr) {
+				retractions[i].message = err.Error()
+			}
+		})
+	}
+
+	// Load deprecations for modules mentioned on the command line. Only load
+	// deprecations for indirect dependencies if they're also direct dependencies
+	// of the main module. Deprecations of purely indirect dependencies are
+	// not actionable.
+	deprecations := make([]modMessage, 0, len(relevantMods))
+	for m, flags := range relevantMods {
+		if flags&(resolved|named) != 0 || flags&(hasPkg|direct) == hasPkg|direct {
+			deprecations = append(deprecations, modMessage{m: m})
+		}
+	}
+	sort.Slice(deprecations, func(i, j int) bool { return deprecations[i].m.Path < deprecations[j].m.Path })
+	for i := range deprecations {
+		i := i
+		r.work.Add(func() {
+			deprecation, err := modload.CheckDeprecation(ctx, deprecations[i].m)
+			if err != nil || deprecation == "" {
+				return
+			}
+			deprecations[i].message = modload.ShortMessage(deprecation, "")
+		})
+	}
+
+	// Load sums for updated modules that had sums before. When we update a
+	// module, we may update another module in the build list that provides a
+	// package in 'all' that wasn't loaded as part of this 'go get' command.
+	// If we don't add a sum for that module, builds may fail later.
+	// Note that an incidentally updated package could still import packages
+	// from unknown modules or from modules in the build list that we didn't
+	// need previously. We can't handle that case without loading 'all'.
+	sumErrs := make([]error, len(r.buildList))
+	for i := range r.buildList {
+		i := i
+		m := r.buildList[i]
+		mActual := m
+		if mRepl := modload.Replacement(m); mRepl.Path != "" {
+			mActual = mRepl
+		}
+		old := module.Version{Path: m.Path, Version: r.initialVersion[m.Path]}
+		if old.Version == "" {
+			continue
+		}
+		oldActual := old
+		if oldRepl := modload.Replacement(old); oldRepl.Path != "" {
+			oldActual = oldRepl
+		}
+		if mActual == oldActual || mActual.Version == "" || !modfetch.HaveSum(oldActual) {
+			continue
+		}
+		r.work.Add(func() {
+			if _, err := modfetch.DownloadZip(ctx, mActual); err != nil {
+				verb := "upgraded"
+				if gover.ModCompare(m.Path, m.Version, old.Version) < 0 {
+					verb = "downgraded"
+				}
+				replaced := ""
+				if mActual != m {
+					replaced = fmt.Sprintf(" (replaced by %s)", mActual)
+				}
+				err = fmt.Errorf("%s %s %s => %s%s: error finding sum for %s: %v", verb, m.Path, old.Version, m.Version, replaced, mActual, err)
+				sumErrs[i] = err
+			}
+		})
+	}
+
+	<-r.work.Idle()
+
+	// Report deprecations, then retractions, then errors fetching sums.
+	// Only errors fetching sums are hard errors.
+	for _, mm := range deprecations {
+		if mm.message != "" {
+			fmt.Fprintf(os.Stderr, "go: module %s is deprecated: %s\n", mm.m.Path, mm.message)
+		}
+	}
+	var retractPath string
+	for _, mm := range retractions {
+		if mm.message != "" {
+			fmt.Fprintf(os.Stderr, "go: warning: %v\n", mm.message)
+			if retractPath == "" {
+				retractPath = mm.m.Path
+			} else {
+				retractPath = "<module>"
+			}
+		}
+	}
+	if retractPath != "" {
+		fmt.Fprintf(os.Stderr, "go: to switch to the latest unretracted version, run:\n\tgo get %s@latest\n", retractPath)
+	}
+	for _, err := range sumErrs {
+		if err != nil {
+			base.Error(err)
+		}
+	}
+	base.ExitIfErrors()
+}
+
+// reportChanges logs version changes to os.Stderr.
+//
+// reportChanges only logs changes to modules named on the command line and to
+// explicitly required modules in go.mod. Most changes to indirect requirements
+// are not relevant to the user and are not logged.
+//
+// reportChanges should be called after WriteGoMod.
+func (r *resolver) reportChanges(oldReqs, newReqs []module.Version) {
+	type change struct {
+		path, old, new string
+	}
+	changes := make(map[string]change)
+
+	// Collect changes in modules matched by command line arguments.
+	for path, reason := range r.resolvedVersion {
+		if gover.IsToolchain(path) {
+			continue
+		}
+		old := r.initialVersion[path]
+		new := reason.version
+		if old != new && (old != "" || new != "none") {
+			changes[path] = change{path, old, new}
+		}
+	}
+
+	// Collect changes to explicit requirements in go.mod.
+	for _, req := range oldReqs {
+		if gover.IsToolchain(req.Path) {
+			continue
+		}
+		path := req.Path
+		old := req.Version
+		new := r.buildListVersion[path]
+		if old != new {
+			changes[path] = change{path, old, new}
+		}
+	}
+	for _, req := range newReqs {
+		if gover.IsToolchain(req.Path) {
+			continue
+		}
+		path := req.Path
+		old := r.initialVersion[path]
+		new := req.Version
+		if old != new {
+			changes[path] = change{path, old, new}
+		}
+	}
+
+	// Toolchain diffs are easier than requirements: diff old and new directly.
+	toolchainVersions := func(reqs []module.Version) (goV, toolchain string) {
+		for _, req := range reqs {
+			if req.Path == "go" {
+				goV = req.Version
+			}
+			if req.Path == "toolchain" {
+				toolchain = req.Version
+			}
+		}
+		return
+	}
+	oldGo, oldToolchain := toolchainVersions(oldReqs)
+	newGo, newToolchain := toolchainVersions(newReqs)
+	if oldGo != newGo {
+		changes["go"] = change{"go", oldGo, newGo}
+	}
+	if oldToolchain != newToolchain {
+		changes["toolchain"] = change{"toolchain", oldToolchain, newToolchain}
+	}
+
+	sortedChanges := make([]change, 0, len(changes))
+	for _, c := range changes {
+		sortedChanges = append(sortedChanges, c)
+	}
+	sort.Slice(sortedChanges, func(i, j int) bool {
+		pi := sortedChanges[i].path
+		pj := sortedChanges[j].path
+		if pi == pj {
+			return false
+		}
+		// go first; toolchain second
+		switch {
+		case pi == "go":
+			return true
+		case pj == "go":
+			return false
+		case pi == "toolchain":
+			return true
+		case pj == "toolchain":
+			return false
+		}
+		return pi < pj
+	})
+
+	for _, c := range sortedChanges {
+		if c.old == "" {
+			fmt.Fprintf(os.Stderr, "go: added %s %s\n", c.path, c.new)
+		} else if c.new == "none" || c.new == "" {
+			fmt.Fprintf(os.Stderr, "go: removed %s %s\n", c.path, c.old)
+		} else if gover.ModCompare(c.path, c.new, c.old) > 0 {
+			fmt.Fprintf(os.Stderr, "go: upgraded %s %s => %s\n", c.path, c.old, c.new)
+			if c.path == "go" && gover.Compare(c.old, gover.ExplicitIndirectVersion) < 0 && gover.Compare(c.new, gover.ExplicitIndirectVersion) >= 0 {
+				fmt.Fprintf(os.Stderr, "\tnote: expanded dependencies to upgrade to go %s or higher; run 'go mod tidy' to clean up\n", gover.ExplicitIndirectVersion)
+			}
+
+		} else {
+			fmt.Fprintf(os.Stderr, "go: downgraded %s %s => %s\n", c.path, c.old, c.new)
+		}
+	}
+
+	// TODO(golang.org/issue/33284): attribute changes to command line arguments.
+	// For modules matched by command line arguments, this probably isn't
+	// necessary, but it would be useful for unmatched direct dependencies of
+	// the main module.
+}
+
+// resolve records that module m must be at its indicated version (which may be
+// "none") due to query q. If some other query forces module m to be at a
+// different version, resolve reports a conflict error.
+func (r *resolver) resolve(q *query, m module.Version) {
+	if m.Path == "" {
+		panic("internal error: resolving a module.Version with an empty path")
+	}
+
+	if modload.MainModules.Contains(m.Path) && m.Version != "" {
+		reportError(q, &modload.QueryMatchesMainModulesError{
+			MainModules: []module.Version{{Path: m.Path}},
+			Pattern:     q.pattern,
+			Query:       q.version,
+		})
+		return
+	}
+
+	vr, ok := r.resolvedVersion[m.Path]
+	if ok && vr.version != m.Version {
+		reportConflict(q, m, vr)
+		return
+	}
+	r.resolvedVersion[m.Path] = versionReason{m.Version, q}
+	q.resolved = append(q.resolved, m)
+}
+
+// updateBuildList updates the module loader's global build list to be
+// consistent with r.resolvedVersion, and to include additional modules
+// provided that they do not conflict with the resolved versions.
+//
+// If the additional modules conflict with the resolved versions, they will be
+// downgraded to a non-conflicting version (possibly "none").
+//
+// If the resulting build list is the same as the one resulting from the last
+// call to updateBuildList, updateBuildList returns with changed=false.
+func (r *resolver) updateBuildList(ctx context.Context, additions []module.Version) (changed bool) {
+	defer base.ExitIfErrors()
+
+	resolved := make([]module.Version, 0, len(r.resolvedVersion))
+	for mPath, rv := range r.resolvedVersion {
+		if !modload.MainModules.Contains(mPath) {
+			resolved = append(resolved, module.Version{Path: mPath, Version: rv.version})
+		}
+	}
+
+	changed, err := modload.EditBuildList(ctx, additions, resolved)
+	if err != nil {
+		if errors.Is(err, gover.ErrTooNew) {
+			toolchain.SwitchOrFatal(ctx, err)
+		}
+
+		var constraint *modload.ConstraintError
+		if !errors.As(err, &constraint) {
+			base.Fatal(err)
+		}
+
+		if cfg.BuildV {
+			// Log complete paths for the conflicts before we summarize them.
+			for _, c := range constraint.Conflicts {
+				fmt.Fprintf(os.Stderr, "go: %v\n", c.String())
+			}
+		}
+
+		// modload.EditBuildList reports constraint errors at
+		// the module level, but 'go get' operates on packages.
+		// Rewrite the errors to explain them in terms of packages.
+		reason := func(m module.Version) string {
+			rv, ok := r.resolvedVersion[m.Path]
+			if !ok {
+				return fmt.Sprintf("(INTERNAL ERROR: no reason found for %v)", m)
+			}
+			return rv.reason.ResolvedString(module.Version{Path: m.Path, Version: rv.version})
+		}
+		for _, c := range constraint.Conflicts {
+			adverb := ""
+			if len(c.Path) > 2 {
+				adverb = "indirectly "
+			}
+			firstReason := reason(c.Path[0])
+			last := c.Path[len(c.Path)-1]
+			if c.Err != nil {
+				base.Errorf("go: %v %srequires %v: %v", firstReason, adverb, last, c.UnwrapModuleError())
+			} else {
+				base.Errorf("go: %v %srequires %v, not %v", firstReason, adverb, last, reason(c.Constraint))
+			}
+		}
+		return false
+	}
+	if !changed {
+		return false
+	}
+
+	mg, err := modload.LoadModGraph(ctx, "")
+	if err != nil {
+		toolchain.SwitchOrFatal(ctx, err)
+	}
+
+	r.buildList = mg.BuildList()
+	r.buildListVersion = make(map[string]string, len(r.buildList))
+	for _, m := range r.buildList {
+		r.buildListVersion[m.Path] = m.Version
+	}
+	return true
+}
+
+func reqsFromGoMod(f *modfile.File) []module.Version {
+	reqs := make([]module.Version, len(f.Require), 2+len(f.Require))
+	for i, r := range f.Require {
+		reqs[i] = r.Mod
+	}
+	if f.Go != nil {
+		reqs = append(reqs, module.Version{Path: "go", Version: f.Go.Version})
+	}
+	if f.Toolchain != nil {
+		reqs = append(reqs, module.Version{Path: "toolchain", Version: f.Toolchain.Name})
+	}
+	return reqs
+}
+
+// isNoSuchModuleVersion reports whether err indicates that the requested module
+// does not exist at the requested version, either because the module does not
+// exist at all or because it does not include that specific version.
+func isNoSuchModuleVersion(err error) bool {
+	var noMatch *modload.NoMatchingVersionError
+	return errors.Is(err, os.ErrNotExist) || errors.As(err, &noMatch)
+}
+
+// isNoSuchPackageVersion reports whether err indicates that the requested
+// package does not exist at the requested version, either because no module
+// that could contain it exists at that version, or because every such module
+// that does exist does not actually contain the package.
+func isNoSuchPackageVersion(err error) bool {
+	var noPackage *modload.PackageNotInModuleError
+	return isNoSuchModuleVersion(err) || errors.As(err, &noPackage)
+}
diff --git a/src/cmd/go/internal/modget/query.go b/src/cmd/go/internal/modget/query.go
new file mode 100644
index 0000000..b78c1c4
--- /dev/null
+++ b/src/cmd/go/internal/modget/query.go
@@ -0,0 +1,358 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modget
+
+import (
+	"fmt"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"sync"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/search"
+	"cmd/go/internal/str"
+	"cmd/internal/pkgpattern"
+
+	"golang.org/x/mod/module"
+)
+
+// A query describes a command-line argument and the modules and/or packages
+// to which that argument may resolve..
+type query struct {
+	// raw is the original argument, to be printed in error messages.
+	raw string
+
+	// rawVersion is the portion of raw corresponding to version, if any
+	rawVersion string
+
+	// pattern is the part of the argument before "@" (or the whole argument
+	// if there is no "@"), which may match either packages (preferred) or
+	// modules (if no matching packages).
+	//
+	// The pattern may also be "-u", for the synthetic query representing the -u
+	// (“upgrade”)flag.
+	pattern string
+
+	// patternIsLocal indicates whether pattern is restricted to match only paths
+	// local to the main module, such as absolute filesystem paths or paths
+	// beginning with './'.
+	//
+	// A local pattern must resolve to one or more packages in the main module.
+	patternIsLocal bool
+
+	// version is the part of the argument after "@", or an implied
+	// "upgrade" or "patch" if there is no "@". version specifies the
+	// module version to get.
+	version string
+
+	// matchWildcard, if non-nil, reports whether pattern, which must be a
+	// wildcard (with the substring "..."), matches the given package or module
+	// path.
+	matchWildcard func(path string) bool
+
+	// canMatchWildcard, if non-nil, reports whether the module with the given
+	// path could lexically contain a package matching pattern, which must be a
+	// wildcard.
+	canMatchWildcardInModule func(mPath string) bool
+
+	// conflict is the first query identified as incompatible with this one.
+	// conflict forces one or more of the modules matching this query to a
+	// version that does not match version.
+	conflict *query
+
+	// candidates is a list of sets of alternatives for a path that matches (or
+	// contains packages that match) the pattern. The query can be resolved by
+	// choosing exactly one alternative from each set in the list.
+	//
+	// A path-literal query results in only one set: the path itself, which
+	// may resolve to either a package path or a module path.
+	//
+	// A wildcard query results in one set for each matching module path, each
+	// module for which the matching version contains at least one matching
+	// package, and (if no other modules match) one candidate set for the pattern
+	// overall if no existing match is identified in the build list.
+	//
+	// A query for pattern "all" results in one set for each package transitively
+	// imported by the main module.
+	//
+	// The special query for the "-u" flag results in one set for each
+	// otherwise-unconstrained package that has available upgrades.
+	candidates   []pathSet
+	candidatesMu sync.Mutex
+
+	// pathSeen ensures that only one pathSet is added to the query per
+	// unique path.
+	pathSeen sync.Map
+
+	// resolved contains the set of modules whose versions have been determined by
+	// this query, in the order in which they were determined.
+	//
+	// The resolver examines the candidate sets for each query, resolving one
+	// module per candidate set in a way that attempts to avoid obvious conflicts
+	// between the versions resolved by different queries.
+	resolved []module.Version
+
+	// matchesPackages is true if the resolved modules provide at least one
+	// package matching q.pattern.
+	matchesPackages bool
+}
+
+// A pathSet describes the possible options for resolving a specific path
+// to a package and/or module.
+type pathSet struct {
+	// path is a package (if "all" or "-u" or a non-wildcard) or module (if
+	// wildcard) path that could be resolved by adding any of the modules in this
+	// set. For a wildcard pattern that so far matches no packages, the path is
+	// the wildcard pattern itself.
+	//
+	// Each path must occur only once in a query's candidate sets, and the path is
+	// added implicitly to each pathSet returned to pathOnce.
+	path string
+
+	// pkgMods is a set of zero or more modules, each of which contains the
+	// package with the indicated path. Due to the requirement that imports be
+	// unambiguous, only one such module can be in the build list, and all others
+	// must be excluded.
+	pkgMods []module.Version
+
+	// mod is either the zero Version, or a module that does not contain any
+	// packages matching the query but for which the module path itself
+	// matches the query pattern.
+	//
+	// We track this module separately from pkgMods because, all else equal, we
+	// prefer to match a query to a package rather than just a module. Also,
+	// unlike the modules in pkgMods, this module does not inherently exclude
+	// any other module in pkgMods.
+	mod module.Version
+
+	err error
+}
+
+// errSet returns a pathSet containing the given error.
+func errSet(err error) pathSet { return pathSet{err: err} }
+
+// newQuery returns a new query parsed from the raw argument,
+// which must be either path or path@version.
+func newQuery(raw string) (*query, error) {
+	pattern, rawVers, found := strings.Cut(raw, "@")
+	if found && (strings.Contains(rawVers, "@") || rawVers == "") {
+		return nil, fmt.Errorf("invalid module version syntax %q", raw)
+	}
+
+	// If no version suffix is specified, assume @upgrade.
+	// If -u=patch was specified, assume @patch instead.
+	version := rawVers
+	if version == "" {
+		if getU.version == "" {
+			version = "upgrade"
+		} else {
+			version = getU.version
+		}
+	}
+
+	q := &query{
+		raw:            raw,
+		rawVersion:     rawVers,
+		pattern:        pattern,
+		patternIsLocal: filepath.IsAbs(pattern) || search.IsRelativePath(pattern),
+		version:        version,
+	}
+	if strings.Contains(q.pattern, "...") {
+		q.matchWildcard = pkgpattern.MatchPattern(q.pattern)
+		q.canMatchWildcardInModule = pkgpattern.TreeCanMatchPattern(q.pattern)
+	}
+	if err := q.validate(); err != nil {
+		return q, err
+	}
+	return q, nil
+}
+
+// validate reports a non-nil error if q is not sensible and well-formed.
+func (q *query) validate() error {
+	if q.patternIsLocal {
+		if q.rawVersion != "" {
+			return fmt.Errorf("can't request explicit version %q of path %q in main module", q.rawVersion, q.pattern)
+		}
+		return nil
+	}
+
+	if q.pattern == "all" {
+		// If there is no main module, "all" is not meaningful.
+		if !modload.HasModRoot() {
+			return fmt.Errorf(`cannot match "all": %v`, modload.ErrNoModRoot)
+		}
+		if !versionOkForMainModule(q.version) {
+			// TODO(bcmills): "all@none" seems like a totally reasonable way to
+			// request that we remove all module requirements, leaving only the main
+			// module and standard library. Perhaps we should implement that someday.
+			return &modload.QueryUpgradesAllError{
+				MainModules: modload.MainModules.Versions(),
+				Query:       q.version,
+			}
+		}
+	}
+
+	if search.IsMetaPackage(q.pattern) && q.pattern != "all" {
+		if q.pattern != q.raw {
+			return fmt.Errorf("can't request explicit version of standard-library pattern %q", q.pattern)
+		}
+	}
+
+	return nil
+}
+
+// String returns the original argument from which q was parsed.
+func (q *query) String() string { return q.raw }
+
+// ResolvedString returns a string describing m as a resolved match for q.
+func (q *query) ResolvedString(m module.Version) string {
+	if m.Path != q.pattern {
+		if m.Version != q.version {
+			return fmt.Sprintf("%v (matching %s@%s)", m, q.pattern, q.version)
+		}
+		return fmt.Sprintf("%v (matching %v)", m, q)
+	}
+	if m.Version != q.version {
+		return fmt.Sprintf("%s@%s (%s)", q.pattern, q.version, m.Version)
+	}
+	return q.String()
+}
+
+// isWildcard reports whether q is a pattern that can match multiple paths.
+func (q *query) isWildcard() bool {
+	return q.matchWildcard != nil || (q.patternIsLocal && strings.Contains(q.pattern, "..."))
+}
+
+// matchesPath reports whether the given path matches q.pattern.
+func (q *query) matchesPath(path string) bool {
+	if q.matchWildcard != nil && !gover.IsToolchain(path) {
+		return q.matchWildcard(path)
+	}
+	return path == q.pattern
+}
+
+// canMatchInModule reports whether the given module path can potentially
+// contain q.pattern.
+func (q *query) canMatchInModule(mPath string) bool {
+	if gover.IsToolchain(mPath) {
+		return false
+	}
+	if q.canMatchWildcardInModule != nil {
+		return q.canMatchWildcardInModule(mPath)
+	}
+	return str.HasPathPrefix(q.pattern, mPath)
+}
+
+// pathOnce invokes f to generate the pathSet for the given path,
+// if one is still needed.
+//
+// Note that, unlike sync.Once, pathOnce does not guarantee that a concurrent
+// call to f for the given path has completed on return.
+//
+// pathOnce is safe for concurrent use by multiple goroutines, but note that
+// multiple concurrent calls will result in the sets being added in
+// nondeterministic order.
+func (q *query) pathOnce(path string, f func() pathSet) {
+	if _, dup := q.pathSeen.LoadOrStore(path, nil); dup {
+		return
+	}
+
+	cs := f()
+
+	if len(cs.pkgMods) > 0 || cs.mod != (module.Version{}) || cs.err != nil {
+		cs.path = path
+		q.candidatesMu.Lock()
+		q.candidates = append(q.candidates, cs)
+		q.candidatesMu.Unlock()
+	}
+}
+
+// reportError logs err concisely using base.Errorf.
+func reportError(q *query, err error) {
+	errStr := err.Error()
+
+	// If err already mentions all of the relevant parts of q, just log err to
+	// reduce stutter. Otherwise, log both q and err.
+	//
+	// TODO(bcmills): Use errors.As to unpack these errors instead of parsing
+	// strings with regular expressions.
+
+	patternRE := regexp.MustCompile("(?m)(?:[ \t(\"`]|^)" + regexp.QuoteMeta(q.pattern) + "(?:[ @:;)\"`]|$)")
+	if patternRE.MatchString(errStr) {
+		if q.rawVersion == "" {
+			base.Errorf("go: %s", errStr)
+			return
+		}
+
+		versionRE := regexp.MustCompile("(?m)(?:[ @(\"`]|^)" + regexp.QuoteMeta(q.version) + "(?:[ :;)\"`]|$)")
+		if versionRE.MatchString(errStr) {
+			base.Errorf("go: %s", errStr)
+			return
+		}
+	}
+
+	if qs := q.String(); qs != "" {
+		base.Errorf("go: %s: %s", qs, errStr)
+	} else {
+		base.Errorf("go: %s", errStr)
+	}
+}
+
+func reportConflict(pq *query, m module.Version, conflict versionReason) {
+	if pq.conflict != nil {
+		// We've already reported a conflict for the proposed query.
+		// Don't report it again, even if it has other conflicts.
+		return
+	}
+	pq.conflict = conflict.reason
+
+	proposed := versionReason{
+		version: m.Version,
+		reason:  pq,
+	}
+	if pq.isWildcard() && !conflict.reason.isWildcard() {
+		// Prefer to report the specific path first and the wildcard second.
+		proposed, conflict = conflict, proposed
+	}
+	reportError(pq, &conflictError{
+		mPath:    m.Path,
+		proposed: proposed,
+		conflict: conflict,
+	})
+}
+
+type conflictError struct {
+	mPath    string
+	proposed versionReason
+	conflict versionReason
+}
+
+func (e *conflictError) Error() string {
+	argStr := func(q *query, v string) string {
+		if v != q.version {
+			return fmt.Sprintf("%s@%s (%s)", q.pattern, q.version, v)
+		}
+		return q.String()
+	}
+
+	pq := e.proposed.reason
+	rq := e.conflict.reason
+	modDetail := ""
+	if e.mPath != pq.pattern {
+		modDetail = fmt.Sprintf("for module %s, ", e.mPath)
+	}
+
+	return fmt.Sprintf("%s%s conflicts with %s",
+		modDetail,
+		argStr(pq, e.proposed.version),
+		argStr(rq, e.conflict.version))
+}
+
+func versionOkForMainModule(version string) bool {
+	return version == "upgrade" || version == "patch"
+}
diff --git a/src/cmd/go/internal/modindex/build.go b/src/cmd/go/internal/modindex/build.go
new file mode 100644
index 0000000..b57f2f6
--- /dev/null
+++ b/src/cmd/go/internal/modindex/build.go
@@ -0,0 +1,950 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file is a lightly modified copy go/build/build.go with unused parts
+// removed.
+
+package modindex
+
+import (
+	"bytes"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/str"
+	"errors"
+	"fmt"
+	"go/ast"
+	"go/build"
+	"go/build/constraint"
+	"go/token"
+	"io"
+	"io/fs"
+	"path/filepath"
+	"sort"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
+// A Context specifies the supporting context for a build.
+type Context struct {
+	GOARCH string // target architecture
+	GOOS   string // target operating system
+	GOROOT string // Go root
+	GOPATH string // Go paths
+
+	// Dir is the caller's working directory, or the empty string to use
+	// the current directory of the running process. In module mode, this is used
+	// to locate the main module.
+	//
+	// If Dir is non-empty, directories passed to Import and ImportDir must
+	// be absolute.
+	Dir string
+
+	CgoEnabled  bool   // whether cgo files are included
+	UseAllFiles bool   // use files regardless of //go:build lines, file names
+	Compiler    string // compiler to assume when computing target paths
+
+	// The build, tool, and release tags specify build constraints
+	// that should be considered satisfied when processing +build lines.
+	// Clients creating a new context may customize BuildTags, which
+	// defaults to empty, but it is usually an error to customize ToolTags or ReleaseTags.
+	// ToolTags defaults to build tags appropriate to the current Go toolchain configuration.
+	// ReleaseTags defaults to the list of Go releases the current release is compatible with.
+	// BuildTags is not set for the Default build Context.
+	// In addition to the BuildTags, ToolTags, and ReleaseTags, build constraints
+	// consider the values of GOARCH and GOOS as satisfied tags.
+	// The last element in ReleaseTags is assumed to be the current release.
+	BuildTags   []string
+	ToolTags    []string
+	ReleaseTags []string
+
+	// The install suffix specifies a suffix to use in the name of the installation
+	// directory. By default it is empty, but custom builds that need to keep
+	// their outputs separate can set InstallSuffix to do so. For example, when
+	// using the race detector, the go command uses InstallSuffix = "race", so
+	// that on a Linux/386 system, packages are written to a directory named
+	// "linux_386_race" instead of the usual "linux_386".
+	InstallSuffix string
+
+	// By default, Import uses the operating system's file system calls
+	// to read directories and files. To read from other sources,
+	// callers can set the following functions. They all have default
+	// behaviors that use the local file system, so clients need only set
+	// the functions whose behaviors they wish to change.
+
+	// JoinPath joins the sequence of path fragments into a single path.
+	// If JoinPath is nil, Import uses filepath.Join.
+	JoinPath func(elem ...string) string
+
+	// SplitPathList splits the path list into a slice of individual paths.
+	// If SplitPathList is nil, Import uses filepath.SplitList.
+	SplitPathList func(list string) []string
+
+	// IsAbsPath reports whether path is an absolute path.
+	// If IsAbsPath is nil, Import uses filepath.IsAbs.
+	IsAbsPath func(path string) bool
+
+	// IsDir reports whether the path names a directory.
+	// If IsDir is nil, Import calls os.Stat and uses the result's IsDir method.
+	IsDir func(path string) bool
+
+	// HasSubdir reports whether dir is lexically a subdirectory of
+	// root, perhaps multiple levels below. It does not try to check
+	// whether dir exists.
+	// If so, HasSubdir sets rel to a slash-separated path that
+	// can be joined to root to produce a path equivalent to dir.
+	// If HasSubdir is nil, Import uses an implementation built on
+	// filepath.EvalSymlinks.
+	HasSubdir func(root, dir string) (rel string, ok bool)
+
+	// ReadDir returns a slice of fs.FileInfo, sorted by Name,
+	// describing the content of the named directory.
+	// If ReadDir is nil, Import uses ioutil.ReadDir.
+	ReadDir func(dir string) ([]fs.FileInfo, error)
+
+	// OpenFile opens a file (not a directory) for reading.
+	// If OpenFile is nil, Import uses os.Open.
+	OpenFile func(path string) (io.ReadCloser, error)
+}
+
+// joinPath calls ctxt.JoinPath (if not nil) or else filepath.Join.
+func (ctxt *Context) joinPath(elem ...string) string {
+	if f := ctxt.JoinPath; f != nil {
+		return f(elem...)
+	}
+	return filepath.Join(elem...)
+}
+
+// splitPathList calls ctxt.SplitPathList (if not nil) or else filepath.SplitList.
+func (ctxt *Context) splitPathList(s string) []string {
+	if f := ctxt.SplitPathList; f != nil {
+		return f(s)
+	}
+	return filepath.SplitList(s)
+}
+
+// isAbsPath calls ctxt.IsAbsPath (if not nil) or else filepath.IsAbs.
+func (ctxt *Context) isAbsPath(path string) bool {
+	if f := ctxt.IsAbsPath; f != nil {
+		return f(path)
+	}
+	return filepath.IsAbs(path)
+}
+
+// isDir calls ctxt.IsDir (if not nil) or else uses fsys.Stat.
+func isDir(path string) bool {
+	fi, err := fsys.Stat(path)
+	return err == nil && fi.IsDir()
+}
+
+// hasSubdir calls ctxt.HasSubdir (if not nil) or else uses
+// the local file system to answer the question.
+func (ctxt *Context) hasSubdir(root, dir string) (rel string, ok bool) {
+	if f := ctxt.HasSubdir; f != nil {
+		return f(root, dir)
+	}
+
+	// Try using paths we received.
+	if rel, ok = hasSubdir(root, dir); ok {
+		return
+	}
+
+	// Try expanding symlinks and comparing
+	// expanded against unexpanded and
+	// expanded against expanded.
+	rootSym, _ := filepath.EvalSymlinks(root)
+	dirSym, _ := filepath.EvalSymlinks(dir)
+
+	if rel, ok = hasSubdir(rootSym, dir); ok {
+		return
+	}
+	if rel, ok = hasSubdir(root, dirSym); ok {
+		return
+	}
+	return hasSubdir(rootSym, dirSym)
+}
+
+// hasSubdir reports if dir is within root by performing lexical analysis only.
+func hasSubdir(root, dir string) (rel string, ok bool) {
+	root = str.WithFilePathSeparator(filepath.Clean(root))
+	dir = filepath.Clean(dir)
+	if !strings.HasPrefix(dir, root) {
+		return "", false
+	}
+	return filepath.ToSlash(dir[len(root):]), true
+}
+
+// gopath returns the list of Go path directories.
+func (ctxt *Context) gopath() []string {
+	var all []string
+	for _, p := range ctxt.splitPathList(ctxt.GOPATH) {
+		if p == "" || p == ctxt.GOROOT {
+			// Empty paths are uninteresting.
+			// If the path is the GOROOT, ignore it.
+			// People sometimes set GOPATH=$GOROOT.
+			// Do not get confused by this common mistake.
+			continue
+		}
+		if strings.HasPrefix(p, "~") {
+			// Path segments starting with ~ on Unix are almost always
+			// users who have incorrectly quoted ~ while setting GOPATH,
+			// preventing it from expanding to $HOME.
+			// The situation is made more confusing by the fact that
+			// bash allows quoted ~ in $PATH (most shells do not).
+			// Do not get confused by this, and do not try to use the path.
+			// It does not exist, and printing errors about it confuses
+			// those users even more, because they think "sure ~ exists!".
+			// The go command diagnoses this situation and prints a
+			// useful error.
+			// On Windows, ~ is used in short names, such as c:\progra~1
+			// for c:\program files.
+			continue
+		}
+		all = append(all, p)
+	}
+	return all
+}
+
+var defaultToolTags, defaultReleaseTags []string
+
+// NoGoError is the error used by Import to describe a directory
+// containing no buildable Go source files. (It may still contain
+// test files, files hidden by build tags, and so on.)
+type NoGoError struct {
+	Dir string
+}
+
+func (e *NoGoError) Error() string {
+	return "no buildable Go source files in " + e.Dir
+}
+
+// MultiplePackageError describes a directory containing
+// multiple buildable Go source files for multiple packages.
+type MultiplePackageError struct {
+	Dir      string   // directory containing files
+	Packages []string // package names found
+	Files    []string // corresponding files: Files[i] declares package Packages[i]
+}
+
+func (e *MultiplePackageError) Error() string {
+	// Error string limited to two entries for compatibility.
+	return fmt.Sprintf("found packages %s (%s) and %s (%s) in %s", e.Packages[0], e.Files[0], e.Packages[1], e.Files[1], e.Dir)
+}
+
+func nameExt(name string) string {
+	i := strings.LastIndex(name, ".")
+	if i < 0 {
+		return ""
+	}
+	return name[i:]
+}
+
+func fileListForExt(p *build.Package, ext string) *[]string {
+	switch ext {
+	case ".c":
+		return &p.CFiles
+	case ".cc", ".cpp", ".cxx":
+		return &p.CXXFiles
+	case ".m":
+		return &p.MFiles
+	case ".h", ".hh", ".hpp", ".hxx":
+		return &p.HFiles
+	case ".f", ".F", ".for", ".f90":
+		return &p.FFiles
+	case ".s", ".S", ".sx":
+		return &p.SFiles
+	case ".swig":
+		return &p.SwigFiles
+	case ".swigcxx":
+		return &p.SwigCXXFiles
+	case ".syso":
+		return &p.SysoFiles
+	}
+	return nil
+}
+
+var errNoModules = errors.New("not using modules")
+
+func findImportComment(data []byte) (s string, line int) {
+	// expect keyword package
+	word, data := parseWord(data)
+	if string(word) != "package" {
+		return "", 0
+	}
+
+	// expect package name
+	_, data = parseWord(data)
+
+	// now ready for import comment, a // or /* */ comment
+	// beginning and ending on the current line.
+	for len(data) > 0 && (data[0] == ' ' || data[0] == '\t' || data[0] == '\r') {
+		data = data[1:]
+	}
+
+	var comment []byte
+	switch {
+	case bytes.HasPrefix(data, slashSlash):
+		comment, _, _ = bytes.Cut(data[2:], newline)
+	case bytes.HasPrefix(data, slashStar):
+		var ok bool
+		comment, _, ok = bytes.Cut(data[2:], starSlash)
+		if !ok {
+			// malformed comment
+			return "", 0
+		}
+		if bytes.Contains(comment, newline) {
+			return "", 0
+		}
+	}
+	comment = bytes.TrimSpace(comment)
+
+	// split comment into `import`, `"pkg"`
+	word, arg := parseWord(comment)
+	if string(word) != "import" {
+		return "", 0
+	}
+
+	line = 1 + bytes.Count(data[:cap(data)-cap(arg)], newline)
+	return strings.TrimSpace(string(arg)), line
+}
+
+var (
+	slashSlash = []byte("//")
+	slashStar  = []byte("/*")
+	starSlash  = []byte("*/")
+	newline    = []byte("\n")
+)
+
+// skipSpaceOrComment returns data with any leading spaces or comments removed.
+func skipSpaceOrComment(data []byte) []byte {
+	for len(data) > 0 {
+		switch data[0] {
+		case ' ', '\t', '\r', '\n':
+			data = data[1:]
+			continue
+		case '/':
+			if bytes.HasPrefix(data, slashSlash) {
+				i := bytes.Index(data, newline)
+				if i < 0 {
+					return nil
+				}
+				data = data[i+1:]
+				continue
+			}
+			if bytes.HasPrefix(data, slashStar) {
+				data = data[2:]
+				i := bytes.Index(data, starSlash)
+				if i < 0 {
+					return nil
+				}
+				data = data[i+2:]
+				continue
+			}
+		}
+		break
+	}
+	return data
+}
+
+// parseWord skips any leading spaces or comments in data
+// and then parses the beginning of data as an identifier or keyword,
+// returning that word and what remains after the word.
+func parseWord(data []byte) (word, rest []byte) {
+	data = skipSpaceOrComment(data)
+
+	// Parse past leading word characters.
+	rest = data
+	for {
+		r, size := utf8.DecodeRune(rest)
+		if unicode.IsLetter(r) || '0' <= r && r <= '9' || r == '_' {
+			rest = rest[size:]
+			continue
+		}
+		break
+	}
+
+	word = data[:len(data)-len(rest)]
+	if len(word) == 0 {
+		return nil, nil
+	}
+
+	return word, rest
+}
+
+var dummyPkg build.Package
+
+// fileInfo records information learned about a file included in a build.
+type fileInfo struct {
+	name       string // full name including dir
+	header     []byte
+	fset       *token.FileSet
+	parsed     *ast.File
+	parseErr   error
+	imports    []fileImport
+	embeds     []fileEmbed
+	directives []build.Directive
+
+	// Additional fields added to go/build's fileinfo for the purposes of the modindex package.
+	binaryOnly           bool
+	goBuildConstraint    string
+	plusBuildConstraints []string
+}
+
+type fileImport struct {
+	path string
+	pos  token.Pos
+	doc  *ast.CommentGroup
+}
+
+type fileEmbed struct {
+	pattern string
+	pos     token.Position
+}
+
+var errNonSource = errors.New("non source file")
+
+// getFileInfo extracts the information needed from each go file for the module
+// index.
+//
+// If Name denotes a Go program, matchFile reads until the end of the
+// Imports and returns that section of the file in the FileInfo's Header field,
+// even though it only considers text until the first non-comment
+// for +build lines.
+//
+// getFileInfo will return errNonSource if the file is not a source or object
+// file and shouldn't even be added to IgnoredFiles.
+func getFileInfo(dir, name string, fset *token.FileSet) (*fileInfo, error) {
+	if strings.HasPrefix(name, "_") ||
+		strings.HasPrefix(name, ".") {
+		return nil, nil
+	}
+
+	i := strings.LastIndex(name, ".")
+	if i < 0 {
+		i = len(name)
+	}
+	ext := name[i:]
+
+	if ext != ".go" && fileListForExt(&dummyPkg, ext) == nil {
+		// skip
+		return nil, errNonSource
+	}
+
+	info := &fileInfo{name: filepath.Join(dir, name), fset: fset}
+	if ext == ".syso" {
+		// binary, no reading
+		return info, nil
+	}
+
+	f, err := fsys.Open(info.name)
+	if err != nil {
+		return nil, err
+	}
+
+	// TODO(matloob) should we decide whether to ignore binary only here or earlier
+	// when we create the index file?
+	var ignoreBinaryOnly bool
+	if strings.HasSuffix(name, ".go") {
+		err = readGoInfo(f, info)
+		if strings.HasSuffix(name, "_test.go") {
+			ignoreBinaryOnly = true // ignore //go:binary-only-package comments in _test.go files
+		}
+	} else {
+		info.header, err = readComments(f)
+	}
+	f.Close()
+	if err != nil {
+		return nil, fmt.Errorf("read %s: %v", info.name, err)
+	}
+
+	// Look for +build comments to accept or reject the file.
+	info.goBuildConstraint, info.plusBuildConstraints, info.binaryOnly, err = getConstraints(info.header)
+	if err != nil {
+		return nil, fmt.Errorf("%s: %v", name, err)
+	}
+
+	if ignoreBinaryOnly && info.binaryOnly {
+		info.binaryOnly = false // override info.binaryOnly
+	}
+
+	return info, nil
+}
+
+func cleanDecls(m map[string][]token.Position) ([]string, map[string][]token.Position) {
+	all := make([]string, 0, len(m))
+	for path := range m {
+		all = append(all, path)
+	}
+	sort.Strings(all)
+	return all, m
+}
+
+var (
+	bSlashSlash = []byte(slashSlash)
+	bStarSlash  = []byte(starSlash)
+	bSlashStar  = []byte(slashStar)
+	bPlusBuild  = []byte("+build")
+
+	goBuildComment = []byte("//go:build")
+
+	errMultipleGoBuild = errors.New("multiple //go:build comments")
+)
+
+func isGoBuildComment(line []byte) bool {
+	if !bytes.HasPrefix(line, goBuildComment) {
+		return false
+	}
+	line = bytes.TrimSpace(line)
+	rest := line[len(goBuildComment):]
+	return len(rest) == 0 || len(bytes.TrimSpace(rest)) < len(rest)
+}
+
+// Special comment denoting a binary-only package.
+// See https://golang.org/design/2775-binary-only-packages
+// for more about the design of binary-only packages.
+var binaryOnlyComment = []byte("//go:binary-only-package")
+
+func getConstraints(content []byte) (goBuild string, plusBuild []string, binaryOnly bool, err error) {
+	// Identify leading run of // comments and blank lines,
+	// which must be followed by a blank line.
+	// Also identify any //go:build comments.
+	content, goBuildBytes, sawBinaryOnly, err := parseFileHeader(content)
+	if err != nil {
+		return "", nil, false, err
+	}
+
+	// If //go:build line is present, it controls, so no need to look for +build .
+	// Otherwise, get plusBuild constraints.
+	if goBuildBytes == nil {
+		p := content
+		for len(p) > 0 {
+			line := p
+			if i := bytes.IndexByte(line, '\n'); i >= 0 {
+				line, p = line[:i], p[i+1:]
+			} else {
+				p = p[len(p):]
+			}
+			line = bytes.TrimSpace(line)
+			if !bytes.HasPrefix(line, bSlashSlash) || !bytes.Contains(line, bPlusBuild) {
+				continue
+			}
+			text := string(line)
+			if !constraint.IsPlusBuild(text) {
+				continue
+			}
+			plusBuild = append(plusBuild, text)
+		}
+	}
+
+	return string(goBuildBytes), plusBuild, sawBinaryOnly, nil
+}
+
+func parseFileHeader(content []byte) (trimmed, goBuild []byte, sawBinaryOnly bool, err error) {
+	end := 0
+	p := content
+	ended := false       // found non-blank, non-// line, so stopped accepting // +build lines
+	inSlashStar := false // in /* */ comment
+
+Lines:
+	for len(p) > 0 {
+		line := p
+		if i := bytes.IndexByte(line, '\n'); i >= 0 {
+			line, p = line[:i], p[i+1:]
+		} else {
+			p = p[len(p):]
+		}
+		line = bytes.TrimSpace(line)
+		if len(line) == 0 && !ended { // Blank line
+			// Remember position of most recent blank line.
+			// When we find the first non-blank, non-// line,
+			// this "end" position marks the latest file position
+			// where a // +build line can appear.
+			// (It must appear _before_ a blank line before the non-blank, non-// line.
+			// Yes, that's confusing, which is part of why we moved to //go:build lines.)
+			// Note that ended==false here means that inSlashStar==false,
+			// since seeing a /* would have set ended==true.
+			end = len(content) - len(p)
+			continue Lines
+		}
+		if !bytes.HasPrefix(line, slashSlash) { // Not comment line
+			ended = true
+		}
+
+		if !inSlashStar && isGoBuildComment(line) {
+			if goBuild != nil {
+				return nil, nil, false, errMultipleGoBuild
+			}
+			goBuild = line
+		}
+		if !inSlashStar && bytes.Equal(line, binaryOnlyComment) {
+			sawBinaryOnly = true
+		}
+
+	Comments:
+		for len(line) > 0 {
+			if inSlashStar {
+				if i := bytes.Index(line, starSlash); i >= 0 {
+					inSlashStar = false
+					line = bytes.TrimSpace(line[i+len(starSlash):])
+					continue Comments
+				}
+				continue Lines
+			}
+			if bytes.HasPrefix(line, bSlashSlash) {
+				continue Lines
+			}
+			if bytes.HasPrefix(line, bSlashStar) {
+				inSlashStar = true
+				line = bytes.TrimSpace(line[len(bSlashStar):])
+				continue Comments
+			}
+			// Found non-comment text.
+			break Lines
+		}
+	}
+
+	return content[:end], goBuild, sawBinaryOnly, nil
+}
+
+// saveCgo saves the information from the #cgo lines in the import "C" comment.
+// These lines set CFLAGS, CPPFLAGS, CXXFLAGS and LDFLAGS and pkg-config directives
+// that affect the way cgo's C code is built.
+func (ctxt *Context) saveCgo(filename string, di *build.Package, text string) error {
+	for _, line := range strings.Split(text, "\n") {
+		orig := line
+
+		// Line is
+		//	#cgo [GOOS/GOARCH...] LDFLAGS: stuff
+		//
+		line = strings.TrimSpace(line)
+		if len(line) < 5 || line[:4] != "#cgo" || (line[4] != ' ' && line[4] != '\t') {
+			continue
+		}
+
+		// Split at colon.
+		line, argstr, ok := strings.Cut(strings.TrimSpace(line[4:]), ":")
+		if !ok {
+			return fmt.Errorf("%s: invalid #cgo line: %s", filename, orig)
+		}
+
+		// Parse GOOS/GOARCH stuff.
+		f := strings.Fields(line)
+		if len(f) < 1 {
+			return fmt.Errorf("%s: invalid #cgo line: %s", filename, orig)
+		}
+
+		cond, verb := f[:len(f)-1], f[len(f)-1]
+		if len(cond) > 0 {
+			ok := false
+			for _, c := range cond {
+				if ctxt.matchAuto(c, nil) {
+					ok = true
+					break
+				}
+			}
+			if !ok {
+				continue
+			}
+		}
+
+		args, err := splitQuoted(argstr)
+		if err != nil {
+			return fmt.Errorf("%s: invalid #cgo line: %s", filename, orig)
+		}
+		for i, arg := range args {
+			if arg, ok = expandSrcDir(arg, di.Dir); !ok {
+				return fmt.Errorf("%s: malformed #cgo argument: %s", filename, arg)
+			}
+			args[i] = arg
+		}
+
+		switch verb {
+		case "CFLAGS", "CPPFLAGS", "CXXFLAGS", "FFLAGS", "LDFLAGS":
+			// Change relative paths to absolute.
+			ctxt.makePathsAbsolute(args, di.Dir)
+		}
+
+		switch verb {
+		case "CFLAGS":
+			di.CgoCFLAGS = append(di.CgoCFLAGS, args...)
+		case "CPPFLAGS":
+			di.CgoCPPFLAGS = append(di.CgoCPPFLAGS, args...)
+		case "CXXFLAGS":
+			di.CgoCXXFLAGS = append(di.CgoCXXFLAGS, args...)
+		case "FFLAGS":
+			di.CgoFFLAGS = append(di.CgoFFLAGS, args...)
+		case "LDFLAGS":
+			di.CgoLDFLAGS = append(di.CgoLDFLAGS, args...)
+		case "pkg-config":
+			di.CgoPkgConfig = append(di.CgoPkgConfig, args...)
+		default:
+			return fmt.Errorf("%s: invalid #cgo verb: %s", filename, orig)
+		}
+	}
+	return nil
+}
+
+// expandSrcDir expands any occurrence of ${SRCDIR}, making sure
+// the result is safe for the shell.
+func expandSrcDir(str string, srcdir string) (string, bool) {
+	// "\" delimited paths cause safeCgoName to fail
+	// so convert native paths with a different delimiter
+	// to "/" before starting (eg: on windows).
+	srcdir = filepath.ToSlash(srcdir)
+
+	chunks := strings.Split(str, "${SRCDIR}")
+	if len(chunks) < 2 {
+		return str, safeCgoName(str)
+	}
+	ok := true
+	for _, chunk := range chunks {
+		ok = ok && (chunk == "" || safeCgoName(chunk))
+	}
+	ok = ok && (srcdir == "" || safeCgoName(srcdir))
+	res := strings.Join(chunks, srcdir)
+	return res, ok && res != ""
+}
+
+// makePathsAbsolute looks for compiler options that take paths and
+// makes them absolute. We do this because through the 1.8 release we
+// ran the compiler in the package directory, so any relative -I or -L
+// options would be relative to that directory. In 1.9 we changed to
+// running the compiler in the build directory, to get consistent
+// build results (issue #19964). To keep builds working, we change any
+// relative -I or -L options to be absolute.
+//
+// Using filepath.IsAbs and filepath.Join here means the results will be
+// different on different systems, but that's OK: -I and -L options are
+// inherently system-dependent.
+func (ctxt *Context) makePathsAbsolute(args []string, srcDir string) {
+	nextPath := false
+	for i, arg := range args {
+		if nextPath {
+			if !filepath.IsAbs(arg) {
+				args[i] = filepath.Join(srcDir, arg)
+			}
+			nextPath = false
+		} else if strings.HasPrefix(arg, "-I") || strings.HasPrefix(arg, "-L") {
+			if len(arg) == 2 {
+				nextPath = true
+			} else {
+				if !filepath.IsAbs(arg[2:]) {
+					args[i] = arg[:2] + filepath.Join(srcDir, arg[2:])
+				}
+			}
+		}
+	}
+}
+
+// NOTE: $ is not safe for the shell, but it is allowed here because of linker options like -Wl,$ORIGIN.
+// We never pass these arguments to a shell (just to programs we construct argv for), so this should be okay.
+// See golang.org/issue/6038.
+// The @ is for OS X. See golang.org/issue/13720.
+// The % is for Jenkins. See golang.org/issue/16959.
+// The ! is because module paths may use them. See golang.org/issue/26716.
+// The ~ and ^ are for sr.ht. See golang.org/issue/32260.
+const safeString = "+-.,/0123456789=ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz:$@%! ~^"
+
+func safeCgoName(s string) bool {
+	if s == "" {
+		return false
+	}
+	for i := 0; i < len(s); i++ {
+		if c := s[i]; c < utf8.RuneSelf && strings.IndexByte(safeString, c) < 0 {
+			return false
+		}
+	}
+	return true
+}
+
+// splitQuoted splits the string s around each instance of one or more consecutive
+// white space characters while taking into account quotes and escaping, and
+// returns an array of substrings of s or an empty list if s contains only white space.
+// Single quotes and double quotes are recognized to prevent splitting within the
+// quoted region, and are removed from the resulting substrings. If a quote in s
+// isn't closed err will be set and r will have the unclosed argument as the
+// last element. The backslash is used for escaping.
+//
+// For example, the following string:
+//
+//	a b:"c d" 'e''f'  "g\""
+//
+// Would be parsed as:
+//
+//	[]string{"a", "b:c d", "ef", `g"`}
+func splitQuoted(s string) (r []string, err error) {
+	var args []string
+	arg := make([]rune, len(s))
+	escaped := false
+	quoted := false
+	quote := '\x00'
+	i := 0
+	for _, rune := range s {
+		switch {
+		case escaped:
+			escaped = false
+		case rune == '\\':
+			escaped = true
+			continue
+		case quote != '\x00':
+			if rune == quote {
+				quote = '\x00'
+				continue
+			}
+		case rune == '"' || rune == '\'':
+			quoted = true
+			quote = rune
+			continue
+		case unicode.IsSpace(rune):
+			if quoted || i > 0 {
+				quoted = false
+				args = append(args, string(arg[:i]))
+				i = 0
+			}
+			continue
+		}
+		arg[i] = rune
+		i++
+	}
+	if quoted || i > 0 {
+		args = append(args, string(arg[:i]))
+	}
+	if quote != 0 {
+		err = errors.New("unclosed quote")
+	} else if escaped {
+		err = errors.New("unfinished escaping")
+	}
+	return args, err
+}
+
+// matchAuto interprets text as either a +build or //go:build expression (whichever works),
+// reporting whether the expression matches the build context.
+//
+// matchAuto is only used for testing of tag evaluation
+// and in #cgo lines, which accept either syntax.
+func (ctxt *Context) matchAuto(text string, allTags map[string]bool) bool {
+	if strings.ContainsAny(text, "&|()") {
+		text = "//go:build " + text
+	} else {
+		text = "// +build " + text
+	}
+	x, err := constraint.Parse(text)
+	if err != nil {
+		return false
+	}
+	return ctxt.eval(x, allTags)
+}
+
+func (ctxt *Context) eval(x constraint.Expr, allTags map[string]bool) bool {
+	return x.Eval(func(tag string) bool { return ctxt.matchTag(tag, allTags) })
+}
+
+// matchTag reports whether the name is one of:
+//
+//	cgo (if cgo is enabled)
+//	$GOOS
+//	$GOARCH
+//	boringcrypto
+//	ctxt.Compiler
+//	linux (if GOOS == android)
+//	solaris (if GOOS == illumos)
+//	tag (if tag is listed in ctxt.BuildTags or ctxt.ReleaseTags)
+//
+// It records all consulted tags in allTags.
+func (ctxt *Context) matchTag(name string, allTags map[string]bool) bool {
+	if allTags != nil {
+		allTags[name] = true
+	}
+
+	// special tags
+	if ctxt.CgoEnabled && name == "cgo" {
+		return true
+	}
+	if name == ctxt.GOOS || name == ctxt.GOARCH || name == ctxt.Compiler {
+		return true
+	}
+	if ctxt.GOOS == "android" && name == "linux" {
+		return true
+	}
+	if ctxt.GOOS == "illumos" && name == "solaris" {
+		return true
+	}
+	if ctxt.GOOS == "ios" && name == "darwin" {
+		return true
+	}
+	if name == "unix" && unixOS[ctxt.GOOS] {
+		return true
+	}
+	if name == "boringcrypto" {
+		name = "goexperiment.boringcrypto" // boringcrypto is an old name for goexperiment.boringcrypto
+	}
+
+	// other tags
+	for _, tag := range ctxt.BuildTags {
+		if tag == name {
+			return true
+		}
+	}
+	for _, tag := range ctxt.ToolTags {
+		if tag == name {
+			return true
+		}
+	}
+	for _, tag := range ctxt.ReleaseTags {
+		if tag == name {
+			return true
+		}
+	}
+
+	return false
+}
+
+// goodOSArchFile returns false if the name contains a $GOOS or $GOARCH
+// suffix which does not match the current system.
+// The recognized name formats are:
+//
+//	name_$(GOOS).*
+//	name_$(GOARCH).*
+//	name_$(GOOS)_$(GOARCH).*
+//	name_$(GOOS)_test.*
+//	name_$(GOARCH)_test.*
+//	name_$(GOOS)_$(GOARCH)_test.*
+//
+// Exceptions:
+// if GOOS=android, then files with GOOS=linux are also matched.
+// if GOOS=illumos, then files with GOOS=solaris are also matched.
+// if GOOS=ios, then files with GOOS=darwin are also matched.
+func (ctxt *Context) goodOSArchFile(name string, allTags map[string]bool) bool {
+	name, _, _ = strings.Cut(name, ".")
+
+	// Before Go 1.4, a file called "linux.go" would be equivalent to having a
+	// build tag "linux" in that file. For Go 1.4 and beyond, we require this
+	// auto-tagging to apply only to files with a non-empty prefix, so
+	// "foo_linux.go" is tagged but "linux.go" is not. This allows new operating
+	// systems, such as android, to arrive without breaking existing code with
+	// innocuous source code in "android.go". The easiest fix: cut everything
+	// in the name before the initial _.
+	i := strings.Index(name, "_")
+	if i < 0 {
+		return true
+	}
+	name = name[i:] // ignore everything before first _
+
+	l := strings.Split(name, "_")
+	if n := len(l); n > 0 && l[n-1] == "test" {
+		l = l[:n-1]
+	}
+	n := len(l)
+	if n >= 2 && knownOS[l[n-2]] && knownArch[l[n-1]] {
+		if allTags != nil {
+			// In case we short-circuit on l[n-1].
+			allTags[l[n-2]] = true
+		}
+		return ctxt.matchTag(l[n-1], allTags) && ctxt.matchTag(l[n-2], allTags)
+	}
+	if n >= 1 && (knownOS[l[n-1]] || knownArch[l[n-1]]) {
+		return ctxt.matchTag(l[n-1], allTags)
+	}
+	return true
+}
diff --git a/src/cmd/go/internal/modindex/build_read.go b/src/cmd/go/internal/modindex/build_read.go
new file mode 100644
index 0000000..9137200
--- /dev/null
+++ b/src/cmd/go/internal/modindex/build_read.go
@@ -0,0 +1,594 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file is a lightly modified copy go/build/read.go with unused parts
+// removed.
+
+package modindex
+
+import (
+	"bufio"
+	"bytes"
+	"errors"
+	"fmt"
+	"go/ast"
+	"go/build"
+	"go/parser"
+	"go/token"
+	"io"
+	"strconv"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
+type importReader struct {
+	b    *bufio.Reader
+	buf  []byte
+	peek byte
+	err  error
+	eof  bool
+	nerr int
+	pos  token.Position
+}
+
+var bom = []byte{0xef, 0xbb, 0xbf}
+
+func newImportReader(name string, r io.Reader) *importReader {
+	b := bufio.NewReader(r)
+	// Remove leading UTF-8 BOM.
+	// Per https://golang.org/ref/spec#Source_code_representation:
+	// a compiler may ignore a UTF-8-encoded byte order mark (U+FEFF)
+	// if it is the first Unicode code point in the source text.
+	if leadingBytes, err := b.Peek(3); err == nil && bytes.Equal(leadingBytes, bom) {
+		b.Discard(3)
+	}
+	return &importReader{
+		b: b,
+		pos: token.Position{
+			Filename: name,
+			Line:     1,
+			Column:   1,
+		},
+	}
+}
+
+func isIdent(c byte) bool {
+	return 'A' <= c && c <= 'Z' || 'a' <= c && c <= 'z' || '0' <= c && c <= '9' || c == '_' || c >= utf8.RuneSelf
+}
+
+var (
+	errSyntax = errors.New("syntax error")
+	errNUL    = errors.New("unexpected NUL in input")
+)
+
+// syntaxError records a syntax error, but only if an I/O error has not already been recorded.
+func (r *importReader) syntaxError() {
+	if r.err == nil {
+		r.err = errSyntax
+	}
+}
+
+// readByte reads the next byte from the input, saves it in buf, and returns it.
+// If an error occurs, readByte records the error in r.err and returns 0.
+func (r *importReader) readByte() byte {
+	c, err := r.b.ReadByte()
+	if err == nil {
+		r.buf = append(r.buf, c)
+		if c == 0 {
+			err = errNUL
+		}
+	}
+	if err != nil {
+		if err == io.EOF {
+			r.eof = true
+		} else if r.err == nil {
+			r.err = err
+		}
+		c = 0
+	}
+	return c
+}
+
+// readByteNoBuf is like readByte but doesn't buffer the byte.
+// It exhausts r.buf before reading from r.b.
+func (r *importReader) readByteNoBuf() byte {
+	var c byte
+	var err error
+	if len(r.buf) > 0 {
+		c = r.buf[0]
+		r.buf = r.buf[1:]
+	} else {
+		c, err = r.b.ReadByte()
+		if err == nil && c == 0 {
+			err = errNUL
+		}
+	}
+
+	if err != nil {
+		if err == io.EOF {
+			r.eof = true
+		} else if r.err == nil {
+			r.err = err
+		}
+		return 0
+	}
+	r.pos.Offset++
+	if c == '\n' {
+		r.pos.Line++
+		r.pos.Column = 1
+	} else {
+		r.pos.Column++
+	}
+	return c
+}
+
+// peekByte returns the next byte from the input reader but does not advance beyond it.
+// If skipSpace is set, peekByte skips leading spaces and comments.
+func (r *importReader) peekByte(skipSpace bool) byte {
+	if r.err != nil {
+		if r.nerr++; r.nerr > 10000 {
+			panic("go/build: import reader looping")
+		}
+		return 0
+	}
+
+	// Use r.peek as first input byte.
+	// Don't just return r.peek here: it might have been left by peekByte(false)
+	// and this might be peekByte(true).
+	c := r.peek
+	if c == 0 {
+		c = r.readByte()
+	}
+	for r.err == nil && !r.eof {
+		if skipSpace {
+			// For the purposes of this reader, semicolons are never necessary to
+			// understand the input and are treated as spaces.
+			switch c {
+			case ' ', '\f', '\t', '\r', '\n', ';':
+				c = r.readByte()
+				continue
+
+			case '/':
+				c = r.readByte()
+				if c == '/' {
+					for c != '\n' && r.err == nil && !r.eof {
+						c = r.readByte()
+					}
+				} else if c == '*' {
+					var c1 byte
+					for (c != '*' || c1 != '/') && r.err == nil {
+						if r.eof {
+							r.syntaxError()
+						}
+						c, c1 = c1, r.readByte()
+					}
+				} else {
+					r.syntaxError()
+				}
+				c = r.readByte()
+				continue
+			}
+		}
+		break
+	}
+	r.peek = c
+	return r.peek
+}
+
+// nextByte is like peekByte but advances beyond the returned byte.
+func (r *importReader) nextByte(skipSpace bool) byte {
+	c := r.peekByte(skipSpace)
+	r.peek = 0
+	return c
+}
+
+var goEmbed = []byte("go:embed")
+
+// findEmbed advances the input reader to the next //go:embed comment.
+// It reports whether it found a comment.
+// (Otherwise it found an error or EOF.)
+func (r *importReader) findEmbed(first bool) bool {
+	// The import block scan stopped after a non-space character,
+	// so the reader is not at the start of a line on the first call.
+	// After that, each //go:embed extraction leaves the reader
+	// at the end of a line.
+	startLine := !first
+	var c byte
+	for r.err == nil && !r.eof {
+		c = r.readByteNoBuf()
+	Reswitch:
+		switch c {
+		default:
+			startLine = false
+
+		case '\n':
+			startLine = true
+
+		case ' ', '\t':
+			// leave startLine alone
+
+		case '"':
+			startLine = false
+			for r.err == nil {
+				if r.eof {
+					r.syntaxError()
+				}
+				c = r.readByteNoBuf()
+				if c == '\\' {
+					r.readByteNoBuf()
+					if r.err != nil {
+						r.syntaxError()
+						return false
+					}
+					continue
+				}
+				if c == '"' {
+					c = r.readByteNoBuf()
+					goto Reswitch
+				}
+			}
+			goto Reswitch
+
+		case '`':
+			startLine = false
+			for r.err == nil {
+				if r.eof {
+					r.syntaxError()
+				}
+				c = r.readByteNoBuf()
+				if c == '`' {
+					c = r.readByteNoBuf()
+					goto Reswitch
+				}
+			}
+
+		case '\'':
+			startLine = false
+			for r.err == nil {
+				if r.eof {
+					r.syntaxError()
+				}
+				c = r.readByteNoBuf()
+				if c == '\\' {
+					r.readByteNoBuf()
+					if r.err != nil {
+						r.syntaxError()
+						return false
+					}
+					continue
+				}
+				if c == '\'' {
+					c = r.readByteNoBuf()
+					goto Reswitch
+				}
+			}
+
+		case '/':
+			c = r.readByteNoBuf()
+			switch c {
+			default:
+				startLine = false
+				goto Reswitch
+
+			case '*':
+				var c1 byte
+				for (c != '*' || c1 != '/') && r.err == nil {
+					if r.eof {
+						r.syntaxError()
+					}
+					c, c1 = c1, r.readByteNoBuf()
+				}
+				startLine = false
+
+			case '/':
+				if startLine {
+					// Try to read this as a //go:embed comment.
+					for i := range goEmbed {
+						c = r.readByteNoBuf()
+						if c != goEmbed[i] {
+							goto SkipSlashSlash
+						}
+					}
+					c = r.readByteNoBuf()
+					if c == ' ' || c == '\t' {
+						// Found one!
+						return true
+					}
+				}
+			SkipSlashSlash:
+				for c != '\n' && r.err == nil && !r.eof {
+					c = r.readByteNoBuf()
+				}
+				startLine = true
+			}
+		}
+	}
+	return false
+}
+
+// readKeyword reads the given keyword from the input.
+// If the keyword is not present, readKeyword records a syntax error.
+func (r *importReader) readKeyword(kw string) {
+	r.peekByte(true)
+	for i := 0; i < len(kw); i++ {
+		if r.nextByte(false) != kw[i] {
+			r.syntaxError()
+			return
+		}
+	}
+	if isIdent(r.peekByte(false)) {
+		r.syntaxError()
+	}
+}
+
+// readIdent reads an identifier from the input.
+// If an identifier is not present, readIdent records a syntax error.
+func (r *importReader) readIdent() {
+	c := r.peekByte(true)
+	if !isIdent(c) {
+		r.syntaxError()
+		return
+	}
+	for isIdent(r.peekByte(false)) {
+		r.peek = 0
+	}
+}
+
+// readString reads a quoted string literal from the input.
+// If an identifier is not present, readString records a syntax error.
+func (r *importReader) readString() {
+	switch r.nextByte(true) {
+	case '`':
+		for r.err == nil {
+			if r.nextByte(false) == '`' {
+				break
+			}
+			if r.eof {
+				r.syntaxError()
+			}
+		}
+	case '"':
+		for r.err == nil {
+			c := r.nextByte(false)
+			if c == '"' {
+				break
+			}
+			if r.eof || c == '\n' {
+				r.syntaxError()
+			}
+			if c == '\\' {
+				r.nextByte(false)
+			}
+		}
+	default:
+		r.syntaxError()
+	}
+}
+
+// readImport reads an import clause - optional identifier followed by quoted string -
+// from the input.
+func (r *importReader) readImport() {
+	c := r.peekByte(true)
+	if c == '.' {
+		r.peek = 0
+	} else if isIdent(c) {
+		r.readIdent()
+	}
+	r.readString()
+}
+
+// readComments is like io.ReadAll, except that it only reads the leading
+// block of comments in the file.
+func readComments(f io.Reader) ([]byte, error) {
+	r := newImportReader("", f)
+	r.peekByte(true)
+	if r.err == nil && !r.eof {
+		// Didn't reach EOF, so must have found a non-space byte. Remove it.
+		r.buf = r.buf[:len(r.buf)-1]
+	}
+	return r.buf, r.err
+}
+
+// readGoInfo expects a Go file as input and reads the file up to and including the import section.
+// It records what it learned in *info.
+// If info.fset is non-nil, readGoInfo parses the file and sets info.parsed, info.parseErr,
+// info.imports and info.embeds.
+//
+// It only returns an error if there are problems reading the file,
+// not for syntax errors in the file itself.
+func readGoInfo(f io.Reader, info *fileInfo) error {
+	r := newImportReader(info.name, f)
+
+	r.readKeyword("package")
+	r.readIdent()
+	for r.peekByte(true) == 'i' {
+		r.readKeyword("import")
+		if r.peekByte(true) == '(' {
+			r.nextByte(false)
+			for r.peekByte(true) != ')' && r.err == nil {
+				r.readImport()
+			}
+			r.nextByte(false)
+		} else {
+			r.readImport()
+		}
+	}
+
+	info.header = r.buf
+
+	// If we stopped successfully before EOF, we read a byte that told us we were done.
+	// Return all but that last byte, which would cause a syntax error if we let it through.
+	if r.err == nil && !r.eof {
+		info.header = r.buf[:len(r.buf)-1]
+	}
+
+	// If we stopped for a syntax error, consume the whole file so that
+	// we are sure we don't change the errors that go/parser returns.
+	if r.err == errSyntax {
+		r.err = nil
+		for r.err == nil && !r.eof {
+			r.readByte()
+		}
+		info.header = r.buf
+	}
+	if r.err != nil {
+		return r.err
+	}
+
+	if info.fset == nil {
+		return nil
+	}
+
+	// Parse file header & record imports.
+	info.parsed, info.parseErr = parser.ParseFile(info.fset, info.name, info.header, parser.ImportsOnly|parser.ParseComments)
+	if info.parseErr != nil {
+		return nil
+	}
+
+	hasEmbed := false
+	for _, decl := range info.parsed.Decls {
+		d, ok := decl.(*ast.GenDecl)
+		if !ok {
+			continue
+		}
+		for _, dspec := range d.Specs {
+			spec, ok := dspec.(*ast.ImportSpec)
+			if !ok {
+				continue
+			}
+			quoted := spec.Path.Value
+			path, err := strconv.Unquote(quoted)
+			if err != nil {
+				return fmt.Errorf("parser returned invalid quoted string: <%s>", quoted)
+			}
+			if path == "embed" {
+				hasEmbed = true
+			}
+
+			doc := spec.Doc
+			if doc == nil && len(d.Specs) == 1 {
+				doc = d.Doc
+			}
+			info.imports = append(info.imports, fileImport{path, spec.Pos(), doc})
+		}
+	}
+
+	// Extract directives.
+	for _, group := range info.parsed.Comments {
+		if group.Pos() >= info.parsed.Package {
+			break
+		}
+		for _, c := range group.List {
+			if strings.HasPrefix(c.Text, "//go:") {
+				info.directives = append(info.directives, build.Directive{Text: c.Text, Pos: info.fset.Position(c.Slash)})
+			}
+		}
+	}
+
+	// If the file imports "embed",
+	// we have to look for //go:embed comments
+	// in the remainder of the file.
+	// The compiler will enforce the mapping of comments to
+	// declared variables. We just need to know the patterns.
+	// If there were //go:embed comments earlier in the file
+	// (near the package statement or imports), the compiler
+	// will reject them. They can be (and have already been) ignored.
+	if hasEmbed {
+		var line []byte
+		for first := true; r.findEmbed(first); first = false {
+			line = line[:0]
+			pos := r.pos
+			for {
+				c := r.readByteNoBuf()
+				if c == '\n' || r.err != nil || r.eof {
+					break
+				}
+				line = append(line, c)
+			}
+			// Add args if line is well-formed.
+			// Ignore badly-formed lines - the compiler will report them when it finds them,
+			// and we can pretend they are not there to help go list succeed with what it knows.
+			embs, err := parseGoEmbed(string(line), pos)
+			if err == nil {
+				info.embeds = append(info.embeds, embs...)
+			}
+		}
+	}
+
+	return nil
+}
+
+// parseGoEmbed parses the text following "//go:embed" to extract the glob patterns.
+// It accepts unquoted space-separated patterns as well as double-quoted and back-quoted Go strings.
+// This is based on a similar function in cmd/compile/internal/gc/noder.go;
+// this version calculates position information as well.
+func parseGoEmbed(args string, pos token.Position) ([]fileEmbed, error) {
+	trimBytes := func(n int) {
+		pos.Offset += n
+		pos.Column += utf8.RuneCountInString(args[:n])
+		args = args[n:]
+	}
+	trimSpace := func() {
+		trim := strings.TrimLeftFunc(args, unicode.IsSpace)
+		trimBytes(len(args) - len(trim))
+	}
+
+	var list []fileEmbed
+	for trimSpace(); args != ""; trimSpace() {
+		var path string
+		pathPos := pos
+	Switch:
+		switch args[0] {
+		default:
+			i := len(args)
+			for j, c := range args {
+				if unicode.IsSpace(c) {
+					i = j
+					break
+				}
+			}
+			path = args[:i]
+			trimBytes(i)
+
+		case '`':
+			var ok bool
+			path, _, ok = strings.Cut(args[1:], "`")
+			if !ok {
+				return nil, fmt.Errorf("invalid quoted string in //go:embed: %s", args)
+			}
+			trimBytes(1 + len(path) + 1)
+
+		case '"':
+			i := 1
+			for ; i < len(args); i++ {
+				if args[i] == '\\' {
+					i++
+					continue
+				}
+				if args[i] == '"' {
+					q, err := strconv.Unquote(args[:i+1])
+					if err != nil {
+						return nil, fmt.Errorf("invalid quoted string in //go:embed: %s", args[:i+1])
+					}
+					path = q
+					trimBytes(i + 1)
+					break Switch
+				}
+			}
+			if i >= len(args) {
+				return nil, fmt.Errorf("invalid quoted string in //go:embed: %s", args)
+			}
+		}
+
+		if args != "" {
+			r, _ := utf8.DecodeRuneInString(args)
+			if !unicode.IsSpace(r) {
+				return nil, fmt.Errorf("invalid quoted string in //go:embed: %s", args)
+			}
+		}
+		list = append(list, fileEmbed{path, pathPos})
+	}
+	return list, nil
+}
diff --git a/src/cmd/go/internal/modindex/index_format.txt b/src/cmd/go/internal/modindex/index_format.txt
new file mode 100644
index 0000000..8b1d2c6
--- /dev/null
+++ b/src/cmd/go/internal/modindex/index_format.txt
@@ -0,0 +1,63 @@
+This file documents the index format that is read and written by this package.
+The index format is an encoding of a series of RawPackage structs
+
+Field names refer to fields on RawPackage and rawFile.
+The file uses little endian encoding for the uint32s.
+Strings are written into the string table at the end of the file.
+Each string is prefixed with a uvarint-encoded length.
+Bools are written as uint32s: 0 for false and 1 for true.
+
+The following is the format for a full module:
+
+“go index v2\n”
+str uint32 - offset of string table
+n uint32 - number of packages
+for each rawPackage:
+	dirname - string offset
+	package - offset where package begins
+for each rawPackage:
+	error uint32 - string offset // error is produced by fsys.ReadDir or fmt.Errorf
+	dir uint32 - string offset (directory path relative to module root)
+	len(sourceFiles) uint32
+	sourceFiles [n]uint32 - offset to source file (relative to start of index file)
+	for each sourceFile:
+		error - string offset // error is either produced by fmt.Errorf,errors.New or is io.EOF
+		parseError - string offset // if non-empty, a json-encoded parseError struct (see below). Is either produced by io.ReadAll,os.ReadFile,errors.New or is scanner.Error,scanner.ErrorList
+		synopsis - string offset
+		name - string offset
+		pkgName - string offset
+		ignoreFile - int32 bool // report the file in Ignored(Go|Other)Files because there was an error reading it or parsing its build constraints.
+		binaryOnly uint32 bool
+		cgoDirectives string offset // the #cgo directive lines in the comment on import "C"
+		goBuildConstraint  - string offset
+		len(plusBuildConstraints) - uint32
+		plusBuildConstraints - [n]uint32 (string offsets)
+		len(imports) uint32
+		for each rawImport:
+			path - string offset
+			position - file, offset, line, column - uint32
+		len(embeds) uint32
+		for each embed:
+			pattern - string offset
+			position - file, offset, line, column - uint32
+		len(directives) uint32
+		for each directive:
+			text - string offset
+			position - file, offset, line, column - uint32
+[string table]
+0xFF (marker)
+
+The following is the format for a single indexed package:
+
+“go index v0\n”
+str uint32 - offset of string table
+for the single RawPackage:
+	[same RawPackage format as above]
+[string table]
+
+The following is the definition of the json-serialized parseError struct:
+
+type parseError struct {
+	ErrorList *scanner.ErrorList // non-nil if the error was an ErrorList, nil otherwise
+	ErrorString string // non-empty for all other cases
+}
diff --git a/src/cmd/go/internal/modindex/index_test.go b/src/cmd/go/internal/modindex/index_test.go
new file mode 100644
index 0000000..6bc62f3
--- /dev/null
+++ b/src/cmd/go/internal/modindex/index_test.go
@@ -0,0 +1,104 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modindex
+
+import (
+	"encoding/hex"
+	"encoding/json"
+	"go/build"
+	"internal/diff"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"testing"
+)
+
+func init() {
+	isTest = true
+	enabled = true // to allow GODEBUG=goindex=0 go test, when things are very broken
+}
+
+func TestIndex(t *testing.T) {
+	src := filepath.Join(runtime.GOROOT(), "src")
+	checkPkg := func(t *testing.T, m *Module, pkg string, data []byte) {
+		p := m.Package(pkg)
+		bp, err := p.Import(build.Default, build.ImportComment)
+		if err != nil {
+			t.Fatal(err)
+		}
+		bp1, err := build.Default.Import(".", filepath.Join(src, pkg), build.ImportComment)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if !reflect.DeepEqual(bp, bp1) {
+			t.Errorf("mismatch")
+			t.Logf("index:\n%s", hex.Dump(data))
+
+			js, err := json.MarshalIndent(bp, "", "\t")
+			if err != nil {
+				t.Fatal(err)
+			}
+			js1, err := json.MarshalIndent(bp1, "", "\t")
+			if err != nil {
+				t.Fatal(err)
+			}
+			t.Logf("diff:\n%s", diff.Diff("index", js, "correct", js1))
+			t.FailNow()
+		}
+	}
+
+	// Check packages in increasing complexity, one at a time.
+	pkgs := []string{
+		"crypto",
+		"encoding",
+		"unsafe",
+		"encoding/json",
+		"runtime",
+		"net",
+	}
+	var raws []*rawPackage
+	for _, pkg := range pkgs {
+		raw := importRaw(src, pkg)
+		raws = append(raws, raw)
+		t.Run(pkg, func(t *testing.T) {
+			data := encodeModuleBytes([]*rawPackage{raw})
+			m, err := fromBytes(src, data)
+			if err != nil {
+				t.Fatal(err)
+			}
+			checkPkg(t, m, pkg, data)
+		})
+	}
+
+	// Check that a multi-package index works too.
+	t.Run("all", func(t *testing.T) {
+		data := encodeModuleBytes(raws)
+		m, err := fromBytes(src, data)
+		if err != nil {
+			t.Fatal(err)
+		}
+		for _, pkg := range pkgs {
+			checkPkg(t, m, pkg, data)
+		}
+	})
+}
+
+func TestImportRaw_IgnoreNonGo(t *testing.T) {
+	path := filepath.Join("testdata", "ignore_non_source")
+	p := importRaw(path, ".")
+
+	wantFiles := []string{"a.syso", "b.go", "c.c"}
+
+	var gotFiles []string
+	for i := range p.sourceFiles {
+		gotFiles = append(gotFiles, p.sourceFiles[i].name)
+	}
+
+	if !reflect.DeepEqual(gotFiles, wantFiles) {
+		t.Errorf("names of files in importRaw(testdata/ignore_non_source): got %v; want %v",
+			gotFiles, wantFiles)
+	}
+}
diff --git a/src/cmd/go/internal/modindex/read.go b/src/cmd/go/internal/modindex/read.go
new file mode 100644
index 0000000..83d5faf
--- /dev/null
+++ b/src/cmd/go/internal/modindex/read.go
@@ -0,0 +1,1037 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modindex
+
+import (
+	"bytes"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"go/build"
+	"go/build/constraint"
+	"go/token"
+	"internal/godebug"
+	"internal/goroot"
+	"path"
+	"path/filepath"
+	"runtime"
+	"runtime/debug"
+	"sort"
+	"strings"
+	"sync"
+	"time"
+	"unsafe"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cache"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/imports"
+	"cmd/go/internal/par"
+	"cmd/go/internal/str"
+)
+
+// enabled is used to flag off the behavior of the module index on tip.
+// It will be removed before the release.
+// TODO(matloob): Remove enabled once we have more confidence on the
+// module index.
+var enabled = godebug.New("#goindex").Value() != "0"
+
+// Module represents and encoded module index file. It is used to
+// do the equivalent of build.Import of packages in the module and answer other
+// questions based on the index file's data.
+type Module struct {
+	modroot string
+	d       *decoder
+	n       int // number of packages
+}
+
+// moduleHash returns an ActionID corresponding to the state of the module
+// located at filesystem path modroot.
+func moduleHash(modroot string, ismodcache bool) (cache.ActionID, error) {
+	// We expect modules stored within the module cache to be checksummed and
+	// immutable, and we expect released modules within GOROOT to change only
+	// infrequently (when the Go version changes).
+	if !ismodcache {
+		// The contents of this module may change over time. We don't want to pay
+		// the cost to detect changes and re-index whenever they occur, so just
+		// don't index it at all.
+		//
+		// Note that this is true even for modules in GOROOT/src: non-release builds
+		// of the Go toolchain may have arbitrary development changes on top of the
+		// commit reported by runtime.Version, or could be completely artificial due
+		// to lacking a `git` binary (like "devel gomote.XXXXX", as synthesized by
+		// "gomote push" as of 2022-06-15). (Release builds shouldn't have
+		// modifications, but we don't want to use a behavior for releases that we
+		// haven't tested during development.)
+		return cache.ActionID{}, ErrNotIndexed
+	}
+
+	h := cache.NewHash("moduleIndex")
+	// TODO(bcmills): Since modules in the index are checksummed, we could
+	// probably improve the cache hit rate by keying off of the module
+	// path@version (perhaps including the checksum?) instead of the module root
+	// directory.
+	fmt.Fprintf(h, "module index %s %s %v\n", runtime.Version(), indexVersion, modroot)
+	return h.Sum(), nil
+}
+
+const modTimeCutoff = 2 * time.Second
+
+// dirHash returns an ActionID corresponding to the state of the package
+// located at filesystem path pkgdir.
+func dirHash(modroot, pkgdir string) (cache.ActionID, error) {
+	h := cache.NewHash("moduleIndex")
+	fmt.Fprintf(h, "modroot %s\n", modroot)
+	fmt.Fprintf(h, "package %s %s %v\n", runtime.Version(), indexVersion, pkgdir)
+	entries, err := fsys.ReadDir(pkgdir)
+	if err != nil {
+		// pkgdir might not be a directory. give up on hashing.
+		return cache.ActionID{}, ErrNotIndexed
+	}
+	cutoff := time.Now().Add(-modTimeCutoff)
+	for _, info := range entries {
+		if info.IsDir() {
+			continue
+		}
+
+		if !info.Mode().IsRegular() {
+			return cache.ActionID{}, ErrNotIndexed
+		}
+		// To avoid problems for very recent files where a new
+		// write might not change the mtime due to file system
+		// mtime precision, reject caching if a file was read that
+		// is less than modTimeCutoff old.
+		//
+		// This is the same strategy used for hashing test inputs.
+		// See hashOpen in cmd/go/internal/test/test.go for the
+		// corresponding code.
+		if info.ModTime().After(cutoff) {
+			return cache.ActionID{}, ErrNotIndexed
+		}
+
+		fmt.Fprintf(h, "file %v %v %v\n", info.Name(), info.ModTime(), info.Size())
+	}
+	return h.Sum(), nil
+}
+
+var ErrNotIndexed = errors.New("not in module index")
+
+var (
+	errDisabled           = fmt.Errorf("%w: module indexing disabled", ErrNotIndexed)
+	errNotFromModuleCache = fmt.Errorf("%w: not from module cache", ErrNotIndexed)
+)
+
+// GetPackage returns the IndexPackage for the package at the given path.
+// It will return ErrNotIndexed if the directory should be read without
+// using the index, for instance because the index is disabled, or the package
+// is not in a module.
+func GetPackage(modroot, pkgdir string) (*IndexPackage, error) {
+	mi, err := GetModule(modroot)
+	if err == nil {
+		return mi.Package(relPath(pkgdir, modroot)), nil
+	}
+	if !errors.Is(err, errNotFromModuleCache) {
+		return nil, err
+	}
+	if cfg.BuildContext.Compiler == "gccgo" && str.HasPathPrefix(modroot, cfg.GOROOTsrc) {
+		return nil, err // gccgo has no sources for GOROOT packages.
+	}
+	return openIndexPackage(modroot, pkgdir)
+}
+
+// GetModule returns the Module for the given modroot.
+// It will return ErrNotIndexed if the directory should be read without
+// using the index, for instance because the index is disabled, or the package
+// is not in a module.
+func GetModule(modroot string) (*Module, error) {
+	if !enabled || cache.DefaultDir() == "off" {
+		return nil, errDisabled
+	}
+	if modroot == "" {
+		panic("modindex.GetPackage called with empty modroot")
+	}
+	if cfg.BuildMod == "vendor" {
+		// Even if the main module is in the module cache,
+		// its vendored dependencies are not loaded from their
+		// usual cached locations.
+		return nil, errNotFromModuleCache
+	}
+	modroot = filepath.Clean(modroot)
+	if str.HasFilePathPrefix(modroot, cfg.GOROOTsrc) || !str.HasFilePathPrefix(modroot, cfg.GOMODCACHE) {
+		return nil, errNotFromModuleCache
+	}
+	return openIndexModule(modroot, true)
+}
+
+var mcache par.ErrCache[string, *Module]
+
+// openIndexModule returns the module index for modPath.
+// It will return ErrNotIndexed if the module can not be read
+// using the index because it contains symlinks.
+func openIndexModule(modroot string, ismodcache bool) (*Module, error) {
+	return mcache.Do(modroot, func() (*Module, error) {
+		fsys.Trace("openIndexModule", modroot)
+		id, err := moduleHash(modroot, ismodcache)
+		if err != nil {
+			return nil, err
+		}
+		data, _, err := cache.GetMmap(cache.Default(), id)
+		if err != nil {
+			// Couldn't read from modindex. Assume we couldn't read from
+			// the index because the module hasn't been indexed yet.
+			data, err = indexModule(modroot)
+			if err != nil {
+				return nil, err
+			}
+			if err = cache.PutBytes(cache.Default(), id, data); err != nil {
+				return nil, err
+			}
+		}
+		mi, err := fromBytes(modroot, data)
+		if err != nil {
+			return nil, err
+		}
+		return mi, nil
+	})
+}
+
+var pcache par.ErrCache[[2]string, *IndexPackage]
+
+func openIndexPackage(modroot, pkgdir string) (*IndexPackage, error) {
+	return pcache.Do([2]string{modroot, pkgdir}, func() (*IndexPackage, error) {
+		fsys.Trace("openIndexPackage", pkgdir)
+		id, err := dirHash(modroot, pkgdir)
+		if err != nil {
+			return nil, err
+		}
+		data, _, err := cache.GetMmap(cache.Default(), id)
+		if err != nil {
+			// Couldn't read from index. Assume we couldn't read from
+			// the index because the package hasn't been indexed yet.
+			data = indexPackage(modroot, pkgdir)
+			if err = cache.PutBytes(cache.Default(), id, data); err != nil {
+				return nil, err
+			}
+		}
+		pkg, err := packageFromBytes(modroot, data)
+		if err != nil {
+			return nil, err
+		}
+		return pkg, nil
+	})
+}
+
+var errCorrupt = errors.New("corrupt index")
+
+// protect marks the start of a large section of code that accesses the index.
+// It should be used as:
+//
+//	defer unprotect(protect, &err)
+//
+// It should not be used for trivial accesses which would be
+// dwarfed by the overhead of the defer.
+func protect() bool {
+	return debug.SetPanicOnFault(true)
+}
+
+var isTest = false
+
+// unprotect marks the end of a large section of code that accesses the index.
+// It should be used as:
+//
+//	defer unprotect(protect, &err)
+//
+// end looks for panics due to errCorrupt or bad mmap accesses.
+// When it finds them, it adds explanatory text, consumes the panic, and sets *errp instead.
+// If errp is nil, end adds the explanatory text but then calls base.Fatalf.
+func unprotect(old bool, errp *error) {
+	// SetPanicOnFault's errors _may_ satisfy this interface. Even though it's not guaranteed
+	// that all its errors satisfy this interface, we'll only check for these errors so that
+	// we don't suppress panics that could have been produced from other sources.
+	type addrer interface {
+		Addr() uintptr
+	}
+
+	debug.SetPanicOnFault(old)
+
+	if e := recover(); e != nil {
+		if _, ok := e.(addrer); ok || e == errCorrupt {
+			// This panic was almost certainly caused by SetPanicOnFault or our panic(errCorrupt).
+			err := fmt.Errorf("error reading module index: %v", e)
+			if errp != nil {
+				*errp = err
+				return
+			}
+			if isTest {
+				panic(err)
+			}
+			base.Fatalf("%v", err)
+		}
+		// The panic was likely not caused by SetPanicOnFault.
+		panic(e)
+	}
+}
+
+// fromBytes returns a *Module given the encoded representation.
+func fromBytes(moddir string, data []byte) (m *Module, err error) {
+	if !enabled {
+		panic("use of index")
+	}
+
+	defer unprotect(protect(), &err)
+
+	if !bytes.HasPrefix(data, []byte(indexVersion+"\n")) {
+		return nil, errCorrupt
+	}
+
+	const hdr = len(indexVersion + "\n")
+	d := &decoder{data: data}
+	str := d.intAt(hdr)
+	if str < hdr+8 || len(d.data) < str {
+		return nil, errCorrupt
+	}
+	d.data, d.str = data[:str], d.data[str:]
+	// Check that string table looks valid.
+	// First string is empty string (length 0),
+	// and we leave a marker byte 0xFF at the end
+	// just to make sure that the file is not truncated.
+	if len(d.str) == 0 || d.str[0] != 0 || d.str[len(d.str)-1] != 0xFF {
+		return nil, errCorrupt
+	}
+
+	n := d.intAt(hdr + 4)
+	if n < 0 || n > (len(d.data)-8)/8 {
+		return nil, errCorrupt
+	}
+
+	m = &Module{
+		moddir,
+		d,
+		n,
+	}
+	return m, nil
+}
+
+// packageFromBytes returns a *IndexPackage given the encoded representation.
+func packageFromBytes(modroot string, data []byte) (p *IndexPackage, err error) {
+	m, err := fromBytes(modroot, data)
+	if err != nil {
+		return nil, err
+	}
+	if m.n != 1 {
+		return nil, fmt.Errorf("corrupt single-package index")
+	}
+	return m.pkg(0), nil
+}
+
+// pkgDir returns the dir string of the i'th package in the index.
+func (m *Module) pkgDir(i int) string {
+	if i < 0 || i >= m.n {
+		panic(errCorrupt)
+	}
+	return m.d.stringAt(12 + 8 + 8*i)
+}
+
+// pkgOff returns the offset of the data for the i'th package in the index.
+func (m *Module) pkgOff(i int) int {
+	if i < 0 || i >= m.n {
+		panic(errCorrupt)
+	}
+	return m.d.intAt(12 + 8 + 8*i + 4)
+}
+
+// Walk calls f for each package in the index, passing the path to that package relative to the module root.
+func (m *Module) Walk(f func(path string)) {
+	defer unprotect(protect(), nil)
+	for i := 0; i < m.n; i++ {
+		f(m.pkgDir(i))
+	}
+}
+
+// relPath returns the path relative to the module's root.
+func relPath(path, modroot string) string {
+	return str.TrimFilePathPrefix(filepath.Clean(path), filepath.Clean(modroot))
+}
+
+var installgorootAll = godebug.New("installgoroot").Value() == "all"
+
+// Import is the equivalent of build.Import given the information in Module.
+func (rp *IndexPackage) Import(bctxt build.Context, mode build.ImportMode) (p *build.Package, err error) {
+	defer unprotect(protect(), &err)
+
+	ctxt := (*Context)(&bctxt)
+
+	p = &build.Package{}
+
+	p.ImportPath = "."
+	p.Dir = filepath.Join(rp.modroot, rp.dir)
+
+	var pkgerr error
+	switch ctxt.Compiler {
+	case "gccgo", "gc":
+	default:
+		// Save error for end of function.
+		pkgerr = fmt.Errorf("import %q: unknown compiler %q", p.Dir, ctxt.Compiler)
+	}
+
+	if p.Dir == "" {
+		return p, fmt.Errorf("import %q: import of unknown directory", p.Dir)
+	}
+
+	// goroot and gopath
+	inTestdata := func(sub string) bool {
+		return strings.Contains(sub, "/testdata/") || strings.HasSuffix(sub, "/testdata") || str.HasPathPrefix(sub, "testdata")
+	}
+	var pkga string
+	if !inTestdata(rp.dir) {
+		// In build.go, p.Root should only be set in the non-local-import case, or in
+		// GOROOT or GOPATH. Since module mode only calls Import with path set to "."
+		// and the module index doesn't apply outside modules, the GOROOT case is
+		// the only case where p.Root needs to be set.
+		if ctxt.GOROOT != "" && str.HasFilePathPrefix(p.Dir, cfg.GOROOTsrc) && p.Dir != cfg.GOROOTsrc {
+			p.Root = ctxt.GOROOT
+			p.Goroot = true
+			modprefix := str.TrimFilePathPrefix(rp.modroot, cfg.GOROOTsrc)
+			p.ImportPath = rp.dir
+			if modprefix != "" {
+				p.ImportPath = filepath.Join(modprefix, p.ImportPath)
+			}
+
+			// Set GOROOT-specific fields (sometimes for modules in a GOPATH directory).
+			// The fields set below (SrcRoot, PkgRoot, BinDir, PkgTargetRoot, and PkgObj)
+			// are only set in build.Import if p.Root != "".
+			var pkgtargetroot string
+			suffix := ""
+			if ctxt.InstallSuffix != "" {
+				suffix = "_" + ctxt.InstallSuffix
+			}
+			switch ctxt.Compiler {
+			case "gccgo":
+				pkgtargetroot = "pkg/gccgo_" + ctxt.GOOS + "_" + ctxt.GOARCH + suffix
+				dir, elem := path.Split(p.ImportPath)
+				pkga = pkgtargetroot + "/" + dir + "lib" + elem + ".a"
+			case "gc":
+				pkgtargetroot = "pkg/" + ctxt.GOOS + "_" + ctxt.GOARCH + suffix
+				pkga = pkgtargetroot + "/" + p.ImportPath + ".a"
+			}
+			p.SrcRoot = ctxt.joinPath(p.Root, "src")
+			p.PkgRoot = ctxt.joinPath(p.Root, "pkg")
+			p.BinDir = ctxt.joinPath(p.Root, "bin")
+			if pkga != "" {
+				// Always set PkgTargetRoot. It might be used when building in shared
+				// mode.
+				p.PkgTargetRoot = ctxt.joinPath(p.Root, pkgtargetroot)
+
+				// Set the install target if applicable.
+				if !p.Goroot || (installgorootAll && p.ImportPath != "unsafe" && p.ImportPath != "builtin") {
+					p.PkgObj = ctxt.joinPath(p.Root, pkga)
+				}
+			}
+		}
+	}
+
+	if rp.error != nil {
+		if errors.Is(rp.error, errCannotFindPackage) && ctxt.Compiler == "gccgo" && p.Goroot {
+			return p, nil
+		}
+		return p, rp.error
+	}
+
+	if mode&build.FindOnly != 0 {
+		return p, pkgerr
+	}
+
+	// We need to do a second round of bad file processing.
+	var badGoError error
+	badGoFiles := make(map[string]bool)
+	badGoFile := func(name string, err error) {
+		if badGoError == nil {
+			badGoError = err
+		}
+		if !badGoFiles[name] {
+			p.InvalidGoFiles = append(p.InvalidGoFiles, name)
+			badGoFiles[name] = true
+		}
+	}
+
+	var Sfiles []string // files with ".S"(capital S)/.sx(capital s equivalent for case insensitive filesystems)
+	var firstFile string
+	embedPos := make(map[string][]token.Position)
+	testEmbedPos := make(map[string][]token.Position)
+	xTestEmbedPos := make(map[string][]token.Position)
+	importPos := make(map[string][]token.Position)
+	testImportPos := make(map[string][]token.Position)
+	xTestImportPos := make(map[string][]token.Position)
+	allTags := make(map[string]bool)
+	for _, tf := range rp.sourceFiles {
+		name := tf.name()
+		// Check errors for go files and call badGoFiles to put them in
+		// InvalidGoFiles if they do have an error.
+		if strings.HasSuffix(name, ".go") {
+			if error := tf.error(); error != "" {
+				badGoFile(name, errors.New(tf.error()))
+				continue
+			} else if parseError := tf.parseError(); parseError != "" {
+				badGoFile(name, parseErrorFromString(tf.parseError()))
+				// Fall through: we still want to list files with parse errors.
+			}
+		}
+
+		var shouldBuild = true
+		if !ctxt.goodOSArchFile(name, allTags) && !ctxt.UseAllFiles {
+			shouldBuild = false
+		} else if goBuildConstraint := tf.goBuildConstraint(); goBuildConstraint != "" {
+			x, err := constraint.Parse(goBuildConstraint)
+			if err != nil {
+				return p, fmt.Errorf("%s: parsing //go:build line: %v", name, err)
+			}
+			shouldBuild = ctxt.eval(x, allTags)
+		} else if plusBuildConstraints := tf.plusBuildConstraints(); len(plusBuildConstraints) > 0 {
+			for _, text := range plusBuildConstraints {
+				if x, err := constraint.Parse(text); err == nil {
+					if !ctxt.eval(x, allTags) {
+						shouldBuild = false
+					}
+				}
+			}
+		}
+
+		ext := nameExt(name)
+		if !shouldBuild || tf.ignoreFile() {
+			if ext == ".go" {
+				p.IgnoredGoFiles = append(p.IgnoredGoFiles, name)
+			} else if fileListForExt(p, ext) != nil {
+				p.IgnoredOtherFiles = append(p.IgnoredOtherFiles, name)
+			}
+			continue
+		}
+
+		// Going to save the file. For non-Go files, can stop here.
+		switch ext {
+		case ".go":
+			// keep going
+		case ".S", ".sx":
+			// special case for cgo, handled at end
+			Sfiles = append(Sfiles, name)
+			continue
+		default:
+			if list := fileListForExt(p, ext); list != nil {
+				*list = append(*list, name)
+			}
+			continue
+		}
+
+		pkg := tf.pkgName()
+		if pkg == "documentation" {
+			p.IgnoredGoFiles = append(p.IgnoredGoFiles, name)
+			continue
+		}
+		isTest := strings.HasSuffix(name, "_test.go")
+		isXTest := false
+		if isTest && strings.HasSuffix(tf.pkgName(), "_test") && p.Name != tf.pkgName() {
+			isXTest = true
+			pkg = pkg[:len(pkg)-len("_test")]
+		}
+
+		if !isTest && tf.binaryOnly() {
+			p.BinaryOnly = true
+		}
+
+		if p.Name == "" {
+			p.Name = pkg
+			firstFile = name
+		} else if pkg != p.Name {
+			// TODO(#45999): The choice of p.Name is arbitrary based on file iteration
+			// order. Instead of resolving p.Name arbitrarily, we should clear out the
+			// existing Name and mark the existing files as also invalid.
+			badGoFile(name, &MultiplePackageError{
+				Dir:      p.Dir,
+				Packages: []string{p.Name, pkg},
+				Files:    []string{firstFile, name},
+			})
+		}
+		// Grab the first package comment as docs, provided it is not from a test file.
+		if p.Doc == "" && !isTest && !isXTest {
+			if synopsis := tf.synopsis(); synopsis != "" {
+				p.Doc = synopsis
+			}
+		}
+
+		// Record Imports and information about cgo.
+		isCgo := false
+		imports := tf.imports()
+		for _, imp := range imports {
+			if imp.path == "C" {
+				if isTest {
+					badGoFile(name, fmt.Errorf("use of cgo in test %s not supported", name))
+					continue
+				}
+				isCgo = true
+			}
+		}
+		if directives := tf.cgoDirectives(); directives != "" {
+			if err := ctxt.saveCgo(name, p, directives); err != nil {
+				badGoFile(name, err)
+			}
+		}
+
+		var fileList *[]string
+		var importMap, embedMap map[string][]token.Position
+		var directives *[]build.Directive
+		switch {
+		case isCgo:
+			allTags["cgo"] = true
+			if ctxt.CgoEnabled {
+				fileList = &p.CgoFiles
+				importMap = importPos
+				embedMap = embedPos
+				directives = &p.Directives
+			} else {
+				// Ignore Imports and Embeds from cgo files if cgo is disabled.
+				fileList = &p.IgnoredGoFiles
+			}
+		case isXTest:
+			fileList = &p.XTestGoFiles
+			importMap = xTestImportPos
+			embedMap = xTestEmbedPos
+			directives = &p.XTestDirectives
+		case isTest:
+			fileList = &p.TestGoFiles
+			importMap = testImportPos
+			embedMap = testEmbedPos
+			directives = &p.TestDirectives
+		default:
+			fileList = &p.GoFiles
+			importMap = importPos
+			embedMap = embedPos
+			directives = &p.Directives
+		}
+		*fileList = append(*fileList, name)
+		if importMap != nil {
+			for _, imp := range imports {
+				importMap[imp.path] = append(importMap[imp.path], imp.position)
+			}
+		}
+		if embedMap != nil {
+			for _, e := range tf.embeds() {
+				embedMap[e.pattern] = append(embedMap[e.pattern], e.position)
+			}
+		}
+		if directives != nil {
+			*directives = append(*directives, tf.directives()...)
+		}
+	}
+
+	p.EmbedPatterns, p.EmbedPatternPos = cleanDecls(embedPos)
+	p.TestEmbedPatterns, p.TestEmbedPatternPos = cleanDecls(testEmbedPos)
+	p.XTestEmbedPatterns, p.XTestEmbedPatternPos = cleanDecls(xTestEmbedPos)
+
+	p.Imports, p.ImportPos = cleanDecls(importPos)
+	p.TestImports, p.TestImportPos = cleanDecls(testImportPos)
+	p.XTestImports, p.XTestImportPos = cleanDecls(xTestImportPos)
+
+	for tag := range allTags {
+		p.AllTags = append(p.AllTags, tag)
+	}
+	sort.Strings(p.AllTags)
+
+	if len(p.CgoFiles) > 0 {
+		p.SFiles = append(p.SFiles, Sfiles...)
+		sort.Strings(p.SFiles)
+	} else {
+		p.IgnoredOtherFiles = append(p.IgnoredOtherFiles, Sfiles...)
+		sort.Strings(p.IgnoredOtherFiles)
+	}
+
+	if badGoError != nil {
+		return p, badGoError
+	}
+	if len(p.GoFiles)+len(p.CgoFiles)+len(p.TestGoFiles)+len(p.XTestGoFiles) == 0 {
+		return p, &build.NoGoError{Dir: p.Dir}
+	}
+	return p, pkgerr
+}
+
+// IsStandardPackage reports whether path is a standard package
+// for the goroot and compiler using the module index if possible,
+// and otherwise falling back to internal/goroot.IsStandardPackage
+func IsStandardPackage(goroot_, compiler, path string) bool {
+	if !enabled || compiler != "gc" {
+		return goroot.IsStandardPackage(goroot_, compiler, path)
+	}
+
+	reldir := filepath.FromSlash(path) // relative dir path in module index for package
+	modroot := filepath.Join(goroot_, "src")
+	if str.HasFilePathPrefix(reldir, "cmd") {
+		reldir = str.TrimFilePathPrefix(reldir, "cmd")
+		modroot = filepath.Join(modroot, "cmd")
+	}
+	if _, err := GetPackage(modroot, filepath.Join(modroot, reldir)); err == nil {
+		// Note that goroot.IsStandardPackage doesn't check that the directory
+		// actually contains any go files-- merely that it exists. GetPackage
+		// returning a nil error is enough for us to know the directory exists.
+		return true
+	} else if errors.Is(err, ErrNotIndexed) {
+		// Fall back because package isn't indexable. (Probably because
+		// a file was modified recently)
+		return goroot.IsStandardPackage(goroot_, compiler, path)
+	}
+	return false
+}
+
+// IsDirWithGoFiles is the equivalent of fsys.IsDirWithGoFiles using the information in the index.
+func (rp *IndexPackage) IsDirWithGoFiles() (_ bool, err error) {
+	defer func() {
+		if e := recover(); e != nil {
+			err = fmt.Errorf("error reading module index: %v", e)
+		}
+	}()
+	for _, sf := range rp.sourceFiles {
+		if strings.HasSuffix(sf.name(), ".go") {
+			return true, nil
+		}
+	}
+	return false, nil
+}
+
+// ScanDir implements imports.ScanDir using the information in the index.
+func (rp *IndexPackage) ScanDir(tags map[string]bool) (sortedImports []string, sortedTestImports []string, err error) {
+	// TODO(matloob) dir should eventually be relative to indexed directory
+	// TODO(matloob): skip reading raw package and jump straight to data we need?
+
+	defer func() {
+		if e := recover(); e != nil {
+			err = fmt.Errorf("error reading module index: %v", e)
+		}
+	}()
+
+	imports_ := make(map[string]bool)
+	testImports := make(map[string]bool)
+	numFiles := 0
+
+Files:
+	for _, sf := range rp.sourceFiles {
+		name := sf.name()
+		if strings.HasPrefix(name, "_") || strings.HasPrefix(name, ".") || !strings.HasSuffix(name, ".go") || !imports.MatchFile(name, tags) {
+			continue
+		}
+
+		// The following section exists for backwards compatibility reasons:
+		// scanDir ignores files with import "C" when collecting the list
+		// of imports unless the "cgo" tag is provided. The following comment
+		// is copied from the original.
+		//
+		// import "C" is implicit requirement of cgo tag.
+		// When listing files on the command line (explicitFiles=true)
+		// we do not apply build tag filtering but we still do apply
+		// cgo filtering, so no explicitFiles check here.
+		// Why? Because we always have, and it's not worth breaking
+		// that behavior now.
+		imps := sf.imports() // TODO(matloob): directly read import paths to avoid the extra strings?
+		for _, imp := range imps {
+			if imp.path == "C" && !tags["cgo"] && !tags["*"] {
+				continue Files
+			}
+		}
+
+		if !shouldBuild(sf, tags) {
+			continue
+		}
+		numFiles++
+		m := imports_
+		if strings.HasSuffix(name, "_test.go") {
+			m = testImports
+		}
+		for _, p := range imps {
+			m[p.path] = true
+		}
+	}
+	if numFiles == 0 {
+		return nil, nil, imports.ErrNoGo
+	}
+	return keys(imports_), keys(testImports), nil
+}
+
+func keys(m map[string]bool) []string {
+	list := make([]string, 0, len(m))
+	for k := range m {
+		list = append(list, k)
+	}
+	sort.Strings(list)
+	return list
+}
+
+// implements imports.ShouldBuild in terms of an index sourcefile.
+func shouldBuild(sf *sourceFile, tags map[string]bool) bool {
+	if goBuildConstraint := sf.goBuildConstraint(); goBuildConstraint != "" {
+		x, err := constraint.Parse(goBuildConstraint)
+		if err != nil {
+			return false
+		}
+		return imports.Eval(x, tags, true)
+	}
+
+	plusBuildConstraints := sf.plusBuildConstraints()
+	for _, text := range plusBuildConstraints {
+		if x, err := constraint.Parse(text); err == nil {
+			if !imports.Eval(x, tags, true) {
+				return false
+			}
+		}
+	}
+
+	return true
+}
+
+// IndexPackage holds the information needed to access information in the
+// index needed to load a package in a specific directory.
+type IndexPackage struct {
+	error error
+	dir   string // directory of the package relative to the modroot
+
+	modroot string
+
+	// Source files
+	sourceFiles []*sourceFile
+}
+
+var errCannotFindPackage = errors.New("cannot find package")
+
+// Package and returns finds the package with the given path (relative to the module root).
+// If the package does not exist, Package returns an IndexPackage that will return an
+// appropriate error from its methods.
+func (m *Module) Package(path string) *IndexPackage {
+	defer unprotect(protect(), nil)
+
+	i, ok := sort.Find(m.n, func(i int) int {
+		return strings.Compare(path, m.pkgDir(i))
+	})
+	if !ok {
+		return &IndexPackage{error: fmt.Errorf("%w %q in:\n\t%s", errCannotFindPackage, path, filepath.Join(m.modroot, path))}
+	}
+	return m.pkg(i)
+}
+
+// pkg returns the i'th IndexPackage in m.
+func (m *Module) pkg(i int) *IndexPackage {
+	r := m.d.readAt(m.pkgOff(i))
+	p := new(IndexPackage)
+	if errstr := r.string(); errstr != "" {
+		p.error = errors.New(errstr)
+	}
+	p.dir = r.string()
+	p.sourceFiles = make([]*sourceFile, r.int())
+	for i := range p.sourceFiles {
+		p.sourceFiles[i] = &sourceFile{
+			d:   m.d,
+			pos: r.int(),
+		}
+	}
+	p.modroot = m.modroot
+	return p
+}
+
+// sourceFile represents the information of a given source file in the module index.
+type sourceFile struct {
+	d               *decoder // encoding of this source file
+	pos             int      // start of sourceFile encoding in d
+	onceReadImports sync.Once
+	savedImports    []rawImport // saved imports so that they're only read once
+}
+
+// Offsets for fields in the sourceFile.
+const (
+	sourceFileError = 4 * iota
+	sourceFileParseError
+	sourceFileSynopsis
+	sourceFileName
+	sourceFilePkgName
+	sourceFileIgnoreFile
+	sourceFileBinaryOnly
+	sourceFileCgoDirectives
+	sourceFileGoBuildConstraint
+	sourceFileNumPlusBuildConstraints
+)
+
+func (sf *sourceFile) error() string {
+	return sf.d.stringAt(sf.pos + sourceFileError)
+}
+func (sf *sourceFile) parseError() string {
+	return sf.d.stringAt(sf.pos + sourceFileParseError)
+}
+func (sf *sourceFile) synopsis() string {
+	return sf.d.stringAt(sf.pos + sourceFileSynopsis)
+}
+func (sf *sourceFile) name() string {
+	return sf.d.stringAt(sf.pos + sourceFileName)
+}
+func (sf *sourceFile) pkgName() string {
+	return sf.d.stringAt(sf.pos + sourceFilePkgName)
+}
+func (sf *sourceFile) ignoreFile() bool {
+	return sf.d.boolAt(sf.pos + sourceFileIgnoreFile)
+}
+func (sf *sourceFile) binaryOnly() bool {
+	return sf.d.boolAt(sf.pos + sourceFileBinaryOnly)
+}
+func (sf *sourceFile) cgoDirectives() string {
+	return sf.d.stringAt(sf.pos + sourceFileCgoDirectives)
+}
+func (sf *sourceFile) goBuildConstraint() string {
+	return sf.d.stringAt(sf.pos + sourceFileGoBuildConstraint)
+}
+
+func (sf *sourceFile) plusBuildConstraints() []string {
+	pos := sf.pos + sourceFileNumPlusBuildConstraints
+	n := sf.d.intAt(pos)
+	pos += 4
+	ret := make([]string, n)
+	for i := 0; i < n; i++ {
+		ret[i] = sf.d.stringAt(pos)
+		pos += 4
+	}
+	return ret
+}
+
+func (sf *sourceFile) importsOffset() int {
+	pos := sf.pos + sourceFileNumPlusBuildConstraints
+	n := sf.d.intAt(pos)
+	// each build constraint is 1 uint32
+	return pos + 4 + n*4
+}
+
+func (sf *sourceFile) embedsOffset() int {
+	pos := sf.importsOffset()
+	n := sf.d.intAt(pos)
+	// each import is 5 uint32s (string + tokpos)
+	return pos + 4 + n*(4*5)
+}
+
+func (sf *sourceFile) directivesOffset() int {
+	pos := sf.embedsOffset()
+	n := sf.d.intAt(pos)
+	// each embed is 5 uint32s (string + tokpos)
+	return pos + 4 + n*(4*5)
+}
+
+func (sf *sourceFile) imports() []rawImport {
+	sf.onceReadImports.Do(func() {
+		importsOffset := sf.importsOffset()
+		r := sf.d.readAt(importsOffset)
+		numImports := r.int()
+		ret := make([]rawImport, numImports)
+		for i := 0; i < numImports; i++ {
+			ret[i] = rawImport{r.string(), r.tokpos()}
+		}
+		sf.savedImports = ret
+	})
+	return sf.savedImports
+}
+
+func (sf *sourceFile) embeds() []embed {
+	embedsOffset := sf.embedsOffset()
+	r := sf.d.readAt(embedsOffset)
+	numEmbeds := r.int()
+	ret := make([]embed, numEmbeds)
+	for i := range ret {
+		ret[i] = embed{r.string(), r.tokpos()}
+	}
+	return ret
+}
+
+func (sf *sourceFile) directives() []build.Directive {
+	directivesOffset := sf.directivesOffset()
+	r := sf.d.readAt(directivesOffset)
+	numDirectives := r.int()
+	ret := make([]build.Directive, numDirectives)
+	for i := range ret {
+		ret[i] = build.Directive{Text: r.string(), Pos: r.tokpos()}
+	}
+	return ret
+}
+
+func asString(b []byte) string {
+	return unsafe.String(unsafe.SliceData(b), len(b))
+}
+
+// A decoder helps decode the index format.
+type decoder struct {
+	data []byte // data after header
+	str  []byte // string table
+}
+
+// intAt returns the int at the given offset in d.data.
+func (d *decoder) intAt(off int) int {
+	if off < 0 || len(d.data)-off < 4 {
+		panic(errCorrupt)
+	}
+	i := binary.LittleEndian.Uint32(d.data[off : off+4])
+	if int32(i)>>31 != 0 {
+		panic(errCorrupt)
+	}
+	return int(i)
+}
+
+// boolAt returns the bool at the given offset in d.data.
+func (d *decoder) boolAt(off int) bool {
+	return d.intAt(off) != 0
+}
+
+// stringAt returns the string pointed at by the int at the given offset in d.data.
+func (d *decoder) stringAt(off int) string {
+	return d.stringTableAt(d.intAt(off))
+}
+
+// stringTableAt returns the string at the given offset in the string table d.str.
+func (d *decoder) stringTableAt(off int) string {
+	if off < 0 || off >= len(d.str) {
+		panic(errCorrupt)
+	}
+	s := d.str[off:]
+	v, n := binary.Uvarint(s)
+	if n <= 0 || v > uint64(len(s[n:])) {
+		panic(errCorrupt)
+	}
+	return asString(s[n : n+int(v)])
+}
+
+// A reader reads sequential fields from a section of the index format.
+type reader struct {
+	d   *decoder
+	pos int
+}
+
+// readAt returns a reader starting at the given position in d.
+func (d *decoder) readAt(pos int) *reader {
+	return &reader{d, pos}
+}
+
+// int reads the next int.
+func (r *reader) int() int {
+	i := r.d.intAt(r.pos)
+	r.pos += 4
+	return i
+}
+
+// string reads the next string.
+func (r *reader) string() string {
+	return r.d.stringTableAt(r.int())
+}
+
+// bool reads the next bool.
+func (r *reader) bool() bool {
+	return r.int() != 0
+}
+
+// tokpos reads the next token.Position.
+func (r *reader) tokpos() token.Position {
+	return token.Position{
+		Filename: r.string(),
+		Offset:   r.int(),
+		Line:     r.int(),
+		Column:   r.int(),
+	}
+}
diff --git a/src/cmd/go/internal/modindex/scan.go b/src/cmd/go/internal/modindex/scan.go
new file mode 100644
index 0000000..6ca73e2
--- /dev/null
+++ b/src/cmd/go/internal/modindex/scan.go
@@ -0,0 +1,290 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modindex
+
+import (
+	"cmd/go/internal/base"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/str"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"go/build"
+	"go/doc"
+	"go/scanner"
+	"go/token"
+	"io/fs"
+	"path/filepath"
+	"strings"
+)
+
+// moduleWalkErr returns filepath.SkipDir if the directory isn't relevant
+// when indexing a module or generating a filehash, ErrNotIndexed,
+// if the module shouldn't be indexed, and nil otherwise.
+func moduleWalkErr(root string, path string, info fs.FileInfo, err error) error {
+	if err != nil {
+		return ErrNotIndexed
+	}
+	// stop at module boundaries
+	if info.IsDir() && path != root {
+		if fi, err := fsys.Stat(filepath.Join(path, "go.mod")); err == nil && !fi.IsDir() {
+			return filepath.SkipDir
+		}
+	}
+	if info.Mode()&fs.ModeSymlink != 0 {
+		if target, err := fsys.Stat(path); err == nil && target.IsDir() {
+			// return an error to make the module hash invalid.
+			// Symlink directories in modules are tricky, so we won't index
+			// modules that contain them.
+			// TODO(matloob): perhaps don't return this error if the symlink leads to
+			// a directory with a go.mod file.
+			return ErrNotIndexed
+		}
+	}
+	return nil
+}
+
+// indexModule indexes the module at the given directory and returns its
+// encoded representation. It returns ErrNotIndexed if the module can't
+// be indexed because it contains symlinks.
+func indexModule(modroot string) ([]byte, error) {
+	fsys.Trace("indexModule", modroot)
+	var packages []*rawPackage
+
+	// If the root itself is a symlink to a directory,
+	// we want to follow it (see https://go.dev/issue/50807).
+	// Add a trailing separator to force that to happen.
+	root := str.WithFilePathSeparator(modroot)
+	err := fsys.Walk(root, func(path string, info fs.FileInfo, err error) error {
+		if err := moduleWalkErr(root, path, info, err); err != nil {
+			return err
+		}
+
+		if !info.IsDir() {
+			return nil
+		}
+		if !strings.HasPrefix(path, root) {
+			panic(fmt.Errorf("path %v in walk doesn't have modroot %v as prefix", path, modroot))
+		}
+		rel := path[len(root):]
+		packages = append(packages, importRaw(modroot, rel))
+		return nil
+	})
+	if err != nil {
+		return nil, err
+	}
+	return encodeModuleBytes(packages), nil
+}
+
+// indexPackage indexes the package at the given directory and returns its
+// encoded representation. It returns ErrNotIndexed if the package can't
+// be indexed.
+func indexPackage(modroot, pkgdir string) []byte {
+	fsys.Trace("indexPackage", pkgdir)
+	p := importRaw(modroot, relPath(pkgdir, modroot))
+	return encodePackageBytes(p)
+}
+
+// rawPackage holds the information from each package that's needed to
+// fill a build.Package once the context is available.
+type rawPackage struct {
+	error string
+	dir   string // directory containing package sources, relative to the module root
+
+	// Source files
+	sourceFiles []*rawFile
+}
+
+type parseError struct {
+	ErrorList   *scanner.ErrorList
+	ErrorString string
+}
+
+// parseErrorToString converts the error from parsing the file into a string
+// representation. A nil error is converted to an empty string, and all other
+// errors are converted to a JSON-marshalled parseError struct, with ErrorList
+// set for errors of type scanner.ErrorList, and ErrorString set to the error's
+// string representation for all other errors.
+func parseErrorToString(err error) string {
+	if err == nil {
+		return ""
+	}
+	var p parseError
+	if e, ok := err.(scanner.ErrorList); ok {
+		p.ErrorList = &e
+	} else {
+		p.ErrorString = e.Error()
+	}
+	s, err := json.Marshal(p)
+	if err != nil {
+		panic(err) // This should be impossible because scanner.Error contains only strings and ints.
+	}
+	return string(s)
+}
+
+// parseErrorFromString converts a string produced by parseErrorToString back
+// to an error.  An empty string is converted to a nil error, and all
+// other strings are expected to be JSON-marshalled parseError structs.
+// The two functions are meant to preserve the structure of an
+// error of type scanner.ErrorList in a round trip, but may not preserve the
+// structure of other errors.
+func parseErrorFromString(s string) error {
+	if s == "" {
+		return nil
+	}
+	var p parseError
+	if err := json.Unmarshal([]byte(s), &p); err != nil {
+		base.Fatalf(`go: invalid parse error value in index: %q. This indicates a corrupted index. Run "go clean -cache" to reset the module cache.`, s)
+	}
+	if p.ErrorList != nil {
+		return *p.ErrorList
+	}
+	return errors.New(p.ErrorString)
+}
+
+// rawFile is the struct representation of the file holding all
+// information in its fields.
+type rawFile struct {
+	error      string
+	parseError string
+
+	name                 string
+	synopsis             string // doc.Synopsis of package comment... Compute synopsis on all of these?
+	pkgName              string
+	ignoreFile           bool   // starts with _ or . or should otherwise always be ignored
+	binaryOnly           bool   // cannot be rebuilt from source (has //go:binary-only-package comment)
+	cgoDirectives        string // the #cgo directive lines in the comment on import "C"
+	goBuildConstraint    string
+	plusBuildConstraints []string
+	imports              []rawImport
+	embeds               []embed
+	directives           []build.Directive
+}
+
+type rawImport struct {
+	path     string
+	position token.Position
+}
+
+type embed struct {
+	pattern  string
+	position token.Position
+}
+
+// importRaw fills the rawPackage from the package files in srcDir.
+// dir is the package's path relative to the modroot.
+func importRaw(modroot, reldir string) *rawPackage {
+	p := &rawPackage{
+		dir: reldir,
+	}
+
+	absdir := filepath.Join(modroot, reldir)
+
+	// We still haven't checked
+	// that p.dir directory exists. This is the right time to do that check.
+	// We can't do it earlier, because we want to gather partial information for the
+	// non-nil *build.Package returned when an error occurs.
+	// We need to do this before we return early on FindOnly flag.
+	if !isDir(absdir) {
+		// package was not found
+		p.error = fmt.Errorf("cannot find package in:\n\t%s", absdir).Error()
+		return p
+	}
+
+	entries, err := fsys.ReadDir(absdir)
+	if err != nil {
+		p.error = err.Error()
+		return p
+	}
+
+	fset := token.NewFileSet()
+	for _, d := range entries {
+		if d.IsDir() {
+			continue
+		}
+		if d.Mode()&fs.ModeSymlink != 0 {
+			if isDir(filepath.Join(absdir, d.Name())) {
+				// Symlinks to directories are not source files.
+				continue
+			}
+		}
+
+		name := d.Name()
+		ext := nameExt(name)
+
+		if strings.HasPrefix(name, "_") || strings.HasPrefix(name, ".") {
+			continue
+		}
+		info, err := getFileInfo(absdir, name, fset)
+		if err == errNonSource {
+			// not a source or object file. completely ignore in the index
+			continue
+		} else if err != nil {
+			p.sourceFiles = append(p.sourceFiles, &rawFile{name: name, error: err.Error()})
+			continue
+		} else if info == nil {
+			p.sourceFiles = append(p.sourceFiles, &rawFile{name: name, ignoreFile: true})
+			continue
+		}
+		rf := &rawFile{
+			name:                 name,
+			goBuildConstraint:    info.goBuildConstraint,
+			plusBuildConstraints: info.plusBuildConstraints,
+			binaryOnly:           info.binaryOnly,
+			directives:           info.directives,
+		}
+		if info.parsed != nil {
+			rf.pkgName = info.parsed.Name.Name
+		}
+
+		// Going to save the file. For non-Go files, can stop here.
+		p.sourceFiles = append(p.sourceFiles, rf)
+		if ext != ".go" {
+			continue
+		}
+
+		if info.parseErr != nil {
+			rf.parseError = parseErrorToString(info.parseErr)
+			// Fall through: we might still have a partial AST in info.Parsed,
+			// and we want to list files with parse errors anyway.
+		}
+
+		if info.parsed != nil && info.parsed.Doc != nil {
+			rf.synopsis = doc.Synopsis(info.parsed.Doc.Text())
+		}
+
+		var cgoDirectives []string
+		for _, imp := range info.imports {
+			if imp.path == "C" {
+				cgoDirectives = append(cgoDirectives, extractCgoDirectives(imp.doc.Text())...)
+			}
+			rf.imports = append(rf.imports, rawImport{path: imp.path, position: fset.Position(imp.pos)})
+		}
+		rf.cgoDirectives = strings.Join(cgoDirectives, "\n")
+		for _, emb := range info.embeds {
+			rf.embeds = append(rf.embeds, embed{emb.pattern, emb.pos})
+		}
+
+	}
+	return p
+}
+
+// extractCgoDirectives filters only the lines containing #cgo directives from the input,
+// which is the comment on import "C".
+func extractCgoDirectives(doc string) []string {
+	var out []string
+	for _, line := range strings.Split(doc, "\n") {
+		// Line is
+		//	#cgo [GOOS/GOARCH...] LDFLAGS: stuff
+		//
+		line = strings.TrimSpace(line)
+		if len(line) < 5 || line[:4] != "#cgo" || (line[4] != ' ' && line[4] != '\t') {
+			continue
+		}
+
+		out = append(out, line)
+	}
+	return out
+}
diff --git a/src/cmd/go/internal/modindex/syslist.go b/src/cmd/go/internal/modindex/syslist.go
new file mode 100644
index 0000000..41adcc5
--- /dev/null
+++ b/src/cmd/go/internal/modindex/syslist.go
@@ -0,0 +1,78 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file is a lightly modified copy go/build/syslist_test.go.
+
+package modindex
+
+// knownOS is the list of past, present, and future known GOOS values.
+// Do not remove from this list, as it is used for filename matching.
+// If you add an entry to this list, look at unixOS, below.
+var knownOS = map[string]bool{
+	"aix":       true,
+	"android":   true,
+	"darwin":    true,
+	"dragonfly": true,
+	"freebsd":   true,
+	"hurd":      true,
+	"illumos":   true,
+	"ios":       true,
+	"js":        true,
+	"linux":     true,
+	"nacl":      true,
+	"netbsd":    true,
+	"openbsd":   true,
+	"plan9":     true,
+	"solaris":   true,
+	"wasip1":    true,
+	"windows":   true,
+	"zos":       true,
+}
+
+// unixOS is the set of GOOS values matched by the "unix" build tag.
+// This is not used for filename matching.
+// This list also appears in cmd/dist/build.go.
+var unixOS = map[string]bool{
+	"aix":       true,
+	"android":   true,
+	"darwin":    true,
+	"dragonfly": true,
+	"freebsd":   true,
+	"hurd":      true,
+	"illumos":   true,
+	"ios":       true,
+	"linux":     true,
+	"netbsd":    true,
+	"openbsd":   true,
+	"solaris":   true,
+}
+
+// knownArch is the list of past, present, and future known GOARCH values.
+// Do not remove from this list, as it is used for filename matching.
+var knownArch = map[string]bool{
+	"386":         true,
+	"amd64":       true,
+	"amd64p32":    true,
+	"arm":         true,
+	"armbe":       true,
+	"arm64":       true,
+	"arm64be":     true,
+	"loong64":     true,
+	"mips":        true,
+	"mipsle":      true,
+	"mips64":      true,
+	"mips64le":    true,
+	"mips64p32":   true,
+	"mips64p32le": true,
+	"ppc":         true,
+	"ppc64":       true,
+	"ppc64le":     true,
+	"riscv":       true,
+	"riscv64":     true,
+	"s390":        true,
+	"s390x":       true,
+	"sparc":       true,
+	"sparc64":     true,
+	"wasm":        true,
+}
diff --git a/src/cmd/go/internal/modindex/syslist_test.go b/src/cmd/go/internal/modindex/syslist_test.go
new file mode 100644
index 0000000..1a61562
--- /dev/null
+++ b/src/cmd/go/internal/modindex/syslist_test.go
@@ -0,0 +1,65 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file is a lightly modified copy go/build/syslist_test.go.
+
+package modindex
+
+import (
+	"go/build"
+	"runtime"
+	"testing"
+)
+
+var (
+	thisOS    = runtime.GOOS
+	thisArch  = runtime.GOARCH
+	otherOS   = anotherOS()
+	otherArch = anotherArch()
+)
+
+func anotherOS() string {
+	if thisOS != "darwin" && thisOS != "ios" {
+		return "darwin"
+	}
+	return "linux"
+}
+
+func anotherArch() string {
+	if thisArch != "amd64" {
+		return "amd64"
+	}
+	return "386"
+}
+
+type GoodFileTest struct {
+	name   string
+	result bool
+}
+
+var tests = []GoodFileTest{
+	{"file.go", true},
+	{"file.c", true},
+	{"file_foo.go", true},
+	{"file_" + thisArch + ".go", true},
+	{"file_" + otherArch + ".go", false},
+	{"file_" + thisOS + ".go", true},
+	{"file_" + otherOS + ".go", false},
+	{"file_" + thisOS + "_" + thisArch + ".go", true},
+	{"file_" + otherOS + "_" + thisArch + ".go", false},
+	{"file_" + thisOS + "_" + otherArch + ".go", false},
+	{"file_" + otherOS + "_" + otherArch + ".go", false},
+	{"file_foo_" + thisArch + ".go", true},
+	{"file_foo_" + otherArch + ".go", false},
+	{"file_" + thisOS + ".c", true},
+	{"file_" + otherOS + ".c", false},
+}
+
+func TestGoodOSArch(t *testing.T) {
+	for _, test := range tests {
+		if (*Context)(&build.Default).goodOSArchFile(test.name, make(map[string]bool)) != test.result {
+			t.Fatalf("goodOSArchFile(%q) != %v", test.name, test.result)
+		}
+	}
+}
diff --git a/src/cmd/go/internal/modindex/testdata/ignore_non_source/a.syso b/src/cmd/go/internal/modindex/testdata/ignore_non_source/a.syso
new file mode 100644
index 0000000..9527d05
--- /dev/null
+++ b/src/cmd/go/internal/modindex/testdata/ignore_non_source/a.syso
@@ -0,0 +1 @@
+package ignore_non_source
diff --git a/src/cmd/go/internal/modindex/testdata/ignore_non_source/b.go b/src/cmd/go/internal/modindex/testdata/ignore_non_source/b.go
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/src/cmd/go/internal/modindex/testdata/ignore_non_source/b.go
diff --git a/src/cmd/go/internal/modindex/testdata/ignore_non_source/bar.json b/src/cmd/go/internal/modindex/testdata/ignore_non_source/bar.json
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/src/cmd/go/internal/modindex/testdata/ignore_non_source/bar.json
diff --git a/src/cmd/go/internal/modindex/testdata/ignore_non_source/baz.log b/src/cmd/go/internal/modindex/testdata/ignore_non_source/baz.log
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/src/cmd/go/internal/modindex/testdata/ignore_non_source/baz.log
diff --git a/src/cmd/go/internal/modindex/testdata/ignore_non_source/c.c b/src/cmd/go/internal/modindex/testdata/ignore_non_source/c.c
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/src/cmd/go/internal/modindex/testdata/ignore_non_source/c.c
diff --git a/src/cmd/go/internal/modindex/write.go b/src/cmd/go/internal/modindex/write.go
new file mode 100644
index 0000000..cd18ad9
--- /dev/null
+++ b/src/cmd/go/internal/modindex/write.go
@@ -0,0 +1,164 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modindex
+
+import (
+	"cmd/go/internal/base"
+	"encoding/binary"
+	"go/token"
+	"sort"
+)
+
+const indexVersion = "go index v2" // 11 bytes (plus \n), to align uint32s in index
+
+// encodeModuleBytes produces the encoded representation of the module index.
+// encodeModuleBytes may modify the packages slice.
+func encodeModuleBytes(packages []*rawPackage) []byte {
+	e := newEncoder()
+	e.Bytes([]byte(indexVersion + "\n"))
+	stringTableOffsetPos := e.Pos() // fill this at the end
+	e.Uint32(0)                     // string table offset
+	sort.Slice(packages, func(i, j int) bool {
+		return packages[i].dir < packages[j].dir
+	})
+	e.Int(len(packages))
+	packagesPos := e.Pos()
+	for _, p := range packages {
+		e.String(p.dir)
+		e.Int(0)
+	}
+	for i, p := range packages {
+		e.IntAt(e.Pos(), packagesPos+8*i+4)
+		encodePackage(e, p)
+	}
+	e.IntAt(e.Pos(), stringTableOffsetPos)
+	e.Bytes(e.stringTable)
+	e.Bytes([]byte{0xFF}) // end of string table marker
+	return e.b
+}
+
+func encodePackageBytes(p *rawPackage) []byte {
+	return encodeModuleBytes([]*rawPackage{p})
+}
+
+func encodePackage(e *encoder, p *rawPackage) {
+	e.String(p.error)
+	e.String(p.dir)
+	e.Int(len(p.sourceFiles))      // number of source files
+	sourceFileOffsetPos := e.Pos() // the pos of the start of the source file offsets
+	for range p.sourceFiles {
+		e.Int(0)
+	}
+	for i, f := range p.sourceFiles {
+		e.IntAt(e.Pos(), sourceFileOffsetPos+4*i)
+		encodeFile(e, f)
+	}
+}
+
+func encodeFile(e *encoder, f *rawFile) {
+	e.String(f.error)
+	e.String(f.parseError)
+	e.String(f.synopsis)
+	e.String(f.name)
+	e.String(f.pkgName)
+	e.Bool(f.ignoreFile)
+	e.Bool(f.binaryOnly)
+	e.String(f.cgoDirectives)
+	e.String(f.goBuildConstraint)
+
+	e.Int(len(f.plusBuildConstraints))
+	for _, s := range f.plusBuildConstraints {
+		e.String(s)
+	}
+
+	e.Int(len(f.imports))
+	for _, m := range f.imports {
+		e.String(m.path)
+		e.Position(m.position)
+	}
+
+	e.Int(len(f.embeds))
+	for _, embed := range f.embeds {
+		e.String(embed.pattern)
+		e.Position(embed.position)
+	}
+
+	e.Int(len(f.directives))
+	for _, d := range f.directives {
+		e.String(d.Text)
+		e.Position(d.Pos)
+	}
+}
+
+func newEncoder() *encoder {
+	e := &encoder{strings: make(map[string]int)}
+
+	// place the empty string at position 0 in the string table
+	e.stringTable = append(e.stringTable, 0)
+	e.strings[""] = 0
+
+	return e
+}
+
+func (e *encoder) Position(position token.Position) {
+	e.String(position.Filename)
+	e.Int(position.Offset)
+	e.Int(position.Line)
+	e.Int(position.Column)
+}
+
+type encoder struct {
+	b           []byte
+	stringTable []byte
+	strings     map[string]int
+}
+
+func (e *encoder) Pos() int {
+	return len(e.b)
+}
+
+func (e *encoder) Bytes(b []byte) {
+	e.b = append(e.b, b...)
+}
+
+func (e *encoder) String(s string) {
+	if n, ok := e.strings[s]; ok {
+		e.Int(n)
+		return
+	}
+	pos := len(e.stringTable)
+	e.strings[s] = pos
+	e.Int(pos)
+	e.stringTable = binary.AppendUvarint(e.stringTable, uint64(len(s)))
+	e.stringTable = append(e.stringTable, s...)
+}
+
+func (e *encoder) Bool(b bool) {
+	if b {
+		e.Uint32(1)
+	} else {
+		e.Uint32(0)
+	}
+}
+
+func (e *encoder) Uint32(n uint32) {
+	e.b = binary.LittleEndian.AppendUint32(e.b, n)
+}
+
+// Int encodes n. Note that all ints are written to the index as uint32s,
+// and to avoid problems on 32-bit systems we require fitting into a 32-bit int.
+func (e *encoder) Int(n int) {
+	if n < 0 || int(int32(n)) != n {
+		base.Fatalf("go: attempting to write an int to the index that overflows int32")
+	}
+	e.Uint32(uint32(n))
+}
+
+func (e *encoder) IntAt(n int, at int) {
+	if n < 0 || int(int32(n)) != n {
+		base.Fatalf("go: attempting to write an int to the index that overflows int32")
+	}
+	binary.LittleEndian.PutUint32(e.b[at:], uint32(n))
+}
diff --git a/src/cmd/go/internal/modinfo/info.go b/src/cmd/go/internal/modinfo/info.go
new file mode 100644
index 0000000..b0adcbc
--- /dev/null
+++ b/src/cmd/go/internal/modinfo/info.go
@@ -0,0 +1,85 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modinfo
+
+import (
+	"cmd/go/internal/modfetch/codehost"
+	"encoding/json"
+	"time"
+)
+
+// Note that these structs are publicly visible (part of go list's API)
+// and the fields are documented in the help text in ../list/list.go
+
+type ModulePublic struct {
+	Path       string        `json:",omitempty"` // module path
+	Version    string        `json:",omitempty"` // module version
+	Query      string        `json:",omitempty"` // version query corresponding to this version
+	Versions   []string      `json:",omitempty"` // available module versions
+	Replace    *ModulePublic `json:",omitempty"` // replaced by this module
+	Time       *time.Time    `json:",omitempty"` // time version was created
+	Update     *ModulePublic `json:",omitempty"` // available update (with -u)
+	Main       bool          `json:",omitempty"` // is this the main module?
+	Indirect   bool          `json:",omitempty"` // module is only indirectly needed by main module
+	Dir        string        `json:",omitempty"` // directory holding local copy of files, if any
+	GoMod      string        `json:",omitempty"` // path to go.mod file describing module, if any
+	GoVersion  string        `json:",omitempty"` // go version used in module
+	Retracted  []string      `json:",omitempty"` // retraction information, if any (with -retracted or -u)
+	Deprecated string        `json:",omitempty"` // deprecation message, if any (with -u)
+	Error      *ModuleError  `json:",omitempty"` // error loading module
+
+	Origin *codehost.Origin `json:",omitempty"` // provenance of module
+	Reuse  bool             `json:",omitempty"` // reuse of old module info is safe
+}
+
+type ModuleError struct {
+	Err string // error text
+}
+
+type moduleErrorNoMethods ModuleError
+
+// UnmarshalJSON accepts both {"Err":"text"} and "text",
+// so that the output of go mod download -json can still
+// be unmarshalled into a ModulePublic during -reuse processing.
+func (e *ModuleError) UnmarshalJSON(data []byte) error {
+	if len(data) > 0 && data[0] == '"' {
+		return json.Unmarshal(data, &e.Err)
+	}
+	return json.Unmarshal(data, (*moduleErrorNoMethods)(e))
+}
+
+func (m *ModulePublic) String() string {
+	s := m.Path
+	versionString := func(mm *ModulePublic) string {
+		v := mm.Version
+		if len(mm.Retracted) == 0 {
+			return v
+		}
+		return v + " (retracted)"
+	}
+
+	if m.Version != "" {
+		s += " " + versionString(m)
+		if m.Update != nil {
+			s += " [" + versionString(m.Update) + "]"
+		}
+	}
+	if m.Deprecated != "" {
+		s += " (deprecated)"
+	}
+	if m.Replace != nil {
+		s += " => " + m.Replace.Path
+		if m.Replace.Version != "" {
+			s += " " + versionString(m.Replace)
+			if m.Replace.Update != nil {
+				s += " [" + versionString(m.Replace.Update) + "]"
+			}
+		}
+		if m.Replace.Deprecated != "" {
+			s += " (deprecated)"
+		}
+	}
+	return s
+}
diff --git a/src/cmd/go/internal/modload/build.go b/src/cmd/go/internal/modload/build.go
new file mode 100644
index 0000000..bb513ea
--- /dev/null
+++ b/src/cmd/go/internal/modload/build.go
@@ -0,0 +1,449 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"context"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/modfetch/codehost"
+	"cmd/go/internal/modindex"
+	"cmd/go/internal/modinfo"
+	"cmd/go/internal/search"
+
+	"golang.org/x/mod/module"
+)
+
+var (
+	infoStart, _ = hex.DecodeString("3077af0c9274080241e1c107e6d618e6")
+	infoEnd, _   = hex.DecodeString("f932433186182072008242104116d8f2")
+)
+
+func isStandardImportPath(path string) bool {
+	return findStandardImportPath(path) != ""
+}
+
+func findStandardImportPath(path string) string {
+	if path == "" {
+		panic("findStandardImportPath called with empty path")
+	}
+	if search.IsStandardImportPath(path) {
+		if modindex.IsStandardPackage(cfg.GOROOT, cfg.BuildContext.Compiler, path) {
+			return filepath.Join(cfg.GOROOT, "src", path)
+		}
+	}
+	return ""
+}
+
+// PackageModuleInfo returns information about the module that provides
+// a given package. If modules are not enabled or if the package is in the
+// standard library or if the package was not successfully loaded with
+// LoadPackages or ImportFromFiles, nil is returned.
+func PackageModuleInfo(ctx context.Context, pkgpath string) *modinfo.ModulePublic {
+	if isStandardImportPath(pkgpath) || !Enabled() {
+		return nil
+	}
+	m, ok := findModule(loaded, pkgpath)
+	if !ok {
+		return nil
+	}
+
+	rs := LoadModFile(ctx)
+	return moduleInfo(ctx, rs, m, 0, nil)
+}
+
+// PackageModRoot returns the module root directory for the module that provides
+// a given package. If modules are not enabled or if the package is in the
+// standard library or if the package was not successfully loaded with
+// LoadPackages or ImportFromFiles, the empty string is returned.
+func PackageModRoot(ctx context.Context, pkgpath string) string {
+	if isStandardImportPath(pkgpath) || !Enabled() || cfg.BuildMod == "vendor" {
+		return ""
+	}
+	m, ok := findModule(loaded, pkgpath)
+	if !ok {
+		return ""
+	}
+	root, _, err := fetch(ctx, m)
+	if err != nil {
+		return ""
+	}
+	return root
+}
+
+func ModuleInfo(ctx context.Context, path string) *modinfo.ModulePublic {
+	if !Enabled() {
+		return nil
+	}
+
+	if path, vers, found := strings.Cut(path, "@"); found {
+		m := module.Version{Path: path, Version: vers}
+		return moduleInfo(ctx, nil, m, 0, nil)
+	}
+
+	rs := LoadModFile(ctx)
+
+	var (
+		v  string
+		ok bool
+	)
+	if rs.pruning == pruned {
+		v, ok = rs.rootSelected(path)
+	}
+	if !ok {
+		mg, err := rs.Graph(ctx)
+		if err != nil {
+			base.Fatal(err)
+		}
+		v = mg.Selected(path)
+	}
+
+	if v == "none" {
+		return &modinfo.ModulePublic{
+			Path: path,
+			Error: &modinfo.ModuleError{
+				Err: "module not in current build",
+			},
+		}
+	}
+
+	return moduleInfo(ctx, rs, module.Version{Path: path, Version: v}, 0, nil)
+}
+
+// addUpdate fills in m.Update if an updated version is available.
+func addUpdate(ctx context.Context, m *modinfo.ModulePublic) {
+	if m.Version == "" {
+		return
+	}
+
+	info, err := Query(ctx, m.Path, "upgrade", m.Version, CheckAllowed)
+	var noVersionErr *NoMatchingVersionError
+	if errors.Is(err, ErrDisallowed) ||
+		errors.Is(err, fs.ErrNotExist) ||
+		errors.As(err, &noVersionErr) {
+		// Ignore "not found" and "no matching version" errors.
+		// This means the proxy has no matching version or no versions at all.
+		//
+		// Ignore "disallowed" errors. This means the current version is
+		// excluded or retracted and there are no higher allowed versions.
+		//
+		// We should report other errors though. An attacker that controls the
+		// network shouldn't be able to hide versions by interfering with
+		// the HTTPS connection. An attacker that controls the proxy may still
+		// hide versions, since the "list" and "latest" endpoints are not
+		// authenticated.
+		return
+	} else if err != nil {
+		if m.Error == nil {
+			m.Error = &modinfo.ModuleError{Err: err.Error()}
+		}
+		return
+	}
+
+	if gover.ModCompare(m.Path, info.Version, m.Version) > 0 {
+		m.Update = &modinfo.ModulePublic{
+			Path:    m.Path,
+			Version: info.Version,
+			Time:    &info.Time,
+		}
+	}
+}
+
+// mergeOrigin merges two origins,
+// returning and possibly modifying one of its arguments.
+// If the two origins conflict, mergeOrigin returns a non-specific one
+// that will not pass CheckReuse.
+// If m1 or m2 is nil, the other is returned unmodified.
+// But if m1 or m2 is non-nil and uncheckable, the result is also uncheckable,
+// to preserve uncheckability.
+func mergeOrigin(m1, m2 *codehost.Origin) *codehost.Origin {
+	if m1 == nil {
+		return m2
+	}
+	if m2 == nil {
+		return m1
+	}
+	if !m1.Checkable() {
+		return m1
+	}
+	if !m2.Checkable() {
+		return m2
+	}
+
+	merged := new(codehost.Origin)
+	*merged = *m1 // Clone to avoid overwriting fields in cached results.
+
+	if m2.TagSum != "" {
+		if m1.TagSum != "" && (m1.TagSum != m2.TagSum || m1.TagPrefix != m2.TagPrefix) {
+			merged.ClearCheckable()
+			return merged
+		}
+		merged.TagSum = m2.TagSum
+		merged.TagPrefix = m2.TagPrefix
+	}
+	if m2.Hash != "" {
+		if m1.Hash != "" && (m1.Hash != m2.Hash || m1.Ref != m2.Ref) {
+			merged.ClearCheckable()
+			return merged
+		}
+		merged.Hash = m2.Hash
+		merged.Ref = m2.Ref
+	}
+	return merged
+}
+
+// addVersions fills in m.Versions with the list of known versions.
+// Excluded versions will be omitted. If listRetracted is false, retracted
+// versions will also be omitted.
+func addVersions(ctx context.Context, m *modinfo.ModulePublic, listRetracted bool) {
+	allowed := CheckAllowed
+	if listRetracted {
+		allowed = CheckExclusions
+	}
+	v, origin, err := versions(ctx, m.Path, allowed)
+	if err != nil && m.Error == nil {
+		m.Error = &modinfo.ModuleError{Err: err.Error()}
+	}
+	m.Versions = v
+	m.Origin = mergeOrigin(m.Origin, origin)
+}
+
+// addRetraction fills in m.Retracted if the module was retracted by its author.
+// m.Error is set if there's an error loading retraction information.
+func addRetraction(ctx context.Context, m *modinfo.ModulePublic) {
+	if m.Version == "" {
+		return
+	}
+
+	err := CheckRetractions(ctx, module.Version{Path: m.Path, Version: m.Version})
+	var noVersionErr *NoMatchingVersionError
+	var retractErr *ModuleRetractedError
+	if err == nil || errors.Is(err, fs.ErrNotExist) || errors.As(err, &noVersionErr) {
+		// Ignore "not found" and "no matching version" errors.
+		// This means the proxy has no matching version or no versions at all.
+		//
+		// We should report other errors though. An attacker that controls the
+		// network shouldn't be able to hide versions by interfering with
+		// the HTTPS connection. An attacker that controls the proxy may still
+		// hide versions, since the "list" and "latest" endpoints are not
+		// authenticated.
+		return
+	} else if errors.As(err, &retractErr) {
+		if len(retractErr.Rationale) == 0 {
+			m.Retracted = []string{"retracted by module author"}
+		} else {
+			m.Retracted = retractErr.Rationale
+		}
+	} else if m.Error == nil {
+		m.Error = &modinfo.ModuleError{Err: err.Error()}
+	}
+}
+
+// addDeprecation fills in m.Deprecated if the module was deprecated by its
+// author. m.Error is set if there's an error loading deprecation information.
+func addDeprecation(ctx context.Context, m *modinfo.ModulePublic) {
+	deprecation, err := CheckDeprecation(ctx, module.Version{Path: m.Path, Version: m.Version})
+	var noVersionErr *NoMatchingVersionError
+	if errors.Is(err, fs.ErrNotExist) || errors.As(err, &noVersionErr) {
+		// Ignore "not found" and "no matching version" errors.
+		// This means the proxy has no matching version or no versions at all.
+		//
+		// We should report other errors though. An attacker that controls the
+		// network shouldn't be able to hide versions by interfering with
+		// the HTTPS connection. An attacker that controls the proxy may still
+		// hide versions, since the "list" and "latest" endpoints are not
+		// authenticated.
+		return
+	}
+	if err != nil {
+		if m.Error == nil {
+			m.Error = &modinfo.ModuleError{Err: err.Error()}
+		}
+		return
+	}
+	m.Deprecated = deprecation
+}
+
+// moduleInfo returns information about module m, loaded from the requirements
+// in rs (which may be nil to indicate that m was not loaded from a requirement
+// graph).
+func moduleInfo(ctx context.Context, rs *Requirements, m module.Version, mode ListMode, reuse map[module.Version]*modinfo.ModulePublic) *modinfo.ModulePublic {
+	if m.Version == "" && MainModules.Contains(m.Path) {
+		info := &modinfo.ModulePublic{
+			Path:    m.Path,
+			Version: m.Version,
+			Main:    true,
+		}
+		if v, ok := rawGoVersion.Load(m); ok {
+			info.GoVersion = v.(string)
+		} else {
+			panic("internal error: GoVersion not set for main module")
+		}
+		if modRoot := MainModules.ModRoot(m); modRoot != "" {
+			info.Dir = modRoot
+			info.GoMod = modFilePath(modRoot)
+		}
+		return info
+	}
+
+	info := &modinfo.ModulePublic{
+		Path:     m.Path,
+		Version:  m.Version,
+		Indirect: rs != nil && !rs.direct[m.Path],
+	}
+	if v, ok := rawGoVersion.Load(m); ok {
+		info.GoVersion = v.(string)
+	}
+
+	// completeFromModCache fills in the extra fields in m using the module cache.
+	completeFromModCache := func(m *modinfo.ModulePublic) {
+		if gover.IsToolchain(m.Path) {
+			return
+		}
+
+		if old := reuse[module.Version{Path: m.Path, Version: m.Version}]; old != nil {
+			if err := checkReuse(ctx, m.Path, old.Origin); err == nil {
+				*m = *old
+				m.Query = ""
+				m.Dir = ""
+				return
+			}
+		}
+
+		checksumOk := func(suffix string) bool {
+			return rs == nil || m.Version == "" || !mustHaveSums() ||
+				modfetch.HaveSum(module.Version{Path: m.Path, Version: m.Version + suffix})
+		}
+
+		if m.Version != "" {
+			if q, err := Query(ctx, m.Path, m.Version, "", nil); err != nil {
+				m.Error = &modinfo.ModuleError{Err: err.Error()}
+			} else {
+				m.Version = q.Version
+				m.Time = &q.Time
+			}
+		}
+		mod := module.Version{Path: m.Path, Version: m.Version}
+
+		if m.GoVersion == "" && checksumOk("/go.mod") {
+			// Load the go.mod file to determine the Go version, since it hasn't
+			// already been populated from rawGoVersion.
+			if summary, err := rawGoModSummary(mod); err == nil && summary.goVersion != "" {
+				m.GoVersion = summary.goVersion
+			}
+		}
+
+		if m.Version != "" {
+			if checksumOk("/go.mod") {
+				gomod, err := modfetch.CachePath(ctx, mod, "mod")
+				if err == nil {
+					if info, err := os.Stat(gomod); err == nil && info.Mode().IsRegular() {
+						m.GoMod = gomod
+					}
+				}
+			}
+			if checksumOk("") {
+				dir, err := modfetch.DownloadDir(ctx, mod)
+				if err == nil {
+					m.Dir = dir
+				}
+			}
+
+			if mode&ListRetracted != 0 {
+				addRetraction(ctx, m)
+			}
+		}
+	}
+
+	if rs == nil {
+		// If this was an explicitly-versioned argument to 'go mod download' or
+		// 'go list -m', report the actual requested version, not its replacement.
+		completeFromModCache(info) // Will set m.Error in vendor mode.
+		return info
+	}
+
+	r := Replacement(m)
+	if r.Path == "" {
+		if cfg.BuildMod == "vendor" {
+			// It's tempting to fill in the "Dir" field to point within the vendor
+			// directory, but that would be misleading: the vendor directory contains
+			// a flattened package tree, not complete modules, and it can even
+			// interleave packages from different modules if one module path is a
+			// prefix of the other.
+		} else {
+			completeFromModCache(info)
+		}
+		return info
+	}
+
+	// Don't hit the network to fill in extra data for replaced modules.
+	// The original resolved Version and Time don't matter enough to be
+	// worth the cost, and we're going to overwrite the GoMod and Dir from the
+	// replacement anyway. See https://golang.org/issue/27859.
+	info.Replace = &modinfo.ModulePublic{
+		Path:    r.Path,
+		Version: r.Version,
+	}
+	if v, ok := rawGoVersion.Load(m); ok {
+		info.Replace.GoVersion = v.(string)
+	}
+	if r.Version == "" {
+		if filepath.IsAbs(r.Path) {
+			info.Replace.Dir = r.Path
+		} else {
+			info.Replace.Dir = filepath.Join(replaceRelativeTo(), r.Path)
+		}
+		info.Replace.GoMod = filepath.Join(info.Replace.Dir, "go.mod")
+	}
+	if cfg.BuildMod != "vendor" {
+		completeFromModCache(info.Replace)
+		info.Dir = info.Replace.Dir
+		info.GoMod = info.Replace.GoMod
+		info.Retracted = info.Replace.Retracted
+	}
+	info.GoVersion = info.Replace.GoVersion
+	return info
+}
+
+// findModule searches for the module that contains the package at path.
+// If the package was loaded, its containing module and true are returned.
+// Otherwise, module.Version{} and false are returned.
+func findModule(ld *loader, path string) (module.Version, bool) {
+	if pkg, ok := ld.pkgCache.Get(path); ok {
+		return pkg.mod, pkg.mod != module.Version{}
+	}
+	return module.Version{}, false
+}
+
+func ModInfoProg(info string, isgccgo bool) []byte {
+	// Inject an init function to set runtime.modinfo.
+	// This is only used for gccgo - with gc we hand the info directly to the linker.
+	// The init function has the drawback that packages may want to
+	// look at the module info in their init functions (see issue 29628),
+	// which won't work. See also issue 30344.
+	if isgccgo {
+		return fmt.Appendf(nil, `package main
+import _ "unsafe"
+//go:linkname __set_debug_modinfo__ runtime.setmodinfo
+func __set_debug_modinfo__(string)
+func init() { __set_debug_modinfo__(%q) }
+`, ModInfoData(info))
+	}
+	return nil
+}
+
+func ModInfoData(info string) []byte {
+	return []byte(string(infoStart) + info + string(infoEnd))
+}
diff --git a/src/cmd/go/internal/modload/buildlist.go b/src/cmd/go/internal/modload/buildlist.go
new file mode 100644
index 0000000..8d3af08
--- /dev/null
+++ b/src/cmd/go/internal/modload/buildlist.go
@@ -0,0 +1,1497 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"reflect"
+	"runtime"
+	"runtime/debug"
+	"slices"
+	"strings"
+	"sync"
+	"sync/atomic"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/mvs"
+	"cmd/go/internal/par"
+
+	"golang.org/x/mod/module"
+)
+
+// A Requirements represents a logically-immutable set of root module requirements.
+type Requirements struct {
+	// pruning is the pruning at which the requirement graph is computed.
+	//
+	// If unpruned, the graph includes all transitive requirements regardless
+	// of whether the requiring module supports pruning.
+	//
+	// If pruned, the graph includes only the root modules, the explicit
+	// requirements of those root modules, and the transitive requirements of only
+	// the root modules that do not support pruning.
+	//
+	// If workspace, the graph includes only the workspace modules, the explicit
+	// requirements of the workspace modules, and the transitive requirements of
+	// the workspace modules that do not support pruning.
+	pruning modPruning
+
+	// rootModules is the set of root modules of the graph, sorted and capped to
+	// length. It may contain duplicates, and may contain multiple versions for a
+	// given module path. The root modules of the graph are the set of main
+	// modules in workspace mode, and the main module's direct requirements
+	// outside workspace mode.
+	//
+	// The roots are always expected to contain an entry for the "go" module,
+	// indicating the Go language version in use.
+	rootModules    []module.Version
+	maxRootVersion map[string]string
+
+	// direct is the set of module paths for which we believe the module provides
+	// a package directly imported by a package or test in the main module.
+	//
+	// The "direct" map controls which modules are annotated with "// indirect"
+	// comments in the go.mod file, and may impact which modules are listed as
+	// explicit roots (vs. indirect-only dependencies). However, it should not
+	// have a semantic effect on the build list overall.
+	//
+	// The initial direct map is populated from the existing "// indirect"
+	// comments (or lack thereof) in the go.mod file. It is updated by the
+	// package loader: dependencies may be promoted to direct if new
+	// direct imports are observed, and may be demoted to indirect during
+	// 'go mod tidy' or 'go mod vendor'.
+	//
+	// The direct map is keyed by module paths, not module versions. When a
+	// module's selected version changes, we assume that it remains direct if the
+	// previous version was a direct dependency. That assumption might not hold in
+	// rare cases (such as if a dependency splits out a nested module, or merges a
+	// nested module back into a parent module).
+	direct map[string]bool
+
+	graphOnce sync.Once // guards writes to (but not reads from) graph
+	graph     atomic.Pointer[cachedGraph]
+}
+
+// A cachedGraph is a non-nil *ModuleGraph, together with any error discovered
+// while loading that graph.
+type cachedGraph struct {
+	mg  *ModuleGraph
+	err error // If err is non-nil, mg may be incomplete (but must still be non-nil).
+}
+
+// requirements is the requirement graph for the main module.
+//
+// It is always non-nil if the main module's go.mod file has been loaded.
+//
+// This variable should only be read from the loadModFile function, and should
+// only be written in the loadModFile and commitRequirements functions.
+// All other functions that need or produce a *Requirements should
+// accept and/or return an explicit parameter.
+var requirements *Requirements
+
+func mustHaveGoRoot(roots []module.Version) {
+	for _, m := range roots {
+		if m.Path == "go" {
+			return
+		}
+	}
+	panic("go: internal error: missing go root module")
+}
+
+// newRequirements returns a new requirement set with the given root modules.
+// The dependencies of the roots will be loaded lazily at the first call to the
+// Graph method.
+//
+// The rootModules slice must be sorted according to gover.ModSort.
+// The caller must not modify the rootModules slice or direct map after passing
+// them to newRequirements.
+//
+// If vendoring is in effect, the caller must invoke initVendor on the returned
+// *Requirements before any other method.
+func newRequirements(pruning modPruning, rootModules []module.Version, direct map[string]bool) *Requirements {
+	mustHaveGoRoot(rootModules)
+
+	if pruning != workspace {
+		if workFilePath != "" {
+			panic("in workspace mode, but pruning is not workspace in newRequirements")
+		}
+	}
+
+	if pruning != workspace {
+		if workFilePath != "" {
+			panic("in workspace mode, but pruning is not workspace in newRequirements")
+		}
+		for i, m := range rootModules {
+			if m.Version == "" && MainModules.Contains(m.Path) {
+				panic(fmt.Sprintf("newRequirements called with untrimmed build list: rootModules[%v] is a main module", i))
+			}
+			if m.Path == "" || m.Version == "" {
+				panic(fmt.Sprintf("bad requirement: rootModules[%v] = %v", i, m))
+			}
+		}
+	}
+
+	rs := &Requirements{
+		pruning:        pruning,
+		rootModules:    rootModules,
+		maxRootVersion: make(map[string]string, len(rootModules)),
+		direct:         direct,
+	}
+
+	for i, m := range rootModules {
+		if i > 0 {
+			prev := rootModules[i-1]
+			if prev.Path > m.Path || (prev.Path == m.Path && gover.ModCompare(m.Path, prev.Version, m.Version) > 0) {
+				panic(fmt.Sprintf("newRequirements called with unsorted roots: %v", rootModules))
+			}
+		}
+
+		if v, ok := rs.maxRootVersion[m.Path]; ok && gover.ModCompare(m.Path, v, m.Version) >= 0 {
+			continue
+		}
+		rs.maxRootVersion[m.Path] = m.Version
+	}
+
+	if rs.maxRootVersion["go"] == "" {
+		panic(`newRequirements called without a "go" version`)
+	}
+	return rs
+}
+
+// String returns a string describing the Requirements for debugging.
+func (rs *Requirements) String() string {
+	return fmt.Sprintf("{%v %v}", rs.pruning, rs.rootModules)
+}
+
+// initVendor initializes rs.graph from the given list of vendored module
+// dependencies, overriding the graph that would normally be loaded from module
+// requirements.
+func (rs *Requirements) initVendor(vendorList []module.Version) {
+	rs.graphOnce.Do(func() {
+		mg := &ModuleGraph{
+			g: mvs.NewGraph(cmpVersion, MainModules.Versions()),
+		}
+
+		if MainModules.Len() != 1 {
+			panic("There should be exactly one main module in Vendor mode.")
+		}
+		mainModule := MainModules.Versions()[0]
+
+		if rs.pruning == pruned {
+			// The roots of a pruned module should already include every module in the
+			// vendor list, because the vendored modules are the same as those needed
+			// for graph pruning.
+			//
+			// Just to be sure, we'll double-check that here.
+			inconsistent := false
+			for _, m := range vendorList {
+				if v, ok := rs.rootSelected(m.Path); !ok || v != m.Version {
+					base.Errorf("go: vendored module %v should be required explicitly in go.mod", m)
+					inconsistent = true
+				}
+			}
+			if inconsistent {
+				base.Fatal(errGoModDirty)
+			}
+
+			// Now we can treat the rest of the module graph as effectively “pruned
+			// out”, as though we are viewing the main module from outside: in vendor
+			// mode, the root requirements *are* the complete module graph.
+			mg.g.Require(mainModule, rs.rootModules)
+		} else {
+			// The transitive requirements of the main module are not in general available
+			// from the vendor directory, and we don't actually know how we got from
+			// the roots to the final build list.
+			//
+			// Instead, we'll inject a fake "vendor/modules.txt" module that provides
+			// those transitive dependencies, and mark it as a dependency of the main
+			// module. That allows us to elide the actual structure of the module
+			// graph, but still distinguishes between direct and indirect
+			// dependencies.
+			vendorMod := module.Version{Path: "vendor/modules.txt", Version: ""}
+			mg.g.Require(mainModule, append(rs.rootModules, vendorMod))
+			mg.g.Require(vendorMod, vendorList)
+		}
+
+		rs.graph.Store(&cachedGraph{mg, nil})
+	})
+}
+
+// GoVersion returns the Go language version for the Requirements.
+func (rs *Requirements) GoVersion() string {
+	v, _ := rs.rootSelected("go")
+	if v == "" {
+		panic("internal error: missing go version in modload.Requirements")
+	}
+	return v
+}
+
+// rootSelected returns the version of the root dependency with the given module
+// path, or the zero module.Version and ok=false if the module is not a root
+// dependency.
+func (rs *Requirements) rootSelected(path string) (version string, ok bool) {
+	if MainModules.Contains(path) {
+		return "", true
+	}
+	if v, ok := rs.maxRootVersion[path]; ok {
+		return v, true
+	}
+	return "", false
+}
+
+// hasRedundantRoot returns true if the root list contains multiple requirements
+// of the same module or a requirement on any version of the main module.
+// Redundant requirements should be pruned, but they may influence version
+// selection.
+func (rs *Requirements) hasRedundantRoot() bool {
+	for i, m := range rs.rootModules {
+		if MainModules.Contains(m.Path) || (i > 0 && m.Path == rs.rootModules[i-1].Path) {
+			return true
+		}
+	}
+	return false
+}
+
+// Graph returns the graph of module requirements loaded from the current
+// root modules (as reported by RootModules).
+//
+// Graph always makes a best effort to load the requirement graph despite any
+// errors, and always returns a non-nil *ModuleGraph.
+//
+// If the requirements of any relevant module fail to load, Graph also
+// returns a non-nil error of type *mvs.BuildListError.
+func (rs *Requirements) Graph(ctx context.Context) (*ModuleGraph, error) {
+	rs.graphOnce.Do(func() {
+		mg, mgErr := readModGraph(ctx, rs.pruning, rs.rootModules, nil)
+		rs.graph.Store(&cachedGraph{mg, mgErr})
+	})
+	cached := rs.graph.Load()
+	return cached.mg, cached.err
+}
+
+// IsDirect returns whether the given module provides a package directly
+// imported by a package or test in the main module.
+func (rs *Requirements) IsDirect(path string) bool {
+	return rs.direct[path]
+}
+
+// A ModuleGraph represents the complete graph of module dependencies
+// of a main module.
+//
+// If the main module supports module graph pruning, the graph does not include
+// transitive dependencies of non-root (implicit) dependencies.
+type ModuleGraph struct {
+	g         *mvs.Graph
+	loadCache par.ErrCache[module.Version, *modFileSummary]
+
+	buildListOnce sync.Once
+	buildList     []module.Version
+}
+
+var readModGraphDebugOnce sync.Once
+
+// readModGraph reads and returns the module dependency graph starting at the
+// given roots.
+//
+// The requirements of the module versions found in the unprune map are included
+// in the graph even if they would normally be pruned out.
+//
+// Unlike LoadModGraph, readModGraph does not attempt to diagnose or update
+// inconsistent roots.
+func readModGraph(ctx context.Context, pruning modPruning, roots []module.Version, unprune map[module.Version]bool) (*ModuleGraph, error) {
+	mustHaveGoRoot(roots)
+	if pruning == pruned {
+		// Enable diagnostics for lazy module loading
+		// (https://golang.org/ref/mod#lazy-loading) only if the module graph is
+		// pruned.
+		//
+		// In unpruned modules,we load the module graph much more aggressively (in
+		// order to detect inconsistencies that wouldn't be feasible to spot-check),
+		// so it wouldn't be useful to log when that occurs (because it happens in
+		// normal operation all the time).
+		readModGraphDebugOnce.Do(func() {
+			for _, f := range strings.Split(os.Getenv("GODEBUG"), ",") {
+				switch f {
+				case "lazymod=log":
+					debug.PrintStack()
+					fmt.Fprintf(os.Stderr, "go: read full module graph.\n")
+				case "lazymod=strict":
+					debug.PrintStack()
+					base.Fatalf("go: read full module graph (forbidden by GODEBUG=lazymod=strict).")
+				}
+			}
+		})
+	}
+
+	var graphRoots []module.Version
+	if inWorkspaceMode() {
+		graphRoots = roots
+	} else {
+		graphRoots = MainModules.Versions()
+	}
+	var (
+		mu       sync.Mutex // guards mg.g and hasError during loading
+		hasError bool
+		mg       = &ModuleGraph{
+			g: mvs.NewGraph(cmpVersion, graphRoots),
+		}
+	)
+
+	if pruning != workspace {
+		if inWorkspaceMode() {
+			panic("pruning is not workspace in workspace mode")
+		}
+		mg.g.Require(MainModules.mustGetSingleMainModule(), roots)
+	}
+
+	type dedupKey struct {
+		m       module.Version
+		pruning modPruning
+	}
+	var (
+		loadQueue = par.NewQueue(runtime.GOMAXPROCS(0))
+		loading   sync.Map // dedupKey → nil; the set of modules that have been or are being loaded
+	)
+
+	// loadOne synchronously loads the explicit requirements for module m.
+	// It does not load the transitive requirements of m even if the go version in
+	// m's go.mod file indicates that it supports graph pruning.
+	loadOne := func(m module.Version) (*modFileSummary, error) {
+		return mg.loadCache.Do(m, func() (*modFileSummary, error) {
+			summary, err := goModSummary(m)
+
+			mu.Lock()
+			if err == nil {
+				mg.g.Require(m, summary.require)
+			} else {
+				hasError = true
+			}
+			mu.Unlock()
+
+			return summary, err
+		})
+	}
+
+	var enqueue func(m module.Version, pruning modPruning)
+	enqueue = func(m module.Version, pruning modPruning) {
+		if m.Version == "none" {
+			return
+		}
+
+		if _, dup := loading.LoadOrStore(dedupKey{m, pruning}, nil); dup {
+			// m has already been enqueued for loading. Since unpruned loading may
+			// follow cycles in the requirement graph, we need to return early
+			// to avoid making the load queue infinitely long.
+			return
+		}
+
+		loadQueue.Add(func() {
+			summary, err := loadOne(m)
+			if err != nil {
+				return // findError will report the error later.
+			}
+
+			// If the version in m's go.mod file does not support pruning, then we
+			// cannot assume that the explicit requirements of m (added by loadOne)
+			// are sufficient to build the packages it contains. We must load its full
+			// transitive dependency graph to be sure that we see all relevant
+			// dependencies. In addition, we must load the requirements of any module
+			// that is explicitly marked as unpruned.
+			nextPruning := summary.pruning
+			if pruning == unpruned {
+				nextPruning = unpruned
+			}
+			for _, r := range summary.require {
+				if pruning != pruned || summary.pruning == unpruned || unprune[r] {
+					enqueue(r, nextPruning)
+				}
+			}
+		})
+	}
+
+	mustHaveGoRoot(roots)
+	for _, m := range roots {
+		enqueue(m, pruning)
+	}
+	<-loadQueue.Idle()
+
+	// Reload any dependencies of the main modules which are not
+	// at their selected versions at workspace mode, because the
+	// requirements don't accurately reflect the transitive imports.
+	if pruning == workspace {
+		// hasDepsInAll contains the set of modules that need to be loaded
+		// at workspace pruning because any of their dependencies may
+		// provide packages in all.
+		hasDepsInAll := make(map[string]bool)
+		seen := map[module.Version]bool{}
+		for _, m := range roots {
+			hasDepsInAll[m.Path] = true
+		}
+		// This loop will terminate because it will call enqueue on each version of
+		// each dependency of the modules in hasDepsInAll at most once (and only
+		// calls enqueue on successively increasing versions of each dependency).
+		for {
+			needsEnqueueing := map[module.Version]bool{}
+			for p := range hasDepsInAll {
+				m := module.Version{Path: p, Version: mg.g.Selected(p)}
+				if !seen[m] {
+					needsEnqueueing[m] = true
+					continue
+				}
+				reqs, _ := mg.g.RequiredBy(m)
+				for _, r := range reqs {
+					s := module.Version{Path: r.Path, Version: mg.g.Selected(r.Path)}
+					if gover.ModCompare(r.Path, s.Version, r.Version) > 0 && !seen[s] {
+						needsEnqueueing[s] = true
+					}
+				}
+			}
+			// add all needs enqueueing to paths we care about
+			if len(needsEnqueueing) == 0 {
+				break
+			}
+
+			for p := range needsEnqueueing {
+				enqueue(p, workspace)
+				seen[p] = true
+				hasDepsInAll[p.Path] = true
+			}
+			<-loadQueue.Idle()
+		}
+	}
+
+	if hasError {
+		return mg, mg.findError()
+	}
+	return mg, nil
+}
+
+// RequiredBy returns the dependencies required by module m in the graph,
+// or ok=false if module m's dependencies are pruned out.
+//
+// The caller must not modify the returned slice, but may safely append to it
+// and may rely on it not to be modified.
+func (mg *ModuleGraph) RequiredBy(m module.Version) (reqs []module.Version, ok bool) {
+	return mg.g.RequiredBy(m)
+}
+
+// Selected returns the selected version of the module with the given path.
+//
+// If no version is selected, Selected returns version "none".
+func (mg *ModuleGraph) Selected(path string) (version string) {
+	return mg.g.Selected(path)
+}
+
+// WalkBreadthFirst invokes f once, in breadth-first order, for each module
+// version other than "none" that appears in the graph, regardless of whether
+// that version is selected.
+func (mg *ModuleGraph) WalkBreadthFirst(f func(m module.Version)) {
+	mg.g.WalkBreadthFirst(f)
+}
+
+// BuildList returns the selected versions of all modules present in the graph,
+// beginning with the main modules.
+//
+// The order of the remaining elements in the list is deterministic
+// but arbitrary.
+//
+// The caller must not modify the returned list, but may safely append to it
+// and may rely on it not to be modified.
+func (mg *ModuleGraph) BuildList() []module.Version {
+	mg.buildListOnce.Do(func() {
+		mg.buildList = slices.Clip(mg.g.BuildList())
+	})
+	return mg.buildList
+}
+
+func (mg *ModuleGraph) findError() error {
+	errStack := mg.g.FindPath(func(m module.Version) bool {
+		_, err := mg.loadCache.Get(m)
+		return err != nil && err != par.ErrCacheEntryNotFound
+	})
+	if len(errStack) > 0 {
+		_, err := mg.loadCache.Get(errStack[len(errStack)-1])
+		var noUpgrade func(from, to module.Version) bool
+		return mvs.NewBuildListError(err, errStack, noUpgrade)
+	}
+
+	return nil
+}
+
+func (mg *ModuleGraph) allRootsSelected() bool {
+	var roots []module.Version
+	if inWorkspaceMode() {
+		roots = MainModules.Versions()
+	} else {
+		roots, _ = mg.g.RequiredBy(MainModules.mustGetSingleMainModule())
+	}
+	for _, m := range roots {
+		if mg.Selected(m.Path) != m.Version {
+			return false
+		}
+	}
+	return true
+}
+
+// LoadModGraph loads and returns the graph of module dependencies of the main module,
+// without loading any packages.
+//
+// If the goVersion string is non-empty, the returned graph is the graph
+// as interpreted by the given Go version (instead of the version indicated
+// in the go.mod file).
+//
+// Modules are loaded automatically (and lazily) in LoadPackages:
+// LoadModGraph need only be called if LoadPackages is not,
+// typically in commands that care about modules but no particular package.
+func LoadModGraph(ctx context.Context, goVersion string) (*ModuleGraph, error) {
+	rs, err := loadModFile(ctx, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	if goVersion != "" {
+		v, _ := rs.rootSelected("go")
+		if gover.Compare(v, gover.GoStrictVersion) >= 0 && gover.Compare(goVersion, v) < 0 {
+			return nil, fmt.Errorf("requested Go version %s cannot load module graph (requires Go >= %s)", goVersion, v)
+		}
+
+		pruning := pruningForGoVersion(goVersion)
+		if pruning == unpruned && rs.pruning != unpruned {
+			// Use newRequirements instead of convertDepth because convertDepth
+			// also updates roots; here, we want to report the unmodified roots
+			// even though they may seem inconsistent.
+			rs = newRequirements(unpruned, rs.rootModules, rs.direct)
+		}
+
+		return rs.Graph(ctx)
+	}
+
+	rs, mg, err := expandGraph(ctx, rs)
+	if err != nil {
+		return nil, err
+	}
+	requirements = rs
+	return mg, err
+}
+
+// expandGraph loads the complete module graph from rs.
+//
+// If the complete graph reveals that some root of rs is not actually the
+// selected version of its path, expandGraph computes a new set of roots that
+// are consistent. (With a pruned module graph, this may result in upgrades to
+// other modules due to requirements that were previously pruned out.)
+//
+// expandGraph returns the updated roots, along with the module graph loaded
+// from those roots and any error encountered while loading that graph.
+// expandGraph returns non-nil requirements and a non-nil graph regardless of
+// errors. On error, the roots might not be updated to be consistent.
+func expandGraph(ctx context.Context, rs *Requirements) (*Requirements, *ModuleGraph, error) {
+	mg, mgErr := rs.Graph(ctx)
+	if mgErr != nil {
+		// Without the graph, we can't update the roots: we don't know which
+		// versions of transitive dependencies would be selected.
+		return rs, mg, mgErr
+	}
+
+	if !mg.allRootsSelected() {
+		// The roots of rs are not consistent with the rest of the graph. Update
+		// them. In an unpruned module this is a no-op for the build list as a whole —
+		// it just promotes what were previously transitive requirements to be
+		// roots — but in a pruned module it may pull in previously-irrelevant
+		// transitive dependencies.
+
+		newRS, rsErr := updateRoots(ctx, rs.direct, rs, nil, nil, false)
+		if rsErr != nil {
+			// Failed to update roots, perhaps because of an error in a transitive
+			// dependency needed for the update. Return the original Requirements
+			// instead.
+			return rs, mg, rsErr
+		}
+		rs = newRS
+		mg, mgErr = rs.Graph(ctx)
+	}
+
+	return rs, mg, mgErr
+}
+
+// EditBuildList edits the global build list by first adding every module in add
+// to the existing build list, then adjusting versions (and adding or removing
+// requirements as needed) until every module in mustSelect is selected at the
+// given version.
+//
+// (Note that the newly-added modules might not be selected in the resulting
+// build list: they could be lower than existing requirements or conflict with
+// versions in mustSelect.)
+//
+// If the versions listed in mustSelect are mutually incompatible (due to one of
+// the listed modules requiring a higher version of another), EditBuildList
+// returns a *ConstraintError and leaves the build list in its previous state.
+//
+// On success, EditBuildList reports whether the selected version of any module
+// in the build list may have been changed (possibly to or from "none") as a
+// result.
+func EditBuildList(ctx context.Context, add, mustSelect []module.Version) (changed bool, err error) {
+	rs, changed, err := editRequirements(ctx, LoadModFile(ctx), add, mustSelect)
+	if err != nil {
+		return false, err
+	}
+	requirements = rs
+	return changed, err
+}
+
+// OverrideRoots edits the global requirement roots by replacing the specific module versions.
+func OverrideRoots(ctx context.Context, replace []module.Version) {
+	requirements = overrideRoots(ctx, requirements, replace)
+}
+
+func overrideRoots(ctx context.Context, rs *Requirements, replace []module.Version) *Requirements {
+	drop := make(map[string]bool)
+	for _, m := range replace {
+		drop[m.Path] = true
+	}
+	var roots []module.Version
+	for _, m := range rs.rootModules {
+		if !drop[m.Path] {
+			roots = append(roots, m)
+		}
+	}
+	roots = append(roots, replace...)
+	gover.ModSort(roots)
+	return newRequirements(rs.pruning, roots, rs.direct)
+}
+
+// A ConstraintError describes inconsistent constraints in EditBuildList
+type ConstraintError struct {
+	// Conflict lists the source of the conflict for each version in mustSelect
+	// that could not be selected due to the requirements of some other version in
+	// mustSelect.
+	Conflicts []Conflict
+}
+
+func (e *ConstraintError) Error() string {
+	b := new(strings.Builder)
+	b.WriteString("version constraints conflict:")
+	for _, c := range e.Conflicts {
+		fmt.Fprintf(b, "\n\t%s", c.Summary())
+	}
+	return b.String()
+}
+
+// A Conflict is a path of requirements starting at a root or proposed root in
+// the requirement graph, explaining why that root either causes a module passed
+// in the mustSelect list to EditBuildList to be unattainable, or introduces an
+// unresolvable error in loading the requirement graph.
+type Conflict struct {
+	// Path is a path of requirements starting at some module version passed in
+	// the mustSelect argument and ending at a module whose requirements make that
+	// version unacceptable. (Path always has len ≥ 1.)
+	Path []module.Version
+
+	// If Err is nil, Constraint is a module version passed in the mustSelect
+	// argument that has the same module path as, and a lower version than,
+	// the last element of the Path slice.
+	Constraint module.Version
+
+	// If Constraint is unset, Err is an error encountered when loading the
+	// requirements of the last element in Path.
+	Err error
+}
+
+// UnwrapModuleError returns c.Err, but unwraps it if it is a module.ModuleError
+// with a version and path matching the last entry in the Path slice.
+func (c Conflict) UnwrapModuleError() error {
+	me, ok := c.Err.(*module.ModuleError)
+	if ok && len(c.Path) > 0 {
+		last := c.Path[len(c.Path)-1]
+		if me.Path == last.Path && me.Version == last.Version {
+			return me.Err
+		}
+	}
+	return c.Err
+}
+
+// Summary returns a string that describes only the first and last modules in
+// the conflict path.
+func (c Conflict) Summary() string {
+	if len(c.Path) == 0 {
+		return "(internal error: invalid Conflict struct)"
+	}
+	first := c.Path[0]
+	last := c.Path[len(c.Path)-1]
+	if len(c.Path) == 1 {
+		if c.Err != nil {
+			return fmt.Sprintf("%s: %v", first, c.UnwrapModuleError())
+		}
+		return fmt.Sprintf("%s is above %s", first, c.Constraint.Version)
+	}
+
+	adverb := ""
+	if len(c.Path) > 2 {
+		adverb = "indirectly "
+	}
+	if c.Err != nil {
+		return fmt.Sprintf("%s %srequires %s: %v", first, adverb, last, c.UnwrapModuleError())
+	}
+	return fmt.Sprintf("%s %srequires %s, but %s is requested", first, adverb, last, c.Constraint.Version)
+}
+
+// String returns a string that describes the full conflict path.
+func (c Conflict) String() string {
+	if len(c.Path) == 0 {
+		return "(internal error: invalid Conflict struct)"
+	}
+	b := new(strings.Builder)
+	fmt.Fprintf(b, "%v", c.Path[0])
+	if len(c.Path) == 1 {
+		fmt.Fprintf(b, " found")
+	} else {
+		for _, r := range c.Path[1:] {
+			fmt.Fprintf(b, " requires\n\t%v", r)
+		}
+	}
+	if c.Constraint != (module.Version{}) {
+		fmt.Fprintf(b, ", but %v is requested", c.Constraint.Version)
+	}
+	if c.Err != nil {
+		fmt.Fprintf(b, ": %v", c.UnwrapModuleError())
+	}
+	return b.String()
+}
+
+// tidyRoots trims the root dependencies to the minimal requirements needed to
+// both retain the same versions of all packages in pkgs and satisfy the
+// graph-pruning invariants (if applicable).
+func tidyRoots(ctx context.Context, rs *Requirements, pkgs []*loadPkg) (*Requirements, error) {
+	mainModule := MainModules.mustGetSingleMainModule()
+	if rs.pruning == unpruned {
+		return tidyUnprunedRoots(ctx, mainModule, rs, pkgs)
+	}
+	return tidyPrunedRoots(ctx, mainModule, rs, pkgs)
+}
+
+func updateRoots(ctx context.Context, direct map[string]bool, rs *Requirements, pkgs []*loadPkg, add []module.Version, rootsImported bool) (*Requirements, error) {
+	switch rs.pruning {
+	case unpruned:
+		return updateUnprunedRoots(ctx, direct, rs, add)
+	case pruned:
+		return updatePrunedRoots(ctx, direct, rs, pkgs, add, rootsImported)
+	case workspace:
+		return updateWorkspaceRoots(ctx, rs, add)
+	default:
+		panic(fmt.Sprintf("unsupported pruning mode: %v", rs.pruning))
+	}
+}
+
+func updateWorkspaceRoots(ctx context.Context, rs *Requirements, add []module.Version) (*Requirements, error) {
+	if len(add) != 0 {
+		// add should be empty in workspace mode because workspace mode implies
+		// -mod=readonly, which in turn implies no new requirements. The code path
+		// that would result in add being non-empty returns an error before it
+		// reaches this point: The set of modules to add comes from
+		// resolveMissingImports, which in turn resolves each package by calling
+		// queryImport. But queryImport explicitly checks for -mod=readonly, and
+		// return an error.
+		panic("add is not empty")
+	}
+	return rs, nil
+}
+
+// tidyPrunedRoots returns a minimal set of root requirements that maintains the
+// invariants of the go.mod file needed to support graph pruning for the given
+// packages:
+//
+//  1. For each package marked with pkgInAll, the module path that provided that
+//     package is included as a root.
+//  2. For all packages, the module that provided that package either remains
+//     selected at the same version or is upgraded by the dependencies of a
+//     root.
+//
+// If any module that provided a package has been upgraded above its previous
+// version, the caller may need to reload and recompute the package graph.
+//
+// To ensure that the loading process eventually converges, the caller should
+// add any needed roots from the tidy root set (without removing existing untidy
+// roots) until the set of roots has converged.
+func tidyPrunedRoots(ctx context.Context, mainModule module.Version, old *Requirements, pkgs []*loadPkg) (*Requirements, error) {
+	var (
+		roots      []module.Version
+		pathIsRoot = map[string]bool{mainModule.Path: true}
+	)
+	if v, ok := old.rootSelected("go"); ok {
+		roots = append(roots, module.Version{Path: "go", Version: v})
+		pathIsRoot["go"] = true
+	}
+	if v, ok := old.rootSelected("toolchain"); ok {
+		roots = append(roots, module.Version{Path: "toolchain", Version: v})
+		pathIsRoot["toolchain"] = true
+	}
+	// We start by adding roots for every package in "all".
+	//
+	// Once that is done, we may still need to add more roots to cover upgraded or
+	// otherwise-missing test dependencies for packages in "all". For those test
+	// dependencies, we prefer to add roots for packages with shorter import
+	// stacks first, on the theory that the module requirements for those will
+	// tend to fill in the requirements for their transitive imports (which have
+	// deeper import stacks). So we add the missing dependencies for one depth at
+	// a time, starting with the packages actually in "all" and expanding outwards
+	// until we have scanned every package that was loaded.
+	var (
+		queue  []*loadPkg
+		queued = map[*loadPkg]bool{}
+	)
+	for _, pkg := range pkgs {
+		if !pkg.flags.has(pkgInAll) {
+			continue
+		}
+		if pkg.fromExternalModule() && !pathIsRoot[pkg.mod.Path] {
+			roots = append(roots, pkg.mod)
+			pathIsRoot[pkg.mod.Path] = true
+		}
+		queue = append(queue, pkg)
+		queued[pkg] = true
+	}
+	gover.ModSort(roots)
+	tidy := newRequirements(pruned, roots, old.direct)
+
+	for len(queue) > 0 {
+		roots = tidy.rootModules
+		mg, err := tidy.Graph(ctx)
+		if err != nil {
+			return nil, err
+		}
+
+		prevQueue := queue
+		queue = nil
+		for _, pkg := range prevQueue {
+			m := pkg.mod
+			if m.Path == "" {
+				continue
+			}
+			for _, dep := range pkg.imports {
+				if !queued[dep] {
+					queue = append(queue, dep)
+					queued[dep] = true
+				}
+			}
+			if pkg.test != nil && !queued[pkg.test] {
+				queue = append(queue, pkg.test)
+				queued[pkg.test] = true
+			}
+
+			if !pathIsRoot[m.Path] {
+				if s := mg.Selected(m.Path); gover.ModCompare(m.Path, s, m.Version) < 0 {
+					roots = append(roots, m)
+					pathIsRoot[m.Path] = true
+				}
+			}
+		}
+
+		if len(roots) > len(tidy.rootModules) {
+			gover.ModSort(roots)
+			tidy = newRequirements(pruned, roots, tidy.direct)
+		}
+	}
+
+	roots = tidy.rootModules
+	_, err := tidy.Graph(ctx)
+	if err != nil {
+		return nil, err
+	}
+
+	// We try to avoid adding explicit requirements for test-only dependencies of
+	// packages in external modules. However, if we drop the explicit
+	// requirements, that may change an import from unambiguous (due to lazy
+	// module loading) to ambiguous (because lazy module loading no longer
+	// disambiguates it). For any package that has become ambiguous, we try
+	// to fix it by promoting its module to an explicit root.
+	// (See https://go.dev/issue/60313.)
+	q := par.NewQueue(runtime.GOMAXPROCS(0))
+	for {
+		var disambiguateRoot sync.Map
+		for _, pkg := range pkgs {
+			if pkg.mod.Path == "" || pathIsRoot[pkg.mod.Path] {
+				// Lazy module loading will cause pkg.mod to be checked before any other modules
+				// that are only indirectly required. It is as unambiguous as possible.
+				continue
+			}
+			pkg := pkg
+			q.Add(func() {
+				skipModFile := true
+				_, _, _, _, err := importFromModules(ctx, pkg.path, tidy, nil, skipModFile)
+				if aie := (*AmbiguousImportError)(nil); errors.As(err, &aie) {
+					disambiguateRoot.Store(pkg.mod, true)
+				}
+			})
+		}
+		<-q.Idle()
+
+		disambiguateRoot.Range(func(k, _ any) bool {
+			m := k.(module.Version)
+			roots = append(roots, m)
+			pathIsRoot[m.Path] = true
+			return true
+		})
+
+		if len(roots) > len(tidy.rootModules) {
+			module.Sort(roots)
+			tidy = newRequirements(pruned, roots, tidy.direct)
+			_, err = tidy.Graph(ctx)
+			if err != nil {
+				return nil, err
+			}
+			// Adding these roots may have pulled additional modules into the module
+			// graph, causing additional packages to become ambiguous. Keep iterating
+			// until we reach a fixed point.
+			continue
+		}
+
+		break
+	}
+
+	return tidy, nil
+}
+
+// updatePrunedRoots returns a set of root requirements that maintains the
+// invariants of the go.mod file needed to support graph pruning:
+//
+//  1. The selected version of the module providing each package marked with
+//     either pkgInAll or pkgIsRoot is included as a root.
+//     Note that certain root patterns (such as '...') may explode the root set
+//     to contain every module that provides any package imported (or merely
+//     required) by any other module.
+//  2. Each root appears only once, at the selected version of its path
+//     (if rs.graph is non-nil) or at the highest version otherwise present as a
+//     root (otherwise).
+//  3. Every module path that appears as a root in rs remains a root.
+//  4. Every version in add is selected at its given version unless upgraded by
+//     (the dependencies of) an existing root or another module in add.
+//
+// The packages in pkgs are assumed to have been loaded from either the roots of
+// rs or the modules selected in the graph of rs.
+//
+// The above invariants together imply the graph-pruning invariants for the
+// go.mod file:
+//
+//  1. (The import invariant.) Every module that provides a package transitively
+//     imported by any package or test in the main module is included as a root.
+//     This follows by induction from (1) and (3) above. Transitively-imported
+//     packages loaded during this invocation are marked with pkgInAll (1),
+//     and by hypothesis any transitively-imported packages loaded in previous
+//     invocations were already roots in rs (3).
+//
+//  2. (The argument invariant.) Every module that provides a package matching
+//     an explicit package pattern is included as a root. This follows directly
+//     from (1): packages matching explicit package patterns are marked with
+//     pkgIsRoot.
+//
+//  3. (The completeness invariant.) Every module that contributed any package
+//     to the build is required by either the main module or one of the modules
+//     it requires explicitly. This invariant is left up to the caller, who must
+//     not load packages from outside the module graph but may add roots to the
+//     graph, but is facilitated by (3). If the caller adds roots to the graph in
+//     order to resolve missing packages, then updatePrunedRoots will retain them,
+//     the selected versions of those roots cannot regress, and they will
+//     eventually be written back to the main module's go.mod file.
+//
+// (See https://golang.org/design/36460-lazy-module-loading#invariants for more
+// detail.)
+func updatePrunedRoots(ctx context.Context, direct map[string]bool, rs *Requirements, pkgs []*loadPkg, add []module.Version, rootsImported bool) (*Requirements, error) {
+	roots := rs.rootModules
+	rootsUpgraded := false
+
+	spotCheckRoot := map[module.Version]bool{}
+
+	// “The selected version of the module providing each package marked with
+	// either pkgInAll or pkgIsRoot is included as a root.”
+	needSort := false
+	for _, pkg := range pkgs {
+		if !pkg.fromExternalModule() {
+			// pkg was not loaded from a module dependency, so we don't need
+			// to do anything special to maintain that dependency.
+			continue
+		}
+
+		switch {
+		case pkg.flags.has(pkgInAll):
+			// pkg is transitively imported by a package or test in the main module.
+			// We need to promote the module that maintains it to a root: if some
+			// other module depends on the main module, and that other module also
+			// uses a pruned module graph, it will expect to find all of our
+			// transitive dependencies by reading just our go.mod file, not the go.mod
+			// files of everything we depend on.
+			//
+			// (This is the “import invariant” that makes graph pruning possible.)
+
+		case rootsImported && pkg.flags.has(pkgFromRoot):
+			// pkg is a transitive dependency of some root, and we are treating the
+			// roots as if they are imported by the main module (as in 'go get').
+
+		case pkg.flags.has(pkgIsRoot):
+			// pkg is a root of the package-import graph. (Generally this means that
+			// it matches a command-line argument.) We want future invocations of the
+			// 'go' command — such as 'go test' on the same package — to continue to
+			// use the same versions of its dependencies that we are using right now.
+			// So we need to bring this package's dependencies inside the pruned
+			// module graph.
+			//
+			// Making the module containing this package a root of the module graph
+			// does exactly that: if the module containing the package supports graph
+			// pruning then it should satisfy the import invariant itself, so all of
+			// its dependencies should be in its go.mod file, and if the module
+			// containing the package does not support pruning then if we make it a
+			// root we will load all of its (unpruned) transitive dependencies into
+			// the module graph.
+			//
+			// (This is the “argument invariant”, and is important for
+			// reproducibility.)
+
+		default:
+			// pkg is a dependency of some other package outside of the main module.
+			// As far as we know it's not relevant to the main module (and thus not
+			// relevant to consumers of the main module either), and its dependencies
+			// should already be in the module graph — included in the dependencies of
+			// the package that imported it.
+			continue
+		}
+
+		if _, ok := rs.rootSelected(pkg.mod.Path); ok {
+			// It is possible that the main module's go.mod file is incomplete or
+			// otherwise erroneous — for example, perhaps the author forgot to 'git
+			// add' their updated go.mod file after adding a new package import, or
+			// perhaps they made an edit to the go.mod file using a third-party tool
+			// ('git merge'?) that doesn't maintain consistency for module
+			// dependencies. If that happens, ideally we want to detect the missing
+			// requirements and fix them up here.
+			//
+			// However, we also need to be careful not to be too aggressive. For
+			// transitive dependencies of external tests, the go.mod file for the
+			// module containing the test itself is expected to provide all of the
+			// relevant dependencies, and we explicitly don't want to pull in
+			// requirements on *irrelevant* requirements that happen to occur in the
+			// go.mod files for these transitive-test-only dependencies. (See the test
+			// in mod_lazy_test_horizon.txt for a concrete example.
+			//
+			// The “goldilocks zone” seems to be to spot-check exactly the same
+			// modules that we promote to explicit roots: namely, those that provide
+			// packages transitively imported by the main module, and those that
+			// provide roots of the package-import graph. That will catch erroneous
+			// edits to the main module's go.mod file and inconsistent requirements in
+			// dependencies that provide imported packages, but will ignore erroneous
+			// or misleading requirements in dependencies that aren't obviously
+			// relevant to the packages in the main module.
+			spotCheckRoot[pkg.mod] = true
+		} else {
+			roots = append(roots, pkg.mod)
+			rootsUpgraded = true
+			// The roots slice was initially sorted because rs.rootModules was sorted,
+			// but the root we just added could be out of order.
+			needSort = true
+		}
+	}
+
+	for _, m := range add {
+		if v, ok := rs.rootSelected(m.Path); !ok || gover.ModCompare(m.Path, v, m.Version) < 0 {
+			roots = append(roots, m)
+			rootsUpgraded = true
+			needSort = true
+		}
+	}
+	if needSort {
+		gover.ModSort(roots)
+	}
+
+	// "Each root appears only once, at the selected version of its path ….”
+	for {
+		var mg *ModuleGraph
+		if rootsUpgraded {
+			// We've added or upgraded one or more roots, so load the full module
+			// graph so that we can update those roots to be consistent with other
+			// requirements.
+			if mustHaveCompleteRequirements() {
+				// Our changes to the roots may have moved dependencies into or out of
+				// the graph-pruning horizon, which could in turn change the selected
+				// versions of other modules. (For pruned modules adding or removing an
+				// explicit root is a semantic change, not just a cosmetic one.)
+				return rs, errGoModDirty
+			}
+
+			rs = newRequirements(pruned, roots, direct)
+			var err error
+			mg, err = rs.Graph(ctx)
+			if err != nil {
+				return rs, err
+			}
+		} else {
+			// Since none of the roots have been upgraded, we have no reason to
+			// suspect that they are inconsistent with the requirements of any other
+			// roots. Only look at the full module graph if we've already loaded it;
+			// otherwise, just spot-check the explicit requirements of the roots from
+			// which we loaded packages.
+			if rs.graph.Load() != nil {
+				// We've already loaded the full module graph, which includes the
+				// requirements of all of the root modules — even the transitive
+				// requirements, if they are unpruned!
+				mg, _ = rs.Graph(ctx)
+			} else if cfg.BuildMod == "vendor" {
+				// We can't spot-check the requirements of other modules because we
+				// don't in general have their go.mod files available in the vendor
+				// directory. (Fortunately this case is impossible, because mg.graph is
+				// always non-nil in vendor mode!)
+				panic("internal error: rs.graph is unexpectedly nil with -mod=vendor")
+			} else if !spotCheckRoots(ctx, rs, spotCheckRoot) {
+				// We spot-checked the explicit requirements of the roots that are
+				// relevant to the packages we've loaded. Unfortunately, they're
+				// inconsistent in some way; we need to load the full module graph
+				// so that we can fix the roots properly.
+				var err error
+				mg, err = rs.Graph(ctx)
+				if err != nil {
+					return rs, err
+				}
+			}
+		}
+
+		roots = make([]module.Version, 0, len(rs.rootModules))
+		rootsUpgraded = false
+		inRootPaths := make(map[string]bool, len(rs.rootModules)+1)
+		for _, mm := range MainModules.Versions() {
+			inRootPaths[mm.Path] = true
+		}
+		for _, m := range rs.rootModules {
+			if inRootPaths[m.Path] {
+				// This root specifies a redundant path. We already retained the
+				// selected version of this path when we saw it before, so omit the
+				// redundant copy regardless of its version.
+				//
+				// When we read the full module graph, we include the dependencies of
+				// every root even if that root is redundant. That better preserves
+				// reproducibility if, say, some automated tool adds a redundant
+				// 'require' line and then runs 'go mod tidy' to try to make everything
+				// consistent, since the requirements of the older version are carried
+				// over.
+				//
+				// So omitting a root that was previously present may *reduce* the
+				// selected versions of non-roots, but merely removing a requirement
+				// cannot *increase* the selected versions of other roots as a result —
+				// we don't need to mark this change as an upgrade. (This particular
+				// change cannot invalidate any other roots.)
+				continue
+			}
+
+			var v string
+			if mg == nil {
+				v, _ = rs.rootSelected(m.Path)
+			} else {
+				v = mg.Selected(m.Path)
+			}
+			roots = append(roots, module.Version{Path: m.Path, Version: v})
+			inRootPaths[m.Path] = true
+			if v != m.Version {
+				rootsUpgraded = true
+			}
+		}
+		// Note that rs.rootModules was already sorted by module path and version,
+		// and we appended to the roots slice in the same order and guaranteed that
+		// each path has only one version, so roots is also sorted by module path
+		// and (trivially) version.
+
+		if !rootsUpgraded {
+			if cfg.BuildMod != "mod" {
+				// The only changes to the root set (if any) were to remove duplicates.
+				// The requirements are consistent (if perhaps redundant), so keep the
+				// original rs to preserve its ModuleGraph.
+				return rs, nil
+			}
+			// The root set has converged: every root going into this iteration was
+			// already at its selected version, although we have have removed other
+			// (redundant) roots for the same path.
+			break
+		}
+	}
+
+	if rs.pruning == pruned && reflect.DeepEqual(roots, rs.rootModules) && reflect.DeepEqual(direct, rs.direct) {
+		// The root set is unchanged and rs was already pruned, so keep rs to
+		// preserve its cached ModuleGraph (if any).
+		return rs, nil
+	}
+	return newRequirements(pruned, roots, direct), nil
+}
+
+// spotCheckRoots reports whether the versions of the roots in rs satisfy the
+// explicit requirements of the modules in mods.
+func spotCheckRoots(ctx context.Context, rs *Requirements, mods map[module.Version]bool) bool {
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel()
+
+	work := par.NewQueue(runtime.GOMAXPROCS(0))
+	for m := range mods {
+		m := m
+		work.Add(func() {
+			if ctx.Err() != nil {
+				return
+			}
+
+			summary, err := goModSummary(m)
+			if err != nil {
+				cancel()
+				return
+			}
+
+			for _, r := range summary.require {
+				if v, ok := rs.rootSelected(r.Path); ok && gover.ModCompare(r.Path, v, r.Version) < 0 {
+					cancel()
+					return
+				}
+			}
+		})
+	}
+	<-work.Idle()
+
+	if ctx.Err() != nil {
+		// Either we failed a spot-check, or the caller no longer cares about our
+		// answer anyway.
+		return false
+	}
+
+	return true
+}
+
+// tidyUnprunedRoots returns a minimal set of root requirements that maintains
+// the selected version of every module that provided or lexically could have
+// provided a package in pkgs, and includes the selected version of every such
+// module in direct as a root.
+func tidyUnprunedRoots(ctx context.Context, mainModule module.Version, old *Requirements, pkgs []*loadPkg) (*Requirements, error) {
+	var (
+		// keep is a set of of modules that provide packages or are needed to
+		// disambiguate imports.
+		keep     []module.Version
+		keptPath = map[string]bool{}
+
+		// rootPaths is a list of module paths that provide packages directly
+		// imported from the main module. They should be included as roots.
+		rootPaths   []string
+		inRootPaths = map[string]bool{}
+
+		// altMods is a set of paths of modules that lexically could have provided
+		// imported packages. It may be okay to remove these from the list of
+		// explicit requirements if that removes them from the module graph. If they
+		// are present in the module graph reachable from rootPaths, they must not
+		// be at a lower version. That could cause a missing sum error or a new
+		// import ambiguity.
+		//
+		// For example, suppose a developer rewrites imports from example.com/m to
+		// example.com/m/v2, then runs 'go mod tidy'. Tidy may delete the
+		// requirement on example.com/m if there is no other transitive requirement
+		// on it. However, if example.com/m were downgraded to a version not in
+		// go.sum, when package example.com/m/v2/p is loaded, we'd get an error
+		// trying to disambiguate the import, since we can't check example.com/m
+		// without its sum. See #47738.
+		altMods = map[string]string{}
+	)
+	if v, ok := old.rootSelected("go"); ok {
+		keep = append(keep, module.Version{Path: "go", Version: v})
+		keptPath["go"] = true
+	}
+	if v, ok := old.rootSelected("toolchain"); ok {
+		keep = append(keep, module.Version{Path: "toolchain", Version: v})
+		keptPath["toolchain"] = true
+	}
+	for _, pkg := range pkgs {
+		if !pkg.fromExternalModule() {
+			continue
+		}
+		if m := pkg.mod; !keptPath[m.Path] {
+			keep = append(keep, m)
+			keptPath[m.Path] = true
+			if old.direct[m.Path] && !inRootPaths[m.Path] {
+				rootPaths = append(rootPaths, m.Path)
+				inRootPaths[m.Path] = true
+			}
+		}
+		for _, m := range pkg.altMods {
+			altMods[m.Path] = m.Version
+		}
+	}
+
+	// Construct a build list with a minimal set of roots.
+	// This may remove or downgrade modules in altMods.
+	reqs := &mvsReqs{roots: keep}
+	min, err := mvs.Req(mainModule, rootPaths, reqs)
+	if err != nil {
+		return nil, err
+	}
+	buildList, err := mvs.BuildList([]module.Version{mainModule}, reqs)
+	if err != nil {
+		return nil, err
+	}
+
+	// Check if modules in altMods were downgraded but not removed.
+	// If so, add them to roots, which will retain an "// indirect" requirement
+	// in go.mod. See comment on altMods above.
+	keptAltMod := false
+	for _, m := range buildList {
+		if v, ok := altMods[m.Path]; ok && gover.ModCompare(m.Path, m.Version, v) < 0 {
+			keep = append(keep, module.Version{Path: m.Path, Version: v})
+			keptAltMod = true
+		}
+	}
+	if keptAltMod {
+		// We must run mvs.Req again instead of simply adding altMods to min.
+		// It's possible that a requirement in altMods makes some other
+		// explicit indirect requirement unnecessary.
+		reqs.roots = keep
+		min, err = mvs.Req(mainModule, rootPaths, reqs)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return newRequirements(unpruned, min, old.direct), nil
+}
+
+// updateUnprunedRoots returns a set of root requirements that includes the selected
+// version of every module path in direct as a root, and maintains the selected
+// version of every module selected in the graph of rs.
+//
+// The roots are updated such that:
+//
+//  1. The selected version of every module path in direct is included as a root
+//     (if it is not "none").
+//  2. Each root is the selected version of its path. (We say that such a root
+//     set is “consistent”.)
+//  3. Every version selected in the graph of rs remains selected unless upgraded
+//     by a dependency in add.
+//  4. Every version in add is selected at its given version unless upgraded by
+//     (the dependencies of) an existing root or another module in add.
+func updateUnprunedRoots(ctx context.Context, direct map[string]bool, rs *Requirements, add []module.Version) (*Requirements, error) {
+	mg, err := rs.Graph(ctx)
+	if err != nil {
+		// We can't ignore errors in the module graph even if the user passed the -e
+		// flag to try to push past them. If we can't load the complete module
+		// dependencies, then we can't reliably compute a minimal subset of them.
+		return rs, err
+	}
+
+	if mustHaveCompleteRequirements() {
+		// Instead of actually updating the requirements, just check that no updates
+		// are needed.
+		if rs == nil {
+			// We're being asked to reconstruct the requirements from scratch,
+			// but we aren't even allowed to modify them.
+			return rs, errGoModDirty
+		}
+		for _, m := range rs.rootModules {
+			if m.Version != mg.Selected(m.Path) {
+				// The root version v is misleading: the actual selected version is higher.
+				return rs, errGoModDirty
+			}
+		}
+		for _, m := range add {
+			if m.Version != mg.Selected(m.Path) {
+				return rs, errGoModDirty
+			}
+		}
+		for mPath := range direct {
+			if _, ok := rs.rootSelected(mPath); !ok {
+				// Module m is supposed to be listed explicitly, but isn't.
+				//
+				// Note that this condition is also detected (and logged with more
+				// detail) earlier during package loading, so it shouldn't actually be
+				// possible at this point — this is just a defense in depth.
+				return rs, errGoModDirty
+			}
+		}
+
+		// No explicit roots are missing and all roots are already at the versions
+		// we want to keep. Any other changes we would make are purely cosmetic,
+		// such as pruning redundant indirect dependencies. Per issue #34822, we
+		// ignore cosmetic changes when we cannot update the go.mod file.
+		return rs, nil
+	}
+
+	var (
+		rootPaths   []string // module paths that should be included as roots
+		inRootPaths = map[string]bool{}
+	)
+	for _, root := range rs.rootModules {
+		// If the selected version of the root is the same as what was already
+		// listed in the go.mod file, retain it as a root (even if redundant) to
+		// avoid unnecessary churn. (See https://golang.org/issue/34822.)
+		//
+		// We do this even for indirect requirements, since we don't know why they
+		// were added and they could become direct at any time.
+		if !inRootPaths[root.Path] && mg.Selected(root.Path) == root.Version {
+			rootPaths = append(rootPaths, root.Path)
+			inRootPaths[root.Path] = true
+		}
+	}
+
+	// “The selected version of every module path in direct is included as a root.”
+	//
+	// This is only for convenience and clarity for end users: in an unpruned module,
+	// the choice of explicit vs. implicit dependency has no impact on MVS
+	// selection (for itself or any other module).
+	keep := append(mg.BuildList()[MainModules.Len():], add...)
+	for _, m := range keep {
+		if direct[m.Path] && !inRootPaths[m.Path] {
+			rootPaths = append(rootPaths, m.Path)
+			inRootPaths[m.Path] = true
+		}
+	}
+
+	var roots []module.Version
+	for _, mainModule := range MainModules.Versions() {
+		min, err := mvs.Req(mainModule, rootPaths, &mvsReqs{roots: keep})
+		if err != nil {
+			return rs, err
+		}
+		roots = append(roots, min...)
+	}
+	if MainModules.Len() > 1 {
+		gover.ModSort(roots)
+	}
+	if rs.pruning == unpruned && reflect.DeepEqual(roots, rs.rootModules) && reflect.DeepEqual(direct, rs.direct) {
+		// The root set is unchanged and rs was already unpruned, so keep rs to
+		// preserve its cached ModuleGraph (if any).
+		return rs, nil
+	}
+
+	return newRequirements(unpruned, roots, direct), nil
+}
+
+// convertPruning returns a version of rs with the given pruning behavior.
+// If rs already has the given pruning, convertPruning returns rs unmodified.
+func convertPruning(ctx context.Context, rs *Requirements, pruning modPruning) (*Requirements, error) {
+	if rs.pruning == pruning {
+		return rs, nil
+	} else if rs.pruning == workspace || pruning == workspace {
+		panic("attempting to convert to/from workspace pruning and another pruning type")
+	}
+
+	if pruning == unpruned {
+		// We are converting a pruned module to an unpruned one. The roots of a
+		// pruned module graph are a superset of the roots of an unpruned one, so
+		// we don't need to add any new roots — we just need to drop the ones that
+		// are redundant, which is exactly what updateUnprunedRoots does.
+		return updateUnprunedRoots(ctx, rs.direct, rs, nil)
+	}
+
+	// We are converting an unpruned module to a pruned one.
+	//
+	// An unpruned module graph includes the transitive dependencies of every
+	// module in the build list. As it turns out, we can express that as a pruned
+	// root set! “Include the transitive dependencies of every module in the build
+	// list” is exactly what happens in a pruned module if we promote every module
+	// in the build list to a root.
+	mg, err := rs.Graph(ctx)
+	if err != nil {
+		return rs, err
+	}
+	return newRequirements(pruned, mg.BuildList()[MainModules.Len():], rs.direct), nil
+}
diff --git a/src/cmd/go/internal/modload/edit.go b/src/cmd/go/internal/modload/edit.go
new file mode 100644
index 0000000..63ee15c
--- /dev/null
+++ b/src/cmd/go/internal/modload/edit.go
@@ -0,0 +1,855 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/mvs"
+	"cmd/go/internal/par"
+	"context"
+	"errors"
+	"fmt"
+	"maps"
+	"os"
+	"slices"
+
+	"golang.org/x/mod/module"
+)
+
+// editRequirements returns an edited version of rs such that:
+//
+//  1. Each module version in mustSelect is selected.
+//
+//  2. Each module version in tryUpgrade is upgraded toward the indicated
+//     version as far as can be done without violating (1).
+//     (Other upgrades are also allowed if they are caused by
+//     transitive requirements of versions in mustSelect or
+//     tryUpgrade.)
+//
+//  3. Each module version in rs.rootModules (or rs.graph, if rs is unpruned)
+//     is downgraded or upgraded from its original version only to the extent
+//     needed to satisfy (1) and (2).
+//
+// Generally, the module versions in mustSelect are due to the module or a
+// package within the module matching an explicit command line argument to 'go
+// get', and the versions in tryUpgrade are transitive dependencies that are
+// either being upgraded by 'go get -u' or being added to satisfy some
+// otherwise-missing package import.
+//
+// If pruning is enabled, the roots of the edited requirements include an
+// explicit entry for each module path in tryUpgrade, mustSelect, and the roots
+// of rs, unless the selected version for the module path is "none".
+func editRequirements(ctx context.Context, rs *Requirements, tryUpgrade, mustSelect []module.Version) (edited *Requirements, changed bool, err error) {
+	if rs.pruning == workspace {
+		panic("editRequirements cannot edit workspace requirements")
+	}
+
+	orig := rs
+	// If we already know what go version we will end up on after the edit, and
+	// the pruning for that version is different, go ahead and apply it now.
+	//
+	// If we are changing from pruned to unpruned, then we MUST check the unpruned
+	// graph for conflicts from the start. (Checking only for pruned conflicts
+	// would miss some that would be introduced later.)
+	//
+	// If we are changing from unpruned to pruned, then we would like to avoid
+	// unnecessary downgrades due to conflicts that would be pruned out of the
+	// final graph anyway.
+	//
+	// Note that even if we don't find a go version in mustSelect, it is possible
+	// that we will switch from unpruned to pruned (but not the other way around!)
+	// after applying the edits if we find a dependency that requires a high
+	// enough go version to trigger an upgrade.
+	rootPruning := orig.pruning
+	for _, m := range mustSelect {
+		if m.Path == "go" {
+			rootPruning = pruningForGoVersion(m.Version)
+			break
+		} else if m.Path == "toolchain" && pruningForGoVersion(gover.FromToolchain(m.Version)) == unpruned {
+			// We don't know exactly what go version we will end up at, but we know
+			// that it must be a version supported by the requested toolchain, and
+			// that toolchain does not support pruning.
+			//
+			// TODO(bcmills): 'go get' ought to reject explicit toolchain versions
+			// older than gover.GoStrictVersion. Once that is fixed, is this still
+			// needed?
+			rootPruning = unpruned
+			break
+		}
+	}
+
+	if rootPruning != rs.pruning {
+		rs, err = convertPruning(ctx, rs, rootPruning)
+		if err != nil {
+			return orig, false, err
+		}
+	}
+
+	// selectedRoot records the edited version (possibly "none") for each module
+	// path that would be a root in the edited requirements.
+	var selectedRoot map[string]string // module path → edited version
+	if rootPruning == pruned {
+		selectedRoot = maps.Clone(rs.maxRootVersion)
+	} else {
+		// In a module without graph pruning, modules that provide packages imported
+		// by the main module may either be explicit roots or implicit transitive
+		// dependencies. To the extent possible, we want to preserve those implicit
+		// dependencies, so we need to treat everything in the build list as
+		// potentially relevant — that is, as what would be a “root” in a module
+		// with graph pruning enabled.
+		mg, err := rs.Graph(ctx)
+		if err != nil {
+			// If we couldn't load the graph, we don't know what its requirements were
+			// to begin with, so we can't edit those requirements in a coherent way.
+			return orig, false, err
+		}
+		bl := mg.BuildList()[MainModules.Len():]
+		selectedRoot = make(map[string]string, len(bl))
+		for _, m := range bl {
+			selectedRoot[m.Path] = m.Version
+		}
+	}
+
+	for _, r := range tryUpgrade {
+		if v, ok := selectedRoot[r.Path]; ok && gover.ModCompare(r.Path, v, r.Version) >= 0 {
+			continue
+		}
+		if cfg.BuildV {
+			fmt.Fprintf(os.Stderr, "go: trying upgrade to %v\n", r)
+		}
+		selectedRoot[r.Path] = r.Version
+	}
+
+	// conflicts is a list of conflicts that we cannot resolve without violating
+	// some version in mustSelect. It may be incomplete, but we want to report
+	// as many conflicts as we can so that the user can solve more of them at once.
+	var conflicts []Conflict
+
+	// mustSelectVersion is an index of the versions in mustSelect.
+	mustSelectVersion := make(map[string]string, len(mustSelect))
+	for _, r := range mustSelect {
+		if v, ok := mustSelectVersion[r.Path]; ok && v != r.Version {
+			prev := module.Version{Path: r.Path, Version: v}
+			if gover.ModCompare(r.Path, v, r.Version) > 0 {
+				conflicts = append(conflicts, Conflict{Path: []module.Version{prev}, Constraint: r})
+			} else {
+				conflicts = append(conflicts, Conflict{Path: []module.Version{r}, Constraint: prev})
+			}
+			continue
+		}
+
+		mustSelectVersion[r.Path] = r.Version
+		selectedRoot[r.Path] = r.Version
+	}
+
+	// We've indexed all of the data we need and we've computed the initial
+	// versions of the roots. Now we need to load the actual module graph and
+	// restore the invariant that every root is the selected version of its path.
+	//
+	// For 'go mod tidy' we would do that using expandGraph, which upgrades the
+	// roots until their requirements are internally consistent and then drops out
+	// the old roots. However, here we need to do more: we also need to make sure
+	// the modules in mustSelect don't get upgraded above their intended versions.
+	// To do that, we repeatedly walk the module graph, identify paths of
+	// requirements that result in versions that are too high, and downgrade the
+	// roots that lead to those paths. When no conflicts remain, we're done.
+	//
+	// Since we want to report accurate paths to each conflict, we don't drop out
+	// older-than-selected roots until the process completes. That might mean that
+	// we do some extra downgrades when they could be skipped, but for the benefit
+	// of being able to explain the reason for every downgrade that seems
+	// worthwhile.
+	//
+	// Graph pruning adds an extra wrinkle: a given node in the module graph
+	// may be reached from a root whose dependencies are pruned, and from a root
+	// whose dependencies are not pruned. It may be the case that the path from
+	// the unpruned root leads to a conflict, while the path from the pruned root
+	// prunes out the requirements that would lead to that conflict.
+	// So we need to track the two kinds of paths independently.
+	// They join back together at the roots of the graph: if a root r1 with pruned
+	// requirements depends on a root r2 with unpruned requirements, then
+	// selecting r1 would cause r2 to become a root and pull in all of its
+	// unpruned dependencies.
+	//
+	// The dqTracker type implements the logic for propagating conflict paths
+	// through the pruned and unpruned parts of the module graph.
+	//
+	// We make a best effort to fix incompatibilities, subject to two properties:
+	//
+	// 	1. If the user runs 'go get' with a set of mutually-compatible module
+	// 	versions, we should accept those versions.
+	//
+	// 	2. If we end up upgrading or downgrading a module, it should be
+	// 	clear why we did so.
+	//
+	// We don't try to find an optimal SAT solution,
+	// especially given the complex interactions with graph pruning.
+
+	var (
+		roots      []module.Version // the current versions in selectedRoot, in sorted order
+		rootsDirty = true           // true if roots does not match selectedRoot
+	)
+
+	// rejectedRoot records the set of module versions that have been disqualified
+	// as roots of the module graph. When downgrading due to a conflict or error,
+	// we skip any version that has already been rejected.
+	//
+	// NOTE(bcmills): I am not sure that the rejectedRoot map is really necessary,
+	// since we normally only downgrade roots or accept indirect upgrades to
+	// known-good versions. However, I am having trouble proving that accepting an
+	// indirect upgrade never introduces a conflict that leads to further
+	// downgrades. I really want to be able to prove that editRequirements
+	// terminates, and the easiest way to prove it is to add this map.
+	//
+	// Then the proof of termination is this:
+	// On every iteration where we mark the roots as dirty, we add some new module
+	// version to the map. The universe of module versions is finite, so we must
+	// eventually reach a state in which we do not add any version to the map.
+	// In that state, we either report a conflict or succeed in the edit.
+	rejectedRoot := map[module.Version]bool{}
+
+	for rootsDirty && len(conflicts) == 0 {
+		roots = roots[:0]
+		for p, v := range selectedRoot {
+			if v != "none" {
+				roots = append(roots, module.Version{Path: p, Version: v})
+			}
+		}
+		gover.ModSort(roots)
+
+		// First, we extend the graph so that it includes the selected version
+		// of every root. The upgraded roots are in addition to the original
+		// roots, so we will have enough information to trace a path to each
+		// conflict we discover from one or more of the original roots.
+		mg, upgradedRoots, err := extendGraph(ctx, rootPruning, roots, selectedRoot)
+		if err != nil {
+			var tooNew *gover.TooNewError
+			if mg == nil || errors.As(err, &tooNew) {
+				return orig, false, err
+			}
+			// We're about to walk the entire extended module graph, so we will find
+			// any error then — and we will either try to resolve it by downgrading
+			// something or report it as a conflict with more detail.
+		}
+
+		// extendedRootPruning is an index of the pruning used to load each root in
+		// the extended module graph.
+		extendedRootPruning := make(map[module.Version]modPruning, len(roots)+len(upgradedRoots))
+		findPruning := func(m module.Version) modPruning {
+			if rootPruning == pruned {
+				summary, _ := mg.loadCache.Get(m)
+				if summary != nil && summary.pruning == unpruned {
+					return unpruned
+				}
+			}
+			return rootPruning
+		}
+		for _, m := range roots {
+			extendedRootPruning[m] = findPruning(m)
+		}
+		for m := range upgradedRoots {
+			extendedRootPruning[m] = findPruning(m)
+		}
+
+		// Now check the resulting extended graph for errors and incompatibilities.
+		t := dqTracker{extendedRootPruning: extendedRootPruning}
+		mg.g.WalkBreadthFirst(func(m module.Version) {
+			if max, ok := mustSelectVersion[m.Path]; ok && gover.ModCompare(m.Path, m.Version, max) > 0 {
+				// m itself violates mustSelect, so it cannot appear in the module graph
+				// even if its transitive dependencies would be pruned out.
+				t.disqualify(m, pruned, dqState{dep: m})
+				return
+			}
+
+			summary, err := mg.loadCache.Get(m)
+			if err != nil && err != par.ErrCacheEntryNotFound {
+				// We can't determine the requirements of m, so we don't know whether
+				// they would be allowed. This may be a transient error reaching the
+				// repository, rather than a permanent error with the retrieved version.
+				//
+				// TODO(golang.org/issue/31730, golang.org/issue/30134):
+				// decide what to do based on the actual error.
+				t.disqualify(m, pruned, dqState{err: err})
+				return
+			}
+
+			reqs, ok := mg.RequiredBy(m)
+			if !ok {
+				// The dependencies of m do not appear in the module graph, so they
+				// can't be causing any problems this time.
+				return
+			}
+
+			if summary == nil {
+				if m.Version != "" {
+					panic(fmt.Sprintf("internal error: %d reqs present for %v, but summary is nil", len(reqs), m))
+				}
+				// m is the main module: we are editing its dependencies, so it cannot
+				// become disqualified.
+				return
+			}
+
+			// Before we check for problems due to transitive dependencies, first
+			// check m's direct requirements. A requirement on a version r that
+			// violates mustSelect disqualifies m, even if the requirements of r are
+			// themselves pruned out.
+			for _, r := range reqs {
+				if max, ok := mustSelectVersion[r.Path]; ok && gover.ModCompare(r.Path, r.Version, max) > 0 {
+					t.disqualify(m, pruned, dqState{dep: r})
+					return
+				}
+			}
+			for _, r := range reqs {
+				if !t.require(m, r) {
+					break
+				}
+			}
+		})
+
+		// We have now marked all of the versions in the graph that have conflicts,
+		// with a path to each conflict from one or more roots that introduce it.
+		// Now we need to identify those roots and change their versions
+		// (if possible) in order to resolve the conflicts.
+		rootsDirty = false
+		for _, m := range roots {
+			path, err := t.path(m, extendedRootPruning[m])
+			if len(path) == 0 && err == nil {
+				continue // Nothing wrong with m; we can keep it.
+			}
+
+			// path leads to a module with a problem: either it violates a constraint,
+			// or some error prevents us from determining whether it violates a
+			// constraint. We might end up logging or returning the conflict
+			// information, so go ahead and fill in the details about it.
+			conflict := Conflict{
+				Path: path,
+				Err:  err,
+			}
+			if err == nil {
+				var last module.Version = path[len(path)-1]
+				mustV, ok := mustSelectVersion[last.Path]
+				if !ok {
+					fmt.Fprintf(os.Stderr, "go: %v\n", conflict)
+					panic("internal error: found a version conflict, but no constraint it violates")
+				}
+				conflict.Constraint = module.Version{
+					Path:    last.Path,
+					Version: mustV,
+				}
+			}
+
+			if v, ok := mustSelectVersion[m.Path]; ok && v == m.Version {
+				// m is in mustSelect, but is marked as disqualified due to a transitive
+				// dependency.
+				//
+				// In theory we could try removing module paths that don't appear in
+				// mustSelect (added by tryUpgrade or already present in rs) in order to
+				// get graph pruning to take effect, but (a) it is likely that 'go mod
+				// tidy' would re-add those roots and reintroduce unwanted upgrades,
+				// causing confusion, and (b) deciding which roots to try to eliminate
+				// would add a lot of complexity.
+				//
+				// Instead, we report the path to the conflict as an error.
+				// If users want to explicitly prune out nodes from the dependency
+				// graph, they can always add an explicit 'exclude' directive.
+				conflicts = append(conflicts, conflict)
+				continue
+			}
+
+			// If m is not the selected version of its path, we have two options: we
+			// can either upgrade to the version that actually is selected (dropping m
+			// itself out of the bottom of the module graph), or we can try
+			// downgrading it.
+			//
+			// If the version we would be upgrading to is ok to use, we will just plan
+			// to do that and avoid the overhead of trying to find some lower version
+			// to downgrade to.
+			//
+			// However, it is possible that m depends on something that leads to its
+			// own upgrade, so if the upgrade isn't viable we should go ahead and try
+			// to downgrade (like with any other root).
+			if v := mg.Selected(m.Path); v != m.Version {
+				u := module.Version{Path: m.Path, Version: v}
+				uPruning, ok := t.extendedRootPruning[m]
+				if !ok {
+					fmt.Fprintf(os.Stderr, "go: %v\n", conflict)
+					panic(fmt.Sprintf("internal error: selected version of root %v is %v, but it was not expanded as a new root", m, u))
+				}
+				if !t.check(u, uPruning).isDisqualified() && !rejectedRoot[u] {
+					// Applying the upgrade from m to u will resolve the conflict,
+					// so plan to do that if there are no other conflicts to resolve.
+					continue
+				}
+			}
+
+			// Figure out what version of m's path was present before we started
+			// the edit. We want to make sure we consider keeping it as-is,
+			// even if it wouldn't normally be included. (For example, it might
+			// be a pseudo-version or pre-release.)
+			origMG, _ := orig.Graph(ctx)
+			origV := origMG.Selected(m.Path)
+
+			if conflict.Err != nil && origV == m.Version {
+				// This version of m.Path was already in the module graph before we
+				// started editing, and the problem with it is that we can't load its
+				// (transitive) requirements.
+				//
+				// If this conflict was just one step in a longer chain of downgrades,
+				// then we would want to keep going past it until we find a version
+				// that doesn't have that problem. However, we only want to downgrade
+				// away from an *existing* requirement if we can confirm that it actually
+				// conflicts with mustSelect. (For example, we don't want
+				// 'go get -u ./...' to incidentally downgrade some dependency whose
+				// go.mod file is unavailable or has a bad checksum.)
+				conflicts = append(conflicts, conflict)
+				continue
+			}
+
+			// We need to downgrade m's path to some lower version to try to resolve
+			// the conflict. Find the next-lowest candidate and apply it.
+			rejectedRoot[m] = true
+			prev := m
+			for {
+				prev, err = previousVersion(ctx, prev)
+				if gover.ModCompare(m.Path, m.Version, origV) > 0 && (gover.ModCompare(m.Path, prev.Version, origV) < 0 || err != nil) {
+					// previousVersion skipped over origV. Insert it into the order.
+					prev.Version = origV
+				} else if err != nil {
+					// We don't know the next downgrade to try. Give up.
+					return orig, false, err
+				}
+				if rejectedRoot[prev] {
+					// We already rejected prev in a previous round.
+					// To ensure that this algorithm terminates, don't try it again.
+					continue
+				}
+				pruning := rootPruning
+				if pruning == pruned {
+					if summary, err := mg.loadCache.Get(m); err == nil {
+						pruning = summary.pruning
+					}
+				}
+				if t.check(prev, pruning).isDisqualified() {
+					// We found a problem with prev this round that would also disqualify
+					// it as a root. Don't bother trying it next round.
+					rejectedRoot[prev] = true
+					continue
+				}
+				break
+			}
+			selectedRoot[m.Path] = prev.Version
+			rootsDirty = true
+
+			// If this downgrade is potentially interesting, log the reason for it.
+			if conflict.Err != nil || cfg.BuildV {
+				var action string
+				if prev.Version == "none" {
+					action = fmt.Sprintf("removing %s", m)
+				} else if prev.Version == origV {
+					action = fmt.Sprintf("restoring %s", prev)
+				} else {
+					action = fmt.Sprintf("trying %s", prev)
+				}
+				fmt.Fprintf(os.Stderr, "go: %s\n\t%s\n", conflict.Summary(), action)
+			}
+		}
+		if rootsDirty {
+			continue
+		}
+
+		// We didn't resolve any issues by downgrading, but we may still need to
+		// resolve some conflicts by locking in upgrades. Do that now.
+		//
+		// We don't do these upgrades until we're done downgrading because the
+		// downgrade process might reveal or remove conflicts (by changing which
+		// requirement edges are pruned out).
+		var upgradedFrom []module.Version // for logging only
+		for p, v := range selectedRoot {
+			if _, ok := mustSelectVersion[p]; !ok {
+				if actual := mg.Selected(p); actual != v {
+					if cfg.BuildV {
+						upgradedFrom = append(upgradedFrom, module.Version{Path: p, Version: v})
+					}
+					selectedRoot[p] = actual
+					// Accepting the upgrade to m.Path might cause the selected versions
+					// of other modules to fall, because they were being increased by
+					// dependencies of m that are no longer present in the graph.
+					//
+					// TODO(bcmills): Can removing m as a root also cause the selected
+					// versions of other modules to rise? I think not: we're strictly
+					// removing non-root nodes from the module graph, which can't cause
+					// any root to decrease (because they're roots), and the dependencies
+					// of non-roots don't matter because they're either always unpruned or
+					// always pruned out.
+					//
+					// At any rate, it shouldn't cost much to reload the module graph one
+					// last time and confirm that it is stable.
+					rootsDirty = true
+				}
+			}
+		}
+		if rootsDirty {
+			if cfg.BuildV {
+				gover.ModSort(upgradedFrom) // Make logging deterministic.
+				for _, m := range upgradedFrom {
+					fmt.Fprintf(os.Stderr, "go: accepting indirect upgrade from %v to %s\n", m, selectedRoot[m.Path])
+				}
+			}
+			continue
+		}
+		break
+	}
+	if len(conflicts) > 0 {
+		return orig, false, &ConstraintError{Conflicts: conflicts}
+	}
+
+	if rootPruning == unpruned {
+		// An unpruned go.mod file lists only a subset of the requirements needed
+		// for building packages. Figure out which requirements need to be explicit.
+		var rootPaths []string
+
+		// The modules in mustSelect are always promoted to be explicit.
+		for _, m := range mustSelect {
+			if m.Version != "none" && !MainModules.Contains(m.Path) {
+				rootPaths = append(rootPaths, m.Path)
+			}
+		}
+
+		for _, m := range roots {
+			if v, ok := rs.rootSelected(m.Path); ok && (v == m.Version || rs.direct[m.Path]) {
+				// m.Path was formerly a root, and either its version hasn't changed or
+				// we believe that it provides a package directly imported by a package
+				// or test in the main module. For now we'll assume that it is still
+				// relevant enough to remain a root. If we actually load all of the
+				// packages and tests in the main module (which we are not doing here),
+				// we can revise the explicit roots at that point.
+				rootPaths = append(rootPaths, m.Path)
+			}
+		}
+
+		roots, err = mvs.Req(MainModules.mustGetSingleMainModule(), rootPaths, &mvsReqs{roots: roots})
+		if err != nil {
+			return nil, false, err
+		}
+	}
+
+	changed = rootPruning != orig.pruning || !slices.Equal(roots, orig.rootModules)
+	if !changed {
+		// Because the roots we just computed are unchanged, the entire graph must
+		// be the same as it was before. Save the original rs, since we have
+		// probably already loaded its requirement graph.
+		return orig, false, nil
+	}
+
+	// A module that is not even in the build list necessarily cannot provide
+	// any imported packages. Mark as direct only the direct modules that are
+	// still in the build list. (We assume that any module path that provided a
+	// direct import before the edit continues to do so after. There are a few
+	// edge cases where that can change, such as if a package moves into or out of
+	// a nested module or disappears entirely. If that happens, the user can run
+	// 'go mod tidy' to clean up the direct/indirect annotations.)
+	//
+	// TODO(bcmills): Would it make more sense to leave the direct map as-is
+	// but allow it to refer to modules that are no longer in the build list?
+	// That might complicate updateRoots, but it may be cleaner in other ways.
+	direct := make(map[string]bool, len(rs.direct))
+	for _, m := range roots {
+		if rs.direct[m.Path] {
+			direct[m.Path] = true
+		}
+	}
+	edited = newRequirements(rootPruning, roots, direct)
+
+	// If we ended up adding a dependency that upgrades our go version far enough
+	// to activate pruning, we must convert the edited Requirements in order to
+	// avoid dropping transitive dependencies from the build list the next time
+	// someone uses the updated go.mod file.
+	//
+	// Note that it isn't possible to go in the other direction (from pruned to
+	// unpruned) unless the "go" or "toolchain" module is explicitly listed in
+	// mustSelect, which we already handled at the very beginning of the edit.
+	// That is because the virtual "go" module only requires a "toolchain",
+	// and the "toolchain" module never requires anything else, which means that
+	// those two modules will never be downgraded due to a conflict with any other
+	// constraint.
+	if rootPruning == unpruned {
+		if v, ok := edited.rootSelected("go"); ok && pruningForGoVersion(v) == pruned {
+			// Since we computed the edit with the unpruned graph, and the pruned
+			// graph is a strict subset of the unpruned graph, this conversion
+			// preserves the exact (edited) build list that we already computed.
+			//
+			// However, it does that by shoving the whole build list into the roots of
+			// the graph. 'go get' will check for that sort of transition and log a
+			// message reminding the user how to clean up this mess we're about to
+			// make. 😅
+			edited, err = convertPruning(ctx, edited, pruned)
+			if err != nil {
+				return orig, false, err
+			}
+		}
+	}
+	return edited, true, nil
+}
+
+// extendGraph loads the module graph from roots, and iteratively extends it by
+// unpruning the selected version of each module path that is a root in rs or in
+// the roots slice until the graph reaches a fixed point.
+//
+// The graph is guaranteed to converge to a fixed point because unpruning a
+// module version can only increase (never decrease) the selected versions,
+// and the set of versions for each module is finite.
+//
+// The extended graph is useful for diagnosing version conflicts: for each
+// selected module version, it can provide a complete path of requirements from
+// some root to that version.
+func extendGraph(ctx context.Context, rootPruning modPruning, roots []module.Version, selectedRoot map[string]string) (mg *ModuleGraph, upgradedRoot map[module.Version]bool, err error) {
+	for {
+		mg, err = readModGraph(ctx, rootPruning, roots, upgradedRoot)
+		// We keep on going even if err is non-nil until we reach a steady state.
+		// (Note that readModGraph returns a non-nil *ModuleGraph even in case of
+		// errors.) The caller may be able to fix the errors by adjusting versions,
+		// so we really want to return as complete a result as we can.
+
+		if rootPruning == unpruned {
+			// Everything is already unpruned, so there isn't anything we can do to
+			// extend it further.
+			break
+		}
+
+		nPrevRoots := len(upgradedRoot)
+		for p := range selectedRoot {
+			// Since p is a root path, when we fix up the module graph to be
+			// consistent with the selected versions, p will be promoted to a root,
+			// which will pull in its dependencies. Ensure that its dependencies are
+			// included in the module graph.
+			v := mg.g.Selected(p)
+			if v == "none" {
+				// Version “none” always has no requirements, so it doesn't need
+				// an explicit node in the module graph.
+				continue
+			}
+			m := module.Version{Path: p, Version: v}
+			if _, ok := mg.g.RequiredBy(m); !ok && !upgradedRoot[m] {
+				// The dependencies of the selected version of p were not loaded.
+				// Mark it as an upgrade so that we will load its dependencies
+				// in the next iteration.
+				//
+				// Note that we don't remove any of the existing roots, even if they are
+				// no longer the selected version: with graph pruning in effect this may
+				// leave some spurious dependencies in the graph, but it at least
+				// preserves enough of the graph to explain why each upgrade occurred:
+				// this way, we can report a complete path from the passed-in roots
+				// to every node in the module graph.
+				//
+				// This process is guaranteed to reach a fixed point: since we are only
+				// adding roots (never removing them), the selected version of each module
+				// can only increase, never decrease, and the set of module versions in the
+				// universe is finite.
+				if upgradedRoot == nil {
+					upgradedRoot = make(map[module.Version]bool)
+				}
+				upgradedRoot[m] = true
+			}
+		}
+		if len(upgradedRoot) == nPrevRoots {
+			break
+		}
+	}
+
+	return mg, upgradedRoot, err
+}
+
+type perPruning[T any] struct {
+	pruned   T
+	unpruned T
+}
+
+func (pp perPruning[T]) from(p modPruning) T {
+	if p == unpruned {
+		return pp.unpruned
+	}
+	return pp.pruned
+}
+
+// A dqTracker tracks and propagates the reason that each module version
+// cannot be included in the module graph.
+type dqTracker struct {
+	// extendedRootPruning is the modPruning given the go.mod file for each root
+	// in the extended module graph.
+	extendedRootPruning map[module.Version]modPruning
+
+	// dqReason records whether and why each each encountered version is
+	// disqualified in a pruned or unpruned context.
+	dqReason map[module.Version]perPruning[dqState]
+
+	// requiring maps each not-yet-disqualified module version to the versions
+	// that would cause that module's requirements to be included in a pruned or
+	// unpruned context. If that version becomes disqualified, the
+	// disqualification will be propagated to all of the versions in the
+	// corresponding list.
+	//
+	// This map is similar to the module requirement graph, but includes more
+	// detail about whether a given dependency edge appears in a pruned or
+	// unpruned context. (Other commands do not need this level of detail.)
+	requiring map[module.Version][]module.Version
+}
+
+// A dqState indicates whether and why a module version is “disqualified” from
+// being used in a way that would incorporate its requirements.
+//
+// The zero dqState indicates that the module version is not known to be
+// disqualified, either because it is ok or because we are currently traversing
+// a cycle that includes it.
+type dqState struct {
+	err error          // if non-nil, disqualified because the requirements of the module could not be read
+	dep module.Version // disqualified because the module is or requires dep
+}
+
+func (dq dqState) isDisqualified() bool {
+	return dq != dqState{}
+}
+
+func (dq dqState) String() string {
+	if dq.err != nil {
+		return dq.err.Error()
+	}
+	if dq.dep != (module.Version{}) {
+		return dq.dep.String()
+	}
+	return "(no conflict)"
+}
+
+// require records that m directly requires r, in case r becomes disqualified.
+// (These edges are in the opposite direction from the edges in an mvs.Graph.)
+//
+// If r is already disqualified, require propagates the disqualification to m
+// and returns the reason for the disqualification.
+func (t *dqTracker) require(m, r module.Version) (ok bool) {
+	rdq := t.dqReason[r]
+	rootPruning, isRoot := t.extendedRootPruning[r]
+	if isRoot && rdq.from(rootPruning).isDisqualified() {
+		// When we pull in m's dependencies, we will have an edge from m to r, and r
+		// is disqualified (it is a root, which causes its problematic dependencies
+		// to always be included). So we cannot pull in m's dependencies at all:
+		// m is completely disqualified.
+		t.disqualify(m, pruned, dqState{dep: r})
+		return false
+	}
+
+	if dq := rdq.from(unpruned); dq.isDisqualified() {
+		t.disqualify(m, unpruned, dqState{dep: r})
+		if _, ok := t.extendedRootPruning[m]; !ok {
+			// Since m is not a root, its dependencies can't be included in the pruned
+			// part of the module graph, and will never be disqualified from a pruned
+			// reason. We've already disqualified everything that matters.
+			return false
+		}
+	}
+
+	// Record that m is a dependant of r, so that if r is later disqualified
+	// m will be disqualified as well.
+	if t.requiring == nil {
+		t.requiring = make(map[module.Version][]module.Version)
+	}
+	t.requiring[r] = append(t.requiring[r], m)
+	return true
+}
+
+// disqualify records why the dependencies of m cannot be included in the module
+// graph if reached from a part of the graph with the given pruning.
+//
+// Since the pruned graph is a subgraph of the unpruned graph, disqualifying a
+// module from a pruned part of the graph also disqualifies it in the unpruned
+// parts.
+func (t *dqTracker) disqualify(m module.Version, fromPruning modPruning, reason dqState) {
+	if !reason.isDisqualified() {
+		panic("internal error: disqualify called with a non-disqualifying dqState")
+	}
+
+	dq := t.dqReason[m]
+	if dq.from(fromPruning).isDisqualified() {
+		return // Already disqualified for some other reason; don't overwrite it.
+	}
+	rootPruning, isRoot := t.extendedRootPruning[m]
+	if fromPruning == pruned {
+		dq.pruned = reason
+		if !dq.unpruned.isDisqualified() {
+			// Since the pruned graph of m is a subgraph of the unpruned graph, if it
+			// is disqualified due to something in the pruned graph, it is certainly
+			// disqualified in the unpruned graph from the same reason.
+			dq.unpruned = reason
+		}
+	} else {
+		dq.unpruned = reason
+		if dq.pruned.isDisqualified() {
+			panic(fmt.Sprintf("internal error: %v is marked as disqualified when pruned, but not when unpruned", m))
+		}
+		if isRoot && rootPruning == unpruned {
+			// Since m is a root that is always unpruned, any other roots — even
+			// pruned ones! — that cause it to be selected would also cause the reason
+			// for is disqualification to be included in the module graph.
+			dq.pruned = reason
+		}
+	}
+	if t.dqReason == nil {
+		t.dqReason = make(map[module.Version]perPruning[dqState])
+	}
+	t.dqReason[m] = dq
+
+	if isRoot && (fromPruning == pruned || rootPruning == unpruned) {
+		// Either m is disqualified even when its dependencies are pruned,
+		// or m's go.mod file causes its dependencies to *always* be unpruned.
+		// Everything that depends on it must be disqualified.
+		for _, p := range t.requiring[m] {
+			t.disqualify(p, pruned, dqState{dep: m})
+			// Note that since the pruned graph is a subset of the unpruned graph,
+			// disqualifying p in the pruned graph also disqualifies it in the
+			// unpruned graph.
+		}
+		// Everything in t.requiring[m] is now fully disqualified.
+		// We won't need to use it again.
+		delete(t.requiring, m)
+		return
+	}
+
+	// Either m is not a root, or it is a pruned root but only being disqualified
+	// when reached from the unpruned parts of the module graph.
+	// Either way, the reason for this disqualification is only visible to the
+	// unpruned parts of the module graph.
+	for _, p := range t.requiring[m] {
+		t.disqualify(p, unpruned, dqState{dep: m})
+	}
+	if !isRoot {
+		// Since m is not a root, its dependencies can't be included in the pruned
+		// part of the module graph, and will never be disqualified from a pruned
+		// reason. We've already disqualified everything that matters.
+		delete(t.requiring, m)
+	}
+}
+
+// check reports whether m is disqualified in the given pruning context.
+func (t *dqTracker) check(m module.Version, pruning modPruning) dqState {
+	return t.dqReason[m].from(pruning)
+}
+
+// path returns the path from m to the reason it is disqualified, which may be
+// either a module that violates constraints or an error in loading
+// requirements.
+//
+// If m is not disqualified, path returns (nil, nil).
+func (t *dqTracker) path(m module.Version, pruning modPruning) (path []module.Version, err error) {
+	for {
+		dq := t.dqReason[m].from(pruning)
+		if !dq.isDisqualified() {
+			return path, nil
+		}
+		path = append(path, m)
+		if dq.err != nil || dq.dep == m {
+			return path, dq.err // m itself is the conflict.
+		}
+		m = dq.dep
+	}
+}
diff --git a/src/cmd/go/internal/modload/help.go b/src/cmd/go/internal/modload/help.go
new file mode 100644
index 0000000..886ad62
--- /dev/null
+++ b/src/cmd/go/internal/modload/help.go
@@ -0,0 +1,64 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import "cmd/go/internal/base"
+
+var HelpModules = &base.Command{
+	UsageLine: "modules",
+	Short:     "modules, module versions, and more",
+	Long: `
+Modules are how Go manages dependencies.
+
+A module is a collection of packages that are released, versioned, and
+distributed together. Modules may be downloaded directly from version control
+repositories or from module proxy servers.
+
+For a series of tutorials on modules, see
+https://golang.org/doc/tutorial/create-module.
+
+For a detailed reference on modules, see https://golang.org/ref/mod.
+
+By default, the go command may download modules from https://proxy.golang.org.
+It may authenticate modules using the checksum database at
+https://sum.golang.org. Both services are operated by the Go team at Google.
+The privacy policies for these services are available at
+https://proxy.golang.org/privacy and https://sum.golang.org/privacy,
+respectively.
+
+The go command's download behavior may be configured using GOPROXY, GOSUMDB,
+GOPRIVATE, and other environment variables. See 'go help environment'
+and https://golang.org/ref/mod#private-module-privacy for more information.
+	`,
+}
+
+var HelpGoMod = &base.Command{
+	UsageLine: "go.mod",
+	Short:     "the go.mod file",
+	Long: `
+A module version is defined by a tree of source files, with a go.mod
+file in its root. When the go command is run, it looks in the current
+directory and then successive parent directories to find the go.mod
+marking the root of the main (current) module.
+
+The go.mod file format is described in detail at
+https://golang.org/ref/mod#go-mod-file.
+
+To create a new go.mod file, use 'go mod init'. For details see
+'go help mod init' or https://golang.org/ref/mod#go-mod-init.
+
+To add missing module requirements or remove unneeded requirements,
+use 'go mod tidy'. For details, see 'go help mod tidy' or
+https://golang.org/ref/mod#go-mod-tidy.
+
+To add, upgrade, downgrade, or remove a specific module requirement, use
+'go get'. For details, see 'go help module-get' or
+https://golang.org/ref/mod#go-get.
+
+To make other changes or to parse go.mod as JSON for use by other tools,
+use 'go mod edit'. See 'go help mod edit' or
+https://golang.org/ref/mod#go-mod-edit.
+	`,
+}
diff --git a/src/cmd/go/internal/modload/import.go b/src/cmd/go/internal/modload/import.go
new file mode 100644
index 0000000..83b9ad4
--- /dev/null
+++ b/src/cmd/go/internal/modload/import.go
@@ -0,0 +1,773 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"go/build"
+	"io/fs"
+	"os"
+	pathpkg "path"
+	"path/filepath"
+	"sort"
+	"strings"
+
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/modindex"
+	"cmd/go/internal/par"
+	"cmd/go/internal/search"
+	"cmd/go/internal/str"
+
+	"golang.org/x/mod/module"
+)
+
+type ImportMissingError struct {
+	Path     string
+	Module   module.Version
+	QueryErr error
+
+	ImportingMainModule module.Version
+
+	// isStd indicates whether we would expect to find the package in the standard
+	// library. This is normally true for all dotless import paths, but replace
+	// directives can cause us to treat the replaced paths as also being in
+	// modules.
+	isStd bool
+
+	// importerGoVersion is the version the module containing the import error
+	// specified. It is only set when isStd is true.
+	importerGoVersion string
+
+	// replaced the highest replaced version of the module where the replacement
+	// contains the package. replaced is only set if the replacement is unused.
+	replaced module.Version
+
+	// newMissingVersion is set to a newer version of Module if one is present
+	// in the build list. When set, we can't automatically upgrade.
+	newMissingVersion string
+}
+
+func (e *ImportMissingError) Error() string {
+	if e.Module.Path == "" {
+		if e.isStd {
+			msg := fmt.Sprintf("package %s is not in std (%s)", e.Path, filepath.Join(cfg.GOROOT, "src", e.Path))
+			if e.importerGoVersion != "" {
+				msg += fmt.Sprintf("\nnote: imported by a module that requires go %s", e.importerGoVersion)
+			}
+			return msg
+		}
+		if e.QueryErr != nil && e.QueryErr != ErrNoModRoot {
+			return fmt.Sprintf("cannot find module providing package %s: %v", e.Path, e.QueryErr)
+		}
+		if cfg.BuildMod == "mod" || (cfg.BuildMod == "readonly" && allowMissingModuleImports) {
+			return "cannot find module providing package " + e.Path
+		}
+
+		if e.replaced.Path != "" {
+			suggestArg := e.replaced.Path
+			if !module.IsZeroPseudoVersion(e.replaced.Version) {
+				suggestArg = e.replaced.String()
+			}
+			return fmt.Sprintf("module %s provides package %s and is replaced but not required; to add it:\n\tgo get %s", e.replaced.Path, e.Path, suggestArg)
+		}
+
+		message := fmt.Sprintf("no required module provides package %s", e.Path)
+		if e.QueryErr != nil {
+			return fmt.Sprintf("%s: %v", message, e.QueryErr)
+		}
+		if e.ImportingMainModule.Path != "" && e.ImportingMainModule != MainModules.ModContainingCWD() {
+			return fmt.Sprintf("%s; to add it:\n\tcd %s\n\tgo get %s", message, MainModules.ModRoot(e.ImportingMainModule), e.Path)
+		}
+		return fmt.Sprintf("%s; to add it:\n\tgo get %s", message, e.Path)
+	}
+
+	if e.newMissingVersion != "" {
+		return fmt.Sprintf("package %s provided by %s at latest version %s but not at required version %s", e.Path, e.Module.Path, e.Module.Version, e.newMissingVersion)
+	}
+
+	return fmt.Sprintf("missing module for import: %s@%s provides %s", e.Module.Path, e.Module.Version, e.Path)
+}
+
+func (e *ImportMissingError) Unwrap() error {
+	return e.QueryErr
+}
+
+func (e *ImportMissingError) ImportPath() string {
+	return e.Path
+}
+
+// An AmbiguousImportError indicates an import of a package found in multiple
+// modules in the build list, or found in both the main module and its vendor
+// directory.
+type AmbiguousImportError struct {
+	importPath string
+	Dirs       []string
+	Modules    []module.Version // Either empty or 1:1 with Dirs.
+}
+
+func (e *AmbiguousImportError) ImportPath() string {
+	return e.importPath
+}
+
+func (e *AmbiguousImportError) Error() string {
+	locType := "modules"
+	if len(e.Modules) == 0 {
+		locType = "directories"
+	}
+
+	var buf strings.Builder
+	fmt.Fprintf(&buf, "ambiguous import: found package %s in multiple %s:", e.importPath, locType)
+
+	for i, dir := range e.Dirs {
+		buf.WriteString("\n\t")
+		if i < len(e.Modules) {
+			m := e.Modules[i]
+			buf.WriteString(m.Path)
+			if m.Version != "" {
+				fmt.Fprintf(&buf, " %s", m.Version)
+			}
+			fmt.Fprintf(&buf, " (%s)", dir)
+		} else {
+			buf.WriteString(dir)
+		}
+	}
+
+	return buf.String()
+}
+
+// A DirectImportFromImplicitDependencyError indicates a package directly
+// imported by a package or test in the main module that is satisfied by a
+// dependency that is not explicit in the main module's go.mod file.
+type DirectImportFromImplicitDependencyError struct {
+	ImporterPath string
+	ImportedPath string
+	Module       module.Version
+}
+
+func (e *DirectImportFromImplicitDependencyError) Error() string {
+	return fmt.Sprintf("package %s imports %s from implicitly required module; to add missing requirements, run:\n\tgo get %s@%s", e.ImporterPath, e.ImportedPath, e.Module.Path, e.Module.Version)
+}
+
+func (e *DirectImportFromImplicitDependencyError) ImportPath() string {
+	return e.ImporterPath
+}
+
+// ImportMissingSumError is reported in readonly mode when we need to check
+// if a module contains a package, but we don't have a sum for its .zip file.
+// We might need sums for multiple modules to verify the package is unique.
+//
+// TODO(#43653): consolidate multiple errors of this type into a single error
+// that suggests a 'go get' command for root packages that transitively import
+// packages from modules with missing sums. load.CheckPackageErrors would be
+// a good place to consolidate errors, but we'll need to attach the import
+// stack here.
+type ImportMissingSumError struct {
+	importPath                string
+	found                     bool
+	mods                      []module.Version
+	importer, importerVersion string // optional, but used for additional context
+	importerIsTest            bool
+}
+
+func (e *ImportMissingSumError) Error() string {
+	var importParen string
+	if e.importer != "" {
+		importParen = fmt.Sprintf(" (imported by %s)", e.importer)
+	}
+	var message string
+	if e.found {
+		message = fmt.Sprintf("missing go.sum entry needed to verify package %s%s is provided by exactly one module", e.importPath, importParen)
+	} else {
+		message = fmt.Sprintf("missing go.sum entry for module providing package %s%s", e.importPath, importParen)
+	}
+	var hint string
+	if e.importer == "" {
+		// Importing package is unknown, or the missing package was named on the
+		// command line. Recommend 'go mod download' for the modules that could
+		// provide the package, since that shouldn't change go.mod.
+		if len(e.mods) > 0 {
+			args := make([]string, len(e.mods))
+			for i, mod := range e.mods {
+				args[i] = mod.Path
+			}
+			hint = fmt.Sprintf("; to add:\n\tgo mod download %s", strings.Join(args, " "))
+		}
+	} else {
+		// Importing package is known (common case). Recommend 'go get' on the
+		// current version of the importing package.
+		tFlag := ""
+		if e.importerIsTest {
+			tFlag = " -t"
+		}
+		version := ""
+		if e.importerVersion != "" {
+			version = "@" + e.importerVersion
+		}
+		hint = fmt.Sprintf("; to add:\n\tgo get%s %s%s", tFlag, e.importer, version)
+	}
+	return message + hint
+}
+
+func (e *ImportMissingSumError) ImportPath() string {
+	return e.importPath
+}
+
+type invalidImportError struct {
+	importPath string
+	err        error
+}
+
+func (e *invalidImportError) ImportPath() string {
+	return e.importPath
+}
+
+func (e *invalidImportError) Error() string {
+	return e.err.Error()
+}
+
+func (e *invalidImportError) Unwrap() error {
+	return e.err
+}
+
+// importFromModules finds the module and directory in the dependency graph of
+// rs containing the package with the given import path. If mg is nil,
+// importFromModules attempts to locate the module using only the main module
+// and the roots of rs before it loads the full graph.
+//
+// The answer must be unique: importFromModules returns an error if multiple
+// modules are observed to provide the same package.
+//
+// importFromModules can return a module with an empty m.Path, for packages in
+// the standard library.
+//
+// importFromModules can return an empty directory string, for fake packages
+// like "C" and "unsafe".
+//
+// If the package is not present in any module selected from the requirement
+// graph, importFromModules returns an *ImportMissingError.
+//
+// If the package is present in exactly one module, importFromModules will
+// return the module, its root directory, and a list of other modules that
+// lexically could have provided the package but did not.
+//
+// If skipModFile is true, the go.mod file for the package is not loaded. This
+// allows 'go mod tidy' to preserve a minor checksum-preservation bug
+// (https://go.dev/issue/56222) for modules with 'go' versions between 1.17 and
+// 1.20, preventing unnecessary go.sum churn and network access in those
+// modules.
+func importFromModules(ctx context.Context, path string, rs *Requirements, mg *ModuleGraph, skipModFile bool) (m module.Version, modroot, dir string, altMods []module.Version, err error) {
+	invalidf := func(format string, args ...interface{}) (module.Version, string, string, []module.Version, error) {
+		return module.Version{}, "", "", nil, &invalidImportError{
+			importPath: path,
+			err:        fmt.Errorf(format, args...),
+		}
+	}
+
+	if strings.Contains(path, "@") {
+		return invalidf("import path %q should not have @version", path)
+	}
+	if build.IsLocalImport(path) {
+		return invalidf("%q is relative, but relative import paths are not supported in module mode", path)
+	}
+	if filepath.IsAbs(path) {
+		return invalidf("%q is not a package path; see 'go help packages'", path)
+	}
+	if search.IsMetaPackage(path) {
+		return invalidf("%q is not an importable package; see 'go help packages'", path)
+	}
+
+	if path == "C" {
+		// There's no directory for import "C".
+		return module.Version{}, "", "", nil, nil
+	}
+	// Before any further lookup, check that the path is valid.
+	if err := module.CheckImportPath(path); err != nil {
+		return module.Version{}, "", "", nil, &invalidImportError{importPath: path, err: err}
+	}
+
+	// Check each module on the build list.
+	var dirs, roots []string
+	var mods []module.Version
+
+	// Is the package in the standard library?
+	pathIsStd := search.IsStandardImportPath(path)
+	if pathIsStd && modindex.IsStandardPackage(cfg.GOROOT, cfg.BuildContext.Compiler, path) {
+		for _, mainModule := range MainModules.Versions() {
+			if MainModules.InGorootSrc(mainModule) {
+				if dir, ok, err := dirInModule(path, MainModules.PathPrefix(mainModule), MainModules.ModRoot(mainModule), true); err != nil {
+					return module.Version{}, MainModules.ModRoot(mainModule), dir, nil, err
+				} else if ok {
+					return mainModule, MainModules.ModRoot(mainModule), dir, nil, nil
+				}
+			}
+		}
+		dir := filepath.Join(cfg.GOROOTsrc, path)
+		modroot = cfg.GOROOTsrc
+		if str.HasPathPrefix(path, "cmd") {
+			modroot = filepath.Join(cfg.GOROOTsrc, "cmd")
+		}
+		dirs = append(dirs, dir)
+		roots = append(roots, modroot)
+		mods = append(mods, module.Version{})
+	}
+	// -mod=vendor is special.
+	// Everything must be in the main module or the main module's vendor directory.
+	if cfg.BuildMod == "vendor" {
+		mainModule := MainModules.mustGetSingleMainModule()
+		modRoot := MainModules.ModRoot(mainModule)
+		var mainErr error
+		if modRoot != "" {
+			mainDir, mainOK, err := dirInModule(path, MainModules.PathPrefix(mainModule), modRoot, true)
+			mainErr = err
+			if mainOK {
+				mods = append(mods, mainModule)
+				dirs = append(dirs, mainDir)
+				roots = append(roots, modRoot)
+			}
+			vendorDir, vendorOK, _ := dirInModule(path, "", filepath.Join(modRoot, "vendor"), false)
+			if vendorOK {
+				readVendorList(mainModule)
+				mods = append(mods, vendorPkgModule[path])
+				dirs = append(dirs, vendorDir)
+				roots = append(roots, modRoot)
+			}
+		}
+
+		if len(dirs) > 1 {
+			return module.Version{}, modRoot, "", nil, &AmbiguousImportError{importPath: path, Dirs: dirs}
+		}
+
+		if mainErr != nil {
+			return module.Version{}, "", "", nil, mainErr
+		}
+
+		if len(dirs) == 0 {
+			return module.Version{}, modRoot, "", nil, &ImportMissingError{Path: path}
+		}
+
+		return mods[0], roots[0], dirs[0], nil, nil
+	}
+
+	// Iterate over possible modules for the path, not all selected modules.
+	// Iterating over selected modules would make the overall loading time
+	// O(M × P) for M modules providing P imported packages, whereas iterating
+	// over path prefixes is only O(P × k) with maximum path depth k. For
+	// large projects both M and P may be very large (note that M ≤ P), but k
+	// will tend to remain smallish (if for no other reason than filesystem
+	// path limitations).
+	//
+	// We perform this iteration either one or two times. If mg is initially nil,
+	// then we first attempt to load the package using only the main module and
+	// its root requirements. If that does not identify the package, or if mg is
+	// already non-nil, then we attempt to load the package using the full
+	// requirements in mg.
+	for {
+		var sumErrMods, altMods []module.Version
+		for prefix := path; prefix != "."; prefix = pathpkg.Dir(prefix) {
+			if gover.IsToolchain(prefix) {
+				// Do not use the synthetic "go" module for "go/ast".
+				continue
+			}
+			var (
+				v  string
+				ok bool
+			)
+			if mg == nil {
+				v, ok = rs.rootSelected(prefix)
+			} else {
+				v, ok = mg.Selected(prefix), true
+			}
+			if !ok || v == "none" {
+				continue
+			}
+			m := module.Version{Path: prefix, Version: v}
+
+			root, isLocal, err := fetch(ctx, m)
+			if err != nil {
+				if sumErr := (*sumMissingError)(nil); errors.As(err, &sumErr) {
+					// We are missing a sum needed to fetch a module in the build list.
+					// We can't verify that the package is unique, and we may not find
+					// the package at all. Keep checking other modules to decide which
+					// error to report. Multiple sums may be missing if we need to look in
+					// multiple nested modules to resolve the import; we'll report them all.
+					sumErrMods = append(sumErrMods, m)
+					continue
+				}
+				// Report fetch error.
+				// Note that we don't know for sure this module is necessary,
+				// but it certainly _could_ provide the package, and even if we
+				// continue the loop and find the package in some other module,
+				// we need to look at this module to make sure the import is
+				// not ambiguous.
+				return module.Version{}, "", "", nil, err
+			}
+			if dir, ok, err := dirInModule(path, m.Path, root, isLocal); err != nil {
+				return module.Version{}, "", "", nil, err
+			} else if ok {
+				mods = append(mods, m)
+				roots = append(roots, root)
+				dirs = append(dirs, dir)
+			} else {
+				altMods = append(altMods, m)
+			}
+		}
+
+		if len(mods) > 1 {
+			// We produce the list of directories from longest to shortest candidate
+			// module path, but the AmbiguousImportError should report them from
+			// shortest to longest. Reverse them now.
+			for i := 0; i < len(mods)/2; i++ {
+				j := len(mods) - 1 - i
+				mods[i], mods[j] = mods[j], mods[i]
+				roots[i], roots[j] = roots[j], roots[i]
+				dirs[i], dirs[j] = dirs[j], dirs[i]
+			}
+			return module.Version{}, "", "", nil, &AmbiguousImportError{importPath: path, Dirs: dirs, Modules: mods}
+		}
+
+		if len(sumErrMods) > 0 {
+			for i := 0; i < len(sumErrMods)/2; i++ {
+				j := len(sumErrMods) - 1 - i
+				sumErrMods[i], sumErrMods[j] = sumErrMods[j], sumErrMods[i]
+			}
+			return module.Version{}, "", "", nil, &ImportMissingSumError{
+				importPath: path,
+				mods:       sumErrMods,
+				found:      len(mods) > 0,
+			}
+		}
+
+		if len(mods) == 1 {
+			// We've found the unique module containing the package.
+			// However, in order to actually compile it we need to know what
+			// Go language version to use, which requires its go.mod file.
+			//
+			// If the module graph is pruned and this is a test-only dependency
+			// of a package in "all", we didn't necessarily load that file
+			// when we read the module graph, so do it now to be sure.
+			if !skipModFile && cfg.BuildMod != "vendor" && mods[0].Path != "" && !MainModules.Contains(mods[0].Path) {
+				if _, err := goModSummary(mods[0]); err != nil {
+					return module.Version{}, "", "", nil, err
+				}
+			}
+			return mods[0], roots[0], dirs[0], altMods, nil
+		}
+
+		if mg != nil {
+			// We checked the full module graph and still didn't find the
+			// requested package.
+			var queryErr error
+			if !HasModRoot() {
+				queryErr = ErrNoModRoot
+			}
+			return module.Version{}, "", "", nil, &ImportMissingError{Path: path, QueryErr: queryErr, isStd: pathIsStd}
+		}
+
+		// So far we've checked the root dependencies.
+		// Load the full module graph and try again.
+		mg, err = rs.Graph(ctx)
+		if err != nil {
+			// We might be missing one or more transitive (implicit) dependencies from
+			// the module graph, so we can't return an ImportMissingError here — one
+			// of the missing modules might actually contain the package in question,
+			// in which case we shouldn't go looking for it in some new dependency.
+			return module.Version{}, "", "", nil, err
+		}
+	}
+}
+
+// queryImport attempts to locate a module that can be added to the current
+// build list to provide the package with the given import path.
+//
+// Unlike QueryPattern, queryImport prefers to add a replaced version of a
+// module *before* checking the proxies for a version to add.
+func queryImport(ctx context.Context, path string, rs *Requirements) (module.Version, error) {
+	// To avoid spurious remote fetches, try the latest replacement for each
+	// module (golang.org/issue/26241).
+	var mods []module.Version
+	if MainModules != nil { // TODO(#48912): Ensure MainModules exists at this point, and remove the check.
+		for mp, mv := range MainModules.HighestReplaced() {
+			if !maybeInModule(path, mp) {
+				continue
+			}
+			if mv == "" {
+				// The only replacement is a wildcard that doesn't specify a version, so
+				// synthesize a pseudo-version with an appropriate major version and a
+				// timestamp below any real timestamp. That way, if the main module is
+				// used from within some other module, the user will be able to upgrade
+				// the requirement to any real version they choose.
+				if _, pathMajor, ok := module.SplitPathVersion(mp); ok && len(pathMajor) > 0 {
+					mv = module.ZeroPseudoVersion(pathMajor[1:])
+				} else {
+					mv = module.ZeroPseudoVersion("v0")
+				}
+			}
+			mg, err := rs.Graph(ctx)
+			if err != nil {
+				return module.Version{}, err
+			}
+			if gover.ModCompare(mp, mg.Selected(mp), mv) >= 0 {
+				// We can't resolve the import by adding mp@mv to the module graph,
+				// because the selected version of mp is already at least mv.
+				continue
+			}
+			mods = append(mods, module.Version{Path: mp, Version: mv})
+		}
+	}
+
+	// Every module path in mods is a prefix of the import path.
+	// As in QueryPattern, prefer the longest prefix that satisfies the import.
+	sort.Slice(mods, func(i, j int) bool {
+		return len(mods[i].Path) > len(mods[j].Path)
+	})
+	for _, m := range mods {
+		root, isLocal, err := fetch(ctx, m)
+		if err != nil {
+			if sumErr := (*sumMissingError)(nil); errors.As(err, &sumErr) {
+				return module.Version{}, &ImportMissingSumError{importPath: path}
+			}
+			return module.Version{}, err
+		}
+		if _, ok, err := dirInModule(path, m.Path, root, isLocal); err != nil {
+			return m, err
+		} else if ok {
+			if cfg.BuildMod == "readonly" {
+				return module.Version{}, &ImportMissingError{Path: path, replaced: m}
+			}
+			return m, nil
+		}
+	}
+	if len(mods) > 0 && module.CheckPath(path) != nil {
+		// The package path is not valid to fetch remotely,
+		// so it can only exist in a replaced module,
+		// and we know from the above loop that it is not.
+		replacement := Replacement(mods[0])
+		return module.Version{}, &PackageNotInModuleError{
+			Mod:         mods[0],
+			Query:       "latest",
+			Pattern:     path,
+			Replacement: replacement,
+		}
+	}
+
+	if search.IsStandardImportPath(path) {
+		// This package isn't in the standard library, isn't in any module already
+		// in the build list, and isn't in any other module that the user has
+		// shimmed in via a "replace" directive.
+		// Moreover, the import path is reserved for the standard library, so
+		// QueryPattern cannot possibly find a module containing this package.
+		//
+		// Instead of trying QueryPattern, report an ImportMissingError immediately.
+		return module.Version{}, &ImportMissingError{Path: path, isStd: true}
+	}
+
+	if (cfg.BuildMod == "readonly" || cfg.BuildMod == "vendor") && !allowMissingModuleImports {
+		// In readonly mode, we can't write go.mod, so we shouldn't try to look up
+		// the module. If readonly mode was enabled explicitly, include that in
+		// the error message.
+		// In vendor mode, we cannot use the network or module cache, so we
+		// shouldn't try to look up the module
+		var queryErr error
+		if cfg.BuildModExplicit {
+			queryErr = fmt.Errorf("import lookup disabled by -mod=%s", cfg.BuildMod)
+		} else if cfg.BuildModReason != "" {
+			queryErr = fmt.Errorf("import lookup disabled by -mod=%s\n\t(%s)", cfg.BuildMod, cfg.BuildModReason)
+		}
+		return module.Version{}, &ImportMissingError{Path: path, QueryErr: queryErr}
+	}
+
+	// Look up module containing the package, for addition to the build list.
+	// Goal is to determine the module, download it to dir,
+	// and return m, dir, ImportMissingError.
+	fmt.Fprintf(os.Stderr, "go: finding module for package %s\n", path)
+
+	mg, err := rs.Graph(ctx)
+	if err != nil {
+		return module.Version{}, err
+	}
+
+	candidates, err := QueryPackages(ctx, path, "latest", mg.Selected, CheckAllowed)
+	if err != nil {
+		if errors.Is(err, fs.ErrNotExist) {
+			// Return "cannot find module providing package […]" instead of whatever
+			// low-level error QueryPattern produced.
+			return module.Version{}, &ImportMissingError{Path: path, QueryErr: err}
+		} else {
+			return module.Version{}, err
+		}
+	}
+
+	candidate0MissingVersion := ""
+	for i, c := range candidates {
+		if v := mg.Selected(c.Mod.Path); gover.ModCompare(c.Mod.Path, v, c.Mod.Version) > 0 {
+			// QueryPattern proposed that we add module c.Mod to provide the package,
+			// but we already depend on a newer version of that module (and that
+			// version doesn't have the package).
+			//
+			// This typically happens when a package is present at the "@latest"
+			// version (e.g., v1.0.0) of a module, but we have a newer version
+			// of the same module in the build list (e.g., v1.0.1-beta), and
+			// the package is not present there.
+			if i == 0 {
+				candidate0MissingVersion = v
+			}
+			continue
+		}
+		return c.Mod, nil
+	}
+	return module.Version{}, &ImportMissingError{
+		Path:              path,
+		Module:            candidates[0].Mod,
+		newMissingVersion: candidate0MissingVersion,
+	}
+}
+
+// maybeInModule reports whether, syntactically,
+// a package with the given import path could be supplied
+// by a module with the given module path (mpath).
+func maybeInModule(path, mpath string) bool {
+	return mpath == path ||
+		len(path) > len(mpath) && path[len(mpath)] == '/' && path[:len(mpath)] == mpath
+}
+
+var (
+	haveGoModCache   par.Cache[string, bool]    // dir → bool
+	haveGoFilesCache par.ErrCache[string, bool] // dir → haveGoFiles
+)
+
+// dirInModule locates the directory that would hold the package named by the given path,
+// if it were in the module with module path mpath and root mdir.
+// If path is syntactically not within mpath,
+// or if mdir is a local file tree (isLocal == true) and the directory
+// that would hold path is in a sub-module (covered by a go.mod below mdir),
+// dirInModule returns "", false, nil.
+//
+// Otherwise, dirInModule returns the name of the directory where
+// Go source files would be expected, along with a boolean indicating
+// whether there are in fact Go source files in that directory.
+// A non-nil error indicates that the existence of the directory and/or
+// source files could not be determined, for example due to a permission error.
+func dirInModule(path, mpath, mdir string, isLocal bool) (dir string, haveGoFiles bool, err error) {
+	// Determine where to expect the package.
+	if path == mpath {
+		dir = mdir
+	} else if mpath == "" { // vendor directory
+		dir = filepath.Join(mdir, path)
+	} else if len(path) > len(mpath) && path[len(mpath)] == '/' && path[:len(mpath)] == mpath {
+		dir = filepath.Join(mdir, path[len(mpath)+1:])
+	} else {
+		return "", false, nil
+	}
+
+	// Check that there aren't other modules in the way.
+	// This check is unnecessary inside the module cache
+	// and important to skip in the vendor directory,
+	// where all the module trees have been overlaid.
+	// So we only check local module trees
+	// (the main module, and any directory trees pointed at by replace directives).
+	if isLocal {
+		for d := dir; d != mdir && len(d) > len(mdir); {
+			haveGoMod := haveGoModCache.Do(d, func() bool {
+				fi, err := fsys.Stat(filepath.Join(d, "go.mod"))
+				return err == nil && !fi.IsDir()
+			})
+
+			if haveGoMod {
+				return "", false, nil
+			}
+			parent := filepath.Dir(d)
+			if parent == d {
+				// Break the loop, as otherwise we'd loop
+				// forever if d=="." and mdir=="".
+				break
+			}
+			d = parent
+		}
+	}
+
+	// Now committed to returning dir (not "").
+
+	// Are there Go source files in the directory?
+	// We don't care about build tags, not even "+build ignore".
+	// We're just looking for a plausible directory.
+	haveGoFiles, err = haveGoFilesCache.Do(dir, func() (bool, error) {
+		// modindex.GetPackage will return ErrNotIndexed for any directories which
+		// are reached through a symlink, so that they will be handled by
+		// fsys.IsDirWithGoFiles below.
+		if ip, err := modindex.GetPackage(mdir, dir); err == nil {
+			return ip.IsDirWithGoFiles()
+		} else if !errors.Is(err, modindex.ErrNotIndexed) {
+			return false, err
+		}
+		return fsys.IsDirWithGoFiles(dir)
+	})
+
+	return dir, haveGoFiles, err
+}
+
+// fetch downloads the given module (or its replacement)
+// and returns its location.
+//
+// The isLocal return value reports whether the replacement,
+// if any, is local to the filesystem.
+func fetch(ctx context.Context, mod module.Version) (dir string, isLocal bool, err error) {
+	if modRoot := MainModules.ModRoot(mod); modRoot != "" {
+		return modRoot, true, nil
+	}
+	if r := Replacement(mod); r.Path != "" {
+		if r.Version == "" {
+			dir = r.Path
+			if !filepath.IsAbs(dir) {
+				dir = filepath.Join(replaceRelativeTo(), dir)
+			}
+			// Ensure that the replacement directory actually exists:
+			// dirInModule does not report errors for missing modules,
+			// so if we don't report the error now, later failures will be
+			// very mysterious.
+			if _, err := fsys.Stat(dir); err != nil {
+				// TODO(bcmills): We should also read dir/go.mod here and check its Go version,
+				// and return a gover.TooNewError if appropriate.
+
+				if os.IsNotExist(err) {
+					// Semantically the module version itself “exists” — we just don't
+					// have its source code. Remove the equivalence to os.ErrNotExist,
+					// and make the message more concise while we're at it.
+					err = fmt.Errorf("replacement directory %s does not exist", r.Path)
+				} else {
+					err = fmt.Errorf("replacement directory %s: %w", r.Path, err)
+				}
+				return dir, true, module.VersionError(mod, err)
+			}
+			return dir, true, nil
+		}
+		mod = r
+	}
+
+	if mustHaveSums() && !modfetch.HaveSum(mod) {
+		return "", false, module.VersionError(mod, &sumMissingError{})
+	}
+
+	dir, err = modfetch.Download(ctx, mod)
+	return dir, false, err
+}
+
+// mustHaveSums reports whether we require that all checksums
+// needed to load or build packages are already present in the go.sum file.
+func mustHaveSums() bool {
+	return HasModRoot() && cfg.BuildMod == "readonly" && !inWorkspaceMode()
+}
+
+type sumMissingError struct {
+	suggestion string
+}
+
+func (e *sumMissingError) Error() string {
+	return "missing go.sum entry" + e.suggestion
+}
diff --git a/src/cmd/go/internal/modload/import_test.go b/src/cmd/go/internal/modload/import_test.go
new file mode 100644
index 0000000..eb4f5d6
--- /dev/null
+++ b/src/cmd/go/internal/modload/import_test.go
@@ -0,0 +1,97 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"context"
+	"internal/testenv"
+	"regexp"
+	"strings"
+	"testing"
+
+	"golang.org/x/mod/module"
+)
+
+var importTests = []struct {
+	path string
+	m    module.Version
+	err  string
+}{
+	{
+		path: "golang.org/x/net/context",
+		m: module.Version{
+			Path: "golang.org/x/net",
+		},
+	},
+	{
+		path: "golang.org/x/net",
+		err:  `module golang.org/x/net@.* found \(v[01]\.\d+\.\d+\), but does not contain package golang.org/x/net`,
+	},
+	{
+		path: "golang.org/x/text",
+		m: module.Version{
+			Path: "golang.org/x/text",
+		},
+	},
+	{
+		path: "github.com/rsc/quote/buggy",
+		m: module.Version{
+			Path:    "github.com/rsc/quote",
+			Version: "v1.5.2",
+		},
+	},
+	{
+		path: "github.com/rsc/quote",
+		m: module.Version{
+			Path:    "github.com/rsc/quote",
+			Version: "v1.5.2",
+		},
+	},
+	{
+		path: "golang.org/x/foo/bar",
+		err:  "cannot find module providing package golang.org/x/foo/bar",
+	},
+}
+
+func TestQueryImport(t *testing.T) {
+	testenv.MustHaveExternalNetwork(t)
+	testenv.MustHaveExecPath(t, "git")
+
+	oldAllowMissingModuleImports := allowMissingModuleImports
+	oldRootMode := RootMode
+	defer func() {
+		allowMissingModuleImports = oldAllowMissingModuleImports
+		RootMode = oldRootMode
+	}()
+	allowMissingModuleImports = true
+	RootMode = NoRoot
+
+	ctx := context.Background()
+	rs := LoadModFile(ctx)
+
+	for _, tt := range importTests {
+		t.Run(strings.ReplaceAll(tt.path, "/", "_"), func(t *testing.T) {
+			// Note that there is no build list, so Import should always fail.
+			m, err := queryImport(ctx, tt.path, rs)
+
+			if tt.err == "" {
+				if err != nil {
+					t.Fatalf("queryImport(_, %q): %v", tt.path, err)
+				}
+			} else {
+				if err == nil {
+					t.Fatalf("queryImport(_, %q) = %v, nil; expected error", tt.path, m)
+				}
+				if !regexp.MustCompile(tt.err).MatchString(err.Error()) {
+					t.Fatalf("queryImport(_, %q): error %q, want error matching %#q", tt.path, err, tt.err)
+				}
+			}
+
+			if m.Path != tt.m.Path || (tt.m.Version != "" && m.Version != tt.m.Version) {
+				t.Errorf("queryImport(_, %q) = %v, _; want %v", tt.path, m, tt.m)
+			}
+		})
+	}
+}
diff --git a/src/cmd/go/internal/modload/init.go b/src/cmd/go/internal/modload/init.go
new file mode 100644
index 0000000..1c6f7d9
--- /dev/null
+++ b/src/cmd/go/internal/modload/init.go
@@ -0,0 +1,1979 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"internal/lazyregexp"
+	"os"
+	"path"
+	"path/filepath"
+	"slices"
+	"strconv"
+	"strings"
+	"sync"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/lockedfile"
+	"cmd/go/internal/modconv"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/search"
+
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/module"
+)
+
+// Variables set by other packages.
+//
+// TODO(#40775): See if these can be plumbed as explicit parameters.
+var (
+	// RootMode determines whether a module root is needed.
+	RootMode Root
+
+	// ForceUseModules may be set to force modules to be enabled when
+	// GO111MODULE=auto or to report an error when GO111MODULE=off.
+	ForceUseModules bool
+
+	allowMissingModuleImports bool
+
+	// ExplicitWriteGoMod prevents LoadPackages, ListModules, and other functions
+	// from updating go.mod and go.sum or reporting errors when updates are
+	// needed. A package should set this if it would cause go.mod to be written
+	// multiple times (for example, 'go get' calls LoadPackages multiple times) or
+	// if it needs some other operation to be successful before go.mod and go.sum
+	// can be written (for example, 'go mod download' must download modules before
+	// adding sums to go.sum). Packages that set this are responsible for calling
+	// WriteGoMod explicitly.
+	ExplicitWriteGoMod bool
+)
+
+// Variables set in Init.
+var (
+	initialized bool
+
+	// These are primarily used to initialize the MainModules, and should be
+	// eventually superseded by them but are still used in cases where the module
+	// roots are required but MainModules hasn't been initialized yet. Set to
+	// the modRoots of the main modules.
+	// modRoots != nil implies len(modRoots) > 0
+	modRoots []string
+	gopath   string
+)
+
+// EnterModule resets MainModules and requirements to refer to just this one module.
+func EnterModule(ctx context.Context, enterModroot string) {
+	MainModules = nil // reset MainModules
+	requirements = nil
+	workFilePath = "" // Force module mode
+	modfetch.Reset()
+
+	modRoots = []string{enterModroot}
+	LoadModFile(ctx)
+}
+
+// Variable set in InitWorkfile
+var (
+	// Set to the path to the go.work file, or "" if workspace mode is disabled.
+	workFilePath string
+)
+
+type MainModuleSet struct {
+	// versions are the module.Version values of each of the main modules.
+	// For each of them, the Path fields are ordinary module paths and the Version
+	// fields are empty strings.
+	// versions is clipped (len=cap).
+	versions []module.Version
+
+	// modRoot maps each module in versions to its absolute filesystem path.
+	modRoot map[module.Version]string
+
+	// pathPrefix is the path prefix for packages in the module, without a trailing
+	// slash. For most modules, pathPrefix is just version.Path, but the
+	// standard-library module "std" has an empty prefix.
+	pathPrefix map[module.Version]string
+
+	// inGorootSrc caches whether modRoot is within GOROOT/src.
+	// The "std" module is special within GOROOT/src, but not otherwise.
+	inGorootSrc map[module.Version]bool
+
+	modFiles map[module.Version]*modfile.File
+
+	modContainingCWD module.Version
+
+	workFile *modfile.WorkFile
+
+	workFileReplaceMap map[module.Version]module.Version
+	// highest replaced version of each module path; empty string for wildcard-only replacements
+	highestReplaced map[string]string
+
+	indexMu sync.Mutex
+	indices map[module.Version]*modFileIndex
+}
+
+func (mms *MainModuleSet) PathPrefix(m module.Version) string {
+	return mms.pathPrefix[m]
+}
+
+// Versions returns the module.Version values of each of the main modules.
+// For each of them, the Path fields are ordinary module paths and the Version
+// fields are empty strings.
+// Callers should not modify the returned slice.
+func (mms *MainModuleSet) Versions() []module.Version {
+	if mms == nil {
+		return nil
+	}
+	return mms.versions
+}
+
+func (mms *MainModuleSet) Contains(path string) bool {
+	if mms == nil {
+		return false
+	}
+	for _, v := range mms.versions {
+		if v.Path == path {
+			return true
+		}
+	}
+	return false
+}
+
+func (mms *MainModuleSet) ModRoot(m module.Version) string {
+	if mms == nil {
+		return ""
+	}
+	return mms.modRoot[m]
+}
+
+func (mms *MainModuleSet) InGorootSrc(m module.Version) bool {
+	if mms == nil {
+		return false
+	}
+	return mms.inGorootSrc[m]
+}
+
+func (mms *MainModuleSet) mustGetSingleMainModule() module.Version {
+	if mms == nil || len(mms.versions) == 0 {
+		panic("internal error: mustGetSingleMainModule called in context with no main modules")
+	}
+	if len(mms.versions) != 1 {
+		if inWorkspaceMode() {
+			panic("internal error: mustGetSingleMainModule called in workspace mode")
+		} else {
+			panic("internal error: multiple main modules present outside of workspace mode")
+		}
+	}
+	return mms.versions[0]
+}
+
+func (mms *MainModuleSet) GetSingleIndexOrNil() *modFileIndex {
+	if mms == nil {
+		return nil
+	}
+	if len(mms.versions) == 0 {
+		return nil
+	}
+	return mms.indices[mms.mustGetSingleMainModule()]
+}
+
+func (mms *MainModuleSet) Index(m module.Version) *modFileIndex {
+	mms.indexMu.Lock()
+	defer mms.indexMu.Unlock()
+	return mms.indices[m]
+}
+
+func (mms *MainModuleSet) SetIndex(m module.Version, index *modFileIndex) {
+	mms.indexMu.Lock()
+	defer mms.indexMu.Unlock()
+	mms.indices[m] = index
+}
+
+func (mms *MainModuleSet) ModFile(m module.Version) *modfile.File {
+	return mms.modFiles[m]
+}
+
+func (mms *MainModuleSet) Len() int {
+	if mms == nil {
+		return 0
+	}
+	return len(mms.versions)
+}
+
+// ModContainingCWD returns the main module containing the working directory,
+// or module.Version{} if none of the main modules contain the working
+// directory.
+func (mms *MainModuleSet) ModContainingCWD() module.Version {
+	return mms.modContainingCWD
+}
+
+func (mms *MainModuleSet) HighestReplaced() map[string]string {
+	return mms.highestReplaced
+}
+
+// GoVersion returns the go version set on the single module, in module mode,
+// or the go.work file in workspace mode.
+func (mms *MainModuleSet) GoVersion() string {
+	if inWorkspaceMode() {
+		return gover.FromGoWork(mms.workFile)
+	}
+	if mms != nil && len(mms.versions) == 1 {
+		f := mms.ModFile(mms.mustGetSingleMainModule())
+		if f == nil {
+			// Special case: we are outside a module, like 'go run x.go'.
+			// Assume the local Go version.
+			// TODO(#49228): Clean this up; see loadModFile.
+			return gover.Local()
+		}
+		return gover.FromGoMod(f)
+	}
+	return gover.DefaultGoModVersion
+}
+
+// Toolchain returns the toolchain set on the single module, in module mode,
+// or the go.work file in workspace mode.
+func (mms *MainModuleSet) Toolchain() string {
+	if inWorkspaceMode() {
+		if mms.workFile != nil && mms.workFile.Toolchain != nil {
+			return mms.workFile.Toolchain.Name
+		}
+		return "go" + mms.GoVersion()
+	}
+	if mms != nil && len(mms.versions) == 1 {
+		f := mms.ModFile(mms.mustGetSingleMainModule())
+		if f == nil {
+			// Special case: we are outside a module, like 'go run x.go'.
+			// Assume the local Go version.
+			// TODO(#49228): Clean this up; see loadModFile.
+			return gover.LocalToolchain()
+		}
+		if f.Toolchain != nil {
+			return f.Toolchain.Name
+		}
+	}
+	return "go" + mms.GoVersion()
+}
+
+func (mms *MainModuleSet) WorkFileReplaceMap() map[module.Version]module.Version {
+	return mms.workFileReplaceMap
+}
+
+var MainModules *MainModuleSet
+
+type Root int
+
+const (
+	// AutoRoot is the default for most commands. modload.Init will look for
+	// a go.mod file in the current directory or any parent. If none is found,
+	// modules may be disabled (GO111MODULE=auto) or commands may run in a
+	// limited module mode.
+	AutoRoot Root = iota
+
+	// NoRoot is used for commands that run in module mode and ignore any go.mod
+	// file the current directory or in parent directories.
+	NoRoot
+
+	// NeedRoot is used for commands that must run in module mode and don't
+	// make sense without a main module.
+	NeedRoot
+)
+
+// ModFile returns the parsed go.mod file.
+//
+// Note that after calling LoadPackages or LoadModGraph,
+// the require statements in the modfile.File are no longer
+// the source of truth and will be ignored: edits made directly
+// will be lost at the next call to WriteGoMod.
+// To make permanent changes to the require statements
+// in go.mod, edit it before loading.
+func ModFile() *modfile.File {
+	Init()
+	modFile := MainModules.ModFile(MainModules.mustGetSingleMainModule())
+	if modFile == nil {
+		die()
+	}
+	return modFile
+}
+
+func BinDir() string {
+	Init()
+	if cfg.GOBIN != "" {
+		return cfg.GOBIN
+	}
+	if gopath == "" {
+		return ""
+	}
+	return filepath.Join(gopath, "bin")
+}
+
+// InitWorkfile initializes the workFilePath variable for commands that
+// operate in workspace mode. It should not be called by other commands,
+// for example 'go mod tidy', that don't operate in workspace mode.
+func InitWorkfile() {
+	workFilePath = FindGoWork(base.Cwd())
+}
+
+// FindGoWork returns the name of the go.work file for this command,
+// or the empty string if there isn't one.
+// Most code should use Init and Enabled rather than use this directly.
+// It is exported mainly for Go toolchain switching, which must process
+// the go.work very early at startup.
+func FindGoWork(wd string) string {
+	if RootMode == NoRoot {
+		return ""
+	}
+
+	switch gowork := cfg.Getenv("GOWORK"); gowork {
+	case "off":
+		return ""
+	case "", "auto":
+		return findWorkspaceFile(wd)
+	default:
+		if !filepath.IsAbs(gowork) {
+			base.Fatalf("go: invalid GOWORK: not an absolute path")
+		}
+		return gowork
+	}
+}
+
+// WorkFilePath returns the absolute path of the go.work file, or "" if not in
+// workspace mode. WorkFilePath must be called after InitWorkfile.
+func WorkFilePath() string {
+	return workFilePath
+}
+
+// Reset clears all the initialized, cached state about the use of modules,
+// so that we can start over.
+func Reset() {
+	initialized = false
+	ForceUseModules = false
+	RootMode = 0
+	modRoots = nil
+	cfg.ModulesEnabled = false
+	MainModules = nil
+	requirements = nil
+	workFilePath = ""
+	modfetch.Reset()
+}
+
+// Init determines whether module mode is enabled, locates the root of the
+// current module (if any), sets environment variables for Git subprocesses, and
+// configures the cfg, codehost, load, modfetch, and search packages for use
+// with modules.
+func Init() {
+	if initialized {
+		return
+	}
+	initialized = true
+
+	// Keep in sync with WillBeEnabled. We perform extra validation here, and
+	// there are lots of diagnostics and side effects, so we can't use
+	// WillBeEnabled directly.
+	var mustUseModules bool
+	env := cfg.Getenv("GO111MODULE")
+	switch env {
+	default:
+		base.Fatalf("go: unknown environment setting GO111MODULE=%s", env)
+	case "auto":
+		mustUseModules = ForceUseModules
+	case "on", "":
+		mustUseModules = true
+	case "off":
+		if ForceUseModules {
+			base.Fatalf("go: modules disabled by GO111MODULE=off; see 'go help modules'")
+		}
+		mustUseModules = false
+		return
+	}
+
+	if err := fsys.Init(base.Cwd()); err != nil {
+		base.Fatal(err)
+	}
+
+	// Disable any prompting for passwords by Git.
+	// Only has an effect for 2.3.0 or later, but avoiding
+	// the prompt in earlier versions is just too hard.
+	// If user has explicitly set GIT_TERMINAL_PROMPT=1, keep
+	// prompting.
+	// See golang.org/issue/9341 and golang.org/issue/12706.
+	if os.Getenv("GIT_TERMINAL_PROMPT") == "" {
+		os.Setenv("GIT_TERMINAL_PROMPT", "0")
+	}
+
+	// Disable any ssh connection pooling by Git.
+	// If a Git subprocess forks a child into the background to cache a new connection,
+	// that child keeps stdout/stderr open. After the Git subprocess exits,
+	// os /exec expects to be able to read from the stdout/stderr pipe
+	// until EOF to get all the data that the Git subprocess wrote before exiting.
+	// The EOF doesn't come until the child exits too, because the child
+	// is holding the write end of the pipe.
+	// This is unfortunate, but it has come up at least twice
+	// (see golang.org/issue/13453 and golang.org/issue/16104)
+	// and confuses users when it does.
+	// If the user has explicitly set GIT_SSH or GIT_SSH_COMMAND,
+	// assume they know what they are doing and don't step on it.
+	// But default to turning off ControlMaster.
+	if os.Getenv("GIT_SSH") == "" && os.Getenv("GIT_SSH_COMMAND") == "" {
+		os.Setenv("GIT_SSH_COMMAND", "ssh -o ControlMaster=no -o BatchMode=yes")
+	}
+
+	if os.Getenv("GCM_INTERACTIVE") == "" {
+		os.Setenv("GCM_INTERACTIVE", "never")
+	}
+	if modRoots != nil {
+		// modRoot set before Init was called ("go mod init" does this).
+		// No need to search for go.mod.
+	} else if RootMode == NoRoot {
+		if cfg.ModFile != "" && !base.InGOFLAGS("-modfile") {
+			base.Fatalf("go: -modfile cannot be used with commands that ignore the current module")
+		}
+		modRoots = nil
+	} else if workFilePath != "" {
+		// We're in workspace mode, which implies module mode.
+		if cfg.ModFile != "" {
+			base.Fatalf("go: -modfile cannot be used in workspace mode")
+		}
+	} else {
+		if modRoot := findModuleRoot(base.Cwd()); modRoot == "" {
+			if cfg.ModFile != "" {
+				base.Fatalf("go: cannot find main module, but -modfile was set.\n\t-modfile cannot be used to set the module root directory.")
+			}
+			if RootMode == NeedRoot {
+				base.Fatal(ErrNoModRoot)
+			}
+			if !mustUseModules {
+				// GO111MODULE is 'auto', and we can't find a module root.
+				// Stay in GOPATH mode.
+				return
+			}
+		} else if search.InDir(modRoot, os.TempDir()) == "." {
+			// If you create /tmp/go.mod for experimenting,
+			// then any tests that create work directories under /tmp
+			// will find it and get modules when they're not expecting them.
+			// It's a bit of a peculiar thing to disallow but quite mysterious
+			// when it happens. See golang.org/issue/26708.
+			fmt.Fprintf(os.Stderr, "go: warning: ignoring go.mod in system temp root %v\n", os.TempDir())
+			if RootMode == NeedRoot {
+				base.Fatal(ErrNoModRoot)
+			}
+			if !mustUseModules {
+				return
+			}
+		} else {
+			modRoots = []string{modRoot}
+		}
+	}
+	if cfg.ModFile != "" && !strings.HasSuffix(cfg.ModFile, ".mod") {
+		base.Fatalf("go: -modfile=%s: file does not have .mod extension", cfg.ModFile)
+	}
+
+	// We're in module mode. Set any global variables that need to be set.
+	cfg.ModulesEnabled = true
+	setDefaultBuildMod()
+	list := filepath.SplitList(cfg.BuildContext.GOPATH)
+	if len(list) > 0 && list[0] != "" {
+		gopath = list[0]
+		if _, err := fsys.Stat(filepath.Join(gopath, "go.mod")); err == nil {
+			base.Fatalf("$GOPATH/go.mod exists but should not")
+		}
+	}
+}
+
+// WillBeEnabled checks whether modules should be enabled but does not
+// initialize modules by installing hooks. If Init has already been called,
+// WillBeEnabled returns the same result as Enabled.
+//
+// This function is needed to break a cycle. The main package needs to know
+// whether modules are enabled in order to install the module or GOPATH version
+// of 'go get', but Init reads the -modfile flag in 'go get', so it shouldn't
+// be called until the command is installed and flags are parsed. Instead of
+// calling Init and Enabled, the main package can call this function.
+func WillBeEnabled() bool {
+	if modRoots != nil || cfg.ModulesEnabled {
+		// Already enabled.
+		return true
+	}
+	if initialized {
+		// Initialized, not enabled.
+		return false
+	}
+
+	// Keep in sync with Init. Init does extra validation and prints warnings or
+	// exits, so it can't call this function directly.
+	env := cfg.Getenv("GO111MODULE")
+	switch env {
+	case "on", "":
+		return true
+	case "auto":
+		break
+	default:
+		return false
+	}
+
+	return FindGoMod(base.Cwd()) != ""
+}
+
+// FindGoMod returns the name of the go.mod file for this command,
+// or the empty string if there isn't one.
+// Most code should use Init and Enabled rather than use this directly.
+// It is exported mainly for Go toolchain switching, which must process
+// the go.mod very early at startup.
+func FindGoMod(wd string) string {
+	modRoot := findModuleRoot(wd)
+	if modRoot == "" {
+		// GO111MODULE is 'auto', and we can't find a module root.
+		// Stay in GOPATH mode.
+		return ""
+	}
+	if search.InDir(modRoot, os.TempDir()) == "." {
+		// If you create /tmp/go.mod for experimenting,
+		// then any tests that create work directories under /tmp
+		// will find it and get modules when they're not expecting them.
+		// It's a bit of a peculiar thing to disallow but quite mysterious
+		// when it happens. See golang.org/issue/26708.
+		return ""
+	}
+	return filepath.Join(modRoot, "go.mod")
+}
+
+// Enabled reports whether modules are (or must be) enabled.
+// If modules are enabled but there is no main module, Enabled returns true
+// and then the first use of module information will call die
+// (usually through MustModRoot).
+func Enabled() bool {
+	Init()
+	return modRoots != nil || cfg.ModulesEnabled
+}
+
+func VendorDir() string {
+	return filepath.Join(MainModules.ModRoot(MainModules.mustGetSingleMainModule()), "vendor")
+}
+
+func inWorkspaceMode() bool {
+	if !initialized {
+		panic("inWorkspaceMode called before modload.Init called")
+	}
+	if !Enabled() {
+		return false
+	}
+	return workFilePath != ""
+}
+
+// HasModRoot reports whether a main module is present.
+// HasModRoot may return false even if Enabled returns true: for example, 'get'
+// does not require a main module.
+func HasModRoot() bool {
+	Init()
+	return modRoots != nil
+}
+
+// MustHaveModRoot checks that a main module or main modules are present,
+// and calls base.Fatalf if there are no main modules.
+func MustHaveModRoot() {
+	Init()
+	if !HasModRoot() {
+		die()
+	}
+}
+
+// ModFilePath returns the path that would be used for the go.mod
+// file, if in module mode. ModFilePath calls base.Fatalf if there is no main
+// module, even if -modfile is set.
+func ModFilePath() string {
+	MustHaveModRoot()
+	return modFilePath(findModuleRoot(base.Cwd()))
+}
+
+func modFilePath(modRoot string) string {
+	if cfg.ModFile != "" {
+		return cfg.ModFile
+	}
+	return filepath.Join(modRoot, "go.mod")
+}
+
+func die() {
+	if cfg.Getenv("GO111MODULE") == "off" {
+		base.Fatalf("go: modules disabled by GO111MODULE=off; see 'go help modules'")
+	}
+	if inWorkspaceMode() {
+		base.Fatalf("go: no modules were found in the current workspace; see 'go help work'")
+	}
+	if dir, name := findAltConfig(base.Cwd()); dir != "" {
+		rel, err := filepath.Rel(base.Cwd(), dir)
+		if err != nil {
+			rel = dir
+		}
+		cdCmd := ""
+		if rel != "." {
+			cdCmd = fmt.Sprintf("cd %s && ", rel)
+		}
+		base.Fatalf("go: cannot find main module, but found %s in %s\n\tto create a module there, run:\n\t%sgo mod init", name, dir, cdCmd)
+	}
+	base.Fatal(ErrNoModRoot)
+}
+
+var ErrNoModRoot = errors.New("go.mod file not found in current directory or any parent directory; see 'go help modules'")
+
+type goModDirtyError struct{}
+
+func (goModDirtyError) Error() string {
+	if cfg.BuildModExplicit {
+		return fmt.Sprintf("updates to go.mod needed, disabled by -mod=%v; to update it:\n\tgo mod tidy", cfg.BuildMod)
+	}
+	if cfg.BuildModReason != "" {
+		return fmt.Sprintf("updates to go.mod needed, disabled by -mod=%s\n\t(%s)\n\tto update it:\n\tgo mod tidy", cfg.BuildMod, cfg.BuildModReason)
+	}
+	return "updates to go.mod needed; to update it:\n\tgo mod tidy"
+}
+
+var errGoModDirty error = goModDirtyError{}
+
+func loadWorkFile(path string) (workFile *modfile.WorkFile, modRoots []string, err error) {
+	workDir := filepath.Dir(path)
+	wf, err := ReadWorkFile(path)
+	if err != nil {
+		return nil, nil, err
+	}
+	seen := map[string]bool{}
+	for _, d := range wf.Use {
+		modRoot := d.Path
+		if !filepath.IsAbs(modRoot) {
+			modRoot = filepath.Join(workDir, modRoot)
+		}
+
+		if seen[modRoot] {
+			return nil, nil, fmt.Errorf("path %s appears multiple times in workspace", modRoot)
+		}
+		seen[modRoot] = true
+		modRoots = append(modRoots, modRoot)
+	}
+
+	return wf, modRoots, nil
+}
+
+// ReadWorkFile reads and parses the go.work file at the given path.
+func ReadWorkFile(path string) (*modfile.WorkFile, error) {
+	workData, err := os.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+
+	f, err := modfile.ParseWork(path, workData, nil)
+	if err != nil {
+		return nil, err
+	}
+	if f.Go != nil && gover.Compare(f.Go.Version, gover.Local()) > 0 && cfg.CmdName != "work edit" {
+		base.Fatal(&gover.TooNewError{What: base.ShortPath(path), GoVersion: f.Go.Version})
+	}
+	return f, nil
+}
+
+// WriteWorkFile cleans and writes out the go.work file to the given path.
+func WriteWorkFile(path string, wf *modfile.WorkFile) error {
+	wf.SortBlocks()
+	wf.Cleanup()
+	out := modfile.Format(wf.Syntax)
+
+	return os.WriteFile(path, out, 0666)
+}
+
+// UpdateWorkGoVersion updates the go line in wf to be at least goVers,
+// reporting whether it changed the file.
+func UpdateWorkGoVersion(wf *modfile.WorkFile, goVers string) (changed bool) {
+	old := gover.FromGoWork(wf)
+	if gover.Compare(old, goVers) >= 0 {
+		return false
+	}
+
+	wf.AddGoStmt(goVers)
+
+	// We wrote a new go line. For reproducibility,
+	// if the toolchain running right now is newer than the new toolchain line,
+	// update the toolchain line to record the newer toolchain.
+	// The user never sets the toolchain explicitly in a 'go work' command,
+	// so this is only happening as a result of a go or toolchain line found
+	// in a module.
+	// If the toolchain running right now is a dev toolchain (like "go1.21")
+	// writing 'toolchain go1.21' will not be useful, since that's not an actual
+	// toolchain you can download and run. In that case fall back to at least
+	// checking that the toolchain is new enough for the Go version.
+	toolchain := "go" + old
+	if wf.Toolchain != nil {
+		toolchain = wf.Toolchain.Name
+	}
+	if gover.IsLang(gover.Local()) {
+		toolchain = gover.ToolchainMax(toolchain, "go"+goVers)
+	} else {
+		toolchain = gover.ToolchainMax(toolchain, "go"+gover.Local())
+	}
+
+	// Drop the toolchain line if it is implied by the go line
+	// or if it is asking for a toolchain older than Go 1.21,
+	// which will not understand the toolchain line.
+	if toolchain == "go"+goVers || gover.Compare(gover.FromToolchain(toolchain), gover.GoStrictVersion) < 0 {
+		wf.DropToolchainStmt()
+	} else {
+		wf.AddToolchainStmt(toolchain)
+	}
+	return true
+}
+
+// UpdateWorkFile updates comments on directory directives in the go.work
+// file to include the associated module path.
+func UpdateWorkFile(wf *modfile.WorkFile) {
+	missingModulePaths := map[string]string{} // module directory listed in file -> abspath modroot
+
+	for _, d := range wf.Use {
+		if d.Path == "" {
+			continue // d is marked for deletion.
+		}
+		modRoot := d.Path
+		if d.ModulePath == "" {
+			missingModulePaths[d.Path] = modRoot
+		}
+	}
+
+	// Clean up and annotate directories.
+	// TODO(matloob): update x/mod to actually add module paths.
+	for moddir, absmodroot := range missingModulePaths {
+		_, f, err := ReadModFile(filepath.Join(absmodroot, "go.mod"), nil)
+		if err != nil {
+			continue // Error will be reported if modules are loaded.
+		}
+		wf.AddUse(moddir, f.Module.Mod.Path)
+	}
+}
+
+// LoadModFile sets Target and, if there is a main module, parses the initial
+// build list from its go.mod file.
+//
+// LoadModFile may make changes in memory, like adding a go directive and
+// ensuring requirements are consistent. The caller is responsible for ensuring
+// those changes are written to disk by calling LoadPackages or ListModules
+// (unless ExplicitWriteGoMod is set) or by calling WriteGoMod directly.
+//
+// As a side-effect, LoadModFile may change cfg.BuildMod to "vendor" if
+// -mod wasn't set explicitly and automatic vendoring should be enabled.
+//
+// If LoadModFile or CreateModFile has already been called, LoadModFile returns
+// the existing in-memory requirements (rather than re-reading them from disk).
+//
+// LoadModFile checks the roots of the module graph for consistency with each
+// other, but unlike LoadModGraph does not load the full module graph or check
+// it for global consistency. Most callers outside of the modload package should
+// use LoadModGraph instead.
+func LoadModFile(ctx context.Context) *Requirements {
+	rs, err := loadModFile(ctx, nil)
+	if err != nil {
+		base.Fatal(err)
+	}
+	return rs
+}
+
+func loadModFile(ctx context.Context, opts *PackageOpts) (*Requirements, error) {
+	if requirements != nil {
+		return requirements, nil
+	}
+
+	Init()
+	var workFile *modfile.WorkFile
+	if inWorkspaceMode() {
+		var err error
+		workFile, modRoots, err = loadWorkFile(workFilePath)
+		if err != nil {
+			return nil, fmt.Errorf("reading go.work: %w", err)
+		}
+		for _, modRoot := range modRoots {
+			sumFile := strings.TrimSuffix(modFilePath(modRoot), ".mod") + ".sum"
+			modfetch.WorkspaceGoSumFiles = append(modfetch.WorkspaceGoSumFiles, sumFile)
+		}
+		modfetch.GoSumFile = workFilePath + ".sum"
+	} else if len(modRoots) == 0 {
+		// We're in module mode, but not inside a module.
+		//
+		// Commands like 'go build', 'go run', 'go list' have no go.mod file to
+		// read or write. They would need to find and download the latest versions
+		// of a potentially large number of modules with no way to save version
+		// information. We can succeed slowly (but not reproducibly), but that's
+		// not usually a good experience.
+		//
+		// Instead, we forbid resolving import paths to modules other than std and
+		// cmd. Users may still build packages specified with .go files on the
+		// command line, but they'll see an error if those files import anything
+		// outside std.
+		//
+		// This can be overridden by calling AllowMissingModuleImports.
+		// For example, 'go get' does this, since it is expected to resolve paths.
+		//
+		// See golang.org/issue/32027.
+	} else {
+		modfetch.GoSumFile = strings.TrimSuffix(modFilePath(modRoots[0]), ".mod") + ".sum"
+	}
+	if len(modRoots) == 0 {
+		// TODO(#49228): Instead of creating a fake module with an empty modroot,
+		// make MainModules.Len() == 0 mean that we're in module mode but not inside
+		// any module.
+		mainModule := module.Version{Path: "command-line-arguments"}
+		MainModules = makeMainModules([]module.Version{mainModule}, []string{""}, []*modfile.File{nil}, []*modFileIndex{nil}, nil)
+		var (
+			goVersion string
+			pruning   modPruning
+			roots     []module.Version
+			direct    = map[string]bool{"go": true}
+		)
+		if inWorkspaceMode() {
+			// Since we are in a workspace, the Go version for the synthetic
+			// "command-line-arguments" module must not exceed the Go version
+			// for the workspace.
+			goVersion = MainModules.GoVersion()
+			pruning = workspace
+			roots = []module.Version{
+				mainModule,
+				{Path: "go", Version: goVersion},
+				{Path: "toolchain", Version: gover.LocalToolchain()},
+			}
+		} else {
+			goVersion = gover.Local()
+			pruning = pruningForGoVersion(goVersion)
+			roots = []module.Version{
+				{Path: "go", Version: goVersion},
+				{Path: "toolchain", Version: gover.LocalToolchain()},
+			}
+		}
+		rawGoVersion.Store(mainModule, goVersion)
+		requirements = newRequirements(pruning, roots, direct)
+		if cfg.BuildMod == "vendor" {
+			// For issue 56536: Some users may have GOFLAGS=-mod=vendor set.
+			// Make sure it behaves as though the fake module is vendored
+			// with no dependencies.
+			requirements.initVendor(nil)
+		}
+		return requirements, nil
+	}
+
+	var modFiles []*modfile.File
+	var mainModules []module.Version
+	var indices []*modFileIndex
+	var errs []error
+	for _, modroot := range modRoots {
+		gomod := modFilePath(modroot)
+		var fixed bool
+		data, f, err := ReadModFile(gomod, fixVersion(ctx, &fixed))
+		if err != nil {
+			if inWorkspaceMode() {
+				if tooNew, ok := err.(*gover.TooNewError); ok && !strings.HasPrefix(cfg.CmdName, "work ") {
+					// Switching to a newer toolchain won't help - the go.work has the wrong version.
+					// Report this more specific error, unless we are a command like 'go work use'
+					// or 'go work sync', which will fix the problem after the caller sees the TooNewError
+					// and switches to a newer toolchain.
+					err = errWorkTooOld(gomod, workFile, tooNew.GoVersion)
+				} else {
+					err = fmt.Errorf("cannot load module %s listed in go.work file: %w",
+						base.ShortPath(filepath.Dir(gomod)), err)
+				}
+			}
+			errs = append(errs, err)
+			continue
+		}
+		if inWorkspaceMode() && !strings.HasPrefix(cfg.CmdName, "work ") {
+			// Refuse to use workspace if its go version is too old.
+			// Disable this check if we are a workspace command like work use or work sync,
+			// which will fix the problem.
+			mv := gover.FromGoMod(f)
+			wv := gover.FromGoWork(workFile)
+			if gover.Compare(mv, wv) > 0 && gover.Compare(mv, gover.GoStrictVersion) >= 0 {
+				errs = append(errs, errWorkTooOld(gomod, workFile, mv))
+				continue
+			}
+		}
+
+		modFiles = append(modFiles, f)
+		mainModule := f.Module.Mod
+		mainModules = append(mainModules, mainModule)
+		indices = append(indices, indexModFile(data, f, mainModule, fixed))
+
+		if err := module.CheckImportPath(f.Module.Mod.Path); err != nil {
+			if pathErr, ok := err.(*module.InvalidPathError); ok {
+				pathErr.Kind = "module"
+			}
+			errs = append(errs, err)
+		}
+	}
+	if len(errs) > 0 {
+		return nil, errors.Join(errs...)
+	}
+
+	MainModules = makeMainModules(mainModules, modRoots, modFiles, indices, workFile)
+	setDefaultBuildMod() // possibly enable automatic vendoring
+	rs := requirementsFromModFiles(ctx, workFile, modFiles, opts)
+
+	if inWorkspaceMode() {
+		// We don't need to do anything for vendor or update the mod file so
+		// return early.
+		requirements = rs
+		return rs, nil
+	}
+
+	mainModule := MainModules.mustGetSingleMainModule()
+
+	if cfg.BuildMod == "vendor" {
+		readVendorList(mainModule)
+		index := MainModules.Index(mainModule)
+		modFile := MainModules.ModFile(mainModule)
+		checkVendorConsistency(index, modFile)
+		rs.initVendor(vendorList)
+	}
+
+	if rs.hasRedundantRoot() {
+		// If any module path appears more than once in the roots, we know that the
+		// go.mod file needs to be updated even though we have not yet loaded any
+		// transitive dependencies.
+		var err error
+		rs, err = updateRoots(ctx, rs.direct, rs, nil, nil, false)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	if MainModules.Index(mainModule).goVersion == "" && rs.pruning != workspace {
+		// TODO(#45551): Do something more principled instead of checking
+		// cfg.CmdName directly here.
+		if cfg.BuildMod == "mod" && cfg.CmdName != "mod graph" && cfg.CmdName != "mod why" {
+			// go line is missing from go.mod; add one there and add to derived requirements.
+			v := gover.Local()
+			if opts != nil && opts.TidyGoVersion != "" {
+				v = opts.TidyGoVersion
+			}
+			addGoStmt(MainModules.ModFile(mainModule), mainModule, v)
+			rs = overrideRoots(ctx, rs, []module.Version{{Path: "go", Version: v}})
+
+			// We need to add a 'go' version to the go.mod file, but we must assume
+			// that its existing contents match something between Go 1.11 and 1.16.
+			// Go 1.11 through 1.16 do not support graph pruning, but the latest Go
+			// version uses a pruned module graph — so we need to convert the
+			// requirements to support pruning.
+			if gover.Compare(v, gover.ExplicitIndirectVersion) >= 0 {
+				var err error
+				rs, err = convertPruning(ctx, rs, pruned)
+				if err != nil {
+					return nil, err
+				}
+			}
+		} else {
+			rawGoVersion.Store(mainModule, gover.DefaultGoModVersion)
+		}
+	}
+
+	requirements = rs
+	return requirements, nil
+}
+
+func errWorkTooOld(gomod string, wf *modfile.WorkFile, goVers string) error {
+	return fmt.Errorf("module %s listed in go.work file requires go >= %s, but go.work lists go %s; to update it:\n\tgo work use",
+		base.ShortPath(filepath.Dir(gomod)), goVers, gover.FromGoWork(wf))
+}
+
+// CreateModFile initializes a new module by creating a go.mod file.
+//
+// If modPath is empty, CreateModFile will attempt to infer the path from the
+// directory location within GOPATH.
+//
+// If a vendoring configuration file is present, CreateModFile will attempt to
+// translate it to go.mod directives. The resulting build list may not be
+// exactly the same as in the legacy configuration (for example, we can't get
+// packages at multiple versions from the same module).
+func CreateModFile(ctx context.Context, modPath string) {
+	modRoot := base.Cwd()
+	modRoots = []string{modRoot}
+	Init()
+	modFilePath := modFilePath(modRoot)
+	if _, err := fsys.Stat(modFilePath); err == nil {
+		base.Fatalf("go: %s already exists", modFilePath)
+	}
+
+	if modPath == "" {
+		var err error
+		modPath, err = findModulePath(modRoot)
+		if err != nil {
+			base.Fatal(err)
+		}
+	} else if err := module.CheckImportPath(modPath); err != nil {
+		if pathErr, ok := err.(*module.InvalidPathError); ok {
+			pathErr.Kind = "module"
+			// Same as build.IsLocalPath()
+			if pathErr.Path == "." || pathErr.Path == ".." ||
+				strings.HasPrefix(pathErr.Path, "./") || strings.HasPrefix(pathErr.Path, "../") {
+				pathErr.Err = errors.New("is a local import path")
+			}
+		}
+		base.Fatal(err)
+	} else if _, _, ok := module.SplitPathVersion(modPath); !ok {
+		if strings.HasPrefix(modPath, "gopkg.in/") {
+			invalidMajorVersionMsg := fmt.Errorf("module paths beginning with gopkg.in/ must always have a major version suffix in the form of .vN:\n\tgo mod init %s", suggestGopkgIn(modPath))
+			base.Fatalf(`go: invalid module path "%v": %v`, modPath, invalidMajorVersionMsg)
+		}
+		invalidMajorVersionMsg := fmt.Errorf("major version suffixes must be in the form of /vN and are only allowed for v2 or later:\n\tgo mod init %s", suggestModulePath(modPath))
+		base.Fatalf(`go: invalid module path "%v": %v`, modPath, invalidMajorVersionMsg)
+	}
+
+	fmt.Fprintf(os.Stderr, "go: creating new go.mod: module %s\n", modPath)
+	modFile := new(modfile.File)
+	modFile.AddModuleStmt(modPath)
+	MainModules = makeMainModules([]module.Version{modFile.Module.Mod}, []string{modRoot}, []*modfile.File{modFile}, []*modFileIndex{nil}, nil)
+	addGoStmt(modFile, modFile.Module.Mod, gover.Local()) // Add the go directive before converted module requirements.
+
+	convertedFrom, err := convertLegacyConfig(modFile, modRoot)
+	if convertedFrom != "" {
+		fmt.Fprintf(os.Stderr, "go: copying requirements from %s\n", base.ShortPath(convertedFrom))
+	}
+	if err != nil {
+		base.Fatal(err)
+	}
+
+	rs := requirementsFromModFiles(ctx, nil, []*modfile.File{modFile}, nil)
+	rs, err = updateRoots(ctx, rs.direct, rs, nil, nil, false)
+	if err != nil {
+		base.Fatal(err)
+	}
+	requirements = rs
+	if err := commitRequirements(ctx, WriteOpts{}); err != nil {
+		base.Fatal(err)
+	}
+
+	// Suggest running 'go mod tidy' unless the project is empty. Even if we
+	// imported all the correct requirements above, we're probably missing
+	// some sums, so the next build command in -mod=readonly will likely fail.
+	//
+	// We look for non-hidden .go files or subdirectories to determine whether
+	// this is an existing project. Walking the tree for packages would be more
+	// accurate, but could take much longer.
+	empty := true
+	files, _ := os.ReadDir(modRoot)
+	for _, f := range files {
+		name := f.Name()
+		if strings.HasPrefix(name, ".") || strings.HasPrefix(name, "_") {
+			continue
+		}
+		if strings.HasSuffix(name, ".go") || f.IsDir() {
+			empty = false
+			break
+		}
+	}
+	if !empty {
+		fmt.Fprintf(os.Stderr, "go: to add module requirements and sums:\n\tgo mod tidy\n")
+	}
+}
+
+// fixVersion returns a modfile.VersionFixer implemented using the Query function.
+//
+// It resolves commit hashes and branch names to versions,
+// canonicalizes versions that appeared in early vgo drafts,
+// and does nothing for versions that already appear to be canonical.
+//
+// The VersionFixer sets 'fixed' if it ever returns a non-canonical version.
+func fixVersion(ctx context.Context, fixed *bool) modfile.VersionFixer {
+	return func(path, vers string) (resolved string, err error) {
+		defer func() {
+			if err == nil && resolved != vers {
+				*fixed = true
+			}
+		}()
+
+		// Special case: remove the old -gopkgin- hack.
+		if strings.HasPrefix(path, "gopkg.in/") && strings.Contains(vers, "-gopkgin-") {
+			vers = vers[strings.Index(vers, "-gopkgin-")+len("-gopkgin-"):]
+		}
+
+		// fixVersion is called speculatively on every
+		// module, version pair from every go.mod file.
+		// Avoid the query if it looks OK.
+		_, pathMajor, ok := module.SplitPathVersion(path)
+		if !ok {
+			return "", &module.ModuleError{
+				Path: path,
+				Err: &module.InvalidVersionError{
+					Version: vers,
+					Err:     fmt.Errorf("malformed module path %q", path),
+				},
+			}
+		}
+		if vers != "" && module.CanonicalVersion(vers) == vers {
+			if err := module.CheckPathMajor(vers, pathMajor); err != nil {
+				return "", module.VersionError(module.Version{Path: path, Version: vers}, err)
+			}
+			return vers, nil
+		}
+
+		info, err := Query(ctx, path, vers, "", nil)
+		if err != nil {
+			return "", err
+		}
+		return info.Version, nil
+	}
+}
+
+// AllowMissingModuleImports allows import paths to be resolved to modules
+// when there is no module root. Normally, this is forbidden because it's slow
+// and there's no way to make the result reproducible, but some commands
+// like 'go get' are expected to do this.
+//
+// This function affects the default cfg.BuildMod when outside of a module,
+// so it can only be called prior to Init.
+func AllowMissingModuleImports() {
+	if initialized {
+		panic("AllowMissingModuleImports after Init")
+	}
+	allowMissingModuleImports = true
+}
+
+// makeMainModules creates a MainModuleSet and associated variables according to
+// the given main modules.
+func makeMainModules(ms []module.Version, rootDirs []string, modFiles []*modfile.File, indices []*modFileIndex, workFile *modfile.WorkFile) *MainModuleSet {
+	for _, m := range ms {
+		if m.Version != "" {
+			panic("mainModulesCalled with module.Version with non empty Version field: " + fmt.Sprintf("%#v", m))
+		}
+	}
+	modRootContainingCWD := findModuleRoot(base.Cwd())
+	mainModules := &MainModuleSet{
+		versions:        slices.Clip(ms),
+		inGorootSrc:     map[module.Version]bool{},
+		pathPrefix:      map[module.Version]string{},
+		modRoot:         map[module.Version]string{},
+		modFiles:        map[module.Version]*modfile.File{},
+		indices:         map[module.Version]*modFileIndex{},
+		highestReplaced: map[string]string{},
+		workFile:        workFile,
+	}
+	var workFileReplaces []*modfile.Replace
+	if workFile != nil {
+		workFileReplaces = workFile.Replace
+		mainModules.workFileReplaceMap = toReplaceMap(workFile.Replace)
+	}
+	mainModulePaths := make(map[string]bool)
+	for _, m := range ms {
+		if mainModulePaths[m.Path] {
+			base.Errorf("go: module %s appears multiple times in workspace", m.Path)
+		}
+		mainModulePaths[m.Path] = true
+	}
+	replacedByWorkFile := make(map[string]bool)
+	replacements := make(map[module.Version]module.Version)
+	for _, r := range workFileReplaces {
+		if mainModulePaths[r.Old.Path] && r.Old.Version == "" {
+			base.Errorf("go: workspace module %v is replaced at all versions in the go.work file. To fix, remove the replacement from the go.work file or specify the version at which to replace the module.", r.Old.Path)
+		}
+		replacedByWorkFile[r.Old.Path] = true
+		v, ok := mainModules.highestReplaced[r.Old.Path]
+		if !ok || gover.ModCompare(r.Old.Path, r.Old.Version, v) > 0 {
+			mainModules.highestReplaced[r.Old.Path] = r.Old.Version
+		}
+		replacements[r.Old] = r.New
+	}
+	for i, m := range ms {
+		mainModules.pathPrefix[m] = m.Path
+		mainModules.modRoot[m] = rootDirs[i]
+		mainModules.modFiles[m] = modFiles[i]
+		mainModules.indices[m] = indices[i]
+
+		if mainModules.modRoot[m] == modRootContainingCWD {
+			mainModules.modContainingCWD = m
+		}
+
+		if rel := search.InDir(rootDirs[i], cfg.GOROOTsrc); rel != "" {
+			mainModules.inGorootSrc[m] = true
+			if m.Path == "std" {
+				// The "std" module in GOROOT/src is the Go standard library. Unlike other
+				// modules, the packages in the "std" module have no import-path prefix.
+				//
+				// Modules named "std" outside of GOROOT/src do not receive this special
+				// treatment, so it is possible to run 'go test .' in other GOROOTs to
+				// test individual packages using a combination of the modified package
+				// and the ordinary standard library.
+				// (See https://golang.org/issue/30756.)
+				mainModules.pathPrefix[m] = ""
+			}
+		}
+
+		if modFiles[i] != nil {
+			curModuleReplaces := make(map[module.Version]bool)
+			for _, r := range modFiles[i].Replace {
+				if replacedByWorkFile[r.Old.Path] {
+					continue
+				}
+				var newV module.Version = r.New
+				if WorkFilePath() != "" && newV.Version == "" && !filepath.IsAbs(newV.Path) {
+					// Since we are in a workspace, we may be loading replacements from
+					// multiple go.mod files. Relative paths in those replacement are
+					// relative to the go.mod file, not the workspace, so the same string
+					// may refer to two different paths and different strings may refer to
+					// the same path. Convert them all to be absolute instead.
+					//
+					// (We could do this outside of a workspace too, but it would mean that
+					// replacement paths in error strings needlessly differ from what's in
+					// the go.mod file.)
+					newV.Path = filepath.Join(rootDirs[i], newV.Path)
+				}
+				if prev, ok := replacements[r.Old]; ok && !curModuleReplaces[r.Old] && prev != newV {
+					base.Fatalf("go: conflicting replacements for %v:\n\t%v\n\t%v\nuse \"go work edit -replace %v=[override]\" to resolve", r.Old, prev, newV, r.Old)
+				}
+				curModuleReplaces[r.Old] = true
+				replacements[r.Old] = newV
+
+				v, ok := mainModules.highestReplaced[r.Old.Path]
+				if !ok || gover.ModCompare(r.Old.Path, r.Old.Version, v) > 0 {
+					mainModules.highestReplaced[r.Old.Path] = r.Old.Version
+				}
+			}
+		}
+	}
+	return mainModules
+}
+
+// requirementsFromModFiles returns the set of non-excluded requirements from
+// the global modFile.
+func requirementsFromModFiles(ctx context.Context, workFile *modfile.WorkFile, modFiles []*modfile.File, opts *PackageOpts) *Requirements {
+	var roots []module.Version
+	direct := map[string]bool{}
+	var pruning modPruning
+	var goVersion, toolchain string
+	if inWorkspaceMode() {
+		pruning = workspace
+		roots = make([]module.Version, len(MainModules.Versions()), 2+len(MainModules.Versions()))
+		copy(roots, MainModules.Versions())
+		goVersion = gover.FromGoWork(workFile)
+		if workFile.Toolchain != nil {
+			toolchain = workFile.Toolchain.Name
+		}
+	} else {
+		pruning = pruningForGoVersion(MainModules.GoVersion())
+		if len(modFiles) != 1 {
+			panic(fmt.Errorf("requirementsFromModFiles called with %v modfiles outside workspace mode", len(modFiles)))
+		}
+		modFile := modFiles[0]
+		roots = make([]module.Version, 0, 2+len(modFile.Require))
+		mm := MainModules.mustGetSingleMainModule()
+		for _, r := range modFile.Require {
+			if index := MainModules.Index(mm); index != nil && index.exclude[r.Mod] {
+				if cfg.BuildMod == "mod" {
+					fmt.Fprintf(os.Stderr, "go: dropping requirement on excluded version %s %s\n", r.Mod.Path, r.Mod.Version)
+				} else {
+					fmt.Fprintf(os.Stderr, "go: ignoring requirement on excluded version %s %s\n", r.Mod.Path, r.Mod.Version)
+				}
+				continue
+			}
+
+			roots = append(roots, r.Mod)
+			if !r.Indirect {
+				direct[r.Mod.Path] = true
+			}
+		}
+		goVersion = gover.FromGoMod(modFile)
+		if modFile.Toolchain != nil {
+			toolchain = modFile.Toolchain.Name
+		}
+	}
+
+	// Add explicit go and toolchain versions, inferring as needed.
+	roots = append(roots, module.Version{Path: "go", Version: goVersion})
+	direct["go"] = true // Every module directly uses the language and runtime.
+
+	if toolchain != "" {
+		roots = append(roots, module.Version{Path: "toolchain", Version: toolchain})
+		// Leave the toolchain as indirect: nothing in the user's module directly
+		// imports a package from the toolchain, and (like an indirect dependency in
+		// a module without graph pruning) we may remove the toolchain line
+		// automatically if the 'go' version is changed so that it implies the exact
+		// same toolchain.
+	}
+
+	gover.ModSort(roots)
+	rs := newRequirements(pruning, roots, direct)
+	return rs
+}
+
+// setDefaultBuildMod sets a default value for cfg.BuildMod if the -mod flag
+// wasn't provided. setDefaultBuildMod may be called multiple times.
+func setDefaultBuildMod() {
+	if cfg.BuildModExplicit {
+		if inWorkspaceMode() && cfg.BuildMod != "readonly" {
+			base.Fatalf("go: -mod may only be set to readonly when in workspace mode, but it is set to %q"+
+				"\n\tRemove the -mod flag to use the default readonly value,"+
+				"\n\tor set GOWORK=off to disable workspace mode.", cfg.BuildMod)
+		}
+		// Don't override an explicit '-mod=' argument.
+		return
+	}
+
+	// TODO(#40775): commands should pass in the module mode as an option
+	// to modload functions instead of relying on an implicit setting
+	// based on command name.
+	switch cfg.CmdName {
+	case "get", "mod download", "mod init", "mod tidy", "work sync":
+		// These commands are intended to update go.mod and go.sum.
+		cfg.BuildMod = "mod"
+		return
+	case "mod graph", "mod verify", "mod why":
+		// These commands should not update go.mod or go.sum, but they should be
+		// able to fetch modules not in go.sum and should not report errors if
+		// go.mod is inconsistent. They're useful for debugging, and they need
+		// to work in buggy situations.
+		cfg.BuildMod = "mod"
+		return
+	case "mod vendor":
+		cfg.BuildMod = "readonly"
+		return
+	}
+	if modRoots == nil {
+		if allowMissingModuleImports {
+			cfg.BuildMod = "mod"
+		} else {
+			cfg.BuildMod = "readonly"
+		}
+		return
+	}
+
+	if len(modRoots) == 1 && !inWorkspaceMode() {
+		index := MainModules.GetSingleIndexOrNil()
+		if fi, err := fsys.Stat(filepath.Join(modRoots[0], "vendor")); err == nil && fi.IsDir() {
+			modGo := "unspecified"
+			if index != nil && index.goVersion != "" {
+				if gover.Compare(index.goVersion, "1.14") >= 0 {
+					// The Go version is at least 1.14, and a vendor directory exists.
+					// Set -mod=vendor by default.
+					cfg.BuildMod = "vendor"
+					cfg.BuildModReason = "Go version in go.mod is at least 1.14 and vendor directory exists."
+					return
+				} else {
+					modGo = index.goVersion
+				}
+			}
+
+			// Since a vendor directory exists, we should record why we didn't use it.
+			// This message won't normally be shown, but it may appear with import errors.
+			cfg.BuildModReason = fmt.Sprintf("Go version in go.mod is %s, so vendor directory was not used.", modGo)
+		}
+	}
+
+	cfg.BuildMod = "readonly"
+}
+
+func mustHaveCompleteRequirements() bool {
+	return cfg.BuildMod != "mod" && !inWorkspaceMode()
+}
+
+// convertLegacyConfig imports module requirements from a legacy vendoring
+// configuration file, if one is present.
+func convertLegacyConfig(modFile *modfile.File, modRoot string) (from string, err error) {
+	noneSelected := func(path string) (version string) { return "none" }
+	queryPackage := func(path, rev string) (module.Version, error) {
+		pkgMods, modOnly, err := QueryPattern(context.Background(), path, rev, noneSelected, nil)
+		if err != nil {
+			return module.Version{}, err
+		}
+		if len(pkgMods) > 0 {
+			return pkgMods[0].Mod, nil
+		}
+		return modOnly.Mod, nil
+	}
+	for _, name := range altConfigs {
+		cfg := filepath.Join(modRoot, name)
+		data, err := os.ReadFile(cfg)
+		if err == nil {
+			convert := modconv.Converters[name]
+			if convert == nil {
+				return "", nil
+			}
+			cfg = filepath.ToSlash(cfg)
+			err := modconv.ConvertLegacyConfig(modFile, cfg, data, queryPackage)
+			return name, err
+		}
+	}
+	return "", nil
+}
+
+// addGoStmt adds a go directive to the go.mod file if it does not already
+// include one. The 'go' version added, if any, is the latest version supported
+// by this toolchain.
+func addGoStmt(modFile *modfile.File, mod module.Version, v string) {
+	if modFile.Go != nil && modFile.Go.Version != "" {
+		return
+	}
+	forceGoStmt(modFile, mod, v)
+}
+
+func forceGoStmt(modFile *modfile.File, mod module.Version, v string) {
+	if err := modFile.AddGoStmt(v); err != nil {
+		base.Fatalf("go: internal error: %v", err)
+	}
+	rawGoVersion.Store(mod, v)
+}
+
+var altConfigs = []string{
+	"Gopkg.lock",
+
+	"GLOCKFILE",
+	"Godeps/Godeps.json",
+	"dependencies.tsv",
+	"glide.lock",
+	"vendor.conf",
+	"vendor.yml",
+	"vendor/manifest",
+	"vendor/vendor.json",
+
+	".git/config",
+}
+
+func findModuleRoot(dir string) (roots string) {
+	if dir == "" {
+		panic("dir not set")
+	}
+	dir = filepath.Clean(dir)
+
+	// Look for enclosing go.mod.
+	for {
+		if fi, err := fsys.Stat(filepath.Join(dir, "go.mod")); err == nil && !fi.IsDir() {
+			return dir
+		}
+		d := filepath.Dir(dir)
+		if d == dir {
+			break
+		}
+		dir = d
+	}
+	return ""
+}
+
+func findWorkspaceFile(dir string) (root string) {
+	if dir == "" {
+		panic("dir not set")
+	}
+	dir = filepath.Clean(dir)
+
+	// Look for enclosing go.mod.
+	for {
+		f := filepath.Join(dir, "go.work")
+		if fi, err := fsys.Stat(f); err == nil && !fi.IsDir() {
+			return f
+		}
+		d := filepath.Dir(dir)
+		if d == dir {
+			break
+		}
+		if d == cfg.GOROOT {
+			// As a special case, don't cross GOROOT to find a go.work file.
+			// The standard library and commands built in go always use the vendored
+			// dependencies, so avoid using a most likely irrelevant go.work file.
+			return ""
+		}
+		dir = d
+	}
+	return ""
+}
+
+func findAltConfig(dir string) (root, name string) {
+	if dir == "" {
+		panic("dir not set")
+	}
+	dir = filepath.Clean(dir)
+	if rel := search.InDir(dir, cfg.BuildContext.GOROOT); rel != "" {
+		// Don't suggest creating a module from $GOROOT/.git/config
+		// or a config file found in any parent of $GOROOT (see #34191).
+		return "", ""
+	}
+	for {
+		for _, name := range altConfigs {
+			if fi, err := fsys.Stat(filepath.Join(dir, name)); err == nil && !fi.IsDir() {
+				return dir, name
+			}
+		}
+		d := filepath.Dir(dir)
+		if d == dir {
+			break
+		}
+		dir = d
+	}
+	return "", ""
+}
+
+func findModulePath(dir string) (string, error) {
+	// TODO(bcmills): once we have located a plausible module path, we should
+	// query version control (if available) to verify that it matches the major
+	// version of the most recent tag.
+	// See https://golang.org/issue/29433, https://golang.org/issue/27009, and
+	// https://golang.org/issue/31549.
+
+	// Cast about for import comments,
+	// first in top-level directory, then in subdirectories.
+	list, _ := os.ReadDir(dir)
+	for _, info := range list {
+		if info.Type().IsRegular() && strings.HasSuffix(info.Name(), ".go") {
+			if com := findImportComment(filepath.Join(dir, info.Name())); com != "" {
+				return com, nil
+			}
+		}
+	}
+	for _, info1 := range list {
+		if info1.IsDir() {
+			files, _ := os.ReadDir(filepath.Join(dir, info1.Name()))
+			for _, info2 := range files {
+				if info2.Type().IsRegular() && strings.HasSuffix(info2.Name(), ".go") {
+					if com := findImportComment(filepath.Join(dir, info1.Name(), info2.Name())); com != "" {
+						return path.Dir(com), nil
+					}
+				}
+			}
+		}
+	}
+
+	// Look for Godeps.json declaring import path.
+	data, _ := os.ReadFile(filepath.Join(dir, "Godeps/Godeps.json"))
+	var cfg1 struct{ ImportPath string }
+	json.Unmarshal(data, &cfg1)
+	if cfg1.ImportPath != "" {
+		return cfg1.ImportPath, nil
+	}
+
+	// Look for vendor.json declaring import path.
+	data, _ = os.ReadFile(filepath.Join(dir, "vendor/vendor.json"))
+	var cfg2 struct{ RootPath string }
+	json.Unmarshal(data, &cfg2)
+	if cfg2.RootPath != "" {
+		return cfg2.RootPath, nil
+	}
+
+	// Look for path in GOPATH.
+	var badPathErr error
+	for _, gpdir := range filepath.SplitList(cfg.BuildContext.GOPATH) {
+		if gpdir == "" {
+			continue
+		}
+		if rel := search.InDir(dir, filepath.Join(gpdir, "src")); rel != "" && rel != "." {
+			path := filepath.ToSlash(rel)
+			// gorelease will alert users publishing their modules to fix their paths.
+			if err := module.CheckImportPath(path); err != nil {
+				badPathErr = err
+				break
+			}
+			return path, nil
+		}
+	}
+
+	reason := "outside GOPATH, module path must be specified"
+	if badPathErr != nil {
+		// return a different error message if the module was in GOPATH, but
+		// the module path determined above would be an invalid path.
+		reason = fmt.Sprintf("bad module path inferred from directory in GOPATH: %v", badPathErr)
+	}
+	msg := `cannot determine module path for source directory %s (%s)
+
+Example usage:
+	'go mod init example.com/m' to initialize a v0 or v1 module
+	'go mod init example.com/m/v2' to initialize a v2 module
+
+Run 'go help mod init' for more information.
+`
+	return "", fmt.Errorf(msg, dir, reason)
+}
+
+var (
+	importCommentRE = lazyregexp.New(`(?m)^package[ \t]+[^ \t\r\n/]+[ \t]+//[ \t]+import[ \t]+(\"[^"]+\")[ \t]*\r?\n`)
+)
+
+func findImportComment(file string) string {
+	data, err := os.ReadFile(file)
+	if err != nil {
+		return ""
+	}
+	m := importCommentRE.FindSubmatch(data)
+	if m == nil {
+		return ""
+	}
+	path, err := strconv.Unquote(string(m[1]))
+	if err != nil {
+		return ""
+	}
+	return path
+}
+
+// WriteOpts control the behavior of WriteGoMod.
+type WriteOpts struct {
+	DropToolchain     bool // go get toolchain@none
+	ExplicitToolchain bool // go get has set explicit toolchain version
+
+	// TODO(bcmills): Make 'go mod tidy' update the go version in the Requirements
+	// instead of writing directly to the modfile.File
+	TidyWroteGo bool // Go.Version field already updated by 'go mod tidy'
+}
+
+// WriteGoMod writes the current build list back to go.mod.
+func WriteGoMod(ctx context.Context, opts WriteOpts) error {
+	requirements = LoadModFile(ctx)
+	return commitRequirements(ctx, opts)
+}
+
+// commitRequirements ensures go.mod and go.sum are up to date with the current
+// requirements.
+//
+// In "mod" mode, commitRequirements writes changes to go.mod and go.sum.
+//
+// In "readonly" and "vendor" modes, commitRequirements returns an error if
+// go.mod or go.sum are out of date in a semantically significant way.
+//
+// In workspace mode, commitRequirements only writes changes to go.work.sum.
+func commitRequirements(ctx context.Context, opts WriteOpts) (err error) {
+	if inWorkspaceMode() {
+		// go.mod files aren't updated in workspace mode, but we still want to
+		// update the go.work.sum file.
+		return modfetch.WriteGoSum(ctx, keepSums(ctx, loaded, requirements, addBuildListZipSums), mustHaveCompleteRequirements())
+	}
+	if MainModules.Len() != 1 || MainModules.ModRoot(MainModules.Versions()[0]) == "" {
+		// We aren't in a module, so we don't have anywhere to write a go.mod file.
+		return nil
+	}
+	mainModule := MainModules.mustGetSingleMainModule()
+	modFile := MainModules.ModFile(mainModule)
+	if modFile == nil {
+		// command-line-arguments has no .mod file to write.
+		return nil
+	}
+	modFilePath := modFilePath(MainModules.ModRoot(mainModule))
+
+	var list []*modfile.Require
+	toolchain := ""
+	goVersion := ""
+	for _, m := range requirements.rootModules {
+		if m.Path == "go" {
+			goVersion = m.Version
+			continue
+		}
+		if m.Path == "toolchain" {
+			toolchain = m.Version
+			continue
+		}
+		list = append(list, &modfile.Require{
+			Mod:      m,
+			Indirect: !requirements.direct[m.Path],
+		})
+	}
+
+	// Update go line.
+	// Every MVS graph we consider should have go as a root,
+	// and toolchain is either implied by the go line or explicitly a root.
+	if goVersion == "" {
+		base.Fatalf("go: internal error: missing go root module in WriteGoMod")
+	}
+	if gover.Compare(goVersion, gover.Local()) > 0 {
+		// We cannot assume that we know how to update a go.mod to a newer version.
+		return &gover.TooNewError{What: "updating go.mod", GoVersion: goVersion}
+	}
+	wroteGo := opts.TidyWroteGo
+	if !wroteGo && modFile.Go == nil || modFile.Go.Version != goVersion {
+		alwaysUpdate := cfg.BuildMod == "mod" || cfg.CmdName == "mod tidy" || cfg.CmdName == "get"
+		if modFile.Go == nil && goVersion == gover.DefaultGoModVersion && !alwaysUpdate {
+			// The go.mod has no go line, the implied default Go version matches
+			// what we've computed for the graph, and we're not in one of the
+			// traditional go.mod-updating programs, so leave it alone.
+		} else {
+			wroteGo = true
+			forceGoStmt(modFile, mainModule, goVersion)
+		}
+	}
+	if toolchain == "" {
+		toolchain = "go" + goVersion
+	}
+
+	// For reproducibility, if we are writing a new go line,
+	// and we're not explicitly modifying the toolchain line with 'go get toolchain@something',
+	// and the go version is one that supports switching toolchains,
+	// and the toolchain running right now is newer than the current toolchain line,
+	// then update the toolchain line to record the newer toolchain.
+	//
+	// TODO(#57001): This condition feels too complicated. Can we simplify it?
+	// TODO(#57001): Add more tests for toolchain lines.
+	toolVers := gover.FromToolchain(toolchain)
+	if wroteGo && !opts.DropToolchain && !opts.ExplicitToolchain &&
+		gover.Compare(goVersion, gover.GoStrictVersion) >= 0 &&
+		(gover.Compare(gover.Local(), toolVers) > 0 && !gover.IsLang(gover.Local())) {
+		toolchain = "go" + gover.Local()
+		toolVers = gover.FromToolchain(toolchain)
+	}
+
+	if opts.DropToolchain || toolchain == "go"+goVersion || (gover.Compare(toolVers, gover.GoStrictVersion) < 0 && !opts.ExplicitToolchain) {
+		// go get toolchain@none or toolchain matches go line or isn't valid; drop it.
+		// TODO(#57001): 'go get' should reject explicit toolchains below GoStrictVersion.
+		modFile.DropToolchainStmt()
+	} else {
+		modFile.AddToolchainStmt(toolchain)
+	}
+
+	// Update require blocks.
+	if gover.Compare(goVersion, gover.SeparateIndirectVersion) < 0 {
+		modFile.SetRequire(list)
+	} else {
+		modFile.SetRequireSeparateIndirect(list)
+	}
+	modFile.Cleanup()
+
+	index := MainModules.GetSingleIndexOrNil()
+	dirty := index.modFileIsDirty(modFile)
+	if dirty && cfg.BuildMod != "mod" {
+		// If we're about to fail due to -mod=readonly,
+		// prefer to report a dirty go.mod over a dirty go.sum
+		return errGoModDirty
+	}
+
+	if !dirty && cfg.CmdName != "mod tidy" {
+		// The go.mod file has the same semantic content that it had before
+		// (but not necessarily the same exact bytes).
+		// Don't write go.mod, but write go.sum in case we added or trimmed sums.
+		// 'go mod init' shouldn't write go.sum, since it will be incomplete.
+		if cfg.CmdName != "mod init" {
+			if err := modfetch.WriteGoSum(ctx, keepSums(ctx, loaded, requirements, addBuildListZipSums), mustHaveCompleteRequirements()); err != nil {
+				return err
+			}
+		}
+		return nil
+	}
+	if _, ok := fsys.OverlayPath(modFilePath); ok {
+		if dirty {
+			return errors.New("updates to go.mod needed, but go.mod is part of the overlay specified with -overlay")
+		}
+		return nil
+	}
+
+	new, err := modFile.Format()
+	if err != nil {
+		return err
+	}
+	defer func() {
+		// At this point we have determined to make the go.mod file on disk equal to new.
+		MainModules.SetIndex(mainModule, indexModFile(new, modFile, mainModule, false))
+
+		// Update go.sum after releasing the side lock and refreshing the index.
+		// 'go mod init' shouldn't write go.sum, since it will be incomplete.
+		if cfg.CmdName != "mod init" {
+			if err == nil {
+				err = modfetch.WriteGoSum(ctx, keepSums(ctx, loaded, requirements, addBuildListZipSums), mustHaveCompleteRequirements())
+			}
+		}
+	}()
+
+	// Make a best-effort attempt to acquire the side lock, only to exclude
+	// previous versions of the 'go' command from making simultaneous edits.
+	if unlock, err := modfetch.SideLock(ctx); err == nil {
+		defer unlock()
+	}
+
+	errNoChange := errors.New("no update needed")
+
+	err = lockedfile.Transform(modFilePath, func(old []byte) ([]byte, error) {
+		if bytes.Equal(old, new) {
+			// The go.mod file is already equal to new, possibly as the result of some
+			// other process.
+			return nil, errNoChange
+		}
+
+		if index != nil && !bytes.Equal(old, index.data) {
+			// The contents of the go.mod file have changed. In theory we could add all
+			// of the new modules to the build list, recompute, and check whether any
+			// module in *our* build list got bumped to a different version, but that's
+			// a lot of work for marginal benefit. Instead, fail the command: if users
+			// want to run concurrent commands, they need to start with a complete,
+			// consistent module definition.
+			return nil, fmt.Errorf("existing contents have changed since last read")
+		}
+
+		return new, nil
+	})
+
+	if err != nil && err != errNoChange {
+		return fmt.Errorf("updating go.mod: %w", err)
+	}
+	return nil
+}
+
+// keepSums returns the set of modules (and go.mod file entries) for which
+// checksums would be needed in order to reload the same set of packages
+// loaded by the most recent call to LoadPackages or ImportFromFiles,
+// including any go.mod files needed to reconstruct the MVS result
+// or identify go versions,
+// in addition to the checksums for every module in keepMods.
+func keepSums(ctx context.Context, ld *loader, rs *Requirements, which whichSums) map[module.Version]bool {
+	// Every module in the full module graph contributes its requirements,
+	// so in order to ensure that the build list itself is reproducible,
+	// we need sums for every go.mod in the graph (regardless of whether
+	// that version is selected).
+	keep := make(map[module.Version]bool)
+
+	// Add entries for modules in the build list with paths that are prefixes of
+	// paths of loaded packages. We need to retain sums for all of these modules —
+	// not just the modules containing the actual packages — in order to rule out
+	// ambiguous import errors the next time we load the package.
+	keepModSumsForZipSums := true
+	if ld == nil {
+		if gover.Compare(MainModules.GoVersion(), gover.TidyGoModSumVersion) < 0 && cfg.BuildMod != "mod" {
+			keepModSumsForZipSums = false
+		}
+	} else {
+		keepPkgGoModSums := true
+		if gover.Compare(ld.requirements.GoVersion(), gover.TidyGoModSumVersion) < 0 && (ld.Tidy || cfg.BuildMod != "mod") {
+			keepPkgGoModSums = false
+			keepModSumsForZipSums = false
+		}
+		for _, pkg := range ld.pkgs {
+			// We check pkg.mod.Path here instead of pkg.inStd because the
+			// pseudo-package "C" is not in std, but not provided by any module (and
+			// shouldn't force loading the whole module graph).
+			if pkg.testOf != nil || (pkg.mod.Path == "" && pkg.err == nil) || module.CheckImportPath(pkg.path) != nil {
+				continue
+			}
+
+			// We need the checksum for the go.mod file for pkg.mod
+			// so that we know what Go version to use to compile pkg.
+			// However, we didn't do so before Go 1.21, and the bug is relatively
+			// minor, so we maintain the previous (buggy) behavior in 'go mod tidy' to
+			// avoid introducing unnecessary churn.
+			if keepPkgGoModSums {
+				r := resolveReplacement(pkg.mod)
+				keep[modkey(r)] = true
+			}
+
+			if rs.pruning == pruned && pkg.mod.Path != "" {
+				if v, ok := rs.rootSelected(pkg.mod.Path); ok && v == pkg.mod.Version {
+					// pkg was loaded from a root module, and because the main module has
+					// a pruned module graph we do not check non-root modules for
+					// conflicts for packages that can be found in roots. So we only need
+					// the checksums for the root modules that may contain pkg, not all
+					// possible modules.
+					for prefix := pkg.path; prefix != "."; prefix = path.Dir(prefix) {
+						if v, ok := rs.rootSelected(prefix); ok && v != "none" {
+							m := module.Version{Path: prefix, Version: v}
+							r := resolveReplacement(m)
+							keep[r] = true
+						}
+					}
+					continue
+				}
+			}
+
+			mg, _ := rs.Graph(ctx)
+			for prefix := pkg.path; prefix != "."; prefix = path.Dir(prefix) {
+				if v := mg.Selected(prefix); v != "none" {
+					m := module.Version{Path: prefix, Version: v}
+					r := resolveReplacement(m)
+					keep[r] = true
+				}
+			}
+		}
+	}
+
+	if rs.graph.Load() == nil {
+		// We haven't needed to load the module graph so far.
+		// Save sums for the root modules (or their replacements), but don't
+		// incur the cost of loading the graph just to find and retain the sums.
+		for _, m := range rs.rootModules {
+			r := resolveReplacement(m)
+			keep[modkey(r)] = true
+			if which == addBuildListZipSums {
+				keep[r] = true
+			}
+		}
+	} else {
+		mg, _ := rs.Graph(ctx)
+		mg.WalkBreadthFirst(func(m module.Version) {
+			if _, ok := mg.RequiredBy(m); ok {
+				// The requirements from m's go.mod file are present in the module graph,
+				// so they are relevant to the MVS result regardless of whether m was
+				// actually selected.
+				r := resolveReplacement(m)
+				keep[modkey(r)] = true
+			}
+		})
+
+		if which == addBuildListZipSums {
+			for _, m := range mg.BuildList() {
+				r := resolveReplacement(m)
+				if keepModSumsForZipSums {
+					keep[modkey(r)] = true // we need the go version from the go.mod file to do anything useful with the zipfile
+				}
+				keep[r] = true
+			}
+		}
+	}
+
+	return keep
+}
+
+type whichSums int8
+
+const (
+	loadedZipSumsOnly = whichSums(iota)
+	addBuildListZipSums
+)
+
+// modkey returns the module.Version under which the checksum for m's go.mod
+// file is stored in the go.sum file.
+func modkey(m module.Version) module.Version {
+	return module.Version{Path: m.Path, Version: m.Version + "/go.mod"}
+}
+
+func suggestModulePath(path string) string {
+	var m string
+
+	i := len(path)
+	for i > 0 && ('0' <= path[i-1] && path[i-1] <= '9' || path[i-1] == '.') {
+		i--
+	}
+	url := path[:i]
+	url = strings.TrimSuffix(url, "/v")
+	url = strings.TrimSuffix(url, "/")
+
+	f := func(c rune) bool {
+		return c > '9' || c < '0'
+	}
+	s := strings.FieldsFunc(path[i:], f)
+	if len(s) > 0 {
+		m = s[0]
+	}
+	m = strings.TrimLeft(m, "0")
+	if m == "" || m == "1" {
+		return url + "/v2"
+	}
+
+	return url + "/v" + m
+}
+
+func suggestGopkgIn(path string) string {
+	var m string
+	i := len(path)
+	for i > 0 && (('0' <= path[i-1] && path[i-1] <= '9') || (path[i-1] == '.')) {
+		i--
+	}
+	url := path[:i]
+	url = strings.TrimSuffix(url, ".v")
+	url = strings.TrimSuffix(url, "/v")
+	url = strings.TrimSuffix(url, "/")
+
+	f := func(c rune) bool {
+		return c > '9' || c < '0'
+	}
+	s := strings.FieldsFunc(path, f)
+	if len(s) > 0 {
+		m = s[0]
+	}
+
+	m = strings.TrimLeft(m, "0")
+
+	if m == "" {
+		return url + ".v1"
+	}
+	return url + ".v" + m
+}
diff --git a/src/cmd/go/internal/modload/list.go b/src/cmd/go/internal/modload/list.go
new file mode 100644
index 0000000..e8872ba
--- /dev/null
+++ b/src/cmd/go/internal/modload/list.go
@@ -0,0 +1,311 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"runtime"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/modfetch/codehost"
+	"cmd/go/internal/modinfo"
+	"cmd/go/internal/search"
+	"cmd/internal/pkgpattern"
+
+	"golang.org/x/mod/module"
+)
+
+type ListMode int
+
+const (
+	ListU ListMode = 1 << iota
+	ListRetracted
+	ListDeprecated
+	ListVersions
+	ListRetractedVersions
+)
+
+// ListModules returns a description of the modules matching args, if known,
+// along with any error preventing additional matches from being identified.
+//
+// The returned slice can be nonempty even if the error is non-nil.
+func ListModules(ctx context.Context, args []string, mode ListMode, reuseFile string) ([]*modinfo.ModulePublic, error) {
+	var reuse map[module.Version]*modinfo.ModulePublic
+	if reuseFile != "" {
+		data, err := os.ReadFile(reuseFile)
+		if err != nil {
+			return nil, err
+		}
+		dec := json.NewDecoder(bytes.NewReader(data))
+		reuse = make(map[module.Version]*modinfo.ModulePublic)
+		for {
+			var m modinfo.ModulePublic
+			if err := dec.Decode(&m); err != nil {
+				if err == io.EOF {
+					break
+				}
+				return nil, fmt.Errorf("parsing %s: %v", reuseFile, err)
+			}
+			if m.Origin == nil || !m.Origin.Checkable() {
+				// Nothing to check to validate reuse.
+				continue
+			}
+			m.Reuse = true
+			reuse[module.Version{Path: m.Path, Version: m.Version}] = &m
+			if m.Query != "" {
+				reuse[module.Version{Path: m.Path, Version: m.Query}] = &m
+			}
+		}
+	}
+
+	rs, mods, err := listModules(ctx, LoadModFile(ctx), args, mode, reuse)
+
+	type token struct{}
+	sem := make(chan token, runtime.GOMAXPROCS(0))
+	if mode != 0 {
+		for _, m := range mods {
+			if m.Reuse {
+				continue
+			}
+			add := func(m *modinfo.ModulePublic) {
+				sem <- token{}
+				go func() {
+					if mode&ListU != 0 {
+						addUpdate(ctx, m)
+					}
+					if mode&ListVersions != 0 {
+						addVersions(ctx, m, mode&ListRetractedVersions != 0)
+					}
+					if mode&ListRetracted != 0 {
+						addRetraction(ctx, m)
+					}
+					if mode&ListDeprecated != 0 {
+						addDeprecation(ctx, m)
+					}
+					<-sem
+				}()
+			}
+
+			add(m)
+			if m.Replace != nil {
+				add(m.Replace)
+			}
+		}
+	}
+	// Fill semaphore channel to wait for all tasks to finish.
+	for n := cap(sem); n > 0; n-- {
+		sem <- token{}
+	}
+
+	if err == nil {
+		requirements = rs
+		// TODO(#61605): The extra ListU clause fixes a problem with Go 1.21rc3
+		// where "go mod tidy" and "go list -m -u all" fight over whether the go.sum
+		// should be considered up-to-date. The fix for now is to always treat the
+		// go.sum as up-to-date during list -m -u. Probably the right fix is more targeted,
+		// but in general list -u is looking up other checksums in the checksum database
+		// that won't be necessary later, so it makes sense not to write the go.sum back out.
+		if !ExplicitWriteGoMod && mode&ListU == 0 {
+			err = commitRequirements(ctx, WriteOpts{})
+		}
+	}
+	return mods, err
+}
+
+func listModules(ctx context.Context, rs *Requirements, args []string, mode ListMode, reuse map[module.Version]*modinfo.ModulePublic) (_ *Requirements, mods []*modinfo.ModulePublic, mgErr error) {
+	if len(args) == 0 {
+		var ms []*modinfo.ModulePublic
+		for _, m := range MainModules.Versions() {
+			if gover.IsToolchain(m.Path) {
+				continue
+			}
+			ms = append(ms, moduleInfo(ctx, rs, m, mode, reuse))
+		}
+		return rs, ms, nil
+	}
+
+	needFullGraph := false
+	for _, arg := range args {
+		if strings.Contains(arg, `\`) {
+			base.Fatalf("go: module paths never use backslash")
+		}
+		if search.IsRelativePath(arg) {
+			base.Fatalf("go: cannot use relative path %s to specify module", arg)
+		}
+		if arg == "all" || strings.Contains(arg, "...") {
+			needFullGraph = true
+			if !HasModRoot() {
+				base.Fatalf("go: cannot match %q: %v", arg, ErrNoModRoot)
+			}
+			continue
+		}
+		if path, vers, found := strings.Cut(arg, "@"); found {
+			if vers == "upgrade" || vers == "patch" {
+				if _, ok := rs.rootSelected(path); !ok || rs.pruning == unpruned {
+					needFullGraph = true
+					if !HasModRoot() {
+						base.Fatalf("go: cannot match %q: %v", arg, ErrNoModRoot)
+					}
+				}
+			}
+			continue
+		}
+		if _, ok := rs.rootSelected(arg); !ok || rs.pruning == unpruned {
+			needFullGraph = true
+			if mode&ListVersions == 0 && !HasModRoot() {
+				base.Fatalf("go: cannot match %q without -versions or an explicit version: %v", arg, ErrNoModRoot)
+			}
+		}
+	}
+
+	var mg *ModuleGraph
+	if needFullGraph {
+		rs, mg, mgErr = expandGraph(ctx, rs)
+	}
+
+	matchedModule := map[module.Version]bool{}
+	for _, arg := range args {
+		if path, vers, found := strings.Cut(arg, "@"); found {
+			var current string
+			if mg == nil {
+				current, _ = rs.rootSelected(path)
+			} else {
+				current = mg.Selected(path)
+			}
+			if current == "none" && mgErr != nil {
+				if vers == "upgrade" || vers == "patch" {
+					// The module graph is incomplete, so we don't know what version we're
+					// actually upgrading from.
+					// mgErr is already set, so just skip this module.
+					continue
+				}
+			}
+
+			allowed := CheckAllowed
+			if IsRevisionQuery(path, vers) || mode&ListRetracted != 0 {
+				// Allow excluded and retracted versions if the user asked for a
+				// specific revision or used 'go list -retracted'.
+				allowed = nil
+			}
+			info, err := queryReuse(ctx, path, vers, current, allowed, reuse)
+			if err != nil {
+				var origin *codehost.Origin
+				if info != nil {
+					origin = info.Origin
+				}
+				mods = append(mods, &modinfo.ModulePublic{
+					Path:    path,
+					Version: vers,
+					Error:   modinfoError(path, vers, err),
+					Origin:  origin,
+				})
+				continue
+			}
+
+			// Indicate that m was resolved from outside of rs by passing a nil
+			// *Requirements instead.
+			var noRS *Requirements
+
+			mod := moduleInfo(ctx, noRS, module.Version{Path: path, Version: info.Version}, mode, reuse)
+			if vers != mod.Version {
+				mod.Query = vers
+			}
+			mod.Origin = info.Origin
+			mods = append(mods, mod)
+			continue
+		}
+
+		// Module path or pattern.
+		var match func(string) bool
+		if arg == "all" {
+			match = func(p string) bool { return !gover.IsToolchain(p) }
+		} else if strings.Contains(arg, "...") {
+			mp := pkgpattern.MatchPattern(arg)
+			match = func(p string) bool { return mp(p) && !gover.IsToolchain(p) }
+		} else {
+			var v string
+			if mg == nil {
+				var ok bool
+				v, ok = rs.rootSelected(arg)
+				if !ok {
+					// We checked rootSelected(arg) in the earlier args loop, so if there
+					// is no such root we should have loaded a non-nil mg.
+					panic(fmt.Sprintf("internal error: root requirement expected but not found for %v", arg))
+				}
+			} else {
+				v = mg.Selected(arg)
+			}
+			if v == "none" && mgErr != nil {
+				// mgErr is already set, so just skip this module.
+				continue
+			}
+			if v != "none" {
+				mods = append(mods, moduleInfo(ctx, rs, module.Version{Path: arg, Version: v}, mode, reuse))
+			} else if cfg.BuildMod == "vendor" {
+				// In vendor mode, we can't determine whether a missing module is “a
+				// known dependency” because the module graph is incomplete.
+				// Give a more explicit error message.
+				mods = append(mods, &modinfo.ModulePublic{
+					Path:  arg,
+					Error: modinfoError(arg, "", errors.New("can't resolve module using the vendor directory\n\t(Use -mod=mod or -mod=readonly to bypass.)")),
+				})
+			} else if mode&ListVersions != 0 {
+				// Don't make the user provide an explicit '@latest' when they're
+				// explicitly asking what the available versions are. Instead, return a
+				// module with version "none", to which we can add the requested list.
+				mods = append(mods, &modinfo.ModulePublic{Path: arg})
+			} else {
+				mods = append(mods, &modinfo.ModulePublic{
+					Path:  arg,
+					Error: modinfoError(arg, "", errors.New("not a known dependency")),
+				})
+			}
+			continue
+		}
+
+		matched := false
+		for _, m := range mg.BuildList() {
+			if match(m.Path) {
+				matched = true
+				if !matchedModule[m] {
+					matchedModule[m] = true
+					mods = append(mods, moduleInfo(ctx, rs, m, mode, reuse))
+				}
+			}
+		}
+		if !matched {
+			fmt.Fprintf(os.Stderr, "warning: pattern %q matched no module dependencies\n", arg)
+		}
+	}
+
+	return rs, mods, mgErr
+}
+
+// modinfoError wraps an error to create an error message in
+// modinfo.ModuleError with minimal redundancy.
+func modinfoError(path, vers string, err error) *modinfo.ModuleError {
+	var nerr *NoMatchingVersionError
+	var merr *module.ModuleError
+	if errors.As(err, &nerr) {
+		// NoMatchingVersionError contains the query, so we don't mention the
+		// query again in ModuleError.
+		err = &module.ModuleError{Path: path, Err: err}
+	} else if !errors.As(err, &merr) {
+		// If the error does not contain path and version, wrap it in a
+		// module.ModuleError.
+		err = &module.ModuleError{Path: path, Version: vers, Err: err}
+	}
+
+	return &modinfo.ModuleError{Err: err.Error()}
+}
diff --git a/src/cmd/go/internal/modload/load.go b/src/cmd/go/internal/modload/load.go
new file mode 100644
index 0000000..a993fe8
--- /dev/null
+++ b/src/cmd/go/internal/modload/load.go
@@ -0,0 +1,2343 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+// This file contains the module-mode package loader, as well as some accessory
+// functions pertaining to the package import graph.
+//
+// There are two exported entry points into package loading — LoadPackages and
+// ImportFromFiles — both implemented in terms of loadFromRoots, which itself
+// manipulates an instance of the loader struct.
+//
+// Although most of the loading state is maintained in the loader struct,
+// one key piece - the build list - is a global, so that it can be modified
+// separate from the loading operation, such as during "go get"
+// upgrades/downgrades or in "go mod" operations.
+// TODO(#40775): It might be nice to make the loader take and return
+// a buildList rather than hard-coding use of the global.
+//
+// Loading is an iterative process. On each iteration, we try to load the
+// requested packages and their transitive imports, then try to resolve modules
+// for any imported packages that are still missing.
+//
+// The first step of each iteration identifies a set of “root” packages.
+// Normally the root packages are exactly those matching the named pattern
+// arguments. However, for the "all" meta-pattern, the final set of packages is
+// computed from the package import graph, and therefore cannot be an initial
+// input to loading that graph. Instead, the root packages for the "all" pattern
+// are those contained in the main module, and allPatternIsRoot parameter to the
+// loader instructs it to dynamically expand those roots to the full "all"
+// pattern as loading progresses.
+//
+// The pkgInAll flag on each loadPkg instance tracks whether that
+// package is known to match the "all" meta-pattern.
+// A package matches the "all" pattern if:
+// 	- it is in the main module, or
+// 	- it is imported by any test in the main module, or
+// 	- it is imported by another package in "all", or
+// 	- the main module specifies a go version ≤ 1.15, and the package is imported
+// 	  by a *test of* another package in "all".
+//
+// When graph pruning is in effect, we want to spot-check the graph-pruning
+// invariants — which depend on which packages are known to be in "all" — even
+// when we are only loading individual packages, so we set the pkgInAll flag
+// regardless of the whether the "all" pattern is a root.
+// (This is necessary to maintain the “import invariant” described in
+// https://golang.org/design/36460-lazy-module-loading.)
+//
+// Because "go mod vendor" prunes out the tests of vendored packages, the
+// behavior of the "all" pattern with -mod=vendor in Go 1.11–1.15 is the same
+// as the "all" pattern (regardless of the -mod flag) in 1.16+.
+// The loader uses the GoVersion parameter to determine whether the "all"
+// pattern should close over tests (as in Go 1.11–1.15) or stop at only those
+// packages transitively imported by the packages and tests in the main module
+// ("all" in Go 1.16+ and "go mod vendor" in Go 1.11+).
+//
+// Note that it is possible for a loaded package NOT to be in "all" even when we
+// are loading the "all" pattern. For example, packages that are transitive
+// dependencies of other roots named on the command line must be loaded, but are
+// not in "all". (The mod_notall test illustrates this behavior.)
+// Similarly, if the LoadTests flag is set but the "all" pattern does not close
+// over test dependencies, then when we load the test of a package that is in
+// "all" but outside the main module, the dependencies of that test will not
+// necessarily themselves be in "all". (That configuration does not arise in Go
+// 1.11–1.15, but it will be possible in Go 1.16+.)
+//
+// Loading proceeds from the roots, using a parallel work-queue with a limit on
+// the amount of active work (to avoid saturating disks, CPU cores, and/or
+// network connections). Each package is added to the queue the first time it is
+// imported by another package. When we have finished identifying the imports of
+// a package, we add the test for that package if it is needed. A test may be
+// needed if:
+// 	- the package matches a root pattern and tests of the roots were requested, or
+// 	- the package is in the main module and the "all" pattern is requested
+// 	  (because the "all" pattern includes the dependencies of tests in the main
+// 	  module), or
+// 	- the package is in "all" and the definition of "all" we are using includes
+// 	  dependencies of tests (as is the case in Go ≤1.15).
+//
+// After all available packages have been loaded, we examine the results to
+// identify any requested or imported packages that are still missing, and if
+// so, which modules we could add to the module graph in order to make the
+// missing packages available. We add those to the module graph and iterate,
+// until either all packages resolve successfully or we cannot identify any
+// module that would resolve any remaining missing package.
+//
+// If the main module is “tidy” (that is, if "go mod tidy" is a no-op for it)
+// and all requested packages are in "all", then loading completes in a single
+// iteration.
+// TODO(bcmills): We should also be able to load in a single iteration if the
+// requested packages all come from modules that are themselves tidy, regardless
+// of whether those packages are in "all". Today, that requires two iterations
+// if those packages are not found in existing dependencies of the main module.
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"go/build"
+	"io/fs"
+	"os"
+	"path"
+	pathpkg "path"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"sort"
+	"strings"
+	"sync"
+	"sync/atomic"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/imports"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/modindex"
+	"cmd/go/internal/mvs"
+	"cmd/go/internal/par"
+	"cmd/go/internal/search"
+	"cmd/go/internal/str"
+
+	"golang.org/x/mod/module"
+)
+
+// loaded is the most recently-used package loader.
+// It holds details about individual packages.
+//
+// This variable should only be accessed directly in top-level exported
+// functions. All other functions that require or produce a *loader should pass
+// or return it as an explicit parameter.
+var loaded *loader
+
+// PackageOpts control the behavior of the LoadPackages function.
+type PackageOpts struct {
+	// TidyGoVersion is the Go version to which the go.mod file should be updated
+	// after packages have been loaded.
+	//
+	// An empty TidyGoVersion means to use the Go version already specified in the
+	// main module's go.mod file, or the latest Go version if there is no main
+	// module.
+	TidyGoVersion string
+
+	// Tags are the build tags in effect (as interpreted by the
+	// cmd/go/internal/imports package).
+	// If nil, treated as equivalent to imports.Tags().
+	Tags map[string]bool
+
+	// Tidy, if true, requests that the build list and go.sum file be reduced to
+	// the minimal dependencies needed to reproducibly reload the requested
+	// packages.
+	Tidy bool
+
+	// TidyCompatibleVersion is the oldest Go version that must be able to
+	// reproducibly reload the requested packages.
+	//
+	// If empty, the compatible version is the Go version immediately prior to the
+	// 'go' version listed in the go.mod file.
+	TidyCompatibleVersion string
+
+	// VendorModulesInGOROOTSrc indicates that if we are within a module in
+	// GOROOT/src, packages in the module's vendor directory should be resolved as
+	// actual module dependencies (instead of standard-library packages).
+	VendorModulesInGOROOTSrc bool
+
+	// ResolveMissingImports indicates that we should attempt to add module
+	// dependencies as needed to resolve imports of packages that are not found.
+	//
+	// For commands that support the -mod flag, resolving imports may still fail
+	// if the flag is set to "readonly" (the default) or "vendor".
+	ResolveMissingImports bool
+
+	// AssumeRootsImported indicates that the transitive dependencies of the root
+	// packages should be treated as if those roots will be imported by the main
+	// module.
+	AssumeRootsImported bool
+
+	// AllowPackage, if non-nil, is called after identifying the module providing
+	// each package. If AllowPackage returns a non-nil error, that error is set
+	// for the package, and the imports and test of that package will not be
+	// loaded.
+	//
+	// AllowPackage may be invoked concurrently by multiple goroutines,
+	// and may be invoked multiple times for a given package path.
+	AllowPackage func(ctx context.Context, path string, mod module.Version) error
+
+	// LoadTests loads the test dependencies of each package matching a requested
+	// pattern. If ResolveMissingImports is also true, test dependencies will be
+	// resolved if missing.
+	LoadTests bool
+
+	// UseVendorAll causes the "all" package pattern to be interpreted as if
+	// running "go mod vendor" (or building with "-mod=vendor").
+	//
+	// This is a no-op for modules that declare 'go 1.16' or higher, for which this
+	// is the default (and only) interpretation of the "all" pattern in module mode.
+	UseVendorAll bool
+
+	// AllowErrors indicates that LoadPackages should not terminate the process if
+	// an error occurs.
+	AllowErrors bool
+
+	// SilencePackageErrors indicates that LoadPackages should not print errors
+	// that occur while matching or loading packages, and should not terminate the
+	// process if such an error occurs.
+	//
+	// Errors encountered in the module graph will still be reported.
+	//
+	// The caller may retrieve the silenced package errors using the Lookup
+	// function, and matching errors are still populated in the Errs field of the
+	// associated search.Match.)
+	SilencePackageErrors bool
+
+	// SilenceMissingStdImports indicates that LoadPackages should not print
+	// errors or terminate the process if an imported package is missing, and the
+	// import path looks like it might be in the standard library (perhaps in a
+	// future version).
+	SilenceMissingStdImports bool
+
+	// SilenceNoGoErrors indicates that LoadPackages should not print
+	// imports.ErrNoGo errors.
+	// This allows the caller to invoke LoadPackages (and report other errors)
+	// without knowing whether the requested packages exist for the given tags.
+	//
+	// Note that if a requested package does not exist *at all*, it will fail
+	// during module resolution and the error will not be suppressed.
+	SilenceNoGoErrors bool
+
+	// SilenceUnmatchedWarnings suppresses the warnings normally emitted for
+	// patterns that did not match any packages.
+	SilenceUnmatchedWarnings bool
+
+	// Resolve the query against this module.
+	MainModule module.Version
+
+	// If Switcher is non-nil, then LoadPackages passes all encountered errors
+	// to Switcher.Error and tries Switcher.Switch before base.ExitIfErrors.
+	Switcher gover.Switcher
+}
+
+// LoadPackages identifies the set of packages matching the given patterns and
+// loads the packages in the import graph rooted at that set.
+func LoadPackages(ctx context.Context, opts PackageOpts, patterns ...string) (matches []*search.Match, loadedPackages []string) {
+	if opts.Tags == nil {
+		opts.Tags = imports.Tags()
+	}
+
+	patterns = search.CleanPatterns(patterns)
+	matches = make([]*search.Match, 0, len(patterns))
+	allPatternIsRoot := false
+	for _, pattern := range patterns {
+		matches = append(matches, search.NewMatch(pattern))
+		if pattern == "all" {
+			allPatternIsRoot = true
+		}
+	}
+
+	updateMatches := func(rs *Requirements, ld *loader) {
+		for _, m := range matches {
+			switch {
+			case m.IsLocal():
+				// Evaluate list of file system directories on first iteration.
+				if m.Dirs == nil {
+					matchModRoots := modRoots
+					if opts.MainModule != (module.Version{}) {
+						matchModRoots = []string{MainModules.ModRoot(opts.MainModule)}
+					}
+					matchLocalDirs(ctx, matchModRoots, m, rs)
+				}
+
+				// Make a copy of the directory list and translate to import paths.
+				// Note that whether a directory corresponds to an import path
+				// changes as the build list is updated, and a directory can change
+				// from not being in the build list to being in it and back as
+				// the exact version of a particular module increases during
+				// the loader iterations.
+				m.Pkgs = m.Pkgs[:0]
+				for _, dir := range m.Dirs {
+					pkg, err := resolveLocalPackage(ctx, dir, rs)
+					if err != nil {
+						if !m.IsLiteral() && (err == errPkgIsBuiltin || err == errPkgIsGorootSrc) {
+							continue // Don't include "builtin" or GOROOT/src in wildcard patterns.
+						}
+
+						// If we're outside of a module, ensure that the failure mode
+						// indicates that.
+						if !HasModRoot() {
+							die()
+						}
+
+						if ld != nil {
+							m.AddError(err)
+						}
+						continue
+					}
+					m.Pkgs = append(m.Pkgs, pkg)
+				}
+
+			case m.IsLiteral():
+				m.Pkgs = []string{m.Pattern()}
+
+			case strings.Contains(m.Pattern(), "..."):
+				m.Errs = m.Errs[:0]
+				mg, err := rs.Graph(ctx)
+				if err != nil {
+					// The module graph is (or may be) incomplete — perhaps we failed to
+					// load the requirements of some module. This is an error in matching
+					// the patterns to packages, because we may be missing some packages
+					// or we may erroneously match packages in the wrong versions of
+					// modules. However, for cases like 'go list -e', the error should not
+					// necessarily prevent us from loading the packages we could find.
+					m.Errs = append(m.Errs, err)
+				}
+				matchPackages(ctx, m, opts.Tags, includeStd, mg.BuildList())
+
+			case m.Pattern() == "all":
+				if ld == nil {
+					// The initial roots are the packages in the main module.
+					// loadFromRoots will expand that to "all".
+					m.Errs = m.Errs[:0]
+					matchModules := MainModules.Versions()
+					if opts.MainModule != (module.Version{}) {
+						matchModules = []module.Version{opts.MainModule}
+					}
+					matchPackages(ctx, m, opts.Tags, omitStd, matchModules)
+				} else {
+					// Starting with the packages in the main module,
+					// enumerate the full list of "all".
+					m.Pkgs = ld.computePatternAll()
+				}
+
+			case m.Pattern() == "std" || m.Pattern() == "cmd":
+				if m.Pkgs == nil {
+					m.MatchPackages() // Locate the packages within GOROOT/src.
+				}
+
+			default:
+				panic(fmt.Sprintf("internal error: modload missing case for pattern %s", m.Pattern()))
+			}
+		}
+	}
+
+	initialRS, err := loadModFile(ctx, &opts)
+	if err != nil {
+		base.Fatal(err)
+	}
+
+	ld := loadFromRoots(ctx, loaderParams{
+		PackageOpts:  opts,
+		requirements: initialRS,
+
+		allPatternIsRoot: allPatternIsRoot,
+
+		listRoots: func(rs *Requirements) (roots []string) {
+			updateMatches(rs, nil)
+			for _, m := range matches {
+				roots = append(roots, m.Pkgs...)
+			}
+			return roots
+		},
+	})
+
+	// One last pass to finalize wildcards.
+	updateMatches(ld.requirements, ld)
+
+	// List errors in matching patterns (such as directory permission
+	// errors for wildcard patterns).
+	if !ld.SilencePackageErrors {
+		for _, match := range matches {
+			for _, err := range match.Errs {
+				ld.error(err)
+			}
+		}
+	}
+	ld.exitIfErrors(ctx)
+
+	if !opts.SilenceUnmatchedWarnings {
+		search.WarnUnmatched(matches)
+	}
+
+	if opts.Tidy {
+		if cfg.BuildV {
+			mg, _ := ld.requirements.Graph(ctx)
+			for _, m := range initialRS.rootModules {
+				var unused bool
+				if ld.requirements.pruning == unpruned {
+					// m is unused if it was dropped from the module graph entirely. If it
+					// was only demoted from direct to indirect, it may still be in use via
+					// a transitive import.
+					unused = mg.Selected(m.Path) == "none"
+				} else {
+					// m is unused if it was dropped from the roots. If it is still present
+					// as a transitive dependency, that transitive dependency is not needed
+					// by any package or test in the main module.
+					_, ok := ld.requirements.rootSelected(m.Path)
+					unused = !ok
+				}
+				if unused {
+					fmt.Fprintf(os.Stderr, "unused %s\n", m.Path)
+				}
+			}
+		}
+
+		keep := keepSums(ctx, ld, ld.requirements, loadedZipSumsOnly)
+		compatVersion := ld.TidyCompatibleVersion
+		goVersion := ld.requirements.GoVersion()
+		if compatVersion == "" {
+			if gover.Compare(goVersion, gover.GoStrictVersion) < 0 {
+				compatVersion = gover.Prev(goVersion)
+			} else {
+				// Starting at GoStrictVersion, we no longer maintain compatibility with
+				// versions older than what is listed in the go.mod file.
+				compatVersion = goVersion
+			}
+		}
+		if gover.Compare(compatVersion, goVersion) > 0 {
+			// Each version of the Go toolchain knows how to interpret go.mod and
+			// go.sum files produced by all previous versions, so a compatibility
+			// version higher than the go.mod version adds nothing.
+			compatVersion = goVersion
+		}
+		if compatPruning := pruningForGoVersion(compatVersion); compatPruning != ld.requirements.pruning {
+			compatRS := newRequirements(compatPruning, ld.requirements.rootModules, ld.requirements.direct)
+			ld.checkTidyCompatibility(ctx, compatRS, compatVersion)
+
+			for m := range keepSums(ctx, ld, compatRS, loadedZipSumsOnly) {
+				keep[m] = true
+			}
+		}
+
+		if !ExplicitWriteGoMod {
+			modfetch.TrimGoSum(keep)
+
+			// commitRequirements below will also call WriteGoSum, but the "keep" map
+			// we have here could be strictly larger: commitRequirements only commits
+			// loaded.requirements, but here we may have also loaded (and want to
+			// preserve checksums for) additional entities from compatRS, which are
+			// only needed for compatibility with ld.TidyCompatibleVersion.
+			if err := modfetch.WriteGoSum(ctx, keep, mustHaveCompleteRequirements()); err != nil {
+				base.Fatal(err)
+			}
+		}
+	}
+
+	// Success! Update go.mod and go.sum (if needed) and return the results.
+	// We'll skip updating if ExplicitWriteGoMod is true (the caller has opted
+	// to call WriteGoMod itself) or if ResolveMissingImports is false (the
+	// command wants to examine the package graph as-is).
+	loaded = ld
+	requirements = loaded.requirements
+
+	for _, pkg := range ld.pkgs {
+		if !pkg.isTest() {
+			loadedPackages = append(loadedPackages, pkg.path)
+		}
+	}
+	sort.Strings(loadedPackages)
+
+	if !ExplicitWriteGoMod && opts.ResolveMissingImports {
+		if err := commitRequirements(ctx, WriteOpts{}); err != nil {
+			base.Fatal(err)
+		}
+	}
+
+	return matches, loadedPackages
+}
+
+// matchLocalDirs is like m.MatchDirs, but tries to avoid scanning directories
+// outside of the standard library and active modules.
+func matchLocalDirs(ctx context.Context, modRoots []string, m *search.Match, rs *Requirements) {
+	if !m.IsLocal() {
+		panic(fmt.Sprintf("internal error: resolveLocalDirs on non-local pattern %s", m.Pattern()))
+	}
+
+	if i := strings.Index(m.Pattern(), "..."); i >= 0 {
+		// The pattern is local, but it is a wildcard. Its packages will
+		// only resolve to paths if they are inside of the standard
+		// library, the main module, or some dependency of the main
+		// module. Verify that before we walk the filesystem: a filesystem
+		// walk in a directory like /var or /etc can be very expensive!
+		dir := filepath.Dir(filepath.Clean(m.Pattern()[:i+3]))
+		absDir := dir
+		if !filepath.IsAbs(dir) {
+			absDir = filepath.Join(base.Cwd(), dir)
+		}
+
+		modRoot := findModuleRoot(absDir)
+		found := false
+		for _, mainModuleRoot := range modRoots {
+			if mainModuleRoot == modRoot {
+				found = true
+				break
+			}
+		}
+		if !found && search.InDir(absDir, cfg.GOROOTsrc) == "" && pathInModuleCache(ctx, absDir, rs) == "" {
+			m.Dirs = []string{}
+			scope := "main module or its selected dependencies"
+			if inWorkspaceMode() {
+				scope = "modules listed in go.work or their selected dependencies"
+			}
+			m.AddError(fmt.Errorf("directory prefix %s does not contain %s", base.ShortPath(absDir), scope))
+			return
+		}
+	}
+
+	m.MatchDirs(modRoots)
+}
+
+// resolveLocalPackage resolves a filesystem path to a package path.
+func resolveLocalPackage(ctx context.Context, dir string, rs *Requirements) (string, error) {
+	var absDir string
+	if filepath.IsAbs(dir) {
+		absDir = filepath.Clean(dir)
+	} else {
+		absDir = filepath.Join(base.Cwd(), dir)
+	}
+
+	bp, err := cfg.BuildContext.ImportDir(absDir, 0)
+	if err != nil && (bp == nil || len(bp.IgnoredGoFiles) == 0) {
+		// golang.org/issue/32917: We should resolve a relative path to a
+		// package path only if the relative path actually contains the code
+		// for that package.
+		//
+		// If the named directory does not exist or contains no Go files,
+		// the package does not exist.
+		// Other errors may affect package loading, but not resolution.
+		if _, err := fsys.Stat(absDir); err != nil {
+			if os.IsNotExist(err) {
+				// Canonicalize OS-specific errors to errDirectoryNotFound so that error
+				// messages will be easier for users to search for.
+				return "", &fs.PathError{Op: "stat", Path: absDir, Err: errDirectoryNotFound}
+			}
+			return "", err
+		}
+		if _, noGo := err.(*build.NoGoError); noGo {
+			// A directory that does not contain any Go source files — even ignored
+			// ones! — is not a Go package, and we can't resolve it to a package
+			// path because that path could plausibly be provided by some other
+			// module.
+			//
+			// Any other error indicates that the package “exists” (at least in the
+			// sense that it cannot exist in any other module), but has some other
+			// problem (such as a syntax error).
+			return "", err
+		}
+	}
+
+	for _, mod := range MainModules.Versions() {
+		modRoot := MainModules.ModRoot(mod)
+		if modRoot != "" && absDir == modRoot {
+			if absDir == cfg.GOROOTsrc {
+				return "", errPkgIsGorootSrc
+			}
+			return MainModules.PathPrefix(mod), nil
+		}
+	}
+
+	// Note: The checks for @ here are just to avoid misinterpreting
+	// the module cache directories (formerly GOPATH/src/mod/foo@v1.5.2/bar).
+	// It's not strictly necessary but helpful to keep the checks.
+	var pkgNotFoundErr error
+	pkgNotFoundLongestPrefix := ""
+	for _, mainModule := range MainModules.Versions() {
+		modRoot := MainModules.ModRoot(mainModule)
+		if modRoot != "" && str.HasFilePathPrefix(absDir, modRoot) && !strings.Contains(absDir[len(modRoot):], "@") {
+			suffix := filepath.ToSlash(str.TrimFilePathPrefix(absDir, modRoot))
+			if pkg, found := strings.CutPrefix(suffix, "vendor/"); found {
+				if cfg.BuildMod != "vendor" {
+					return "", fmt.Errorf("without -mod=vendor, directory %s has no package path", absDir)
+				}
+
+				readVendorList(mainModule)
+				if _, ok := vendorPkgModule[pkg]; !ok {
+					return "", fmt.Errorf("directory %s is not a package listed in vendor/modules.txt", absDir)
+				}
+				return pkg, nil
+			}
+
+			mainModulePrefix := MainModules.PathPrefix(mainModule)
+			if mainModulePrefix == "" {
+				pkg := suffix
+				if pkg == "builtin" {
+					// "builtin" is a pseudo-package with a real source file.
+					// It's not included in "std", so it shouldn't resolve from "."
+					// within module "std" either.
+					return "", errPkgIsBuiltin
+				}
+				return pkg, nil
+			}
+
+			pkg := pathpkg.Join(mainModulePrefix, suffix)
+			if _, ok, err := dirInModule(pkg, mainModulePrefix, modRoot, true); err != nil {
+				return "", err
+			} else if !ok {
+				// This main module could contain the directory but doesn't. Other main
+				// modules might contain the directory, so wait till we finish the loop
+				// to see if another main module contains directory. But if not,
+				// return an error.
+				if len(mainModulePrefix) > len(pkgNotFoundLongestPrefix) {
+					pkgNotFoundLongestPrefix = mainModulePrefix
+					pkgNotFoundErr = &PackageNotInModuleError{MainModules: []module.Version{mainModule}, Pattern: pkg}
+				}
+				continue
+			}
+			return pkg, nil
+		}
+	}
+	if pkgNotFoundErr != nil {
+		return "", pkgNotFoundErr
+	}
+
+	if sub := search.InDir(absDir, cfg.GOROOTsrc); sub != "" && sub != "." && !strings.Contains(sub, "@") {
+		pkg := filepath.ToSlash(sub)
+		if pkg == "builtin" {
+			return "", errPkgIsBuiltin
+		}
+		return pkg, nil
+	}
+
+	pkg := pathInModuleCache(ctx, absDir, rs)
+	if pkg == "" {
+		dirstr := fmt.Sprintf("directory %s", base.ShortPath(absDir))
+		if dirstr == "directory ." {
+			dirstr = "current directory"
+		}
+		if inWorkspaceMode() {
+			if mr := findModuleRoot(absDir); mr != "" {
+				return "", fmt.Errorf("%s is contained in a module that is not one of the workspace modules listed in go.work. You can add the module to the workspace using:\n\tgo work use %s", dirstr, base.ShortPath(mr))
+			}
+			return "", fmt.Errorf("%s outside modules listed in go.work or their selected dependencies", dirstr)
+		}
+		return "", fmt.Errorf("%s outside main module or its selected dependencies", dirstr)
+	}
+	return pkg, nil
+}
+
+var (
+	errDirectoryNotFound = errors.New("directory not found")
+	errPkgIsGorootSrc    = errors.New("GOROOT/src is not an importable package")
+	errPkgIsBuiltin      = errors.New(`"builtin" is a pseudo-package, not an importable package`)
+)
+
+// pathInModuleCache returns the import path of the directory dir,
+// if dir is in the module cache copy of a module in our build list.
+func pathInModuleCache(ctx context.Context, dir string, rs *Requirements) string {
+	tryMod := func(m module.Version) (string, bool) {
+		if gover.IsToolchain(m.Path) {
+			return "", false
+		}
+		var root string
+		var err error
+		if repl := Replacement(m); repl.Path != "" && repl.Version == "" {
+			root = repl.Path
+			if !filepath.IsAbs(root) {
+				root = filepath.Join(replaceRelativeTo(), root)
+			}
+		} else if repl.Path != "" {
+			root, err = modfetch.DownloadDir(ctx, repl)
+		} else {
+			root, err = modfetch.DownloadDir(ctx, m)
+		}
+		if err != nil {
+			return "", false
+		}
+
+		sub := search.InDir(dir, root)
+		if sub == "" {
+			return "", false
+		}
+		sub = filepath.ToSlash(sub)
+		if strings.Contains(sub, "/vendor/") || strings.HasPrefix(sub, "vendor/") || strings.Contains(sub, "@") {
+			return "", false
+		}
+
+		return path.Join(m.Path, filepath.ToSlash(sub)), true
+	}
+
+	if rs.pruning == pruned {
+		for _, m := range rs.rootModules {
+			if v, _ := rs.rootSelected(m.Path); v != m.Version {
+				continue // m is a root, but we have a higher root for the same path.
+			}
+			if importPath, ok := tryMod(m); ok {
+				// checkMultiplePaths ensures that a module can be used for at most one
+				// requirement, so this must be it.
+				return importPath
+			}
+		}
+	}
+
+	// None of the roots contained dir, or the graph is unpruned (so we don't want
+	// to distinguish between roots and transitive dependencies). Either way,
+	// check the full graph to see if the directory is a non-root dependency.
+	//
+	// If the roots are not consistent with the full module graph, the selected
+	// versions of root modules may differ from what we already checked above.
+	// Re-check those paths too.
+
+	mg, _ := rs.Graph(ctx)
+	var importPath string
+	for _, m := range mg.BuildList() {
+		var found bool
+		importPath, found = tryMod(m)
+		if found {
+			break
+		}
+	}
+	return importPath
+}
+
+// ImportFromFiles adds modules to the build list as needed
+// to satisfy the imports in the named Go source files.
+//
+// Errors in missing dependencies are silenced.
+//
+// TODO(bcmills): Silencing errors seems off. Take a closer look at this and
+// figure out what the error-reporting actually ought to be.
+func ImportFromFiles(ctx context.Context, gofiles []string) {
+	rs := LoadModFile(ctx)
+
+	tags := imports.Tags()
+	imports, testImports, err := imports.ScanFiles(gofiles, tags)
+	if err != nil {
+		base.Fatal(err)
+	}
+
+	loaded = loadFromRoots(ctx, loaderParams{
+		PackageOpts: PackageOpts{
+			Tags:                  tags,
+			ResolveMissingImports: true,
+			SilencePackageErrors:  true,
+		},
+		requirements: rs,
+		listRoots: func(*Requirements) (roots []string) {
+			roots = append(roots, imports...)
+			roots = append(roots, testImports...)
+			return roots
+		},
+	})
+	requirements = loaded.requirements
+
+	if !ExplicitWriteGoMod {
+		if err := commitRequirements(ctx, WriteOpts{}); err != nil {
+			base.Fatal(err)
+		}
+	}
+}
+
+// DirImportPath returns the effective import path for dir,
+// provided it is within a main module, or else returns ".".
+func (mms *MainModuleSet) DirImportPath(ctx context.Context, dir string) (path string, m module.Version) {
+	if !HasModRoot() {
+		return ".", module.Version{}
+	}
+	LoadModFile(ctx) // Sets targetPrefix.
+
+	if !filepath.IsAbs(dir) {
+		dir = filepath.Join(base.Cwd(), dir)
+	} else {
+		dir = filepath.Clean(dir)
+	}
+
+	var longestPrefix string
+	var longestPrefixPath string
+	var longestPrefixVersion module.Version
+	for _, v := range mms.Versions() {
+		modRoot := mms.ModRoot(v)
+		if dir == modRoot {
+			return mms.PathPrefix(v), v
+		}
+		if str.HasFilePathPrefix(dir, modRoot) {
+			pathPrefix := MainModules.PathPrefix(v)
+			if pathPrefix > longestPrefix {
+				longestPrefix = pathPrefix
+				longestPrefixVersion = v
+				suffix := filepath.ToSlash(str.TrimFilePathPrefix(dir, modRoot))
+				if strings.HasPrefix(suffix, "vendor/") {
+					longestPrefixPath = strings.TrimPrefix(suffix, "vendor/")
+					continue
+				}
+				longestPrefixPath = pathpkg.Join(mms.PathPrefix(v), suffix)
+			}
+		}
+	}
+	if len(longestPrefix) > 0 {
+		return longestPrefixPath, longestPrefixVersion
+	}
+
+	return ".", module.Version{}
+}
+
+// PackageModule returns the module providing the package named by the import path.
+func PackageModule(path string) module.Version {
+	pkg, ok := loaded.pkgCache.Get(path)
+	if !ok {
+		return module.Version{}
+	}
+	return pkg.mod
+}
+
+// Lookup returns the source directory, import path, and any loading error for
+// the package at path as imported from the package in parentDir.
+// Lookup requires that one of the Load functions in this package has already
+// been called.
+func Lookup(parentPath string, parentIsStd bool, path string) (dir, realPath string, err error) {
+	if path == "" {
+		panic("Lookup called with empty package path")
+	}
+
+	if parentIsStd {
+		path = loaded.stdVendor(parentPath, path)
+	}
+	pkg, ok := loaded.pkgCache.Get(path)
+	if !ok {
+		// The loader should have found all the relevant paths.
+		// There are a few exceptions, though:
+		//	- during go list without -test, the p.Resolve calls to process p.TestImports and p.XTestImports
+		//	  end up here to canonicalize the import paths.
+		//	- during any load, non-loaded packages like "unsafe" end up here.
+		//	- during any load, build-injected dependencies like "runtime/cgo" end up here.
+		//	- because we ignore appengine/* in the module loader,
+		//	  the dependencies of any actual appengine/* library end up here.
+		dir := findStandardImportPath(path)
+		if dir != "" {
+			return dir, path, nil
+		}
+		return "", "", errMissing
+	}
+	return pkg.dir, pkg.path, pkg.err
+}
+
+// A loader manages the process of loading information about
+// the required packages for a particular build,
+// checking that the packages are available in the module set,
+// and updating the module set if needed.
+type loader struct {
+	loaderParams
+
+	// allClosesOverTests indicates whether the "all" pattern includes
+	// dependencies of tests outside the main module (as in Go 1.11–1.15).
+	// (Otherwise — as in Go 1.16+ — the "all" pattern includes only the packages
+	// transitively *imported by* the packages and tests in the main module.)
+	allClosesOverTests bool
+
+	// skipImportModFiles indicates whether we may skip loading go.mod files
+	// for imported packages (as in 'go mod tidy' in Go 1.17–1.20).
+	skipImportModFiles bool
+
+	work *par.Queue
+
+	// reset on each iteration
+	roots    []*loadPkg
+	pkgCache *par.Cache[string, *loadPkg]
+	pkgs     []*loadPkg // transitive closure of loaded packages and tests; populated in buildStacks
+}
+
+// loaderParams configure the packages loaded by, and the properties reported
+// by, a loader instance.
+type loaderParams struct {
+	PackageOpts
+	requirements *Requirements
+
+	allPatternIsRoot bool // Is the "all" pattern an additional root?
+
+	listRoots func(rs *Requirements) []string
+}
+
+func (ld *loader) reset() {
+	select {
+	case <-ld.work.Idle():
+	default:
+		panic("loader.reset when not idle")
+	}
+
+	ld.roots = nil
+	ld.pkgCache = new(par.Cache[string, *loadPkg])
+	ld.pkgs = nil
+}
+
+// error reports an error via either os.Stderr or base.Error,
+// according to whether ld.AllowErrors is set.
+func (ld *loader) error(err error) {
+	if ld.AllowErrors {
+		fmt.Fprintf(os.Stderr, "go: %v\n", err)
+	} else if ld.Switcher != nil {
+		ld.Switcher.Error(err)
+	} else {
+		base.Error(err)
+	}
+}
+
+// switchIfErrors switches toolchains if a switch is needed.
+func (ld *loader) switchIfErrors(ctx context.Context) {
+	if ld.Switcher != nil {
+		ld.Switcher.Switch(ctx)
+	}
+}
+
+// exitIfErrors switches toolchains if a switch is needed
+// or else exits if any errors have been reported.
+func (ld *loader) exitIfErrors(ctx context.Context) {
+	ld.switchIfErrors(ctx)
+	base.ExitIfErrors()
+}
+
+// goVersion reports the Go version that should be used for the loader's
+// requirements: ld.TidyGoVersion if set, or ld.requirements.GoVersion()
+// otherwise.
+func (ld *loader) goVersion() string {
+	if ld.TidyGoVersion != "" {
+		return ld.TidyGoVersion
+	}
+	return ld.requirements.GoVersion()
+}
+
+// A loadPkg records information about a single loaded package.
+type loadPkg struct {
+	// Populated at construction time:
+	path   string // import path
+	testOf *loadPkg
+
+	// Populated at construction time and updated by (*loader).applyPkgFlags:
+	flags atomicLoadPkgFlags
+
+	// Populated by (*loader).load:
+	mod         module.Version // module providing package
+	dir         string         // directory containing source code
+	err         error          // error loading package
+	imports     []*loadPkg     // packages imported by this one
+	testImports []string       // test-only imports, saved for use by pkg.test.
+	inStd       bool
+	altMods     []module.Version // modules that could have contained the package but did not
+
+	// Populated by (*loader).pkgTest:
+	testOnce sync.Once
+	test     *loadPkg
+
+	// Populated by postprocessing in (*loader).buildStacks:
+	stack *loadPkg // package importing this one in minimal import stack for this pkg
+}
+
+// loadPkgFlags is a set of flags tracking metadata about a package.
+type loadPkgFlags int8
+
+const (
+	// pkgInAll indicates that the package is in the "all" package pattern,
+	// regardless of whether we are loading the "all" package pattern.
+	//
+	// When the pkgInAll flag and pkgImportsLoaded flags are both set, the caller
+	// who set the last of those flags must propagate the pkgInAll marking to all
+	// of the imports of the marked package.
+	//
+	// A test is marked with pkgInAll if that test would promote the packages it
+	// imports to be in "all" (such as when the test is itself within the main
+	// module, or when ld.allClosesOverTests is true).
+	pkgInAll loadPkgFlags = 1 << iota
+
+	// pkgIsRoot indicates that the package matches one of the root package
+	// patterns requested by the caller.
+	//
+	// If LoadTests is set, then when pkgIsRoot and pkgImportsLoaded are both set,
+	// the caller who set the last of those flags must populate a test for the
+	// package (in the pkg.test field).
+	//
+	// If the "all" pattern is included as a root, then non-test packages in "all"
+	// are also roots (and must be marked pkgIsRoot).
+	pkgIsRoot
+
+	// pkgFromRoot indicates that the package is in the transitive closure of
+	// imports starting at the roots. (Note that every package marked as pkgIsRoot
+	// is also trivially marked pkgFromRoot.)
+	pkgFromRoot
+
+	// pkgImportsLoaded indicates that the imports and testImports fields of a
+	// loadPkg have been populated.
+	pkgImportsLoaded
+)
+
+// has reports whether all of the flags in cond are set in f.
+func (f loadPkgFlags) has(cond loadPkgFlags) bool {
+	return f&cond == cond
+}
+
+// An atomicLoadPkgFlags stores a loadPkgFlags for which individual flags can be
+// added atomically.
+type atomicLoadPkgFlags struct {
+	bits atomic.Int32
+}
+
+// update sets the given flags in af (in addition to any flags already set).
+//
+// update returns the previous flag state so that the caller may determine which
+// flags were newly-set.
+func (af *atomicLoadPkgFlags) update(flags loadPkgFlags) (old loadPkgFlags) {
+	for {
+		old := af.bits.Load()
+		new := old | int32(flags)
+		if new == old || af.bits.CompareAndSwap(old, new) {
+			return loadPkgFlags(old)
+		}
+	}
+}
+
+// has reports whether all of the flags in cond are set in af.
+func (af *atomicLoadPkgFlags) has(cond loadPkgFlags) bool {
+	return loadPkgFlags(af.bits.Load())&cond == cond
+}
+
+// isTest reports whether pkg is a test of another package.
+func (pkg *loadPkg) isTest() bool {
+	return pkg.testOf != nil
+}
+
+// fromExternalModule reports whether pkg was loaded from a module other than
+// the main module.
+func (pkg *loadPkg) fromExternalModule() bool {
+	if pkg.mod.Path == "" {
+		return false // loaded from the standard library, not a module
+	}
+	return !MainModules.Contains(pkg.mod.Path)
+}
+
+var errMissing = errors.New("cannot find package")
+
+// loadFromRoots attempts to load the build graph needed to process a set of
+// root packages and their dependencies.
+//
+// The set of root packages is returned by the params.listRoots function, and
+// expanded to the full set of packages by tracing imports (and possibly tests)
+// as needed.
+func loadFromRoots(ctx context.Context, params loaderParams) *loader {
+	ld := &loader{
+		loaderParams: params,
+		work:         par.NewQueue(runtime.GOMAXPROCS(0)),
+	}
+
+	if ld.requirements.pruning == unpruned {
+		// If the module graph does not support pruning, we assume that we will need
+		// the full module graph in order to load package dependencies.
+		//
+		// This might not be strictly necessary, but it matches the historical
+		// behavior of the 'go' command and keeps the go.mod file more consistent in
+		// case of erroneous hand-edits — which are less likely to be detected by
+		// spot-checks in modules that do not maintain the expanded go.mod
+		// requirements needed for graph pruning.
+		var err error
+		ld.requirements, _, err = expandGraph(ctx, ld.requirements)
+		if err != nil {
+			ld.error(err)
+		}
+	}
+	ld.exitIfErrors(ctx)
+
+	updateGoVersion := func() {
+		goVersion := ld.goVersion()
+
+		if ld.requirements.pruning != workspace {
+			var err error
+			ld.requirements, err = convertPruning(ctx, ld.requirements, pruningForGoVersion(goVersion))
+			if err != nil {
+				ld.error(err)
+				ld.exitIfErrors(ctx)
+			}
+		}
+
+		// If the module's Go version omits go.sum entries for go.mod files for test
+		// dependencies of external packages, avoid loading those files in the first
+		// place.
+		ld.skipImportModFiles = ld.Tidy && gover.Compare(goVersion, gover.TidyGoModSumVersion) < 0
+
+		// If the module's go version explicitly predates the change in "all" for
+		// graph pruning, continue to use the older interpretation.
+		ld.allClosesOverTests = gover.Compare(goVersion, gover.NarrowAllVersion) < 0 && !ld.UseVendorAll
+	}
+
+	for {
+		ld.reset()
+		updateGoVersion()
+
+		// Load the root packages and their imports.
+		// Note: the returned roots can change on each iteration,
+		// since the expansion of package patterns depends on the
+		// build list we're using.
+		rootPkgs := ld.listRoots(ld.requirements)
+
+		if ld.requirements.pruning == pruned && cfg.BuildMod == "mod" {
+			// Before we start loading transitive imports of packages, locate all of
+			// the root packages and promote their containing modules to root modules
+			// dependencies. If their go.mod files are tidy (the common case) and the
+			// set of root packages does not change then we can select the correct
+			// versions of all transitive imports on the first try and complete
+			// loading in a single iteration.
+			changedBuildList := ld.preloadRootModules(ctx, rootPkgs)
+			if changedBuildList {
+				// The build list has changed, so the set of root packages may have also
+				// changed. Start over to pick up the changes. (Preloading roots is much
+				// cheaper than loading the full import graph, so we would rather pay
+				// for an extra iteration of preloading than potentially end up
+				// discarding the result of a full iteration of loading.)
+				continue
+			}
+		}
+
+		inRoots := map[*loadPkg]bool{}
+		for _, path := range rootPkgs {
+			root := ld.pkg(ctx, path, pkgIsRoot)
+			if !inRoots[root] {
+				ld.roots = append(ld.roots, root)
+				inRoots[root] = true
+			}
+		}
+
+		// ld.pkg adds imported packages to the work queue and calls applyPkgFlags,
+		// which adds tests (and test dependencies) as needed.
+		//
+		// When all of the work in the queue has completed, we'll know that the
+		// transitive closure of dependencies has been loaded.
+		<-ld.work.Idle()
+
+		ld.buildStacks()
+
+		changed, err := ld.updateRequirements(ctx)
+		if err != nil {
+			ld.error(err)
+			break
+		}
+		if changed {
+			// Don't resolve missing imports until the module graph has stabilized.
+			// If the roots are still changing, they may turn out to specify a
+			// requirement on the missing package(s), and we would rather use a
+			// version specified by a new root than add a new dependency on an
+			// unrelated version.
+			continue
+		}
+
+		if !ld.ResolveMissingImports || (!HasModRoot() && !allowMissingModuleImports) {
+			// We've loaded as much as we can without resolving missing imports.
+			break
+		}
+
+		modAddedBy, err := ld.resolveMissingImports(ctx)
+		if err != nil {
+			ld.error(err)
+			break
+		}
+		if len(modAddedBy) == 0 {
+			// The roots are stable, and we've resolved all of the missing packages
+			// that we can.
+			break
+		}
+
+		toAdd := make([]module.Version, 0, len(modAddedBy))
+		for m := range modAddedBy {
+			toAdd = append(toAdd, m)
+		}
+		gover.ModSort(toAdd) // to make errors deterministic
+
+		// We ran updateRequirements before resolving missing imports and it didn't
+		// make any changes, so we know that the requirement graph is already
+		// consistent with ld.pkgs: we don't need to pass ld.pkgs to updateRoots
+		// again. (That would waste time looking for changes that we have already
+		// applied.)
+		var noPkgs []*loadPkg
+		// We also know that we're going to call updateRequirements again next
+		// iteration so we don't need to also update it here. (That would waste time
+		// computing a "direct" map that we'll have to recompute later anyway.)
+		direct := ld.requirements.direct
+		rs, err := updateRoots(ctx, direct, ld.requirements, noPkgs, toAdd, ld.AssumeRootsImported)
+		if err != nil {
+			// If an error was found in a newly added module, report the package
+			// import stack instead of the module requirement stack. Packages
+			// are more descriptive.
+			if err, ok := err.(*mvs.BuildListError); ok {
+				if pkg := modAddedBy[err.Module()]; pkg != nil {
+					ld.error(fmt.Errorf("%s: %w", pkg.stackText(), err.Err))
+					break
+				}
+			}
+			ld.error(err)
+			break
+		}
+		if reflect.DeepEqual(rs.rootModules, ld.requirements.rootModules) {
+			// Something is deeply wrong. resolveMissingImports gave us a non-empty
+			// set of modules to add to the graph, but adding those modules had no
+			// effect — either they were already in the graph, or updateRoots did not
+			// add them as requested.
+			panic(fmt.Sprintf("internal error: adding %v to module graph had no effect on root requirements (%v)", toAdd, rs.rootModules))
+		}
+		ld.requirements = rs
+	}
+	ld.exitIfErrors(ctx)
+
+	// Tidy the build list, if applicable, before we report errors.
+	// (The process of tidying may remove errors from irrelevant dependencies.)
+	if ld.Tidy {
+		rs, err := tidyRoots(ctx, ld.requirements, ld.pkgs)
+		if err != nil {
+			ld.error(err)
+		} else {
+			if ld.TidyGoVersion != "" {
+				// Attempt to switch to the requested Go version. We have been using its
+				// pruning and semantics all along, but there may have been — and may
+				// still be — requirements on higher versions in the graph.
+				tidy := overrideRoots(ctx, rs, []module.Version{{Path: "go", Version: ld.TidyGoVersion}})
+				mg, err := tidy.Graph(ctx)
+				if err != nil {
+					ld.error(err)
+				}
+				if v := mg.Selected("go"); v == ld.TidyGoVersion {
+					rs = tidy
+				} else {
+					conflict := Conflict{
+						Path: mg.g.FindPath(func(m module.Version) bool {
+							return m.Path == "go" && m.Version == v
+						})[1:],
+						Constraint: module.Version{Path: "go", Version: ld.TidyGoVersion},
+					}
+					msg := conflict.Summary()
+					if cfg.BuildV {
+						msg = conflict.String()
+					}
+					ld.error(errors.New(msg))
+				}
+			}
+
+			if ld.requirements.pruning == pruned {
+				// We continuously add tidy roots to ld.requirements during loading, so
+				// at this point the tidy roots (other than possibly the "go" version
+				// edited above) should be a subset of the roots of ld.requirements,
+				// ensuring that no new dependencies are brought inside the
+				// graph-pruning horizon.
+				// If that is not the case, there is a bug in the loading loop above.
+				for _, m := range rs.rootModules {
+					if m.Path == "go" && ld.TidyGoVersion != "" {
+						continue
+					}
+					if v, ok := ld.requirements.rootSelected(m.Path); !ok || v != m.Version {
+						ld.error(fmt.Errorf("internal error: a requirement on %v is needed but was not added during package loading (selected %s)", m, v))
+					}
+				}
+			}
+
+			ld.requirements = rs
+		}
+
+		ld.exitIfErrors(ctx)
+	}
+
+	// Report errors, if any.
+	for _, pkg := range ld.pkgs {
+		if pkg.err == nil {
+			continue
+		}
+
+		// Add importer information to checksum errors.
+		if sumErr := (*ImportMissingSumError)(nil); errors.As(pkg.err, &sumErr) {
+			if importer := pkg.stack; importer != nil {
+				sumErr.importer = importer.path
+				sumErr.importerVersion = importer.mod.Version
+				sumErr.importerIsTest = importer.testOf != nil
+			}
+		}
+
+		if stdErr := (*ImportMissingError)(nil); errors.As(pkg.err, &stdErr) && stdErr.isStd {
+			// Add importer go version information to import errors of standard
+			// library packages arising from newer releases.
+			if importer := pkg.stack; importer != nil {
+				if v, ok := rawGoVersion.Load(importer.mod); ok && gover.Compare(gover.Local(), v.(string)) < 0 {
+					stdErr.importerGoVersion = v.(string)
+				}
+			}
+			if ld.SilenceMissingStdImports {
+				continue
+			}
+		}
+		if ld.SilencePackageErrors {
+			continue
+		}
+		if ld.SilenceNoGoErrors && errors.Is(pkg.err, imports.ErrNoGo) {
+			continue
+		}
+
+		ld.error(fmt.Errorf("%s: %w", pkg.stackText(), pkg.err))
+	}
+
+	ld.checkMultiplePaths()
+	return ld
+}
+
+// updateRequirements ensures that ld.requirements is consistent with the
+// information gained from ld.pkgs.
+//
+// In particular:
+//
+//   - Modules that provide packages directly imported from the main module are
+//     marked as direct, and are promoted to explicit roots. If a needed root
+//     cannot be promoted due to -mod=readonly or -mod=vendor, the importing
+//     package is marked with an error.
+//
+//   - If ld scanned the "all" pattern independent of build constraints, it is
+//     guaranteed to have seen every direct import. Module dependencies that did
+//     not provide any directly-imported package are then marked as indirect.
+//
+//   - Root dependencies are updated to their selected versions.
+//
+// The "changed" return value reports whether the update changed the selected
+// version of any module that either provided a loaded package or may now
+// provide a package that was previously unresolved.
+func (ld *loader) updateRequirements(ctx context.Context) (changed bool, err error) {
+	rs := ld.requirements
+
+	// direct contains the set of modules believed to provide packages directly
+	// imported by the main module.
+	var direct map[string]bool
+
+	// If we didn't scan all of the imports from the main module, or didn't use
+	// imports.AnyTags, then we didn't necessarily load every package that
+	// contributes “direct” imports — so we can't safely mark existing direct
+	// dependencies in ld.requirements as indirect-only. Propagate them as direct.
+	loadedDirect := ld.allPatternIsRoot && reflect.DeepEqual(ld.Tags, imports.AnyTags())
+	if loadedDirect {
+		direct = make(map[string]bool)
+	} else {
+		// TODO(bcmills): It seems like a shame to allocate and copy a map here when
+		// it will only rarely actually vary from rs.direct. Measure this cost and
+		// maybe avoid the copy.
+		direct = make(map[string]bool, len(rs.direct))
+		for mPath := range rs.direct {
+			direct[mPath] = true
+		}
+	}
+
+	var maxTooNew *gover.TooNewError
+	for _, pkg := range ld.pkgs {
+		if pkg.err != nil {
+			if tooNew := (*gover.TooNewError)(nil); errors.As(pkg.err, &tooNew) {
+				if maxTooNew == nil || gover.Compare(tooNew.GoVersion, maxTooNew.GoVersion) > 0 {
+					maxTooNew = tooNew
+				}
+			}
+		}
+		if pkg.mod.Version != "" || !MainModules.Contains(pkg.mod.Path) {
+			continue
+		}
+
+		for _, dep := range pkg.imports {
+			if !dep.fromExternalModule() {
+				continue
+			}
+
+			if inWorkspaceMode() {
+				// In workspace mode / workspace pruning mode, the roots are the main modules
+				// rather than the main module's direct dependencies. The check below on the selected
+				// roots does not apply.
+				if mg, err := rs.Graph(ctx); err != nil {
+					return false, err
+				} else if _, ok := mg.RequiredBy(dep.mod); !ok {
+					// dep.mod is not an explicit dependency, but needs to be.
+					// See comment on error returned below.
+					pkg.err = &DirectImportFromImplicitDependencyError{
+						ImporterPath: pkg.path,
+						ImportedPath: dep.path,
+						Module:       dep.mod,
+					}
+				}
+				continue
+			}
+
+			if pkg.err == nil && cfg.BuildMod != "mod" {
+				if v, ok := rs.rootSelected(dep.mod.Path); !ok || v != dep.mod.Version {
+					// dep.mod is not an explicit dependency, but needs to be.
+					// Because we are not in "mod" mode, we will not be able to update it.
+					// Instead, mark the importing package with an error.
+					//
+					// TODO(#41688): The resulting error message fails to include the file
+					// position of the import statement (because that information is not
+					// tracked by the module loader). Figure out how to plumb the import
+					// position through.
+					pkg.err = &DirectImportFromImplicitDependencyError{
+						ImporterPath: pkg.path,
+						ImportedPath: dep.path,
+						Module:       dep.mod,
+					}
+					// cfg.BuildMod does not allow us to change dep.mod to be a direct
+					// dependency, so don't mark it as such.
+					continue
+				}
+			}
+
+			// dep is a package directly imported by a package or test in the main
+			// module and loaded from some other module (not the standard library).
+			// Mark its module as a direct dependency.
+			direct[dep.mod.Path] = true
+		}
+	}
+	if maxTooNew != nil {
+		return false, maxTooNew
+	}
+
+	var addRoots []module.Version
+	if ld.Tidy {
+		// When we are tidying a module with a pruned dependency graph, we may need
+		// to add roots to preserve the versions of indirect, test-only dependencies
+		// that are upgraded above or otherwise missing from the go.mod files of
+		// direct dependencies. (For example, the direct dependency might be a very
+		// stable codebase that predates modules and thus lacks a go.mod file, or
+		// the author of the direct dependency may have forgotten to commit a change
+		// to the go.mod file, or may have made an erroneous hand-edit that causes
+		// it to be untidy.)
+		//
+		// Promoting an indirect dependency to a root adds the next layer of its
+		// dependencies to the module graph, which may increase the selected
+		// versions of other modules from which we have already loaded packages.
+		// So after we promote an indirect dependency to a root, we need to reload
+		// packages, which means another iteration of loading.
+		//
+		// As an extra wrinkle, the upgrades due to promoting a root can cause
+		// previously-resolved packages to become unresolved. For example, the
+		// module providing an unstable package might be upgraded to a version
+		// that no longer contains that package. If we then resolve the missing
+		// package, we might add yet another root that upgrades away some other
+		// dependency. (The tests in mod_tidy_convergence*.txt illustrate some
+		// particularly worrisome cases.)
+		//
+		// To ensure that this process of promoting, adding, and upgrading roots
+		// eventually terminates, during iteration we only ever add modules to the
+		// root set — we only remove irrelevant roots at the very end of
+		// iteration, after we have already added every root that we plan to need
+		// in the (eventual) tidy root set.
+		//
+		// Since we do not remove any roots during iteration, even if they no
+		// longer provide any imported packages, the selected versions of the
+		// roots can only increase and the set of roots can only expand. The set
+		// of extant root paths is finite and the set of versions of each path is
+		// finite, so the iteration *must* reach a stable fixed-point.
+		tidy, err := tidyRoots(ctx, rs, ld.pkgs)
+		if err != nil {
+			return false, err
+		}
+		addRoots = tidy.rootModules
+	}
+
+	rs, err = updateRoots(ctx, direct, rs, ld.pkgs, addRoots, ld.AssumeRootsImported)
+	if err != nil {
+		// We don't actually know what even the root requirements are supposed to be,
+		// so we can't proceed with loading. Return the error to the caller
+		return false, err
+	}
+
+	if rs.GoVersion() != ld.requirements.GoVersion() {
+		// A change in the selected Go version may or may not affect the set of
+		// loaded packages, but in some cases it can change the meaning of the "all"
+		// pattern, the level of pruning in the module graph, and even the set of
+		// packages present in the standard library. If it has changed, it's best to
+		// reload packages once more to be sure everything is stable.
+		changed = true
+	} else if rs != ld.requirements && !reflect.DeepEqual(rs.rootModules, ld.requirements.rootModules) {
+		// The roots of the module graph have changed in some way (not just the
+		// "direct" markings). Check whether the changes affected any of the loaded
+		// packages.
+		mg, err := rs.Graph(ctx)
+		if err != nil {
+			return false, err
+		}
+		for _, pkg := range ld.pkgs {
+			if pkg.fromExternalModule() && mg.Selected(pkg.mod.Path) != pkg.mod.Version {
+				changed = true
+				break
+			}
+			if pkg.err != nil {
+				// Promoting a module to a root may resolve an import that was
+				// previously missing (by pulling in a previously-prune dependency that
+				// provides it) or ambiguous (by promoting exactly one of the
+				// alternatives to a root and ignoring the second-level alternatives) or
+				// otherwise errored out (by upgrading from a version that cannot be
+				// fetched to one that can be).
+				//
+				// Instead of enumerating all of the possible errors, we'll just check
+				// whether importFromModules returns nil for the package.
+				// False-positives are ok: if we have a false-positive here, we'll do an
+				// extra iteration of package loading this time, but we'll still
+				// converge when the root set stops changing.
+				//
+				// In some sense, we can think of this as ‘upgraded the module providing
+				// pkg.path from "none" to a version higher than "none"’.
+				if _, _, _, _, err = importFromModules(ctx, pkg.path, rs, nil, ld.skipImportModFiles); err == nil {
+					changed = true
+					break
+				}
+			}
+		}
+	}
+
+	ld.requirements = rs
+	return changed, nil
+}
+
+// resolveMissingImports returns a set of modules that could be added as
+// dependencies in order to resolve missing packages from pkgs.
+//
+// The newly-resolved packages are added to the addedModuleFor map, and
+// resolveMissingImports returns a map from each new module version to
+// the first missing package that module would resolve.
+func (ld *loader) resolveMissingImports(ctx context.Context) (modAddedBy map[module.Version]*loadPkg, err error) {
+	type pkgMod struct {
+		pkg *loadPkg
+		mod *module.Version
+	}
+	var pkgMods []pkgMod
+	for _, pkg := range ld.pkgs {
+		if pkg.err == nil {
+			continue
+		}
+		if pkg.isTest() {
+			// If we are missing a test, we are also missing its non-test version, and
+			// we should only add the missing import once.
+			continue
+		}
+		if !errors.As(pkg.err, new(*ImportMissingError)) {
+			// Leave other errors for Import or load.Packages to report.
+			continue
+		}
+
+		pkg := pkg
+		var mod module.Version
+		ld.work.Add(func() {
+			var err error
+			mod, err = queryImport(ctx, pkg.path, ld.requirements)
+			if err != nil {
+				var ime *ImportMissingError
+				if errors.As(err, &ime) {
+					for curstack := pkg.stack; curstack != nil; curstack = curstack.stack {
+						if MainModules.Contains(curstack.mod.Path) {
+							ime.ImportingMainModule = curstack.mod
+							break
+						}
+					}
+				}
+				// pkg.err was already non-nil, so we can reasonably attribute the error
+				// for pkg to either the original error or the one returned by
+				// queryImport. The existing error indicates only that we couldn't find
+				// the package, whereas the query error also explains why we didn't fix
+				// the problem — so we prefer the latter.
+				pkg.err = err
+			}
+
+			// err is nil, but we intentionally leave pkg.err non-nil and pkg.mod
+			// unset: we still haven't satisfied other invariants of a
+			// successfully-loaded package, such as scanning and loading the imports
+			// of that package. If we succeed in resolving the new dependency graph,
+			// the caller can reload pkg and update the error at that point.
+			//
+			// Even then, the package might not be loaded from the version we've
+			// identified here. The module may be upgraded by some other dependency,
+			// or by a transitive dependency of mod itself, or — less likely — the
+			// package may be rejected by an AllowPackage hook or rendered ambiguous
+			// by some other newly-added or newly-upgraded dependency.
+		})
+
+		pkgMods = append(pkgMods, pkgMod{pkg: pkg, mod: &mod})
+	}
+	<-ld.work.Idle()
+
+	modAddedBy = map[module.Version]*loadPkg{}
+
+	var (
+		maxTooNew    *gover.TooNewError
+		maxTooNewPkg *loadPkg
+	)
+	for _, pm := range pkgMods {
+		if tooNew := (*gover.TooNewError)(nil); errors.As(pm.pkg.err, &tooNew) {
+			if maxTooNew == nil || gover.Compare(tooNew.GoVersion, maxTooNew.GoVersion) > 0 {
+				maxTooNew = tooNew
+				maxTooNewPkg = pm.pkg
+			}
+		}
+	}
+	if maxTooNew != nil {
+		fmt.Fprintf(os.Stderr, "go: toolchain upgrade needed to resolve %s\n", maxTooNewPkg.path)
+		return nil, maxTooNew
+	}
+
+	for _, pm := range pkgMods {
+		pkg, mod := pm.pkg, *pm.mod
+		if mod.Path == "" {
+			continue
+		}
+
+		fmt.Fprintf(os.Stderr, "go: found %s in %s %s\n", pkg.path, mod.Path, mod.Version)
+		if modAddedBy[mod] == nil {
+			modAddedBy[mod] = pkg
+		}
+	}
+
+	return modAddedBy, nil
+}
+
+// pkg locates the *loadPkg for path, creating and queuing it for loading if
+// needed, and updates its state to reflect the given flags.
+//
+// The imports of the returned *loadPkg will be loaded asynchronously in the
+// ld.work queue, and its test (if requested) will also be populated once
+// imports have been resolved. When ld.work goes idle, all transitive imports of
+// the requested package (and its test, if requested) will have been loaded.
+func (ld *loader) pkg(ctx context.Context, path string, flags loadPkgFlags) *loadPkg {
+	if flags.has(pkgImportsLoaded) {
+		panic("internal error: (*loader).pkg called with pkgImportsLoaded flag set")
+	}
+
+	pkg := ld.pkgCache.Do(path, func() *loadPkg {
+		pkg := &loadPkg{
+			path: path,
+		}
+		ld.applyPkgFlags(ctx, pkg, flags)
+
+		ld.work.Add(func() { ld.load(ctx, pkg) })
+		return pkg
+	})
+
+	ld.applyPkgFlags(ctx, pkg, flags)
+	return pkg
+}
+
+// applyPkgFlags updates pkg.flags to set the given flags and propagate the
+// (transitive) effects of those flags, possibly loading or enqueueing further
+// packages as a result.
+func (ld *loader) applyPkgFlags(ctx context.Context, pkg *loadPkg, flags loadPkgFlags) {
+	if flags == 0 {
+		return
+	}
+
+	if flags.has(pkgInAll) && ld.allPatternIsRoot && !pkg.isTest() {
+		// This package matches a root pattern by virtue of being in "all".
+		flags |= pkgIsRoot
+	}
+	if flags.has(pkgIsRoot) {
+		flags |= pkgFromRoot
+	}
+
+	old := pkg.flags.update(flags)
+	new := old | flags
+	if new == old || !new.has(pkgImportsLoaded) {
+		// We either didn't change the state of pkg, or we don't know anything about
+		// its dependencies yet. Either way, we can't usefully load its test or
+		// update its dependencies.
+		return
+	}
+
+	if !pkg.isTest() {
+		// Check whether we should add (or update the flags for) a test for pkg.
+		// ld.pkgTest is idempotent and extra invocations are inexpensive,
+		// so it's ok if we call it more than is strictly necessary.
+		wantTest := false
+		switch {
+		case ld.allPatternIsRoot && MainModules.Contains(pkg.mod.Path):
+			// We are loading the "all" pattern, which includes packages imported by
+			// tests in the main module. This package is in the main module, so we
+			// need to identify the imports of its test even if LoadTests is not set.
+			//
+			// (We will filter out the extra tests explicitly in computePatternAll.)
+			wantTest = true
+
+		case ld.allPatternIsRoot && ld.allClosesOverTests && new.has(pkgInAll):
+			// This variant of the "all" pattern includes imports of tests of every
+			// package that is itself in "all", and pkg is in "all", so its test is
+			// also in "all" (as above).
+			wantTest = true
+
+		case ld.LoadTests && new.has(pkgIsRoot):
+			// LoadTest explicitly requests tests of “the root packages”.
+			wantTest = true
+		}
+
+		if wantTest {
+			var testFlags loadPkgFlags
+			if MainModules.Contains(pkg.mod.Path) || (ld.allClosesOverTests && new.has(pkgInAll)) {
+				// Tests of packages in the main module are in "all", in the sense that
+				// they cause the packages they import to also be in "all". So are tests
+				// of packages in "all" if "all" closes over test dependencies.
+				testFlags |= pkgInAll
+			}
+			ld.pkgTest(ctx, pkg, testFlags)
+		}
+	}
+
+	if new.has(pkgInAll) && !old.has(pkgInAll|pkgImportsLoaded) {
+		// We have just marked pkg with pkgInAll, or we have just loaded its
+		// imports, or both. Now is the time to propagate pkgInAll to the imports.
+		for _, dep := range pkg.imports {
+			ld.applyPkgFlags(ctx, dep, pkgInAll)
+		}
+	}
+
+	if new.has(pkgFromRoot) && !old.has(pkgFromRoot|pkgImportsLoaded) {
+		for _, dep := range pkg.imports {
+			ld.applyPkgFlags(ctx, dep, pkgFromRoot)
+		}
+	}
+}
+
+// preloadRootModules loads the module requirements needed to identify the
+// selected version of each module providing a package in rootPkgs,
+// adding new root modules to the module graph if needed.
+func (ld *loader) preloadRootModules(ctx context.Context, rootPkgs []string) (changedBuildList bool) {
+	needc := make(chan map[module.Version]bool, 1)
+	needc <- map[module.Version]bool{}
+	for _, path := range rootPkgs {
+		path := path
+		ld.work.Add(func() {
+			// First, try to identify the module containing the package using only roots.
+			//
+			// If the main module is tidy and the package is in "all" — or if we're
+			// lucky — we can identify all of its imports without actually loading the
+			// full module graph.
+			m, _, _, _, err := importFromModules(ctx, path, ld.requirements, nil, ld.skipImportModFiles)
+			if err != nil {
+				var missing *ImportMissingError
+				if errors.As(err, &missing) && ld.ResolveMissingImports {
+					// This package isn't provided by any selected module.
+					// If we can find it, it will be a new root dependency.
+					m, err = queryImport(ctx, path, ld.requirements)
+				}
+				if err != nil {
+					// We couldn't identify the root module containing this package.
+					// Leave it unresolved; we will report it during loading.
+					return
+				}
+			}
+			if m.Path == "" {
+				// The package is in std or cmd. We don't need to change the root set.
+				return
+			}
+
+			v, ok := ld.requirements.rootSelected(m.Path)
+			if !ok || v != m.Version {
+				// We found the requested package in m, but m is not a root, so
+				// loadModGraph will not load its requirements. We need to promote the
+				// module to a root to ensure that any other packages this package
+				// imports are resolved from correct dependency versions.
+				//
+				// (This is the “argument invariant” from
+				// https://golang.org/design/36460-lazy-module-loading.)
+				need := <-needc
+				need[m] = true
+				needc <- need
+			}
+		})
+	}
+	<-ld.work.Idle()
+
+	need := <-needc
+	if len(need) == 0 {
+		return false // No roots to add.
+	}
+
+	toAdd := make([]module.Version, 0, len(need))
+	for m := range need {
+		toAdd = append(toAdd, m)
+	}
+	gover.ModSort(toAdd)
+
+	rs, err := updateRoots(ctx, ld.requirements.direct, ld.requirements, nil, toAdd, ld.AssumeRootsImported)
+	if err != nil {
+		// We are missing some root dependency, and for some reason we can't load
+		// enough of the module dependency graph to add the missing root. Package
+		// loading is doomed to fail, so fail quickly.
+		ld.error(err)
+		ld.exitIfErrors(ctx)
+		return false
+	}
+	if reflect.DeepEqual(rs.rootModules, ld.requirements.rootModules) {
+		// Something is deeply wrong. resolveMissingImports gave us a non-empty
+		// set of modules to add to the graph, but adding those modules had no
+		// effect — either they were already in the graph, or updateRoots did not
+		// add them as requested.
+		panic(fmt.Sprintf("internal error: adding %v to module graph had no effect on root requirements (%v)", toAdd, rs.rootModules))
+	}
+
+	ld.requirements = rs
+	return true
+}
+
+// load loads an individual package.
+func (ld *loader) load(ctx context.Context, pkg *loadPkg) {
+	var mg *ModuleGraph
+	if ld.requirements.pruning == unpruned {
+		var err error
+		mg, err = ld.requirements.Graph(ctx)
+		if err != nil {
+			// We already checked the error from Graph in loadFromRoots and/or
+			// updateRequirements, so we ignored the error on purpose and we should
+			// keep trying to push past it.
+			//
+			// However, because mg may be incomplete (and thus may select inaccurate
+			// versions), we shouldn't use it to load packages. Instead, we pass a nil
+			// *ModuleGraph, which will cause mg to first try loading from only the
+			// main module and root dependencies.
+			mg = nil
+		}
+	}
+
+	var modroot string
+	pkg.mod, modroot, pkg.dir, pkg.altMods, pkg.err = importFromModules(ctx, pkg.path, ld.requirements, mg, ld.skipImportModFiles)
+	if pkg.dir == "" {
+		return
+	}
+	if MainModules.Contains(pkg.mod.Path) {
+		// Go ahead and mark pkg as in "all". This provides the invariant that a
+		// package that is *only* imported by other packages in "all" is always
+		// marked as such before loading its imports.
+		//
+		// We don't actually rely on that invariant at the moment, but it may
+		// improve efficiency somewhat and makes the behavior a bit easier to reason
+		// about (by reducing churn on the flag bits of dependencies), and costs
+		// essentially nothing (these atomic flag ops are essentially free compared
+		// to scanning source code for imports).
+		ld.applyPkgFlags(ctx, pkg, pkgInAll)
+	}
+	if ld.AllowPackage != nil {
+		if err := ld.AllowPackage(ctx, pkg.path, pkg.mod); err != nil {
+			pkg.err = err
+		}
+	}
+
+	pkg.inStd = (search.IsStandardImportPath(pkg.path) && search.InDir(pkg.dir, cfg.GOROOTsrc) != "")
+
+	var imports, testImports []string
+
+	if cfg.BuildContext.Compiler == "gccgo" && pkg.inStd {
+		// We can't scan standard packages for gccgo.
+	} else {
+		var err error
+		imports, testImports, err = scanDir(modroot, pkg.dir, ld.Tags)
+		if err != nil {
+			pkg.err = err
+			return
+		}
+	}
+
+	pkg.imports = make([]*loadPkg, 0, len(imports))
+	var importFlags loadPkgFlags
+	if pkg.flags.has(pkgInAll) {
+		importFlags = pkgInAll
+	}
+	for _, path := range imports {
+		if pkg.inStd {
+			// Imports from packages in "std" and "cmd" should resolve using
+			// GOROOT/src/vendor even when "std" is not the main module.
+			path = ld.stdVendor(pkg.path, path)
+		}
+		pkg.imports = append(pkg.imports, ld.pkg(ctx, path, importFlags))
+	}
+	pkg.testImports = testImports
+
+	ld.applyPkgFlags(ctx, pkg, pkgImportsLoaded)
+}
+
+// pkgTest locates the test of pkg, creating it if needed, and updates its state
+// to reflect the given flags.
+//
+// pkgTest requires that the imports of pkg have already been loaded (flagged
+// with pkgImportsLoaded).
+func (ld *loader) pkgTest(ctx context.Context, pkg *loadPkg, testFlags loadPkgFlags) *loadPkg {
+	if pkg.isTest() {
+		panic("pkgTest called on a test package")
+	}
+
+	createdTest := false
+	pkg.testOnce.Do(func() {
+		pkg.test = &loadPkg{
+			path:   pkg.path,
+			testOf: pkg,
+			mod:    pkg.mod,
+			dir:    pkg.dir,
+			err:    pkg.err,
+			inStd:  pkg.inStd,
+		}
+		ld.applyPkgFlags(ctx, pkg.test, testFlags)
+		createdTest = true
+	})
+
+	test := pkg.test
+	if createdTest {
+		test.imports = make([]*loadPkg, 0, len(pkg.testImports))
+		var importFlags loadPkgFlags
+		if test.flags.has(pkgInAll) {
+			importFlags = pkgInAll
+		}
+		for _, path := range pkg.testImports {
+			if pkg.inStd {
+				path = ld.stdVendor(test.path, path)
+			}
+			test.imports = append(test.imports, ld.pkg(ctx, path, importFlags))
+		}
+		pkg.testImports = nil
+		ld.applyPkgFlags(ctx, test, pkgImportsLoaded)
+	} else {
+		ld.applyPkgFlags(ctx, test, testFlags)
+	}
+
+	return test
+}
+
+// stdVendor returns the canonical import path for the package with the given
+// path when imported from the standard-library package at parentPath.
+func (ld *loader) stdVendor(parentPath, path string) string {
+	if search.IsStandardImportPath(path) {
+		return path
+	}
+
+	if str.HasPathPrefix(parentPath, "cmd") {
+		if !ld.VendorModulesInGOROOTSrc || !MainModules.Contains("cmd") {
+			vendorPath := pathpkg.Join("cmd", "vendor", path)
+
+			if _, err := os.Stat(filepath.Join(cfg.GOROOTsrc, filepath.FromSlash(vendorPath))); err == nil {
+				return vendorPath
+			}
+		}
+	} else if !ld.VendorModulesInGOROOTSrc || !MainModules.Contains("std") || str.HasPathPrefix(parentPath, "vendor") {
+		// If we are outside of the 'std' module, resolve imports from within 'std'
+		// to the vendor directory.
+		//
+		// Do the same for importers beginning with the prefix 'vendor/' even if we
+		// are *inside* of the 'std' module: the 'vendor/' packages that resolve
+		// globally from GOROOT/src/vendor (and are listed as part of 'go list std')
+		// are distinct from the real module dependencies, and cannot import
+		// internal packages from the real module.
+		//
+		// (Note that although the 'vendor/' packages match the 'std' *package*
+		// pattern, they are not part of the std *module*, and do not affect
+		// 'go mod tidy' and similar module commands when working within std.)
+		vendorPath := pathpkg.Join("vendor", path)
+		if _, err := os.Stat(filepath.Join(cfg.GOROOTsrc, filepath.FromSlash(vendorPath))); err == nil {
+			return vendorPath
+		}
+	}
+
+	// Not vendored: resolve from modules.
+	return path
+}
+
+// computePatternAll returns the list of packages matching pattern "all",
+// starting with a list of the import paths for the packages in the main module.
+func (ld *loader) computePatternAll() (all []string) {
+	for _, pkg := range ld.pkgs {
+		if pkg.flags.has(pkgInAll) && !pkg.isTest() {
+			all = append(all, pkg.path)
+		}
+	}
+	sort.Strings(all)
+	return all
+}
+
+// checkMultiplePaths verifies that a given module path is used as itself
+// or as a replacement for another module, but not both at the same time.
+//
+// (See https://golang.org/issue/26607 and https://golang.org/issue/34650.)
+func (ld *loader) checkMultiplePaths() {
+	mods := ld.requirements.rootModules
+	if cached := ld.requirements.graph.Load(); cached != nil {
+		if mg := cached.mg; mg != nil {
+			mods = mg.BuildList()
+		}
+	}
+
+	firstPath := map[module.Version]string{}
+	for _, mod := range mods {
+		src := resolveReplacement(mod)
+		if prev, ok := firstPath[src]; !ok {
+			firstPath[src] = mod.Path
+		} else if prev != mod.Path {
+			ld.error(fmt.Errorf("%s@%s used for two different module paths (%s and %s)", src.Path, src.Version, prev, mod.Path))
+		}
+	}
+}
+
+// checkTidyCompatibility emits an error if any package would be loaded from a
+// different module under rs than under ld.requirements.
+func (ld *loader) checkTidyCompatibility(ctx context.Context, rs *Requirements, compatVersion string) {
+	goVersion := rs.GoVersion()
+	suggestUpgrade := false
+	suggestEFlag := false
+	suggestFixes := func() {
+		if ld.AllowErrors {
+			// The user is explicitly ignoring these errors, so don't bother them with
+			// other options.
+			return
+		}
+
+		// We print directly to os.Stderr because this information is advice about
+		// how to fix errors, not actually an error itself.
+		// (The actual errors should have been logged already.)
+
+		fmt.Fprintln(os.Stderr)
+
+		goFlag := ""
+		if goVersion != MainModules.GoVersion() {
+			goFlag = " -go=" + goVersion
+		}
+
+		compatFlag := ""
+		if compatVersion != gover.Prev(goVersion) {
+			compatFlag = " -compat=" + compatVersion
+		}
+		if suggestUpgrade {
+			eDesc := ""
+			eFlag := ""
+			if suggestEFlag {
+				eDesc = ", leaving some packages unresolved"
+				eFlag = " -e"
+			}
+			fmt.Fprintf(os.Stderr, "To upgrade to the versions selected by go %s%s:\n\tgo mod tidy%s -go=%s && go mod tidy%s -go=%s%s\n", compatVersion, eDesc, eFlag, compatVersion, eFlag, goVersion, compatFlag)
+		} else if suggestEFlag {
+			// If some packages are missing but no package is upgraded, then we
+			// shouldn't suggest upgrading to the Go 1.16 versions explicitly — that
+			// wouldn't actually fix anything for Go 1.16 users, and *would* break
+			// something for Go 1.17 users.
+			fmt.Fprintf(os.Stderr, "To proceed despite packages unresolved in go %s:\n\tgo mod tidy -e%s%s\n", compatVersion, goFlag, compatFlag)
+		}
+
+		fmt.Fprintf(os.Stderr, "If reproducibility with go %s is not needed:\n\tgo mod tidy%s -compat=%s\n", compatVersion, goFlag, goVersion)
+
+		// TODO(#46141): Populate the linked wiki page.
+		fmt.Fprintf(os.Stderr, "For other options, see:\n\thttps://golang.org/doc/modules/pruning\n")
+	}
+
+	mg, err := rs.Graph(ctx)
+	if err != nil {
+		ld.error(fmt.Errorf("error loading go %s module graph: %w", compatVersion, err))
+		ld.switchIfErrors(ctx)
+		suggestFixes()
+		ld.exitIfErrors(ctx)
+		return
+	}
+
+	// Re-resolve packages in parallel.
+	//
+	// We re-resolve each package — rather than just checking versions — to ensure
+	// that we have fetched module source code (and, importantly, checksums for
+	// that source code) for all modules that are necessary to ensure that imports
+	// are unambiguous. That also produces clearer diagnostics, since we can say
+	// exactly what happened to the package if it became ambiguous or disappeared
+	// entirely.
+	//
+	// We re-resolve the packages in parallel because this process involves disk
+	// I/O to check for package sources, and because the process of checking for
+	// ambiguous imports may require us to download additional modules that are
+	// otherwise pruned out in Go 1.17 — we don't want to block progress on other
+	// packages while we wait for a single new download.
+	type mismatch struct {
+		mod module.Version
+		err error
+	}
+	mismatchMu := make(chan map[*loadPkg]mismatch, 1)
+	mismatchMu <- map[*loadPkg]mismatch{}
+	for _, pkg := range ld.pkgs {
+		if pkg.mod.Path == "" && pkg.err == nil {
+			// This package is from the standard library (which does not vary based on
+			// the module graph).
+			continue
+		}
+
+		pkg := pkg
+		ld.work.Add(func() {
+			mod, _, _, _, err := importFromModules(ctx, pkg.path, rs, mg, ld.skipImportModFiles)
+			if mod != pkg.mod {
+				mismatches := <-mismatchMu
+				mismatches[pkg] = mismatch{mod: mod, err: err}
+				mismatchMu <- mismatches
+			}
+		})
+	}
+	<-ld.work.Idle()
+
+	mismatches := <-mismatchMu
+	if len(mismatches) == 0 {
+		// Since we're running as part of 'go mod tidy', the roots of the module
+		// graph should contain only modules that are relevant to some package in
+		// the package graph. We checked every package in the package graph and
+		// didn't find any mismatches, so that must mean that all of the roots of
+		// the module graph are also consistent.
+		//
+		// If we're wrong, Go 1.16 in -mod=readonly mode will error out with
+		// "updates to go.mod needed", which would be very confusing. So instead,
+		// we'll double-check that our reasoning above actually holds — if it
+		// doesn't, we'll emit an internal error and hopefully the user will report
+		// it as a bug.
+		for _, m := range ld.requirements.rootModules {
+			if v := mg.Selected(m.Path); v != m.Version {
+				fmt.Fprintln(os.Stderr)
+				base.Fatalf("go: internal error: failed to diagnose selected-version mismatch for module %s: go %s selects %s, but go %s selects %s\n\tPlease report this at https://golang.org/issue.", m.Path, goVersion, m.Version, compatVersion, v)
+			}
+		}
+		return
+	}
+
+	// Iterate over the packages (instead of the mismatches map) to emit errors in
+	// deterministic order.
+	for _, pkg := range ld.pkgs {
+		mismatch, ok := mismatches[pkg]
+		if !ok {
+			continue
+		}
+
+		if pkg.isTest() {
+			// We already did (or will) report an error for the package itself,
+			// so don't report a duplicate (and more verbose) error for its test.
+			if _, ok := mismatches[pkg.testOf]; !ok {
+				base.Fatalf("go: internal error: mismatch recorded for test %s, but not its non-test package", pkg.path)
+			}
+			continue
+		}
+
+		switch {
+		case mismatch.err != nil:
+			// pkg resolved successfully, but errors out using the requirements in rs.
+			//
+			// This could occur because the import is provided by a single root (and
+			// is thus unambiguous in a main module with a pruned module graph) and
+			// also one or more transitive dependencies (and is ambiguous with an
+			// unpruned graph).
+			//
+			// It could also occur because some transitive dependency upgrades the
+			// module that previously provided the package to a version that no
+			// longer does, or to a version for which the module source code (but
+			// not the go.mod file in isolation) has a checksum error.
+			if missing := (*ImportMissingError)(nil); errors.As(mismatch.err, &missing) {
+				selected := module.Version{
+					Path:    pkg.mod.Path,
+					Version: mg.Selected(pkg.mod.Path),
+				}
+				ld.error(fmt.Errorf("%s loaded from %v,\n\tbut go %s would fail to locate it in %s", pkg.stackText(), pkg.mod, compatVersion, selected))
+			} else {
+				if ambiguous := (*AmbiguousImportError)(nil); errors.As(mismatch.err, &ambiguous) {
+					// TODO: Is this check needed?
+				}
+				ld.error(fmt.Errorf("%s loaded from %v,\n\tbut go %s would fail to locate it:\n\t%v", pkg.stackText(), pkg.mod, compatVersion, mismatch.err))
+			}
+
+			suggestEFlag = true
+
+			// Even if we press ahead with the '-e' flag, the older version will
+			// error out in readonly mode if it thinks the go.mod file contains
+			// any *explicit* dependency that is not at its selected version,
+			// even if that dependency is not relevant to any package being loaded.
+			//
+			// We check for that condition here. If all of the roots are consistent
+			// the '-e' flag suffices, but otherwise we need to suggest an upgrade.
+			if !suggestUpgrade {
+				for _, m := range ld.requirements.rootModules {
+					if v := mg.Selected(m.Path); v != m.Version {
+						suggestUpgrade = true
+						break
+					}
+				}
+			}
+
+		case pkg.err != nil:
+			// pkg had an error in with a pruned module graph (presumably suppressed
+			// with the -e flag), but the error went away using an unpruned graph.
+			//
+			// This is possible, if, say, the import is unresolved in the pruned graph
+			// (because the "latest" version of each candidate module either is
+			// unavailable or does not contain the package), but is resolved in the
+			// unpruned graph due to a newer-than-latest dependency that is normally
+			// pruned out.
+			//
+			// This could also occur if the source code for the module providing the
+			// package in the pruned graph has a checksum error, but the unpruned
+			// graph upgrades that module to a version with a correct checksum.
+			//
+			// pkg.err should have already been logged elsewhere — along with a
+			// stack trace — so log only the import path and non-error info here.
+			suggestUpgrade = true
+			ld.error(fmt.Errorf("%s failed to load from any module,\n\tbut go %s would load it from %v", pkg.path, compatVersion, mismatch.mod))
+
+		case pkg.mod != mismatch.mod:
+			// The package is loaded successfully by both Go versions, but from a
+			// different module in each. This could lead to subtle (and perhaps even
+			// unnoticed!) variations in behavior between builds with different
+			// toolchains.
+			suggestUpgrade = true
+			ld.error(fmt.Errorf("%s loaded from %v,\n\tbut go %s would select %v\n", pkg.stackText(), pkg.mod, compatVersion, mismatch.mod.Version))
+
+		default:
+			base.Fatalf("go: internal error: mismatch recorded for package %s, but no differences found", pkg.path)
+		}
+	}
+
+	ld.switchIfErrors(ctx)
+	suggestFixes()
+	ld.exitIfErrors(ctx)
+}
+
+// scanDir is like imports.ScanDir but elides known magic imports from the list,
+// so that we do not go looking for packages that don't really exist.
+//
+// The standard magic import is "C", for cgo.
+//
+// The only other known magic imports are appengine and appengine/*.
+// These are so old that they predate "go get" and did not use URL-like paths.
+// Most code today now uses google.golang.org/appengine instead,
+// but not all code has been so updated. When we mostly ignore build tags
+// during "go vendor", we look into "// +build appengine" files and
+// may see these legacy imports. We drop them so that the module
+// search does not look for modules to try to satisfy them.
+func scanDir(modroot string, dir string, tags map[string]bool) (imports_, testImports []string, err error) {
+	if ip, mierr := modindex.GetPackage(modroot, dir); mierr == nil {
+		imports_, testImports, err = ip.ScanDir(tags)
+		goto Happy
+	} else if !errors.Is(mierr, modindex.ErrNotIndexed) {
+		return nil, nil, mierr
+	}
+
+	imports_, testImports, err = imports.ScanDir(dir, tags)
+Happy:
+
+	filter := func(x []string) []string {
+		w := 0
+		for _, pkg := range x {
+			if pkg != "C" && pkg != "appengine" && !strings.HasPrefix(pkg, "appengine/") &&
+				pkg != "appengine_internal" && !strings.HasPrefix(pkg, "appengine_internal/") {
+				x[w] = pkg
+				w++
+			}
+		}
+		return x[:w]
+	}
+
+	return filter(imports_), filter(testImports), err
+}
+
+// buildStacks computes minimal import stacks for each package,
+// for use in error messages. When it completes, packages that
+// are part of the original root set have pkg.stack == nil,
+// and other packages have pkg.stack pointing at the next
+// package up the import stack in their minimal chain.
+// As a side effect, buildStacks also constructs ld.pkgs,
+// the list of all packages loaded.
+func (ld *loader) buildStacks() {
+	if len(ld.pkgs) > 0 {
+		panic("buildStacks")
+	}
+	for _, pkg := range ld.roots {
+		pkg.stack = pkg // sentinel to avoid processing in next loop
+		ld.pkgs = append(ld.pkgs, pkg)
+	}
+	for i := 0; i < len(ld.pkgs); i++ { // not range: appending to ld.pkgs in loop
+		pkg := ld.pkgs[i]
+		for _, next := range pkg.imports {
+			if next.stack == nil {
+				next.stack = pkg
+				ld.pkgs = append(ld.pkgs, next)
+			}
+		}
+		if next := pkg.test; next != nil && next.stack == nil {
+			next.stack = pkg
+			ld.pkgs = append(ld.pkgs, next)
+		}
+	}
+	for _, pkg := range ld.roots {
+		pkg.stack = nil
+	}
+}
+
+// stackText builds the import stack text to use when
+// reporting an error in pkg. It has the general form
+//
+//	root imports
+//		other imports
+//		other2 tested by
+//		other2.test imports
+//		pkg
+func (pkg *loadPkg) stackText() string {
+	var stack []*loadPkg
+	for p := pkg; p != nil; p = p.stack {
+		stack = append(stack, p)
+	}
+
+	var buf strings.Builder
+	for i := len(stack) - 1; i >= 0; i-- {
+		p := stack[i]
+		fmt.Fprint(&buf, p.path)
+		if p.testOf != nil {
+			fmt.Fprint(&buf, ".test")
+		}
+		if i > 0 {
+			if stack[i-1].testOf == p {
+				fmt.Fprint(&buf, " tested by\n\t")
+			} else {
+				fmt.Fprint(&buf, " imports\n\t")
+			}
+		}
+	}
+	return buf.String()
+}
+
+// why returns the text to use in "go mod why" output about the given package.
+// It is less ornate than the stackText but contains the same information.
+func (pkg *loadPkg) why() string {
+	var buf strings.Builder
+	var stack []*loadPkg
+	for p := pkg; p != nil; p = p.stack {
+		stack = append(stack, p)
+	}
+
+	for i := len(stack) - 1; i >= 0; i-- {
+		p := stack[i]
+		if p.testOf != nil {
+			fmt.Fprintf(&buf, "%s.test\n", p.testOf.path)
+		} else {
+			fmt.Fprintf(&buf, "%s\n", p.path)
+		}
+	}
+	return buf.String()
+}
+
+// Why returns the "go mod why" output stanza for the given package,
+// without the leading # comment.
+// The package graph must have been loaded already, usually by LoadPackages.
+// If there is no reason for the package to be in the current build,
+// Why returns an empty string.
+func Why(path string) string {
+	pkg, ok := loaded.pkgCache.Get(path)
+	if !ok {
+		return ""
+	}
+	return pkg.why()
+}
+
+// WhyDepth returns the number of steps in the Why listing.
+// If there is no reason for the package to be in the current build,
+// WhyDepth returns 0.
+func WhyDepth(path string) int {
+	n := 0
+	pkg, _ := loaded.pkgCache.Get(path)
+	for p := pkg; p != nil; p = p.stack {
+		n++
+	}
+	return n
+}
diff --git a/src/cmd/go/internal/modload/modfile.go b/src/cmd/go/internal/modload/modfile.go
new file mode 100644
index 0000000..d6c395f
--- /dev/null
+++ b/src/cmd/go/internal/modload/modfile.go
@@ -0,0 +1,806 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+	"unicode"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/lockedfile"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/par"
+	"cmd/go/internal/trace"
+
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/module"
+)
+
+// ReadModFile reads and parses the mod file at gomod. ReadModFile properly applies the
+// overlay, locks the file while reading, and applies fix, if applicable.
+func ReadModFile(gomod string, fix modfile.VersionFixer) (data []byte, f *modfile.File, err error) {
+	gomod = base.ShortPath(gomod) // use short path in any errors
+	if gomodActual, ok := fsys.OverlayPath(gomod); ok {
+		// Don't lock go.mod if it's part of the overlay.
+		// On Plan 9, locking requires chmod, and we don't want to modify any file
+		// in the overlay. See #44700.
+		data, err = os.ReadFile(gomodActual)
+	} else {
+		data, err = lockedfile.Read(gomodActual)
+	}
+	if err != nil {
+		return nil, nil, err
+	}
+
+	f, err = modfile.Parse(gomod, data, fix)
+	if err != nil {
+		// Errors returned by modfile.Parse begin with file:line.
+		return nil, nil, fmt.Errorf("errors parsing %s:\n%w", gomod, err)
+	}
+	if f.Go != nil && gover.Compare(f.Go.Version, gover.Local()) > 0 {
+		toolchain := ""
+		if f.Toolchain != nil {
+			toolchain = f.Toolchain.Name
+		}
+		return nil, nil, &gover.TooNewError{What: gomod, GoVersion: f.Go.Version, Toolchain: toolchain}
+	}
+	if f.Module == nil {
+		// No module declaration. Must add module path.
+		return nil, nil, fmt.Errorf("error reading %s: missing module declaration. To specify the module path:\n\tgo mod edit -module=example.com/mod", gomod)
+	}
+
+	return data, f, err
+}
+
+// A modFileIndex is an index of data corresponding to a modFile
+// at a specific point in time.
+type modFileIndex struct {
+	data         []byte
+	dataNeedsFix bool // true if fixVersion applied a change while parsing data
+	module       module.Version
+	goVersion    string // Go version (no "v" or "go" prefix)
+	toolchain    string
+	require      map[module.Version]requireMeta
+	replace      map[module.Version]module.Version
+	exclude      map[module.Version]bool
+}
+
+type requireMeta struct {
+	indirect bool
+}
+
+// A modPruning indicates whether transitive dependencies of Go 1.17 dependencies
+// are pruned out of the module subgraph rooted at a given module.
+// (See https://golang.org/ref/mod#graph-pruning.)
+type modPruning uint8
+
+const (
+	pruned    modPruning = iota // transitive dependencies of modules at go 1.17 and higher are pruned out
+	unpruned                    // no transitive dependencies are pruned out
+	workspace                   // pruned to the union of modules in the workspace
+)
+
+func (p modPruning) String() string {
+	switch p {
+	case pruned:
+		return "pruned"
+	case unpruned:
+		return "unpruned"
+	case workspace:
+		return "workspace"
+	default:
+		return fmt.Sprintf("%T(%d)", p, p)
+	}
+}
+
+func pruningForGoVersion(goVersion string) modPruning {
+	if gover.Compare(goVersion, gover.ExplicitIndirectVersion) < 0 {
+		// The go.mod file does not duplicate relevant information about transitive
+		// dependencies, so they cannot be pruned out.
+		return unpruned
+	}
+	return pruned
+}
+
+// CheckAllowed returns an error equivalent to ErrDisallowed if m is excluded by
+// the main module's go.mod or retracted by its author. Most version queries use
+// this to filter out versions that should not be used.
+func CheckAllowed(ctx context.Context, m module.Version) error {
+	if err := CheckExclusions(ctx, m); err != nil {
+		return err
+	}
+	if err := CheckRetractions(ctx, m); err != nil {
+		return err
+	}
+	return nil
+}
+
+// ErrDisallowed is returned by version predicates passed to Query and similar
+// functions to indicate that a version should not be considered.
+var ErrDisallowed = errors.New("disallowed module version")
+
+// CheckExclusions returns an error equivalent to ErrDisallowed if module m is
+// excluded by the main module's go.mod file.
+func CheckExclusions(ctx context.Context, m module.Version) error {
+	for _, mainModule := range MainModules.Versions() {
+		if index := MainModules.Index(mainModule); index != nil && index.exclude[m] {
+			return module.VersionError(m, errExcluded)
+		}
+	}
+	return nil
+}
+
+var errExcluded = &excludedError{}
+
+type excludedError struct{}
+
+func (e *excludedError) Error() string     { return "excluded by go.mod" }
+func (e *excludedError) Is(err error) bool { return err == ErrDisallowed }
+
+// CheckRetractions returns an error if module m has been retracted by
+// its author.
+func CheckRetractions(ctx context.Context, m module.Version) (err error) {
+	defer func() {
+		if retractErr := (*ModuleRetractedError)(nil); err == nil || errors.As(err, &retractErr) {
+			return
+		}
+		// Attribute the error to the version being checked, not the version from
+		// which the retractions were to be loaded.
+		if mErr := (*module.ModuleError)(nil); errors.As(err, &mErr) {
+			err = mErr.Err
+		}
+		err = &retractionLoadingError{m: m, err: err}
+	}()
+
+	if m.Version == "" {
+		// Main module, standard library, or file replacement module.
+		// Cannot be retracted.
+		return nil
+	}
+	if repl := Replacement(module.Version{Path: m.Path}); repl.Path != "" {
+		// All versions of the module were replaced.
+		// Don't load retractions, since we'd just load the replacement.
+		return nil
+	}
+
+	// Find the latest available version of the module, and load its go.mod. If
+	// the latest version is replaced, we'll load the replacement.
+	//
+	// If there's an error loading the go.mod, we'll return it here. These errors
+	// should generally be ignored by callers since they happen frequently when
+	// we're offline. These errors are not equivalent to ErrDisallowed, so they
+	// may be distinguished from retraction errors.
+	//
+	// We load the raw file here: the go.mod file may have a different module
+	// path that we expect if the module or its repository was renamed.
+	// We still want to apply retractions to other aliases of the module.
+	rm, err := queryLatestVersionIgnoringRetractions(ctx, m.Path)
+	if err != nil {
+		return err
+	}
+	summary, err := rawGoModSummary(rm)
+	if err != nil {
+		return err
+	}
+
+	var rationale []string
+	isRetracted := false
+	for _, r := range summary.retract {
+		if gover.ModCompare(m.Path, r.Low, m.Version) <= 0 && gover.ModCompare(m.Path, m.Version, r.High) <= 0 {
+			isRetracted = true
+			if r.Rationale != "" {
+				rationale = append(rationale, r.Rationale)
+			}
+		}
+	}
+	if isRetracted {
+		return module.VersionError(m, &ModuleRetractedError{Rationale: rationale})
+	}
+	return nil
+}
+
+type ModuleRetractedError struct {
+	Rationale []string
+}
+
+func (e *ModuleRetractedError) Error() string {
+	msg := "retracted by module author"
+	if len(e.Rationale) > 0 {
+		// This is meant to be a short error printed on a terminal, so just
+		// print the first rationale.
+		msg += ": " + ShortMessage(e.Rationale[0], "retracted by module author")
+	}
+	return msg
+}
+
+func (e *ModuleRetractedError) Is(err error) bool {
+	return err == ErrDisallowed
+}
+
+type retractionLoadingError struct {
+	m   module.Version
+	err error
+}
+
+func (e *retractionLoadingError) Error() string {
+	return fmt.Sprintf("loading module retractions for %v: %v", e.m, e.err)
+}
+
+func (e *retractionLoadingError) Unwrap() error {
+	return e.err
+}
+
+// ShortMessage returns a string from go.mod (for example, a retraction
+// rationale or deprecation message) that is safe to print in a terminal.
+//
+// If the given string is empty, ShortMessage returns the given default. If the
+// given string is too long or contains non-printable characters, ShortMessage
+// returns a hard-coded string.
+func ShortMessage(message, emptyDefault string) string {
+	const maxLen = 500
+	if i := strings.Index(message, "\n"); i >= 0 {
+		message = message[:i]
+	}
+	message = strings.TrimSpace(message)
+	if message == "" {
+		return emptyDefault
+	}
+	if len(message) > maxLen {
+		return "(message omitted: too long)"
+	}
+	for _, r := range message {
+		if !unicode.IsGraphic(r) && !unicode.IsSpace(r) {
+			return "(message omitted: contains non-printable characters)"
+		}
+	}
+	// NOTE: the go.mod parser rejects invalid UTF-8, so we don't check that here.
+	return message
+}
+
+// CheckDeprecation returns a deprecation message from the go.mod file of the
+// latest version of the given module. Deprecation messages are comments
+// before or on the same line as the module directives that start with
+// "Deprecated:" and run until the end of the paragraph.
+//
+// CheckDeprecation returns an error if the message can't be loaded.
+// CheckDeprecation returns "", nil if there is no deprecation message.
+func CheckDeprecation(ctx context.Context, m module.Version) (deprecation string, err error) {
+	defer func() {
+		if err != nil {
+			err = fmt.Errorf("loading deprecation for %s: %w", m.Path, err)
+		}
+	}()
+
+	if m.Version == "" {
+		// Main module, standard library, or file replacement module.
+		// Don't look up deprecation.
+		return "", nil
+	}
+	if repl := Replacement(module.Version{Path: m.Path}); repl.Path != "" {
+		// All versions of the module were replaced.
+		// We'll look up deprecation separately for the replacement.
+		return "", nil
+	}
+
+	latest, err := queryLatestVersionIgnoringRetractions(ctx, m.Path)
+	if err != nil {
+		return "", err
+	}
+	summary, err := rawGoModSummary(latest)
+	if err != nil {
+		return "", err
+	}
+	return summary.deprecated, nil
+}
+
+func replacement(mod module.Version, replace map[module.Version]module.Version) (fromVersion string, to module.Version, ok bool) {
+	if r, ok := replace[mod]; ok {
+		return mod.Version, r, true
+	}
+	if r, ok := replace[module.Version{Path: mod.Path}]; ok {
+		return "", r, true
+	}
+	return "", module.Version{}, false
+}
+
+// Replacement returns the replacement for mod, if any. If the path in the
+// module.Version is relative it's relative to the single main module outside
+// workspace mode, or the workspace's directory in workspace mode.
+func Replacement(mod module.Version) module.Version {
+	foundFrom, found, foundModRoot := "", module.Version{}, ""
+	if MainModules == nil {
+		return module.Version{}
+	} else if MainModules.Contains(mod.Path) && mod.Version == "" {
+		// Don't replace the workspace version of the main module.
+		return module.Version{}
+	}
+	if _, r, ok := replacement(mod, MainModules.WorkFileReplaceMap()); ok {
+		return r
+	}
+	for _, v := range MainModules.Versions() {
+		if index := MainModules.Index(v); index != nil {
+			if from, r, ok := replacement(mod, index.replace); ok {
+				modRoot := MainModules.ModRoot(v)
+				if foundModRoot != "" && foundFrom != from && found != r {
+					base.Errorf("conflicting replacements found for %v in workspace modules defined by %v and %v",
+						mod, modFilePath(foundModRoot), modFilePath(modRoot))
+					return canonicalizeReplacePath(found, foundModRoot)
+				}
+				found, foundModRoot = r, modRoot
+			}
+		}
+	}
+	return canonicalizeReplacePath(found, foundModRoot)
+}
+
+func replaceRelativeTo() string {
+	if workFilePath := WorkFilePath(); workFilePath != "" {
+		return filepath.Dir(workFilePath)
+	}
+	return MainModules.ModRoot(MainModules.mustGetSingleMainModule())
+}
+
+// canonicalizeReplacePath ensures that relative, on-disk, replaced module paths
+// are relative to the workspace directory (in workspace mode) or to the module's
+// directory (in module mode, as they already are).
+func canonicalizeReplacePath(r module.Version, modRoot string) module.Version {
+	if filepath.IsAbs(r.Path) || r.Version != "" {
+		return r
+	}
+	workFilePath := WorkFilePath()
+	if workFilePath == "" {
+		return r
+	}
+	abs := filepath.Join(modRoot, r.Path)
+	if rel, err := filepath.Rel(filepath.Dir(workFilePath), abs); err == nil {
+		return module.Version{Path: rel, Version: r.Version}
+	}
+	// We couldn't make the version's path relative to the workspace's path,
+	// so just return the absolute path. It's the best we can do.
+	return module.Version{Path: abs, Version: r.Version}
+}
+
+// resolveReplacement returns the module actually used to load the source code
+// for m: either m itself, or the replacement for m (iff m is replaced).
+// It also returns the modroot of the module providing the replacement if
+// one was found.
+func resolveReplacement(m module.Version) module.Version {
+	if r := Replacement(m); r.Path != "" {
+		return r
+	}
+	return m
+}
+
+func toReplaceMap(replacements []*modfile.Replace) map[module.Version]module.Version {
+	replaceMap := make(map[module.Version]module.Version, len(replacements))
+	for _, r := range replacements {
+		if prev, dup := replaceMap[r.Old]; dup && prev != r.New {
+			base.Fatalf("go: conflicting replacements for %v:\n\t%v\n\t%v", r.Old, prev, r.New)
+		}
+		replaceMap[r.Old] = r.New
+	}
+	return replaceMap
+}
+
+// indexModFile rebuilds the index of modFile.
+// If modFile has been changed since it was first read,
+// modFile.Cleanup must be called before indexModFile.
+func indexModFile(data []byte, modFile *modfile.File, mod module.Version, needsFix bool) *modFileIndex {
+	i := new(modFileIndex)
+	i.data = data
+	i.dataNeedsFix = needsFix
+
+	i.module = module.Version{}
+	if modFile.Module != nil {
+		i.module = modFile.Module.Mod
+	}
+
+	i.goVersion = ""
+	if modFile.Go == nil {
+		rawGoVersion.Store(mod, "")
+	} else {
+		i.goVersion = modFile.Go.Version
+		rawGoVersion.Store(mod, modFile.Go.Version)
+	}
+	if modFile.Toolchain != nil {
+		i.toolchain = modFile.Toolchain.Name
+	}
+
+	i.require = make(map[module.Version]requireMeta, len(modFile.Require))
+	for _, r := range modFile.Require {
+		i.require[r.Mod] = requireMeta{indirect: r.Indirect}
+	}
+
+	i.replace = toReplaceMap(modFile.Replace)
+
+	i.exclude = make(map[module.Version]bool, len(modFile.Exclude))
+	for _, x := range modFile.Exclude {
+		i.exclude[x.Mod] = true
+	}
+
+	return i
+}
+
+// modFileIsDirty reports whether the go.mod file differs meaningfully
+// from what was indexed.
+// If modFile has been changed (even cosmetically) since it was first read,
+// modFile.Cleanup must be called before modFileIsDirty.
+func (i *modFileIndex) modFileIsDirty(modFile *modfile.File) bool {
+	if i == nil {
+		return modFile != nil
+	}
+
+	if i.dataNeedsFix {
+		return true
+	}
+
+	if modFile.Module == nil {
+		if i.module != (module.Version{}) {
+			return true
+		}
+	} else if modFile.Module.Mod != i.module {
+		return true
+	}
+
+	var goV, toolchain string
+	if modFile.Go != nil {
+		goV = modFile.Go.Version
+	}
+	if modFile.Toolchain != nil {
+		toolchain = modFile.Toolchain.Name
+	}
+
+	if goV != i.goVersion ||
+		toolchain != i.toolchain ||
+		len(modFile.Require) != len(i.require) ||
+		len(modFile.Replace) != len(i.replace) ||
+		len(modFile.Exclude) != len(i.exclude) {
+		return true
+	}
+
+	for _, r := range modFile.Require {
+		if meta, ok := i.require[r.Mod]; !ok {
+			return true
+		} else if r.Indirect != meta.indirect {
+			if cfg.BuildMod == "readonly" {
+				// The module's requirements are consistent; only the "// indirect"
+				// comments that are wrong. But those are only guaranteed to be accurate
+				// after a "go mod tidy" — it's a good idea to run those before
+				// committing a change, but it's certainly not mandatory.
+			} else {
+				return true
+			}
+		}
+	}
+
+	for _, r := range modFile.Replace {
+		if r.New != i.replace[r.Old] {
+			return true
+		}
+	}
+
+	for _, x := range modFile.Exclude {
+		if !i.exclude[x.Mod] {
+			return true
+		}
+	}
+
+	return false
+}
+
+// rawGoVersion records the Go version parsed from each module's go.mod file.
+//
+// If a module is replaced, the version of the replacement is keyed by the
+// replacement module.Version, not the version being replaced.
+var rawGoVersion sync.Map // map[module.Version]string
+
+// A modFileSummary is a summary of a go.mod file for which we do not need to
+// retain complete information — for example, the go.mod file of a dependency
+// module.
+type modFileSummary struct {
+	module     module.Version
+	goVersion  string
+	toolchain  string
+	pruning    modPruning
+	require    []module.Version
+	retract    []retraction
+	deprecated string
+}
+
+// A retraction consists of a retracted version interval and rationale.
+// retraction is like modfile.Retract, but it doesn't point to the syntax tree.
+type retraction struct {
+	modfile.VersionInterval
+	Rationale string
+}
+
+// goModSummary returns a summary of the go.mod file for module m,
+// taking into account any replacements for m, exclusions of its dependencies,
+// and/or vendoring.
+//
+// m must be a version in the module graph, reachable from the Target module.
+// In readonly mode, the go.sum file must contain an entry for m's go.mod file
+// (or its replacement). goModSummary must not be called for the Target module
+// itself, as its requirements may change. Use rawGoModSummary for other
+// module versions.
+//
+// The caller must not modify the returned summary.
+func goModSummary(m module.Version) (*modFileSummary, error) {
+	if m.Version == "" && !inWorkspaceMode() && MainModules.Contains(m.Path) {
+		panic("internal error: goModSummary called on a main module")
+	}
+	if gover.IsToolchain(m.Path) {
+		return rawGoModSummary(m)
+	}
+
+	if cfg.BuildMod == "vendor" {
+		summary := &modFileSummary{
+			module: module.Version{Path: m.Path},
+		}
+
+		readVendorList(MainModules.mustGetSingleMainModule())
+		if vendorVersion[m.Path] != m.Version {
+			// This module is not vendored, so packages cannot be loaded from it and
+			// it cannot be relevant to the build.
+			return summary, nil
+		}
+
+		// For every module other than the target,
+		// return the full list of modules from modules.txt.
+		// We don't know what versions the vendored module actually relies on,
+		// so assume that it requires everything.
+		summary.require = vendorList
+		return summary, nil
+	}
+
+	actual := resolveReplacement(m)
+	if mustHaveSums() && actual.Version != "" {
+		key := module.Version{Path: actual.Path, Version: actual.Version + "/go.mod"}
+		if !modfetch.HaveSum(key) {
+			suggestion := fmt.Sprintf(" for go.mod file; to add it:\n\tgo mod download %s", m.Path)
+			return nil, module.VersionError(actual, &sumMissingError{suggestion: suggestion})
+		}
+	}
+	summary, err := rawGoModSummary(actual)
+	if err != nil {
+		return nil, err
+	}
+
+	if actual.Version == "" {
+		// The actual module is a filesystem-local replacement, for which we have
+		// unfortunately not enforced any sort of invariants about module lines or
+		// matching module paths. Anything goes.
+		//
+		// TODO(bcmills): Remove this special-case, update tests, and add a
+		// release note.
+	} else {
+		if summary.module.Path == "" {
+			return nil, module.VersionError(actual, errors.New("parsing go.mod: missing module line"))
+		}
+
+		// In theory we should only allow mpath to be unequal to m.Path here if the
+		// version that we fetched lacks an explicit go.mod file: if the go.mod file
+		// is explicit, then it should match exactly (to ensure that imports of other
+		// packages within the module are interpreted correctly). Unfortunately, we
+		// can't determine that information from the module proxy protocol: we'll have
+		// to leave that validation for when we load actual packages from within the
+		// module.
+		if mpath := summary.module.Path; mpath != m.Path && mpath != actual.Path {
+			return nil, module.VersionError(actual,
+				fmt.Errorf("parsing go.mod:\n"+
+					"\tmodule declares its path as: %s\n"+
+					"\t        but was required as: %s", mpath, m.Path))
+		}
+	}
+
+	for _, mainModule := range MainModules.Versions() {
+		if index := MainModules.Index(mainModule); index != nil && len(index.exclude) > 0 {
+			// Drop any requirements on excluded versions.
+			// Don't modify the cached summary though, since we might need the raw
+			// summary separately.
+			haveExcludedReqs := false
+			for _, r := range summary.require {
+				if index.exclude[r] {
+					haveExcludedReqs = true
+					break
+				}
+			}
+			if haveExcludedReqs {
+				s := new(modFileSummary)
+				*s = *summary
+				s.require = make([]module.Version, 0, len(summary.require))
+				for _, r := range summary.require {
+					if !index.exclude[r] {
+						s.require = append(s.require, r)
+					}
+				}
+				summary = s
+			}
+		}
+	}
+	return summary, nil
+}
+
+// rawGoModSummary returns a new summary of the go.mod file for module m,
+// ignoring all replacements that may apply to m and excludes that may apply to
+// its dependencies.
+//
+// rawGoModSummary cannot be used on the main module outside of workspace mode.
+func rawGoModSummary(m module.Version) (*modFileSummary, error) {
+	if gover.IsToolchain(m.Path) {
+		if m.Path == "go" && gover.Compare(m.Version, gover.GoStrictVersion) >= 0 {
+			// Declare that go 1.21.3 requires toolchain 1.21.3,
+			// so that go get knows that downgrading toolchain implies downgrading go
+			// and similarly upgrading go requires upgrading the toolchain.
+			return &modFileSummary{module: m, require: []module.Version{{Path: "toolchain", Version: "go" + m.Version}}}, nil
+		}
+		return &modFileSummary{module: m}, nil
+	}
+	if m.Version == "" && !inWorkspaceMode() && MainModules.Contains(m.Path) {
+		// Calling rawGoModSummary implies that we are treating m as a module whose
+		// requirements aren't the roots of the module graph and can't be modified.
+		//
+		// If we are not in workspace mode, then the requirements of the main module
+		// are the roots of the module graph and we expect them to be kept consistent.
+		panic("internal error: rawGoModSummary called on a main module")
+	}
+	if m.Version == "" && inWorkspaceMode() && m.Path == "command-line-arguments" {
+		// "go work sync" calls LoadModGraph to make sure the module graph is valid.
+		// If there are no modules in the workspace, we synthesize an empty
+		// command-line-arguments module, which rawGoModData cannot read a go.mod for.
+		return &modFileSummary{module: m}, nil
+	}
+	return rawGoModSummaryCache.Do(m, func() (*modFileSummary, error) {
+		summary := new(modFileSummary)
+		name, data, err := rawGoModData(m)
+		if err != nil {
+			return nil, err
+		}
+		f, err := modfile.ParseLax(name, data, nil)
+		if err != nil {
+			return nil, module.VersionError(m, fmt.Errorf("parsing %s: %v", base.ShortPath(name), err))
+		}
+		if f.Module != nil {
+			summary.module = f.Module.Mod
+			summary.deprecated = f.Module.Deprecated
+		}
+		if f.Go != nil {
+			rawGoVersion.LoadOrStore(m, f.Go.Version)
+			summary.goVersion = f.Go.Version
+			summary.pruning = pruningForGoVersion(f.Go.Version)
+		} else {
+			summary.pruning = unpruned
+		}
+		if f.Toolchain != nil {
+			summary.toolchain = f.Toolchain.Name
+		}
+		if len(f.Require) > 0 {
+			summary.require = make([]module.Version, 0, len(f.Require)+1)
+			for _, req := range f.Require {
+				summary.require = append(summary.require, req.Mod)
+			}
+		}
+		if summary.goVersion != "" && gover.Compare(summary.goVersion, gover.GoStrictVersion) >= 0 {
+			if gover.Compare(summary.goVersion, gover.Local()) > 0 {
+				return nil, &gover.TooNewError{What: "module " + m.String(), GoVersion: summary.goVersion}
+			}
+			summary.require = append(summary.require, module.Version{Path: "go", Version: summary.goVersion})
+		}
+		if len(f.Retract) > 0 {
+			summary.retract = make([]retraction, 0, len(f.Retract))
+			for _, ret := range f.Retract {
+				summary.retract = append(summary.retract, retraction{
+					VersionInterval: ret.VersionInterval,
+					Rationale:       ret.Rationale,
+				})
+			}
+		}
+
+		return summary, nil
+	})
+}
+
+var rawGoModSummaryCache par.ErrCache[module.Version, *modFileSummary]
+
+// rawGoModData returns the content of the go.mod file for module m, ignoring
+// all replacements that may apply to m.
+//
+// rawGoModData cannot be used on the main module outside of workspace mode.
+//
+// Unlike rawGoModSummary, rawGoModData does not cache its results in memory.
+// Use rawGoModSummary instead unless you specifically need these bytes.
+func rawGoModData(m module.Version) (name string, data []byte, err error) {
+	if m.Version == "" {
+		dir := m.Path
+		if !filepath.IsAbs(dir) {
+			if inWorkspaceMode() && MainModules.Contains(m.Path) {
+				dir = MainModules.ModRoot(m)
+			} else {
+				// m is a replacement module with only a file path.
+				dir = filepath.Join(replaceRelativeTo(), dir)
+			}
+		}
+		name = filepath.Join(dir, "go.mod")
+		if gomodActual, ok := fsys.OverlayPath(name); ok {
+			// Don't lock go.mod if it's part of the overlay.
+			// On Plan 9, locking requires chmod, and we don't want to modify any file
+			// in the overlay. See #44700.
+			data, err = os.ReadFile(gomodActual)
+		} else {
+			data, err = lockedfile.Read(gomodActual)
+		}
+		if err != nil {
+			return "", nil, module.VersionError(m, fmt.Errorf("reading %s: %v", base.ShortPath(name), err))
+		}
+	} else {
+		if !gover.ModIsValid(m.Path, m.Version) {
+			// Disallow the broader queries supported by fetch.Lookup.
+			base.Fatalf("go: internal error: %s@%s: unexpected invalid semantic version", m.Path, m.Version)
+		}
+		name = "go.mod"
+		data, err = modfetch.GoMod(context.TODO(), m.Path, m.Version)
+	}
+	return name, data, err
+}
+
+// queryLatestVersionIgnoringRetractions looks up the latest version of the
+// module with the given path without considering retracted or excluded
+// versions.
+//
+// If all versions of the module are replaced,
+// queryLatestVersionIgnoringRetractions returns the replacement without making
+// a query.
+//
+// If the queried latest version is replaced,
+// queryLatestVersionIgnoringRetractions returns the replacement.
+func queryLatestVersionIgnoringRetractions(ctx context.Context, path string) (latest module.Version, err error) {
+	return latestVersionIgnoringRetractionsCache.Do(path, func() (module.Version, error) {
+		ctx, span := trace.StartSpan(ctx, "queryLatestVersionIgnoringRetractions "+path)
+		defer span.Done()
+
+		if repl := Replacement(module.Version{Path: path}); repl.Path != "" {
+			// All versions of the module were replaced.
+			// No need to query.
+			return repl, nil
+		}
+
+		// Find the latest version of the module.
+		// Ignore exclusions from the main module's go.mod.
+		const ignoreSelected = ""
+		var allowAll AllowedFunc
+		rev, err := Query(ctx, path, "latest", ignoreSelected, allowAll)
+		if err != nil {
+			return module.Version{}, err
+		}
+		latest := module.Version{Path: path, Version: rev.Version}
+		if repl := resolveReplacement(latest); repl.Path != "" {
+			latest = repl
+		}
+		return latest, nil
+	})
+}
+
+var latestVersionIgnoringRetractionsCache par.ErrCache[string, module.Version] // path → queryLatestVersionIgnoringRetractions result
+
+// ToDirectoryPath adds a prefix if necessary so that path in unambiguously
+// an absolute path or a relative path starting with a '.' or '..'
+// path component.
+func ToDirectoryPath(path string) string {
+	if path == "." || modfile.IsDirectoryPath(path) {
+		return path
+	}
+	// The path is not a relative path or an absolute path, so make it relative
+	// to the current directory.
+	return "./" + filepath.ToSlash(filepath.Clean(path))
+}
diff --git a/src/cmd/go/internal/modload/mvs.go b/src/cmd/go/internal/modload/mvs.go
new file mode 100644
index 0000000..8ae2dbf
--- /dev/null
+++ b/src/cmd/go/internal/modload/mvs.go
@@ -0,0 +1,136 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"context"
+	"errors"
+	"os"
+	"sort"
+
+	"cmd/go/internal/gover"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/modfetch/codehost"
+
+	"golang.org/x/mod/module"
+)
+
+// cmpVersion implements the comparison for versions in the module loader.
+//
+// It is consistent with gover.ModCompare except that as a special case,
+// the version "" is considered higher than all other versions.
+// The main module (also known as the target) has no version and must be chosen
+// over other versions of the same module in the module dependency graph.
+func cmpVersion(p string, v1, v2 string) int {
+	if v2 == "" {
+		if v1 == "" {
+			return 0
+		}
+		return -1
+	}
+	if v1 == "" {
+		return 1
+	}
+	return gover.ModCompare(p, v1, v2)
+}
+
+// mvsReqs implements mvs.Reqs for module semantic versions,
+// with any exclusions or replacements applied internally.
+type mvsReqs struct {
+	roots []module.Version
+}
+
+func (r *mvsReqs) Required(mod module.Version) ([]module.Version, error) {
+	if mod.Version == "" && MainModules.Contains(mod.Path) {
+		// Use the build list as it existed when r was constructed, not the current
+		// global build list.
+		return r.roots, nil
+	}
+
+	if mod.Version == "none" {
+		return nil, nil
+	}
+
+	summary, err := goModSummary(mod)
+	if err != nil {
+		return nil, err
+	}
+	return summary.require, nil
+}
+
+// Max returns the maximum of v1 and v2 according to gover.ModCompare.
+//
+// As a special case, the version "" is considered higher than all other
+// versions. The main module (also known as the target) has no version and must
+// be chosen over other versions of the same module in the module dependency
+// graph.
+func (*mvsReqs) Max(p, v1, v2 string) string {
+	if cmpVersion(p, v1, v2) < 0 {
+		return v2
+	}
+	return v1
+}
+
+// Upgrade is a no-op, here to implement mvs.Reqs.
+// The upgrade logic for go get -u is in ../modget/get.go.
+func (*mvsReqs) Upgrade(m module.Version) (module.Version, error) {
+	return m, nil
+}
+
+func versions(ctx context.Context, path string, allowed AllowedFunc) (versions []string, origin *codehost.Origin, err error) {
+	// Note: modfetch.Lookup and repo.Versions are cached,
+	// so there's no need for us to add extra caching here.
+	err = modfetch.TryProxies(func(proxy string) error {
+		repo, err := lookupRepo(ctx, proxy, path)
+		if err != nil {
+			return err
+		}
+		allVersions, err := repo.Versions(ctx, "")
+		if err != nil {
+			return err
+		}
+		allowedVersions := make([]string, 0, len(allVersions.List))
+		for _, v := range allVersions.List {
+			if err := allowed(ctx, module.Version{Path: path, Version: v}); err == nil {
+				allowedVersions = append(allowedVersions, v)
+			} else if !errors.Is(err, ErrDisallowed) {
+				return err
+			}
+		}
+		versions = allowedVersions
+		origin = allVersions.Origin
+		return nil
+	})
+	return versions, origin, err
+}
+
+// previousVersion returns the tagged version of m.Path immediately prior to
+// m.Version, or version "none" if no prior version is tagged.
+//
+// Since the version of a main module is not found in the version list,
+// it has no previous version.
+func previousVersion(ctx context.Context, m module.Version) (module.Version, error) {
+	if m.Version == "" && MainModules.Contains(m.Path) {
+		return module.Version{Path: m.Path, Version: "none"}, nil
+	}
+
+	list, _, err := versions(ctx, m.Path, CheckAllowed)
+	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return module.Version{Path: m.Path, Version: "none"}, nil
+		}
+		return module.Version{}, err
+	}
+	i := sort.Search(len(list), func(i int) bool { return gover.ModCompare(m.Path, list[i], m.Version) >= 0 })
+	if i > 0 {
+		return module.Version{Path: m.Path, Version: list[i-1]}, nil
+	}
+	return module.Version{Path: m.Path, Version: "none"}, nil
+}
+
+func (*mvsReqs) Previous(m module.Version) (module.Version, error) {
+	// TODO(golang.org/issue/38714): thread tracing context through MVS.
+	return previousVersion(context.TODO(), m)
+}
diff --git a/src/cmd/go/internal/modload/mvs_test.go b/src/cmd/go/internal/modload/mvs_test.go
new file mode 100644
index 0000000..e0a38b9
--- /dev/null
+++ b/src/cmd/go/internal/modload/mvs_test.go
@@ -0,0 +1,31 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"testing"
+)
+
+func TestReqsMax(t *testing.T) {
+	type testCase struct {
+		a, b, want string
+	}
+	reqs := new(mvsReqs)
+	for _, tc := range []testCase{
+		{a: "v0.1.0", b: "v0.2.0", want: "v0.2.0"},
+		{a: "v0.2.0", b: "v0.1.0", want: "v0.2.0"},
+		{a: "", b: "v0.1.0", want: ""}, // "" is Target.Version
+		{a: "v0.1.0", b: "", want: ""},
+		{a: "none", b: "v0.1.0", want: "v0.1.0"},
+		{a: "v0.1.0", b: "none", want: "v0.1.0"},
+		{a: "none", b: "", want: ""},
+		{a: "", b: "none", want: ""},
+	} {
+		max := reqs.Max("", tc.a, tc.b)
+		if max != tc.want {
+			t.Errorf("(%T).Max(%q, %q) = %q; want %q", reqs, tc.a, tc.b, max, tc.want)
+		}
+	}
+}
diff --git a/src/cmd/go/internal/modload/query.go b/src/cmd/go/internal/modload/query.go
new file mode 100644
index 0000000..f8ddf11
--- /dev/null
+++ b/src/cmd/go/internal/modload/query.go
@@ -0,0 +1,1265 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	pathpkg "path"
+	"slices"
+	"sort"
+	"strings"
+	"sync"
+	"time"
+
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/imports"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/modfetch/codehost"
+	"cmd/go/internal/modinfo"
+	"cmd/go/internal/search"
+	"cmd/go/internal/str"
+	"cmd/go/internal/trace"
+	"cmd/internal/pkgpattern"
+
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
+)
+
+// Query looks up a revision of a given module given a version query string.
+// The module must be a complete module path.
+// The version must take one of the following forms:
+//
+//   - the literal string "latest", denoting the latest available, allowed
+//     tagged version, with non-prereleases preferred over prereleases.
+//     If there are no tagged versions in the repo, latest returns the most
+//     recent commit.
+//
+//   - the literal string "upgrade", equivalent to "latest" except that if
+//     current is a newer version, current will be returned (see below).
+//
+//   - the literal string "patch", denoting the latest available tagged version
+//     with the same major and minor number as current (see below).
+//
+//   - v1, denoting the latest available tagged version v1.x.x.
+//
+//   - v1.2, denoting the latest available tagged version v1.2.x.
+//
+//   - v1.2.3, a semantic version string denoting that tagged version.
+//
+//   - <v1.2.3, <=v1.2.3, >v1.2.3, >=v1.2.3,
+//     denoting the version closest to the target and satisfying the given operator,
+//     with non-prereleases preferred over prereleases.
+//
+//   - a repository commit identifier or tag, denoting that commit.
+//
+// current denotes the currently-selected version of the module; it may be
+// "none" if no version is currently selected, or "" if the currently-selected
+// version is unknown or should not be considered. If query is
+// "upgrade" or "patch", current will be returned if it is a newer
+// semantic version or a chronologically later pseudo-version than the
+// version that would otherwise be chosen. This prevents accidental downgrades
+// from newer pre-release or development versions.
+//
+// The allowed function (which may be nil) is used to filter out unsuitable
+// versions (see AllowedFunc documentation for details). If the query refers to
+// a specific revision (for example, "master"; see IsRevisionQuery), and the
+// revision is disallowed by allowed, Query returns the error. If the query
+// does not refer to a specific revision (for example, "latest"), Query
+// acts as if versions disallowed by allowed do not exist.
+//
+// If path is the path of the main module and the query is "latest",
+// Query returns Target.Version as the version.
+//
+// Query often returns a non-nil *RevInfo with a non-nil error,
+// to provide an info.Origin that can allow the error to be cached.
+func Query(ctx context.Context, path, query, current string, allowed AllowedFunc) (*modfetch.RevInfo, error) {
+	ctx, span := trace.StartSpan(ctx, "modload.Query "+path)
+	defer span.Done()
+
+	return queryReuse(ctx, path, query, current, allowed, nil)
+}
+
+// queryReuse is like Query but also takes a map of module info that can be reused
+// if the validation criteria in Origin are met.
+func queryReuse(ctx context.Context, path, query, current string, allowed AllowedFunc, reuse map[module.Version]*modinfo.ModulePublic) (*modfetch.RevInfo, error) {
+	var info *modfetch.RevInfo
+	err := modfetch.TryProxies(func(proxy string) (err error) {
+		info, err = queryProxy(ctx, proxy, path, query, current, allowed, reuse)
+		return err
+	})
+	return info, err
+}
+
+// checkReuse checks whether a revision of a given module or a version list
+// for a given module may be reused, according to the information in origin.
+func checkReuse(ctx context.Context, path string, old *codehost.Origin) error {
+	return modfetch.TryProxies(func(proxy string) error {
+		repo, err := lookupRepo(ctx, proxy, path)
+		if err != nil {
+			return err
+		}
+		return repo.CheckReuse(ctx, old)
+	})
+}
+
+// AllowedFunc is used by Query and other functions to filter out unsuitable
+// versions, for example, those listed in exclude directives in the main
+// module's go.mod file.
+//
+// An AllowedFunc returns an error equivalent to ErrDisallowed for an unsuitable
+// version. Any other error indicates the function was unable to determine
+// whether the version should be allowed, for example, the function was unable
+// to fetch or parse a go.mod file containing retractions. Typically, errors
+// other than ErrDisallowed may be ignored.
+type AllowedFunc func(context.Context, module.Version) error
+
+var errQueryDisabled error = queryDisabledError{}
+
+type queryDisabledError struct{}
+
+func (queryDisabledError) Error() string {
+	if cfg.BuildModReason == "" {
+		return fmt.Sprintf("cannot query module due to -mod=%s", cfg.BuildMod)
+	}
+	return fmt.Sprintf("cannot query module due to -mod=%s\n\t(%s)", cfg.BuildMod, cfg.BuildModReason)
+}
+
+func queryProxy(ctx context.Context, proxy, path, query, current string, allowed AllowedFunc, reuse map[module.Version]*modinfo.ModulePublic) (*modfetch.RevInfo, error) {
+	ctx, span := trace.StartSpan(ctx, "modload.queryProxy "+path+" "+query)
+	defer span.Done()
+
+	if current != "" && current != "none" && !gover.ModIsValid(path, current) {
+		return nil, fmt.Errorf("invalid previous version %v@%v", path, current)
+	}
+	if cfg.BuildMod == "vendor" {
+		return nil, errQueryDisabled
+	}
+	if allowed == nil {
+		allowed = func(context.Context, module.Version) error { return nil }
+	}
+
+	if MainModules.Contains(path) && (query == "upgrade" || query == "patch") {
+		m := module.Version{Path: path}
+		if err := allowed(ctx, m); err != nil {
+			return nil, fmt.Errorf("internal error: main module version is not allowed: %w", err)
+		}
+		return &modfetch.RevInfo{Version: m.Version}, nil
+	}
+
+	if path == "std" || path == "cmd" {
+		return nil, fmt.Errorf("can't query specific version (%q) of standard-library module %q", query, path)
+	}
+
+	repo, err := lookupRepo(ctx, proxy, path)
+	if err != nil {
+		return nil, err
+	}
+
+	if old := reuse[module.Version{Path: path, Version: query}]; old != nil {
+		if err := repo.CheckReuse(ctx, old.Origin); err == nil {
+			info := &modfetch.RevInfo{
+				Version: old.Version,
+				Origin:  old.Origin,
+			}
+			if old.Time != nil {
+				info.Time = *old.Time
+			}
+			return info, nil
+		}
+	}
+
+	// Parse query to detect parse errors (and possibly handle query)
+	// before any network I/O.
+	qm, err := newQueryMatcher(path, query, current, allowed)
+	if (err == nil && qm.canStat) || err == errRevQuery {
+		// Direct lookup of a commit identifier or complete (non-prefix) semantic
+		// version.
+
+		// If the identifier is not a canonical semver tag — including if it's a
+		// semver tag with a +metadata suffix — then modfetch.Stat will populate
+		// info.Version with a suitable pseudo-version.
+		info, err := repo.Stat(ctx, query)
+		if err != nil {
+			queryErr := err
+			// The full query doesn't correspond to a tag. If it is a semantic version
+			// with a +metadata suffix, see if there is a tag without that suffix:
+			// semantic versioning defines them to be equivalent.
+			canonicalQuery := module.CanonicalVersion(query)
+			if canonicalQuery != "" && query != canonicalQuery {
+				info, err = repo.Stat(ctx, canonicalQuery)
+				if err != nil && !errors.Is(err, fs.ErrNotExist) {
+					return info, err
+				}
+			}
+			if err != nil {
+				return info, queryErr
+			}
+		}
+		if err := allowed(ctx, module.Version{Path: path, Version: info.Version}); errors.Is(err, ErrDisallowed) {
+			return nil, err
+		}
+		return info, nil
+	} else if err != nil {
+		return nil, err
+	}
+
+	// Load versions and execute query.
+	versions, err := repo.Versions(ctx, qm.prefix)
+	if err != nil {
+		return nil, err
+	}
+	revErr := &modfetch.RevInfo{Origin: versions.Origin} // RevInfo to return with error
+
+	releases, prereleases, err := qm.filterVersions(ctx, versions.List)
+	if err != nil {
+		return revErr, err
+	}
+
+	mergeRevOrigin := func(rev *modfetch.RevInfo, origin *codehost.Origin) *modfetch.RevInfo {
+		merged := mergeOrigin(rev.Origin, origin)
+		if merged == rev.Origin {
+			return rev
+		}
+		clone := new(modfetch.RevInfo)
+		*clone = *rev
+		clone.Origin = merged
+		return clone
+	}
+
+	lookup := func(v string) (*modfetch.RevInfo, error) {
+		rev, err := repo.Stat(ctx, v)
+		// Stat can return a non-nil rev and a non-nil err,
+		// in order to provide origin information to make the error cacheable.
+		if rev == nil && err != nil {
+			return revErr, err
+		}
+		rev = mergeRevOrigin(rev, versions.Origin)
+		if err != nil {
+			return rev, err
+		}
+
+		if (query == "upgrade" || query == "patch") && module.IsPseudoVersion(current) && !rev.Time.IsZero() {
+			// Don't allow "upgrade" or "patch" to move from a pseudo-version
+			// to a chronologically older version or pseudo-version.
+			//
+			// If the current version is a pseudo-version from an untagged branch, it
+			// may be semantically lower than the "latest" release or the latest
+			// pseudo-version on the main branch. A user on such a version is unlikely
+			// to intend to “upgrade” to a version that already existed at that point
+			// in time.
+			//
+			// We do this only if the current version is a pseudo-version: if the
+			// version is tagged, the author of the dependency module has given us
+			// explicit information about their intended precedence of this version
+			// relative to other versions, and we shouldn't contradict that
+			// information. (For example, v1.0.1 might be a backport of a fix already
+			// incorporated into v1.1.0, in which case v1.0.1 would be chronologically
+			// newer but v1.1.0 is still an “upgrade”; or v1.0.2 might be a revert of
+			// an unsuccessful fix in v1.0.1, in which case the v1.0.2 commit may be
+			// older than the v1.0.1 commit despite the tag itself being newer.)
+			currentTime, err := module.PseudoVersionTime(current)
+			if err == nil && rev.Time.Before(currentTime) {
+				if err := allowed(ctx, module.Version{Path: path, Version: current}); errors.Is(err, ErrDisallowed) {
+					return revErr, err
+				}
+				rev, err = repo.Stat(ctx, current)
+				if rev == nil && err != nil {
+					return revErr, err
+				}
+				rev = mergeRevOrigin(rev, versions.Origin)
+				return rev, err
+			}
+		}
+
+		return rev, nil
+	}
+
+	if qm.preferLower {
+		if len(releases) > 0 {
+			return lookup(releases[0])
+		}
+		if len(prereleases) > 0 {
+			return lookup(prereleases[0])
+		}
+	} else {
+		if len(releases) > 0 {
+			return lookup(releases[len(releases)-1])
+		}
+		if len(prereleases) > 0 {
+			return lookup(prereleases[len(prereleases)-1])
+		}
+	}
+
+	if qm.mayUseLatest {
+		latest, err := repo.Latest(ctx)
+		if err == nil {
+			if qm.allowsVersion(ctx, latest.Version) {
+				return lookup(latest.Version)
+			}
+		} else if !errors.Is(err, fs.ErrNotExist) {
+			return revErr, err
+		}
+	}
+
+	if (query == "upgrade" || query == "patch") && current != "" && current != "none" {
+		// "upgrade" and "patch" may stay on the current version if allowed.
+		if err := allowed(ctx, module.Version{Path: path, Version: current}); errors.Is(err, ErrDisallowed) {
+			return nil, err
+		}
+		return lookup(current)
+	}
+
+	return revErr, &NoMatchingVersionError{query: query, current: current}
+}
+
+// IsRevisionQuery returns true if vers is a version query that may refer to
+// a particular version or revision in a repository like "v1.0.0", "master",
+// or "0123abcd". IsRevisionQuery returns false if vers is a query that
+// chooses from among available versions like "latest" or ">v1.0.0".
+func IsRevisionQuery(path, vers string) bool {
+	if vers == "latest" ||
+		vers == "upgrade" ||
+		vers == "patch" ||
+		strings.HasPrefix(vers, "<") ||
+		strings.HasPrefix(vers, ">") ||
+		(gover.ModIsValid(path, vers) && gover.ModIsPrefix(path, vers)) {
+		return false
+	}
+	return true
+}
+
+type queryMatcher struct {
+	path               string
+	prefix             string
+	filter             func(version string) bool
+	allowed            AllowedFunc
+	canStat            bool // if true, the query can be resolved by repo.Stat
+	preferLower        bool // if true, choose the lowest matching version
+	mayUseLatest       bool
+	preferIncompatible bool
+}
+
+var errRevQuery = errors.New("query refers to a non-semver revision")
+
+// newQueryMatcher returns a new queryMatcher that matches the versions
+// specified by the given query on the module with the given path.
+//
+// If the query can only be resolved by statting a non-SemVer revision,
+// newQueryMatcher returns errRevQuery.
+func newQueryMatcher(path string, query, current string, allowed AllowedFunc) (*queryMatcher, error) {
+	badVersion := func(v string) (*queryMatcher, error) {
+		return nil, fmt.Errorf("invalid semantic version %q in range %q", v, query)
+	}
+
+	matchesMajor := func(v string) bool {
+		_, pathMajor, ok := module.SplitPathVersion(path)
+		if !ok {
+			return false
+		}
+		return module.CheckPathMajor(v, pathMajor) == nil
+	}
+
+	qm := &queryMatcher{
+		path:               path,
+		allowed:            allowed,
+		preferIncompatible: strings.HasSuffix(current, "+incompatible"),
+	}
+
+	switch {
+	case query == "latest":
+		qm.mayUseLatest = true
+
+	case query == "upgrade":
+		if current == "" || current == "none" {
+			qm.mayUseLatest = true
+		} else {
+			qm.mayUseLatest = module.IsPseudoVersion(current)
+			qm.filter = func(mv string) bool { return gover.ModCompare(qm.path, mv, current) >= 0 }
+		}
+
+	case query == "patch":
+		if current == "" || current == "none" {
+			return nil, &NoPatchBaseError{path}
+		}
+		if current == "" {
+			qm.mayUseLatest = true
+		} else {
+			qm.mayUseLatest = module.IsPseudoVersion(current)
+			qm.prefix = gover.ModMajorMinor(qm.path, current) + "."
+			qm.filter = func(mv string) bool { return gover.ModCompare(qm.path, mv, current) >= 0 }
+		}
+
+	case strings.HasPrefix(query, "<="):
+		v := query[len("<="):]
+		if !gover.ModIsValid(path, v) {
+			return badVersion(v)
+		}
+		if gover.ModIsPrefix(path, v) {
+			// Refuse to say whether <=v1.2 allows v1.2.3 (remember, @v1.2 might mean v1.2.3).
+			return nil, fmt.Errorf("ambiguous semantic version %q in range %q", v, query)
+		}
+		qm.filter = func(mv string) bool { return gover.ModCompare(qm.path, mv, v) <= 0 }
+		if !matchesMajor(v) {
+			qm.preferIncompatible = true
+		}
+
+	case strings.HasPrefix(query, "<"):
+		v := query[len("<"):]
+		if !gover.ModIsValid(path, v) {
+			return badVersion(v)
+		}
+		qm.filter = func(mv string) bool { return gover.ModCompare(qm.path, mv, v) < 0 }
+		if !matchesMajor(v) {
+			qm.preferIncompatible = true
+		}
+
+	case strings.HasPrefix(query, ">="):
+		v := query[len(">="):]
+		if !gover.ModIsValid(path, v) {
+			return badVersion(v)
+		}
+		qm.filter = func(mv string) bool { return gover.ModCompare(qm.path, mv, v) >= 0 }
+		qm.preferLower = true
+		if !matchesMajor(v) {
+			qm.preferIncompatible = true
+		}
+
+	case strings.HasPrefix(query, ">"):
+		v := query[len(">"):]
+		if !gover.ModIsValid(path, v) {
+			return badVersion(v)
+		}
+		if gover.ModIsPrefix(path, v) {
+			// Refuse to say whether >v1.2 allows v1.2.3 (remember, @v1.2 might mean v1.2.3).
+			return nil, fmt.Errorf("ambiguous semantic version %q in range %q", v, query)
+		}
+		qm.filter = func(mv string) bool { return gover.ModCompare(qm.path, mv, v) > 0 }
+		qm.preferLower = true
+		if !matchesMajor(v) {
+			qm.preferIncompatible = true
+		}
+
+	case gover.ModIsValid(path, query):
+		if gover.ModIsPrefix(path, query) {
+			qm.prefix = query + "."
+			// Do not allow the query "v1.2" to match versions lower than "v1.2.0",
+			// such as prereleases for that version. (https://golang.org/issue/31972)
+			qm.filter = func(mv string) bool { return gover.ModCompare(qm.path, mv, query) >= 0 }
+		} else {
+			qm.canStat = true
+			qm.filter = func(mv string) bool { return gover.ModCompare(qm.path, mv, query) == 0 }
+			qm.prefix = semver.Canonical(query)
+		}
+		if !matchesMajor(query) {
+			qm.preferIncompatible = true
+		}
+
+	default:
+		return nil, errRevQuery
+	}
+
+	return qm, nil
+}
+
+// allowsVersion reports whether version v is allowed by the prefix, filter, and
+// AllowedFunc of qm.
+func (qm *queryMatcher) allowsVersion(ctx context.Context, v string) bool {
+	if qm.prefix != "" && !strings.HasPrefix(v, qm.prefix) {
+		if gover.IsToolchain(qm.path) && strings.TrimSuffix(qm.prefix, ".") == v {
+			// Allow 1.21 to match "1.21." prefix.
+		} else {
+			return false
+		}
+	}
+	if qm.filter != nil && !qm.filter(v) {
+		return false
+	}
+	if qm.allowed != nil {
+		if err := qm.allowed(ctx, module.Version{Path: qm.path, Version: v}); errors.Is(err, ErrDisallowed) {
+			return false
+		}
+	}
+	return true
+}
+
+// filterVersions classifies versions into releases and pre-releases, filtering
+// out:
+//  1. versions that do not satisfy the 'allowed' predicate, and
+//  2. "+incompatible" versions, if a compatible one satisfies the predicate
+//     and the incompatible version is not preferred.
+//
+// If the allowed predicate returns an error not equivalent to ErrDisallowed,
+// filterVersions returns that error.
+func (qm *queryMatcher) filterVersions(ctx context.Context, versions []string) (releases, prereleases []string, err error) {
+	needIncompatible := qm.preferIncompatible
+
+	var lastCompatible string
+	for _, v := range versions {
+		if !qm.allowsVersion(ctx, v) {
+			continue
+		}
+
+		if !needIncompatible {
+			// We're not yet sure whether we need to include +incompatible versions.
+			// Keep track of the last compatible version we've seen, and use the
+			// presence (or absence) of a go.mod file in that version to decide: a
+			// go.mod file implies that the module author is supporting modules at a
+			// compatible version (and we should ignore +incompatible versions unless
+			// requested explicitly), while a lack of go.mod file implies the
+			// potential for legacy (pre-modules) versioning without semantic import
+			// paths (and thus *with* +incompatible versions).
+			//
+			// This isn't strictly accurate if the latest compatible version has been
+			// replaced by a local file path, because we do not allow file-path
+			// replacements without a go.mod file: the user would have needed to add
+			// one. However, replacing the last compatible version while
+			// simultaneously expecting to upgrade implicitly to a +incompatible
+			// version seems like an extreme enough corner case to ignore for now.
+
+			if !strings.HasSuffix(v, "+incompatible") {
+				lastCompatible = v
+			} else if lastCompatible != "" {
+				// If the latest compatible version is allowed and has a go.mod file,
+				// ignore any version with a higher (+incompatible) major version. (See
+				// https://golang.org/issue/34165.) Note that we even prefer a
+				// compatible pre-release over an incompatible release.
+				ok, err := versionHasGoMod(ctx, module.Version{Path: qm.path, Version: lastCompatible})
+				if err != nil {
+					return nil, nil, err
+				}
+				if ok {
+					// The last compatible version has a go.mod file, so that's the
+					// highest version we're willing to consider. Don't bother even
+					// looking at higher versions, because they're all +incompatible from
+					// here onward.
+					break
+				}
+
+				// No acceptable compatible release has a go.mod file, so the versioning
+				// for the module might not be module-aware, and we should respect
+				// legacy major-version tags.
+				needIncompatible = true
+			}
+		}
+
+		if gover.ModIsPrerelease(qm.path, v) {
+			prereleases = append(prereleases, v)
+		} else {
+			releases = append(releases, v)
+		}
+	}
+
+	return releases, prereleases, nil
+}
+
+type QueryResult struct {
+	Mod      module.Version
+	Rev      *modfetch.RevInfo
+	Packages []string
+}
+
+// QueryPackages is like QueryPattern, but requires that the pattern match at
+// least one package and omits the non-package result (if any).
+func QueryPackages(ctx context.Context, pattern, query string, current func(string) string, allowed AllowedFunc) ([]QueryResult, error) {
+	pkgMods, modOnly, err := QueryPattern(ctx, pattern, query, current, allowed)
+
+	if len(pkgMods) == 0 && err == nil {
+		replacement := Replacement(modOnly.Mod)
+		return nil, &PackageNotInModuleError{
+			Mod:         modOnly.Mod,
+			Replacement: replacement,
+			Query:       query,
+			Pattern:     pattern,
+		}
+	}
+
+	return pkgMods, err
+}
+
+// QueryPattern looks up the module(s) containing at least one package matching
+// the given pattern at the given version. The results are sorted by module path
+// length in descending order. If any proxy provides a non-empty set of candidate
+// modules, no further proxies are tried.
+//
+// For wildcard patterns, QueryPattern looks in modules with package paths up to
+// the first "..." in the pattern. For the pattern "example.com/a/b.../c",
+// QueryPattern would consider prefixes of "example.com/a".
+//
+// If any matching package is in the main module, QueryPattern considers only
+// the main module and only the version "latest", without checking for other
+// possible modules.
+//
+// QueryPattern always returns at least one QueryResult (which may be only
+// modOnly) or a non-nil error.
+func QueryPattern(ctx context.Context, pattern, query string, current func(string) string, allowed AllowedFunc) (pkgMods []QueryResult, modOnly *QueryResult, err error) {
+	ctx, span := trace.StartSpan(ctx, "modload.QueryPattern "+pattern+" "+query)
+	defer span.Done()
+
+	base := pattern
+
+	firstError := func(m *search.Match) error {
+		if len(m.Errs) == 0 {
+			return nil
+		}
+		return m.Errs[0]
+	}
+
+	var match func(mod module.Version, roots []string, isLocal bool) *search.Match
+	matchPattern := pkgpattern.MatchPattern(pattern)
+
+	if i := strings.Index(pattern, "..."); i >= 0 {
+		base = pathpkg.Dir(pattern[:i+3])
+		if base == "." {
+			return nil, nil, &WildcardInFirstElementError{Pattern: pattern, Query: query}
+		}
+		match = func(mod module.Version, roots []string, isLocal bool) *search.Match {
+			m := search.NewMatch(pattern)
+			matchPackages(ctx, m, imports.AnyTags(), omitStd, []module.Version{mod})
+			return m
+		}
+	} else {
+		match = func(mod module.Version, roots []string, isLocal bool) *search.Match {
+			m := search.NewMatch(pattern)
+			prefix := mod.Path
+			if MainModules.Contains(mod.Path) {
+				prefix = MainModules.PathPrefix(module.Version{Path: mod.Path})
+			}
+			for _, root := range roots {
+				if _, ok, err := dirInModule(pattern, prefix, root, isLocal); err != nil {
+					m.AddError(err)
+				} else if ok {
+					m.Pkgs = []string{pattern}
+				}
+			}
+			return m
+		}
+	}
+
+	var mainModuleMatches []module.Version
+	for _, mainModule := range MainModules.Versions() {
+		m := match(mainModule, modRoots, true)
+		if len(m.Pkgs) > 0 {
+			if query != "upgrade" && query != "patch" {
+				return nil, nil, &QueryMatchesPackagesInMainModuleError{
+					Pattern:  pattern,
+					Query:    query,
+					Packages: m.Pkgs,
+				}
+			}
+			if err := allowed(ctx, mainModule); err != nil {
+				return nil, nil, fmt.Errorf("internal error: package %s is in the main module (%s), but version is not allowed: %w", pattern, mainModule.Path, err)
+			}
+			return []QueryResult{{
+				Mod:      mainModule,
+				Rev:      &modfetch.RevInfo{Version: mainModule.Version},
+				Packages: m.Pkgs,
+			}}, nil, nil
+		}
+		if err := firstError(m); err != nil {
+			return nil, nil, err
+		}
+
+		var matchesMainModule bool
+		if matchPattern(mainModule.Path) {
+			mainModuleMatches = append(mainModuleMatches, mainModule)
+			matchesMainModule = true
+		}
+
+		if (query == "upgrade" || query == "patch") && matchesMainModule {
+			if err := allowed(ctx, mainModule); err == nil {
+				modOnly = &QueryResult{
+					Mod: mainModule,
+					Rev: &modfetch.RevInfo{Version: mainModule.Version},
+				}
+			}
+		}
+	}
+
+	var (
+		results          []QueryResult
+		candidateModules = modulePrefixesExcludingTarget(base)
+	)
+	if len(candidateModules) == 0 {
+		if modOnly != nil {
+			return nil, modOnly, nil
+		} else if len(mainModuleMatches) != 0 {
+			return nil, nil, &QueryMatchesMainModulesError{
+				MainModules: mainModuleMatches,
+				Pattern:     pattern,
+				Query:       query,
+			}
+		} else {
+			return nil, nil, &PackageNotInModuleError{
+				MainModules: mainModuleMatches,
+				Query:       query,
+				Pattern:     pattern,
+			}
+		}
+	}
+
+	err = modfetch.TryProxies(func(proxy string) error {
+		queryModule := func(ctx context.Context, path string) (r QueryResult, err error) {
+			ctx, span := trace.StartSpan(ctx, "modload.QueryPattern.queryModule ["+proxy+"] "+path)
+			defer span.Done()
+
+			pathCurrent := current(path)
+			r.Mod.Path = path
+			r.Rev, err = queryProxy(ctx, proxy, path, query, pathCurrent, allowed, nil)
+			if err != nil {
+				return r, err
+			}
+			r.Mod.Version = r.Rev.Version
+			if gover.IsToolchain(r.Mod.Path) {
+				return r, nil
+			}
+			root, isLocal, err := fetch(ctx, r.Mod)
+			if err != nil {
+				return r, err
+			}
+			m := match(r.Mod, []string{root}, isLocal)
+			r.Packages = m.Pkgs
+			if len(r.Packages) == 0 && !matchPattern(path) {
+				if err := firstError(m); err != nil {
+					return r, err
+				}
+				replacement := Replacement(r.Mod)
+				return r, &PackageNotInModuleError{
+					Mod:         r.Mod,
+					Replacement: replacement,
+					Query:       query,
+					Pattern:     pattern,
+				}
+			}
+			return r, nil
+		}
+
+		allResults, err := queryPrefixModules(ctx, candidateModules, queryModule)
+		results = allResults[:0]
+		for _, r := range allResults {
+			if len(r.Packages) == 0 {
+				modOnly = &r
+			} else {
+				results = append(results, r)
+			}
+		}
+		return err
+	})
+
+	if len(mainModuleMatches) > 0 && len(results) == 0 && modOnly == nil && errors.Is(err, fs.ErrNotExist) {
+		return nil, nil, &QueryMatchesMainModulesError{
+			Pattern: pattern,
+			Query:   query,
+		}
+	}
+	return slices.Clip(results), modOnly, err
+}
+
+// modulePrefixesExcludingTarget returns all prefixes of path that may plausibly
+// exist as a module, excluding targetPrefix but otherwise including path
+// itself, sorted by descending length. Prefixes that are not valid module paths
+// but are valid package paths (like "m" or "example.com/.gen") are included,
+// since they might be replaced.
+func modulePrefixesExcludingTarget(path string) []string {
+	prefixes := make([]string, 0, strings.Count(path, "/")+1)
+
+	mainModulePrefixes := make(map[string]bool)
+	for _, m := range MainModules.Versions() {
+		mainModulePrefixes[m.Path] = true
+	}
+
+	for {
+		if !mainModulePrefixes[path] {
+			if _, _, ok := module.SplitPathVersion(path); ok {
+				prefixes = append(prefixes, path)
+			}
+		}
+
+		j := strings.LastIndexByte(path, '/')
+		if j < 0 {
+			break
+		}
+		path = path[:j]
+	}
+
+	return prefixes
+}
+
+func queryPrefixModules(ctx context.Context, candidateModules []string, queryModule func(ctx context.Context, path string) (QueryResult, error)) (found []QueryResult, err error) {
+	ctx, span := trace.StartSpan(ctx, "modload.queryPrefixModules")
+	defer span.Done()
+
+	// If the path we're attempting is not in the module cache and we don't have a
+	// fetch result cached either, we'll end up making a (potentially slow)
+	// request to the proxy or (often even slower) the origin server.
+	// To minimize latency, execute all of those requests in parallel.
+	type result struct {
+		QueryResult
+		err error
+	}
+	results := make([]result, len(candidateModules))
+	var wg sync.WaitGroup
+	wg.Add(len(candidateModules))
+	for i, p := range candidateModules {
+		ctx := trace.StartGoroutine(ctx)
+		go func(p string, r *result) {
+			r.QueryResult, r.err = queryModule(ctx, p)
+			wg.Done()
+		}(p, &results[i])
+	}
+	wg.Wait()
+
+	// Classify the results. In case of failure, identify the error that the user
+	// is most likely to find helpful: the most useful class of error at the
+	// longest matching path.
+	var (
+		noPackage   *PackageNotInModuleError
+		noVersion   *NoMatchingVersionError
+		noPatchBase *NoPatchBaseError
+		invalidPath *module.InvalidPathError // see comment in case below
+		notExistErr error
+	)
+	for _, r := range results {
+		switch rErr := r.err.(type) {
+		case nil:
+			found = append(found, r.QueryResult)
+		case *PackageNotInModuleError:
+			// Given the option, prefer to attribute “package not in module”
+			// to modules other than the main one.
+			if noPackage == nil || MainModules.Contains(noPackage.Mod.Path) {
+				noPackage = rErr
+			}
+		case *NoMatchingVersionError:
+			if noVersion == nil {
+				noVersion = rErr
+			}
+		case *NoPatchBaseError:
+			if noPatchBase == nil {
+				noPatchBase = rErr
+			}
+		case *module.InvalidPathError:
+			// The prefix was not a valid module path, and there was no replacement.
+			// Prefixes like this may appear in candidateModules, since we handle
+			// replaced modules that weren't required in the repo lookup process
+			// (see lookupRepo).
+			//
+			// A shorter prefix may be a valid module path and may contain a valid
+			// import path, so this is a low-priority error.
+			if invalidPath == nil {
+				invalidPath = rErr
+			}
+		default:
+			if errors.Is(rErr, fs.ErrNotExist) {
+				if notExistErr == nil {
+					notExistErr = rErr
+				}
+			} else if err == nil {
+				if len(found) > 0 || noPackage != nil {
+					// golang.org/issue/34094: If we have already found a module that
+					// could potentially contain the target package, ignore unclassified
+					// errors for modules with shorter paths.
+
+					// golang.org/issue/34383 is a special case of this: if we have
+					// already found example.com/foo/v2@v2.0.0 with a matching go.mod
+					// file, ignore the error from example.com/foo@v2.0.0.
+				} else {
+					err = r.err
+				}
+			}
+		}
+	}
+
+	// TODO(#26232): If len(found) == 0 and some of the errors are 4xx HTTP
+	// codes, have the auth package recheck the failed paths.
+	// If we obtain new credentials for any of them, re-run the above loop.
+
+	if len(found) == 0 && err == nil {
+		switch {
+		case noPackage != nil:
+			err = noPackage
+		case noVersion != nil:
+			err = noVersion
+		case noPatchBase != nil:
+			err = noPatchBase
+		case invalidPath != nil:
+			err = invalidPath
+		case notExistErr != nil:
+			err = notExistErr
+		default:
+			panic("queryPrefixModules: no modules found, but no error detected")
+		}
+	}
+
+	return found, err
+}
+
+// A NoMatchingVersionError indicates that Query found a module at the requested
+// path, but not at any versions satisfying the query string and allow-function.
+//
+// NOTE: NoMatchingVersionError MUST NOT implement Is(fs.ErrNotExist).
+//
+// If the module came from a proxy, that proxy had to return a successful status
+// code for the versions it knows about, and thus did not have the opportunity
+// to return a non-400 status code to suppress fallback.
+type NoMatchingVersionError struct {
+	query, current string
+}
+
+func (e *NoMatchingVersionError) Error() string {
+	currentSuffix := ""
+	if (e.query == "upgrade" || e.query == "patch") && e.current != "" && e.current != "none" {
+		currentSuffix = fmt.Sprintf(" (current version is %s)", e.current)
+	}
+	return fmt.Sprintf("no matching versions for query %q", e.query) + currentSuffix
+}
+
+// A NoPatchBaseError indicates that Query was called with the query "patch"
+// but with a current version of "" or "none".
+type NoPatchBaseError struct {
+	path string
+}
+
+func (e *NoPatchBaseError) Error() string {
+	return fmt.Sprintf(`can't query version "patch" of module %s: no existing version is required`, e.path)
+}
+
+// A WildcardInFirstElementError indicates that a pattern passed to QueryPattern
+// had a wildcard in its first path element, and therefore had no pattern-prefix
+// modules to search in.
+type WildcardInFirstElementError struct {
+	Pattern string
+	Query   string
+}
+
+func (e *WildcardInFirstElementError) Error() string {
+	return fmt.Sprintf("no modules to query for %s@%s because first path element contains a wildcard", e.Pattern, e.Query)
+}
+
+// A PackageNotInModuleError indicates that QueryPattern found a candidate
+// module at the requested version, but that module did not contain any packages
+// matching the requested pattern.
+//
+// NOTE: PackageNotInModuleError MUST NOT implement Is(fs.ErrNotExist).
+//
+// If the module came from a proxy, that proxy had to return a successful status
+// code for the versions it knows about, and thus did not have the opportunity
+// to return a non-400 status code to suppress fallback.
+type PackageNotInModuleError struct {
+	MainModules []module.Version
+	Mod         module.Version
+	Replacement module.Version
+	Query       string
+	Pattern     string
+}
+
+func (e *PackageNotInModuleError) Error() string {
+	if len(e.MainModules) > 0 {
+		prefix := "workspace modules do"
+		if len(e.MainModules) == 1 {
+			prefix = fmt.Sprintf("main module (%s) does", e.MainModules[0])
+		}
+		if strings.Contains(e.Pattern, "...") {
+			return fmt.Sprintf("%s not contain packages matching %s", prefix, e.Pattern)
+		}
+		return fmt.Sprintf("%s not contain package %s", prefix, e.Pattern)
+	}
+
+	found := ""
+	if r := e.Replacement; r.Path != "" {
+		replacement := r.Path
+		if r.Version != "" {
+			replacement = fmt.Sprintf("%s@%s", r.Path, r.Version)
+		}
+		if e.Query == e.Mod.Version {
+			found = fmt.Sprintf(" (replaced by %s)", replacement)
+		} else {
+			found = fmt.Sprintf(" (%s, replaced by %s)", e.Mod.Version, replacement)
+		}
+	} else if e.Query != e.Mod.Version {
+		found = fmt.Sprintf(" (%s)", e.Mod.Version)
+	}
+
+	if strings.Contains(e.Pattern, "...") {
+		return fmt.Sprintf("module %s@%s found%s, but does not contain packages matching %s", e.Mod.Path, e.Query, found, e.Pattern)
+	}
+	return fmt.Sprintf("module %s@%s found%s, but does not contain package %s", e.Mod.Path, e.Query, found, e.Pattern)
+}
+
+func (e *PackageNotInModuleError) ImportPath() string {
+	if !strings.Contains(e.Pattern, "...") {
+		return e.Pattern
+	}
+	return ""
+}
+
+// versionHasGoMod returns whether a version has a go.mod file.
+//
+// versionHasGoMod fetches the go.mod file (possibly a fake) and true if it
+// contains anything other than a module directive with the same path. When a
+// module does not have a real go.mod file, the go command acts as if it had one
+// that only contained a module directive. Normal go.mod files created after
+// 1.12 at least have a go directive.
+//
+// This function is a heuristic, since it's possible to commit a file that would
+// pass this test. However, we only need a heuristic for determining whether
+// +incompatible versions may be "latest", which is what this function is used
+// for.
+//
+// This heuristic is useful for two reasons: first, when using a proxy,
+// this lets us fetch from the .mod endpoint which is much faster than the .zip
+// endpoint. The .mod file is used anyway, even if the .zip file contains a
+// go.mod with different content. Second, if we don't fetch the .zip, then
+// we don't need to verify it in go.sum. This makes 'go list -m -u' faster
+// and simpler.
+func versionHasGoMod(_ context.Context, m module.Version) (bool, error) {
+	_, data, err := rawGoModData(m)
+	if err != nil {
+		return false, err
+	}
+	isFake := bytes.Equal(data, modfetch.LegacyGoMod(m.Path))
+	return !isFake, nil
+}
+
+// A versionRepo is a subset of modfetch.Repo that can report information about
+// available versions, but cannot fetch specific source files.
+type versionRepo interface {
+	ModulePath() string
+	CheckReuse(context.Context, *codehost.Origin) error
+	Versions(ctx context.Context, prefix string) (*modfetch.Versions, error)
+	Stat(ctx context.Context, rev string) (*modfetch.RevInfo, error)
+	Latest(context.Context) (*modfetch.RevInfo, error)
+}
+
+var _ versionRepo = modfetch.Repo(nil)
+
+func lookupRepo(ctx context.Context, proxy, path string) (repo versionRepo, err error) {
+	if path != "go" && path != "toolchain" {
+		err = module.CheckPath(path)
+	}
+	if err == nil {
+		repo = modfetch.Lookup(ctx, proxy, path)
+	} else {
+		repo = emptyRepo{path: path, err: err}
+	}
+
+	if MainModules == nil {
+		return repo, err
+	} else if _, ok := MainModules.HighestReplaced()[path]; ok {
+		return &replacementRepo{repo: repo}, nil
+	}
+
+	return repo, err
+}
+
+// An emptyRepo is a versionRepo that contains no versions.
+type emptyRepo struct {
+	path string
+	err  error
+}
+
+var _ versionRepo = emptyRepo{}
+
+func (er emptyRepo) ModulePath() string { return er.path }
+func (er emptyRepo) CheckReuse(ctx context.Context, old *codehost.Origin) error {
+	return fmt.Errorf("empty repo")
+}
+func (er emptyRepo) Versions(ctx context.Context, prefix string) (*modfetch.Versions, error) {
+	return &modfetch.Versions{}, nil
+}
+func (er emptyRepo) Stat(ctx context.Context, rev string) (*modfetch.RevInfo, error) {
+	return nil, er.err
+}
+func (er emptyRepo) Latest(ctx context.Context) (*modfetch.RevInfo, error) { return nil, er.err }
+
+// A replacementRepo augments a versionRepo to include the replacement versions
+// (if any) found in the main module's go.mod file.
+//
+// A replacementRepo suppresses "not found" errors for otherwise-nonexistent
+// modules, so a replacementRepo should only be constructed for a module that
+// actually has one or more valid replacements.
+type replacementRepo struct {
+	repo versionRepo
+}
+
+var _ versionRepo = (*replacementRepo)(nil)
+
+func (rr *replacementRepo) ModulePath() string { return rr.repo.ModulePath() }
+
+func (rr *replacementRepo) CheckReuse(ctx context.Context, old *codehost.Origin) error {
+	return fmt.Errorf("replacement repo")
+}
+
+// Versions returns the versions from rr.repo augmented with any matching
+// replacement versions.
+func (rr *replacementRepo) Versions(ctx context.Context, prefix string) (*modfetch.Versions, error) {
+	repoVersions, err := rr.repo.Versions(ctx, prefix)
+	if err != nil {
+		if !errors.Is(err, os.ErrNotExist) {
+			return nil, err
+		}
+		repoVersions = new(modfetch.Versions)
+	}
+
+	versions := repoVersions.List
+	for _, mm := range MainModules.Versions() {
+		if index := MainModules.Index(mm); index != nil && len(index.replace) > 0 {
+			path := rr.ModulePath()
+			for m := range index.replace {
+				if m.Path == path && strings.HasPrefix(m.Version, prefix) && m.Version != "" && !module.IsPseudoVersion(m.Version) {
+					versions = append(versions, m.Version)
+				}
+			}
+		}
+	}
+
+	if len(versions) == len(repoVersions.List) { // replacement versions added
+		return repoVersions, nil
+	}
+
+	path := rr.ModulePath()
+	sort.Slice(versions, func(i, j int) bool {
+		return gover.ModCompare(path, versions[i], versions[j]) < 0
+	})
+	str.Uniq(&versions)
+	return &modfetch.Versions{List: versions}, nil
+}
+
+func (rr *replacementRepo) Stat(ctx context.Context, rev string) (*modfetch.RevInfo, error) {
+	info, err := rr.repo.Stat(ctx, rev)
+	if err == nil {
+		return info, err
+	}
+	var hasReplacements bool
+	for _, v := range MainModules.Versions() {
+		if index := MainModules.Index(v); index != nil && len(index.replace) > 0 {
+			hasReplacements = true
+		}
+	}
+	if !hasReplacements {
+		return info, err
+	}
+
+	v := module.CanonicalVersion(rev)
+	if v != rev {
+		// The replacements in the go.mod file list only canonical semantic versions,
+		// so a non-canonical version can't possibly have a replacement.
+		return info, err
+	}
+
+	path := rr.ModulePath()
+	_, pathMajor, ok := module.SplitPathVersion(path)
+	if ok && pathMajor == "" {
+		if err := module.CheckPathMajor(v, pathMajor); err != nil && semver.Build(v) == "" {
+			v += "+incompatible"
+		}
+	}
+
+	if r := Replacement(module.Version{Path: path, Version: v}); r.Path == "" {
+		return info, err
+	}
+	return rr.replacementStat(v)
+}
+
+func (rr *replacementRepo) Latest(ctx context.Context) (*modfetch.RevInfo, error) {
+	info, err := rr.repo.Latest(ctx)
+	path := rr.ModulePath()
+
+	if v, ok := MainModules.HighestReplaced()[path]; ok {
+		if v == "" {
+			// The only replacement is a wildcard that doesn't specify a version, so
+			// synthesize a pseudo-version with an appropriate major version and a
+			// timestamp below any real timestamp. That way, if the main module is
+			// used from within some other module, the user will be able to upgrade
+			// the requirement to any real version they choose.
+			if _, pathMajor, ok := module.SplitPathVersion(path); ok && len(pathMajor) > 0 {
+				v = module.PseudoVersion(pathMajor[1:], "", time.Time{}, "000000000000")
+			} else {
+				v = module.PseudoVersion("v0", "", time.Time{}, "000000000000")
+			}
+		}
+
+		if err != nil || gover.ModCompare(path, v, info.Version) > 0 {
+			return rr.replacementStat(v)
+		}
+	}
+
+	return info, err
+}
+
+func (rr *replacementRepo) replacementStat(v string) (*modfetch.RevInfo, error) {
+	rev := &modfetch.RevInfo{Version: v}
+	if module.IsPseudoVersion(v) {
+		rev.Time, _ = module.PseudoVersionTime(v)
+		rev.Short, _ = module.PseudoVersionRev(v)
+	}
+	return rev, nil
+}
+
+// A QueryMatchesMainModulesError indicates that a query requests
+// a version of the main module that cannot be satisfied.
+// (The main module's version cannot be changed.)
+type QueryMatchesMainModulesError struct {
+	MainModules []module.Version
+	Pattern     string
+	Query       string
+}
+
+func (e *QueryMatchesMainModulesError) Error() string {
+	if MainModules.Contains(e.Pattern) {
+		return fmt.Sprintf("can't request version %q of the main module (%s)", e.Query, e.Pattern)
+	}
+
+	plural := ""
+	mainModulePaths := make([]string, len(e.MainModules))
+	for i := range e.MainModules {
+		mainModulePaths[i] = e.MainModules[i].Path
+	}
+	if len(e.MainModules) > 1 {
+		plural = "s"
+	}
+	return fmt.Sprintf("can't request version %q of pattern %q that includes the main module%s (%s)", e.Query, e.Pattern, plural, strings.Join(mainModulePaths, ", "))
+}
+
+// A QueryUpgradesAllError indicates that a query requests
+// an upgrade on the all pattern.
+// (The main module's version cannot be changed.)
+type QueryUpgradesAllError struct {
+	MainModules []module.Version
+	Query       string
+}
+
+func (e *QueryUpgradesAllError) Error() string {
+	var plural string = ""
+	if len(e.MainModules) != 1 {
+		plural = "s"
+	}
+
+	return fmt.Sprintf("can't request version %q of pattern \"all\" that includes the main module%s", e.Query, plural)
+}
+
+// A QueryMatchesPackagesInMainModuleError indicates that a query cannot be
+// satisfied because it matches one or more packages found in the main module.
+type QueryMatchesPackagesInMainModuleError struct {
+	Pattern  string
+	Query    string
+	Packages []string
+}
+
+func (e *QueryMatchesPackagesInMainModuleError) Error() string {
+	if len(e.Packages) > 1 {
+		return fmt.Sprintf("pattern %s matches %d packages in the main module, so can't request version %s", e.Pattern, len(e.Packages), e.Query)
+	}
+
+	if search.IsMetaPackage(e.Pattern) || strings.Contains(e.Pattern, "...") {
+		return fmt.Sprintf("pattern %s matches package %s in the main module, so can't request version %s", e.Pattern, e.Packages[0], e.Query)
+	}
+
+	return fmt.Sprintf("package %s is in the main module, so can't request version %s", e.Packages[0], e.Query)
+}
diff --git a/src/cmd/go/internal/modload/query_test.go b/src/cmd/go/internal/modload/query_test.go
new file mode 100644
index 0000000..93f8f0d
--- /dev/null
+++ b/src/cmd/go/internal/modload/query_test.go
@@ -0,0 +1,202 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"context"
+	"flag"
+	"internal/testenv"
+	"log"
+	"os"
+	"path"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/vcweb/vcstest"
+
+	"golang.org/x/mod/module"
+)
+
+func TestMain(m *testing.M) {
+	flag.Parse()
+	if err := testMain(m); err != nil {
+		log.Fatal(err)
+	}
+}
+
+func testMain(m *testing.M) (err error) {
+	cfg.GOPROXY = "direct"
+	cfg.ModCacheRW = true
+
+	srv, err := vcstest.NewServer()
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if closeErr := srv.Close(); err == nil {
+			err = closeErr
+		}
+	}()
+
+	dir, err := os.MkdirTemp("", "modload-test-")
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if rmErr := os.RemoveAll(dir); err == nil {
+			err = rmErr
+		}
+	}()
+
+	os.Setenv("GOPATH", dir)
+	cfg.BuildContext.GOPATH = dir
+	cfg.GOMODCACHE = filepath.Join(dir, "pkg/mod")
+	cfg.SumdbDir = filepath.Join(dir, "pkg/sumdb")
+	m.Run()
+	return nil
+}
+
+var (
+	queryRepo   = "vcs-test.golang.org/git/querytest.git"
+	queryRepoV2 = queryRepo + "/v2"
+	queryRepoV3 = queryRepo + "/v3"
+
+	// Empty version list (no semver tags), not actually empty.
+	emptyRepoPath = "vcs-test.golang.org/git/emptytest.git"
+)
+
+var queryTests = []struct {
+	path    string
+	query   string
+	current string
+	allow   string
+	vers    string
+	err     string
+}{
+	{path: queryRepo, query: "<v0.0.0", vers: "v0.0.0-pre1"},
+	{path: queryRepo, query: "<v0.0.0-pre1", err: `no matching versions for query "<v0.0.0-pre1"`},
+	{path: queryRepo, query: "<=v0.0.0", vers: "v0.0.0"},
+	{path: queryRepo, query: ">v0.0.0", vers: "v0.0.1"},
+	{path: queryRepo, query: ">=v0.0.0", vers: "v0.0.0"},
+	{path: queryRepo, query: "v0.0.1", vers: "v0.0.1"},
+	{path: queryRepo, query: "v0.0.1+foo", vers: "v0.0.1"},
+	{path: queryRepo, query: "v0.0.99", err: `vcs-test.golang.org/git/querytest.git@v0.0.99: invalid version: unknown revision v0.0.99`},
+	{path: queryRepo, query: "v0", vers: "v0.3.0"},
+	{path: queryRepo, query: "v0.1", vers: "v0.1.2"},
+	{path: queryRepo, query: "v0.2", err: `no matching versions for query "v0.2"`},
+	{path: queryRepo, query: "v0.0", vers: "v0.0.3"},
+	{path: queryRepo, query: "v1.9.10-pre2+metadata", vers: "v1.9.10-pre2.0.20190513201126-42abcb6df8ee"},
+	{path: queryRepo, query: "ed5ffdaa", vers: "v1.9.10-pre2.0.20191220134614-ed5ffdaa1f5e"},
+
+	// golang.org/issue/29262: The major version for a module without a suffix
+	// should be based on the most recent tag (v1 as appropriate, not v0
+	// unconditionally).
+	{path: queryRepo, query: "42abcb6df8ee", vers: "v1.9.10-pre2.0.20190513201126-42abcb6df8ee"},
+
+	{path: queryRepo, query: "v1.9.10-pre2+wrongmetadata", err: `vcs-test.golang.org/git/querytest.git@v1.9.10-pre2+wrongmetadata: invalid version: unknown revision v1.9.10-pre2+wrongmetadata`},
+	{path: queryRepo, query: "v1.9.10-pre2", err: `vcs-test.golang.org/git/querytest.git@v1.9.10-pre2: invalid version: unknown revision v1.9.10-pre2`},
+	{path: queryRepo, query: "latest", vers: "v1.9.9"},
+	{path: queryRepo, query: "latest", current: "v1.9.10-pre1", vers: "v1.9.9"},
+	{path: queryRepo, query: "upgrade", vers: "v1.9.9"},
+	{path: queryRepo, query: "upgrade", current: "v1.9.10-pre1", vers: "v1.9.10-pre1"},
+	{path: queryRepo, query: "upgrade", current: "v1.9.10-pre2+metadata", vers: "v1.9.10-pre2.0.20190513201126-42abcb6df8ee"},
+	{path: queryRepo, query: "upgrade", current: "v0.0.0-20190513201126-42abcb6df8ee", vers: "v0.0.0-20190513201126-42abcb6df8ee"},
+	{path: queryRepo, query: "upgrade", allow: "NOMATCH", err: `no matching versions for query "upgrade"`},
+	{path: queryRepo, query: "upgrade", current: "v1.9.9", allow: "NOMATCH", err: `vcs-test.golang.org/git/querytest.git@v1.9.9: disallowed module version`},
+	{path: queryRepo, query: "upgrade", current: "v1.99.99", err: `vcs-test.golang.org/git/querytest.git@v1.99.99: invalid version: unknown revision v1.99.99`},
+	{path: queryRepo, query: "patch", current: "", err: `can't query version "patch" of module vcs-test.golang.org/git/querytest.git: no existing version is required`},
+	{path: queryRepo, query: "patch", current: "v0.1.0", vers: "v0.1.2"},
+	{path: queryRepo, query: "patch", current: "v1.9.0", vers: "v1.9.9"},
+	{path: queryRepo, query: "patch", current: "v1.9.10-pre1", vers: "v1.9.10-pre1"},
+	{path: queryRepo, query: "patch", current: "v1.9.10-pre2+metadata", vers: "v1.9.10-pre2.0.20190513201126-42abcb6df8ee"},
+	{path: queryRepo, query: "patch", current: "v1.99.99", err: `vcs-test.golang.org/git/querytest.git@v1.99.99: invalid version: unknown revision v1.99.99`},
+	{path: queryRepo, query: ">v1.9.9", vers: "v1.9.10-pre1"},
+	{path: queryRepo, query: ">v1.10.0", err: `no matching versions for query ">v1.10.0"`},
+	{path: queryRepo, query: ">=v1.10.0", err: `no matching versions for query ">=v1.10.0"`},
+	{path: queryRepo, query: "6cf84eb", vers: "v0.0.2-0.20180704023347-6cf84ebaea54"},
+
+	// golang.org/issue/27173: A pseudo-version may be based on the highest tag on
+	// any parent commit, or any existing semantically-lower tag: a given commit
+	// could have been a pre-release for a backport tag at any point.
+	{path: queryRepo, query: "3ef0cec634e0", vers: "v0.1.2-0.20180704023347-3ef0cec634e0"},
+	{path: queryRepo, query: "v0.1.2-0.20180704023347-3ef0cec634e0", vers: "v0.1.2-0.20180704023347-3ef0cec634e0"},
+	{path: queryRepo, query: "v0.1.1-0.20180704023347-3ef0cec634e0", vers: "v0.1.1-0.20180704023347-3ef0cec634e0"},
+	{path: queryRepo, query: "v0.0.4-0.20180704023347-3ef0cec634e0", vers: "v0.0.4-0.20180704023347-3ef0cec634e0"},
+
+	// Invalid tags are tested in cmd/go/testdata/script/mod_pseudo_invalid.txt.
+
+	{path: queryRepo, query: "start", vers: "v0.0.0-20180704023101-5e9e31667ddf"},
+	{path: queryRepo, query: "5e9e31667ddf", vers: "v0.0.0-20180704023101-5e9e31667ddf"},
+	{path: queryRepo, query: "v0.0.0-20180704023101-5e9e31667ddf", vers: "v0.0.0-20180704023101-5e9e31667ddf"},
+
+	{path: queryRepo, query: "7a1b6bf", vers: "v0.1.0"},
+
+	{path: queryRepoV2, query: "<v0.0.0", err: `no matching versions for query "<v0.0.0"`},
+	{path: queryRepoV2, query: "<=v0.0.0", err: `no matching versions for query "<=v0.0.0"`},
+	{path: queryRepoV2, query: ">v0.0.0", vers: "v2.0.0"},
+	{path: queryRepoV2, query: ">=v0.0.0", vers: "v2.0.0"},
+
+	{path: queryRepoV2, query: "v2", vers: "v2.5.5"},
+	{path: queryRepoV2, query: "v2.5", vers: "v2.5.5"},
+	{path: queryRepoV2, query: "v2.6", err: `no matching versions for query "v2.6"`},
+	{path: queryRepoV2, query: "v2.6.0-pre1", vers: "v2.6.0-pre1"},
+	{path: queryRepoV2, query: "latest", vers: "v2.5.5"},
+
+	// Commit e0cf3de987e6 is actually v1.19.10-pre1, not anything resembling v3,
+	// and it has a go.mod file with a non-v3 module path. Attempting to query it
+	// as the v3 module should fail.
+	{path: queryRepoV3, query: "e0cf3de987e6", err: `vcs-test.golang.org/git/querytest.git/v3@v3.0.0-20180704024501-e0cf3de987e6: invalid version: go.mod has non-.../v3 module path "vcs-test.golang.org/git/querytest.git" (and .../v3/go.mod does not exist) at revision e0cf3de987e6`},
+
+	// The querytest repo does not have any commits tagged with major version 3,
+	// and the latest commit in the repo has a go.mod file specifying a non-v3 path.
+	// That should prevent us from resolving any version for the /v3 path.
+	{path: queryRepoV3, query: "latest", err: `no matching versions for query "latest"`},
+
+	{path: emptyRepoPath, query: "latest", vers: "v0.0.0-20180704023549-7bb914627242"},
+	{path: emptyRepoPath, query: ">v0.0.0", err: `no matching versions for query ">v0.0.0"`},
+	{path: emptyRepoPath, query: "<v10.0.0", err: `no matching versions for query "<v10.0.0"`},
+}
+
+func TestQuery(t *testing.T) {
+	testenv.MustHaveExternalNetwork(t)
+	testenv.MustHaveExecPath(t, "git")
+
+	ctx := context.Background()
+
+	for _, tt := range queryTests {
+		allow := tt.allow
+		if allow == "" {
+			allow = "*"
+		}
+		allowed := func(ctx context.Context, m module.Version) error {
+			if ok, _ := path.Match(allow, m.Version); !ok {
+				return module.VersionError(m, ErrDisallowed)
+			}
+			return nil
+		}
+		tt := tt
+		t.Run(strings.ReplaceAll(tt.path, "/", "_")+"/"+tt.query+"/"+tt.current+"/"+allow, func(t *testing.T) {
+			t.Parallel()
+
+			info, err := Query(ctx, tt.path, tt.query, tt.current, allowed)
+			if tt.err != "" {
+				if err == nil {
+					t.Errorf("Query(_, %q, %q, %q, %v) = %v, want error %q", tt.path, tt.query, tt.current, allow, info.Version, tt.err)
+				} else if err.Error() != tt.err {
+					t.Errorf("Query(_, %q, %q, %q, %v): %v\nwant error %q", tt.path, tt.query, tt.current, allow, err, tt.err)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("Query(_, %q, %q, %q, %v): %v\nwant %v", tt.path, tt.query, tt.current, allow, err, tt.vers)
+			}
+			if info.Version != tt.vers {
+				t.Errorf("Query(_, %q, %q, %q, %v) = %v, want %v", tt.path, tt.query, tt.current, allow, info.Version, tt.vers)
+			}
+		})
+	}
+}
diff --git a/src/cmd/go/internal/modload/search.go b/src/cmd/go/internal/modload/search.go
new file mode 100644
index 0000000..cb03b69
--- /dev/null
+++ b/src/cmd/go/internal/modload/search.go
@@ -0,0 +1,304 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"runtime"
+	"sort"
+	"strings"
+	"sync"
+
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/imports"
+	"cmd/go/internal/modindex"
+	"cmd/go/internal/par"
+	"cmd/go/internal/search"
+	"cmd/go/internal/str"
+	"cmd/go/internal/trace"
+	"cmd/internal/pkgpattern"
+
+	"golang.org/x/mod/module"
+)
+
+type stdFilter int8
+
+const (
+	omitStd = stdFilter(iota)
+	includeStd
+)
+
+// matchPackages is like m.MatchPackages, but uses a local variable (rather than
+// a global) for tags, can include or exclude packages in the standard library,
+// and is restricted to the given list of modules.
+func matchPackages(ctx context.Context, m *search.Match, tags map[string]bool, filter stdFilter, modules []module.Version) {
+	ctx, span := trace.StartSpan(ctx, "modload.matchPackages")
+	defer span.Done()
+
+	m.Pkgs = []string{}
+
+	isMatch := func(string) bool { return true }
+	treeCanMatch := func(string) bool { return true }
+	if !m.IsMeta() {
+		isMatch = pkgpattern.MatchPattern(m.Pattern())
+		treeCanMatch = pkgpattern.TreeCanMatchPattern(m.Pattern())
+	}
+
+	var mu sync.Mutex
+	have := map[string]bool{
+		"builtin": true, // ignore pseudo-package that exists only for documentation
+	}
+	addPkg := func(p string) {
+		mu.Lock()
+		m.Pkgs = append(m.Pkgs, p)
+		mu.Unlock()
+	}
+	if !cfg.BuildContext.CgoEnabled {
+		have["runtime/cgo"] = true // ignore during walk
+	}
+
+	type pruning int8
+	const (
+		pruneVendor = pruning(1 << iota)
+		pruneGoMod
+	)
+
+	q := par.NewQueue(runtime.GOMAXPROCS(0))
+
+	walkPkgs := func(root, importPathRoot string, prune pruning) {
+		_, span := trace.StartSpan(ctx, "walkPkgs "+root)
+		defer span.Done()
+
+		// If the root itself is a symlink to a directory,
+		// we want to follow it (see https://go.dev/issue/50807).
+		// Add a trailing separator to force that to happen.
+		root = str.WithFilePathSeparator(filepath.Clean(root))
+		err := fsys.Walk(root, func(pkgDir string, fi fs.FileInfo, err error) error {
+			if err != nil {
+				m.AddError(err)
+				return nil
+			}
+
+			want := true
+			elem := ""
+
+			// Don't use GOROOT/src but do walk down into it.
+			if pkgDir == root {
+				if importPathRoot == "" {
+					return nil
+				}
+			} else {
+				// Avoid .foo, _foo, and testdata subdirectory trees.
+				_, elem = filepath.Split(pkgDir)
+				if strings.HasPrefix(elem, ".") || strings.HasPrefix(elem, "_") || elem == "testdata" {
+					want = false
+				}
+			}
+
+			name := path.Join(importPathRoot, filepath.ToSlash(pkgDir[len(root):]))
+			if !treeCanMatch(name) {
+				want = false
+			}
+
+			if !fi.IsDir() {
+				if fi.Mode()&fs.ModeSymlink != 0 && want && strings.Contains(m.Pattern(), "...") {
+					if target, err := fsys.Stat(pkgDir); err == nil && target.IsDir() {
+						fmt.Fprintf(os.Stderr, "warning: ignoring symlink %s\n", pkgDir)
+					}
+				}
+				return nil
+			}
+
+			if !want {
+				return filepath.SkipDir
+			}
+			// Stop at module boundaries.
+			if (prune&pruneGoMod != 0) && pkgDir != root {
+				if fi, err := os.Stat(filepath.Join(pkgDir, "go.mod")); err == nil && !fi.IsDir() {
+					return filepath.SkipDir
+				}
+			}
+
+			if !have[name] {
+				have[name] = true
+				if isMatch(name) {
+					q.Add(func() {
+						if _, _, err := scanDir(root, pkgDir, tags); err != imports.ErrNoGo {
+							addPkg(name)
+						}
+					})
+				}
+			}
+
+			if elem == "vendor" && (prune&pruneVendor != 0) {
+				return filepath.SkipDir
+			}
+			return nil
+		})
+		if err != nil {
+			m.AddError(err)
+		}
+	}
+
+	// Wait for all in-flight operations to complete before returning.
+	defer func() {
+		<-q.Idle()
+		sort.Strings(m.Pkgs) // sort everything we added for determinism
+	}()
+
+	if filter == includeStd {
+		walkPkgs(cfg.GOROOTsrc, "", pruneGoMod)
+		if treeCanMatch("cmd") {
+			walkPkgs(filepath.Join(cfg.GOROOTsrc, "cmd"), "cmd", pruneGoMod)
+		}
+	}
+
+	if cfg.BuildMod == "vendor" {
+		mod := MainModules.mustGetSingleMainModule()
+		if modRoot := MainModules.ModRoot(mod); modRoot != "" {
+			walkPkgs(modRoot, MainModules.PathPrefix(mod), pruneGoMod|pruneVendor)
+			walkPkgs(filepath.Join(modRoot, "vendor"), "", pruneVendor)
+		}
+		return
+	}
+
+	for _, mod := range modules {
+		if gover.IsToolchain(mod.Path) || !treeCanMatch(mod.Path) {
+			continue
+		}
+
+		var (
+			root, modPrefix string
+			isLocal         bool
+		)
+		if MainModules.Contains(mod.Path) {
+			if MainModules.ModRoot(mod) == "" {
+				continue // If there is no main module, we can't search in it.
+			}
+			root = MainModules.ModRoot(mod)
+			modPrefix = MainModules.PathPrefix(mod)
+			isLocal = true
+		} else {
+			var err error
+			root, isLocal, err = fetch(ctx, mod)
+			if err != nil {
+				m.AddError(err)
+				continue
+			}
+			modPrefix = mod.Path
+		}
+		if mi, err := modindex.GetModule(root); err == nil {
+			walkFromIndex(mi, modPrefix, isMatch, treeCanMatch, tags, have, addPkg)
+			continue
+		} else if !errors.Is(err, modindex.ErrNotIndexed) {
+			m.AddError(err)
+		}
+
+		prune := pruneVendor
+		if isLocal {
+			prune |= pruneGoMod
+		}
+		walkPkgs(root, modPrefix, prune)
+	}
+
+	return
+}
+
+// walkFromIndex matches packages in a module using the module index. modroot
+// is the module's root directory on disk, index is the modindex.Module for the
+// module, and importPathRoot is the module's path prefix.
+func walkFromIndex(index *modindex.Module, importPathRoot string, isMatch, treeCanMatch func(string) bool, tags, have map[string]bool, addPkg func(string)) {
+	index.Walk(func(reldir string) {
+		// Avoid .foo, _foo, and testdata subdirectory trees.
+		p := reldir
+		for {
+			elem, rest, found := strings.Cut(p, string(filepath.Separator))
+			if strings.HasPrefix(elem, ".") || strings.HasPrefix(elem, "_") || elem == "testdata" {
+				return
+			}
+			if found && elem == "vendor" {
+				// Ignore this path if it contains the element "vendor" anywhere
+				// except for the last element (packages named vendor are allowed
+				// for historical reasons). Note that found is true when this
+				// isn't the last path element.
+				return
+			}
+			if !found {
+				// Didn't find the separator, so we're considering the last element.
+				break
+			}
+			p = rest
+		}
+
+		// Don't use GOROOT/src.
+		if reldir == "" && importPathRoot == "" {
+			return
+		}
+
+		name := path.Join(importPathRoot, filepath.ToSlash(reldir))
+		if !treeCanMatch(name) {
+			return
+		}
+
+		if !have[name] {
+			have[name] = true
+			if isMatch(name) {
+				if _, _, err := index.Package(reldir).ScanDir(tags); err != imports.ErrNoGo {
+					addPkg(name)
+				}
+			}
+		}
+	})
+}
+
+// MatchInModule identifies the packages matching the given pattern within the
+// given module version, which does not need to be in the build list or module
+// requirement graph.
+//
+// If m is the zero module.Version, MatchInModule matches the pattern
+// against the standard library (std and cmd) in GOROOT/src.
+func MatchInModule(ctx context.Context, pattern string, m module.Version, tags map[string]bool) *search.Match {
+	match := search.NewMatch(pattern)
+	if m == (module.Version{}) {
+		matchPackages(ctx, match, tags, includeStd, nil)
+	}
+
+	LoadModFile(ctx) // Sets Target, needed by fetch and matchPackages.
+
+	if !match.IsLiteral() {
+		matchPackages(ctx, match, tags, omitStd, []module.Version{m})
+		return match
+	}
+
+	root, isLocal, err := fetch(ctx, m)
+	if err != nil {
+		match.Errs = []error{err}
+		return match
+	}
+
+	dir, haveGoFiles, err := dirInModule(pattern, m.Path, root, isLocal)
+	if err != nil {
+		match.Errs = []error{err}
+		return match
+	}
+	if haveGoFiles {
+		if _, _, err := scanDir(root, dir, tags); err != imports.ErrNoGo {
+			// ErrNoGo indicates that the directory is not actually a Go package,
+			// perhaps due to the tags in use. Any other non-nil error indicates a
+			// problem with one or more of the Go source files, but such an error does
+			// not stop the package from existing, so it has no impact on matching.
+			match.Pkgs = []string{pattern}
+		}
+	}
+	return match
+}
diff --git a/src/cmd/go/internal/modload/stat_openfile.go b/src/cmd/go/internal/modload/stat_openfile.go
new file mode 100644
index 0000000..5773073
--- /dev/null
+++ b/src/cmd/go/internal/modload/stat_openfile.go
@@ -0,0 +1,28 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build (js && wasm) || plan9
+
+// On plan9, per http://9p.io/magic/man2html/2/access: “Since file permissions
+// are checked by the server and group information is not known to the client,
+// access must open the file to check permissions.”
+//
+// js,wasm is similar, in that it does not define syscall.Access.
+
+package modload
+
+import (
+	"io/fs"
+	"os"
+)
+
+// hasWritePerm reports whether the current user has permission to write to the
+// file with the given info.
+func hasWritePerm(path string, _ fs.FileInfo) bool {
+	if f, err := os.OpenFile(path, os.O_WRONLY, 0); err == nil {
+		f.Close()
+		return true
+	}
+	return false
+}
diff --git a/src/cmd/go/internal/modload/stat_unix.go b/src/cmd/go/internal/modload/stat_unix.go
new file mode 100644
index 0000000..a0d5f4d
--- /dev/null
+++ b/src/cmd/go/internal/modload/stat_unix.go
@@ -0,0 +1,32 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix
+
+package modload
+
+import (
+	"io/fs"
+	"os"
+	"syscall"
+)
+
+// hasWritePerm reports whether the current user has permission to write to the
+// file with the given info.
+//
+// Although the root user on most Unix systems can write to files even without
+// permission, hasWritePerm reports false if no appropriate permission bit is
+// set even if the current user is root.
+func hasWritePerm(path string, fi fs.FileInfo) bool {
+	if os.Getuid() == 0 {
+		// The root user can access any file, but we still want to default to
+		// read-only mode if the go.mod file is marked as globally non-writable.
+		// (If the user really intends not to be in readonly mode, they can
+		// pass -mod=mod explicitly.)
+		return fi.Mode()&0222 != 0
+	}
+
+	const W_OK = 0x2
+	return syscall.Access(path, W_OK) == nil
+}
diff --git a/src/cmd/go/internal/modload/stat_windows.go b/src/cmd/go/internal/modload/stat_windows.go
new file mode 100644
index 0000000..f29a991
--- /dev/null
+++ b/src/cmd/go/internal/modload/stat_windows.go
@@ -0,0 +1,21 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build windows
+
+package modload
+
+import "io/fs"
+
+// hasWritePerm reports whether the current user has permission to write to the
+// file with the given info.
+func hasWritePerm(_ string, fi fs.FileInfo) bool {
+	// Windows has a read-only attribute independent of ACLs, so use that to
+	// determine whether the file is intended to be overwritten.
+	//
+	// Per https://golang.org/pkg/os/#Chmod:
+	// “On Windows, only the 0200 bit (owner writable) of mode is used; it
+	// controls whether the file's read-only attribute is set or cleared.”
+	return fi.Mode()&0200 != 0
+}
diff --git a/src/cmd/go/internal/modload/vendor.go b/src/cmd/go/internal/modload/vendor.go
new file mode 100644
index 0000000..ffc79bb
--- /dev/null
+++ b/src/cmd/go/internal/modload/vendor.go
@@ -0,0 +1,231 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/gover"
+
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
+)
+
+var (
+	vendorOnce      sync.Once
+	vendorList      []module.Version          // modules that contribute packages to the build, in order of appearance
+	vendorReplaced  []module.Version          // all replaced modules; may or may not also contribute packages
+	vendorVersion   map[string]string         // module path → selected version (if known)
+	vendorPkgModule map[string]module.Version // package → containing module
+	vendorMeta      map[module.Version]vendorMetadata
+)
+
+type vendorMetadata struct {
+	Explicit    bool
+	Replacement module.Version
+	GoVersion   string
+}
+
+// readVendorList reads the list of vendored modules from vendor/modules.txt.
+func readVendorList(mainModule module.Version) {
+	vendorOnce.Do(func() {
+		vendorList = nil
+		vendorPkgModule = make(map[string]module.Version)
+		vendorVersion = make(map[string]string)
+		vendorMeta = make(map[module.Version]vendorMetadata)
+		vendorFile := filepath.Join(MainModules.ModRoot(mainModule), "vendor/modules.txt")
+		data, err := os.ReadFile(vendorFile)
+		if err != nil {
+			if !errors.Is(err, fs.ErrNotExist) {
+				base.Fatalf("go: %s", err)
+			}
+			return
+		}
+
+		var mod module.Version
+		for _, line := range strings.Split(string(data), "\n") {
+			if strings.HasPrefix(line, "# ") {
+				f := strings.Fields(line)
+
+				if len(f) < 3 {
+					continue
+				}
+				if semver.IsValid(f[2]) {
+					// A module, but we don't yet know whether it is in the build list or
+					// only included to indicate a replacement.
+					mod = module.Version{Path: f[1], Version: f[2]}
+					f = f[3:]
+				} else if f[2] == "=>" {
+					// A wildcard replacement found in the main module's go.mod file.
+					mod = module.Version{Path: f[1]}
+					f = f[2:]
+				} else {
+					// Not a version or a wildcard replacement.
+					// We don't know how to interpret this module line, so ignore it.
+					mod = module.Version{}
+					continue
+				}
+
+				if len(f) >= 2 && f[0] == "=>" {
+					meta := vendorMeta[mod]
+					if len(f) == 2 {
+						// File replacement.
+						meta.Replacement = module.Version{Path: f[1]}
+						vendorReplaced = append(vendorReplaced, mod)
+					} else if len(f) == 3 && semver.IsValid(f[2]) {
+						// Path and version replacement.
+						meta.Replacement = module.Version{Path: f[1], Version: f[2]}
+						vendorReplaced = append(vendorReplaced, mod)
+					} else {
+						// We don't understand this replacement. Ignore it.
+					}
+					vendorMeta[mod] = meta
+				}
+				continue
+			}
+
+			// Not a module line. Must be a package within a module or a metadata
+			// directive, either of which requires a preceding module line.
+			if mod.Path == "" {
+				continue
+			}
+
+			if annotations, ok := strings.CutPrefix(line, "## "); ok {
+				// Metadata. Take the union of annotations across multiple lines, if present.
+				meta := vendorMeta[mod]
+				for _, entry := range strings.Split(annotations, ";") {
+					entry = strings.TrimSpace(entry)
+					if entry == "explicit" {
+						meta.Explicit = true
+					}
+					if goVersion, ok := strings.CutPrefix(entry, "go "); ok {
+						meta.GoVersion = goVersion
+						rawGoVersion.Store(mod, meta.GoVersion)
+						if gover.Compare(goVersion, gover.Local()) > 0 {
+							base.Fatal(&gover.TooNewError{What: mod.Path + " in " + base.ShortPath(vendorFile), GoVersion: goVersion})
+						}
+					}
+					// All other tokens are reserved for future use.
+				}
+				vendorMeta[mod] = meta
+				continue
+			}
+
+			if f := strings.Fields(line); len(f) == 1 && module.CheckImportPath(f[0]) == nil {
+				// A package within the current module.
+				vendorPkgModule[f[0]] = mod
+
+				// Since this module provides a package for the build, we know that it
+				// is in the build list and is the selected version of its path.
+				// If this information is new, record it.
+				if v, ok := vendorVersion[mod.Path]; !ok || gover.ModCompare(mod.Path, v, mod.Version) < 0 {
+					vendorList = append(vendorList, mod)
+					vendorVersion[mod.Path] = mod.Version
+				}
+			}
+		}
+	})
+}
+
+// checkVendorConsistency verifies that the vendor/modules.txt file matches (if
+// go 1.14) or at least does not contradict (go 1.13 or earlier) the
+// requirements and replacements listed in the main module's go.mod file.
+func checkVendorConsistency(index *modFileIndex, modFile *modfile.File) {
+	readVendorList(MainModules.mustGetSingleMainModule())
+
+	pre114 := false
+	if gover.Compare(index.goVersion, "1.14") < 0 {
+		// Go versions before 1.14 did not include enough information in
+		// vendor/modules.txt to check for consistency.
+		// If we know that we're on an earlier version, relax the consistency check.
+		pre114 = true
+	}
+
+	vendErrors := new(strings.Builder)
+	vendErrorf := func(mod module.Version, format string, args ...any) {
+		detail := fmt.Sprintf(format, args...)
+		if mod.Version == "" {
+			fmt.Fprintf(vendErrors, "\n\t%s: %s", mod.Path, detail)
+		} else {
+			fmt.Fprintf(vendErrors, "\n\t%s@%s: %s", mod.Path, mod.Version, detail)
+		}
+	}
+
+	// Iterate over the Require directives in their original (not indexed) order
+	// so that the errors match the original file.
+	for _, r := range modFile.Require {
+		if !vendorMeta[r.Mod].Explicit {
+			if pre114 {
+				// Before 1.14, modules.txt did not indicate whether modules were listed
+				// explicitly in the main module's go.mod file.
+				// However, we can at least detect a version mismatch if packages were
+				// vendored from a non-matching version.
+				if vv, ok := vendorVersion[r.Mod.Path]; ok && vv != r.Mod.Version {
+					vendErrorf(r.Mod, fmt.Sprintf("is explicitly required in go.mod, but vendor/modules.txt indicates %s@%s", r.Mod.Path, vv))
+				}
+			} else {
+				vendErrorf(r.Mod, "is explicitly required in go.mod, but not marked as explicit in vendor/modules.txt")
+			}
+		}
+	}
+
+	describe := func(m module.Version) string {
+		if m.Version == "" {
+			return m.Path
+		}
+		return m.Path + "@" + m.Version
+	}
+
+	// We need to verify *all* replacements that occur in modfile: even if they
+	// don't directly apply to any module in the vendor list, the replacement
+	// go.mod file can affect the selected versions of other (transitive)
+	// dependencies
+	for _, r := range modFile.Replace {
+		vr := vendorMeta[r.Old].Replacement
+		if vr == (module.Version{}) {
+			if pre114 && (r.Old.Version == "" || vendorVersion[r.Old.Path] != r.Old.Version) {
+				// Before 1.14, modules.txt omitted wildcard replacements and
+				// replacements for modules that did not have any packages to vendor.
+			} else {
+				vendErrorf(r.Old, "is replaced in go.mod, but not marked as replaced in vendor/modules.txt")
+			}
+		} else if vr != r.New {
+			vendErrorf(r.Old, "is replaced by %s in go.mod, but marked as replaced by %s in vendor/modules.txt", describe(r.New), describe(vr))
+		}
+	}
+
+	for _, mod := range vendorList {
+		meta := vendorMeta[mod]
+		if meta.Explicit {
+			if _, inGoMod := index.require[mod]; !inGoMod {
+				vendErrorf(mod, "is marked as explicit in vendor/modules.txt, but not explicitly required in go.mod")
+			}
+		}
+	}
+
+	for _, mod := range vendorReplaced {
+		r := Replacement(mod)
+		if r == (module.Version{}) {
+			vendErrorf(mod, "is marked as replaced in vendor/modules.txt, but not replaced in go.mod")
+			continue
+		}
+		if meta := vendorMeta[mod]; r != meta.Replacement {
+			vendErrorf(mod, "is marked as replaced by %s in vendor/modules.txt, but replaced by %s in go.mod", describe(meta.Replacement), describe(r))
+		}
+	}
+
+	if vendErrors.Len() > 0 {
+		modRoot := MainModules.ModRoot(MainModules.mustGetSingleMainModule())
+		base.Fatalf("go: inconsistent vendoring in %s:%s\n\n\tTo ignore the vendor directory, use -mod=readonly or -mod=mod.\n\tTo sync the vendor directory, run:\n\t\tgo mod vendor", modRoot, vendErrors)
+	}
+}
diff --git a/src/cmd/go/internal/mvs/errors.go b/src/cmd/go/internal/mvs/errors.go
new file mode 100644
index 0000000..8db65d6
--- /dev/null
+++ b/src/cmd/go/internal/mvs/errors.go
@@ -0,0 +1,105 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package mvs
+
+import (
+	"fmt"
+	"strings"
+
+	"golang.org/x/mod/module"
+)
+
+// BuildListError decorates an error that occurred gathering requirements
+// while constructing a build list. BuildListError prints the chain
+// of requirements to the module where the error occurred.
+type BuildListError struct {
+	Err   error
+	stack []buildListErrorElem
+}
+
+type buildListErrorElem struct {
+	m module.Version
+
+	// nextReason is the reason this module depends on the next module in the
+	// stack. Typically either "requires", or "updating to".
+	nextReason string
+}
+
+// NewBuildListError returns a new BuildListError wrapping an error that
+// occurred at a module found along the given path of requirements and/or
+// upgrades, which must be non-empty.
+//
+// The isVersionChange function reports whether a path step is due to an
+// explicit upgrade or downgrade (as opposed to an existing requirement in a
+// go.mod file). A nil isVersionChange function indicates that none of the path
+// steps are due to explicit version changes.
+func NewBuildListError(err error, path []module.Version, isVersionChange func(from, to module.Version) bool) *BuildListError {
+	stack := make([]buildListErrorElem, 0, len(path))
+	for len(path) > 1 {
+		reason := "requires"
+		if isVersionChange != nil && isVersionChange(path[0], path[1]) {
+			reason = "updating to"
+		}
+		stack = append(stack, buildListErrorElem{
+			m:          path[0],
+			nextReason: reason,
+		})
+		path = path[1:]
+	}
+	stack = append(stack, buildListErrorElem{m: path[0]})
+
+	return &BuildListError{
+		Err:   err,
+		stack: stack,
+	}
+}
+
+// Module returns the module where the error occurred. If the module stack
+// is empty, this returns a zero value.
+func (e *BuildListError) Module() module.Version {
+	if len(e.stack) == 0 {
+		return module.Version{}
+	}
+	return e.stack[len(e.stack)-1].m
+}
+
+func (e *BuildListError) Error() string {
+	b := &strings.Builder{}
+	stack := e.stack
+
+	// Don't print modules at the beginning of the chain without a
+	// version. These always seem to be the main module or a
+	// synthetic module ("target@").
+	for len(stack) > 0 && stack[0].m.Version == "" {
+		stack = stack[1:]
+	}
+
+	if len(stack) == 0 {
+		b.WriteString(e.Err.Error())
+	} else {
+		for _, elem := range stack[:len(stack)-1] {
+			fmt.Fprintf(b, "%s %s\n\t", elem.m, elem.nextReason)
+		}
+		// Ensure that the final module path and version are included as part of the
+		// error message.
+		m := stack[len(stack)-1].m
+		if mErr, ok := e.Err.(*module.ModuleError); ok {
+			actual := module.Version{Path: mErr.Path, Version: mErr.Version}
+			if v, ok := mErr.Err.(*module.InvalidVersionError); ok {
+				actual.Version = v.Version
+			}
+			if actual == m {
+				fmt.Fprintf(b, "%v", e.Err)
+			} else {
+				fmt.Fprintf(b, "%s (replaced by %s): %v", m, actual, mErr.Err)
+			}
+		} else {
+			fmt.Fprintf(b, "%v", module.VersionError(m, e.Err))
+		}
+	}
+	return b.String()
+}
+
+func (e *BuildListError) Unwrap() error { return e.Err }
diff --git a/src/cmd/go/internal/mvs/graph.go b/src/cmd/go/internal/mvs/graph.go
new file mode 100644
index 0000000..56b3c60
--- /dev/null
+++ b/src/cmd/go/internal/mvs/graph.go
@@ -0,0 +1,226 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package mvs
+
+import (
+	"fmt"
+	"slices"
+
+	"cmd/go/internal/gover"
+
+	"golang.org/x/mod/module"
+)
+
+// Graph implements an incremental version of the MVS algorithm, with the
+// requirements pushed by the caller instead of pulled by the MVS traversal.
+type Graph struct {
+	cmp   func(p, v1, v2 string) int
+	roots []module.Version
+
+	required map[module.Version][]module.Version
+
+	isRoot   map[module.Version]bool // contains true for roots and false for reachable non-roots
+	selected map[string]string       // path → version
+}
+
+// NewGraph returns an incremental MVS graph containing only a set of root
+// dependencies and using the given max function for version strings.
+//
+// The caller must ensure that the root slice is not modified while the Graph
+// may be in use.
+func NewGraph(cmp func(p, v1, v2 string) int, roots []module.Version) *Graph {
+	g := &Graph{
+		cmp:      cmp,
+		roots:    slices.Clip(roots),
+		required: make(map[module.Version][]module.Version),
+		isRoot:   make(map[module.Version]bool),
+		selected: make(map[string]string),
+	}
+
+	for _, m := range roots {
+		g.isRoot[m] = true
+		if g.cmp(m.Path, g.Selected(m.Path), m.Version) < 0 {
+			g.selected[m.Path] = m.Version
+		}
+	}
+
+	return g
+}
+
+// Require adds the information that module m requires all modules in reqs.
+// The reqs slice must not be modified after it is passed to Require.
+//
+// m must be reachable by some existing chain of requirements from g's target,
+// and Require must not have been called for it already.
+//
+// If any of the modules in reqs has the same path as g's target,
+// the target must have higher precedence than the version in req.
+func (g *Graph) Require(m module.Version, reqs []module.Version) {
+	// To help catch disconnected-graph bugs, enforce that all required versions
+	// are actually reachable from the roots (and therefore should affect the
+	// selected versions of the modules they name).
+	if _, reachable := g.isRoot[m]; !reachable {
+		panic(fmt.Sprintf("%v is not reachable from any root", m))
+	}
+
+	// Truncate reqs to its capacity to avoid aliasing bugs if it is later
+	// returned from RequiredBy and appended to.
+	reqs = slices.Clip(reqs)
+
+	if _, dup := g.required[m]; dup {
+		panic(fmt.Sprintf("requirements of %v have already been set", m))
+	}
+	g.required[m] = reqs
+
+	for _, dep := range reqs {
+		// Mark dep reachable, regardless of whether it is selected.
+		if _, ok := g.isRoot[dep]; !ok {
+			g.isRoot[dep] = false
+		}
+
+		if g.cmp(dep.Path, g.Selected(dep.Path), dep.Version) < 0 {
+			g.selected[dep.Path] = dep.Version
+		}
+	}
+}
+
+// RequiredBy returns the slice of requirements passed to Require for m, if any,
+// with its capacity reduced to its length.
+// If Require has not been called for m, RequiredBy(m) returns ok=false.
+//
+// The caller must not modify the returned slice, but may safely append to it
+// and may rely on it not to be modified.
+func (g *Graph) RequiredBy(m module.Version) (reqs []module.Version, ok bool) {
+	reqs, ok = g.required[m]
+	return reqs, ok
+}
+
+// Selected returns the selected version of the given module path.
+//
+// If no version is selected, Selected returns version "none".
+func (g *Graph) Selected(path string) (version string) {
+	v, ok := g.selected[path]
+	if !ok {
+		return "none"
+	}
+	return v
+}
+
+// BuildList returns the selected versions of all modules present in the Graph,
+// beginning with the selected versions of each module path in the roots of g.
+//
+// The order of the remaining elements in the list is deterministic
+// but arbitrary.
+func (g *Graph) BuildList() []module.Version {
+	seenRoot := make(map[string]bool, len(g.roots))
+
+	var list []module.Version
+	for _, r := range g.roots {
+		if seenRoot[r.Path] {
+			// Multiple copies of the same root, with the same or different versions,
+			// are a bit of a degenerate case: we will take the transitive
+			// requirements of both roots into account, but only the higher one can
+			// possibly be selected. However — especially given that we need the
+			// seenRoot map for later anyway — it is simpler to support this
+			// degenerate case than to forbid it.
+			continue
+		}
+
+		if v := g.Selected(r.Path); v != "none" {
+			list = append(list, module.Version{Path: r.Path, Version: v})
+		}
+		seenRoot[r.Path] = true
+	}
+	uniqueRoots := list
+
+	for path, version := range g.selected {
+		if !seenRoot[path] {
+			list = append(list, module.Version{Path: path, Version: version})
+		}
+	}
+	gover.ModSort(list[len(uniqueRoots):])
+
+	return list
+}
+
+// WalkBreadthFirst invokes f once, in breadth-first order, for each module
+// version other than "none" that appears in the graph, regardless of whether
+// that version is selected.
+func (g *Graph) WalkBreadthFirst(f func(m module.Version)) {
+	var queue []module.Version
+	enqueued := make(map[module.Version]bool)
+	for _, m := range g.roots {
+		if m.Version != "none" {
+			queue = append(queue, m)
+			enqueued[m] = true
+		}
+	}
+
+	for len(queue) > 0 {
+		m := queue[0]
+		queue = queue[1:]
+
+		f(m)
+
+		reqs, _ := g.RequiredBy(m)
+		for _, r := range reqs {
+			if !enqueued[r] && r.Version != "none" {
+				queue = append(queue, r)
+				enqueued[r] = true
+			}
+		}
+	}
+}
+
+// FindPath reports a shortest requirement path starting at one of the roots of
+// the graph and ending at a module version m for which f(m) returns true, or
+// nil if no such path exists.
+func (g *Graph) FindPath(f func(module.Version) bool) []module.Version {
+	// firstRequires[a] = b means that in a breadth-first traversal of the
+	// requirement graph, the module version a was first required by b.
+	firstRequires := make(map[module.Version]module.Version)
+
+	queue := g.roots
+	for _, m := range g.roots {
+		firstRequires[m] = module.Version{}
+	}
+
+	for len(queue) > 0 {
+		m := queue[0]
+		queue = queue[1:]
+
+		if f(m) {
+			// Construct the path reversed (because we're starting from the far
+			// endpoint), then reverse it.
+			path := []module.Version{m}
+			for {
+				m = firstRequires[m]
+				if m.Path == "" {
+					break
+				}
+				path = append(path, m)
+			}
+
+			i, j := 0, len(path)-1
+			for i < j {
+				path[i], path[j] = path[j], path[i]
+				i++
+				j--
+			}
+
+			return path
+		}
+
+		reqs, _ := g.RequiredBy(m)
+		for _, r := range reqs {
+			if _, seen := firstRequires[r]; !seen {
+				queue = append(queue, r)
+				firstRequires[r] = m
+			}
+		}
+	}
+
+	return nil
+}
diff --git a/src/cmd/go/internal/mvs/mvs.go b/src/cmd/go/internal/mvs/mvs.go
new file mode 100644
index 0000000..468a985
--- /dev/null
+++ b/src/cmd/go/internal/mvs/mvs.go
@@ -0,0 +1,488 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package mvs implements Minimal Version Selection.
+// See https://research.swtch.com/vgo-mvs.
+package mvs
+
+import (
+	"fmt"
+	"reflect"
+	"sort"
+	"sync"
+
+	"cmd/go/internal/par"
+
+	"golang.org/x/mod/module"
+)
+
+// A Reqs is the requirement graph on which Minimal Version Selection (MVS) operates.
+//
+// The version strings are opaque except for the special version "none"
+// (see the documentation for module.Version). In particular, MVS does not
+// assume that the version strings are semantic versions; instead, the Max method
+// gives access to the comparison operation.
+//
+// It must be safe to call methods on a Reqs from multiple goroutines simultaneously.
+// Because a Reqs may read the underlying graph from the network on demand,
+// the MVS algorithms parallelize the traversal to overlap network delays.
+type Reqs interface {
+	// Required returns the module versions explicitly required by m itself.
+	// The caller must not modify the returned list.
+	Required(m module.Version) ([]module.Version, error)
+
+	// Max returns the maximum of v1 and v2 (it returns either v1 or v2)
+	// in the module with path p.
+	//
+	// For all versions v, Max(v, "none") must be v,
+	// and for the target passed as the first argument to MVS functions,
+	// Max(target, v) must be target.
+	//
+	// Note that v1 < v2 can be written Max(v1, v2) != v1
+	// and similarly v1 <= v2 can be written Max(v1, v2) == v2.
+	Max(p, v1, v2 string) string
+}
+
+// An UpgradeReqs is a Reqs that can also identify available upgrades.
+type UpgradeReqs interface {
+	Reqs
+
+	// Upgrade returns the upgraded version of m,
+	// for use during an UpgradeAll operation.
+	// If m should be kept as is, Upgrade returns m.
+	// If m is not yet used in the build, then m.Version will be "none".
+	// More typically, m.Version will be the version required
+	// by some other module in the build.
+	//
+	// If no module version is available for the given path,
+	// Upgrade returns a non-nil error.
+	// TODO(rsc): Upgrade must be able to return errors,
+	// but should "no latest version" just return m instead?
+	Upgrade(m module.Version) (module.Version, error)
+}
+
+// A DowngradeReqs is a Reqs that can also identify available downgrades.
+type DowngradeReqs interface {
+	Reqs
+
+	// Previous returns the version of m.Path immediately prior to m.Version,
+	// or "none" if no such version is known.
+	Previous(m module.Version) (module.Version, error)
+}
+
+// BuildList returns the build list for the target module.
+//
+// target is the root vertex of a module requirement graph. For cmd/go, this is
+// typically the main module, but note that this algorithm is not intended to
+// be Go-specific: module paths and versions are treated as opaque values.
+//
+// reqs describes the module requirement graph and provides an opaque method
+// for comparing versions.
+//
+// BuildList traverses the graph and returns a list containing the highest
+// version for each visited module. The first element of the returned list is
+// target itself; reqs.Max requires target.Version to compare higher than all
+// other versions, so no other version can be selected. The remaining elements
+// of the list are sorted by path.
+//
+// See https://research.swtch.com/vgo-mvs for details.
+func BuildList(targets []module.Version, reqs Reqs) ([]module.Version, error) {
+	return buildList(targets, reqs, nil)
+}
+
+func buildList(targets []module.Version, reqs Reqs, upgrade func(module.Version) (module.Version, error)) ([]module.Version, error) {
+	cmp := func(p, v1, v2 string) int {
+		if reqs.Max(p, v1, v2) != v1 {
+			return -1
+		}
+		if reqs.Max(p, v2, v1) != v2 {
+			return 1
+		}
+		return 0
+	}
+
+	var (
+		mu       sync.Mutex
+		g        = NewGraph(cmp, targets)
+		upgrades = map[module.Version]module.Version{}
+		errs     = map[module.Version]error{} // (non-nil errors only)
+	)
+
+	// Explore work graph in parallel in case reqs.Required
+	// does high-latency network operations.
+	var work par.Work[module.Version]
+	for _, target := range targets {
+		work.Add(target)
+	}
+	work.Do(10, func(m module.Version) {
+
+		var required []module.Version
+		var err error
+		if m.Version != "none" {
+			required, err = reqs.Required(m)
+		}
+
+		u := m
+		if upgrade != nil {
+			upgradeTo, upErr := upgrade(m)
+			if upErr == nil {
+				u = upgradeTo
+			} else if err == nil {
+				err = upErr
+			}
+		}
+
+		mu.Lock()
+		if err != nil {
+			errs[m] = err
+		}
+		if u != m {
+			upgrades[m] = u
+			required = append([]module.Version{u}, required...)
+		}
+		g.Require(m, required)
+		mu.Unlock()
+
+		for _, r := range required {
+			work.Add(r)
+		}
+	})
+
+	// If there was an error, find the shortest path from the target to the
+	// node where the error occurred so we can report a useful error message.
+	if len(errs) > 0 {
+		errPath := g.FindPath(func(m module.Version) bool {
+			return errs[m] != nil
+		})
+		if len(errPath) == 0 {
+			panic("internal error: could not reconstruct path to module with error")
+		}
+
+		err := errs[errPath[len(errPath)-1]]
+		isUpgrade := func(from, to module.Version) bool {
+			if u, ok := upgrades[from]; ok {
+				return u == to
+			}
+			return false
+		}
+		return nil, NewBuildListError(err, errPath, isUpgrade)
+	}
+
+	// The final list is the minimum version of each module found in the graph.
+	list := g.BuildList()
+	if vs := list[:len(targets)]; !reflect.DeepEqual(vs, targets) {
+		// target.Version will be "" for modload, the main client of MVS.
+		// "" denotes the main module, which has no version. However, MVS treats
+		// version strings as opaque, so "" is not a special value here.
+		// See golang.org/issue/31491, golang.org/issue/29773.
+		panic(fmt.Sprintf("mistake: chose versions %+v instead of targets %+v", vs, targets))
+	}
+	return list, nil
+}
+
+// Req returns the minimal requirement list for the target module,
+// with the constraint that all module paths listed in base must
+// appear in the returned list.
+func Req(mainModule module.Version, base []string, reqs Reqs) ([]module.Version, error) {
+	list, err := BuildList([]module.Version{mainModule}, reqs)
+	if err != nil {
+		return nil, err
+	}
+
+	// Note: Not running in parallel because we assume
+	// that list came from a previous operation that paged
+	// in all the requirements, so there's no I/O to overlap now.
+
+	max := map[string]string{}
+	for _, m := range list {
+		max[m.Path] = m.Version
+	}
+
+	// Compute postorder, cache requirements.
+	var postorder []module.Version
+	reqCache := map[module.Version][]module.Version{}
+	reqCache[mainModule] = nil
+
+	var walk func(module.Version) error
+	walk = func(m module.Version) error {
+		_, ok := reqCache[m]
+		if ok {
+			return nil
+		}
+		required, err := reqs.Required(m)
+		if err != nil {
+			return err
+		}
+		reqCache[m] = required
+		for _, m1 := range required {
+			if err := walk(m1); err != nil {
+				return err
+			}
+		}
+		postorder = append(postorder, m)
+		return nil
+	}
+	for _, m := range list {
+		if err := walk(m); err != nil {
+			return nil, err
+		}
+	}
+
+	// Walk modules in reverse post-order, only adding those not implied already.
+	have := map[module.Version]bool{}
+	walk = func(m module.Version) error {
+		if have[m] {
+			return nil
+		}
+		have[m] = true
+		for _, m1 := range reqCache[m] {
+			walk(m1)
+		}
+		return nil
+	}
+	// First walk the base modules that must be listed.
+	var min []module.Version
+	haveBase := map[string]bool{}
+	for _, path := range base {
+		if haveBase[path] {
+			continue
+		}
+		m := module.Version{Path: path, Version: max[path]}
+		min = append(min, m)
+		walk(m)
+		haveBase[path] = true
+	}
+	// Now the reverse postorder to bring in anything else.
+	for i := len(postorder) - 1; i >= 0; i-- {
+		m := postorder[i]
+		if max[m.Path] != m.Version {
+			// Older version.
+			continue
+		}
+		if !have[m] {
+			min = append(min, m)
+			walk(m)
+		}
+	}
+	sort.Slice(min, func(i, j int) bool {
+		return min[i].Path < min[j].Path
+	})
+	return min, nil
+}
+
+// UpgradeAll returns a build list for the target module
+// in which every module is upgraded to its latest version.
+func UpgradeAll(target module.Version, reqs UpgradeReqs) ([]module.Version, error) {
+	return buildList([]module.Version{target}, reqs, func(m module.Version) (module.Version, error) {
+		if m.Path == target.Path {
+			return target, nil
+		}
+
+		return reqs.Upgrade(m)
+	})
+}
+
+// Upgrade returns a build list for the target module
+// in which the given additional modules are upgraded.
+func Upgrade(target module.Version, reqs UpgradeReqs, upgrade ...module.Version) ([]module.Version, error) {
+	list, err := reqs.Required(target)
+	if err != nil {
+		return nil, err
+	}
+
+	pathInList := make(map[string]bool, len(list))
+	for _, m := range list {
+		pathInList[m.Path] = true
+	}
+	list = append([]module.Version(nil), list...)
+
+	upgradeTo := make(map[string]string, len(upgrade))
+	for _, u := range upgrade {
+		if !pathInList[u.Path] {
+			list = append(list, module.Version{Path: u.Path, Version: "none"})
+		}
+		if prev, dup := upgradeTo[u.Path]; dup {
+			upgradeTo[u.Path] = reqs.Max(u.Path, prev, u.Version)
+		} else {
+			upgradeTo[u.Path] = u.Version
+		}
+	}
+
+	return buildList([]module.Version{target}, &override{target, list, reqs}, func(m module.Version) (module.Version, error) {
+		if v, ok := upgradeTo[m.Path]; ok {
+			return module.Version{Path: m.Path, Version: v}, nil
+		}
+		return m, nil
+	})
+}
+
+// Downgrade returns a build list for the target module
+// in which the given additional modules are downgraded,
+// potentially overriding the requirements of the target.
+//
+// The versions to be downgraded may be unreachable from reqs.Latest and
+// reqs.Previous, but the methods of reqs must otherwise handle such versions
+// correctly.
+func Downgrade(target module.Version, reqs DowngradeReqs, downgrade ...module.Version) ([]module.Version, error) {
+	// Per https://research.swtch.com/vgo-mvs#algorithm_4:
+	// “To avoid an unnecessary downgrade to E 1.1, we must also add a new
+	// requirement on E 1.2. We can apply Algorithm R to find the minimal set of
+	// new requirements to write to go.mod.”
+	//
+	// In order to generate those new requirements, we need to identify versions
+	// for every module in the build list — not just reqs.Required(target).
+	list, err := BuildList([]module.Version{target}, reqs)
+	if err != nil {
+		return nil, err
+	}
+	list = list[1:] // remove target
+
+	max := make(map[string]string)
+	for _, r := range list {
+		max[r.Path] = r.Version
+	}
+	for _, d := range downgrade {
+		if v, ok := max[d.Path]; !ok || reqs.Max(d.Path, v, d.Version) != d.Version {
+			max[d.Path] = d.Version
+		}
+	}
+
+	var (
+		added    = make(map[module.Version]bool)
+		rdeps    = make(map[module.Version][]module.Version)
+		excluded = make(map[module.Version]bool)
+	)
+	var exclude func(module.Version)
+	exclude = func(m module.Version) {
+		if excluded[m] {
+			return
+		}
+		excluded[m] = true
+		for _, p := range rdeps[m] {
+			exclude(p)
+		}
+	}
+	var add func(module.Version)
+	add = func(m module.Version) {
+		if added[m] {
+			return
+		}
+		added[m] = true
+		if v, ok := max[m.Path]; ok && reqs.Max(m.Path, m.Version, v) != v {
+			// m would upgrade an existing dependency — it is not a strict downgrade,
+			// and because it was already present as a dependency, it could affect the
+			// behavior of other relevant packages.
+			exclude(m)
+			return
+		}
+		list, err := reqs.Required(m)
+		if err != nil {
+			// If we can't load the requirements, we couldn't load the go.mod file.
+			// There are a number of reasons this can happen, but this usually
+			// means an older version of the module had a missing or invalid
+			// go.mod file. For example, if example.com/mod released v2.0.0 before
+			// migrating to modules (v2.0.0+incompatible), then added a valid go.mod
+			// in v2.0.1, downgrading from v2.0.1 would cause this error.
+			//
+			// TODO(golang.org/issue/31730, golang.org/issue/30134): if the error
+			// is transient (we couldn't download go.mod), return the error from
+			// Downgrade. Currently, we can't tell what kind of error it is.
+			exclude(m)
+			return
+		}
+		for _, r := range list {
+			add(r)
+			if excluded[r] {
+				exclude(m)
+				return
+			}
+			rdeps[r] = append(rdeps[r], m)
+		}
+	}
+
+	downgraded := make([]module.Version, 0, len(list)+1)
+	downgraded = append(downgraded, target)
+List:
+	for _, r := range list {
+		add(r)
+		for excluded[r] {
+			p, err := reqs.Previous(r)
+			if err != nil {
+				// This is likely a transient error reaching the repository,
+				// rather than a permanent error with the retrieved version.
+				//
+				// TODO(golang.org/issue/31730, golang.org/issue/30134):
+				// decode what to do based on the actual error.
+				return nil, err
+			}
+			// If the target version is a pseudo-version, it may not be
+			// included when iterating over prior versions using reqs.Previous.
+			// Insert it into the right place in the iteration.
+			// If v is excluded, p should be returned again by reqs.Previous on the next iteration.
+			if v := max[r.Path]; reqs.Max(r.Path, v, r.Version) != v && reqs.Max(r.Path, p.Version, v) != p.Version {
+				p.Version = v
+			}
+			if p.Version == "none" {
+				continue List
+			}
+			add(p)
+			r = p
+		}
+		downgraded = append(downgraded, r)
+	}
+
+	// The downgrades we computed above only downgrade to versions enumerated by
+	// reqs.Previous. However, reqs.Previous omits some versions — such as
+	// pseudo-versions and retracted versions — that may be selected as transitive
+	// requirements of other modules.
+	//
+	// If one of those requirements pulls the version back up above the version
+	// identified by reqs.Previous, then the transitive dependencies of that that
+	// initially-downgraded version should no longer matter — in particular, we
+	// should not add new dependencies on module paths that nothing else in the
+	// updated module graph even requires.
+	//
+	// In order to eliminate those spurious dependencies, we recompute the build
+	// list with the actual versions of the downgraded modules as selected by MVS,
+	// instead of our initial downgrades.
+	// (See the downhiddenartifact and downhiddencross test cases).
+	actual, err := BuildList([]module.Version{target}, &override{
+		target: target,
+		list:   downgraded,
+		Reqs:   reqs,
+	})
+	if err != nil {
+		return nil, err
+	}
+	actualVersion := make(map[string]string, len(actual))
+	for _, m := range actual {
+		actualVersion[m.Path] = m.Version
+	}
+
+	downgraded = downgraded[:0]
+	for _, m := range list {
+		if v, ok := actualVersion[m.Path]; ok {
+			downgraded = append(downgraded, module.Version{Path: m.Path, Version: v})
+		}
+	}
+
+	return BuildList([]module.Version{target}, &override{
+		target: target,
+		list:   downgraded,
+		Reqs:   reqs,
+	})
+}
+
+type override struct {
+	target module.Version
+	list   []module.Version
+	Reqs
+}
+
+func (r *override) Required(m module.Version) ([]module.Version, error) {
+	if m == r.target {
+		return r.list, nil
+	}
+	return r.Reqs.Required(m)
+}
diff --git a/src/cmd/go/internal/mvs/mvs_test.go b/src/cmd/go/internal/mvs/mvs_test.go
new file mode 100644
index 0000000..6e1e71c
--- /dev/null
+++ b/src/cmd/go/internal/mvs/mvs_test.go
@@ -0,0 +1,635 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package mvs
+
+import (
+	"fmt"
+	"reflect"
+	"strings"
+	"testing"
+
+	"golang.org/x/mod/module"
+)
+
+var tests = `
+# Scenario from blog.
+name: blog
+A: B1 C2
+B1: D3
+C1: D2
+C2: D4
+C3: D5
+C4: G1
+D2: E1
+D3: E2
+D4: E2 F1
+D5: E2
+G1: C4
+A2: B1 C4 D4
+build A:       A B1 C2 D4 E2 F1
+upgrade* A:    A B1 C4 D5 E2 F1 G1
+upgrade A C4:  A B1 C4 D4 E2 F1 G1
+build A2:     A2 B1 C4 D4 E2 F1 G1
+downgrade A2 D2: A2 C4 D2 E2 F1 G1
+
+name: trim
+A: B1 C2
+B1: D3
+C2: B2
+B2:
+build A: A B2 C2 D3
+
+# Cross-dependency between D and E.
+# No matter how it arises, should get result of merging all build lists via max,
+# which leads to including both D2 and E2.
+
+name: cross1
+A: B C
+B: D1
+C: D2
+D1: E2
+D2: E1
+build A: A B C D2 E2
+
+name: cross1V
+A: B2 C D2 E1
+B1:
+B2: D1
+C: D2
+D1: E2
+D2: E1
+build A: A B2 C D2 E2
+
+name: cross1U
+A: B1 C
+B1:
+B2: D1
+C: D2
+D1: E2
+D2: E1
+build A:      A B1 C D2 E1
+upgrade A B2: A B2 C D2 E2
+
+name: cross1R
+A: B C
+B: D2
+C: D1
+D1: E2
+D2: E1
+build A: A B C D2 E2
+
+name: cross1X
+A: B C
+B: D1 E2
+C: D2
+D1: E2
+D2: E1
+build A: A B C D2 E2
+
+name: cross2
+A: B D2
+B: D1
+D1: E2
+D2: E1
+build A: A B D2 E2
+
+name: cross2X
+A: B D2
+B: D1 E2
+C: D2
+D1: E2
+D2: E1
+build A: A B D2 E2
+
+name: cross3
+A: B D2 E1
+B: D1
+D1: E2
+D2: E1
+build A: A B D2 E2
+
+name: cross3X
+A: B D2 E1
+B: D1 E2
+D1: E2
+D2: E1
+build A: A B D2 E2
+
+# Should not get E2 here, because B has been updated
+# not to depend on D1 anymore.
+name: cross4
+A1: B1 D2
+A2: B2 D2
+B1: D1
+B2: D2
+D1: E2
+D2: E1
+build A1: A1 B1 D2 E2
+build A2: A2 B2 D2 E1
+
+# But the upgrade from A1 preserves the E2 dep explicitly.
+upgrade A1 B2: A1 B2 D2 E2
+upgradereq A1 B2: B2 E2
+
+name: cross5
+A: D1
+D1: E2
+D2: E1
+build A:       A D1 E2
+upgrade* A:    A D2 E2
+upgrade A D2:  A D2 E2
+upgradereq A D2: D2 E2
+
+name: cross6
+A: D2
+D1: E2
+D2: E1
+build A:      A D2 E1
+upgrade* A:   A D2 E2
+upgrade A E2: A D2 E2
+
+name: cross7
+A: B C
+B: D1
+C: E1
+D1: E2
+E1: D2
+build A: A B C D2 E2
+
+# golang.org/issue/31248:
+# Even though we select X2, the requirement on I1
+# via X1 should be preserved.
+name: cross8
+M: A1 B1
+A1: X1
+B1: X2
+X1: I1
+X2:
+build M: M A1 B1 I1 X2
+
+# Upgrade from B1 to B2 should not drop the transitive dep on D.
+name: drop
+A: B1 C1
+B1: D1
+B2:
+C2:
+D2:
+build A:    A B1 C1 D1
+upgrade* A: A B2 C2 D2
+
+name: simplify
+A: B1 C1
+B1: C2
+C1: D1
+C2:
+build A: A B1 C2 D1
+
+name: up1
+A: B1 C1
+B1:
+B2:
+B3:
+B4:
+B5.hidden:
+C2:
+C3:
+build A:    A B1 C1
+upgrade* A: A B4 C3
+
+name: up2
+A: B5.hidden C1
+B1:
+B2:
+B3:
+B4:
+B5.hidden:
+C2:
+C3:
+build A:    A B5.hidden C1
+upgrade* A: A B5.hidden C3
+
+name: down1
+A: B2
+B1: C1
+B2: C2
+build A:        A B2 C2
+downgrade A C1: A B1 C1
+
+name: down2
+A: B2 E2
+B1:
+B2: C2 F2
+C1:
+D1:
+C2: D2 E2
+D2: B2
+E2: D2
+E1:
+F1:
+build A:        A B2 C2 D2 E2 F2
+downgrade A F1: A B1 C1 D1 E1 F1
+
+# https://research.swtch.com/vgo-mvs#algorithm_4:
+# “[D]owngrades are constrained to only downgrade packages, not also upgrade
+# them; if an upgrade before downgrade is needed, the user must ask for it
+# explicitly.”
+#
+# Here, downgrading B2 to B1 upgrades C1 to C2, and C2 does not depend on D2.
+# However, C2 would be an upgrade — not a downgrade — so B1 must also be
+# rejected.
+name: downcross1
+A: B2 C1
+B1: C2
+B2: C1
+C1: D2
+C2:
+D1:
+D2:
+build A:        A B2 C1 D2
+downgrade A D1: A       D1
+
+# https://research.swtch.com/vgo-mvs#algorithm_4:
+# “Unlike upgrades, downgrades must work by removing requirements, not adding
+# them.”
+#
+# However, downgrading a requirement may introduce a new requirement on a
+# previously-unrequired module. If each dependency's requirements are complete
+# (“tidy”), that can't change the behavior of any other package whose version is
+# not also being downgraded, so we should allow it.
+name: downcross2
+A: B2
+B1: C1
+B2: D2
+C1:
+D1:
+D2:
+build A:        A B2    D2
+downgrade A D1: A B1 C1 D1
+
+name: downcycle
+A: A B2
+B2: A
+B1:
+build A:        A B2
+downgrade A B1: A B1
+
+# Both B3 and C2 require D2.
+# If we downgrade D to D1, then in isolation B3 would downgrade to B1,
+# because B2 is hidden — B1 is the next-highest version that is not hidden.
+# However, if we downgrade D, we will also downgrade C to C1.
+# And C1 requires B2.hidden, and B2.hidden also meets our requirements:
+# it is compatible with D1 and a strict downgrade from B3.
+#
+# Since neither the initial nor the final build list includes B1,
+# and the nothing in the final downgraded build list requires E at all,
+# no dependency on E1 (required by only B1) should be introduced.
+#
+name: downhiddenartifact
+A: B3 C2
+A1: B3
+B1: E1
+B2.hidden:
+B3: D2
+C1: B2.hidden
+C2: D2
+D1:
+D2:
+build A1: A1 B3 D2
+downgrade A1 D1: A1 B1 D1 E1
+build A: A B3 C2 D2
+downgrade A D1: A B2.hidden C1 D1
+
+# Both B3 and C3 require D2.
+# If we downgrade D to D1, then in isolation B3 would downgrade to B1,
+# and C3 would downgrade to C1.
+# But C1 requires B2.hidden, and B1 requires C2.hidden, so we can't
+# downgrade to either of those without pulling the other back up a little.
+#
+# B2.hidden and C2.hidden are both compatible with D1, so that still
+# meets our requirements — but then we're in an odd state in which
+# B and C have both been downgraded to hidden versions, without any
+# remaining requirements to explain how those hidden versions got there.
+#
+# TODO(bcmills): Would it be better to force downgrades to land on non-hidden
+# versions?
+# In this case, that would remove the dependencies on B and C entirely.
+#
+name: downhiddencross
+A: B3 C3
+B1: C2.hidden
+B2.hidden:
+B3: D2
+C1: B2.hidden
+C2.hidden:
+C3: D2
+D1:
+D2:
+build A: A B3 C3 D2
+downgrade A D1: A B2.hidden C2.hidden D1
+
+# golang.org/issue/25542.
+name: noprev1
+A: B4 C2
+B2.hidden:
+C2:
+build A:               A B4        C2
+downgrade A B2.hidden: A B2.hidden C2
+
+name: noprev2
+A: B4 C2
+B2.hidden:
+B1:
+C2:
+build A:               A B4        C2
+downgrade A B2.hidden: A B2.hidden C2
+
+name: noprev3
+A: B4 C2
+B3:
+B2.hidden:
+C2:
+build A:               A B4        C2
+downgrade A B2.hidden: A B2.hidden C2
+
+# Cycles involving the target.
+
+# The target must be the newest version of itself.
+name: cycle1
+A: B1
+B1: A1
+B2: A2
+B3: A3
+build A:      A B1
+upgrade A B2: A B2
+upgrade* A:   A B3
+
+# golang.org/issue/29773:
+# Requirements of older versions of the target
+# must be carried over.
+name: cycle2
+A: B1
+A1: C1
+A2: D1
+B1: A1
+B2: A2
+C1: A2
+C2:
+D2:
+build A:    A B1 C1 D1
+upgrade* A: A B2 C2 D2
+
+# Cycles with multiple possible solutions.
+# (golang.org/issue/34086)
+name: cycle3
+M: A1 C2
+A1: B1
+B1: C1
+B2: C2
+C1:
+C2: B2
+build M: M A1 B2 C2
+req M:     A1 B2
+req M A:   A1 B2
+req M C:   A1 C2
+
+# Requirement minimization.
+
+name: req1
+A: B1 C1 D1 E1 F1
+B1: C1 E1 F1
+req A:   B1    D1
+req A C: B1 C1 D1
+
+name: req2
+A: G1 H1
+G1: H1
+H1: G1
+req A:   G1
+req A G: G1
+req A H: H1
+
+name: req3
+M: A1 B1
+A1: X1
+B1: X2
+X1: I1
+X2:
+req M: A1 B1
+
+name: reqnone
+M: Anone B1 D1 E1
+B1: Cnone D1
+E1: Fnone
+build M: M B1 D1 E1
+req M:     B1    E1
+
+name: reqdup
+M: A1 B1
+A1: B1
+B1:
+req M A A: A1
+
+name: reqcross
+M: A1 B1 C1
+A1: B1 C1
+B1: C1
+C1:
+req M A B: A1 B1
+`
+
+func Test(t *testing.T) {
+	var (
+		name string
+		reqs reqsMap
+		fns  []func(*testing.T)
+	)
+	flush := func() {
+		if name != "" {
+			t.Run(name, func(t *testing.T) {
+				for _, fn := range fns {
+					fn(t)
+				}
+				if len(fns) == 0 {
+					t.Errorf("no functions tested")
+				}
+			})
+		}
+	}
+	m := func(s string) module.Version {
+		return module.Version{Path: s[:1], Version: s[1:]}
+	}
+	ms := func(list []string) []module.Version {
+		var mlist []module.Version
+		for _, s := range list {
+			mlist = append(mlist, m(s))
+		}
+		return mlist
+	}
+	checkList := func(t *testing.T, desc string, list []module.Version, err error, val string) {
+		if err != nil {
+			t.Fatalf("%s: %v", desc, err)
+		}
+		vs := ms(strings.Fields(val))
+		if !reflect.DeepEqual(list, vs) {
+			t.Errorf("%s = %v, want %v", desc, list, vs)
+		}
+	}
+
+	for _, line := range strings.Split(tests, "\n") {
+		line = strings.TrimSpace(line)
+		if strings.HasPrefix(line, "#") || line == "" {
+			continue
+		}
+		i := strings.Index(line, ":")
+		if i < 0 {
+			t.Fatalf("missing colon: %q", line)
+		}
+		key := strings.TrimSpace(line[:i])
+		val := strings.TrimSpace(line[i+1:])
+		if key == "" {
+			t.Fatalf("missing key: %q", line)
+		}
+		kf := strings.Fields(key)
+		switch kf[0] {
+		case "name":
+			if len(kf) != 1 {
+				t.Fatalf("name takes no arguments: %q", line)
+			}
+			flush()
+			reqs = make(reqsMap)
+			fns = nil
+			name = val
+			continue
+		case "build":
+			if len(kf) != 2 {
+				t.Fatalf("build takes one argument: %q", line)
+			}
+			fns = append(fns, func(t *testing.T) {
+				list, err := BuildList([]module.Version{m(kf[1])}, reqs)
+				checkList(t, key, list, err, val)
+			})
+			continue
+		case "upgrade*":
+			if len(kf) != 2 {
+				t.Fatalf("upgrade* takes one argument: %q", line)
+			}
+			fns = append(fns, func(t *testing.T) {
+				list, err := UpgradeAll(m(kf[1]), reqs)
+				checkList(t, key, list, err, val)
+			})
+			continue
+		case "upgradereq":
+			if len(kf) < 2 {
+				t.Fatalf("upgrade takes at least one argument: %q", line)
+			}
+			fns = append(fns, func(t *testing.T) {
+				list, err := Upgrade(m(kf[1]), reqs, ms(kf[2:])...)
+				if err == nil {
+					// Copy the reqs map, but substitute the upgraded requirements in
+					// place of the target's original requirements.
+					upReqs := make(reqsMap, len(reqs))
+					for m, r := range reqs {
+						upReqs[m] = r
+					}
+					upReqs[m(kf[1])] = list
+
+					list, err = Req(m(kf[1]), nil, upReqs)
+				}
+				checkList(t, key, list, err, val)
+			})
+			continue
+		case "upgrade":
+			if len(kf) < 2 {
+				t.Fatalf("upgrade takes at least one argument: %q", line)
+			}
+			fns = append(fns, func(t *testing.T) {
+				list, err := Upgrade(m(kf[1]), reqs, ms(kf[2:])...)
+				checkList(t, key, list, err, val)
+			})
+			continue
+		case "downgrade":
+			if len(kf) < 2 {
+				t.Fatalf("downgrade takes at least one argument: %q", line)
+			}
+			fns = append(fns, func(t *testing.T) {
+				list, err := Downgrade(m(kf[1]), reqs, ms(kf[1:])...)
+				checkList(t, key, list, err, val)
+			})
+			continue
+		case "req":
+			if len(kf) < 2 {
+				t.Fatalf("req takes at least one argument: %q", line)
+			}
+			fns = append(fns, func(t *testing.T) {
+				list, err := Req(m(kf[1]), kf[2:], reqs)
+				checkList(t, key, list, err, val)
+			})
+			continue
+		}
+		if len(kf) == 1 && 'A' <= key[0] && key[0] <= 'Z' {
+			var rs []module.Version
+			for _, f := range strings.Fields(val) {
+				r := m(f)
+				if reqs[r] == nil {
+					reqs[r] = []module.Version{}
+				}
+				rs = append(rs, r)
+			}
+			reqs[m(key)] = rs
+			continue
+		}
+		t.Fatalf("bad line: %q", line)
+	}
+	flush()
+}
+
+type reqsMap map[module.Version][]module.Version
+
+func (r reqsMap) Max(_, v1, v2 string) string {
+	if v1 == "none" || v2 == "" {
+		return v2
+	}
+	if v2 == "none" || v1 == "" {
+		return v1
+	}
+	if v1 < v2 {
+		return v2
+	}
+	return v1
+}
+
+func (r reqsMap) Upgrade(m module.Version) (module.Version, error) {
+	u := module.Version{Version: "none"}
+	for k := range r {
+		if k.Path == m.Path && r.Max(k.Path, u.Version, k.Version) == k.Version && !strings.HasSuffix(k.Version, ".hidden") {
+			u = k
+		}
+	}
+	if u.Path == "" {
+		return module.Version{}, fmt.Errorf("missing module: %v", module.Version{Path: m.Path})
+	}
+	return u, nil
+}
+
+func (r reqsMap) Previous(m module.Version) (module.Version, error) {
+	var p module.Version
+	for k := range r {
+		if k.Path == m.Path && p.Version < k.Version && k.Version < m.Version && !strings.HasSuffix(k.Version, ".hidden") {
+			p = k
+		}
+	}
+	if p.Path == "" {
+		return module.Version{Path: m.Path, Version: "none"}, nil
+	}
+	return p, nil
+}
+
+func (r reqsMap) Required(m module.Version) ([]module.Version, error) {
+	rr, ok := r[m]
+	if !ok {
+		return nil, fmt.Errorf("missing module: %v", m)
+	}
+	return rr, nil
+}
diff --git a/src/cmd/go/internal/par/queue.go b/src/cmd/go/internal/par/queue.go
new file mode 100644
index 0000000..180bc75
--- /dev/null
+++ b/src/cmd/go/internal/par/queue.go
@@ -0,0 +1,88 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package par
+
+import "fmt"
+
+// Queue manages a set of work items to be executed in parallel. The number of
+// active work items is limited, and excess items are queued sequentially.
+type Queue struct {
+	maxActive int
+	st        chan queueState
+}
+
+type queueState struct {
+	active  int // number of goroutines processing work; always nonzero when len(backlog) > 0
+	backlog []func()
+	idle    chan struct{} // if non-nil, closed when active becomes 0
+}
+
+// NewQueue returns a Queue that executes up to maxActive items in parallel.
+//
+// maxActive must be positive.
+func NewQueue(maxActive int) *Queue {
+	if maxActive < 1 {
+		panic(fmt.Sprintf("par.NewQueue called with nonpositive limit (%d)", maxActive))
+	}
+
+	q := &Queue{
+		maxActive: maxActive,
+		st:        make(chan queueState, 1),
+	}
+	q.st <- queueState{}
+	return q
+}
+
+// Add adds f as a work item in the queue.
+//
+// Add returns immediately, but the queue will be marked as non-idle until after
+// f (and any subsequently-added work) has completed.
+func (q *Queue) Add(f func()) {
+	st := <-q.st
+	if st.active == q.maxActive {
+		st.backlog = append(st.backlog, f)
+		q.st <- st
+		return
+	}
+	if st.active == 0 {
+		// Mark q as non-idle.
+		st.idle = nil
+	}
+	st.active++
+	q.st <- st
+
+	go func() {
+		for {
+			f()
+
+			st := <-q.st
+			if len(st.backlog) == 0 {
+				if st.active--; st.active == 0 && st.idle != nil {
+					close(st.idle)
+				}
+				q.st <- st
+				return
+			}
+			f, st.backlog = st.backlog[0], st.backlog[1:]
+			q.st <- st
+		}
+	}()
+}
+
+// Idle returns a channel that will be closed when q has no (active or enqueued)
+// work outstanding.
+func (q *Queue) Idle() <-chan struct{} {
+	st := <-q.st
+	defer func() { q.st <- st }()
+
+	if st.idle == nil {
+		st.idle = make(chan struct{})
+		if st.active == 0 {
+			close(st.idle)
+		}
+	}
+
+	return st.idle
+}
diff --git a/src/cmd/go/internal/par/queue_test.go b/src/cmd/go/internal/par/queue_test.go
new file mode 100644
index 0000000..1331e65
--- /dev/null
+++ b/src/cmd/go/internal/par/queue_test.go
@@ -0,0 +1,79 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package par
+
+import (
+	"sync"
+	"testing"
+)
+
+func TestQueueIdle(t *testing.T) {
+	q := NewQueue(1)
+	select {
+	case <-q.Idle():
+	default:
+		t.Errorf("NewQueue(1) is not initially idle.")
+	}
+
+	started := make(chan struct{})
+	unblock := make(chan struct{})
+	q.Add(func() {
+		close(started)
+		<-unblock
+	})
+
+	<-started
+	idle := q.Idle()
+	select {
+	case <-idle:
+		t.Errorf("NewQueue(1) is marked idle while processing work.")
+	default:
+	}
+
+	close(unblock)
+	<-idle // Should be closed as soon as the Add callback returns.
+}
+
+func TestQueueBacklog(t *testing.T) {
+	const (
+		maxActive = 2
+		totalWork = 3 * maxActive
+	)
+
+	q := NewQueue(maxActive)
+	t.Logf("q = NewQueue(%d)", maxActive)
+
+	var wg sync.WaitGroup
+	wg.Add(totalWork)
+	started := make([]chan struct{}, totalWork)
+	unblock := make(chan struct{})
+	for i := range started {
+		started[i] = make(chan struct{})
+		i := i
+		q.Add(func() {
+			close(started[i])
+			<-unblock
+			wg.Done()
+		})
+	}
+
+	for i, c := range started {
+		if i < maxActive {
+			<-c // Work item i should be started immediately.
+		} else {
+			select {
+			case <-c:
+				t.Errorf("Work item %d started before previous items finished.", i)
+			default:
+			}
+		}
+	}
+
+	close(unblock)
+	for _, c := range started[maxActive:] {
+		<-c
+	}
+	wg.Wait()
+}
diff --git a/src/cmd/go/internal/par/work.go b/src/cmd/go/internal/par/work.go
new file mode 100644
index 0000000..5b6de94
--- /dev/null
+++ b/src/cmd/go/internal/par/work.go
@@ -0,0 +1,223 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package par implements parallel execution helpers.
+package par
+
+import (
+	"errors"
+	"math/rand"
+	"sync"
+	"sync/atomic"
+)
+
+// Work manages a set of work items to be executed in parallel, at most once each.
+// The items in the set must all be valid map keys.
+type Work[T comparable] struct {
+	f       func(T) // function to run for each item
+	running int     // total number of runners
+
+	mu      sync.Mutex
+	added   map[T]bool // items added to set
+	todo    []T        // items yet to be run
+	wait    sync.Cond  // wait when todo is empty
+	waiting int        // number of runners waiting for todo
+}
+
+func (w *Work[T]) init() {
+	if w.added == nil {
+		w.added = make(map[T]bool)
+	}
+}
+
+// Add adds item to the work set, if it hasn't already been added.
+func (w *Work[T]) Add(item T) {
+	w.mu.Lock()
+	w.init()
+	if !w.added[item] {
+		w.added[item] = true
+		w.todo = append(w.todo, item)
+		if w.waiting > 0 {
+			w.wait.Signal()
+		}
+	}
+	w.mu.Unlock()
+}
+
+// Do runs f in parallel on items from the work set,
+// with at most n invocations of f running at a time.
+// It returns when everything added to the work set has been processed.
+// At least one item should have been added to the work set
+// before calling Do (or else Do returns immediately),
+// but it is allowed for f(item) to add new items to the set.
+// Do should only be used once on a given Work.
+func (w *Work[T]) Do(n int, f func(item T)) {
+	if n < 1 {
+		panic("par.Work.Do: n < 1")
+	}
+	if w.running >= 1 {
+		panic("par.Work.Do: already called Do")
+	}
+
+	w.running = n
+	w.f = f
+	w.wait.L = &w.mu
+
+	for i := 0; i < n-1; i++ {
+		go w.runner()
+	}
+	w.runner()
+}
+
+// runner executes work in w until both nothing is left to do
+// and all the runners are waiting for work.
+// (Then all the runners return.)
+func (w *Work[T]) runner() {
+	for {
+		// Wait for something to do.
+		w.mu.Lock()
+		for len(w.todo) == 0 {
+			w.waiting++
+			if w.waiting == w.running {
+				// All done.
+				w.wait.Broadcast()
+				w.mu.Unlock()
+				return
+			}
+			w.wait.Wait()
+			w.waiting--
+		}
+
+		// Pick something to do at random,
+		// to eliminate pathological contention
+		// in case items added at about the same time
+		// are most likely to contend.
+		i := rand.Intn(len(w.todo))
+		item := w.todo[i]
+		w.todo[i] = w.todo[len(w.todo)-1]
+		w.todo = w.todo[:len(w.todo)-1]
+		w.mu.Unlock()
+
+		w.f(item)
+	}
+}
+
+// ErrCache is like Cache except that it also stores
+// an error value alongside the cached value V.
+type ErrCache[K comparable, V any] struct {
+	Cache[K, errValue[V]]
+}
+
+type errValue[V any] struct {
+	v   V
+	err error
+}
+
+func (c *ErrCache[K, V]) Do(key K, f func() (V, error)) (V, error) {
+	v := c.Cache.Do(key, func() errValue[V] {
+		v, err := f()
+		return errValue[V]{v, err}
+	})
+	return v.v, v.err
+}
+
+var ErrCacheEntryNotFound = errors.New("cache entry not found")
+
+// Get returns the cached result associated with key.
+// It returns ErrCacheEntryNotFound if there is no such result.
+func (c *ErrCache[K, V]) Get(key K) (V, error) {
+	v, ok := c.Cache.Get(key)
+	if !ok {
+		v.err = ErrCacheEntryNotFound
+	}
+	return v.v, v.err
+}
+
+// Cache runs an action once per key and caches the result.
+type Cache[K comparable, V any] struct {
+	m sync.Map
+}
+
+type cacheEntry[V any] struct {
+	done   atomic.Bool
+	mu     sync.Mutex
+	result V
+}
+
+// Do calls the function f if and only if Do is being called for the first time with this key.
+// No call to Do with a given key returns until the one call to f returns.
+// Do returns the value returned by the one call to f.
+func (c *Cache[K, V]) Do(key K, f func() V) V {
+	entryIface, ok := c.m.Load(key)
+	if !ok {
+		entryIface, _ = c.m.LoadOrStore(key, new(cacheEntry[V]))
+	}
+	e := entryIface.(*cacheEntry[V])
+	if !e.done.Load() {
+		e.mu.Lock()
+		if !e.done.Load() {
+			e.result = f()
+			e.done.Store(true)
+		}
+		e.mu.Unlock()
+	}
+	return e.result
+}
+
+// Get returns the cached result associated with key
+// and reports whether there is such a result.
+//
+// If the result for key is being computed, Get does not wait for the computation to finish.
+func (c *Cache[K, V]) Get(key K) (V, bool) {
+	entryIface, ok := c.m.Load(key)
+	if !ok {
+		return *new(V), false
+	}
+	e := entryIface.(*cacheEntry[V])
+	if !e.done.Load() {
+		return *new(V), false
+	}
+	return e.result, true
+}
+
+// Clear removes all entries in the cache.
+//
+// Concurrent calls to Get may return old values. Concurrent calls to Do
+// may return old values or store results in entries that have been deleted.
+//
+// TODO(jayconrod): Delete this after the package cache clearing functions
+// in internal/load have been removed.
+func (c *Cache[K, V]) Clear() {
+	c.m.Range(func(key, value any) bool {
+		c.m.Delete(key)
+		return true
+	})
+}
+
+// Delete removes an entry from the map. It is safe to call Delete for an
+// entry that does not exist. Delete will return quickly, even if the result
+// for a key is still being computed; the computation will finish, but the
+// result won't be accessible through the cache.
+//
+// TODO(jayconrod): Delete this after the package cache clearing functions
+// in internal/load have been removed.
+func (c *Cache[K, V]) Delete(key K) {
+	c.m.Delete(key)
+}
+
+// DeleteIf calls pred for each key in the map. If pred returns true for a key,
+// DeleteIf removes the corresponding entry. If the result for a key is
+// still being computed, DeleteIf will remove the entry without waiting for
+// the computation to finish. The result won't be accessible through the cache.
+//
+// TODO(jayconrod): Delete this after the package cache clearing functions
+// in internal/load have been removed.
+func (c *Cache[K, V]) DeleteIf(pred func(key K) bool) {
+	c.m.Range(func(key, _ any) bool {
+		if key := key.(K); pred(key) {
+			c.Delete(key)
+		}
+		return true
+	})
+}
diff --git a/src/cmd/go/internal/par/work_test.go b/src/cmd/go/internal/par/work_test.go
new file mode 100644
index 0000000..9d96ffa
--- /dev/null
+++ b/src/cmd/go/internal/par/work_test.go
@@ -0,0 +1,76 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package par
+
+import (
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestWork(t *testing.T) {
+	var w Work[int]
+
+	const N = 10000
+	n := int32(0)
+	w.Add(N)
+	w.Do(100, func(i int) {
+		atomic.AddInt32(&n, 1)
+		if i >= 2 {
+			w.Add(i - 1)
+			w.Add(i - 2)
+		}
+		w.Add(i >> 1)
+		w.Add((i >> 1) ^ 1)
+	})
+	if n != N+1 {
+		t.Fatalf("ran %d items, expected %d", n, N+1)
+	}
+}
+
+func TestWorkParallel(t *testing.T) {
+	for tries := 0; tries < 10; tries++ {
+		var w Work[int]
+		const N = 100
+		for i := 0; i < N; i++ {
+			w.Add(i)
+		}
+		start := time.Now()
+		var n int32
+		w.Do(N, func(x int) {
+			time.Sleep(1 * time.Millisecond)
+			atomic.AddInt32(&n, +1)
+		})
+		if n != N {
+			t.Fatalf("par.Work.Do did not do all the work")
+		}
+		if time.Since(start) < N/2*time.Millisecond {
+			return
+		}
+	}
+	t.Fatalf("par.Work.Do does not seem to be parallel")
+}
+
+func TestCache(t *testing.T) {
+	var cache Cache[int, int]
+
+	n := 1
+	v := cache.Do(1, func() int { n++; return n })
+	if v != 2 {
+		t.Fatalf("cache.Do(1) did not run f")
+	}
+	v = cache.Do(1, func() int { n++; return n })
+	if v != 2 {
+		t.Fatalf("cache.Do(1) ran f again!")
+	}
+	v = cache.Do(2, func() int { n++; return n })
+	if v != 3 {
+		t.Fatalf("cache.Do(2) did not run f")
+	}
+	v = cache.Do(1, func() int { n++; return n })
+	if v != 2 {
+		t.Fatalf("cache.Do(1) did not returned saved value from original cache.Do(1)")
+	}
+}
diff --git a/src/cmd/go/internal/robustio/robustio.go b/src/cmd/go/internal/robustio/robustio.go
new file mode 100644
index 0000000..15b3377
--- /dev/null
+++ b/src/cmd/go/internal/robustio/robustio.go
@@ -0,0 +1,53 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package robustio wraps I/O functions that are prone to failure on Windows,
+// transparently retrying errors up to an arbitrary timeout.
+//
+// Errors are classified heuristically and retries are bounded, so the functions
+// in this package do not completely eliminate spurious errors. However, they do
+// significantly reduce the rate of failure in practice.
+//
+// If so, the error will likely wrap one of:
+// The functions in this package do not completely eliminate spurious errors,
+// but substantially reduce their rate of occurrence in practice.
+package robustio
+
+// Rename is like os.Rename, but on Windows retries errors that may occur if the
+// file is concurrently read or overwritten.
+//
+// (See golang.org/issue/31247 and golang.org/issue/32188.)
+func Rename(oldpath, newpath string) error {
+	return rename(oldpath, newpath)
+}
+
+// ReadFile is like os.ReadFile, but on Windows retries errors that may
+// occur if the file is concurrently replaced.
+//
+// (See golang.org/issue/31247 and golang.org/issue/32188.)
+func ReadFile(filename string) ([]byte, error) {
+	return readFile(filename)
+}
+
+// RemoveAll is like os.RemoveAll, but on Windows retries errors that may occur
+// if an executable file in the directory has recently been executed.
+//
+// (See golang.org/issue/19491.)
+func RemoveAll(path string) error {
+	return removeAll(path)
+}
+
+// IsEphemeralError reports whether err is one of the errors that the functions
+// in this package attempt to mitigate.
+//
+// Errors considered ephemeral include:
+//   - syscall.ERROR_ACCESS_DENIED
+//   - syscall.ERROR_FILE_NOT_FOUND
+//   - internal/syscall/windows.ERROR_SHARING_VIOLATION
+//
+// This set may be expanded in the future; programs must not rely on the
+// non-ephemerality of any given error.
+func IsEphemeralError(err error) bool {
+	return isEphemeralError(err)
+}
diff --git a/src/cmd/go/internal/robustio/robustio_darwin.go b/src/cmd/go/internal/robustio/robustio_darwin.go
new file mode 100644
index 0000000..99fd8eb
--- /dev/null
+++ b/src/cmd/go/internal/robustio/robustio_darwin.go
@@ -0,0 +1,21 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package robustio
+
+import (
+	"errors"
+	"syscall"
+)
+
+const errFileNotFound = syscall.ENOENT
+
+// isEphemeralError returns true if err may be resolved by waiting.
+func isEphemeralError(err error) bool {
+	var errno syscall.Errno
+	if errors.As(err, &errno) {
+		return errno == errFileNotFound
+	}
+	return false
+}
diff --git a/src/cmd/go/internal/robustio/robustio_flaky.go b/src/cmd/go/internal/robustio/robustio_flaky.go
new file mode 100644
index 0000000..c56e36c
--- /dev/null
+++ b/src/cmd/go/internal/robustio/robustio_flaky.go
@@ -0,0 +1,91 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build windows || darwin
+
+package robustio
+
+import (
+	"errors"
+	"math/rand"
+	"os"
+	"syscall"
+	"time"
+)
+
+const arbitraryTimeout = 2000 * time.Millisecond
+
+// retry retries ephemeral errors from f up to an arbitrary timeout
+// to work around filesystem flakiness on Windows and Darwin.
+func retry(f func() (err error, mayRetry bool)) error {
+	var (
+		bestErr     error
+		lowestErrno syscall.Errno
+		start       time.Time
+		nextSleep   time.Duration = 1 * time.Millisecond
+	)
+	for {
+		err, mayRetry := f()
+		if err == nil || !mayRetry {
+			return err
+		}
+
+		var errno syscall.Errno
+		if errors.As(err, &errno) && (lowestErrno == 0 || errno < lowestErrno) {
+			bestErr = err
+			lowestErrno = errno
+		} else if bestErr == nil {
+			bestErr = err
+		}
+
+		if start.IsZero() {
+			start = time.Now()
+		} else if d := time.Since(start) + nextSleep; d >= arbitraryTimeout {
+			break
+		}
+		time.Sleep(nextSleep)
+		nextSleep += time.Duration(rand.Int63n(int64(nextSleep)))
+	}
+
+	return bestErr
+}
+
+// rename is like os.Rename, but retries ephemeral errors.
+//
+// On Windows it wraps os.Rename, which (as of 2019-06-04) uses MoveFileEx with
+// MOVEFILE_REPLACE_EXISTING.
+//
+// Windows also provides a different system call, ReplaceFile,
+// that provides similar semantics, but perhaps preserves more metadata. (The
+// documentation on the differences between the two is very sparse.)
+//
+// Empirical error rates with MoveFileEx are lower under modest concurrency, so
+// for now we're sticking with what the os package already provides.
+func rename(oldpath, newpath string) (err error) {
+	return retry(func() (err error, mayRetry bool) {
+		err = os.Rename(oldpath, newpath)
+		return err, isEphemeralError(err)
+	})
+}
+
+// readFile is like os.ReadFile, but retries ephemeral errors.
+func readFile(filename string) ([]byte, error) {
+	var b []byte
+	err := retry(func() (err error, mayRetry bool) {
+		b, err = os.ReadFile(filename)
+
+		// Unlike in rename, we do not retry errFileNotFound here: it can occur
+		// as a spurious error, but the file may also genuinely not exist, so the
+		// increase in robustness is probably not worth the extra latency.
+		return err, isEphemeralError(err) && !errors.Is(err, errFileNotFound)
+	})
+	return b, err
+}
+
+func removeAll(path string) error {
+	return retry(func() (err error, mayRetry bool) {
+		err = os.RemoveAll(path)
+		return err, isEphemeralError(err)
+	})
+}
diff --git a/src/cmd/go/internal/robustio/robustio_other.go b/src/cmd/go/internal/robustio/robustio_other.go
new file mode 100644
index 0000000..da9a46e
--- /dev/null
+++ b/src/cmd/go/internal/robustio/robustio_other.go
@@ -0,0 +1,27 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !windows && !darwin
+
+package robustio
+
+import (
+	"os"
+)
+
+func rename(oldpath, newpath string) error {
+	return os.Rename(oldpath, newpath)
+}
+
+func readFile(filename string) ([]byte, error) {
+	return os.ReadFile(filename)
+}
+
+func removeAll(path string) error {
+	return os.RemoveAll(path)
+}
+
+func isEphemeralError(err error) bool {
+	return false
+}
diff --git a/src/cmd/go/internal/robustio/robustio_windows.go b/src/cmd/go/internal/robustio/robustio_windows.go
new file mode 100644
index 0000000..687dcb6
--- /dev/null
+++ b/src/cmd/go/internal/robustio/robustio_windows.go
@@ -0,0 +1,27 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package robustio
+
+import (
+	"errors"
+	"internal/syscall/windows"
+	"syscall"
+)
+
+const errFileNotFound = syscall.ERROR_FILE_NOT_FOUND
+
+// isEphemeralError returns true if err may be resolved by waiting.
+func isEphemeralError(err error) bool {
+	var errno syscall.Errno
+	if errors.As(err, &errno) {
+		switch errno {
+		case syscall.ERROR_ACCESS_DENIED,
+			syscall.ERROR_FILE_NOT_FOUND,
+			windows.ERROR_SHARING_VIOLATION:
+			return true
+		}
+	}
+	return false
+}
diff --git a/src/cmd/go/internal/run/run.go b/src/cmd/go/internal/run/run.go
new file mode 100644
index 0000000..4a3dcf0
--- /dev/null
+++ b/src/cmd/go/internal/run/run.go
@@ -0,0 +1,219 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package run implements the “go run” command.
+package run
+
+import (
+	"context"
+	"fmt"
+	"go/build"
+	"os"
+	"path"
+	"path/filepath"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/load"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/str"
+	"cmd/go/internal/work"
+)
+
+var CmdRun = &base.Command{
+	UsageLine: "go run [build flags] [-exec xprog] package [arguments...]",
+	Short:     "compile and run Go program",
+	Long: `
+Run compiles and runs the named main Go package.
+Typically the package is specified as a list of .go source files from a single
+directory, but it may also be an import path, file system path, or pattern
+matching a single known package, as in 'go run .' or 'go run my/cmd'.
+
+If the package argument has a version suffix (like @latest or @v1.0.0),
+"go run" builds the program in module-aware mode, ignoring the go.mod file in
+the current directory or any parent directory, if there is one. This is useful
+for running programs without affecting the dependencies of the main module.
+
+If the package argument doesn't have a version suffix, "go run" may run in
+module-aware mode or GOPATH mode, depending on the GO111MODULE environment
+variable and the presence of a go.mod file. See 'go help modules' for details.
+If module-aware mode is enabled, "go run" runs in the context of the main
+module.
+
+By default, 'go run' runs the compiled binary directly: 'a.out arguments...'.
+If the -exec flag is given, 'go run' invokes the binary using xprog:
+	'xprog a.out arguments...'.
+If the -exec flag is not given, GOOS or GOARCH is different from the system
+default, and a program named go_$GOOS_$GOARCH_exec can be found
+on the current search path, 'go run' invokes the binary using that program,
+for example 'go_js_wasm_exec a.out arguments...'. This allows execution of
+cross-compiled programs when a simulator or other execution method is
+available.
+
+By default, 'go run' compiles the binary without generating the information
+used by debuggers, to reduce build time. To include debugger information in
+the binary, use 'go build'.
+
+The exit status of Run is not the exit status of the compiled binary.
+
+For more about build flags, see 'go help build'.
+For more about specifying packages, see 'go help packages'.
+
+See also: go build.
+	`,
+}
+
+func init() {
+	CmdRun.Run = runRun // break init loop
+
+	work.AddBuildFlags(CmdRun, work.DefaultBuildFlags)
+	if cfg.Experiment != nil && cfg.Experiment.CoverageRedesign {
+		work.AddCoverFlags(CmdRun, nil)
+	}
+	CmdRun.Flag.Var((*base.StringsFlag)(&work.ExecCmd), "exec", "")
+}
+
+func printStderr(args ...any) (int, error) {
+	return fmt.Fprint(os.Stderr, args...)
+}
+
+func runRun(ctx context.Context, cmd *base.Command, args []string) {
+	if shouldUseOutsideModuleMode(args) {
+		// Set global module flags for 'go run cmd@version'.
+		// This must be done before modload.Init, but we need to call work.BuildInit
+		// before loading packages, since it affects package locations, e.g.,
+		// for -race and -msan.
+		modload.ForceUseModules = true
+		modload.RootMode = modload.NoRoot
+		modload.AllowMissingModuleImports()
+		modload.Init()
+	} else {
+		modload.InitWorkfile()
+	}
+
+	work.BuildInit()
+	b := work.NewBuilder("")
+	defer func() {
+		if err := b.Close(); err != nil {
+			base.Fatal(err)
+		}
+	}()
+	b.Print = printStderr
+
+	i := 0
+	for i < len(args) && strings.HasSuffix(args[i], ".go") {
+		i++
+	}
+	pkgOpts := load.PackageOpts{MainOnly: true}
+	var p *load.Package
+	if i > 0 {
+		files := args[:i]
+		for _, file := range files {
+			if strings.HasSuffix(file, "_test.go") {
+				// GoFilesPackage is going to assign this to TestGoFiles.
+				// Reject since it won't be part of the build.
+				base.Fatalf("go: cannot run *_test.go files (%s)", file)
+			}
+		}
+		p = load.GoFilesPackage(ctx, pkgOpts, files)
+	} else if len(args) > 0 && !strings.HasPrefix(args[0], "-") {
+		arg := args[0]
+		var pkgs []*load.Package
+		if strings.Contains(arg, "@") && !build.IsLocalImport(arg) && !filepath.IsAbs(arg) {
+			var err error
+			pkgs, err = load.PackagesAndErrorsOutsideModule(ctx, pkgOpts, args[:1])
+			if err != nil {
+				base.Fatal(err)
+			}
+		} else {
+			pkgs = load.PackagesAndErrors(ctx, pkgOpts, args[:1])
+		}
+
+		if len(pkgs) == 0 {
+			base.Fatalf("go: no packages loaded from %s", arg)
+		}
+		if len(pkgs) > 1 {
+			var names []string
+			for _, p := range pkgs {
+				names = append(names, p.ImportPath)
+			}
+			base.Fatalf("go: pattern %s matches multiple packages:\n\t%s", arg, strings.Join(names, "\n\t"))
+		}
+		p = pkgs[0]
+		i++
+	} else {
+		base.Fatalf("go: no go files listed")
+	}
+	cmdArgs := args[i:]
+	load.CheckPackageErrors([]*load.Package{p})
+
+	if cfg.Experiment.CoverageRedesign && cfg.BuildCover {
+		load.PrepareForCoverageBuild([]*load.Package{p})
+	}
+
+	p.Internal.OmitDebug = true
+	p.Target = "" // must build - not up to date
+	if p.Internal.CmdlineFiles {
+		//set executable name if go file is given as cmd-argument
+		var src string
+		if len(p.GoFiles) > 0 {
+			src = p.GoFiles[0]
+		} else if len(p.CgoFiles) > 0 {
+			src = p.CgoFiles[0]
+		} else {
+			// this case could only happen if the provided source uses cgo
+			// while cgo is disabled.
+			hint := ""
+			if !cfg.BuildContext.CgoEnabled {
+				hint = " (cgo is disabled)"
+			}
+			base.Fatalf("go: no suitable source files%s", hint)
+		}
+		p.Internal.ExeName = src[:len(src)-len(".go")]
+	} else {
+		p.Internal.ExeName = path.Base(p.ImportPath)
+	}
+
+	a1 := b.LinkAction(work.ModeBuild, work.ModeBuild, p)
+	a := &work.Action{Mode: "go run", Actor: work.ActorFunc(buildRunProgram), Args: cmdArgs, Deps: []*work.Action{a1}}
+	b.Do(ctx, a)
+}
+
+// shouldUseOutsideModuleMode returns whether 'go run' will load packages in
+// module-aware mode, ignoring the go.mod file in the current directory. It
+// returns true if the first argument contains "@", does not begin with "-"
+// (resembling a flag) or end with ".go" (a file). The argument must not be a
+// local or absolute file path.
+//
+// These rules are slightly different than other commands. Whether or not
+// 'go run' uses this mode, it interprets arguments ending with ".go" as files
+// and uses arguments up to the last ".go" argument to comprise the package.
+// If there are no ".go" arguments, only the first argument is interpreted
+// as a package path, since there can be only one package.
+func shouldUseOutsideModuleMode(args []string) bool {
+	// NOTE: "@" not allowed in import paths, but it is allowed in non-canonical
+	// versions.
+	return len(args) > 0 &&
+		!strings.HasSuffix(args[0], ".go") &&
+		!strings.HasPrefix(args[0], "-") &&
+		strings.Contains(args[0], "@") &&
+		!build.IsLocalImport(args[0]) &&
+		!filepath.IsAbs(args[0])
+}
+
+// buildRunProgram is the action for running a binary that has already
+// been compiled. We ignore exit status.
+func buildRunProgram(b *work.Builder, ctx context.Context, a *work.Action) error {
+	cmdline := str.StringList(work.FindExecCmd(), a.Deps[0].Target, a.Args)
+	if cfg.BuildN || cfg.BuildX {
+		b.Showcmd("", "%s", strings.Join(cmdline, " "))
+		if cfg.BuildN {
+			return nil
+		}
+	}
+
+	base.RunStdin(cmdline)
+	return nil
+}
diff --git a/src/cmd/go/internal/script/cmds.go b/src/cmd/go/internal/script/cmds.go
new file mode 100644
index 0000000..36e16c5
--- /dev/null
+++ b/src/cmd/go/internal/script/cmds.go
@@ -0,0 +1,1125 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package script
+
+import (
+	"cmd/go/internal/robustio"
+	"errors"
+	"fmt"
+	"internal/diff"
+	"io/fs"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
+	"runtime"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+)
+
+// DefaultCmds returns a set of broadly useful script commands.
+//
+// Run the 'help' command within a script engine to view a list of the available
+// commands.
+func DefaultCmds() map[string]Cmd {
+	return map[string]Cmd{
+		"cat":     Cat(),
+		"cd":      Cd(),
+		"chmod":   Chmod(),
+		"cmp":     Cmp(),
+		"cmpenv":  Cmpenv(),
+		"cp":      Cp(),
+		"echo":    Echo(),
+		"env":     Env(),
+		"exec":    Exec(func(cmd *exec.Cmd) error { return cmd.Process.Signal(os.Interrupt) }, 100*time.Millisecond), // arbitrary grace period
+		"exists":  Exists(),
+		"grep":    Grep(),
+		"help":    Help(),
+		"mkdir":   Mkdir(),
+		"mv":      Mv(),
+		"rm":      Rm(),
+		"replace": Replace(),
+		"sleep":   Sleep(),
+		"stderr":  Stderr(),
+		"stdout":  Stdout(),
+		"stop":    Stop(),
+		"symlink": Symlink(),
+		"wait":    Wait(),
+	}
+}
+
+// Command returns a new Cmd with a Usage method that returns a copy of the
+// given CmdUsage and a Run method calls the given function.
+func Command(usage CmdUsage, run func(*State, ...string) (WaitFunc, error)) Cmd {
+	return &funcCmd{
+		usage: usage,
+		run:   run,
+	}
+}
+
+// A funcCmd implements Cmd using a function value.
+type funcCmd struct {
+	usage CmdUsage
+	run   func(*State, ...string) (WaitFunc, error)
+}
+
+func (c *funcCmd) Run(s *State, args ...string) (WaitFunc, error) {
+	return c.run(s, args...)
+}
+
+func (c *funcCmd) Usage() *CmdUsage { return &c.usage }
+
+// firstNonFlag returns a slice containing the index of the first argument in
+// rawArgs that is not a flag, or nil if all arguments are flags.
+func firstNonFlag(rawArgs ...string) []int {
+	for i, arg := range rawArgs {
+		if !strings.HasPrefix(arg, "-") {
+			return []int{i}
+		}
+		if arg == "--" {
+			return []int{i + 1}
+		}
+	}
+	return nil
+}
+
+// Cat writes the concatenated contents of the named file(s) to the script's
+// stdout buffer.
+func Cat() Cmd {
+	return Command(
+		CmdUsage{
+			Summary: "concatenate files and print to the script's stdout buffer",
+			Args:    "files...",
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			if len(args) == 0 {
+				return nil, ErrUsage
+			}
+
+			paths := make([]string, 0, len(args))
+			for _, arg := range args {
+				paths = append(paths, s.Path(arg))
+			}
+
+			var buf strings.Builder
+			errc := make(chan error, 1)
+			go func() {
+				for _, p := range paths {
+					b, err := os.ReadFile(p)
+					buf.Write(b)
+					if err != nil {
+						errc <- err
+						return
+					}
+				}
+				errc <- nil
+			}()
+
+			wait := func(*State) (stdout, stderr string, err error) {
+				err = <-errc
+				return buf.String(), "", err
+			}
+			return wait, nil
+		})
+}
+
+// Cd changes the current working directory.
+func Cd() Cmd {
+	return Command(
+		CmdUsage{
+			Summary: "change the working directory",
+			Args:    "dir",
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			if len(args) != 1 {
+				return nil, ErrUsage
+			}
+			return nil, s.Chdir(args[0])
+		})
+}
+
+// Chmod changes the permissions of a file or a directory..
+func Chmod() Cmd {
+	return Command(
+		CmdUsage{
+			Summary: "change file mode bits",
+			Args:    "perm paths...",
+			Detail: []string{
+				"Changes the permissions of the named files or directories to be equal to perm.",
+				"Only numerical permissions are supported.",
+			},
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			if len(args) < 2 {
+				return nil, ErrUsage
+			}
+
+			perm, err := strconv.ParseUint(args[0], 0, 32)
+			if err != nil || perm&uint64(fs.ModePerm) != perm {
+				return nil, fmt.Errorf("invalid mode: %s", args[0])
+			}
+
+			for _, arg := range args[1:] {
+				err := os.Chmod(s.Path(arg), fs.FileMode(perm))
+				if err != nil {
+					return nil, err
+				}
+			}
+			return nil, nil
+		})
+}
+
+// Cmp compares the contents of two files, or the contents of either the
+// "stdout" or "stderr" buffer and a file, returning a non-nil error if the
+// contents differ.
+func Cmp() Cmd {
+	return Command(
+		CmdUsage{
+			Args:    "[-q] file1 file2",
+			Summary: "compare files for differences",
+			Detail: []string{
+				"By convention, file1 is the actual data and file2 is the expected data.",
+				"The command succeeds if the file contents are identical.",
+				"File1 can be 'stdout' or 'stderr' to compare the stdout or stderr buffer from the most recent command.",
+			},
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			return nil, doCompare(s, false, args...)
+		})
+}
+
+// Cmpenv is like Compare, but also performs environment substitutions
+// on the contents of both arguments.
+func Cmpenv() Cmd {
+	return Command(
+		CmdUsage{
+			Args:    "[-q] file1 file2",
+			Summary: "compare files for differences, with environment expansion",
+			Detail: []string{
+				"By convention, file1 is the actual data and file2 is the expected data.",
+				"The command succeeds if the file contents are identical after substituting variables from the script environment.",
+				"File1 can be 'stdout' or 'stderr' to compare the script's stdout or stderr buffer.",
+			},
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			return nil, doCompare(s, true, args...)
+		})
+}
+
+func doCompare(s *State, env bool, args ...string) error {
+	quiet := false
+	if len(args) > 0 && args[0] == "-q" {
+		quiet = true
+		args = args[1:]
+	}
+	if len(args) != 2 {
+		return ErrUsage
+	}
+
+	name1, name2 := args[0], args[1]
+	var text1, text2 string
+	switch name1 {
+	case "stdout":
+		text1 = s.Stdout()
+	case "stderr":
+		text1 = s.Stderr()
+	default:
+		data, err := os.ReadFile(s.Path(name1))
+		if err != nil {
+			return err
+		}
+		text1 = string(data)
+	}
+
+	data, err := os.ReadFile(s.Path(name2))
+	if err != nil {
+		return err
+	}
+	text2 = string(data)
+
+	if env {
+		text1 = s.ExpandEnv(text1, false)
+		text2 = s.ExpandEnv(text2, false)
+	}
+
+	if text1 != text2 {
+		if !quiet {
+			diffText := diff.Diff(name1, []byte(text1), name2, []byte(text2))
+			s.Logf("%s\n", diffText)
+		}
+		return fmt.Errorf("%s and %s differ", name1, name2)
+	}
+	return nil
+}
+
+// Cp copies one or more files to a new location.
+func Cp() Cmd {
+	return Command(
+		CmdUsage{
+			Summary: "copy files to a target file or directory",
+			Args:    "src... dst",
+			Detail: []string{
+				"src can include 'stdout' or 'stderr' to copy from the script's stdout or stderr buffer.",
+			},
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			if len(args) < 2 {
+				return nil, ErrUsage
+			}
+
+			dst := s.Path(args[len(args)-1])
+			info, err := os.Stat(dst)
+			dstDir := err == nil && info.IsDir()
+			if len(args) > 2 && !dstDir {
+				return nil, &fs.PathError{Op: "cp", Path: dst, Err: errors.New("destination is not a directory")}
+			}
+
+			for _, arg := range args[:len(args)-1] {
+				var (
+					src  string
+					data []byte
+					mode fs.FileMode
+				)
+				switch arg {
+				case "stdout":
+					src = arg
+					data = []byte(s.Stdout())
+					mode = 0666
+				case "stderr":
+					src = arg
+					data = []byte(s.Stderr())
+					mode = 0666
+				default:
+					src = s.Path(arg)
+					info, err := os.Stat(src)
+					if err != nil {
+						return nil, err
+					}
+					mode = info.Mode() & 0777
+					data, err = os.ReadFile(src)
+					if err != nil {
+						return nil, err
+					}
+				}
+				targ := dst
+				if dstDir {
+					targ = filepath.Join(dst, filepath.Base(src))
+				}
+				err := os.WriteFile(targ, data, mode)
+				if err != nil {
+					return nil, err
+				}
+			}
+
+			return nil, nil
+		})
+}
+
+// Echo writes its arguments to stdout, followed by a newline.
+func Echo() Cmd {
+	return Command(
+		CmdUsage{
+			Summary: "display a line of text",
+			Args:    "string...",
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			var buf strings.Builder
+			for i, arg := range args {
+				if i > 0 {
+					buf.WriteString(" ")
+				}
+				buf.WriteString(arg)
+			}
+			buf.WriteString("\n")
+			out := buf.String()
+
+			// Stuff the result into a callback to satisfy the OutputCommandFunc
+			// interface, even though it isn't really asynchronous even if run in the
+			// background.
+			//
+			// Nobody should be running 'echo' as a background command, but it's not worth
+			// defining yet another interface, and also doesn't seem worth shoehorning
+			// into a SimpleCommand the way we did with Wait.
+			return func(*State) (stdout, stderr string, err error) {
+				return out, "", nil
+			}, nil
+		})
+}
+
+// Env sets or logs the values of environment variables.
+//
+// With no arguments, Env reports all variables in the environment.
+// "key=value" arguments set variables, and arguments without "="
+// cause the corresponding value to be printed to the stdout buffer.
+func Env() Cmd {
+	return Command(
+		CmdUsage{
+			Summary: "set or log the values of environment variables",
+			Args:    "[key[=value]...]",
+			Detail: []string{
+				"With no arguments, print the script environment to the log.",
+				"Otherwise, add the listed key=value pairs to the environment or print the listed keys.",
+			},
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			out := new(strings.Builder)
+			if len(args) == 0 {
+				for _, kv := range s.env {
+					fmt.Fprintf(out, "%s\n", kv)
+				}
+			} else {
+				for _, env := range args {
+					i := strings.Index(env, "=")
+					if i < 0 {
+						// Display value instead of setting it.
+						fmt.Fprintf(out, "%s=%s\n", env, s.envMap[env])
+						continue
+					}
+					if err := s.Setenv(env[:i], env[i+1:]); err != nil {
+						return nil, err
+					}
+				}
+			}
+			var wait WaitFunc
+			if out.Len() > 0 || len(args) == 0 {
+				wait = func(*State) (stdout, stderr string, err error) {
+					return out.String(), "", nil
+				}
+			}
+			return wait, nil
+		})
+}
+
+// Exec runs an arbitrary executable as a subprocess.
+//
+// When the Script's context is canceled, Exec sends the interrupt signal, then
+// waits for up to the given delay for the subprocess to flush output before
+// terminating it with os.Kill.
+func Exec(cancel func(*exec.Cmd) error, waitDelay time.Duration) Cmd {
+	return Command(
+		CmdUsage{
+			Summary: "run an executable program with arguments",
+			Args:    "program [args...]",
+			Detail: []string{
+				"Note that 'exec' does not terminate the script (unlike Unix shells).",
+			},
+			Async: true,
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			if len(args) < 1 {
+				return nil, ErrUsage
+			}
+
+			// Use the script's PATH to look up the command (if it does not contain a separator)
+			// instead of the test process's PATH (see lookPath).
+			// Don't use filepath.Clean, since that changes "./foo" to "foo".
+			name := filepath.FromSlash(args[0])
+			path := name
+			if !strings.Contains(name, string(filepath.Separator)) {
+				var err error
+				path, err = lookPath(s, name)
+				if err != nil {
+					return nil, err
+				}
+			}
+
+			return startCommand(s, name, path, args[1:], cancel, waitDelay)
+		})
+}
+
+func startCommand(s *State, name, path string, args []string, cancel func(*exec.Cmd) error, waitDelay time.Duration) (WaitFunc, error) {
+	var (
+		cmd                  *exec.Cmd
+		stdoutBuf, stderrBuf strings.Builder
+	)
+	for {
+		cmd = exec.CommandContext(s.Context(), path, args...)
+		if cancel == nil {
+			cmd.Cancel = nil
+		} else {
+			cmd.Cancel = func() error { return cancel(cmd) }
+		}
+		cmd.WaitDelay = waitDelay
+		cmd.Args[0] = name
+		cmd.Dir = s.Getwd()
+		cmd.Env = s.env
+		cmd.Stdout = &stdoutBuf
+		cmd.Stderr = &stderrBuf
+		err := cmd.Start()
+		if err == nil {
+			break
+		}
+		if isETXTBSY(err) {
+			// If the script (or its host process) just wrote the executable we're
+			// trying to run, a fork+exec in another thread may be holding open the FD
+			// that we used to write the executable (see https://go.dev/issue/22315).
+			// Since the descriptor should have CLOEXEC set, the problem should
+			// resolve as soon as the forked child reaches its exec call.
+			// Keep retrying until that happens.
+		} else {
+			return nil, err
+		}
+	}
+
+	wait := func(s *State) (stdout, stderr string, err error) {
+		err = cmd.Wait()
+		return stdoutBuf.String(), stderrBuf.String(), err
+	}
+	return wait, nil
+}
+
+// lookPath is (roughly) like exec.LookPath, but it uses the script's current
+// PATH to find the executable.
+func lookPath(s *State, command string) (string, error) {
+	var strEqual func(string, string) bool
+	if runtime.GOOS == "windows" || runtime.GOOS == "darwin" {
+		// Using GOOS as a proxy for case-insensitive file system.
+		// TODO(bcmills): Remove this assumption.
+		strEqual = strings.EqualFold
+	} else {
+		strEqual = func(a, b string) bool { return a == b }
+	}
+
+	var pathExt []string
+	var searchExt bool
+	var isExecutable func(os.FileInfo) bool
+	if runtime.GOOS == "windows" {
+		// Use the test process's PathExt instead of the script's.
+		// If PathExt is set in the command's environment, cmd.Start fails with
+		// "parameter is invalid". Not sure why.
+		// If the command already has an extension in PathExt (like "cmd.exe")
+		// don't search for other extensions (not "cmd.bat.exe").
+		pathExt = strings.Split(os.Getenv("PathExt"), string(filepath.ListSeparator))
+		searchExt = true
+		cmdExt := filepath.Ext(command)
+		for _, ext := range pathExt {
+			if strEqual(cmdExt, ext) {
+				searchExt = false
+				break
+			}
+		}
+		isExecutable = func(fi os.FileInfo) bool {
+			return fi.Mode().IsRegular()
+		}
+	} else {
+		isExecutable = func(fi os.FileInfo) bool {
+			return fi.Mode().IsRegular() && fi.Mode().Perm()&0111 != 0
+		}
+	}
+
+	pathEnv, _ := s.LookupEnv(pathEnvName())
+	for _, dir := range strings.Split(pathEnv, string(filepath.ListSeparator)) {
+		if dir == "" {
+			continue
+		}
+
+		// Determine whether dir needs a trailing path separator.
+		// Note: we avoid filepath.Join in this function because it cleans the
+		// result: we want to preserve the exact dir prefix from the environment.
+		sep := string(filepath.Separator)
+		if os.IsPathSeparator(dir[len(dir)-1]) {
+			sep = ""
+		}
+
+		if searchExt {
+			ents, err := os.ReadDir(dir)
+			if err != nil {
+				continue
+			}
+			for _, ent := range ents {
+				for _, ext := range pathExt {
+					if !ent.IsDir() && strEqual(ent.Name(), command+ext) {
+						return dir + sep + ent.Name(), nil
+					}
+				}
+			}
+		} else {
+			path := dir + sep + command
+			if fi, err := os.Stat(path); err == nil && isExecutable(fi) {
+				return path, nil
+			}
+		}
+	}
+	return "", &exec.Error{Name: command, Err: exec.ErrNotFound}
+}
+
+// pathEnvName returns the platform-specific variable used by os/exec.LookPath
+// to look up executable names (either "PATH" or "path").
+//
+// TODO(bcmills): Investigate whether we can instead use PATH uniformly and
+// rewrite it to $path when executing subprocesses.
+func pathEnvName() string {
+	switch runtime.GOOS {
+	case "plan9":
+		return "path"
+	default:
+		return "PATH"
+	}
+}
+
+// Exists checks that the named file(s) exist.
+func Exists() Cmd {
+	return Command(
+		CmdUsage{
+			Summary: "check that files exist",
+			Args:    "[-readonly] [-exec] file...",
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			var readonly, exec bool
+		loop:
+			for len(args) > 0 {
+				switch args[0] {
+				case "-readonly":
+					readonly = true
+					args = args[1:]
+				case "-exec":
+					exec = true
+					args = args[1:]
+				default:
+					break loop
+				}
+			}
+			if len(args) == 0 {
+				return nil, ErrUsage
+			}
+
+			for _, file := range args {
+				file = s.Path(file)
+				info, err := os.Stat(file)
+				if err != nil {
+					return nil, err
+				}
+				if readonly && info.Mode()&0222 != 0 {
+					return nil, fmt.Errorf("%s exists but is writable", file)
+				}
+				if exec && runtime.GOOS != "windows" && info.Mode()&0111 == 0 {
+					return nil, fmt.Errorf("%s exists but is not executable", file)
+				}
+			}
+
+			return nil, nil
+		})
+}
+
+// Grep checks that file content matches a regexp.
+// Like stdout/stderr and unlike Unix grep, it accepts Go regexp syntax.
+//
+// Grep does not modify the State's stdout or stderr buffers.
+// (Its output goes to the script log, not stdout.)
+func Grep() Cmd {
+	return Command(
+		CmdUsage{
+			Summary: "find lines in a file that match a pattern",
+			Args:    matchUsage + " file",
+			Detail: []string{
+				"The command succeeds if at least one match (or the exact count, if given) is found.",
+				"The -q flag suppresses printing of matches.",
+			},
+			RegexpArgs: firstNonFlag,
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			return nil, match(s, args, "", "grep")
+		})
+}
+
+const matchUsage = "[-count=N] [-q] 'pattern'"
+
+// match implements the Grep, Stdout, and Stderr commands.
+func match(s *State, args []string, text, name string) error {
+	n := 0
+	if len(args) >= 1 && strings.HasPrefix(args[0], "-count=") {
+		var err error
+		n, err = strconv.Atoi(args[0][len("-count="):])
+		if err != nil {
+			return fmt.Errorf("bad -count=: %v", err)
+		}
+		if n < 1 {
+			return fmt.Errorf("bad -count=: must be at least 1")
+		}
+		args = args[1:]
+	}
+	quiet := false
+	if len(args) >= 1 && args[0] == "-q" {
+		quiet = true
+		args = args[1:]
+	}
+
+	isGrep := name == "grep"
+
+	wantArgs := 1
+	if isGrep {
+		wantArgs = 2
+	}
+	if len(args) != wantArgs {
+		return ErrUsage
+	}
+
+	pattern := `(?m)` + args[0]
+	re, err := regexp.Compile(pattern)
+	if err != nil {
+		return err
+	}
+
+	if isGrep {
+		name = args[1] // for error messages
+		data, err := os.ReadFile(s.Path(args[1]))
+		if err != nil {
+			return err
+		}
+		text = string(data)
+	}
+
+	if n > 0 {
+		count := len(re.FindAllString(text, -1))
+		if count != n {
+			return fmt.Errorf("found %d matches for %#q in %s", count, pattern, name)
+		}
+		return nil
+	}
+
+	if !re.MatchString(text) {
+		return fmt.Errorf("no match for %#q in %s", pattern, name)
+	}
+
+	if !quiet {
+		// Print the lines containing the match.
+		loc := re.FindStringIndex(text)
+		for loc[0] > 0 && text[loc[0]-1] != '\n' {
+			loc[0]--
+		}
+		for loc[1] < len(text) && text[loc[1]] != '\n' {
+			loc[1]++
+		}
+		lines := strings.TrimSuffix(text[loc[0]:loc[1]], "\n")
+		s.Logf("matched: %s\n", lines)
+	}
+	return nil
+}
+
+// Help writes command documentation to the script log.
+func Help() Cmd {
+	return Command(
+		CmdUsage{
+			Summary: "log help text for commands and conditions",
+			Args:    "[-v] name...",
+			Detail: []string{
+				"To display help for a specific condition, enclose it in brackets: 'help [amd64]'.",
+				"To display complete documentation when listing all commands, pass the -v flag.",
+			},
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			if s.engine == nil {
+				return nil, errors.New("no engine configured")
+			}
+
+			verbose := false
+			if len(args) > 0 {
+				verbose = true
+				if args[0] == "-v" {
+					args = args[1:]
+				}
+			}
+
+			var cmds, conds []string
+			for _, arg := range args {
+				if strings.HasPrefix(arg, "[") && strings.HasSuffix(arg, "]") {
+					conds = append(conds, arg[1:len(arg)-1])
+				} else {
+					cmds = append(cmds, arg)
+				}
+			}
+
+			out := new(strings.Builder)
+
+			if len(conds) > 0 || (len(args) == 0 && len(s.engine.Conds) > 0) {
+				if conds == nil {
+					out.WriteString("conditions:\n\n")
+				}
+				s.engine.ListConds(out, s, conds...)
+			}
+
+			if len(cmds) > 0 || len(args) == 0 {
+				if len(args) == 0 {
+					out.WriteString("\ncommands:\n\n")
+				}
+				s.engine.ListCmds(out, verbose, cmds...)
+			}
+
+			wait := func(*State) (stdout, stderr string, err error) {
+				return out.String(), "", nil
+			}
+			return wait, nil
+		})
+}
+
+// Mkdir creates a directory and any needed parent directories.
+func Mkdir() Cmd {
+	return Command(
+		CmdUsage{
+			Summary: "create directories, if they do not already exist",
+			Args:    "path...",
+			Detail: []string{
+				"Unlike Unix mkdir, parent directories are always created if needed.",
+			},
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			if len(args) < 1 {
+				return nil, ErrUsage
+			}
+			for _, arg := range args {
+				if err := os.MkdirAll(s.Path(arg), 0777); err != nil {
+					return nil, err
+				}
+			}
+			return nil, nil
+		})
+}
+
+// Mv renames an existing file or directory to a new path.
+func Mv() Cmd {
+	return Command(
+		CmdUsage{
+			Summary: "rename a file or directory to a new path",
+			Args:    "old new",
+			Detail: []string{
+				"OS-specific restrictions may apply when old and new are in different directories.",
+			},
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			if len(args) != 2 {
+				return nil, ErrUsage
+			}
+			return nil, os.Rename(s.Path(args[0]), s.Path(args[1]))
+		})
+}
+
+// Program returns a new command that runs the named program, found from the
+// host process's PATH (not looked up in the script's PATH).
+func Program(name string, cancel func(*exec.Cmd) error, waitDelay time.Duration) Cmd {
+	var (
+		shortName    string
+		summary      string
+		lookPathOnce sync.Once
+		path         string
+		pathErr      error
+	)
+	if filepath.IsAbs(name) {
+		lookPathOnce.Do(func() { path = filepath.Clean(name) })
+		shortName = strings.TrimSuffix(filepath.Base(path), ".exe")
+		summary = "run the '" + shortName + "' program provided by the script host"
+	} else {
+		shortName = name
+		summary = "run the '" + shortName + "' program from the script host's PATH"
+	}
+
+	return Command(
+		CmdUsage{
+			Summary: summary,
+			Args:    "[args...]",
+			Async:   true,
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			lookPathOnce.Do(func() {
+				path, pathErr = exec.LookPath(name)
+			})
+			if pathErr != nil {
+				return nil, pathErr
+			}
+			return startCommand(s, shortName, path, args, cancel, waitDelay)
+		})
+}
+
+// Replace replaces all occurrences of a string in a file with another string.
+func Replace() Cmd {
+	return Command(
+		CmdUsage{
+			Summary: "replace strings in a file",
+			Args:    "[old new]... file",
+			Detail: []string{
+				"The 'old' and 'new' arguments are unquoted as if in quoted Go strings.",
+			},
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			if len(args)%2 != 1 {
+				return nil, ErrUsage
+			}
+
+			oldNew := make([]string, 0, len(args)-1)
+			for _, arg := range args[:len(args)-1] {
+				s, err := strconv.Unquote(`"` + arg + `"`)
+				if err != nil {
+					return nil, err
+				}
+				oldNew = append(oldNew, s)
+			}
+
+			r := strings.NewReplacer(oldNew...)
+			file := s.Path(args[len(args)-1])
+
+			data, err := os.ReadFile(file)
+			if err != nil {
+				return nil, err
+			}
+			replaced := r.Replace(string(data))
+
+			return nil, os.WriteFile(file, []byte(replaced), 0666)
+		})
+}
+
+// Rm removes a file or directory.
+//
+// If a directory, Rm also recursively removes that directory's
+// contents.
+func Rm() Cmd {
+	return Command(
+		CmdUsage{
+			Summary: "remove a file or directory",
+			Args:    "path...",
+			Detail: []string{
+				"If the path is a directory, its contents are removed recursively.",
+			},
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			if len(args) < 1 {
+				return nil, ErrUsage
+			}
+			for _, arg := range args {
+				if err := removeAll(s.Path(arg)); err != nil {
+					return nil, err
+				}
+			}
+			return nil, nil
+		})
+}
+
+// removeAll removes dir and all files and directories it contains.
+//
+// Unlike os.RemoveAll, removeAll attempts to make the directories writable if
+// needed in order to remove their contents.
+func removeAll(dir string) error {
+	// module cache has 0444 directories;
+	// make them writable in order to remove content.
+	filepath.WalkDir(dir, func(path string, info fs.DirEntry, err error) error {
+		// chmod not only directories, but also things that we couldn't even stat
+		// due to permission errors: they may also be unreadable directories.
+		if err != nil || info.IsDir() {
+			os.Chmod(path, 0777)
+		}
+		return nil
+	})
+	return robustio.RemoveAll(dir)
+}
+
+// Sleep sleeps for the given Go duration or until the script's context is
+// cancelled, whichever happens first.
+func Sleep() Cmd {
+	return Command(
+		CmdUsage{
+			Summary: "sleep for a specified duration",
+			Args:    "duration",
+			Detail: []string{
+				"The duration must be given as a Go time.Duration string.",
+			},
+			Async: true,
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			if len(args) != 1 {
+				return nil, ErrUsage
+			}
+
+			d, err := time.ParseDuration(args[0])
+			if err != nil {
+				return nil, err
+			}
+
+			timer := time.NewTimer(d)
+			wait := func(s *State) (stdout, stderr string, err error) {
+				ctx := s.Context()
+				select {
+				case <-ctx.Done():
+					timer.Stop()
+					return "", "", ctx.Err()
+				case <-timer.C:
+					return "", "", nil
+				}
+			}
+			return wait, nil
+		})
+}
+
+// Stderr searches for a regular expression in the stderr buffer.
+func Stderr() Cmd {
+	return Command(
+		CmdUsage{
+			Summary: "find lines in the stderr buffer that match a pattern",
+			Args:    matchUsage + " file",
+			Detail: []string{
+				"The command succeeds if at least one match (or the exact count, if given) is found.",
+				"The -q flag suppresses printing of matches.",
+			},
+			RegexpArgs: firstNonFlag,
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			return nil, match(s, args, s.Stderr(), "stderr")
+		})
+}
+
+// Stdout searches for a regular expression in the stdout buffer.
+func Stdout() Cmd {
+	return Command(
+		CmdUsage{
+			Summary: "find lines in the stdout buffer that match a pattern",
+			Args:    matchUsage + " file",
+			Detail: []string{
+				"The command succeeds if at least one match (or the exact count, if given) is found.",
+				"The -q flag suppresses printing of matches.",
+			},
+			RegexpArgs: firstNonFlag,
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			return nil, match(s, args, s.Stdout(), "stdout")
+		})
+}
+
+// Stop returns a sentinel error that causes script execution to halt
+// and s.Execute to return with a nil error.
+func Stop() Cmd {
+	return Command(
+		CmdUsage{
+			Summary: "stop execution of the script",
+			Args:    "[msg]",
+			Detail: []string{
+				"The message is written to the script log, but no error is reported from the script engine.",
+			},
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			if len(args) > 1 {
+				return nil, ErrUsage
+			}
+			// TODO(bcmills): The argument passed to stop seems redundant with comments.
+			// Either use it systematically or remove it.
+			if len(args) == 1 {
+				return nil, stopError{msg: args[0]}
+			}
+			return nil, stopError{}
+		})
+}
+
+// stopError is the sentinel error type returned by the Stop command.
+type stopError struct {
+	msg string
+}
+
+func (s stopError) Error() string {
+	if s.msg == "" {
+		return "stop"
+	}
+	return "stop: " + s.msg
+}
+
+// Symlink creates a symbolic link.
+func Symlink() Cmd {
+	return Command(
+		CmdUsage{
+			Summary: "create a symlink",
+			Args:    "path -> target",
+			Detail: []string{
+				"Creates path as a symlink to target.",
+				"The '->' token (like in 'ls -l' output on Unix) is required.",
+			},
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			if len(args) != 3 || args[1] != "->" {
+				return nil, ErrUsage
+			}
+
+			// Note that the link target args[2] is not interpreted with s.Path:
+			// it will be interpreted relative to the directory file is in.
+			return nil, os.Symlink(filepath.FromSlash(args[2]), s.Path(args[0]))
+		})
+}
+
+// Wait waits for the completion of background commands.
+//
+// When Wait returns, the stdout and stderr buffers contain the concatenation of
+// the background commands' respective outputs in the order in which those
+// commands were started.
+func Wait() Cmd {
+	return Command(
+		CmdUsage{
+			Summary: "wait for completion of background commands",
+			Args:    "",
+			Detail: []string{
+				"Waits for all background commands to complete.",
+				"The output (and any error) from each command is printed to the log in the order in which the commands were started.",
+				"After the call to 'wait', the script's stdout and stderr buffers contain the concatenation of the background commands' outputs.",
+			},
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			if len(args) > 0 {
+				return nil, ErrUsage
+			}
+
+			var stdouts, stderrs []string
+			var errs []*CommandError
+			for _, bg := range s.background {
+				stdout, stderr, err := bg.wait(s)
+
+				beforeArgs := ""
+				if len(bg.args) > 0 {
+					beforeArgs = " "
+				}
+				s.Logf("[background] %s%s%s\n", bg.name, beforeArgs, quoteArgs(bg.args))
+
+				if stdout != "" {
+					s.Logf("[stdout]\n%s", stdout)
+					stdouts = append(stdouts, stdout)
+				}
+				if stderr != "" {
+					s.Logf("[stderr]\n%s", stderr)
+					stderrs = append(stderrs, stderr)
+				}
+				if err != nil {
+					s.Logf("[%v]\n", err)
+				}
+				if cmdErr := checkStatus(bg.command, err); cmdErr != nil {
+					errs = append(errs, cmdErr.(*CommandError))
+				}
+			}
+
+			s.stdout = strings.Join(stdouts, "")
+			s.stderr = strings.Join(stderrs, "")
+			s.background = nil
+			if len(errs) > 0 {
+				return nil, waitError{errs: errs}
+			}
+			return nil, nil
+		})
+}
+
+// A waitError wraps one or more errors returned by background commands.
+type waitError struct {
+	errs []*CommandError
+}
+
+func (w waitError) Error() string {
+	b := new(strings.Builder)
+	for i, err := range w.errs {
+		if i != 0 {
+			b.WriteString("\n")
+		}
+		b.WriteString(err.Error())
+	}
+	return b.String()
+}
+
+func (w waitError) Unwrap() error {
+	if len(w.errs) == 1 {
+		return w.errs[0]
+	}
+	return nil
+}
diff --git a/src/cmd/go/internal/script/cmds_other.go b/src/cmd/go/internal/script/cmds_other.go
new file mode 100644
index 0000000..847b225
--- /dev/null
+++ b/src/cmd/go/internal/script/cmds_other.go
@@ -0,0 +1,11 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !(unix || windows)
+
+package script
+
+func isETXTBSY(err error) bool {
+	return false
+}
diff --git a/src/cmd/go/internal/script/cmds_posix.go b/src/cmd/go/internal/script/cmds_posix.go
new file mode 100644
index 0000000..2525f6e
--- /dev/null
+++ b/src/cmd/go/internal/script/cmds_posix.go
@@ -0,0 +1,16 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix || windows
+
+package script
+
+import (
+	"errors"
+	"syscall"
+)
+
+func isETXTBSY(err error) bool {
+	return errors.Is(err, syscall.ETXTBSY)
+}
diff --git a/src/cmd/go/internal/script/conds.go b/src/cmd/go/internal/script/conds.go
new file mode 100644
index 0000000..d70f274
--- /dev/null
+++ b/src/cmd/go/internal/script/conds.go
@@ -0,0 +1,205 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package script
+
+import (
+	"cmd/go/internal/imports"
+	"fmt"
+	"os"
+	"runtime"
+	"sync"
+)
+
+// DefaultConds returns a set of broadly useful script conditions.
+//
+// Run the 'help' command within a script engine to view a list of the available
+// conditions.
+func DefaultConds() map[string]Cond {
+	conds := make(map[string]Cond)
+
+	conds["GOOS"] = PrefixCondition(
+		"runtime.GOOS == <suffix>",
+		func(_ *State, suffix string) (bool, error) {
+			if suffix == runtime.GOOS {
+				return true, nil
+			}
+			if _, ok := imports.KnownOS[suffix]; !ok {
+				return false, fmt.Errorf("unrecognized GOOS %q", suffix)
+			}
+			return false, nil
+		})
+
+	conds["GOARCH"] = PrefixCondition(
+		"runtime.GOARCH == <suffix>",
+		func(_ *State, suffix string) (bool, error) {
+			if suffix == runtime.GOARCH {
+				return true, nil
+			}
+			if _, ok := imports.KnownArch[suffix]; !ok {
+				return false, fmt.Errorf("unrecognized GOOS %q", suffix)
+			}
+			return false, nil
+		})
+
+	conds["compiler"] = PrefixCondition(
+		"runtime.Compiler == <suffix>",
+		func(_ *State, suffix string) (bool, error) {
+			if suffix == runtime.Compiler {
+				return true, nil
+			}
+			switch suffix {
+			case "gc", "gccgo":
+				return false, nil
+			default:
+				return false, fmt.Errorf("unrecognized compiler %q", suffix)
+			}
+		})
+
+	conds["root"] = BoolCondition("os.Geteuid() == 0", os.Geteuid() == 0)
+
+	return conds
+}
+
+// Condition returns a Cond with the given summary and evaluation function.
+func Condition(summary string, eval func(*State) (bool, error)) Cond {
+	return &funcCond{eval: eval, usage: CondUsage{Summary: summary}}
+}
+
+type funcCond struct {
+	eval  func(*State) (bool, error)
+	usage CondUsage
+}
+
+func (c *funcCond) Usage() *CondUsage { return &c.usage }
+
+func (c *funcCond) Eval(s *State, suffix string) (bool, error) {
+	if suffix != "" {
+		return false, ErrUsage
+	}
+	return c.eval(s)
+}
+
+// PrefixCondition returns a Cond with the given summary and evaluation function.
+func PrefixCondition(summary string, eval func(*State, string) (bool, error)) Cond {
+	return &prefixCond{eval: eval, usage: CondUsage{Summary: summary, Prefix: true}}
+}
+
+type prefixCond struct {
+	eval  func(*State, string) (bool, error)
+	usage CondUsage
+}
+
+func (c *prefixCond) Usage() *CondUsage { return &c.usage }
+
+func (c *prefixCond) Eval(s *State, suffix string) (bool, error) {
+	return c.eval(s, suffix)
+}
+
+// BoolCondition returns a Cond with the given truth value and summary.
+// The Cond rejects the use of condition suffixes.
+func BoolCondition(summary string, v bool) Cond {
+	return &boolCond{v: v, usage: CondUsage{Summary: summary}}
+}
+
+type boolCond struct {
+	v     bool
+	usage CondUsage
+}
+
+func (b *boolCond) Usage() *CondUsage { return &b.usage }
+
+func (b *boolCond) Eval(s *State, suffix string) (bool, error) {
+	if suffix != "" {
+		return false, ErrUsage
+	}
+	return b.v, nil
+}
+
+// OnceCondition returns a Cond that calls eval the first time the condition is
+// evaluated. Future calls reuse the same result.
+//
+// The eval function is not passed a *State because the condition is cached
+// across all execution states and must not vary by state.
+func OnceCondition(summary string, eval func() (bool, error)) Cond {
+	return &onceCond{eval: eval, usage: CondUsage{Summary: summary}}
+}
+
+type onceCond struct {
+	once  sync.Once
+	v     bool
+	err   error
+	eval  func() (bool, error)
+	usage CondUsage
+}
+
+func (l *onceCond) Usage() *CondUsage { return &l.usage }
+
+func (l *onceCond) Eval(s *State, suffix string) (bool, error) {
+	if suffix != "" {
+		return false, ErrUsage
+	}
+	l.once.Do(func() { l.v, l.err = l.eval() })
+	return l.v, l.err
+}
+
+// CachedCondition is like Condition but only calls eval the first time the
+// condition is evaluated for a given suffix.
+// Future calls with the same suffix reuse the earlier result.
+//
+// The eval function is not passed a *State because the condition is cached
+// across all execution states and must not vary by state.
+func CachedCondition(summary string, eval func(string) (bool, error)) Cond {
+	return &cachedCond{eval: eval, usage: CondUsage{Summary: summary, Prefix: true}}
+}
+
+type cachedCond struct {
+	m     sync.Map
+	eval  func(string) (bool, error)
+	usage CondUsage
+}
+
+func (c *cachedCond) Usage() *CondUsage { return &c.usage }
+
+func (c *cachedCond) Eval(_ *State, suffix string) (bool, error) {
+	for {
+		var ready chan struct{}
+
+		v, loaded := c.m.Load(suffix)
+		if !loaded {
+			ready = make(chan struct{})
+			v, loaded = c.m.LoadOrStore(suffix, (<-chan struct{})(ready))
+
+			if !loaded {
+				inPanic := true
+				defer func() {
+					if inPanic {
+						c.m.Delete(suffix)
+					}
+					close(ready)
+				}()
+
+				b, err := c.eval(suffix)
+				inPanic = false
+
+				if err == nil {
+					c.m.Store(suffix, b)
+					return b, nil
+				} else {
+					c.m.Store(suffix, err)
+					return false, err
+				}
+			}
+		}
+
+		switch v := v.(type) {
+		case bool:
+			return v, nil
+		case error:
+			return false, v
+		case <-chan struct{}:
+			<-v
+		}
+	}
+}
diff --git a/src/cmd/go/internal/script/engine.go b/src/cmd/go/internal/script/engine.go
new file mode 100644
index 0000000..43054a2
--- /dev/null
+++ b/src/cmd/go/internal/script/engine.go
@@ -0,0 +1,788 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package script implements a small, customizable, platform-agnostic scripting
+// language.
+//
+// Scripts are run by an [Engine] configured with a set of available commands
+// and conditions that guard those commands. Each script has an associated
+// working directory and environment, along with a buffer containing the stdout
+// and stderr output of a prior command, tracked in a [State] that commands can
+// inspect and modify.
+//
+// The default commands configured by [NewEngine] resemble a simplified Unix
+// shell.
+//
+// # Script Language
+//
+// Each line of a script is parsed into a sequence of space-separated command
+// words, with environment variable expansion within each word and # marking an
+// end-of-line comment. Additional variables named ':' and '/' are expanded
+// within script arguments (expanding to the value of os.PathListSeparator and
+// os.PathSeparator respectively) but are not inherited in subprocess
+// environments.
+//
+// Adding single quotes around text keeps spaces in that text from being treated
+// as word separators and also disables environment variable expansion.
+// Inside a single-quoted block of text, a repeated single quote indicates
+// a literal single quote, as in:
+//
+//	'Don''t communicate by sharing memory.'
+//
+// A line beginning with # is a comment and conventionally explains what is
+// being done or tested at the start of a new section of the script.
+//
+// Commands are executed one at a time, and errors are checked for each command;
+// if any command fails unexpectedly, no subsequent commands in the script are
+// executed. The command prefix ! indicates that the command on the rest of the
+// line (typically go or a matching predicate) must fail instead of succeeding.
+// The command prefix ? indicates that the command may or may not succeed, but
+// the script should continue regardless.
+//
+// The command prefix [cond] indicates that the command on the rest of the line
+// should only run when the condition is satisfied.
+//
+// A condition can be negated: [!root] means to run the rest of the line only if
+// the user is not root. Multiple conditions may be given for a single command,
+// for example, '[linux] [amd64] skip'. The command will run if all conditions
+// are satisfied.
+package script
+
+import (
+	"bufio"
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"sort"
+	"strings"
+	"time"
+)
+
+// An Engine stores the configuration for executing a set of scripts.
+//
+// The same Engine may execute multiple scripts concurrently.
+type Engine struct {
+	Cmds  map[string]Cmd
+	Conds map[string]Cond
+
+	// If Quiet is true, Execute deletes log prints from the previous
+	// section when starting a new section.
+	Quiet bool
+}
+
+// NewEngine returns an Engine configured with a basic set of commands and conditions.
+func NewEngine() *Engine {
+	return &Engine{
+		Cmds:  DefaultCmds(),
+		Conds: DefaultConds(),
+	}
+}
+
+// A Cmd is a command that is available to a script.
+type Cmd interface {
+	// Run begins running the command.
+	//
+	// If the command produces output or can be run in the background, run returns
+	// a WaitFunc that will be called to obtain the result of the command and
+	// update the engine's stdout and stderr buffers.
+	//
+	// Run itself and the returned WaitFunc may inspect and/or modify the State,
+	// but the State's methods must not be called concurrently after Run has
+	// returned.
+	//
+	// Run may retain and access the args slice until the WaitFunc has returned.
+	Run(s *State, args ...string) (WaitFunc, error)
+
+	// Usage returns the usage for the command, which the caller must not modify.
+	Usage() *CmdUsage
+}
+
+// A WaitFunc is a function called to retrieve the results of a Cmd.
+type WaitFunc func(*State) (stdout, stderr string, err error)
+
+// A CmdUsage describes the usage of a Cmd, independent of its name
+// (which can change based on its registration).
+type CmdUsage struct {
+	Summary string   // in the style of the Name section of a Unix 'man' page, omitting the name
+	Args    string   // a brief synopsis of the command's arguments (only)
+	Detail  []string // zero or more sentences in the style of the Description section of a Unix 'man' page
+
+	// If Async is true, the Cmd is meaningful to run in the background, and its
+	// Run method must return either a non-nil WaitFunc or a non-nil error.
+	Async bool
+
+	// RegexpArgs reports which arguments, if any, should be treated as regular
+	// expressions. It takes as input the raw, unexpanded arguments and returns
+	// the list of argument indices that will be interpreted as regular
+	// expressions.
+	//
+	// If RegexpArgs is nil, all arguments are assumed not to be regular
+	// expressions.
+	RegexpArgs func(rawArgs ...string) []int
+}
+
+// A Cond is a condition deciding whether a command should be run.
+type Cond interface {
+	// Eval reports whether the condition applies to the given State.
+	//
+	// If the condition's usage reports that it is a prefix,
+	// the condition must be used with a suffix.
+	// Otherwise, the passed-in suffix argument is always the empty string.
+	Eval(s *State, suffix string) (bool, error)
+
+	// Usage returns the usage for the condition, which the caller must not modify.
+	Usage() *CondUsage
+}
+
+// A CondUsage describes the usage of a Cond, independent of its name
+// (which can change based on its registration).
+type CondUsage struct {
+	Summary string // a single-line summary of when the condition is true
+
+	// If Prefix is true, the condition is a prefix and requires a
+	// colon-separated suffix (like "[GOOS:linux]" for the "GOOS" condition).
+	// The suffix may be the empty string (like "[prefix:]").
+	Prefix bool
+}
+
+// Execute reads and executes script, writing the output to log.
+//
+// Execute stops and returns an error at the first command that does not succeed.
+// The returned error's text begins with "file:line: ".
+//
+// If the script runs to completion or ends by a 'stop' command,
+// Execute returns nil.
+//
+// Execute does not stop background commands started by the script
+// before returning. To stop those, use [State.CloseAndWait] or the
+// [Wait] command.
+func (e *Engine) Execute(s *State, file string, script *bufio.Reader, log io.Writer) (err error) {
+	defer func(prev *Engine) { s.engine = prev }(s.engine)
+	s.engine = e
+
+	var sectionStart time.Time
+	// endSection flushes the logs for the current section from s.log to log.
+	// ok indicates whether all commands in the section succeeded.
+	endSection := func(ok bool) error {
+		var err error
+		if sectionStart.IsZero() {
+			// We didn't write a section header or record a timestamp, so just dump the
+			// whole log without those.
+			if s.log.Len() > 0 {
+				err = s.flushLog(log)
+			}
+		} else if s.log.Len() == 0 {
+			// Adding elapsed time for doing nothing is meaningless, so don't.
+			_, err = io.WriteString(log, "\n")
+		} else {
+			// Insert elapsed time for section at the end of the section's comment.
+			_, err = fmt.Fprintf(log, " (%.3fs)\n", time.Since(sectionStart).Seconds())
+
+			if err == nil && (!ok || !e.Quiet) {
+				err = s.flushLog(log)
+			} else {
+				s.log.Reset()
+			}
+		}
+
+		sectionStart = time.Time{}
+		return err
+	}
+
+	var lineno int
+	lineErr := func(err error) error {
+		if errors.As(err, new(*CommandError)) {
+			return err
+		}
+		return fmt.Errorf("%s:%d: %w", file, lineno, err)
+	}
+
+	// In case of failure or panic, flush any pending logs for the section.
+	defer func() {
+		if sErr := endSection(false); sErr != nil && err == nil {
+			err = lineErr(sErr)
+		}
+	}()
+
+	for {
+		if err := s.ctx.Err(); err != nil {
+			// This error wasn't produced by any particular command,
+			// so don't wrap it in a CommandError.
+			return lineErr(err)
+		}
+
+		line, err := script.ReadString('\n')
+		if err == io.EOF {
+			if line == "" {
+				break // Reached the end of the script.
+			}
+			// If the script doesn't end in a newline, interpret the final line.
+		} else if err != nil {
+			return lineErr(err)
+		}
+		line = strings.TrimSuffix(line, "\n")
+		lineno++
+
+		// The comment character "#" at the start of the line delimits a section of
+		// the script.
+		if strings.HasPrefix(line, "#") {
+			// If there was a previous section, the fact that we are starting a new
+			// one implies the success of the previous one.
+			//
+			// At the start of the script, the state may also contain accumulated logs
+			// from commands executed on the State outside of the engine in order to
+			// set it up; flush those logs too.
+			if err := endSection(true); err != nil {
+				return lineErr(err)
+			}
+
+			// Log the section start without a newline so that we can add
+			// a timestamp for the section when it ends.
+			_, err = fmt.Fprintf(log, "%s", line)
+			sectionStart = time.Now()
+			if err != nil {
+				return lineErr(err)
+			}
+			continue
+		}
+
+		cmd, err := parse(file, lineno, line)
+		if cmd == nil && err == nil {
+			continue // Ignore blank lines.
+		}
+		s.Logf("> %s\n", line)
+		if err != nil {
+			return lineErr(err)
+		}
+
+		// Evaluate condition guards.
+		ok, err := e.conditionsActive(s, cmd.conds)
+		if err != nil {
+			return lineErr(err)
+		}
+		if !ok {
+			s.Logf("[condition not met]\n")
+			continue
+		}
+
+		impl := e.Cmds[cmd.name]
+
+		// Expand variables in arguments.
+		var regexpArgs []int
+		if impl != nil {
+			usage := impl.Usage()
+			if usage.RegexpArgs != nil {
+				// First join rawArgs without expansion to pass to RegexpArgs.
+				rawArgs := make([]string, 0, len(cmd.rawArgs))
+				for _, frags := range cmd.rawArgs {
+					var b strings.Builder
+					for _, frag := range frags {
+						b.WriteString(frag.s)
+					}
+					rawArgs = append(rawArgs, b.String())
+				}
+				regexpArgs = usage.RegexpArgs(rawArgs...)
+			}
+		}
+		cmd.args = expandArgs(s, cmd.rawArgs, regexpArgs)
+
+		// Run the command.
+		err = e.runCommand(s, cmd, impl)
+		if err != nil {
+			if stop := (stopError{}); errors.As(err, &stop) {
+				// Since the 'stop' command halts execution of the entire script,
+				// log its message separately from the section in which it appears.
+				err = endSection(true)
+				s.Logf("%v\n", stop)
+				if err == nil {
+					return nil
+				}
+			}
+			return lineErr(err)
+		}
+	}
+
+	if err := endSection(true); err != nil {
+		return lineErr(err)
+	}
+	return nil
+}
+
+// A command is a complete command parsed from a script.
+type command struct {
+	file       string
+	line       int
+	want       expectedStatus
+	conds      []condition // all must be satisfied
+	name       string      // the name of the command; must be non-empty
+	rawArgs    [][]argFragment
+	args       []string // shell-expanded arguments following name
+	background bool     // command should run in background (ends with a trailing &)
+}
+
+// A expectedStatus describes the expected outcome of a command.
+// Script execution halts when a command does not match its expected status.
+type expectedStatus string
+
+const (
+	success          expectedStatus = ""
+	failure          expectedStatus = "!"
+	successOrFailure expectedStatus = "?"
+)
+
+type argFragment struct {
+	s      string
+	quoted bool // if true, disable variable expansion for this fragment
+}
+
+type condition struct {
+	want bool
+	tag  string
+}
+
+const argSepChars = " \t\r\n#"
+
+// parse parses a single line as a list of space-separated arguments.
+// subject to environment variable expansion (but not resplitting).
+// Single quotes around text disable splitting and expansion.
+// To embed a single quote, double it:
+//
+//	'Don''t communicate by sharing memory.'
+func parse(filename string, lineno int, line string) (cmd *command, err error) {
+	cmd = &command{file: filename, line: lineno}
+	var (
+		rawArg []argFragment // text fragments of current arg so far (need to add line[start:i])
+		start  = -1          // if >= 0, position where current arg text chunk starts
+		quoted = false       // currently processing quoted text
+	)
+
+	flushArg := func() error {
+		if len(rawArg) == 0 {
+			return nil // Nothing to flush.
+		}
+		defer func() { rawArg = nil }()
+
+		if cmd.name == "" && len(rawArg) == 1 && !rawArg[0].quoted {
+			arg := rawArg[0].s
+
+			// Command prefix ! means negate the expectations about this command:
+			// go command should fail, match should not be found, etc.
+			// Prefix ? means allow either success or failure.
+			switch want := expectedStatus(arg); want {
+			case failure, successOrFailure:
+				if cmd.want != "" {
+					return errors.New("duplicated '!' or '?' token")
+				}
+				cmd.want = want
+				return nil
+			}
+
+			// Command prefix [cond] means only run this command if cond is satisfied.
+			if strings.HasPrefix(arg, "[") && strings.HasSuffix(arg, "]") {
+				want := true
+				arg = strings.TrimSpace(arg[1 : len(arg)-1])
+				if strings.HasPrefix(arg, "!") {
+					want = false
+					arg = strings.TrimSpace(arg[1:])
+				}
+				if arg == "" {
+					return errors.New("empty condition")
+				}
+				cmd.conds = append(cmd.conds, condition{want: want, tag: arg})
+				return nil
+			}
+
+			if arg == "" {
+				return errors.New("empty command")
+			}
+			cmd.name = arg
+			return nil
+		}
+
+		cmd.rawArgs = append(cmd.rawArgs, rawArg)
+		return nil
+	}
+
+	for i := 0; ; i++ {
+		if !quoted && (i >= len(line) || strings.ContainsRune(argSepChars, rune(line[i]))) {
+			// Found arg-separating space.
+			if start >= 0 {
+				rawArg = append(rawArg, argFragment{s: line[start:i], quoted: false})
+				start = -1
+			}
+			if err := flushArg(); err != nil {
+				return nil, err
+			}
+			if i >= len(line) || line[i] == '#' {
+				break
+			}
+			continue
+		}
+		if i >= len(line) {
+			return nil, errors.New("unterminated quoted argument")
+		}
+		if line[i] == '\'' {
+			if !quoted {
+				// starting a quoted chunk
+				if start >= 0 {
+					rawArg = append(rawArg, argFragment{s: line[start:i], quoted: false})
+				}
+				start = i + 1
+				quoted = true
+				continue
+			}
+			// 'foo''bar' means foo'bar, like in rc shell and Pascal.
+			if i+1 < len(line) && line[i+1] == '\'' {
+				rawArg = append(rawArg, argFragment{s: line[start:i], quoted: true})
+				start = i + 1
+				i++ // skip over second ' before next iteration
+				continue
+			}
+			// ending a quoted chunk
+			rawArg = append(rawArg, argFragment{s: line[start:i], quoted: true})
+			start = i + 1
+			quoted = false
+			continue
+		}
+		// found character worth saving; make sure we're saving
+		if start < 0 {
+			start = i
+		}
+	}
+
+	if cmd.name == "" {
+		if cmd.want != "" || len(cmd.conds) > 0 || len(cmd.rawArgs) > 0 || cmd.background {
+			// The line contains a command prefix or suffix, but no actual command.
+			return nil, errors.New("missing command")
+		}
+
+		// The line is blank, or contains only a comment.
+		return nil, nil
+	}
+
+	if n := len(cmd.rawArgs); n > 0 {
+		last := cmd.rawArgs[n-1]
+		if len(last) == 1 && !last[0].quoted && last[0].s == "&" {
+			cmd.background = true
+			cmd.rawArgs = cmd.rawArgs[:n-1]
+		}
+	}
+	return cmd, nil
+}
+
+// expandArgs expands the shell variables in rawArgs and joins them to form the
+// final arguments to pass to a command.
+func expandArgs(s *State, rawArgs [][]argFragment, regexpArgs []int) []string {
+	args := make([]string, 0, len(rawArgs))
+	for i, frags := range rawArgs {
+		isRegexp := false
+		for _, j := range regexpArgs {
+			if i == j {
+				isRegexp = true
+				break
+			}
+		}
+
+		var b strings.Builder
+		for _, frag := range frags {
+			if frag.quoted {
+				b.WriteString(frag.s)
+			} else {
+				b.WriteString(s.ExpandEnv(frag.s, isRegexp))
+			}
+		}
+		args = append(args, b.String())
+	}
+	return args
+}
+
+// quoteArgs returns a string that parse would parse as args when passed to a command.
+//
+// TODO(bcmills): This function should have a fuzz test.
+func quoteArgs(args []string) string {
+	var b strings.Builder
+	for i, arg := range args {
+		if i > 0 {
+			b.WriteString(" ")
+		}
+		if strings.ContainsAny(arg, "'"+argSepChars) {
+			// Quote the argument to a form that would be parsed as a single argument.
+			b.WriteString("'")
+			b.WriteString(strings.ReplaceAll(arg, "'", "''"))
+			b.WriteString("'")
+		} else {
+			b.WriteString(arg)
+		}
+	}
+	return b.String()
+}
+
+func (e *Engine) conditionsActive(s *State, conds []condition) (bool, error) {
+	for _, cond := range conds {
+		var impl Cond
+		prefix, suffix, ok := strings.Cut(cond.tag, ":")
+		if ok {
+			impl = e.Conds[prefix]
+			if impl == nil {
+				return false, fmt.Errorf("unknown condition prefix %q", prefix)
+			}
+			if !impl.Usage().Prefix {
+				return false, fmt.Errorf("condition %q cannot be used with a suffix", prefix)
+			}
+		} else {
+			impl = e.Conds[cond.tag]
+			if impl == nil {
+				return false, fmt.Errorf("unknown condition %q", cond.tag)
+			}
+			if impl.Usage().Prefix {
+				return false, fmt.Errorf("condition %q requires a suffix", cond.tag)
+			}
+		}
+		active, err := impl.Eval(s, suffix)
+
+		if err != nil {
+			return false, fmt.Errorf("evaluating condition %q: %w", cond.tag, err)
+		}
+		if active != cond.want {
+			return false, nil
+		}
+	}
+
+	return true, nil
+}
+
+func (e *Engine) runCommand(s *State, cmd *command, impl Cmd) error {
+	if impl == nil {
+		return cmdError(cmd, errors.New("unknown command"))
+	}
+
+	async := impl.Usage().Async
+	if cmd.background && !async {
+		return cmdError(cmd, errors.New("command cannot be run in background"))
+	}
+
+	wait, runErr := impl.Run(s, cmd.args...)
+	if wait == nil {
+		if async && runErr == nil {
+			return cmdError(cmd, errors.New("internal error: async command returned a nil WaitFunc"))
+		}
+		return checkStatus(cmd, runErr)
+	}
+	if runErr != nil {
+		return cmdError(cmd, errors.New("internal error: command returned both an error and a WaitFunc"))
+	}
+
+	if cmd.background {
+		s.background = append(s.background, backgroundCmd{
+			command: cmd,
+			wait:    wait,
+		})
+		// Clear stdout and stderr, since they no longer correspond to the last
+		// command executed.
+		s.stdout = ""
+		s.stderr = ""
+		return nil
+	}
+
+	if wait != nil {
+		stdout, stderr, waitErr := wait(s)
+		s.stdout = stdout
+		s.stderr = stderr
+		if stdout != "" {
+			s.Logf("[stdout]\n%s", stdout)
+		}
+		if stderr != "" {
+			s.Logf("[stderr]\n%s", stderr)
+		}
+		if cmdErr := checkStatus(cmd, waitErr); cmdErr != nil {
+			return cmdErr
+		}
+		if waitErr != nil {
+			// waitErr was expected (by cmd.want), so log it instead of returning it.
+			s.Logf("[%v]\n", waitErr)
+		}
+	}
+	return nil
+}
+
+func checkStatus(cmd *command, err error) error {
+	if err == nil {
+		if cmd.want == failure {
+			return cmdError(cmd, ErrUnexpectedSuccess)
+		}
+		return nil
+	}
+
+	if s := (stopError{}); errors.As(err, &s) {
+		// This error originated in the Stop command.
+		// Propagate it as-is.
+		return cmdError(cmd, err)
+	}
+
+	if w := (waitError{}); errors.As(err, &w) {
+		// This error was surfaced from a background process by a call to Wait.
+		// Add a call frame for Wait itself, but ignore its "want" field.
+		// (Wait itself cannot fail to wait on commands or else it would leak
+		// processes and/or goroutines — so a negative assertion for it would be at
+		// best ambiguous.)
+		return cmdError(cmd, err)
+	}
+
+	if cmd.want == success {
+		return cmdError(cmd, err)
+	}
+
+	if cmd.want == failure && (errors.Is(err, context.DeadlineExceeded) || errors.Is(err, context.Canceled)) {
+		// The command was terminated because the script is no longer interested in
+		// its output, so we don't know what it would have done had it run to
+		// completion — for all we know, it could have exited without error if it
+		// ran just a smidge faster.
+		return cmdError(cmd, err)
+	}
+
+	return nil
+}
+
+// ListCmds prints to w a list of the named commands,
+// annotating each with its arguments and a short usage summary.
+// If verbose is true, ListCmds prints full details for each command.
+//
+// Each of the name arguments should be a command name.
+// If no names are passed as arguments, ListCmds lists all the
+// commands registered in e.
+func (e *Engine) ListCmds(w io.Writer, verbose bool, names ...string) error {
+	if names == nil {
+		names = make([]string, 0, len(e.Cmds))
+		for name := range e.Cmds {
+			names = append(names, name)
+		}
+		sort.Strings(names)
+	}
+
+	for _, name := range names {
+		cmd := e.Cmds[name]
+		usage := cmd.Usage()
+
+		suffix := ""
+		if usage.Async {
+			suffix = " [&]"
+		}
+
+		_, err := fmt.Fprintf(w, "%s %s%s\n\t%s\n", name, usage.Args, suffix, usage.Summary)
+		if err != nil {
+			return err
+		}
+
+		if verbose {
+			if _, err := io.WriteString(w, "\n"); err != nil {
+				return err
+			}
+			for _, line := range usage.Detail {
+				if err := wrapLine(w, line, 60, "\t"); err != nil {
+					return err
+				}
+			}
+			if _, err := io.WriteString(w, "\n"); err != nil {
+				return err
+			}
+		}
+	}
+
+	return nil
+}
+
+func wrapLine(w io.Writer, line string, cols int, indent string) error {
+	line = strings.TrimLeft(line, " ")
+	for len(line) > cols {
+		bestSpace := -1
+		for i, r := range line {
+			if r == ' ' {
+				if i <= cols || bestSpace < 0 {
+					bestSpace = i
+				}
+				if i > cols {
+					break
+				}
+			}
+		}
+		if bestSpace < 0 {
+			break
+		}
+
+		if _, err := fmt.Fprintf(w, "%s%s\n", indent, line[:bestSpace]); err != nil {
+			return err
+		}
+		line = line[bestSpace+1:]
+	}
+
+	_, err := fmt.Fprintf(w, "%s%s\n", indent, line)
+	return err
+}
+
+// ListConds prints to w a list of conditions, one per line,
+// annotating each with a description and whether the condition
+// is true in the state s (if s is non-nil).
+//
+// Each of the tag arguments should be a condition string of
+// the form "name" or "name:suffix". If no tags are passed as
+// arguments, ListConds lists all conditions registered in
+// the engine e.
+func (e *Engine) ListConds(w io.Writer, s *State, tags ...string) error {
+	if tags == nil {
+		tags = make([]string, 0, len(e.Conds))
+		for name := range e.Conds {
+			tags = append(tags, name)
+		}
+		sort.Strings(tags)
+	}
+
+	for _, tag := range tags {
+		if prefix, suffix, ok := strings.Cut(tag, ":"); ok {
+			cond := e.Conds[prefix]
+			if cond == nil {
+				return fmt.Errorf("unknown condition prefix %q", prefix)
+			}
+			usage := cond.Usage()
+			if !usage.Prefix {
+				return fmt.Errorf("condition %q cannot be used with a suffix", prefix)
+			}
+
+			activeStr := ""
+			if s != nil {
+				if active, _ := cond.Eval(s, suffix); active {
+					activeStr = " (active)"
+				}
+			}
+			_, err := fmt.Fprintf(w, "[%s]%s\n\t%s\n", tag, activeStr, usage.Summary)
+			if err != nil {
+				return err
+			}
+			continue
+		}
+
+		cond := e.Conds[tag]
+		if cond == nil {
+			return fmt.Errorf("unknown condition %q", tag)
+		}
+		var err error
+		usage := cond.Usage()
+		if usage.Prefix {
+			_, err = fmt.Fprintf(w, "[%s:*]\n\t%s\n", tag, usage.Summary)
+		} else {
+			activeStr := ""
+			if s != nil {
+				if ok, _ := cond.Eval(s, ""); ok {
+					activeStr = " (active)"
+				}
+			}
+			_, err = fmt.Fprintf(w, "[%s]%s\n\t%s\n", tag, activeStr, usage.Summary)
+		}
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
diff --git a/src/cmd/go/internal/script/errors.go b/src/cmd/go/internal/script/errors.go
new file mode 100644
index 0000000..7f43e72
--- /dev/null
+++ b/src/cmd/go/internal/script/errors.go
@@ -0,0 +1,64 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package script
+
+import (
+	"errors"
+	"fmt"
+)
+
+// ErrUnexpectedSuccess indicates that a script command that was expected to
+// fail (as indicated by a "!" prefix) instead completed successfully.
+var ErrUnexpectedSuccess = errors.New("unexpected success")
+
+// A CommandError describes an error resulting from attempting to execute a
+// specific command.
+type CommandError struct {
+	File string
+	Line int
+	Op   string
+	Args []string
+	Err  error
+}
+
+func cmdError(cmd *command, err error) *CommandError {
+	return &CommandError{
+		File: cmd.file,
+		Line: cmd.line,
+		Op:   cmd.name,
+		Args: cmd.args,
+		Err:  err,
+	}
+}
+
+func (e *CommandError) Error() string {
+	if len(e.Args) == 0 {
+		return fmt.Sprintf("%s:%d: %s: %v", e.File, e.Line, e.Op, e.Err)
+	}
+	return fmt.Sprintf("%s:%d: %s %s: %v", e.File, e.Line, e.Op, quoteArgs(e.Args), e.Err)
+}
+
+func (e *CommandError) Unwrap() error { return e.Err }
+
+// A UsageError reports the valid arguments for a command.
+//
+// It may be returned in response to invalid arguments.
+type UsageError struct {
+	Name    string
+	Command Cmd
+}
+
+func (e *UsageError) Error() string {
+	usage := e.Command.Usage()
+	suffix := ""
+	if usage.Async {
+		suffix = " [&]"
+	}
+	return fmt.Sprintf("usage: %s %s%s", e.Name, usage.Args, suffix)
+}
+
+// ErrUsage may be returned by a Command to indicate that it was called with
+// invalid arguments; its Usage method may be called to obtain details.
+var ErrUsage = errors.New("invalid usage")
diff --git a/src/cmd/go/internal/script/scripttest/scripttest.go b/src/cmd/go/internal/script/scripttest/scripttest.go
new file mode 100644
index 0000000..0696624
--- /dev/null
+++ b/src/cmd/go/internal/script/scripttest/scripttest.go
@@ -0,0 +1,143 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package scripttest adapts the script engine for use in tests.
+package scripttest
+
+import (
+	"bufio"
+	"cmd/go/internal/script"
+	"errors"
+	"io"
+	"os/exec"
+	"strings"
+	"testing"
+)
+
+// DefaultCmds returns a set of broadly useful script commands.
+//
+// This set includes all of the commands in script.DefaultCmds,
+// as well as a "skip" command that halts the script and causes the
+// testing.TB passed to Run to be skipped.
+func DefaultCmds() map[string]script.Cmd {
+	cmds := script.DefaultCmds()
+	cmds["skip"] = Skip()
+	return cmds
+}
+
+// DefaultConds returns a set of broadly useful script conditions.
+//
+// This set includes all of the conditions in script.DefaultConds,
+// as well as:
+//
+//   - Conditions of the form "exec:foo" are active when the executable "foo" is
+//     found in the test process's PATH, and inactive when the executable is
+//     not found.
+//
+//   - "short" is active when testing.Short() is true.
+//
+//   - "verbose" is active when testing.Verbose() is true.
+func DefaultConds() map[string]script.Cond {
+	conds := script.DefaultConds()
+	conds["exec"] = CachedExec()
+	conds["short"] = script.BoolCondition("testing.Short()", testing.Short())
+	conds["verbose"] = script.BoolCondition("testing.Verbose()", testing.Verbose())
+	return conds
+}
+
+// Run runs the script from the given filename starting at the given initial state.
+// When the script completes, Run closes the state.
+func Run(t testing.TB, e *script.Engine, s *script.State, filename string, testScript io.Reader) {
+	t.Helper()
+	err := func() (err error) {
+		log := new(strings.Builder)
+		log.WriteString("\n") // Start output on a new line for consistent indentation.
+
+		// Defer writing to the test log in case the script engine panics during execution,
+		// but write the log before we write the final "skip" or "FAIL" line.
+		t.Helper()
+		defer func() {
+			t.Helper()
+
+			if closeErr := s.CloseAndWait(log); err == nil {
+				err = closeErr
+			}
+
+			if log.Len() > 0 {
+				t.Log(strings.TrimSuffix(log.String(), "\n"))
+			}
+		}()
+
+		if testing.Verbose() {
+			// Add the environment to the start of the script log.
+			wait, err := script.Env().Run(s)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if wait != nil {
+				stdout, stderr, err := wait(s)
+				if err != nil {
+					t.Fatalf("env: %v\n%s", err, stderr)
+				}
+				if len(stdout) > 0 {
+					s.Logf("%s\n", stdout)
+				}
+			}
+		}
+
+		return e.Execute(s, filename, bufio.NewReader(testScript), log)
+	}()
+
+	if skip := (skipError{}); errors.As(err, &skip) {
+		if skip.msg == "" {
+			t.Skip("SKIP")
+		} else {
+			t.Skipf("SKIP: %v", skip.msg)
+		}
+	}
+	if err != nil {
+		t.Errorf("FAIL: %v", err)
+	}
+}
+
+// Skip returns a sentinel error that causes Run to mark the test as skipped.
+func Skip() script.Cmd {
+	return script.Command(
+		script.CmdUsage{
+			Summary: "skip the current test",
+			Args:    "[msg]",
+		},
+		func(_ *script.State, args ...string) (script.WaitFunc, error) {
+			if len(args) > 1 {
+				return nil, script.ErrUsage
+			}
+			if len(args) == 0 {
+				return nil, skipError{""}
+			}
+			return nil, skipError{args[0]}
+		})
+}
+
+type skipError struct {
+	msg string
+}
+
+func (s skipError) Error() string {
+	if s.msg == "" {
+		return "skip"
+	}
+	return s.msg
+}
+
+// CachedExec returns a Condition that reports whether the PATH of the test
+// binary itself (not the script's current environment) contains the named
+// executable.
+func CachedExec() script.Cond {
+	return script.CachedCondition(
+		"<suffix> names an executable in the test binary's PATH",
+		func(name string) (bool, error) {
+			_, err := exec.LookPath(name)
+			return err == nil, nil
+		})
+}
diff --git a/src/cmd/go/internal/script/state.go b/src/cmd/go/internal/script/state.go
new file mode 100644
index 0000000..548f673
--- /dev/null
+++ b/src/cmd/go/internal/script/state.go
@@ -0,0 +1,236 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package script
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"internal/txtar"
+	"io"
+	"io/fs"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
+	"strings"
+)
+
+// A State encapsulates the current state of a running script engine,
+// including the script environment and any running background commands.
+type State struct {
+	engine *Engine // the engine currently executing the script, if any
+
+	ctx    context.Context
+	cancel context.CancelFunc
+	file   string
+	log    bytes.Buffer
+
+	workdir string            // initial working directory
+	pwd     string            // current working directory during execution
+	env     []string          // environment list (for os/exec)
+	envMap  map[string]string // environment mapping (matches env)
+	stdout  string            // standard output from last 'go' command; for 'stdout' command
+	stderr  string            // standard error from last 'go' command; for 'stderr' command
+
+	background []backgroundCmd
+}
+
+type backgroundCmd struct {
+	*command
+	wait WaitFunc
+}
+
+// NewState returns a new State permanently associated with ctx, with its
+// initial working directory in workdir and its initial environment set to
+// initialEnv (or os.Environ(), if initialEnv is nil).
+//
+// The new State also contains pseudo-environment-variables for
+// ${/} and ${:} (for the platform's path and list separators respectively),
+// but does not pass those to subprocesses.
+func NewState(ctx context.Context, workdir string, initialEnv []string) (*State, error) {
+	absWork, err := filepath.Abs(workdir)
+	if err != nil {
+		return nil, err
+	}
+
+	ctx, cancel := context.WithCancel(ctx)
+
+	// Make a fresh copy of the env slice to avoid aliasing bugs if we ever
+	// start modifying it in place; this also establishes the invariant that
+	// s.env contains no duplicates.
+	env := cleanEnv(initialEnv, absWork)
+
+	envMap := make(map[string]string, len(env))
+
+	// Add entries for ${:} and ${/} to make it easier to write platform-independent
+	// paths in scripts.
+	envMap["/"] = string(os.PathSeparator)
+	envMap[":"] = string(os.PathListSeparator)
+
+	for _, kv := range env {
+		if k, v, ok := strings.Cut(kv, "="); ok {
+			envMap[k] = v
+		}
+	}
+
+	s := &State{
+		ctx:     ctx,
+		cancel:  cancel,
+		workdir: absWork,
+		pwd:     absWork,
+		env:     env,
+		envMap:  envMap,
+	}
+	s.Setenv("PWD", absWork)
+	return s, nil
+}
+
+// CloseAndWait cancels the State's Context and waits for any background commands to
+// finish. If any remaining background command ended in an unexpected state,
+// Close returns a non-nil error.
+func (s *State) CloseAndWait(log io.Writer) error {
+	s.cancel()
+	wait, err := Wait().Run(s)
+	if wait != nil {
+		panic("script: internal error: Wait unexpectedly returns its own WaitFunc")
+	}
+	if flushErr := s.flushLog(log); err == nil {
+		err = flushErr
+	}
+	return err
+}
+
+// Chdir changes the State's working directory to the given path.
+func (s *State) Chdir(path string) error {
+	dir := s.Path(path)
+	if _, err := os.Stat(dir); err != nil {
+		return &fs.PathError{Op: "Chdir", Path: dir, Err: err}
+	}
+	s.pwd = dir
+	s.Setenv("PWD", dir)
+	return nil
+}
+
+// Context returns the Context with which the State was created.
+func (s *State) Context() context.Context {
+	return s.ctx
+}
+
+// Environ returns a copy of the current script environment,
+// in the form "key=value".
+func (s *State) Environ() []string {
+	return append([]string(nil), s.env...)
+}
+
+// ExpandEnv replaces ${var} or $var in the string according to the values of
+// the environment variables in s. References to undefined variables are
+// replaced by the empty string.
+func (s *State) ExpandEnv(str string, inRegexp bool) string {
+	return os.Expand(str, func(key string) string {
+		e := s.envMap[key]
+		if inRegexp {
+			// Quote to literal strings: we want paths like C:\work\go1.4 to remain
+			// paths rather than regular expressions.
+			e = regexp.QuoteMeta(e)
+		}
+		return e
+	})
+}
+
+// ExtractFiles extracts the files in ar to the state's current directory,
+// expanding any environment variables within each name.
+//
+// The files must reside within the working directory with which the State was
+// originally created.
+func (s *State) ExtractFiles(ar *txtar.Archive) error {
+	wd := s.workdir
+
+	// Add trailing separator to terminate wd.
+	// This prevents extracting to outside paths which prefix wd,
+	// e.g. extracting to /home/foobar when wd is /home/foo
+	if wd == "" {
+		panic("s.workdir is unexpectedly empty")
+	}
+	if !os.IsPathSeparator(wd[len(wd)-1]) {
+		wd += string(filepath.Separator)
+	}
+
+	for _, f := range ar.Files {
+		name := s.Path(s.ExpandEnv(f.Name, false))
+
+		if !strings.HasPrefix(name, wd) {
+			return fmt.Errorf("file %#q is outside working directory", f.Name)
+		}
+
+		if err := os.MkdirAll(filepath.Dir(name), 0777); err != nil {
+			return err
+		}
+		if err := os.WriteFile(name, f.Data, 0666); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// Getwd returns the directory in which to run the next script command.
+func (s *State) Getwd() string { return s.pwd }
+
+// Logf writes output to the script's log without updating its stdout or stderr
+// buffers. (The output log functions as a kind of meta-stderr.)
+func (s *State) Logf(format string, args ...any) {
+	fmt.Fprintf(&s.log, format, args...)
+}
+
+// flushLog writes the contents of the script's log to w and clears the log.
+func (s *State) flushLog(w io.Writer) error {
+	_, err := w.Write(s.log.Bytes())
+	s.log.Reset()
+	return err
+}
+
+// LookupEnv retrieves the value of the environment variable in s named by the key.
+func (s *State) LookupEnv(key string) (string, bool) {
+	v, ok := s.envMap[key]
+	return v, ok
+}
+
+// Path returns the absolute path in the host operating system for a
+// script-based (generally slash-separated and relative) path.
+func (s *State) Path(path string) string {
+	if filepath.IsAbs(path) {
+		return filepath.Clean(path)
+	}
+	return filepath.Join(s.pwd, path)
+}
+
+// Setenv sets the value of the environment variable in s named by the key.
+func (s *State) Setenv(key, value string) error {
+	s.env = cleanEnv(append(s.env, key+"="+value), s.pwd)
+	s.envMap[key] = value
+	return nil
+}
+
+// Stdout returns the stdout output of the last command run,
+// or the empty string if no command has been run.
+func (s *State) Stdout() string { return s.stdout }
+
+// Stderr returns the stderr output of the last command run,
+// or the empty string if no command has been run.
+func (s *State) Stderr() string { return s.stderr }
+
+// cleanEnv returns a copy of env with any duplicates removed in favor of
+// later values and any required system variables defined.
+//
+// If env is nil, cleanEnv copies the environment from os.Environ().
+func cleanEnv(env []string, pwd string) []string {
+	// There are some funky edge-cases in this logic, especially on Windows (with
+	// case-insensitive environment variables and variables with keys like "=C:").
+	// Rather than duplicating exec.dedupEnv here, cheat and use exec.Cmd directly.
+	cmd := &exec.Cmd{Env: env}
+	cmd.Dir = pwd
+	return cmd.Environ()
+}
diff --git a/src/cmd/go/internal/search/search.go b/src/cmd/go/internal/search/search.go
new file mode 100644
index 0000000..9f216d5
--- /dev/null
+++ b/src/cmd/go/internal/search/search.go
@@ -0,0 +1,512 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package search
+
+import (
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/str"
+	"cmd/internal/pkgpattern"
+	"fmt"
+	"go/build"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"strings"
+)
+
+// A Match represents the result of matching a single package pattern.
+type Match struct {
+	pattern string   // the pattern itself
+	Dirs    []string // if the pattern is local, directories that potentially contain matching packages
+	Pkgs    []string // matching packages (import paths)
+	Errs    []error  // errors matching the patterns to packages, NOT errors loading those packages
+
+	// Errs may be non-empty even if len(Pkgs) > 0, indicating that some matching
+	// packages could be located but results may be incomplete.
+	// If len(Pkgs) == 0 && len(Errs) == 0, the pattern is well-formed but did not
+	// match any packages.
+}
+
+// NewMatch returns a Match describing the given pattern,
+// without resolving its packages or errors.
+func NewMatch(pattern string) *Match {
+	return &Match{pattern: pattern}
+}
+
+// Pattern returns the pattern to be matched.
+func (m *Match) Pattern() string { return m.pattern }
+
+// AddError appends a MatchError wrapping err to m.Errs.
+func (m *Match) AddError(err error) {
+	m.Errs = append(m.Errs, &MatchError{Match: m, Err: err})
+}
+
+// IsLiteral reports whether the pattern is free of wildcards and meta-patterns.
+//
+// A literal pattern must match at most one package.
+func (m *Match) IsLiteral() bool {
+	return !strings.Contains(m.pattern, "...") && !m.IsMeta()
+}
+
+// IsLocal reports whether the pattern must be resolved from a specific root or
+// directory, such as a filesystem path or a single module.
+func (m *Match) IsLocal() bool {
+	return build.IsLocalImport(m.pattern) || filepath.IsAbs(m.pattern)
+}
+
+// IsMeta reports whether the pattern is a “meta-package” keyword that represents
+// multiple packages, such as "std", "cmd", or "all".
+func (m *Match) IsMeta() bool {
+	return IsMetaPackage(m.pattern)
+}
+
+// IsMetaPackage checks if name is a reserved package name that expands to multiple packages.
+func IsMetaPackage(name string) bool {
+	return name == "std" || name == "cmd" || name == "all"
+}
+
+// A MatchError indicates an error that occurred while attempting to match a
+// pattern.
+type MatchError struct {
+	Match *Match
+	Err   error
+}
+
+func (e *MatchError) Error() string {
+	if e.Match.IsLiteral() {
+		return fmt.Sprintf("%s: %v", e.Match.Pattern(), e.Err)
+	}
+	return fmt.Sprintf("pattern %s: %v", e.Match.Pattern(), e.Err)
+}
+
+func (e *MatchError) Unwrap() error {
+	return e.Err
+}
+
+// MatchPackages sets m.Pkgs to a non-nil slice containing all the packages that
+// can be found under the $GOPATH directories and $GOROOT that match the
+// pattern. The pattern must be either "all" (all packages), "std" (standard
+// packages), "cmd" (standard commands), or a path including "...".
+//
+// If any errors may have caused the set of packages to be incomplete,
+// MatchPackages appends those errors to m.Errs.
+func (m *Match) MatchPackages() {
+	m.Pkgs = []string{}
+	if m.IsLocal() {
+		m.AddError(fmt.Errorf("internal error: MatchPackages: %s is not a valid package pattern", m.pattern))
+		return
+	}
+
+	if m.IsLiteral() {
+		m.Pkgs = []string{m.pattern}
+		return
+	}
+
+	match := func(string) bool { return true }
+	treeCanMatch := func(string) bool { return true }
+	if !m.IsMeta() {
+		match = pkgpattern.MatchPattern(m.pattern)
+		treeCanMatch = pkgpattern.TreeCanMatchPattern(m.pattern)
+	}
+
+	have := map[string]bool{
+		"builtin": true, // ignore pseudo-package that exists only for documentation
+	}
+	if !cfg.BuildContext.CgoEnabled {
+		have["runtime/cgo"] = true // ignore during walk
+	}
+
+	for _, src := range cfg.BuildContext.SrcDirs() {
+		if (m.pattern == "std" || m.pattern == "cmd") && src != cfg.GOROOTsrc {
+			continue
+		}
+
+		// If the root itself is a symlink to a directory,
+		// we want to follow it (see https://go.dev/issue/50807).
+		// Add a trailing separator to force that to happen.
+		src = str.WithFilePathSeparator(filepath.Clean(src))
+		root := src
+		if m.pattern == "cmd" {
+			root += "cmd" + string(filepath.Separator)
+		}
+
+		err := fsys.Walk(root, func(path string, fi fs.FileInfo, err error) error {
+			if err != nil {
+				return err // Likely a permission error, which could interfere with matching.
+			}
+			if path == src {
+				return nil // GOROOT/src and GOPATH/src cannot contain packages.
+			}
+
+			want := true
+			// Avoid .foo, _foo, and testdata directory trees.
+			_, elem := filepath.Split(path)
+			if strings.HasPrefix(elem, ".") || strings.HasPrefix(elem, "_") || elem == "testdata" {
+				want = false
+			}
+
+			name := filepath.ToSlash(path[len(src):])
+			if m.pattern == "std" && (!IsStandardImportPath(name) || name == "cmd") {
+				// The name "std" is only the standard library.
+				// If the name is cmd, it's the root of the command tree.
+				want = false
+			}
+			if !treeCanMatch(name) {
+				want = false
+			}
+
+			if !fi.IsDir() {
+				if fi.Mode()&fs.ModeSymlink != 0 && want && strings.Contains(m.pattern, "...") {
+					if target, err := fsys.Stat(path); err == nil && target.IsDir() {
+						fmt.Fprintf(os.Stderr, "warning: ignoring symlink %s\n", path)
+					}
+				}
+				return nil
+			}
+			if !want {
+				return filepath.SkipDir
+			}
+
+			if have[name] {
+				return nil
+			}
+			have[name] = true
+			if !match(name) {
+				return nil
+			}
+			pkg, err := cfg.BuildContext.ImportDir(path, 0)
+			if err != nil {
+				if _, noGo := err.(*build.NoGoError); noGo {
+					// The package does not actually exist, so record neither the package
+					// nor the error.
+					return nil
+				}
+				// There was an error importing path, but not matching it,
+				// which is all that Match promises to do.
+				// Ignore the import error.
+			}
+
+			// If we are expanding "cmd", skip main
+			// packages under cmd/vendor. At least as of
+			// March, 2017, there is one there for the
+			// vendored pprof tool.
+			if m.pattern == "cmd" && pkg != nil && strings.HasPrefix(pkg.ImportPath, "cmd/vendor") && pkg.Name == "main" {
+				return nil
+			}
+
+			m.Pkgs = append(m.Pkgs, name)
+			return nil
+		})
+		if err != nil {
+			m.AddError(err)
+		}
+	}
+}
+
+// MatchDirs sets m.Dirs to a non-nil slice containing all directories that
+// potentially match a local pattern. The pattern must begin with an absolute
+// path, or "./", or "../". On Windows, the pattern may use slash or backslash
+// separators or a mix of both.
+//
+// If any errors may have caused the set of directories to be incomplete,
+// MatchDirs appends those errors to m.Errs.
+func (m *Match) MatchDirs(modRoots []string) {
+	m.Dirs = []string{}
+	if !m.IsLocal() {
+		m.AddError(fmt.Errorf("internal error: MatchDirs: %s is not a valid filesystem pattern", m.pattern))
+		return
+	}
+
+	if m.IsLiteral() {
+		m.Dirs = []string{m.pattern}
+		return
+	}
+
+	// Clean the path and create a matching predicate.
+	// filepath.Clean removes "./" prefixes (and ".\" on Windows). We need to
+	// preserve these, since they are meaningful in MatchPattern and in
+	// returned import paths.
+	cleanPattern := filepath.Clean(m.pattern)
+	isLocal := strings.HasPrefix(m.pattern, "./") || (os.PathSeparator == '\\' && strings.HasPrefix(m.pattern, `.\`))
+	prefix := ""
+	if cleanPattern != "." && isLocal {
+		prefix = "./"
+		cleanPattern = "." + string(os.PathSeparator) + cleanPattern
+	}
+	slashPattern := filepath.ToSlash(cleanPattern)
+	match := pkgpattern.MatchPattern(slashPattern)
+
+	// Find directory to begin the scan.
+	// Could be smarter but this one optimization
+	// is enough for now, since ... is usually at the
+	// end of a path.
+	i := strings.Index(cleanPattern, "...")
+	dir, _ := filepath.Split(cleanPattern[:i])
+
+	// pattern begins with ./ or ../.
+	// path.Clean will discard the ./ but not the ../.
+	// We need to preserve the ./ for pattern matching
+	// and in the returned import paths.
+
+	if len(modRoots) > 1 {
+		abs, err := filepath.Abs(dir)
+		if err != nil {
+			m.AddError(err)
+			return
+		}
+		var found bool
+		for _, modRoot := range modRoots {
+			if modRoot != "" && str.HasFilePathPrefix(abs, modRoot) {
+				found = true
+			}
+		}
+		if !found {
+			plural := ""
+			if len(modRoots) > 1 {
+				plural = "s"
+			}
+			m.AddError(fmt.Errorf("directory %s is outside module root%s (%s)", abs, plural, strings.Join(modRoots, ", ")))
+		}
+	}
+
+	// If dir is actually a symlink to a directory,
+	// we want to follow it (see https://go.dev/issue/50807).
+	// Add a trailing separator to force that to happen.
+	dir = str.WithFilePathSeparator(dir)
+	err := fsys.Walk(dir, func(path string, fi fs.FileInfo, err error) error {
+		if err != nil {
+			return err // Likely a permission error, which could interfere with matching.
+		}
+		if !fi.IsDir() {
+			return nil
+		}
+		top := false
+		if path == dir {
+			// Walk starts at dir and recurses. For the recursive case,
+			// the path is the result of filepath.Join, which calls filepath.Clean.
+			// The initial case is not Cleaned, though, so we do this explicitly.
+			//
+			// This converts a path like "./io/" to "io". Without this step, running
+			// "cd $GOROOT/src; go list ./io/..." would incorrectly skip the io
+			// package, because prepending the prefix "./" to the unclean path would
+			// result in "././io", and match("././io") returns false.
+			top = true
+			path = filepath.Clean(path)
+		}
+
+		// Avoid .foo, _foo, and testdata directory trees, but do not avoid "." or "..".
+		_, elem := filepath.Split(path)
+		dot := strings.HasPrefix(elem, ".") && elem != "." && elem != ".."
+		if dot || strings.HasPrefix(elem, "_") || elem == "testdata" {
+			return filepath.SkipDir
+		}
+
+		if !top && cfg.ModulesEnabled {
+			// Ignore other modules found in subdirectories.
+			if fi, err := fsys.Stat(filepath.Join(path, "go.mod")); err == nil && !fi.IsDir() {
+				return filepath.SkipDir
+			}
+		}
+
+		name := prefix + filepath.ToSlash(path)
+		if !match(name) {
+			return nil
+		}
+
+		// We keep the directory if we can import it, or if we can't import it
+		// due to invalid Go source files. This means that directories containing
+		// parse errors will be built (and fail) instead of being silently skipped
+		// as not matching the pattern. Go 1.5 and earlier skipped, but that
+		// behavior means people miss serious mistakes.
+		// See golang.org/issue/11407.
+		if p, err := cfg.BuildContext.ImportDir(path, 0); err != nil && (p == nil || len(p.InvalidGoFiles) == 0) {
+			if _, noGo := err.(*build.NoGoError); noGo {
+				// The package does not actually exist, so record neither the package
+				// nor the error.
+				return nil
+			}
+			// There was an error importing path, but not matching it,
+			// which is all that Match promises to do.
+			// Ignore the import error.
+		}
+		m.Dirs = append(m.Dirs, name)
+		return nil
+	})
+	if err != nil {
+		m.AddError(err)
+	}
+}
+
+// WarnUnmatched warns about patterns that didn't match any packages.
+func WarnUnmatched(matches []*Match) {
+	for _, m := range matches {
+		if len(m.Pkgs) == 0 && len(m.Errs) == 0 {
+			fmt.Fprintf(os.Stderr, "go: warning: %q matched no packages\n", m.pattern)
+		}
+	}
+}
+
+// ImportPaths returns the matching paths to use for the given command line.
+// It calls ImportPathsQuiet and then WarnUnmatched.
+func ImportPaths(patterns, modRoots []string) []*Match {
+	matches := ImportPathsQuiet(patterns, modRoots)
+	WarnUnmatched(matches)
+	return matches
+}
+
+// ImportPathsQuiet is like ImportPaths but does not warn about patterns with no matches.
+func ImportPathsQuiet(patterns, modRoots []string) []*Match {
+	var out []*Match
+	for _, a := range CleanPatterns(patterns) {
+		m := NewMatch(a)
+		if m.IsLocal() {
+			m.MatchDirs(modRoots)
+
+			// Change the file import path to a regular import path if the package
+			// is in GOPATH or GOROOT. We don't report errors here; LoadImport
+			// (or something similar) will report them later.
+			m.Pkgs = make([]string, len(m.Dirs))
+			for i, dir := range m.Dirs {
+				absDir := dir
+				if !filepath.IsAbs(dir) {
+					absDir = filepath.Join(base.Cwd(), dir)
+				}
+				if bp, _ := cfg.BuildContext.ImportDir(absDir, build.FindOnly); bp.ImportPath != "" && bp.ImportPath != "." {
+					m.Pkgs[i] = bp.ImportPath
+				} else {
+					m.Pkgs[i] = dir
+				}
+			}
+		} else {
+			m.MatchPackages()
+		}
+
+		out = append(out, m)
+	}
+	return out
+}
+
+// CleanPatterns returns the patterns to use for the given command line. It
+// canonicalizes the patterns but does not evaluate any matches. For patterns
+// that are not local or absolute paths, it preserves text after '@' to avoid
+// modifying version queries.
+func CleanPatterns(patterns []string) []string {
+	if len(patterns) == 0 {
+		return []string{"."}
+	}
+	var out []string
+	for _, a := range patterns {
+		var p, v string
+		if build.IsLocalImport(a) || filepath.IsAbs(a) {
+			p = a
+		} else if i := strings.IndexByte(a, '@'); i < 0 {
+			p = a
+		} else {
+			p = a[:i]
+			v = a[i:]
+		}
+
+		// Arguments may be either file paths or import paths.
+		// As a courtesy to Windows developers, rewrite \ to /
+		// in arguments that look like import paths.
+		// Don't replace slashes in absolute paths.
+		if filepath.IsAbs(p) {
+			p = filepath.Clean(p)
+		} else {
+			if filepath.Separator == '\\' {
+				p = strings.ReplaceAll(p, `\`, `/`)
+			}
+
+			// Put argument in canonical form, but preserve leading ./.
+			if strings.HasPrefix(p, "./") {
+				p = "./" + path.Clean(p)
+				if p == "./." {
+					p = "."
+				}
+			} else {
+				p = path.Clean(p)
+			}
+		}
+
+		out = append(out, p+v)
+	}
+	return out
+}
+
+// IsStandardImportPath reports whether $GOROOT/src/path should be considered
+// part of the standard distribution. For historical reasons we allow people to add
+// their own code to $GOROOT instead of using $GOPATH, but we assume that
+// code will start with a domain name (dot in the first element).
+//
+// Note that this function is meant to evaluate whether a directory found in GOROOT
+// should be treated as part of the standard library. It should not be used to decide
+// that a directory found in GOPATH should be rejected: directories in GOPATH
+// need not have dots in the first element, and they just take their chances
+// with future collisions in the standard library.
+func IsStandardImportPath(path string) bool {
+	i := strings.Index(path, "/")
+	if i < 0 {
+		i = len(path)
+	}
+	elem := path[:i]
+	return !strings.Contains(elem, ".")
+}
+
+// IsRelativePath reports whether pattern should be interpreted as a directory
+// path relative to the current directory, as opposed to a pattern matching
+// import paths.
+func IsRelativePath(pattern string) bool {
+	return strings.HasPrefix(pattern, "./") || strings.HasPrefix(pattern, "../") || pattern == "." || pattern == ".."
+}
+
+// InDir checks whether path is in the file tree rooted at dir.
+// If so, InDir returns an equivalent path relative to dir.
+// If not, InDir returns an empty string.
+// InDir makes some effort to succeed even in the presence of symbolic links.
+func InDir(path, dir string) string {
+	// inDirLex reports whether path is lexically in dir,
+	// without considering symbolic or hard links.
+	inDirLex := func(path, dir string) (string, bool) {
+		if dir == "" {
+			return path, true
+		}
+		rel := str.TrimFilePathPrefix(path, dir)
+		if rel == path {
+			return "", false
+		}
+		if rel == "" {
+			return ".", true
+		}
+		return rel, true
+	}
+
+	if rel, ok := inDirLex(path, dir); ok {
+		return rel
+	}
+	xpath, err := filepath.EvalSymlinks(path)
+	if err != nil || xpath == path {
+		xpath = ""
+	} else {
+		if rel, ok := inDirLex(xpath, dir); ok {
+			return rel
+		}
+	}
+
+	xdir, err := filepath.EvalSymlinks(dir)
+	if err == nil && xdir != dir {
+		if rel, ok := inDirLex(path, xdir); ok {
+			return rel
+		}
+		if xpath != "" {
+			if rel, ok := inDirLex(xpath, xdir); ok {
+				return rel
+			}
+		}
+	}
+	return ""
+}
diff --git a/src/cmd/go/internal/str/path.go b/src/cmd/go/internal/str/path.go
new file mode 100644
index 0000000..83a3d0e
--- /dev/null
+++ b/src/cmd/go/internal/str/path.go
@@ -0,0 +1,133 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package str
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+)
+
+// HasPathPrefix reports whether the slash-separated path s
+// begins with the elements in prefix.
+func HasPathPrefix(s, prefix string) bool {
+	if len(s) == len(prefix) {
+		return s == prefix
+	}
+	if prefix == "" {
+		return true
+	}
+	if len(s) > len(prefix) {
+		if prefix[len(prefix)-1] == '/' || s[len(prefix)] == '/' {
+			return s[:len(prefix)] == prefix
+		}
+	}
+	return false
+}
+
+// HasFilePathPrefix reports whether the filesystem path s
+// begins with the elements in prefix.
+//
+// HasFilePathPrefix is case-sensitive (except for volume names) even if the
+// filesystem is not, does not apply Unicode normalization even if the
+// filesystem does, and assumes that all path separators are canonicalized to
+// filepath.Separator (as returned by filepath.Clean).
+func HasFilePathPrefix(s, prefix string) bool {
+	sv := filepath.VolumeName(s)
+	pv := filepath.VolumeName(prefix)
+
+	// Strip the volume from both paths before canonicalizing sv and pv:
+	// it's unlikely that strings.ToUpper will change the length of the string,
+	// but doesn't seem impossible.
+	s = s[len(sv):]
+	prefix = prefix[len(pv):]
+
+	// Always treat Windows volume names as case-insensitive, even though
+	// we don't treat the rest of the path as such.
+	//
+	// TODO(bcmills): Why do we care about case only for the volume name? It's
+	// been this way since https://go.dev/cl/11316, but I don't understand why
+	// that problem doesn't apply to case differences in the entire path.
+	if sv != pv {
+		sv = strings.ToUpper(sv)
+		pv = strings.ToUpper(pv)
+	}
+
+	switch {
+	default:
+		return false
+	case sv != pv:
+		return false
+	case len(s) == len(prefix):
+		return s == prefix
+	case prefix == "":
+		return true
+	case len(s) > len(prefix):
+		if prefix[len(prefix)-1] == filepath.Separator {
+			return strings.HasPrefix(s, prefix)
+		}
+		return s[len(prefix)] == filepath.Separator && s[:len(prefix)] == prefix
+	}
+}
+
+// TrimFilePathPrefix returns s without the leading path elements in prefix,
+// such that joining the string to prefix produces s.
+//
+// If s does not start with prefix (HasFilePathPrefix with the same arguments
+// returns false), TrimFilePathPrefix returns s. If s equals prefix,
+// TrimFilePathPrefix returns "".
+func TrimFilePathPrefix(s, prefix string) string {
+	if prefix == "" {
+		// Trimming the empty string from a path should join to produce that path.
+		// (Trim("/tmp/foo", "") should give "/tmp/foo", not "tmp/foo".)
+		return s
+	}
+	if !HasFilePathPrefix(s, prefix) {
+		return s
+	}
+
+	trimmed := s[len(prefix):]
+	if len(trimmed) > 0 && os.IsPathSeparator(trimmed[0]) {
+		if runtime.GOOS == "windows" && prefix == filepath.VolumeName(prefix) && len(prefix) == 2 && prefix[1] == ':' {
+			// Joining a relative path to a bare Windows drive letter produces a path
+			// relative to the working directory on that drive, but the original path
+			// was absolute, not relative. Keep the leading path separator so that it
+			// remains absolute when joined to prefix.
+		} else {
+			// Prefix ends in a regular path element, so strip the path separator that
+			// follows it.
+			trimmed = trimmed[1:]
+		}
+	}
+	return trimmed
+}
+
+// WithFilePathSeparator returns s with a trailing path separator, or the empty
+// string if s is empty.
+func WithFilePathSeparator(s string) string {
+	if s == "" || os.IsPathSeparator(s[len(s)-1]) {
+		return s
+	}
+	return s + string(filepath.Separator)
+}
+
+// QuoteGlob returns s with all Glob metacharacters quoted.
+// We don't try to handle backslash here, as that can appear in a
+// file path on Windows.
+func QuoteGlob(s string) string {
+	if !strings.ContainsAny(s, `*?[]`) {
+		return s
+	}
+	var sb strings.Builder
+	for _, c := range s {
+		switch c {
+		case '*', '?', '[', ']':
+			sb.WriteByte('\\')
+		}
+		sb.WriteRune(c)
+	}
+	return sb.String()
+}
diff --git a/src/cmd/go/internal/str/str.go b/src/cmd/go/internal/str/str.go
new file mode 100644
index 0000000..af7c699
--- /dev/null
+++ b/src/cmd/go/internal/str/str.go
@@ -0,0 +1,113 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package str provides string manipulation utilities.
+package str
+
+import (
+	"fmt"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
+// StringList flattens its arguments into a single []string.
+// Each argument in args must have type string or []string.
+func StringList(args ...any) []string {
+	var x []string
+	for _, arg := range args {
+		switch arg := arg.(type) {
+		case []string:
+			x = append(x, arg...)
+		case string:
+			x = append(x, arg)
+		default:
+			panic("stringList: invalid argument of type " + fmt.Sprintf("%T", arg))
+		}
+	}
+	return x
+}
+
+// ToFold returns a string with the property that
+//
+//	strings.EqualFold(s, t) iff ToFold(s) == ToFold(t)
+//
+// This lets us test a large set of strings for fold-equivalent
+// duplicates without making a quadratic number of calls
+// to EqualFold. Note that strings.ToUpper and strings.ToLower
+// do not have the desired property in some corner cases.
+func ToFold(s string) string {
+	// Fast path: all ASCII, no upper case.
+	// Most paths look like this already.
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		if c >= utf8.RuneSelf || 'A' <= c && c <= 'Z' {
+			goto Slow
+		}
+	}
+	return s
+
+Slow:
+	var b strings.Builder
+	for _, r := range s {
+		// SimpleFold(x) cycles to the next equivalent rune > x
+		// or wraps around to smaller values. Iterate until it wraps,
+		// and we've found the minimum value.
+		for {
+			r0 := r
+			r = unicode.SimpleFold(r0)
+			if r <= r0 {
+				break
+			}
+		}
+		// Exception to allow fast path above: A-Z => a-z
+		if 'A' <= r && r <= 'Z' {
+			r += 'a' - 'A'
+		}
+		b.WriteRune(r)
+	}
+	return b.String()
+}
+
+// FoldDup reports a pair of strings from the list that are
+// equal according to strings.EqualFold.
+// It returns "", "" if there are no such strings.
+func FoldDup(list []string) (string, string) {
+	clash := map[string]string{}
+	for _, s := range list {
+		fold := ToFold(s)
+		if t := clash[fold]; t != "" {
+			if s > t {
+				s, t = t, s
+			}
+			return s, t
+		}
+		clash[fold] = s
+	}
+	return "", ""
+}
+
+// Contains reports whether x contains s.
+func Contains(x []string, s string) bool {
+	for _, t := range x {
+		if t == s {
+			return true
+		}
+	}
+	return false
+}
+
+// Uniq removes consecutive duplicate strings from ss.
+func Uniq(ss *[]string) {
+	if len(*ss) <= 1 {
+		return
+	}
+	uniq := (*ss)[:1]
+	for _, s := range *ss {
+		if s != uniq[len(uniq)-1] {
+			uniq = append(uniq, s)
+		}
+	}
+	*ss = uniq
+}
diff --git a/src/cmd/go/internal/str/str_test.go b/src/cmd/go/internal/str/str_test.go
new file mode 100644
index 0000000..7c19877
--- /dev/null
+++ b/src/cmd/go/internal/str/str_test.go
@@ -0,0 +1,185 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package str
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+)
+
+var foldDupTests = []struct {
+	list   []string
+	f1, f2 string
+}{
+	{StringList("math/rand", "math/big"), "", ""},
+	{StringList("math", "strings"), "", ""},
+	{StringList("strings"), "", ""},
+	{StringList("strings", "strings"), "strings", "strings"},
+	{StringList("Rand", "rand", "math", "math/rand", "math/Rand"), "Rand", "rand"},
+}
+
+func TestFoldDup(t *testing.T) {
+	for _, tt := range foldDupTests {
+		f1, f2 := FoldDup(tt.list)
+		if f1 != tt.f1 || f2 != tt.f2 {
+			t.Errorf("foldDup(%q) = %q, %q, want %q, %q", tt.list, f1, f2, tt.f1, tt.f2)
+		}
+	}
+}
+
+func TestHasPathPrefix(t *testing.T) {
+	type testCase struct {
+		s, prefix string
+		want      bool
+	}
+	for _, tt := range []testCase{
+		{"", "", true},
+		{"", "/", false},
+		{"foo", "", true},
+		{"foo", "/", false},
+		{"foo", "foo", true},
+		{"foo", "foo/", false},
+		{"foo", "/foo", false},
+		{"foo/bar", "", true},
+		{"foo/bar", "foo", true},
+		{"foo/bar", "foo/", true},
+		{"foo/bar", "/foo", false},
+		{"foo/bar", "foo/bar", true},
+		{"foo/bar", "foo/bar/", false},
+		{"foo/bar", "/foo/bar", false},
+	} {
+		got := HasPathPrefix(tt.s, tt.prefix)
+		if got != tt.want {
+			t.Errorf("HasPathPrefix(%q, %q) = %v; want %v", tt.s, tt.prefix, got, tt.want)
+		}
+	}
+}
+
+func TestTrimFilePathPrefixSlash(t *testing.T) {
+	if os.PathSeparator != '/' {
+		t.Skipf("test requires slash-separated file paths")
+	}
+
+	type testCase struct {
+		s, prefix, want string
+	}
+	for _, tt := range []testCase{
+		{"/", "", "/"},
+		{"/", "/", ""},
+		{"/foo", "", "/foo"},
+		{"/foo", "/", "foo"},
+		{"/foo", "/foo", ""},
+		{"/foo/bar", "/foo", "bar"},
+		{"/foo/bar", "/foo/", "bar"},
+		{"/foo/", "/", "foo/"},
+		{"/foo/", "/foo", ""},
+		{"/foo/", "/foo/", ""},
+
+		// if prefix is not s's prefix, return s
+		{"", "/", ""},
+		{"/foo", "/bar", "/foo"},
+		{"/foo", "/foo/bar", "/foo"},
+		{"foo", "/foo", "foo"},
+		{"/foo", "foo", "/foo"},
+		{"/foo", "/foo/", "/foo"},
+	} {
+		got := TrimFilePathPrefix(tt.s, tt.prefix)
+		if got == tt.want {
+			t.Logf("TrimFilePathPrefix(%q, %q) = %q", tt.s, tt.prefix, got)
+		} else {
+			t.Errorf("TrimFilePathPrefix(%q, %q) = %q, want %q", tt.s, tt.prefix, got, tt.want)
+		}
+
+		if HasFilePathPrefix(tt.s, tt.prefix) {
+			joined := filepath.Join(tt.prefix, got)
+			if clean := filepath.Clean(tt.s); joined != clean {
+				t.Errorf("filepath.Join(%q, %q) = %q, want %q", tt.prefix, got, joined, clean)
+			}
+		}
+	}
+}
+
+func TestTrimFilePathPrefixWindows(t *testing.T) {
+	if runtime.GOOS != "windows" {
+		t.Skipf("test requires Windows file paths")
+	}
+	type testCase struct {
+		s, prefix, want string
+	}
+	for _, tt := range []testCase{
+		{`\`, ``, `\`},
+		{`\`, `\`, ``},
+		{`C:`, `C:`, ``},
+		{`C:\`, `C:`, `\`},
+		{`C:\`, `C:\`, ``},
+		{`C:\foo`, ``, `C:\foo`},
+		{`C:\foo`, `C:`, `\foo`},
+		{`C:\foo`, `C:\`, `foo`},
+		{`C:\foo`, `C:\foo`, ``},
+		{`C:\foo\`, `C:\foo`, ``},
+		{`C:\foo\bar`, `C:\foo`, `bar`},
+		{`C:\foo\bar`, `C:\foo\`, `bar`},
+		// if prefix is not s's prefix, return s
+		{`C:\foo`, `C:\bar`, `C:\foo`},
+		{`C:\foo`, `C:\foo\bar`, `C:\foo`},
+		{`C:`, `C:\`, `C:`},
+		// if volumes are different, return s
+		{`C:`, ``, `C:`},
+		{`C:\`, ``, `C:\`},
+		{`C:\foo`, ``, `C:\foo`},
+		{`C:\foo`, `\foo`, `C:\foo`},
+		{`C:\foo`, `D:\foo`, `C:\foo`},
+
+		//UNC path
+		{`\\host\share\foo`, `\\host\share`, `foo`},
+		{`\\host\share\foo`, `\\host\share\`, `foo`},
+		{`\\host\share\foo`, `\\host\share\foo`, ``},
+		{`\\host\share\foo\bar`, `\\host\share\foo`, `bar`},
+		{`\\host\share\foo\bar`, `\\host\share\foo\`, `bar`},
+		// if prefix is not s's prefix, return s
+		{`\\host\share\foo`, `\\host\share\bar`, `\\host\share\foo`},
+		{`\\host\share\foo`, `\\host\share\foo\bar`, `\\host\share\foo`},
+		// if either host or share name is different, return s
+		{`\\host\share\foo`, ``, `\\host\share\foo`},
+		{`\\host\share\foo`, `\foo`, `\\host\share\foo`},
+		{`\\host\share\foo`, `\\host\other\`, `\\host\share\foo`},
+		{`\\host\share\foo`, `\\other\share\`, `\\host\share\foo`},
+		{`\\host\share\foo`, `\\host\`, `\\host\share\foo`},
+		{`\\host\share\foo`, `\share\`, `\\host\share\foo`},
+
+		// only volume names are case-insensitive
+		{`C:\foo`, `c:`, `\foo`},
+		{`C:\foo`, `c:\foo`, ``},
+		{`c:\foo`, `C:`, `\foo`},
+		{`c:\foo`, `C:\foo`, ``},
+		{`C:\foo`, `C:\Foo`, `C:\foo`},
+		{`\\Host\Share\foo`, `\\host\share`, `foo`},
+		{`\\Host\Share\foo`, `\\host\share\foo`, ``},
+		{`\\host\share\foo`, `\\Host\Share`, `foo`},
+		{`\\host\share\foo`, `\\Host\Share\foo`, ``},
+		{`\\Host\Share\foo`, `\\Host\Share\Foo`, `\\Host\Share\foo`},
+	} {
+		got := TrimFilePathPrefix(tt.s, tt.prefix)
+		if got == tt.want {
+			t.Logf("TrimFilePathPrefix(%#q, %#q) = %#q", tt.s, tt.prefix, got)
+		} else {
+			t.Errorf("TrimFilePathPrefix(%#q, %#q) = %#q, want %#q", tt.s, tt.prefix, got, tt.want)
+		}
+
+		if HasFilePathPrefix(tt.s, tt.prefix) {
+			// Although TrimFilePathPrefix is only case-insensitive in the volume name,
+			// what we care about in testing Join is that absolute paths remain
+			// absolute and relative paths remaining relative — there is no harm in
+			// over-normalizing letters in the comparison, so we use EqualFold.
+			joined := filepath.Join(tt.prefix, got)
+			if clean := filepath.Clean(tt.s); !strings.EqualFold(joined, clean) {
+				t.Errorf("filepath.Join(%#q, %#q) = %#q, want %#q", tt.prefix, got, joined, clean)
+			}
+		}
+	}
+}
diff --git a/src/cmd/go/internal/test/cover.go b/src/cmd/go/internal/test/cover.go
new file mode 100644
index 0000000..f614458
--- /dev/null
+++ b/src/cmd/go/internal/test/cover.go
@@ -0,0 +1,85 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package test
+
+import (
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"sync"
+)
+
+var coverMerge struct {
+	f          *os.File
+	sync.Mutex // for f.Write
+}
+
+// initCoverProfile initializes the test coverage profile.
+// It must be run before any calls to mergeCoverProfile or closeCoverProfile.
+// Using this function clears the profile in case it existed from a previous run,
+// or in case it doesn't exist and the test is going to fail to create it (or not run).
+func initCoverProfile() {
+	if testCoverProfile == "" || testC {
+		return
+	}
+	if !filepath.IsAbs(testCoverProfile) {
+		testCoverProfile = filepath.Join(testOutputDir.getAbs(), testCoverProfile)
+	}
+
+	// No mutex - caller's responsibility to call with no racing goroutines.
+	f, err := os.Create(testCoverProfile)
+	if err != nil {
+		base.Fatalf("%v", err)
+	}
+	_, err = fmt.Fprintf(f, "mode: %s\n", cfg.BuildCoverMode)
+	if err != nil {
+		base.Fatalf("%v", err)
+	}
+	coverMerge.f = f
+}
+
+// mergeCoverProfile merges file into the profile stored in testCoverProfile.
+// It prints any errors it encounters to ew.
+func mergeCoverProfile(ew io.Writer, file string) {
+	if coverMerge.f == nil {
+		return
+	}
+	coverMerge.Lock()
+	defer coverMerge.Unlock()
+
+	expect := fmt.Sprintf("mode: %s\n", cfg.BuildCoverMode)
+	buf := make([]byte, len(expect))
+	r, err := os.Open(file)
+	if err != nil {
+		// Test did not create profile, which is OK.
+		return
+	}
+	defer r.Close()
+
+	n, err := io.ReadFull(r, buf)
+	if n == 0 {
+		return
+	}
+	if err != nil || string(buf) != expect {
+		fmt.Fprintf(ew, "error: test wrote malformed coverage profile %s.\n", file)
+		return
+	}
+	_, err = io.Copy(coverMerge.f, r)
+	if err != nil {
+		fmt.Fprintf(ew, "error: saving coverage profile: %v\n", err)
+	}
+}
+
+func closeCoverProfile() {
+	if coverMerge.f == nil {
+		return
+	}
+	if err := coverMerge.f.Close(); err != nil {
+		base.Errorf("closing coverage profile: %v", err)
+	}
+}
diff --git a/src/cmd/go/internal/test/flagdefs.go b/src/cmd/go/internal/test/flagdefs.go
new file mode 100644
index 0000000..947c27e
--- /dev/null
+++ b/src/cmd/go/internal/test/flagdefs.go
@@ -0,0 +1,77 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Code generated by genflags.go — DO NOT EDIT.
+
+package test
+
+// passFlagToTest contains the flags that should be forwarded to
+// the test binary with the prefix "test.".
+var passFlagToTest = map[string]bool{
+	"bench":                true,
+	"benchmem":             true,
+	"benchtime":            true,
+	"blockprofile":         true,
+	"blockprofilerate":     true,
+	"count":                true,
+	"coverprofile":         true,
+	"cpu":                  true,
+	"cpuprofile":           true,
+	"failfast":             true,
+	"fullpath":             true,
+	"fuzz":                 true,
+	"fuzzminimizetime":     true,
+	"fuzztime":             true,
+	"list":                 true,
+	"memprofile":           true,
+	"memprofilerate":       true,
+	"mutexprofile":         true,
+	"mutexprofilefraction": true,
+	"outputdir":            true,
+	"parallel":             true,
+	"run":                  true,
+	"short":                true,
+	"shuffle":              true,
+	"skip":                 true,
+	"timeout":              true,
+	"trace":                true,
+	"v":                    true,
+}
+
+var passAnalyzersToVet = map[string]bool{
+	"asmdecl":          true,
+	"assign":           true,
+	"atomic":           true,
+	"bool":             true,
+	"bools":            true,
+	"buildtag":         true,
+	"buildtags":        true,
+	"cgocall":          true,
+	"composites":       true,
+	"copylocks":        true,
+	"directive":        true,
+	"errorsas":         true,
+	"framepointer":     true,
+	"httpresponse":     true,
+	"ifaceassert":      true,
+	"loopclosure":      true,
+	"lostcancel":       true,
+	"methods":          true,
+	"nilfunc":          true,
+	"printf":           true,
+	"rangeloops":       true,
+	"shift":            true,
+	"sigchanyzer":      true,
+	"slog":             true,
+	"stdmethods":       true,
+	"stringintconv":    true,
+	"structtag":        true,
+	"testinggoroutine": true,
+	"tests":            true,
+	"timeformat":       true,
+	"unmarshal":        true,
+	"unreachable":      true,
+	"unsafeptr":        true,
+	"unusedresult":     true,
+}
diff --git a/src/cmd/go/internal/test/flagdefs_test.go b/src/cmd/go/internal/test/flagdefs_test.go
new file mode 100644
index 0000000..5461b2d
--- /dev/null
+++ b/src/cmd/go/internal/test/flagdefs_test.go
@@ -0,0 +1,76 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package test
+
+import (
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/test/internal/genflags"
+	"internal/testenv"
+	"maps"
+	"os"
+	"testing"
+)
+
+func TestMain(m *testing.M) {
+	cfg.SetGOROOT(testenv.GOROOT(nil), false)
+	os.Exit(m.Run())
+}
+
+func TestPassFlagToTest(t *testing.T) {
+	wantNames := genflags.ShortTestFlags()
+
+	missing := map[string]bool{}
+	for _, name := range wantNames {
+		if !passFlagToTest[name] {
+			missing[name] = true
+		}
+	}
+	if len(missing) > 0 {
+		t.Errorf("passFlagToTest is missing entries: %v", missing)
+	}
+
+	extra := maps.Clone(passFlagToTest)
+	for _, name := range wantNames {
+		delete(extra, name)
+	}
+	if len(extra) > 0 {
+		t.Errorf("passFlagToTest contains extra entries: %v", extra)
+	}
+
+	if t.Failed() {
+		t.Logf("To regenerate:\n\tgo generate cmd/go/internal/test")
+	}
+}
+
+func TestPassAnalyzersToVet(t *testing.T) {
+	testenv.MustHaveGoBuild(t) // runs 'go tool vet -flags'
+
+	wantNames, err := genflags.VetAnalyzers()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	missing := map[string]bool{}
+	for _, name := range wantNames {
+		if !passAnalyzersToVet[name] {
+			missing[name] = true
+		}
+	}
+	if len(missing) > 0 {
+		t.Errorf("passAnalyzersToVet is missing entries: %v", missing)
+	}
+
+	extra := maps.Clone(passAnalyzersToVet)
+	for _, name := range wantNames {
+		delete(extra, name)
+	}
+	if len(extra) > 0 {
+		t.Errorf("passFlagToTest contains extra entries: %v", extra)
+	}
+
+	if t.Failed() {
+		t.Logf("To regenerate:\n\tgo generate cmd/go/internal/test")
+	}
+}
diff --git a/src/cmd/go/internal/test/genflags.go b/src/cmd/go/internal/test/genflags.go
new file mode 100644
index 0000000..bb5ceb6
--- /dev/null
+++ b/src/cmd/go/internal/test/genflags.go
@@ -0,0 +1,84 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ignore
+
+package main
+
+import (
+	"bytes"
+	"log"
+	"os"
+	"os/exec"
+	"text/template"
+
+	"cmd/go/internal/test/internal/genflags"
+)
+
+func main() {
+	if err := regenerate(); err != nil {
+		log.Fatal(err)
+	}
+}
+
+func regenerate() error {
+	vetAnalyzers, err := genflags.VetAnalyzers()
+	if err != nil {
+		return err
+	}
+
+	t := template.Must(template.New("fileTemplate").Parse(fileTemplate))
+	tData := map[string][]string{
+		"testFlags":    genflags.ShortTestFlags(),
+		"vetAnalyzers": vetAnalyzers,
+	}
+	buf := bytes.NewBuffer(nil)
+	if err := t.Execute(buf, tData); err != nil {
+		return err
+	}
+
+	f, err := os.Create("flagdefs.go")
+	if err != nil {
+		return err
+	}
+
+	cmd := exec.Command("gofmt")
+	cmd.Stdin = buf
+	cmd.Stdout = f
+	cmd.Stderr = os.Stderr
+	cmdErr := cmd.Run()
+
+	if err := f.Close(); err != nil {
+		return err
+	}
+	if cmdErr != nil {
+		os.Remove(f.Name())
+		return cmdErr
+	}
+
+	return nil
+}
+
+const fileTemplate = `// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Code generated by genflags.go — DO NOT EDIT.
+
+package test
+
+// passFlagToTest contains the flags that should be forwarded to
+// the test binary with the prefix "test.".
+var passFlagToTest = map[string]bool {
+{{- range .testFlags}}
+	"{{.}}": true,
+{{- end }}
+}
+
+var passAnalyzersToVet = map[string]bool {
+{{- range .vetAnalyzers}}
+	"{{.}}": true,
+{{- end }}
+}
+`
diff --git a/src/cmd/go/internal/test/internal/genflags/testflag.go b/src/cmd/go/internal/test/internal/genflags/testflag.go
new file mode 100644
index 0000000..712428d
--- /dev/null
+++ b/src/cmd/go/internal/test/internal/genflags/testflag.go
@@ -0,0 +1,35 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package genflags
+
+import (
+	"flag"
+	"strings"
+	"testing"
+)
+
+// ShortTestFlags returns the set of "-test." flag shorthand names that end
+// users may pass to 'go test'.
+func ShortTestFlags() []string {
+	testing.Init()
+
+	var names []string
+	flag.VisitAll(func(f *flag.Flag) {
+		var name string
+		var found bool
+		if name, found = strings.CutPrefix(f.Name, "test."); !found {
+			return
+		}
+
+		switch name {
+		case "testlogfile", "paniconexit0", "fuzzcachedir", "fuzzworker", "gocoverdir":
+			// These flags are only for use by cmd/go.
+		default:
+			names = append(names, name)
+		}
+	})
+
+	return names
+}
diff --git a/src/cmd/go/internal/test/internal/genflags/vetflag.go b/src/cmd/go/internal/test/internal/genflags/vetflag.go
new file mode 100644
index 0000000..1448811
--- /dev/null
+++ b/src/cmd/go/internal/test/internal/genflags/vetflag.go
@@ -0,0 +1,68 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package genflags
+
+import (
+	"bytes"
+	"cmd/go/internal/base"
+	"encoding/json"
+	"fmt"
+	"os/exec"
+	"regexp"
+	"sort"
+)
+
+// VetAnalyzers computes analyzers and their aliases supported by vet.
+func VetAnalyzers() ([]string, error) {
+	// get supported vet flag information
+	tool := base.Tool("vet")
+	vetcmd := exec.Command(tool, "-flags")
+	out := new(bytes.Buffer)
+	vetcmd.Stdout = out
+	if err := vetcmd.Run(); err != nil {
+		return nil, fmt.Errorf("go vet: can't execute %s -flags: %v\n", tool, err)
+	}
+	var analysisFlags []struct {
+		Name  string
+		Bool  bool
+		Usage string
+	}
+	if err := json.Unmarshal(out.Bytes(), &analysisFlags); err != nil {
+		return nil, fmt.Errorf("go vet: can't unmarshal JSON from %s -flags: %v", tool, err)
+	}
+
+	// parse the flags to figure out which ones stand for analyses
+	analyzerSet := make(map[string]bool)
+	rEnable := regexp.MustCompile("^enable .+ analysis$")
+	for _, flag := range analysisFlags {
+		if rEnable.MatchString(flag.Usage) {
+			analyzerSet[flag.Name] = true
+		}
+	}
+
+	rDeprecated := regexp.MustCompile("^deprecated alias for -(?P<analyzer>(.+))$")
+	// Returns the original value matched by rDeprecated on input value.
+	// If there is no match, "" is returned.
+	originalValue := func(value string) string {
+		match := rDeprecated.FindStringSubmatch(value)
+		if len(match) < 2 {
+			return ""
+		}
+		return match[1]
+	}
+	// extract deprecated aliases for existing analyses
+	for _, flag := range analysisFlags {
+		if o := originalValue(flag.Usage); analyzerSet[o] {
+			analyzerSet[flag.Name] = true
+		}
+	}
+
+	var analyzers []string
+	for a := range analyzerSet {
+		analyzers = append(analyzers, a)
+	}
+	sort.Strings(analyzers)
+	return analyzers, nil
+}
diff --git a/src/cmd/go/internal/test/test.go b/src/cmd/go/internal/test/test.go
new file mode 100644
index 0000000..3bce026
--- /dev/null
+++ b/src/cmd/go/internal/test/test.go
@@ -0,0 +1,1942 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package test
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"internal/platform"
+	"io"
+	"io/fs"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
+	"slices"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cache"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/load"
+	"cmd/go/internal/lockedfile"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/search"
+	"cmd/go/internal/str"
+	"cmd/go/internal/trace"
+	"cmd/go/internal/work"
+	"cmd/internal/test2json"
+
+	"golang.org/x/mod/module"
+)
+
+// Break init loop.
+func init() {
+	CmdTest.Run = runTest
+}
+
+const testUsage = "go test [build/test flags] [packages] [build/test flags & test binary flags]"
+
+var CmdTest = &base.Command{
+	CustomFlags: true,
+	UsageLine:   testUsage,
+	Short:       "test packages",
+	Long: `
+'Go test' automates testing the packages named by the import paths.
+It prints a summary of the test results in the format:
+
+	ok   archive/tar   0.011s
+	FAIL archive/zip   0.022s
+	ok   compress/gzip 0.033s
+	...
+
+followed by detailed output for each failed package.
+
+'Go test' recompiles each package along with any files with names matching
+the file pattern "*_test.go".
+These additional files can contain test functions, benchmark functions, fuzz
+tests and example functions. See 'go help testfunc' for more.
+Each listed package causes the execution of a separate test binary.
+Files whose names begin with "_" (including "_test.go") or "." are ignored.
+
+Test files that declare a package with the suffix "_test" will be compiled as a
+separate package, and then linked and run with the main test binary.
+
+The go tool will ignore a directory named "testdata", making it available
+to hold ancillary data needed by the tests.
+
+As part of building a test binary, go test runs go vet on the package
+and its test source files to identify significant problems. If go vet
+finds any problems, go test reports those and does not run the test
+binary. Only a high-confidence subset of the default go vet checks are
+used. That subset is: atomic, bool, buildtags, directive, errorsas,
+ifaceassert, nilfunc, printf, and stringintconv. You can see
+the documentation for these and other vet tests via "go doc cmd/vet".
+To disable the running of go vet, use the -vet=off flag. To run all
+checks, use the -vet=all flag.
+
+All test output and summary lines are printed to the go command's
+standard output, even if the test printed them to its own standard
+error. (The go command's standard error is reserved for printing
+errors building the tests.)
+
+The go command places $GOROOT/bin at the beginning of $PATH
+in the test's environment, so that tests that execute
+'go' commands use the same 'go' as the parent 'go test' command.
+
+Go test runs in two different modes:
+
+The first, called local directory mode, occurs when go test is
+invoked with no package arguments (for example, 'go test' or 'go
+test -v'). In this mode, go test compiles the package sources and
+tests found in the current directory and then runs the resulting
+test binary. In this mode, caching (discussed below) is disabled.
+After the package test finishes, go test prints a summary line
+showing the test status ('ok' or 'FAIL'), package name, and elapsed
+time.
+
+The second, called package list mode, occurs when go test is invoked
+with explicit package arguments (for example 'go test math', 'go
+test ./...', and even 'go test .'). In this mode, go test compiles
+and tests each of the packages listed on the command line. If a
+package test passes, go test prints only the final 'ok' summary
+line. If a package test fails, go test prints the full test output.
+If invoked with the -bench or -v flag, go test prints the full
+output even for passing package tests, in order to display the
+requested benchmark results or verbose logging. After the package
+tests for all of the listed packages finish, and their output is
+printed, go test prints a final 'FAIL' status if any package test
+has failed.
+
+In package list mode only, go test caches successful package test
+results to avoid unnecessary repeated running of tests. When the
+result of a test can be recovered from the cache, go test will
+redisplay the previous output instead of running the test binary
+again. When this happens, go test prints '(cached)' in place of the
+elapsed time in the summary line.
+
+The rule for a match in the cache is that the run involves the same
+test binary and the flags on the command line come entirely from a
+restricted set of 'cacheable' test flags, defined as -benchtime, -cpu,
+-list, -parallel, -run, -short, -timeout, -failfast, and -v.
+If a run of go test has any test or non-test flags outside this set,
+the result is not cached. To disable test caching, use any test flag
+or argument other than the cacheable flags. The idiomatic way to disable
+test caching explicitly is to use -count=1. Tests that open files within
+the package's source root (usually $GOPATH) or that consult environment
+variables only match future runs in which the files and environment
+variables are unchanged. A cached test result is treated as executing
+in no time at all, so a successful package test result will be cached and
+reused regardless of -timeout setting.
+
+In addition to the build flags, the flags handled by 'go test' itself are:
+
+	-args
+	    Pass the remainder of the command line (everything after -args)
+	    to the test binary, uninterpreted and unchanged.
+	    Because this flag consumes the remainder of the command line,
+	    the package list (if present) must appear before this flag.
+
+	-c
+	    Compile the test binary to pkg.test in the current directory but do not run it
+	    (where pkg is the last element of the package's import path).
+	    The file name or target directory can be changed with the -o flag.
+
+	-exec xprog
+	    Run the test binary using xprog. The behavior is the same as
+	    in 'go run'. See 'go help run' for details.
+
+	-json
+	    Convert test output to JSON suitable for automated processing.
+	    See 'go doc test2json' for the encoding details.
+
+	-o file
+	    Compile the test binary to the named file.
+	    The test still runs (unless -c or -i is specified).
+	    If file ends in a slash or names an existing directory,
+	    the test is written to pkg.test in that directory.
+
+The test binary also accepts flags that control execution of the test; these
+flags are also accessible by 'go test'. See 'go help testflag' for details.
+
+For more about build flags, see 'go help build'.
+For more about specifying packages, see 'go help packages'.
+
+See also: go build, go vet.
+`,
+}
+
+var HelpTestflag = &base.Command{
+	UsageLine: "testflag",
+	Short:     "testing flags",
+	Long: `
+The 'go test' command takes both flags that apply to 'go test' itself
+and flags that apply to the resulting test binary.
+
+Several of the flags control profiling and write an execution profile
+suitable for "go tool pprof"; run "go tool pprof -h" for more
+information. The --alloc_space, --alloc_objects, and --show_bytes
+options of pprof control how the information is presented.
+
+The following flags are recognized by the 'go test' command and
+control the execution of any test:
+
+	-bench regexp
+	    Run only those benchmarks matching a regular expression.
+	    By default, no benchmarks are run.
+	    To run all benchmarks, use '-bench .' or '-bench=.'.
+	    The regular expression is split by unbracketed slash (/)
+	    characters into a sequence of regular expressions, and each
+	    part of a benchmark's identifier must match the corresponding
+	    element in the sequence, if any. Possible parents of matches
+	    are run with b.N=1 to identify sub-benchmarks. For example,
+	    given -bench=X/Y, top-level benchmarks matching X are run
+	    with b.N=1 to find any sub-benchmarks matching Y, which are
+	    then run in full.
+
+	-benchtime t
+	    Run enough iterations of each benchmark to take t, specified
+	    as a time.Duration (for example, -benchtime 1h30s).
+	    The default is 1 second (1s).
+	    The special syntax Nx means to run the benchmark N times
+	    (for example, -benchtime 100x).
+
+	-count n
+	    Run each test, benchmark, and fuzz seed n times (default 1).
+	    If -cpu is set, run n times for each GOMAXPROCS value.
+	    Examples are always run once. -count does not apply to
+	    fuzz tests matched by -fuzz.
+
+	-cover
+	    Enable coverage analysis.
+	    Note that because coverage works by annotating the source
+	    code before compilation, compilation and test failures with
+	    coverage enabled may report line numbers that don't correspond
+	    to the original sources.
+
+	-covermode set,count,atomic
+	    Set the mode for coverage analysis for the package[s]
+	    being tested. The default is "set" unless -race is enabled,
+	    in which case it is "atomic".
+	    The values:
+		set: bool: does this statement run?
+		count: int: how many times does this statement run?
+		atomic: int: count, but correct in multithreaded tests;
+			significantly more expensive.
+	    Sets -cover.
+
+	-coverpkg pattern1,pattern2,pattern3
+	    Apply coverage analysis in each test to packages matching the patterns.
+	    The default is for each test to analyze only the package being tested.
+	    See 'go help packages' for a description of package patterns.
+	    Sets -cover.
+
+	-cpu 1,2,4
+	    Specify a list of GOMAXPROCS values for which the tests, benchmarks or
+	    fuzz tests should be executed. The default is the current value
+	    of GOMAXPROCS. -cpu does not apply to fuzz tests matched by -fuzz.
+
+	-failfast
+	    Do not start new tests after the first test failure.
+
+	-fullpath
+	    Show full file names in the error messages.
+
+	-fuzz regexp
+	    Run the fuzz test matching the regular expression. When specified,
+	    the command line argument must match exactly one package within the
+	    main module, and regexp must match exactly one fuzz test within
+	    that package. Fuzzing will occur after tests, benchmarks, seed corpora
+	    of other fuzz tests, and examples have completed. See the Fuzzing
+	    section of the testing package documentation for details.
+
+	-fuzztime t
+	    Run enough iterations of the fuzz target during fuzzing to take t,
+	    specified as a time.Duration (for example, -fuzztime 1h30s).
+		The default is to run forever.
+	    The special syntax Nx means to run the fuzz target N times
+	    (for example, -fuzztime 1000x).
+
+	-fuzzminimizetime t
+	    Run enough iterations of the fuzz target during each minimization
+	    attempt to take t, as specified as a time.Duration (for example,
+	    -fuzzminimizetime 30s).
+		The default is 60s.
+	    The special syntax Nx means to run the fuzz target N times
+	    (for example, -fuzzminimizetime 100x).
+
+	-json
+	    Log verbose output and test results in JSON. This presents the
+	    same information as the -v flag in a machine-readable format.
+
+	-list regexp
+	    List tests, benchmarks, fuzz tests, or examples matching the regular
+	    expression. No tests, benchmarks, fuzz tests, or examples will be run.
+	    This will only list top-level tests. No subtest or subbenchmarks will be
+	    shown.
+
+	-parallel n
+	    Allow parallel execution of test functions that call t.Parallel, and
+	    fuzz targets that call t.Parallel when running the seed corpus.
+	    The value of this flag is the maximum number of tests to run
+	    simultaneously.
+	    While fuzzing, the value of this flag is the maximum number of
+	    subprocesses that may call the fuzz function simultaneously, regardless of
+	    whether T.Parallel is called.
+	    By default, -parallel is set to the value of GOMAXPROCS.
+	    Setting -parallel to values higher than GOMAXPROCS may cause degraded
+	    performance due to CPU contention, especially when fuzzing.
+	    Note that -parallel only applies within a single test binary.
+	    The 'go test' command may run tests for different packages
+	    in parallel as well, according to the setting of the -p flag
+	    (see 'go help build').
+
+	-run regexp
+	    Run only those tests, examples, and fuzz tests matching the regular
+	    expression. For tests, the regular expression is split by unbracketed
+	    slash (/) characters into a sequence of regular expressions, and each
+	    part of a test's identifier must match the corresponding element in
+	    the sequence, if any. Note that possible parents of matches are
+	    run too, so that -run=X/Y matches and runs and reports the result
+	    of all tests matching X, even those without sub-tests matching Y,
+	    because it must run them to look for those sub-tests.
+	    See also -skip.
+
+	-short
+	    Tell long-running tests to shorten their run time.
+	    It is off by default but set during all.bash so that installing
+	    the Go tree can run a sanity check but not spend time running
+	    exhaustive tests.
+
+	-shuffle off,on,N
+	    Randomize the execution order of tests and benchmarks.
+	    It is off by default. If -shuffle is set to on, then it will seed
+	    the randomizer using the system clock. If -shuffle is set to an
+	    integer N, then N will be used as the seed value. In both cases,
+	    the seed will be reported for reproducibility.
+
+	-skip regexp
+	    Run only those tests, examples, fuzz tests, and benchmarks that
+	    do not match the regular expression. Like for -run and -bench,
+	    for tests and benchmarks, the regular expression is split by unbracketed
+	    slash (/) characters into a sequence of regular expressions, and each
+	    part of a test's identifier must match the corresponding element in
+	    the sequence, if any.
+
+	-timeout d
+	    If a test binary runs longer than duration d, panic.
+	    If d is 0, the timeout is disabled.
+	    The default is 10 minutes (10m).
+
+	-v
+	    Verbose output: log all tests as they are run. Also print all
+	    text from Log and Logf calls even if the test succeeds.
+
+	-vet list
+	    Configure the invocation of "go vet" during "go test"
+	    to use the comma-separated list of vet checks.
+	    If list is empty, "go test" runs "go vet" with a curated list of
+	    checks believed to be always worth addressing.
+	    If list is "off", "go test" does not run "go vet" at all.
+
+The following flags are also recognized by 'go test' and can be used to
+profile the tests during execution:
+
+	-benchmem
+	    Print memory allocation statistics for benchmarks.
+
+	-blockprofile block.out
+	    Write a goroutine blocking profile to the specified file
+	    when all tests are complete.
+	    Writes test binary as -c would.
+
+	-blockprofilerate n
+	    Control the detail provided in goroutine blocking profiles by
+	    calling runtime.SetBlockProfileRate with n.
+	    See 'go doc runtime.SetBlockProfileRate'.
+	    The profiler aims to sample, on average, one blocking event every
+	    n nanoseconds the program spends blocked. By default,
+	    if -test.blockprofile is set without this flag, all blocking events
+	    are recorded, equivalent to -test.blockprofilerate=1.
+
+	-coverprofile cover.out
+	    Write a coverage profile to the file after all tests have passed.
+	    Sets -cover.
+
+	-cpuprofile cpu.out
+	    Write a CPU profile to the specified file before exiting.
+	    Writes test binary as -c would.
+
+	-memprofile mem.out
+	    Write an allocation profile to the file after all tests have passed.
+	    Writes test binary as -c would.
+
+	-memprofilerate n
+	    Enable more precise (and expensive) memory allocation profiles by
+	    setting runtime.MemProfileRate. See 'go doc runtime.MemProfileRate'.
+	    To profile all memory allocations, use -test.memprofilerate=1.
+
+	-mutexprofile mutex.out
+	    Write a mutex contention profile to the specified file
+	    when all tests are complete.
+	    Writes test binary as -c would.
+
+	-mutexprofilefraction n
+	    Sample 1 in n stack traces of goroutines holding a
+	    contended mutex.
+
+	-outputdir directory
+	    Place output files from profiling in the specified directory,
+	    by default the directory in which "go test" is running.
+
+	-trace trace.out
+	    Write an execution trace to the specified file before exiting.
+
+Each of these flags is also recognized with an optional 'test.' prefix,
+as in -test.v. When invoking the generated test binary (the result of
+'go test -c') directly, however, the prefix is mandatory.
+
+The 'go test' command rewrites or removes recognized flags,
+as appropriate, both before and after the optional package list,
+before invoking the test binary.
+
+For instance, the command
+
+	go test -v -myflag testdata -cpuprofile=prof.out -x
+
+will compile the test binary and then run it as
+
+	pkg.test -test.v -myflag testdata -test.cpuprofile=prof.out
+
+(The -x flag is removed because it applies only to the go command's
+execution, not to the test itself.)
+
+The test flags that generate profiles (other than for coverage) also
+leave the test binary in pkg.test for use when analyzing the profiles.
+
+When 'go test' runs a test binary, it does so from within the
+corresponding package's source code directory. Depending on the test,
+it may be necessary to do the same when invoking a generated test
+binary directly. Because that directory may be located within the
+module cache, which may be read-only and is verified by checksums, the
+test must not write to it or any other directory within the module
+unless explicitly requested by the user (such as with the -fuzz flag,
+which writes failures to testdata/fuzz).
+
+The command-line package list, if present, must appear before any
+flag not known to the go test command. Continuing the example above,
+the package list would have to appear before -myflag, but could appear
+on either side of -v.
+
+When 'go test' runs in package list mode, 'go test' caches successful
+package test results to avoid unnecessary repeated running of tests. To
+disable test caching, use any test flag or argument other than the
+cacheable flags. The idiomatic way to disable test caching explicitly
+is to use -count=1.
+
+To keep an argument for a test binary from being interpreted as a
+known flag or a package name, use -args (see 'go help test') which
+passes the remainder of the command line through to the test binary
+uninterpreted and unaltered.
+
+For instance, the command
+
+	go test -v -args -x -v
+
+will compile the test binary and then run it as
+
+	pkg.test -test.v -x -v
+
+Similarly,
+
+	go test -args math
+
+will compile the test binary and then run it as
+
+	pkg.test math
+
+In the first example, the -x and the second -v are passed through to the
+test binary unchanged and with no effect on the go command itself.
+In the second example, the argument math is passed through to the test
+binary, instead of being interpreted as the package list.
+`,
+}
+
+var HelpTestfunc = &base.Command{
+	UsageLine: "testfunc",
+	Short:     "testing functions",
+	Long: `
+The 'go test' command expects to find test, benchmark, and example functions
+in the "*_test.go" files corresponding to the package under test.
+
+A test function is one named TestXxx (where Xxx does not start with a
+lower case letter) and should have the signature,
+
+	func TestXxx(t *testing.T) { ... }
+
+A benchmark function is one named BenchmarkXxx and should have the signature,
+
+	func BenchmarkXxx(b *testing.B) { ... }
+
+A fuzz test is one named FuzzXxx and should have the signature,
+
+	func FuzzXxx(f *testing.F) { ... }
+
+An example function is similar to a test function but, instead of using
+*testing.T to report success or failure, prints output to os.Stdout.
+If the last comment in the function starts with "Output:" then the output
+is compared exactly against the comment (see examples below). If the last
+comment begins with "Unordered output:" then the output is compared to the
+comment, however the order of the lines is ignored. An example with no such
+comment is compiled but not executed. An example with no text after
+"Output:" is compiled, executed, and expected to produce no output.
+
+Godoc displays the body of ExampleXxx to demonstrate the use
+of the function, constant, or variable Xxx. An example of a method M with
+receiver type T or *T is named ExampleT_M. There may be multiple examples
+for a given function, constant, or variable, distinguished by a trailing _xxx,
+where xxx is a suffix not beginning with an upper case letter.
+
+Here is an example of an example:
+
+	func ExamplePrintln() {
+		Println("The output of\nthis example.")
+		// Output: The output of
+		// this example.
+	}
+
+Here is another example where the ordering of the output is ignored:
+
+	func ExamplePerm() {
+		for _, value := range Perm(4) {
+			fmt.Println(value)
+		}
+
+		// Unordered output: 4
+		// 2
+		// 1
+		// 3
+		// 0
+	}
+
+The entire test file is presented as the example when it contains a single
+example function, at least one other function, type, variable, or constant
+declaration, and no tests, benchmarks, or fuzz tests.
+
+See the documentation of the testing package for more information.
+`,
+}
+
+var (
+	testBench        string                            // -bench flag
+	testC            bool                              // -c flag
+	testCoverPkgs    []*load.Package                   // -coverpkg flag
+	testCoverProfile string                            // -coverprofile flag
+	testFuzz         string                            // -fuzz flag
+	testJSON         bool                              // -json flag
+	testList         string                            // -list flag
+	testO            string                            // -o flag
+	testOutputDir    outputdirFlag                     // -outputdir flag
+	testShuffle      shuffleFlag                       // -shuffle flag
+	testTimeout      time.Duration                     // -timeout flag
+	testV            testVFlag                         // -v flag
+	testVet          = vetFlag{flags: defaultVetFlags} // -vet flag
+)
+
+type testVFlag struct {
+	on   bool // -v is set in some form
+	json bool // -v=test2json is set, to make output better for test2json
+}
+
+func (*testVFlag) IsBoolFlag() bool { return true }
+
+func (f *testVFlag) Set(arg string) error {
+	if v, err := strconv.ParseBool(arg); err == nil {
+		f.on = v
+		f.json = false
+		return nil
+	}
+	if arg == "test2json" {
+		f.on = true
+		f.json = arg == "test2json"
+		return nil
+	}
+	return fmt.Errorf("invalid flag -test.v=%s", arg)
+}
+
+func (f *testVFlag) String() string {
+	if f.json {
+		return "test2json"
+	}
+	if f.on {
+		return "true"
+	}
+	return "false"
+}
+
+var (
+	testArgs []string
+	pkgArgs  []string
+	pkgs     []*load.Package
+
+	testHelp bool // -help option passed to test via -args
+
+	testKillTimeout = 100 * 365 * 24 * time.Hour // backup alarm; defaults to about a century if no timeout is set
+	testWaitDelay   time.Duration                // how long to wait for output to close after a test binary exits; zero means unlimited
+	testCacheExpire time.Time                    // ignore cached test results before this time
+
+	testBlockProfile, testCPUProfile, testMemProfile, testMutexProfile, testTrace string // profiling flag that limits test to one package
+
+	testODir = false
+)
+
+// testProfile returns the name of an arbitrary single-package profiling flag
+// that is set, if any.
+func testProfile() string {
+	switch {
+	case testBlockProfile != "":
+		return "-blockprofile"
+	case testCPUProfile != "":
+		return "-cpuprofile"
+	case testMemProfile != "":
+		return "-memprofile"
+	case testMutexProfile != "":
+		return "-mutexprofile"
+	case testTrace != "":
+		return "-trace"
+	default:
+		return ""
+	}
+}
+
+// testNeedBinary reports whether the test needs to keep the binary around.
+func testNeedBinary() bool {
+	switch {
+	case testBlockProfile != "":
+		return true
+	case testCPUProfile != "":
+		return true
+	case testMemProfile != "":
+		return true
+	case testMutexProfile != "":
+		return true
+	case testO != "":
+		return true
+	default:
+		return false
+	}
+}
+
+// testShowPass reports whether the output for a passing test should be shown.
+func testShowPass() bool {
+	return testV.on || testList != "" || testHelp
+}
+
+var defaultVetFlags = []string{
+	// TODO(rsc): Decide which tests are enabled by default.
+	// See golang.org/issue/18085.
+	// "-asmdecl",
+	// "-assign",
+	"-atomic",
+	"-bool",
+	"-buildtags",
+	// "-cgocall",
+	// "-composites",
+	// "-copylocks",
+	"-directive",
+	"-errorsas",
+	// "-httpresponse",
+	"-ifaceassert",
+	// "-lostcancel",
+	// "-methods",
+	"-nilfunc",
+	"-printf",
+	// "-rangeloops",
+	// "-shift",
+	"-slog",
+	"-stringintconv",
+	// "-structtags",
+	// "-tests",
+	// "-unreachable",
+	// "-unsafeptr",
+	// "-unusedresult",
+}
+
+func runTest(ctx context.Context, cmd *base.Command, args []string) {
+	pkgArgs, testArgs = testFlags(args)
+	modload.InitWorkfile() // The test command does custom flag processing; initialize workspaces after that.
+
+	if cfg.DebugTrace != "" {
+		var close func() error
+		var err error
+		ctx, close, err = trace.Start(ctx, cfg.DebugTrace)
+		if err != nil {
+			base.Fatalf("failed to start trace: %v", err)
+		}
+		defer func() {
+			if err := close(); err != nil {
+				base.Fatalf("failed to stop trace: %v", err)
+			}
+		}()
+	}
+
+	ctx, span := trace.StartSpan(ctx, fmt.Sprint("Running ", cmd.Name(), " command"))
+	defer span.Done()
+
+	work.FindExecCmd() // initialize cached result
+
+	work.BuildInit()
+	work.VetFlags = testVet.flags
+	work.VetExplicit = testVet.explicit
+
+	pkgOpts := load.PackageOpts{ModResolveTests: true}
+	pkgs = load.PackagesAndErrors(ctx, pkgOpts, pkgArgs)
+	load.CheckPackageErrors(pkgs)
+	if len(pkgs) == 0 {
+		base.Fatalf("no packages to test")
+	}
+
+	if testFuzz != "" {
+		if !platform.FuzzSupported(cfg.Goos, cfg.Goarch) {
+			base.Fatalf("-fuzz flag is not supported on %s/%s", cfg.Goos, cfg.Goarch)
+		}
+		if len(pkgs) != 1 {
+			base.Fatalf("cannot use -fuzz flag with multiple packages")
+		}
+		if testCoverProfile != "" {
+			base.Fatalf("cannot use -coverprofile flag with -fuzz flag")
+		}
+		if profileFlag := testProfile(); profileFlag != "" {
+			base.Fatalf("cannot use %s flag with -fuzz flag", profileFlag)
+		}
+
+		// Reject the '-fuzz' flag if the package is outside the main module.
+		// Otherwise, if fuzzing identifies a failure it could corrupt checksums in
+		// the module cache (or permanently alter the behavior of std tests for all
+		// users) by writing the failing input to the package's testdata directory.
+		// (See https://golang.org/issue/48495 and test_fuzz_modcache.txt.)
+		mainMods := modload.MainModules
+		if m := pkgs[0].Module; m != nil && m.Path != "" {
+			if !mainMods.Contains(m.Path) {
+				base.Fatalf("cannot use -fuzz flag on package outside the main module")
+			}
+		} else if pkgs[0].Standard && modload.Enabled() {
+			// Because packages in 'std' and 'cmd' are part of the standard library,
+			// they are only treated as part of a module in 'go mod' subcommands and
+			// 'go get'. However, we still don't want to accidentally corrupt their
+			// testdata during fuzzing, nor do we want to fail with surprising errors
+			// if GOROOT isn't writable (as is often the case for Go toolchains
+			// installed through package managers).
+			//
+			// If the user is requesting to fuzz a standard-library package, ensure
+			// that they are in the same module as that package (just like when
+			// fuzzing any other package).
+			if strings.HasPrefix(pkgs[0].ImportPath, "cmd/") {
+				if !mainMods.Contains("cmd") || !mainMods.InGorootSrc(module.Version{Path: "cmd"}) {
+					base.Fatalf("cannot use -fuzz flag on package outside the main module")
+				}
+			} else {
+				if !mainMods.Contains("std") || !mainMods.InGorootSrc(module.Version{Path: "std"}) {
+					base.Fatalf("cannot use -fuzz flag on package outside the main module")
+				}
+			}
+		}
+	}
+	if testProfile() != "" && len(pkgs) != 1 {
+		base.Fatalf("cannot use %s flag with multiple packages", testProfile())
+	}
+
+	if testO != "" {
+		if strings.HasSuffix(testO, "/") || strings.HasSuffix(testO, string(os.PathSeparator)) {
+			testODir = true
+		} else if fi, err := os.Stat(testO); err == nil && fi.IsDir() {
+			testODir = true
+		}
+	}
+
+	if len(pkgs) > 1 && (testC || testO != "") && !base.IsNull(testO) {
+		if testO != "" && !testODir {
+			base.Fatalf("with multiple packages, -o must refer to a directory or %s", os.DevNull)
+		}
+
+		pkgsForBinary := map[string][]*load.Package{}
+
+		for _, p := range pkgs {
+			testBinary := testBinaryName(p)
+			pkgsForBinary[testBinary] = append(pkgsForBinary[testBinary], p)
+		}
+
+		for testBinary, pkgs := range pkgsForBinary {
+			if len(pkgs) > 1 {
+				var buf strings.Builder
+				for _, pkg := range pkgs {
+					buf.WriteString(pkg.ImportPath)
+					buf.WriteString("\n")
+				}
+
+				base.Errorf("cannot write test binary %s for multiple packages:\n%s", testBinary, buf.String())
+			}
+		}
+
+		base.ExitIfErrors()
+	}
+
+	initCoverProfile()
+	defer closeCoverProfile()
+
+	// If a test timeout is finite, set our kill timeout
+	// to that timeout plus one minute. This is a backup alarm in case
+	// the test wedges with a goroutine spinning and its background
+	// timer does not get a chance to fire.
+	// Don't set this if fuzzing, since it should be able to run
+	// indefinitely.
+	if testTimeout > 0 && testFuzz == "" {
+		// The WaitDelay for the test process depends on both the OS I/O and
+		// scheduling overhead and the amount of I/O generated by the test just
+		// before it exits. We set the minimum at 5 seconds to account for the OS
+		// overhead, and scale it up from there proportional to the overall test
+		// timeout on the assumption that the time to write and read a goroutine
+		// dump from a timed-out test process scales roughly with the overall
+		// running time of the test.
+		//
+		// This is probably too generous when the timeout is very long, but it seems
+		// better to hard-code a scale factor than to hard-code a constant delay.
+		if wd := testTimeout / 10; wd < 5*time.Second {
+			testWaitDelay = 5 * time.Second
+		} else {
+			testWaitDelay = wd
+		}
+
+		// We expect the test binary to terminate itself (and dump stacks) after
+		// exactly testTimeout. We give it up to one WaitDelay or one minute,
+		// whichever is longer, to finish dumping stacks before we send it an
+		// external signal: if the process has a lot of goroutines, dumping stacks
+		// after the timeout can take a while.
+		//
+		// After the signal is delivered, the test process may have up to one
+		// additional WaitDelay to finish writing its output streams.
+		if testWaitDelay < 1*time.Minute {
+			testKillTimeout = testTimeout + 1*time.Minute
+		} else {
+			testKillTimeout = testTimeout + testWaitDelay
+		}
+	}
+
+	// Read testcache expiration time, if present.
+	// (We implement go clean -testcache by writing an expiration date
+	// instead of searching out and deleting test result cache entries.)
+	if dir := cache.DefaultDir(); dir != "off" {
+		if data, _ := lockedfile.Read(filepath.Join(dir, "testexpire.txt")); len(data) > 0 && data[len(data)-1] == '\n' {
+			if t, err := strconv.ParseInt(string(data[:len(data)-1]), 10, 64); err == nil {
+				testCacheExpire = time.Unix(0, t)
+			}
+		}
+	}
+
+	b := work.NewBuilder("")
+	defer func() {
+		if err := b.Close(); err != nil {
+			base.Fatal(err)
+		}
+	}()
+
+	var builds, runs, prints []*work.Action
+
+	if cfg.BuildCoverPkg != nil {
+		match := make([]func(*load.Package) bool, len(cfg.BuildCoverPkg))
+		for i := range cfg.BuildCoverPkg {
+			match[i] = load.MatchPackage(cfg.BuildCoverPkg[i], base.Cwd())
+		}
+
+		// Select for coverage all dependencies matching the -coverpkg
+		// patterns.
+		plist := load.TestPackageList(ctx, pkgOpts, pkgs)
+		testCoverPkgs = load.SelectCoverPackages(plist, match, "test")
+	}
+
+	// Inform the compiler that it should instrument the binary at
+	// build-time when fuzzing is enabled.
+	if testFuzz != "" {
+		// Don't instrument packages which may affect coverage guidance but are
+		// unlikely to be useful. Most of these are used by the testing or
+		// internal/fuzz packages concurrently with fuzzing.
+		var skipInstrumentation = map[string]bool{
+			"context":       true,
+			"internal/fuzz": true,
+			"reflect":       true,
+			"runtime":       true,
+			"sync":          true,
+			"sync/atomic":   true,
+			"syscall":       true,
+			"testing":       true,
+			"time":          true,
+		}
+		for _, p := range load.TestPackageList(ctx, pkgOpts, pkgs) {
+			if !skipInstrumentation[p.ImportPath] {
+				p.Internal.FuzzInstrument = true
+			}
+		}
+	}
+
+	// Collect all the packages imported by the packages being tested.
+	allImports := make(map[*load.Package]bool)
+	for _, p := range pkgs {
+		if p.Error != nil && p.Error.IsImportCycle {
+			continue
+		}
+		for _, p1 := range p.Internal.Imports {
+			allImports[p1] = true
+		}
+	}
+
+	// Prepare build + run + print actions for all packages being tested.
+	for _, p := range pkgs {
+		// sync/atomic import is inserted by the cover tool if we're
+		// using atomic mode (and not compiling sync/atomic package itself).
+		// See #18486 and #57445.
+		if cfg.BuildCover && cfg.BuildCoverMode == "atomic" &&
+			p.ImportPath != "sync/atomic" {
+			load.EnsureImport(p, "sync/atomic")
+		}
+
+		buildTest, runTest, printTest, err := builderTest(b, ctx, pkgOpts, p, allImports[p])
+		if err != nil {
+			str := err.Error()
+			str = strings.TrimPrefix(str, "\n")
+			if p.ImportPath != "" {
+				base.Errorf("# %s\n%s", p.ImportPath, str)
+			} else {
+				base.Errorf("%s", str)
+			}
+			fmt.Printf("FAIL\t%s [setup failed]\n", p.ImportPath)
+			continue
+		}
+		builds = append(builds, buildTest)
+		runs = append(runs, runTest)
+		prints = append(prints, printTest)
+	}
+
+	// Order runs for coordinating start JSON prints.
+	ch := make(chan struct{})
+	close(ch)
+	for _, a := range runs {
+		if r, ok := a.Actor.(*runTestActor); ok {
+			r.prev = ch
+			ch = make(chan struct{})
+			r.next = ch
+		}
+	}
+
+	// Ultimately the goal is to print the output.
+	root := &work.Action{Mode: "go test", Actor: work.ActorFunc(printExitStatus), Deps: prints}
+
+	// Force the printing of results to happen in order,
+	// one at a time.
+	for i, a := range prints {
+		if i > 0 {
+			a.Deps = append(a.Deps, prints[i-1])
+		}
+	}
+
+	// Force benchmarks to run in serial.
+	if !testC && (testBench != "") {
+		// The first run must wait for all builds.
+		// Later runs must wait for the previous run's print.
+		for i, run := range runs {
+			if i == 0 {
+				run.Deps = append(run.Deps, builds...)
+			} else {
+				run.Deps = append(run.Deps, prints[i-1])
+			}
+		}
+	}
+
+	b.Do(ctx, root)
+}
+
+var windowsBadWords = []string{
+	"install",
+	"patch",
+	"setup",
+	"update",
+}
+
+func builderTest(b *work.Builder, ctx context.Context, pkgOpts load.PackageOpts, p *load.Package, imported bool) (buildAction, runAction, printAction *work.Action, err error) {
+	if len(p.TestGoFiles)+len(p.XTestGoFiles) == 0 {
+		build := b.CompileAction(work.ModeBuild, work.ModeBuild, p)
+		run := &work.Action{
+			Mode:       "test run",
+			Actor:      new(runTestActor),
+			Deps:       []*work.Action{build},
+			Package:    p,
+			IgnoreFail: true, // run (prepare output) even if build failed
+		}
+		addTestVet(b, p, run, nil)
+		print := &work.Action{
+			Mode:       "test print",
+			Actor:      work.ActorFunc(builderPrintTest),
+			Deps:       []*work.Action{run},
+			Package:    p,
+			IgnoreFail: true, // print even if test failed
+		}
+		return build, run, print, nil
+	}
+
+	// Build Package structs describing:
+	//	pmain - pkg.test binary
+	//	ptest - package + test files
+	//	pxtest - package of external test files
+	var cover *load.TestCover
+	if cfg.BuildCover {
+		cover = &load.TestCover{
+			Mode:  cfg.BuildCoverMode,
+			Local: cfg.BuildCoverPkg == nil,
+			Pkgs:  testCoverPkgs,
+			Paths: cfg.BuildCoverPkg,
+		}
+	}
+	pmain, ptest, pxtest, err := load.TestPackagesFor(ctx, pkgOpts, p, cover)
+	if err != nil {
+		return nil, nil, nil, err
+	}
+
+	// If imported is true then this package is imported by some
+	// package being tested. Make building the test version of the
+	// package depend on building the non-test version, so that we
+	// only report build errors once. Issue #44624.
+	if imported && ptest != p {
+		buildTest := b.CompileAction(work.ModeBuild, work.ModeBuild, ptest)
+		buildP := b.CompileAction(work.ModeBuild, work.ModeBuild, p)
+		buildTest.Deps = append(buildTest.Deps, buildP)
+	}
+
+	testBinary := testBinaryName(p)
+
+	testDir := b.NewObjdir()
+	if err := b.Mkdir(testDir); err != nil {
+		return nil, nil, nil, err
+	}
+
+	pmain.Dir = testDir
+	pmain.Internal.OmitDebug = !testC && !testNeedBinary()
+
+	if !cfg.BuildN {
+		// writeTestmain writes _testmain.go,
+		// using the test description gathered in t.
+		if err := os.WriteFile(testDir+"_testmain.go", *pmain.Internal.TestmainGo, 0666); err != nil {
+			return nil, nil, nil, err
+		}
+	}
+
+	// Set compile objdir to testDir we've already created,
+	// so that the default file path stripping applies to _testmain.go.
+	b.CompileAction(work.ModeBuild, work.ModeBuild, pmain).Objdir = testDir
+
+	a := b.LinkAction(work.ModeBuild, work.ModeBuild, pmain)
+	a.Target = testDir + testBinary + cfg.ExeSuffix
+	if cfg.Goos == "windows" {
+		// There are many reserved words on Windows that,
+		// if used in the name of an executable, cause Windows
+		// to try to ask for extra permissions.
+		// The word list includes setup, install, update, and patch,
+		// but it does not appear to be defined anywhere.
+		// We have run into this trying to run the
+		// go.codereview/patch tests.
+		// For package names containing those words, use test.test.exe
+		// instead of pkgname.test.exe.
+		// Note that this file name is only used in the Go command's
+		// temporary directory. If the -c or other flags are
+		// given, the code below will still use pkgname.test.exe.
+		// There are two user-visible effects of this change.
+		// First, you can actually run 'go test' in directories that
+		// have names that Windows thinks are installer-like,
+		// without getting a dialog box asking for more permissions.
+		// Second, in the Windows process listing during go test,
+		// the test shows up as test.test.exe, not pkgname.test.exe.
+		// That second one is a drawback, but it seems a small
+		// price to pay for the test running at all.
+		// If maintaining the list of bad words is too onerous,
+		// we could just do this always on Windows.
+		for _, bad := range windowsBadWords {
+			if strings.Contains(testBinary, bad) {
+				a.Target = testDir + "test.test" + cfg.ExeSuffix
+				break
+			}
+		}
+	}
+	buildAction = a
+	var installAction, cleanAction *work.Action
+	if testC || testNeedBinary() {
+		// -c or profiling flag: create action to copy binary to ./test.out.
+		target := filepath.Join(base.Cwd(), testBinary+cfg.ExeSuffix)
+		isNull := false
+
+		if testO != "" {
+			target = testO
+
+			if testODir {
+				if filepath.IsAbs(target) {
+					target = filepath.Join(target, testBinary+cfg.ExeSuffix)
+				} else {
+					target = filepath.Join(base.Cwd(), target, testBinary+cfg.ExeSuffix)
+				}
+			} else {
+				if base.IsNull(target) {
+					isNull = true
+				} else if !filepath.IsAbs(target) {
+					target = filepath.Join(base.Cwd(), target)
+				}
+			}
+		}
+
+		if isNull {
+			runAction = buildAction
+		} else {
+			pmain.Target = target
+			installAction = &work.Action{
+				Mode:    "test build",
+				Actor:   work.ActorFunc(work.BuildInstallFunc),
+				Deps:    []*work.Action{buildAction},
+				Package: pmain,
+				Target:  target,
+			}
+			runAction = installAction // make sure runAction != nil even if not running test
+		}
+	}
+	var vetRunAction *work.Action
+	if testC {
+		printAction = &work.Action{Mode: "test print (nop)", Package: p, Deps: []*work.Action{runAction}} // nop
+		vetRunAction = printAction
+	} else {
+		// run test
+		r := new(runTestActor)
+		runAction = &work.Action{
+			Mode:       "test run",
+			Actor:      r,
+			Deps:       []*work.Action{buildAction},
+			Package:    p,
+			IgnoreFail: true, // run (prepare output) even if build failed
+			TryCache:   r.c.tryCache,
+			Objdir:     testDir,
+		}
+		vetRunAction = runAction
+		cleanAction = &work.Action{
+			Mode:       "test clean",
+			Actor:      work.ActorFunc(builderCleanTest),
+			Deps:       []*work.Action{runAction},
+			Package:    p,
+			IgnoreFail: true, // clean even if test failed
+			Objdir:     testDir,
+		}
+		printAction = &work.Action{
+			Mode:       "test print",
+			Actor:      work.ActorFunc(builderPrintTest),
+			Deps:       []*work.Action{cleanAction},
+			Package:    p,
+			IgnoreFail: true, // print even if test failed
+		}
+	}
+
+	if len(ptest.GoFiles)+len(ptest.CgoFiles) > 0 {
+		addTestVet(b, ptest, vetRunAction, installAction)
+	}
+	if pxtest != nil {
+		addTestVet(b, pxtest, vetRunAction, installAction)
+	}
+
+	if installAction != nil {
+		if runAction != installAction {
+			installAction.Deps = append(installAction.Deps, runAction)
+		}
+		if cleanAction != nil {
+			cleanAction.Deps = append(cleanAction.Deps, installAction)
+		}
+	}
+
+	return buildAction, runAction, printAction, nil
+}
+
+func addTestVet(b *work.Builder, p *load.Package, runAction, installAction *work.Action) {
+	if testVet.off {
+		return
+	}
+
+	vet := b.VetAction(work.ModeBuild, work.ModeBuild, p)
+	runAction.Deps = append(runAction.Deps, vet)
+	// Install will clean the build directory.
+	// Make sure vet runs first.
+	// The install ordering in b.VetAction does not apply here
+	// because we are using a custom installAction (created above).
+	if installAction != nil {
+		installAction.Deps = append(installAction.Deps, vet)
+	}
+}
+
+var noTestsToRun = []byte("\ntesting: warning: no tests to run\n")
+var noFuzzTestsToFuzz = []byte("\ntesting: warning: no fuzz tests to fuzz\n")
+var tooManyFuzzTestsToFuzz = []byte("\ntesting: warning: -fuzz matches more than one fuzz test, won't fuzz\n")
+
+// runTestActor is the actor for running a test.
+type runTestActor struct {
+	c runCache
+
+	// sequencing of json start messages, to preserve test order
+	prev <-chan struct{} // wait to start until prev is closed
+	next chan<- struct{} // close next once the next test can start.
+}
+
+// runCache is the cache for running a single test.
+type runCache struct {
+	disableCache bool // cache should be disabled for this run
+
+	buf *bytes.Buffer
+	id1 cache.ActionID
+	id2 cache.ActionID
+}
+
+// stdoutMu and lockedStdout provide a locked standard output
+// that guarantees never to interlace writes from multiple
+// goroutines, so that we can have multiple JSON streams writing
+// to a lockedStdout simultaneously and know that events will
+// still be intelligible.
+var stdoutMu sync.Mutex
+
+type lockedStdout struct{}
+
+func (lockedStdout) Write(b []byte) (int, error) {
+	stdoutMu.Lock()
+	defer stdoutMu.Unlock()
+	return os.Stdout.Write(b)
+}
+
+func (r *runTestActor) Act(b *work.Builder, ctx context.Context, a *work.Action) error {
+	// Wait for previous test to get started and print its first json line.
+	select {
+	case <-r.prev:
+	case <-base.Interrupted:
+		// We can't wait for the previous test action to complete: we don't start
+		// new actions after an interrupt, so if that action wasn't already running
+		// it might never happen. Instead, just don't log anything for this action.
+		base.SetExitStatus(1)
+		return nil
+	}
+
+	if a.Failed {
+		// We were unable to build the binary.
+		a.Failed = false
+		a.TestOutput = new(bytes.Buffer)
+		fmt.Fprintf(a.TestOutput, "FAIL\t%s [build failed]\n", a.Package.ImportPath)
+		base.SetExitStatus(1)
+
+		// release next test to start
+		close(r.next)
+		return nil
+	}
+
+	var stdout io.Writer = os.Stdout
+	var err error
+	if testJSON {
+		json := test2json.NewConverter(lockedStdout{}, a.Package.ImportPath, test2json.Timestamp)
+		defer func() {
+			json.Exited(err)
+			json.Close()
+		}()
+		stdout = json
+	}
+
+	// Release next test to start (test2json.NewConverter writes the start event).
+	close(r.next)
+
+	if p := a.Package; len(p.TestGoFiles)+len(p.XTestGoFiles) == 0 {
+		fmt.Fprintf(stdout, "?   \t%s\t[no test files]\n", p.ImportPath)
+		return nil
+	}
+
+	var buf bytes.Buffer
+	if len(pkgArgs) == 0 || testBench != "" || testFuzz != "" {
+		// Stream test output (no buffering) when no package has
+		// been given on the command line (implicit current directory)
+		// or when benchmarking or fuzzing.
+		// No change to stdout.
+	} else {
+		// If we're only running a single package under test or if parallelism is
+		// set to 1, and if we're displaying all output (testShowPass), we can
+		// hurry the output along, echoing it as soon as it comes in.
+		// We still have to copy to &buf for caching the result. This special
+		// case was introduced in Go 1.5 and is intentionally undocumented:
+		// the exact details of output buffering are up to the go command and
+		// subject to change. It would be nice to remove this special case
+		// entirely, but it is surely very helpful to see progress being made
+		// when tests are run on slow single-CPU ARM systems.
+		//
+		// If we're showing JSON output, then display output as soon as
+		// possible even when multiple tests are being run: the JSON output
+		// events are attributed to specific package tests, so interlacing them
+		// is OK.
+		if testShowPass() && (len(pkgs) == 1 || cfg.BuildP == 1) || testJSON {
+			// Write both to stdout and buf, for possible saving
+			// to cache, and for looking for the "no tests to run" message.
+			stdout = io.MultiWriter(stdout, &buf)
+		} else {
+			stdout = &buf
+		}
+	}
+
+	if r.c.buf == nil {
+		// We did not find a cached result using the link step action ID,
+		// so we ran the link step. Try again now with the link output
+		// content ID. The attempt using the action ID makes sure that
+		// if the link inputs don't change, we reuse the cached test
+		// result without even rerunning the linker. The attempt using
+		// the link output (test binary) content ID makes sure that if
+		// we have different link inputs but the same final binary,
+		// we still reuse the cached test result.
+		// c.saveOutput will store the result under both IDs.
+		r.c.tryCacheWithID(b, a, a.Deps[0].BuildContentID())
+	}
+	if r.c.buf != nil {
+		if stdout != &buf {
+			stdout.Write(r.c.buf.Bytes())
+			r.c.buf.Reset()
+		}
+		a.TestOutput = r.c.buf
+		return nil
+	}
+
+	execCmd := work.FindExecCmd()
+	testlogArg := []string{}
+	if !r.c.disableCache && len(execCmd) == 0 {
+		testlogArg = []string{"-test.testlogfile=" + a.Objdir + "testlog.txt"}
+	}
+	panicArg := "-test.paniconexit0"
+	fuzzArg := []string{}
+	if testFuzz != "" {
+		fuzzCacheDir := filepath.Join(cache.Default().FuzzDir(), a.Package.ImportPath)
+		fuzzArg = []string{"-test.fuzzcachedir=" + fuzzCacheDir}
+	}
+	coverdirArg := []string{}
+	addToEnv := ""
+	if cfg.BuildCover {
+		gcd := filepath.Join(a.Objdir, "gocoverdir")
+		if err := b.Mkdir(gcd); err != nil {
+			// If we can't create a temp dir, terminate immediately
+			// with an error as opposed to returning an error to the
+			// caller; failed MkDir most likely indicates that we're
+			// out of disk space or there is some other systemic error
+			// that will make forward progress unlikely.
+			base.Fatalf("failed to create temporary dir: %v", err)
+		}
+		coverdirArg = append(coverdirArg, "-test.gocoverdir="+gcd)
+		// Even though we are passing the -test.gocoverdir option to
+		// the test binary, also set GOCOVERDIR as well. This is
+		// intended to help with tests that run "go build" to build
+		// fresh copies of tools to test as part of the testing.
+		addToEnv = "GOCOVERDIR=" + gcd
+	}
+	args := str.StringList(execCmd, a.Deps[0].BuiltTarget(), testlogArg, panicArg, fuzzArg, coverdirArg, testArgs)
+
+	if testCoverProfile != "" {
+		// Write coverage to temporary profile, for merging later.
+		for i, arg := range args {
+			if strings.HasPrefix(arg, "-test.coverprofile=") {
+				args[i] = "-test.coverprofile=" + a.Objdir + "_cover_.out"
+			}
+		}
+	}
+
+	if cfg.BuildN || cfg.BuildX {
+		b.Showcmd("", "%s", strings.Join(args, " "))
+		if cfg.BuildN {
+			return nil
+		}
+	}
+
+	// Normally, the test will terminate itself when the timeout expires,
+	// but add a last-ditch deadline to detect and stop wedged binaries.
+	ctx, cancel := context.WithTimeout(ctx, testKillTimeout)
+	defer cancel()
+
+	// Now we're ready to actually run the command.
+	//
+	// If the -o flag is set, or if at some point we change cmd/go to start
+	// copying test executables into the build cache, we may run into spurious
+	// ETXTBSY errors on Unix platforms (see https://go.dev/issue/22315).
+	//
+	// Since we know what causes those, and we know that they should resolve
+	// quickly (the ETXTBSY error will resolve as soon as the subprocess
+	// holding the descriptor open reaches its 'exec' call), we retry them
+	// in a loop.
+
+	var (
+		cmd            *exec.Cmd
+		t0             time.Time
+		cancelKilled   = false
+		cancelSignaled = false
+	)
+	for {
+		cmd = exec.CommandContext(ctx, args[0], args[1:]...)
+		cmd.Dir = a.Package.Dir
+
+		env := slices.Clip(cfg.OrigEnv)
+		env = base.AppendPATH(env)
+		env = base.AppendPWD(env, cmd.Dir)
+		cmd.Env = env
+		if addToEnv != "" {
+			cmd.Env = append(cmd.Env, addToEnv)
+		}
+
+		cmd.Stdout = stdout
+		cmd.Stderr = stdout
+
+		// If there are any local SWIG dependencies, we want to load
+		// the shared library from the build directory.
+		if a.Package.UsesSwig() {
+			env := cmd.Env
+			found := false
+			prefix := "LD_LIBRARY_PATH="
+			for i, v := range env {
+				if strings.HasPrefix(v, prefix) {
+					env[i] = v + ":."
+					found = true
+					break
+				}
+			}
+			if !found {
+				env = append(env, "LD_LIBRARY_PATH=.")
+			}
+			cmd.Env = env
+		}
+
+		cmd.Cancel = func() error {
+			if base.SignalTrace == nil {
+				err := cmd.Process.Kill()
+				if err == nil {
+					cancelKilled = true
+				}
+				return err
+			}
+
+			// Send a quit signal in the hope that the program will print
+			// a stack trace and exit.
+			err := cmd.Process.Signal(base.SignalTrace)
+			if err == nil {
+				cancelSignaled = true
+			}
+			return err
+		}
+		cmd.WaitDelay = testWaitDelay
+
+		base.StartSigHandlers()
+		t0 = time.Now()
+		err = cmd.Run()
+
+		if !isETXTBSY(err) {
+			// We didn't hit the race in #22315, so there is no reason to retry the
+			// command.
+			break
+		}
+	}
+
+	out := buf.Bytes()
+	a.TestOutput = &buf
+	t := fmt.Sprintf("%.3fs", time.Since(t0).Seconds())
+
+	mergeCoverProfile(cmd.Stdout, a.Objdir+"_cover_.out")
+
+	if err == nil {
+		norun := ""
+		if !testShowPass() && !testJSON {
+			buf.Reset()
+		}
+		if bytes.HasPrefix(out, noTestsToRun[1:]) || bytes.Contains(out, noTestsToRun) {
+			norun = " [no tests to run]"
+		}
+		if bytes.HasPrefix(out, noFuzzTestsToFuzz[1:]) || bytes.Contains(out, noFuzzTestsToFuzz) {
+			norun = " [no fuzz tests to fuzz]"
+		}
+		if bytes.HasPrefix(out, tooManyFuzzTestsToFuzz[1:]) || bytes.Contains(out, tooManyFuzzTestsToFuzz) {
+			norun = "[-fuzz matches more than one fuzz test, won't fuzz]"
+		}
+		if len(out) > 0 && !bytes.HasSuffix(out, []byte("\n")) {
+			// Ensure that the output ends with a newline before the "ok"
+			// line we're about to print (https://golang.org/issue/49317).
+			cmd.Stdout.Write([]byte("\n"))
+		}
+		fmt.Fprintf(cmd.Stdout, "ok  \t%s\t%s%s%s\n", a.Package.ImportPath, t, coveragePercentage(out), norun)
+		r.c.saveOutput(a)
+	} else {
+		base.SetExitStatus(1)
+		if cancelSignaled {
+			fmt.Fprintf(cmd.Stdout, "*** Test killed with %v: ran too long (%v).\n", base.SignalTrace, testKillTimeout)
+		} else if cancelKilled {
+			fmt.Fprintf(cmd.Stdout, "*** Test killed: ran too long (%v).\n", testKillTimeout)
+		} else if errors.Is(err, exec.ErrWaitDelay) {
+			fmt.Fprintf(cmd.Stdout, "*** Test I/O incomplete %v after exiting.\n", cmd.WaitDelay)
+		}
+		var ee *exec.ExitError
+		if len(out) == 0 || !errors.As(err, &ee) || !ee.Exited() {
+			// If there was no test output, print the exit status so that the reason
+			// for failure is clear.
+			fmt.Fprintf(cmd.Stdout, "%s\n", err)
+		} else if !bytes.HasSuffix(out, []byte("\n")) {
+			// Otherwise, ensure that the output ends with a newline before the FAIL
+			// line we're about to print (https://golang.org/issue/49317).
+			cmd.Stdout.Write([]byte("\n"))
+		}
+
+		// NOTE(golang.org/issue/37555): test2json reports that a test passes
+		// unless "FAIL" is printed at the beginning of a line. The test may not
+		// actually print that if it panics, exits, or terminates abnormally,
+		// so we print it here. We can't always check whether it was printed
+		// because some tests need stdout to be a terminal (golang.org/issue/34791),
+		// not a pipe.
+		// TODO(golang.org/issue/29062): tests that exit with status 0 without
+		// printing a final result should fail.
+		prefix := ""
+		if testJSON || testV.json {
+			prefix = "\x16"
+		}
+		fmt.Fprintf(cmd.Stdout, "%sFAIL\t%s\t%s\n", prefix, a.Package.ImportPath, t)
+	}
+
+	if cmd.Stdout != &buf {
+		buf.Reset() // cmd.Stdout was going to os.Stdout already
+	}
+	return nil
+}
+
+// tryCache is called just before the link attempt,
+// to see if the test result is cached and therefore the link is unneeded.
+// It reports whether the result can be satisfied from cache.
+func (c *runCache) tryCache(b *work.Builder, a *work.Action) bool {
+	return c.tryCacheWithID(b, a, a.Deps[0].BuildActionID())
+}
+
+func (c *runCache) tryCacheWithID(b *work.Builder, a *work.Action, id string) bool {
+	if len(pkgArgs) == 0 {
+		// Caching does not apply to "go test",
+		// only to "go test foo" (including "go test .").
+		if cache.DebugTest {
+			fmt.Fprintf(os.Stderr, "testcache: caching disabled in local directory mode\n")
+		}
+		c.disableCache = true
+		return false
+	}
+
+	if a.Package.Root == "" {
+		// Caching does not apply to tests outside of any module, GOPATH, or GOROOT.
+		if cache.DebugTest {
+			fmt.Fprintf(os.Stderr, "testcache: caching disabled for package outside of module root, GOPATH, or GOROOT: %s\n", a.Package.ImportPath)
+		}
+		c.disableCache = true
+		return false
+	}
+
+	var cacheArgs []string
+	for _, arg := range testArgs {
+		i := strings.Index(arg, "=")
+		if i < 0 || !strings.HasPrefix(arg, "-test.") {
+			if cache.DebugTest {
+				fmt.Fprintf(os.Stderr, "testcache: caching disabled for test argument: %s\n", arg)
+			}
+			c.disableCache = true
+			return false
+		}
+		switch arg[:i] {
+		case "-test.benchtime",
+			"-test.cpu",
+			"-test.list",
+			"-test.parallel",
+			"-test.run",
+			"-test.short",
+			"-test.timeout",
+			"-test.failfast",
+			"-test.v":
+			// These are cacheable.
+			// Note that this list is documented above,
+			// so if you add to this list, update the docs too.
+			cacheArgs = append(cacheArgs, arg)
+
+		default:
+			// nothing else is cacheable
+			if cache.DebugTest {
+				fmt.Fprintf(os.Stderr, "testcache: caching disabled for test argument: %s\n", arg)
+			}
+			c.disableCache = true
+			return false
+		}
+	}
+
+	// The test cache result fetch is a two-level lookup.
+	//
+	// First, we use the content hash of the test binary
+	// and its command-line arguments to find the
+	// list of environment variables and files consulted
+	// the last time the test was run with those arguments.
+	// (To avoid unnecessary links, we store this entry
+	// under two hashes: id1 uses the linker inputs as a
+	// proxy for the test binary, and id2 uses the actual
+	// test binary. If the linker inputs are unchanged,
+	// this way we avoid the link step, even though we
+	// do not cache link outputs.)
+	//
+	// Second, we compute a hash of the values of the
+	// environment variables and the content of the files
+	// listed in the log from the previous run.
+	// Then we look up test output using a combination of
+	// the hash from the first part (testID) and the hash of the
+	// test inputs (testInputsID).
+	//
+	// In order to store a new test result, we must redo the
+	// testInputsID computation using the log from the run
+	// we want to cache, and then we store that new log and
+	// the new outputs.
+
+	h := cache.NewHash("testResult")
+	fmt.Fprintf(h, "test binary %s args %q execcmd %q", id, cacheArgs, work.ExecCmd)
+	testID := h.Sum()
+	if c.id1 == (cache.ActionID{}) {
+		c.id1 = testID
+	} else {
+		c.id2 = testID
+	}
+	if cache.DebugTest {
+		fmt.Fprintf(os.Stderr, "testcache: %s: test ID %x => %x\n", a.Package.ImportPath, id, testID)
+	}
+
+	// Load list of referenced environment variables and files
+	// from last run of testID, and compute hash of that content.
+	data, entry, err := cache.GetBytes(cache.Default(), testID)
+	if !bytes.HasPrefix(data, testlogMagic) || data[len(data)-1] != '\n' {
+		if cache.DebugTest {
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "testcache: %s: input list not found: %v\n", a.Package.ImportPath, err)
+			} else {
+				fmt.Fprintf(os.Stderr, "testcache: %s: input list malformed\n", a.Package.ImportPath)
+			}
+		}
+		return false
+	}
+	testInputsID, err := computeTestInputsID(a, data)
+	if err != nil {
+		return false
+	}
+	if cache.DebugTest {
+		fmt.Fprintf(os.Stderr, "testcache: %s: test ID %x => input ID %x => %x\n", a.Package.ImportPath, testID, testInputsID, testAndInputKey(testID, testInputsID))
+	}
+
+	// Parse cached result in preparation for changing run time to "(cached)".
+	// If we can't parse the cached result, don't use it.
+	data, entry, err = cache.GetBytes(cache.Default(), testAndInputKey(testID, testInputsID))
+	if len(data) == 0 || data[len(data)-1] != '\n' {
+		if cache.DebugTest {
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "testcache: %s: test output not found: %v\n", a.Package.ImportPath, err)
+			} else {
+				fmt.Fprintf(os.Stderr, "testcache: %s: test output malformed\n", a.Package.ImportPath)
+			}
+		}
+		return false
+	}
+	if entry.Time.Before(testCacheExpire) {
+		if cache.DebugTest {
+			fmt.Fprintf(os.Stderr, "testcache: %s: test output expired due to go clean -testcache\n", a.Package.ImportPath)
+		}
+		return false
+	}
+	i := bytes.LastIndexByte(data[:len(data)-1], '\n') + 1
+	if !bytes.HasPrefix(data[i:], []byte("ok  \t")) {
+		if cache.DebugTest {
+			fmt.Fprintf(os.Stderr, "testcache: %s: test output malformed\n", a.Package.ImportPath)
+		}
+		return false
+	}
+	j := bytes.IndexByte(data[i+len("ok  \t"):], '\t')
+	if j < 0 {
+		if cache.DebugTest {
+			fmt.Fprintf(os.Stderr, "testcache: %s: test output malformed\n", a.Package.ImportPath)
+		}
+		return false
+	}
+	j += i + len("ok  \t") + 1
+
+	// Committed to printing.
+	c.buf = new(bytes.Buffer)
+	c.buf.Write(data[:j])
+	c.buf.WriteString("(cached)")
+	for j < len(data) && ('0' <= data[j] && data[j] <= '9' || data[j] == '.' || data[j] == 's') {
+		j++
+	}
+	c.buf.Write(data[j:])
+	return true
+}
+
+var errBadTestInputs = errors.New("error parsing test inputs")
+var testlogMagic = []byte("# test log\n") // known to testing/internal/testdeps/deps.go
+
+// computeTestInputsID computes the "test inputs ID"
+// (see comment in tryCacheWithID above) for the
+// test log.
+func computeTestInputsID(a *work.Action, testlog []byte) (cache.ActionID, error) {
+	testlog = bytes.TrimPrefix(testlog, testlogMagic)
+	h := cache.NewHash("testInputs")
+	pwd := a.Package.Dir
+	for _, line := range bytes.Split(testlog, []byte("\n")) {
+		if len(line) == 0 {
+			continue
+		}
+		s := string(line)
+		op, name, found := strings.Cut(s, " ")
+		if !found {
+			if cache.DebugTest {
+				fmt.Fprintf(os.Stderr, "testcache: %s: input list malformed (%q)\n", a.Package.ImportPath, line)
+			}
+			return cache.ActionID{}, errBadTestInputs
+		}
+		switch op {
+		default:
+			if cache.DebugTest {
+				fmt.Fprintf(os.Stderr, "testcache: %s: input list malformed (%q)\n", a.Package.ImportPath, line)
+			}
+			return cache.ActionID{}, errBadTestInputs
+		case "getenv":
+			fmt.Fprintf(h, "env %s %x\n", name, hashGetenv(name))
+		case "chdir":
+			pwd = name // always absolute
+			fmt.Fprintf(h, "chdir %s %x\n", name, hashStat(name))
+		case "stat":
+			if !filepath.IsAbs(name) {
+				name = filepath.Join(pwd, name)
+			}
+			if a.Package.Root == "" || search.InDir(name, a.Package.Root) == "" {
+				// Do not recheck files outside the module, GOPATH, or GOROOT root.
+				break
+			}
+			fmt.Fprintf(h, "stat %s %x\n", name, hashStat(name))
+		case "open":
+			if !filepath.IsAbs(name) {
+				name = filepath.Join(pwd, name)
+			}
+			if a.Package.Root == "" || search.InDir(name, a.Package.Root) == "" {
+				// Do not recheck files outside the module, GOPATH, or GOROOT root.
+				break
+			}
+			fh, err := hashOpen(name)
+			if err != nil {
+				if cache.DebugTest {
+					fmt.Fprintf(os.Stderr, "testcache: %s: input file %s: %s\n", a.Package.ImportPath, name, err)
+				}
+				return cache.ActionID{}, err
+			}
+			fmt.Fprintf(h, "open %s %x\n", name, fh)
+		}
+	}
+	sum := h.Sum()
+	return sum, nil
+}
+
+func hashGetenv(name string) cache.ActionID {
+	h := cache.NewHash("getenv")
+	v, ok := os.LookupEnv(name)
+	if !ok {
+		h.Write([]byte{0})
+	} else {
+		h.Write([]byte{1})
+		h.Write([]byte(v))
+	}
+	return h.Sum()
+}
+
+const modTimeCutoff = 2 * time.Second
+
+var errFileTooNew = errors.New("file used as input is too new")
+
+func hashOpen(name string) (cache.ActionID, error) {
+	h := cache.NewHash("open")
+	info, err := os.Stat(name)
+	if err != nil {
+		fmt.Fprintf(h, "err %v\n", err)
+		return h.Sum(), nil
+	}
+	hashWriteStat(h, info)
+	if info.IsDir() {
+		files, err := os.ReadDir(name)
+		if err != nil {
+			fmt.Fprintf(h, "err %v\n", err)
+		}
+		for _, f := range files {
+			fmt.Fprintf(h, "file %s ", f.Name())
+			finfo, err := f.Info()
+			if err != nil {
+				fmt.Fprintf(h, "err %v\n", err)
+			} else {
+				hashWriteStat(h, finfo)
+			}
+		}
+	} else if info.Mode().IsRegular() {
+		// Because files might be very large, do not attempt
+		// to hash the entirety of their content. Instead assume
+		// the mtime and size recorded in hashWriteStat above
+		// are good enough.
+		//
+		// To avoid problems for very recent files where a new
+		// write might not change the mtime due to file system
+		// mtime precision, reject caching if a file was read that
+		// is less than modTimeCutoff old.
+		if time.Since(info.ModTime()) < modTimeCutoff {
+			return cache.ActionID{}, errFileTooNew
+		}
+	}
+	return h.Sum(), nil
+}
+
+func hashStat(name string) cache.ActionID {
+	h := cache.NewHash("stat")
+	if info, err := os.Stat(name); err != nil {
+		fmt.Fprintf(h, "err %v\n", err)
+	} else {
+		hashWriteStat(h, info)
+	}
+	if info, err := os.Lstat(name); err != nil {
+		fmt.Fprintf(h, "err %v\n", err)
+	} else {
+		hashWriteStat(h, info)
+	}
+	return h.Sum()
+}
+
+func hashWriteStat(h io.Writer, info fs.FileInfo) {
+	fmt.Fprintf(h, "stat %d %x %v %v\n", info.Size(), uint64(info.Mode()), info.ModTime(), info.IsDir())
+}
+
+// testAndInputKey returns the actual cache key for the pair (testID, testInputsID).
+func testAndInputKey(testID, testInputsID cache.ActionID) cache.ActionID {
+	return cache.Subkey(testID, fmt.Sprintf("inputs:%x", testInputsID))
+}
+
+func (c *runCache) saveOutput(a *work.Action) {
+	if c.id1 == (cache.ActionID{}) && c.id2 == (cache.ActionID{}) {
+		return
+	}
+
+	// See comment about two-level lookup in tryCacheWithID above.
+	testlog, err := os.ReadFile(a.Objdir + "testlog.txt")
+	if err != nil || !bytes.HasPrefix(testlog, testlogMagic) || testlog[len(testlog)-1] != '\n' {
+		if cache.DebugTest {
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "testcache: %s: reading testlog: %v\n", a.Package.ImportPath, err)
+			} else {
+				fmt.Fprintf(os.Stderr, "testcache: %s: reading testlog: malformed\n", a.Package.ImportPath)
+			}
+		}
+		return
+	}
+	testInputsID, err := computeTestInputsID(a, testlog)
+	if err != nil {
+		return
+	}
+	if c.id1 != (cache.ActionID{}) {
+		if cache.DebugTest {
+			fmt.Fprintf(os.Stderr, "testcache: %s: save test ID %x => input ID %x => %x\n", a.Package.ImportPath, c.id1, testInputsID, testAndInputKey(c.id1, testInputsID))
+		}
+		cache.PutNoVerify(cache.Default(), c.id1, bytes.NewReader(testlog))
+		cache.PutNoVerify(cache.Default(), testAndInputKey(c.id1, testInputsID), bytes.NewReader(a.TestOutput.Bytes()))
+	}
+	if c.id2 != (cache.ActionID{}) {
+		if cache.DebugTest {
+			fmt.Fprintf(os.Stderr, "testcache: %s: save test ID %x => input ID %x => %x\n", a.Package.ImportPath, c.id2, testInputsID, testAndInputKey(c.id2, testInputsID))
+		}
+		cache.PutNoVerify(cache.Default(), c.id2, bytes.NewReader(testlog))
+		cache.PutNoVerify(cache.Default(), testAndInputKey(c.id2, testInputsID), bytes.NewReader(a.TestOutput.Bytes()))
+	}
+}
+
+// coveragePercentage returns the coverage results (if enabled) for the
+// test. It uncovers the data by scanning the output from the test run.
+func coveragePercentage(out []byte) string {
+	if !cfg.BuildCover {
+		return ""
+	}
+	// The string looks like
+	//	test coverage for encoding/binary: 79.9% of statements
+	// Extract the piece from the percentage to the end of the line.
+	re := regexp.MustCompile(`coverage: (.*)\n`)
+	matches := re.FindSubmatch(out)
+	if matches == nil {
+		// Probably running "go test -cover" not "go test -cover fmt".
+		// The coverage output will appear in the output directly.
+		return ""
+	}
+	return fmt.Sprintf("\tcoverage: %s", matches[1])
+}
+
+// builderCleanTest is the action for cleaning up after a test.
+func builderCleanTest(b *work.Builder, ctx context.Context, a *work.Action) error {
+	if cfg.BuildWork {
+		return nil
+	}
+	if cfg.BuildX {
+		b.Showcmd("", "rm -r %s", a.Objdir)
+	}
+	os.RemoveAll(a.Objdir)
+	return nil
+}
+
+// builderPrintTest is the action for printing a test result.
+func builderPrintTest(b *work.Builder, ctx context.Context, a *work.Action) error {
+	clean := a.Deps[0]
+	run := clean.Deps[0]
+	if run.TestOutput != nil {
+		os.Stdout.Write(run.TestOutput.Bytes())
+		run.TestOutput = nil
+	}
+	return nil
+}
+
+// printExitStatus is the action for printing the final exit status.
+// If we are running multiple test targets, print a final "FAIL"
+// in case a failure in an early package has already scrolled
+// off of the user's terminal.
+// (See https://golang.org/issue/30507#issuecomment-470593235.)
+//
+// In JSON mode, we need to maintain valid JSON output and
+// we assume that the test output is being parsed by a tool
+// anyway, so the failure will not be missed and would be
+// awkward to try to wedge into the JSON stream.
+//
+// In fuzz mode, we only allow a single package for now
+// (see CL 350156 and https://golang.org/issue/46312),
+// so there is no possibility of scrolling off and no need
+// to print the final status.
+func printExitStatus(b *work.Builder, ctx context.Context, a *work.Action) error {
+	if !testJSON && testFuzz == "" && len(pkgArgs) != 0 {
+		if base.GetExitStatus() != 0 {
+			fmt.Println("FAIL")
+			return nil
+		}
+	}
+	return nil
+}
+
+// testBinaryName can be used to create name for test binary executable.
+// Use last element of import path, not package name.
+// They differ when package name is "main".
+// But if the import path is "command-line-arguments",
+// like it is during 'go run', use the package name.
+func testBinaryName(p *load.Package) string {
+	var elem string
+	if p.ImportPath == "command-line-arguments" {
+		elem = p.Name
+	} else {
+		elem = p.DefaultExecName()
+	}
+
+	return elem + ".test"
+}
diff --git a/src/cmd/go/internal/test/test_nonunix.go b/src/cmd/go/internal/test/test_nonunix.go
new file mode 100644
index 0000000..df84487
--- /dev/null
+++ b/src/cmd/go/internal/test/test_nonunix.go
@@ -0,0 +1,12 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !unix
+
+package test
+
+func isETXTBSY(err error) bool {
+	// syscall.ETXTBSY is only meaningful on Unix platforms.
+	return false
+}
diff --git a/src/cmd/go/internal/test/test_unix.go b/src/cmd/go/internal/test/test_unix.go
new file mode 100644
index 0000000..f50ef98
--- /dev/null
+++ b/src/cmd/go/internal/test/test_unix.go
@@ -0,0 +1,16 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix
+
+package test
+
+import (
+	"errors"
+	"syscall"
+)
+
+func isETXTBSY(err error) bool {
+	return errors.Is(err, syscall.ETXTBSY)
+}
diff --git a/src/cmd/go/internal/test/testflag.go b/src/cmd/go/internal/test/testflag.go
new file mode 100644
index 0000000..4253788
--- /dev/null
+++ b/src/cmd/go/internal/test/testflag.go
@@ -0,0 +1,416 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package test
+
+import (
+	"cmd/go/internal/base"
+	"cmd/go/internal/cmdflag"
+	"cmd/go/internal/work"
+	"errors"
+	"flag"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"time"
+)
+
+//go:generate go run ./genflags.go
+
+// The flag handling part of go test is large and distracting.
+// We can't use (*flag.FlagSet).Parse because some of the flags from
+// our command line are for us, and some are for the test binary, and
+// some are for both.
+
+func init() {
+	work.AddBuildFlags(CmdTest, work.OmitVFlag)
+
+	cf := CmdTest.Flag
+	cf.BoolVar(&testC, "c", false, "")
+	cf.StringVar(&testO, "o", "", "")
+	work.AddCoverFlags(CmdTest, &testCoverProfile)
+	cf.Var((*base.StringsFlag)(&work.ExecCmd), "exec", "")
+	cf.BoolVar(&testJSON, "json", false, "")
+	cf.Var(&testVet, "vet", "")
+
+	// Register flags to be forwarded to the test binary. We retain variables for
+	// some of them so that cmd/go knows what to do with the test output, or knows
+	// to build the test in a way that supports the use of the flag.
+
+	cf.StringVar(&testBench, "bench", "", "")
+	cf.Bool("benchmem", false, "")
+	cf.String("benchtime", "", "")
+	cf.StringVar(&testBlockProfile, "blockprofile", "", "")
+	cf.String("blockprofilerate", "", "")
+	cf.Int("count", 0, "")
+	cf.String("cpu", "", "")
+	cf.StringVar(&testCPUProfile, "cpuprofile", "", "")
+	cf.Bool("failfast", false, "")
+	cf.StringVar(&testFuzz, "fuzz", "", "")
+	cf.Bool("fullpath", false, "")
+	cf.StringVar(&testList, "list", "", "")
+	cf.StringVar(&testMemProfile, "memprofile", "", "")
+	cf.String("memprofilerate", "", "")
+	cf.StringVar(&testMutexProfile, "mutexprofile", "", "")
+	cf.String("mutexprofilefraction", "", "")
+	cf.Var(&testOutputDir, "outputdir", "")
+	cf.Int("parallel", 0, "")
+	cf.String("run", "", "")
+	cf.Bool("short", false, "")
+	cf.String("skip", "", "")
+	cf.DurationVar(&testTimeout, "timeout", 10*time.Minute, "") // known to cmd/dist
+	cf.String("fuzztime", "", "")
+	cf.String("fuzzminimizetime", "", "")
+	cf.StringVar(&testTrace, "trace", "", "")
+	cf.Var(&testV, "v", "")
+	cf.Var(&testShuffle, "shuffle", "")
+
+	for name, ok := range passFlagToTest {
+		if ok {
+			cf.Var(cf.Lookup(name).Value, "test."+name, "")
+		}
+	}
+}
+
+// outputdirFlag implements the -outputdir flag.
+// It interprets an empty value as the working directory of the 'go' command.
+type outputdirFlag struct {
+	abs string
+}
+
+func (f *outputdirFlag) String() string {
+	return f.abs
+}
+
+func (f *outputdirFlag) Set(value string) (err error) {
+	if value == "" {
+		f.abs = ""
+	} else {
+		f.abs, err = filepath.Abs(value)
+	}
+	return err
+}
+
+func (f *outputdirFlag) getAbs() string {
+	if f.abs == "" {
+		return base.Cwd()
+	}
+	return f.abs
+}
+
+// vetFlag implements the special parsing logic for the -vet flag:
+// a comma-separated list, with distinguished values "all" and
+// "off", plus a boolean tracking whether it was set explicitly.
+//
+// "all" is encoded as vetFlag{true, false, nil}, since it will
+// pass no flags to the vet binary, and by default, it runs all
+// analyzers.
+type vetFlag struct {
+	explicit bool
+	off      bool
+	flags    []string // passed to vet when invoked automatically during 'go test'
+}
+
+func (f *vetFlag) String() string {
+	switch {
+	case !f.off && !f.explicit && len(f.flags) == 0:
+		return "all"
+	case f.off:
+		return "off"
+	}
+
+	var buf strings.Builder
+	for i, f := range f.flags {
+		if i > 0 {
+			buf.WriteByte(',')
+		}
+		buf.WriteString(f)
+	}
+	return buf.String()
+}
+
+func (f *vetFlag) Set(value string) error {
+	switch {
+	case value == "":
+		*f = vetFlag{flags: defaultVetFlags}
+		return nil
+	case strings.Contains(value, "="):
+		return fmt.Errorf("-vet argument cannot contain equal signs")
+	case strings.Contains(value, " "):
+		return fmt.Errorf("-vet argument is comma-separated list, cannot contain spaces")
+	}
+
+	*f = vetFlag{explicit: true}
+	var single string
+	for _, arg := range strings.Split(value, ",") {
+		switch arg {
+		case "":
+			return fmt.Errorf("-vet argument contains empty list element")
+		case "all":
+			single = arg
+			*f = vetFlag{explicit: true}
+			continue
+		case "off":
+			single = arg
+			*f = vetFlag{
+				explicit: true,
+				off:      true,
+			}
+			continue
+		default:
+			if _, ok := passAnalyzersToVet[arg]; !ok {
+				return fmt.Errorf("-vet argument must be a supported analyzer or a distinguished value; found %s", arg)
+			}
+			f.flags = append(f.flags, "-"+arg)
+		}
+	}
+	if len(f.flags) > 1 && single != "" {
+		return fmt.Errorf("-vet does not accept %q in a list with other analyzers", single)
+	}
+	if len(f.flags) > 1 && single != "" {
+		return fmt.Errorf("-vet does not accept %q in a list with other analyzers", single)
+	}
+	return nil
+}
+
+type shuffleFlag struct {
+	on   bool
+	seed *int64
+}
+
+func (f *shuffleFlag) String() string {
+	if !f.on {
+		return "off"
+	}
+	if f.seed == nil {
+		return "on"
+	}
+	return fmt.Sprintf("%d", *f.seed)
+}
+
+func (f *shuffleFlag) Set(value string) error {
+	if value == "off" {
+		*f = shuffleFlag{on: false}
+		return nil
+	}
+
+	if value == "on" {
+		*f = shuffleFlag{on: true}
+		return nil
+	}
+
+	seed, err := strconv.ParseInt(value, 10, 64)
+	if err != nil {
+		return fmt.Errorf(`-shuffle argument must be "on", "off", or an int64: %v`, err)
+	}
+
+	*f = shuffleFlag{on: true, seed: &seed}
+	return nil
+}
+
+// testFlags processes the command line, grabbing -x and -c, rewriting known flags
+// to have "test" before them, and reading the command line for the test binary.
+// Unfortunately for us, we need to do our own flag processing because go test
+// grabs some flags but otherwise its command line is just a holding place for
+// pkg.test's arguments.
+// We allow known flags both before and after the package name list,
+// to allow both
+//
+//	go test fmt -custom-flag-for-fmt-test
+//	go test -x math
+func testFlags(args []string) (packageNames, passToTest []string) {
+	base.SetFromGOFLAGS(&CmdTest.Flag)
+	addFromGOFLAGS := map[string]bool{}
+	CmdTest.Flag.Visit(func(f *flag.Flag) {
+		if short := strings.TrimPrefix(f.Name, "test."); passFlagToTest[short] {
+			addFromGOFLAGS[f.Name] = true
+		}
+	})
+
+	// firstUnknownFlag helps us report an error when flags not known to 'go
+	// test' are used along with -i or -c.
+	firstUnknownFlag := ""
+
+	explicitArgs := make([]string, 0, len(args))
+	inPkgList := false
+	afterFlagWithoutValue := false
+	for len(args) > 0 {
+		f, remainingArgs, err := cmdflag.ParseOne(&CmdTest.Flag, args)
+
+		wasAfterFlagWithoutValue := afterFlagWithoutValue
+		afterFlagWithoutValue = false // provisionally
+
+		if errors.Is(err, flag.ErrHelp) {
+			exitWithUsage()
+		}
+
+		if errors.Is(err, cmdflag.ErrFlagTerminator) {
+			// 'go list' allows package arguments to be named either before or after
+			// the terminator, but 'go test' has historically allowed them only
+			// before. Preserve that behavior and treat all remaining arguments —
+			// including the terminator itself! — as arguments to the test.
+			explicitArgs = append(explicitArgs, args...)
+			break
+		}
+
+		if nf := (cmdflag.NonFlagError{}); errors.As(err, &nf) {
+			if !inPkgList && packageNames != nil {
+				// We already saw the package list previously, and this argument is not
+				// a flag, so it — and everything after it — must be either a value for
+				// a preceding flag or a literal argument to the test binary.
+				if wasAfterFlagWithoutValue {
+					// This argument could syntactically be a flag value, so
+					// optimistically assume that it is and keep looking for go command
+					// flags after it.
+					//
+					// (If we're wrong, we'll at least be consistent with historical
+					// behavior; see https://golang.org/issue/40763.)
+					explicitArgs = append(explicitArgs, nf.RawArg)
+					args = remainingArgs
+					continue
+				} else {
+					// This argument syntactically cannot be a flag value, so it must be a
+					// positional argument, and so must everything after it.
+					explicitArgs = append(explicitArgs, args...)
+					break
+				}
+			}
+
+			inPkgList = true
+			packageNames = append(packageNames, nf.RawArg)
+			args = remainingArgs // Consume the package name.
+			continue
+		}
+
+		if inPkgList {
+			// This argument is syntactically a flag, so if we were in the package
+			// list we're not anymore.
+			inPkgList = false
+		}
+
+		if nd := (cmdflag.FlagNotDefinedError{}); errors.As(err, &nd) {
+			// This is a flag we do not know. We must assume that any args we see
+			// after this might be flag arguments, not package names, so make
+			// packageNames non-nil to indicate that the package list is complete.
+			//
+			// (Actually, we only strictly need to assume that if the flag is not of
+			// the form -x=value, but making this more precise would be a breaking
+			// change in the command line API.)
+			if packageNames == nil {
+				packageNames = []string{}
+			}
+
+			if nd.RawArg == "-args" || nd.RawArg == "--args" {
+				// -args or --args signals that everything that follows
+				// should be passed to the test.
+				explicitArgs = append(explicitArgs, remainingArgs...)
+				break
+			}
+
+			if firstUnknownFlag == "" {
+				firstUnknownFlag = nd.RawArg
+			}
+
+			explicitArgs = append(explicitArgs, nd.RawArg)
+			args = remainingArgs
+			if !nd.HasValue {
+				afterFlagWithoutValue = true
+			}
+			continue
+		}
+
+		if err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			exitWithUsage()
+		}
+
+		if short := strings.TrimPrefix(f.Name, "test."); passFlagToTest[short] {
+			explicitArgs = append(explicitArgs, fmt.Sprintf("-test.%s=%v", short, f.Value))
+
+			// This flag has been overridden explicitly, so don't forward its implicit
+			// value from GOFLAGS.
+			delete(addFromGOFLAGS, short)
+			delete(addFromGOFLAGS, "test."+short)
+		}
+
+		args = remainingArgs
+	}
+	if firstUnknownFlag != "" && testC {
+		fmt.Fprintf(os.Stderr, "go: unknown flag %s cannot be used with -c\n", firstUnknownFlag)
+		exitWithUsage()
+	}
+
+	var injectedFlags []string
+	if testJSON {
+		// If converting to JSON, we need the full output in order to pipe it to test2json.
+		// The -test.v=test2json flag is like -test.v=true but causes the test to add
+		// extra ^V characters before testing output lines and other framing,
+		// which helps test2json do a better job creating the JSON events.
+		injectedFlags = append(injectedFlags, "-test.v=test2json")
+		delete(addFromGOFLAGS, "v")
+		delete(addFromGOFLAGS, "test.v")
+	}
+
+	// Inject flags from GOFLAGS before the explicit command-line arguments.
+	// (They must appear before the flag terminator or first non-flag argument.)
+	// Also determine whether flags with awkward defaults have already been set.
+	var timeoutSet, outputDirSet bool
+	CmdTest.Flag.Visit(func(f *flag.Flag) {
+		short := strings.TrimPrefix(f.Name, "test.")
+		if addFromGOFLAGS[f.Name] {
+			injectedFlags = append(injectedFlags, fmt.Sprintf("-test.%s=%v", short, f.Value))
+		}
+		switch short {
+		case "timeout":
+			timeoutSet = true
+		case "outputdir":
+			outputDirSet = true
+		}
+	})
+
+	// 'go test' has a default timeout, but the test binary itself does not.
+	// If the timeout wasn't set (and forwarded) explicitly, add the default
+	// timeout to the command line.
+	if testTimeout > 0 && !timeoutSet {
+		injectedFlags = append(injectedFlags, fmt.Sprintf("-test.timeout=%v", testTimeout))
+	}
+
+	// Similarly, the test binary defaults -test.outputdir to its own working
+	// directory, but 'go test' defaults it to the working directory of the 'go'
+	// command. Set it explicitly if it is needed due to some other flag that
+	// requests output.
+	if testProfile() != "" && !outputDirSet {
+		injectedFlags = append(injectedFlags, "-test.outputdir="+testOutputDir.getAbs())
+	}
+
+	// If the user is explicitly passing -help or -h, show output
+	// of the test binary so that the help output is displayed
+	// even though the test will exit with success.
+	// This loop is imperfect: it will do the wrong thing for a case
+	// like -args -test.outputdir -help. Such cases are probably rare,
+	// and getting this wrong doesn't do too much harm.
+helpLoop:
+	for _, arg := range explicitArgs {
+		switch arg {
+		case "--":
+			break helpLoop
+		case "-h", "-help", "--help":
+			testHelp = true
+			break helpLoop
+		}
+	}
+
+	// Forward any unparsed arguments (following --args) to the test binary.
+	return packageNames, append(injectedFlags, explicitArgs...)
+}
+
+func exitWithUsage() {
+	fmt.Fprintf(os.Stderr, "usage: %s\n", CmdTest.UsageLine)
+	fmt.Fprintf(os.Stderr, "Run 'go help %s' and 'go help %s' for details.\n", CmdTest.LongName(), HelpTestflag.LongName())
+
+	base.SetExitStatus(2)
+	base.Exit()
+}
diff --git a/src/cmd/go/internal/tool/tool.go b/src/cmd/go/internal/tool/tool.go
new file mode 100644
index 0000000..ebe189b
--- /dev/null
+++ b/src/cmd/go/internal/tool/tool.go
@@ -0,0 +1,224 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package tool implements the “go tool” command.
+package tool
+
+import (
+	"context"
+	"encoding/json"
+	"flag"
+	"fmt"
+	"go/build"
+	"internal/platform"
+	"os"
+	"os/exec"
+	"os/signal"
+	"sort"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+)
+
+var CmdTool = &base.Command{
+	Run:       runTool,
+	UsageLine: "go tool [-n] command [args...]",
+	Short:     "run specified go tool",
+	Long: `
+Tool runs the go tool command identified by the arguments.
+With no arguments it prints the list of known tools.
+
+The -n flag causes tool to print the command that would be
+executed but not execute it.
+
+For more about each tool command, see 'go doc cmd/<command>'.
+`,
+}
+
+var toolN bool
+
+// Return whether tool can be expected in the gccgo tool directory.
+// Other binaries could be in the same directory so don't
+// show those with the 'go tool' command.
+func isGccgoTool(tool string) bool {
+	switch tool {
+	case "cgo", "fix", "cover", "godoc", "vet":
+		return true
+	}
+	return false
+}
+
+func init() {
+	base.AddChdirFlag(&CmdTool.Flag)
+	CmdTool.Flag.BoolVar(&toolN, "n", false, "")
+}
+
+func runTool(ctx context.Context, cmd *base.Command, args []string) {
+	if len(args) == 0 {
+		listTools()
+		return
+	}
+	toolName := args[0]
+	// The tool name must be lower-case letters, numbers or underscores.
+	for _, c := range toolName {
+		switch {
+		case 'a' <= c && c <= 'z', '0' <= c && c <= '9', c == '_':
+		default:
+			fmt.Fprintf(os.Stderr, "go: bad tool name %q\n", toolName)
+			base.SetExitStatus(2)
+			return
+		}
+	}
+
+	toolPath, err := base.ToolPath(toolName)
+	if err != nil {
+		if toolName == "dist" && len(args) > 1 && args[1] == "list" {
+			// cmd/distpack removes the 'dist' tool from the toolchain to save space,
+			// since it is normally only used for building the toolchain in the first
+			// place. However, 'go tool dist list' is useful for listing all supported
+			// platforms.
+			//
+			// If the dist tool does not exist, impersonate this command.
+			if impersonateDistList(args[2:]) {
+				return
+			}
+		}
+
+		// Emit the usual error for the missing tool.
+		_ = base.Tool(toolName)
+	}
+
+	if toolN {
+		cmd := toolPath
+		if len(args) > 1 {
+			cmd += " " + strings.Join(args[1:], " ")
+		}
+		fmt.Printf("%s\n", cmd)
+		return
+	}
+	args[0] = toolPath // in case the tool wants to re-exec itself, e.g. cmd/dist
+	toolCmd := &exec.Cmd{
+		Path:   toolPath,
+		Args:   args,
+		Stdin:  os.Stdin,
+		Stdout: os.Stdout,
+		Stderr: os.Stderr,
+	}
+	err = toolCmd.Start()
+	if err == nil {
+		c := make(chan os.Signal, 100)
+		signal.Notify(c)
+		go func() {
+			for sig := range c {
+				toolCmd.Process.Signal(sig)
+			}
+		}()
+		err = toolCmd.Wait()
+		signal.Stop(c)
+		close(c)
+	}
+	if err != nil {
+		// Only print about the exit status if the command
+		// didn't even run (not an ExitError) or it didn't exit cleanly
+		// or we're printing command lines too (-x mode).
+		// Assume if command exited cleanly (even with non-zero status)
+		// it printed any messages it wanted to print.
+		if e, ok := err.(*exec.ExitError); !ok || !e.Exited() || cfg.BuildX {
+			fmt.Fprintf(os.Stderr, "go tool %s: %s\n", toolName, err)
+		}
+		base.SetExitStatus(1)
+		return
+	}
+}
+
+// listTools prints a list of the available tools in the tools directory.
+func listTools() {
+	f, err := os.Open(build.ToolDir)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "go: no tool directory: %s\n", err)
+		base.SetExitStatus(2)
+		return
+	}
+	defer f.Close()
+	names, err := f.Readdirnames(-1)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "go: can't read tool directory: %s\n", err)
+		base.SetExitStatus(2)
+		return
+	}
+
+	sort.Strings(names)
+	for _, name := range names {
+		// Unify presentation by going to lower case.
+		// If it's windows, don't show the .exe suffix.
+		name = strings.TrimSuffix(strings.ToLower(name), cfg.ToolExeSuffix())
+
+		// The tool directory used by gccgo will have other binaries
+		// in addition to go tools. Only display go tools here.
+		if cfg.BuildToolchainName == "gccgo" && !isGccgoTool(name) {
+			continue
+		}
+		fmt.Println(name)
+	}
+}
+
+func impersonateDistList(args []string) (handled bool) {
+	fs := flag.NewFlagSet("go tool dist list", flag.ContinueOnError)
+	jsonFlag := fs.Bool("json", false, "produce JSON output")
+	brokenFlag := fs.Bool("broken", false, "include broken ports")
+
+	// The usage for 'go tool dist' claims that
+	// “All commands take -v flags to emit extra information”,
+	// but list -v appears not to have any effect.
+	_ = fs.Bool("v", false, "emit extra information")
+
+	if err := fs.Parse(args); err != nil || len(fs.Args()) > 0 {
+		// Unrecognized flag or argument.
+		// Force fallback to the real 'go tool dist'.
+		return false
+	}
+
+	if !*jsonFlag {
+		for _, p := range platform.List {
+			if !*brokenFlag && platform.Broken(p.GOOS, p.GOARCH) {
+				continue
+			}
+			fmt.Println(p)
+		}
+		return true
+	}
+
+	type jsonResult struct {
+		GOOS         string
+		GOARCH       string
+		CgoSupported bool
+		FirstClass   bool
+		Broken       bool `json:",omitempty"`
+	}
+
+	var results []jsonResult
+	for _, p := range platform.List {
+		broken := platform.Broken(p.GOOS, p.GOARCH)
+		if broken && !*brokenFlag {
+			continue
+		}
+		if *jsonFlag {
+			results = append(results, jsonResult{
+				GOOS:         p.GOOS,
+				GOARCH:       p.GOARCH,
+				CgoSupported: platform.CgoSupported(p.GOOS, p.GOARCH),
+				FirstClass:   platform.FirstClass(p.GOOS, p.GOARCH),
+				Broken:       broken,
+			})
+		}
+	}
+	out, err := json.MarshalIndent(results, "", "\t")
+	if err != nil {
+		return false
+	}
+
+	os.Stdout.Write(out)
+	return true
+}
diff --git a/src/cmd/go/internal/toolchain/exec.go b/src/cmd/go/internal/toolchain/exec.go
new file mode 100644
index 0000000..820fe93
--- /dev/null
+++ b/src/cmd/go/internal/toolchain/exec.go
@@ -0,0 +1,55 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !js && !wasip1
+
+package toolchain
+
+import (
+	"cmd/go/internal/base"
+	"internal/godebug"
+	"os"
+	"os/exec"
+	"runtime"
+	"syscall"
+)
+
+// execGoToolchain execs the Go toolchain with the given name (gotoolchain),
+// GOROOT directory, and go command executable.
+// The GOROOT directory is empty if we are invoking a command named
+// gotoolchain found in $PATH.
+func execGoToolchain(gotoolchain, dir, exe string) {
+	os.Setenv(targetEnv, gotoolchain)
+	if dir == "" {
+		os.Unsetenv("GOROOT")
+	} else {
+		os.Setenv("GOROOT", dir)
+	}
+
+	// On Windows, there is no syscall.Exec, so the best we can do
+	// is run a subprocess and exit with the same status.
+	// Doing the same on Unix would be a problem because it wouldn't
+	// propagate signals and such, but there are no signals on Windows.
+	// We also use the exec case when GODEBUG=gotoolchainexec=0,
+	// to allow testing this code even when not on Windows.
+	if godebug.New("#gotoolchainexec").Value() == "0" || runtime.GOOS == "windows" {
+		cmd := exec.Command(exe, os.Args[1:]...)
+		cmd.Stdin = os.Stdin
+		cmd.Stdout = os.Stdout
+		cmd.Stderr = os.Stderr
+		err := cmd.Run()
+		if err != nil {
+			if e, ok := err.(*exec.ExitError); ok && e.ProcessState != nil {
+				if e.ProcessState.Exited() {
+					os.Exit(e.ProcessState.ExitCode())
+				}
+				base.Fatalf("exec %s: %s", gotoolchain, e.ProcessState)
+			}
+			base.Fatalf("exec %s: %s", exe, err)
+		}
+		os.Exit(0)
+	}
+	err := syscall.Exec(exe, os.Args, os.Environ())
+	base.Fatalf("exec %s: %v", gotoolchain, err)
+}
diff --git a/src/cmd/go/internal/toolchain/exec_stub.go b/src/cmd/go/internal/toolchain/exec_stub.go
new file mode 100644
index 0000000..e212379
--- /dev/null
+++ b/src/cmd/go/internal/toolchain/exec_stub.go
@@ -0,0 +1,13 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build js || wasip1
+
+package toolchain
+
+import "cmd/go/internal/base"
+
+func execGoToolchain(gotoolchain, dir, exe string) {
+	base.Fatalf("execGoToolchain unsupported")
+}
diff --git a/src/cmd/go/internal/toolchain/path_none.go b/src/cmd/go/internal/toolchain/path_none.go
new file mode 100644
index 0000000..8fdf71a
--- /dev/null
+++ b/src/cmd/go/internal/toolchain/path_none.go
@@ -0,0 +1,21 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !unix && !plan9 && !windows
+
+package toolchain
+
+import "io/fs"
+
+// pathDirs returns the directories in the system search path.
+func pathDirs() []string {
+	return nil
+}
+
+// pathVersion returns the Go version implemented by the file
+// described by de and info in directory dir.
+// The analysis only uses the name itself; it does not run the program.
+func pathVersion(dir string, de fs.DirEntry, info fs.FileInfo) (string, bool) {
+	return "", false
+}
diff --git a/src/cmd/go/internal/toolchain/path_plan9.go b/src/cmd/go/internal/toolchain/path_plan9.go
new file mode 100644
index 0000000..3f836a0
--- /dev/null
+++ b/src/cmd/go/internal/toolchain/path_plan9.go
@@ -0,0 +1,29 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package toolchain
+
+import (
+	"io/fs"
+	"os"
+	"path/filepath"
+
+	"cmd/go/internal/gover"
+)
+
+// pathDirs returns the directories in the system search path.
+func pathDirs() []string {
+	return filepath.SplitList(os.Getenv("path"))
+}
+
+// pathVersion returns the Go version implemented by the file
+// described by de and info in directory dir.
+// The analysis only uses the name itself; it does not run the program.
+func pathVersion(dir string, de fs.DirEntry, info fs.FileInfo) (string, bool) {
+	v := gover.FromToolchain(de.Name())
+	if v == "" || info.Mode()&0111 == 0 {
+		return "", false
+	}
+	return v, true
+}
diff --git a/src/cmd/go/internal/toolchain/path_unix.go b/src/cmd/go/internal/toolchain/path_unix.go
new file mode 100644
index 0000000..519c53e
--- /dev/null
+++ b/src/cmd/go/internal/toolchain/path_unix.go
@@ -0,0 +1,46 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix
+
+package toolchain
+
+import (
+	"internal/syscall/unix"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"syscall"
+
+	"cmd/go/internal/gover"
+)
+
+// pathDirs returns the directories in the system search path.
+func pathDirs() []string {
+	return filepath.SplitList(os.Getenv("PATH"))
+}
+
+// pathVersion returns the Go version implemented by the file
+// described by de and info in directory dir.
+// The analysis only uses the name itself; it does not run the program.
+func pathVersion(dir string, de fs.DirEntry, info fs.FileInfo) (string, bool) {
+	v := gover.FromToolchain(de.Name())
+	if v == "" {
+		return "", false
+	}
+
+	// Mimicking exec.findExecutable here.
+	// ENOSYS means Eaccess is not available or not implemented.
+	// EPERM can be returned by Linux containers employing seccomp.
+	// In both cases, fall back to checking the permission bits.
+	err := unix.Eaccess(filepath.Join(dir, de.Name()), unix.X_OK)
+	if (err == syscall.ENOSYS || err == syscall.EPERM) && info.Mode()&0111 != 0 {
+		err = nil
+	}
+	if err != nil {
+		return "", false
+	}
+
+	return v, true
+}
diff --git a/src/cmd/go/internal/toolchain/path_windows.go b/src/cmd/go/internal/toolchain/path_windows.go
new file mode 100644
index 0000000..086c591
--- /dev/null
+++ b/src/cmd/go/internal/toolchain/path_windows.go
@@ -0,0 +1,78 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package toolchain
+
+import (
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+
+	"cmd/go/internal/gover"
+)
+
+// pathExts is a cached PATHEXT list.
+var pathExts struct {
+	once sync.Once
+	list []string
+}
+
+func initPathExts() {
+	var exts []string
+	x := os.Getenv(`PATHEXT`)
+	if x != "" {
+		for _, e := range strings.Split(strings.ToLower(x), `;`) {
+			if e == "" {
+				continue
+			}
+			if e[0] != '.' {
+				e = "." + e
+			}
+			exts = append(exts, e)
+		}
+	} else {
+		exts = []string{".com", ".exe", ".bat", ".cmd"}
+	}
+	pathExts.list = exts
+}
+
+// pathDirs returns the directories in the system search path.
+func pathDirs() []string {
+	return filepath.SplitList(os.Getenv("PATH"))
+}
+
+// pathVersion returns the Go version implemented by the file
+// described by de and info in directory dir.
+// The analysis only uses the name itself; it does not run the program.
+func pathVersion(dir string, de fs.DirEntry, info fs.FileInfo) (string, bool) {
+	pathExts.once.Do(initPathExts)
+	name, _, ok := cutExt(de.Name(), pathExts.list)
+	if !ok {
+		return "", false
+	}
+	v := gover.FromToolchain(name)
+	if v == "" {
+		return "", false
+	}
+	return v, true
+}
+
+// cutExt looks for any of the known extensions at the end of file.
+// If one is found, cutExt returns the file name with the extension trimmed,
+// the extension itself, and true to signal that an extension was found.
+// Otherwise cutExt returns file, "", false.
+func cutExt(file string, exts []string) (name, ext string, found bool) {
+	i := strings.LastIndex(file, ".")
+	if i < 0 {
+		return file, "", false
+	}
+	for _, x := range exts {
+		if strings.EqualFold(file[i:], x) {
+			return file[:i], file[i:], true
+		}
+	}
+	return file, "", false
+}
diff --git a/src/cmd/go/internal/toolchain/select.go b/src/cmd/go/internal/toolchain/select.go
new file mode 100644
index 0000000..3446a48
--- /dev/null
+++ b/src/cmd/go/internal/toolchain/select.go
@@ -0,0 +1,649 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package toolchain implements dynamic switching of Go toolchains.
+package toolchain
+
+import (
+	"context"
+	"errors"
+	"flag"
+	"fmt"
+	"go/build"
+	"io/fs"
+	"log"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"strconv"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/run"
+	"cmd/go/internal/work"
+
+	"golang.org/x/mod/module"
+)
+
+const (
+	// We download golang.org/toolchain version v0.0.1-<gotoolchain>.<goos>-<goarch>.
+	// If the 0.0.1 indicates anything at all, its the version of the toolchain packaging:
+	// if for some reason we needed to change the way toolchains are packaged into
+	// module zip files in a future version of Go, we could switch to v0.0.2 and then
+	// older versions expecting the old format could use v0.0.1 and newer versions
+	// would use v0.0.2. Of course, then we'd also have to publish two of each
+	// module zip file. It's not likely we'll ever need to change this.
+	gotoolchainModule  = "golang.org/toolchain"
+	gotoolchainVersion = "v0.0.1"
+
+	// targetEnv is a special environment variable set to the expected
+	// toolchain version during the toolchain switch by the parent
+	// process and cleared in the child process. When set, that indicates
+	// to the child to confirm that it provides the expected toolchain version.
+	targetEnv = "GOTOOLCHAIN_INTERNAL_SWITCH_VERSION"
+
+	// countEnv is a special environment variable
+	// that is incremented during each toolchain switch, to detect loops.
+	// It is cleared before invoking programs in 'go run', 'go test', 'go generate', and 'go tool'
+	// by invoking them in an environment filtered with FilterEnv,
+	// so user programs should not see this in their environment.
+	countEnv = "GOTOOLCHAIN_INTERNAL_SWITCH_COUNT"
+
+	// maxSwitch is the maximum toolchain switching depth.
+	// Most uses should never see more than three.
+	// (Perhaps one for the initial GOTOOLCHAIN dispatch,
+	// a second for go get doing an upgrade, and a third if
+	// for some reason the chosen upgrade version is too small
+	// by a little.)
+	// When the count reaches maxSwitch - 10, we start logging
+	// the switched versions for debugging before crashing with
+	// a fatal error upon reaching maxSwitch.
+	// That should be enough to see the repetition.
+	maxSwitch = 100
+)
+
+// FilterEnv returns a copy of env with internal GOTOOLCHAIN environment
+// variables filtered out.
+func FilterEnv(env []string) []string {
+	// Note: Don't need to filter out targetEnv because Switch does that.
+	var out []string
+	for _, e := range env {
+		if strings.HasPrefix(e, countEnv+"=") {
+			continue
+		}
+		out = append(out, e)
+	}
+	return out
+}
+
+// Select invokes a different Go toolchain if directed by
+// the GOTOOLCHAIN environment variable or the user's configuration
+// or go.mod file.
+// It must be called early in startup.
+// See https://go.dev/doc/toolchain#select.
+func Select() {
+	log.SetPrefix("go: ")
+	defer log.SetPrefix("")
+
+	if !modload.WillBeEnabled() {
+		return
+	}
+
+	// As a special case, let "go env GOTOOLCHAIN" and "go env -w GOTOOLCHAIN=..."
+	// be handled by the local toolchain, since an older toolchain may not understand it.
+	// This provides an easy way out of "go env -w GOTOOLCHAIN=go1.19" and makes
+	// sure that "go env GOTOOLCHAIN" always prints the local go command's interpretation of it.
+	// We look for these specific command lines in order to avoid mishandling
+	//
+	//	GOTOOLCHAIN=go1.999 go env -newflag GOTOOLCHAIN
+	//
+	// where -newflag is a flag known to Go 1.999 but not known to us.
+	if (len(os.Args) == 3 && os.Args[1] == "env" && os.Args[2] == "GOTOOLCHAIN") ||
+		(len(os.Args) == 4 && os.Args[1] == "env" && os.Args[2] == "-w" && strings.HasPrefix(os.Args[3], "GOTOOLCHAIN=")) {
+		return
+	}
+
+	// Interpret GOTOOLCHAIN to select the Go toolchain to run.
+	gotoolchain := cfg.Getenv("GOTOOLCHAIN")
+	gover.Startup.GOTOOLCHAIN = gotoolchain
+	if gotoolchain == "" {
+		// cfg.Getenv should fall back to $GOROOT/go.env,
+		// so this should not happen, unless a packager
+		// has deleted the GOTOOLCHAIN line from go.env.
+		// It can also happen if GOROOT is missing or broken,
+		// in which case best to let the go command keep running
+		// and diagnose the problem.
+		return
+	}
+
+	// Note: minToolchain is what https://go.dev/doc/toolchain#select calls the default toolchain.
+	minToolchain := gover.LocalToolchain()
+	minVers := gover.Local()
+	var mode string
+	if gotoolchain == "auto" {
+		mode = "auto"
+	} else if gotoolchain == "path" {
+		mode = "path"
+	} else {
+		min, suffix, plus := strings.Cut(gotoolchain, "+") // go1.2.3+auto
+		if min != "local" {
+			v := gover.FromToolchain(min)
+			if v == "" {
+				if plus {
+					base.Fatalf("invalid GOTOOLCHAIN %q: invalid minimum toolchain %q", gotoolchain, min)
+				}
+				base.Fatalf("invalid GOTOOLCHAIN %q", gotoolchain)
+			}
+			minToolchain = min
+			minVers = v
+		}
+		if plus && suffix != "auto" && suffix != "path" {
+			base.Fatalf("invalid GOTOOLCHAIN %q: only version suffixes are +auto and +path", gotoolchain)
+		}
+		mode = suffix
+	}
+
+	gotoolchain = minToolchain
+	if (mode == "auto" || mode == "path") && !goInstallVersion() {
+		// Read go.mod to find new minimum and suggested toolchain.
+		file, goVers, toolchain := modGoToolchain()
+		gover.Startup.AutoFile = file
+		if toolchain == "default" {
+			// "default" means always use the default toolchain,
+			// which is already set, so nothing to do here.
+			// Note that if we have Go 1.21 installed originally,
+			// GOTOOLCHAIN=go1.30.0+auto or GOTOOLCHAIN=go1.30.0,
+			// and the go.mod  says "toolchain default", we use Go 1.30, not Go 1.21.
+			// That is, default overrides the "auto" part of the calculation
+			// but not the minimum that the user has set.
+			// Of course, if the go.mod also says "go 1.35", using Go 1.30
+			// will provoke an error about the toolchain being too old.
+			// That's what people who use toolchain default want:
+			// only ever use the toolchain configured by the user
+			// (including its environment and go env -w file).
+			gover.Startup.AutoToolchain = toolchain
+		} else {
+			if toolchain != "" {
+				// Accept toolchain only if it is >= our min.
+				toolVers := gover.FromToolchain(toolchain)
+				if toolVers == "" || (!strings.HasPrefix(toolchain, "go") && !strings.Contains(toolchain, "-go")) {
+					base.Fatalf("invalid toolchain %q in %s", toolchain, base.ShortPath(file))
+				}
+				if gover.Compare(toolVers, minVers) >= 0 {
+					gotoolchain = toolchain
+					minVers = toolVers
+					gover.Startup.AutoToolchain = toolchain
+				}
+			}
+			if gover.Compare(goVers, minVers) > 0 {
+				gotoolchain = "go" + goVers
+				gover.Startup.AutoGoVersion = goVers
+				gover.Startup.AutoToolchain = "" // in case we are overriding it for being too old
+			}
+		}
+	}
+
+	// If we are invoked as a target toolchain, confirm that
+	// we provide the expected version and then run.
+	// This check is delayed until after the handling of auto and path
+	// so that we have initialized gover.Startup for use in error messages.
+	if target := os.Getenv(targetEnv); target != "" && TestVersionSwitch != "loop" {
+		if gover.LocalToolchain() != target {
+			base.Fatalf("toolchain %v invoked to provide %v", gover.LocalToolchain(), target)
+		}
+		os.Unsetenv(targetEnv)
+
+		// Note: It is tempting to check that if gotoolchain != "local"
+		// then target == gotoolchain here, as a sanity check that
+		// the child has made the same version determination as the parent.
+		// This turns out not always to be the case. Specifically, if we are
+		// running Go 1.21 with GOTOOLCHAIN=go1.22+auto, which invokes
+		// Go 1.22, then 'go get go@1.23.0' or 'go get needs_go_1_23'
+		// will invoke Go 1.23, but as the Go 1.23 child the reason for that
+		// will not be apparent here: it will look like we should be using Go 1.22.
+		// We rely on the targetEnv being set to know not to downgrade.
+		// A longer term problem with the sanity check is that the exact details
+		// may change over time: there may be other reasons that a future Go
+		// version might invoke an older one, and the older one won't know why.
+		// Best to just accept that we were invoked to provide a specific toolchain
+		// (which we just checked) and leave it at that.
+		return
+	}
+
+	if gotoolchain == "local" || gotoolchain == gover.LocalToolchain() {
+		// Let the current binary handle the command.
+		return
+	}
+
+	// Minimal sanity check of GOTOOLCHAIN setting before search.
+	// We want to allow things like go1.20.3 but also gccgo-go1.20.3.
+	// We want to disallow mistakes / bad ideas like GOTOOLCHAIN=bash,
+	// since we will find that in the path lookup.
+	if !strings.HasPrefix(gotoolchain, "go1") && !strings.Contains(gotoolchain, "-go1") {
+		base.Fatalf("invalid GOTOOLCHAIN %q", gotoolchain)
+	}
+
+	Exec(gotoolchain)
+}
+
+// TestVersionSwitch is set in the test go binary to the value in $TESTGO_VERSION_SWITCH.
+// Valid settings are:
+//
+//	"switch" - simulate version switches by reinvoking the test go binary with a different TESTGO_VERSION.
+//	"mismatch" - like "switch" but forget to set TESTGO_VERSION, so it looks like we invoked a mismatched toolchain
+//	"loop" - like "mismatch" but forget the target check, causing a toolchain switching loop
+var TestVersionSwitch string
+
+// Exec invokes the specified Go toolchain or else prints an error and exits the process.
+// If $GOTOOLCHAIN is set to path or min+path, Exec only considers the PATH
+// as a source of Go toolchains. Otherwise Exec tries the PATH but then downloads
+// a toolchain if necessary.
+func Exec(gotoolchain string) {
+	log.SetPrefix("go: ")
+
+	writeBits = sysWriteBits()
+
+	count, _ := strconv.Atoi(os.Getenv(countEnv))
+	if count >= maxSwitch-10 {
+		fmt.Fprintf(os.Stderr, "go: switching from go%v to %v [depth %d]\n", gover.Local(), gotoolchain, count)
+	}
+	if count >= maxSwitch {
+		base.Fatalf("too many toolchain switches")
+	}
+	os.Setenv(countEnv, fmt.Sprint(count+1))
+
+	env := cfg.Getenv("GOTOOLCHAIN")
+	pathOnly := env == "path" || strings.HasSuffix(env, "+path")
+
+	// For testing, if TESTGO_VERSION is already in use
+	// (only happens in the cmd/go test binary)
+	// and TESTGO_VERSION_SWITCH=switch is set,
+	// "switch" toolchains by changing TESTGO_VERSION
+	// and reinvoking the current binary.
+	// The special cases =loop and =mismatch skip the
+	// setting of TESTGO_VERSION so that it looks like we
+	// accidentally invoked the wrong toolchain,
+	// to test detection of that failure mode.
+	switch TestVersionSwitch {
+	case "switch":
+		os.Setenv("TESTGO_VERSION", gotoolchain)
+		fallthrough
+	case "loop", "mismatch":
+		exe, err := os.Executable()
+		if err != nil {
+			base.Fatalf("%v", err)
+		}
+		execGoToolchain(gotoolchain, os.Getenv("GOROOT"), exe)
+	}
+
+	// Look in PATH for the toolchain before we download one.
+	// This allows custom toolchains as well as reuse of toolchains
+	// already installed using go install golang.org/dl/go1.2.3@latest.
+	if exe, err := exec.LookPath(gotoolchain); err == nil {
+		execGoToolchain(gotoolchain, "", exe)
+	}
+
+	// GOTOOLCHAIN=auto looks in PATH and then falls back to download.
+	// GOTOOLCHAIN=path only looks in PATH.
+	if pathOnly {
+		base.Fatalf("cannot find %q in PATH", gotoolchain)
+	}
+
+	// Set up modules without an explicit go.mod, to download distribution.
+	modload.Reset()
+	modload.ForceUseModules = true
+	modload.RootMode = modload.NoRoot
+	modload.Init()
+
+	// Download and unpack toolchain module into module cache.
+	// Note that multiple go commands might be doing this at the same time,
+	// and that's OK: the module cache handles that case correctly.
+	m := module.Version{
+		Path:    gotoolchainModule,
+		Version: gotoolchainVersion + "-" + gotoolchain + "." + runtime.GOOS + "-" + runtime.GOARCH,
+	}
+	dir, err := modfetch.Download(context.Background(), m)
+	if err != nil {
+		if errors.Is(err, fs.ErrNotExist) {
+			base.Fatalf("download %s for %s/%s: toolchain not available", gotoolchain, runtime.GOOS, runtime.GOARCH)
+		}
+		base.Fatalf("download %s: %v", gotoolchain, err)
+	}
+
+	// On first use after download, set the execute bits on the commands
+	// so that we can run them. Note that multiple go commands might be
+	// doing this at the same time, but if so no harm done.
+	if runtime.GOOS != "windows" {
+		info, err := os.Stat(filepath.Join(dir, "bin/go"))
+		if err != nil {
+			base.Fatalf("download %s: %v", gotoolchain, err)
+		}
+		if info.Mode()&0111 == 0 {
+			// allowExec sets the exec permission bits on all files found in dir.
+			allowExec := func(dir string) {
+				err := filepath.WalkDir(dir, func(path string, d fs.DirEntry, err error) error {
+					if err != nil {
+						return err
+					}
+					if !d.IsDir() {
+						info, err := os.Stat(path)
+						if err != nil {
+							return err
+						}
+						if err := os.Chmod(path, info.Mode()&0777|0111); err != nil {
+							return err
+						}
+					}
+					return nil
+				})
+				if err != nil {
+					base.Fatalf("download %s: %v", gotoolchain, err)
+				}
+			}
+
+			// Set the bits in pkg/tool before bin/go.
+			// If we are racing with another go command and do bin/go first,
+			// then the check of bin/go above might succeed, the other go command
+			// would skip its own mode-setting, and then the go command might
+			// try to run a tool before we get to setting the bits on pkg/tool.
+			// Setting pkg/tool before bin/go avoids that ordering problem.
+			// The only other tool the go command invokes is gofmt,
+			// so we set that one explicitly before handling bin (which will include bin/go).
+			allowExec(filepath.Join(dir, "pkg/tool"))
+			allowExec(filepath.Join(dir, "bin/gofmt"))
+			allowExec(filepath.Join(dir, "bin"))
+		}
+	}
+
+	srcUGoMod := filepath.Join(dir, "src/_go.mod")
+	srcGoMod := filepath.Join(dir, "src/go.mod")
+	if size(srcGoMod) != size(srcUGoMod) {
+		err := filepath.WalkDir(dir, func(path string, d fs.DirEntry, err error) error {
+			if err != nil {
+				return err
+			}
+			if path == srcUGoMod {
+				// Leave for last, in case we are racing with another go command.
+				return nil
+			}
+			if pdir, name := filepath.Split(path); name == "_go.mod" {
+				if err := raceSafeCopy(path, pdir+"go.mod"); err != nil {
+					return err
+				}
+			}
+			return nil
+		})
+		// Handle src/go.mod; this is the signal to other racing go commands
+		// that everything is okay and they can skip this step.
+		if err == nil {
+			err = raceSafeCopy(srcUGoMod, srcGoMod)
+		}
+		if err != nil {
+			base.Fatalf("download %s: %v", gotoolchain, err)
+		}
+	}
+
+	// Reinvoke the go command.
+	execGoToolchain(gotoolchain, dir, filepath.Join(dir, "bin/go"))
+}
+
+func size(path string) int64 {
+	info, err := os.Stat(path)
+	if err != nil {
+		return -1
+	}
+	return info.Size()
+}
+
+var writeBits fs.FileMode
+
+// raceSafeCopy copies the file old to the file new, being careful to ensure
+// that if multiple go commands call raceSafeCopy(old, new) at the same time,
+// they don't interfere with each other: both will succeed and return and
+// later observe the correct content in new. Like in the build cache, we arrange
+// this by opening new without truncation and then writing the content.
+// Both go commands can do this simultaneously and will write the same thing
+// (old never changes content).
+func raceSafeCopy(old, new string) error {
+	oldInfo, err := os.Stat(old)
+	if err != nil {
+		return err
+	}
+	newInfo, err := os.Stat(new)
+	if err == nil && newInfo.Size() == oldInfo.Size() {
+		return nil
+	}
+	data, err := os.ReadFile(old)
+	if err != nil {
+		return err
+	}
+	// The module cache has unwritable directories by default.
+	// Restore the user write bit in the directory so we can create
+	// the new go.mod file. We clear it again at the end on a
+	// best-effort basis (ignoring failures).
+	dir := filepath.Dir(old)
+	info, err := os.Stat(dir)
+	if err != nil {
+		return err
+	}
+	if err := os.Chmod(dir, info.Mode()|writeBits); err != nil {
+		return err
+	}
+	defer os.Chmod(dir, info.Mode())
+	// Note: create the file writable, so that a racing go command
+	// doesn't get an error before we store the actual data.
+	f, err := os.OpenFile(new, os.O_CREATE|os.O_WRONLY, writeBits&^0o111)
+	if err != nil {
+		// If OpenFile failed because a racing go command completed our work
+		// (and then OpenFile failed because the directory or file is now read-only),
+		// count that as a success.
+		if size(old) == size(new) {
+			return nil
+		}
+		return err
+	}
+	defer os.Chmod(new, oldInfo.Mode())
+	if _, err := f.Write(data); err != nil {
+		f.Close()
+		return err
+	}
+	return f.Close()
+}
+
+// modGoToolchain finds the enclosing go.work or go.mod file
+// and returns the go version and toolchain lines from the file.
+// The toolchain line overrides the version line
+func modGoToolchain() (file, goVers, toolchain string) {
+	wd := base.UncachedCwd()
+	file = modload.FindGoWork(wd)
+	// $GOWORK can be set to a file that does not yet exist, if we are running 'go work init'.
+	// Do not try to load the file in that case
+	if _, err := os.Stat(file); err != nil {
+		file = ""
+	}
+	if file == "" {
+		file = modload.FindGoMod(wd)
+	}
+	if file == "" {
+		return "", "", ""
+	}
+
+	data, err := os.ReadFile(file)
+	if err != nil {
+		base.Fatalf("%v", err)
+	}
+	return file, gover.GoModLookup(data, "go"), gover.GoModLookup(data, "toolchain")
+}
+
+// goInstallVersion reports whether the command line is go install m@v or go run m@v.
+// If so, Select must not read the go.mod or go.work file in "auto" or "path" mode.
+func goInstallVersion() bool {
+	// Note: We assume there are no flags between 'go' and 'install' or 'run'.
+	// During testing there are some debugging flags that are accepted
+	// in that position, but in production go binaries there are not.
+	if len(os.Args) < 3 {
+		return false
+	}
+
+	var cmdFlags *flag.FlagSet
+	switch os.Args[1] {
+	default:
+		// Command doesn't support a pkg@version as the main module.
+		return false
+	case "install":
+		cmdFlags = &work.CmdInstall.Flag
+	case "run":
+		cmdFlags = &run.CmdRun.Flag
+	}
+
+	// The modcachrw flag is unique, in that it affects how we fetch the
+	// requested module to even figure out what toolchain it needs.
+	// We need to actually set it before we check the toolchain version.
+	// (See https://go.dev/issue/64282.)
+	modcacherwFlag := cmdFlags.Lookup("modcacherw")
+	if modcacherwFlag == nil {
+		base.Fatalf("internal error: modcacherw flag not registered for command")
+	}
+	modcacherwVal, ok := modcacherwFlag.Value.(interface {
+		IsBoolFlag() bool
+		flag.Value
+	})
+	if !ok || !modcacherwVal.IsBoolFlag() {
+		base.Fatalf("internal error: modcacherw is not a boolean flag")
+	}
+
+	// Make a best effort to parse the command's args to find the pkg@version
+	// argument and the -modcacherw flag.
+	var (
+		pkgArg         string
+		modcacherwSeen bool
+	)
+	for args := os.Args[2:]; len(args) > 0; {
+		a := args[0]
+		args = args[1:]
+		if a == "--" {
+			if len(args) == 0 {
+				return false
+			}
+			pkgArg = args[0]
+			break
+		}
+
+		a, ok := strings.CutPrefix(a, "-")
+		if !ok {
+			// Not a flag argument. Must be a package.
+			pkgArg = a
+			break
+		}
+		a = strings.TrimPrefix(a, "-") // Treat --flag as -flag.
+
+		name, val, hasEq := strings.Cut(a, "=")
+
+		if name == "modcacherw" {
+			if !hasEq {
+				val = "true"
+			}
+			if err := modcacherwVal.Set(val); err != nil {
+				return false
+			}
+			modcacherwSeen = true
+			continue
+		}
+
+		if hasEq {
+			// Already has a value; don't bother parsing it.
+			continue
+		}
+
+		f := run.CmdRun.Flag.Lookup(a)
+		if f == nil {
+			// We don't know whether this flag is a boolean.
+			if os.Args[1] == "run" {
+				// We don't know where to find the pkg@version argument.
+				// For run, the pkg@version can be anywhere on the command line,
+				// because it is preceded by run flags and followed by arguments to the
+				// program being run. Since we don't know whether this flag takes
+				// an argument, we can't reliably identify the end of the run flags.
+				// Just give up and let the user clarify using the "=" form..
+				return false
+			}
+
+			// We would like to let 'go install -newflag pkg@version' work even
+			// across a toolchain switch. To make that work, assume by default that
+			// the pkg@version is the last argument and skip the remaining args unless
+			// we spot a plausible "-modcacherw" flag.
+			for len(args) > 0 {
+				a := args[0]
+				name, _, _ := strings.Cut(a, "=")
+				if name == "-modcacherw" || name == "--modcacherw" {
+					break
+				}
+				if len(args) == 1 && !strings.HasPrefix(a, "-") {
+					pkgArg = a
+				}
+				args = args[1:]
+			}
+			continue
+		}
+
+		if bf, ok := f.Value.(interface{ IsBoolFlag() bool }); !ok || !bf.IsBoolFlag() {
+			// The next arg is the value for this flag. Skip it.
+			args = args[1:]
+			continue
+		}
+	}
+
+	if !strings.Contains(pkgArg, "@") || build.IsLocalImport(pkgArg) || filepath.IsAbs(pkgArg) {
+		return false
+	}
+	path, version, _ := strings.Cut(pkgArg, "@")
+	if path == "" || version == "" || gover.IsToolchain(path) {
+		return false
+	}
+
+	if !modcacherwSeen && base.InGOFLAGS("-modcacherw") {
+		fs := flag.NewFlagSet("goInstallVersion", flag.ExitOnError)
+		fs.Var(modcacherwVal, "modcacherw", modcacherwFlag.Usage)
+		base.SetFromGOFLAGS(fs)
+	}
+
+	// It would be correct to simply return true here, bypassing use
+	// of the current go.mod or go.work, and let "go run" or "go install"
+	// do the rest, including a toolchain switch.
+	// Our goal instead is, since we have gone to the trouble of handling
+	// unknown flags to some degree, to run the switch now, so that
+	// these commands can switch to a newer toolchain directed by the
+	// go.mod which may actually understand the flag.
+	// This was brought up during the go.dev/issue/57001 proposal discussion
+	// and may end up being common in self-contained "go install" or "go run"
+	// command lines if we add new flags in the future.
+
+	// Set up modules without an explicit go.mod, to download go.mod.
+	modload.ForceUseModules = true
+	modload.RootMode = modload.NoRoot
+	modload.Init()
+	defer modload.Reset()
+
+	// See internal/load.PackagesAndErrorsOutsideModule
+	ctx := context.Background()
+	allowed := modload.CheckAllowed
+	if modload.IsRevisionQuery(path, version) {
+		// Don't check for retractions if a specific revision is requested.
+		allowed = nil
+	}
+	noneSelected := func(path string) (version string) { return "none" }
+	_, err := modload.QueryPackages(ctx, path, version, noneSelected, allowed)
+	if errors.Is(err, gover.ErrTooNew) {
+		// Run early switch, same one go install or go run would eventually do,
+		// if it understood all the command-line flags.
+		SwitchOrFatal(ctx, err)
+	}
+
+	return true // pkg@version found
+}
diff --git a/src/cmd/go/internal/toolchain/switch.go b/src/cmd/go/internal/toolchain/switch.go
new file mode 100644
index 0000000..2c6a2b8
--- /dev/null
+++ b/src/cmd/go/internal/toolchain/switch.go
@@ -0,0 +1,231 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package toolchain
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/modfetch"
+)
+
+// A Switcher collects errors to be reported and then decides
+// between reporting the errors or switching to a new toolchain
+// to resolve them.
+//
+// The client calls [Switcher.Error] repeatedly with errors encountered
+// and then calls [Switcher.Switch]. If the errors included any
+// *gover.TooNewErrors (potentially wrapped) and switching is
+// permitted by GOTOOLCHAIN, Switch switches to a new toolchain.
+// Otherwise Switch prints all the errors using base.Error.
+//
+// See https://go.dev/doc/toolchain#switch.
+type Switcher struct {
+	TooNew *gover.TooNewError // max go requirement observed
+	Errors []error            // errors collected so far
+}
+
+// Error reports the error to the Switcher,
+// which saves it for processing during Switch.
+func (s *Switcher) Error(err error) {
+	s.Errors = append(s.Errors, err)
+	s.addTooNew(err)
+}
+
+// addTooNew adds any TooNew errors that can be found in err.
+func (s *Switcher) addTooNew(err error) {
+	switch err := err.(type) {
+	case interface{ Unwrap() []error }:
+		for _, e := range err.Unwrap() {
+			s.addTooNew(e)
+		}
+
+	case interface{ Unwrap() error }:
+		s.addTooNew(err.Unwrap())
+
+	case *gover.TooNewError:
+		if s.TooNew == nil ||
+			gover.Compare(err.GoVersion, s.TooNew.GoVersion) > 0 ||
+			gover.Compare(err.GoVersion, s.TooNew.GoVersion) == 0 && err.What < s.TooNew.What {
+			s.TooNew = err
+		}
+	}
+}
+
+// NeedSwitch reports whether Switch would attempt to switch toolchains.
+func (s *Switcher) NeedSwitch() bool {
+	return s.TooNew != nil && (HasAuto() || HasPath())
+}
+
+// Switch decides whether to switch to a newer toolchain
+// to resolve any of the saved errors.
+// It switches if toolchain switches are permitted and there is at least one TooNewError.
+//
+// If Switch decides not to switch toolchains, it prints the errors using base.Error and returns.
+//
+// If Switch decides to switch toolchains but cannot identify a toolchain to use.
+// it prints the errors along with one more about not being able to find the toolchain
+// and returns.
+//
+// Otherwise, Switch prints an informational message giving a reason for the
+// switch and the toolchain being invoked and then switches toolchains.
+// This operation never returns.
+func (s *Switcher) Switch(ctx context.Context) {
+	if !s.NeedSwitch() {
+		for _, err := range s.Errors {
+			base.Error(err)
+		}
+		return
+	}
+
+	// Switch to newer Go toolchain if necessary and possible.
+	tv, err := NewerToolchain(ctx, s.TooNew.GoVersion)
+	if err != nil {
+		for _, err := range s.Errors {
+			base.Error(err)
+		}
+		base.Error(fmt.Errorf("switching to go >= %v: %w", s.TooNew.GoVersion, err))
+		return
+	}
+
+	fmt.Fprintf(os.Stderr, "go: %v requires go >= %v; switching to %v\n", s.TooNew.What, s.TooNew.GoVersion, tv)
+	Exec(tv)
+	panic("unreachable")
+}
+
+// SwitchOrFatal attempts a toolchain switch based on the information in err
+// and otherwise falls back to base.Fatal(err).
+func SwitchOrFatal(ctx context.Context, err error) {
+	var s Switcher
+	s.Error(err)
+	s.Switch(ctx)
+	base.Exit()
+}
+
+// NewerToolchain returns the name of the toolchain to use when we need
+// to switch to a newer toolchain that must support at least the given Go version.
+// See https://go.dev/doc/toolchain#switch.
+//
+// If the latest major release is 1.N.0, we use the latest patch release of 1.(N-1) if that's >= version.
+// Otherwise we use the latest 1.N if that's allowed.
+// Otherwise we use the latest release.
+func NewerToolchain(ctx context.Context, version string) (string, error) {
+	fetch := autoToolchains
+	if !HasAuto() {
+		fetch = pathToolchains
+	}
+	list, err := fetch(ctx)
+	if err != nil {
+		return "", err
+	}
+	return newerToolchain(version, list)
+}
+
+// autoToolchains returns the list of toolchain versions available to GOTOOLCHAIN=auto or =min+auto mode.
+func autoToolchains(ctx context.Context) ([]string, error) {
+	var versions *modfetch.Versions
+	err := modfetch.TryProxies(func(proxy string) error {
+		v, err := modfetch.Lookup(ctx, proxy, "go").Versions(ctx, "")
+		if err != nil {
+			return err
+		}
+		versions = v
+		return nil
+	})
+	if err != nil {
+		return nil, err
+	}
+	return versions.List, nil
+}
+
+// pathToolchains returns the list of toolchain versions available to GOTOOLCHAIN=path or =min+path mode.
+func pathToolchains(ctx context.Context) ([]string, error) {
+	have := make(map[string]bool)
+	var list []string
+	for _, dir := range pathDirs() {
+		if dir == "" || !filepath.IsAbs(dir) {
+			// Refuse to use local directories in $PATH (hard-coding exec.ErrDot).
+			continue
+		}
+		entries, err := os.ReadDir(dir)
+		if err != nil {
+			continue
+		}
+		for _, de := range entries {
+			if de.IsDir() || !strings.HasPrefix(de.Name(), "go1.") {
+				continue
+			}
+			info, err := de.Info()
+			if err != nil {
+				continue
+			}
+			v, ok := pathVersion(dir, de, info)
+			if !ok || !strings.HasPrefix(v, "1.") || have[v] {
+				continue
+			}
+			have[v] = true
+			list = append(list, v)
+		}
+	}
+	sort.Slice(list, func(i, j int) bool {
+		return gover.Compare(list[i], list[j]) < 0
+	})
+	return list, nil
+}
+
+// newerToolchain implements NewerToolchain where the list of choices is known.
+// It is separated out for easier testing of this logic.
+func newerToolchain(need string, list []string) (string, error) {
+	// Consider each release in the list, from newest to oldest,
+	// considering only entries >= need and then only entries
+	// that are the latest in their language family
+	// (the latest 1.40, the latest 1.39, and so on).
+	// We prefer the latest patch release before the most recent release family,
+	// so if the latest release is 1.40.1 we'll take the latest 1.39.X.
+	// Failing that, we prefer the latest patch release before the most recent
+	// prerelease family, so if the latest release is 1.40rc1 is out but 1.39 is okay,
+	// we'll still take 1.39.X.
+	// Failing that we'll take the latest release.
+	latest := ""
+	for i := len(list) - 1; i >= 0; i-- {
+		v := list[i]
+		if gover.Compare(v, need) < 0 {
+			break
+		}
+		if gover.Lang(latest) == gover.Lang(v) {
+			continue
+		}
+		newer := latest
+		latest = v
+		if newer != "" && !gover.IsPrerelease(newer) {
+			// latest is the last patch release of Go 1.X, and we saw a non-prerelease of Go 1.(X+1),
+			// so latest is the one we want.
+			break
+		}
+	}
+	if latest == "" {
+		return "", fmt.Errorf("no releases found for go >= %v", need)
+	}
+	return "go" + latest, nil
+}
+
+// HasAuto reports whether the GOTOOLCHAIN setting allows "auto" upgrades.
+func HasAuto() bool {
+	env := cfg.Getenv("GOTOOLCHAIN")
+	return env == "auto" || strings.HasSuffix(env, "+auto")
+}
+
+// HasPath reports whether the GOTOOLCHAIN setting allows "path" upgrades.
+func HasPath() bool {
+	env := cfg.Getenv("GOTOOLCHAIN")
+	return env == "path" || strings.HasSuffix(env, "+path")
+}
diff --git a/src/cmd/go/internal/toolchain/toolchain_test.go b/src/cmd/go/internal/toolchain/toolchain_test.go
new file mode 100644
index 0000000..e8ed566
--- /dev/null
+++ b/src/cmd/go/internal/toolchain/toolchain_test.go
@@ -0,0 +1,66 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package toolchain
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestNewerToolchain(t *testing.T) {
+	for _, tt := range newerToolchainTests {
+		out, err := newerToolchain(tt.need, tt.list)
+		if (err != nil) != (out == "") {
+			t.Errorf("newerToolchain(%v, %v) = %v, %v, want error", tt.need, tt.list, out, err)
+			continue
+		}
+		if out != tt.out {
+			t.Errorf("newerToolchain(%v, %v) = %v, %v want %v, nil", tt.need, tt.list, out, err, tt.out)
+		}
+	}
+}
+
+var f = strings.Fields
+
+var relRC = []string{"1.39.0", "1.39.1", "1.39.2", "1.40.0", "1.40.1", "1.40.2", "1.41rc1"}
+var rel2 = []string{"1.39.0", "1.39.1", "1.39.2", "1.40.0", "1.40.1", "1.40.2"}
+var rel0 = []string{"1.39.0", "1.39.1", "1.39.2", "1.40.0"}
+var newerToolchainTests = []struct {
+	need string
+	list []string
+	out  string
+}{
+	{"1.30", rel0, "go1.39.2"},
+	{"1.30", rel2, "go1.39.2"},
+	{"1.30", relRC, "go1.39.2"},
+	{"1.38", rel0, "go1.39.2"},
+	{"1.38", rel2, "go1.39.2"},
+	{"1.38", relRC, "go1.39.2"},
+	{"1.38.1", rel0, "go1.39.2"},
+	{"1.38.1", rel2, "go1.39.2"},
+	{"1.38.1", relRC, "go1.39.2"},
+	{"1.39", rel0, "go1.39.2"},
+	{"1.39", rel2, "go1.39.2"},
+	{"1.39", relRC, "go1.39.2"},
+	{"1.39.2", rel0, "go1.39.2"},
+	{"1.39.2", rel2, "go1.39.2"},
+	{"1.39.2", relRC, "go1.39.2"},
+	{"1.39.3", rel0, "go1.40.0"},
+	{"1.39.3", rel2, "go1.40.2"},
+	{"1.39.3", relRC, "go1.40.2"},
+	{"1.40", rel0, "go1.40.0"},
+	{"1.40", rel2, "go1.40.2"},
+	{"1.40", relRC, "go1.40.2"},
+	{"1.40.1", rel0, ""},
+	{"1.40.1", rel2, "go1.40.2"},
+	{"1.40.1", relRC, "go1.40.2"},
+	{"1.41", rel0, ""},
+	{"1.41", rel2, ""},
+	{"1.41", relRC, "go1.41rc1"},
+	{"1.41.0", rel0, ""},
+	{"1.41.0", rel2, ""},
+	{"1.41.0", relRC, ""},
+	{"1.40", nil, ""},
+}
diff --git a/src/cmd/go/internal/toolchain/umask_none.go b/src/cmd/go/internal/toolchain/umask_none.go
new file mode 100644
index 0000000..b092fe8
--- /dev/null
+++ b/src/cmd/go/internal/toolchain/umask_none.go
@@ -0,0 +1,13 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !(darwin || freebsd || linux || netbsd || openbsd)
+
+package toolchain
+
+import "io/fs"
+
+func sysWriteBits() fs.FileMode {
+	return 0700
+}
diff --git a/src/cmd/go/internal/toolchain/umask_unix.go b/src/cmd/go/internal/toolchain/umask_unix.go
new file mode 100644
index 0000000..cbe4307
--- /dev/null
+++ b/src/cmd/go/internal/toolchain/umask_unix.go
@@ -0,0 +1,28 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build darwin || freebsd || linux || netbsd || openbsd
+
+package toolchain
+
+import (
+	"io/fs"
+	"syscall"
+)
+
+// sysWriteBits determines which bits to OR into the mode to make a directory writable.
+// It must be called when there are no other file system operations happening.
+func sysWriteBits() fs.FileMode {
+	// Read current umask. There's no way to read it without also setting it,
+	// so set it conservatively and then restore the original one.
+	m := syscall.Umask(0o777)
+	syscall.Umask(m)    // restore bits
+	if m&0o22 == 0o22 { // group and world are unwritable by default
+		return 0o700
+	}
+	if m&0o2 == 0o2 { // group is writable by default, but not world
+		return 0o770
+	}
+	return 0o777 // everything is writable by default
+}
diff --git a/src/cmd/go/internal/trace/trace.go b/src/cmd/go/internal/trace/trace.go
new file mode 100644
index 0000000..d69dc4f
--- /dev/null
+++ b/src/cmd/go/internal/trace/trace.go
@@ -0,0 +1,206 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package trace
+
+import (
+	"cmd/internal/traceviewer"
+	"context"
+	"encoding/json"
+	"errors"
+	"os"
+	"strings"
+	"sync/atomic"
+	"time"
+)
+
+// Constants used in event fields.
+// See https://docs.google.com/document/d/1CvAClvFfyA5R-PhYUmn5OOQtYMH4h6I0nSsKchNAySU
+// for more details.
+const (
+	phaseDurationBegin = "B"
+	phaseDurationEnd   = "E"
+	phaseFlowStart     = "s"
+	phaseFlowEnd       = "f"
+
+	bindEnclosingSlice = "e"
+)
+
+var traceStarted atomic.Bool
+
+func getTraceContext(ctx context.Context) (traceContext, bool) {
+	if !traceStarted.Load() {
+		return traceContext{}, false
+	}
+	v := ctx.Value(traceKey{})
+	if v == nil {
+		return traceContext{}, false
+	}
+	return v.(traceContext), true
+}
+
+// StartSpan starts a trace event with the given name. The Span ends when its Done method is called.
+func StartSpan(ctx context.Context, name string) (context.Context, *Span) {
+	tc, ok := getTraceContext(ctx)
+	if !ok {
+		return ctx, nil
+	}
+	childSpan := &Span{t: tc.t, name: name, tid: tc.tid, start: time.Now()}
+	tc.t.writeEvent(&traceviewer.Event{
+		Name:  childSpan.name,
+		Time:  float64(childSpan.start.UnixNano()) / float64(time.Microsecond),
+		TID:   childSpan.tid,
+		Phase: phaseDurationBegin,
+	})
+	ctx = context.WithValue(ctx, traceKey{}, traceContext{tc.t, tc.tid})
+	return ctx, childSpan
+}
+
+// StartGoroutine associates the context with a new Thread ID. The Chrome trace viewer associates each
+// trace event with a thread, and doesn't expect events with the same thread id to happen at the
+// same time.
+func StartGoroutine(ctx context.Context) context.Context {
+	tc, ok := getTraceContext(ctx)
+	if !ok {
+		return ctx
+	}
+	return context.WithValue(ctx, traceKey{}, traceContext{tc.t, tc.t.getNextTID()})
+}
+
+// Flow marks a flow indicating that the 'to' span depends on the 'from' span.
+// Flow should be called while the 'to' span is in progress.
+func Flow(ctx context.Context, from *Span, to *Span) {
+	tc, ok := getTraceContext(ctx)
+	if !ok || from == nil || to == nil {
+		return
+	}
+
+	id := tc.t.getNextFlowID()
+	tc.t.writeEvent(&traceviewer.Event{
+		Name:     from.name + " -> " + to.name,
+		Category: "flow",
+		ID:       id,
+		Time:     float64(from.end.UnixNano()) / float64(time.Microsecond),
+		Phase:    phaseFlowStart,
+		TID:      from.tid,
+	})
+	tc.t.writeEvent(&traceviewer.Event{
+		Name:      from.name + " -> " + to.name,
+		Category:  "flow", // TODO(matloob): Add Category to Flow?
+		ID:        id,
+		Time:      float64(to.start.UnixNano()) / float64(time.Microsecond),
+		Phase:     phaseFlowEnd,
+		TID:       to.tid,
+		BindPoint: bindEnclosingSlice,
+	})
+}
+
+type Span struct {
+	t *tracer
+
+	name  string
+	tid   uint64
+	start time.Time
+	end   time.Time
+}
+
+func (s *Span) Done() {
+	if s == nil {
+		return
+	}
+	s.end = time.Now()
+	s.t.writeEvent(&traceviewer.Event{
+		Name:  s.name,
+		Time:  float64(s.end.UnixNano()) / float64(time.Microsecond),
+		TID:   s.tid,
+		Phase: phaseDurationEnd,
+	})
+}
+
+type tracer struct {
+	file chan traceFile // 1-buffered
+
+	nextTID    uint64
+	nextFlowID uint64
+}
+
+func (t *tracer) writeEvent(ev *traceviewer.Event) error {
+	f := <-t.file
+	defer func() { t.file <- f }()
+	var err error
+	if f.entries == 0 {
+		_, err = f.sb.WriteString("[\n")
+	} else {
+		_, err = f.sb.WriteString(",")
+	}
+	f.entries++
+	if err != nil {
+		return nil
+	}
+
+	if err := f.enc.Encode(ev); err != nil {
+		return err
+	}
+
+	// Write event string to output file.
+	_, err = f.f.WriteString(f.sb.String())
+	f.sb.Reset()
+	return err
+}
+
+func (t *tracer) Close() error {
+	f := <-t.file
+	defer func() { t.file <- f }()
+
+	_, firstErr := f.f.WriteString("]")
+	if err := f.f.Close(); firstErr == nil {
+		firstErr = err
+	}
+	return firstErr
+}
+
+func (t *tracer) getNextTID() uint64 {
+	return atomic.AddUint64(&t.nextTID, 1)
+}
+
+func (t *tracer) getNextFlowID() uint64 {
+	return atomic.AddUint64(&t.nextFlowID, 1)
+}
+
+// traceKey is the context key for tracing information. It is unexported to prevent collisions with context keys defined in
+// other packages.
+type traceKey struct{}
+
+type traceContext struct {
+	t   *tracer
+	tid uint64
+}
+
+// Start starts a trace which writes to the given file.
+func Start(ctx context.Context, file string) (context.Context, func() error, error) {
+	traceStarted.Store(true)
+	if file == "" {
+		return nil, nil, errors.New("no trace file supplied")
+	}
+	f, err := os.Create(file)
+	if err != nil {
+		return nil, nil, err
+	}
+	t := &tracer{file: make(chan traceFile, 1)}
+	sb := new(strings.Builder)
+	t.file <- traceFile{
+		f:   f,
+		sb:  sb,
+		enc: json.NewEncoder(sb),
+	}
+	ctx = context.WithValue(ctx, traceKey{}, traceContext{t: t})
+	return ctx, t.Close, nil
+}
+
+type traceFile struct {
+	f       *os.File
+	sb      *strings.Builder
+	enc     *json.Encoder
+	entries int64
+}
diff --git a/src/cmd/go/internal/vcs/discovery.go b/src/cmd/go/internal/vcs/discovery.go
new file mode 100644
index 0000000..327b44c
--- /dev/null
+++ b/src/cmd/go/internal/vcs/discovery.go
@@ -0,0 +1,97 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package vcs
+
+import (
+	"encoding/xml"
+	"fmt"
+	"io"
+	"strings"
+)
+
+// charsetReader returns a reader that converts from the given charset to UTF-8.
+// Currently it only supports UTF-8 and ASCII. Otherwise, it returns a meaningful
+// error which is printed by go get, so the user can find why the package
+// wasn't downloaded if the encoding is not supported. Note that, in
+// order to reduce potential errors, ASCII is treated as UTF-8 (i.e. characters
+// greater than 0x7f are not rejected).
+func charsetReader(charset string, input io.Reader) (io.Reader, error) {
+	switch strings.ToLower(charset) {
+	case "utf-8", "ascii":
+		return input, nil
+	default:
+		return nil, fmt.Errorf("can't decode XML document using charset %q", charset)
+	}
+}
+
+// parseMetaGoImports returns meta imports from the HTML in r.
+// Parsing ends at the end of the <head> section or the beginning of the <body>.
+func parseMetaGoImports(r io.Reader, mod ModuleMode) ([]metaImport, error) {
+	d := xml.NewDecoder(r)
+	d.CharsetReader = charsetReader
+	d.Strict = false
+	var imports []metaImport
+	for {
+		t, err := d.RawToken()
+		if err != nil {
+			if err != io.EOF && len(imports) == 0 {
+				return nil, err
+			}
+			break
+		}
+		if e, ok := t.(xml.StartElement); ok && strings.EqualFold(e.Name.Local, "body") {
+			break
+		}
+		if e, ok := t.(xml.EndElement); ok && strings.EqualFold(e.Name.Local, "head") {
+			break
+		}
+		e, ok := t.(xml.StartElement)
+		if !ok || !strings.EqualFold(e.Name.Local, "meta") {
+			continue
+		}
+		if attrValue(e.Attr, "name") != "go-import" {
+			continue
+		}
+		if f := strings.Fields(attrValue(e.Attr, "content")); len(f) == 3 {
+			imports = append(imports, metaImport{
+				Prefix:   f[0],
+				VCS:      f[1],
+				RepoRoot: f[2],
+			})
+		}
+	}
+
+	// Extract mod entries if we are paying attention to them.
+	var list []metaImport
+	var have map[string]bool
+	if mod == PreferMod {
+		have = make(map[string]bool)
+		for _, m := range imports {
+			if m.VCS == "mod" {
+				have[m.Prefix] = true
+				list = append(list, m)
+			}
+		}
+	}
+
+	// Append non-mod entries, ignoring those superseded by a mod entry.
+	for _, m := range imports {
+		if m.VCS != "mod" && !have[m.Prefix] {
+			list = append(list, m)
+		}
+	}
+	return list, nil
+}
+
+// attrValue returns the attribute value for the case-insensitive key
+// `name', or the empty string if nothing is found.
+func attrValue(attrs []xml.Attr, name string) string {
+	for _, a := range attrs {
+		if strings.EqualFold(a.Name.Local, name) {
+			return a.Value
+		}
+	}
+	return ""
+}
diff --git a/src/cmd/go/internal/vcs/discovery_test.go b/src/cmd/go/internal/vcs/discovery_test.go
new file mode 100644
index 0000000..eb99fdf
--- /dev/null
+++ b/src/cmd/go/internal/vcs/discovery_test.go
@@ -0,0 +1,110 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package vcs
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+)
+
+var parseMetaGoImportsTests = []struct {
+	in  string
+	mod ModuleMode
+	out []metaImport
+}{
+	{
+		`<meta name="go-import" content="foo/bar git https://github.com/rsc/foo/bar">`,
+		IgnoreMod,
+		[]metaImport{{"foo/bar", "git", "https://github.com/rsc/foo/bar"}},
+	},
+	{
+		`<meta name="go-import" content="foo/bar git https://github.com/rsc/foo/bar">
+		<meta name="go-import" content="baz/quux git http://github.com/rsc/baz/quux">`,
+		IgnoreMod,
+		[]metaImport{
+			{"foo/bar", "git", "https://github.com/rsc/foo/bar"},
+			{"baz/quux", "git", "http://github.com/rsc/baz/quux"},
+		},
+	},
+	{
+		`<meta name="go-import" content="foo/bar git https://github.com/rsc/foo/bar">
+		<meta name="go-import" content="foo/bar mod http://github.com/rsc/baz/quux">`,
+		IgnoreMod,
+		[]metaImport{
+			{"foo/bar", "git", "https://github.com/rsc/foo/bar"},
+		},
+	},
+	{
+		`<meta name="go-import" content="foo/bar mod http://github.com/rsc/baz/quux">
+		<meta name="go-import" content="foo/bar git https://github.com/rsc/foo/bar">`,
+		IgnoreMod,
+		[]metaImport{
+			{"foo/bar", "git", "https://github.com/rsc/foo/bar"},
+		},
+	},
+	{
+		`<meta name="go-import" content="foo/bar mod http://github.com/rsc/baz/quux">
+		<meta name="go-import" content="foo/bar git https://github.com/rsc/foo/bar">`,
+		PreferMod,
+		[]metaImport{
+			{"foo/bar", "mod", "http://github.com/rsc/baz/quux"},
+		},
+	},
+	{
+		`<head>
+		<meta name="go-import" content="foo/bar git https://github.com/rsc/foo/bar">
+		</head>`,
+		IgnoreMod,
+		[]metaImport{{"foo/bar", "git", "https://github.com/rsc/foo/bar"}},
+	},
+	{
+		`<meta name="go-import" content="foo/bar git https://github.com/rsc/foo/bar">
+		<body>`,
+		IgnoreMod,
+		[]metaImport{{"foo/bar", "git", "https://github.com/rsc/foo/bar"}},
+	},
+	{
+		`<!doctype html><meta name="go-import" content="foo/bar git https://github.com/rsc/foo/bar">`,
+		IgnoreMod,
+		[]metaImport{{"foo/bar", "git", "https://github.com/rsc/foo/bar"}},
+	},
+	{
+		// XML doesn't like <div style=position:relative>.
+		`<!doctype html><title>Page Not Found</title><meta name=go-import content="chitin.io/chitin git https://github.com/chitin-io/chitin"><div style=position:relative>DRAFT</div>`,
+		IgnoreMod,
+		[]metaImport{{"chitin.io/chitin", "git", "https://github.com/chitin-io/chitin"}},
+	},
+	{
+		`<meta name="go-import" content="myitcv.io git https://github.com/myitcv/x">
+	        <meta name="go-import" content="myitcv.io/blah2 mod https://raw.githubusercontent.com/myitcv/pubx/master">
+	        `,
+		IgnoreMod,
+		[]metaImport{{"myitcv.io", "git", "https://github.com/myitcv/x"}},
+	},
+	{
+		`<meta name="go-import" content="myitcv.io git https://github.com/myitcv/x">
+	        <meta name="go-import" content="myitcv.io/blah2 mod https://raw.githubusercontent.com/myitcv/pubx/master">
+	        `,
+		PreferMod,
+		[]metaImport{
+			{"myitcv.io/blah2", "mod", "https://raw.githubusercontent.com/myitcv/pubx/master"},
+			{"myitcv.io", "git", "https://github.com/myitcv/x"},
+		},
+	},
+}
+
+func TestParseMetaGoImports(t *testing.T) {
+	for i, tt := range parseMetaGoImportsTests {
+		out, err := parseMetaGoImports(strings.NewReader(tt.in), tt.mod)
+		if err != nil {
+			t.Errorf("test#%d: %v", i, err)
+			continue
+		}
+		if !reflect.DeepEqual(out, tt.out) {
+			t.Errorf("test#%d:\n\thave %q\n\twant %q", i, out, tt.out)
+		}
+	}
+}
diff --git a/src/cmd/go/internal/vcs/vcs.go b/src/cmd/go/internal/vcs/vcs.go
new file mode 100644
index 0000000..dbf16d1
--- /dev/null
+++ b/src/cmd/go/internal/vcs/vcs.go
@@ -0,0 +1,1688 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package vcs
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"internal/lazyregexp"
+	"internal/singleflight"
+	"io/fs"
+	"log"
+	urlpkg "net/url"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/search"
+	"cmd/go/internal/str"
+	"cmd/go/internal/web"
+
+	"golang.org/x/mod/module"
+)
+
+// A Cmd describes how to use a version control system
+// like Mercurial, Git, or Subversion.
+type Cmd struct {
+	Name      string
+	Cmd       string     // name of binary to invoke command
+	RootNames []rootName // filename and mode indicating the root of a checkout directory
+
+	CreateCmd   []string // commands to download a fresh copy of a repository
+	DownloadCmd []string // commands to download updates into an existing repository
+
+	TagCmd         []tagCmd // commands to list tags
+	TagLookupCmd   []tagCmd // commands to lookup tags before running tagSyncCmd
+	TagSyncCmd     []string // commands to sync to specific tag
+	TagSyncDefault []string // commands to sync to default tag
+
+	Scheme  []string
+	PingCmd string
+
+	RemoteRepo  func(v *Cmd, rootDir string) (remoteRepo string, err error)
+	ResolveRepo func(v *Cmd, rootDir, remoteRepo string) (realRepo string, err error)
+	Status      func(v *Cmd, rootDir string) (Status, error)
+}
+
+// Status is the current state of a local repository.
+type Status struct {
+	Revision    string    // Optional.
+	CommitTime  time.Time // Optional.
+	Uncommitted bool      // Required.
+}
+
+var (
+	// VCSTestRepoURL is the URL of the HTTP server that serves the repos for
+	// vcs-test.golang.org.
+	//
+	// In tests, this is set to the URL of an httptest.Server hosting a
+	// cmd/go/internal/vcweb.Server.
+	VCSTestRepoURL string
+
+	// VCSTestHosts is the set of hosts supported by the vcs-test server.
+	VCSTestHosts []string
+
+	// VCSTestIsLocalHost reports whether the given URL refers to a local
+	// (loopback) host, such as "localhost" or "127.0.0.1:8080".
+	VCSTestIsLocalHost func(*urlpkg.URL) bool
+)
+
+var defaultSecureScheme = map[string]bool{
+	"https":   true,
+	"git+ssh": true,
+	"bzr+ssh": true,
+	"svn+ssh": true,
+	"ssh":     true,
+}
+
+func (v *Cmd) IsSecure(repo string) bool {
+	u, err := urlpkg.Parse(repo)
+	if err != nil {
+		// If repo is not a URL, it's not secure.
+		return false
+	}
+	if VCSTestRepoURL != "" && web.IsLocalHost(u) {
+		// If the vcstest server is in use, it may redirect to other local ports for
+		// other protocols (such as svn). Assume that all loopback addresses are
+		// secure during testing.
+		return true
+	}
+	return v.isSecureScheme(u.Scheme)
+}
+
+func (v *Cmd) isSecureScheme(scheme string) bool {
+	switch v.Cmd {
+	case "git":
+		// GIT_ALLOW_PROTOCOL is an environment variable defined by Git. It is a
+		// colon-separated list of schemes that are allowed to be used with git
+		// fetch/clone. Any scheme not mentioned will be considered insecure.
+		if allow := os.Getenv("GIT_ALLOW_PROTOCOL"); allow != "" {
+			for _, s := range strings.Split(allow, ":") {
+				if s == scheme {
+					return true
+				}
+			}
+			return false
+		}
+	}
+	return defaultSecureScheme[scheme]
+}
+
+// A tagCmd describes a command to list available tags
+// that can be passed to tagSyncCmd.
+type tagCmd struct {
+	cmd     string // command to list tags
+	pattern string // regexp to extract tags from list
+}
+
+// vcsList lists the known version control systems
+var vcsList = []*Cmd{
+	vcsHg,
+	vcsGit,
+	vcsSvn,
+	vcsBzr,
+	vcsFossil,
+}
+
+// vcsMod is a stub for the "mod" scheme. It's returned by
+// repoRootForImportPathDynamic, but is otherwise not treated as a VCS command.
+var vcsMod = &Cmd{Name: "mod"}
+
+// vcsByCmd returns the version control system for the given
+// command name (hg, git, svn, bzr).
+func vcsByCmd(cmd string) *Cmd {
+	for _, vcs := range vcsList {
+		if vcs.Cmd == cmd {
+			return vcs
+		}
+	}
+	return nil
+}
+
+// vcsHg describes how to use Mercurial.
+var vcsHg = &Cmd{
+	Name: "Mercurial",
+	Cmd:  "hg",
+	RootNames: []rootName{
+		{filename: ".hg", isDir: true},
+	},
+
+	CreateCmd:   []string{"clone -U -- {repo} {dir}"},
+	DownloadCmd: []string{"pull"},
+
+	// We allow both tag and branch names as 'tags'
+	// for selecting a version. This lets people have
+	// a go.release.r60 branch and a go1 branch
+	// and make changes in both, without constantly
+	// editing .hgtags.
+	TagCmd: []tagCmd{
+		{"tags", `^(\S+)`},
+		{"branches", `^(\S+)`},
+	},
+	TagSyncCmd:     []string{"update -r {tag}"},
+	TagSyncDefault: []string{"update default"},
+
+	Scheme:     []string{"https", "http", "ssh"},
+	PingCmd:    "identify -- {scheme}://{repo}",
+	RemoteRepo: hgRemoteRepo,
+	Status:     hgStatus,
+}
+
+func hgRemoteRepo(vcsHg *Cmd, rootDir string) (remoteRepo string, err error) {
+	out, err := vcsHg.runOutput(rootDir, "paths default")
+	if err != nil {
+		return "", err
+	}
+	return strings.TrimSpace(string(out)), nil
+}
+
+func hgStatus(vcsHg *Cmd, rootDir string) (Status, error) {
+	// Output changeset ID and seconds since epoch.
+	out, err := vcsHg.runOutputVerboseOnly(rootDir, `log -l1 -T {node}:{date|hgdate}`)
+	if err != nil {
+		return Status{}, err
+	}
+
+	// Successful execution without output indicates an empty repo (no commits).
+	var rev string
+	var commitTime time.Time
+	if len(out) > 0 {
+		// Strip trailing timezone offset.
+		if i := bytes.IndexByte(out, ' '); i > 0 {
+			out = out[:i]
+		}
+		rev, commitTime, err = parseRevTime(out)
+		if err != nil {
+			return Status{}, err
+		}
+	}
+
+	// Also look for untracked files.
+	out, err = vcsHg.runOutputVerboseOnly(rootDir, "status")
+	if err != nil {
+		return Status{}, err
+	}
+	uncommitted := len(out) > 0
+
+	return Status{
+		Revision:    rev,
+		CommitTime:  commitTime,
+		Uncommitted: uncommitted,
+	}, nil
+}
+
+// parseRevTime parses commit details in "revision:seconds" format.
+func parseRevTime(out []byte) (string, time.Time, error) {
+	buf := string(bytes.TrimSpace(out))
+
+	i := strings.IndexByte(buf, ':')
+	if i < 1 {
+		return "", time.Time{}, errors.New("unrecognized VCS tool output")
+	}
+	rev := buf[:i]
+
+	secs, err := strconv.ParseInt(string(buf[i+1:]), 10, 64)
+	if err != nil {
+		return "", time.Time{}, fmt.Errorf("unrecognized VCS tool output: %v", err)
+	}
+
+	return rev, time.Unix(secs, 0), nil
+}
+
+// vcsGit describes how to use Git.
+var vcsGit = &Cmd{
+	Name: "Git",
+	Cmd:  "git",
+	RootNames: []rootName{
+		{filename: ".git", isDir: true},
+	},
+
+	CreateCmd:   []string{"clone -- {repo} {dir}", "-go-internal-cd {dir} submodule update --init --recursive"},
+	DownloadCmd: []string{"pull --ff-only", "submodule update --init --recursive"},
+
+	TagCmd: []tagCmd{
+		// tags/xxx matches a git tag named xxx
+		// origin/xxx matches a git branch named xxx on the default remote repository
+		{"show-ref", `(?:tags|origin)/(\S+)$`},
+	},
+	TagLookupCmd: []tagCmd{
+		{"show-ref tags/{tag} origin/{tag}", `((?:tags|origin)/\S+)$`},
+	},
+	TagSyncCmd: []string{"checkout {tag}", "submodule update --init --recursive"},
+	// both createCmd and downloadCmd update the working dir.
+	// No need to do more here. We used to 'checkout master'
+	// but that doesn't work if the default branch is not named master.
+	// DO NOT add 'checkout master' here.
+	// See golang.org/issue/9032.
+	TagSyncDefault: []string{"submodule update --init --recursive"},
+
+	Scheme: []string{"git", "https", "http", "git+ssh", "ssh"},
+
+	// Leave out the '--' separator in the ls-remote command: git 2.7.4 does not
+	// support such a separator for that command, and this use should be safe
+	// without it because the {scheme} value comes from the predefined list above.
+	// See golang.org/issue/33836.
+	PingCmd: "ls-remote {scheme}://{repo}",
+
+	RemoteRepo: gitRemoteRepo,
+	Status:     gitStatus,
+}
+
+// scpSyntaxRe matches the SCP-like addresses used by Git to access
+// repositories by SSH.
+var scpSyntaxRe = lazyregexp.New(`^(\w+)@([\w.-]+):(.*)$`)
+
+func gitRemoteRepo(vcsGit *Cmd, rootDir string) (remoteRepo string, err error) {
+	cmd := "config remote.origin.url"
+	errParse := errors.New("unable to parse output of git " + cmd)
+	errRemoteOriginNotFound := errors.New("remote origin not found")
+	outb, err := vcsGit.run1(rootDir, cmd, nil, false)
+	if err != nil {
+		// if it doesn't output any message, it means the config argument is correct,
+		// but the config value itself doesn't exist
+		if outb != nil && len(outb) == 0 {
+			return "", errRemoteOriginNotFound
+		}
+		return "", err
+	}
+	out := strings.TrimSpace(string(outb))
+
+	var repoURL *urlpkg.URL
+	if m := scpSyntaxRe.FindStringSubmatch(out); m != nil {
+		// Match SCP-like syntax and convert it to a URL.
+		// Eg, "git@github.com:user/repo" becomes
+		// "ssh://git@github.com/user/repo".
+		repoURL = &urlpkg.URL{
+			Scheme: "ssh",
+			User:   urlpkg.User(m[1]),
+			Host:   m[2],
+			Path:   m[3],
+		}
+	} else {
+		repoURL, err = urlpkg.Parse(out)
+		if err != nil {
+			return "", err
+		}
+	}
+
+	// Iterate over insecure schemes too, because this function simply
+	// reports the state of the repo. If we can't see insecure schemes then
+	// we can't report the actual repo URL.
+	for _, s := range vcsGit.Scheme {
+		if repoURL.Scheme == s {
+			return repoURL.String(), nil
+		}
+	}
+	return "", errParse
+}
+
+func gitStatus(vcsGit *Cmd, rootDir string) (Status, error) {
+	out, err := vcsGit.runOutputVerboseOnly(rootDir, "status --porcelain")
+	if err != nil {
+		return Status{}, err
+	}
+	uncommitted := len(out) > 0
+
+	// "git status" works for empty repositories, but "git show" does not.
+	// Assume there are no commits in the repo when "git show" fails with
+	// uncommitted files and skip tagging revision / committime.
+	var rev string
+	var commitTime time.Time
+	out, err = vcsGit.runOutputVerboseOnly(rootDir, "-c log.showsignature=false show -s --format=%H:%ct")
+	if err != nil && !uncommitted {
+		return Status{}, err
+	} else if err == nil {
+		rev, commitTime, err = parseRevTime(out)
+		if err != nil {
+			return Status{}, err
+		}
+	}
+
+	return Status{
+		Revision:    rev,
+		CommitTime:  commitTime,
+		Uncommitted: uncommitted,
+	}, nil
+}
+
+// vcsBzr describes how to use Bazaar.
+var vcsBzr = &Cmd{
+	Name: "Bazaar",
+	Cmd:  "bzr",
+	RootNames: []rootName{
+		{filename: ".bzr", isDir: true},
+	},
+
+	CreateCmd: []string{"branch -- {repo} {dir}"},
+
+	// Without --overwrite bzr will not pull tags that changed.
+	// Replace by --overwrite-tags after http://pad.lv/681792 goes in.
+	DownloadCmd: []string{"pull --overwrite"},
+
+	TagCmd:         []tagCmd{{"tags", `^(\S+)`}},
+	TagSyncCmd:     []string{"update -r {tag}"},
+	TagSyncDefault: []string{"update -r revno:-1"},
+
+	Scheme:      []string{"https", "http", "bzr", "bzr+ssh"},
+	PingCmd:     "info -- {scheme}://{repo}",
+	RemoteRepo:  bzrRemoteRepo,
+	ResolveRepo: bzrResolveRepo,
+	Status:      bzrStatus,
+}
+
+func bzrRemoteRepo(vcsBzr *Cmd, rootDir string) (remoteRepo string, err error) {
+	outb, err := vcsBzr.runOutput(rootDir, "config parent_location")
+	if err != nil {
+		return "", err
+	}
+	return strings.TrimSpace(string(outb)), nil
+}
+
+func bzrResolveRepo(vcsBzr *Cmd, rootDir, remoteRepo string) (realRepo string, err error) {
+	outb, err := vcsBzr.runOutput(rootDir, "info "+remoteRepo)
+	if err != nil {
+		return "", err
+	}
+	out := string(outb)
+
+	// Expect:
+	// ...
+	//   (branch root|repository branch): <URL>
+	// ...
+
+	found := false
+	for _, prefix := range []string{"\n  branch root: ", "\n  repository branch: "} {
+		i := strings.Index(out, prefix)
+		if i >= 0 {
+			out = out[i+len(prefix):]
+			found = true
+			break
+		}
+	}
+	if !found {
+		return "", fmt.Errorf("unable to parse output of bzr info")
+	}
+
+	i := strings.Index(out, "\n")
+	if i < 0 {
+		return "", fmt.Errorf("unable to parse output of bzr info")
+	}
+	out = out[:i]
+	return strings.TrimSpace(out), nil
+}
+
+func bzrStatus(vcsBzr *Cmd, rootDir string) (Status, error) {
+	outb, err := vcsBzr.runOutputVerboseOnly(rootDir, "version-info")
+	if err != nil {
+		return Status{}, err
+	}
+	out := string(outb)
+
+	// Expect (non-empty repositories only):
+	//
+	// revision-id: gopher@gopher.net-20211021072330-qshok76wfypw9lpm
+	// date: 2021-09-21 12:00:00 +1000
+	// ...
+	var rev string
+	var commitTime time.Time
+
+	for _, line := range strings.Split(out, "\n") {
+		i := strings.IndexByte(line, ':')
+		if i < 0 {
+			continue
+		}
+		key := line[:i]
+		value := strings.TrimSpace(line[i+1:])
+
+		switch key {
+		case "revision-id":
+			rev = value
+		case "date":
+			var err error
+			commitTime, err = time.Parse("2006-01-02 15:04:05 -0700", value)
+			if err != nil {
+				return Status{}, errors.New("unable to parse output of bzr version-info")
+			}
+		}
+	}
+
+	outb, err = vcsBzr.runOutputVerboseOnly(rootDir, "status")
+	if err != nil {
+		return Status{}, err
+	}
+
+	// Skip warning when working directory is set to an older revision.
+	if bytes.HasPrefix(outb, []byte("working tree is out of date")) {
+		i := bytes.IndexByte(outb, '\n')
+		if i < 0 {
+			i = len(outb)
+		}
+		outb = outb[:i]
+	}
+	uncommitted := len(outb) > 0
+
+	return Status{
+		Revision:    rev,
+		CommitTime:  commitTime,
+		Uncommitted: uncommitted,
+	}, nil
+}
+
+// vcsSvn describes how to use Subversion.
+var vcsSvn = &Cmd{
+	Name: "Subversion",
+	Cmd:  "svn",
+	RootNames: []rootName{
+		{filename: ".svn", isDir: true},
+	},
+
+	CreateCmd:   []string{"checkout -- {repo} {dir}"},
+	DownloadCmd: []string{"update"},
+
+	// There is no tag command in subversion.
+	// The branch information is all in the path names.
+
+	Scheme:     []string{"https", "http", "svn", "svn+ssh"},
+	PingCmd:    "info -- {scheme}://{repo}",
+	RemoteRepo: svnRemoteRepo,
+}
+
+func svnRemoteRepo(vcsSvn *Cmd, rootDir string) (remoteRepo string, err error) {
+	outb, err := vcsSvn.runOutput(rootDir, "info")
+	if err != nil {
+		return "", err
+	}
+	out := string(outb)
+
+	// Expect:
+	//
+	//	 ...
+	// 	URL: <URL>
+	// 	...
+	//
+	// Note that we're not using the Repository Root line,
+	// because svn allows checking out subtrees.
+	// The URL will be the URL of the subtree (what we used with 'svn co')
+	// while the Repository Root may be a much higher parent.
+	i := strings.Index(out, "\nURL: ")
+	if i < 0 {
+		return "", fmt.Errorf("unable to parse output of svn info")
+	}
+	out = out[i+len("\nURL: "):]
+	i = strings.Index(out, "\n")
+	if i < 0 {
+		return "", fmt.Errorf("unable to parse output of svn info")
+	}
+	out = out[:i]
+	return strings.TrimSpace(out), nil
+}
+
+// fossilRepoName is the name go get associates with a fossil repository. In the
+// real world the file can be named anything.
+const fossilRepoName = ".fossil"
+
+// vcsFossil describes how to use Fossil (fossil-scm.org)
+var vcsFossil = &Cmd{
+	Name: "Fossil",
+	Cmd:  "fossil",
+	RootNames: []rootName{
+		{filename: ".fslckout", isDir: false},
+		{filename: "_FOSSIL_", isDir: false},
+	},
+
+	CreateCmd:   []string{"-go-internal-mkdir {dir} clone -- {repo} " + filepath.Join("{dir}", fossilRepoName), "-go-internal-cd {dir} open .fossil"},
+	DownloadCmd: []string{"up"},
+
+	TagCmd:         []tagCmd{{"tag ls", `(.*)`}},
+	TagSyncCmd:     []string{"up tag:{tag}"},
+	TagSyncDefault: []string{"up trunk"},
+
+	Scheme:     []string{"https", "http"},
+	RemoteRepo: fossilRemoteRepo,
+	Status:     fossilStatus,
+}
+
+func fossilRemoteRepo(vcsFossil *Cmd, rootDir string) (remoteRepo string, err error) {
+	out, err := vcsFossil.runOutput(rootDir, "remote-url")
+	if err != nil {
+		return "", err
+	}
+	return strings.TrimSpace(string(out)), nil
+}
+
+var errFossilInfo = errors.New("unable to parse output of fossil info")
+
+func fossilStatus(vcsFossil *Cmd, rootDir string) (Status, error) {
+	outb, err := vcsFossil.runOutputVerboseOnly(rootDir, "info")
+	if err != nil {
+		return Status{}, err
+	}
+	out := string(outb)
+
+	// Expect:
+	// ...
+	// checkout:     91ed71f22c77be0c3e250920f47bfd4e1f9024d2 2021-09-21 12:00:00 UTC
+	// ...
+
+	// Extract revision and commit time.
+	// Ensure line ends with UTC (known timezone offset).
+	const prefix = "\ncheckout:"
+	const suffix = " UTC"
+	i := strings.Index(out, prefix)
+	if i < 0 {
+		return Status{}, errFossilInfo
+	}
+	checkout := out[i+len(prefix):]
+	i = strings.Index(checkout, suffix)
+	if i < 0 {
+		return Status{}, errFossilInfo
+	}
+	checkout = strings.TrimSpace(checkout[:i])
+
+	i = strings.IndexByte(checkout, ' ')
+	if i < 0 {
+		return Status{}, errFossilInfo
+	}
+	rev := checkout[:i]
+
+	commitTime, err := time.ParseInLocation(time.DateTime, checkout[i+1:], time.UTC)
+	if err != nil {
+		return Status{}, fmt.Errorf("%v: %v", errFossilInfo, err)
+	}
+
+	// Also look for untracked changes.
+	outb, err = vcsFossil.runOutputVerboseOnly(rootDir, "changes --differ")
+	if err != nil {
+		return Status{}, err
+	}
+	uncommitted := len(outb) > 0
+
+	return Status{
+		Revision:    rev,
+		CommitTime:  commitTime,
+		Uncommitted: uncommitted,
+	}, nil
+}
+
+func (v *Cmd) String() string {
+	return v.Name
+}
+
+// run runs the command line cmd in the given directory.
+// keyval is a list of key, value pairs. run expands
+// instances of {key} in cmd into value, but only after
+// splitting cmd into individual arguments.
+// If an error occurs, run prints the command line and the
+// command's combined stdout+stderr to standard error.
+// Otherwise run discards the command's output.
+func (v *Cmd) run(dir string, cmd string, keyval ...string) error {
+	_, err := v.run1(dir, cmd, keyval, true)
+	return err
+}
+
+// runVerboseOnly is like run but only generates error output to standard error in verbose mode.
+func (v *Cmd) runVerboseOnly(dir string, cmd string, keyval ...string) error {
+	_, err := v.run1(dir, cmd, keyval, false)
+	return err
+}
+
+// runOutput is like run but returns the output of the command.
+func (v *Cmd) runOutput(dir string, cmd string, keyval ...string) ([]byte, error) {
+	return v.run1(dir, cmd, keyval, true)
+}
+
+// runOutputVerboseOnly is like runOutput but only generates error output to
+// standard error in verbose mode.
+func (v *Cmd) runOutputVerboseOnly(dir string, cmd string, keyval ...string) ([]byte, error) {
+	return v.run1(dir, cmd, keyval, false)
+}
+
+// run1 is the generalized implementation of run and runOutput.
+func (v *Cmd) run1(dir string, cmdline string, keyval []string, verbose bool) ([]byte, error) {
+	m := make(map[string]string)
+	for i := 0; i < len(keyval); i += 2 {
+		m[keyval[i]] = keyval[i+1]
+	}
+	args := strings.Fields(cmdline)
+	for i, arg := range args {
+		args[i] = expand(m, arg)
+	}
+
+	if len(args) >= 2 && args[0] == "-go-internal-mkdir" {
+		var err error
+		if filepath.IsAbs(args[1]) {
+			err = os.Mkdir(args[1], fs.ModePerm)
+		} else {
+			err = os.Mkdir(filepath.Join(dir, args[1]), fs.ModePerm)
+		}
+		if err != nil {
+			return nil, err
+		}
+		args = args[2:]
+	}
+
+	if len(args) >= 2 && args[0] == "-go-internal-cd" {
+		if filepath.IsAbs(args[1]) {
+			dir = args[1]
+		} else {
+			dir = filepath.Join(dir, args[1])
+		}
+		args = args[2:]
+	}
+
+	_, err := exec.LookPath(v.Cmd)
+	if err != nil {
+		fmt.Fprintf(os.Stderr,
+			"go: missing %s command. See https://golang.org/s/gogetcmd\n",
+			v.Name)
+		return nil, err
+	}
+
+	cmd := exec.Command(v.Cmd, args...)
+	cmd.Dir = dir
+	if cfg.BuildX {
+		fmt.Fprintf(os.Stderr, "cd %s\n", dir)
+		fmt.Fprintf(os.Stderr, "%s %s\n", v.Cmd, strings.Join(args, " "))
+	}
+	out, err := cmd.Output()
+	if err != nil {
+		if verbose || cfg.BuildV {
+			fmt.Fprintf(os.Stderr, "# cd %s; %s %s\n", dir, v.Cmd, strings.Join(args, " "))
+			if ee, ok := err.(*exec.ExitError); ok && len(ee.Stderr) > 0 {
+				os.Stderr.Write(ee.Stderr)
+			} else {
+				fmt.Fprintln(os.Stderr, err.Error())
+			}
+		}
+	}
+	return out, err
+}
+
+// Ping pings to determine scheme to use.
+func (v *Cmd) Ping(scheme, repo string) error {
+	// Run the ping command in an arbitrary working directory,
+	// but don't let the current working directory pollute the results.
+	// In module mode, we expect GOMODCACHE to exist and be a safe place for
+	// commands; in GOPATH mode, we expect that to be true of GOPATH/src.
+	dir := cfg.GOMODCACHE
+	if !cfg.ModulesEnabled {
+		dir = filepath.Join(cfg.BuildContext.GOPATH, "src")
+	}
+	os.MkdirAll(dir, 0777) // Ignore errors — if unsuccessful, the command will likely fail.
+
+	release, err := base.AcquireNet()
+	if err != nil {
+		return err
+	}
+	defer release()
+
+	return v.runVerboseOnly(dir, v.PingCmd, "scheme", scheme, "repo", repo)
+}
+
+// Create creates a new copy of repo in dir.
+// The parent of dir must exist; dir must not.
+func (v *Cmd) Create(dir, repo string) error {
+	release, err := base.AcquireNet()
+	if err != nil {
+		return err
+	}
+	defer release()
+
+	for _, cmd := range v.CreateCmd {
+		if err := v.run(filepath.Dir(dir), cmd, "dir", dir, "repo", repo); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// Download downloads any new changes for the repo in dir.
+func (v *Cmd) Download(dir string) error {
+	release, err := base.AcquireNet()
+	if err != nil {
+		return err
+	}
+	defer release()
+
+	for _, cmd := range v.DownloadCmd {
+		if err := v.run(dir, cmd); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// Tags returns the list of available tags for the repo in dir.
+func (v *Cmd) Tags(dir string) ([]string, error) {
+	var tags []string
+	for _, tc := range v.TagCmd {
+		out, err := v.runOutput(dir, tc.cmd)
+		if err != nil {
+			return nil, err
+		}
+		re := regexp.MustCompile(`(?m-s)` + tc.pattern)
+		for _, m := range re.FindAllStringSubmatch(string(out), -1) {
+			tags = append(tags, m[1])
+		}
+	}
+	return tags, nil
+}
+
+// TagSync syncs the repo in dir to the named tag,
+// which either is a tag returned by tags or is v.tagDefault.
+func (v *Cmd) TagSync(dir, tag string) error {
+	if v.TagSyncCmd == nil {
+		return nil
+	}
+	if tag != "" {
+		for _, tc := range v.TagLookupCmd {
+			out, err := v.runOutput(dir, tc.cmd, "tag", tag)
+			if err != nil {
+				return err
+			}
+			re := regexp.MustCompile(`(?m-s)` + tc.pattern)
+			m := re.FindStringSubmatch(string(out))
+			if len(m) > 1 {
+				tag = m[1]
+				break
+			}
+		}
+	}
+
+	release, err := base.AcquireNet()
+	if err != nil {
+		return err
+	}
+	defer release()
+
+	if tag == "" && v.TagSyncDefault != nil {
+		for _, cmd := range v.TagSyncDefault {
+			if err := v.run(dir, cmd); err != nil {
+				return err
+			}
+		}
+		return nil
+	}
+
+	for _, cmd := range v.TagSyncCmd {
+		if err := v.run(dir, cmd, "tag", tag); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// A vcsPath describes how to convert an import path into a
+// version control system and repository name.
+type vcsPath struct {
+	pathPrefix     string                              // prefix this description applies to
+	regexp         *lazyregexp.Regexp                  // compiled pattern for import path
+	repo           string                              // repository to use (expand with match of re)
+	vcs            string                              // version control system to use (expand with match of re)
+	check          func(match map[string]string) error // additional checks
+	schemelessRepo bool                                // if true, the repo pattern lacks a scheme
+}
+
+// FromDir inspects dir and its parents to determine the
+// version control system and code repository to use.
+// If no repository is found, FromDir returns an error
+// equivalent to os.ErrNotExist.
+func FromDir(dir, srcRoot string, allowNesting bool) (repoDir string, vcsCmd *Cmd, err error) {
+	// Clean and double-check that dir is in (a subdirectory of) srcRoot.
+	dir = filepath.Clean(dir)
+	if srcRoot != "" {
+		srcRoot = filepath.Clean(srcRoot)
+		if len(dir) <= len(srcRoot) || dir[len(srcRoot)] != filepath.Separator {
+			return "", nil, fmt.Errorf("directory %q is outside source root %q", dir, srcRoot)
+		}
+	}
+
+	origDir := dir
+	for len(dir) > len(srcRoot) {
+		for _, vcs := range vcsList {
+			if isVCSRoot(dir, vcs.RootNames) {
+				// Record first VCS we find.
+				// If allowNesting is false (as it is in GOPATH), keep looking for
+				// repositories in parent directories and report an error if one is
+				// found to mitigate VCS injection attacks.
+				if vcsCmd == nil {
+					vcsCmd = vcs
+					repoDir = dir
+					if allowNesting {
+						return repoDir, vcsCmd, nil
+					}
+					continue
+				}
+				// Otherwise, we have one VCS inside a different VCS.
+				return "", nil, fmt.Errorf("directory %q uses %s, but parent %q uses %s",
+					repoDir, vcsCmd.Cmd, dir, vcs.Cmd)
+			}
+		}
+
+		// Move to parent.
+		ndir := filepath.Dir(dir)
+		if len(ndir) >= len(dir) {
+			break
+		}
+		dir = ndir
+	}
+	if vcsCmd == nil {
+		return "", nil, &vcsNotFoundError{dir: origDir}
+	}
+	return repoDir, vcsCmd, nil
+}
+
+// isVCSRoot identifies a VCS root by checking whether the directory contains
+// any of the listed root names.
+func isVCSRoot(dir string, rootNames []rootName) bool {
+	for _, root := range rootNames {
+		fi, err := os.Stat(filepath.Join(dir, root.filename))
+		if err == nil && fi.IsDir() == root.isDir {
+			return true
+		}
+	}
+
+	return false
+}
+
+type rootName struct {
+	filename string
+	isDir    bool
+}
+
+type vcsNotFoundError struct {
+	dir string
+}
+
+func (e *vcsNotFoundError) Error() string {
+	return fmt.Sprintf("directory %q is not using a known version control system", e.dir)
+}
+
+func (e *vcsNotFoundError) Is(err error) bool {
+	return err == os.ErrNotExist
+}
+
+// A govcsRule is a single GOVCS rule like private:hg|svn.
+type govcsRule struct {
+	pattern string
+	allowed []string
+}
+
+// A govcsConfig is a full GOVCS configuration.
+type govcsConfig []govcsRule
+
+func parseGOVCS(s string) (govcsConfig, error) {
+	s = strings.TrimSpace(s)
+	if s == "" {
+		return nil, nil
+	}
+	var cfg govcsConfig
+	have := make(map[string]string)
+	for _, item := range strings.Split(s, ",") {
+		item = strings.TrimSpace(item)
+		if item == "" {
+			return nil, fmt.Errorf("empty entry in GOVCS")
+		}
+		pattern, list, found := strings.Cut(item, ":")
+		if !found {
+			return nil, fmt.Errorf("malformed entry in GOVCS (missing colon): %q", item)
+		}
+		pattern, list = strings.TrimSpace(pattern), strings.TrimSpace(list)
+		if pattern == "" {
+			return nil, fmt.Errorf("empty pattern in GOVCS: %q", item)
+		}
+		if list == "" {
+			return nil, fmt.Errorf("empty VCS list in GOVCS: %q", item)
+		}
+		if search.IsRelativePath(pattern) {
+			return nil, fmt.Errorf("relative pattern not allowed in GOVCS: %q", pattern)
+		}
+		if old := have[pattern]; old != "" {
+			return nil, fmt.Errorf("unreachable pattern in GOVCS: %q after %q", item, old)
+		}
+		have[pattern] = item
+		allowed := strings.Split(list, "|")
+		for i, a := range allowed {
+			a = strings.TrimSpace(a)
+			if a == "" {
+				return nil, fmt.Errorf("empty VCS name in GOVCS: %q", item)
+			}
+			allowed[i] = a
+		}
+		cfg = append(cfg, govcsRule{pattern, allowed})
+	}
+	return cfg, nil
+}
+
+func (c *govcsConfig) allow(path string, private bool, vcs string) bool {
+	for _, rule := range *c {
+		match := false
+		switch rule.pattern {
+		case "private":
+			match = private
+		case "public":
+			match = !private
+		default:
+			// Note: rule.pattern is known to be comma-free,
+			// so MatchPrefixPatterns is only matching a single pattern for us.
+			match = module.MatchPrefixPatterns(rule.pattern, path)
+		}
+		if !match {
+			continue
+		}
+		for _, allow := range rule.allowed {
+			if allow == vcs || allow == "all" {
+				return true
+			}
+		}
+		return false
+	}
+
+	// By default, nothing is allowed.
+	return false
+}
+
+var (
+	govcs     govcsConfig
+	govcsErr  error
+	govcsOnce sync.Once
+)
+
+// defaultGOVCS is the default setting for GOVCS.
+// Setting GOVCS adds entries ahead of these but does not remove them.
+// (They are appended to the parsed GOVCS setting.)
+//
+// The rationale behind allowing only Git and Mercurial is that
+// these two systems have had the most attention to issues
+// of being run as clients of untrusted servers. In contrast,
+// Bazaar, Fossil, and Subversion have primarily been used
+// in trusted, authenticated environments and are not as well
+// scrutinized as attack surfaces.
+//
+// See golang.org/issue/41730 for details.
+var defaultGOVCS = govcsConfig{
+	{"private", []string{"all"}},
+	{"public", []string{"git", "hg"}},
+}
+
+// CheckGOVCS checks whether the policy defined by the environment variable
+// GOVCS allows the given vcs command to be used with the given repository
+// root path. Note that root may not be a real package or module path; it's
+// the same as the root path in the go-import meta tag.
+func CheckGOVCS(vcs *Cmd, root string) error {
+	if vcs == vcsMod {
+		// Direct module (proxy protocol) fetches don't
+		// involve an external version control system
+		// and are always allowed.
+		return nil
+	}
+
+	govcsOnce.Do(func() {
+		govcs, govcsErr = parseGOVCS(os.Getenv("GOVCS"))
+		govcs = append(govcs, defaultGOVCS...)
+	})
+	if govcsErr != nil {
+		return govcsErr
+	}
+
+	private := module.MatchPrefixPatterns(cfg.GOPRIVATE, root)
+	if !govcs.allow(root, private, vcs.Cmd) {
+		what := "public"
+		if private {
+			what = "private"
+		}
+		return fmt.Errorf("GOVCS disallows using %s for %s %s; see 'go help vcs'", vcs.Cmd, what, root)
+	}
+
+	return nil
+}
+
+// CheckNested checks for an incorrectly-nested VCS-inside-VCS
+// situation for dir, checking parents up until srcRoot.
+func CheckNested(vcs *Cmd, dir, srcRoot string) error {
+	if len(dir) <= len(srcRoot) || dir[len(srcRoot)] != filepath.Separator {
+		return fmt.Errorf("directory %q is outside source root %q", dir, srcRoot)
+	}
+
+	otherDir := dir
+	for len(otherDir) > len(srcRoot) {
+		for _, otherVCS := range vcsList {
+			if isVCSRoot(otherDir, otherVCS.RootNames) {
+				// Allow expected vcs in original dir.
+				if otherDir == dir && otherVCS == vcs {
+					continue
+				}
+				// Otherwise, we have one VCS inside a different VCS.
+				return fmt.Errorf("directory %q uses %s, but parent %q uses %s", dir, vcs.Cmd, otherDir, otherVCS.Cmd)
+			}
+		}
+		// Move to parent.
+		newDir := filepath.Dir(otherDir)
+		if len(newDir) >= len(otherDir) {
+			// Shouldn't happen, but just in case, stop.
+			break
+		}
+		otherDir = newDir
+	}
+
+	return nil
+}
+
+// RepoRoot describes the repository root for a tree of source code.
+type RepoRoot struct {
+	Repo     string // repository URL, including scheme
+	Root     string // import path corresponding to root of repo
+	IsCustom bool   // defined by served <meta> tags (as opposed to hard-coded pattern)
+	VCS      *Cmd
+}
+
+func httpPrefix(s string) string {
+	for _, prefix := range [...]string{"http:", "https:"} {
+		if strings.HasPrefix(s, prefix) {
+			return prefix
+		}
+	}
+	return ""
+}
+
+// ModuleMode specifies whether to prefer modules when looking up code sources.
+type ModuleMode int
+
+const (
+	IgnoreMod ModuleMode = iota
+	PreferMod
+)
+
+// RepoRootForImportPath analyzes importPath to determine the
+// version control system, and code repository to use.
+func RepoRootForImportPath(importPath string, mod ModuleMode, security web.SecurityMode) (*RepoRoot, error) {
+	rr, err := repoRootFromVCSPaths(importPath, security, vcsPaths)
+	if err == errUnknownSite {
+		rr, err = repoRootForImportDynamic(importPath, mod, security)
+		if err != nil {
+			err = importErrorf(importPath, "unrecognized import path %q: %v", importPath, err)
+		}
+	}
+	if err != nil {
+		rr1, err1 := repoRootFromVCSPaths(importPath, security, vcsPathsAfterDynamic)
+		if err1 == nil {
+			rr = rr1
+			err = nil
+		}
+	}
+
+	// Should have been taken care of above, but make sure.
+	if err == nil && strings.Contains(importPath, "...") && strings.Contains(rr.Root, "...") {
+		// Do not allow wildcards in the repo root.
+		rr = nil
+		err = importErrorf(importPath, "cannot expand ... in %q", importPath)
+	}
+	return rr, err
+}
+
+var errUnknownSite = errors.New("dynamic lookup required to find mapping")
+
+// repoRootFromVCSPaths attempts to map importPath to a repoRoot
+// using the mappings defined in vcsPaths.
+func repoRootFromVCSPaths(importPath string, security web.SecurityMode, vcsPaths []*vcsPath) (*RepoRoot, error) {
+	if str.HasPathPrefix(importPath, "example.net") {
+		// TODO(rsc): This should not be necessary, but it's required to keep
+		// tests like ../../testdata/script/mod_get_extra.txt from using the network.
+		// That script has everything it needs in the replacement set, but it is still
+		// doing network calls.
+		return nil, fmt.Errorf("no modules on example.net")
+	}
+	if importPath == "rsc.io" {
+		// This special case allows tests like ../../testdata/script/govcs.txt
+		// to avoid making any network calls. The module lookup for a path
+		// like rsc.io/nonexist.svn/foo needs to not make a network call for
+		// a lookup on rsc.io.
+		return nil, fmt.Errorf("rsc.io is not a module")
+	}
+	// A common error is to use https://packagepath because that's what
+	// hg and git require. Diagnose this helpfully.
+	if prefix := httpPrefix(importPath); prefix != "" {
+		// The importPath has been cleaned, so has only one slash. The pattern
+		// ignores the slashes; the error message puts them back on the RHS at least.
+		return nil, fmt.Errorf("%q not allowed in import path", prefix+"//")
+	}
+	for _, srv := range vcsPaths {
+		if !str.HasPathPrefix(importPath, srv.pathPrefix) {
+			continue
+		}
+		m := srv.regexp.FindStringSubmatch(importPath)
+		if m == nil {
+			if srv.pathPrefix != "" {
+				return nil, importErrorf(importPath, "invalid %s import path %q", srv.pathPrefix, importPath)
+			}
+			continue
+		}
+
+		// Build map of named subexpression matches for expand.
+		match := map[string]string{
+			"prefix": srv.pathPrefix + "/",
+			"import": importPath,
+		}
+		for i, name := range srv.regexp.SubexpNames() {
+			if name != "" && match[name] == "" {
+				match[name] = m[i]
+			}
+		}
+		if srv.vcs != "" {
+			match["vcs"] = expand(match, srv.vcs)
+		}
+		if srv.repo != "" {
+			match["repo"] = expand(match, srv.repo)
+		}
+		if srv.check != nil {
+			if err := srv.check(match); err != nil {
+				return nil, err
+			}
+		}
+		vcs := vcsByCmd(match["vcs"])
+		if vcs == nil {
+			return nil, fmt.Errorf("unknown version control system %q", match["vcs"])
+		}
+		if err := CheckGOVCS(vcs, match["root"]); err != nil {
+			return nil, err
+		}
+		var repoURL string
+		if !srv.schemelessRepo {
+			repoURL = match["repo"]
+		} else {
+			repo := match["repo"]
+			var ok bool
+			repoURL, ok = interceptVCSTest(repo, vcs, security)
+			if !ok {
+				scheme, err := func() (string, error) {
+					for _, s := range vcs.Scheme {
+						if security == web.SecureOnly && !vcs.isSecureScheme(s) {
+							continue
+						}
+
+						// If we know how to ping URL schemes for this VCS,
+						// check that this repo works.
+						// Otherwise, default to the first scheme
+						// that meets the requested security level.
+						if vcs.PingCmd == "" {
+							return s, nil
+						}
+						if err := vcs.Ping(s, repo); err == nil {
+							return s, nil
+						}
+					}
+					securityFrag := ""
+					if security == web.SecureOnly {
+						securityFrag = "secure "
+					}
+					return "", fmt.Errorf("no %sprotocol found for repository", securityFrag)
+				}()
+				if err != nil {
+					return nil, err
+				}
+				repoURL = scheme + "://" + repo
+			}
+		}
+		rr := &RepoRoot{
+			Repo: repoURL,
+			Root: match["root"],
+			VCS:  vcs,
+		}
+		return rr, nil
+	}
+	return nil, errUnknownSite
+}
+
+func interceptVCSTest(repo string, vcs *Cmd, security web.SecurityMode) (repoURL string, ok bool) {
+	if VCSTestRepoURL == "" {
+		return "", false
+	}
+	if vcs == vcsMod {
+		// Since the "mod" protocol is implemented internally,
+		// requests will be intercepted at a lower level (in cmd/go/internal/web).
+		return "", false
+	}
+
+	if scheme, path, ok := strings.Cut(repo, "://"); ok {
+		if security == web.SecureOnly && !vcs.isSecureScheme(scheme) {
+			return "", false // Let the caller reject the original URL.
+		}
+		repo = path // Remove leading URL scheme if present.
+	}
+	for _, host := range VCSTestHosts {
+		if !str.HasPathPrefix(repo, host) {
+			continue
+		}
+
+		httpURL := VCSTestRepoURL + strings.TrimPrefix(repo, host)
+
+		if vcs == vcsSvn {
+			// Ping the vcweb HTTP server to tell it to initialize the SVN repository
+			// and get the SVN server URL.
+			u, err := urlpkg.Parse(httpURL + "?vcwebsvn=1")
+			if err != nil {
+				panic(fmt.Sprintf("invalid vcs-test repo URL: %v", err))
+			}
+			svnURL, err := web.GetBytes(u)
+			svnURL = bytes.TrimSpace(svnURL)
+			if err == nil && len(svnURL) > 0 {
+				return string(svnURL) + strings.TrimPrefix(repo, host), true
+			}
+
+			// vcs-test doesn't have a svn handler for the given path,
+			// so resolve the repo to HTTPS instead.
+		}
+
+		return httpURL, true
+	}
+	return "", false
+}
+
+// urlForImportPath returns a partially-populated URL for the given Go import path.
+//
+// The URL leaves the Scheme field blank so that web.Get will try any scheme
+// allowed by the selected security mode.
+func urlForImportPath(importPath string) (*urlpkg.URL, error) {
+	slash := strings.Index(importPath, "/")
+	if slash < 0 {
+		slash = len(importPath)
+	}
+	host, path := importPath[:slash], importPath[slash:]
+	if !strings.Contains(host, ".") {
+		return nil, errors.New("import path does not begin with hostname")
+	}
+	if len(path) == 0 {
+		path = "/"
+	}
+	return &urlpkg.URL{Host: host, Path: path, RawQuery: "go-get=1"}, nil
+}
+
+// repoRootForImportDynamic finds a *RepoRoot for a custom domain that's not
+// statically known by repoRootFromVCSPaths.
+//
+// This handles custom import paths like "name.tld/pkg/foo" or just "name.tld".
+func repoRootForImportDynamic(importPath string, mod ModuleMode, security web.SecurityMode) (*RepoRoot, error) {
+	url, err := urlForImportPath(importPath)
+	if err != nil {
+		return nil, err
+	}
+	resp, err := web.Get(security, url)
+	if err != nil {
+		msg := "https fetch: %v"
+		if security == web.Insecure {
+			msg = "http/" + msg
+		}
+		return nil, fmt.Errorf(msg, err)
+	}
+	body := resp.Body
+	defer body.Close()
+	imports, err := parseMetaGoImports(body, mod)
+	if len(imports) == 0 {
+		if respErr := resp.Err(); respErr != nil {
+			// If the server's status was not OK, prefer to report that instead of
+			// an XML parse error.
+			return nil, respErr
+		}
+	}
+	if err != nil {
+		return nil, fmt.Errorf("parsing %s: %v", importPath, err)
+	}
+	// Find the matched meta import.
+	mmi, err := matchGoImport(imports, importPath)
+	if err != nil {
+		if _, ok := err.(ImportMismatchError); !ok {
+			return nil, fmt.Errorf("parse %s: %v", url, err)
+		}
+		return nil, fmt.Errorf("parse %s: no go-import meta tags (%s)", resp.URL, err)
+	}
+	if cfg.BuildV {
+		log.Printf("get %q: found meta tag %#v at %s", importPath, mmi, url)
+	}
+	// If the import was "uni.edu/bob/project", which said the
+	// prefix was "uni.edu" and the RepoRoot was "evilroot.com",
+	// make sure we don't trust Bob and check out evilroot.com to
+	// "uni.edu" yet (possibly overwriting/preempting another
+	// non-evil student). Instead, first verify the root and see
+	// if it matches Bob's claim.
+	if mmi.Prefix != importPath {
+		if cfg.BuildV {
+			log.Printf("get %q: verifying non-authoritative meta tag", importPath)
+		}
+		var imports []metaImport
+		url, imports, err = metaImportsForPrefix(mmi.Prefix, mod, security)
+		if err != nil {
+			return nil, err
+		}
+		metaImport2, err := matchGoImport(imports, importPath)
+		if err != nil || mmi != metaImport2 {
+			return nil, fmt.Errorf("%s and %s disagree about go-import for %s", resp.URL, url, mmi.Prefix)
+		}
+	}
+
+	if err := validateRepoRoot(mmi.RepoRoot); err != nil {
+		return nil, fmt.Errorf("%s: invalid repo root %q: %v", resp.URL, mmi.RepoRoot, err)
+	}
+	var vcs *Cmd
+	if mmi.VCS == "mod" {
+		vcs = vcsMod
+	} else {
+		vcs = vcsByCmd(mmi.VCS)
+		if vcs == nil {
+			return nil, fmt.Errorf("%s: unknown vcs %q", resp.URL, mmi.VCS)
+		}
+	}
+
+	if err := CheckGOVCS(vcs, mmi.Prefix); err != nil {
+		return nil, err
+	}
+
+	repoURL, ok := interceptVCSTest(mmi.RepoRoot, vcs, security)
+	if !ok {
+		repoURL = mmi.RepoRoot
+	}
+	rr := &RepoRoot{
+		Repo:     repoURL,
+		Root:     mmi.Prefix,
+		IsCustom: true,
+		VCS:      vcs,
+	}
+	return rr, nil
+}
+
+// validateRepoRoot returns an error if repoRoot does not seem to be
+// a valid URL with scheme.
+func validateRepoRoot(repoRoot string) error {
+	url, err := urlpkg.Parse(repoRoot)
+	if err != nil {
+		return err
+	}
+	if url.Scheme == "" {
+		return errors.New("no scheme")
+	}
+	if url.Scheme == "file" {
+		return errors.New("file scheme disallowed")
+	}
+	return nil
+}
+
+var fetchGroup singleflight.Group
+var (
+	fetchCacheMu sync.Mutex
+	fetchCache   = map[string]fetchResult{} // key is metaImportsForPrefix's importPrefix
+)
+
+// metaImportsForPrefix takes a package's root import path as declared in a <meta> tag
+// and returns its HTML discovery URL and the parsed metaImport lines
+// found on the page.
+//
+// The importPath is of the form "golang.org/x/tools".
+// It is an error if no imports are found.
+// url will still be valid if err != nil.
+// The returned url will be of the form "https://golang.org/x/tools?go-get=1"
+func metaImportsForPrefix(importPrefix string, mod ModuleMode, security web.SecurityMode) (*urlpkg.URL, []metaImport, error) {
+	setCache := func(res fetchResult) (fetchResult, error) {
+		fetchCacheMu.Lock()
+		defer fetchCacheMu.Unlock()
+		fetchCache[importPrefix] = res
+		return res, nil
+	}
+
+	resi, _, _ := fetchGroup.Do(importPrefix, func() (resi any, err error) {
+		fetchCacheMu.Lock()
+		if res, ok := fetchCache[importPrefix]; ok {
+			fetchCacheMu.Unlock()
+			return res, nil
+		}
+		fetchCacheMu.Unlock()
+
+		url, err := urlForImportPath(importPrefix)
+		if err != nil {
+			return setCache(fetchResult{err: err})
+		}
+		resp, err := web.Get(security, url)
+		if err != nil {
+			return setCache(fetchResult{url: url, err: fmt.Errorf("fetching %s: %v", importPrefix, err)})
+		}
+		body := resp.Body
+		defer body.Close()
+		imports, err := parseMetaGoImports(body, mod)
+		if len(imports) == 0 {
+			if respErr := resp.Err(); respErr != nil {
+				// If the server's status was not OK, prefer to report that instead of
+				// an XML parse error.
+				return setCache(fetchResult{url: url, err: respErr})
+			}
+		}
+		if err != nil {
+			return setCache(fetchResult{url: url, err: fmt.Errorf("parsing %s: %v", resp.URL, err)})
+		}
+		if len(imports) == 0 {
+			err = fmt.Errorf("fetching %s: no go-import meta tag found in %s", importPrefix, resp.URL)
+		}
+		return setCache(fetchResult{url: url, imports: imports, err: err})
+	})
+	res := resi.(fetchResult)
+	return res.url, res.imports, res.err
+}
+
+type fetchResult struct {
+	url     *urlpkg.URL
+	imports []metaImport
+	err     error
+}
+
+// metaImport represents the parsed <meta name="go-import"
+// content="prefix vcs reporoot" /> tags from HTML files.
+type metaImport struct {
+	Prefix, VCS, RepoRoot string
+}
+
+// A ImportMismatchError is returned where metaImport/s are present
+// but none match our import path.
+type ImportMismatchError struct {
+	importPath string
+	mismatches []string // the meta imports that were discarded for not matching our importPath
+}
+
+func (m ImportMismatchError) Error() string {
+	formattedStrings := make([]string, len(m.mismatches))
+	for i, pre := range m.mismatches {
+		formattedStrings[i] = fmt.Sprintf("meta tag %s did not match import path %s", pre, m.importPath)
+	}
+	return strings.Join(formattedStrings, ", ")
+}
+
+// matchGoImport returns the metaImport from imports matching importPath.
+// An error is returned if there are multiple matches.
+// An ImportMismatchError is returned if none match.
+func matchGoImport(imports []metaImport, importPath string) (metaImport, error) {
+	match := -1
+
+	errImportMismatch := ImportMismatchError{importPath: importPath}
+	for i, im := range imports {
+		if !str.HasPathPrefix(importPath, im.Prefix) {
+			errImportMismatch.mismatches = append(errImportMismatch.mismatches, im.Prefix)
+			continue
+		}
+
+		if match >= 0 {
+			if imports[match].VCS == "mod" && im.VCS != "mod" {
+				// All the mod entries precede all the non-mod entries.
+				// We have a mod entry and don't care about the rest,
+				// matching or not.
+				break
+			}
+			return metaImport{}, fmt.Errorf("multiple meta tags match import path %q", importPath)
+		}
+		match = i
+	}
+
+	if match == -1 {
+		return metaImport{}, errImportMismatch
+	}
+	return imports[match], nil
+}
+
+// expand rewrites s to replace {k} with match[k] for each key k in match.
+func expand(match map[string]string, s string) string {
+	// We want to replace each match exactly once, and the result of expansion
+	// must not depend on the iteration order through the map.
+	// A strings.Replacer has exactly the properties we're looking for.
+	oldNew := make([]string, 0, 2*len(match))
+	for k, v := range match {
+		oldNew = append(oldNew, "{"+k+"}", v)
+	}
+	return strings.NewReplacer(oldNew...).Replace(s)
+}
+
+// vcsPaths defines the meaning of import paths referring to
+// commonly-used VCS hosting sites (github.com/user/dir)
+// and import paths referring to a fully-qualified importPath
+// containing a VCS type (foo.com/repo.git/dir)
+var vcsPaths = []*vcsPath{
+	// GitHub
+	{
+		pathPrefix: "github.com",
+		regexp:     lazyregexp.New(`^(?P<root>github\.com/[\w.\-]+/[\w.\-]+)(/[\w.\-]+)*$`),
+		vcs:        "git",
+		repo:       "https://{root}",
+		check:      noVCSSuffix,
+	},
+
+	// Bitbucket
+	{
+		pathPrefix: "bitbucket.org",
+		regexp:     lazyregexp.New(`^(?P<root>bitbucket\.org/(?P<bitname>[\w.\-]+/[\w.\-]+))(/[\w.\-]+)*$`),
+		vcs:        "git",
+		repo:       "https://{root}",
+		check:      noVCSSuffix,
+	},
+
+	// IBM DevOps Services (JazzHub)
+	{
+		pathPrefix: "hub.jazz.net/git",
+		regexp:     lazyregexp.New(`^(?P<root>hub\.jazz\.net/git/[a-z0-9]+/[\w.\-]+)(/[\w.\-]+)*$`),
+		vcs:        "git",
+		repo:       "https://{root}",
+		check:      noVCSSuffix,
+	},
+
+	// Git at Apache
+	{
+		pathPrefix: "git.apache.org",
+		regexp:     lazyregexp.New(`^(?P<root>git\.apache\.org/[a-z0-9_.\-]+\.git)(/[\w.\-]+)*$`),
+		vcs:        "git",
+		repo:       "https://{root}",
+	},
+
+	// Git at OpenStack
+	{
+		pathPrefix: "git.openstack.org",
+		regexp:     lazyregexp.New(`^(?P<root>git\.openstack\.org/[\w.\-]+/[\w.\-]+)(\.git)?(/[\w.\-]+)*$`),
+		vcs:        "git",
+		repo:       "https://{root}",
+	},
+
+	// chiselapp.com for fossil
+	{
+		pathPrefix: "chiselapp.com",
+		regexp:     lazyregexp.New(`^(?P<root>chiselapp\.com/user/[A-Za-z0-9]+/repository/[\w.\-]+)$`),
+		vcs:        "fossil",
+		repo:       "https://{root}",
+	},
+
+	// General syntax for any server.
+	// Must be last.
+	{
+		regexp:         lazyregexp.New(`(?P<root>(?P<repo>([a-z0-9.\-]+\.)+[a-z0-9.\-]+(:[0-9]+)?(/~?[\w.\-]+)+?)\.(?P<vcs>bzr|fossil|git|hg|svn))(/~?[\w.\-]+)*$`),
+		schemelessRepo: true,
+	},
+}
+
+// vcsPathsAfterDynamic gives additional vcsPaths entries
+// to try after the dynamic HTML check.
+// This gives those sites a chance to introduce <meta> tags
+// as part of a graceful transition away from the hard-coded logic.
+var vcsPathsAfterDynamic = []*vcsPath{
+	// Launchpad. See golang.org/issue/11436.
+	{
+		pathPrefix: "launchpad.net",
+		regexp:     lazyregexp.New(`^(?P<root>launchpad\.net/((?P<project>[\w.\-]+)(?P<series>/[\w.\-]+)?|~[\w.\-]+/(\+junk|[\w.\-]+)/[\w.\-]+))(/[\w.\-]+)*$`),
+		vcs:        "bzr",
+		repo:       "https://{root}",
+		check:      launchpadVCS,
+	},
+}
+
+// noVCSSuffix checks that the repository name does not
+// end in .foo for any version control system foo.
+// The usual culprit is ".git".
+func noVCSSuffix(match map[string]string) error {
+	repo := match["repo"]
+	for _, vcs := range vcsList {
+		if strings.HasSuffix(repo, "."+vcs.Cmd) {
+			return fmt.Errorf("invalid version control suffix in %s path", match["prefix"])
+		}
+	}
+	return nil
+}
+
+// launchpadVCS solves the ambiguity for "lp.net/project/foo". In this case,
+// "foo" could be a series name registered in Launchpad with its own branch,
+// and it could also be the name of a directory within the main project
+// branch one level up.
+func launchpadVCS(match map[string]string) error {
+	if match["project"] == "" || match["series"] == "" {
+		return nil
+	}
+	url := &urlpkg.URL{
+		Scheme: "https",
+		Host:   "code.launchpad.net",
+		Path:   expand(match, "/{project}{series}/.bzr/branch-format"),
+	}
+	_, err := web.GetBytes(url)
+	if err != nil {
+		match["root"] = expand(match, "launchpad.net/{project}")
+		match["repo"] = expand(match, "https://{root}")
+	}
+	return nil
+}
+
+// importError is a copy of load.importError, made to avoid a dependency cycle
+// on cmd/go/internal/load. It just needs to satisfy load.ImportPathError.
+type importError struct {
+	importPath string
+	err        error
+}
+
+func importErrorf(path, format string, args ...any) error {
+	err := &importError{importPath: path, err: fmt.Errorf(format, args...)}
+	if errStr := err.Error(); !strings.Contains(errStr, path) {
+		panic(fmt.Sprintf("path %q not in error %q", path, errStr))
+	}
+	return err
+}
+
+func (e *importError) Error() string {
+	return e.err.Error()
+}
+
+func (e *importError) Unwrap() error {
+	// Don't return e.err directly, since we're only wrapping an error if %w
+	// was passed to ImportErrorf.
+	return errors.Unwrap(e.err)
+}
+
+func (e *importError) ImportPath() string {
+	return e.importPath
+}
diff --git a/src/cmd/go/internal/vcs/vcs_test.go b/src/cmd/go/internal/vcs/vcs_test.go
new file mode 100644
index 0000000..2ce85ea
--- /dev/null
+++ b/src/cmd/go/internal/vcs/vcs_test.go
@@ -0,0 +1,581 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package vcs
+
+import (
+	"errors"
+	"fmt"
+	"internal/testenv"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"cmd/go/internal/web"
+)
+
+func init() {
+	// GOVCS defaults to public:git|hg,private:all,
+	// which breaks many tests here - they can't use non-git, non-hg VCS at all!
+	// Change to fully permissive.
+	// The tests of the GOVCS setting itself are in ../../testdata/script/govcs.txt.
+	os.Setenv("GOVCS", "*:all")
+}
+
+// Test that RepoRootForImportPath determines the correct RepoRoot for a given importPath.
+// TODO(cmang): Add tests for SVN and BZR.
+func TestRepoRootForImportPath(t *testing.T) {
+	testenv.MustHaveExternalNetwork(t)
+
+	tests := []struct {
+		path string
+		want *RepoRoot
+	}{
+		{
+			"github.com/golang/groupcache",
+			&RepoRoot{
+				VCS:  vcsGit,
+				Repo: "https://github.com/golang/groupcache",
+			},
+		},
+		// Unicode letters in directories are not valid.
+		{
+			"github.com/user/unicode/испытание",
+			nil,
+		},
+		// IBM DevOps Services tests
+		{
+			"hub.jazz.net/git/user1/pkgname",
+			&RepoRoot{
+				VCS:  vcsGit,
+				Repo: "https://hub.jazz.net/git/user1/pkgname",
+			},
+		},
+		{
+			"hub.jazz.net/git/user1/pkgname/submodule/submodule/submodule",
+			&RepoRoot{
+				VCS:  vcsGit,
+				Repo: "https://hub.jazz.net/git/user1/pkgname",
+			},
+		},
+		{
+			"hub.jazz.net",
+			nil,
+		},
+		{
+			"hubajazz.net",
+			nil,
+		},
+		{
+			"hub2.jazz.net",
+			nil,
+		},
+		{
+			"hub.jazz.net/someotherprefix",
+			nil,
+		},
+		{
+			"hub.jazz.net/someotherprefix/user1/pkgname",
+			nil,
+		},
+		// Spaces are not valid in user names or package names
+		{
+			"hub.jazz.net/git/User 1/pkgname",
+			nil,
+		},
+		{
+			"hub.jazz.net/git/user1/pkg name",
+			nil,
+		},
+		// Dots are not valid in user names
+		{
+			"hub.jazz.net/git/user.1/pkgname",
+			nil,
+		},
+		{
+			"hub.jazz.net/git/user/pkg.name",
+			&RepoRoot{
+				VCS:  vcsGit,
+				Repo: "https://hub.jazz.net/git/user/pkg.name",
+			},
+		},
+		// User names cannot have uppercase letters
+		{
+			"hub.jazz.net/git/USER/pkgname",
+			nil,
+		},
+		// OpenStack tests
+		{
+			"git.openstack.org/openstack/swift",
+			&RepoRoot{
+				VCS:  vcsGit,
+				Repo: "https://git.openstack.org/openstack/swift",
+			},
+		},
+		// Trailing .git is less preferred but included for
+		// compatibility purposes while the same source needs to
+		// be compilable on both old and new go
+		{
+			"git.openstack.org/openstack/swift.git",
+			&RepoRoot{
+				VCS:  vcsGit,
+				Repo: "https://git.openstack.org/openstack/swift.git",
+			},
+		},
+		{
+			"git.openstack.org/openstack/swift/go/hummingbird",
+			&RepoRoot{
+				VCS:  vcsGit,
+				Repo: "https://git.openstack.org/openstack/swift",
+			},
+		},
+		{
+			"git.openstack.org",
+			nil,
+		},
+		{
+			"git.openstack.org/openstack",
+			nil,
+		},
+		// Spaces are not valid in package name
+		{
+			"git.apache.org/package name/path/to/lib",
+			nil,
+		},
+		// Should have ".git" suffix
+		{
+			"git.apache.org/package-name/path/to/lib",
+			nil,
+		},
+		{
+			"gitbapache.org",
+			nil,
+		},
+		{
+			"git.apache.org/package-name.git",
+			&RepoRoot{
+				VCS:  vcsGit,
+				Repo: "https://git.apache.org/package-name.git",
+			},
+		},
+		{
+			"git.apache.org/package-name_2.x.git/path/to/lib",
+			&RepoRoot{
+				VCS:  vcsGit,
+				Repo: "https://git.apache.org/package-name_2.x.git",
+			},
+		},
+		{
+			"chiselapp.com/user/kyle/repository/fossilgg",
+			&RepoRoot{
+				VCS:  vcsFossil,
+				Repo: "https://chiselapp.com/user/kyle/repository/fossilgg",
+			},
+		},
+		{
+			// must have a user/$name/repository/$repo path
+			"chiselapp.com/kyle/repository/fossilgg",
+			nil,
+		},
+		{
+			"chiselapp.com/user/kyle/fossilgg",
+			nil,
+		},
+		{
+			"bitbucket.org/workspace/pkgname",
+			&RepoRoot{
+				VCS:  vcsGit,
+				Repo: "https://bitbucket.org/workspace/pkgname",
+			},
+		},
+	}
+
+	for _, test := range tests {
+		got, err := RepoRootForImportPath(test.path, IgnoreMod, web.SecureOnly)
+		want := test.want
+
+		if want == nil {
+			if err == nil {
+				t.Errorf("RepoRootForImportPath(%q): Error expected but not received", test.path)
+			}
+			continue
+		}
+		if err != nil {
+			t.Errorf("RepoRootForImportPath(%q): %v", test.path, err)
+			continue
+		}
+		if got.VCS.Name != want.VCS.Name || got.Repo != want.Repo {
+			t.Errorf("RepoRootForImportPath(%q) = VCS(%s) Repo(%s), want VCS(%s) Repo(%s)", test.path, got.VCS, got.Repo, want.VCS, want.Repo)
+		}
+	}
+}
+
+// Test that vcs.FromDir correctly inspects a given directory and returns the
+// right VCS and repo directory.
+func TestFromDir(t *testing.T) {
+	tempDir := t.TempDir()
+
+	for _, vcs := range vcsList {
+		for r, root := range vcs.RootNames {
+			vcsName := fmt.Sprint(vcs.Name, r)
+			dir := filepath.Join(tempDir, "example.com", vcsName, root.filename)
+			if root.isDir {
+				err := os.MkdirAll(dir, 0755)
+				if err != nil {
+					t.Fatal(err)
+				}
+			} else {
+				err := os.MkdirAll(filepath.Dir(dir), 0755)
+				if err != nil {
+					t.Fatal(err)
+				}
+				f, err := os.Create(dir)
+				if err != nil {
+					t.Fatal(err)
+				}
+				f.Close()
+			}
+
+			wantRepoDir := filepath.Dir(dir)
+			gotRepoDir, gotVCS, err := FromDir(dir, tempDir, false)
+			if err != nil {
+				t.Errorf("FromDir(%q, %q): %v", dir, tempDir, err)
+				continue
+			}
+			if gotRepoDir != wantRepoDir || gotVCS.Name != vcs.Name {
+				t.Errorf("FromDir(%q, %q) = RepoDir(%s), VCS(%s); want RepoDir(%s), VCS(%s)", dir, tempDir, gotRepoDir, gotVCS.Name, wantRepoDir, vcs.Name)
+			}
+		}
+	}
+}
+
+func TestIsSecure(t *testing.T) {
+	tests := []struct {
+		vcs    *Cmd
+		url    string
+		secure bool
+	}{
+		{vcsGit, "http://example.com/foo.git", false},
+		{vcsGit, "https://example.com/foo.git", true},
+		{vcsBzr, "http://example.com/foo.bzr", false},
+		{vcsBzr, "https://example.com/foo.bzr", true},
+		{vcsSvn, "http://example.com/svn", false},
+		{vcsSvn, "https://example.com/svn", true},
+		{vcsHg, "http://example.com/foo.hg", false},
+		{vcsHg, "https://example.com/foo.hg", true},
+		{vcsGit, "ssh://user@example.com/foo.git", true},
+		{vcsGit, "user@server:path/to/repo.git", false},
+		{vcsGit, "user@server:", false},
+		{vcsGit, "server:repo.git", false},
+		{vcsGit, "server:path/to/repo.git", false},
+		{vcsGit, "example.com:path/to/repo.git", false},
+		{vcsGit, "path/that/contains/a:colon/repo.git", false},
+		{vcsHg, "ssh://user@example.com/path/to/repo.hg", true},
+		{vcsFossil, "http://example.com/foo", false},
+		{vcsFossil, "https://example.com/foo", true},
+	}
+
+	for _, test := range tests {
+		secure := test.vcs.IsSecure(test.url)
+		if secure != test.secure {
+			t.Errorf("%s isSecure(%q) = %t; want %t", test.vcs, test.url, secure, test.secure)
+		}
+	}
+}
+
+func TestIsSecureGitAllowProtocol(t *testing.T) {
+	tests := []struct {
+		vcs    *Cmd
+		url    string
+		secure bool
+	}{
+		// Same as TestIsSecure to verify same behavior.
+		{vcsGit, "http://example.com/foo.git", false},
+		{vcsGit, "https://example.com/foo.git", true},
+		{vcsBzr, "http://example.com/foo.bzr", false},
+		{vcsBzr, "https://example.com/foo.bzr", true},
+		{vcsSvn, "http://example.com/svn", false},
+		{vcsSvn, "https://example.com/svn", true},
+		{vcsHg, "http://example.com/foo.hg", false},
+		{vcsHg, "https://example.com/foo.hg", true},
+		{vcsGit, "user@server:path/to/repo.git", false},
+		{vcsGit, "user@server:", false},
+		{vcsGit, "server:repo.git", false},
+		{vcsGit, "server:path/to/repo.git", false},
+		{vcsGit, "example.com:path/to/repo.git", false},
+		{vcsGit, "path/that/contains/a:colon/repo.git", false},
+		{vcsHg, "ssh://user@example.com/path/to/repo.hg", true},
+		// New behavior.
+		{vcsGit, "ssh://user@example.com/foo.git", false},
+		{vcsGit, "foo://example.com/bar.git", true},
+		{vcsHg, "foo://example.com/bar.hg", false},
+		{vcsSvn, "foo://example.com/svn", false},
+		{vcsBzr, "foo://example.com/bar.bzr", false},
+	}
+
+	defer os.Unsetenv("GIT_ALLOW_PROTOCOL")
+	os.Setenv("GIT_ALLOW_PROTOCOL", "https:foo")
+	for _, test := range tests {
+		secure := test.vcs.IsSecure(test.url)
+		if secure != test.secure {
+			t.Errorf("%s isSecure(%q) = %t; want %t", test.vcs, test.url, secure, test.secure)
+		}
+	}
+}
+
+func TestMatchGoImport(t *testing.T) {
+	tests := []struct {
+		imports []metaImport
+		path    string
+		mi      metaImport
+		err     error
+	}{
+		{
+			imports: []metaImport{
+				{Prefix: "example.com/user/foo", VCS: "git", RepoRoot: "https://example.com/repo/target"},
+			},
+			path: "example.com/user/foo",
+			mi:   metaImport{Prefix: "example.com/user/foo", VCS: "git", RepoRoot: "https://example.com/repo/target"},
+		},
+		{
+			imports: []metaImport{
+				{Prefix: "example.com/user/foo", VCS: "git", RepoRoot: "https://example.com/repo/target"},
+			},
+			path: "example.com/user/foo/",
+			mi:   metaImport{Prefix: "example.com/user/foo", VCS: "git", RepoRoot: "https://example.com/repo/target"},
+		},
+		{
+			imports: []metaImport{
+				{Prefix: "example.com/user/foo", VCS: "git", RepoRoot: "https://example.com/repo/target"},
+				{Prefix: "example.com/user/fooa", VCS: "git", RepoRoot: "https://example.com/repo/target"},
+			},
+			path: "example.com/user/foo",
+			mi:   metaImport{Prefix: "example.com/user/foo", VCS: "git", RepoRoot: "https://example.com/repo/target"},
+		},
+		{
+			imports: []metaImport{
+				{Prefix: "example.com/user/foo", VCS: "git", RepoRoot: "https://example.com/repo/target"},
+				{Prefix: "example.com/user/fooa", VCS: "git", RepoRoot: "https://example.com/repo/target"},
+			},
+			path: "example.com/user/fooa",
+			mi:   metaImport{Prefix: "example.com/user/fooa", VCS: "git", RepoRoot: "https://example.com/repo/target"},
+		},
+		{
+			imports: []metaImport{
+				{Prefix: "example.com/user/foo", VCS: "git", RepoRoot: "https://example.com/repo/target"},
+				{Prefix: "example.com/user/foo/bar", VCS: "git", RepoRoot: "https://example.com/repo/target"},
+			},
+			path: "example.com/user/foo/bar",
+			err:  errors.New("should not be allowed to create nested repo"),
+		},
+		{
+			imports: []metaImport{
+				{Prefix: "example.com/user/foo", VCS: "git", RepoRoot: "https://example.com/repo/target"},
+				{Prefix: "example.com/user/foo/bar", VCS: "git", RepoRoot: "https://example.com/repo/target"},
+			},
+			path: "example.com/user/foo/bar/baz",
+			err:  errors.New("should not be allowed to create nested repo"),
+		},
+		{
+			imports: []metaImport{
+				{Prefix: "example.com/user/foo", VCS: "git", RepoRoot: "https://example.com/repo/target"},
+				{Prefix: "example.com/user/foo/bar", VCS: "git", RepoRoot: "https://example.com/repo/target"},
+			},
+			path: "example.com/user/foo/bar/baz/qux",
+			err:  errors.New("should not be allowed to create nested repo"),
+		},
+		{
+			imports: []metaImport{
+				{Prefix: "example.com/user/foo", VCS: "git", RepoRoot: "https://example.com/repo/target"},
+				{Prefix: "example.com/user/foo/bar", VCS: "git", RepoRoot: "https://example.com/repo/target"},
+			},
+			path: "example.com/user/foo/bar/baz/",
+			err:  errors.New("should not be allowed to create nested repo"),
+		},
+		{
+			imports: []metaImport{
+				{Prefix: "example.com/user/foo", VCS: "git", RepoRoot: "https://example.com/repo/target"},
+				{Prefix: "example.com/user/foo/bar", VCS: "git", RepoRoot: "https://example.com/repo/target"},
+			},
+			path: "example.com",
+			err:  errors.New("pathologically short path"),
+		},
+		{
+			imports: []metaImport{
+				{Prefix: "example.com/user/foo", VCS: "git", RepoRoot: "https://example.com/repo/target"},
+			},
+			path: "different.example.com/user/foo",
+			err:  errors.New("meta tags do not match import path"),
+		},
+		{
+			imports: []metaImport{
+				{Prefix: "myitcv.io/blah2", VCS: "mod", RepoRoot: "https://raw.githubusercontent.com/myitcv/pubx/master"},
+				{Prefix: "myitcv.io", VCS: "git", RepoRoot: "https://github.com/myitcv/x"},
+			},
+			path: "myitcv.io/blah2/foo",
+			mi:   metaImport{Prefix: "myitcv.io/blah2", VCS: "mod", RepoRoot: "https://raw.githubusercontent.com/myitcv/pubx/master"},
+		},
+		{
+			imports: []metaImport{
+				{Prefix: "myitcv.io/blah2", VCS: "mod", RepoRoot: "https://raw.githubusercontent.com/myitcv/pubx/master"},
+				{Prefix: "myitcv.io", VCS: "git", RepoRoot: "https://github.com/myitcv/x"},
+			},
+			path: "myitcv.io/other",
+			mi:   metaImport{Prefix: "myitcv.io", VCS: "git", RepoRoot: "https://github.com/myitcv/x"},
+		},
+	}
+
+	for _, test := range tests {
+		mi, err := matchGoImport(test.imports, test.path)
+		if mi != test.mi {
+			t.Errorf("unexpected metaImport; got %v, want %v", mi, test.mi)
+		}
+
+		got := err
+		want := test.err
+		if (got == nil) != (want == nil) {
+			t.Errorf("unexpected error; got %v, want %v", got, want)
+		}
+	}
+}
+
+func TestValidateRepoRoot(t *testing.T) {
+	tests := []struct {
+		root string
+		ok   bool
+	}{
+		{
+			root: "",
+			ok:   false,
+		},
+		{
+			root: "http://",
+			ok:   true,
+		},
+		{
+			root: "git+ssh://",
+			ok:   true,
+		},
+		{
+			root: "http#://",
+			ok:   false,
+		},
+		{
+			root: "-config",
+			ok:   false,
+		},
+		{
+			root: "-config://",
+			ok:   false,
+		},
+	}
+
+	for _, test := range tests {
+		err := validateRepoRoot(test.root)
+		ok := err == nil
+		if ok != test.ok {
+			want := "error"
+			if test.ok {
+				want = "nil"
+			}
+			t.Errorf("validateRepoRoot(%q) = %q, want %s", test.root, err, want)
+		}
+	}
+}
+
+var govcsTests = []struct {
+	govcs string
+	path  string
+	vcs   string
+	ok    bool
+}{
+	{"private:all", "is-public.com/foo", "zzz", false},
+	{"private:all", "is-private.com/foo", "zzz", true},
+	{"public:all", "is-public.com/foo", "zzz", true},
+	{"public:all", "is-private.com/foo", "zzz", false},
+	{"public:all,private:none", "is-public.com/foo", "zzz", true},
+	{"public:all,private:none", "is-private.com/foo", "zzz", false},
+	{"*:all", "is-public.com/foo", "zzz", true},
+	{"golang.org:git", "golang.org/x/text", "zzz", false},
+	{"golang.org:git", "golang.org/x/text", "git", true},
+	{"golang.org:zzz", "golang.org/x/text", "zzz", true},
+	{"golang.org:zzz", "golang.org/x/text", "git", false},
+	{"golang.org:zzz", "golang.org/x/text", "zzz", true},
+	{"golang.org:zzz", "golang.org/x/text", "git", false},
+	{"golang.org:git|hg", "golang.org/x/text", "hg", true},
+	{"golang.org:git|hg", "golang.org/x/text", "git", true},
+	{"golang.org:git|hg", "golang.org/x/text", "zzz", false},
+	{"golang.org:all", "golang.org/x/text", "hg", true},
+	{"golang.org:all", "golang.org/x/text", "git", true},
+	{"golang.org:all", "golang.org/x/text", "zzz", true},
+	{"other.xyz/p:none,golang.org/x:git", "other.xyz/p/x", "git", false},
+	{"other.xyz/p:none,golang.org/x:git", "unexpected.com", "git", false},
+	{"other.xyz/p:none,golang.org/x:git", "golang.org/x/text", "zzz", false},
+	{"other.xyz/p:none,golang.org/x:git", "golang.org/x/text", "git", true},
+	{"other.xyz/p:none,golang.org/x:zzz", "golang.org/x/text", "zzz", true},
+	{"other.xyz/p:none,golang.org/x:zzz", "golang.org/x/text", "git", false},
+	{"other.xyz/p:none,golang.org/x:git|hg", "golang.org/x/text", "hg", true},
+	{"other.xyz/p:none,golang.org/x:git|hg", "golang.org/x/text", "git", true},
+	{"other.xyz/p:none,golang.org/x:git|hg", "golang.org/x/text", "zzz", false},
+	{"other.xyz/p:none,golang.org/x:all", "golang.org/x/text", "hg", true},
+	{"other.xyz/p:none,golang.org/x:all", "golang.org/x/text", "git", true},
+	{"other.xyz/p:none,golang.org/x:all", "golang.org/x/text", "zzz", true},
+	{"other.xyz/p:none,golang.org/x:git", "golang.org/y/text", "zzz", false},
+	{"other.xyz/p:none,golang.org/x:git", "golang.org/y/text", "git", false},
+	{"other.xyz/p:none,golang.org/x:zzz", "golang.org/y/text", "zzz", false},
+	{"other.xyz/p:none,golang.org/x:zzz", "golang.org/y/text", "git", false},
+	{"other.xyz/p:none,golang.org/x:git|hg", "golang.org/y/text", "hg", false},
+	{"other.xyz/p:none,golang.org/x:git|hg", "golang.org/y/text", "git", false},
+	{"other.xyz/p:none,golang.org/x:git|hg", "golang.org/y/text", "zzz", false},
+	{"other.xyz/p:none,golang.org/x:all", "golang.org/y/text", "hg", false},
+	{"other.xyz/p:none,golang.org/x:all", "golang.org/y/text", "git", false},
+	{"other.xyz/p:none,golang.org/x:all", "golang.org/y/text", "zzz", false},
+}
+
+func TestGOVCS(t *testing.T) {
+	for _, tt := range govcsTests {
+		cfg, err := parseGOVCS(tt.govcs)
+		if err != nil {
+			t.Errorf("parseGOVCS(%q): %v", tt.govcs, err)
+			continue
+		}
+		private := strings.HasPrefix(tt.path, "is-private")
+		ok := cfg.allow(tt.path, private, tt.vcs)
+		if ok != tt.ok {
+			t.Errorf("parseGOVCS(%q).allow(%q, %v, %q) = %v, want %v",
+				tt.govcs, tt.path, private, tt.vcs, ok, tt.ok)
+		}
+	}
+}
+
+var govcsErrors = []struct {
+	s   string
+	err string
+}{
+	{`,`, `empty entry in GOVCS`},
+	{`,x`, `empty entry in GOVCS`},
+	{`x,`, `malformed entry in GOVCS (missing colon): "x"`},
+	{`x:y,`, `empty entry in GOVCS`},
+	{`x`, `malformed entry in GOVCS (missing colon): "x"`},
+	{`x:`, `empty VCS list in GOVCS: "x:"`},
+	{`x:|`, `empty VCS name in GOVCS: "x:|"`},
+	{`x:y|`, `empty VCS name in GOVCS: "x:y|"`},
+	{`x:|y`, `empty VCS name in GOVCS: "x:|y"`},
+	{`x:y,z:`, `empty VCS list in GOVCS: "z:"`},
+	{`x:y,z:|`, `empty VCS name in GOVCS: "z:|"`},
+	{`x:y,z:|w`, `empty VCS name in GOVCS: "z:|w"`},
+	{`x:y,z:w|`, `empty VCS name in GOVCS: "z:w|"`},
+	{`x:y,z:w||v`, `empty VCS name in GOVCS: "z:w||v"`},
+	{`x:y,x:z`, `unreachable pattern in GOVCS: "x:z" after "x:y"`},
+}
+
+func TestGOVCSErrors(t *testing.T) {
+	for _, tt := range govcsErrors {
+		_, err := parseGOVCS(tt.s)
+		if err == nil || !strings.Contains(err.Error(), tt.err) {
+			t.Errorf("parseGOVCS(%s): err=%v, want %v", tt.s, err, tt.err)
+		}
+	}
+}
diff --git a/src/cmd/go/internal/vcweb/auth.go b/src/cmd/go/internal/vcweb/auth.go
new file mode 100644
index 0000000..383bf75
--- /dev/null
+++ b/src/cmd/go/internal/vcweb/auth.go
@@ -0,0 +1,108 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package vcweb
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"log"
+	"net/http"
+	"os"
+	"path"
+	"strings"
+)
+
+// authHandler serves requests only if the Basic Auth data sent with the request
+// matches the contents of a ".access" file in the requested directory.
+//
+// For each request, the handler looks for a file named ".access" and parses it
+// as a JSON-serialized accessToken. If the credentials from the request match
+// the accessToken, the file is served normally; otherwise, it is rejected with
+// the StatusCode and Message provided by the token.
+type authHandler struct{}
+
+type accessToken struct {
+	Username, Password string
+	StatusCode         int // defaults to 401.
+	Message            string
+}
+
+func (h *authHandler) Available() bool { return true }
+
+func (h *authHandler) Handler(dir string, env []string, logger *log.Logger) (http.Handler, error) {
+	fs := http.Dir(dir)
+
+	handler := http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+		urlPath := req.URL.Path
+		if urlPath != "" && strings.HasPrefix(path.Base(urlPath), ".") {
+			http.Error(w, "filename contains leading dot", http.StatusBadRequest)
+			return
+		}
+
+		f, err := fs.Open(urlPath)
+		if err != nil {
+			if os.IsNotExist(err) {
+				http.NotFound(w, req)
+			} else {
+				http.Error(w, err.Error(), http.StatusInternalServerError)
+			}
+			return
+		}
+
+		accessDir := urlPath
+		if fi, err := f.Stat(); err == nil && !fi.IsDir() {
+			accessDir = path.Dir(urlPath)
+		}
+		f.Close()
+
+		var accessFile http.File
+		for {
+			var err error
+			accessFile, err = fs.Open(path.Join(accessDir, ".access"))
+			if err == nil {
+				break
+			}
+
+			if !os.IsNotExist(err) {
+				http.Error(w, err.Error(), http.StatusInternalServerError)
+				return
+			}
+			if accessDir == "." {
+				http.Error(w, "failed to locate access file", http.StatusInternalServerError)
+				return
+			}
+			accessDir = path.Dir(accessDir)
+		}
+
+		data, err := io.ReadAll(accessFile)
+		if err != nil {
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+			return
+		}
+
+		var token accessToken
+		if err := json.Unmarshal(data, &token); err != nil {
+			logger.Print(err)
+			http.Error(w, "malformed access file", http.StatusInternalServerError)
+			return
+		}
+		if username, password, ok := req.BasicAuth(); !ok || username != token.Username || password != token.Password {
+			code := token.StatusCode
+			if code == 0 {
+				code = http.StatusUnauthorized
+			}
+			if code == http.StatusUnauthorized {
+				w.Header().Add("WWW-Authenticate", fmt.Sprintf("basic realm=%s", accessDir))
+			}
+			http.Error(w, token.Message, code)
+			return
+		}
+
+		http.FileServer(fs).ServeHTTP(w, req)
+	})
+
+	return handler, nil
+}
diff --git a/src/cmd/go/internal/vcweb/bzr.go b/src/cmd/go/internal/vcweb/bzr.go
new file mode 100644
index 0000000..a915fb2
--- /dev/null
+++ b/src/cmd/go/internal/vcweb/bzr.go
@@ -0,0 +1,18 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package vcweb
+
+import (
+	"log"
+	"net/http"
+)
+
+type bzrHandler struct{}
+
+func (*bzrHandler) Available() bool { return true }
+
+func (*bzrHandler) Handler(dir string, env []string, logger *log.Logger) (http.Handler, error) {
+	return http.FileServer(http.Dir(dir)), nil
+}
diff --git a/src/cmd/go/internal/vcweb/dir.go b/src/cmd/go/internal/vcweb/dir.go
new file mode 100644
index 0000000..2f122f4
--- /dev/null
+++ b/src/cmd/go/internal/vcweb/dir.go
@@ -0,0 +1,19 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package vcweb
+
+import (
+	"log"
+	"net/http"
+)
+
+// dirHandler is a vcsHandler that serves the raw contents of a directory.
+type dirHandler struct{}
+
+func (*dirHandler) Available() bool { return true }
+
+func (*dirHandler) Handler(dir string, env []string, logger *log.Logger) (http.Handler, error) {
+	return http.FileServer(http.Dir(dir)), nil
+}
diff --git a/src/cmd/go/internal/vcweb/fossil.go b/src/cmd/go/internal/vcweb/fossil.go
new file mode 100644
index 0000000..cc24f2f
--- /dev/null
+++ b/src/cmd/go/internal/vcweb/fossil.go
@@ -0,0 +1,61 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package vcweb
+
+import (
+	"fmt"
+	"log"
+	"net/http"
+	"net/http/cgi"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"sync"
+)
+
+type fossilHandler struct {
+	once          sync.Once
+	fossilPath    string
+	fossilPathErr error
+}
+
+func (h *fossilHandler) Available() bool {
+	h.once.Do(func() {
+		h.fossilPath, h.fossilPathErr = exec.LookPath("fossil")
+	})
+	return h.fossilPathErr == nil
+}
+
+func (h *fossilHandler) Handler(dir string, env []string, logger *log.Logger) (http.Handler, error) {
+	if !h.Available() {
+		return nil, ServerNotInstalledError{name: "fossil"}
+	}
+
+	name := filepath.Base(dir)
+	db := filepath.Join(dir, name+".fossil")
+
+	cgiPath := db + ".cgi"
+	cgiScript := fmt.Sprintf("#!%s\nrepository: %s\n", h.fossilPath, db)
+	if err := os.WriteFile(cgiPath, []byte(cgiScript), 0755); err != nil {
+		return nil, err
+	}
+
+	handler := http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+		if _, err := os.Stat(db); err != nil {
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+			return
+		}
+		ch := &cgi.Handler{
+			Env:    env,
+			Logger: logger,
+			Path:   h.fossilPath,
+			Args:   []string{cgiPath},
+			Dir:    dir,
+		}
+		ch.ServeHTTP(w, req)
+	})
+
+	return handler, nil
+}
diff --git a/src/cmd/go/internal/vcweb/git.go b/src/cmd/go/internal/vcweb/git.go
new file mode 100644
index 0000000..316c238
--- /dev/null
+++ b/src/cmd/go/internal/vcweb/git.go
@@ -0,0 +1,52 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package vcweb
+
+import (
+	"log"
+	"net/http"
+	"net/http/cgi"
+	"os/exec"
+	"runtime"
+	"slices"
+	"sync"
+)
+
+type gitHandler struct {
+	once       sync.Once
+	gitPath    string
+	gitPathErr error
+}
+
+func (h *gitHandler) Available() bool {
+	if runtime.GOOS == "plan9" {
+		// The Git command is usually not the real Git on Plan 9.
+		// See https://golang.org/issues/29640.
+		return false
+	}
+	h.once.Do(func() {
+		h.gitPath, h.gitPathErr = exec.LookPath("git")
+	})
+	return h.gitPathErr == nil
+}
+
+func (h *gitHandler) Handler(dir string, env []string, logger *log.Logger) (http.Handler, error) {
+	if !h.Available() {
+		return nil, ServerNotInstalledError{name: "git"}
+	}
+
+	handler := &cgi.Handler{
+		Path:   h.gitPath,
+		Logger: logger,
+		Args:   []string{"http-backend"},
+		Dir:    dir,
+		Env: append(slices.Clip(env),
+			"GIT_PROJECT_ROOT="+dir,
+			"GIT_HTTP_EXPORT_ALL=1",
+		),
+	}
+
+	return handler, nil
+}
diff --git a/src/cmd/go/internal/vcweb/hg.go b/src/cmd/go/internal/vcweb/hg.go
new file mode 100644
index 0000000..4571277
--- /dev/null
+++ b/src/cmd/go/internal/vcweb/hg.go
@@ -0,0 +1,123 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package vcweb
+
+import (
+	"bufio"
+	"context"
+	"errors"
+	"io"
+	"log"
+	"net/http"
+	"net/http/httputil"
+	"net/url"
+	"os"
+	"os/exec"
+	"slices"
+	"strings"
+	"sync"
+	"time"
+)
+
+type hgHandler struct {
+	once      sync.Once
+	hgPath    string
+	hgPathErr error
+}
+
+func (h *hgHandler) Available() bool {
+	h.once.Do(func() {
+		h.hgPath, h.hgPathErr = exec.LookPath("hg")
+	})
+	return h.hgPathErr == nil
+}
+
+func (h *hgHandler) Handler(dir string, env []string, logger *log.Logger) (http.Handler, error) {
+	if !h.Available() {
+		return nil, ServerNotInstalledError{name: "hg"}
+	}
+
+	handler := http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+		// Mercurial has a CGI server implementation (called hgweb). In theory we
+		// could use that — however, assuming that hgweb is even installed, the
+		// configuration for hgweb varies by Python version (2 vs 3), and we would
+		// rather not go rooting around trying to find the right Python version to
+		// run.
+		//
+		// Instead, we'll take a somewhat more roundabout approach: we assume that
+		// if "hg" works at all then "hg serve" works too, and we'll execute that as
+		// a subprocess, using a reverse proxy to forward the request and response.
+
+		ctx, cancel := context.WithCancel(req.Context())
+		defer cancel()
+
+		cmd := exec.CommandContext(ctx, h.hgPath, "serve", "--port", "0", "--address", "localhost", "--accesslog", os.DevNull, "--name", "vcweb", "--print-url")
+		cmd.Dir = dir
+		cmd.Env = append(slices.Clip(env), "PWD="+dir)
+
+		cmd.Cancel = func() error {
+			err := cmd.Process.Signal(os.Interrupt)
+			if err != nil && !errors.Is(err, os.ErrProcessDone) {
+				err = cmd.Process.Kill()
+			}
+			return err
+		}
+		// This WaitDelay is arbitrary. After 'hg serve' prints its URL, any further
+		// I/O is only for debugging. (The actual output goes through the HTTP URL,
+		// not the standard I/O streams.)
+		cmd.WaitDelay = 10 * time.Second
+
+		stderr := new(strings.Builder)
+		cmd.Stderr = stderr
+
+		stdout, err := cmd.StdoutPipe()
+		if err != nil {
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+			return
+		}
+
+		if err := cmd.Start(); err != nil {
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+			return
+		}
+		var wg sync.WaitGroup
+		defer func() {
+			cancel()
+			err := cmd.Wait()
+			if out := strings.TrimSuffix(stderr.String(), "interrupted!\n"); out != "" {
+				logger.Printf("%v: %v\n%s", cmd, err, out)
+			} else {
+				logger.Printf("%v", cmd)
+			}
+			wg.Wait()
+		}()
+
+		r := bufio.NewReader(stdout)
+		line, err := r.ReadString('\n')
+		if err != nil {
+			return
+		}
+		// We have read what should be the server URL. 'hg serve' shouldn't need to
+		// write anything else to stdout, but it's not a big deal if it does anyway.
+		// Keep the stdout pipe open so that 'hg serve' won't get a SIGPIPE, but
+		// actively discard its output so that it won't hang on a blocking write.
+		wg.Add(1)
+		go func() {
+			io.Copy(io.Discard, r)
+			wg.Done()
+		}()
+
+		u, err := url.Parse(strings.TrimSpace(line))
+		if err != nil {
+			logger.Printf("%v: %v", cmd, err)
+			http.Error(w, err.Error(), http.StatusBadGateway)
+			return
+		}
+		logger.Printf("proxying hg request to %s", u)
+		httputil.NewSingleHostReverseProxy(u).ServeHTTP(w, req)
+	})
+
+	return handler, nil
+}
diff --git a/src/cmd/go/internal/vcweb/insecure.go b/src/cmd/go/internal/vcweb/insecure.go
new file mode 100644
index 0000000..1d6af25
--- /dev/null
+++ b/src/cmd/go/internal/vcweb/insecure.go
@@ -0,0 +1,42 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package vcweb
+
+import (
+	"log"
+	"net/http"
+)
+
+// insecureHandler redirects requests to the same host and path but using the
+// "http" scheme instead of "https".
+type insecureHandler struct{}
+
+func (h *insecureHandler) Available() bool { return true }
+
+func (h *insecureHandler) Handler(dir string, env []string, logger *log.Logger) (http.Handler, error) {
+	// The insecure-redirect handler implementation doesn't depend or dir or env.
+	//
+	// The only effect of the directory is to determine which prefix the caller
+	// will strip from the request before passing it on to this handler.
+	return h, nil
+}
+
+func (h *insecureHandler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
+	if req.Host == "" && req.URL.Host == "" {
+		http.Error(w, "no Host provided in request", http.StatusBadRequest)
+		return
+	}
+
+	// Note that if the handler is wrapped with http.StripPrefix, the prefix
+	// will remain stripped in the redirected URL, preventing redirect loops
+	// if the scheme is already "http".
+
+	u := *req.URL
+	u.Scheme = "http"
+	u.User = nil
+	u.Host = req.Host
+
+	http.Redirect(w, req, u.String(), http.StatusFound)
+}
diff --git a/src/cmd/go/internal/vcweb/script.go b/src/cmd/go/internal/vcweb/script.go
new file mode 100644
index 0000000..c35b46f
--- /dev/null
+++ b/src/cmd/go/internal/vcweb/script.go
@@ -0,0 +1,345 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package vcweb
+
+import (
+	"bufio"
+	"bytes"
+	"cmd/go/internal/script"
+	"context"
+	"errors"
+	"fmt"
+	"internal/txtar"
+	"io"
+	"log"
+	"net/http"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"strconv"
+	"strings"
+	"time"
+
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/zip"
+)
+
+// newScriptEngine returns a script engine augmented with commands for
+// reproducing version-control repositories by replaying commits.
+func newScriptEngine() *script.Engine {
+	conds := script.DefaultConds()
+
+	interrupt := func(cmd *exec.Cmd) error { return cmd.Process.Signal(os.Interrupt) }
+	gracePeriod := 30 * time.Second // arbitrary
+
+	cmds := script.DefaultCmds()
+	cmds["at"] = scriptAt()
+	cmds["bzr"] = script.Program("bzr", interrupt, gracePeriod)
+	cmds["fossil"] = script.Program("fossil", interrupt, gracePeriod)
+	cmds["git"] = script.Program("git", interrupt, gracePeriod)
+	cmds["hg"] = script.Program("hg", interrupt, gracePeriod)
+	cmds["handle"] = scriptHandle()
+	cmds["modzip"] = scriptModzip()
+	cmds["svnadmin"] = script.Program("svnadmin", interrupt, gracePeriod)
+	cmds["svn"] = script.Program("svn", interrupt, gracePeriod)
+	cmds["unquote"] = scriptUnquote()
+
+	return &script.Engine{
+		Cmds:  cmds,
+		Conds: conds,
+	}
+}
+
+// loadScript interprets the given script content using the vcweb script engine.
+// loadScript always returns either a non-nil handler or a non-nil error.
+//
+// The script content must be a txtar archive with a comment containing a script
+// with exactly one "handle" command and zero or more VCS commands to prepare
+// the repository to be served.
+func (s *Server) loadScript(ctx context.Context, logger *log.Logger, scriptPath string, scriptContent []byte, workDir string) (http.Handler, error) {
+	ar := txtar.Parse(scriptContent)
+
+	if err := os.MkdirAll(workDir, 0755); err != nil {
+		return nil, err
+	}
+
+	st, err := s.newState(ctx, workDir)
+	if err != nil {
+		return nil, err
+	}
+	if err := st.ExtractFiles(ar); err != nil {
+		return nil, err
+	}
+
+	scriptName := filepath.Base(scriptPath)
+	scriptLog := new(strings.Builder)
+	err = s.engine.Execute(st, scriptName, bufio.NewReader(bytes.NewReader(ar.Comment)), scriptLog)
+	closeErr := st.CloseAndWait(scriptLog)
+	logger.Printf("%s:", scriptName)
+	io.WriteString(logger.Writer(), scriptLog.String())
+	io.WriteString(logger.Writer(), "\n")
+	if err != nil {
+		return nil, err
+	}
+	if closeErr != nil {
+		return nil, err
+	}
+
+	sc, err := getScriptCtx(st)
+	if err != nil {
+		return nil, err
+	}
+	if sc.handler == nil {
+		return nil, errors.New("script completed without setting handler")
+	}
+	return sc.handler, nil
+}
+
+// newState returns a new script.State for executing scripts in workDir.
+func (s *Server) newState(ctx context.Context, workDir string) (*script.State, error) {
+	ctx = &scriptCtx{
+		Context: ctx,
+		server:  s,
+	}
+
+	st, err := script.NewState(ctx, workDir, s.env)
+	if err != nil {
+		return nil, err
+	}
+	return st, nil
+}
+
+// scriptEnviron returns a new environment that attempts to provide predictable
+// behavior for the supported version-control tools.
+func scriptEnviron(homeDir string) []string {
+	env := []string{
+		"USER=gopher",
+		homeEnvName() + "=" + homeDir,
+		"GIT_CONFIG_NOSYSTEM=1",
+		"HGRCPATH=" + filepath.Join(homeDir, ".hgrc"),
+		"HGENCODING=utf-8",
+	}
+	// Preserve additional environment variables that may be needed by VCS tools.
+	for _, k := range []string{
+		pathEnvName(),
+		tempEnvName(),
+		"SYSTEMROOT",        // must be preserved on Windows to find DLLs; golang.org/issue/25210
+		"WINDIR",            // must be preserved on Windows to be able to run PowerShell command; golang.org/issue/30711
+		"ComSpec",           // must be preserved on Windows to be able to run Batch files; golang.org/issue/56555
+		"DYLD_LIBRARY_PATH", // must be preserved on macOS systems to find shared libraries
+		"LD_LIBRARY_PATH",   // must be preserved on Unix systems to find shared libraries
+		"LIBRARY_PATH",      // allow override of non-standard static library paths
+		"PYTHONPATH",        // may be needed by hg to find imported modules
+	} {
+		if v, ok := os.LookupEnv(k); ok {
+			env = append(env, k+"="+v)
+		}
+	}
+
+	if os.Getenv("GO_BUILDER_NAME") != "" || os.Getenv("GIT_TRACE_CURL") == "1" {
+		// To help diagnose https://go.dev/issue/52545,
+		// enable tracing for Git HTTPS requests.
+		env = append(env,
+			"GIT_TRACE_CURL=1",
+			"GIT_TRACE_CURL_NO_DATA=1",
+			"GIT_REDACT_COOKIES=o,SSO,GSSO_Uberproxy")
+	}
+
+	return env
+}
+
+// homeEnvName returns the environment variable used by os.UserHomeDir
+// to locate the user's home directory.
+func homeEnvName() string {
+	switch runtime.GOOS {
+	case "windows":
+		return "USERPROFILE"
+	case "plan9":
+		return "home"
+	default:
+		return "HOME"
+	}
+}
+
+// tempEnvName returns the environment variable used by os.TempDir
+// to locate the default directory for temporary files.
+func tempEnvName() string {
+	switch runtime.GOOS {
+	case "windows":
+		return "TMP"
+	case "plan9":
+		return "TMPDIR" // actually plan 9 doesn't have one at all but this is fine
+	default:
+		return "TMPDIR"
+	}
+}
+
+// pathEnvName returns the environment variable used by exec.LookPath to
+// identify directories to search for executables.
+func pathEnvName() string {
+	switch runtime.GOOS {
+	case "plan9":
+		return "path"
+	default:
+		return "PATH"
+	}
+}
+
+// A scriptCtx is a context.Context that stores additional state for script
+// commands.
+type scriptCtx struct {
+	context.Context
+	server      *Server
+	commitTime  time.Time
+	handlerName string
+	handler     http.Handler
+}
+
+// scriptCtxKey is the key associating the *scriptCtx in a script's Context..
+type scriptCtxKey struct{}
+
+func (sc *scriptCtx) Value(key any) any {
+	if key == (scriptCtxKey{}) {
+		return sc
+	}
+	return sc.Context.Value(key)
+}
+
+func getScriptCtx(st *script.State) (*scriptCtx, error) {
+	sc, ok := st.Context().Value(scriptCtxKey{}).(*scriptCtx)
+	if !ok {
+		return nil, errors.New("scriptCtx not found in State.Context")
+	}
+	return sc, nil
+}
+
+func scriptAt() script.Cmd {
+	return script.Command(
+		script.CmdUsage{
+			Summary: "set the current commit time for all version control systems",
+			Args:    "time",
+			Detail: []string{
+				"The argument must be an absolute timestamp in RFC3339 format.",
+			},
+		},
+		func(st *script.State, args ...string) (script.WaitFunc, error) {
+			if len(args) != 1 {
+				return nil, script.ErrUsage
+			}
+
+			sc, err := getScriptCtx(st)
+			if err != nil {
+				return nil, err
+			}
+
+			sc.commitTime, err = time.ParseInLocation(time.RFC3339, args[0], time.UTC)
+			if err == nil {
+				st.Setenv("GIT_COMMITTER_DATE", args[0])
+				st.Setenv("GIT_AUTHOR_DATE", args[0])
+			}
+			return nil, err
+		})
+}
+
+func scriptHandle() script.Cmd {
+	return script.Command(
+		script.CmdUsage{
+			Summary: "set the HTTP handler that will serve the script's output",
+			Args:    "handler [dir]",
+			Detail: []string{
+				"The handler will be passed the script's current working directory and environment as arguments.",
+				"Valid handlers include 'dir' (for general http.Dir serving), 'bzr', 'fossil', 'git', and 'hg'",
+			},
+		},
+		func(st *script.State, args ...string) (script.WaitFunc, error) {
+			if len(args) == 0 || len(args) > 2 {
+				return nil, script.ErrUsage
+			}
+
+			sc, err := getScriptCtx(st)
+			if err != nil {
+				return nil, err
+			}
+
+			if sc.handler != nil {
+				return nil, fmt.Errorf("server handler already set to %s", sc.handlerName)
+			}
+
+			name := args[0]
+			h, ok := sc.server.vcsHandlers[name]
+			if !ok {
+				return nil, fmt.Errorf("unrecognized VCS %q", name)
+			}
+			sc.handlerName = name
+			if !h.Available() {
+				return nil, ServerNotInstalledError{name}
+			}
+
+			dir := st.Getwd()
+			if len(args) >= 2 {
+				dir = st.Path(args[1])
+			}
+			sc.handler, err = h.Handler(dir, st.Environ(), sc.server.logger)
+			return nil, err
+		})
+}
+
+func scriptModzip() script.Cmd {
+	return script.Command(
+		script.CmdUsage{
+			Summary: "create a Go module zip file from a directory",
+			Args:    "zipfile path@version dir",
+		},
+		func(st *script.State, args ...string) (wait script.WaitFunc, err error) {
+			if len(args) != 3 {
+				return nil, script.ErrUsage
+			}
+			zipPath := st.Path(args[0])
+			mPath, version, ok := strings.Cut(args[1], "@")
+			if !ok {
+				return nil, script.ErrUsage
+			}
+			dir := st.Path(args[2])
+
+			if err := os.MkdirAll(filepath.Dir(zipPath), 0755); err != nil {
+				return nil, err
+			}
+			f, err := os.Create(zipPath)
+			if err != nil {
+				return nil, err
+			}
+			defer func() {
+				if closeErr := f.Close(); err == nil {
+					err = closeErr
+				}
+			}()
+
+			return nil, zip.CreateFromDir(f, module.Version{Path: mPath, Version: version}, dir)
+		})
+}
+
+func scriptUnquote() script.Cmd {
+	return script.Command(
+		script.CmdUsage{
+			Summary: "unquote the argument as a Go string",
+			Args:    "string",
+		},
+		func(st *script.State, args ...string) (script.WaitFunc, error) {
+			if len(args) != 1 {
+				return nil, script.ErrUsage
+			}
+
+			s, err := strconv.Unquote(`"` + args[0] + `"`)
+			if err != nil {
+				return nil, err
+			}
+
+			wait := func(*script.State) (stdout, stderr string, err error) {
+				return s, "", nil
+			}
+			return wait, nil
+		})
+}
diff --git a/src/cmd/go/internal/vcweb/svn.go b/src/cmd/go/internal/vcweb/svn.go
new file mode 100644
index 0000000..60222f1
--- /dev/null
+++ b/src/cmd/go/internal/vcweb/svn.go
@@ -0,0 +1,199 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package vcweb
+
+import (
+	"io"
+	"log"
+	"net"
+	"net/http"
+	"os/exec"
+	"strings"
+	"sync"
+)
+
+// An svnHandler serves requests for Subversion repos.
+//
+// Unlike the other vcweb handlers, svnHandler does not serve the Subversion
+// protocol directly over the HTTP connection. Instead, it opens a separate port
+// that serves the (non-HTTP) 'svn' protocol. The test binary can retrieve the
+// URL for that port by sending an HTTP request with the query parameter
+// "vcwebsvn=1".
+//
+// We take this approach because the 'svn' protocol is implemented by a
+// lightweight 'svnserve' binary that is usually packaged along with the 'svn'
+// client binary, whereas only known implementation of the Subversion HTTP
+// protocol is the mod_dav_svn apache2 module. Apache2 has a lot of dependencies
+// and also seems to rely on global configuration via well-known file paths, so
+// implementing a hermetic test using apache2 would require the test to run in a
+// complicated container environment, which wouldn't be nearly as
+// straightforward for Go contributors to set up and test against on their local
+// machine.
+type svnHandler struct {
+	svnRoot string // a directory containing all svn repos to be served
+	logger  *log.Logger
+
+	pathOnce     sync.Once
+	svnservePath string // the path to the 'svnserve' executable
+	svnserveErr  error
+
+	listenOnce sync.Once
+	s          chan *svnState // 1-buffered
+}
+
+// An svnState describes the state of a port serving the 'svn://' protocol.
+type svnState struct {
+	listener  net.Listener
+	listenErr error
+	conns     map[net.Conn]struct{}
+	closing   bool
+	done      chan struct{}
+}
+
+func (h *svnHandler) Available() bool {
+	h.pathOnce.Do(func() {
+		h.svnservePath, h.svnserveErr = exec.LookPath("svnserve")
+	})
+	return h.svnserveErr == nil
+}
+
+// Handler returns an http.Handler that checks for the "vcwebsvn" query
+// parameter and then serves the 'svn://' URL for the repository at the
+// requested path.
+// The HTTP client is expected to read that URL and pass it to the 'svn' client.
+func (h *svnHandler) Handler(dir string, env []string, logger *log.Logger) (http.Handler, error) {
+	if !h.Available() {
+		return nil, ServerNotInstalledError{name: "svn"}
+	}
+
+	// Go ahead and start the listener now, so that if it fails (for example, due
+	// to port exhaustion) we can return an error from the Handler method instead
+	// of serving an error for each individual HTTP request.
+	h.listenOnce.Do(func() {
+		h.s = make(chan *svnState, 1)
+		l, err := net.Listen("tcp", "localhost:0")
+		done := make(chan struct{})
+
+		h.s <- &svnState{
+			listener:  l,
+			listenErr: err,
+			conns:     map[net.Conn]struct{}{},
+			done:      done,
+		}
+		if err != nil {
+			close(done)
+			return
+		}
+
+		h.logger.Printf("serving svn on svn://%v", l.Addr())
+
+		go func() {
+			for {
+				c, err := l.Accept()
+
+				s := <-h.s
+				if err != nil {
+					s.listenErr = err
+					if len(s.conns) == 0 {
+						close(s.done)
+					}
+					h.s <- s
+					return
+				}
+				if s.closing {
+					c.Close()
+				} else {
+					s.conns[c] = struct{}{}
+					go h.serve(c)
+				}
+				h.s <- s
+			}
+		}()
+	})
+
+	s := <-h.s
+	addr := ""
+	if s.listener != nil {
+		addr = s.listener.Addr().String()
+	}
+	err := s.listenErr
+	h.s <- s
+	if err != nil {
+		return nil, err
+	}
+
+	handler := http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+		if req.FormValue("vcwebsvn") != "" {
+			w.Header().Add("Content-Type", "text/plain; charset=UTF-8")
+			io.WriteString(w, "svn://"+addr+"\n")
+			return
+		}
+		http.NotFound(w, req)
+	})
+
+	return handler, nil
+}
+
+// serve serves a single 'svn://' connection on c.
+func (h *svnHandler) serve(c net.Conn) {
+	defer func() {
+		c.Close()
+
+		s := <-h.s
+		delete(s.conns, c)
+		if len(s.conns) == 0 && s.listenErr != nil {
+			close(s.done)
+		}
+		h.s <- s
+	}()
+
+	// The "--inetd" flag causes svnserve to speak the 'svn' protocol over its
+	// stdin and stdout streams as if invoked by the Unix "inetd" service.
+	// We aren't using inetd, but we are implementing essentially the same
+	// approach: using a host process to listen for connections and spawn
+	// subprocesses to serve them.
+	cmd := exec.Command(h.svnservePath, "--read-only", "--root="+h.svnRoot, "--inetd")
+	cmd.Stdin = c
+	cmd.Stdout = c
+	stderr := new(strings.Builder)
+	cmd.Stderr = stderr
+	err := cmd.Run()
+
+	var errFrag any = "ok"
+	if err != nil {
+		errFrag = err
+	}
+	stderrFrag := ""
+	if stderr.Len() > 0 {
+		stderrFrag = "\n" + stderr.String()
+	}
+	h.logger.Printf("%v: %s%s", cmd, errFrag, stderrFrag)
+}
+
+// Close stops accepting new svn:// connections and terminates the existing
+// ones, then waits for the 'svnserve' subprocesses to complete.
+func (h *svnHandler) Close() error {
+	h.listenOnce.Do(func() {})
+	if h.s == nil {
+		return nil
+	}
+
+	var err error
+	s := <-h.s
+	s.closing = true
+	if s.listener == nil {
+		err = s.listenErr
+	} else {
+		err = s.listener.Close()
+	}
+	for c := range s.conns {
+		c.Close()
+	}
+	done := s.done
+	h.s <- s
+
+	<-done
+	return err
+}
diff --git a/src/cmd/go/internal/vcweb/vcstest/vcstest.go b/src/cmd/go/internal/vcweb/vcstest/vcstest.go
new file mode 100644
index 0000000..d460259
--- /dev/null
+++ b/src/cmd/go/internal/vcweb/vcstest/vcstest.go
@@ -0,0 +1,169 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package vcstest serves the repository scripts in cmd/go/testdata/vcstest
+// using the [vcweb] script engine.
+package vcstest
+
+import (
+	"cmd/go/internal/vcs"
+	"cmd/go/internal/vcweb"
+	"cmd/go/internal/web"
+	"crypto/tls"
+	"crypto/x509"
+	"encoding/pem"
+	"fmt"
+	"internal/testenv"
+	"io"
+	"log"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+var Hosts = []string{
+	"vcs-test.golang.org",
+}
+
+type Server struct {
+	vcweb   *vcweb.Server
+	workDir string
+	HTTP    *httptest.Server
+	HTTPS   *httptest.Server
+}
+
+// NewServer returns a new test-local vcweb server that serves VCS requests
+// for modules with paths that begin with "vcs-test.golang.org" using the
+// scripts in cmd/go/testdata/vcstest.
+func NewServer() (srv *Server, err error) {
+	if vcs.VCSTestRepoURL != "" {
+		panic("vcs URL hooks already set")
+	}
+
+	scriptDir := filepath.Join(testenv.GOROOT(nil), "src/cmd/go/testdata/vcstest")
+
+	workDir, err := os.MkdirTemp("", "vcstest")
+	if err != nil {
+		return nil, err
+	}
+	defer func() {
+		if err != nil {
+			os.RemoveAll(workDir)
+		}
+	}()
+
+	logger := log.Default()
+	if !testing.Verbose() {
+		logger = log.New(io.Discard, "", log.LstdFlags)
+	}
+	handler, err := vcweb.NewServer(scriptDir, workDir, logger)
+	if err != nil {
+		return nil, err
+	}
+	defer func() {
+		if err != nil {
+			handler.Close()
+		}
+	}()
+
+	srvHTTP := httptest.NewServer(handler)
+	httpURL, err := url.Parse(srvHTTP.URL)
+	if err != nil {
+		return nil, err
+	}
+	defer func() {
+		if err != nil {
+			srvHTTP.Close()
+		}
+	}()
+
+	srvHTTPS := httptest.NewTLSServer(handler)
+	httpsURL, err := url.Parse(srvHTTPS.URL)
+	if err != nil {
+		return nil, err
+	}
+	defer func() {
+		if err != nil {
+			srvHTTPS.Close()
+		}
+	}()
+
+	srv = &Server{
+		vcweb:   handler,
+		workDir: workDir,
+		HTTP:    srvHTTP,
+		HTTPS:   srvHTTPS,
+	}
+	vcs.VCSTestRepoURL = srv.HTTP.URL
+	vcs.VCSTestHosts = Hosts
+
+	var interceptors []web.Interceptor
+	for _, host := range Hosts {
+		interceptors = append(interceptors,
+			web.Interceptor{Scheme: "http", FromHost: host, ToHost: httpURL.Host, Client: srv.HTTP.Client()},
+			web.Interceptor{Scheme: "https", FromHost: host, ToHost: httpsURL.Host, Client: srv.HTTPS.Client()})
+	}
+	web.EnableTestHooks(interceptors)
+
+	fmt.Fprintln(os.Stderr, "vcs-test.golang.org rerouted to "+srv.HTTP.URL)
+	fmt.Fprintln(os.Stderr, "https://vcs-test.golang.org rerouted to "+srv.HTTPS.URL)
+
+	return srv, nil
+}
+
+func (srv *Server) Close() error {
+	if vcs.VCSTestRepoURL != srv.HTTP.URL {
+		panic("vcs URL hooks modified before Close")
+	}
+	vcs.VCSTestRepoURL = ""
+	vcs.VCSTestHosts = nil
+	web.DisableTestHooks()
+
+	srv.HTTP.Close()
+	srv.HTTPS.Close()
+	err := srv.vcweb.Close()
+	if rmErr := os.RemoveAll(srv.workDir); err == nil {
+		err = rmErr
+	}
+	return err
+}
+
+func (srv *Server) WriteCertificateFile() (string, error) {
+	b := pem.EncodeToMemory(&pem.Block{
+		Type:  "CERTIFICATE",
+		Bytes: srv.HTTPS.Certificate().Raw,
+	})
+
+	filename := filepath.Join(srv.workDir, "cert.pem")
+	if err := os.WriteFile(filename, b, 0644); err != nil {
+		return "", err
+	}
+	return filename, nil
+}
+
+// TLSClient returns an http.Client that can talk to the httptest.Server
+// whose certificate is written to the given file path.
+func TLSClient(certFile string) (*http.Client, error) {
+	client := &http.Client{
+		Transport: http.DefaultTransport.(*http.Transport).Clone(),
+	}
+
+	pemBytes, err := os.ReadFile(certFile)
+	if err != nil {
+		return nil, err
+	}
+
+	certpool := x509.NewCertPool()
+	if !certpool.AppendCertsFromPEM(pemBytes) {
+		return nil, fmt.Errorf("no certificates found in %s", certFile)
+	}
+	client.Transport.(*http.Transport).TLSClientConfig = &tls.Config{
+		RootCAs: certpool,
+	}
+
+	return client, nil
+}
diff --git a/src/cmd/go/internal/vcweb/vcstest/vcstest_test.go b/src/cmd/go/internal/vcweb/vcstest/vcstest_test.go
new file mode 100644
index 0000000..4a6d600
--- /dev/null
+++ b/src/cmd/go/internal/vcweb/vcstest/vcstest_test.go
@@ -0,0 +1,170 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package vcstest_test
+
+import (
+	"cmd/go/internal/vcweb"
+	"errors"
+	"flag"
+	"fmt"
+	"io"
+	"io/fs"
+	"log"
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+)
+
+var (
+	dir  = flag.String("dir", "../../../testdata/vcstest", "directory containing scripts to serve")
+	host = flag.String("host", "localhost", "hostname on which to serve HTTP")
+	port = flag.Int("port", -1, "port on which to serve HTTP; if nonnegative, skips running tests")
+)
+
+func TestMain(m *testing.M) {
+	flag.Parse()
+
+	if *port >= 0 {
+		err := serveStandalone(*host, *port)
+		if err != nil {
+			log.Fatal(err)
+		}
+		os.Exit(0)
+	}
+
+	m.Run()
+}
+
+// serveStandalone serves the vcweb testdata in a standalone HTTP server.
+func serveStandalone(host string, port int) (err error) {
+	scriptDir, err := filepath.Abs(*dir)
+	if err != nil {
+		return err
+	}
+	work, err := os.MkdirTemp("", "vcweb")
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if rmErr := os.RemoveAll(work); err == nil {
+			err = rmErr
+		}
+	}()
+
+	log.Printf("running scripts in %s", work)
+
+	v, err := vcweb.NewServer(scriptDir, work, log.Default())
+	if err != nil {
+		return err
+	}
+
+	l, err := net.Listen("tcp", fmt.Sprintf("%s:%d", host, port))
+	if err != nil {
+		return err
+	}
+	log.Printf("serving on http://%s:%d/", host, l.Addr().(*net.TCPAddr).Port)
+
+	return http.Serve(l, v)
+}
+
+// TestScripts verifies that the VCS setup scripts in cmd/go/testdata/vcstest
+// run successfully.
+func TestScripts(t *testing.T) {
+	scriptDir, err := filepath.Abs(*dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	s, err := vcweb.NewServer(scriptDir, t.TempDir(), log.Default())
+	if err != nil {
+		t.Fatal(err)
+	}
+	srv := httptest.NewServer(s)
+
+	// To check for data races in the handler, run the root handler to produce an
+	// overview of the script status at an arbitrary point during the test.
+	// (We ignore the output because the expected failure mode is a friendly stack
+	// dump from the race detector.)
+	t.Run("overview", func(t *testing.T) {
+		t.Parallel()
+
+		time.Sleep(1 * time.Millisecond) // Give the other handlers time to race.
+
+		resp, err := http.Get(srv.URL)
+		if err == nil {
+			io.Copy(io.Discard, resp.Body)
+			resp.Body.Close()
+		} else {
+			t.Error(err)
+		}
+	})
+
+	t.Cleanup(func() {
+		// The subtests spawned by WalkDir run in parallel. When they complete, this
+		// Cleanup callback will run. At that point we fetch the root URL (which
+		// contains a status page), both to test that the root handler runs without
+		// crashing and to display a nice summary of the server's view of the test
+		// coverage.
+		resp, err := http.Get(srv.URL)
+		if err == nil {
+			var body []byte
+			body, err = io.ReadAll(resp.Body)
+			if err == nil && testing.Verbose() {
+				t.Logf("GET %s:\n%s", srv.URL, body)
+			}
+			resp.Body.Close()
+		}
+		if err != nil {
+			t.Error(err)
+		}
+
+		srv.Close()
+	})
+
+	err = filepath.WalkDir(scriptDir, func(path string, d fs.DirEntry, err error) error {
+		if err != nil || d.IsDir() {
+			return err
+		}
+
+		rel, err := filepath.Rel(scriptDir, path)
+		if err != nil {
+			return err
+		}
+		if rel == "README" {
+			return nil
+		}
+
+		t.Run(filepath.ToSlash(rel), func(t *testing.T) {
+			t.Parallel()
+
+			buf := new(strings.Builder)
+			logger := log.New(buf, "", log.LstdFlags)
+			// Load the script but don't try to serve the results:
+			// different VCS tools have different handler protocols,
+			// and the tests that actually use these repos will ensure
+			// that they are served correctly as a side effect anyway.
+			err := s.HandleScript(rel, logger, func(http.Handler) {})
+			if buf.Len() > 0 {
+				t.Log(buf)
+			}
+			if err != nil {
+				if notInstalled := (vcweb.ServerNotInstalledError{}); errors.As(err, &notInstalled) || errors.Is(err, exec.ErrNotFound) {
+					t.Skip(err)
+				}
+				t.Error(err)
+			}
+		})
+		return nil
+	})
+
+	if err != nil {
+		t.Error(err)
+	}
+}
diff --git a/src/cmd/go/internal/vcweb/vcweb.go b/src/cmd/go/internal/vcweb/vcweb.go
new file mode 100644
index 0000000..f748b34
--- /dev/null
+++ b/src/cmd/go/internal/vcweb/vcweb.go
@@ -0,0 +1,425 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package vcweb serves version control repos for testing the go command.
+//
+// It is loosely derived from golang.org/x/build/vcs-test/vcweb,
+// which ran as a service hosted at vcs-test.golang.org.
+//
+// When a repository URL is first requested, the vcweb [Server] dynamically
+// regenerates the repository using a script interpreted by a [script.Engine].
+// The script produces the server's contents for a corresponding root URL and
+// all subdirectories of that URL, which are then cached: subsequent requests
+// for any URL generated by the script will serve the script's previous output
+// until the script is modified.
+//
+// The script engine includes all of the engine's default commands and
+// conditions, as well as commands for each supported VCS binary (bzr, fossil,
+// git, hg, and svn), a "handle" command that informs the script which protocol
+// or handler to use to serve the request, and utilities "at" (which sets
+// environment variables for Git timestamps) and "unquote" (which unquotes its
+// argument as if it were a Go string literal).
+//
+// The server's "/" endpoint provides a summary of the available scripts,
+// and "/help" provides documentation for the script environment.
+//
+// To run a standalone server based on the vcweb engine, use:
+//
+//	go test cmd/go/internal/vcweb/vcstest -v --port=0
+package vcweb
+
+import (
+	"bufio"
+	"cmd/go/internal/script"
+	"context"
+	"crypto/sha256"
+	"errors"
+	"fmt"
+	"io"
+	"io/fs"
+	"log"
+	"net/http"
+	"os"
+	"os/exec"
+	"path"
+	"path/filepath"
+	"runtime/debug"
+	"strings"
+	"sync"
+	"text/tabwriter"
+	"time"
+)
+
+// A Server serves cached, dynamically-generated version control repositories.
+type Server struct {
+	env    []string
+	logger *log.Logger
+
+	scriptDir string
+	workDir   string
+	homeDir   string // $workdir/home
+	engine    *script.Engine
+
+	scriptCache sync.Map // script path → *scriptResult
+
+	vcsHandlers map[string]vcsHandler
+}
+
+// A vcsHandler serves repositories over HTTP for a known version-control tool.
+type vcsHandler interface {
+	Available() bool
+	Handler(dir string, env []string, logger *log.Logger) (http.Handler, error)
+}
+
+// A scriptResult describes the cached result of executing a vcweb script.
+type scriptResult struct {
+	mu sync.RWMutex
+
+	hash     [sha256.Size]byte // hash of the script file, for cache invalidation
+	hashTime time.Time         // timestamp at which the script was run, for diagnostics
+
+	handler http.Handler // HTTP handler configured by the script
+	err     error        // error from executing the script, if any
+}
+
+// NewServer returns a Server that generates and serves repositories in workDir
+// using the scripts found in scriptDir and its subdirectories.
+//
+// A request for the path /foo/bar/baz will be handled by the first script along
+// that path that exists: $scriptDir/foo.txt, $scriptDir/foo/bar.txt, or
+// $scriptDir/foo/bar/baz.txt.
+func NewServer(scriptDir, workDir string, logger *log.Logger) (*Server, error) {
+	if scriptDir == "" {
+		panic("vcweb.NewServer: scriptDir is required")
+	}
+	var err error
+	scriptDir, err = filepath.Abs(scriptDir)
+	if err != nil {
+		return nil, err
+	}
+
+	if workDir == "" {
+		workDir, err = os.MkdirTemp("", "vcweb-*")
+		if err != nil {
+			return nil, err
+		}
+		logger.Printf("vcweb work directory: %s", workDir)
+	} else {
+		workDir, err = filepath.Abs(workDir)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	homeDir := filepath.Join(workDir, "home")
+	if err := os.MkdirAll(homeDir, 0755); err != nil {
+		return nil, err
+	}
+
+	env := scriptEnviron(homeDir)
+
+	s := &Server{
+		env:       env,
+		logger:    logger,
+		scriptDir: scriptDir,
+		workDir:   workDir,
+		homeDir:   homeDir,
+		engine:    newScriptEngine(),
+		vcsHandlers: map[string]vcsHandler{
+			"auth":     new(authHandler),
+			"dir":      new(dirHandler),
+			"bzr":      new(bzrHandler),
+			"fossil":   new(fossilHandler),
+			"git":      new(gitHandler),
+			"hg":       new(hgHandler),
+			"insecure": new(insecureHandler),
+			"svn":      &svnHandler{svnRoot: workDir, logger: logger},
+		},
+	}
+
+	if err := os.WriteFile(filepath.Join(s.homeDir, ".gitconfig"), []byte(gitConfig), 0644); err != nil {
+		return nil, err
+	}
+	gitConfigDir := filepath.Join(s.homeDir, ".config", "git")
+	if err := os.MkdirAll(gitConfigDir, 0755); err != nil {
+		return nil, err
+	}
+	if err := os.WriteFile(filepath.Join(gitConfigDir, "ignore"), []byte(""), 0644); err != nil {
+		return nil, err
+	}
+
+	if err := os.WriteFile(filepath.Join(s.homeDir, ".hgrc"), []byte(hgrc), 0644); err != nil {
+		return nil, err
+	}
+
+	return s, nil
+}
+
+func (s *Server) Close() error {
+	var firstErr error
+	for _, h := range s.vcsHandlers {
+		if c, ok := h.(io.Closer); ok {
+			if closeErr := c.Close(); firstErr == nil {
+				firstErr = closeErr
+			}
+		}
+	}
+	return firstErr
+}
+
+// gitConfig contains a ~/.gitconfg file that attempts to provide
+// deterministic, platform-agnostic behavior for the 'git' command.
+var gitConfig = `
+[user]
+	name = Go Gopher
+	email = gopher@golang.org
+[init]
+	defaultBranch = main
+[core]
+	eol = lf
+[gui]
+	encoding = utf-8
+`[1:]
+
+// hgrc contains a ~/.hgrc file that attempts to provide
+// deterministic, platform-agnostic behavior for the 'hg' command.
+var hgrc = `
+[ui]
+username=Go Gopher <gopher@golang.org>
+[phases]
+new-commit=public
+[extensions]
+convert=
+`[1:]
+
+// ServeHTTP implements [http.Handler] for version-control repositories.
+func (s *Server) ServeHTTP(w http.ResponseWriter, req *http.Request) {
+	s.logger.Printf("serving %s", req.URL)
+
+	defer func() {
+		if v := recover(); v != nil {
+			debug.PrintStack()
+			s.logger.Fatal(v)
+		}
+	}()
+
+	urlPath := req.URL.Path
+	if !strings.HasPrefix(urlPath, "/") {
+		urlPath = "/" + urlPath
+	}
+	clean := path.Clean(urlPath)[1:]
+	if clean == "" {
+		s.overview(w, req)
+		return
+	}
+	if clean == "help" {
+		s.help(w, req)
+		return
+	}
+
+	// Locate the script that generates the requested path.
+	// We follow directories all the way to the end, then look for a ".txt" file
+	// matching the first component that doesn't exist. That guarantees
+	// uniqueness: if a path exists as a directory, then it cannot exist as a
+	// ".txt" script (because the search would ignore that file).
+	scriptPath := "."
+	for _, part := range strings.Split(clean, "/") {
+		scriptPath = filepath.Join(scriptPath, part)
+		dir := filepath.Join(s.scriptDir, scriptPath)
+		if _, err := os.Stat(dir); err != nil {
+			if !os.IsNotExist(err) {
+				http.Error(w, err.Error(), http.StatusInternalServerError)
+				return
+			}
+			// scriptPath does not exist as a directory, so it either is the script
+			// location or the script doesn't exist.
+			break
+		}
+	}
+	scriptPath += ".txt"
+
+	err := s.HandleScript(scriptPath, s.logger, func(handler http.Handler) {
+		handler.ServeHTTP(w, req)
+	})
+	if err != nil {
+		s.logger.Print(err)
+		if notFound := (ScriptNotFoundError{}); errors.As(err, &notFound) {
+			http.NotFound(w, req)
+		} else if notInstalled := (ServerNotInstalledError{}); errors.As(err, &notInstalled) || errors.Is(err, exec.ErrNotFound) {
+			http.Error(w, err.Error(), http.StatusNotImplemented)
+		} else {
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+		}
+	}
+}
+
+// A ScriptNotFoundError indicates that the requested script file does not exist.
+// (It typically wraps a "stat" error for the script file.)
+type ScriptNotFoundError struct{ err error }
+
+func (e ScriptNotFoundError) Error() string { return e.err.Error() }
+func (e ScriptNotFoundError) Unwrap() error { return e.err }
+
+// A ServerNotInstalledError indicates that the server binary required for the
+// indicated VCS does not exist.
+type ServerNotInstalledError struct{ name string }
+
+func (v ServerNotInstalledError) Error() string {
+	return fmt.Sprintf("server for %#q VCS is not installed", v.name)
+}
+
+// HandleScript ensures that the script at scriptRelPath has been evaluated
+// with its current contents.
+//
+// If the script completed successfully, HandleScript invokes f on the handler
+// with the script's result still read-locked, and waits for it to return. (That
+// ensures that cache invalidation does not race with an in-flight handler.)
+//
+// Otherwise, HandleScript returns the (cached) error from executing the script.
+func (s *Server) HandleScript(scriptRelPath string, logger *log.Logger, f func(http.Handler)) error {
+	ri, ok := s.scriptCache.Load(scriptRelPath)
+	if !ok {
+		ri, _ = s.scriptCache.LoadOrStore(scriptRelPath, new(scriptResult))
+	}
+	r := ri.(*scriptResult)
+
+	relDir := strings.TrimSuffix(scriptRelPath, filepath.Ext(scriptRelPath))
+	workDir := filepath.Join(s.workDir, relDir)
+	prefix := path.Join("/", filepath.ToSlash(relDir))
+
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+	for {
+		// For efficiency, we cache the script's output (in the work directory)
+		// across invocations. However, to allow for rapid iteration, we hash the
+		// script's contents and regenerate its output if the contents change.
+		//
+		// That way, one can use 'go run main.go' in this directory to stand up a
+		// server and see the output of the test script in order to fine-tune it.
+		content, err := os.ReadFile(filepath.Join(s.scriptDir, scriptRelPath))
+		if err != nil {
+			if !os.IsNotExist(err) {
+				return err
+			}
+			return ScriptNotFoundError{err}
+		}
+
+		hash := sha256.Sum256(content)
+		if prevHash := r.hash; prevHash != hash {
+			// The script's hash has changed, so regenerate its output.
+			func() {
+				r.mu.RUnlock()
+				r.mu.Lock()
+				defer func() {
+					r.mu.Unlock()
+					r.mu.RLock()
+				}()
+				if r.hash != prevHash {
+					// The cached result changed while we were waiting on the lock.
+					// It may have been updated to our hash or something even newer,
+					// so don't overwrite it.
+					return
+				}
+
+				r.hash = hash
+				r.hashTime = time.Now()
+				r.handler, r.err = nil, nil
+
+				if err := os.RemoveAll(workDir); err != nil {
+					r.err = err
+					return
+				}
+
+				// Note: we use context.Background here instead of req.Context() so that we
+				// don't cache a spurious error (and lose work) if the request is canceled
+				// while the script is still running.
+				scriptHandler, err := s.loadScript(context.Background(), logger, scriptRelPath, content, workDir)
+				if err != nil {
+					r.err = err
+					return
+				}
+				r.handler = http.StripPrefix(prefix, scriptHandler)
+			}()
+		}
+
+		if r.hash != hash {
+			continue // Raced with an update from another handler; try again.
+		}
+
+		if r.err != nil {
+			return r.err
+		}
+		f(r.handler)
+		return nil
+	}
+}
+
+// overview serves an HTML summary of the status of the scripts in the server's
+// script directory.
+func (s *Server) overview(w http.ResponseWriter, r *http.Request) {
+	fmt.Fprintf(w, "<html>\n")
+	fmt.Fprintf(w, "<title>vcweb</title>\n<pre>\n")
+	fmt.Fprintf(w, "<b>vcweb</b>\n\n")
+	fmt.Fprintf(w, "This server serves various version control repos for testing the go command.\n\n")
+	fmt.Fprintf(w, "For an overview of the script language, see <a href=\"/help\">/help</a>.\n\n")
+
+	fmt.Fprintf(w, "<b>cache</b>\n")
+
+	tw := tabwriter.NewWriter(w, 1, 8, 1, '\t', 0)
+	err := filepath.WalkDir(s.scriptDir, func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		if filepath.Ext(path) != ".txt" {
+			return nil
+		}
+
+		rel, err := filepath.Rel(s.scriptDir, path)
+		if err != nil {
+			return err
+		}
+		hashTime := "(not loaded)"
+		status := ""
+		if ri, ok := s.scriptCache.Load(rel); ok {
+			r := ri.(*scriptResult)
+			r.mu.RLock()
+			defer r.mu.RUnlock()
+
+			if !r.hashTime.IsZero() {
+				hashTime = r.hashTime.Format(time.RFC3339)
+			}
+			if r.err == nil {
+				status = "ok"
+			} else {
+				status = r.err.Error()
+			}
+		}
+		fmt.Fprintf(tw, "%s\t%s\t%s\n", rel, hashTime, status)
+		return nil
+	})
+	tw.Flush()
+
+	if err != nil {
+		fmt.Fprintln(w, err)
+	}
+}
+
+// help serves a plain-text summary of the server's supported script language.
+func (s *Server) help(w http.ResponseWriter, req *http.Request) {
+	st, err := s.newState(req.Context(), s.workDir)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+
+	scriptLog := new(strings.Builder)
+	err = s.engine.Execute(st, "help", bufio.NewReader(strings.NewReader("help")), scriptLog)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+
+	w.Header().Set("Content-Type", "text/plain; charset=UTF-8")
+	io.WriteString(w, scriptLog.String())
+}
diff --git a/src/cmd/go/internal/vcweb/vcweb_test.go b/src/cmd/go/internal/vcweb/vcweb_test.go
new file mode 100644
index 0000000..20b2137
--- /dev/null
+++ b/src/cmd/go/internal/vcweb/vcweb_test.go
@@ -0,0 +1,63 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package vcweb_test
+
+import (
+	"cmd/go/internal/vcweb"
+	"io"
+	"log"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"testing"
+)
+
+func TestHelp(t *testing.T) {
+	s, err := vcweb.NewServer(os.DevNull, t.TempDir(), log.Default())
+	if err != nil {
+		t.Fatal(err)
+	}
+	srv := httptest.NewServer(s)
+	defer srv.Close()
+
+	resp, err := http.Get(srv.URL + "/help")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != 200 {
+		t.Fatal(resp.Status)
+	}
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("%s", body)
+}
+
+func TestOverview(t *testing.T) {
+	s, err := vcweb.NewServer(os.DevNull, t.TempDir(), log.Default())
+	if err != nil {
+		t.Fatal(err)
+	}
+	srv := httptest.NewServer(s)
+	defer srv.Close()
+
+	resp, err := http.Get(srv.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != 200 {
+		t.Fatal(resp.Status)
+	}
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("%s", body)
+}
diff --git a/src/cmd/go/internal/version/version.go b/src/cmd/go/internal/version/version.go
new file mode 100644
index 0000000..4a0132a
--- /dev/null
+++ b/src/cmd/go/internal/version/version.go
@@ -0,0 +1,173 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package version implements the “go version” command.
+package version
+
+import (
+	"context"
+	"debug/buildinfo"
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/gover"
+)
+
+var CmdVersion = &base.Command{
+	UsageLine: "go version [-m] [-v] [file ...]",
+	Short:     "print Go version",
+	Long: `Version prints the build information for Go binary files.
+
+Go version reports the Go version used to build each of the named files.
+
+If no files are named on the command line, go version prints its own
+version information.
+
+If a directory is named, go version walks that directory, recursively,
+looking for recognized Go binaries and reporting their versions.
+By default, go version does not report unrecognized files found
+during a directory scan. The -v flag causes it to report unrecognized files.
+
+The -m flag causes go version to print each file's embedded
+module version information, when available. In the output, the module
+information consists of multiple lines following the version line, each
+indented by a leading tab character.
+
+See also: go doc runtime/debug.BuildInfo.
+`,
+}
+
+func init() {
+	base.AddChdirFlag(&CmdVersion.Flag)
+	CmdVersion.Run = runVersion // break init cycle
+}
+
+var (
+	versionM = CmdVersion.Flag.Bool("m", false, "")
+	versionV = CmdVersion.Flag.Bool("v", false, "")
+)
+
+func runVersion(ctx context.Context, cmd *base.Command, args []string) {
+	if len(args) == 0 {
+		// If any of this command's flags were passed explicitly, error
+		// out, because they only make sense with arguments.
+		//
+		// Don't error if the flags came from GOFLAGS, since that can be
+		// a reasonable use case. For example, imagine GOFLAGS=-v to
+		// turn "verbose mode" on for all Go commands, which should not
+		// break "go version".
+		var argOnlyFlag string
+		if !base.InGOFLAGS("-m") && *versionM {
+			argOnlyFlag = "-m"
+		} else if !base.InGOFLAGS("-v") && *versionV {
+			argOnlyFlag = "-v"
+		}
+		if argOnlyFlag != "" {
+			fmt.Fprintf(os.Stderr, "go: 'go version' only accepts %s flag with arguments\n", argOnlyFlag)
+			base.SetExitStatus(2)
+			return
+		}
+		v := runtime.Version()
+		if gover.TestVersion != "" {
+			v = gover.TestVersion + " (TESTGO_VERSION)"
+		}
+		fmt.Printf("go version %s %s/%s\n", v, runtime.GOOS, runtime.GOARCH)
+		return
+	}
+
+	for _, arg := range args {
+		info, err := os.Stat(arg)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "%v\n", err)
+			base.SetExitStatus(1)
+			continue
+		}
+		if info.IsDir() {
+			scanDir(arg)
+		} else {
+			scanFile(arg, info, true)
+		}
+	}
+}
+
+// scanDir scans a directory for binary to run scanFile on.
+func scanDir(dir string) {
+	filepath.WalkDir(dir, func(path string, d fs.DirEntry, err error) error {
+		if d.Type().IsRegular() || d.Type()&fs.ModeSymlink != 0 {
+			info, err := d.Info()
+			if err != nil {
+				if *versionV {
+					fmt.Fprintf(os.Stderr, "%s: %v\n", path, err)
+				}
+				return nil
+			}
+			scanFile(path, info, *versionV)
+		}
+		return nil
+	})
+}
+
+// isGoBinaryCandidate reports whether the file is a candidate to be a Go binary.
+func isGoBinaryCandidate(file string, info fs.FileInfo) bool {
+	if info.Mode().IsRegular() && info.Mode()&0111 != 0 {
+		return true
+	}
+	name := strings.ToLower(file)
+	switch filepath.Ext(name) {
+	case ".so", ".exe", ".dll":
+		return true
+	default:
+		return strings.Contains(name, ".so.")
+	}
+}
+
+// scanFile scans file to try to report the Go and module versions.
+// If mustPrint is true, scanFile will report any error reading file.
+// Otherwise (mustPrint is false, because scanFile is being called
+// by scanDir) scanFile prints nothing for non-Go binaries.
+func scanFile(file string, info fs.FileInfo, mustPrint bool) {
+	if info.Mode()&fs.ModeSymlink != 0 {
+		// Accept file symlinks only.
+		i, err := os.Stat(file)
+		if err != nil || !i.Mode().IsRegular() {
+			if mustPrint {
+				fmt.Fprintf(os.Stderr, "%s: symlink\n", file)
+			}
+			return
+		}
+		info = i
+	}
+
+	bi, err := buildinfo.ReadFile(file)
+	if err != nil {
+		if mustPrint {
+			if pathErr := (*os.PathError)(nil); errors.As(err, &pathErr) && filepath.Clean(pathErr.Path) == filepath.Clean(file) {
+				fmt.Fprintf(os.Stderr, "%v\n", file)
+			} else {
+
+				// Skip errors for non-Go binaries.
+				// buildinfo.ReadFile errors are not fine-grained enough
+				// to know if the file is a Go binary or not,
+				// so try to infer it from the file mode and extension.
+				if isGoBinaryCandidate(file, info) {
+					fmt.Fprintf(os.Stderr, "%s: %v\n", file, err)
+				}
+			}
+		}
+		return
+	}
+
+	fmt.Printf("%s: %s\n", file, bi.GoVersion)
+	bi.GoVersion = "" // suppress printing go version again
+	mod := bi.String()
+	if *versionM && len(mod) > 0 {
+		fmt.Printf("\t%s\n", strings.ReplaceAll(mod[:len(mod)-1], "\n", "\n\t"))
+	}
+}
diff --git a/src/cmd/go/internal/vet/vet.go b/src/cmd/go/internal/vet/vet.go
new file mode 100644
index 0000000..2d42097
--- /dev/null
+++ b/src/cmd/go/internal/vet/vet.go
@@ -0,0 +1,120 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package vet implements the “go vet” command.
+package vet
+
+import (
+	"context"
+	"fmt"
+	"path/filepath"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/load"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/trace"
+	"cmd/go/internal/work"
+)
+
+// Break init loop.
+func init() {
+	CmdVet.Run = runVet
+}
+
+var CmdVet = &base.Command{
+	CustomFlags: true,
+	UsageLine:   "go vet [build flags] [-vettool prog] [vet flags] [packages]",
+	Short:       "report likely mistakes in packages",
+	Long: `
+Vet runs the Go vet command on the packages named by the import paths.
+
+For more about vet and its flags, see 'go doc cmd/vet'.
+For more about specifying packages, see 'go help packages'.
+For a list of checkers and their flags, see 'go tool vet help'.
+For details of a specific checker such as 'printf', see 'go tool vet help printf'.
+
+The -vettool=prog flag selects a different analysis tool with alternative
+or additional checks.
+For example, the 'shadow' analyzer can be built and run using these commands:
+
+  go install golang.org/x/tools/go/analysis/passes/shadow/cmd/shadow@latest
+  go vet -vettool=$(which shadow)
+
+The build flags supported by go vet are those that control package resolution
+and execution, such as -C, -n, -x, -v, -tags, and -toolexec.
+For more about these flags, see 'go help build'.
+
+See also: go fmt, go fix.
+	`,
+}
+
+func runVet(ctx context.Context, cmd *base.Command, args []string) {
+	vetFlags, pkgArgs := vetFlags(args)
+	modload.InitWorkfile() // The vet command does custom flag processing; initialize workspaces after that.
+
+	if cfg.DebugTrace != "" {
+		var close func() error
+		var err error
+		ctx, close, err = trace.Start(ctx, cfg.DebugTrace)
+		if err != nil {
+			base.Fatalf("failed to start trace: %v", err)
+		}
+		defer func() {
+			if err := close(); err != nil {
+				base.Fatalf("failed to stop trace: %v", err)
+			}
+		}()
+	}
+
+	ctx, span := trace.StartSpan(ctx, fmt.Sprint("Running ", cmd.Name(), " command"))
+	defer span.Done()
+
+	work.BuildInit()
+	work.VetFlags = vetFlags
+	if len(vetFlags) > 0 {
+		work.VetExplicit = true
+	}
+	if vetTool != "" {
+		var err error
+		work.VetTool, err = filepath.Abs(vetTool)
+		if err != nil {
+			base.Fatalf("%v", err)
+		}
+	}
+
+	pkgOpts := load.PackageOpts{ModResolveTests: true}
+	pkgs := load.PackagesAndErrors(ctx, pkgOpts, pkgArgs)
+	load.CheckPackageErrors(pkgs)
+	if len(pkgs) == 0 {
+		base.Fatalf("no packages to vet")
+	}
+
+	b := work.NewBuilder("")
+	defer func() {
+		if err := b.Close(); err != nil {
+			base.Fatal(err)
+		}
+	}()
+
+	root := &work.Action{Mode: "go vet"}
+	for _, p := range pkgs {
+		_, ptest, pxtest, err := load.TestPackagesFor(ctx, pkgOpts, p, nil)
+		if err != nil {
+			base.Errorf("%v", err)
+			continue
+		}
+		if len(ptest.GoFiles) == 0 && len(ptest.CgoFiles) == 0 && pxtest == nil {
+			base.Errorf("go: can't vet %s: no Go files in %s", p.ImportPath, p.Dir)
+			continue
+		}
+		if len(ptest.GoFiles) > 0 || len(ptest.CgoFiles) > 0 {
+			root.Deps = append(root.Deps, b.VetAction(work.ModeBuild, work.ModeBuild, ptest))
+		}
+		if pxtest != nil {
+			root.Deps = append(root.Deps, b.VetAction(work.ModeBuild, work.ModeBuild, pxtest))
+		}
+	}
+	b.Do(ctx, root)
+}
diff --git a/src/cmd/go/internal/vet/vetflag.go b/src/cmd/go/internal/vet/vetflag.go
new file mode 100644
index 0000000..eb7af65
--- /dev/null
+++ b/src/cmd/go/internal/vet/vetflag.go
@@ -0,0 +1,191 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package vet
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"flag"
+	"fmt"
+	"log"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cmdflag"
+	"cmd/go/internal/work"
+)
+
+// go vet flag processing
+//
+// We query the flags of the tool specified by -vettool and accept any
+// of those flags plus any flag valid for 'go build'. The tool must
+// support -flags, which prints a description of its flags in JSON to
+// stdout.
+
+// vetTool specifies the vet command to run.
+// Any tool that supports the (still unpublished) vet
+// command-line protocol may be supplied; see
+// golang.org/x/tools/go/analysis/unitchecker for one
+// implementation. It is also used by tests.
+//
+// The default behavior (vetTool=="") runs 'go tool vet'.
+var vetTool string // -vettool
+
+func init() {
+	work.AddBuildFlags(CmdVet, work.DefaultBuildFlags)
+	CmdVet.Flag.StringVar(&vetTool, "vettool", "", "")
+}
+
+func parseVettoolFlag(args []string) {
+	// Extract -vettool by ad hoc flag processing:
+	// its value is needed even before we can declare
+	// the flags available during main flag processing.
+	for i, arg := range args {
+		if arg == "-vettool" || arg == "--vettool" {
+			if i+1 >= len(args) {
+				log.Fatalf("%s requires a filename", arg)
+			}
+			vetTool = args[i+1]
+			return
+		} else if strings.HasPrefix(arg, "-vettool=") ||
+			strings.HasPrefix(arg, "--vettool=") {
+			vetTool = arg[strings.IndexByte(arg, '=')+1:]
+			return
+		}
+	}
+}
+
+// vetFlags processes the command line, splitting it at the first non-flag
+// into the list of flags and list of packages.
+func vetFlags(args []string) (passToVet, packageNames []string) {
+	parseVettoolFlag(args)
+
+	// Query the vet command for its flags.
+	var tool string
+	if vetTool == "" {
+		tool = base.Tool("vet")
+	} else {
+		var err error
+		tool, err = filepath.Abs(vetTool)
+		if err != nil {
+			log.Fatal(err)
+		}
+	}
+	out := new(bytes.Buffer)
+	vetcmd := exec.Command(tool, "-flags")
+	vetcmd.Stdout = out
+	if err := vetcmd.Run(); err != nil {
+		fmt.Fprintf(os.Stderr, "go: can't execute %s -flags: %v\n", tool, err)
+		base.SetExitStatus(2)
+		base.Exit()
+	}
+	var analysisFlags []struct {
+		Name  string
+		Bool  bool
+		Usage string
+	}
+	if err := json.Unmarshal(out.Bytes(), &analysisFlags); err != nil {
+		fmt.Fprintf(os.Stderr, "go: can't unmarshal JSON from %s -flags: %v", tool, err)
+		base.SetExitStatus(2)
+		base.Exit()
+	}
+
+	// Add vet's flags to CmdVet.Flag.
+	//
+	// Some flags, in particular -tags and -v, are known to vet but
+	// also defined as build flags. This works fine, so we omit duplicates here.
+	// However some, like -x, are known to the build but not to vet.
+	isVetFlag := make(map[string]bool, len(analysisFlags))
+	cf := CmdVet.Flag
+	for _, f := range analysisFlags {
+		isVetFlag[f.Name] = true
+		if cf.Lookup(f.Name) == nil {
+			if f.Bool {
+				cf.Bool(f.Name, false, "")
+			} else {
+				cf.String(f.Name, "", "")
+			}
+		}
+	}
+
+	// Record the set of vet tool flags set by GOFLAGS. We want to pass them to
+	// the vet tool, but only if they aren't overridden by an explicit argument.
+	base.SetFromGOFLAGS(&CmdVet.Flag)
+	addFromGOFLAGS := map[string]bool{}
+	CmdVet.Flag.Visit(func(f *flag.Flag) {
+		if isVetFlag[f.Name] {
+			addFromGOFLAGS[f.Name] = true
+		}
+	})
+
+	explicitFlags := make([]string, 0, len(args))
+	for len(args) > 0 {
+		f, remainingArgs, err := cmdflag.ParseOne(&CmdVet.Flag, args)
+
+		if errors.Is(err, flag.ErrHelp) {
+			exitWithUsage()
+		}
+
+		if errors.Is(err, cmdflag.ErrFlagTerminator) {
+			// All remaining args must be package names, but the flag terminator is
+			// not included.
+			packageNames = remainingArgs
+			break
+		}
+
+		if nf := (cmdflag.NonFlagError{}); errors.As(err, &nf) {
+			// Everything from here on out — including the argument we just consumed —
+			// must be a package name.
+			packageNames = args
+			break
+		}
+
+		if err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			exitWithUsage()
+		}
+
+		if isVetFlag[f.Name] {
+			// Forward the raw arguments rather than cleaned equivalents, just in
+			// case the vet tool parses them idiosyncratically.
+			explicitFlags = append(explicitFlags, args[:len(args)-len(remainingArgs)]...)
+
+			// This flag has been overridden explicitly, so don't forward its implicit
+			// value from GOFLAGS.
+			delete(addFromGOFLAGS, f.Name)
+		}
+
+		args = remainingArgs
+	}
+
+	// Prepend arguments from GOFLAGS before other arguments.
+	CmdVet.Flag.Visit(func(f *flag.Flag) {
+		if addFromGOFLAGS[f.Name] {
+			passToVet = append(passToVet, fmt.Sprintf("-%s=%s", f.Name, f.Value))
+		}
+	})
+	passToVet = append(passToVet, explicitFlags...)
+	return passToVet, packageNames
+}
+
+func exitWithUsage() {
+	fmt.Fprintf(os.Stderr, "usage: %s\n", CmdVet.UsageLine)
+	fmt.Fprintf(os.Stderr, "Run 'go help %s' for details.\n", CmdVet.LongName())
+
+	// This part is additional to what (*Command).Usage does:
+	cmd := "go tool vet"
+	if vetTool != "" {
+		cmd = vetTool
+	}
+	fmt.Fprintf(os.Stderr, "Run '%s help' for a full list of flags and analyzers.\n", cmd)
+	fmt.Fprintf(os.Stderr, "Run '%s -help' for an overview.\n", cmd)
+
+	base.SetExitStatus(2)
+	base.Exit()
+}
diff --git a/src/cmd/go/internal/web/api.go b/src/cmd/go/internal/web/api.go
new file mode 100644
index 0000000..7a6e0c3
--- /dev/null
+++ b/src/cmd/go/internal/web/api.go
@@ -0,0 +1,246 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package web defines minimal helper routines for accessing HTTP/HTTPS
+// resources without requiring external dependencies on the net package.
+//
+// If the cmd_go_bootstrap build tag is present, web avoids the use of the net
+// package and returns errors for all network operations.
+package web
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"io/fs"
+	"net/url"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
+// SecurityMode specifies whether a function should make network
+// calls using insecure transports (eg, plain text HTTP).
+// The zero value is "secure".
+type SecurityMode int
+
+const (
+	SecureOnly      SecurityMode = iota // Reject plain HTTP; validate HTTPS.
+	DefaultSecurity                     // Allow plain HTTP if explicit; validate HTTPS.
+	Insecure                            // Allow plain HTTP if not explicitly HTTPS; skip HTTPS validation.
+)
+
+// An HTTPError describes an HTTP error response (non-200 result).
+type HTTPError struct {
+	URL        string // redacted
+	Status     string
+	StatusCode int
+	Err        error  // underlying error, if known
+	Detail     string // limited to maxErrorDetailLines and maxErrorDetailBytes
+}
+
+const (
+	maxErrorDetailLines = 8
+	maxErrorDetailBytes = maxErrorDetailLines * 81
+)
+
+func (e *HTTPError) Error() string {
+	if e.Detail != "" {
+		detailSep := " "
+		if strings.ContainsRune(e.Detail, '\n') {
+			detailSep = "\n\t"
+		}
+		return fmt.Sprintf("reading %s: %v\n\tserver response:%s%s", e.URL, e.Status, detailSep, e.Detail)
+	}
+
+	if eErr := e.Err; eErr != nil {
+		if pErr, ok := e.Err.(*fs.PathError); ok {
+			if u, err := url.Parse(e.URL); err == nil {
+				if fp, err := urlToFilePath(u); err == nil && pErr.Path == fp {
+					// Remove the redundant copy of the path.
+					eErr = pErr.Err
+				}
+			}
+		}
+		return fmt.Sprintf("reading %s: %v", e.URL, eErr)
+	}
+
+	return fmt.Sprintf("reading %s: %v", e.URL, e.Status)
+}
+
+func (e *HTTPError) Is(target error) bool {
+	return target == fs.ErrNotExist && (e.StatusCode == 404 || e.StatusCode == 410)
+}
+
+func (e *HTTPError) Unwrap() error {
+	return e.Err
+}
+
+// GetBytes returns the body of the requested resource, or an error if the
+// response status was not http.StatusOK.
+//
+// GetBytes is a convenience wrapper around Get and Response.Err.
+func GetBytes(u *url.URL) ([]byte, error) {
+	resp, err := Get(DefaultSecurity, u)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	if err := resp.Err(); err != nil {
+		return nil, err
+	}
+	b, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("reading %s: %v", u.Redacted(), err)
+	}
+	return b, nil
+}
+
+type Response struct {
+	URL        string // redacted
+	Status     string
+	StatusCode int
+	Header     map[string][]string
+	Body       io.ReadCloser // Either the original body or &errorDetail.
+
+	fileErr     error
+	errorDetail errorDetailBuffer
+}
+
+// Err returns an *HTTPError corresponding to the response r.
+// If the response r has StatusCode 200 or 0 (unset), Err returns nil.
+// Otherwise, Err may read from r.Body in order to extract relevant error detail.
+func (r *Response) Err() error {
+	if r.StatusCode == 200 || r.StatusCode == 0 {
+		return nil
+	}
+
+	return &HTTPError{
+		URL:        r.URL,
+		Status:     r.Status,
+		StatusCode: r.StatusCode,
+		Err:        r.fileErr,
+		Detail:     r.formatErrorDetail(),
+	}
+}
+
+// formatErrorDetail converts r.errorDetail (a prefix of the output of r.Body)
+// into a short, tab-indented summary.
+func (r *Response) formatErrorDetail() string {
+	if r.Body != &r.errorDetail {
+		return "" // Error detail collection not enabled.
+	}
+
+	// Ensure that r.errorDetail has been populated.
+	_, _ = io.Copy(io.Discard, r.Body)
+
+	s := r.errorDetail.buf.String()
+	if !utf8.ValidString(s) {
+		return "" // Don't try to recover non-UTF-8 error messages.
+	}
+	for _, r := range s {
+		if !unicode.IsGraphic(r) && !unicode.IsSpace(r) {
+			return "" // Don't let the server do any funny business with the user's terminal.
+		}
+	}
+
+	var detail strings.Builder
+	for i, line := range strings.Split(s, "\n") {
+		if strings.TrimSpace(line) == "" {
+			break // Stop at the first blank line.
+		}
+		if i > 0 {
+			detail.WriteString("\n\t")
+		}
+		if i >= maxErrorDetailLines {
+			detail.WriteString("[Truncated: too many lines.]")
+			break
+		}
+		if detail.Len()+len(line) > maxErrorDetailBytes {
+			detail.WriteString("[Truncated: too long.]")
+			break
+		}
+		detail.WriteString(line)
+	}
+
+	return detail.String()
+}
+
+// Get returns the body of the HTTP or HTTPS resource specified at the given URL.
+//
+// If the URL does not include an explicit scheme, Get first tries "https".
+// If the server does not respond under that scheme and the security mode is
+// Insecure, Get then tries "http".
+// The URL included in the response indicates which scheme was actually used,
+// and it is a redacted URL suitable for use in error messages.
+//
+// For the "https" scheme only, credentials are attached using the
+// cmd/go/internal/auth package. If the URL itself includes a username and
+// password, it will not be attempted under the "http" scheme unless the
+// security mode is Insecure.
+//
+// Get returns a non-nil error only if the request did not receive a response
+// under any applicable scheme. (A non-2xx response does not cause an error.)
+func Get(security SecurityMode, u *url.URL) (*Response, error) {
+	return get(security, u)
+}
+
+// OpenBrowser attempts to open the requested URL in a web browser.
+func OpenBrowser(url string) (opened bool) {
+	return openBrowser(url)
+}
+
+// Join returns the result of adding the slash-separated
+// path elements to the end of u's path.
+func Join(u *url.URL, path string) *url.URL {
+	j := *u
+	if path == "" {
+		return &j
+	}
+	j.Path = strings.TrimSuffix(u.Path, "/") + "/" + strings.TrimPrefix(path, "/")
+	j.RawPath = strings.TrimSuffix(u.RawPath, "/") + "/" + strings.TrimPrefix(path, "/")
+	return &j
+}
+
+// An errorDetailBuffer is an io.ReadCloser that copies up to
+// maxErrorDetailLines into a buffer for later inspection.
+type errorDetailBuffer struct {
+	r        io.ReadCloser
+	buf      strings.Builder
+	bufLines int
+}
+
+func (b *errorDetailBuffer) Close() error {
+	return b.r.Close()
+}
+
+func (b *errorDetailBuffer) Read(p []byte) (n int, err error) {
+	n, err = b.r.Read(p)
+
+	// Copy the first maxErrorDetailLines+1 lines into b.buf,
+	// discarding any further lines.
+	//
+	// Note that the read may begin or end in the middle of a UTF-8 character,
+	// so don't try to do anything fancy with characters that encode to larger
+	// than one byte.
+	if b.bufLines <= maxErrorDetailLines {
+		for _, line := range bytes.SplitAfterN(p[:n], []byte("\n"), maxErrorDetailLines-b.bufLines) {
+			b.buf.Write(line)
+			if len(line) > 0 && line[len(line)-1] == '\n' {
+				b.bufLines++
+				if b.bufLines > maxErrorDetailLines {
+					break
+				}
+			}
+		}
+	}
+
+	return n, err
+}
+
+// IsLocalHost reports whether the given URL refers to a local
+// (loopback) host, such as "localhost" or "127.0.0.1:8080".
+func IsLocalHost(u *url.URL) bool {
+	return isLocalHost(u)
+}
diff --git a/src/cmd/go/internal/web/bootstrap.go b/src/cmd/go/internal/web/bootstrap.go
new file mode 100644
index 0000000..6312169
--- /dev/null
+++ b/src/cmd/go/internal/web/bootstrap.go
@@ -0,0 +1,25 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build cmd_go_bootstrap
+
+// This code is compiled only into the bootstrap 'go' binary.
+// These stubs avoid importing packages with large dependency
+// trees that potentially require C linking,
+// like the use of "net/http" in vcs.go.
+
+package web
+
+import (
+	"errors"
+	urlpkg "net/url"
+)
+
+func get(security SecurityMode, url *urlpkg.URL) (*Response, error) {
+	return nil, errors.New("no http in bootstrap go command")
+}
+
+func openBrowser(url string) bool { return false }
+
+func isLocalHost(u *urlpkg.URL) bool { return false }
diff --git a/src/cmd/go/internal/web/file_test.go b/src/cmd/go/internal/web/file_test.go
new file mode 100644
index 0000000..3734df5
--- /dev/null
+++ b/src/cmd/go/internal/web/file_test.go
@@ -0,0 +1,60 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package web
+
+import (
+	"errors"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestGetFileURL(t *testing.T) {
+	const content = "Hello, file!\n"
+
+	f, err := os.CreateTemp("", "web-TestGetFileURL")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.Remove(f.Name())
+
+	if _, err := f.WriteString(content); err != nil {
+		t.Error(err)
+	}
+	if err := f.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	u, err := urlFromFilePath(f.Name())
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	b, err := GetBytes(u)
+	if err != nil {
+		t.Fatalf("GetBytes(%v) = _, %v", u, err)
+	}
+	if string(b) != content {
+		t.Fatalf("after writing %q to %s, GetBytes(%v) read %q", content, f.Name(), u, b)
+	}
+}
+
+func TestGetNonexistentFile(t *testing.T) {
+	path, err := filepath.Abs("nonexistent")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	u, err := urlFromFilePath(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	b, err := GetBytes(u)
+	if !errors.Is(err, fs.ErrNotExist) {
+		t.Fatalf("GetBytes(%v) = %q, %v; want _, fs.ErrNotExist", u, b, err)
+	}
+}
diff --git a/src/cmd/go/internal/web/http.go b/src/cmd/go/internal/web/http.go
new file mode 100644
index 0000000..4fc939a
--- /dev/null
+++ b/src/cmd/go/internal/web/http.go
@@ -0,0 +1,395 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !cmd_go_bootstrap
+
+// This code is compiled into the real 'go' binary, but it is not
+// compiled into the binary that is built during all.bash, so as
+// to avoid needing to build net (and thus use cgo) during the
+// bootstrap process.
+
+package web
+
+import (
+	"crypto/tls"
+	"errors"
+	"fmt"
+	"io"
+	"mime"
+	"net"
+	"net/http"
+	urlpkg "net/url"
+	"os"
+	"strings"
+	"time"
+
+	"cmd/go/internal/auth"
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/internal/browser"
+)
+
+// impatientInsecureHTTPClient is used with GOINSECURE,
+// when we're connecting to https servers that might not be there
+// or might be using self-signed certificates.
+var impatientInsecureHTTPClient = &http.Client{
+	CheckRedirect: checkRedirect,
+	Timeout:       5 * time.Second,
+	Transport: &http.Transport{
+		Proxy: http.ProxyFromEnvironment,
+		TLSClientConfig: &tls.Config{
+			InsecureSkipVerify: true,
+		},
+	},
+}
+
+var securityPreservingDefaultClient = securityPreservingHTTPClient(http.DefaultClient)
+
+// securityPreservingHTTPClient returns a client that is like the original
+// but rejects redirects to plain-HTTP URLs if the original URL was secure.
+func securityPreservingHTTPClient(original *http.Client) *http.Client {
+	c := new(http.Client)
+	*c = *original
+	c.CheckRedirect = func(req *http.Request, via []*http.Request) error {
+		if len(via) > 0 && via[0].URL.Scheme == "https" && req.URL.Scheme != "https" {
+			lastHop := via[len(via)-1].URL
+			return fmt.Errorf("redirected from secure URL %s to insecure URL %s", lastHop, req.URL)
+		}
+		return checkRedirect(req, via)
+	}
+	return c
+}
+
+func checkRedirect(req *http.Request, via []*http.Request) error {
+	// Go's http.DefaultClient allows 10 redirects before returning an error.
+	// Mimic that behavior here.
+	if len(via) >= 10 {
+		return errors.New("stopped after 10 redirects")
+	}
+
+	interceptRequest(req)
+	return nil
+}
+
+type Interceptor struct {
+	Scheme   string
+	FromHost string
+	ToHost   string
+	Client   *http.Client
+}
+
+func EnableTestHooks(interceptors []Interceptor) error {
+	if enableTestHooks {
+		return errors.New("web: test hooks already enabled")
+	}
+
+	for _, t := range interceptors {
+		if t.FromHost == "" {
+			panic("EnableTestHooks: missing FromHost")
+		}
+		if t.ToHost == "" {
+			panic("EnableTestHooks: missing ToHost")
+		}
+	}
+
+	testInterceptors = interceptors
+	enableTestHooks = true
+	return nil
+}
+
+func DisableTestHooks() {
+	if !enableTestHooks {
+		panic("web: test hooks not enabled")
+	}
+	enableTestHooks = false
+	testInterceptors = nil
+}
+
+var (
+	enableTestHooks  = false
+	testInterceptors []Interceptor
+)
+
+func interceptURL(u *urlpkg.URL) (*Interceptor, bool) {
+	if !enableTestHooks {
+		return nil, false
+	}
+	for i, t := range testInterceptors {
+		if u.Host == t.FromHost && (u.Scheme == "" || u.Scheme == t.Scheme) {
+			return &testInterceptors[i], true
+		}
+	}
+	return nil, false
+}
+
+func interceptRequest(req *http.Request) {
+	if t, ok := interceptURL(req.URL); ok {
+		req.Host = req.URL.Host
+		req.URL.Host = t.ToHost
+	}
+}
+
+func get(security SecurityMode, url *urlpkg.URL) (*Response, error) {
+	start := time.Now()
+
+	if url.Scheme == "file" {
+		return getFile(url)
+	}
+
+	if enableTestHooks {
+		switch url.Host {
+		case "proxy.golang.org":
+			if os.Getenv("TESTGOPROXY404") == "1" {
+				res := &Response{
+					URL:        url.Redacted(),
+					Status:     "404 testing",
+					StatusCode: 404,
+					Header:     make(map[string][]string),
+					Body:       http.NoBody,
+				}
+				if cfg.BuildX {
+					fmt.Fprintf(os.Stderr, "# get %s: %v (%.3fs)\n", url.Redacted(), res.Status, time.Since(start).Seconds())
+				}
+				return res, nil
+			}
+
+		case "localhost.localdev":
+			return nil, fmt.Errorf("no such host localhost.localdev")
+
+		default:
+			if os.Getenv("TESTGONETWORK") == "panic" {
+				if _, ok := interceptURL(url); !ok {
+					host := url.Host
+					if h, _, err := net.SplitHostPort(url.Host); err == nil && h != "" {
+						host = h
+					}
+					addr := net.ParseIP(host)
+					if addr == nil || (!addr.IsLoopback() && !addr.IsUnspecified()) {
+						panic("use of network: " + url.String())
+					}
+				}
+			}
+		}
+	}
+
+	fetch := func(url *urlpkg.URL) (*urlpkg.URL, *http.Response, error) {
+		// Note: The -v build flag does not mean "print logging information",
+		// despite its historical misuse for this in GOPATH-based go get.
+		// We print extra logging in -x mode instead, which traces what
+		// commands are executed.
+		if cfg.BuildX {
+			fmt.Fprintf(os.Stderr, "# get %s\n", url.Redacted())
+		}
+
+		req, err := http.NewRequest("GET", url.String(), nil)
+		if err != nil {
+			return nil, nil, err
+		}
+		if url.Scheme == "https" {
+			auth.AddCredentials(req)
+		}
+		t, intercepted := interceptURL(req.URL)
+		if intercepted {
+			req.Host = req.URL.Host
+			req.URL.Host = t.ToHost
+		}
+
+		release, err := base.AcquireNet()
+		if err != nil {
+			return nil, nil, err
+		}
+
+		var res *http.Response
+		if security == Insecure && url.Scheme == "https" { // fail earlier
+			res, err = impatientInsecureHTTPClient.Do(req)
+		} else {
+			if intercepted && t.Client != nil {
+				client := securityPreservingHTTPClient(t.Client)
+				res, err = client.Do(req)
+			} else {
+				res, err = securityPreservingDefaultClient.Do(req)
+			}
+		}
+
+		if err != nil {
+			// Per the docs for [net/http.Client.Do], “On error, any Response can be
+			// ignored. A non-nil Response with a non-nil error only occurs when
+			// CheckRedirect fails, and even then the returned Response.Body is
+			// already closed.”
+			release()
+			return nil, nil, err
+		}
+
+		// “If the returned error is nil, the Response will contain a non-nil Body
+		// which the user is expected to close.”
+		body := res.Body
+		res.Body = hookCloser{
+			ReadCloser: body,
+			afterClose: release,
+		}
+		return url, res, err
+	}
+
+	var (
+		fetched *urlpkg.URL
+		res     *http.Response
+		err     error
+	)
+	if url.Scheme == "" || url.Scheme == "https" {
+		secure := new(urlpkg.URL)
+		*secure = *url
+		secure.Scheme = "https"
+
+		fetched, res, err = fetch(secure)
+		if err != nil {
+			if cfg.BuildX {
+				fmt.Fprintf(os.Stderr, "# get %s: %v\n", secure.Redacted(), err)
+			}
+			if security != Insecure || url.Scheme == "https" {
+				// HTTPS failed, and we can't fall back to plain HTTP.
+				// Report the error from the HTTPS attempt.
+				return nil, err
+			}
+		}
+	}
+
+	if res == nil {
+		switch url.Scheme {
+		case "http":
+			if security == SecureOnly {
+				if cfg.BuildX {
+					fmt.Fprintf(os.Stderr, "# get %s: insecure\n", url.Redacted())
+				}
+				return nil, fmt.Errorf("insecure URL: %s", url.Redacted())
+			}
+		case "":
+			if security != Insecure {
+				panic("should have returned after HTTPS failure")
+			}
+		default:
+			if cfg.BuildX {
+				fmt.Fprintf(os.Stderr, "# get %s: unsupported\n", url.Redacted())
+			}
+			return nil, fmt.Errorf("unsupported scheme: %s", url.Redacted())
+		}
+
+		insecure := new(urlpkg.URL)
+		*insecure = *url
+		insecure.Scheme = "http"
+		if insecure.User != nil && security != Insecure {
+			if cfg.BuildX {
+				fmt.Fprintf(os.Stderr, "# get %s: insecure credentials\n", insecure.Redacted())
+			}
+			return nil, fmt.Errorf("refusing to pass credentials to insecure URL: %s", insecure.Redacted())
+		}
+
+		fetched, res, err = fetch(insecure)
+		if err != nil {
+			if cfg.BuildX {
+				fmt.Fprintf(os.Stderr, "# get %s: %v\n", insecure.Redacted(), err)
+			}
+			// HTTP failed, and we already tried HTTPS if applicable.
+			// Report the error from the HTTP attempt.
+			return nil, err
+		}
+	}
+
+	// Note: accepting a non-200 OK here, so people can serve a
+	// meta import in their http 404 page.
+	if cfg.BuildX {
+		fmt.Fprintf(os.Stderr, "# get %s: %v (%.3fs)\n", fetched.Redacted(), res.Status, time.Since(start).Seconds())
+	}
+
+	r := &Response{
+		URL:        fetched.Redacted(),
+		Status:     res.Status,
+		StatusCode: res.StatusCode,
+		Header:     map[string][]string(res.Header),
+		Body:       res.Body,
+	}
+
+	if res.StatusCode != http.StatusOK {
+		contentType := res.Header.Get("Content-Type")
+		if mediaType, params, _ := mime.ParseMediaType(contentType); mediaType == "text/plain" {
+			switch charset := strings.ToLower(params["charset"]); charset {
+			case "us-ascii", "utf-8", "":
+				// Body claims to be plain text in UTF-8 or a subset thereof.
+				// Try to extract a useful error message from it.
+				r.errorDetail.r = res.Body
+				r.Body = &r.errorDetail
+			}
+		}
+	}
+
+	return r, nil
+}
+
+func getFile(u *urlpkg.URL) (*Response, error) {
+	path, err := urlToFilePath(u)
+	if err != nil {
+		return nil, err
+	}
+	f, err := os.Open(path)
+
+	if os.IsNotExist(err) {
+		return &Response{
+			URL:        u.Redacted(),
+			Status:     http.StatusText(http.StatusNotFound),
+			StatusCode: http.StatusNotFound,
+			Body:       http.NoBody,
+			fileErr:    err,
+		}, nil
+	}
+
+	if os.IsPermission(err) {
+		return &Response{
+			URL:        u.Redacted(),
+			Status:     http.StatusText(http.StatusForbidden),
+			StatusCode: http.StatusForbidden,
+			Body:       http.NoBody,
+			fileErr:    err,
+		}, nil
+	}
+
+	if err != nil {
+		return nil, err
+	}
+
+	return &Response{
+		URL:        u.Redacted(),
+		Status:     http.StatusText(http.StatusOK),
+		StatusCode: http.StatusOK,
+		Body:       f,
+	}, nil
+}
+
+func openBrowser(url string) bool { return browser.Open(url) }
+
+func isLocalHost(u *urlpkg.URL) bool {
+	// VCSTestRepoURL itself is secure, and it may redirect requests to other
+	// ports (such as a port serving the "svn" protocol) which should also be
+	// considered secure.
+	host, _, err := net.SplitHostPort(u.Host)
+	if err != nil {
+		host = u.Host
+	}
+	if host == "localhost" {
+		return true
+	}
+	if ip := net.ParseIP(host); ip != nil && ip.IsLoopback() {
+		return true
+	}
+	return false
+}
+
+type hookCloser struct {
+	io.ReadCloser
+	afterClose func()
+}
+
+func (c hookCloser) Close() error {
+	err := c.ReadCloser.Close()
+	c.afterClose()
+	return err
+}
diff --git a/src/cmd/go/internal/web/url.go b/src/cmd/go/internal/web/url.go
new file mode 100644
index 0000000..146c51f
--- /dev/null
+++ b/src/cmd/go/internal/web/url.go
@@ -0,0 +1,95 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package web
+
+import (
+	"errors"
+	"net/url"
+	"path/filepath"
+	"strings"
+)
+
+// TODO(golang.org/issue/32456): If accepted, move these functions into the
+// net/url package.
+
+var errNotAbsolute = errors.New("path is not absolute")
+
+func urlToFilePath(u *url.URL) (string, error) {
+	if u.Scheme != "file" {
+		return "", errors.New("non-file URL")
+	}
+
+	checkAbs := func(path string) (string, error) {
+		if !filepath.IsAbs(path) {
+			return "", errNotAbsolute
+		}
+		return path, nil
+	}
+
+	if u.Path == "" {
+		if u.Host != "" || u.Opaque == "" {
+			return "", errors.New("file URL missing path")
+		}
+		return checkAbs(filepath.FromSlash(u.Opaque))
+	}
+
+	path, err := convertFileURLPath(u.Host, u.Path)
+	if err != nil {
+		return path, err
+	}
+	return checkAbs(path)
+}
+
+func urlFromFilePath(path string) (*url.URL, error) {
+	if !filepath.IsAbs(path) {
+		return nil, errNotAbsolute
+	}
+
+	// If path has a Windows volume name, convert the volume to a host and prefix
+	// per https://blogs.msdn.microsoft.com/ie/2006/12/06/file-uris-in-windows/.
+	if vol := filepath.VolumeName(path); vol != "" {
+		if strings.HasPrefix(vol, `\\`) {
+			path = filepath.ToSlash(path[2:])
+			i := strings.IndexByte(path, '/')
+
+			if i < 0 {
+				// A degenerate case.
+				// \\host.example.com (without a share name)
+				// becomes
+				// file://host.example.com/
+				return &url.URL{
+					Scheme: "file",
+					Host:   path,
+					Path:   "/",
+				}, nil
+			}
+
+			// \\host.example.com\Share\path\to\file
+			// becomes
+			// file://host.example.com/Share/path/to/file
+			return &url.URL{
+				Scheme: "file",
+				Host:   path[:i],
+				Path:   filepath.ToSlash(path[i:]),
+			}, nil
+		}
+
+		// C:\path\to\file
+		// becomes
+		// file:///C:/path/to/file
+		return &url.URL{
+			Scheme: "file",
+			Path:   "/" + filepath.ToSlash(path),
+		}, nil
+	}
+
+	// /path/to/file
+	// becomes
+	// file:///path/to/file
+	return &url.URL{
+		Scheme: "file",
+		Path:   filepath.ToSlash(path),
+	}, nil
+}
diff --git a/src/cmd/go/internal/web/url_other.go b/src/cmd/go/internal/web/url_other.go
new file mode 100644
index 0000000..84bbd72
--- /dev/null
+++ b/src/cmd/go/internal/web/url_other.go
@@ -0,0 +1,21 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !windows
+
+package web
+
+import (
+	"errors"
+	"path/filepath"
+)
+
+func convertFileURLPath(host, path string) (string, error) {
+	switch host {
+	case "", "localhost":
+	default:
+		return "", errors.New("file URL specifies non-local host")
+	}
+	return filepath.FromSlash(path), nil
+}
diff --git a/src/cmd/go/internal/web/url_other_test.go b/src/cmd/go/internal/web/url_other_test.go
new file mode 100644
index 0000000..5c197de
--- /dev/null
+++ b/src/cmd/go/internal/web/url_other_test.go
@@ -0,0 +1,36 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !windows
+
+package web
+
+var urlTests = []struct {
+	url          string
+	filePath     string
+	canonicalURL string // If empty, assume equal to url.
+	wantErr      string
+}{
+	// Examples from RFC 8089:
+	{
+		url:      `file:///path/to/file`,
+		filePath: `/path/to/file`,
+	},
+	{
+		url:          `file:/path/to/file`,
+		filePath:     `/path/to/file`,
+		canonicalURL: `file:///path/to/file`,
+	},
+	{
+		url:          `file://localhost/path/to/file`,
+		filePath:     `/path/to/file`,
+		canonicalURL: `file:///path/to/file`,
+	},
+
+	// We reject non-local files.
+	{
+		url:     `file://host.example.com/path/to/file`,
+		wantErr: "file URL specifies non-local host",
+	},
+}
diff --git a/src/cmd/go/internal/web/url_test.go b/src/cmd/go/internal/web/url_test.go
new file mode 100644
index 0000000..8f462f5
--- /dev/null
+++ b/src/cmd/go/internal/web/url_test.go
@@ -0,0 +1,77 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package web
+
+import (
+	"net/url"
+	"testing"
+)
+
+func TestURLToFilePath(t *testing.T) {
+	for _, tc := range urlTests {
+		if tc.url == "" {
+			continue
+		}
+		tc := tc
+
+		t.Run(tc.url, func(t *testing.T) {
+			u, err := url.Parse(tc.url)
+			if err != nil {
+				t.Fatalf("url.Parse(%q): %v", tc.url, err)
+			}
+
+			path, err := urlToFilePath(u)
+			if err != nil {
+				if err.Error() == tc.wantErr {
+					return
+				}
+				if tc.wantErr == "" {
+					t.Fatalf("urlToFilePath(%v): %v; want <nil>", u, err)
+				} else {
+					t.Fatalf("urlToFilePath(%v): %v; want %s", u, err, tc.wantErr)
+				}
+			}
+
+			if path != tc.filePath || tc.wantErr != "" {
+				t.Fatalf("urlToFilePath(%v) = %q, <nil>; want %q, %s", u, path, tc.filePath, tc.wantErr)
+			}
+		})
+	}
+}
+
+func TestURLFromFilePath(t *testing.T) {
+	for _, tc := range urlTests {
+		if tc.filePath == "" {
+			continue
+		}
+		tc := tc
+
+		t.Run(tc.filePath, func(t *testing.T) {
+			u, err := urlFromFilePath(tc.filePath)
+			if err != nil {
+				if err.Error() == tc.wantErr {
+					return
+				}
+				if tc.wantErr == "" {
+					t.Fatalf("urlFromFilePath(%v): %v; want <nil>", tc.filePath, err)
+				} else {
+					t.Fatalf("urlFromFilePath(%v): %v; want %s", tc.filePath, err, tc.wantErr)
+				}
+			}
+
+			if tc.wantErr != "" {
+				t.Fatalf("urlFromFilePath(%v) = <nil>; want error: %s", tc.filePath, tc.wantErr)
+			}
+
+			wantURL := tc.url
+			if tc.canonicalURL != "" {
+				wantURL = tc.canonicalURL
+			}
+			if u.String() != wantURL {
+				t.Errorf("urlFromFilePath(%v) = %v; want %s", tc.filePath, u, wantURL)
+			}
+		})
+	}
+}
diff --git a/src/cmd/go/internal/web/url_windows.go b/src/cmd/go/internal/web/url_windows.go
new file mode 100644
index 0000000..2a65ec8
--- /dev/null
+++ b/src/cmd/go/internal/web/url_windows.go
@@ -0,0 +1,43 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package web
+
+import (
+	"errors"
+	"path/filepath"
+	"strings"
+)
+
+func convertFileURLPath(host, path string) (string, error) {
+	if len(path) == 0 || path[0] != '/' {
+		return "", errNotAbsolute
+	}
+
+	path = filepath.FromSlash(path)
+
+	// We interpret Windows file URLs per the description in
+	// https://blogs.msdn.microsoft.com/ie/2006/12/06/file-uris-in-windows/.
+
+	// The host part of a file URL (if any) is the UNC volume name,
+	// but RFC 8089 reserves the authority "localhost" for the local machine.
+	if host != "" && host != "localhost" {
+		// A common "legacy" format omits the leading slash before a drive letter,
+		// encoding the drive letter as the host instead of part of the path.
+		// (See https://blogs.msdn.microsoft.com/freeassociations/2005/05/19/the-bizarre-and-unhappy-story-of-file-urls/.)
+		// We do not support that format, but we should at least emit a more
+		// helpful error message for it.
+		if filepath.VolumeName(host) != "" {
+			return "", errors.New("file URL encodes volume in host field: too few slashes?")
+		}
+		return `\\` + host + path, nil
+	}
+
+	// If host is empty, path must contain an initial slash followed by a
+	// drive letter and path. Remove the slash and verify that the path is valid.
+	if vol := filepath.VolumeName(path[1:]); vol == "" || strings.HasPrefix(vol, `\\`) {
+		return "", errors.New("file URL missing drive letter")
+	}
+	return path[1:], nil
+}
diff --git a/src/cmd/go/internal/web/url_windows_test.go b/src/cmd/go/internal/web/url_windows_test.go
new file mode 100644
index 0000000..06386a0
--- /dev/null
+++ b/src/cmd/go/internal/web/url_windows_test.go
@@ -0,0 +1,94 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package web
+
+var urlTests = []struct {
+	url          string
+	filePath     string
+	canonicalURL string // If empty, assume equal to url.
+	wantErr      string
+}{
+	// Examples from https://blogs.msdn.microsoft.com/ie/2006/12/06/file-uris-in-windows/:
+
+	{
+		url:      `file://laptop/My%20Documents/FileSchemeURIs.doc`,
+		filePath: `\\laptop\My Documents\FileSchemeURIs.doc`,
+	},
+	{
+		url:      `file:///C:/Documents%20and%20Settings/davris/FileSchemeURIs.doc`,
+		filePath: `C:\Documents and Settings\davris\FileSchemeURIs.doc`,
+	},
+	{
+		url:      `file:///D:/Program%20Files/Viewer/startup.htm`,
+		filePath: `D:\Program Files\Viewer\startup.htm`,
+	},
+	{
+		url:          `file:///C:/Program%20Files/Music/Web%20Sys/main.html?REQUEST=RADIO`,
+		filePath:     `C:\Program Files\Music\Web Sys\main.html`,
+		canonicalURL: `file:///C:/Program%20Files/Music/Web%20Sys/main.html`,
+	},
+	{
+		url:      `file://applib/products/a-b/abc_9/4148.920a/media/start.swf`,
+		filePath: `\\applib\products\a-b\abc_9\4148.920a\media\start.swf`,
+	},
+	{
+		url:     `file:////applib/products/a%2Db/abc%5F9/4148.920a/media/start.swf`,
+		wantErr: "file URL missing drive letter",
+	},
+	{
+		url:     `C:\Program Files\Music\Web Sys\main.html?REQUEST=RADIO`,
+		wantErr: "non-file URL",
+	},
+
+	// The example "file://D:\Program Files\Viewer\startup.htm" errors out in
+	// url.Parse, so we substitute a slash-based path for testing instead.
+	{
+		url:     `file://D:/Program Files/Viewer/startup.htm`,
+		wantErr: "file URL encodes volume in host field: too few slashes?",
+	},
+
+	// The blog post discourages the use of non-ASCII characters because they
+	// depend on the user's current codepage. However, when we are working with Go
+	// strings we assume UTF-8 encoding, and our url package refuses to encode
+	// URLs to non-ASCII strings.
+	{
+		url:          `file:///C:/exampleㄓ.txt`,
+		filePath:     `C:\exampleㄓ.txt`,
+		canonicalURL: `file:///C:/example%E3%84%93.txt`,
+	},
+	{
+		url:      `file:///C:/example%E3%84%93.txt`,
+		filePath: `C:\exampleㄓ.txt`,
+	},
+
+	// Examples from RFC 8089:
+
+	// We allow the drive-letter variation from section E.2, because it is
+	// simpler to support than not to. However, we do not generate the shorter
+	// form in the reverse direction.
+	{
+		url:          `file:c:/path/to/file`,
+		filePath:     `c:\path\to\file`,
+		canonicalURL: `file:///c:/path/to/file`,
+	},
+
+	// We encode the UNC share name as the authority following section E.3.1,
+	// because that is what the Microsoft blog post explicitly recommends.
+	{
+		url:      `file://host.example.com/Share/path/to/file.txt`,
+		filePath: `\\host.example.com\Share\path\to\file.txt`,
+	},
+
+	// We decline the four- and five-slash variations from section E.3.2.
+	// The paths in these URLs would change meaning under path.Clean.
+	{
+		url:     `file:////host.example.com/path/to/file`,
+		wantErr: "file URL missing drive letter",
+	},
+	{
+		url:     `file://///host.example.com/path/to/file`,
+		wantErr: "file URL missing drive letter",
+	},
+}
diff --git a/src/cmd/go/internal/work/action.go b/src/cmd/go/internal/work/action.go
new file mode 100644
index 0000000..d4d0a71
--- /dev/null
+++ b/src/cmd/go/internal/work/action.go
@@ -0,0 +1,917 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Action graph creation (planning).
+
+package work
+
+import (
+	"bufio"
+	"bytes"
+	"container/heap"
+	"context"
+	"debug/elf"
+	"encoding/json"
+	"fmt"
+	"internal/platform"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+	"time"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cache"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/load"
+	"cmd/go/internal/robustio"
+	"cmd/go/internal/str"
+	"cmd/go/internal/trace"
+	"cmd/internal/buildid"
+)
+
+// A Builder holds global state about a build.
+// It does not hold per-package state, because we
+// build packages in parallel, and the builder is shared.
+type Builder struct {
+	WorkDir            string                    // the temporary work directory (ends in filepath.Separator)
+	actionCache        map[cacheKey]*Action      // a cache of already-constructed actions
+	mkdirCache         map[string]bool           // a cache of created directories
+	flagCache          map[[2]string]bool        // a cache of supported compiler flags
+	gccCompilerIDCache map[string]cache.ActionID // cache for gccCompilerID
+	Print              func(args ...any) (int, error)
+
+	IsCmdList           bool // running as part of go list; set p.Stale and additional fields below
+	NeedError           bool // list needs p.Error
+	NeedExport          bool // list needs p.Export
+	NeedCompiledGoFiles bool // list needs p.CompiledGoFiles
+	AllowErrors         bool // errors don't immediately exit the program
+
+	objdirSeq int // counter for NewObjdir
+	pkgSeq    int
+
+	output    sync.Mutex
+	scriptDir string // current directory in printed script
+
+	exec      sync.Mutex
+	readySema chan bool
+	ready     actionQueue
+
+	id           sync.Mutex
+	toolIDCache  map[string]string // tool name -> tool ID
+	buildIDCache map[string]string // file name -> build ID
+}
+
+// NOTE: Much of Action would not need to be exported if not for test.
+// Maybe test functionality should move into this package too?
+
+// An Actor runs an action.
+type Actor interface {
+	Act(*Builder, context.Context, *Action) error
+}
+
+// An ActorFunc is an Actor that calls the function.
+type ActorFunc func(*Builder, context.Context, *Action) error
+
+func (f ActorFunc) Act(b *Builder, ctx context.Context, a *Action) error {
+	return f(b, ctx, a)
+}
+
+// An Action represents a single action in the action graph.
+type Action struct {
+	Mode       string        // description of action operation
+	Package    *load.Package // the package this action works on
+	Deps       []*Action     // actions that must happen before this one
+	Actor      Actor         // the action itself (nil = no-op)
+	IgnoreFail bool          // whether to run f even if dependencies fail
+	TestOutput *bytes.Buffer // test output buffer
+	Args       []string      // additional args for runProgram
+
+	triggers []*Action // inverse of deps
+
+	buggyInstall bool // is this a buggy install (see -linkshared)?
+
+	TryCache func(*Builder, *Action) bool // callback for cache bypass
+
+	// Generated files, directories.
+	Objdir   string         // directory for intermediate objects
+	Target   string         // goal of the action: the created package or executable
+	built    string         // the actual created package or executable
+	actionID cache.ActionID // cache ID of action input
+	buildID  string         // build ID of action output
+
+	VetxOnly  bool       // Mode=="vet": only being called to supply info about dependencies
+	needVet   bool       // Mode=="build": need to fill in vet config
+	needBuild bool       // Mode=="build": need to do actual build (can be false if needVet is true)
+	vetCfg    *vetConfig // vet config
+	output    []byte     // output redirect buffer (nil means use b.Print)
+
+	// Execution state.
+	pending      int               // number of deps yet to complete
+	priority     int               // relative execution priority
+	Failed       bool              // whether the action failed
+	json         *actionJSON       // action graph information
+	nonGoOverlay map[string]string // map from non-.go source files to copied files in objdir. Nil if no overlay is used.
+	traceSpan    *trace.Span
+}
+
+// BuildActionID returns the action ID section of a's build ID.
+func (a *Action) BuildActionID() string { return actionID(a.buildID) }
+
+// BuildContentID returns the content ID section of a's build ID.
+func (a *Action) BuildContentID() string { return contentID(a.buildID) }
+
+// BuildID returns a's build ID.
+func (a *Action) BuildID() string { return a.buildID }
+
+// BuiltTarget returns the actual file that was built. This differs
+// from Target when the result was cached.
+func (a *Action) BuiltTarget() string { return a.built }
+
+// An actionQueue is a priority queue of actions.
+type actionQueue []*Action
+
+// Implement heap.Interface
+func (q *actionQueue) Len() int           { return len(*q) }
+func (q *actionQueue) Swap(i, j int)      { (*q)[i], (*q)[j] = (*q)[j], (*q)[i] }
+func (q *actionQueue) Less(i, j int) bool { return (*q)[i].priority < (*q)[j].priority }
+func (q *actionQueue) Push(x any)         { *q = append(*q, x.(*Action)) }
+func (q *actionQueue) Pop() any {
+	n := len(*q) - 1
+	x := (*q)[n]
+	*q = (*q)[:n]
+	return x
+}
+
+func (q *actionQueue) push(a *Action) {
+	if a.json != nil {
+		a.json.TimeReady = time.Now()
+	}
+	heap.Push(q, a)
+}
+
+func (q *actionQueue) pop() *Action {
+	return heap.Pop(q).(*Action)
+}
+
+type actionJSON struct {
+	ID         int
+	Mode       string
+	Package    string
+	Deps       []int     `json:",omitempty"`
+	IgnoreFail bool      `json:",omitempty"`
+	Args       []string  `json:",omitempty"`
+	Link       bool      `json:",omitempty"`
+	Objdir     string    `json:",omitempty"`
+	Target     string    `json:",omitempty"`
+	Priority   int       `json:",omitempty"`
+	Failed     bool      `json:",omitempty"`
+	Built      string    `json:",omitempty"`
+	VetxOnly   bool      `json:",omitempty"`
+	NeedVet    bool      `json:",omitempty"`
+	NeedBuild  bool      `json:",omitempty"`
+	ActionID   string    `json:",omitempty"`
+	BuildID    string    `json:",omitempty"`
+	TimeReady  time.Time `json:",omitempty"`
+	TimeStart  time.Time `json:",omitempty"`
+	TimeDone   time.Time `json:",omitempty"`
+
+	Cmd     []string      // `json:",omitempty"`
+	CmdReal time.Duration `json:",omitempty"`
+	CmdUser time.Duration `json:",omitempty"`
+	CmdSys  time.Duration `json:",omitempty"`
+}
+
+// cacheKey is the key for the action cache.
+type cacheKey struct {
+	mode string
+	p    *load.Package
+}
+
+func actionGraphJSON(a *Action) string {
+	var workq []*Action
+	var inWorkq = make(map[*Action]int)
+
+	add := func(a *Action) {
+		if _, ok := inWorkq[a]; ok {
+			return
+		}
+		inWorkq[a] = len(workq)
+		workq = append(workq, a)
+	}
+	add(a)
+
+	for i := 0; i < len(workq); i++ {
+		for _, dep := range workq[i].Deps {
+			add(dep)
+		}
+	}
+
+	var list []*actionJSON
+	for id, a := range workq {
+		if a.json == nil {
+			a.json = &actionJSON{
+				Mode:       a.Mode,
+				ID:         id,
+				IgnoreFail: a.IgnoreFail,
+				Args:       a.Args,
+				Objdir:     a.Objdir,
+				Target:     a.Target,
+				Failed:     a.Failed,
+				Priority:   a.priority,
+				Built:      a.built,
+				VetxOnly:   a.VetxOnly,
+				NeedBuild:  a.needBuild,
+				NeedVet:    a.needVet,
+			}
+			if a.Package != nil {
+				// TODO(rsc): Make this a unique key for a.Package somehow.
+				a.json.Package = a.Package.ImportPath
+			}
+			for _, a1 := range a.Deps {
+				a.json.Deps = append(a.json.Deps, inWorkq[a1])
+			}
+		}
+		list = append(list, a.json)
+	}
+
+	js, err := json.MarshalIndent(list, "", "\t")
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "go: writing debug action graph: %v\n", err)
+		return ""
+	}
+	return string(js)
+}
+
+// BuildMode specifies the build mode:
+// are we just building things or also installing the results?
+type BuildMode int
+
+const (
+	ModeBuild BuildMode = iota
+	ModeInstall
+	ModeBuggyInstall
+
+	ModeVetOnly = 1 << 8
+)
+
+// NewBuilder returns a new Builder ready for use.
+//
+// If workDir is the empty string, NewBuilder creates a WorkDir if needed
+// and arranges for it to be removed in case of an unclean exit.
+// The caller must Close the builder explicitly to clean up the WorkDir
+// before a clean exit.
+func NewBuilder(workDir string) *Builder {
+	b := new(Builder)
+
+	b.Print = func(a ...any) (int, error) {
+		return fmt.Fprint(os.Stderr, a...)
+	}
+	b.actionCache = make(map[cacheKey]*Action)
+	b.mkdirCache = make(map[string]bool)
+	b.toolIDCache = make(map[string]string)
+	b.buildIDCache = make(map[string]string)
+
+	if workDir != "" {
+		b.WorkDir = workDir
+	} else if cfg.BuildN {
+		b.WorkDir = "$WORK"
+	} else {
+		if !buildInitStarted {
+			panic("internal error: NewBuilder called before BuildInit")
+		}
+		tmp, err := os.MkdirTemp(cfg.Getenv("GOTMPDIR"), "go-build")
+		if err != nil {
+			base.Fatalf("go: creating work dir: %v", err)
+		}
+		if !filepath.IsAbs(tmp) {
+			abs, err := filepath.Abs(tmp)
+			if err != nil {
+				os.RemoveAll(tmp)
+				base.Fatalf("go: creating work dir: %v", err)
+			}
+			tmp = abs
+		}
+		b.WorkDir = tmp
+		builderWorkDirs.Store(b, b.WorkDir)
+		if cfg.BuildX || cfg.BuildWork {
+			fmt.Fprintf(os.Stderr, "WORK=%s\n", b.WorkDir)
+		}
+	}
+
+	if err := CheckGOOSARCHPair(cfg.Goos, cfg.Goarch); err != nil {
+		fmt.Fprintf(os.Stderr, "go: %v\n", err)
+		base.SetExitStatus(2)
+		base.Exit()
+	}
+
+	for _, tag := range cfg.BuildContext.BuildTags {
+		if strings.Contains(tag, ",") {
+			fmt.Fprintf(os.Stderr, "go: -tags space-separated list contains comma\n")
+			base.SetExitStatus(2)
+			base.Exit()
+		}
+	}
+
+	return b
+}
+
+var builderWorkDirs sync.Map // *Builder → WorkDir
+
+func (b *Builder) Close() error {
+	wd, ok := builderWorkDirs.Load(b)
+	if !ok {
+		return nil
+	}
+	defer builderWorkDirs.Delete(b)
+
+	if b.WorkDir != wd.(string) {
+		base.Errorf("go: internal error: Builder WorkDir unexpectedly changed from %s to %s", wd, b.WorkDir)
+	}
+
+	if !cfg.BuildWork {
+		if err := robustio.RemoveAll(b.WorkDir); err != nil {
+			return err
+		}
+	}
+	b.WorkDir = ""
+	return nil
+}
+
+func closeBuilders() {
+	leakedBuilders := 0
+	builderWorkDirs.Range(func(bi, _ any) bool {
+		leakedBuilders++
+		if err := bi.(*Builder).Close(); err != nil {
+			base.Error(err)
+		}
+		return true
+	})
+
+	if leakedBuilders > 0 && base.GetExitStatus() == 0 {
+		fmt.Fprintf(os.Stderr, "go: internal error: Builder leaked on successful exit\n")
+		base.SetExitStatus(1)
+	}
+}
+
+func CheckGOOSARCHPair(goos, goarch string) error {
+	if !platform.BuildModeSupported(cfg.BuildContext.Compiler, "default", goos, goarch) {
+		return fmt.Errorf("unsupported GOOS/GOARCH pair %s/%s", goos, goarch)
+	}
+	return nil
+}
+
+// NewObjdir returns the name of a fresh object directory under b.WorkDir.
+// It is up to the caller to call b.Mkdir on the result at an appropriate time.
+// The result ends in a slash, so that file names in that directory
+// can be constructed with direct string addition.
+//
+// NewObjdir must be called only from a single goroutine at a time,
+// so it is safe to call during action graph construction, but it must not
+// be called during action graph execution.
+func (b *Builder) NewObjdir() string {
+	b.objdirSeq++
+	return str.WithFilePathSeparator(filepath.Join(b.WorkDir, fmt.Sprintf("b%03d", b.objdirSeq)))
+}
+
+// readpkglist returns the list of packages that were built into the shared library
+// at shlibpath. For the native toolchain this list is stored, newline separated, in
+// an ELF note with name "Go\x00\x00" and type 1. For GCCGO it is extracted from the
+// .go_export section.
+func readpkglist(shlibpath string) (pkgs []*load.Package) {
+	var stk load.ImportStack
+	if cfg.BuildToolchainName == "gccgo" {
+		f, err := elf.Open(shlibpath)
+		if err != nil {
+			base.Fatal(fmt.Errorf("failed to open shared library: %v", err))
+		}
+		sect := f.Section(".go_export")
+		if sect == nil {
+			base.Fatal(fmt.Errorf("%s: missing .go_export section", shlibpath))
+		}
+		data, err := sect.Data()
+		if err != nil {
+			base.Fatal(fmt.Errorf("%s: failed to read .go_export section: %v", shlibpath, err))
+		}
+		pkgpath := []byte("pkgpath ")
+		for _, line := range bytes.Split(data, []byte{'\n'}) {
+			if path, found := bytes.CutPrefix(line, pkgpath); found {
+				path = bytes.TrimSuffix(path, []byte{';'})
+				pkgs = append(pkgs, load.LoadPackageWithFlags(string(path), base.Cwd(), &stk, nil, 0))
+			}
+		}
+	} else {
+		pkglistbytes, err := buildid.ReadELFNote(shlibpath, "Go\x00\x00", 1)
+		if err != nil {
+			base.Fatalf("readELFNote failed: %v", err)
+		}
+		scanner := bufio.NewScanner(bytes.NewBuffer(pkglistbytes))
+		for scanner.Scan() {
+			t := scanner.Text()
+			pkgs = append(pkgs, load.LoadPackageWithFlags(t, base.Cwd(), &stk, nil, 0))
+		}
+	}
+	return
+}
+
+// cacheAction looks up {mode, p} in the cache and returns the resulting action.
+// If the cache has no such action, f() is recorded and returned.
+// TODO(rsc): Change the second key from *load.Package to interface{},
+// to make the caching in linkShared less awkward?
+func (b *Builder) cacheAction(mode string, p *load.Package, f func() *Action) *Action {
+	a := b.actionCache[cacheKey{mode, p}]
+	if a == nil {
+		a = f()
+		b.actionCache[cacheKey{mode, p}] = a
+	}
+	return a
+}
+
+// AutoAction returns the "right" action for go build or go install of p.
+func (b *Builder) AutoAction(mode, depMode BuildMode, p *load.Package) *Action {
+	if p.Name == "main" {
+		return b.LinkAction(mode, depMode, p)
+	}
+	return b.CompileAction(mode, depMode, p)
+}
+
+// CompileAction returns the action for compiling and possibly installing
+// (according to mode) the given package. The resulting action is only
+// for building packages (archives), never for linking executables.
+// depMode is the action (build or install) to use when building dependencies.
+// To turn package main into an executable, call b.Link instead.
+func (b *Builder) CompileAction(mode, depMode BuildMode, p *load.Package) *Action {
+	vetOnly := mode&ModeVetOnly != 0
+	mode &^= ModeVetOnly
+
+	if mode != ModeBuild && p.Target == "" {
+		// No permanent target.
+		mode = ModeBuild
+	}
+	if mode != ModeBuild && p.Name == "main" {
+		// We never install the .a file for a main package.
+		mode = ModeBuild
+	}
+
+	// Construct package build action.
+	a := b.cacheAction("build", p, func() *Action {
+		a := &Action{
+			Mode:    "build",
+			Package: p,
+			Actor:   ActorFunc((*Builder).build),
+			Objdir:  b.NewObjdir(),
+		}
+
+		if p.Error == nil || !p.Error.IsImportCycle {
+			for _, p1 := range p.Internal.Imports {
+				a.Deps = append(a.Deps, b.CompileAction(depMode, depMode, p1))
+			}
+		}
+
+		if p.Standard {
+			switch p.ImportPath {
+			case "builtin", "unsafe":
+				// Fake packages - nothing to build.
+				a.Mode = "built-in package"
+				a.Actor = nil
+				return a
+			}
+
+			// gccgo standard library is "fake" too.
+			if cfg.BuildToolchainName == "gccgo" {
+				// the target name is needed for cgo.
+				a.Mode = "gccgo stdlib"
+				a.Target = p.Target
+				a.Actor = nil
+				return a
+			}
+		}
+
+		return a
+	})
+
+	// Find the build action; the cache entry may have been replaced
+	// by the install action during (*Builder).installAction.
+	buildAction := a
+	switch buildAction.Mode {
+	case "build", "built-in package", "gccgo stdlib":
+		// ok
+	case "build-install":
+		buildAction = a.Deps[0]
+	default:
+		panic("lost build action: " + buildAction.Mode)
+	}
+	buildAction.needBuild = buildAction.needBuild || !vetOnly
+
+	// Construct install action.
+	if mode == ModeInstall || mode == ModeBuggyInstall {
+		a = b.installAction(a, mode)
+	}
+
+	return a
+}
+
+// VetAction returns the action for running go vet on package p.
+// It depends on the action for compiling p.
+// If the caller may be causing p to be installed, it is up to the caller
+// to make sure that the install depends on (runs after) vet.
+func (b *Builder) VetAction(mode, depMode BuildMode, p *load.Package) *Action {
+	a := b.vetAction(mode, depMode, p)
+	a.VetxOnly = false
+	return a
+}
+
+func (b *Builder) vetAction(mode, depMode BuildMode, p *load.Package) *Action {
+	// Construct vet action.
+	a := b.cacheAction("vet", p, func() *Action {
+		a1 := b.CompileAction(mode|ModeVetOnly, depMode, p)
+
+		// vet expects to be able to import "fmt".
+		var stk load.ImportStack
+		stk.Push("vet")
+		p1, err := load.LoadImportWithFlags("fmt", p.Dir, p, &stk, nil, 0)
+		if err != nil {
+			base.Fatalf("unexpected error loading fmt package from package %s: %v", p.ImportPath, err)
+		}
+		stk.Pop()
+		aFmt := b.CompileAction(ModeBuild, depMode, p1)
+
+		var deps []*Action
+		if a1.buggyInstall {
+			// (*Builder).vet expects deps[0] to be the package
+			// and deps[1] to be "fmt". If we see buggyInstall
+			// here then a1 is an install of a shared library,
+			// and the real package is a1.Deps[0].
+			deps = []*Action{a1.Deps[0], aFmt, a1}
+		} else {
+			deps = []*Action{a1, aFmt}
+		}
+		for _, p1 := range p.Internal.Imports {
+			deps = append(deps, b.vetAction(mode, depMode, p1))
+		}
+
+		a := &Action{
+			Mode:       "vet",
+			Package:    p,
+			Deps:       deps,
+			Objdir:     a1.Objdir,
+			VetxOnly:   true,
+			IgnoreFail: true, // it's OK if vet of dependencies "fails" (reports problems)
+		}
+		if a1.Actor == nil {
+			// Built-in packages like unsafe.
+			return a
+		}
+		deps[0].needVet = true
+		a.Actor = ActorFunc((*Builder).vet)
+		return a
+	})
+	return a
+}
+
+// LinkAction returns the action for linking p into an executable
+// and possibly installing the result (according to mode).
+// depMode is the action (build or install) to use when compiling dependencies.
+func (b *Builder) LinkAction(mode, depMode BuildMode, p *load.Package) *Action {
+	// Construct link action.
+	a := b.cacheAction("link", p, func() *Action {
+		a := &Action{
+			Mode:    "link",
+			Package: p,
+		}
+
+		a1 := b.CompileAction(ModeBuild, depMode, p)
+		a.Actor = ActorFunc((*Builder).link)
+		a.Deps = []*Action{a1}
+		a.Objdir = a1.Objdir
+
+		// An executable file. (This is the name of a temporary file.)
+		// Because we run the temporary file in 'go run' and 'go test',
+		// the name will show up in ps listings. If the caller has specified
+		// a name, use that instead of a.out. The binary is generated
+		// in an otherwise empty subdirectory named exe to avoid
+		// naming conflicts. The only possible conflict is if we were
+		// to create a top-level package named exe.
+		name := "a.out"
+		if p.Internal.ExeName != "" {
+			name = p.Internal.ExeName
+		} else if (cfg.Goos == "darwin" || cfg.Goos == "windows") && cfg.BuildBuildmode == "c-shared" && p.Target != "" {
+			// On OS X, the linker output name gets recorded in the
+			// shared library's LC_ID_DYLIB load command.
+			// The code invoking the linker knows to pass only the final
+			// path element. Arrange that the path element matches what
+			// we'll install it as; otherwise the library is only loadable as "a.out".
+			// On Windows, DLL file name is recorded in PE file
+			// export section, so do like on OS X.
+			_, name = filepath.Split(p.Target)
+		}
+		a.Target = a.Objdir + filepath.Join("exe", name) + cfg.ExeSuffix
+		a.built = a.Target
+		b.addTransitiveLinkDeps(a, a1, "")
+
+		// Sequence the build of the main package (a1) strictly after the build
+		// of all other dependencies that go into the link. It is likely to be after
+		// them anyway, but just make sure. This is required by the build ID-based
+		// shortcut in (*Builder).useCache(a1), which will call b.linkActionID(a).
+		// In order for that linkActionID call to compute the right action ID, all the
+		// dependencies of a (except a1) must have completed building and have
+		// recorded their build IDs.
+		a1.Deps = append(a1.Deps, &Action{Mode: "nop", Deps: a.Deps[1:]})
+		return a
+	})
+
+	if mode == ModeInstall || mode == ModeBuggyInstall {
+		a = b.installAction(a, mode)
+	}
+
+	return a
+}
+
+// installAction returns the action for installing the result of a1.
+func (b *Builder) installAction(a1 *Action, mode BuildMode) *Action {
+	// Because we overwrite the build action with the install action below,
+	// a1 may already be an install action fetched from the "build" cache key,
+	// and the caller just doesn't realize.
+	if strings.HasSuffix(a1.Mode, "-install") {
+		if a1.buggyInstall && mode == ModeInstall {
+			//  Congratulations! The buggy install is now a proper install.
+			a1.buggyInstall = false
+		}
+		return a1
+	}
+
+	// If there's no actual action to build a1,
+	// there's nothing to install either.
+	// This happens if a1 corresponds to reusing an already-built object.
+	if a1.Actor == nil {
+		return a1
+	}
+
+	p := a1.Package
+	return b.cacheAction(a1.Mode+"-install", p, func() *Action {
+		// The install deletes the temporary build result,
+		// so we need all other actions, both past and future,
+		// that attempt to depend on the build to depend instead
+		// on the install.
+
+		// Make a private copy of a1 (the build action),
+		// no longer accessible to any other rules.
+		buildAction := new(Action)
+		*buildAction = *a1
+
+		// Overwrite a1 with the install action.
+		// This takes care of updating past actions that
+		// point at a1 for the build action; now they will
+		// point at a1 and get the install action.
+		// We also leave a1 in the action cache as the result
+		// for "build", so that actions not yet created that
+		// try to depend on the build will instead depend
+		// on the install.
+		*a1 = Action{
+			Mode:    buildAction.Mode + "-install",
+			Actor:   ActorFunc(BuildInstallFunc),
+			Package: p,
+			Objdir:  buildAction.Objdir,
+			Deps:    []*Action{buildAction},
+			Target:  p.Target,
+			built:   p.Target,
+
+			buggyInstall: mode == ModeBuggyInstall,
+		}
+
+		b.addInstallHeaderAction(a1)
+		return a1
+	})
+}
+
+// addTransitiveLinkDeps adds to the link action a all packages
+// that are transitive dependencies of a1.Deps.
+// That is, if a is a link of package main, a1 is the compile of package main
+// and a1.Deps is the actions for building packages directly imported by
+// package main (what the compiler needs). The linker needs all packages
+// transitively imported by the whole program; addTransitiveLinkDeps
+// makes sure those are present in a.Deps.
+// If shlib is non-empty, then a corresponds to the build and installation of shlib,
+// so any rebuild of shlib should not be added as a dependency.
+func (b *Builder) addTransitiveLinkDeps(a, a1 *Action, shlib string) {
+	// Expand Deps to include all built packages, for the linker.
+	// Use breadth-first search to find rebuilt-for-test packages
+	// before the standard ones.
+	// TODO(rsc): Eliminate the standard ones from the action graph,
+	// which will require doing a little bit more rebuilding.
+	workq := []*Action{a1}
+	haveDep := map[string]bool{}
+	if a1.Package != nil {
+		haveDep[a1.Package.ImportPath] = true
+	}
+	for i := 0; i < len(workq); i++ {
+		a1 := workq[i]
+		for _, a2 := range a1.Deps {
+			// TODO(rsc): Find a better discriminator than the Mode strings, once the dust settles.
+			if a2.Package == nil || (a2.Mode != "build-install" && a2.Mode != "build") || haveDep[a2.Package.ImportPath] {
+				continue
+			}
+			haveDep[a2.Package.ImportPath] = true
+			a.Deps = append(a.Deps, a2)
+			if a2.Mode == "build-install" {
+				a2 = a2.Deps[0] // walk children of "build" action
+			}
+			workq = append(workq, a2)
+		}
+	}
+
+	// If this is go build -linkshared, then the link depends on the shared libraries
+	// in addition to the packages themselves. (The compile steps do not.)
+	if cfg.BuildLinkshared {
+		haveShlib := map[string]bool{shlib: true}
+		for _, a1 := range a.Deps {
+			p1 := a1.Package
+			if p1 == nil || p1.Shlib == "" || haveShlib[filepath.Base(p1.Shlib)] {
+				continue
+			}
+			haveShlib[filepath.Base(p1.Shlib)] = true
+			// TODO(rsc): The use of ModeInstall here is suspect, but if we only do ModeBuild,
+			// we'll end up building an overall library or executable that depends at runtime
+			// on other libraries that are out-of-date, which is clearly not good either.
+			// We call it ModeBuggyInstall to make clear that this is not right.
+			a.Deps = append(a.Deps, b.linkSharedAction(ModeBuggyInstall, ModeBuggyInstall, p1.Shlib, nil))
+		}
+	}
+}
+
+// addInstallHeaderAction adds an install header action to a, if needed.
+// The action a should be an install action as generated by either
+// b.CompileAction or b.LinkAction with mode=ModeInstall,
+// and so a.Deps[0] is the corresponding build action.
+func (b *Builder) addInstallHeaderAction(a *Action) {
+	// Install header for cgo in c-archive and c-shared modes.
+	p := a.Package
+	if p.UsesCgo() && (cfg.BuildBuildmode == "c-archive" || cfg.BuildBuildmode == "c-shared") {
+		hdrTarget := a.Target[:len(a.Target)-len(filepath.Ext(a.Target))] + ".h"
+		if cfg.BuildContext.Compiler == "gccgo" && cfg.BuildO == "" {
+			// For the header file, remove the "lib"
+			// added by go/build, so we generate pkg.h
+			// rather than libpkg.h.
+			dir, file := filepath.Split(hdrTarget)
+			file = strings.TrimPrefix(file, "lib")
+			hdrTarget = filepath.Join(dir, file)
+		}
+		ah := &Action{
+			Mode:    "install header",
+			Package: a.Package,
+			Deps:    []*Action{a.Deps[0]},
+			Actor:   ActorFunc((*Builder).installHeader),
+			Objdir:  a.Deps[0].Objdir,
+			Target:  hdrTarget,
+		}
+		a.Deps = append(a.Deps, ah)
+	}
+}
+
+// buildmodeShared takes the "go build" action a1 into the building of a shared library of a1.Deps.
+// That is, the input a1 represents "go build pkgs" and the result represents "go build -buildmode=shared pkgs".
+func (b *Builder) buildmodeShared(mode, depMode BuildMode, args []string, pkgs []*load.Package, a1 *Action) *Action {
+	name, err := libname(args, pkgs)
+	if err != nil {
+		base.Fatalf("%v", err)
+	}
+	return b.linkSharedAction(mode, depMode, name, a1)
+}
+
+// linkSharedAction takes a grouping action a1 corresponding to a list of built packages
+// and returns an action that links them together into a shared library with the name shlib.
+// If a1 is nil, shlib should be an absolute path to an existing shared library,
+// and then linkSharedAction reads that library to find out the package list.
+func (b *Builder) linkSharedAction(mode, depMode BuildMode, shlib string, a1 *Action) *Action {
+	fullShlib := shlib
+	shlib = filepath.Base(shlib)
+	a := b.cacheAction("build-shlib "+shlib, nil, func() *Action {
+		if a1 == nil {
+			// TODO(rsc): Need to find some other place to store config,
+			// not in pkg directory. See golang.org/issue/22196.
+			pkgs := readpkglist(fullShlib)
+			a1 = &Action{
+				Mode: "shlib packages",
+			}
+			for _, p := range pkgs {
+				a1.Deps = append(a1.Deps, b.CompileAction(mode, depMode, p))
+			}
+		}
+
+		// Fake package to hold ldflags.
+		// As usual shared libraries are a kludgy, abstraction-violating special case:
+		// we let them use the flags specified for the command-line arguments.
+		p := &load.Package{}
+		p.Internal.CmdlinePkg = true
+		p.Internal.Ldflags = load.BuildLdflags.For(p)
+		p.Internal.Gccgoflags = load.BuildGccgoflags.For(p)
+
+		// Add implicit dependencies to pkgs list.
+		// Currently buildmode=shared forces external linking mode, and
+		// external linking mode forces an import of runtime/cgo (and
+		// math on arm). So if it was not passed on the command line and
+		// it is not present in another shared library, add it here.
+		// TODO(rsc): Maybe this should only happen if "runtime" is in the original package set.
+		// TODO(rsc): This should probably be changed to use load.LinkerDeps(p).
+		// TODO(rsc): We don't add standard library imports for gccgo
+		// because they are all always linked in anyhow.
+		// Maybe load.LinkerDeps should be used and updated.
+		a := &Action{
+			Mode:    "go build -buildmode=shared",
+			Package: p,
+			Objdir:  b.NewObjdir(),
+			Actor:   ActorFunc((*Builder).linkShared),
+			Deps:    []*Action{a1},
+		}
+		a.Target = filepath.Join(a.Objdir, shlib)
+		if cfg.BuildToolchainName != "gccgo" {
+			add := func(a1 *Action, pkg string, force bool) {
+				for _, a2 := range a1.Deps {
+					if a2.Package != nil && a2.Package.ImportPath == pkg {
+						return
+					}
+				}
+				var stk load.ImportStack
+				p := load.LoadPackageWithFlags(pkg, base.Cwd(), &stk, nil, 0)
+				if p.Error != nil {
+					base.Fatalf("load %s: %v", pkg, p.Error)
+				}
+				// Assume that if pkg (runtime/cgo or math)
+				// is already accounted for in a different shared library,
+				// then that shared library also contains runtime,
+				// so that anything we do will depend on that library,
+				// so we don't need to include pkg in our shared library.
+				if force || p.Shlib == "" || filepath.Base(p.Shlib) == pkg {
+					a1.Deps = append(a1.Deps, b.CompileAction(depMode, depMode, p))
+				}
+			}
+			add(a1, "runtime/cgo", false)
+			if cfg.Goarch == "arm" {
+				add(a1, "math", false)
+			}
+
+			// The linker step still needs all the usual linker deps.
+			// (For example, the linker always opens runtime.a.)
+			for _, dep := range load.LinkerDeps(nil) {
+				add(a, dep, true)
+			}
+		}
+		b.addTransitiveLinkDeps(a, a1, shlib)
+		return a
+	})
+
+	// Install result.
+	if (mode == ModeInstall || mode == ModeBuggyInstall) && a.Actor != nil {
+		buildAction := a
+
+		a = b.cacheAction("install-shlib "+shlib, nil, func() *Action {
+			// Determine the eventual install target.
+			// The install target is root/pkg/shlib, where root is the source root
+			// in which all the packages lie.
+			// TODO(rsc): Perhaps this cross-root check should apply to the full
+			// transitive package dependency list, not just the ones named
+			// on the command line?
+			pkgDir := a1.Deps[0].Package.Internal.Build.PkgTargetRoot
+			for _, a2 := range a1.Deps {
+				if dir := a2.Package.Internal.Build.PkgTargetRoot; dir != pkgDir {
+					base.Fatalf("installing shared library: cannot use packages %s and %s from different roots %s and %s",
+						a1.Deps[0].Package.ImportPath,
+						a2.Package.ImportPath,
+						pkgDir,
+						dir)
+				}
+			}
+			// TODO(rsc): Find out and explain here why gccgo is different.
+			if cfg.BuildToolchainName == "gccgo" {
+				pkgDir = filepath.Join(pkgDir, "shlibs")
+			}
+			target := filepath.Join(pkgDir, shlib)
+
+			a := &Action{
+				Mode:   "go install -buildmode=shared",
+				Objdir: buildAction.Objdir,
+				Actor:  ActorFunc(BuildInstallFunc),
+				Deps:   []*Action{buildAction},
+				Target: target,
+			}
+			for _, a2 := range buildAction.Deps[0].Deps {
+				p := a2.Package
+				pkgTargetRoot := p.Internal.Build.PkgTargetRoot
+				if pkgTargetRoot == "" {
+					continue
+				}
+				a.Deps = append(a.Deps, &Action{
+					Mode:    "shlibname",
+					Package: p,
+					Actor:   ActorFunc((*Builder).installShlibname),
+					Target:  filepath.Join(pkgTargetRoot, p.ImportPath+".shlibname"),
+					Deps:    []*Action{a.Deps[0]},
+				})
+			}
+			return a
+		})
+	}
+
+	return a
+}
diff --git a/src/cmd/go/internal/work/build.go b/src/cmd/go/internal/work/build.go
new file mode 100644
index 0000000..e2e0e07
--- /dev/null
+++ b/src/cmd/go/internal/work/build.go
@@ -0,0 +1,956 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package work
+
+import (
+	"context"
+	"errors"
+	"flag"
+	"fmt"
+	"go/build"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"strconv"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/load"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/search"
+	"cmd/go/internal/trace"
+)
+
+var CmdBuild = &base.Command{
+	UsageLine: "go build [-o output] [build flags] [packages]",
+	Short:     "compile packages and dependencies",
+	Long: `
+Build compiles the packages named by the import paths,
+along with their dependencies, but it does not install the results.
+
+If the arguments to build are a list of .go files from a single directory,
+build treats them as a list of source files specifying a single package.
+
+When compiling packages, build ignores files that end in '_test.go'.
+
+When compiling a single main package, build writes
+the resulting executable to an output file named after
+the first source file ('go build ed.go rx.go' writes 'ed' or 'ed.exe')
+or the source code directory ('go build unix/sam' writes 'sam' or 'sam.exe').
+The '.exe' suffix is added when writing a Windows executable.
+
+When compiling multiple packages or a single non-main package,
+build compiles the packages but discards the resulting object,
+serving only as a check that the packages can be built.
+
+The -o flag forces build to write the resulting executable or object
+to the named output file or directory, instead of the default behavior described
+in the last two paragraphs. If the named output is an existing directory or
+ends with a slash or backslash, then any resulting executables
+will be written to that directory.
+
+The build flags are shared by the build, clean, get, install, list, run,
+and test commands:
+
+	-C dir
+		Change to dir before running the command.
+		Any files named on the command line are interpreted after
+		changing directories.
+		If used, this flag must be the first one in the command line.
+	-a
+		force rebuilding of packages that are already up-to-date.
+	-n
+		print the commands but do not run them.
+	-p n
+		the number of programs, such as build commands or
+		test binaries, that can be run in parallel.
+		The default is GOMAXPROCS, normally the number of CPUs available.
+	-race
+		enable data race detection.
+		Supported only on linux/amd64, freebsd/amd64, darwin/amd64, darwin/arm64, windows/amd64,
+		linux/ppc64le and linux/arm64 (only for 48-bit VMA).
+	-msan
+		enable interoperation with memory sanitizer.
+		Supported only on linux/amd64, linux/arm64, freebsd/amd64
+		and only with Clang/LLVM as the host C compiler.
+		PIE build mode will be used on all platforms except linux/amd64.
+	-asan
+		enable interoperation with address sanitizer.
+		Supported only on linux/arm64, linux/amd64.
+		Supported only on linux/amd64 or linux/arm64 and only with GCC 7 and higher
+		or Clang/LLVM 9 and higher.
+	-cover
+		enable code coverage instrumentation.
+	-covermode set,count,atomic
+		set the mode for coverage analysis.
+		The default is "set" unless -race is enabled,
+		in which case it is "atomic".
+		The values:
+		set: bool: does this statement run?
+		count: int: how many times does this statement run?
+		atomic: int: count, but correct in multithreaded tests;
+			significantly more expensive.
+		Sets -cover.
+	-coverpkg pattern1,pattern2,pattern3
+		For a build that targets package 'main' (e.g. building a Go
+		executable), apply coverage analysis to each package matching
+		the patterns. The default is to apply coverage analysis to
+		packages in the main Go module. See 'go help packages' for a
+		description of package patterns.  Sets -cover.
+	-v
+		print the names of packages as they are compiled.
+	-work
+		print the name of the temporary work directory and
+		do not delete it when exiting.
+	-x
+		print the commands.
+	-asmflags '[pattern=]arg list'
+		arguments to pass on each go tool asm invocation.
+	-buildmode mode
+		build mode to use. See 'go help buildmode' for more.
+	-buildvcs
+		Whether to stamp binaries with version control information
+		("true", "false", or "auto"). By default ("auto"), version control
+		information is stamped into a binary if the main package, the main module
+		containing it, and the current directory are all in the same repository.
+		Use -buildvcs=false to always omit version control information, or
+		-buildvcs=true to error out if version control information is available but
+		cannot be included due to a missing tool or ambiguous directory structure.
+	-compiler name
+		name of compiler to use, as in runtime.Compiler (gccgo or gc).
+	-gccgoflags '[pattern=]arg list'
+		arguments to pass on each gccgo compiler/linker invocation.
+	-gcflags '[pattern=]arg list'
+		arguments to pass on each go tool compile invocation.
+	-installsuffix suffix
+		a suffix to use in the name of the package installation directory,
+		in order to keep output separate from default builds.
+		If using the -race flag, the install suffix is automatically set to race
+		or, if set explicitly, has _race appended to it. Likewise for the -msan
+		and -asan flags. Using a -buildmode option that requires non-default compile
+		flags has a similar effect.
+	-ldflags '[pattern=]arg list'
+		arguments to pass on each go tool link invocation.
+	-linkshared
+		build code that will be linked against shared libraries previously
+		created with -buildmode=shared.
+	-mod mode
+		module download mode to use: readonly, vendor, or mod.
+		By default, if a vendor directory is present and the go version in go.mod
+		is 1.14 or higher, the go command acts as if -mod=vendor were set.
+		Otherwise, the go command acts as if -mod=readonly were set.
+		See https://golang.org/ref/mod#build-commands for details.
+	-modcacherw
+		leave newly-created directories in the module cache read-write
+		instead of making them read-only.
+	-modfile file
+		in module aware mode, read (and possibly write) an alternate go.mod
+		file instead of the one in the module root directory. A file named
+		"go.mod" must still be present in order to determine the module root
+		directory, but it is not accessed. When -modfile is specified, an
+		alternate go.sum file is also used: its path is derived from the
+		-modfile flag by trimming the ".mod" extension and appending ".sum".
+	-overlay file
+		read a JSON config file that provides an overlay for build operations.
+		The file is a JSON struct with a single field, named 'Replace', that
+		maps each disk file path (a string) to its backing file path, so that
+		a build will run as if the disk file path exists with the contents
+		given by the backing file paths, or as if the disk file path does not
+		exist if its backing file path is empty. Support for the -overlay flag
+		has some limitations: importantly, cgo files included from outside the
+		include path must be in the same directory as the Go package they are
+		included from, and overlays will not appear when binaries and tests are
+		run through go run and go test respectively.
+	-pgo file
+		specify the file path of a profile for profile-guided optimization (PGO).
+		When the special name "auto" is specified, for each main package in the
+		build, the go command selects a file named "default.pgo" in the package's
+		directory if that file exists, and applies it to the (transitive)
+		dependencies of the main package (other packages are not affected).
+		Special name "off" turns off PGO. The default is "auto".
+	-pkgdir dir
+		install and load all packages from dir instead of the usual locations.
+		For example, when building with a non-standard configuration,
+		use -pkgdir to keep generated packages in a separate location.
+	-tags tag,list
+		a comma-separated list of additional build tags to consider satisfied
+		during the build. For more information about build tags, see
+		'go help buildconstraint'. (Earlier versions of Go used a
+		space-separated list, and that form is deprecated but still recognized.)
+	-trimpath
+		remove all file system paths from the resulting executable.
+		Instead of absolute file system paths, the recorded file names
+		will begin either a module path@version (when using modules),
+		or a plain import path (when using the standard library, or GOPATH).
+	-toolexec 'cmd args'
+		a program to use to invoke toolchain programs like vet and asm.
+		For example, instead of running asm, the go command will run
+		'cmd args /path/to/asm <arguments for asm>'.
+		The TOOLEXEC_IMPORTPATH environment variable will be set,
+		matching 'go list -f {{.ImportPath}}' for the package being built.
+
+The -asmflags, -gccgoflags, -gcflags, and -ldflags flags accept a
+space-separated list of arguments to pass to an underlying tool
+during the build. To embed spaces in an element in the list, surround
+it with either single or double quotes. The argument list may be
+preceded by a package pattern and an equal sign, which restricts
+the use of that argument list to the building of packages matching
+that pattern (see 'go help packages' for a description of package
+patterns). Without a pattern, the argument list applies only to the
+packages named on the command line. The flags may be repeated
+with different patterns in order to specify different arguments for
+different sets of packages. If a package matches patterns given in
+multiple flags, the latest match on the command line wins.
+For example, 'go build -gcflags=-S fmt' prints the disassembly
+only for package fmt, while 'go build -gcflags=all=-S fmt'
+prints the disassembly for fmt and all its dependencies.
+
+For more about specifying packages, see 'go help packages'.
+For more about where packages and binaries are installed,
+run 'go help gopath'.
+For more about calling between Go and C/C++, run 'go help c'.
+
+Note: Build adheres to certain conventions such as those described
+by 'go help gopath'. Not all projects can follow these conventions,
+however. Installations that have their own conventions or that use
+a separate software build system may choose to use lower-level
+invocations such as 'go tool compile' and 'go tool link' to avoid
+some of the overheads and design decisions of the build tool.
+
+See also: go install, go get, go clean.
+	`,
+}
+
+const concurrentGCBackendCompilationEnabledByDefault = true
+
+func init() {
+	// break init cycle
+	CmdBuild.Run = runBuild
+	CmdInstall.Run = runInstall
+
+	CmdBuild.Flag.StringVar(&cfg.BuildO, "o", "", "output file or directory")
+
+	AddBuildFlags(CmdBuild, DefaultBuildFlags)
+	AddBuildFlags(CmdInstall, DefaultBuildFlags)
+	if cfg.Experiment != nil && cfg.Experiment.CoverageRedesign {
+		AddCoverFlags(CmdBuild, nil)
+		AddCoverFlags(CmdInstall, nil)
+	}
+}
+
+// Note that flags consulted by other parts of the code
+// (for example, buildV) are in cmd/go/internal/cfg.
+
+var (
+	forcedAsmflags   []string // internally-forced flags for cmd/asm
+	forcedGcflags    []string // internally-forced flags for cmd/compile
+	forcedLdflags    []string // internally-forced flags for cmd/link
+	forcedGccgoflags []string // internally-forced flags for gccgo
+)
+
+var BuildToolchain toolchain = noToolchain{}
+var ldBuildmode string
+
+// buildCompiler implements flag.Var.
+// It implements Set by updating both
+// BuildToolchain and buildContext.Compiler.
+type buildCompiler struct{}
+
+func (c buildCompiler) Set(value string) error {
+	switch value {
+	case "gc":
+		BuildToolchain = gcToolchain{}
+	case "gccgo":
+		BuildToolchain = gccgoToolchain{}
+	default:
+		return fmt.Errorf("unknown compiler %q", value)
+	}
+	cfg.BuildToolchainName = value
+	cfg.BuildToolchainCompiler = BuildToolchain.compiler
+	cfg.BuildToolchainLinker = BuildToolchain.linker
+	cfg.BuildContext.Compiler = value
+	return nil
+}
+
+func (c buildCompiler) String() string {
+	return cfg.BuildContext.Compiler
+}
+
+func init() {
+	switch build.Default.Compiler {
+	case "gc", "gccgo":
+		buildCompiler{}.Set(build.Default.Compiler)
+	}
+}
+
+type BuildFlagMask int
+
+const (
+	DefaultBuildFlags BuildFlagMask = 0
+	OmitModFlag       BuildFlagMask = 1 << iota
+	OmitModCommonFlags
+	OmitVFlag
+)
+
+// AddBuildFlags adds the flags common to the build, clean, get,
+// install, list, run, and test commands.
+func AddBuildFlags(cmd *base.Command, mask BuildFlagMask) {
+	base.AddBuildFlagsNX(&cmd.Flag)
+	base.AddChdirFlag(&cmd.Flag)
+	cmd.Flag.BoolVar(&cfg.BuildA, "a", false, "")
+	cmd.Flag.IntVar(&cfg.BuildP, "p", cfg.BuildP, "")
+	if mask&OmitVFlag == 0 {
+		cmd.Flag.BoolVar(&cfg.BuildV, "v", false, "")
+	}
+
+	cmd.Flag.Var(&load.BuildAsmflags, "asmflags", "")
+	cmd.Flag.Var(buildCompiler{}, "compiler", "")
+	cmd.Flag.StringVar(&cfg.BuildBuildmode, "buildmode", "default", "")
+	cmd.Flag.Var(&load.BuildGcflags, "gcflags", "")
+	cmd.Flag.Var(&load.BuildGccgoflags, "gccgoflags", "")
+	if mask&OmitModFlag == 0 {
+		base.AddModFlag(&cmd.Flag)
+	}
+	if mask&OmitModCommonFlags == 0 {
+		base.AddModCommonFlags(&cmd.Flag)
+	} else {
+		// Add the overlay flag even when we don't add the rest of the mod common flags.
+		// This only affects 'go get' in GOPATH mode, but add the flag anyway for
+		// consistency.
+		cmd.Flag.StringVar(&fsys.OverlayFile, "overlay", "", "")
+	}
+	cmd.Flag.StringVar(&cfg.BuildContext.InstallSuffix, "installsuffix", "", "")
+	cmd.Flag.Var(&load.BuildLdflags, "ldflags", "")
+	cmd.Flag.BoolVar(&cfg.BuildLinkshared, "linkshared", false, "")
+	cmd.Flag.StringVar(&cfg.BuildPGO, "pgo", "auto", "")
+	cmd.Flag.StringVar(&cfg.BuildPkgdir, "pkgdir", "", "")
+	cmd.Flag.BoolVar(&cfg.BuildRace, "race", false, "")
+	cmd.Flag.BoolVar(&cfg.BuildMSan, "msan", false, "")
+	cmd.Flag.BoolVar(&cfg.BuildASan, "asan", false, "")
+	cmd.Flag.Var((*tagsFlag)(&cfg.BuildContext.BuildTags), "tags", "")
+	cmd.Flag.Var((*base.StringsFlag)(&cfg.BuildToolexec), "toolexec", "")
+	cmd.Flag.BoolVar(&cfg.BuildTrimpath, "trimpath", false, "")
+	cmd.Flag.BoolVar(&cfg.BuildWork, "work", false, "")
+	cmd.Flag.Var((*buildvcsFlag)(&cfg.BuildBuildvcs), "buildvcs", "")
+
+	// Undocumented, unstable debugging flags.
+	cmd.Flag.StringVar(&cfg.DebugActiongraph, "debug-actiongraph", "", "")
+	cmd.Flag.StringVar(&cfg.DebugTrace, "debug-trace", "", "")
+	cmd.Flag.StringVar(&cfg.DebugRuntimeTrace, "debug-runtime-trace", "", "")
+}
+
+// AddCoverFlags adds coverage-related flags to "cmd". If the
+// CoverageRedesign experiment is enabled, we add -cover{mode,pkg} to
+// the build command and only -coverprofile to the test command. If
+// the CoverageRedesign experiment is disabled, -cover* flags are
+// added only to the test command.
+func AddCoverFlags(cmd *base.Command, coverProfileFlag *string) {
+	addCover := false
+	if cfg.Experiment != nil && cfg.Experiment.CoverageRedesign {
+		// New coverage enabled: both build and test commands get
+		// coverage flags.
+		addCover = true
+	} else {
+		// New coverage disabled: only test command gets cover flags.
+		addCover = coverProfileFlag != nil
+	}
+	if addCover {
+		cmd.Flag.BoolVar(&cfg.BuildCover, "cover", false, "")
+		cmd.Flag.Var(coverFlag{(*coverModeFlag)(&cfg.BuildCoverMode)}, "covermode", "")
+		cmd.Flag.Var(coverFlag{commaListFlag{&cfg.BuildCoverPkg}}, "coverpkg", "")
+	}
+	if coverProfileFlag != nil {
+		cmd.Flag.Var(coverFlag{V: stringFlag{coverProfileFlag}}, "coverprofile", "")
+	}
+}
+
+// tagsFlag is the implementation of the -tags flag.
+type tagsFlag []string
+
+func (v *tagsFlag) Set(s string) error {
+	// For compatibility with Go 1.12 and earlier, allow "-tags='a b c'" or even just "-tags='a'".
+	if strings.Contains(s, " ") || strings.Contains(s, "'") {
+		return (*base.StringsFlag)(v).Set(s)
+	}
+
+	// Split on commas, ignore empty strings.
+	*v = []string{}
+	for _, s := range strings.Split(s, ",") {
+		if s != "" {
+			*v = append(*v, s)
+		}
+	}
+	return nil
+}
+
+func (v *tagsFlag) String() string {
+	return "<TagsFlag>"
+}
+
+// buildvcsFlag is the implementation of the -buildvcs flag.
+type buildvcsFlag string
+
+func (f *buildvcsFlag) IsBoolFlag() bool { return true } // allow -buildvcs (without arguments)
+
+func (f *buildvcsFlag) Set(s string) error {
+	// https://go.dev/issue/51748: allow "-buildvcs=auto",
+	// in addition to the usual "true" and "false".
+	if s == "" || s == "auto" {
+		*f = "auto"
+		return nil
+	}
+
+	b, err := strconv.ParseBool(s)
+	if err != nil {
+		return errors.New("value is neither 'auto' nor a valid bool")
+	}
+	*f = (buildvcsFlag)(strconv.FormatBool(b)) // convert to canonical "true" or "false"
+	return nil
+}
+
+func (f *buildvcsFlag) String() string { return string(*f) }
+
+// fileExtSplit expects a filename and returns the name
+// and ext (without the dot). If the file has no
+// extension, ext will be empty.
+func fileExtSplit(file string) (name, ext string) {
+	dotExt := filepath.Ext(file)
+	name = file[:len(file)-len(dotExt)]
+	if dotExt != "" {
+		ext = dotExt[1:]
+	}
+	return
+}
+
+func pkgsMain(pkgs []*load.Package) (res []*load.Package) {
+	for _, p := range pkgs {
+		if p.Name == "main" {
+			res = append(res, p)
+		}
+	}
+	return res
+}
+
+func pkgsNotMain(pkgs []*load.Package) (res []*load.Package) {
+	for _, p := range pkgs {
+		if p.Name != "main" {
+			res = append(res, p)
+		}
+	}
+	return res
+}
+
+func oneMainPkg(pkgs []*load.Package) []*load.Package {
+	if len(pkgs) != 1 || pkgs[0].Name != "main" {
+		base.Fatalf("-buildmode=%s requires exactly one main package", cfg.BuildBuildmode)
+	}
+	return pkgs
+}
+
+var pkgsFilter = func(pkgs []*load.Package) []*load.Package { return pkgs }
+
+func runBuild(ctx context.Context, cmd *base.Command, args []string) {
+	modload.InitWorkfile()
+	BuildInit()
+	b := NewBuilder("")
+	defer func() {
+		if err := b.Close(); err != nil {
+			base.Fatal(err)
+		}
+	}()
+
+	pkgs := load.PackagesAndErrors(ctx, load.PackageOpts{AutoVCS: true}, args)
+	load.CheckPackageErrors(pkgs)
+
+	explicitO := len(cfg.BuildO) > 0
+
+	if len(pkgs) == 1 && pkgs[0].Name == "main" && cfg.BuildO == "" {
+		cfg.BuildO = pkgs[0].DefaultExecName()
+		cfg.BuildO += cfg.ExeSuffix
+	}
+
+	// sanity check some often mis-used options
+	switch cfg.BuildContext.Compiler {
+	case "gccgo":
+		if load.BuildGcflags.Present() {
+			fmt.Println("go build: when using gccgo toolchain, please pass compiler flags using -gccgoflags, not -gcflags")
+		}
+		if load.BuildLdflags.Present() {
+			fmt.Println("go build: when using gccgo toolchain, please pass linker flags using -gccgoflags, not -ldflags")
+		}
+	case "gc":
+		if load.BuildGccgoflags.Present() {
+			fmt.Println("go build: when using gc toolchain, please pass compile flags using -gcflags, and linker flags using -ldflags")
+		}
+	}
+
+	depMode := ModeBuild
+
+	pkgs = omitTestOnly(pkgsFilter(pkgs))
+
+	// Special case -o /dev/null by not writing at all.
+	if base.IsNull(cfg.BuildO) {
+		cfg.BuildO = ""
+	}
+
+	if cfg.Experiment.CoverageRedesign && cfg.BuildCover {
+		load.PrepareForCoverageBuild(pkgs)
+	}
+
+	if cfg.BuildO != "" {
+		// If the -o name exists and is a directory or
+		// ends with a slash or backslash, then
+		// write all main packages to that directory.
+		// Otherwise require only a single package be built.
+		if fi, err := os.Stat(cfg.BuildO); (err == nil && fi.IsDir()) ||
+			strings.HasSuffix(cfg.BuildO, "/") ||
+			strings.HasSuffix(cfg.BuildO, string(os.PathSeparator)) {
+			if !explicitO {
+				base.Fatalf("go: build output %q already exists and is a directory", cfg.BuildO)
+			}
+			a := &Action{Mode: "go build"}
+			for _, p := range pkgs {
+				if p.Name != "main" {
+					continue
+				}
+
+				p.Target = filepath.Join(cfg.BuildO, p.DefaultExecName())
+				p.Target += cfg.ExeSuffix
+				p.Stale = true
+				p.StaleReason = "build -o flag in use"
+				a.Deps = append(a.Deps, b.AutoAction(ModeInstall, depMode, p))
+			}
+			if len(a.Deps) == 0 {
+				base.Fatalf("go: no main packages to build")
+			}
+			b.Do(ctx, a)
+			return
+		}
+		if len(pkgs) > 1 {
+			base.Fatalf("go: cannot write multiple packages to non-directory %s", cfg.BuildO)
+		} else if len(pkgs) == 0 {
+			base.Fatalf("no packages to build")
+		}
+		p := pkgs[0]
+		p.Target = cfg.BuildO
+		p.Stale = true // must build - not up to date
+		p.StaleReason = "build -o flag in use"
+		a := b.AutoAction(ModeInstall, depMode, p)
+		b.Do(ctx, a)
+		return
+	}
+
+	a := &Action{Mode: "go build"}
+	for _, p := range pkgs {
+		a.Deps = append(a.Deps, b.AutoAction(ModeBuild, depMode, p))
+	}
+	if cfg.BuildBuildmode == "shared" {
+		a = b.buildmodeShared(ModeBuild, depMode, args, pkgs, a)
+	}
+	b.Do(ctx, a)
+}
+
+var CmdInstall = &base.Command{
+	UsageLine: "go install [build flags] [packages]",
+	Short:     "compile and install packages and dependencies",
+	Long: `
+Install compiles and installs the packages named by the import paths.
+
+Executables are installed in the directory named by the GOBIN environment
+variable, which defaults to $GOPATH/bin or $HOME/go/bin if the GOPATH
+environment variable is not set. Executables in $GOROOT
+are installed in $GOROOT/bin or $GOTOOLDIR instead of $GOBIN.
+
+If the arguments have version suffixes (like @latest or @v1.0.0), "go install"
+builds packages in module-aware mode, ignoring the go.mod file in the current
+directory or any parent directory, if there is one. This is useful for
+installing executables without affecting the dependencies of the main module.
+To eliminate ambiguity about which module versions are used in the build, the
+arguments must satisfy the following constraints:
+
+- Arguments must be package paths or package patterns (with "..." wildcards).
+They must not be standard packages (like fmt), meta-patterns (std, cmd,
+all), or relative or absolute file paths.
+
+- All arguments must have the same version suffix. Different queries are not
+allowed, even if they refer to the same version.
+
+- All arguments must refer to packages in the same module at the same version.
+
+- Package path arguments must refer to main packages. Pattern arguments
+will only match main packages.
+
+- No module is considered the "main" module. If the module containing
+packages named on the command line has a go.mod file, it must not contain
+directives (replace and exclude) that would cause it to be interpreted
+differently than if it were the main module. The module must not require
+a higher version of itself.
+
+- Vendor directories are not used in any module. (Vendor directories are not
+included in the module zip files downloaded by 'go install'.)
+
+If the arguments don't have version suffixes, "go install" may run in
+module-aware mode or GOPATH mode, depending on the GO111MODULE environment
+variable and the presence of a go.mod file. See 'go help modules' for details.
+If module-aware mode is enabled, "go install" runs in the context of the main
+module.
+
+When module-aware mode is disabled, non-main packages are installed in the
+directory $GOPATH/pkg/$GOOS_$GOARCH. When module-aware mode is enabled,
+non-main packages are built and cached but not installed.
+
+Before Go 1.20, the standard library was installed to
+$GOROOT/pkg/$GOOS_$GOARCH.
+Starting in Go 1.20, the standard library is built and cached but not installed.
+Setting GODEBUG=installgoroot=all restores the use of
+$GOROOT/pkg/$GOOS_$GOARCH.
+
+For more about build flags, see 'go help build'.
+
+For more about specifying packages, see 'go help packages'.
+
+See also: go build, go get, go clean.
+	`,
+}
+
+// libname returns the filename to use for the shared library when using
+// -buildmode=shared. The rules we use are:
+// Use arguments for special 'meta' packages:
+//
+//	std --> libstd.so
+//	std cmd --> libstd,cmd.so
+//
+// A single non-meta argument with trailing "/..." is special cased:
+//
+//	foo/... --> libfoo.so
+//	(A relative path like "./..."  expands the "." first)
+//
+// Use import paths for other cases, changing '/' to '-':
+//
+//	somelib --> libsubdir-somelib.so
+//	./ or ../ --> libsubdir-somelib.so
+//	gopkg.in/tomb.v2 -> libgopkg.in-tomb.v2.so
+//	a/... b/... ---> liba/c,b/d.so - all matching import paths
+//
+// Name parts are joined with ','.
+func libname(args []string, pkgs []*load.Package) (string, error) {
+	var libname string
+	appendName := func(arg string) {
+		if libname == "" {
+			libname = arg
+		} else {
+			libname += "," + arg
+		}
+	}
+	var haveNonMeta bool
+	for _, arg := range args {
+		if search.IsMetaPackage(arg) {
+			appendName(arg)
+		} else {
+			haveNonMeta = true
+		}
+	}
+	if len(libname) == 0 { // non-meta packages only. use import paths
+		if len(args) == 1 && strings.HasSuffix(args[0], "/...") {
+			// Special case of "foo/..." as mentioned above.
+			arg := strings.TrimSuffix(args[0], "/...")
+			if build.IsLocalImport(arg) {
+				cwd, _ := os.Getwd()
+				bp, _ := cfg.BuildContext.ImportDir(filepath.Join(cwd, arg), build.FindOnly)
+				if bp.ImportPath != "" && bp.ImportPath != "." {
+					arg = bp.ImportPath
+				}
+			}
+			appendName(strings.ReplaceAll(arg, "/", "-"))
+		} else {
+			for _, pkg := range pkgs {
+				appendName(strings.ReplaceAll(pkg.ImportPath, "/", "-"))
+			}
+		}
+	} else if haveNonMeta { // have both meta package and a non-meta one
+		return "", errors.New("mixing of meta and non-meta packages is not allowed")
+	}
+	// TODO(mwhudson): Needs to change for platforms that use different naming
+	// conventions...
+	return "lib" + libname + ".so", nil
+}
+
+func runInstall(ctx context.Context, cmd *base.Command, args []string) {
+	for _, arg := range args {
+		if strings.Contains(arg, "@") && !build.IsLocalImport(arg) && !filepath.IsAbs(arg) {
+			installOutsideModule(ctx, args)
+			return
+		}
+	}
+
+	modload.InitWorkfile()
+	BuildInit()
+	pkgs := load.PackagesAndErrors(ctx, load.PackageOpts{AutoVCS: true}, args)
+	if cfg.ModulesEnabled && !modload.HasModRoot() {
+		haveErrors := false
+		allMissingErrors := true
+		for _, pkg := range pkgs {
+			if pkg.Error == nil {
+				continue
+			}
+			haveErrors = true
+			if missingErr := (*modload.ImportMissingError)(nil); !errors.As(pkg.Error, &missingErr) {
+				allMissingErrors = false
+				break
+			}
+		}
+		if haveErrors && allMissingErrors {
+			latestArgs := make([]string, len(args))
+			for i := range args {
+				latestArgs[i] = args[i] + "@latest"
+			}
+			hint := strings.Join(latestArgs, " ")
+			base.Fatalf("go: 'go install' requires a version when current directory is not in a module\n\tTry 'go install %s' to install the latest version", hint)
+		}
+	}
+	load.CheckPackageErrors(pkgs)
+
+	if cfg.Experiment.CoverageRedesign && cfg.BuildCover {
+		load.PrepareForCoverageBuild(pkgs)
+	}
+
+	InstallPackages(ctx, args, pkgs)
+}
+
+// omitTestOnly returns pkgs with test-only packages removed.
+func omitTestOnly(pkgs []*load.Package) []*load.Package {
+	var list []*load.Package
+	for _, p := range pkgs {
+		if len(p.GoFiles)+len(p.CgoFiles) == 0 && !p.Internal.CmdlinePkgLiteral {
+			// Package has no source files,
+			// perhaps due to build tags or perhaps due to only having *_test.go files.
+			// Also, it is only being processed as the result of a wildcard match
+			// like ./..., not because it was listed as a literal path on the command line.
+			// Ignore it.
+			continue
+		}
+		list = append(list, p)
+	}
+	return list
+}
+
+func InstallPackages(ctx context.Context, patterns []string, pkgs []*load.Package) {
+	ctx, span := trace.StartSpan(ctx, "InstallPackages "+strings.Join(patterns, " "))
+	defer span.Done()
+
+	if cfg.GOBIN != "" && !filepath.IsAbs(cfg.GOBIN) {
+		base.Fatalf("cannot install, GOBIN must be an absolute path")
+	}
+
+	pkgs = omitTestOnly(pkgsFilter(pkgs))
+	for _, p := range pkgs {
+		if p.Target == "" {
+			switch {
+			case p.Name != "main" && p.Internal.Local && p.ConflictDir == "":
+				// Non-executables outside GOPATH need not have a target:
+				// we can use the cache to hold the built package archive for use in future builds.
+				// The ones inside GOPATH should have a target (in GOPATH/pkg)
+				// or else something is wrong and worth reporting (like a ConflictDir).
+			case p.Name != "main" && p.Module != nil:
+				// Non-executables have no target (except the cache) when building with modules.
+			case p.Name != "main" && p.Standard && p.Internal.Build.PkgObj == "":
+				// Most packages in std do not need an installed .a, because they can be
+				// rebuilt and used directly from the build cache.
+				// A few targets (notably those using cgo) still do need to be installed
+				// in case the user's environment lacks a C compiler.
+			case p.Internal.GobinSubdir:
+				base.Errorf("go: cannot install cross-compiled binaries when GOBIN is set")
+			case p.Internal.CmdlineFiles:
+				base.Errorf("go: no install location for .go files listed on command line (GOBIN not set)")
+			case p.ConflictDir != "":
+				base.Errorf("go: no install location for %s: hidden by %s", p.Dir, p.ConflictDir)
+			default:
+				base.Errorf("go: no install location for directory %s outside GOPATH\n"+
+					"\tFor more details see: 'go help gopath'", p.Dir)
+			}
+		}
+	}
+	base.ExitIfErrors()
+
+	b := NewBuilder("")
+	defer func() {
+		if err := b.Close(); err != nil {
+			base.Fatal(err)
+		}
+	}()
+
+	depMode := ModeBuild
+	a := &Action{Mode: "go install"}
+	var tools []*Action
+	for _, p := range pkgs {
+		// If p is a tool, delay the installation until the end of the build.
+		// This avoids installing assemblers/compilers that are being executed
+		// by other steps in the build.
+		a1 := b.AutoAction(ModeInstall, depMode, p)
+		if load.InstallTargetDir(p) == load.ToTool {
+			a.Deps = append(a.Deps, a1.Deps...)
+			a1.Deps = append(a1.Deps, a)
+			tools = append(tools, a1)
+			continue
+		}
+		a.Deps = append(a.Deps, a1)
+	}
+	if len(tools) > 0 {
+		a = &Action{
+			Mode: "go install (tools)",
+			Deps: tools,
+		}
+	}
+
+	if cfg.BuildBuildmode == "shared" {
+		// Note: If buildmode=shared then only non-main packages
+		// are present in the pkgs list, so all the special case code about
+		// tools above did not apply, and a is just a simple Action
+		// with a list of Deps, one per package named in pkgs,
+		// the same as in runBuild.
+		a = b.buildmodeShared(ModeInstall, ModeInstall, patterns, pkgs, a)
+	}
+
+	b.Do(ctx, a)
+	base.ExitIfErrors()
+
+	// Success. If this command is 'go install' with no arguments
+	// and the current directory (the implicit argument) is a command,
+	// remove any leftover command binary from a previous 'go build'.
+	// The binary is installed; it's not needed here anymore.
+	// And worse it might be a stale copy, which you don't want to find
+	// instead of the installed one if $PATH contains dot.
+	// One way to view this behavior is that it is as if 'go install' first
+	// runs 'go build' and the moves the generated file to the install dir.
+	// See issue 9645.
+	if len(patterns) == 0 && len(pkgs) == 1 && pkgs[0].Name == "main" {
+		// Compute file 'go build' would have created.
+		// If it exists and is an executable file, remove it.
+		targ := pkgs[0].DefaultExecName()
+		targ += cfg.ExeSuffix
+		if filepath.Join(pkgs[0].Dir, targ) != pkgs[0].Target { // maybe $GOBIN is the current directory
+			fi, err := os.Stat(targ)
+			if err == nil {
+				m := fi.Mode()
+				if m.IsRegular() {
+					if m&0111 != 0 || cfg.Goos == "windows" { // windows never sets executable bit
+						os.Remove(targ)
+					}
+				}
+			}
+		}
+	}
+}
+
+// installOutsideModule implements 'go install pkg@version'. It builds and
+// installs one or more main packages in module mode while ignoring any go.mod
+// in the current directory or parent directories.
+//
+// See golang.org/issue/40276 for details and rationale.
+func installOutsideModule(ctx context.Context, args []string) {
+	modload.ForceUseModules = true
+	modload.RootMode = modload.NoRoot
+	modload.AllowMissingModuleImports()
+	modload.Init()
+	BuildInit()
+
+	// Load packages. Ignore non-main packages.
+	// Print a warning if an argument contains "..." and matches no main packages.
+	// PackagesAndErrors already prints warnings for patterns that don't match any
+	// packages, so be careful not to double print.
+	// TODO(golang.org/issue/40276): don't report errors loading non-main packages
+	// matched by a pattern.
+	pkgOpts := load.PackageOpts{MainOnly: true}
+	pkgs, err := load.PackagesAndErrorsOutsideModule(ctx, pkgOpts, args)
+	if err != nil {
+		base.Fatal(err)
+	}
+	load.CheckPackageErrors(pkgs)
+	patterns := make([]string, len(args))
+	for i, arg := range args {
+		patterns[i] = arg[:strings.Index(arg, "@")]
+	}
+
+	// Build and install the packages.
+	InstallPackages(ctx, patterns, pkgs)
+}
+
+// ExecCmd is the command to use to run user binaries.
+// Normally it is empty, meaning run the binaries directly.
+// If cross-compiling and running on a remote system or
+// simulator, it is typically go_GOOS_GOARCH_exec, with
+// the target GOOS and GOARCH substituted.
+// The -exec flag overrides these defaults.
+var ExecCmd []string
+
+// FindExecCmd derives the value of ExecCmd to use.
+// It returns that value and leaves ExecCmd set for direct use.
+func FindExecCmd() []string {
+	if ExecCmd != nil {
+		return ExecCmd
+	}
+	ExecCmd = []string{} // avoid work the second time
+	if cfg.Goos == runtime.GOOS && cfg.Goarch == runtime.GOARCH {
+		return ExecCmd
+	}
+	path, err := exec.LookPath(fmt.Sprintf("go_%s_%s_exec", cfg.Goos, cfg.Goarch))
+	if err == nil {
+		ExecCmd = []string{path}
+	}
+	return ExecCmd
+}
+
+// A coverFlag is a flag.Value that also implies -cover.
+type coverFlag struct{ V flag.Value }
+
+func (f coverFlag) String() string { return f.V.String() }
+
+func (f coverFlag) Set(value string) error {
+	if err := f.V.Set(value); err != nil {
+		return err
+	}
+	cfg.BuildCover = true
+	return nil
+}
+
+type coverModeFlag string
+
+func (f *coverModeFlag) String() string { return string(*f) }
+func (f *coverModeFlag) Set(value string) error {
+	switch value {
+	case "", "set", "count", "atomic":
+		*f = coverModeFlag(value)
+		cfg.BuildCoverMode = value
+		return nil
+	default:
+		return errors.New(`valid modes are "set", "count", or "atomic"`)
+	}
+}
+
+// A commaListFlag is a flag.Value representing a comma-separated list.
+type commaListFlag struct{ Vals *[]string }
+
+func (f commaListFlag) String() string { return strings.Join(*f.Vals, ",") }
+
+func (f commaListFlag) Set(value string) error {
+	if value == "" {
+		*f.Vals = nil
+	} else {
+		*f.Vals = strings.Split(value, ",")
+	}
+	return nil
+}
+
+// A stringFlag is a flag.Value representing a single string.
+type stringFlag struct{ val *string }
+
+func (f stringFlag) String() string { return *f.val }
+func (f stringFlag) Set(value string) error {
+	*f.val = value
+	return nil
+}
diff --git a/src/cmd/go/internal/work/build_test.go b/src/cmd/go/internal/work/build_test.go
new file mode 100644
index 0000000..91648a3
--- /dev/null
+++ b/src/cmd/go/internal/work/build_test.go
@@ -0,0 +1,283 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package work
+
+import (
+	"fmt"
+	"internal/testenv"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"strings"
+	"testing"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/load"
+)
+
+func TestRemoveDevNull(t *testing.T) {
+	fi, err := os.Lstat(os.DevNull)
+	if err != nil {
+		t.Skip(err)
+	}
+	if fi.Mode().IsRegular() {
+		t.Errorf("Lstat(%s).Mode().IsRegular() = true; expected false", os.DevNull)
+	}
+	mayberemovefile(os.DevNull)
+	_, err = os.Lstat(os.DevNull)
+	if err != nil {
+		t.Errorf("mayberemovefile(%s) did remove it; oops", os.DevNull)
+	}
+}
+
+func TestSplitPkgConfigOutput(t *testing.T) {
+	for _, test := range []struct {
+		in   []byte
+		want []string
+	}{
+		{[]byte(`-r:foo -L/usr/white\ space/lib -lfoo\ bar -lbar\ baz`), []string{"-r:foo", "-L/usr/white space/lib", "-lfoo bar", "-lbar baz"}},
+		{[]byte(`-lextra\ fun\ arg\\`), []string{`-lextra fun arg\`}},
+		{[]byte("\textra     whitespace\r\n"), []string{"extra", "whitespace\r"}},
+		{[]byte("     \r\n      "), []string{"\r"}},
+		{[]byte(`"-r:foo" "-L/usr/white space/lib" "-lfoo bar" "-lbar baz"`), []string{"-r:foo", "-L/usr/white space/lib", "-lfoo bar", "-lbar baz"}},
+		{[]byte(`"-lextra fun arg\\"`), []string{`-lextra fun arg\`}},
+		{[]byte(`"     \r\n\      "`), []string{`     \r\n\      `}},
+		{[]byte(`""`), []string{""}},
+		{[]byte(``), nil},
+		{[]byte(`"\\"`), []string{`\`}},
+		{[]byte(`"\x"`), []string{`\x`}},
+		{[]byte(`"\\x"`), []string{`\x`}},
+		{[]byte(`'\\'`), []string{`\\`}},
+		{[]byte(`'\x'`), []string{`\x`}},
+		{[]byte(`"\\x"`), []string{`\x`}},
+		{[]byte("\\\n"), nil},
+		{[]byte(`-fPIC -I/test/include/foo -DQUOTED='"/test/share/doc"'`), []string{"-fPIC", "-I/test/include/foo", `-DQUOTED="/test/share/doc"`}},
+		{[]byte(`-fPIC -I/test/include/foo -DQUOTED="/test/share/doc"`), []string{"-fPIC", "-I/test/include/foo", "-DQUOTED=/test/share/doc"}},
+		{[]byte(`-fPIC -I/test/include/foo -DQUOTED=\"/test/share/doc\"`), []string{"-fPIC", "-I/test/include/foo", `-DQUOTED="/test/share/doc"`}},
+		{[]byte(`-fPIC -I/test/include/foo -DQUOTED='/test/share/doc'`), []string{"-fPIC", "-I/test/include/foo", "-DQUOTED=/test/share/doc"}},
+		{[]byte(`-DQUOTED='/te\st/share/d\oc'`), []string{`-DQUOTED=/te\st/share/d\oc`}},
+		{[]byte(`-Dhello=10 -Dworld=+32 -DDEFINED_FROM_PKG_CONFIG=hello\ world`), []string{"-Dhello=10", "-Dworld=+32", "-DDEFINED_FROM_PKG_CONFIG=hello world"}},
+		{[]byte(`"broken\"" \\\a "a"`), []string{"broken\"", "\\a", "a"}},
+	} {
+		got, err := splitPkgConfigOutput(test.in)
+		if err != nil {
+			t.Errorf("splitPkgConfigOutput on %#q failed with error %v", test.in, err)
+			continue
+		}
+		if !reflect.DeepEqual(got, test.want) {
+			t.Errorf("splitPkgConfigOutput(%#q) = %#q; want %#q", test.in, got, test.want)
+		}
+	}
+
+	for _, test := range []struct {
+		in   []byte
+		want []string
+	}{
+		// broken quotation
+		{[]byte(`"     \r\n      `), nil},
+		{[]byte(`"-r:foo" "-L/usr/white space/lib "-lfoo bar" "-lbar baz"`), nil},
+		{[]byte(`"-lextra fun arg\\`), nil},
+		// broken char escaping
+		{[]byte(`broken flag\`), nil},
+		{[]byte(`extra broken flag \`), nil},
+		{[]byte(`\`), nil},
+		{[]byte(`"broken\"" "extra" \`), nil},
+	} {
+		got, err := splitPkgConfigOutput(test.in)
+		if err == nil {
+			t.Errorf("splitPkgConfigOutput(%v) = %v; haven't failed with error as expected.", test.in, got)
+		}
+		if !reflect.DeepEqual(got, test.want) {
+			t.Errorf("splitPkgConfigOutput(%v) = %v; want %v", test.in, got, test.want)
+		}
+	}
+
+}
+
+func TestSharedLibName(t *testing.T) {
+	// TODO(avdva) - make these values platform-specific
+	prefix := "lib"
+	suffix := ".so"
+	testData := []struct {
+		args      []string
+		pkgs      []*load.Package
+		expected  string
+		expectErr bool
+		rootedAt  string
+	}{
+		{
+			args:     []string{"std"},
+			pkgs:     []*load.Package{},
+			expected: "std",
+		},
+		{
+			args:     []string{"std", "cmd"},
+			pkgs:     []*load.Package{},
+			expected: "std,cmd",
+		},
+		{
+			args:     []string{},
+			pkgs:     []*load.Package{pkgImportPath("gopkg.in/somelib")},
+			expected: "gopkg.in-somelib",
+		},
+		{
+			args:     []string{"./..."},
+			pkgs:     []*load.Package{pkgImportPath("somelib")},
+			expected: "somelib",
+			rootedAt: "somelib",
+		},
+		{
+			args:     []string{"../somelib", "../somelib"},
+			pkgs:     []*load.Package{pkgImportPath("somelib")},
+			expected: "somelib",
+		},
+		{
+			args:     []string{"../lib1", "../lib2"},
+			pkgs:     []*load.Package{pkgImportPath("gopkg.in/lib1"), pkgImportPath("gopkg.in/lib2")},
+			expected: "gopkg.in-lib1,gopkg.in-lib2",
+		},
+		{
+			args: []string{"./..."},
+			pkgs: []*load.Package{
+				pkgImportPath("gopkg.in/dir/lib1"),
+				pkgImportPath("gopkg.in/lib2"),
+				pkgImportPath("gopkg.in/lib3"),
+			},
+			expected: "gopkg.in",
+			rootedAt: "gopkg.in",
+		},
+		{
+			args:      []string{"std", "../lib2"},
+			pkgs:      []*load.Package{},
+			expectErr: true,
+		},
+		{
+			args:      []string{"all", "./"},
+			pkgs:      []*load.Package{},
+			expectErr: true,
+		},
+		{
+			args:      []string{"cmd", "fmt"},
+			pkgs:      []*load.Package{},
+			expectErr: true,
+		},
+	}
+	for _, data := range testData {
+		func() {
+			if data.rootedAt != "" {
+				tmpGopath, err := os.MkdirTemp("", "gopath")
+				if err != nil {
+					t.Fatal(err)
+				}
+				cwd := base.Cwd()
+				oldGopath := cfg.BuildContext.GOPATH
+				defer func() {
+					cfg.BuildContext.GOPATH = oldGopath
+					os.Chdir(cwd)
+					err := os.RemoveAll(tmpGopath)
+					if err != nil {
+						t.Error(err)
+					}
+				}()
+				root := filepath.Join(tmpGopath, "src", data.rootedAt)
+				err = os.MkdirAll(root, 0755)
+				if err != nil {
+					t.Fatal(err)
+				}
+				cfg.BuildContext.GOPATH = tmpGopath
+				os.Chdir(root)
+			}
+			computed, err := libname(data.args, data.pkgs)
+			if err != nil {
+				if !data.expectErr {
+					t.Errorf("libname returned an error %q, expected a name", err.Error())
+				}
+			} else if data.expectErr {
+				t.Errorf("libname returned %q, expected an error", computed)
+			} else {
+				expected := prefix + data.expected + suffix
+				if expected != computed {
+					t.Errorf("libname returned %q, expected %q", computed, expected)
+				}
+			}
+		}()
+	}
+}
+
+func pkgImportPath(pkgpath string) *load.Package {
+	return &load.Package{
+		PackagePublic: load.PackagePublic{
+			ImportPath: pkgpath,
+		},
+	}
+}
+
+// When installing packages, the installed package directory should
+// respect the SetGID bit and group name of the destination
+// directory.
+// See https://golang.org/issue/18878.
+func TestRespectSetgidDir(t *testing.T) {
+	var b Builder
+
+	// Check that `cp` is called instead of `mv` by looking at the output
+	// of `(*Builder).ShowCmd` afterwards as a sanity check.
+	cfg.BuildX = true
+	var cmdBuf strings.Builder
+	b.Print = func(a ...any) (int, error) {
+		return cmdBuf.WriteString(fmt.Sprint(a...))
+	}
+
+	setgiddir, err := os.MkdirTemp("", "SetGroupID")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(setgiddir)
+
+	// BSD mkdir(2) inherits the parent directory group, and other platforms
+	// can inherit the parent directory group via setgid. The test setup (chmod
+	// setgid) will fail if the process does not have the group permission to
+	// the new temporary directory.
+	err = os.Chown(setgiddir, os.Getuid(), os.Getgid())
+	if err != nil {
+		if testenv.SyscallIsNotSupported(err) {
+			t.Skip("skipping: chown is not supported on " + runtime.GOOS)
+		}
+		t.Fatal(err)
+	}
+
+	// Change setgiddir's permissions to include the SetGID bit.
+	if err := os.Chmod(setgiddir, 0755|fs.ModeSetgid); err != nil {
+		if testenv.SyscallIsNotSupported(err) {
+			t.Skip("skipping: chmod is not supported on " + runtime.GOOS)
+		}
+		t.Fatal(err)
+	}
+	if fi, err := os.Stat(setgiddir); err != nil {
+		t.Fatal(err)
+	} else if fi.Mode()&fs.ModeSetgid == 0 {
+		t.Skip("skipping: Chmod ignored ModeSetgid on " + runtime.GOOS)
+	}
+
+	pkgfile, err := os.CreateTemp("", "pkgfile")
+	if err != nil {
+		t.Fatalf("os.CreateTemp(\"\", \"pkgfile\"): %v", err)
+	}
+	defer os.Remove(pkgfile.Name())
+	defer pkgfile.Close()
+
+	dirGIDFile := filepath.Join(setgiddir, "setgid")
+	if err := b.moveOrCopyFile(dirGIDFile, pkgfile.Name(), 0666, true); err != nil {
+		t.Fatalf("moveOrCopyFile: %v", err)
+	}
+
+	got := strings.TrimSpace(cmdBuf.String())
+	want := b.fmtcmd("", "cp %s %s", pkgfile.Name(), dirGIDFile)
+	if got != want {
+		t.Fatalf("moveOrCopyFile(%q, %q): want %q, got %q", dirGIDFile, pkgfile.Name(), want, got)
+	}
+}
diff --git a/src/cmd/go/internal/work/buildid.go b/src/cmd/go/internal/work/buildid.go
new file mode 100644
index 0000000..a1d7599
--- /dev/null
+++ b/src/cmd/go/internal/work/buildid.go
@@ -0,0 +1,703 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package work
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"os/exec"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cache"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/str"
+	"cmd/internal/buildid"
+	"cmd/internal/quoted"
+)
+
+// Build IDs
+//
+// Go packages and binaries are stamped with build IDs that record both
+// the action ID, which is a hash of the inputs to the action that produced
+// the packages or binary, and the content ID, which is a hash of the action
+// output, namely the archive or binary itself. The hash is the same one
+// used by the build artifact cache (see cmd/go/internal/cache), but
+// truncated when stored in packages and binaries, as the full length is not
+// needed and is a bit unwieldy. The precise form is
+//
+//	actionID/[.../]contentID
+//
+// where the actionID and contentID are prepared by buildid.HashToString below.
+// and are found by looking for the first or last slash.
+// Usually the buildID is simply actionID/contentID, but see below for an
+// exception.
+//
+// The build ID serves two primary purposes.
+//
+// 1. The action ID half allows installed packages and binaries to serve as
+// one-element cache entries. If we intend to build math.a with a given
+// set of inputs summarized in the action ID, and the installed math.a already
+// has that action ID, we can reuse the installed math.a instead of rebuilding it.
+//
+// 2. The content ID half allows the easy preparation of action IDs for steps
+// that consume a particular package or binary. The content hash of every
+// input file for a given action must be included in the action ID hash.
+// Storing the content ID in the build ID lets us read it from the file with
+// minimal I/O, instead of reading and hashing the entire file.
+// This is especially effective since packages and binaries are typically
+// the largest inputs to an action.
+//
+// Separating action ID from content ID is important for reproducible builds.
+// The compiler is compiled with itself. If an output were represented by its
+// own action ID (instead of content ID) when computing the action ID of
+// the next step in the build process, then the compiler could never have its
+// own input action ID as its output action ID (short of a miraculous hash collision).
+// Instead we use the content IDs to compute the next action ID, and because
+// the content IDs converge, so too do the action IDs and therefore the
+// build IDs and the overall compiler binary. See cmd/dist's cmdbootstrap
+// for the actual convergence sequence.
+//
+// The “one-element cache” purpose is a bit more complex for installed
+// binaries. For a binary, like cmd/gofmt, there are two steps: compile
+// cmd/gofmt/*.go into main.a, and then link main.a into the gofmt binary.
+// We do not install gofmt's main.a, only the gofmt binary. Being able to
+// decide that the gofmt binary is up-to-date means computing the action ID
+// for the final link of the gofmt binary and comparing it against the
+// already-installed gofmt binary. But computing the action ID for the link
+// means knowing the content ID of main.a, which we did not keep.
+// To sidestep this problem, each binary actually stores an expanded build ID:
+//
+//	actionID(binary)/actionID(main.a)/contentID(main.a)/contentID(binary)
+//
+// (Note that this can be viewed equivalently as:
+//
+//	actionID(binary)/buildID(main.a)/contentID(binary)
+//
+// Storing the buildID(main.a) in the middle lets the computations that care
+// about the prefix or suffix halves ignore the middle and preserves the
+// original build ID as a contiguous string.)
+//
+// During the build, when it's time to build main.a, the gofmt binary has the
+// information needed to decide whether the eventual link would produce
+// the same binary: if the action ID for main.a's inputs matches and then
+// the action ID for the link step matches when assuming the given main.a
+// content ID, then the binary as a whole is up-to-date and need not be rebuilt.
+//
+// This is all a bit complex and may be simplified once we can rely on the
+// main cache, but at least at the start we will be using the content-based
+// staleness determination without a cache beyond the usual installed
+// package and binary locations.
+
+const buildIDSeparator = "/"
+
+// actionID returns the action ID half of a build ID.
+func actionID(buildID string) string {
+	i := strings.Index(buildID, buildIDSeparator)
+	if i < 0 {
+		return buildID
+	}
+	return buildID[:i]
+}
+
+// contentID returns the content ID half of a build ID.
+func contentID(buildID string) string {
+	return buildID[strings.LastIndex(buildID, buildIDSeparator)+1:]
+}
+
+// toolID returns the unique ID to use for the current copy of the
+// named tool (asm, compile, cover, link).
+//
+// It is important that if the tool changes (for example a compiler bug is fixed
+// and the compiler reinstalled), toolID returns a different string, so that old
+// package archives look stale and are rebuilt (with the fixed compiler).
+// This suggests using a content hash of the tool binary, as stored in the build ID.
+//
+// Unfortunately, we can't just open the tool binary, because the tool might be
+// invoked via a wrapper program specified by -toolexec and we don't know
+// what the wrapper program does. In particular, we want "-toolexec toolstash"
+// to continue working: it does no good if "-toolexec toolstash" is executing a
+// stashed copy of the compiler but the go command is acting as if it will run
+// the standard copy of the compiler. The solution is to ask the tool binary to tell
+// us its own build ID using the "-V=full" flag now supported by all tools.
+// Then we know we're getting the build ID of the compiler that will actually run
+// during the build. (How does the compiler binary know its own content hash?
+// We store it there using updateBuildID after the standard link step.)
+//
+// A final twist is that we'd prefer to have reproducible builds for release toolchains.
+// It should be possible to cross-compile for Windows from either Linux or Mac
+// or Windows itself and produce the same binaries, bit for bit. If the tool ID,
+// which influences the action ID half of the build ID, is based on the content ID,
+// then the Linux compiler binary and Mac compiler binary will have different tool IDs
+// and therefore produce executables with different action IDs.
+// To avoid this problem, for releases we use the release version string instead
+// of the compiler binary's content hash. This assumes that all compilers built
+// on all different systems are semantically equivalent, which is of course only true
+// modulo bugs. (Producing the exact same executables also requires that the different
+// build setups agree on details like $GOROOT and file name paths, but at least the
+// tool IDs do not make it impossible.)
+func (b *Builder) toolID(name string) string {
+	b.id.Lock()
+	id := b.toolIDCache[name]
+	b.id.Unlock()
+
+	if id != "" {
+		return id
+	}
+
+	path := base.Tool(name)
+	desc := "go tool " + name
+
+	// Special case: undocumented -vettool overrides usual vet,
+	// for testing vet or supplying an alternative analysis tool.
+	if name == "vet" && VetTool != "" {
+		path = VetTool
+		desc = VetTool
+	}
+
+	cmdline := str.StringList(cfg.BuildToolexec, path, "-V=full")
+	cmd := exec.Command(cmdline[0], cmdline[1:]...)
+	var stdout, stderr strings.Builder
+	cmd.Stdout = &stdout
+	cmd.Stderr = &stderr
+	if err := cmd.Run(); err != nil {
+		if stderr.Len() > 0 {
+			os.Stderr.WriteString(stderr.String())
+		}
+		base.Fatalf("go: error obtaining buildID for %s: %v", desc, err)
+	}
+
+	line := stdout.String()
+	f := strings.Fields(line)
+	if len(f) < 3 || f[0] != name && path != VetTool || f[1] != "version" || f[2] == "devel" && !strings.HasPrefix(f[len(f)-1], "buildID=") {
+		base.Fatalf("go: parsing buildID from %s -V=full: unexpected output:\n\t%s", desc, line)
+	}
+	if f[2] == "devel" {
+		// On the development branch, use the content ID part of the build ID.
+		id = contentID(f[len(f)-1])
+	} else {
+		// For a release, the output is like: "compile version go1.9.1 X:framepointer".
+		// Use the whole line.
+		id = strings.TrimSpace(line)
+	}
+
+	b.id.Lock()
+	b.toolIDCache[name] = id
+	b.id.Unlock()
+
+	return id
+}
+
+// gccToolID returns the unique ID to use for a tool that is invoked
+// by the GCC driver. This is used particularly for gccgo, but this can also
+// be used for gcc, g++, gfortran, etc.; those tools all use the GCC
+// driver under different names. The approach used here should also
+// work for sufficiently new versions of clang. Unlike toolID, the
+// name argument is the program to run. The language argument is the
+// type of input file as passed to the GCC driver's -x option.
+//
+// For these tools we have no -V=full option to dump the build ID,
+// but we can run the tool with -v -### to reliably get the compiler proper
+// and hash that. That will work in the presence of -toolexec.
+//
+// In order to get reproducible builds for released compilers, we
+// detect a released compiler by the absence of "experimental" in the
+// --version output, and in that case we just use the version string.
+//
+// gccToolID also returns the underlying executable for the compiler.
+// The caller assumes that stat of the exe can be used, combined with the id,
+// to detect changes in the underlying compiler. The returned exe can be empty,
+// which means to rely only on the id.
+func (b *Builder) gccToolID(name, language string) (id, exe string, err error) {
+	key := name + "." + language
+	b.id.Lock()
+	id = b.toolIDCache[key]
+	exe = b.toolIDCache[key+".exe"]
+	b.id.Unlock()
+
+	if id != "" {
+		return id, exe, nil
+	}
+
+	// Invoke the driver with -### to see the subcommands and the
+	// version strings. Use -x to set the language. Pretend to
+	// compile an empty file on standard input.
+	cmdline := str.StringList(cfg.BuildToolexec, name, "-###", "-x", language, "-c", "-")
+	cmd := exec.Command(cmdline[0], cmdline[1:]...)
+	// Force untranslated output so that we see the string "version".
+	cmd.Env = append(os.Environ(), "LC_ALL=C")
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return "", "", fmt.Errorf("%s: %v; output: %q", name, err, out)
+	}
+
+	version := ""
+	lines := strings.Split(string(out), "\n")
+	for _, line := range lines {
+		if fields := strings.Fields(line); len(fields) > 1 && fields[1] == "version" || len(fields) > 2 && fields[2] == "version" {
+			version = line
+			break
+		}
+	}
+	if version == "" {
+		return "", "", fmt.Errorf("%s: can not find version number in %q", name, out)
+	}
+
+	if !strings.Contains(version, "experimental") {
+		// This is a release. Use this line as the tool ID.
+		id = version
+	} else {
+		// This is a development version. The first line with
+		// a leading space is the compiler proper.
+		compiler := ""
+		for _, line := range lines {
+			if strings.HasPrefix(line, " ") && !strings.HasPrefix(line, " (in-process)") {
+				compiler = line
+				break
+			}
+		}
+		if compiler == "" {
+			return "", "", fmt.Errorf("%s: can not find compilation command in %q", name, out)
+		}
+
+		fields, _ := quoted.Split(compiler)
+		if len(fields) == 0 {
+			return "", "", fmt.Errorf("%s: compilation command confusion %q", name, out)
+		}
+		exe = fields[0]
+		if !strings.ContainsAny(exe, `/\`) {
+			if lp, err := exec.LookPath(exe); err == nil {
+				exe = lp
+			}
+		}
+		id, err = buildid.ReadFile(exe)
+		if err != nil {
+			return "", "", err
+		}
+
+		// If we can't find a build ID, use a hash.
+		if id == "" {
+			id = b.fileHash(exe)
+		}
+	}
+
+	b.id.Lock()
+	b.toolIDCache[key] = id
+	b.toolIDCache[key+".exe"] = exe
+	b.id.Unlock()
+
+	return id, exe, nil
+}
+
+// Check if assembler used by gccgo is GNU as.
+func assemblerIsGas() bool {
+	cmd := exec.Command(BuildToolchain.compiler(), "-print-prog-name=as")
+	assembler, err := cmd.Output()
+	if err == nil {
+		cmd := exec.Command(strings.TrimSpace(string(assembler)), "--version")
+		out, err := cmd.Output()
+		return err == nil && strings.Contains(string(out), "GNU")
+	} else {
+		return false
+	}
+}
+
+// gccgoBuildIDFile creates an assembler file that records the
+// action's build ID in an SHF_EXCLUDE section for ELF files or
+// in a CSECT in XCOFF files.
+func (b *Builder) gccgoBuildIDFile(a *Action) (string, error) {
+	sfile := a.Objdir + "_buildid.s"
+
+	var buf bytes.Buffer
+	if cfg.Goos == "aix" {
+		fmt.Fprintf(&buf, "\t.csect .go.buildid[XO]\n")
+	} else if (cfg.Goos != "solaris" && cfg.Goos != "illumos") || assemblerIsGas() {
+		fmt.Fprintf(&buf, "\t"+`.section .go.buildid,"e"`+"\n")
+	} else if cfg.Goarch == "sparc" || cfg.Goarch == "sparc64" {
+		fmt.Fprintf(&buf, "\t"+`.section ".go.buildid",#exclude`+"\n")
+	} else { // cfg.Goarch == "386" || cfg.Goarch == "amd64"
+		fmt.Fprintf(&buf, "\t"+`.section .go.buildid,#exclude`+"\n")
+	}
+	fmt.Fprintf(&buf, "\t.byte ")
+	for i := 0; i < len(a.buildID); i++ {
+		if i > 0 {
+			if i%8 == 0 {
+				fmt.Fprintf(&buf, "\n\t.byte ")
+			} else {
+				fmt.Fprintf(&buf, ",")
+			}
+		}
+		fmt.Fprintf(&buf, "%#02x", a.buildID[i])
+	}
+	fmt.Fprintf(&buf, "\n")
+	if cfg.Goos != "solaris" && cfg.Goos != "illumos" && cfg.Goos != "aix" {
+		secType := "@progbits"
+		if cfg.Goarch == "arm" {
+			secType = "%progbits"
+		}
+		fmt.Fprintf(&buf, "\t"+`.section .note.GNU-stack,"",%s`+"\n", secType)
+		fmt.Fprintf(&buf, "\t"+`.section .note.GNU-split-stack,"",%s`+"\n", secType)
+	}
+
+	if cfg.BuildN || cfg.BuildX {
+		for _, line := range bytes.Split(buf.Bytes(), []byte("\n")) {
+			b.Showcmd("", "echo '%s' >> %s", line, sfile)
+		}
+		if cfg.BuildN {
+			return sfile, nil
+		}
+	}
+
+	if err := os.WriteFile(sfile, buf.Bytes(), 0666); err != nil {
+		return "", err
+	}
+
+	return sfile, nil
+}
+
+// buildID returns the build ID found in the given file.
+// If no build ID is found, buildID returns the content hash of the file.
+func (b *Builder) buildID(file string) string {
+	b.id.Lock()
+	id := b.buildIDCache[file]
+	b.id.Unlock()
+
+	if id != "" {
+		return id
+	}
+
+	id, err := buildid.ReadFile(file)
+	if err != nil {
+		id = b.fileHash(file)
+	}
+
+	b.id.Lock()
+	b.buildIDCache[file] = id
+	b.id.Unlock()
+
+	return id
+}
+
+// fileHash returns the content hash of the named file.
+func (b *Builder) fileHash(file string) string {
+	file, _ = fsys.OverlayPath(file)
+	sum, err := cache.FileHash(file)
+	if err != nil {
+		return ""
+	}
+	return buildid.HashToString(sum)
+}
+
+// useCache tries to satisfy the action a, which has action ID actionHash,
+// by using a cached result from an earlier build. At the moment, the only
+// cached result is the installed package or binary at target.
+// If useCache decides that the cache can be used, it sets a.buildID
+// and a.built for use by parent actions and then returns true.
+// Otherwise it sets a.buildID to a temporary build ID for use in the build
+// and returns false. When useCache returns false the expectation is that
+// the caller will build the target and then call updateBuildID to finish the
+// build ID computation.
+// When useCache returns false, it may have initiated buffering of output
+// during a's work. The caller should defer b.flushOutput(a), to make sure
+// that flushOutput is eventually called regardless of whether the action
+// succeeds. The flushOutput call must happen after updateBuildID.
+func (b *Builder) useCache(a *Action, actionHash cache.ActionID, target string, printOutput bool) bool {
+	// The second half of the build ID here is a placeholder for the content hash.
+	// It's important that the overall buildID be unlikely verging on impossible
+	// to appear in the output by chance, but that should be taken care of by
+	// the actionID half; if it also appeared in the input that would be like an
+	// engineered 120-bit partial SHA256 collision.
+	a.actionID = actionHash
+	actionID := buildid.HashToString(actionHash)
+	if a.json != nil {
+		a.json.ActionID = actionID
+	}
+	contentID := actionID // temporary placeholder, likely unique
+	a.buildID = actionID + buildIDSeparator + contentID
+
+	// Executable binaries also record the main build ID in the middle.
+	// See "Build IDs" comment above.
+	if a.Mode == "link" {
+		mainpkg := a.Deps[0]
+		a.buildID = actionID + buildIDSeparator + mainpkg.buildID + buildIDSeparator + contentID
+	}
+
+	// If user requested -a, we force a rebuild, so don't use the cache.
+	if cfg.BuildA {
+		if p := a.Package; p != nil && !p.Stale {
+			p.Stale = true
+			p.StaleReason = "build -a flag in use"
+		}
+		// Begin saving output for later writing to cache.
+		a.output = []byte{}
+		return false
+	}
+
+	c := cache.Default()
+
+	if target != "" {
+		buildID, _ := buildid.ReadFile(target)
+		if strings.HasPrefix(buildID, actionID+buildIDSeparator) {
+			a.buildID = buildID
+			if a.json != nil {
+				a.json.BuildID = a.buildID
+			}
+			a.built = target
+			// Poison a.Target to catch uses later in the build.
+			a.Target = "DO NOT USE - " + a.Mode
+			return true
+		}
+		// Special case for building a main package: if the only thing we
+		// want the package for is to link a binary, and the binary is
+		// already up-to-date, then to avoid a rebuild, report the package
+		// as up-to-date as well. See "Build IDs" comment above.
+		// TODO(rsc): Rewrite this code to use a TryCache func on the link action.
+		if !b.NeedExport && a.Mode == "build" && len(a.triggers) == 1 && a.triggers[0].Mode == "link" {
+			if id := strings.Split(buildID, buildIDSeparator); len(id) == 4 && id[1] == actionID {
+				// Temporarily assume a.buildID is the package build ID
+				// stored in the installed binary, and see if that makes
+				// the upcoming link action ID a match. If so, report that
+				// we built the package, safe in the knowledge that the
+				// link step will not ask us for the actual package file.
+				// Note that (*Builder).LinkAction arranged that all of
+				// a.triggers[0]'s dependencies other than a are also
+				// dependencies of a, so that we can be sure that,
+				// other than a.buildID, b.linkActionID is only accessing
+				// build IDs of completed actions.
+				oldBuildID := a.buildID
+				a.buildID = id[1] + buildIDSeparator + id[2]
+				linkID := buildid.HashToString(b.linkActionID(a.triggers[0]))
+				if id[0] == linkID {
+					// Best effort attempt to display output from the compile and link steps.
+					// If it doesn't work, it doesn't work: reusing the cached binary is more
+					// important than reprinting diagnostic information.
+					if printOutput {
+						showStdout(b, c, a.actionID, "stdout")      // compile output
+						showStdout(b, c, a.actionID, "link-stdout") // link output
+					}
+
+					// Poison a.Target to catch uses later in the build.
+					a.Target = "DO NOT USE - main build pseudo-cache Target"
+					a.built = "DO NOT USE - main build pseudo-cache built"
+					if a.json != nil {
+						a.json.BuildID = a.buildID
+					}
+					return true
+				}
+				// Otherwise restore old build ID for main build.
+				a.buildID = oldBuildID
+			}
+		}
+	}
+
+	// Special case for linking a test binary: if the only thing we
+	// want the binary for is to run the test, and the test result is cached,
+	// then to avoid the link step, report the link as up-to-date.
+	// We avoid the nested build ID problem in the previous special case
+	// by recording the test results in the cache under the action ID half.
+	if len(a.triggers) == 1 && a.triggers[0].TryCache != nil && a.triggers[0].TryCache(b, a.triggers[0]) {
+		// Best effort attempt to display output from the compile and link steps.
+		// If it doesn't work, it doesn't work: reusing the test result is more
+		// important than reprinting diagnostic information.
+		if printOutput {
+			showStdout(b, c, a.Deps[0].actionID, "stdout")      // compile output
+			showStdout(b, c, a.Deps[0].actionID, "link-stdout") // link output
+		}
+
+		// Poison a.Target to catch uses later in the build.
+		a.Target = "DO NOT USE -  pseudo-cache Target"
+		a.built = "DO NOT USE - pseudo-cache built"
+		return true
+	}
+
+	// Check to see if the action output is cached.
+	if file, _, err := cache.GetFile(c, actionHash); err == nil {
+		if buildID, err := buildid.ReadFile(file); err == nil {
+			if printOutput {
+				showStdout(b, c, a.actionID, "stdout")
+			}
+			a.built = file
+			a.Target = "DO NOT USE - using cache"
+			a.buildID = buildID
+			if a.json != nil {
+				a.json.BuildID = a.buildID
+			}
+			if p := a.Package; p != nil && target != "" {
+				p.Stale = true
+				// Clearer than explaining that something else is stale.
+				p.StaleReason = "not installed but available in build cache"
+			}
+			return true
+		}
+	}
+
+	// If we've reached this point, we can't use the cache for the action.
+	if p := a.Package; p != nil && !p.Stale {
+		p.Stale = true
+		p.StaleReason = "build ID mismatch"
+		if b.IsCmdList {
+			// Since we may end up printing StaleReason, include more detail.
+			for _, p1 := range p.Internal.Imports {
+				if p1.Stale && p1.StaleReason != "" {
+					if strings.HasPrefix(p1.StaleReason, "stale dependency: ") {
+						p.StaleReason = p1.StaleReason
+						break
+					}
+					if strings.HasPrefix(p.StaleReason, "build ID mismatch") {
+						p.StaleReason = "stale dependency: " + p1.ImportPath
+					}
+				}
+			}
+		}
+	}
+
+	// Begin saving output for later writing to cache.
+	a.output = []byte{}
+	return false
+}
+
+func showStdout(b *Builder, c cache.Cache, actionID cache.ActionID, key string) error {
+	stdout, stdoutEntry, err := cache.GetBytes(c, cache.Subkey(actionID, key))
+	if err != nil {
+		return err
+	}
+
+	if len(stdout) > 0 {
+		if cfg.BuildX || cfg.BuildN {
+			b.Showcmd("", "%s  # internal", joinUnambiguously(str.StringList("cat", c.OutputFile(stdoutEntry.OutputID))))
+		}
+		if !cfg.BuildN {
+			b.output.Lock()
+			defer b.output.Unlock()
+			b.Print(string(stdout))
+		}
+	}
+	return nil
+}
+
+// flushOutput flushes the output being queued in a.
+func (b *Builder) flushOutput(a *Action) {
+	b.output.Lock()
+	defer b.output.Unlock()
+	b.Print(string(a.output))
+	a.output = nil
+}
+
+// updateBuildID updates the build ID in the target written by action a.
+// It requires that useCache was called for action a and returned false,
+// and that the build was then carried out and given the temporary
+// a.buildID to record as the build ID in the resulting package or binary.
+// updateBuildID computes the final content ID and updates the build IDs
+// in the binary.
+//
+// Keep in sync with src/cmd/buildid/buildid.go
+func (b *Builder) updateBuildID(a *Action, target string, rewrite bool) error {
+	if cfg.BuildX || cfg.BuildN {
+		if rewrite {
+			b.Showcmd("", "%s # internal", joinUnambiguously(str.StringList(base.Tool("buildid"), "-w", target)))
+		}
+		if cfg.BuildN {
+			return nil
+		}
+	}
+
+	c := cache.Default()
+
+	// Cache output from compile/link, even if we don't do the rest.
+	switch a.Mode {
+	case "build":
+		cache.PutBytes(c, cache.Subkey(a.actionID, "stdout"), a.output)
+	case "link":
+		// Even though we don't cache the binary, cache the linker text output.
+		// We might notice that an installed binary is up-to-date but still
+		// want to pretend to have run the linker.
+		// Store it under the main package's action ID
+		// to make it easier to find when that's all we have.
+		for _, a1 := range a.Deps {
+			if p1 := a1.Package; p1 != nil && p1.Name == "main" {
+				cache.PutBytes(c, cache.Subkey(a1.actionID, "link-stdout"), a.output)
+				break
+			}
+		}
+	}
+
+	// Find occurrences of old ID and compute new content-based ID.
+	r, err := os.Open(target)
+	if err != nil {
+		return err
+	}
+	matches, hash, err := buildid.FindAndHash(r, a.buildID, 0)
+	r.Close()
+	if err != nil {
+		return err
+	}
+	newID := a.buildID[:strings.LastIndex(a.buildID, buildIDSeparator)] + buildIDSeparator + buildid.HashToString(hash)
+	if len(newID) != len(a.buildID) {
+		return fmt.Errorf("internal error: build ID length mismatch %q vs %q", a.buildID, newID)
+	}
+
+	// Replace with new content-based ID.
+	a.buildID = newID
+	if a.json != nil {
+		a.json.BuildID = a.buildID
+	}
+	if len(matches) == 0 {
+		// Assume the user specified -buildid= to override what we were going to choose.
+		return nil
+	}
+
+	if rewrite {
+		w, err := os.OpenFile(target, os.O_RDWR, 0)
+		if err != nil {
+			return err
+		}
+		err = buildid.Rewrite(w, matches, newID)
+		if err != nil {
+			w.Close()
+			return err
+		}
+		if err := w.Close(); err != nil {
+			return err
+		}
+	}
+
+	// Cache package builds, but not binaries (link steps).
+	// The expectation is that binaries are not reused
+	// nearly as often as individual packages, and they're
+	// much larger, so the cache-footprint-to-utility ratio
+	// of binaries is much lower for binaries.
+	// Not caching the link step also makes sure that repeated "go run" at least
+	// always rerun the linker, so that they don't get too fast.
+	// (We don't want people thinking go is a scripting language.)
+	// Note also that if we start caching binaries, then we will
+	// copy the binaries out of the cache to run them, and then
+	// that will mean the go process is itself writing a binary
+	// and then executing it, so we will need to defend against
+	// ETXTBSY problems as discussed in exec.go and golang.org/issue/22220.
+	if a.Mode == "build" {
+		r, err := os.Open(target)
+		if err == nil {
+			if a.output == nil {
+				panic("internal error: a.output not set")
+			}
+			outputID, _, err := c.Put(a.actionID, r)
+			r.Close()
+			if err == nil && cfg.BuildX {
+				b.Showcmd("", "%s # internal", joinUnambiguously(str.StringList("cp", target, c.OutputFile(outputID))))
+			}
+			if b.NeedExport {
+				if err != nil {
+					return err
+				}
+				a.Package.Export = c.OutputFile(outputID)
+				a.Package.BuildID = a.buildID
+			}
+		}
+	}
+
+	return nil
+}
diff --git a/src/cmd/go/internal/work/exec.go b/src/cmd/go/internal/work/exec.go
new file mode 100644
index 0000000..13d2a78
--- /dev/null
+++ b/src/cmd/go/internal/work/exec.go
@@ -0,0 +1,3936 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Action graph execution.
+
+package work
+
+import (
+	"bytes"
+	"context"
+	"crypto/sha256"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"go/token"
+	"internal/coverage"
+	"internal/lazyregexp"
+	"io"
+	"io/fs"
+	"log"
+	"math/rand"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
+	"runtime"
+	"slices"
+	"sort"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cache"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/load"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/str"
+	"cmd/go/internal/trace"
+	"cmd/internal/buildid"
+	"cmd/internal/quoted"
+	"cmd/internal/sys"
+)
+
+const defaultCFlags = "-O2 -g"
+
+// actionList returns the list of actions in the dag rooted at root
+// as visited in a depth-first post-order traversal.
+func actionList(root *Action) []*Action {
+	seen := map[*Action]bool{}
+	all := []*Action{}
+	var walk func(*Action)
+	walk = func(a *Action) {
+		if seen[a] {
+			return
+		}
+		seen[a] = true
+		for _, a1 := range a.Deps {
+			walk(a1)
+		}
+		all = append(all, a)
+	}
+	walk(root)
+	return all
+}
+
+// Do runs the action graph rooted at root.
+func (b *Builder) Do(ctx context.Context, root *Action) {
+	ctx, span := trace.StartSpan(ctx, "exec.Builder.Do ("+root.Mode+" "+root.Target+")")
+	defer span.Done()
+
+	if !b.IsCmdList {
+		// If we're doing real work, take time at the end to trim the cache.
+		c := cache.Default()
+		defer func() {
+			if err := c.Close(); err != nil {
+				base.Fatalf("go: failed to trim cache: %v", err)
+			}
+		}()
+	}
+
+	// Build list of all actions, assigning depth-first post-order priority.
+	// The original implementation here was a true queue
+	// (using a channel) but it had the effect of getting
+	// distracted by low-level leaf actions to the detriment
+	// of completing higher-level actions. The order of
+	// work does not matter much to overall execution time,
+	// but when running "go test std" it is nice to see each test
+	// results as soon as possible. The priorities assigned
+	// ensure that, all else being equal, the execution prefers
+	// to do what it would have done first in a simple depth-first
+	// dependency order traversal.
+	all := actionList(root)
+	for i, a := range all {
+		a.priority = i
+	}
+
+	// Write action graph, without timing information, in case we fail and exit early.
+	writeActionGraph := func() {
+		if file := cfg.DebugActiongraph; file != "" {
+			if strings.HasSuffix(file, ".go") {
+				// Do not overwrite Go source code in:
+				//	go build -debug-actiongraph x.go
+				base.Fatalf("go: refusing to write action graph to %v\n", file)
+			}
+			js := actionGraphJSON(root)
+			if err := os.WriteFile(file, []byte(js), 0666); err != nil {
+				fmt.Fprintf(os.Stderr, "go: writing action graph: %v\n", err)
+				base.SetExitStatus(1)
+			}
+		}
+	}
+	writeActionGraph()
+
+	b.readySema = make(chan bool, len(all))
+
+	// Initialize per-action execution state.
+	for _, a := range all {
+		for _, a1 := range a.Deps {
+			a1.triggers = append(a1.triggers, a)
+		}
+		a.pending = len(a.Deps)
+		if a.pending == 0 {
+			b.ready.push(a)
+			b.readySema <- true
+		}
+	}
+
+	// Handle runs a single action and takes care of triggering
+	// any actions that are runnable as a result.
+	handle := func(ctx context.Context, a *Action) {
+		if a.json != nil {
+			a.json.TimeStart = time.Now()
+		}
+		var err error
+		if a.Actor != nil && (!a.Failed || a.IgnoreFail) {
+			// TODO(matloob): Better action descriptions
+			desc := "Executing action "
+			if a.Package != nil {
+				desc += "(" + a.Mode + " " + a.Package.Desc() + ")"
+			}
+			ctx, span := trace.StartSpan(ctx, desc)
+			a.traceSpan = span
+			for _, d := range a.Deps {
+				trace.Flow(ctx, d.traceSpan, a.traceSpan)
+			}
+			err = a.Actor.Act(b, ctx, a)
+			span.Done()
+		}
+		if a.json != nil {
+			a.json.TimeDone = time.Now()
+		}
+
+		// The actions run in parallel but all the updates to the
+		// shared work state are serialized through b.exec.
+		b.exec.Lock()
+		defer b.exec.Unlock()
+
+		if err != nil {
+			if b.AllowErrors && a.Package != nil {
+				if a.Package.Error == nil {
+					a.Package.Error = &load.PackageError{Err: err}
+				}
+			} else {
+				var ipe load.ImportPathError
+				if a.Package != nil && (!errors.As(err, &ipe) || ipe.ImportPath() != a.Package.ImportPath) {
+					err = fmt.Errorf("%s: %v", a.Package.ImportPath, err)
+				}
+				base.Errorf("%s", err)
+			}
+			a.Failed = true
+		}
+
+		for _, a0 := range a.triggers {
+			if a.Failed {
+				a0.Failed = true
+			}
+			if a0.pending--; a0.pending == 0 {
+				b.ready.push(a0)
+				b.readySema <- true
+			}
+		}
+
+		if a == root {
+			close(b.readySema)
+		}
+	}
+
+	var wg sync.WaitGroup
+
+	// Kick off goroutines according to parallelism.
+	// If we are using the -n flag (just printing commands)
+	// drop the parallelism to 1, both to make the output
+	// deterministic and because there is no real work anyway.
+	par := cfg.BuildP
+	if cfg.BuildN {
+		par = 1
+	}
+	for i := 0; i < par; i++ {
+		wg.Add(1)
+		go func() {
+			ctx := trace.StartGoroutine(ctx)
+			defer wg.Done()
+			for {
+				select {
+				case _, ok := <-b.readySema:
+					if !ok {
+						return
+					}
+					// Receiving a value from b.readySema entitles
+					// us to take from the ready queue.
+					b.exec.Lock()
+					a := b.ready.pop()
+					b.exec.Unlock()
+					handle(ctx, a)
+				case <-base.Interrupted:
+					base.SetExitStatus(1)
+					return
+				}
+			}
+		}()
+	}
+
+	wg.Wait()
+
+	// Write action graph again, this time with timing information.
+	writeActionGraph()
+}
+
+// buildActionID computes the action ID for a build action.
+func (b *Builder) buildActionID(a *Action) cache.ActionID {
+	p := a.Package
+	h := cache.NewHash("build " + p.ImportPath)
+
+	// Configuration independent of compiler toolchain.
+	// Note: buildmode has already been accounted for in buildGcflags
+	// and should not be inserted explicitly. Most buildmodes use the
+	// same compiler settings and can reuse each other's results.
+	// If not, the reason is already recorded in buildGcflags.
+	fmt.Fprintf(h, "compile\n")
+
+	// Include information about the origin of the package that
+	// may be embedded in the debug info for the object file.
+	if cfg.BuildTrimpath {
+		// When -trimpath is used with a package built from the module cache,
+		// its debug information refers to the module path and version
+		// instead of the directory.
+		if p.Module != nil {
+			fmt.Fprintf(h, "module %s@%s\n", p.Module.Path, p.Module.Version)
+		}
+	} else if p.Goroot {
+		// The Go compiler always hides the exact value of $GOROOT
+		// when building things in GOROOT.
+		//
+		// The C compiler does not, but for packages in GOROOT we rewrite the path
+		// as though -trimpath were set, so that we don't invalidate the build cache
+		// (and especially any precompiled C archive files) when changing
+		// GOROOT_FINAL. (See https://go.dev/issue/50183.)
+		//
+		// b.WorkDir is always either trimmed or rewritten to
+		// the literal string "/tmp/go-build".
+	} else if !strings.HasPrefix(p.Dir, b.WorkDir) {
+		// -trimpath is not set and no other rewrite rules apply,
+		// so the object file may refer to the absolute directory
+		// containing the package.
+		fmt.Fprintf(h, "dir %s\n", p.Dir)
+	}
+
+	if p.Module != nil {
+		fmt.Fprintf(h, "go %s\n", p.Module.GoVersion)
+	}
+	fmt.Fprintf(h, "goos %s goarch %s\n", cfg.Goos, cfg.Goarch)
+	fmt.Fprintf(h, "import %q\n", p.ImportPath)
+	fmt.Fprintf(h, "omitdebug %v standard %v local %v prefix %q\n", p.Internal.OmitDebug, p.Standard, p.Internal.Local, p.Internal.LocalPrefix)
+	if cfg.BuildTrimpath {
+		fmt.Fprintln(h, "trimpath")
+	}
+	if p.Internal.ForceLibrary {
+		fmt.Fprintf(h, "forcelibrary\n")
+	}
+	if len(p.CgoFiles)+len(p.SwigFiles)+len(p.SwigCXXFiles) > 0 {
+		fmt.Fprintf(h, "cgo %q\n", b.toolID("cgo"))
+		cppflags, cflags, cxxflags, fflags, ldflags, _ := b.CFlags(p)
+
+		ccExe := b.ccExe()
+		fmt.Fprintf(h, "CC=%q %q %q %q\n", ccExe, cppflags, cflags, ldflags)
+		// Include the C compiler tool ID so that if the C
+		// compiler changes we rebuild the package.
+		if ccID, _, err := b.gccToolID(ccExe[0], "c"); err == nil {
+			fmt.Fprintf(h, "CC ID=%q\n", ccID)
+		}
+		if len(p.CXXFiles)+len(p.SwigCXXFiles) > 0 {
+			cxxExe := b.cxxExe()
+			fmt.Fprintf(h, "CXX=%q %q\n", cxxExe, cxxflags)
+			if cxxID, _, err := b.gccToolID(cxxExe[0], "c++"); err == nil {
+				fmt.Fprintf(h, "CXX ID=%q\n", cxxID)
+			}
+		}
+		if len(p.FFiles) > 0 {
+			fcExe := b.fcExe()
+			fmt.Fprintf(h, "FC=%q %q\n", fcExe, fflags)
+			if fcID, _, err := b.gccToolID(fcExe[0], "f95"); err == nil {
+				fmt.Fprintf(h, "FC ID=%q\n", fcID)
+			}
+		}
+		// TODO(rsc): Should we include the SWIG version?
+	}
+	if p.Internal.CoverMode != "" {
+		fmt.Fprintf(h, "cover %q %q\n", p.Internal.CoverMode, b.toolID("cover"))
+	}
+	if p.Internal.FuzzInstrument {
+		if fuzzFlags := fuzzInstrumentFlags(); fuzzFlags != nil {
+			fmt.Fprintf(h, "fuzz %q\n", fuzzFlags)
+		}
+	}
+	if p.Internal.BuildInfo != nil {
+		fmt.Fprintf(h, "modinfo %q\n", p.Internal.BuildInfo.String())
+	}
+
+	// Configuration specific to compiler toolchain.
+	switch cfg.BuildToolchainName {
+	default:
+		base.Fatalf("buildActionID: unknown build toolchain %q", cfg.BuildToolchainName)
+	case "gc":
+		fmt.Fprintf(h, "compile %s %q %q\n", b.toolID("compile"), forcedGcflags, p.Internal.Gcflags)
+		if len(p.SFiles) > 0 {
+			fmt.Fprintf(h, "asm %q %q %q\n", b.toolID("asm"), forcedAsmflags, p.Internal.Asmflags)
+		}
+
+		// GOARM, GOMIPS, etc.
+		key, val := cfg.GetArchEnv()
+		fmt.Fprintf(h, "%s=%s\n", key, val)
+
+		if cfg.CleanGOEXPERIMENT != "" {
+			fmt.Fprintf(h, "GOEXPERIMENT=%q\n", cfg.CleanGOEXPERIMENT)
+		}
+
+		// TODO(rsc): Convince compiler team not to add more magic environment variables,
+		// or perhaps restrict the environment variables passed to subprocesses.
+		// Because these are clumsy, undocumented special-case hacks
+		// for debugging the compiler, they are not settable using 'go env -w',
+		// and so here we use os.Getenv, not cfg.Getenv.
+		magic := []string{
+			"GOCLOBBERDEADHASH",
+			"GOSSAFUNC",
+			"GOSSADIR",
+			"GOCOMPILEDEBUG",
+		}
+		for _, env := range magic {
+			if x := os.Getenv(env); x != "" {
+				fmt.Fprintf(h, "magic %s=%s\n", env, x)
+			}
+		}
+
+	case "gccgo":
+		id, _, err := b.gccToolID(BuildToolchain.compiler(), "go")
+		if err != nil {
+			base.Fatalf("%v", err)
+		}
+		fmt.Fprintf(h, "compile %s %q %q\n", id, forcedGccgoflags, p.Internal.Gccgoflags)
+		fmt.Fprintf(h, "pkgpath %s\n", gccgoPkgpath(p))
+		fmt.Fprintf(h, "ar %q\n", BuildToolchain.(gccgoToolchain).ar())
+		if len(p.SFiles) > 0 {
+			id, _, _ = b.gccToolID(BuildToolchain.compiler(), "assembler-with-cpp")
+			// Ignore error; different assembler versions
+			// are unlikely to make any difference anyhow.
+			fmt.Fprintf(h, "asm %q\n", id)
+		}
+	}
+
+	// Input files.
+	inputFiles := str.StringList(
+		p.GoFiles,
+		p.CgoFiles,
+		p.CFiles,
+		p.CXXFiles,
+		p.FFiles,
+		p.MFiles,
+		p.HFiles,
+		p.SFiles,
+		p.SysoFiles,
+		p.SwigFiles,
+		p.SwigCXXFiles,
+		p.EmbedFiles,
+	)
+	for _, file := range inputFiles {
+		fmt.Fprintf(h, "file %s %s\n", file, b.fileHash(filepath.Join(p.Dir, file)))
+	}
+	if p.Internal.PGOProfile != "" {
+		fmt.Fprintf(h, "pgofile %s\n", b.fileHash(p.Internal.PGOProfile))
+	}
+	for _, a1 := range a.Deps {
+		p1 := a1.Package
+		if p1 != nil {
+			fmt.Fprintf(h, "import %s %s\n", p1.ImportPath, contentID(a1.buildID))
+		}
+	}
+
+	return h.Sum()
+}
+
+// needCgoHdr reports whether the actions triggered by this one
+// expect to be able to access the cgo-generated header file.
+func (b *Builder) needCgoHdr(a *Action) bool {
+	// If this build triggers a header install, run cgo to get the header.
+	if !b.IsCmdList && (a.Package.UsesCgo() || a.Package.UsesSwig()) && (cfg.BuildBuildmode == "c-archive" || cfg.BuildBuildmode == "c-shared") {
+		for _, t1 := range a.triggers {
+			if t1.Mode == "install header" {
+				return true
+			}
+		}
+		for _, t1 := range a.triggers {
+			for _, t2 := range t1.triggers {
+				if t2.Mode == "install header" {
+					return true
+				}
+			}
+		}
+	}
+	return false
+}
+
+// allowedVersion reports whether the version v is an allowed version of go
+// (one that we can compile).
+// v is known to be of the form "1.23".
+func allowedVersion(v string) bool {
+	// Special case: no requirement.
+	if v == "" {
+		return true
+	}
+	return gover.Compare(gover.Local(), v) >= 0
+}
+
+const (
+	needBuild uint32 = 1 << iota
+	needCgoHdr
+	needVet
+	needCompiledGoFiles
+	needStale
+)
+
+// build is the action for building a single package.
+// Note that any new influence on this logic must be reported in b.buildActionID above as well.
+func (b *Builder) build(ctx context.Context, a *Action) (err error) {
+	p := a.Package
+
+	bit := func(x uint32, b bool) uint32 {
+		if b {
+			return x
+		}
+		return 0
+	}
+
+	cachedBuild := false
+	need := bit(needBuild, !b.IsCmdList && a.needBuild || b.NeedExport) |
+		bit(needCgoHdr, b.needCgoHdr(a)) |
+		bit(needVet, a.needVet) |
+		bit(needCompiledGoFiles, b.NeedCompiledGoFiles)
+
+	if !p.BinaryOnly {
+		if b.useCache(a, b.buildActionID(a), p.Target, need&needBuild != 0) {
+			// We found the main output in the cache.
+			// If we don't need any other outputs, we can stop.
+			// Otherwise, we need to write files to a.Objdir (needVet, needCgoHdr).
+			// Remember that we might have them in cache
+			// and check again after we create a.Objdir.
+			cachedBuild = true
+			a.output = []byte{} // start saving output in case we miss any cache results
+			need &^= needBuild
+			if b.NeedExport {
+				p.Export = a.built
+				p.BuildID = a.buildID
+			}
+			if need&needCompiledGoFiles != 0 {
+				if err := b.loadCachedCompiledGoFiles(a); err == nil {
+					need &^= needCompiledGoFiles
+				}
+			}
+		}
+
+		// Source files might be cached, even if the full action is not
+		// (e.g., go list -compiled -find).
+		if !cachedBuild && need&needCompiledGoFiles != 0 {
+			if err := b.loadCachedCompiledGoFiles(a); err == nil {
+				need &^= needCompiledGoFiles
+			}
+		}
+
+		if need == 0 {
+			return nil
+		}
+		defer b.flushOutput(a)
+	}
+
+	defer func() {
+		if err != nil && b.IsCmdList && b.NeedError && p.Error == nil {
+			p.Error = &load.PackageError{Err: err}
+		}
+	}()
+	if cfg.BuildN {
+		// In -n mode, print a banner between packages.
+		// The banner is five lines so that when changes to
+		// different sections of the bootstrap script have to
+		// be merged, the banners give patch something
+		// to use to find its context.
+		b.Print("\n#\n# " + p.ImportPath + "\n#\n\n")
+	}
+
+	if cfg.BuildV {
+		b.Print(p.ImportPath + "\n")
+	}
+
+	if p.Error != nil {
+		// Don't try to build anything for packages with errors. There may be a
+		// problem with the inputs that makes the package unsafe to build.
+		return p.Error
+	}
+
+	if p.BinaryOnly {
+		p.Stale = true
+		p.StaleReason = "binary-only packages are no longer supported"
+		if b.IsCmdList {
+			return nil
+		}
+		return errors.New("binary-only packages are no longer supported")
+	}
+
+	if p.Module != nil && !allowedVersion(p.Module.GoVersion) {
+		return errors.New("module requires Go " + p.Module.GoVersion + " or later")
+	}
+
+	if err := b.checkDirectives(a); err != nil {
+		return err
+	}
+
+	if err := b.Mkdir(a.Objdir); err != nil {
+		return err
+	}
+	objdir := a.Objdir
+
+	// Load cached cgo header, but only if we're skipping the main build (cachedBuild==true).
+	if cachedBuild && need&needCgoHdr != 0 {
+		if err := b.loadCachedCgoHdr(a); err == nil {
+			need &^= needCgoHdr
+		}
+	}
+
+	// Load cached vet config, but only if that's all we have left
+	// (need == needVet, not testing just the one bit).
+	// If we are going to do a full build anyway,
+	// we're going to regenerate the files below anyway.
+	if need == needVet {
+		if err := b.loadCachedVet(a); err == nil {
+			need &^= needVet
+		}
+	}
+	if need == 0 {
+		return nil
+	}
+
+	if err := AllowInstall(a); err != nil {
+		return err
+	}
+
+	// make target directory
+	dir, _ := filepath.Split(a.Target)
+	if dir != "" {
+		if err := b.Mkdir(dir); err != nil {
+			return err
+		}
+	}
+
+	gofiles := str.StringList(p.GoFiles)
+	cgofiles := str.StringList(p.CgoFiles)
+	cfiles := str.StringList(p.CFiles)
+	sfiles := str.StringList(p.SFiles)
+	cxxfiles := str.StringList(p.CXXFiles)
+	var objects, cgoObjects, pcCFLAGS, pcLDFLAGS []string
+
+	if p.UsesCgo() || p.UsesSwig() {
+		if pcCFLAGS, pcLDFLAGS, err = b.getPkgConfigFlags(p); err != nil {
+			return
+		}
+	}
+
+	// Compute overlays for .c/.cc/.h/etc. and if there are any overlays
+	// put correct contents of all those files in the objdir, to ensure
+	// the correct headers are included. nonGoOverlay is the overlay that
+	// points from nongo files to the copied files in objdir.
+	nonGoFileLists := [][]string{p.CFiles, p.SFiles, p.CXXFiles, p.HFiles, p.FFiles}
+OverlayLoop:
+	for _, fs := range nonGoFileLists {
+		for _, f := range fs {
+			if _, ok := fsys.OverlayPath(mkAbs(p.Dir, f)); ok {
+				a.nonGoOverlay = make(map[string]string)
+				break OverlayLoop
+			}
+		}
+	}
+	if a.nonGoOverlay != nil {
+		for _, fs := range nonGoFileLists {
+			for i := range fs {
+				from := mkAbs(p.Dir, fs[i])
+				opath, _ := fsys.OverlayPath(from)
+				dst := objdir + filepath.Base(fs[i])
+				if err := b.copyFile(dst, opath, 0666, false); err != nil {
+					return err
+				}
+				a.nonGoOverlay[from] = dst
+			}
+		}
+	}
+
+	// Run SWIG on each .swig and .swigcxx file.
+	// Each run will generate two files, a .go file and a .c or .cxx file.
+	// The .go file will use import "C" and is to be processed by cgo.
+	if p.UsesSwig() {
+		outGo, outC, outCXX, err := b.swig(a, p, objdir, pcCFLAGS)
+		if err != nil {
+			return err
+		}
+		cgofiles = append(cgofiles, outGo...)
+		cfiles = append(cfiles, outC...)
+		cxxfiles = append(cxxfiles, outCXX...)
+	}
+
+	// If we're doing coverage, preprocess the .go files and put them in the work directory
+	if p.Internal.CoverMode != "" {
+		outfiles := []string{}
+		infiles := []string{}
+		for i, file := range str.StringList(gofiles, cgofiles) {
+			if base.IsTestFile(file) {
+				continue // Not covering this file.
+			}
+
+			var sourceFile string
+			var coverFile string
+			var key string
+			if base, found := strings.CutSuffix(file, ".cgo1.go"); found {
+				// cgo files have absolute paths
+				base = filepath.Base(base)
+				sourceFile = file
+				coverFile = objdir + base + ".cgo1.go"
+				key = base + ".go"
+			} else {
+				sourceFile = filepath.Join(p.Dir, file)
+				coverFile = objdir + file
+				key = file
+			}
+			coverFile = strings.TrimSuffix(coverFile, ".go") + ".cover.go"
+			if cfg.Experiment.CoverageRedesign {
+				infiles = append(infiles, sourceFile)
+				outfiles = append(outfiles, coverFile)
+			} else {
+				cover := p.Internal.CoverVars[key]
+				if cover == nil {
+					continue // Not covering this file.
+				}
+				if err := b.cover(a, coverFile, sourceFile, cover.Var); err != nil {
+					return err
+				}
+			}
+			if i < len(gofiles) {
+				gofiles[i] = coverFile
+			} else {
+				cgofiles[i-len(gofiles)] = coverFile
+			}
+		}
+
+		if cfg.Experiment.CoverageRedesign {
+			if len(infiles) != 0 {
+				// Coverage instrumentation creates new top level
+				// variables in the target package for things like
+				// meta-data containers, counter vars, etc. To avoid
+				// collisions with user variables, suffix the var name
+				// with 12 hex digits from the SHA-256 hash of the
+				// import path. Choice of 12 digits is historical/arbitrary,
+				// we just need enough of the hash to avoid accidents,
+				// as opposed to precluding determined attempts by
+				// users to break things.
+				sum := sha256.Sum256([]byte(a.Package.ImportPath))
+				coverVar := fmt.Sprintf("goCover_%x_", sum[:6])
+				mode := a.Package.Internal.CoverMode
+				if mode == "" {
+					panic("covermode should be set at this point")
+				}
+				if newoutfiles, err := b.cover2(a, infiles, outfiles, coverVar, mode); err != nil {
+					return err
+				} else {
+					outfiles = newoutfiles
+					gofiles = append([]string{newoutfiles[0]}, gofiles...)
+				}
+			} else {
+				// If there are no input files passed to cmd/cover,
+				// then we don't want to pass -covercfg when building
+				// the package with the compiler, so set covermode to
+				// the empty string so as to signal that we need to do
+				// that.
+				p.Internal.CoverMode = ""
+			}
+		}
+	}
+
+	// Run cgo.
+	if p.UsesCgo() || p.UsesSwig() {
+		// In a package using cgo, cgo compiles the C, C++ and assembly files with gcc.
+		// There is one exception: runtime/cgo's job is to bridge the
+		// cgo and non-cgo worlds, so it necessarily has files in both.
+		// In that case gcc only gets the gcc_* files.
+		var gccfiles []string
+		gccfiles = append(gccfiles, cfiles...)
+		cfiles = nil
+		if p.Standard && p.ImportPath == "runtime/cgo" {
+			filter := func(files, nongcc, gcc []string) ([]string, []string) {
+				for _, f := range files {
+					if strings.HasPrefix(f, "gcc_") {
+						gcc = append(gcc, f)
+					} else {
+						nongcc = append(nongcc, f)
+					}
+				}
+				return nongcc, gcc
+			}
+			sfiles, gccfiles = filter(sfiles, sfiles[:0], gccfiles)
+		} else {
+			for _, sfile := range sfiles {
+				data, err := os.ReadFile(filepath.Join(p.Dir, sfile))
+				if err == nil {
+					if bytes.HasPrefix(data, []byte("TEXT")) || bytes.Contains(data, []byte("\nTEXT")) ||
+						bytes.HasPrefix(data, []byte("DATA")) || bytes.Contains(data, []byte("\nDATA")) ||
+						bytes.HasPrefix(data, []byte("GLOBL")) || bytes.Contains(data, []byte("\nGLOBL")) {
+						return fmt.Errorf("package using cgo has Go assembly file %s", sfile)
+					}
+				}
+			}
+			gccfiles = append(gccfiles, sfiles...)
+			sfiles = nil
+		}
+
+		outGo, outObj, err := b.cgo(a, base.Tool("cgo"), objdir, pcCFLAGS, pcLDFLAGS, mkAbsFiles(p.Dir, cgofiles), gccfiles, cxxfiles, p.MFiles, p.FFiles)
+
+		// The files in cxxfiles have now been handled by b.cgo.
+		cxxfiles = nil
+
+		if err != nil {
+			return err
+		}
+		if cfg.BuildToolchainName == "gccgo" {
+			cgoObjects = append(cgoObjects, a.Objdir+"_cgo_flags")
+		}
+		cgoObjects = append(cgoObjects, outObj...)
+		gofiles = append(gofiles, outGo...)
+
+		switch cfg.BuildBuildmode {
+		case "c-archive", "c-shared":
+			b.cacheCgoHdr(a)
+		}
+	}
+
+	var srcfiles []string // .go and non-.go
+	srcfiles = append(srcfiles, gofiles...)
+	srcfiles = append(srcfiles, sfiles...)
+	srcfiles = append(srcfiles, cfiles...)
+	srcfiles = append(srcfiles, cxxfiles...)
+	b.cacheSrcFiles(a, srcfiles)
+
+	// Running cgo generated the cgo header.
+	need &^= needCgoHdr
+
+	// Sanity check only, since Package.load already checked as well.
+	if len(gofiles) == 0 {
+		return &load.NoGoError{Package: p}
+	}
+
+	// Prepare Go vet config if needed.
+	if need&needVet != 0 {
+		buildVetConfig(a, srcfiles)
+		need &^= needVet
+	}
+	if need&needCompiledGoFiles != 0 {
+		if err := b.loadCachedCompiledGoFiles(a); err != nil {
+			return fmt.Errorf("loading compiled Go files from cache: %w", err)
+		}
+		need &^= needCompiledGoFiles
+	}
+	if need == 0 {
+		// Nothing left to do.
+		return nil
+	}
+
+	// Collect symbol ABI requirements from assembly.
+	symabis, err := BuildToolchain.symabis(b, a, sfiles)
+	if err != nil {
+		return err
+	}
+
+	// Prepare Go import config.
+	// We start it off with a comment so it can't be empty, so icfg.Bytes() below is never nil.
+	// It should never be empty anyway, but there have been bugs in the past that resulted
+	// in empty configs, which then unfortunately turn into "no config passed to compiler",
+	// and the compiler falls back to looking in pkg itself, which mostly works,
+	// except when it doesn't.
+	var icfg bytes.Buffer
+	fmt.Fprintf(&icfg, "# import config\n")
+	for i, raw := range p.Internal.RawImports {
+		final := p.Imports[i]
+		if final != raw {
+			fmt.Fprintf(&icfg, "importmap %s=%s\n", raw, final)
+		}
+	}
+	for _, a1 := range a.Deps {
+		p1 := a1.Package
+		if p1 == nil || p1.ImportPath == "" || a1.built == "" {
+			continue
+		}
+		fmt.Fprintf(&icfg, "packagefile %s=%s\n", p1.ImportPath, a1.built)
+	}
+
+	// Prepare Go embed config if needed.
+	// Unlike the import config, it's okay for the embed config to be empty.
+	var embedcfg []byte
+	if len(p.Internal.Embed) > 0 {
+		var embed struct {
+			Patterns map[string][]string
+			Files    map[string]string
+		}
+		embed.Patterns = p.Internal.Embed
+		embed.Files = make(map[string]string)
+		for _, file := range p.EmbedFiles {
+			embed.Files[file] = filepath.Join(p.Dir, file)
+		}
+		js, err := json.MarshalIndent(&embed, "", "\t")
+		if err != nil {
+			return fmt.Errorf("marshal embedcfg: %v", err)
+		}
+		embedcfg = js
+	}
+
+	if p.Internal.BuildInfo != nil && cfg.ModulesEnabled {
+		prog := modload.ModInfoProg(p.Internal.BuildInfo.String(), cfg.BuildToolchainName == "gccgo")
+		if len(prog) > 0 {
+			if err := b.writeFile(objdir+"_gomod_.go", prog); err != nil {
+				return err
+			}
+			gofiles = append(gofiles, objdir+"_gomod_.go")
+		}
+	}
+
+	// Compile Go.
+	objpkg := objdir + "_pkg_.a"
+	ofile, out, err := BuildToolchain.gc(b, a, objpkg, icfg.Bytes(), embedcfg, symabis, len(sfiles) > 0, gofiles)
+	if len(out) > 0 {
+		output := b.processOutput(out)
+		if err != nil {
+			return formatOutput(b.WorkDir, p.Dir, p.ImportPath, p.Desc(), output)
+		} else {
+			b.showOutput(a, p.Dir, p.Desc(), output)
+		}
+	}
+	if err != nil {
+		return err
+	}
+	if ofile != objpkg {
+		objects = append(objects, ofile)
+	}
+
+	// Copy .h files named for goos or goarch or goos_goarch
+	// to names using GOOS and GOARCH.
+	// For example, defs_linux_amd64.h becomes defs_GOOS_GOARCH.h.
+	_goos_goarch := "_" + cfg.Goos + "_" + cfg.Goarch
+	_goos := "_" + cfg.Goos
+	_goarch := "_" + cfg.Goarch
+	for _, file := range p.HFiles {
+		name, ext := fileExtSplit(file)
+		switch {
+		case strings.HasSuffix(name, _goos_goarch):
+			targ := file[:len(name)-len(_goos_goarch)] + "_GOOS_GOARCH." + ext
+			if err := b.copyFile(objdir+targ, filepath.Join(p.Dir, file), 0666, true); err != nil {
+				return err
+			}
+		case strings.HasSuffix(name, _goarch):
+			targ := file[:len(name)-len(_goarch)] + "_GOARCH." + ext
+			if err := b.copyFile(objdir+targ, filepath.Join(p.Dir, file), 0666, true); err != nil {
+				return err
+			}
+		case strings.HasSuffix(name, _goos):
+			targ := file[:len(name)-len(_goos)] + "_GOOS." + ext
+			if err := b.copyFile(objdir+targ, filepath.Join(p.Dir, file), 0666, true); err != nil {
+				return err
+			}
+		}
+	}
+
+	for _, file := range cfiles {
+		out := file[:len(file)-len(".c")] + ".o"
+		if err := BuildToolchain.cc(b, a, objdir+out, file); err != nil {
+			return err
+		}
+		objects = append(objects, out)
+	}
+
+	// Assemble .s files.
+	if len(sfiles) > 0 {
+		ofiles, err := BuildToolchain.asm(b, a, sfiles)
+		if err != nil {
+			return err
+		}
+		objects = append(objects, ofiles...)
+	}
+
+	// For gccgo on ELF systems, we write the build ID as an assembler file.
+	// This lets us set the SHF_EXCLUDE flag.
+	// This is read by readGccgoArchive in cmd/internal/buildid/buildid.go.
+	if a.buildID != "" && cfg.BuildToolchainName == "gccgo" {
+		switch cfg.Goos {
+		case "aix", "android", "dragonfly", "freebsd", "illumos", "linux", "netbsd", "openbsd", "solaris":
+			asmfile, err := b.gccgoBuildIDFile(a)
+			if err != nil {
+				return err
+			}
+			ofiles, err := BuildToolchain.asm(b, a, []string{asmfile})
+			if err != nil {
+				return err
+			}
+			objects = append(objects, ofiles...)
+		}
+	}
+
+	// NOTE(rsc): On Windows, it is critically important that the
+	// gcc-compiled objects (cgoObjects) be listed after the ordinary
+	// objects in the archive. I do not know why this is.
+	// https://golang.org/issue/2601
+	objects = append(objects, cgoObjects...)
+
+	// Add system object files.
+	for _, syso := range p.SysoFiles {
+		objects = append(objects, filepath.Join(p.Dir, syso))
+	}
+
+	// Pack into archive in objdir directory.
+	// If the Go compiler wrote an archive, we only need to add the
+	// object files for non-Go sources to the archive.
+	// If the Go compiler wrote an archive and the package is entirely
+	// Go sources, there is no pack to execute at all.
+	if len(objects) > 0 {
+		if err := BuildToolchain.pack(b, a, objpkg, objects); err != nil {
+			return err
+		}
+	}
+
+	if err := b.updateBuildID(a, objpkg, true); err != nil {
+		return err
+	}
+
+	a.built = objpkg
+	return nil
+}
+
+func (b *Builder) checkDirectives(a *Action) error {
+	var msg *bytes.Buffer
+	p := a.Package
+	var seen map[string]token.Position
+	for _, d := range p.Internal.Build.Directives {
+		if strings.HasPrefix(d.Text, "//go:debug") {
+			key, _, err := load.ParseGoDebug(d.Text)
+			if err != nil && err != load.ErrNotGoDebug {
+				if msg == nil {
+					msg = new(bytes.Buffer)
+				}
+				fmt.Fprintf(msg, "%s: invalid //go:debug: %v\n", d.Pos, err)
+				continue
+			}
+			if pos, ok := seen[key]; ok {
+				fmt.Fprintf(msg, "%s: repeated //go:debug for %v\n\t%s: previous //go:debug\n", d.Pos, key, pos)
+				continue
+			}
+			if seen == nil {
+				seen = make(map[string]token.Position)
+			}
+			seen[key] = d.Pos
+		}
+	}
+	if msg != nil {
+		return formatOutput(b.WorkDir, p.Dir, p.ImportPath, p.Desc(), b.processOutput(msg.Bytes()))
+
+	}
+	return nil
+}
+
+func (b *Builder) cacheObjdirFile(a *Action, c cache.Cache, name string) error {
+	f, err := os.Open(a.Objdir + name)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+	_, _, err = c.Put(cache.Subkey(a.actionID, name), f)
+	return err
+}
+
+func (b *Builder) findCachedObjdirFile(a *Action, c cache.Cache, name string) (string, error) {
+	file, _, err := cache.GetFile(c, cache.Subkey(a.actionID, name))
+	if err != nil {
+		return "", fmt.Errorf("loading cached file %s: %w", name, err)
+	}
+	return file, nil
+}
+
+func (b *Builder) loadCachedObjdirFile(a *Action, c cache.Cache, name string) error {
+	cached, err := b.findCachedObjdirFile(a, c, name)
+	if err != nil {
+		return err
+	}
+	return b.copyFile(a.Objdir+name, cached, 0666, true)
+}
+
+func (b *Builder) cacheCgoHdr(a *Action) {
+	c := cache.Default()
+	b.cacheObjdirFile(a, c, "_cgo_install.h")
+}
+
+func (b *Builder) loadCachedCgoHdr(a *Action) error {
+	c := cache.Default()
+	return b.loadCachedObjdirFile(a, c, "_cgo_install.h")
+}
+
+func (b *Builder) cacheSrcFiles(a *Action, srcfiles []string) {
+	c := cache.Default()
+	var buf bytes.Buffer
+	for _, file := range srcfiles {
+		if !strings.HasPrefix(file, a.Objdir) {
+			// not generated
+			buf.WriteString("./")
+			buf.WriteString(file)
+			buf.WriteString("\n")
+			continue
+		}
+		name := file[len(a.Objdir):]
+		buf.WriteString(name)
+		buf.WriteString("\n")
+		if err := b.cacheObjdirFile(a, c, name); err != nil {
+			return
+		}
+	}
+	cache.PutBytes(c, cache.Subkey(a.actionID, "srcfiles"), buf.Bytes())
+}
+
+func (b *Builder) loadCachedVet(a *Action) error {
+	c := cache.Default()
+	list, _, err := cache.GetBytes(c, cache.Subkey(a.actionID, "srcfiles"))
+	if err != nil {
+		return fmt.Errorf("reading srcfiles list: %w", err)
+	}
+	var srcfiles []string
+	for _, name := range strings.Split(string(list), "\n") {
+		if name == "" { // end of list
+			continue
+		}
+		if strings.HasPrefix(name, "./") {
+			srcfiles = append(srcfiles, name[2:])
+			continue
+		}
+		if err := b.loadCachedObjdirFile(a, c, name); err != nil {
+			return err
+		}
+		srcfiles = append(srcfiles, a.Objdir+name)
+	}
+	buildVetConfig(a, srcfiles)
+	return nil
+}
+
+func (b *Builder) loadCachedCompiledGoFiles(a *Action) error {
+	c := cache.Default()
+	list, _, err := cache.GetBytes(c, cache.Subkey(a.actionID, "srcfiles"))
+	if err != nil {
+		return fmt.Errorf("reading srcfiles list: %w", err)
+	}
+	var gofiles []string
+	for _, name := range strings.Split(string(list), "\n") {
+		if name == "" { // end of list
+			continue
+		} else if !strings.HasSuffix(name, ".go") {
+			continue
+		}
+		if strings.HasPrefix(name, "./") {
+			gofiles = append(gofiles, name[len("./"):])
+			continue
+		}
+		file, err := b.findCachedObjdirFile(a, c, name)
+		if err != nil {
+			return fmt.Errorf("finding %s: %w", name, err)
+		}
+		gofiles = append(gofiles, file)
+	}
+	a.Package.CompiledGoFiles = gofiles
+	return nil
+}
+
+// vetConfig is the configuration passed to vet describing a single package.
+type vetConfig struct {
+	ID           string   // package ID (example: "fmt [fmt.test]")
+	Compiler     string   // compiler name (gc, gccgo)
+	Dir          string   // directory containing package
+	ImportPath   string   // canonical import path ("package path")
+	GoFiles      []string // absolute paths to package source files
+	NonGoFiles   []string // absolute paths to package non-Go files
+	IgnoredFiles []string // absolute paths to ignored source files
+
+	ImportMap   map[string]string // map import path in source code to package path
+	PackageFile map[string]string // map package path to .a file with export data
+	Standard    map[string]bool   // map package path to whether it's in the standard library
+	PackageVetx map[string]string // map package path to vetx data from earlier vet run
+	VetxOnly    bool              // only compute vetx data; don't report detected problems
+	VetxOutput  string            // write vetx data to this output file
+	GoVersion   string            // Go version for package
+
+	SucceedOnTypecheckFailure bool // awful hack; see #18395 and below
+}
+
+func buildVetConfig(a *Action, srcfiles []string) {
+	// Classify files based on .go extension.
+	// srcfiles does not include raw cgo files.
+	var gofiles, nongofiles []string
+	for _, name := range srcfiles {
+		if strings.HasSuffix(name, ".go") {
+			gofiles = append(gofiles, name)
+		} else {
+			nongofiles = append(nongofiles, name)
+		}
+	}
+
+	ignored := str.StringList(a.Package.IgnoredGoFiles, a.Package.IgnoredOtherFiles)
+
+	// Pass list of absolute paths to vet,
+	// so that vet's error messages will use absolute paths,
+	// so that we can reformat them relative to the directory
+	// in which the go command is invoked.
+	vcfg := &vetConfig{
+		ID:           a.Package.ImportPath,
+		Compiler:     cfg.BuildToolchainName,
+		Dir:          a.Package.Dir,
+		GoFiles:      mkAbsFiles(a.Package.Dir, gofiles),
+		NonGoFiles:   mkAbsFiles(a.Package.Dir, nongofiles),
+		IgnoredFiles: mkAbsFiles(a.Package.Dir, ignored),
+		ImportPath:   a.Package.ImportPath,
+		ImportMap:    make(map[string]string),
+		PackageFile:  make(map[string]string),
+		Standard:     make(map[string]bool),
+	}
+	if a.Package.Module != nil {
+		v := a.Package.Module.GoVersion
+		if v == "" {
+			v = gover.DefaultGoModVersion
+		}
+		vcfg.GoVersion = "go" + v
+	}
+	a.vetCfg = vcfg
+	for i, raw := range a.Package.Internal.RawImports {
+		final := a.Package.Imports[i]
+		vcfg.ImportMap[raw] = final
+	}
+
+	// Compute the list of mapped imports in the vet config
+	// so that we can add any missing mappings below.
+	vcfgMapped := make(map[string]bool)
+	for _, p := range vcfg.ImportMap {
+		vcfgMapped[p] = true
+	}
+
+	for _, a1 := range a.Deps {
+		p1 := a1.Package
+		if p1 == nil || p1.ImportPath == "" {
+			continue
+		}
+		// Add import mapping if needed
+		// (for imports like "runtime/cgo" that appear only in generated code).
+		if !vcfgMapped[p1.ImportPath] {
+			vcfg.ImportMap[p1.ImportPath] = p1.ImportPath
+		}
+		if a1.built != "" {
+			vcfg.PackageFile[p1.ImportPath] = a1.built
+		}
+		if p1.Standard {
+			vcfg.Standard[p1.ImportPath] = true
+		}
+	}
+}
+
+// VetTool is the path to an alternate vet tool binary.
+// The caller is expected to set it (if needed) before executing any vet actions.
+var VetTool string
+
+// VetFlags are the default flags to pass to vet.
+// The caller is expected to set them before executing any vet actions.
+var VetFlags []string
+
+// VetExplicit records whether the vet flags were set explicitly on the command line.
+var VetExplicit bool
+
+func (b *Builder) vet(ctx context.Context, a *Action) error {
+	// a.Deps[0] is the build of the package being vetted.
+	// a.Deps[1] is the build of the "fmt" package.
+
+	a.Failed = false // vet of dependency may have failed but we can still succeed
+
+	if a.Deps[0].Failed {
+		// The build of the package has failed. Skip vet check.
+		// Vet could return export data for non-typecheck errors,
+		// but we ignore it because the package cannot be compiled.
+		return nil
+	}
+
+	vcfg := a.Deps[0].vetCfg
+	if vcfg == nil {
+		// Vet config should only be missing if the build failed.
+		return fmt.Errorf("vet config not found")
+	}
+
+	vcfg.VetxOnly = a.VetxOnly
+	vcfg.VetxOutput = a.Objdir + "vet.out"
+	vcfg.PackageVetx = make(map[string]string)
+
+	h := cache.NewHash("vet " + a.Package.ImportPath)
+	fmt.Fprintf(h, "vet %q\n", b.toolID("vet"))
+
+	vetFlags := VetFlags
+
+	// In GOROOT, we enable all the vet tests during 'go test',
+	// not just the high-confidence subset. This gets us extra
+	// checking for the standard library (at some compliance cost)
+	// and helps us gain experience about how well the checks
+	// work, to help decide which should be turned on by default.
+	// The command-line still wins.
+	//
+	// Note that this flag change applies even when running vet as
+	// a dependency of vetting a package outside std.
+	// (Otherwise we'd have to introduce a whole separate
+	// space of "vet fmt as a dependency of a std top-level vet"
+	// versus "vet fmt as a dependency of a non-std top-level vet".)
+	// This is OK as long as the packages that are farther down the
+	// dependency tree turn on *more* analysis, as here.
+	// (The unsafeptr check does not write any facts for use by
+	// later vet runs, nor does unreachable.)
+	if a.Package.Goroot && !VetExplicit && VetTool == "" {
+		// Turn off -unsafeptr checks.
+		// There's too much unsafe.Pointer code
+		// that vet doesn't like in low-level packages
+		// like runtime, sync, and reflect.
+		// Note that $GOROOT/src/buildall.bash
+		// does the same for the misc-compile trybots
+		// and should be updated if these flags are
+		// changed here.
+		vetFlags = []string{"-unsafeptr=false"}
+
+		// Also turn off -unreachable checks during go test.
+		// During testing it is very common to make changes
+		// like hard-coded forced returns or panics that make
+		// code unreachable. It's unreasonable to insist on files
+		// not having any unreachable code during "go test".
+		// (buildall.bash still runs with -unreachable enabled
+		// for the overall whole-tree scan.)
+		if cfg.CmdName == "test" {
+			vetFlags = append(vetFlags, "-unreachable=false")
+		}
+	}
+
+	// Note: We could decide that vet should compute export data for
+	// all analyses, in which case we don't need to include the flags here.
+	// But that would mean that if an analysis causes problems like
+	// unexpected crashes there would be no way to turn it off.
+	// It seems better to let the flags disable export analysis too.
+	fmt.Fprintf(h, "vetflags %q\n", vetFlags)
+
+	fmt.Fprintf(h, "pkg %q\n", a.Deps[0].actionID)
+	for _, a1 := range a.Deps {
+		if a1.Mode == "vet" && a1.built != "" {
+			fmt.Fprintf(h, "vetout %q %s\n", a1.Package.ImportPath, b.fileHash(a1.built))
+			vcfg.PackageVetx[a1.Package.ImportPath] = a1.built
+		}
+	}
+	key := cache.ActionID(h.Sum())
+
+	if vcfg.VetxOnly && !cfg.BuildA {
+		c := cache.Default()
+		if file, _, err := cache.GetFile(c, key); err == nil {
+			a.built = file
+			return nil
+		}
+	}
+
+	js, err := json.MarshalIndent(vcfg, "", "\t")
+	if err != nil {
+		return fmt.Errorf("internal error marshaling vet config: %v", err)
+	}
+	js = append(js, '\n')
+	if err := b.writeFile(a.Objdir+"vet.cfg", js); err != nil {
+		return err
+	}
+
+	// TODO(rsc): Why do we pass $GCCGO to go vet?
+	env := b.cCompilerEnv()
+	if cfg.BuildToolchainName == "gccgo" {
+		env = append(env, "GCCGO="+BuildToolchain.compiler())
+	}
+
+	p := a.Package
+	tool := VetTool
+	if tool == "" {
+		tool = base.Tool("vet")
+	}
+	runErr := b.run(a, p.Dir, p.ImportPath, env, cfg.BuildToolexec, tool, vetFlags, a.Objdir+"vet.cfg")
+
+	// If vet wrote export data, save it for input to future vets.
+	if f, err := os.Open(vcfg.VetxOutput); err == nil {
+		a.built = vcfg.VetxOutput
+		cache.Default().Put(key, f)
+		f.Close()
+	}
+
+	return runErr
+}
+
+// linkActionID computes the action ID for a link action.
+func (b *Builder) linkActionID(a *Action) cache.ActionID {
+	p := a.Package
+	h := cache.NewHash("link " + p.ImportPath)
+
+	// Toolchain-independent configuration.
+	fmt.Fprintf(h, "link\n")
+	fmt.Fprintf(h, "buildmode %s goos %s goarch %s\n", cfg.BuildBuildmode, cfg.Goos, cfg.Goarch)
+	fmt.Fprintf(h, "import %q\n", p.ImportPath)
+	fmt.Fprintf(h, "omitdebug %v standard %v local %v prefix %q\n", p.Internal.OmitDebug, p.Standard, p.Internal.Local, p.Internal.LocalPrefix)
+	if cfg.BuildTrimpath {
+		fmt.Fprintln(h, "trimpath")
+	}
+
+	// Toolchain-dependent configuration, shared with b.linkSharedActionID.
+	b.printLinkerConfig(h, p)
+
+	// Input files.
+	for _, a1 := range a.Deps {
+		p1 := a1.Package
+		if p1 != nil {
+			if a1.built != "" || a1.buildID != "" {
+				buildID := a1.buildID
+				if buildID == "" {
+					buildID = b.buildID(a1.built)
+				}
+				fmt.Fprintf(h, "packagefile %s=%s\n", p1.ImportPath, contentID(buildID))
+			}
+			// Because we put package main's full action ID into the binary's build ID,
+			// we must also put the full action ID into the binary's action ID hash.
+			if p1.Name == "main" {
+				fmt.Fprintf(h, "packagemain %s\n", a1.buildID)
+			}
+			if p1.Shlib != "" {
+				fmt.Fprintf(h, "packageshlib %s=%s\n", p1.ImportPath, contentID(b.buildID(p1.Shlib)))
+			}
+		}
+	}
+
+	return h.Sum()
+}
+
+// printLinkerConfig prints the linker config into the hash h,
+// as part of the computation of a linker-related action ID.
+func (b *Builder) printLinkerConfig(h io.Writer, p *load.Package) {
+	switch cfg.BuildToolchainName {
+	default:
+		base.Fatalf("linkActionID: unknown toolchain %q", cfg.BuildToolchainName)
+
+	case "gc":
+		fmt.Fprintf(h, "link %s %q %s\n", b.toolID("link"), forcedLdflags, ldBuildmode)
+		if p != nil {
+			fmt.Fprintf(h, "linkflags %q\n", p.Internal.Ldflags)
+		}
+
+		// GOARM, GOMIPS, etc.
+		key, val := cfg.GetArchEnv()
+		fmt.Fprintf(h, "%s=%s\n", key, val)
+
+		if cfg.CleanGOEXPERIMENT != "" {
+			fmt.Fprintf(h, "GOEXPERIMENT=%q\n", cfg.CleanGOEXPERIMENT)
+		}
+
+		// The linker writes source file paths that say GOROOT_FINAL, but
+		// only if -trimpath is not specified (see ld() in gc.go).
+		gorootFinal := cfg.GOROOT_FINAL
+		if cfg.BuildTrimpath {
+			gorootFinal = trimPathGoRootFinal
+		}
+		fmt.Fprintf(h, "GOROOT=%s\n", gorootFinal)
+
+		// GO_EXTLINK_ENABLED controls whether the external linker is used.
+		fmt.Fprintf(h, "GO_EXTLINK_ENABLED=%s\n", cfg.Getenv("GO_EXTLINK_ENABLED"))
+
+		// TODO(rsc): Do cgo settings and flags need to be included?
+		// Or external linker settings and flags?
+
+	case "gccgo":
+		id, _, err := b.gccToolID(BuildToolchain.linker(), "go")
+		if err != nil {
+			base.Fatalf("%v", err)
+		}
+		fmt.Fprintf(h, "link %s %s\n", id, ldBuildmode)
+		// TODO(iant): Should probably include cgo flags here.
+	}
+}
+
+// link is the action for linking a single command.
+// Note that any new influence on this logic must be reported in b.linkActionID above as well.
+func (b *Builder) link(ctx context.Context, a *Action) (err error) {
+	if b.useCache(a, b.linkActionID(a), a.Package.Target, !b.IsCmdList) || b.IsCmdList {
+		return nil
+	}
+	defer b.flushOutput(a)
+
+	if err := b.Mkdir(a.Objdir); err != nil {
+		return err
+	}
+
+	importcfg := a.Objdir + "importcfg.link"
+	if err := b.writeLinkImportcfg(a, importcfg); err != nil {
+		return err
+	}
+
+	if err := AllowInstall(a); err != nil {
+		return err
+	}
+
+	// make target directory
+	dir, _ := filepath.Split(a.Target)
+	if dir != "" {
+		if err := b.Mkdir(dir); err != nil {
+			return err
+		}
+	}
+
+	if err := BuildToolchain.ld(b, a, a.Target, importcfg, a.Deps[0].built); err != nil {
+		return err
+	}
+
+	// Update the binary with the final build ID.
+	// But if OmitDebug is set, don't rewrite the binary, because we set OmitDebug
+	// on binaries that we are going to run and then delete.
+	// There's no point in doing work on such a binary.
+	// Worse, opening the binary for write here makes it
+	// essentially impossible to safely fork+exec due to a fundamental
+	// incompatibility between ETXTBSY and threads on modern Unix systems.
+	// See golang.org/issue/22220.
+	// We still call updateBuildID to update a.buildID, which is important
+	// for test result caching, but passing rewrite=false (final arg)
+	// means we don't actually rewrite the binary, nor store the
+	// result into the cache. That's probably a net win:
+	// less cache space wasted on large binaries we are not likely to
+	// need again. (On the other hand it does make repeated go test slower.)
+	// It also makes repeated go run slower, which is a win in itself:
+	// we don't want people to treat go run like a scripting environment.
+	if err := b.updateBuildID(a, a.Target, !a.Package.Internal.OmitDebug); err != nil {
+		return err
+	}
+
+	a.built = a.Target
+	return nil
+}
+
+func (b *Builder) writeLinkImportcfg(a *Action, file string) error {
+	// Prepare Go import cfg.
+	var icfg bytes.Buffer
+	for _, a1 := range a.Deps {
+		p1 := a1.Package
+		if p1 == nil {
+			continue
+		}
+		fmt.Fprintf(&icfg, "packagefile %s=%s\n", p1.ImportPath, a1.built)
+		if p1.Shlib != "" {
+			fmt.Fprintf(&icfg, "packageshlib %s=%s\n", p1.ImportPath, p1.Shlib)
+		}
+	}
+	info := ""
+	if a.Package.Internal.BuildInfo != nil {
+		info = a.Package.Internal.BuildInfo.String()
+	}
+	fmt.Fprintf(&icfg, "modinfo %q\n", modload.ModInfoData(info))
+	return b.writeFile(file, icfg.Bytes())
+}
+
+// PkgconfigCmd returns a pkg-config binary name
+// defaultPkgConfig is defined in zdefaultcc.go, written by cmd/dist.
+func (b *Builder) PkgconfigCmd() string {
+	return envList("PKG_CONFIG", cfg.DefaultPkgConfig)[0]
+}
+
+// splitPkgConfigOutput parses the pkg-config output into a slice of flags.
+// This implements the shell quoting semantics described in
+// https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_02,
+// except that it does not support parameter or arithmetic expansion or command
+// substitution and hard-codes the <blank> delimiters instead of reading them
+// from LC_LOCALE.
+func splitPkgConfigOutput(out []byte) ([]string, error) {
+	if len(out) == 0 {
+		return nil, nil
+	}
+	var flags []string
+	flag := make([]byte, 0, len(out))
+	didQuote := false // was the current flag parsed from a quoted string?
+	escaped := false  // did we just read `\` in a non-single-quoted context?
+	quote := byte(0)  // what is the quote character around the current string?
+
+	for _, c := range out {
+		if escaped {
+			if quote == '"' {
+				// “The <backslash> shall retain its special meaning as an escape
+				// character … only when followed by one of the following characters
+				// when considered special:”
+				switch c {
+				case '$', '`', '"', '\\', '\n':
+					// Handle the escaped character normally.
+				default:
+					// Not an escape character after all.
+					flag = append(flag, '\\', c)
+					escaped = false
+					continue
+				}
+			}
+
+			if c == '\n' {
+				// “If a <newline> follows the <backslash>, the shell shall interpret
+				// this as line continuation.”
+			} else {
+				flag = append(flag, c)
+			}
+			escaped = false
+			continue
+		}
+
+		if quote != 0 && c == quote {
+			quote = 0
+			continue
+		}
+		switch quote {
+		case '\'':
+			// “preserve the literal value of each character”
+			flag = append(flag, c)
+			continue
+		case '"':
+			// “preserve the literal value of all characters within the double-quotes,
+			// with the exception of …”
+			switch c {
+			case '`', '$', '\\':
+			default:
+				flag = append(flag, c)
+				continue
+			}
+		}
+
+		// “The application shall quote the following characters if they are to
+		// represent themselves:”
+		switch c {
+		case '|', '&', ';', '<', '>', '(', ')', '$', '`':
+			return nil, fmt.Errorf("unexpected shell character %q in pkgconf output", c)
+
+		case '\\':
+			// “A <backslash> that is not quoted shall preserve the literal value of
+			// the following character, with the exception of a <newline>.”
+			escaped = true
+			continue
+
+		case '"', '\'':
+			quote = c
+			didQuote = true
+			continue
+
+		case ' ', '\t', '\n':
+			if len(flag) > 0 || didQuote {
+				flags = append(flags, string(flag))
+			}
+			flag, didQuote = flag[:0], false
+			continue
+		}
+
+		flag = append(flag, c)
+	}
+
+	// Prefer to report a missing quote instead of a missing escape. If the string
+	// is something like `"foo\`, it's ambiguous as to whether the trailing
+	// backslash is really an escape at all.
+	if quote != 0 {
+		return nil, errors.New("unterminated quoted string in pkgconf output")
+	}
+	if escaped {
+		return nil, errors.New("broken character escaping in pkgconf output")
+	}
+
+	if len(flag) > 0 || didQuote {
+		flags = append(flags, string(flag))
+	}
+	return flags, nil
+}
+
+// Calls pkg-config if needed and returns the cflags/ldflags needed to build the package.
+func (b *Builder) getPkgConfigFlags(p *load.Package) (cflags, ldflags []string, err error) {
+	if pcargs := p.CgoPkgConfig; len(pcargs) > 0 {
+		// pkg-config permits arguments to appear anywhere in
+		// the command line. Move them all to the front, before --.
+		var pcflags []string
+		var pkgs []string
+		for _, pcarg := range pcargs {
+			if pcarg == "--" {
+				// We're going to add our own "--" argument.
+			} else if strings.HasPrefix(pcarg, "--") {
+				pcflags = append(pcflags, pcarg)
+			} else {
+				pkgs = append(pkgs, pcarg)
+			}
+		}
+		for _, pkg := range pkgs {
+			if !load.SafeArg(pkg) {
+				return nil, nil, fmt.Errorf("invalid pkg-config package name: %s", pkg)
+			}
+		}
+		var out []byte
+		out, err = b.runOut(nil, p.Dir, nil, b.PkgconfigCmd(), "--cflags", pcflags, "--", pkgs)
+		if err != nil {
+			err = formatOutput(b.WorkDir, p.Dir, p.ImportPath, b.PkgconfigCmd()+" --cflags "+strings.Join(pcflags, " ")+" -- "+strings.Join(pkgs, " "), string(out)+err.Error())
+			return nil, nil, err
+		}
+		if len(out) > 0 {
+			cflags, err = splitPkgConfigOutput(bytes.TrimSpace(out))
+			if err != nil {
+				return nil, nil, err
+			}
+			if err := checkCompilerFlags("CFLAGS", "pkg-config --cflags", cflags); err != nil {
+				return nil, nil, err
+			}
+		}
+		out, err = b.runOut(nil, p.Dir, nil, b.PkgconfigCmd(), "--libs", pcflags, "--", pkgs)
+		if err != nil {
+			err = formatOutput(b.WorkDir, p.Dir, p.ImportPath, b.PkgconfigCmd()+" --libs "+strings.Join(pcflags, " ")+" -- "+strings.Join(pkgs, " "), string(out)+err.Error())
+			return nil, nil, err
+		}
+		if len(out) > 0 {
+			// We need to handle path with spaces so that C:/Program\ Files can pass
+			// checkLinkerFlags. Use splitPkgConfigOutput here just like we treat cflags.
+			ldflags, err = splitPkgConfigOutput(bytes.TrimSpace(out))
+			if err != nil {
+				return nil, nil, err
+			}
+			if err := checkLinkerFlags("LDFLAGS", "pkg-config --libs", ldflags); err != nil {
+				return nil, nil, err
+			}
+		}
+	}
+
+	return
+}
+
+func (b *Builder) installShlibname(ctx context.Context, a *Action) error {
+	if err := AllowInstall(a); err != nil {
+		return err
+	}
+
+	// TODO: BuildN
+	a1 := a.Deps[0]
+	if err := b.Mkdir(filepath.Dir(a.Target)); err != nil {
+		return err
+	}
+	err := os.WriteFile(a.Target, []byte(filepath.Base(a1.Target)+"\n"), 0666)
+	if err != nil {
+		return err
+	}
+	if cfg.BuildX {
+		b.Showcmd("", "echo '%s' > %s # internal", filepath.Base(a1.Target), a.Target)
+	}
+	return nil
+}
+
+func (b *Builder) linkSharedActionID(a *Action) cache.ActionID {
+	h := cache.NewHash("linkShared")
+
+	// Toolchain-independent configuration.
+	fmt.Fprintf(h, "linkShared\n")
+	fmt.Fprintf(h, "goos %s goarch %s\n", cfg.Goos, cfg.Goarch)
+
+	// Toolchain-dependent configuration, shared with b.linkActionID.
+	b.printLinkerConfig(h, nil)
+
+	// Input files.
+	for _, a1 := range a.Deps {
+		p1 := a1.Package
+		if a1.built == "" {
+			continue
+		}
+		if p1 != nil {
+			fmt.Fprintf(h, "packagefile %s=%s\n", p1.ImportPath, contentID(b.buildID(a1.built)))
+			if p1.Shlib != "" {
+				fmt.Fprintf(h, "packageshlib %s=%s\n", p1.ImportPath, contentID(b.buildID(p1.Shlib)))
+			}
+		}
+	}
+	// Files named on command line are special.
+	for _, a1 := range a.Deps[0].Deps {
+		p1 := a1.Package
+		fmt.Fprintf(h, "top %s=%s\n", p1.ImportPath, contentID(b.buildID(a1.built)))
+	}
+
+	return h.Sum()
+}
+
+func (b *Builder) linkShared(ctx context.Context, a *Action) (err error) {
+	if b.useCache(a, b.linkSharedActionID(a), a.Target, !b.IsCmdList) || b.IsCmdList {
+		return nil
+	}
+	defer b.flushOutput(a)
+
+	if err := AllowInstall(a); err != nil {
+		return err
+	}
+
+	if err := b.Mkdir(a.Objdir); err != nil {
+		return err
+	}
+
+	importcfg := a.Objdir + "importcfg.link"
+	if err := b.writeLinkImportcfg(a, importcfg); err != nil {
+		return err
+	}
+
+	// TODO(rsc): There is a missing updateBuildID here,
+	// but we have to decide where to store the build ID in these files.
+	a.built = a.Target
+	return BuildToolchain.ldShared(b, a, a.Deps[0].Deps, a.Target, importcfg, a.Deps)
+}
+
+// BuildInstallFunc is the action for installing a single package or executable.
+func BuildInstallFunc(b *Builder, ctx context.Context, a *Action) (err error) {
+	defer func() {
+		if err != nil {
+			// a.Package == nil is possible for the go install -buildmode=shared
+			// action that installs libmangledname.so, which corresponds to
+			// a list of packages, not just one.
+			sep, path := "", ""
+			if a.Package != nil {
+				sep, path = " ", a.Package.ImportPath
+			}
+			err = fmt.Errorf("go %s%s%s: %v", cfg.CmdName, sep, path, err)
+		}
+	}()
+
+	a1 := a.Deps[0]
+	a.buildID = a1.buildID
+	if a.json != nil {
+		a.json.BuildID = a.buildID
+	}
+
+	// If we are using the eventual install target as an up-to-date
+	// cached copy of the thing we built, then there's no need to
+	// copy it into itself (and that would probably fail anyway).
+	// In this case a1.built == a.Target because a1.built == p.Target,
+	// so the built target is not in the a1.Objdir tree that b.cleanup(a1) removes.
+	if a1.built == a.Target {
+		a.built = a.Target
+		if !a.buggyInstall {
+			b.cleanup(a1)
+		}
+		// Whether we're smart enough to avoid a complete rebuild
+		// depends on exactly what the staleness and rebuild algorithms
+		// are, as well as potentially the state of the Go build cache.
+		// We don't really want users to be able to infer (or worse start depending on)
+		// those details from whether the modification time changes during
+		// "go install", so do a best-effort update of the file times to make it
+		// look like we rewrote a.Target even if we did not. Updating the mtime
+		// may also help other mtime-based systems that depend on our
+		// previous mtime updates that happened more often.
+		// This is still not perfect - we ignore the error result, and if the file was
+		// unwritable for some reason then pretending to have written it is also
+		// confusing - but it's probably better than not doing the mtime update.
+		//
+		// But don't do that for the special case where building an executable
+		// with -linkshared implicitly installs all its dependent libraries.
+		// We want to hide that awful detail as much as possible, so don't
+		// advertise it by touching the mtimes (usually the libraries are up
+		// to date).
+		if !a.buggyInstall && !b.IsCmdList {
+			if cfg.BuildN {
+				b.Showcmd("", "touch %s", a.Target)
+			} else if err := AllowInstall(a); err == nil {
+				now := time.Now()
+				os.Chtimes(a.Target, now, now)
+			}
+		}
+		return nil
+	}
+
+	// If we're building for go list -export,
+	// never install anything; just keep the cache reference.
+	if b.IsCmdList {
+		a.built = a1.built
+		return nil
+	}
+	if err := AllowInstall(a); err != nil {
+		return err
+	}
+
+	if err := b.Mkdir(a.Objdir); err != nil {
+		return err
+	}
+
+	perm := fs.FileMode(0666)
+	if a1.Mode == "link" {
+		switch cfg.BuildBuildmode {
+		case "c-archive", "c-shared", "plugin":
+		default:
+			perm = 0777
+		}
+	}
+
+	// make target directory
+	dir, _ := filepath.Split(a.Target)
+	if dir != "" {
+		if err := b.Mkdir(dir); err != nil {
+			return err
+		}
+	}
+
+	if !a.buggyInstall {
+		defer b.cleanup(a1)
+	}
+
+	return b.moveOrCopyFile(a.Target, a1.built, perm, false)
+}
+
+// AllowInstall returns a non-nil error if this invocation of the go command is
+// allowed to install a.Target.
+//
+// The build of cmd/go running under its own test is forbidden from installing
+// to its original GOROOT. The var is exported so it can be set by TestMain.
+var AllowInstall = func(*Action) error { return nil }
+
+// cleanup removes a's object dir to keep the amount of
+// on-disk garbage down in a large build. On an operating system
+// with aggressive buffering, cleaning incrementally like
+// this keeps the intermediate objects from hitting the disk.
+func (b *Builder) cleanup(a *Action) {
+	if !cfg.BuildWork {
+		if cfg.BuildX {
+			// Don't say we are removing the directory if
+			// we never created it.
+			if _, err := os.Stat(a.Objdir); err == nil || cfg.BuildN {
+				b.Showcmd("", "rm -r %s", a.Objdir)
+			}
+		}
+		os.RemoveAll(a.Objdir)
+	}
+}
+
+// moveOrCopyFile is like 'mv src dst' or 'cp src dst'.
+func (b *Builder) moveOrCopyFile(dst, src string, perm fs.FileMode, force bool) error {
+	if cfg.BuildN {
+		b.Showcmd("", "mv %s %s", src, dst)
+		return nil
+	}
+
+	// If we can update the mode and rename to the dst, do it.
+	// Otherwise fall back to standard copy.
+
+	// If the source is in the build cache, we need to copy it.
+	if strings.HasPrefix(src, cache.DefaultDir()) {
+		return b.copyFile(dst, src, perm, force)
+	}
+
+	// On Windows, always copy the file, so that we respect the NTFS
+	// permissions of the parent folder. https://golang.org/issue/22343.
+	// What matters here is not cfg.Goos (the system we are building
+	// for) but runtime.GOOS (the system we are building on).
+	if runtime.GOOS == "windows" {
+		return b.copyFile(dst, src, perm, force)
+	}
+
+	// If the destination directory has the group sticky bit set,
+	// we have to copy the file to retain the correct permissions.
+	// https://golang.org/issue/18878
+	if fi, err := os.Stat(filepath.Dir(dst)); err == nil {
+		if fi.IsDir() && (fi.Mode()&fs.ModeSetgid) != 0 {
+			return b.copyFile(dst, src, perm, force)
+		}
+	}
+
+	// The perm argument is meant to be adjusted according to umask,
+	// but we don't know what the umask is.
+	// Create a dummy file to find out.
+	// This avoids build tags and works even on systems like Plan 9
+	// where the file mask computation incorporates other information.
+	mode := perm
+	f, err := os.OpenFile(filepath.Clean(dst)+"-go-tmp-umask", os.O_WRONLY|os.O_CREATE|os.O_EXCL, perm)
+	if err == nil {
+		fi, err := f.Stat()
+		if err == nil {
+			mode = fi.Mode() & 0777
+		}
+		name := f.Name()
+		f.Close()
+		os.Remove(name)
+	}
+
+	if err := os.Chmod(src, mode); err == nil {
+		if err := os.Rename(src, dst); err == nil {
+			if cfg.BuildX {
+				b.Showcmd("", "mv %s %s", src, dst)
+			}
+			return nil
+		}
+	}
+
+	return b.copyFile(dst, src, perm, force)
+}
+
+// copyFile is like 'cp src dst'.
+func (b *Builder) copyFile(dst, src string, perm fs.FileMode, force bool) error {
+	if cfg.BuildN || cfg.BuildX {
+		b.Showcmd("", "cp %s %s", src, dst)
+		if cfg.BuildN {
+			return nil
+		}
+	}
+
+	sf, err := os.Open(src)
+	if err != nil {
+		return err
+	}
+	defer sf.Close()
+
+	// Be careful about removing/overwriting dst.
+	// Do not remove/overwrite if dst exists and is a directory
+	// or a non-empty non-object file.
+	if fi, err := os.Stat(dst); err == nil {
+		if fi.IsDir() {
+			return fmt.Errorf("build output %q already exists and is a directory", dst)
+		}
+		if !force && fi.Mode().IsRegular() && fi.Size() != 0 && !isObject(dst) {
+			return fmt.Errorf("build output %q already exists and is not an object file", dst)
+		}
+	}
+
+	// On Windows, remove lingering ~ file from last attempt.
+	if runtime.GOOS == "windows" {
+		if _, err := os.Stat(dst + "~"); err == nil {
+			os.Remove(dst + "~")
+		}
+	}
+
+	mayberemovefile(dst)
+	df, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
+	if err != nil && runtime.GOOS == "windows" {
+		// Windows does not allow deletion of a binary file
+		// while it is executing. Try to move it out of the way.
+		// If the move fails, which is likely, we'll try again the
+		// next time we do an install of this binary.
+		if err := os.Rename(dst, dst+"~"); err == nil {
+			os.Remove(dst + "~")
+		}
+		df, err = os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
+	}
+	if err != nil {
+		return fmt.Errorf("copying %s: %w", src, err) // err should already refer to dst
+	}
+
+	_, err = io.Copy(df, sf)
+	df.Close()
+	if err != nil {
+		mayberemovefile(dst)
+		return fmt.Errorf("copying %s to %s: %v", src, dst, err)
+	}
+	return nil
+}
+
+// writeFile writes the text to file.
+func (b *Builder) writeFile(file string, text []byte) error {
+	if cfg.BuildN || cfg.BuildX {
+		b.Showcmd("", "cat >%s << 'EOF' # internal\n%sEOF", file, text)
+	}
+	if cfg.BuildN {
+		return nil
+	}
+	return os.WriteFile(file, text, 0666)
+}
+
+// Install the cgo export header file, if there is one.
+func (b *Builder) installHeader(ctx context.Context, a *Action) error {
+	src := a.Objdir + "_cgo_install.h"
+	if _, err := os.Stat(src); os.IsNotExist(err) {
+		// If the file does not exist, there are no exported
+		// functions, and we do not install anything.
+		// TODO(rsc): Once we know that caching is rebuilding
+		// at the right times (not missing rebuilds), here we should
+		// probably delete the installed header, if any.
+		if cfg.BuildX {
+			b.Showcmd("", "# %s not created", src)
+		}
+		return nil
+	}
+
+	if err := AllowInstall(a); err != nil {
+		return err
+	}
+
+	dir, _ := filepath.Split(a.Target)
+	if dir != "" {
+		if err := b.Mkdir(dir); err != nil {
+			return err
+		}
+	}
+
+	return b.moveOrCopyFile(a.Target, src, 0666, true)
+}
+
+// cover runs, in effect,
+//
+//	go tool cover -mode=b.coverMode -var="varName" -o dst.go src.go
+func (b *Builder) cover(a *Action, dst, src string, varName string) error {
+	return b.run(a, a.Objdir, "cover "+a.Package.ImportPath, nil,
+		cfg.BuildToolexec,
+		base.Tool("cover"),
+		"-mode", a.Package.Internal.CoverMode,
+		"-var", varName,
+		"-o", dst,
+		src)
+}
+
+// cover2 runs, in effect,
+//
+//	go tool cover -pkgcfg=<config file> -mode=b.coverMode -var="varName" -o <outfiles> <infiles>
+//
+// Return value is an updated output files list; in addition to the
+// regular outputs (instrumented source files) the cover tool also
+// writes a separate file (appearing first in the list of outputs)
+// that will contain coverage counters and meta-data.
+func (b *Builder) cover2(a *Action, infiles, outfiles []string, varName string, mode string) ([]string, error) {
+	pkgcfg := a.Objdir + "pkgcfg.txt"
+	covoutputs := a.Objdir + "coveroutfiles.txt"
+	odir := filepath.Dir(outfiles[0])
+	cv := filepath.Join(odir, "covervars.go")
+	outfiles = append([]string{cv}, outfiles...)
+	if err := b.writeCoverPkgInputs(a, pkgcfg, covoutputs, outfiles); err != nil {
+		return nil, err
+	}
+	args := []string{base.Tool("cover"),
+		"-pkgcfg", pkgcfg,
+		"-mode", mode,
+		"-var", varName,
+		"-outfilelist", covoutputs,
+	}
+	args = append(args, infiles...)
+	if err := b.run(a, a.Objdir, "cover "+a.Package.ImportPath, nil,
+		cfg.BuildToolexec, args); err != nil {
+		return nil, err
+	}
+	return outfiles, nil
+}
+
+func (b *Builder) writeCoverPkgInputs(a *Action, pconfigfile string, covoutputsfile string, outfiles []string) error {
+	p := a.Package
+	p.Internal.CoverageCfg = a.Objdir + "coveragecfg"
+	pcfg := coverage.CoverPkgConfig{
+		PkgPath: p.ImportPath,
+		PkgName: p.Name,
+		// Note: coverage granularity is currently hard-wired to
+		// 'perblock'; there isn't a way using "go build -cover" or "go
+		// test -cover" to select it. This may change in the future
+		// depending on user demand.
+		Granularity: "perblock",
+		OutConfig:   p.Internal.CoverageCfg,
+		Local:       p.Internal.Local,
+	}
+	if a.Package.Module != nil {
+		pcfg.ModulePath = a.Package.Module.Path
+	}
+	data, err := json.Marshal(pcfg)
+	if err != nil {
+		return err
+	}
+	if err := b.writeFile(pconfigfile, data); err != nil {
+		return err
+	}
+	var sb strings.Builder
+	for i := range outfiles {
+		fmt.Fprintf(&sb, "%s\n", outfiles[i])
+	}
+	return b.writeFile(covoutputsfile, []byte(sb.String()))
+}
+
+var objectMagic = [][]byte{
+	{'!', '<', 'a', 'r', 'c', 'h', '>', '\n'}, // Package archive
+	{'<', 'b', 'i', 'g', 'a', 'f', '>', '\n'}, // Package AIX big archive
+	{'\x7F', 'E', 'L', 'F'},                   // ELF
+	{0xFE, 0xED, 0xFA, 0xCE},                  // Mach-O big-endian 32-bit
+	{0xFE, 0xED, 0xFA, 0xCF},                  // Mach-O big-endian 64-bit
+	{0xCE, 0xFA, 0xED, 0xFE},                  // Mach-O little-endian 32-bit
+	{0xCF, 0xFA, 0xED, 0xFE},                  // Mach-O little-endian 64-bit
+	{0x4d, 0x5a, 0x90, 0x00, 0x03, 0x00},      // PE (Windows) as generated by 6l/8l and gcc
+	{0x4d, 0x5a, 0x78, 0x00, 0x01, 0x00},      // PE (Windows) as generated by llvm for dll
+	{0x00, 0x00, 0x01, 0xEB},                  // Plan 9 i386
+	{0x00, 0x00, 0x8a, 0x97},                  // Plan 9 amd64
+	{0x00, 0x00, 0x06, 0x47},                  // Plan 9 arm
+	{0x00, 0x61, 0x73, 0x6D},                  // WASM
+	{0x01, 0xDF},                              // XCOFF 32bit
+	{0x01, 0xF7},                              // XCOFF 64bit
+}
+
+func isObject(s string) bool {
+	f, err := os.Open(s)
+	if err != nil {
+		return false
+	}
+	defer f.Close()
+	buf := make([]byte, 64)
+	io.ReadFull(f, buf)
+	for _, magic := range objectMagic {
+		if bytes.HasPrefix(buf, magic) {
+			return true
+		}
+	}
+	return false
+}
+
+// mayberemovefile removes a file only if it is a regular file
+// When running as a user with sufficient privileges, we may delete
+// even device files, for example, which is not intended.
+func mayberemovefile(s string) {
+	if fi, err := os.Lstat(s); err == nil && !fi.Mode().IsRegular() {
+		return
+	}
+	os.Remove(s)
+}
+
+// fmtcmd formats a command in the manner of fmt.Sprintf but also:
+//
+//	If dir is non-empty and the script is not in dir right now,
+//	fmtcmd inserts "cd dir\n" before the command.
+//
+//	fmtcmd replaces the value of b.WorkDir with $WORK.
+//	fmtcmd replaces the value of goroot with $GOROOT.
+//	fmtcmd replaces the value of b.gobin with $GOBIN.
+//
+//	fmtcmd replaces the name of the current directory with dot (.)
+//	but only when it is at the beginning of a space-separated token.
+func (b *Builder) fmtcmd(dir string, format string, args ...any) string {
+	cmd := fmt.Sprintf(format, args...)
+	if dir != "" && dir != "/" {
+		dot := " ."
+		if dir[len(dir)-1] == filepath.Separator {
+			dot += string(filepath.Separator)
+		}
+		cmd = strings.ReplaceAll(" "+cmd, " "+dir, dot)[1:]
+		if b.scriptDir != dir {
+			b.scriptDir = dir
+			cmd = "cd " + dir + "\n" + cmd
+		}
+	}
+	if b.WorkDir != "" && !strings.HasPrefix(cmd, "cat ") {
+		cmd = strings.ReplaceAll(cmd, b.WorkDir, "$WORK")
+		escaped := strconv.Quote(b.WorkDir)
+		escaped = escaped[1 : len(escaped)-1] // strip quote characters
+		if escaped != b.WorkDir {
+			cmd = strings.ReplaceAll(cmd, escaped, "$WORK")
+		}
+	}
+	return cmd
+}
+
+// Showcmd prints the given command to standard output
+// for the implementation of -n or -x.
+func (b *Builder) Showcmd(dir string, format string, args ...any) {
+	b.output.Lock()
+	defer b.output.Unlock()
+	b.Print(b.fmtcmd(dir, format, args...) + "\n")
+}
+
+// showOutput prints "# desc" followed by the given output.
+// The output is expected to contain references to 'dir', usually
+// the source directory for the package that has failed to build.
+// showOutput rewrites mentions of dir with a relative path to dir
+// when the relative path is shorter. This is usually more pleasant.
+// For example, if fmt doesn't compile and we are in src/html,
+// the output is
+//
+//	$ go build
+//	# fmt
+//	../fmt/print.go:1090: undefined: asdf
+//	$
+//
+// instead of
+//
+//	$ go build
+//	# fmt
+//	/usr/gopher/go/src/fmt/print.go:1090: undefined: asdf
+//	$
+//
+// showOutput also replaces references to the work directory with $WORK.
+//
+// If a is not nil and a.output is not nil, showOutput appends to that slice instead of
+// printing to b.Print.
+func (b *Builder) showOutput(a *Action, dir, desc, out string) {
+	importPath := ""
+	if a != nil && a.Package != nil {
+		importPath = a.Package.ImportPath
+	}
+	psErr := formatOutput(b.WorkDir, dir, importPath, desc, out)
+	if a != nil && a.output != nil {
+		a.output = append(a.output, psErr.prefix...)
+		a.output = append(a.output, psErr.suffix...)
+		return
+	}
+
+	b.output.Lock()
+	defer b.output.Unlock()
+	b.Print(psErr.prefix, psErr.suffix)
+}
+
+// A prefixSuffixError is an error formatted by formatOutput.
+type prefixSuffixError struct {
+	importPath     string
+	prefix, suffix string
+}
+
+func (e *prefixSuffixError) Error() string {
+	if e.importPath != "" && !strings.HasPrefix(strings.TrimPrefix(e.prefix, "# "), e.importPath) {
+		return fmt.Sprintf("go build %s:\n%s%s", e.importPath, e.prefix, e.suffix)
+	}
+	return e.prefix + e.suffix
+}
+
+func (e *prefixSuffixError) ImportPath() string {
+	return e.importPath
+}
+
+// formatOutput prints "# desc" followed by the given output.
+// The output is expected to contain references to 'dir', usually
+// the source directory for the package that has failed to build.
+// formatOutput rewrites mentions of dir with a relative path to dir
+// when the relative path is shorter. This is usually more pleasant.
+// For example, if fmt doesn't compile and we are in src/html,
+// the output is
+//
+//	$ go build
+//	# fmt
+//	../fmt/print.go:1090: undefined: asdf
+//	$
+//
+// instead of
+//
+//	$ go build
+//	# fmt
+//	/usr/gopher/go/src/fmt/print.go:1090: undefined: asdf
+//	$
+//
+// formatOutput also replaces references to the work directory with $WORK.
+// formatOutput returns the output in a prefix with the description and a
+// suffix with the actual output.
+func formatOutput(workDir, dir, importPath, desc, out string) *prefixSuffixError {
+	prefix := "# " + desc
+	suffix := "\n" + out
+
+	suffix = strings.ReplaceAll(suffix, " "+workDir, " $WORK")
+
+	for {
+		// Note that dir starts out long, something like
+		// /foo/bar/baz/root/a
+		// The target string to be reduced is something like
+		// (blah-blah-blah) /foo/bar/baz/root/sibling/whatever.go:blah:blah
+		// /foo/bar/baz/root/a doesn't match /foo/bar/baz/root/sibling, but the prefix
+		// /foo/bar/baz/root does.  And there may be other niblings sharing shorter
+		// prefixes, the only way to find them is to look.
+		// This doesn't always produce a relative path --
+		// /foo is shorter than ../../.., for example.
+		//
+		if reldir := base.ShortPath(dir); reldir != dir {
+			suffix = strings.ReplaceAll(suffix, " "+dir, " "+reldir)
+			suffix = strings.ReplaceAll(suffix, "\n"+dir, "\n"+reldir)
+			suffix = strings.ReplaceAll(suffix, "\n\t"+dir, "\n\t"+reldir)
+			if filepath.Separator == '\\' {
+				// Don't know why, sometimes this comes out with slashes, not backslashes.
+				wdir := strings.ReplaceAll(dir, "\\", "/")
+				suffix = strings.ReplaceAll(suffix, " "+wdir, " "+reldir)
+				suffix = strings.ReplaceAll(suffix, "\n"+wdir, "\n"+reldir)
+				suffix = strings.ReplaceAll(suffix, "\n\t"+wdir, "\n\t"+reldir)
+			}
+		}
+		dirP := filepath.Dir(dir)
+		if dir == dirP {
+			break
+		}
+		dir = dirP
+	}
+
+	return &prefixSuffixError{importPath: importPath, prefix: prefix, suffix: suffix}
+}
+
+var cgoLine = lazyregexp.New(`\[[^\[\]]+\.(cgo1|cover)\.go:[0-9]+(:[0-9]+)?\]`)
+var cgoTypeSigRe = lazyregexp.New(`\b_C2?(type|func|var|macro)_\B`)
+
+// run runs the command given by cmdline in the directory dir.
+// If the command fails, run prints information about the failure
+// and returns a non-nil error.
+func (b *Builder) run(a *Action, dir string, desc string, env []string, cmdargs ...any) error {
+	out, err := b.runOut(a, dir, env, cmdargs...)
+	if len(out) > 0 {
+		if desc == "" {
+			desc = b.fmtcmd(dir, "%s", strings.Join(str.StringList(cmdargs...), " "))
+		}
+		if err != nil {
+			err = formatOutput(b.WorkDir, dir, a.Package.ImportPath, desc, b.processOutput(out))
+		} else {
+			b.showOutput(a, dir, desc, b.processOutput(out))
+		}
+	}
+	return err
+}
+
+// processOutput prepares the output of runOut to be output to the console.
+func (b *Builder) processOutput(out []byte) string {
+	if out[len(out)-1] != '\n' {
+		out = append(out, '\n')
+	}
+	messages := string(out)
+	// Fix up output referring to cgo-generated code to be more readable.
+	// Replace x.go:19[/tmp/.../x.cgo1.go:18] with x.go:19.
+	// Replace *[100]_Ctype_foo with *[100]C.foo.
+	// If we're using -x, assume we're debugging and want the full dump, so disable the rewrite.
+	if !cfg.BuildX && cgoLine.MatchString(messages) {
+		messages = cgoLine.ReplaceAllString(messages, "")
+		messages = cgoTypeSigRe.ReplaceAllString(messages, "C.")
+	}
+	return messages
+}
+
+// runOut runs the command given by cmdline in the directory dir.
+// It returns the command output and any errors that occurred.
+// It accumulates execution time in a.
+func (b *Builder) runOut(a *Action, dir string, env []string, cmdargs ...any) ([]byte, error) {
+	cmdline := str.StringList(cmdargs...)
+
+	for _, arg := range cmdline {
+		// GNU binutils commands, including gcc and gccgo, interpret an argument
+		// @foo anywhere in the command line (even following --) as meaning
+		// "read and insert arguments from the file named foo."
+		// Don't say anything that might be misinterpreted that way.
+		if strings.HasPrefix(arg, "@") {
+			return nil, fmt.Errorf("invalid command-line argument %s in command: %s", arg, joinUnambiguously(cmdline))
+		}
+	}
+
+	if cfg.BuildN || cfg.BuildX {
+		var envcmdline string
+		for _, e := range env {
+			if j := strings.IndexByte(e, '='); j != -1 {
+				if strings.ContainsRune(e[j+1:], '\'') {
+					envcmdline += fmt.Sprintf("%s=%q", e[:j], e[j+1:])
+				} else {
+					envcmdline += fmt.Sprintf("%s='%s'", e[:j], e[j+1:])
+				}
+				envcmdline += " "
+			}
+		}
+		envcmdline += joinUnambiguously(cmdline)
+		b.Showcmd(dir, "%s", envcmdline)
+		if cfg.BuildN {
+			return nil, nil
+		}
+	}
+
+	var buf bytes.Buffer
+	cmd := exec.Command(cmdline[0], cmdline[1:]...)
+	if cmd.Path != "" {
+		cmd.Args[0] = cmd.Path
+	}
+	cmd.Stdout = &buf
+	cmd.Stderr = &buf
+	cleanup := passLongArgsInResponseFiles(cmd)
+	defer cleanup()
+	if dir != "." {
+		cmd.Dir = dir
+	}
+	cmd.Env = cmd.Environ() // Pre-allocate with correct PWD.
+
+	// Add the TOOLEXEC_IMPORTPATH environment variable for -toolexec tools.
+	// It doesn't really matter if -toolexec isn't being used.
+	// Note that a.Package.Desc is not really an import path,
+	// but this is consistent with 'go list -f {{.ImportPath}}'.
+	// Plus, it is useful to uniquely identify packages in 'go list -json'.
+	if a != nil && a.Package != nil {
+		cmd.Env = append(cmd.Env, "TOOLEXEC_IMPORTPATH="+a.Package.Desc())
+	}
+
+	cmd.Env = append(cmd.Env, env...)
+	start := time.Now()
+	err := cmd.Run()
+	if a != nil && a.json != nil {
+		aj := a.json
+		aj.Cmd = append(aj.Cmd, joinUnambiguously(cmdline))
+		aj.CmdReal += time.Since(start)
+		if ps := cmd.ProcessState; ps != nil {
+			aj.CmdUser += ps.UserTime()
+			aj.CmdSys += ps.SystemTime()
+		}
+	}
+
+	// err can be something like 'exit status 1'.
+	// Add information about what program was running.
+	// Note that if buf.Bytes() is non-empty, the caller usually
+	// shows buf.Bytes() and does not print err at all, so the
+	// prefix here does not make most output any more verbose.
+	if err != nil {
+		err = errors.New(cmdline[0] + ": " + err.Error())
+	}
+	return buf.Bytes(), err
+}
+
+// joinUnambiguously prints the slice, quoting where necessary to make the
+// output unambiguous.
+// TODO: See issue 5279. The printing of commands needs a complete redo.
+func joinUnambiguously(a []string) string {
+	var buf strings.Builder
+	for i, s := range a {
+		if i > 0 {
+			buf.WriteByte(' ')
+		}
+		q := strconv.Quote(s)
+		// A gccgo command line can contain -( and -).
+		// Make sure we quote them since they are special to the shell.
+		// The trimpath argument can also contain > (part of =>) and ;. Quote those too.
+		if s == "" || strings.ContainsAny(s, " ()>;") || len(q) > len(s)+2 {
+			buf.WriteString(q)
+		} else {
+			buf.WriteString(s)
+		}
+	}
+	return buf.String()
+}
+
+// cCompilerEnv returns environment variables to set when running the
+// C compiler. This is needed to disable escape codes in clang error
+// messages that confuse tools like cgo.
+func (b *Builder) cCompilerEnv() []string {
+	return []string{"TERM=dumb"}
+}
+
+// Mkdir makes the named directory.
+func (b *Builder) Mkdir(dir string) error {
+	// Make Mkdir(a.Objdir) a no-op instead of an error when a.Objdir == "".
+	if dir == "" {
+		return nil
+	}
+
+	b.exec.Lock()
+	defer b.exec.Unlock()
+	// We can be a little aggressive about being
+	// sure directories exist. Skip repeated calls.
+	if b.mkdirCache[dir] {
+		return nil
+	}
+	b.mkdirCache[dir] = true
+
+	if cfg.BuildN || cfg.BuildX {
+		b.Showcmd("", "mkdir -p %s", dir)
+		if cfg.BuildN {
+			return nil
+		}
+	}
+
+	if err := os.MkdirAll(dir, 0777); err != nil {
+		return err
+	}
+	return nil
+}
+
+// Symlink creates a symlink newname -> oldname.
+func (b *Builder) Symlink(oldname, newname string) error {
+	// It's not an error to try to recreate an existing symlink.
+	if link, err := os.Readlink(newname); err == nil && link == oldname {
+		return nil
+	}
+
+	if cfg.BuildN || cfg.BuildX {
+		b.Showcmd("", "ln -s %s %s", oldname, newname)
+		if cfg.BuildN {
+			return nil
+		}
+	}
+	return os.Symlink(oldname, newname)
+}
+
+// mkAbs returns an absolute path corresponding to
+// evaluating f in the directory dir.
+// We always pass absolute paths of source files so that
+// the error messages will include the full path to a file
+// in need of attention.
+func mkAbs(dir, f string) string {
+	// Leave absolute paths alone.
+	// Also, during -n mode we use the pseudo-directory $WORK
+	// instead of creating an actual work directory that won't be used.
+	// Leave paths beginning with $WORK alone too.
+	if filepath.IsAbs(f) || strings.HasPrefix(f, "$WORK") {
+		return f
+	}
+	return filepath.Join(dir, f)
+}
+
+type toolchain interface {
+	// gc runs the compiler in a specific directory on a set of files
+	// and returns the name of the generated output file.
+	gc(b *Builder, a *Action, archive string, importcfg, embedcfg []byte, symabis string, asmhdr bool, gofiles []string) (ofile string, out []byte, err error)
+	// cc runs the toolchain's C compiler in a directory on a C file
+	// to produce an output file.
+	cc(b *Builder, a *Action, ofile, cfile string) error
+	// asm runs the assembler in a specific directory on specific files
+	// and returns a list of named output files.
+	asm(b *Builder, a *Action, sfiles []string) ([]string, error)
+	// symabis scans the symbol ABIs from sfiles and returns the
+	// path to the output symbol ABIs file, or "" if none.
+	symabis(b *Builder, a *Action, sfiles []string) (string, error)
+	// pack runs the archive packer in a specific directory to create
+	// an archive from a set of object files.
+	// typically it is run in the object directory.
+	pack(b *Builder, a *Action, afile string, ofiles []string) error
+	// ld runs the linker to create an executable starting at mainpkg.
+	ld(b *Builder, root *Action, out, importcfg, mainpkg string) error
+	// ldShared runs the linker to create a shared library containing the pkgs built by toplevelactions
+	ldShared(b *Builder, root *Action, toplevelactions []*Action, out, importcfg string, allactions []*Action) error
+
+	compiler() string
+	linker() string
+}
+
+type noToolchain struct{}
+
+func noCompiler() error {
+	log.Fatalf("unknown compiler %q", cfg.BuildContext.Compiler)
+	return nil
+}
+
+func (noToolchain) compiler() string {
+	noCompiler()
+	return ""
+}
+
+func (noToolchain) linker() string {
+	noCompiler()
+	return ""
+}
+
+func (noToolchain) gc(b *Builder, a *Action, archive string, importcfg, embedcfg []byte, symabis string, asmhdr bool, gofiles []string) (ofile string, out []byte, err error) {
+	return "", nil, noCompiler()
+}
+
+func (noToolchain) asm(b *Builder, a *Action, sfiles []string) ([]string, error) {
+	return nil, noCompiler()
+}
+
+func (noToolchain) symabis(b *Builder, a *Action, sfiles []string) (string, error) {
+	return "", noCompiler()
+}
+
+func (noToolchain) pack(b *Builder, a *Action, afile string, ofiles []string) error {
+	return noCompiler()
+}
+
+func (noToolchain) ld(b *Builder, root *Action, out, importcfg, mainpkg string) error {
+	return noCompiler()
+}
+
+func (noToolchain) ldShared(b *Builder, root *Action, toplevelactions []*Action, out, importcfg string, allactions []*Action) error {
+	return noCompiler()
+}
+
+func (noToolchain) cc(b *Builder, a *Action, ofile, cfile string) error {
+	return noCompiler()
+}
+
+// gcc runs the gcc C compiler to create an object from a single C file.
+func (b *Builder) gcc(a *Action, p *load.Package, workdir, out string, flags []string, cfile string) error {
+	return b.ccompile(a, p, out, flags, cfile, b.GccCmd(p.Dir, workdir))
+}
+
+// gxx runs the g++ C++ compiler to create an object from a single C++ file.
+func (b *Builder) gxx(a *Action, p *load.Package, workdir, out string, flags []string, cxxfile string) error {
+	return b.ccompile(a, p, out, flags, cxxfile, b.GxxCmd(p.Dir, workdir))
+}
+
+// gfortran runs the gfortran Fortran compiler to create an object from a single Fortran file.
+func (b *Builder) gfortran(a *Action, p *load.Package, workdir, out string, flags []string, ffile string) error {
+	return b.ccompile(a, p, out, flags, ffile, b.gfortranCmd(p.Dir, workdir))
+}
+
+// ccompile runs the given C or C++ compiler and creates an object from a single source file.
+func (b *Builder) ccompile(a *Action, p *load.Package, outfile string, flags []string, file string, compiler []string) error {
+	file = mkAbs(p.Dir, file)
+	outfile = mkAbs(p.Dir, outfile)
+
+	// Elide source directory paths if -trimpath or GOROOT_FINAL is set.
+	// This is needed for source files (e.g., a .c file in a package directory).
+	// TODO(golang.org/issue/36072): cgo also generates files with #line
+	// directives pointing to the source directory. It should not generate those
+	// when -trimpath is enabled.
+	if b.gccSupportsFlag(compiler, "-fdebug-prefix-map=a=b") {
+		if cfg.BuildTrimpath || p.Goroot {
+			prefixMapFlag := "-fdebug-prefix-map"
+			if b.gccSupportsFlag(compiler, "-ffile-prefix-map=a=b") {
+				prefixMapFlag = "-ffile-prefix-map"
+			}
+			// Keep in sync with Action.trimpath.
+			// The trimmed paths are a little different, but we need to trim in mostly the
+			// same situations.
+			var from, toPath string
+			if m := p.Module; m == nil {
+				if p.Root == "" { // command-line-arguments in GOPATH mode, maybe?
+					from = p.Dir
+					toPath = p.ImportPath
+				} else if p.Goroot {
+					from = p.Root
+					toPath = "GOROOT"
+				} else {
+					from = p.Root
+					toPath = "GOPATH"
+				}
+			} else if m.Dir == "" {
+				// The module is in the vendor directory. Replace the entire vendor
+				// directory path, because the module's Dir is not filled in.
+				from = modload.VendorDir()
+				toPath = "vendor"
+			} else {
+				from = m.Dir
+				toPath = m.Path
+				if m.Version != "" {
+					toPath += "@" + m.Version
+				}
+			}
+			// -fdebug-prefix-map (or -ffile-prefix-map) requires an absolute "to"
+			// path (or it joins the path  with the working directory). Pick something
+			// that makes sense for the target platform.
+			var to string
+			if cfg.BuildContext.GOOS == "windows" {
+				to = filepath.Join(`\\_\_`, toPath)
+			} else {
+				to = filepath.Join("/_", toPath)
+			}
+			flags = append(slices.Clip(flags), prefixMapFlag+"="+from+"="+to)
+		}
+	}
+
+	// Tell gcc to not insert truly random numbers into the build process
+	// this ensures LTO won't create random numbers for symbols.
+	if b.gccSupportsFlag(compiler, "-frandom-seed=1") {
+		flags = append(flags, "-frandom-seed="+buildid.HashToString(a.actionID))
+	}
+
+	overlayPath := file
+	if p, ok := a.nonGoOverlay[overlayPath]; ok {
+		overlayPath = p
+	}
+	output, err := b.runOut(a, filepath.Dir(overlayPath), b.cCompilerEnv(), compiler, flags, "-o", outfile, "-c", filepath.Base(overlayPath))
+	if len(output) > 0 {
+		// On FreeBSD 11, when we pass -g to clang 3.8 it
+		// invokes its internal assembler with -dwarf-version=2.
+		// When it sees .section .note.GNU-stack, it warns
+		// "DWARF2 only supports one section per compilation unit".
+		// This warning makes no sense, since the section is empty,
+		// but it confuses people.
+		// We work around the problem by detecting the warning
+		// and dropping -g and trying again.
+		if bytes.Contains(output, []byte("DWARF2 only supports one section per compilation unit")) {
+			newFlags := make([]string, 0, len(flags))
+			for _, f := range flags {
+				if !strings.HasPrefix(f, "-g") {
+					newFlags = append(newFlags, f)
+				}
+			}
+			if len(newFlags) < len(flags) {
+				return b.ccompile(a, p, outfile, newFlags, file, compiler)
+			}
+		}
+
+		if err != nil || os.Getenv("GO_BUILDER_NAME") != "" {
+			err = formatOutput(b.WorkDir, p.Dir, p.ImportPath, p.Desc(), b.processOutput(output))
+		} else {
+			b.showOutput(a, p.Dir, p.Desc(), b.processOutput(output))
+		}
+	}
+	return err
+}
+
+// gccld runs the gcc linker to create an executable from a set of object files.
+// Any error output is only displayed for BuildN or BuildX.
+func (b *Builder) gccld(a *Action, p *load.Package, objdir, outfile string, flags []string, objs []string) error {
+	var cmd []string
+	if len(p.CXXFiles) > 0 || len(p.SwigCXXFiles) > 0 {
+		cmd = b.GxxCmd(p.Dir, objdir)
+	} else {
+		cmd = b.GccCmd(p.Dir, objdir)
+	}
+
+	cmdargs := []any{cmd, "-o", outfile, objs, flags}
+	dir := p.Dir
+	out, err := b.runOut(a, base.Cwd(), b.cCompilerEnv(), cmdargs...)
+
+	if len(out) > 0 {
+		// Filter out useless linker warnings caused by bugs outside Go.
+		// See also cmd/link/internal/ld's hostlink method.
+		var save [][]byte
+		var skipLines int
+		for _, line := range bytes.SplitAfter(out, []byte("\n")) {
+			// golang.org/issue/26073 - Apple Xcode bug
+			if bytes.Contains(line, []byte("ld: warning: text-based stub file")) {
+				continue
+			}
+
+			if skipLines > 0 {
+				skipLines--
+				continue
+			}
+
+			// Remove duplicate main symbol with runtime/cgo on AIX.
+			// With runtime/cgo, two main are available:
+			// One is generated by cgo tool with {return 0;}.
+			// The other one is the main calling runtime.rt0_go
+			// in runtime/cgo.
+			// The second can't be used by cgo programs because
+			// runtime.rt0_go is unknown to them.
+			// Therefore, we let ld remove this main version
+			// and used the cgo generated one.
+			if p.ImportPath == "runtime/cgo" && bytes.Contains(line, []byte("ld: 0711-224 WARNING: Duplicate symbol: .main")) {
+				skipLines = 1
+				continue
+			}
+
+			save = append(save, line)
+		}
+		out = bytes.Join(save, nil)
+		if len(out) > 0 && (cfg.BuildN || cfg.BuildX) {
+			b.showOutput(nil, dir, p.ImportPath, b.processOutput(out))
+		}
+	}
+	return err
+}
+
+// GccCmd returns a gcc command line prefix
+// defaultCC is defined in zdefaultcc.go, written by cmd/dist.
+func (b *Builder) GccCmd(incdir, workdir string) []string {
+	return b.compilerCmd(b.ccExe(), incdir, workdir)
+}
+
+// GxxCmd returns a g++ command line prefix
+// defaultCXX is defined in zdefaultcc.go, written by cmd/dist.
+func (b *Builder) GxxCmd(incdir, workdir string) []string {
+	return b.compilerCmd(b.cxxExe(), incdir, workdir)
+}
+
+// gfortranCmd returns a gfortran command line prefix.
+func (b *Builder) gfortranCmd(incdir, workdir string) []string {
+	return b.compilerCmd(b.fcExe(), incdir, workdir)
+}
+
+// ccExe returns the CC compiler setting without all the extra flags we add implicitly.
+func (b *Builder) ccExe() []string {
+	return envList("CC", cfg.DefaultCC(cfg.Goos, cfg.Goarch))
+}
+
+// cxxExe returns the CXX compiler setting without all the extra flags we add implicitly.
+func (b *Builder) cxxExe() []string {
+	return envList("CXX", cfg.DefaultCXX(cfg.Goos, cfg.Goarch))
+}
+
+// fcExe returns the FC compiler setting without all the extra flags we add implicitly.
+func (b *Builder) fcExe() []string {
+	return envList("FC", "gfortran")
+}
+
+// compilerCmd returns a command line prefix for the given environment
+// variable and using the default command when the variable is empty.
+func (b *Builder) compilerCmd(compiler []string, incdir, workdir string) []string {
+	a := append(compiler, "-I", incdir)
+
+	// Definitely want -fPIC but on Windows gcc complains
+	// "-fPIC ignored for target (all code is position independent)"
+	if cfg.Goos != "windows" {
+		a = append(a, "-fPIC")
+	}
+	a = append(a, b.gccArchArgs()...)
+	// gcc-4.5 and beyond require explicit "-pthread" flag
+	// for multithreading with pthread library.
+	if cfg.BuildContext.CgoEnabled {
+		switch cfg.Goos {
+		case "windows":
+			a = append(a, "-mthreads")
+		default:
+			a = append(a, "-pthread")
+		}
+	}
+
+	if cfg.Goos == "aix" {
+		// mcmodel=large must always be enabled to allow large TOC.
+		a = append(a, "-mcmodel=large")
+	}
+
+	// disable ASCII art in clang errors, if possible
+	if b.gccSupportsFlag(compiler, "-fno-caret-diagnostics") {
+		a = append(a, "-fno-caret-diagnostics")
+	}
+	// clang is too smart about command-line arguments
+	if b.gccSupportsFlag(compiler, "-Qunused-arguments") {
+		a = append(a, "-Qunused-arguments")
+	}
+
+	// zig cc passes --gc-sections to the underlying linker, which then causes
+	// undefined symbol errors when compiling with cgo but without C code.
+	// https://github.com/golang/go/issues/52690
+	if b.gccSupportsFlag(compiler, "-Wl,--no-gc-sections") {
+		a = append(a, "-Wl,--no-gc-sections")
+	}
+
+	// disable word wrapping in error messages
+	a = append(a, "-fmessage-length=0")
+
+	// Tell gcc not to include the work directory in object files.
+	if b.gccSupportsFlag(compiler, "-fdebug-prefix-map=a=b") {
+		if workdir == "" {
+			workdir = b.WorkDir
+		}
+		workdir = strings.TrimSuffix(workdir, string(filepath.Separator))
+		if b.gccSupportsFlag(compiler, "-ffile-prefix-map=a=b") {
+			a = append(a, "-ffile-prefix-map="+workdir+"=/tmp/go-build")
+		} else {
+			a = append(a, "-fdebug-prefix-map="+workdir+"=/tmp/go-build")
+		}
+	}
+
+	// Tell gcc not to include flags in object files, which defeats the
+	// point of -fdebug-prefix-map above.
+	if b.gccSupportsFlag(compiler, "-gno-record-gcc-switches") {
+		a = append(a, "-gno-record-gcc-switches")
+	}
+
+	// On OS X, some of the compilers behave as if -fno-common
+	// is always set, and the Mach-O linker in 6l/8l assumes this.
+	// See https://golang.org/issue/3253.
+	if cfg.Goos == "darwin" || cfg.Goos == "ios" {
+		a = append(a, "-fno-common")
+	}
+
+	return a
+}
+
+// gccNoPie returns the flag to use to request non-PIE. On systems
+// with PIE (position independent executables) enabled by default,
+// -no-pie must be passed when doing a partial link with -Wl,-r.
+// But -no-pie is not supported by all compilers, and clang spells it -nopie.
+func (b *Builder) gccNoPie(linker []string) string {
+	if b.gccSupportsFlag(linker, "-no-pie") {
+		return "-no-pie"
+	}
+	if b.gccSupportsFlag(linker, "-nopie") {
+		return "-nopie"
+	}
+	return ""
+}
+
+// gccSupportsFlag checks to see if the compiler supports a flag.
+func (b *Builder) gccSupportsFlag(compiler []string, flag string) bool {
+	key := [2]string{compiler[0], flag}
+
+	// We used to write an empty C file, but that gets complicated with go
+	// build -n. We tried using a file that does not exist, but that fails on
+	// systems with GCC version 4.2.1; that is the last GPLv2 version of GCC,
+	// so some systems have frozen on it. Now we pass an empty file on stdin,
+	// which should work at least for GCC and clang.
+	//
+	// If the argument is "-Wl,", then it is testing the linker. In that case,
+	// skip "-c". If it's not "-Wl,", then we are testing the compiler and can
+	// omit the linking step with "-c".
+	//
+	// Using the same CFLAGS/LDFLAGS here and for building the program.
+
+	// On the iOS builder the command
+	//   $CC -Wl,--no-gc-sections -x c - -o /dev/null < /dev/null
+	// is failing with:
+	//   Unable to remove existing file: Invalid argument
+	tmp := os.DevNull
+	if runtime.GOOS == "windows" || runtime.GOOS == "ios" {
+		f, err := os.CreateTemp(b.WorkDir, "")
+		if err != nil {
+			return false
+		}
+		f.Close()
+		tmp = f.Name()
+		defer os.Remove(tmp)
+	}
+
+	cmdArgs := str.StringList(compiler, flag)
+	if strings.HasPrefix(flag, "-Wl,") /* linker flag */ {
+		ldflags, err := buildFlags("LDFLAGS", defaultCFlags, nil, checkLinkerFlags)
+		if err != nil {
+			return false
+		}
+		cmdArgs = append(cmdArgs, ldflags...)
+	} else { /* compiler flag, add "-c" */
+		cflags, err := buildFlags("CFLAGS", defaultCFlags, nil, checkCompilerFlags)
+		if err != nil {
+			return false
+		}
+		cmdArgs = append(cmdArgs, cflags...)
+		cmdArgs = append(cmdArgs, "-c")
+	}
+
+	cmdArgs = append(cmdArgs, "-x", "c", "-", "-o", tmp)
+
+	if cfg.BuildN {
+		b.Showcmd(b.WorkDir, "%s || true", joinUnambiguously(cmdArgs))
+		return false
+	}
+
+	// gccCompilerID acquires b.exec, so do before acquiring lock.
+	compilerID, cacheOK := b.gccCompilerID(compiler[0])
+
+	b.exec.Lock()
+	defer b.exec.Unlock()
+	if b, ok := b.flagCache[key]; ok {
+		return b
+	}
+	if b.flagCache == nil {
+		b.flagCache = make(map[[2]string]bool)
+	}
+
+	// Look in build cache.
+	var flagID cache.ActionID
+	if cacheOK {
+		flagID = cache.Subkey(compilerID, "gccSupportsFlag "+flag)
+		if data, _, err := cache.GetBytes(cache.Default(), flagID); err == nil {
+			supported := string(data) == "true"
+			b.flagCache[key] = supported
+			return supported
+		}
+	}
+
+	if cfg.BuildX {
+		b.Showcmd(b.WorkDir, "%s || true", joinUnambiguously(cmdArgs))
+	}
+	cmd := exec.Command(cmdArgs[0], cmdArgs[1:]...)
+	cmd.Dir = b.WorkDir
+	cmd.Env = append(cmd.Environ(), "LC_ALL=C")
+	out, _ := cmd.CombinedOutput()
+	// GCC says "unrecognized command line option".
+	// clang says "unknown argument".
+	// tcc says "unsupported"
+	// AIX says "not recognized"
+	// Older versions of GCC say "unrecognised debug output level".
+	// For -fsplit-stack GCC says "'-fsplit-stack' is not supported".
+	supported := !bytes.Contains(out, []byte("unrecognized")) &&
+		!bytes.Contains(out, []byte("unknown")) &&
+		!bytes.Contains(out, []byte("unrecognised")) &&
+		!bytes.Contains(out, []byte("is not supported")) &&
+		!bytes.Contains(out, []byte("not recognized")) &&
+		!bytes.Contains(out, []byte("unsupported"))
+
+	if cacheOK {
+		s := "false"
+		if supported {
+			s = "true"
+		}
+		cache.PutBytes(cache.Default(), flagID, []byte(s))
+	}
+
+	b.flagCache[key] = supported
+	return supported
+}
+
+// statString returns a string form of an os.FileInfo, for serializing and comparison.
+func statString(info os.FileInfo) string {
+	return fmt.Sprintf("stat %d %x %v %v\n", info.Size(), uint64(info.Mode()), info.ModTime(), info.IsDir())
+}
+
+// gccCompilerID returns a build cache key for the current gcc,
+// as identified by running 'compiler'.
+// The caller can use subkeys of the key.
+// Other parts of cmd/go can use the id as a hash
+// of the installed compiler version.
+func (b *Builder) gccCompilerID(compiler string) (id cache.ActionID, ok bool) {
+	if cfg.BuildN {
+		b.Showcmd(b.WorkDir, "%s || true", joinUnambiguously([]string{compiler, "--version"}))
+		return cache.ActionID{}, false
+	}
+
+	b.exec.Lock()
+	defer b.exec.Unlock()
+
+	if id, ok := b.gccCompilerIDCache[compiler]; ok {
+		return id, ok
+	}
+
+	// We hash the compiler's full path to get a cache entry key.
+	// That cache entry holds a validation description,
+	// which is of the form:
+	//
+	//	filename \x00 statinfo \x00
+	//	...
+	//	compiler id
+	//
+	// If os.Stat of each filename matches statinfo,
+	// then the entry is still valid, and we can use the
+	// compiler id without any further expense.
+	//
+	// Otherwise, we compute a new validation description
+	// and compiler id (below).
+	exe, err := exec.LookPath(compiler)
+	if err != nil {
+		return cache.ActionID{}, false
+	}
+
+	h := cache.NewHash("gccCompilerID")
+	fmt.Fprintf(h, "gccCompilerID %q", exe)
+	key := h.Sum()
+	data, _, err := cache.GetBytes(cache.Default(), key)
+	if err == nil && len(data) > len(id) {
+		stats := strings.Split(string(data[:len(data)-len(id)]), "\x00")
+		if len(stats)%2 != 0 {
+			goto Miss
+		}
+		for i := 0; i+2 <= len(stats); i++ {
+			info, err := os.Stat(stats[i])
+			if err != nil || statString(info) != stats[i+1] {
+				goto Miss
+			}
+		}
+		copy(id[:], data[len(data)-len(id):])
+		return id, true
+	Miss:
+	}
+
+	// Validation failed. Compute a new description (in buf) and compiler ID (in h).
+	// For now, there are only at most two filenames in the stat information.
+	// The first one is the compiler executable we invoke.
+	// The second is the underlying compiler as reported by -v -###
+	// (see b.gccToolID implementation in buildid.go).
+	toolID, exe2, err := b.gccToolID(compiler, "c")
+	if err != nil {
+		return cache.ActionID{}, false
+	}
+
+	exes := []string{exe, exe2}
+	str.Uniq(&exes)
+	fmt.Fprintf(h, "gccCompilerID %q %q\n", exes, toolID)
+	id = h.Sum()
+
+	var buf bytes.Buffer
+	for _, exe := range exes {
+		if exe == "" {
+			continue
+		}
+		info, err := os.Stat(exe)
+		if err != nil {
+			return cache.ActionID{}, false
+		}
+		buf.WriteString(exe)
+		buf.WriteString("\x00")
+		buf.WriteString(statString(info))
+		buf.WriteString("\x00")
+	}
+	buf.Write(id[:])
+
+	cache.PutBytes(cache.Default(), key, buf.Bytes())
+	if b.gccCompilerIDCache == nil {
+		b.gccCompilerIDCache = make(map[string]cache.ActionID)
+	}
+	b.gccCompilerIDCache[compiler] = id
+	return id, true
+}
+
+// gccArchArgs returns arguments to pass to gcc based on the architecture.
+func (b *Builder) gccArchArgs() []string {
+	switch cfg.Goarch {
+	case "386":
+		return []string{"-m32"}
+	case "amd64":
+		if cfg.Goos == "darwin" {
+			return []string{"-arch", "x86_64", "-m64"}
+		}
+		return []string{"-m64"}
+	case "arm64":
+		if cfg.Goos == "darwin" {
+			return []string{"-arch", "arm64"}
+		}
+	case "arm":
+		return []string{"-marm"} // not thumb
+	case "s390x":
+		return []string{"-m64", "-march=z196"}
+	case "mips64", "mips64le":
+		args := []string{"-mabi=64"}
+		if cfg.GOMIPS64 == "hardfloat" {
+			return append(args, "-mhard-float")
+		} else if cfg.GOMIPS64 == "softfloat" {
+			return append(args, "-msoft-float")
+		}
+	case "mips", "mipsle":
+		args := []string{"-mabi=32", "-march=mips32"}
+		if cfg.GOMIPS == "hardfloat" {
+			return append(args, "-mhard-float", "-mfp32", "-mno-odd-spreg")
+		} else if cfg.GOMIPS == "softfloat" {
+			return append(args, "-msoft-float")
+		}
+	case "loong64":
+		return []string{"-mabi=lp64d"}
+	case "ppc64":
+		if cfg.Goos == "aix" {
+			return []string{"-maix64"}
+		}
+	}
+	return nil
+}
+
+// envList returns the value of the given environment variable broken
+// into fields, using the default value when the variable is empty.
+//
+// The environment variable must be quoted correctly for
+// quoted.Split. This should be done before building
+// anything, for example, in BuildInit.
+func envList(key, def string) []string {
+	v := cfg.Getenv(key)
+	if v == "" {
+		v = def
+	}
+	args, err := quoted.Split(v)
+	if err != nil {
+		panic(fmt.Sprintf("could not parse environment variable %s with value %q: %v", key, v, err))
+	}
+	return args
+}
+
+// CFlags returns the flags to use when invoking the C, C++ or Fortran compilers, or cgo.
+func (b *Builder) CFlags(p *load.Package) (cppflags, cflags, cxxflags, fflags, ldflags []string, err error) {
+	if cppflags, err = buildFlags("CPPFLAGS", "", p.CgoCPPFLAGS, checkCompilerFlags); err != nil {
+		return
+	}
+	if cflags, err = buildFlags("CFLAGS", defaultCFlags, p.CgoCFLAGS, checkCompilerFlags); err != nil {
+		return
+	}
+	if cxxflags, err = buildFlags("CXXFLAGS", defaultCFlags, p.CgoCXXFLAGS, checkCompilerFlags); err != nil {
+		return
+	}
+	if fflags, err = buildFlags("FFLAGS", defaultCFlags, p.CgoFFLAGS, checkCompilerFlags); err != nil {
+		return
+	}
+	if ldflags, err = buildFlags("LDFLAGS", defaultCFlags, p.CgoLDFLAGS, checkLinkerFlags); err != nil {
+		return
+	}
+
+	return
+}
+
+func buildFlags(name, defaults string, fromPackage []string, check func(string, string, []string) error) ([]string, error) {
+	if err := check(name, "#cgo "+name, fromPackage); err != nil {
+		return nil, err
+	}
+	return str.StringList(envList("CGO_"+name, defaults), fromPackage), nil
+}
+
+var cgoRe = lazyregexp.New(`[/\\:]`)
+
+func (b *Builder) cgo(a *Action, cgoExe, objdir string, pcCFLAGS, pcLDFLAGS, cgofiles, gccfiles, gxxfiles, mfiles, ffiles []string) (outGo, outObj []string, err error) {
+	p := a.Package
+	cgoCPPFLAGS, cgoCFLAGS, cgoCXXFLAGS, cgoFFLAGS, cgoLDFLAGS, err := b.CFlags(p)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	cgoCPPFLAGS = append(cgoCPPFLAGS, pcCFLAGS...)
+	cgoLDFLAGS = append(cgoLDFLAGS, pcLDFLAGS...)
+	// If we are compiling Objective-C code, then we need to link against libobjc
+	if len(mfiles) > 0 {
+		cgoLDFLAGS = append(cgoLDFLAGS, "-lobjc")
+	}
+
+	// Likewise for Fortran, except there are many Fortran compilers.
+	// Support gfortran out of the box and let others pass the correct link options
+	// via CGO_LDFLAGS
+	if len(ffiles) > 0 {
+		fc := cfg.Getenv("FC")
+		if fc == "" {
+			fc = "gfortran"
+		}
+		if strings.Contains(fc, "gfortran") {
+			cgoLDFLAGS = append(cgoLDFLAGS, "-lgfortran")
+		}
+	}
+
+	// Scrutinize CFLAGS and related for flags that might cause
+	// problems if we are using internal linking (for example, use of
+	// plugins, LTO, etc) by calling a helper routine that builds on
+	// the existing CGO flags allow-lists. If we see anything
+	// suspicious, emit a special token file "preferlinkext" (known to
+	// the linker) in the object file to signal the that it should not
+	// try to link internally and should revert to external linking.
+	// The token we pass is a suggestion, not a mandate; if a user is
+	// explicitly asking for a specific linkmode via the "-linkmode"
+	// flag, the token will be ignored. NB: in theory we could ditch
+	// the token approach and just pass a flag to the linker when we
+	// eventually invoke it, and the linker flag could then be
+	// documented (although coming up with a simple explanation of the
+	// flag might be challenging). For more context see issues #58619,
+	// #58620, and #58848.
+	flagSources := []string{"CGO_CFLAGS", "CGO_CXXFLAGS", "CGO_FFLAGS"}
+	flagLists := [][]string{cgoCFLAGS, cgoCXXFLAGS, cgoFFLAGS}
+	if flagsNotCompatibleWithInternalLinking(flagSources, flagLists) {
+		tokenFile := objdir + "preferlinkext"
+		if cfg.BuildN || cfg.BuildX {
+			b.Showcmd("", "echo > %s", tokenFile)
+		}
+		if !cfg.BuildN {
+			if err := os.WriteFile(tokenFile, nil, 0666); err != nil {
+				return nil, nil, err
+			}
+		}
+		outObj = append(outObj, tokenFile)
+	}
+
+	if cfg.BuildMSan {
+		cgoCFLAGS = append([]string{"-fsanitize=memory"}, cgoCFLAGS...)
+		cgoLDFLAGS = append([]string{"-fsanitize=memory"}, cgoLDFLAGS...)
+	}
+	if cfg.BuildASan {
+		cgoCFLAGS = append([]string{"-fsanitize=address"}, cgoCFLAGS...)
+		cgoLDFLAGS = append([]string{"-fsanitize=address"}, cgoLDFLAGS...)
+	}
+
+	// Allows including _cgo_export.h, as well as the user's .h files,
+	// from .[ch] files in the package.
+	cgoCPPFLAGS = append(cgoCPPFLAGS, "-I", objdir)
+
+	// cgo
+	// TODO: CGO_FLAGS?
+	gofiles := []string{objdir + "_cgo_gotypes.go"}
+	cfiles := []string{"_cgo_export.c"}
+	for _, fn := range cgofiles {
+		f := strings.TrimSuffix(filepath.Base(fn), ".go")
+		gofiles = append(gofiles, objdir+f+".cgo1.go")
+		cfiles = append(cfiles, f+".cgo2.c")
+	}
+
+	// TODO: make cgo not depend on $GOARCH?
+
+	cgoflags := []string{}
+	if p.Standard && p.ImportPath == "runtime/cgo" {
+		cgoflags = append(cgoflags, "-import_runtime_cgo=false")
+	}
+	if p.Standard && (p.ImportPath == "runtime/race" || p.ImportPath == "runtime/msan" || p.ImportPath == "runtime/cgo" || p.ImportPath == "runtime/asan") {
+		cgoflags = append(cgoflags, "-import_syscall=false")
+	}
+
+	// Update $CGO_LDFLAGS with p.CgoLDFLAGS.
+	// These flags are recorded in the generated _cgo_gotypes.go file
+	// using //go:cgo_ldflag directives, the compiler records them in the
+	// object file for the package, and then the Go linker passes them
+	// along to the host linker. At this point in the code, cgoLDFLAGS
+	// consists of the original $CGO_LDFLAGS (unchecked) and all the
+	// flags put together from source code (checked).
+	cgoenv := b.cCompilerEnv()
+	if len(cgoLDFLAGS) > 0 {
+		flags := make([]string, len(cgoLDFLAGS))
+		for i, f := range cgoLDFLAGS {
+			flags[i] = strconv.Quote(f)
+		}
+		cgoenv = append(cgoenv, "CGO_LDFLAGS="+strings.Join(flags, " "))
+	}
+
+	if cfg.BuildToolchainName == "gccgo" {
+		if b.gccSupportsFlag([]string{BuildToolchain.compiler()}, "-fsplit-stack") {
+			cgoCFLAGS = append(cgoCFLAGS, "-fsplit-stack")
+		}
+		cgoflags = append(cgoflags, "-gccgo")
+		if pkgpath := gccgoPkgpath(p); pkgpath != "" {
+			cgoflags = append(cgoflags, "-gccgopkgpath="+pkgpath)
+		}
+		if !BuildToolchain.(gccgoToolchain).supportsCgoIncomplete(b) {
+			cgoflags = append(cgoflags, "-gccgo_define_cgoincomplete")
+		}
+	}
+
+	switch cfg.BuildBuildmode {
+	case "c-archive", "c-shared":
+		// Tell cgo that if there are any exported functions
+		// it should generate a header file that C code can
+		// #include.
+		cgoflags = append(cgoflags, "-exportheader="+objdir+"_cgo_install.h")
+	}
+
+	// Rewrite overlaid paths in cgo files.
+	// cgo adds //line and #line pragmas in generated files with these paths.
+	var trimpath []string
+	for i := range cgofiles {
+		path := mkAbs(p.Dir, cgofiles[i])
+		if opath, ok := fsys.OverlayPath(path); ok {
+			cgofiles[i] = opath
+			trimpath = append(trimpath, opath+"=>"+path)
+		}
+	}
+	if len(trimpath) > 0 {
+		cgoflags = append(cgoflags, "-trimpath", strings.Join(trimpath, ";"))
+	}
+
+	if err := b.run(a, p.Dir, p.ImportPath, cgoenv, cfg.BuildToolexec, cgoExe, "-objdir", objdir, "-importpath", p.ImportPath, cgoflags, "--", cgoCPPFLAGS, cgoCFLAGS, cgofiles); err != nil {
+		return nil, nil, err
+	}
+	outGo = append(outGo, gofiles...)
+
+	// Use sequential object file names to keep them distinct
+	// and short enough to fit in the .a header file name slots.
+	// We no longer collect them all into _all.o, and we'd like
+	// tools to see both the .o suffix and unique names, so
+	// we need to make them short enough not to be truncated
+	// in the final archive.
+	oseq := 0
+	nextOfile := func() string {
+		oseq++
+		return objdir + fmt.Sprintf("_x%03d.o", oseq)
+	}
+
+	// gcc
+	cflags := str.StringList(cgoCPPFLAGS, cgoCFLAGS)
+	for _, cfile := range cfiles {
+		ofile := nextOfile()
+		if err := b.gcc(a, p, a.Objdir, ofile, cflags, objdir+cfile); err != nil {
+			return nil, nil, err
+		}
+		outObj = append(outObj, ofile)
+	}
+
+	for _, file := range gccfiles {
+		ofile := nextOfile()
+		if err := b.gcc(a, p, a.Objdir, ofile, cflags, file); err != nil {
+			return nil, nil, err
+		}
+		outObj = append(outObj, ofile)
+	}
+
+	cxxflags := str.StringList(cgoCPPFLAGS, cgoCXXFLAGS)
+	for _, file := range gxxfiles {
+		ofile := nextOfile()
+		if err := b.gxx(a, p, a.Objdir, ofile, cxxflags, file); err != nil {
+			return nil, nil, err
+		}
+		outObj = append(outObj, ofile)
+	}
+
+	for _, file := range mfiles {
+		ofile := nextOfile()
+		if err := b.gcc(a, p, a.Objdir, ofile, cflags, file); err != nil {
+			return nil, nil, err
+		}
+		outObj = append(outObj, ofile)
+	}
+
+	fflags := str.StringList(cgoCPPFLAGS, cgoFFLAGS)
+	for _, file := range ffiles {
+		ofile := nextOfile()
+		if err := b.gfortran(a, p, a.Objdir, ofile, fflags, file); err != nil {
+			return nil, nil, err
+		}
+		outObj = append(outObj, ofile)
+	}
+
+	switch cfg.BuildToolchainName {
+	case "gc":
+		importGo := objdir + "_cgo_import.go"
+		dynOutGo, dynOutObj, err := b.dynimport(a, p, objdir, importGo, cgoExe, cflags, cgoLDFLAGS, outObj)
+		if err != nil {
+			return nil, nil, err
+		}
+		if dynOutGo != "" {
+			outGo = append(outGo, dynOutGo)
+		}
+		if dynOutObj != "" {
+			outObj = append(outObj, dynOutObj)
+		}
+
+	case "gccgo":
+		defunC := objdir + "_cgo_defun.c"
+		defunObj := objdir + "_cgo_defun.o"
+		if err := BuildToolchain.cc(b, a, defunObj, defunC); err != nil {
+			return nil, nil, err
+		}
+		outObj = append(outObj, defunObj)
+
+	default:
+		noCompiler()
+	}
+
+	// Double check the //go:cgo_ldflag comments in the generated files.
+	// The compiler only permits such comments in files whose base name
+	// starts with "_cgo_". Make sure that the comments in those files
+	// are safe. This is a backstop against people somehow smuggling
+	// such a comment into a file generated by cgo.
+	if cfg.BuildToolchainName == "gc" && !cfg.BuildN {
+		var flags []string
+		for _, f := range outGo {
+			if !strings.HasPrefix(filepath.Base(f), "_cgo_") {
+				continue
+			}
+
+			src, err := os.ReadFile(f)
+			if err != nil {
+				return nil, nil, err
+			}
+
+			const cgoLdflag = "//go:cgo_ldflag"
+			idx := bytes.Index(src, []byte(cgoLdflag))
+			for idx >= 0 {
+				// We are looking at //go:cgo_ldflag.
+				// Find start of line.
+				start := bytes.LastIndex(src[:idx], []byte("\n"))
+				if start == -1 {
+					start = 0
+				}
+
+				// Find end of line.
+				end := bytes.Index(src[idx:], []byte("\n"))
+				if end == -1 {
+					end = len(src)
+				} else {
+					end += idx
+				}
+
+				// Check for first line comment in line.
+				// We don't worry about /* */ comments,
+				// which normally won't appear in files
+				// generated by cgo.
+				commentStart := bytes.Index(src[start:], []byte("//"))
+				commentStart += start
+				// If that line comment is //go:cgo_ldflag,
+				// it's a match.
+				if bytes.HasPrefix(src[commentStart:], []byte(cgoLdflag)) {
+					// Pull out the flag, and unquote it.
+					// This is what the compiler does.
+					flag := string(src[idx+len(cgoLdflag) : end])
+					flag = strings.TrimSpace(flag)
+					flag = strings.Trim(flag, `"`)
+					flags = append(flags, flag)
+				}
+				src = src[end:]
+				idx = bytes.Index(src, []byte(cgoLdflag))
+			}
+		}
+
+		// We expect to find the contents of cgoLDFLAGS in flags.
+		if len(cgoLDFLAGS) > 0 {
+		outer:
+			for i := range flags {
+				for j, f := range cgoLDFLAGS {
+					if f != flags[i+j] {
+						continue outer
+					}
+				}
+				flags = append(flags[:i], flags[i+len(cgoLDFLAGS):]...)
+				break
+			}
+		}
+
+		if err := checkLinkerFlags("LDFLAGS", "go:cgo_ldflag", flags); err != nil {
+			return nil, nil, err
+		}
+	}
+
+	return outGo, outObj, nil
+}
+
+// flagsNotCompatibleWithInternalLinking scans the list of cgo
+// compiler flags (C/C++/Fortran) looking for flags that might cause
+// problems if the build in question uses internal linking. The
+// primary culprits are use of plugins or use of LTO, but we err on
+// the side of caution, supporting only those flags that are on the
+// allow-list for safe flags from security perspective. Return is TRUE
+// if a sensitive flag is found, FALSE otherwise.
+func flagsNotCompatibleWithInternalLinking(sourceList []string, flagListList [][]string) bool {
+	for i := range sourceList {
+		sn := sourceList[i]
+		fll := flagListList[i]
+		if err := checkCompilerFlagsForInternalLink(sn, sn, fll); err != nil {
+			return true
+		}
+	}
+	return false
+}
+
+// dynimport creates a Go source file named importGo containing
+// //go:cgo_import_dynamic directives for each symbol or library
+// dynamically imported by the object files outObj.
+// dynOutGo, if not empty, is a new Go file to build as part of the package.
+// dynOutObj, if not empty, is a new file to add to the generated archive.
+func (b *Builder) dynimport(a *Action, p *load.Package, objdir, importGo, cgoExe string, cflags, cgoLDFLAGS, outObj []string) (dynOutGo, dynOutObj string, err error) {
+	cfile := objdir + "_cgo_main.c"
+	ofile := objdir + "_cgo_main.o"
+	if err := b.gcc(a, p, objdir, ofile, cflags, cfile); err != nil {
+		return "", "", err
+	}
+
+	// Gather .syso files from this package and all (transitive) dependencies.
+	var syso []string
+	seen := make(map[*Action]bool)
+	var gatherSyso func(*Action)
+	gatherSyso = func(a1 *Action) {
+		if seen[a1] {
+			return
+		}
+		seen[a1] = true
+		if p1 := a1.Package; p1 != nil {
+			syso = append(syso, mkAbsFiles(p1.Dir, p1.SysoFiles)...)
+		}
+		for _, a2 := range a1.Deps {
+			gatherSyso(a2)
+		}
+	}
+	gatherSyso(a)
+	sort.Strings(syso)
+	str.Uniq(&syso)
+	linkobj := str.StringList(ofile, outObj, syso)
+	dynobj := objdir + "_cgo_.o"
+
+	ldflags := cgoLDFLAGS
+	if (cfg.Goarch == "arm" && cfg.Goos == "linux") || cfg.Goos == "android" {
+		if !str.Contains(ldflags, "-no-pie") {
+			// we need to use -pie for Linux/ARM to get accurate imported sym (added in https://golang.org/cl/5989058)
+			// this seems to be outdated, but we don't want to break existing builds depending on this (Issue 45940)
+			ldflags = append(ldflags, "-pie")
+		}
+		if str.Contains(ldflags, "-pie") && str.Contains(ldflags, "-static") {
+			// -static -pie doesn't make sense, and causes link errors.
+			// Issue 26197.
+			n := make([]string, 0, len(ldflags)-1)
+			for _, flag := range ldflags {
+				if flag != "-static" {
+					n = append(n, flag)
+				}
+			}
+			ldflags = n
+		}
+	}
+	if err := b.gccld(a, p, objdir, dynobj, ldflags, linkobj); err != nil {
+		// We only need this information for internal linking.
+		// If this link fails, mark the object as requiring
+		// external linking. This link can fail for things like
+		// syso files that have unexpected dependencies.
+		// cmd/link explicitly looks for the name "dynimportfail".
+		// See issue #52863.
+		fail := objdir + "dynimportfail"
+		if cfg.BuildN || cfg.BuildX {
+			b.Showcmd("", "echo > %s", fail)
+		}
+		if !cfg.BuildN {
+			if err := os.WriteFile(fail, nil, 0666); err != nil {
+				return "", "", err
+			}
+		}
+		return "", fail, nil
+	}
+
+	// cgo -dynimport
+	var cgoflags []string
+	if p.Standard && p.ImportPath == "runtime/cgo" {
+		cgoflags = []string{"-dynlinker"} // record path to dynamic linker
+	}
+	err = b.run(a, base.Cwd(), p.ImportPath, b.cCompilerEnv(), cfg.BuildToolexec, cgoExe, "-dynpackage", p.Name, "-dynimport", dynobj, "-dynout", importGo, cgoflags)
+	if err != nil {
+		return "", "", err
+	}
+	return importGo, "", nil
+}
+
+// Run SWIG on all SWIG input files.
+// TODO: Don't build a shared library, once SWIG emits the necessary
+// pragmas for external linking.
+func (b *Builder) swig(a *Action, p *load.Package, objdir string, pcCFLAGS []string) (outGo, outC, outCXX []string, err error) {
+	if err := b.swigVersionCheck(); err != nil {
+		return nil, nil, nil, err
+	}
+
+	intgosize, err := b.swigIntSize(objdir)
+	if err != nil {
+		return nil, nil, nil, err
+	}
+
+	for _, f := range p.SwigFiles {
+		goFile, cFile, err := b.swigOne(a, p, f, objdir, pcCFLAGS, false, intgosize)
+		if err != nil {
+			return nil, nil, nil, err
+		}
+		if goFile != "" {
+			outGo = append(outGo, goFile)
+		}
+		if cFile != "" {
+			outC = append(outC, cFile)
+		}
+	}
+	for _, f := range p.SwigCXXFiles {
+		goFile, cxxFile, err := b.swigOne(a, p, f, objdir, pcCFLAGS, true, intgosize)
+		if err != nil {
+			return nil, nil, nil, err
+		}
+		if goFile != "" {
+			outGo = append(outGo, goFile)
+		}
+		if cxxFile != "" {
+			outCXX = append(outCXX, cxxFile)
+		}
+	}
+	return outGo, outC, outCXX, nil
+}
+
+// Make sure SWIG is new enough.
+var (
+	swigCheckOnce sync.Once
+	swigCheck     error
+)
+
+func (b *Builder) swigDoVersionCheck() error {
+	out, err := b.runOut(nil, ".", nil, "swig", "-version")
+	if err != nil {
+		return err
+	}
+	re := regexp.MustCompile(`[vV]ersion +(\d+)([.]\d+)?([.]\d+)?`)
+	matches := re.FindSubmatch(out)
+	if matches == nil {
+		// Can't find version number; hope for the best.
+		return nil
+	}
+
+	major, err := strconv.Atoi(string(matches[1]))
+	if err != nil {
+		// Can't find version number; hope for the best.
+		return nil
+	}
+	const errmsg = "must have SWIG version >= 3.0.6"
+	if major < 3 {
+		return errors.New(errmsg)
+	}
+	if major > 3 {
+		// 4.0 or later
+		return nil
+	}
+
+	// We have SWIG version 3.x.
+	if len(matches[2]) > 0 {
+		minor, err := strconv.Atoi(string(matches[2][1:]))
+		if err != nil {
+			return nil
+		}
+		if minor > 0 {
+			// 3.1 or later
+			return nil
+		}
+	}
+
+	// We have SWIG version 3.0.x.
+	if len(matches[3]) > 0 {
+		patch, err := strconv.Atoi(string(matches[3][1:]))
+		if err != nil {
+			return nil
+		}
+		if patch < 6 {
+			// Before 3.0.6.
+			return errors.New(errmsg)
+		}
+	}
+
+	return nil
+}
+
+func (b *Builder) swigVersionCheck() error {
+	swigCheckOnce.Do(func() {
+		swigCheck = b.swigDoVersionCheck()
+	})
+	return swigCheck
+}
+
+// Find the value to pass for the -intgosize option to swig.
+var (
+	swigIntSizeOnce  sync.Once
+	swigIntSize      string
+	swigIntSizeError error
+)
+
+// This code fails to build if sizeof(int) <= 32
+const swigIntSizeCode = `
+package main
+const i int = 1 << 32
+`
+
+// Determine the size of int on the target system for the -intgosize option
+// of swig >= 2.0.9. Run only once.
+func (b *Builder) swigDoIntSize(objdir string) (intsize string, err error) {
+	if cfg.BuildN {
+		return "$INTBITS", nil
+	}
+	src := filepath.Join(b.WorkDir, "swig_intsize.go")
+	if err = os.WriteFile(src, []byte(swigIntSizeCode), 0666); err != nil {
+		return
+	}
+	srcs := []string{src}
+
+	p := load.GoFilesPackage(context.TODO(), load.PackageOpts{}, srcs)
+
+	if _, _, e := BuildToolchain.gc(b, &Action{Mode: "swigDoIntSize", Package: p, Objdir: objdir}, "", nil, nil, "", false, srcs); e != nil {
+		return "32", nil
+	}
+	return "64", nil
+}
+
+// Determine the size of int on the target system for the -intgosize option
+// of swig >= 2.0.9.
+func (b *Builder) swigIntSize(objdir string) (intsize string, err error) {
+	swigIntSizeOnce.Do(func() {
+		swigIntSize, swigIntSizeError = b.swigDoIntSize(objdir)
+	})
+	return swigIntSize, swigIntSizeError
+}
+
+// Run SWIG on one SWIG input file.
+func (b *Builder) swigOne(a *Action, p *load.Package, file, objdir string, pcCFLAGS []string, cxx bool, intgosize string) (outGo, outC string, err error) {
+	cgoCPPFLAGS, cgoCFLAGS, cgoCXXFLAGS, _, _, err := b.CFlags(p)
+	if err != nil {
+		return "", "", err
+	}
+
+	var cflags []string
+	if cxx {
+		cflags = str.StringList(cgoCPPFLAGS, pcCFLAGS, cgoCXXFLAGS)
+	} else {
+		cflags = str.StringList(cgoCPPFLAGS, pcCFLAGS, cgoCFLAGS)
+	}
+
+	n := 5 // length of ".swig"
+	if cxx {
+		n = 8 // length of ".swigcxx"
+	}
+	base := file[:len(file)-n]
+	goFile := base + ".go"
+	gccBase := base + "_wrap."
+	gccExt := "c"
+	if cxx {
+		gccExt = "cxx"
+	}
+
+	gccgo := cfg.BuildToolchainName == "gccgo"
+
+	// swig
+	args := []string{
+		"-go",
+		"-cgo",
+		"-intgosize", intgosize,
+		"-module", base,
+		"-o", objdir + gccBase + gccExt,
+		"-outdir", objdir,
+	}
+
+	for _, f := range cflags {
+		if len(f) > 3 && f[:2] == "-I" {
+			args = append(args, f)
+		}
+	}
+
+	if gccgo {
+		args = append(args, "-gccgo")
+		if pkgpath := gccgoPkgpath(p); pkgpath != "" {
+			args = append(args, "-go-pkgpath", pkgpath)
+		}
+	}
+	if cxx {
+		args = append(args, "-c++")
+	}
+
+	out, err := b.runOut(a, p.Dir, nil, "swig", args, file)
+	if err != nil {
+		if len(out) > 0 {
+			if bytes.Contains(out, []byte("-intgosize")) || bytes.Contains(out, []byte("-cgo")) {
+				return "", "", errors.New("must have SWIG version >= 3.0.6")
+			}
+			// swig error
+			err = formatOutput(b.WorkDir, p.Dir, p.ImportPath, p.Desc(), b.processOutput(out))
+		}
+		return "", "", err
+	}
+	if len(out) > 0 {
+		b.showOutput(a, p.Dir, p.Desc(), b.processOutput(out)) // swig warning
+	}
+
+	// If the input was x.swig, the output is x.go in the objdir.
+	// But there might be an x.go in the original dir too, and if it
+	// uses cgo as well, cgo will be processing both and will
+	// translate both into x.cgo1.go in the objdir, overwriting one.
+	// Rename x.go to _x_swig.go to avoid this problem.
+	// We ignore files in the original dir that begin with underscore
+	// so _x_swig.go cannot conflict with an original file we were
+	// going to compile.
+	goFile = objdir + goFile
+	newGoFile := objdir + "_" + base + "_swig.go"
+	if cfg.BuildX || cfg.BuildN {
+		b.Showcmd("", "mv %s %s", goFile, newGoFile)
+	}
+	if !cfg.BuildN {
+		if err := os.Rename(goFile, newGoFile); err != nil {
+			return "", "", err
+		}
+	}
+	return newGoFile, objdir + gccBase + gccExt, nil
+}
+
+// disableBuildID adjusts a linker command line to avoid creating a
+// build ID when creating an object file rather than an executable or
+// shared library. Some systems, such as Ubuntu, always add
+// --build-id to every link, but we don't want a build ID when we are
+// producing an object file. On some of those system a plain -r (not
+// -Wl,-r) will turn off --build-id, but clang 3.0 doesn't support a
+// plain -r. I don't know how to turn off --build-id when using clang
+// other than passing a trailing --build-id=none. So that is what we
+// do, but only on systems likely to support it, which is to say,
+// systems that normally use gold or the GNU linker.
+func (b *Builder) disableBuildID(ldflags []string) []string {
+	switch cfg.Goos {
+	case "android", "dragonfly", "linux", "netbsd":
+		ldflags = append(ldflags, "-Wl,--build-id=none")
+	}
+	return ldflags
+}
+
+// mkAbsFiles converts files into a list of absolute files,
+// assuming they were originally relative to dir,
+// and returns that new list.
+func mkAbsFiles(dir string, files []string) []string {
+	abs := make([]string, len(files))
+	for i, f := range files {
+		if !filepath.IsAbs(f) {
+			f = filepath.Join(dir, f)
+		}
+		abs[i] = f
+	}
+	return abs
+}
+
+// passLongArgsInResponseFiles modifies cmd such that, for
+// certain programs, long arguments are passed in "response files", a
+// file on disk with the arguments, with one arg per line. An actual
+// argument starting with '@' means that the rest of the argument is
+// a filename of arguments to expand.
+//
+// See issues 18468 (Windows) and 37768 (Darwin).
+func passLongArgsInResponseFiles(cmd *exec.Cmd) (cleanup func()) {
+	cleanup = func() {} // no cleanup by default
+
+	var argLen int
+	for _, arg := range cmd.Args {
+		argLen += len(arg)
+	}
+
+	// If we're not approaching 32KB of args, just pass args normally.
+	// (use 30KB instead to be conservative; not sure how accounting is done)
+	if !useResponseFile(cmd.Path, argLen) {
+		return
+	}
+
+	tf, err := os.CreateTemp("", "args")
+	if err != nil {
+		log.Fatalf("error writing long arguments to response file: %v", err)
+	}
+	cleanup = func() { os.Remove(tf.Name()) }
+	var buf bytes.Buffer
+	for _, arg := range cmd.Args[1:] {
+		fmt.Fprintf(&buf, "%s\n", encodeArg(arg))
+	}
+	if _, err := tf.Write(buf.Bytes()); err != nil {
+		tf.Close()
+		cleanup()
+		log.Fatalf("error writing long arguments to response file: %v", err)
+	}
+	if err := tf.Close(); err != nil {
+		cleanup()
+		log.Fatalf("error writing long arguments to response file: %v", err)
+	}
+	cmd.Args = []string{cmd.Args[0], "@" + tf.Name()}
+	return cleanup
+}
+
+func useResponseFile(path string, argLen int) bool {
+	// Unless the program uses objabi.Flagparse, which understands
+	// response files, don't use response files.
+	// TODO: Note that other toolchains like CC are missing here for now.
+	prog := strings.TrimSuffix(filepath.Base(path), ".exe")
+	switch prog {
+	case "compile", "link", "cgo", "asm", "cover":
+	default:
+		return false
+	}
+
+	if argLen > sys.ExecArgLengthLimit {
+		return true
+	}
+
+	// On the Go build system, use response files about 10% of the
+	// time, just to exercise this codepath.
+	isBuilder := os.Getenv("GO_BUILDER_NAME") != ""
+	if isBuilder && rand.Intn(10) == 0 {
+		return true
+	}
+
+	return false
+}
+
+// encodeArg encodes an argument for response file writing.
+func encodeArg(arg string) string {
+	// If there aren't any characters we need to reencode, fastpath out.
+	if !strings.ContainsAny(arg, "\\\n") {
+		return arg
+	}
+	var b strings.Builder
+	for _, r := range arg {
+		switch r {
+		case '\\':
+			b.WriteByte('\\')
+			b.WriteByte('\\')
+		case '\n':
+			b.WriteByte('\\')
+			b.WriteByte('n')
+		default:
+			b.WriteRune(r)
+		}
+	}
+	return b.String()
+}
diff --git a/src/cmd/go/internal/work/exec_test.go b/src/cmd/go/internal/work/exec_test.go
new file mode 100644
index 0000000..8bbf25b
--- /dev/null
+++ b/src/cmd/go/internal/work/exec_test.go
@@ -0,0 +1,87 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package work
+
+import (
+	"bytes"
+	"cmd/internal/objabi"
+	"cmd/internal/sys"
+	"fmt"
+	"math/rand"
+	"testing"
+	"time"
+	"unicode/utf8"
+)
+
+func TestEncodeArgs(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		arg, want string
+	}{
+		{"", ""},
+		{"hello", "hello"},
+		{"hello\n", "hello\\n"},
+		{"hello\\", "hello\\\\"},
+		{"hello\nthere", "hello\\nthere"},
+		{"\\\n", "\\\\\\n"},
+	}
+	for _, test := range tests {
+		if got := encodeArg(test.arg); got != test.want {
+			t.Errorf("encodeArg(%q) = %q, want %q", test.arg, got, test.want)
+		}
+	}
+}
+
+func TestEncodeDecode(t *testing.T) {
+	t.Parallel()
+	tests := []string{
+		"",
+		"hello",
+		"hello\\there",
+		"hello\nthere",
+		"hello 中国",
+		"hello \n中\\国",
+	}
+	for _, arg := range tests {
+		if got := objabi.DecodeArg(encodeArg(arg)); got != arg {
+			t.Errorf("objabi.DecodeArg(encodeArg(%q)) = %q", arg, got)
+		}
+	}
+}
+
+func TestEncodeDecodeFuzz(t *testing.T) {
+	if testing.Short() {
+		t.Skip("fuzz test is slow")
+	}
+	t.Parallel()
+
+	nRunes := sys.ExecArgLengthLimit + 100
+	rBuffer := make([]rune, nRunes)
+	buf := bytes.NewBuffer([]byte(string(rBuffer)))
+
+	seed := time.Now().UnixNano()
+	t.Logf("rand seed: %v", seed)
+	rng := rand.New(rand.NewSource(seed))
+
+	for i := 0; i < 50; i++ {
+		// Generate a random string of runes.
+		buf.Reset()
+		for buf.Len() < sys.ExecArgLengthLimit+1 {
+			var r rune
+			for {
+				r = rune(rng.Intn(utf8.MaxRune + 1))
+				if utf8.ValidRune(r) {
+					break
+				}
+			}
+			fmt.Fprintf(buf, "%c", r)
+		}
+		arg := buf.String()
+
+		if got := objabi.DecodeArg(encodeArg(arg)); got != arg {
+			t.Errorf("[%d] objabi.DecodeArg(encodeArg(%q)) = %q [seed: %v]", i, arg, got, seed)
+		}
+	}
+}
diff --git a/src/cmd/go/internal/work/gc.go b/src/cmd/go/internal/work/gc.go
new file mode 100644
index 0000000..26b4e0f
--- /dev/null
+++ b/src/cmd/go/internal/work/gc.go
@@ -0,0 +1,728 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package work
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"internal/platform"
+	"io"
+	"log"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/load"
+	"cmd/go/internal/str"
+	"cmd/internal/objabi"
+	"cmd/internal/quoted"
+	"crypto/sha1"
+)
+
+// Tests can override this by setting $TESTGO_TOOLCHAIN_VERSION.
+var ToolchainVersion = runtime.Version()
+
+// The 'path' used for GOROOT_FINAL when -trimpath is specified
+const trimPathGoRootFinal string = "$GOROOT"
+
+var runtimePackages = map[string]struct{}{
+	"internal/abi":             struct{}{},
+	"internal/bytealg":         struct{}{},
+	"internal/coverage/rtcov":  struct{}{},
+	"internal/cpu":             struct{}{},
+	"internal/goarch":          struct{}{},
+	"internal/goos":            struct{}{},
+	"runtime":                  struct{}{},
+	"runtime/internal/atomic":  struct{}{},
+	"runtime/internal/math":    struct{}{},
+	"runtime/internal/sys":     struct{}{},
+	"runtime/internal/syscall": struct{}{},
+}
+
+// The Go toolchain.
+
+type gcToolchain struct{}
+
+func (gcToolchain) compiler() string {
+	return base.Tool("compile")
+}
+
+func (gcToolchain) linker() string {
+	return base.Tool("link")
+}
+
+func pkgPath(a *Action) string {
+	p := a.Package
+	ppath := p.ImportPath
+	if cfg.BuildBuildmode == "plugin" {
+		ppath = pluginPath(a)
+	} else if p.Name == "main" && !p.Internal.ForceLibrary {
+		ppath = "main"
+	}
+	return ppath
+}
+
+func (gcToolchain) gc(b *Builder, a *Action, archive string, importcfg, embedcfg []byte, symabis string, asmhdr bool, gofiles []string) (ofile string, output []byte, err error) {
+	p := a.Package
+	objdir := a.Objdir
+	if archive != "" {
+		ofile = archive
+	} else {
+		out := "_go_.o"
+		ofile = objdir + out
+	}
+
+	pkgpath := pkgPath(a)
+	defaultGcFlags := []string{"-p", pkgpath}
+	if p.Module != nil {
+		v := p.Module.GoVersion
+		if v == "" {
+			v = gover.DefaultGoModVersion
+		}
+		if allowedVersion(v) {
+			defaultGcFlags = append(defaultGcFlags, "-lang=go"+gover.Lang(v))
+		}
+	}
+	if p.Standard {
+		defaultGcFlags = append(defaultGcFlags, "-std")
+	}
+	_, compilingRuntime := runtimePackages[p.ImportPath]
+	compilingRuntime = compilingRuntime && p.Standard
+	if compilingRuntime {
+		// runtime compiles with a special gc flag to check for
+		// memory allocations that are invalid in the runtime package,
+		// and to implement some special compiler pragmas.
+		defaultGcFlags = append(defaultGcFlags, "-+")
+	}
+
+	// If we're giving the compiler the entire package (no C etc files), tell it that,
+	// so that it can give good error messages about forward declarations.
+	// Exceptions: a few standard packages have forward declarations for
+	// pieces supplied behind-the-scenes by package runtime.
+	extFiles := len(p.CgoFiles) + len(p.CFiles) + len(p.CXXFiles) + len(p.MFiles) + len(p.FFiles) + len(p.SFiles) + len(p.SysoFiles) + len(p.SwigFiles) + len(p.SwigCXXFiles)
+	if p.Standard {
+		switch p.ImportPath {
+		case "bytes", "internal/poll", "net", "os":
+			fallthrough
+		case "runtime/metrics", "runtime/pprof", "runtime/trace":
+			fallthrough
+		case "sync", "syscall", "time":
+			extFiles++
+		}
+	}
+	if extFiles == 0 {
+		defaultGcFlags = append(defaultGcFlags, "-complete")
+	}
+	if cfg.BuildContext.InstallSuffix != "" {
+		defaultGcFlags = append(defaultGcFlags, "-installsuffix", cfg.BuildContext.InstallSuffix)
+	}
+	if a.buildID != "" {
+		defaultGcFlags = append(defaultGcFlags, "-buildid", a.buildID)
+	}
+	if p.Internal.OmitDebug || cfg.Goos == "plan9" || cfg.Goarch == "wasm" {
+		defaultGcFlags = append(defaultGcFlags, "-dwarf=false")
+	}
+	if strings.HasPrefix(ToolchainVersion, "go1") && !strings.Contains(os.Args[0], "go_bootstrap") {
+		defaultGcFlags = append(defaultGcFlags, "-goversion", ToolchainVersion)
+	}
+	if p.Internal.CoverageCfg != "" {
+		defaultGcFlags = append(defaultGcFlags, "-coveragecfg="+p.Internal.CoverageCfg)
+	}
+	if p.Internal.PGOProfile != "" {
+		defaultGcFlags = append(defaultGcFlags, "-pgoprofile="+p.Internal.PGOProfile)
+	}
+	if symabis != "" {
+		defaultGcFlags = append(defaultGcFlags, "-symabis", symabis)
+	}
+
+	gcflags := str.StringList(forcedGcflags, p.Internal.Gcflags)
+	if p.Internal.FuzzInstrument {
+		gcflags = append(gcflags, fuzzInstrumentFlags()...)
+	}
+	if compilingRuntime {
+		// Remove -N, if present.
+		// It is not possible to build the runtime with no optimizations,
+		// because the compiler cannot eliminate enough write barriers.
+		for i := 0; i < len(gcflags); i++ {
+			if gcflags[i] == "-N" {
+				copy(gcflags[i:], gcflags[i+1:])
+				gcflags = gcflags[:len(gcflags)-1]
+				i--
+			}
+		}
+	}
+	// Add -c=N to use concurrent backend compilation, if possible.
+	if c := gcBackendConcurrency(gcflags); c > 1 {
+		defaultGcFlags = append(defaultGcFlags, fmt.Sprintf("-c=%d", c))
+	}
+
+	args := []any{cfg.BuildToolexec, base.Tool("compile"), "-o", ofile, "-trimpath", a.trimpath(), defaultGcFlags, gcflags}
+	if p.Internal.LocalPrefix == "" {
+		args = append(args, "-nolocalimports")
+	} else {
+		args = append(args, "-D", p.Internal.LocalPrefix)
+	}
+	if importcfg != nil {
+		if err := b.writeFile(objdir+"importcfg", importcfg); err != nil {
+			return "", nil, err
+		}
+		args = append(args, "-importcfg", objdir+"importcfg")
+	}
+	if embedcfg != nil {
+		if err := b.writeFile(objdir+"embedcfg", embedcfg); err != nil {
+			return "", nil, err
+		}
+		args = append(args, "-embedcfg", objdir+"embedcfg")
+	}
+	if ofile == archive {
+		args = append(args, "-pack")
+	}
+	if asmhdr {
+		args = append(args, "-asmhdr", objdir+"go_asm.h")
+	}
+
+	for _, f := range gofiles {
+		f := mkAbs(p.Dir, f)
+
+		// Handle overlays. Convert path names using OverlayPath
+		// so these paths can be handed directly to tools.
+		// Deleted files won't show up in when scanning directories earlier,
+		// so OverlayPath will never return "" (meaning a deleted file) here.
+		// TODO(#39958): Handle cases where the package directory
+		// doesn't exist on disk (this can happen when all the package's
+		// files are in an overlay): the code expects the package directory
+		// to exist and runs some tools in that directory.
+		// TODO(#39958): Process the overlays when the
+		// gofiles, cgofiles, cfiles, sfiles, and cxxfiles variables are
+		// created in (*Builder).build. Doing that requires rewriting the
+		// code that uses those values to expect absolute paths.
+		f, _ = fsys.OverlayPath(f)
+
+		args = append(args, f)
+	}
+
+	output, err = b.runOut(a, base.Cwd(), nil, args...)
+	return ofile, output, err
+}
+
+// gcBackendConcurrency returns the backend compiler concurrency level for a package compilation.
+func gcBackendConcurrency(gcflags []string) int {
+	// First, check whether we can use -c at all for this compilation.
+	canDashC := concurrentGCBackendCompilationEnabledByDefault
+
+	switch e := os.Getenv("GO19CONCURRENTCOMPILATION"); e {
+	case "0":
+		canDashC = false
+	case "1":
+		canDashC = true
+	case "":
+		// Not set. Use default.
+	default:
+		log.Fatalf("GO19CONCURRENTCOMPILATION must be 0, 1, or unset, got %q", e)
+	}
+
+	// TODO: Test and delete these conditions.
+	if cfg.ExperimentErr != nil || cfg.Experiment.FieldTrack || cfg.Experiment.PreemptibleLoops {
+		canDashC = false
+	}
+
+	if !canDashC {
+		return 1
+	}
+
+	// Decide how many concurrent backend compilations to allow.
+	//
+	// If we allow too many, in theory we might end up with p concurrent processes,
+	// each with c concurrent backend compiles, all fighting over the same resources.
+	// However, in practice, that seems not to happen too much.
+	// Most build graphs are surprisingly serial, so p==1 for much of the build.
+	// Furthermore, concurrent backend compilation is only enabled for a part
+	// of the overall compiler execution, so c==1 for much of the build.
+	// So don't worry too much about that interaction for now.
+	//
+	// However, in practice, setting c above 4 tends not to help very much.
+	// See the analysis in CL 41192.
+	//
+	// TODO(josharian): attempt to detect whether this particular compilation
+	// is likely to be a bottleneck, e.g. when:
+	//   - it has no successor packages to compile (usually package main)
+	//   - all paths through the build graph pass through it
+	//   - critical path scheduling says it is high priority
+	// and in such a case, set c to runtime.GOMAXPROCS(0).
+	// By default this is the same as runtime.NumCPU.
+	// We do this now when p==1.
+	// To limit parallelism, set GOMAXPROCS below numCPU; this may be useful
+	// on a low-memory builder, or if a deterministic build order is required.
+	c := runtime.GOMAXPROCS(0)
+	if cfg.BuildP == 1 {
+		// No process parallelism, do not cap compiler parallelism.
+		return c
+	}
+	// Some process parallelism. Set c to min(4, maxprocs).
+	if c > 4 {
+		c = 4
+	}
+	return c
+}
+
+// trimpath returns the -trimpath argument to use
+// when compiling the action.
+func (a *Action) trimpath() string {
+	// Keep in sync with Builder.ccompile
+	// The trimmed paths are a little different, but we need to trim in the
+	// same situations.
+
+	// Strip the object directory entirely.
+	objdir := a.Objdir
+	if len(objdir) > 1 && objdir[len(objdir)-1] == filepath.Separator {
+		objdir = objdir[:len(objdir)-1]
+	}
+	rewrite := ""
+
+	rewriteDir := a.Package.Dir
+	if cfg.BuildTrimpath {
+		importPath := a.Package.Internal.OrigImportPath
+		if m := a.Package.Module; m != nil && m.Version != "" {
+			rewriteDir = m.Path + "@" + m.Version + strings.TrimPrefix(importPath, m.Path)
+		} else {
+			rewriteDir = importPath
+		}
+		rewrite += a.Package.Dir + "=>" + rewriteDir + ";"
+	}
+
+	// Add rewrites for overlays. The 'from' and 'to' paths in overlays don't need to have
+	// same basename, so go from the overlay contents file path (passed to the compiler)
+	// to the path the disk path would be rewritten to.
+
+	cgoFiles := make(map[string]bool)
+	for _, f := range a.Package.CgoFiles {
+		cgoFiles[f] = true
+	}
+
+	// TODO(matloob): Higher up in the stack, when the logic for deciding when to make copies
+	// of c/c++/m/f/hfiles is consolidated, use the same logic that Build uses to determine
+	// whether to create the copies in objdir to decide whether to rewrite objdir to the
+	// package directory here.
+	var overlayNonGoRewrites string // rewrites for non-go files
+	hasCgoOverlay := false
+	if fsys.OverlayFile != "" {
+		for _, filename := range a.Package.AllFiles() {
+			path := filename
+			if !filepath.IsAbs(path) {
+				path = filepath.Join(a.Package.Dir, path)
+			}
+			base := filepath.Base(path)
+			isGo := strings.HasSuffix(filename, ".go") || strings.HasSuffix(filename, ".s")
+			isCgo := cgoFiles[filename] || !isGo
+			overlayPath, isOverlay := fsys.OverlayPath(path)
+			if isCgo && isOverlay {
+				hasCgoOverlay = true
+			}
+			if !isCgo && isOverlay {
+				rewrite += overlayPath + "=>" + filepath.Join(rewriteDir, base) + ";"
+			} else if isCgo {
+				// Generate rewrites for non-Go files copied to files in objdir.
+				if filepath.Dir(path) == a.Package.Dir {
+					// This is a file copied to objdir.
+					overlayNonGoRewrites += filepath.Join(objdir, base) + "=>" + filepath.Join(rewriteDir, base) + ";"
+				}
+			} else {
+				// Non-overlay Go files are covered by the a.Package.Dir rewrite rule above.
+			}
+		}
+	}
+	if hasCgoOverlay {
+		rewrite += overlayNonGoRewrites
+	}
+	rewrite += objdir + "=>"
+
+	return rewrite
+}
+
+func asmArgs(a *Action, p *load.Package) []any {
+	// Add -I pkg/GOOS_GOARCH so #include "textflag.h" works in .s files.
+	inc := filepath.Join(cfg.GOROOT, "pkg", "include")
+	pkgpath := pkgPath(a)
+	args := []any{cfg.BuildToolexec, base.Tool("asm"), "-p", pkgpath, "-trimpath", a.trimpath(), "-I", a.Objdir, "-I", inc, "-D", "GOOS_" + cfg.Goos, "-D", "GOARCH_" + cfg.Goarch, forcedAsmflags, p.Internal.Asmflags}
+	if p.ImportPath == "runtime" && cfg.Goarch == "386" {
+		for _, arg := range forcedAsmflags {
+			if arg == "-dynlink" {
+				args = append(args, "-D=GOBUILDMODE_shared=1")
+			}
+		}
+	}
+	if objabi.IsRuntimePackagePath(pkgpath) {
+		args = append(args, "-compiling-runtime")
+	}
+
+	if cfg.Goarch == "386" {
+		// Define GO386_value from cfg.GO386.
+		args = append(args, "-D", "GO386_"+cfg.GO386)
+	}
+
+	if cfg.Goarch == "amd64" {
+		// Define GOAMD64_value from cfg.GOAMD64.
+		args = append(args, "-D", "GOAMD64_"+cfg.GOAMD64)
+	}
+
+	if cfg.Goarch == "mips" || cfg.Goarch == "mipsle" {
+		// Define GOMIPS_value from cfg.GOMIPS.
+		args = append(args, "-D", "GOMIPS_"+cfg.GOMIPS)
+	}
+
+	if cfg.Goarch == "mips64" || cfg.Goarch == "mips64le" {
+		// Define GOMIPS64_value from cfg.GOMIPS64.
+		args = append(args, "-D", "GOMIPS64_"+cfg.GOMIPS64)
+	}
+
+	if cfg.Goarch == "ppc64" || cfg.Goarch == "ppc64le" {
+		// Define GOPPC64_power8..N from cfg.PPC64.
+		// We treat each powerpc version as a superset of functionality.
+		switch cfg.GOPPC64 {
+		case "power10":
+			args = append(args, "-D", "GOPPC64_power10")
+			fallthrough
+		case "power9":
+			args = append(args, "-D", "GOPPC64_power9")
+			fallthrough
+		default: // This should always be power8.
+			args = append(args, "-D", "GOPPC64_power8")
+		}
+	}
+
+	return args
+}
+
+func (gcToolchain) asm(b *Builder, a *Action, sfiles []string) ([]string, error) {
+	p := a.Package
+	args := asmArgs(a, p)
+
+	var ofiles []string
+	for _, sfile := range sfiles {
+		overlayPath, _ := fsys.OverlayPath(mkAbs(p.Dir, sfile))
+		ofile := a.Objdir + sfile[:len(sfile)-len(".s")] + ".o"
+		ofiles = append(ofiles, ofile)
+		args1 := append(args, "-o", ofile, overlayPath)
+		if err := b.run(a, p.Dir, p.ImportPath, nil, args1...); err != nil {
+			return nil, err
+		}
+	}
+	return ofiles, nil
+}
+
+func (gcToolchain) symabis(b *Builder, a *Action, sfiles []string) (string, error) {
+	mkSymabis := func(p *load.Package, sfiles []string, path string) error {
+		args := asmArgs(a, p)
+		args = append(args, "-gensymabis", "-o", path)
+		for _, sfile := range sfiles {
+			if p.ImportPath == "runtime/cgo" && strings.HasPrefix(sfile, "gcc_") {
+				continue
+			}
+			op, _ := fsys.OverlayPath(mkAbs(p.Dir, sfile))
+			args = append(args, op)
+		}
+
+		// Supply an empty go_asm.h as if the compiler had been run.
+		// -gensymabis parsing is lax enough that we don't need the
+		// actual definitions that would appear in go_asm.h.
+		if err := b.writeFile(a.Objdir+"go_asm.h", nil); err != nil {
+			return err
+		}
+
+		return b.run(a, p.Dir, p.ImportPath, nil, args...)
+	}
+
+	var symabis string // Only set if we actually create the file
+	p := a.Package
+	if len(sfiles) != 0 {
+		symabis = a.Objdir + "symabis"
+		if err := mkSymabis(p, sfiles, symabis); err != nil {
+			return "", err
+		}
+	}
+
+	return symabis, nil
+}
+
+// toolVerify checks that the command line args writes the same output file
+// if run using newTool instead.
+// Unused now but kept around for future use.
+func toolVerify(a *Action, b *Builder, p *load.Package, newTool string, ofile string, args []any) error {
+	newArgs := make([]any, len(args))
+	copy(newArgs, args)
+	newArgs[1] = base.Tool(newTool)
+	newArgs[3] = ofile + ".new" // x.6 becomes x.6.new
+	if err := b.run(a, p.Dir, p.ImportPath, nil, newArgs...); err != nil {
+		return err
+	}
+	data1, err := os.ReadFile(ofile)
+	if err != nil {
+		return err
+	}
+	data2, err := os.ReadFile(ofile + ".new")
+	if err != nil {
+		return err
+	}
+	if !bytes.Equal(data1, data2) {
+		return fmt.Errorf("%s and %s produced different output files:\n%s\n%s", filepath.Base(args[1].(string)), newTool, strings.Join(str.StringList(args...), " "), strings.Join(str.StringList(newArgs...), " "))
+	}
+	os.Remove(ofile + ".new")
+	return nil
+}
+
+func (gcToolchain) pack(b *Builder, a *Action, afile string, ofiles []string) error {
+	var absOfiles []string
+	for _, f := range ofiles {
+		absOfiles = append(absOfiles, mkAbs(a.Objdir, f))
+	}
+	absAfile := mkAbs(a.Objdir, afile)
+
+	// The archive file should have been created by the compiler.
+	// Since it used to not work that way, verify.
+	if !cfg.BuildN {
+		if _, err := os.Stat(absAfile); err != nil {
+			base.Fatalf("os.Stat of archive file failed: %v", err)
+		}
+	}
+
+	p := a.Package
+	if cfg.BuildN || cfg.BuildX {
+		cmdline := str.StringList(base.Tool("pack"), "r", absAfile, absOfiles)
+		b.Showcmd(p.Dir, "%s # internal", joinUnambiguously(cmdline))
+	}
+	if cfg.BuildN {
+		return nil
+	}
+	if err := packInternal(absAfile, absOfiles); err != nil {
+		return formatOutput(b.WorkDir, p.Dir, p.ImportPath, p.Desc(), err.Error()+"\n")
+	}
+	return nil
+}
+
+func packInternal(afile string, ofiles []string) error {
+	dst, err := os.OpenFile(afile, os.O_WRONLY|os.O_APPEND, 0)
+	if err != nil {
+		return err
+	}
+	defer dst.Close() // only for error returns or panics
+	w := bufio.NewWriter(dst)
+
+	for _, ofile := range ofiles {
+		src, err := os.Open(ofile)
+		if err != nil {
+			return err
+		}
+		fi, err := src.Stat()
+		if err != nil {
+			src.Close()
+			return err
+		}
+		// Note: Not using %-16.16s format because we care
+		// about bytes, not runes.
+		name := fi.Name()
+		if len(name) > 16 {
+			name = name[:16]
+		} else {
+			name += strings.Repeat(" ", 16-len(name))
+		}
+		size := fi.Size()
+		fmt.Fprintf(w, "%s%-12d%-6d%-6d%-8o%-10d`\n",
+			name, 0, 0, 0, 0644, size)
+		n, err := io.Copy(w, src)
+		src.Close()
+		if err == nil && n < size {
+			err = io.ErrUnexpectedEOF
+		} else if err == nil && n > size {
+			err = fmt.Errorf("file larger than size reported by stat")
+		}
+		if err != nil {
+			return fmt.Errorf("copying %s to %s: %v", ofile, afile, err)
+		}
+		if size&1 != 0 {
+			w.WriteByte(0)
+		}
+	}
+
+	if err := w.Flush(); err != nil {
+		return err
+	}
+	return dst.Close()
+}
+
+// setextld sets the appropriate linker flags for the specified compiler.
+func setextld(ldflags []string, compiler []string) ([]string, error) {
+	for _, f := range ldflags {
+		if f == "-extld" || strings.HasPrefix(f, "-extld=") {
+			// don't override -extld if supplied
+			return ldflags, nil
+		}
+	}
+	joined, err := quoted.Join(compiler)
+	if err != nil {
+		return nil, err
+	}
+	return append(ldflags, "-extld="+joined), nil
+}
+
+// pluginPath computes the package path for a plugin main package.
+//
+// This is typically the import path of the main package p, unless the
+// plugin is being built directly from source files. In that case we
+// combine the package build ID with the contents of the main package
+// source files. This allows us to identify two different plugins
+// built from two source files with the same name.
+func pluginPath(a *Action) string {
+	p := a.Package
+	if p.ImportPath != "command-line-arguments" {
+		return p.ImportPath
+	}
+	h := sha1.New()
+	buildID := a.buildID
+	if a.Mode == "link" {
+		// For linking, use the main package's build ID instead of
+		// the binary's build ID, so it is the same hash used in
+		// compiling and linking.
+		// When compiling, we use actionID/actionID (instead of
+		// actionID/contentID) as a temporary build ID to compute
+		// the hash. Do the same here. (See buildid.go:useCache)
+		// The build ID matters because it affects the overall hash
+		// in the plugin's pseudo-import path returned below.
+		// We need to use the same import path when compiling and linking.
+		id := strings.Split(buildID, buildIDSeparator)
+		buildID = id[1] + buildIDSeparator + id[1]
+	}
+	fmt.Fprintf(h, "build ID: %s\n", buildID)
+	for _, file := range str.StringList(p.GoFiles, p.CgoFiles, p.SFiles) {
+		data, err := os.ReadFile(filepath.Join(p.Dir, file))
+		if err != nil {
+			base.Fatalf("go: %s", err)
+		}
+		h.Write(data)
+	}
+	return fmt.Sprintf("plugin/unnamed-%x", h.Sum(nil))
+}
+
+func (gcToolchain) ld(b *Builder, root *Action, out, importcfg, mainpkg string) error {
+	cxx := len(root.Package.CXXFiles) > 0 || len(root.Package.SwigCXXFiles) > 0
+	for _, a := range root.Deps {
+		if a.Package != nil && (len(a.Package.CXXFiles) > 0 || len(a.Package.SwigCXXFiles) > 0) {
+			cxx = true
+		}
+	}
+	var ldflags []string
+	if cfg.BuildContext.InstallSuffix != "" {
+		ldflags = append(ldflags, "-installsuffix", cfg.BuildContext.InstallSuffix)
+	}
+	if root.Package.Internal.OmitDebug {
+		ldflags = append(ldflags, "-s", "-w")
+	}
+	if cfg.BuildBuildmode == "plugin" {
+		ldflags = append(ldflags, "-pluginpath", pluginPath(root))
+	}
+
+	// Store BuildID inside toolchain binaries as a unique identifier of the
+	// tool being run, for use by content-based staleness determination.
+	if root.Package.Goroot && strings.HasPrefix(root.Package.ImportPath, "cmd/") {
+		// External linking will include our build id in the external
+		// linker's build id, which will cause our build id to not
+		// match the next time the tool is built.
+		// Rely on the external build id instead.
+		if !platform.MustLinkExternal(cfg.Goos, cfg.Goarch, false) {
+			ldflags = append(ldflags, "-X=cmd/internal/objabi.buildID="+root.buildID)
+		}
+	}
+
+	// Store default GODEBUG in binaries.
+	if root.Package.DefaultGODEBUG != "" {
+		ldflags = append(ldflags, "-X=runtime.godebugDefault="+root.Package.DefaultGODEBUG)
+	}
+
+	// If the user has not specified the -extld option, then specify the
+	// appropriate linker. In case of C++ code, use the compiler named
+	// by the CXX environment variable or defaultCXX if CXX is not set.
+	// Else, use the CC environment variable and defaultCC as fallback.
+	var compiler []string
+	if cxx {
+		compiler = envList("CXX", cfg.DefaultCXX(cfg.Goos, cfg.Goarch))
+	} else {
+		compiler = envList("CC", cfg.DefaultCC(cfg.Goos, cfg.Goarch))
+	}
+	ldflags = append(ldflags, "-buildmode="+ldBuildmode)
+	if root.buildID != "" {
+		ldflags = append(ldflags, "-buildid="+root.buildID)
+	}
+	ldflags = append(ldflags, forcedLdflags...)
+	ldflags = append(ldflags, root.Package.Internal.Ldflags...)
+	ldflags, err := setextld(ldflags, compiler)
+	if err != nil {
+		return err
+	}
+
+	// On OS X when using external linking to build a shared library,
+	// the argument passed here to -o ends up recorded in the final
+	// shared library in the LC_ID_DYLIB load command.
+	// To avoid putting the temporary output directory name there
+	// (and making the resulting shared library useless),
+	// run the link in the output directory so that -o can name
+	// just the final path element.
+	// On Windows, DLL file name is recorded in PE file
+	// export section, so do like on OS X.
+	// On Linux, for a shared object, at least with the Gold linker,
+	// the output file path is recorded in the .gnu.version_d section.
+	dir := "."
+	if cfg.BuildBuildmode == "c-shared" || cfg.BuildBuildmode == "plugin" {
+		dir, out = filepath.Split(out)
+	}
+
+	env := []string{}
+	if cfg.BuildTrimpath {
+		env = append(env, "GOROOT_FINAL="+trimPathGoRootFinal)
+	}
+	return b.run(root, dir, root.Package.ImportPath, env, cfg.BuildToolexec, base.Tool("link"), "-o", out, "-importcfg", importcfg, ldflags, mainpkg)
+}
+
+func (gcToolchain) ldShared(b *Builder, root *Action, toplevelactions []*Action, out, importcfg string, allactions []*Action) error {
+	ldflags := []string{"-installsuffix", cfg.BuildContext.InstallSuffix}
+	ldflags = append(ldflags, "-buildmode=shared")
+	ldflags = append(ldflags, forcedLdflags...)
+	ldflags = append(ldflags, root.Package.Internal.Ldflags...)
+	cxx := false
+	for _, a := range allactions {
+		if a.Package != nil && (len(a.Package.CXXFiles) > 0 || len(a.Package.SwigCXXFiles) > 0) {
+			cxx = true
+		}
+	}
+	// If the user has not specified the -extld option, then specify the
+	// appropriate linker. In case of C++ code, use the compiler named
+	// by the CXX environment variable or defaultCXX if CXX is not set.
+	// Else, use the CC environment variable and defaultCC as fallback.
+	var compiler []string
+	if cxx {
+		compiler = envList("CXX", cfg.DefaultCXX(cfg.Goos, cfg.Goarch))
+	} else {
+		compiler = envList("CC", cfg.DefaultCC(cfg.Goos, cfg.Goarch))
+	}
+	ldflags, err := setextld(ldflags, compiler)
+	if err != nil {
+		return err
+	}
+	for _, d := range toplevelactions {
+		if !strings.HasSuffix(d.Target, ".a") { // omit unsafe etc and actions for other shared libraries
+			continue
+		}
+		ldflags = append(ldflags, d.Package.ImportPath+"="+d.Target)
+	}
+	return b.run(root, ".", out, nil, cfg.BuildToolexec, base.Tool("link"), "-o", out, "-importcfg", importcfg, ldflags)
+}
+
+func (gcToolchain) cc(b *Builder, a *Action, ofile, cfile string) error {
+	return fmt.Errorf("%s: C source files not supported without cgo", mkAbs(a.Package.Dir, cfile))
+}
diff --git a/src/cmd/go/internal/work/gccgo.go b/src/cmd/go/internal/work/gccgo.go
new file mode 100644
index 0000000..a048b7f
--- /dev/null
+++ b/src/cmd/go/internal/work/gccgo.go
@@ -0,0 +1,677 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package work
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"sync"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/load"
+	"cmd/go/internal/str"
+	"cmd/internal/pkgpath"
+)
+
+// The Gccgo toolchain.
+
+type gccgoToolchain struct{}
+
+var GccgoName, GccgoBin string
+var gccgoErr error
+
+func init() {
+	GccgoName = cfg.Getenv("GCCGO")
+	if GccgoName == "" {
+		GccgoName = "gccgo"
+	}
+	GccgoBin, gccgoErr = exec.LookPath(GccgoName)
+}
+
+func (gccgoToolchain) compiler() string {
+	checkGccgoBin()
+	return GccgoBin
+}
+
+func (gccgoToolchain) linker() string {
+	checkGccgoBin()
+	return GccgoBin
+}
+
+func (gccgoToolchain) ar() string {
+	ar := cfg.Getenv("AR")
+	if ar == "" {
+		ar = "ar"
+	}
+	return ar
+}
+
+func checkGccgoBin() {
+	if gccgoErr == nil {
+		return
+	}
+	fmt.Fprintf(os.Stderr, "cmd/go: gccgo: %s\n", gccgoErr)
+	base.SetExitStatus(2)
+	base.Exit()
+}
+
+func (tools gccgoToolchain) gc(b *Builder, a *Action, archive string, importcfg, embedcfg []byte, symabis string, asmhdr bool, gofiles []string) (ofile string, output []byte, err error) {
+	p := a.Package
+	objdir := a.Objdir
+	out := "_go_.o"
+	ofile = objdir + out
+	gcargs := []string{"-g"}
+	gcargs = append(gcargs, b.gccArchArgs()...)
+	gcargs = append(gcargs, "-fdebug-prefix-map="+b.WorkDir+"=/tmp/go-build")
+	gcargs = append(gcargs, "-gno-record-gcc-switches")
+	if pkgpath := gccgoPkgpath(p); pkgpath != "" {
+		gcargs = append(gcargs, "-fgo-pkgpath="+pkgpath)
+	}
+	if p.Internal.LocalPrefix != "" {
+		gcargs = append(gcargs, "-fgo-relative-import-path="+p.Internal.LocalPrefix)
+	}
+
+	args := str.StringList(tools.compiler(), "-c", gcargs, "-o", ofile, forcedGccgoflags)
+	if importcfg != nil {
+		if b.gccSupportsFlag(args[:1], "-fgo-importcfg=/dev/null") {
+			if err := b.writeFile(objdir+"importcfg", importcfg); err != nil {
+				return "", nil, err
+			}
+			args = append(args, "-fgo-importcfg="+objdir+"importcfg")
+		} else {
+			root := objdir + "_importcfgroot_"
+			if err := buildImportcfgSymlinks(b, root, importcfg); err != nil {
+				return "", nil, err
+			}
+			args = append(args, "-I", root)
+		}
+	}
+	if embedcfg != nil && b.gccSupportsFlag(args[:1], "-fgo-embedcfg=/dev/null") {
+		if err := b.writeFile(objdir+"embedcfg", embedcfg); err != nil {
+			return "", nil, err
+		}
+		args = append(args, "-fgo-embedcfg="+objdir+"embedcfg")
+	}
+
+	if b.gccSupportsFlag(args[:1], "-ffile-prefix-map=a=b") {
+		if cfg.BuildTrimpath {
+			args = append(args, "-ffile-prefix-map="+base.Cwd()+"=.")
+			args = append(args, "-ffile-prefix-map="+b.WorkDir+"=/tmp/go-build")
+		}
+		if fsys.OverlayFile != "" {
+			for _, name := range gofiles {
+				absPath := mkAbs(p.Dir, name)
+				overlayPath, ok := fsys.OverlayPath(absPath)
+				if !ok {
+					continue
+				}
+				toPath := absPath
+				// gccgo only applies the last matching rule, so also handle the case where
+				// BuildTrimpath is true and the path is relative to base.Cwd().
+				if cfg.BuildTrimpath && str.HasFilePathPrefix(toPath, base.Cwd()) {
+					toPath = "." + toPath[len(base.Cwd()):]
+				}
+				args = append(args, "-ffile-prefix-map="+overlayPath+"="+toPath)
+			}
+		}
+	}
+
+	args = append(args, a.Package.Internal.Gccgoflags...)
+	for _, f := range gofiles {
+		f := mkAbs(p.Dir, f)
+		// Overlay files if necessary.
+		// See comment on gctoolchain.gc about overlay TODOs
+		f, _ = fsys.OverlayPath(f)
+		args = append(args, f)
+	}
+
+	output, err = b.runOut(a, p.Dir, nil, args)
+	return ofile, output, err
+}
+
+// buildImportcfgSymlinks builds in root a tree of symlinks
+// implementing the directives from importcfg.
+// This serves as a temporary transition mechanism until
+// we can depend on gccgo reading an importcfg directly.
+// (The Go 1.9 and later gc compilers already do.)
+func buildImportcfgSymlinks(b *Builder, root string, importcfg []byte) error {
+	for lineNum, line := range strings.Split(string(importcfg), "\n") {
+		lineNum++ // 1-based
+		line = strings.TrimSpace(line)
+		if line == "" {
+			continue
+		}
+		if line == "" || strings.HasPrefix(line, "#") {
+			continue
+		}
+		var verb, args string
+		if i := strings.Index(line, " "); i < 0 {
+			verb = line
+		} else {
+			verb, args = line[:i], strings.TrimSpace(line[i+1:])
+		}
+		before, after, _ := strings.Cut(args, "=")
+		switch verb {
+		default:
+			base.Fatalf("importcfg:%d: unknown directive %q", lineNum, verb)
+		case "packagefile":
+			if before == "" || after == "" {
+				return fmt.Errorf(`importcfg:%d: invalid packagefile: syntax is "packagefile path=filename": %s`, lineNum, line)
+			}
+			archive := gccgoArchive(root, before)
+			if err := b.Mkdir(filepath.Dir(archive)); err != nil {
+				return err
+			}
+			if err := b.Symlink(after, archive); err != nil {
+				return err
+			}
+		case "importmap":
+			if before == "" || after == "" {
+				return fmt.Errorf(`importcfg:%d: invalid importmap: syntax is "importmap old=new": %s`, lineNum, line)
+			}
+			beforeA := gccgoArchive(root, before)
+			afterA := gccgoArchive(root, after)
+			if err := b.Mkdir(filepath.Dir(beforeA)); err != nil {
+				return err
+			}
+			if err := b.Mkdir(filepath.Dir(afterA)); err != nil {
+				return err
+			}
+			if err := b.Symlink(afterA, beforeA); err != nil {
+				return err
+			}
+		case "packageshlib":
+			return fmt.Errorf("gccgo -importcfg does not support shared libraries")
+		}
+	}
+	return nil
+}
+
+func (tools gccgoToolchain) asm(b *Builder, a *Action, sfiles []string) ([]string, error) {
+	p := a.Package
+	var ofiles []string
+	for _, sfile := range sfiles {
+		base := filepath.Base(sfile)
+		ofile := a.Objdir + base[:len(base)-len(".s")] + ".o"
+		ofiles = append(ofiles, ofile)
+		sfile, _ = fsys.OverlayPath(mkAbs(p.Dir, sfile))
+		defs := []string{"-D", "GOOS_" + cfg.Goos, "-D", "GOARCH_" + cfg.Goarch}
+		if pkgpath := tools.gccgoCleanPkgpath(b, p); pkgpath != "" {
+			defs = append(defs, `-D`, `GOPKGPATH=`+pkgpath)
+		}
+		defs = tools.maybePIC(defs)
+		defs = append(defs, b.gccArchArgs()...)
+		err := b.run(a, p.Dir, p.ImportPath, nil, tools.compiler(), "-xassembler-with-cpp", "-I", a.Objdir, "-c", "-o", ofile, defs, sfile)
+		if err != nil {
+			return nil, err
+		}
+	}
+	return ofiles, nil
+}
+
+func (gccgoToolchain) symabis(b *Builder, a *Action, sfiles []string) (string, error) {
+	return "", nil
+}
+
+func gccgoArchive(basedir, imp string) string {
+	end := filepath.FromSlash(imp + ".a")
+	afile := filepath.Join(basedir, end)
+	// add "lib" to the final element
+	return filepath.Join(filepath.Dir(afile), "lib"+filepath.Base(afile))
+}
+
+func (tools gccgoToolchain) pack(b *Builder, a *Action, afile string, ofiles []string) error {
+	p := a.Package
+	objdir := a.Objdir
+	var absOfiles []string
+	for _, f := range ofiles {
+		absOfiles = append(absOfiles, mkAbs(objdir, f))
+	}
+	var arArgs []string
+	if cfg.Goos == "aix" && cfg.Goarch == "ppc64" {
+		// AIX puts both 32-bit and 64-bit objects in the same archive.
+		// Tell the AIX "ar" command to only care about 64-bit objects.
+		arArgs = []string{"-X64"}
+	}
+	absAfile := mkAbs(objdir, afile)
+	// Try with D modifier first, then without if that fails.
+	output, err := b.runOut(a, p.Dir, nil, tools.ar(), arArgs, "rcD", absAfile, absOfiles)
+	if err != nil {
+		return b.run(a, p.Dir, p.ImportPath, nil, tools.ar(), arArgs, "rc", absAfile, absOfiles)
+	}
+
+	if len(output) > 0 {
+		// Show the output if there is any even without errors.
+		b.showOutput(a, p.Dir, p.ImportPath, b.processOutput(output))
+	}
+
+	return nil
+}
+
+func (tools gccgoToolchain) link(b *Builder, root *Action, out, importcfg string, allactions []*Action, buildmode, desc string) error {
+	// gccgo needs explicit linking with all package dependencies,
+	// and all LDFLAGS from cgo dependencies.
+	afiles := []string{}
+	shlibs := []string{}
+	ldflags := b.gccArchArgs()
+	cgoldflags := []string{}
+	usesCgo := false
+	cxx := false
+	objc := false
+	fortran := false
+	if root.Package != nil {
+		cxx = len(root.Package.CXXFiles) > 0 || len(root.Package.SwigCXXFiles) > 0
+		objc = len(root.Package.MFiles) > 0
+		fortran = len(root.Package.FFiles) > 0
+	}
+
+	readCgoFlags := func(flagsFile string) error {
+		flags, err := os.ReadFile(flagsFile)
+		if err != nil {
+			return err
+		}
+		const ldflagsPrefix = "_CGO_LDFLAGS="
+		for _, line := range strings.Split(string(flags), "\n") {
+			if strings.HasPrefix(line, ldflagsPrefix) {
+				flag := line[len(ldflagsPrefix):]
+				// Every _cgo_flags file has -g and -O2 in _CGO_LDFLAGS
+				// but they don't mean anything to the linker so filter
+				// them out.
+				if flag != "-g" && !strings.HasPrefix(flag, "-O") {
+					cgoldflags = append(cgoldflags, flag)
+				}
+			}
+		}
+		return nil
+	}
+
+	var arArgs []string
+	if cfg.Goos == "aix" && cfg.Goarch == "ppc64" {
+		// AIX puts both 32-bit and 64-bit objects in the same archive.
+		// Tell the AIX "ar" command to only care about 64-bit objects.
+		arArgs = []string{"-X64"}
+	}
+
+	newID := 0
+	readAndRemoveCgoFlags := func(archive string) (string, error) {
+		newID++
+		newArchive := root.Objdir + fmt.Sprintf("_pkg%d_.a", newID)
+		if err := b.copyFile(newArchive, archive, 0666, false); err != nil {
+			return "", err
+		}
+		if cfg.BuildN || cfg.BuildX {
+			b.Showcmd("", "ar d %s _cgo_flags", newArchive)
+			if cfg.BuildN {
+				// TODO(rsc): We could do better about showing the right _cgo_flags even in -n mode.
+				// Either the archive is already built and we can read them out,
+				// or we're printing commands to build the archive and can
+				// forward the _cgo_flags directly to this step.
+				return "", nil
+			}
+		}
+		err := b.run(root, root.Objdir, desc, nil, tools.ar(), arArgs, "x", newArchive, "_cgo_flags")
+		if err != nil {
+			return "", err
+		}
+		err = b.run(root, ".", desc, nil, tools.ar(), arArgs, "d", newArchive, "_cgo_flags")
+		if err != nil {
+			return "", err
+		}
+		err = readCgoFlags(filepath.Join(root.Objdir, "_cgo_flags"))
+		if err != nil {
+			return "", err
+		}
+		return newArchive, nil
+	}
+
+	// If using -linkshared, find the shared library deps.
+	haveShlib := make(map[string]bool)
+	targetBase := filepath.Base(root.Target)
+	if cfg.BuildLinkshared {
+		for _, a := range root.Deps {
+			p := a.Package
+			if p == nil || p.Shlib == "" {
+				continue
+			}
+
+			// The .a we are linking into this .so
+			// will have its Shlib set to this .so.
+			// Don't start thinking we want to link
+			// this .so into itself.
+			base := filepath.Base(p.Shlib)
+			if base != targetBase {
+				haveShlib[base] = true
+			}
+		}
+	}
+
+	// Arrange the deps into afiles and shlibs.
+	addedShlib := make(map[string]bool)
+	for _, a := range root.Deps {
+		p := a.Package
+		if p != nil && p.Shlib != "" && haveShlib[filepath.Base(p.Shlib)] {
+			// This is a package linked into a shared
+			// library that we will put into shlibs.
+			continue
+		}
+
+		if haveShlib[filepath.Base(a.Target)] {
+			// This is a shared library we want to link against.
+			if !addedShlib[a.Target] {
+				shlibs = append(shlibs, a.Target)
+				addedShlib[a.Target] = true
+			}
+			continue
+		}
+
+		if p != nil {
+			target := a.built
+			if p.UsesCgo() || p.UsesSwig() {
+				var err error
+				target, err = readAndRemoveCgoFlags(target)
+				if err != nil {
+					continue
+				}
+			}
+
+			afiles = append(afiles, target)
+		}
+	}
+
+	for _, a := range allactions {
+		// Gather CgoLDFLAGS, but not from standard packages.
+		// The go tool can dig up runtime/cgo from GOROOT and
+		// think that it should use its CgoLDFLAGS, but gccgo
+		// doesn't use runtime/cgo.
+		if a.Package == nil {
+			continue
+		}
+		if !a.Package.Standard {
+			cgoldflags = append(cgoldflags, a.Package.CgoLDFLAGS...)
+		}
+		if len(a.Package.CgoFiles) > 0 {
+			usesCgo = true
+		}
+		if a.Package.UsesSwig() {
+			usesCgo = true
+		}
+		if len(a.Package.CXXFiles) > 0 || len(a.Package.SwigCXXFiles) > 0 {
+			cxx = true
+		}
+		if len(a.Package.MFiles) > 0 {
+			objc = true
+		}
+		if len(a.Package.FFiles) > 0 {
+			fortran = true
+		}
+	}
+
+	wholeArchive := []string{"-Wl,--whole-archive"}
+	noWholeArchive := []string{"-Wl,--no-whole-archive"}
+	if cfg.Goos == "aix" {
+		wholeArchive = nil
+		noWholeArchive = nil
+	}
+	ldflags = append(ldflags, wholeArchive...)
+	ldflags = append(ldflags, afiles...)
+	ldflags = append(ldflags, noWholeArchive...)
+
+	ldflags = append(ldflags, cgoldflags...)
+	ldflags = append(ldflags, envList("CGO_LDFLAGS", "")...)
+	if root.Package != nil {
+		ldflags = append(ldflags, root.Package.CgoLDFLAGS...)
+	}
+	if cfg.Goos != "aix" {
+		ldflags = str.StringList("-Wl,-(", ldflags, "-Wl,-)")
+	}
+
+	if root.buildID != "" {
+		// On systems that normally use gold or the GNU linker,
+		// use the --build-id option to write a GNU build ID note.
+		switch cfg.Goos {
+		case "android", "dragonfly", "linux", "netbsd":
+			ldflags = append(ldflags, fmt.Sprintf("-Wl,--build-id=0x%x", root.buildID))
+		}
+	}
+
+	var rLibPath string
+	if cfg.Goos == "aix" {
+		rLibPath = "-Wl,-blibpath="
+	} else {
+		rLibPath = "-Wl,-rpath="
+	}
+	for _, shlib := range shlibs {
+		ldflags = append(
+			ldflags,
+			"-L"+filepath.Dir(shlib),
+			rLibPath+filepath.Dir(shlib),
+			"-l"+strings.TrimSuffix(
+				strings.TrimPrefix(filepath.Base(shlib), "lib"),
+				".so"))
+	}
+
+	var realOut string
+	goLibBegin := str.StringList(wholeArchive, "-lgolibbegin", noWholeArchive)
+	switch buildmode {
+	case "exe":
+		if usesCgo && cfg.Goos == "linux" {
+			ldflags = append(ldflags, "-Wl,-E")
+		}
+
+	case "c-archive":
+		// Link the Go files into a single .o, and also link
+		// in -lgolibbegin.
+		//
+		// We need to use --whole-archive with -lgolibbegin
+		// because it doesn't define any symbols that will
+		// cause the contents to be pulled in; it's just
+		// initialization code.
+		//
+		// The user remains responsible for linking against
+		// -lgo -lpthread -lm in the final link. We can't use
+		// -r to pick them up because we can't combine
+		// split-stack and non-split-stack code in a single -r
+		// link, and libgo picks up non-split-stack code from
+		// libffi.
+		ldflags = append(ldflags, "-Wl,-r", "-nostdlib")
+		ldflags = append(ldflags, goLibBegin...)
+
+		if nopie := b.gccNoPie([]string{tools.linker()}); nopie != "" {
+			ldflags = append(ldflags, nopie)
+		}
+
+		// We are creating an object file, so we don't want a build ID.
+		if root.buildID == "" {
+			ldflags = b.disableBuildID(ldflags)
+		}
+
+		realOut = out
+		out = out + ".o"
+
+	case "c-shared":
+		ldflags = append(ldflags, "-shared", "-nostdlib")
+		ldflags = append(ldflags, goLibBegin...)
+		ldflags = append(ldflags, "-lgo", "-lgcc_s", "-lgcc", "-lc", "-lgcc")
+
+	case "shared":
+		if cfg.Goos != "aix" {
+			ldflags = append(ldflags, "-zdefs")
+		}
+		ldflags = append(ldflags, "-shared", "-nostdlib", "-lgo", "-lgcc_s", "-lgcc", "-lc")
+
+	default:
+		base.Fatalf("-buildmode=%s not supported for gccgo", buildmode)
+	}
+
+	switch buildmode {
+	case "exe", "c-shared":
+		if cxx {
+			ldflags = append(ldflags, "-lstdc++")
+		}
+		if objc {
+			ldflags = append(ldflags, "-lobjc")
+		}
+		if fortran {
+			fc := cfg.Getenv("FC")
+			if fc == "" {
+				fc = "gfortran"
+			}
+			// support gfortran out of the box and let others pass the correct link options
+			// via CGO_LDFLAGS
+			if strings.Contains(fc, "gfortran") {
+				ldflags = append(ldflags, "-lgfortran")
+			}
+		}
+	}
+
+	if err := b.run(root, ".", desc, nil, tools.linker(), "-o", out, ldflags, forcedGccgoflags, root.Package.Internal.Gccgoflags); err != nil {
+		return err
+	}
+
+	switch buildmode {
+	case "c-archive":
+		if err := b.run(root, ".", desc, nil, tools.ar(), arArgs, "rc", realOut, out); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (tools gccgoToolchain) ld(b *Builder, root *Action, out, importcfg, mainpkg string) error {
+	return tools.link(b, root, out, importcfg, root.Deps, ldBuildmode, root.Package.ImportPath)
+}
+
+func (tools gccgoToolchain) ldShared(b *Builder, root *Action, toplevelactions []*Action, out, importcfg string, allactions []*Action) error {
+	return tools.link(b, root, out, importcfg, allactions, "shared", out)
+}
+
+func (tools gccgoToolchain) cc(b *Builder, a *Action, ofile, cfile string) error {
+	p := a.Package
+	inc := filepath.Join(cfg.GOROOT, "pkg", "include")
+	cfile = mkAbs(p.Dir, cfile)
+	defs := []string{"-D", "GOOS_" + cfg.Goos, "-D", "GOARCH_" + cfg.Goarch}
+	defs = append(defs, b.gccArchArgs()...)
+	if pkgpath := tools.gccgoCleanPkgpath(b, p); pkgpath != "" {
+		defs = append(defs, `-D`, `GOPKGPATH="`+pkgpath+`"`)
+	}
+	compiler := envList("CC", cfg.DefaultCC(cfg.Goos, cfg.Goarch))
+	if b.gccSupportsFlag(compiler, "-fsplit-stack") {
+		defs = append(defs, "-fsplit-stack")
+	}
+	defs = tools.maybePIC(defs)
+	if b.gccSupportsFlag(compiler, "-ffile-prefix-map=a=b") {
+		defs = append(defs, "-ffile-prefix-map="+base.Cwd()+"=.")
+		defs = append(defs, "-ffile-prefix-map="+b.WorkDir+"=/tmp/go-build")
+	} else if b.gccSupportsFlag(compiler, "-fdebug-prefix-map=a=b") {
+		defs = append(defs, "-fdebug-prefix-map="+b.WorkDir+"=/tmp/go-build")
+	}
+	if b.gccSupportsFlag(compiler, "-gno-record-gcc-switches") {
+		defs = append(defs, "-gno-record-gcc-switches")
+	}
+	return b.run(a, p.Dir, p.ImportPath, nil, compiler, "-Wall", "-g",
+		"-I", a.Objdir, "-I", inc, "-o", ofile, defs, "-c", cfile)
+}
+
+// maybePIC adds -fPIC to the list of arguments if needed.
+func (tools gccgoToolchain) maybePIC(args []string) []string {
+	switch cfg.BuildBuildmode {
+	case "c-shared", "shared", "plugin":
+		args = append(args, "-fPIC")
+	}
+	return args
+}
+
+func gccgoPkgpath(p *load.Package) string {
+	if p.Internal.Build.IsCommand() && !p.Internal.ForceLibrary {
+		return ""
+	}
+	return p.ImportPath
+}
+
+var gccgoToSymbolFuncOnce sync.Once
+var gccgoToSymbolFunc func(string) string
+
+func (tools gccgoToolchain) gccgoCleanPkgpath(b *Builder, p *load.Package) string {
+	gccgoToSymbolFuncOnce.Do(func() {
+		tmpdir := b.WorkDir
+		if cfg.BuildN {
+			tmpdir = os.TempDir()
+		}
+		fn, err := pkgpath.ToSymbolFunc(tools.compiler(), tmpdir)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "cmd/go: %v\n", err)
+			base.SetExitStatus(2)
+			base.Exit()
+		}
+		gccgoToSymbolFunc = fn
+	})
+
+	return gccgoToSymbolFunc(gccgoPkgpath(p))
+}
+
+var (
+	gccgoSupportsCgoIncompleteOnce sync.Once
+	gccgoSupportsCgoIncomplete     bool
+)
+
+const gccgoSupportsCgoIncompleteCode = `
+package p
+
+import "runtime/cgo"
+
+type I cgo.Incomplete
+`
+
+// supportsCgoIncomplete reports whether the gccgo/GoLLVM compiler
+// being used supports cgo.Incomplete, which was added in GCC 13.
+func (tools gccgoToolchain) supportsCgoIncomplete(b *Builder) bool {
+	gccgoSupportsCgoIncompleteOnce.Do(func() {
+		fail := func(err error) {
+			fmt.Fprintf(os.Stderr, "cmd/go: %v\n", err)
+			base.SetExitStatus(2)
+			base.Exit()
+		}
+
+		tmpdir := b.WorkDir
+		if cfg.BuildN {
+			tmpdir = os.TempDir()
+		}
+		f, err := os.CreateTemp(tmpdir, "*_gccgo_cgoincomplete.go")
+		if err != nil {
+			fail(err)
+		}
+		fn := f.Name()
+		f.Close()
+		defer os.Remove(fn)
+
+		if err := os.WriteFile(fn, []byte(gccgoSupportsCgoIncompleteCode), 0644); err != nil {
+			fail(err)
+		}
+
+		on := strings.TrimSuffix(fn, ".go") + ".o"
+		if cfg.BuildN || cfg.BuildX {
+			b.Showcmd(tmpdir, "%s -c -o %s %s || true", tools.compiler(), on, fn)
+			// Since this function affects later builds,
+			// and only generates temporary files,
+			// we run the command even with -n.
+		}
+		cmd := exec.Command(tools.compiler(), "-c", "-o", on, fn)
+		cmd.Dir = tmpdir
+		var buf strings.Builder
+		cmd.Stdout = &buf
+		cmd.Stderr = &buf
+		err = cmd.Run()
+		if out := buf.String(); len(out) > 0 {
+			b.showOutput(nil, tmpdir, b.fmtcmd(tmpdir, "%s -c -o %s %s", tools.compiler(), on, fn), buf.String())
+		}
+		gccgoSupportsCgoIncomplete = err == nil
+	})
+	return gccgoSupportsCgoIncomplete
+}
diff --git a/src/cmd/go/internal/work/init.go b/src/cmd/go/internal/work/init.go
new file mode 100644
index 0000000..29116cb
--- /dev/null
+++ b/src/cmd/go/internal/work/init.go
@@ -0,0 +1,424 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Build initialization (after flag parsing).
+
+package work
+
+import (
+	"bytes"
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/modload"
+	"cmd/internal/quoted"
+	"fmt"
+	"internal/platform"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
+	"runtime"
+	"strconv"
+	"sync"
+)
+
+var buildInitStarted = false
+
+func BuildInit() {
+	if buildInitStarted {
+		base.Fatalf("go: internal error: work.BuildInit called more than once")
+	}
+	buildInitStarted = true
+	base.AtExit(closeBuilders)
+
+	modload.Init()
+	instrumentInit()
+	buildModeInit()
+	if err := fsys.Init(base.Cwd()); err != nil {
+		base.Fatal(err)
+	}
+
+	// Make sure -pkgdir is absolute, because we run commands
+	// in different directories.
+	if cfg.BuildPkgdir != "" && !filepath.IsAbs(cfg.BuildPkgdir) {
+		p, err := filepath.Abs(cfg.BuildPkgdir)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "go: evaluating -pkgdir: %v\n", err)
+			base.SetExitStatus(2)
+			base.Exit()
+		}
+		cfg.BuildPkgdir = p
+	}
+
+	if cfg.BuildP <= 0 {
+		base.Fatalf("go: -p must be a positive integer: %v\n", cfg.BuildP)
+	}
+
+	// Make sure CC, CXX, and FC are absolute paths.
+	for _, key := range []string{"CC", "CXX", "FC"} {
+		value := cfg.Getenv(key)
+		args, err := quoted.Split(value)
+		if err != nil {
+			base.Fatalf("go: %s environment variable could not be parsed: %v", key, err)
+		}
+		if len(args) == 0 {
+			continue
+		}
+		path := args[0]
+		if !filepath.IsAbs(path) && path != filepath.Base(path) {
+			base.Fatalf("go: %s environment variable is relative; must be absolute path: %s\n", key, path)
+		}
+	}
+
+	// Set covermode if not already set.
+	// Ensure that -race and -covermode are compatible.
+	if cfg.BuildCoverMode == "" {
+		cfg.BuildCoverMode = "set"
+		if cfg.BuildRace {
+			// Default coverage mode is atomic when -race is set.
+			cfg.BuildCoverMode = "atomic"
+		}
+	}
+	if cfg.BuildRace && cfg.BuildCoverMode != "atomic" {
+		base.Fatalf(`-covermode must be "atomic", not %q, when -race is enabled`, cfg.BuildCoverMode)
+	}
+}
+
+// fuzzInstrumentFlags returns compiler flags that enable fuzzing instrumentation
+// on supported platforms.
+//
+// On unsupported platforms, fuzzInstrumentFlags returns nil, meaning no
+// instrumentation is added. 'go test -fuzz' still works without coverage,
+// but it generates random inputs without guidance, so it's much less effective.
+func fuzzInstrumentFlags() []string {
+	if !platform.FuzzInstrumented(cfg.Goos, cfg.Goarch) {
+		return nil
+	}
+	return []string{"-d=libfuzzer"}
+}
+
+func instrumentInit() {
+	if !cfg.BuildRace && !cfg.BuildMSan && !cfg.BuildASan {
+		return
+	}
+	if cfg.BuildRace && cfg.BuildMSan {
+		fmt.Fprintf(os.Stderr, "go: may not use -race and -msan simultaneously\n")
+		base.SetExitStatus(2)
+		base.Exit()
+	}
+	if cfg.BuildRace && cfg.BuildASan {
+		fmt.Fprintf(os.Stderr, "go: may not use -race and -asan simultaneously\n")
+		base.SetExitStatus(2)
+		base.Exit()
+	}
+	if cfg.BuildMSan && cfg.BuildASan {
+		fmt.Fprintf(os.Stderr, "go: may not use -msan and -asan simultaneously\n")
+		base.SetExitStatus(2)
+		base.Exit()
+	}
+	if cfg.BuildMSan && !platform.MSanSupported(cfg.Goos, cfg.Goarch) {
+		fmt.Fprintf(os.Stderr, "-msan is not supported on %s/%s\n", cfg.Goos, cfg.Goarch)
+		base.SetExitStatus(2)
+		base.Exit()
+	}
+	if cfg.BuildRace && !platform.RaceDetectorSupported(cfg.Goos, cfg.Goarch) {
+		fmt.Fprintf(os.Stderr, "-race is not supported on %s/%s\n", cfg.Goos, cfg.Goarch)
+		base.SetExitStatus(2)
+		base.Exit()
+	}
+	if cfg.BuildASan && !platform.ASanSupported(cfg.Goos, cfg.Goarch) {
+		fmt.Fprintf(os.Stderr, "-asan is not supported on %s/%s\n", cfg.Goos, cfg.Goarch)
+		base.SetExitStatus(2)
+		base.Exit()
+	}
+	// The current implementation is only compatible with the ASan library from version
+	// v7 to v9 (See the description in src/runtime/asan/asan.go). Therefore, using the
+	// -asan option must use a compatible version of ASan library, which requires that
+	// the gcc version is not less than 7 and the clang version is not less than 9,
+	// otherwise a segmentation fault will occur.
+	if cfg.BuildASan {
+		if err := compilerRequiredAsanVersion(); err != nil {
+			fmt.Fprintf(os.Stderr, "%v\n", err)
+			base.SetExitStatus(2)
+			base.Exit()
+		}
+	}
+
+	mode := "race"
+	if cfg.BuildMSan {
+		mode = "msan"
+		// MSAN needs PIE on all platforms except linux/amd64.
+		// https://github.com/llvm/llvm-project/blob/llvmorg-13.0.1/clang/lib/Driver/SanitizerArgs.cpp#L621
+		if cfg.BuildBuildmode == "default" && (cfg.Goos != "linux" || cfg.Goarch != "amd64") {
+			cfg.BuildBuildmode = "pie"
+		}
+	}
+	if cfg.BuildASan {
+		mode = "asan"
+	}
+	modeFlag := "-" + mode
+
+	// Check that cgo is enabled.
+	// Note: On macOS, -race does not require cgo. -asan and -msan still do.
+	if !cfg.BuildContext.CgoEnabled && (cfg.Goos != "darwin" || cfg.BuildASan || cfg.BuildMSan) {
+		if runtime.GOOS != cfg.Goos || runtime.GOARCH != cfg.Goarch {
+			fmt.Fprintf(os.Stderr, "go: %s requires cgo\n", modeFlag)
+		} else {
+			fmt.Fprintf(os.Stderr, "go: %s requires cgo; enable cgo by setting CGO_ENABLED=1\n", modeFlag)
+		}
+
+		base.SetExitStatus(2)
+		base.Exit()
+	}
+	forcedGcflags = append(forcedGcflags, modeFlag)
+	forcedLdflags = append(forcedLdflags, modeFlag)
+
+	if cfg.BuildContext.InstallSuffix != "" {
+		cfg.BuildContext.InstallSuffix += "_"
+	}
+	cfg.BuildContext.InstallSuffix += mode
+	cfg.BuildContext.ToolTags = append(cfg.BuildContext.ToolTags, mode)
+}
+
+func buildModeInit() {
+	gccgo := cfg.BuildToolchainName == "gccgo"
+	var codegenArg string
+
+	// Configure the build mode first, then verify that it is supported.
+	// That way, if the flag is completely bogus we will prefer to error out with
+	// "-buildmode=%s not supported" instead of naming the specific platform.
+
+	switch cfg.BuildBuildmode {
+	case "archive":
+		pkgsFilter = pkgsNotMain
+	case "c-archive":
+		pkgsFilter = oneMainPkg
+		if gccgo {
+			codegenArg = "-fPIC"
+		} else {
+			switch cfg.Goos {
+			case "darwin", "ios":
+				switch cfg.Goarch {
+				case "arm64":
+					codegenArg = "-shared"
+				}
+
+			case "dragonfly", "freebsd", "illumos", "linux", "netbsd", "openbsd", "solaris":
+				// Use -shared so that the result is
+				// suitable for inclusion in a PIE or
+				// shared library.
+				codegenArg = "-shared"
+			}
+		}
+		cfg.ExeSuffix = ".a"
+		ldBuildmode = "c-archive"
+	case "c-shared":
+		pkgsFilter = oneMainPkg
+		if gccgo {
+			codegenArg = "-fPIC"
+		} else {
+			switch cfg.Goos {
+			case "linux", "android", "freebsd":
+				codegenArg = "-shared"
+			case "windows":
+				// Do not add usual .exe suffix to the .dll file.
+				cfg.ExeSuffix = ""
+			}
+		}
+		ldBuildmode = "c-shared"
+	case "default":
+		ldBuildmode = "exe"
+		if platform.DefaultPIE(cfg.Goos, cfg.Goarch, cfg.BuildRace) {
+			ldBuildmode = "pie"
+			if cfg.Goos != "windows" && !gccgo {
+				codegenArg = "-shared"
+			}
+		}
+	case "exe":
+		pkgsFilter = pkgsMain
+		ldBuildmode = "exe"
+		// Set the pkgsFilter to oneMainPkg if the user passed a specific binary output
+		// and is using buildmode=exe for a better error message.
+		// See issue #20017.
+		if cfg.BuildO != "" {
+			pkgsFilter = oneMainPkg
+		}
+	case "pie":
+		if cfg.BuildRace {
+			base.Fatalf("-buildmode=pie not supported when -race is enabled")
+		}
+		if gccgo {
+			codegenArg = "-fPIE"
+		} else {
+			switch cfg.Goos {
+			case "aix", "windows":
+			default:
+				codegenArg = "-shared"
+			}
+		}
+		ldBuildmode = "pie"
+	case "shared":
+		pkgsFilter = pkgsNotMain
+		if gccgo {
+			codegenArg = "-fPIC"
+		} else {
+			codegenArg = "-dynlink"
+		}
+		if cfg.BuildO != "" {
+			base.Fatalf("-buildmode=shared and -o not supported together")
+		}
+		ldBuildmode = "shared"
+	case "plugin":
+		pkgsFilter = oneMainPkg
+		if gccgo {
+			codegenArg = "-fPIC"
+		} else {
+			codegenArg = "-dynlink"
+		}
+		cfg.ExeSuffix = ".so"
+		ldBuildmode = "plugin"
+	default:
+		base.Fatalf("buildmode=%s not supported", cfg.BuildBuildmode)
+	}
+
+	if cfg.BuildBuildmode != "default" && !platform.BuildModeSupported(cfg.BuildToolchainName, cfg.BuildBuildmode, cfg.Goos, cfg.Goarch) {
+		base.Fatalf("-buildmode=%s not supported on %s/%s\n", cfg.BuildBuildmode, cfg.Goos, cfg.Goarch)
+	}
+
+	if cfg.BuildLinkshared {
+		if !platform.BuildModeSupported(cfg.BuildToolchainName, "shared", cfg.Goos, cfg.Goarch) {
+			base.Fatalf("-linkshared not supported on %s/%s\n", cfg.Goos, cfg.Goarch)
+		}
+		if gccgo {
+			codegenArg = "-fPIC"
+		} else {
+			forcedAsmflags = append(forcedAsmflags, "-D=GOBUILDMODE_shared=1",
+				"-linkshared")
+			codegenArg = "-dynlink"
+			forcedGcflags = append(forcedGcflags, "-linkshared")
+			// TODO(mwhudson): remove -w when that gets fixed in linker.
+			forcedLdflags = append(forcedLdflags, "-linkshared", "-w")
+		}
+	}
+	if codegenArg != "" {
+		if gccgo {
+			forcedGccgoflags = append([]string{codegenArg}, forcedGccgoflags...)
+		} else {
+			forcedAsmflags = append([]string{codegenArg}, forcedAsmflags...)
+			forcedGcflags = append([]string{codegenArg}, forcedGcflags...)
+		}
+		// Don't alter InstallSuffix when modifying default codegen args.
+		if cfg.BuildBuildmode != "default" || cfg.BuildLinkshared {
+			if cfg.BuildContext.InstallSuffix != "" {
+				cfg.BuildContext.InstallSuffix += "_"
+			}
+			cfg.BuildContext.InstallSuffix += codegenArg[1:]
+		}
+	}
+
+	switch cfg.BuildMod {
+	case "":
+		// Behavior will be determined automatically, as if no flag were passed.
+	case "readonly", "vendor", "mod":
+		if !cfg.ModulesEnabled && !base.InGOFLAGS("-mod") {
+			base.Fatalf("build flag -mod=%s only valid when using modules", cfg.BuildMod)
+		}
+	default:
+		base.Fatalf("-mod=%s not supported (can be '', 'mod', 'readonly', or 'vendor')", cfg.BuildMod)
+	}
+	if !cfg.ModulesEnabled {
+		if cfg.ModCacheRW && !base.InGOFLAGS("-modcacherw") {
+			base.Fatalf("build flag -modcacherw only valid when using modules")
+		}
+		if cfg.ModFile != "" && !base.InGOFLAGS("-mod") {
+			base.Fatalf("build flag -modfile only valid when using modules")
+		}
+	}
+}
+
+type version struct {
+	name         string
+	major, minor int
+}
+
+var compiler struct {
+	sync.Once
+	version
+	err error
+}
+
+// compilerVersion detects the version of $(go env CC).
+// It returns a non-nil error if the compiler matches a known version schema but
+// the version could not be parsed, or if $(go env CC) could not be determined.
+func compilerVersion() (version, error) {
+	compiler.Once.Do(func() {
+		compiler.err = func() error {
+			compiler.name = "unknown"
+			cc := os.Getenv("CC")
+			out, err := exec.Command(cc, "--version").Output()
+			if err != nil {
+				// Compiler does not support "--version" flag: not Clang or GCC.
+				return err
+			}
+
+			var match [][]byte
+			if bytes.HasPrefix(out, []byte("gcc")) {
+				compiler.name = "gcc"
+				out, err := exec.Command(cc, "-v").CombinedOutput()
+				if err != nil {
+					// gcc, but does not support gcc's "-v" flag?!
+					return err
+				}
+				gccRE := regexp.MustCompile(`gcc version (\d+)\.(\d+)`)
+				match = gccRE.FindSubmatch(out)
+			} else {
+				clangRE := regexp.MustCompile(`clang version (\d+)\.(\d+)`)
+				if match = clangRE.FindSubmatch(out); len(match) > 0 {
+					compiler.name = "clang"
+				}
+			}
+
+			if len(match) < 3 {
+				return nil // "unknown"
+			}
+			if compiler.major, err = strconv.Atoi(string(match[1])); err != nil {
+				return err
+			}
+			if compiler.minor, err = strconv.Atoi(string(match[2])); err != nil {
+				return err
+			}
+			return nil
+		}()
+	})
+	return compiler.version, compiler.err
+}
+
+// compilerRequiredAsanVersion is a copy of the function defined in
+// cmd/cgo/internal/testsanitizers/cc_test.go
+// compilerRequiredAsanVersion reports whether the compiler is the version
+// required by Asan.
+func compilerRequiredAsanVersion() error {
+	compiler, err := compilerVersion()
+	if err != nil {
+		return fmt.Errorf("-asan: the version of $(go env CC) could not be parsed")
+	}
+
+	switch compiler.name {
+	case "gcc":
+		if runtime.GOARCH == "ppc64le" && compiler.major < 9 {
+			return fmt.Errorf("-asan is not supported with %s compiler %d.%d\n", compiler.name, compiler.major, compiler.minor)
+		}
+		if compiler.major < 7 {
+			return fmt.Errorf("-asan is not supported with %s compiler %d.%d\n", compiler.name, compiler.major, compiler.minor)
+		}
+	case "clang":
+		if compiler.major < 9 {
+			return fmt.Errorf("-asan is not supported with %s compiler %d.%d\n", compiler.name, compiler.major, compiler.minor)
+		}
+	default:
+		return fmt.Errorf("-asan: C compiler is not gcc or clang")
+	}
+	return nil
+}
diff --git a/src/cmd/go/internal/work/security.go b/src/cmd/go/internal/work/security.go
new file mode 100644
index 0000000..270a34e
--- /dev/null
+++ b/src/cmd/go/internal/work/security.go
@@ -0,0 +1,334 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Checking of compiler and linker flags.
+// We must avoid flags like -fplugin=, which can allow
+// arbitrary code execution during the build.
+// Do not make changes here without carefully
+// considering the implications.
+// (That's why the code is isolated in a file named security.go.)
+//
+// Note that -Wl,foo means split foo on commas and pass to
+// the linker, so that -Wl,-foo,bar means pass -foo bar to
+// the linker. Similarly -Wa,foo for the assembler and so on.
+// If any of these are permitted, the wildcard portion must
+// disallow commas.
+//
+// Note also that GNU binutils accept any argument @foo
+// as meaning "read more flags from the file foo", so we must
+// guard against any command-line argument beginning with @,
+// even things like "-I @foo".
+// We use load.SafeArg (which is even more conservative)
+// to reject these.
+//
+// Even worse, gcc -I@foo (one arg) turns into cc1 -I @foo (two args),
+// so although gcc doesn't expand the @foo, cc1 will.
+// So out of paranoia, we reject @ at the beginning of every
+// flag argument that might be split into its own argument.
+
+package work
+
+import (
+	"fmt"
+	"internal/lazyregexp"
+	"regexp"
+	"strings"
+
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/load"
+)
+
+var re = lazyregexp.New
+
+var validCompilerFlags = []*lazyregexp.Regexp{
+	re(`-D([A-Za-z_][A-Za-z0-9_]*)(=[^@\-]*)?`),
+	re(`-U([A-Za-z_][A-Za-z0-9_]*)`),
+	re(`-F([^@\-].*)`),
+	re(`-I([^@\-].*)`),
+	re(`-O`),
+	re(`-O([^@\-].*)`),
+	re(`-W`),
+	re(`-W([^@,]+)`), // -Wall but not -Wa,-foo.
+	re(`-Wa,-mbig-obj`),
+	re(`-Wp,-D([A-Za-z_][A-Za-z0-9_]*)(=[^@,\-]*)?`),
+	re(`-Wp,-U([A-Za-z_][A-Za-z0-9_]*)`),
+	re(`-ansi`),
+	re(`-f(no-)?asynchronous-unwind-tables`),
+	re(`-f(no-)?blocks`),
+	re(`-f(no-)builtin-[a-zA-Z0-9_]*`),
+	re(`-f(no-)?common`),
+	re(`-f(no-)?constant-cfstrings`),
+	re(`-fdiagnostics-show-note-include-stack`),
+	re(`-f(no-)?eliminate-unused-debug-types`),
+	re(`-f(no-)?exceptions`),
+	re(`-f(no-)?fast-math`),
+	re(`-f(no-)?inline-functions`),
+	re(`-finput-charset=([^@\-].*)`),
+	re(`-f(no-)?fat-lto-objects`),
+	re(`-f(no-)?keep-inline-dllexport`),
+	re(`-f(no-)?lto`),
+	re(`-fmacro-backtrace-limit=(.+)`),
+	re(`-fmessage-length=(.+)`),
+	re(`-f(no-)?modules`),
+	re(`-f(no-)?objc-arc`),
+	re(`-f(no-)?objc-nonfragile-abi`),
+	re(`-f(no-)?objc-legacy-dispatch`),
+	re(`-f(no-)?omit-frame-pointer`),
+	re(`-f(no-)?openmp(-simd)?`),
+	re(`-f(no-)?permissive`),
+	re(`-f(no-)?(pic|PIC|pie|PIE)`),
+	re(`-f(no-)?plt`),
+	re(`-f(no-)?rtti`),
+	re(`-f(no-)?split-stack`),
+	re(`-f(no-)?stack-(.+)`),
+	re(`-f(no-)?strict-aliasing`),
+	re(`-f(un)signed-char`),
+	re(`-f(no-)?use-linker-plugin`), // safe if -B is not used; we don't permit -B
+	re(`-f(no-)?visibility-inlines-hidden`),
+	re(`-fsanitize=(.+)`),
+	re(`-ftemplate-depth-(.+)`),
+	re(`-fvisibility=(.+)`),
+	re(`-g([^@\-].*)?`),
+	re(`-m32`),
+	re(`-m64`),
+	re(`-m(abi|arch|cpu|fpu|tune)=([^@\-].*)`),
+	re(`-m(no-)?v?aes`),
+	re(`-marm`),
+	re(`-m(no-)?avx[0-9a-z]*`),
+	re(`-mfloat-abi=([^@\-].*)`),
+	re(`-mfpmath=[0-9a-z,+]*`),
+	re(`-m(no-)?avx[0-9a-z.]*`),
+	re(`-m(no-)?ms-bitfields`),
+	re(`-m(no-)?stack-(.+)`),
+	re(`-mmacosx-(.+)`),
+	re(`-mios-simulator-version-min=(.+)`),
+	re(`-miphoneos-version-min=(.+)`),
+	re(`-mtvos-simulator-version-min=(.+)`),
+	re(`-mtvos-version-min=(.+)`),
+	re(`-mwatchos-simulator-version-min=(.+)`),
+	re(`-mwatchos-version-min=(.+)`),
+	re(`-mnop-fun-dllimport`),
+	re(`-m(no-)?sse[0-9.]*`),
+	re(`-m(no-)?ssse3`),
+	re(`-mthumb(-interwork)?`),
+	re(`-mthreads`),
+	re(`-mwindows`),
+	re(`--param=ssp-buffer-size=[0-9]*`),
+	re(`-pedantic(-errors)?`),
+	re(`-pipe`),
+	re(`-pthread`),
+	re(`-?-std=([^@\-].*)`),
+	re(`-?-stdlib=([^@\-].*)`),
+	re(`--sysroot=([^@\-].*)`),
+	re(`-w`),
+	re(`-x([^@\-].*)`),
+	re(`-v`),
+}
+
+var validCompilerFlagsWithNextArg = []string{
+	"-arch",
+	"-D",
+	"-U",
+	"-I",
+	"-F",
+	"-framework",
+	"-include",
+	"-isysroot",
+	"-isystem",
+	"--sysroot",
+	"-target",
+	"-x",
+}
+
+var validLinkerFlags = []*lazyregexp.Regexp{
+	re(`-F([^@\-].*)`),
+	re(`-l([^@\-].*)`),
+	re(`-L([^@\-].*)`),
+	re(`-O`),
+	re(`-O([^@\-].*)`),
+	re(`-f(no-)?(pic|PIC|pie|PIE)`),
+	re(`-f(no-)?openmp(-simd)?`),
+	re(`-fsanitize=([^@\-].*)`),
+	re(`-flat_namespace`),
+	re(`-g([^@\-].*)?`),
+	re(`-headerpad_max_install_names`),
+	re(`-m(abi|arch|cpu|fpu|tune)=([^@\-].*)`),
+	re(`-mfloat-abi=([^@\-].*)`),
+	re(`-mmacosx-(.+)`),
+	re(`-mios-simulator-version-min=(.+)`),
+	re(`-miphoneos-version-min=(.+)`),
+	re(`-mthreads`),
+	re(`-mwindows`),
+	re(`-(pic|PIC|pie|PIE)`),
+	re(`-pthread`),
+	re(`-rdynamic`),
+	re(`-shared`),
+	re(`-?-static([-a-z0-9+]*)`),
+	re(`-?-stdlib=([^@\-].*)`),
+	re(`-v`),
+
+	// Note that any wildcards in -Wl need to exclude comma,
+	// since -Wl splits its argument at commas and passes
+	// them all to the linker uninterpreted. Allowing comma
+	// in a wildcard would allow tunneling arbitrary additional
+	// linker arguments through one of these.
+	re(`-Wl,--(no-)?allow-multiple-definition`),
+	re(`-Wl,--(no-)?allow-shlib-undefined`),
+	re(`-Wl,--(no-)?as-needed`),
+	re(`-Wl,-Bdynamic`),
+	re(`-Wl,-berok`),
+	re(`-Wl,-Bstatic`),
+	re(`-Wl,-Bsymbolic-functions`),
+	re(`-Wl,-O[0-9]+`),
+	re(`-Wl,-d[ny]`),
+	re(`-Wl,--disable-new-dtags`),
+	re(`-Wl,-e[=,][a-zA-Z0-9]+`),
+	re(`-Wl,--enable-new-dtags`),
+	re(`-Wl,--end-group`),
+	re(`-Wl,--(no-)?export-dynamic`),
+	re(`-Wl,-E`),
+	re(`-Wl,-framework,[^,@\-][^,]+`),
+	re(`-Wl,--hash-style=(sysv|gnu|both)`),
+	re(`-Wl,-headerpad_max_install_names`),
+	re(`-Wl,--no-undefined`),
+	re(`-Wl,-R,?([^@\-,][^,@]*$)`),
+	re(`-Wl,--just-symbols[=,]([^,@\-][^,@]+)`),
+	re(`-Wl,-rpath(-link)?[=,]([^,@\-][^,]+)`),
+	re(`-Wl,-s`),
+	re(`-Wl,-search_paths_first`),
+	re(`-Wl,-sectcreate,([^,@\-][^,]+),([^,@\-][^,]+),([^,@\-][^,]+)`),
+	re(`-Wl,--start-group`),
+	re(`-Wl,-?-static`),
+	re(`-Wl,-?-subsystem,(native|windows|console|posix|xbox)`),
+	re(`-Wl,-syslibroot[=,]([^,@\-][^,]+)`),
+	re(`-Wl,-undefined[=,]([^,@\-][^,]+)`),
+	re(`-Wl,-?-unresolved-symbols=[^,]+`),
+	re(`-Wl,--(no-)?warn-([^,]+)`),
+	re(`-Wl,-?-wrap[=,][^,@\-][^,]*`),
+	re(`-Wl,-z,(no)?execstack`),
+	re(`-Wl,-z,relro`),
+
+	re(`[a-zA-Z0-9_/].*\.(a|o|obj|dll|dylib|so|tbd)`), // direct linker inputs: x.o or libfoo.so (but not -foo.o or @foo.o)
+	re(`\./.*\.(a|o|obj|dll|dylib|so|tbd)`),
+}
+
+var validLinkerFlagsWithNextArg = []string{
+	"-arch",
+	"-F",
+	"-l",
+	"-L",
+	"-framework",
+	"-isysroot",
+	"--sysroot",
+	"-target",
+	"-Wl,-framework",
+	"-Wl,-rpath",
+	"-Wl,-R",
+	"-Wl,--just-symbols",
+	"-Wl,-undefined",
+}
+
+func checkCompilerFlags(name, source string, list []string) error {
+	checkOverrides := true
+	return checkFlags(name, source, list, validCompilerFlags, validCompilerFlagsWithNextArg, checkOverrides)
+}
+
+func checkLinkerFlags(name, source string, list []string) error {
+	checkOverrides := true
+	return checkFlags(name, source, list, validLinkerFlags, validLinkerFlagsWithNextArg, checkOverrides)
+}
+
+// checkCompilerFlagsForInternalLink returns an error if 'list'
+// contains a flag or flags that may not be fully supported by
+// internal linking (meaning that we should punt the link to the
+// external linker).
+func checkCompilerFlagsForInternalLink(name, source string, list []string) error {
+	checkOverrides := false
+	if err := checkFlags(name, source, list, validCompilerFlags, validCompilerFlagsWithNextArg, checkOverrides); err != nil {
+		return err
+	}
+	// Currently the only flag on the allow list that causes problems
+	// for the linker is "-flto"; check for it manually here.
+	for _, fl := range list {
+		if strings.HasPrefix(fl, "-flto") {
+			return fmt.Errorf("flag %q triggers external linking", fl)
+		}
+	}
+	return nil
+}
+
+func checkFlags(name, source string, list []string, valid []*lazyregexp.Regexp, validNext []string, checkOverrides bool) error {
+	// Let users override rules with $CGO_CFLAGS_ALLOW, $CGO_CFLAGS_DISALLOW, etc.
+	var (
+		allow    *regexp.Regexp
+		disallow *regexp.Regexp
+	)
+	if checkOverrides {
+		if env := cfg.Getenv("CGO_" + name + "_ALLOW"); env != "" {
+			r, err := regexp.Compile(env)
+			if err != nil {
+				return fmt.Errorf("parsing $CGO_%s_ALLOW: %v", name, err)
+			}
+			allow = r
+		}
+		if env := cfg.Getenv("CGO_" + name + "_DISALLOW"); env != "" {
+			r, err := regexp.Compile(env)
+			if err != nil {
+				return fmt.Errorf("parsing $CGO_%s_DISALLOW: %v", name, err)
+			}
+			disallow = r
+		}
+	}
+
+Args:
+	for i := 0; i < len(list); i++ {
+		arg := list[i]
+		if disallow != nil && disallow.FindString(arg) == arg {
+			goto Bad
+		}
+		if allow != nil && allow.FindString(arg) == arg {
+			continue Args
+		}
+		for _, re := range valid {
+			if re.FindString(arg) == arg { // must be complete match
+				continue Args
+			}
+		}
+		for _, x := range validNext {
+			if arg == x {
+				if i+1 < len(list) && load.SafeArg(list[i+1]) {
+					i++
+					continue Args
+				}
+
+				// Permit -Wl,-framework -Wl,name.
+				if i+1 < len(list) &&
+					strings.HasPrefix(arg, "-Wl,") &&
+					strings.HasPrefix(list[i+1], "-Wl,") &&
+					load.SafeArg(list[i+1][4:]) &&
+					!strings.Contains(list[i+1][4:], ",") {
+					i++
+					continue Args
+				}
+
+				// Permit -I= /path, -I $SYSROOT.
+				if i+1 < len(list) && arg == "-I" {
+					if (strings.HasPrefix(list[i+1], "=") || strings.HasPrefix(list[i+1], "$SYSROOT")) &&
+						load.SafeArg(list[i+1][1:]) {
+						i++
+						continue Args
+					}
+				}
+
+				if i+1 < len(list) {
+					return fmt.Errorf("invalid flag in %s: %s %s (see https://golang.org/s/invalidflag)", source, arg, list[i+1])
+				}
+				return fmt.Errorf("invalid flag in %s: %s without argument (see https://golang.org/s/invalidflag)", source, arg)
+			}
+		}
+	Bad:
+		return fmt.Errorf("invalid flag in %s: %s", source, arg)
+	}
+	return nil
+}
diff --git a/src/cmd/go/internal/work/security_test.go b/src/cmd/go/internal/work/security_test.go
new file mode 100644
index 0000000..c05ba7b
--- /dev/null
+++ b/src/cmd/go/internal/work/security_test.go
@@ -0,0 +1,318 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package work
+
+import (
+	"os"
+	"strings"
+	"testing"
+)
+
+var goodCompilerFlags = [][]string{
+	{"-DFOO"},
+	{"-Dfoo=bar"},
+	{"-Ufoo"},
+	{"-Ufoo1"},
+	{"-F/Qt"},
+	{"-F", "/Qt"},
+	{"-I/"},
+	{"-I/etc/passwd"},
+	{"-I."},
+	{"-O"},
+	{"-O2"},
+	{"-Osmall"},
+	{"-W"},
+	{"-Wall"},
+	{"-Wp,-Dfoo=bar"},
+	{"-Wp,-Ufoo"},
+	{"-Wp,-Dfoo1"},
+	{"-Wp,-Ufoo1"},
+	{"-flto"},
+	{"-fobjc-arc"},
+	{"-fno-objc-arc"},
+	{"-fomit-frame-pointer"},
+	{"-fno-omit-frame-pointer"},
+	{"-fpic"},
+	{"-fno-pic"},
+	{"-fPIC"},
+	{"-fno-PIC"},
+	{"-fpie"},
+	{"-fno-pie"},
+	{"-fPIE"},
+	{"-fno-PIE"},
+	{"-fsplit-stack"},
+	{"-fno-split-stack"},
+	{"-fstack-xxx"},
+	{"-fno-stack-xxx"},
+	{"-fsanitize=hands"},
+	{"-g"},
+	{"-ggdb"},
+	{"-march=souza"},
+	{"-mcpu=123"},
+	{"-mfpu=123"},
+	{"-mtune=happybirthday"},
+	{"-mstack-overflow"},
+	{"-mno-stack-overflow"},
+	{"-mmacosx-version"},
+	{"-mnop-fun-dllimport"},
+	{"-pthread"},
+	{"-std=c99"},
+	{"-xc"},
+	{"-D", "FOO"},
+	{"-D", "foo=bar"},
+	{"-I", "."},
+	{"-I", "/etc/passwd"},
+	{"-I", "世界"},
+	{"-I", "=/usr/include/libxml2"},
+	{"-I", "dir"},
+	{"-I", "$SYSROOT/dir"},
+	{"-isystem", "/usr/include/mozjs-68"},
+	{"-include", "/usr/include/mozjs-68/RequiredDefines.h"},
+	{"-framework", "Chocolate"},
+	{"-x", "c"},
+	{"-v"},
+}
+
+var badCompilerFlags = [][]string{
+	{"-D@X"},
+	{"-D-X"},
+	{"-Ufoo=bar"},
+	{"-F@dir"},
+	{"-F-dir"},
+	{"-I@dir"},
+	{"-I-dir"},
+	{"-O@1"},
+	{"-Wa,-foo"},
+	{"-W@foo"},
+	{"-Wp,-DX,-D@X"},
+	{"-Wp,-UX,-U@X"},
+	{"-g@gdb"},
+	{"-g-gdb"},
+	{"-march=@dawn"},
+	{"-march=-dawn"},
+	{"-std=@c99"},
+	{"-std=-c99"},
+	{"-x@c"},
+	{"-x-c"},
+	{"-D", "@foo"},
+	{"-D", "-foo"},
+	{"-I", "@foo"},
+	{"-I", "-foo"},
+	{"-I", "=@obj"},
+	{"-include", "@foo"},
+	{"-framework", "-Caffeine"},
+	{"-framework", "@Home"},
+	{"-x", "--c"},
+	{"-x", "@obj"},
+}
+
+func TestCheckCompilerFlags(t *testing.T) {
+	for _, f := range goodCompilerFlags {
+		if err := checkCompilerFlags("test", "test", f); err != nil {
+			t.Errorf("unexpected error for %q: %v", f, err)
+		}
+	}
+	for _, f := range badCompilerFlags {
+		if err := checkCompilerFlags("test", "test", f); err == nil {
+			t.Errorf("missing error for %q", f)
+		}
+	}
+}
+
+var goodLinkerFlags = [][]string{
+	{"-Fbar"},
+	{"-lbar"},
+	{"-Lbar"},
+	{"-fpic"},
+	{"-fno-pic"},
+	{"-fPIC"},
+	{"-fno-PIC"},
+	{"-fpie"},
+	{"-fno-pie"},
+	{"-fPIE"},
+	{"-fno-PIE"},
+	{"-fsanitize=hands"},
+	{"-g"},
+	{"-ggdb"},
+	{"-march=souza"},
+	{"-mcpu=123"},
+	{"-mfpu=123"},
+	{"-mtune=happybirthday"},
+	{"-pic"},
+	{"-pthread"},
+	{"-Wl,--hash-style=both"},
+	{"-Wl,-rpath,foo"},
+	{"-Wl,-rpath,$ORIGIN/foo"},
+	{"-Wl,-R", "/foo"},
+	{"-Wl,-R", "foo"},
+	{"-Wl,-R,foo"},
+	{"-Wl,--just-symbols=foo"},
+	{"-Wl,--just-symbols,foo"},
+	{"-Wl,--warn-error"},
+	{"-Wl,--no-warn-error"},
+	{"foo.so"},
+	{"_世界.dll"},
+	{"./x.o"},
+	{"libcgosotest.dylib"},
+	{"-F", "framework"},
+	{"-l", "."},
+	{"-l", "/etc/passwd"},
+	{"-l", "世界"},
+	{"-L", "framework"},
+	{"-framework", "Chocolate"},
+	{"-v"},
+	{"-Wl,-sectcreate,__TEXT,__info_plist,${SRCDIR}/Info.plist"},
+	{"-Wl,-framework", "-Wl,Chocolate"},
+	{"-Wl,-framework,Chocolate"},
+	{"-Wl,-unresolved-symbols=ignore-all"},
+	{"libcgotbdtest.tbd"},
+	{"./libcgotbdtest.tbd"},
+}
+
+var badLinkerFlags = [][]string{
+	{"-DFOO"},
+	{"-Dfoo=bar"},
+	{"-W"},
+	{"-Wall"},
+	{"-fobjc-arc"},
+	{"-fno-objc-arc"},
+	{"-fomit-frame-pointer"},
+	{"-fno-omit-frame-pointer"},
+	{"-fsplit-stack"},
+	{"-fno-split-stack"},
+	{"-fstack-xxx"},
+	{"-fno-stack-xxx"},
+	{"-mstack-overflow"},
+	{"-mno-stack-overflow"},
+	{"-mnop-fun-dllimport"},
+	{"-std=c99"},
+	{"-xc"},
+	{"-D", "FOO"},
+	{"-D", "foo=bar"},
+	{"-I", "FOO"},
+	{"-L", "@foo"},
+	{"-L", "-foo"},
+	{"-x", "c"},
+	{"-D@X"},
+	{"-D-X"},
+	{"-I@dir"},
+	{"-I-dir"},
+	{"-O@1"},
+	{"-Wa,-foo"},
+	{"-W@foo"},
+	{"-g@gdb"},
+	{"-g-gdb"},
+	{"-march=@dawn"},
+	{"-march=-dawn"},
+	{"-std=@c99"},
+	{"-std=-c99"},
+	{"-x@c"},
+	{"-x-c"},
+	{"-D", "@foo"},
+	{"-D", "-foo"},
+	{"-I", "@foo"},
+	{"-I", "-foo"},
+	{"-l", "@foo"},
+	{"-l", "-foo"},
+	{"-framework", "-Caffeine"},
+	{"-framework", "@Home"},
+	{"-Wl,-framework,-Caffeine"},
+	{"-Wl,-framework", "-Wl,@Home"},
+	{"-Wl,-framework", "@Home"},
+	{"-Wl,-framework,Chocolate,@Home"},
+	{"-Wl,--hash-style=foo"},
+	{"-x", "--c"},
+	{"-x", "@obj"},
+	{"-Wl,-rpath,@foo"},
+	{"-Wl,-R,foo,bar"},
+	{"-Wl,-R,@foo"},
+	{"-Wl,--just-symbols,@foo"},
+	{"../x.o"},
+	{"-Wl,-R,"},
+	{"-Wl,-O"},
+	{"-Wl,-e="},
+	{"-Wl,-e,"},
+	{"-Wl,-R,-flag"},
+}
+
+func TestCheckLinkerFlags(t *testing.T) {
+	for _, f := range goodLinkerFlags {
+		if err := checkLinkerFlags("test", "test", f); err != nil {
+			t.Errorf("unexpected error for %q: %v", f, err)
+		}
+	}
+	for _, f := range badLinkerFlags {
+		if err := checkLinkerFlags("test", "test", f); err == nil {
+			t.Errorf("missing error for %q", f)
+		}
+	}
+}
+
+func TestCheckFlagAllowDisallow(t *testing.T) {
+	if err := checkCompilerFlags("TEST", "test", []string{"-disallow"}); err == nil {
+		t.Fatalf("missing error for -disallow")
+	}
+	os.Setenv("CGO_TEST_ALLOW", "-disallo")
+	if err := checkCompilerFlags("TEST", "test", []string{"-disallow"}); err == nil {
+		t.Fatalf("missing error for -disallow with CGO_TEST_ALLOW=-disallo")
+	}
+	os.Setenv("CGO_TEST_ALLOW", "-disallow")
+	if err := checkCompilerFlags("TEST", "test", []string{"-disallow"}); err != nil {
+		t.Fatalf("unexpected error for -disallow with CGO_TEST_ALLOW=-disallow: %v", err)
+	}
+	os.Unsetenv("CGO_TEST_ALLOW")
+
+	if err := checkCompilerFlags("TEST", "test", []string{"-Wall"}); err != nil {
+		t.Fatalf("unexpected error for -Wall: %v", err)
+	}
+	os.Setenv("CGO_TEST_DISALLOW", "-Wall")
+	if err := checkCompilerFlags("TEST", "test", []string{"-Wall"}); err == nil {
+		t.Fatalf("missing error for -Wall with CGO_TEST_DISALLOW=-Wall")
+	}
+	os.Setenv("CGO_TEST_ALLOW", "-Wall") // disallow wins
+	if err := checkCompilerFlags("TEST", "test", []string{"-Wall"}); err == nil {
+		t.Fatalf("missing error for -Wall with CGO_TEST_DISALLOW=-Wall and CGO_TEST_ALLOW=-Wall")
+	}
+
+	os.Setenv("CGO_TEST_ALLOW", "-fplugin.*")
+	os.Setenv("CGO_TEST_DISALLOW", "-fplugin=lint.so")
+	if err := checkCompilerFlags("TEST", "test", []string{"-fplugin=faster.so"}); err != nil {
+		t.Fatalf("unexpected error for -fplugin=faster.so: %v", err)
+	}
+	if err := checkCompilerFlags("TEST", "test", []string{"-fplugin=lint.so"}); err == nil {
+		t.Fatalf("missing error for -fplugin=lint.so: %v", err)
+	}
+}
+
+func TestCheckCompilerFlagsForInternalLink(t *testing.T) {
+	// Any "bad" compiler flag should trigger external linking.
+	for _, f := range badCompilerFlags {
+		if err := checkCompilerFlagsForInternalLink("test", "test", f); err == nil {
+			t.Errorf("missing error for %q", f)
+		}
+	}
+
+	// All "good" compiler flags should not trigger external linking,
+	// except for anything that begins with "-flto".
+	for _, f := range goodCompilerFlags {
+		foundLTO := false
+		for _, s := range f {
+			if strings.Contains(s, "-flto") {
+				foundLTO = true
+			}
+		}
+		if err := checkCompilerFlagsForInternalLink("test", "test", f); err != nil {
+			// expect error for -flto
+			if !foundLTO {
+				t.Errorf("unexpected error for %q: %v", f, err)
+			}
+		} else {
+			// expect no error for everything else
+			if foundLTO {
+				t.Errorf("missing error for %q: %v", f, err)
+			}
+		}
+	}
+}
diff --git a/src/cmd/go/internal/work/shell_test.go b/src/cmd/go/internal/work/shell_test.go
new file mode 100644
index 0000000..24bef4e
--- /dev/null
+++ b/src/cmd/go/internal/work/shell_test.go
@@ -0,0 +1,139 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix
+
+package work
+
+import (
+	"bytes"
+	"internal/testenv"
+	"strings"
+	"testing"
+	"unicode"
+)
+
+func FuzzSplitPkgConfigOutput(f *testing.F) {
+	testenv.MustHaveExecPath(f, "/bin/sh")
+
+	f.Add([]byte(`$FOO`))
+	f.Add([]byte(`\$FOO`))
+	f.Add([]byte(`${FOO}`))
+	f.Add([]byte(`\${FOO}`))
+	f.Add([]byte(`$(/bin/false)`))
+	f.Add([]byte(`\$(/bin/false)`))
+	f.Add([]byte(`$((0))`))
+	f.Add([]byte(`\$((0))`))
+	f.Add([]byte(`unescaped space`))
+	f.Add([]byte(`escaped\ space`))
+	f.Add([]byte(`"unterminated quote`))
+	f.Add([]byte(`'unterminated quote`))
+	f.Add([]byte(`unterminated escape\`))
+	f.Add([]byte(`"quote with unterminated escape\`))
+	f.Add([]byte(`'quoted "double quotes"'`))
+	f.Add([]byte(`"quoted 'single quotes'"`))
+	f.Add([]byte(`"\$0"`))
+	f.Add([]byte(`"\$\0"`))
+	f.Add([]byte(`"\$"`))
+	f.Add([]byte(`"\$ "`))
+
+	// Example positive inputs from TestSplitPkgConfigOutput.
+	// Some bare newlines have been removed so that the inputs
+	// are valid in the shell script we use for comparison.
+	f.Add([]byte(`-r:foo -L/usr/white\ space/lib -lfoo\ bar -lbar\ baz`))
+	f.Add([]byte(`-lextra\ fun\ arg\\`))
+	f.Add([]byte("\textra     whitespace\r"))
+	f.Add([]byte("     \r      "))
+	f.Add([]byte(`"-r:foo" "-L/usr/white space/lib" "-lfoo bar" "-lbar baz"`))
+	f.Add([]byte(`"-lextra fun arg\\"`))
+	f.Add([]byte(`"     \r\n\      "`))
+	f.Add([]byte(`""`))
+	f.Add([]byte(``))
+	f.Add([]byte(`"\\"`))
+	f.Add([]byte(`"\x"`))
+	f.Add([]byte(`"\\x"`))
+	f.Add([]byte(`'\\'`))
+	f.Add([]byte(`'\x'`))
+	f.Add([]byte(`"\\x"`))
+	f.Add([]byte("\\\n"))
+	f.Add([]byte(`-fPIC -I/test/include/foo -DQUOTED='"/test/share/doc"'`))
+	f.Add([]byte(`-fPIC -I/test/include/foo -DQUOTED="/test/share/doc"`))
+	f.Add([]byte(`-fPIC -I/test/include/foo -DQUOTED=\"/test/share/doc\"`))
+	f.Add([]byte(`-fPIC -I/test/include/foo -DQUOTED='/test/share/doc'`))
+	f.Add([]byte(`-DQUOTED='/te\st/share/d\oc'`))
+	f.Add([]byte(`-Dhello=10 -Dworld=+32 -DDEFINED_FROM_PKG_CONFIG=hello\ world`))
+	f.Add([]byte(`"broken\"" \\\a "a"`))
+
+	// Example negative inputs from TestSplitPkgConfigOutput.
+	f.Add([]byte(`"     \r\n      `))
+	f.Add([]byte(`"-r:foo" "-L/usr/white space/lib "-lfoo bar" "-lbar baz"`))
+	f.Add([]byte(`"-lextra fun arg\\`))
+	f.Add([]byte(`broken flag\`))
+	f.Add([]byte(`extra broken flag \`))
+	f.Add([]byte(`\`))
+	f.Add([]byte(`"broken\"" "extra" \`))
+
+	f.Fuzz(func(t *testing.T, b []byte) {
+		t.Parallel()
+
+		if bytes.ContainsAny(b, "*?[#~%\x00{}!") {
+			t.Skipf("skipping %#q: contains a sometimes-quoted character", b)
+		}
+		// splitPkgConfigOutput itself rejects inputs that contain unquoted
+		// shell operator characters. (Quoted shell characters are fine.)
+
+		for _, c := range b {
+			if c > unicode.MaxASCII {
+				t.Skipf("skipping %#q: contains a non-ASCII character %q", b, c)
+			}
+			if !unicode.IsGraphic(rune(c)) && !unicode.IsSpace(rune(c)) {
+				t.Skipf("skipping %#q: contains non-graphic character %q", b, c)
+			}
+		}
+
+		args, err := splitPkgConfigOutput(b)
+		if err != nil {
+			// We haven't checked that the shell would actually reject this input too,
+			// but if splitPkgConfigOutput rejected it it's probably too dangerous to
+			// run in the script.
+			t.Logf("%#q: %v", b, err)
+			return
+		}
+		t.Logf("splitPkgConfigOutput(%#q) = %#q", b, args)
+		if len(args) == 0 {
+			t.Skipf("skipping %#q: contains no arguments", b)
+		}
+
+		var buf strings.Builder
+		for _, arg := range args {
+			buf.WriteString(arg)
+			buf.WriteString("\n")
+		}
+		wantOut := buf.String()
+
+		if strings.Count(wantOut, "\n") != len(args)+bytes.Count(b, []byte("\n")) {
+			// One of the newlines in b was treated as a delimiter and not part of an
+			// argument. Our bash test script would interpret that as a syntax error.
+			t.Skipf("skipping %#q: contains a bare newline", b)
+		}
+
+		// We use the printf shell command to echo the arguments because, per
+		// https://pubs.opengroup.org/onlinepubs/9699919799/utilities/echo.html#tag_20_37_16:
+		// “It is not possible to use echo portably across all POSIX systems unless
+		// both -n (as the first argument) and escape sequences are omitted.”
+		cmd := testenv.Command(t, "/bin/sh", "-c", "printf '%s\n' "+string(b))
+		cmd.Env = append(cmd.Environ(), "LC_ALL=POSIX", "POSIXLY_CORRECT=1")
+		cmd.Stderr = new(strings.Builder)
+		out, err := cmd.Output()
+		if err != nil {
+			t.Fatalf("%#q: %v\n%s", cmd.Args, err, cmd.Stderr)
+		}
+
+		if string(out) != wantOut {
+			t.Logf("%#q:\n%#q", cmd.Args, out)
+			t.Logf("want:\n%#q", wantOut)
+			t.Errorf("parsed args do not match")
+		}
+	})
+}
diff --git a/src/cmd/go/internal/workcmd/edit.go b/src/cmd/go/internal/workcmd/edit.go
new file mode 100644
index 0000000..8d975b0
--- /dev/null
+++ b/src/cmd/go/internal/workcmd/edit.go
@@ -0,0 +1,340 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// go work edit
+
+package workcmd
+
+import (
+	"cmd/go/internal/base"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/modload"
+	"context"
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"golang.org/x/mod/module"
+
+	"golang.org/x/mod/modfile"
+)
+
+var cmdEdit = &base.Command{
+	UsageLine: "go work edit [editing flags] [go.work]",
+	Short:     "edit go.work from tools or scripts",
+	Long: `Edit provides a command-line interface for editing go.work,
+for use primarily by tools or scripts. It only reads go.work;
+it does not look up information about the modules involved.
+If no file is specified, Edit looks for a go.work file in the current
+directory and its parent directories
+
+The editing flags specify a sequence of editing operations.
+
+The -fmt flag reformats the go.work file without making other changes.
+This reformatting is also implied by any other modifications that use or
+rewrite the go.mod file. The only time this flag is needed is if no other
+flags are specified, as in 'go work edit -fmt'.
+
+The -use=path and -dropuse=path flags
+add and drop a use directive from the go.work file's set of module directories.
+
+The -replace=old[@v]=new[@v] flag adds a replacement of the given
+module path and version pair. If the @v in old@v is omitted, a
+replacement without a version on the left side is added, which applies
+to all versions of the old module path. If the @v in new@v is omitted,
+the new path should be a local module root directory, not a module
+path. Note that -replace overrides any redundant replacements for old[@v],
+so omitting @v will drop existing replacements for specific versions.
+
+The -dropreplace=old[@v] flag drops a replacement of the given
+module path and version pair. If the @v is omitted, a replacement without
+a version on the left side is dropped.
+
+The -use, -dropuse, -replace, and -dropreplace,
+editing flags may be repeated, and the changes are applied in the order given.
+
+The -go=version flag sets the expected Go language version.
+
+The -toolchain=name flag sets the Go toolchain to use.
+
+The -print flag prints the final go.work in its text format instead of
+writing it back to go.mod.
+
+The -json flag prints the final go.work file in JSON format instead of
+writing it back to go.mod. The JSON output corresponds to these Go types:
+
+	type GoWork struct {
+		Go        string
+		Toolchain string
+		Use       []Use
+		Replace   []Replace
+	}
+
+	type Use struct {
+		DiskPath   string
+		ModulePath string
+	}
+
+	type Replace struct {
+		Old Module
+		New Module
+	}
+
+	type Module struct {
+		Path    string
+		Version string
+	}
+
+See the workspaces reference at https://go.dev/ref/mod#workspaces
+for more information.
+`,
+}
+
+var (
+	editFmt       = cmdEdit.Flag.Bool("fmt", false, "")
+	editGo        = cmdEdit.Flag.String("go", "", "")
+	editToolchain = cmdEdit.Flag.String("toolchain", "", "")
+	editJSON      = cmdEdit.Flag.Bool("json", false, "")
+	editPrint     = cmdEdit.Flag.Bool("print", false, "")
+	workedits     []func(file *modfile.WorkFile) // edits specified in flags
+)
+
+type flagFunc func(string)
+
+func (f flagFunc) String() string     { return "" }
+func (f flagFunc) Set(s string) error { f(s); return nil }
+
+func init() {
+	cmdEdit.Run = runEditwork // break init cycle
+
+	cmdEdit.Flag.Var(flagFunc(flagEditworkUse), "use", "")
+	cmdEdit.Flag.Var(flagFunc(flagEditworkDropUse), "dropuse", "")
+	cmdEdit.Flag.Var(flagFunc(flagEditworkReplace), "replace", "")
+	cmdEdit.Flag.Var(flagFunc(flagEditworkDropReplace), "dropreplace", "")
+	base.AddChdirFlag(&cmdEdit.Flag)
+}
+
+func runEditwork(ctx context.Context, cmd *base.Command, args []string) {
+	if *editJSON && *editPrint {
+		base.Fatalf("go: cannot use both -json and -print")
+	}
+
+	if len(args) > 1 {
+		base.Fatalf("go: 'go help work edit' accepts at most one argument")
+	}
+	var gowork string
+	if len(args) == 1 {
+		gowork = args[0]
+	} else {
+		modload.InitWorkfile()
+		gowork = modload.WorkFilePath()
+	}
+	if gowork == "" {
+		base.Fatalf("go: no go.work file found\n\t(run 'go work init' first or specify path using GOWORK environment variable)")
+	}
+
+	if *editGo != "" && *editGo != "none" {
+		if !modfile.GoVersionRE.MatchString(*editGo) {
+			base.Fatalf(`go work: invalid -go option; expecting something like "-go %s"`, gover.Local())
+		}
+	}
+	if *editToolchain != "" && *editToolchain != "none" {
+		if !modfile.ToolchainRE.MatchString(*editToolchain) {
+			base.Fatalf(`go work: invalid -toolchain option; expecting something like "-toolchain go%s"`, gover.Local())
+		}
+	}
+
+	anyFlags := *editGo != "" ||
+		*editToolchain != "" ||
+		*editJSON ||
+		*editPrint ||
+		*editFmt ||
+		len(workedits) > 0
+
+	if !anyFlags {
+		base.Fatalf("go: no flags specified (see 'go help work edit').")
+	}
+
+	workFile, err := modload.ReadWorkFile(gowork)
+	if err != nil {
+		base.Fatalf("go: errors parsing %s:\n%s", base.ShortPath(gowork), err)
+	}
+
+	if *editGo == "none" {
+		workFile.DropGoStmt()
+	} else if *editGo != "" {
+		if err := workFile.AddGoStmt(*editGo); err != nil {
+			base.Fatalf("go: internal error: %v", err)
+		}
+	}
+	if *editToolchain == "none" {
+		workFile.DropToolchainStmt()
+	} else if *editToolchain != "" {
+		if err := workFile.AddToolchainStmt(*editToolchain); err != nil {
+			base.Fatalf("go: internal error: %v", err)
+		}
+	}
+
+	if len(workedits) > 0 {
+		for _, edit := range workedits {
+			edit(workFile)
+		}
+	}
+
+	workFile.SortBlocks()
+	workFile.Cleanup() // clean file after edits
+
+	// Note: No call to modload.UpdateWorkFile here.
+	// Edit's job is only to make the edits on the command line,
+	// not to apply the kinds of semantic changes that
+	// UpdateWorkFile does (or would eventually do, if we
+	// decide to add the module comments in go.work).
+
+	if *editJSON {
+		editPrintJSON(workFile)
+		return
+	}
+
+	if *editPrint {
+		os.Stdout.Write(modfile.Format(workFile.Syntax))
+		return
+	}
+
+	modload.WriteWorkFile(gowork, workFile)
+}
+
+// flagEditworkUse implements the -use flag.
+func flagEditworkUse(arg string) {
+	workedits = append(workedits, func(f *modfile.WorkFile) {
+		_, mf, err := modload.ReadModFile(filepath.Join(arg, "go.mod"), nil)
+		modulePath := ""
+		if err == nil {
+			modulePath = mf.Module.Mod.Path
+		}
+		f.AddUse(modload.ToDirectoryPath(arg), modulePath)
+		if err := f.AddUse(modload.ToDirectoryPath(arg), ""); err != nil {
+			base.Fatalf("go: -use=%s: %v", arg, err)
+		}
+	})
+}
+
+// flagEditworkDropUse implements the -dropuse flag.
+func flagEditworkDropUse(arg string) {
+	workedits = append(workedits, func(f *modfile.WorkFile) {
+		if err := f.DropUse(modload.ToDirectoryPath(arg)); err != nil {
+			base.Fatalf("go: -dropdirectory=%s: %v", arg, err)
+		}
+	})
+}
+
+// allowedVersionArg returns whether a token may be used as a version in go.mod.
+// We don't call modfile.CheckPathVersion, because that insists on versions
+// being in semver form, but here we want to allow versions like "master" or
+// "1234abcdef", which the go command will resolve the next time it runs (or
+// during -fix).  Even so, we need to make sure the version is a valid token.
+func allowedVersionArg(arg string) bool {
+	return !modfile.MustQuote(arg)
+}
+
+// parsePathVersionOptional parses path[@version], using adj to
+// describe any errors.
+func parsePathVersionOptional(adj, arg string, allowDirPath bool) (path, version string, err error) {
+	before, after, found := strings.Cut(arg, "@")
+	if !found {
+		path = arg
+	} else {
+		path, version = strings.TrimSpace(before), strings.TrimSpace(after)
+	}
+	if err := module.CheckImportPath(path); err != nil {
+		if !allowDirPath || !modfile.IsDirectoryPath(path) {
+			return path, version, fmt.Errorf("invalid %s path: %v", adj, err)
+		}
+	}
+	if path != arg && !allowedVersionArg(version) {
+		return path, version, fmt.Errorf("invalid %s version: %q", adj, version)
+	}
+	return path, version, nil
+}
+
+// flagEditworkReplace implements the -replace flag.
+func flagEditworkReplace(arg string) {
+	before, after, found := strings.Cut(arg, "=")
+	if !found {
+		base.Fatalf("go: -replace=%s: need old[@v]=new[@w] (missing =)", arg)
+	}
+	old, new := strings.TrimSpace(before), strings.TrimSpace(after)
+	if strings.HasPrefix(new, ">") {
+		base.Fatalf("go: -replace=%s: separator between old and new is =, not =>", arg)
+	}
+	oldPath, oldVersion, err := parsePathVersionOptional("old", old, false)
+	if err != nil {
+		base.Fatalf("go: -replace=%s: %v", arg, err)
+	}
+	newPath, newVersion, err := parsePathVersionOptional("new", new, true)
+	if err != nil {
+		base.Fatalf("go: -replace=%s: %v", arg, err)
+	}
+	if newPath == new && !modfile.IsDirectoryPath(new) {
+		base.Fatalf("go: -replace=%s: unversioned new path must be local directory", arg)
+	}
+
+	workedits = append(workedits, func(f *modfile.WorkFile) {
+		if err := f.AddReplace(oldPath, oldVersion, newPath, newVersion); err != nil {
+			base.Fatalf("go: -replace=%s: %v", arg, err)
+		}
+	})
+}
+
+// flagEditworkDropReplace implements the -dropreplace flag.
+func flagEditworkDropReplace(arg string) {
+	path, version, err := parsePathVersionOptional("old", arg, true)
+	if err != nil {
+		base.Fatalf("go: -dropreplace=%s: %v", arg, err)
+	}
+	workedits = append(workedits, func(f *modfile.WorkFile) {
+		if err := f.DropReplace(path, version); err != nil {
+			base.Fatalf("go: -dropreplace=%s: %v", arg, err)
+		}
+	})
+}
+
+type replaceJSON struct {
+	Old module.Version
+	New module.Version
+}
+
+// editPrintJSON prints the -json output.
+func editPrintJSON(workFile *modfile.WorkFile) {
+	var f workfileJSON
+	if workFile.Go != nil {
+		f.Go = workFile.Go.Version
+	}
+	for _, d := range workFile.Use {
+		f.Use = append(f.Use, useJSON{DiskPath: d.Path, ModPath: d.ModulePath})
+	}
+
+	for _, r := range workFile.Replace {
+		f.Replace = append(f.Replace, replaceJSON{r.Old, r.New})
+	}
+	data, err := json.MarshalIndent(&f, "", "\t")
+	if err != nil {
+		base.Fatalf("go: internal error: %v", err)
+	}
+	data = append(data, '\n')
+	os.Stdout.Write(data)
+}
+
+// workfileJSON is the -json output data structure.
+type workfileJSON struct {
+	Go      string `json:",omitempty"`
+	Use     []useJSON
+	Replace []replaceJSON
+}
+
+type useJSON struct {
+	DiskPath string
+	ModPath  string `json:",omitempty"`
+}
diff --git a/src/cmd/go/internal/workcmd/init.go b/src/cmd/go/internal/workcmd/init.go
new file mode 100644
index 0000000..02240b8
--- /dev/null
+++ b/src/cmd/go/internal/workcmd/init.go
@@ -0,0 +1,66 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// go work init
+
+package workcmd
+
+import (
+	"context"
+	"path/filepath"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/modload"
+
+	"golang.org/x/mod/modfile"
+)
+
+var cmdInit = &base.Command{
+	UsageLine: "go work init [moddirs]",
+	Short:     "initialize workspace file",
+	Long: `Init initializes and writes a new go.work file in the
+current directory, in effect creating a new workspace at the current
+directory.
+
+go work init optionally accepts paths to the workspace modules as
+arguments. If the argument is omitted, an empty workspace with no
+modules will be created.
+
+Each argument path is added to a use directive in the go.work file. The
+current go version will also be listed in the go.work file.
+
+See the workspaces reference at https://go.dev/ref/mod#workspaces
+for more information.
+`,
+	Run: runInit,
+}
+
+func init() {
+	base.AddChdirFlag(&cmdInit.Flag)
+	base.AddModCommonFlags(&cmdInit.Flag)
+}
+
+func runInit(ctx context.Context, cmd *base.Command, args []string) {
+	modload.InitWorkfile()
+
+	modload.ForceUseModules = true
+
+	gowork := modload.WorkFilePath()
+	if gowork == "" {
+		gowork = filepath.Join(base.Cwd(), "go.work")
+	}
+
+	if _, err := fsys.Stat(gowork); err == nil {
+		base.Fatalf("go: %s already exists", gowork)
+	}
+
+	goV := gover.Local() // Use current Go version by default
+	wf := new(modfile.WorkFile)
+	wf.Syntax = new(modfile.FileSyntax)
+	wf.AddGoStmt(goV)
+	workUse(ctx, gowork, wf, args)
+	modload.WriteWorkFile(gowork, wf)
+}
diff --git a/src/cmd/go/internal/workcmd/sync.go b/src/cmd/go/internal/workcmd/sync.go
new file mode 100644
index 0000000..719cf76
--- /dev/null
+++ b/src/cmd/go/internal/workcmd/sync.go
@@ -0,0 +1,146 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// go work sync
+
+package workcmd
+
+import (
+	"cmd/go/internal/base"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/imports"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/toolchain"
+	"context"
+
+	"golang.org/x/mod/module"
+)
+
+var cmdSync = &base.Command{
+	UsageLine: "go work sync",
+	Short:     "sync workspace build list to modules",
+	Long: `Sync syncs the workspace's build list back to the
+workspace's modules
+
+The workspace's build list is the set of versions of all the
+(transitive) dependency modules used to do builds in the workspace. go
+work sync generates that build list using the Minimal Version Selection
+algorithm, and then syncs those versions back to each of modules
+specified in the workspace (with use directives).
+
+The syncing is done by sequentially upgrading each of the dependency
+modules specified in a workspace module to the version in the build list
+if the dependency module's version is not already the same as the build
+list's version. Note that Minimal Version Selection guarantees that the
+build list's version of each module is always the same or higher than
+that in each workspace module.
+
+See the workspaces reference at https://go.dev/ref/mod#workspaces
+for more information.
+`,
+	Run: runSync,
+}
+
+func init() {
+	base.AddChdirFlag(&cmdSync.Flag)
+	base.AddModCommonFlags(&cmdSync.Flag)
+}
+
+func runSync(ctx context.Context, cmd *base.Command, args []string) {
+	modload.ForceUseModules = true
+	modload.InitWorkfile()
+	if modload.WorkFilePath() == "" {
+		base.Fatalf("go: no go.work file found\n\t(run 'go work init' first or specify path using GOWORK environment variable)")
+	}
+
+	_, err := modload.LoadModGraph(ctx, "")
+	if err != nil {
+		toolchain.SwitchOrFatal(ctx, err)
+	}
+	mustSelectFor := map[module.Version][]module.Version{}
+
+	mms := modload.MainModules
+
+	opts := modload.PackageOpts{
+		Tags:                     imports.AnyTags(),
+		VendorModulesInGOROOTSrc: true,
+		ResolveMissingImports:    false,
+		LoadTests:                true,
+		AllowErrors:              true,
+		SilencePackageErrors:     true,
+		SilenceUnmatchedWarnings: true,
+	}
+	for _, m := range mms.Versions() {
+		opts.MainModule = m
+		_, pkgs := modload.LoadPackages(ctx, opts, "all")
+		opts.MainModule = module.Version{} // reset
+
+		var (
+			mustSelect   []module.Version
+			inMustSelect = map[module.Version]bool{}
+		)
+		for _, pkg := range pkgs {
+			if r := modload.PackageModule(pkg); r.Version != "" && !inMustSelect[r] {
+				// r has a known version, so force that version.
+				mustSelect = append(mustSelect, r)
+				inMustSelect[r] = true
+			}
+		}
+		gover.ModSort(mustSelect) // ensure determinism
+		mustSelectFor[m] = mustSelect
+	}
+
+	workFilePath := modload.WorkFilePath() // save go.work path because EnterModule clobbers it.
+
+	var goV string
+	for _, m := range mms.Versions() {
+		if mms.ModRoot(m) == "" && m.Path == "command-line-arguments" {
+			// This is not a real module.
+			// TODO(#49228): Remove this special case once the special
+			// command-line-arguments module is gone.
+			continue
+		}
+
+		// Use EnterModule to reset the global state in modload to be in
+		// single-module mode using the modroot of m.
+		modload.EnterModule(ctx, mms.ModRoot(m))
+
+		// Edit the build list in the same way that 'go get' would if we
+		// requested the relevant module versions explicitly.
+		// TODO(#57001): Do we need a toolchain.SwitchOrFatal here,
+		// and do we need to pass a toolchain.Switcher in LoadPackages?
+		// If so, think about saving the WriteGoMods for after the loop,
+		// so we don't write some go.mods with the "before" toolchain
+		// and others with the "after" toolchain. If nothing else, that
+		// discrepancy could show up in auto-recorded toolchain lines.
+		changed, err := modload.EditBuildList(ctx, nil, mustSelectFor[m])
+		if err != nil {
+			continue
+		}
+		if changed {
+			modload.LoadPackages(ctx, modload.PackageOpts{
+				Tags:                     imports.AnyTags(),
+				Tidy:                     true,
+				VendorModulesInGOROOTSrc: true,
+				ResolveMissingImports:    false,
+				LoadTests:                true,
+				AllowErrors:              true,
+				SilenceMissingStdImports: true,
+				SilencePackageErrors:     true,
+			}, "all")
+			modload.WriteGoMod(ctx, modload.WriteOpts{})
+		}
+		goV = gover.Max(goV, modload.MainModules.GoVersion())
+	}
+
+	wf, err := modload.ReadWorkFile(workFilePath)
+	if err != nil {
+		base.Fatal(err)
+	}
+	modload.UpdateWorkGoVersion(wf, goV)
+	modload.UpdateWorkFile(wf)
+	if err := modload.WriteWorkFile(workFilePath, wf); err != nil {
+		base.Fatal(err)
+	}
+}
diff --git a/src/cmd/go/internal/workcmd/use.go b/src/cmd/go/internal/workcmd/use.go
new file mode 100644
index 0000000..5547711
--- /dev/null
+++ b/src/cmd/go/internal/workcmd/use.go
@@ -0,0 +1,254 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// go work use
+
+package workcmd
+
+import (
+	"context"
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/str"
+	"cmd/go/internal/toolchain"
+
+	"golang.org/x/mod/modfile"
+)
+
+var cmdUse = &base.Command{
+	UsageLine: "go work use [-r] [moddirs]",
+	Short:     "add modules to workspace file",
+	Long: `Use provides a command-line interface for adding
+directories, optionally recursively, to a go.work file.
+
+A use directive will be added to the go.work file for each argument
+directory listed on the command line go.work file, if it exists,
+or removed from the go.work file if it does not exist.
+Use fails if any remaining use directives refer to modules that
+do not exist.
+
+Use updates the go line in go.work to specify a version at least as
+new as all the go lines in the used modules, both preexisting ones
+and newly added ones. With no arguments, this update is the only
+thing that go work use does.
+
+The -r flag searches recursively for modules in the argument
+directories, and the use command operates as if each of the directories
+were specified as arguments: namely, use directives will be added for
+directories that exist, and removed for directories that do not exist.
+
+
+
+See the workspaces reference at https://go.dev/ref/mod#workspaces
+for more information.
+`,
+}
+
+var useR = cmdUse.Flag.Bool("r", false, "")
+
+func init() {
+	cmdUse.Run = runUse // break init cycle
+
+	base.AddChdirFlag(&cmdUse.Flag)
+	base.AddModCommonFlags(&cmdUse.Flag)
+}
+
+func runUse(ctx context.Context, cmd *base.Command, args []string) {
+	modload.ForceUseModules = true
+	modload.InitWorkfile()
+	gowork := modload.WorkFilePath()
+	if gowork == "" {
+		base.Fatalf("go: no go.work file found\n\t(run 'go work init' first or specify path using GOWORK environment variable)")
+	}
+	wf, err := modload.ReadWorkFile(gowork)
+	if err != nil {
+		base.Fatal(err)
+	}
+	workUse(ctx, gowork, wf, args)
+	modload.WriteWorkFile(gowork, wf)
+}
+
+func workUse(ctx context.Context, gowork string, wf *modfile.WorkFile, args []string) {
+	workDir := filepath.Dir(gowork) // absolute, since gowork itself is absolute
+
+	haveDirs := make(map[string][]string) // absolute → original(s)
+	for _, use := range wf.Use {
+		var abs string
+		if filepath.IsAbs(use.Path) {
+			abs = filepath.Clean(use.Path)
+		} else {
+			abs = filepath.Join(workDir, use.Path)
+		}
+		haveDirs[abs] = append(haveDirs[abs], use.Path)
+	}
+
+	// keepDirs maps each absolute path to keep to the literal string to use for
+	// that path (either an absolute or a relative path), or the empty string if
+	// all entries for the absolute path should be removed.
+	keepDirs := make(map[string]string)
+
+	var sw toolchain.Switcher
+
+	// lookDir updates the entry in keepDirs for the directory dir,
+	// which is either absolute or relative to the current working directory
+	// (not necessarily the directory containing the workfile).
+	lookDir := func(dir string) {
+		absDir, dir := pathRel(workDir, dir)
+
+		file := base.ShortPath(filepath.Join(absDir, "go.mod"))
+		fi, err := fsys.Stat(file)
+		if err != nil {
+			if os.IsNotExist(err) {
+				keepDirs[absDir] = ""
+			} else {
+				sw.Error(err)
+			}
+			return
+		}
+
+		if !fi.Mode().IsRegular() {
+			sw.Error(fmt.Errorf("%v is not a regular file", file))
+			return
+		}
+
+		if dup := keepDirs[absDir]; dup != "" && dup != dir {
+			base.Errorf(`go: already added "%s" as "%s"`, dir, dup)
+		}
+		keepDirs[absDir] = dir
+	}
+
+	for _, useDir := range args {
+		absArg, _ := pathRel(workDir, useDir)
+
+		info, err := fsys.Stat(base.ShortPath(absArg))
+		if err != nil {
+			// Errors raised from os.Stat are formatted to be more user-friendly.
+			if os.IsNotExist(err) {
+				err = fmt.Errorf("directory %v does not exist", base.ShortPath(absArg))
+			}
+			sw.Error(err)
+			continue
+		} else if !info.IsDir() {
+			sw.Error(fmt.Errorf("%s is not a directory", base.ShortPath(absArg)))
+			continue
+		}
+
+		if !*useR {
+			lookDir(useDir)
+			continue
+		}
+
+		// Add or remove entries for any subdirectories that still exist.
+		// If the root itself is a symlink to a directory,
+		// we want to follow it (see https://go.dev/issue/50807).
+		// Add a trailing separator to force that to happen.
+		fsys.Walk(str.WithFilePathSeparator(useDir), func(path string, info fs.FileInfo, err error) error {
+			if err != nil {
+				return err
+			}
+
+			if !info.IsDir() {
+				if info.Mode()&fs.ModeSymlink != 0 {
+					if target, err := fsys.Stat(path); err == nil && target.IsDir() {
+						fmt.Fprintf(os.Stderr, "warning: ignoring symlink %s\n", base.ShortPath(path))
+					}
+				}
+				return nil
+			}
+			lookDir(path)
+			return nil
+		})
+
+		// Remove entries for subdirectories that no longer exist.
+		// Because they don't exist, they will be skipped by Walk.
+		for absDir := range haveDirs {
+			if str.HasFilePathPrefix(absDir, absArg) {
+				if _, ok := keepDirs[absDir]; !ok {
+					keepDirs[absDir] = "" // Mark for deletion.
+				}
+			}
+		}
+	}
+
+	// Update the work file.
+	for absDir, keepDir := range keepDirs {
+		nKept := 0
+		for _, dir := range haveDirs[absDir] {
+			if dir == keepDir { // (note that dir is always non-empty)
+				nKept++
+			} else {
+				wf.DropUse(dir)
+			}
+		}
+		if keepDir != "" && nKept != 1 {
+			// If we kept more than one copy, delete them all.
+			// We'll recreate a unique copy with AddUse.
+			if nKept > 1 {
+				wf.DropUse(keepDir)
+			}
+			wf.AddUse(keepDir, "")
+		}
+	}
+
+	// Read the Go versions from all the use entries, old and new (but not dropped).
+	goV := gover.FromGoWork(wf)
+	for _, use := range wf.Use {
+		if use.Path == "" { // deleted
+			continue
+		}
+		var abs string
+		if filepath.IsAbs(use.Path) {
+			abs = filepath.Clean(use.Path)
+		} else {
+			abs = filepath.Join(workDir, use.Path)
+		}
+		_, mf, err := modload.ReadModFile(base.ShortPath(filepath.Join(abs, "go.mod")), nil)
+		if err != nil {
+			sw.Error(err)
+			continue
+		}
+		goV = gover.Max(goV, gover.FromGoMod(mf))
+	}
+	sw.Switch(ctx)
+	base.ExitIfErrors()
+
+	modload.UpdateWorkGoVersion(wf, goV)
+	modload.UpdateWorkFile(wf)
+}
+
+// pathRel returns the absolute and canonical forms of dir for use in a
+// go.work file located in directory workDir.
+//
+// If dir is relative, it is interpreted relative to base.Cwd()
+// and its canonical form is relative to workDir if possible.
+// If dir is absolute or cannot be made relative to workDir,
+// its canonical form is absolute.
+//
+// Canonical absolute paths are clean.
+// Canonical relative paths are clean and slash-separated.
+func pathRel(workDir, dir string) (abs, canonical string) {
+	if filepath.IsAbs(dir) {
+		abs = filepath.Clean(dir)
+		return abs, abs
+	}
+
+	abs = filepath.Join(base.Cwd(), dir)
+	rel, err := filepath.Rel(workDir, abs)
+	if err != nil {
+		// The path can't be made relative to the go.work file,
+		// so it must be kept absolute instead.
+		return abs, abs
+	}
+
+	// Normalize relative paths to use slashes, so that checked-in go.work
+	// files with relative paths within the repo are platform-independent.
+	return abs, modload.ToDirectoryPath(rel)
+}
diff --git a/src/cmd/go/internal/workcmd/work.go b/src/cmd/go/internal/workcmd/work.go
new file mode 100644
index 0000000..c99cc2a
--- /dev/null
+++ b/src/cmd/go/internal/workcmd/work.go
@@ -0,0 +1,78 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package workcmd implements the “go work” command.
+package workcmd
+
+import (
+	"cmd/go/internal/base"
+)
+
+var CmdWork = &base.Command{
+	UsageLine: "go work",
+	Short:     "workspace maintenance",
+	Long: `Work provides access to operations on workspaces.
+
+Note that support for workspaces is built into many other commands, not
+just 'go work'.
+
+See 'go help modules' for information about Go's module system of which
+workspaces are a part.
+
+See https://go.dev/ref/mod#workspaces for an in-depth reference on
+workspaces.
+
+See https://go.dev/doc/tutorial/workspaces for an introductory
+tutorial on workspaces.
+
+A workspace is specified by a go.work file that specifies a set of
+module directories with the "use" directive. These modules are used as
+root modules by the go command for builds and related operations.  A
+workspace that does not specify modules to be used cannot be used to do
+builds from local modules.
+
+go.work files are line-oriented. Each line holds a single directive,
+made up of a keyword followed by arguments. For example:
+
+	go 1.18
+
+	use ../foo/bar
+	use ./baz
+
+	replace example.com/foo v1.2.3 => example.com/bar v1.4.5
+
+The leading keyword can be factored out of adjacent lines to create a block,
+like in Go imports.
+
+	use (
+	  ../foo/bar
+	  ./baz
+	)
+
+The use directive specifies a module to be included in the workspace's
+set of main modules. The argument to the use directive is the directory
+containing the module's go.mod file.
+
+The go directive specifies the version of Go the file was written at. It
+is possible there may be future changes in the semantics of workspaces
+that could be controlled by this version, but for now the version
+specified has no effect.
+
+The replace directive has the same syntax as the replace directive in a
+go.mod file and takes precedence over replaces in go.mod files.  It is
+primarily intended to override conflicting replaces in different workspace
+modules.
+
+To determine whether the go command is operating in workspace mode, use
+the "go env GOWORK" command. This will specify the workspace file being
+used.
+`,
+
+	Commands: []*base.Command{
+		cmdEdit,
+		cmdInit,
+		cmdSync,
+		cmdUse,
+	},
+}
diff --git a/src/cmd/go/main.go b/src/cmd/go/main.go
new file mode 100644
index 0000000..7b73642
--- /dev/null
+++ b/src/cmd/go/main.go
@@ -0,0 +1,338 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:generate go test cmd/go -v -run=TestDocsUpToDate -fixdocs
+
+package main
+
+import (
+	"cmd/go/internal/toolchain"
+	"cmd/go/internal/workcmd"
+	"context"
+	"flag"
+	"fmt"
+	"internal/buildcfg"
+	"log"
+	"os"
+	"path/filepath"
+	"runtime"
+	rtrace "runtime/trace"
+	"slices"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/bug"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/clean"
+	"cmd/go/internal/doc"
+	"cmd/go/internal/envcmd"
+	"cmd/go/internal/fix"
+	"cmd/go/internal/fmtcmd"
+	"cmd/go/internal/generate"
+	"cmd/go/internal/get"
+	"cmd/go/internal/help"
+	"cmd/go/internal/list"
+	"cmd/go/internal/modcmd"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/modget"
+	"cmd/go/internal/modload"
+	"cmd/go/internal/run"
+	"cmd/go/internal/test"
+	"cmd/go/internal/tool"
+	"cmd/go/internal/trace"
+	"cmd/go/internal/version"
+	"cmd/go/internal/vet"
+	"cmd/go/internal/work"
+)
+
+func init() {
+	base.Go.Commands = []*base.Command{
+		bug.CmdBug,
+		work.CmdBuild,
+		clean.CmdClean,
+		doc.CmdDoc,
+		envcmd.CmdEnv,
+		fix.CmdFix,
+		fmtcmd.CmdFmt,
+		generate.CmdGenerate,
+		modget.CmdGet,
+		work.CmdInstall,
+		list.CmdList,
+		modcmd.CmdMod,
+		workcmd.CmdWork,
+		run.CmdRun,
+		test.CmdTest,
+		tool.CmdTool,
+		version.CmdVersion,
+		vet.CmdVet,
+
+		help.HelpBuildConstraint,
+		help.HelpBuildmode,
+		help.HelpC,
+		help.HelpCache,
+		help.HelpEnvironment,
+		help.HelpFileType,
+		modload.HelpGoMod,
+		help.HelpGopath,
+		get.HelpGopathGet,
+		modfetch.HelpGoproxy,
+		help.HelpImportPath,
+		modload.HelpModules,
+		modget.HelpModuleGet,
+		modfetch.HelpModuleAuth,
+		help.HelpPackages,
+		modfetch.HelpPrivate,
+		test.HelpTestflag,
+		test.HelpTestfunc,
+		modget.HelpVCS,
+	}
+}
+
+var _ = go11tag
+
+func main() {
+	log.SetFlags(0)
+	handleChdirFlag()
+	toolchain.Select()
+
+	flag.Usage = base.Usage
+	flag.Parse()
+
+	args := flag.Args()
+	if len(args) < 1 {
+		base.Usage()
+	}
+
+	if args[0] == "get" || args[0] == "help" {
+		if !modload.WillBeEnabled() {
+			// Replace module-aware get with GOPATH get if appropriate.
+			*modget.CmdGet = *get.CmdGet
+		}
+	}
+
+	cfg.CmdName = args[0] // for error messages
+	if args[0] == "help" {
+		help.Help(os.Stdout, args[1:])
+		return
+	}
+
+	// Diagnose common mistake: GOPATH==GOROOT.
+	// This setting is equivalent to not setting GOPATH at all,
+	// which is not what most people want when they do it.
+	if gopath := cfg.BuildContext.GOPATH; filepath.Clean(gopath) == filepath.Clean(runtime.GOROOT()) {
+		fmt.Fprintf(os.Stderr, "warning: GOPATH set to GOROOT (%s) has no effect\n", gopath)
+	} else {
+		for _, p := range filepath.SplitList(gopath) {
+			// Some GOPATHs have empty directory elements - ignore them.
+			// See issue 21928 for details.
+			if p == "" {
+				continue
+			}
+			// Note: using HasPrefix instead of Contains because a ~ can appear
+			// in the middle of directory elements, such as /tmp/git-1.8.2~rc3
+			// or C:\PROGRA~1. Only ~ as a path prefix has meaning to the shell.
+			if strings.HasPrefix(p, "~") {
+				fmt.Fprintf(os.Stderr, "go: GOPATH entry cannot start with shell metacharacter '~': %q\n", p)
+				os.Exit(2)
+			}
+			if !filepath.IsAbs(p) {
+				if cfg.Getenv("GOPATH") == "" {
+					// We inferred $GOPATH from $HOME and did a bad job at it.
+					// Instead of dying, uninfer it.
+					cfg.BuildContext.GOPATH = ""
+				} else {
+					fmt.Fprintf(os.Stderr, "go: GOPATH entry is relative; must be absolute path: %q.\nFor more details see: 'go help gopath'\n", p)
+					os.Exit(2)
+				}
+			}
+		}
+	}
+
+	if cfg.GOROOT == "" {
+		fmt.Fprintf(os.Stderr, "go: cannot find GOROOT directory: 'go' binary is trimmed and GOROOT is not set\n")
+		os.Exit(2)
+	}
+	if fi, err := os.Stat(cfg.GOROOT); err != nil || !fi.IsDir() {
+		fmt.Fprintf(os.Stderr, "go: cannot find GOROOT directory: %v\n", cfg.GOROOT)
+		os.Exit(2)
+	}
+
+	cmd, used := lookupCmd(args)
+	cfg.CmdName = strings.Join(args[:used], " ")
+	if len(cmd.Commands) > 0 {
+		if used >= len(args) {
+			help.PrintUsage(os.Stderr, cmd)
+			base.SetExitStatus(2)
+			base.Exit()
+		}
+		if args[used] == "help" {
+			// Accept 'go mod help' and 'go mod help foo' for 'go help mod' and 'go help mod foo'.
+			help.Help(os.Stdout, append(slices.Clip(args[:used]), args[used+1:]...))
+			base.Exit()
+		}
+		helpArg := ""
+		if used > 0 {
+			helpArg += " " + strings.Join(args[:used], " ")
+		}
+		cmdName := cfg.CmdName
+		if cmdName == "" {
+			cmdName = args[0]
+		}
+		fmt.Fprintf(os.Stderr, "go %s: unknown command\nRun 'go help%s' for usage.\n", cmdName, helpArg)
+		base.SetExitStatus(2)
+		base.Exit()
+	}
+	invoke(cmd, args[used-1:])
+	base.Exit()
+}
+
+// lookupCmd interprets the initial elements of args
+// to find a command to run (cmd.Runnable() == true)
+// or else a command group that ran out of arguments
+// or had an unknown subcommand (len(cmd.Commands) > 0).
+// It returns that command and the number of elements of args
+// that it took to arrive at that command.
+func lookupCmd(args []string) (cmd *base.Command, used int) {
+	cmd = base.Go
+	for used < len(args) {
+		c := cmd.Lookup(args[used])
+		if c == nil {
+			break
+		}
+		if c.Runnable() {
+			cmd = c
+			used++
+			break
+		}
+		if len(c.Commands) > 0 {
+			cmd = c
+			used++
+			if used >= len(args) || args[0] == "help" {
+				break
+			}
+			continue
+		}
+		// len(c.Commands) == 0 && !c.Runnable() => help text; stop at "help"
+		break
+	}
+	return cmd, used
+}
+
+func invoke(cmd *base.Command, args []string) {
+	// 'go env' handles checking the build config
+	if cmd != envcmd.CmdEnv {
+		buildcfg.Check()
+		if cfg.ExperimentErr != nil {
+			base.Fatal(cfg.ExperimentErr)
+		}
+	}
+
+	// Set environment (GOOS, GOARCH, etc) explicitly.
+	// In theory all the commands we invoke should have
+	// the same default computation of these as we do,
+	// but in practice there might be skew
+	// This makes sure we all agree.
+	cfg.OrigEnv = toolchain.FilterEnv(os.Environ())
+	cfg.CmdEnv = envcmd.MkEnv()
+	for _, env := range cfg.CmdEnv {
+		if os.Getenv(env.Name) != env.Value {
+			os.Setenv(env.Name, env.Value)
+		}
+	}
+
+	cmd.Flag.Usage = func() { cmd.Usage() }
+	if cmd.CustomFlags {
+		args = args[1:]
+	} else {
+		base.SetFromGOFLAGS(&cmd.Flag)
+		cmd.Flag.Parse(args[1:])
+		args = cmd.Flag.Args()
+	}
+
+	if cfg.DebugRuntimeTrace != "" {
+		f, err := os.Create(cfg.DebugRuntimeTrace)
+		if err != nil {
+			base.Fatalf("creating trace file: %v", err)
+		}
+		if err := rtrace.Start(f); err != nil {
+			base.Fatalf("starting event trace: %v", err)
+		}
+		defer func() {
+			rtrace.Stop()
+		}()
+	}
+
+	ctx := maybeStartTrace(context.Background())
+	ctx, span := trace.StartSpan(ctx, fmt.Sprint("Running ", cmd.Name(), " command"))
+	cmd.Run(ctx, cmd, args)
+	span.Done()
+}
+
+func init() {
+	base.Usage = mainUsage
+}
+
+func mainUsage() {
+	help.PrintUsage(os.Stderr, base.Go)
+	os.Exit(2)
+}
+
+func maybeStartTrace(pctx context.Context) context.Context {
+	if cfg.DebugTrace == "" {
+		return pctx
+	}
+
+	ctx, close, err := trace.Start(pctx, cfg.DebugTrace)
+	if err != nil {
+		base.Fatalf("failed to start trace: %v", err)
+	}
+	base.AtExit(func() {
+		if err := close(); err != nil {
+			base.Fatalf("failed to stop trace: %v", err)
+		}
+	})
+
+	return ctx
+}
+
+// handleChdirFlag handles the -C flag before doing anything else.
+// The -C flag must be the first flag on the command line, to make it easy to find
+// even with commands that have custom flag parsing.
+// handleChdirFlag handles the flag by chdir'ing to the directory
+// and then removing that flag from the command line entirely.
+//
+// We have to handle the -C flag this way for two reasons:
+//
+//  1. Toolchain selection needs to be in the right directory to look for go.mod and go.work.
+//
+//  2. A toolchain switch later on reinvokes the new go command with the same arguments.
+//     The parent toolchain has already done the chdir; the child must not try to do it again.
+func handleChdirFlag() {
+	_, used := lookupCmd(os.Args[1:])
+	used++ // because of [1:]
+	if used >= len(os.Args) {
+		return
+	}
+
+	var dir string
+	switch a := os.Args[used]; {
+	default:
+		return
+
+	case a == "-C", a == "--C":
+		if used+1 >= len(os.Args) {
+			return
+		}
+		dir = os.Args[used+1]
+		os.Args = slices.Delete(os.Args, used, used+2)
+
+	case strings.HasPrefix(a, "-C="), strings.HasPrefix(a, "--C="):
+		_, dir, _ = strings.Cut(a, "=")
+		os.Args = slices.Delete(os.Args, used, used+1)
+	}
+
+	if err := os.Chdir(dir); err != nil {
+		base.Fatalf("go: %v", err)
+	}
+}
diff --git a/src/cmd/go/note_test.go b/src/cmd/go/note_test.go
new file mode 100644
index 0000000..ba7ec2a
--- /dev/null
+++ b/src/cmd/go/note_test.go
@@ -0,0 +1,72 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main_test
+
+import (
+	"internal/testenv"
+	"runtime"
+	"testing"
+
+	"cmd/internal/buildid"
+)
+
+func TestNoteReading(t *testing.T) {
+	// cmd/internal/buildid already has tests that the basic reading works.
+	// This test is essentially checking that -ldflags=-buildid=XXX works,
+	// both in internal and external linking mode.
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+
+	tg.tempFile("hello.go", `package main; func main() { print("hello, world\n") }`)
+	const buildID = "TestNoteReading-Build-ID"
+	tg.run("build", "-ldflags", "-buildid="+buildID, "-o", tg.path("hello.exe"), tg.path("hello.go"))
+	id, err := buildid.ReadFile(tg.path("hello.exe"))
+	if err != nil {
+		t.Fatalf("reading build ID from hello binary: %v", err)
+	}
+	if id != buildID {
+		t.Fatalf("buildID in hello binary = %q, want %q", id, buildID)
+	}
+
+	switch {
+	case !testenv.HasCGO():
+		t.Skipf("skipping - no cgo, so assuming external linking not available")
+	case runtime.GOOS == "plan9":
+		t.Skipf("skipping - external linking not supported")
+	}
+
+	tg.run("build", "-ldflags", "-buildid="+buildID+" -linkmode=external", "-o", tg.path("hello2.exe"), tg.path("hello.go"))
+	id, err = buildid.ReadFile(tg.path("hello2.exe"))
+	if err != nil {
+		t.Fatalf("reading build ID from hello binary (linkmode=external): %v", err)
+	}
+	if id != buildID {
+		t.Fatalf("buildID in hello binary = %q, want %q (linkmode=external)", id, buildID)
+	}
+
+	switch runtime.GOOS {
+	case "dragonfly", "freebsd", "linux", "netbsd", "openbsd":
+		// Test while forcing use of the gold linker, since in the past
+		// we've had trouble reading the notes generated by gold.
+		err := tg.doRun([]string{"build", "-ldflags", "-buildid=" + buildID + " -linkmode=external -extldflags=-fuse-ld=gold", "-o", tg.path("hello3.exe"), tg.path("hello.go")})
+		if err != nil {
+			if tg.grepCountBoth("(invalid linker|gold|cannot find [‘']ld[’'])") > 0 {
+				// It's not an error if gold isn't there. gcc claims it "cannot find 'ld'" if
+				// ld.gold is missing, see issue #22340.
+				t.Log("skipping gold test")
+				break
+			}
+			t.Fatalf("building hello binary: %v", err)
+		}
+		id, err = buildid.ReadFile(tg.path("hello3.exe"))
+		if err != nil {
+			t.Fatalf("reading build ID from hello binary (linkmode=external -extldflags=-fuse-ld=gold): %v", err)
+		}
+		if id != buildID {
+			t.Fatalf("buildID in hello binary = %q, want %q (linkmode=external -extldflags=-fuse-ld=gold)", id, buildID)
+		}
+	}
+}
diff --git a/src/cmd/go/proxy_test.go b/src/cmd/go/proxy_test.go
new file mode 100644
index 0000000..cb3d9f9
--- /dev/null
+++ b/src/cmd/go/proxy_test.go
@@ -0,0 +1,487 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main_test
+
+import (
+	"archive/zip"
+	"bytes"
+	"encoding/json"
+	"errors"
+	"flag"
+	"fmt"
+	"internal/txtar"
+	"io"
+	"io/fs"
+	"log"
+	"net"
+	"net/http"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"sync"
+	"testing"
+
+	"cmd/go/internal/modfetch/codehost"
+	"cmd/go/internal/par"
+
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
+	"golang.org/x/mod/sumdb"
+	"golang.org/x/mod/sumdb/dirhash"
+)
+
+var (
+	proxyAddr = flag.String("proxy", "", "run proxy on this network address instead of running any tests")
+	proxyURL  string
+)
+
+var proxyOnce sync.Once
+
+// StartProxy starts the Go module proxy running on *proxyAddr (like "localhost:1234")
+// and sets proxyURL to the GOPROXY setting to use to access the proxy.
+// Subsequent calls are no-ops.
+//
+// The proxy serves from testdata/mod. See testdata/mod/README.
+func StartProxy() {
+	proxyOnce.Do(func() {
+		readModList()
+		addr := *proxyAddr
+		if addr == "" {
+			addr = "localhost:0"
+		}
+		l, err := net.Listen("tcp", addr)
+		if err != nil {
+			log.Fatal(err)
+		}
+		*proxyAddr = l.Addr().String()
+		proxyURL = "http://" + *proxyAddr + "/mod"
+		fmt.Fprintf(os.Stderr, "go test proxy running at GOPROXY=%s\n", proxyURL)
+		go func() {
+			log.Fatalf("go proxy: http.Serve: %v", http.Serve(l, http.HandlerFunc(proxyHandler)))
+		}()
+
+		// Prepopulate main sumdb.
+		for _, mod := range modList {
+			sumdbOps.Lookup(nil, mod)
+		}
+	})
+}
+
+var modList []module.Version
+
+func readModList() {
+	files, err := os.ReadDir("testdata/mod")
+	if err != nil {
+		log.Fatal(err)
+	}
+	for _, f := range files {
+		name := f.Name()
+		if !strings.HasSuffix(name, ".txt") {
+			continue
+		}
+		name = strings.TrimSuffix(name, ".txt")
+		i := strings.LastIndex(name, "_v")
+		if i < 0 {
+			continue
+		}
+		encPath := strings.ReplaceAll(name[:i], "_", "/")
+		path, err := module.UnescapePath(encPath)
+		if err != nil {
+			if testing.Verbose() && encPath != "example.com/invalidpath/v1" {
+				fmt.Fprintf(os.Stderr, "go proxy_test: %v\n", err)
+			}
+			continue
+		}
+		encVers := name[i+1:]
+		vers, err := module.UnescapeVersion(encVers)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "go proxy_test: %v\n", err)
+			continue
+		}
+		modList = append(modList, module.Version{Path: path, Version: vers})
+	}
+}
+
+var zipCache par.ErrCache[*txtar.Archive, []byte]
+
+const (
+	testSumDBName        = "localhost.localdev/sumdb"
+	testSumDBVerifierKey = "localhost.localdev/sumdb+00000c67+AcTrnkbUA+TU4heY3hkjiSES/DSQniBqIeQ/YppAUtK6"
+	testSumDBSignerKey   = "PRIVATE+KEY+localhost.localdev/sumdb+00000c67+AXu6+oaVaOYuQOFrf1V59JK1owcFlJcHwwXHDfDGxSPk"
+)
+
+var (
+	sumdbOps    = sumdb.NewTestServer(testSumDBSignerKey, proxyGoSum)
+	sumdbServer = sumdb.NewServer(sumdbOps)
+
+	sumdbWrongOps    = sumdb.NewTestServer(testSumDBSignerKey, proxyGoSumWrong)
+	sumdbWrongServer = sumdb.NewServer(sumdbWrongOps)
+)
+
+// proxyHandler serves the Go module proxy protocol.
+// See the proxy section of https://research.swtch.com/vgo-module.
+func proxyHandler(w http.ResponseWriter, r *http.Request) {
+	if !strings.HasPrefix(r.URL.Path, "/mod/") {
+		http.NotFound(w, r)
+		return
+	}
+	path := r.URL.Path[len("/mod/"):]
+
+	// /mod/invalid returns faulty responses.
+	if strings.HasPrefix(path, "invalid/") {
+		w.Write([]byte("invalid"))
+		return
+	}
+
+	// Next element may opt into special behavior.
+	if j := strings.Index(path, "/"); j >= 0 {
+		n, err := strconv.Atoi(path[:j])
+		if err == nil && n >= 200 {
+			w.WriteHeader(n)
+			return
+		}
+		if strings.HasPrefix(path, "sumdb-") {
+			n, err := strconv.Atoi(path[len("sumdb-"):j])
+			if err == nil && n >= 200 {
+				if strings.HasPrefix(path[j:], "/sumdb/") {
+					w.WriteHeader(n)
+					return
+				}
+				path = path[j+1:]
+			}
+		}
+	}
+
+	// Request for $GOPROXY/sumdb-direct is direct sumdb access.
+	// (Client thinks it is talking directly to a sumdb.)
+	if strings.HasPrefix(path, "sumdb-direct/") {
+		r.URL.Path = path[len("sumdb-direct"):]
+		sumdbServer.ServeHTTP(w, r)
+		return
+	}
+
+	// Request for $GOPROXY/sumdb-wrong is direct sumdb access
+	// but all the hashes are wrong.
+	// (Client thinks it is talking directly to a sumdb.)
+	if strings.HasPrefix(path, "sumdb-wrong/") {
+		r.URL.Path = path[len("sumdb-wrong"):]
+		sumdbWrongServer.ServeHTTP(w, r)
+		return
+	}
+
+	// Request for $GOPROXY/redirect/<count>/... goes to redirects.
+	if strings.HasPrefix(path, "redirect/") {
+		path = path[len("redirect/"):]
+		if j := strings.Index(path, "/"); j >= 0 {
+			count, err := strconv.Atoi(path[:j])
+			if err != nil {
+				return
+			}
+
+			// The last redirect.
+			if count <= 1 {
+				http.Redirect(w, r, fmt.Sprintf("/mod/%s", path[j+1:]), 302)
+				return
+			}
+			http.Redirect(w, r, fmt.Sprintf("/mod/redirect/%d/%s", count-1, path[j+1:]), 302)
+			return
+		}
+	}
+
+	// Request for $GOPROXY/sumdb/<name>/supported
+	// is checking whether it's OK to access sumdb via the proxy.
+	if path == "sumdb/"+testSumDBName+"/supported" {
+		w.WriteHeader(200)
+		return
+	}
+
+	// Request for $GOPROXY/sumdb/<name>/... goes to sumdb.
+	if sumdbPrefix := "sumdb/" + testSumDBName + "/"; strings.HasPrefix(path, sumdbPrefix) {
+		r.URL.Path = path[len(sumdbPrefix)-1:]
+		sumdbServer.ServeHTTP(w, r)
+		return
+	}
+
+	// Module proxy request: /mod/path/@latest
+	// Rewrite to /mod/path/@v/<latest>.info where <latest> is the semantically
+	// latest version, including pseudo-versions.
+	if i := strings.LastIndex(path, "/@latest"); i >= 0 {
+		enc := path[:i]
+		modPath, err := module.UnescapePath(enc)
+		if err != nil {
+			if testing.Verbose() {
+				fmt.Fprintf(os.Stderr, "go proxy_test: %v\n", err)
+			}
+			http.NotFound(w, r)
+			return
+		}
+
+		// Imitate what "latest" does in direct mode and what proxy.golang.org does.
+		// Use the latest released version.
+		// If there is no released version, use the latest prereleased version.
+		// Otherwise, use the latest pseudoversion.
+		var latestRelease, latestPrerelease, latestPseudo string
+		for _, m := range modList {
+			if m.Path != modPath {
+				continue
+			}
+			if module.IsPseudoVersion(m.Version) && (latestPseudo == "" || semver.Compare(latestPseudo, m.Version) > 0) {
+				latestPseudo = m.Version
+			} else if semver.Prerelease(m.Version) != "" && (latestPrerelease == "" || semver.Compare(latestPrerelease, m.Version) > 0) {
+				latestPrerelease = m.Version
+			} else if latestRelease == "" || semver.Compare(latestRelease, m.Version) > 0 {
+				latestRelease = m.Version
+			}
+		}
+		var latest string
+		if latestRelease != "" {
+			latest = latestRelease
+		} else if latestPrerelease != "" {
+			latest = latestPrerelease
+		} else if latestPseudo != "" {
+			latest = latestPseudo
+		} else {
+			http.NotFound(w, r)
+			return
+		}
+
+		encVers, err := module.EscapeVersion(latest)
+		if err != nil {
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+			return
+		}
+		path = fmt.Sprintf("%s/@v/%s.info", enc, encVers)
+	}
+
+	// Module proxy request: /mod/path/@v/version[.suffix]
+	i := strings.Index(path, "/@v/")
+	if i < 0 {
+		http.NotFound(w, r)
+		return
+	}
+	enc, file := path[:i], path[i+len("/@v/"):]
+	path, err := module.UnescapePath(enc)
+	if err != nil {
+		if testing.Verbose() {
+			fmt.Fprintf(os.Stderr, "go proxy_test: %v\n", err)
+		}
+		http.NotFound(w, r)
+		return
+	}
+	if file == "list" {
+		// list returns a list of versions, not including pseudo-versions.
+		// If the module has no tagged versions, we should serve an empty 200.
+		// If the module doesn't exist, we should serve 404 or 410.
+		found := false
+		for _, m := range modList {
+			if m.Path != path {
+				continue
+			}
+			found = true
+			if !module.IsPseudoVersion(m.Version) {
+				if err := module.Check(m.Path, m.Version); err == nil {
+					fmt.Fprintf(w, "%s\n", m.Version)
+				}
+			}
+		}
+		if !found {
+			http.NotFound(w, r)
+		}
+		return
+	}
+
+	i = strings.LastIndex(file, ".")
+	if i < 0 {
+		http.NotFound(w, r)
+		return
+	}
+	encVers, ext := file[:i], file[i+1:]
+	vers, err := module.UnescapeVersion(encVers)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "go proxy_test: %v\n", err)
+		http.NotFound(w, r)
+		return
+	}
+
+	if codehost.AllHex(vers) {
+		var best string
+		// Convert commit hash (only) to known version.
+		// Use latest version in semver priority, to match similar logic
+		// in the repo-based module server (see modfetch.(*codeRepo).convert).
+		for _, m := range modList {
+			if m.Path == path && semver.Compare(best, m.Version) < 0 {
+				var hash string
+				if module.IsPseudoVersion(m.Version) {
+					hash = m.Version[strings.LastIndex(m.Version, "-")+1:]
+				} else {
+					hash = findHash(m)
+				}
+				if strings.HasPrefix(hash, vers) || strings.HasPrefix(vers, hash) {
+					best = m.Version
+				}
+			}
+		}
+		if best != "" {
+			vers = best
+		}
+	}
+
+	a, err := readArchive(path, vers)
+	if err != nil {
+		if testing.Verbose() {
+			fmt.Fprintf(os.Stderr, "go proxy: no archive %s %s: %v\n", path, vers, err)
+		}
+		if errors.Is(err, fs.ErrNotExist) {
+			http.NotFound(w, r)
+		} else {
+			http.Error(w, "cannot load archive", 500)
+		}
+		return
+	}
+
+	switch ext {
+	case "info", "mod":
+		want := "." + ext
+		for _, f := range a.Files {
+			if f.Name == want {
+				w.Write(f.Data)
+				return
+			}
+		}
+
+	case "zip":
+		zipBytes, err := zipCache.Do(a, func() ([]byte, error) {
+			var buf bytes.Buffer
+			z := zip.NewWriter(&buf)
+			for _, f := range a.Files {
+				if f.Name == ".info" || f.Name == ".mod" || f.Name == ".zip" {
+					continue
+				}
+				var zipName string
+				if strings.HasPrefix(f.Name, "/") {
+					zipName = f.Name[1:]
+				} else {
+					zipName = path + "@" + vers + "/" + f.Name
+				}
+				zf, err := z.Create(zipName)
+				if err != nil {
+					return nil, err
+				}
+				if _, err := zf.Write(f.Data); err != nil {
+					return nil, err
+				}
+			}
+			if err := z.Close(); err != nil {
+				return nil, err
+			}
+			return buf.Bytes(), nil
+		})
+
+		if err != nil {
+			if testing.Verbose() {
+				fmt.Fprintf(os.Stderr, "go proxy: %v\n", err)
+			}
+			http.Error(w, err.Error(), 500)
+			return
+		}
+		w.Write(zipBytes)
+		return
+
+	}
+	http.NotFound(w, r)
+}
+
+func findHash(m module.Version) string {
+	a, err := readArchive(m.Path, m.Version)
+	if err != nil {
+		return ""
+	}
+	var data []byte
+	for _, f := range a.Files {
+		if f.Name == ".info" {
+			data = f.Data
+			break
+		}
+	}
+	var info struct{ Short string }
+	json.Unmarshal(data, &info)
+	return info.Short
+}
+
+var archiveCache par.Cache[string, *txtar.Archive]
+
+var cmdGoDir, _ = os.Getwd()
+
+func readArchive(path, vers string) (*txtar.Archive, error) {
+	enc, err := module.EscapePath(path)
+	if err != nil {
+		return nil, err
+	}
+	encVers, err := module.EscapeVersion(vers)
+	if err != nil {
+		return nil, err
+	}
+
+	prefix := strings.ReplaceAll(enc, "/", "_")
+	name := filepath.Join(cmdGoDir, "testdata/mod", prefix+"_"+encVers+".txt")
+	a := archiveCache.Do(name, func() *txtar.Archive {
+		a, err := txtar.ParseFile(name)
+		if err != nil {
+			if testing.Verbose() || !os.IsNotExist(err) {
+				fmt.Fprintf(os.Stderr, "go proxy: %v\n", err)
+			}
+			a = nil
+		}
+		return a
+	})
+	if a == nil {
+		return nil, fs.ErrNotExist
+	}
+	return a, nil
+}
+
+// proxyGoSum returns the two go.sum lines for path@vers.
+func proxyGoSum(path, vers string) ([]byte, error) {
+	a, err := readArchive(path, vers)
+	if err != nil {
+		return nil, err
+	}
+	var names []string
+	files := make(map[string][]byte)
+	var gomod []byte
+	for _, f := range a.Files {
+		if strings.HasPrefix(f.Name, ".") {
+			if f.Name == ".mod" {
+				gomod = f.Data
+			}
+			continue
+		}
+		name := path + "@" + vers + "/" + f.Name
+		names = append(names, name)
+		files[name] = f.Data
+	}
+	h1, err := dirhash.Hash1(names, func(name string) (io.ReadCloser, error) {
+		data := files[name]
+		return io.NopCloser(bytes.NewReader(data)), nil
+	})
+	if err != nil {
+		return nil, err
+	}
+	h1mod, err := dirhash.Hash1([]string{"go.mod"}, func(string) (io.ReadCloser, error) {
+		return io.NopCloser(bytes.NewReader(gomod)), nil
+	})
+	if err != nil {
+		return nil, err
+	}
+	data := []byte(fmt.Sprintf("%s %s %s\n%s %s/go.mod %s\n", path, vers, h1, path, vers, h1mod))
+	return data, nil
+}
+
+// proxyGoSumWrong returns the wrong lines.
+func proxyGoSumWrong(path, vers string) ([]byte, error) {
+	data := []byte(fmt.Sprintf("%s %s %s\n%s %s/go.mod %s\n", path, vers, "h1:wrong", path, vers, "h1:wrong"))
+	return data, nil
+}
diff --git a/src/cmd/go/script_test.go b/src/cmd/go/script_test.go
new file mode 100644
index 0000000..624c5bf
--- /dev/null
+++ b/src/cmd/go/script_test.go
@@ -0,0 +1,360 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Script-driven tests.
+// See testdata/script/README for an overview.
+
+//go:generate go test cmd/go -v -run=TestScript/README --fixreadme
+
+package main_test
+
+import (
+	"bufio"
+	"bytes"
+	"context"
+	"flag"
+	"internal/testenv"
+	"internal/txtar"
+	"net/url"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+	"time"
+
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/gover"
+	"cmd/go/internal/script"
+	"cmd/go/internal/script/scripttest"
+	"cmd/go/internal/vcweb/vcstest"
+)
+
+var testSum = flag.String("testsum", "", `may be tidy, listm, or listall. If set, TestScript generates a go.sum file at the beginning of each test and updates test files if they pass.`)
+
+// TestScript runs the tests in testdata/script/*.txt.
+func TestScript(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+	testenv.SkipIfShortAndSlow(t)
+
+	srv, err := vcstest.NewServer()
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() {
+		if err := srv.Close(); err != nil {
+			t.Fatal(err)
+		}
+	})
+	certFile, err := srv.WriteCertificateFile()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	StartProxy()
+
+	var (
+		ctx         = context.Background()
+		gracePeriod = 100 * time.Millisecond
+	)
+	if deadline, ok := t.Deadline(); ok {
+		timeout := time.Until(deadline)
+
+		// If time allows, increase the termination grace period to 5% of the
+		// remaining time.
+		if gp := timeout / 20; gp > gracePeriod {
+			gracePeriod = gp
+		}
+
+		// When we run commands that execute subprocesses, we want to reserve two
+		// grace periods to clean up. We will send the first termination signal when
+		// the context expires, then wait one grace period for the process to
+		// produce whatever useful output it can (such as a stack trace). After the
+		// first grace period expires, we'll escalate to os.Kill, leaving the second
+		// grace period for the test function to record its output before the test
+		// process itself terminates.
+		timeout -= 2 * gracePeriod
+
+		var cancel context.CancelFunc
+		ctx, cancel = context.WithTimeout(ctx, timeout)
+		t.Cleanup(cancel)
+	}
+
+	env, err := scriptEnv(srv, certFile)
+	if err != nil {
+		t.Fatal(err)
+	}
+	engine := &script.Engine{
+		Conds: scriptConditions(),
+		Cmds:  scriptCommands(quitSignal(), gracePeriod),
+		Quiet: !testing.Verbose(),
+	}
+
+	t.Run("README", func(t *testing.T) {
+		checkScriptReadme(t, engine, env)
+	})
+
+	files, err := filepath.Glob("testdata/script/*.txt")
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, file := range files {
+		file := file
+		name := strings.TrimSuffix(filepath.Base(file), ".txt")
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+			StartProxy()
+
+			workdir, err := os.MkdirTemp(testTmpDir, name)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if !*testWork {
+				defer removeAll(workdir)
+			}
+
+			s, err := script.NewState(tbContext(ctx, t), workdir, env)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			// Unpack archive.
+			a, err := txtar.ParseFile(file)
+			if err != nil {
+				t.Fatal(err)
+			}
+			initScriptDirs(t, s)
+			if err := s.ExtractFiles(a); err != nil {
+				t.Fatal(err)
+			}
+
+			t.Log(time.Now().UTC().Format(time.RFC3339))
+			work, _ := s.LookupEnv("WORK")
+			t.Logf("$WORK=%s", work)
+
+			// With -testsum, if a go.mod file is present in the test's initial
+			// working directory, run 'go mod tidy'.
+			if *testSum != "" {
+				if updateSum(t, engine, s, a) {
+					defer func() {
+						if t.Failed() {
+							return
+						}
+						data := txtar.Format(a)
+						if err := os.WriteFile(file, data, 0666); err != nil {
+							t.Errorf("rewriting test file: %v", err)
+						}
+					}()
+				}
+			}
+
+			// Note: Do not use filepath.Base(file) here:
+			// editors that can jump to file:line references in the output
+			// will work better seeing the full path relative to cmd/go
+			// (where the "go test" command is usually run).
+			scripttest.Run(t, engine, s, file, bytes.NewReader(a.Comment))
+		})
+	}
+}
+
+// testingTBKey is the Context key for a testing.TB.
+type testingTBKey struct{}
+
+// tbContext returns a Context derived from ctx and associated with t.
+func tbContext(ctx context.Context, t testing.TB) context.Context {
+	return context.WithValue(ctx, testingTBKey{}, t)
+}
+
+// tbFromContext returns the testing.TB associated with ctx, if any.
+func tbFromContext(ctx context.Context) (testing.TB, bool) {
+	t := ctx.Value(testingTBKey{})
+	if t == nil {
+		return nil, false
+	}
+	return t.(testing.TB), true
+}
+
+// initScriptState creates the initial directory structure in s for unpacking a
+// cmd/go script.
+func initScriptDirs(t testing.TB, s *script.State) {
+	must := func(err error) {
+		if err != nil {
+			t.Helper()
+			t.Fatal(err)
+		}
+	}
+
+	work := s.Getwd()
+	must(s.Setenv("WORK", work))
+
+	must(os.MkdirAll(filepath.Join(work, "tmp"), 0777))
+	must(s.Setenv(tempEnvName(), filepath.Join(work, "tmp")))
+
+	gopath := filepath.Join(work, "gopath")
+	must(s.Setenv("GOPATH", gopath))
+	gopathSrc := filepath.Join(gopath, "src")
+	must(os.MkdirAll(gopathSrc, 0777))
+	must(s.Chdir(gopathSrc))
+}
+
+func scriptEnv(srv *vcstest.Server, srvCertFile string) ([]string, error) {
+	httpURL, err := url.Parse(srv.HTTP.URL)
+	if err != nil {
+		return nil, err
+	}
+	httpsURL, err := url.Parse(srv.HTTPS.URL)
+	if err != nil {
+		return nil, err
+	}
+	env := []string{
+		pathEnvName() + "=" + testBin + string(filepath.ListSeparator) + os.Getenv(pathEnvName()),
+		homeEnvName() + "=/no-home",
+		"CCACHE_DISABLE=1", // ccache breaks with non-existent HOME
+		"GOARCH=" + runtime.GOARCH,
+		"TESTGO_GOHOSTARCH=" + goHostArch,
+		"GOCACHE=" + testGOCACHE,
+		"GOCOVERDIR=" + os.Getenv("GOCOVERDIR"),
+		"GODEBUG=" + os.Getenv("GODEBUG"),
+		"GOEXE=" + cfg.ExeSuffix,
+		"GOEXPERIMENT=" + os.Getenv("GOEXPERIMENT"),
+		"GOOS=" + runtime.GOOS,
+		"TESTGO_GOHOSTOS=" + goHostOS,
+		"GOPROXY=" + proxyURL,
+		"GOPRIVATE=",
+		"GOROOT=" + testGOROOT,
+		"GOROOT_FINAL=" + testGOROOT_FINAL, // causes spurious rebuilds and breaks the "stale" built-in if not propagated
+		"GOTRACEBACK=system",
+		"TESTGONETWORK=panic", // allow only local connections by default; the [net] condition resets this
+		"TESTGO_GOROOT=" + testGOROOT,
+		"TESTGO_EXE=" + testGo,
+		"TESTGO_VCSTEST_HOST=" + httpURL.Host,
+		"TESTGO_VCSTEST_TLS_HOST=" + httpsURL.Host,
+		"TESTGO_VCSTEST_CERT=" + srvCertFile,
+		"TESTGONETWORK=panic", // cleared by the [net] condition
+		"GOSUMDB=" + testSumDBVerifierKey,
+		"GONOPROXY=",
+		"GONOSUMDB=",
+		"GOVCS=*:all",
+		"devnull=" + os.DevNull,
+		"goversion=" + gover.Local(),
+		"CMDGO_TEST_RUN_MAIN=true",
+		"HGRCPATH=",
+		"GOTOOLCHAIN=auto",
+		"newline=\n",
+	}
+
+	if testenv.Builder() != "" || os.Getenv("GIT_TRACE_CURL") == "1" {
+		// To help diagnose https://go.dev/issue/52545,
+		// enable tracing for Git HTTPS requests.
+		env = append(env,
+			"GIT_TRACE_CURL=1",
+			"GIT_TRACE_CURL_NO_DATA=1",
+			"GIT_REDACT_COOKIES=o,SSO,GSSO_Uberproxy")
+	}
+	if testing.Short() {
+		// VCS commands are always somewhat slow: they either require access to external hosts,
+		// or they require our intercepted vcs-test.golang.org to regenerate the repository.
+		// Require all tests that use VCS commands to be skipped in short mode.
+		env = append(env, "TESTGOVCS=panic")
+	}
+
+	if os.Getenv("CGO_ENABLED") != "" || runtime.GOOS != goHostOS || runtime.GOARCH != goHostArch {
+		// If the actual CGO_ENABLED might not match the cmd/go default, set it
+		// explicitly in the environment. Otherwise, leave it unset so that we also
+		// cover the default behaviors.
+		env = append(env, "CGO_ENABLED="+cgoEnabled)
+	}
+
+	for _, key := range extraEnvKeys {
+		if val, ok := os.LookupEnv(key); ok {
+			env = append(env, key+"="+val)
+		}
+	}
+
+	return env, nil
+}
+
+var extraEnvKeys = []string{
+	"SYSTEMROOT",         // must be preserved on Windows to find DLLs; golang.org/issue/25210
+	"WINDIR",             // must be preserved on Windows to be able to run PowerShell command; golang.org/issue/30711
+	"LD_LIBRARY_PATH",    // must be preserved on Unix systems to find shared libraries
+	"LIBRARY_PATH",       // allow override of non-standard static library paths
+	"C_INCLUDE_PATH",     // allow override non-standard include paths
+	"CC",                 // don't lose user settings when invoking cgo
+	"GO_TESTING_GOTOOLS", // for gccgo testing
+	"GCCGO",              // for gccgo testing
+	"GCCGOTOOLDIR",       // for gccgo testing
+}
+
+// updateSum runs 'go mod tidy', 'go list -mod=mod -m all', or
+// 'go list -mod=mod all' in the test's current directory if a file named
+// "go.mod" is present after the archive has been extracted. updateSum modifies
+// archive and returns true if go.mod or go.sum were changed.
+func updateSum(t testing.TB, e *script.Engine, s *script.State, archive *txtar.Archive) (rewrite bool) {
+	gomodIdx, gosumIdx := -1, -1
+	for i := range archive.Files {
+		switch archive.Files[i].Name {
+		case "go.mod":
+			gomodIdx = i
+		case "go.sum":
+			gosumIdx = i
+		}
+	}
+	if gomodIdx < 0 {
+		return false
+	}
+
+	var cmd string
+	switch *testSum {
+	case "tidy":
+		cmd = "go mod tidy"
+	case "listm":
+		cmd = "go list -m -mod=mod all"
+	case "listall":
+		cmd = "go list -mod=mod all"
+	default:
+		t.Fatalf(`unknown value for -testsum %q; may be "tidy", "listm", or "listall"`, *testSum)
+	}
+
+	log := new(strings.Builder)
+	err := e.Execute(s, "updateSum", bufio.NewReader(strings.NewReader(cmd)), log)
+	if log.Len() > 0 {
+		t.Logf("%s", log)
+	}
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	newGomodData, err := os.ReadFile(s.Path("go.mod"))
+	if err != nil {
+		t.Fatalf("reading go.mod after -testsum: %v", err)
+	}
+	if !bytes.Equal(newGomodData, archive.Files[gomodIdx].Data) {
+		archive.Files[gomodIdx].Data = newGomodData
+		rewrite = true
+	}
+
+	newGosumData, err := os.ReadFile(s.Path("go.sum"))
+	if err != nil && !os.IsNotExist(err) {
+		t.Fatalf("reading go.sum after -testsum: %v", err)
+	}
+	switch {
+	case os.IsNotExist(err) && gosumIdx >= 0:
+		// go.sum was deleted.
+		rewrite = true
+		archive.Files = append(archive.Files[:gosumIdx], archive.Files[gosumIdx+1:]...)
+	case err == nil && gosumIdx < 0:
+		// go.sum was created.
+		rewrite = true
+		gosumIdx = gomodIdx + 1
+		archive.Files = append(archive.Files, txtar.File{})
+		copy(archive.Files[gosumIdx+1:], archive.Files[gosumIdx:])
+		archive.Files[gosumIdx] = txtar.File{Name: "go.sum", Data: newGosumData}
+	case err == nil && gosumIdx >= 0 && !bytes.Equal(newGosumData, archive.Files[gosumIdx].Data):
+		// go.sum was changed.
+		rewrite = true
+		archive.Files[gosumIdx].Data = newGosumData
+	}
+	return rewrite
+}
diff --git a/src/cmd/go/scriptcmds_test.go b/src/cmd/go/scriptcmds_test.go
new file mode 100644
index 0000000..db5e6ca
--- /dev/null
+++ b/src/cmd/go/scriptcmds_test.go
@@ -0,0 +1,109 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main_test
+
+import (
+	"cmd/go/internal/script"
+	"cmd/go/internal/script/scripttest"
+	"cmd/go/internal/work"
+	"errors"
+	"fmt"
+	"os"
+	"os/exec"
+	"strings"
+	"time"
+)
+
+func scriptCommands(interrupt os.Signal, waitDelay time.Duration) map[string]script.Cmd {
+	cmds := scripttest.DefaultCmds()
+
+	// Customize the "exec" interrupt signal and grace period.
+	var cancel func(cmd *exec.Cmd) error
+	if interrupt != nil {
+		cancel = func(cmd *exec.Cmd) error {
+			return cmd.Process.Signal(interrupt)
+		}
+	}
+
+	cmdExec := script.Exec(cancel, waitDelay)
+	cmds["exec"] = cmdExec
+
+	add := func(name string, cmd script.Cmd) {
+		if _, ok := cmds[name]; ok {
+			panic(fmt.Sprintf("command %q is already registered", name))
+		}
+		cmds[name] = cmd
+	}
+
+	add("cc", scriptCC(cmdExec))
+	cmdGo := scriptGo(cancel, waitDelay)
+	add("go", cmdGo)
+	add("stale", scriptStale(cmdGo))
+
+	return cmds
+}
+
+// scriptCC runs the C compiler along with platform specific options.
+func scriptCC(cmdExec script.Cmd) script.Cmd {
+	return script.Command(
+		script.CmdUsage{
+			Summary: "run the platform C compiler",
+			Args:    "args...",
+		},
+		func(s *script.State, args ...string) (script.WaitFunc, error) {
+			b := work.NewBuilder(s.Getwd())
+			wait, err := cmdExec.Run(s, append(b.GccCmd(".", ""), args...)...)
+			if err != nil {
+				return wait, err
+			}
+			waitAndClean := func(s *script.State) (stdout, stderr string, err error) {
+				stdout, stderr, err = wait(s)
+				if closeErr := b.Close(); err == nil {
+					err = closeErr
+				}
+				return stdout, stderr, err
+			}
+			return waitAndClean, nil
+		})
+}
+
+// scriptGo runs the go command.
+func scriptGo(cancel func(*exec.Cmd) error, waitDelay time.Duration) script.Cmd {
+	return script.Program(testGo, cancel, waitDelay)
+}
+
+// scriptStale checks that the named build targets are stale.
+func scriptStale(cmdGo script.Cmd) script.Cmd {
+	return script.Command(
+		script.CmdUsage{
+			Summary: "check that build targets are stale",
+			Args:    "target...",
+		},
+		func(s *script.State, args ...string) (script.WaitFunc, error) {
+			if len(args) == 0 {
+				return nil, script.ErrUsage
+			}
+			tmpl := "{{if .Error}}{{.ImportPath}}: {{.Error.Err}}" +
+				"{{else}}{{if not .Stale}}{{.ImportPath}} ({{.Target}}) is not stale{{end}}" +
+				"{{end}}"
+
+			wait, err := cmdGo.Run(s, append([]string{"list", "-e", "-f=" + tmpl}, args...)...)
+			if err != nil {
+				return nil, err
+			}
+
+			stdout, stderr, err := wait(s)
+			if len(stderr) != 0 {
+				s.Logf("%s", stderr)
+			}
+			if err != nil {
+				return nil, err
+			}
+			if out := strings.TrimSpace(stdout); out != "" {
+				return nil, errors.New(out)
+			}
+			return nil, nil
+		})
+}
diff --git a/src/cmd/go/scriptconds_test.go b/src/cmd/go/scriptconds_test.go
new file mode 100644
index 0000000..8dd9b0d
--- /dev/null
+++ b/src/cmd/go/scriptconds_test.go
@@ -0,0 +1,235 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main_test
+
+import (
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/script"
+	"cmd/go/internal/script/scripttest"
+	"errors"
+	"fmt"
+	"internal/buildcfg"
+	"internal/platform"
+	"internal/testenv"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"runtime/debug"
+	"strings"
+	"sync"
+)
+
+func scriptConditions() map[string]script.Cond {
+	conds := scripttest.DefaultConds()
+
+	add := func(name string, cond script.Cond) {
+		if _, ok := conds[name]; ok {
+			panic(fmt.Sprintf("condition %q is already registered", name))
+		}
+		conds[name] = cond
+	}
+
+	lazyBool := func(summary string, f func() bool) script.Cond {
+		return script.OnceCondition(summary, func() (bool, error) { return f(), nil })
+	}
+
+	add("abscc", script.Condition("default $CC path is absolute and exists", defaultCCIsAbsolute))
+	add("asan", sysCondition("-asan", platform.ASanSupported, true))
+	add("buildmode", script.PrefixCondition("go supports -buildmode=<suffix>", hasBuildmode))
+	add("case-sensitive", script.OnceCondition("$WORK filesystem is case-sensitive", isCaseSensitive))
+	add("cc", script.PrefixCondition("go env CC = <suffix> (ignoring the go/env file)", ccIs))
+	add("cgo", script.BoolCondition("host CGO_ENABLED", testenv.HasCGO()))
+	add("cgolinkext", script.Condition("platform requires external linking for cgo", cgoLinkExt))
+	add("cross", script.BoolCondition("cmd/go GOOS/GOARCH != GOHOSTOS/GOHOSTARCH", goHostOS != runtime.GOOS || goHostArch != runtime.GOARCH))
+	add("fuzz", sysCondition("-fuzz", platform.FuzzSupported, false))
+	add("fuzz-instrumented", sysCondition("-fuzz with instrumentation", platform.FuzzInstrumented, false))
+	add("git", lazyBool("the 'git' executable exists and provides the standard CLI", hasWorkingGit))
+	add("GODEBUG", script.PrefixCondition("GODEBUG contains <suffix>", hasGodebug))
+	add("GOEXPERIMENT", script.PrefixCondition("GOEXPERIMENT <suffix> is enabled", hasGoexperiment))
+	add("go-builder", script.BoolCondition("GO_BUILDER_NAME is non-empty", testenv.Builder() != ""))
+	add("link", lazyBool("testenv.HasLink()", testenv.HasLink))
+	add("mismatched-goroot", script.Condition("test's GOROOT_FINAL does not match the real GOROOT", isMismatchedGoroot))
+	add("msan", sysCondition("-msan", platform.MSanSupported, true))
+	add("mustlinkext", script.Condition("platform always requires external linking", mustLinkExt))
+	add("net", script.PrefixCondition("can connect to external network host <suffix>", hasNet))
+	add("race", sysCondition("-race", platform.RaceDetectorSupported, true))
+	add("symlink", lazyBool("testenv.HasSymlink()", testenv.HasSymlink))
+	add("trimpath", script.OnceCondition("test binary was built with -trimpath", isTrimpath))
+
+	return conds
+}
+
+func defaultCCIsAbsolute(s *script.State) (bool, error) {
+	GOOS, _ := s.LookupEnv("GOOS")
+	GOARCH, _ := s.LookupEnv("GOARCH")
+	defaultCC := cfg.DefaultCC(GOOS, GOARCH)
+	if filepath.IsAbs(defaultCC) {
+		if _, err := exec.LookPath(defaultCC); err == nil {
+			return true, nil
+		}
+	}
+	return false, nil
+}
+
+func ccIs(s *script.State, want string) (bool, error) {
+	CC, _ := s.LookupEnv("CC")
+	if CC != "" {
+		return CC == want, nil
+	}
+	GOOS, _ := s.LookupEnv("GOOS")
+	GOARCH, _ := s.LookupEnv("GOARCH")
+	return cfg.DefaultCC(GOOS, GOARCH) == want, nil
+}
+
+func isMismatchedGoroot(s *script.State) (bool, error) {
+	gorootFinal, _ := s.LookupEnv("GOROOT_FINAL")
+	if gorootFinal == "" {
+		gorootFinal, _ = s.LookupEnv("GOROOT")
+	}
+	return gorootFinal != testGOROOT, nil
+}
+
+func sysCondition(flag string, f func(goos, goarch string) bool, needsCgo bool) script.Cond {
+	return script.Condition(
+		"GOOS/GOARCH supports "+flag,
+		func(s *script.State) (bool, error) {
+			GOOS, _ := s.LookupEnv("GOOS")
+			GOARCH, _ := s.LookupEnv("GOARCH")
+			cross := goHostOS != GOOS || goHostArch != GOARCH
+			return (!needsCgo || (testenv.HasCGO() && !cross)) && f(GOOS, GOARCH), nil
+		})
+}
+
+func hasBuildmode(s *script.State, mode string) (bool, error) {
+	GOOS, _ := s.LookupEnv("GOOS")
+	GOARCH, _ := s.LookupEnv("GOARCH")
+	return platform.BuildModeSupported(runtime.Compiler, mode, GOOS, GOARCH), nil
+}
+
+var scriptNetEnabled sync.Map // testing.TB → already enabled
+
+func hasNet(s *script.State, host string) (bool, error) {
+	if !testenv.HasExternalNetwork() {
+		return false, nil
+	}
+
+	// TODO(bcmills): Add a flag or environment variable to allow skipping tests
+	// for specific hosts and/or skipping all net tests except for specific hosts.
+
+	t, ok := tbFromContext(s.Context())
+	if !ok {
+		return false, errors.New("script Context unexpectedly missing testing.TB key")
+	}
+
+	if netTestSem != nil {
+		// When the number of external network connections is limited, we limit the
+		// number of net tests that can run concurrently so that the overall number
+		// of network connections won't exceed the limit.
+		_, dup := scriptNetEnabled.LoadOrStore(t, true)
+		if !dup {
+			// Acquire a net token for this test until the test completes.
+			netTestSem <- struct{}{}
+			t.Cleanup(func() {
+				<-netTestSem
+				scriptNetEnabled.Delete(t)
+			})
+		}
+	}
+
+	// Since we have confirmed that the network is available,
+	// allow cmd/go to use it.
+	s.Setenv("TESTGONETWORK", "")
+	return true, nil
+}
+
+func hasGodebug(s *script.State, value string) (bool, error) {
+	godebug, _ := s.LookupEnv("GODEBUG")
+	for _, p := range strings.Split(godebug, ",") {
+		if strings.TrimSpace(p) == value {
+			return true, nil
+		}
+	}
+	return false, nil
+}
+
+func hasGoexperiment(s *script.State, value string) (bool, error) {
+	GOOS, _ := s.LookupEnv("GOOS")
+	GOARCH, _ := s.LookupEnv("GOARCH")
+	goexp, _ := s.LookupEnv("GOEXPERIMENT")
+	flags, err := buildcfg.ParseGOEXPERIMENT(GOOS, GOARCH, goexp)
+	if err != nil {
+		return false, err
+	}
+	for _, exp := range flags.All() {
+		if value == exp {
+			return true, nil
+		}
+		if strings.TrimPrefix(value, "no") == strings.TrimPrefix(exp, "no") {
+			return false, nil
+		}
+	}
+	return false, fmt.Errorf("unrecognized GOEXPERIMENT %q", value)
+}
+
+func isCaseSensitive() (bool, error) {
+	tmpdir, err := os.MkdirTemp(testTmpDir, "case-sensitive")
+	if err != nil {
+		return false, fmt.Errorf("failed to create directory to determine case-sensitivity: %w", err)
+	}
+	defer os.RemoveAll(tmpdir)
+
+	fcap := filepath.Join(tmpdir, "FILE")
+	if err := os.WriteFile(fcap, []byte{}, 0644); err != nil {
+		return false, fmt.Errorf("error writing file to determine case-sensitivity: %w", err)
+	}
+
+	flow := filepath.Join(tmpdir, "file")
+	_, err = os.ReadFile(flow)
+	switch {
+	case err == nil:
+		return false, nil
+	case os.IsNotExist(err):
+		return true, nil
+	default:
+		return false, fmt.Errorf("unexpected error reading file when determining case-sensitivity: %w", err)
+	}
+}
+
+func isTrimpath() (bool, error) {
+	info, _ := debug.ReadBuildInfo()
+	if info == nil {
+		return false, errors.New("missing build info")
+	}
+
+	for _, s := range info.Settings {
+		if s.Key == "-trimpath" && s.Value == "true" {
+			return true, nil
+		}
+	}
+	return false, nil
+}
+
+func hasWorkingGit() bool {
+	if runtime.GOOS == "plan9" {
+		// The Git command is usually not the real Git on Plan 9.
+		// See https://golang.org/issues/29640.
+		return false
+	}
+	_, err := exec.LookPath("git")
+	return err == nil
+}
+
+func cgoLinkExt(s *script.State) (bool, error) {
+	GOOS, _ := s.LookupEnv("GOOS")
+	GOARCH, _ := s.LookupEnv("GOARCH")
+	return platform.MustLinkExternal(GOOS, GOARCH, true), nil
+}
+
+func mustLinkExt(s *script.State) (bool, error) {
+	GOOS, _ := s.LookupEnv("GOOS")
+	GOARCH, _ := s.LookupEnv("GOARCH")
+	return platform.MustLinkExternal(GOOS, GOARCH, false), nil
+}
diff --git a/src/cmd/go/scriptreadme_test.go b/src/cmd/go/scriptreadme_test.go
new file mode 100644
index 0000000..2a842fb
--- /dev/null
+++ b/src/cmd/go/scriptreadme_test.go
@@ -0,0 +1,267 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main_test
+
+import (
+	"bytes"
+	"cmd/go/internal/script"
+	"flag"
+	"internal/diff"
+	"internal/testenv"
+	"os"
+	"strings"
+	"testing"
+	"text/template"
+)
+
+var fixReadme = flag.Bool("fixreadme", false, "if true, update ../testdata/script/README")
+
+func checkScriptReadme(t *testing.T, engine *script.Engine, env []string) {
+	var args struct {
+		Language   string
+		Commands   string
+		Conditions string
+	}
+
+	cmds := new(strings.Builder)
+	if err := engine.ListCmds(cmds, true); err != nil {
+		t.Fatal(err)
+	}
+	args.Commands = cmds.String()
+
+	conds := new(strings.Builder)
+	if err := engine.ListConds(conds, nil); err != nil {
+		t.Fatal(err)
+	}
+	args.Conditions = conds.String()
+
+	doc := new(strings.Builder)
+	cmd := testenv.Command(t, testGo, "doc", "cmd/go/internal/script")
+	cmd.Env = env
+	cmd.Stdout = doc
+	if err := cmd.Run(); err != nil {
+		t.Fatal(cmd, ":", err)
+	}
+	_, lang, ok := strings.Cut(doc.String(), "# Script Language\n\n")
+	if !ok {
+		t.Fatalf("%q did not include Script Language section", cmd)
+	}
+	lang, _, ok = strings.Cut(lang, "\n\nvar ")
+	if !ok {
+		t.Fatalf("%q did not include vars after Script Language section", cmd)
+	}
+	args.Language = lang
+
+	tmpl := template.Must(template.New("README").Parse(readmeTmpl[1:]))
+	buf := new(bytes.Buffer)
+	if err := tmpl.Execute(buf, args); err != nil {
+		t.Fatal(err)
+	}
+
+	const readmePath = "testdata/script/README"
+	old, err := os.ReadFile(readmePath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	diff := diff.Diff(readmePath, old, "readmeTmpl", buf.Bytes())
+	if diff == nil {
+		t.Logf("%s is up to date.", readmePath)
+		return
+	}
+
+	if *fixReadme {
+		if err := os.WriteFile(readmePath, buf.Bytes(), 0666); err != nil {
+			t.Fatal(err)
+		}
+		t.Logf("wrote %d bytes to %s", buf.Len(), readmePath)
+	} else {
+		t.Logf("\n%s", diff)
+		t.Errorf("%s is stale. To update, run 'go generate cmd/go'.", readmePath)
+	}
+}
+
+const readmeTmpl = `
+This file is generated by 'go generate cmd/go'. DO NOT EDIT.
+
+This directory holds test scripts *.txt run during 'go test cmd/go'.
+To run a specific script foo.txt
+
+	go test cmd/go -run=Script/^foo$
+
+In general script files should have short names: a few words, not whole sentences.
+The first word should be the general category of behavior being tested,
+often the name of a go subcommand (list, build, test, ...) or concept (vendor, pattern).
+
+Each script is a text archive (go doc internal/txtar).
+The script begins with an actual command script to run
+followed by the content of zero or more supporting files to
+create in the script's temporary file system before it starts executing.
+
+As an example, run_hello.txt says:
+
+	# hello world
+	go run hello.go
+	stderr 'hello world'
+	! stdout .
+
+	-- hello.go --
+	package main
+	func main() { println("hello world") }
+
+Each script runs in a fresh temporary work directory tree, available to scripts as $WORK.
+Scripts also have access to other environment variables, including:
+
+	GOARCH=<target GOARCH>
+	GOCACHE=<actual GOCACHE being used outside the test>
+	GOEXE=<executable file suffix: .exe on Windows, empty on other systems>
+	GOOS=<target GOOS>
+	GOPATH=$WORK/gopath
+	GOPROXY=<local module proxy serving from cmd/go/testdata/mod>
+	GOROOT=<actual GOROOT>
+	GOROOT_FINAL=<actual GOROOT_FINAL>
+	TESTGO_GOROOT=<GOROOT used to build cmd/go, for use in tests that may change GOROOT>
+	HOME=/no-home
+	PATH=<actual PATH>
+	TMPDIR=$WORK/tmp
+	GODEBUG=<actual GODEBUG>
+	devnull=<value of os.DevNull>
+	goversion=<current Go version; for example, 1.12>
+
+On Plan 9, the variables $path and $home are set instead of $PATH and $HOME.
+On Windows, the variables $USERPROFILE and $TMP are set instead of
+$HOME and $TMPDIR.
+
+The lines at the top of the script are a sequence of commands to be executed by
+a small script engine configured in ../../script_test.go (not the system shell).
+
+The scripts' supporting files are unpacked relative to $GOPATH/src
+(aka $WORK/gopath/src) and then the script begins execution in that directory as
+well. Thus the example above runs in $WORK/gopath/src with GOPATH=$WORK/gopath
+and $WORK/gopath/src/hello.go containing the listed contents.
+
+{{.Language}}
+
+When TestScript runs a script and the script fails, by default TestScript shows
+the execution of the most recent phase of the script (since the last # comment)
+and only shows the # comments for earlier phases. For example, here is a
+multi-phase script with a bug in it:
+
+	# GOPATH with p1 in d2, p2 in d2
+	env GOPATH=$WORK${/}d1${:}$WORK${/}d2
+
+	# build & install p1
+	env
+	go install -i p1
+	! stale p1
+	! stale p2
+
+	# modify p2 - p1 should appear stale
+	cp $WORK/p2x.go $WORK/d2/src/p2/p2.go
+	stale p1 p2
+
+	# build & install p1 again
+	go install -i p11
+	! stale p1
+	! stale p2
+
+	-- $WORK/d1/src/p1/p1.go --
+	package p1
+	import "p2"
+	func F() { p2.F() }
+	-- $WORK/d2/src/p2/p2.go --
+	package p2
+	func F() {}
+	-- $WORK/p2x.go --
+	package p2
+	func F() {}
+	func G() {}
+
+The bug is that the final phase installs p11 instead of p1. The test failure looks like:
+
+	$ go test -run=Script
+	--- FAIL: TestScript (3.75s)
+	    --- FAIL: TestScript/install_rebuild_gopath (0.16s)
+	        script_test.go:223:
+	            # GOPATH with p1 in d2, p2 in d2 (0.000s)
+	            # build & install p1 (0.087s)
+	            # modify p2 - p1 should appear stale (0.029s)
+	            # build & install p1 again (0.022s)
+	            > go install -i p11
+	            [stderr]
+	            can't load package: package p11: cannot find package "p11" in any of:
+	            	/Users/rsc/go/src/p11 (from $GOROOT)
+	            	$WORK/d1/src/p11 (from $GOPATH)
+	            	$WORK/d2/src/p11
+	            [exit status 1]
+	            FAIL: unexpected go command failure
+
+	        script_test.go:73: failed at testdata/script/install_rebuild_gopath.txt:15 in $WORK/gopath/src
+
+	FAIL
+	exit status 1
+	FAIL	cmd/go	4.875s
+	$
+
+Note that the commands in earlier phases have been hidden, so that the relevant
+commands are more easily found, and the elapsed time for a completed phase
+is shown next to the phase heading. To see the entire execution, use "go test -v",
+which also adds an initial environment dump to the beginning of the log.
+
+Note also that in reported output, the actual name of the per-script temporary directory
+has been consistently replaced with the literal string $WORK.
+
+The cmd/go test flag -testwork (which must appear on the "go test" command line after
+standard test flags) causes each test to log the name of its $WORK directory and other
+environment variable settings and also to leave that directory behind when it exits,
+for manual debugging of failing tests:
+
+	$ go test -run=Script -work
+	--- FAIL: TestScript (3.75s)
+	    --- FAIL: TestScript/install_rebuild_gopath (0.16s)
+	        script_test.go:223:
+	            WORK=/tmp/cmd-go-test-745953508/script-install_rebuild_gopath
+	            GOARCH=
+	            GOCACHE=/Users/rsc/Library/Caches/go-build
+	            GOOS=
+	            GOPATH=$WORK/gopath
+	            GOROOT=/Users/rsc/go
+	            HOME=/no-home
+	            TMPDIR=$WORK/tmp
+	            exe=
+
+	            # GOPATH with p1 in d2, p2 in d2 (0.000s)
+	            # build & install p1 (0.085s)
+	            # modify p2 - p1 should appear stale (0.030s)
+	            # build & install p1 again (0.019s)
+	            > go install -i p11
+	            [stderr]
+	            can't load package: package p11: cannot find package "p11" in any of:
+	            	/Users/rsc/go/src/p11 (from $GOROOT)
+	            	$WORK/d1/src/p11 (from $GOPATH)
+	            	$WORK/d2/src/p11
+	            [exit status 1]
+	            FAIL: unexpected go command failure
+
+	        script_test.go:73: failed at testdata/script/install_rebuild_gopath.txt:15 in $WORK/gopath/src
+
+	FAIL
+	exit status 1
+	FAIL	cmd/go	4.875s
+	$
+
+	$ WORK=/tmp/cmd-go-test-745953508/script-install_rebuild_gopath
+	$ cd $WORK/d1/src/p1
+	$ cat p1.go
+	package p1
+	import "p2"
+	func F() { p2.F() }
+	$
+
+The available commands are:
+{{.Commands}}
+
+The available conditions are:
+{{.Conditions}}
+`
diff --git a/src/cmd/go/stop_other_test.go b/src/cmd/go/stop_other_test.go
new file mode 100644
index 0000000..cb4569b
--- /dev/null
+++ b/src/cmd/go/stop_other_test.go
@@ -0,0 +1,24 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !(unix || (js && wasm))
+
+package main_test
+
+import (
+	"os"
+	"runtime"
+)
+
+// quitSignal returns the appropriate signal to use to request that a process
+// quit execution.
+func quitSignal() os.Signal {
+	if runtime.GOOS == "windows" {
+		// Per https://golang.org/pkg/os/#Signal, “Interrupt is not implemented on
+		// Windows; using it with os.Process.Signal will return an error.”
+		// Fall back to Kill instead.
+		return os.Kill
+	}
+	return os.Interrupt
+}
diff --git a/src/cmd/go/stop_unix_test.go b/src/cmd/go/stop_unix_test.go
new file mode 100644
index 0000000..baa1427
--- /dev/null
+++ b/src/cmd/go/stop_unix_test.go
@@ -0,0 +1,16 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix || (js && wasm)
+
+package main_test
+
+import (
+	"os"
+	"syscall"
+)
+
+func quitSignal() os.Signal {
+	return syscall.SIGQUIT
+}
diff --git a/src/cmd/go/terminal_test.go b/src/cmd/go/terminal_test.go
new file mode 100644
index 0000000..a5ad919
--- /dev/null
+++ b/src/cmd/go/terminal_test.go
@@ -0,0 +1,130 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main_test
+
+import (
+	"errors"
+	"internal/testenv"
+	"internal/testpty"
+	"io"
+	"os"
+	"testing"
+
+	"golang.org/x/term"
+)
+
+func TestTerminalPassthrough(t *testing.T) {
+	// Check that if 'go test' is run with a terminal connected to stdin/stdout,
+	// then the go command passes that terminal down to the test binary
+	// invocation (rather than, e.g., putting a pipe in the way).
+	//
+	// See issue 18153.
+	testenv.MustHaveGoBuild(t)
+
+	// Start with a "self test" to make sure that if we *don't* pass in a
+	// terminal, the test can correctly detect that. (cmd/go doesn't guarantee
+	// that it won't add a terminal in the middle, but that would be pretty weird.)
+	t.Run("pipe", func(t *testing.T) {
+		r, w, err := os.Pipe()
+		if err != nil {
+			t.Fatalf("pipe failed: %s", err)
+		}
+		defer r.Close()
+		defer w.Close()
+		stdout, stderr := runTerminalPassthrough(t, r, w)
+		if stdout {
+			t.Errorf("stdout is unexpectedly a terminal")
+		}
+		if stderr {
+			t.Errorf("stderr is unexpectedly a terminal")
+		}
+	})
+
+	// Now test with a read PTY.
+	t.Run("pty", func(t *testing.T) {
+		r, processTTY, err := testpty.Open()
+		if errors.Is(err, testpty.ErrNotSupported) {
+			t.Skipf("%s", err)
+		} else if err != nil {
+			t.Fatalf("failed to open test PTY: %s", err)
+		}
+		defer r.Close()
+		w, err := os.OpenFile(processTTY, os.O_RDWR, 0)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer w.Close()
+		stdout, stderr := runTerminalPassthrough(t, r, w)
+		if !stdout {
+			t.Errorf("stdout is not a terminal")
+		}
+		if !stderr {
+			t.Errorf("stderr is not a terminal")
+		}
+	})
+}
+
+func runTerminalPassthrough(t *testing.T, r, w *os.File) (stdout, stderr bool) {
+	cmd := testenv.Command(t, testGo, "test", "-run=^$")
+	cmd.Env = append(cmd.Environ(), "GO_TEST_TERMINAL_PASSTHROUGH=1")
+	cmd.Stdout = w
+	cmd.Stderr = w
+
+	// The behavior of reading from a PTY after the child closes it is very
+	// strange: on Linux, Read returns EIO, and on at least some versions of
+	// macOS, unread output may be discarded (see https://go.dev/issue/57141).
+	//
+	// To avoid that situation, we keep the child process running until the
+	// parent has finished reading from the PTY, at which point we unblock the
+	// child by closing its stdin pipe.
+	stdin, err := cmd.StdinPipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	t.Logf("running %s", cmd)
+	err = cmd.Start()
+	if err != nil {
+		t.Fatalf("starting subprocess: %s", err)
+	}
+	w.Close()
+	t.Cleanup(func() {
+		stdin.Close()
+		if err := cmd.Wait(); err != nil {
+			t.Errorf("suprocess failed with: %s", err)
+		}
+	})
+
+	buf := make([]byte, 2)
+	n, err := io.ReadFull(r, buf)
+	if err != nil || !(buf[0] == '1' || buf[0] == 'X') || !(buf[1] == '2' || buf[1] == 'X') {
+		t.Logf("read error: %v", err)
+		t.Fatalf("expected 2 bytes matching `[1X][2X]`; got %q", buf[:n])
+	}
+	return buf[0] == '1', buf[1] == '2'
+}
+
+func init() {
+	if os.Getenv("GO_TEST_TERMINAL_PASSTHROUGH") == "" {
+		return
+	}
+
+	if term.IsTerminal(1) {
+		os.Stdout.WriteString("1")
+	} else {
+		os.Stdout.WriteString("X")
+	}
+	if term.IsTerminal(2) {
+		os.Stdout.WriteString("2")
+	} else {
+		os.Stdout.WriteString("X")
+	}
+
+	// Before exiting, wait for the parent process to read the PTY output,
+	// at which point it will close stdin.
+	io.Copy(io.Discard, os.Stdin)
+
+	os.Exit(0)
+}
diff --git a/src/cmd/go/testdata/addmod.go b/src/cmd/go/testdata/addmod.go
new file mode 100644
index 0000000..0045d50
--- /dev/null
+++ b/src/cmd/go/testdata/addmod.go
@@ -0,0 +1,153 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ignore
+// +build ignore
+
+// Addmod adds a module as a txtar archive to the testdata/mod directory.
+//
+// Usage:
+//
+//	go run addmod.go path@version...
+//
+// It should only be used for very small modules - we do not want to check
+// very large files into testdata/mod.
+//
+// It is acceptable to edit the archive afterward to remove or shorten files.
+// See mod/README for more information.
+package main
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"internal/txtar"
+	"io/fs"
+	"log"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+)
+
+func usage() {
+	fmt.Fprintf(os.Stderr, "usage: go run addmod.go path@version...\n")
+	os.Exit(2)
+}
+
+var tmpdir string
+
+func fatalf(format string, args ...any) {
+	os.RemoveAll(tmpdir)
+	log.Fatalf(format, args...)
+}
+
+const goCmd = "go"
+
+func main() {
+	flag.Usage = usage
+	flag.Parse()
+	if flag.NArg() == 0 {
+		usage()
+	}
+
+	log.SetPrefix("addmod: ")
+	log.SetFlags(0)
+
+	var err error
+	tmpdir, err = os.MkdirTemp("", "addmod-")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	run := func(command string, args ...string) string {
+		cmd := exec.Command(command, args...)
+		cmd.Dir = tmpdir
+		var stderr bytes.Buffer
+		cmd.Stderr = &stderr
+		out, err := cmd.Output()
+		if err != nil {
+			fatalf("%s %s: %v\n%s", command, strings.Join(args, " "), err, stderr.Bytes())
+		}
+		return string(out)
+	}
+
+	gopath := strings.TrimSpace(run("go", "env", "GOPATH"))
+	if gopath == "" {
+		fatalf("cannot find GOPATH")
+	}
+
+	exitCode := 0
+	for _, arg := range flag.Args() {
+		if err := os.WriteFile(filepath.Join(tmpdir, "go.mod"), []byte("module m\n"), 0666); err != nil {
+			fatalf("%v", err)
+		}
+		run(goCmd, "get", "-d", arg)
+		path := arg
+		if i := strings.Index(path, "@"); i >= 0 {
+			path = path[:i]
+		}
+		out := run(goCmd, "list", "-m", "-f={{.Path}} {{.Version}} {{.Dir}}", path)
+		f := strings.Fields(out)
+		if len(f) != 3 {
+			log.Printf("go list -m %s: unexpected output %q", arg, out)
+			exitCode = 1
+			continue
+		}
+		path, vers, dir := f[0], f[1], f[2]
+		mod, err := os.ReadFile(filepath.Join(gopath, "pkg/mod/cache/download", path, "@v", vers+".mod"))
+		if err != nil {
+			log.Printf("%s: %v", arg, err)
+			exitCode = 1
+			continue
+		}
+		info, err := os.ReadFile(filepath.Join(gopath, "pkg/mod/cache/download", path, "@v", vers+".info"))
+		if err != nil {
+			log.Printf("%s: %v", arg, err)
+			exitCode = 1
+			continue
+		}
+
+		a := new(txtar.Archive)
+		title := arg
+		if !strings.Contains(arg, "@") {
+			title += "@" + vers
+		}
+		a.Comment = []byte(fmt.Sprintf("module %s\n\n", title))
+		a.Files = []txtar.File{
+			{Name: ".mod", Data: mod},
+			{Name: ".info", Data: info},
+		}
+		dir = filepath.Clean(dir)
+		err = filepath.WalkDir(dir, func(path string, info fs.DirEntry, err error) error {
+			if !info.Type().IsRegular() {
+				return nil
+			}
+			name := info.Name()
+			if name == "go.mod" || strings.HasSuffix(name, ".go") {
+				data, err := os.ReadFile(path)
+				if err != nil {
+					return err
+				}
+				a.Files = append(a.Files, txtar.File{Name: str.TrimFilePathPrefix(path, dir), Data: data})
+			}
+			return nil
+		})
+		if err != nil {
+			log.Printf("%s: %v", arg, err)
+			exitCode = 1
+			continue
+		}
+
+		data := txtar.Format(a)
+		target := filepath.Join("mod", strings.ReplaceAll(path, "/", "_")+"_"+vers+".txt")
+		if err := os.WriteFile(target, data, 0666); err != nil {
+			log.Printf("%s: %v", arg, err)
+			exitCode = 1
+			continue
+		}
+	}
+	os.RemoveAll(tmpdir)
+	os.Exit(exitCode)
+}
diff --git a/src/cmd/go/testdata/failssh/ssh b/src/cmd/go/testdata/failssh/ssh
new file mode 100644
index 0000000..ecdbef9
--- /dev/null
+++ b/src/cmd/go/testdata/failssh/ssh
@@ -0,0 +1,2 @@
+#!/bin/sh
+exit 1
diff --git a/src/cmd/go/testdata/mod/README b/src/cmd/go/testdata/mod/README
new file mode 100644
index 0000000..43ddf77
--- /dev/null
+++ b/src/cmd/go/testdata/mod/README
@@ -0,0 +1,36 @@
+This directory holds Go modules served by a Go module proxy
+that runs on localhost during tests, both to make tests avoid
+requiring specific network servers and also to make them 
+significantly faster.
+
+A small go get'able test module can be added here by running
+
+	cd cmd/go/testdata
+	go run addmod.go path@vers
+
+where path and vers are the module path and version to add here.
+
+For interactive experimentation using this set of modules, run:
+
+	cd cmd/go
+	go test -proxy=localhost:1234 &
+	export GOPROXY=http://localhost:1234/mod
+
+and then run go commands as usual.
+
+Modules saved to this directory should be small: a few kilobytes at most.
+It is acceptable to edit the archives created by addmod.go to remove
+or shorten files. It is also acceptable to write module archives by hand: 
+they need not be backed by some public git repo.
+
+Each module archive is named path_vers.txt, where slashes in path
+have been replaced with underscores. The archive must contain
+two files ".info" and ".mod", to be served as the info and mod files
+in the proxy protocol (see https://research.swtch.com/vgo-module).
+The remaining files are served as the content of the module zip file.
+The path@vers prefix required of files in the zip file is added
+automatically by the proxy: the files in the archive have names without
+the prefix, like plain "go.mod", "x.go", and so on.
+
+See ../addmod.go and ../savedir.go for tools to generate txtar files,
+although again it is also fine to write them by hand.
diff --git a/src/cmd/go/testdata/mod/example.com_ambiguous_a_b_v0.0.0-empty.txt b/src/cmd/go/testdata/mod/example.com_ambiguous_a_b_v0.0.0-empty.txt
new file mode 100644
index 0000000..a869519
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_ambiguous_a_b_v0.0.0-empty.txt
@@ -0,0 +1,12 @@
+Module example.com/ambiguous/a/b is a suffix of example.com/a.
+This version contains no package.
+-- .mod --
+module example.com/ambiguous/a/b
+
+go 1.16
+-- .info --
+{"Version":"v0.0.0-empty"}
+-- go.mod --
+module example.com/ambiguous/a/b
+
+go 1.16
diff --git a/src/cmd/go/testdata/mod/example.com_ambiguous_a_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_ambiguous_a_v1.0.0.txt
new file mode 100644
index 0000000..bb43826
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_ambiguous_a_v1.0.0.txt
@@ -0,0 +1,18 @@
+Module example.com/ambiguous/a is a prefix of example.com/a/b.
+It contains package example.com/a/b.
+-- .mod --
+module example.com/ambiguous/a
+
+go 1.16
+
+require example.com/ambiguous/a/b v0.0.0-empty
+-- .info --
+{"Version":"v1.0.0"}
+-- go.mod --
+module example.com/ambiguous/a
+
+go 1.16
+
+require example.com/ambiguous/a/b v0.0.0-empty
+-- b/b.go --
+package b
diff --git a/src/cmd/go/testdata/mod/example.com_badchain_a_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_badchain_a_v1.0.0.txt
new file mode 100644
index 0000000..d7bf647
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_badchain_a_v1.0.0.txt
@@ -0,0 +1,12 @@
+example.com/badchain/a v1.0.0
+
+-- .mod --
+module example.com/badchain/a
+
+require example.com/badchain/b v1.0.0
+-- .info --
+{"Version":"v1.0.0"}
+-- a.go --
+package a
+
+import _ "example.com/badchain/b"
diff --git a/src/cmd/go/testdata/mod/example.com_badchain_a_v1.1.0.txt b/src/cmd/go/testdata/mod/example.com_badchain_a_v1.1.0.txt
new file mode 100644
index 0000000..92190d8
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_badchain_a_v1.1.0.txt
@@ -0,0 +1,12 @@
+example.com/badchain/a v1.1.0
+
+-- .mod --
+module example.com/badchain/a
+
+require example.com/badchain/b v1.1.0
+-- .info --
+{"Version":"v1.1.0"}
+-- a.go --
+package a
+
+import _ "example.com/badchain/b"
diff --git a/src/cmd/go/testdata/mod/example.com_badchain_b_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_badchain_b_v1.0.0.txt
new file mode 100644
index 0000000..d42b8aa
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_badchain_b_v1.0.0.txt
@@ -0,0 +1,12 @@
+example.com/badchain/b v1.0.0
+
+-- .mod --
+module example.com/badchain/b
+
+require example.com/badchain/c v1.0.0
+-- .info --
+{"Version":"v1.0.0"}
+-- b.go --
+package b
+
+import _ "example.com/badchain/c"
diff --git a/src/cmd/go/testdata/mod/example.com_badchain_b_v1.1.0.txt b/src/cmd/go/testdata/mod/example.com_badchain_b_v1.1.0.txt
new file mode 100644
index 0000000..6648184
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_badchain_b_v1.1.0.txt
@@ -0,0 +1,12 @@
+example.com/badchain/b v1.1.0
+
+-- .mod --
+module example.com/badchain/b
+
+require example.com/badchain/c v1.1.0
+-- .info --
+{"Version":"v1.1.0"}
+-- b.go --
+package b
+
+import _ "example.com/badchain/c"
diff --git a/src/cmd/go/testdata/mod/example.com_badchain_c_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_badchain_c_v1.0.0.txt
new file mode 100644
index 0000000..9c717cb
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_badchain_c_v1.0.0.txt
@@ -0,0 +1,8 @@
+example.com/badchain/c v1.0.0
+
+-- .mod --
+module example.com/badchain/c
+-- .info --
+{"Version":"v1.0.0"}
+-- c.go --
+package c
diff --git a/src/cmd/go/testdata/mod/example.com_badchain_c_v1.1.0.txt b/src/cmd/go/testdata/mod/example.com_badchain_c_v1.1.0.txt
new file mode 100644
index 0000000..36bc2c6
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_badchain_c_v1.1.0.txt
@@ -0,0 +1,8 @@
+example.com/badchain/c v1.1.0
+
+-- .mod --
+module badchain.example.com/c
+-- .info --
+{"Version":"v1.1.0"}
+-- c.go --
+package c
diff --git a/src/cmd/go/testdata/mod/example.com_cmd_v1.0.0-exclude.txt b/src/cmd/go/testdata/mod/example.com_cmd_v1.0.0-exclude.txt
new file mode 100644
index 0000000..c883d8a
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_cmd_v1.0.0-exclude.txt
@@ -0,0 +1,28 @@
+example.com/cmd contains main packages.
+
+-- .info --
+{"Version":"v1.0.0-exclude"}
+-- .mod --
+module example.com/cmd
+
+go 1.16
+
+exclude rsc.io/quote v1.5.2
+-- go.mod --
+module example.com/cmd
+
+go 1.16
+
+exclude rsc.io/quote v1.5.2
+-- a/a.go --
+package main
+
+func main() {}
+-- b/b.go --
+package main
+
+func main() {}
+-- err/err.go --
+package err
+
+var X = DoesNotCompile
diff --git a/src/cmd/go/testdata/mod/example.com_cmd_v1.0.0-newerself.txt b/src/cmd/go/testdata/mod/example.com_cmd_v1.0.0-newerself.txt
new file mode 100644
index 0000000..7670f29
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_cmd_v1.0.0-newerself.txt
@@ -0,0 +1,28 @@
+example.com/cmd contains main packages.
+
+-- .info --
+{"Version":"v1.0.0-newerself"}
+-- .mod --
+module example.com/cmd
+
+go 1.16
+
+require example.com/cmd v1.0.0
+-- go.mod --
+module example.com/cmd
+
+go 1.16
+
+require example.com/cmd v1.0.0
+-- a/a.go --
+package main
+
+func main() {}
+-- b/b.go --
+package main
+
+func main() {}
+-- err/err.go --
+package err
+
+var X = DoesNotCompile
diff --git a/src/cmd/go/testdata/mod/example.com_cmd_v1.0.0-replace.txt b/src/cmd/go/testdata/mod/example.com_cmd_v1.0.0-replace.txt
new file mode 100644
index 0000000..581a496
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_cmd_v1.0.0-replace.txt
@@ -0,0 +1,28 @@
+example.com/cmd contains main packages.
+
+-- .info --
+{"Version":"v1.0.0-replace"}
+-- .mod --
+module example.com/cmd
+
+go 1.16
+
+replace rsc.io/quote => rsc.io/quote v1.5.2
+-- go.mod --
+module example.com/cmd
+
+go 1.16
+
+replace rsc.io/quote => rsc.io/quote v1.5.2
+-- a/a.go --
+package main
+
+func main() {}
+-- b/b.go --
+package main
+
+func main() {}
+-- err/err.go --
+package err
+
+var X = DoesNotCompile
diff --git a/src/cmd/go/testdata/mod/example.com_cmd_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_cmd_v1.0.0.txt
new file mode 100644
index 0000000..c198139
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_cmd_v1.0.0.txt
@@ -0,0 +1,31 @@
+example.com/cmd contains main packages.
+
+v1.0.0 is the latest non-retracted version. Other versions contain errors or
+detectable problems.
+
+-- .info --
+{"Version":"v1.0.0"}
+-- .mod --
+module example.com/cmd
+
+go 1.16
+-- go.mod --
+module example.com/cmd
+
+go 1.16
+-- a/a.go --
+package main
+
+import "fmt"
+
+func main() { fmt.Println("a@v1.0.0") }
+-- b/b.go --
+package main
+
+import "fmt"
+
+func main() { fmt.Println("b@v1.0.0") }
+-- err/err.go --
+package err
+
+var X = DoesNotCompile
diff --git a/src/cmd/go/testdata/mod/example.com_cmd_v1.9.0.txt b/src/cmd/go/testdata/mod/example.com_cmd_v1.9.0.txt
new file mode 100644
index 0000000..9298afb
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_cmd_v1.9.0.txt
@@ -0,0 +1,30 @@
+example.com/cmd contains main packages.
+
+-- .info --
+{"Version":"v1.9.0"}
+-- .mod --
+module example.com/cmd
+
+go 1.16
+
+// this is a bad version
+retract v1.9.0
+-- go.mod --
+module example.com/cmd
+
+go 1.16
+
+// this is a bad version
+retract v1.9.0
+-- a/a.go --
+package main
+
+func main() {}
+-- b/b.go --
+package main
+
+func main() {}
+-- err/err.go --
+package err
+
+var X = DoesNotCompile
diff --git a/src/cmd/go/testdata/mod/example.com_depends_on_generics_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_depends_on_generics_v1.0.0.txt
new file mode 100644
index 0000000..80d3095
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_depends_on_generics_v1.0.0.txt
@@ -0,0 +1,23 @@
+example.com/depends/on/generics v1.0.0
+written by hand
+
+-- .mod --
+module example.com/depends/on/generics
+
+go 1.18
+
+require example.com/generics v1.0.0
+-- .info --
+{"Version":"v1.0.0"}
+-- go.mod --
+module example.com/depends/on/generics
+
+go 1.18
+
+require example.com/generics v1.0.0
+-- main.go --
+package main
+
+import "example.com/generics"
+
+func main() {generics.Bar()}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/mod/example.com_deprecated_a_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_deprecated_a_v1.0.0.txt
new file mode 100644
index 0000000..7c29621
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_deprecated_a_v1.0.0.txt
@@ -0,0 +1,12 @@
+-- .info --
+{"Version":"v1.0.0"}
+-- .mod --
+module example.com/deprecated/a
+
+go 1.17
+-- go.mod --
+module example.com/deprecated/a
+
+go 1.17
+-- a.go --
+package a
diff --git a/src/cmd/go/testdata/mod/example.com_deprecated_a_v1.9.0.txt b/src/cmd/go/testdata/mod/example.com_deprecated_a_v1.9.0.txt
new file mode 100644
index 0000000..0613389
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_deprecated_a_v1.9.0.txt
@@ -0,0 +1,14 @@
+-- .info --
+{"Version":"v1.9.0"}
+-- .mod --
+// Deprecated: in example.com/deprecated/a@v1.9.0
+module example.com/deprecated/a
+
+go 1.17
+-- go.mod --
+// Deprecated: in example.com/deprecated/a@v1.9.0
+module example.com/deprecated/a
+
+go 1.17
+-- a.go --
+package a
diff --git a/src/cmd/go/testdata/mod/example.com_deprecated_b_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_deprecated_b_v1.0.0.txt
new file mode 100644
index 0000000..50006ae
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_deprecated_b_v1.0.0.txt
@@ -0,0 +1,12 @@
+-- .info --
+{"Version":"v1.0.0"}
+-- .mod --
+module example.com/deprecated/b
+
+go 1.17
+-- go.mod --
+module example.com/deprecated/b
+
+go 1.17
+-- b.go --
+package b
diff --git a/src/cmd/go/testdata/mod/example.com_deprecated_b_v1.9.0.txt b/src/cmd/go/testdata/mod/example.com_deprecated_b_v1.9.0.txt
new file mode 100644
index 0000000..163d6b5
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_deprecated_b_v1.9.0.txt
@@ -0,0 +1,14 @@
+-- .info --
+{"Version":"v1.9.0"}
+-- .mod --
+// Deprecated: in example.com/deprecated/b@v1.9.0
+module example.com/deprecated/b
+
+go 1.17
+-- go.mod --
+// Deprecated: in example.com/deprecated/b@v1.9.0
+module example.com/deprecated/b
+
+go 1.17
+-- b.go --
+package b
diff --git a/src/cmd/go/testdata/mod/example.com_dotgo.go_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_dotgo.go_v1.0.0.txt
new file mode 100644
index 0000000..4f7f4d7
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_dotgo.go_v1.0.0.txt
@@ -0,0 +1,16 @@
+This module's path ends with ".go".
+Based on github.com/nats-io/nats.go.
+Used in regression tests for golang.org/issue/32483.
+
+-- .mod --
+module example.com/dotgo.go
+
+go 1.13
+-- .info --
+{"Version":"v1.0.0"}
+-- go.mod --
+module example.com/dotgo.go
+
+go 1.13
+-- dotgo.go --
+package dotgo
diff --git a/src/cmd/go/testdata/mod/example.com_dotname_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_dotname_v1.0.0.txt
new file mode 100644
index 0000000..2ada3a3
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_dotname_v1.0.0.txt
@@ -0,0 +1,12 @@
+-- .info --
+{"Version":"v1.0.0"}
+-- .mod --
+module example.com/dotname
+
+go 1.16
+-- go.mod --
+module example.com/dotname
+
+go 1.16
+-- .dot/dot.go --
+package dot
diff --git a/src/cmd/go/testdata/mod/example.com_downgrade_v2.0.0.txt b/src/cmd/go/testdata/mod/example.com_downgrade_v2.0.0.txt
new file mode 100644
index 0000000..88d50e5
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_downgrade_v2.0.0.txt
@@ -0,0 +1,9 @@
+example.com/downgrade v2.0.0
+written by hand
+
+-- .mod --
+module example.com/downgrade
+
+require rsc.io/quote v1.5.2
+-- .info --
+{"Version":"v2.0.0"}
diff --git a/src/cmd/go/testdata/mod/example.com_downgrade_v2_v2.0.1.txt b/src/cmd/go/testdata/mod/example.com_downgrade_v2_v2.0.1.txt
new file mode 100644
index 0000000..a4d665f
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_downgrade_v2_v2.0.1.txt
@@ -0,0 +1,13 @@
+example.com/downgrade/v2 v2.0.1
+written by hand
+
+-- .mod --
+module example.com/downgrade/v2
+
+require rsc.io/quote v1.5.2
+-- .info --
+{"Version":"v2.0.1"}
+-- go.mod --
+module example.com/downgrade/v2
+
+require rsc.io/quote v1.5.2
diff --git a/src/cmd/go/testdata/mod/example.com_fuzzfail_v0.1.0.txt b/src/cmd/go/testdata/mod/example.com_fuzzfail_v0.1.0.txt
new file mode 100644
index 0000000..af005ff
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_fuzzfail_v0.1.0.txt
@@ -0,0 +1,20 @@
+-- .mod --
+module example.com/fuzzfail
+
+go 1.18
+-- .info --
+{"Version":"v0.1.0"}
+-- go.mod --
+module example.com/fuzzfail
+
+go 1.18
+-- fuzzfail_test.go --
+package fuzzfail
+
+import "testing"
+
+func FuzzFail(f *testing.F) {
+	f.Fuzz(func(t *testing.T, b []byte) {
+		t.Fatalf("oops: %q", b)
+	})
+}
diff --git a/src/cmd/go/testdata/mod/example.com_fuzzfail_v0.2.0.txt b/src/cmd/go/testdata/mod/example.com_fuzzfail_v0.2.0.txt
new file mode 100644
index 0000000..ea599aa
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_fuzzfail_v0.2.0.txt
@@ -0,0 +1,23 @@
+-- .mod --
+module example.com/fuzzfail
+
+go 1.18
+-- .info --
+{"Version":"v0.2.0"}
+-- go.mod --
+module example.com/fuzzfail
+
+go 1.18
+-- fuzzfail_test.go --
+package fuzzfail
+
+import "testing"
+
+func FuzzFail(f *testing.F) {
+	f.Fuzz(func(t *testing.T, b []byte) {
+		t.Fatalf("oops: %q", b)
+	})
+}
+-- testdata/fuzz/FuzzFail/bbb0c2d22aa1a24617301566dc7486f8b625d38024603ba62757c1124013b49a --
+go test fuzz v1
+[]byte("\x05")
diff --git a/src/cmd/go/testdata/mod/example.com_generics_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_generics_v1.0.0.txt
new file mode 100644
index 0000000..092241e
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_generics_v1.0.0.txt
@@ -0,0 +1,21 @@
+example.com/generics v1.0.0
+written by hand
+
+-- .mod --
+module example.com/generics
+
+go 1.18
+-- .info --
+{"Version":"v1.0.0"}
+-- go.mod --
+module example.com/generics
+
+go 1.18
+-- generics.go --
+package generics
+
+type Int interface {
+	~int
+}
+
+func Bar() {}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/mod/example.com_incompatiblewithsub_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_incompatiblewithsub_v1.0.0.txt
new file mode 100644
index 0000000..435578d
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_incompatiblewithsub_v1.0.0.txt
@@ -0,0 +1,8 @@
+Module example.com/incompatiblewithsub has an incompatible version
+and a package in a subdirectory.
+-- .info --
+{"Version":"v1.0.0"}
+-- .mod --
+module example.com/incompatiblewithsub
+-- sub/sub.go --
+package sub
diff --git a/src/cmd/go/testdata/mod/example.com_incompatiblewithsub_v2.0.0+incompatible.txt b/src/cmd/go/testdata/mod/example.com_incompatiblewithsub_v2.0.0+incompatible.txt
new file mode 100644
index 0000000..198ec17
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_incompatiblewithsub_v2.0.0+incompatible.txt
@@ -0,0 +1,8 @@
+Module example.com/incompatiblewithsub has an incompatible version
+and a package in a subdirectory.
+-- .info --
+{"Version":"v2.0.0+incompatible"}
+-- .mod --
+module example.com/incompatiblewithsub
+-- sub/sub.go --
+package sub
diff --git a/src/cmd/go/testdata/mod/example.com_invalidpath_v1_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_invalidpath_v1_v1.0.0.txt
new file mode 100644
index 0000000..7d9d130
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_invalidpath_v1_v1.0.0.txt
@@ -0,0 +1,13 @@
+example.com/invalidpath/v1 v1.0.0
+written by hand
+
+-- .mod --
+module example.com/invalidpath/v1
+-- .info --
+{"Version":"v1.0.0"}
+-- go.mod --
+module example.com/invalidpath/v1
+-- version.go --
+package version
+
+const V = "v1.0.0"
diff --git a/src/cmd/go/testdata/mod/example.com_join_subpkg_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_join_subpkg_v1.0.0.txt
new file mode 100644
index 0000000..1ecfa0b
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_join_subpkg_v1.0.0.txt
@@ -0,0 +1,9 @@
+Written by hand.
+Test case for package moved into a parent module.
+
+-- .mod --
+module example.com/join/subpkg
+-- .info --
+{"Version": "v1.0.0"}
+-- x.go --
+package subpkg
diff --git a/src/cmd/go/testdata/mod/example.com_join_subpkg_v1.1.0.txt b/src/cmd/go/testdata/mod/example.com_join_subpkg_v1.1.0.txt
new file mode 100644
index 0000000..9eb823a
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_join_subpkg_v1.1.0.txt
@@ -0,0 +1,9 @@
+Written by hand.
+Test case for package moved into a parent module.
+
+-- .mod --
+module example.com/join/subpkg
+
+require example.com/join v1.1.0
+-- .info --
+{"Version": "v1.1.0"}
diff --git a/src/cmd/go/testdata/mod/example.com_join_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_join_v1.0.0.txt
new file mode 100644
index 0000000..84c68b1
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_join_v1.0.0.txt
@@ -0,0 +1,7 @@
+Written by hand.
+Test case for package moved into a parent module.
+
+-- .mod --
+module example.com/join
+-- .info --
+{"Version": "v1.0.0"}
diff --git a/src/cmd/go/testdata/mod/example.com_join_v1.1.0.txt b/src/cmd/go/testdata/mod/example.com_join_v1.1.0.txt
new file mode 100644
index 0000000..5f92036
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_join_v1.1.0.txt
@@ -0,0 +1,9 @@
+Written by hand.
+Test case for package moved into a parent module.
+
+-- .mod --
+module example.com/join
+-- .info --
+{"Version": "v1.1.0"}
+-- subpkg/x.go --
+package subpkg
diff --git a/src/cmd/go/testdata/mod/example.com_latemigrate_v2_v2.0.0.txt b/src/cmd/go/testdata/mod/example.com_latemigrate_v2_v2.0.0.txt
new file mode 100644
index 0000000..25bd3d9
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_latemigrate_v2_v2.0.0.txt
@@ -0,0 +1,14 @@
+example.com/latemigrate/v2 v2.0.0
+written by hand
+
+This repository migrated to modules in v2.0.1 after v2.0.0 was already tagged.
+All versions require rsc.io/quote so we can test downgrades.
+
+v2.0.0 is technically part of example.com/latemigrate as v2.0.0+incompatible.
+Proxies may serve it as part of the version list for example.com/latemigrate/v2.
+'go get' must be able to ignore these versions.
+
+-- .mod --
+module example.com/latemigrate
+-- .info --
+{"Version":"v2.0.0"}
diff --git a/src/cmd/go/testdata/mod/example.com_latemigrate_v2_v2.0.1.txt b/src/cmd/go/testdata/mod/example.com_latemigrate_v2_v2.0.1.txt
new file mode 100644
index 0000000..be427a3
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_latemigrate_v2_v2.0.1.txt
@@ -0,0 +1,20 @@
+example.com/latemigrate/v2 v2.0.1
+written by hand
+
+This repository migrated to modules in v2.0.1 after v2.0.0 was already tagged.
+All versions require rsc.io/quote so we can test downgrades.
+
+v2.0.1 belongs to example.com/latemigrate/v2.
+
+-- .mod --
+module example.com/latemigrate/v2
+
+require rsc.io/quote v1.3.0
+-- .info --
+{"Version":"v2.0.1"}
+-- go.mod --
+module example.com/latemigrate/v2
+
+require rsc.io/quote v1.3.0
+-- late.go --
+package late
diff --git a/src/cmd/go/testdata/mod/example.com_missingpkg_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_missingpkg_v1.0.0.txt
new file mode 100644
index 0000000..15f3f69
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_missingpkg_v1.0.0.txt
@@ -0,0 +1,11 @@
+The deprecated package is present in this version (which is @latest) but
+is deleted in a newer prerelease version.
+
+-- .mod --
+module example.com/missingpkg
+-- .info --
+{"Version":"v1.0.0"}
+-- lib.go --
+package lib
+-- deprecated/deprecated.go --
+package deprecated
diff --git a/src/cmd/go/testdata/mod/example.com_missingpkg_v1.0.1-beta.txt b/src/cmd/go/testdata/mod/example.com_missingpkg_v1.0.1-beta.txt
new file mode 100644
index 0000000..44580fe
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_missingpkg_v1.0.1-beta.txt
@@ -0,0 +1,8 @@
+The deprecated package is deleted in this version.
+
+-- .mod --
+module example.com/missingpkg
+-- .info --
+{"Version":"v1.0.1-beta"}
+-- lib.go --
+package lib
diff --git a/src/cmd/go/testdata/mod/example.com_nest_sub_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_nest_sub_v1.0.0.txt
new file mode 100644
index 0000000..90f1459
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_nest_sub_v1.0.0.txt
@@ -0,0 +1,12 @@
+Written by hand.
+Test case for nested modules without an explicit relationship.
+This is nested below the top-level module.
+
+-- .mod --
+module example.com/nest/sub
+-- .info --
+{"Version": "v1.0.0"}
+-- go.mod --
+module example.com/nest/sub
+-- y/y.go --
+package y
diff --git a/src/cmd/go/testdata/mod/example.com_nest_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_nest_v1.0.0.txt
new file mode 100644
index 0000000..593caf1
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_nest_v1.0.0.txt
@@ -0,0 +1,12 @@
+Written by hand.
+Test case for nested modules without an explicit relationship.
+This is the top-level module.
+
+-- .mod --
+module example.com/nest
+-- .info --
+{"Version": "v1.0.0"}
+-- go.mod --
+module example.com/nest
+-- sub/x/x.go --
+package x
diff --git a/src/cmd/go/testdata/mod/example.com_nest_v1.1.0.txt b/src/cmd/go/testdata/mod/example.com_nest_v1.1.0.txt
new file mode 100644
index 0000000..5a01550
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_nest_v1.1.0.txt
@@ -0,0 +1,12 @@
+Written by hand.
+Test case for nested modules without an explicit relationship.
+This is the top-level module.
+
+-- .mod --
+module example.com/nest
+-- .info --
+{"Version": "v1.1.0"}
+-- go.mod --
+module example.com/nest
+-- sub/x/x.go --
+package x
diff --git a/src/cmd/go/testdata/mod/example.com_newcycle_a_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_newcycle_a_v1.0.0.txt
new file mode 100644
index 0000000..829065d
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_newcycle_a_v1.0.0.txt
@@ -0,0 +1,10 @@
+example.com/newcycle/a v1.0.0
+
+Transitively requires v1.0.1 of itself via example.com/newcycle/b
+
+-- .mod --
+module example.com/newcycle/a
+
+require example.com/newcycle/b v1.0.0
+-- .info --
+{"Version":"v1.0.0"}
diff --git a/src/cmd/go/testdata/mod/example.com_newcycle_a_v1.0.1.txt b/src/cmd/go/testdata/mod/example.com_newcycle_a_v1.0.1.txt
new file mode 100644
index 0000000..a03f4b4
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_newcycle_a_v1.0.1.txt
@@ -0,0 +1,10 @@
+example.com/newcycle/a v1.0.1
+
+Transitively requires itself via example.com/newcycle/b
+
+-- .mod --
+module example.com/newcycle/a
+
+require example.com/newcycle/b v1.0.0
+-- .info --
+{"Version":"v1.0.1"}
diff --git a/src/cmd/go/testdata/mod/example.com_newcycle_b_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_newcycle_b_v1.0.0.txt
new file mode 100644
index 0000000..ff9e1f5
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_newcycle_b_v1.0.0.txt
@@ -0,0 +1,8 @@
+example.com/newcycle/b v1.0.0
+
+-- .mod --
+module example.com/newcycle/b
+
+require example.com/newcycle/a v1.0.1
+-- .info --
+{"Version":"v1.0.0"}
diff --git a/src/cmd/go/testdata/mod/example.com_noroot_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_noroot_v1.0.0.txt
new file mode 100644
index 0000000..aa5febf
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_noroot_v1.0.0.txt
@@ -0,0 +1,8 @@
+A module which has no root package.
+
+-- .mod --
+module example.com/noroot
+-- .info --
+{"Version":"v1.0.0"}
+-- pkg/pkg.go --
+package pkg
diff --git a/src/cmd/go/testdata/mod/example.com_noroot_v1.0.1.txt b/src/cmd/go/testdata/mod/example.com_noroot_v1.0.1.txt
new file mode 100644
index 0000000..9b93717
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_noroot_v1.0.1.txt
@@ -0,0 +1,8 @@
+A module which has no root package.
+
+-- .mod --
+module example.com/noroot
+-- .info --
+{"Version":"v1.0.1"}
+-- pkg/pkg.go --
+package pkg
diff --git a/src/cmd/go/testdata/mod/example.com_notags_v0.0.0-20190507143103-cc8cbe209b64.txt b/src/cmd/go/testdata/mod/example.com_notags_v0.0.0-20190507143103-cc8cbe209b64.txt
new file mode 100644
index 0000000..259774d
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_notags_v0.0.0-20190507143103-cc8cbe209b64.txt
@@ -0,0 +1,9 @@
+Written by hand.
+The "latest" version of a module without any tags.
+
+-- .mod --
+module example.com/notags
+-- .info --
+{"Version":"v0.0.0-20190507143103-cc8cbe209b64","Time":"2019-05-07T07:31:03-07:00"}
+-- notags.go --
+package notags
diff --git a/src/cmd/go/testdata/mod/example.com_printversion_v0.1.0.txt b/src/cmd/go/testdata/mod/example.com_printversion_v0.1.0.txt
new file mode 100644
index 0000000..606322a
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_printversion_v0.1.0.txt
@@ -0,0 +1,33 @@
+example.com/printversion v0.1.0
+
+-- .mod --
+module example.com/printversion
+-- .info --
+{"Version":"v0.1.0"}
+-- README.txt --
+There is no go.mod file for this version of the module.
+-- printversion.go --
+package main
+
+import (
+	"fmt"
+	"os"
+	"runtime/debug"
+
+	_ "example.com/version"
+)
+
+func main() {
+	info, _ := debug.ReadBuildInfo()
+	fmt.Fprintf(os.Stdout, "path is %s\n", info.Path)
+	fmt.Fprintf(os.Stdout, "main is %s %s\n", info.Main.Path, info.Main.Version)
+	if r := info.Main.Replace; r != nil {
+		fmt.Fprintf(os.Stdout, "\t(replaced by %s %s)\n", r.Path, r.Version)
+	}
+	for _, m := range info.Deps {
+		fmt.Fprintf(os.Stdout, "using %s %s\n", m.Path, m.Version)
+		if r := m.Replace; r != nil {
+			fmt.Fprintf(os.Stdout, "\t(replaced by %s %s)\n", r.Path, r.Version)
+		}
+	}
+}
diff --git a/src/cmd/go/testdata/mod/example.com_printversion_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_printversion_v1.0.0.txt
new file mode 100644
index 0000000..b9b71e9
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_printversion_v1.0.0.txt
@@ -0,0 +1,41 @@
+example.com/printversion v1.0.0
+
+-- .mod --
+module example.com/printversion
+
+require example.com/version v1.0.0
+replace example.com/version v1.0.0 => ../oops v0.0.0
+exclude example.com/version v1.1.0
+-- .info --
+{"Version":"v1.0.0"}
+-- go.mod --
+module example.com/printversion
+
+require example.com/version v1.0.0
+replace example.com/version v1.0.0 => ../oops v0.0.0
+exclude example.com/version v1.0.1
+-- printversion.go --
+package main
+
+import (
+	"fmt"
+	"os"
+	"runtime/debug"
+
+	_ "example.com/version"
+)
+
+func main() {
+	info, _ := debug.ReadBuildInfo()
+	fmt.Fprintf(os.Stdout, "path is %s\n", info.Path)
+	fmt.Fprintf(os.Stdout, "main is %s %s\n", info.Main.Path, info.Main.Version)
+	if r := info.Main.Replace; r != nil {
+		fmt.Fprintf(os.Stdout, "\t(replaced by %s %s)\n", r.Path, r.Version)
+	}
+	for _, m := range info.Deps {
+		fmt.Fprintf(os.Stdout, "using %s %s\n", m.Path, m.Version)
+		if r := m.Replace; r != nil {
+			fmt.Fprintf(os.Stdout, "\t(replaced by %s %s)\n", r.Path, r.Version)
+		}
+	}
+}
diff --git a/src/cmd/go/testdata/mod/example.com_pseudoupgrade_v0.0.0-20190430073000-30950c05d534.txt b/src/cmd/go/testdata/mod/example.com_pseudoupgrade_v0.0.0-20190430073000-30950c05d534.txt
new file mode 100644
index 0000000..047ceb6
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_pseudoupgrade_v0.0.0-20190430073000-30950c05d534.txt
@@ -0,0 +1,13 @@
+example.com/pseudoupgrade v0.0.0-20190429073000-30950c05d534
+written by hand
+
+-- .mod --
+module example.com/pseudoupgrade
+
+-- .info --
+{"Version":"v0.0.0-20190430073000-30950c05d534","Name":"v0.0.0-20190430073000-30950c05d534","Short":"30950c05d534","Time":"2019-04-30T07:30:00Z"}
+
+-- pseudoupgrade.go --
+package pseudoupgrade
+
+const X = 1
diff --git a/src/cmd/go/testdata/mod/example.com_pseudoupgrade_v0.1.0.txt b/src/cmd/go/testdata/mod/example.com_pseudoupgrade_v0.1.0.txt
new file mode 100644
index 0000000..7ddb0dc
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_pseudoupgrade_v0.1.0.txt
@@ -0,0 +1,13 @@
+example.com/pseudoupgrade v0.1.0
+written by hand
+
+-- .mod --
+module example.com/pseudoupgrade
+
+-- .info --
+{"Version":"v0.1.0","Name":"","Short":"","Time":"2019-04-29T07:30:30Z"}
+
+-- pseudoupgrade.go --
+package pseudoupgrade
+
+const X = 1
diff --git a/src/cmd/go/testdata/mod/example.com_pseudoupgrade_v0.1.1-0.20190429073117-b5426c86b553.txt b/src/cmd/go/testdata/mod/example.com_pseudoupgrade_v0.1.1-0.20190429073117-b5426c86b553.txt
new file mode 100644
index 0000000..b3f48bb
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_pseudoupgrade_v0.1.1-0.20190429073117-b5426c86b553.txt
@@ -0,0 +1,13 @@
+example.com/pseudoupgrade v0.1.1-0.20190429073117-b5426c86b553
+written by hand
+
+-- .mod --
+module example.com/pseudoupgrade
+
+-- .info --
+{"Version":"v0.1.1-0.20190429073117-b5426c86b553","Name":"v0.1.1-0.20190429073117-b5426c86b553","Short":"b5426c86b553","Time":"2019-04-29T07:31:00Z"}
+
+-- pseudoupgrade.go --
+package pseudoupgrade
+
+const X = 1
diff --git a/src/cmd/go/testdata/mod/example.com_quote_v1.5.2.txt b/src/cmd/go/testdata/mod/example.com_quote_v1.5.2.txt
new file mode 100644
index 0000000..05f7ae2
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_quote_v1.5.2.txt
@@ -0,0 +1,9 @@
+This module is a replacement for rsc.io/quote, but its go.mod file declares
+a module path different from its location and the original module.
+
+-- .mod --
+module rsc.io/Quote
+
+go 1.14
+-- .info --
+{"Version":"v1.5.2"}
diff --git a/src/cmd/go/testdata/mod/example.com_retract_ambiguous_nested_v1.9.0-bad.txt b/src/cmd/go/testdata/mod/example.com_retract_ambiguous_nested_v1.9.0-bad.txt
new file mode 100644
index 0000000..f8e623d
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_ambiguous_nested_v1.9.0-bad.txt
@@ -0,0 +1,10 @@
+-- .mod --
+module example.com/retract/ambiguous/nested
+
+go 1.16
+
+retract v1.9.0-bad // nested modules are bad
+-- .info --
+{"Version":"v1.9.0-bad"}
+-- nested.go --
+package nested
diff --git a/src/cmd/go/testdata/mod/example.com_retract_ambiguous_other_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_retract_ambiguous_other_v1.0.0.txt
new file mode 100644
index 0000000..5ee0139
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_ambiguous_other_v1.0.0.txt
@@ -0,0 +1,12 @@
+-- .mod --
+module example.com/retract/ambiguous/other
+
+go 1.16
+
+require example.com/retract/ambiguous v1.0.0
+-- .info --
+{"Version":"v1.0.0"}
+-- other.go --
+package other
+
+import _ "example.com/retract/ambiguous/nested"
diff --git a/src/cmd/go/testdata/mod/example.com_retract_ambiguous_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_retract_ambiguous_v1.0.0.txt
new file mode 100644
index 0000000..c8eeb16
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_ambiguous_v1.0.0.txt
@@ -0,0 +1,9 @@
+-- .mod --
+module example.com/retract/ambiguous
+
+go 1.16
+-- .info --
+{"Version":"v1.0.0"}
+-- nested/nested.go --
+package nested
+
diff --git a/src/cmd/go/testdata/mod/example.com_retract_incompatible_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_retract_incompatible_v1.0.0.txt
new file mode 100644
index 0000000..a987685
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_incompatible_v1.0.0.txt
@@ -0,0 +1,19 @@
+The v1.0.0 release of example.com/retract/incompatible retracts
+v2.0.0+incompatible.
+
+-- .mod --
+module example.com/retract/incompatible
+
+go 1.16
+
+retract v2.0.0+incompatible
+-- .info --
+{"Version":"v1.0.0"}
+-- go.mod --
+module example.com/retract/incompatible
+
+go 1.16
+
+retract v2.0.0+incompatible
+-- incompatible.go --
+package incompatible
diff --git a/src/cmd/go/testdata/mod/example.com_retract_incompatible_v2.0.0+incompatible.txt b/src/cmd/go/testdata/mod/example.com_retract_incompatible_v2.0.0+incompatible.txt
new file mode 100644
index 0000000..c668dbb
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_incompatible_v2.0.0+incompatible.txt
@@ -0,0 +1,9 @@
+The v1.0.0 release of example.com/retract/incompatible retracts
+v2.0.0+incompatible.
+
+-- .mod --
+module example.com/retract/incompatible
+-- .info --
+{"Version":"v2.0.0+incompatible"}
+-- incompatible.go --
+package incompatible
diff --git a/src/cmd/go/testdata/mod/example.com_retract_missingmod_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_retract_missingmod_v1.0.0.txt
new file mode 100644
index 0000000..1d8d810
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_missingmod_v1.0.0.txt
@@ -0,0 +1,10 @@
+This version should be retracted, but the go.mod file for the version that would
+contain the retraction is not available.
+-- .mod --
+module example.com/retract/missingmod
+
+go 1.14
+-- .info --
+{"Version":"v1.0.0"}
+-- missingmod.go --
+package missingmod
diff --git a/src/cmd/go/testdata/mod/example.com_retract_missingmod_v1.9.0.txt b/src/cmd/go/testdata/mod/example.com_retract_missingmod_v1.9.0.txt
new file mode 100644
index 0000000..bba919e
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_missingmod_v1.9.0.txt
@@ -0,0 +1,4 @@
+The go.mod file at this version will be loaded to check for retractions
+of earlier versions. However, the .mod file is not available.
+-- .info --
+{"Version":"v1.9.0"}
diff --git a/src/cmd/go/testdata/mod/example.com_retract_noupgrade_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_retract_noupgrade_v1.0.0.txt
new file mode 100644
index 0000000..466afc5
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_noupgrade_v1.0.0.txt
@@ -0,0 +1,9 @@
+-- .mod --
+module example.com/retract/noupgrade
+
+go 1.19
+
+retract v1.0.0 // bad
+
+-- .info --
+{"Version":"v1.0.0"}
diff --git a/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-block.txt b/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-block.txt
new file mode 100644
index 0000000..c4a53e1
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-block.txt
@@ -0,0 +1,6 @@
+-- .mod --
+module example.com/retract/rationale
+
+go 1.14
+-- .info --
+{"Version":"v1.0.0-block"}
diff --git a/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-blockwithcomment.txt b/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-blockwithcomment.txt
new file mode 100644
index 0000000..92573b6
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-blockwithcomment.txt
@@ -0,0 +1,6 @@
+-- .mod --
+module example.com/retract/rationale
+
+go 1.14
+-- .info --
+{"Version":"v1.0.0-blockwithcomment"}
diff --git a/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-empty.txt b/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-empty.txt
new file mode 100644
index 0000000..1f0894a
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-empty.txt
@@ -0,0 +1,8 @@
+-- .mod --
+module example.com/retract/rationale
+
+go 1.14
+-- .info --
+{"Version":"v1.0.0-empty"}
+-- empty.go --
+package empty
diff --git a/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-long.txt b/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-long.txt
new file mode 100644
index 0000000..1b5e753
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-long.txt
@@ -0,0 +1,8 @@
+-- .mod --
+module example.com/retract/rationale
+
+go 1.14
+-- .info --
+{"Version":"v1.0.0-long"}
+-- empty.go --
+package empty
diff --git a/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-multiline1.txt b/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-multiline1.txt
new file mode 100644
index 0000000..b1ffe27
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-multiline1.txt
@@ -0,0 +1,8 @@
+-- .mod --
+module example.com/retract/rationale
+
+go 1.14
+-- .info --
+{"Version":"v1.0.0-multiline1"}
+-- empty.go --
+package empty
diff --git a/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-multiline2.txt b/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-multiline2.txt
new file mode 100644
index 0000000..72f80b3
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-multiline2.txt
@@ -0,0 +1,8 @@
+-- .mod --
+module example.com/retract/rationale
+
+go 1.14
+-- .info --
+{"Version":"v1.0.0-multiline2"}
+-- empty.go --
+package empty
diff --git a/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-order.txt b/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-order.txt
new file mode 100644
index 0000000..1b04504
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-order.txt
@@ -0,0 +1,6 @@
+-- .mod --
+module example.com/retract/rationale
+
+go 1.14
+-- .info --
+{"Version":"v1.0.0-order"}
diff --git a/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-unprintable.txt b/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-unprintable.txt
new file mode 100644
index 0000000..9496124
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.0-unprintable.txt
@@ -0,0 +1,8 @@
+-- .mod --
+module example.com/retract/rationale
+
+go 1.14
+-- .info --
+{"Version":"v1.0.0-unprintable"}
+-- empty.go --
+package empty
diff --git a/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.1-order.txt b/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.1-order.txt
new file mode 100644
index 0000000..3be7d5b
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.0.1-order.txt
@@ -0,0 +1,6 @@
+-- .mod --
+module example.com/retract/rationale
+
+go 1.14
+-- .info --
+{"Version":"v1.0.1-order"}
diff --git a/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.9.0.txt b/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.9.0.txt
new file mode 100644
index 0000000..6975d4e
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_rationale_v1.9.0.txt
@@ -0,0 +1,48 @@
+Module example.com/retract/description retracts all versions of itself.
+The rationale comments have various problems.
+
+-- .mod --
+module example.com/retract/rationale
+
+go 1.14
+
+retract (
+	v1.0.0-empty
+
+	// short description
+	// more
+	//
+	// detail
+	v1.0.0-multiline1 // suffix
+	// after not included
+)
+
+// short description
+// more
+//
+// detail
+retract v1.0.0-multiline2 // suffix
+
+// loooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooong
+retract v1.0.0-long
+
+// Ends with a BEL character. Beep!
+retract v1.0.0-unprintable
+
+// block comment
+retract (
+	v1.0.0-block
+
+	// inner comment
+	v1.0.0-blockwithcomment
+)
+
+retract (
+	[v1.0.0-order, v1.0.0-order] // degenerate range
+	v1.0.0-order // single version
+
+	v1.0.1-order // single version
+	[v1.0.1-order, v1.0.1-order] // degenerate range
+)
+-- .info --
+{"Version":"v1.9.0"}
diff --git a/src/cmd/go/testdata/mod/example.com_retract_rename_v1.0.0-bad.txt b/src/cmd/go/testdata/mod/example.com_retract_rename_v1.0.0-bad.txt
new file mode 100644
index 0000000..25c4ff1
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_rename_v1.0.0-bad.txt
@@ -0,0 +1,16 @@
+Module example.com/retract/rename is renamed in a later version.
+
+This happens frequently when a repository is renamed or when a go.mod file
+is added for the first time with a custom module path.
+-- .info --
+{"Version":"v1.0.0-bad"}
+-- .mod --
+module example.com/retract/rename
+
+go 1.16
+-- go.mod --
+module example.com/retract/rename
+
+go 1.16
+-- rename.go --
+package rename
diff --git a/src/cmd/go/testdata/mod/example.com_retract_rename_v1.9.0-new.txt b/src/cmd/go/testdata/mod/example.com_retract_rename_v1.9.0-new.txt
new file mode 100644
index 0000000..9c08f71
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_rename_v1.9.0-new.txt
@@ -0,0 +1,22 @@
+Module example.com/retract/rename is renamed in this version.
+
+This happens frequently when a repository is renamed or when a go.mod file
+is added for the first time with a custom module path.
+-- .info --
+{"Version":"v1.9.0-new"}
+-- .mod --
+module example.com/retract/newname
+
+go 1.16
+
+// bad
+retract v1.0.0-bad
+-- go.mod --
+module example.com/retract/newname
+
+go 1.16
+
+// bad
+retract v1.0.0-bad
+-- newname.go --
+package newname
diff --git a/src/cmd/go/testdata/mod/example.com_retract_self_all_v1.9.0.txt b/src/cmd/go/testdata/mod/example.com_retract_self_all_v1.9.0.txt
new file mode 100644
index 0000000..4dc486b
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_self_all_v1.9.0.txt
@@ -0,0 +1,14 @@
+Module example.com/retract/self/prev is a module that retracts its own
+latest version.
+
+No unretracted versions are available.
+
+-- .mod --
+module example.com/retract/self/all
+
+go 1.15
+
+retract v1.9.0 // bad
+
+-- .info --
+{"Version":"v1.9.0"}
diff --git a/src/cmd/go/testdata/mod/example.com_retract_self_prerelease_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_retract_self_prerelease_v1.0.0.txt
new file mode 100644
index 0000000..04c2845
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_self_prerelease_v1.0.0.txt
@@ -0,0 +1,16 @@
+Module example.com/retract/self/prerelease is a module that retracts its own
+latest version and all other release version.
+
+A pre-release version higher than the highest release version is still
+available, and that should be matched by @latest.
+
+-- .mod --
+module example.com/retract/self/prerelease
+
+go 1.15
+
+-- .info --
+{"Version":"v1.0.0"}
+
+-- p.go --
+package p
diff --git a/src/cmd/go/testdata/mod/example.com_retract_self_prerelease_v1.9.0.txt b/src/cmd/go/testdata/mod/example.com_retract_self_prerelease_v1.9.0.txt
new file mode 100644
index 0000000..7c1c047
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_self_prerelease_v1.9.0.txt
@@ -0,0 +1,19 @@
+Module example.com/retract/self/prerelease is a module that retracts its own
+latest version and all other release version.
+
+A pre-release version higher than the highest release version is still
+available, and that should be matched by @latest.
+
+-- .mod --
+module example.com/retract/self/prerelease
+
+go 1.15
+
+retract v1.0.0 // bad
+retract v1.9.0 // self
+
+-- .info --
+{"Version":"v1.9.0"}
+
+-- p.go --
+package p
diff --git a/src/cmd/go/testdata/mod/example.com_retract_self_prerelease_v1.9.1-pre.txt b/src/cmd/go/testdata/mod/example.com_retract_self_prerelease_v1.9.1-pre.txt
new file mode 100644
index 0000000..abf44fd
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_self_prerelease_v1.9.1-pre.txt
@@ -0,0 +1,16 @@
+Module example.com/retract/self/prerelease is a module that retracts its own
+latest version and all other release version.
+
+A pre-release version higher than the highest release version is still
+available, and that should be matched by @latest.
+
+-- .mod --
+module example.com/retract/self/prerelease
+
+go 1.15
+
+-- .info --
+{"Version":"v1.9.1-pre"}
+
+-- p.go --
+package p
diff --git a/src/cmd/go/testdata/mod/example.com_retract_self_prev_v1.0.0-bad.txt b/src/cmd/go/testdata/mod/example.com_retract_self_prev_v1.0.0-bad.txt
new file mode 100644
index 0000000..095063d
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_self_prev_v1.0.0-bad.txt
@@ -0,0 +1,14 @@
+See example.com_retract_self_prev_v1.9.0.txt.
+
+This version is retracted.
+
+-- .mod --
+module example.com/retract/self/prev
+
+go 1.15
+
+-- .info --
+{"Version":"v1.0.0-bad"}
+
+-- p.go --
+package p
diff --git a/src/cmd/go/testdata/mod/example.com_retract_self_prev_v1.1.0.txt b/src/cmd/go/testdata/mod/example.com_retract_self_prev_v1.1.0.txt
new file mode 100644
index 0000000..27c3a39
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_self_prev_v1.1.0.txt
@@ -0,0 +1,14 @@
+See example.com_retract_self_pref_v1.9.0.txt.
+
+This version is the latest (only) non-retracted version.
+
+-- .mod --
+module example.com/retract/self/prev
+
+go 1.15
+
+-- .info --
+{"Version":"v1.1.0"}
+
+-- p.go --
+package p
diff --git a/src/cmd/go/testdata/mod/example.com_retract_self_prev_v1.9.0.txt b/src/cmd/go/testdata/mod/example.com_retract_self_prev_v1.9.0.txt
new file mode 100644
index 0000000..03d6168
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_self_prev_v1.9.0.txt
@@ -0,0 +1,18 @@
+Module example.com/retract/self/prev is a module that retracts its own
+latest version, as well as an earlier version.
+
+A previous unretracted release version, v1.1.0, is still available.
+
+-- .mod --
+module example.com/retract/self/prev
+
+go 1.15
+
+retract v1.0.0-bad // bad
+retract v1.9.0 // self
+
+-- .info --
+{"Version":"v1.9.0"}
+
+-- p.go --
+package p
diff --git a/src/cmd/go/testdata/mod/example.com_retract_self_pseudo_v0.0.0-20200325131415-0123456789ab b/src/cmd/go/testdata/mod/example.com_retract_self_pseudo_v0.0.0-20200325131415-0123456789ab
new file mode 100644
index 0000000..f9ab41e
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_self_pseudo_v0.0.0-20200325131415-0123456789ab
@@ -0,0 +1,20 @@
+See example.com_retract_self_pseudo_v1.9.0.txt.
+
+This version is not retracted. It should be returned by the proxy's
+@latest endpoint. It should match the @latest version query.
+
+TODO(golang.org/issue/24031): the proxy and proxy.golang.org both return
+the highest release version from the @latest endpoint, even if that
+version is retracted, so there is no way for the go command to
+discover an unretracted pseudo-version.
+
+-- .mod --
+module example.com/retract/self/pseudo
+
+go 1.15
+
+-- .info --
+{"Version":"v0.0.0-20200325131415-01234567890ab"}
+
+-- p.go --
+package p
diff --git a/src/cmd/go/testdata/mod/example.com_retract_self_pseudo_v1.0.0-bad.txt b/src/cmd/go/testdata/mod/example.com_retract_self_pseudo_v1.0.0-bad.txt
new file mode 100644
index 0000000..d47eda0
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_self_pseudo_v1.0.0-bad.txt
@@ -0,0 +1,14 @@
+See example.com_retract_self_pseudo_v1.9.0.txt.
+
+This version is retracted.
+
+-- .mod --
+module example.com/retract/self/pseudo
+
+go 1.15
+
+-- .info --
+{"Version":"v1.0.0-bad"}
+
+-- p.go --
+package p
diff --git a/src/cmd/go/testdata/mod/example.com_retract_self_pseudo_v1.9.0.txt b/src/cmd/go/testdata/mod/example.com_retract_self_pseudo_v1.9.0.txt
new file mode 100644
index 0000000..db09cc6
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_self_pseudo_v1.9.0.txt
@@ -0,0 +1,16 @@
+Module example.com/retract/self/pseudo is a module that retracts its own
+latest version, as well as an earlier version.
+
+An unretracted pseudo-version is available.
+
+-- .mod --
+module example.com/retract/self/pseudo
+
+go 1.15
+
+retract v1.0.0-bad // bad
+retract v1.9.0 // self
+
+-- .info --
+{"Version":"v1.9.0"}
+
diff --git a/src/cmd/go/testdata/mod/example.com_retract_v1.0.0-bad.txt b/src/cmd/go/testdata/mod/example.com_retract_v1.0.0-bad.txt
new file mode 100644
index 0000000..2f996cf
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_v1.0.0-bad.txt
@@ -0,0 +1,10 @@
+-- .mod --
+module example.com/retract
+
+go 1.15
+
+-- .info --
+{"Version":"v1.0.0-bad"}
+
+-- retract.go --
+package retract
diff --git a/src/cmd/go/testdata/mod/example.com_retract_v1.0.0-good.txt b/src/cmd/go/testdata/mod/example.com_retract_v1.0.0-good.txt
new file mode 100644
index 0000000..78152bb
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_v1.0.0-good.txt
@@ -0,0 +1,10 @@
+-- .mod --
+module example.com/retract
+
+go 1.15
+
+-- .info --
+{"Version":"v1.0.0-good"}
+
+-- retract.go --
+package retract
diff --git a/src/cmd/go/testdata/mod/example.com_retract_v1.0.0-unused.txt b/src/cmd/go/testdata/mod/example.com_retract_v1.0.0-unused.txt
new file mode 100644
index 0000000..3bc9e35
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_v1.0.0-unused.txt
@@ -0,0 +1,10 @@
+-- .mod --
+module example.com/retract
+
+go 1.15
+
+-- .info --
+{"Version":"v1.0.0-unused"}
+
+-- retract.go --
+package retract
diff --git a/src/cmd/go/testdata/mod/example.com_retract_v1.1.0.txt b/src/cmd/go/testdata/mod/example.com_retract_v1.1.0.txt
new file mode 100644
index 0000000..18d6d83
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_retract_v1.1.0.txt
@@ -0,0 +1,13 @@
+-- .mod --
+module example.com/retract
+
+go 1.15
+
+retract v1.0.0-bad // bad
+retract v1.0.0-unused // bad
+
+-- .info --
+{"Version":"v1.1.0"}
+
+-- retract.go --
+package retract
diff --git a/src/cmd/go/testdata/mod/example.com_split-incompatible_subpkg_v0.1.0.txt b/src/cmd/go/testdata/mod/example.com_split-incompatible_subpkg_v0.1.0.txt
new file mode 100644
index 0000000..edf5d48
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_split-incompatible_subpkg_v0.1.0.txt
@@ -0,0 +1,14 @@
+Written by hand.
+Test case for getting a package that has been moved to a nested module,
+with a +incompatible version (and thus no go.mod file) at the root module.
+
+-- .mod --
+module example.com/split-incompatible/subpkg
+-- .info --
+{"Version": "v0.1.0"}
+-- go.mod --
+module example.com/split-incompatible/subpkg
+
+go 1.16
+-- subpkg.go --
+package subpkg
diff --git a/src/cmd/go/testdata/mod/example.com_split-incompatible_v2.0.0+incompatible.txt b/src/cmd/go/testdata/mod/example.com_split-incompatible_v2.0.0+incompatible.txt
new file mode 100644
index 0000000..00076d7
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_split-incompatible_v2.0.0+incompatible.txt
@@ -0,0 +1,10 @@
+Written by hand.
+Test case for getting a package that has been moved to a nested module,
+with a +incompatible version (and thus no go.mod file) at the root module.
+
+-- .mod --
+module example.com/split-incompatible
+-- .info --
+{"Version": "v2.0.0+incompatible"}
+-- subpkg/subpkg.go --
+package subpkg
diff --git a/src/cmd/go/testdata/mod/example.com_split-incompatible_v2.1.0-pre+incompatible.txt b/src/cmd/go/testdata/mod/example.com_split-incompatible_v2.1.0-pre+incompatible.txt
new file mode 100644
index 0000000..bb1c1fe
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_split-incompatible_v2.1.0-pre+incompatible.txt
@@ -0,0 +1,10 @@
+Written by hand.
+Test case for getting a package that has been moved to a nested module,
+with a +incompatible version (and thus no go.mod file) at the root module.
+
+-- .mod --
+module example.com/split-incompatible
+-- .info --
+{"Version": "v2.1.0-pre+incompatible"}
+-- README.txt --
+subpkg has moved to module example.com/split-incompatible/subpkg
diff --git a/src/cmd/go/testdata/mod/example.com_split_subpkg_v1.1.0.txt b/src/cmd/go/testdata/mod/example.com_split_subpkg_v1.1.0.txt
new file mode 100644
index 0000000..b197b66
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_split_subpkg_v1.1.0.txt
@@ -0,0 +1,11 @@
+Written by hand.
+Test case for getting a package that has been moved to a different module.
+
+-- .mod --
+module example.com/split/subpkg
+
+require example.com/split v1.1.0
+-- .info --
+{"Version": "v1.1.0"}
+-- x.go --
+package subpkg
diff --git a/src/cmd/go/testdata/mod/example.com_split_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_split_v1.0.0.txt
new file mode 100644
index 0000000..b706e59
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_split_v1.0.0.txt
@@ -0,0 +1,9 @@
+Written by hand.
+Test case for getting a package that has been moved to a different module.
+
+-- .mod --
+module example.com/split
+-- .info --
+{"Version": "v1.0.0"}
+-- subpkg/x.go --
+package subpkg
diff --git a/src/cmd/go/testdata/mod/example.com_split_v1.1.0.txt b/src/cmd/go/testdata/mod/example.com_split_v1.1.0.txt
new file mode 100644
index 0000000..d38971f
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_split_v1.1.0.txt
@@ -0,0 +1,9 @@
+Written by hand.
+Test case for getting a package that has been moved to a different module.
+
+-- .mod --
+module example.com/split
+
+require example.com/split/subpkg v1.1.0
+-- .info --
+{"Version": "v1.1.0"}
diff --git a/src/cmd/go/testdata/mod/example.com_stack_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_stack_v1.0.0.txt
new file mode 100644
index 0000000..787b7ae
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_stack_v1.0.0.txt
@@ -0,0 +1,18 @@
+Module with a function that prints file name for the top stack frame.
+Different versions of this module are identical, but they should return
+different file names with -trimpath.
+-- .mod --
+module example.com/stack
+
+go 1.14
+-- .info --
+{"Version":"v1.0.0"}
+-- stack.go --
+package stack
+
+import "runtime"
+
+func TopFile() string {
+	_, file, _, _ := runtime.Caller(0)
+	return file
+}
diff --git a/src/cmd/go/testdata/mod/example.com_stack_v1.0.1.txt b/src/cmd/go/testdata/mod/example.com_stack_v1.0.1.txt
new file mode 100644
index 0000000..c715dd2
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_stack_v1.0.1.txt
@@ -0,0 +1,18 @@
+Module with a function that prints file name for the top stack frame.
+Different versions of this module are identical, but they should return
+different file names with -trimpath.
+-- .mod --
+module example.com/stack
+
+go 1.14
+-- .info --
+{"Version":"v1.0.1"}
+-- stack.go --
+package stack
+
+import "runtime"
+
+func TopFile() string {
+	_, file, _, _ := runtime.Caller(0)
+	return file
+}
diff --git a/src/cmd/go/testdata/mod/example.com_tools_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_tools_v1.0.0.txt
new file mode 100644
index 0000000..22e36b9
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_tools_v1.0.0.txt
@@ -0,0 +1,12 @@
+-- .info --
+{"Version": "v1.0.0"}
+-- .mod --
+module example.com/tools
+-- cmd/hello/hello.go --
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("hello")
+}
diff --git a/src/cmd/go/testdata/mod/example.com_undeprecated_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_undeprecated_v1.0.0.txt
new file mode 100644
index 0000000..a68588e
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_undeprecated_v1.0.0.txt
@@ -0,0 +1,14 @@
+-- .info --
+{"Version":"v1.0.0"}
+-- .mod --
+// Deprecated: in v1.0.0
+module example.com/undeprecated
+
+go 1.17
+-- go.mod --
+// Deprecated: in v1.0.0
+module example.com/undeprecated
+
+go 1.17
+-- undeprecated.go --
+package undeprecated
diff --git a/src/cmd/go/testdata/mod/example.com_undeprecated_v1.0.1.txt b/src/cmd/go/testdata/mod/example.com_undeprecated_v1.0.1.txt
new file mode 100644
index 0000000..ecabf32
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_undeprecated_v1.0.1.txt
@@ -0,0 +1,14 @@
+-- .info --
+{"Version":"v1.0.1"}
+-- .mod --
+// no longer deprecated
+module example.com/undeprecated
+
+go 1.17
+-- go.mod --
+// no longer deprecated
+module example.com/undeprecated
+
+go 1.17
+-- undeprecated.go --
+package undeprecated
diff --git a/src/cmd/go/testdata/mod/example.com_usemissingpre_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_usemissingpre_v1.0.0.txt
new file mode 100644
index 0000000..5e1c5c8
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_usemissingpre_v1.0.0.txt
@@ -0,0 +1,13 @@
+This module requires example.com/missingpkg at a prerelease version, which
+is newer than @latest.
+
+-- .mod --
+module example.com/usemissingpre
+
+require example.com/missingpkg v1.0.1-beta
+-- .info --
+{"Version":"v1.0.0"}
+-- use.go --
+package use
+
+import _ "example.com/missingpkg"
diff --git a/src/cmd/go/testdata/mod/example.com_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_v1.0.0.txt
new file mode 100644
index 0000000..263287d
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_v1.0.0.txt
@@ -0,0 +1,9 @@
+Written by hand.
+Test case for module at root of domain.
+
+-- .mod --
+module example.com
+-- .info --
+{"Version": "v1.0.0"}
+-- x.go --
+package x
diff --git a/src/cmd/go/testdata/mod/example.com_version_v1.0.0.txt b/src/cmd/go/testdata/mod/example.com_version_v1.0.0.txt
new file mode 100644
index 0000000..d8c45b5
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_version_v1.0.0.txt
@@ -0,0 +1,11 @@
+example.com/version v1.0.0
+written by hand
+
+-- .mod --
+module example.com/version
+-- .info --
+{"Version":"v1.0.0"}
+-- version.go --
+package version
+
+const V = "v1.0.0"
diff --git a/src/cmd/go/testdata/mod/example.com_version_v1.0.1.txt b/src/cmd/go/testdata/mod/example.com_version_v1.0.1.txt
new file mode 100644
index 0000000..3bfdb0e
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_version_v1.0.1.txt
@@ -0,0 +1,11 @@
+example.com/version v1.0.1
+written by hand
+
+-- .mod --
+module example.com/version
+-- .info --
+{"Version":"v1.0.1"}
+-- version.go --
+package version
+
+const V = "v1.0.1"
diff --git a/src/cmd/go/testdata/mod/example.com_version_v1.1.0.txt b/src/cmd/go/testdata/mod/example.com_version_v1.1.0.txt
new file mode 100644
index 0000000..8109a9a
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.com_version_v1.1.0.txt
@@ -0,0 +1,11 @@
+example.com/version v1.1.0
+written by hand
+
+-- .mod --
+module example.com/version
+-- .info --
+{"Version":"v1.1.0"}
+-- version.go --
+package version
+
+const V = "v1.1.0"
diff --git a/src/cmd/go/testdata/mod/example.net_ambiguous_nested_v0.1.0.txt b/src/cmd/go/testdata/mod/example.net_ambiguous_nested_v0.1.0.txt
new file mode 100644
index 0000000..8c9de7a
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.net_ambiguous_nested_v0.1.0.txt
@@ -0,0 +1,19 @@
+Written by hand.
+
+Test module containing a package that is also provided by a nested module tagged
+with the same version.
+
+-- .mod --
+module example.net/ambiguous/nested
+
+go 1.16
+-- .info --
+{"Version": "v0.1.0"}
+-- go.mod --
+module example.net/ambiguous/nested
+
+go 1.16
+-- pkg/pkg.go --
+// Package pkg exists in both example.net/ambiguous v0.1.0
+// and example.net/ambiguous/nested v0.1.0
+package pkg
diff --git a/src/cmd/go/testdata/mod/example.net_ambiguous_v0.1.0.txt b/src/cmd/go/testdata/mod/example.net_ambiguous_v0.1.0.txt
new file mode 100644
index 0000000..8fa6d83
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.net_ambiguous_v0.1.0.txt
@@ -0,0 +1,19 @@
+Written by hand.
+
+Test module containing a package that is also provided by a nested module tagged
+with the same version.
+
+-- .mod --
+module example.net/ambiguous
+
+go 1.16
+-- .info --
+{"Version": "v0.1.0"}
+-- go.mod --
+module example.net/ambiguous
+
+go 1.16
+-- nested/pkg/pkg.go --
+// Package pkg exists in both example.net/ambiguous v0.1.0
+// and example.net/ambiguous/nested v0.1.0
+package pkg
diff --git a/src/cmd/go/testdata/mod/example.net_ambiguous_v0.2.0.txt b/src/cmd/go/testdata/mod/example.net_ambiguous_v0.2.0.txt
new file mode 100644
index 0000000..7589ad7
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.net_ambiguous_v0.2.0.txt
@@ -0,0 +1,18 @@
+Written by hand.
+
+Test module containing a package that is also provided by a nested module tagged
+with the same version.
+
+-- .mod --
+module example.net/ambiguous
+
+go 1.16
+-- .info --
+{"Version": "v0.2.0"}
+-- go.mod --
+module example.net/ambiguous
+
+go 1.16
+-- nested/pkg/README.txt --
+// Package pkg no longer exists in this module at v0.2.0.
+// Find it in module example.net/ambiguous/nested instead.
diff --git a/src/cmd/go/testdata/mod/example.net_pkgadded_v1.0.0.txt b/src/cmd/go/testdata/mod/example.net_pkgadded_v1.0.0.txt
new file mode 100644
index 0000000..207e86a
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.net_pkgadded_v1.0.0.txt
@@ -0,0 +1,17 @@
+Written by hand.
+Test module with a root package added in v1.1.0
+and a subpackage added in v1.2.0.
+
+-- .mod --
+module example.net/pkgadded
+
+go 1.16
+-- .info --
+{"Version":"v1.0.0"}
+-- go.mod --
+module example.net/pkgadded
+
+go 1.16
+-- README.txt --
+We will add the package example.net/pkgadded in v1.1.0,
+and example.net/pkgadded/subpkg in v1.2.0.
diff --git a/src/cmd/go/testdata/mod/example.net_pkgadded_v1.1.0.txt b/src/cmd/go/testdata/mod/example.net_pkgadded_v1.1.0.txt
new file mode 100644
index 0000000..1c88de2
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.net_pkgadded_v1.1.0.txt
@@ -0,0 +1,19 @@
+Written by hand.
+Test module with a root package added in v1.1.0
+and a subpackage added in v1.2.0.
+
+-- .mod --
+module example.net/pkgadded
+
+go 1.16
+-- .info --
+{"Version":"v1.1.0"}
+-- go.mod --
+module example.net/pkgadded
+
+go 1.16
+-- README.txt --
+We will add the package example.net/pkgadded/subpkg in v1.2.0.
+-- pkgadded.go --
+// Package pkgadded was added in v1.1.0.
+package pkgadded
diff --git a/src/cmd/go/testdata/mod/example.net_pkgadded_v1.2.0.txt b/src/cmd/go/testdata/mod/example.net_pkgadded_v1.2.0.txt
new file mode 100644
index 0000000..922951a
--- /dev/null
+++ b/src/cmd/go/testdata/mod/example.net_pkgadded_v1.2.0.txt
@@ -0,0 +1,20 @@
+Written by hand.
+Test module with a root package added in v1.1.0
+and a subpackage added in v1.2.0.
+
+-- .mod --
+module example.net/pkgadded
+
+go 1.16
+-- .info --
+{"Version":"v1.2.0"}
+-- go.mod --
+module example.net/pkgadded
+
+go 1.16
+-- pkgadded.go --
+// Package pkgadded was added in v1.1.0.
+package pkgadded
+-- subpkg/subpkg.go --
+// Package subpkg was added in v1.2.0.
+package subpkg
diff --git a/src/cmd/go/testdata/mod/github.com_dmitshur-test_modtest5_v0.0.0-20190619020302-197a620e0c9a.txt b/src/cmd/go/testdata/mod/github.com_dmitshur-test_modtest5_v0.0.0-20190619020302-197a620e0c9a.txt
new file mode 100644
index 0000000..c2709c1
--- /dev/null
+++ b/src/cmd/go/testdata/mod/github.com_dmitshur-test_modtest5_v0.0.0-20190619020302-197a620e0c9a.txt
@@ -0,0 +1,10 @@
+module github.com/dmitshur-test/modtest5@v0.0.0-20190619020302-197a620e0c9a
+
+-- .mod --
+module github.com/dmitshur-test/modtest5
+-- .info --
+{"Version":"v0.0.0-20190619020302-197a620e0c9a","Time":"2019-06-18T19:03:02-07:00"}
+-- p.go --
+package p
+
+const v = 1
diff --git a/src/cmd/go/testdata/mod/github.com_dmitshur-test_modtest5_v0.5.0-alpha.0.20190619023908-3da23a9deb9e.txt b/src/cmd/go/testdata/mod/github.com_dmitshur-test_modtest5_v0.5.0-alpha.0.20190619023908-3da23a9deb9e.txt
new file mode 100644
index 0000000..22e47f3
--- /dev/null
+++ b/src/cmd/go/testdata/mod/github.com_dmitshur-test_modtest5_v0.5.0-alpha.0.20190619023908-3da23a9deb9e.txt
@@ -0,0 +1,10 @@
+module github.com/dmitshur-test/modtest5@v0.5.0-alpha.0.20190619023908-3da23a9deb9e
+
+-- .mod --
+module github.com/dmitshur-test/modtest5
+-- .info --
+{"Version":"v0.5.0-alpha.0.20190619023908-3da23a9deb9e","Time":"2019-06-18T19:39:08-07:00"}
+-- p.go --
+package p
+
+const v = 3
diff --git a/src/cmd/go/testdata/mod/github.com_dmitshur-test_modtest5_v0.5.0-alpha.txt b/src/cmd/go/testdata/mod/github.com_dmitshur-test_modtest5_v0.5.0-alpha.txt
new file mode 100644
index 0000000..4f088cc
--- /dev/null
+++ b/src/cmd/go/testdata/mod/github.com_dmitshur-test_modtest5_v0.5.0-alpha.txt
@@ -0,0 +1,10 @@
+module github.com/dmitshur-test/modtest5@v0.5.0-alpha
+
+-- .mod --
+module github.com/dmitshur-test/modtest5
+-- .info --
+{"Version":"v0.5.0-alpha","Time":"2019-06-18T19:04:46-07:00"}
+-- p.go --
+package p
+
+const v = 2
diff --git a/src/cmd/go/testdata/mod/golang.org_notx_useinternal_v0.1.0.txt b/src/cmd/go/testdata/mod/golang.org_notx_useinternal_v0.1.0.txt
new file mode 100644
index 0000000..0420a1a
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_notx_useinternal_v0.1.0.txt
@@ -0,0 +1,13 @@
+written by hand — attempts to use a prohibited internal package
+(https://golang.org/s/go14internal)
+
+-- .mod --
+module golang.org/notx/useinternal
+-- .info --
+{"Version":"v0.1.0","Name":"","Short":"","Time":"2018-07-25T17:24:00Z"}
+-- go.mod --
+module golang.org/notx/useinternal
+-- useinternal.go --
+package useinternal
+
+import _ "golang.org/x/internal/subtle"
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.18.1.linux-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.18.1.linux-amd64.txt
new file mode 100644
index 0000000..3713de3
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.18.1.linux-amd64.txt
@@ -0,0 +1,8 @@
+golang.org/toolchain v0.0.1-go1.18.1.linux-amd64
+written by hand
+-- .info --
+{"Version":"v0.0.1-go1.18.1.linux-amd64"}
+-- .mod --
+golang.org/toolchain
+-- go.mod --
+golang.org/toolchain
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.18.3.linux-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.18.3.linux-amd64.txt
new file mode 100644
index 0000000..8eda1ee
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.18.3.linux-amd64.txt
@@ -0,0 +1,8 @@
+golang.org/toolchain v0.0.1-go1.18.3.linux-amd64
+written by hand
+-- .info --
+{"Version":"v0.0.1-go1.18.3.linux-amd64"}
+-- .mod --
+golang.org/toolchain
+-- go.mod --
+golang.org/toolchain
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.18.5.linux-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.18.5.linux-amd64.txt
new file mode 100644
index 0000000..d74ef25
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.18.5.linux-amd64.txt
@@ -0,0 +1,8 @@
+golang.org/toolchain v0.0.1-go1.18.5.linux-amd64
+written by hand
+-- .info --
+{"Version":"v0.0.1-go1.18.5.linux-amd64"}
+-- .mod --
+golang.org/toolchain
+-- go.mod --
+golang.org/toolchain
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.18.7.linux-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.18.7.linux-amd64.txt
new file mode 100644
index 0000000..2fc7f85
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.18.7.linux-amd64.txt
@@ -0,0 +1,8 @@
+golang.org/toolchain v0.0.1-go1.18.7.linux-amd64
+written by hand
+-- .info --
+{"Version":"v0.0.1-go1.18.7.linux-amd64"}
+-- .mod --
+golang.org/toolchain
+-- go.mod --
+golang.org/toolchain
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.18.9.linux-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.18.9.linux-amd64.txt
new file mode 100644
index 0000000..7b07851
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.18.9.linux-amd64.txt
@@ -0,0 +1,8 @@
+golang.org/toolchain v0.0.1-go1.18.9.linux-amd64
+written by hand
+-- .info --
+{"Version":"v0.0.1-go1.18.9.linux-amd64"}
+-- .mod --
+golang.org/toolchain
+-- go.mod --
+golang.org/toolchain
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.18.linux-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.18.linux-amd64.txt
new file mode 100644
index 0000000..2c80ce5
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.18.linux-amd64.txt
@@ -0,0 +1,8 @@
+golang.org/toolchain v0.0.1-go1.18.linux-amd64
+written by hand
+-- .info --
+{"Version":"v0.0.1-go1.18.linux-amd64"}
+-- .mod --
+golang.org/toolchain
+-- go.mod --
+golang.org/toolchain
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.22.0.linux-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.22.0.linux-amd64.txt
new file mode 100644
index 0000000..215c547
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.22.0.linux-amd64.txt
@@ -0,0 +1,8 @@
+golang.org/toolchain v0.0.1-go1.22.0.linux-amd64
+written by hand
+-- .info --
+{"Version":"v0.0.1-go1.22.0.linux-amd64"}
+-- .mod --
+golang.org/toolchain
+-- go.mod --
+golang.org/toolchain
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.22.1.linux-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.22.1.linux-amd64.txt
new file mode 100644
index 0000000..ac36e3f
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.22.1.linux-amd64.txt
@@ -0,0 +1,8 @@
+golang.org/toolchain v0.0.1-go1.22.1.linux-amd64
+written by hand
+-- .info --
+{"Version":"v0.0.1-go1.22.1.linux-amd64"}
+-- .mod --
+golang.org/toolchain
+-- go.mod --
+golang.org/toolchain
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.22.3.linux-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.22.3.linux-amd64.txt
new file mode 100644
index 0000000..1178a48
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.22.3.linux-amd64.txt
@@ -0,0 +1,8 @@
+golang.org/toolchain v0.0.1-go1.22.3.linux-amd64
+written by hand
+-- .info --
+{"Version":"v0.0.1-go1.22.3.linux-amd64"}
+-- .mod --
+golang.org/toolchain
+-- go.mod --
+golang.org/toolchain
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.22.5.linux-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.22.5.linux-amd64.txt
new file mode 100644
index 0000000..d330127
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.22.5.linux-amd64.txt
@@ -0,0 +1,8 @@
+golang.org/toolchain v0.0.1-go1.22.5.linux-amd64
+written by hand
+-- .info --
+{"Version":"v0.0.1-go1.22.5.linux-amd64"}
+-- .mod --
+golang.org/toolchain
+-- go.mod --
+golang.org/toolchain
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.22.7.linux-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.22.7.linux-amd64.txt
new file mode 100644
index 0000000..a72863b
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.22.7.linux-amd64.txt
@@ -0,0 +1,8 @@
+golang.org/toolchain v0.0.1-go1.22.7.linux-amd64
+written by hand
+-- .info --
+{"Version":"v0.0.1-go1.22.7.linux-amd64"}
+-- .mod --
+golang.org/toolchain
+-- go.mod --
+golang.org/toolchain
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.22.9.linux-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.22.9.linux-amd64.txt
new file mode 100644
index 0000000..ac55849
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.22.9.linux-amd64.txt
@@ -0,0 +1,8 @@
+golang.org/toolchain v0.0.1-go1.22.9.linux-amd64
+written by hand
+-- .info --
+{"Version":"v0.0.1-go1.22.9.linux-amd64"}
+-- .mod --
+golang.org/toolchain
+-- go.mod --
+golang.org/toolchain
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.22rc1.linux-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.22rc1.linux-amd64.txt
new file mode 100644
index 0000000..b384f28
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.22rc1.linux-amd64.txt
@@ -0,0 +1,8 @@
+golang.org/toolchain v0.0.1-go1.22rc1.linux-amd64
+written by hand
+-- .info --
+{"Version":"v0.0.1-go1.22rc1.linux-amd64"}
+-- .mod --
+golang.org/toolchain
+-- go.mod --
+golang.org/toolchain
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.23.0.linux-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.23.0.linux-amd64.txt
new file mode 100644
index 0000000..bbc1377
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.23.0.linux-amd64.txt
@@ -0,0 +1,8 @@
+golang.org/toolchain v0.0.1-go1.23.0.linux-amd64
+written by hand
+-- .info --
+{"Version":"v0.0.1-go1.23.0.linux-amd64"}
+-- .mod --
+golang.org/toolchain
+-- go.mod --
+golang.org/toolchain
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.23.5.linux-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.23.5.linux-amd64.txt
new file mode 100644
index 0000000..206e8ad
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.23.5.linux-amd64.txt
@@ -0,0 +1,8 @@
+golang.org/toolchain v0.0.1-go1.23.5.linux-amd64
+written by hand
+-- .info --
+{"Version":"v0.0.1-go1.23.5.linux-amd64"}
+-- .mod --
+golang.org/toolchain
+-- go.mod --
+golang.org/toolchain
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.23.9.linux-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.23.9.linux-amd64.txt
new file mode 100644
index 0000000..7d03776
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.23.9.linux-amd64.txt
@@ -0,0 +1,8 @@
+golang.org/toolchain v0.0.1-go1.23.9.linux-amd64
+written by hand
+-- .info --
+{"Version":"v0.0.1-go1.23.9.linux-amd64"}
+-- .mod --
+golang.org/toolchain
+-- go.mod --
+golang.org/toolchain
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.24rc1.linux-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.24rc1.linux-amd64.txt
new file mode 100644
index 0000000..4b61ddd
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.24rc1.linux-amd64.txt
@@ -0,0 +1,8 @@
+golang.org/toolchain v0.0.1-go1.24rc1.linux-amd64
+written by hand
+-- .info --
+{"Version":"v0.0.1-go1.24rc1.linux-amd64"}
+-- .mod --
+golang.org/toolchain
+-- go.mod --
+golang.org/toolchain
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.aix-ppc64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.aix-ppc64.txt
new file mode 100644
index 0000000..e70c4d7
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.aix-ppc64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.aix-ppc64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.aix-ppc64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.android-386.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.android-386.txt
new file mode 100644
index 0000000..981334e
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.android-386.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.android-386
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.android-386"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.android-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.android-amd64.txt
new file mode 100644
index 0000000..a01fce8
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.android-amd64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.android-amd64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.android-amd64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.android-arm.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.android-arm.txt
new file mode 100644
index 0000000..0de1cec
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.android-arm.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.android-arm
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.android-arm"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.android-arm64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.android-arm64.txt
new file mode 100644
index 0000000..1ebeadc
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.android-arm64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.android-arm64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.android-arm64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.darwin-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.darwin-amd64.txt
new file mode 100644
index 0000000..5091853
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.darwin-amd64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.darwin-amd64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.darwin-amd64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.darwin-arm64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.darwin-arm64.txt
new file mode 100644
index 0000000..6b2b132
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.darwin-arm64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.darwin-arm64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.darwin-arm64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.dragonfly-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.dragonfly-amd64.txt
new file mode 100644
index 0000000..814180a
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.dragonfly-amd64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.dragonfly-amd64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.dragonfly-amd64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.freebsd-386.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.freebsd-386.txt
new file mode 100644
index 0000000..12e0df4
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.freebsd-386.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.freebsd-386
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.freebsd-386"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.freebsd-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.freebsd-amd64.txt
new file mode 100644
index 0000000..bf470a5
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.freebsd-amd64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.freebsd-amd64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.freebsd-amd64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.freebsd-arm.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.freebsd-arm.txt
new file mode 100644
index 0000000..dc32e0e
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.freebsd-arm.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.freebsd-arm
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.freebsd-arm"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.freebsd-arm64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.freebsd-arm64.txt
new file mode 100644
index 0000000..4335ba6
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.freebsd-arm64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.freebsd-arm64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.freebsd-arm64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.freebsd-riscv64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.freebsd-riscv64.txt
new file mode 100644
index 0000000..6161fe2
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.freebsd-riscv64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.freebsd-riscv64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.freebsd-riscv64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.illumos-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.illumos-amd64.txt
new file mode 100644
index 0000000..b677457
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.illumos-amd64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.illumos-amd64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.illumos-amd64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.ios-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.ios-amd64.txt
new file mode 100644
index 0000000..e8363bc
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.ios-amd64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.ios-amd64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.ios-amd64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.ios-arm64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.ios-arm64.txt
new file mode 100644
index 0000000..9585966
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.ios-arm64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.ios-arm64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.ios-arm64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.js-wasm.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.js-wasm.txt
new file mode 100644
index 0000000..37fa6c0
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.js-wasm.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.js-wasm
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.js-wasm"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-386.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-386.txt
new file mode 100644
index 0000000..ee96638
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-386.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.linux-386
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.linux-386"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-amd64.txt
new file mode 100644
index 0000000..6277341
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-amd64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.linux-amd64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.linux-amd64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-arm.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-arm.txt
new file mode 100644
index 0000000..678711b
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-arm.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.linux-arm
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.linux-arm"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-arm64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-arm64.txt
new file mode 100644
index 0000000..bb305ab
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-arm64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.linux-arm64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.linux-arm64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-loong64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-loong64.txt
new file mode 100644
index 0000000..52a23d4
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-loong64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.linux-loong64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.linux-loong64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-mips64x.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-mips64x.txt
new file mode 100644
index 0000000..79fff13
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-mips64x.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.linux-mips64x
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.linux-mips64x"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-mipsx.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-mipsx.txt
new file mode 100644
index 0000000..a725626
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-mipsx.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.linux-mipsx
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.linux-mipsx"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-ppc64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-ppc64.txt
new file mode 100644
index 0000000..f25ae8e
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-ppc64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.linux-ppc64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.linux-ppc64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-ppc64le.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-ppc64le.txt
new file mode 100644
index 0000000..e22b8ee
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-ppc64le.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.linux-ppc64le
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.linux-ppc64le"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-riscv64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-riscv64.txt
new file mode 100644
index 0000000..2e15fe3
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-riscv64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.linux-riscv64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.linux-riscv64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-s390x.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-s390x.txt
new file mode 100644
index 0000000..1022ee4
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.linux-s390x.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.linux-s390x
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.linux-s390x"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.netbsd-386.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.netbsd-386.txt
new file mode 100644
index 0000000..8b7b156
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.netbsd-386.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.netbsd-386
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.netbsd-386"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.netbsd-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.netbsd-amd64.txt
new file mode 100644
index 0000000..ebdb407
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.netbsd-amd64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.netbsd-amd64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.netbsd-amd64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.netbsd-arm.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.netbsd-arm.txt
new file mode 100644
index 0000000..fcacf08
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.netbsd-arm.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.netbsd-arm
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.netbsd-arm"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.netbsd-arm64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.netbsd-arm64.txt
new file mode 100644
index 0000000..c2bd257
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.netbsd-arm64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.netbsd-arm64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.netbsd-arm64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.openbsd-386.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.openbsd-386.txt
new file mode 100644
index 0000000..965a054
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.openbsd-386.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.openbsd-386
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.openbsd-386"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.openbsd-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.openbsd-amd64.txt
new file mode 100644
index 0000000..1acb7a3
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.openbsd-amd64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.openbsd-amd64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.openbsd-amd64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.openbsd-arm.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.openbsd-arm.txt
new file mode 100644
index 0000000..0e47d9c
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.openbsd-arm.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.openbsd-arm
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.openbsd-arm"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.openbsd-arm64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.openbsd-arm64.txt
new file mode 100644
index 0000000..8c99161
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.openbsd-arm64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.openbsd-arm64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.openbsd-arm64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.openbsd-mips64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.openbsd-mips64.txt
new file mode 100644
index 0000000..6c7cdfb
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.openbsd-mips64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.openbsd-mips64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.openbsd-mips64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.plan9-386.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.plan9-386.txt
new file mode 100644
index 0000000..26f7209
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.plan9-386.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.plan9-386
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.plan9-386"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/rc
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.plan9-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.plan9-amd64.txt
new file mode 100644
index 0000000..7cf1dde
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.plan9-amd64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.plan9-amd64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.plan9-amd64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/rc
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.plan9-arm.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.plan9-arm.txt
new file mode 100644
index 0000000..3c3b6a7
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.plan9-arm.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.plan9-arm
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.plan9-arm"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/rc
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.solaris-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.solaris-amd64.txt
new file mode 100644
index 0000000..a750aaf
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.solaris-amd64.txt
@@ -0,0 +1,14 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.solaris-amd64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.solaris-amd64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go --
+#!/bin/sh
+echo go1.999testmod here!
+-- bin/gofmt --
+echo i am unused
+-- pkg/tool/fake --
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.windows-386.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.windows-386.txt
new file mode 100644
index 0000000..ca0f7aa
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.windows-386.txt
@@ -0,0 +1,10 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.windows-386
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.windows-386"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go.bat --
+@echo go1.999testmod here!
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.windows-amd64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.windows-amd64.txt
new file mode 100644
index 0000000..44e16c7
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.windows-amd64.txt
@@ -0,0 +1,10 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.windows-amd64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.windows-amd64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go.bat --
+@echo go1.999testmod here!
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.windows-arm.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.windows-arm.txt
new file mode 100644
index 0000000..ee4e016
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.windows-arm.txt
@@ -0,0 +1,10 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.windows-arm
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.windows-arm"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go.bat --
+@echo go1.999testmod here!
diff --git a/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.windows-arm64.txt b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.windows-arm64.txt
new file mode 100644
index 0000000..be3ff80
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_toolchain_v0.0.1-go1.999testmod.windows-arm64.txt
@@ -0,0 +1,10 @@
+golang.org/toolchain@v0.0.1-go1.999testmod.windows-arm64
+
+-- .mod --
+module golang.org/toolchain
+-- .info --
+{"Version":"v0.0.1-go1.999testmod.windows-arm64"}
+-- go.mod --
+module golang.org/toolchain
+-- bin/go.bat --
+@echo go1.999testmod here!
diff --git a/src/cmd/go/testdata/mod/golang.org_x_internal_v0.1.0.txt b/src/cmd/go/testdata/mod/golang.org_x_internal_v0.1.0.txt
new file mode 100644
index 0000000..5737e95
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_x_internal_v0.1.0.txt
@@ -0,0 +1,43 @@
+written by hand — loosely derived from golang.org/x/crypto/internal/subtle,
+but splitting the internal package across a module boundary
+
+-- .mod --
+module golang.org/x/internal
+-- .info --
+{"Version":"v0.1.0","Name":"","Short":"","Time":"2018-07-25T17:24:00Z"}
+-- go.mod --
+module golang.org/x/internal
+-- subtle/aliasing.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !appengine
+
+// This is a tiny version of golang.org/x/crypto/internal/subtle.
+
+package subtle
+
+import "unsafe"
+
+func AnyOverlap(x, y []byte) bool {
+	return len(x) > 0 && len(y) > 0 &&
+		uintptr(unsafe.Pointer(&x[0])) <= uintptr(unsafe.Pointer(&y[len(y)-1])) &&
+		uintptr(unsafe.Pointer(&y[0])) <= uintptr(unsafe.Pointer(&x[len(x)-1]))
+}
+-- subtle/aliasing_appengine.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build appengine
+
+package subtle
+
+import "reflect"
+
+func AnyOverlap(x, y []byte) bool {
+	return len(x) > 0 && len(y) > 0 &&
+		reflect.ValueOf(&x[0]).Pointer() <= reflect.ValueOf(&y[len(y)-1]).Pointer() &&
+		reflect.ValueOf(&y[0]).Pointer() <= reflect.ValueOf(&x[len(x)-1]).Pointer()
+}
diff --git a/src/cmd/go/testdata/mod/golang.org_x_text_v0.0.0-20170915032832-14c0d48ead0c.txt b/src/cmd/go/testdata/mod/golang.org_x_text_v0.0.0-20170915032832-14c0d48ead0c.txt
new file mode 100644
index 0000000..f4f50cd
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_x_text_v0.0.0-20170915032832-14c0d48ead0c.txt
@@ -0,0 +1,47 @@
+written by hand - just enough to compile rsc.io/sampler, rsc.io/quote
+
+-- .mod --
+module golang.org/x/text
+-- .info --
+{"Version":"v0.0.0-20170915032832-14c0d48ead0c","Name":"v0.0.0-20170915032832-14c0d48ead0c","Short":"14c0d48ead0c","Time":"2017-09-15T03:28:32Z"}
+-- go.mod --
+module golang.org/x/text
+-- unused/unused.go --
+package unused
+-- language/lang.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This is a tiny version of golang.org/x/text.
+
+package language
+
+import "strings"
+
+type Tag string
+
+func Make(s string) Tag { return Tag(s) }
+
+func (t Tag) String() string { return string(t) }
+
+func NewMatcher(tags []Tag) Matcher { return &matcher{tags} }
+
+type Matcher interface {
+	Match(...Tag) (Tag, int, int)
+}
+
+type matcher struct {
+	tags []Tag
+}
+
+func (m *matcher) Match(prefs ...Tag) (Tag, int, int) {
+	for _, pref := range prefs {
+		for _, tag := range m.tags {
+			if tag == pref || strings.HasPrefix(string(pref), string(tag+"-")) || strings.HasPrefix(string(tag), string(pref+"-")) {
+				return tag, 0, 0
+			}
+		}
+	}
+	return m.tags[0], 0, 0
+}
diff --git a/src/cmd/go/testdata/mod/golang.org_x_text_v0.3.0.txt b/src/cmd/go/testdata/mod/golang.org_x_text_v0.3.0.txt
new file mode 100644
index 0000000..5561afa
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_x_text_v0.3.0.txt
@@ -0,0 +1,47 @@
+written by hand - just enough to compile rsc.io/sampler, rsc.io/quote
+
+-- .mod --
+module golang.org/x/text
+-- .info --
+{"Version":"v0.3.0","Name":"","Short":"","Time":"2017-09-16T03:28:32Z"}
+-- go.mod --
+module golang.org/x/text
+-- unused/unused.go --
+package unused
+-- language/lang.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This is a tiny version of golang.org/x/text.
+
+package language
+
+import "strings"
+
+type Tag string
+
+func Make(s string) Tag { return Tag(s) }
+
+func (t Tag) String() string { return string(t) }
+
+func NewMatcher(tags []Tag) Matcher { return &matcher{tags} }
+
+type Matcher interface {
+	Match(...Tag) (Tag, int, int)
+}
+
+type matcher struct {
+	tags []Tag
+}
+
+func (m *matcher) Match(prefs ...Tag) (Tag, int, int) {
+	for _, pref := range prefs {
+		for _, tag := range m.tags {
+			if tag == pref || strings.HasPrefix(string(pref), string(tag+"-")) || strings.HasPrefix(string(tag), string(pref+"-")) {
+				return tag, 0, 0
+			}
+		}
+	}
+	return m.tags[0], 0, 0
+}
diff --git a/src/cmd/go/testdata/mod/golang.org_x_useinternal_v0.1.0.txt b/src/cmd/go/testdata/mod/golang.org_x_useinternal_v0.1.0.txt
new file mode 100644
index 0000000..3fcba44
--- /dev/null
+++ b/src/cmd/go/testdata/mod/golang.org_x_useinternal_v0.1.0.txt
@@ -0,0 +1,13 @@
+written by hand — uses an internal package from another module
+(https://golang.org/s/go14internal)
+
+-- .mod --
+module golang.org/x/useinternal
+-- .info --
+{"Version":"v0.1.0","Name":"","Short":"","Time":"2018-07-25T17:24:00Z"}
+-- go.mod --
+module golang.org/x/useinternal
+-- useinternal.go --
+package useinternal
+
+import _ "golang.org/x/internal/subtle"
diff --git a/src/cmd/go/testdata/mod/gopkg.in_dummy.v2-unstable_v2.0.0.txt b/src/cmd/go/testdata/mod/gopkg.in_dummy.v2-unstable_v2.0.0.txt
new file mode 100644
index 0000000..f174159
--- /dev/null
+++ b/src/cmd/go/testdata/mod/gopkg.in_dummy.v2-unstable_v2.0.0.txt
@@ -0,0 +1,9 @@
+gopkg.in/dummy.v2-unstable v2.0.0
+written by hand
+
+-- .mod --
+module gopkg.in/dummy.v2-unstable
+-- .info --
+{"Version":"v2.0.0"}
+-- dummy.go --
+package dummy
diff --git a/src/cmd/go/testdata/mod/not-rsc.io_quote_v0.1.0-nomod.txt b/src/cmd/go/testdata/mod/not-rsc.io_quote_v0.1.0-nomod.txt
new file mode 100644
index 0000000..efff088
--- /dev/null
+++ b/src/cmd/go/testdata/mod/not-rsc.io_quote_v0.1.0-nomod.txt
@@ -0,0 +1,59 @@
+Constructed by hand.
+(derived from rsc.io/quote@e7a685a342, but without an explicit go.mod file.)
+
+-- .mod --
+module "not-rsc.io/quote"
+-- .info --
+{"Version":"v0.1.0-nomod","Time":"2018-02-14T00:51:33Z"}
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+// Hello returns a greeting.
+func Hello() string {
+	return "Hello, world."
+}
+
+// Glass returns a useful phrase for world travelers.
+func Glass() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return "I can eat glass and it doesn't hurt me."
+}
+
+// Go returns a Go proverb.
+func Go() string {
+	return "Don't communicate by sharing memory, share memory by communicating."
+}
+-- quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quote
+
+import "testing"
+
+func TestHello(t *testing.T) {
+	hello := "Hello, world."
+	if out := Hello(); out != hello {
+		t.Errorf("Hello() = %q, want %q", out, hello)
+	}
+}
+
+func TestGlass(t *testing.T) {
+	glass := "I can eat glass and it doesn't hurt me."
+	if out := Glass(); out != glass {
+		t.Errorf("Glass() = %q, want %q", out, glass)
+	}
+}
+
+func TestGo(t *testing.T) {
+	go1 := "Don't communicate by sharing memory. Share memory by communicating."
+	if out := Go(); out != go1 {
+		t.Errorf("Go() = %q, want %q", out, go1)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/patch.example.com_depofdirectpatch_v1.0.0.txt b/src/cmd/go/testdata/mod/patch.example.com_depofdirectpatch_v1.0.0.txt
new file mode 100644
index 0000000..40616c6
--- /dev/null
+++ b/src/cmd/go/testdata/mod/patch.example.com_depofdirectpatch_v1.0.0.txt
@@ -0,0 +1,11 @@
+patch.example.com/depofdirectpatch v1.0.0
+written by hand
+
+-- .mod --
+module patch.example.com/depofdirectpatch
+-- .info --
+{"Version":"v1.0.0"}
+-- go.mod --
+module patch.example.com/depofdirectpatch
+-- depofdirectpatch.go --
+package depofdirectpatch
diff --git a/src/cmd/go/testdata/mod/patch.example.com_depofdirectpatch_v1.0.1.txt b/src/cmd/go/testdata/mod/patch.example.com_depofdirectpatch_v1.0.1.txt
new file mode 100644
index 0000000..e075028
--- /dev/null
+++ b/src/cmd/go/testdata/mod/patch.example.com_depofdirectpatch_v1.0.1.txt
@@ -0,0 +1,11 @@
+patch.example.com/depofdirectpatch v1.0.1
+written by hand
+
+-- .mod --
+module patch.example.com/depofdirectpatch
+-- .info --
+{"Version":"v1.0.1"}
+-- go.mod --
+module patch.example.com/depofdirectpatch
+-- depofdirectpatch.go --
+package depofdirectpatch
diff --git a/src/cmd/go/testdata/mod/patch.example.com_direct_v1.0.0.txt b/src/cmd/go/testdata/mod/patch.example.com_direct_v1.0.0.txt
new file mode 100644
index 0000000..1e775fb
--- /dev/null
+++ b/src/cmd/go/testdata/mod/patch.example.com_direct_v1.0.0.txt
@@ -0,0 +1,21 @@
+patch.example.com/direct v1.0.0
+written by hand
+
+-- .mod --
+module patch.example.com/direct
+
+require (
+	patch.example.com/indirect v1.0.0
+)
+-- .info --
+{"Version":"v1.0.0"}
+-- go.mod --
+module patch.example.com/direct
+
+require (
+	patch.example.com/indirect v1.0.0
+)
+-- direct.go --
+package direct
+
+import _ "patch.example.com/indirect"
diff --git a/src/cmd/go/testdata/mod/patch.example.com_direct_v1.0.1.txt b/src/cmd/go/testdata/mod/patch.example.com_direct_v1.0.1.txt
new file mode 100644
index 0000000..64912b7
--- /dev/null
+++ b/src/cmd/go/testdata/mod/patch.example.com_direct_v1.0.1.txt
@@ -0,0 +1,27 @@
+patch.example.com/direct v1.0.1
+written by hand
+
+-- .mod --
+module patch.example.com/direct
+
+require (
+	patch.example.com/indirect v1.0.0
+	patch.example.com/depofdirectpatch v1.0.0
+)
+-- .info --
+{"Version":"v1.0.1"}
+-- go.mod --
+module patch.example.com/direct
+
+require (
+	patch.example.com/indirect v1.0.0
+	patch.example.com/depofdirectpatch v1.0.0
+)
+-- direct.go --
+package direct
+
+import _ "patch.example.com/indirect"
+-- usedepofdirectpatch/unused.go --
+package usedepofdirectpatch
+
+import _ "patch.example.com/depofdirectpatch"
diff --git a/src/cmd/go/testdata/mod/patch.example.com_direct_v1.1.0.txt b/src/cmd/go/testdata/mod/patch.example.com_direct_v1.1.0.txt
new file mode 100644
index 0000000..406e3b9
--- /dev/null
+++ b/src/cmd/go/testdata/mod/patch.example.com_direct_v1.1.0.txt
@@ -0,0 +1,21 @@
+patch.example.com/direct v1.1.0
+written by hand
+
+-- .mod --
+module patch.example.com/direct
+
+require (
+	patch.example.com/indirect v1.0.0
+)
+-- .info --
+{"Version":"v1.1.0"}
+-- go.mod --
+module patch.example.com/direct
+
+require (
+	patch.example.com/indirect v1.0.0
+)
+-- direct.go --
+package direct
+
+import _ "patch.example.com/indirect"
diff --git a/src/cmd/go/testdata/mod/patch.example.com_indirect_v1.0.0.txt b/src/cmd/go/testdata/mod/patch.example.com_indirect_v1.0.0.txt
new file mode 100644
index 0000000..ea7f5e2
--- /dev/null
+++ b/src/cmd/go/testdata/mod/patch.example.com_indirect_v1.0.0.txt
@@ -0,0 +1,11 @@
+patch.example.com/indirect v1.0.0
+written by hand
+
+-- .mod --
+module patch.example.com/indirect
+-- .info --
+{"Version":"v1.0.0"}
+-- go.mod --
+module patch.example.com/indirect
+-- direct.go --
+package indirect
diff --git a/src/cmd/go/testdata/mod/patch.example.com_indirect_v1.0.1.txt b/src/cmd/go/testdata/mod/patch.example.com_indirect_v1.0.1.txt
new file mode 100644
index 0000000..8c6cf8e
--- /dev/null
+++ b/src/cmd/go/testdata/mod/patch.example.com_indirect_v1.0.1.txt
@@ -0,0 +1,11 @@
+patch.example.com/indirect v1.0.1
+written by hand
+
+-- .mod --
+module patch.example.com/indirect
+-- .info --
+{"Version":"v1.0.1"}
+-- go.mod --
+module patch.example.com/indirect
+-- direct.go --
+package indirect
diff --git a/src/cmd/go/testdata/mod/patch.example.com_indirect_v1.1.0.txt b/src/cmd/go/testdata/mod/patch.example.com_indirect_v1.1.0.txt
new file mode 100644
index 0000000..f7229d4
--- /dev/null
+++ b/src/cmd/go/testdata/mod/patch.example.com_indirect_v1.1.0.txt
@@ -0,0 +1,11 @@
+patch.example.com/indirect v1.1.0
+written by hand
+
+-- .mod --
+module patch.example.com/indirect
+-- .info --
+{"Version":"v1.1.0"}
+-- go.mod --
+module patch.example.com/indirect
+-- direct.go --
+package indirect
diff --git a/src/cmd/go/testdata/mod/rsc.io_!c!g!o_v1.0.0.txt b/src/cmd/go/testdata/mod/rsc.io_!c!g!o_v1.0.0.txt
new file mode 100644
index 0000000..6276147
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_!c!g!o_v1.0.0.txt
@@ -0,0 +1,19 @@
+rsc.io/CGO v1.0.0
+
+-- .mod --
+module rsc.io/CGO
+-- .info --
+{"Version":"v1.0.0","Name":"","Short":"","Time":"2018-08-01T18:23:45Z"}
+-- go.mod --
+module rsc.io/CGO
+-- cgo.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package CGO
+
+// #cgo CFLAGS: -I${SRCDIR}
+import "C"
+
+var V = 0
diff --git a/src/cmd/go/testdata/mod/rsc.io_!q!u!o!t!e_v1.5.2.txt b/src/cmd/go/testdata/mod/rsc.io_!q!u!o!t!e_v1.5.2.txt
new file mode 100644
index 0000000..21185c3
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_!q!u!o!t!e_v1.5.2.txt
@@ -0,0 +1,88 @@
+rsc.io/QUOTE v1.5.2
+
+-- .mod --
+module rsc.io/QUOTE
+
+require rsc.io/quote v1.5.2
+-- .info --
+{"Version":"v1.5.2","Name":"","Short":"","Time":"2018-07-15T16:25:34Z"}
+-- go.mod --
+module rsc.io/QUOTE
+
+require rsc.io/quote v1.5.2
+-- QUOTE/quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// PACKAGE QUOTE COLLECTS LOUD SAYINGS.
+package QUOTE
+
+import (
+	"strings"
+
+	"rsc.io/quote"
+)
+
+// HELLO RETURNS A GREETING.
+func HELLO() string {
+	return strings.ToUpper(quote.Hello())
+}
+
+// GLASS RETURNS A USEFUL PHRASE FOR WORLD TRAVELERS.
+func GLASS() string {
+	return strings.ToUpper(quote.GLASS())
+}
+
+// GO RETURNS A GO PROVERB.
+func GO() string {
+	return strings.ToUpper(quote.GO())
+}
+
+// OPT RETURNS AN OPTIMIZATION TRUTH.
+func OPT() string {
+	return strings.ToUpper(quote.OPT())
+}
+-- QUOTE/quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package QUOTE
+
+import (
+	"os"
+	"testing"
+)
+
+func init() {
+	os.Setenv("LC_ALL", "en")
+}
+
+func TestHELLO(t *testing.T) {
+	hello := "HELLO, WORLD"
+	if out := HELLO(); out != hello {
+		t.Errorf("HELLO() = %q, want %q", out, hello)
+	}
+}
+
+func TestGLASS(t *testing.T) {
+	glass := "I CAN EAT GLASS AND IT DOESN'T HURT ME."
+	if out := GLASS(); out != glass {
+		t.Errorf("GLASS() = %q, want %q", out, glass)
+	}
+}
+
+func TestGO(t *testing.T) {
+	go1 := "DON'T COMMUNICATE BY SHARING MEMORY, SHARE MEMORY BY COMMUNICATING."
+	if out := GO(); out != go1 {
+		t.Errorf("GO() = %q, want %q", out, go1)
+	}
+}
+
+func TestOPT(t *testing.T) {
+	opt := "IF A PROGRAM IS TOO SLOW, IT MUST HAVE A LOOP."
+	if out := OPT(); out != opt {
+		t.Errorf("OPT() = %q, want %q", out, opt)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_!q!u!o!t!e_v1.5.3-!p!r!e.txt b/src/cmd/go/testdata/mod/rsc.io_!q!u!o!t!e_v1.5.3-!p!r!e.txt
new file mode 100644
index 0000000..54bac2d
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_!q!u!o!t!e_v1.5.3-!p!r!e.txt
@@ -0,0 +1,88 @@
+rsc.io/QUOTE v1.5.3-PRE (sigh)
+
+-- .mod --
+module rsc.io/QUOTE
+
+require rsc.io/quote v1.5.2
+-- .info --
+{"Version":"v1.5.3-PRE","Name":"","Short":"","Time":"2018-07-15T16:25:34Z"}
+-- go.mod --
+module rsc.io/QUOTE
+
+require rsc.io/quote v1.5.2
+-- QUOTE/quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// PACKAGE QUOTE COLLECTS LOUD SAYINGS.
+package QUOTE
+
+import (
+	"strings"
+
+	"rsc.io/quote"
+)
+
+// HELLO RETURNS A GREETING.
+func HELLO() string {
+	return strings.ToUpper(quote.Hello())
+}
+
+// GLASS RETURNS A USEFUL PHRASE FOR WORLD TRAVELERS.
+func GLASS() string {
+	return strings.ToUpper(quote.GLASS())
+}
+
+// GO RETURNS A GO PROVERB.
+func GO() string {
+	return strings.ToUpper(quote.GO())
+}
+
+// OPT RETURNS AN OPTIMIZATION TRUTH.
+func OPT() string {
+	return strings.ToUpper(quote.OPT())
+}
+-- QUOTE/quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package QUOTE
+
+import (
+	"os"
+	"testing"
+)
+
+func init() {
+	os.Setenv("LC_ALL", "en")
+}
+
+func TestHELLO(t *testing.T) {
+	hello := "HELLO, WORLD"
+	if out := HELLO(); out != hello {
+		t.Errorf("HELLO() = %q, want %q", out, hello)
+	}
+}
+
+func TestGLASS(t *testing.T) {
+	glass := "I CAN EAT GLASS AND IT DOESN'T HURT ME."
+	if out := GLASS(); out != glass {
+		t.Errorf("GLASS() = %q, want %q", out, glass)
+	}
+}
+
+func TestGO(t *testing.T) {
+	go1 := "DON'T COMMUNICATE BY SHARING MEMORY, SHARE MEMORY BY COMMUNICATING."
+	if out := GO(); out != go1 {
+		t.Errorf("GO() = %q, want %q", out, go1)
+	}
+}
+
+func TestOPT(t *testing.T) {
+	opt := "IF A PROGRAM IS TOO SLOW, IT MUST HAVE A LOOP."
+	if out := OPT(); out != opt {
+		t.Errorf("OPT() = %q, want %q", out, opt)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_badfile1_v1.0.0.txt b/src/cmd/go/testdata/mod/rsc.io_badfile1_v1.0.0.txt
new file mode 100644
index 0000000..9d23e7d
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_badfile1_v1.0.0.txt
@@ -0,0 +1,14 @@
+rsc.io/badfile1 v1.0.0
+written by hand
+this is part of the badfile test but is a valid zip file.
+
+-- .mod --
+module rsc.io/badfile1
+-- .info --
+{"Version":"v1.0.0"}
+-- go.mod --
+module rsc.io/badfile1
+-- α.go --
+package α
+-- .gitignore --
+-- x/y/z/.gitignore --
diff --git a/src/cmd/go/testdata/mod/rsc.io_badfile2_v1.0.0.txt b/src/cmd/go/testdata/mod/rsc.io_badfile2_v1.0.0.txt
new file mode 100644
index 0000000..58e1e1c
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_badfile2_v1.0.0.txt
@@ -0,0 +1,12 @@
+rsc.io/badfile1 v1.0.0
+written by hand
+
+-- .mod --
+module rsc.io/badfile2
+-- .info --
+{"Version":"v1.0.0"}
+-- go.mod --
+module rsc.io/badfile2
+-- ☺.go --
+package smiley
+
diff --git a/src/cmd/go/testdata/mod/rsc.io_badfile3_v1.0.0.txt b/src/cmd/go/testdata/mod/rsc.io_badfile3_v1.0.0.txt
new file mode 100644
index 0000000..a008448
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_badfile3_v1.0.0.txt
@@ -0,0 +1,12 @@
+rsc.io/badfile3 v1.0.0
+written by hand
+
+-- .mod --
+module rsc.io/badfile3
+-- .info --
+{"Version":"v1.0.0"}
+-- go.mod --
+module rsc.io/badfile3
+-- x?y.go --
+package x
+
diff --git a/src/cmd/go/testdata/mod/rsc.io_badfile4_v1.0.0.txt b/src/cmd/go/testdata/mod/rsc.io_badfile4_v1.0.0.txt
new file mode 100644
index 0000000..e28844d
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_badfile4_v1.0.0.txt
@@ -0,0 +1,15 @@
+rsc.io/badfile4 v1.0.0
+written by hand
+
+-- .mod --
+module rsc.io/badfile4
+-- .info --
+{"Version":"v1.0.0"}
+-- go.mod --
+module rsc.io/badfile4
+-- x/Y.go --
+package x
+-- x/y.go --
+package x
+
+
diff --git a/src/cmd/go/testdata/mod/rsc.io_badfile5_v1.0.0.txt b/src/cmd/go/testdata/mod/rsc.io_badfile5_v1.0.0.txt
new file mode 100644
index 0000000..3c7903a
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_badfile5_v1.0.0.txt
@@ -0,0 +1,13 @@
+rsc.io/badfile5 v1.0.0
+written by hand
+
+-- .mod --
+module rsc.io/badfile5
+-- .info --
+{"Version":"v1.0.0"}
+-- go.mod --
+module rsc.io/badfile5
+-- x/y/z/w.go --
+package z
+-- x/Y/zz/ww.go --
+package zz
diff --git a/src/cmd/go/testdata/mod/rsc.io_badmod_v1.0.0.txt b/src/cmd/go/testdata/mod/rsc.io_badmod_v1.0.0.txt
new file mode 100644
index 0000000..993ceb7
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_badmod_v1.0.0.txt
@@ -0,0 +1,11 @@
+rsc.io/badmod v1.0.0
+written by hand
+
+-- .mod --
+module rsc.io/badmod
+hello world
+-- .info --
+{"Version":"v1.0.0"}
+-- x.go --
+package x
+
diff --git a/src/cmd/go/testdata/mod/rsc.io_badsum_v1.0.0.txt b/src/cmd/go/testdata/mod/rsc.io_badsum_v1.0.0.txt
new file mode 100644
index 0000000..d62db26
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_badsum_v1.0.0.txt
@@ -0,0 +1,14 @@
+rsc.io/badsum@v1.0.0
+
+This module would match the hard-coded hash for rsc.io/badsum v1.0.0
+in modfetch/notary.go if not for the "break hash" line.
+
+-- .mod --
+module "rsc.io/badsum"
+-- .info --
+{"Version":"v1.0.0","Time":"2018-02-14T00:45:20Z"}
+-- go.mod --
+module "rsc.io/badsum"
+-- badsum.go --
+package badsum
+// break hash
diff --git a/src/cmd/go/testdata/mod/rsc.io_badsum_v1.0.1.txt b/src/cmd/go/testdata/mod/rsc.io_badsum_v1.0.1.txt
new file mode 100644
index 0000000..5fea50a
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_badsum_v1.0.1.txt
@@ -0,0 +1,14 @@
+rsc.io/badsum@v1.0.1
+
+This module would match the hard-coded hash for rsc.io/badsum v1.0.1/go.mod
+in modfetch/notary.go if not for the "break hash" line.
+
+-- .mod --
+module "rsc.io/badsum"
+# break hash
+-- .info --
+{"Version":"v1.0.1","Time":"2018-02-14T00:45:20Z"}
+-- go.mod --
+module "rsc.io/badsum"
+-- badsum.go --
+package badsum
diff --git a/src/cmd/go/testdata/mod/rsc.io_badzip_v1.0.0.txt b/src/cmd/go/testdata/mod/rsc.io_badzip_v1.0.0.txt
new file mode 100644
index 0000000..07a38fa
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_badzip_v1.0.0.txt
@@ -0,0 +1,11 @@
+rsc.io/badzip v1.0.0
+written by hand
+
+-- .mod --
+module rsc.io/badzip
+-- .info --
+{"Version":"v1.0.0"}
+-- x.go --
+package x
+-- /rsc.io/badzip@v1.0.0.txt --
+This file should not be here.
diff --git a/src/cmd/go/testdata/mod/rsc.io_breaker_v1.0.0.txt b/src/cmd/go/testdata/mod/rsc.io_breaker_v1.0.0.txt
new file mode 100644
index 0000000..a103e3f
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_breaker_v1.0.0.txt
@@ -0,0 +1,11 @@
+rsc.io/breaker v1.0.0
+written by hand
+
+-- .mod --
+module rsc.io/breaker
+-- .info --
+{"Version":"v1.0.0"}
+-- breaker.go --
+package breaker
+
+const X = 1
diff --git a/src/cmd/go/testdata/mod/rsc.io_breaker_v2.0.0+incompatible.txt b/src/cmd/go/testdata/mod/rsc.io_breaker_v2.0.0+incompatible.txt
new file mode 100644
index 0000000..59d8bac
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_breaker_v2.0.0+incompatible.txt
@@ -0,0 +1,11 @@
+rsc.io/breaker v2.0.0+incompatible
+written by hand
+
+-- .mod --
+module rsc.io/breaker
+-- .info --
+{"Version":"v2.0.0+incompatible", "Name": "7307b307f4f0dde421900f8e5126fadac1e13aed", "Short": "7307b307f4f0"}
+-- breaker.go --
+package breaker
+
+const XX = 2
diff --git a/src/cmd/go/testdata/mod/rsc.io_breaker_v2.0.0.txt b/src/cmd/go/testdata/mod/rsc.io_breaker_v2.0.0.txt
new file mode 100644
index 0000000..59d8bac
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_breaker_v2.0.0.txt
@@ -0,0 +1,11 @@
+rsc.io/breaker v2.0.0+incompatible
+written by hand
+
+-- .mod --
+module rsc.io/breaker
+-- .info --
+{"Version":"v2.0.0+incompatible", "Name": "7307b307f4f0dde421900f8e5126fadac1e13aed", "Short": "7307b307f4f0"}
+-- breaker.go --
+package breaker
+
+const XX = 2
diff --git a/src/cmd/go/testdata/mod/rsc.io_fortune_v0.0.1.txt b/src/cmd/go/testdata/mod/rsc.io_fortune_v0.0.1.txt
new file mode 100644
index 0000000..644695c
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_fortune_v0.0.1.txt
@@ -0,0 +1,21 @@
+rsc.io/fortune 0.0.1
+written by hand
+
+-- .mod --
+module rsc.io/fortune
+go 1.21rc999
+
+-- go.mod --
+module rsc.io/fortune
+go 1.21rc999
+
+-- .info --
+{"Version":"v0.0.1"}
+-- fortune.go --
+package main
+
+import "rsc.io/quote"
+
+func main() {
+	println(quote.Hello())
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_fortune_v1.0.0.txt b/src/cmd/go/testdata/mod/rsc.io_fortune_v1.0.0.txt
new file mode 100644
index 0000000..d8a71f3
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_fortune_v1.0.0.txt
@@ -0,0 +1,15 @@
+rsc.io/fortune v1.0.0
+written by hand
+
+-- .mod --
+module rsc.io/fortune
+-- .info --
+{"Version":"v1.0.0"}
+-- fortune.go --
+package main
+
+import "rsc.io/quote"
+
+func main() {
+	println(quote.Hello())
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_fortune_v2_v2.0.0.txt b/src/cmd/go/testdata/mod/rsc.io_fortune_v2_v2.0.0.txt
new file mode 100644
index 0000000..3acd637
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_fortune_v2_v2.0.0.txt
@@ -0,0 +1,21 @@
+rsc.io/fortune v2.0.0
+written by hand
+
+-- .mod --
+module rsc.io/fortune/v2
+-- .info --
+{"Version":"v2.0.0"}
+-- fortune.go --
+package main
+
+import "rsc.io/quote"
+
+func main() {
+	println(quote.Hello())
+}
+-- fortune_test.go --
+package main
+
+import "testing"
+
+func TestFortuneV2(t *testing.T) {}
diff --git a/src/cmd/go/testdata/mod/rsc.io_future_v1.0.0.txt b/src/cmd/go/testdata/mod/rsc.io_future_v1.0.0.txt
new file mode 100644
index 0000000..d3826a3
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_future_v1.0.0.txt
@@ -0,0 +1,16 @@
+rsc.io/future v1.0.0
+written by hand
+
+-- .mod --
+module rsc.io/future
+go 1.999
+-- .info --
+{"Version":"v1.0.0"}
+-- main.go --
+package main
+
+func main() {
+}
+-- go.mod --
+module rsc.io/future
+go 1.999
diff --git a/src/cmd/go/testdata/mod/rsc.io_needall_v0.0.1.txt b/src/cmd/go/testdata/mod/rsc.io_needall_v0.0.1.txt
new file mode 100644
index 0000000..0a1582a
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_needall_v0.0.1.txt
@@ -0,0 +1,25 @@
+rsc.io/needall 0.0.1
+written by hand
+
+-- .mod --
+module rsc.io/needall
+go 1.23
+
+require rsc.io/needgo121 v0.0.1
+require rsc.io/needgo122 v0.0.1
+require rsc.io/needgo123 v0.0.1
+
+-- go.mod --
+module rsc.io/needall
+go 1.23
+
+require rsc.io/needgo121 v0.0.1
+require rsc.io/needgo122 v0.0.1
+require rsc.io/needgo123 v0.0.1
+
+-- .info --
+{"Version":"v0.0.1"}
+-- p.go --
+package p
+
+func F() {}
diff --git a/src/cmd/go/testdata/mod/rsc.io_needgo1183_v0.0.1.txt b/src/cmd/go/testdata/mod/rsc.io_needgo1183_v0.0.1.txt
new file mode 100644
index 0000000..a41296e
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_needgo1183_v0.0.1.txt
@@ -0,0 +1,17 @@
+rsc.io/needgo1183 v0.0.1
+written by hand
+
+-- .mod --
+module rsc.io/needgo1183
+go 1.18.3
+
+-- go.mod --
+module rsc.io/needgo1183
+go 1.18.3
+
+-- .info --
+{"Version":"v0.0.1"}
+-- p.go --
+package p
+
+func F() {}
diff --git a/src/cmd/go/testdata/mod/rsc.io_needgo118_v0.0.1.txt b/src/cmd/go/testdata/mod/rsc.io_needgo118_v0.0.1.txt
new file mode 100644
index 0000000..805eac7
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_needgo118_v0.0.1.txt
@@ -0,0 +1,17 @@
+rsc.io/needgo118 0.0.1
+written by hand
+
+-- .mod --
+module rsc.io/needgo118
+go 1.18
+
+-- go.mod --
+module rsc.io/needgo118
+go 1.18
+
+-- .info --
+{"Version":"v0.0.1"}
+-- p.go --
+package p
+
+func F() {}
diff --git a/src/cmd/go/testdata/mod/rsc.io_needgo121_v0.0.1.txt b/src/cmd/go/testdata/mod/rsc.io_needgo121_v0.0.1.txt
new file mode 100644
index 0000000..5b05960
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_needgo121_v0.0.1.txt
@@ -0,0 +1,17 @@
+rsc.io/needgo121 0.0.1
+written by hand
+
+-- .mod --
+module rsc.io/needgo121
+go 1.21
+
+-- go.mod --
+module rsc.io/needgo121
+go 1.21
+
+-- .info --
+{"Version":"v0.0.1"}
+-- p.go --
+package p
+
+func F() {}
diff --git a/src/cmd/go/testdata/mod/rsc.io_needgo1223_v0.0.1.txt b/src/cmd/go/testdata/mod/rsc.io_needgo1223_v0.0.1.txt
new file mode 100644
index 0000000..f166a82
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_needgo1223_v0.0.1.txt
@@ -0,0 +1,17 @@
+rsc.io/needgo1223 0.0.1
+written by hand
+
+-- .mod --
+module rsc.io/needgo1223
+go 1.22.3
+
+-- go.mod --
+module rsc.io/needgo1223
+go 1.22.3
+
+-- .info --
+{"Version":"v0.0.1"}
+-- p.go --
+package p
+
+func F() {}
diff --git a/src/cmd/go/testdata/mod/rsc.io_needgo122_v0.0.1.txt b/src/cmd/go/testdata/mod/rsc.io_needgo122_v0.0.1.txt
new file mode 100644
index 0000000..59116eb
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_needgo122_v0.0.1.txt
@@ -0,0 +1,17 @@
+rsc.io/needgo122 0.0.1
+written by hand
+
+-- .mod --
+module rsc.io/needgo122
+go 1.22
+
+-- go.mod --
+module rsc.io/needgo122
+go 1.22
+
+-- .info --
+{"Version":"v0.0.1"}
+-- p.go --
+package p
+
+func F() {}
diff --git a/src/cmd/go/testdata/mod/rsc.io_needgo123_v0.0.1.txt b/src/cmd/go/testdata/mod/rsc.io_needgo123_v0.0.1.txt
new file mode 100644
index 0000000..0ec5571
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_needgo123_v0.0.1.txt
@@ -0,0 +1,17 @@
+rsc.io/needgo123 0.0.1
+written by hand
+
+-- .mod --
+module rsc.io/needgo123
+go 1.23
+
+-- go.mod --
+module rsc.io/needgo123
+go 1.23
+
+-- .info --
+{"Version":"v0.0.1"}
+-- p.go --
+package p
+
+func F() {}
diff --git a/src/cmd/go/testdata/mod/rsc.io_needgo124_v0.0.1.txt b/src/cmd/go/testdata/mod/rsc.io_needgo124_v0.0.1.txt
new file mode 100644
index 0000000..634f504
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_needgo124_v0.0.1.txt
@@ -0,0 +1,17 @@
+rsc.io/needgo124 0.0.1
+written by hand
+
+-- .mod --
+module rsc.io/needgo124
+go 1.24
+
+-- go.mod --
+module rsc.io/needgo124
+go 1.24
+
+-- .info --
+{"Version":"v0.0.1"}
+-- p.go --
+package p
+
+func F() {}
diff --git a/src/cmd/go/testdata/mod/rsc.io_panicnil_v1.0.0.txt b/src/cmd/go/testdata/mod/rsc.io_panicnil_v1.0.0.txt
new file mode 100644
index 0000000..6ea1b22
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_panicnil_v1.0.0.txt
@@ -0,0 +1,13 @@
+rsc.io/panicnil v1.0.0
+written by hand
+
+-- .mod --
+module rsc.io/panicnil
+-- .info --
+{"Version":"v1.0.0"}
+-- fortune.go --
+package main
+
+func main() {
+	panic(nil)
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_panicnil_v1.1.0.txt b/src/cmd/go/testdata/mod/rsc.io_panicnil_v1.1.0.txt
new file mode 100644
index 0000000..fe67a88
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_panicnil_v1.1.0.txt
@@ -0,0 +1,14 @@
+rsc.io/panicnil v1.1.0
+written by hand
+
+-- .mod --
+module rsc.io/panicnil
+go 1.21
+-- .info --
+{"Version":"v1.1.0"}
+-- fortune.go --
+package main
+
+func main() {
+	panic(nil)
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180214005133-e7a685a342c0.txt b/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180214005133-e7a685a342c0.txt
new file mode 100644
index 0000000..8ae173e
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180214005133-e7a685a342c0.txt
@@ -0,0 +1,60 @@
+rsc.io/quote@e7a685a342
+
+-- .mod --
+module "rsc.io/quote"
+-- .info --
+{"Version":"v0.0.0-20180214005133-e7a685a342c0","Name":"e7a685a342c001acc3eb7f5eafa82980480042c7","Short":"e7a685a342c0","Time":"2018-02-14T00:51:33Z"}
+-- go.mod --
+module "rsc.io/quote"
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+// Hello returns a greeting.
+func Hello() string {
+	return "Hello, world."
+}
+
+// Glass returns a useful phrase for world travelers.
+func Glass() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return "I can eat glass and it doesn't hurt me."
+}
+
+// Go returns a Go proverb.
+func Go() string {
+	return "Don't communicate by sharing memory, share memory by communicating."
+}
+-- quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quote
+
+import "testing"
+
+func TestHello(t *testing.T) {
+	hello := "Hello, world."
+	if out := Hello(); out != hello {
+		t.Errorf("Hello() = %q, want %q", out, hello)
+	}
+}
+
+func TestGlass(t *testing.T) {
+	glass := "I can eat glass and it doesn't hurt me."
+	if out := Glass(); out != glass {
+		t.Errorf("Glass() = %q, want %q", out, glass)
+	}
+}
+
+func TestGo(t *testing.T) {
+	go1 := "Don't communicate by sharing memory. Share memory by communicating."
+	if out := Go(); out != go1 {
+		t.Errorf("Go() = %q, want %q", out, go1)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180214005840-23179ee8a569.txt b/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180214005840-23179ee8a569.txt
new file mode 100644
index 0000000..bc626ba
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180214005840-23179ee8a569.txt
@@ -0,0 +1,86 @@
+rsc.io/quote@v0.0.0-20180214005840-23179ee8a569
+
+-- .mod --
+module "rsc.io/quote"
+
+require "rsc.io/sampler" v1.3.0
+-- .info --
+{"Version":"v0.0.0-20180214005840-23179ee8a569","Name":"23179ee8a569bb05d896ae05c6503ec69a19f99f","Short":"23179ee8a569","Time":"2018-02-14T00:58:40Z"}
+-- go.mod --
+module "rsc.io/quote"
+
+require "rsc.io/sampler" v1.3.0
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+import "rsc.io/sampler"
+
+// Hello returns a greeting.
+func Hello() string {
+	return sampler.Hello()
+}
+
+// Glass returns a useful phrase for world travelers.
+func Glass() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return "I can eat glass and it doesn't hurt me."
+}
+
+// Go returns a Go proverb.
+func Go() string {
+	return "Don't communicate by sharing memory, share memory by communicating."
+}
+
+// Opt returns an optimization truth.
+func Opt() string {
+	// Wisdom from ken.
+	return "If a program is too slow, it must have a loop."
+}
+-- quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quote
+
+import (
+	"os"
+	"testing"
+)
+
+func init() {
+	os.Setenv("LC_ALL", "en")
+}
+
+func TestHello(t *testing.T) {
+	hello := "Hello, world."
+	if out := Hello(); out != hello {
+		t.Errorf("Hello() = %q, want %q", out, hello)
+	}
+}
+
+func TestGlass(t *testing.T) {
+	glass := "I can eat glass and it doesn't hurt me."
+	if out := Glass(); out != glass {
+		t.Errorf("Glass() = %q, want %q", out, glass)
+	}
+}
+
+func TestGo(t *testing.T) {
+	go1 := "Don't communicate by sharing memory, share memory by communicating."
+	if out := Go(); out != go1 {
+		t.Errorf("Go() = %q, want %q", out, go1)
+	}
+}
+
+func TestOpt(t *testing.T) {
+	opt := "If a program is too slow, it must have a loop."
+	if out := Opt(); out != opt {
+		t.Errorf("Opt() = %q, want %q", out, opt)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180628003336-dd9747d19b04.txt b/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180628003336-dd9747d19b04.txt
new file mode 100644
index 0000000..bbc8097
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180628003336-dd9747d19b04.txt
@@ -0,0 +1,100 @@
+rsc.io/quote@dd9747d
+
+-- .mod --
+module "rsc.io/quote"
+
+require "rsc.io/sampler" v1.3.0
+-- .info --
+{"Version":"v0.0.0-20180628003336-dd9747d19b04","Name":"dd9747d19b041365fbddf0399ddba6bff5eb1b3e","Short":"dd9747d19b04","Time":"2018-06-28T00:33:36Z"}
+-- buggy/buggy_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package buggy
+
+import "testing"
+
+func Test(t *testing.T) {
+	t.Fatal("buggy!")
+}
+-- go.mod --
+module "rsc.io/quote"
+
+require "rsc.io/sampler" v1.3.0
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+import "rsc.io/sampler"
+
+AN EVEN WORSE CHANGE!
+
+// Hello returns a greeting.
+func Hello() string {
+	return sampler.Hello()
+}
+
+// Glass returns a useful phrase for world travelers.
+func Glass() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return "I can eat glass and it doesn't hurt me."
+}
+
+// Go returns a Go proverb.
+func Go() string {
+	return "Don't communicate by sharing memory, share memory by communicating."
+}
+
+// Opt returns an optimization truth.
+func Opt() string {
+	// Wisdom from ken.
+	return "If a program is too slow, it must have a loop."
+}
+-- quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quote
+
+import (
+	"os"
+	"testing"
+)
+
+func init() {
+	os.Setenv("LC_ALL", "en")
+}
+
+func TestHello(t *testing.T) {
+	hello := "Hello, world."
+	if out := Hello(); out != hello {
+		t.Errorf("Hello() = %q, want %q", out, hello)
+	}
+}
+
+func TestGlass(t *testing.T) {
+	glass := "I can eat glass and it doesn't hurt me."
+	if out := Glass(); out != glass {
+		t.Errorf("Glass() = %q, want %q", out, glass)
+	}
+}
+
+func TestGo(t *testing.T) {
+	go1 := "Don't communicate by sharing memory, share memory by communicating."
+	if out := Go(); out != go1 {
+		t.Errorf("Go() = %q, want %q", out, go1)
+	}
+}
+
+func TestOpt(t *testing.T) {
+	opt := "If a program is too slow, it must have a loop."
+	if out := Opt(); out != opt {
+		t.Errorf("Opt() = %q, want %q", out, opt)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180709153244-fd906ed3b100.txt b/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180709153244-fd906ed3b100.txt
new file mode 100644
index 0000000..e461ed4
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180709153244-fd906ed3b100.txt
@@ -0,0 +1,86 @@
+rsc.io/quote@v2.0.0
+
+-- .mod --
+module "rsc.io/quote"
+
+require "rsc.io/sampler" v1.3.0
+-- .info --
+{"Version":"v0.0.0-20180709153244-fd906ed3b100","Name":"fd906ed3b100e47181ffa9ec36d82294525c9109","Short":"fd906ed3b100","Time":"2018-07-09T15:32:44Z"}
+-- go.mod --
+module "rsc.io/quote"
+
+require "rsc.io/sampler" v1.3.0
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+import "rsc.io/sampler"
+
+// Hello returns a greeting.
+func HelloV2() string {
+	return sampler.Hello()
+}
+
+// Glass returns a useful phrase for world travelers.
+func GlassV2() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return "I can eat glass and it doesn't hurt me."
+}
+
+// Go returns a Go proverb.
+func GoV2() string {
+	return "Don't communicate by sharing memory, share memory by communicating."
+}
+
+// Opt returns an optimization truth.
+func OptV2() string {
+	// Wisdom from ken.
+	return "If a program is too slow, it must have a loop."
+}
+-- quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quote
+
+import (
+	"os"
+	"testing"
+)
+
+func init() {
+	os.Setenv("LC_ALL", "en")
+}
+
+func TestHello(t *testing.T) {
+	hello := "Hello, world."
+	if out := Hello(); out != hello {
+		t.Errorf("Hello() = %q, want %q", out, hello)
+	}
+}
+
+func TestGlass(t *testing.T) {
+	glass := "I can eat glass and it doesn't hurt me."
+	if out := Glass(); out != glass {
+		t.Errorf("Glass() = %q, want %q", out, glass)
+	}
+}
+
+func TestGo(t *testing.T) {
+	go1 := "Don't communicate by sharing memory, share memory by communicating."
+	if out := Go(); out != go1 {
+		t.Errorf("Go() = %q, want %q", out, go1)
+	}
+}
+
+func TestOpt(t *testing.T) {
+	opt := "If a program is too slow, it must have a loop."
+	if out := Opt(); out != opt {
+		t.Errorf("Opt() = %q, want %q", out, opt)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180709160352-0d003b9c4bfa.txt b/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180709160352-0d003b9c4bfa.txt
new file mode 100644
index 0000000..c1d511f
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180709160352-0d003b9c4bfa.txt
@@ -0,0 +1,98 @@
+rsc.io/quote@v0.0.0-20180709160352-0d003b9c4bfa
+
+-- .mod --
+module rsc.io/quote
+
+require rsc.io/sampler v1.3.0
+-- .info --
+{"Version":"v0.0.0-20180709160352-0d003b9c4bfa","Name":"0d003b9c4bfac881641be8eb1598b782a467a97f","Short":"0d003b9c4bfa","Time":"2018-07-09T16:03:52Z"}
+-- buggy/buggy_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package buggy
+
+import "testing"
+
+func Test(t *testing.T) {
+	t.Fatal("buggy!")
+}
+-- go.mod --
+module rsc.io/quote
+
+require rsc.io/sampler v1.3.0
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+import "rsc.io/quote/v2"
+
+// Hello returns a greeting.
+func Hello() string {
+	return quote.HelloV2()
+}
+
+// Glass returns a useful phrase for world travelers.
+func Glass() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return quote.GlassV2()
+}
+
+// Go returns a Go proverb.
+func Go() string {
+	return quote.GoV2()
+}
+
+// Opt returns an optimization truth.
+func Opt() string {
+	// Wisdom from ken.
+	return quote.OptV2()
+}
+-- quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quote
+
+import (
+	"os"
+	"testing"
+)
+
+func init() {
+	os.Setenv("LC_ALL", "en")
+}
+
+func TestHello(t *testing.T) {
+	hello := "Hello, world."
+	if out := Hello(); out != hello {
+		t.Errorf("Hello() = %q, want %q", out, hello)
+	}
+}
+
+func TestGlass(t *testing.T) {
+	glass := "I can eat glass and it doesn't hurt me."
+	if out := Glass(); out != glass {
+		t.Errorf("Glass() = %q, want %q", out, glass)
+	}
+}
+
+func TestGo(t *testing.T) {
+	go1 := "Don't communicate by sharing memory, share memory by communicating."
+	if out := Go(); out != go1 {
+		t.Errorf("Go() = %q, want %q", out, go1)
+	}
+}
+
+func TestOpt(t *testing.T) {
+	opt := "If a program is too slow, it must have a loop."
+	if out := Opt(); out != opt {
+		t.Errorf("Opt() = %q, want %q", out, opt)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180709162749-b44a0b17b2d1.txt b/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180709162749-b44a0b17b2d1.txt
new file mode 100644
index 0000000..f7f794d
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180709162749-b44a0b17b2d1.txt
@@ -0,0 +1,104 @@
+rsc.io/quote@v0.0.0-20180709162749-b44a0b17b2d1
+
+-- .mod --
+module rsc.io/quote
+
+require (
+	rsc.io/quote/v2 v2.0.1
+	rsc.io/sampler v1.3.0
+)
+-- .info --
+{"Version":"v0.0.0-20180709162749-b44a0b17b2d1","Name":"b44a0b17b2d1fe4c98a8d0e7a68c9bf9e762799a","Short":"b44a0b17b2d1","Time":"2018-07-09T16:27:49Z"}
+-- buggy/buggy_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package buggy
+
+import "testing"
+
+func Test(t *testing.T) {
+	t.Fatal("buggy!")
+}
+-- go.mod --
+module rsc.io/quote
+
+require (
+	rsc.io/quote/v2 v2.0.1
+	rsc.io/sampler v1.3.0
+)
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+import "rsc.io/quote/v2"
+
+// Hello returns a greeting.
+func Hello() string {
+	return quote.HelloV2()
+}
+
+// Glass returns a useful phrase for world travelers.
+func Glass() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return quote.GlassV2()
+}
+
+// Go returns a Go proverb.
+func Go() string {
+	return quote.GoV2()
+}
+
+// Opt returns an optimization truth.
+func Opt() string {
+	// Wisdom from ken.
+	return quote.OptV2()
+}
+-- quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quote
+
+import (
+	"os"
+	"testing"
+)
+
+func init() {
+	os.Setenv("LC_ALL", "en")
+}
+
+func TestHello(t *testing.T) {
+	hello := "Hello, world."
+	if out := Hello(); out != hello {
+		t.Errorf("Hello() = %q, want %q", out, hello)
+	}
+}
+
+func TestGlass(t *testing.T) {
+	glass := "I can eat glass and it doesn't hurt me."
+	if out := Glass(); out != glass {
+		t.Errorf("Glass() = %q, want %q", out, glass)
+	}
+}
+
+func TestGo(t *testing.T) {
+	go1 := "Don't communicate by sharing memory, share memory by communicating."
+	if out := Go(); out != go1 {
+		t.Errorf("Go() = %q, want %q", out, go1)
+	}
+}
+
+func TestOpt(t *testing.T) {
+	opt := "If a program is too slow, it must have a loop."
+	if out := Opt(); out != opt {
+		t.Errorf("Opt() = %q, want %q", out, opt)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180709162816-fe488b867524.txt b/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180709162816-fe488b867524.txt
new file mode 100644
index 0000000..2d5d8b4
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180709162816-fe488b867524.txt
@@ -0,0 +1,104 @@
+rsc.io/quote@v0.0.0-20180709162816-fe488b867524
+
+-- .mod --
+module rsc.io/quote
+
+require (
+	rsc.io/quote/v2 v2.0.1
+	rsc.io/sampler v1.3.0
+)
+-- .info --
+{"Version":"v0.0.0-20180709162816-fe488b867524","Name":"fe488b867524806e861c3f4f43ae6946a42ca3f1","Short":"fe488b867524","Time":"2018-07-09T16:28:16Z"}
+-- buggy/buggy_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package buggy
+
+import "testing"
+
+func Test(t *testing.T) {
+	t.Fatal("buggy!")
+}
+-- go.mod --
+module rsc.io/quote
+
+require (
+	rsc.io/quote/v2 v2.0.1
+	rsc.io/sampler v1.3.0
+)
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+import "rsc.io/quote/v2"
+
+// Hello returns a greeting.
+func Hello() string {
+	return quote.HelloV2()
+}
+
+// Glass returns a useful phrase for world travelers.
+func Glass() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return quote.GlassV2()
+}
+
+// Go returns a Go proverb.
+func Go() string {
+	return quote.GoV2()
+}
+
+// Opt returns an optimization truth.
+func Opt() string {
+	// Wisdom from ken.
+	return quote.OptV2()
+}
+-- quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quote
+
+import (
+	"os"
+	"testing"
+)
+
+func init() {
+	os.Setenv("LC_ALL", "en")
+}
+
+func TestHello(t *testing.T) {
+	hello := "Hello, world."
+	if out := Hello(); out != hello {
+		t.Errorf("Hello() = %q, want %q", out, hello)
+	}
+}
+
+func TestGlass(t *testing.T) {
+	glass := "I can eat glass and it doesn't hurt me."
+	if out := Glass(); out != glass {
+		t.Errorf("Glass() = %q, want %q", out, glass)
+	}
+}
+
+func TestGo(t *testing.T) {
+	go1 := "Don't communicate by sharing memory, share memory by communicating."
+	if out := Go(); out != go1 {
+		t.Errorf("Go() = %q, want %q", out, go1)
+	}
+}
+
+func TestOpt(t *testing.T) {
+	opt := "If a program is too slow, it must have a loop."
+	if out := Opt(); out != opt {
+		t.Errorf("Opt() = %q, want %q", out, opt)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180709162918-a91498bed0a7.txt b/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180709162918-a91498bed0a7.txt
new file mode 100644
index 0000000..853a8c2
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180709162918-a91498bed0a7.txt
@@ -0,0 +1,98 @@
+rsc.io/quote@v0.0.0-20180709162918-a91498bed0a7
+
+-- .mod --
+module rsc.io/quote
+
+require rsc.io/sampler v1.3.0
+-- .info --
+{"Version":"v0.0.0-20180709162918-a91498bed0a7","Name":"a91498bed0a73d4bb9c1fb2597925f7883bc40a7","Short":"a91498bed0a7","Time":"2018-07-09T16:29:18Z"}
+-- buggy/buggy_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package buggy
+
+import "testing"
+
+func Test(t *testing.T) {
+	t.Fatal("buggy!")
+}
+-- go.mod --
+module rsc.io/quote
+
+require rsc.io/sampler v1.3.0
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+import "rsc.io/quote/v3"
+
+// Hello returns a greeting.
+func Hello() string {
+	return quote.HelloV3()
+}
+
+// Glass returns a useful phrase for world travelers.
+func Glass() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return quote.GlassV3()
+}
+
+// Go returns a Go proverb.
+func Go() string {
+	return quote.GoV3()
+}
+
+// Opt returns an optimization truth.
+func Opt() string {
+	// Wisdom from ken.
+	return quote.OptV3()
+}
+-- quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quote
+
+import (
+	"os"
+	"testing"
+)
+
+func init() {
+	os.Setenv("LC_ALL", "en")
+}
+
+func TestHello(t *testing.T) {
+	hello := "Hello, world."
+	if out := Hello(); out != hello {
+		t.Errorf("Hello() = %q, want %q", out, hello)
+	}
+}
+
+func TestGlass(t *testing.T) {
+	glass := "I can eat glass and it doesn't hurt me."
+	if out := Glass(); out != glass {
+		t.Errorf("Glass() = %q, want %q", out, glass)
+	}
+}
+
+func TestGo(t *testing.T) {
+	go1 := "Don't communicate by sharing memory, share memory by communicating."
+	if out := Go(); out != go1 {
+		t.Errorf("Go() = %q, want %q", out, go1)
+	}
+}
+
+func TestOpt(t *testing.T) {
+	opt := "If a program is too slow, it must have a loop."
+	if out := Opt(); out != opt {
+		t.Errorf("Opt() = %q, want %q", out, opt)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180710144737-5d9f230bcfba.txt b/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180710144737-5d9f230bcfba.txt
new file mode 100644
index 0000000..2ebeac3
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_quote_v0.0.0-20180710144737-5d9f230bcfba.txt
@@ -0,0 +1,104 @@
+rsc.io/quote@v0.0.0-20180710144737-5d9f230bcfba
+
+-- .mod --
+module rsc.io/quote
+
+require (
+	rsc.io/quote/v3 v3.0.0
+	rsc.io/sampler v1.3.0
+)
+-- .info --
+{"Version":"v0.0.0-20180710144737-5d9f230bcfba","Name":"5d9f230bcfbae514bb6c2215694c2ce7273fc604","Short":"5d9f230bcfba","Time":"2018-07-10T14:47:37Z"}
+-- buggy/buggy_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package buggy
+
+import "testing"
+
+func Test(t *testing.T) {
+	t.Fatal("buggy!")
+}
+-- go.mod --
+module rsc.io/quote
+
+require (
+	rsc.io/quote/v3 v3.0.0
+	rsc.io/sampler v1.3.0
+)
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+import "rsc.io/quote/v3"
+
+// Hello returns a greeting.
+func Hello() string {
+	return quote.HelloV3()
+}
+
+// Glass returns a useful phrase for world travelers.
+func Glass() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return quote.GlassV3()
+}
+
+// Go returns a Go proverb.
+func Go() string {
+	return quote.GoV3()
+}
+
+// Opt returns an optimization truth.
+func Opt() string {
+	// Wisdom from ken.
+	return quote.OptV3()
+}
+-- quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quote
+
+import (
+	"os"
+	"testing"
+)
+
+func init() {
+	os.Setenv("LC_ALL", "en")
+}
+
+func TestHello(t *testing.T) {
+	hello := "Hello, world."
+	if out := Hello(); out != hello {
+		t.Errorf("Hello() = %q, want %q", out, hello)
+	}
+}
+
+func TestGlass(t *testing.T) {
+	glass := "I can eat glass and it doesn't hurt me."
+	if out := Glass(); out != glass {
+		t.Errorf("Glass() = %q, want %q", out, glass)
+	}
+}
+
+func TestGo(t *testing.T) {
+	go1 := "Don't communicate by sharing memory, share memory by communicating."
+	if out := Go(); out != go1 {
+		t.Errorf("Go() = %q, want %q", out, go1)
+	}
+}
+
+func TestOpt(t *testing.T) {
+	opt := "If a program is too slow, it must have a loop."
+	if out := Opt(); out != opt {
+		t.Errorf("Opt() = %q, want %q", out, opt)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_quote_v1.0.0.txt b/src/cmd/go/testdata/mod/rsc.io_quote_v1.0.0.txt
new file mode 100644
index 0000000..9a07937
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_quote_v1.0.0.txt
@@ -0,0 +1,35 @@
+rsc.io/quote@v1.0.0
+
+-- .mod --
+module "rsc.io/quote"
+-- .info --
+{"Version":"v1.0.0","Name":"f488df80bcdbd3e5bafdc24ad7d1e79e83edd7e6","Short":"f488df80bcdb","Time":"2018-02-14T00:45:20Z"}
+-- go.mod --
+module "rsc.io/quote"
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+// Hello returns a greeting.
+func Hello() string {
+	return "Hello, world."
+}
+-- quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quote
+
+import "testing"
+
+func TestHello(t *testing.T) {
+	hello := "Hello, world."
+	if out := Hello(); out != hello {
+		t.Errorf("Hello() = %q, want %q", out, hello)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_quote_v1.1.0.txt b/src/cmd/go/testdata/mod/rsc.io_quote_v1.1.0.txt
new file mode 100644
index 0000000..0c41605
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_quote_v1.1.0.txt
@@ -0,0 +1,48 @@
+rsc.io/quote@v1.1.0
+
+-- .mod --
+module "rsc.io/quote"
+-- .info --
+{"Version":"v1.1.0","Name":"cfd7145f43f92a8d56b4a3dd603795a3291381a9","Short":"cfd7145f43f9","Time":"2018-02-14T00:46:44Z"}
+-- go.mod --
+module "rsc.io/quote"
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+// Hello returns a greeting.
+func Hello() string {
+	return "Hello, world."
+}
+
+// Glass returns a useful phrase for world travelers.
+func Glass() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return "I can eat glass and it doesn't hurt me."
+}
+-- quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quote
+
+import "testing"
+
+func TestHello(t *testing.T) {
+	hello := "Hello, world."
+	if out := Hello(); out != hello {
+		t.Errorf("Hello() = %q, want %q", out, hello)
+	}
+}
+
+func TestGlass(t *testing.T) {
+	glass := "I can eat glass and it doesn't hurt me."
+	if out := Glass(); out != glass {
+		t.Errorf("Glass() = %q, want %q", out, glass)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_quote_v1.2.0.txt b/src/cmd/go/testdata/mod/rsc.io_quote_v1.2.0.txt
new file mode 100644
index 0000000..e714f0b
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_quote_v1.2.0.txt
@@ -0,0 +1,61 @@
+rsc.io/quote@v1.2.0
+
+-- .mod --
+module "rsc.io/quote"
+-- .info --
+{"Version":"v1.2.0","Name":"d8a3de91045c932a1c71e545308fe97571d6d65c","Short":"d8a3de91045c","Time":"2018-02-14T00:47:51Z"}
+-- go.mod --
+module "rsc.io/quote"
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+// Hello returns a greeting.
+func Hello() string {
+	return "Hello, world."
+}
+
+// Glass returns a useful phrase for world travelers.
+func Glass() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return "I can eat glass and it doesn't hurt me."
+}
+
+// Go returns a Go proverb.
+func Go() string {
+	return "Don't communicate by sharing memory, share memory by communicating."
+}
+-- quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quote
+
+import "testing"
+
+func TestHello(t *testing.T) {
+	hello := "Hello, world."
+	if out := Hello(); out != hello {
+		t.Errorf("Hello() = %q, want %q", out, hello)
+	}
+}
+
+func TestGlass(t *testing.T) {
+	glass := "I can eat glass and it doesn't hurt me."
+	if out := Glass(); out != glass {
+		t.Errorf("Glass() = %q, want %q", out, glass)
+	}
+}
+
+// Go returns a Go proverb.
+func TestGo(t *testing.T) {
+	go1 := "Don't communicate by sharing memory. Share memory by communicating."
+	if out := Go(); out != go1 {
+		t.Errorf("Go() = %q, want %q", out, go1)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_quote_v1.2.1.txt b/src/cmd/go/testdata/mod/rsc.io_quote_v1.2.1.txt
new file mode 100644
index 0000000..89d5191
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_quote_v1.2.1.txt
@@ -0,0 +1,60 @@
+rsc.io/quote@v1.2.1
+
+-- .mod --
+module "rsc.io/quote"
+-- .info --
+{"Version":"v1.2.1","Name":"5c1f03b64ab7aa958798a569a31924655dc41e76","Short":"5c1f03b64ab7","Time":"2018-02-14T00:54:20Z"}
+-- go.mod --
+module "rsc.io/quote"
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+// Hello returns a greeting.
+func Hello() string {
+	return "Hello, world."
+}
+
+// Glass returns a useful phrase for world travelers.
+func Glass() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return "I can eat glass and it doesn't hurt me."
+}
+
+// Go returns a Go proverb.
+func Go() string {
+	return "Don't communicate by sharing memory, share memory by communicating."
+}
+-- quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quote
+
+import "testing"
+
+func TestHello(t *testing.T) {
+	hello := "Hello, world."
+	if out := Hello(); out != hello {
+		t.Errorf("Hello() = %q, want %q", out, hello)
+	}
+}
+
+func TestGlass(t *testing.T) {
+	glass := "I can eat glass and it doesn't hurt me."
+	if out := Glass(); out != glass {
+		t.Errorf("Glass() = %q, want %q", out, glass)
+	}
+}
+
+func TestGo(t *testing.T) {
+	go1 := "Don't communicate by sharing memory, share memory by communicating."
+	if out := Go(); out != go1 {
+		t.Errorf("Go() = %q, want %q", out, go1)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_quote_v1.3.0.txt b/src/cmd/go/testdata/mod/rsc.io_quote_v1.3.0.txt
new file mode 100644
index 0000000..d62766c
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_quote_v1.3.0.txt
@@ -0,0 +1,73 @@
+rsc.io/quote@v1.3.0
+
+-- .mod --
+module "rsc.io/quote"
+-- .info --
+{"Version":"v1.3.0","Name":"84de74b35823c1e49634f2262f1a58cfc951ebae","Short":"84de74b35823","Time":"2018-02-14T00:54:53Z"}
+-- go.mod --
+module "rsc.io/quote"
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+// Hello returns a greeting.
+func Hello() string {
+	return "Hello, world."
+}
+
+// Glass returns a useful phrase for world travelers.
+func Glass() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return "I can eat glass and it doesn't hurt me."
+}
+
+// Go returns a Go proverb.
+func Go() string {
+	return "Don't communicate by sharing memory, share memory by communicating."
+}
+
+// Opt returns an optimization truth.
+func Opt() string {
+	// Wisdom from ken.
+	return "If a program is too slow, it must have a loop."
+}
+-- quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quote
+
+import "testing"
+
+func TestHello(t *testing.T) {
+	hello := "Hello, world."
+	if out := Hello(); out != hello {
+		t.Errorf("Hello() = %q, want %q", out, hello)
+	}
+}
+
+func TestGlass(t *testing.T) {
+	glass := "I can eat glass and it doesn't hurt me."
+	if out := Glass(); out != glass {
+		t.Errorf("Glass() = %q, want %q", out, glass)
+	}
+}
+
+func TestGo(t *testing.T) {
+	go1 := "Don't communicate by sharing memory, share memory by communicating."
+	if out := Go(); out != go1 {
+		t.Errorf("Go() = %q, want %q", out, go1)
+	}
+}
+
+func TestOpt(t *testing.T) {
+	opt := "If a program is too slow, it must have a loop."
+	if out := Opt(); out != opt {
+		t.Errorf("Opt() = %q, want %q", out, opt)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_quote_v1.4.0.txt b/src/cmd/go/testdata/mod/rsc.io_quote_v1.4.0.txt
new file mode 100644
index 0000000..698ff8d
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_quote_v1.4.0.txt
@@ -0,0 +1,79 @@
+rsc.io/quote@v1.4.0
+
+-- .mod --
+module "rsc.io/quote"
+
+require "rsc.io/sampler" v1.0.0
+-- .info --
+{"Version":"v1.4.0","Name":"19e8b977bd2f437798c2cc2dcfe8a1c0f169481b","Short":"19e8b977bd2f","Time":"2018-02-14T00:56:05Z"}
+-- go.mod --
+module "rsc.io/quote"
+
+require "rsc.io/sampler" v1.0.0
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+import "rsc.io/sampler"
+
+// Hello returns a greeting.
+func Hello() string {
+	return sampler.Hello()
+}
+
+// Glass returns a useful phrase for world travelers.
+func Glass() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return "I can eat glass and it doesn't hurt me."
+}
+
+// Go returns a Go proverb.
+func Go() string {
+	return "Don't communicate by sharing memory, share memory by communicating."
+}
+
+// Opt returns an optimization truth.
+func Opt() string {
+	// Wisdom from ken.
+	return "If a program is too slow, it must have a loop."
+}
+-- quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quote
+
+import "testing"
+
+func TestHello(t *testing.T) {
+	hello := "Hello, world."
+	if out := Hello(); out != hello {
+		t.Errorf("Hello() = %q, want %q", out, hello)
+	}
+}
+
+func TestGlass(t *testing.T) {
+	glass := "I can eat glass and it doesn't hurt me."
+	if out := Glass(); out != glass {
+		t.Errorf("Glass() = %q, want %q", out, glass)
+	}
+}
+
+func TestGo(t *testing.T) {
+	go1 := "Don't communicate by sharing memory, share memory by communicating."
+	if out := Go(); out != go1 {
+		t.Errorf("Go() = %q, want %q", out, go1)
+	}
+}
+
+func TestOpt(t *testing.T) {
+	opt := "If a program is too slow, it must have a loop."
+	if out := Opt(); out != opt {
+		t.Errorf("Opt() = %q, want %q", out, opt)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_quote_v1.5.0.txt b/src/cmd/go/testdata/mod/rsc.io_quote_v1.5.0.txt
new file mode 100644
index 0000000..e7fcdbc
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_quote_v1.5.0.txt
@@ -0,0 +1,79 @@
+rsc.io/quote@v1.5.0
+
+-- .mod --
+module "rsc.io/quote"
+
+require "rsc.io/sampler" v1.3.0
+-- .info --
+{"Version":"v1.5.0","Name":"3ba1e30dc83bd52c990132b9dfb1688a9d22de13","Short":"3ba1e30dc83b","Time":"2018-02-14T00:58:15Z"}
+-- go.mod --
+module "rsc.io/quote"
+
+require "rsc.io/sampler" v1.3.0
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+import "rsc.io/sampler"
+
+// Hello returns a greeting.
+func Hello() string {
+	return sampler.Hello()
+}
+
+// Glass returns a useful phrase for world travelers.
+func Glass() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return "I can eat glass and it doesn't hurt me."
+}
+
+// Go returns a Go proverb.
+func Go() string {
+	return "Don't communicate by sharing memory, share memory by communicating."
+}
+
+// Opt returns an optimization truth.
+func Opt() string {
+	// Wisdom from ken.
+	return "If a program is too slow, it must have a loop."
+}
+-- quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quote
+
+import "testing"
+
+func TestHello(t *testing.T) {
+	hello := "Hello, world."
+	if out := Hello(); out != hello {
+		t.Errorf("Hello() = %q, want %q", out, hello)
+	}
+}
+
+func TestGlass(t *testing.T) {
+	glass := "I can eat glass and it doesn't hurt me."
+	if out := Glass(); out != glass {
+		t.Errorf("Glass() = %q, want %q", out, glass)
+	}
+}
+
+func TestGo(t *testing.T) {
+	go1 := "Don't communicate by sharing memory, share memory by communicating."
+	if out := Go(); out != go1 {
+		t.Errorf("Go() = %q, want %q", out, go1)
+	}
+}
+
+func TestOpt(t *testing.T) {
+	opt := "If a program is too slow, it must have a loop."
+	if out := Opt(); out != opt {
+		t.Errorf("Opt() = %q, want %q", out, opt)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_quote_v1.5.1.txt b/src/cmd/go/testdata/mod/rsc.io_quote_v1.5.1.txt
new file mode 100644
index 0000000..eed051b
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_quote_v1.5.1.txt
@@ -0,0 +1,86 @@
+rsc.io/quote@23179ee8a569
+
+-- .mod --
+module "rsc.io/quote"
+
+require "rsc.io/sampler" v1.3.0
+-- .info --
+{"Version":"v1.5.1","Name":"23179ee8a569bb05d896ae05c6503ec69a19f99f","Short":"23179ee8a569","Time":"2018-02-14T00:58:40Z"}
+-- go.mod --
+module "rsc.io/quote"
+
+require "rsc.io/sampler" v1.3.0
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+import "rsc.io/sampler"
+
+// Hello returns a greeting.
+func Hello() string {
+	return sampler.Hello()
+}
+
+// Glass returns a useful phrase for world travelers.
+func Glass() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return "I can eat glass and it doesn't hurt me."
+}
+
+// Go returns a Go proverb.
+func Go() string {
+	return "Don't communicate by sharing memory, share memory by communicating."
+}
+
+// Opt returns an optimization truth.
+func Opt() string {
+	// Wisdom from ken.
+	return "If a program is too slow, it must have a loop."
+}
+-- quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quote
+
+import (
+	"os"
+	"testing"
+)
+
+func init() {
+	os.Setenv("LC_ALL", "en")
+}
+
+func TestHello(t *testing.T) {
+	hello := "Hello, world."
+	if out := Hello(); out != hello {
+		t.Errorf("Hello() = %q, want %q", out, hello)
+	}
+}
+
+func TestGlass(t *testing.T) {
+	glass := "I can eat glass and it doesn't hurt me."
+	if out := Glass(); out != glass {
+		t.Errorf("Glass() = %q, want %q", out, glass)
+	}
+}
+
+func TestGo(t *testing.T) {
+	go1 := "Don't communicate by sharing memory, share memory by communicating."
+	if out := Go(); out != go1 {
+		t.Errorf("Go() = %q, want %q", out, go1)
+	}
+}
+
+func TestOpt(t *testing.T) {
+	opt := "If a program is too slow, it must have a loop."
+	if out := Opt(); out != opt {
+		t.Errorf("Opt() = %q, want %q", out, opt)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_quote_v1.5.2.txt b/src/cmd/go/testdata/mod/rsc.io_quote_v1.5.2.txt
new file mode 100644
index 0000000..8671f6f
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_quote_v1.5.2.txt
@@ -0,0 +1,98 @@
+rsc.io/quote@v1.5.2
+
+-- .mod --
+module "rsc.io/quote"
+
+require "rsc.io/sampler" v1.3.0
+-- .info --
+{"Version":"v1.5.2","Name":"c4d4236f92427c64bfbcf1cc3f8142ab18f30b22","Short":"c4d4236f9242","Time":"2018-02-14T15:44:20Z"}
+-- buggy/buggy_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package buggy
+
+import "testing"
+
+func Test(t *testing.T) {
+	t.Fatal("buggy!")
+}
+-- go.mod --
+module "rsc.io/quote"
+
+require "rsc.io/sampler" v1.3.0
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+import "rsc.io/sampler"
+
+// Hello returns a greeting.
+func Hello() string {
+	return sampler.Hello()
+}
+
+// Glass returns a useful phrase for world travelers.
+func Glass() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return "I can eat glass and it doesn't hurt me."
+}
+
+// Go returns a Go proverb.
+func Go() string {
+	return "Don't communicate by sharing memory, share memory by communicating."
+}
+
+// Opt returns an optimization truth.
+func Opt() string {
+	// Wisdom from ken.
+	return "If a program is too slow, it must have a loop."
+}
+-- quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quote
+
+import (
+	"os"
+	"testing"
+)
+
+func init() {
+	os.Setenv("LC_ALL", "en")
+}
+
+func TestHello(t *testing.T) {
+	hello := "Hello, world."
+	if out := Hello(); out != hello {
+		t.Errorf("Hello() = %q, want %q", out, hello)
+	}
+}
+
+func TestGlass(t *testing.T) {
+	glass := "I can eat glass and it doesn't hurt me."
+	if out := Glass(); out != glass {
+		t.Errorf("Glass() = %q, want %q", out, glass)
+	}
+}
+
+func TestGo(t *testing.T) {
+	go1 := "Don't communicate by sharing memory, share memory by communicating."
+	if out := Go(); out != go1 {
+		t.Errorf("Go() = %q, want %q", out, go1)
+	}
+}
+
+func TestOpt(t *testing.T) {
+	opt := "If a program is too slow, it must have a loop."
+	if out := Opt(); out != opt {
+		t.Errorf("Opt() = %q, want %q", out, opt)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_quote_v1.5.3-pre1.txt b/src/cmd/go/testdata/mod/rsc.io_quote_v1.5.3-pre1.txt
new file mode 100644
index 0000000..212ef13
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_quote_v1.5.3-pre1.txt
@@ -0,0 +1,100 @@
+rsc.io/quote@v1.5.3-pre1
+
+-- .mod --
+module "rsc.io/quote"
+
+require "rsc.io/sampler" v1.3.0
+-- .info --
+{"Version":"v1.5.3-pre1","Name":"2473dfd877c95382420e47686aa9076bf58c79e0","Short":"2473dfd877c9","Time":"2018-06-28T00:32:53Z"}
+-- buggy/buggy_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package buggy
+
+import "testing"
+
+func Test(t *testing.T) {
+	t.Fatal("buggy!")
+}
+-- go.mod --
+module "rsc.io/quote"
+
+require "rsc.io/sampler" v1.3.0
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+import "rsc.io/sampler"
+
+// A CHANGE!
+
+// Hello returns a greeting.
+func Hello() string {
+	return sampler.Hello()
+}
+
+// Glass returns a useful phrase for world travelers.
+func Glass() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return "I can eat glass and it doesn't hurt me."
+}
+
+// Go returns a Go proverb.
+func Go() string {
+	return "Don't communicate by sharing memory, share memory by communicating."
+}
+
+// Opt returns an optimization truth.
+func Opt() string {
+	// Wisdom from ken.
+	return "If a program is too slow, it must have a loop."
+}
+-- quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quote
+
+import (
+	"os"
+	"testing"
+)
+
+func init() {
+	os.Setenv("LC_ALL", "en")
+}
+
+func TestHello(t *testing.T) {
+	hello := "Hello, world."
+	if out := Hello(); out != hello {
+		t.Errorf("Hello() = %q, want %q", out, hello)
+	}
+}
+
+func TestGlass(t *testing.T) {
+	glass := "I can eat glass and it doesn't hurt me."
+	if out := Glass(); out != glass {
+		t.Errorf("Glass() = %q, want %q", out, glass)
+	}
+}
+
+func TestGo(t *testing.T) {
+	go1 := "Don't communicate by sharing memory, share memory by communicating."
+	if out := Go(); out != go1 {
+		t.Errorf("Go() = %q, want %q", out, go1)
+	}
+}
+
+func TestOpt(t *testing.T) {
+	opt := "If a program is too slow, it must have a loop."
+	if out := Opt(); out != opt {
+		t.Errorf("Opt() = %q, want %q", out, opt)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_quote_v2.0.0.txt b/src/cmd/go/testdata/mod/rsc.io_quote_v2.0.0.txt
new file mode 100644
index 0000000..e461ed4
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_quote_v2.0.0.txt
@@ -0,0 +1,86 @@
+rsc.io/quote@v2.0.0
+
+-- .mod --
+module "rsc.io/quote"
+
+require "rsc.io/sampler" v1.3.0
+-- .info --
+{"Version":"v0.0.0-20180709153244-fd906ed3b100","Name":"fd906ed3b100e47181ffa9ec36d82294525c9109","Short":"fd906ed3b100","Time":"2018-07-09T15:32:44Z"}
+-- go.mod --
+module "rsc.io/quote"
+
+require "rsc.io/sampler" v1.3.0
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+import "rsc.io/sampler"
+
+// Hello returns a greeting.
+func HelloV2() string {
+	return sampler.Hello()
+}
+
+// Glass returns a useful phrase for world travelers.
+func GlassV2() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return "I can eat glass and it doesn't hurt me."
+}
+
+// Go returns a Go proverb.
+func GoV2() string {
+	return "Don't communicate by sharing memory, share memory by communicating."
+}
+
+// Opt returns an optimization truth.
+func OptV2() string {
+	// Wisdom from ken.
+	return "If a program is too slow, it must have a loop."
+}
+-- quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quote
+
+import (
+	"os"
+	"testing"
+)
+
+func init() {
+	os.Setenv("LC_ALL", "en")
+}
+
+func TestHello(t *testing.T) {
+	hello := "Hello, world."
+	if out := Hello(); out != hello {
+		t.Errorf("Hello() = %q, want %q", out, hello)
+	}
+}
+
+func TestGlass(t *testing.T) {
+	glass := "I can eat glass and it doesn't hurt me."
+	if out := Glass(); out != glass {
+		t.Errorf("Glass() = %q, want %q", out, glass)
+	}
+}
+
+func TestGo(t *testing.T) {
+	go1 := "Don't communicate by sharing memory, share memory by communicating."
+	if out := Go(); out != go1 {
+		t.Errorf("Go() = %q, want %q", out, go1)
+	}
+}
+
+func TestOpt(t *testing.T) {
+	opt := "If a program is too slow, it must have a loop."
+	if out := Opt(); out != opt {
+		t.Errorf("Opt() = %q, want %q", out, opt)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_quote_v2_v2.0.1.txt b/src/cmd/go/testdata/mod/rsc.io_quote_v2_v2.0.1.txt
new file mode 100644
index 0000000..d51128c
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_quote_v2_v2.0.1.txt
@@ -0,0 +1,86 @@
+rsc.io/quote/v2@v2.0.1
+
+-- .mod --
+module rsc.io/quote/v2
+
+require rsc.io/sampler v1.3.0
+-- .info --
+{"Version":"v2.0.1","Name":"754f68430672776c84704e2d10209a6ec700cd64","Short":"754f68430672","Time":"2018-07-09T16:25:34Z"}
+-- go.mod --
+module rsc.io/quote/v2
+
+require rsc.io/sampler v1.3.0
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+import "rsc.io/sampler"
+
+// Hello returns a greeting.
+func HelloV2() string {
+	return sampler.Hello()
+}
+
+// Glass returns a useful phrase for world travelers.
+func GlassV2() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return "I can eat glass and it doesn't hurt me."
+}
+
+// Go returns a Go proverb.
+func GoV2() string {
+	return "Don't communicate by sharing memory, share memory by communicating."
+}
+
+// Opt returns an optimization truth.
+func OptV2() string {
+	// Wisdom from ken.
+	return "If a program is too slow, it must have a loop."
+}
+-- quote_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package quote
+
+import (
+	"os"
+	"testing"
+)
+
+func init() {
+	os.Setenv("LC_ALL", "en")
+}
+
+func TestHello(t *testing.T) {
+	hello := "Hello, world."
+	if out := Hello(); out != hello {
+		t.Errorf("Hello() = %q, want %q", out, hello)
+	}
+}
+
+func TestGlass(t *testing.T) {
+	glass := "I can eat glass and it doesn't hurt me."
+	if out := Glass(); out != glass {
+		t.Errorf("Glass() = %q, want %q", out, glass)
+	}
+}
+
+func TestGo(t *testing.T) {
+	go1 := "Don't communicate by sharing memory, share memory by communicating."
+	if out := Go(); out != go1 {
+		t.Errorf("Go() = %q, want %q", out, go1)
+	}
+}
+
+func TestOpt(t *testing.T) {
+	opt := "If a program is too slow, it must have a loop."
+	if out := Opt(); out != opt {
+		t.Errorf("Opt() = %q, want %q", out, opt)
+	}
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_quote_v3_v3.0.0.txt b/src/cmd/go/testdata/mod/rsc.io_quote_v3_v3.0.0.txt
new file mode 100644
index 0000000..0afe1f0
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_quote_v3_v3.0.0.txt
@@ -0,0 +1,45 @@
+rsc.io/quote/v3@v3.0.0
+
+-- .mod --
+module rsc.io/quote/v3
+
+require rsc.io/sampler v1.3.0
+
+-- .info --
+{"Version":"v3.0.0","Name":"d88915d7e77ed0fd35d0a022a2f244e2202fd8c8","Short":"d88915d7e77e","Time":"2018-07-09T15:34:46Z"}
+-- go.mod --
+module rsc.io/quote/v3
+
+require rsc.io/sampler v1.3.0
+
+-- quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote // import "rsc.io/quote"
+
+import "rsc.io/sampler"
+
+// Hello returns a greeting.
+func HelloV3() string {
+	return sampler.Hello()
+}
+
+// Glass returns a useful phrase for world travelers.
+func GlassV3() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return "I can eat glass and it doesn't hurt me."
+}
+
+// Go returns a Go proverb.
+func GoV3() string {
+	return "Don't communicate by sharing memory, share memory by communicating."
+}
+
+// Opt returns an optimization truth.
+func OptV3() string {
+	// Wisdom from ken.
+	return "If a program is too slow, it must have a loop."
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_sampler_v1.0.0.txt b/src/cmd/go/testdata/mod/rsc.io_sampler_v1.0.0.txt
new file mode 100644
index 0000000..c4b6a71
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_sampler_v1.0.0.txt
@@ -0,0 +1,20 @@
+rsc.io/sampler@v1.0.0
+
+-- .mod --
+module "rsc.io/sampler"
+-- .info --
+{"Version":"v1.0.0","Name":"60bef405c52117ad21d2adb10872b95cf17f8fca","Short":"60bef405c521","Time":"2018-02-13T18:05:54Z"}
+-- go.mod --
+module "rsc.io/sampler"
+-- sampler.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package sampler shows simple texts.
+package sampler // import "rsc.io/sampler"
+
+// Hello returns a greeting.
+func Hello() string {
+	return "Hello, world."
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_sampler_v1.2.0.txt b/src/cmd/go/testdata/mod/rsc.io_sampler_v1.2.0.txt
new file mode 100644
index 0000000..98c35fa
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_sampler_v1.2.0.txt
@@ -0,0 +1,138 @@
+rsc.io/sampler@v1.2.0
+
+-- .mod --
+module "rsc.io/sampler"
+
+require "golang.org/x/text" v0.0.0-20170915032832-14c0d48ead0c
+-- .info --
+{"Version":"v1.2.0","Name":"25f24110b153246056eccc14a3a4cd81afaff586","Short":"25f24110b153","Time":"2018-02-13T18:13:45Z"}
+-- go.mod --
+module "rsc.io/sampler"
+
+require "golang.org/x/text" v0.0.0-20170915032832-14c0d48ead0c
+-- hello.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Translations by Google Translate.
+
+package sampler
+
+var hello = newText(`
+
+English: en: Hello, world.
+French: fr: Bonjour le monde.
+Spanish: es: Hola Mundo.
+
+`)
+-- hello_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sampler
+
+import (
+	"testing"
+
+	"golang.org/x/text/language"
+)
+
+var helloTests = []struct {
+	prefs []language.Tag
+	text  string
+}{
+	{
+		[]language.Tag{language.Make("en-US"), language.Make("fr")},
+		"Hello, world.",
+	},
+	{
+		[]language.Tag{language.Make("fr"), language.Make("en-US")},
+		"Bonjour la monde.",
+	},
+}
+
+func TestHello(t *testing.T) {
+	for _, tt := range helloTests {
+		text := Hello(tt.prefs...)
+		if text != tt.text {
+			t.Errorf("Hello(%v) = %q, want %q", tt.prefs, text, tt.text)
+		}
+	}
+}
+-- sampler.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package sampler shows simple texts.
+package sampler // import "rsc.io/sampler"
+
+import (
+	"os"
+	"strings"
+
+	"golang.org/x/text/language"
+)
+
+// DefaultUserPrefs returns the default user language preferences.
+// It consults the $LC_ALL, $LC_MESSAGES, and $LANG environment
+// variables, in that order.
+func DefaultUserPrefs() []language.Tag {
+	var prefs []language.Tag
+	for _, k := range []string{"LC_ALL", "LC_MESSAGES", "LANG"} {
+		if env := os.Getenv(k); env != "" {
+			prefs = append(prefs, language.Make(env))
+		}
+	}
+	return prefs
+}
+
+// Hello returns a localized greeting.
+// If no prefs are given, Hello uses DefaultUserPrefs.
+func Hello(prefs ...language.Tag) string {
+	if len(prefs) == 0 {
+		prefs = DefaultUserPrefs()
+	}
+	return hello.find(prefs)
+}
+
+// A text is a localized text.
+type text struct {
+	byTag   map[string]string
+	matcher language.Matcher
+}
+
+// newText creates a new localized text, given a list of translations.
+func newText(s string) *text {
+	t := &text{
+		byTag: make(map[string]string),
+	}
+	var tags []language.Tag
+	for _, line := range strings.Split(s, "\n") {
+		line = strings.TrimSpace(line)
+		if line == "" {
+			continue
+		}
+		f := strings.Split(line, ": ")
+		if len(f) != 3 {
+			continue
+		}
+		tag := language.Make(f[1])
+		tags = append(tags, tag)
+		t.byTag[tag.String()] = f[2]
+	}
+	t.matcher = language.NewMatcher(tags)
+	return t
+}
+
+// find finds the text to use for the given language tag preferences.
+func (t *text) find(prefs []language.Tag) string {
+	tag, _, _ := t.matcher.Match(prefs...)
+	s := t.byTag[tag.String()]
+	if strings.HasPrefix(s, "RTL ") {
+		s = "\u200F" + strings.TrimPrefix(s, "RTL ") + "\u200E"
+	}
+	return s
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_sampler_v1.2.1.txt b/src/cmd/go/testdata/mod/rsc.io_sampler_v1.2.1.txt
new file mode 100644
index 0000000..7982ccc
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_sampler_v1.2.1.txt
@@ -0,0 +1,134 @@
+generated by ./addmod.bash rsc.io/sampler@v1.2.1
+
+-- .mod --
+module "rsc.io/sampler"
+
+require "golang.org/x/text" v0.0.0-20170915032832-14c0d48ead0c
+-- .info --
+{"Version":"v1.2.1","Name":"cac3af4f8a0ab40054fa6f8d423108a63a1255bb","Short":"cac3af4f8a0a","Time":"2018-02-13T18:16:22Z"}
+-- hello.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Translations by Google Translate.
+
+package sampler
+
+var hello = newText(`
+
+English: en: Hello, world.
+French: fr: Bonjour le monde.
+Spanish: es: Hola Mundo.
+
+`)
+-- hello_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sampler
+
+import (
+	"testing"
+
+	"golang.org/x/text/language"
+)
+
+var helloTests = []struct {
+	prefs []language.Tag
+	text  string
+}{
+	{
+		[]language.Tag{language.Make("en-US"), language.Make("fr")},
+		"Hello, world.",
+	},
+	{
+		[]language.Tag{language.Make("fr"), language.Make("en-US")},
+		"Bonjour le monde.",
+	},
+}
+
+func TestHello(t *testing.T) {
+	for _, tt := range helloTests {
+		text := Hello(tt.prefs...)
+		if text != tt.text {
+			t.Errorf("Hello(%v) = %q, want %q", tt.prefs, text, tt.text)
+		}
+	}
+}
+-- sampler.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package sampler shows simple texts.
+package sampler // import "rsc.io/sampler"
+
+import (
+	"os"
+	"strings"
+
+	"golang.org/x/text/language"
+)
+
+// DefaultUserPrefs returns the default user language preferences.
+// It consults the $LC_ALL, $LC_MESSAGES, and $LANG environment
+// variables, in that order.
+func DefaultUserPrefs() []language.Tag {
+	var prefs []language.Tag
+	for _, k := range []string{"LC_ALL", "LC_MESSAGES", "LANG"} {
+		if env := os.Getenv(k); env != "" {
+			prefs = append(prefs, language.Make(env))
+		}
+	}
+	return prefs
+}
+
+// Hello returns a localized greeting.
+// If no prefs are given, Hello uses DefaultUserPrefs.
+func Hello(prefs ...language.Tag) string {
+	if len(prefs) == 0 {
+		prefs = DefaultUserPrefs()
+	}
+	return hello.find(prefs)
+}
+
+// A text is a localized text.
+type text struct {
+	byTag   map[string]string
+	matcher language.Matcher
+}
+
+// newText creates a new localized text, given a list of translations.
+func newText(s string) *text {
+	t := &text{
+		byTag: make(map[string]string),
+	}
+	var tags []language.Tag
+	for _, line := range strings.Split(s, "\n") {
+		line = strings.TrimSpace(line)
+		if line == "" {
+			continue
+		}
+		f := strings.Split(line, ": ")
+		if len(f) != 3 {
+			continue
+		}
+		tag := language.Make(f[1])
+		tags = append(tags, tag)
+		t.byTag[tag.String()] = f[2]
+	}
+	t.matcher = language.NewMatcher(tags)
+	return t
+}
+
+// find finds the text to use for the given language tag preferences.
+func (t *text) find(prefs []language.Tag) string {
+	tag, _, _ := t.matcher.Match(prefs...)
+	s := t.byTag[tag.String()]
+	if strings.HasPrefix(s, "RTL ") {
+		s = "\u200F" + strings.TrimPrefix(s, "RTL ") + "\u200E"
+	}
+	return s
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_sampler_v1.3.0.txt b/src/cmd/go/testdata/mod/rsc.io_sampler_v1.3.0.txt
new file mode 100644
index 0000000..febe51f
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_sampler_v1.3.0.txt
@@ -0,0 +1,202 @@
+rsc.io/sampler@v1.3.0
+
+-- .mod --
+module "rsc.io/sampler"
+
+require "golang.org/x/text" v0.0.0-20170915032832-14c0d48ead0c
+-- .info --
+{"Version":"v1.3.0","Name":"0cc034b51e57ed7832d4c67d526f75a900996e5c","Short":"0cc034b51e57","Time":"2018-02-13T19:05:03Z"}
+-- glass.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Translations from Frank da Cruz, Ethan Mollick, and many others.
+// See http://kermitproject.org/utf8.html.
+// http://www.oocities.org/nodotus/hbglass.html
+// https://en.wikipedia.org/wiki/I_Can_Eat_Glass
+
+package sampler
+
+var glass = newText(`
+
+English: en: I can eat glass and it doesn't hurt me.
+French: fr: Je peux manger du verre, ça ne me fait pas mal.
+Spanish: es: Puedo comer vidrio, no me hace daño.
+
+`)
+-- glass_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sampler
+
+import (
+	"testing"
+
+	"golang.org/x/text/language"
+	_ "rsc.io/testonly"
+)
+
+var glassTests = []struct {
+	prefs []language.Tag
+	text  string
+}{
+	{
+		[]language.Tag{language.Make("en-US"), language.Make("fr")},
+		"I can eat glass and it doesn't hurt me.",
+	},
+	{
+		[]language.Tag{language.Make("fr"), language.Make("en-US")},
+		"Je peux manger du verre, ça ne me fait pas mal.",
+	},
+}
+
+func TestGlass(t *testing.T) {
+	for _, tt := range glassTests {
+		text := Glass(tt.prefs...)
+		if text != tt.text {
+			t.Errorf("Glass(%v) = %q, want %q", tt.prefs, text, tt.text)
+		}
+	}
+}
+-- go.mod --
+module "rsc.io/sampler"
+
+require "golang.org/x/text" v0.0.0-20170915032832-14c0d48ead0c
+-- hello.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Translations by Google Translate.
+
+package sampler
+
+var hello = newText(`
+
+English: en: Hello, world.
+French: fr: Bonjour le monde.
+Spanish: es: Hola Mundo.
+
+`)
+-- hello_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sampler
+
+import (
+	"testing"
+
+	"golang.org/x/text/language"
+)
+
+var helloTests = []struct {
+	prefs []language.Tag
+	text  string
+}{
+	{
+		[]language.Tag{language.Make("en-US"), language.Make("fr")},
+		"Hello, world.",
+	},
+	{
+		[]language.Tag{language.Make("fr"), language.Make("en-US")},
+		"Bonjour le monde.",
+	},
+}
+
+func TestHello(t *testing.T) {
+	for _, tt := range helloTests {
+		text := Hello(tt.prefs...)
+		if text != tt.text {
+			t.Errorf("Hello(%v) = %q, want %q", tt.prefs, text, tt.text)
+		}
+	}
+}
+-- sampler.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package sampler shows simple texts.
+package sampler // import "rsc.io/sampler"
+
+import (
+	"os"
+	"strings"
+
+	"golang.org/x/text/language"
+)
+
+// DefaultUserPrefs returns the default user language preferences.
+// It consults the $LC_ALL, $LC_MESSAGES, and $LANG environment
+// variables, in that order.
+func DefaultUserPrefs() []language.Tag {
+	var prefs []language.Tag
+	for _, k := range []string{"LC_ALL", "LC_MESSAGES", "LANG"} {
+		if env := os.Getenv(k); env != "" {
+			prefs = append(prefs, language.Make(env))
+		}
+	}
+	return prefs
+}
+
+// Hello returns a localized greeting.
+// If no prefs are given, Hello uses DefaultUserPrefs.
+func Hello(prefs ...language.Tag) string {
+	if len(prefs) == 0 {
+		prefs = DefaultUserPrefs()
+	}
+	return hello.find(prefs)
+}
+
+// Glass returns a localized silly phrase.
+// If no prefs are given, Glass uses DefaultUserPrefs.
+func Glass(prefs ...language.Tag) string {
+	if len(prefs) == 0 {
+		prefs = DefaultUserPrefs()
+	}
+	return glass.find(prefs)
+}
+
+// A text is a localized text.
+type text struct {
+	byTag   map[string]string
+	matcher language.Matcher
+}
+
+// newText creates a new localized text, given a list of translations.
+func newText(s string) *text {
+	t := &text{
+		byTag: make(map[string]string),
+	}
+	var tags []language.Tag
+	for _, line := range strings.Split(s, "\n") {
+		line = strings.TrimSpace(line)
+		if line == "" {
+			continue
+		}
+		f := strings.Split(line, ": ")
+		if len(f) != 3 {
+			continue
+		}
+		tag := language.Make(f[1])
+		tags = append(tags, tag)
+		t.byTag[tag.String()] = f[2]
+	}
+	t.matcher = language.NewMatcher(tags)
+	return t
+}
+
+// find finds the text to use for the given language tag preferences.
+func (t *text) find(prefs []language.Tag) string {
+	tag, _, _ := t.matcher.Match(prefs...)
+	s := t.byTag[tag.String()]
+	if strings.HasPrefix(s, "RTL ") {
+		s = "\u200F" + strings.TrimPrefix(s, "RTL ") + "\u200E"
+	}
+	return s
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_sampler_v1.3.1.txt b/src/cmd/go/testdata/mod/rsc.io_sampler_v1.3.1.txt
new file mode 100644
index 0000000..a293f10
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_sampler_v1.3.1.txt
@@ -0,0 +1,201 @@
+rsc.io/sampler@v1.3.1
+
+-- .mod --
+module "rsc.io/sampler"
+
+require "golang.org/x/text" v0.0.0-20170915032832-14c0d48ead0c
+-- .info --
+{"Version":"v1.3.1","Name":"f545d0289d06e2add4556ea6a15fc4938014bf87","Short":"f545d0289d06","Time":"2018-02-14T16:34:12Z"}
+-- glass.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Translations from Frank da Cruz, Ethan Mollick, and many others.
+// See http://kermitproject.org/utf8.html.
+// http://www.oocities.org/nodotus/hbglass.html
+// https://en.wikipedia.org/wiki/I_Can_Eat_Glass
+
+package sampler
+
+var glass = newText(`
+
+English: en: I can eat glass and it doesn't hurt me.
+French: fr: Je peux manger du verre, ça ne me fait pas mal.
+Spanish: es: Puedo comer vidrio, no me hace daño.
+
+`)
+-- glass_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sampler
+
+import (
+	"testing"
+
+	"golang.org/x/text/language"
+)
+
+var glassTests = []struct {
+	prefs []language.Tag
+	text  string
+}{
+	{
+		[]language.Tag{language.Make("en-US"), language.Make("fr")},
+		"I can eat glass and it doesn't hurt me.",
+	},
+	{
+		[]language.Tag{language.Make("fr"), language.Make("en-US")},
+		"Je peux manger du verre, ça ne me fait pas mal.",
+	},
+}
+
+func TestGlass(t *testing.T) {
+	for _, tt := range glassTests {
+		text := Glass(tt.prefs...)
+		if text != tt.text {
+			t.Errorf("Glass(%v) = %q, want %q", tt.prefs, text, tt.text)
+		}
+	}
+}
+-- go.mod --
+module "rsc.io/sampler"
+
+require "golang.org/x/text" v0.0.0-20170915032832-14c0d48ead0c
+-- hello.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Translations by Google Translate.
+
+package sampler
+
+var hello = newText(`
+
+English: en: Hello, world.
+French: fr: Bonjour le monde.
+Spanish: es: Hola Mundo.
+
+`)
+-- hello_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sampler
+
+import (
+	"testing"
+
+	"golang.org/x/text/language"
+)
+
+var helloTests = []struct {
+	prefs []language.Tag
+	text  string
+}{
+	{
+		[]language.Tag{language.Make("en-US"), language.Make("fr")},
+		"Hello, world.",
+	},
+	{
+		[]language.Tag{language.Make("fr"), language.Make("en-US")},
+		"Bonjour le monde.",
+	},
+}
+
+func TestHello(t *testing.T) {
+	for _, tt := range helloTests {
+		text := Hello(tt.prefs...)
+		if text != tt.text {
+			t.Errorf("Hello(%v) = %q, want %q", tt.prefs, text, tt.text)
+		}
+	}
+}
+-- sampler.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package sampler shows simple texts in a variety of languages.
+package sampler // import "rsc.io/sampler"
+
+import (
+	"os"
+	"strings"
+
+	"golang.org/x/text/language"
+)
+
+// DefaultUserPrefs returns the default user language preferences.
+// It consults the $LC_ALL, $LC_MESSAGES, and $LANG environment
+// variables, in that order.
+func DefaultUserPrefs() []language.Tag {
+	var prefs []language.Tag
+	for _, k := range []string{"LC_ALL", "LC_MESSAGES", "LANG"} {
+		if env := os.Getenv(k); env != "" {
+			prefs = append(prefs, language.Make(env))
+		}
+	}
+	return prefs
+}
+
+// Hello returns a localized greeting.
+// If no prefs are given, Hello uses DefaultUserPrefs.
+func Hello(prefs ...language.Tag) string {
+	if len(prefs) == 0 {
+		prefs = DefaultUserPrefs()
+	}
+	return hello.find(prefs)
+}
+
+// Glass returns a localized silly phrase.
+// If no prefs are given, Glass uses DefaultUserPrefs.
+func Glass(prefs ...language.Tag) string {
+	if len(prefs) == 0 {
+		prefs = DefaultUserPrefs()
+	}
+	return glass.find(prefs)
+}
+
+// A text is a localized text.
+type text struct {
+	byTag   map[string]string
+	matcher language.Matcher
+}
+
+// newText creates a new localized text, given a list of translations.
+func newText(s string) *text {
+	t := &text{
+		byTag: make(map[string]string),
+	}
+	var tags []language.Tag
+	for _, line := range strings.Split(s, "\n") {
+		line = strings.TrimSpace(line)
+		if line == "" {
+			continue
+		}
+		f := strings.Split(line, ": ")
+		if len(f) != 3 {
+			continue
+		}
+		tag := language.Make(f[1])
+		tags = append(tags, tag)
+		t.byTag[tag.String()] = f[2]
+	}
+	t.matcher = language.NewMatcher(tags)
+	return t
+}
+
+// find finds the text to use for the given language tag preferences.
+func (t *text) find(prefs []language.Tag) string {
+	tag, _, _ := t.matcher.Match(prefs...)
+	s := t.byTag[tag.String()]
+	if strings.HasPrefix(s, "RTL ") {
+		s = "\u200F" + strings.TrimPrefix(s, "RTL ") + "\u200E"
+	}
+	return s
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_sampler_v1.99.99.txt b/src/cmd/go/testdata/mod/rsc.io_sampler_v1.99.99.txt
new file mode 100644
index 0000000..5036d20
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_sampler_v1.99.99.txt
@@ -0,0 +1,140 @@
+rsc.io/sampler@v1.99.99
+
+-- .mod --
+module "rsc.io/sampler"
+
+require "golang.org/x/text" v0.0.0-20170915032832-14c0d48ead0c
+-- .info --
+{"Version":"v1.99.99","Name":"732a3c400797d8835f2af34a9561f155bef85435","Short":"732a3c400797","Time":"2018-02-13T22:20:19Z"}
+-- go.mod --
+module "rsc.io/sampler"
+
+require "golang.org/x/text" v0.0.0-20170915032832-14c0d48ead0c
+-- hello.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Translations by Google Translate.
+
+package sampler
+
+var hello = newText(`
+
+English: en: 99 bottles of beer on the wall, 99 bottles of beer, ...
+
+`)
+-- hello_test.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sampler
+
+import (
+	"testing"
+
+	"golang.org/x/text/language"
+)
+
+var helloTests = []struct {
+	prefs []language.Tag
+	text  string
+}{
+	{
+		[]language.Tag{language.Make("en-US"), language.Make("fr")},
+		"Hello, world.",
+	},
+	{
+		[]language.Tag{language.Make("fr"), language.Make("en-US")},
+		"Bonjour le monde.",
+	},
+}
+
+func TestHello(t *testing.T) {
+	for _, tt := range helloTests {
+		text := Hello(tt.prefs...)
+		if text != tt.text {
+			t.Errorf("Hello(%v) = %q, want %q", tt.prefs, text, tt.text)
+		}
+	}
+}
+-- sampler.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package sampler shows simple texts.
+package sampler // import "rsc.io/sampler"
+
+import (
+	"os"
+	"strings"
+
+	"golang.org/x/text/language"
+)
+
+// DefaultUserPrefs returns the default user language preferences.
+// It consults the $LC_ALL, $LC_MESSAGES, and $LANG environment
+// variables, in that order.
+func DefaultUserPrefs() []language.Tag {
+	var prefs []language.Tag
+	for _, k := range []string{"LC_ALL", "LC_MESSAGES", "LANG"} {
+		if env := os.Getenv(k); env != "" {
+			prefs = append(prefs, language.Make(env))
+		}
+	}
+	return prefs
+}
+
+// Hello returns a localized greeting.
+// If no prefs are given, Hello uses DefaultUserPrefs.
+func Hello(prefs ...language.Tag) string {
+	if len(prefs) == 0 {
+		prefs = DefaultUserPrefs()
+	}
+	return hello.find(prefs)
+}
+
+func Glass() string {
+	return "I can eat glass and it doesn't hurt me."
+}
+
+// A text is a localized text.
+type text struct {
+	byTag   map[string]string
+	matcher language.Matcher
+}
+
+// newText creates a new localized text, given a list of translations.
+func newText(s string) *text {
+	t := &text{
+		byTag: make(map[string]string),
+	}
+	var tags []language.Tag
+	for _, line := range strings.Split(s, "\n") {
+		line = strings.TrimSpace(line)
+		if line == "" {
+			continue
+		}
+		f := strings.Split(line, ": ")
+		if len(f) != 3 {
+			continue
+		}
+		tag := language.Make(f[1])
+		tags = append(tags, tag)
+		t.byTag[tag.String()] = f[2]
+	}
+	t.matcher = language.NewMatcher(tags)
+	return t
+}
+
+// find finds the text to use for the given language tag preferences.
+func (t *text) find(prefs []language.Tag) string {
+	tag, _, _ := t.matcher.Match(prefs...)
+	s := t.byTag[tag.String()]
+	if strings.HasPrefix(s, "RTL ") {
+		s = "\u200F" + strings.TrimPrefix(s, "RTL ") + "\u200E"
+	}
+	return s
+}
diff --git a/src/cmd/go/testdata/mod/rsc.io_testonly_v1.0.0.txt b/src/cmd/go/testdata/mod/rsc.io_testonly_v1.0.0.txt
new file mode 100644
index 0000000..dfb8ca2
--- /dev/null
+++ b/src/cmd/go/testdata/mod/rsc.io_testonly_v1.0.0.txt
@@ -0,0 +1,9 @@
+rsc.io/testonly v1.0.0
+written by hand
+
+-- .mod --
+module rsc.io/testonly
+-- .info --
+{"Version":"v1.0.0"}
+-- testonly.go --
+package testonly
diff --git a/src/cmd/go/testdata/savedir.go b/src/cmd/go/testdata/savedir.go
new file mode 100644
index 0000000..9a3ed50
--- /dev/null
+++ b/src/cmd/go/testdata/savedir.go
@@ -0,0 +1,78 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build ignore
+// +build ignore
+
+// Savedir archives a directory tree as a txtar archive printed to standard output.
+//
+// Usage:
+//
+//	go run savedir.go /path/to/dir >saved.txt
+//
+// Typically the tree is later extracted during a test with tg.extract("testdata/saved.txt").
+package main
+
+import (
+	"flag"
+	"fmt"
+	"internal/txtar"
+	"io/fs"
+	"log"
+	"os"
+	"path/filepath"
+	"strings"
+	"unicode/utf8"
+)
+
+func usage() {
+	fmt.Fprintf(os.Stderr, "usage: go run savedir.go dir >saved.txt\n")
+	os.Exit(2)
+}
+
+const goCmd = "vgo"
+
+func main() {
+	flag.Usage = usage
+	flag.Parse()
+	if flag.NArg() != 1 {
+		usage()
+	}
+
+	log.SetPrefix("savedir: ")
+	log.SetFlags(0)
+
+	dir := flag.Arg(0)
+
+	a := new(txtar.Archive)
+	dir = filepath.Clean(dir)
+	filepath.WalkDir(dir, func(path string, info fs.DirEntry, err error) error {
+		if path == dir {
+			return nil
+		}
+		name := info.Name()
+		if strings.HasPrefix(name, ".") {
+			if info.IsDir() {
+				return filepath.SkipDir
+			}
+			return nil
+		}
+		if !info.Type().IsRegular() {
+			return nil
+		}
+		data, err := os.ReadFile(path)
+		if err != nil {
+			log.Fatal(err)
+		}
+		if !utf8.Valid(data) {
+			log.Printf("%s: ignoring invalid UTF-8 data", path)
+			return nil
+		}
+		a.Files = append(a.Files, txtar.File{Name: str.TrimFilePathPrefix(path, dir), Data: data})
+		return nil
+	})
+
+	data := txtar.Format(a)
+	os.Stdout.Write(data)
+}
diff --git a/src/cmd/go/testdata/script/README b/src/cmd/go/testdata/script/README
new file mode 100644
index 0000000..792a158
--- /dev/null
+++ b/src/cmd/go/testdata/script/README
@@ -0,0 +1,425 @@
+This file is generated by 'go generate cmd/go'. DO NOT EDIT.
+
+This directory holds test scripts *.txt run during 'go test cmd/go'.
+To run a specific script foo.txt
+
+	go test cmd/go -run=Script/^foo$
+
+In general script files should have short names: a few words, not whole sentences.
+The first word should be the general category of behavior being tested,
+often the name of a go subcommand (list, build, test, ...) or concept (vendor, pattern).
+
+Each script is a text archive (go doc internal/txtar).
+The script begins with an actual command script to run
+followed by the content of zero or more supporting files to
+create in the script's temporary file system before it starts executing.
+
+As an example, run_hello.txt says:
+
+	# hello world
+	go run hello.go
+	stderr 'hello world'
+	! stdout .
+
+	-- hello.go --
+	package main
+	func main() { println("hello world") }
+
+Each script runs in a fresh temporary work directory tree, available to scripts as $WORK.
+Scripts also have access to other environment variables, including:
+
+	GOARCH=<target GOARCH>
+	GOCACHE=<actual GOCACHE being used outside the test>
+	GOEXE=<executable file suffix: .exe on Windows, empty on other systems>
+	GOOS=<target GOOS>
+	GOPATH=$WORK/gopath
+	GOPROXY=<local module proxy serving from cmd/go/testdata/mod>
+	GOROOT=<actual GOROOT>
+	GOROOT_FINAL=<actual GOROOT_FINAL>
+	TESTGO_GOROOT=<GOROOT used to build cmd/go, for use in tests that may change GOROOT>
+	HOME=/no-home
+	PATH=<actual PATH>
+	TMPDIR=$WORK/tmp
+	GODEBUG=<actual GODEBUG>
+	devnull=<value of os.DevNull>
+	goversion=<current Go version; for example, 1.12>
+
+On Plan 9, the variables $path and $home are set instead of $PATH and $HOME.
+On Windows, the variables $USERPROFILE and $TMP are set instead of
+$HOME and $TMPDIR.
+
+The lines at the top of the script are a sequence of commands to be executed by
+a small script engine configured in ../../script_test.go (not the system shell).
+
+The scripts' supporting files are unpacked relative to $GOPATH/src
+(aka $WORK/gopath/src) and then the script begins execution in that directory as
+well. Thus the example above runs in $WORK/gopath/src with GOPATH=$WORK/gopath
+and $WORK/gopath/src/hello.go containing the listed contents.
+
+Each line of a script is parsed into a sequence of space-separated command
+words, with environment variable expansion within each word and # marking
+an end-of-line comment. Additional variables named ':' and '/' are expanded
+within script arguments (expanding to the value of os.PathListSeparator and
+os.PathSeparator respectively) but are not inherited in subprocess environments.
+
+Adding single quotes around text keeps spaces in that text from being treated
+as word separators and also disables environment variable expansion. Inside a
+single-quoted block of text, a repeated single quote indicates a literal single
+quote, as in:
+
+    'Don''t communicate by sharing memory.'
+
+A line beginning with # is a comment and conventionally explains what is being
+done or tested at the start of a new section of the script.
+
+Commands are executed one at a time, and errors are checked for each command;
+if any command fails unexpectedly, no subsequent commands in the script are
+executed. The command prefix ! indicates that the command on the rest of the
+line (typically go or a matching predicate) must fail instead of succeeding.
+The command prefix ? indicates that the command may or may not succeed, but the
+script should continue regardless.
+
+The command prefix [cond] indicates that the command on the rest of the line
+should only run when the condition is satisfied.
+
+A condition can be negated: [!root] means to run the rest of the line only if
+the user is not root. Multiple conditions may be given for a single command,
+for example, '[linux] [amd64] skip'. The command will run if all conditions are
+satisfied.
+
+When TestScript runs a script and the script fails, by default TestScript shows
+the execution of the most recent phase of the script (since the last # comment)
+and only shows the # comments for earlier phases. For example, here is a
+multi-phase script with a bug in it:
+
+	# GOPATH with p1 in d2, p2 in d2
+	env GOPATH=$WORK${/}d1${:}$WORK${/}d2
+
+	# build & install p1
+	env
+	go install -i p1
+	! stale p1
+	! stale p2
+
+	# modify p2 - p1 should appear stale
+	cp $WORK/p2x.go $WORK/d2/src/p2/p2.go
+	stale p1 p2
+
+	# build & install p1 again
+	go install -i p11
+	! stale p1
+	! stale p2
+
+	-- $WORK/d1/src/p1/p1.go --
+	package p1
+	import "p2"
+	func F() { p2.F() }
+	-- $WORK/d2/src/p2/p2.go --
+	package p2
+	func F() {}
+	-- $WORK/p2x.go --
+	package p2
+	func F() {}
+	func G() {}
+
+The bug is that the final phase installs p11 instead of p1. The test failure looks like:
+
+	$ go test -run=Script
+	--- FAIL: TestScript (3.75s)
+	    --- FAIL: TestScript/install_rebuild_gopath (0.16s)
+	        script_test.go:223:
+	            # GOPATH with p1 in d2, p2 in d2 (0.000s)
+	            # build & install p1 (0.087s)
+	            # modify p2 - p1 should appear stale (0.029s)
+	            # build & install p1 again (0.022s)
+	            > go install -i p11
+	            [stderr]
+	            can't load package: package p11: cannot find package "p11" in any of:
+	            	/Users/rsc/go/src/p11 (from $GOROOT)
+	            	$WORK/d1/src/p11 (from $GOPATH)
+	            	$WORK/d2/src/p11
+	            [exit status 1]
+	            FAIL: unexpected go command failure
+
+	        script_test.go:73: failed at testdata/script/install_rebuild_gopath.txt:15 in $WORK/gopath/src
+
+	FAIL
+	exit status 1
+	FAIL	cmd/go	4.875s
+	$
+
+Note that the commands in earlier phases have been hidden, so that the relevant
+commands are more easily found, and the elapsed time for a completed phase
+is shown next to the phase heading. To see the entire execution, use "go test -v",
+which also adds an initial environment dump to the beginning of the log.
+
+Note also that in reported output, the actual name of the per-script temporary directory
+has been consistently replaced with the literal string $WORK.
+
+The cmd/go test flag -testwork (which must appear on the "go test" command line after
+standard test flags) causes each test to log the name of its $WORK directory and other
+environment variable settings and also to leave that directory behind when it exits,
+for manual debugging of failing tests:
+
+	$ go test -run=Script -work
+	--- FAIL: TestScript (3.75s)
+	    --- FAIL: TestScript/install_rebuild_gopath (0.16s)
+	        script_test.go:223:
+	            WORK=/tmp/cmd-go-test-745953508/script-install_rebuild_gopath
+	            GOARCH=
+	            GOCACHE=/Users/rsc/Library/Caches/go-build
+	            GOOS=
+	            GOPATH=$WORK/gopath
+	            GOROOT=/Users/rsc/go
+	            HOME=/no-home
+	            TMPDIR=$WORK/tmp
+	            exe=
+
+	            # GOPATH with p1 in d2, p2 in d2 (0.000s)
+	            # build & install p1 (0.085s)
+	            # modify p2 - p1 should appear stale (0.030s)
+	            # build & install p1 again (0.019s)
+	            > go install -i p11
+	            [stderr]
+	            can't load package: package p11: cannot find package "p11" in any of:
+	            	/Users/rsc/go/src/p11 (from $GOROOT)
+	            	$WORK/d1/src/p11 (from $GOPATH)
+	            	$WORK/d2/src/p11
+	            [exit status 1]
+	            FAIL: unexpected go command failure
+
+	        script_test.go:73: failed at testdata/script/install_rebuild_gopath.txt:15 in $WORK/gopath/src
+
+	FAIL
+	exit status 1
+	FAIL	cmd/go	4.875s
+	$
+
+	$ WORK=/tmp/cmd-go-test-745953508/script-install_rebuild_gopath
+	$ cd $WORK/d1/src/p1
+	$ cat p1.go
+	package p1
+	import "p2"
+	func F() { p2.F() }
+	$
+
+The available commands are:
+cat files...
+	concatenate files and print to the script's stdout buffer
+
+
+cc args...
+	run the platform C compiler
+
+
+cd dir
+	change the working directory
+
+
+chmod perm paths...
+	change file mode bits
+
+	Changes the permissions of the named files or directories to
+	be equal to perm.
+	Only numerical permissions are supported.
+
+cmp [-q] file1 file2
+	compare files for differences
+
+	By convention, file1 is the actual data and file2 is the
+	expected data.
+	The command succeeds if the file contents are identical.
+	File1 can be 'stdout' or 'stderr' to compare the stdout or
+	stderr buffer from the most recent command.
+
+cmpenv [-q] file1 file2
+	compare files for differences, with environment expansion
+
+	By convention, file1 is the actual data and file2 is the
+	expected data.
+	The command succeeds if the file contents are identical
+	after substituting variables from the script environment.
+	File1 can be 'stdout' or 'stderr' to compare the script's
+	stdout or stderr buffer.
+
+cp src... dst
+	copy files to a target file or directory
+
+	src can include 'stdout' or 'stderr' to copy from the
+	script's stdout or stderr buffer.
+
+echo string...
+	display a line of text
+
+
+env [key[=value]...]
+	set or log the values of environment variables
+
+	With no arguments, print the script environment to the log.
+	Otherwise, add the listed key=value pairs to the environment
+	or print the listed keys.
+
+exec program [args...] [&]
+	run an executable program with arguments
+
+	Note that 'exec' does not terminate the script (unlike Unix
+	shells).
+
+exists [-readonly] [-exec] file...
+	check that files exist
+
+
+go [args...] [&]
+	run the 'go' program provided by the script host
+
+
+grep [-count=N] [-q] 'pattern' file
+	find lines in a file that match a pattern
+
+	The command succeeds if at least one match (or the exact
+	count, if given) is found.
+	The -q flag suppresses printing of matches.
+
+help [-v] name...
+	log help text for commands and conditions
+
+	To display help for a specific condition, enclose it in
+	brackets: 'help [amd64]'.
+	To display complete documentation when listing all commands,
+	pass the -v flag.
+
+mkdir path...
+	create directories, if they do not already exist
+
+	Unlike Unix mkdir, parent directories are always created if
+	needed.
+
+mv old new
+	rename a file or directory to a new path
+
+	OS-specific restrictions may apply when old and new are in
+	different directories.
+
+replace [old new]... file
+	replace strings in a file
+
+	The 'old' and 'new' arguments are unquoted as if in quoted
+	Go strings.
+
+rm path...
+	remove a file or directory
+
+	If the path is a directory, its contents are removed
+	recursively.
+
+skip [msg]
+	skip the current test
+
+
+sleep duration [&]
+	sleep for a specified duration
+
+	The duration must be given as a Go time.Duration string.
+
+stale target...
+	check that build targets are stale
+
+
+stderr [-count=N] [-q] 'pattern' file
+	find lines in the stderr buffer that match a pattern
+
+	The command succeeds if at least one match (or the exact
+	count, if given) is found.
+	The -q flag suppresses printing of matches.
+
+stdout [-count=N] [-q] 'pattern' file
+	find lines in the stdout buffer that match a pattern
+
+	The command succeeds if at least one match (or the exact
+	count, if given) is found.
+	The -q flag suppresses printing of matches.
+
+stop [msg]
+	stop execution of the script
+
+	The message is written to the script log, but no error is
+	reported from the script engine.
+
+symlink path -> target
+	create a symlink
+
+	Creates path as a symlink to target.
+	The '->' token (like in 'ls -l' output on Unix) is required.
+
+wait 
+	wait for completion of background commands
+
+	Waits for all background commands to complete.
+	The output (and any error) from each command is printed to
+	the log in the order in which the commands were started.
+	After the call to 'wait', the script's stdout and stderr
+	buffers contain the concatenation of the background
+	commands' outputs.
+
+
+
+The available conditions are:
+[GOARCH:*]
+	runtime.GOARCH == <suffix>
+[GODEBUG:*]
+	GODEBUG contains <suffix>
+[GOEXPERIMENT:*]
+	GOEXPERIMENT <suffix> is enabled
+[GOOS:*]
+	runtime.GOOS == <suffix>
+[abscc]
+	default $CC path is absolute and exists
+[asan]
+	GOOS/GOARCH supports -asan
+[buildmode:*]
+	go supports -buildmode=<suffix>
+[case-sensitive]
+	$WORK filesystem is case-sensitive
+[cc:*]
+	go env CC = <suffix> (ignoring the go/env file)
+[cgo]
+	host CGO_ENABLED
+[cgolinkext]
+	platform requires external linking for cgo
+[compiler:*]
+	runtime.Compiler == <suffix>
+[cross]
+	cmd/go GOOS/GOARCH != GOHOSTOS/GOHOSTARCH
+[exec:*]
+	<suffix> names an executable in the test binary's PATH
+[fuzz]
+	GOOS/GOARCH supports -fuzz
+[fuzz-instrumented]
+	GOOS/GOARCH supports -fuzz with instrumentation
+[git]
+	the 'git' executable exists and provides the standard CLI
+[go-builder]
+	GO_BUILDER_NAME is non-empty
+[link]
+	testenv.HasLink()
+[mismatched-goroot]
+	test's GOROOT_FINAL does not match the real GOROOT
+[msan]
+	GOOS/GOARCH supports -msan
+[mustlinkext]
+	platform always requires external linking
+[net:*]
+	can connect to external network host <suffix>
+[race]
+	GOOS/GOARCH supports -race
+[root]
+	os.Geteuid() == 0
+[short]
+	testing.Short()
+[symlink]
+	testenv.HasSymlink()
+[trimpath]
+	test binary was built with -trimpath
+[verbose]
+	testing.Verbose()
+
diff --git a/src/cmd/go/testdata/script/autocgo.txt b/src/cmd/go/testdata/script/autocgo.txt
new file mode 100644
index 0000000..586c802
--- /dev/null
+++ b/src/cmd/go/testdata/script/autocgo.txt
@@ -0,0 +1,29 @@
+# Test automatic setting of CGO_ENABLED based on $CC and what's in $PATH.
+
+[!cgo] skip
+[cross] skip
+
+# Assume we're on a system that can enable cgo normally.
+env CGO_ENABLED=
+go env CGO_ENABLED
+stdout 1
+
+# Clearing CC and removing everything but Go from the PATH should usually
+# disable cgo: no C compiler anymore (unless the baked-in defaultCC is an
+# absolute path and exists.
+env CC=
+env PATH=$GOROOT/bin
+go env CGO_ENABLED
+[!abscc] stdout 0
+[abscc] stdout 1
+
+# Setting CC should re-enable cgo.
+env CC=cc
+go env CGO_ENABLED
+stdout 1
+
+# So should setting CGO_ENABLED.
+env CC=
+env CGO_ENABLED=1
+go env CGO_ENABLED
+stdout 1
diff --git a/src/cmd/go/testdata/script/badgo.txt b/src/cmd/go/testdata/script/badgo.txt
new file mode 100644
index 0000000..cf4e258
--- /dev/null
+++ b/src/cmd/go/testdata/script/badgo.txt
@@ -0,0 +1,50 @@
+go get example.net/badgo@v1.0.0
+go get example.net/badgo@v1.1.0
+go get example.net/badgo@v1.2.0
+go get example.net/badgo@v1.3.0
+go get example.net/badgo@v1.4.0
+go get example.net/badgo@v1.5.0
+! go get example.net/badgo@v1.6.0
+stderr 'invalid go version .X.Y.: must match format 1.23'
+
+-- go.mod --
+module m
+
+replace (
+	example.net/badgo v1.0.0 => ./v1.0.0
+	example.net/badgo v1.1.0 => ./v1.1.0
+	example.net/badgo v1.2.0 => ./v1.2.0
+	example.net/badgo v1.3.0 => ./v1.3.0
+	example.net/badgo v1.4.0 => ./v1.4.0
+	example.net/badgo v1.5.0 => ./v1.5.0
+	example.net/badgo v1.6.0 => ./v1.6.0
+)
+
+-- v1.0.0/go.mod --
+module example.net/badgo
+go 1.17.0
+
+-- v1.1.0/go.mod --
+module example.net/badgo
+go 1.17rc2
+
+-- v1.2.0/go.mod --
+module example.net/badgo
+go 1.17.1
+
+-- v1.3.0/go.mod --
+module example.net/badgo
+go v1.17.0
+
+-- v1.4.0/go.mod --
+module example.net/badgo
+go v1.17.0-rc.2
+
+-- v1.5.0/go.mod --
+module example.net/badgo
+go v1.17.1
+
+-- v1.6.0/go.mod --
+module example.net/badgo
+go X.Y
+
diff --git a/src/cmd/go/testdata/script/bug.txt b/src/cmd/go/testdata/script/bug.txt
new file mode 100644
index 0000000..f64fb85
--- /dev/null
+++ b/src/cmd/go/testdata/script/bug.txt
@@ -0,0 +1,58 @@
+# Verify that go bug creates the appropriate URL issue body
+
+[!GOOS:linux] skip
+[short] skip
+
+go install
+go build -o $TMPDIR/go ./go
+env BROWSER=$GOPATH/bin/browser PATH=$TMPDIR:$PATH
+go bug
+exists $TMPDIR/browser
+grep '^go version' $TMPDIR/browser
+grep '^GOROOT/bin/go version: go version' $TMPDIR/browser
+grep '^GOROOT/bin/go tool compile -V: compile version' $TMPDIR/browser
+grep '^uname -sr: Linux' $TMPDIR/browser
+
+-- go.mod --
+module browser
+
+-- main.go --
+package main
+
+import (
+	"fmt"
+	"net/url"
+	"os"
+	"path/filepath"
+)
+
+func main() {
+	u, err := url.Parse(os.Args[1])
+	if err != nil {
+		panic(err)
+	}
+	body, err := url.PathUnescape(u.Query().Get("body"))
+	if err != nil {
+		panic(err)
+	}
+	out := filepath.Join(os.TempDir(), "browser")
+	f, err := os.Create(out)
+	if err != nil {
+		panic(err)
+	}
+	fmt.Fprintln(f, body)
+	if err := f.Close(); err != nil {
+		panic(err)
+	}
+}
+
+-- go/main.go --
+package main
+
+import (
+    "os"
+)
+
+func main() {
+    os.Exit(1)
+}
diff --git a/src/cmd/go/testdata/script/build_GOTMPDIR.txt b/src/cmd/go/testdata/script/build_GOTMPDIR.txt
new file mode 100644
index 0000000..4c9129e
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_GOTMPDIR.txt
@@ -0,0 +1,50 @@
+# Set GOCACHE to a clean directory to ensure that 'go build' has work to report.
+[!GOOS:windows] env GOCACHE=$WORK/gocache
+[GOOS:windows] env GOCACHE=$WORK\gocache
+
+# 'go build' should use GOTMPDIR if set.
+[!GOOS:windows] env GOTMPDIR=$WORK/my-favorite-tmpdir
+[GOOS:windows] env GOTMPDIR=$WORK\my-favorite-tmpdir
+mkdir $GOTMPDIR
+go build -x hello.go
+stderr ^WORK=.*my-favorite-tmpdir
+
+# Make GOTMPDIR a regular file. This prevents the creation of work directories,
+# so we can check that certain commands don't create them.
+# This simulates running on a full disk or a read-only volume.
+rm $GOTMPDIR
+cp hello.go $GOTMPDIR # any file will do
+
+# 'go build' should fail if GOTMPDIR is read-only.
+! go build -x .
+stderr '^go: creating work dir: \w+ '$GOTMPDIR
+
+# 'go list' should only fail if it needs to build something.
+go list -x .
+! stderr 'creating work dir'
+stdout m
+go list -m all
+stdout m
+! go list -x -export .
+stderr '^go: creating work dir: \w+ '$GOTMPDIR
+
+# 'go clean -cache' and 'go clean -modcache' should not fail.
+go clean -x -cache
+! stderr 'creating work dir'
+go clean -x -modcache
+! stderr 'creating work dir'
+
+# 'go env' should not fail for specific variables.
+# Without arguments, it needs to initialize a builder to load cgo flags, and
+# that uses a temporary directory.
+! go env
+stderr '^go: creating work dir: \w+ '$GOTMPDIR
+go env GOROOT
+
+-- go.mod --
+module m
+
+go 1.15
+-- hello.go --
+package main
+func main() { println("hello") }
diff --git a/src/cmd/go/testdata/script/build_acl_windows.txt b/src/cmd/go/testdata/script/build_acl_windows.txt
new file mode 100644
index 0000000..2cb60e0
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_acl_windows.txt
@@ -0,0 +1,44 @@
+[!GOOS:windows] stop
+[!exec:icacls] skip
+[!exec:powershell] skip
+
+# Create $WORK\guest and give the Guests group full access.
+# Files created within that directory will have different security attributes by default.
+mkdir $WORK\guest
+exec icacls $WORK\guest /grant '*S-1-5-32-546:(oi)(ci)f'
+
+env TMP=$WORK\guest
+env TEMP=$WORK\guest
+
+# Build a binary using the guest directory as an intermediate
+cd TestACL
+go build -o main.exe main.go
+# Build the same binary, but write it to the guest directory.
+go build -o $TMP\main.exe main.go
+
+# Read ACLs for the files.
+exec powershell -Command 'Get-Acl main.exe | Select -expand AccessToString'
+cp stdout $WORK\exe-acl.txt
+exec powershell -Command 'Get-Acl main.go | Select -expand AccessToString'
+cp stdout $WORK\src-acl.txt
+cd $TMP
+exec powershell -Command 'Get-Acl main.exe | Select -expand AccessToString'
+cp stdout $WORK\guest-acl.txt
+
+cd $WORK
+
+# The executable written to the source directory should have the same ACL as the source file.
+cmp $WORK\exe-acl.txt $WORK\src-acl.txt
+
+# The file written to the guest-allowed directory should give Guests control.
+grep 'BUILTIN\\Guests\s+Allow' $WORK\guest-acl.txt
+
+# The file written to the ordinary directory should not.
+! grep 'BUILTIN\\Guests\s+Allow' $WORK\exe-acl.txt
+
+
+-- TestACL/go.mod --
+module TestACL
+-- TestACL/main.go --
+package main
+func main() {}
diff --git a/src/cmd/go/testdata/script/build_arm.txt b/src/cmd/go/testdata/script/build_arm.txt
new file mode 100644
index 0000000..ff2a364
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_arm.txt
@@ -0,0 +1,13 @@
+[short] skip 'skipping cross-compile in short mode'
+
+env GOARCH=arm
+env GOOS=linux
+env GOARM=5
+
+go build hello.go
+! stderr 'unable to find math.a'
+
+-- hello.go --
+package main
+
+func main() {}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/build_buildvcs_auto.txt b/src/cmd/go/testdata/script/build_buildvcs_auto.txt
new file mode 100644
index 0000000..cfd5d82
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_buildvcs_auto.txt
@@ -0,0 +1,91 @@
+# Regression test for https://go.dev/issue/51748: by default, 'go build' should
+# not attempt to stamp VCS information when the VCS tool is not present.
+
+[short] skip
+[!git] skip
+
+cd sub
+exec git init .
+exec git config user.name 'Nameless Gopher'
+exec git config user.email 'nobody@golang.org'
+exec git add sub.go
+exec git commit -m 'initial state'
+cd ..
+
+exec git init
+exec git config user.name 'Nameless Gopher'
+exec git config user.email 'nobody@golang.org'
+exec git submodule add ./sub
+exec git add go.mod example.go
+exec git commit -m 'initial state'
+
+
+# Control case: with a git binary in $PATH,
+# 'go build' on a package in the same git repo
+# succeeds and stamps VCS metadata by default.
+
+go build -o example.exe .
+go version -m example.exe
+stdout '^\tbuild\tvcs=git$'
+stdout '^\tbuild\tvcs.modified=false$'
+
+
+# Building a binary from a different (nested) VCS repo should not stamp VCS
+# info. It should be an error if VCS stamps are requested explicitly with
+# '-buildvcs' (since we know the VCS metadata exists), but not an error
+# with '-buildvcs=auto'.
+
+go build -o sub.exe ./sub
+go version -m sub.exe
+! stdout '^\tbuild\tvcs'
+
+! go build -buildvcs -o sub.exe ./sub
+stderr '\Aerror obtaining VCS status: main package is in repository ".*" but current directory is in repository ".*"\n\tUse -buildvcs=false to disable VCS stamping.\n\z'
+
+cd ./sub
+go build -o sub.exe .
+go version -m sub.exe
+! stdout '^\tbuild\tvcs'
+
+! go build -buildvcs -o sub.exe .
+stderr '\Aerror obtaining VCS status: main module is in repository ".*" but current directory is in repository ".*"\n\tUse -buildvcs=false to disable VCS stamping.\n\z'
+cd ..
+
+
+# After removing 'git' from $PATH, 'go build -buildvcs' should fail...
+
+env PATH=
+env path=
+! go build -buildvcs -o example.exe .
+stderr 'go: missing Git command\. See https://golang\.org/s/gogetcmd$'
+
+# ...but by default we should omit VCS metadata when the tool is missing.
+
+go build -o example.exe .
+go version -m example.exe
+! stdout '^\tbuild\tvcs'
+
+# The default behavior can be explicitly set with '-buildvcs=auto'.
+
+go build -buildvcs=auto -o example.exe .
+go version -m example.exe
+! stdout '^\tbuild\tvcs'
+
+# Other flag values should be rejected with a useful error message.
+
+! go build -buildvcs=hg -o example.exe .
+stderr '\Ainvalid boolean value "hg" for -buildvcs: value is neither ''auto'' nor a valid bool\nusage: go build .*\nRun ''go help build'' for details.\n\z'
+
+
+-- go.mod --
+module example
+
+go 1.18
+-- example.go --
+package main
+
+func main() {}
+-- sub/sub.go --
+package main
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/build_cache_arch_mode.txt b/src/cmd/go/testdata/script/build_cache_arch_mode.txt
new file mode 100644
index 0000000..931827f
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_cache_arch_mode.txt
@@ -0,0 +1,21 @@
+# Issue 9737: verify that GOARM affects the computed build ID
+
+[short] skip
+
+# arm
+env GOOS=linux
+env GOARCH=arm
+env GOARM=5
+go install mycmd
+env GOARM=7
+stale mycmd
+
+
+-- go.mod --
+module mycmd
+
+go 1.16
+-- x.go --
+package main
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/build_cache_compile.txt b/src/cmd/go/testdata/script/build_cache_compile.txt
new file mode 100644
index 0000000..64b391f
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_cache_compile.txt
@@ -0,0 +1,21 @@
+env GO111MODULE=off
+[short] skip
+
+# Set up fresh GOCACHE.
+env GOCACHE=$WORK/gocache
+mkdir $GOCACHE
+
+# Building trivial non-main package should run compiler the first time.
+go build -x lib.go
+stderr '(compile|gccgo)( |\.exe).*lib\.go'
+
+# ... but not again ...
+go build -x lib.go
+! stderr '(compile|gccgo)( |\.exe).*lib\.go'
+
+# ... unless we use -a.
+go build -a -x lib.go
+stderr '(compile|gccgo)( |\.exe)'
+
+-- lib.go --
+package lib
diff --git a/src/cmd/go/testdata/script/build_cache_disabled.txt b/src/cmd/go/testdata/script/build_cache_disabled.txt
new file mode 100644
index 0000000..cb1a755
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_cache_disabled.txt
@@ -0,0 +1,50 @@
+# The build cache is required to build anything. It also may be needed to
+# initialize the build system, which is needed for commands like 'go env'.
+# However, there are lots of commands the cache is not needed for, and we
+# shouldn't require it when it won't be used.
+#
+# TODO(golang.org/issue/39882): commands below should work, too.
+# * go clean -modcache
+# * go env
+# * go fix
+# * go fmt
+# * go generate
+# * go get
+# * go list (without -export or -compiled)
+
+env GOCACHE=off
+
+# Commands that don't completely load packages should work.
+go doc fmt
+stdout Printf
+
+! go tool compile -h
+stderr usage:
+
+go version
+stdout '^go version'
+
+
+# Module commands that don't load packages should work.
+go mod init m
+exists go.mod
+
+go mod edit -require rsc.io/quote@v1.5.2
+
+go mod download rsc.io/quote
+
+go mod graph
+stdout rsc.io/quote
+
+go mod verify
+
+
+# Commands that load but don't build packages should work.
+go fmt .
+
+go doc .
+
+-- main.go --
+package main
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/build_cache_gomips.txt b/src/cmd/go/testdata/script/build_cache_gomips.txt
new file mode 100644
index 0000000..0cbf16a
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_cache_gomips.txt
@@ -0,0 +1,40 @@
+env GO111MODULE=off
+[short] skip # rebuilds std for mips
+
+# Set up fresh GOCACHE.
+env GOCACHE=$WORK/gocache
+mkdir $GOCACHE
+
+# Building for mipsle without setting GOMIPS will use floating point registers.
+env GOARCH=mipsle
+env GOOS=linux
+go build -gcflags=-S f.go
+stderr ADDD.F[0-9]+,.F[0-9]+,.F[0-9]+
+
+# Clean cache
+go clean -cache
+
+# Building with GOMIPS=softfloat will not use floating point registers
+env GOMIPS=softfloat
+go build -gcflags=-S f.go
+! stderr ADDD.F[0-9]+,.F[0-9]+,.F[0-9]+
+
+# Clean cache
+go clean -cache
+
+# Build without setting GOMIPS
+env GOMIPS=
+go build -gcflags=-S f.go
+stderr ADDD.F[0-9]+,.F[0-9]+,.F[0-9]+
+
+# Building with GOMIPS should still not use floating point registers.
+env GOMIPS=softfloat
+go build -gcflags=-S f.go
+! stderr ADDD.F[0-9]+,.F[0-9]+,.F[0-9]+
+
+-- f.go --
+package f
+
+func F(x float64) float64 {
+     return x + x
+}
diff --git a/src/cmd/go/testdata/script/build_cache_link.txt b/src/cmd/go/testdata/script/build_cache_link.txt
new file mode 100644
index 0000000..b9c740a
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_cache_link.txt
@@ -0,0 +1,26 @@
+env GO111MODULE=off
+[short] skip
+
+# Set up fresh GOCACHE.
+env GOCACHE=$WORK/gocache
+mkdir $GOCACHE
+
+# Building a main package should run the compiler and linker ...
+go build -o $devnull -x main.go
+stderr '(compile|gccgo)( |\.exe).*main\.go'
+stderr '(link|gccgo)( |\.exe)'
+
+# ... and then the linker again ...
+go build -o $devnull -x main.go
+! stderr '(compile|gccgo)( |\.exe).*main\.go'
+stderr '(link|gccgo)( |\.exe)'
+
+# ... but the output binary can serve as a cache.
+go build -o main$GOEXE -x main.go
+stderr '(link|gccgo)( |\.exe)'
+go build -o main$GOEXE -x main.go
+! stderr '(link|gccgo)( |\.exe)'
+
+-- main.go --
+package main
+func main() {}
diff --git a/src/cmd/go/testdata/script/build_cache_output.txt b/src/cmd/go/testdata/script/build_cache_output.txt
new file mode 100644
index 0000000..fc040b4
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_cache_output.txt
@@ -0,0 +1,67 @@
+env GO111MODULE=off
+env GODEBUG=gocachetest=1
+
+[!compiler:gc] skip
+[short] skip # clears cache, rebuilds too much
+
+# Set up fresh GOCACHE.
+env GOCACHE=$WORK/gocache
+mkdir $GOCACHE
+
+# Building a trivial non-main package should run compiler the first time.
+go build -x -gcflags=-m lib.go
+stderr 'compile( |\.exe"?)'
+stderr 'lib.go:2.* can inline f'
+
+# ... but not the second, even though it still prints the compiler output.
+go build -x -gcflags=-m lib.go
+! stderr 'compile( |\.exe"?)'
+stderr 'lib.go:2.* can inline f'
+
+# Building a trivial main package should run the compiler and linker the first time.
+go build -x -gcflags=-m -ldflags='-v -w' main.go
+stderr 'compile( |\.exe"?)'
+stderr 'main.go:2.* can inline main' # from compiler
+stderr 'link(\.exe"?)? -'
+stderr '\d+ symbols' # from linker
+
+# ... but not the second, even though it still prints the compiler and linker output.
+go build -x -gcflags=-m -ldflags='-v -w' main.go
+! stderr 'compile( |\.exe"?)'
+stderr 'main.go:2.* can inline main' # from compiler
+! stderr 'link(\.exe"?)? -'
+stderr '\d+ symbols' # from linker
+
+# Running a test should run the compiler, linker, and the test the first time.
+go test -v -x -gcflags=-m -ldflags=-v p
+stderr 'compile( |\.exe"?)'
+stderr 'p_test.go:.*can inline Test' # from compile of p_test
+stderr 'testmain\.go:.*inlin' # from compile of testmain
+stderr 'link(\.exe"?)? -'
+stderr '\d+ symbols' # from linker
+stderr 'p\.test( |\.exe"?)'
+stdout 'TEST' # from test
+
+# ... but not the second, even though it still prints the compiler, linker, and test output.
+go test -v -x -gcflags=-m -ldflags=-v p
+! stderr 'compile( |\.exe"?)'
+stderr 'p_test.go:.*can inline Test' # from compile of p_test
+stderr 'testmain\.go:.*inlin' # from compile of testmain
+! stderr 'link(\.exe"?)? -'
+stderr '\d+ symbols' # from linker
+! stderr 'p\.test( |\.exe"?)'
+stdout 'TEST' # from test
+
+
+-- lib.go --
+package p
+func f(x *int) *int { return x }
+
+-- main.go --
+package main
+func main() {}
+
+-- p/p_test.go --
+package p
+import "testing"
+func Test(t *testing.T) {println("TEST")}
diff --git a/src/cmd/go/testdata/script/build_cache_trimpath.txt b/src/cmd/go/testdata/script/build_cache_trimpath.txt
new file mode 100644
index 0000000..7ee3c3b
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_cache_trimpath.txt
@@ -0,0 +1,47 @@
+[short] skip
+env GO111MODULE=on
+
+# Set up fresh GOCACHE.
+env GOCACHE=$WORK/gocache
+mkdir $GOCACHE
+
+cd $WORK
+go build -o a.out
+
+# Varying -trimpath should cause a rebuild.
+go build -x -o a.out -trimpath
+stderr '(compile|gccgo)( |\.exe)'
+stderr 'link( |\.exe)'
+
+# Two distinct versions of the same module with identical content should
+# still be cached separately.
+# Verifies golang.org/issue/35412.
+go get example.com/stack@v1.0.0
+go run -trimpath printstack.go
+stdout '^example.com/stack@v1.0.0/stack.go$'
+go get example.com/stack@v1.0.1
+go run -trimpath printstack.go
+stdout '^example.com/stack@v1.0.1/stack.go$'
+
+-- $WORK/hello.go --
+package main
+func main() { println("hello") }
+
+-- $WORK/printstack.go --
+// +build ignore
+
+package main
+
+import (
+	"fmt"
+
+	"example.com/stack"
+)
+
+func main() {
+	fmt.Println(stack.TopFile())
+}
+-- $WORK/go.mod --
+module m
+
+go 1.14
diff --git a/src/cmd/go/testdata/script/build_cd_gopath_different.txt b/src/cmd/go/testdata/script/build_cd_gopath_different.txt
new file mode 100644
index 0000000..64d7d74
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_cd_gopath_different.txt
@@ -0,0 +1,73 @@
+[compiler:gccgo] skip 'gccgo does not support -ldflags -X'
+env GO111MODULE=off
+go build run_go.go
+
+# Apply identity function to GOPATH
+exec ./run_go$GOEXE $GOPATH/src/my.pkg/main $GOPATH IDENTITY build -o $WORK/tmp/a.exe -ldflags -X=my.pkg.Text=linkXworked
+exec $WORK/tmp/a.exe
+stderr 'linkXworked'
+rm $WORK/tmp/a.exe
+
+[!GOOS:windows] stop 'rest of the tests only apply to Windows'
+
+# Replace '\' with '/' in GOPATH
+exec ./run_go$GOEXE $GOPATH/src/my.pkg/main $GOPATH REPLACE_SLASH build -o $WORK/tmp/a.exe -ldflags -X=my.pkg.Text=linkXworked
+exec $WORK/tmp/a.exe
+stderr 'linkXworked'
+rm $WORK/tmp/a.exe
+
+# Apply identity function to GOPATH
+exec ./run_go$GOEXE $GOPATH/src/my.pkg/main $GOPATH UPPER build -o $WORK/tmp/a.exe -ldflags -X=my.pkg.Text=linkXworked
+exec $WORK/tmp/a.exe
+stderr 'linkXworked'
+rm $WORK/tmp/a.exe
+
+# Apply identity function to GOPATH
+exec ./run_go$GOEXE $GOPATH/src/my.pkg/main $GOPATH LOWER build -o $WORK/tmp/a.exe -ldflags -X=my.pkg.Text=linkXworked
+exec $WORK/tmp/a.exe
+stderr 'linkXworked'
+rm $WORK/tmp/a.exe
+
+-- run_go.go --
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"strings"
+)
+
+func main() {
+	dir := os.Args[1]
+	gopath := os.Args[2]
+	switch os.Args[3] {
+		case "IDENTITY":
+		case "REPLACE_SLASH": gopath = strings.ReplaceAll(gopath, `\`, `/`)
+		case "UPPER": gopath = strings.ToUpper(gopath)
+		case "LOWER": gopath = strings.ToLower(gopath)
+		default: fmt.Fprintln(os.Stderr, "bad op"); os.Exit(1)
+	}
+	cmd := exec.Command("go", os.Args[4:]...)
+	cmd.Dir = dir
+	cmd.Env = append(os.Environ(), "GOPATH="+gopath)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	if err := cmd.Run(); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}
+
+-- my.pkg/main/main.go --
+package main
+
+import "my.pkg"
+
+func main() {
+	println(pkg.Text)
+}
+-- my.pkg/pkg.go --
+package pkg
+
+var Text = "unset"
diff --git a/src/cmd/go/testdata/script/build_cgo_consistent_results.txt b/src/cmd/go/testdata/script/build_cgo_consistent_results.txt
new file mode 100644
index 0000000..f22994f
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_cgo_consistent_results.txt
@@ -0,0 +1,23 @@
+[short] skip
+[!cgo] skip
+
+[GOOS:solaris] skip "skipping on Solaris; see golang.org/issue/13247"
+[GOOS:illumos] skip "skipping on Solaris; see golang.org/issue/13247"
+
+go build -o $WORK/exe1$GOEXE cgotest
+go build -x -o $WORK/exe2$GOEXE cgotest
+
+# TODO(matloob): skip if stderr does not contain '-fdebug-prefix-map=\$WORK'
+
+cmp $WORK/exe1$GOEXE $WORK/exe2$GOEXE
+
+-- go.mod --
+module cgotest
+
+go 1.16
+-- m.go --
+package cgotest
+
+import "C"
+
+var _ C.int
diff --git a/src/cmd/go/testdata/script/build_cgo_error.txt b/src/cmd/go/testdata/script/build_cgo_error.txt
new file mode 100644
index 0000000..c11ab46
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_cgo_error.txt
@@ -0,0 +1,17 @@
+[short] skip
+[!cgo] skip
+
+! go build .
+stderr '# foo\nfoo.c:'
+! stderr 'EXTRA string'
+
+-- go.mod --
+module foo
+
+go 1.20
+-- foo.go --
+package foo
+
+import "C"
+-- foo.c --
+#include "doesnotexist.h"
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/build_concurrent_backend.txt b/src/cmd/go/testdata/script/build_concurrent_backend.txt
new file mode 100644
index 0000000..9cac635
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_concurrent_backend.txt
@@ -0,0 +1,10 @@
+# Tests golang.org/issue/48490
+# cmd/go should enable concurrent compilation by default
+
+# Reset all experiments, since one of them can disable
+# concurrent compilation, e.g: fieldtrack.
+env GOEXPERIMENT=none
+
+env GOMAXPROCS=4
+go build -n -x -a fmt
+stderr ' -c=4 '
diff --git a/src/cmd/go/testdata/script/build_cwd_newline.txt b/src/cmd/go/testdata/script/build_cwd_newline.txt
new file mode 100644
index 0000000..91cb57f
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_cwd_newline.txt
@@ -0,0 +1,138 @@
+[GOOS:windows] skip 'filesystem normalizes / to \'
+[GOOS:plan9] skip 'filesystem disallows \n in paths'
+
+# If the directory path containing a package to be built includes a newline,
+# the go command should refuse to even try to build the package.
+
+env DIR=$WORK${/}${newline}'package main'${newline}'func main() { panic("uh-oh")'${newline}'/*'
+
+mkdir $DIR
+cd $DIR
+exec pwd
+cp $WORK/go.mod ./go.mod
+cp $WORK/main.go ./main.go
+cp $WORK/main_nocgo.go ./main_nocgo.go
+cp $WORK/main_test.go ./main_test.go
+
+! go build -o $devnull .
+stderr 'package example: invalid package directory .*uh-oh'
+
+[cgo] ! go build -o $devnull main.go
+[!cgo] ! go build -o $devnull main_nocgo.go
+stderr 'package command-line-arguments: invalid package directory .*uh-oh'
+
+! go run .
+stderr 'package example: invalid package directory .*uh-oh'
+
+[cgo] ! go run main.go
+[!cgo] ! go run main_nocgo.go
+stderr 'package command-line-arguments: invalid package directory .*uh-oh'
+
+! go test .
+stderr 'package example: invalid package directory .*uh-oh'
+
+[cgo] ! go test -v main.go main_test.go
+[!cgo] ! go test -v main_nocgo.go main_test.go
+stderr 'package command-line-arguments: invalid package directory .*uh-oh'
+
+go list -compiled -e -f '{{with .CompiledGoFiles}}{{.}}{{end}}' .
+! stdout .
+! stderr .
+! exists obj_
+
+
+# The cgo tool should only accept the source file if the working directory
+# is not written in line directives in the resulting files.
+
+[cgo] ! go tool cgo main.go
+[cgo] stderr 'cgo: input path contains newline character: .*uh-oh'
+[cgo] ! exists _obj
+
+[cgo] go tool cgo -trimpath=$PWD main.go
+[cgo] grep '//line main\.go:1:1' _obj/main.cgo1.go
+[cgo] ! grep 'uh-oh' _obj/main.cgo1.go
+[cgo] rm _obj
+
+
+# Since we do preserve $PWD (or set it appropriately) for commands, and we do
+# not resolve symlinks unnecessarily, referring to the contents of the unsafe
+# directory via a safe symlink should be ok, and should not inject the data from
+# the symlink target path.
+
+[!symlink] stop 'remainder of test checks symlink behavior'
+[short] stop 'links and runs binaries'
+
+symlink $WORK${/}link -> $DIR
+
+[cgo] go run $WORK${/}link${/}main.go
+[!cgo] go run $WORK${/}link${/}main_nocgo.go
+! stdout panic
+! stderr panic
+stderr '^ok$'
+
+[cgo] go test -v $WORK${/}link${/}main.go $WORK${/}link${/}main_test.go
+[!cgo] go test -v $WORK${/}link${/}main_nocgo.go $WORK${/}link${/}main_test.go
+! stdout panic
+! stderr panic
+stdout '^ok$'   # 'go test' combines the test's stdout into stderr
+
+cd $WORK/link
+
+[cgo] ! go run $DIR${/}main.go
+[!cgo] ! go run $DIR${/}main_nocgo.go
+stderr 'package command-line-arguments: invalid package directory .*uh-oh'
+
+go run .
+! stdout panic
+! stderr panic
+stderr '^ok$'
+
+[cgo] go run main.go
+[!cgo] go run main_nocgo.go
+! stdout panic
+! stderr panic
+stderr '^ok$'
+
+go test -v
+! stdout panic
+! stderr panic
+stdout '^ok$'  # 'go test' combines the test's stdout into stderr
+
+go test -v .
+! stdout panic
+! stderr panic
+stdout '^ok$'  # 'go test' combines the test's stdout into stderr
+
+[cgo] go tool cgo main.go
+[cgo] grep '//line .*'${/}'link'${/}'main\.go:1:1' _obj/main.cgo1.go
+[cgo] ! grep 'uh-oh' _obj/main.cgo1.go
+
+-- $WORK/go.mod --
+module example
+go 1.19
+-- $WORK/main.go --
+package main
+
+import "C"
+
+func main() {
+	/* nothing here */
+	println("ok")
+}
+-- $WORK/main_nocgo.go --
+//go:build !cgo
+
+package main
+
+func main() {
+	/* nothing here */
+	println("ok")
+}
+-- $WORK/main_test.go --
+package main
+
+import "testing"
+
+func TestMain(*testing.M) {
+	main()
+}
diff --git a/src/cmd/go/testdata/script/build_darwin_cc_arch.txt b/src/cmd/go/testdata/script/build_darwin_cc_arch.txt
new file mode 100644
index 0000000..df3fa5b
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_darwin_cc_arch.txt
@@ -0,0 +1,24 @@
+# Test that we pass -arch flag to C compiler on Darwin (issue 43692).
+
+[!GOOS:darwin] skip
+[!cgo] skip
+
+# clear CC, in case user sets it
+env CC=
+
+env CGO_ENABLED=1
+
+env GOARCH=amd64
+go build -n -x c.go
+stderr 'clang.*-arch x86_64'
+
+env GOARCH=arm64
+go build -n -x c.go
+stderr 'clang.*-arch arm64'
+
+-- c.go --
+package main
+
+import "C"
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/build_dash_n_cgo.txt b/src/cmd/go/testdata/script/build_dash_n_cgo.txt
new file mode 100644
index 0000000..3f49ef6
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_dash_n_cgo.txt
@@ -0,0 +1,18 @@
+# Tests golang.org/issue/14944
+
+[!cgo] skip
+
+go build -n foo.go
+! stderr 'os.Stat .* no such file or directory' # there shouldn't be a stat of the archive file
+
+-- foo.go --
+package main
+
+/*
+#include <limits.h>
+*/
+import "C"
+
+func main() {
+        println(C.INT_MAX)
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/build_dash_o_dev_null.txt b/src/cmd/go/testdata/script/build_dash_o_dev_null.txt
new file mode 100644
index 0000000..e415fc2
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_dash_o_dev_null.txt
@@ -0,0 +1,13 @@
+# Issue #25579
+
+[short] skip
+
+go build -o $devnull hello.go
+! exists 'hello'$GOEXE
+
+-- hello.go --
+package main
+
+func main() {
+	println("hello, world")
+}
diff --git a/src/cmd/go/testdata/script/build_dash_x.txt b/src/cmd/go/testdata/script/build_dash_x.txt
new file mode 100644
index 0000000..e5580a2
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_dash_x.txt
@@ -0,0 +1,48 @@
+[short] skip
+[!cgo] skip
+
+[!exec:/usr/bin/env] skip
+[!exec:bash] skip
+
+mkdir $WORK/tmp/cache
+env GOCACHE=$WORK/tmp/cache
+
+# Before building our test main.go, ensure that an up-to-date copy of
+# runtime/cgo is present in the cache. If it isn't, the 'go build' step below
+# will fail with "can't open import". See golang.org/issue/29004.
+#
+# (The fix in golang.org/issue/29004 didn't completely fix the underlying issue:
+# cmd/go/internal/load adds a bunch of implicit dependencies
+# based on various heuristics, and, due to a bug described in
+# https://golang.org/issue/31544#issuecomment-490607180,
+# those implicit dependencies are not added early enough during
+# loading to properly affect the import graph.)
+go build runtime/cgo
+
+go build -x -o main main.go
+cp stderr commands.txt
+cat header.txt commands.txt
+cp stdout test.sh
+
+exec ./main
+cmp stderr hello.txt
+rm ./main
+
+exec /usr/bin/env bash -x test.sh
+exec ./main
+cmp stderr hello.txt
+
+grep '^WORK=(.*)\n' commands.txt
+
+-- main.go --
+package main
+
+import "C"
+
+func main() {
+	print("hello\n")
+}
+-- header.txt --
+set -e
+-- hello.txt --
+hello
diff --git a/src/cmd/go/testdata/script/build_exe.txt b/src/cmd/go/testdata/script/build_exe.txt
new file mode 100644
index 0000000..a994d17
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_exe.txt
@@ -0,0 +1,25 @@
+# go build with -o and -buildmode=exe should report an error on a non-main package.
+
+! go build -buildmode=exe -o out$GOEXE ./not_main
+stderr '-buildmode=exe requires exactly one main package'
+! exists out$GOEXE
+! go build -buildmode=exe -o out$GOEXE ./main_one ./main_two
+stderr '-buildmode=exe requires exactly one main package'
+! exists out$GOEXE
+
+-- go.mod --
+module m
+
+go 1.16
+-- not_main/not_main.go --
+package not_main
+
+func F() {}
+-- main_one/main_one.go --
+package main
+
+func main() {}
+-- main_two/main_two.go --
+package main
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/build_gcflags.txt b/src/cmd/go/testdata/script/build_gcflags.txt
new file mode 100644
index 0000000..9603e0b
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_gcflags.txt
@@ -0,0 +1,22 @@
+env GO111MODULE=off
+
+# Test that the user can override default code generation flags.
+
+[compiler:gccgo] skip  # gccgo does not use -gcflags
+[!cgo] skip
+[!GOOS:linux] skip  # test only works if c-archive implies -shared
+[short] skip
+
+env GOCACHE=$WORK/gocache  # Looking for compile commands, so need a clean cache.
+go build -x -n -buildmode=c-archive -gcflags=all=-shared=false ./override.go
+stderr '^.*/compile (.* )?-shared (.* )?-shared=false'
+
+-- override.go --
+package main
+
+import "C"
+
+//export GoFunc
+func GoFunc() {}
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/build_gcflags_order.txt b/src/cmd/go/testdata/script/build_gcflags_order.txt
new file mode 100644
index 0000000..3725c89
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_gcflags_order.txt
@@ -0,0 +1,22 @@
+# Tests golang.org/issue/47682
+# Flags specified with -gcflags should appear after other flags generated by cmd/go.
+
+cd m
+go build -n -gcflags=-lang=go1.17
+stderr ' -lang=go1.16.* -lang=go1.17'
+! go build -gcflags='-c 0'
+stderr 'compile: -c must be at least 1, got 0'
+
+-- m/go.mod --
+module example.com
+
+go 1.16
+
+-- m/main.go --
+package main
+
+func main() {
+    var s = []int{1, 2, 3}
+    var pa = (*[2]int)(s[1:])
+    println(pa[1])
+}
diff --git a/src/cmd/go/testdata/script/build_gopath_order.txt b/src/cmd/go/testdata/script/build_gopath_order.txt
new file mode 100644
index 0000000..caf2502
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_gopath_order.txt
@@ -0,0 +1,36 @@
+# golang.org/issue/14176#issuecomment-179895769
+# golang.org/issue/14192
+# -I arguments to compiler could end up not in GOPATH order,
+# leading to unexpected import resolution in the compiler.
+
+env GO111MODULE=off
+env GOPATH=$WORK/p1${:}$WORK/p2
+mkdir $WORK/p1/src/foo $WORK/p2/src/baz
+mkdir $WORK/p2/pkg/${GOOS}_${GOARCH} $WORK/p1/src/bar
+cp foo.go $WORK/p1/src/foo/foo.go
+cp baz.go $WORK/p2/src/baz/baz.go
+cp foo.a $WORK/p2/pkg/${GOOS}_${GOARCH}/foo.a
+cp bar.go $WORK/p1/src/bar/bar.go
+
+go install -x bar
+
+# add in baz.a to the mix
+mkdir $WORK/p1/pkg/${GOOS}_${GOARCH}
+cp baz.a $WORK/p1/pkg/${GOOS}_${GOARCH}/baz.a
+env GOPATH=$WORK/p1${:}$WORK/p2
+go install -x bar
+env GOPATH=$WORK/p2${:}$WORK/p1
+go install -x bar
+
+-- foo.go --
+package foo
+-- baz.go --
+package baz
+-- foo.a --
+bad
+-- baz.a --
+bad
+-- bar.go --
+package bar
+import _ "baz"
+import _ "foo"
diff --git a/src/cmd/go/testdata/script/build_ignore_leading_bom.txt b/src/cmd/go/testdata/script/build_ignore_leading_bom.txt
new file mode 100644
index 0000000..37141f3
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_ignore_leading_bom.txt
@@ -0,0 +1,27 @@
+# Per https://golang.org/ref/spec#Source_code_representation:
+# a compiler may ignore a UTF-8-encoded byte order mark (U+FEFF)
+# if it is the first Unicode code point in the source text.
+
+go list -f 'Imports: {{.Imports}} EmbedFiles: {{.EmbedFiles}}' .
+stdout '^Imports: \[embed m/hello\] EmbedFiles: \[.*file\]$'
+
+-- go.mod --
+module m
+
+go 1.16
+-- m.go --
+package main
+
+import (
+	_ "embed"
+
+	"m/hello"
+)
+
+//go:embed file
+var s string
+
+-- hello/hello.go --
+package hello
+
+-- file --
diff --git a/src/cmd/go/testdata/script/build_import_comment.txt b/src/cmd/go/testdata/script/build_import_comment.txt
new file mode 100644
index 0000000..b500340
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_import_comment.txt
@@ -0,0 +1,68 @@
+# Test in GOPATH mode first.
+env GO111MODULE=off
+cd m
+
+# Import comment matches
+go build -n works.go
+
+# Import comment mismatch
+! go build -n wrongplace.go
+stderr 'wrongplace expects import "my/x"'
+
+# Import comment syntax error
+! go build -n bad.go
+stderr 'cannot parse import comment'
+
+# Import comment conflict
+! go build -n conflict.go
+stderr 'found import comments'
+
+
+# Test in module mode.
+# We ignore import comments, so these commands should succeed.
+env GO111MODULE=on
+
+# Import comment matches
+go build -n works.go
+
+# Import comment mismatch
+go build -n wrongplace.go
+
+# Import comment syntax error
+go build -n bad.go
+
+# Import comment conflict
+go build -n conflict.go
+
+-- m/go.mod --
+module m
+
+go 1.16
+-- m/bad.go --
+package p
+
+import "m/bad"
+-- m/conflict.go --
+package p
+
+import "m/conflict"
+-- m/works.go --
+package p
+
+import _ "m/works/x"
+-- m/wrongplace.go --
+package p
+
+import "m/wrongplace"
+-- m/bad/bad.go --
+package bad // import
+-- m/conflict/a.go --
+package conflict // import "a"
+-- m/conflict/b.go --
+package conflict /* import "b" */
+-- m/works/x/x.go --
+package x // import "m/works/x"
+-- m/works/x/x1.go --
+package x // important! not an import comment
+-- m/wrongplace/x.go --
+package x // import "my/x"
diff --git a/src/cmd/go/testdata/script/build_import_cycle.txt b/src/cmd/go/testdata/script/build_import_cycle.txt
new file mode 100644
index 0000000..16e4e87
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_import_cycle.txt
@@ -0,0 +1,13 @@
+# mod_import_cycle covers this error in module mode.
+env GO111MODULE=off
+
+! go build selfimport
+stderr -count=1 'import cycle not allowed'
+
+go list -e -f '{{.Error}}' selfimport # Don't hang forever
+stdout -count=1 'import cycle not allowed'
+
+-- selfimport/selfimport.go --
+package selfimport
+
+import "selfimport"
diff --git a/src/cmd/go/testdata/script/build_internal.txt b/src/cmd/go/testdata/script/build_internal.txt
new file mode 100644
index 0000000..0a37d65
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_internal.txt
@@ -0,0 +1,65 @@
+# Test internal package errors are handled
+cd testinternal3
+go list .
+stdout 'testinternal3'
+
+# Test internal cache
+cd ../testinternal4
+! go build testinternal4/p
+stderr 'internal'
+
+# Test internal packages outside GOROOT are respected
+cd ../testinternal2
+env GO111MODULE=off
+! go build -v .
+stderr 'p\.go:3:8: use of internal package .*internal/w not allowed'
+env GO111MODULE=''
+
+[compiler:gccgo] skip # gccgo does not have GOROOT
+cd ../testinternal
+! go build -v .
+stderr 'p\.go:3:8: use of internal package net/http/internal not allowed'
+
+-- testinternal/go.mod --
+module testinternal
+
+go 1.16
+-- testinternal/p.go --
+package p
+
+import _ "net/http/internal"
+-- testinternal2/go.mod --
+module testinternal2
+
+go 1.16
+-- testinternal2/p.go --
+package p
+
+import _ "./x/y/z/internal/w"
+-- testinternal2/x/y/z/internal/w/w.go --
+package w
+-- testinternal3/go.mod --
+module testinternal3
+
+go 1.16
+-- testinternal3/t.go --
+package t
+
+import _ "internal/does-not-exist"
+-- testinternal4/go.mod --
+module testinternal4
+
+go 1.16
+-- testinternal4/p/p.go --
+package p
+
+import (
+	_ "testinternal4/q/internal/x"
+	_ "testinternal4/q/j"
+)
+-- testinternal4/q/internal/x/x.go --
+package x
+-- testinternal4/q/j/j.go --
+package j
+
+import _ "testinternal4/q/internal/x"
diff --git a/src/cmd/go/testdata/script/build_issue48319.txt b/src/cmd/go/testdata/script/build_issue48319.txt
new file mode 100644
index 0000000..4543303
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_issue48319.txt
@@ -0,0 +1,46 @@
+# Regression test for https://go.dev/issue/48319:
+# cgo builds should not include debug information from a stale GOROOT_FINAL.
+
+[short] skip
+[!cgo] skip
+
+# This test is sensitive to cache invalidation,
+# so use a separate build cache that we can control.
+env GOCACHE=$WORK/gocache
+mkdir $GOCACHE
+
+# Build a binary using a specific value of GOROOT_FINAL.
+env GOROOT_FINAL=$WORK${/}goroot1
+go build -o main.exe
+mv main.exe main1.exe
+
+# Now clean the cache and build using a different GOROOT_FINAL.
+# The resulting binaries should differ in their debug metadata.
+go clean -cache
+env GOROOT_FINAL=$WORK${/}goroot2
+go build -o main.exe
+mv main.exe main2.exe
+! cmp -q main2.exe main1.exe
+
+# Set GOROOT_FINAL back to the first value.
+# If the build is properly reproducible, the two binaries should match.
+env GOROOT_FINAL=$WORK${/}goroot1
+go build -o main.exe
+cmp -q main.exe main1.exe
+
+-- go.mod --
+module main
+
+go 1.18
+-- main.go --
+package main
+
+import "C"
+
+import "runtime"
+
+var _ C.int
+
+func main() {
+	println(runtime.GOROOT())
+}
diff --git a/src/cmd/go/testdata/script/build_issue59571.txt b/src/cmd/go/testdata/script/build_issue59571.txt
new file mode 100644
index 0000000..2cf3259
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_issue59571.txt
@@ -0,0 +1,40 @@
+# Regression test for https://go.dev/issue/59571
+# Build should be reproducible, even with aliased generic types.
+
+go build -a -o 1.a
+go build -a -o 2.a
+cmp -q 1.a 2.a
+
+-- go.mod --
+module m
+
+go 1.20
+-- m.go --
+package m
+
+type (
+	SliceFlag[T any] struct{}
+
+	Alias1  = SliceFlag[[1]int]
+	Alias2  = SliceFlag[[2]int]
+	Alias3  = SliceFlag[[3]int]
+	Alias4  = SliceFlag[[4]int]
+	Alias5  = SliceFlag[[5]int]
+	Alias6  = SliceFlag[[6]int]
+	Alias7  = SliceFlag[[7]int]
+	Alias8  = SliceFlag[[8]int]
+	Alias9  = SliceFlag[[9]int]
+	Alias10 = SliceFlag[[10]int]
+	Alias11 = SliceFlag[[11]int]
+	Alias12 = SliceFlag[[12]int]
+	Alias13 = SliceFlag[[13]int]
+	Alias14 = SliceFlag[[14]int]
+	Alias15 = SliceFlag[[15]int]
+	Alias16 = SliceFlag[[16]int]
+	Alias17 = SliceFlag[[17]int]
+	Alias18 = SliceFlag[[18]int]
+	Alias19 = SliceFlag[[19]int]
+	Alias20 = SliceFlag[[20]int]
+)
+
+func (x *SliceFlag[T]) String() string { return "zzz" }
diff --git a/src/cmd/go/testdata/script/build_issue6480.txt b/src/cmd/go/testdata/script/build_issue6480.txt
new file mode 100644
index 0000000..991112f
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_issue6480.txt
@@ -0,0 +1,128 @@
+# "go test -c -test.bench=XXX errors" should not hang.
+# "go test -c" should also produce reproducible binaries.
+# "go test -c" should also appear to write a new binary every time,
+# even if it's really just updating the mtime on an existing up-to-date binary.
+
+[compiler:gccgo] skip
+[short] skip
+
+# Install some commands to compare mtimes
+env GOBIN=$WORK/tmp/bin
+go install m/now m/mtime m/before
+
+# Initial builds
+go test -c -test.bench=XXX errors
+go test -c -o errors2.test errors
+cmp errors.test$GOEXE errors2.test # // errors2.test has no exeSuffix because -o above doesn't have it
+
+# Check errors.test mtime is updated
+exec $GOBIN/now
+cp stdout start_time.txt
+go test -x -c -test.bench=XXX errors
+! stderr '[\\/]link|gccgo' # make sure up-to-date test binary is not relinked
+exec $GOBIN/mtime errors.test$GOEXE
+cp stdout errors1_mod_time.txt
+exec $GOBIN/before start_time.txt errors1_mod_time.txt
+rm start_time.txt errors1_mod_time.txt
+
+# Check errors2.test mtime is updated
+exec $GOBIN/now
+cp stdout start_time.txt
+go test -x -c -o errors2.test errors
+! stderr '[\\/]link|gccgo' # make sure up-to-date test binary is not relinked
+exec $GOBIN/mtime errors2.test
+cp stdout errors2_mod_time.txt
+exec $GOBIN/before start_time.txt errors2_mod_time.txt
+
+-- go.mod --
+module m
+
+go 1.16
+-- now/now.go --
+// Writes time.Now() to a file
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"time"
+)
+
+func main() {
+	if err := json.NewEncoder(os.Stdout).Encode(time.Now()); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}
+-- mtime/mtime.go --
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+)
+
+func main() {
+	info, err := os.Stat(os.Args[1])
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	if err := json.NewEncoder(os.Stdout).Encode(info.ModTime()); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}
+-- before/before.go --
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"time"
+)
+
+func truncateLike(t, p time.Time) time.Time {
+	nano := p.UnixNano()
+	d := 1 * time.Nanosecond
+	for nano%int64(d) == 0 && d < 1*time.Second {
+		d *= 10
+	}
+	for nano%int64(d) == 0 && d < 2*time.Second {
+		d *= 2
+	}
+	return t.Truncate(d)
+}
+
+func main() {
+	var t1 time.Time
+	b1, err := os.ReadFile(os.Args[1])
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	if err := json.Unmarshal(b1, &t1); err != nil  {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+
+	var t2 time.Time
+	b2, err := os.ReadFile(os.Args[2])
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	if err := json.Unmarshal(b2, &t2); err != nil  {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+
+	t1 = truncateLike(t1, t2)
+	if !t1.Before(t2) {
+		fmt.Fprintf(os.Stderr, "time in %v (%v) is not before time in %v (%v)", os.Args[1], t1, os.Args[2], t2)
+		os.Exit(1)
+	}
+}
diff --git a/src/cmd/go/testdata/script/build_link_x_import_path_escape.txt b/src/cmd/go/testdata/script/build_link_x_import_path_escape.txt
new file mode 100644
index 0000000..d47c482
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_link_x_import_path_escape.txt
@@ -0,0 +1,22 @@
+[compiler:gccgo] skip 'gccgo does not support -ldflags -X'
+
+go build -o linkx$GOEXE -ldflags -X=my.pkg.Text=linkXworked my.pkg/main
+exec ./linkx$GOEXE
+stderr '^linkXworked$'
+
+-- go.mod --
+module my.pkg
+
+go 1.16
+-- main/main.go --
+package main
+
+import "my.pkg"
+
+func main() {
+	println(pkg.Text)
+}
+-- pkg.go --
+package pkg
+
+var Text = "unset"
diff --git a/src/cmd/go/testdata/script/build_multi_main.txt b/src/cmd/go/testdata/script/build_multi_main.txt
new file mode 100644
index 0000000..8afd8b8
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_multi_main.txt
@@ -0,0 +1,43 @@
+# Verify build -o can output multiple executables to a directory.
+
+mkdir $WORK/bin
+go build -o $WORK/bin ./cmd/c1 ./cmd/c2
+! stderr 'multiple packages'
+
+! go build -o $WORK/bin ./pkg1 ./pkg1
+stderr 'no main packages'
+
+! go build ./cmd/c1
+stderr 'already exists and is a directory'
+
+# Verify build -o output correctly local packages
+mkdir $WORK/local
+go build -o $WORK/local ./exec.go
+exists $WORK/local/exec$GOEXE
+
+-- go.mod --
+module exmod
+
+-- cmd/c1/main.go --
+package main
+
+func main() {}
+
+-- cmd/c2/main.go --
+package main
+
+func main() {}
+
+-- pkg1/pkg1.go --
+package pkg1
+
+-- pkg2/pkg2.go --
+package pkg2
+
+-- exec.go --
+package main
+
+func main() {}
+
+-- c1$GOEXE/keep.txt --
+Create c1 directory.
diff --git a/src/cmd/go/testdata/script/build_n_cgo.txt b/src/cmd/go/testdata/script/build_n_cgo.txt
new file mode 100644
index 0000000..fa01927
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_n_cgo.txt
@@ -0,0 +1,18 @@
+[!cgo] skip
+
+# Test that nothing is prepended to $WORK path prefix.
+# See issue golang.org/issue/37012.
+go build -n
+! stderr '[/\\]\$WORK'
+stderr '[ =]\$WORK'
+
+-- go.mod --
+module m
+
+go 1.16
+-- main.go --
+package main
+
+import "C"
+
+var _ C.int
diff --git a/src/cmd/go/testdata/script/build_negative_p.txt b/src/cmd/go/testdata/script/build_negative_p.txt
new file mode 100644
index 0000000..9123907
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_negative_p.txt
@@ -0,0 +1,5 @@
+! go build -p=-1 example.go
+stderr 'go: -p must be a positive integer: -1'
+
+-- example.go --
+package example
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/build_no_go.txt b/src/cmd/go/testdata/script/build_no_go.txt
new file mode 100644
index 0000000..b61d752
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_no_go.txt
@@ -0,0 +1,41 @@
+! go build ./empty/test
+stderr 'no non-test Go files in '
+
+! go build ./empty/xtest
+stderr 'no non-test Go files in '
+
+! go build ./empty/testxtest
+stderr 'no non-test Go files in '
+
+! go build ./exclude
+stderr 'build constraints exclude all Go files in '
+
+! go build ./exclude/ignore
+stderr 'no Go files in '
+
+! go build ./exclude/empty
+stderr 'no Go files in '
+
+-- go.mod --
+module m
+
+go 1.16
+-- empty/test/test_test.go --
+package p
+-- empty/testxtest/test_test.go --
+package p
+-- empty/testxtest/xtest_test.go --
+package p_test
+-- empty/xtest/xtest_test.go --
+package p_test
+-- exclude/empty/x.txt --
+-- exclude/ignore/_x.go --
+package x
+-- exclude/x.go --
+// +build linux,!linux
+
+package x
+-- exclude/x_linux.go --
+// +build windows
+
+package x
diff --git a/src/cmd/go/testdata/script/build_nocache.txt b/src/cmd/go/testdata/script/build_nocache.txt
new file mode 100644
index 0000000..b21e755
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_nocache.txt
@@ -0,0 +1,40 @@
+env GO111MODULE=off
+
+# As of Go 1.12, the module cache is required.
+
+# If none of the variables we use to locate GOCACHE are set, the cache is off
+# and we cannot build.
+env GOCACHE=
+env XDG_CACHE_HOME=
+env HOME=
+[GOOS:plan9] env home=
+[GOOS:windows] env LocalAppData=
+! go build -o triv triv.go
+stderr 'build cache is required, but could not be located: GOCACHE is not defined and .*'
+
+# If GOCACHE is set but is not an absolute path, and we cannot build.
+env GOCACHE=test
+! go build -o triv triv.go
+stderr 'build cache is required, but could not be located: GOCACHE is not an absolute path'
+
+# An explicit GOCACHE=off also disables builds.
+env GOCACHE=off
+! go build -o triv triv.go
+stderr 'build cache is disabled by GOCACHE=off'
+
+# If GOCACHE is set to an unwritable directory, we should diagnose it as such.
+[GOOS:windows] stop # Does not support unwritable directories.
+[root] skip # Can write to unwritable directories.
+
+mkdir $WORK/unwritable/home
+chmod 0555 $WORK/unwritable/home
+[!GOOS:plan9] env HOME=$WORK/unwritable/home
+[GOOS:plan9] env home=$WORK/unwritable/home
+
+env GOCACHE=$WORK/unwritable/home
+! go build -o triv triv.go
+stderr 'failed to initialize build cache.* permission denied'
+
+-- triv.go --
+package main
+func main() {}
diff --git a/src/cmd/go/testdata/script/build_output.txt b/src/cmd/go/testdata/script/build_output.txt
new file mode 100644
index 0000000..457960f
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_output.txt
@@ -0,0 +1,117 @@
+[compiler:gccgo] skip 'gccgo has no standard packages'
+[short] skip
+
+[!GOOS:windows] env NONEXE='.exe'
+[GOOS:windows] env NONEXE=''
+
+env GOBIN=$WORK/tmp/bin
+go install m/isarchive &
+
+go build x.go
+exists -exec x$GOEXE
+rm x$GOEXE
+! exists x$NONEXE
+
+go build -o myprog x.go
+! exists x
+! exists x.exe
+exists -exec myprog
+! exists myprogr.exe
+
+! exists bin
+go build -o bin/x x.go
+exists -exec bin/x
+rm bin
+
+! exists bin
+go build -o bin/ x.go
+exists -exec bin/x$GOEXE
+rm bin
+
+[GOOS:windows] ! exists bin
+[GOOS:windows] go build -o bin\x x.go
+[GOOS:windows] exists -exec bin\x
+[GOOS:windows] rm bin
+
+[GOOS:windows] ! exists bin
+[GOOS:windows] go build -o bin\ x.go
+[GOOS:windows] exists -exec bin\x.exe
+[GOOS:windows] rm bin
+
+! exists bin
+mkdir bin
+go build -o bin x.go
+exists -exec bin/x$GOEXE
+rm bin
+
+go build p.go
+! exists p
+! exists p.a
+! exists p.o
+! exists p.exe
+
+wait # for isarchive
+
+go build -o p.a p.go
+exists p.a
+exec $GOBIN/isarchive p.a
+
+go build cmd/gofmt
+exists -exec gofmt$GOEXE
+rm gofmt$GOEXE
+! exists gofmt$NONEXE
+
+go build -o mygofmt cmd/gofmt
+exists -exec mygofmt
+! exists mygofmt.exe
+! exists gofmt
+! exists gofmt.exe
+
+go build sync/atomic
+! exists atomic
+! exists atomic.exe
+
+go build -o myatomic.a sync/atomic
+exists myatomic.a
+exec $GOBIN/isarchive myatomic.a
+! exists atomic
+! exists atomic.a
+! exists atomic.exe
+
+! go build -o whatever cmd/gofmt sync/atomic
+stderr 'multiple packages'
+
+-- go.mod --
+module m
+
+go 1.16
+-- x.go --
+package main
+
+func main() {}
+-- p.go --
+package p
+-- isarchive/isarchive.go --
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+)
+
+func main() {
+	f, err := os.Open(os.Args[1])
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	buf := make([]byte, 100)
+	io.ReadFull(f, buf)
+	f.Close()
+	if !bytes.HasPrefix(buf, []byte("!<arch>\n")) {
+		fmt.Fprintf(os.Stderr, "file %s exists but is not an archive\n", os.Args[1])
+		os.Exit(1)
+	}
+}
diff --git a/src/cmd/go/testdata/script/build_overlay.txt b/src/cmd/go/testdata/script/build_overlay.txt
new file mode 100644
index 0000000..b64bc02
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_overlay.txt
@@ -0,0 +1,313 @@
+[short] skip
+
+# Test building in overlays.
+# TODO(#39958): add a test case where the destination file in the replace map
+#   isn't a go file. Either completely exclude that case in fs.IsDirWithGoFiles
+#   if the compiler doesn't allow it, or test that it works all the way.
+# TODO(#39958): add a test that both gc and gccgo assembly files can include .h
+#   files.
+
+# The main package (m) is contained in an overlay. It imports m/dir2 which has one
+# file in an overlay and one file outside the overlay, which in turn imports m/dir,
+# which only has source files in the overlay.
+
+cd m
+
+! go build .
+go build -overlay overlay.json -o main$GOEXE .
+exec ./main$goexe
+stdout '^hello$'
+
+go build -overlay overlay.json -o print_abspath$GOEXE ./printpath
+exec ./print_abspath$GOEXE
+stdout $WORK[/\\]gopath[/\\]src[/\\]m[/\\]printpath[/\\]main.go
+
+go build -overlay overlay.json -o print_trimpath$GOEXE -trimpath ./printpath
+exec ./print_trimpath$GOEXE
+stdout ^m[/\\]printpath[/\\]main.go
+
+go build -overlay overlay.json -o print_trimpath_two_files$GOEXE printpath/main.go printpath/other.go
+exec ./print_trimpath_two_files$GOEXE
+stdout $WORK[/\\]gopath[/\\]src[/\\]m[/\\]printpath[/\\]main.go
+stdout $WORK[/\\]gopath[/\\]src[/\\]m[/\\]printpath[/\\]other.go
+
+[cgo] go build -overlay overlay.json -o main_cgo_replace$GOEXE ./cgo_hello_replace
+[cgo] exec ./main_cgo_replace$GOEXE
+[cgo] stdout '^hello cgo\r?\n'
+
+[cgo] go build -overlay overlay.json -o main_cgo_quote$GOEXE ./cgo_hello_quote
+[cgo] exec ./main_cgo_quote$GOEXE
+[cgo] stdout '^hello cgo\r?\n'
+
+[cgo] go build -overlay overlay.json -o main_cgo_angle$GOEXE ./cgo_hello_angle
+[cgo] exec ./main_cgo_angle$GOEXE
+[cgo] stdout '^hello cgo\r?\n'
+
+go build -overlay overlay.json -o main_call_asm$GOEXE ./call_asm
+exec ./main_call_asm$GOEXE
+! stdout .
+
+[cgo] go list -compiled -overlay overlay.json -f '{{range .CompiledGoFiles}}{{. | printf "%s\n"}}{{end}}' ./cgo_hello_replace
+[cgo] cp stdout compiled_cgo_sources.txt
+[cgo] go run ../print_line_comments.go compiled_cgo_sources.txt
+[cgo] stdout $GOPATH[/\\]src[/\\]m[/\\]cgo_hello_replace[/\\]cgo_hello_replace.go
+[cgo] ! stdout $GOPATH[/\\]src[/\\]m[/\\]overlay[/\\]hello.c
+
+# Change the contents of a file in the overlay and ensure that makes the target stale
+env OLD_GOCACHE=$GOCACHE
+env GOCACHE=$WORK/cache  # use a fresh cache so that multiple runs of the test don't interfere
+go build -x -overlay overlay.json ./test_cache
+stderr '(compile|gccgo)( |\.exe).*test_cache.go'
+go build -x -overlay overlay.json ./test_cache
+! stderr '(compile|gccgo)( |\.exe).*test_cache.go'  # cached
+cp overlay/test_cache_different.go overlay/test_cache.go
+go build -x -overlay overlay.json ./test_cache
+stderr '(compile|gccgo)( |\.exe).*test_cache.go'  # not cached
+env CACHE=$OLD_GOCACHE
+
+# Run same tests but with gccgo.
+env GO111MODULE=off
+[!exec:gccgo] stop
+[cross] stop  # gccgo can't necessarily cross-compile
+
+! go build -compiler=gccgo .
+go build -compiler=gccgo -overlay overlay.json -o main_gccgo$GOEXE .
+exec ./main_gccgo$goexe
+stdout '^hello$'
+
+go build -compiler=gccgo -overlay overlay.json -o print_abspath_gccgo$GOEXE ./printpath
+exec ./print_abspath_gccgo$GOEXE
+stdout $WORK[/\\]gopath[/\\]src[/\\]m[/\\]printpath[/\\]main.go
+
+go build -compiler=gccgo -overlay overlay.json -o print_trimpath_gccgo$GOEXE -trimpath ./printpath
+exec ./print_trimpath_gccgo$GOEXE
+stdout ^\.[/\\]printpath[/\\]main.go
+
+
+go build -compiler=gccgo  -overlay overlay.json -o main_cgo_replace_gccgo$GOEXE ./cgo_hello_replace
+exec ./main_cgo_replace_gccgo$GOEXE
+stdout '^hello cgo\r?\n'
+
+go build -compiler=gccgo  -overlay overlay.json -o main_cgo_quote_gccgo$GOEXE ./cgo_hello_quote
+exec ./main_cgo_quote_gccgo$GOEXE
+stdout '^hello cgo\r?\n'
+
+go build -compiler=gccgo  -overlay overlay.json -o main_cgo_angle_gccgo$GOEXE ./cgo_hello_angle
+exec ./main_cgo_angle_gccgo$GOEXE
+stdout '^hello cgo\r?\n'
+
+go build -compiler=gccgo -overlay overlay.json -o main_call_asm_gccgo$GOEXE ./call_asm
+exec ./main_call_asm_gccgo$GOEXE
+! stdout .
+
+
+-- m/go.mod --
+// TODO(matloob): how do overlays work with go.mod (especially if mod=readonly)
+module m
+
+go 1.16
+
+-- m/dir2/h.go --
+package dir2
+
+func PrintMessage() {
+	printMessage()
+}
+-- m/dir/foo.txt --
+The build action code currently expects the package directory
+to exist, so it can run the compiler in that directory.
+TODO(matloob): Remove this requirement.
+-- m/printpath/about.txt --
+the actual code is in the overlay
+-- m/overlay.json --
+{
+	"Replace": {
+		"f.go": "overlay/f.go",
+		"dir/g.go": "overlay/dir_g.go",
+		"dir2/i.go": "overlay/dir2_i.go",
+		"printpath/main.go": "overlay/printpath.go",
+		"printpath/other.go": "overlay2/printpath2.go",
+		"call_asm/asm_gc.s": "overlay/asm_gc.s",
+		"call_asm/asm_gccgo.s": "overlay/asm_gccgo.s",
+		"test_cache/main.go": "overlay/test_cache.go",
+		"cgo_hello_replace/cgo_header.h": "overlay/cgo_head.h",
+		"cgo_hello_replace/hello.c": "overlay/hello.c",
+		"cgo_hello_quote/cgo_hello.go": "overlay/cgo_hello_quote.go",
+		"cgo_hello_quote/cgo_header.h": "overlay/cgo_head.h",
+		"cgo_hello_angle/cgo_hello.go": "overlay/cgo_hello_angle.go",
+		"cgo_hello_angle/cgo_header.h": "overlay/cgo_head.h"
+	}
+}
+-- m/cgo_hello_replace/cgo_hello_replace.go --
+package main
+
+// #include "cgo_header.h"
+import "C"
+
+func main() {
+	C.say_hello()
+}
+-- m/cgo_hello_replace/cgo_header.h --
+ // Test that this header is replaced with one that has the proper declaration.
+void say_goodbye();
+
+-- m/cgo_hello_replace/hello.c --
+#include <stdio.h>
+
+void say_goodbye() { puts("goodbye cgo\n"); fflush(stdout); }
+
+-- m/overlay/f.go --
+package main
+
+import "m/dir2"
+
+func main() {
+	dir2.PrintMessage()
+}
+-- m/call_asm/main.go --
+package main
+
+func foo() // There will be a "missing function body" error if the assembly file isn't found.
+
+func main() {
+	foo()
+}
+-- m/overlay/dir_g.go --
+package dir
+
+import "fmt"
+
+func PrintMessage() {
+	fmt.Println("hello")
+}
+-- m/overlay/printpath.go --
+package main
+
+import (
+	"fmt"
+	"path/filepath"
+	"runtime"
+)
+
+func main() {
+	_, file, _, _ := runtime.Caller(0)
+
+	// Since https://golang.org/cl/214286, the runtime's debug paths are
+	// slash-separated regardless of platform, so normalize them to system file
+	// paths.
+	fmt.Println(filepath.FromSlash(file))
+}
+-- m/overlay2/printpath2.go --
+package main
+
+import (
+	"fmt"
+	"path/filepath"
+	"runtime"
+)
+
+func init() {
+	_, file, _, _ := runtime.Caller(0)
+	fmt.Println(filepath.FromSlash(file))
+}
+-- m/overlay/dir2_i.go --
+package dir2
+
+import "m/dir"
+
+func printMessage() {
+	dir.PrintMessage()
+}
+-- m/overlay/cgo_hello_quote.go --
+package main
+
+// #include "cgo_header.h"
+import "C"
+
+func main() {
+	C.say_hello()
+}
+-- m/overlay/cgo_hello_angle.go --
+package main
+
+// #include <cgo_header.h>
+import "C"
+
+func main() {
+	C.say_hello()
+}
+-- m/overlay/cgo_head.h --
+void say_hello();
+-- m/overlay/hello.c --
+#include <stdio.h>
+
+void say_hello() { puts("hello cgo\n"); fflush(stdout); }
+-- m/overlay/asm_gc.s --
+// +build gc
+
+TEXT ·foo(SB),0,$0
+	RET
+
+-- m/overlay/asm_gccgo.s --
+// +build gccgo
+
+.globl main.foo
+.text
+main.foo:
+	ret
+
+-- m/overlay/test_cache.go --
+package foo
+
+import "fmt"
+
+func bar() {
+	fmt.Println("something")
+}
+-- m/overlay/test_cache_different.go --
+package foo
+
+import "fmt"
+
+func bar() {
+	fmt.Println("different")
+}
+-- m/cgo_hello_quote/hello.c --
+#include <stdio.h>
+
+void say_hello() { puts("hello cgo\n"); fflush(stdout); }
+-- m/cgo_hello_angle/hello.c --
+#include <stdio.h>
+
+void say_hello() { puts("hello cgo\n"); fflush(stdout); }
+
+-- print_line_comments.go --
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"log"
+	"os"
+	"strings"
+)
+
+func main() {
+	compiledGoFilesArg := os.Args[1]
+	b, err := ioutil.ReadFile(compiledGoFilesArg)
+	if err != nil {
+		log.Fatal(err)
+	}
+	compiledGoFiles := strings.Split(strings.TrimSpace(string(b)), "\n")
+	for _, f := range compiledGoFiles {
+		b, err := ioutil.ReadFile(f)
+		if err != nil {
+			log.Fatal(err)
+		}
+		for _, line := range strings.Split(string(b), "\n") {
+			if strings.HasPrefix(line, "#line") || strings.HasPrefix(line, "//line") {
+				fmt.Println(line)
+			}
+		}
+	}
+}
diff --git a/src/cmd/go/testdata/script/build_patterns_outside_gopath.txt b/src/cmd/go/testdata/script/build_patterns_outside_gopath.txt
new file mode 100644
index 0000000..6a600cf
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_patterns_outside_gopath.txt
@@ -0,0 +1,36 @@
+# Tests issue #18778
+[short] skip
+
+cd pkgs
+
+env GO111MODULE=off
+go build ./...
+! stdout .
+go test ./...
+stdout '^ok'
+go list ./...
+stdout 'pkgs$'
+stdout 'pkgs/a'
+
+-- pkgs/go.mod --
+module pkgs
+
+go 1.16
+-- pkgs/a.go --
+package x
+-- pkgs/a_test.go --
+package x_test
+
+import "testing"
+
+func TestX(t *testing.T) {
+}
+-- pkgs/a/a.go --
+package a
+-- pkgs/a/a_test.go --
+package a_test
+
+import "testing"
+
+func TestA(t *testing.T) {
+}
diff --git a/src/cmd/go/testdata/script/build_pgo.txt b/src/cmd/go/testdata/script/build_pgo.txt
new file mode 100644
index 0000000..3b0804b
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_pgo.txt
@@ -0,0 +1,74 @@
+# Test go build -pgo flag.
+# Specifically, the build cache handles profile content correctly.
+
+[short] skip 'compiles and links executables'
+
+# build without PGO
+go build triv.go
+
+# build with PGO, should trigger rebuild
+# starting with an empty profile (the compiler accepts it)
+go build -x -pgo=prof -o triv.exe triv.go
+stderr 'compile.*-pgoprofile=.*prof.*triv.go'
+
+# check that PGO appears in build info
+# N.B. we can't start the stdout check with -pgo because the script assumes that
+# if the first arg starts with - it is a grep flag.
+go version -m triv.exe
+stdout 'build\s+-pgo=.*'${/}'prof'
+
+# store the build ID
+go list -export -json=BuildID -pgo=prof triv.go
+stdout '"BuildID":' # check that output actually contains a build ID
+cp stdout list.out
+
+# build again with the same profile, should be cached
+go build -x -pgo=prof -o triv.exe triv.go
+! stderr 'compile.*triv.go'
+
+# check that the build ID is the same
+go list -export -json=BuildID -pgo=prof triv.go
+cmp stdout list.out
+
+# overwrite the prof
+go run overwrite.go
+
+# build again, profile content changed, should trigger rebuild
+go build -n -pgo=prof triv.go
+stderr 'compile.*-pgoprofile=.*prof.*p.go'
+
+# check that the build ID is different
+go list -export -json=BuildID -pgo=prof triv.go
+! cmp stdout list.out
+
+# build with trimpath, buildinfo path should be trimmed
+go build -x -pgo=prof -trimpath -o triv.exe triv.go
+
+# check that path is trimmed
+go version -m triv.exe
+stdout 'build\s+-pgo=prof'
+
+-- prof --
+-- triv.go --
+package main
+func main() {}
+-- overwrite.go --
+package main
+
+import (
+	"os"
+	"runtime/pprof"
+)
+
+func main() {
+	f, err := os.Create("prof")
+	if err != nil {
+		panic(err)
+	}
+	err = pprof.StartCPUProfile(f)
+	if err != nil {
+		panic(err)
+	}
+	pprof.StopCPUProfile()
+	f.Close()
+}
diff --git a/src/cmd/go/testdata/script/build_pgo_auto.txt b/src/cmd/go/testdata/script/build_pgo_auto.txt
new file mode 100644
index 0000000..5dd799a
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_pgo_auto.txt
@@ -0,0 +1,94 @@
+# Test go build -pgo=auto flag.
+
+[short] skip 'compiles and links executables'
+
+# use default.pgo for a single main package
+go build -n -pgo=auto -o a1.exe ./a/a1
+stderr 'compile.*-pgoprofile=.*default\.pgo.*a1.go'
+
+# check that pgo applied to dependencies
+stderr 'compile.*-p test/dep.*-pgoprofile=.*default\.pgo'
+
+# check that pgo appears in build info
+# N.B. we can't start the stdout check with -pgo because the script assumes that
+# if the first arg starts with - it is a grep flag.
+stderr 'build\\t-pgo=.*default\.pgo'
+
+# use default.pgo for ... with a single main package
+go build -n -pgo=auto ./a/...
+stderr 'compile.*-pgoprofile=.*default\.pgo.*a1.go'
+
+# check that pgo appears in build info
+stderr 'build\\t-pgo=.*default\.pgo'
+
+# build succeeds without PGO when default.pgo file is absent
+go build -n -pgo=auto -o nopgo.exe ./nopgo
+stderr 'compile.*nopgo.go'
+! stderr 'compile.*-pgoprofile'
+
+# check that pgo doesn't appear in build info
+! stderr 'build\\t-pgo='
+
+# other build-related commands
+go install -a -n -pgo=auto ./a/a1
+stderr 'compile.*-pgoprofile=.*default\.pgo.*a1.go'
+
+go run -a -n -pgo=auto ./a/a1
+stderr 'compile.*-pgoprofile=.*default\.pgo.*a1.go'
+
+go test -a -n -pgo=auto ./a/a1
+stderr 'compile.*-pgoprofile=.*default\.pgo.*a1.go.*a1_test.go'
+stderr 'compile.*-pgoprofile=.*default\.pgo.*external_test.go'
+
+# go list commands should succeed as usual
+go list -pgo=auto ./a/a1
+
+go list -test -pgo=auto ./a/a1
+
+go list -deps -pgo=auto ./a/a1
+
+# -pgo=auto is the default. Commands without explicit -pgo=auto
+# should work as -pgo=auto.
+go build -a -n -o a1.exe ./a/a1
+stderr 'compile.*-pgoprofile=.*default\.pgo.*a1.go'
+stderr 'compile.*-p test/dep.*-pgoprofile=.*default\.pgo'
+
+# check that pgo appears in build info
+stderr 'build\\t-pgo=.*default\.pgo'
+
+go build -a -n -o nopgo.exe ./nopgo
+stderr 'compile.*nopgo.go'
+! stderr 'compile.*-pgoprofile'
+
+# check that pgo doesn't appear in build info
+! stderr 'build\\t-pgo='
+
+# -pgo=off should turn off PGO.
+go build -a -n -pgo=off -o a1.exe ./a/a1
+stderr 'compile.*a1.go'
+! stderr 'compile.*-pgoprofile'
+
+# check that pgo doesn't appear in build info
+! stderr 'build\\t-pgo='
+
+-- go.mod --
+module test
+go 1.20
+-- a/a1/a1.go --
+package main
+import _ "test/dep"
+func main() {}
+-- a/a1/a1_test.go --
+package main
+import "testing"
+func TestA(*testing.T) {}
+-- a/a1/external_test.go --
+package main_test
+import "testing"
+func TestExternal(*testing.T) {}
+-- a/a1/default.pgo --
+-- nopgo/nopgo.go --
+package main
+func main() {}
+-- dep/dep.go --
+package dep
diff --git a/src/cmd/go/testdata/script/build_pgo_auto_multi.txt b/src/cmd/go/testdata/script/build_pgo_auto_multi.txt
new file mode 100644
index 0000000..991b72c
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_pgo_auto_multi.txt
@@ -0,0 +1,109 @@
+# Test go build -pgo=auto flag with multiple main packages.
+
+go install -a -n -pgo=auto ./a ./b ./nopgo
+
+# a/default.pgo applies to package a and (transitive)
+# dependencies.
+stderr 'compile.*-pgoprofile=.*a(/|\\\\)default\.pgo.*a(/|\\\\)a\.go'
+stderr 'compile.*-pgoprofile=.*a(/|\\\\)default\.pgo.*dep(/|\\\\)dep\.go'
+stderr 'compile.*-pgoprofile=.*a(/|\\\\)default\.pgo.*dep2(/|\\\\)dep2\.go'
+stderr -count=1 'compile.*-pgoprofile=.*a(/|\\\\)default\.pgo.*dep3(/|\\\\)dep3\.go'
+
+# b/default.pgo applies to package b and (transitive)
+# dependencies.
+stderr 'compile.*-pgoprofile=.*b(/|\\\\)default\.pgo.*b(/|\\\\)b\.go'
+stderr 'compile.*-pgoprofile=.*b(/|\\\\)default\.pgo.*dep(/|\\\\)dep\.go'
+stderr 'compile.*-pgoprofile=.*b(/|\\\\)default\.pgo.*dep2(/|\\\\)dep2\.go'
+stderr -count=1 'compile.*-pgoprofile=.*b(/|\\\\)default\.pgo.*dep3(/|\\\\)dep3\.go'
+
+# nopgo should be built without PGO.
+! stderr 'compile.*-pgoprofile=.*nopgo(/|\\\\)nopgo\.go'
+
+# Dependencies should also be built without PGO.
+# Here we want to match a compile action without -pgoprofile,
+# by matching 3 occurrences of "compile dep.go", among which
+# 2 of them have -pgoprofile (therefore one without).
+stderr -count=3 'compile.*dep(/|\\\\)dep.go'
+stderr -count=2 'compile.*-pgoprofile=.*dep(/|\\\\)dep\.go'
+
+stderr -count=3 'compile.*dep2(/|\\\\)dep2.go'
+stderr -count=2 'compile.*-pgoprofile=.*dep2(/|\\\\)dep2\.go'
+
+stderr -count=3 'compile.*dep3(/|\\\\)dep3.go'
+stderr -count=2 'compile.*-pgoprofile=.*dep3(/|\\\\)dep3\.go'
+
+# check that pgo appears or not in build info as expected
+stderr 'path\\ttest/a\\n.*build\\t-pgo=.*a(/|\\\\)default\.pgo'
+stderr 'path\\ttest/b\\n.*build\\t-pgo=.*b(/|\\\\)default\.pgo'
+! stderr 'path\\ttest/nopgo\\n.*build\\t-pgo='
+
+# go test works the same way
+go test -a -n -pgo=auto ./a ./b ./nopgo
+stderr 'compile.*-pgoprofile=.*a(/|\\\\)default\.pgo.*a(/|\\\\)a_test\.go'
+stderr 'compile.*-pgoprofile=.*a(/|\\\\)default\.pgo.*dep(/|\\\\)dep\.go'
+stderr 'compile.*-pgoprofile=.*b(/|\\\\)default\.pgo.*b(/|\\\\)b_test\.go'
+stderr 'compile.*-pgoprofile=.*b(/|\\\\)default\.pgo.*dep(/|\\\\)dep\.go'
+! stderr 'compile.*-pgoprofile=.*nopgo(/|\\\\)nopgo_test\.go'
+
+# test-only dependencies also have profiles attached
+stderr 'compile.*-pgoprofile=.*a(/|\\\\)default\.pgo.*testdep(/|\\\\)testdep\.go'
+stderr 'compile.*-pgoprofile=.*b(/|\\\\)default\.pgo.*testdep(/|\\\\)testdep\.go'
+stderr 'compile.*-pgoprofile=.*a(/|\\\\)default\.pgo.*testdep2(/|\\\\)testdep2\.go'
+stderr 'compile.*-pgoprofile=.*b(/|\\\\)default\.pgo.*testdep2(/|\\\\)testdep2\.go'
+
+# go list -deps prints packages built multiple times.
+go list -pgo=auto -deps ./a ./b ./nopgo
+stdout 'test/dep \[test/a\]'
+stdout 'test/dep \[test/b\]'
+stdout 'test/dep$'
+
+# Here we have 3 main packages, a, b, and nopgo, where a and b each has
+# its own default.pgo profile, and nopgo has none.
+# All 3 main packages import dep and dep2, both of which then import dep3
+# (a diamond-shape import graph).
+-- go.mod --
+module test
+go 1.20
+-- a/a.go --
+package main
+import _ "test/dep"
+import _ "test/dep2"
+func main() {}
+-- a/a_test.go --
+package main
+import "testing"
+import _ "test/testdep"
+func TestA(*testing.T) {}
+-- a/default.pgo --
+-- b/b.go --
+package main
+import _ "test/dep"
+import _ "test/dep2"
+func main() {}
+-- b/b_test.go --
+package main
+import "testing"
+import _ "test/testdep"
+func TestB(*testing.T) {}
+-- b/default.pgo --
+-- nopgo/nopgo.go --
+package main
+import _ "test/dep"
+import _ "test/dep2"
+func main() {}
+-- nopgo/nopgo_test.go --
+package main
+import "testing"
+func TestNopgo(*testing.T) {}
+-- dep/dep.go --
+package dep
+import _ "test/dep3"
+-- dep2/dep2.go --
+package dep2
+-- dep3/dep3.go --
+package dep3
+-- testdep/testdep.go --
+package testdep
+import _ "test/testdep2"
+-- testdep2/testdep2.go --
+package testdep2
diff --git a/src/cmd/go/testdata/script/build_plugin_non_main.txt b/src/cmd/go/testdata/script/build_plugin_non_main.txt
new file mode 100644
index 0000000..e0bbbef
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_plugin_non_main.txt
@@ -0,0 +1,13 @@
+# Plugins are not supported on all platforms.
+[!buildmode:plugin] skip
+
+go build -n testdep
+! go build -buildmode=plugin testdep
+stderr '-buildmode=plugin requires exactly one main package'
+
+-- go.mod --
+module testdep
+
+go 1.16
+-- testdep.go --
+package p
diff --git a/src/cmd/go/testdata/script/build_plugin_reproducible.txt b/src/cmd/go/testdata/script/build_plugin_reproducible.txt
new file mode 100644
index 0000000..5369954
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_plugin_reproducible.txt
@@ -0,0 +1,11 @@
+[!buildmode:plugin] skip
+[short] skip
+
+go build -trimpath -buildvcs=false -buildmode=plugin -o a.so main.go
+go build -trimpath -buildvcs=false -buildmode=plugin -o b.so main.go
+cmp -q a.so b.so
+
+-- main.go --
+package main
+
+func main() {}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/build_relative_pkgdir.txt b/src/cmd/go/testdata/script/build_relative_pkgdir.txt
new file mode 100644
index 0000000..57f18ee
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_relative_pkgdir.txt
@@ -0,0 +1,9 @@
+env GO111MODULE=off
+
+# Regression test for golang.org/issue/21309: accept relative -pkgdir argument.
+
+[short] skip
+
+mkdir $WORK/gocache
+env GOCACHE=$WORK/gocache
+go build -pkgdir=. runtime
diff --git a/src/cmd/go/testdata/script/build_relative_tmpdir.txt b/src/cmd/go/testdata/script/build_relative_tmpdir.txt
new file mode 100644
index 0000000..ea7412e
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_relative_tmpdir.txt
@@ -0,0 +1,18 @@
+env GO111MODULE=off
+
+# If GOTMPDIR is relative, 'go build' should derive an absolute $WORK directory.
+cd $WORK
+mkdir tmp
+env GOTMPDIR=tmp
+go build -work a
+stderr 'WORK='$WORK
+
+# Similarly if TMP/TMPDIR is relative.
+env GOTMPDIR=
+env TMP=tmp    # Windows
+env TMPDIR=tmp # Unix
+go build -work a
+stderr 'WORK='$WORK
+
+-- a/a.go --
+package a
diff --git a/src/cmd/go/testdata/script/build_runtime_gcflags.txt b/src/cmd/go/testdata/script/build_runtime_gcflags.txt
new file mode 100644
index 0000000..c87e480
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_runtime_gcflags.txt
@@ -0,0 +1,11 @@
+env GO111MODULE=off
+[short] skip # rebuilds all of std
+
+# Set up fresh GOCACHE.
+env GOCACHE=$WORK/gocache
+mkdir $GOCACHE
+
+# Verify the standard library (specifically runtime/internal/atomic) can be
+# built with -gcflags when -n is given. See golang.org/issue/29346.
+go build -n -gcflags=all='-l' std
+stderr 'compile.* runtime/internal/atomic .* -l'
diff --git a/src/cmd/go/testdata/script/build_shorten_pkg.txt b/src/cmd/go/testdata/script/build_shorten_pkg.txt
new file mode 100644
index 0000000..38672b6
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_shorten_pkg.txt
@@ -0,0 +1,81 @@
+[short] skip
+
+# This test may go away when the loopvar experiment goes away.
+# Accurate reporting of notable loops in the presence of inlining
+# can create warnings in sibling directories, and it's nice if those
+# can be trimmed like subdirectory paths are.
+
+env GOEXPERIMENT=loopvar
+go build -gcflags=inlines/a=-d=loopvar=2 .
+stderr ^\.[\\/]b[\\/]b\.go:12:6:.*loop.inlined.into.a[\\/]a\.go
+stderr ^\.[\\/]b[\\/]b\.go:12:9:.*loop.inlined.into.a[\\/]a\.go
+
+-- go.mod --
+module inlines
+
+go 1.21
+-- a/a.go --
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package a
+
+import "inlines/b"
+
+func F() []*int {
+	var s []*int
+	for i := 0; i < 10; i++ {
+		s = append(s, &i)
+	}
+	return s
+}
+
+func Fb() []*int {
+	bf, _ := b.F()
+	return bf
+}
+-- b/b.go --
+package b
+
+var slice = []int{1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024}
+
+func F() ([]*int, []*int) {
+	return g()
+}
+
+func g() ([]*int, []*int) {
+	var s []*int
+	var t []*int
+	for i, j := range slice {
+		s = append(s, &i)
+		t = append(t, &j)
+	}
+	return s[:len(s)-1], t
+}
+-- main.go --
+package main
+
+import (
+	"fmt"
+	"inlines/a"
+	"inlines/b"
+)
+
+func sum(s []*int) int {
+	sum := 0
+	for _, pi := range s {
+		sum += *pi
+	}
+	return sum
+}
+
+func main() {
+	af := a.F()
+	bf, _ := b.F()
+	abf := a.Fb()
+
+	saf, sbf, sabf := sum(af), sum(bf), sum(abf)
+
+	fmt.Printf("af, bf, abf sums = %d, %d, %d\n", saf, sbf, sabf)
+}
diff --git a/src/cmd/go/testdata/script/build_single_error.txt b/src/cmd/go/testdata/script/build_single_error.txt
new file mode 100644
index 0000000..241cdb9
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_single_error.txt
@@ -0,0 +1,18 @@
+# go test ./... with a bad package should report the error once (#44624).
+! go test ./...
+stderr -count=1 undefined
+
+-- go.mod --
+module example.com
+
+go 1.18
+-- a/a.go --
+package a
+
+import "example.com/b"
+-- b/b.go --
+package b
+
+var X = Y
+-- b/b_test.go --
+package b
diff --git a/src/cmd/go/testdata/script/build_static.txt b/src/cmd/go/testdata/script/build_static.txt
new file mode 100644
index 0000000..7db90a1
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_static.txt
@@ -0,0 +1,44 @@
+[short] skip 'links and runs binaries'
+
+# This test requires external linking. Assume that if cgo is supported
+# then external linking works.
+[!cgo] skip 'requires a C linker'
+
+# Only run on Unix systems.
+[GOOS:windows] skip
+[GOOS:plan9] skip
+
+# Ordinary build should work.
+go build
+exec ./hello
+stdout Hello
+
+# Building with -linkmode=external should not say anything about
+# runtime/cgo (issue #31544).
+go build -ldflags=-linkmode=external
+! stderr runtime/cgo
+exec ./hello
+stdout Hello
+
+# Some targets don't support -static
+[GOOS:darwin] skip 'no static linking on Darwin'
+[GOOS:solaris] skip 'no static linking on Solaris'
+
+# Building with -linkmode=external -extldflags=-static should work.
+go build -ldflags='-linkmode=external -extldflags=-static'
+! stderr runtime/cgo
+exec ./hello
+stdout Hello
+
+-- go.mod --
+module hello
+
+go 1.20
+-- hello.go --
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("Hello, world")
+}
diff --git a/src/cmd/go/testdata/script/build_tag_goexperiment.txt b/src/cmd/go/testdata/script/build_tag_goexperiment.txt
new file mode 100644
index 0000000..bee218f
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_tag_goexperiment.txt
@@ -0,0 +1,31 @@
+[short] skip
+# Reset all experiments so fieldtrack is definitely off.
+env GOEXPERIMENT=none
+go run m
+stderr 'fieldtrack off'
+# Turn fieldtrack on.
+env GOEXPERIMENT=none,fieldtrack
+go run m
+stderr 'fieldtrack on'
+
+-- ft_off.go --
+// +build !goexperiment.fieldtrack
+
+package main
+
+func main() {
+	println("fieldtrack off")
+}
+
+-- ft_on.go --
+// +build goexperiment.fieldtrack
+
+package main
+
+func main() {
+	println("fieldtrack on")
+}
+
+-- go.mod --
+module m
+go 1.14
diff --git a/src/cmd/go/testdata/script/build_tags_no_comma.txt b/src/cmd/go/testdata/script/build_tags_no_comma.txt
new file mode 100644
index 0000000..a14a200
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_tags_no_comma.txt
@@ -0,0 +1,4 @@
+[compiler:gccgo] skip 'gccgo has no standard packages'
+go build -tags 'tag1 tag2' math
+! go build -tags 'tag1,tag2 tag3' math
+stderr 'space-separated list contains comma'
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/build_test_only.txt b/src/cmd/go/testdata/script/build_test_only.txt
new file mode 100644
index 0000000..8693a80
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_test_only.txt
@@ -0,0 +1,19 @@
+# Named explicitly, test-only packages should be reported as
+# unbuildable/uninstallable, even if there is a wildcard also matching.
+! go build m/testonly m/testonly...
+stderr 'no non-test Go files in'
+! go install ./testonly
+stderr 'no non-test Go files in'
+
+# Named through a wildcard, the test-only packages should be silently ignored.
+go build m/testonly...
+go install ./testonly...
+
+-- go.mod --
+module m
+
+go 1.16
+-- testonly/t_test.go --
+package testonly
+-- testonly2/t.go --
+package testonly2
diff --git a/src/cmd/go/testdata/script/build_trimpath.txt b/src/cmd/go/testdata/script/build_trimpath.txt
new file mode 100644
index 0000000..2a2aa20
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_trimpath.txt
@@ -0,0 +1,164 @@
+[short] skip
+
+# If GOROOT_FINAL is set, 'go build -trimpath' bakes that into the resulting
+# binary instead of GOROOT. Explicitly unset it here.
+env GOROOT_FINAL=
+
+# Set up two identical directories that can be used as GOPATH.
+env GO111MODULE=on
+mkdir $WORK/a/src/paths $WORK/b/src/paths
+cp paths.go $WORK/a/src/paths
+cp paths.go $WORK/b/src/paths
+cp overlay.json $WORK/a/src/paths
+cp overlay.json $WORK/b/src/paths
+cp go.mod $WORK/a/src/paths/
+cp go.mod $WORK/b/src/paths/
+
+
+# A binary built without -trimpath should contain the module root dir
+# and GOROOT for debugging and stack traces.
+cd $WORK/a/src/paths
+go build -o $WORK/paths-dbg.exe .
+exec $WORK/paths-dbg.exe $WORK/paths-dbg.exe
+stdout 'binary contains module root: true'
+stdout 'binary contains GOROOT: true'
+
+# A binary built with -trimpath should not contain the current workspace
+# or GOROOT.
+go build -trimpath -o $WORK/paths-a.exe .
+exec $WORK/paths-a.exe $WORK/paths-a.exe
+stdout 'binary contains module root: false'
+stdout 'binary contains GOROOT: false'
+
+# A binary from an external module built with -trimpath should not contain
+# the current workspace or GOROOT.
+go get rsc.io/fortune
+go install -trimpath rsc.io/fortune
+exec $WORK/paths-a.exe $GOPATH/bin/fortune$GOEXE
+stdout 'binary contains module root: false'
+stdout 'binary contains GOROOT: false'
+go mod edit -droprequire rsc.io/fortune
+
+# Two binaries built from identical packages in different directories
+# should be identical.
+cd $WORK/b/src/paths
+go build -trimpath -o $WORK/paths-b.exe
+cmp -q $WORK/paths-a.exe $WORK/paths-b.exe
+
+
+# Same sequence of tests but with overlays.
+# A binary built without -trimpath should contain the module root dir
+# and GOROOT for debugging and stack traces.
+cd $WORK/a/src/paths
+go build -overlay overlay.json -o $WORK/paths-dbg.exe ./overlaydir
+exec $WORK/paths-dbg.exe $WORK/paths-dbg.exe
+stdout 'binary contains module root: true'
+stdout 'binary contains GOROOT: true'
+
+# A binary built with -trimpath should not contain the current workspace
+# or GOROOT.
+go build -overlay overlay.json -trimpath -o $WORK/paths-a.exe ./overlaydir
+exec $WORK/paths-a.exe $WORK/paths-a.exe
+stdout 'binary contains module root: false'
+stdout 'binary contains GOROOT: false'
+
+# Two binaries built from identical packages in different directories
+# should be identical.
+cd $WORK/b/src/paths
+go build -overlay overlay.json -trimpath -o $WORK/paths-b.exe ./overlaydir
+cmp -q $WORK/paths-a.exe $WORK/paths-b.exe
+
+
+# Same sequence of tests but in GOPATH mode.
+# A binary built without -trimpath should contain GOPATH and GOROOT.
+env GO111MODULE=off
+cd $WORK
+env GOPATH=$WORK/a
+go build -o paths-dbg.exe paths
+exec ./paths-dbg.exe paths-dbg.exe
+stdout 'binary contains GOPATH: true'
+stdout 'binary contains GOROOT: true'
+
+# A binary built with -trimpath should not contain GOPATH or GOROOT.
+go build -trimpath -o paths-a.exe paths
+exec ./paths-a.exe paths-a.exe
+stdout 'binary contains GOPATH: false'
+stdout 'binary contains GOROOT: false'
+
+# Two binaries built from identical packages in different GOPATH roots
+# should be identical.
+env GOPATH=$WORK/b
+go build -trimpath -o paths-b.exe paths
+cmp -q paths-a.exe paths-b.exe
+
+
+# Same sequence of tests but with gccgo.
+# gccgo does not support builds in module mode.
+[!exec:gccgo] stop
+[cross] stop  # gccgo can't necessarily cross-compile
+env GOPATH=$WORK/a
+
+# A binary built with gccgo without -trimpath should contain the current
+# GOPATH and GOROOT.
+go build -compiler=gccgo -o paths-dbg.exe paths
+exec ./paths-dbg.exe paths-dbg.exe
+stdout 'binary contains GOPATH: true'
+stdout 'binary contains GOROOT: false' # gccgo doesn't load std from GOROOT.
+
+# A binary built with gccgo with -trimpath should not contain GOPATH or GOROOT.
+go build -compiler=gccgo -trimpath -o paths-a.exe paths
+exec ./paths-a.exe paths-a.exe
+stdout 'binary contains GOPATH: false'
+stdout 'binary contains GOROOT: false'
+
+# Two binaries built from identical packages in different directories
+# should be identical.
+env GOPATH=$WORK/b
+go build -compiler=gccgo -trimpath -o paths-b.exe paths
+cmp -q paths-a.exe paths-b.exe
+
+-- paths.go --
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"log"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+)
+
+func main() {
+	exe := os.Args[1]
+	data, err := ioutil.ReadFile(exe)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	if os.Getenv("GO111MODULE") == "on" {
+		out, err := exec.Command("go", "env", "GOMOD").Output()
+		if err != nil {
+			log.Fatal(err)
+		}
+		modRoot := filepath.Dir(strings.TrimSpace(string(out)))
+		check(data, "module root", modRoot)
+	} else {
+		check(data, "GOPATH", os.Getenv("GOPATH"))
+	}
+	check(data, "GOROOT", os.Getenv("GOROOT"))
+}
+
+func check(data []byte, desc, dir string) {
+	containsDir := bytes.Contains(data, []byte(dir))
+	containsSlashDir := bytes.Contains(data, []byte(filepath.ToSlash(dir)))
+	fmt.Printf("binary contains %s: %v\n", desc, containsDir || containsSlashDir)
+}
+-- overlay.json --
+{ "Replace": { "overlaydir/paths.go": "paths.go" } }
+-- go.mod --
+module paths
+
+go 1.14
diff --git a/src/cmd/go/testdata/script/build_trimpath_cgo.txt b/src/cmd/go/testdata/script/build_trimpath_cgo.txt
new file mode 100644
index 0000000..5289824
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_trimpath_cgo.txt
@@ -0,0 +1,184 @@
+# This test builds a cgo binary and verifies the source directory path
+# does not appear in the binary, either literally or in compressed DWARF.
+# TODO(golang.org/issue/36072): ideally we should build a binary from identical
+# sources in different directories and verify the binary and all intermediate
+# files are identical.
+
+[short] skip
+[!cgo] skip
+
+# Check that the source path appears when -trimpath is not used.
+go build -o hello.exe .
+grep -q gopath[/\\]src hello.exe
+go run ./list-dwarf hello.exe
+stdout gopath[/\\]src
+
+# Check that the source path does not appear when -trimpath is used.
+[GOOS:aix] stop # can't inspect XCOFF binaries
+go build -trimpath -o hello.exe .
+! grep -q gopath[/\\]src hello.exe
+go run ./list-dwarf hello.exe
+! stdout gopath/src
+
+
+# Do the above, with the cgo (but not .c) sources in an overlay
+# Check that the source path appears when -trimpath is not used.
+mkdir $WORK/overlay
+cp hello.go $WORK/overlay/hello.go
+mkdir hello_overlay
+cp hello.c hello_overlay/hello.c
+go build -overlay overlay.json -o hello_overlay.exe ./hello_overlay
+grep -q gopath[/\\]src hello_overlay.exe
+! grep -q $WORK[/\\]overlay hello_overlay.exe
+go run ./list-dwarf hello_overlay.exe
+stdout gopath[/\\]src
+! stdout $WORK[/\\]overlay
+
+# Check that the source path does not appear when -trimpath is used.
+go build -overlay overlay.json -trimpath -o hello_overlay.exe ./hello_overlay
+! grep -q gopath[/\\]src hello_overlay.exe
+! grep -q $WORK[/\\]overlay hello_overlay.exe
+go run ./list-dwarf hello_overlay.exe
+! stdout gopath/src
+! stdout $WORK[/\\]overlay
+
+-- go.mod --
+module m
+
+go 1.14
+-- overlay.json --
+{
+	"Replace": {
+		"hello_overlay/hello.go": "../../overlay/hello.go"
+	}
+}
+-- hello.c --
+#include <stdio.h>
+
+void say_hello() { puts("Hello, world!\n"); }
+
+-- hello.go --
+package main
+
+// void say_hello();
+import "C"
+
+func main() {
+	C.say_hello()
+}
+
+-- list-dwarf/list-dwarf.go --
+package main
+
+import (
+	"debug/dwarf"
+	"fmt"
+	"io"
+	"log"
+	"os"
+	"sort"
+)
+
+func main() {
+	files, err := run(os.Args[1])
+	if err != nil {
+		log.Fatal(err)
+	}
+	for _, file := range files {
+		fmt.Println(file)
+	}
+}
+
+func run(exePath string) ([]string, error) {
+	dwarfData, err := readDWARF(exePath)
+	if err != nil {
+		return nil, err
+	}
+
+	dwarfReader := dwarfData.Reader()
+	files := make(map[string]bool)
+	for {
+		e, err := dwarfReader.Next()
+		if err != nil {
+			return nil, err
+		}
+		if e == nil {
+			break
+		}
+		lr, err := dwarfData.LineReader(e)
+		if err != nil {
+			return nil, err
+		}
+		if lr == nil {
+			continue
+		}
+
+		var le dwarf.LineEntry
+		for {
+			if err := lr.Next(&le); err != nil {
+				if err == io.EOF {
+					break
+				}
+				return nil, err
+			}
+			files[le.File.Name] = true
+		}
+	}
+
+	sortedFiles := make([]string, 0, len(files))
+	for file := range files {
+		sortedFiles = append(sortedFiles, file)
+	}
+	sort.Strings(sortedFiles)
+	return sortedFiles, nil
+}
+-- list-dwarf/read_darwin.go --
+package main
+
+import (
+	"debug/dwarf"
+	"debug/macho"
+)
+
+func readDWARF(exePath string) (*dwarf.Data, error) {
+	machoFile, err := macho.Open(exePath)
+	if err != nil {
+		return nil, err
+	}
+	defer machoFile.Close()
+	return machoFile.DWARF()
+}
+-- list-dwarf/read_elf.go --
+// +build android dragonfly freebsd illumos linux netbsd openbsd solaris
+
+package main
+
+import (
+	"debug/dwarf"
+	"debug/elf"
+)
+
+func readDWARF(exePath string) (*dwarf.Data, error) {
+	elfFile, err := elf.Open(exePath)
+	if err != nil {
+		return nil, err
+	}
+	defer elfFile.Close()
+	return elfFile.DWARF()
+}
+-- list-dwarf/read_windows.go --
+package main
+
+import (
+	"debug/dwarf"
+	"debug/pe"
+)
+
+func readDWARF(exePath string) (*dwarf.Data, error) {
+	peFile, err := pe.Open(exePath)
+	if err != nil {
+		return nil, err
+	}
+	defer peFile.Close()
+	return peFile.DWARF()
+}
diff --git a/src/cmd/go/testdata/script/build_trimpath_goroot.txt b/src/cmd/go/testdata/script/build_trimpath_goroot.txt
new file mode 100644
index 0000000..a26cfd2
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_trimpath_goroot.txt
@@ -0,0 +1,103 @@
+# Regression test for https://go.dev/issue/51461 and https://go.dev/issue/51483.
+#
+# When built with -trimpath, runtime.GOROOT() returned the bogus string "go"
+# if GOROOT was not set explicitly in the environment.
+# It should instead return the empty string, since we know that we don't
+# have a valid path to return.
+#
+# TODO(#51483): when runtime.GOROOT() returns the empty string,
+# go/build should default to 'go env GOROOT' instead.
+
+env GOROOT_FINAL=
+
+[trimpath] env GOROOT=
+[trimpath] ! go env GOROOT
+[trimpath] stderr '^go: cannot find GOROOT directory: ''go'' binary is trimmed and GOROOT is not set$'
+[trimpath] env GOROOT=$TESTGO_GOROOT
+
+[short] stop
+
+# With GOROOT still set but GOROOT_FINAL unset, 'go build' and 'go test -c'
+# should cause runtime.GOROOT() to report either the correct GOROOT
+# (without -trimpath) or no GOROOT at all (with -trimpath).
+
+go build -o example.exe .
+go build -trimpath -o example-trimpath.exe .
+go test -c -o example.test.exe .
+go test -trimpath -c -o example.test-trimpath.exe .
+
+env GOROOT=
+
+exec ./example.exe
+stdout '^GOROOT '$TESTGO_GOROOT'$'
+stdout '^runtime '$TESTGO_GOROOT${/}src${/}runtime'$'
+
+! exec ./example-trimpath.exe
+stdout '^GOROOT $'
+stderr 'cannot find package "runtime" in any of:\n\t\(\$GOROOT not set\)\n\t'$WORK${/}gopath${/}src${/}runtime' \(from \$GOPATH\)\n\z'
+
+exec ./example.test.exe -test.v
+stdout '^GOROOT '$TESTGO_GOROOT'$'
+stdout '^runtime '$TESTGO_GOROOT${/}src${/}runtime'$'
+
+! exec ./example.test-trimpath.exe -test.v
+stdout '^GOROOT $'
+stderr 'cannot find package "runtime" in any of:\n\t\(\$GOROOT not set\)\n\t'$WORK${/}gopath${/}src${/}runtime' \(from \$GOPATH\)$'
+
+# If a correct GOROOT is baked in to the 'go' command itself, 'go run' and
+# 'go test' should not implicitly set GOROOT in the process environment
+# (because that could mask an unexpected production dependency on the GOROOT
+# environment variable), but 'go generate' should (because the generator may
+# reasonably expect to be able to locate the GOROOT for which it is generating
+# code).
+
+[trimpath] stop
+[mismatched-goroot] stop
+
+! go run -trimpath .
+stdout '^GOROOT $'
+stderr 'cannot find package "runtime" in any of:\n\t\(\$GOROOT not set\)\n\t'$WORK${/}gopath${/}src${/}runtime' \(from \$GOPATH\)\nexit status 1\n\z'
+
+! go test -trimpath -v .
+stdout '^GOROOT $'
+stdout 'cannot find package "runtime" in any of:\n\t\(\$GOROOT not set\)\n\t'$WORK${/}gopath${/}src${/}runtime' \(from \$GOPATH\)$'
+
+env GOFLAGS=-trimpath
+go generate .
+stdout '^GOROOT '$TESTGO_GOROOT'$'
+stdout '^runtime '$TESTGO_GOROOT${/}src${/}runtime'$'
+
+-- go.mod --
+module example
+
+go 1.19
+-- main.go --
+package main
+
+//go:generate go run .
+
+import (
+	"fmt"
+	"go/build"
+	"os"
+	"runtime"
+)
+
+func main() {
+	fmt.Println("GOROOT", runtime.GOROOT())
+
+	p, err := build.Default.Import("runtime", "", build.FindOnly)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	fmt.Println("runtime", p.Dir)
+}
+-- main_test.go --
+package main
+
+import "testing"
+
+func TestMain(*testing.M) {
+	main()
+}
diff --git a/src/cmd/go/testdata/script/build_unsupported_goos.txt b/src/cmd/go/testdata/script/build_unsupported_goos.txt
new file mode 100644
index 0000000..c94d6d2
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_unsupported_goos.txt
@@ -0,0 +1,6 @@
+[compiler:gccgo] skip # gccgo assumes cross-compilation is always possible
+
+env GOOS=windwos # intentional misspelling of windows
+
+! go build -n exclude
+stderr 'unsupported GOOS/GOARCH pair'
diff --git a/src/cmd/go/testdata/script/build_vendor.txt b/src/cmd/go/testdata/script/build_vendor.txt
new file mode 100644
index 0000000..f430ff2
--- /dev/null
+++ b/src/cmd/go/testdata/script/build_vendor.txt
@@ -0,0 +1,42 @@
+# Build
+env GO111MODULE=off
+go build vend/x
+! stdout .
+! stderr .
+
+-- vend/dir1/dir1.go --
+package dir1
+-- vend/subdir/bad.go --
+package subdir
+
+import _ "r"
+-- vend/subdir/good.go --
+package subdir
+
+import _ "p"
+-- vend/vendor/p/p.go --
+package p
+-- vend/vendor/q/q.go --
+package q
+-- vend/vendor/vend/dir1/dir2/dir2.go --
+package dir2
+-- vend/x/invalid/invalid.go --
+package invalid
+
+import "vend/x/invalid/vendor/foo"
+-- vend/x/vendor/p/p/p.go --
+package p
+
+import _ "notfound"
+-- vend/x/vendor/p/p.go --
+package p
+-- vend/x/vendor/r/r.go --
+package r
+-- vend/x/x.go --
+package x
+
+import _ "p"
+import _ "q"
+import _ "r"
+import _ "vend/dir1"      // not vendored
+import _ "vend/dir1/dir2" // vendored
diff --git a/src/cmd/go/testdata/script/cache_unix.txt b/src/cmd/go/testdata/script/cache_unix.txt
new file mode 100644
index 0000000..e11804d
--- /dev/null
+++ b/src/cmd/go/testdata/script/cache_unix.txt
@@ -0,0 +1,37 @@
+env GO111MODULE=off
+
+# Integration test for cache directory calculation (cmd/go/internal/cache).
+
+[GOOS:windows] skip 'windows does not use XDG_CACHE_HOME'
+[GOOS:darwin]  skip 'darwin does not use XDG_CACHE_HOME'
+[GOOS:ios]     skip 'ios does not use XDG_CACHE_HOME'
+[GOOS:plan9]   skip 'plan9 does not use XDG_CACHE_HOME'
+
+mkdir $WORK/gocache
+mkdir $WORK/xdg
+mkdir $WORK/home
+
+# Set GOCACHE, XDG_CACHE_HOME, and HOME.
+env GOCACHE=$WORK/gocache
+env XDG_CACHE_HOME=$WORK/xdg
+env HOME=$WORK/home
+
+# With all three set, we should prefer GOCACHE.
+go env GOCACHE
+stdout $WORK'/gocache$'
+
+# Without GOCACHE, we should prefer XDG_CACHE_HOME over HOME.
+env GOCACHE=
+go env GOCACHE
+stdout $WORK'/xdg/go-build$$'
+
+# With only HOME set, we should use $HOME/.cache.
+env XDG_CACHE_HOME=
+go env GOCACHE
+stdout $WORK'/home/.cache/go-build$'
+
+# With no guidance from the environment, we must disable the cache, but that
+# should not cause commands that do not write to the cache to fail.
+env HOME=
+go env GOCACHE
+stdout 'off'
diff --git a/src/cmd/go/testdata/script/cache_vet.txt b/src/cmd/go/testdata/script/cache_vet.txt
new file mode 100644
index 0000000..6689048
--- /dev/null
+++ b/src/cmd/go/testdata/script/cache_vet.txt
@@ -0,0 +1,22 @@
+env GO111MODULE=off
+
+[short] skip
+[GODEBUG:gocacheverify=1] skip
+[compiler:gccgo] skip  # gccgo has no standard packages
+
+# Start with a clean build cache:
+# test failures may be masked if the cache has just the right entries already.
+env GOCACHE=$WORK/cache
+
+# Run 'go vet os/user' once to warm up the cache.
+go vet os/user
+
+# Check that second vet reuses cgo-derived inputs.
+# The first command could be build instead of vet,
+# except that if the cache is empty and there's a net.a
+# in GOROOT/pkg, the build will not bother to regenerate
+# and cache the cgo outputs, whereas vet always will.
+
+go vet -x os/user
+! stderr '^(clang|gcc)'  # should not have run compiler
+! stderr '[\\/]cgo '     # should not have run cgo
diff --git a/src/cmd/go/testdata/script/cgo_asm_error.txt b/src/cmd/go/testdata/script/cgo_asm_error.txt
new file mode 100644
index 0000000..7aaa713
--- /dev/null
+++ b/src/cmd/go/testdata/script/cgo_asm_error.txt
@@ -0,0 +1,25 @@
+[!cgo] skip
+
+# Test that cgo package can't contain a go assembly file.
+
+# Ensure the build fails and reports that the package has a Go assembly file.
+! go build cgoasm
+stderr 'package using cgo has Go assembly file'
+
+-- go.mod --
+module cgoasm
+
+go 1.16
+-- p.go --
+package p
+
+/*
+// hi
+*/
+import "C"
+
+func F() {}
+-- p.s --
+TEXT asm(SB),$0
+	RET
+
diff --git a/src/cmd/go/testdata/script/cgo_bad_directives.txt b/src/cmd/go/testdata/script/cgo_bad_directives.txt
new file mode 100644
index 0000000..7d28171
--- /dev/null
+++ b/src/cmd/go/testdata/script/cgo_bad_directives.txt
@@ -0,0 +1,129 @@
+[!cgo] skip
+[short] skip
+
+cp x.go.txt x.go
+
+# Only allow //go:cgo_ldflag .* in cgo-generated code
+[compiler:gc] cp x_gc.go.txt x.go
+[compiler:gc] ! go build x
+[compiler:gc] stderr '//go:cgo_ldflag .* only allowed in cgo-generated code'
+
+# Ignore _* files
+rm x.go
+! go build .
+stderr 'no Go files'
+cp cgo_yy.go.txt _cgo_yy.go
+! go build .
+stderr 'no Go files' #_* files are ignored...
+
+[compiler:gc] ! go build _cgo_yy.go # ... but if forced, the comment is rejected
+# Actually, today there is a separate issue that _ files named
+# on the command line are ignored. Once that is fixed,
+# we want to see the cgo_ldflag error.
+[compiler:gc] stderr '//go:cgo_ldflag only allowed in cgo-generated code|no Go files'
+
+rm _cgo_yy.go
+
+# Reject #cgo CFLAGS: -fplugin=foo.so
+cp x.go.txt x.go
+cp y_fplugin.go.txt y.go
+! go build x
+stderr 'invalid flag in #cgo CFLAGS: -fplugin=foo.so'
+
+# Reject #cgo CFLAGS: -lbar -fplugin=foo.so
+cp y_lbar_fplugin.go.txt y.go
+! go build x
+stderr 'invalid flag in #cgo CFLAGS: -fplugin=foo.so'
+
+# Reject #cgo pkg-config: -foo
+cp y_pkgconfig_dash_foo.txt y.go
+! go build x
+stderr 'invalid pkg-config package name: -foo'
+
+# Reject #cgo pkg-config: @foo
+cp y_pkgconfig_at_foo.txt y.go
+! go build x
+stderr 'invalid pkg-config package name: @foo'
+
+# Reject #cgo CFLAGS: @foo
+cp y_cflags_at_foo.txt y.go
+! go build x
+stderr 'invalid flag in #cgo CFLAGS: @foo'
+
+# Reject #cgo CFLAGS: -D
+cp y_cflags_dash_d.txt y.go
+! go build x
+stderr 'invalid flag in #cgo CFLAGS: -D without argument'
+
+# Note that -I @foo is allowed because we rewrite it into -I /path/to/src/@foo
+# before the check is applied. There's no such rewrite for -D.
+
+# Reject #cgo CFLAGS: -D @foo
+cp y_cflags_dash_d_space_at_foo.txt y.go
+! go build x
+stderr 'invalid flag in #cgo CFLAGS: -D @foo'
+
+# Reject #cgo CFLAGS -D@foo
+cp y_cflags_dash_d_at_foo.txt y.go
+! go build x
+stderr 'invalid flag in #cgo CFLAGS: -D@foo'
+
+# Check for CFLAGS in commands
+env CGO_CFLAGS=-D@foo
+cp y_no_cflags.txt y.go
+go build -n x
+stderr '-D@foo'
+
+-- go.mod --
+module x
+
+go 1.16
+-- x_gc.go.txt --
+package x
+
+//go:cgo_ldflag "-fplugin=foo.so"
+
+import "C"
+-- cgo_yy.go.txt --
+package x
+
+//go:cgo_ldflag "-fplugin=foo.so"
+
+import "C"
+-- x.go.txt --
+package x
+-- y_fplugin.go.txt --
+package x
+// #cgo CFLAGS: -fplugin=foo.so
+import "C"
+-- y_lbar_fplugin.go.txt --
+package x
+// #cgo CFLAGS: -Ibar -fplugin=foo.so
+import "C"
+-- y_pkgconfig_dash_foo.txt --
+package x
+// #cgo pkg-config: -foo
+import "C"
+-- y_pkgconfig_at_foo.txt --
+package x
+// #cgo pkg-config: @foo
+import "C"
+-- y_cflags_at_foo.txt --
+package x
+// #cgo CFLAGS: @foo
+import "C"
+-- y_cflags_dash_d.txt --
+package x
+// #cgo CFLAGS: -D
+import "C"
+-- y_cflags_dash_d_space_at_foo.txt --
+package x
+// #cgo CFLAGS: -D @foo
+import "C"
+-- y_cflags_dash_d_at_foo.txt --
+package x
+// #cgo CFLAGS: -D@foo
+import "C"
+-- y_no_cflags.txt --
+package x
+import "C"
diff --git a/src/cmd/go/testdata/script/cgo_badmethod_issue57926.txt b/src/cmd/go/testdata/script/cgo_badmethod_issue57926.txt
new file mode 100644
index 0000000..81ef850
--- /dev/null
+++ b/src/cmd/go/testdata/script/cgo_badmethod_issue57926.txt
@@ -0,0 +1,31 @@
+[short] skip
+[!cgo] skip
+
+# Test that cgo rejects attempts to declare methods
+# on the types C.T or *C.T; see issue #57926.
+
+! go build
+stderr 'cannot define new methods on non-local type C.T'
+stderr 'cannot define new methods on non-local type \*C.T'
+! stderr 'Alias'
+
+-- go.mod --
+module example.com
+go 1.12
+
+-- a.go --
+package a
+
+/*
+typedef int T;
+*/
+import "C"
+
+func (C.T) f() {}
+func (recv *C.T) g() {}
+
+// The check is more education than enforcement,
+// and is easily defeated using a type alias.
+type Alias = C.T
+func (Alias) h() {}
+func (*Alias) i() {}
diff --git a/src/cmd/go/testdata/script/cgo_depends_on_syscall.txt b/src/cmd/go/testdata/script/cgo_depends_on_syscall.txt
new file mode 100644
index 0000000..bd4777c
--- /dev/null
+++ b/src/cmd/go/testdata/script/cgo_depends_on_syscall.txt
@@ -0,0 +1,15 @@
+[!cgo] skip
+[!race] skip
+
+go list -race -deps foo
+stdout syscall
+
+-- go.mod --
+module foo
+
+go 1.16
+-- foo.go --
+package foo
+
+// #include <stdio.h>
+import "C"
diff --git a/src/cmd/go/testdata/script/cgo_flag_contains_space.txt b/src/cmd/go/testdata/script/cgo_flag_contains_space.txt
new file mode 100644
index 0000000..a3372bb
--- /dev/null
+++ b/src/cmd/go/testdata/script/cgo_flag_contains_space.txt
@@ -0,0 +1,16 @@
+[short] skip
+[!cgo] skip
+
+env GOCACHE=$WORK/gocache  # Looking for compile flags, so need a clean cache.
+go build -x -n main.go
+stderr '"-I[^"]+c flags"' # find quoted c flags
+! stderr '"-I[^"]+c flags".*"-I[^"]+c flags"' # don't find too many quoted c flags per line
+stderr '"-L[^"]+ld flags"' # find quoted ld flags
+! stderr '"-L[^"]+c flags".*"-L[^"]+c flags"' # don't find too many quoted ld flags per line
+
+-- main.go --
+package main
+// #cgo CFLAGS: -I"c flags"
+// #cgo LDFLAGS: -L"ld flags"
+import "C"
+func main() {}
diff --git a/src/cmd/go/testdata/script/cgo_path.txt b/src/cmd/go/testdata/script/cgo_path.txt
new file mode 100644
index 0000000..be23893
--- /dev/null
+++ b/src/cmd/go/testdata/script/cgo_path.txt
@@ -0,0 +1,41 @@
+[!cgo] skip
+
+# Require that CC is something that requires a PATH lookup.
+# Normally, the default is gcc or clang, but if CC was set during make.bash,
+# that becomes the default.
+[!cc:clang] [!cc:gcc] skip 'C compiler is not gcc or clang'
+
+env GOCACHE=$WORK/gocache  # Looking for compile flags, so need a clean cache.
+[!GOOS:windows] env PATH=.:$PATH
+[!GOOS:windows] chmod 0755 p/gcc p/clang
+[!GOOS:windows] exists -exec p/gcc p/clang
+[GOOS:windows] exists -exec p/gcc.bat p/clang.bat
+! exists p/bug.txt
+! go build -x
+stderr '^cgo: C compiler "(clang|gcc)" not found: exec: "(clang|gcc)": cannot run executable found relative to current directory'
+! exists p/bug.txt
+
+-- go.mod --
+module m
+
+-- m.go --
+package m
+
+import _ "m/p"
+
+-- p/p.go --
+package p
+
+// #define X 1
+import "C"
+
+-- p/gcc --
+#!/bin/sh
+echo ran gcc >bug.txt
+-- p/clang --
+#!/bin/sh
+echo ran clang >bug.txt
+-- p/gcc.bat --
+echo ran gcc >bug.txt
+-- p/clang.bat --
+echo ran clang >bug.txt
diff --git a/src/cmd/go/testdata/script/cgo_path_space.txt b/src/cmd/go/testdata/script/cgo_path_space.txt
new file mode 100644
index 0000000..1a78902
--- /dev/null
+++ b/src/cmd/go/testdata/script/cgo_path_space.txt
@@ -0,0 +1,56 @@
+# Check that if the PATH directory containing the C compiler has a space,
+# we can still use that compiler with cgo.
+# Verifies #43808.
+[!cgo] skip
+
+# Set CC explicitly to something that requires a PATH lookup.
+# Normally, the default is gcc or clang, but if CC was set during make.bash,
+# that becomes the default.
+[exec:clang] env CC=clang
+[exec:gcc] env CC=gcc
+[!exec:clang] [!exec:gcc] skip 'Unknown C compiler'
+
+[!GOOS:windows] chmod 0755 $WORK/'program files'/clang
+[!GOOS:windows] chmod 0755 $WORK/'program files'/gcc
+[!GOOS:windows] exists -exec $WORK/'program files'/clang
+[!GOOS:windows] exists -exec $WORK/'program files'/gcc
+[!GOOS:windows] env PATH=$WORK/'program files':$PATH
+[GOOS:windows] exists -exec $WORK/'program files'/gcc.bat
+[GOOS:windows] exists -exec $WORK/'program files'/clang.bat
+[GOOS:windows] env PATH=$WORK\'program files';%PATH%
+
+! exists $WORK/log.txt
+? go build -x
+exists $WORK/log.txt
+rm $WORK/log.txt
+
+# TODO(#41400, #43078): when CC is set explicitly, it should be allowed to
+# contain spaces separating arguments, and it should be possible to quote
+# arguments with spaces (including the path), as in CGO_CFLAGS and other
+# variables. For now, this doesn't work.
+[!GOOS:windows] env CC=$WORK/'program files'/gcc
+[GOOS:windows] env CC=$WORK\'program files'\gcc.bat
+! go build -x
+! exists $WORK/log.txt
+
+-- go.mod --
+module m
+
+-- m.go --
+package m
+
+// #define X 1
+import "C"
+
+-- $WORK/program files/gcc --
+#!/bin/sh
+
+echo ok >$WORK/log.txt
+-- $WORK/program files/clang --
+#!/bin/sh
+
+echo ok >$WORK/log.txt
+-- $WORK/program files/gcc.bat --
+echo ok >%WORK%\log.txt
+-- $WORK/program files/clang.bat --
+echo ok >%WORK%\log.txt
diff --git a/src/cmd/go/testdata/script/cgo_path_space_quote.txt b/src/cmd/go/testdata/script/cgo_path_space_quote.txt
new file mode 100644
index 0000000..9556101
--- /dev/null
+++ b/src/cmd/go/testdata/script/cgo_path_space_quote.txt
@@ -0,0 +1,58 @@
+# This test checks that the CC environment variable may contain quotes and
+# spaces. Arguments are normally split on spaces, tabs, newlines. If an
+# argument contains these characters, the entire argument may be quoted
+# with single or double quotes. This is the same as -gcflags and similar
+# options.
+
+[short] skip
+[!exec:clang] [!exec:gcc] skip
+[!cgo] skip
+
+env GOENV=$WORK/go.env
+mkdir 'program files'
+go build -o 'program files' './which cc/which cc.go'
+[exec:clang] env CC='"'$PWD${/}program' 'files${/}which' 'cc"' 'clang
+[!exec:clang] env CC='"'$PWD${/}program' 'files${/}which' 'cc"' 'gcc
+go env CC
+stdout 'program files[/\\]which cc" (clang|gcc)$'
+go env -w CC=$CC
+env CC=
+go env CC
+stdout 'program files[/\\]which cc" (clang|gcc)$'
+
+go run .
+stdout 1
+
+-- go.mod --
+module test
+
+go 1.17
+-- which cc/which cc.go --
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+)
+
+func main() {
+	args := append([]string{"-DWRAPPER_WAS_USED=1"}, os.Args[2:]...)
+	cmd := exec.Command(os.Args[1], args...)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	if err := cmd.Run(); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}
+-- hello.go --
+package main
+
+// int x = WRAPPER_WAS_USED;
+import "C"
+import "fmt"
+
+func main() {
+	fmt.Println(C.x)
+}
diff --git a/src/cmd/go/testdata/script/cgo_stale.txt b/src/cmd/go/testdata/script/cgo_stale.txt
new file mode 100644
index 0000000..0d30aea
--- /dev/null
+++ b/src/cmd/go/testdata/script/cgo_stale.txt
@@ -0,0 +1,39 @@
+# golang.org/issue/46347: a stale runtime/cgo should only force a single rebuild
+
+[!cgo] skip
+[short] skip
+
+
+# If we set a unique CGO_CFLAGS, the installed copy of runtime/cgo
+# should be reported as stale.
+
+env CGO_CFLAGS=-DTestScript_cgo_stale=true
+stale runtime/cgo
+
+
+# If we then build a package that uses cgo, runtime/cgo should be rebuilt and
+# cached with the new flag, but not installed to GOROOT.
+# It has no install target, and thus is never stale.
+
+env GOCACHE=$WORK/cache  # Use a fresh cache to avoid interference between runs.
+
+go build -x .
+stderr '[/\\]cgo'$GOEXE'["]? .* -importpath runtime/cgo'
+! stale runtime/cgo
+
+
+# After runtime/cgo has been rebuilt and cached, it should not be rebuilt again.
+
+go build -x .
+! stderr '[/\\]cgo'$GOEXE'["]? .* -importpath runtime/cgo'
+! stale runtime/cgo
+
+
+-- go.mod --
+module example.com/m
+
+go 1.17
+-- m.go --
+package m
+
+import "C"
diff --git a/src/cmd/go/testdata/script/cgo_stale_precompiled.txt b/src/cmd/go/testdata/script/cgo_stale_precompiled.txt
new file mode 100644
index 0000000..eb7e105
--- /dev/null
+++ b/src/cmd/go/testdata/script/cgo_stale_precompiled.txt
@@ -0,0 +1,36 @@
+# Regression test for https://go.dev/issue/47215 and https://go.dev/issue/50183:
+# A mismatched $GOROOT_FINAL or missing $CC caused the C dependencies of the net
+# package to appear stale, and it could not be rebuilt due to a missing $CC.
+
+[!cgo] skip
+
+# This test may start with the runtime/cgo package already stale.
+# Explicitly rebuild it to ensure that it is cached.
+# (See https://go.dev/issue/50892.)
+#
+# If running in non-short mode, explicitly vary CGO_CFLAGS
+# as a control case (to ensure that our regexps do catch rebuilds).
+
+[!short] env GOCACHE=$WORK/cache
+[!short] env CGO_CFLAGS=-DTestScript_cgo_stale_precompiled=true
+go build -x runtime/cgo
+[!short] stderr '[/\\]cgo'$GOEXE'["]? .* -importpath runtime/cgo'
+
+# https://go.dev/issue/50183: a mismatched GOROOT_FINAL caused net to be stale.
+env oldGOROOT_FINAL=$GOROOT_FINAL
+env GOROOT_FINAL=$WORK${/}goroot
+go build -x runtime/cgo
+! stderr '[/\\]cgo'$GOEXE'["]? .* -importpath runtime/cgo'
+
+env GOROOT_FINAL=$oldGOROOT_FINAL
+
+# https://go.dev/issue/47215: a missing $(go env CC) caused the precompiled net
+# to be stale. But as of https://go.dev/cl/452457 the precompiled libraries are
+# no longer installed anyway! Since we're requiring a C compiler in order to
+# build and use cgo libraries in the standard library, we should make sure it
+# matches what's in the cache.
+[!abscc] env CGO_ENABLED=1
+[!abscc] [!GOOS:plan9] env PATH=''  # Guaranteed not to include $(go env CC)!
+[!abscc] [GOOS:plan9] env path=''
+[!abscc] ! go build -x runtime/cgo
+[!abscc] stderr 'C compiler .* not found'
diff --git a/src/cmd/go/testdata/script/cgo_suspect_flag_force_external.txt b/src/cmd/go/testdata/script/cgo_suspect_flag_force_external.txt
new file mode 100644
index 0000000..e5bcdc6
--- /dev/null
+++ b/src/cmd/go/testdata/script/cgo_suspect_flag_force_external.txt
@@ -0,0 +1,155 @@
+# Test case to verify that when we have a package that uses CGO in
+# combination with selected "unusual" flags (involving plugins, LTO)
+# that we force external linking.  See related
+# issues 58619,  58620, and 58848.
+
+[compiler:gccgo] skip # only external linking for gccgo
+
+[!cgo] skip 'test verifies behavior that depends on CGO_CFLAGS'
+[mustlinkext] skip 'test expects internal linking for non-cgo programs'
+
+# Here we build three program: one with explicit CGO use, one with no
+# CGO use, and one that uses a stdlib package ("runtime/cgo") that has
+# CGO in it. It used to be that only the explicit use of CGO would
+# trigger external linking, and that the program that only used
+# "runtime/cgo" would always be handled with internal linking. This caused
+# issues when users included odd/unusual flags (ex: -fplugin, -flto)
+# in CGO_CFLAGS, causing the Go linker to have to read and interpret
+# non-standard host objects.
+#
+# As of 1.21 we continue to use internal linking for programs whose
+# CGO use comes only from stdlib packages in the absence of any flag
+# funny business, however if the Go command sees flags that may be suspicious,
+# it signals the Go linker to invoke the external linker.
+
+# The next few tests run builds passing "-n" to the Go command, then
+# checking the output to see if the Go command is trying to pass a
+# "preferlinkext" token to the linker to request external linking.
+
+#-----------------------
+
+# Use a fresh GOCACHE for these next steps, so as to have the real
+# actions for the runtime/cgo package appear in the "-n -x" output.
+env GOCACHE=$WORK/gocache
+mkdir $GOCACHE
+
+# First build: there is no CGO in use, so no token should be present regardless
+# of weird CGO flags.
+go build -x -n -o dummy.exe ./noUseOfCgo
+! stderr preferlinkext
+env CGO_CFLAGS=-flto
+go build -x -n -o dummy.exe ./noUseOfCgo
+! stderr preferlinkext
+env CGO_CFLAGS=
+
+# Second build uses CGO, so we expect to see the token present in the
+# -n output only when strange flags are used.
+go build -x -n -o dummy.exe ./usesInternalCgo
+! stderr preferlinkext
+env CGO_CFLAGS=-flto
+go build -x -n -o dummy.exe ./usesInternalCgo
+stderr preferlinkext
+env CGO_CFLAGS=-fplugin
+go build -x -n -o dummy.exe ./usesInternalCgo
+stderr preferlinkext
+env CGO_CFLAGS=-fprofile-instr-generate
+go build -x -n -o dummy.exe ./usesInternalCgo
+stderr preferlinkext
+env CGO_CFLAGS=
+
+[short] skip
+
+# In the remaining tests below we do actual builds (without -n) to
+# verify that the Go linker is going the right thing in addition to the
+# Go command. Here the idea is to pass "-tmpdir" to the linker, then
+# check after the link is done for the presence of the file
+# <tmpdir>/go.o, which the Go linker creates prior to kicking off the
+# external linker.
+
+mkdir tmp1
+mkdir tmp2
+mkdir tmp3
+mkdir tmp4
+mkdir tmp5
+
+# First build: no external linking expected
+go build -ldflags=-tmpdir=tmp1 -o $devnull ./noUseOfCgo &
+
+# Second build: using only "runtime/cgo", expect internal linking.
+go build -ldflags=-tmpdir=tmp2 -o $devnull ./usesInternalCgo &
+
+# Third build: program uses only "runtime/cgo", so we would normally
+# expect internal linking, except that cflags contain suspicious entries
+# (in this case, a flag that does not appear on the allow list).
+env CGO_CFLAGS=-fmerge-all-constants
+env CGO_LDFLAGS=-fmerge-all-constants
+go build -ldflags=-tmpdir=tmp3 -o $devnull ./usesInternalCgo &
+env CGO_CFLAGS=
+env CGO_LDFLAGS=
+
+# Fourth build: explicit CGO, expect external linking.
+go build -ldflags=-tmpdir=tmp4 -o $devnull ./usesExplicitCgo &
+
+# Fifth build: explicit CGO, but we specifically asked for internal linking
+# via a flag, so using internal linking it is.
+[cgolinkext] go list ./usesInternalCgo
+[!cgolinkext] go build '-ldflags=-tmpdir=tmp5 -linkmode=internal' -o $devnull ./usesInternalCgo &
+
+wait
+
+# Check first build: no external linking expected
+! exists tmp1/go.o
+
+# Check second build: using only "runtime/cgo", expect internal linking.
+[!cgolinkext] ! exists tmp2/go.o
+[cgolinkext] exists tmp2/go.o
+
+# Check third build: has suspicious flag.
+exists tmp3/go.o
+
+# Fourth build: explicit CGO, expect external linking.
+exists tmp4/go.o
+
+# Fifth build: explicit CGO, -linkmode=internal.
+! exists tmp5/go.o
+
+-- go.mod --
+
+module cgo.example
+
+go 1.20
+
+-- noUseOfCgo/main.go --
+
+package main
+
+func main() {
+	println("clean as a whistle")
+}
+
+-- usesInternalCgo/main.go --
+
+package main
+
+import (
+	"runtime/cgo"
+)
+
+func main() {
+	q := "hello"
+	h := cgo.NewHandle(q)
+	h.Delete()
+}
+
+-- usesExplicitCgo/main.go --
+
+package main
+
+/*
+int meaningOfLife() { return 42; }
+*/
+import "C"
+
+func main() {
+     println(C.meaningOfLife())
+}
diff --git a/src/cmd/go/testdata/script/cgo_syso_issue29253.txt b/src/cmd/go/testdata/script/cgo_syso_issue29253.txt
new file mode 100644
index 0000000..18526c6
--- /dev/null
+++ b/src/cmd/go/testdata/script/cgo_syso_issue29253.txt
@@ -0,0 +1,31 @@
+env GO111MODULE=off
+[short] skip
+
+# This test tests that we can link in-package syso files that provides symbols
+# for cgo. See issue 29253.
+[!cgo] stop
+[!compiler:gc] stop
+cc -c -o pkg/o.syso ext.c
+go build main.go
+
+-- ext.c --
+// +build ignore
+
+int f() { return 42; }
+-- pkg/pkg.go --
+package pkg
+
+// extern int f(void);
+import "C"
+
+func init() {
+	if v := C.f(); v != 42 {
+		panic(v)
+	}
+}
+-- main.go --
+package main
+
+import _ "pkg"
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/cgo_trimpath_macro.txt b/src/cmd/go/testdata/script/cgo_trimpath_macro.txt
new file mode 100644
index 0000000..b5cc116
--- /dev/null
+++ b/src/cmd/go/testdata/script/cgo_trimpath_macro.txt
@@ -0,0 +1,71 @@
+# This is a test that -trimpath trims the paths of every directory
+# of Cgo dependencies in the module, and trims file paths included
+# through the __FILE__ macro using --file-prefix-map.
+
+[!cgo] skip
+[short] skip 'links and runs binaries'
+
+# Test in main module.
+go run -trimpath -mod=vendor ./main
+stdout '(\\_\\_|/_)[\\/]m[\\/]c[\\/]bar.h'
+
+# Test in vendored module.
+go run -trimpath -mod=vendor v.com/main
+stdout '(\\_\\_|/_)[\\/]vendor[\\/]v.com[\\/]c[\\/]bar.h'
+
+# Test in GOPATH mode.
+env GO111MODULE=off
+go run -trimpath ./main
+stdout '(\\_\\_|/_)[\\/]GOPATH[\\/]src[\\/]c[\\/]bar.h'
+
+-- go.mod --
+module m
+
+require v.com v1.0.0
+-- go.sum --
+v.com v1.0.0 h1:xxx
+v.com v1.0.0/go.mod h1:xxx
+-- vendor/modules.txt --
+# v.com v1.0.0
+## explicit; go 1.20
+v.com/main
+-- vendor/v.com/main/main.go --
+package main
+
+// #cgo CFLAGS: -I../c
+// #include "stdio.h"
+// void printfile();
+import "C"
+
+func main() {
+    C.printfile()
+    C.fflush(C.stdout)
+}
+-- vendor/v.com/main/foo.c --
+#include "bar.h"
+-- vendor/v.com/c/bar.h --
+#include "stdio.h"
+
+void printfile() {
+    printf("%s\n", __FILE__);
+}
+-- main/main.go --
+package main
+
+// #cgo CFLAGS: -I../c
+// #include "stdio.h"
+// void printfile();
+import "C"
+
+func main() {
+    C.printfile()
+    C.fflush(C.stdout)
+}
+-- main/foo.c --
+#include "bar.h"
+-- c/bar.h --
+#include "stdio.h"
+
+void printfile() {
+    printf("%s\n", __FILE__);
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/cgo_undef.txt b/src/cmd/go/testdata/script/cgo_undef.txt
new file mode 100644
index 0000000..30034fb
--- /dev/null
+++ b/src/cmd/go/testdata/script/cgo_undef.txt
@@ -0,0 +1,68 @@
+# Issue 52863.
+
+# We manually create a .syso and a .a file in package a,
+# such that the .syso file only works when linked against the .a file.
+# Package a has #cgo LDFLAGS to make this happen.
+#
+# Package c imports package a, and uses cgo itself.
+# The generation of the _cgo_import.go for package c will fail,
+# because it won't know that it has to link against a/libb.a
+# (because we don't gather the #cgo LDFLAGS from all transitively
+# imported packages).
+#
+# The _cgo_import.go file is only needed for internal linking.
+# When generating _cgo_import.go for package c fails, an ordinary
+# external link should still work. But an internal link is expected
+# to fail, because the failure to create _cgo_import.go should cause
+# the linker to report an inability to internally link.
+
+[short] skip
+[!cgo] skip
+[!exec:ar] skip
+
+cc -c -o a/b.syso b/b.c
+cc -c -o b/lib.o b/lib.c
+exec ar rc a/libb.a b/lib.o
+go build
+! go build -ldflags=-linkmode=internal
+stderr 'some packages could not be built to support internal linking.*m/c|requires external linking|does not support internal cgo'
+
+-- go.mod --
+module m
+
+-- a/a.go --
+package a
+
+// #cgo LDFLAGS: -L. -lb
+// extern int CFn(int);
+import "C"
+
+func GoFn(v int) int { return int(C.CFn(C.int(v))) }
+
+-- b/b.c --
+extern int LibFn(int);
+int CFn(int i) { return LibFn(i); }
+
+-- b/lib.c --
+int LibFn(int i) { return i; }
+
+-- c/c.go --
+package c
+
+// static int D(int i) { return i; }
+import "C"
+
+import "m/a"
+
+func Fn(i int) (int, int) {
+     return a.GoFn(i), int(C.D(C.int(i)))
+}
+
+-- main.go --
+package main
+
+import "m/c"
+
+func main() {
+	println(c.Fn(0))
+}
diff --git a/src/cmd/go/testdata/script/chdir.txt b/src/cmd/go/testdata/script/chdir.txt
new file mode 100644
index 0000000..a6feed6
--- /dev/null
+++ b/src/cmd/go/testdata/script/chdir.txt
@@ -0,0 +1,35 @@
+env OLD=$PWD
+
+# basic -C functionality
+cd $GOROOT/src/math
+go list -C ../strings
+stdout strings
+! go list -C ../nonexist
+stderr 'chdir.*nonexist'
+
+# check for -C in subcommands with custom flag parsing
+# cmd/go/chdir_test.go handles the normal ones more directly.
+
+# go doc
+go doc -C ../strings HasPrefix
+
+# go env
+go env -C $OLD/custom GOMOD
+stdout 'custom[\\/]go.mod'
+! go env -C ../nonexist
+stderr '^go: chdir ../nonexist: '
+
+# go test
+go test -C ../strings -n
+stderr 'strings\.test'
+
+# go vet
+go vet -C ../strings -n
+stderr strings_test
+
+# -C must be first on command line (as of Go 1.21)
+! go test -n -C ../strings
+stderr '^invalid value "../strings" for flag -C: -C flag must be first flag on command line$'
+
+-- custom/go.mod --
+module m
diff --git a/src/cmd/go/testdata/script/check_goexperiment.txt b/src/cmd/go/testdata/script/check_goexperiment.txt
new file mode 100644
index 0000000..3434cb9
--- /dev/null
+++ b/src/cmd/go/testdata/script/check_goexperiment.txt
@@ -0,0 +1,10 @@
+# Test that [GOEXPERIMENT:x] is accepted.
+# Here fieldtrack is picked arbitrarily.
+
+[GOEXPERIMENT:nofieldtrack] env
+
+[GOEXPERIMENT:fieldtrack] env
+
+#[GOEXPERIMENT:crashme] env
+
+
diff --git a/src/cmd/go/testdata/script/clean_binary.txt b/src/cmd/go/testdata/script/clean_binary.txt
new file mode 100644
index 0000000..7335f8a
--- /dev/null
+++ b/src/cmd/go/testdata/script/clean_binary.txt
@@ -0,0 +1,78 @@
+# Build something to create the executable, including several cases
+[short] skip
+
+# --------------------- clean executables -------------------------
+
+# case1: test file-named executable 'main'
+env GO111MODULE=on
+
+! exists main$GOEXE
+go build main.go
+exists -exec main$GOEXE
+go clean
+! exists main$GOEXE
+
+# case2: test module-named executable 'a.b.c'
+! exists a.b.c$GOEXE
+go build
+exists -exec a.b.c$GOEXE
+go clean
+! exists a.b.c$GOEXE
+
+# case3: directory-named executable 'src'
+env GO111MODULE=off
+
+! exists src$GOEXE
+go build
+exists -exec src$GOEXE
+go clean
+! exists src$GOEXE
+
+# --------------------- clean test files -------------------------
+
+# case1: test file-named test file
+env GO111MODULE=on
+
+! exists main.test$GOEXE
+go test -c main_test.go
+exists -exec main.test$GOEXE
+go clean
+! exists main.test$GOEXE
+
+# case2: test module-named test file
+! exists a.b.c.test$GOEXE
+go test -c
+exists -exec a.b.c.test$GOEXE
+go clean
+! exists a.b.c.test$GOEXE
+
+# case3: test directory-based test file
+env GO111MODULE=off
+
+! exists src.test$GOEXE
+go test -c
+exists -exec src.test$GOEXE
+go clean
+! exists src.test$GOEXE
+
+-- main.go --
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("hello!")
+}
+
+-- main_test.go --
+package main
+
+import "testing"
+
+func TestSomething(t *testing.T) {
+}
+
+-- go.mod --
+module example.com/a.b.c/v2
+
+go 1.12
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/clean_cache_n.txt b/src/cmd/go/testdata/script/clean_cache_n.txt
new file mode 100644
index 0000000..72f9abf
--- /dev/null
+++ b/src/cmd/go/testdata/script/clean_cache_n.txt
@@ -0,0 +1,28 @@
+# We're testing cache behavior, so start with a clean GOCACHE.
+env GOCACHE=$WORK/cache
+
+# Build something so that the cache gets populates
+go build main.go
+
+# Check that cache contains directories before running
+exists $GOCACHE/00
+
+# Run go clean -cache -n and ensure that directories weren't deleted
+go clean -cache -n
+exists $GOCACHE/00
+
+# Re-run go clean cache without the -n flag go ensure that directories were properly removed
+go clean -cache
+! exists $GOCACHE/00
+
+! go clean -cache .
+stderr 'go: clean -cache cannot be used with package arguments'
+
+-- main.go --
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("hello!")
+}
diff --git a/src/cmd/go/testdata/script/clean_testcache.txt b/src/cmd/go/testdata/script/clean_testcache.txt
new file mode 100644
index 0000000..3f98602
--- /dev/null
+++ b/src/cmd/go/testdata/script/clean_testcache.txt
@@ -0,0 +1,28 @@
+env GO111MODULE=off
+[short] skip
+
+# go clean -testcache
+# should work (see golang.org/issue/29757).
+cd x
+go test x_test.go
+go clean -testcache
+go test x_test.go
+! stdout 'cached'
+! go clean -testcache ../x
+stderr 'go: clean -testcache cannot be used with package arguments'
+
+# golang.org/issue/29100: 'go clean -testcache' should succeed
+# if the cache directory doesn't exist at all.
+# It should not write a testexpire.txt file, since there are no
+# test results that need to be invalidated in the first place.
+env GOCACHE=$WORK/nonexistent
+go clean -testcache
+! exists $WORK/nonexistent
+
+-- x/x_test.go --
+package x_test
+import (
+    "testing"
+)
+func TestMain(t *testing.T) {
+}
diff --git a/src/cmd/go/testdata/script/cmd_import_error.txt b/src/cmd/go/testdata/script/cmd_import_error.txt
new file mode 100644
index 0000000..89e1dbb
--- /dev/null
+++ b/src/cmd/go/testdata/script/cmd_import_error.txt
@@ -0,0 +1,16 @@
+env GO111MODULE=on
+
+# Regression test for golang.org/issue/31031:
+# Importing or loading a non-existent package in cmd/ should print
+# a clear error in module mode.
+
+! go list cmd/unknown
+stderr '^package cmd/unknown is not in std \('$GOROOT'[/\\]src[/\\]cmd[/\\]unknown\)$'
+
+go list -f '{{range .DepsErrors}}{{.Err}}{{end}}' x.go
+stdout '^package cmd/unknown is not in std \('$GOROOT'[/\\]src[/\\]cmd[/\\]unknown\)$'
+
+-- x.go --
+package x
+
+import _ "cmd/unknown"
diff --git a/src/cmd/go/testdata/script/cover_asm.txt b/src/cmd/go/testdata/script/cover_asm.txt
new file mode 100644
index 0000000..c7b7114
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_asm.txt
@@ -0,0 +1,33 @@
+[short] skip
+[compiler:gccgo] skip # gccgo has no cover tool
+
+# Test cover for a package that has an assembly function.
+
+go test -outputdir=$WORK -coverprofile=cover.out coverasm
+go tool cover -func=$WORK/cover.out
+stdout '\tg\t*100.0%' # Check g is 100% covered.
+! stdout '\tf\t*[0-9]' # Check for no coverage on the assembly function
+
+-- go.mod --
+module coverasm
+
+go 1.16
+-- p.go --
+package p
+
+func f()
+
+func g() {
+	println("g")
+}
+-- p.s --
+// empty asm file,
+// so go test doesn't complain about declaration of f in p.go.
+-- p_test.go --
+package p
+
+import "testing"
+
+func Test(t *testing.T) {
+	g()
+}
diff --git a/src/cmd/go/testdata/script/cover_atomic_pkgall.txt b/src/cmd/go/testdata/script/cover_atomic_pkgall.txt
new file mode 100644
index 0000000..c3bc67d
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_atomic_pkgall.txt
@@ -0,0 +1,25 @@
+env GO111MODULE=off
+
+[short] skip
+
+go test -coverpkg=all -covermode=atomic x
+stdout ok[\s\S]+?coverage
+
+[!race] stop
+
+go test -coverpkg=all -race x
+stdout ok[\s\S]+?coverage
+
+-- x/x.go --
+package x
+
+import _ "sync/atomic"
+
+func F() {}
+
+-- x/x_test.go --
+package x
+
+import "testing"
+
+func TestF(t *testing.T) { F() }
diff --git a/src/cmd/go/testdata/script/cover_blank_func_decl.txt b/src/cmd/go/testdata/script/cover_blank_func_decl.txt
new file mode 100644
index 0000000..e7d5250
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_blank_func_decl.txt
@@ -0,0 +1,35 @@
+[short] skip
+go test -cover coverblank
+stdout 'coverage: 100.0% of statements'
+
+
+-- go.mod --
+module coverblank
+
+go 1.16
+-- a.go --
+package coverblank
+
+func _() {
+	println("unreachable")
+}
+
+type X int
+
+func (x X) Print() {
+	println(x)
+}
+
+func (x X) _() {
+	println("unreachable")
+}
+
+-- a_test.go --
+package coverblank
+
+import "testing"
+
+func TestX(t *testing.T) {
+	var x X
+	x.Print()
+}
diff --git a/src/cmd/go/testdata/script/cover_build_cmdline_pkgs.txt b/src/cmd/go/testdata/script/cover_build_cmdline_pkgs.txt
new file mode 100644
index 0000000..e14a078
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_build_cmdline_pkgs.txt
@@ -0,0 +1,73 @@
+
+# This test is intended to verify that when a user does "go run -cover ..."
+# or "go build -cover ...", packages named on the command line are
+# always instrumented (but not their dependencies). This rule applies
+# inside and outside the standard library.
+
+[short] skip
+[!GOEXPERIMENT:coverageredesign] skip
+
+# Compile an object.
+go tool compile -p tiny tiny/tiny.go tiny/tiny2.go
+
+# Build a stdlib command with coverage.
+go build -o $WORK/nm.exe -cover cmd/nm 
+
+# Save off old GOCOVERDIR setting
+env SAVEGOCOVERDIR=$GOCOVERDIR
+
+# Collect a coverage profile from running 'cmd/nm' on the object.
+mkdir $WORK/covdata
+env GOCOVERDIR=$WORK/covdata
+exec $WORK/nm.exe tiny.o
+
+# Restore previous GOCOVERDIR setting
+env GOCOVERDIR=$SAVEGOCOVERDIR
+
+# Check to make sure we instrumented just the main package, not
+# any dependencies.
+go tool covdata pkglist -i=$WORK/covdata
+stdout cmd/nm
+! stdout cmd/internal/goobj pkglist.txt
+
+# ... now collect a coverage profile from a Go file
+# listed on the command line.
+go build -cover -o $WORK/another.exe testdata/another.go
+mkdir $WORK/covdata2
+env GOCOVERDIR=$WORK/covdata2
+exec $WORK/another.exe 
+
+# Restore previous GOCOVERDIR setting
+env GOCOVERDIR=$SAVEGOCOVERDIR
+
+# Check to make sure we instrumented just the main package.
+go tool covdata pkglist -i=$WORK/covdata2
+stdout command-line-arguments
+! stdout fmt
+
+-- go.mod --
+
+module example.prog
+
+-- testdata/another.go --
+
+package main
+
+import "fmt"
+
+func main() {
+  fmt.Println("Hi dad")
+}
+
+-- tiny/tiny.go --
+
+package tiny
+
+var Tvar int
+
+-- tiny/tiny2.go --
+
+package tiny
+
+var Tvar2 bool
+
diff --git a/src/cmd/go/testdata/script/cover_build_pkg_select.txt b/src/cmd/go/testdata/script/cover_build_pkg_select.txt
new file mode 100644
index 0000000..447ca77
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_build_pkg_select.txt
@@ -0,0 +1,114 @@
+# This test checks more of the "go build -cover" functionality,
+# specifically which packages get selected when building.
+
+[short] skip
+
+# Skip if new coverage is not enabled.
+[!GOEXPERIMENT:coverageredesign] skip
+
+#-------------------------------------------
+
+# Build for coverage.
+go build -mod=mod -o $WORK/modex.exe -cover mod.example/main
+
+# Save off old GOCOVERDIR setting
+env SAVEGOCOVERDIR=$GOCOVERDIR
+
+# Execute.
+mkdir $WORK/covdata
+env GOCOVERDIR=$WORK/covdata
+exec $WORK/modex.exe
+
+# Restore previous GOCOVERDIR setting
+env GOCOVERDIR=$SAVEGOCOVERDIR
+
+# Examine the result.
+go tool covdata percent -i=$WORK/covdata
+stdout 'coverage: 100.0% of statements'
+
+# By default we want to see packages resident in the module covered,
+# but not dependencies.
+go tool covdata textfmt -i=$WORK/covdata -o=$WORK/covdata/out.txt
+grep 'mode: set' $WORK/covdata/out.txt
+grep 'mod.example/main/main.go:' $WORK/covdata/out.txt
+grep 'mod.example/sub/sub.go:' $WORK/covdata/out.txt
+! grep 'rsc.io' $WORK/covdata/out.txt
+
+rm $WORK/covdata
+rm $WORK/modex.exe
+
+#-------------------------------------------
+
+# Repeat the build but with -coverpkg=all
+
+go build -mod=mod -coverpkg=all -o $WORK/modex.exe -cover mod.example/main
+
+# Execute.
+mkdir $WORK/covdata
+env GOCOVERDIR=$WORK/covdata
+exec $WORK/modex.exe
+
+# Restore previous GOCOVERDIR setting
+env GOCOVERDIR=$SAVEGOCOVERDIR
+
+# Examine the result.
+go tool covdata percent -i=$WORK/covdata
+stdout  'coverage:.*[1-9][0-9.]+%'
+
+# The whole enchilada.
+go tool covdata textfmt -i=$WORK/covdata -o=$WORK/covdata/out.txt
+grep 'mode: set' $WORK/covdata/out.txt
+grep 'mod.example/main/main.go:' $WORK/covdata/out.txt
+grep 'mod.example/sub/sub.go:' $WORK/covdata/out.txt
+grep 'rsc.io' $WORK/covdata/out.txt
+grep 'bufio/bufio.go:' $WORK/covdata/out.txt
+
+# Use the covdata tool to select a specific set of module paths
+mkdir $WORK/covdata2
+go tool covdata merge -pkg=rsc.io/quote -i=$WORK/covdata -o=$WORK/covdata2
+
+# Examine the result.
+go tool covdata percent -i=$WORK/covdata2
+stdout  'coverage:.*[1-9][0-9.]+%'
+
+# Check for expected packages + check that we don't see things from stdlib.
+go tool covdata textfmt -i=$WORK/covdata2 -o=$WORK/covdata2/out.txt
+grep 'mode: set' $WORK/covdata2/out.txt
+! grep 'mod.example/main/main.go:' $WORK/covdata2/out.txt
+! grep 'mod.example/sub/sub.go:' $WORK/covdata2/out.txt
+grep 'rsc.io' $WORK/covdata2/out.txt
+! grep 'bufio/bufio.go:' $WORK/covdata2/out.txt
+
+#-------------------------------------------
+# end of test cmds, start of harness and related files.
+
+-- go.mod --
+module mod.example
+
+go 1.20
+
+require rsc.io/quote/v3 v3.0.0
+
+-- main/main.go --
+package main
+
+import (
+	"fmt"
+	"mod.example/sub"
+
+	"rsc.io/quote"
+)
+
+func main() {
+	fmt.Println(quote.Go(), sub.F())
+}
+
+-- sub/sub.go --
+
+package sub
+
+func F() int {
+	return 42
+}
+
+
diff --git a/src/cmd/go/testdata/script/cover_build_simple.txt b/src/cmd/go/testdata/script/cover_build_simple.txt
new file mode 100644
index 0000000..b61e631
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_build_simple.txt
@@ -0,0 +1,149 @@
+# This test checks basic "go build -cover" functionality.
+
+[short] skip
+
+# Hard-wire new coverage for this test.
+env GOEXPERIMENT=coverageredesign
+
+# Build for coverage.
+go build -gcflags=-m -o example.exe -cover example/main &
+[race] go build -o examplewithrace.exe -race -cover example/main &
+wait
+
+# First execute without GOCOVERDIR set...
+env GOCOVERDIR=
+exec ./example.exe normal
+stderr '^warning: GOCOVERDIR not set, no coverage data emitted'
+
+# ... then with GOCOVERDIR set.
+env GOCOVERDIR=data/normal
+exec ./example.exe normal
+! stderr '^warning: GOCOVERDIR not set, no coverage data emitted'
+go tool covdata percent -i=data/normal
+stdout  'coverage:.*[1-9][0-9.]+%'
+
+# Program makes a direct call to os.Exit(0).
+env GOCOVERDIR=data/goodexit
+exec ./example.exe goodexit
+! stderr '^warning: GOCOVERDIR not set, no coverage data emitted'
+go tool covdata percent -i=data/goodexit
+stdout  'coverage:.*[1-9][0-9.]+%'
+
+# Program makes a direct call to os.Exit(1).
+env GOCOVERDIR=data/badexit
+! exec ./example.exe badexit
+! stderr '^warning: GOCOVERDIR not set, no coverage data emitted'
+go tool covdata percent -i=data/badexit
+stdout  'coverage:.*[1-9][0-9.]+%'
+
+# Program invokes panic.
+env GOCOVERDIR=data/panic
+! exec ./example.exe panic
+! stderr '^warning: GOCOVERDIR not set, no coverage data emitted'
+go tool covdata percent -i=data/panic
+stdout  'coverage:.*[0-9.]+%'
+
+# Skip remainder if no race detector support.
+[!race] skip
+
+env GOCOVERDIR=data2/normal
+exec ./examplewithrace.exe normal
+! stderr '^warning: GOCOVERDIR not set, no coverage data emitted'
+go tool covdata percent -i=data2/normal
+stdout  'coverage:.*[1-9][0-9.]+%'
+
+# Program makes a direct call to os.Exit(0).
+env GOCOVERDIR=data2/goodexit
+exec ./examplewithrace.exe goodexit
+! stderr '^warning: GOCOVERDIR not set, no coverage data emitted'
+go tool covdata percent -i=data2/goodexit
+stdout  'coverage:.*[1-9][0-9.]+%'
+
+# Program makes a direct call to os.Exit(1).
+env GOCOVERDIR=data2/badexit
+! exec ./examplewithrace.exe badexit
+! stderr '^warning: GOCOVERDIR not set, no coverage data emitted'
+go tool covdata percent -i=data2/badexit
+stdout  'coverage:.*[1-9][0-9.]+%'
+
+# Program invokes panic.
+env GOCOVERDIR=data2/panic
+! exec ./examplewithrace.exe panic
+! stderr '^warning: GOCOVERDIR not set, no coverage data emitted'
+go tool covdata percent -i=data2/panic
+stdout  'coverage:.*[0-9.]+%'
+
+# end of test cmds, start of harness and related files.
+
+-- go.mod --
+module example
+
+go 1.18
+
+-- main/example.go --
+package main
+
+import "example/sub"
+
+func main() {
+	sub.S()
+}
+
+-- sub/sub.go --
+
+package sub
+
+import "os"
+
+func S() {
+	switch os.Args[1] {
+	case "normal":
+		println("hi")
+	case "goodexit":
+		os.Exit(0)
+	case "badexit":
+		os.Exit(1)
+	case "panic":
+		panic("something bad happened")
+	}
+}
+
+-- data/README.txt --
+
+Just a location where we can write coverage profiles.
+
+-- data/normal/f.txt --
+
+X
+
+-- data/goodexit/f.txt --
+
+X
+
+-- data/badexit/f.txt --
+
+X
+
+-- data/panic/f.txt --
+
+X
+
+-- data2/README.txt --
+
+Just a location where we can write coverage profiles.
+
+-- data2/normal/f.txt --
+
+X
+
+-- data2/goodexit/f.txt --
+
+X
+
+-- data2/badexit/f.txt --
+
+X
+
+-- data2/panic/f.txt --
+
+X
diff --git a/src/cmd/go/testdata/script/cover_cgo.txt b/src/cmd/go/testdata/script/cover_cgo.txt
new file mode 100644
index 0000000..85754b5
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_cgo.txt
@@ -0,0 +1,42 @@
+[short] skip
+[!cgo] skip
+[compiler:gccgo] skip # gccgo has no cover tool
+
+# Test coverage on cgo code.
+
+go test -short -cover cgocover
+stdout  'coverage:.*[1-9][0-9.]+%'
+! stderr '[^0-9]0\.0%'
+
+-- go.mod --
+module cgocover
+
+go 1.16
+-- p.go --
+package p
+
+/*
+void
+f(void)
+{
+}
+*/
+import "C"
+
+var b bool
+
+func F() {
+	if b {
+		for {
+		}
+	}
+	C.f()
+}
+-- p_test.go --
+package p
+
+import "testing"
+
+func TestF(t *testing.T) {
+	F()
+}
diff --git a/src/cmd/go/testdata/script/cover_cgo_extra_file.txt b/src/cmd/go/testdata/script/cover_cgo_extra_file.txt
new file mode 100644
index 0000000..afc2178
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_cgo_extra_file.txt
@@ -0,0 +1,48 @@
+[short] skip
+[!cgo] skip
+[compiler:gccgo] skip # gccgo has no cover tool
+
+# Test coverage on cgo code. This test case includes an
+# extra empty non-cgo file in the package being checked.
+
+go test -short -cover cgocover4
+stdout  'coverage:.*[1-9][0-9.]+%'
+! stderr '[^0-9]0\.0%'
+
+-- go.mod --
+module cgocover4
+
+go 1.16
+-- notcgo.go --
+package p
+-- p.go --
+package p
+
+/*
+void
+f(void)
+{
+}
+*/
+import "C"
+
+var b bool
+
+func F() {
+	if b {
+		for {
+		}
+	}
+	C.f()
+}
+-- x_test.go --
+package p_test
+
+import (
+	. "cgocover4"
+	"testing"
+)
+
+func TestF(t *testing.T) {
+	F()
+}
diff --git a/src/cmd/go/testdata/script/cover_cgo_extra_test.txt b/src/cmd/go/testdata/script/cover_cgo_extra_test.txt
new file mode 100644
index 0000000..ad09bab
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_cgo_extra_test.txt
@@ -0,0 +1,49 @@
+[short] skip
+[!cgo] skip
+[compiler:gccgo] skip # gccgo has no cover tool
+
+# Test coverage on cgo code. This test case has an external
+# test that tests the code and an in-package test file with
+# no test cases.
+
+go test -short -cover cgocover3
+stdout  'coverage:.*[1-9][0-9.]+%'
+! stderr '[^0-9]0\.0%'
+
+-- go.mod --
+module cgocover3
+
+go 1.16
+-- p.go --
+package p
+
+/*
+void
+f(void)
+{
+}
+*/
+import "C"
+
+var b bool
+
+func F() {
+	if b {
+		for {
+		}
+	}
+	C.f()
+}
+-- p_test.go --
+package p
+-- x_test.go --
+package p_test
+
+import (
+	. "cgocover3"
+	"testing"
+)
+
+func TestF(t *testing.T) {
+	F()
+}
diff --git a/src/cmd/go/testdata/script/cover_cgo_xtest.txt b/src/cmd/go/testdata/script/cover_cgo_xtest.txt
new file mode 100644
index 0000000..0900a48
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_cgo_xtest.txt
@@ -0,0 +1,45 @@
+[short] skip
+[!cgo] skip
+[compiler:gccgo] skip # gccgo has no cover tool
+
+# Test cgo coverage with an external test.
+
+go test -short -cover cgocover2
+stdout  'coverage:.*[1-9][0-9.]+%'
+! stderr '[^0-9]0\.0%'
+
+-- go.mod --
+module cgocover2
+
+go 1.16
+-- p.go --
+package p
+
+/*
+void
+f(void)
+{
+}
+*/
+import "C"
+
+var b bool
+
+func F() {
+	if b {
+		for {
+		}
+	}
+	C.f()
+}
+-- x_test.go --
+package p_test
+
+import (
+	. "cgocover2"
+	"testing"
+)
+
+func TestF(t *testing.T) {
+	F()
+}
diff --git a/src/cmd/go/testdata/script/cover_dash_c.txt b/src/cmd/go/testdata/script/cover_dash_c.txt
new file mode 100644
index 0000000..f28c69e
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_dash_c.txt
@@ -0,0 +1,31 @@
+[short] skip
+[compiler:gccgo] skip
+
+# Test for issue 24588
+
+go test -c -o $WORK/coverdep -coverprofile=$WORK/no/such/dir/cover.out coverdep
+exists -exec $WORK/coverdep
+
+-- go.mod --
+module coverdep
+
+go 1.16
+-- p.go --
+package p
+
+import _ "coverdep/p1"
+
+func F() {
+}
+-- p1/p1.go --
+package p1
+
+import _ "errors"
+-- p_test.go --
+package p
+
+import "testing"
+
+func Test(t *testing.T) {
+	F()
+}
diff --git a/src/cmd/go/testdata/script/cover_dep_loop.txt b/src/cmd/go/testdata/script/cover_dep_loop.txt
new file mode 100644
index 0000000..46748e1
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_dep_loop.txt
@@ -0,0 +1,36 @@
+[short] skip
+[compiler:gccgo] skip
+
+# coverdep2/p1's xtest imports coverdep2/p2 which imports coverdep2/p1.
+# Make sure that coverage on coverdep2/p2 recompiles coverdep2/p2.
+
+go test -short -cover coverdep2/p1
+stdout 'coverage: 100.0% of statements' # expect 100.0% coverage
+
+-- go.mod --
+module coverdep2
+
+go 1.16
+-- p1/p.go --
+package p1
+
+func F() int { return 1 }
+-- p1/p_test.go --
+package p1_test
+
+import (
+	"coverdep2/p2"
+	"testing"
+)
+
+func Test(t *testing.T) {
+	p2.F()
+}
+-- p2/p2.go --
+package p2
+
+import "coverdep2/p1"
+
+func F() {
+	p1.F()
+}
diff --git a/src/cmd/go/testdata/script/cover_dot_import.txt b/src/cmd/go/testdata/script/cover_dot_import.txt
new file mode 100644
index 0000000..a336b1c
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_dot_import.txt
@@ -0,0 +1,29 @@
+[short] skip
+[compiler:gccgo] skip # gccgo has no cover tool
+
+go test -coverpkg=coverdot/a,coverdot/b coverdot/b
+! stderr '[^0-9]0\.0%'
+! stdout '[^0-9]0\.0%'
+
+-- go.mod --
+module coverdot
+
+go 1.16
+-- a/a.go --
+package a
+
+func F() {}
+-- b/b.go --
+package b
+
+import . "coverdot/a"
+
+func G() { F() }
+-- b/b_test.go --
+package b
+
+import "testing"
+
+func TestG(t *testing.T) {
+	G()
+}
diff --git a/src/cmd/go/testdata/script/cover_error.txt b/src/cmd/go/testdata/script/cover_error.txt
new file mode 100644
index 0000000..fa4b58b
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_error.txt
@@ -0,0 +1,45 @@
+[short] skip
+[compiler:gccgo] skip
+
+# Test line numbers in cover errors.
+
+# Get errors from a go test into stderr.txt
+! go test coverbad
+stderr 'p\.go:4:2' # look for error at coverbad/p.go:4
+[cgo] stderr 'p1\.go:6:2' # look for error at coverbad/p.go:6
+! stderr $WORK # make sure temporary directory isn't in error
+
+cp stderr $WORK/stderr.txt
+
+# Get errors from coverage into stderr2.txt
+! go test -cover coverbad
+cp stderr $WORK/stderr2.txt
+
+wait # for go run above
+
+cmp $WORK/stderr.txt $WORK/stderr2.txt
+
+-- go.mod --
+module coverbad
+
+go 1.16
+-- p.go --
+package p
+
+func f() {
+	g()
+}
+-- p1.go --
+package p
+
+import "C"
+
+func h() {
+	j()
+}
+-- p_test.go --
+package p
+
+import "testing"
+
+func Test(t *testing.T) {}
diff --git a/src/cmd/go/testdata/script/cover_import_main_loop.txt b/src/cmd/go/testdata/script/cover_import_main_loop.txt
new file mode 100644
index 0000000..36a09c2
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_import_main_loop.txt
@@ -0,0 +1,26 @@
+[compiler:gccgo] skip # gccgo has no cover tool
+
+! go test -n importmain/test
+stderr 'not an importable package' # check that import main was detected
+! go test -n -cover importmain/test
+stderr 'not an importable package' # check that import main was detected
+
+-- go.mod --
+module importmain
+
+go 1.16
+-- ismain/main.go --
+package main
+
+import _ "importmain/test"
+
+func main() {}
+-- test/test.go --
+package test
+-- test/test_test.go --
+package test_test
+
+import "testing"
+import _ "importmain/ismain"
+
+func TestCase(t *testing.T) {}
diff --git a/src/cmd/go/testdata/script/cover_list.txt b/src/cmd/go/testdata/script/cover_list.txt
new file mode 100644
index 0000000..6b8aaf4
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_list.txt
@@ -0,0 +1,65 @@
+
+# This test is intended to verify that "go list" accepts coverage related
+# build arguments (such as -cover, -covermode). See issue #57785.
+
+[short] skip
+[!GOEXPERIMENT:coverageredesign] skip
+
+env GOBIN=$WORK/bin
+
+# Install a target and then do an ordinary staleness check on it.
+go install m/example
+! stale m/example
+
+# Run a second staleness check with "-cover" as a build flag. The
+# installed target should indeed be stale, since we didn't build it
+# with -cover.
+stale -cover m/example
+
+# Collect build ID from for m/example built with -cover.
+go list -cover -export -f '{{.BuildID}}' m/example
+cp stdout $WORK/listbuildid.txt
+
+# Now build the m/example binary with coverage.
+go build -cover -o $WORK/m.exe m/example
+
+# Ask for the binary build ID by running "go tool buildid".
+go tool buildid $WORK/m.exe
+cp stdout $WORK/rawtoolbuildid.txt
+
+# Make sure that the two build IDs agree with respect to the
+# m/example package. Build IDs from binaries are of the form X/Y/Z/W
+# where Y/Z is the package build ID; running the program below will
+# pick out the parts of the ID that we want.
+env GOCOVERDIR=$WORK
+exec $WORK/m.exe $WORK/rawtoolbuildid.txt
+cp stdout $WORK/toolbuildid.txt
+
+# Build IDs should match here.
+cmp $WORK/toolbuildid.txt $WORK/listbuildid.txt
+
+-- go.mod --
+module m
+
+go 1.20
+-- example/main.go --
+package main
+
+import (
+	"fmt"
+	"os"
+	"strings"
+)
+
+func main() {
+	println(os.Args[1])
+	content, err := os.ReadFile(os.Args[1])
+	if err != nil {
+		os.Exit(1)
+	}
+	fields := strings.Split(strings.TrimSpace(string(content)), "/")
+	if len(fields) != 4 {
+		os.Exit(2)
+	}
+	fmt.Println(fields[1] + "/" + fields[2])
+}
diff --git a/src/cmd/go/testdata/script/cover_main_import_path.txt b/src/cmd/go/testdata/script/cover_main_import_path.txt
new file mode 100644
index 0000000..e8696e2
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_main_import_path.txt
@@ -0,0 +1,55 @@
+
+# This test is intended to verify that coverage reporting is consistent
+# between "go test -cover" and "go build -cover" with respect to how
+# the "main" package is handled. See issue 57169 for details.
+
+[short] skip
+[!GOEXPERIMENT:coverageredesign] skip
+
+# Build this program with -cover and run to collect a profile.
+
+go build -cover -o $WORK/prog.exe .
+
+# Save off old GOCOVERDIR setting
+env SAVEGOCOVERDIR=$GOCOVERDIR
+
+mkdir $WORK/covdata
+env GOCOVERDIR=$WORK/covdata
+exec $WORK/prog.exe
+
+# Restore previous GOCOVERDIR setting
+env GOCOVERDIR=$SAVEGOCOVERDIR
+
+# Report percent lines covered.
+go tool covdata percent -i=$WORK/covdata
+stdout '\s*mainwithtest\s+coverage:'
+! stdout 'main\s+coverage:'
+
+# Go test -cover should behave the same way.
+go test -cover .
+stdout 'ok\s+mainwithtest\s+\S+\s+coverage:'
+! stdout 'ok\s+main\s+.*'
+
+
+-- go.mod --
+module mainwithtest
+
+go 1.20
+-- mymain.go --
+package main
+
+func main() {
+	println("hi mom")
+}
+
+func Mainer() int {
+	return 42
+}
+-- main_test.go --
+package main
+
+import "testing"
+
+func TestCoverage(t *testing.T) {
+	println(Mainer())
+}
diff --git a/src/cmd/go/testdata/script/cover_mod_empty.txt b/src/cmd/go/testdata/script/cover_mod_empty.txt
new file mode 100644
index 0000000..3c45243
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_mod_empty.txt
@@ -0,0 +1,9 @@
+go tool cover -func=cover.out
+stdout total.*statements.*0.0%
+
+go mod init golang.org/issue/33855
+
+go tool cover -func=cover.out
+stdout total.*statements.*0.0%
+
+-- cover.out --
diff --git a/src/cmd/go/testdata/script/cover_modes.txt b/src/cmd/go/testdata/script/cover_modes.txt
new file mode 100644
index 0000000..a27296e
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_modes.txt
@@ -0,0 +1,25 @@
+env GO111MODULE=off
+
+# Coverage analysis should use 'set' mode by default,
+# and should merge coverage profiles correctly.
+
+[short] skip
+[compiler:gccgo] skip # gccgo has no cover tool
+
+go test -short -cover encoding/binary errors -coverprofile=$WORK/cover.out
+! stderr '[^0-9]0\.0%'
+! stdout '[^0-9]0\.0%'
+
+grep -count=1 '^mode: set$' $WORK/cover.out
+grep 'errors\.go' $WORK/cover.out
+grep 'binary\.go' $WORK/cover.out
+
+[!race] stop
+
+go test -short -race -cover encoding/binary errors -coverprofile=$WORK/cover.out
+! stderr '[^0-9]0\.0%'
+! stdout '[^0-9]0\.0%'
+
+grep -count=1 '^mode: atomic$' $WORK/cover.out
+grep 'errors\.go' $WORK/cover.out
+grep 'binary\.go' $WORK/cover.out
diff --git a/src/cmd/go/testdata/script/cover_pattern.txt b/src/cmd/go/testdata/script/cover_pattern.txt
new file mode 100644
index 0000000..6c79c10
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_pattern.txt
@@ -0,0 +1,41 @@
+[compiler:gccgo] skip
+
+# If coverpkg=m/sleepy... expands by package loading
+# (as opposed to pattern matching on deps)
+# then it will try to load sleepybad, which does not compile,
+# and the test command will fail.
+! go list m/sleepy...
+go test -c -n -coverprofile=$TMPDIR/cover.out -coverpkg=m/sleepy... -run=^$ m/sleepy1
+
+-- go.mod --
+module m
+
+go 1.16
+-- sleepy1/p_test.go --
+package p
+
+import (
+	"testing"
+	"time"
+)
+
+func Test1(t *testing.T) {
+	time.Sleep(200 * time.Millisecond)
+}
+-- sleepy2/p_test.go --
+package p
+
+import (
+	"testing"
+	"time"
+)
+
+func Test1(t *testing.T) {
+	time.Sleep(200 * time.Millisecond)
+}
+-- sleepybad/p.go --
+package p
+
+import ^
+
+var _ = io.DoesNotExist
diff --git a/src/cmd/go/testdata/script/cover_pkgall_imports.txt b/src/cmd/go/testdata/script/cover_pkgall_imports.txt
new file mode 100644
index 0000000..4e51726
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_pkgall_imports.txt
@@ -0,0 +1,48 @@
+# This test checks that -coverpkg=all can be used
+# when the package pattern includes packages
+# which only have tests.
+# Verifies golang.org/issue/27333, golang.org/issue/43242.
+
+[short] skip
+cd $GOPATH/src/example.com/cov
+
+env GO111MODULE=on
+go test -coverpkg=all ./...
+
+env GO111MODULE=off
+go test -coverpkg=all ./...
+
+-- $GOPATH/src/example.com/cov/go.mod --
+module example.com/cov
+
+-- $GOPATH/src/example.com/cov/notest/notest.go --
+package notest
+
+func Foo() {}
+
+-- $GOPATH/src/example.com/cov/onlytest/onlytest_test.go --
+package onlytest_test
+
+import (
+	"testing"
+
+	"example.com/cov/notest"
+)
+
+func TestFoo(t *testing.T) {
+	notest.Foo()
+}
+
+-- $GOPATH/src/example.com/cov/withtest/withtest.go --
+package withtest
+
+func Bar() {}
+
+-- $GOPATH/src/example.com/cov/withtest/withtest_test.go --
+package withtest
+
+import "testing"
+
+func TestBar(t *testing.T) {
+	Bar()
+}
diff --git a/src/cmd/go/testdata/script/cover_pkgall_multiple_mains.txt b/src/cmd/go/testdata/script/cover_pkgall_multiple_mains.txt
new file mode 100644
index 0000000..f21cd8b
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_pkgall_multiple_mains.txt
@@ -0,0 +1,46 @@
+# This test checks that multiple main packages can be tested
+# with -coverpkg=all without duplicate symbol errors.
+# Verifies golang.org/issue/30374, golang.org/issue/34114.
+
+[short] skip
+cd $GOPATH/src/example.com/cov
+
+env GO111MODULE=on
+go test -coverpkg=all ./...
+
+env GO111MODULE=off
+go test -coverpkg=all ./...
+
+-- $GOPATH/src/example.com/cov/go.mod --
+module example.com/cov
+
+-- $GOPATH/src/example.com/cov/mainonly/mainonly.go --
+package main
+
+func main() {}
+
+-- $GOPATH/src/example.com/cov/mainwithtest/mainwithtest.go --
+package main
+
+func main() {}
+
+func Foo() {}
+
+-- $GOPATH/src/example.com/cov/mainwithtest/mainwithtest_test.go --
+package main
+
+import "testing"
+
+func TestFoo(t *testing.T) {
+  Foo()
+}
+
+-- $GOPATH/src/example.com/cov/xtest/x.go --
+package x
+
+-- $GOPATH/src/example.com/cov/xtest/x_test.go --
+package x_test
+
+import "testing"
+
+func TestX(t *testing.T) {}
diff --git a/src/cmd/go/testdata/script/cover_pkgall_runtime.txt b/src/cmd/go/testdata/script/cover_pkgall_runtime.txt
new file mode 100644
index 0000000..9927c30
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_pkgall_runtime.txt
@@ -0,0 +1,23 @@
+env GO111MODULE=off
+
+# Issue 23882
+
+[short] skip
+
+go test -coverpkg=all x
+stdout ok[\s\S]+?coverage
+
+[!race] stop
+
+go test -coverpkg=all -race x
+stdout ok[\s\S]+?coverage
+
+-- x/x.go --
+package x
+import _ "runtime"
+func F() {}
+
+-- x/x_test.go --
+package x
+import "testing"
+func TestF(t *testing.T) { F() }
diff --git a/src/cmd/go/testdata/script/cover_runs.txt b/src/cmd/go/testdata/script/cover_runs.txt
new file mode 100644
index 0000000..6df6096
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_runs.txt
@@ -0,0 +1,13 @@
+[compiler:gccgo] skip 'gccgo has no cover tool'
+[short] skip
+
+go test -short -coverpkg=strings strings regexp
+! stdout '[^0-9]0\.0%'
+stdout  'strings.*coverage:.*[1-9][0-9.]+%'
+stdout  'regexp.*coverage:.*[1-9][0-9.]+%'
+
+go test -short -cover strings math regexp
+! stdout '[^0-9]0\.0%'
+stdout  'strings.*coverage:.*[1-9][0-9.]+%'
+stdout  'math.*coverage:.*[1-9][0-9.]+%'
+stdout  'regexp.*coverage:.*[1-9][0-9.]+%'
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/cover_statements.txt b/src/cmd/go/testdata/script/cover_statements.txt
new file mode 100644
index 0000000..4f3c9ca
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_statements.txt
@@ -0,0 +1,61 @@
+[short] skip
+go test -cover ./pkg1 ./pkg2 ./pkg3 ./pkg4
+stdout 'pkg1	\[no test files\]'
+stdout 'pkg2	\S+	coverage: 0.0% of statements \[no tests to run\]'
+stdout 'pkg3	\S+	coverage: 100.0% of statements'
+stdout 'pkg4	\S+	coverage: \[no statements\]'
+
+-- go.mod --
+module m
+
+go 1.16
+-- pkg1/a.go --
+package pkg1
+
+import "fmt"
+
+func F() {
+	fmt.Println("pkg1")
+}
+-- pkg2/a.go --
+package pkg2
+
+import "fmt"
+
+func F() {
+	fmt.Println("pkg2")
+}
+-- pkg2/a_test.go --
+package pkg2
+-- pkg3/a.go --
+package pkg3
+
+import "fmt"
+
+func F() {
+	fmt.Println("pkg3")
+}
+-- pkg3/a_test.go --
+package pkg3
+
+import "testing"
+
+func TestF(t *testing.T) {
+	F()
+}
+-- pkg4/a.go --
+package pkg4
+
+type T struct {
+	X bool
+}
+-- pkg4/a_test.go --
+package pkg4
+
+import (
+	"testing"
+)
+
+func TestT(t *testing.T) {
+	_ = T{}
+}
diff --git a/src/cmd/go/testdata/script/cover_sync_atomic_import.txt b/src/cmd/go/testdata/script/cover_sync_atomic_import.txt
new file mode 100644
index 0000000..283db3e
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_sync_atomic_import.txt
@@ -0,0 +1,44 @@
+[short] skip
+[compiler:gccgo] skip # gccgo has no cover tool
+[!GOEXPERIMENT:coverageredesign] skip
+
+go test -short -cover -covermode=atomic -coverpkg=coverdep/p1 coverdep
+
+# In addition to the above, test to make sure there is no funny
+# business if we try "go test -cover" in atomic mode targeting
+# sync/atomic itself (see #57445). Just a short test run is needed
+# since we're mainly interested in making sure the test builds and can
+# execute at least one test.
+
+go test -short -covermode=atomic -run=TestStoreInt64 sync/atomic
+go test -short -covermode=atomic -run=TestAnd8 runtime/internal/atomic
+
+# Skip remainder if no race detector support.
+[!race] skip
+
+go test -short -cover -race -run=TestStoreInt64 sync/atomic
+go test -short -cover -race -run=TestAnd8 runtime/internal/atomic
+
+-- go.mod --
+module coverdep
+
+go 1.16
+-- p.go --
+package p
+
+import _ "coverdep/p1"
+
+func F() {
+}
+-- p1/p1.go --
+package p1
+
+import _ "errors"
+-- p_test.go --
+package p
+
+import "testing"
+
+func Test(t *testing.T) {
+	F()
+}
diff --git a/src/cmd/go/testdata/script/cover_test_localpkg_filepath.txt b/src/cmd/go/testdata/script/cover_test_localpkg_filepath.txt
new file mode 100644
index 0000000..17c5808
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_test_localpkg_filepath.txt
@@ -0,0 +1,40 @@
+
+[short] skip
+
+# collect coverage profile in text format
+go test -coverprofile=blah.prof prog.go prog_test.go
+
+# should not contain cmd-line pseudo-import-path
+grep prog.go blah.prof
+grep $PWD blah.prof
+! grep command-line-arguments blah.prof
+
+-- prog.go --
+package main
+
+func Mumble(x int) int {
+	if x < 0 {
+		return -x
+	}
+	return 42
+}
+
+func Grumble(y int) int {
+	return -y
+}
+
+func main() {
+}
+
+-- prog_test.go --
+package main
+
+import (
+	"testing"
+)
+
+func TestMumble(t *testing.T) {
+	if x := Mumble(10); x != 42 {
+		t.Errorf("Mumble(%d): got %d want %d", 10, x, 42)
+	}
+}
diff --git a/src/cmd/go/testdata/script/cover_test_pkgselect.txt b/src/cmd/go/testdata/script/cover_test_pkgselect.txt
new file mode 100644
index 0000000..97a1d2c
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_test_pkgselect.txt
@@ -0,0 +1,80 @@
+
+[short] skip
+
+# Hard-wire new coverage for this test.
+env GOEXPERIMENT=coverageredesign
+
+# Baseline run.
+go test -cover example/foo
+stdout 'coverage: 50.0% of statements$'
+
+# Coverage percentage output should mention -coverpkg selection.
+go test -coverpkg=example/foo example/foo
+stdout 'coverage: 50.0% of statements in example/foo'
+
+# Try to ask for coverage of a package that doesn't exist.
+go test -coverpkg nonexistent example/bar
+stderr 'no packages being tested depend on matches for pattern nonexistent'
+stdout 'coverage: \[no statements\]'
+
+# Ask for foo coverage, but test bar.
+go test -coverpkg=example/foo example/bar
+stdout 'coverage: 50.0% of statements in example/foo'
+
+# end of test cmds, start of harness and related files.
+
+-- go.mod --
+module example
+
+go 1.18
+
+-- foo/foo.go --
+package foo
+
+func FooFunc() int {
+	return 42
+}
+func FooFunc2() int {
+	return 42
+}
+
+-- foo/foo_test.go --
+package foo
+
+import "testing"
+
+func TestFoo(t *testing.T) {
+	if FooFunc() != 42 {
+		t.Fatalf("bad")
+	}
+}
+
+-- bar/bar.go --
+package bar
+
+import "example/foo"
+
+func BarFunc() int {
+	return foo.FooFunc2()
+}
+
+-- bar/bar_test.go --
+package bar_test
+
+import (
+	"example/bar"
+	"testing"
+)
+
+func TestBar(t *testing.T) {
+	if bar.BarFunc() != 42 {
+		t.Fatalf("bad")
+	}
+}
+
+-- baz/baz.go --
+package baz
+
+func BazFunc() int {
+	return -42
+}
diff --git a/src/cmd/go/testdata/script/cover_test_race_issue56370.txt b/src/cmd/go/testdata/script/cover_test_race_issue56370.txt
new file mode 100644
index 0000000..2e55f10
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_test_race_issue56370.txt
@@ -0,0 +1,54 @@
+[short] skip
+[!race] skip
+
+go test -race -cover issue.56370/filter
+
+-- go.mod --
+module issue.56370
+
+go 1.20
+
+-- filter/filter.go --
+
+package filter
+
+func New() func(error) bool {
+	return func(error) bool {
+		return false
+	}
+}
+
+-- filter/filter_test.go --
+
+package filter_test
+
+import (
+	"testing"
+
+	"issue.56370/filter"
+)
+
+func Test1(t *testing.T) {
+	t.Parallel()
+
+	_ = filter.New()
+}
+
+func Test2(t *testing.T) {
+	t.Parallel()
+
+	_ = filter.New()
+}
+
+func Test3(t *testing.T) {
+	t.Parallel()
+
+	_ = filter.New()
+}
+
+func Test4(t *testing.T) {
+	t.Parallel()
+
+	_ = filter.New()
+}
+
diff --git a/src/cmd/go/testdata/script/cover_var_init_order.txt b/src/cmd/go/testdata/script/cover_var_init_order.txt
new file mode 100644
index 0000000..37e07b7
--- /dev/null
+++ b/src/cmd/go/testdata/script/cover_var_init_order.txt
@@ -0,0 +1,55 @@
+# This test verifies that issue 56293 has been fixed, and that the
+# insertion of coverage instrumentation doesn't perturb package
+# initialization order.
+
+[short] skip
+
+# Skip if new coverage is turned off.
+[!GOEXPERIMENT:coverageredesign] skip
+
+go test -cover example
+
+-- go.mod --
+module example
+
+go 1.20
+
+-- m.go --
+
+package main
+
+import (
+	"flag"
+)
+
+var (
+	fooFlag = flag.String("foo", "", "this should be ok")
+	foo     = flag.Lookup("foo")
+
+	barFlag = flag.String("bar", "", "this should be also ok, but is "+notOK()+".")
+	bar     = flag.Lookup("bar")
+)
+
+func notOK() string {
+	return "not OK"
+}
+
+-- m_test.go --
+
+package main
+
+import (
+	"testing"
+)
+
+func TestFoo(t *testing.T) {
+	if foo == nil {
+		t.Fatal()
+	}
+}
+
+func TestBar(t *testing.T) {
+	if bar == nil {
+		t.Fatal()
+	}
+}
diff --git a/src/cmd/go/testdata/script/cpu_profile_twice.txt b/src/cmd/go/testdata/script/cpu_profile_twice.txt
new file mode 100644
index 0000000..38d6439
--- /dev/null
+++ b/src/cmd/go/testdata/script/cpu_profile_twice.txt
@@ -0,0 +1,22 @@
+env GO111MODULE=off
+
+# Issue 23150
+
+[short] skip
+
+go test -o=$WORK/x.test -cpuprofile=$WORK/cpu_profile_twice.out x
+rm $WORK/cpu_profile_twice.out
+
+go test -o=$WORK/x.test -cpuprofile=$WORK/cpu_profile_twice.out x
+exists $WORK/cpu_profile_twice.out
+
+
+-- x/x_test.go --
+package x_test
+import (
+    "testing"
+    "time"
+)
+func TestSleep(t *testing.T) {
+    time.Sleep(10 * time.Millisecond)
+}
diff --git a/src/cmd/go/testdata/script/devnull.txt b/src/cmd/go/testdata/script/devnull.txt
new file mode 100644
index 0000000..ccb866a
--- /dev/null
+++ b/src/cmd/go/testdata/script/devnull.txt
@@ -0,0 +1,26 @@
+env GO111MODULE=off
+
+# Issue 28035: go test -c -o NUL should work.
+# Issue 28549: go test -c -o /dev/null should not overwrite /dev/null when run as root.
+cd x
+cmp $devnull $WORK/empty.txt
+go test -o=$devnull -c
+! exists x.test$GOEXE
+cmp $devnull $WORK/empty.txt
+
+# Issue 12407: go build -o /dev/null should succeed.
+cd ..
+go build -o $devnull y
+cmp $devnull $WORK/empty.txt
+
+-- x/x_test.go --
+package x_test
+import (
+    "testing"
+)
+func TestNUL(t *testing.T) {
+}
+-- y/y.go --
+package y
+func main() {}
+-- $WORK/empty.txt --
diff --git a/src/cmd/go/testdata/script/dist_list_missing.txt b/src/cmd/go/testdata/script/dist_list_missing.txt
new file mode 100644
index 0000000..affaa00
--- /dev/null
+++ b/src/cmd/go/testdata/script/dist_list_missing.txt
@@ -0,0 +1,57 @@
+# Regression test for #60939: when 'go tool dist' is missing,
+# 'go tool dist list' should inject its output.
+
+
+# Set GOROOT to a directory that definitely does not include
+# a compiled 'dist' tool. 'go tool dist list' should still
+# work, because 'cmd/go' itself can impersonate this command.
+
+mkdir $WORK/goroot/bin
+mkdir $WORK/goroot/pkg/tool/${GOOS}_${GOARCH}
+env GOROOT=$WORK/goroot
+
+! go tool -n dist
+stderr 'go: no such tool "dist"'
+
+go tool dist list
+stdout linux/amd64
+cp stdout tool.txt
+
+go tool dist list -v
+stdout linux/amd64
+cp stdout tool-v.txt
+
+go tool dist list -broken
+stdout $GOOS/$GOARCH
+cp stdout tool-broken.txt
+
+go tool dist list -json
+stdout '"GOOS": "linux",\n\s*"GOARCH": "amd64",\n'
+cp stdout tool-json.txt
+
+go tool dist list -json -broken
+stdout '"GOOS": "'$GOOS'",\n\s*"GOARCH": "'$GOARCH'",\n'
+cp stdout tool-json-broken.txt
+
+[short] stop
+
+
+# Check against the real cmd/dist as the source of truth.
+
+env GOROOT=$TESTGO_GOROOT
+go build -o dist.exe cmd/dist
+
+exec ./dist.exe list
+cmp stdout tool.txt
+
+exec ./dist.exe list -v
+cmp stdout tool-v.txt
+
+exec ./dist.exe list -broken
+cmp stdout tool-broken.txt
+
+exec ./dist.exe list -json
+cmp stdout tool-json.txt
+
+exec ./dist.exe list -json -broken
+cmp stdout tool-json-broken.txt
diff --git a/src/cmd/go/testdata/script/doc.txt b/src/cmd/go/testdata/script/doc.txt
new file mode 100644
index 0000000..3ff1aab
--- /dev/null
+++ b/src/cmd/go/testdata/script/doc.txt
@@ -0,0 +1,75 @@
+# go doc --help
+! go doc --help
+stderr 'go doc'
+stderr 'go doc <pkg>'
+stderr 'go doc <sym>\[\.<methodOrField>\]'
+stderr 'go doc \[<pkg>\.\]<sym>\[\.<methodOrField>\]'
+stderr 'go doc \[<pkg>\.\]\[<sym>\.\]<methodOrField>'
+stderr 'go doc <pkg> <sym>\[\.<methodOrField>\]'
+
+# go help doc
+go help doc
+stdout 'go doc'
+stdout 'go doc <pkg>'
+stdout 'go doc <sym>\[\.<methodOrField>\]'
+stdout 'go doc \[<pkg>\.\]<sym>\[\.<methodOrField>\]'
+stdout 'go doc \[<pkg>\.\]\[<sym>\.\]<methodOrField>'
+stdout 'go doc <pkg> <sym>\[\.<methodOrField>\]'
+
+# go doc <pkg>
+go doc p/v2
+stdout .
+
+# go doc <pkg> <sym>
+go doc p/v2 Symbol
+stdout .
+
+# go doc <pkg> <sym> <method>
+! go doc p/v2 Symbol Method
+stderr .
+
+# go doc <pkg>.<sym>
+go doc p/v2.Symbol
+stdout .
+
+# go doc <pkg>.<sym>.<method>
+go doc p/v2.Symbol.Method
+stdout .
+
+# go doc <sym>
+go doc Symbol
+stdout .
+
+# go doc <sym> <method>
+! go doc Symbol Method
+stderr .
+
+# go doc <sym>.<method>
+go doc Symbol.Method
+stdout .
+
+# go doc <pkg>.<method>
+go doc p/v2.Method
+stdout .
+
+# go doc <pkg> <method>
+go doc p/v2 Method
+stdout .
+
+# go doc <method>
+go doc Method
+stdout .
+
+-- go.mod --
+module p/v2
+
+go 1.13
+
+-- p.go --
+package p
+
+type Symbol struct{}
+
+func (Symbol) Method() error {
+	return nil
+}
diff --git a/src/cmd/go/testdata/script/embed.txt b/src/cmd/go/testdata/script/embed.txt
new file mode 100644
index 0000000..5f7f6ed
--- /dev/null
+++ b/src/cmd/go/testdata/script/embed.txt
@@ -0,0 +1,130 @@
+# go list shows patterns and files
+go list -f '{{.EmbedPatterns}}'
+stdout '\[x\*t\*t\]'
+go list -f '{{.EmbedFiles}}'
+stdout '\[x.txt\]'
+go list -test -f '{{.TestEmbedPatterns}}'
+stdout '\[y\*t\*t\]'
+go list -test -f '{{.TestEmbedFiles}}'
+stdout '\[y.txt\]'
+go list -test -f '{{.XTestEmbedPatterns}}'
+stdout '\[z\*t\*t\]'
+go list -test -f '{{.XTestEmbedFiles}}'
+stdout '\[z.txt\]'
+
+# build embeds x.txt
+go build -x
+stderr 'x.txt'
+
+# build uses cache correctly
+go build -x
+! stderr 'x.txt'
+cp x.txt2 x.txt
+go build -x
+stderr 'x.txt'
+
+# build rejects invalid names
+cp x.go2 x.go
+go build -x
+cp x.txt .git
+! go build -x
+stderr '^x.go:5:12: pattern [*]t: cannot embed file [.]git: invalid name [.]git$'
+rm .git
+
+# build rejects symlinks
+[symlink] symlink x.tzt -> x.txt
+[symlink] ! go build -x
+[symlink] stderr 'pattern [*]t: cannot embed irregular file x.tzt'
+[symlink] rm x.tzt
+
+# build rejects empty directories
+mkdir t
+! go build -x
+stderr '^x.go:5:12: pattern [*]t: cannot embed directory t: contains no embeddable files$'
+
+# build ignores symlinks and invalid names in directories
+cp x.txt t/.git
+! go build -x
+stderr '^x.go:5:12: pattern [*]t: cannot embed directory t: contains no embeddable files$'
+go list -e -f '{{.Incomplete}}'
+stdout 'true'
+[symlink] symlink t/x.link -> ../x.txt
+[symlink] ! go build -x
+[symlink] stderr '^x.go:5:12: pattern [*]t: cannot embed directory t: contains no embeddable files$'
+
+cp x.txt t/x.txt
+go build -x
+
+# build reports errors with positions in imported packages
+rm t/x.txt
+! go build m/use
+stderr '^x.go:5:12: pattern [*]t: cannot embed directory t: contains no embeddable files$'
+
+# all still ignores .git and symlinks
+cp x.go3 x.go
+! go build -x
+stderr '^x.go:5:12: pattern all:t: cannot embed directory t: contains no embeddable files$'
+
+# all finds dot files and underscore files
+cp x.txt t/.x.txt
+go build -x
+rm t/.x.txt
+cp x.txt t/_x.txt
+go build -x
+
+-- x.go --
+package p
+
+import "embed"
+
+//go:embed x*t*t
+var X embed.FS
+
+-- x_test.go --
+package p
+
+import "embed"
+
+//go:embed y*t*t
+var Y string
+
+-- x_x_test.go --
+package p_test
+
+import "embed"
+
+//go:embed z*t*t
+var Z string
+
+-- x.go2 --
+package p
+
+import "embed"
+
+//go:embed *t
+var X embed.FS
+
+-- x.go3 --
+package p
+
+import "embed"
+
+//go:embed all:t
+var X embed.FS
+
+-- x.txt --
+hello
+
+-- y.txt --
+-- z.txt --
+-- x.txt2 --
+not hello
+
+-- use/use.go --
+package use
+
+import _ "m"
+-- go.mod --
+module m
+
+go 1.16
diff --git a/src/cmd/go/testdata/script/embed_brackets.txt b/src/cmd/go/testdata/script/embed_brackets.txt
new file mode 100644
index 0000000..ec17ff3
--- /dev/null
+++ b/src/cmd/go/testdata/script/embed_brackets.txt
@@ -0,0 +1,18 @@
+# issue 53314
+[GOOS:windows] skip
+cd [pkg]
+go build
+
+-- [pkg]/go.mod --
+module m
+
+go 1.19
+-- [pkg]/x.go --
+package p
+
+import _ "embed"
+
+//go:embed t.txt
+var S string
+
+-- [pkg]//t.txt --
diff --git a/src/cmd/go/testdata/script/embed_fmt.txt b/src/cmd/go/testdata/script/embed_fmt.txt
new file mode 100644
index 0000000..8a16afe
--- /dev/null
+++ b/src/cmd/go/testdata/script/embed_fmt.txt
@@ -0,0 +1,22 @@
+# go fmt ignores file not found
+go fmt xnofmt.go
+cmp xnofmt.go xfmt.ref
+! go build xnofmt.go
+stderr 'xnofmt.go:5:12: pattern missing.txt: no matching files found'
+
+-- xnofmt.go --
+package p
+
+import "embed"
+
+//go:embed missing.txt
+var X  embed.FS
+-- xfmt.ref --
+package p
+
+import "embed"
+
+//go:embed missing.txt
+var X embed.FS
+-- go.mod --
+module m
diff --git a/src/cmd/go/testdata/script/env_cache.txt b/src/cmd/go/testdata/script/env_cache.txt
new file mode 100644
index 0000000..f2af7ee
--- /dev/null
+++ b/src/cmd/go/testdata/script/env_cache.txt
@@ -0,0 +1,5 @@
+# go env should caches compiler results
+go env
+go env -x
+! stdout '\|\| true'
+
diff --git a/src/cmd/go/testdata/script/env_cross_build.txt b/src/cmd/go/testdata/script/env_cross_build.txt
new file mode 100644
index 0000000..91d1cb9
--- /dev/null
+++ b/src/cmd/go/testdata/script/env_cross_build.txt
@@ -0,0 +1,29 @@
+# Test that the correct default GOEXPERIMENT is used when cross
+# building with GOENV (#46815).
+
+# Unset variables set by the TestScript harness. Users typically won't
+# explicitly configure these, and #46815 doesn't repro if they are.
+env GOOS=
+env GOARCH=
+env GOEXPERIMENT=
+
+env GOENV=windows-amd64
+go build internal/abi
+
+env GOENV=ios-arm64
+go build internal/abi
+
+env GOENV=linux-mips
+go build internal/abi
+
+-- windows-amd64 --
+GOOS=windows
+GOARCH=amd64
+
+-- ios-arm64 --
+GOOS=ios
+GOARCH=arm64
+
+-- linux-mips --
+GOOS=linux
+GOARCH=mips
diff --git a/src/cmd/go/testdata/script/env_exp.txt b/src/cmd/go/testdata/script/env_exp.txt
new file mode 100644
index 0000000..681512d
--- /dev/null
+++ b/src/cmd/go/testdata/script/env_exp.txt
@@ -0,0 +1,17 @@
+# Test GOEXPERIMENT variable
+
+# go env shows default empty GOEXPERIMENT
+go env
+stdout GOEXPERIMENT=
+
+# go env shows valid experiments
+env GOEXPERIMENT=fieldtrack,staticlockranking
+go env GOEXPERIMENT
+stdout '.*fieldtrack.*staticlockranking.*'
+go env
+stdout 'GOEXPERIMENT=.*fieldtrack.*staticlockranking.*'
+
+# go env rejects unknown experiments
+env GOEXPERIMENT=bad
+! go env GOEXPERIMENT
+stderr 'unknown GOEXPERIMENT bad'
diff --git a/src/cmd/go/testdata/script/env_sanitize.txt b/src/cmd/go/testdata/script/env_sanitize.txt
new file mode 100644
index 0000000..cc4d23a
--- /dev/null
+++ b/src/cmd/go/testdata/script/env_sanitize.txt
@@ -0,0 +1,5 @@
+env GOFLAGS='$(echo ''cc"''; echo ''OOPS="oops'')'
+go env
+[GOOS:darwin] stdout 'GOFLAGS=''\$\(echo ''\\''''cc"''\\''''; echo ''\\''''OOPS="oops''\\''''\)'''
+[GOOS:linux] stdout 'GOFLAGS=''\$\(echo ''\\''''cc"''\\''''; echo ''\\''''OOPS="oops''\\''''\)'''
+[GOOS:windows] stdout 'set GOFLAGS=\$\(echo ''cc"''; echo ''OOPS="oops''\)'
diff --git a/src/cmd/go/testdata/script/env_unset.txt b/src/cmd/go/testdata/script/env_unset.txt
new file mode 100644
index 0000000..22bc845
--- /dev/null
+++ b/src/cmd/go/testdata/script/env_unset.txt
@@ -0,0 +1,30 @@
+# Test that we can unset variables, even if initially invalid,
+# as long as resulting config is valid.
+
+env GOENV=badenv
+env GOOS=
+env GOARCH=
+env GOEXPERIMENT=
+
+! go env
+stderr '^go(\.exe)?: unknown GOEXPERIMENT badexp$'
+
+go env -u GOEXPERIMENT
+
+! go env
+stderr '^go: unsupported GOOS/GOARCH pair bados/badarch$'
+
+! go env -u GOOS
+stderr '^go: unsupported GOOS/GOARCH pair \w+/badarch$'
+
+! go env -u GOARCH
+stderr '^go: unsupported GOOS/GOARCH pair bados/\w+$'
+
+go env -u GOOS GOARCH
+
+go env
+
+-- badenv --
+GOOS=bados
+GOARCH=badarch
+GOEXPERIMENT=badexp
diff --git a/src/cmd/go/testdata/script/env_write.txt b/src/cmd/go/testdata/script/env_write.txt
new file mode 100644
index 0000000..5d40949
--- /dev/null
+++ b/src/cmd/go/testdata/script/env_write.txt
@@ -0,0 +1,206 @@
+env GO111MODULE=off
+
+# go env should default to the right places
+env AppData=$HOME/windowsappdata
+env home=$HOME/plan9home
+go env GOENV
+[GOOS:aix] stdout $HOME/.config/go/env
+[GOOS:darwin] stdout $HOME'/Library/Application Support/go/env'
+[GOOS:freebsd] stdout $HOME/.config/go/env
+[GOOS:linux] stdout $HOME/.config/go/env
+[GOOS:netbsd] stdout $HOME/.config/go/env
+[GOOS:openbsd] stdout $HOME/.config/go/env
+[GOOS:plan9] stdout $HOME/plan9home/lib/go/env
+[GOOS:windows] stdout $HOME\\windowsappdata\\go\\env
+
+# Now override it to something writable.
+env GOENV=$WORK/envdir/go/env
+go env GOENV
+stdout envdir[\\/]go[\\/]env
+
+# go env shows all variables
+go env
+stdout GOARCH=
+stdout GOOS=
+stdout GOROOT=
+
+# go env ignores invalid flag in GOFLAGS environment variable
+env GOFLAGS='=true'
+go env
+
+# checking errors
+! go env -w
+stderr 'go: no KEY=VALUE arguments given'
+! go env -u
+stderr 'go: ''go env -u'' requires an argument'
+
+# go env -w changes default setting
+env root=
+[GOOS:windows] env root=c:
+env GOPATH=
+go env -w GOPATH=$root/non-exist/gopath
+! stderr .+
+grep GOPATH=$root/non-exist/gopath $WORK/envdir/go/env
+go env GOPATH
+stdout /non-exist/gopath
+
+# go env -w does not override OS environment, and warns about that
+env GOPATH=$root/other
+go env -w GOPATH=$root/non-exist/gopath2
+stderr 'warning: go env -w GOPATH=... does not override conflicting OS environment variable'
+go env GOPATH
+stdout $root/other
+
+# but go env -w does do the update, and unsetting the env var exposes the change
+env GOPATH=
+go env GOPATH
+stdout $root/non-exist/gopath2
+
+# unsetting with go env -u does not warn about OS environment overrides,
+# nor does it warn about variables that haven't been set by go env -w.
+env GOPATH=$root/other
+go env -u GOPATH
+! stderr .+
+go env -u GOPATH
+! stderr .+
+
+# go env -w rejects unknown or bad variables
+! go env -w GODEBUG=gctrace=1
+stderr 'unknown go command variable GODEBUG'
+! go env -w GOEXE=.bat
+stderr 'GOEXE cannot be modified'
+! go env -w GOVERSION=customversion
+stderr 'GOVERSION cannot be modified'
+! go env -w GOENV=/env
+stderr 'GOENV can only be set using the OS environment'
+
+# go env -w can set multiple variables
+env CC=
+go env CC
+! stdout ^xyc$
+go env -w GOOS=$GOOS CC=xyc
+grep CC=xyc $GOENV
+# file is maintained in sorted order
+grep 'CC=xyc\nGOOS=' $GOENV
+go env CC
+stdout ^xyc$
+
+# go env -u unsets effect of go env -w.
+go env -u CC
+go env CC
+! stdout ^xyc$
+
+# go env -w rejects double-set variables
+! go env -w GOOS=$GOOS GOOS=$GOOS
+stderr 'multiple values for key: GOOS'
+
+# go env -w rejects missing variables
+! go env -w GOOS
+stderr 'arguments must be KEY=VALUE: invalid argument: GOOS'
+
+# go env -w rejects invalid GO111MODULE values, as otherwise cmd/go would break
+! go env -w GO111MODULE=badvalue
+stderr 'invalid GO111MODULE value "badvalue"'
+
+# go env -w rejects invalid GOPATH values
+! go env -w GOPATH=~/go
+stderr 'GOPATH entry cannot start with shell metacharacter'
+
+! go env -w GOPATH=./go
+stderr 'GOPATH entry is relative; must be absolute path'
+
+# go env -w rejects invalid GOTMPDIR values
+! go env -w GOTMPDIR=x
+stderr 'go: GOTMPDIR must be an absolute path'
+
+# go env -w should accept absolute GOTMPDIR value
+# and should not create it
+[GOOS:windows] go env -w GOTMPDIR=$WORK\x\y\z
+[!GOOS:windows] go env -w GOTMPDIR=$WORK/x/y/z
+! exists $WORK/x/y/z
+# we should be able to clear an env
+go env -u GOTMPDIR
+go env GOTMPDIR
+stdout ^$
+
+[GOOS:windows] go env -w GOTMPDIR=$WORK\x\y\z
+[!GOOS:windows] go env -w GOTMPDIR=$WORK/x/y/z
+go env -w GOTMPDIR=
+go env GOTMPDIR
+stdout ^$
+
+# go env -w rejects relative CC values
+[!GOOS:windows] go env -w CC=/usr/bin/clang
+go env -w CC=clang
+[!GOOS:windows] ! go env -w CC=./clang
+[!GOOS:windows] ! go env -w CC=bin/clang
+[!GOOS:windows] stderr 'go: CC entry is relative; must be absolute path'
+
+[GOOS:windows] go env -w CC=$WORK\bin\clang
+[GOOS:windows] ! go env -w CC=.\clang
+[GOOS:windows] ! go env -w CC=bin\clang
+[GOOS:windows] stderr 'go: CC entry is relative; must be absolute path'
+
+# go env -w rejects relative CXX values
+[!GOOS:windows] go env -w CC=/usr/bin/cpp
+go env -w CXX=cpp
+[!GOOS:windows] ! go env -w CXX=./cpp
+[!GOOS:windows] ! go env -w CXX=bin/cpp
+[!GOOS:windows] stderr 'go: CXX entry is relative; must be absolute path'
+
+[GOOS:windows] go env -w CXX=$WORK\bin\cpp
+[GOOS:windows] ! go env -w CXX=.\cpp
+[GOOS:windows] ! go env -w CXX=bin\cpp
+[GOOS:windows] stderr 'go: CXX entry is relative; must be absolute path'
+
+# go env -w/-u checks validity of GOOS/ARCH combinations
+env GOOS=
+env GOARCH=
+# check -w doesn't allow invalid GOOS
+! go env -w GOOS=linuxx
+stderr 'unsupported GOOS/GOARCH pair linuxx'
+# check -w doesn't allow invalid GOARCH
+! go env -w GOARCH=amd644
+stderr 'unsupported GOOS/GOARCH.*/amd644$'
+# check -w doesn't allow invalid GOOS with valid GOARCH
+! go env -w GOOS=linuxx GOARCH=amd64
+stderr 'unsupported GOOS/GOARCH pair linuxx'
+# check a valid GOOS and GOARCH values but an incompatible combinations
+! go env -w GOOS=android GOARCH=s390x
+stderr 'unsupported GOOS/GOARCH pair android/s390x'
+# check that -u considers explicit envs
+go env -w GOOS=linux GOARCH=mips
+env GOOS=windows
+! go env -u GOOS
+stderr 'unsupported GOOS/GOARCH.*windows/mips$'
+env GOOS=
+
+# go env -w should reject relative paths in GOMODCACHE environment.
+! go env -w GOMODCACHE=~/test
+stderr 'go: GOMODCACHE entry is relative; must be absolute path: "~/test"'
+! go env -w GOMODCACHE=./test
+stderr 'go: GOMODCACHE entry is relative; must be absolute path: "./test"'
+
+# go env -w checks validity of GOEXPERIMENT
+env GOEXPERIMENT=
+! go env -w GOEXPERIMENT=badexp
+stderr 'unknown GOEXPERIMENT badexp'
+go env -w GOEXPERIMENT=fieldtrack
+
+# go env -w and go env -u work on unknown fields already in the go/env file
+cp bad.env $GOENV
+go env GOENV
+cat $GOENV
+go env
+! stdout UNKNOWN
+go env UNKNOWN
+stdout yes
+go env -w UNKNOWN=maybe
+go env UNKNOWN
+stdout maybe
+go env -u UNKNOWN
+go env UNKNOWN
+! stdout . # gone
+
+-- bad.env --
+UNKNOWN=yes
diff --git a/src/cmd/go/testdata/script/fileline.txt b/src/cmd/go/testdata/script/fileline.txt
new file mode 100644
index 0000000..5cb35f0
--- /dev/null
+++ b/src/cmd/go/testdata/script/fileline.txt
@@ -0,0 +1,8 @@
+env GO111MODULE=off
+
+# look for short, relative file:line in error message
+! go run ../../gopath/x/y/z/err.go
+stderr ^..[\\/]x[\\/]y[\\/]z[\\/]err.go:
+
+-- ../x/y/z/err.go --
+package main; import "bar"
diff --git a/src/cmd/go/testdata/script/fmt_load_errors.txt b/src/cmd/go/testdata/script/fmt_load_errors.txt
new file mode 100644
index 0000000..e3a9034
--- /dev/null
+++ b/src/cmd/go/testdata/script/fmt_load_errors.txt
@@ -0,0 +1,38 @@
+env GO111MODULE=off
+
+! go fmt does-not-exist
+
+go fmt -n exclude
+stdout 'exclude[/\\]x\.go'
+stdout 'exclude[/\\]x_linux\.go'
+
+# Test edge cases with gofmt.
+
+! exec $GOROOT/bin/gofmt does-not-exist
+
+exec $GOROOT/bin/gofmt gofmt-dir/no-extension
+stdout 'package x'
+
+exec $GOROOT/bin/gofmt gofmt-dir
+! stdout 'package x'
+
+! exec $GOROOT/bin/gofmt empty.go nopackage.go
+stderr -count=1 'empty\.go:1:1: expected .package., found .EOF.'
+stderr -count=1 'nopackage\.go:1:1: expected .package., found not'
+
+-- exclude/empty/x.txt --
+-- exclude/ignore/_x.go --
+package x
+-- exclude/x.go --
+// +build linux,!linux
+
+package x
+-- exclude/x_linux.go --
+// +build windows
+
+package x
+-- gofmt-dir/no-extension --
+package x
+-- empty.go --
+-- nopackage.go --
+not the proper start to a Go file
diff --git a/src/cmd/go/testdata/script/fsys_walk.txt b/src/cmd/go/testdata/script/fsys_walk.txt
new file mode 100644
index 0000000..9d1a945
--- /dev/null
+++ b/src/cmd/go/testdata/script/fsys_walk.txt
@@ -0,0 +1,6 @@
+# Test that go list prefix... does not read directories not beginning with prefix.
+env GODEBUG=gofsystrace=1
+go list m...
+stderr mime
+stderr mime[\\/]multipart
+! stderr archive
diff --git a/src/cmd/go/testdata/script/gccgo_link_c.txt b/src/cmd/go/testdata/script/gccgo_link_c.txt
new file mode 100644
index 0000000..b9a4c70
--- /dev/null
+++ b/src/cmd/go/testdata/script/gccgo_link_c.txt
@@ -0,0 +1,18 @@
+# Issue #7573
+# cmd/cgo: undefined reference when linking a C-library using gccgo
+
+[!cgo] skip
+[!exec:gccgo] skip
+
+go build -n -compiler gccgo
+stderr 'gccgo.*\-L [^ ]*alibpath \-lalib' # make sure that Go-inline "#cgo LDFLAGS:" ("-L alibpath -lalib") passed to gccgo linking stage
+
+-- go.mod --
+module m
+-- cgoref.go --
+package main
+// #cgo LDFLAGS: -L alibpath -lalib
+// void f(void) {}
+import "C"
+
+func main() { C.f() }
diff --git a/src/cmd/go/testdata/script/gccgo_link_ldflags.txt b/src/cmd/go/testdata/script/gccgo_link_ldflags.txt
new file mode 100644
index 0000000..80526c6
--- /dev/null
+++ b/src/cmd/go/testdata/script/gccgo_link_ldflags.txt
@@ -0,0 +1,23 @@
+# Test that #cgo LDFLAGS are properly quoted.
+# The #cgo LDFLAGS below should pass a string with spaces to -L,
+# as though searching a directory with a space in its name.
+# It should not pass --nosuchoption to the external linker.
+
+[!cgo] skip
+
+go build
+
+[!exec:gccgo] skip
+
+# TODO: remove once gccgo on builder is updated
+[GOOS:aix] [GOARCH:ppc64] skip
+
+go build -compiler gccgo
+
+-- go.mod --
+module m
+-- cgo.go --
+package main
+// #cgo LDFLAGS: -L "./ -Wl,--nosuchoption"
+import "C"
+func main() {}
diff --git a/src/cmd/go/testdata/script/gccgo_m.txt b/src/cmd/go/testdata/script/gccgo_m.txt
new file mode 100644
index 0000000..beb9c50
--- /dev/null
+++ b/src/cmd/go/testdata/script/gccgo_m.txt
@@ -0,0 +1,20 @@
+# It's absurd, but builds with -compiler=gccgo used to fail to build module m.
+# golang.org/issue/34358
+
+env GO111MODULE=off
+
+[short] skip
+[cross] skip # gccgo can't necessarily cross-compile
+
+cd m
+go build
+exists m$GOEXE
+rm m$GOEXE
+[exec:gccgo] go build -compiler=gccgo
+[exec:gccgo] exists m$GOEXE
+
+-- m/go.mod --
+module m
+-- m/main.go --
+package main
+func main() {}
diff --git a/src/cmd/go/testdata/script/gccgo_mangle.txt b/src/cmd/go/testdata/script/gccgo_mangle.txt
new file mode 100644
index 0000000..7a09a80
--- /dev/null
+++ b/src/cmd/go/testdata/script/gccgo_mangle.txt
@@ -0,0 +1,15 @@
+# Issue 33871.
+
+cd m/a.0
+go build
+
+-- m/go.mod --
+module m
+-- m/a.0/a.go --
+package a
+
+type T int
+
+func (t T) M() int {
+	return int(t)
+}
diff --git a/src/cmd/go/testdata/script/gcflags_patterns.txt b/src/cmd/go/testdata/script/gcflags_patterns.txt
new file mode 100644
index 0000000..cc7b2fc
--- /dev/null
+++ b/src/cmd/go/testdata/script/gcflags_patterns.txt
@@ -0,0 +1,93 @@
+env GO111MODULE=off
+
+[!compiler:gc] skip 'using -gcflags and -ldflags'
+[short] skip
+
+# -gcflags=-e applies to named packages, not dependencies
+go build -a -n -v -gcflags=-e z1 z2
+stderr 'compile.* -p z1.* -e '
+stderr 'compile.* -p z2.* -e '
+stderr 'compile.* -p y'
+! stderr 'compile.* -p [^z].* -e '
+
+# -gcflags can specify package=flags, and can be repeated; last match wins
+go build -a -n -v -gcflags=-e -gcflags=z1=-N z1 z2
+stderr 'compile.* -p z1.* -N '
+! stderr 'compile.* -p z1.* -e '
+! stderr 'compile.* -p z2.* -N '
+stderr 'compile.* -p z2.* -e '
+stderr 'compile.* -p y'
+! stderr 'compile.* -p [^z].* -e '
+! stderr 'compile.* -p [^z].* -N '
+
+# -gcflags can have arbitrary spaces around the flags
+go build -a -n -v -gcflags='  z1 =  	-e 	' z1
+stderr 'compile.* -p z1.* -e '
+
+# -gcflags='all=-e' should apply to all packages, even with go test
+go test -a -c -n -gcflags='all=-e' z1
+stderr 'compile.* -p z3.* -e '
+
+# this particular -gcflags argument made the compiler crash
+! go build -gcflags=-d=ssa/ z1
+stderr 'PhaseOptions usage'
+
+# check for valid -ldflags parameter
+! go build '-ldflags="-X main.X=Hello"'
+stderr 'invalid value'
+
+# -ldflags for implicit test package applies to test binary
+go test -a -c -n -gcflags=-N -ldflags=-X=x.y=z z1
+stderr 'compile.* -N .*z_test.go'
+stderr 'link.* -X=x.y=z'
+
+# -ldflags for explicit test package applies to test binary
+go test -a -c -n -gcflags=z1=-N -ldflags=z1=-X=x.y=z z1
+stderr 'compile.* -N .*z_test.go'
+stderr 'link.* -X=x.y=z'
+
+# -ldflags applies to link of command
+go build -a -n -ldflags=-X=math.pi=3 my/cmd/prog
+stderr 'link.* -X=math.pi=3'
+
+# -ldflags applies to link of command even with strange directory name
+go build -a -n -ldflags=-X=math.pi=3 my/cmd/prog/
+stderr 'link.* -X=math.pi=3'
+
+# -ldflags applies to current directory
+cd my/cmd/prog
+go build -a -n -ldflags=-X=math.pi=3
+stderr 'link.* -X=math.pi=3'
+
+# -ldflags applies to current directory even if GOPATH is funny
+[!case-sensitive] cd $WORK/GoPath/src/my/cmd/prog
+go build -a -n -ldflags=-X=math.pi=3
+stderr 'link.* -X=math.pi=3'
+
+# cgo.a should not be a dependency of internally-linked go package
+go build -ldflags='-linkmode=external -linkmode=internal' -n prog.go
+! stderr 'packagefile .*runtime/cgo.a'
+
+-- z1/z.go --
+package z1
+import _ "y"
+import _ "z2"
+
+-- z1/z_test.go --
+package z1_test
+import "testing"
+import _ "z3"
+func Test(t *testing.T) {}
+
+-- z2/z.go --
+package z2
+
+-- z3/z.go --
+package z3
+
+-- y/y.go --
+package y
+
+-- my/cmd/prog/prog.go --
+package main
+func main() {}
diff --git a/src/cmd/go/testdata/script/generate.txt b/src/cmd/go/testdata/script/generate.txt
new file mode 100644
index 0000000..58777c5
--- /dev/null
+++ b/src/cmd/go/testdata/script/generate.txt
@@ -0,0 +1,107 @@
+[short] skip
+
+# Install an echo command because Windows doesn't have it.
+env GOBIN=$WORK/tmp/bin
+go install echo.go
+env PATH=$GOBIN${:}$PATH
+
+# Test go generate handles a simple command
+go generate ./generate/simple.go
+stdout 'Success'
+
+# Test go generate handles a command alias
+go generate './generate/alias.go'
+stdout 'Now is the time for all good men'
+
+# Test go generate's variable substitution
+go generate './generate/substitution.go'
+stdout $GOARCH' substitution.go:7 pabc xyzp/substitution.go/123'
+
+# Test go generate's run and skip flags
+go generate -run y.s './generate/flag.go'
+stdout 'yes' # flag.go should select yes
+! stdout 'no' # flag.go should not select no
+
+go generate -skip th..sand './generate/flag.go'
+stdout 'yes' # flag.go should select yes
+! stdout 'no' # flag.go should not select no
+
+go generate -run . -skip th..sand './generate/flag.go'
+stdout 'yes' # flag.go should select yes
+! stdout 'no' # flag.go should not select no
+
+# Test go generate provides the right "$GOPACKAGE" name in an x_test
+go generate './generate/env_test.go'
+stdout 'main_test'
+
+# Test go generate provides the right "$PWD"
+go generate './generate/env_pwd.go'
+stdout $WORK'[/\\]gopath[/\\]src[/\\]generate'
+
+-- echo.go --
+package main
+
+import (
+	"fmt"
+	"os"
+	"strings"
+)
+
+func main() {
+	fmt.Println(strings.Join(os.Args[1:], " "))
+	fmt.Println()
+}
+-- generate/simple.go --
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Simple test for go generate.
+
+// We include a build tag that go generate should ignore.
+
+// +build ignore
+
+//go:generate echo Success
+
+package p
+-- generate/alias.go --
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test that go generate handles command aliases.
+
+//go:generate -command run echo Now is the time
+//go:generate run for all good men
+
+package p
+-- generate/substitution.go --
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test go generate variable substitution.
+
+//go:generate echo $GOARCH $GOFILE:$GOLINE ${GOPACKAGE}abc xyz$GOPACKAGE/$GOFILE/123
+
+package p
+-- generate/flag.go --
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test -run flag
+
+//go:generate echo oh yes my man
+//go:generate echo no, no, a thousand times no
+
+package p
+-- generate/env_test.go --
+package main_test
+
+//go:generate echo $GOPACKAGE
+-- generate/env_pwd.go --
+package p
+
+//go:generate echo $PWD
diff --git a/src/cmd/go/testdata/script/generate_bad_imports.txt b/src/cmd/go/testdata/script/generate_bad_imports.txt
new file mode 100644
index 0000000..3da391e
--- /dev/null
+++ b/src/cmd/go/testdata/script/generate_bad_imports.txt
@@ -0,0 +1,15 @@
+[GOOS:windows] skip # skip because windows has no echo command
+
+go generate gencycle
+stdout 'hello world' # check go generate gencycle ran the generator
+
+-- go.mod --
+module gencycle
+
+go 1.16
+-- gencycle.go --
+//go:generate echo hello world
+
+package gencycle
+
+import _ "gencycle"
diff --git a/src/cmd/go/testdata/script/generate_env.txt b/src/cmd/go/testdata/script/generate_env.txt
new file mode 100644
index 0000000..cb76d30
--- /dev/null
+++ b/src/cmd/go/testdata/script/generate_env.txt
@@ -0,0 +1,32 @@
+# Install an env command because Windows and plan9 don't have it.
+env GOBIN=$WORK/tmp/bin
+go install env.go
+[GOOS:plan9] env path=$GOBIN${:}$path
+[!GOOS:plan9] env PATH=$GOBIN${:}$PATH
+
+# Test generators have access to the environment
+go generate ./printenv.go
+stdout '^GOARCH='$GOARCH
+stdout '^GOOS='$GOOS
+stdout '^GOFILE='
+stdout '^GOLINE='
+stdout '^GOPACKAGE='
+stdout '^DOLLAR='
+
+-- env.go --
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func main() {
+	for _, v := range os.Environ() {
+		fmt.Println(v)
+	}
+}
+-- printenv.go --
+package main
+
+//go:generate env
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/generate_goroot_PATH.txt b/src/cmd/go/testdata/script/generate_goroot_PATH.txt
new file mode 100644
index 0000000..24591f0
--- /dev/null
+++ b/src/cmd/go/testdata/script/generate_goroot_PATH.txt
@@ -0,0 +1,38 @@
+# https://go.dev/issue/51473: to avoid the need for generators to rely on
+# runtime.GOROOT, 'go generate' should run the test with its own GOROOT/bin
+# at the beginning of $PATH.
+
+[short] skip
+
+[!GOOS:plan9] env PATH=
+[GOOS:plan9] env path=
+go generate .
+
+[!GOOS:plan9] env PATH=$WORK${/}bin
+[GOOS:plan9] env path=$WORK${/}bin
+go generate .
+
+-- go.mod --
+module example
+
+go 1.19
+-- main.go --
+//go:generate go run .
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+)
+
+func main() {
+	_, err := exec.LookPath("go")
+	if err != nil {
+		fmt.Println(err)
+		os.Exit(1)
+	}
+}
+-- $WORK/bin/README.txt --
+This directory contains no executables.
diff --git a/src/cmd/go/testdata/script/generate_invalid.txt b/src/cmd/go/testdata/script/generate_invalid.txt
new file mode 100644
index 0000000..e18e62c
--- /dev/null
+++ b/src/cmd/go/testdata/script/generate_invalid.txt
@@ -0,0 +1,203 @@
+[short] skip
+
+# Install an echo command because Windows doesn't have it.
+env GOBIN=$WORK/tmp/bin
+go install echo.go
+env PATH=$GOBIN${:}$PATH
+
+# Test go generate for directory with no go files
+go generate ./nogo
+! stdout 'Fail'
+
+# Test go generate for package where all .go files are excluded by build
+# constraints
+go generate -v ./excluded
+! stdout 'Fail'
+! stderr 'go' # -v shouldn't list any files
+
+# Test go generate for "package" with no package clause in any file
+go generate ./nopkg
+stdout 'Success a'
+! stdout 'Fail'
+
+# Test go generate for package with inconsistent package clauses
+# $GOPACKAGE should depend on each file's package clause
+go generate ./inconsistent
+stdout 'Success a'
+stdout 'Success b'
+stdout -count=2 'Success c'
+! stdout 'Fail'
+
+# Test go generate for syntax errors before and after package clauses
+go generate ./syntax
+stdout 'Success a'
+stdout 'Success b'
+! stdout 'Fail'
+
+# Test go generate for files importing non-existent packages
+go generate ./importerr
+stdout 'Success a'
+stdout 'Success b'
+stdout 'Success c'
+
+-- echo.go --
+package main
+
+import (
+	"fmt"
+	"os"
+	"strings"
+)
+
+func main() {
+	fmt.Println(strings.Join(os.Args[1:], " "))
+	fmt.Println()
+}
+
+-- go.mod --
+module m
+
+go 1.16
+-- nogo/foo.txt --
+Text file in a directory without go files.
+Go generate should ignore this directory.
+//go:generate echo Fail nogo
+
+-- excluded/a.go --
+// Include a build tag that go generate should exclude.
+// Go generate should ignore this file.
+
+// +build a
+
+//go:generate echo Fail a
+
+package excluded
+
+-- excluded/b.go --
+// Include a build tag that go generate should exclude.
+// Go generate should ignore this file.
+
+//go:generate echo Fail b
+
+// +build b
+
+package excluded
+
+
+-- nopkg/a.go --
+// Go file with package clause after comment.
+// Go generate should process this file.
+
+/* Pre-comment */ package nopkg
+//go:generate echo Success a
+
+-- nopkg/b.go --
+// Go file with commented package clause.
+// Go generate should ignore this file.
+
+//package nopkg
+
+//go:generate echo Fail b
+
+-- nopkg/c.go --
+// Go file with package clause inside multiline comment.
+// Go generate should ignore this file.
+
+/*
+package nopkg
+*/
+
+//go:generate echo Fail c
+
+-- nopkg/d.go --
+// Go file with package clause inside raw string literal.
+// Go generate should ignore this file.
+
+const foo = `
+package nopkg
+`
+//go:generate echo Fail d
+
+-- nopkg/e.go --
+// Go file without package clause.
+// Go generate should ignore this file.
+
+//go:generate echo Fail e
+
+-- inconsistent/a.go --
+// Valid go file with inconsistent package name.
+// Go generate should process this file with GOPACKAGE=a
+
+package a
+//go:generate echo Success $GOPACKAGE
+
+-- inconsistent/b.go --
+// Valid go file with inconsistent package name.
+// Go generate should process this file with GOPACKAGE=b
+
+//go:generate echo Success $GOPACKAGE
+package b
+
+-- inconsistent/c.go --
+// Go file with two package clauses.
+// Go generate should process this file with GOPACKAGE=c
+
+//go:generate echo Success $GOPACKAGE
+package c
+// Invalid package clause, should be ignored:
+package cinvalid
+//go:generate echo Success $GOPACKAGE
+
+-- inconsistent/d.go --
+// Go file with invalid package name.
+// Go generate should ignore this file.
+
+package +d+
+//go:generate echo Fail $GOPACKAGE
+
+-- syntax/a.go --
+// Go file with syntax error after package clause.
+// Go generate should process this file.
+
+package syntax
+123
+//go:generate echo Success a
+
+-- syntax/b.go --
+// Go file with syntax error after package clause.
+// Go generate should process this file.
+
+package syntax; 123
+//go:generate echo Success b
+
+-- syntax/c.go --
+// Go file with syntax error before package clause.
+// Go generate should ignore this file.
+
+foo
+package syntax
+//go:generate echo Fail c
+
+-- importerr/a.go --
+// Go file which imports non-existing package.
+// Go generate should process this file.
+
+package importerr
+//go:generate echo Success a
+import "foo"
+
+-- importerr/b.go --
+// Go file which imports non-existing package.
+// Go generate should process this file.
+
+//go:generate echo Success b
+package importerr
+import "bar"
+
+-- importerr/c.go --
+// Go file which imports non-existing package.
+// Go generate should process this file.
+
+package importerr
+import "moo"
+//go:generate echo Success c
diff --git a/src/cmd/go/testdata/script/generate_workspace.txt b/src/cmd/go/testdata/script/generate_workspace.txt
new file mode 100644
index 0000000..eda8a4f
--- /dev/null
+++ b/src/cmd/go/testdata/script/generate_workspace.txt
@@ -0,0 +1,27 @@
+# This is a regression test for Issue #56098: Go generate
+# wasn't initializing workspace mode
+
+[short] skip
+
+go generate ./mod
+cmp ./mod/got.txt want.txt
+
+-- go.work --
+go 1.21
+
+use ./mod
+-- mod/go.mod --
+module example.com/mod
+-- mod/gen.go --
+//go:generate go run gen.go got.txt
+
+package main
+
+import "os"
+
+func main() {
+    outfile := os.Args[1]
+    os.WriteFile(outfile, []byte("Hello World!\n"), 0644)
+}
+-- want.txt --
+Hello World!
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/get_404_meta.txt b/src/cmd/go/testdata/script/get_404_meta.txt
new file mode 100644
index 0000000..4ffbdeb
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_404_meta.txt
@@ -0,0 +1,18 @@
+# golang.org/issue/13037: 'go get' was not parsing <meta> tags in 404 served over HTTPS.
+
+[!net:bazil.org] skip
+[!git] skip
+
+env GONOSUMDB=bazil.org,github.com,golang.org
+env GO111MODULE=off
+go get -d bazil.org/fuse/fs/fstestutil
+
+env GO111MODULE=on
+env GOPROXY=direct
+go get bazil.org/fuse/fs/fstestutil
+
+
+-- go.mod --
+module m
+
+go 1.18
diff --git a/src/cmd/go/testdata/script/get_brace.txt b/src/cmd/go/testdata/script/get_brace.txt
new file mode 100644
index 0000000..34f66a6
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_brace.txt
@@ -0,0 +1,51 @@
+env GO111MODULE=off
+
+[!git] skip
+
+# Set up some empty repositories.
+cd $WORK/_origin/foo
+exec git init
+exec git config user.name 'Nameless Gopher'
+exec git config user.email 'nobody@golang.org'
+exec git commit --allow-empty -m 'create master branch'
+
+cd $WORK
+cd '_origin/{confusing}'
+exec git init
+exec git config user.name 'Nameless Gopher'
+exec git config user.email 'nobody@golang.org'
+exec git commit --allow-empty -m 'create master branch'
+
+# Clone the empty repositories into GOPATH.
+# This tells the Go command where to find them: it takes the place of a user's meta-tag redirector.
+mkdir $GOPATH/src/example.com
+cd $GOPATH/src/example.com
+exec git clone $WORK/_origin/foo
+exec git clone $WORK/_origin/{confusing}
+
+# Commit contents to the repositories.
+cd $WORK/_origin/foo
+exec git add main.go
+exec git commit -m 'add main'
+
+cd $WORK
+cd '_origin/{confusing}'
+exec git add confusing.go
+exec git commit -m 'just try to delete this!'
+
+# 'go get' should refuse to download or update the confusingly-named repo.
+cd $GOPATH/src/example.com/foo
+! go get -u 'example.com/{confusing}'
+stderr 'invalid char'
+! go get -u example.com/foo
+stderr 'invalid import path'
+! exists example.com/{confusing}
+
+-- $WORK/_origin/foo/main.go --
+package main
+import _ "example.com/{confusing}"
+
+func main() {}
+
+-- $WORK/_origin/{confusing}/confusing.go --
+package confusing
diff --git a/src/cmd/go/testdata/script/get_custom_domain_wildcard.txt b/src/cmd/go/testdata/script/get_custom_domain_wildcard.txt
new file mode 100644
index 0000000..45ab524
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_custom_domain_wildcard.txt
@@ -0,0 +1,6 @@
+[!net:rsc.io] skip
+[!git] skip
+env GO111MODULE=off
+
+go get -u rsc.io/pdf/...
+exists $GOPATH/bin/pdfpasswd$GOEXE
diff --git a/src/cmd/go/testdata/script/get_dash_t.txt b/src/cmd/go/testdata/script/get_dash_t.txt
new file mode 100644
index 0000000..8f3a036
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_dash_t.txt
@@ -0,0 +1,9 @@
+# Tests issue 8181
+
+[!net:github.com] skip
+[!git] skip
+env GO111MODULE=off
+
+go get -v -t github.com/rsc/go-get-issue-8181/a github.com/rsc/go-get-issue-8181/b
+go list -test -deps github.com/rsc/go-get-issue-8181/b
+stdout 'x/build/gerrit'
diff --git a/src/cmd/go/testdata/script/get_domain_root.txt b/src/cmd/go/testdata/script/get_domain_root.txt
new file mode 100644
index 0000000..dfcea86
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_domain_root.txt
@@ -0,0 +1,20 @@
+# Tests issue #9357
+# go get foo.io (not foo.io/subdir) was not working consistently.
+
+[!net:go-get-issue-9357.appspot.com] skip
+[!git] skip
+env GO111MODULE=off
+
+# go-get-issue-9357.appspot.com is running
+# the code at github.com/rsc/go-get-issue-9357,
+# a trivial Go on App Engine app that serves a
+# <meta> tag for the domain root.
+go get -d go-get-issue-9357.appspot.com
+go get go-get-issue-9357.appspot.com
+go get -u go-get-issue-9357.appspot.com
+
+rm $GOPATH/src/go-get-issue-9357.appspot.com
+go get go-get-issue-9357.appspot.com
+
+rm $GOPATH/src/go-get-issue-9357.appspot.com
+go get -u go-get-issue-9357.appspot.com
diff --git a/src/cmd/go/testdata/script/get_dot_slash_download.txt b/src/cmd/go/testdata/script/get_dot_slash_download.txt
new file mode 100644
index 0000000..6dbd118
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_dot_slash_download.txt
@@ -0,0 +1,10 @@
+[!net:rsc.io] skip
+[!git] skip
+env GO111MODULE=off
+
+# Tests Issues #9797 and #19769
+
+mkdir $WORK/tmp/src/rsc.io
+env GOPATH=$WORK/tmp
+cd $WORK/tmp/src/rsc.io
+go get ./pprof_mac_fix
diff --git a/src/cmd/go/testdata/script/get_dotfiles.txt b/src/cmd/go/testdata/script/get_dotfiles.txt
new file mode 100644
index 0000000..676a044
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_dotfiles.txt
@@ -0,0 +1,64 @@
+env GO111MODULE=off
+[short] skip
+
+[!git] skip
+
+# Set up a benign repository and a repository with a dotfile name.
+cd $WORK/_origin/foo
+exec git init
+exec git config user.name 'Nameless Gopher'
+exec git config user.email 'nobody@golang.org'
+exec git commit --allow-empty -m 'create master branch'
+
+cd $WORK/_origin/.hidden
+exec git init
+exec git config user.name 'Nameless Gopher'
+exec git config user.email 'nobody@golang.org'
+exec git commit --allow-empty -m 'create master branch'
+
+# Clone the empty repositories into GOPATH.
+# This tells the Go command where to find them: it takes the place of a user's meta-tag redirector.
+mkdir $GOPATH/src/example.com
+cd $GOPATH/src/example.com
+exec git clone $WORK/_origin/foo
+exec git clone $WORK/_origin/.hidden
+
+# Add a benign commit.
+cd $WORK/_origin/foo
+cp _ok/main.go main.go
+exec git add main.go
+exec git commit -m 'add ok'
+
+# 'go get' should install the benign commit.
+cd $GOPATH
+go get -u example.com/foo
+
+# Now sneak in an import of a dotfile path.
+cd $WORK/_origin/.hidden
+exec git add hidden.go
+exec git commit -m 'nothing to see here, move along'
+
+cd $WORK/_origin/foo
+cp _sneaky/main.go main.go
+exec git add main.go
+exec git commit -m 'fix typo (heh heh heh)'
+
+# 'go get -u' should refuse to download or update the dotfile-named repo.
+cd $GOPATH/src/example.com/foo
+! go get -u example.com/foo
+stderr 'leading dot'
+! exists example.com/.hidden/hidden.go
+
+-- $WORK/_origin/foo/_ok/main.go --
+package main
+
+func main() {}
+
+-- $WORK/_origin/foo/_sneaky/main.go --
+package main
+import _ "example.com/.hidden"
+
+func main() {}
+
+-- $WORK/_origin/.hidden/hidden.go --
+package hidden
diff --git a/src/cmd/go/testdata/script/get_go_file.txt b/src/cmd/go/testdata/script/get_go_file.txt
new file mode 100644
index 0000000..f6d0de4
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_go_file.txt
@@ -0,0 +1,60 @@
+# Tests Issue #38478
+# Tests that go get in GOPATH mode returns a specific error if the argument
+# ends with '.go', and either has no slash or refers to an existing file.
+
+env GO111MODULE=off
+
+# argument doesn't have .go suffix
+go get -d test
+
+# argument has .go suffix, is a file and exists
+! go get -d test.go
+stderr 'go: test.go: arguments must be package or module paths'
+
+# argument has .go suffix, doesn't exist and has no slashes
+! go get -d test_missing.go
+stderr 'go: test_missing.go: arguments must be package or module paths'
+
+# argument has .go suffix, is a file and exists in sub-directory
+! go get -d test/test.go
+stderr 'go: test/test.go exists as a file, but ''go get'' requires package arguments'
+
+# argument has .go suffix, doesn't exist and has slashes
+! go get -d test/test_missing.go
+! stderr 'arguments must be package or module paths'
+! stderr 'exists as a file, but ''go get'' requires package arguments'
+
+# argument has .go suffix, is a symlink and exists
+[symlink] symlink test_sym.go -> test.go
+[symlink] ! go get -d test_sym.go
+[symlink] stderr 'go: test_sym.go: arguments must be package or module paths'
+[symlink] rm test_sym.go
+
+# argument has .go suffix, is a symlink and exists in sub-directory
+[symlink] symlink test/test_sym.go -> test.go
+[symlink] ! go get -d test/test_sym.go
+[symlink] stderr 'go: test/test_sym.go exists as a file, but ''go get'' requires package arguments'
+[symlink] rm test_sym.go
+
+# argument has .go suffix, is a directory and exists
+mkdir test_dir.go
+! go get -d test_dir.go
+stderr 'go: test_dir.go: arguments must be package or module paths'
+rm test_dir.go
+
+# argument has .go suffix, is a directory and exists in sub-directory
+mkdir test/test_dir.go
+! go get -d test/test_dir.go
+! stderr 'arguments must be package or module paths'
+! stderr 'exists as a file, but ''go get'' requires package arguments'
+rm test/test_dir.go
+
+
+-- test.go --
+package main
+func main() {println("test")}
+
+
+-- test/test.go --
+package main
+func main() {println("test")}
diff --git a/src/cmd/go/testdata/script/get_goroot.txt b/src/cmd/go/testdata/script/get_goroot.txt
new file mode 100644
index 0000000..7510692
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_goroot.txt
@@ -0,0 +1,53 @@
+[!net:github.com] skip
+env GO111MODULE=off
+
+# Issue 4186. go get cannot be used to download packages to $GOROOT.
+# Test that without GOPATH set, go get should fail.
+
+# Fails because GOROOT=GOPATH
+env GOPATH=$WORK/tmp
+env GOROOT=$WORK/tmp
+! go get -d github.com/golang/example/hello
+stderr 'warning: GOPATH set to GOROOT'
+stderr '\$GOPATH must not be set to \$GOROOT'
+
+# Fails because GOROOT=GOPATH after cleaning.
+env GOPATH=$WORK/tmp/
+env GOROOT=$WORK/tmp
+! go get -d github.com/golang/example/hello
+stderr 'warning: GOPATH set to GOROOT'
+stderr '\$GOPATH must not be set to \$GOROOT'
+
+env GOPATH=$WORK/tmp
+env GOROOT=$WORK/tmp/
+! go get -d github.com/golang/example/hello
+stderr 'warning: GOPATH set to GOROOT'
+stderr '\$GOPATH must not be set to \$GOROOT'
+
+# Make a home directory
+mkdir $WORK/home/go
+
+# Fails because GOROOT=$HOME/go so default GOPATH unset.
+[GOOS:windows] env USERPROFILE=$WORK/home
+[GOOS:plan9] env home=$WORK/home
+[!GOOS:windows] [!GOOS:plan9] env HOME=$WORK/home
+env GOPATH=
+env GOROOT=$WORK/home/go
+! go get -d github.com/golang/example/hello
+stderr '\$GOPATH not set'
+
+[GOOS:windows] env USERPROFILE=$WORK/home/
+[GOOS:plan9] env home=$WORK/home/
+[!GOOS:windows] [!GOOS:plan9] env HOME=$WORK/home/
+env GOPATH=
+env GOROOT=$WORK/home/go
+! go get -d github.com/golang/example/hello
+stderr '\$GOPATH not set'
+
+[GOOS:windows] env USERPROFILE=$WORK/home
+[GOOS:plan9] env home=$WORK/home
+[!GOOS:windows] [!GOOS:plan9] env HOME=$WORK/home
+env GOPATH=
+env GOROOT=$WORK/home/go/
+! go get -d github.com/golang/example/hello
+stderr '\$GOPATH not set'
diff --git a/src/cmd/go/testdata/script/get_insecure.txt b/src/cmd/go/testdata/script/get_insecure.txt
new file mode 100644
index 0000000..afe64b8
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_insecure.txt
@@ -0,0 +1,57 @@
+# TODO(matloob): Split this test into two? It's one of the slowest tests we have.
+
+[!net:insecure.go-get-issue-15410.appspot.com] skip
+[!git] skip
+
+env PATH=$WORK/tmp/bin${:}$PATH
+go build -o $WORK/tmp/bin/ssh ssh.go
+
+# GOPATH: Set up
+env GO111MODULE=off
+
+# GOPATH: Try go get -d of HTTP-only repo (should fail).
+! go get -d insecure.go-get-issue-15410.appspot.com/pkg/p
+
+# GOPATH: Try again with GOINSECURE (should succeed).
+env GOINSECURE=insecure.go-get-issue-15410.appspot.com
+go get -d insecure.go-get-issue-15410.appspot.com/pkg/p
+
+# GOPATH: Try updating without GOINSECURE (should fail).
+env GOINSECURE=''
+! go get -d -u -f insecure.go-get-issue-15410.appspot.com/pkg/p
+
+# Modules: Set up
+env GOPATH=$WORK/m/gp
+mkdir $WORK/m
+cp module_file $WORK/m/go.mod
+cd $WORK/m
+env GO111MODULE=on
+env GOPROXY=''
+
+# Modules: Try go get -d of HTTP-only repo (should fail).
+! go get -d insecure.go-get-issue-15410.appspot.com/pkg/p
+
+# Modules: Try again with GOINSECURE (should succeed).
+env GOINSECURE=insecure.go-get-issue-15410.appspot.com
+env GONOSUMDB=insecure.go-get-issue-15410.appspot.com
+go get -d insecure.go-get-issue-15410.appspot.com/pkg/p
+
+# Modules: Try updating without GOINSECURE (should fail).
+env GOINSECURE=''
+env GONOSUMDB=''
+! go get -d -u -f insecure.go-get-issue-15410.appspot.com/pkg/p
+
+go list -m ...
+stdout 'insecure.go-get-issue'
+
+-- ssh.go --
+// stub out uses of ssh by go get
+package main
+
+import "os"
+
+func main() {
+	os.Exit(1)
+}
+-- module_file --
+module m
diff --git a/src/cmd/go/testdata/script/get_insecure_custom_domain.txt b/src/cmd/go/testdata/script/get_insecure_custom_domain.txt
new file mode 100644
index 0000000..4b3c9d6
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_insecure_custom_domain.txt
@@ -0,0 +1,8 @@
+[!net:insecure.go-get-issue-15410.appspot.com] skip
+[!git] skip
+env GO111MODULE=off
+
+! go get -d insecure.go-get-issue-15410.appspot.com/pkg/p
+
+env GOINSECURE=insecure.go-get-issue-15410.appspot.com
+go get -d insecure.go-get-issue-15410.appspot.com/pkg/p
diff --git a/src/cmd/go/testdata/script/get_insecure_env.txt b/src/cmd/go/testdata/script/get_insecure_env.txt
new file mode 100644
index 0000000..98e7053
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_insecure_env.txt
@@ -0,0 +1,29 @@
+[!net:insecure.go-get-issue-15410.appspot.com] skip
+[!git] skip
+
+# GOPATH: Set up
+env GO111MODULE=off
+
+# GOPATH: Try go get -d of HTTP-only repo (should fail).
+! go get -d insecure.go-get-issue-15410.appspot.com/pkg/p
+
+# GOPATH: Try again with invalid GOINSECURE (should fail).
+env GOINSECURE=insecure.go-get-issue-15410.appspot.com/pkg/q
+! go get -d insecure.go-get-issue-15410.appspot.com/pkg/p
+
+# GOPATH: Try with correct GOINSECURE (should succeed).
+env GOINSECURE=insecure.go-get-issue-15410.appspot.com/pkg/p
+go get -d insecure.go-get-issue-15410.appspot.com/pkg/p
+
+# GOPATH: Try updating without GOINSECURE (should fail).
+env GOINSECURE=
+! go get -d -u -f insecure.go-get-issue-15410.appspot.com/pkg/p
+
+# GOPATH: Try updating with GOINSECURE glob (should succeed).
+env GOINSECURE=*.go-get-*.appspot.com
+go get -d -u -f insecure.go-get-issue-15410.appspot.com/pkg/p
+
+# GOPATH: Try updating with GOINSECURE base URL (should succeed).
+env GOINSECURE=insecure.go-get-issue-15410.appspot.com
+go get -d -u -f insecure.go-get-issue-15410.appspot.com/pkg/p
+
diff --git a/src/cmd/go/testdata/script/get_insecure_no_longer_supported.txt b/src/cmd/go/testdata/script/get_insecure_no_longer_supported.txt
new file mode 100644
index 0000000..00bf32f
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_insecure_no_longer_supported.txt
@@ -0,0 +1,13 @@
+# GOPATH: Set up
+env GO111MODULE=off
+
+# GOPATH: Fetch with insecure, should error
+! go get -insecure test
+stderr 'go: -insecure flag is no longer supported; use GOINSECURE instead'
+
+# Modules: Set up
+env GO111MODULE=on
+
+# Modules: Fetch with insecure, should error
+! go get -insecure test
+stderr 'go: -insecure flag is no longer supported; use GOINSECURE instead'
diff --git a/src/cmd/go/testdata/script/get_insecure_redirect.txt b/src/cmd/go/testdata/script/get_insecure_redirect.txt
new file mode 100644
index 0000000..2e53c58
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_insecure_redirect.txt
@@ -0,0 +1,14 @@
+# golang.org/issue/29591: 'go get' was following plain-HTTP redirects even without -insecure (now replaced by GOINSECURE).
+# golang.org/issue/34049: 'go get' would panic in case of an insecure redirect in GOPATH mode
+
+[!git] skip
+
+env GO111MODULE=off
+
+! go get -d vcs-test.golang.org/insecure/go/insecure
+stderr 'redirected .* to insecure URL'
+
+[short] stop 'builds a git repo'
+
+env GOINSECURE=vcs-test.golang.org/insecure/go/insecure
+go get -d vcs-test.golang.org/insecure/go/insecure
diff --git a/src/cmd/go/testdata/script/get_insecure_update.txt b/src/cmd/go/testdata/script/get_insecure_update.txt
new file mode 100644
index 0000000..7cddd6b
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_insecure_update.txt
@@ -0,0 +1,14 @@
+[!net:github.com] skip
+[!git] skip
+env GO111MODULE=off
+
+# Clone the repo via HTTP manually.
+exec git clone -q http://github.com/golang/example github.com/golang/example
+
+# Update without GOINSECURE should fail.
+# We need -f to ignore import comments.
+! go get -d -u -f github.com/golang/example/hello
+
+# Update with GOINSECURE should succeed.
+env GOINSECURE=github.com/golang/example/hello
+go get -d -u -f github.com/golang/example/hello
diff --git a/src/cmd/go/testdata/script/get_internal_wildcard.txt b/src/cmd/go/testdata/script/get_internal_wildcard.txt
new file mode 100644
index 0000000..b25e746
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_internal_wildcard.txt
@@ -0,0 +1,6 @@
+[!net:github.com] skip
+[!git] skip
+env GO111MODULE=off
+
+# This used to fail with errors about internal packages
+go get github.com/rsc/go-get-issue-11960/...
diff --git a/src/cmd/go/testdata/script/get_issue11307.txt b/src/cmd/go/testdata/script/get_issue11307.txt
new file mode 100644
index 0000000..d490959
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_issue11307.txt
@@ -0,0 +1,9 @@
+# go get -u was not working except in checkout directory
+
+[!net:github.com] skip
+[!git] skip
+env GO111MODULE=off
+
+env GOPATH=$WORK/tmp/gopath
+go get github.com/rsc/go-get-issue-11307
+go get -u github.com/rsc/go-get-issue-11307 # was failing
diff --git a/src/cmd/go/testdata/script/get_issue16471.txt b/src/cmd/go/testdata/script/get_issue16471.txt
new file mode 100644
index 0000000..1aeae58
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_issue16471.txt
@@ -0,0 +1,23 @@
+[!net:rsc.io] skip
+[!net:github.com] skip
+[!git] skip
+
+env GO111MODULE=off
+
+cd rsc.io/go-get-issue-10952
+
+exec git init
+exec git add foo.go
+exec git config user.name Gopher
+exec git config user.email gopher@golang.org
+exec git commit -a -m 'initial commit'
+exec git remote add origin https://github.com/golang/go-get-issue-10952
+
+exec git status
+
+! go get -x -u rsc.io/go-get-issue-10952
+stderr '^package rsc.io/go-get-issue-10952: rsc\.io/go-get-issue-10952 is a custom import path for https://github.com/rsc/go-get-issue-10952, but .* is checked out from https://github.com/golang/go-get-issue-10952$'
+
+-- rsc.io/go-get-issue-10952/foo.go --
+// Junk package to test go get.
+package foo
diff --git a/src/cmd/go/testdata/script/get_issue22125.txt b/src/cmd/go/testdata/script/get_issue22125.txt
new file mode 100644
index 0000000..086081f
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_issue22125.txt
@@ -0,0 +1,14 @@
+# This test verifies a fix for a security issue; see https://go.dev/issue/22125.
+
+[short] skip
+[!git] skip
+[!exec:svn] skip
+
+env GO111MODULE=off
+
+cd $GOPATH
+! go get -u vcs-test.golang.org/go/test1-svn-git
+stderr 'src'${/}'vcs-test.* uses git, but parent .*src'${/}'vcs-test.* uses svn'
+
+[!case-sensitive] ! go get -u vcs-test.golang.org/go/test2-svn-git/test2main
+[!case-sensitive] stderr 'src'${/}'vcs-test.* uses git, but parent .*src'${/}'vcs-test.* uses svn'
diff --git a/src/cmd/go/testdata/script/get_legacy.txt b/src/cmd/go/testdata/script/get_legacy.txt
new file mode 100644
index 0000000..2909b73
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_legacy.txt
@@ -0,0 +1,58 @@
+# This test was converted from a test in vendor_test.go (which no longer exists).
+# That seems to imply that it's about vendoring semantics, but the test doesn't
+# use 'go -mod=vendor' (and none of the fetched repos have vendor folders).
+# The test still seems to be useful as a test of direct-mode go get.
+
+[short] skip
+[!git] skip
+env GO111MODULE=off
+
+env GOPATH=$WORK/tmp/d1
+go get vcs-test.golang.org/git/modlegacy1-old.git/p1
+go list -f '{{.Deps}}' vcs-test.golang.org/git/modlegacy1-old.git/p1
+stdout 'new.git/p2' # old/p1 should depend on new/p2
+! stdout new.git/v2/p2 # old/p1 should NOT depend on new/v2/p2
+go build vcs-test.golang.org/git/modlegacy1-old.git/p1 vcs-test.golang.org/git/modlegacy1-new.git/p1
+! stdout .
+
+env GOPATH=$WORK/tmp/d2
+
+rm $GOPATH
+go get github.com/rsc/vgotest5
+go get github.com/rsc/vgotest4
+go get github.com/myitcv/vgo_example_compat
+
+rm $GOPATH
+go get github.com/rsc/vgotest4
+go get github.com/rsc/vgotest5
+go get github.com/myitcv/vgo_example_compat
+
+rm $GOPATH
+go get github.com/rsc/vgotest4 github.com/rsc/vgotest5
+go get github.com/myitcv/vgo_example_compat
+
+rm $GOPATH
+go get github.com/rsc/vgotest5 github.com/rsc/vgotest4
+go get github.com/myitcv/vgo_example_compat
+
+rm $GOPATH
+go get github.com/myitcv/vgo_example_compat
+go get github.com/rsc/vgotest5 github.com/rsc/vgotest4
+
+rm $GOPATH
+go get github.com/myitcv/vgo_example_compat github.com/rsc/vgotest4 github.com/rsc/vgotest5
+
+rm $GOPATH
+go get github.com/myitcv/vgo_example_compat github.com/rsc/vgotest5 github.com/rsc/vgotest4
+
+rm $GOPATH
+go get github.com/rsc/vgotest4 github.com/myitcv/vgo_example_compat github.com/rsc/vgotest5
+
+rm $GOPATH
+go get github.com/rsc/vgotest4 github.com/rsc/vgotest5 github.com/myitcv/vgo_example_compat
+
+rm $GOPATH
+go get github.com/rsc/vgotest5 github.com/myitcv/vgo_example_compat github.com/rsc/vgotest4
+
+rm $GOPATH
+go get github.com/rsc/vgotest5 github.com/rsc/vgotest4 github.com/myitcv/vgo_example_compat
diff --git a/src/cmd/go/testdata/script/get_non_pkg.txt b/src/cmd/go/testdata/script/get_non_pkg.txt
new file mode 100644
index 0000000..5202e88
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_non_pkg.txt
@@ -0,0 +1,14 @@
+[!net:golang.org] skip
+[!git] skip
+
+env GOBIN=$WORK/tmp/gobin
+env GO111MODULE=off
+
+! go get -d golang.org/x/tools
+stderr 'golang.org/x/tools: no Go files'
+
+! go get -d -u golang.org/x/tools
+stderr 'golang.org/x/tools: no Go files'
+
+! go get -d golang.org/x/tools
+stderr 'golang.org/x/tools: no Go files'
diff --git a/src/cmd/go/testdata/script/get_race.txt b/src/cmd/go/testdata/script/get_race.txt
new file mode 100644
index 0000000..1e06c80
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_race.txt
@@ -0,0 +1,8 @@
+# Tests issue #20502
+
+[!net:github.com] skip
+[!git] skip
+[!race] skip
+env GO111MODULE=off
+
+go get -race github.com/rsc/go-get-issue-9224-cmd
diff --git a/src/cmd/go/testdata/script/get_test_only.txt b/src/cmd/go/testdata/script/get_test_only.txt
new file mode 100644
index 0000000..af90f74
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_test_only.txt
@@ -0,0 +1,6 @@
+[!net:golang.org] skip
+[!git] skip
+env GO111MODULE=off
+
+go get golang.org/x/tour/content...
+go get -t golang.org/x/tour/content...
diff --git a/src/cmd/go/testdata/script/get_tilde.txt b/src/cmd/go/testdata/script/get_tilde.txt
new file mode 100644
index 0000000..1c3a029
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_tilde.txt
@@ -0,0 +1,25 @@
+env GO111MODULE=off
+
+# Paths containing windows short names should be rejected before attempting to fetch.
+! go get vcs-test.golang.org/longna~1.dir/thing
+stderr 'trailing tilde and digits'
+! go get vcs-test.golang.org/longna~1/thing
+stderr 'trailing tilde and digits'
+! go get vcs-test.golang.org/~9999999/thing
+stderr 'trailing tilde and digits'
+
+[short] stop
+
+# A path containing an element that is just a tilde, or a tilde followed by non-digits,
+# should attempt to resolve.
+! go get vcs-test.golang.org/~glenda/notfound
+! stderr 'trailing tilde and digits'
+stderr 'unrecognized import path'
+
+! go get vcs-test.golang.org/~glenda2/notfound
+! stderr 'trailing tilde and digits'
+stderr 'unrecognized import path'
+
+! go get vcs-test.golang.org/~/notfound
+! stderr 'trailing tilde and digits'
+stderr 'unrecognized import path'
diff --git a/src/cmd/go/testdata/script/get_update.txt b/src/cmd/go/testdata/script/get_update.txt
new file mode 100644
index 0000000..a70a80d
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_update.txt
@@ -0,0 +1,25 @@
+# Tests Issue #9224
+# The recursive updating was trying to walk to
+# former dependencies, not current ones.
+
+[!net:github.com] skip
+[!git] skip
+env GO111MODULE=off
+
+# Rewind
+go get github.com/rsc/go-get-issue-9224-cmd
+cd $GOPATH/src/github.com/rsc/go-get-issue-9224-lib
+exec git reset --hard HEAD~
+cd $GOPATH/src
+
+# Run get
+go get -u 'github.com/rsc/go-get-issue-9224-cmd'
+
+# (Again with -d -u) Rewind
+go get github.com/rsc/go-get-issue-9224-cmd
+cd $GOPATH/src/github.com/rsc/go-get-issue-9224-lib
+exec git reset --hard HEAD~
+cd $GOPATH/src
+
+# (Again with -d -u) Run get
+go get -d -u 'github.com/rsc/go-get-issue-9224-cmd'
diff --git a/src/cmd/go/testdata/script/get_update_all.txt b/src/cmd/go/testdata/script/get_update_all.txt
new file mode 100644
index 0000000..22fe3ed
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_update_all.txt
@@ -0,0 +1,7 @@
+# Issue 14444: go get -u .../ duplicate loads errors
+# Check that go get update -u ... does not try to load duplicates
+
+env GO111MODULE=off
+
+go get -u -n .../
+! stderr 'duplicate loads of' # make sure old packages are removed from cache
diff --git a/src/cmd/go/testdata/script/get_update_unknown_protocol.txt b/src/cmd/go/testdata/script/get_update_unknown_protocol.txt
new file mode 100644
index 0000000..714ed6a
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_update_unknown_protocol.txt
@@ -0,0 +1,14 @@
+[!net:github.com] skip
+[!git] skip
+env GO111MODULE=off
+
+# Clone the repo via HTTPS manually.
+exec git clone -q https://github.com/golang/example github.com/golang/example
+
+# Configure the repo to use a protocol unknown to cmd/go
+# that still actually works.
+cd github.com/golang/example
+exec git remote set-url origin xyz://github.com/golang/example
+exec git config --local url.https://github.com/.insteadOf xyz://github.com/
+
+go get -d -u -f github.com/golang/example/hello
diff --git a/src/cmd/go/testdata/script/get_update_wildcard.txt b/src/cmd/go/testdata/script/get_update_wildcard.txt
new file mode 100644
index 0000000..c833783
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_update_wildcard.txt
@@ -0,0 +1,16 @@
+# Issue 14450: go get -u .../ tried to import not downloaded package
+
+[!net:github.com] skip
+[!git] skip
+env GO111MODULE=off
+
+go get github.com/tmwh/go-get-issue-14450/a
+! go get -u .../
+stderr 'cannot find package.*d-dependency/e'
+
+# Even though get -u failed, the source for others should be downloaded.
+exists github.com/tmwh/go-get-issue-14450/b
+exists github.com/tmwh/go-get-issue-14450-b-dependency/c
+exists github.com/tmwh/go-get-issue-14450-b-dependency/d
+
+! exists github.com/tmwh/go-get-issue-14450-c-dependency/e
diff --git a/src/cmd/go/testdata/script/get_vcs_error_message.txt b/src/cmd/go/testdata/script/get_vcs_error_message.txt
new file mode 100644
index 0000000..8dc84fc
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_vcs_error_message.txt
@@ -0,0 +1,9 @@
+# Test that the Version Control error message includes the correct directory
+env GO111MODULE=off
+! go get -u foo
+stderr gopath(\\\\|/)src(\\\\|/)foo
+
+-- foo/foo.go --
+package foo
+-- math/math.go --
+package math
diff --git a/src/cmd/go/testdata/script/get_vendor.txt b/src/cmd/go/testdata/script/get_vendor.txt
new file mode 100644
index 0000000..179456d
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_vendor.txt
@@ -0,0 +1,95 @@
+[short] skip
+env GO111MODULE=off
+
+cd $GOPATH/src/v
+go run m.go
+go test
+go list -f '{{.Imports}}'
+stdout 'v/vendor/vendor.org/p'
+go list -f '{{.TestImports}}'
+stdout 'v/vendor/vendor.org/p'
+go get -d
+go get -t -d
+
+[!net:github.com] stop
+[!git] stop
+
+cd $GOPATH/src
+
+# Update
+go get 'github.com/rsc/go-get-issue-11864'
+go get -u 'github.com/rsc/go-get-issue-11864'
+exists github.com/rsc/go-get-issue-11864/vendor
+
+# get -u
+rm $GOPATH
+mkdir $GOPATH/src
+go get -u 'github.com/rsc/go-get-issue-11864'
+exists github.com/rsc/go-get-issue-11864/vendor
+
+# get -t -u
+rm $GOPATH
+mkdir $GOPATH/src
+go get -t -u 'github.com/rsc/go-get-issue-11864/...'
+exists github.com/rsc/go-get-issue-11864/vendor
+
+# Submodules
+rm $GOPATH
+mkdir $GOPATH/src
+go get -d 'github.com/rsc/go-get-issue-12612'
+go get -u -d 'github.com/rsc/go-get-issue-12612'
+exists github.com/rsc/go-get-issue-12612/vendor/golang.org/x/crypto/.git
+
+# Bad vendor (bad/imp)
+rm $GOPATH
+mkdir $GOPATH/src
+! go get -t -u 'github.com/rsc/go-get-issue-18219/bad/imp'
+stderr 'must be imported as'
+! exists github.com/rsc/go-get-issue-11864/vendor
+
+# Bad vendor (bad/imp2)
+rm $GOPATH
+mkdir $GOPATH/src
+! go get -t -u 'github.com/rsc/go-get-issue-18219/bad/imp2'
+stderr 'must be imported as'
+! exists github.com/rsc/go-get-issue-11864/vendor
+
+# Bad vendor (bad/imp3)
+rm $GOPATH
+mkdir $GOPATH/src
+! go get -t -u 'github.com/rsc/go-get-issue-18219/bad/imp3'
+stderr 'must be imported as'
+! exists github.com/rsc/go-get-issue-11864/vendor
+
+# Bad vendor (bad/...)
+rm $GOPATH
+mkdir $GOPATH/src
+! go get -t -u 'github.com/rsc/go-get-issue-18219/bad/...'
+stderr 'must be imported as'
+! exists github.com/rsc/go-get-issue-11864/vendor
+
+-- v/m.go --
+package main
+
+import (
+	"fmt"
+	"vendor.org/p"
+)
+
+func main() {
+	fmt.Println(p.C)
+}
+-- v/m_test.go --
+package main
+import (
+	"fmt"
+	"testing"
+	"vendor.org/p"
+)
+
+func TestNothing(t *testing.T) {
+	fmt.Println(p.C)
+}
+-- v/vendor/vendor.org/p/p.go --
+package p
+const C = 1
diff --git a/src/cmd/go/testdata/script/get_with_git_trace.txt b/src/cmd/go/testdata/script/get_with_git_trace.txt
new file mode 100644
index 0000000..6f1305a
--- /dev/null
+++ b/src/cmd/go/testdata/script/get_with_git_trace.txt
@@ -0,0 +1,9 @@
+env GO111MODULE=off
+
+env GIT_TRACE=1
+
+[!net:golang.org] skip
+[!git] skip
+
+# go get should be success when GIT_TRACE set
+go get golang.org/x/text
diff --git a/src/cmd/go/testdata/script/go_badcmd.txt b/src/cmd/go/testdata/script/go_badcmd.txt
new file mode 100644
index 0000000..661375a
--- /dev/null
+++ b/src/cmd/go/testdata/script/go_badcmd.txt
@@ -0,0 +1,2 @@
+! go asdf
+stderr '^go asdf: unknown command'
diff --git a/src/cmd/go/testdata/script/go_version.txt b/src/cmd/go/testdata/script/go_version.txt
new file mode 100644
index 0000000..1a787e1
--- /dev/null
+++ b/src/cmd/go/testdata/script/go_version.txt
@@ -0,0 +1,9 @@
+# test that go version doesn't panic on non-go binaries

+# See Issue #49181

+

+[exec:/bin/true] cp /bin/true true

+[exec:C:\windows\system32\help.exe] cp C:\windows\system32\help.exe help.exe

+

+go version -m .

+! stdout .

+! stderr .

diff --git a/src/cmd/go/testdata/script/godebug_default.txt b/src/cmd/go/testdata/script/godebug_default.txt
new file mode 100644
index 0000000..5bb8cac
--- /dev/null
+++ b/src/cmd/go/testdata/script/godebug_default.txt
@@ -0,0 +1,114 @@
+env GO111MODULE=on
+env GOTRACEBACK=single
+
+# Go 1.21 work module should leave panicnil with an implicit default.
+cp go.mod.21 go.mod
+go list -f '{{.Module.GoVersion}} {{.DefaultGODEBUG}}'
+! stdout panicnil
+stdout randautoseed=0
+
+# Go 1.21 work module should NOT set panicnil=1 in Go 1.20 dependency.
+cp go.mod.21 go.mod
+go list -f '{{.Module.GoVersion}} {{.DefaultGODEBUG}}' q
+! stdout panicnil=1
+! stdout randautoseed
+
+go mod download rsc.io/panicnil # for go.sum
+go list -f '{{.Module.GoVersion}} {{.DefaultGODEBUG}}' rsc.io/panicnil
+! stdout panicnil=1
+! stdout randautoseed
+
+# Go 1.20 work module should set panicnil=1.
+cp go.mod.20 go.mod
+go list -f '{{.Module.GoVersion}} {{.DefaultGODEBUG}}'
+stdout panicnil=1
+stdout randautoseed=0
+
+# Go 1.20 work module should set panicnil=1 in Go 1.20 dependency.
+cp go.mod.20 go.mod
+go list -f '{{.Module.GoVersion}} {{.DefaultGODEBUG}}' q
+stdout panicnil=1
+! stdout randautoseed
+
+# Go 1.21 workspace should leave panicnil with an implicit default.
+cat q/go.mod
+cp go.work.21 go.work
+go list -f '{{.Module.GoVersion}} {{.DefaultGODEBUG}}'
+! stdout panicnil
+stdout randautoseed=0
+rm go.work
+
+# Go 1.20 workspace with Go 1.21 module cannot happen.
+cp go.work.20 go.work
+cp go.mod.21 go.mod
+! go list -f '{{.Module.GoVersion}} {{.DefaultGODEBUG}}'
+stderr 'go: module . listed in go.work file requires go >= 1.21'
+rm go.work
+
+[short] skip
+
+# Programs in Go 1.21 work module should trigger run-time error.
+cp go.mod.21 go.mod
+! go run .
+stderr 'panic: panic called with nil argument'
+
+! go run rsc.io/panicnil
+stderr 'panic: panic called with nil argument'
+
+# Programs in Go 1.20 work module use old panic nil behavior.
+cp go.mod.20 go.mod
+! go run .
+stderr 'panic: nil'
+
+! go run rsc.io/panicnil
+stderr 'panic: nil'
+
+# Programs in no module at all should use their go.mod file.
+rm go.mod
+! go run rsc.io/panicnil@v1.0.0
+stderr 'panic: nil'
+
+rm go.mod
+! go run rsc.io/panicnil@v1.1.0
+stderr 'panic: panic called with nil argument'
+
+-- go.work.21 --
+go 1.21
+use .
+use ./q
+
+-- go.work.20 --
+go 1.20
+use .
+use ./q
+
+-- go.mod.21 --
+go 1.21
+module m
+require q v1.0.0
+replace q => ./q
+require rsc.io/panicnil v1.0.0
+
+-- go.mod.20 --
+go 1.20
+module m
+require q v1.0.0
+replace q => ./q
+require rsc.io/panicnil v1.0.0
+
+-- p.go --
+//go:debug randautoseed=0
+
+package main
+
+func main() {
+	panic(nil)
+}
+
+-- q/go.mod --
+go 1.20
+module q
+
+-- q/q.go --
+package main
+func main() {}
diff --git a/src/cmd/go/testdata/script/godebug_unknown.txt b/src/cmd/go/testdata/script/godebug_unknown.txt
new file mode 100644
index 0000000..57dacbc
--- /dev/null
+++ b/src/cmd/go/testdata/script/godebug_unknown.txt
@@ -0,0 +1,9 @@
+! go build
+stderr 'p.go:1:1: invalid //go:debug: unknown //go:debug setting "x"'
+
+-- go.mod --
+module m
+-- p.go --
+//go:debug x=y
+package main
+func main() {}
diff --git a/src/cmd/go/testdata/script/goflags.txt b/src/cmd/go/testdata/script/goflags.txt
new file mode 100644
index 0000000..1120860
--- /dev/null
+++ b/src/cmd/go/testdata/script/goflags.txt
@@ -0,0 +1,64 @@
+env GO111MODULE=off
+
+# GOFLAGS sets flags for commands
+
+env GOFLAGS='-e -f={{.Dir}} --test.benchtime=1s -count=10'
+go list asdfasdfasdf  # succeeds because of -e
+go list runtime
+stdout '[\\/]runtime$'
+
+env GOFLAGS=-race OLDGOARCH=$GOARCH OLDGOOS=$GOOS GOARCH=386 GOOS=linux
+! go list runtime
+stderr 'race is not supported on linux/386'
+
+env GOARCH=$OLDGOARCH GOOS=$OLDGOOS
+
+# go env succeeds even though -f={{.Dir}} is inappropriate
+go env
+
+# bad flags are diagnosed
+env GOFLAGS=-typoflag
+! go list runtime
+stderr 'unknown flag -typoflag'
+
+env GOFLAGS=-
+! go list runtime
+stderr '^go: parsing \$GOFLAGS: non-flag "-"'
+
+env GOFLAGS=--
+! go list runtime
+stderr '^go: parsing \$GOFLAGS: non-flag "--"'
+
+env GOFLAGS=---oops
+! go list runtime
+stderr '^go: parsing \$GOFLAGS: non-flag "---oops"'
+
+env GOFLAGS=-=noname
+! go list runtime
+stderr '^go: parsing \$GOFLAGS: non-flag "-=noname"'
+
+env GOFLAGS=-f
+! go list runtime
+stderr '^go: flag needs an argument: -f \(from (\$GOFLAGS|%GOFLAGS%)\)$'
+
+env GOFLAGS=-e=asdf
+! go list runtime
+stderr '^go: invalid boolean value \"asdf\" for flag -e \(from (\$GOFLAGS|%GOFLAGS%)\)'
+
+# except in go bug (untested) and go env
+go env
+stdout GOFLAGS
+
+# Flags listed in GOFLAGS should be safe to duplicate on the command line.
+env GOFLAGS=-tags=magic
+go list -tags=magic
+go test -tags=magic -c -o $devnull
+go vet -tags=magic
+
+# GOFLAGS uses the same quoting rules (quoted.Split) as the rest of
+# the go command env variables
+env GOFLAGS='"-tags=magic wizardry"'
+go list
+
+-- foo_test.go --
+package foo
diff --git a/src/cmd/go/testdata/script/goline_order.txt b/src/cmd/go/testdata/script/goline_order.txt
new file mode 100644
index 0000000..6212cd6
--- /dev/null
+++ b/src/cmd/go/testdata/script/goline_order.txt
@@ -0,0 +1,74 @@
+# Check that go lines are always >= go lines of dependencies.
+
+# Using too old a release cannot even complete module load.
+env TESTGO_VERSION=go1.21.1
+env TESTGO_VERSION_SWITCH=switch
+cp go.mod go.mod.orig
+
+# If the offending module is not imported, it's not detected.
+go list
+cmp go.mod go.mod.orig
+
+# Adding the import produces the error.
+# Maybe this should auto-switch, but it requires more plumbing to get this error through,
+# and it's a misconfigured system that should not arise in practice, so not switching is fine.
+! go list -deps -tags usem1
+cmp go.mod go.mod.orig
+stderr '^go: module ./m1 requires go >= 1.21.2 \(running go 1.21.1\)$'
+
+# go get go@1.21.2 fixes the error.
+cp go.mod.orig go.mod
+go get go@1.21.2
+go list -deps -tags usem1
+
+# go get -tags usem1 fixes the error.
+cp go.mod.orig go.mod
+go get -tags usem1
+go list -deps -tags usem1
+
+# go get fixes the error.
+cp go.mod.orig go.mod
+go get
+go list -deps -tags usem1
+
+# Using a new enough release reports the error after module load and suggests 'go mod tidy'
+env TESTGO_VERSION=go1.21.2
+cp go.mod.orig go.mod
+! go list -deps -tags usem1
+stderr 'updates to go.mod needed'
+stderr 'go mod tidy'
+go mod tidy
+go list -deps -tags usem1
+
+# go get also works
+cp go.mod.orig go.mod
+! go list -deps -tags usem1
+stderr 'updates to go.mod needed'
+stderr 'go mod tidy'
+go get go@1.21.2
+go list -deps -tags usem1
+
+
+-- go.mod --
+module m
+go 1.21.1
+
+require m1 v0.0.1
+
+replace m1 => ./m1
+
+-- m1/go.mod --
+go 1.21.2
+
+-- p.go --
+//go:build usem1
+
+package p
+
+import _ "m1"
+
+-- p1.go --
+package p
+
+-- m1/p.go --
+package p
diff --git a/src/cmd/go/testdata/script/gopath_install.txt b/src/cmd/go/testdata/script/gopath_install.txt
new file mode 100644
index 0000000..6c572ea
--- /dev/null
+++ b/src/cmd/go/testdata/script/gopath_install.txt
@@ -0,0 +1,53 @@
+# Regression test for 'go install' locations in GOPATH mode.
+env GO111MODULE=off
+[short] skip
+
+# Without $GOBIN set, binaries should be installed into the GOPATH bin directory.
+env GOBIN=
+rm $GOPATH/bin/go-cmd-test$GOEXE
+go install go-cmd-test
+exists $GOPATH/bin/go-cmd-test$GOEXE
+
+# With $GOBIN set, binaries should be installed to $GOBIN.
+env GOBIN=$WORK/bin1
+mkdir -p $GOBIN
+go install go-cmd-test
+exists $GOBIN/go-cmd-test$GOEXE
+
+# Issue 11065: installing to the current directory should create an executable.
+cd go-cmd-test
+env GOBIN=$PWD
+go install
+exists ./go-cmd-test$GOEXE
+cd ..
+
+# Without $GOBIN set, installing a program outside $GOPATH should fail
+# (there is nowhere to install it).
+env GOPATH= # reset to default ($HOME/go, which does not exist)
+env GOBIN=
+! go install go-cmd-test/helloworld.go
+stderr '^go: no install location for \.go files listed on command line \(GOBIN not set\)$'
+
+# With $GOBIN set, should install there.
+env GOBIN=$WORK/bin1
+go install go-cmd-test/helloworld.go
+exists $GOBIN/helloworld$GOEXE
+
+# We can't assume that we can write to GOROOT, because it may not be writable.
+# However, we can check its install location using 'go list'.
+# cmd/fix should be installed to GOROOT/pkg, not GOPATH/bin.
+env GOPATH=$PWD
+go list -f '{{.Target}}' cmd/fix
+stdout $GOROOT'[/\\]pkg[/\\]tool[/\\]'$GOOS'_'$GOARCH'[/\\]fix'$GOEXE'$'
+
+# GOBIN should not affect toolchain install locations.
+env GOBIN=$WORK/bin1
+go list -f '{{.Target}}' cmd/fix
+stdout $GOROOT'[/\\]pkg[/\\]tool[/\\]'$GOOS'_'$GOARCH'[/\\]fix'$GOEXE'$'
+
+-- go-cmd-test/helloworld.go --
+package main
+
+func main() {
+	println("hello world")
+}
diff --git a/src/cmd/go/testdata/script/gopath_local.txt b/src/cmd/go/testdata/script/gopath_local.txt
new file mode 100644
index 0000000..efde0e7
--- /dev/null
+++ b/src/cmd/go/testdata/script/gopath_local.txt
@@ -0,0 +1,117 @@
+env GO111MODULE=off  # Relative imports only work in GOPATH mode.
+
+[short] skip
+
+# Imports should be resolved relative to the source file.
+go build testdata/local/easy.go
+exec ./easy$GOEXE
+stdout '^easysub\.Hello'
+
+# Ignored files should be able to import the package built from
+# non-ignored files in the same directory.
+go build -o easysub$GOEXE testdata/local/easysub/main.go
+exec ./easysub$GOEXE
+stdout '^easysub\.Hello'
+
+# Files in relative-imported packages should be able to
+# use relative imports themselves.
+go build testdata/local/hard.go
+exec ./hard$GOEXE
+stdout '^sub\.Hello'
+
+# Explicit source files listed on the command line should not install without
+# GOBIN set, since individual source files aren't part of the containing GOPATH.
+! go install testdata/local/easy.go
+stderr '^go: no install location for \.go files listed on command line \(GOBIN not set\)$'
+
+[GOOS:windows] stop  # Windows does not allow the ridiculous directory name we're about to use.
+
+env BAD_DIR_NAME='#$%:, &()*;<=>?\^{}'
+
+mkdir -p testdata/$BAD_DIR_NAME/easysub
+mkdir -p testdata/$BAD_DIR_NAME/sub/sub
+
+cp testdata/local/easy.go testdata/$BAD_DIR_NAME/easy.go
+cp testdata/local/easysub/easysub.go testdata/$BAD_DIR_NAME/easysub/easysub.go
+cp testdata/local/easysub/main.go testdata/$BAD_DIR_NAME/easysub/main.go
+cp testdata/local/hard.go testdata/$BAD_DIR_NAME/hard.go
+cp testdata/local/sub/sub.go testdata/$BAD_DIR_NAME/sub/sub.go
+cp testdata/local/sub/sub/subsub.go testdata/$BAD_DIR_NAME/sub/sub/subsub.go
+
+# Imports should be resolved relative to the source file.
+go build testdata/$BAD_DIR_NAME/easy.go
+exec ./easy$GOEXE
+stdout '^easysub\.Hello'
+
+# Ignored files should be able to import the package built from
+# non-ignored files in the same directory.
+go build -o easysub$GOEXE testdata/$BAD_DIR_NAME/easysub/main.go
+exec ./easysub$GOEXE
+stdout '^easysub\.Hello'
+
+# Files in relative-imported packages should be able to
+# use relative imports themselves.
+go build testdata/$BAD_DIR_NAME/hard.go
+exec ./hard$GOEXE
+stdout '^sub\.Hello'
+
+# Explicit source files listed on the command line should not install without
+# GOBIN set, since individual source files aren't part of the containing GOPATH.
+! go install testdata/$BAD_DIR_NAME/easy.go
+stderr '^go: no install location for \.go files listed on command line \(GOBIN not set\)$'
+
+-- testdata/local/easy.go --
+package main
+
+import "./easysub"
+
+func main() {
+	easysub.Hello()
+}
+-- testdata/local/easysub/easysub.go --
+package easysub
+
+import "fmt"
+
+func Hello() {
+	fmt.Println("easysub.Hello")
+}
+-- testdata/local/easysub/main.go --
+// +build ignore
+
+package main
+
+import "."
+
+func main() {
+	easysub.Hello()
+}
+-- testdata/local/hard.go --
+package main
+
+import "./sub"
+
+func main() {
+	sub.Hello()
+}
+-- testdata/local/sub/sub.go --
+package sub
+
+import (
+	"fmt"
+
+	subsub "./sub"
+)
+
+func Hello() {
+	fmt.Println("sub.Hello")
+	subsub.Hello()
+}
+-- testdata/local/sub/sub/subsub.go --
+package subsub
+
+import "fmt"
+
+func Hello() {
+	fmt.Println("subsub.Hello")
+}
diff --git a/src/cmd/go/testdata/script/gopath_moved_repo.txt b/src/cmd/go/testdata/script/gopath_moved_repo.txt
new file mode 100644
index 0000000..8108d9b
--- /dev/null
+++ b/src/cmd/go/testdata/script/gopath_moved_repo.txt
@@ -0,0 +1,67 @@
+env GO111MODULE=off
+
+# Test that 'go get -u' reports packages whose VCS configurations do not
+# match their import paths.
+
+[!net:rsc.io] skip
+[short] skip
+
+# We need to execute a custom Go program to break the config files.
+#
+# git will ask for a username and password when we run 'go get -d -f -u',
+# so we also need to set GIT_ASKPASS. Conveniently, a single binary can
+# perform both tasks!
+
+go build -o replace.exe replace
+env GIT_ASKPASS=$PWD/replace.exe
+
+
+# Test that 'go get -u' reports moved git packages.
+
+[git] go get -d rsc.io/pdf
+[git] go get -d -u rsc.io/pdf
+[git] exec ./replace.exe pdf rsc.io/pdf/.git/config
+
+[git] ! go get -d -u rsc.io/pdf
+[git] stderr 'is a custom import path for'
+[git] ! go get -d -f -u rsc.io/pdf
+[git] stderr 'validating server certificate|[nN]ot [fF]ound'
+
+
+# Test that 'go get -u' reports moved Mercurial packages.
+
+[exec:hg] go get -d vcs-test.golang.org/go/custom-hg-hello
+[exec:hg] go get -d -u vcs-test.golang.org/go/custom-hg-hello
+[exec:hg] exec ./replace.exe custom-hg-hello vcs-test.golang.org/go/custom-hg-hello/.hg/hgrc
+
+[exec:hg] ! go get -d -u vcs-test.golang.org/go/custom-hg-hello
+[exec:hg] stderr 'is a custom import path for'
+[exec:hg] ! go get -d -f -u vcs-test.golang.org/go/custom-hg-hello
+[exec:hg] stderr 'validating server certificate|[nN]ot [fF]ound'
+
+
+-- replace/replace.go --
+package main
+
+import (
+	"bytes"
+	"log"
+	"os"
+)
+
+func main() {
+	if len(os.Args) < 3 {
+		return
+	}
+
+	base := []byte(os.Args[1])
+	path := os.Args[2]
+	data, err := os.ReadFile(path)
+	if err != nil {
+		log.Fatal(err)
+	}
+	err = os.WriteFile(path, bytes.ReplaceAll(data, base, append(base, "XXX"...)), 0644)
+	if err != nil {
+		log.Fatal(err)
+	}
+}
diff --git a/src/cmd/go/testdata/script/gopath_paths.txt b/src/cmd/go/testdata/script/gopath_paths.txt
new file mode 100644
index 0000000..04265b1
--- /dev/null
+++ b/src/cmd/go/testdata/script/gopath_paths.txt
@@ -0,0 +1,43 @@
+# Regression test for GOPATH validation in GOPATH mode.
+env GO111MODULE=off
+
+env ORIG_GOPATH=$GOPATH
+
+# The literal path '.' in GOPATH should be rejected.
+env GOPATH=.
+! go build go-cmd-test/helloworld.go
+stderr 'GOPATH entry is relative'
+
+# It should still be rejected if the requested package can be
+# found using another entry.
+env GOPATH=${:}$ORIG_GOPATH${:}.
+! go build go-cmd-test
+stderr 'GOPATH entry is relative'
+
+# GOPATH cannot be a relative subdirectory of the working directory.
+env ORIG_GOPATH
+stdout 'ORIG_GOPATH='$WORK[/\\]gopath
+cd $WORK
+env GOPATH=gopath
+! go build gopath/src/go-cmd-test/helloworld.go
+stderr 'GOPATH entry is relative'
+
+# Blank paths in GOPATH should be rejected as relative (issue 21928).
+env GOPATH=' '${:}$ORIG_GOPATH
+! go build go-cmd-test
+stderr 'GOPATH entry is relative'
+
+[short] stop
+
+# Empty paths in GOPATH should be ignored (issue 21928).
+env GOPATH=${:}$ORIG_GOPATH
+env GOPATH
+go install go-cmd-test
+exists $ORIG_GOPATH/bin/go-cmd-test$GOEXE
+
+-- go-cmd-test/helloworld.go --
+package main
+
+func main() {
+	println("hello world")
+}
diff --git a/src/cmd/go/testdata/script/gopath_std_vendor.txt b/src/cmd/go/testdata/script/gopath_std_vendor.txt
new file mode 100644
index 0000000..4aaf46b
--- /dev/null
+++ b/src/cmd/go/testdata/script/gopath_std_vendor.txt
@@ -0,0 +1,44 @@
+env GO111MODULE=off
+
+[!compiler:gc] skip
+
+go list -f '{{.Dir}}' vendor/golang.org/x/net/http2/hpack
+stdout $GOPATH[/\\]src[/\\]vendor
+
+# A package importing 'net/http' should resolve its dependencies
+# to the package 'vendor/golang.org/x/net/http2/hpack' within GOROOT.
+cd importnethttp
+go list -deps -f '{{.ImportPath}} {{.Dir}}'
+stdout ^vendor/golang.org/x/net/http2/hpack
+stdout $GOROOT[/\\]src[/\\]vendor[/\\]golang.org[/\\]x[/\\]net[/\\]http2[/\\]hpack
+! stdout $GOPATH[/\\]src[/\\]vendor
+
+# In the presence of $GOPATH/src/vendor/golang.org/x/net/http2/hpack,
+# a package in GOPATH importing 'golang.org/x/net/http2/hpack' should
+# resolve its dependencies in GOPATH/src.
+cd ../issue16333
+go build .
+
+go list -deps -f '{{.ImportPath}} {{.Dir}}' .
+stdout $GOPATH[/\\]src[/\\]vendor[/\\]golang.org[/\\]x[/\\]net[/\\]http2[/\\]hpack
+! stdout $GOROOT[/\\]src[/\\]vendor
+
+go list -test -deps -f '{{.ImportPath}} {{.Dir}}' .
+stdout $GOPATH[/\\]src[/\\]vendor[/\\]golang.org[/\\]x[/\\]net[/\\]http2[/\\]hpack
+! stdout $GOROOT[/\\]src[/\\]vendor
+
+-- issue16333/issue16333.go --
+package vendoring17
+
+import _ "golang.org/x/net/http2/hpack"
+-- issue16333/issue16333_test.go --
+package vendoring17
+
+import _ "testing"
+import _ "golang.org/x/net/http2/hpack"
+-- importnethttp/http.go --
+package importnethttp
+
+import _ "net/http"
+-- $GOPATH/src/vendor/golang.org/x/net/http2/hpack/hpack.go --
+package hpack
diff --git a/src/cmd/go/testdata/script/gopath_vendor_dup_err.txt b/src/cmd/go/testdata/script/gopath_vendor_dup_err.txt
new file mode 100644
index 0000000..5f1e09a
--- /dev/null
+++ b/src/cmd/go/testdata/script/gopath_vendor_dup_err.txt
@@ -0,0 +1,24 @@
+env GO111MODULE=off
+
+# Issue 17119: Test more duplicate load errors.
+! go build dupload
+! stderr 'duplicate load|internal error'
+stderr 'dupload/vendor/p must be imported as p'
+
+-- dupload/dupload.go --
+package main
+
+import (
+	_ "dupload/p2"
+	_ "p"
+)
+
+func main() {}
+-- dupload/p/p.go --
+package p
+-- dupload/p2/p2.go --
+package p2
+
+import _ "dupload/vendor/p"
+-- dupload/vendor/p/p.go --
+package p
diff --git a/src/cmd/go/testdata/script/goroot_executable.txt b/src/cmd/go/testdata/script/goroot_executable.txt
new file mode 100644
index 0000000..e20dbd8
--- /dev/null
+++ b/src/cmd/go/testdata/script/goroot_executable.txt
@@ -0,0 +1,115 @@
+[compiler:gccgo] skip
+[short] skip 'builds and links another cmd/go'
+
+mkdir $WORK/new/bin
+
+# In this test, we are specifically checking the logic for deriving
+# the value of GOROOT from runtime.GOROOT.
+# GOROOT_FINAL changes the default behavior of runtime.GOROOT,
+# and will thus cause the test to fail if it is set when our
+# new cmd/go is built.
+env GOROOT_FINAL=
+
+# $GOROOT/bin/go is whatever the user has already installed
+# (using make.bash or similar). We can't make assumptions about what
+# options it may have been built with, such as -trimpath or GOROOT_FINAL.
+# Instead, we build a fresh copy of the binary with known settings.
+go build -o $WORK/new/bin/go$GOEXE cmd/go &
+go build -trimpath -o $WORK/bin/check$GOEXE check.go &
+wait
+
+env TESTGOROOT=$GOROOT
+env GOROOT=
+
+# Relocated Executable
+exec $WORK/bin/check$GOEXE $WORK/new/bin/go$GOEXE $TESTGOROOT
+
+# Relocated Tree:
+# If the binary is sitting in a bin dir next to ../pkg/tool, that counts as a GOROOT,
+# so it should find the new tree.
+mkdir $WORK/new/pkg/tool
+exec $WORK/bin/check$GOEXE $WORK/new/bin/go$GOEXE $WORK/new
+
+[!symlink] stop 'The rest of the test cases require symlinks'
+
+# Symlinked Executable:
+# With a symlink into go tree, we should still find the go tree.
+mkdir $WORK/other/bin
+symlink $WORK/other/bin/go$GOEXE -> $WORK/new/bin/go$GOEXE
+exec $WORK/bin/check$GOEXE $WORK/new/bin/go$GOEXE $WORK/new
+
+rm $WORK/new/pkg
+
+# Runtime GOROOT:
+# Binaries built in the new tree should report the
+# new tree when they call runtime.GOROOT.
+symlink $WORK/new/src -> $TESTGOROOT/src
+symlink $WORK/new/pkg -> $TESTGOROOT/pkg
+exec $WORK/new/bin/go$GOEXE run check_runtime_goroot.go $WORK/new
+
+-- check.go --
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+)
+
+func main() {
+	exe := os.Args[1]
+	want := os.Args[2]
+	cmd := exec.Command(exe, "env", "GOROOT")
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "%s env GOROOT: %v, %s\n", exe, err, out)
+		os.Exit(1)
+	}
+	goroot, err := filepath.EvalSymlinks(strings.TrimSpace(string(out)))
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	want, err = filepath.EvalSymlinks(want)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	if !strings.EqualFold(goroot, want) {
+		fmt.Fprintf(os.Stderr, "go env GOROOT:\nhave %s\nwant %s\n", goroot, want)
+		os.Exit(1)
+	}
+	fmt.Fprintf(os.Stderr, "go env GOROOT: %s\n", goroot)
+
+}
+-- check_runtime_goroot.go --
+package main
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+)
+
+func main() {
+	goroot, err := filepath.EvalSymlinks(runtime.GOROOT())
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	want, err := filepath.EvalSymlinks(os.Args[1])
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	if !strings.EqualFold(goroot, want) {
+		fmt.Fprintf(os.Stderr, "go env GOROOT:\nhave %s\nwant %s\n", goroot, want)
+		os.Exit(1)
+	}
+	fmt.Fprintf(os.Stderr, "go env GOROOT: %s\n", goroot)
+
+}
diff --git a/src/cmd/go/testdata/script/goroot_executable_trimpath.txt b/src/cmd/go/testdata/script/goroot_executable_trimpath.txt
new file mode 100644
index 0000000..dc1e25e
--- /dev/null
+++ b/src/cmd/go/testdata/script/goroot_executable_trimpath.txt
@@ -0,0 +1,91 @@
+# Regression test for https://go.dev/issue/62119:
+# A 'go' command cross-compiled with a different GOHOSTOS
+# should be able to locate its GOROOT using os.Executable.
+#
+# (This also tests a 'go' command built with -trimpath
+# that is not cross-compiled, since we need to build that
+# configuration for the test anyway.)
+
+[short] skip 'builds and links another cmd/go'
+
+mkdir $WORK/new/bin
+mkdir $WORK/new/bin/${GOOS}_${GOARCH}
+
+# In this test, we are specifically checking the logic for deriving
+# the value of GOROOT from os.Executable when runtime.GOROOT is
+# trimmed away.
+# GOROOT_FINAL changes the default behavior of runtime.GOROOT,
+# so we explicitly clear it to remove it as a confounding variable.
+env GOROOT_FINAL=
+
+# $GOROOT/bin/go is whatever the user has already installed
+# (using make.bash or similar). We can't make assumptions about what
+# options it may have been built with, such as -trimpath or GOROOT_FINAL.
+# Instead, we build a fresh copy of the binary with known settings.
+go build -trimpath -o $WORK/new/bin/go$GOEXE cmd/go &
+go build -trimpath -o $WORK/bin/check$GOEXE check.go &
+wait
+
+env TESTGOROOT=$GOROOT
+env GOROOT=
+
+# Relocated Executable
+# Since we built with -trimpath and the binary isn't installed in a
+# normal-looking GOROOT, this command should fail.
+
+! exec $WORK/new/bin/go$GOEXE env GOROOT
+stderr '^go: cannot find GOROOT directory: ''go'' binary is trimmed and GOROOT is not set$'
+
+# Cross-compiled binaries in cmd are installed to a ${GOOS}_${GOARCH} subdirectory,
+# so we also want to try a copy there.
+# (Note that the script engine's 'exec' engine already works around
+# https://go.dev/issue/22315, so we don't have to do that explicitly in the
+# 'check' program we use later.)
+cp $WORK/new/bin/go$GOEXE $WORK/new/bin/${GOOS}_${GOARCH}/go$GOEXE
+! exec $WORK/new/bin/${GOOS}_${GOARCH}/go$GOEXE env GOROOT
+stderr '^go: cannot find GOROOT directory: ''go'' binary is trimmed and GOROOT is not set$'
+
+# Relocated Tree:
+# If the binary is sitting in a bin dir next to ../pkg/tool, that counts as a GOROOT,
+# so it should find the new tree.
+mkdir $WORK/new/pkg/tool
+exec $WORK/bin/check$GOEXE $WORK/new/bin/go$GOEXE $WORK/new
+exec $WORK/bin/check$GOEXE $WORK/new/bin/${GOOS}_${GOARCH}/go$GOEXE $WORK/new
+
+-- check.go --
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+)
+
+func main() {
+	exe := os.Args[1]
+	want := os.Args[2]
+	cmd := exec.Command(exe, "env", "GOROOT")
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "%s env GOROOT: %v, %s\n", exe, err, out)
+		os.Exit(1)
+	}
+	goroot, err := filepath.EvalSymlinks(strings.TrimSpace(string(out)))
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	want, err = filepath.EvalSymlinks(want)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	if !strings.EqualFold(goroot, want) {
+		fmt.Fprintf(os.Stderr, "go env GOROOT:\nhave %s\nwant %s\n", goroot, want)
+		os.Exit(1)
+	}
+	fmt.Fprintf(os.Stderr, "go env GOROOT: %s\n", goroot)
+
+}
diff --git a/src/cmd/go/testdata/script/gotoolchain_local.txt b/src/cmd/go/testdata/script/gotoolchain_local.txt
new file mode 100644
index 0000000..93f5570
--- /dev/null
+++ b/src/cmd/go/testdata/script/gotoolchain_local.txt
@@ -0,0 +1,243 @@
+# This test uses the fake toolchain switch support in cmd/go/internal/toolchain.Switch
+# to exercise all the version selection logic without needing actual toolchains.
+# See gotoolchain_net.txt and gotoolchain_path.txt for tests of network and PATH toolchains.
+
+env TESTGO_VERSION=go1.500
+env TESTGO_VERSION_SWITCH=switch
+
+# GOTOOLCHAIN=auto runs default toolchain without a go.mod or go.work
+env GOTOOLCHAIN=auto
+go version
+stdout go1.500
+
+# GOTOOLCHAIN=path runs default toolchain without a go.mod or go.work
+env GOTOOLCHAIN=path
+go version
+stdout go1.500
+
+# GOTOOLCHAIN=asdf is a syntax error
+env GOTOOLCHAIN=asdf
+! go version
+stderr '^go: invalid GOTOOLCHAIN "asdf"$'
+
+# GOTOOLCHAIN=version is used directly.
+env GOTOOLCHAIN=go1.600
+go version
+stdout go1.600
+
+env GOTOOLCHAIN=go1.400
+go version
+stdout go1.400
+
+# GOTOOLCHAIN=version+auto sets a minimum.
+env GOTOOLCHAIN=go1.600+auto
+go version
+stdout go1.600
+
+env GOTOOLCHAIN=go1.400.0+auto
+go version
+stdout go1.400.0
+
+# GOTOOLCHAIN=version+path sets a minimum too.
+env GOTOOLCHAIN=go1.600+path
+go version
+stdout go1.600
+
+env GOTOOLCHAIN=go1.400+path
+go version
+stdout go1.400
+
+# Create a go.mod file and test interactions with auto and path.
+
+# GOTOOLCHAIN=auto uses go line if newer than local toolchain.
+env GOTOOLCHAIN=auto
+go mod init m
+go mod edit -go=1.700 -toolchain=none
+go version
+stdout 1.700
+
+go mod edit -go=1.300 -toolchain=none
+go version
+stdout 1.500 # local toolchain is newer
+
+go mod edit -go=1.700 -toolchain=go1.300
+go version
+stdout go1.700 # toolchain too old, ignored
+
+go mod edit -go=1.300 -toolchain=default
+go version
+stdout go1.500
+
+go mod edit -go=1.700 -toolchain=default
+go version
+stdout go1.500 # toolchain local is like GOTOOLCHAIN=local and wins
+! go build
+stderr '^go: go.mod requires go >= 1.700 \(running go 1.500; go.mod sets toolchain default\)'
+
+# GOTOOLCHAIN=path does the same.
+env GOTOOLCHAIN=path
+go mod edit -go=1.700 -toolchain=none
+go version
+stdout 1.700
+
+go mod edit -go=1.300 -toolchain=none
+go version
+stdout 1.500 # local toolchain is newer
+
+go mod edit -go=1.700 -toolchain=go1.300
+go version
+stdout go1.700 # toolchain too old, ignored
+
+go mod edit -go=1.300 -toolchain=default
+go version
+stdout go1.500
+
+go mod edit -go=1.700 -toolchain=default
+go version
+stdout go1.500 # toolchain default applies even if older than go line
+! go build
+stderr '^go: go.mod requires go >= 1.700 \(running go 1.500; GOTOOLCHAIN=path; go.mod sets toolchain default\)'
+
+# GOTOOLCHAIN=min+auto with toolchain default uses min, not local
+
+env GOTOOLCHAIN=go1.400+auto
+go mod edit -go=1.300 -toolchain=default
+go version
+stdout 1.400 # not 1.500 local toolchain
+
+env GOTOOLCHAIN=go1.600+auto
+go mod edit -go=1.300 -toolchain=default
+go version
+stdout 1.600 # not 1.500 local toolchain
+
+# GOTOOLCHAIN names can have -suffix
+env GOTOOLCHAIN=go1.800-bigcorp
+go version
+stdout go1.800-bigcorp
+
+env GOTOOLCHAIN=auto
+go mod edit -go=1.999 -toolchain=go1.800-bigcorp
+go version
+stdout go1.999
+
+go mod edit -go=1.777 -toolchain=go1.800-bigcorp
+go version
+stdout go1.800-bigcorp
+
+# go.work takes priority over go.mod
+go mod edit -go=1.700 -toolchain=go1.999-wrong
+go work init
+go work edit -go=1.400 -toolchain=go1.600-right
+go version
+stdout go1.600-right
+
+go work edit -go=1.400 -toolchain=default
+go version
+stdout go1.500
+
+# go.work misconfiguration does not break go work edit
+# ('go 1.600 / toolchain local' forces use of 1.500 which can't normally load that go.work; allow work edit to fix it.)
+go work edit -go=1.600 -toolchain=default
+go version
+stdout go1.500
+
+go work edit -toolchain=none
+go version
+stdout go1.600
+
+rm go.work
+
+# go.mod misconfiguration does not break go mod edit
+go mod edit -go=1.600 -toolchain=default
+go version
+stdout go1.500
+
+go mod edit -toolchain=none
+go version
+stdout go1.600
+
+# toolchain built with a custom version should know how it compares to others
+
+env TESTGO_VERSION=go1.500-bigcorp
+go mod edit -go=1.499 -toolchain=none
+go version
+stdout go1.500-bigcorp
+
+go mod edit -go=1.500 -toolchain=none
+go version
+stdout go1.500-bigcorp
+
+go mod edit -go=1.501 -toolchain=none
+go version
+stdout go1.501
+
+env TESTGO_VERSION='go1.500 (bigcorp)'
+go mod edit -go=1.499 -toolchain=none
+go version
+stdout 'go1.500 \(bigcorp\)'
+
+go mod edit -go=1.500 -toolchain=none
+go version
+stdout 'go1.500 \(bigcorp\)'
+
+go mod edit -go=1.501 -toolchain=none
+go version
+stdout go1.501
+
+# go install m@v and go run m@v should ignore go.mod and use m@v
+env TESTGO_VERSION=go1.2.3
+go mod edit -go=1.999 -toolchain=go1.998
+
+! go install rsc.io/fortune/nonexist@v0.0.1
+stderr '^go: rsc.io/fortune@v0.0.1 requires go >= 1.21rc999; switching to go1.22.9$'
+stderr '^go: rsc.io/fortune/nonexist@v0.0.1: module rsc.io/fortune@v0.0.1 found, but does not contain package rsc.io/fortune/nonexist'
+
+! go run rsc.io/fortune/nonexist@v0.0.1
+stderr '^go: rsc.io/fortune@v0.0.1 requires go >= 1.21rc999; switching to go1.22.9$'
+stderr '^go: rsc.io/fortune/nonexist@v0.0.1: module rsc.io/fortune@v0.0.1 found, but does not contain package rsc.io/fortune/nonexist'
+
+# go install should handle unknown flags to find m@v
+! go install -unknownflag rsc.io/fortune/nonexist@v0.0.1
+stderr '^go: rsc.io/fortune@v0.0.1 requires go >= 1.21rc999; switching to go1.22.9$'
+stderr '^flag provided but not defined: -unknownflag'
+
+! go install -unknownflag arg rsc.io/fortune/nonexist@v0.0.1
+stderr '^go: rsc.io/fortune@v0.0.1 requires go >= 1.21rc999; switching to go1.22.9$'
+stderr '^flag provided but not defined: -unknownflag'
+
+# go run cannot handle unknown boolean flags
+! go run -unknownflag rsc.io/fortune/nonexist@v0.0.1
+! stderr switching
+stderr '^flag provided but not defined: -unknownflag'
+
+! go run -unknownflag oops rsc.io/fortune/nonexist@v0.0.1
+! stderr switching
+stderr '^flag provided but not defined: -unknownflag'
+
+# go run can handle unknown flag with argument.
+! go run -unknown=flag rsc.io/fortune/nonexist@v0.0.1
+stderr '^go: rsc.io/fortune@v0.0.1 requires go >= 1.21rc999; switching to go1.22.9$'
+stderr '^flag provided but not defined: -unknown'
+
+# go install m@v should handle queries
+! go install rsc.io/fortune/nonexist@v0.0
+stderr '^go: rsc.io/fortune@v0.0.1 requires go >= 1.21rc999; switching to go1.22.9$'
+stderr '^go: rsc.io/fortune/nonexist@v0.0: module rsc.io/fortune@v0.0 found \(v0.0.1\), but does not contain package rsc.io/fortune/nonexist'
+
+# go run m@v should handle queries
+! go install rsc.io/fortune/nonexist@v0
+stderr '^go: rsc.io/fortune@v0.0.1 requires go >= 1.21rc999; switching to go1.22.9$'
+stderr '^go: rsc.io/fortune/nonexist@v0: module rsc.io/fortune@v0 found \(v0.0.1\), but does not contain package rsc.io/fortune/nonexist'
+
+# go install m@v should use local toolchain if not upgrading
+! go install rsc.io/fortune/nonexist@v1
+! stderr go1.22.9
+! stderr switching
+stderr '^go: downloading rsc.io/fortune v1.0.0$'
+stderr '^go: rsc.io/fortune/nonexist@v1: module rsc.io/fortune@v1 found \(v1.0.0\), but does not contain package rsc.io/fortune/nonexist'
+
+# go run m@v should use local toolchain if not upgrading
+! go run rsc.io/fortune/nonexist@v1
+! stderr go1.22.9
+! stderr switching
+stderr '^go: rsc.io/fortune/nonexist@v1: module rsc.io/fortune@v1 found \(v1.0.0\), but does not contain package rsc.io/fortune/nonexist'
diff --git a/src/cmd/go/testdata/script/gotoolchain_loop.txt b/src/cmd/go/testdata/script/gotoolchain_loop.txt
new file mode 100644
index 0000000..a803d2e
--- /dev/null
+++ b/src/cmd/go/testdata/script/gotoolchain_loop.txt
@@ -0,0 +1,65 @@
+env GOTOOLCHAIN=auto
+env TESTGO_VERSION=go1.21.1
+
+# Basic switch should work.
+env TESTGO_VERSION_SWITCH=switch
+go version
+stdout go1.21.99
+
+# Toolchain target mismatch should be detected.
+env TESTGO_VERSION_SWITCH=mismatch
+! go version
+stderr '^go: toolchain go1.21.1 invoked to provide go1.21.99$'
+
+# Toolchain loop should be detected.
+env TESTGO_VERSION_SWITCH=loop
+! go version
+stderr -count=10 '^go: switching from go1.21.1 to go1.21.99 \[depth 9[0-9]\]$'
+stderr -count=1 '^go: switching from go1.21.1 to go1.21.99 \[depth 100\]$'
+stderr '^go: too many toolchain switches$'
+
+[short] skip
+
+# Internal env vars should not leak to go test or go run.
+env TESTGO_VERSION_SWITCH=switch
+go version
+stdout go1.21.99
+go test
+stdout clean
+go run .
+stdout clean
+
+-- go.mod --
+module m
+go 1.21.99
+
+-- m_test.go --
+package main
+
+import "testing"
+
+func TestEnv(t *testing.T) {
+	// the check is in func init in m.go
+}
+
+-- m.go --
+package main
+
+import "os"
+
+func init() {
+	envs := []string{
+		"GOTOOLCHAIN_INTERNAL_SWITCH_COUNT",
+		"GOTOOLCHAIN_INTERNAL_SWITCH_VERSION",
+	}
+	for _, e := range envs {
+		if v := os.Getenv(e); v != "" {
+			panic("$"+e+"="+v)
+		}
+	}
+	os.Stdout.WriteString("clean\n")
+}
+
+func main() {
+}
+
diff --git a/src/cmd/go/testdata/script/gotoolchain_modcmds.txt b/src/cmd/go/testdata/script/gotoolchain_modcmds.txt
new file mode 100644
index 0000000..1edd6d8
--- /dev/null
+++ b/src/cmd/go/testdata/script/gotoolchain_modcmds.txt
@@ -0,0 +1,45 @@
+env TESTGO_VERSION=go1.21.0
+env TESTGO_VERSION_SWITCH=switch
+
+# If the main module's go.mod file lists a version lower than the version
+# required by its dependencies, the commands that fetch and diagnose the module
+# graph (such as 'go mod graph' and 'go mod verify') should fail explicitly:
+# they can't interpret the graph themselves, and they aren't allowed to update
+# the go.mod file to record a specific, stable toolchain version that can.
+
+! go mod verify
+stderr '^go: rsc.io/future@v1.0.0: module rsc.io/future@v1.0.0 requires go >= 1.999 \(running go 1.21.0\)'
+
+! go mod graph
+stderr '^go: rsc.io/future@v1.0.0: module rsc.io/future@v1.0.0 requires go >= 1.999 \(running go 1.21.0\)'
+
+# TODO(#64008): 'go mod download' without arguments should fail too.
+
+
+# 'go get' should update the main module's go.mod file to a version compatible with the
+# go version required for rsc.io/future, not fail.
+go get .
+stderr '^go: module rsc.io/future@v1.0.0 requires go >= 1.999; switching to go1.999testmod$'
+stderr '^go: upgraded go 1.21 => 1.999$'
+stderr '^go: added toolchain go1.999testmod$'
+
+
+# Now, the various 'go mod' subcommands should succeed.
+
+go mod download
+
+go mod verify
+
+go mod graph
+
+
+-- go.mod --
+module example
+
+go 1.21
+
+require rsc.io/future v1.0.0
+-- example.go --
+package example
+
+import _ "rsc.io/future"
diff --git a/src/cmd/go/testdata/script/gotoolchain_net.txt b/src/cmd/go/testdata/script/gotoolchain_net.txt
new file mode 100644
index 0000000..35f6696
--- /dev/null
+++ b/src/cmd/go/testdata/script/gotoolchain_net.txt
@@ -0,0 +1,65 @@
+# This test only checks that basic network lookups work.
+# The full test of toolchain version selection is in gotoolchain.txt.
+
+env TESTGO_VERSION=go1.21actual
+
+# GOTOOLCHAIN from network, does not exist
+env GOTOOLCHAIN=go1.9999x
+! go version
+stderr 'go: download go1.9999x for .*: toolchain not available'
+
+# GOTOOLCHAIN from network
+[!exec:/bin/sh] stop 'the fake proxy serves shell scripts instead of binaries'
+env GOTOOLCHAIN=go1.999testmod
+go version
+stderr 'go: downloading go1.999testmod \(.*/.*\)'
+
+# GOTOOLCHAIN cached from network
+go version
+! stderr downloading
+stdout go1.999testmod
+
+# GOTOOLCHAIN with GOSUMDB enabled but at a bad URL should operate in cache and not try badurl
+env oldsumdb=$GOSUMDB
+env GOSUMDB=$oldsumdb' http://badurl'
+go version
+! stderr downloading
+stdout go1.999testmod
+
+# GOTOOLCHAIN with GOSUMB=off should fail, because it cannot access even the cached sumdb info
+# without the sumdb name.
+env GOSUMDB=off
+! go version
+stderr '^go: golang.org/toolchain@v0.0.1-go1.999testmod.[a-z0-9\-]*: verifying module: checksum database disabled by GOSUMDB=off$'
+
+# GOTOOLCHAIN with GOSUMDB enabled but at a bad URL should fail if cache is incomplete
+env GOSUMDB=$oldsumdb' http://badurl'
+rm $GOPATH/pkg/mod/cache/download/sumdb
+! go version
+! stderr downloading
+stderr 'panic: use of network' # test catches network access
+env GOSUMDB=$oldsumdb
+
+# Test a real GOTOOLCHAIN
+[short] skip
+[!net:golang.org] skip
+[!net:sum.golang.org] skip
+[!GOOS:darwin] [!GOOS:windows] [!GOOS:linux] skip
+[!GOARCH:amd64] [!GOARCH:arm64] skip
+
+env GOPROXY=
+[go-builder] env GOSUMDB=
+[!go-builder] env GOSUMDB=sum.golang.org  # Set explicitly in case GOROOT/go.env is modified.
+env GOTOOLCHAIN=go1.20.1
+
+	# Avoid resolving a "go1.20.1" from the user's real $PATH.
+	# That would not only cause the "downloading go1.20.1" message
+	# to be suppressed, but may spuriously fail:
+	# golang.org/dl/go1.20.1 expects to find its GOROOT in $HOME/sdk,
+	# but the script environment sets HOME=/no-home.
+env PATH=
+env path=
+
+go version
+stderr '^go: downloading go1.20.1 '
+stdout go1.20.1
diff --git a/src/cmd/go/testdata/script/gotoolchain_path.txt b/src/cmd/go/testdata/script/gotoolchain_path.txt
new file mode 100644
index 0000000..9628348
--- /dev/null
+++ b/src/cmd/go/testdata/script/gotoolchain_path.txt
@@ -0,0 +1,86 @@
+# This test only checks that basic PATH lookups work.
+# The full test of toolchain version selection is in gotoolchain.txt.
+
+[short] skip
+
+env TESTGO_VERSION=go1.21pre3
+
+# Compile a fake toolchain to put in the path under various names.
+env GOTOOLCHAIN=
+mkdir $WORK/bin
+go build -o $WORK/bin/ ./fakego.go  # adds .exe extension implicitly on Windows
+cp $WORK/bin/fakego$GOEXE $WORK/bin/go1.50.0$GOEXE
+
+[!GOOS:plan9] env PATH=$WORK/bin
+[GOOS:plan9] env path=$WORK/bin
+
+go version
+stdout go1.21pre3
+
+# GOTOOLCHAIN=go1.50.0
+env GOTOOLCHAIN=go1.50.0
+! go version
+stderr 'running go1.50.0 from PATH'
+
+# GOTOOLCHAIN=path with toolchain line
+env GOTOOLCHAIN=local
+go mod init m
+go mod edit -toolchain=go1.50.0
+grep go1.50.0 go.mod
+env GOTOOLCHAIN=path
+! go version
+stderr 'running go1.50.0 from PATH'
+
+# GOTOOLCHAIN=path with go line
+env GOTOOLCHAIN=local
+go mod edit -toolchain=none -go=1.50.0
+grep 'go 1.50.0' go.mod
+! grep toolchain go.mod
+env GOTOOLCHAIN=path
+! go version
+stderr 'running go1.50.0 from PATH'
+
+# GOTOOLCHAIN=auto with toolchain line
+env GOTOOLCHAIN=local
+go mod edit -toolchain=go1.50.0 -go=1.21
+grep 'go 1.21$' go.mod
+grep 'toolchain go1.50.0' go.mod
+env GOTOOLCHAIN=auto
+! go version
+stderr 'running go1.50.0 from PATH'
+
+# GOTOOLCHAIN=auto with go line
+env GOTOOLCHAIN=local
+go mod edit -toolchain=none -go=1.50.0
+grep 'go 1.50.0$' go.mod
+! grep toolchain go.mod
+env GOTOOLCHAIN=auto
+! go version
+stderr 'running go1.50.0 from PATH'
+
+# NewerToolchain should find Go 1.50.0.
+env GOTOOLCHAIN=local
+go mod edit -toolchain=none -go=1.22
+grep 'go 1.22$' go.mod
+! grep toolchain go.mod
+env GOTOOLCHAIN=path
+! go run rsc.io/fortune@v0.0.1
+stderr 'running go1.50.0 from PATH'
+
+-- fakego.go --
+package main
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+func main() {
+	exe, _ := os.Executable()
+	name := filepath.Base(exe)
+	name = strings.TrimSuffix(name, ".exe")
+	fmt.Fprintf(os.Stderr, "running %s from PATH\n", name)
+	os.Exit(1) // fail in case we are running this accidentally (like in "go mod edit")
+}
diff --git a/src/cmd/go/testdata/script/gotoolchain_version.txt b/src/cmd/go/testdata/script/gotoolchain_version.txt
new file mode 100644
index 0000000..e0736ff
--- /dev/null
+++ b/src/cmd/go/testdata/script/gotoolchain_version.txt
@@ -0,0 +1,22 @@
+[!net:proxy.golang.org] skip
+
+	# In the Go project's official release GOPROXY defaults to proxy.golang.org,
+	# but it may be changed in GOROOT/go.env (such as in third-party
+	# distributions).
+	#
+	# Make sure it is in use here, because the server for releases not served
+	# through the proxy (https://golang.org/toolchain?go-get=1) currently only
+	# serves the latest patch release for each of the supported stable releases.
+
+[go-builder] env GOPROXY=
+[!go-builder] env GOPROXY=https://proxy.golang.org
+
+go list -m -versions go
+stdout 1.20.1 # among others
+stdout 1.19rc2
+! stdout go1.20.1 # no go prefixes
+! stdout go1.19rc2
+
+go list -m -versions toolchain
+stdout go1.20.1 # among others
+stdout go1.19rc2
diff --git a/src/cmd/go/testdata/script/govcs.txt b/src/cmd/go/testdata/script/govcs.txt
new file mode 100644
index 0000000..dd128cc
--- /dev/null
+++ b/src/cmd/go/testdata/script/govcs.txt
@@ -0,0 +1,174 @@
+env GO111MODULE=on
+env proxy=$GOPROXY
+env GOPROXY=direct
+
+# GOVCS stops go get
+env GOVCS='*:none'
+! go get github.com/google/go-cmp
+stderr '^go: GOVCS disallows using git for public github.com/google/go-cmp; see ''go help vcs''$'
+env GOPRIVATE='github.com/google'
+! go get github.com/google/go-cmp
+stderr '^go: GOVCS disallows using git for private github.com/google/go-cmp; see ''go help vcs''$'
+
+# public pattern works
+env GOPRIVATE='github.com/google'
+env GOVCS='public:all,private:none'
+! go get github.com/google/go-cmp
+stderr '^go: GOVCS disallows using git for private github.com/google/go-cmp; see ''go help vcs''$'
+
+# private pattern works
+env GOPRIVATE='hubgit.com/google'
+env GOVCS='private:all,public:none'
+! go get github.com/google/go-cmp
+stderr '^go: GOVCS disallows using git for public github.com/google/go-cmp; see ''go help vcs''$'
+
+# other patterns work (for more patterns, see TestGOVCS)
+env GOPRIVATE=
+env GOVCS='github.com:svn|hg'
+! go get github.com/google/go-cmp
+stderr '^go: GOVCS disallows using git for public github.com/google/go-cmp; see ''go help vcs''$'
+env GOVCS='github.com/google/go-cmp/inner:git,github.com:svn|hg'
+! go get github.com/google/go-cmp
+stderr '^go: GOVCS disallows using git for public github.com/google/go-cmp; see ''go help vcs''$'
+
+# bad patterns are reported (for more bad patterns, see TestGOVCSErrors)
+env GOVCS='git'
+! go get github.com/google/go-cmp
+stderr '^go: github.com/google/go-cmp: malformed entry in GOVCS \(missing colon\): "git"$'
+
+env GOVCS=github.com:hg,github.com:git
+! go get github.com/google/go-cmp
+stderr '^go: github.com/google/go-cmp: unreachable pattern in GOVCS: "github.com:git" after "github.com:hg"$'
+
+# bad GOVCS patterns do not stop commands that do not need to check VCS
+go list
+env GOPROXY=$proxy
+go get rsc.io/quote # ok because used proxy
+env GOPROXY=direct
+
+# svn is disallowed by default
+env GOPRIVATE=
+env GOVCS=
+! go get rsc.io/nonexist.svn/hello
+stderr '^go: rsc.io/nonexist.svn/hello: GOVCS disallows using svn for public rsc.io/nonexist.svn; see ''go help vcs''$'
+
+# fossil is disallowed by default
+env GOPRIVATE=
+env GOVCS=
+! go get rsc.io/nonexist.fossil/hello
+stderr '^go: rsc.io/nonexist.fossil/hello: GOVCS disallows using fossil for public rsc.io/nonexist.fossil; see ''go help vcs''$'
+
+# bzr is disallowed by default
+env GOPRIVATE=
+env GOVCS=
+! go get rsc.io/nonexist.bzr/hello
+stderr '^go: rsc.io/nonexist.bzr/hello: GOVCS disallows using bzr for public rsc.io/nonexist.bzr; see ''go help vcs''$'
+
+# git is OK by default
+env GOVCS=
+env GONOSUMDB='*'
+[net:rsc.io] [git] [!short] go get rsc.io/sampler
+
+# hg is OK by default
+env GOVCS=
+env GONOSUMDB='*'
+[exec:hg] [!short] go get vcs-test.golang.org/go/custom-hg-hello
+
+# git can be disallowed
+env GOVCS=public:hg
+! go get rsc.io/nonexist.git/hello
+stderr '^go: rsc.io/nonexist.git/hello: GOVCS disallows using git for public rsc.io/nonexist.git; see ''go help vcs''$'
+
+# hg can be disallowed
+env GOVCS=public:git
+! go get rsc.io/nonexist.hg/hello
+stderr '^go: rsc.io/nonexist.hg/hello: GOVCS disallows using hg for public rsc.io/nonexist.hg; see ''go help vcs''$'
+
+# Repeat in GOPATH mode. Error texts slightly different.
+
+env GO111MODULE=off
+
+# GOVCS stops go get
+env GOVCS='*:none'
+! go get github.com/google/go-cmp
+stderr '^package github.com/google/go-cmp: GOVCS disallows using git for public github.com/google/go-cmp; see ''go help vcs''$'
+env GOPRIVATE='github.com/google'
+! go get github.com/google/go-cmp
+stderr '^package github.com/google/go-cmp: GOVCS disallows using git for private github.com/google/go-cmp; see ''go help vcs''$'
+
+# public pattern works
+env GOPRIVATE='github.com/google'
+env GOVCS='public:all,private:none'
+! go get github.com/google/go-cmp
+stderr '^package github.com/google/go-cmp: GOVCS disallows using git for private github.com/google/go-cmp; see ''go help vcs''$'
+
+# private pattern works
+env GOPRIVATE='hubgit.com/google'
+env GOVCS='private:all,public:none'
+! go get github.com/google/go-cmp
+stderr '^package github.com/google/go-cmp: GOVCS disallows using git for public github.com/google/go-cmp; see ''go help vcs''$'
+
+# other patterns work (for more patterns, see TestGOVCS)
+env GOPRIVATE=
+env GOVCS='github.com:svn|hg'
+! go get github.com/google/go-cmp
+stderr '^package github.com/google/go-cmp: GOVCS disallows using git for public github.com/google/go-cmp; see ''go help vcs''$'
+env GOVCS='github.com/google/go-cmp/inner:git,github.com:svn|hg'
+! go get github.com/google/go-cmp
+stderr '^package github.com/google/go-cmp: GOVCS disallows using git for public github.com/google/go-cmp; see ''go help vcs''$'
+
+# bad patterns are reported (for more bad patterns, see TestGOVCSErrors)
+env GOVCS='git'
+! go get github.com/google/go-cmp
+stderr '^package github.com/google/go-cmp: malformed entry in GOVCS \(missing colon\): "git"$'
+
+env GOVCS=github.com:hg,github.com:git
+! go get github.com/google/go-cmp
+stderr '^package github.com/google/go-cmp: unreachable pattern in GOVCS: "github.com:git" after "github.com:hg"$'
+
+# bad GOVCS patterns do not stop commands that do not need to check VCS
+go list
+
+# svn is disallowed by default
+env GOPRIVATE=
+env GOVCS=
+! go get rsc.io/nonexist.svn/hello
+stderr '^package rsc.io/nonexist.svn/hello: GOVCS disallows using svn for public rsc.io/nonexist.svn; see ''go help vcs''$'
+
+# fossil is disallowed by default
+env GOPRIVATE=
+env GOVCS=
+! go get rsc.io/nonexist.fossil/hello
+stderr '^package rsc.io/nonexist.fossil/hello: GOVCS disallows using fossil for public rsc.io/nonexist.fossil; see ''go help vcs''$'
+
+# bzr is disallowed by default
+env GOPRIVATE=
+env GOVCS=
+! go get rsc.io/nonexist.bzr/hello
+stderr '^package rsc.io/nonexist.bzr/hello: GOVCS disallows using bzr for public rsc.io/nonexist.bzr; see ''go help vcs''$'
+
+# git is OK by default
+env GOVCS=
+env GONOSUMDB='*'
+[net:rsc.io] [git] [!short] go get rsc.io/sampler
+
+# hg is OK by default
+env GOVCS=
+env GONOSUMDB='*'
+[exec:hg] [!short] go get vcs-test.golang.org/go/custom-hg-hello
+
+# git can be disallowed
+env GOVCS=public:hg
+! go get rsc.io/nonexist.git/hello
+stderr '^package rsc.io/nonexist.git/hello: GOVCS disallows using git for public rsc.io/nonexist.git; see ''go help vcs''$'
+
+# hg can be disallowed
+env GOVCS=public:git
+! go get rsc.io/nonexist.hg/hello
+stderr '^package rsc.io/nonexist.hg/hello: GOVCS disallows using hg for public rsc.io/nonexist.hg; see ''go help vcs''$'
+
+-- go.mod --
+module m
+
+-- p.go --
+package p
diff --git a/src/cmd/go/testdata/script/help.txt b/src/cmd/go/testdata/script/help.txt
new file mode 100644
index 0000000..26a0194
--- /dev/null
+++ b/src/cmd/go/testdata/script/help.txt
@@ -0,0 +1,51 @@
+env GO111MODULE=off
+
+# go help shows overview.
+go help
+stdout 'Go is a tool'
+stdout 'bug.*start a bug report'
+
+# go help bug shows usage for bug
+go help bug
+stdout 'usage: go bug'
+stdout 'bug report'
+
+# go bug help is an error (bug takes no arguments)
+! go bug help
+stderr 'bug takes no arguments'
+
+# go help mod shows mod subcommands
+go help mod
+stdout 'go mod <command>'
+stdout tidy
+
+# go help mod tidy explains tidy
+go help mod tidy
+stdout 'usage: go mod tidy'
+
+# go mod help tidy does too
+go mod help tidy
+stdout 'usage: go mod tidy'
+
+# go mod --help doesn't print help but at least suggests it.
+! go mod --help
+stderr 'Run ''go help mod'' for usage.'
+
+# Earlier versions of Go printed the same as 'go -h' here.
+# Also make sure we print the short help line.
+! go vet -h
+stderr 'usage: go vet .*'
+stderr 'Run ''go help vet'' for details.'
+stderr 'Run ''go tool vet help'' for a full list of flags and analyzers.'
+stderr 'Run ''go tool vet -help'' for an overview.'
+
+# Earlier versions of Go printed a large document here, instead of these two
+# lines.
+! go test -h
+stderr 'usage: go test'
+stderr 'Run ''go help test'' and ''go help testflag'' for details.'
+
+# go help get shows usage for get
+go help get
+stdout 'usage: go get'
+stdout 'get when using GOPATH'
diff --git a/src/cmd/go/testdata/script/import_cycle.txt b/src/cmd/go/testdata/script/import_cycle.txt
new file mode 100644
index 0000000..901f43c
--- /dev/null
+++ b/src/cmd/go/testdata/script/import_cycle.txt
@@ -0,0 +1,12 @@
+env GO111MODULE=off
+
+! go build selfimport
+stderr -count=1 'import cycle not allowed'
+
+# 'go list' shouldn't hang forever.
+go list -e -json selfimport
+
+-- $GOPATH/src/selfimport/selfimport.go --
+package selfimport
+
+import "selfimport"
diff --git a/src/cmd/go/testdata/script/import_ignore.txt b/src/cmd/go/testdata/script/import_ignore.txt
new file mode 100644
index 0000000..83a39a0
--- /dev/null
+++ b/src/cmd/go/testdata/script/import_ignore.txt
@@ -0,0 +1,11 @@
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod go.mod.orig
+
+-- go.mod --
+module m.test
+
+go 1.16
+-- .ignore.go --
+package p
+import _ "golang.org/x/mod/modfile"
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/import_main.txt b/src/cmd/go/testdata/script/import_main.txt
new file mode 100644
index 0000000..bc2cc4d
--- /dev/null
+++ b/src/cmd/go/testdata/script/import_main.txt
@@ -0,0 +1,114 @@
+env GO111MODULE=off
+
+# Test that you cannot import a main package.
+# See golang.org/issue/4210 and golang.org/issue/17475.
+
+[short] skip
+cd $WORK
+
+# Importing package main from that package main's test should work.
+go build x
+go test -c x
+
+# Importing package main from another package should fail.
+! go build p1
+stderr 'import "x" is a program, not an importable package'
+
+# ... even in that package's test.
+go build p2
+! go test -c p2
+stderr 'import "x" is a program, not an importable package'
+
+# ... even if that package's test is an xtest.
+go build p3
+! go test p3
+stderr 'import "x" is a program, not an importable package'
+
+# ... even if that package is a package main
+go build p4
+! go test -c p4
+stderr 'import "x" is a program, not an importable package'
+
+# ... even if that package is a package main using an xtest.
+go build p5
+! go test -c p5
+stderr 'import "x" is a program, not an importable package'
+
+-- x/main.go --
+package main
+
+var X int
+
+func main() {}
+-- x/main_test.go --
+package main_test
+
+import (
+	"testing"
+	xmain "x"
+)
+
+var _ = xmain.X
+
+func TestFoo(t *testing.T) {}
+-- p1/p.go --
+package p1
+
+import xmain "x"
+
+var _ = xmain.X
+-- p2/p.go --
+package p2
+-- p2/p_test.go --
+package p2
+
+import (
+	"testing"
+	xmain "x"
+)
+
+var _ = xmain.X
+
+func TestFoo(t *testing.T) {}
+-- p3/p.go --
+package p
+-- p3/p_test.go --
+package p_test
+
+import (
+	"testing"
+	xmain "x"
+)
+
+var _ = xmain.X
+
+func TestFoo(t *testing.T) {}
+-- p4/p.go --
+package main
+
+func main() {}
+-- p4/p_test.go --
+package main
+
+import (
+	"testing"
+	xmain "x"
+)
+
+var _ = xmain.X
+
+func TestFoo(t *testing.T) {}
+-- p5/p.go --
+package main
+func main() {}
+-- p5/p_test.go --
+package main_test
+
+import (
+	"testing"
+	xmain "x"
+)
+
+var _ = xmain.X
+
+func TestFoo(t *testing.T) {}
diff --git a/src/cmd/go/testdata/script/import_unix_tag.txt b/src/cmd/go/testdata/script/import_unix_tag.txt
new file mode 100644
index 0000000..b88ca1e
--- /dev/null
+++ b/src/cmd/go/testdata/script/import_unix_tag.txt
@@ -0,0 +1,32 @@
+# Regression test for https://go.dev/issue/54712: the "unix" build constraint
+# was not applied consistently during package loading.
+
+go list -x -f '{{if .Module}}{{.ImportPath}}{{end}}' -deps .
+stdout 'example.com/version'
+
+-- go.mod --
+module example
+
+go 1.19
+
+require example.com/version v1.1.0
+-- go.sum --
+example.com/version v1.1.0 h1:VdPnGmIF1NJrntStkxGrF3L/OfhaL567VzCjncGUgtM=
+example.com/version v1.1.0/go.mod h1:S7K9BnT4o5wT4PCczXPfWVzpjD4ud4e7AJMQJEgiu2Q=
+-- main_notunix.go --
+//go:build !(aix || darwin || dragonfly || freebsd || hurd || linux || netbsd || openbsd || solaris)
+
+package main
+
+import _ "example.com/version"
+
+func main() {}
+
+-- main_unix.go --
+//go:build unix
+
+package main
+
+import _ "example.com/version"
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/index.txt b/src/cmd/go/testdata/script/index.txt
new file mode 100644
index 0000000..6a2d13c
--- /dev/null
+++ b/src/cmd/go/testdata/script/index.txt
@@ -0,0 +1,6 @@
+# Check that standard library packages are cached.
+go list -json math # refresh cache
+env GODEBUG=gofsystrace=1,gofsystracelog=fsys.log
+go list -json math
+! grep math/abs.go fsys.log
+grep 'openIndexPackage .*[\\/]math$' fsys.log
diff --git a/src/cmd/go/testdata/script/install_cgo_excluded.txt b/src/cmd/go/testdata/script/install_cgo_excluded.txt
new file mode 100644
index 0000000..5a2b460
--- /dev/null
+++ b/src/cmd/go/testdata/script/install_cgo_excluded.txt
@@ -0,0 +1,15 @@
+env CGO_ENABLED=0
+
+! go install cgotest
+stderr 'build constraints exclude all Go files'
+
+-- go.mod --
+module cgotest
+
+go 1.16
+-- m.go --
+package cgotest
+
+import "C"
+
+var _ C.int
diff --git a/src/cmd/go/testdata/script/install_cleans_build.txt b/src/cmd/go/testdata/script/install_cleans_build.txt
new file mode 100644
index 0000000..dc85eb8
--- /dev/null
+++ b/src/cmd/go/testdata/script/install_cleans_build.txt
@@ -0,0 +1,25 @@
+env GO111MODULE=off
+[short] skip
+
+# 'go install' with no arguments should clean up after go build
+cd mycmd
+go build
+exists mycmd$GOEXE
+go install
+! exists mycmd$GOEXE
+
+# 'go install mycmd' does not clean up, even in the mycmd directory
+go build
+exists mycmd$GOEXE
+go install mycmd
+exists mycmd$GOEXE
+
+# 'go install mycmd' should not clean up in an unrelated current directory either
+cd ..
+cp mycmd/mycmd$GOEXE mycmd$GOEXE
+go install mycmd
+exists mycmd$GOEXE
+
+-- mycmd/main.go --
+package main
+func main() {}
diff --git a/src/cmd/go/testdata/script/install_cmd_gobin.txt b/src/cmd/go/testdata/script/install_cmd_gobin.txt
new file mode 100644
index 0000000..049bf41
--- /dev/null
+++ b/src/cmd/go/testdata/script/install_cmd_gobin.txt
@@ -0,0 +1,10 @@
+# Check that commands in cmd are install to $GOROOT/bin, not $GOBIN.
+# Verifies golang.org/issue/32674.
+env GOBIN=gobin
+mkdir gobin
+go list -f '{{.Target}}' cmd/go
+stdout $GOROOT${/}bin${/}go$GOEXE
+
+# Check that tools are installed to $GOTOOLDIR, not $GOBIN.
+go list -f '{{.Target}}' cmd/compile
+stdout $GOROOT${/}pkg${/}tool${/}${GOOS}_${GOARCH}${/}compile$GOEXE
diff --git a/src/cmd/go/testdata/script/install_cross_gobin.txt b/src/cmd/go/testdata/script/install_cross_gobin.txt
new file mode 100644
index 0000000..7679a7e
--- /dev/null
+++ b/src/cmd/go/testdata/script/install_cross_gobin.txt
@@ -0,0 +1,25 @@
+env GO111MODULE=off
+[short] skip # rebuilds std for alternate architecture
+
+cd mycmd
+go build mycmd
+
+# cross-compile install with implicit GOBIN=$GOPATH/bin can make subdirectory
+env GOARCH=386
+[GOARCH:386] env GOARCH=amd64
+env GOOS=linux
+go install mycmd
+exists $GOPATH/bin/linux_$GOARCH/mycmd
+
+# cross-compile install with explicit GOBIN cannot make subdirectory
+env GOBIN=$WORK/bin
+! go install mycmd
+! exists $GOBIN/linux_$GOARCH
+
+# The install directory for a cross-compiled standard command should include GOARCH.
+go list -f '{{.Target}}'  cmd/pack
+stdout ${GOROOT}[/\\]pkg[/\\]tool[/\\]${GOOS}_${GOARCH}[/\\]pack$
+
+-- mycmd/x.go --
+package main
+func main() {}
diff --git a/src/cmd/go/testdata/script/install_dep_version.txt b/src/cmd/go/testdata/script/install_dep_version.txt
new file mode 100644
index 0000000..58330e6
--- /dev/null
+++ b/src/cmd/go/testdata/script/install_dep_version.txt
@@ -0,0 +1,8 @@
+# Regression test for Issue #54908. When running a go install module@version
+# with --mod=readonly moduleInfo was not setting the GoVersion for the module
+# because the checksumOk function was failing because modfetch.GoSumFile
+# was not set when running outside of a module.
+
+env GOTOOLCHAIN=local
+
+go install --mod=readonly example.com/depends/on/generics@v1.0.0
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/install_goroot_targets.txt b/src/cmd/go/testdata/script/install_goroot_targets.txt
new file mode 100644
index 0000000..f26ee82
--- /dev/null
+++ b/src/cmd/go/testdata/script/install_goroot_targets.txt
@@ -0,0 +1,27 @@
+[short] skip
+
+# Packages in std do not have an install target.
+go list -f '{{.Target}}' fmt
+! stdout .
+go list -export -f '{{.Export}}' fmt
+stdout $GOCACHE
+
+# With GODEBUG=installgoroot=all, fmt has a target.
+# (Though we can't try installing it without modifying goroot).
+env GODEBUG=installgoroot=all
+go list -f '{{.Target}}' fmt
+stdout fmt\.a
+
+# However, the fake packages "builtin" and "unsafe" do not.
+go list -f '{{.Target}}' builtin unsafe
+! stdout .
+go install builtin unsafe  # Should succeed as no-ops.
+
+# With CGO_ENABLED=0, packages that would have
+# an install target with cgo on no longer do.
+env GODEBUG=
+env CGO_ENABLED=0
+go list -f '{{.Target}}' runtime/cgo
+! stdout .
+go list -export -f '{{.Export}}' runtime/cgo
+stdout $GOCACHE
diff --git a/src/cmd/go/testdata/script/install_modcacherw_issue64282.txt b/src/cmd/go/testdata/script/install_modcacherw_issue64282.txt
new file mode 100644
index 0000000..3e1e6e5
--- /dev/null
+++ b/src/cmd/go/testdata/script/install_modcacherw_issue64282.txt
@@ -0,0 +1,45 @@
+# Regression test for https://go.dev/issue/64282.
+#
+# 'go install' and 'go run' with pkg@version arguments should make
+# a best effort to parse flags relevant to downloading modules
+# (currently only -modcacherw) before actually downloading the module
+# to identify which toolchain version to use.
+#
+# However, the best-effort flag parsing should not interfere with
+# actual flag parsing if we don't switch toolchains. In particular,
+# unrecognized flags should still be diagnosed after the module for
+# the requested package has been downloaded and checked for toolchain
+# upgrades.
+
+
+! go install -cake=delicious -modcacherw example.com/printversion@v0.1.0
+stderr '^flag provided but not defined: -cake$'
+	# Because the -modcacherw flag was set, we should be able to modify the contents
+	# of a directory within the module cache.
+cp $WORK/extraneous.txt $GOPATH/pkg/mod/example.com/printversion@v0.1.0/extraneous_file.go
+go clean -modcache
+
+
+! go install -unknownflag -tags -modcacherw example.com/printversion@v0.1.0
+stderr '^flag provided but not defined: -unknownflag$'
+cp $WORK/extraneous.txt $GOPATH/pkg/mod/example.com/printversion@v0.1.0/extraneous_file.go
+go clean -modcache
+
+
+# Also try it with a 'go install' that succeeds.
+# (But skip in short mode, because linking a binary is expensive.)
+[!short] go install -modcacherw example.com/printversion@v0.1.0
+[!short] cp $WORK/extraneous.txt $GOPATH/pkg/mod/example.com/printversion@v0.1.0/extraneous_file.go
+[!short] go clean -modcache
+
+
+# The flag should also be applied if given in GOFLAGS
+# instead of on the command line.
+env GOFLAGS=-modcacherw
+! go install -cake=delicious example.com/printversion@v0.1.0
+stderr '^flag provided but not defined: -cake$'
+cp $WORK/extraneous.txt $GOPATH/pkg/mod/example.com/printversion@v0.1.0/extraneous_file.go
+
+
+-- $WORK/extraneous.txt --
+This is not a Go source file.
diff --git a/src/cmd/go/testdata/script/install_move_not_stale.txt b/src/cmd/go/testdata/script/install_move_not_stale.txt
new file mode 100644
index 0000000..de191cf
--- /dev/null
+++ b/src/cmd/go/testdata/script/install_move_not_stale.txt
@@ -0,0 +1,26 @@
+# Check to see that the distribution is not stale
+# even when it's been moved to a different directory.
+# Simulate that by creating a symlink to the tree.
+
+# We use net instead of std because stale std has
+# the behavior of checking that all std targets
+# are stale rather than any of them.
+
+[!symlink] skip
+[short] skip
+
+go build net
+! stale net
+
+symlink new -> $GOROOT
+env OLDGOROOT=$GOROOT
+env GOROOT=$WORK${/}gopath${/}src${/}new
+go env GOROOT
+stdout $WORK[\\/]gopath[\\/]src[\\/]new
+cd new
+! stale net
+
+# Add a control case to check that std is
+# stale with an empty cache
+env GOCACHE=$WORK${/}gopath${/}cache
+stale net
diff --git a/src/cmd/go/testdata/script/install_msan_and_race_and_asan_require_cgo.txt b/src/cmd/go/testdata/script/install_msan_and_race_and_asan_require_cgo.txt
new file mode 100644
index 0000000..d8f2aba
--- /dev/null
+++ b/src/cmd/go/testdata/script/install_msan_and_race_and_asan_require_cgo.txt
@@ -0,0 +1,22 @@
+# Tests Issue #21895
+
+env CGO_ENABLED=0
+
+[GOOS:darwin] [!short] [race] go build -race triv.go
+
+[!GOOS:darwin] [race] ! go install -race triv.go
+[!GOOS:darwin] [race] stderr '-race requires cgo'
+[!GOOS:darwin] [race] ! stderr '-msan'
+
+[msan] ! go install -msan triv.go
+[msan] stderr '-msan requires cgo'
+[msan] ! stderr '-race'
+
+[asan] ! go install -asan triv.go
+[asan] stderr '(-asan: the version of $(go env CC) could not be parsed)|(-asan: C compiler is not gcc or clang)|(-asan is not supported with [A-Za-z]+ compiler (\d+)\.(\d+))|(-asan requires cgo)'
+[asan] ! stderr '-msan'
+
+-- triv.go --
+package main
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/install_rebuild_removed.txt b/src/cmd/go/testdata/script/install_rebuild_removed.txt
new file mode 100644
index 0000000..5db3778
--- /dev/null
+++ b/src/cmd/go/testdata/script/install_rebuild_removed.txt
@@ -0,0 +1,44 @@
+env GO111MODULE=off
+
+# go command should detect package staleness as source file set changes
+go install mypkg
+! stale mypkg
+
+# z.go was not compiled; removing it should NOT make mypkg stale
+rm mypkg/z.go
+! stale mypkg
+
+# y.go was compiled; removing it should make mypkg stale
+rm mypkg/y.go
+stale mypkg
+
+# go command should detect executable staleness too
+go install mycmd
+! stale mycmd
+rm mycmd/z.go
+! stale mycmd
+rm mycmd/y.go
+stale mycmd
+
+-- mypkg/x.go --
+package mypkg
+
+-- mypkg/y.go --
+package mypkg
+
+-- mypkg/z.go --
+// +build missingtag
+
+package mypkg
+
+-- mycmd/x.go --
+package main
+func main() {}
+
+-- mycmd/y.go --
+package main
+
+-- mycmd/z.go --
+// +build missingtag
+
+package main
diff --git a/src/cmd/go/testdata/script/install_relative_gobin_fail.txt b/src/cmd/go/testdata/script/install_relative_gobin_fail.txt
new file mode 100644
index 0000000..aa14524
--- /dev/null
+++ b/src/cmd/go/testdata/script/install_relative_gobin_fail.txt
@@ -0,0 +1,12 @@
+env GOBIN=.
+! go install
+stderr 'cannot install, GOBIN must be an absolute path'
+
+-- go.mod --
+module triv
+
+go 1.16
+-- triv.go --
+package main
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/install_shadow_gopath.txt b/src/cmd/go/testdata/script/install_shadow_gopath.txt
new file mode 100644
index 0000000..148e6cc
--- /dev/null
+++ b/src/cmd/go/testdata/script/install_shadow_gopath.txt
@@ -0,0 +1,18 @@
+# Tests Issue #3562
+# go get foo.io (not foo.io/subdir) was not working consistently.
+
+env GO111MODULE=off
+env GOPATH=$WORK/gopath1${:}$WORK/gopath2
+
+mkdir $WORK/gopath1/src/test
+mkdir $WORK/gopath2/src/test
+cp main.go $WORK/gopath2/src/test/main.go
+cd $WORK/gopath2/src/test
+
+! go install
+stderr 'no install location for.*gopath2.src.test: hidden by .*gopath1.src.test'
+
+-- main.go --
+package main
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/issue36000.txt b/src/cmd/go/testdata/script/issue36000.txt
new file mode 100644
index 0000000..4173275
--- /dev/null
+++ b/src/cmd/go/testdata/script/issue36000.txt
@@ -0,0 +1,6 @@
+# Tests golang.org/issue/36000
+
+[!cgo] skip
+
+# go env with CGO flags should not make NUL file
+go env CGO_CFLAGS
diff --git a/src/cmd/go/testdata/script/issue53586.txt b/src/cmd/go/testdata/script/issue53586.txt
new file mode 100644
index 0000000..db405cd
--- /dev/null
+++ b/src/cmd/go/testdata/script/issue53586.txt
@@ -0,0 +1,18 @@
+[short] skip  # sleeps to make mtime cacheable
+
+go mod init example
+
+cd subdir
+go mod init example/subdir
+sleep 2s  # allow go.mod mtime to be cached
+
+go list -f '{{.Dir}}: {{.ImportPath}}' ./pkg
+stdout $PWD${/}pkg': example/subdir/pkg$'
+
+rm go.mod  # expose ../go.mod
+
+go list -f '{{.Dir}}: {{.ImportPath}}' ./pkg
+stdout $PWD${/}pkg': example/subdir/pkg$'
+
+-- subdir/pkg/pkg.go --
+package pkg
diff --git a/src/cmd/go/testdata/script/ldflag.txt b/src/cmd/go/testdata/script/ldflag.txt
new file mode 100644
index 0000000..6ceb33b
--- /dev/null
+++ b/src/cmd/go/testdata/script/ldflag.txt
@@ -0,0 +1,44 @@
+# Issue #42565
+
+[!cgo] skip
+
+# We can't build package bad, which uses #cgo LDFLAGS.
+cd bad
+! go build
+stderr no-such-warning
+
+# We can build package ok with the same flags in CGO_LDFLAGS.
+env CGO_LDFLAGS=-Wno-such-warning -Wno-unknown-warning-option
+cd ../ok
+go build
+
+# Build a main program that actually uses LDFLAGS.
+cd ..
+go build -ldflags=-v
+
+# Because we passed -v the Go linker should print the external linker
+# command which should include the flag we passed in CGO_LDFLAGS.
+stderr no-such-warning
+
+-- go.mod --
+module ldflag
+
+-- bad/bad.go --
+package bad
+
+// #cgo LDFLAGS: -Wno-such-warning -Wno-unknown-warning
+import "C"
+
+func F() {}
+-- ok/ok.go --
+package ok
+
+import "C"
+
+func F() {}
+-- main.go --
+package main
+
+import _ "ldflag/ok"
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/link_external_undef.txt b/src/cmd/go/testdata/script/link_external_undef.txt
new file mode 100644
index 0000000..f320505
--- /dev/null
+++ b/src/cmd/go/testdata/script/link_external_undef.txt
@@ -0,0 +1,48 @@
+
+# Test case for issue 47993, in which the linker crashes
+# on a bad input instead of issuing an error and exiting.
+
+# This test requires external linking, so use cgo as a proxy 
+[!cgo] skip
+
+! go build -ldflags='-linkmode=external' .
+! stderr 'panic'
+stderr '^.*undefined symbol in relocation.*'
+
+-- go.mod --
+
+module issue47993
+
+go 1.16
+
+-- main.go --
+
+package main
+
+type M struct {
+	b bool
+}
+
+// Note the body-less func def here. This is what causes the problems.
+func (m *M) run(fp func())
+
+func doit(m *M) {
+        InAsm()
+	m.run(func() {
+	})
+}
+
+func main() {
+     m := &M{true}
+     doit(m)
+}
+
+func InAsm() 
+
+-- main.s --
+
+// Add an assembly function so as to leave open the possibility
+// that body-less functions in Go might be defined in assembly.
+
+// Currently we just need an empty file here.
+
diff --git a/src/cmd/go/testdata/script/link_matching_actionid.txt b/src/cmd/go/testdata/script/link_matching_actionid.txt
new file mode 100644
index 0000000..b8d423d
--- /dev/null
+++ b/src/cmd/go/testdata/script/link_matching_actionid.txt
@@ -0,0 +1,38 @@
+# Checks that an identical binary is built with -trimpath from the same
+# source files, with GOROOT in two different locations.
+# Verifies golang.org/issue/38989
+
+[short] skip
+[!symlink] skip
+
+# Symlink the compiler to a local path
+env GOROOT=$WORK/goroot1
+symlink $GOROOT -> $TESTGO_GOROOT
+
+# Set up fresh GOCACHE
+env GOCACHE=$WORK/gocache1
+mkdir $GOCACHE
+
+# Build a simple binary
+go build -o binary1 -trimpath -x main.go
+
+# Now repeat the same process with the compiler at a different local path
+env GOROOT=$WORK/goroot2
+symlink $GOROOT -> $TESTGO_GOROOT
+
+env GOCACHE=$WORK/gocache2
+mkdir $GOCACHE
+
+go build -o binary2 -trimpath -x main.go
+
+# Check that the binaries match exactly
+go tool buildid binary1
+cp stdout buildid1
+go tool buildid binary2
+cp stdout buildid2
+cmp buildid1 buildid2
+
+
+-- main.go --
+package main
+func main() {}
diff --git a/src/cmd/go/testdata/script/link_syso_deps.txt b/src/cmd/go/testdata/script/link_syso_deps.txt
new file mode 100644
index 0000000..6bb9005
--- /dev/null
+++ b/src/cmd/go/testdata/script/link_syso_deps.txt
@@ -0,0 +1,55 @@
+# Test that syso in deps is available to cgo.
+
+[!compiler:gc] skip 'requires syso support'
+[!cgo] skip
+[short] skip 'invokes system C compiler'
+
+# External linking is not supported on linux/ppc64.
+# See: https://github.com/golang/go/issues/8912
+[GOOS:linux] [GOARCH:ppc64] skip
+
+cc -c -o syso/x.syso syso/x.c
+cc -c -o syso2/x.syso syso2/x.c
+go build m/cgo
+
+-- go.mod --
+module m
+
+go 1.18
+-- cgo/x.go --
+package cgo
+
+// extern void f(void);
+// extern void g(void);
+import "C"
+
+func F() {
+	C.f()
+}
+
+func G() {
+	C.g()
+}
+
+-- cgo/x2.go --
+package cgo
+
+import _ "m/syso"
+
+-- syso/x.c --
+//go:build ignore
+
+void f() {}
+
+-- syso/x.go --
+package syso
+
+import _ "m/syso2"
+
+-- syso2/x.c --
+//go:build ignore
+
+void g() {}
+
+-- syso2/x.go --
+package syso2
diff --git a/src/cmd/go/testdata/script/link_syso_issue33139.txt b/src/cmd/go/testdata/script/link_syso_issue33139.txt
new file mode 100644
index 0000000..b0d4a7c
--- /dev/null
+++ b/src/cmd/go/testdata/script/link_syso_issue33139.txt
@@ -0,0 +1,45 @@
+# Test that we can use the external linker with a host syso file that is
+# embedded in a package, that is referenced by a Go assembly function.
+# See issue 33139.
+
+[!compiler:gc] skip
+[!cgo] skip
+[short] skip 'invokes system C compiler'
+
+# External linking is not supported on linux/ppc64.
+# See: https://github.com/golang/go/issues/8912
+[GOOS:linux] [GOARCH:ppc64] skip
+
+cc -c -o syso/objTestImpl.syso syso/src/objTestImpl.c
+go build -ldflags='-linkmode=external' ./cmd/main.go
+
+-- go.mod --
+module m
+
+go 1.16
+-- syso/objTest.s --
+#include "textflag.h"
+
+TEXT ·ObjTest(SB), NOSPLIT, $0
+	// We do not actually execute this function in the test above, thus
+	// there is no stack frame setup here.
+	// We only care about Go build and/or link errors when referencing
+	// the objTestImpl symbol in the syso file.
+	JMP objTestImpl(SB)
+
+-- syso/pkg.go --
+package syso
+
+func ObjTest()
+
+-- syso/src/objTestImpl.c --
+void objTestImpl() { /* Empty */ }
+
+-- cmd/main.go --
+package main
+
+import "m/syso"
+
+func main() {
+	syso.ObjTest()
+}
diff --git a/src/cmd/go/testdata/script/linkname.txt b/src/cmd/go/testdata/script/linkname.txt
new file mode 100644
index 0000000..36ac8eb
--- /dev/null
+++ b/src/cmd/go/testdata/script/linkname.txt
@@ -0,0 +1,9 @@
+env GO111MODULE=off
+
+# check for linker name in error message about linker crash
+[!compiler:gc] skip
+! go build -ldflags=-crash_for_testing x.go
+stderr [\\/]tool[\\/].*[\\/]link
+
+-- x.go --
+package main; func main() {}
diff --git a/src/cmd/go/testdata/script/list_all_gobuild.txt b/src/cmd/go/testdata/script/list_all_gobuild.txt
new file mode 100644
index 0000000..92ad7d8
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_all_gobuild.txt
@@ -0,0 +1,42 @@
+# go list all should work with GOOS=linux because all packages build on Linux
+env GOOS=linux
+env GOARCH=amd64
+go list all
+
+# go list all should work with GOOS=darwin, but it used to fail because
+# in the absence of //go:build support, p looked like it needed q
+# (p_test.go was not properly excluded), and q was Linux-only.
+#
+# Also testing with r and s that +build lines keep working.
+env GOOS=darwin
+go list all
+
+-- go.mod --
+go 1.17
+module m
+
+-- p/p.go --
+package p
+
+-- p/p_test.go --
+//go:build linux
+
+package p
+
+import "m/q"
+
+-- q/q_linux.go --
+package q
+
+-- r/r.go --
+package r
+
+-- r/r_test.go --
+// +build linux
+
+package r
+
+import "m/s"
+
+-- s/s_linux.go --
+package s
diff --git a/src/cmd/go/testdata/script/list_ambiguous_path.txt b/src/cmd/go/testdata/script/list_ambiguous_path.txt
new file mode 100644
index 0000000..6f22314
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_ambiguous_path.txt
@@ -0,0 +1,38 @@
+# Ensures that we can correctly list package patterns ending in '.go'.
+# See golang.org/issue/34653.
+
+# A single pattern for a package ending in '.go'.
+go list ./foo.go
+stdout '^test/foo.go$'
+
+# Multiple patterns for packages including one ending in '.go'.
+go list ./bar ./foo.go
+stdout '^test/bar$'
+stdout '^test/foo.go$'
+
+# A single pattern for a Go file.
+go list ./a.go
+stdout '^command-line-arguments$'
+
+# A single typo-ed pattern for a Go file. This should
+# treat the wrong pattern as if it were a package.
+! go list ./foo.go/b.go
+stderr '^stat .*[/\\]foo\.go[/\\]b\.go: directory not found$'
+
+# Multiple patterns for Go files with a typo. This should
+# treat the wrong pattern as if it were a nonexistent file.
+! go list ./foo.go/a.go ./foo.go/b.go
+[GOOS:plan9] stderr 'stat ./foo.go/b.go: ''./foo.go/b.go'' does not exist'
+[GOOS:windows] stderr './foo.go/b.go: The system cannot find the file specified'
+[!GOOS:plan9] [!GOOS:windows] stderr './foo.go/b.go: no such file or directory'
+
+-- a.go --
+package main
+-- bar/a.go --
+package bar
+-- foo.go/a.go --
+package foo.go
+-- go.mod --
+module "test"
+
+go 1.13
diff --git a/src/cmd/go/testdata/script/list_bad_import.txt b/src/cmd/go/testdata/script/list_bad_import.txt
new file mode 100644
index 0000000..dbec350
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_bad_import.txt
@@ -0,0 +1,72 @@
+env GO111MODULE=off
+[short] skip
+
+# This test matches mod_list_bad_import, but in GOPATH mode.
+# Please keep them in sync.
+
+env GO111MODULE=off
+cd example.com
+
+# Without -e, listing an otherwise-valid package with an unsatisfied direct import should fail.
+# BUG: Today it succeeds.
+go list -f '{{if .Error}}error{{end}} {{if .Incomplete}}incomplete{{end}} {{range .DepsErrors}}bad dep: {{.Err}}{{end}}' example.com/direct
+! stdout ^error
+stdout 'incomplete'
+stdout 'bad dep: .*example.com[/\\]notfound'
+
+# Listing with -deps should also fail.
+! go list -deps example.com/direct
+stderr example.com[/\\]notfound
+
+# But -e -deps should succeed.
+go list -e -deps example.com/direct
+stdout example.com/notfound
+
+
+# Listing an otherwise-valid package that imports some *other* package with an
+# unsatisfied import should also fail.
+# BUG: Today, it succeeds.
+go list -f '{{if .Error}}error{{end}} {{if .Incomplete}}incomplete{{end}} {{range .DepsErrors}}bad dep: {{.Err}}{{end}}' example.com/indirect
+! stdout ^error
+stdout incomplete
+stdout 'bad dep: .*example.com[/\\]notfound'
+
+# Again, -deps should fail.
+! go list -deps example.com/indirect
+stderr example.com[/\\]notfound
+
+# But -deps -e should succeed.
+go list -e -deps example.com/indirect
+stdout example.com/notfound
+
+
+# Listing the missing dependency directly should fail outright...
+! go list -f '{{if .Error}}error{{end}} {{if .Incomplete}}incomplete{{end}}' example.com/notfound
+stderr 'no Go files in .*example.com[/\\]notfound'
+! stdout error
+! stdout incomplete
+
+# ...but listing with -e should succeed.
+go list -e -f '{{if .Error}}error{{end}} {{if .Incomplete}}incomplete{{end}}' example.com/notfound
+stdout error
+stdout incomplete
+
+
+# The pattern "all" should match only packages that actually exist,
+# ignoring those whose existence is merely implied by imports.
+go list -e -f '{{.ImportPath}}' all
+stdout example.com/direct
+stdout example.com/indirect
+! stdout example.com/notfound
+
+
+-- example.com/direct/direct.go --
+package direct
+import _ "example.com/notfound"
+
+-- example.com/indirect/indirect.go --
+package indirect
+import _ "example.com/direct"
+
+-- example.com/notfound/README --
+This directory intentionally left blank.
diff --git a/src/cmd/go/testdata/script/list_case_collision.txt b/src/cmd/go/testdata/script/list_case_collision.txt
new file mode 100644
index 0000000..181a202
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_case_collision.txt
@@ -0,0 +1,41 @@
+# Tests golang.org/issue/4773
+
+go list -json example/a
+stdout 'case-insensitive import collision'
+
+! go build example/a
+stderr 'case-insensitive import collision'
+
+# List files explicitly on command line, to encounter case-checking
+# logic even on case-insensitive filesystems.
+cp b/file.go b/FILE.go  # no-op on case-insensitive filesystems
+! go list b/file.go b/FILE.go
+stderr 'case-insensitive file name collision'
+
+mkdir a/Pkg  # no-op on case-insensitive filesystems
+cp a/pkg/pkg.go a/Pkg/pkg.go  # no-op on case-insensitive filesystems
+! go list example/a/pkg example/a/Pkg
+
+# Test that the path reported with an indirect import is correct.
+cp b/file.go b/FILE.go
+[case-sensitive] ! go build example/c
+[case-sensitive] stderr '^package example/c\n\timports example/b: case-insensitive file name collision: "FILE.go" and "file.go"$'
+
+-- go.mod --
+module example
+
+go 1.16
+-- a/a.go --
+package p
+import (
+	_ "example/a/pkg"
+	_ "example/a/Pkg"
+)
+-- a/pkg/pkg.go --
+package pkg
+-- b/file.go --
+package b
+-- c/c.go --
+package c
+
+import _ "example/b"
diff --git a/src/cmd/go/testdata/script/list_cgo_compiled_importmap.txt b/src/cmd/go/testdata/script/list_cgo_compiled_importmap.txt
new file mode 100644
index 0000000..8693339
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_cgo_compiled_importmap.txt
@@ -0,0 +1,41 @@
+# Regression test for https://golang.org/issue/46462.
+#
+# The "runtime/cgo" import found in synthesized .go files (reported in
+# the CompiledGoFiles field) should have a corresponding entry in the
+# ImportMap field when a runtime/cgo variant (such as a test variant)
+# will be used.
+
+[short] skip  # -compiled can be slow (because it compiles things)
+[!cgo] skip
+[GOOS:darwin] skip # net package does not import "C" on Darwin
+[GOOS:windows] skip # net package does not import "C" on Windows
+[GOOS:plan9] skip # net package does not import "C" on Plan 9
+
+env CGO_ENABLED=1
+env GOFLAGS=-tags=netcgo  # Force net to use cgo
+
+
+# "runtime/cgo [runtime.test]" appears in the test dependencies of "runtime",
+# because "runtime/cgo" itself depends on "runtime"
+
+go list -deps -test -compiled -f '{{if eq .ImportPath "net [runtime.test]"}}{{printf "%q" .Imports}}{{end}}' runtime
+
+	# Control case: the explicitly-imported package "sync" is a test variant,
+	# because "sync" depends on "runtime".
+stdout '"sync \[runtime\.test\]"'
+! stdout '"sync"'
+
+	# Experiment: the implicitly-imported package "runtime/cgo" is also a test variant,
+	# because "runtime/cgo" also depends on "runtime".
+stdout '"runtime/cgo \[runtime\.test\]"'
+! stdout '"runtime/cgo"'
+
+
+# Because the import of "runtime/cgo" in the cgo-generated file actually refers
+# to "runtime/cgo [runtime.test]", the latter should be listed in the ImportMap.
+# BUG(#46462): Today, it is not.
+
+go list -deps -test -compiled -f '{{if eq .ImportPath "net [runtime.test]"}}{{printf "%q" .ImportMap}}{{end}}' runtime
+
+stdout '"sync":"sync \[runtime\.test\]"'                # control
+stdout '"runtime/cgo":"runtime/cgo \[runtime\.test\]"'  # experiment
diff --git a/src/cmd/go/testdata/script/list_compiled_files_issue28749.txt b/src/cmd/go/testdata/script/list_compiled_files_issue28749.txt
new file mode 100644
index 0000000..e0fb977
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_compiled_files_issue28749.txt
@@ -0,0 +1,10 @@
+go list -compiled -f {{.CompiledGoFiles}} .
+! stdout 'foo.s'
+
+-- go.mod --
+module example.com/foo
+
+go 1.20
+-- foo.go --
+package foo
+-- foo.s --
diff --git a/src/cmd/go/testdata/script/list_compiled_imports.txt b/src/cmd/go/testdata/script/list_compiled_imports.txt
new file mode 100644
index 0000000..7780b07
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_compiled_imports.txt
@@ -0,0 +1,31 @@
+env GO111MODULE=off
+
+[!cgo] skip
+
+# go list should report import "C"
+cd x
+go list -f '{{.Imports}}'
+! stdout runtime/cgo
+! stdout unsafe
+! stdout syscall
+stdout C
+stdout unicode
+stdout unicode/utf16
+
+# go list -compiled should report imports in compiled files as well,
+# adding "runtime/cgo", "unsafe", and "syscall" but not dropping "C".
+go list -compiled -f '{{.Imports}}'
+stdout runtime/cgo
+stdout unsafe
+stdout syscall
+stdout C
+stdout unicode
+stdout unicode/utf16
+
+-- x/x.go --
+package x
+import "C"
+import "unicode" // does not use unsafe, syscall, runtime/cgo, unicode/utf16
+-- x/x1.go --
+package x
+import "unicode/utf16" // does not use unsafe, syscall, runtime/cgo, unicode
diff --git a/src/cmd/go/testdata/script/list_compiler_output.txt b/src/cmd/go/testdata/script/list_compiler_output.txt
new file mode 100644
index 0000000..5230bab
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_compiler_output.txt
@@ -0,0 +1,16 @@
+[short] skip
+
+go install -gcflags=-m .
+stderr 'can inline main'
+go list -gcflags=-m -f '{{.Stale}}' .
+stdout 'false'
+! stderr 'can inline main'
+
+-- go.mod --
+module example.com/foo
+
+go 1.20
+-- main.go --
+package main
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/list_constraints.txt b/src/cmd/go/testdata/script/list_constraints.txt
new file mode 100644
index 0000000..7115c36
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_constraints.txt
@@ -0,0 +1,86 @@
+# Check that files and their imports are not included in 'go list' output
+# when they are excluded by build constraints.
+
+# Linux and cgo files should be included when building in that configuration.
+env GOOS=linux
+env GOARCH=amd64
+env CGO_ENABLED=1
+go list -f '{{range .GoFiles}}{{.}} {{end}}'
+stdout '^cgotag.go empty.go suffix_linux.go tag.go $'
+go list -f '{{range .CgoFiles}}{{.}} {{end}}'
+stdout '^cgoimport.go $'
+go list -f '{{range .Imports}}{{.}} {{end}}'
+stdout '^C cgoimport cgotag suffix tag $'
+
+# Disabling cgo should exclude cgo files and their imports.
+env CGO_ENABLED=0
+go list -f '{{range .GoFiles}}{{.}} {{end}}'
+stdout 'empty.go suffix_linux.go tag.go'
+go list -f '{{range .CgoFiles}}{{.}} {{end}}'
+! stdout .
+go list -f '{{range .Imports}}{{.}} {{end}}'
+stdout '^suffix tag $'
+
+# Changing OS should exclude linux sources.
+env GOOS=darwin
+go list -f '{{range .GoFiles}}{{.}} {{end}}'
+stdout '^empty.go $'
+go list -f '{{range .Imports}}{{.}} {{end}}'
+stdout '^$'
+
+# Enabling a tag should include files that require it.
+go list -tags=extra -f '{{range .GoFiles}}{{.}} {{end}}'
+stdout '^empty.go extra.go $'
+go list -tags=extra -f '{{range .Imports}}{{.}} {{end}}'
+stdout '^extra $'
+
+# Packages that require a tag should not be listed unless the tag is on.
+! go list ./tagonly
+go list -tags=extra ./tagonly
+stdout m/tagonly
+
+-- go.mod --
+module m
+
+go 1.13
+
+-- empty.go --
+package p
+
+-- extra.go --
+// +build extra
+
+package p
+
+import _ "extra"
+
+-- suffix_linux.go --
+package p
+
+import _ "suffix"
+
+-- tag.go --
+// +build linux
+
+package p
+
+import _ "tag"
+
+-- cgotag.go --
+// +build cgo
+
+package p
+
+import _ "cgotag"
+
+-- cgoimport.go --
+package p
+
+import "C"
+
+import _ "cgoimport"
+
+-- tagonly/tagonly.go --
+// +build extra
+
+package tagonly
diff --git a/src/cmd/go/testdata/script/list_dedup_packages.txt b/src/cmd/go/testdata/script/list_dedup_packages.txt
new file mode 100644
index 0000000..30c68dd
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_dedup_packages.txt
@@ -0,0 +1,31 @@
+# Setup
+env GO111MODULE=off
+mkdir $WORK/tmp/testdata/src/xtestonly
+cp f.go $WORK/tmp/testdata/src/xtestonly/f.go
+cp f_test.go $WORK/tmp/testdata/src/xtestonly/f_test.go
+env GOPATH=$WORK/tmp/testdata
+cd $WORK
+
+# Check output of go list to ensure no duplicates
+go list xtestonly ./tmp/testdata/src/xtestonly/...
+cmp stdout $WORK/gopath/src/wantstdout
+
+-- wantstdout --
+xtestonly
+-- f.go --
+package xtestonly
+
+func F() int { return 42 }
+-- f_test.go --
+package xtestonly_test
+
+import (
+	"testing"
+	"xtestonly"
+)
+
+func TestF(t *testing.T) {
+	if x := xtestonly.F(); x != 42 {
+		t.Errorf("f.F() = %d, want 42", x)
+	}
+}
diff --git a/src/cmd/go/testdata/script/list_empty_import.txt b/src/cmd/go/testdata/script/list_empty_import.txt
new file mode 100644
index 0000000..4d76f09
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_empty_import.txt
@@ -0,0 +1,9 @@
+! go list a.go
+! stdout .
+stderr 'invalid import path'
+! stderr panic
+
+-- a.go --
+package a
+
+import ""
diff --git a/src/cmd/go/testdata/script/list_err_cycle.txt b/src/cmd/go/testdata/script/list_err_cycle.txt
new file mode 100644
index 0000000..44b82a6
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_err_cycle.txt
@@ -0,0 +1,15 @@
+# Check that we don't get infinite recursion when loading a package with
+# an import cycle and another error. Verifies #25830.
+! go list
+stderr 'found packages a \(a.go\) and b \(b.go\)'
+
+-- go.mod --
+module errcycle
+
+go 1.16
+-- a.go --
+package a
+
+import _ "errcycle"
+-- b.go --
+package b
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/list_err_stack.txt b/src/cmd/go/testdata/script/list_err_stack.txt
new file mode 100644
index 0000000..a7be9fd
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_err_stack.txt
@@ -0,0 +1,27 @@
+
+# golang.org/issue/40544: regression in error stacks for parse errors
+
+env GO111MODULE=off
+cd sandbox/foo
+go list -e -json .
+stdout '"sandbox/foo"'
+stdout '"sandbox/bar"'
+stdout '"Pos": "..(/|\\\\)bar(/|\\\\)bar.go:1:1"'
+stdout '"Err": "expected ''package'', found ackage"'
+
+env GO111MODULE=on
+go list -e -json .
+stdout '"sandbox/foo"'
+stdout '"sandbox/bar"'
+stdout '"Pos": "..(/|\\\\)bar(/|\\\\)bar.go:1:1"'
+stdout '"Err": "expected ''package'', found ackage"'
+
+-- sandbox/go.mod --
+module sandbox
+
+-- sandbox/foo/foo.go --
+package pkg
+
+import "sandbox/bar"
+-- sandbox/bar/bar.go --
+ackage bar
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/list_export_e.txt b/src/cmd/go/testdata/script/list_export_e.txt
new file mode 100644
index 0000000..8e4c361
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_export_e.txt
@@ -0,0 +1,26 @@
+! go list -export ./...
+stderr '^# example.com/p2\np2'${/}'main\.go:7:.*'
+! stderr '^go build '
+
+go list -f '{{with .Error}}{{.}}{{end}}' -e -export ./...
+! stderr '.'
+stdout '^# example.com/p2\np2'${/}'main\.go:7:.*'
+
+go list -e -export -json=Error ./...
+stdout '"Err": "# example.com/p2'
+
+-- go.mod --
+module example.com
+-- p1/p1.go --
+package p1
+
+const Name = "p1"
+-- p2/main.go --
+package main
+
+import "fmt"
+import "example.com/p1"
+
+func main() {
+	fmt.Println(p1.Name == 5)
+}
diff --git a/src/cmd/go/testdata/script/list_export_embed.txt b/src/cmd/go/testdata/script/list_export_embed.txt
new file mode 100644
index 0000000..da74998
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_export_embed.txt
@@ -0,0 +1,17 @@
+# Regression test for https://go.dev/issue/58885:
+# 'go list -json=Export' should not fail due to missing go:embed metadata.
+
+[short] skip 'runs the compiler to produce export data'
+
+go list -json=Export -export .
+
+-- go.mod --
+module example
+go 1.20
+-- example.go --
+package example
+
+import _ "embed"
+
+//go:embed example.go
+var src string
diff --git a/src/cmd/go/testdata/script/list_find.txt b/src/cmd/go/testdata/script/list_find.txt
new file mode 100644
index 0000000..d450fc9
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_find.txt
@@ -0,0 +1,22 @@
+env GO111MODULE=off
+
+# go list -find should not report imports
+
+go list -f {{.Incomplete}} x/y/z...  # should probably exit non-zero but never has
+stdout true
+go list -find -f '{{.Incomplete}} {{.Imports}}' x/y/z...
+stdout '^false \[\]'
+
+# go list -find -compiled should use cached sources the second time it's run.
+# It might not find the same cached sources as "go build", but the sources
+# should be identical. "go build" derives action IDs (which are used as cache
+# keys) from dependencies' action IDs. "go list -find" won't know what the
+# dependencies are, so it's can't construct the same action IDs.
+[short] skip
+go list -find -compiled net
+go list -find -compiled -x net
+! stderr 'cgo'
+
+-- x/y/z/z.go --
+package z
+import "does/not/exist"
diff --git a/src/cmd/go/testdata/script/list_find_nodeps.txt b/src/cmd/go/testdata/script/list_find_nodeps.txt
new file mode 100644
index 0000000..e08ce78
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_find_nodeps.txt
@@ -0,0 +1,49 @@
+# Issue #46092
+# go list -find should always return a package with an empty Deps list
+
+# The linker loads implicit dependencies
+go list -find -f {{.Deps}} ./cmd
+stdout '\[\]'
+
+# Cgo translation may add imports of "unsafe", "runtime/cgo" and "syscall"
+go list -find -f {{.Deps}} ./cgo
+stdout '\[\]'
+
+# SWIG adds imports of some standard packages
+go list -find -f {{.Deps}} ./swig
+stdout '\[\]'
+
+-- go.mod --
+module listfind
+
+-- cmd/main.go --
+package main
+
+func main() {}
+
+-- cgo/pkg.go --
+package cgopkg
+
+/*
+#include <limits.h>
+*/
+import "C"
+
+func F() {
+    println(C.INT_MAX)
+}
+
+-- cgo/pkg_notcgo.go --
+//go:build !cgo
+// +build !cgo
+
+package cgopkg
+
+func F() {
+    println(0)
+}
+
+-- swig/pkg.go --
+package swigpkg
+
+-- swig/a.swigcxx --
diff --git a/src/cmd/go/testdata/script/list_gofile_in_goroot.txt b/src/cmd/go/testdata/script/list_gofile_in_goroot.txt
new file mode 100644
index 0000000..b4ff3cb
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_gofile_in_goroot.txt
@@ -0,0 +1,76 @@
+# Return an error if the user tries to list a go source file directly in $GOROOT/src.
+# Tests golang.org/issue/36587
+
+env GOROOT=$WORK/goroot
+env GOPATH=$WORK/gopath
+
+go env GOROOT
+stdout $WORK[/\\]goroot
+
+# switch to GOROOT/src
+cd $GOROOT/src
+
+# In module mode, 'go list ./...' should not treat .go files in GOROOT/src as an
+# importable package, since that directory has no valid import path.
+env GO111MODULE=on
+go list ...
+stdout -count=1 '^.+$'
+stdout '^fmt$'
+! stdout foo
+
+go list ./...
+stdout -count=1 '^.+$'
+stdout '^fmt$'
+! stdout foo
+
+go list std
+stdout -count=1 '^.+$'
+stdout '^fmt$'
+
+! go list .
+stderr '^GOROOT/src is not an importable package$'
+
+# In GOPATH mode, 'go list ./...' should synthesize a legacy GOPATH-mode path —
+# not a standard-library or empty path — for the errant package.
+env GO111MODULE=off
+go list ./...
+stdout -count=2 '^.+$' # Both 'fmt' and GOROOT/src should be listed.
+stdout '^fmt$'
+[!GOOS:windows] stdout ^_$WORK/goroot/src$
+[GOOS:windows] stdout goroot/src$ # On windows the ":" in the volume name is mangled
+
+go list ...
+! stdout goroot/src
+
+go list std
+! stdout goroot/src
+
+go list .
+[!GOOS:windows] stdout ^_$WORK/goroot/src$
+[GOOS:windows] stdout goroot/src$
+
+# switch to GOPATH/src
+cd $GOPATH/src
+
+# GO111MODULE=off,GOPATH
+env GO111MODULE=off
+go list ./...
+[!GOOS:windows] stdout ^_$WORK/gopath/src$
+[GOOS:windows] stdout gopath/src$
+
+go list all
+! stdout gopath/src
+
+-- $WORK/goroot/src/go.mod --
+module std
+
+go 1.14
+-- $WORK/goroot/src/foo.go --
+package foo
+-- $WORK/goroot/src/fmt/fmt.go --
+package fmt
+-- $WORK/goroot/src/cmd/README --
+This directory must exist in order for the 'cmd' pattern to have something to
+match against.
+-- $GOPATH/src/foo.go --
+package foo
diff --git a/src/cmd/go/testdata/script/list_gomod_in_gopath.txt b/src/cmd/go/testdata/script/list_gomod_in_gopath.txt
new file mode 100644
index 0000000..064f33a
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_gomod_in_gopath.txt
@@ -0,0 +1,23 @@
+# Issue 46119
+
+# When a module is inside a GOPATH workspace, Package.Root should be set to
+# Module.Dir instead of $GOPATH/src.
+
+env GOPATH=$WORK/tmp
+cd $WORK/tmp/src/test
+
+go list -f {{.Root}}
+stdout ^$PWD$
+
+# Were we really inside a GOPATH workspace?
+env GO111MODULE=off
+go list -f {{.Root}}
+stdout ^$WORK/tmp$
+
+-- $WORK/tmp/src/test/go.mod --
+module test
+
+-- $WORK/tmp/src/test/main.go --
+package main
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/list_goroot_symlink.txt b/src/cmd/go/testdata/script/list_goroot_symlink.txt
new file mode 100644
index 0000000..041ae55
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_goroot_symlink.txt
@@ -0,0 +1,62 @@
+# Regression test for https://go.dev/issue/57754: 'go list' failed if ../src
+# relative to the location of the go executable was a symlink to the real src
+# directory. (cmd/go expects that ../src is GOROOT/src, but it appears that the
+# Debian build of the Go toolchain is attempting to split GOROOT into binary and
+# source artifacts in different parent directories.)
+
+[short] skip 'copies the cmd/go binary'
+[!symlink] skip 'tests symlink-specific behavior'
+[GOOS:darwin] skip 'Lstat on darwin does not conform to POSIX pathname resolution; see #59586'
+[GOOS:ios] skip 'Lstat on ios does not conform to POSIX pathname resolution; see #59586'
+
+# Ensure that the relative path to $WORK/lib/goroot/src from $PWD is a different
+# number of ".." hops than the relative path to it from $WORK/share/goroot/src.
+
+cd $WORK
+
+# Construct a fake GOROOT in $WORK/lib/goroot whose src directory is a symlink
+# to a subdirectory of $WORK/share. This mimics the directory structure reported
+# in https://go.dev/issue/57754.
+#
+# Symlink everything else to the original $GOROOT to avoid needless copying work.
+
+mkdir $WORK/lib/goroot
+mkdir $WORK/share/goroot
+symlink $WORK/share/goroot/src -> $GOROOT${/}src
+symlink $WORK/lib/goroot/src -> ../../share/goroot/src
+symlink $WORK/lib/goroot/pkg -> $GOROOT${/}pkg
+
+# Verify that our symlink shenanigans don't prevent cmd/go from finding its
+# GOROOT using os.Executable.
+#
+# To do so, we copy the actual cmd/go executable — which is implemented as the
+# cmd/go test binary instead of the original $GOROOT/bin/go, which may be
+# arbitrarily stale — into the bin subdirectory of the fake GOROOT, causing
+# os.Executable to report a path in that directory.
+
+mkdir $WORK/lib/goroot/bin
+cp $TESTGO_EXE $WORK/lib/goroot/bin/go$GOEXE
+
+env GOROOT=''  # Clear to force cmd/go to find GOROOT itself.
+exec $WORK/lib/goroot/bin/go env GOROOT
+stdout $WORK${/}lib${/}goroot
+
+# Now verify that 'go list' can find standard-library packages in the symlinked
+# source tree, with paths matching the one reported by 'go env GOROOT'.
+
+exec $WORK/lib/goroot/bin/go list -f '{{.ImportPath}}: {{.Dir}}' encoding/binary
+stdout '^encoding/binary: '$WORK${/}lib${/}goroot${/}src${/}encoding${/}binary'$'
+
+exec $WORK/lib/goroot/bin/go list -f '{{.ImportPath}}: {{.Dir}}' std
+stdout '^encoding/binary: '$WORK${/}lib${/}goroot${/}src${/}encoding${/}binary'$'
+
+# Most path lookups in GOROOT are not sensitive to symlinks. However, patterns
+# involving '...' wildcards must use Walk to check the GOROOT tree, which makes
+# them more sensitive to symlinks (because Walk doesn't follow them).
+#
+# So we check such a pattern to confirm that it works and reports a path relative
+# to $GOROOT/src (and not the symlink target).
+
+exec $WORK/lib/goroot/bin/go list -f '{{.ImportPath}}: {{.Dir}}' .../binary
+stdout '^encoding/binary: '$WORK${/}lib${/}goroot${/}src${/}encoding${/}binary'$'
+! stderr .
diff --git a/src/cmd/go/testdata/script/list_import_cycle_deps_errors.txt b/src/cmd/go/testdata/script/list_import_cycle_deps_errors.txt
new file mode 100644
index 0000000..e2c5cf9
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_import_cycle_deps_errors.txt
@@ -0,0 +1,75 @@
+go list -e -deps -json=ImportPath,Error,DepsErrors m/a
+cmp stdout want
+
+-- want --
+{
+	"ImportPath": "m/c",
+	"DepsErrors": [
+		{
+			"ImportStack": [
+				"m/a",
+				"m/b",
+				"m/c",
+				"m/a"
+			],
+			"Pos": "",
+			"Err": "import cycle not allowed"
+		}
+	]
+}
+{
+	"ImportPath": "m/b",
+	"DepsErrors": [
+		{
+			"ImportStack": [
+				"m/a",
+				"m/b",
+				"m/c",
+				"m/a"
+			],
+			"Pos": "",
+			"Err": "import cycle not allowed"
+		}
+	]
+}
+{
+	"ImportPath": "m/a",
+	"Error": {
+		"ImportStack": [
+			"m/a",
+			"m/b",
+			"m/c",
+			"m/a"
+		],
+		"Pos": "",
+		"Err": "import cycle not allowed"
+	},
+	"DepsErrors": [
+		{
+			"ImportStack": [
+				"m/a",
+				"m/b",
+				"m/c",
+				"m/a"
+			],
+			"Pos": "",
+			"Err": "import cycle not allowed"
+		}
+	]
+}
+-- go.mod --
+module m
+
+go 1.21
+-- a/a.go --
+package a
+
+import _ "m/b"
+-- b/b.go --
+package b
+
+import _ "m/c"
+-- c/c.go --
+package c
+
+import _ "m/a"
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/list_import_err.txt b/src/cmd/go/testdata/script/list_import_err.txt
new file mode 100644
index 0000000..c2b7d7c
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_import_err.txt
@@ -0,0 +1,22 @@
+# Test that errors importing packages are reported on the importing package,
+# not the imported package.
+
+env GO111MODULE=off # simplify vendor layout for test
+
+go list -e -deps -f '{{.ImportPath}}: {{.Error}}' ./importvendor
+stdout 'importvendor: importvendor[\\/]p.go:2:8: vendor/p must be imported as p'
+stdout 'vendor/p: <nil>'
+
+go list -e -deps -f '{{.ImportPath}}: {{.Error}}' ./importinternal
+stdout 'importinternal: package importinternal\n\timportinternal[\\/]p.go:2:8: use of internal package other/internal/p not allowed'
+stdout 'other/internal/p: <nil>'
+-- importvendor/p.go --
+package importvendor
+import "vendor/p"
+-- importinternal/p.go --
+package importinternal
+import "other/internal/p"
+-- other/internal/p/p.go --
+package p
+-- vendor/p/p.go --
+package p
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/list_importmap.txt b/src/cmd/go/testdata/script/list_importmap.txt
new file mode 100644
index 0000000..3c59cfd
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_importmap.txt
@@ -0,0 +1,27 @@
+env GO111MODULE=off
+
+# gccgo does not have standard packages.
+[compiler:gccgo] skip
+
+# fmt should have no rewritten imports.
+# The import from a/b should map c/d to a's vendor directory.
+go list -f '{{.ImportPath}}: {{.ImportMap}}' fmt a/b
+stdout 'fmt: map\[\]'
+stdout 'a/b: map\[c/d:a/vendor/c/d\]'
+
+# flag [fmt.test] should import fmt [fmt.test] as fmt
+# fmt.test should import testing [fmt.test] as testing
+# fmt.test should not import a modified os
+go list -deps -test -f '{{.ImportPath}} MAP: {{.ImportMap}}{{"\n"}}{{.ImportPath}} IMPORT: {{.Imports}}' fmt
+stdout '^flag \[fmt\.test\] MAP: map\[fmt:fmt \[fmt\.test\]\]'
+stdout '^fmt\.test MAP: map\[(.* )?testing:testing \[fmt\.test\]'
+! stdout '^fmt\.test MAP: map\[(.* )?os:'
+stdout '^fmt\.test IMPORT: \[fmt \[fmt\.test\] fmt_test \[fmt\.test\] os reflect testing \[fmt\.test\] testing/internal/testdeps \[fmt\.test\]\]'
+
+
+-- a/b/b.go --
+package b
+
+import _ "c/d"
+-- a/vendor/c/d/d.go --
+package d
diff --git a/src/cmd/go/testdata/script/list_issue_56509.txt b/src/cmd/go/testdata/script/list_issue_56509.txt
new file mode 100644
index 0000000..b402b2b
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_issue_56509.txt
@@ -0,0 +1,35 @@
+# Test that a directory with an .s file that has a comment that can't
+# be parsed isn't matched as a go directory. (This was happening because
+# non-go files with unparsable comments were being added to InvalidGoFiles
+# leading the package matching code to think there were Go files in the
+# directory.)
+
+cd bar
+go list ./...
+! stdout .
+cd ..
+
+[short] skip
+
+# Test that an unparsable .s file is completely ignored when its name
+# has build tags that cause it to be filtered out, but produces an error
+# when it is included
+
+env GOARCH=arm64
+env GOOS=linux
+go build ./baz
+
+env GOARCH=amd64
+env GOOS=linux
+! go build ./baz
+
+-- go.mod --
+module example.com/foo
+
+go 1.20
+-- bar/bar.s --
+;/
+-- baz/baz.go --
+package bar
+-- baz/baz_amd64.s --
+;/
diff --git a/src/cmd/go/testdata/script/list_issue_59905.txt b/src/cmd/go/testdata/script/list_issue_59905.txt
new file mode 100644
index 0000000..48c40d0
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_issue_59905.txt
@@ -0,0 +1,88 @@
+# Expect no panic
+go list -f '{{if .DepsErrors}}{{.DepsErrors}}{{end}}' -export -e -deps
+cmpenv stdout wanterr_59905
+
+# Expect no panic (Issue 61816)
+cp level1b_61816.txt level1b/pkg.go
+go list -f '{{if .DepsErrors}}{{.DepsErrors}}{{end}}' -export -e -deps
+cmpenv stdout wanterr_61816
+
+-- wanterr_59905 --
+[# test/main/level1a
+level1a${/}pkg.go:5:2: level2x redeclared in this block
+	level1a${/}pkg.go:4:2: other declaration of level2x
+level1a${/}pkg.go:5:2: "test/main/level1a/level2y" imported as level2x and not used
+level1a${/}pkg.go:8:39: undefined: level2y
+ # test/main/level1b
+level1b${/}pkg.go:5:2: level2x redeclared in this block
+	level1b${/}pkg.go:4:2: other declaration of level2x
+level1b${/}pkg.go:5:2: "test/main/level1b/level2y" imported as level2x and not used
+level1b${/}pkg.go:8:39: undefined: level2y
+]
+-- wanterr_61816 --
+[level1b${/}pkg.go:4:2: package foo is not in std ($GOROOT${/}src${/}foo)]
+[# test/main/level1a
+level1a${/}pkg.go:5:2: level2x redeclared in this block
+	level1a${/}pkg.go:4:2: other declaration of level2x
+level1a${/}pkg.go:5:2: "test/main/level1a/level2y" imported as level2x and not used
+level1a${/}pkg.go:8:39: undefined: level2y
+ level1b${/}pkg.go:4:2: package foo is not in std ($GOROOT${/}src${/}foo)]
+-- level1b_61816.txt --
+package level1b
+
+import (
+	"foo"
+)
+
+func Print() { println(level2x.Value, level2y.Value) }
+
+-- go.mod --
+module test/main
+
+go 1.20
+-- main.go --
+package main
+
+import (
+	"test/main/level1a"
+	"test/main/level1b"
+)
+
+func main() {
+	level1a.Print()
+	level1b.Print()
+}
+-- level1a/pkg.go --
+package level1a
+
+import (
+	"test/main/level1a/level2x"
+	"test/main/level1a/level2y"
+)
+
+func Print() { println(level2x.Value, level2y.Value) }
+-- level1a/level2x/pkg.go --
+package level2x
+
+var Value = "1a/2x"
+-- level1a/level2y/pkg.go --
+package level2x
+
+var Value = "1a/2y"
+-- level1b/pkg.go --
+package level1b
+
+import (
+	"test/main/level1b/level2x"
+	"test/main/level1b/level2y"
+)
+
+func Print() { println(level2x.Value, level2y.Value) }
+-- level1b/level2x/pkg.go --
+package level2x
+
+var Value = "1b/2x"
+-- level1b/level2y/pkg.go --
+package level2x
+
+var Value = "1b/2y"
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/list_json_fields.txt b/src/cmd/go/testdata/script/list_json_fields.txt
new file mode 100644
index 0000000..7e008ea
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_json_fields.txt
@@ -0,0 +1,113 @@
+# Test using -json flag to specify specific fields.
+
+# Test -json produces "full" output by looking for multiple fields present.
+go list -json .
+stdout '"Name": "a"'
+stdout '"Stale": true'
+# Same thing for -json=true
+go list -json=true .
+stdout '"Name": "a"'
+stdout '"Stale": true'
+
+# Test -json=false produces non-json output.
+go list -json=false
+cmp stdout want-non-json.txt
+
+# Test -json=<field> keeps only that field.
+go list -json=Name
+cmp stdout want-json-name.txt
+
+# Test -json=<field> with multiple fields.
+go list -json=ImportPath,Name,GoFiles,Imports
+cmp stdout want-json-multiple.txt
+
+# Test -json=<field> with Deps outputs the Deps field.
+go list -json=Deps
+stdout '"Deps": \['
+stdout '"errors",'
+
+# Test -json=<field> with *EmbedPatterns outputs embed patterns.
+cd embed
+go list -json=EmbedPatterns,TestEmbedPatterns,XTestEmbedPatterns
+stdout '"EmbedPatterns": \['
+stdout '"TestEmbedPatterns": \['
+stdout '"XTestEmbedPatterns": \['
+# Test -json=<field> with *EmbedFiles fails due to broken file reference.
+! go list -json=EmbedFiles
+stderr 'no matching files found'
+! go list -json=TestEmbedFiles
+stderr 'no matching files found'
+! go list -json=XTestEmbedFiles
+stderr 'no matching files found'
+cd ..
+
+[!git] skip
+
+# Test -json=<field> without Stale skips computing buildinfo
+cd repo
+exec git init
+# Control case: with -json=Stale cmd/go executes git status to compute buildinfo
+go list -json=Stale -x
+stderr 'git status'
+# Test case: without -json=Stale cmd/go skips git status
+go list -json=Name -x
+! stderr 'git status'
+
+-- go.mod --
+module example.com/a
+
+go 1.18
+-- a.go --
+package a
+
+import "fmt"
+
+func F() {
+    fmt.Println("hey there")
+}
+-- want-non-json.txt --
+example.com/a
+-- want-json-name.txt --
+{
+	"Name": "a"
+}
+-- want-json-multiple.txt --
+{
+	"ImportPath": "example.com/a",
+	"Name": "a",
+	"GoFiles": [
+		"a.go"
+	],
+	"Imports": [
+		"fmt"
+	]
+}
+-- repo/go.mod --
+module example.com/repo
+-- repo/main.go --
+package main
+
+func main() {}
+-- embed/go.mod --
+module example.com/embed
+-- embed/embed.go --
+package embed
+
+import _ "embed"
+
+//go:embed non-existing-file.txt
+var s string
+-- embed/embed_test.go --
+package embed
+
+import _ "embed"
+
+//go:embed non-existing-file.txt
+var s string
+-- embed/embed_xtest_test.go --
+package embed_test
+
+import _ "embed"
+
+//go:embed non-existing-file.txt
+var s string
diff --git a/src/cmd/go/testdata/script/list_json_with_f.txt b/src/cmd/go/testdata/script/list_json_with_f.txt
new file mode 100644
index 0000000..2011a6e
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_json_with_f.txt
@@ -0,0 +1,20 @@
+[short] skip
+
+# list -json should generate output on stdout
+go list -json ./...
+stdout .
+# list -f should generate output on stdout
+go list -f '{{.}}' ./...
+stdout .
+
+# test passing first -json then -f
+! go list -json -f '{{.}}' ./...
+stderr '^go list -f cannot be used with -json$'
+
+# test passing first -f then -json
+! go list -f '{{.}}' -json ./...
+stderr '^go list -f cannot be used with -json$'
+-- go.mod --
+module m
+-- list_test.go --
+package list_test
diff --git a/src/cmd/go/testdata/script/list_legacy_mod.txt b/src/cmd/go/testdata/script/list_legacy_mod.txt
new file mode 100644
index 0000000..ab901d7
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_legacy_mod.txt
@@ -0,0 +1,48 @@
+# In GOPATH mode, module legacy support does path rewriting very similar to vendoring.
+
+env GO111MODULE=off
+
+go list -f '{{range .Imports}}{{.}}{{"\n"}}{{end}}' old/p1
+stdout ^new/p1$
+
+go list -f '{{range .Imports}}{{.}}{{"\n"}}{{end}}' new/p1
+stdout ^new/p2$           # not new/v2/p2
+! stdout ^new/v2
+stdout ^new/sub/x/v1/y$   # not new/sub/v2/x/v1/y
+! stdout ^new/sub/v2
+stdout ^new/sub/inner/x # not new/sub/v2/inner/x
+
+go build old/p1 new/p1
+
+-- new/go.mod --
+module "new/v2"
+-- new/new.go --
+package new
+
+import _ "new/v2/p2"
+-- new/p1/p1.go --
+package p1
+
+import _ "old/p2"
+import _ "new/v2"
+import _ "new/v2/p2"
+import _ "new/sub/v2/x/v1/y" // v2 is module, v1 is directory in module
+import _ "new/sub/inner/x"   // new/sub/inner/go.mod overrides new/sub/go.mod
+-- new/p2/p2.go --
+package p2
+-- new/sub/go.mod --
+module new/sub/v2
+-- new/sub/inner/go.mod --
+module new/sub/inner
+-- new/sub/inner/x/x.go --
+package x
+-- new/sub/x/v1/y/y.go --
+package y
+-- old/p1/p1.go --
+package p1
+
+import _ "old/p2"
+import _ "new/p1"
+import _ "new"
+-- old/p2/p2.go --
+package p2
diff --git a/src/cmd/go/testdata/script/list_linkshared.txt b/src/cmd/go/testdata/script/list_linkshared.txt
new file mode 100644
index 0000000..baae1e2
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_linkshared.txt
@@ -0,0 +1,16 @@
+env GO111MODULE=on
+
+# golang.org/issue/35759: 'go list -linkshared'
+# panicked if invoked on a test-only package.
+
+[!buildmode:shared] skip
+
+go list -f '{{.ImportPath}}: {{.Target}} {{.Shlib}}' -linkshared .
+stdout '^example.com:  $'
+
+-- go.mod --
+module example.com
+
+go 1.14
+-- x.go --
+package x
diff --git a/src/cmd/go/testdata/script/list_load_err.txt b/src/cmd/go/testdata/script/list_load_err.txt
new file mode 100644
index 0000000..f1b9205
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_load_err.txt
@@ -0,0 +1,95 @@
+# go list -e -deps should list imports from any file it can read, even if
+# other files in the same package cause go/build.Import to return an error.
+# Verifies golang.org/issue/38568
+
+go list -e -deps ./scan
+stdout m/want
+
+go list -e -deps ./multi
+stdout m/want
+
+go list -e -deps ./constraint
+stdout m/want
+
+[cgo] go list -e -test -deps ./cgotest
+[cgo] stdout m/want
+
+[cgo] go list -e -deps ./cgoflag
+[cgo] stdout m/want
+
+
+# go list -e should include files with errors in GoFiles, TestGoFiles, and
+# other lists, assuming they match constraints.
+# Verifies golang.org/issue/39986
+go list -e -f '{{range .GoFiles}}{{.}},{{end}}' ./scan
+stdout '^good.go,scan.go,$'
+
+go list -e -f '{{range .GoFiles}}{{.}},{{end}}' ./multi
+stdout '^a.go,b.go,$'
+
+go list -e -f '{{range .GoFiles}}{{.}},{{end}}' ./constraint
+stdout '^good.go,$'
+go list -e -f '{{range .IgnoredGoFiles}}{{.}},{{end}}' ./constraint
+stdout '^constraint.go,$'
+
+[cgo] go list -e -f '{{range .XTestGoFiles}}{{.}},{{end}}' ./cgotest
+[cgo] stdout '^cgo_test.go,$'
+
+[cgo] go list -e -f '{{range .GoFiles}}{{.}},{{end}}' ./cgoflag
+[cgo] stdout '^cgoflag.go,$'
+
+-- go.mod --
+module m
+
+go 1.14
+
+-- want/want.go --
+package want
+
+-- scan/scan.go --
+// scan error
+ʕ◔ϖ◔ʔ
+
+-- scan/good.go --
+package scan
+
+import _ "m/want"
+
+-- multi/a.go --
+package a
+
+-- multi/b.go --
+package b
+
+import _ "m/want"
+
+-- constraint/constraint.go --
+// +build !!nope
+
+package constraint
+
+-- constraint/good.go --
+package constraint
+
+import _ "m/want"
+
+-- cgotest/cgo_test.go --
+package cgo_test
+
+// cgo is not allowed in tests.
+// See golang.org/issue/18647
+
+import "C"
+import (
+	"testing"
+	_ "m/want"
+)
+
+func Test(t *testing.T) {}
+
+-- cgoflag/cgoflag.go --
+package cgoflag
+
+// #cgo ʕ◔ϖ◔ʔ:
+
+import _ "m/want"
diff --git a/src/cmd/go/testdata/script/list_module_when_error.txt b/src/cmd/go/testdata/script/list_module_when_error.txt
new file mode 100644
index 0000000..844164c
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_module_when_error.txt
@@ -0,0 +1,19 @@
+# The Module field should be populated even if there is an error loading the package.
+
+env GO111MODULE=on
+
+go list -e -f {{.Module}}
+stdout '^mod.com$'
+
+-- go.mod --
+module mod.com
+
+go 1.16
+
+-- blah.go --
+package blah
+
+import _ "embed"
+
+//go:embed README.md
+var readme string
diff --git a/src/cmd/go/testdata/script/list_overlay.txt b/src/cmd/go/testdata/script/list_overlay.txt
new file mode 100644
index 0000000..1153975
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_overlay.txt
@@ -0,0 +1,63 @@
+# Test listing with overlays
+
+# Overlay in an existing directory
+go list -overlay overlay.json  -f '{{.GoFiles}}' .
+stdout '^\[f.go\]$'
+
+# Overlays in a non-existing directory
+go list -overlay overlay.json -f '{{.GoFiles}}' ./dir
+stdout '^\[g.go\]$'
+
+# Overlays in an existing directory with already existing files
+go list -overlay overlay.json -f '{{.GoFiles}}' ./dir2
+stdout '^\[h.go i.go\]$'
+
+# Overlay that removes a file from a directory
+! go list ./dir3 # contains a file without a package statement
+go list -overlay overlay.json -f '{{.GoFiles}}' ./dir3 # overlay removes that file
+
+# Walking through an overlay
+go list -overlay overlay.json ./...
+cmp stdout want-list.txt
+
+# TODO(#39958): assembly files, C files, files that require cgo preprocessing
+
+-- want-list.txt --
+m
+m/dir
+m/dir2
+m/dir3
+-- go.mod --
+// TODO(#39958): Support and test overlays including go.mod itself (especially if mod=readonly)
+module m
+
+go 1.16
+
+-- dir2/h.go --
+package dir2
+
+-- dir3/good.go --
+package dir3
+-- dir3/bad.go --
+// no package statement
+-- overlay.json --
+{
+    "Replace": {
+        "f.go": "overlay/f_go",
+        "dir/g.go": "overlay/dir_g_go",
+        "dir2/i.go": "overlay/dir2_i_go",
+        "dir3/bad.go": ""
+    }
+}
+-- overlay/f_go --
+package m
+
+func f() {
+}
+-- overlay/dir_g_go --
+package m
+
+func g() {
+}
+-- overlay/dir2_i_go --
+package dir2
diff --git a/src/cmd/go/testdata/script/list_parse_err.txt b/src/cmd/go/testdata/script/list_parse_err.txt
new file mode 100644
index 0000000..0a3eb02
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_parse_err.txt
@@ -0,0 +1,45 @@
+# 'go list' without -e should fail and print errors on stderr.
+! go list ./p
+stderr '^p[/\\]b.go:2:2: expected ''package'', found ''EOF''$'
+! go list -f '{{range .Imports}}{{.}} {{end}}' ./p
+stderr '^p[/\\]b.go:2:2: expected ''package'', found ''EOF''$'
+! go list -test ./t
+stderr '^go: can''t load test package: t[/\\]t_test.go:8:1: expected declaration, found ʕ'
+! go list -test -f '{{range .Imports}}{{.}} {{end}}' ./t
+stderr '^go: can''t load test package: t[/\\]t_test.go:8:1: expected declaration, found ʕ'
+
+# 'go list -e' should report imports, even if some files have parse errors
+# before the import block.
+go list -e -f '{{range .Imports}}{{.}} {{end}}' ./p
+stdout '^fmt '
+
+# 'go list' should report the position of the error if there's only one.
+go list -e -f '{{.Error.Pos}} => {{.Error.Err}}' ./p
+stdout 'b.go:[0-9:]+ => expected ''package'', found ''EOF'''
+
+# 'go test' should report the position of the error if there's only one.
+go list -e -test -f '{{if .Error}}{{.Error.Pos}} => {{.Error.Err}}{{end}}' ./t
+stdout 't_test.go:[0-9:]+ => expected declaration, found ʕ'
+
+-- go.mod --
+module m
+
+go 1.13
+
+-- p/a.go --
+package a
+
+import "fmt"
+
+-- p/b.go --
+// no package statement
+
+-- t/t_test.go --
+package t
+
+import "testing"
+
+func Test(t *testing.T) {}
+
+// scan error
+ʕ◔ϖ◔ʔ
diff --git a/src/cmd/go/testdata/script/list_perm.txt b/src/cmd/go/testdata/script/list_perm.txt
new file mode 100644
index 0000000..14d6f72
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_perm.txt
@@ -0,0 +1,83 @@
+env GO111MODULE=on
+
+# Establish baseline behavior, before mucking with file permissions.
+
+go list ./noread/...
+stdout '^example.com/noread$'
+
+go list example.com/noread/...
+stdout '^example.com/noread$'
+
+go list ./empty/...
+stderr 'matched no packages'
+
+# Make the directory ./noread unreadable, and verify that 'go list' reports an
+# explicit error for a pattern that should match it (rather than treating it as
+# equivalent to an empty directory).
+
+[root] stop # Root typically ignores file permissions.
+[GOOS:windows] skip # Does not have Unix-style directory permissions.
+[GOOS:plan9] skip   # Might not have Unix-style directory permissions.
+
+chmod 000 noread
+
+# Check explicit paths.
+
+! go list ./noread
+! stdout '^example.com/noread$'
+! stderr 'matched no packages'
+
+! go list example.com/noread
+! stdout '^example.com/noread$'
+! stderr 'matched no packages'
+
+# Check filesystem-relative patterns.
+
+! go list ./...
+! stdout '^example.com/noread$'
+! stderr 'matched no packages'
+stderr '^pattern ./...: '
+
+! go list ./noread/...
+! stdout '^example.com/noread$'
+! stderr 'matched no packages'
+stderr '^pattern ./noread/...: '
+
+
+# Check module-prefix patterns.
+
+! go list example.com/...
+! stdout '^example.com/noread$'
+! stderr 'matched no packages'
+stderr '^pattern example.com/...: '
+
+! go list example.com/noread/...
+! stdout '^example.com/noread$'
+! stderr 'matched no packages'
+stderr '^pattern example.com/noread/...: '
+
+
+[short] stop
+
+# Check global patterns, which should still
+# fail due to errors in the local module.
+
+! go list all
+! stdout '^example.com/noread$'
+! stderr 'matched no packages'
+stderr '^pattern all: '
+
+! go list ...
+! stdout '^example.com/noread$'
+! stderr 'matched no packages'
+stderr '^pattern ...: '
+
+
+-- go.mod --
+module example.com
+go 1.15
+-- noread/noread.go --
+// Package noread exists, but will be made unreadable.
+package noread
+-- empty/README.txt --
+This directory intentionally left empty.
diff --git a/src/cmd/go/testdata/script/list_pkgconfig_error.txt b/src/cmd/go/testdata/script/list_pkgconfig_error.txt
new file mode 100644
index 0000000..7d671a6
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_pkgconfig_error.txt
@@ -0,0 +1,16 @@
+[!cgo] skip 'test verifies cgo pkg-config errors'
+[!exec:pkg-config] skip 'test requires pkg-config tool'
+
+! go list -export .
+stderr '^go build example:\n# pkg-config (.*\n)+pkg-config: exit status \d+$'
+
+-- go.mod --
+module example
+go 1.20
+-- example.go --
+package example
+
+// #cgo pkg-config: libnot-a-valid-cgo-library
+import "C"
+
+package main() {}
diff --git a/src/cmd/go/testdata/script/list_replace_absolute_windows.txt b/src/cmd/go/testdata/script/list_replace_absolute_windows.txt
new file mode 100644
index 0000000..8b71921
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_replace_absolute_windows.txt
@@ -0,0 +1,38 @@
+# Test a replacement with an absolute path (so the path isn't
+# cleaned by having filepath.Abs called on it). This checks
+# whether the modindex logic cleans the modroot path before using
+# it.
+
+[!GOOS:windows] skip
+[short] skip
+
+go run print_go_mod.go # use this program to write a go.mod with an absolute path
+cp stdout go.mod
+
+go list -modfile=go.mod all
+-- print_go_mod.go --
+//go:build ignore
+package main
+
+import (
+    "fmt"
+    "os"
+)
+
+func main() {
+    work := os.Getenv("WORK")
+fmt.Printf(`module example.com/mod
+
+require b.com v0.0.0
+
+replace b.com => %s\gopath\src/modb
+`,  work)
+}
+-- a.go --
+package a
+
+import _ "b.com/b"
+-- modb/go.mod --
+module b.com
+-- modb/b/b.go --
+package b
diff --git a/src/cmd/go/testdata/script/list_reserved.txt b/src/cmd/go/testdata/script/list_reserved.txt
new file mode 100644
index 0000000..b9c5361
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_reserved.txt
@@ -0,0 +1,7 @@
+# https://golang.org/issue/37641: the paths "example" and "test" are reserved
+# for end users, and must never exist in the standard library.
+
+go list example/... test/...
+stderr 'go: warning: "example/..." matched no packages$'
+stderr 'go: warning: "test/..." matched no packages$'
+! stdout .
diff --git a/src/cmd/go/testdata/script/list_shadow.txt b/src/cmd/go/testdata/script/list_shadow.txt
new file mode 100644
index 0000000..660508d
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_shadow.txt
@@ -0,0 +1,25 @@
+env GO111MODULE=off
+env GOPATH=$WORK/gopath/src/shadow/root1${:}$WORK/gopath/src/shadow/root2
+
+# The math in root1 is not "math" because the standard math is.
+go list -f '({{.ImportPath}}) ({{.ConflictDir}})' ./shadow/root1/src/math
+stdout '^\(.*(\\|/)src(\\|/)shadow(\\|/)root1(\\|/)src(\\|/)math\) \('$GOROOT'(\\|/)?src(\\|/)math\)$'
+
+# The foo in root1 is "foo".
+go list -f '({{.ImportPath}}) ({{.ConflictDir}})' ./shadow/root1/src/foo
+stdout '^\(foo\) \(\)$'
+
+# The foo in root2 is not "foo" because the foo in root1 got there first.
+go list -f '({{.ImportPath}}) ({{.ConflictDir}})' ./shadow/root2/src/foo
+stdout '^\(.*gopath(\\|/)src(\\|/)shadow(\\|/)root2(\\|/)src(\\|/)foo\) \('$WORK'(\\|/)?gopath(\\|/)src(\\|/)shadow(\\|/)root1(\\|/)src(\\|/)foo\)$'
+
+# The error for go install should mention the conflicting directory.
+! go install -n ./shadow/root2/src/foo
+stderr 'go: no install location for '$WORK'(\\|/)?gopath(\\|/)src(\\|/)shadow(\\|/)root2(\\|/)src(\\|/)foo: hidden by '$WORK'(\\|/)?gopath(\\|/)src(\\|/)shadow(\\|/)root1(\\|/)src(\\|/)foo'
+
+-- shadow/root1/src/foo/foo.go --
+package foo
+-- shadow/root1/src/math/math.go --
+package math
+-- shadow/root2/src/foo/foo.go --
+package foo
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/list_split_main.txt b/src/cmd/go/testdata/script/list_split_main.txt
new file mode 100644
index 0000000..74e7d5d
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_split_main.txt
@@ -0,0 +1,25 @@
+# This test checks that a "main" package with an external test package
+# is recompiled only once.
+# Verifies golang.org/issue/34321.
+
+env GO111MODULE=off
+
+go list -e -test -deps -f '{{if not .Standard}}{{.ImportPath}}{{end}}' pkg
+cmp stdout want
+
+-- $GOPATH/src/pkg/pkg.go --
+package main
+
+func main() {}
+
+-- $GOPATH/src/pkg/pkg_test.go --
+package main
+
+import "testing"
+
+func Test(t *testing.T) {}
+
+-- want --
+pkg
+pkg [pkg.test]
+pkg.test
diff --git a/src/cmd/go/testdata/script/list_std.txt b/src/cmd/go/testdata/script/list_std.txt
new file mode 100644
index 0000000..64c4766
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_std.txt
@@ -0,0 +1,25 @@
+env GO111MODULE=off
+
+[!compiler:gc] skip
+[short] skip
+
+# Listing GOROOT should only find standard packages.
+cd $GOROOT/src
+go list -f '{{if not .Standard}}{{.ImportPath}}{{end}}' ./...
+! stdout .
+
+# Standard packages should include cmd, but not cmd/vendor.
+go list ./...
+stdout cmd/compile
+! stdout vendor/golang.org
+! stdout cmd/vendor
+
+# In GOPATH mode, packages vendored into GOROOT should be reported as standard.
+go list -f '{{if .Standard}}{{.ImportPath}}{{end}}' std cmd
+stdout golang.org/x/net/http2/hpack
+stdout cmd/vendor/golang\.org/x/arch/x86/x86asm
+
+# However, vendored packages should not match wildcard patterns beginning with cmd.
+go list cmd/...
+stdout cmd/compile
+! stdout cmd/vendor
diff --git a/src/cmd/go/testdata/script/list_std_vendor.txt b/src/cmd/go/testdata/script/list_std_vendor.txt
new file mode 100644
index 0000000..834babe
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_std_vendor.txt
@@ -0,0 +1,33 @@
+# https://golang.org/issue/44725: packages in std should have the same
+# dependencies regardless of whether they are listed from within or outside
+# GOROOT/src.
+
+# Control case: net, viewed from outside the 'std' module,
+# should depend on vendor/golang.org/… instead of golang.org/….
+
+go list -deps net
+stdout '^vendor/golang.org/x/net'
+! stdout '^golang.org/x/net'
+cp stdout $WORK/net-deps.txt
+
+
+# It should still report the same package dependencies when viewed from
+# within GOROOT/src.
+
+cd $GOROOT/src
+
+go list -deps net
+stdout '^vendor/golang.org/x/net'
+! stdout '^golang.org/x/net'
+cmp stdout $WORK/net-deps.txt
+
+
+# However, 'go mod' and 'go get' subcommands should report the original module
+# dependencies, not the vendored packages.
+
+[!net:golang.org] stop
+
+env GOPROXY=
+env GOWORK=off
+go mod why -m golang.org/x/net
+stdout '^# golang.org/x/net\nnet\ngolang.org/x/net'
diff --git a/src/cmd/go/testdata/script/list_swigcxx.txt b/src/cmd/go/testdata/script/list_swigcxx.txt
new file mode 100644
index 0000000..731c1e5
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_swigcxx.txt
@@ -0,0 +1,30 @@
+# go list should not report SWIG-generated C++ files in CompiledGoFiles.
+
+[!exec:swig] skip
+[!exec:g++] skip
+[!cgo] skip
+
+# CompiledGoFiles should contain 4 files:
+#  a.go
+#  _cgo_import.go [gc only]
+#  _cgo_gotypes.go
+#  a.cgo1.go
+#
+# These names we see here, other than a.go, will be from the build cache,
+# so we just count them.
+
+go list -f '{{.CompiledGoFiles}}' -compiled=true example/swig
+
+stdout a\.go
+[compiler:gc] stdout -count=3 $GOCACHE
+[compiler:gccgo] stdout -count=2 $GOCACHE
+
+-- go.mod --
+module example
+
+go 1.16
+
+-- swig/a.go --
+package swig
+
+-- swig/a.swigcxx --
diff --git a/src/cmd/go/testdata/script/list_symlink.txt b/src/cmd/go/testdata/script/list_symlink.txt
new file mode 100644
index 0000000..f74ca86
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_symlink.txt
@@ -0,0 +1,12 @@
+[!symlink] skip
+env GO111MODULE=off
+
+mkdir $WORK/tmp/src
+symlink $WORK/tmp/src/dir1 -> $WORK/tmp
+cp p.go $WORK/tmp/src/dir1/p.go
+env GOPATH=$WORK/tmp
+go list -f '{{.Root}}' dir1
+stdout '^'$WORK/tmp'$'
+
+-- p.go --
+package p
diff --git a/src/cmd/go/testdata/script/list_symlink_dotdotdot.txt b/src/cmd/go/testdata/script/list_symlink_dotdotdot.txt
new file mode 100644
index 0000000..8df1982
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_symlink_dotdotdot.txt
@@ -0,0 +1,20 @@
+[!symlink] skip
+
+symlink $WORK/gopath/src/sym -> $WORK/gopath/src/tree
+symlink $WORK/gopath/src/tree/squirrel -> $WORK/gopath/src/dir2 # this symlink should not be followed
+cd sym
+go list ./...
+cmp stdout $WORK/gopath/src/want_list.txt
+-- tree/go.mod --
+module example.com/tree
+
+go 1.20
+-- tree/tree.go --
+package tree
+-- tree/branch/branch.go --
+package branch
+-- dir2/squirrel.go --
+package squirrel
+-- want_list.txt --
+example.com/tree
+example.com/tree/branch
diff --git a/src/cmd/go/testdata/script/list_symlink_internal.txt b/src/cmd/go/testdata/script/list_symlink_internal.txt
new file mode 100644
index 0000000..f756a56
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_symlink_internal.txt
@@ -0,0 +1,27 @@
+[!symlink] skip
+env GO111MODULE=off
+
+mkdir $WORK/tmp/gopath/src/dir1/internal/v
+cp p.go $WORK/tmp/gopath/src/dir1/p.go
+cp v.go $WORK/tmp/gopath/src/dir1/internal/v/v.go
+symlink $WORK/tmp/symdir1 -> $WORK/tmp/gopath/src/dir1
+env GOPATH=$WORK/tmp/gopath
+cd $WORK/tmp/symdir1
+go list -f '{{.Root}}' .
+stdout '^'$WORK/tmp/gopath'$'
+
+# All of these should succeed, not die in internal-handling code.
+go run p.go &
+go build &
+go install &
+
+wait
+
+-- p.go --
+package main
+
+import _ `dir1/internal/v`
+
+func main() {}
+-- v.go --
+package v
diff --git a/src/cmd/go/testdata/script/list_symlink_issue35941.txt b/src/cmd/go/testdata/script/list_symlink_issue35941.txt
new file mode 100644
index 0000000..eb12bde
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_symlink_issue35941.txt
@@ -0,0 +1,18 @@
+[!symlink] skip
+env GO111MODULE=off
+
+# Issue 35941: suppress symlink warnings when running 'go list all'.
+symlink goproj/css -> $GOPATH/src/css
+
+go list all
+! stderr 'warning: ignoring symlink'
+
+# Show symlink warnings when patterns contain '...'.
+go list goproj/...
+stderr 'warning: ignoring symlink'
+
+-- goproj/a.go --
+package a
+
+-- css/index.css --
+body {}
diff --git a/src/cmd/go/testdata/script/list_symlink_vendor_issue14054.txt b/src/cmd/go/testdata/script/list_symlink_vendor_issue14054.txt
new file mode 100644
index 0000000..8e63a5a
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_symlink_vendor_issue14054.txt
@@ -0,0 +1,28 @@
+[!symlink] skip
+env GO111MODULE=off
+
+mkdir $WORK/tmp/gopath/src/dir1/vendor/v
+cp p.go $WORK/tmp/gopath/src/dir1/p.go
+cp v.go $WORK/tmp/gopath/src/dir1/vendor/v/v.go
+symlink $WORK/tmp/symdir1 -> $WORK/tmp/gopath/src/dir1
+env GOPATH=$WORK/tmp/gopath
+cd $WORK/tmp/symdir1
+
+go list -f '{{.Root}}' .
+stdout '^'$WORK/tmp/gopath'$'
+
+# All of these should succeed, not die in vendor-handling code.
+go run p.go &
+go build &
+go install &
+
+wait
+
+-- p.go --
+package main
+
+import _ `v`
+
+func main () {}
+-- v.go --
+package v
diff --git a/src/cmd/go/testdata/script/list_symlink_vendor_issue15201.txt b/src/cmd/go/testdata/script/list_symlink_vendor_issue15201.txt
new file mode 100644
index 0000000..19f2138
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_symlink_vendor_issue15201.txt
@@ -0,0 +1,21 @@
+[!symlink] skip
+env GO111MODULE=off
+
+mkdir $WORK/tmp/gopath/src/x/y/_vendor/src/x
+symlink $WORK/tmp/gopath/src/x/y/_vendor/src/x/y -> ../../..
+mkdir $WORK/tmp/gopath/src/x/y/_vendor/src/x/y/w
+cp w.go $WORK/tmp/gopath/src/x/y/w/w.go
+symlink $WORK/tmp/gopath/src/x/y/w/vendor -> ../_vendor/src
+mkdir $WORK/tmp/gopath/src/x/y/_vendor/src/x/y/z
+cp z.go $WORK/tmp/gopath/src/x/y/z/z.go
+
+env GOPATH=$WORK/tmp/gopath/src/x/y/_vendor${:}$WORK/tmp/gopath
+cd $WORK/tmp/gopath/src
+go list ./...
+
+-- w.go --
+package w
+
+import "x/y/z"
+-- z.go --
+package z
diff --git a/src/cmd/go/testdata/script/list_test_cycle.txt b/src/cmd/go/testdata/script/list_test_cycle.txt
new file mode 100644
index 0000000..67edf18
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_test_cycle.txt
@@ -0,0 +1,34 @@
+go list ./p
+stdout 'example/p'
+
+! go list -json=ImportPath -test ./p
+cmp stderr wanterr.txt
+
+! go list -json=ImportPath,Deps -test ./p
+cmp stderr wanterr.txt
+
+! go list -json=ImportPath,Deps -deps -test ./p
+cmp stderr wanterr.txt
+
+! go list -json=ImportPath -deps -test ./p
+cmp stderr wanterr.txt
+
+-- wanterr.txt --
+go: can't load test package: package example/p
+	imports example/q
+	imports example/r
+	imports example/p: import cycle not allowed in test
+-- go.mod --
+module example
+go 1.20
+-- p/p.go --
+package p
+-- p/p_test.go --
+package p
+import "example/q"
+-- q/q.go --
+package q
+import "example/r"
+-- r/r.go --
+package r
+import "example/p"
diff --git a/src/cmd/go/testdata/script/list_test_e.txt b/src/cmd/go/testdata/script/list_test_e.txt
new file mode 100644
index 0000000..263892b
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_test_e.txt
@@ -0,0 +1,11 @@
+env GO111MODULE=off
+
+# issue 25980: crash in go list -e -test
+go list -e -test -deps -f '{{.Error}}' p
+stdout '^p[/\\]d_test.go:2:8: cannot find package "d" in any of:'
+
+-- p/d.go --
+package d
+-- p/d_test.go --
+package d_test
+import _ "d"
diff --git a/src/cmd/go/testdata/script/list_test_err.txt b/src/cmd/go/testdata/script/list_test_err.txt
new file mode 100644
index 0000000..02bd6a1
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_test_err.txt
@@ -0,0 +1,118 @@
+env GO111MODULE=off
+
+# issue 28491: errors in test source files should not prevent
+# "go list -test" from returning useful information.
+
+# go list -e prints information for all test packages.
+# The syntax error is shown in the package error field.
+go list -e -test -deps -f '{{.ImportPath}} {{.Error | printf "%q"}}' syntaxerr
+stdout 'pkgdep <nil>'
+stdout 'testdep_a <nil>'
+stdout 'testdep_b <nil>'
+stdout 'syntaxerr <nil>'
+stdout 'syntaxerr \[syntaxerr.test\] <nil>'
+stdout 'syntaxerr_test \[syntaxerr.test\] <nil>'
+stdout 'syntaxerr\.test "[^"]*expected declaration'
+! stderr 'expected declaration'
+
+[short] stop
+
+go list -e -test -deps -f '{{.ImportPath}} {{.Error | printf "%q"}}' nameerr
+stdout 'pkgdep <nil>'
+stdout 'testdep_a <nil>'
+stdout 'testdep_b <nil>'
+stdout 'nameerr\.test "[^"]*wrong signature for TestBad'
+! stderr 'wrong signature for TestBad'
+
+# go list prints a useful error for generic test functions
+! go list -test -deps genericerr
+stderr 'wrong signature for TestGeneric, test functions cannot have type parameters'
+
+go list -e -test -deps -f '{{.ImportPath}} {{.Error | printf "%q"}}' cycleerr
+stdout 'cycleerr <nil>'
+stdout 'testdep_a <nil>'
+stdout 'testdep_cycle \[cycleerr.test\] <nil>'
+stdout 'cycleerr \[cycleerr.test\] "[^"]*import cycle not allowed in test'
+! stderr 'import cycle not allowed in test'
+
+-- syntaxerr/syntaxerr.go --
+package syntaxerr
+
+import _ "pkgdep"
+
+-- syntaxerr/syntaxerr_ie_test.go --
+package syntaxerr
+
+!!!syntax error
+
+-- syntaxerr/syntaxerr_xe_test.go --
+package syntaxerr_test
+
+!!!syntax error
+
+-- syntaxerr/syntaxerr_i_test.go --
+package syntaxerr
+
+import _ "testdep_a"
+
+-- syntaxerr/syntaxerr_x_test.go --
+package syntaxerr
+
+import _ "testdep_b"
+
+-- nameerr/nameerr.go --
+package nameerr
+
+import _ "pkgdep"
+
+-- nameerr/nameerr_i_test.go --
+package nameerr
+
+import (
+  _ "testdep_a"
+  "testing"
+)
+
+func TestBad(t *testing.B) {}
+
+-- nameerr/nameerr_x_test.go --
+package nameerr_test
+
+import (
+  _ "testdep_b"
+  "testing"
+)
+
+func TestBad(t *testing.B) {}
+
+-- genericerr/genericerr.go --
+package genericerr
+
+-- genericerr/genericerr_test.go --
+package genericerr
+
+import "testing"
+
+func TestGeneric[T any](t *testing.T) {}
+
+-- cycleerr/cycleerr_test.go --
+package cycleerr
+
+import (
+  _ "testdep_a"
+  _ "testdep_cycle"
+)
+
+-- pkgdep/pkgdep.go --
+package pkgdep
+
+-- testdep_a/testdep_a.go --
+package testdep_a
+
+-- testdep_b/testdep_b.go --
+package testdep_b
+
+-- testdep_cycle/testdep_cycle.go --
+package testdep_cycle
+
+import _ "cycleerr"
diff --git a/src/cmd/go/testdata/script/list_test_imports.txt b/src/cmd/go/testdata/script/list_test_imports.txt
new file mode 100644
index 0000000..0342eba
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_test_imports.txt
@@ -0,0 +1,21 @@
+env GO111MODULE=off
+
+# issue 26880: list with tests has wrong variant in imports
+go list -test -f '{{.ImportPath}}:{{with .Imports}} {{join . ", "}}{{end}}' a b
+cmp stdout imports.txt
+
+-- a/a.go --
+package a; import _ "b"
+-- b/b.go --
+package b
+-- b/b_test.go --
+package b
+-- b/b_x_test.go --
+package b_test; import _ "a"
+
+-- imports.txt --
+a: b
+b:
+b.test: b [b.test], b_test [b.test], os, reflect, testing, testing/internal/testdeps
+b [b.test]:
+b_test [b.test]: a [b.test]
diff --git a/src/cmd/go/testdata/script/list_test_non_go_files.txt b/src/cmd/go/testdata/script/list_test_non_go_files.txt
new file mode 100644
index 0000000..6b2b633
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_test_non_go_files.txt
@@ -0,0 +1,13 @@
+env GO111MODULE=off
+
+# issue 29899: handling files with non-Go extension
+go list -e -test -json -- c.c x.go
+stdout '"Err": "named files must be .go files: c.c"'
+
+! go list -test -json -- c.c x.go
+stderr '^named files must be \.go files: c\.c$'
+
+-- x.go --
+package main
+-- c.c --
+package c
diff --git a/src/cmd/go/testdata/script/list_test_simple.txt b/src/cmd/go/testdata/script/list_test_simple.txt
new file mode 100644
index 0000000..954897c
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_test_simple.txt
@@ -0,0 +1,67 @@
+[short] skip
+
+# Test
+go test -list=Test
+stdout TestSimple
+
+# Benchmark
+go test -list=Benchmark
+stdout BenchmarkSimple
+
+# Examples
+go test -list=Example
+stdout ExampleSimple
+stdout ExampleWithEmptyOutput
+
+-- go.mod --
+module m
+
+go 1.16
+-- bench_test.go --
+package testlist
+
+import (
+	"fmt"
+	"testing"
+)
+
+func BenchmarkSimplefunc(b *testing.B) {
+	b.StopTimer()
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		_ = fmt.Sprint("Test for bench")
+	}
+}
+-- example_test.go --
+package testlist
+
+import (
+	"fmt"
+)
+
+func ExampleSimple() {
+	fmt.Println("Test with Output.")
+
+	// Output: Test with Output.
+}
+
+func ExampleWithEmptyOutput() {
+	fmt.Println("")
+
+	// Output:
+}
+
+func ExampleNoOutput() {
+	_ = fmt.Sprint("Test with no output")
+}
+-- test_test.go --
+package testlist
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestSimple(t *testing.T) {
+	_ = fmt.Sprint("Test simple")
+}
diff --git a/src/cmd/go/testdata/script/list_wildcard_skip_nonmatching.txt b/src/cmd/go/testdata/script/list_wildcard_skip_nonmatching.txt
new file mode 100644
index 0000000..02b1088
--- /dev/null
+++ b/src/cmd/go/testdata/script/list_wildcard_skip_nonmatching.txt
@@ -0,0 +1,17 @@
+# Test that wildcards don't look in useless directories.
+
+# First make sure that badpkg fails the list of '...'.
+! go list ./...
+stderr badpkg
+
+# Check that the list of './goodpkg...' succeeds. That implies badpkg was skipped.
+go list ./goodpkg...
+
+-- go.mod --
+module m
+
+go 1.16
+-- goodpkg/x.go --
+package goodpkg
+-- badpkg/x.go --
+pkg badpkg
diff --git a/src/cmd/go/testdata/script/load_test_pkg_err.txt b/src/cmd/go/testdata/script/load_test_pkg_err.txt
new file mode 100644
index 0000000..d088ee4
--- /dev/null
+++ b/src/cmd/go/testdata/script/load_test_pkg_err.txt
@@ -0,0 +1,30 @@
+# Tests issue 37971. Check that tests are still loaded even when the package has an error.
+
+go list -e -test d
+cmp stdout want_stdout
+
+go list -e -test -deps d
+stdout golang.org/fake/d
+
+-- want_stdout --
+d
+d.test
+d_test [d.test]
+-- go.mod --
+module d
+
+go 1.16
+-- d.go --
+package d
+
+import "net/http"
+
+const d = http.MethodGet
+func Get() string { return d; }
+-- d2.go --
+-- d_test.go --
+package d_test
+
+import "testing"
+import "golang.org/fake/d"
+func TestD(t *testing.T) { d.Get(); }
diff --git a/src/cmd/go/testdata/script/mod_all.txt b/src/cmd/go/testdata/script/mod_all.txt
new file mode 100644
index 0000000..b71a920
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_all.txt
@@ -0,0 +1,531 @@
+# This test illustrates the relationship between the 'all' pattern and
+# the dependencies of the main module.
+
+# The package import graph used in this test looks like:
+#
+# main --------- a --------- b
+# |              |
+# |              a_test ---- c
+# |                          |
+# |                          c_test ---- d
+# |
+# main_test ---- t --------- u
+#                |
+#                t_test ---- w
+#                            |
+#                            w_test ---- x
+#
+# main/testonly_test ---- q --------- r
+#                         |
+#                         q_test ---- s
+#
+# And the module dependency graph looks like:
+#
+# main --- a.1 ---- b.1
+#  \   \    \
+#   \   \    c.1 -- d.1
+#    \   \
+#     \    t.1 ---- u.1
+#      \    \
+#       \    w.1 -- x.1
+#        \
+#          q.1 ---- r.1
+#           \
+#            s.1
+
+env PKGFMT='{{if .Module}}{{.ImportPath}}{{end}}'
+env MODFMT='{{.Path}}'
+
+
+# 'go list -deps' lists packages and tests in the main module,
+# along with their transitive dependencies.
+
+go list -f $PKGFMT -deps ./...
+stdout -count=4 '^.'
+stdout '^example.com/a$'
+stdout '^example.com/b$'
+stdout '^example.com/main$'
+stdout '^example.com/main/testonly'
+
+
+# 'go list -deps -test' lists transitive imports of tests and non-tests in the
+# main module.
+
+go list -f $PKGFMT -deps -test ./...
+stdout -count=13 '^.'
+stdout '^example.com/a$'
+stdout '^example.com/b$'
+stdout '^example.com/main$'
+stdout '^example.com/main.test$'
+stdout '^example.com/main \[example.com/main.test\]$'
+stdout '^example.com/main_test \[example.com/main.test\]$'
+stdout '^example.com/main/testonly$'
+stdout '^example.com/main/testonly.test$'
+stdout '^example.com/main/testonly_test \[example.com/main/testonly.test\]$'
+stdout '^example.com/q$'
+stdout '^example.com/r$'
+stdout '^example.com/t$'
+stdout '^example.com/u$'
+
+
+# 'go list all' lists the fixpoint of iterating 'go list -deps -test' starting
+# with the packages in the main module, then reducing to only the non-test
+# variants of those packages.
+
+go list -f $PKGFMT all
+stdout -count=13 '^.'
+stdout '^example.com/a$'
+stdout '^example.com/b$'
+stdout '^example.com/c$'
+stdout '^example.com/d$'
+stdout '^example.com/main$'
+stdout '^example.com/main/testonly$'
+stdout '^example.com/q$'
+stdout '^example.com/r$'
+stdout '^example.com/s$'
+stdout '^example.com/t$'
+stdout '^example.com/u$'
+stdout '^example.com/w$'
+stdout '^example.com/x$'
+
+
+# 'go list -test all' is equivalent to 'go list -test $(go list all)'
+# and both should include tests for every package in 'all'.
+
+go list -test -f $PKGFMT example.com/a example.com/b example.com/c example.com/d example.com/main example.com/main/testonly example.com/q example.com/r example.com/s example.com/t example.com/u example.com/w example.com/x
+cp stdout list-test-explicit.txt
+
+go list -test -f $PKGFMT all
+cmp stdout list-test-explicit.txt
+stdout -count=36 '^.'
+stdout '^example.com/a$'
+stdout '^example.com/b$'
+stdout '^example.com/c$'
+stdout '^example.com/d$'
+stdout '^example.com/main$'
+stdout '^example.com/main/testonly$'
+stdout '^example.com/q$'
+stdout '^example.com/r$'
+stdout '^example.com/s$'
+stdout '^example.com/t$'
+stdout '^example.com/u$'
+stdout '^example.com/w$'
+stdout '^example.com/x$'
+stdout '^example.com/a.test$'
+stdout '^example.com/a_test \[example.com/a.test\]$'
+stdout '^example.com/b.test$'
+stdout '^example.com/b_test \[example.com/b.test\]$'
+stdout '^example.com/c.test$'
+stdout '^example.com/c_test \[example.com/c.test\]$'
+stdout '^example.com/main.test$'
+stdout '^example.com/main \[example.com/main.test\]$'
+stdout '^example.com/main_test \[example.com/main.test\]$'
+stdout '^example.com/main/testonly.test$'
+stdout '^example.com/main/testonly_test \[example.com/main/testonly.test\]$'
+stdout '^example.com/q.test$'
+stdout '^example.com/q_test \[example.com/q.test\]$'
+stdout '^example.com/r.test$'
+stdout '^example.com/r_test \[example.com/r.test\]$'
+stdout '^example.com/s.test$'
+stdout '^example.com/s_test \[example.com/s.test\]$'
+stdout '^example.com/t.test$'
+stdout '^example.com/t_test \[example.com/t.test\]$'
+stdout '^example.com/u.test$'
+stdout '^example.com/u_test \[example.com/u.test\]$'
+stdout '^example.com/w.test$'
+stdout '^example.com/w_test \[example.com/w.test\]$'
+
+
+# 'go list -m all' covers the packages in 'go list -test -deps all'.
+
+go list -m -f $MODFMT all
+stdout -count=12 '^.'
+stdout '^example.com/a$'
+stdout '^example.com/b$'
+stdout '^example.com/c$'
+stdout '^example.com/d$'
+stdout '^example.com/main$'
+stdout '^example.com/q$'
+stdout '^example.com/r$'
+stdout '^example.com/s$'
+stdout '^example.com/t$'
+stdout '^example.com/u$'
+stdout '^example.com/w$'
+stdout '^example.com/x$'
+
+
+# 'go mod vendor' copies in only the packages transitively imported by the main
+# module, and omits their tests. As a result, the 'all' and '...' patterns
+# report fewer packages when using '-mod=vendor'.
+
+go mod vendor
+
+go list -f $PKGFMT -mod=vendor all
+stdout -count=8 '^.'
+stdout '^example.com/a$'
+stdout '^example.com/b$'
+stdout '^example.com/main$'
+stdout '^example.com/main/testonly$'
+stdout '^example.com/q$'
+stdout '^example.com/r$'
+stdout '^example.com/t$'
+stdout '^example.com/u$'
+
+go list -test -f $PKGFMT -mod=vendor all
+stdout -count=13 '^.'
+stdout '^example.com/a$'
+stdout '^example.com/b$'
+stdout '^example.com/main$'
+stdout '^example.com/main/testonly$'
+stdout '^example.com/q$'
+stdout '^example.com/r$'
+stdout '^example.com/t$'
+stdout '^example.com/u$'
+stdout '^example.com/main.test$'
+stdout '^example.com/main \[example.com/main.test\]$'
+stdout '^example.com/main_test \[example.com/main.test\]$'
+stdout '^example.com/main/testonly.test$'
+stdout '^example.com/main/testonly_test \[example.com/main/testonly.test\]$'
+
+rm vendor
+
+# Convert all modules to go 1.17 to enable lazy loading.
+go mod edit -go=1.17 a/go.mod
+go mod edit -go=1.17 b/go.mod
+go mod edit -go=1.17 c/go.mod
+go mod edit -go=1.17 d/go.mod
+go mod edit -go=1.17 q/go.mod
+go mod edit -go=1.17 r/go.mod
+go mod edit -go=1.17 s/go.mod
+go mod edit -go=1.17 t/go.mod
+go mod edit -go=1.17 u/go.mod
+go mod edit -go=1.17 w/go.mod
+go mod edit -go=1.17 x/go.mod
+go mod edit -go=1.17
+cmp go.mod go.mod.beforetidy
+go mod tidy
+cmp go.mod go.mod.aftertidy
+
+# With lazy loading, 'go list all' with neither -mod=vendor nor -test should
+# match -mod=vendor without -test in 1.15.
+
+go list -f $PKGFMT all
+stdout -count=8 '^.'
+stdout '^example.com/a$'
+stdout '^example.com/b$'
+stdout '^example.com/main$'
+stdout '^example.com/main/testonly$'
+stdout '^example.com/q$'
+stdout '^example.com/r$'
+stdout '^example.com/t$'
+stdout '^example.com/u$'
+
+# 'go list -test all' should expand that to include the test variants of the
+# packages in 'all', but not the dependencies of outside tests.
+
+go list -test -f $PKGFMT all
+stdout -count=25 '^.'
+stdout '^example.com/a$'
+stdout '^example.com/b$'
+stdout '^example.com/main$'
+stdout '^example.com/main/testonly$'
+stdout '^example.com/q$'
+stdout '^example.com/r$'
+stdout '^example.com/t$'
+stdout '^example.com/u$'
+stdout '^example.com/a.test$'
+stdout '^example.com/a_test \[example.com/a.test\]$'
+stdout '^example.com/b.test$'
+stdout '^example.com/b_test \[example.com/b.test\]$'
+stdout '^example.com/main.test$'
+stdout '^example.com/main \[example.com/main.test\]$'
+stdout '^example.com/main_test \[example.com/main.test\]$'
+stdout '^example.com/main/testonly.test$'
+stdout '^example.com/main/testonly_test \[example.com/main/testonly.test\]$'
+stdout '^example.com/q.test$'
+stdout '^example.com/q_test \[example.com/q.test\]$'
+stdout '^example.com/r.test$'
+stdout '^example.com/r_test \[example.com/r.test\]$'
+stdout '^example.com/t.test$'
+stdout '^example.com/t_test \[example.com/t.test\]$'
+stdout '^example.com/u.test$'
+stdout '^example.com/u_test \[example.com/u.test\]$'
+
+# 'go list -test -deps all' should include the dependencies of those tests,
+# but not the tests of the dependencies of outside tests.
+
+go list -test -deps -f $PKGFMT all
+stdout -count=28 '^.'
+stdout '^example.com/a$'
+stdout '^example.com/b$'
+stdout '^example.com/c$'
+stdout '^example.com/main$'
+stdout '^example.com/main/testonly$'
+stdout '^example.com/q$'
+stdout '^example.com/r$'
+stdout '^example.com/s$'
+stdout '^example.com/t$'
+stdout '^example.com/u$'
+stdout '^example.com/w$'
+stdout '^example.com/a.test$'
+stdout '^example.com/a_test \[example.com/a.test\]$'
+stdout '^example.com/b.test$'
+stdout '^example.com/b_test \[example.com/b.test\]$'
+stdout '^example.com/main.test$'
+stdout '^example.com/main \[example.com/main.test\]$'
+stdout '^example.com/main_test \[example.com/main.test\]$'
+stdout '^example.com/main/testonly.test$'
+stdout '^example.com/main/testonly_test \[example.com/main/testonly.test\]$'
+stdout '^example.com/q.test$'
+stdout '^example.com/q_test \[example.com/q.test\]$'
+stdout '^example.com/r.test$'
+stdout '^example.com/r_test \[example.com/r.test\]$'
+stdout '^example.com/t.test$'
+stdout '^example.com/t_test \[example.com/t.test\]$'
+stdout '^example.com/u.test$'
+stdout '^example.com/u_test \[example.com/u.test\]$'
+
+# 'go list -m all' should cover all of the modules providing packages in
+# 'go list -test -deps all', but should exclude modules d and x,
+# which are not relevant to the main module and are outside of the
+# lazy-loading horizon.
+
+go list -m -f $MODFMT all
+stdout -count=10 '^.'
+stdout '^example.com/a$'
+stdout '^example.com/b$'
+stdout '^example.com/c$'
+! stdout '^example.com/d$'
+stdout '^example.com/main$'
+stdout '^example.com/q$'
+stdout '^example.com/r$'
+stdout '^example.com/s$'
+stdout '^example.com/t$'
+stdout '^example.com/u$'
+stdout '^example.com/w$'
+! stdout '^example.com/x$'
+
+-- go.mod --
+module example.com/main
+
+// Note: this go.mod file initially specifies go 1.15,
+// but includes some redundant roots so that it
+// also already obeys the 1.17 lazy loading invariants.
+go 1.15
+
+require (
+	example.com/a v0.1.0
+	example.com/b v0.1.0 // indirect
+	example.com/q v0.1.0
+	example.com/r v0.1.0 // indirect
+	example.com/t v0.1.0
+	example.com/u v0.1.0 // indirect
+)
+
+replace (
+	example.com/a v0.1.0 => ./a
+	example.com/b v0.1.0 => ./b
+	example.com/c v0.1.0 => ./c
+	example.com/d v0.1.0 => ./d
+	example.com/q v0.1.0 => ./q
+	example.com/r v0.1.0 => ./r
+	example.com/s v0.1.0 => ./s
+	example.com/t v0.1.0 => ./t
+	example.com/u v0.1.0 => ./u
+	example.com/w v0.1.0 => ./w
+	example.com/x v0.1.0 => ./x
+)
+-- main.go --
+package main
+
+import _ "example.com/a"
+
+func main() {}
+-- main_test.go --
+package main_test
+
+import _ "example.com/t"
+-- testonly/testonly_test.go --
+package testonly_test
+
+import _ "example.com/q"
+-- a/go.mod --
+module example.com/a
+
+go 1.15
+
+require (
+	example.com/b v0.1.0
+	example.com/c v0.1.0
+)
+-- a/a.go --
+package a
+
+import _ "example.com/b"
+-- a/a_test.go --
+package a_test
+
+import _ "example.com/c"
+-- b/go.mod --
+module example.com/b
+
+go 1.15
+-- b/b.go --
+package b
+-- b/b_test.go --
+package b_test
+-- c/go.mod --
+module example.com/c
+
+go 1.15
+
+require example.com/d v0.1.0
+-- c/c.go --
+package c
+-- c/c_test.go --
+package c_test
+
+import _ "example.com/d"
+-- d/go.mod --
+module example.com/d
+
+go 1.15
+-- d/d.go --
+package d
+-- q/go.mod --
+module example.com/q
+
+go 1.15
+
+require (
+	example.com/r v0.1.0
+	example.com/s v0.1.0
+)
+-- q/q.go --
+package q
+import _ "example.com/r"
+-- q/q_test.go --
+package q_test
+import _ "example.com/s"
+-- r/go.mod --
+module example.com/r
+
+go 1.15
+-- r/r.go --
+package r
+-- r/r_test.go --
+package r_test
+-- s/go.mod --
+module example.com/s
+
+go 1.15
+-- s/s.go --
+package s
+-- s/s_test.go --
+package s_test
+-- t/go.mod --
+module example.com/t
+
+go 1.15
+
+require (
+	example.com/u v0.1.0
+	example.com/w v0.1.0
+)
+-- t/t.go --
+package t
+
+import _ "example.com/u"
+-- t/t_test.go --
+package t_test
+
+import _ "example.com/w"
+-- u/go.mod --
+module example.com/u
+
+go 1.15
+-- u/u.go --
+package u
+-- u/u_test.go --
+package u_test
+-- w/go.mod --
+module example.com/w
+
+go 1.15
+
+require example.com/x v0.1.0
+-- w/w.go --
+package w
+-- w/w_test.go --
+package w_test
+
+import _ "example.com/x"
+-- x/go.mod --
+module example.com/x
+
+go 1.15
+-- x/x.go --
+package x
+-- go.mod.beforetidy --
+module example.com/main
+
+// Note: this go.mod file initially specifies go 1.15,
+// but includes some redundant roots so that it
+// also already obeys the 1.17 lazy loading invariants.
+go 1.17
+
+require (
+	example.com/a v0.1.0
+	example.com/b v0.1.0 // indirect
+	example.com/q v0.1.0
+	example.com/r v0.1.0 // indirect
+	example.com/t v0.1.0
+	example.com/u v0.1.0 // indirect
+)
+
+replace (
+	example.com/a v0.1.0 => ./a
+	example.com/b v0.1.0 => ./b
+	example.com/c v0.1.0 => ./c
+	example.com/d v0.1.0 => ./d
+	example.com/q v0.1.0 => ./q
+	example.com/r v0.1.0 => ./r
+	example.com/s v0.1.0 => ./s
+	example.com/t v0.1.0 => ./t
+	example.com/u v0.1.0 => ./u
+	example.com/w v0.1.0 => ./w
+	example.com/x v0.1.0 => ./x
+)
+-- go.mod.aftertidy --
+module example.com/main
+
+// Note: this go.mod file initially specifies go 1.15,
+// but includes some redundant roots so that it
+// also already obeys the 1.17 lazy loading invariants.
+go 1.17
+
+require (
+	example.com/a v0.1.0
+	example.com/q v0.1.0
+	example.com/t v0.1.0
+)
+
+require (
+	example.com/b v0.1.0 // indirect
+	example.com/r v0.1.0 // indirect
+	example.com/u v0.1.0 // indirect
+)
+
+replace (
+	example.com/a v0.1.0 => ./a
+	example.com/b v0.1.0 => ./b
+	example.com/c v0.1.0 => ./c
+	example.com/d v0.1.0 => ./d
+	example.com/q v0.1.0 => ./q
+	example.com/r v0.1.0 => ./r
+	example.com/s v0.1.0 => ./s
+	example.com/t v0.1.0 => ./t
+	example.com/u v0.1.0 => ./u
+	example.com/w v0.1.0 => ./w
+	example.com/x v0.1.0 => ./x
+)
diff --git a/src/cmd/go/testdata/script/mod_alt_goroot.txt b/src/cmd/go/testdata/script/mod_alt_goroot.txt
new file mode 100644
index 0000000..32f94c5
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_alt_goroot.txt
@@ -0,0 +1,20 @@
+env GO111MODULE=on
+
+# If the working directory is a different GOROOT, then the 'std' module should be
+# treated as an ordinary module (with an ordinary module prefix).
+# It should not override packages in GOROOT, but should not fail the command.
+# See golang.org/issue/30756.
+go list -e -deps -f '{{.ImportPath}} {{.Dir}}' ./bytes
+stdout ^std/bytes.*$PWD[/\\]bytes
+stdout '^bytes/modified'
+
+-- go.mod --
+module std
+
+go 1.12
+-- bytes/bytes.go --
+package bytes
+
+import _"bytes/modified"
+-- bytes/modified/modified.go --
+package modified
diff --git a/src/cmd/go/testdata/script/mod_ambiguous_import.txt b/src/cmd/go/testdata/script/mod_ambiguous_import.txt
new file mode 100644
index 0000000..feaf5d2
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_ambiguous_import.txt
@@ -0,0 +1,48 @@
+env GO111MODULE=on
+
+cd $WORK
+
+# An import provided by two different modules should be flagged as an error.
+! go build ./importx
+stderr '^importx[/\\]importx.go:2:8: ambiguous import: found package example.com/a/x in multiple modules:\n\texample.com/a v0.1.0 \('$WORK'[/\\]a[/\\]x\)\n\texample.com/a/x v0.1.0 \('$WORK'[/\\]ax\)$'
+
+# However, it should not be an error if that import is unused.
+go build ./importy
+
+# An import provided by both the main module and the vendor directory
+# should be flagged as an error only when -mod=vendor is set.
+mkdir vendor/example.com/m/importy
+cp $WORK/importy/importy.go vendor/example.com/m/importy/importy.go
+go build example.com/m/importy
+! go build -mod=vendor example.com/m/importy
+stderr '^ambiguous import: found package example.com/m/importy in multiple directories:\n\t'$WORK'[/\\]importy\n\t'$WORK'[/\\]vendor[/\\]example.com[/\\]m[/\\]importy$'
+
+-- $WORK/go.mod --
+module example.com/m
+go 1.13
+require (
+	example.com/a v0.1.0
+	example.com/a/x v0.1.0
+)
+replace (
+	example.com/a v0.1.0 => ./a
+	example.com/a/x v0.1.0 => ./ax
+)
+-- $WORK/importx/importx.go --
+package importx
+import _ "example.com/a/x"
+-- $WORK/importy/importy.go --
+package importy
+import _ "example.com/a/y"
+-- $WORK/a/go.mod --
+module example.com/a
+go 1.14
+-- $WORK/a/x/x.go --
+package x
+-- $WORK/a/y/y.go --
+package y
+-- $WORK/ax/go.mod --
+module example.com/a/x
+go 1.14
+-- $WORK/ax/x.go --
+package x
diff --git a/src/cmd/go/testdata/script/mod_auth.txt b/src/cmd/go/testdata/script/mod_auth.txt
new file mode 100644
index 0000000..5e2b7a9
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_auth.txt
@@ -0,0 +1,35 @@
+[short] skip
+
+env GO111MODULE=on
+env GOPROXY=direct
+env GOSUMDB=off
+env GOVCS='*:off'
+
+# Without credentials, downloading a module from a path that requires HTTPS
+# basic auth should fail.
+env NETRC=$WORK/empty
+! go mod tidy
+stderr '^\tserver response: ACCESS DENIED, buddy$'
+stderr '^\tserver response: File\? What file\?$'
+
+# With credentials from a netrc file, it should succeed.
+env NETRC=$WORK/netrc
+go mod tidy
+go list all
+stdout vcs-test.golang.org/auth/or401
+stdout vcs-test.golang.org/auth/or404
+
+-- go.mod --
+module private.example.com
+-- main.go --
+package useprivate
+
+import (
+	_ "vcs-test.golang.org/auth/or401"
+	_ "vcs-test.golang.org/auth/or404"
+)
+-- $WORK/empty --
+-- $WORK/netrc --
+machine vcs-test.golang.org
+	login aladdin
+	password opensesame
diff --git a/src/cmd/go/testdata/script/mod_bad_domain.txt b/src/cmd/go/testdata/script/mod_bad_domain.txt
new file mode 100644
index 0000000..86aa96e
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_bad_domain.txt
@@ -0,0 +1,47 @@
+env GO111MODULE=on
+
+# explicit get should report errors about bad names
+! go get appengine
+stderr '^go: malformed module path "appengine": missing dot in first path element$'
+! go get x/y.z
+stderr 'malformed module path "x/y.z": missing dot in first path element'
+
+
+# 'go list -m' should report errors about module names, never GOROOT.
+! go list -m -versions appengine
+stderr 'malformed module path "appengine": missing dot in first path element'
+! go list -m -versions x/y.z
+stderr 'malformed module path "x/y.z": missing dot in first path element'
+
+
+# build should report all unsatisfied imports,
+# but should be more definitive about non-module import paths
+! go build ./useappengine
+stderr '^useappengine[/\\]x.go:2:8: cannot find package$'
+! go build ./usenonexistent
+stderr '^usenonexistent[/\\]x.go:2:8: no required module provides package nonexistent.rsc.io; to add it:\n\tgo get nonexistent.rsc.io$'
+
+
+# 'get -d' should be similarly definitive
+
+go get ./useappengine  # TODO(#41315): This should fail.
+ # stderr '^useappengine[/\\]x.go:2:8: cannot find package$'
+
+! go get  ./usenonexistent
+stderr '^go: x/usenonexistent imports\n\tnonexistent.rsc.io: cannot find module providing package nonexistent.rsc.io$'
+
+
+# go mod vendor and go mod tidy should ignore appengine imports.
+rm usenonexistent/x.go
+go mod tidy
+go mod vendor
+
+-- go.mod --
+module x
+
+-- useappengine/x.go --
+package useappengine
+import _ "appengine" // package does not exist
+-- usenonexistent/x.go --
+package usenonexistent
+import _ "nonexistent.rsc.io" // domain does not exist
diff --git a/src/cmd/go/testdata/script/mod_bad_filenames.txt b/src/cmd/go/testdata/script/mod_bad_filenames.txt
new file mode 100644
index 0000000..eb556f4
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_bad_filenames.txt
@@ -0,0 +1,11 @@
+env GO111MODULE=on
+
+! go get rsc.io/badfile1 rsc.io/badfile2 rsc.io/badfile3 rsc.io/badfile4 rsc.io/badfile5
+! stderr 'unzip.*badfile1'
+stderr 'unzip.*badfile2[\\/]@v[\\/]v1.0.0.zip:.*malformed file path "☺.go": invalid char ''☺'''
+stderr 'unzip.*badfile3[\\/]@v[\\/]v1.0.0.zip: rsc.io[\\/]badfile3@v1.0.0[\\/]x\?y.go: malformed file path "x\?y.go": invalid char ''\?'''
+stderr 'unzip.*badfile4[\\/]@v[\\/]v1.0.0.zip: rsc.io[\\/]badfile4@v1.0.0[\\/]x[\\/]y.go: case-insensitive file name collision: "x/Y.go" and "x/y.go"'
+stderr 'unzip.*badfile5[\\/]@v[\\/]v1.0.0.zip: rsc.io[\\/]badfile5@v1.0.0[\\/]x[\\/]Y[\\/]zz[\\/]ww.go: case-insensitive file name collision: "x/y" and "x/Y"'
+
+-- go.mod --
+module x
diff --git a/src/cmd/go/testdata/script/mod_build_info_err.txt b/src/cmd/go/testdata/script/mod_build_info_err.txt
new file mode 100644
index 0000000..8d7bd0c
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_build_info_err.txt
@@ -0,0 +1,33 @@
+# This test verifies that line numbers are included in module import errors.
+# Verifies golang.org/issue/34393.
+
+go list -e -mod=mod -deps -f '{{with .Error}}{{.Pos}}: {{.Err}}{{end}}' ./main
+stdout '^bad[/\\]bad.go:3:8: malformed import path "🐧.example.com/string": invalid char ''🐧''$'
+
+# TODO(#26909): This should include an import stack.
+# (Today it includes only a file and line.)
+! go build ./main
+stderr '^bad[/\\]bad.go:3:8: malformed import path "🐧.example.com/string": invalid char ''🐧''$'
+
+# TODO(#41688): This should include a file and line, and report the reason for the error..
+# (Today it includes only an import stack.)
+! go get ./main
+stderr '^go: m/main imports\n\tm/bad imports\n\t🐧.example.com/string: malformed import path "🐧.example.com/string": invalid char ''🐧''$'
+
+
+-- go.mod --
+module m
+
+go 1.13
+
+-- main/main.go --
+package main
+
+import _ "m/bad"
+
+func main() {}
+
+-- bad/bad.go --
+package bad
+
+import _ "🐧.example.com/string"
diff --git a/src/cmd/go/testdata/script/mod_build_tags.txt b/src/cmd/go/testdata/script/mod_build_tags.txt
new file mode 100644
index 0000000..ae1d605
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_build_tags.txt
@@ -0,0 +1,33 @@
+# Test that build tags are used.
+# golang.org/issue/24053.
+
+env GO111MODULE=on
+
+cd x
+! go list -f {{.GoFiles}}
+stderr 'build constraints exclude all Go files'
+
+go list -f {{.GoFiles}} -tags tag1
+stdout '\[x.go\]'
+
+go list -f {{.GoFiles}} -tags tag2
+stdout '\[y\.go\]'
+
+go list -f {{.GoFiles}} -tags 'tag1 tag2'
+stdout '\[x\.go y\.go\]'
+
+go list -f {{.GoFiles}} -tags tag1,tag2 # commas allowed as of Go 1.13
+stdout '\[x\.go y\.go\]'
+
+-- x/go.mod --
+module x
+
+-- x/x.go --
+// +build tag1
+
+package y
+
+-- x/y.go --
+// +build tag2
+
+package y
diff --git a/src/cmd/go/testdata/script/mod_build_trimpath_issue48557.txt b/src/cmd/go/testdata/script/mod_build_trimpath_issue48557.txt
new file mode 100644
index 0000000..859eafc
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_build_trimpath_issue48557.txt
@@ -0,0 +1,52 @@
+# Regression test for issue #48557.
+# Since builds in module mode do not support relative imports at all, the build
+# ID for (and other contents of) a binary built with -trimpath in module mode
+# should not depend on its working directory, even if the binary is specified as
+# a list of relative source files.
+
+[short] skip  # links and runs binaries
+
+env GOFLAGS=-trimpath
+env GOCACHE=$WORK/gocache
+
+
+# When we build a binary in module mode with -trimpath, the -D flag (for the
+# "local import prefix") should not be passed to it.
+
+cd $WORK/tmp/foo
+go build -x -o a.exe main.go
+stderr ${/}compile$GOEXE.*' -nolocalimports'
+! stderr ${/}compile$GOEXE.*' -D[ =]'
+
+go tool buildid a.exe
+cp stdout ../foo-buildid.txt
+go version a.exe
+cp stdout ../foo-version.txt
+cd ..
+
+
+# On the second build — in a different directory but with -trimpath — the
+# compiler should not be invoked, since the cache key should be identical.
+# Only the linker and buildid tool should be needed.
+
+mkdir bar
+cp foo/main.go bar/main.go
+cd bar
+go build -x -o a.exe main.go
+! stderr ${/}compile$GOEXE
+
+go tool buildid a.exe
+cp stdout ../bar-buildid.txt
+go version a.exe
+cp stdout ../bar-version.txt
+cd ..
+
+cmp bar-buildid.txt foo-buildid.txt
+cmp bar-version.txt foo-version.txt
+cmp bar/a.exe foo/a.exe
+
+
+-- $WORK/tmp/foo/main.go --
+package main
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/mod_build_versioned.txt b/src/cmd/go/testdata/script/mod_build_versioned.txt
new file mode 100644
index 0000000..f550418
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_build_versioned.txt
@@ -0,0 +1,17 @@
+env GO111MODULE=on
+[short] skip
+
+go get rsc.io/fortune/v2
+
+# The default executable name shouldn't be v2$GOEXE
+go build rsc.io/fortune/v2
+! exists v2$GOEXE
+exists fortune$GOEXE
+
+# The default test binary name shouldn't be v2.test$GOEXE
+go test -c rsc.io/fortune/v2
+! exists v2.test$GOEXE
+exists fortune.test$GOEXE
+
+-- go.mod --
+module scratch
diff --git a/src/cmd/go/testdata/script/mod_cache_dir.txt b/src/cmd/go/testdata/script/mod_cache_dir.txt
new file mode 100644
index 0000000..4045928
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_cache_dir.txt
@@ -0,0 +1,11 @@
+env GO111MODULE=on
+
+# Go should reject relative paths in GOMODCACHE environment.
+
+env GOMODCACHE="~/test"
+! go install example.com/tools/cmd/hello@latest
+stderr 'must be absolute path'
+
+env GOMODCACHE="./test"
+! go install example.com/tools/cmd/hello@latest
+stderr 'must be absolute path'
diff --git a/src/cmd/go/testdata/script/mod_cache_rw.txt b/src/cmd/go/testdata/script/mod_cache_rw.txt
new file mode 100644
index 0000000..87f27e8
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_cache_rw.txt
@@ -0,0 +1,49 @@
+# Regression test for golang.org/issue/31481.
+
+env GO111MODULE=on
+
+# golang.org/issue/31481: an explicit flag should make directories in the module
+# cache writable in order to work around the historical inability of 'rm -rf' to
+# forcibly remove files in unwritable directories.
+go get -modcacherw rsc.io/quote@v1.5.2
+cp $WORK/extraneous.txt $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/extraneous_file.go
+
+# After adding an extraneous file, 'go mod verify' should fail.
+! go mod verify
+
+# However, files within those directories should still be read-only to avoid
+# accidental mutations.
+[!root] ! cp $WORK/extraneous.txt $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/go.mod
+
+# If all 'go' commands ran with the flag, the system's 'rm' binary
+# should be able to remove the module cache if the '-rf' flags are set.
+[!GOOS:windows] [exec:rm] exec rm -rf $GOPATH/pkg/mod
+[!GOOS:windows] [!exec:rm] go clean -modcache
+[GOOS:windows] [exec:cmd.exe] exec cmd.exe /c rmdir /s /q $GOPATH\pkg\mod
+[GOOS:windows] [!exec:cmd.exe] go clean -modcache
+! exists $GOPATH/pkg/mod
+
+# The directories in the module cache should by default be unwritable,
+# so that tests and tools will not accidentally add extraneous files to them.
+# Windows does not respect FILE_ATTRIBUTE_READONLY on directories, according
+# to MSDN, so there we disable testing whether the directory itself is
+# unwritable.
+go get rsc.io/quote@latest
+[!root] ! cp $WORK/extraneous.txt $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/go.mod
+[!GOOS:windows] [!root] ! cp $WORK/extraneous.txt $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/extraneous_file.go
+! exists $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/extraneous_file.go
+
+
+# Repeat part of the test with 'go mod download' instead of 'go get' to verify
+# -modcacherw is supported on 'go mod' subcommands.
+go clean -modcache
+go mod download -modcacherw rsc.io/quote
+cp $WORK/extraneous.txt $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/extraneous_file.go
+! go mod verify
+[!root] ! cp $WORK/extraneous.txt $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/go.mod
+
+
+-- $WORK/extraneous.txt --
+module oops
+-- go.mod --
+module golang.org/issue/31481
diff --git a/src/cmd/go/testdata/script/mod_case.txt b/src/cmd/go/testdata/script/mod_case.txt
new file mode 100644
index 0000000..d3fb11d
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_case.txt
@@ -0,0 +1,25 @@
+env GO111MODULE=on
+
+go get
+go list -m all
+stdout '^rsc.io/quote v1.5.2'
+stdout '^rsc.io/QUOTE v1.5.2'
+
+go list -f 'DIR {{.Dir}} DEPS {{.Deps}}' rsc.io/QUOTE/QUOTE
+stdout 'DEPS.*rsc.io/quote'
+stdout 'DIR.*!q!u!o!t!e'
+
+go get rsc.io/QUOTE@v1.5.3-PRE
+go list -m all
+stdout '^rsc.io/QUOTE v1.5.3-PRE'
+
+go list -f '{{.Dir}}' rsc.io/QUOTE/QUOTE
+stdout '!q!u!o!t!e@v1.5.3-!p!r!e'
+
+-- go.mod --
+module x
+
+-- use.go --
+package use
+
+import _ "rsc.io/QUOTE/QUOTE"
diff --git a/src/cmd/go/testdata/script/mod_case_cgo.txt b/src/cmd/go/testdata/script/mod_case_cgo.txt
new file mode 100644
index 0000000..7c768a0
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_case_cgo.txt
@@ -0,0 +1,11 @@
+[!cgo] skip
+
+env GO111MODULE=on
+
+go get rsc.io/CGO
+[short] stop
+
+go build rsc.io/CGO
+
+-- go.mod --
+module x
diff --git a/src/cmd/go/testdata/script/mod_clean_cache.txt b/src/cmd/go/testdata/script/mod_clean_cache.txt
new file mode 100644
index 0000000..2b8e820
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_clean_cache.txt
@@ -0,0 +1,61 @@
+env GO111MODULE=on
+
+# 'mod download' should download the module to the cache.
+go mod download rsc.io/quote@v1.5.0
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.info
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.mod
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.zip
+
+# '-n' should print commands but not actually execute them.
+go clean -modcache -n
+stdout '^rm -rf .*pkg.mod$'
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.info
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.mod
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.zip
+
+# 'go clean -modcache' should actually delete the files.
+go clean -modcache
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.info
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.mod
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.zip
+
+# 'go clean -r -modcache' should clean only the dependencies that are within the
+# main module.
+# BUG(golang.org/issue/28680): Today, it cleans across module boundaries.
+cd r
+exists ./test.out
+exists ../replaced/test.out
+go clean -r -modcache
+! exists ./test.out
+! exists ../replaced/test.out  # BUG: should still exist
+
+# 'go clean -modcache' should not download anything before cleaning.
+go mod edit -require rsc.io/quote@v1.99999999.0-not-a-real-version
+go clean -modcache
+! stderr 'finding rsc.io'
+go mod edit -droprequire rsc.io/quote
+
+! go clean -modcache m
+stderr 'go: clean -modcache cannot be used with package arguments'
+
+-- go.mod --
+module m
+-- m.go --
+package m
+
+-- r/go.mod --
+module example.com/r
+require example.com/r/replaced v0.0.0
+replace example.com/r/replaced => ../replaced
+-- r/r.go --
+package r
+import _ "example.com/r/replaced"
+-- r/test.out --
+DELETE ME
+
+-- replaced/go.mod --
+module example.com/r/replaced
+-- replaced/replaced.go --
+package replaced
+-- replaced/test.out --
+DO NOT DELETE
diff --git a/src/cmd/go/testdata/script/mod_concurrent.txt b/src/cmd/go/testdata/script/mod_concurrent.txt
new file mode 100644
index 0000000..e2224d6
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_concurrent.txt
@@ -0,0 +1,32 @@
+env GO111MODULE=on
+
+# Concurrent builds should succeed, even if they need to download modules.
+go get ./x ./y
+go build ./x &
+go build ./y
+wait
+
+# Concurrent builds should update go.sum to the union of the hashes for the
+# modules they read.
+cmp go.sum go.sum.want
+
+-- go.mod --
+module golang.org/issue/26794
+
+require (
+	golang.org/x/text v0.3.0
+	rsc.io/sampler v1.0.0
+)
+-- x/x.go --
+package x
+
+import _ "golang.org/x/text/language"
+-- y/y.go --
+package y
+
+import _ "rsc.io/sampler"
+-- go.sum.want --
+golang.org/x/text v0.3.0 h1:ivTorhoiROmZ1mcs15mO2czVF0uy0tnezXpBVNzgrmA=
+golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/sampler v1.0.0 h1:SRJnjyQ07sAtq6G4RcfJEmz8JxqLyj3PoGXG2VhbDWo=
+rsc.io/sampler v1.0.0/go.mod h1:cqxpM3ZVz9VtirqxZPmrWzkQ+UkiNiGtkrN+B+i8kx8=
diff --git a/src/cmd/go/testdata/script/mod_convert.txt b/src/cmd/go/testdata/script/mod_convert.txt
new file mode 100644
index 0000000..922d924
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_convert.txt
@@ -0,0 +1,70 @@
+[!net:github.com] skip
+[!net:golang.org] skip
+[!net:gopkg.in] skip
+[!git] skip
+
+env GO111MODULE=on
+env GOPROXY=
+env GOSUMDB=
+
+go mod download github.com/docker/distribution@v0.0.0-20150410205453-85de3967aa93
+mkdir x/Godeps
+cp $GOPATH/pkg/mod/github.com/docker/distribution@v0.0.0-20150410205453-85de3967aa93/Godeps/Godeps.json x/Godeps
+cd x
+go mod init github.com/docker/distribution
+cmpenv go.mod go.mod.want
+
+[!net:google.golang.org] skip
+[!net:cloud.google.com] skip
+
+go mod download github.com/fishy/gcsbucket@v0.0.0-20180217031846-618d60fe84e0
+cp $GOPATH/pkg/mod/github.com/fishy/gcsbucket@v0.0.0-20180217031846-618d60fe84e0/Gopkg.lock ../y
+cd ../y
+go mod init github.com/fishy/gcsbucket
+cmpenv go.mod go.mod.want
+
+-- x/go.mod.want --
+module github.com/docker/distribution
+
+go $goversion
+
+require (
+	github.com/AdRoll/goamz v0.0.0-20150130162828-d3664b76d905
+	github.com/MSOpenTech/azure-sdk-for-go v0.0.0-20150323223030-d90753bcad2e
+	github.com/Sirupsen/logrus v0.7.3
+	github.com/bugsnag/bugsnag-go v1.0.3-0.20141110184014-b1d153021fcd
+	github.com/bugsnag/osext v0.0.0-20130617224835-0dd3f918b21b
+	github.com/bugsnag/panicwrap v0.0.0-20141110184334-e5f9854865b9
+	github.com/codegangsta/cli v1.4.2-0.20150131031259-6086d7927ec3
+	github.com/docker/docker v1.4.2-0.20150204013315-165ea5c158cf
+	github.com/docker/libtrust v0.0.0-20150114040149-fa567046d9b1
+	github.com/garyburd/redigo v0.0.0-20150301180006-535138d7bcd7
+	github.com/gorilla/context v0.0.0-20140604161150-14f550f51af5
+	github.com/gorilla/handlers v0.0.0-20140825150757-0e84b7d810c1
+	github.com/gorilla/mux v0.0.0-20140926153814-e444e69cbd2e
+	github.com/jlhawn/go-crypto v0.0.0-20150401213827-cd738dde20f0
+	github.com/yvasiyarov/go-metrics v0.0.0-20140926110328-57bccd1ccd43
+	github.com/yvasiyarov/gorelic v0.0.7-0.20141212073537-a9bba5b9ab50
+	github.com/yvasiyarov/newrelic_platform_go v0.0.0-20140908184405-b21fdbd4370f
+	golang.org/x/net v0.0.0-20150202051010-1dfe7915deaf
+	gopkg.in/check.v1 v1.0.0-20141024133853-64131543e789
+	gopkg.in/yaml.v2 v2.0.0-20150116202057-bef53efd0c76
+)
+-- y/go.mod.want --
+module github.com/fishy/gcsbucket
+
+go $goversion
+
+require (
+	cloud.google.com/go v0.18.0
+	github.com/fishy/fsdb v0.0.0-20180217030800-5527ded01371
+	github.com/golang/protobuf v1.0.0
+	github.com/googleapis/gax-go v2.0.0+incompatible
+	golang.org/x/net v0.0.0-20180216171745-136a25c244d3
+	golang.org/x/oauth2 v0.0.0-20180207181906-543e37812f10
+	golang.org/x/text v0.3.1-0.20180208041248-4e4a3210bb54
+	google.golang.org/api v0.0.0-20180217000815-c7a403bb5fe1
+	google.golang.org/appengine v1.0.0
+	google.golang.org/genproto v0.0.0-20180206005123-2b5a72b8730b
+	google.golang.org/grpc v1.10.0
+)
diff --git a/src/cmd/go/testdata/script/mod_convert_dep.txt b/src/cmd/go/testdata/script/mod_convert_dep.txt
new file mode 100644
index 0000000..875a836
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_convert_dep.txt
@@ -0,0 +1,30 @@
+env GO111MODULE=on
+
+# We should not create a go.mod file unless the user ran 'go mod init' explicitly.
+# However, we should suggest 'go mod init' if we can find an alternate config file.
+cd $WORK/test/x
+! go list .
+stderr 'found Gopkg.lock in .*[/\\]test'
+stderr '\s*cd \.\. && go mod init'
+
+# The command we suggested should succeed.
+cd ..
+go mod init
+go list -m all
+stdout '^m$'
+
+# In Plan 9, directories are automatically created in /n.
+# For example, /n/Gopkg.lock always exists, but it's a directory.
+# Test that we ignore directories when trying to find alternate config files.
+cd $WORK/gopkgdir/x
+! go list .
+stderr '^go: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+! stderr 'Gopkg.lock'
+
+-- $WORK/test/Gopkg.lock --
+-- $WORK/test/x/x.go --
+package x // import "m/x"
+-- $WORK/gopkgdir/Gopkg.lock/README.txt --
+../Gopkg.lock is a directory, not a file.
+-- $WORK/gopkgdir/x/x.go --
+package x // import "m/x"
diff --git a/src/cmd/go/testdata/script/mod_convert_git.txt b/src/cmd/go/testdata/script/mod_convert_git.txt
new file mode 100644
index 0000000..6ff1eb5
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_convert_git.txt
@@ -0,0 +1,40 @@
+env GO111MODULE=on
+
+# We should not create a go.mod file unless the user ran 'go mod init' explicitly.
+# However, we should suggest 'go mod init' if we can find an alternate config file.
+cd $WORK/test/x
+! go list .
+stderr 'found .git/config in .*[/\\]test'
+stderr '\s*cd \.\. && go mod init'
+
+# The command we suggested should succeed.
+cd ..
+go mod init
+go list -m all
+stdout '^m$'
+
+# We should not suggest creating a go.mod file in $GOROOT, even though there may be a .git/config there.
+cd $GOROOT
+! go list .
+! stderr 'go mod init'
+
+# We should also not suggest creating a go.mod file in $GOROOT if its own
+# .git/config has been stripped away and we find one in a parent directory.
+# (https://golang.org/issue/34191)
+env GOROOT=$WORK/parent/goroot
+cd $GOROOT
+! go list .
+! stderr 'go mod init'
+
+cd $GOROOT/doc
+! go list .
+! stderr 'go mod init'
+
+-- $WORK/test/.git/config --
+-- $WORK/test/x/x.go --
+package x // import "m/x"
+-- $WORK/parent/.git/config --
+-- $WORK/parent/goroot/README --
+This directory isn't really a GOROOT, but let's pretend that it is.
+-- $WORK/parent/goroot/doc/README --
+This is a subdirectory of our fake GOROOT.
diff --git a/src/cmd/go/testdata/script/mod_convert_glide.txt b/src/cmd/go/testdata/script/mod_convert_glide.txt
new file mode 100644
index 0000000..9f1fff5
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_convert_glide.txt
@@ -0,0 +1,18 @@
+env GO111MODULE=on
+
+# We should not create a go.mod file unless the user ran 'go mod init' explicitly.
+# However, we should suggest 'go mod init' if we can find an alternate config file.
+cd $WORK/test/x
+! go list .
+stderr 'found glide.lock in .*[/\\]test'
+stderr '\s*cd \.\. && go mod init'
+
+# The command we suggested should succeed.
+cd ..
+go mod init
+go list -m all
+stdout '^m$'
+
+-- $WORK/test/glide.lock --
+-- $WORK/test/x/x.go --
+package x // import "m/x"
diff --git a/src/cmd/go/testdata/script/mod_convert_glockfile.txt b/src/cmd/go/testdata/script/mod_convert_glockfile.txt
new file mode 100644
index 0000000..6aa0794
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_convert_glockfile.txt
@@ -0,0 +1,18 @@
+env GO111MODULE=on
+
+# We should not create a go.mod file unless the user ran 'go mod init' explicitly.
+# However, we should suggest 'go mod init' if we can find an alternate config file.
+cd $WORK/test/x
+! go list .
+stderr 'found GLOCKFILE in .*[/\\]test'
+stderr '\s*cd \.\. && go mod init'
+
+# The command we suggested should succeed.
+cd ..
+go mod init
+go list -m all
+stdout '^m$'
+
+-- $WORK/test/GLOCKFILE --
+-- $WORK/test/x/x.go --
+package x // import "m/x"
diff --git a/src/cmd/go/testdata/script/mod_convert_godeps.txt b/src/cmd/go/testdata/script/mod_convert_godeps.txt
new file mode 100644
index 0000000..da7b6c1
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_convert_godeps.txt
@@ -0,0 +1,19 @@
+env GO111MODULE=on
+
+# We should not create a go.mod file unless the user ran 'go mod init' explicitly.
+# However, we should suggest 'go mod init' if we can find an alternate config file.
+cd $WORK/test/x
+! go list .
+stderr 'found Godeps/Godeps.json in .*[/\\]test'
+stderr '\s*cd \.\. && go mod init'
+
+# The command we suggested should succeed.
+cd ..
+go mod init
+go list -m all
+stdout '^m$'
+
+-- $WORK/test/Godeps/Godeps.json --
+{}
+-- $WORK/test/x/x.go --
+package x // import "m/x"
diff --git a/src/cmd/go/testdata/script/mod_convert_tsv.txt b/src/cmd/go/testdata/script/mod_convert_tsv.txt
new file mode 100644
index 0000000..6015ac8
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_convert_tsv.txt
@@ -0,0 +1,18 @@
+env GO111MODULE=on
+
+# We should not create a go.mod file unless the user ran 'go mod init' explicitly.
+# However, we should suggest 'go mod init' if we can find an alternate config file.
+cd $WORK/test/x
+! go list .
+stderr 'found dependencies.tsv in .*[/\\]test'
+stderr '\s*cd \.\. && go mod init'
+
+# The command we suggested should succeed.
+cd ..
+go mod init
+go list -m all
+stdout '^m$'
+
+-- $WORK/test/dependencies.tsv --
+-- $WORK/test/x/x.go --
+package x // import "m/x"
diff --git a/src/cmd/go/testdata/script/mod_convert_tsv_insecure.txt b/src/cmd/go/testdata/script/mod_convert_tsv_insecure.txt
new file mode 100644
index 0000000..6ff6993
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_convert_tsv_insecure.txt
@@ -0,0 +1,29 @@
+env GO111MODULE=on
+env GOPROXY=direct
+env GOSUMDB=off
+
+[short] skip
+[!git] skip
+
+# secure fetch should report insecure warning
+cd $WORK/test
+go mod init
+stderr 'redirected .* to insecure URL'
+
+# insecure fetch should not
+env GOINSECURE=*.golang.org
+rm go.mod
+go mod init
+! stderr 'redirected .* to insecure URL'
+
+# insecure fetch invalid path should report insecure warning
+env GOINSECURE=foo.golang.org
+rm go.mod
+go mod init
+stderr 'redirected .* to insecure URL'
+
+-- $WORK/test/dependencies.tsv --
+vcs-test.golang.org/insecure/go/insecure	git	6fecd21f7c0c	2019-09-04T18:39:48Z 
+
+-- $WORK/test/x.go --
+package x // import "m"
diff --git a/src/cmd/go/testdata/script/mod_convert_vendor_conf.txt b/src/cmd/go/testdata/script/mod_convert_vendor_conf.txt
new file mode 100644
index 0000000..57ec419
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_convert_vendor_conf.txt
@@ -0,0 +1,18 @@
+env GO111MODULE=on
+
+# We should not create a go.mod file unless the user ran 'go mod init' explicitly.
+# However, we should suggest 'go mod init' if we can find an alternate config file.
+cd $WORK/test/x
+! go list .
+stderr 'found vendor.conf in .*[/\\]test'
+stderr '\s*cd \.\. && go mod init'
+
+# The command we suggested should succeed.
+cd ..
+go mod init
+go list -m all
+stdout '^m$'
+
+-- $WORK/test/vendor.conf --
+-- $WORK/test/x/x.go --
+package x // import "m/x"
diff --git a/src/cmd/go/testdata/script/mod_convert_vendor_json.txt b/src/cmd/go/testdata/script/mod_convert_vendor_json.txt
new file mode 100644
index 0000000..df6db36
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_convert_vendor_json.txt
@@ -0,0 +1,19 @@
+env GO111MODULE=on
+
+# We should not create a go.mod file unless the user ran 'go mod init' explicitly.
+# However, we should suggest 'go mod init' if we can find an alternate config file.
+cd $WORK/test/x
+! go list .
+stderr 'found vendor/vendor.json in .*[/\\]test'
+stderr '\s*cd \.\. && go mod init'
+
+# The command we suggested should succeed.
+cd ..
+go mod init
+go list -m
+stdout '^m$'
+
+-- $WORK/test/vendor/vendor.json --
+{}
+-- $WORK/test/x/x.go --
+package x // import "m/x"
diff --git a/src/cmd/go/testdata/script/mod_convert_vendor_manifest.txt b/src/cmd/go/testdata/script/mod_convert_vendor_manifest.txt
new file mode 100644
index 0000000..8b6a141
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_convert_vendor_manifest.txt
@@ -0,0 +1,19 @@
+env GO111MODULE=on
+
+# We should not create a go.mod file unless the user ran 'go mod init' explicitly.
+# However, we should suggest 'go mod init' if we can find an alternate config file.
+cd $WORK/test/x
+! go list .
+stderr 'found vendor/manifest in .*[/\\]test'
+stderr '\s*cd \.\. && go mod init'
+
+# The command we suggested should succeed.
+cd ..
+go mod init
+go list -m
+stdout '^m$'
+
+-- $WORK/test/vendor/manifest --
+{}
+-- $WORK/test/x/x.go --
+package x // import "m/x"
diff --git a/src/cmd/go/testdata/script/mod_convert_vendor_yml.txt b/src/cmd/go/testdata/script/mod_convert_vendor_yml.txt
new file mode 100644
index 0000000..4ed140a
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_convert_vendor_yml.txt
@@ -0,0 +1,18 @@
+env GO111MODULE=on
+
+# We should not create a go.mod file unless the user ran 'go mod init' explicitly.
+# However, we should suggest 'go mod init' if we can find an alternate config file.
+cd $WORK/test/x
+! go list .
+stderr 'found vendor.yml in .*[/\\]test'
+stderr '\s*cd \.\. && go mod init'
+
+# The command we suggested should succeed.
+cd ..
+go mod init
+go list -m all
+stdout '^m$'
+
+-- $WORK/test/vendor.yml --
+-- $WORK/test/x/x.go --
+package x // import "m/x"
diff --git a/src/cmd/go/testdata/script/mod_deprecate_message.txt b/src/cmd/go/testdata/script/mod_deprecate_message.txt
new file mode 100644
index 0000000..1fb1246
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_deprecate_message.txt
@@ -0,0 +1,73 @@
+# When there is a short single-line message, 'go get' should print it all.
+go get short
+stderr '^go: module short is deprecated: short$'
+go list -m -u -f '{{.Deprecated}}' short
+stdout '^short$'
+
+# When there is a multi-line message, 'go get' should print the first line.
+go get multiline
+stderr '^go: module multiline is deprecated: first line$'
+! stderr 'second line'
+go list -m -u -f '{{.Deprecated}}' multiline
+stdout '^first line\nsecond line.$'
+
+# When there is a long message, 'go get' should print a placeholder.
+go get long
+stderr '^go: module long is deprecated: \(message omitted: too long\)$'
+go list -m -u -f '{{.Deprecated}}' long
+stdout '^aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa$'
+
+# When a message contains unprintable characters, 'go get' should say that
+# without printing the message.
+go get unprintable
+stderr '^go: module unprintable is deprecated: \(message omitted: contains non-printable characters\)$'
+go list -m -u -f '{{.Deprecated}}' unprintable
+stdout '^message contains ASCII BEL\x07$'
+
+-- go.mod --
+module use
+
+go 1.16
+
+require (
+	short v0.0.0
+	multiline v0.0.0
+	long v0.0.0
+	unprintable v0.0.0
+)
+
+replace (
+	short v0.0.0 => ./short
+	multiline v0.0.0 => ./multiline
+	long v0.0.0 => ./long
+	unprintable v0.0.0 => ./unprintable
+)
+-- short/go.mod --
+// Deprecated: short
+module short
+
+go 1.16
+-- short/short.go --
+package short
+-- multiline/go.mod --
+// Deprecated: first line
+// second line.
+module multiline
+
+go 1.16
+-- multiline/multiline.go --
+package multiline
+-- long/go.mod --
+// Deprecated: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+module long
+
+go 1.16
+-- long/long.go --
+package long
+-- unprintable/go.mod --
+// Deprecated: message contains ASCII BEL
+module unprintable
+
+go 1.16
+-- unprintable/unprintable.go --
+package unprintable
diff --git a/src/cmd/go/testdata/script/mod_dir.txt b/src/cmd/go/testdata/script/mod_dir.txt
new file mode 100644
index 0000000..05548f6
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_dir.txt
@@ -0,0 +1,20 @@
+# The directory named go.mod should be ignored
+
+env GO111MODULE=on
+
+cd $WORK/sub
+
+go list .
+stdout 'x/sub'
+
+mkdir go.mod
+exists go.mod
+
+go list .
+stdout 'x/sub'
+
+-- $WORK/go.mod --
+module x
+
+-- $WORK/sub/x.go --
+package x
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/mod_doc.txt b/src/cmd/go/testdata/script/mod_doc.txt
new file mode 100644
index 0000000..bf0a19d
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_doc.txt
@@ -0,0 +1,90 @@
+# go doc should find module documentation
+
+env GO111MODULE=on
+env GOFLAGS=-mod=mod
+[short] skip
+
+# Check when module x is inside GOPATH/src.
+go doc y
+stdout 'Package y is.*alphabet'
+stdout 'import "x/y"'
+go doc x/y
+stdout 'Package y is.*alphabet'
+! go doc quote.Hello
+stderr 'doc: symbol quote is not a type' # because quote is not in local cache
+go list rsc.io/quote # now it is
+go doc quote.Hello
+stdout 'Hello returns a greeting'
+go doc quote
+stdout 'Package quote collects pithy sayings.'
+
+# Double-check when module x is outside GOPATH/src.
+env GOPATH=$WORK/emptygopath
+go doc x/y
+stdout 'Package y is.*alphabet'
+go doc y
+stdout 'Package y is.*alphabet'
+
+# Triple-check when module x is outside GOPATH/src,
+# but other packages with same import paths are in GOPATH/src.
+# Since go doc is running in module mode here, packages in active module
+# should be preferred over packages in GOPATH. See golang.org/issue/28992.
+env GOPATH=$WORK/gopath2
+go doc x/y
+! stdout 'Package y is.*GOPATH'
+stdout 'Package y is.*alphabet'
+go doc rsc.io/quote
+! stdout 'Package quote is located in a GOPATH workspace.'
+stdout 'Package quote collects pithy sayings.'
+
+# Check that a sensible error message is printed when a package is not found.
+env GOPROXY=off
+! go doc example.com/hello
+stderr '^doc: cannot find module providing package example.com/hello: module lookup disabled by GOPROXY=off$'
+
+# When in a module with a vendor directory, doc should use the vendored copies
+# of the packages. 'std' and 'cmd' are convenient examples of such modules.
+#
+# When in those modules, the "// import" comment should refer to the same import
+# path used in source code, not to the absolute path relative to GOROOT.
+
+cd $GOROOT/src
+env GOFLAGS=
+env GOWORK=off
+go doc cryptobyte
+stdout '// import "golang.org/x/crypto/cryptobyte"'
+
+cd $GOROOT/src/cmd/go
+go doc modfile
+stdout '// import "golang.org/x/mod/modfile"'
+
+# When outside of the 'std' module, its vendored packages
+# remain accessible using the 'vendor/' prefix, but report
+# the correct "// import" comment as used within std.
+cd $GOPATH
+go doc vendor/golang.org/x/crypto/cryptobyte
+stdout '// import "vendor/golang.org/x/crypto/cryptobyte"'
+
+go doc cmd/vendor/golang.org/x/mod/modfile
+stdout '// import "cmd/vendor/golang.org/x/mod/modfile"'
+
+-- go.mod --
+module x
+require rsc.io/quote v1.5.2
+
+-- y/y.go --
+// Package y is the next to last package of the alphabet.
+package y
+
+-- x.go --
+package x
+
+-- $WORK/gopath2/src/x/y/y.go --
+// Package y is located in a GOPATH workspace.
+package y
+-- $WORK/gopath2/src/rsc.io/quote/quote.go --
+// Package quote is located in a GOPATH workspace.
+package quote
+
+// Hello is located in a GOPATH workspace.
+func Hello() string { return "" }
diff --git a/src/cmd/go/testdata/script/mod_doc_path.txt b/src/cmd/go/testdata/script/mod_doc_path.txt
new file mode 100644
index 0000000..83d5392
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_doc_path.txt
@@ -0,0 +1,30 @@
+# cmd/doc should use GOROOT to locate the 'go' command,
+# not use whatever is in $PATH.
+
+# Remove 'go' from $PATH. (It can still be located via $GOROOT/bin/go, and the
+# test script's built-in 'go' command still knows where to find it.)
+env PATH=''
+[GOOS:plan9] env path=''
+
+go doc p.X
+
+-- go.mod --
+module example
+
+go 1.19
+
+require example.com/p v0.1.0
+
+replace example.com/p => ./pfork
+-- example.go --
+package example
+
+import _ "example.com/p"
+-- pfork/go.mod --
+module example.com/p
+
+go 1.19
+-- pfork/p.go --
+package p
+
+const X = 42
diff --git a/src/cmd/go/testdata/script/mod_domain_root.txt b/src/cmd/go/testdata/script/mod_domain_root.txt
new file mode 100644
index 0000000..c13029d
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_domain_root.txt
@@ -0,0 +1,12 @@
+# Module paths that are domain roots should resolve.
+# (example.com not example.com/something)
+
+env GO111MODULE=on
+go get
+
+-- go.mod --
+module x
+
+-- x.go --
+package x
+import _ "example.com"
diff --git a/src/cmd/go/testdata/script/mod_dot.txt b/src/cmd/go/testdata/script/mod_dot.txt
new file mode 100644
index 0000000..aa24986
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_dot.txt
@@ -0,0 +1,132 @@
+env GOWORK=off
+env GO111MODULE=on
+
+# golang.org/issue/32917 and golang.org/issue/28459: 'go build' and 'go test'
+# in an empty directory should refer to the path '.' and should not attempt
+# to resolve an external module.
+cd dir
+! go get
+stderr '^go: no package to get in current directory$'
+! go get .
+stderr '^go: .: no package to get in current directory$'
+! go get ./subdir
+stderr '^go: \.[/\\]subdir \('$WORK'[/\\]gopath[/\\]src[/\\]dir[/\\]subdir\) is not a package in module rooted at '$WORK'[/\\]gopath[/\\]src[/\\]dir$'
+! go list
+! stderr 'cannot find module providing package'
+stderr '^no Go files in '$WORK'[/\\]gopath[/\\]src[/\\]dir$'
+
+cd subdir
+! go list
+! stderr 'cannot find module providing package'
+stderr '^no Go files in '$WORK'[/\\]gopath[/\\]src[/\\]dir[/\\]subdir$'
+cd ..
+
+# golang.org/issue/30590: if a package is found in the filesystem
+# but is not in the main module, the error message should not say
+# "cannot find module providing package", and we shouldn't try
+# to find a module providing the package.
+! go list ./othermodule
+! stderr 'cannot find module providing package'
+stderr '^main module \(example\.com\) does not contain package example.com/othermodule$'
+
+# golang.org/issue/27122: 'go build' of a nonexistent directory should produce
+# a helpful "no Go files" error message, not a generic "unknown import path".
+! go list ./subdir
+stderr '^no Go files in '$WORK'[/\\]gopath[/\\]src[/\\]dir[/\\]subdir$'
+
+# golang.org/issue/29280: 'go list -e' for a nonexistent directory should
+# report a nonexistent package with an error.
+go list -e -json ./subdir
+stdout '"Incomplete": true'
+
+# golang.org/issue/28155: 'go list ./testdata' should not synthesize underscores.
+go list ./testdata
+stdout '^example.com/testdata'
+
+# golang.org/issue/32921: vendor directories should only be accepted as directories
+# if the directory would actually be used to load the package.
+! go list ./vendor/nonexist
+stderr '^no Go files in '$WORK'[/\\]gopath[/\\]src[/\\]dir[/\\]vendor[/\\]nonexist$'
+
+! go list ./vendor/pkg
+stderr '^without -mod=vendor, directory '$WORK'[/\\]gopath[/\\]src[/\\]dir[/\\]vendor[/\\]pkg has no package path$'
+
+! go list -mod=vendor ./vendor/nonexist
+stderr '^no Go files in '$WORK'[/\\]gopath[/\\]src[/\\]dir[/\\]vendor[/\\]nonexist$'
+
+! go list -mod=vendor ./vendor/unlisted
+stderr '^directory '$WORK'[/\\]gopath[/\\]src[/\\]dir[/\\]vendor[/\\]unlisted is not a package listed in vendor/modules.txt$'
+
+go list -mod=vendor ./vendor/pkg
+stdout '^pkg$'
+
+# Packages within GOROOT should resolve as in any other module,
+# except that -mod=vendor is implied by default.
+cd $GOROOT/src
+! go list .
+stderr '^no Go files in '$GOROOT'[/\\]src$'
+
+! go list ./builtin
+stderr '^"builtin" is a pseudo-package, not an importable package$'
+
+! go list ./debug
+! stderr 'cannot find module providing package'
+stderr '^no Go files in '$GOROOT'[/\\]src[/\\]debug$'
+
+! go list ./golang.org/x/tools/cmd/goimports
+! stderr 'cannot find module providing package'
+stderr '^stat '$GOROOT'[/\\]src[/\\]golang.org[/\\]x[/\\]tools[/\\]cmd[/\\]goimports: directory not found'
+
+go list ./vendor/golang.org/x/net/http2/hpack
+stdout '^golang.org/x/net/http2/hpack$'
+
+# golang.org/issue/30756: packages in other GOROOTs should not get the special
+# prefixless treatment of GOROOT itself.
+cd $WORK/othergoroot/src
+! go list .
+stderr '^no Go files in '$WORK'[/\\]othergoroot[/\\]src$'
+
+go list ./builtin
+stdout '^std/builtin$'  # Only the "std" in actual $GOROOT is special, and only its "builtin" is special.
+
+! go list ./bytes
+! stderr 'cannot find module providing package'
+stderr '^no Go files in '$WORK'[/\\]othergoroot[/\\]src[/\\]bytes$'
+
+! go list ./vendor/golang.org/x/net/http2/hpack
+stderr '^without -mod=vendor, directory '$WORK'[/\\]othergoroot[/\\]src[/\\]vendor[/\\]golang.org[/\\]x[/\\]net[/\\]http2[/\\]hpack has no package path$'
+
+-- dir/go.mod --
+module example.com
+go 1.13
+-- dir/subdir/README --
+There are no Go source files in this directory.
+-- dir/othermodule/go.mod --
+module example.com/othermodule
+go 1.13
+-- dir/othermodule/om.go --
+package othermodule
+-- dir/testdata/td.go --
+package testdata
+-- dir/vendor/modules.txt --
+# pkg v0.0.0
+pkg
+-- dir/vendor/nonexist/README --
+There are no Go source files here either.
+-- dir/vendor/pkg/pkg.go --
+package pkg
+-- dir/vendor/unlisted/unlisted.go --
+package unlisted
+-- emptyroot/go.mod --
+module example.com/emptyroot
+-- emptyroot/pkg/pkg.go --
+package pkg
+-- $WORK/othergoroot/src/go.mod --
+module std
+go 1.13
+-- $WORK/othergoroot/src/builtin/builtin.go --
+package builtin
+-- $WORK/othergoroot/src/bytes/README --
+There are no Go source files in this directory.
+-- $WORK/othergoroot/src/vendor/golang.org/x/net/http2/hpack --
+package hpack
diff --git a/src/cmd/go/testdata/script/mod_download.txt b/src/cmd/go/testdata/script/mod_download.txt
new file mode 100644
index 0000000..154e683
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_download.txt
@@ -0,0 +1,214 @@
+env GO111MODULE=on
+
+# download with version should print nothing.
+# It should not load retractions from the .mod file from the latest version.
+go mod download rsc.io/quote@v1.5.0
+! stdout .
+! stderr .
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.info
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.mod
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.zip
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.info
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.mod
+
+# download of an invalid path should report the error
+[short] skip
+! go mod download this.domain.is.invalid/somemodule@v1.0.0
+stderr 'this.domain.is.invalid'
+! go mod download -json this.domain.is.invalid/somemodule@v1.0.0
+stdout '"Error": ".*this.domain.is.invalid.*"'
+
+# download -json with version should print JSON
+go mod download -json 'rsc.io/quote@<=v1.5.0'
+stdout '^\t"Path": "rsc.io/quote"'
+stdout '^\t"Version": "v1.5.0"'
+stdout '^\t"Info": ".*(\\\\|/)pkg(\\\\|/)mod(\\\\|/)cache(\\\\|/)download(\\\\|/)rsc.io(\\\\|/)quote(\\\\|/)@v(\\\\|/)v1.5.0.info"'
+stdout '^\t"GoMod": ".*(\\\\|/)pkg(\\\\|/)mod(\\\\|/)cache(\\\\|/)download(\\\\|/)rsc.io(\\\\|/)quote(\\\\|/)@v(\\\\|/)v1.5.0.mod"'
+stdout '^\t"Zip": ".*(\\\\|/)pkg(\\\\|/)mod(\\\\|/)cache(\\\\|/)download(\\\\|/)rsc.io(\\\\|/)quote(\\\\|/)@v(\\\\|/)v1.5.0.zip"'
+stdout '^\t"Sum": "h1:6fJa6E\+wGadANKkUMlZ0DhXFpoKlslOQDCo259XtdIE="'  # hash of testdata/mod version, not real version!
+stdout '^\t"GoModSum": "h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe\+TKr0="'
+! stdout '"Error"'
+
+# download queries above should not have added to go.mod.
+go list -m all
+! stdout rsc.io
+
+# download query should have downloaded go.mod for the highest release version
+# in order to find retractions when resolving the query '@<=v1.5.0'.
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.info
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.mod
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.zip
+
+# add to go.mod so we can test non-query downloads
+go mod edit -require rsc.io/quote@v1.5.3-pre1
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-pre1.info
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-pre1.mod
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-pre1.zip
+
+# module loading will page in the info and mod files
+go list -m -mod=mod all
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-pre1.info
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-pre1.mod
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-pre1.zip
+
+# download will fetch and unpack the zip file
+go mod download
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-pre1.info
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-pre1.mod
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-pre1.zip
+exists $GOPATH/pkg/mod/rsc.io/quote@v1.5.3-pre1
+
+# download repopulates deleted files and directories independently.
+rm $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-pre1.info
+go mod download
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-pre1.info
+rm $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-pre1.mod
+go mod download
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-pre1.mod
+rm $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-pre1.zip
+go mod download
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-pre1.zip
+rm -r $GOPATH/pkg/mod/rsc.io/quote@v1.5.3-pre1
+go mod download
+exists $GOPATH/pkg/mod/rsc.io/quote@v1.5.3-pre1
+
+# download reports the locations of downloaded files
+go mod download -json
+stdout '^\t"Path": "rsc.io/quote"'
+stdout '^\t"Version": "v1.5.3-pre1"'
+stdout '^\t"Info": ".*(\\\\|/)pkg(\\\\|/)mod(\\\\|/)cache(\\\\|/)download(\\\\|/)rsc.io(\\\\|/)quote(\\\\|/)@v(\\\\|/)v1.5.3-pre1.info"'
+stdout '^\t"GoMod": ".*(\\\\|/)pkg(\\\\|/)mod(\\\\|/)cache(\\\\|/)download(\\\\|/)rsc.io(\\\\|/)quote(\\\\|/)@v(\\\\|/)v1.5.3-pre1.mod"'
+stdout '^\t"Zip": ".*(\\\\|/)pkg(\\\\|/)mod(\\\\|/)cache(\\\\|/)download(\\\\|/)rsc.io(\\\\|/)quote(\\\\|/)@v(\\\\|/)v1.5.3-pre1.zip"'
+stdout '^\t"Dir": ".*(\\\\|/)pkg(\\\\|/)mod(\\\\|/)rsc.io(\\\\|/)quote@v1.5.3-pre1"'
+
+# download will follow replacements
+go mod edit -require rsc.io/quote@v1.5.1 -replace rsc.io/quote@v1.5.1=rsc.io/quote@v1.5.2
+go mod download
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.1.zip
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.zip
+
+# download will not follow replacements for explicit module queries
+go mod download -json rsc.io/quote@v1.5.1
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.1.zip
+
+# download reports errors encountered when locating modules
+! go mod download bad/path
+stderr '^go: module bad/path: not a known dependency$'
+! go mod download bad/path@latest
+stderr '^go: bad/path@latest: malformed module path "bad/path": missing dot in first path element$'
+! go mod download rsc.io/quote@v1.999.999
+stderr '^go: rsc.io/quote@v1.999.999: reading .*/v1.999.999.info: 404 Not Found$'
+! go mod download -json bad/path
+stdout '^\t"Error": "module bad/path: not a known dependency"'
+
+# download main module produces a warning or error
+go mod download m
+stderr '^go: skipping download of m that resolves to the main module\n'
+! go mod download m@latest
+stderr '^go: m@latest: malformed module path "m": missing dot in first path element$'
+
+# download without arguments updates go.mod and go.sum after loading the
+# build list, but does not save sums for downloaded zips.
+cd update
+cp go.mod.orig go.mod
+! exists go.sum
+go mod download
+cmp go.mod.update go.mod
+cmp go.sum.update go.sum
+cp go.mod.orig go.mod
+rm go.sum
+
+# download with arguments (even "all") does update go.mod and go.sum.
+go mod download rsc.io/sampler
+cmp go.mod.update go.mod
+grep '^rsc.io/sampler v1.3.0 ' go.sum
+cp go.mod.orig go.mod
+rm go.sum
+
+go mod download all
+cmp go.mod.update go.mod
+grep '^rsc.io/sampler v1.3.0 ' go.sum
+
+# https://golang.org/issue/44435: At go 1.17 or higher, 'go mod download'
+# (without arguments) should only download the modules explicitly required in
+# the go.mod file, not (presumed-irrelevant) transitive dependencies.
+#
+# (If the go.mod file is inconsistent, the version downloaded should be the
+# selected version from the broader graph, but the go.mod file will also be
+# updated to list the correct versions. If at some point we change 'go mod
+# download' to stop updating for consistency, then it should fail if the
+# requirements are inconsistent.)
+
+rm go.sum
+cp go.mod.orig go.mod
+go mod edit -go=1.17
+cp go.mod.update go.mod.go117
+go mod edit -go=1.17 go.mod.go117
+
+go clean -modcache
+go mod download
+cmp go.mod go.mod.go117
+
+go list -e -m all
+stdout '^rsc.io/quote v1.5.2$'
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.zip
+stdout '^rsc.io/sampler v1.3.0$'
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/sampler/@v/v1.2.1.zip
+exists $GOPATH/pkg/mod/cache/download/rsc.io/sampler/@v/v1.3.0.zip
+stdout '^golang\.org/x/text v0.0.0-20170915032832-14c0d48ead0c$'
+! exists $GOPATH/pkg/mod/cache/download/golang.org/x/text/@v/v0.0.0-20170915032832-14c0d48ead0c.zip
+cmp go.mod go.mod.go117
+
+# However, 'go mod download all' continues to download the selected version
+# of every module reported by 'go list -m all'.
+
+cp go.mod.orig go.mod
+go mod edit -go=1.17
+go clean -modcache
+go mod download all
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.zip
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/sampler/@v/v1.2.1.zip
+exists $GOPATH/pkg/mod/cache/download/rsc.io/sampler/@v/v1.3.0.zip
+exists $GOPATH/pkg/mod/cache/download/golang.org/x/text/@v/v0.0.0-20170915032832-14c0d48ead0c.zip
+cmp go.mod go.mod.go117
+
+cd ..
+
+# allow go mod download without go.mod
+env GO111MODULE=auto
+rm go.mod
+rm $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.2.1.zip
+go mod download rsc.io/quote@v1.2.1
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.2.1.zip
+
+# download -x with version should print
+# the underlying commands such as contacting GOPROXY.
+go mod download -x rsc.io/quote@v1.0.0
+! stdout .
+stderr 'get '$GOPROXY
+
+-- go.mod --
+module m
+
+-- update/go.mod.orig --
+module m
+
+go 1.16
+
+require (
+	rsc.io/quote v1.5.2
+	rsc.io/sampler v1.2.1 // older version than in build list
+)
+-- update/go.mod.update --
+module m
+
+go 1.16
+
+require (
+	rsc.io/quote v1.5.2
+	rsc.io/sampler v1.3.0 // older version than in build list
+)
+-- update/go.sum.update --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
diff --git a/src/cmd/go/testdata/script/mod_download_concurrent_read.txt b/src/cmd/go/testdata/script/mod_download_concurrent_read.txt
new file mode 100644
index 0000000..c2c8b18
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_download_concurrent_read.txt
@@ -0,0 +1,110 @@
+# This test simulates a process watching for changes and reading files in
+# module cache as a module is extracted.
+#
+# Before Go 1.16, we extracted each module zip to a temporary directory with
+# a random name, then renamed that into place with os.Rename. On Windows,
+# this failed with ERROR_ACCESS_DENIED when another process (usually an
+# anti-virus scanner) opened files in the temporary directory. This test
+# simulates that behavior, verifying golang.org/issue/36568.
+#
+# Since 1.16, we extract to the final directory, but we create a .partial file
+# so that if we crash, other processes know the directory is incomplete.
+
+[!GOOS:windows] skip
+[short] skip
+
+go run downloader.go
+
+-- go.mod --
+module example.com/m
+
+go 1.14
+
+-- downloader.go --
+package main
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"os/exec"
+	"path/filepath"
+)
+
+func main() {
+	if err := run(); err != nil {
+		log.Fatal(err)
+	}
+}
+
+// run repeatedly downloads a module while opening files in the module cache
+// in a background goroutine.
+//
+// run uses a different temporary module cache in each iteration so that we
+// don't need to clean the cache or synchronize closing files after each
+// iteration.
+func run() (err error) {
+	tmpDir, err := os.MkdirTemp("", "")
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if rmErr := os.RemoveAll(tmpDir); err == nil && rmErr != nil {
+			err = rmErr
+		}
+	}()
+	for i := 0; i < 10; i++ {
+    gopath := filepath.Join(tmpDir, fmt.Sprintf("gopath%d", i))
+		var err error
+		done := make(chan struct{})
+		go func() {
+			err = download(gopath)
+			close(done)
+		}()
+		readCache(gopath, done)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// download downloads a module into the given cache using 'go mod download'.
+func download(gopath string) error {
+	cmd := exec.Command("go", "mod", "download", "-modcacherw", "rsc.io/quote@v1.5.2")
+	cmd.Stderr = os.Stderr
+	cmd.Env = append(os.Environ(), "GOPATH="+gopath)
+	return cmd.Run()
+}
+
+// readCache repeatedly globs for go.mod files in the given cache, then opens
+// those files for reading. When the done chan is closed, readCache closes
+// files and returns.
+func readCache(gopath string, done <-chan struct{}) {
+	files := make(map[string]*os.File)
+	defer func() {
+		for _, f := range files {
+			f.Close()
+		}
+	}()
+
+	pattern := filepath.Join(gopath, "pkg/mod/rsc.io/quote@v1.5.2*/go.mod")
+	for {
+		select {
+		case <-done:
+			return
+		default:
+		}
+
+		names, _ := filepath.Glob(pattern)
+		for _, name := range names {
+			if files[name] != nil {
+				continue
+			}
+			f, _ := os.Open(name)
+			if f != nil {
+				files[name] = f
+			}
+		}
+	}
+}
diff --git a/src/cmd/go/testdata/script/mod_download_exec_toolchain.txt b/src/cmd/go/testdata/script/mod_download_exec_toolchain.txt
new file mode 100644
index 0000000..6cf863b
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_download_exec_toolchain.txt
@@ -0,0 +1,107 @@
+env TESTGO_VERSION=go1.21
+env TESTGO_VERSION_SWITCH=switch
+
+# First, test 'go mod download' outside of a module.
+#
+# There is no go.mod file into which we can record the selected toolchain,
+# so unfortunately these version switches won't be as reproducible as other
+# go commands, but that's still preferable to failing entirely or downloading
+# a module zip that we don't understand.
+
+# GOTOOLCHAIN=auto should run the newer toolchain
+env GOTOOLCHAIN=auto
+go mod download rsc.io/needgo121@latest rsc.io/needgo122@latest rsc.io/needgo123@latest rsc.io/needall@latest
+stderr '^go: rsc.io/needall@v0.0.1 requires go >= 1.23; switching to go1.23.9$'
+! stderr '\(running'
+
+# GOTOOLCHAIN=min+auto should run the newer toolchain
+env GOTOOLCHAIN=go1.21+auto
+go mod download rsc.io/needgo121@latest rsc.io/needgo122@latest rsc.io/needgo123@latest rsc.io/needall@latest
+stderr '^go: rsc.io/needall@v0.0.1 requires go >= 1.23; switching to go1.23.9$'
+! stderr '\(running'
+
+# GOTOOLCHAIN=go1.21 should NOT run the newer toolchain
+env GOTOOLCHAIN=go1.21
+! go mod download rsc.io/needgo121@latest rsc.io/needgo122@latest rsc.io/needgo123@latest rsc.io/needall@latest
+! stderr switching
+stderr 'rsc.io/needgo122@v0.0.1 requires go >= 1.22'
+stderr 'rsc.io/needgo123@v0.0.1 requires go >= 1.23'
+stderr 'rsc.io/needall@v0.0.1 requires go >= 1.23'
+stderr 'requires go >= 1.23'
+! stderr 'requires go >= 1.21' # that's us!
+
+
+# JSON output should be emitted exactly once,
+# and non-JSON output should go to stderr instead of stdout.
+env GOTOOLCHAIN=auto
+go mod download -json rsc.io/needgo121@latest rsc.io/needgo122@latest rsc.io/needgo123@latest rsc.io/needall@latest
+stderr '^go: rsc.io/needall@v0.0.1 requires go >= 1.23; switching to go1.23.9$'
+! stderr '\(running'
+stdout -count=1 '"Path": "rsc.io/needgo121",'
+stdout -count=1 '"Path": "rsc.io/needgo122",'
+stdout -count=1 '"Path": "rsc.io/needgo123",'
+stdout -count=1 '"Path": "rsc.io/needall",'
+
+# GOTOOLCHAIN=go1.21 should write the errors in the JSON Error fields, not to stderr.
+env GOTOOLCHAIN=go1.21
+! go mod download -json rsc.io/needgo121@latest rsc.io/needgo122@latest rsc.io/needgo123@latest rsc.io/needall@latest
+! stderr switching
+stdout -count=1 '"Error": "rsc.io/needgo122@v0.0.1 requires go .*= 1.22 \(running go 1.21; GOTOOLCHAIN=go1.21\)"'
+stdout -count=1 '"Error": "rsc.io/needgo123@v0.0.1 requires go .*= 1.23 \(running go 1.21; GOTOOLCHAIN=go1.21\)"'
+stdout -count=1 '"Error": "rsc.io/needall@v0.0.1 requires go .*= 1.23 \(running go 1.21; GOTOOLCHAIN=go1.21\)"'
+! stdout '"Error": "rsc.io/needgo121'  # We can handle this one.
+! stderr .
+
+
+# Within a module, 'go mod download' of explicit versions should upgrade if
+# needed to perform the download, but should not change the main module's
+# toolchain version (because the downloaded modules are still not required by
+# the main module).
+
+cd example
+cp go.mod go.mod.orig
+
+env GOTOOLCHAIN=auto
+go mod download rsc.io/needgo121@latest rsc.io/needgo122@latest rsc.io/needgo123@latest rsc.io/needall@latest
+stderr '^go: rsc.io/needall@v0.0.1 requires go >= 1.23; switching to go1.23.9$'
+! stderr '\(running'
+cmp go.mod go.mod.orig
+
+
+# However, 'go mod download' without arguments should fix up the
+# 'go' and 'toolchain' lines to be consistent with the existing
+# requirements in the module graph.
+
+go mod edit -require=rsc.io/needall@v0.0.1
+cp go.mod go.mod.121
+
+# If an upgrade is needed, GOTOOLCHAIN=go1.21 should cause
+# the command to fail without changing go.mod.
+
+env GOTOOLCHAIN=go1.21
+! go mod download
+stderr 'rsc.io/needall@v0.0.1 requires go >= 1.23'
+! stderr switching
+cmp go.mod go.mod.121
+
+# If an upgrade is needed, GOTOOLCHAIN=auto should perform
+# the upgrade and record the resulting toolchain version.
+
+env GOTOOLCHAIN=auto
+go mod download
+stderr '^go: module rsc.io/needall@v0.0.1 requires go >= 1.23; switching to go1.23.9$'
+cmp go.mod go.mod.final
+
+
+-- example/go.mod --
+module example
+
+go 1.21
+-- example/go.mod.final --
+module example
+
+go 1.23
+
+toolchain go1.23.9
+
+require rsc.io/needall v0.0.1
diff --git a/src/cmd/go/testdata/script/mod_download_git_bareRepository.txt b/src/cmd/go/testdata/script/mod_download_git_bareRepository.txt
new file mode 100644
index 0000000..8050461
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_download_git_bareRepository.txt
@@ -0,0 +1,19 @@
+[short] skip
+[!git] skip
+[!GOOS:linux] skip  # Uses XDG_CONFIG_HOME
+
+env GIT_CONFIG_GLOBAL=$WORK/.gitconfig
+env GOPRIVATE=vcs-test.golang.org
+
+go mod download -x
+
+-- go.mod --
+module test
+
+go 1.18
+
+require vcs-test.golang.org/git/gitrepo1.git v1.2.3
+
+-- $WORK/.gitconfig --
+[safe]
+bareRepository = explicit
diff --git a/src/cmd/go/testdata/script/mod_download_git_decorate_full.txt b/src/cmd/go/testdata/script/mod_download_git_decorate_full.txt
new file mode 100644
index 0000000..080ccf0
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_download_git_decorate_full.txt
@@ -0,0 +1,28 @@
+env GO111MODULE=on
+
+[short] skip
+[!git] skip
+
+env GOPROXY=direct
+env HOME=$WORK/home/gopher
+
+
+go env GOPROXY
+stdout 'direct'
+
+exec git config --get log.decorate
+stdout 'full'
+
+# Test that Git log with user's global config '~/gitconfig' has log.decorate=full
+# go mod download has an error 'v1.x.y is not a tag'
+# with go1.16.14: 
+# `go1.16.14 list -m vcs-test.golang.org/git/gitrepo1.git@v1.2.3`
+# will output with error:
+# go list -m: vcs-test.golang.org/git/gitrepo1.git@v1.2.3: invalid version: unknown revision v1.2.3
+# See golang/go#51312.
+go list -m vcs-test.golang.org/git/gitrepo1.git@v1.2.3
+stdout 'vcs-test.golang.org/git/gitrepo1.git v1.2.3'
+
+-- $WORK/home/gopher/.gitconfig --
+[log]
+	decorate = full
diff --git a/src/cmd/go/testdata/script/mod_download_hash.txt b/src/cmd/go/testdata/script/mod_download_hash.txt
new file mode 100644
index 0000000..5677e69
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_download_hash.txt
@@ -0,0 +1,24 @@
+env GO111MODULE=on
+
+# Testing mod download with non semantic versions; turn off proxy.
+[!net:rsc.io] skip
+[!git] skip
+env GOPROXY=direct
+env GOSUMDB=off
+
+go mod download rsc.io/quote@a91498bed0a73d4bb9c1fb2597925f7883bc40a7
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-0.20180709162918-a91498bed0a7.info
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-0.20180709162918-a91498bed0a7.mod
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-0.20180709162918-a91498bed0a7.zip
+
+go mod download rsc.io/quote@master
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-0.20180709162918-a91498bed0a7.info
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-0.20180709162918-a91498bed0a7.mod
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.3-0.20180709162918-a91498bed0a7.zip
+
+
+-- go.mod --
+module m
+
+-- m.go --
+package m
diff --git a/src/cmd/go/testdata/script/mod_download_insecure_redirect.txt b/src/cmd/go/testdata/script/mod_download_insecure_redirect.txt
new file mode 100644
index 0000000..20a6ac2
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_download_insecure_redirect.txt
@@ -0,0 +1,32 @@
+# golang.org/issue/29591: 'go get' was following plain-HTTP redirects even without -insecure (now replaced by GOINSECURE).
+
+[short] skip
+[!git] skip
+
+env GO111MODULE=on
+env GOPROXY=direct
+env GOSUMDB=off
+
+! go mod download vcs-test.golang.org/insecure/go/insecure@latest
+stderr 'redirected .* to insecure URL'
+
+# insecure host
+env GOINSECURE=vcs-test.golang.org
+go clean -modcache
+go mod download vcs-test.golang.org/insecure/go/insecure@latest
+
+# insecure glob host
+env GOINSECURE=*.golang.org
+go clean -modcache
+go mod download vcs-test.golang.org/insecure/go/insecure@latest
+
+# insecure multiple host
+env GOINSECURE=somewhere-else.com,*.golang.org
+go clean -modcache
+go mod download vcs-test.golang.org/insecure/go/insecure@latest
+
+# different insecure host does not fetch
+env GOINSECURE=somewhere-else.com
+go clean -modcache
+! go mod download vcs-test.golang.org/insecure/go/insecure@latest
+stderr 'redirected .* to insecure URL'
diff --git a/src/cmd/go/testdata/script/mod_download_issue51114.txt b/src/cmd/go/testdata/script/mod_download_issue51114.txt
new file mode 100644
index 0000000..4d274d6
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_download_issue51114.txt
@@ -0,0 +1,20 @@
+[!net:github.com] skip
+[!git] skip
+[!GOOS:linux] skip  # Uses XDG_CONFIG_HOME
+
+env GIT_CONFIG_GLOBAL=$WORK/.gitconfig
+env GOPROXY=direct
+
+! go mod download
+stderr '^go: github\.com/golang/notexist/subdir@v0.1.0: reading github\.com/golang/notexist/subdir/go\.mod at revision subdir/v0\.1\.0: '
+
+-- go.mod --
+module test
+
+go 1.18
+
+require github.com/golang/notexist/subdir v0.1.0
+
+-- $WORK/.gitconfig --
+[url "git@github.com:"]
+	insteadOf = https://github.com/
diff --git a/src/cmd/go/testdata/script/mod_download_json.txt b/src/cmd/go/testdata/script/mod_download_json.txt
new file mode 100644
index 0000000..9555adf
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_download_json.txt
@@ -0,0 +1,9 @@
+env GO111MODULE=on
+env GOSUMDB=$sumdb' '$proxy/sumdb-wrong
+
+# download -json with version should print JSON on sumdb failure
+! go mod download -json 'rsc.io/quote@<=v1.5.0'
+stdout '"Error": ".*verifying (module|go.mod)'
+
+-- go.mod --
+module m
diff --git a/src/cmd/go/testdata/script/mod_download_partial.txt b/src/cmd/go/testdata/script/mod_download_partial.txt
new file mode 100644
index 0000000..617b1fd
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_download_partial.txt
@@ -0,0 +1,69 @@
+# Download modules and populate go.sum.
+go get -modcacherw
+exists $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/go.mod
+
+# 'go mod verify' should fail if we delete a file.
+go mod verify
+rm $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/go.mod
+! go mod verify
+
+# Create a .partial file to simulate an failure extracting the zip file.
+cp empty $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.partial
+
+# 'go mod verify' should not fail, since the module hasn't been completely
+# ingested into the cache.
+go mod verify
+
+# 'go list' should not load packages from the directory.
+# NOTE: the message "directory $dir outside main module or its selected dependencies"
+# is reported for directories not in the main module, active modules in the
+# module cache, or local replacements. In this case, the directory is in the
+# right place, but it's incomplete, so 'go list' acts as if it's not an
+# active module.
+! go list $GOPATH/pkg/mod/rsc.io/quote@v1.5.2
+stderr 'outside main module or its selected dependencies'
+
+# 'go list -m' should not print the directory.
+go list -m -f '{{.Dir}}' rsc.io/quote
+! stdout .
+
+# 'go mod download' should re-extract the module and remove the .partial file.
+go mod download -modcacherw rsc.io/quote
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.partial
+exists $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/go.mod
+
+# 'go list' should succeed.
+go list $GOPATH/pkg/mod/rsc.io/quote@v1.5.2
+stdout '^rsc.io/quote$'
+
+# 'go list -m' should print the directory.
+go list -m -f '{{.Dir}}' rsc.io/quote
+stdout 'pkg[/\\]mod[/\\]rsc.io[/\\]quote@v1.5.2'
+
+# go mod verify should fail if we delete a file.
+go mod verify
+rm $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/go.mod
+! go mod verify
+
+# 'go mod download' should not leave behind a directory or a .partial file
+# if there is an error extracting the zip file.
+rm $GOPATH/pkg/mod/rsc.io/quote@v1.5.2
+cp empty $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.zip
+! go mod download
+stderr 'not a valid zip file'
+! exists $GOPATH/pkg/mod/rsc.io/quote@v1.5.2
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.partial
+
+-- go.mod --
+module m
+
+go 1.14
+
+require rsc.io/quote v1.5.2
+
+-- use.go --
+package use
+
+import _ "rsc.io/quote"
+
+-- empty --
diff --git a/src/cmd/go/testdata/script/mod_download_private_vcs.txt b/src/cmd/go/testdata/script/mod_download_private_vcs.txt
new file mode 100644
index 0000000..2f72a42
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_download_private_vcs.txt
@@ -0,0 +1,39 @@
+env GO111MODULE=on
+
+# Testing stderr for git ls-remote; turn off proxy.
+[!net:github.com] skip
+[!git] skip
+env GOPROXY=direct
+
+! go mod download github.com/golang/nonexist@latest
+stderr 'Confirm the import path was entered correctly.'
+stderr 'If this is a private repository, see https://golang.org/doc/faq#git_https for additional information.'
+! stdout .
+
+# Fetching a nonexistent commit should return an "unknown revision"
+# error message.
+! go mod download github.com/golang/term@86186f3aba07ed0212cfb944f3398997d2d07c6b
+stderr '^go: github.com/golang/term@86186f3aba07ed0212cfb944f3398997d2d07c6b: invalid version: unknown revision 86186f3aba07ed0212cfb944f3398997d2d07c6b$'
+! stdout .
+
+! go mod download github.com/golang/nonexist@master
+stderr '^Confirm the import path was entered correctly.$'
+stderr '^If this is a private repository, see https://golang.org/doc/faq#git_https for additional information.$'
+! stderr 'unknown revision'
+! stdout .
+
+[!exec:false] stop
+
+# Test that Git clone errors will be shown to the user instead of a generic
+# "unknown revision" error. To do this we want to force git ls-remote to return
+# an error we don't already have special handling for. See golang/go#42751.
+env HOME=$WORK${/}home${/}gopher
+env GIT_SSH_COMMAND=false
+! go install github.com/golang/nonexist@master
+stderr 'fatal: Could not read from remote repository.'
+! stderr 'unknown revision'
+! stdout .
+
+-- $WORK/home/gopher/.gitconfig --
+[url "git@github.com:"]
+	insteadOf = https://github.com/
diff --git a/src/cmd/go/testdata/script/mod_download_replace_file.txt b/src/cmd/go/testdata/script/mod_download_replace_file.txt
new file mode 100644
index 0000000..f6ab4fe
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_download_replace_file.txt
@@ -0,0 +1,16 @@
+# This test checks that 'go mod download' produces no output for
+# the main module (when specified implicitly) and for a module replaced
+# with a file path.
+# Verifies golang.org/issue/35505.
+go mod download -json all
+cmp stdout no-output
+
+-- go.mod --
+module example.com/a
+
+require example.com/b v1.0.0
+
+replace example.com/b => ./local/b
+-- local/b/go.mod --
+module example.com/b
+-- no-output --
diff --git a/src/cmd/go/testdata/script/mod_download_svn.txt b/src/cmd/go/testdata/script/mod_download_svn.txt
new file mode 100644
index 0000000..c11b4f9
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_download_svn.txt
@@ -0,0 +1,29 @@
+[short] skip
+[!exec:svn] skip
+
+# 'go mod download' will fall back to svn+ssh once svn fails over protocols like https.
+# If vcs-test.golang.org isn't in the user's known_hosts file, this will result
+# in an ssh prompt, which will stop 'go test' entirely
+#
+# Unfortunately, there isn't a way to globally disable host checking for ssh,
+# without modifying the real system's or user's configs. Changing $HOME won't
+# affect ssh either, as it ignores the environment variable entirely.
+#
+# However, a useful trick is pointing SVN_SSH to a program that doesn't exist,
+# resulting in svn skipping ssh entirely. Alternatives like
+# SVN_SSH="ssh -o StrictHostKeyChecking=no" didn't avoid the prompt.
+env SVN_SSH="svn_do_not_use_ssh"
+
+env GO111MODULE=on
+env GOPROXY=direct
+env GOSUMDB=off
+
+# Attempting to get a module zip using svn should succeed.
+go mod download vcs-test.golang.org/svn/hello.svn@000000000001
+exists $GOPATH/pkg/mod/cache/download/vcs-test.golang.org/svn/hello.svn/@v/v0.0.0-20170922011245-000000000001.zip
+
+# Attempting to get a nonexistent module using svn should fail with a
+# reasonable message instead of a panic.
+! go mod download vcs-test.golang.org/svn/nonexistent.svn@latest
+! stderr panic
+stderr 'go: module vcs-test.golang.org/svn/nonexistent.svn: no matching versions for query "latest"$'
diff --git a/src/cmd/go/testdata/script/mod_download_too_many_redirects.txt b/src/cmd/go/testdata/script/mod_download_too_many_redirects.txt
new file mode 100644
index 0000000..a6b5a59
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_download_too_many_redirects.txt
@@ -0,0 +1,10 @@
+env GO111MODULE=on
+env GOPROXYBASE=$GOPROXY
+env GOPROXY=$GOPROXYBASE/redirect/11
+env GOSUMDB=off
+
+! go mod download rsc.io/quote@v1.2.0
+stderr 'stopped after 10 redirects'
+
+env GOPROXY=$GOPROXYBASE/redirect/9
+go mod download rsc.io/quote@v1.2.0
diff --git a/src/cmd/go/testdata/script/mod_e.txt b/src/cmd/go/testdata/script/mod_e.txt
new file mode 100644
index 0000000..6497e6c
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_e.txt
@@ -0,0 +1,96 @@
+cp go.mod go.mod.orig
+
+
+# If a dependency cannot be resolved, 'go mod tidy' fails with an error message
+# explaining the problem and does not update the go.mod file.
+# TODO(bcmills): Ideally, with less redundancy than these error messages!
+
+! go mod tidy
+
+stderr '^go: example.com/untidy imports\n\texample.net/directnotfound: cannot find module providing package example.net/directnotfound: module example.net/directnotfound: reading http://.*: 404 Not Found$'
+
+stderr '^go: example.com/untidy imports\n\texample.net/m imports\n\texample.net/indirectnotfound: cannot find module providing package example.net/indirectnotfound: module example.net/indirectnotfound: reading http://.*: 404 Not Found$'
+
+stderr '^go: example.com/untidy tested by\n\texample.com/untidy.test imports\n\texample.net/directtestnotfound: cannot find module providing package example.net/directtestnotfound: module example.net/directtestnotfound: reading http://.*: 404 Not Found$'
+
+stderr '^go: example.com/untidy imports\n\texample.net/m tested by\n\texample.net/m.test imports\n\texample.net/indirecttestnotfound: cannot find module providing package example.net/indirecttestnotfound: module example.net/indirecttestnotfound: reading http://.*: 404 Not Found$'
+
+cmp go.mod.orig go.mod
+
+
+# If a dependency cannot be resolved, 'go mod vendor' fails with an error message
+# explaining the problem, does not update the go.mod file, and does not create
+# the vendor directory.
+
+! go mod vendor
+
+stderr '^go: example.com/untidy imports\n\texample.net/directnotfound: no required module provides package example.net/directnotfound; to add it:\n\tgo get example.net/directnotfound$'
+
+stderr '^go: example.com/untidy imports\n\texample.net/m: module example.net/m provides package example.net/m and is replaced but not required; to add it:\n\tgo get example.net/m@v0.1.0$'
+
+stderr '^go: example.com/untidy tested by\n\texample.com/untidy.test imports\n\texample.net/directtestnotfound: no required module provides package example.net/directtestnotfound; to add it:\n\tgo get example.net/directtestnotfound$'
+
+! stderr 'indirecttestnotfound'  # Vendor prunes test dependencies.
+
+cmp go.mod.orig go.mod
+! exists vendor
+
+
+# 'go mod tidy' still logs the errors, but succeeds and updates go.mod.
+
+go mod tidy -e
+stderr -count=4 'cannot find module providing package'
+cmp go.mod.final go.mod
+
+
+# 'go mod vendor -e' still logs the errors, but creates a vendor directory
+# and exits with status 0.
+# 'go mod vendor -e' does not update go.mod and will not vendor packages that
+# would require changing go.mod, for example, by adding a requirement.
+cp go.mod.orig go.mod
+go mod vendor -e
+stderr -count=2 'no required module provides package'
+stderr '^go: example.com/untidy imports\n\texample.net/m: module example.net/m provides package example.net/m and is replaced but not required; to add it:\n\tgo get example.net/m@v0.1.0$'
+exists vendor/modules.txt
+! exists vendor/example.net
+
+go mod edit -require example.net/m@v0.1.0
+go mod vendor -e
+stderr -count=3 'no required module provides package'
+exists vendor/modules.txt
+exists vendor/example.net/m/m.go
+
+-- go.mod --
+module example.com/untidy
+go 1.16
+replace example.net/m v0.1.0 => ./m
+-- go.mod.final --
+module example.com/untidy
+
+go 1.16
+
+replace example.net/m v0.1.0 => ./m
+
+require example.net/m v0.1.0
+-- untidy.go --
+package untidy
+
+import (
+	_ "example.net/m"
+	_ "example.net/directnotfound"
+)
+-- untidy_test.go --
+package untidy_test
+
+import _ "example.net/directtestnotfound"
+-- m/go.mod --
+module example.net/m
+go 1.16
+-- m/m.go --
+package m
+
+import _ "example.net/indirectnotfound"
+-- m/m_test.go --
+package m_test
+
+import _ "example.net/indirecttestnotfound"
diff --git a/src/cmd/go/testdata/script/mod_edit.txt b/src/cmd/go/testdata/script/mod_edit.txt
new file mode 100644
index 0000000..ebc032a
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_edit.txt
@@ -0,0 +1,321 @@
+env GO111MODULE=on
+
+# Test that go mod edits and related mod flags work.
+# Also test that they can use a dummy name that isn't resolvable. golang.org/issue/24100
+
+# go mod init
+! go mod init
+stderr 'cannot determine module path'
+! exists go.mod
+
+go mod init x.x/y/z
+stderr 'creating new go.mod: module x.x/y/z'
+cmpenv go.mod $WORK/go.mod.init
+
+! go mod init
+cmpenv go.mod $WORK/go.mod.init
+
+# go mod edits
+go mod edit -droprequire=x.1 -require=x.1@v1.0.0 -require=x.2@v1.1.0 -droprequire=x.2 -exclude='x.1 @ v1.2.0' -exclude=x.1@v1.2.1 -exclude=x.1@v2.0.0+incompatible -replace=x.1@v1.3.0=y.1@v1.4.0 -replace='x.1@v1.4.0 = ../z' -retract=v1.6.0 -retract=[v1.1.0,v1.2.0] -retract=[v1.3.0,v1.4.0] -retract=v1.0.0
+cmpenv go.mod $WORK/go.mod.edit1
+go mod edit -droprequire=x.1 -dropexclude=x.1@v1.2.1 -dropexclude=x.1@v2.0.0+incompatible -dropreplace=x.1@v1.3.0 -require=x.3@v1.99.0 -dropretract=v1.0.0 -dropretract=[v1.1.0,v1.2.0]
+cmpenv go.mod $WORK/go.mod.edit2
+
+# -exclude and -retract reject invalid versions.
+! go mod edit -exclude=example.com/m@bad
+stderr '^go: -exclude=example.com/m@bad: version "bad" invalid: must be of the form v1.2.3$'
+! go mod edit -retract=bad
+stderr '^go: -retract=bad: version "bad" invalid: must be of the form v1.2.3$'
+
+! go mod edit -exclude=example.com/m@v2.0.0
+stderr '^go: -exclude=example.com/m@v2\.0\.0: version "v2\.0\.0" invalid: should be v2\.0\.0\+incompatible \(or module example\.com/m/v2\)$'
+
+! go mod edit -exclude=example.com/m/v2@v1.0.0
+stderr '^go: -exclude=example.com/m/v2@v1\.0\.0: version "v1\.0\.0" invalid: should be v2, not v1$'
+
+! go mod edit -exclude=gopkg.in/example.v1@v2.0.0
+stderr '^go: -exclude=gopkg\.in/example\.v1@v2\.0\.0: version "v2\.0\.0" invalid: should be v1, not v2$'
+
+cmpenv go.mod $WORK/go.mod.edit2
+
+# go mod edit -json
+go mod edit -json
+cmpenv stdout $WORK/go.mod.json
+
+# go mod edit -json (retractions with rationales)
+go mod edit -json $WORK/go.mod.retractrationale
+cmp stdout $WORK/go.mod.retractrationale.json
+
+# go mod edit -json (deprecation)
+go mod edit -json $WORK/go.mod.deprecation
+cmp stdout $WORK/go.mod.deprecation.json
+
+# go mod edit -json (empty mod file)
+go mod edit -json $WORK/go.mod.empty
+cmp stdout $WORK/go.mod.empty.json
+
+# go mod edit -replace
+go mod edit -replace=x.1@v1.3.0=y.1/v2@v2.3.5 -replace=x.1@v1.4.0=y.1/v2@v2.3.5
+cmpenv go.mod $WORK/go.mod.edit3
+go mod edit -replace=x.1=y.1/v2@v2.3.6
+cmpenv go.mod $WORK/go.mod.edit4
+go mod edit -dropreplace=x.1
+cmpenv go.mod $WORK/go.mod.edit5
+
+# go mod edit -fmt
+cp $WORK/go.mod.badfmt go.mod
+go mod edit -fmt -print # -print should avoid writing file
+cmpenv stdout $WORK/go.mod.goodfmt
+cmp go.mod $WORK/go.mod.badfmt
+go mod edit -fmt # without -print, should write file (and nothing to stdout)
+! stdout .
+cmpenv go.mod $WORK/go.mod.goodfmt
+
+# go mod edit -module
+cd $WORK/m
+go mod init a.a/b/c
+go mod edit -module x.x/y/z
+cmpenv go.mod go.mod.edit
+
+# golang.org/issue/30513: don't require go-gettable module paths.
+cd $WORK/local
+go mod init foo
+go mod edit -module local-only -require=other-local@v1.0.0 -replace other-local@v1.0.0=./other
+cmpenv go.mod go.mod.edit
+
+-- x.go --
+package x
+
+-- w/w.go --
+package w
+
+-- $WORK/go.mod.init --
+module x.x/y/z
+
+go $goversion
+-- $WORK/go.mod.edit1 --
+module x.x/y/z
+
+go $goversion
+
+require x.1 v1.0.0
+
+exclude (
+	x.1 v1.2.0
+	x.1 v1.2.1
+	x.1 v2.0.0+incompatible
+)
+
+replace (
+	x.1 v1.3.0 => y.1 v1.4.0
+	x.1 v1.4.0 => ../z
+)
+
+retract (
+	v1.6.0
+	[v1.3.0, v1.4.0]
+	[v1.1.0, v1.2.0]
+	v1.0.0
+)
+-- $WORK/go.mod.edit2 --
+module x.x/y/z
+
+go $goversion
+
+exclude x.1 v1.2.0
+
+replace x.1 v1.4.0 => ../z
+
+retract (
+	v1.6.0
+	[v1.3.0, v1.4.0]
+)
+
+require x.3 v1.99.0
+-- $WORK/go.mod.json --
+{
+	"Module": {
+		"Path": "x.x/y/z"
+	},
+	"Go": "$goversion",
+	"Require": [
+		{
+			"Path": "x.3",
+			"Version": "v1.99.0"
+		}
+	],
+	"Exclude": [
+		{
+			"Path": "x.1",
+			"Version": "v1.2.0"
+		}
+	],
+	"Replace": [
+		{
+			"Old": {
+				"Path": "x.1",
+				"Version": "v1.4.0"
+			},
+			"New": {
+				"Path": "../z"
+			}
+		}
+	],
+	"Retract": [
+		{
+			"Low": "v1.6.0",
+			"High": "v1.6.0"
+		},
+		{
+			"Low": "v1.3.0",
+			"High": "v1.4.0"
+		}
+	]
+}
+-- $WORK/go.mod.edit3 --
+module x.x/y/z
+
+go $goversion
+
+exclude x.1 v1.2.0
+
+replace (
+	x.1 v1.3.0 => y.1/v2 v2.3.5
+	x.1 v1.4.0 => y.1/v2 v2.3.5
+)
+
+retract (
+	v1.6.0
+	[v1.3.0, v1.4.0]
+)
+
+require x.3 v1.99.0
+-- $WORK/go.mod.edit4 --
+module x.x/y/z
+
+go $goversion
+
+exclude x.1 v1.2.0
+
+replace x.1 => y.1/v2 v2.3.6
+
+retract (
+	v1.6.0
+	[v1.3.0, v1.4.0]
+)
+
+require x.3 v1.99.0
+-- $WORK/go.mod.edit5 --
+module x.x/y/z
+
+go $goversion
+
+exclude x.1 v1.2.0
+
+retract (
+	v1.6.0
+	[v1.3.0, v1.4.0]
+)
+
+require x.3 v1.99.0
+-- $WORK/local/go.mod.edit --
+module local-only
+
+go $goversion
+
+require other-local v1.0.0
+
+replace other-local v1.0.0 => ./other
+-- $WORK/go.mod.badfmt --
+module     x.x/y/z
+
+go 1.10
+
+exclude x.1     v1.2.0
+
+replace x.1    =>   y.1/v2 v2.3.6
+
+require x.3   v1.99.0
+
+retract [  "v1.8.1" , "v1.8.2" ]
+-- $WORK/go.mod.goodfmt --
+module x.x/y/z
+
+go 1.10
+
+exclude x.1 v1.2.0
+
+replace x.1 => y.1/v2 v2.3.6
+
+require x.3 v1.99.0
+
+retract [v1.8.1, v1.8.2]
+-- $WORK/m/go.mod.edit --
+module x.x/y/z
+
+go $goversion
+-- $WORK/go.mod.retractrationale --
+module x.x/y/z
+
+go 1.15
+
+// a
+retract v1.0.0
+
+// b
+retract (
+  v1.0.1
+  v1.0.2 // c
+)
+-- $WORK/go.mod.retractrationale.json --
+{
+	"Module": {
+		"Path": "x.x/y/z"
+	},
+	"Go": "1.15",
+	"Require": null,
+	"Exclude": null,
+	"Replace": null,
+	"Retract": [
+		{
+			"Low": "v1.0.0",
+			"High": "v1.0.0",
+			"Rationale": "a"
+		},
+		{
+			"Low": "v1.0.1",
+			"High": "v1.0.1",
+			"Rationale": "b"
+		},
+		{
+			"Low": "v1.0.2",
+			"High": "v1.0.2",
+			"Rationale": "c"
+		}
+	]
+}
+-- $WORK/go.mod.deprecation --
+// Deprecated: and the new one is not ready yet
+module m
+-- $WORK/go.mod.deprecation.json --
+{
+	"Module": {
+		"Path": "m",
+		"Deprecated": "and the new one is not ready yet"
+	},
+	"Require": null,
+	"Exclude": null,
+	"Replace": null,
+	"Retract": null
+}
+-- $WORK/go.mod.empty --
+-- $WORK/go.mod.empty.json --
+{
+	"Module": {
+		"Path": ""
+	},
+	"Require": null,
+	"Exclude": null,
+	"Replace": null,
+	"Retract": null
+}
diff --git a/src/cmd/go/testdata/script/mod_edit_go.txt b/src/cmd/go/testdata/script/mod_edit_go.txt
new file mode 100644
index 0000000..ec04f40
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_edit_go.txt
@@ -0,0 +1,26 @@
+# Test support for go mod edit -go to set language version.
+
+env GO111MODULE=on
+! go build
+stderr ' type aliases requires'
+go mod edit -go=1.9
+grep 'go 1.9' go.mod
+go build
+
+# Reverting the version should force a rebuild and error instead of using
+# the cached 1.9 build. (https://golang.org/issue/37804)
+go mod edit -go=1.8
+! go build
+stderr 'type aliases requires'
+
+# go=none should drop the line
+go mod edit -go=none
+! grep go go.mod
+
+-- go.mod --
+module m
+go 1.8
+
+-- alias.go --
+package alias
+type T = int
diff --git a/src/cmd/go/testdata/script/mod_edit_no_modcache.txt b/src/cmd/go/testdata/script/mod_edit_no_modcache.txt
new file mode 100644
index 0000000..ced15bb
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_edit_no_modcache.txt
@@ -0,0 +1,15 @@
+# 'go mod edit' opportunistically locks the side-lock file in the module cache,
+# for compatibility with older versions of the 'go' command.
+# It does not otherwise depend on the module cache, so it should not
+# fail if the module cache directory cannot be created.
+
+[root] skip
+
+mkdir $WORK/readonly
+chmod 0555 $WORK/readonly
+env GOPATH=$WORK/readonly/nonexist
+
+go mod edit -go=1.17
+
+-- go.mod --
+module example.com/m
diff --git a/src/cmd/go/testdata/script/mod_edit_toolchain.txt b/src/cmd/go/testdata/script/mod_edit_toolchain.txt
new file mode 100644
index 0000000..bb544be
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_edit_toolchain.txt
@@ -0,0 +1,18 @@
+# Test support for go mod edit -toolchain to set toolchain to use
+
+env GOTOOLCHAIN=local
+env GO111MODULE=on
+
+! grep toolchain go.mod
+go mod edit -toolchain=go1.9
+grep 'toolchain go1.9' go.mod
+
+go mod edit -toolchain=default
+grep 'toolchain default' go.mod
+
+go mod edit -toolchain=none
+! grep toolchain go.mod
+
+-- go.mod --
+module m
+go 1.8
diff --git a/src/cmd/go/testdata/script/mod_empty_err.txt b/src/cmd/go/testdata/script/mod_empty_err.txt
new file mode 100644
index 0000000..4b4a007
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_empty_err.txt
@@ -0,0 +1,36 @@
+# This test checks error messages for non-existent packages in module mode.
+# Veries golang.org/issue/35414
+env GO111MODULE=on
+cd $WORK
+
+go list -e -f {{.Error}} .
+stdout 'no Go files in '$WORK
+
+go list -e -f {{.Error}} ./empty
+stdout 'no Go files in '$WORK${/}'empty'
+
+go list -e -f {{.Error}} ./exclude
+stdout 'build constraints exclude all Go files in '$WORK${/}'exclude'
+
+go list -e -f {{.Error}} ./missing
+stdout 'stat '$WORK'[/\\]missing: directory not found'
+
+# use 'go build -n' because 'go list' reports no error.
+! go build -n ./testonly
+stderr 'example.com/m/testonly: no non-test Go files in '$WORK${/}'testonly'
+
+-- $WORK/go.mod --
+module example.com/m
+
+go 1.14
+
+-- $WORK/empty/empty.txt --
+-- $WORK/exclude/exclude.go --
+// +build exclude
+
+package exclude
+-- $WORK/testonly/testonly_test.go --
+package testonly_test
+-- $WORK/excluded-stdout --
+package ./excluded: cannot find package "." in:
+	$WORK/excluded
diff --git a/src/cmd/go/testdata/script/mod_enabled.txt b/src/cmd/go/testdata/script/mod_enabled.txt
new file mode 100644
index 0000000..39f1ece
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_enabled.txt
@@ -0,0 +1,93 @@
+# GO111MODULE=auto should trigger any time a go.mod exists in a parent directory.
+env GO111MODULE=auto
+
+cd $GOPATH/src/x/y/z
+go env GOMOD
+stdout $GOPATH[/\\]src[/\\]x[/\\]y[/\\]z[/\\]go.mod
+go list -m -f {{.GoMod}}
+stdout $GOPATH[/\\]src[/\\]x[/\\]y[/\\]z[/\\]go.mod
+
+cd $GOPATH/src/x/y/z/w
+go env GOMOD
+stdout $GOPATH[/\\]src[/\\]x[/\\]y[/\\]z[/\\]go.mod
+
+cd $GOPATH/src/x/y
+go env GOMOD
+! stdout .
+
+cd $GOPATH/foo
+go env GOMOD
+stdout foo[/\\]go.mod
+go list -m -f {{.GoMod}}
+stdout foo[/\\]go.mod
+
+cd $GOPATH/foo/bar/baz
+go env GOMOD
+stdout foo[/\\]go.mod
+
+# GO111MODULE unset should be equivalent to on.
+env GO111MODULE=
+
+cd $GOPATH/src/x/y/z
+go env GOMOD
+stdout $GOPATH[/\\]src[/\\]x[/\\]y[/\\]z[/\\]go.mod
+
+cd $GOPATH/src/x/y
+go env GOMOD
+stdout 'NUL|/dev/null'
+
+# GO111MODULE=on should trigger everywhere
+env GO111MODULE=on
+
+cd $GOPATH/src/x/y/z
+go env GOMOD
+stdout z[/\\]go.mod
+
+cd $GOPATH/src/x/y/z/w
+go env GOMOD
+stdout z[/\\]go.mod
+
+cd $GOPATH/src/x/y
+go env GOMOD
+stdout 'NUL|/dev/null'
+go list -m
+stdout '^command-line-arguments$'
+
+cd $GOPATH/foo
+go env GOMOD
+stdout foo[/\\]go.mod
+
+cd $GOPATH/foo/bar/baz
+go env GOMOD
+stdout foo[/\\]go.mod
+
+# GO111MODULE=off should trigger nowhere
+env GO111MODULE=off
+
+cd $GOPATH/src/x/y/z
+go env GOMOD
+! stdout .+
+
+cd $GOPATH/foo
+go env GOMOD
+! stdout .+
+
+cd $GOPATH/foo/bar/baz
+go env GOMOD
+! stdout .+
+
+# GO111MODULE=auto should ignore and warn about /tmp/go.mod
+env GO111MODULE=auto
+cp $GOPATH/src/x/y/z/go.mod $WORK/tmp/go.mod
+mkdir $WORK/tmp/mydir
+cd $WORK/tmp/mydir
+go env GOMOD
+! stdout .+
+stderr '^go: warning: ignoring go.mod in system temp root '
+
+-- $GOPATH/src/x/y/z/go.mod --
+module x/y/z
+-- $GOPATH/src/x/y/z/w/w.txt --
+-- $GOPATH/foo/go.mod --
+module example.com/mod
+-- $GOPATH/foo/bar/baz/quux.txt --
diff --git a/src/cmd/go/testdata/script/mod_exclude_go121.txt b/src/cmd/go/testdata/script/mod_exclude_go121.txt
new file mode 100644
index 0000000..51c8a00
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_exclude_go121.txt
@@ -0,0 +1,34 @@
+# go.dev/issue/60028: use semver sort in exclude block in 1.21
+cp $WORK/go.mod.badfmtexclude go.mod
+go mod edit -go=1.20
+cmp go.mod $WORK/go.mod.goodfmtexclude120
+go mod edit -go=1.21
+cmp go.mod $WORK/go.mod.goodfmtexclude121
+
+-- $WORK/go.mod.badfmtexclude --
+module     x.x/y/z
+exclude  (
+	x.1   v1.11.0
+	x.1    v1.10.0
+	x.1     v1.9.0
+)
+-- $WORK/go.mod.goodfmtexclude120 --
+module x.x/y/z
+
+go 1.20
+
+exclude (
+	x.1 v1.10.0
+	x.1 v1.11.0
+	x.1 v1.9.0
+)
+-- $WORK/go.mod.goodfmtexclude121 --
+module x.x/y/z
+
+go 1.21
+
+exclude (
+	x.1 v1.9.0
+	x.1 v1.10.0
+	x.1 v1.11.0
+)
diff --git a/src/cmd/go/testdata/script/mod_file_proxy.txt b/src/cmd/go/testdata/script/mod_file_proxy.txt
new file mode 100644
index 0000000..ebc28a0
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_file_proxy.txt
@@ -0,0 +1,36 @@
+[short] skip
+
+# Allow (cached) downloads for -mod=readonly.
+env GO111MODULE=on
+env GOPATH=$WORK/gopath1
+cd $WORK/x
+go mod edit -fmt
+go list -mod=readonly
+env GOPROXY=file:///nonexist
+go list
+grep v1.5.1 $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/list
+
+# Use download cache as file:/// proxy.
+env GOPATH=$WORK/gopath2
+[GOOS:windows] env GOPROXY=file:///C:/nonexist
+[!GOOS:windows] env GOPROXY=file:///nonexist
+! go list
+[GOOS:windows] env GOPROXY=file:///$WORK/gopath1/pkg/mod/cache/download
+[!GOOS:windows] env GOPROXY=file://$WORK/gopath1/pkg/mod/cache/download
+go list
+grep v1.5.1 $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/list
+
+-- $WORK/x/go.mod --
+module x
+go 1.13
+require rsc.io/quote v1.5.1
+-- $WORK/x/x.go --
+package x
+import _ "rsc.io/quote"
+-- $WORK/x/go.sum --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c h1:pvCbr/wm8HzDD3fVywevekufpn6tCGPY3spdHeZJEsw=
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.5.1 h1:ZE3OgnVGrhXtFkGw90HwW992ZRqcdli/33DLqEYsoxA=
+rsc.io/quote v1.5.1/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0 h1:HLGR/BgEtI3r0uymSP/nl2uPLsUnNJX8toRyhfpBTII=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
diff --git a/src/cmd/go/testdata/script/mod_fileproxy_vcs_missing_issue51589.txt b/src/cmd/go/testdata/script/mod_fileproxy_vcs_missing_issue51589.txt
new file mode 100644
index 0000000..633bd8b
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_fileproxy_vcs_missing_issue51589.txt
@@ -0,0 +1,42 @@
+# This test checks that "go mod tidy -e" do not panic when
+# using a file goproxy that is missing some modules.
+# Verifies golang.org/issue/51589
+
+# download the modules first
+env GO111MODULE=on
+env GOPATH=$WORK/gopath
+cd $WORK/x
+go mod tidy
+
+# Use download cache as file:/// proxy.
+[GOOS:windows] env GOPROXY=file:///$WORK/gopath/pkg/mod/cache/download
+[!GOOS:windows] env GOPROXY=file://$WORK/gopath/pkg/mod/cache/download
+rm $WORK/gopath/pkg/mod/cache/download/golang.org/x/text/
+go mod tidy -e
+stderr '^go: rsc.io/sampler@v1.3.0 requires\n\tgolang.org/x/text@.*: reading file://.*/pkg/mod/cache/download/golang.org/x/text/.*'
+! stderr 'signal SIGSEGV: segmentation violation'
+
+-- $WORK/x/go.mod --
+module example.com/mod
+
+go 1.17
+
+require rsc.io/quote v1.5.2
+
+require (
+	golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c // indirect
+	rsc.io/sampler v1.3.0 // indirect
+)
+
+-- $WORK/x/x.go --
+package mod
+
+import (
+	"fmt"
+
+	"rsc.io/quote"
+)
+
+func Echo() {
+	fmt.Println(quote.Hello())
+}
diff --git a/src/cmd/go/testdata/script/mod_find.txt b/src/cmd/go/testdata/script/mod_find.txt
new file mode 100644
index 0000000..9c2037b
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_find.txt
@@ -0,0 +1,104 @@
+env GO111MODULE=on
+
+# Derive module path from import comment.
+cd $WORK/x
+exists x.go
+go mod init
+stderr 'module x'
+
+# Import comment works even with CRLF line endings.
+rm go.mod
+replace '\n' '\r\n' x.go
+go mod init
+stderr 'module x'
+
+# Derive module path from location inside GOPATH.
+# 'go mod init' should succeed if modules are not explicitly disabled.
+cd $GOPATH/src/example.com/x/y
+go mod init
+stderr 'module example.com/x/y$'
+rm go.mod
+
+# go mod init rejects a zero-length go.mod file
+cp $devnull go.mod # can't use touch to create it because Windows
+! go mod init
+stderr 'go.mod already exists'
+
+# Module path from Godeps/Godeps.json overrides GOPATH.
+cd $GOPATH/src/example.com/x/y/z
+go mod init
+stderr 'unexpected.com/z'
+rm go.mod
+
+# Empty directory outside GOPATH fails.
+mkdir $WORK/empty
+cd $WORK/empty
+! go mod init
+stderr 'cannot determine module path for source directory'
+rm go.mod
+
+# Empty directory inside GOPATH/src uses location inside GOPATH.
+mkdir $GOPATH/src/empty
+cd $GOPATH/src/empty
+go mod init
+stderr 'empty'
+rm go.mod
+
+# In Plan 9, directories are automatically created in /n.
+# For example, /n/go.mod always exist, but it's a directory.
+# Test that we ignore directories when trying to find go.mod.
+cd $WORK/gomoddir
+! go list .
+stderr '^go: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+
+[!symlink] stop
+
+# gplink1/src/empty where gopathlink -> GOPATH
+symlink $WORK/gopathlink -> gopath
+cd $WORK/gopathlink/src/empty
+go mod init
+rm go.mod
+
+# GOPATH/src/link where link -> out of GOPATH
+symlink $GOPATH/src/link -> $WORK/empty
+cd $WORK/empty
+! go mod init
+cd $GOPATH/src/link
+go mod init
+stderr link
+rm go.mod
+
+# GOPATH/src/empty where GOPATH itself is a symlink
+env GOPATH=$WORK/gopathlink
+cd $GOPATH/src/empty
+go mod init
+rm go.mod
+cd $WORK/gopath/src/empty
+go mod init
+rm go.mod
+
+# GOPATH/src/link where GOPATH and link are both symlinks
+cd $GOPATH/src/link
+go mod init
+stderr link
+rm go.mod
+
+# Too hard: doesn't match unevaluated nor completely evaluated. (Only partially evaluated.)
+# Whether this works depends on which OS we are running on.
+# cd $WORK/gopath/src/link
+# ! go mod init
+
+-- $WORK/x/x.go --
+package x // import "x"
+
+-- $GOPATH/src/example.com/x/y/y.go --
+package y
+-- $GOPATH/src/example.com/x/y/z/z.go --
+package z
+-- $GOPATH/src/example.com/x/y/z/Godeps/Godeps.json --
+{"ImportPath": "unexpected.com/z"}
+
+-- $WORK/gomoddir/go.mod/README.txt --
+../go.mod is a directory, not a file.
+-- $WORK/gomoddir/p.go --
+package p
diff --git a/src/cmd/go/testdata/script/mod_fs_patterns.txt b/src/cmd/go/testdata/script/mod_fs_patterns.txt
new file mode 100644
index 0000000..c834ce8
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_fs_patterns.txt
@@ -0,0 +1,97 @@
+env GO111MODULE=on
+
+# File system pattern searches should skip sub-modules and vendor directories.
+cd x
+
+# all packages
+go list all
+stdout ^m$
+stdout ^m/vendor$
+! stdout vendor/
+stdout ^m/y$
+! stdout ^m/y/z
+
+# path pattern
+go list m/...
+stdout ^m$
+stdout ^m/vendor$
+! stdout vendor/
+stdout ^m/y$
+! stdout ^m/y/z
+
+# directory pattern
+go list ./...
+stdout ^m$
+stdout ^m/vendor$
+! stdout vendor/
+stdout ^m/y$
+! stdout ^m/y/z
+
+# non-existent directory should not prompt lookups
+! go build -mod=readonly example.com/nonexist
+stderr 'import lookup disabled'
+
+! go build -mod=readonly ./nonexist
+! stderr 'import lookup disabled'
+stderr '^stat '$GOPATH'[/\\]src[/\\]x[/\\]nonexist: directory not found'
+
+! go build -mod=readonly ./go.mod
+! stderr 'import lookup disabled'
+stderr 'main module \(m\) does not contain package m/go.mod'
+
+
+# File system paths and patterns should allow the '@' character.
+cd ../@at
+go list $PWD
+stdout '^at$'
+go list $PWD/...
+stdout '^at$'
+
+# The '@' character is not allowed in directory paths that are part of
+# a package path.
+cd ../badat/bad@
+! go list .
+stderr 'current directory outside main module or its selected dependencies'
+! go list $PWD
+stderr 'current directory outside main module or its selected dependencies'
+! go list $PWD/...
+stderr 'current directory outside main module or its selected dependencies'
+
+-- x/go.mod --
+module m
+
+-- x/x.go --
+package x
+
+-- x/vendor/v/v.go --
+package v
+import _ "golang.org/x/crypto"
+
+-- x/vendor/v.go --
+package main
+
+-- x/y/y.go --
+package y
+
+-- x/y/z/go.mod --
+syntax error!
+
+-- x/y/z/z.go --
+package z
+
+-- x/y/z/w/w.go --
+package w
+
+-- @at/go.mod --
+module at
+
+go 1.14
+-- @at/at.go --
+package at
+
+-- badat/go.mod --
+module badat
+
+go 1.14
+-- badat/bad@/bad.go --
+package bad
diff --git a/src/cmd/go/testdata/script/mod_get_ambiguous_arg.txt b/src/cmd/go/testdata/script/mod_get_ambiguous_arg.txt
new file mode 100644
index 0000000..9214993
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_ambiguous_arg.txt
@@ -0,0 +1,107 @@
+go mod tidy
+cp go.mod go.mod.orig
+
+# If there is no sensible *package* meaning for 'm/p', it should refer
+# to *module* m/p.
+
+go get m/p  # @latest
+go list -m all
+stdout '^m/p v0.3.0 '
+! stdout '^m '
+
+cp go.mod.orig go.mod
+
+go get m/p@v0.1.0
+go list -m all
+stdout '^m/p v0.1.0 '
+! stdout '^m '
+
+# When feasible, the argument 'm/p' in 'go get m/p' refers to *package* m/p,
+# which is in module m.
+#
+# (It only refers to *module* m/p if there is no such package at the
+# requested version.)
+
+go get m/p@v0.2.0
+go list -m all
+stdout '^m v0.2.0 '
+stdout '^m/p v0.1.0 '  # unchanged from the previous case
+
+# Repeating the above with module m/p already in the module graph does not
+# change its meaning.
+
+go get m/p@v0.2.0
+go list -m all
+stdout '^m v0.2.0 '
+stdout '^m/p v0.1.0 '
+
+-- go.mod --
+module example.com
+
+go 1.16
+
+replace (
+	m v0.1.0 => ./m01
+	m v0.2.0 => ./m02
+	m v0.3.0 => ./m03
+	m/p v0.1.0 => ./mp01
+	m/p v0.2.0 => ./mp02
+	m/p v0.3.0 => ./mp03
+)
+-- m01/go.mod --
+module m
+
+go 1.16
+-- m01/README.txt --
+Module m at v0.1.0 does not yet contain package p.
+
+-- m02/go.mod --
+module m
+
+go 1.16
+
+require m/p v0.1.0
+-- m02/p/p.go --
+// Package p is present in module m, but not module m/p.
+package p
+
+-- m03/go.mod --
+module m
+
+go 1.16
+
+require m/p v0.1.0
+-- m03/README.txt --
+Module m at v0.3.0 no longer contains package p.
+
+-- mv2/go.mod --
+module m/v2
+
+go 1.16
+-- mv2/README.txt --
+This module is m/v2. It doesn't actually need to exist,
+but it explains how module m could plausibly exist
+and still contain package p at 'latest' even when module
+m/p also exists.
+
+-- mp01/go.mod --
+module m/p
+
+go 1.16
+-- mp01/README.txt --
+This module is m/p.
+Package m/p does not exist in this module.
+-- mp02/go.mod --
+module m/p
+
+go 1.16
+-- mp02/README.txt --
+This module is m/p.
+Package m/p does not exist in this module.
+-- mp03/go.mod --
+module m/p
+
+go 1.16
+-- mp03/README.txt --
+This module is m/p.
+Package m/p does not exist in this module.
diff --git a/src/cmd/go/testdata/script/mod_get_ambiguous_import.txt b/src/cmd/go/testdata/script/mod_get_ambiguous_import.txt
new file mode 100644
index 0000000..f08e540
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_ambiguous_import.txt
@@ -0,0 +1,60 @@
+go list -m all
+stdout '^example.net/m v0.1.0 '
+! stdout '^example.net/m/p '
+cp go.mod go.mod.orig
+
+# Upgrading example.net/m/p without also upgrading example.net/m
+# causes the import of package example.net/m/p to be ambiguous.
+#
+# TODO(#27899): Should we automatically upgrade example.net/m to v0.2.0
+# to resolve the conflict?
+! go get example.net/m/p@v1.0.0
+stderr '^go: example.net/m/p: ambiguous import: found package example.net/m/p in multiple modules:\n\texample.net/m v0.1.0 \(.*[/\\]m1[/\\]p\)\n\texample.net/m/p v1.0.0 \(.*[/\\]p0\)\n\z'
+cmp go.mod go.mod.orig
+
+# Upgrading both modules simultaneously resolves the ambiguous upgrade.
+# Note that this command line mixes a module path (example.net/m)
+# and a package path (example.net/m/p) in the same command.
+go get example.net/m@v0.2.0 example.net/m/p@v1.0.0
+
+go list -m all
+stdout '^example.net/m v0.2.0 '
+stdout '^example.net/m/p v1.0.0 '
+
+-- go.mod --
+module example.net/importer
+
+go 1.16
+
+require (
+	example.net/m v0.1.0
+)
+
+replace (
+	example.net/m v0.1.0 => ./m1
+	example.net/m v0.2.0 => ./m2
+	example.net/m/p v1.0.0 => ./p0
+)
+-- importer.go --
+package importer
+import _ "example.net/m/p"
+-- m1/go.mod --
+module example.net/m
+
+go 1.16
+-- m1/p/p.go --
+package p
+-- m2/go.mod --
+module example.net/m
+
+go 1.16
+-- m2/README.txt --
+Package p has been moved to module …/m/p.
+Module …/m/p does not require any version of module …/m.
+
+-- p0/go.mod --
+module example.net/m/p
+
+go 1.16
+-- p0/p.go --
+package p
diff --git a/src/cmd/go/testdata/script/mod_get_ambiguous_pkg.txt b/src/cmd/go/testdata/script/mod_get_ambiguous_pkg.txt
new file mode 100644
index 0000000..1641196
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_ambiguous_pkg.txt
@@ -0,0 +1,87 @@
+# Both example.net/ambiguous v0.1.0 and example.net/ambiguous/pkg v0.1.0 exist.
+# 'go mod tidy' would arbitrarily choose the one with the longer path,
+# but 'go mod tidy' also arbitrarily chooses the latest version.
+
+cp go.mod go.mod.orig
+
+
+# From a clean slate, 'go get' currently does the same thing as 'go mod tidy':
+# it resolves the package from the module with the longest matching prefix.
+
+go get example.net/ambiguous/nested/pkg@v0.1.0
+go list -m all
+stdout '^example.net/ambiguous/nested v0.1.0$'
+! stdout '^example.net/ambiguous '
+
+
+# From an initial state that already depends on the shorter path,
+# the same 'go get' command should (somewhat arbitrarily) keep the
+# existing path, since it is a valid interpretation of the command.
+
+cp go.mod.orig go.mod
+go mod edit -require=example.net/ambiguous@v0.1.0
+
+go get example.net/ambiguous/nested/pkg@v0.1.0
+go list -m all
+stdout '^example.net/ambiguous v0.1.0$'
+! stdout '^example.net/ambiguous/nested '
+
+
+# The user should be able to make the command unambiguous by explicitly
+# upgrading the conflicting module...
+
+go get example.net/ambiguous@v0.2.0 example.net/ambiguous/nested/pkg@v0.1.0
+go list -m all
+stdout '^example.net/ambiguous/nested v0.1.0$'
+stdout '^example.net/ambiguous v0.2.0$'
+
+
+# ...or by explicitly NOT adding the conflicting module.
+
+cp go.mod.orig go.mod
+go mod edit -require=example.net/ambiguous@v0.1.0
+
+go get example.net/ambiguous/nested/pkg@v0.1.0 example.net/ambiguous/nested@none
+go list -m all
+! stdout '^example.net/ambiguous/nested '
+stdout '^example.net/ambiguous v0.1.0$'
+
+
+# The user should also be able to fix it by *downgrading* the conflicting module
+# away.
+
+cp go.mod.orig go.mod
+go mod edit -require=example.net/ambiguous@v0.1.0
+
+go get example.net/ambiguous@none example.net/ambiguous/nested/pkg@v0.1.0
+go list -m all
+stdout '^example.net/ambiguous/nested v0.1.0$'
+! stdout '^example.net/ambiguous '
+
+
+# In contrast, if we do the same thing tacking a wildcard pattern ('/...') on
+# the end of the package path, we get different behaviors depending on the
+# initial state, and no error. (This seems to contradict the “same meaning
+# regardless of the initial state” point above, but maybe that's ok?)
+
+cp go.mod.orig go.mod
+
+go get example.net/ambiguous/nested/pkg/...@v0.1.0
+go list -m all
+stdout '^example.net/ambiguous/nested v0.1.0$'
+! stdout '^example.net/ambiguous '
+
+
+cp go.mod.orig go.mod
+go mod edit -require=example.net/ambiguous@v0.1.0
+
+go get example.net/ambiguous/nested/pkg/...@v0.1.0
+go list -m all
+! stdout '^example.net/ambiguous/nested '
+stdout '^example.net/ambiguous v0.1.0$'
+
+
+-- go.mod --
+module test
+
+go 1.16
diff --git a/src/cmd/go/testdata/script/mod_get_boost.txt b/src/cmd/go/testdata/script/mod_get_boost.txt
new file mode 100644
index 0000000..105dc2e
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_boost.txt
@@ -0,0 +1,96 @@
+# If 'go get -u' finds an upgrade candidate that isn't viable,
+# but some other upgraded module's requirement moves past it
+# (for example, to a higher prerelease), then we should accept
+# the transitive upgrade instead of trying lower roots.
+
+go get -v -u . example.net/b@v0.1.0
+cmp go.mod go.mod.want
+
+-- go.mod --
+module example
+
+go 1.17
+
+require (
+	example.net/a v0.1.0
+	example.net/b v0.1.0
+	example.net/c v0.1.0
+)
+
+replace (
+	example.net/a v0.1.0 => ./a1
+	example.net/a v0.2.0-pre => ./a2p
+	example.net/b v0.1.0 => ./b
+	example.net/b v0.2.0 => ./b
+	example.net/c v0.1.0 => ./c1
+	example.net/c v0.2.0 => ./c2
+)
+-- go.mod.want --
+module example
+
+go 1.17
+
+require (
+	example.net/a v0.2.0-pre
+	example.net/b v0.1.0
+	example.net/c v0.2.0
+)
+
+replace (
+	example.net/a v0.1.0 => ./a1
+	example.net/a v0.2.0-pre => ./a2p
+	example.net/b v0.1.0 => ./b
+	example.net/b v0.2.0 => ./b
+	example.net/c v0.1.0 => ./c1
+	example.net/c v0.2.0 => ./c2
+)
+-- example.go --
+package example
+
+import (
+	_ "example.net/a"
+	_ "example.net/b"
+	_ "example.net/c"
+)
+-- a1/go.mod --
+module example.net/a
+
+go 1.17
+
+require example.net/b v0.2.0
+-- a1/a.go --
+package a
+
+import _ "example.net/b"
+-- a2p/go.mod --
+module example.net/a
+
+go 1.17
+-- a2p/a.go --
+package a
+-- b/go.mod --
+module example.net/b
+
+go 1.17
+-- b/b.go --
+package b
+-- c1/go.mod --
+module example.net/c
+
+go 1.17
+
+require example.net/a v0.1.0
+-- c1/c.go --
+package c
+
+import _ "example.net/a"
+-- c2/go.mod --
+module example.net/c
+
+go 1.17
+
+require example.net/a v0.2.0-pre
+-- c2/c.go --
+package c
+
+import _ "example.net/c"
diff --git a/src/cmd/go/testdata/script/mod_get_changes.txt b/src/cmd/go/testdata/script/mod_get_changes.txt
new file mode 100644
index 0000000..12a112b
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_changes.txt
@@ -0,0 +1,70 @@
+# When adding a requirement, 'go get' prints a message for the requirement
+# and for changed explicit dependencies. 'go get' does not print messages
+# for changed indirect dependencies.
+go list -m all
+! stdout golang.org/x/text
+go get rsc.io/quote@v1.5.2
+stderr '^go: added rsc.io/quote v1.5.2$'
+stderr '^go: upgraded rsc.io/sampler v1.0.0 => v1.3.0$'
+! stderr '^go get.*golang.org/x/text'
+go list -m all
+stdout golang.org/x/text
+cmp go.mod go.mod.upgrade
+
+# When removing a requirement, 'go get' prints a message for the requiremnent
+# and for changed explicit dependencies. 'go get' does not print messages
+# for changed indirect dependencies.
+go get rsc.io/sampler@none
+stderr '^go: downgraded rsc.io/quote v1.5.2 => v1.3.0$'
+stderr '^go: removed rsc.io/sampler v1.3.0$'
+! stderr '^go get.*golang.org/x/text'
+cmp go.mod go.mod.downgrade
+
+# When removing or downgrading a requirement, 'go get' also prints a message
+# for explicit dependencies removed as a consequence.
+cp go.mod.usequote go.mod
+go get rsc.io/quote@v1.5.1
+stderr '^go: downgraded rsc.io/quote v1.5.2 => v1.5.1$'
+stderr '^go: removed usequote v0.0.0$'
+
+-- go.mod --
+module m
+
+go 1.16
+
+require rsc.io/sampler v1.0.0
+-- go.sum --
+rsc.io/sampler v1.0.0 h1:SRJnjyQ07sAtq6G4RcfJEmz8JxqLyj3PoGXG2VhbDWo=
+rsc.io/sampler v1.0.0/go.mod h1:cqxpM3ZVz9VtirqxZPmrWzkQ+UkiNiGtkrN+B+i8kx8=
+-- go.mod.upgrade --
+module m
+
+go 1.16
+
+require (
+	rsc.io/quote v1.5.2 // indirect
+	rsc.io/sampler v1.3.0
+)
+-- go.mod.downgrade --
+module m
+
+go 1.16
+
+require (
+	golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c // indirect
+	rsc.io/quote v1.3.0 // indirect
+)
+-- go.mod.usequote --
+module m
+
+go 1.16
+
+require usequote v0.0.0
+
+replace usequote => ./usequote
+-- usequote/go.mod --
+module usequote
+
+go 1.16
+
+require rsc.io/quote v1.5.2
diff --git a/src/cmd/go/testdata/script/mod_get_commit.txt b/src/cmd/go/testdata/script/mod_get_commit.txt
new file mode 100644
index 0000000..76650f3
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_commit.txt
@@ -0,0 +1,50 @@
+env GO111MODULE=on
+[short] skip
+
+# @commit should resolve
+
+# golang.org/x/text/language@commit should resolve.
+# Because of -d, the compiler should not run.
+go get -x golang.org/x/text/language@14c0d48
+! stderr 'compile|cp|gccgo .*language\.a$'
+
+# go get should skip build with no Go files in root
+go get golang.org/x/text@14c0d48
+
+# dropping -d, we should see a build.
+[short] skip
+
+env GOCACHE=$WORK/gocache  # Looking for compile commands, so need a clean cache.
+
+go build -x golang.org/x/text/language
+stderr 'compile|cp|gccgo .*language\.a$'
+
+go list -f '{{.Stale}}' golang.org/x/text/language
+stdout ^false
+
+# install after build should not run the compiler again.
+go install -x golang.org/x/text/language
+! stderr 'compile|cp|gccgo .*language\.a$'
+
+# we should see an error for unknown packages.
+! go get -x golang.org/x/text/foo@14c0d48
+stderr '^go: module golang.org/x/text@14c0d48 found \(v0.3.0\), but does not contain package golang.org/x/text/foo$'
+
+# get pseudo-version should record that version
+go get rsc.io/quote@v0.0.0-20180214005840-23179ee8a569
+grep 'rsc.io/quote v0.0.0-20180214005840-23179ee8a569' go.mod
+
+# but as commit should record as v1.5.1
+go get rsc.io/quote@23179ee8
+grep 'rsc.io/quote v1.5.1' go.mod
+
+# go mod edit -require does not interpret commits
+go mod edit -require rsc.io/quote@23179ee
+grep 'rsc.io/quote 23179ee' go.mod
+
+# but other commands fix them
+go list -m -mod=mod all
+grep 'rsc.io/quote v1.5.1' go.mod
+
+-- go.mod --
+module x
diff --git a/src/cmd/go/testdata/script/mod_get_deprecate_install.txt b/src/cmd/go/testdata/script/mod_get_deprecate_install.txt
new file mode 100644
index 0000000..03258f5
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_deprecate_install.txt
@@ -0,0 +1,42 @@
+[short] skip
+
+env GO111MODULE=on
+
+# 'go get' outside a module prints an error.
+! go get example.com/cmd/a
+stderr '^go: go.mod file not found in current directory or any parent directory.$'
+stderr '^\t''go get'' is no longer supported outside a module.$'
+
+cp go.mod.orig go.mod
+
+# 'go get' inside a module with a non-main package does not print a message.
+# This will stop building in the future, but it's the command we want to use.
+go get rsc.io/quote
+! stderr deprecated
+! stderr 'no longer installs'
+cp go.mod.orig go.mod
+
+# 'go get' inside a module with an executable does not print a message.
+# In 1.16 and 1.17, 'go get' did print a message in this case suggesting the
+# use of -d. In 1.18, -d is a no-op, and we'd like to begin discouraging
+# its use.
+go get example.com/cmd/a
+! stderr deprecated
+! stderr 'no longer installs'
+cp go.mod.orig go.mod
+
+# 'go get' should not print a warning for a main package inside the main module.
+# The intent is most likely to update the dependencies of that package.
+# 'go install' would be used otherwise.
+go get m
+! stderr .
+cp go.mod.orig go.mod
+
+-- go.mod.orig --
+module m
+
+go 1.17
+-- main.go --
+package main
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/mod_get_deprecated.txt b/src/cmd/go/testdata/script/mod_get_deprecated.txt
new file mode 100644
index 0000000..ec7bcfd
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_deprecated.txt
@@ -0,0 +1,66 @@
+# 'go get pkg' should not show a deprecation message for an unrelated module.
+go get ./use/nothing
+! stderr 'module.*is deprecated'
+
+# 'go get pkg' should show a deprecation message for the module providing pkg.
+go get example.com/deprecated/a
+stderr '^go: module example.com/deprecated/a is deprecated: in example.com/deprecated/a@v1.9.0$'
+go get example.com/deprecated/a@v1.0.0
+stderr '^go: module example.com/deprecated/a is deprecated: in example.com/deprecated/a@v1.9.0$'
+
+# 'go get pkg' should show a deprecation message for a module providing
+# packages directly imported by pkg.
+go get ./use/a
+stderr '^go: module example.com/deprecated/a is deprecated: in example.com/deprecated/a@v1.9.0$'
+
+# 'go get pkg' may show a deprecation message for an indirectly required module
+# if it provides a package named on the command line.
+go get ./use/b
+! stderr 'module.*is deprecated'
+go get local/use
+! stderr 'module.*is deprecated'
+go get example.com/deprecated/b
+stderr '^go: module example.com/deprecated/b is deprecated: in example.com/deprecated/b@v1.9.0$'
+
+# 'go get pkg' does not show a deprecation message for a module providing a
+# directly imported package if the module is no longer deprecated in its
+# latest version, even if the module is deprecated in its current version.
+go get ./use/undeprecated
+! stderr 'module.*is deprecated'
+
+-- go.mod --
+module m
+
+go 1.17
+
+require (
+	example.com/deprecated/a v1.0.0
+	example.com/undeprecated v1.0.0
+	local v0.0.0
+)
+
+replace local v0.0.0 => ./local
+-- use/nothing/nothing.go --
+package nothing
+-- use/a/a.go --
+package a
+
+import _ "example.com/deprecated/a"
+-- use/b/b.go --
+package b
+
+import _ "local/use"
+-- use/undeprecated/undeprecated.go --
+package undeprecated
+
+import _ "example.com/undeprecated"
+-- local/go.mod --
+module local
+
+go 1.17
+
+require example.com/deprecated/b v1.0.0
+-- local/use/use.go --
+package use
+
+import _ "example.com/deprecated/b"
diff --git a/src/cmd/go/testdata/script/mod_get_direct.txt b/src/cmd/go/testdata/script/mod_get_direct.txt
new file mode 100644
index 0000000..02b10ab
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_direct.txt
@@ -0,0 +1,19 @@
+# Regression test for golang.org/issue/34092: with an empty module cache,
+# 'GOPROXY=direct go get golang.org/x/tools/gopls@master' did not correctly
+# resolve the pseudo-version for its dependency on golang.org/x/tools.
+
+[!net:cloud.google.com] skip
+[!git] skip
+
+env GO111MODULE=on
+env GOPROXY=direct
+env GOSUMDB=off
+
+go list -m cloud.google.com/go@main
+! stdout 'v0.0.0-'
+
+-- go.mod --
+module example.com
+
+go 1.14
+-- go.sum --
diff --git a/src/cmd/go/testdata/script/mod_get_downadd_indirect.txt b/src/cmd/go/testdata/script/mod_get_downadd_indirect.txt
new file mode 100644
index 0000000..0f5ba99
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_downadd_indirect.txt
@@ -0,0 +1,81 @@
+# This test illustrates a case where downgrading one module may upgrade another.
+# Compare to the downcross2 test case in cmd/go/internal/mvs/mvs_test.go.
+
+# The initial package import graph used in this test looks like:
+#
+# a ---- b ---- d
+#
+# The module dependency graph originally looks like:
+#
+# a ---- b.2 ---- d.2
+#
+# b.1 ---- c.1
+#
+# If we downgrade module d to version 1, we must downgrade b as well.
+# If that downgrade selects b version 1, we will add a new dependency on module c.
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod.orig go.mod
+
+go get example.com/d@v0.1.0
+go list -m all
+stdout '^example.com/b v0.1.0 '
+stdout '^example.com/c v0.1.0 '
+stdout '^example.com/d v0.1.0 '
+
+-- go.mod --
+module example.com/a
+
+go 1.15
+
+require example.com/b v0.2.0
+
+replace (
+	example.com/b v0.1.0 => ./b1
+	example.com/b v0.2.0 => ./b2
+	example.com/c v0.1.0 => ./c
+	example.com/d v0.1.0 => ./d
+	example.com/d v0.2.0 => ./d
+)
+-- a.go --
+package a
+
+import _ "example.com/b"
+
+-- b1/go.mod --
+module example.com/b
+
+go 1.15
+
+require example.com/c v0.1.0
+-- b1/b.go --
+package b
+
+import _ "example.com/c"
+
+-- b2/go.mod --
+module example.com/b
+
+go 1.15
+
+require example.com/d v0.2.0
+-- b2/b.go --
+package b
+
+import _ "example.com/d"
+
+-- c/go.mod --
+module example.com/c
+
+go 1.15
+
+-- c/c.go --
+package c
+
+-- d/go.mod --
+module example.com/d
+
+go 1.15
+-- d/d.go --
+package d
diff --git a/src/cmd/go/testdata/script/mod_get_downgrade.txt b/src/cmd/go/testdata/script/mod_get_downgrade.txt
new file mode 100644
index 0000000..2eed56d
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_downgrade.txt
@@ -0,0 +1,56 @@
+env GO111MODULE=on
+[short] skip
+
+# downgrade sampler should downgrade quote
+cp go.mod.orig go.mod
+go get rsc.io/sampler@v1.0.0
+go list -m all
+stdout 'rsc.io/quote v1.4.0'
+stdout 'rsc.io/sampler v1.0.0'
+
+# downgrade sampler away should downgrade quote further
+go get rsc.io/sampler@none
+go list -m all
+stdout 'rsc.io/quote v1.3.0'
+
+# downgrade should report inconsistencies and not change go.mod
+go get rsc.io/quote@v1.5.1
+go list -m all
+stdout 'rsc.io/quote v1.5.1'
+stdout 'rsc.io/sampler v1.3.0'
+
+! go get rsc.io/sampler@v1.0.0 rsc.io/quote@v1.5.2 golang.org/x/text@none
+! stderr add|remove|upgrad|downgrad
+stderr '^go: rsc.io/quote@v1.5.2 requires rsc.io/sampler@v1.3.0, not rsc.io/sampler@v1.0.0$'
+
+go list -m all
+stdout 'rsc.io/quote v1.5.1'
+stdout 'rsc.io/sampler v1.3.0'
+
+# go get -u args should limit upgrades
+cp go.mod.empty go.mod
+go get -u rsc.io/quote@v1.4.0 rsc.io/sampler@v1.0.0
+go list -m all
+stdout 'rsc.io/quote v1.4.0'
+stdout 'rsc.io/sampler v1.0.0'
+! stdout golang.org/x/text
+
+# downgrading away quote should also downgrade away latemigrate/v2,
+# since there are no older versions. v2.0.0 is incompatible.
+cp go.mod.orig go.mod
+go list -m -versions example.com/latemigrate/v2
+stdout v2.0.0 # proxy may serve incompatible versions
+go get rsc.io/quote@none
+go list -m all
+! stdout 'example.com/latemigrate/v2'
+
+-- go.mod.orig --
+module x
+require (
+	rsc.io/quote v1.5.1
+	example.com/latemigrate/v2 v2.0.1
+)
+-- go.mod.empty --
+module x
+-- x.go --
+package x
diff --git a/src/cmd/go/testdata/script/mod_get_downgrade_missing.txt b/src/cmd/go/testdata/script/mod_get_downgrade_missing.txt
new file mode 100644
index 0000000..4486a67
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_downgrade_missing.txt
@@ -0,0 +1,43 @@
+cp go.mod go.mod.orig
+
+# getting a specific version of a module along with a pattern
+# not yet present in that module should report the version mismatch
+# rather than a "matched no packages" warning.
+
+! go get example.net/pkgadded@v1.1.0 example.net/pkgadded/subpkg/...
+stderr '^go: example.net/pkgadded@v1.1.0 conflicts with example.net/pkgadded/subpkg/...@upgrade \(v1.2.0\)$'
+! stderr 'matched no packages'
+cmp go.mod.orig go.mod
+
+
+# A wildcard pattern should match the pattern with that path.
+
+go get example.net/pkgadded/...@v1.0.0
+go list -m all
+stdout '^example.net/pkgadded v1.0.0'
+cp go.mod.orig go.mod
+
+
+# If we need to resolve a transitive dependency of a package,
+# and another argument constrains away the version that provides that
+# package, then 'go get' should fail with a useful error message.
+
+! go get example.net/pkgadded@v1.0.0 .
+stderr '^go: example.com/m imports\n\texample.net/pkgadded/subpkg: cannot find module providing package example.net/pkgadded/subpkg$'
+! stderr 'example.net/pkgadded v1\.2\.0'
+cmp go.mod.orig go.mod
+
+go get example.net/pkgadded@v1.0.0
+! go list -deps -mod=readonly .
+stderr '^m.go:3:8: cannot find module providing package example\.net/pkgadded/subpkg: '
+
+-- go.mod --
+module example.com/m
+
+go 1.16
+
+require example.net/pkgadded v1.2.0
+-- m.go --
+package m
+
+import _ "example.net/pkgadded/subpkg"
diff --git a/src/cmd/go/testdata/script/mod_get_downup_artifact.txt b/src/cmd/go/testdata/script/mod_get_downup_artifact.txt
new file mode 100644
index 0000000..111a54f
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_downup_artifact.txt
@@ -0,0 +1,159 @@
+# This test illustrates a case where an upgrade–downgrade–upgrade cycle can
+# result in upgrades of otherwise-irrelevant dependencies.
+#
+# This case has no corresponding test in the mvs package, because it is an
+# artifact that results from the composition of *multiple* MVS operations.
+
+# The initial package import graph used in the test looks like:
+#
+# m ---- a
+# |      |
+# +----- b
+# |      |
+# +----- c
+# |
+# +----- d
+#
+# b version 2 adds its own import of package d.
+#
+# The module dependency graph initially looks like:
+#
+# m ---- a.1
+# |       |
+# +----- b.1
+# |       |
+# +----- c.1
+# |
+# +----- d.1
+#
+# b.2 ---- c.2
+#  |
+#  +------ d.2
+#  |
+#  +------ e.1
+#
+# If we upgrade module b to version 2, we will upgrade c and d and add a new
+# dependency on e. If b version 2 is disallowed because of any of those
+# dependencies, the other dependencies should not be upgraded as a side-effect.
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod go.mod.orig
+
+go list -m all
+stdout '^example.com/a v0.1.0 '
+stdout '^example.com/b v0.1.0 '
+stdout '^example.com/c v0.1.0 '
+stdout '^example.com/d v0.1.0 '
+! stdout '^example.com/e '
+
+# b is imported by a, so the -u flag would normally upgrade it to v0.2.0.
+# However, that would conflict with the explicit c@v0.1.0 constraint,
+# so b must remain at v0.1.0.
+#
+# If we're not careful, we might temporarily add b@v0.2.0 and pull in its
+# upgrades of module d and addition of module e, which are not relevant to
+# b@v0.1.0 and should not be added to the main module's dependencies.
+
+go get -u example.com/a@latest example.com/c@v0.1.0
+
+go list -m all
+stdout '^example.com/a v0.1.0 '
+stdout '^example.com/b v0.1.0 '
+stdout '^example.com/c v0.1.0 '
+stdout '^example.com/d v0.1.0 '
+! stdout '^example.com/e '
+
+-- go.mod --
+module example.com/m
+
+go 1.16
+
+require (
+	example.com/a v0.1.0
+	example.com/b v0.1.0
+	example.com/c v0.1.0
+	example.com/d v0.1.0
+)
+
+replace (
+	example.com/a v0.1.0 => ./a1
+	example.com/b v0.1.0 => ./b1
+	example.com/b v0.2.0 => ./b2
+	example.com/c v0.1.0 => ./c
+	example.com/c v0.2.0 => ./c
+	example.com/d v0.1.0 => ./d
+	example.com/d v0.2.0 => ./d
+	example.com/e v0.1.0 => ./e
+)
+-- m.go --
+package m
+
+import (
+	_ "example.com/a"
+	_ "example.com/b"
+	_ "example.com/c"
+	_ "example.com/d"
+)
+
+-- a1/go.mod --
+module example.com/a
+
+go 1.16
+
+require example.com/b v0.1.0
+-- a1/a.go --
+package a
+
+import _ "example.com/b"
+
+-- b1/go.mod --
+module example.com/b
+
+go 1.16
+
+require example.com/c v0.1.0
+-- b1/b.go --
+package b
+
+import _ "example.com/c"
+
+-- b2/go.mod --
+module example.com/b
+
+go 1.16
+
+require (
+	example.com/c v0.2.0
+	example.com/d v0.2.0
+	example.com/e v0.1.0
+)
+-- b2/b.go --
+package b
+
+import (
+	"example.com/c"
+	"example.com/d"
+	"example.com/e"
+)
+
+-- c/go.mod --
+module example.com/c
+
+go 1.16
+-- c/c.go --
+package c
+
+-- d/go.mod --
+module example.com/d
+
+go 1.16
+-- d/d.go --
+package d
+
+-- e/go.mod --
+module example.com/e
+
+go 1.16
+-- e/e.go --
+package e
diff --git a/src/cmd/go/testdata/script/mod_get_downup_indirect.txt b/src/cmd/go/testdata/script/mod_get_downup_indirect.txt
new file mode 100644
index 0000000..432e626
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_downup_indirect.txt
@@ -0,0 +1,149 @@
+# This test illustrates a case where downgrading one module may upgrade another.
+# Compare to the downcross1 test case in cmd/go/internal/mvs/mvs_test.go.
+
+# The package import graph used in this test looks like:
+#
+# a ---- b
+#   \     \
+#    \     \
+#     ----- c ---- d
+#
+# The module dependency graph originally looks like:
+#
+# a ---- b.2
+#   \      \
+#    \      \
+#     ----- c.1 ---- d.2
+#
+# b.1 ---- c.2
+#
+# If we downgrade module d to version 1, we must downgrade b as well.
+# If that downgrade selects b version 1, we will upgrade module c to version 2.
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod.orig go.mod
+
+# Downgrading d to version 1 downgrades b, which upgrades c.
+go get example.com/d@v0.1.0
+go list -m all
+stdout '^example.com/b v0.1.0 '
+stdout '^example.com/c v0.2.0 '
+stdout '^example.com/d v0.1.0 '
+cmp go.mod go.mod.down1
+
+# Restoring c to version 1 upgrades d to meet c's requirements.
+go get example.com/c@v0.1.0
+go list -m all
+! stdout '^example.com/b '
+stdout '^example.com/c v0.1.0 '
+stdout '^example.com/d v0.2.0 '
+cmp go.mod go.mod.down2
+
+# If a user explicitly requests the incompatible versions together,
+# 'go get' should explain why they are not compatible.
+! go get example.com/c@v0.1.0 example.com/d@v0.1.0
+stderr '^go: example\.com/c@v0\.1\.0 requires example\.com/d@v0\.2\.0, not example\.com/d@v0\.1\.0'
+
+-- go.mod --
+module example.com/a
+
+go 1.15
+
+require (
+	example.com/b v0.2.0
+	example.com/c v0.1.0
+)
+
+replace (
+	example.com/b v0.1.0 => ./b1
+	example.com/b v0.2.0 => ./b2
+	example.com/c v0.1.0 => ./c1
+	example.com/c v0.2.0 => ./c2
+	example.com/d v0.1.0 => ./d
+	example.com/d v0.2.0 => ./d
+)
+-- go.mod.down1 --
+module example.com/a
+
+go 1.15
+
+require (
+	example.com/b v0.1.0
+	example.com/c v0.2.0
+	example.com/d v0.1.0 // indirect
+)
+
+replace (
+	example.com/b v0.1.0 => ./b1
+	example.com/b v0.2.0 => ./b2
+	example.com/c v0.1.0 => ./c1
+	example.com/c v0.2.0 => ./c2
+	example.com/d v0.1.0 => ./d
+	example.com/d v0.2.0 => ./d
+)
+-- go.mod.down2 --
+module example.com/a
+
+go 1.15
+
+require example.com/c v0.1.0
+
+replace (
+	example.com/b v0.1.0 => ./b1
+	example.com/b v0.2.0 => ./b2
+	example.com/c v0.1.0 => ./c1
+	example.com/c v0.2.0 => ./c2
+	example.com/d v0.1.0 => ./d
+	example.com/d v0.2.0 => ./d
+)
+-- a.go --
+package a
+
+import (
+	_ "example.com/b"
+	_ "example.com/c"
+)
+
+-- b1/go.mod --
+module example.com/b
+
+go 1.15
+
+require example.com/c v0.2.0
+-- b1/b.go --
+package b
+
+import _ "example.com/c"
+
+-- b2/go.mod --
+module example.com/b
+
+go 1.15
+
+require example.com/c v0.1.0
+-- b2/b.go --
+package b
+
+import _ "example.com/c"
+
+-- c1/go.mod --
+module example.com/c
+
+go 1.15
+
+require example.com/d v0.2.0
+-- c1/c.go --
+package c
+
+-- c2/go.mod --
+module example.com/c
+
+go 1.15
+-- c2/c.go --
+package c
+
+-- d/go.mod --
+module example.com/d
+
+go 1.15
diff --git a/src/cmd/go/testdata/script/mod_get_downup_indirect_pruned.txt b/src/cmd/go/testdata/script/mod_get_downup_indirect_pruned.txt
new file mode 100644
index 0000000..cac6b96
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_downup_indirect_pruned.txt
@@ -0,0 +1,154 @@
+# This test illustrates a case where downgrading one module may upgrade another.
+# This is the same as mod_get_downup_indirect, but using modules
+# with graph pruning enabled (go ≥ 1.17).
+# Compare to the downcross1 test case in cmd/go/internal/mvs/mvs_test.go.
+
+# The package import graph used in this test looks like:
+#
+# a ---- b
+#   \     \
+#    \     \
+#     ----- c ---- d
+#
+# The module dependency graph originally looks like:
+#
+# a ---- b.2
+#   \      \
+#    \      \
+#     ----- c.1 ---- d.2
+#
+# b.1 ---- c.2
+#
+# If we downgrade module d to version 1, we must downgrade b as well.
+# If that downgrade selects b version 1, we will upgrade module c to version 2.
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod.orig go.mod
+
+# Downgrading d to version 1 downgrades b, which upgrades c.
+go get -v example.com/d@v0.1.0
+go list -m all
+stdout '^example.com/b v0.1.0 '
+stdout '^example.com/c v0.2.0 '
+stdout '^example.com/d v0.1.0 '
+cmp go.mod go.mod.down1
+
+# Restoring c to version 1 upgrades d to meet c's requirements.
+go get example.com/c@v0.1.0
+go list -m all
+! stdout '^example.com/b '
+stdout '^example.com/c v0.1.0 '
+stdout '^example.com/d v0.2.0 '
+cmp go.mod go.mod.down2
+
+# If a user explicitly requests the incompatible versions together,
+# 'go get' should explain why they are not compatible.
+! go get example.com/c@v0.1.0 example.com/d@v0.1.0
+stderr '^go: example\.com/c@v0\.1\.0 requires example\.com/d@v0\.2\.0, not example\.com/d@v0\.1\.0'
+
+-- go.mod --
+module example.com/a
+
+go 1.17
+
+require (
+	example.com/b v0.2.0
+	example.com/c v0.1.0
+)
+
+replace (
+	example.com/b v0.1.0 => ./b1
+	example.com/b v0.2.0 => ./b2
+	example.com/c v0.1.0 => ./c1
+	example.com/c v0.2.0 => ./c2
+	example.com/d v0.1.0 => ./d
+	example.com/d v0.2.0 => ./d
+)
+-- go.mod.down1 --
+module example.com/a
+
+go 1.17
+
+require (
+	example.com/b v0.1.0
+	example.com/c v0.2.0
+)
+
+require example.com/d v0.1.0 // indirect
+
+replace (
+	example.com/b v0.1.0 => ./b1
+	example.com/b v0.2.0 => ./b2
+	example.com/c v0.1.0 => ./c1
+	example.com/c v0.2.0 => ./c2
+	example.com/d v0.1.0 => ./d
+	example.com/d v0.2.0 => ./d
+)
+-- go.mod.down2 --
+module example.com/a
+
+go 1.17
+
+require example.com/c v0.1.0
+
+require example.com/d v0.2.0 // indirect
+
+replace (
+	example.com/b v0.1.0 => ./b1
+	example.com/b v0.2.0 => ./b2
+	example.com/c v0.1.0 => ./c1
+	example.com/c v0.2.0 => ./c2
+	example.com/d v0.1.0 => ./d
+	example.com/d v0.2.0 => ./d
+)
+-- a.go --
+package a
+
+import (
+	_ "example.com/b"
+	_ "example.com/c"
+)
+
+-- b1/go.mod --
+module example.com/b
+
+go 1.17
+
+require example.com/c v0.2.0
+-- b1/b.go --
+package b
+
+import _ "example.com/c"
+
+-- b2/go.mod --
+module example.com/b
+
+go 1.17
+
+require example.com/c v0.1.0
+-- b2/b.go --
+package b
+
+import _ "example.com/c"
+
+-- c1/go.mod --
+module example.com/c
+
+go 1.17
+
+require example.com/d v0.2.0
+-- c1/c.go --
+package c
+
+-- c2/go.mod --
+module example.com/c
+
+go 1.17
+-- c2/c.go --
+package c
+
+-- d/go.mod --
+module example.com/d
+
+go 1.17
diff --git a/src/cmd/go/testdata/script/mod_get_downup_pseudo_artifact.txt b/src/cmd/go/testdata/script/mod_get_downup_pseudo_artifact.txt
new file mode 100644
index 0000000..b678a17
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_downup_pseudo_artifact.txt
@@ -0,0 +1,129 @@
+# This test illustrates a case where an upgrade–downgrade–upgrade cycle could
+# add extraneous dependencies due to another module depending on an
+# otherwise-unlisted version (such as a pseudo-version).
+#
+# This case corresponds to the "downhiddenartifact" test in the mvs package.
+
+# The initial package import graph used in the test looks like:
+#
+# a --- b
+#  \     \
+#   \     \
+#    c --- d
+#
+# The module dependency graph initially looks like:
+#
+# a --- b.3
+#  \      \
+#   \      \
+#   c.2 --- d.2
+#
+# c.1 --- b.2 (pseudo)
+#
+# b.1 --- e.1
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod.orig go.mod
+
+# When we downgrade d.2 to d.1, no dependency on e should be added
+# because nothing else in the module or import graph requires it.
+go get example.net/d@v0.1.0
+
+go list -m all
+stdout '^example.net/b v0.2.1-0.20210219000000-000000000000 '
+stdout '^example.net/c v0.1.0 '
+stdout '^example.net/d v0.1.0 '
+! stdout '^example.net/e '
+
+-- go.mod --
+module example.net/a
+
+go 1.16
+
+require (
+	example.net/b v0.3.0
+	example.net/c v0.2.0
+)
+
+replace (
+	example.net/b v0.1.0 => ./b1
+	example.net/b v0.2.1-0.20210219000000-000000000000 => ./b2
+	example.net/b v0.3.0 => ./b3
+	example.net/c v0.1.0 => ./c1
+	example.net/c v0.2.0 => ./c2
+	example.net/d v0.1.0 => ./d
+	example.net/d v0.2.0 => ./d
+	example.net/e v0.1.0 => ./e
+)
+-- a.go --
+package a
+
+import (
+	_ "example.net/b"
+	_ "example.net/c"
+)
+
+-- b1/go.mod --
+module example.net/b
+
+go 1.16
+
+require example.net/e v0.1.0
+-- b1/b.go --
+package b
+
+import _ "example.net/e"
+
+-- b2/go.mod --
+module example.net/b
+
+go 1.16
+-- b2/b.go --
+package b
+
+-- b3/go.mod --
+module example.net/b
+
+go 1.16
+
+require example.net/d v0.2.0
+-- b3/b.go --
+package b
+
+import _ "example.net/d"
+-- c1/go.mod --
+module example.net/c
+
+go 1.16
+
+require example.net/b v0.2.1-0.20210219000000-000000000000
+-- c1/c.go --
+package c
+
+import _ "example.net/b"
+
+-- c2/go.mod --
+module example.net/c
+
+go 1.16
+
+require example.net/d v0.2.0
+-- c2/c.go --
+package c
+
+import _ "example.net/d"
+
+-- d/go.mod --
+module example.net/d
+
+go 1.16
+-- d/d.go --
+package d
+
+-- e/go.mod --
+module example.net/e
+
+go 1.16
+-- e/e.go --
+package e
diff --git a/src/cmd/go/testdata/script/mod_get_errors.txt b/src/cmd/go/testdata/script/mod_get_errors.txt
new file mode 100644
index 0000000..981d6f0
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_errors.txt
@@ -0,0 +1,57 @@
+cp go.mod go.mod.orig
+
+
+# 'go get' should fail, without updating go.mod, if the transitive dependencies
+# of the requested package (by default, the package in the current directory)
+# cannot be resolved.
+
+! go get
+stderr '^go: example.com/m imports\n\texample.com/badimport imports\n\texample.net/oops: cannot find module providing package example.net/oops$'
+cmp go.mod.orig go.mod
+
+cd importsyntax
+
+
+# A syntax error in a dependency prevents the compiler from needing that
+# dependency's imports, so 'go get' should not report an error when those
+# imports cannot be resolved: it has all of the dependencies that the compiler
+# needs, and the user did not request to run the compiler.
+
+go get
+cmp ../go.mod.syntax-d ../go.mod
+
+
+-- go.mod --
+module example.com/m
+
+go 1.16
+
+replace example.com/badimport v0.1.0 => ./badimport
+-- go.mod.syntax-d --
+module example.com/m
+
+go 1.16
+
+replace example.com/badimport v0.1.0 => ./badimport
+
+require example.com/badimport v0.1.0
+-- m.go --
+package m
+
+import _ "example.com/badimport"
+-- importsyntax/importsyntax.go --
+package importsyntax
+
+import _ "example.com/badimport/syntaxerror"
+-- badimport/go.mod --
+module example.com/badimport
+
+go 1.16
+-- badimport/badimport.go --
+package badimport
+
+import "example.net/oops"
+-- badimport/syntaxerror/syntaxerror.go --
+pack-age syntaxerror // sic
+
+import "example.net/oops"
diff --git a/src/cmd/go/testdata/script/mod_get_exec_toolchain.txt b/src/cmd/go/testdata/script/mod_get_exec_toolchain.txt
new file mode 100644
index 0000000..497fe36
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_exec_toolchain.txt
@@ -0,0 +1,145 @@
+env TESTGO_VERSION=go1.21
+env TESTGO_VERSION_SWITCH=switch
+
+# GOTOOLCHAIN=auto should run the newer toolchain
+env GOTOOLCHAIN=auto
+cp go.mod.new go.mod
+go get rsc.io/needgo121 rsc.io/needgo122 rsc.io/needgo123 rsc.io/needall
+stderr '^go: rsc.io/needall@v0.0.1 requires go >= 1.23; switching to go1.23.9$'
+! stderr '\(running'
+stderr '^go: added rsc.io/needall v0.0.1'
+grep 'go 1.23' go.mod
+grep 'toolchain go1.23.9' go.mod
+
+# GOTOOLCHAIN=min+auto should run the newer toolchain
+env GOTOOLCHAIN=go1.21+auto
+cp go.mod.new go.mod
+go get rsc.io/needgo121 rsc.io/needgo122 rsc.io/needgo123 rsc.io/needall
+stderr '^go: rsc.io/needall@v0.0.1 requires go >= 1.23; switching to go1.23.9$'
+! stderr '\(running'
+stderr '^go: added rsc.io/needall v0.0.1'
+grep 'go 1.23' go.mod
+grep 'toolchain go1.23.9' go.mod
+
+# GOTOOLCHAIN=go1.21 should NOT run the newer toolchain
+env GOTOOLCHAIN=go1.21
+cp go.mod.new go.mod
+! go get rsc.io/needgo121 rsc.io/needgo122 rsc.io/needgo123 rsc.io/needall
+! stderr switching
+stderr 'rsc.io/needgo122@v0.0.1 requires go >= 1.22'
+stderr 'rsc.io/needgo123@v0.0.1 requires go >= 1.23'
+stderr 'rsc.io/needall@v0.0.1 requires go >= 1.23'
+stderr 'requires go >= 1.23'
+! stderr 'requires go >= 1.21' # that's us!
+cmp go.mod go.mod.new
+
+# GOTOOLCHAIN=local should NOT run the newer toolchain
+env GOTOOLCHAIN=local
+cp go.mod.new go.mod
+! go get rsc.io/needgo121 rsc.io/needgo122 rsc.io/needgo123 rsc.io/needall
+! stderr switching
+stderr 'rsc.io/needgo122@v0.0.1 requires go >= 1.22'
+stderr 'rsc.io/needgo123@v0.0.1 requires go >= 1.23'
+stderr 'rsc.io/needall@v0.0.1 requires go >= 1.23'
+stderr 'requires go >= 1.23'
+! stderr 'requires go >= 1.21' # that's us!
+cmp go.mod go.mod.new
+
+# go get go@1.22 should resolve to the latest 1.22
+env GOTOOLCHAIN=local
+cp go.mod.new go.mod
+! go get go@1.22
+stderr '^go: updating go.mod requires go >= 1.22.9 \(running go 1.21; GOTOOLCHAIN=local\)'
+
+env GOTOOLCHAIN=auto
+cp go.mod.new go.mod
+go get go@1.22
+stderr '^go: updating go.mod requires go >= 1.22.9; switching to go1.22.9$'
+
+# go get go@1.22rc1 should use 1.22rc1 exactly, not a later release.
+env GOTOOLCHAIN=local
+cp go.mod.new go.mod
+! go get go@1.22rc1
+stderr '^go: updating go.mod requires go >= 1.22rc1 \(running go 1.21; GOTOOLCHAIN=local\)'
+
+env GOTOOLCHAIN=auto
+cp go.mod.new go.mod
+go get go@1.22rc1
+stderr '^go: updating go.mod requires go >= 1.22rc1; switching to go1.22.9$'
+stderr '^go: upgraded go 1.1 => 1.22rc1$'
+stderr '^go: added toolchain go1.22.9$'
+
+# go get go@1.22.1 should use 1.22.1 exactly, not a later release.
+env GOTOOLCHAIN=local
+cp go.mod.new go.mod
+! go get go@1.22.1
+stderr '^go: updating go.mod requires go >= 1.22.1 \(running go 1.21; GOTOOLCHAIN=local\)'
+
+env GOTOOLCHAIN=auto
+cp go.mod.new go.mod
+go get go@1.22.1
+stderr '^go: updating go.mod requires go >= 1.22.1; switching to go1.22.9$'
+stderr '^go: upgraded go 1.1 => 1.22.1$'
+stderr '^go: added toolchain go1.22.9$'
+
+# go get needgo122 (says 'go 1.22') should use 1.22.0, the earliest release we have available
+# (ignoring prereleases).
+env GOTOOLCHAIN=local
+cp go.mod.new go.mod
+! go get rsc.io/needgo122
+stderr '^go: rsc.io/needgo122@v0.0.1 requires go >= 1.22 \(running go 1.21; GOTOOLCHAIN=local\)'
+
+env GOTOOLCHAIN=auto
+cp go.mod.new go.mod
+go get rsc.io/needgo122
+stderr '^go: upgraded go 1.1 => 1.22$'
+stderr '^go: rsc.io/needgo122@v0.0.1 requires go >= 1.22; switching to go1.22.9$'
+stderr '^go: added toolchain go1.22.9$'
+
+# go get needgo1223 (says 'go 1.22.3') should use go 1.22.3
+env GOTOOLCHAIN=local
+cp go.mod.new go.mod
+! go get rsc.io/needgo1223
+stderr '^go: rsc.io/needgo1223@v0.0.1 requires go >= 1.22.3 \(running go 1.21; GOTOOLCHAIN=local\)'
+
+env GOTOOLCHAIN=auto
+cp go.mod.new go.mod
+go get rsc.io/needgo1223
+stderr '^go: upgraded go 1.1 => 1.22.3$'
+stderr '^go: rsc.io/needgo1223@v0.0.1 requires go >= 1.22.3; switching to go1.22.9$'
+stderr '^go: added toolchain go1.22.9$'
+
+# go get needgo124 (says 'go 1.24') should use go 1.24rc1, the only version available
+env GOTOOLCHAIN=local
+cp go.mod.new go.mod
+! go get rsc.io/needgo124
+stderr '^go: rsc.io/needgo124@v0.0.1 requires go >= 1.24 \(running go 1.21; GOTOOLCHAIN=local\)'
+
+env GOTOOLCHAIN=auto
+cp go.mod.new go.mod
+go get rsc.io/needgo124
+stderr '^go: rsc.io/needgo124@v0.0.1 requires go >= 1.24; switching to go1.24rc1$'
+stderr '^go: upgraded go 1.1 => 1.24$'
+stderr '^go: added toolchain go1.24rc1$'
+
+# The -C flag should not happen more than once due to switching.
+mkdir dir dir/dir
+cp go.mod.new go.mod
+cp go.mod.new dir/go.mod
+cp go.mod.new dir/dir/go.mod
+cp p.go dir/p.go
+cp p.go dir/dir/p.go
+go get -C dir rsc.io/needgo124
+stderr '^go: rsc.io/needgo124@v0.0.1 requires go >= 1.24; switching to go1.24rc1$'
+stderr '^go: upgraded go 1.1 => 1.24$'
+stderr '^go: added toolchain go1.24rc1$'
+cmp go.mod.new go.mod
+cmp go.mod.new dir/dir/go.mod
+grep 'go 1.24$' dir/go.mod
+
+-- go.mod.new --
+module m
+go 1.1
+
+-- p.go --
+package p
diff --git a/src/cmd/go/testdata/script/mod_get_extra.txt b/src/cmd/go/testdata/script/mod_get_extra.txt
new file mode 100644
index 0000000..083e036
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_extra.txt
@@ -0,0 +1,69 @@
+cp go.mod go.mod.orig
+
+# The -u flag should not (even temporarily) upgrade modules whose versions are
+# determined by explicit queries to any version other than the explicit one.
+# Otherwise, 'go get -u' could introduce spurious dependencies.
+
+go get -u example.net/a@v0.1.0 example.net/b@v0.1.0
+go list -m all
+stdout '^example.net/a v0.1.0 '
+stdout '^example.net/b v0.1.0 '
+! stdout '^example.net/c '
+
+
+# TODO(bcmills): This property does not yet hold for modules added for
+# missing packages when the newly-added module matches a wildcard.
+
+cp go.mod.orig go.mod
+
+go get -u example.net/a@v0.1.0 example.net/b/...@v0.1.0
+go list -m all
+stdout '^example.net/a v0.1.0 '
+stdout '^example.net/b v0.1.0 '
+stdout '^example.net/c '  # BUG, but a minor and rare one
+
+
+-- go.mod --
+module example
+
+go 1.15
+
+replace (
+	example.net/a v0.1.0 => ./a1
+	example.net/b v0.1.0 => ./b1
+	example.net/b v0.2.0 => ./b2
+	example.net/c v0.1.0 => ./c1
+	example.net/c v0.2.0 => ./c1
+)
+
+-- a1/go.mod --
+module example.net/a
+
+go 1.15
+
+// example.net/a needs a dependency on example.net/b, but lacks a requirement
+// on it (perhaps due to a missed file in a VCS commit).
+-- a1/a.go --
+package a
+import _ "example.net/b"
+
+-- b1/go.mod --
+module example.net/b
+
+go 1.15
+-- b1/b.go --
+package b
+
+-- b2/go.mod --
+module example.net/b
+
+go 1.15
+
+require example.net/c v0.1.0
+-- b2/b.go --
+package b
+
+-- c1/go.mod --
+module example.net/c
+
+go 1.15
diff --git a/src/cmd/go/testdata/script/mod_get_fallback.txt b/src/cmd/go/testdata/script/mod_get_fallback.txt
new file mode 100644
index 0000000..a5119b6
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_fallback.txt
@@ -0,0 +1,16 @@
+env GO111MODULE=on
+
+[!net:golang.org] skip
+[!net:proxy.golang.org] skip
+
+env GOPROXY=https://proxy.golang.org,direct
+env GOSUMDB=off
+
+go get -x -v golang.org/x/tools/cmd/goimports
+stderr '# get https://proxy.golang.org/golang.org/x/tools/@v/list'
+! stderr '# get https://golang.org'
+
+-- go.mod --
+module m
+
+go 1.18
diff --git a/src/cmd/go/testdata/script/mod_get_fossil.txt b/src/cmd/go/testdata/script/mod_get_fossil.txt
new file mode 100644
index 0000000..830e0de
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_fossil.txt
@@ -0,0 +1,28 @@
+[short] skip
+[!exec:fossil] skip
+
+# Regression test for 'go get' to ensure repositories
+# provided by fossil v2.12 and up are able to be fetched
+# and parsed correctly.
+# Verifies golang.org/issue/42323.
+
+
+env GO111MODULE=on
+env GOPROXY=direct
+env GOSUMDB=off
+
+# 'go get' for the fossil repo will fail if fossil
+# is unable to determine your fossil user. Easiest
+# way to set it for use by 'go get' is specifying
+# a any non-empty $USER; the value doesn't otherwise matter.
+env USER=fossiluser
+env FOSSIL_HOME=$WORK/home
+
+# Attempt to get the latest version of a fossil repo.
+go get vcs-test.golang.org/fossil/hello.fossil
+! stderr 'unexpected response from fossil info'
+grep 'vcs-test.golang.org/fossil/hello.fossil' go.mod
+
+-- go.mod --
+module x
+-- $WORK/home/.fossil --
diff --git a/src/cmd/go/testdata/script/mod_get_future.txt b/src/cmd/go/testdata/script/mod_get_future.txt
new file mode 100644
index 0000000..3f1c777
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_future.txt
@@ -0,0 +1,13 @@
+env TESTGO_VERSION=go1.21
+env GOTOOLCHAIN=local
+! go mod download rsc.io/future@v1.0.0
+stderr '^go: rsc.io/future@v1.0.0 requires go >= 1.999 \(running go 1.21; GOTOOLCHAIN=local\)$'
+
+-- go.mod --
+module m
+go 1.21
+
+-- x.go --
+package p
+
+import "rsc.io/future/foo"
diff --git a/src/cmd/go/testdata/script/mod_get_go_file.txt b/src/cmd/go/testdata/script/mod_get_go_file.txt
new file mode 100644
index 0000000..c81e491
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_go_file.txt
@@ -0,0 +1,73 @@
+# Tests Issue #38478
+# Tests that go get in GOMOD mode returns a specific error if the argument
+# ends with '.go', has no version, and either has no slash or refers to an
+# existing file.
+
+env GO111MODULE=on
+
+# argument doesn't have .go suffix and has no version
+! go get test
+! stderr 'arguments must be package or module paths'
+! stderr 'exists as a file, but ''go get'' requires package arguments'
+
+# argument has .go suffix and has version
+! go get test.go@v1.0.0
+! stderr 'arguments must be package or module paths'
+! stderr 'exists as a file, but ''go get'' requires package arguments'
+
+# argument has .go suffix, is a file and exists
+! go get test.go
+stderr 'go: test.go: arguments must be package or module paths'
+
+# argument has .go suffix, doesn't exist and has no slashes
+! go get test_missing.go
+stderr 'arguments must be package or module paths'
+
+# argument has .go suffix, is a file and exists in sub-directory
+! go get test/test.go
+stderr 'go: test/test.go exists as a file, but ''go get'' requires package arguments'
+
+# argument has .go suffix, doesn't exist and has slashes
+! go get test/test_missing.go
+! stderr 'arguments must be package or module paths'
+! stderr 'exists as a file, but ''go get'' requires package arguments'
+
+# argument has .go suffix, is a symlink and exists
+[symlink] symlink test_sym.go -> test.go
+[symlink] ! go get test_sym.go
+[symlink] stderr 'go: test_sym.go: arguments must be package or module paths'
+[symlink] rm test_sym.go
+
+# argument has .go suffix, is a symlink and exists in sub-directory
+[symlink] symlink test/test_sym.go -> test.go
+[symlink] ! go get test/test_sym.go
+[symlink] stderr 'go: test/test_sym.go exists as a file, but ''go get'' requires package arguments'
+[symlink] rm test_sym.go
+
+# argument has .go suffix, is a directory and exists
+mkdir test_dir.go
+! go get test_dir.go
+stderr 'go: test_dir.go: arguments must be package or module paths'
+rm test_dir.go
+
+# argument has .go suffix, is a directory and exists in sub-directory
+mkdir test/test_dir.go
+! go get test/test_dir.go
+! stderr 'arguments must be package or module paths'
+! stderr 'exists as a file, but ''go get'' requires package arguments'
+rm test/test_dir.go
+
+
+-- go.mod --
+module m
+
+go 1.18
+
+-- test.go --
+package main
+func main() {println("test")}
+
+
+-- test/test.go --
+package main
+func main() {println("test")}
diff --git a/src/cmd/go/testdata/script/mod_get_hash.txt b/src/cmd/go/testdata/script/mod_get_hash.txt
new file mode 100644
index 0000000..ec5549d
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_hash.txt
@@ -0,0 +1,19 @@
+env GO111MODULE=on
+env GOPROXY=direct
+env GOSUMDB=off
+[!net:golang.org] skip
+[!git] skip
+
+# fetch commit hash reachable from refs/heads/* and refs/tags/* is OK
+go list -m golang.org/x/time@8be79e1e0910c292df4e79c241bb7e8f7e725959 # on master branch
+
+# fetch other commit hash, even with a non-standard ref, is not OK
+! go list -m golang.org/x/time@334d83c35137ac2b376c1dc3e4c7733791855a3a # refs/changes/24/41624/3
+stderr 'unknown revision'
+! go list -m golang.org/x/time@v0.0.0-20170424233410-334d83c35137
+stderr 'unknown revision'
+! go list -m golang.org/x/time@334d83c35137
+stderr 'unknown revision'
+
+-- go.mod --
+module m
diff --git a/src/cmd/go/testdata/script/mod_get_incompatible.txt b/src/cmd/go/testdata/script/mod_get_incompatible.txt
new file mode 100644
index 0000000..5a7d706
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_incompatible.txt
@@ -0,0 +1,26 @@
+env GO111MODULE=on
+
+go get x
+go list -m all
+stdout 'rsc.io/breaker v2.0.0\+incompatible'
+
+cp go.mod2 go.mod
+go get rsc.io/breaker@7307b30
+go list -m all
+stdout 'rsc.io/breaker v2.0.0\+incompatible'
+
+go get rsc.io/breaker@v2.0.0
+go list -m all
+stdout 'rsc.io/breaker v2.0.0\+incompatible'
+
+-- go.mod --
+module x
+
+-- go.mod2 --
+module x
+require rsc.io/breaker v1.0.0
+
+-- x.go --
+package x
+import "rsc.io/breaker"
+var _ = breaker.XX
diff --git a/src/cmd/go/testdata/script/mod_get_indirect.txt b/src/cmd/go/testdata/script/mod_get_indirect.txt
new file mode 100644
index 0000000..fa7edf2
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_indirect.txt
@@ -0,0 +1,59 @@
+env GO111MODULE=on
+[short] skip
+
+# get -u should not upgrade anything, since the package
+# in the current directory doesn't import anything.
+go get -u
+go list -m all
+stdout 'quote v1.5.1$'
+grep 'rsc.io/quote v1.5.1$' go.mod
+
+# get -u should find quote v1.5.2 once there is a use.
+cp $WORK/tmp/usequote.go x.go
+go get -u
+go list -m all
+stdout 'quote v1.5.2$'
+grep 'rsc.io/quote v1.5.2$' go.mod
+
+# it should also update x/text later than requested by v1.5.2
+go list -m -f '{{.Path}} {{.Version}}{{if .Indirect}} // indirect{{end}}' all
+stdout '^golang.org/x/text [v0-9a-f\.-]+ // indirect'
+grep 'golang.org/x/text [v0-9a-f\.-]+ // indirect' go.mod
+
+# importing an empty module root as a package does not remove indirect tag.
+cp $WORK/tmp/usetext.go x.go
+go list -e
+grep 'golang.org/x/text v0.3.0 // indirect$' go.mod
+
+# indirect tag should be removed upon seeing direct import.
+cp $WORK/tmp/uselang.go x.go
+go get
+grep 'rsc.io/quote v1.5.2$' go.mod
+grep 'golang.org/x/text [v0-9a-f\.-]+$' go.mod
+
+# indirect tag should be added by go mod tidy
+cp $WORK/tmp/usequote.go x.go
+go mod tidy
+grep 'rsc.io/quote v1.5.2$' go.mod
+grep 'golang.org/x/text [v0-9a-f\.-]+ // indirect' go.mod
+
+# requirement should be dropped entirely if not needed
+cp $WORK/tmp/uselang.go x.go
+go mod tidy
+! grep rsc.io/quote go.mod
+grep 'golang.org/x/text [v0-9a-f\.-]+$' go.mod
+
+-- go.mod --
+module x
+require rsc.io/quote v1.5.1
+-- x.go --
+package x
+-- $WORK/tmp/usetext.go --
+package x
+import _ "golang.org/x/text"
+-- $WORK/tmp/uselang.go --
+package x
+import _ "golang.org/x/text/language"
+-- $WORK/tmp/usequote.go --
+package x
+import _ "rsc.io/quote"
diff --git a/src/cmd/go/testdata/script/mod_get_insecure_redirect.txt b/src/cmd/go/testdata/script/mod_get_insecure_redirect.txt
new file mode 100644
index 0000000..a503c91
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_insecure_redirect.txt
@@ -0,0 +1,19 @@
+# golang.org/issue/29591: 'go get' was following plain-HTTP redirects even without -insecure (now replaced by GOINSECURE).
+# golang.org/issue/61877: 'go get' would panic in case of an insecure redirect in module mode
+
+[!git] skip
+
+env GOPRIVATE=vcs-test.golang.org
+
+! go get -d vcs-test.golang.org/insecure/go/insecure
+stderr 'redirected .* to insecure URL'
+
+[short] stop 'builds a git repo'
+
+env GOINSECURE=vcs-test.golang.org/insecure/go/insecure
+go get -d vcs-test.golang.org/insecure/go/insecure
+
+-- go.mod --
+module example
+go 1.21
+
diff --git a/src/cmd/go/testdata/script/mod_get_issue37438.txt b/src/cmd/go/testdata/script/mod_get_issue37438.txt
new file mode 100644
index 0000000..9392e73
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_issue37438.txt
@@ -0,0 +1,37 @@
+# Regression test for https://golang.org/issue/37438.
+#
+# If a path exists at the requested version, but does not exist at the
+# version of the module that is already required and does not exist at
+# the version that would be selected by 'go mod tidy', then
+# 'go get foo@requested' should resolve the requested version,
+# not error out on the (unrelated) latest one.
+
+go get example.net/a/p@v0.2.0
+
+-- go.mod --
+module example
+
+go 1.15
+
+require example.net/a v0.1.0
+
+replace (
+	example.net/a v0.1.0 => ./a1
+	example.net/a v0.2.0 => ./a2
+	example.net/a v0.3.0 => ./a1
+)
+
+-- a1/go.mod --
+module example.net/a
+
+go 1.15
+-- a1/README --
+package example.net/a/p does not exist at this version.
+
+-- a2/go.mod --
+module example.net/a
+
+go 1.15
+-- a2/p/p.go --
+// Package p exists only at v0.2.0.
+package p
diff --git a/src/cmd/go/testdata/script/mod_get_issue47979.txt b/src/cmd/go/testdata/script/mod_get_issue47979.txt
new file mode 100644
index 0000000..848ee3a
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_issue47979.txt
@@ -0,0 +1,117 @@
+# Regression test for https://golang.org/issue/47979:
+#
+# An argument to 'go get' that results in an upgrade to a different existing
+# root should be allowed, and should not panic the 'go' command.
+
+cp go.mod go.mod.orig
+
+
+# Transitive upgrades from upgraded roots should not prevent
+# 'go get -u' from performing upgrades.
+
+cp go.mod.orig go.mod
+go get -u .
+cmp go.mod go.mod.want
+
+
+# 'go get' of a specific version should allow upgrades of
+# every dependency (transitively) required by that version,
+# including dependencies that are pulled into the module
+# graph by upgrading other root requirements
+# (in this case, example.net/indirect).
+
+cp go.mod.orig go.mod
+go get example.net/a@v0.2.0
+cmp go.mod go.mod.want
+
+
+-- go.mod --
+module golang.org/issue47979
+
+go 1.17
+
+replace (
+	example.net/a v0.1.0 => ./a1
+	example.net/a v0.2.0 => ./a2
+	example.net/indirect v0.1.0 => ./indirect1
+	example.net/indirect v0.2.0 => ./indirect2
+	example.net/other v0.1.0 => ./other
+	example.net/other v0.2.0 => ./other
+)
+
+require (
+	example.net/a v0.1.0
+	example.net/other v0.1.0
+)
+
+require example.net/indirect v0.1.0 // indirect
+-- go.mod.want --
+module golang.org/issue47979
+
+go 1.17
+
+replace (
+	example.net/a v0.1.0 => ./a1
+	example.net/a v0.2.0 => ./a2
+	example.net/indirect v0.1.0 => ./indirect1
+	example.net/indirect v0.2.0 => ./indirect2
+	example.net/other v0.1.0 => ./other
+	example.net/other v0.2.0 => ./other
+)
+
+require (
+	example.net/a v0.2.0
+	example.net/other v0.2.0
+)
+
+require example.net/indirect v0.2.0 // indirect
+-- issue.go --
+package issue
+
+import _ "example.net/a"
+-- useother/useother.go --
+package useother
+
+import _ "example.net/other"
+-- a1/go.mod --
+module example.net/a
+
+go 1.17
+
+require example.net/indirect v0.1.0
+-- a1/a.go --
+package a
+-- a2/go.mod --
+module example.net/a
+
+go 1.17
+
+require example.net/indirect v0.2.0
+-- a2/a.go --
+package a
+
+import "example.net/indirect"
+-- indirect1/go.mod --
+module example.net/indirect
+
+go 1.17
+
+require example.net/other v0.1.0
+-- indirect1/indirect.go --
+package indirect
+-- indirect2/go.mod --
+module example.net/indirect
+
+go 1.17
+
+require example.net/other v0.2.0
+-- indirect2/indirect.go --
+package indirect
+
+import "example.net/other"
+-- other/go.mod --
+module example.net/other
+
+go 1.17
+-- other/other.go --
+package other
diff --git a/src/cmd/go/testdata/script/mod_get_issue48511.txt b/src/cmd/go/testdata/script/mod_get_issue48511.txt
new file mode 100644
index 0000000..0ba486d
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_issue48511.txt
@@ -0,0 +1,68 @@
+# Regression test for https://golang.org/issue/48511:
+# requirement minimization was accidentally replacing previous
+# versions of the main module, causing dependencies to be
+# spuriously dropping during requirement minimization and
+# leading to an infinite loop.
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod go.mod.orig
+
+go get -u=patch ./...
+cmp go.mod go.mod.want
+
+-- go.mod --
+module example.net/m
+
+go 1.16
+
+replace (
+	example.net/a v0.1.0 => ./a
+	example.net/b v0.1.0 => ./b
+	example.net/b v0.1.1 => ./b
+	example.net/m v0.1.0 => ./m1
+)
+
+require example.net/a v0.1.0
+-- go.mod.want --
+module example.net/m
+
+go 1.16
+
+replace (
+	example.net/a v0.1.0 => ./a
+	example.net/b v0.1.0 => ./b
+	example.net/b v0.1.1 => ./b
+	example.net/m v0.1.0 => ./m1
+)
+
+require (
+	example.net/a v0.1.0
+	example.net/b v0.1.1 // indirect
+)
+-- m.go --
+package m
+
+import "example.net/a"
+-- m1/go.mod --
+module example.net/m
+
+go 1.16
+
+require example.net/b v0.1.0
+-- a/go.mod --
+module example.net/a
+
+go 1.16
+
+require example.net/m v0.1.0
+-- a/a.go --
+package a
+
+import "example.net/b"
+-- b/go.mod --
+module example.net/b
+
+go 1.16
+-- b/b.go --
+package b
diff --git a/src/cmd/go/testdata/script/mod_get_issue56494.txt b/src/cmd/go/testdata/script/mod_get_issue56494.txt
new file mode 100644
index 0000000..3e4d4af
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_issue56494.txt
@@ -0,0 +1,134 @@
+# Regression test for https://go.dev/issue/56494:
+# 'go get' in module mode was failing to prune out dependencies
+# through modules whose versions are too low to be selected.
+
+# Initially, modules "a", "b", and "c" are unrelated.
+#
+# The package import graph at v1 of everything looks like:
+#
+# m --- a
+# |
+# + --- b
+# |
+# + --- c
+#
+# At v2, package "a" adds imports of "b" and "c"
+# (and a requirement on "c" v2):
+#
+# a --- b
+# |
+# + --- c
+#
+# And "b" adds an import of "a/sub" (in module "a"):
+#
+# b --- a/sub
+#
+# At v3, "a" no longer imports (nor requires) "c":
+#
+# a --- b
+
+# So upgrading to a3 adds a dependency on b2,
+# b2 adds a dependency on a2 (for "a/sub"),
+# and a2 (but not a3) would add a dependency on c2.
+# Since a2 is lower than a3 it cannot possibly be selected when
+# upgrading to a3: normally a2 is pruned out of a3's module graph,
+# so 'go get' should prune it out too, and c should remain at c1
+# without error.
+
+go get a@v0.3.0
+
+go list -m c
+stdout '^c v0.1.0 '
+
+-- go.mod --
+module m
+
+go 1.19
+
+require (
+	a v0.1.0
+	b v0.1.0
+	c v0.1.0
+)
+
+replace (
+	a v0.1.0 => ./a1
+	a v0.2.0 => ./a2
+	a v0.3.0 => ./a3
+	b v0.1.0 => ./b1
+	b v0.2.0 => ./b2
+	c v0.1.0 => ./c1
+	c v0.2.0 => ./c2
+)
+-- m.go --
+package m
+
+import (
+	_ "a"
+	_ "b"
+	_ "c"
+)
+-- a1/go.mod --
+module a
+
+go 1.19
+-- a1/a.go --
+package a
+-- a2/go.mod --
+module a
+
+go 1.19
+
+require (
+	b v0.1.0
+	c v0.2.0
+)
+-- a2/a.go --
+package a
+
+import (
+	_ "b"
+	_ "c"
+)
+-- a2/sub/sub.go --
+package sub
+-- a3/go.mod --
+module a
+
+go 1.19
+
+require b v0.2.0
+-- a3/a.go --
+package a
+
+import _ "b"
+-- a3/sub/sub.go --
+package sub
+-- b1/go.mod --
+module b
+
+go 1.19
+-- b1/b.go --
+package b
+-- b2/go.mod --
+module b
+
+go 1.19
+
+require a v0.2.0
+-- b2/b.go --
+package b
+
+import "a/sub"
+-- c1/go.mod --
+module c
+
+go 1.19
+-- c1/c.go --
+package c
+-- c2/go.mod --
+module c
+
+go 1.19
+-- c2/c.go --
+package c
diff --git a/src/cmd/go/testdata/script/mod_get_issue60490.txt b/src/cmd/go/testdata/script/mod_get_issue60490.txt
new file mode 100644
index 0000000..e0ac26a
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_issue60490.txt
@@ -0,0 +1,48 @@
+# Regression test for https://go.dev/issue/60490: 'go get' should not cause an
+# infinite loop for cycles introduced in the pruned module graph.
+
+go get example.net/c@v0.1.0
+
+-- go.mod --
+module example
+
+go 1.19
+
+require (
+	example.net/a v0.1.0
+	example.net/b v0.1.0
+)
+
+replace (
+	example.net/a v0.1.0 => ./a1
+	example.net/a v0.2.0 => ./a2
+	example.net/b v0.1.0 => ./b1
+	example.net/b v0.2.0 => ./b2
+	example.net/c v0.1.0 => ./c1
+)
+-- a1/go.mod --
+module example.net/a
+
+go 1.19
+-- a2/go.mod --
+module example.net/a
+
+go 1.19
+
+require example.net/b v0.2.0
+-- b1/go.mod --
+module example.net/b
+
+go 1.19
+-- b2/go.mod --
+module example.net/b
+
+go 1.19
+
+require example.net/a v0.2.0
+-- c1/go.mod --
+module example.net/c
+
+go 1.19
+
+require example.net/a v0.2.0
diff --git a/src/cmd/go/testdata/script/mod_get_latest_pseudo.txt b/src/cmd/go/testdata/script/mod_get_latest_pseudo.txt
new file mode 100644
index 0000000..00da0c3
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_latest_pseudo.txt
@@ -0,0 +1,10 @@
+# Check that we can build a module with no tagged versions by querying
+# "@latest" through a proxy.
+# Verifies golang.org/issue/32636
+
+env GO111MODULE=on
+
+go mod init m
+go get example.com/notags
+go list -m all
+stdout '^example.com/notags v0.0.0-20190507143103-cc8cbe209b64$'
diff --git a/src/cmd/go/testdata/script/mod_get_lazy_indirect.txt b/src/cmd/go/testdata/script/mod_get_lazy_indirect.txt
new file mode 100644
index 0000000..1cef9d1
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_lazy_indirect.txt
@@ -0,0 +1,44 @@
+# https://golang.org/issue/45979: after 'go get' on a package,
+# that package should be importable without error.
+
+
+# We start out with an unresolved dependency.
+# 'go list' suggests that we run 'go get' on that dependency.
+
+! go list -deps .
+stderr '^m.go:3:8: no required module provides package rsc\.io/quote; to add it:\n\tgo get rsc.io/quote$'
+
+
+# When we run the suggested 'go get' command, the new dependency can be used
+# immediately.
+#
+# 'go get' marks the new dependency as 'indirect', because it doesn't scan
+# enough source code to know whether it is direct, and it is easier and less
+# invasive to remove an incorrect indirect mark (e.g. using 'go get') than to
+# add one that is missing ('go mod tidy' or 'go mod vendor').
+
+go get rsc.io/quote
+grep 'rsc.io/quote v\d+\.\d+\.\d+ // indirect$' go.mod
+! grep 'rsc.io/quote v\d+\.\d+\.\d+$' go.mod
+
+go list -deps .
+! stderr .
+[!short] go build .
+[!short] ! stderr .
+
+
+# 'go get .' (or 'go mod tidy') removes the indirect mark.
+
+go get .
+grep 'rsc.io/quote v\d+\.\d+\.\d+$' go.mod
+! grep 'rsc.io/quote v\d+\.\d+\.\d+ // indirect$' go.mod
+
+
+-- go.mod --
+module example.com/m
+
+go 1.17
+-- m.go --
+package m
+
+import _ "rsc.io/quote"
diff --git a/src/cmd/go/testdata/script/mod_get_lazy_upgrade_lazy.txt b/src/cmd/go/testdata/script/mod_get_lazy_upgrade_lazy.txt
new file mode 100644
index 0000000..3dae383
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_lazy_upgrade_lazy.txt
@@ -0,0 +1,68 @@
+# Check that 'go get -u' will upgrade a dependency (direct or indirect)
+# when the main module and the dependency are both lazy.
+# Verifies #47768.
+
+# Check that go.mod is tidy, and an upgrade is available.
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod go.mod.orig
+
+go list -m -u example.com/lazyupgrade
+stdout '^example.com/lazyupgrade v0.1.0 \[v0.1.1\] => ./lazyupgrade@v0.1.0$'
+
+# 'go get -u' on a package that directly imports the dependency should upgrade.
+go get -u ./usedirect
+go list -m example.com/lazyupgrade
+stdout '^example.com/lazyupgrade v0.1.1 => ./lazyupgrade@v0.1.1$'
+cp go.mod.orig go.mod
+
+# 'go get -u' on a package that indirectly imports the dependency should upgrade.
+go get -u ./useindirect
+go list -m example.com/lazyupgrade
+stdout '^example.com/lazyupgrade v0.1.1 => ./lazyupgrade@v0.1.1$'
+
+-- go.mod --
+module use
+
+go 1.17
+
+require (
+	direct v0.0.0
+	example.com/lazyupgrade v0.1.0
+)
+
+replace (
+	direct => ./direct
+	example.com/lazyupgrade v0.1.0 => ./lazyupgrade@v0.1.0
+	example.com/lazyupgrade v0.1.1 => ./lazyupgrade@v0.1.1
+)
+-- usedirect/usedirect.go --
+package use
+
+import _ "example.com/lazyupgrade"
+-- useindirect/useindirect.go --
+package use
+
+import _ "direct"
+-- direct/go.mod --
+module direct
+
+go 1.17
+
+require example.com/lazyupgrade v0.1.0
+-- direct/direct.go --
+package direct
+
+import _ "example.com/lazyupgrade"
+-- lazyupgrade@v0.1.0/go.mod --
+module example.com/lazyupgrade
+
+go 1.17
+-- lazyupgrade@v0.1.0/lazyupgrade.go --
+package lazyupgrade
+-- lazyupgrade@v0.1.1/go.mod --
+module example.com/lazyupgrade
+
+go 1.17
+-- lazyupgrade@v0.1.1/lazyupgrade.go --
+package lazyupgrade
diff --git a/src/cmd/go/testdata/script/mod_get_local.txt b/src/cmd/go/testdata/script/mod_get_local.txt
new file mode 100644
index 0000000..4c81d16
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_local.txt
@@ -0,0 +1,62 @@
+# Test 'go get' with a local module with a name that is not valid for network lookup.
+[short] skip
+
+env GO111MODULE=on
+go mod edit -fmt
+cp go.mod go.mod.orig
+
+# 'go get -u' within the main module should work, even if it has a local-only name.
+cp go.mod.orig go.mod
+go get -u ./...
+grep 'rsc.io/quote.*v1.5.2' go.mod
+grep 'golang.org/x/text.*v0.3.0' go.mod
+cp go.mod go.mod.implicitmod
+
+# 'go get -u local/...' should be equivalent to 'go get -u ./...'
+# (assuming no nested modules)
+cp go.mod.orig go.mod
+go get -u local/...
+cmp go.mod go.mod.implicitmod
+
+# For the main module, @patch should be a no-op.
+cp go.mod.orig go.mod
+go get -u local/...@patch
+cmp go.mod go.mod.implicitmod
+
+# 'go get -u' in the empty root of the main module should fail.
+# 'go get -u .' should also fail.
+cp go.mod.orig go.mod
+! go get -u
+! go get -u .
+
+# 'go get -u .' within a package in the main module updates the dependencies
+# of that package.
+cp go.mod.orig go.mod
+cd uselang
+go get -u .
+cd ..
+grep 'rsc.io/quote.*v1.3.0' go.mod
+grep 'golang.org/x/text.*v0.3.0' go.mod
+cp go.mod go.mod.dotpkg
+
+# 'go get -u' with an explicit package in the main module updates the
+# dependencies of that package.
+cp go.mod.orig go.mod
+go get -u local/uselang
+cmp go.mod go.mod.dotpkg
+
+-- go.mod --
+module local
+
+require (
+	golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c
+	rsc.io/quote v1.3.0
+)
+
+-- uselang/uselang.go --
+package uselang
+import _ "golang.org/x/text/language"
+
+-- usequote/usequote.go --
+package usequote
+import _ "rsc.io/quote"
diff --git a/src/cmd/go/testdata/script/mod_get_main.txt b/src/cmd/go/testdata/script/mod_get_main.txt
new file mode 100644
index 0000000..cddd5f7
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_main.txt
@@ -0,0 +1,51 @@
+env GO111MODULE=on
+cp go.mod.orig go.mod
+
+# relative and absolute paths must be within the main module.
+! go get ..
+stderr '^go: \.\. \('$WORK'[/\\]gopath\) is not within module rooted at '$WORK'[/\\]gopath[/\\]src$'
+! go get $WORK
+stderr '^go: '$WORK' is not within module rooted at '$WORK'[/\\]gopath[/\\]src$'
+! go get ../...
+stderr '^go: \.\./\.\.\. \('$WORK'[/\\]gopath([/\\]...)?\) is not within module rooted at '$WORK'[/\\]gopath[/\\]src$'
+! go get $WORK/...
+stderr '^go: '$WORK'[/\\]\.\.\. is not within module rooted at '$WORK'[/\\]gopath[/\\]src$'
+
+# @patch and @latest within the main module refer to the current version.
+# The main module won't be upgraded, but missing dependencies will be added.
+go get rsc.io/x
+grep 'rsc.io/quote v1.5.2' go.mod
+go get rsc.io/x@upgrade
+grep 'rsc.io/quote v1.5.2' go.mod
+cp go.mod.orig go.mod
+go get rsc.io/x@patch
+grep 'rsc.io/quote v1.5.2' go.mod
+cp go.mod.orig go.mod
+
+
+# Upgrading a package pattern not contained in the main module should not
+# attempt to upgrade the main module.
+go get rsc.io/quote/...@v1.5.1
+grep 'rsc.io/quote v1.5.1' go.mod
+
+
+# The main module cannot be updated to a specific version.
+! go get rsc.io@v0.1.0
+stderr '^go: can''t request version "v0.1.0" of the main module \(rsc.io\)$'
+
+# A package in the main module can't be upgraded either.
+! go get rsc.io/x@v0.1.0
+stderr '^go: package rsc.io/x is in the main module, so can''t request version v0.1.0$'
+
+# Nor can a pattern matching packages in the main module.
+! go get rsc.io/x/...@latest
+stderr '^go: pattern rsc.io/x/... matches package rsc.io/x in the main module, so can''t request version latest$'
+
+-- go.mod.orig --
+module rsc.io
+
+go 1.13
+-- x/x.go --
+package x
+
+import _ "rsc.io/quote"
diff --git a/src/cmd/go/testdata/script/mod_get_major.txt b/src/cmd/go/testdata/script/mod_get_major.txt
new file mode 100644
index 0000000..4e0febb
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_major.txt
@@ -0,0 +1,23 @@
+[short] skip
+[!git] skip
+
+env GO111MODULE=on
+env GOPROXY=direct
+env GOSUMDB=off
+
+# golang.org/issue/34383: if a module path ends in a major-version suffix,
+# ensure that 'direct' mode can resolve the package to a module.
+
+go get vcs-test.golang.org/git/v3pkg.git/v3@v3.0.0
+
+go list -m vcs-test.golang.org/git/v3pkg.git/v3
+stdout '^vcs-test.golang.org/git/v3pkg.git/v3 v3.0.0$'
+
+go get vcs-test.golang.org/git/empty-v2-without-v1.git/v2@v2.0.0
+
+go list -m vcs-test.golang.org/git/empty-v2-without-v1.git/v2
+stdout '^vcs-test.golang.org/git/empty-v2-without-v1.git/v2 v2.0.0$'
+
+-- go.mod --
+module example.com
+go 1.13
diff --git a/src/cmd/go/testdata/script/mod_get_missing_ziphash.txt b/src/cmd/go/testdata/script/mod_get_missing_ziphash.txt
new file mode 100644
index 0000000..5934251
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_missing_ziphash.txt
@@ -0,0 +1,55 @@
+# Test that if the module cache contains an extracted source directory but not
+# a ziphash, 'go build' complains about a missing sum, and 'go get' adds
+# the sum. Verifies #44749.
+
+# With a tidy go.sum, go build succeeds. This also populates the module cache.
+cp go.sum.tidy go.sum
+go build -n use
+env GOPROXY=off
+env GOSUMDB=off
+
+# Control case: if we delete the hash for rsc.io/quote v1.5.2,
+# 'go build' reports an error. 'go get' adds the sum.
+cp go.sum.bug go.sum
+! go build -n use
+stderr '^use.go:3:8: missing go.sum entry for module providing package rsc.io/quote \(imported by use\); to add:\n\tgo get use$'
+go get use
+cmp go.sum go.sum.tidy
+go build -n use
+
+# If we delete the hash *and* the ziphash file, we should see the same behavior.
+cp go.sum.bug go.sum
+rm $WORK/gopath/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.ziphash
+! go build -n use
+stderr '^use.go:3:8: missing go.sum entry for module providing package rsc.io/quote \(imported by use\); to add:\n\tgo get use$'
+go get use
+cmp go.sum go.sum.tidy
+go build -n use
+
+-- go.mod --
+module use
+
+go 1.16
+
+require rsc.io/quote v1.5.2
+-- go.sum.tidy --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c h1:pvCbr/wm8HzDD3fVywevekufpn6tCGPY3spdHeZJEsw=
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.5.2 h1:3fEykkD9k7lYzXqCYrwGAf7iNhbk4yCjHmKBN9td4L0=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0 h1:HLGR/BgEtI3r0uymSP/nl2uPLsUnNJX8toRyhfpBTII=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+rsc.io/testonly v1.0.0 h1:K/VWHdO+Jv7woUXG0GzVNx1czBXUt3Ib1deaMn+xk64=
+rsc.io/testonly v1.0.0/go.mod h1:OqmGbIFOcF+XrFReLOGZ6BhMM7uMBiQwZsyNmh74SzY=
+-- go.sum.bug --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c h1:pvCbr/wm8HzDD3fVywevekufpn6tCGPY3spdHeZJEsw=
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0 h1:HLGR/BgEtI3r0uymSP/nl2uPLsUnNJX8toRyhfpBTII=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+rsc.io/testonly v1.0.0 h1:K/VWHdO+Jv7woUXG0GzVNx1czBXUt3Ib1deaMn+xk64=
+rsc.io/testonly v1.0.0/go.mod h1:OqmGbIFOcF+XrFReLOGZ6BhMM7uMBiQwZsyNmh74SzY=
+-- use.go --
+package use
+
+import _ "rsc.io/quote"
diff --git a/src/cmd/go/testdata/script/mod_get_moved.txt b/src/cmd/go/testdata/script/mod_get_moved.txt
new file mode 100644
index 0000000..cc35b4c
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_moved.txt
@@ -0,0 +1,40 @@
+env GO111MODULE=on
+[short] skip
+
+# A 'go get' that worked at a previous version should continue to work at that version,
+# even if the package was subsequently moved into a submodule.
+go mod init example.com/foo
+go get example.com/split/subpkg@v1.0.0
+go list -m all
+stdout 'example.com/split v1.0.0'
+
+# A 'go get' that simultaneously upgrades away conflicting package definitions is not ambiguous.
+go get example.com/split/subpkg@v1.1.0
+
+# A 'go get' without an upgrade should find the package.
+rm go.mod
+go mod init example.com/foo
+go get example.com/split/subpkg
+go list -m all
+stdout 'example.com/split/subpkg v1.1.0'
+
+
+# A 'go get' that worked at a previous version should continue to work at that version,
+# even if the package was subsequently moved into a parent module.
+rm go.mod
+go mod init example.com/foo
+go get example.com/join/subpkg@v1.0.0
+go list -m all
+stdout 'example.com/join/subpkg v1.0.0'
+
+# A 'go get' that simultaneously upgrades away conflicting package definitions is not ambiguous.
+# (A wildcard pattern applies to both packages and modules,
+# because we define wildcard matching to apply after version resolution.)
+go get example.com/join/subpkg/...@v1.1.0
+
+# A 'go get' without an upgrade should find the package.
+rm go.mod
+go mod init example.com/foo
+go get example.com/join/subpkg@v1.1.0
+go list -m all
+stdout 'example.com/join v1.1.0'
diff --git a/src/cmd/go/testdata/script/mod_get_newcycle.txt b/src/cmd/go/testdata/script/mod_get_newcycle.txt
new file mode 100644
index 0000000..4f5229e
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_newcycle.txt
@@ -0,0 +1,14 @@
+env GO111MODULE=on
+
+# Download modules to avoid stderr chatter
+go mod download example.com@v1.0.0
+go mod download example.com/newcycle/a@v1.0.0
+go mod download example.com/newcycle/a@v1.0.1
+go mod download example.com/newcycle/b@v1.0.0
+
+go mod init m
+! go get example.com/newcycle/a@v1.0.0
+cmp stderr stderr-expected
+
+-- stderr-expected --
+go: example.com/newcycle/a@v1.0.0 indirectly requires example.com/newcycle/a@v1.0.1, not example.com/newcycle/a@v1.0.0
diff --git a/src/cmd/go/testdata/script/mod_get_none.txt b/src/cmd/go/testdata/script/mod_get_none.txt
new file mode 100644
index 0000000..5aec209
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_none.txt
@@ -0,0 +1,12 @@
+env GO111MODULE=on
+
+go mod init example.com/foo
+
+# 'go get bar@none' should be a no-op if module bar is not active.
+go get example.com/bar@none
+go list -m all
+! stdout example.com/bar
+
+go get example.com/bar@none
+go list -m all
+! stdout example.com/bar
diff --git a/src/cmd/go/testdata/script/mod_get_nopkgs.txt b/src/cmd/go/testdata/script/mod_get_nopkgs.txt
new file mode 100644
index 0000000..14176a7
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_nopkgs.txt
@@ -0,0 +1,40 @@
+cd subdir
+
+# 'go get' on empty patterns that are necessarily local to the module
+# should warn that the patterns are empty, exactly once.
+
+go get ./...
+stderr -count=1 'matched no packages'
+
+go get ./...
+stderr -count=1 'matched no packages'
+
+# 'go get' on patterns that could conceivably match nested modules
+# should report a module resolution error.
+
+go get example.net/emptysubdir/... # control case
+
+! go get example.net/emptysubdir/subdir/...
+! stderr 'matched no packages'
+stderr '^go: example\.net/emptysubdir/subdir/\.\.\.: module example\.net/emptysubdir/subdir: reading http://.*: 404 Not Found\n\tserver response: 404 page not found\n\z'
+
+# It doesn't make sense to 'go get' a path in the standard library,
+# since the standard library necessarily can't have unresolved imports.
+#
+# TODO(#30241): Maybe that won't always be the case?
+#
+# For that case, we emit a "malformed module path" error message,
+# which isn't ideal either.
+
+! go get builtin/...  # in GOROOT/src, but contains no packages
+stderr '^go: builtin/...: malformed module path "builtin": missing dot in first path element$'
+
+-- go.mod --
+module example.net/emptysubdir
+
+go 1.16
+-- emptysubdir.go --
+// Package emptysubdir has a subdirectory containing no packages.
+package emptysubdir
+-- subdir/README.txt --
+This module intentionally does not contain any p
diff --git a/src/cmd/go/testdata/script/mod_get_patch.txt b/src/cmd/go/testdata/script/mod_get_patch.txt
new file mode 100644
index 0000000..35cc276
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_patch.txt
@@ -0,0 +1,130 @@
+# This test examines the behavior of 'go get …@patch'
+# See also mod_upgrade_patch.txt (focused on "-u=patch" specifically)
+# and mod_get_patchmod.txt (focused on module/package ambiguities).
+
+cp go.mod go.mod.orig
+
+# example.net/b@patch refers to the patch for the version of b that was selected
+# at the start of 'go get', not the version after applying other changes.
+
+! go get example.net/a@v0.2.0 example.net/b@patch
+stderr '^go: example.net/a@v0.2.0 requires example.net/b@v0.2.0, not example.net/b@patch \(v0.1.1\)$'
+cmp go.mod go.mod.orig
+
+
+# -u=patch changes the default version for other arguments to '@patch',
+# but they continue to be resolved against the originally-selected version,
+# not the updated one.
+#
+# TODO(#42360): Reconsider the change in defaults.
+
+! go get -u=patch example.net/a@v0.2.0 example.net/b
+stderr '^go: example.net/a@v0.2.0 requires example.net/b@v0.2.0, not example.net/b@patch \(v0.1.1\)$'
+cmp go.mod go.mod.orig
+
+
+# -u=patch refers to the patches for the selected versions of dependencies *after*
+# applying other version changes, not the versions that were selected at the start.
+# However, it should not patch versions determined by explicit arguments.
+
+go get -u=patch example.net/a@v0.2.0
+go list -m all
+stdout '^example.net/a v0.2.0 '
+stdout '^example.net/b v0.2.1 '
+
+
+# "-u=patch all" should be equivalent to "all@patch", and should fail if the
+# patched versions result in a higher-than-patch upgrade.
+
+cp go.mod.orig go.mod
+! go get -u=patch all
+stderr '^go: example.net/a@v0.1.1 \(matching all@patch\) requires example.net/b@v0.2.0, not example.net/b@v0.1.1 \(matching all@patch\)$'
+cmp go.mod go.mod.orig
+
+
+# On the other hand, "-u=patch ./..." should patch-upgrade dependencies until
+# they reach a fixed point, even if that results in higher-than-patch upgrades.
+
+go get -u=patch ./...
+go list -m all
+stdout '^example.net/a v0.1.1 '
+stdout '^example.net/b v0.2.1 '
+
+
+-- go.mod --
+module example
+
+go 1.16
+
+require (
+	example.net/a v0.1.0
+	example.net/b v0.1.0  // indirect
+)
+
+replace (
+	example.net/a v0.1.0 => ./a10
+	example.net/a v0.1.1 => ./a11
+	example.net/a v0.2.0 => ./a20
+	example.net/a v0.2.1 => ./a21
+	example.net/b v0.1.0 => ./b
+	example.net/b v0.1.1 => ./b
+	example.net/b v0.2.0 => ./b
+	example.net/b v0.2.1 => ./b
+	example.net/b v0.3.0 => ./b
+	example.net/b v0.3.1 => ./b
+)
+-- example.go --
+package example
+
+import _ "example.net/a"
+
+-- a10/go.mod --
+module example.net/a
+
+go 1.16
+
+require example.net/b v0.1.0
+-- a10/a.go --
+package a
+
+import _ "example.net/b"
+
+-- a11/go.mod --
+module example.net/a
+
+go 1.16
+
+require example.net/b v0.2.0  // upgraded
+-- a11/a.go --
+package a
+
+import _ "example.net/b"
+
+-- a20/go.mod --
+module example.net/a
+
+go 1.16
+
+require example.net/b v0.2.0
+-- a20/a.go --
+package a
+
+import _ "example.net/b"
+
+-- a21/go.mod --
+module example.net/a
+
+go 1.16
+
+require example.net/b v0.2.0  // not upgraded
+-- a21/a.go --
+package a
+
+import _ "example.net/b"
+
+-- b/go.mod --
+module example.net/b
+
+go 1.16
+-- b/b.go --
+package b
diff --git a/src/cmd/go/testdata/script/mod_get_patchbound.txt b/src/cmd/go/testdata/script/mod_get_patchbound.txt
new file mode 100644
index 0000000..e4d3c49
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_patchbound.txt
@@ -0,0 +1,84 @@
+# -u=patch will patch dependencies as far as possible, but not so far that they
+# conflict with other command-line arguments.
+
+go list -m all
+stdout '^example.net/a v0.1.0 '
+stdout '^example.net/b v0.1.0 '
+
+go get -u=patch example.net/a@v0.2.0
+go list -m all
+stdout '^example.net/a v0.2.0 '
+stdout '^example.net/b v0.1.1 '  # not v0.1.2, which requires …/a v0.3.0.
+
+-- go.mod --
+module example
+
+go 1.16
+
+require (
+	example.net/a v0.1.0
+	example.net/b v0.1.0 // indirect
+)
+
+replace (
+	example.net/a v0.1.0 => ./a
+	example.net/a v0.2.0 => ./a
+	example.net/a v0.3.0 => ./a
+	example.net/b v0.1.0 => ./b10
+	example.net/b v0.1.1 => ./b11
+	example.net/b v0.1.2 => ./b12
+)
+-- example.go --
+package example
+
+import _ "example.net/a"
+
+-- a/go.mod --
+module example.net/a
+
+go 1.16
+
+require example.net/b v0.1.0
+-- a/a.go --
+package a
+
+import _ "example.net/b"
+
+-- b10/go.mod --
+module example.net/b
+
+go 1.16
+
+require example.net/a v0.1.0
+-- b10/b.go --
+package b
+-- b10/b_test.go --
+package b_test
+
+import _ "example.net/a"
+
+-- b11/go.mod --
+module example.net/b
+
+go 1.16
+
+require example.net/a v0.2.0
+-- b11/b.go --
+package b
+-- b11/b_test.go --
+package b_test
+
+import _ "example.net/a"
+
+-- b12/go.mod --
+module example.net/b
+
+go 1.16
+
+require example.net/a v0.3.0
+-- b12/b.go --
+package b
+-- b12/b_test.go --
+package b_test
+
+import _ "example.net/a"
diff --git a/src/cmd/go/testdata/script/mod_get_patchcycle.txt b/src/cmd/go/testdata/script/mod_get_patchcycle.txt
new file mode 100644
index 0000000..9f180d6
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_patchcycle.txt
@@ -0,0 +1,64 @@
+# If a patch of a module requires a higher version of itself,
+# it should be reported as its own conflict.
+#
+# This case is weird and unlikely to occur often at all, but it should not
+# spuriously succeed.
+# (It used to print v0.1.1 but then silently upgrade to v0.2.0.)
+
+! go get example.net/a@patch
+stderr '^go: example.net/a@patch \(v0.1.1\) indirectly requires example.net/a@v0.2.0, not example.net/a@patch \(v0.1.1\)$'  # TODO: A mention of b v0.1.0 would be nice.
+
+-- go.mod --
+module example
+
+go 1.16
+
+require example.net/a v0.1.0
+
+replace (
+	example.net/a v0.1.0 => ./a10
+	example.net/a v0.1.1 => ./a11
+	example.net/a v0.2.0 => ./a20
+	example.net/b v0.1.0 => ./b10
+)
+-- example.go --
+package example
+
+import _ "example.net/a"
+
+-- a10/go.mod --
+module example.net/a
+
+go 1.16
+-- a10/a.go --
+package a
+
+-- a11/go.mod --
+module example.net/a
+
+go 1.16
+
+require example.net/b v0.1.0
+-- a11/a.go --
+package a
+
+import _ "example.net/b"
+
+-- a20/go.mod --
+module example.net/a
+
+go 1.16
+-- a20/a.go --
+package a
+
+
+-- b10/go.mod --
+module example.net/b
+
+go 1.16
+
+require example.net/a v0.2.0
+-- b10/b.go --
+package b
+
+import _ "example.net/a"
diff --git a/src/cmd/go/testdata/script/mod_get_patchmod.txt b/src/cmd/go/testdata/script/mod_get_patchmod.txt
new file mode 100644
index 0000000..2827731
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_patchmod.txt
@@ -0,0 +1,82 @@
+# example.net/pkgremoved@v0.1.0 refers to a package.
+go get example.net/pkgremoved@v0.1.0
+
+go list example.net/pkgremoved
+stdout '^example.net/pkgremoved'
+
+cp go.mod go.mod.orig
+
+
+# When we resolve a new dependency on example.net/other,
+# it will change the meaning of the path "example.net/pkgremoved"
+# from a package (at v0.1.0) to only a module (at v0.2.0).
+#
+# If we simultaneously 'get' that module at the query "patch", the module should
+# be constrained to the latest patch of its originally-selected version (v0.1.0),
+# not upgraded to the latest patch of the new transitive dependency.
+
+! go get example.net/pkgremoved@patch example.net/other@v0.1.0
+stderr '^go: example.net/other@v0.1.0 requires example.net/pkgremoved@v0.2.0, not example.net/pkgremoved@patch \(v0.1.1\)$'
+cmp go.mod.orig go.mod
+
+
+# However, we should be able to patch from a package to a module and vice-versa.
+
+# Package to module ...
+
+go get example.net/pkgremoved@v0.3.0
+go list example.net/pkgremoved
+stdout 'example.net/pkgremoved'
+
+go get example.net/pkgremoved@patch
+! go list example.net/pkgremoved
+
+# ... and module to package.
+
+go get example.net/pkgremoved@v0.4.0
+! go list example.net/pkgremoved
+
+go get example.net/pkgremoved@patch
+go list example.net/pkgremoved
+stdout 'example.net/pkgremoved'
+
+
+-- go.mod --
+module example
+
+go 1.16
+
+replace (
+	example.net/other v0.1.0 => ./other
+
+	example.net/pkgremoved v0.1.0 => ./prpkg
+	example.net/pkgremoved v0.1.1 => ./prpkg
+
+	example.net/pkgremoved v0.2.0 => ./prmod
+	example.net/pkgremoved v0.2.1 => ./prmod
+
+	example.net/pkgremoved v0.3.0 => ./prpkg
+	example.net/pkgremoved v0.3.1 => ./prmod
+
+	example.net/pkgremoved v0.4.0 => ./prmod
+	example.net/pkgremoved v0.4.1 => ./prpkg
+)
+-- other/go.mod --
+module example.net/other
+
+go 1.16
+
+require example.net/pkgremoved v0.2.0
+-- other/other.go --
+package other
+-- prpkg/go.mod --
+module example.net/pkgremoved
+
+go 1.16
+-- prpkg/pkgremoved.go --
+package pkgremoved
+-- prmod/go.mod --
+module example.net/pkgremoved
+-- prmod/README.txt --
+Package pkgremoved was removed in v0.2.0 and v0.3.1,
+and added in v0.1.0 and v0.4.1.
diff --git a/src/cmd/go/testdata/script/mod_get_patterns.txt b/src/cmd/go/testdata/script/mod_get_patterns.txt
new file mode 100644
index 0000000..891353f
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_patterns.txt
@@ -0,0 +1,42 @@
+env GO111MODULE=on
+[short] skip
+
+# If a pattern doesn't match any packages provided by modules
+# in the build list, we assume the pattern matches a single module
+# whose path is a prefix of the part of the pattern before "...".
+cp go.mod.orig go.mod
+go get rsc.io/quote/...
+grep 'require rsc.io/quote' go.mod
+
+cp go.mod.orig go.mod
+! go get rsc.io/quote/x...
+stderr 'go: module rsc.io/quote@upgrade found \(v1.5.2\), but does not contain packages matching rsc.io/quote/x...'
+! grep 'require rsc.io/quote' go.mod
+
+! go get rsc.io/quote/x/...
+stderr 'go: module rsc.io/quote@upgrade found \(v1.5.2\), but does not contain packages matching rsc.io/quote/x/...'
+! grep 'require rsc.io/quote' go.mod
+
+# If a pattern matches no packages within a module, the module should not
+# be upgraded, even if the module path is a prefix of the pattern.
+cp go.mod.orig go.mod
+go mod edit -require example.com/nest@v1.0.0
+go get example.com/nest/sub/y...
+grep 'example.com/nest/sub v1.0.0' go.mod
+grep 'example.com/nest v1.0.0' go.mod
+
+# However, if the pattern matches the module path itself, the module
+# should be upgraded even if it contains no matching packages.
+go get example.com/n...t
+grep 'example.com/nest v1.1.0' go.mod
+grep 'example.com/nest/sub v1.0.0' go.mod
+
+-- go.mod.orig --
+module m
+
+go 1.13
+
+-- use/use.go --
+package use
+
+import _ "rsc.io/quote"
diff --git a/src/cmd/go/testdata/script/mod_get_pkgtags.txt b/src/cmd/go/testdata/script/mod_get_pkgtags.txt
new file mode 100644
index 0000000..8cb41ad
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_pkgtags.txt
@@ -0,0 +1,130 @@
+# https://golang.org/issue/44106
+# 'go get' should fetch the transitive dependencies of packages regardless of
+# tags, but shouldn't error out if the package is missing tag-guarded
+# dependencies.
+
+# Control case: just adding the top-level module to the go.mod file does not
+# fetch its dependencies.
+
+go mod edit -require example.net/tools@v0.1.0
+! go list -deps example.net/cmd/tool
+stderr '^module example\.net/cmd provides package example\.net/cmd/tool and is replaced but not required; to add it:\n\tgo get example\.net/cmd@v0\.1\.0$'
+go mod edit -droprequire example.net/tools
+
+
+# 'go get' makes a best effort to fetch those dependencies, but shouldn't
+# error out if dependencies of tag-guarded files are missing.
+
+go get example.net/tools@v0.1.0
+! stderr 'no Go source files'
+
+! go list example.net/tools
+stderr '^package example.net/tools: build constraints exclude all Go files in .*[/\\]tools$'
+
+go list -tags=tools -e -deps example.net/tools
+stdout '^example.net/cmd/tool$'
+stdout '^example.net/missing$'
+
+go list -deps example.net/cmd/tool
+
+! go list example.net/missing
+stderr '^no required module provides package example.net/missing; to add it:\n\tgo get example.net/missing$'
+
+
+# https://golang.org/issue/33526: 'go get' without '-d' should succeed
+# for a module whose root is a constrained-out package.
+#
+# Ideally it should silently succeed, but today it logs the "no Go source files"
+# error and succeeds anyway.
+
+go get example.net/tools@v0.1.0
+! stderr .
+
+! go build example.net/tools
+stderr '^package example.net/tools: build constraints exclude all Go files in .*[/\\]tools$'
+
+
+# https://golang.org/issue/29268
+# 'go get' should fetch modules whose roots contain test-only packages, but
+# without the -t flag shouldn't error out if the test has missing dependencies.
+
+go get example.net/testonly@v0.1.0
+
+# With the -t flag, the test dependencies must resolve successfully.
+! go get -t example.net/testonly@v0.1.0
+stderr '^go: example.net/testonly tested by\n\texample.net/testonly\.test imports\n\texample.net/missing: cannot find module providing package example.net/missing$'
+
+
+# 'go get' should succeed for a module path that does not contain a package,
+# but fail for a non-package subdirectory of a module.
+
+! go get example.net/missing/subdir@v0.1.0
+stderr '^go: module example.net/missing@v0.1.0 found \(replaced by ./missing\), but does not contain package example.net/missing/subdir$'
+
+go get example.net/missing@v0.1.0
+
+
+# Getting the subdirectory should continue to fail even if the corresponding
+# module is already present in the build list.
+
+! go get example.net/missing/subdir@v0.1.0
+stderr '^go: module example.net/missing@v0.1.0 found \(replaced by ./missing\), but does not contain package example.net/missing/subdir$'
+
+
+-- go.mod --
+module example.net/m
+
+go 1.15
+
+replace (
+	example.net/tools v0.1.0 => ./tools
+	example.net/cmd v0.1.0 => ./cmd
+	example.net/testonly v0.1.0 => ./testonly
+	example.net/missing v0.1.0 => ./missing
+)
+
+-- tools/go.mod --
+module example.net/tools
+
+go 1.15
+
+// Requirements intentionally omitted.
+
+-- tools/tools.go --
+// +build tools
+
+package tools
+
+import (
+	_ "example.net/cmd/tool"
+	_ "example.net/missing"
+)
+
+-- cmd/go.mod --
+module example.net/cmd
+
+go 1.16
+-- cmd/tool/tool.go --
+package main
+
+func main() {}
+
+-- testonly/go.mod --
+module example.net/testonly
+
+go 1.15
+-- testonly/testonly_test.go --
+package testonly_test
+
+import _ "example.net/missing"
+
+func Test(t *testing.T) {}
+
+-- missing/go.mod --
+module example.net/missing
+
+go 1.15
+-- missing/README.txt --
+There are no Go source files here.
+-- missing/subdir/README.txt --
+There are no Go source files here either.
diff --git a/src/cmd/go/testdata/script/mod_get_prefer_incompatible.txt b/src/cmd/go/testdata/script/mod_get_prefer_incompatible.txt
new file mode 100644
index 0000000..06e2fc6
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_prefer_incompatible.txt
@@ -0,0 +1,29 @@
+# Verifies golang.org/issue/37574.
+
+# If we are already using an +incompatible version, we shouldn't look up
+# a lower compatible version when upgrading.
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod.orig go.mod
+grep '^example.com/incompatiblewithsub v2\.0\.0\+incompatible' go.sum
+! grep '^example.com/incompatiblewithsub v1.0.0' go.sum
+
+go get example.com/incompatiblewithsub/sub
+cmp go.mod.orig go.mod
+! grep '^example.com/incompatiblewithsub v1.0.0' go.sum
+
+# TODO(golang.org/issue/31580): the 'go get' command above should not change
+# go.sum. However, as part of the query above, we download example.com@v1.0.0,
+# an unrelated module, since it's a possible prefix. The sum for that module
+# should not be written to go.sum.
+
+-- go.mod --
+module m
+
+go 1.15
+
+require example.com/incompatiblewithsub v2.0.0+incompatible
+-- use.go --
+package use
+
+import _ "example.com/incompatiblewithsub/sub"
diff --git a/src/cmd/go/testdata/script/mod_get_promote_implicit.txt b/src/cmd/go/testdata/script/mod_get_promote_implicit.txt
new file mode 100644
index 0000000..03f6810
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_promote_implicit.txt
@@ -0,0 +1,92 @@
+cp go.mod.orig go.mod
+
+# If we list a package in an implicit dependency imported from the main module,
+# we should get an error because the dependency should have an explicit
+# requirement.
+go list -m indirect-with-pkg
+stdout '^indirect-with-pkg v1.0.0 => ./indirect-with-pkg$'
+! go list ./use-indirect
+stderr '^package m/use-indirect imports indirect-with-pkg from implicitly required module; to add missing requirements, run:\n\tgo get indirect-with-pkg@v1.0.0$'
+
+# We can promote the implicit requirement by getting the importing package.
+# NOTE: the hint recommends getting the imported package (tested below) since
+# it's more obvious and doesn't require -d. However, that adds an '// indirect'
+# comment on the requirement.
+go get m/use-indirect
+cmp go.mod go.mod.use
+cp go.mod.orig go.mod
+
+# We can also promote implicit requirements using 'go get' on them, or their
+# packages. This gives us "// indirect" requirements, since 'go get' doesn't
+# know they're needed by the main module. See #43131 for the rationale.
+# The hint above recommends this because it's more obvious usage and doesn't
+# require the -d flag.
+go get indirect-with-pkg indirect-without-pkg
+cmp go.mod go.mod.indirect
+
+-- go.mod.orig --
+module m
+
+go 1.16
+
+require direct v1.0.0
+
+replace (
+	direct v1.0.0 => ./direct
+	indirect-with-pkg v1.0.0 => ./indirect-with-pkg
+	indirect-without-pkg v1.0.0 => ./indirect-without-pkg
+)
+-- go.mod.use --
+module m
+
+go 1.16
+
+require (
+	direct v1.0.0
+	indirect-with-pkg v1.0.0
+)
+
+replace (
+	direct v1.0.0 => ./direct
+	indirect-with-pkg v1.0.0 => ./indirect-with-pkg
+	indirect-without-pkg v1.0.0 => ./indirect-without-pkg
+)
+-- go.mod.indirect --
+module m
+
+go 1.16
+
+require (
+	direct v1.0.0
+	indirect-with-pkg v1.0.0 // indirect
+	indirect-without-pkg v1.0.0 // indirect
+)
+
+replace (
+	direct v1.0.0 => ./direct
+	indirect-with-pkg v1.0.0 => ./indirect-with-pkg
+	indirect-without-pkg v1.0.0 => ./indirect-without-pkg
+)
+-- use-indirect/use-indirect.go --
+package use
+
+import _ "indirect-with-pkg"
+-- direct/go.mod --
+module direct
+
+go 1.16
+
+require (
+	indirect-with-pkg v1.0.0
+	indirect-without-pkg v1.0.0
+)
+-- indirect-with-pkg/go.mod --
+module indirect-with-pkg
+
+go 1.16
+-- indirect-with-pkg/p.go --
+package p
+-- indirect-without-pkg/go.mod --
+module indirect-without-pkg
+
+go 1.16
diff --git a/src/cmd/go/testdata/script/mod_get_pseudo.txt b/src/cmd/go/testdata/script/mod_get_pseudo.txt
new file mode 100644
index 0000000..47ad54e
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_pseudo.txt
@@ -0,0 +1,82 @@
+env GO111MODULE=on
+
+# Testing git->module converter's generation of +incompatible tags; turn off proxy.
+[!net:github.com] skip
+[!git] skip
+env GOPROXY=direct
+env GOSUMDB=off
+
+# We can resolve the @master branch without unshallowing the local repository
+# (even with older gits), so try that before we do anything else.
+# (This replicates https://golang.org/issue/26713 with git 2.7.4.)
+go get github.com/rsc/legacytest@master
+go list -m all
+stdout '^github.com/rsc/legacytest v2\.0\.1-0\.\d{14}-7303f7796364\+incompatible$'
+
+# get should include incompatible tags in "latest" calculation.
+go mod edit -droprequire github.com/rsc/legacytest
+go get github.com/rsc/legacytest@latest
+go list
+go list -m all
+stdout '^github.com/rsc/legacytest v2\.0\.0\+incompatible$'
+
+# v2.0.1-0.pseudo+incompatible
+go get ...test@7303f77
+go list -m all
+stdout '^github.com/rsc/legacytest v2\.0\.1-0\.\d{14}-7303f7796364\+incompatible$'
+
+# v2.0.0+incompatible by tag+incompatible
+go get ...test@v2.0.0+incompatible
+go list -m all
+stdout '^github.com/rsc/legacytest v2\.0\.0\+incompatible$'
+
+# v2.0.0+incompatible by tag
+go get ...test@v2.0.0
+go list -m all
+stdout '^github.com/rsc/legacytest v2\.0\.0\+incompatible$'
+
+# v2.0.0+incompatible by hash (back on master)
+go get ...test@d7ae1e4
+go list -m all
+stdout '^github.com/rsc/legacytest v2\.0\.0\+incompatible$'
+
+# v1.2.1-0.pseudo
+go get ...test@d2d4c3e
+go list -m all
+stdout '^github.com/rsc/legacytest v1\.2\.1-0\.\d{14}-d2d4c3ea6623$'
+
+# v1.2.0
+go get ...test@9f6f860
+go list -m all
+stdout '^github.com/rsc/legacytest v1\.2\.0$'
+
+# v1.1.0-pre.0.pseudo
+go get ...test@fb3c628
+go list -m all
+stdout '^github.com/rsc/legacytest v1\.1\.0-pre\.0\.\d{14}-fb3c628075e3$'
+
+# v1.1.0-pre (no longer on master)
+go get ...test@731e3b1
+go list -m all
+stdout '^github.com/rsc/legacytest v1\.1\.0-pre$'
+
+# v1.0.1-0.pseudo
+go get ...test@fa4f5d6
+go list -m all
+stdout '^github.com/rsc/legacytest v1\.0\.1-0\.\d{14}-fa4f5d6a71c6$'
+
+# v1.0.0
+go get ...test@7fff7f3
+go list -m all
+stdout '^github.com/rsc/legacytest v1\.0\.0$'
+
+# v0.0.0-pseudo
+go get ...test@52853eb
+go list -m all
+stdout '^github.com/rsc/legacytest v0\.0\.0-\d{14}-52853eb7b552$'
+
+-- go.mod --
+module x
+-- x.go --
+package x
+import "github.com/rsc/legacytest"
diff --git a/src/cmd/go/testdata/script/mod_get_pseudo_other_branch.txt b/src/cmd/go/testdata/script/mod_get_pseudo_other_branch.txt
new file mode 100644
index 0000000..6019b45
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_pseudo_other_branch.txt
@@ -0,0 +1,67 @@
+env GO111MODULE=on
+env GOPROXY=direct
+env GOSUMDB=off
+
+# Testing that a pseudo-version is based on the semantically-latest
+# tag that appears in any commit that is a (transitive) parent of the commit
+# supplied to 'go get', regardless of branches
+
+[short] skip
+[!git] skip
+
+# For this test repository:
+#  tag v0.2.1 is most recent tag on master itself
+#  tag v0.2.2 is on branch2, which was then merged to master
+#  master is a merge commit with both tags as parents
+#
+# The pseudo-version hence sorts immediately after v0.2.2 rather
+# than v0.2.1, even though the v0.2.2 tag is not on master.
+
+go get vcs-test.golang.org/git/tagtests.git@master
+go list -m all
+stdout '^vcs-test.golang.org/git/tagtests.git v0.2.3-0\.'
+
+-- go.mod --
+module x
+
+go 1.12
+-- x.go --
+package x
+
+import _ "vcs-test.golang.org/git/tagtests.git"
+-- gen_testtags.sh --
+#!/bin/bash
+
+# This is not part of the test.
+# Run this to generate and update the repository on vcs-test.golang.org.
+
+set -euo pipefail
+cd "$(dirname "$0")"
+rm -rf tagtests
+mkdir tagtests
+cd tagtests
+
+git init
+echo module vcs-test.golang.org/git/tagtests.git >go.mod
+echo package tagtests >tagtests.go
+git add go.mod tagtests.go
+git commit -m 'create module tagtests'
+
+git branch b
+
+echo v0.2.1 >v0.2.1
+git add v0.2.1
+git commit -m v0.2.1
+git tag v0.2.1
+
+git checkout b
+echo v0.2.2 >v0.2.2
+git add v0.2.2
+git commit -m v0.2.2
+git tag v0.2.2
+
+git checkout master
+git merge b -m merge
+
+zip -r ../tagtests.zip .
+gsutil cp ../tagtests.zip gs://vcs-test/git/tagtests.zip
diff --git a/src/cmd/go/testdata/script/mod_get_pseudo_prefix.txt b/src/cmd/go/testdata/script/mod_get_pseudo_prefix.txt
new file mode 100644
index 0000000..ac3233e
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_pseudo_prefix.txt
@@ -0,0 +1,64 @@
+env GO111MODULE=on
+env GOPROXY=direct
+env GOSUMDB=off
+
+# Testing that a pseudo-version is based on the semantically-latest
+# prefixed tag in any commit that is a parent of the commit supplied
+# to 'go get', when using a repo with go.mod in a sub directory.
+
+[short] skip
+[!git] skip
+
+# For this test repository go.mod resides in sub/ (only):
+#  master is not tagged
+#  tag v0.2.0 is most recent tag before master
+#  tag sub/v0.0.10 is most recent tag before v0.2.0
+#
+# The pseudo-version is based on sub/v0.0.10, since v0.2.0 doesn't
+# contain the prefix.
+go get vcs-test.golang.org/git/prefixtagtests.git/sub
+go list -m all
+stdout '^vcs-test.golang.org/git/prefixtagtests.git/sub v0.0.10$'
+
+go get -u vcs-test.golang.org/git/prefixtagtests.git/sub@master
+go list -m all
+stdout '^vcs-test.golang.org/git/prefixtagtests.git/sub v0.0.11-0\.'
+
+-- go.mod --
+module x
+
+go 1.12
+-- x.go --
+package x
+
+import _ "vcs-test.golang.org/prefixtagtests.git/sub"
+-- gen_prefixtagtests.sh --
+#!/bin/bash
+
+# This is not part of the test.
+# Run this to generate and update the repository on vcs-test.golang.org.
+
+set -euo pipefail
+cd "$(dirname "$0")"
+rm -rf prefixtagtests
+mkdir prefixtagtests
+cd prefixtagtests
+
+git init
+mkdir sub
+echo module vcs-test.golang.org/git/prefixtagtests.git/sub >sub/go.mod
+echo package sub >sub/sub.go
+git add sub
+git commit -m 'create module sub'
+for i in v0.1.0 sub/v0.0.9 sub/v0.0.10 v0.2.0; do
+  echo $i >status
+  git add status
+  git commit -m $i
+  git tag $i
+done
+echo 'after last tag' >status
+git add status
+git commit -m 'after last tag'
+
+zip -r ../prefixtagtests.zip .
+gsutil cp ../prefixtagtests.zip gs://vcs-test/git/prefixtagtests.zip
diff --git a/src/cmd/go/testdata/script/mod_get_replaced.txt b/src/cmd/go/testdata/script/mod_get_replaced.txt
new file mode 100644
index 0000000..c31d5be
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_replaced.txt
@@ -0,0 +1,111 @@
+cp go.mod go.mod.orig
+
+env oldGOPROXY=$GOPROXY
+
+# If a wildcard replacement exists for an otherwise-nonexistent module,
+# 'go get' should resolve it to the minimum valid pseudo-version.
+
+go mod edit -replace=example.com/x=./x
+go get example.com/x
+
+go list -m example.com/x
+stdout '^example.com/x v0.0.0-00010101000000-000000000000 '
+
+# If specific-version replacements exist, the highest matching version should be used.
+go mod edit -replace=example.com/x@v0.1.0=./x
+go mod edit -replace=example.com/x@v0.2.0=./x
+
+go get example.com/x
+go list -m example.com/x
+stdout '^example.com/x v0.2.0 '
+
+go get example.com/x@<v0.2.0
+go list -m example.com/x
+stdout '^example.com/x v0.1.0 '
+
+
+# The same should work with GOPROXY=off.
+
+env GOPROXY=off
+cp go.mod.orig go.mod
+
+go mod edit -replace=example.com/x=./x
+go get example.com/x
+
+go list -m example.com/x
+stdout '^example.com/x v0.0.0-00010101000000-000000000000 '
+
+# If specific-version replacements exist, the highest matching version should be used.
+go mod edit -replace=example.com/x@v0.1.0=./x
+go mod edit -replace=example.com/x@v0.2.0=./x
+
+go get example.com/x
+go list -m example.com/x
+stdout '^example.com/x v0.2.0 '
+
+go get example.com/x@<v0.2.0
+go list -m example.com/x
+stdout '^example.com/x v0.1.0 '
+
+
+# Replacements should also be listed as known versions, and 'go get' should sort
+# them in with ordinary versions.
+
+env GOPROXY=$oldGOPROXY
+
+cp go.mod.orig go.mod
+go list -versions -m rsc.io/quote
+stdout 'v1.3.0 v1.4.0'
+
+go get rsc.io/quote@v1.3
+go list -m rsc.io/quote
+stdout '^rsc.io/quote v1.3.0'
+
+go mod edit -replace rsc.io/quote@v1.3.1=rsc.io/quote@v1.4.0
+
+go list -versions -m rsc.io/quote
+stdout 'v1.3.0 v1.3.1 v1.4.0'
+
+go get rsc.io/quote@v1.3
+go list -m rsc.io/quote
+stdout '^rsc.io/quote v1.3.1 '
+
+go get rsc.io/quote@>v1.3.1
+go list -m rsc.io/quote
+stdout '^rsc.io/quote v1.4.0'
+
+
+# Replacements should allow 'go get' to work even with dotless module paths.
+
+cp go.mod.orig go.mod
+
+! go list example
+stderr '^package example is not in std \(.*\)$'
+! go get example
+stderr '^go: malformed module path "example": missing dot in first path element$'
+
+go mod edit -replace example@v0.1.0=./example
+
+! go list example
+stderr '^module example provides package example and is replaced but not required; to add it:\n\tgo get example@v0.1.0$'
+
+go get example
+go list -m example
+stdout '^example v0.1.0 '
+
+
+-- go.mod --
+module example.com
+
+go 1.16
+-- x/go.mod --
+module example.com/x
+
+go 1.16
+-- x/x.go --
+package x
+-- example/go.mod --
+module example
+go 1.16
+-- example/example.go --
+package example
diff --git a/src/cmd/go/testdata/script/mod_get_retract.txt b/src/cmd/go/testdata/script/mod_get_retract.txt
new file mode 100644
index 0000000..9757989
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_retract.txt
@@ -0,0 +1,59 @@
+# 'go get pkg' should not upgrade to a retracted version.
+cp go.mod.orig go.mod
+go mod edit -require example.com/retract/self/prev@v1.1.0
+go get example.com/retract/self/prev
+go list -m example.com/retract/self/prev
+stdout '^example.com/retract/self/prev v1.1.0$'
+
+# 'go get pkg' should not downgrade from a retracted version when no higher
+# version is available.
+cp go.mod.orig go.mod
+go mod edit -require example.com/retract/self/prev@v1.9.0
+go get example.com/retract/self/prev
+stderr '^go: warning: example.com/retract/self/prev@v1.9.0: retracted by module author: self$'
+stderr '^go: to switch to the latest unretracted version, run:\n\tgo get example.com/retract/self/prev@latest\n$'
+go list -m example.com/retract/self/prev
+stdout '^example.com/retract/self/prev v1.9.0$'
+
+# 'go get pkg@latest' should downgrade from a retracted version.
+cp go.mod.orig go.mod
+go mod edit -require example.com/retract/self/prev@v1.9.0
+go get example.com/retract/self/prev@latest
+go list -m example.com/retract/self/prev
+stdout '^example.com/retract/self/prev v1.1.0$'
+
+# 'go get pkg@version' should update to a specific version, even if that
+# version is retracted.
+cp go.mod.orig go.mod
+go get example.com/retract@v1.0.0-bad
+stderr '^go: warning: example.com/retract@v1.0.0-bad: retracted by module author: bad$'
+go list -m example.com/retract
+stdout '^example.com/retract v1.0.0-bad$'
+
+# 'go get -u' should not downgrade from a retracted version when no higher
+# version is available.
+cp go.mod.orig go.mod
+go mod edit -require example.com/retract/self/prev@v1.9.0
+go get -u ./use
+stderr '^go: warning: example.com/retract/self/prev@v1.9.0: retracted by module author: self$'
+go list -m example.com/retract/self/prev
+stdout '^example.com/retract/self/prev v1.9.0$'
+
+# 'go get' should warn if a module needed to build named packages is retracted.
+# 'go get' should not warn about unrelated modules.
+go get ./empty
+! stderr retracted
+go get ./use
+stderr '^go: warning: example.com/retract/self/prev@v1.9.0: retracted by module author: self$'
+
+-- go.mod.orig --
+module example.com/use
+
+go 1.15
+
+-- use/use.go --
+package use
+
+import _ "example.com/retract/self/prev"
+-- empty/empty.go --
+package empty
diff --git a/src/cmd/go/testdata/script/mod_get_retract_ambiguous.txt b/src/cmd/go/testdata/script/mod_get_retract_ambiguous.txt
new file mode 100644
index 0000000..4b4f5da
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_retract_ambiguous.txt
@@ -0,0 +1,10 @@
+! go get example.com/retract/ambiguous/other
+stderr 'ambiguous import: found package example.com/retract/ambiguous/nested in multiple modules:'
+stderr '^go: warning: example.com/retract/ambiguous/nested@v1.9.0-bad: retracted by module author: nested modules are bad$'
+
+-- go.mod --
+module example.com/use
+
+go 1.16
+
+require example.com/retract/ambiguous/nested v1.9.0-bad
diff --git a/src/cmd/go/testdata/script/mod_get_split.txt b/src/cmd/go/testdata/script/mod_get_split.txt
new file mode 100644
index 0000000..6c2f00d
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_split.txt
@@ -0,0 +1,157 @@
+cp go.mod go.mod.orig
+
+
+# 'go get' on a package already provided by the build list should update
+# the module already in the build list, not fail with an ambiguous import error.
+
+go get example.net/split/nested@patch
+go list -m all
+stdout '^example.net/split v0.2.1 '
+! stdout '^example.net/split/nested'
+
+# We should get the same behavior if we use a pattern that matches only that package.
+
+cp go.mod.orig go.mod
+
+go get example.net/split/nested/...@patch
+go list -m all
+stdout '^example.net/split v0.2.1 '
+! stdout '^example.net/split/nested'
+
+
+# If we request a version for which the package only exists in one particular module,
+# we should add that one particular module but not resolve import ambiguities.
+#
+# In particular, if the module that previously provided the package has a
+# matching version, but does not itself match the pattern and contains no
+# matching packages, we should not change its version. (We should *not* downgrade
+# module example.net/split to v0.1.0, despite the fact that
+# example.net/split v0.2.0 currently provides the package with the requested path.)
+#
+# TODO(#27899): Maybe we should resolve the ambiguities by upgrading.
+
+cp go.mod.orig go.mod
+
+! go get example.net/split/nested@v0.1.0
+stderr '^go: example.net/split/nested: ambiguous import: found package example.net/split/nested in multiple modules:\n\texample.net/split v0.2.0 \(.*split.2[/\\]nested\)\n\texample.net/split/nested v0.1.0 \(.*nested.1\)$'
+
+# A wildcard that matches packages in some module at its selected version
+# but not at the requested version should fail.
+#
+# We can't set the module to the selected version, because that version doesn't
+# even match the query: if we ran the same query twice, we wouldn't consider the
+# module to match the wildcard during the second call, so why should we consider
+# it to match during the first one?  ('go get' should be idempotent, and if we
+# did that then it would not be.)
+#
+# But we also can't leave it where it is: the user requested that we set everything
+# matching the pattern to the given version, and right now we have packages
+# that match the pattern but *not* the version.
+#
+# That only leaves two options: we can set the module to an arbitrary version
+# (perhaps 'latest' or 'none'), or we can report an error and the let the user
+# disambiguate. We would rather not choose arbitrarily, so we do the latter.
+#
+# TODO(#27899): Should we instead upgrade or downgrade to an arbitrary version?
+
+! go get example.net/split/nested/...@v0.1.0
+stderr '^go: example.net/split/nested/\.\.\.@v0.1.0 matches packages in example.net/split@v0.2.0 but not example.net/split@v0.1.0: specify a different version for module example.net/split$'
+
+cmp go.mod go.mod.orig
+
+
+# If another argument resolves the ambiguity, we should be ok again.
+
+go get example.net/split@none example.net/split/nested@v0.1.0
+go list -m all
+! stdout '^example.net/split '
+stdout '^example.net/split/nested v0.1.0 '
+
+cp go.mod.orig go.mod
+
+go get example.net/split@v0.3.0 example.net/split/nested@v0.1.0
+go list -m all
+stdout '^example.net/split v0.3.0 '
+stdout '^example.net/split/nested v0.1.0 '
+
+
+# If a pattern applies to modules and to packages, we should set all matching
+# modules to the version indicated by the pattern, and also resolve packages
+# to match the pattern if possible.
+
+cp go.mod.orig go.mod
+go get example.net/split/nested@v0.0.0
+
+go get example.net/...@v0.1.0
+go list -m all
+stdout '^example.net/split v0.1.0 '
+stdout '^example.net/split/nested v0.1.0 '
+
+go get example.net/...
+go list -m all
+stdout '^example.net/split v0.3.0 '
+stdout '^example.net/split/nested v0.2.0 '
+
+
+# @none applies to all matching module paths,
+# regardless of whether they contain any packages.
+
+go get example.net/...@none
+go list -m all
+! stdout '^example.net'
+
+# Starting from no dependencies, a wildcard can resolve to an empty module with
+# the same prefix even if it contains no packages.
+
+go get example.net/...@none
+go get example.net/split/...@v0.1.0
+go list -m all
+stdout '^example.net/split v0.1.0 '
+
+
+-- go.mod --
+module m
+
+go 1.16
+
+require example.net/split v0.2.0
+
+replace (
+	example.net/split v0.1.0 => ./split.1
+	example.net/split v0.2.0 => ./split.2
+	example.net/split v0.2.1 => ./split.2
+	example.net/split v0.3.0 => ./split.3
+	example.net/split/nested v0.0.0 => ./nested.0
+	example.net/split/nested v0.1.0 => ./nested.1
+	example.net/split/nested v0.2.0 => ./nested.2
+)
+-- split.1/go.mod --
+module example.net/split
+
+go 1.16
+-- split.2/go.mod --
+module example.net/split
+
+go 1.16
+-- split.2/nested/nested.go --
+package nested
+-- split.3/go.mod --
+module example.net/split
+
+go 1.16
+-- nested.0/go.mod --
+module example.net/split/nested
+
+go 1.16
+-- nested.1/go.mod --
+module example.net/split/nested
+
+go 1.16
+-- nested.1/nested.go --
+package nested
+-- nested.2/go.mod --
+module example.net/split/nested
+
+go 1.16
+-- nested.2/nested.go --
+package nested
diff --git a/src/cmd/go/testdata/script/mod_get_sum_noroot.txt b/src/cmd/go/testdata/script/mod_get_sum_noroot.txt
new file mode 100644
index 0000000..0d9a840
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_sum_noroot.txt
@@ -0,0 +1,11 @@
+# When 'go get' is invoked on a module without a package in the root directory,
+# it should add sums for the module's go.mod file and its content to go.sum.
+# Verifies golang.org/issue/41103.
+go mod init m
+go get rsc.io/QUOTE
+grep '^rsc.io/QUOTE v1.5.2/go.mod ' go.sum
+grep '^rsc.io/QUOTE v1.5.2 ' go.sum
+
+# Double-check rsc.io/QUOTE does not have a root package.
+! go list -mod=readonly rsc.io/QUOTE
+stderr '^cannot find module providing package rsc.io/QUOTE: import lookup disabled by -mod=readonly$'
diff --git a/src/cmd/go/testdata/script/mod_get_tags.txt b/src/cmd/go/testdata/script/mod_get_tags.txt
new file mode 100644
index 0000000..e4fb6c4
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_tags.txt
@@ -0,0 +1,34 @@
+env GO111MODULE=on
+
+# get should add modules needed to build packages, even if those
+# dependencies are in sources excluded by build tags.
+# All build tags are considered true except "ignore".
+go mod init m
+go get .
+go list -m all
+stdout 'example.com/version v1.1.0'
+stdout 'rsc.io/quote v1.5.2'
+
+-- empty.go --
+package m
+
+-- excluded.go --
+// +build windows,mips
+
+package m
+
+import _ "example.com/version"
+
+-- tools.go --
+// +build tools
+
+package tools
+
+import _ "rsc.io/quote"
+
+-- ignore.go --
+// +build ignore
+
+package ignore
+
+import _ "example.com/doesnotexist"
diff --git a/src/cmd/go/testdata/script/mod_get_test.txt b/src/cmd/go/testdata/script/mod_get_test.txt
new file mode 100644
index 0000000..0fa7cd9
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_test.txt
@@ -0,0 +1,58 @@
+env GO111MODULE=on
+
+# By default, 'go get' should ignore tests
+cp go.mod.empty go.mod
+go get m/a
+! grep rsc.io/quote go.mod
+
+# 'go get -t' should consider test dependencies of the named package.
+cp go.mod.empty go.mod
+go get -t m/a
+grep 'rsc.io/quote v1.5.2$' go.mod
+
+# 'go get -t' should not consider test dependencies of imported packages,
+# including packages imported from tests.
+cp go.mod.empty go.mod
+go get -t m/b
+! grep rsc.io/quote go.mod
+
+# 'go get -t -u' should update test dependencies of the named package.
+cp go.mod.empty go.mod
+go mod edit -require=rsc.io/quote@v1.5.1
+go get -t -u m/a
+grep 'rsc.io/quote v1.5.2$' go.mod
+
+# 'go get -t -u' should not add or update test dependencies
+# of imported packages, including packages imported from tests.
+cp go.mod.empty go.mod
+go get -t -u m/b
+! grep rsc.io/quote go.mod
+go mod edit -require=rsc.io/quote@v1.5.1
+go get -t -u m/b
+grep 'rsc.io/quote v1.5.1$' go.mod
+
+# 'go get all' should consider test dependencies with or without -t.
+cp go.mod.empty go.mod
+go get all
+grep 'rsc.io/quote v1.5.2$' go.mod
+
+-- go.mod.empty --
+module m
+
+-- a/a.go --
+package a
+
+-- a/a_test.go --
+package a_test
+
+import _ "rsc.io/quote"
+
+-- b/b.go --
+package b
+
+import _ "m/a"
+
+-- b/b_test.go --
+package b_test
+
+import _ "m/a"
diff --git a/src/cmd/go/testdata/script/mod_get_toolchain.txt b/src/cmd/go/testdata/script/mod_get_toolchain.txt
new file mode 100644
index 0000000..758142d
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_toolchain.txt
@@ -0,0 +1,129 @@
+# setup
+env TESTGO_VERSION=go1.99rc1
+env TESTGO_VERSION_SWITCH=switch
+
+# go get go should use the latest Go 1.23
+cp go.mod.orig go.mod
+go get go
+stderr '^go: upgraded go 1.21 => 1.23.9$'
+grep 'go 1.23.9' go.mod
+grep 'toolchain go1.99rc1' go.mod
+
+# go get go@1.23 should use the latest Go 1.23
+cp go.mod.orig go.mod
+go get go@1.23
+stderr '^go: upgraded go 1.21 => 1.23.9$'
+grep 'go 1.23.9' go.mod
+grep 'toolchain go1.99rc1' go.mod
+
+# go get go@1.22 should use the latest Go 1.22
+cp go.mod.orig go.mod
+go get go@1.22
+stderr '^go: upgraded go 1.21 => 1.22.9$'
+grep 'go 1.22.9' go.mod
+grep 'toolchain go1.99rc1' go.mod
+
+# go get go@patch should use the latest patch release
+go get go@1.22.1
+go get go@patch
+stderr '^go: upgraded go 1.22.1 => 1.22.9$'
+grep 'go 1.22.9' go.mod
+grep 'toolchain go1.99rc1' go.mod
+
+# go get go@1.24 does NOT find the release candidate
+cp go.mod.orig go.mod
+! go get go@1.24
+stderr '^go: go@1.24: no matching versions for query "1.24"$'
+
+# go get go@1.24rc1 works
+cp go.mod.orig go.mod
+go get go@1.24rc1
+stderr '^go: upgraded go 1.21 => 1.24rc1$'
+grep 'go 1.24rc1' go.mod
+grep 'toolchain go1.99rc1' go.mod
+
+# go get go@latest finds the latest Go 1.23
+cp go.mod.orig go.mod
+go get go@latest
+stderr '^go: upgraded go 1.21 => 1.23.9$'
+grep 'go 1.23.9' go.mod
+grep 'toolchain go1.99rc1' go.mod
+
+# Again, with toolchains.
+
+# go get toolchain should find go1.999testmod.
+go get toolchain
+stderr '^go: upgraded toolchain go1.99rc1 => go1.999testmod$'
+grep 'go 1.23.9' go.mod
+grep 'toolchain go1.999testmod' go.mod
+
+# go get toolchain@go1.23 should use the latest Go 1.23
+go get toolchain@go1.23
+stderr '^go: removed toolchain go1.999testmod$'
+grep 'go 1.23.9' go.mod
+! grep 'toolchain go1.23.9' go.mod  # implied
+
+# go get toolchain@go1.22 should use the latest Go 1.22 and downgrade go.
+go get toolchain@go1.22
+stderr '^go: downgraded go 1.23.9 => 1.22.9$'
+grep 'go 1.22.9' go.mod
+! grep 'toolchain go1.22.9' go.mod # implied
+
+# go get toolchain@patch should use the latest patch release
+go get toolchain@go1.22.1
+go get toolchain@patch
+stderr '^go: added toolchain go1.22.9$'
+grep 'go 1.22.1' go.mod
+grep 'toolchain go1.22.9' go.mod
+go get go@1.22.9 toolchain@none
+grep 'go 1.22.9' go.mod
+! grep 'toolchain go1.22.9' go.mod
+
+# go get toolchain@go1.24 does NOT find the release candidate
+! go get toolchain@go1.24
+stderr '^go: toolchain@go1.24: no matching versions for query "go1.24"$'
+
+# go get toolchain@go1.24rc1 works
+go get toolchain@go1.24rc1
+stderr '^go: added toolchain go1.24rc1$'
+grep 'go 1.22.9' go.mod  # no longer implied
+grep 'toolchain go1.24rc1' go.mod
+
+# go get toolchain@latest finds go1.999testmod.
+cp go.mod.orig go.mod
+go get toolchain@latest
+stderr '^go: added toolchain go1.999testmod$'
+grep 'go 1.21' go.mod
+grep 'toolchain go1.999testmod' go.mod
+
+# Bug fixes.
+
+# go get go@garbage should fail but not crash
+! go get go@garbage
+! stderr panic
+stderr '^go: invalid go version garbage$'
+
+# go get go@go1.21.0 is OK - we silently correct to 1.21.0
+go get go@1.19
+go get go@go1.21.0
+stderr '^go: upgraded go 1.19 => 1.21.0'
+
+# go get toolchain@1.24rc1 is OK too.
+go get toolchain@1.24rc1
+stderr '^go: downgraded toolchain go1.999testmod => go1.24rc1$'
+
+# go get go@1.21 should work if we are the Go 1.21 language version,
+# even though there's no toolchain for it.
+# (Older versions resolve to the latest release in that version, so for example
+# go get go@1.20 might resolve to 1.20.9, but if we're the devel copy of
+# Go 1.21, there's no release yet to resolve to, so we resolve to ourselves.)
+env TESTGO_VERSION=go1.21
+go get go@1.19 toolchain@none
+go get go@1.21
+grep 'go 1.21$' go.mod
+! grep toolchain go.mod
+
+-- go.mod.orig --
+module m
+
+go 1.21
diff --git a/src/cmd/go/testdata/script/mod_get_trailing_slash.txt b/src/cmd/go/testdata/script/mod_get_trailing_slash.txt
new file mode 100644
index 0000000..7b46900
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_trailing_slash.txt
@@ -0,0 +1,30 @@
+# go list should succeed to load a package ending with ".go" if the path does
+# not correspond to an existing local file. Listing a pattern ending with
+# ".go/" should try to list a package regardless of whether a file exists at the
+# path without the suffixed "/" or not.
+go list example.com/dotgo.go
+stdout ^example.com/dotgo.go$
+go list example.com/dotgo.go/
+stdout ^example.com/dotgo.go$
+
+# go get should succeed in either case, with or without a version.
+# Arguments are interpreted as packages or package patterns with versions,
+# not source files.
+go get example.com/dotgo.go
+go get example.com/dotgo.go/
+go get example.com/dotgo.go@v1.0.0
+go get example.com/dotgo.go/@v1.0.0
+
+-- go.mod --
+module m
+
+go 1.13
+
+require example.com/dotgo.go v1.0.0
+-- go.sum --
+example.com/dotgo.go v1.0.0 h1:XKJfs0V8x2PvY2tX8bJBCEbCDLnt15ma2onwhVpew/I=
+example.com/dotgo.go v1.0.0/go.mod h1:Qi6z/X3AC5vHiuMt6HF2ICx3KhIBGrMdrA7YoPDKqR0=
+-- use.go --
+package use
+
+import _ "example.com/dotgo.go"
diff --git a/src/cmd/go/testdata/script/mod_get_update_unrelated_sum.txt b/src/cmd/go/testdata/script/mod_get_update_unrelated_sum.txt
new file mode 100644
index 0000000..a5651e9
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_update_unrelated_sum.txt
@@ -0,0 +1,120 @@
+# Check that 'go get' adds sums for updated modules if we had sums before,
+# even if we didn't load packages from them.
+# Verifies #44129.
+
+env fmt='{{.ImportPath}}: {{if .Error}}{{.Error.Err}}{{else}}ok{{end}}'
+
+# Control case: before upgrading, we have the sums we need.
+# go list -deps -e -f $fmt .
+# stdout '^rsc.io/quote: ok$'
+# ! stdout rsc.io/sampler  # not imported by quote in this version
+cp go.mod.orig go.mod
+cp go.sum.orig go.sum
+go mod tidy
+cmp go.mod.orig go.mod
+cmp go.sum.orig go.sum
+
+
+# Upgrade a module. This also upgrades rsc.io/quote, and though we didn't load
+# a package from it, we had the sum for its old version, so we need the
+# sum for the new version, too.
+go get example.com/upgrade@v0.0.2
+grep '^rsc.io/quote v1.5.2 ' go.sum
+
+# The upgrade still breaks the build because the new version of quote imports
+# rsc.io/sampler, and we don't have its zip sum.
+go list -deps -e -f $fmt
+stdout 'rsc.io/quote: ok'
+stdout 'rsc.io/sampler: missing go.sum entry for module providing package rsc.io/sampler'
+cp go.mod.orig go.mod
+cp go.sum.orig go.sum
+
+
+# Replace the old version with a directory before upgrading.
+# We didn't need a sum for it before (even though we had one), so we won't
+# fetch a new sum.
+go mod edit -replace rsc.io/quote@v1.0.0=./dummy
+go get example.com/upgrade@v0.0.2
+! grep '^rsc.io/quote v1.5.2 ' go.sum
+cp go.mod.orig go.mod
+cp go.sum.orig go.sum
+
+
+# Replace the new version with a directory before upgrading.
+# We can't get a sum for a directory.
+go mod edit -replace rsc.io/quote@v1.5.2=./dummy
+go get example.com/upgrade@v0.0.2
+! grep '^rsc.io/quote v1.5.2 ' go.sum
+cp go.mod.orig go.mod
+cp go.sum.orig go.sum
+
+
+# Replace the new version with a different version.
+# We should get a sum for that version.
+go mod edit -replace rsc.io/quote@v1.5.2=rsc.io/quote@v1.5.1
+go get example.com/upgrade@v0.0.2
+! grep '^rsc.io/quote v1.5.2 ' go.sum
+grep '^rsc.io/quote v1.5.1 ' go.sum
+cp go.mod.orig go.mod
+cp go.sum.orig go.sum
+
+
+# Delete the new version's zip (but not mod) from the cache and go offline.
+# 'go get' should fail when fetching the zip.
+rm $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.zip
+env GOPROXY=off
+! go get example.com/upgrade@v0.0.2
+stderr '^go: upgraded rsc.io/quote v1.0.0 => v1.5.2: error finding sum for rsc.io/quote@v1.5.2: module lookup disabled by GOPROXY=off$'
+
+-- go.mod.orig --
+module m
+
+go 1.16
+
+require (
+	example.com/upgrade v0.0.1
+	rsc.io/quote v1.0.0
+)
+
+replace (
+	example.com/upgrade v0.0.1 => ./upgrade1
+	example.com/upgrade v0.0.2 => ./upgrade2
+)
+-- go.sum.orig --
+rsc.io/quote v1.0.0 h1:kQ3IZQzPTiDJxSZI98YaWgxFEhlNdYASHvh+MplbViw=
+rsc.io/quote v1.0.0/go.mod h1:v83Ri/njykPcgJltBc/gEkJTmjTsNgtO1Y7vyIK1CQA=
+-- go.sum.want --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.0.0 h1:kQ3IZQzPTiDJxSZI98YaWgxFEhlNdYASHvh+MplbViw=
+rsc.io/quote v1.0.0/go.mod h1:v83Ri/njykPcgJltBc/gEkJTmjTsNgtO1Y7vyIK1CQA=
+rsc.io/quote v1.5.2 h1:3fEykkD9k7lYzXqCYrwGAf7iNhbk4yCjHmKBN9td4L0=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+-- use.go --
+package use
+
+import (
+	_ "example.com/upgrade"
+	_ "rsc.io/quote"
+)
+-- upgrade1/go.mod --
+module example.com/upgrade
+
+go 1.16
+-- upgrade1/upgrade.go --
+package upgrade
+-- upgrade2/go.mod --
+module example.com/upgrade
+
+go 1.16
+
+require rsc.io/quote v1.5.2 // indirect
+-- upgrade2/upgrade.go --
+package upgrade
+-- dummy/go.mod --
+module rsc.io/quote
+
+go 1.16
+-- dummy/quote.go --
+package quote
+
diff --git a/src/cmd/go/testdata/script/mod_get_upgrade.txt b/src/cmd/go/testdata/script/mod_get_upgrade.txt
new file mode 100644
index 0000000..51d5990
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_upgrade.txt
@@ -0,0 +1,46 @@
+env GO111MODULE=on
+
+go get rsc.io/quote@v1.5.1
+go list -m all
+stdout 'rsc.io/quote v1.5.1'
+grep 'rsc.io/quote v1.5.1$' go.mod
+
+# get -u should update dependencies of the package in the current directory
+go get -u
+grep 'rsc.io/quote v1.5.2$' go.mod
+grep 'golang.org/x/text [v0-9a-f\.-]+ // indirect' go.mod
+
+# get -u rsc.io/sampler should update only sampler's dependencies
+cp go.mod-v1.5.1 go.mod
+go get -u rsc.io/sampler
+grep 'rsc.io/quote v1.5.1$' go.mod
+grep 'golang.org/x/text [v0-9a-f\.-]+ // indirect' go.mod
+
+# move to a pseudo-version after any tags
+go get rsc.io/quote@dd9747d
+grep 'rsc.io/quote v0.0.0-20180628003336-dd9747d19b04' go.mod
+
+# get -u should not jump off newer pseudo-version to earlier tag
+go get -u
+grep 'rsc.io/quote v0.0.0-20180628003336-dd9747d19b04' go.mod
+
+# move to earlier pseudo-version
+go get rsc.io/quote@e7a685a342
+grep 'rsc.io/quote v0.0.0-20180214005133-e7a685a342c0' go.mod
+
+# get -u should jump off earlier pseudo-version to newer tag
+go get -u
+grep 'rsc.io/quote v1.5.2' go.mod
+
+-- go.mod --
+module x
+require rsc.io/quote v1.1.0
+
+-- go.mod-v1.5.1 --
+module x
+require rsc.io/quote v1.5.1
+
+-- use.go --
+package use
+
+import _ "rsc.io/quote"
diff --git a/src/cmd/go/testdata/script/mod_get_upgrade_pseudo.txt b/src/cmd/go/testdata/script/mod_get_upgrade_pseudo.txt
new file mode 100644
index 0000000..deff935
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_upgrade_pseudo.txt
@@ -0,0 +1,70 @@
+env GO111MODULE=on
+
+# For this test module there are three versions:
+#   * v0.1.1-0.20190429073117-b5426c86b553
+#   * v0.1.0
+#   * v0.0.0-20190430073000-30950c05d534
+# Only v0.1.0 is tagged.
+#
+# The v0.1.1 pseudo-version is semantically higher than the latest tag.
+# The v0.0.0 pseudo-version is chronologically newer.
+
+# Start at v0.1.1-0.20190429073117-b5426c86b553
+go get example.com/pseudoupgrade@b5426c8
+go list -m -u all
+stdout '^example.com/pseudoupgrade v0.1.1-0.20190429073117-b5426c86b553$'
+
+# 'get -u' should not downgrade to the (lower) tagged version.
+go get -u
+go list -m -u all
+stdout '^example.com/pseudoupgrade v0.1.1-0.20190429073117-b5426c86b553$'
+
+# 'get example.com/pseudoupgrade@upgrade' should not downgrade.
+go get example.com/pseudoupgrade@upgrade
+go list -m all
+stdout '^example.com/pseudoupgrade v0.1.1-0.20190429073117-b5426c86b553$'
+
+# 'get example.com/pseudoupgrade' should not downgrade.
+# This is equivalent to 'get example.com/pseudoupgrade@upgrade'.
+go get example.com/pseudoupgrade
+go list -m all
+stdout '^example.com/pseudoupgrade v0.1.1-0.20190429073117-b5426c86b553$'
+
+# 'get example.com/pseudoupgrade@latest' should downgrade.
+# @latest should not consider the current version.
+go get example.com/pseudoupgrade@latest
+go list -m all
+stdout '^example.com/pseudoupgrade v0.1.0$'
+
+# We should observe the same behavior with the newer pseudo-version.
+go get example.com/pseudoupgrade@v0.0.0-20190430073000-30950c05d534
+
+# 'get -u' should not downgrade to the chronologically older tagged version.
+go get -u
+go list -m -u all
+stdout '^example.com/pseudoupgrade v0.0.0-20190430073000-30950c05d534$'
+
+# 'get example.com/pseudoupgrade@upgrade should not downgrade.
+go get example.com/pseudoupgrade@upgrade
+go list -m -u all
+stdout '^example.com/pseudoupgrade v0.0.0-20190430073000-30950c05d534$'
+
+# 'get example.com/pseudoupgrade' should not downgrade.
+go get example.com/pseudoupgrade
+go list -m -u all
+stdout '^example.com/pseudoupgrade v0.0.0-20190430073000-30950c05d534$'
+
+# 'get example.com/pseudoupgrade@latest' should downgrade.
+go get example.com/pseudoupgrade@latest
+go list -m -u all
+stdout '^example.com/pseudoupgrade v0.1.0$'
+
+-- go.mod --
+module x
+
+go 1.12
+
+-- main.go --
+package x
+
+import _ "example.com/pseudoupgrade"
diff --git a/src/cmd/go/testdata/script/mod_get_wild.txt b/src/cmd/go/testdata/script/mod_get_wild.txt
new file mode 100644
index 0000000..06f9973
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_wild.txt
@@ -0,0 +1,95 @@
+# This test covers a crazy edge-case involving wildcards and multiple passes of
+# patch-upgrades, but if we get it right we probably get many other edge-cases
+# right too.
+
+go list -m all
+stdout '^example.net/a v0.1.0 '
+! stdout '^example.net/b '
+
+
+# Requesting pattern example.../b by itself fails: there is no such module
+# already in the build list, and the wildcard in the first element prevents us
+# from attempting to resolve a new module whose path is a prefix of the pattern.
+
+! go get -u=patch example.../b@upgrade
+stderr '^go: no modules to query for example\.\.\./b@upgrade because first path element contains a wildcard$'
+
+
+# Patching . causes a patch to example.net/a, which introduces a new match
+# for example.net/b/..., which is itself patched and causes another upgrade to
+# example.net/a, which is then patched again.
+
+go get -u=patch . example.../b@upgrade
+go list -m all
+stdout '^example.net/a v0.2.1 '  # upgraded by dependency of b and -u=patch
+stdout '^example.net/b v0.2.0 '  # introduced by patch of a and upgraded by wildcard
+
+
+-- go.mod --
+module example
+
+go 1.16
+
+require example.net/a v0.1.0
+
+replace (
+	example.net/a v0.1.0 => ./a10
+	example.net/a v0.1.1 => ./a11
+	example.net/a v0.2.0 => ./a20
+	example.net/a v0.2.1 => ./a20
+	example.net/b v0.1.0 => ./b1
+	example.net/b v0.1.1 => ./b1
+	example.net/b v0.2.0 => ./b2
+)
+-- example.go --
+package example
+
+import _ "example.net/a"
+
+-- a10/go.mod --
+module example.net/a
+
+go 1.16
+-- a10/a.go --
+package a
+
+-- a11/go.mod --
+module example.net/a
+
+go 1.16
+
+require example.net/b v0.1.0
+-- a11/a.go --
+package a
+-- a11/unimported/unimported.go --
+package unimported
+
+import _ "example.net/b"
+
+
+-- a20/go.mod --
+module example.net/a
+
+go 1.16
+-- a20/a.go --
+package a
+
+-- b1/go.mod --
+module example.net/b
+
+go 1.16
+-- b1/b.go --
+package b
+
+-- b2/go.mod --
+module example.net/b
+
+go 1.16
+
+require example.net/a v0.2.0
+-- b2/b.go --
+package b
+-- b2/b_test.go --
+package b_test
+
+import _ "example.net/a"
diff --git a/src/cmd/go/testdata/script/mod_getmode_vendor.txt b/src/cmd/go/testdata/script/mod_getmode_vendor.txt
new file mode 100644
index 0000000..aaf526b
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_getmode_vendor.txt
@@ -0,0 +1,31 @@
+env GO111MODULE=on
+
+go get rsc.io/quote@v1.5.1
+go mod vendor
+env GOPATH=$WORK/empty
+env GOPROXY=file:///nonexist
+
+go list -mod=vendor
+go list -mod=vendor -f '{{with .Module}}{{.Path}} {{.Version}}{{end}} {{.Dir}}' all
+stdout '^rsc.io/quote v1.5.1 .*vendor[\\/]rsc.io[\\/]quote$'
+stdout '^golang.org/x/text v0.0.0.* .*vendor[\\/]golang.org[\\/]x[\\/]text[\\/]language$'
+
+! go list -mod=vendor -m rsc.io/quote@latest
+stderr 'go: rsc.io/quote@latest: cannot query module due to -mod=vendor'
+! go get -mod=vendor -u
+stderr 'flag provided but not defined: -mod'
+
+# Since we don't have a complete module graph, 'go list -m' queries
+# that require the complete graph should fail with a useful error.
+! go list -mod=vendor -m all
+stderr 'go: can''t compute ''all'' using the vendor directory\n\t\(Use -mod=mod or -mod=readonly to bypass.\)'
+! go list -mod=vendor -m ...
+stderr 'go: can''t match module patterns using the vendor directory\n\t\(Use -mod=mod or -mod=readonly to bypass.\)'
+
+-- go.mod --
+module x
+
+go 1.16
+-- x.go --
+package x
+import _ "rsc.io/quote"
diff --git a/src/cmd/go/testdata/script/mod_getx.txt b/src/cmd/go/testdata/script/mod_getx.txt
new file mode 100644
index 0000000..46bb95b
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_getx.txt
@@ -0,0 +1,18 @@
+[!net:golang.org] skip
+[!git] skip
+
+env GO111MODULE=on
+env GOPROXY=direct
+env GOSUMDB=off
+
+# 'go get -x' should log URLs with an HTTP or HTTPS scheme.
+# A bug had caused us to log schemeless URLs instead.
+go get -x golang.org/x/text@v0.1.0
+stderr '^# get https://golang.org/x/text\?go-get=1$'
+stderr '^# get https://golang.org/x/text\?go-get=1: 200 OK \([0-9.]+s\)$'
+! stderr '^# get //.*'
+
+-- go.mod --
+module m
+
+go 1.18
diff --git a/src/cmd/go/testdata/script/mod_git_export_subst.txt b/src/cmd/go/testdata/script/mod_git_export_subst.txt
new file mode 100644
index 0000000..740ccbd
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_git_export_subst.txt
@@ -0,0 +1,21 @@
+env GO111MODULE=on
+env GOPROXY=direct
+
+# Testing that git export-subst is disabled
+[!net:github.com] skip
+[!git] skip
+go build
+
+-- x.go --
+package x
+
+import _ "github.com/jasonkeene/export-subst"
+
+-- go.mod --
+module x
+
+require github.com/jasonkeene/export-subst v0.0.0-20180927204031-5845945ec626
+
+-- go.sum --
+github.com/jasonkeene/export-subst v0.0.0-20180927204031-5845945ec626 h1:AUkXi/xFnm7lH2pgtvVkGb7buRn1ywFHw+xDpZ29Rz0=
+github.com/jasonkeene/export-subst v0.0.0-20180927204031-5845945ec626/go.mod h1:DwJXqVtrgrQkv3Giuf2Jh4YyubVe7y41S1eOIaysTJw=
diff --git a/src/cmd/go/testdata/script/mod_go_version.txt b/src/cmd/go/testdata/script/mod_go_version.txt
new file mode 100644
index 0000000..82b55d8
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_go_version.txt
@@ -0,0 +1,29 @@
+# Test support for declaring needed Go version in module.
+
+env GO111MODULE=on
+env TESTGO_VERSION=go1.21
+
+! go list
+stderr -count=1 '^go: sub@v1.0.0: module ./sub requires go >= 1.999 \(running go 1.21\)$'
+! go build sub
+stderr -count=1 '^go: sub@v1.0.0: module ./sub requires go >= 1.999 \(running go 1.21\)$'
+
+-- go.mod --
+module m
+go 1.1
+require (
+	sub v1.0.0
+)
+replace (
+	sub => ./sub
+)
+
+-- x.go --
+package x
+
+-- sub/go.mod --
+module sub
+go 1.999
+
+-- sub/x.go --
+package x
diff --git a/src/cmd/go/testdata/script/mod_go_version_missing.txt b/src/cmd/go/testdata/script/mod_go_version_missing.txt
new file mode 100644
index 0000000..e9a8e72
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_go_version_missing.txt
@@ -0,0 +1,122 @@
+cp go.mod go.mod.orig
+
+# For modules whose go.mod file does not include a 'go' directive,
+# we assume the language and dependency semantics of Go 1.16,
+# but do not trigger “automatic vendoring” mode (-mod=vendor),
+# which was added in Go 1.14 and was not triggered
+# under the same conditions in Go 1.16 (which would instead
+# default to -mod=readonly when no 'go' directive is present).
+
+# For Go 1.16 modules, 'all' should prune out dependencies of tests,
+# even if the 'go' directive is missing.
+
+go list -mod=readonly all
+stdout '^example.com/dep$'
+! stdout '^example.com/testdep$'
+cp stdout list-1.txt
+cmp go.mod go.mod.orig
+
+# We should only default to -mod=vendor if the 'go' directive is explicit in the
+# go.mod file. Otherwise, we don't actually know whether the module was written
+# against Go 1.11 or 1.16. We would have to update the go.mod file to clarify,
+# and as of Go 1.16 we don't update the go.mod file by default.
+#
+# If we set -mod=vendor explicitly, we shouldn't apply the Go 1.14
+# consistency check, because — again — we don't know whether we're in a 1.11
+# module or a bad-script-edited 1.16 module.
+
+! go list -mod=vendor all
+! stderr '^go: inconsistent vendoring'
+stderr 'cannot find module providing package example.com/badedit: import lookup disabled by -mod=vendor'
+
+# When we set -mod=mod, the go version should be updated immediately,
+# to the current version, converting the requirements from eager to lazy.
+#
+# Since we don't know which requirements are actually relevant to the main
+# module, all requirements are added as roots, making the requirements untidy.
+
+go list -mod=mod all
+! stdout '^example.com/testdep$'
+cmp stdout list-1.txt
+cmpenv go.mod go.mod.untidy
+
+go mod tidy
+cmpenv go.mod go.mod.tidy
+
+# On the other hand, if we jump straight to 'go mod tidy',
+# the requirements remain tidy from the start.
+
+cp go.mod.orig go.mod
+go mod tidy
+cmpenv go.mod go.mod.tidy
+
+
+# The updated version should have been written back to go.mod, so now the 'go'
+# directive is explicit. -mod=vendor should trigger by default, and the stronger
+# Go 1.14 consistency check should apply.
+! go list all
+stderr '^go: inconsistent vendoring'
+! stderr badedit
+
+
+-- go.mod --
+module example.com/m
+
+require example.com/dep v0.1.0
+
+replace (
+	example.com/dep v0.1.0 => ./dep
+	example.com/testdep v0.1.0 => ./testdep
+)
+-- go.mod.untidy --
+module example.com/m
+
+go $goversion
+
+require example.com/dep v0.1.0
+
+require example.com/testdep v0.1.0 // indirect
+
+replace (
+	example.com/dep v0.1.0 => ./dep
+	example.com/testdep v0.1.0 => ./testdep
+)
+-- go.mod.tidy --
+module example.com/m
+
+go $goversion
+
+require example.com/dep v0.1.0
+
+replace (
+	example.com/dep v0.1.0 => ./dep
+	example.com/testdep v0.1.0 => ./testdep
+)
+-- vendor/example.com/dep/dep.go --
+package dep
+import _ "example.com/badedit"
+-- vendor/modules.txt --
+HAHAHA this is broken.
+
+-- m.go --
+package m
+
+import _ "example.com/dep"
+
+const x = 1_000
+
+-- dep/go.mod --
+module example.com/dep
+
+require example.com/testdep v0.1.0
+-- dep/dep.go --
+package dep
+-- dep/dep_test.go --
+package dep_test
+
+import _ "example.com/testdep"
+
+-- testdep/go.mod --
+module example.com/testdep
+-- testdep/testdep.go --
+package testdep
diff --git a/src/cmd/go/testdata/script/mod_go_version_mixed.txt b/src/cmd/go/testdata/script/mod_go_version_mixed.txt
new file mode 100644
index 0000000..d6216ae
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_go_version_mixed.txt
@@ -0,0 +1,43 @@
+# Test that dependencies can use Go language features newer than the
+# Go version specified by the main module.
+
+env GO111MODULE=on
+
+go build
+
+-- go.mod --
+module m
+go 1.12
+require (
+	sub.1 v1.0.0
+)
+replace (
+	sub.1 => ./sub
+)
+
+-- x.go --
+package x
+
+import "sub.1"
+
+func F() { sub.F(0, 0) }
+
+var A sub.Alias
+var D sub.Defined
+
+-- sub/go.mod --
+module m
+go 1.14
+
+-- sub/sub.go --
+package sub
+
+// signed shift counts added in Go 1.13
+func F(l, r int) int { return l << r }
+
+type m1 interface { M() }
+type m2 interface { M() }
+
+// overlapping interfaces added in Go 1.14
+type Alias = interface { m1; m2; M() }
+type Defined interface { m1; m2; M() }
diff --git a/src/cmd/go/testdata/script/mod_gobuild_import.txt b/src/cmd/go/testdata/script/mod_gobuild_import.txt
new file mode 100644
index 0000000..70af331
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_gobuild_import.txt
@@ -0,0 +1,133 @@
+[short] skip
+
+# go/build's Import should find modules by invoking the go command
+
+go build -o $WORK ./testimport ./testfindonly
+
+# GO111MODULE=off
+env GO111MODULE=off
+! exec $WORK/testimport$GOEXE gobuild.example.com/x/y/z/w .
+
+# GO111MODULE=auto in GOPATH/src
+env GO111MODULE=auto
+exec $WORK/testimport$GOEXE gobuild.example.com/x/y/z/w .
+
+# GO111MODULE=auto outside GOPATH/src
+cd $GOPATH/other
+env GO111MODULE=auto
+exec $WORK/testimport$GOEXE other/x/y/z/w .
+stdout w2.go
+
+! exec $WORK/testimport$GOEXE gobuild.example.com/x/y/z/w .
+stderr 'no required module provides package gobuild.example.com/x/y/z/w; to add it:\n\tgo get gobuild.example.com/x/y/z/w'
+
+cd z
+exec $WORK/testimport$GOEXE other/x/y/z/w .
+stdout w2.go
+
+# GO111MODULE=on outside GOPATH/src
+env GO111MODULE=
+exec $WORK/testimport$GOEXE other/x/y/z/w .
+stdout w2.go
+env GO111MODULE=on
+exec $WORK/testimport$GOEXE other/x/y/z/w .
+stdout w2.go
+
+# GO111MODULE=on in GOPATH/src
+cd $GOPATH/src
+env GO111MODULE=
+exec $WORK/testimport$GOEXE gobuild.example.com/x/y/z/w .
+stdout w1.go
+env GO111MODULE=on
+exec $WORK/testimport$GOEXE gobuild.example.com/x/y/z/w .
+stdout w1.go
+cd w
+exec $WORK/testimport$GOEXE gobuild.example.com/x/y/z/w ..
+stdout w1.go
+
+# go/build's Import in FindOnly mode should find directories by invoking the go command
+#
+# Calling build.Import in build.FindOnly mode on an import path of a Go package
+# that produces errors when loading (e.g., due to build constraints not matching
+# the current build context) should return the package directory and nil error.
+
+# Issue 31603: Import with non-empty srcDir should work.
+env GO111MODULE=on
+exec $WORK/testfindonly$GOEXE gobuild.example.com/x/y/z/i $WORK
+! stdout 'build constraints'
+stdout '^dir='$WORK'.+i err=<nil>$'
+
+# Issue 37153: Import with empty srcDir should work.
+env GO111MODULE=on
+exec $WORK/testfindonly$GOEXE gobuild.example.com/x/y/z/i ''
+! stdout 'build constraints'
+stdout '^dir='$WORK'.+i err=<nil>$'
+
+-- go.mod --
+module gobuild.example.com/x/y/z
+
+-- z.go --
+package z
+
+-- w/w1.go --
+package w
+
+-- i/i.go --
+// +build i
+
+package i
+
+-- testimport/x.go --
+package main
+
+import (
+	"fmt"
+	"go/build"
+	"log"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+func main() {
+	// build.Import should support relative and absolute source dir paths.
+	path := os.Args[1]
+	srcDir := os.Args[2]
+	p1, err := build.Import(path, srcDir, 0)
+	if err != nil {
+		log.Fatal(err)
+	}
+	absSrcDir, err := filepath.Abs(srcDir)
+	if err != nil {
+		log.Fatal(err)
+	}
+	p2, err := build.Import(path, absSrcDir, 0)
+	if err != nil {
+		log.Fatal(err)
+	}
+	if p1.Dir != p2.Dir {
+		log.Fatalf("different packages loaded with relative and absolute paths:\n\t%s\n\t%s", p1.Dir, p2.Dir)
+	}
+
+	fmt.Printf("%s\n%s\n", p1.Dir, strings.Join(p1.GoFiles, " "))
+}
+
+-- testfindonly/x.go --
+package main
+
+import (
+	"fmt"
+	"go/build"
+	"os"
+)
+
+func main() {
+	p, err := build.Import(os.Args[1], os.Args[2], build.FindOnly)
+	fmt.Printf("dir=%s err=%v\n", p.Dir, err)
+}
+
+-- $GOPATH/other/go.mod --
+module other/x/y
+
+-- $GOPATH/other/z/w/w2.go --
+package w
diff --git a/src/cmd/go/testdata/script/mod_gofmt_invalid.txt b/src/cmd/go/testdata/script/mod_gofmt_invalid.txt
new file mode 100644
index 0000000..21edc7d
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_gofmt_invalid.txt
@@ -0,0 +1,13 @@
+# Test for a crash in go fmt on invalid input when using modules.
+# Issue 26792.
+
+env GO111MODULE=on
+! go fmt x.go
+! stderr panic
+
+-- go.mod --
+module x
+
+-- x.go --
+// Missing package declaration.
+var V int
diff --git a/src/cmd/go/testdata/script/mod_goline.txt b/src/cmd/go/testdata/script/mod_goline.txt
new file mode 100644
index 0000000..6b2eab0
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_goline.txt
@@ -0,0 +1,117 @@
+env TESTGO_VERSION=go1.99
+
+! go list -f '{{.Module.GoVersion}}'
+stderr 'go: updates to go.mod needed'
+stderr 'go mod tidy'
+
+go mod tidy
+cat go.mod
+go list -f '{{.Module.GoVersion}}'
+stdout 1.22
+
+# Adding a@v1.0.01 should upgrade to Go 1.23rc1.
+cp go.mod go.mod1
+go get example.com/a@v1.0.1
+stderr '^go: upgraded go 1.22 => 1.23rc1\ngo: upgraded example.com/a v1.0.0 => v1.0.1\ngo: upgraded example.com/b v1.0.0 => v1.0.1$'
+go list -f '{{.Module.GoVersion}}'
+stdout 1.23rc1
+
+# Repeating the update with go@1.24.0 should use that Go version.
+cp go.mod1 go.mod
+go get example.com/a@v1.0.1 go@1.24.0
+go list -f '{{.Module.GoVersion}}'
+stdout 1.24.0
+
+# Go version-constrained updates should report the problems.
+cp go.mod1 go.mod
+! go get example.com/a@v1.0.2 go@1.24.2
+stderr '^go: example.com/a@v1.0.2 requires go@1.25, not go@1.24.2$'
+! go get example.com/a@v1.0.2 go@1.26.3
+stderr '^go: example.com/a@v1.0.2 indirectly requires go@1.27, not go@1.26.3$'
+go get example.com/a@v1.0.2 go@1.28rc1
+go list -f '{{.Module.GoVersion}}'
+stdout 1.28rc1
+go get go@1.24.2
+stderr '^go: downgraded go 1.28rc1 => 1.24.2$'
+stderr '^go: downgraded example.com/a v1.0.2 => v1.0.1$'
+stderr '^go: downgraded example.com/b v1.0.2 => v1.0.1$'
+go list -f '{{.Module.GoVersion}}'
+stdout 1.24.2
+
+-- go.mod --
+module m
+go 1.21
+
+require (
+	example.com/a v1.0.0
+	example.com/b v0.9.0
+)
+
+replace example.com/a v1.0.0 => ./a100
+replace example.com/a v1.0.1 => ./a101
+replace example.com/a v1.0.2 => ./a102
+replace example.com/b v1.0.1 => ./b101
+replace example.com/b v1.0.2 => ./b102
+replace example.com/b v1.0.0 => ./b100
+replace example.com/b v0.9.0 => ./b100
+
+-- x.go --
+package m
+
+import (
+	_ "example.com/a"
+	_ "example.com/b"
+)
+
+-- a100/go.mod --
+module example.com/a
+go 1.22
+
+require example.com/b v1.0.0
+
+-- a100/a.go --
+package a
+
+-- a101/go.mod --
+// this module is technically invalid, since the dep example.com/b has a newer go line than this module,
+// but we should still be able to handle it.
+module example.com/a
+go 1.22
+
+require example.com/b v1.0.1
+
+-- a101/a.go --
+package a
+
+-- a102/go.mod --
+// this module is technically invalid, since the dep example.com/b has a newer go line than this module,
+// but we should still be able to handle it.
+module example.com/a
+go 1.25
+
+require example.com/b v1.0.2
+
+-- a102/a.go --
+package a
+
+-- b100/go.mod --
+module example.com/b
+go 1.22
+
+-- b100/b.go --
+package b
+
+-- b101/go.mod --
+module example.com/b
+go 1.23rc1
+
+-- b101/b.go --
+package b
+
+-- b102/go.mod --
+module example.com/b
+go 1.27
+
+-- b102/b.go --
+package b
+
diff --git a/src/cmd/go/testdata/script/mod_goline_old.txt b/src/cmd/go/testdata/script/mod_goline_old.txt
new file mode 100644
index 0000000..bbe611b
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_goline_old.txt
@@ -0,0 +1,72 @@
+env TESTGO_VERSION=go1.24
+
+go list -f '{{.Module.GoVersion}}'
+stdout 1.15
+
+go mod tidy
+go list -f '{{.Module.GoVersion}}'
+stdout 1.15
+
+go get example.com/a@v1.0.1
+go list -f '{{.Module.GoVersion}}'
+stdout 1.15
+
+go get example.com/a@v1.0.1 go@1.16
+go list -f '{{.Module.GoVersion}}'
+stdout 1.16
+
+-- go.mod --
+module m
+go 1.15
+
+require (
+	example.com/a v1.0.0
+	example.com/b v1.0.0
+)
+
+replace example.com/a v1.0.0 => ./a100
+replace example.com/a v1.0.1 => ./a101
+replace example.com/b v1.0.1 => ./b101
+replace example.com/b v1.0.0 => ./b100
+replace example.com/b v0.9.0 => ./b100
+
+-- x.go --
+package m
+
+import (
+	_ "example.com/a"
+	_ "example.com/b"
+)
+
+-- a100/go.mod --
+module example.com/a
+go 1.16
+
+require example.com/b v1.0.0
+
+-- a100/a.go --
+package a
+
+-- a101/go.mod --
+module example.com/a
+go 1.17
+
+require example.com/b v1.0.1
+
+-- a101/a.go --
+package a
+
+-- b100/go.mod --
+module example.com/b
+go 1.18
+
+-- b100/b.go --
+package b
+
+-- b101/go.mod --
+module example.com/b
+go 1.19
+
+-- b101/b.go --
+package b
+
diff --git a/src/cmd/go/testdata/script/mod_goline_too_new.txt b/src/cmd/go/testdata/script/mod_goline_too_new.txt
new file mode 100644
index 0000000..6d26679
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_goline_too_new.txt
@@ -0,0 +1,50 @@
+# Go should refuse to build code that is too new according to go.mod.
+
+# go.mod too new
+env GOTOOLCHAIN=local
+! go build .
+stderr '^go: go.mod requires go >= 1.99999 \(running go 1\..+\)$'
+
+# go.mod referenced from go.work too new
+cp go.work.old go.work
+! go build .
+stderr '^go: module . listed in go.work file requires go >= 1.99999, but go.work lists go 1.10; to update it:\n\tgo work use$'
+
+! go work sync
+stderr '^go: cannot load module . listed in go.work file: go.mod requires go >= 1.99999 \(running go 1\..+\)$'
+
+# go.work too new
+cp go.work.new go.work
+cp go.mod.old go.mod
+! go build .
+stderr '^go: go.work requires go >= 1.99999 \(running go 1\..+\)$'
+
+# vendor too new
+rm go.work
+mv notvendor vendor
+! go build -mod=vendor .
+stderr '^go: golang.org/x/text in vendor'${/}'modules.txt requires go >= 1.99999 \(running go 1\..+\)$'
+
+-- go.mod --
+module example
+go 1.99999
+
+-- p.go --
+package p
+
+-- go.mod.old --
+module example
+go 1.10
+
+-- go.work.new --
+go 1.99999
+use .
+
+-- go.work.old --
+go 1.10
+use .
+
+-- notvendor/modules.txt --
+# golang.org/x/text v0.9.0
+## explicit; go 1.99999
+golang.org/x/text/internal/language
diff --git a/src/cmd/go/testdata/script/mod_gomodcache.txt b/src/cmd/go/testdata/script/mod_gomodcache.txt
new file mode 100644
index 0000000..bfc6cb1
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_gomodcache.txt
@@ -0,0 +1,74 @@
+# Test GOMODCACHE
+env GO111MODULE=on
+
+# Explicitly set GOMODCACHE
+env GOMODCACHE=$WORK/modcache
+go env GOMODCACHE
+stdout $WORK[/\\]modcache
+go get rsc.io/quote@v1.0.0
+exists $WORK/modcache/cache/download/rsc.io/quote/@v/v1.0.0.info
+grep '{"Version":"v1.0.0","Time":"2018-02-14T00:45:20Z"}' $WORK/modcache/cache/download/rsc.io/quote/@v/v1.0.0.info
+
+# Ensure GOMODCACHE doesn't affect location of sumdb, but $GOMODCACHE/cache/download/sumdb is still written
+exists $GOPATH/pkg/sumdb
+! exists $WORK/modcache/sumdb
+exists $WORK/modcache/cache/download/sumdb
+
+# Test that the default GOMODCACHE is $GOPATH[0]/pkg/mod
+env GOMODCACHE=
+go env GOMODCACHE
+stdout $GOPATH[/\\]pkg[/\\]mod
+go get rsc.io/quote@v1.0.0
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.0.0.info
+grep '{"Version":"v1.0.0","Time":"2018-02-14T00:45:20Z"}' $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.0.0.info
+
+# If neither GOMODCACHE or GOPATH are set, GOPATH defaults to the user's $HOME/go, so GOMODCACHE becomes $HOME/go/pkg/mod
+[GOOS:windows] env USERPROFILE=$WORK/home # Ensure USERPROFILE is a valid path (rather than /no-home/ so we don't run into the logic that "uninfers" GOPATH in cmd/go/main.go
+[GOOS:plan9] env home=$WORK/home
+[!GOOS:windows] [!GOOS:plan9] env HOME=$WORK/home
+env GOMODCACHE=
+env GOPATH=
+go env GOMODCACHE
+stdout $HOME[/\\]go[/\\]pkg[/\\]mod
+
+# If GOMODCACHE isn't set and GOPATH starts with the path list separator,
+# GOMODCACHE is empty and any command that needs it errors out.
+env GOMODCACHE=
+env GOPATH=${:}$WORK/this/is/ignored
+
+go env GOMODCACHE
+stdout '^$'
+! stdout .
+! stderr .
+
+! go mod download rsc.io/quote@v1.0.0
+stderr '^go: module cache not found: neither GOMODCACHE nor GOPATH is set$'
+
+# If GOMODCACHE isn't set and GOPATH has multiple elements only the first is used.
+env GOMODCACHE=
+env GOPATH=$WORK/first/path${:}$WORK/this/is/ignored
+go env GOMODCACHE
+stdout $WORK[/\\]first[/\\]path[/\\]pkg[/\\]mod
+
+env GOMODCACHE=$WORK/modcache
+go mod download rsc.io/quote@v1.0.0
+exists $WORK/modcache/cache/download/rsc.io/quote/@v/v1.0.0.info
+
+# Test error when cannot create GOMODCACHE directory
+env GOMODCACHE=$WORK/modcachefile
+! go install example.com/cmd/a@v1.0.0
+stderr 'go: could not create module cache'
+
+# Test that the following work even with GO111MODULE=off
+env GO111MODULE=off
+
+# Cleaning modcache
+exists $WORK/modcache
+env GOMODCACHE=$WORK/modcache
+go clean -modcache
+! exists $WORK/modcache
+
+-- go.mod --
+module m
+
+-- $WORK/modcachefile --
diff --git a/src/cmd/go/testdata/script/mod_gonoproxy.txt b/src/cmd/go/testdata/script/mod_gonoproxy.txt
new file mode 100644
index 0000000..94d03f2
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_gonoproxy.txt
@@ -0,0 +1,55 @@
+env GO111MODULE=on
+env sumdb=$GOSUMDB
+env proxy=$GOPROXY
+env GOPRIVATE GOPROXY GONOPROXY GOSUMDB GONOSUMDB
+env dbname=localhost.localdev/sumdb
+
+# disagree with sumdb fails
+cp go.mod.orig go.mod
+env GOSUMDB=$sumdb' '$proxy/sumdb-wrong
+! go get rsc.io/quote
+stderr 'SECURITY ERROR'
+
+# GONOSUMDB bypasses sumdb, for rsc.io/quote, rsc.io/sampler, golang.org/x/text
+env GONOSUMDB='*/quote,*/*mple*,golang.org/x'
+go get rsc.io/quote
+rm go.sum
+env GOPRIVATE='*/quote,*/*mple*,golang.org/x'
+env GONOPROXY=none # that is, proxy all despite GOPRIVATE
+go get rsc.io/quote
+
+# Download .info files needed for 'go list -m all' later.
+# TODO(#42723): either 'go list -m' should not read these files,
+# or 'go get' and 'go mod tidy' should download them.
+go list -m all
+stdout '^golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c$'
+
+# When GOPROXY is not empty but contains no entries, an error should be reported.
+env GOPROXY=','
+! go get golang.org/x/text
+stderr '^go: golang.org/x/text: GOPROXY list is not the empty string, but contains no entries$'
+
+# When GOPROXY=off, fetching modules not matched by GONOPROXY fails.
+env GONOPROXY=*/fortune
+env GOPROXY=off
+! go get golang.org/x/text
+stderr '^go: golang.org/x/text: module lookup disabled by GOPROXY=off$'
+
+# GONOPROXY bypasses proxy
+[!net:rsc.io] skip
+[!git] skip
+env GOPRIVATE=none
+env GONOPROXY='*/fortune'
+! go get rsc.io/fortune # does not exist in real world, only on test proxy
+stderr 'git ls-remote'
+
+[!net:golang.org] skip
+env GOSUMDB=
+env GONOPROXY=
+env GOPRIVATE='*/x'
+go get golang.org/x/text
+go list -m all
+! stdout 'text.*v0.0.0-2017' # should not have the version from the proxy
+
+-- go.mod.orig --
+module m
diff --git a/src/cmd/go/testdata/script/mod_gopkg_unstable.txt b/src/cmd/go/testdata/script/mod_gopkg_unstable.txt
new file mode 100644
index 0000000..856f493
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_gopkg_unstable.txt
@@ -0,0 +1,26 @@
+env GO111MODULE=on
+
+cp go.mod.empty go.mod
+go get gopkg.in/dummy.v2-unstable
+
+cp x.go.txt x.go
+cp go.mod.empty go.mod
+go list
+
+[!net:gopkg.in] skip
+[!git] skip
+
+skip  # TODO(#54503): redirect gopkg.in requests to a local server and re-enable.
+
+env GOPROXY=direct
+env GOSUMDB=off
+go get gopkg.in/macaroon-bakery.v2-unstable/bakery
+go list -m all
+stdout 'gopkg.in/macaroon-bakery.v2-unstable v2.0.0-[0-9]+-[0-9a-f]+$'
+
+-- go.mod.empty --
+module m
+
+-- x.go.txt --
+package x
+import _ "gopkg.in/dummy.v2-unstable"
diff --git a/src/cmd/go/testdata/script/mod_goroot_errors.txt b/src/cmd/go/testdata/script/mod_goroot_errors.txt
new file mode 100644
index 0000000..110a196
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_goroot_errors.txt
@@ -0,0 +1,53 @@
+env GO111MODULE=on
+
+# Regression test for https://golang.org/issue/34769.
+# Missing standard-library imports should refer to GOROOT rather than
+# complaining about a malformed module path.
+# This is especially important when GOROOT is set incorrectly,
+# since such an error will occur for every package in std.
+
+# Building a nonexistent std package directly should fail usefully.
+
+! go build -mod=readonly nonexist
+! stderr 'import lookup disabled'
+! stderr 'missing dot'
+stderr '^package nonexist is not in std \('$GOROOT'[/\\]src[/\\]nonexist\)$'
+
+! go build nonexist
+! stderr 'import lookup disabled'
+! stderr 'missing dot'
+stderr '^package nonexist is not in std \('$GOROOT'[/\\]src[/\\]nonexist\)$'
+
+# Building a nonexistent std package indirectly should also fail usefully.
+
+! go build -mod=readonly ./importnonexist
+! stderr 'import lookup disabled'
+! stderr 'missing dot'
+stderr '^importnonexist[/\\]x.go:2:8: package nonexist is not in std \('$GOROOT'[/\\]src[/\\]nonexist\)$'
+
+! go build ./importnonexist
+! stderr 'import lookup disabled'
+! stderr 'missing dot'
+stderr '^importnonexist[/\\]x.go:2:8: package nonexist is not in std \('$GOROOT'[/\\]src[/\\]nonexist\)$'
+
+# Building an *actual* std package should fail if GOROOT is set to something bogus.
+
+[!short] go build ./importjson  # Prove that it works when GOROOT is valid.
+
+env GOROOT=$WORK/not-a-valid-goroot
+! go build ./importjson
+! stderr 'import lookup disabled'
+! stderr 'missing dot'
+stderr 'importjson[/\\]x.go:2:8: package encoding/json is not in std \('$WORK'[/\\]not-a-valid-goroot[/\\]src[/\\]encoding[/\\]json\)$'
+
+-- go.mod --
+module example.com
+go 1.14
+-- importnonexist/x.go --
+package importnonexist
+import _ "nonexist"
+-- importjson/x.go --
+package importjson
+import _ "encoding/json"
+-- $WORK/not-a-valid-goroot/README --
+This directory is not a valid GOROOT.
diff --git a/src/cmd/go/testdata/script/mod_graph.txt b/src/cmd/go/testdata/script/mod_graph.txt
new file mode 100644
index 0000000..8d51439
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_graph.txt
@@ -0,0 +1,15 @@
+env GO111MODULE=on
+
+go mod graph
+stdout '^m rsc.io/quote@v1.5.2$'
+stdout '^rsc.io/quote@v1.5.2 rsc.io/sampler@v1.3.0$'
+! stdout '^m rsc.io/sampler@v1.3.0$'
+! stderr 'get '$GOPROXY
+
+rm $GOPATH/pkg/mod/cache/download/rsc.io/quote
+go mod graph -x
+stderr 'get '$GOPROXY
+
+-- go.mod --
+module m
+require rsc.io/quote v1.5.2
diff --git a/src/cmd/go/testdata/script/mod_graph_version.txt b/src/cmd/go/testdata/script/mod_graph_version.txt
new file mode 100644
index 0000000..ed7e399
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_graph_version.txt
@@ -0,0 +1,101 @@
+# For this module, Go 1.17 prunes out a (transitive and otherwise-irrelevant)
+# requirement on a retracted higher version of a dependency.
+# However, when Go 1.16 reads the same requirements from the go.mod file,
+# it does not prune out that requirement, and selects the retracted version.
+#
+# The Go 1.16 module graph looks like:
+#
+# m ---- lazy v0.1.0 ---- requireincompatible v0.1.0 ---- incompatible v2.0.0+incompatible
+# |        |
+# + -------+------------- incompatible v1.0.0
+#
+# The Go 1.17 module graph is the same except that the dependencies of
+# requireincompatible are pruned out (because the module that requires
+# it — lazy v0.1.0 — specifies 'go 1.17', and it is not otherwise relevant to
+# the main module).
+
+cp go.mod go.mod.orig
+
+go mod graph
+cp stdout graph-1.17.txt
+stdout '^example\.com/m example\.com/retract/incompatible@v1\.0\.0$'
+stdout '^example\.net/lazy@v0\.1\.0 example\.com/retract/incompatible@v1\.0\.0$'
+! stdout 'example\.com/retract/incompatible@v2\.0\.0\+incompatible'
+
+go mod graph -go=1.17
+cmp stdout graph-1.17.txt
+
+cmp go.mod go.mod.orig
+
+
+# Setting -go=1.16 should report the graph as viewed by Go 1.16,
+# but should not edit the go.mod file.
+
+go mod graph -go=1.16
+cp stdout graph-1.16.txt
+stdout '^example\.com/m example\.com/retract/incompatible@v1\.0\.0$'
+stdout '^example\.net/lazy@v0\.1\.0 example.com/retract/incompatible@v1\.0\.0$'
+stdout '^example.net/requireincompatible@v0.1.0 example.com/retract/incompatible@v2\.0\.0\+incompatible$'
+
+cmp go.mod go.mod.orig
+
+
+# If we actually update the go.mod file to the requested go version,
+# we should get the same selected versions, but the roots of the graph
+# may be updated.
+#
+# TODO(#45551): The roots should not be updated.
+
+go mod edit -go=1.16
+go mod graph
+! stdout '^example\.com/m example\.com/retract/incompatible@v1\.0\.0$'
+stdout '^example\.net/lazy@v0.1.0 example.com/retract/incompatible@v1\.0\.0$'
+stdout '^example.net/requireincompatible@v0.1.0 example.com/retract/incompatible@v2\.0\.0\+incompatible$'
+	# TODO(#45551): cmp stdout graph-1.16.txt
+
+
+# Unsupported go versions should be rejected, since we don't know
+# what versions they would report.
+! go mod graph -go=1.99999999999
+stderr '^invalid value "1\.99999999999" for flag -go: maximum supported Go version is '$goversion'\nusage: go mod graph \[-go=version\] \[-x\]\nRun ''go help mod graph'' for details.$'
+
+
+-- go.mod --
+// Module m indirectly imports a package from
+// example.com/retract/incompatible. Its selected version of
+// that module is lower under Go 1.17 semantics than under Go 1.16.
+module example.com/m
+
+go 1.17
+
+replace (
+	example.net/lazy v0.1.0 => ./lazy
+	example.net/requireincompatible v0.1.0 => ./requireincompatible
+)
+
+require (
+	example.com/retract/incompatible v1.0.0 // indirect
+	example.net/lazy v0.1.0
+)
+-- lazy/go.mod --
+// Module lazy requires example.com/retract/incompatible v1.0.0.
+//
+// When viewed from the outside it also has a transitive dependency
+// on v2.0.0+incompatible, but in lazy mode that transitive dependency
+// is pruned out.
+module example.net/lazy
+
+go 1.17
+
+exclude example.com/retract/incompatible v2.0.0+incompatible
+
+require (
+	example.com/retract/incompatible v1.0.0
+	example.net/requireincompatible v0.1.0
+)
+-- requireincompatible/go.mod --
+module example.net/requireincompatible
+
+go 1.15
+
+require example.com/retract/incompatible v2.0.0+incompatible
diff --git a/src/cmd/go/testdata/script/mod_help.txt b/src/cmd/go/testdata/script/mod_help.txt
new file mode 100644
index 0000000..b5cd30c
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_help.txt
@@ -0,0 +1,6 @@
+env GO111MODULE=on
+
+# go help get shows usage for get
+go help get
+stdout 'usage: go get'
+stdout 'get using modules to manage source'
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/mod_import.txt b/src/cmd/go/testdata/script/mod_import.txt
new file mode 100644
index 0000000..07714e9
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_import.txt
@@ -0,0 +1,18 @@
+env GO111MODULE=on
+
+# latest rsc.io/quote should be v1.5.2 not v1.5.3-pre1
+go get
+go list -m all
+stdout 'rsc.io/quote v1.5.2'
+
+# but v1.5.3-pre1 should be a known version
+go list -m -versions rsc.io/quote
+stdout '^rsc.io/quote v1.0.0 v1.1.0 v1.2.0 v1.2.1 v1.3.0 v1.4.0 v1.5.0 v1.5.1 v1.5.2 v1.5.3-pre1$'
+
+-- go.mod --
+module x
+
+-- x.go --
+package x
+import _ "rsc.io/quote"
+
diff --git a/src/cmd/go/testdata/script/mod_import_cycle.txt b/src/cmd/go/testdata/script/mod_import_cycle.txt
new file mode 100644
index 0000000..7be0749
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_import_cycle.txt
@@ -0,0 +1,40 @@
+env GO111MODULE=on
+
+# 'go list all' should fail with a reasonable error message
+! go list all
+stderr '^package m\n\timports m/a\n\timports m/b\n\timports m/a: import cycle not allowed'
+
+# 'go list -e' should not print to stderr, but should mark all three
+# packages (m, m/a, and m/b) as Incomplete.
+go list -e -json all
+! stderr .
+stdout -count=3 '"Incomplete": true,'
+
+-- go.mod --
+module m
+
+require (
+	m/a v0.0.0
+	m/b v0.0.0
+)
+
+replace (
+	m/a => ./a
+	m/b => ./b
+)
+-- m.go --
+package m
+import (
+	_ "m/a"
+	_ "m/b"
+)
+-- a/go.mod --
+module m/a
+-- a/a.go --
+package a
+import _ "m/b"
+-- b/go.mod --
+module m/b
+-- b/b.go --
+package b
+import _ "m/a"
diff --git a/src/cmd/go/testdata/script/mod_import_issue41113.txt b/src/cmd/go/testdata/script/mod_import_issue41113.txt
new file mode 100644
index 0000000..fed2510
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_import_issue41113.txt
@@ -0,0 +1,28 @@
+# Regression test for https://golang.org/issue/41113.
+#
+# When resolving a missing import path, the inability to add the package from
+# one module path should not interfere with adding a nested path.
+
+# Initially, our module depends on split-incompatible v2.1.0-pre+incompatible,
+# from which an imported package has been removed (and relocated to the nested
+# split-incompatible/subpkg module). modload.QueryPattern will suggest
+# split-incompatible v2.0.0+incompatible, which we cannot use (because it would
+# be an implicit downgrade), and split-incompatible/subpkg v0.1.0, which we
+# *should* use.
+
+go mod tidy
+
+go list -m all
+stdout '^example.com/split-incompatible/subpkg v0\.1\.0$'
+! stdout '^example.com/split-incompatible .*'
+
+-- go.mod --
+module golang.org/issue/41113
+
+go 1.16
+
+require example.com/split-incompatible v2.1.0-pre+incompatible
+-- x.go --
+package issue41113
+
+import _ "example.com/split-incompatible/subpkg"
diff --git a/src/cmd/go/testdata/script/mod_import_issue42891.txt b/src/cmd/go/testdata/script/mod_import_issue42891.txt
new file mode 100644
index 0000000..a78cab2
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_import_issue42891.txt
@@ -0,0 +1,14 @@
+# If an import declaration is an absolute path, most commands should report
+# an error instead of going into an infinite loop.
+# Verifies golang.org/issue/42891.
+go list .
+stdout '^m$'
+
+-- go.mod --
+module m
+
+go 1.16
+-- m.go --
+package m
+
+import "/"
diff --git a/src/cmd/go/testdata/script/mod_import_meta.txt b/src/cmd/go/testdata/script/mod_import_meta.txt
new file mode 100644
index 0000000..0e469d0
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_import_meta.txt
@@ -0,0 +1,45 @@
+# The loader should not attempt to resolve imports of the "all", "std", and "cmd" meta-packages.
+
+! go list -deps ./importall
+! stderr 'internal error'
+stderr '^importall[/\\]x.go:3:8: "all" is not an importable package; see ''go help packages''$'
+
+! go list -deps ./importcmd
+! stderr 'internal error'
+stderr '^importcmd[/\\]x.go:3:8: "cmd" is not an importable package; see ''go help packages''$'
+
+! go list -deps ./importstd
+! stderr 'internal error'
+stderr '^importstd[/\\]x.go:3:8: "std" is not an importable package; see ''go help packages''$'
+
+
+# Not even if such a path is theoretically provided by a (necessarily replaced) module.
+
+go mod edit -replace std@v0.1.0=./modstd
+go mod edit -require std@v0.1.0
+
+! go list -deps ./importstd
+stderr '^importstd[/\\]x.go:3:8: "std" is not an importable package; see ''go help packages''$'
+
+
+-- go.mod --
+module example.com
+go 1.16
+-- importall/x.go --
+package importall
+
+import _ "all"
+-- importcmd/x.go --
+package importcmd
+
+import _ "cmd"
+-- importstd/x.go --
+package importstd
+
+import _ "std"
+-- modstd/go.mod --
+module std
+go 1.16
+-- modstd/std.go --
+// Package std is an incredibly confusingly-named package.
+package std
diff --git a/src/cmd/go/testdata/script/mod_import_mod.txt b/src/cmd/go/testdata/script/mod_import_mod.txt
new file mode 100644
index 0000000..b035e3d
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_import_mod.txt
@@ -0,0 +1,7 @@
+# Test that GOPATH/pkg/mod is excluded
+env GO111MODULE=off
+! go list mod/foo
+stderr 'disallowed import path'
+
+-- mod/foo/foo.go --
+package foo
diff --git a/src/cmd/go/testdata/script/mod_import_toolchain.txt b/src/cmd/go/testdata/script/mod_import_toolchain.txt
new file mode 100644
index 0000000..42c12c1
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_import_toolchain.txt
@@ -0,0 +1,192 @@
+# This test verifies that 'go get' and 'go mod tidy' switch to a newer toolchain
+# if needed to process newly-reolved imports.
+
+env TESTGO_VERSION=go1.21.0
+env TESTGO_VERSION_SWITCH=switch
+
+cp go.mod go.mod.orig
+
+# tidy reports needing 1.22.0 for b1
+env GOTOOLCHAIN=local
+! go mod tidy
+stderr '^go: example imports\n\texample.net/b: module ./b1 requires go >= 1.22.0 \(running go 1.21.0; GOTOOLCHAIN=local\)$'
+env GOTOOLCHAIN=auto
+go mod tidy
+
+cmp stderr tidy-stderr.want
+cmp go.mod go.mod.tidy
+
+cp go.mod.orig go.mod
+env GOTOOLCHAIN=local
+! go get -v .
+stderr '^go: example.net/b@v0.1.0: module ./b1 requires go >= 1.22.0 \(running go 1.21.0; GOTOOLCHAIN=local\)$'
+env GOTOOLCHAIN=auto
+go get -v .
+cmp stderr get-v-stderr.want
+cmp go.mod go.mod.tidy
+
+cp go.mod.orig go.mod
+env GOTOOLCHAIN=local
+! go get -u -v .
+stderr '^go: example.net/a@v0.2.0: module ./a2 requires go >= 1.22.0 \(running go 1.21.0; GOTOOLCHAIN=local\)$'
+env GOTOOLCHAIN=auto
+go get -u -v .
+cmp stderr get-u-v-stderr.want
+cmp go.mod go.mod.upgraded
+
+-- tidy-stderr.want --
+go: found example.net/b in example.net/b v0.1.0
+go: module ./b1 requires go >= 1.22.0; switching to go1.22.9
+go: found example.net/b in example.net/b v0.1.0
+go: found example.net/c in example.net/c v0.1.0
+-- get-v-stderr.want --
+go: trying upgrade to example.net/b@v0.1.0
+go: module ./b1 requires go >= 1.22.0; switching to go1.22.9
+go: trying upgrade to example.net/b@v0.1.0
+go: accepting indirect upgrade from go@1.20 to 1.22.0
+go: trying upgrade to example.net/c@v0.1.0
+go: upgraded go 1.20 => 1.22.0
+go: added toolchain go1.22.9
+go: added example.net/b v0.1.0
+go: added example.net/c v0.1.0
+go: added example.net/d v0.1.0
+-- get-u-v-stderr.want --
+go: trying upgrade to example.net/a@v0.2.0
+go: trying upgrade to example.net/b@v0.1.0
+go: module ./a2 requires go >= 1.22.0; switching to go1.22.9
+go: trying upgrade to example.net/a@v0.2.0
+go: trying upgrade to example.net/b@v0.1.0
+go: accepting indirect upgrade from go@1.20 to 1.22.0
+go: trying upgrade to example.net/c@v0.1.0
+go: trying upgrade to example.net/d@v0.2.0
+go: module ./d2 requires go >= 1.23.0; switching to go1.23.9
+go: trying upgrade to example.net/a@v0.2.0
+go: trying upgrade to example.net/b@v0.1.0
+go: accepting indirect upgrade from go@1.20 to 1.22.0
+go: trying upgrade to example.net/c@v0.1.0
+go: trying upgrade to example.net/d@v0.2.0
+go: accepting indirect upgrade from go@1.22.0 to 1.23.0
+go: upgraded go 1.20 => 1.23.0
+go: added toolchain go1.23.9
+go: upgraded example.net/a v0.1.0 => v0.2.0
+go: added example.net/b v0.1.0
+go: added example.net/c v0.1.0
+go: added example.net/d v0.2.0
+-- go.mod --
+module example
+
+go 1.20
+
+require example.net/a v0.1.0
+
+replace (
+	example.net/a v0.1.0 => ./a1
+	example.net/a v0.2.0 => ./a2
+	example.net/b v0.1.0 => ./b1
+	example.net/c v0.1.0 => ./c1
+	example.net/d v0.1.0 => ./d1
+	example.net/d v0.2.0 => ./d2
+)
+-- go.mod.tidy --
+module example
+
+go 1.22.0
+
+toolchain go1.22.9
+
+require (
+	example.net/a v0.1.0
+	example.net/b v0.1.0
+)
+
+require (
+	example.net/c v0.1.0 // indirect
+	example.net/d v0.1.0 // indirect
+)
+
+replace (
+	example.net/a v0.1.0 => ./a1
+	example.net/a v0.2.0 => ./a2
+	example.net/b v0.1.0 => ./b1
+	example.net/c v0.1.0 => ./c1
+	example.net/d v0.1.0 => ./d1
+	example.net/d v0.2.0 => ./d2
+)
+-- go.mod.upgraded --
+module example
+
+go 1.23.0
+
+toolchain go1.23.9
+
+require (
+	example.net/a v0.2.0
+	example.net/b v0.1.0
+)
+
+require (
+	example.net/c v0.1.0 // indirect
+	example.net/d v0.2.0 // indirect
+)
+
+replace (
+	example.net/a v0.1.0 => ./a1
+	example.net/a v0.2.0 => ./a2
+	example.net/b v0.1.0 => ./b1
+	example.net/c v0.1.0 => ./c1
+	example.net/d v0.1.0 => ./d1
+	example.net/d v0.2.0 => ./d2
+)
+-- example.go --
+package example
+
+import (
+	_ "example.net/a"
+	_ "example.net/b"
+)
+-- a1/go.mod --
+module example.net/a
+
+go 1.20
+-- a1/a.go --
+package a
+-- a2/go.mod --
+module example.net/a
+
+go 1.22.0
+
+toolchain go1.23.0
+-- a2/a.go --
+package a
+-- b1/go.mod --
+module example.net/b
+
+go 1.22.0
+
+toolchain go1.23.0
+-- b1/b.go --
+package b
+
+import _ "example.net/c"  // Note: module b is intentionally untidy, as if due to a bad git merge
+-- c1/go.mod --
+module example.net/c
+
+go 1.22.0
+
+require example.net/d v0.1.0
+-- c1/c.go --
+package c
+
+import _ "example.net/d"
+-- d1/go.mod --
+module example.net/d
+
+go 1.22.0
+-- d1/d.go --
+package d
+-- d2/go.mod --
+module example.net/d
+
+go 1.23.0
+-- d2/d.go --
+package d
diff --git a/src/cmd/go/testdata/script/mod_import_v1suffix.txt b/src/cmd/go/testdata/script/mod_import_v1suffix.txt
new file mode 100644
index 0000000..75b3374
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_import_v1suffix.txt
@@ -0,0 +1,11 @@
+env GO111MODULE=on
+
+! go get example.com/invalidpath/v1
+! go install .
+
+-- go.mod --
+module example.com
+-- main.go --
+package main
+import _ "example.com/invalidpath/v1"
+func main() {}
diff --git a/src/cmd/go/testdata/script/mod_in_testdata_dir.txt b/src/cmd/go/testdata/script/mod_in_testdata_dir.txt
new file mode 100644
index 0000000..866f784
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_in_testdata_dir.txt
@@ -0,0 +1,45 @@
+# Regression test for golang.org/issue/28481:
+# 'mod tidy' removed dependencies if the module root was
+# within a directory named 'testdata' or '_foo'.
+
+env GO111MODULE=on
+
+# A module should be allowed in a directory named testdata.
+cd $WORK/testdata
+go mod init testdata.tld/foo
+
+# Getting a package within that module should resolve its dependencies.
+go get
+grep 'rsc.io/quote' go.mod
+
+# Tidying the module should preserve those dependencies.
+go mod tidy
+grep 'rsc.io/quote' go.mod
+
+[short] stop
+
+# Vendoring the module's dependencies should work too.
+go mod vendor
+exists vendor/rsc.io/quote
+
+# The same should work in directories with names starting with underscores.
+cd $WORK/_ignored
+go mod init testdata.tld/foo
+
+go get
+grep 'rsc.io/quote' go.mod
+
+go mod tidy
+grep 'rsc.io/quote' go.mod
+
+go mod vendor
+exists vendor/rsc.io/quote
+
+-- $WORK/testdata/main.go --
+package foo
+
+import _ "rsc.io/quote"
+-- $WORK/_ignored/main.go --
+package foo
+
+import _ "rsc.io/quote"
diff --git a/src/cmd/go/testdata/script/mod_indirect.txt b/src/cmd/go/testdata/script/mod_indirect.txt
new file mode 100644
index 0000000..6ea1cae
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_indirect.txt
@@ -0,0 +1,81 @@
+env GO111MODULE=on
+
+# golang.org/issue/31248: required modules imposed by dependency versions
+# older than the selected version must still be taken into account.
+
+env GOFLAGS=-mod=readonly
+
+# Indirect dependencies required via older-than-selected versions must exist in
+# the module graph, but do not need to be listed explicitly in the go.mod file
+# (since they are implied).
+go mod graph
+stdout i@v0.1.0
+
+# The modules must also appear in the build list, not just the graph.
+go list -m all
+stdout '^i v0.1.0'
+
+# The packages provided by those dependencies must resolve.
+go list all
+stdout '^i$'
+
+-- go.mod --
+module main
+
+go 1.13
+
+require (
+	a v0.0.0
+	b v0.0.0
+	c v0.0.0
+)
+
+// Apply replacements so that the test can be self-contained.
+// (It's easier to see all of the modules here than to go
+// rooting around in testdata/mod.)
+replace (
+	a => ./a
+	b => ./b
+	c => ./c
+	x v0.1.0 => ./x1
+	x v0.2.0 => ./x2
+	i => ./i
+)
+-- main.go --
+package main
+
+import (
+	_ "a"
+	_ "b"
+	_ "c"
+)
+
+func main() {}
+-- a/go.mod --
+module a
+go 1.13
+require x v0.1.0
+-- a/a.go --
+package a
+-- b/go.mod --
+module b
+go 1.13
+require x v0.2.0
+-- b/b.go --
+package b
+-- c/go.mod --
+module c
+go 1.13
+-- c/c.go --
+package c
+import _ "i"
+-- x1/go.mod --
+module x
+go1.13
+require i v0.1.0
+-- x2/go.mod --
+module x
+go1.13
+-- i/go.mod --
+-- i/i.go --
+package i
diff --git a/src/cmd/go/testdata/script/mod_indirect_main.txt b/src/cmd/go/testdata/script/mod_indirect_main.txt
new file mode 100644
index 0000000..e84eb9c
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_indirect_main.txt
@@ -0,0 +1,66 @@
+env GO111MODULE=on
+
+# Regression test for golang.org/issue/29773: 'go list -m' was not following
+# dependencies through older versions of the main module.
+
+go list -f '{{with .Module}}{{.Path}}{{with .Version}} {{.}}{{end}}{{end}}' all
+cmp stdout pkgmods.txt
+
+go list -m all
+cmp stdout mods.txt
+
+go mod graph
+cmp stdout graph.txt
+
+-- go.mod --
+module golang.org/issue/root
+
+go 1.12
+
+replace (
+	golang.org/issue/mirror v0.1.0 => ./mirror-v0.1.0
+	golang.org/issue/pkg v0.1.0 => ./pkg-v0.1.0
+	golang.org/issue/root v0.1.0 => ./root-v0.1.0
+)
+
+require golang.org/issue/mirror v0.1.0
+
+-- root.go --
+package root
+
+import _ "golang.org/issue/mirror"
+
+-- mirror-v0.1.0/go.mod --
+module golang.org/issue/mirror
+
+require golang.org/issue/root v0.1.0
+
+-- mirror-v0.1.0/mirror.go --
+package mirror
+
+import _ "golang.org/issue/pkg"
+
+-- pkg-v0.1.0/go.mod --
+module golang.org/issue/pkg
+
+-- pkg-v0.1.0/pkg.go --
+package pkg
+
+-- root-v0.1.0/go.mod --
+module golang.org/issue/root
+
+require golang.org/issue/pkg v0.1.0
+
+-- pkgmods.txt --
+golang.org/issue/mirror v0.1.0
+golang.org/issue/pkg v0.1.0
+golang.org/issue/root
+-- mods.txt --
+golang.org/issue/root
+golang.org/issue/mirror v0.1.0 => ./mirror-v0.1.0
+golang.org/issue/pkg v0.1.0 => ./pkg-v0.1.0
+-- graph.txt --
+golang.org/issue/root go@1.12
+golang.org/issue/root golang.org/issue/mirror@v0.1.0
+golang.org/issue/mirror@v0.1.0 golang.org/issue/root@v0.1.0
+golang.org/issue/root@v0.1.0 golang.org/issue/pkg@v0.1.0
diff --git a/src/cmd/go/testdata/script/mod_indirect_nospace.txt b/src/cmd/go/testdata/script/mod_indirect_nospace.txt
new file mode 100644
index 0000000..f4fb6a8
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_indirect_nospace.txt
@@ -0,0 +1,32 @@
+# https://golang.org/issue/45932: "indirect" comments missing spaces
+# should not be corrupted when the comment is removed.
+
+go mod tidy
+cmp go.mod go.mod.direct
+
+-- go.mod --
+module example.net/m
+
+go 1.16
+
+require example.net/x v0.1.0 //indirect
+
+replace example.net/x v0.1.0 => ./x
+-- go.mod.direct --
+module example.net/m
+
+go 1.16
+
+require example.net/x v0.1.0
+
+replace example.net/x v0.1.0 => ./x
+-- m.go --
+package m
+import _ "example.net/x"
+
+-- x/go.mod --
+module example.net/x
+
+go 1.16
+-- x/x.go --
+package x
diff --git a/src/cmd/go/testdata/script/mod_indirect_tidy.txt b/src/cmd/go/testdata/script/mod_indirect_tidy.txt
new file mode 100644
index 0000000..a12b35c
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_indirect_tidy.txt
@@ -0,0 +1,60 @@
+env GO111MODULE=on
+
+# golang.org/issue/31248: loading the build list must not add explicit entries
+# for indirect dependencies already implied by older-than-selected versions
+# already in the build list.
+
+cp go.mod.orig go.mod
+go mod tidy
+cmp go.mod go.mod.orig
+
+cp go.mod.orig go.mod
+go list -m all
+cmp go.mod go.mod.orig
+
+-- go.mod.orig --
+module main
+
+go 1.13
+
+require a v0.0.0
+
+replace (
+	a v0.0.0 => ./a
+	b v0.0.0 => ./b
+	i v0.0.0 => ./i
+	x v0.1.0 => ./x1
+	x v0.2.0 => ./x2
+)
+-- main.go --
+package main
+
+import _ "a"
+
+func main() {}
+-- a/go.mod --
+module a
+go 1.13
+require (
+	x v0.2.0
+	b v0.0.0
+)
+-- a/a.go --
+package a
+-- b/go.mod --
+module b
+go 1.13
+require x v0.1.0
+-- x1/go.mod --
+module x
+go 1.13
+require (
+	b v0.0.0
+	i v0.0.0
+)
+-- x2/go.mod --
+module x
+go 1.13
+-- i/go.mod --
+module i
+go 1.13
diff --git a/src/cmd/go/testdata/script/mod_init_dep.txt b/src/cmd/go/testdata/script/mod_init_dep.txt
new file mode 100644
index 0000000..76b4867
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_init_dep.txt
@@ -0,0 +1,40 @@
+env GO111MODULE=on
+env GOFLAGS=-mod=mod
+
+# go mod init should populate go.mod from Gopkg.lock
+go mod init x
+stderr 'copying requirements from Gopkg.lock'
+go list -m all
+stdout 'rsc.io/sampler v1.0.0'
+
+# test dep replacement
+cd y
+go mod init
+cmpenv go.mod go.mod.replace
+
+-- x.go --
+package x
+
+-- Gopkg.lock --
+[[projects]]
+  name = "rsc.io/sampler"
+  version = "v1.0.0"
+
+-- y/Gopkg.lock --
+[[projects]]
+  name = "z"
+  revision = "v1.0.0"
+  source = "rsc.io/quote"
+
+-- y/y.go --
+package y // import "y"
+import _ "z"
+
+-- y/go.mod.replace --
+module y
+
+go $goversion
+
+replace z v1.0.0 => rsc.io/quote v1.0.0
+
+require rsc.io/quote v1.0.0
diff --git a/src/cmd/go/testdata/script/mod_init_empty.txt b/src/cmd/go/testdata/script/mod_init_empty.txt
new file mode 100644
index 0000000..d197a79
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_init_empty.txt
@@ -0,0 +1,21 @@
+env GO111MODULE=on
+
+env GOPATH=$WORK${/}invalid-gopath
+
+go list -m
+stdout '^example.com$'
+
+go list
+stdout '^example.com$'
+
+-- go.mod --
+module example.com
+
+go 1.13
+-- main.go --
+package main
+
+func main() {}
+
+-- $WORK/invalid-gopath
+This is a text file, not a directory.
diff --git a/src/cmd/go/testdata/script/mod_init_glide.txt b/src/cmd/go/testdata/script/mod_init_glide.txt
new file mode 100644
index 0000000..0d087eb
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_init_glide.txt
@@ -0,0 +1,35 @@
+[!net:github.com] skip
+[!git] skip
+
+env GO111MODULE=on
+env GOPROXY=direct
+env GOSUMDB=
+
+# Regression test for golang.org/issue/32161:
+# 'go mod init' did not locate tags when resolving a commit to a pseudo-version.
+go mod init x
+cmpenv go.mod go.mod.out
+
+-- main.go --
+package main
+
+import (
+	_ "github.com/rsc/legacytest"
+)
+
+func main() {}
+
+-- glide.lock --
+imports:
+- name: github.com/rsc/legacytest
+  version: fb3c628075e32f7f3c248a3abbdafd69ad6e21e1
+
+-- glide.yaml --
+package: x
+
+-- go.mod.out --
+module x
+
+go $goversion
+
+require github.com/rsc/legacytest v1.1.0-pre.0.20180717164849-fb3c628075e3
diff --git a/src/cmd/go/testdata/script/mod_init_invalid_major.txt b/src/cmd/go/testdata/script/mod_init_invalid_major.txt
new file mode 100644
index 0000000..ae93e70
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_init_invalid_major.txt
@@ -0,0 +1,82 @@
+env GO111MODULE=on
+env GOFLAGS=-mod=mod
+
+! go mod init example.com/user/repo/v0
+stderr '(?s)^go: invalid module path "example.com/user/repo/v0": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v2$'
+
+! go mod init example.com/user/repo/v02
+stderr '(?s)^go: invalid module path "example.com/user/repo/v02": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v2$'
+
+! go mod init example.com/user/repo/v023
+stderr '(?s)^go: invalid module path "example.com/user/repo/v023": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v23$'
+
+! go mod init example.com/user/repo/v1
+stderr '(?s)^go: invalid module path "example.com/user/repo/v1": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v2$'
+
+! go mod init example.com/user/repo/v2.0
+stderr '(?s)^go: invalid module path "example.com/user/repo/v2.0": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v2$'
+
+! go mod init example.com/user/repo/v2.1.4
+stderr '(?s)^go: invalid module path "example.com/user/repo/v2.1.4": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v2$'
+
+! go mod init example.com/user/repo/v3.5
+stderr '(?s)^go: invalid module path "example.com/user/repo/v3.5": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v3$'
+
+! go mod init example.com/user/repo/v4.1.4
+stderr '(?s)^go: invalid module path "example.com/user/repo/v4.1.4": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v4$'
+
+! go mod init example.com/user/repo/v.2.3
+stderr '(?s)^go: invalid module path "example.com/user/repo/v.2.3": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v2$'
+
+! go mod init example.com/user/repo/v.5.3
+stderr '(?s)^go: invalid module path "example.com/user/repo/v.5.3": major version suffixes must be in the form of /vN and are only allowed for v2 or later(.*)go mod init example.com/user/repo/v5$'
+
+! go mod init gopkg.in/pkg
+stderr '(?s)^go: invalid module path "gopkg.in/pkg": module paths beginning with gopkg.in/ must always have a major version suffix in the form of .vN(.*)go mod init gopkg.in/pkg.v1$'
+
+! go mod init gopkg.in/user/pkg
+stderr '(?s)^go: invalid module path "gopkg.in/user/pkg": module paths beginning with gopkg.in/ must always have a major version suffix in the form of .vN(.*)go mod init gopkg.in/user/pkg.v1$'
+
+! go mod init gopkg.in/user/pkg/v0
+stderr '(?s)^go: invalid module path "gopkg.in/user/pkg/v0": module paths beginning with gopkg.in/ must always have a major version suffix in the form of .vN(.*)go mod init gopkg.in/user/pkg.v1$'
+
+! go mod init gopkg.in/user/pkg/v1
+stderr '(?s)^go: invalid module path "gopkg.in/user/pkg/v1": module paths beginning with gopkg.in/ must always have a major version suffix in the form of .vN(.*)go mod init gopkg.in/user/pkg.v1$'
+
+! go mod init gopkg.in/user/pkg/v2
+stderr '(?s)^go: invalid module path "gopkg.in/user/pkg/v2": module paths beginning with gopkg.in/ must always have a major version suffix in the form of .vN(.*)go mod init gopkg.in/user/pkg.v2$'
+
+! go mod init gopkg.in/user/pkg.v
+stderr '(?s)^go: invalid module path "gopkg.in/user/pkg.v": module paths beginning with gopkg.in/ must always have a major version suffix in the form of .vN(.*)go mod init gopkg.in/user/pkg.v1$'
+
+! go mod init gopkg.in/user/pkg.v0.1
+stderr '(?s)^go: invalid module path "gopkg.in/user/pkg.v0.1": module paths beginning with gopkg.in/ must always have a major version suffix in the form of .vN(.*)go mod init gopkg.in/user/pkg.v1$'
+
+! go mod init gopkg.in/user/pkg.v.1
+stderr '(?s)^go: invalid module path "gopkg.in/user/pkg.v.1": module paths beginning with gopkg.in/ must always have a major version suffix in the form of .vN(.*)go mod init gopkg.in/user/pkg.v1$'
+
+! go mod init gopkg.in/user/pkg.v01
+stderr '(?s)^go: invalid module path "gopkg.in/user/pkg.v01": module paths beginning with gopkg.in/ must always have a major version suffix in the form of .vN(.*)go mod init gopkg.in/user/pkg.v1$'
+
+! go mod init gopkg.in/user/pkg.v.2.3
+stderr '(?s)^go: invalid module path "gopkg.in/user/pkg.v.2.3": module paths beginning with gopkg.in/ must always have a major version suffix in the form of .vN(.*)go mod init gopkg.in/user/pkg.v2$'
+
+# module paths with a trailing dot are rejected as invalid import paths
+! go mod init example.com/user/repo/v2.
+stderr '(?s)^go: malformed module path "example.com/user/repo/v2.": trailing dot in path element$'
+
+! go mod init example.com/user/repo/v2..
+stderr '(?s)^go: malformed module path "example.com/user/repo/v2..": trailing dot in path element$'
+
+! go mod init gopkg.in/user/pkg.v.2.
+stderr '(?s)^go: malformed module path "gopkg.in/user/pkg.v.2.": trailing dot in path element$'
+
+! go mod init gopkg.in/user/pkg.v.2..
+stderr '(?s)^go: malformed module path "gopkg.in/user/pkg.v.2..": trailing dot in path element$'
+
+# module paths with spaces are also rejected
+! go mod init 'foo bar'
+stderr '(?s)^go: malformed module path "foo bar": invalid char '' ''$'
+
+! go mod init 'foo  bar baz'
+stderr '(?s)^go: malformed module path "foo  bar baz": invalid char '' ''$'
diff --git a/src/cmd/go/testdata/script/mod_init_path.txt b/src/cmd/go/testdata/script/mod_init_path.txt
new file mode 100644
index 0000000..e5fd4dd
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_init_path.txt
@@ -0,0 +1,20 @@
+env GO111MODULE=on
+
+! go mod init .
+stderr '^go: malformed module path ".": is a local import path$'
+
+cd x
+go mod init example.com/x
+
+cd ../y
+go mod init m
+
+-- x/main.go --
+package main
+
+func main() {}
+
+-- y/main.go --
+package main
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/mod_init_tidy.txt b/src/cmd/go/testdata/script/mod_init_tidy.txt
new file mode 100644
index 0000000..4a52590
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_init_tidy.txt
@@ -0,0 +1,30 @@
+# 'go mod init' should not recommend 'go mod tidy' in an empty directory
+# (one that contains no non-hidden .go files or subdirectories).
+cd empty
+go mod init m
+! stderr tidy
+cd ..
+
+# 'go mod init' should recommend 'go mod tidy' if the directory has a .go file.
+cd pkginroot
+go mod init m
+stderr '^go: to add module requirements and sums:\n\tgo mod tidy$'
+cd ..
+
+# 'go mod init' should recommend 'go mod tidy' if the directory has a
+# subdirectory. We don't walk the tree to see if it has .go files.
+cd subdir
+go mod init m
+stderr '^go: to add module requirements and sums:\n\tgo mod tidy$'
+cd ..
+
+-- empty/empty.txt --
+Not a .go file. Still counts as an empty project.
+-- empty/.hidden/empty.go --
+File in hidden directory. Still as an empty project.
+-- empty/_hidden/empty.go --
+File in hidden directory. Still as an empty project.
+-- pkginroot/hello.go --
+package vendorimport
+-- subdir/sub/empty.txt --
+Subdirectory doesn't need to contain a package.
diff --git a/src/cmd/go/testdata/script/mod_insecure_issue63845.txt b/src/cmd/go/testdata/script/mod_insecure_issue63845.txt
new file mode 100644
index 0000000..c051c05
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_insecure_issue63845.txt
@@ -0,0 +1,28 @@
+# Regression test for https://go.dev/issue/63845:
+# If 'git ls-remote' fails for all secure protocols,
+# we should fail instead of falling back to an arbitrary protocol.
+#
+# Note that this test does not use the local vcweb test server
+# (vcs-test.golang.org), because the hook for redirecting to that
+# server bypasses the "ping to determine protocol" logic
+# in cmd/go/internal/vcs.
+
+[!net:golang.org] skip
+[!git] skip
+[short] skip 'tries to access a nonexistent external Git repo'
+
+env GOPRIVATE=golang.org
+env CURLOPT_TIMEOUT_MS=100
+env GIT_SSH_COMMAND=false
+
+! go get -x golang.org/nonexist.git@latest
+stderr '^git ls-remote https://golang.org/nonexist$'
+stderr '^git ls-remote git\+ssh://golang.org/nonexist'
+stderr '^git ls-remote ssh://golang.org/nonexist$'
+! stderr 'git://'
+stderr '^go: golang.org/nonexist.git@latest: no secure protocol found for repository$'
+
+-- go.mod --
+module example
+
+go 1.19
diff --git a/src/cmd/go/testdata/script/mod_install_hint.txt b/src/cmd/go/testdata/script/mod_install_hint.txt
new file mode 100644
index 0000000..ab02840
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_install_hint.txt
@@ -0,0 +1,5 @@
+# Module is replaced but not required. No hint appears as no module is suggested.

+go mod init m

+go mod edit -replace=github.com/notrequired@v0.5.0=github.com/doesnotexist@v0.5.0

+! go install github.com/notrequired

+! stderr 'to add it:'
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/mod_install_pkg_version.txt b/src/cmd/go/testdata/script/mod_install_pkg_version.txt
new file mode 100644
index 0000000..89dfc14
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_install_pkg_version.txt
@@ -0,0 +1,204 @@
+# 'go install pkg@version' works outside a module.
+env GO111MODULE=auto
+go install example.com/cmd/a@v1.0.0
+exists $GOPATH/bin/a$GOEXE
+rm $GOPATH/bin
+
+
+# 'go install pkg@version' reports an error if modules are disabled.
+env GO111MODULE=off
+! go install example.com/cmd/a@v1.0.0
+stderr '^go: modules disabled by GO111MODULE=off; see ''go help modules''$'
+env GO111MODULE=auto
+
+
+# 'go install pkg@version' ignores go.mod in current directory.
+cd m
+cp go.mod go.mod.orig
+! go list -m all
+stderr '^go: example.com/cmd@v1.1.0-doesnotexist: reading http.*/mod/example.com/cmd/@v/v1.1.0-doesnotexist.info: 404 Not Found\n\tserver response: 404 page not found$'
+stderr '^go: example.com/cmd@v1.1.0-doesnotexist: missing go.sum entry for go.mod file; to add it:\n\tgo mod download example.com/cmd$'
+go install example.com/cmd/a@latest
+cmp go.mod go.mod.orig
+exists $GOPATH/bin/a$GOEXE
+go version -m $GOPATH/bin/a$GOEXE
+stdout '^\tmod\texample.com/cmd\tv1.0.0\t' # "latest", not from go.mod
+rm $GOPATH/bin/a
+cd ..
+
+
+# 'go install -modfile=x.mod pkg@version' reports an error, but only if
+# -modfile is specified explicitly on the command line.
+cd m
+env GOFLAGS=-modfile=go.mod
+go install example.com/cmd/a@latest  # same as above
+env GOFLAGS=
+! go install -modfile=go.mod example.com/cmd/a@latest
+stderr '^go: -modfile cannot be used with commands that ignore the current module$'
+cd ..
+
+
+# Every test case requires linking, so we only cover the most important cases
+# when -short is set.
+[short] stop
+
+
+# 'go install pkg@version' works on a module that doesn't have a go.mod file
+# and with a module whose go.mod file has missing requirements.
+# With a proxy, the two cases are indistinguishable.
+go install rsc.io/fortune@v1.0.0
+stderr '^go: found rsc.io/quote in rsc.io/quote v1.5.2$'
+exists $GOPATH/bin/fortune$GOEXE
+! exists $GOPATH/pkg/mod/rsc.io/fortune@v1.0.0/go.mod # no go.mod file
+go version -m $GOPATH/bin/fortune$GOEXE
+stdout '^\tdep\trsc.io/quote\tv1.5.2\t' # latest version of fortune's dependency
+rm $GOPATH/bin
+
+
+# 'go install dir@version' works like a normal 'go install' command if
+# dir is a relative or absolute path.
+env GO111MODULE=on
+go mod download rsc.io/fortune@v1.0.0
+! go install $GOPATH/pkg/mod/rsc.io/fortune@v1.0.0
+stderr '^go: go\.mod file not found in current directory or any parent directory; see ''go help modules''$'
+! go install ../pkg/mod/rsc.io/fortune@v1.0.0
+stderr '^go: go\.mod file not found in current directory or any parent directory; see ''go help modules''$'
+mkdir tmp
+cd tmp
+go mod init tmp
+go mod edit -require=rsc.io/fortune@v1.0.0
+! go install -mod=readonly $GOPATH/pkg/mod/rsc.io/fortune@v1.0.0
+stderr '^missing go\.sum entry for module providing package rsc\.io/fortune; to add:\n\tgo mod download rsc\.io/fortune$'
+! go install -mod=readonly ../../pkg/mod/rsc.io/fortune@v1.0.0
+stderr '^missing go\.sum entry for module providing package rsc\.io/fortune; to add:\n\tgo mod download rsc\.io/fortune$'
+go get rsc.io/fortune@v1.0.0
+go install -mod=readonly $GOPATH/pkg/mod/rsc.io/fortune@v1.0.0
+exists $GOPATH/bin/fortune$GOEXE
+cd ..
+rm tmp
+rm $GOPATH/bin
+env GO111MODULE=auto
+
+# 'go install pkg@version' reports errors for meta packages, std packages,
+# and directories.
+! go install std@v1.0.0
+stderr '^go: std@v1.0.0: argument must be a package path, not a meta-package$'
+! go install fmt@v1.0.0
+stderr '^go: fmt@v1.0.0: argument must not be a package in the standard library$'
+! go install example.com//cmd/a@v1.0.0
+stderr '^go: example.com//cmd/a@v1.0.0: argument must be a clean package path$'
+! go install example.com/cmd/a@v1.0.0 ./x@v1.0.0
+stderr '^go: ./x@v1.0.0: argument must be a package path, not a relative path$'
+! go install example.com/cmd/a@v1.0.0 $GOPATH/src/x@v1.0.0
+stderr '^go: '$WORK'[/\\]gopath/src/x@v1.0.0: argument must be a package path, not an absolute path$'
+! go install example.com/cmd/a@v1.0.0 cmd/...@v1.0.0
+stderr '^package cmd/go not provided by module example.com/cmd@v1.0.0$'
+
+# 'go install pkg@version' should accept multiple arguments but report an error
+# if the version suffixes are different, even if they refer to the same version.
+go install example.com/cmd/a@v1.0.0 example.com/cmd/b@v1.0.0
+exists $GOPATH/bin/a$GOEXE
+exists $GOPATH/bin/b$GOEXE
+rm $GOPATH/bin
+
+env GO111MODULE=on
+go list -m example.com/cmd@latest
+stdout '^example.com/cmd v1.0.0$'
+env GO111MODULE=auto
+
+! go install example.com/cmd/a@v1.0.0 example.com/cmd/b@latest
+stderr '^go: example.com/cmd/b@latest: all arguments must refer to packages in the same module at the same version \(@v1.0.0\)$'
+
+
+# 'go install pkg@version' should report an error if the arguments are in
+# different modules.
+! go install example.com/cmd/a@v1.0.0 rsc.io/fortune@v1.0.0
+stderr '^package rsc.io/fortune provided by module rsc.io/fortune@v1.0.0\n\tAll packages must be provided by the same module \(example.com/cmd@v1.0.0\).$'
+
+
+# 'go install pkg@version' should report an error if an argument is not
+# a main package.
+! go install example.com/cmd/a@v1.0.0 example.com/cmd/err@v1.0.0
+stderr '^package example.com/cmd/err is not a main package$'
+
+# Wildcards should match only main packages. This module has a non-main package
+# with an error, so we'll know if that gets built.
+mkdir tmp
+cd tmp
+go mod init m
+go get example.com/cmd@v1.0.0
+! go build example.com/cmd/...
+stderr 'err[/\\]err.go:3:9: undefined: DoesNotCompile( .*)?$'
+cd ..
+
+go install example.com/cmd/...@v1.0.0
+exists $GOPATH/bin/a$GOEXE
+exists $GOPATH/bin/b$GOEXE
+rm $GOPATH/bin
+
+# If a wildcard matches no packages, we should see a warning.
+! go install example.com/cmd/nomatch...@v1.0.0
+stderr '^go: example.com/cmd/nomatch\.\.\.@v1.0.0: module example.com/cmd@v1.0.0 found, but does not contain packages matching example.com/cmd/nomatch\.\.\.$'
+go install example.com/cmd/a@v1.0.0 example.com/cmd/nomatch...@v1.0.0
+stderr '^go: warning: "example.com/cmd/nomatch\.\.\." matched no packages$'
+
+# If a wildcard matches only non-main packages, we should see a different warning.
+go install example.com/cmd/err...@v1.0.0
+stderr '^go: warning: "example.com/cmd/err\.\.\." matched only non-main packages$'
+
+
+# 'go install pkg@version' should report errors if the module contains
+# replace or exclude directives.
+go mod download example.com/cmd@v1.0.0-replace
+! go install example.com/cmd/a@v1.0.0-replace
+cmp stderr replace-err
+
+go mod download example.com/cmd@v1.0.0-exclude
+! go install example.com/cmd/a@v1.0.0-exclude
+cmp stderr exclude-err
+
+# 'go install pkg@version' should report an error if the module requires a
+# higher version of itself.
+! go install example.com/cmd/a@v1.0.0-newerself
+stderr '^go: example.com/cmd/a@v1.0.0-newerself: version constraints conflict:\n\texample.com/cmd@v1.0.0-newerself requires example.com/cmd@v1.0.0, but v1.0.0-newerself is requested$'
+
+
+# 'go install pkg@version' will only match a retracted version if it's
+# explicitly requested.
+env GO111MODULE=on
+go list -m -versions example.com/cmd
+! stdout v1.9.0
+go list -m -versions -retracted example.com/cmd
+stdout v1.9.0
+go install example.com/cmd/a@latest
+go version -m $GOPATH/bin/a$GOEXE
+stdout '^\tmod\texample.com/cmd\tv1.0.0\t'
+go install example.com/cmd/a@v1.9.0
+go version -m $GOPATH/bin/a$GOEXE
+stdout '^\tmod\texample.com/cmd\tv1.9.0\t'
+env GO111MODULE=
+
+# 'go install pkg@version' succeeds when -mod=readonly is set explicitly.
+# Verifies #43278.
+go install -mod=readonly example.com/cmd/a@v1.0.0
+
+-- m/go.mod --
+module m
+
+go 1.16
+
+require example.com/cmd v1.1.0-doesnotexist
+-- x/x.go --
+package main
+
+func main() {}
+-- replace-err --
+go: example.com/cmd/a@v1.0.0-replace (in example.com/cmd@v1.0.0-replace):
+	The go.mod file for the module providing named packages contains one or
+	more replace directives. It must not contain directives that would cause
+	it to be interpreted differently than if it were the main module.
+-- exclude-err --
+go: example.com/cmd/a@v1.0.0-exclude (in example.com/cmd@v1.0.0-exclude):
+	The go.mod file for the module providing named packages contains one or
+	more exclude directives. It must not contain directives that would cause
+	it to be interpreted differently than if it were the main module.
diff --git a/src/cmd/go/testdata/script/mod_install_versioned.txt b/src/cmd/go/testdata/script/mod_install_versioned.txt
new file mode 100644
index 0000000..627a9a8
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_install_versioned.txt
@@ -0,0 +1,14 @@
+env GO111MODULE=on
+
+go get rsc.io/fortune
+go list -f '{{.Target}}' rsc.io/fortune
+! stdout fortune@v1
+stdout 'fortune(\.exe)?$'
+
+go get rsc.io/fortune/v2
+go list -f '{{.Target}}' rsc.io/fortune/v2
+! stdout v2
+stdout 'fortune(\.exe)?$'
+
+-- go.mod --
+module m
diff --git a/src/cmd/go/testdata/script/mod_internal.txt b/src/cmd/go/testdata/script/mod_internal.txt
new file mode 100644
index 0000000..787b21f
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_internal.txt
@@ -0,0 +1,96 @@
+env GO111MODULE=on
+[short] skip
+
+# golang.org/x/internal should be importable from other golang.org/x modules.
+go mod edit -module=golang.org/x/anything
+go get .
+
+# ...and their tests...
+go test
+stdout PASS
+
+# ...but that should not leak into other modules.
+go get ./baddep
+! go build ./baddep
+stderr golang.org[/\\]notx[/\\]useinternal
+stderr 'use of internal package golang.org/x/.* not allowed'
+
+# Internal packages in the standard library should not leak into modules.
+go get ./fromstd
+! go build ./fromstd
+stderr 'use of internal package internal/testenv not allowed'
+
+# Dependencies should be able to use their own internal modules...
+go mod edit -module=golang.org/notx
+go get ./throughdep
+
+# ... but other modules should not, even if they have transitive dependencies.
+go get .
+! go build .
+stderr 'use of internal package golang.org/x/.* not allowed'
+
+# And transitive dependencies still should not leak.
+go get ./baddep
+! go build ./baddep
+stderr golang.org[/\\]notx[/\\]useinternal
+stderr 'use of internal package golang.org/x/.* not allowed'
+
+# Replacing an internal module should keep it internal to the same paths.
+go mod edit -module=golang.org/notx
+go mod edit -replace golang.org/x/internal=./replace/golang.org/notx/internal
+go get ./throughdep
+
+go get ./baddep
+! go build ./baddep
+stderr golang.org[/\\]notx[/\\]useinternal
+stderr 'use of internal package golang.org/x/.* not allowed'
+
+go mod edit -replace golang.org/x/internal=./vendor/golang.org/x/internal
+go get ./throughdep
+
+go get ./baddep
+! go build ./baddep
+stderr golang.org[/\\]notx[/\\]useinternal
+stderr 'use of internal package golang.org/x/.* not allowed'
+
+-- go.mod --
+module TBD
+go 1.12
+-- useinternal.go --
+package useinternal
+import _ "golang.org/x/internal/subtle"
+
+-- useinternal_test.go --
+package useinternal_test
+import (
+	"testing"
+	_ "golang.org/x/internal/subtle"
+)
+
+func Test(*testing.T) {}
+
+-- throughdep/useinternal.go --
+package throughdep
+import _ "golang.org/x/useinternal"
+
+-- baddep/useinternal.go --
+package baddep
+import _ "golang.org/notx/useinternal"
+
+-- fromstd/useinternal.go --
+package fromstd
+import _ "internal/testenv"
+
+-- replace/golang.org/notx/internal/go.mod --
+module golang.org/x/internal
+
+-- replace/golang.org/notx/internal/subtle/subtle.go --
+package subtle
+// Ha ha! Nothing here!
+
+-- vendor/golang.org/x/internal/go.mod --
+module golang.org/x/internal
+
+-- vendor/golang.org/x/internal/subtle/subtle.go --
+package subtle
+// Ha ha! Nothing here!
diff --git a/src/cmd/go/testdata/script/mod_invalid_path.txt b/src/cmd/go/testdata/script/mod_invalid_path.txt
new file mode 100644
index 0000000..975de5e
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_invalid_path.txt
@@ -0,0 +1,61 @@
+# Test that mod files with invalid or missing paths produce an error.
+
+# Test that go list fails on a go.mod with no module declaration.
+cd $WORK/gopath/src/mod
+! go list .
+stderr '^go: error reading go.mod: missing module declaration. To specify the module path:\n\tgo mod edit -module=example.com/mod$'
+
+# Test that go mod init in GOPATH doesn't add a module declaration
+# with a path that can't possibly be a module path, because
+# it isn't even a valid import path.
+# The single quote and backtick are the only characters which are not allowed
+# but are a valid Windows file name.
+cd $WORK/'gopath/src/m''d'
+! go mod init
+stderr 'cannot determine module path'
+
+# Test that a go.mod file is rejected when its module declaration has a path that can't
+# possibly be a module path, because it isn't even a valid import path
+cd $WORK/gopath/src/badname
+! go list .
+stderr 'malformed module path'
+
+# Test that an import path containing an element with a leading dot is valid,
+# but such a module path is not.
+# Verifies #43985.
+cd $WORK/gopath/src/dotname
+go list ./.dot
+stdout '^example.com/dotname/.dot$'
+go list ./use
+stdout '^example.com/dotname/use$'
+! go list -m example.com/dotname/.dot@latest
+stderr '^go: example.com/dotname/.dot@latest: malformed module path "example.com/dotname/.dot": leading dot in path element$'
+go get example.com/dotname/.dot
+go get example.com/dotname/use
+go mod tidy
+
+-- mod/go.mod --
+
+-- mod/foo.go --
+package foo
+
+-- m'd/foo.go --
+package mad
+
+-- badname/go.mod --
+
+module .\.
+
+-- badname/foo.go --
+package badname
+
+-- dotname/go.mod --
+module example.com/dotname
+
+go 1.16
+-- dotname/.dot/dot.go --
+package dot
+-- dotname/use/use.go --
+package use
+
+import _ "example.com/dotname/.dot"
diff --git a/src/cmd/go/testdata/script/mod_invalid_path_dotname.txt b/src/cmd/go/testdata/script/mod_invalid_path_dotname.txt
new file mode 100644
index 0000000..484c208
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_invalid_path_dotname.txt
@@ -0,0 +1,46 @@
+# Test that an import path containing an element with a leading dot
+# in another module is valid.
+
+# 'go get' works with no version query.
+cp go.mod.empty go.mod
+go get example.com/dotname/.dot
+go list -m example.com/dotname
+stdout '^example.com/dotname v1.0.0$'
+
+# 'go get' works with a version query.
+cp go.mod.empty go.mod
+go get example.com/dotname/.dot@latest
+go list -m example.com/dotname
+stdout '^example.com/dotname v1.0.0$'
+
+# 'go get' works on an importing package.
+cp go.mod.empty go.mod
+go get .
+go list -m example.com/dotname
+stdout '^example.com/dotname v1.0.0$'
+
+# 'go list' works on the dotted package.
+go list example.com/dotname/.dot
+stdout '^example.com/dotname/.dot$'
+
+# 'go list' works on an importing package.
+go list .
+stdout '^m$'
+
+# 'go mod tidy' works.
+cp go.mod.empty go.mod
+go mod tidy
+go list -m example.com/dotname
+stdout '^example.com/dotname v1.0.0$'
+
+-- go.mod.empty --
+module m
+
+go 1.16
+-- go.sum --
+example.com/dotname v1.0.0 h1:Q0JMAn464CnwFVCshs1n4+f5EFiW/eRhnx/fTWjw2Ag=
+example.com/dotname v1.0.0/go.mod h1:7K4VLT7QylRI8H7yZwUkeDH2s19wQnyfp/3oBlItWJ0=
+-- use.go --
+package use
+
+import _ "example.com/dotname/.dot"
diff --git a/src/cmd/go/testdata/script/mod_invalid_path_plus.txt b/src/cmd/go/testdata/script/mod_invalid_path_plus.txt
new file mode 100644
index 0000000..dd59eb1
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_invalid_path_plus.txt
@@ -0,0 +1,36 @@
+# https://golang.org/issue/44776
+# The '+' character should be disallowed in module paths, but allowed in package
+# paths within valid modules.
+
+# 'go list' accepts package paths with pluses.
+cp go.mod.orig go.mod
+go get example.net/cmd
+go list example.net/cmd/x++
+
+# 'go list -m' rejects module paths with pluses.
+! go list -versions -m 'example.net/bad++'
+stderr '^go: malformed module path "example.net/bad\+\+": invalid char ''\+''$'
+
+# 'go get' accepts package paths with pluses.
+cp go.mod.orig go.mod
+go get example.net/cmd/x++
+go list -m example.net/cmd
+stdout '^example.net/cmd v0.0.0-00010101000000-000000000000 => ./cmd$'
+
+-- go.mod.orig --
+module example.com/m
+
+go 1.16
+
+replace (
+	example.net/cmd => ./cmd
+)
+
+-- cmd/go.mod --
+module example.net/cmd
+
+go 1.16
+-- cmd/x++/main.go --
+package main
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/mod_invalid_version.txt b/src/cmd/go/testdata/script/mod_invalid_version.txt
new file mode 100644
index 0000000..a0427b3
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_invalid_version.txt
@@ -0,0 +1,253 @@
+[!net:golang.org] skip
+[!git] skip
+
+env GO111MODULE=on
+env GOPROXY=direct
+env GOSUMDB=off
+env GOFLAGS=-mod=mod
+
+# Regression test for golang.org/issue/27173: if the user (or go.mod file)
+# requests a pseudo-version that does not match both the module path and commit
+# metadata, reject it with a helpful error message.
+#
+# TODO(bcmills): Replace the github.com/pierrec/lz4 examples with something
+# equivalent on vcs-test.golang.org.
+
+# An incomplete commit hash is not a valid semantic version,
+# but can appear in the main go.mod file anyway and should be resolved.
+cp go.mod.orig go.mod
+go mod edit -require golang.org/x/text@14c0d48ead0c
+cd outside
+! go list -m golang.org/x/text
+stderr 'go: example.com@v0.0.0 \(replaced by \./\.\.\): parsing ..[/\\]go.mod: '$WORK'[/\\]gopath[/\\]src[/\\]go.mod:5: require golang.org/x/text: version "14c0d48ead0c" invalid: must be of the form v1.2.3'
+cd ..
+go list -m golang.org/x/text
+stdout 'golang.org/x/text v0.1.1-0.20170915032832-14c0d48ead0c'
+grep 'golang.org/x/text v0.1.1-0.20170915032832-14c0d48ead0c' go.mod
+
+# A module path below the repo root that does not contain a go.mod file is invalid.
+cp go.mod.orig go.mod
+go mod edit -require golang.org/x/text/unicode@v0.0.0-20170915032832-14c0d48ead0c
+cd outside
+! go list -m golang.org/x/text
+stderr 'go: example.com@v0.0.0 requires\n\tgolang.org/x/text/unicode@v0.0.0-20170915032832-14c0d48ead0c: invalid version: missing golang.org/x/text/unicode/go.mod at revision 14c0d48ead0c'
+cd ..
+! go list -m golang.org/x/text
+stderr 'golang.org/x/text/unicode@v0.0.0-20170915032832-14c0d48ead0c: invalid version: missing golang.org/x/text/unicode/go.mod at revision 14c0d48ead0c'
+
+# However, arguments to 'go get' can name packages above the root.
+cp go.mod.orig go.mod
+go get golang.org/x/text/unicode@v0.0.0-20170915032832-14c0d48ead0c
+go list -m golang.org/x/text/...
+stdout 'golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c'
+! stdout 'golang.org/x/text/unicode'
+
+# A major version that does not match the module path is invalid.
+cp go.mod.orig go.mod
+go mod edit -require golang.org/x/text@v2.1.1-0.20170915032832-14c0d48ead0c
+cd outside
+! go list -m golang.org/x/text
+stderr 'go: example.com@v0.0.0 \(replaced by \./\.\.\): parsing ..[/\\]go.mod: '$WORK'[/\\]gopath[/\\]src[/\\]go.mod:5: require golang.org/x/text: version "v2.1.1-0.20170915032832-14c0d48ead0c" invalid: should be v0 or v1, not v2'
+cd ..
+! go list -m golang.org/x/text
+stderr '^go.mod:5: require golang.org/x/text: version "v2.1.1-0.20170915032832-14c0d48ead0c" invalid: should be v0 or v1, not v2'
+
+# A pseudo-version with fewer than 12 digits of SHA-1 prefix is invalid.
+cp go.mod.orig go.mod
+go mod edit -require golang.org/x/text@v0.1.1-0.20170915032832-14c0d48ead0
+cd outside
+! go list -m golang.org/x/text
+stderr 'go: example.com@v0.0.0 requires\n\tgolang.org/x/text@v0.1.1-0.20170915032832-14c0d48ead0: invalid pseudo-version: revision is shorter than canonical \(expected 14c0d48ead0c\)'
+cd ..
+! go list -m golang.org/x/text
+stderr 'golang.org/x/text@v0.1.1-0.20170915032832-14c0d48ead0: invalid pseudo-version: revision is shorter than canonical \(expected 14c0d48ead0c\)'
+
+# A pseudo-version with more than 12 digits of SHA-1 prefix is invalid.
+cp go.mod.orig go.mod
+go mod edit -require golang.org/x/text@v0.1.1-0.20170915032832-14c0d48ead0cd47e3104ada247d91be04afc7a5a
+cd outside
+! go list -m golang.org/x/text
+stderr 'go: example.com@v0.0.0 requires\n\tgolang.org/x/text@v0.1.1-0.20170915032832-14c0d48ead0cd47e3104ada247d91be04afc7a5a: invalid pseudo-version: revision is longer than canonical \(expected 14c0d48ead0c\)'
+cd ..
+! go list -m golang.org/x/text
+stderr 'golang.org/x/text@v0.1.1-0.20170915032832-14c0d48ead0cd47e3104ada247d91be04afc7a5a: invalid pseudo-version: revision is longer than canonical \(expected 14c0d48ead0c\)'
+
+# A pseudo-version that does not match the commit timestamp is invalid.
+cp go.mod.orig go.mod
+go mod edit -require golang.org/x/text@v0.1.1-0.20190915032832-14c0d48ead0c
+cd outside
+! go list -m golang.org/x/text
+stderr 'go: example.com@v0.0.0 requires\n\tgolang.org/x/text@v0.1.1-0.20190915032832-14c0d48ead0c: invalid pseudo-version: does not match version-control timestamp \(expected 20170915032832\)'
+cd ..
+! go list -m golang.org/x/text
+stderr 'golang.org/x/text@v0.1.1-0.20190915032832-14c0d48ead0c: invalid pseudo-version: does not match version-control timestamp \(expected 20170915032832\)'
+
+# A 'replace' directive in the main module can replace an invalid timestamp
+# with a valid one.
+go mod edit -replace golang.org/x/text@v0.1.1-0.20190915032832-14c0d48ead0c=golang.org/x/text@14c0d48ead0c
+cd outside
+! go list -m golang.org/x/text
+stderr 'go: example.com@v0.0.0 requires\n\tgolang.org/x/text@v0.1.1-0.20190915032832-14c0d48ead0c: invalid pseudo-version: does not match version-control timestamp \(expected 20170915032832\)'
+cd ..
+go list -m golang.org/x/text
+stdout 'golang.org/x/text v0.1.1-0.20190915032832-14c0d48ead0c => golang.org/x/text v0.1.1-0.20170915032832-14c0d48ead0c'
+
+# A pseudo-version that is not derived from a tag is invalid.
+cp go.mod.orig go.mod
+go mod edit -require golang.org/x/text@v1.999.999-0.20170915032832-14c0d48ead0c
+cd outside
+! go list -m golang.org/x/text
+stderr 'go: example.com@v0.0.0 requires\n\tgolang.org/x/text@v1.999.999-0.20170915032832-14c0d48ead0c: invalid pseudo-version: preceding tag \(v1.999.998\) not found'
+cd ..
+! go list -m golang.org/x/text
+stderr 'golang.org/x/text@v1.999.999-0.20170915032832-14c0d48ead0c: invalid pseudo-version: preceding tag \(v1.999.998\) not found'
+
+# A v1.0.0- pseudo-version that is not derived from a tag is invalid:
+# v1.0.0- implies no tag, but the correct no-tag prefix for a module path
+# without a major-version suffix is v0.0.0-.
+cp go.mod.orig go.mod
+go mod edit -require golang.org/x/text@v1.0.0-20170915032832-14c0d48ead0c
+cd outside
+! go list -m golang.org/x/text
+stderr 'go: example.com@v0.0.0 requires\n\tgolang.org/x/text@v1.0.0-20170915032832-14c0d48ead0c: invalid pseudo-version: major version without preceding tag must be v0, not v1'
+cd ..
+! go list -m golang.org/x/text
+stderr 'golang.org/x/text@v1.0.0-20170915032832-14c0d48ead0c: invalid pseudo-version: major version without preceding tag must be v0, not v1'
+
+# A pseudo-version vX.Y.Z+1 cannot have Z+1 == 0, since that would
+# imply a base tag with a negative patch field.
+cp go.mod.orig go.mod
+go mod edit -require golang.org/x/text@v0.0.0-0.20170915032832-14c0d48ead0c
+cd outside
+! go list -m golang.org/x/text
+stderr 'go: example.com@v0.0.0 requires\n\tgolang.org/x/text@v0.0.0-0.20170915032832-14c0d48ead0c: invalid pseudo-version: version before v0.0.0 would have negative patch number'
+cd ..
+! go list -m golang.org/x/text
+stderr 'golang.org/x/text@v0.0.0-0.20170915032832-14c0d48ead0c: invalid pseudo-version: version before v0.0.0 would have negative patch number'
+
+# A 'replace' directive in the main module can replace an
+# invalid pseudo-version base with a valid one.
+go mod edit -replace golang.org/x/text@v0.0.0-0.20170915032832-14c0d48ead0c=golang.org/x/text@v0.0.0-20170915032832-14c0d48ead0c
+cd outside
+! go list -m golang.org/x/text
+stderr 'go: example.com@v0.0.0 requires\n\tgolang.org/x/text@v0.0.0-0.20170915032832-14c0d48ead0c: invalid pseudo-version: version before v0.0.0 would have negative patch number'
+cd ..
+go list -m golang.org/x/text
+stdout 'golang.org/x/text v0.0.0-0.20170915032832-14c0d48ead0c => golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c'
+
+# A 'replace' directive can replace an invalid 'latest' version, and
+# should suppress errors for that version in 'go get -u'
+cp go.mod.orig go.mod
+go mod edit -require golang.org/x/text@v1.999999.0
+go mod edit -replace golang.org/x/text@v1.999999.0=golang.org/x/text@v0.0.0-20170915032832-14c0d48ead0c
+cd outside
+! go get golang.org/x/text@upgrade
+stderr 'go: example.com@v0.0.0 requires\n\tgolang.org/x/text@v1.999999.0: reading golang.org/x/text/go.mod at revision v1.999999.0: unknown revision v1.999999.0'
+cd ..
+go get golang.org/x/text@upgrade
+go list -m golang.org/x/text
+stdout 'golang.org/x/text v1.999999.0 => golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c'
+
+# A pseudo-version derived from a non-ancestor tag is invalid.
+cp go.mod.orig go.mod
+go mod edit -require golang.org/x/text@v0.2.1-0.20170915032832-14c0d48ead0c
+cd outside
+! go list -m golang.org/x/text
+stderr 'go: example.com@v0.0.0 requires\n\tgolang.org/x/text@v0.2.1-0.20170915032832-14c0d48ead0c: invalid pseudo-version: revision 14c0d48ead0c is not a descendent of preceding tag \(v0.2.0\)'
+cd ..
+! go list -m golang.org/x/text
+stderr 'golang.org/x/text@v0.2.1-0.20170915032832-14c0d48ead0c: invalid pseudo-version: revision 14c0d48ead0c is not a descendent of preceding tag \(v0.2.0\)'
+
+# A pseudo-version derived from a canonical tag on the same revision is invalid.
+cp go.mod.orig go.mod
+go mod edit -require golang.org/x/text@v0.2.1-0.20171213102548-c4d099d611ac
+cd outside
+! go list -m golang.org/x/text
+stderr 'go: example.com@v0.0.0 requires\n\tgolang.org/x/text@v0.2.1-0.20171213102548-c4d099d611ac: invalid pseudo-version: tag \(v0.2.0\) found on revision c4d099d611ac is already canonical, so should not be replaced with a pseudo-version derived from that tag'
+cd ..
+! go list -m golang.org/x/text
+stderr 'golang.org/x/text@v0.2.1-0.20171213102548-c4d099d611ac: invalid pseudo-version: tag \(v0.2.0\) found on revision c4d099d611ac is already canonical, so should not be replaced with a pseudo-version derived from that tag'
+
+# A +incompatible suffix is not allowed on a version that is actually compatible.
+cp go.mod.orig go.mod
+go mod edit -require golang.org/x/text@v0.1.1-0.20170915032832-14c0d48ead0c+incompatible
+cd outside
+! go list -m golang.org/x/text
+stderr 'go: example.com@v0.0.0 requires\n\tgolang.org/x/text@v0.1.1-0.20170915032832-14c0d48ead0c\+incompatible: invalid version: \+incompatible suffix not allowed: major version v0 is compatible'
+cd ..
+! go list -m golang.org/x/text
+stderr 'golang.org/x/text@v0.1.1-0.20170915032832-14c0d48ead0c\+incompatible: invalid version: \+incompatible suffix not allowed: major version v0 is compatible'
+
+[!net:github.com] stop
+
+# The pseudo-version for a commit after a tag with a non-matching major version
+# should instead be based on the last matching tag.
+cp go.mod.orig go.mod
+go mod edit -require github.com/pierrec/lz4@473cd7ce01a1
+go list -m github.com/pierrec/lz4
+stdout 'github.com/pierrec/lz4 v1.0.2-0.20190131084431-473cd7ce01a1'
+cd outside
+go list -m github.com/pierrec/lz4
+stdout 'github.com/pierrec/lz4 v1.0.2-0.20190131084431-473cd7ce01a1'
+cd ..
+
+# A +incompatible pseudo-version for a module that has an explicit go.mod file is invalid.
+cp go.mod.orig go.mod
+go mod edit -require github.com/pierrec/lz4@v2.0.9-0.20190209155647-9a39efadad3d+incompatible
+cd outside
+! go list -m github.com/pierrec/lz4
+stderr '^go: example.com@v0.0.0 requires\n\tgithub.com/pierrec/lz4@v2.0.9-0.20190209155647-9a39efadad3d\+incompatible: invalid version: module contains a go.mod file, so module path must match major version \("github.com/pierrec/lz4/v2"\)$'
+cd ..
+! go list -m github.com/pierrec/lz4
+stderr '^go: github.com/pierrec/lz4@v2.0.9-0.20190209155647-9a39efadad3d\+incompatible: invalid version: module contains a go.mod file, so module path must match major version \("github.com/pierrec/lz4/v2"\)$'
+
+# A +incompatible pseudo-version is valid for a revision of the module
+# that lacks a go.mod file.
+cp go.mod.orig go.mod
+go mod edit -require github.com/pierrec/lz4@v2.0.4-0.20180826165652-dbe9298ce099+incompatible
+cd outside
+go list -m github.com/pierrec/lz4
+stdout 'github.com/pierrec/lz4 v2.0.4-0.20180826165652-dbe9298ce099\+incompatible'
+cd ..
+go list -m github.com/pierrec/lz4
+stdout 'github.com/pierrec/lz4 v2.0.4-0.20180826165652-dbe9298ce099\+incompatible'
+
+# 'go get' for a mismatched major version without a go.mod file should resolve
+# to the equivalent +incompatible version, not a pseudo-version with a different
+# major version.
+cp go.mod.orig go.mod
+go get github.com/pierrec/lz4@v2.0.5
+go list -m github.com/pierrec/lz4
+stdout 'github.com/pierrec/lz4 v2.0.5\+incompatible'
+
+# 'go get' for a mismatched major version with a go.mod file should error out,
+# not resolve to a pseudo-version with a different major version.
+cp go.mod.orig go.mod
+! go get github.com/pierrec/lz4@v2.0.8
+stderr 'go: github.com/pierrec/lz4@v2.0.8: invalid version: module contains a go.mod file, so module path must match major version \("github.com/pierrec/lz4/v2"\)$'
+
+# An invalid +incompatible suffix for a canonical version should error out,
+# not resolve to a pseudo-version.
+#
+# TODO(bcmills): The "outside" view for this failure mode is missing its import stack.
+# Figure out why and fix it.
+cp go.mod.orig go.mod
+go mod edit -require github.com/pierrec/lz4@v2.0.8+incompatible
+cd outside
+! go list -m github.com/pierrec/lz4
+stderr '^go: github.com/pierrec/lz4@v2.0.8\+incompatible: invalid version: module contains a go.mod file, so module path must match major version \("github.com/pierrec/lz4/v2"\)$'
+cd ..
+! go list -m github.com/pierrec/lz4
+stderr '^go: github.com/pierrec/lz4@v2.0.8\+incompatible: invalid version: module contains a go.mod file, so module path must match major version \("github.com/pierrec/lz4/v2"\)$'
+
+-- go.mod.orig --
+module example.com
+
+go 1.13
+-- outside/go.mod --
+module example.com/outside
+
+go 1.13
+
+require example.com v0.0.0
+replace example.com v0.0.0 => ./..
diff --git a/src/cmd/go/testdata/script/mod_issue35270.txt b/src/cmd/go/testdata/script/mod_issue35270.txt
new file mode 100644
index 0000000..27b9226
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_issue35270.txt
@@ -0,0 +1,57 @@
+
+cd a
+! go build
+stderr '^ambiguous import: found package image in multiple modules:\s+image\s+.+\s.+image.+\s$'
+
+
+cd ../b
+! go build -mod=vendor
+stderr '^main.go:4:5: ambiguous import: found package image in multiple directories:\s+.+image\s+.+image\s+$'
+
+cd ../c
+! go build -mod=vendor
+stderr 'main.go:4:5: package p is not in std'
+
+-- a/go.mod --
+module image
+
+-- a/main.go --
+package main
+
+func main() {
+    println("hello world!")
+}
+
+-- b/go.mod --
+module test
+
+-- b/vendor/image/b.go --
+package image
+func Add(a, b int) int {
+    return a + b
+}
+
+-- b/main.go --
+package main
+
+import (
+    "image"
+)
+
+func main() {
+    println(image.Add(1,1))
+}
+
+-- c/go.mod --
+module test
+
+-- c/main.go --
+package main
+
+import (
+    "p"
+)
+
+func main() {
+    println(p.Add(1,1))
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/mod_issue35317.txt b/src/cmd/go/testdata/script/mod_issue35317.txt
new file mode 100644
index 0000000..92416a5
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_issue35317.txt
@@ -0,0 +1,8 @@
+# Regression test for golang.org/issue/35317:
+# 'go get' with multiple module-only arguments was racy.
+
+env GO111MODULE=on
+[short] skip
+
+go mod init example.com
+go get golang.org/x/text@v0.3.0 golang.org/x/internal@v0.1.0 golang.org/x/exp@none
diff --git a/src/cmd/go/testdata/script/mod_lazy_consistency.txt b/src/cmd/go/testdata/script/mod_lazy_consistency.txt
new file mode 100644
index 0000000..1bf3e31
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_lazy_consistency.txt
@@ -0,0 +1,95 @@
+# If the root requirements in a lazy module are inconsistent
+# (for example, due to a bad hand-edit or git merge),
+# they can go unnoticed as long as the module with the violated
+# requirement is not used.
+# When we load a package from that module, we should spot-check its
+# requirements and either emit an error or update the go.mod file.
+
+cp go.mod go.mod.orig
+
+
+# If we load package x from x.1, we only check the requirements of x,
+# which are fine: loading succeeds.
+
+go list -deps ./usex
+stdout '^example.net/x$'
+cmp go.mod go.mod.orig
+
+
+# However, if we load needx2, we should load the requirements of needx2.
+# Those requirements indicate x.2, not x.1, so the module graph is
+# inconsistent and needs to be fixed.
+
+! go list -deps ./useneedx2
+stderr '^go: updates to go.mod needed; to update it:\n\tgo mod tidy$'
+
+! go list -deps example.net/needx2
+stderr '^go: updates to go.mod needed; to update it:\n\tgo mod tidy$'
+
+
+# The command printed in the error message should fix the problem.
+
+go mod tidy
+go list -deps ./useneedx2
+stdout '^example.net/m/useneedx2$'
+stdout '^example.net/needx2$'
+stdout '^example.net/x$'
+
+go list -m all
+stdout '^example.net/needx2 v0\.1\.0 '
+stdout '^example.net/x v0\.2\.0 '
+
+
+-- go.mod --
+module example.net/m
+
+go 1.17
+
+require (
+	example.net/needx2 v0.1.0
+	example.net/x v0.1.0
+)
+
+replace (
+	example.net/needx2 v0.1.0 => ./needx2.1
+	example.net/x v0.1.0 => ./x.1
+	example.net/x v0.2.0 => ./x.2
+)
+-- useneedx2/useneedx2.go --
+package useneedx2
+
+import _ "example.net/needx2"
+-- usex/usex.go --
+package usex
+
+import _ "example.net/x"
+
+-- x.1/go.mod --
+module example.com/x
+
+go 1.17
+-- x.1/x.go --
+package x
+
+-- x.2/go.mod --
+module example.com/x
+
+go 1.17
+-- x.2/x.go --
+package x
+
+const AddedInV2 = true
+
+-- needx2.1/go.mod --
+module example.com/x
+
+go 1.17
+
+require example.net/x v0.2.0
+-- needx2.1/needx2.go --
+// Package needx2 needs x v0.2.0 or higher.
+package needx2
+
+import "example.net/x"
+
+var _ = x.AddedInV2
diff --git a/src/cmd/go/testdata/script/mod_lazy_downgrade.txt b/src/cmd/go/testdata/script/mod_lazy_downgrade.txt
new file mode 100644
index 0000000..eb69d2e
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_lazy_downgrade.txt
@@ -0,0 +1,183 @@
+# This test illustrates the interaction between lazy loading and downgrading in
+# 'go get'.
+
+# The package import graph used in this test looks like:
+#
+# lazy ---- a
+#           |
+#           a_test ---- b
+#                       b_test ---- c
+#
+# The module dependency graph initially looks like:
+#
+# lazy ---- a.1 ---- b.1 ---- c.1
+#      \                     /
+#        b.3 ---- c.2    b.2
+#
+# (Note that lazy loading will prune out the dependency from b.1 on c.1.)
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod.orig go.mod
+
+go list -m all
+stdout '^example.com/a v0.1.0 '
+stdout '^example.com/b v0.3.0 '
+stdout '^example.com/c v0.2.0 '
+
+# Downgrading c should also downgrade the b that requires it.
+
+go get example.com/c@v0.1.0
+go list -m all
+stdout '^example.com/a v0.1.0 '
+stdout '^example.com/b v0.2.0 '
+stdout '^example.com/c v0.1.0 '
+
+# Removing c entirely should also remove the a and b that require it.
+
+go get example.com/c@none
+go list -m all
+! stdout '^example.com/a '
+! stdout '^example.com/b '
+! stdout '^example.com/c '
+
+
+# With lazy loading, downgrading c should work the same way, but dependencies
+# outside of the deepening scan should not affect the downgrade.
+
+cp go.mod.orig go.mod
+go mod edit -go=1.17
+
+go list -m all
+stdout '^example.com/a v0.1.0 '
+stdout '^example.com/b v0.3.0 '
+stdout '^example.com/c v0.2.0 '
+
+go get example.com/c@v0.1.0
+go list -m all
+stdout '^example.com/a v0.1.0 '
+stdout '^example.com/b v0.2.0 '
+stdout '^example.com/c v0.1.0 '
+
+# At this point, b.2 is still an explicit root, so its dependency on c
+# is still tracked, and it will still be downgraded away if we remove c.
+# ('go get' never makes a root into a non-root. Only 'go mod tidy' does that.)
+
+go get example.com/c@none
+go list -m all
+! stdout '^example.com/a '
+! stdout '^example.com/b '
+! stdout '^example.com/c '
+
+
+# This time, we drop the explicit 'b' root by downgrading it to v0.1.0
+# (the version required by a.1) and running 'go mod tidy'.
+# It is still selected at v0.1.0 (as a dependency of a),
+# but its dependency on c is now pruned from the module graph, so it doesn't
+# result in any downgrades to b or a if we run 'go get c@none'.
+
+cp go.mod.orig go.mod
+go mod edit -go=1.17
+
+go list -m all
+stdout '^example.com/a v0.1.0 '
+stdout '^example.com/b v0.3.0 '
+stdout '^example.com/c v0.2.0 '
+
+go get example.com/c@v0.1.0 example.com/b@v0.1.0
+go list -m all
+stdout '^example.com/a v0.1.0 '
+stdout '^example.com/b v0.1.0 '
+stdout '^example.com/c v0.1.0 '
+
+go mod tidy
+go list -m all
+stdout '^example.com/a v0.1.0 '
+stdout '^example.com/b v0.1.0 '
+! stdout '^example.com/c '
+
+go get example.com/c@none
+go list -m all
+stdout '^example.com/a v0.1.0'
+stdout '^example.com/b v0.1.0'
+! stdout '^example.com/c '
+
+
+-- go.mod --
+module example.com/lazy
+
+go 1.15
+
+require (
+	example.com/a v0.1.0
+	example.com/b v0.3.0 // indirect
+)
+
+replace (
+	example.com/a v0.1.0 => ./a
+	example.com/b v0.1.0 => ./b1
+	example.com/b v0.2.0 => ./b2
+	example.com/b v0.3.0 => ./b3
+	example.com/c v0.1.0 => ./c
+	example.com/c v0.2.0 => ./c
+)
+-- lazy.go --
+package lazy
+
+import _ "example.com/a"
+
+-- a/go.mod --
+module example.com/a
+
+go 1.17
+
+require example.com/b v0.1.0
+-- a/a.go --
+package a
+-- a/a_test.go --
+package a_test
+
+import _ "example.com/b"
+
+-- b1/go.mod --
+module example.com/b
+
+go 1.17
+
+require example.com/c v0.1.0
+-- b1/b.go --
+package b
+-- b1/b_test.go --
+package b_test
+import _ "example.com/c"
+
+-- b2/go.mod --
+module example.com/b
+
+go 1.17
+
+require example.com/c v0.1.0
+-- b2/b.go --
+package b
+-- b2/b_test.go --
+package b_test
+import _ "example.com/c"
+
+-- b3/go.mod --
+module example.com/b
+
+go 1.17
+
+require example.com/c v0.2.0
+-- b3/b.go --
+package b
+-- b3/b_test.go --
+package b_test
+import _ "example.com/c"
+
+-- c/go.mod --
+module example.com/c
+
+go 1.17
+-- c/c.go --
+package c
diff --git a/src/cmd/go/testdata/script/mod_lazy_import_allmod.txt b/src/cmd/go/testdata/script/mod_lazy_import_allmod.txt
new file mode 100644
index 0000000..60d4187
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_lazy_import_allmod.txt
@@ -0,0 +1,191 @@
+# This test demonstrates dependency resolution when the main module imports a
+# new package from a previously-test-only dependency.
+#
+# When lazy loading is active, the loader will not load dependencies of any
+# module whose packages are *only* imported by tests outside the main module. If
+# the main module is changed to import a package from such a module, the
+# dependencies of that module will need to be reloaded.
+
+# The import graph used in this test looks like:
+#
+# m ---- a
+#  \     |
+#   \    a_test ---- b/x
+#    \
+#      --------------b/y (new) ---- c
+#
+# Where b/x and b/y are disjoint packages, but both contained in module b.
+#
+# The module dependency graph initially looks like:
+#
+# m ---- a.1 ---- b.1 ---- c.1
+#
+# This configuration is similar to that used in mod_lazy_new_import,
+# but the new import is from what is initially a test-only dependency.
+
+# Control case: in Go 1.14, the original go.mod is tidy,
+# and the dependency on c is eagerly loaded.
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod.orig go.mod
+
+go list -m all
+stdout '^a v0.1.0 '
+stdout '^b v0.1.0 '
+stdout '^c v0.1.0 '
+
+# After adding a new import of b/y,
+# the import of c from b/y should resolve to the version required by b.
+
+cp m.go m.go.orig
+cp m.go.new m.go
+go mod tidy
+cmp go.mod.new go.mod
+
+go list -m all
+stdout '^a v0.1.0 '
+stdout '^b v0.1.0 '
+stdout '^c v0.1.0 '
+
+# With lazy loading, the go.mod requirements are the same,
+# but the dependency on c is initially pruned out.
+
+cp m.go.orig m.go
+cp go.mod.orig go.mod
+go mod edit -go=1.17
+go mod edit -go=1.17 go.mod.new
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod.orig go.mod
+
+go list -m all
+stdout '^a v0.1.0 '
+stdout '^b v0.1.0 '
+! stdout '^c '
+
+# After adding a new direct import of b/y,
+# the existing version of b should be promoted to a root,
+# bringing the version of c required by b into the build list.
+
+cp m.go.new m.go
+go mod tidy
+cmp go.mod.lazy go.mod
+
+go list -m all
+stdout '^a v0.1.0 '
+stdout '^b v0.1.0 '
+stdout '^c v0.1.0 '
+
+-- m.go --
+package main
+
+import (
+	"fmt"
+
+	_ "a"  // a_test imports b/x.
+)
+
+func main() {
+}
+-- m.go.new --
+package main
+
+import (
+	"fmt"
+
+	_ "a"  // a_test imports b/x.
+	"b/y"  // This is a new import, not yet reflected in the go.mod file.
+)
+
+func main() {
+	fmt.Println(b.CVersion())
+}
+-- go.mod --
+module m
+
+go 1.14
+
+require a v0.1.0
+
+replace (
+	a v0.1.0 => ./a1
+	b v0.1.0 => ./b1
+	c v0.1.0 => ./c1
+	c v0.2.0 => ./c2
+)
+-- go.mod.new --
+module m
+
+go 1.14
+
+require (
+	a v0.1.0
+	b v0.1.0
+)
+
+replace (
+	a v0.1.0 => ./a1
+	b v0.1.0 => ./b1
+	c v0.1.0 => ./c1
+	c v0.2.0 => ./c2
+)
+-- go.mod.lazy --
+module m
+
+go 1.17
+
+require (
+	a v0.1.0
+	b v0.1.0
+)
+
+require c v0.1.0 // indirect
+
+replace (
+	a v0.1.0 => ./a1
+	b v0.1.0 => ./b1
+	c v0.1.0 => ./c1
+	c v0.2.0 => ./c2
+)
+-- a1/go.mod --
+module a
+
+go 1.17
+
+require b v0.1.0
+-- a1/a.go --
+package a
+-- a1/a_test.go --
+package a_test
+
+import _ "b/x"
+-- b1/go.mod --
+module b
+
+go 1.17
+
+require c v0.1.0
+-- b1/x/x.go --
+package x
+-- b1/y/y.go --
+package y
+
+import "c"
+
+func CVersion() string {
+	return c.Version
+}
+-- c1/go.mod --
+module c
+
+go 1.17
+-- c1/c.go --
+package c
+
+const Version = "v0.1.0"
+-- c2/go.mod --
+This file should be unused.
+-- c2/c.go --
+This file should be unused.
diff --git a/src/cmd/go/testdata/script/mod_lazy_new_import.txt b/src/cmd/go/testdata/script/mod_lazy_new_import.txt
new file mode 100644
index 0000000..520d845
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_lazy_new_import.txt
@@ -0,0 +1,155 @@
+# This test illustrates the use of a deepening scan to resolve transitive
+# imports of imports of new packages from within existing dependencies.
+
+# The package import graph used in this test looks like:
+#
+# lazy ---- a/x ---- b
+#     \
+#      ---- a/y (new) ---- c
+#
+# Where a/x and a/y are disjoint packages, but both contained in module a.
+#
+# The module dependency graph initially looks like:
+#
+# lazy ---- a.1 ---- b.1
+#              \
+#               c.1
+
+
+cp go.mod go.mod.old
+cp lazy.go lazy.go.old
+go mod tidy
+cmp go.mod go.mod.old
+
+# Before adding a new import, the go.mod file should
+# enumerate modules for all packages already imported.
+go list all
+cmp go.mod go.mod.old
+
+# When we add a new import of a package in an existing dependency,
+# and that dependency is already tidy, its transitive dependencies
+# should already be present.
+cp lazy.go.new lazy.go
+go list all
+go list -m all
+stdout '^example.com/c v0.1.0' # not v0.2.0 as would be resolved by 'latest'
+cmp go.mod go.mod.old
+
+# Now, we repeat the test with a lazy main module.
+cp lazy.go.old lazy.go
+cp go.mod.117 go.mod
+
+# Before adding a new import, the go.mod file should
+# enumerate modules for all packages already imported.
+go list all
+cmp go.mod go.mod.117
+
+# When a new import is found, we should perform a deepening scan of the existing
+# dependencies and add a requirement on the version required by those
+# dependencies — not re-resolve 'latest'.
+cp lazy.go.new lazy.go
+
+! go list all
+stderr '^go: updates to go.mod needed; to update it:\n\tgo mod tidy$'
+
+go mod tidy
+go list all
+go list -m all
+stdout '^example.com/c v0.1.0' # not v0.2.0 as would be resolved by 'latest'
+
+cmp go.mod go.mod.new
+
+
+-- go.mod --
+module example.com/lazy
+
+go 1.15
+
+require example.com/a v0.1.0
+
+replace (
+	example.com/a v0.1.0 => ./a
+	example.com/b v0.1.0 => ./b
+	example.com/c v0.1.0 => ./c1
+	example.com/c v0.2.0 => ./c2
+)
+-- go.mod.117 --
+module example.com/lazy
+
+go 1.17
+
+require example.com/a v0.1.0
+
+require example.com/b v0.1.0 // indirect
+
+replace (
+	example.com/a v0.1.0 => ./a
+	example.com/b v0.1.0 => ./b
+	example.com/c v0.1.0 => ./c1
+	example.com/c v0.2.0 => ./c2
+)
+-- go.mod.new --
+module example.com/lazy
+
+go 1.17
+
+require example.com/a v0.1.0
+
+require (
+	example.com/b v0.1.0 // indirect
+	example.com/c v0.1.0 // indirect
+)
+
+replace (
+	example.com/a v0.1.0 => ./a
+	example.com/b v0.1.0 => ./b
+	example.com/c v0.1.0 => ./c1
+	example.com/c v0.2.0 => ./c2
+)
+-- lazy.go --
+package lazy
+
+import (
+	_ "example.com/a/x"
+)
+-- lazy.go.new --
+package lazy
+
+import (
+	_ "example.com/a/x"
+	_ "example.com/a/y"
+)
+-- a/go.mod --
+module example.com/a
+
+go 1.15
+
+require (
+	example.com/b v0.1.0
+	example.com/c v0.1.0
+)
+-- a/x/x.go --
+package x
+import _ "example.com/b"
+-- a/y/y.go --
+package y
+import _ "example.com/c"
+-- b/go.mod --
+module example.com/b
+
+go 1.15
+-- b/b.go --
+package b
+-- c1/go.mod --
+module example.com/c
+
+go 1.15
+-- c1/c.go --
+package c
+-- c2/go.mod --
+module example.com/c
+
+go 1.15
+-- c2/c.go --
+package c
+This file should not be used, so this syntax error should be ignored.
diff --git a/src/cmd/go/testdata/script/mod_lazy_test_horizon.txt b/src/cmd/go/testdata/script/mod_lazy_test_horizon.txt
new file mode 100644
index 0000000..7d07eb6
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_lazy_test_horizon.txt
@@ -0,0 +1,131 @@
+# This file demonstrates the effect of lazy loading on the selected
+# versions of test dependencies.
+
+# The package import graph used in this test looks like:
+#
+# m ---- a
+#  \     |
+#   \    a_test ---- b
+#    \               |
+#     x              b_test
+#     |                    \
+#     x_test -------------- c
+#
+# And the module dependency graph looks like:
+#
+# m -- a.1 -- b.1 -- c.2
+#  \
+#   x.1 ------------ c.1
+
+# Control case: in Go 1.15, the version of c imported by 'go test x' is the
+# version required by module b, even though b_test is not relevant to the main
+# module. (The main module imports a, and a_test imports b, but all of the
+# packages and tests in the main module can be built without b.)
+
+go list -m c
+stdout '^c v0.2.0 '
+
+[!short] go test -v x
+[!short] stdout ' c v0.2.0$'
+
+# With lazy loading, the go.mod requirements are the same,
+# but the irrelevant dependency on c v0.2.0 should be pruned out,
+# leaving only the relevant dependency on c v0.1.0.
+
+go mod edit -go=1.17
+go list -m c
+stdout '^c v0.1.0'
+
+[!short] go test -v x
+[!short] stdout ' c v0.1.0$'
+
+-- m.go --
+package m
+
+import (
+	_ "a"
+	_ "x"
+)
+-- go.mod --
+module m
+
+go 1.15
+
+require (
+	a v0.1.0
+	x v0.1.0
+)
+
+replace (
+	a v0.1.0 => ./a1
+	b v0.1.0 => ./b1
+	c v0.1.0 => ./c1
+	c v0.2.0 => ./c2
+	x v0.1.0 => ./x1
+)
+-- a1/go.mod --
+module a
+
+go 1.17
+
+require b v0.1.0
+-- a1/a.go --
+package a
+-- a1/a_test.go --
+package a_test
+
+import _ "b"
+-- b1/go.mod --
+module b
+
+go 1.17
+
+require c v0.2.0
+-- b1/b.go --
+package b
+-- b1/b_test.go --
+package b_test
+
+import (
+	"c"
+	"testing"
+)
+
+func TestCVersion(t *testing.T) {
+	t.Log(c.Version)
+}
+-- c1/go.mod --
+module c
+
+go 1.17
+-- c1/c.go --
+package c
+
+const Version = "v0.1.0"
+-- c2/go.mod --
+module c
+
+go 1.17
+-- c2/c.go --
+package c
+
+const Version = "v0.2.0"
+-- x1/go.mod --
+module x
+
+go 1.17
+
+require c v0.1.0
+-- x1/x.go --
+package x
+-- x1/x_test.go --
+package x_test
+
+import (
+	"c"
+	"testing"
+)
+
+func TestCVersion(t *testing.T) {
+	t.Log("c", c.Version)
+}
diff --git a/src/cmd/go/testdata/script/mod_lazy_test_of_test_dep.txt b/src/cmd/go/testdata/script/mod_lazy_test_of_test_dep.txt
new file mode 100644
index 0000000..68a5b6d
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_lazy_test_of_test_dep.txt
@@ -0,0 +1,224 @@
+# This file demonstrates the effect of lazy loading on the reproducibility of
+# tests (and tests of test dependencies) outside the main module.
+#
+# It is similar to the cases in mod_all.txt and mod_lazy_test_horizon.txt, but
+# focuses on the effect of "go test" on specific packages instead of the "all"
+# pattern.
+
+# The package import graph used in this test looks like:
+#
+# lazy ---- a
+#           |
+#           a_test ---- b
+#                       |
+#                       b_test ---- c
+#
+# And the non-lazy module dependency graph looks like:
+#
+# lazy ---- a.1 ---- b.1 ---- c.1
+
+cp go.mod go.mod.old
+go mod tidy
+cmp go.mod go.mod.old
+
+
+# In Go 1.15 mode, 'go list -m all' includes modules needed by the
+# transitive closure of tests of dependencies of tests of dependencies of ….
+
+go list -m all
+stdout '^example.com/b v0.1.0 '
+stdout '^example.com/c v0.1.0 '
+cmp go.mod go.mod.old
+
+# 'go test' (or equivalent) of any such dependency, no matter how remote, does
+# not update the go.mod file.
+
+go list -test -deps example.com/a
+stdout example.com/b
+! stdout example.com/c
+
+[!short] go test -c -o $devnull example.com/a
+[!short] cmp go.mod go.mod.old
+
+go list -test -deps example.com/b
+stdout example.com/c
+
+[!short] go test -c -o $devnull example.com/b
+[!short] cmp go.mod go.mod.old
+
+go mod edit -go=1.17 a/go.mod
+go mod edit -go=1.17 b1/go.mod
+go mod edit -go=1.17 b2/go.mod
+go mod edit -go=1.17 c1/go.mod
+go mod edit -go=1.17 c2/go.mod
+go mod edit -go=1.17
+
+
+# After changing to 'go 1.17` uniformly, 'go list -m all' should prune out
+# example.com/c, because it is not imported by any package (or test of a package)
+# transitively imported by the main module.
+#
+# example.com/a is imported,
+# and example.com/b is needed in order to run 'go test example.com/a',
+# but example.com/c is not needed because we don't expect the user to need to run
+# 'go test example.com/b'.
+
+# If we skip directly to adding a new import of c, the dependency is too far
+# away for a deepening scan to find, which is fine because the package whose
+# test imported it wasn't even it "all". It should resolve from the latest
+# version of its module.
+
+# However, if we reach c by running successive tests starting from the main
+# module, we should end up with exactly the version required by b, with an update
+# to the go.mod file as soon as we test a test dependency that is not itself in
+# "all".
+
+cp go.mod go.mod.117
+go mod tidy
+cmp go.mod go.mod.117
+
+go list -m all
+stdout '^example.com/b v0.1.0 '
+! stdout '^example.com/c '
+
+# 'go test' of a package (transitively) imported by the main module
+# should work without changes to the go.mod file.
+
+go list -test -deps example.com/a
+stdout example.com/b
+! stdout example.com/c
+
+[!short] go test -c -o $devnull example.com/a
+
+# However, 'go test' of a package that is itself a dependency should require an
+# update to the go.mod file.
+! go list -test -deps example.com/b
+
+	# TODO(#36460): The hint here is wrong. We should suggest
+	# 'go get -t example.com/b@v0.1.0' instead of 'go mod tidy'.
+stderr '^go: updates to go\.mod needed; to update it:\n\tgo mod tidy$'
+
+[!short] ! go test -c -o $devnull example.com/b
+[!short] stderr '^go: updates to go\.mod needed; to update it:\n\tgo mod tidy$'
+
+go get -t example.com/b@v0.1.0
+go list -test -deps example.com/b
+stdout example.com/c
+
+[!short] go test -c -o $devnull example.com/b
+
+# The update should bring the version required by b, not the latest version of c.
+
+go list -m example.com/c
+stdout '^example.com/c v0.1.0 '
+
+cmp go.mod go.mod.b
+
+
+# We should reach the same state if we arrive at it via `go test -mod=mod`.
+
+cp go.mod.117 go.mod
+
+[short] go list -mod=mod -test -deps example.com/a
+[!short] go test -mod=mod -c -o $devnull example.com/a
+
+[short] go list -mod=mod -test -deps example.com/b
+[!short] go test -mod=mod -c -o $devnull example.com/b
+
+cmp go.mod go.mod.b
+
+
+
+-- go.mod --
+module example.com/lazy
+
+go 1.15
+
+require example.com/a v0.1.0
+
+replace (
+	example.com/a v0.1.0 => ./a
+	example.com/b v0.1.0 => ./b1
+	example.com/b v0.2.0 => ./b2
+	example.com/c v0.1.0 => ./c1
+	example.com/c v0.2.0 => ./c2
+)
+-- go.mod.b --
+module example.com/lazy
+
+go 1.17
+
+require example.com/a v0.1.0
+
+require example.com/b v0.1.0 // indirect
+
+replace (
+	example.com/a v0.1.0 => ./a
+	example.com/b v0.1.0 => ./b1
+	example.com/b v0.2.0 => ./b2
+	example.com/c v0.1.0 => ./c1
+	example.com/c v0.2.0 => ./c2
+)
+-- lazy.go --
+package lazy
+
+import (
+	_ "example.com/a"
+)
+-- a/go.mod --
+module example.com/a
+
+go 1.15
+
+require example.com/b v0.1.0
+-- a/a.go --
+package a
+-- a/a_test.go --
+package a
+
+import (
+	"testing"
+
+	_ "example.com/b"
+)
+
+func TestUsingB(t *testing.T) {
+	// …
+}
+-- b1/go.mod --
+module example.com/b
+
+go 1.15
+
+require example.com/c v0.1.0
+-- b1/b.go --
+package b
+-- b1/b_test.go --
+package b
+
+import _ "example.com/c"
+-- b2/go.mod --
+module example.com/b
+
+go 1.15
+
+require example.com/c v0.1.0
+-- b2/b.go --
+package b
+This file should not be used, so this syntax error should be ignored.
+-- b2/b_test.go --
+package b
+This file should not be used, so this syntax error should be ignored.
+-- c1/go.mod --
+module example.com/c
+
+go 1.15
+-- c1/c.go --
+package c
+-- c2/go.mod --
+module example.com/c
+
+go 1.15
+-- c2/c.go --
+package c
+This file should not be used, so this syntax error should be ignored.
diff --git a/src/cmd/go/testdata/script/mod_list.txt b/src/cmd/go/testdata/script/mod_list.txt
new file mode 100644
index 0000000..06316cc
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_list.txt
@@ -0,0 +1,62 @@
+env GO111MODULE=on
+[short] skip
+
+# list {{.Dir}} shows main module and go.mod but not not-yet-downloaded dependency dir.
+go list -mod=mod -m -f '{{.Path}} {{.Main}} {{.GoMod}} {{.Dir}}' all
+stdout '^x true .*[\\/]src[\\/]go.mod .*[\\/]src$'
+stdout '^rsc.io/quote false .*[\\/]v1.5.2.mod $'
+
+# list {{.Dir}} shows dependency after download (and go list without -m downloads it)
+go list -mod=mod -f '{{.Dir}}' rsc.io/quote
+stdout '.*mod[\\/]rsc.io[\\/]quote@v1.5.2$'
+
+# downloaded dependencies are read-only
+exists -readonly $GOPATH/pkg/mod/rsc.io/quote@v1.5.2
+exists -readonly $GOPATH/pkg/mod/rsc.io/quote@v1.5.2/buggy
+
+# go clean -modcache can delete read-only dependencies
+go clean -modcache
+! exists $GOPATH/pkg/mod/rsc.io/quote@v1.5.2
+
+# list {{.Dir}} shows replaced directories
+cp go.mod2 go.mod
+go list -mod=mod -f {{.Dir}} rsc.io/quote
+go list -m -f '{{.Path}} {{.Version}} {{.Dir}}{{with .Replace}} {{.GoMod}} => {{.Version}} {{.Dir}} {{.GoMod}}{{end}}' all
+stdout 'mod[\\/]rsc.io[\\/]quote@v1.5.1'
+stdout 'v1.3.0.*mod[\\/]rsc.io[\\/]sampler@v1.3.1 .*[\\/]v1.3.1.mod => v1.3.1.*sampler@v1.3.1 .*[\\/]v1.3.1.mod'
+
+# list std should work
+go list std
+stdout ^math/big
+
+# rsc.io/quote/buggy should be listable as a package,
+# even though it is only a test.
+go list -mod=mod rsc.io/quote/buggy
+
+# rsc.io/quote/buggy should not be listable as a module
+go list -m -e -f '{{.Error.Err}}' nonexist rsc.io/quote/buggy
+stdout '^module nonexist: not a known dependency$'
+stdout '^module rsc.io/quote/buggy: not a known dependency$'
+
+! go list -m nonexist rsc.io/quote/buggy
+stderr '^go: module nonexist: not a known dependency'
+stderr '^go: module rsc.io/quote/buggy: not a known dependency'
+
+# Module loader does not interfere with list -e (golang.org/issue/24149).
+go list -e -f '{{.Error.Err}}' database
+stdout 'no Go files in '
+! go list database
+stderr 'no Go files in '
+
+-- go.mod --
+module x
+require rsc.io/quote v1.5.2
+
+-- go.mod2 --
+module x
+require rsc.io/quote v1.5.1
+replace rsc.io/sampler v1.3.0 => rsc.io/sampler v1.3.1
+
+-- x.go --
+package x
+import _ "rsc.io/quote"
diff --git a/src/cmd/go/testdata/script/mod_list_bad_import.txt b/src/cmd/go/testdata/script/mod_list_bad_import.txt
new file mode 100644
index 0000000..b128408
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_list_bad_import.txt
@@ -0,0 +1,75 @@
+# This test matches list_bad_import, but in module mode.
+# Please keep them in sync.
+
+env GO111MODULE=on
+cd example.com
+
+# Without -e, listing an otherwise-valid package with an unsatisfied direct import should fail.
+# BUG: Today it succeeds.
+go list -f '{{if .Error}}error{{end}} {{if .Incomplete}}incomplete{{end}} {{range .DepsErrors}}bad dep: {{.Err}}{{end}}' example.com/direct
+! stdout ^error
+stdout 'incomplete'
+stdout 'bad dep: .*example.com/notfound'
+
+# Listing with -deps should also fail.
+! go list -deps example.com/direct
+stderr example.com/notfound
+
+# But -e -deps should succeed.
+go list -e -deps example.com/direct
+stdout example.com/notfound
+
+
+# Listing an otherwise-valid package that imports some *other* package with an
+# unsatisfied import should also fail.
+# BUG: Today, it succeeds.
+go list -f '{{if .Error}}error{{end}} {{if .Incomplete}}incomplete{{end}} {{range .DepsErrors}}bad dep: {{.Err}}{{end}}' example.com/indirect
+! stdout ^error
+stdout incomplete
+stdout 'bad dep: .*example.com/notfound'
+
+# Again, -deps should fail.
+! go list -deps example.com/indirect
+stderr example.com/notfound
+
+# But -e -deps should succeed.
+go list -e -deps example.com/indirect
+stdout example.com/notfound
+
+
+# Listing the missing dependency directly should fail outright...
+! go list -f '{{if .Error}}error{{end}} {{if .Incomplete}}incomplete{{end}}' example.com/notfound
+stderr 'no required module provides package example.com/notfound; to add it:\n\tgo get example.com/notfound'
+! stdout error
+! stdout incomplete
+
+# ...but listing with -e should succeed.
+go list -e -f '{{if .Error}}error{{end}} {{if .Incomplete}}incomplete{{end}}' example.com/notfound
+stdout error
+stdout incomplete
+
+
+# The pattern "all" should match only packages that actually exist,
+# ignoring those whose existence is merely implied by imports.
+go list -e -f '{{.ImportPath}} {{.Error}}' all
+stdout example.com/direct
+stdout example.com/indirect
+# TODO: go list creates a dummy package with the import-not-found
+# but really the Error belongs on example.com/direct, and this package
+# should not be printed.
+# ! stdout example.com/notfound
+
+
+-- example.com/go.mod --
+module example.com
+
+-- example.com/direct/direct.go --
+package direct
+import _ "example.com/notfound"
+
+-- example.com/indirect/indirect.go --
+package indirect
+import _ "example.com/direct"
+
+-- example.com/notfound/README --
+This directory intentionally left blank.
diff --git a/src/cmd/go/testdata/script/mod_list_command_line_arguments.txt b/src/cmd/go/testdata/script/mod_list_command_line_arguments.txt
new file mode 100644
index 0000000..25c68c5
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_list_command_line_arguments.txt
@@ -0,0 +1,35 @@
+# The command-line-arguments package does not belong to a module...
+cd a
+go list -f '{{.Module}}' ../b/b.go
+stdout '^<nil>$'
+
+# ... even if the arguments are sources from that module
+go list -f '{{.Module}}' a.go
+stdout '^<nil>$'
+
+[short] skip
+
+# check that the version of command-line-arguments doesn't include a module
+go build -o a.exe a.go
+go version -m a.exe
+stdout '^\tpath\tcommand-line-arguments$'
+stdout '^\tdep\ta\t\(devel\)\t$'
+! stdout mod[^e]
+
+-- a/go.mod --
+module a
+go 1.17
+-- a/a.go --
+package main
+
+import "a/dep"
+
+func main() {
+    dep.D()
+}
+-- a/dep/dep.go --
+package dep
+
+func D() {}
+-- b/b.go --
+package b
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/mod_list_compiled_concurrent.txt b/src/cmd/go/testdata/script/mod_list_compiled_concurrent.txt
new file mode 100644
index 0000000..195f7b1
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_list_compiled_concurrent.txt
@@ -0,0 +1,21 @@
+env GO111MODULE=on
+
+[short] skip
+[!cgo] skip
+
+# Regression test for golang.org/issue/29667:
+# spurious 'failed to cache compiled Go files' errors.
+
+env GOCACHE=$WORK/gocache
+mkdir $GOCACHE
+
+go list -json -compiled -test=false -export=false -deps=true -- . &
+go list -json -compiled -test=false -export=false -deps=true -- . &
+wait
+
+-- go.mod --
+module sandbox/bar
+-- bar.go --
+package bar
+
+import "C"
diff --git a/src/cmd/go/testdata/script/mod_list_deprecated.txt b/src/cmd/go/testdata/script/mod_list_deprecated.txt
new file mode 100644
index 0000000..ee985cc
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_list_deprecated.txt
@@ -0,0 +1,52 @@
+# 'go list pkg' does not show deprecation.
+go list example.com/deprecated/a
+stdout '^example.com/deprecated/a$'
+
+# 'go list -m' does not show deprecation.
+go list -m example.com/deprecated/a
+stdout '^example.com/deprecated/a v1.9.0$'
+
+# 'go list -m -versions' does not show deprecation.
+go list -m -versions example.com/deprecated/a
+stdout '^example.com/deprecated/a v1.0.0 v1.9.0$'
+
+# 'go list -m -u' shows deprecation.
+go list -m -u example.com/deprecated/a
+stdout '^example.com/deprecated/a v1.9.0 \(deprecated\)$'
+
+# 'go list -m -u -f' exposes the deprecation message.
+go list -m -u -f {{.Deprecated}} example.com/deprecated/a
+stdout '^in example.com/deprecated/a@v1.9.0$'
+
+# This works even if we use an old version that does not have the deprecation
+# message in its go.mod file.
+go get example.com/deprecated/a@v1.0.0
+! grep Deprecated: $WORK/gopath/pkg/mod/cache/download/example.com/deprecated/a/@v/v1.0.0.mod
+go list -m -u -f {{.Deprecated}} example.com/deprecated/a
+stdout '^in example.com/deprecated/a@v1.9.0$'
+
+# 'go list -m -u' does not show deprecation for the main module.
+go list -m -u
+! stdout deprecated
+go list -m -u -f '{{if not .Deprecated}}ok{{end}}'
+stdout ok
+
+# 'go list -m -u' does not show a deprecation message for a module that is not
+# deprecated at the latest version, even if it is deprecated at the current
+# version.
+go list -m -u example.com/undeprecated
+stdout '^example.com/undeprecated v1.0.0 \[v1.0.1\]$'
+-- go.mod --
+// Deprecated: main module is deprecated, too!
+module example.com/use
+
+go 1.17
+
+require (
+	example.com/deprecated/a v1.9.0
+	example.com/undeprecated v1.0.0
+)
+-- go.sum --
+example.com/deprecated/a v1.9.0 h1:pRyvBIZheJpQVVnNW4Fdg8QuoqDgtkCreqZZbASV3BE=
+example.com/deprecated/a v1.9.0/go.mod h1:Z1uUVshSY9kh6l/2hZ8oA9SBviX2yfaeEpcLDz6AZwY=
+example.com/undeprecated v1.0.0/go.mod h1:1qiRbdA9VzJXDqlG26Y41O5Z7YyO+jAD9do8XCZQ+Gg=
diff --git a/src/cmd/go/testdata/script/mod_list_deprecated_replace.txt b/src/cmd/go/testdata/script/mod_list_deprecated_replace.txt
new file mode 100644
index 0000000..48b991f
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_list_deprecated_replace.txt
@@ -0,0 +1,68 @@
+# When all versions are replaced, we should not look up a deprecation message.
+# We will still look up a deprecation message for the replacement.
+cp go.mod.allreplaced go.mod
+go list -m -u -f '{{.Path}}@{{.Version}} <{{.Deprecated}}>{{with .Replace}} => {{.Path}}@{{.Version}} <{{.Deprecated}}>{{end}}' all
+stdout '^example.com/deprecated/a@v1.0.0 <> => example.com/deprecated/b@v1.0.0 <in example.com/deprecated/b@v1.9.0>$'
+
+# When one version is replaced, we should see a deprecation message.
+cp go.mod.onereplaced go.mod
+go list -m -u -f '{{.Path}}@{{.Version}} <{{.Deprecated}}>{{with .Replace}} => {{.Path}}@{{.Version}} <{{.Deprecated}}>{{end}}' all
+stdout '^example.com/deprecated/a@v1.0.0 <in example.com/deprecated/a@v1.9.0> => example.com/deprecated/b@v1.0.0 <in example.com/deprecated/b@v1.9.0>$'
+
+# If the replacement is a directory, we won't look that up.
+cp go.mod.dirreplacement go.mod
+go list -m -u -f '{{.Path}}@{{.Version}} <{{.Deprecated}}>{{with .Replace}} => {{.Path}}@{{.Version}} <{{.Deprecated}}>{{end}}' all
+stdout '^example.com/deprecated/a@v1.0.0 <> => ./a@ <>$'
+
+# If the latest version of the replacement is replaced, we'll use the content
+# from that replacement.
+cp go.mod.latestreplaced go.mod
+go list -m -u -f '{{.Path}}@{{.Version}} <{{.Deprecated}}>{{with .Replace}} => {{.Path}}@{{.Version}} <{{.Deprecated}}>{{end}}' all
+stdout '^example.com/deprecated/a@v1.0.0 <> => example.com/deprecated/b@v1.0.0 <in ./b>$'
+
+-- go.mod.allreplaced --
+module m
+
+go 1.17
+
+require example.com/deprecated/a v1.0.0
+
+replace example.com/deprecated/a => example.com/deprecated/b v1.0.0
+-- go.mod.onereplaced --
+module m
+
+go 1.17
+
+require example.com/deprecated/a v1.0.0
+
+replace example.com/deprecated/a v1.0.0 => example.com/deprecated/b v1.0.0
+-- go.mod.dirreplacement --
+module m
+
+go 1.17
+
+require example.com/deprecated/a v1.0.0
+
+replace example.com/deprecated/a => ./a
+-- go.mod.latestreplaced --
+module m
+
+go 1.17
+
+require example.com/deprecated/a v1.0.0
+
+replace (
+	example.com/deprecated/a => example.com/deprecated/b v1.0.0
+	example.com/deprecated/b v1.9.0 => ./b
+)
+-- go.sum --
+example.com/deprecated/b v1.0.0/go.mod h1:b19J9ywRGviY7Nq4aJ1WBJ+A7qUlEY9ihp22yI4/F6M=
+-- a/go.mod --
+module example.com/deprecated/a
+
+go 1.17
+-- b/go.mod --
+// Deprecated: in ./b
+module example.com/deprecated/b
+
+go 1.17
diff --git a/src/cmd/go/testdata/script/mod_list_dir.txt b/src/cmd/go/testdata/script/mod_list_dir.txt
new file mode 100644
index 0000000..157d3b6
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_list_dir.txt
@@ -0,0 +1,36 @@
+[short] skip
+
+# go list with path to directory should work
+
+# populate go.sum
+go get
+
+env GO111MODULE=off
+go list -f '{{.ImportPath}}' $GOROOT/src/math
+stdout ^math$
+
+env GO111MODULE=on
+go list -f '{{.ImportPath}}' $GOROOT/src/math
+stdout ^math$
+go list -f '{{.ImportPath}}' .
+stdout ^x$
+
+go mod download rsc.io/quote@v1.5.2
+go list -f '{{.ImportPath}}' $GOPATH/pkg/mod/rsc.io/quote@v1.5.2
+stdout '^rsc.io/quote$'
+go list -f '{{.ImportPath}}' $GOPATH/pkg/mod/rsc.io/sampler@v1.3.0
+stdout '^rsc.io/sampler$'
+go get rsc.io/sampler@v1.3.1
+go list -f '{{.ImportPath}}' $GOPATH/pkg/mod/rsc.io/sampler@v1.3.1
+stdout '^rsc.io/sampler$'
+! go list -f '{{.ImportPath}}' $GOPATH/pkg/mod/rsc.io/sampler@v1.3.0
+stderr 'outside main module or its selected dependencies'
+
+-- go.mod --
+module x
+require rsc.io/quote v1.5.2
+
+-- x.go --
+package x
+
+import _ "rsc.io/quote"
diff --git a/src/cmd/go/testdata/script/mod_list_direct.txt b/src/cmd/go/testdata/script/mod_list_direct.txt
new file mode 100644
index 0000000..8bab330
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_list_direct.txt
@@ -0,0 +1,24 @@
+env GO111MODULE=on
+env GOPROXY=direct
+env GOSUMDB=off
+
+[short] skip
+[!git] skip
+
+# golang.org/issue/33099: if an import path ends in a major-version suffix,
+# ensure that 'direct' mode can resolve the package to the module.
+# For a while, (*modfetch.codeRepo).Stat was not checking for a go.mod file,
+# which would produce a hard error at the subsequent call to GoMod.
+
+go get -v
+
+-- go.mod --
+module example.com
+go 1.13
+
+-- main.go --
+package main
+
+import _ "vcs-test.golang.org/git/v3pkg.git/v3"
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/mod_list_e_readonly.txt b/src/cmd/go/testdata/script/mod_list_e_readonly.txt
new file mode 100644
index 0000000..4969434
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_list_e_readonly.txt
@@ -0,0 +1,15 @@
+# 'go list -mod=readonly -e should attribute errors
+# to individual missing packages.
+# Verifies golang.org/issue/34829.
+go list -mod=readonly -e -deps -f '{{if .Error}}{{.ImportPath}}: {{.Error}}{{end}}' .
+stdout 'example.com/missing: use.go:3:8: cannot find module providing package example.com/missing: import lookup disabled by -mod=readonly'
+
+-- go.mod --
+module example.com/m
+
+go 1.14
+
+-- use.go --
+package use
+
+import _ "example.com/missing"
diff --git a/src/cmd/go/testdata/script/mod_list_odd_tags.txt b/src/cmd/go/testdata/script/mod_list_odd_tags.txt
new file mode 100644
index 0000000..232754e
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_list_odd_tags.txt
@@ -0,0 +1,12 @@
+[short] skip
+[!git] skip
+
+env GOPROXY=direct
+
+go list -m vcs-test.golang.org/git/odd-tags.git@latest
+stdout -count=1 '^.'
+stdout '^vcs-test.golang.org/git/odd-tags.git v0.1.1-0.20220223184835-9d863d525bbf$'
+
+go list -m -versions vcs-test.golang.org/git/odd-tags.git
+stdout -count=1 '^.'
+stdout '^vcs-test.golang.org/git/odd-tags.git$'  # No versions listed — the odd tags are filtered out.
diff --git a/src/cmd/go/testdata/script/mod_list_pseudo.txt b/src/cmd/go/testdata/script/mod_list_pseudo.txt
new file mode 100644
index 0000000..056c093
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_list_pseudo.txt
@@ -0,0 +1,39 @@
+env GO111MODULE=on
+
+# Regression test for golang.org/issue/32715.
+
+# When using $GOPATH/pkg/mod/cache/download as a proxy,
+# 'latest' queries should prefer tagged versions over pseudo-versions.
+
+go mod download github.com/dmitshur-test/modtest5@v0.0.0-20190619020302-197a620e0c9a
+go mod download github.com/dmitshur-test/modtest5@v0.5.0-alpha
+go mod download github.com/dmitshur-test/modtest5@v0.5.0-alpha.0.20190619023908-3da23a9deb9e
+cmp $GOPATH/pkg/mod/cache/download/github.com/dmitshur-test/modtest5/@v/list $WORK/modtest5.list
+
+env GOSUMDB=off # don't verify go.mod files when loading retractions
+env GOPROXY=file:///$GOPATH/pkg/mod/cache/download
+env GOPATH=$WORK/gopath2
+mkdir $GOPATH
+
+go list -m -f '{{.Path}} {{.Version}} {{.Time.Format "2006-01-02"}}' github.com/dmitshur-test/modtest5@latest
+stdout '^github.com/dmitshur-test/modtest5 v0.5.0-alpha 2019-06-18$'
+
+# If the module proxy contains only pseudo-versions, 'latest' should stat
+# the version with the most recent timestamp — not the highest semantic
+# version — and return its metadata.
+env GOPROXY=file:///$WORK/tinyproxy
+go list -m -f '{{.Path}} {{.Version}} {{.Time.Format "2006-01-02"}}' dmitri.shuralyov.com/test/modtest3@latest
+stdout '^dmitri.shuralyov.com/test/modtest3 v0.0.0-20181023043359-a85b471d5412 2018-10-22$'
+
+-- $WORK/modtest5.list --
+v0.0.0-20190619020302-197a620e0c9a
+v0.5.0-alpha
+v0.5.0-alpha.0.20190619023908-3da23a9deb9e
+-- $WORK/tinyproxy/dmitri.shuralyov.com/test/modtest3/@v/list --
+v0.1.0-0.20161023043300-000000000000
+v0.0.0-20181023043359-a85b471d5412
+-- $WORK/tinyproxy/dmitri.shuralyov.com/test/modtest3/@v/v0.0.0-20181023043359-a85b471d5412.info --
+{
+	"Version": "v0.0.0-20181023043359-a85b471d5412",
+	"Time": "2018-10-22T21:33:59-07:00"
+}
diff --git a/src/cmd/go/testdata/script/mod_list_replace_dir.txt b/src/cmd/go/testdata/script/mod_list_replace_dir.txt
new file mode 100644
index 0000000..b446543
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_list_replace_dir.txt
@@ -0,0 +1,27 @@
+# Test that "go list" succeeds when given a directory in a replacement
+# module within the module cache.
+# Verifies golang.org/issue/29548
+
+# Populate go.sum and download dependencies.
+go get
+
+# Ensure v1.5.2 is also in the cache so we can list it.
+go mod download rsc.io/quote@v1.5.2
+
+! go list $GOPATH/pkg/mod/rsc.io/quote@v1.5.2
+stderr '^directory ..[/\\]pkg[/\\]mod[/\\]rsc.io[/\\]quote@v1.5.2 outside main module or its selected dependencies$'
+
+go list $GOPATH/pkg/mod/rsc.io/quote@v1.5.1
+stdout 'rsc.io/quote'
+
+-- go.mod --
+module example.com/quoter
+
+require rsc.io/quote v1.5.2
+
+replace rsc.io/quote => rsc.io/quote v1.5.1
+
+-- use.go --
+package use
+
+import _ "rsc.io/quote"
diff --git a/src/cmd/go/testdata/script/mod_list_retract.txt b/src/cmd/go/testdata/script/mod_list_retract.txt
new file mode 100644
index 0000000..b7147aa
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_list_retract.txt
@@ -0,0 +1,110 @@
+# 'go list -mod=vendor -retracted' reports an error.
+go mod vendor
+! go list -m -retracted -mod=vendor
+stderr '^go list -retracted cannot be used when vendoring is enabled$'
+rm vendor
+
+# 'go list -retracted' reports an error in GOPATH mode.
+env GO111MODULE=off
+! go list -retracted
+stderr '^go list -retracted can only be used in module-aware mode$'
+env GO111MODULE=
+
+# 'go list pkg' does not show retraction.
+go list -f '{{with .Module}}{{with .Retracted}}retracted{{end}}{{end}}' example.com/retract
+! stdout .
+
+# 'go list -retracted pkg' shows retraction.
+go list -retracted -f '{{with .Module}}{{with .Retracted}}retracted{{end}}{{end}}' example.com/retract
+stdout retracted
+
+# 'go list -m' does not show retraction.
+go list -m -f '{{with .Retracted}}retracted{{end}}' example.com/retract
+! stdout .
+
+# 'go list -m -retracted' shows retraction.
+go list -m -retracted -f '{{with .Retracted}}retracted{{end}}' example.com/retract
+
+# 'go list -m mod@version' does not show retraction.
+go list -m -f '{{with .Retracted}}retracted{{end}}' example.com/retract@v1.0.0-unused
+! stdout .
+
+# 'go list -m -retracted mod@version' does not show an error if the module
+# that would contain the retraction is unavailable. See #45305.
+go list -m -retracted -f '{{.Path}} {{.Version}} {{.Error}}' example.com/retract/missingmod@v1.0.0
+stdout '^example.com/retract/missingmod v1.0.0 <nil>$'
+exists $GOPATH/pkg/mod/cache/download/example.com/retract/missingmod/@v/v1.9.0.info
+! exists $GOPATH/pkg/mod/cache/download/example.com/retract/missingmod/@v/v1.9.0.mod
+
+# 'go list -m -retracted mod@version' shows retractions.
+go list -m -retracted example.com/retract@v1.0.0-unused
+stdout '^example.com/retract v1.0.0-unused \(retracted\)$'
+go list -m -retracted -f '{{with .Retracted}}retracted{{end}}' example.com/retract@v1.0.0-unused
+stdout retracted
+
+# 'go list -m mod@latest' selects a previous release version, not self-retracted latest.
+go list -m -f '{{.Version}}{{with .Retracted}} retracted{{end}}' example.com/retract/self/prev@latest
+stdout '^v1.1.0$'
+
+# 'go list -m -retracted mod@latest' selects the self-retracted latest version.
+go list -m -retracted -f '{{.Version}}{{with .Retracted}} retracted{{end}}' example.com/retract/self/prev@latest
+stdout '^v1.9.0 retracted$'
+
+# 'go list -m mod@latest' selects a pre-release version if all release versions are retracted.
+go list -m -f '{{.Version}}{{with .Retracted}} retracted{{end}}' example.com/retract/self/prerelease@latest
+stdout '^v1.9.1-pre$'
+
+# 'go list -m -retracted mod@latest' selects the self-retracted latest version.
+go list -m -retracted -f '{{.Version}}{{with .Retracted}} retracted{{end}}' example.com/retract/self/prerelease@latest
+stdout '^v1.9.0 retracted$'
+
+# 'go list -m mod@latest' selects a pseudo-version if all versions are retracted.
+# TODO(golang.org/issue/24031): the proxy does not expose the pseudo-version,
+# even if all release versions are retracted.
+go list -m -e -f '{{.Error.Err}}' example.com/retract/self/pseudo@latest
+stdout '^module example.com/retract/self/pseudo: no matching versions for query "latest"$'
+
+# 'go list -m mod@latest' reports an error if all versions are retracted.
+go list -m -e -f '{{.Error.Err}}' example.com/retract/self/all@latest
+stdout '^module example.com/retract/self/all: no matching versions for query "latest"$'
+
+# 'go list -m mod@<v1.10' selects a previous release version, not self-retracted latest.
+# The @latest query is not special with respect to retractions.
+go list -m -f '{{.Version}}{{with .Retracted}} retracted{{end}}' example.com/retract/self/prev@<v1.10
+stdout '^v1.1.0$'
+
+# 'go list -m -versions' hides retracted versions.
+go list -m -versions example.com/retract
+stdout '^example.com/retract v1.0.0-good v1.1.0$'
+
+# 'go list -m -retracted -versions' shows retracted versions.
+go list -m -retracted -versions example.com/retract
+stdout '^example.com/retract v1.0.0-bad v1.0.0-good v1.0.0-unused v1.1.0$'
+
+# 'go list -m -u -versions' loads retractions and does not show retracted versions.
+go list -m -u -versions example.com/retract
+stdout '^example.com/retract v1.0.0-good v1.1.0$'
+go list -m -u -versions -f '{{with .Retracted}}retracted{{end}}' example.com/retract
+stdout retracted
+
+# 'go list -m -u' shows retraction.
+go list -m -u -f '{{with .Retracted}}retracted{{end}}' example.com/retract
+stdout retracted
+
+# 'go list -m -u' does not suggest an update to a self-retracted latest version.
+go list -m -u -f '{{with .Update}}{{.Version}}{{with .Retracted}} retracted{{end}}{{end}}' example.com/retract/self/prev@v1.0.0-bad
+stdout '^v1.1.0$'
+
+-- go.mod --
+module example.com/use
+
+go 1.15
+
+require example.com/retract v1.0.0-bad
+-- go.sum --
+example.com/retract v1.0.0-bad h1:liAW69rbtjY67x2CcNzat668L/w+YGgNX3lhJsWIJis=
+example.com/retract v1.0.0-bad/go.mod h1:0DvGGofJ9hr1q63cBrOY/jSY52OwhRGA0K47NE80I5Y=
+-- use.go --
+package use
+
+import _ "example.com/retract"
diff --git a/src/cmd/go/testdata/script/mod_list_std.txt b/src/cmd/go/testdata/script/mod_list_std.txt
new file mode 100644
index 0000000..7a4fe21
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_list_std.txt
@@ -0,0 +1,93 @@
+env GO111MODULE=on
+env GOPROXY=off
+
+[!compiler:gc] skip
+[short] skip
+
+# Outside of GOROOT, our vendored packages should be reported as part of the standard library.
+go list -f '{{if .Standard}}{{.ImportPath}}{{end}}' std cmd
+stdout ^vendor/golang\.org/x/net/http2/hpack
+stdout ^cmd/vendor/golang\.org/x/arch/x86/x86asm
+! stdout ^golang\.org/x/
+
+# The dependencies of those packages should also be vendored.
+go list -deps vendor/golang.org/x/crypto/chacha20
+stdout ^vendor/golang\.org/x/crypto/internal/alias
+
+# cmd/... should match the same packages it used to match in GOPATH mode.
+go list cmd/...
+stdout ^cmd/compile
+! stdout ^cmd/vendor/golang\.org/x/arch/x86/x86asm
+
+# GOROOT/src/... should list the packages in std as if it were a module
+# dependency: omitting vendored dependencies and stopping at the 'cmd' module
+# boundary.
+
+go list $GOROOT/src/...
+stdout ^bytes$
+! stdout ^builtin$
+! stdout ^cmd/
+! stdout ^vendor/
+! stdout ^golang\.org/x/
+
+
+# Vendored dependencies should appear with their 'vendor/' paths in std (they're
+# in GOROOT/src, but not in the 'std' module following the usual module-boundary
+# rules).
+
+cd $GOROOT/src
+env GOWORK=off
+
+go list std
+stdout ^vendor/golang.org/x/net/http2/hpack
+! stdout ^golang\.org/x
+
+# The dependencies of packages with an explicit 'vendor/' prefix should
+# still themselves resolve to vendored packages.
+go list -deps vendor/golang.org/x/crypto/chacha20
+stdout ^vendor/golang.org/x/crypto/internal/alias
+! stdout ^golang\.org/x
+
+# Within the std module, the dependencies of the non-vendored packages within
+# std should appear to be packages beginning with 'vendor/', not 'golang.org/…'
+# module dependencies.
+
+go list all
+! stdout ^golang.org/x/
+! stdout ^std/
+! stdout ^cmd/
+stdout ^vendor/
+
+go list -deps -f '{{if not .Standard}}{{.ImportPath}}{{end}}' std
+! stdout .
+
+# However, the 'golang.org/…' module dependencies should resolve to those same
+# directories.
+
+go list -f '{{.Dir}}' golang.org/x/net/http2/hpack
+stdout $GOROOT[/\\]src[/\\]vendor
+
+# Within the std module, the packages within the module should omit the 'std/'
+# prefix (they retain their own identities), but should respect normal module
+# boundaries (vendored packages are not included in the module, even though they
+# are included in the 'std' pattern).
+
+go list ./...
+stdout ^bytes$
+! stdout ^builtin$
+! stdout ^cmd/
+! stdout ^vendor/
+! stdout ^golang\.org/x/
+
+
+# Within std, the vendored dependencies of cmd should still appear to be part of cmd.
+
+go list -f '{{if .Standard}}{{.ImportPath}}{{end}}' cmd
+stdout ^cmd/vendor/golang\.org/x/arch/x86/x86asm
+
+go list -f '{{if not .Standard}}{{.ImportPath}}{{end}}' cmd
+! stdout .
+
+go list cmd/...
+stdout ^cmd/compile
+! stdout ^cmd/vendor/golang\.org/x/arch/x86/x86asm
diff --git a/src/cmd/go/testdata/script/mod_list_sums.txt b/src/cmd/go/testdata/script/mod_list_sums.txt
new file mode 100644
index 0000000..cf1c18b
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_list_sums.txt
@@ -0,0 +1,32 @@
+# https://golang.org/issue/41297: 'go list -m' should not require go.sum with
+# -versions or when all args are version queries.
+
+go mod init m
+go mod edit -require=rsc.io/quote@v1.5.1
+
+go list -m -mod=readonly rsc.io/quote@latest
+stdout '^rsc\.io/quote v1\.5\.2$'
+! stderr .
+
+go list -m -mod=readonly -versions rsc.io/quote
+stdout 'rsc\.io/quote v1\.0\.0 .* v1\.5\.3-pre1$'
+! stderr .
+
+# Incidentally fetching the required version of a module records its checksum,
+# just because it happens to be in the build list, and recording the checksum
+# triggers an error under -mod=readonly.
+#
+# TODO(#41297): This should not be an error.
+! go list -m -mod=readonly rsc.io/quote@<v1.5.2
+stderr '^go: updates to go.sum needed, disabled by -mod=readonly$'
+! stderr 'missing go.sum entry'
+
+# Attempting to list the versions of a module that is not a root dependency
+# causes the build list to be resolved (so that the selected version can *also*
+# be identified, even though it is not relevant to this particular output).
+# That, in turn, causes us to need checksums for the go.sum files for the
+# modules in the module graph.
+#
+# TODO(#41297): This should not be an error either.
+! go list -m -mod=readonly -versions rsc.io/sampler
+stderr '^go: rsc\.io/quote@v1\.5\.1: missing go\.sum entry for go.mod file; to add it:\n\tgo mod download rsc\.io/quote$'
diff --git a/src/cmd/go/testdata/script/mod_list_test.txt b/src/cmd/go/testdata/script/mod_list_test.txt
new file mode 100644
index 0000000..f697af6
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_list_test.txt
@@ -0,0 +1,30 @@
+env GO111MODULE=on
+
+# go list -compiled -test must handle test-only packages
+# golang.org/issue/27097.
+go list -compiled -test
+stdout -count=4 '^.' # 4 lines
+stdout '^m$'
+stdout '^m\.test$'
+stdout '^m \[m\.test\]$'
+stdout '^m_test \[m\.test\]$'
+
+# https://golang.org/issue/39974: test packages should have the Module field populated.
+go list -test -f '{{.ImportPath}}{{with .Module}}: {{.Path}}{{end}}'
+stdout -count=4 '^.' # 4 lines
+stdout '^m: m$'
+stdout '^m\.test: m$'
+stdout '^m \[m\.test\]: m$'
+stdout '^m_test \[m\.test\]: m$'
+
+-- go.mod --
+module m
+
+-- x_test.go --
+package x
+import "testing"
+func Test(t *testing.T) {}
+-- x_x_test.go --
+package x_test
+import "testing"
+func Test(t *testing.T) {}
diff --git a/src/cmd/go/testdata/script/mod_list_test_cycle.txt b/src/cmd/go/testdata/script/mod_list_test_cycle.txt
new file mode 100644
index 0000000..755e50b
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_list_test_cycle.txt
@@ -0,0 +1,23 @@
+# https://golang.org/issue/45863: a typo in a test package leading to an
+# import cycle should be diagnosed, instead of causing an infinite loop.
+# The failure mode of this test prior to the fix was a timeout or OOM crash.
+
+go list -e -test -deps ./datastore/sql
+
+-- go.mod --
+module golang.org/issue45863
+
+go 1.17
+-- datastore/datastore_health.go --
+package datastore
+
+import (
+	"golang.org/issue45863/datastore"
+	"golang.org/issue45863/datastore/sql"
+)
+-- datastore/sql/sql.go --
+package sql
+-- datastore/sql/sql_test.go --
+package sql
+
+import _ "golang.org/issue45863/datastore"
diff --git a/src/cmd/go/testdata/script/mod_list_update_nolatest.txt b/src/cmd/go/testdata/script/mod_list_update_nolatest.txt
new file mode 100644
index 0000000..7eebe26
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_list_update_nolatest.txt
@@ -0,0 +1,55 @@
+# Check that if a proxy does not have a version of a module that could be
+# an upgrade, 'go list -m -u' still succeeds.
+# We use a local file proxy, since our test proxy doesn't have the behavior
+# we want to test, and we don't want it to be too clever.
+# Verifies #45305, where proxy.golang.org serves an empty /@v/list (200)
+# but has no /@latest (410) because the go.mod at the tip of the default
+# branch has a different major version suffix.
+env testproxy=$GOPROXY
+env GOPROXY=file:///$WORK/proxy
+env GOSUMDB=off
+
+# If the proxy does not return a list of versions (404/410)
+# or a latest version (404/410), we should see no error.
+go list -m example.com/noversion
+stdout '^example.com/noversion v0.0.0$'
+go list -m -u example.com/noversion
+stdout '^example.com/noversion v0.0.0$'
+
+# If the proxy returns an empty list of versions (200, not 404/410)
+# but does not have a latest version (404/410), we should see no error.
+go list -m example.com/nolatest
+stdout '^example.com/nolatest v0.0.0$'
+go list -m -u example.com/nolatest
+stdout '^example.com/nolatest v0.0.0$'
+
+# If proxy returns an invalid response, we should see an error.
+env GOPROXY=$testproxy/invalid
+! go list -m -u example.com/nolatest
+stderr '^go: loading module retractions for example.com/nolatest@v0.0.0: invalid response from proxy "[^"]*": invalid character ''i'' looking for beginning of value$'
+
+-- go.mod --
+module m
+
+go 1.17
+
+require (
+	example.com/nolatest v0.0.0
+	example.com/noversion v0.0.0
+)
+-- go.sum --
+example.com/nolatest v0.0.0/go.mod h1:HnLrCt6SJga5tCtJ7IzG9dOOCniY3G5C0VT7jfMdS0M=
+example.com/noversion v0.0.0/go.mod h1:2RUfWiCYsygSXPM2Igxx0FD3Kq33OnVdxm34eDDhXbQ=
+-- $WORK/proxy/example.com/nolatest/@v/list --
+-- $WORK/proxy/example.com/nolatest/@v/v0.0.0.info --
+{"Version":"v0.0.0"}
+-- $WORK/proxy/example.com/nolatest/@v/v0.0.0.mod --
+module example.com/nolatest
+
+go 1.17
+-- $WORK/proxy/example.com/noversion/@v/v0.0.0.info --
+{"Version":"v0.0.0"}
+-- $WORK/proxy/example.com/noversion/@v/v0.0.0.mod --
+module example.com/noversion
+
+go 1.17
diff --git a/src/cmd/go/testdata/script/mod_list_upgrade.txt b/src/cmd/go/testdata/script/mod_list_upgrade.txt
new file mode 100644
index 0000000..0cef04b
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_list_upgrade.txt
@@ -0,0 +1,12 @@
+env GO111MODULE=on
+
+# Populate go.sum
+go list -m -mod=mod all
+
+# Check for upgrades.
+go list -m -u all
+stdout 'rsc.io/quote v1.2.0 \[v1\.5\.2\]'
+
+-- go.mod --
+module x
+require rsc.io/quote v1.2.0
diff --git a/src/cmd/go/testdata/script/mod_list_upgrade_pseudo.txt b/src/cmd/go/testdata/script/mod_list_upgrade_pseudo.txt
new file mode 100644
index 0000000..8e51dfc
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_list_upgrade_pseudo.txt
@@ -0,0 +1,26 @@
+env GO111MODULE=on
+
+# For this test module there are three versions:
+#   * v0.1.1-0.20190429073117-b5426c86b553
+#   * v0.1.0
+#   * v0.0.0-20190430073000-30950c05d534
+# Only v0.1.0 is tagged.
+#
+# The v0.1.1 pseudo-version is semantically higher than the latest tag.
+# The v0.0.0 pseudo-version is chronologically newer.
+
+# The latest pseudo-version is semantically higher than the latest tag.
+# 'list -u' should not suggest a lower version as an upgrade.
+
+go get example.com/pseudoupgrade@b5426c8
+go list -m -u all
+stdout '^example.com/pseudoupgrade v0.1.1-0.20190429073117-b5426c86b553$'
+
+go get example.com/pseudoupgrade@v0.0.0-20190430073000-30950c05d534
+go list -m -u all
+stdout '^example.com/pseudoupgrade v0.0.0-20190430073000-30950c05d534$'
+
+-- go.mod --
+module x
+
+go 1.12
diff --git a/src/cmd/go/testdata/script/mod_load_badchain.txt b/src/cmd/go/testdata/script/mod_load_badchain.txt
new file mode 100644
index 0000000..500a954
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_load_badchain.txt
@@ -0,0 +1,110 @@
+[short] skip
+env GO111MODULE=on
+
+# Download everything to avoid "finding" messages in stderr later.
+cp go.mod.orig go.mod
+go mod download
+go mod download example.com@v1.0.0
+go mod download example.com/badchain/a@v1.1.0
+go mod download example.com/badchain/b@v1.1.0
+go mod download example.com/badchain/c@v1.1.0
+
+# Try to update example.com/badchain/a (and its dependencies).
+! go get example.com/badchain/a
+cmp stderr update-a-expected
+cmp go.mod go.mod.orig
+
+# Try to update the main module. This updates everything, including
+# modules that aren't direct requirements, so the error stack is shorter.
+go get -u ./...
+cmp stderr update-main-expected
+cmp go.mod go.mod.withc
+
+# Update manually. Listing modules should produce an error.
+cp go.mod.orig go.mod
+go mod edit -require=example.com/badchain/a@v1.1.0
+! go list -m all
+cmp stderr list-expected
+
+# Try listing a package that imports a package
+# in a module without a requirement.
+go mod edit -droprequire example.com/badchain/a
+! go list -mod=mod m/use
+cmp stderr list-missing-expected
+
+! go list -mod=mod -test m/testuse
+cmp stderr list-missing-test-expected
+
+-- go.mod.orig --
+module m
+
+go 1.13
+
+require example.com/badchain/a v1.0.0
+-- go.mod.withc --
+module m
+
+go 1.13
+
+require (
+	example.com/badchain/a v1.0.0
+	example.com/badchain/c v1.0.0
+)
+-- go.sum --
+example.com/badchain/a v1.0.0 h1:iJDLiHLmpQgr9Zrv+44UqywAE2IG6WkHnH4uG08vf+s=
+example.com/badchain/a v1.0.0/go.mod h1:6/gnCYHdVrs6mUgatUYUSbuHxEY+/yWedmTggLz23EI=
+example.com/badchain/a v1.1.0 h1:cPxQpsOjaIrn05yDfl4dFFgGSbjYmytLqtIIBfTsEqA=
+example.com/badchain/a v1.1.0/go.mod h1:T15b2BEK+RY7h7Lr2dgS38p1pgH5/t7Kf5nQXBlcW/A=
+example.com/badchain/b v1.0.0 h1:kjDVlBxpjQavYxHE7ECCyyXhfwsfhWIqvghfRgPktSA=
+example.com/badchain/b v1.0.0/go.mod h1:sYsH934pMc3/A2vQZh019qrWmp4+k87l3O0VFUYqL+I=
+example.com/badchain/b v1.1.0 h1:iEALV+DRN62FArnYylBR4YwCALn/hCdITvhdagHa0L4=
+example.com/badchain/b v1.1.0/go.mod h1:mlCgKO7lRZ+ijwMFIBFRPCGt5r5oqCcHdhSSE0VL4uY=
+example.com/badchain/c v1.0.0 h1:lOeUHQKR7SboSH7Bj6eIDWoNHaDQXI0T2GfaH2x9fNA=
+example.com/badchain/c v1.0.0/go.mod h1:4U3gzno17SaQ2koSVNxITu9r60CeLSgye9y4/5LnfOE=
+example.com/badchain/c v1.1.0 h1:VtTg1g7fOutWKHQf+ag04KLRpdMGSfQ9s9tagVtGW14=
+example.com/badchain/c v1.1.0/go.mod h1:tyoJj5qh+qtb48sflwdVvk4R+OjPQEY2UJOoibsVLPk=
+-- use/use.go --
+package use
+
+import _ "example.com/badchain/c"
+-- testuse/testuse.go --
+package testuse
+-- testuse/testuse_test.go --
+package testuse
+
+import (
+	"testing"
+	_ "example.com/badchain/c"
+)
+
+func Test(t *testing.T) {}
+-- update-main-expected --
+go: example.com/badchain/c@v1.1.0: parsing go.mod:
+	module declares its path as: badchain.example.com/c
+	        but was required as: example.com/badchain/c
+	restoring example.com/badchain/c@v1.0.0
+-- update-a-expected --
+go: example.com/badchain/a@upgrade (v1.1.0) indirectly requires example.com/badchain/c@v1.1.0: parsing go.mod:
+	module declares its path as: badchain.example.com/c
+	        but was required as: example.com/badchain/c
+-- list-expected --
+go: example.com/badchain/a@v1.1.0 requires
+	example.com/badchain/b@v1.1.0 requires
+	example.com/badchain/c@v1.1.0: parsing go.mod:
+	module declares its path as: badchain.example.com/c
+	        but was required as: example.com/badchain/c
+-- list-missing-expected --
+go: finding module for package example.com/badchain/c
+go: found example.com/badchain/c in example.com/badchain/c v1.1.0
+go: m/use imports
+	example.com/badchain/c: example.com/badchain/c@v1.1.0: parsing go.mod:
+	module declares its path as: badchain.example.com/c
+	        but was required as: example.com/badchain/c
+-- list-missing-test-expected --
+go: finding module for package example.com/badchain/c
+go: found example.com/badchain/c in example.com/badchain/c v1.1.0
+go: m/testuse tested by
+	m/testuse.test imports
+	example.com/badchain/c: example.com/badchain/c@v1.1.0: parsing go.mod:
+	module declares its path as: badchain.example.com/c
+	        but was required as: example.com/badchain/c
diff --git a/src/cmd/go/testdata/script/mod_load_badmod.txt b/src/cmd/go/testdata/script/mod_load_badmod.txt
new file mode 100644
index 0000000..fa22e18
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_load_badmod.txt
@@ -0,0 +1,25 @@
+# Unknown lines should be ignored in dependency go.mod files.
+go list -m -mod=mod all
+
+# ... and in replaced dependency go.mod files.
+cp go.mod go.mod.usesub
+go list -m -mod=mod all
+
+# ... but not in the main module.
+cp go.mod.bad go.mod
+! go list -m -mod=mod all
+stderr 'unknown directive: hello'
+
+-- go.mod --
+module m
+require rsc.io/badmod v1.0.0
+-- go.mod.bad --
+module m
+hello world
+-- go.mod.usesub --
+module m
+require rsc.io/badmod v1.0.0
+replace rsc.io/badmod v1.0.0 => ./sub
+-- sub/go.mod --
+module sub
+hello world
diff --git a/src/cmd/go/testdata/script/mod_load_badzip.txt b/src/cmd/go/testdata/script/mod_load_badzip.txt
new file mode 100644
index 0000000..58160b4
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_load_badzip.txt
@@ -0,0 +1,13 @@
+# Zip files with unexpected file names inside should be rejected.
+env GO111MODULE=on
+
+! go get rsc.io/badzip
+stderr 'zip for rsc.io/badzip@v1.0.0 has unexpected file rsc.io/badzip@v1.0.0.txt'
+! grep rsc.io/badzip go.mod
+
+go mod edit -require rsc.io/badzip@v1.0.0
+! go build -mod=mod rsc.io/badzip
+stderr 'zip for rsc.io/badzip@v1.0.0 has unexpected file rsc.io/badzip@v1.0.0.txt'
+
+-- go.mod --
+module m
diff --git a/src/cmd/go/testdata/script/mod_load_replace_mismatch.txt b/src/cmd/go/testdata/script/mod_load_replace_mismatch.txt
new file mode 100644
index 0000000..2ca8b3c
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_load_replace_mismatch.txt
@@ -0,0 +1,23 @@
+# If a replacement module declares a module path different from both
+# the original module and its location, report an error with all three paths.
+# In particular, the "required as" path should be the original.
+# Verifies golang.org/issue/38220.
+! go mod download
+cmp stderr want
+
+-- go.mod --
+module m
+
+require rsc.io/quote v1.5.2
+
+replace rsc.io/quote v1.5.2 => example.com/quote v1.5.2
+
+-- use.go --
+package use
+
+import _ "rsc.io/quote"
+
+-- want --
+go: rsc.io/quote@v1.5.2 (replaced by example.com/quote@v1.5.2): parsing go.mod:
+	module declares its path as: rsc.io/Quote
+	        but was required as: rsc.io/quote
diff --git a/src/cmd/go/testdata/script/mod_local_replace.txt b/src/cmd/go/testdata/script/mod_local_replace.txt
new file mode 100644
index 0000000..19bc8f3
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_local_replace.txt
@@ -0,0 +1,23 @@
+# Test that local replacements work even with dummy module names.
+# golang.org/issue/24100.
+
+env GO111MODULE=on
+
+cd x/y
+go list -f '{{.Dir}}' zz
+stdout x[/\\]z$
+
+-- x/y/go.mod --
+module x/y
+require zz v1.0.0
+replace zz v1.0.0 => ../z
+
+-- x/y/y.go --
+package y
+import _ "zz"
+
+-- x/z/go.mod --
+module x/z
+
+-- x/z/z.go --
+package z
diff --git a/src/cmd/go/testdata/script/mod_missing_repo.txt b/src/cmd/go/testdata/script/mod_missing_repo.txt
new file mode 100644
index 0000000..d0076f7
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_missing_repo.txt
@@ -0,0 +1,15 @@
+# Regression test for golang.org/issue/34094: modules hosted within gitlab.com
+# subgroups could not be fetched because the server returned bogus go-import
+# tags for prefixes of the module path.
+
+[short] skip
+[!git] skip
+
+env GO111MODULE=on
+env GOPROXY=direct
+env GOSUMDB=off
+
+! go mod download vcs-test.golang.org/go/missingrepo/missingrepo-git@latest
+stderr 'vcs-test.golang.org/go/missingrepo/missingrepo-git: git ls-remote .*: exit status .*'
+
+go mod download vcs-test.golang.org/go/missingrepo/missingrepo-git/notmissing@latest
diff --git a/src/cmd/go/testdata/script/mod_missingpkg_prerelease.txt b/src/cmd/go/testdata/script/mod_missingpkg_prerelease.txt
new file mode 100644
index 0000000..9c250e7
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_missingpkg_prerelease.txt
@@ -0,0 +1,17 @@
+env GO111MODULE=on
+
+! go list -mod=mod -deps use.go
+stderr '^use.go:4:2: package example.com/missingpkg/deprecated provided by example.com/missingpkg at latest version v1.0.0 but not at required version v1.0.1-beta$'
+
+-- go.mod --
+module m
+
+go 1.14
+
+-- use.go --
+package use
+
+import (
+	_ "example.com/missingpkg/deprecated"
+	_ "example.com/usemissingpre"
+)
diff --git a/src/cmd/go/testdata/script/mod_modinfo.txt b/src/cmd/go/testdata/script/mod_modinfo.txt
new file mode 100644
index 0000000..8d77e22
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_modinfo.txt
@@ -0,0 +1,89 @@
+# Test to ensure runtime/debug.ReadBuildInfo parses
+# the modinfo embedded in a binary by the go tool
+# when module is enabled.
+env GO111MODULE=on
+
+cd x
+go mod edit -require=rsc.io/quote@v1.5.2
+go mod edit -replace=rsc.io/quote@v1.5.2=rsc.io/quote@v1.0.0
+go mod tidy # populate go.sum
+
+# Build a binary and ensure that it can output its own debug info.
+# The debug info should be accessible before main starts (golang.org/issue/29628).
+go build
+exec ./x$GOEXE
+stderr 'mod\s+x\s+\(devel\)'
+stderr 'dep\s+rsc.io/quote\s+v1.5.2\s+'
+stderr '=>\s+rsc.io/quote\s+v1.0.0\s+h1:'
+stderr 'Hello, world.'
+
+[short] skip
+
+# Build a binary that accesses its debug info by reading the binary directly
+# (rather than through debug.ReadBuildInfo).
+# The debug info should still be present (golang.org/issue/28753).
+cd unused
+go build
+exec ./unused$GOEXE
+
+-- x/go.mod --
+module x
+
+-- x/lib/lib.go --
+// Package lib accesses runtime/debug.modinfo before package main's init
+// functions have run.
+package lib
+
+import "runtime/debug"
+
+func init() {
+	m, ok := debug.ReadBuildInfo()
+	if !ok {
+		panic("failed debug.ReadBuildInfo")
+	}
+	println("mod", m.Main.Path, m.Main.Version)
+	for _, d := range m.Deps {
+		println("dep", d.Path, d.Version, d.Sum)
+		if r := d.Replace; r != nil {
+			println("=>", r.Path, r.Version, r.Sum)
+		}
+	}
+}
+
+-- x/main.go --
+package main
+
+import (
+	"rsc.io/quote"
+	_ "x/lib"
+)
+
+func main() {
+	println(quote.Hello())
+}
+
+-- x/unused/main.go --
+// The unused binary does not access runtime/debug.modinfo.
+package main
+
+import (
+	"bytes"
+	"encoding/hex"
+	"log"
+	"os"
+
+	_ "rsc.io/quote"
+)
+
+func main() {
+	b, err := os.ReadFile(os.Args[0])
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	infoStart, _ := hex.DecodeString("3077af0c9274080241e1c107e6d618e6")
+	if !bytes.Contains(b, infoStart) {
+		log.Fatal("infoStart not found in binary")
+	}
+	log.Println("ok")
+}
diff --git a/src/cmd/go/testdata/script/mod_multirepo.txt b/src/cmd/go/testdata/script/mod_multirepo.txt
new file mode 100644
index 0000000..bbefb78
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_multirepo.txt
@@ -0,0 +1,41 @@
+env GO111MODULE=on
+
+# initial standalone module should use no downloaded modules
+go list -deps -f {{.Dir}}
+! stdout 'pkg[\\/]mod'
+
+# v2 import should use a downloaded module
+# both without an explicit go.mod entry ...
+cp tmp/use_v2.go x.go
+go get .
+go list -deps -f {{.Dir}}
+stdout 'pkg[\\/]mod[\\/]rsc.io[\\/]quote[\\/]v2@v2.0.1$'
+
+# ... and with one ...
+cp tmp/use_v2.mod go.mod
+go list -deps -f {{.Dir}}
+stdout 'pkg[\\/]mod[\\/]rsc.io[\\/]quote[\\/]v2@v2.0.1$'
+
+# ... and even if there is a v2 module in a subdirectory.
+mkdir v2
+cp x.go v2/x.go
+cp tmp/v2.mod v2/go.mod
+go list -deps -f {{.Dir}}
+stdout 'pkg[\\/]mod[\\/]rsc.io[\\/]quote[\\/]v2@v2.0.1$'
+
+-- go.mod --
+module rsc.io/quote
+
+-- x.go --
+package quote
+
+-- tmp/use_v2.go --
+package quote
+import _ "rsc.io/quote/v2"
+
+-- tmp/use_v2.mod --
+module rsc.io/quote
+require rsc.io/quote/v2 v2.0.1
+
+-- tmp/v2.mod --
+package rsc.io/quote/v2
diff --git a/src/cmd/go/testdata/script/mod_no_gopath.txt b/src/cmd/go/testdata/script/mod_no_gopath.txt
new file mode 100644
index 0000000..ed91f5d
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_no_gopath.txt
@@ -0,0 +1,15 @@
+# https://golang.org/issue/43938: 'go build' should succeed
+# if GOPATH and the variables needed for its default value
+# are all unset but not relevant to the specific command.
+
+env HOME=''
+env home=''
+env GOPATH=''
+
+go list -deps main.go
+stdout '^io$'
+
+-- main.go --
+package main
+
+import _ "io"
diff --git a/src/cmd/go/testdata/script/mod_nomod.txt b/src/cmd/go/testdata/script/mod_nomod.txt
new file mode 100644
index 0000000..7e0f55a
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_nomod.txt
@@ -0,0 +1,43 @@
+# Test go commands with no module.
+env GO111MODULE=on
+
+# go mod edit fails unless given explicit mod file argument
+! go mod edit -json
+go mod edit -json x.mod
+
+# bug succeeds
+[exec:echo] env BROWSER=echo
+[exec:echo] go bug
+
+# commands that load the package in the current directory fail
+! go build
+! go fmt
+! go generate
+! go get
+! go install
+! go list
+! go run
+! go test
+! go vet
+
+# clean succeeds, even with -modcache
+go clean -modcache
+
+# doc succeeds for standard library
+go doc unsafe
+
+# env succeeds
+go env
+
+# tool succeeds
+go tool -n test2json
+
+# version succeeds
+go version
+
+-- x.mod --
+module m
+
+-- x.go --
+package main
+func main() {}
diff --git a/src/cmd/go/testdata/script/mod_notall.txt b/src/cmd/go/testdata/script/mod_notall.txt
new file mode 100644
index 0000000..1657c8d
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_notall.txt
@@ -0,0 +1,99 @@
+# This test demonstrates go commands that combine the 'all' pattern
+# with packages outside of 'all'.
+
+# With -deps, 'all' should include test dependencies of packages in the main
+# module, but not should not include test dependencies of packages imported only
+# by other root patterns.
+
+env GOFLAGS=-mod=mod
+cp go.mod go.mod.orig
+
+go list -deps all x/otherroot
+
+stdout '^x/inall$'
+stdout '^x/inall/fromtest$'
+stdout '^x/inall/fromtestinall$'
+stdout '^x/otherroot$'
+stdout '^x/otherdep$'
+
+! stdout '^x/fromotherroottest$'
+! stdout '^y/fromotherdeptest$'
+
+cmp go.mod go.mod.orig
+
+# With -deps -test, test dependencies of other roots should be included,
+# but test dependencies of non-roots should not.
+
+go list -deps -test all x/otherroot
+stdout '^x/inall$'
+stdout '^x/inall/fromtest$'
+stdout '^x/inall/fromtestinall$'
+stdout '^x/otherroot$'
+stdout '^x/otherdep$'
+
+stdout '^x/fromotherroottest$'
+! stdout '^y/fromotherdeptest$'
+
+cmp go.mod go.mod.orig
+
+-- m.go --
+package m
+
+import _ "x/inall"
+-- m_test.go --
+package m_test
+
+import _ "x/inall/fromtest"
+-- go.mod --
+module m
+
+go 1.15
+
+require x v0.1.0
+
+replace (
+	x v0.1.0 => ./x
+	y v0.1.0 => ./y
+)
+-- x/go.mod --
+module x
+
+go 1.15
+-- x/inall/inall.go --
+package inall
+-- x/inall/inall_test.go --
+package inall_test
+
+import _ "x/inall/fromtestinall"
+-- x/inall/fromtest/fromtest.go --
+package fromtest
+-- x/inall/fromtestinall/fromtestinall.go --
+package fromtestinall
+-- x/otherroot/otherroot.go --
+package otherroot
+
+import _ "x/otherdep"
+-- x/otherroot/otherroot_test.go --
+package otherroot_test
+
+import _ "x/fromotherroottest"
+-- x/fromotherroottest/fromotherroottest.go --
+package fromotherroottest
+-- x/otherdep/otherdep.go --
+package otherdep
+-- x/otherdep/otherdep_test.go --
+package otherdep_test
+
+import _ "y/fromotherdeptest"
+-- x/otherroot/testonly/testonly.go --
+package testonly
+-- y/go.mod --
+module y
+
+go 1.15
+-- y/fromotherdeptest/fromotherdeptest.go --
+// Package fromotherdeptest is a test dependency of x/otherdep that is
+// not declared in x/go.mod. If the loader resolves this package,
+// it will add this module to the main module's go.mod file,
+// and we can detect the mistake.
+package fromotherdeptest
diff --git a/src/cmd/go/testdata/script/mod_off.txt b/src/cmd/go/testdata/script/mod_off.txt
new file mode 100644
index 0000000..a73a58d
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_off.txt
@@ -0,0 +1,35 @@
+env GO111MODULE=off
+
+# This script tests that running go mod with
+# GO111MODULE=off when outside of GOPATH will fatal
+# with an error message, even with some source code in the directory and a go.mod.
+! go mod init
+stderr 'go: modules disabled by GO111MODULE=off; see ''go help modules'''
+! go mod graph
+stderr 'go: modules disabled by GO111MODULE=off; see ''go help modules'''
+! go mod verify
+stderr 'go: modules disabled by GO111MODULE=off; see ''go help modules'''
+! go mod download
+stderr 'go: modules disabled by GO111MODULE=off; see ''go help modules'''
+
+# Same result in an empty directory
+mkdir z
+cd z
+! go mod init
+stderr 'go: modules disabled by GO111MODULE=off; see ''go help modules'''
+! go mod graph
+stderr 'go: modules disabled by GO111MODULE=off; see ''go help modules'''
+! go mod verify
+stderr 'go: modules disabled by GO111MODULE=off; see ''go help modules'''
+! go mod download
+stderr 'go: modules disabled by GO111MODULE=off; see ''go help modules'''
+
+-- sample.go --
+package sample
+
+func main() {}
+
+-- go.mod --
+module sample
+
+go 1.12
diff --git a/src/cmd/go/testdata/script/mod_off_init.txt b/src/cmd/go/testdata/script/mod_off_init.txt
new file mode 100644
index 0000000..2aec0b3
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_off_init.txt
@@ -0,0 +1,5 @@
+# 'go mod init' should refuse to initialize a module if it will be
+# ignored anyway due to GO111MODULE=off.
+env GO111MODULE=off
+! go mod init
+stderr 'go: modules disabled by GO111MODULE=off; see ''go help modules'''
diff --git a/src/cmd/go/testdata/script/mod_outside.txt b/src/cmd/go/testdata/script/mod_outside.txt
new file mode 100644
index 0000000..7a0dc9f
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_outside.txt
@@ -0,0 +1,315 @@
+env GO111MODULE=on
+[short] skip
+
+# This script tests commands in module mode outside of any module.
+#
+# First, ensure that we really are in module mode, and that we really don't have
+# a go.mod file.
+go env GOMOD
+stdout 'NUL|/dev/null'
+
+
+# 'go list' without arguments implicitly operates on the current directory,
+# which is not in a module.
+! go list
+stderr '^go: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+go list -m
+stdout '^command-line-arguments$'
+# 'go list' in the working directory should fail even if there is a a 'package
+# main' present: without a main module, we do not know its package path.
+! go list ./needmod
+stderr '^go: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+
+# 'go list all' lists the transitive import graph of the main module,
+# which is empty if there is no main module.
+go list all
+! stdout .
+stderr 'warning: "all" matched no packages'
+
+# 'go list' on standard-library packages should work, since they do not depend
+# on the contents of any module.
+go list -deps cmd
+stdout '^fmt$'
+stdout '^cmd/go$'
+
+go list $GOROOT/src/fmt
+stdout '^fmt$'
+
+# 'go list' should work with file arguments.
+go list ./needmod/needmod.go
+stdout 'command-line-arguments'
+
+# 'go list' on a package from a module should fail.
+! go list example.com/printversion
+stderr '^no required module provides package example.com/printversion: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+
+
+# 'go list -m' with an explicit version should resolve that version.
+go list -m example.com/version@latest
+stdout 'example.com/version v1.1.0'
+
+# 'go list -m -versions' should succeed even without an explicit version.
+go list -m -versions example.com/version
+stdout 'v1.0.0\s+v1.0.1\s+v1.1.0'
+
+# 'go list -m all' should fail. "all" is not meaningful outside of a module.
+! go list -m all
+stderr 'go: cannot match "all": go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+
+# 'go list -m <mods> all' should also fail.
+! go list -m example.com/printversion@v1.0.0 all
+stderr 'go: cannot match "all": go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+! stdout 'example.com/version'
+
+# 'go list -m <mods>' should fail if any of the mods lacks an explicit version.
+! go list -m example.com/printversion
+stderr 'go: cannot match "example.com/printversion" without -versions or an explicit version: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+! stdout 'example.com/version'
+
+# 'go list -m' with wildcards should fail. Wildcards match modules in the
+# build list, so they aren't meaningful outside a module.
+! go list -m ...
+stderr 'go: cannot match "...": go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+! go list -m rsc.io/quote/...
+stderr 'go: cannot match "rsc.io/quote/...": go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+
+
+# 'go clean' should skip the current directory if it isn't in a module.
+go clean -n
+! stdout .
+! stderr .
+
+# 'go mod graph' should fail, since there's no module graph.
+! go mod graph
+stderr '^go: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+
+# 'go mod why' should fail, since there is no main module to depend on anything.
+! go mod why -m example.com/version
+stderr '^go: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+
+# 'go mod edit', 'go mod tidy', and 'go mod fmt' should fail:
+# there is no go.mod file to edit.
+! go mod tidy
+stderr '^go: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+! go mod edit -fmt
+stderr '^go: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+! go mod edit -require example.com/version@v1.0.0
+stderr '^go: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+
+
+# 'go mod download' without arguments should report an error.
+! go mod download
+stderr 'no modules specified'
+
+# 'go mod download' should download exactly the requested module without dependencies.
+rm -r $GOPATH/pkg/mod/cache/download/example.com
+go mod download example.com/printversion@v1.0.0
+exists $GOPATH/pkg/mod/cache/download/example.com/printversion/@v/v1.0.0.zip
+! exists $GOPATH/pkg/mod/cache/download/example.com/version/@v/v1.0.0.zip
+
+# 'go mod download all' should fail. "all" is not meaningful outside of a module.
+! go mod download all
+stderr 'go: cannot match "all": go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+
+
+# 'go mod vendor' should fail: it starts by clearing the existing vendor
+# directory, and we don't know where that is.
+! go mod vendor
+stderr '^go: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+
+
+# 'go mod verify' should fail: we have no modules to verify.
+! go mod verify
+stderr '^go: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+
+
+# 'go get' has no go.mod file to update outside a module and should fail.
+! go get
+stderr '^go: go.mod file not found in current directory or any parent directory.$'
+stderr '^\t''go get'' is no longer supported outside a module.$'
+! go get -u
+stderr '^go: go.mod file not found in current directory or any parent directory.$'
+stderr '^\t''go get'' is no longer supported outside a module.$'
+! go get -u ./needmod
+stderr '^go: go.mod file not found in current directory or any parent directory.$'
+stderr '^\t''go get'' is no longer supported outside a module.$'
+! go get -u all
+stderr '^go: go.mod file not found in current directory or any parent directory.$'
+stderr '^\t''go get'' is no longer supported outside a module.$'
+! go get example.com/printversion@v1.0.0 example.com/version@none
+stderr '^go: go.mod file not found in current directory or any parent directory.$'
+stderr '^\t''go get'' is no longer supported outside a module.$'
+
+# 'go get' should not download anything.
+go clean -modcache
+! go get example.com/printversion@v1.0.0
+stderr '^go: go.mod file not found in current directory or any parent directory.$'
+stderr '^\t''go get'' is no longer supported outside a module.$'
+! exists $GOPATH/pkg/mod/example.com/printversion@v1.0.0
+! exists $GOPATH/pkg/mod/example.com/version@v1.0.0
+
+
+# 'go build' without arguments implicitly operates on the current directory, and should fail.
+cd needmod
+! go build
+stderr '^go: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+cd ..
+
+# 'go build' of a non-module directory should fail too.
+! go build ./needmod
+stderr '^go: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+
+# 'go build' of source files should fail if they import anything outside std.
+! go build -n ./needmod/needmod.go
+stderr '^needmod[/\\]needmod.go:10:2: no required module provides package example.com/version: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+
+# 'go build' of source files should succeed if they do not import anything outside std.
+go build -n -o ignore ./stdonly/stdonly.go
+
+# 'go build' should succeed for standard-library packages.
+go build -n fmt
+
+# 'go build' should use the latest version of the Go language.
+go build ./newgo/newgo.go
+
+# 'go doc' without arguments implicitly operates on the current directory, and should fail.
+# TODO(golang.org/issue/32027): currently, it succeeds.
+cd needmod
+go doc
+cd ..
+
+# 'go doc' of a non-module directory should also succeed.
+go doc ./needmod
+
+# 'go doc' should succeed for standard-library packages.
+go doc fmt
+
+# 'go doc' should fail for a package path outside a module.
+! go doc example.com/version
+stderr 'doc: no required module provides package example.com/version: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+
+# 'go install' with a version should succeed if all constraints are met.
+# See mod_install_pkg_version.
+rm $GOPATH/bin
+go install example.com/printversion@v0.1.0
+exists $GOPATH/bin/printversion$GOEXE
+
+# 'go install' should fail if a package argument must be resolved to a module.
+! go install example.com/printversion
+stderr '^go: ''go install'' requires a version when current directory is not in a module\n\tTry ''go install example.com/printversion@latest'' to install the latest version$'
+
+# 'go install' should fail if a source file imports a package that must be
+# resolved to a module.
+! go install ./needmod/needmod.go
+stderr 'needmod[/\\]needmod.go:10:2: no required module provides package example.com/version: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+
+# 'go run' should fail if a package argument must be resolved to a module.
+! go run example.com/printversion
+stderr '^no required module provides package example.com/printversion: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+
+# 'go run' should fail if a source file imports a package that must be
+# resolved to a module.
+! go run ./needmod/needmod.go
+stderr '^needmod[/\\]needmod.go:10:2: no required module provides package example.com/version: go.mod file not found in current directory or any parent directory; see ''go help modules''$'
+
+
+# 'go fmt' should be able to format files outside of a module.
+go fmt needmod/needmod.go
+
+
+# The remainder of the test checks dependencies by linking and running binaries.
+
+# 'go run' should work with file arguments if they don't import anything
+# outside std.
+go run ./stdonly/stdonly.go
+stdout 'path is command-line-arguments$'
+stdout 'main is  $'
+
+# 'go generate' should work with file arguments.
+[exec:touch] go generate ./needmod/needmod.go
+[exec:touch] exists ./needmod/gen.txt
+
+# 'go install' should work with file arguments.
+go install ./stdonly/stdonly.go
+
+# 'go test' should work with file arguments.
+go test -v ./stdonly/stdonly_test.go
+stdout 'stdonly was tested'
+
+# 'go vet' should work with file arguments.
+go vet ./stdonly/stdonly.go
+
+
+-- README.txt --
+There is no go.mod file in the working directory.
+
+-- needmod/needmod.go --
+//go:generate touch gen.txt
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"runtime/debug"
+
+	_ "example.com/version"
+)
+
+func main() {
+	info, ok := debug.ReadBuildInfo()
+	if !ok {
+		panic("missing build info")
+	}
+	fmt.Fprintf(os.Stdout, "path is %s\n", info.Path)
+	fmt.Fprintf(os.Stdout, "main is %s %s\n", info.Main.Path, info.Main.Version)
+	for _, m := range info.Deps {
+		fmt.Fprintf(os.Stdout, "using %s %s\n", m.Path, m.Version)
+	}
+}
+
+-- stdonly/stdonly.go --
+package main
+
+import (
+	"fmt"
+	"os"
+	"runtime/debug"
+)
+
+func main() {
+	info, ok := debug.ReadBuildInfo()
+	if !ok {
+		panic("missing build info")
+	}
+	fmt.Fprintf(os.Stdout, "path is %s\n", info.Path)
+	fmt.Fprintf(os.Stdout, "main is %s %s\n", info.Main.Path, info.Main.Version)
+	for _, m := range info.Deps {
+		fmt.Fprintf(os.Stdout, "using %s %s\n", m.Path, m.Version)
+	}
+}
+
+-- stdonly/stdonly_test.go --
+package main
+
+import (
+	"fmt"
+	"testing"
+)
+
+func Test(t *testing.T) {
+	fmt.Println("stdonly was tested")
+}
+
+-- newgo/newgo.go --
+// Package newgo requires Go 1.14 or newer.
+package newgo
+
+import "io"
+
+const C = 299_792_458
+
+type ReadWriteCloser interface {
+	io.ReadCloser
+	io.WriteCloser
+}
diff --git a/src/cmd/go/testdata/script/mod_overlay.txt b/src/cmd/go/testdata/script/mod_overlay.txt
new file mode 100644
index 0000000..da35be6
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_overlay.txt
@@ -0,0 +1,254 @@
+# Test overlays that affect go.mod files
+
+# The go.mod file can exist only in the overlay.
+cd $WORK/gopath/src/no-go-mod
+go list -overlay overlay.json .
+stdout example.com/simple
+
+# Check content of overlaid go.mod is used.
+cd $WORK/gopath/src/overlay-go-mod
+go list -overlay overlay.json .
+stdout use.this/module/name
+
+# Check content of overlaid go.mod in a replacement module is used.
+# The go.mod in the replacement module is missing a requirement
+# that the overlay has, so it will fail to list without the overlay.
+cd $WORK/gopath/src/overlay-replaced-go-mod
+! go list -deps .
+go list -deps -overlay overlay.json .
+
+# Overlaid go.mod is not rewritten by 'go get'.
+cd $WORK/gopath/src/get-doesnt-add-dep
+cp $WORK/overlay/get_doesnt_add_dep_go_mod $WORK/want_go_mod
+! go get -overlay overlay.json .
+stderr '^go: updates to go.mod needed, but go.mod is part of the overlay specified with -overlay$'
+cmp $WORK/overlay/get_doesnt_add_dep_go_mod $WORK/want_go_mod
+
+# Content of overlaid go.sum is used.
+# The go.sum in the module directory has garbage values for its
+# hashes, but the overlaid file has the correct values. If
+# the correct go.sum is used with the overlay, 'go get .' should
+# not report a security error.
+cd $WORK/gopath/src/overlay-sum-used
+! go get .
+stderr 'SECURITY ERROR'
+! go mod verify
+stderr 'SECURITY ERROR'
+go get -overlay overlay.json .
+go mod verify -overlay overlay.json
+# Overlaid go.sum is not rewritten.
+# Copy an incomplete file to the overlay file, and expect an error
+# attempting to update the file
+cp incomplete-sum-file $WORK/overlay/overlay-sum-used-correct-sums
+! go get -overlay overlay.json .
+stderr '^go: updates to go.sum needed, but go.sum is part of the overlay specified with -overlay$'
+cmp incomplete-sum-file $WORK/overlay/overlay-sum-used-correct-sums
+! go mod tidy -overlay overlay.json
+stderr '^go: updates to go.sum needed, but go.sum is part of the overlay specified with -overlay$'
+cmp incomplete-sum-file $WORK/overlay/overlay-sum-used-correct-sums
+
+# -overlay works with -modfile.
+# There's an empty go.mod file in the directory, and the file alternate.mod is
+# overlaid to the true go.mod file, so the -modfile flag and the overlay
+# mechanism need to work together to determine the name of the module.
+cd $WORK/gopath/src/overlay-and-dash-modfile
+go list -modfile=alternate.mod -overlay overlay.json .
+stdout 'found.the/module'
+# Even with -modfile, overlaid files can't be opened for write.
+! go get -modfile=alternate.mod -overlay overlay.json rsc.io/quote
+stderr '^go: updates to go.mod needed, but go.mod is part of the overlay specified with -overlay$'
+
+# Carving out a module by adding an overlaid go.mod file
+cd $WORK/gopath/src/carve
+go list ./... # without an overlay, hasmod is carved out and nomod isn't
+stdout carve/nomod
+! stdout carve/hasmod
+go list -overlay overlay_carve_module.json ./... # The overlay carves out nomod, leaving nothing
+! stdout .
+stderr 'matched no packages'
+go list -overlay overlay_uncarve_module.json ./... # The overlay uncarves out hasmod
+stdout carve/nomod
+stdout carve/hasmod
+
+# Carving out a module by adding an overlaid go.mod file and using
+# -modfile to write to that file.
+cd $WORK/gopath/src/carve2/nomod
+go list -overlay overlay.json all
+! stdout ^carve2$
+stdout ^carve2/nomod$
+# Editing go.mod file fails because overlay is read only
+! go get -overlay overlay.json rsc.io/quote
+stderr '^go: updates to go.mod needed, but go.mod is part of the overlay specified with -overlay$'
+! grep rsc.io/quote $WORK/overlay/carve2-nomod-go.mod
+# Editing go.mod file succeeds because we use -modfile to redirect to same file
+go get -overlay overlay.json -modfile $WORK/overlay/carve2-nomod-go.mod rsc.io/quote
+grep rsc.io/quote $WORK/overlay/carve2-nomod-go.mod
+
+-- no-go-mod/file.go --
+package simple
+-- no-go-mod/overlay.json --
+{
+	"Replace": {
+		"go.mod": "../../../overlay/simple_go_mod"
+	}
+}
+-- $WORK/overlay/simple_go_mod --
+module example.com/simple
+-- overlay-go-mod/file.go --
+package name
+-- overlay-go-mod/go.mod --
+module dont.use/this/module/name
+-- overlay-go-mod/overlay.json --
+{
+	"Replace": {
+		"go.mod": "../../../overlay/use_this_go_mod"
+	}
+}
+-- $WORK/overlay/use_this_go_mod --
+module use.this/module/name
+-- overlay-replaced-go-mod/go.mod --
+module m
+
+go 1.15
+
+require replaced/mod v1.0.0
+replace replaced/mod v1.0.0 => ../replaced-mod
+replace dep/mod v1.0.0 => ../dep-mod
+-- overlay-replaced-go-mod/source.go --
+package m
+
+import "replaced/mod/foo"
+
+func main() {
+	foo.f()
+}
+-- overlay-replaced-go-mod/overlay.json --
+{
+	"Replace": {
+		"../replaced-mod/go.mod": "../../../overlay/replacement_module_go_mod"
+	}
+}
+-- replaced-mod/go.mod --
+module replaced/mod
+-- replaced-mod/foo/foo.go --
+package foo
+
+import "dep/mod/foo"
+
+func f() { foo.g() }
+-- dep-mod/go.mod --
+invalid
+-- dep-mod/foo/foo.go --
+package foo
+
+func g() { fmt.Println("hello") }
+-- $WORK/overlay/replacement_module_go_mod --
+module replaced/mod
+
+require dep/mod v1.0.0
+
+-- get-doesnt-add-dep/overlay.json --
+{
+	"Replace": {
+		"go.mod": "../../../overlay/get_doesnt_add_dep_go_mod"
+	}
+}
+-- get-doesnt-add-dep/p.go --
+package p
+
+import "dependency/mod"
+
+func f() { mod.G() }
+-- get-doesnt-add-dep-dependency/go.mod --
+module dependency/mod
+-- get-doesnt-add-dep-dependency/mod.go --
+package mod
+
+func G() {}
+-- $WORK/overlay/get_doesnt_add_dep_go_mod --
+module get.doesnt/add/dep
+
+replace dependency/mod v1.0.0 => ../get-doesnt-add-dep-dependency
+-- overlay-sum-used/go.mod --
+module overlay.sum/used
+
+require rsc.io/quote v1.5.0
+-- overlay-sum-used/p.go --
+package p
+
+import "rsc.io/quote"
+
+func f() string {
+	return quote.Hello()
+}
+-- overlay-sum-used/incomplete-sum-file --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c h1:pvCbr/wm8HzDD3fVywevekufpn6tCGPY3spdHeZJEsw=
+rsc.io/quote v1.5.0 h1:6fJa6E+wGadANKkUMlZ0DhXFpoKlslOQDCo259XtdIE=
+rsc.io/sampler v1.3.0 h1:HLGR/BgEtI3r0uymSP/nl2uPLsUnNJX8toRyhfpBTII=
+-- overlay-sum-used/overlay.json --
+{
+	"Replace": {
+		"go.sum": "../../../overlay/overlay-sum-used-correct-sums"
+	}
+}
+-- overlay-sum-used/go.sum --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c h1:garbage+hash
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:garbage+hash
+rsc.io/quote v1.5.0 h1:garbage+hash
+rsc.io/quote v1.5.0/go.mod h1:garbage+hash
+rsc.io/sampler v1.3.0 h1:garbage+hash
+rsc.io/sampler v1.3.0/go.mod h1:garbage+hash
+-- $WORK/overlay/overlay-sum-used-correct-sums --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c h1:pvCbr/wm8HzDD3fVywevekufpn6tCGPY3spdHeZJEsw=
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.5.0 h1:6fJa6E+wGadANKkUMlZ0DhXFpoKlslOQDCo259XtdIE=
+rsc.io/quote v1.5.0/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0 h1:HLGR/BgEtI3r0uymSP/nl2uPLsUnNJX8toRyhfpBTII=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+-- overlay-and-dash-modfile/p.go --
+package module
+-- overlay-and-dash-modfile/go.mod --
+-- overlay-and-dash-modfile/overlay.json --
+{
+	"Replace": {
+		"alternate.mod": "../../../overlay/overlay-and-dash-modfile-alternate-mod"
+	}
+}
+-- $WORK/overlay/overlay-and-dash-modfile-alternate-mod --
+module found.the/module
+-- carve/go.mod --
+module carve
+-- carve/overlay_carve_module.json --
+{
+	"Replace": {
+		"nomod/go.mod": "../../../overlay/carve-nomod-go-mod"
+	}
+}
+-- carve/overlay_uncarve_module.json --
+{
+	"Replace": {
+		"hasmod/go.mod": ""
+	}
+}
+-- carve/hasmod/a.go --
+package hasmod
+-- carve/hasmod/go.mod --
+module carve/hasmod
+-- carve/nomod/b.go --
+package nomod
+-- $WORK/overlay/carve-nomod-go-mod --
+module carve/nomod
+-- carve2/go.mod --
+module carve2
+-- carve2/p.go --
+package p
+-- carve2/nomod/overlay.json --
+{
+	"Replace": {
+		"go.mod": "../../../../overlay/carve2-nomod-go.mod"
+	}
+}
+-- carve2/nomod/b.go --
+package nomod
+-- $WORK/overlay/carve2-nomod-go.mod --
+module carve2/nomod
diff --git a/src/cmd/go/testdata/script/mod_patterns.txt b/src/cmd/go/testdata/script/mod_patterns.txt
new file mode 100644
index 0000000..1b4b438
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_patterns.txt
@@ -0,0 +1,78 @@
+env GO111MODULE=on
+[short] skip
+
+cd m
+
+# 'go list all' should list all of the packages used (directly or indirectly) by
+# the packages in the main module, but no other packages from the standard
+# library or active modules.
+#
+# 'go list ...' should list packages in all active modules and the standard library.
+#
+# 'go list example.com/m/...' should list packages in all modules that begin with 'example.com/m/'.
+#
+# 'go list ./...' should list only packages in the current module, not other active modules.
+#
+# Warnings about unmatched patterns should only be printed once.
+#
+# And the go command should be able to keep track of all this!
+go list -f '{{.ImportPath}}: {{.Match}}' all ... example.com/m/... ./... ./xyz...
+stdout 'example.com/m/useunicode: \[all \.\.\. example.com/m/... ./...\]'
+stdout 'example.com/m/useunsafe: \[all \.\.\. example.com/m/... ./...\]'
+[cgo] stdout 'example.com/m/useC: \[all \.\.\. example.com/m/... ./...\]'
+[!cgo] ! stdout example.com/m/useC
+stdout 'example.com/unused/useerrors: \[\.\.\.\]' # but not "all"
+stdout 'example.com/m/nested/useencoding: \[\.\.\. example.com/m/...\]' # but NOT "all" or "./..."
+stdout '^unicode: \[all \.\.\.\]'
+stdout '^unsafe: \[all \.\.\.\]'
+stdout 'index/suffixarray: \[\.\.\.\]'
+stdout 'cmd/pprof: \[\.\.\.\]'
+
+stderr -count=1 '^go: warning: "./xyz..." matched no packages$'
+
+# 'go list ./...' should not try to resolve the main module.
+cd ../empty
+go list -deps ./...
+! stdout .
+! stderr 'finding'
+stderr -count=1 '^go: warning: "./..." matched no packages'
+
+# disabling cgo should drop useC
+[short] skip
+env CGO_ENABLED=0
+go list -f '{{.ImportPath}}: {{.Match}}' all ... example.com/m/... ./... ./xyz...
+! stdout example.com/m/useC
+
+-- m/go.mod --
+module example.com/m
+
+require example.com/unused v0.0.0 // indirect
+replace example.com/unused => ../unused
+
+require example.com/m/nested v0.0.0 // indirect
+replace example.com/m/nested => ../nested
+
+-- m/useC/useC.go --
+package useC
+import _ "C" // "C" is a pseudo-package, not an actual one
+-- m/useunicode/useunicode.go --
+package useunicode
+import _ "unicode"
+-- m/useunsafe/useunsafe.go --
+package useunsafe
+import _ "unsafe"
+
+-- unused/go.mod --
+module example.com/unused
+-- unused/useerrors/useerrors.go --
+package useerrors
+import _ "errors"
+
+-- nested/go.mod --
+module example.com/m/nested
+-- nested/useencoding/useencoding.go --
+package useencoding
+import _ "encoding"
+
+-- empty/go.mod --
+module example.com/empty
diff --git a/src/cmd/go/testdata/script/mod_patterns_vendor.txt b/src/cmd/go/testdata/script/mod_patterns_vendor.txt
new file mode 100644
index 0000000..b4dc401
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_patterns_vendor.txt
@@ -0,0 +1,28 @@
+env GO111MODULE=on
+
+go list -mod=vendor example.com/...
+stdout ^example.com/x$
+stdout ^example.com/x/y$
+! stdout ^example.com/x/vendor
+
+-- go.mod --
+module example.com/m
+
+-- vendor/modules.txt --
+# example.com/x v0.0.0
+example.com/x
+# example.com/x/y v0.1.0
+example.com/x/y
+
+-- vendor/example.com/x/go.mod --
+module example.com/x
+-- vendor/example.com/x/x.go --
+package x
+
+-- vendor/example.com/x/y/go.mod --
+module example.com/x/y
+-- vendor/example.com/x/y/y.go --
+package y
+
+-- vendor/example.com/x/vendor/z/z.go --
+package z
diff --git a/src/cmd/go/testdata/script/mod_perm.txt b/src/cmd/go/testdata/script/mod_perm.txt
new file mode 100644
index 0000000..2972f46
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_perm.txt
@@ -0,0 +1,23 @@
+# go list should work in ordinary conditions.
+go list ./...
+! stdout _data
+
+# skip in conditions where chmod 0 may not work.
+# plan9 should be fine, but copied from list_perm.txt unchanged.
+[root] skip
+[GOOS:windows] skip
+[GOOS:plan9] skip
+
+# go list should work with unreadable _data directory.
+chmod 0 _data
+go list ./...
+! stdout _data
+
+-- go.mod --
+module m
+
+-- x.go --
+package m
+
+-- _data/x.go --
+package p
diff --git a/src/cmd/go/testdata/script/mod_permissions.txt b/src/cmd/go/testdata/script/mod_permissions.txt
new file mode 100644
index 0000000..b523e6a
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_permissions.txt
@@ -0,0 +1,57 @@
+# Regression test for golang.org/issue/34634: permissions for the go.sum and
+# go.mod files should be preserved when overwriting them.
+
+env GO111MODULE=on
+[short] skip
+
+# Skip platforms that do not have Unix-style file permissions.
+[GOOS:windows] skip
+[GOOS:plan9] skip
+
+chmod 0640 go.mod
+chmod 0604 go.sum
+go mod edit -module=golang.org/issue/34634
+
+go get
+cmp go.mod go.mod.want
+cmp go.sum go.sum.want
+
+go run .
+stdout 'go.mod: 0640'
+stdout 'go.sum: 0604'
+
+-- read_perm.go --
+package main
+
+import (
+	"fmt"
+	"os"
+	_ "rsc.io/sampler"
+)
+
+func main() {
+	for _, name := range []string{"go.mod", "go.sum"} {
+		fi, err := os.Stat(name)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "%s: %v\n", err)
+			continue
+		}
+		fmt.Printf("%s: 0%o\n", name, fi.Mode().Perm())
+	}
+}
+-- go.mod --
+module TODO
+
+go 1.14
+-- go.sum --
+-- go.mod.want --
+module golang.org/issue/34634
+
+go 1.14
+
+require rsc.io/sampler v1.99.99
+-- go.sum.want --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c h1:pvCbr/wm8HzDD3fVywevekufpn6tCGPY3spdHeZJEsw=
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/sampler v1.99.99 h1:iMG9lbEG/8MdeR4lgL+Q8IcwbLNw7ijW7fTiK8Miqts=
+rsc.io/sampler v1.99.99/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
diff --git a/src/cmd/go/testdata/script/mod_prefer_compatible.txt b/src/cmd/go/testdata/script/mod_prefer_compatible.txt
new file mode 100644
index 0000000..4d583ff
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_prefer_compatible.txt
@@ -0,0 +1,80 @@
+# Regression test for golang.org/issue/34189 and golang.org/issue/34165:
+# @latest, @upgrade, and @patch should prefer compatible versions over
+# +incompatible ones, even if offered by a proxy.
+
+[!net:github.com] skip
+[!net:proxy.golang.org] skip
+
+env GO111MODULE=on
+env GOPROXY=
+env GOSUMDB=
+
+# github.com/russross/blackfriday v2.0.0+incompatible exists,
+# and should be resolved if we ask for it explicitly.
+
+go list -m github.com/russross/blackfriday@v2.0.0+incompatible
+stdout '^github.com/russross/blackfriday v2\.0\.0\+incompatible$'
+
+# blackfriday v1.5.2 has a go.mod file, so v1.5.2 should be preferred over
+# v2.0.0+incompatible when resolving latest, upgrade, and patch.
+
+go list -m github.com/russross/blackfriday@latest
+stdout '^github.com/russross/blackfriday v1\.'
+
+go list -m github.com/russross/blackfriday@upgrade
+stdout '^github.com/russross/blackfriday v1\.'
+
+! go list -m github.com/russross/blackfriday@patch
+stderr '^go: github.com/russross/blackfriday@patch: can''t query version "patch" of module github.com/russross/blackfriday: no existing version is required$'
+
+
+# If we're fetching directly from version control, ignored +incompatible
+# versions should also be omitted by 'go list'.
+
+# (Note that they may still be included in results from a proxy: in proxy mode,
+# we would need to fetch the whole zipfile for the latest compatible version in
+# order to determine whether it contains a go.mod file, and part of the point of
+# the proxy is to avoid fetching unnecessary data.)
+
+[!git] stop
+env GOPROXY=direct
+
+go list -versions -m github.com/russross/blackfriday
+stdout '^github.com/russross/blackfriday v1\.5\.1 v1\.5\.2' # and possibly others
+! stdout ' v2\.'
+
+# For this module, v2.1.0 exists and has a go.mod file.
+# 'go list -m github.com/russross/blackfriday@v2.0' will check
+# the latest v2.0 tag, discover that it isn't the right module, and stop there
+# (instead of spending the time to check O(N) previous tags).
+
+! go list -m github.com/russross/blackfriday@v2.0
+stderr '^go: module github.com/russross/blackfriday: no matching versions for query "v2\.0\"'
+
+# (But asking for exactly v2.0.0+incompatible should still succeed.)
+go list -m github.com/russross/blackfriday@v2.0.0+incompatible
+stdout '^github.com/russross/blackfriday v2\.0\.0\+incompatible$'
+
+
+# However, if the latest compatible version does not include a go.mod file,
+# +incompatible versions should still be listed, as they may still reflect the
+# intent of the module author.
+
+go list -versions -m github.com/rsc/legacytest
+stdout '^github.com/rsc/legacytest v1\.0\.0 v1\.1\.0-pre v1\.2\.0 v2\.0\.0\+incompatible'
+
+# If we're fetching directly from version control, asking for a commit hash
+# corresponding to a +incompatible version should continue to produce the
+# +incompatible version tagged for that commit, even if it is no longer listed.
+
+go list -m github.com/russross/blackfriday@cadec560ec52
+stdout '^github.com/russross/blackfriday v2\.0\.0\+incompatible$'
+
+# Similarly, requesting an untagged commit should continue to produce a +incompatible
+# pseudo-version.
+
+go list -m github.com/rsc/legacytest@7303f7796364
+stdout '^github.com/rsc/legacytest v2\.0\.1-0\.20180717164253-7303f7796364\+incompatible$'
+
+-- go.mod --
+module github.com/golang.org/issue/34165
diff --git a/src/cmd/go/testdata/script/mod_proxy_errors.txt b/src/cmd/go/testdata/script/mod_proxy_errors.txt
new file mode 100644
index 0000000..99a4ef1
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_proxy_errors.txt
@@ -0,0 +1,19 @@
+[short] skip
+
+env GO111MODULE=on
+env GOSUMDB=off
+env GOPROXY=direct
+
+# Server responses should be truncated to some reasonable number of lines.
+# (For now, exactly eight.)
+! go list -m vcs-test.golang.org/auth/ormanylines@latest
+stderr '\tserver response:\n(.|\n)*\tline 8\n\t\[Truncated: too many lines.\]$'
+
+# Server responses should be truncated to some reasonable number of characters.
+! go list -m vcs-test.golang.org/auth/oronelongline@latest
+! stderr 'blah{40}'
+stderr '\tserver response: \[Truncated: too long\.\]$'
+
+# Responses from servers using the 'mod' protocol should be propagated.
+! go list -m vcs-test.golang.org/go/modauth404@latest
+stderr '\tserver response: File\? What file\?'
diff --git a/src/cmd/go/testdata/script/mod_proxy_https.txt b/src/cmd/go/testdata/script/mod_proxy_https.txt
new file mode 100644
index 0000000..c87a0d9
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_proxy_https.txt
@@ -0,0 +1,20 @@
+env GO111MODULE=on
+
+# GOPROXY file paths must provide the "file://" prefix explicitly.
+env GOPROXY=$WORK/proxydir
+! go list -versions -m golang.org/x/text
+stderr 'invalid proxy URL.*proxydir'
+
+[!net:proxy.golang.org] stop
+
+# GOPROXY HTTPS paths may elide the "https://" prefix.
+# (See golang.org/issue/32191.)
+env GOPROXY=proxy.golang.org
+env GOSUMDB=
+go list -versions -m golang.org/x/text
+
+-- go.mod --
+module example.com
+go 1.13
+-- $WORK/proxydir/README.md --
+This proxy contains no data.
diff --git a/src/cmd/go/testdata/script/mod_proxy_invalid.txt b/src/cmd/go/testdata/script/mod_proxy_invalid.txt
new file mode 100644
index 0000000..63980b8
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_proxy_invalid.txt
@@ -0,0 +1,8 @@
+env GO111MODULE=on
+env GOPROXY=$GOPROXY/invalid
+
+! go list -m rsc.io/quote@latest
+stderr '^go: module rsc.io/quote: invalid response from proxy "'$GOPROXY'": invalid character ''i'' looking for beginning of value$'
+
+! go list -m rsc.io/quote@1.5.2
+stderr '^go: rsc.io/quote@1.5.2: invalid version: invalid response from proxy "'$GOPROXY'": invalid character ''i'' looking for beginning of value$'
diff --git a/src/cmd/go/testdata/script/mod_proxy_list.txt b/src/cmd/go/testdata/script/mod_proxy_list.txt
new file mode 100644
index 0000000..849cf2c
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_proxy_list.txt
@@ -0,0 +1,37 @@
+env GO111MODULE=on
+env proxy=$GOPROXY
+
+# Proxy that can't serve should fail.
+env GOPROXY=$proxy/404
+! go get rsc.io/quote@v1.0.0
+stderr '404 Not Found'
+
+# get should walk down the proxy list past 404 and 410 responses.
+env GOPROXY=$proxy/404,$proxy/410,$proxy
+go get rsc.io/quote@v1.1.0
+
+# get should not walk past other 4xx errors if proxies are separated with ','.
+env GOPROXY=$proxy/403,$proxy
+! go get rsc.io/quote@v1.2.0
+stderr 'reading.*/403/rsc.io/.*: 403 Forbidden'
+
+# get should not walk past non-4xx errors if proxies are separated with ','.
+env GOPROXY=$proxy/500,$proxy
+! go get rsc.io/quote@v1.3.0
+stderr 'reading.*/500/rsc.io/.*: 500 Internal Server Error'
+
+# get should walk past other 4xx errors if proxies are separated with '|'.
+env GOPROXY=$proxy/403|https://0.0.0.0|$proxy
+go get rsc.io/quote@v1.2.0
+
+# get should walk past non-4xx errors if proxies are separated with '|'.
+env GOPROXY=$proxy/500|https://0.0.0.0|$proxy
+go get rsc.io/quote@v1.3.0
+
+# get should return the final error if that's all we have.
+env GOPROXY=$proxy/404,$proxy/410
+! go get rsc.io/quote@v1.4.0
+stderr 'reading.*/410/rsc.io/.*: 410 Gone'
+
+-- go.mod --
+module x
diff --git a/src/cmd/go/testdata/script/mod_pseudo_cache.txt b/src/cmd/go/testdata/script/mod_pseudo_cache.txt
new file mode 100644
index 0000000..bcaefa2
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_pseudo_cache.txt
@@ -0,0 +1,29 @@
+[!net:golang.org] skip
+[!git] skip
+
+env GO111MODULE=on
+env GOPROXY=direct
+env GOSUMDB=off
+
+# Regression test for golang.org/issue/27171: after resolving an older
+# pseudo-version of a commit, future resolution of that commit by hash should
+# choose the highest appropriate pseudo-version instead of the cached one.
+
+go mod download -json golang.org/x/text@v0.0.0-20171215141712-a1b916ed6726
+stdout '"Version": "v0.0.0-20171215141712-a1b916ed6726",'
+
+# If GOPROXY is 'off', lookups should use whatever pseudo-version is available.
+env GOPROXY=off
+go mod download -json golang.org/x/text@a1b916ed6726
+stdout '"Version": "v0.0.0-20171215141712-a1b916ed6726",'
+
+# If we can re-resolve the commit to a pseudo-version, fetching the commit by
+# hash should use the highest such pseudo-version appropriate to the commit.
+env GOPROXY=direct
+go mod download -json golang.org/x/text@a1b916ed6726
+stdout '"Version": "v0.3.1-0.20171215141712-a1b916ed6726",'
+
+# If GOPROXY is 'off', lookups should use the highest pseudo-version in the cache.
+env GOPROXY=off
+go mod download -json golang.org/x/text@a1b916ed6726
+stdout '"Version": "v0.3.1-0.20171215141712-a1b916ed6726",'
diff --git a/src/cmd/go/testdata/script/mod_query.txt b/src/cmd/go/testdata/script/mod_query.txt
new file mode 100644
index 0000000..3758732
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_query.txt
@@ -0,0 +1,43 @@
+env GO111MODULE=on
+
+# TODO(golang.org/issue/41297): we shouldn't need go.sum. None of the commands
+# below depend on the build list.
+
+go list -m -versions rsc.io/quote
+stdout '^rsc.io/quote v1.0.0 v1.1.0 v1.2.0 v1.2.1 v1.3.0 v1.4.0 v1.5.0 v1.5.1 v1.5.2 v1.5.3-pre1$'
+
+# Latest rsc.io/quote should be v1.5.2, not v1.5.3-pre1.
+go list -m rsc.io/quote@latest
+stdout 'rsc.io/quote v1.5.2$'
+
+# Same for rsc.io/quote@v1 and rsc.io/quote@v1.5 (with no patch version).
+go list -m rsc.io/quote@v1
+stdout 'rsc.io/quote v1.5.2$'
+go list -m rsc.io/quote@v1.5
+stdout 'rsc.io/quote v1.5.2$'
+
+# We should fall back to prereleases if no release tags match...
+go list -m rsc.io/quote@>v1.5.2
+stdout 'rsc.io/quote v1.5.3-pre1$'
+
+# ...but prefer release versions when given the option.
+go list -m rsc.io/quote@<v1.5.4
+stdout 'rsc.io/quote v1.5.2$'
+
+! go list -m rsc.io/quote@>v1.5.3
+stderr 'go: module rsc.io/quote: no matching versions for query ">v1.5.3"'
+
+go list -m -e -f '{{.Error.Err}}' rsc.io/quote@>v1.5.3
+stdout 'no matching versions for query ">v1.5.3"'
+
+-- go.mod --
+module x
+require rsc.io/quote v1.0.0
+
+-- go.sum --
+rsc.io/quote v1.0.0 h1:kQ3IZQzPTiDJxSZI98YaWgxFEhlNdYASHvh+MplbViw=
+rsc.io/quote v1.0.0/go.mod h1:v83Ri/njykPcgJltBc/gEkJTmjTsNgtO1Y7vyIK1CQA=
+-- use.go --
+package use
+
+import _ "rsc.io/quote"
diff --git a/src/cmd/go/testdata/script/mod_query_empty.txt b/src/cmd/go/testdata/script/mod_query_empty.txt
new file mode 100644
index 0000000..1c39eae
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_query_empty.txt
@@ -0,0 +1,73 @@
+env GO111MODULE=on
+env GOSUMDB=off
+
+go mod download example.com/join@v1.1.0
+
+# If the proxy serves a bogus result for the @latest version,
+# reading that version should cause 'go get' to fail.
+env GOPROXY=file:///$WORK/badproxy
+cp go.mod.orig go.mod
+! go get example.com/join/subpkg
+stderr 'go: example.com/join/subpkg@v0.0.0-20190624000000-123456abcdef: .*'
+
+# If @v/list is empty, the 'go' command should still try to resolve
+# other module paths.
+env GOPROXY=file:///$WORK/emptysub
+cp go.mod.orig go.mod
+go get example.com/join/subpkg
+go list -m example.com/join/...
+! stdout 'example.com/join/subpkg'
+stdout 'example.com/join v1.1.0'
+
+# If @v/list includes a version that the proxy does not actually serve,
+# that version is treated as nonexistent.
+env GOPROXY=file:///$WORK/notfound
+cp go.mod.orig go.mod
+go get example.com/join/subpkg
+go list -m example.com/join/...
+! stdout 'example.com/join/subpkg'
+stdout 'example.com/join v1.1.0'
+
+# If the proxy provides an empty @v/list but rejects @latest with
+# some other explicit error (for example, a "permission denied" error),
+# that error should be reported to the user (and override a successful
+# result for other possible module paths).
+#
+# Depending on how the specific platform enforces permissions, the 'go get' may
+# fail either due to the intended permission error or due to a parse error.
+# We accept either failure message.
+env GOPROXY=file:///$WORK/gatekeeper
+chmod 0000 $WORK/gatekeeper/example.com/join/subpkg/@latest
+cp go.mod.orig go.mod
+! go get example.com/join/subpkg
+stderr 'go: module example.com/join/subpkg: (invalid response from proxy ".+": invalid character .+|reading file://.*/gatekeeper/example.com/join/subpkg/@latest: .+)'
+
+-- go.mod.orig --
+module example.com/othermodule
+go 1.13
+-- $WORK/badproxy/example.com/join/subpkg/@v/list --
+v0.0.0-20190624000000-123456abcdef
+-- $WORK/badproxy/example.com/join/subpkg/@v/v0.0.0-20190624000000-123456abcdef.info --
+This file is not valid JSON.
+-- $WORK/badproxy/example.com/join/@v/list --
+v1.1.0
+-- $WORK/badproxy/example.com/join/@v/v1.1.0.info --
+{"Version": "v1.1.0"}
+-- $WORK/emptysub/example.com/join/subpkg/@v/list --
+-- $WORK/emptysub/example.com/join/@v/list --
+v1.1.0
+-- $WORK/emptysub/example.com/join/@v/v1.1.0.info --
+{"Version": "v1.1.0"}
+-- $WORK/notfound/example.com/join/subpkg/@v/list --
+v1.0.0-does-not-exist
+-- $WORK/notfound/example.com/join/@v/list --
+v1.1.0
+-- $WORK/notfound/example.com/join/@v/v1.1.0.info --
+{"Version": "v1.1.0"}
+-- $WORK/gatekeeper/example.com/join/subpkg/@v/list --
+-- $WORK/gatekeeper/example.com/join/subpkg/@latest --
+ERROR: Latest version is forbidden.
+-- $WORK/gatekeeper/example.com/join/@v/list --
+v1.1.0
+-- $WORK/gatekeeper/example.com/join/@v/v1.1.0.info --
+{"Version": "v1.1.0"}
diff --git a/src/cmd/go/testdata/script/mod_query_exclude.txt b/src/cmd/go/testdata/script/mod_query_exclude.txt
new file mode 100644
index 0000000..f76b20c
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_query_exclude.txt
@@ -0,0 +1,46 @@
+env GO111MODULE=on
+
+# list excluded version
+go list -modfile=go.exclude.mod -m rsc.io/quote@v1.5.0
+stdout '^rsc.io/quote v1.5.0$'
+
+# list versions should not print excluded versions
+go list -m -versions rsc.io/quote
+stdout '\bv1.5.0\b'
+go list -modfile=go.exclude.mod -m -versions rsc.io/quote
+! stdout '\bv1.5.0\b'
+
+# list query with excluded version
+go list -m rsc.io/quote@>=v1.5
+stdout '^rsc.io/quote v1.5.0$'
+go list -modfile=go.exclude.mod -m rsc.io/quote@>=v1.5
+stdout '^rsc.io/quote v1.5.1$'
+
+# get excluded version
+cp go.exclude.mod go.exclude.mod.orig
+! go get -modfile=go.exclude.mod rsc.io/quote@v1.5.0
+stderr '^go: rsc.io/quote@v1.5.0: excluded by go.mod$'
+
+# get non-excluded version
+cp go.exclude.mod.orig go.exclude.mod
+go get -modfile=go.exclude.mod rsc.io/quote@v1.5.1
+stderr 'rsc.io/quote v1.5.1'
+
+# get query with excluded version
+cp go.exclude.mod.orig go.exclude.mod
+go get -modfile=go.exclude.mod rsc.io/quote@>=v1.5
+go list -modfile=go.exclude.mod -m ...quote
+stdout 'rsc.io/quote v1.5.[1-9]'
+
+-- go.mod --
+module x
+
+-- go.exclude.mod --
+module x
+
+exclude rsc.io/quote v1.5.0
+
+-- x.go --
+package x
+import _ "rsc.io/quote"
+
diff --git a/src/cmd/go/testdata/script/mod_query_main.txt b/src/cmd/go/testdata/script/mod_query_main.txt
new file mode 100644
index 0000000..2a2fa42
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_query_main.txt
@@ -0,0 +1,43 @@
+# 'go mod download' can download specific versions of the main module.
+go mod download rsc.io/quote@5d9f230b
+go mod download rsc.io/quote@v1.5.2
+go mod download rsc.io/quote@latest
+
+# 'go mod download' will not download @upgrade or @patch, since they always
+# resolve to the main module.
+go mod download rsc.io/quote@upgrade
+stderr '^go: skipping download of rsc.io/quote@upgrade that resolves to the main module$'
+go mod download rsc.io/quote@patch
+stderr '^go: skipping download of rsc.io/quote@patch that resolves to the main module$'
+
+# 'go list -m' can show a version of the main module.
+go list -m rsc.io/quote@5d9f230b
+stdout '^rsc.io/quote v0.0.0-20180710144737-5d9f230bcfba$'
+go list -m rsc.io/quote@v1.5.2
+stdout '^rsc.io/quote v1.5.2$'
+go list -m rsc.io/quote@latest
+stdout '^rsc.io/quote v1.5.2$'
+
+# 'go list -m -versions' shows available versions.
+go list -m -versions rsc.io/quote
+stdout '^rsc.io/quote.*v1.5.2'
+
+# 'go list -m' resolves @upgrade and @patch to the main module.
+go list -m rsc.io/quote@upgrade
+stdout '^rsc.io/quote$'
+go list -m rsc.io/quote@patch
+stdout '^rsc.io/quote$'
+
+# 'go get' will not attempt to upgrade the main module to any specific version.
+# See also: mod_get_main.txt.
+! go get rsc.io/quote@5d9f230b
+stderr '^go: can''t request version "5d9f230b" of the main module \(rsc.io/quote\)$'
+! go get rsc.io/quote@v1.5.2
+stderr '^go: can''t request version "v1.5.2" of the main module \(rsc.io/quote\)$'
+! go get rsc.io/quote@latest
+stderr '^go: can''t request version "latest" of the main module \(rsc.io/quote\)$'
+
+-- go.mod --
+module rsc.io/quote
+
+go 1.16
diff --git a/src/cmd/go/testdata/script/mod_readonly.txt b/src/cmd/go/testdata/script/mod_readonly.txt
new file mode 100644
index 0000000..9e950c3
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_readonly.txt
@@ -0,0 +1,131 @@
+env GO111MODULE=on
+[short] skip
+
+# -mod=readonly must not resolve missing modules nor update go.mod
+env GOFLAGS=-mod=readonly
+go mod edit -fmt
+cp go.mod go.mod.empty
+! go list all
+stderr '^x.go:2:8: cannot find module providing package rsc\.io/quote: import lookup disabled by -mod=readonly'
+! stderr '\(\)' # If we don't have a reason for -mod=readonly, don't log an empty one.
+cmp go.mod go.mod.empty
+
+# -mod=readonly should be set by default.
+env GOFLAGS=
+! go list all
+stderr '^x.go:2:8: no required module provides package rsc\.io/quote; to add it:\n\tgo get rsc\.io/quote$'
+cmp go.mod go.mod.empty
+
+env GOFLAGS=-mod=readonly
+
+# update go.mod - go get allowed
+go get rsc.io/quote
+grep rsc.io/quote go.mod
+
+# update go.mod - go mod tidy allowed
+cp go.mod.empty go.mod
+go mod tidy
+cp go.mod go.mod.tidy
+
+# -mod=readonly must succeed once go.mod is up-to-date...
+go list all
+
+# ... even if it needs downloads
+go clean -modcache
+go list all
+
+# -mod=readonly must not cause 'go list -m' to fail.
+# (golang.org/issue/36478)
+go list -m all
+! stderr 'cannot query module'
+
+# -mod=readonly should reject inconsistent go.mod files
+# (ones that would be rewritten).
+go get rsc.io/sampler@v1.2.0
+go mod edit -require rsc.io/quote@v1.5.2
+cp go.mod go.mod.inconsistent
+! go list
+stderr 'go: updates to go.mod needed, disabled by -mod=readonly'
+cmp go.mod go.mod.inconsistent
+
+# We get a different message when -mod=readonly is used by default.
+env GOFLAGS=
+! go list
+stderr '^go: updates to go.mod needed; to update it:\n\tgo mod tidy'
+
+# However, it should not reject files missing a 'go' directive,
+# since that was not always required.
+cp go.mod.nogo go.mod
+go list all
+cmp go.mod go.mod.nogo
+
+# Nor should it reject files with redundant (not incorrect)
+# requirements.
+cp go.mod.redundant go.mod
+go list all
+cmp go.mod go.mod.redundant
+
+cp go.mod.indirect go.mod
+go list all
+cmp go.mod go.mod.indirect
+
+
+# If we identify a missing package as a dependency of some other package in the
+# main module, we should suggest 'go mod tidy' instead of resolving it.
+
+cp go.mod.untidy go.mod
+! go list all
+stderr '^x.go:2:8: no required module provides package rsc.io/quote; to add it:\n\tgo get rsc.io/quote$'
+
+! go list -deps .
+stderr '^x.go:2:8: no required module provides package rsc.io/quote; to add it:\n\tgo get rsc.io/quote$'
+
+# However, if we didn't see an import from the main module, we should suggest
+# 'go get' instead, because we don't know whether 'go mod tidy' would add it.
+! go list rsc.io/quote
+stderr '^no required module provides package rsc.io/quote; to add it:\n\tgo get rsc.io/quote$'
+
+
+-- go.mod --
+module m
+
+go 1.16
+
+-- x.go --
+package x
+import _ "rsc.io/quote"
+-- go.mod.nogo --
+module m
+
+require (
+	rsc.io/quote v1.5.2
+	rsc.io/testonly v1.0.0 // indirect
+)
+-- go.mod.redundant --
+module m
+
+go 1.16
+
+require (
+	rsc.io/quote v1.5.2
+	rsc.io/sampler v1.3.0 // indirect
+	rsc.io/testonly v1.0.0 // indirect
+)
+-- go.mod.indirect --
+module m
+
+go 1.16
+
+require (
+	rsc.io/quote v1.5.2 // indirect
+	rsc.io/sampler v1.3.0 // indirect
+	rsc.io/testonly v1.0.0 // indirect
+)
+-- go.mod.untidy --
+module m
+
+go 1.16
+
+require (
+	rsc.io/sampler v1.3.0 // indirect
+)
diff --git a/src/cmd/go/testdata/script/mod_replace.txt b/src/cmd/go/testdata/script/mod_replace.txt
new file mode 100644
index 0000000..26b1551
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_replace.txt
@@ -0,0 +1,129 @@
+env GO111MODULE=on
+[short] skip
+
+cp go.mod go.mod.orig
+
+# Make sure the test builds without replacement.
+go build -mod=mod -o a1.exe .
+exec ./a1.exe
+stdout 'Don''t communicate by sharing memory'
+
+# Modules can be replaced by local packages.
+cp go.mod.orig go.mod
+go mod edit -replace=rsc.io/quote/v3=./local/rsc.io/quote/v3
+go build -o a2.exe .
+exec ./a2.exe
+stdout 'Concurrency is not parallelism.'
+
+# The module path of the replacement doesn't need to match.
+# (For example, it could be a long-running fork with its own import path.)
+cp go.mod.orig go.mod
+go mod edit -replace=rsc.io/quote/v3=./local/not-rsc.io/quote/v3
+go build -o a3.exe .
+exec ./a3.exe
+stdout 'Clear is better than clever.'
+
+# However, the same module can't be used as two different paths.
+cp go.mod.orig go.mod
+go mod edit -replace=not-rsc.io/quote/v3@v3.0.0=rsc.io/quote/v3@v3.0.0 -require=not-rsc.io/quote/v3@v3.0.0
+! go build -o a4.exe .
+stderr 'rsc.io/quote/v3@v3.0.0 used for two different module paths \(not-rsc.io/quote/v3 and rsc.io/quote/v3\)'
+
+# Modules that do not (yet) exist upstream can be replaced too.
+cp go.mod.orig go.mod
+go mod edit -replace=not-rsc.io/quote/v3@v3.1.0=./local/rsc.io/quote/v3
+go build -mod=mod -o a5.exe ./usenewmodule
+! stderr 'finding not-rsc.io/quote/v3'
+grep 'not-rsc.io/quote/v3 v3.1.0' go.mod
+exec ./a5.exe
+stdout 'Concurrency is not parallelism.'
+
+# Error messages for modules not found in replacements should
+# indicate the replacement module.
+cp go.mod.orig go.mod
+go mod edit -replace=rsc.io/quote/v3=./local/rsc.io/quote/v3
+! go get rsc.io/quote/v3/missing-package
+stderr 'module rsc.io/quote/v3@upgrade found \(v3.0.0, replaced by ./local/rsc.io/quote/v3\), but does not contain package'
+
+# The reported Dir and GoMod for a replaced module should be accurate.
+cp go.mod.orig go.mod
+go mod edit -replace=rsc.io/quote/v3=not-rsc.io/quote@v0.1.0-nomod
+go mod download rsc.io/quote/v3
+go list -m -f '{{.Path}} {{.Version}} {{.Dir}} {{.GoMod}}{{with .Replace}} => {{.Path}} {{.Version}} {{.Dir}} {{.GoMod}}{{end}}' rsc.io/quote/v3
+stdout '^rsc.io/quote/v3 v3.0.0 '$GOPATH'[/\\]pkg[/\\]mod[/\\]not-rsc.io[/\\]quote@v0.1.0-nomod '$GOPATH'[/\\]pkg[/\\]mod[/\\]cache[/\\]download[/\\]not-rsc.io[/\\]quote[/\\]@v[/\\]v0.1.0-nomod.mod => not-rsc.io/quote v0.1.0-nomod '$GOPATH'[/\\]pkg[/\\]mod[/\\]not-rsc.io[/\\]quote@v0.1.0-nomod '$GOPATH'[/\\]pkg[/\\]mod[/\\]cache[/\\]download[/\\]not-rsc.io[/\\]quote[/\\]@v[/\\]v0.1.0-nomod.mod$'
+
+-- go.mod --
+module quoter
+
+require rsc.io/quote/v3 v3.0.0
+
+-- main.go --
+package main
+
+import (
+	"fmt"
+	"rsc.io/quote/v3"
+)
+
+func main() {
+	fmt.Println(quote.GoV3())
+}
+
+-- usenewmodule/main.go --
+package main
+
+import (
+	"fmt"
+	"not-rsc.io/quote/v3"
+)
+
+func main() {
+	fmt.Println(quote.GoV3())
+}
+
+-- local/rsc.io/quote/v3/go.mod --
+module rsc.io/quote/v3
+
+require rsc.io/sampler v1.3.0
+
+-- local/rsc.io/quote/v3/quote.go --
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package quote collects pithy sayings.
+package quote
+
+import "rsc.io/sampler"
+
+// Hello returns a greeting.
+func HelloV3() string {
+	return sampler.Hello()
+}
+
+// Glass returns a useful phrase for world travelers.
+func GlassV3() string {
+	// See http://www.oocities.org/nodotus/hbglass.html.
+	return "I can eat glass and it doesn't hurt me."
+}
+
+// Go returns a REPLACED Go proverb.
+func GoV3() string {
+	return "Concurrency is not parallelism."
+}
+
+// Opt returns a optimization truth.
+func OptV3() string {
+	// Wisdom from ken.
+	return "If a program is too slow, it must have a loop."
+}
+
+-- local/not-rsc.io/quote/v3/go.mod --
+module not-rsc.io/quote/v3
+
+-- local/not-rsc.io/quote/v3/quote.go --
+package quote
+
+func GoV3() string {
+	return "Clear is better than clever."
+}
diff --git a/src/cmd/go/testdata/script/mod_replace_gopkgin.txt b/src/cmd/go/testdata/script/mod_replace_gopkgin.txt
new file mode 100644
index 0000000..91008f9
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_replace_gopkgin.txt
@@ -0,0 +1,82 @@
+# Regression test for golang.org/issue/34254:
+# a clone of gopkg.in/[…].vN should be replaceable by
+# a fork hosted at corp.example.com/[…]/vN,
+# even if there is an explicit go.mod file containing the
+# gopkg.in path.
+
+skip 'skipping test that depends on an unreliable third-party server; see https://go.dev/issue/54503'
+	# TODO(#54043): Make this test hermetic and re-enable it.
+
+[!net:gopkg.in] skip
+[!git] skip
+
+env GO111MODULE=on
+env GOPROXY=direct
+env GOSUMDB=off
+env GOFLAGS=-mod=mod
+
+# Replacing gopkg.in/[…].vN with a repository with a root go.mod file
+# specifying […].vN and a compatible version should succeed, even if
+# the replacement path is not a gopkg.in path.
+cd 4-to-4
+go list -m gopkg.in/src-d/go-git.v4
+
+# Previous versions of the "go" command accepted v0 and v1 pseudo-versions
+# as replacements for gopkg.in/[…].v4.
+# As a special case, we continue to accept those.
+
+cd ../4-to-0
+go list -m gopkg.in/src-d/go-git.v4
+
+cd ../4-to-1
+go list -m gopkg.in/src-d/go-git.v4
+
+cd ../4-to-incompatible
+go list -m gopkg.in/src-d/go-git.v4
+
+# A mismatched gopkg.in path should not be able to replace a different major version.
+cd ../3-to-gomod-4
+! go list -m gopkg.in/src-d/go-git.v3
+stderr '^go: gopkg\.in/src-d/go-git\.v3@v3\.2\.0 \(replaced by gopkg\.in/src-d/go-git\.v3@v3\.0\.0-20190801152248-0d1a009cbb60\): version "v3\.0\.0-20190801152248-0d1a009cbb60" invalid: go\.mod has non-\.\.\.\.v3 module path "gopkg\.in/src-d/go-git\.v4" at revision 0d1a009cbb60$'
+
+-- 4-to-4/go.mod --
+module golang.org/issue/34254
+
+go 1.13
+
+require gopkg.in/src-d/go-git.v4 v4.13.1
+
+replace gopkg.in/src-d/go-git.v4 v4.13.1 => github.com/src-d/go-git/v4 v4.13.1
+-- 4-to-1/go.mod --
+module golang.org/issue/34254
+
+go 1.13
+
+require gopkg.in/src-d/go-git.v4 v4.13.1
+
+replace gopkg.in/src-d/go-git.v4 v4.13.1 => github.com/src-d/go-git v1.0.1-0.20190801152248-0d1a009cbb60
+-- 4-to-0/go.mod --
+module golang.org/issue/34254
+
+go 1.13
+
+require gopkg.in/src-d/go-git.v4 v4.13.1
+
+replace gopkg.in/src-d/go-git.v4 v4.13.1 => github.com/src-d/go-git v0.0.0-20190801152248-0d1a009cbb60
+-- 4-to-incompatible/go.mod --
+module golang.org/issue/34254
+
+go 1.13
+
+require gopkg.in/src-d/go-git.v4 v4.13.1
+
+replace gopkg.in/src-d/go-git.v4 v4.13.1 => github.com/src-d/go-git v4.6.0+incompatible
+-- 3-to-gomod-4/go.mod --
+module golang.org/issue/34254
+go 1.13
+
+require gopkg.in/src-d/go-git.v3 v3.2.0
+
+// This replacement has a go.mod file declaring its path to be
+// gopkg.in/src-d/go-git.v4, so it cannot be used as a replacement for v3.
+replace gopkg.in/src-d/go-git.v3 v3.2.0 => gopkg.in/src-d/go-git.v3 v3.0.0-20190801152248-0d1a009cbb60
diff --git a/src/cmd/go/testdata/script/mod_replace_import.txt b/src/cmd/go/testdata/script/mod_replace_import.txt
new file mode 100644
index 0000000..753071f
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_replace_import.txt
@@ -0,0 +1,144 @@
+env GO111MODULE=on
+
+# 'go list' should not add requirements even if they can be resolved locally.
+cp go.mod go.mod.orig
+! go list all
+cmp go.mod go.mod.orig
+
+# 'go list' should resolve imports using replacements.
+go get
+go list all
+stdout 'example.com/a/b$'
+stdout 'example.com/x/v3$'
+stdout 'example.com/y/z/w$'
+stdout 'example.com/v'
+
+# The selected modules should prefer longer paths,
+# but should try shorter paths if needed.
+# Modules with a major-version suffix should have a corresponding pseudo-version.
+# Replacements that specify a version should use the latest such version.
+go list -m all
+stdout 'example.com/a/b v0.0.0-00010101000000-000000000000 => ./b'
+stdout 'example.com/y v0.0.0-00010101000000-000000000000 => ./y'
+stdout 'example.com/x/v3 v3.0.0-00010101000000-000000000000 => ./v3'
+stdout 'example.com/v v1.12.0 => ./v12'
+
+# The go command should print an informative error when the matched
+# module does not contain a package.
+# TODO(#26909): Ideally these errors should include line numbers for the imports within the main module.
+cd fail
+! go mod tidy
+stderr '^go: localhost.fail imports\n\tw: module w@latest found \(v0.0.0-00010101000000-000000000000, replaced by ../w\), but does not contain package w$'
+stderr '^go: localhost.fail imports\n\tnonexist: nonexist@v0.1.0: replacement directory ../nonexist does not exist$'
+
+-- go.mod --
+module example.com/m
+
+replace (
+	example.com/a => ./a
+	example.com/a/b => ./b
+)
+
+replace (
+	example.com/x => ./x
+	example.com/x/v3 => ./v3
+)
+
+replace (
+	example.com/y/z/w => ./w
+	example.com/y => ./y
+)
+
+replace (
+	example.com/v v1.11.0 => ./v11
+	example.com/v v1.12.0 => ./v12
+	example.com/v => ./v
+)
+
+replace (
+	example.com/i v2.0.0+incompatible => ./i2
+)
+
+-- m.go --
+package main
+import (
+	_ "example.com/a/b"
+	_ "example.com/x/v3"
+	_ "example.com/y/z/w"
+	_ "example.com/v"
+	_ "example.com/i"
+)
+func main() {}
+
+-- a/go.mod --
+module a.localhost
+-- a/a.go --
+package a
+-- a/b/b.go--
+package b
+
+-- b/go.mod --
+module a.localhost/b
+-- b/b.go --
+package b
+
+-- x/go.mod --
+module x.localhost
+-- x/x.go --
+package x
+-- x/v3.go --
+package v3
+import _ "x.localhost/v3"
+
+-- v3/go.mod --
+module x.localhost/v3
+-- v3/x.go --
+package x
+
+-- w/go.mod --
+module w.localhost
+-- w/skip/skip.go --
+// Package skip is nested below nonexistent package w.
+package skip
+
+-- y/go.mod --
+module y.localhost
+-- y/z/w/w.go --
+package w
+
+-- v12/go.mod --
+module v.localhost
+-- v12/v.go --
+package v
+
+-- v11/go.mod --
+module v.localhost
+-- v11/v.go --
+package v
+
+-- v/go.mod --
+module v.localhost
+-- v/v.go --
+package v
+
+-- i2/go.mod --
+module example.com/i
+-- i2/i.go --
+package i
+
+-- fail/m.go --
+package main
+
+import (
+	_ "w"
+	_ "nonexist"
+)
+
+func main() {}
+
+-- fail/go.mod --
+module localhost.fail
+
+replace w => ../w
+
+replace nonexist v0.1.0 => ../nonexist
diff --git a/src/cmd/go/testdata/script/mod_replace_readonly.txt b/src/cmd/go/testdata/script/mod_replace_readonly.txt
new file mode 100644
index 0000000..5c1226b
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_replace_readonly.txt
@@ -0,0 +1,62 @@
+# Check that with -mod=readonly, when we load a package in a module that is
+# replaced but not required, we emit an error with the command to add the
+# requirement.
+# Verifies golang.org/issue/41416, golang.org/issue/41577.
+cp go.mod go.mod.orig
+
+# Replace all versions of a module without requiring it.
+# With -mod=mod, we'd add a requirement for a "zero" pseudo-version, but we
+# can't in readonly mode, since its go.mod may alter the build list.
+go mod edit -replace rsc.io/quote=./quote
+! go list rsc.io/quote
+stderr '^module rsc.io/quote provides package rsc.io/quote and is replaced but not required; to add it:\n\tgo get rsc.io/quote$'
+go get rsc.io/quote
+cmp go.mod go.mod.latest
+go list rsc.io/quote
+cp go.mod.orig go.mod
+
+# Same test with a specific version.
+go mod edit -replace rsc.io/quote@v1.0.0-doesnotexist=./quote
+! go list rsc.io/quote
+stderr '^module rsc.io/quote provides package rsc.io/quote and is replaced but not required; to add it:\n\tgo get rsc.io/quote@v1.0.0-doesnotexist$'
+go get rsc.io/quote@v1.0.0-doesnotexist
+cmp go.mod go.mod.specific
+go list rsc.io/quote
+cp go.mod.orig go.mod
+
+# If there are multiple versions, the highest is suggested.
+go mod edit -replace rsc.io/quote@v1.0.0-doesnotexist=./quote
+go mod edit -replace rsc.io/quote@v1.1.0-doesnotexist=./quote
+! go list rsc.io/quote
+stderr '^module rsc.io/quote provides package rsc.io/quote and is replaced but not required; to add it:\n\tgo get rsc.io/quote@v1.1.0-doesnotexist$'
+
+-- go.mod --
+module m
+
+go 1.16
+-- go.mod.latest --
+module m
+
+go 1.16
+
+replace rsc.io/quote => ./quote
+
+require rsc.io/quote v1.5.2 // indirect
+-- go.mod.specific --
+module m
+
+go 1.16
+
+replace rsc.io/quote v1.0.0-doesnotexist => ./quote
+
+require rsc.io/quote v1.0.0-doesnotexist // indirect
+-- use.go --
+package use
+
+import _ "rsc.io/quote"
+-- quote/go.mod --
+module rsc.io/quote
+
+go 1.16
+-- quote/quote.go --
+package quote
diff --git a/src/cmd/go/testdata/script/mod_require_exclude.txt b/src/cmd/go/testdata/script/mod_require_exclude.txt
new file mode 100644
index 0000000..0946dbf
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_require_exclude.txt
@@ -0,0 +1,96 @@
+# build with no newer version to satisfy exclude
+env GO111MODULE=on
+cp go.mod go.mod.orig
+
+# With the selected version excluded, commands that query that version without
+# updating go.mod should fail.
+
+! go list -mod=readonly -m all
+stderr '^go: ignoring requirement on excluded version rsc.io/sampler v1\.99\.99$'
+stderr '^go: updates to go.mod needed, disabled by -mod=readonly; to update it:\n\tgo mod tidy$'
+! stdout '^rsc.io/sampler v1.99.99'
+cmp go.mod go.mod.orig
+
+! go list -mod=vendor -m rsc.io/sampler
+stderr '^go: ignoring requirement on excluded version rsc.io/sampler v1\.99\.99$'
+stderr '^go: updates to go.mod needed, disabled by -mod=vendor; to update it:\n\tgo mod tidy$'
+! stdout '^rsc.io/sampler v1.99.99'
+cmp go.mod go.mod.orig
+
+# The failure message should be clear when -mod=vendor is implicit.
+
+go mod edit -go=1.14
+! go list -m rsc.io/sampler
+stderr '^go: ignoring requirement on excluded version rsc.io/sampler v1\.99\.99$'
+stderr '^go: updates to go.mod needed, disabled by -mod=vendor\n\t\(Go version in go.mod is at least 1.14 and vendor directory exists\.\)\n\tto update it:\n\tgo mod tidy$'
+! stdout '^rsc.io/sampler v1.99.99'
+go mod edit -go=1.13
+cmp go.mod go.mod.orig
+
+
+# With the selected version excluded, commands that load only modules should
+# drop the excluded module.
+
+go list -m -mod=mod all
+stderr '^go: dropping requirement on excluded version rsc.io/sampler v1\.99\.99$'
+stdout '^x$'
+! stdout '^rsc.io/sampler'
+cmp go.mod go.moddrop
+
+# With the latest version excluded, 'go list' should resolve needed packages
+# from the next-highest version.
+
+cp go.mod.orig go.mod
+go list -mod=mod -f '{{with .Module}}{{.Path}} {{.Version}}{{end}}' all
+stderr '^go: dropping requirement on excluded version rsc.io/sampler v1\.99\.99$'
+stdout '^x $'
+! stdout '^rsc.io/sampler v1.99.99'
+stdout '^rsc.io/sampler v1.3.0'
+
+# build with newer version available
+cp go.mod2 go.mod
+go list -mod=mod -f '{{with .Module}}{{.Path}} {{.Version}}{{end}}' all
+stderr '^go: dropping requirement on excluded version rsc.io/quote v1\.5\.1$'
+stdout 'rsc.io/quote v1.5.2'
+
+# build with excluded newer version
+cp go.mod3 go.mod
+go list -mod=mod -f '{{with .Module}}{{.Path}} {{.Version}}{{end}}' all
+! stderr '^go: dropping requirement'
+stdout 'rsc.io/quote v1.5.1'
+
+-- x.go --
+package x
+import _ "rsc.io/quote"
+
+-- go.mod --
+module x
+
+go 1.13
+
+exclude rsc.io/sampler v1.99.99
+
+require rsc.io/sampler v1.99.99
+-- vendor/modules.txt --
+# rsc.io/sampler v1.99.99
+## explicit
+-- go.moddrop --
+module x
+
+go 1.13
+
+exclude rsc.io/sampler v1.99.99
+-- go.mod2 --
+module x
+
+go 1.13
+
+exclude rsc.io/quote v1.5.1
+require rsc.io/quote v1.5.1
+-- go.mod3 --
+module x
+
+go 1.13
+
+exclude rsc.io/quote v1.5.2
+require rsc.io/quote v1.5.1
diff --git a/src/cmd/go/testdata/script/mod_retention.txt b/src/cmd/go/testdata/script/mod_retention.txt
new file mode 100644
index 0000000..9d30026
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_retention.txt
@@ -0,0 +1,150 @@
+# Regression test for golang.org/issue/34822: the 'go' command should prefer not
+# to update the go.mod file if the changes only affect formatting, and should only
+# remove redundant requirements in 'go mod tidy'.
+
+env GO111MODULE=on
+[short] skip
+
+# Control case: verify that go.mod.tidy is actually tidy.
+cp go.mod.tidy go.mod
+go list -mod=mod all
+cmp go.mod go.mod.tidy
+
+
+# If the only difference in the go.mod file is the line endings,
+# it should not be overwritten automatically.
+cp go.mod.crlf go.mod
+go list all
+cmp go.mod go.mod.crlf
+
+# However, 'go mod tidy' should fix whitespace even if there are no other changes.
+go mod tidy
+cmp go.mod go.mod.tidy
+
+
+# Out-of-order requirements should not be overwritten automatically...
+cp go.mod.unsorted go.mod
+go list all
+cmp go.mod go.mod.unsorted
+
+# ...but 'go mod edit -fmt' should sort them.
+go mod edit -fmt
+cmp go.mod go.mod.tidy
+
+
+# "// indirect" comments should be removed if direct dependencies are seen.
+# changes.
+cp go.mod.indirect go.mod
+go list -mod=mod all
+cmp go.mod go.mod.tidy
+
+# "// indirect" comments should be added if appropriate.
+# TODO(#42504): add case for 'go list -mod=mod -tags=any all' when -tags=any
+# is supported. Only a command that loads "all" without build constraints
+# (except "ignore") has enough information to add "// indirect" comments.
+# 'go mod tidy' and 'go mod vendor' are the only commands that do that,
+# but 'go mod vendor' cannot write go.mod.
+cp go.mod.toodirect go.mod
+go list all
+cmp go.mod go.mod.toodirect
+
+
+# Redundant requirements should be preserved...
+cp go.mod.redundant go.mod
+go list all
+cmp go.mod go.mod.redundant
+go mod vendor
+cmp go.mod go.mod.redundant
+rm -r vendor
+
+# ...except by 'go mod tidy'.
+go mod tidy
+cmp go.mod go.mod.tidy
+
+
+# A missing "go" version directive should be added.
+# However, that should not remove other redundant requirements.
+# In fact, it may *add* redundant requirements due to activating lazy loading.
+cp go.mod.nogo go.mod
+go list -mod=mod all
+cmpenv go.mod go.mod.addedgo
+
+
+-- go.mod.tidy --
+module m
+
+go 1.14
+
+require (
+	rsc.io/quote v1.5.2
+	rsc.io/testonly v1.0.0 // indirect
+)
+-- x.go --
+package x
+import _ "rsc.io/quote"
+-- go.mod.crlf --
+module m
+
+go 1.14
+
+require (
+	rsc.io/quote v1.5.2
+	rsc.io/testonly v1.0.0 // indirect
+)
+-- go.mod.unsorted --
+module m
+
+go 1.14
+
+require (
+	rsc.io/testonly v1.0.0 // indirect
+	rsc.io/quote v1.5.2
+)
+-- go.mod.indirect --
+module m
+
+go 1.14
+
+require (
+	rsc.io/quote v1.5.2 // indirect
+	rsc.io/testonly v1.0.0 // indirect
+)
+-- go.mod.toodirect --
+module m
+
+go 1.14
+
+require (
+	rsc.io/quote v1.5.2
+	rsc.io/testonly v1.0.0
+)
+-- go.mod.redundant --
+module m
+
+go 1.14
+
+require (
+	rsc.io/quote v1.5.2
+	rsc.io/sampler v1.3.0 // indirect
+	rsc.io/testonly v1.0.0 // indirect
+)
+-- go.mod.nogo --
+module m
+
+require (
+	rsc.io/quote v1.5.2
+	rsc.io/sampler v1.3.0 // indirect
+	rsc.io/testonly v1.0.0 // indirect
+)
+-- go.mod.addedgo --
+module m
+
+go $goversion
+
+require rsc.io/quote v1.5.2
+
+require (
+	golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c // indirect
+	rsc.io/sampler v1.3.0 // indirect
+	rsc.io/testonly v1.0.0 // indirect
+)
diff --git a/src/cmd/go/testdata/script/mod_retract.txt b/src/cmd/go/testdata/script/mod_retract.txt
new file mode 100644
index 0000000..37aae48
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_retract.txt
@@ -0,0 +1,45 @@
+cp go.mod go.mod.orig
+
+# 'go list pkg' does not report an error when a retracted version is used.
+go list -e -f '{{if .Error}}{{.Error}}{{end}}' ./use
+! stdout .
+cmp go.mod go.mod.orig
+
+# Nor does 'go build'.
+[!short] go build ./use
+[!short] ! stderr .
+[!short] cmp go.mod go.mod.orig
+
+# Neither 'go list' nor 'go build' should download go.mod from the version
+# that would list retractions.
+exists $GOPATH/pkg/mod/cache/download/example.com/retract/@v/v1.0.0-bad.mod
+! exists $GOPATH/pkg/mod/cache/download/example.com/retract/@v/v1.1.0.mod
+
+# Importing a package from a module with a retracted latest version will
+# select the latest non-retracted version.
+go get ./use_self_prev
+go list -m example.com/retract/self/prev
+stdout '^example.com/retract/self/prev v1.1.0$'
+exists $GOPATH/pkg/mod/cache/download/example.com/retract/self/prev/@v/v1.9.0.mod
+
+-- go.mod --
+module example.com/use
+
+go 1.15
+
+require example.com/retract v1.0.0-bad
+
+-- go.sum --
+example.com/retract v1.0.0-bad h1:liAW69rbtjY67x2CcNzat668L/w+YGgNX3lhJsWIJis=
+example.com/retract v1.0.0-bad/go.mod h1:0DvGGofJ9hr1q63cBrOY/jSY52OwhRGA0K47NE80I5Y=
+example.com/retract/self/prev v1.1.0 h1:0/8I/GTG+1eJTFeDQ/fUbgrMsVHHyKhh3Z8DSZp1fuA=
+example.com/retract/self/prev v1.1.0/go.mod h1:xl2EcklWuZZHVtHWcpzfSJQmnzAGpKZYpA/Wto7SZN4=
+-- use/use.go --
+package use
+
+import _ "example.com/retract"
+
+-- use_self_prev/use.go --
+package use_self_prev
+
+import _ "example.com/retract/self/prev"
diff --git a/src/cmd/go/testdata/script/mod_retract_fix_version.txt b/src/cmd/go/testdata/script/mod_retract_fix_version.txt
new file mode 100644
index 0000000..9ae49f5
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_retract_fix_version.txt
@@ -0,0 +1,48 @@
+# retract must not be used without a module directive.
+! go list -m all
+stderr 'go.mod:3: no module directive found, so retract cannot be used$'
+
+# Commands that update go.mod should fix non-canonical versions in
+# retract directives.
+# Verifies #44494.
+go mod edit -module=rsc.io/quote/v2
+! go list -m all
+stderr '^go: updates to go.mod needed; to update it:\n\tgo mod tidy$'
+go mod tidy
+go list -m all
+cmp go.mod go.mod.want
+
+# If a retracted version doesn't match the module's major version suffx,
+# an error should be reported.
+! go mod edit -retract=v3.0.1
+stderr '^go: -retract=v3.0.1: version "v3.0.1" invalid: should be v2, not v3$'
+cp go.mod.mismatch-v2 go.mod
+! go list -m all
+stderr 'go.mod:3: retract rsc.io/quote/v2: version "v3.0.1" invalid: should be v2, not v3$'
+
+cp go.mod.mismatch-v1 go.mod
+! go list -m all
+stderr 'go.mod:3: retract rsc.io/quote: version "v3.0.1" invalid: should be v0 or v1, not v3$'
+
+-- go.mod --
+go 1.16
+
+retract latest
+-- go.mod.want --
+go 1.16
+
+retract v2.0.1
+
+module rsc.io/quote/v2
+-- go.mod.mismatch-v2 --
+go 1.16
+
+retract v3.0.1
+
+module rsc.io/quote/v2
+-- go.mod.mismatch-v1 --
+go 1.16
+
+retract v3.0.1
+
+module rsc.io/quote
diff --git a/src/cmd/go/testdata/script/mod_retract_incompatible.txt b/src/cmd/go/testdata/script/mod_retract_incompatible.txt
new file mode 100644
index 0000000..5d09532
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_retract_incompatible.txt
@@ -0,0 +1,15 @@
+# The current version of a module should not be considered when loading
+# retractions. If the current version is +incompatible, we should not prefer
+# +incompatible versions when looking for retractions.
+# Verifies #42601.
+
+go mod init m
+
+# Request a +incompatible version retracted in v1.0.0.
+go get example.com/retract/incompatible@v2.0.0+incompatible
+stderr '^go: warning: example.com/retract/incompatible@v2.0.0\+incompatible: retracted by module author$'
+
+# We should still see a warning if the +incompatible was previously in the
+# build list.
+go get example.com/retract/incompatible@v2.0.0+incompatible
+stderr '^go: warning: example.com/retract/incompatible@v2.0.0\+incompatible: retracted by module author$'
diff --git a/src/cmd/go/testdata/script/mod_retract_noupgrade.txt b/src/cmd/go/testdata/script/mod_retract_noupgrade.txt
new file mode 100644
index 0000000..67de79f
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_retract_noupgrade.txt
@@ -0,0 +1,11 @@
+go list -m -u example.com/retract/noupgrade
+stdout '^example.com/retract/noupgrade v1.0.0 \(retracted\)$'
+
+-- go.mod --
+module use
+
+go 1.19
+
+require example.com/retract/noupgrade v1.0.0
+-- go.sum --
+example.com/retract/noupgrade v1.0.0/go.mod h1:q2/HnBejUQ83RcUo4stf2U++/Zr9R/Ky3BsodjKBkQ4=
diff --git a/src/cmd/go/testdata/script/mod_retract_pseudo_base.txt b/src/cmd/go/testdata/script/mod_retract_pseudo_base.txt
new file mode 100644
index 0000000..87b440d
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_retract_pseudo_base.txt
@@ -0,0 +1,62 @@
+# When converting a commit to a pseudo-version, don't use a retracted version
+# as the base.
+# Verifies golang.org/issue/41700.
+
+[short] skip
+[!git] skip
+env GOPROXY=direct
+env GOSUMDB=off
+go mod init m
+
+# Control: check that v1.0.0 is the only version and is retracted.
+go list -m -versions vcs-test.golang.org/git/retract-pseudo.git
+stdout '^vcs-test.golang.org/git/retract-pseudo.git$'
+go list -m -versions -retracted vcs-test.golang.org/git/retract-pseudo.git
+stdout '^vcs-test.golang.org/git/retract-pseudo.git v1.0.0$'
+
+# 713affd19d7b is a commit after v1.0.0. Don't use v1.0.0 as the base.
+go list -m vcs-test.golang.org/git/retract-pseudo.git@713affd19d7b
+stdout '^vcs-test.golang.org/git/retract-pseudo.git v0.0.0-20201009173747-713affd19d7b$'
+
+# 64c061ed4371 is the commit v1.0.0 refers to. Don't convert to v1.0.0.
+go list -m vcs-test.golang.org/git/retract-pseudo.git@64c061ed4371
+stdout '^vcs-test.golang.org/git/retract-pseudo.git v0.0.0-20201009173747-64c061ed4371'
+
+# A retracted version is a valid base. Retraction should not validate existing
+# pseudo-versions, nor should it turn invalid pseudo-versions valid.
+go get vcs-test.golang.org/git/retract-pseudo.git@v1.0.1-0.20201009173747-713affd19d7b
+go list -m vcs-test.golang.org/git/retract-pseudo.git
+stdout '^vcs-test.golang.org/git/retract-pseudo.git v1.0.1-0.20201009173747-713affd19d7b$'
+
+! go get vcs-test.golang.org/git/retract-pseudo.git@v1.0.1-0.20201009173747-64c061ed4371
+stderr '^go: vcs-test.golang.org/git/retract-pseudo.git@v1.0.1-0.20201009173747-64c061ed4371: invalid pseudo-version: tag \(v1.0.0\) found on revision 64c061ed4371 is already canonical, so should not be replaced with a pseudo-version derived from that tag$'
+
+-- retract-pseudo.sh --
+#!/bin/bash
+
+# This is not part of the test.
+# Run this to generate and update the repository on vcs-test.golang.org.
+
+set -euo pipefail
+
+rm -rf retract-pseudo
+mkdir retract-pseudo
+cd retract-pseudo
+git init
+
+# Create the module.
+# Retract v1.0.0 and tag v1.0.0 at the same commit.
+# The module has no unretracted release versions.
+go mod init vcs-test.golang.org/git/retract-pseudo.git
+go mod edit -retract v1.0.0
+echo 'package p' >p.go
+git add -A
+git commit -m 'create module retract-pseudo'
+git tag v1.0.0
+
+# Commit a trivial change so the default branch does not point to v1.0.0.
+git mv p.go q.go
+git commit -m 'trivial change'
+
+zip -r ../retract-pseudo.zip .
+gsutil cp ../retract-pseudo.zip gs://vcs-test/git/retract-pseudo.zip
diff --git a/src/cmd/go/testdata/script/mod_retract_rationale.txt b/src/cmd/go/testdata/script/mod_retract_rationale.txt
new file mode 100644
index 0000000..92e9b7d
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_retract_rationale.txt
@@ -0,0 +1,79 @@
+# When there is no rationale, 'go get' should print a hard-coded message.
+go get example.com/retract/rationale@v1.0.0-empty
+stderr '^go: warning: example.com/retract/rationale@v1.0.0-empty: retracted by module author$'
+
+# 'go list' should print the same hard-coded message.
+go list -m -retracted -f '{{.Retracted}}' example.com/retract/rationale
+stdout '^\[retracted by module author\]$'
+
+
+# When there is a multi-line message, 'go get' should print the first line.
+go get example.com/retract/rationale@v1.0.0-multiline1
+stderr '^go: warning: example.com/retract/rationale@v1.0.0-multiline1: retracted by module author: short description$'
+! stderr 'detail'
+
+# 'go list' should show the full message.
+go list -m -retracted -f '{{.Retracted}}' example.com/retract/rationale
+cmp stdout multiline
+
+# 'go get' output should be the same whether the retraction appears at top-level
+# or in a block.
+go get example.com/retract/rationale@v1.0.0-multiline2
+stderr '^go: warning: example.com/retract/rationale@v1.0.0-multiline2: retracted by module author: short description$'
+! stderr 'detail'
+
+# Same for 'go list'.
+go list -m -retracted -f '{{.Retracted}}' example.com/retract/rationale
+cmp stdout multiline
+
+
+# 'go get' should omit long messages.
+go get example.com/retract/rationale@v1.0.0-long
+stderr '^go: warning: example.com/retract/rationale@v1.0.0-long: retracted by module author: \(message omitted: too long\)'
+
+# 'go list' should show the full message.
+go list -m -retracted -f '{{.Retracted}}' example.com/retract/rationale
+stdout '^\[lo{500}ng\]$'
+
+
+# 'go get' should omit messages with unprintable characters.
+go get example.com/retract/rationale@v1.0.0-unprintable
+stderr '^go: warning: example.com/retract/rationale@v1.0.0-unprintable: retracted by module author: \(message omitted: contains non-printable characters\)'
+
+# 'go list' should show the full message.
+go list -m -retracted -f '{{.Retracted}}' example.com/retract/rationale
+stdout '^\[Ends with a BEL character. Beep!\x07\]$'
+
+
+# When there is a comment on a block, but not on individual retractions within
+# the block, the rationale should come from the block comment.
+go list -m -retracted -f '{{.Retracted}}' example.com/retract/rationale@v1.0.0-block
+stdout '^\[block comment\]$'
+go list -m -retracted -f '{{.Retracted}}' example.com/retract/rationale@v1.0.0-blockwithcomment
+stdout '^\[inner comment\]$'
+
+
+# When a version is covered by multiple retractions, all retractions should
+# be reported in the order they appear in the file.
+go list -m -retracted -f '{{range .Retracted}}{{.}},{{end}}' example.com/retract/rationale@v1.0.0-order
+stdout '^degenerate range,single version,$'
+go list -m -retracted -f '{{range .Retracted}}{{.}},{{end}}' example.com/retract/rationale@v1.0.1-order
+stdout '^single version,degenerate range,$'
+
+# 'go get' will only report the first retraction to avoid being too verbose.
+go get example.com/retract/rationale@v1.0.0-order
+stderr '^go: warning: example.com/retract/rationale@v1.0.0-order: retracted by module author: degenerate range$'
+go get example.com/retract/rationale@v1.0.1-order
+stderr '^go: warning: example.com/retract/rationale@v1.0.1-order: retracted by module author: single version$'
+
+-- go.mod --
+module m
+
+go 1.14
+
+-- multiline --
+[short description
+more
+
+detail
+suffix]
diff --git a/src/cmd/go/testdata/script/mod_retract_rename.txt b/src/cmd/go/testdata/script/mod_retract_rename.txt
new file mode 100644
index 0000000..38986f3
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_retract_rename.txt
@@ -0,0 +1,28 @@
+# Populate go.sum.
+go get
+
+# 'go list -m -retracted' should load retractions, even if the version
+# containing retractions has a different module path.
+go list -m -retracted -f '{{with .Retracted}}retracted{{end}}' example.com/retract/rename
+
+# 'go list -m -u' should load retractions, too.
+go list -m -u -f '{{with .Retracted}}retracted{{end}}' example.com/retract/rename
+
+# 'go get' should warn about the retracted version.
+go get
+stderr '^go: warning: example.com/retract/rename@v1.0.0-bad: retracted by module author: bad$'
+
+# We can't upgrade, since this latest version has a different module path.
+! go get example.com/retract/rename
+stderr 'module declares its path as: example.com/retract/newname'
+
+-- go.mod --
+module example.com/use
+
+go 1.16
+
+require example.com/retract/rename v1.0.0-bad
+-- use.go --
+package use
+
+import _ "example.com/retract/rename"
diff --git a/src/cmd/go/testdata/script/mod_retract_replace.txt b/src/cmd/go/testdata/script/mod_retract_replace.txt
new file mode 100644
index 0000000..788968f
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_retract_replace.txt
@@ -0,0 +1,63 @@
+# If the latest unretracted version of a module is replaced, 'go list' should
+# obtain retractions from the replacement.
+
+# Populate go.sum.
+go get
+
+# The latest version, v1.9.0, is not available on the proxy.
+go list -m -retracted example.com/retract/missingmod
+stdout '^example.com/retract/missingmod v1.0.0$'
+exists $GOPATH/pkg/mod/cache/download/example.com/retract/missingmod/@v/v1.9.0.info
+! exists $GOPATH/pkg/mod/cache/download/example.com/retract/missingmod/@v/v1.9.0.mod
+
+# If we replace that version, we should see retractions.
+go mod edit -replace=example.com/retract/missingmod@v1.9.0=./missingmod-v1.9.0
+go list -m -retracted -f '{{range .Retracted}}{{.}}{{end}}' example.com/retract/missingmod
+stdout '^bad version$'
+
+# If we replace the retracted version, we should not see a retraction.
+go mod edit -replace=example.com/retract/missingmod=./missingmod-v1.9.0
+go list -m -retracted -f '{{if not .Retracted}}good version{{end}}' example.com/retract/missingmod
+stdout '^good version$'
+
+
+# If a replacement version is retracted, we should see a retraction.
+# It should appear in both the replaced module and the replacement, as other
+# fields like GoMod do.
+go list -m -retracted -f '{{range .Retracted}}{{.}}{{end}}' example.com/retract
+! stdout .
+go list -m -retracted -f '{{if .Replace}}replaced{{end}}' example.com/retract
+! stdout .
+go mod edit -replace example.com/retract@v1.0.0-good=example.com/retract@v1.0.0-bad
+go list -m -mod=mod -retracted -f '{{range .Retracted}}{{.}}{{end}}' example.com/retract
+stdout '^bad$'
+go list -m -mod=mod -retracted -f '{{with .Replace}}{{range .Retracted}}{{.}}{{end}}{{end}}' example.com/retract
+stdout '^bad$'
+
+-- go.mod --
+module m
+
+go 1.14
+
+require (
+	example.com/retract v1.0.0-good
+	example.com/retract/missingmod v1.0.0
+)
+-- use.go --
+package use
+
+import (
+	_ "example.com/retract"
+	_ "example.com/retract/missingmod"
+)
+-- missingmod-v1.0.0/go.mod --
+module example.com/retract/missingmod
+
+go 1.14
+-- missingmod-v1.9.0/go.mod --
+module example.com/retract/missingmod
+
+go 1.14
+
+// bad version
+retract v1.0.0
diff --git a/src/cmd/go/testdata/script/mod_retract_versions.txt b/src/cmd/go/testdata/script/mod_retract_versions.txt
new file mode 100644
index 0000000..012fa15
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_retract_versions.txt
@@ -0,0 +1,22 @@
+# https://golang.org/issue/44296: the --versions flag should not affect
+# the version reported by 'go list' in case of retractions.
+
+env FMT='{{.Path}}{{with .Error}}: {{printf "%q" .Err}}{{end}} {{printf "%q" .Version}}{{with .Versions}} {{.}}{{end}}'
+
+go list -m -e -f $FMT example.com/retract/self/pseudo
+stdout '^example.com/retract/self/pseudo: "module example.com/retract/self/pseudo: not a known dependency" ""$'
+
+go list -m -e -f $FMT example.com/retract/self/pseudo@latest
+stdout '^example.com/retract/self/pseudo: "module example.com/retract/self/pseudo: no matching versions for query \\"latest\\"" "latest"$'
+
+
+go list -m -e -f $FMT --versions example.com/retract/self/pseudo
+stdout '^example.com/retract/self/pseudo ""$'
+
+go list -m -e -f $FMT --versions example.com/retract/self/pseudo@latest
+stdout '^example.com/retract/self/pseudo: "module example.com/retract/self/pseudo: no matching versions for query \\"latest\\"" "latest"$'
+
+-- go.mod --
+module test
+
+go 1.17
diff --git a/src/cmd/go/testdata/script/mod_run_issue52331.txt b/src/cmd/go/testdata/script/mod_run_issue52331.txt
new file mode 100644
index 0000000..917e890
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_run_issue52331.txt
@@ -0,0 +1,35 @@
+# Regression test for https://go.dev/issue/52331: 'go run -mod=mod'
+# failed to write go.mod and go.sum with the resolved dependencies.
+
+[short] skip
+
+! go run main.go
+# stderr '^main\.go:6:2: no required module provides package example\.com/version; to add it:\n\tgo get example\.com/version\n\z'
+
+go run -mod=mod main.go
+cmp go.mod go.mod.want
+grep -count=1 '^example\.com/version v1.1.0 h1:' go.sum
+grep -count=1 '^example\.com/version v1.1.0/go.mod h1:' go.sum
+
+-- go.mod --
+module example
+
+go 1.17
+-- go.mod.want --
+module example
+
+go 1.17
+
+require example.com/version v1.1.0 // indirect
+-- main.go --
+package main
+
+import (
+	"fmt"
+
+	"example.com/version"
+)
+
+func main() {
+	fmt.Println(version.V)
+}
diff --git a/src/cmd/go/testdata/script/mod_run_nonmain.txt b/src/cmd/go/testdata/script/mod_run_nonmain.txt
new file mode 100644
index 0000000..8435fc0
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_run_nonmain.txt
@@ -0,0 +1,18 @@
+! go run $PWD
+! stderr 'no packages loaded'
+stderr '^package example.net/nonmain is not a main package$'
+
+! go run .
+stderr '^package example.net/nonmain is not a main package$'
+
+! go run ./...
+stderr '^go: warning: "\./\.\.\." matched only non-main packages$'
+stderr '^go: no packages loaded from \./\.\.\.$'
+
+-- go.mod --
+module example.net/nonmain
+
+go 1.17
+-- nonmain.go --
+// Package nonmain is not a main package.
+package nonmain
diff --git a/src/cmd/go/testdata/script/mod_run_path.txt b/src/cmd/go/testdata/script/mod_run_path.txt
new file mode 100644
index 0000000..4369ee4
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_run_path.txt
@@ -0,0 +1,15 @@
+# Test that go run does not get confused by conflict
+# between go.mod's module path and what you'd
+# expect from GOPATH. golang.org/issue/26046.
+
+env GO111MODULE=on
+
+cd $GOPATH/src/example.com/hello
+go run main.go
+
+-- $GOPATH/src/example.com/hello/go.mod --
+module example.com/hello/v2
+
+-- $GOPATH/src/example.com/hello/main.go --
+package main
+func main() {}
diff --git a/src/cmd/go/testdata/script/mod_run_pkg_version.txt b/src/cmd/go/testdata/script/mod_run_pkg_version.txt
new file mode 100644
index 0000000..969852c
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_run_pkg_version.txt
@@ -0,0 +1,104 @@
+# This test checks the behavior of 'go run' with a 'cmd@version' argument.
+# Most of 'go run' is covered in other tests.
+# mod_install_pkg_version covers most of the package loading functionality.
+# This test focuses on 'go run' behavior specific to this mode.
+[short] skip
+
+# 'go run pkg@version' works outside a module.
+env GO111MODULE=auto
+go run example.com/cmd/a@v1.0.0
+stdout '^a@v1.0.0$'
+
+
+# 'go run pkg@version' reports an error if modules are disabled.
+env GO111MODULE=off
+! go run example.com/cmd/a@v1.0.0
+stderr '^go: modules disabled by GO111MODULE=off; see ''go help modules''$'
+env GO111MODULE=on
+
+
+# 'go run pkg@version' ignores go.mod in the current directory.
+cd m
+cp go.mod go.mod.orig
+! go list -m all
+stderr '^go: example.com/cmd@v1.1.0-doesnotexist: reading http.*/mod/example\.com/cmd/@v/v1.1.0-doesnotexist.info: 404 Not Found\n\tserver response: 404 page not found$'
+stderr '^go: example.com/cmd@v1.1.0-doesnotexist: missing go.sum entry for go.mod file; to add it:\n\tgo mod download example.com/cmd$'
+go run example.com/cmd/a@v1.0.0
+stdout '^a@v1.0.0$'
+cmp go.mod go.mod.orig
+cd ..
+
+
+# 'go install pkg@version' works on a module that doesn't have a go.mod file
+# and with a module whose go.mod file has missing requirements.
+# With a proxy, the two cases are indistinguishable.
+go run rsc.io/fortune@v1.0.0
+stderr '^go: found rsc.io/quote in rsc.io/quote v1.5.2$'
+stderr '^Hello, world.$'
+
+
+# 'go run pkg@version' should report an error if pkg is not a main package.
+! go run example.com/cmd/err@v1.0.0
+stderr '^package example.com/cmd/err is not a main package$'
+
+
+# 'go run pkg@version' should report errors if the module contains
+# replace or exclude directives.
+go mod download example.com/cmd@v1.0.0-replace
+! go run example.com/cmd/a@v1.0.0-replace
+cmp stderr replace-err
+
+go mod download example.com/cmd@v1.0.0-exclude
+! go run example.com/cmd/a@v1.0.0-exclude
+cmp stderr exclude-err
+
+
+# 'go run dir@version' works like a normal 'go run' command if
+# dir is a relative or absolute path.
+go mod download rsc.io/fortune@v1.0.0
+! go run $GOPATH/pkg/mod/rsc.io/fortune@v1.0.0
+stderr '^go: go\.mod file not found in current directory or any parent directory; see ''go help modules''$'
+! go run ../pkg/mod/rsc.io/fortune@v1.0.0
+stderr '^go: go\.mod file not found in current directory or any parent directory; see ''go help modules''$'
+mkdir tmp
+cd tmp
+go mod init tmp
+go mod edit -require=rsc.io/fortune@v1.0.0
+! go run -mod=readonly $GOPATH/pkg/mod/rsc.io/fortune@v1.0.0
+stderr '^missing go\.sum entry for module providing package rsc\.io/fortune; to add:\n\tgo mod download rsc\.io/fortune$'
+! go run -mod=readonly ../../pkg/mod/rsc.io/fortune@v1.0.0
+stderr '^missing go\.sum entry for module providing package rsc\.io/fortune; to add:\n\tgo mod download rsc\.io/fortune$'
+cd ..
+rm tmp
+
+
+# 'go run' does not interpret @version arguments after the first.
+go run example.com/cmd/a@v1.0.0 example.com/doesnotexist@v1.0.0
+stdout '^a@v1.0.0$'
+
+
+# 'go run pkg@version' succeeds when -mod=readonly is set explicitly.
+# Verifies #43278.
+go run -mod=readonly example.com/cmd/a@v1.0.0
+stdout '^a@v1.0.0$'
+
+-- m/go.mod --
+module m
+
+go 1.16
+
+require example.com/cmd v1.1.0-doesnotexist
+-- x/x.go --
+package main
+
+func main() {}
+-- replace-err --
+go: example.com/cmd/a@v1.0.0-replace (in example.com/cmd@v1.0.0-replace):
+	The go.mod file for the module providing named packages contains one or
+	more replace directives. It must not contain directives that would cause
+	it to be interpreted differently than if it were the main module.
+-- exclude-err --
+go: example.com/cmd/a@v1.0.0-exclude (in example.com/cmd@v1.0.0-exclude):
+	The go.mod file for the module providing named packages contains one or
+	more exclude directives. It must not contain directives that would cause
+	it to be interpreted differently than if it were the main module.
diff --git a/src/cmd/go/testdata/script/mod_run_pkgerror.txt b/src/cmd/go/testdata/script/mod_run_pkgerror.txt
new file mode 100644
index 0000000..48f900d
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_run_pkgerror.txt
@@ -0,0 +1,32 @@
+# https://golang.org/issue/39986: files reported as invalid by go/build should
+# be listed in InvalidGoFiles.
+
+go list -e -f '{{.Incomplete}}{{"\n"}}{{.Error}}{{"\n"}}{{.InvalidGoFiles}}{{"\n"}}' .
+stdout '^true\nfound packages m \(m\.go\) and main \(main\.go\) in '$PWD'\n\[main.go\]\n'
+
+
+# https://golang.org/issue/45827: 'go run .' should report the same package
+# errors as 'go build' and 'go list'.
+
+! go build
+stderr '^found packages m \(m\.go\) and main \(main\.go\) in '$PWD'$'
+
+! go list .
+stderr '^found packages m \(m\.go\) and main \(main\.go\) in '$PWD'$'
+
+! go run .
+! stderr 'no packages loaded'
+stderr '^found packages m \(m\.go\) and main \(main\.go\) in '$PWD'$'
+
+! go run ./...
+! stderr 'no packages loaded'
+stderr '^found packages m \(m\.go\) and main \(main\.go\) in '$PWD'$'
+
+-- go.mod --
+module m
+
+go 1.17
+-- m.go --
+package m
+-- main.go --
+package main
diff --git a/src/cmd/go/testdata/script/mod_skip_write.txt b/src/cmd/go/testdata/script/mod_skip_write.txt
new file mode 100644
index 0000000..db47b9c
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_skip_write.txt
@@ -0,0 +1,93 @@
+# Commands used to debug the module graph should not write go.mod or go.sum
+# or report errors when those files need to be updated.
+
+# Everything's okay initially.
+go list -m all
+
+# Downgrading sampler makes go.mod inconsistent, but 'go mod graph',
+# 'go mod verify', and 'go mod why' still work.
+cp go.mod go.mod.orig
+go mod edit -require=rsc.io/sampler@v1.2.0
+cp go.mod go.mod.edit
+! go list -m all
+stderr 'updates to go.mod needed'
+
+go mod graph
+cmp stdout graph.want
+cmp go.mod go.mod.edit
+
+go mod verify
+stdout '^all modules verified$'
+cmp go.mod go.mod.edit
+
+go mod why rsc.io/sampler
+cmp stdout why.want
+cmp go.mod go.mod.edit
+
+go mod why -m rsc.io/sampler
+cmp stdout why.want
+cmp go.mod go.mod.edit
+
+cp go.mod.orig go.mod
+
+# Removing go.sum breaks other commands, but 'go mod graph' and
+# 'go mod why' still work.
+rm go.sum
+! go list -m all
+stderr 'missing go.sum entry'
+
+go mod graph
+cmp stdout graph.want
+! exists go.sum
+
+go mod verify
+stdout '^all modules verified$'
+! exists go.sum
+
+go mod why rsc.io/sampler
+cmp stdout why.want
+! exists go.sum
+
+go mod why -m rsc.io/sampler
+cmp stdout why.want
+! exists go.sum
+
+-- go.mod --
+module m
+
+go 1.18
+
+require rsc.io/quote v1.5.2
+
+require (
+	golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c // indirect
+	rsc.io/sampler v1.3.0 // indirect
+	rsc.io/testonly v1.0.0 // indirect
+)
+-- go.sum --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c h1:pvCbr/wm8HzDD3fVywevekufpn6tCGPY3spdHeZJEsw=
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.5.2 h1:3fEykkD9k7lYzXqCYrwGAf7iNhbk4yCjHmKBN9td4L0=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.2.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+rsc.io/sampler v1.3.0 h1:HLGR/BgEtI3r0uymSP/nl2uPLsUnNJX8toRyhfpBTII=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+rsc.io/testonly v1.0.0 h1:K/VWHdO+Jv7woUXG0GzVNx1czBXUt3Ib1deaMn+xk64=
+rsc.io/testonly v1.0.0/go.mod h1:OqmGbIFOcF+XrFReLOGZ6BhMM7uMBiQwZsyNmh74SzY=
+-- use.go --
+package use
+
+import _ "rsc.io/quote"
+-- graph.want --
+m go@1.18
+m golang.org/x/text@v0.0.0-20170915032832-14c0d48ead0c
+m rsc.io/quote@v1.5.2
+m rsc.io/sampler@v1.3.0
+m rsc.io/testonly@v1.0.0
+rsc.io/quote@v1.5.2 rsc.io/sampler@v1.3.0
+rsc.io/sampler@v1.3.0 golang.org/x/text@v0.0.0-20170915032832-14c0d48ead0c
+-- why.want --
+# rsc.io/sampler
+m
+rsc.io/quote
+rsc.io/sampler
diff --git a/src/cmd/go/testdata/script/mod_stale.txt b/src/cmd/go/testdata/script/mod_stale.txt
new file mode 100644
index 0000000..c6ab27d
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_stale.txt
@@ -0,0 +1,15 @@
+[short] skip
+
+env GOCACHE=$WORK/cache
+go list -f '{{.Stale}}' .
+stdout true
+go install .
+go list -f '{{.Stale}}' .
+stdout false
+
+-- go.mod --
+module example.com/mod
+
+go 1.20
+-- m.go --
+package m
diff --git a/src/cmd/go/testdata/script/mod_std_vendor.txt b/src/cmd/go/testdata/script/mod_std_vendor.txt
new file mode 100644
index 0000000..ed47542
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_std_vendor.txt
@@ -0,0 +1,77 @@
+env GO111MODULE=on
+env GOPROXY=off
+
+[!compiler:gc] skip
+
+# 'go list' should report imports from _test.go in the TestImports field.
+go list -f '{{.TestImports}}'
+stdout net/http # from .TestImports
+
+# 'go list' should find standard-vendored packages.
+go list -f '{{.Dir}}' vendor/golang.org/x/net/http2/hpack
+stdout $GOROOT[/\\]src[/\\]vendor
+
+# 'go list -test' should report vendored transitive dependencies of _test.go
+# imports in the Deps field.
+go list -test -f '{{range .Deps}}{{.}}{{"\n"}}{{end}}'
+stdout ^vendor/golang.org/x/crypto # dep of .TestImports
+
+
+# Modules outside the standard library should not use the packages vendored there...
+cd broken
+! go build -mod=readonly
+stderr 'disabled by -mod=readonly'
+! go build -mod=vendor
+stderr 'http.go:5:2: cannot find module providing package golang.org/x/net/http2/hpack: import lookup disabled by -mod=vendor'
+
+# ...even if they explicitly use the "cmd/vendor/" or "vendor/" prefix.
+cd ../importcmd
+! go build .
+stderr 'use of vendored package'
+
+cd ../importstd
+! go build .
+stderr 'use of vendored package'
+
+
+# When run within the 'std' module, 'go list -test' should report vendored
+# transitive dependencies at their vendored paths.
+cd $GOROOT/src
+go list -test -f '{{range .Deps}}{{.}}{{"\n"}}{{end}}' net/http
+! stdout ^golang.org/x/net/http2/hpack
+stdout ^vendor/golang.org/x/net/http2/hpack
+
+-- go.mod --
+module m
+
+-- x.go --
+package x
+
+-- x_test.go --
+package x
+import "testing"
+import _ "net/http"
+func Test(t *testing.T) {}
+
+-- broken/go.mod --
+module broken
+-- broken/http.go --
+package broken
+
+import (
+	_ "net/http"
+	_ "golang.org/x/net/http2/hpack"
+)
+
+-- importcmd/go.mod --
+module importcmd
+-- importcmd/x.go --
+package importcmd
+
+import _ "cmd/vendor/golang.org/x/tools/go/analysis"
+-- importstd/go.mod --
+module importvendor
+-- importstd/x.go --
+package importstd
+
+import _ "vendor/golang.org/x/net/http2/hpack"
diff --git a/src/cmd/go/testdata/script/mod_string_alias.txt b/src/cmd/go/testdata/script/mod_string_alias.txt
new file mode 100644
index 0000000..5c3d428
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_string_alias.txt
@@ -0,0 +1,14 @@
+[short] skip
+
+env GO111MODULE=on
+
+go mod init golang.org/issue/27584
+
+go build .
+
+-- main.go --
+package main
+
+type string = []int
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/mod_sum_ambiguous.txt b/src/cmd/go/testdata/script/mod_sum_ambiguous.txt
new file mode 100644
index 0000000..07c6659
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_sum_ambiguous.txt
@@ -0,0 +1,62 @@
+# Confirm our build list.
+cp go.sum.buildlist-only go.sum
+go list -m all
+stdout '^example.com/ambiguous/a v1.0.0$'
+stdout '^example.com/ambiguous/a/b v0.0.0-empty$'
+
+# If two modules could provide a package, but only one does,
+# 'go mod tidy' should retain sums for both zips.
+go mod tidy
+grep '^example.com/ambiguous/a v1.0.0 h1:' go.sum
+grep '^example.com/ambiguous/a/b v0.0.0-empty h1:' go.sum
+
+# 'go mod download' should also add sums.
+cp go.sum.buildlist-only go.sum
+go mod download example.com/ambiguous/a
+grep '^example.com/ambiguous/a v1.0.0 h1:' go.sum
+! grep '^example.com/ambiguous/a/b v0.0.0-empty h1:' go.sum
+go mod download example.com/ambiguous/a/b
+grep '^example.com/ambiguous/a/b v0.0.0-empty h1:' go.sum
+
+# If two modules could provide a package, and we're missing a sum for one,
+# we should see a missing sum error, even if we have a sum for a module that
+# provides the package.
+cp go.sum.a-only go.sum
+! go list example.com/ambiguous/a/b
+stderr '^missing go.sum entry needed to verify package example.com/ambiguous/a/b is provided by exactly one module; to add:\n\tgo mod download example.com/ambiguous/a/b$'
+! go list -deps .
+stderr '^use.go:3:8: missing go.sum entry needed to verify package example.com/ambiguous/a/b \(imported by m\) is provided by exactly one module; to add:\n\tgo get m$'
+
+cp go.sum.b-only go.sum
+! go list example.com/ambiguous/a/b
+stderr '^missing go.sum entry for module providing package example.com/ambiguous/a/b; to add:\n\tgo mod download example.com/ambiguous/a$'
+! go list -deps .
+stderr '^use.go:3:8: missing go.sum entry for module providing package example.com/ambiguous/a/b \(imported by m\); to add:\n\tgo get m$'
+
+cp go.sum.buildlist-only go.sum
+! go list example.com/ambiguous/a/b
+stderr '^missing go.sum entry for module providing package example.com/ambiguous/a/b; to add:\n\tgo mod download example.com/ambiguous/a example.com/ambiguous/a/b$'
+! go list -deps .
+stderr '^use.go:3:8: missing go.sum entry for module providing package example.com/ambiguous/a/b \(imported by m\); to add:\n\tgo get m$'
+
+-- go.mod --
+module m
+
+go 1.15
+
+require example.com/ambiguous/a v1.0.0
+-- go.sum.buildlist-only --
+example.com/ambiguous/a v1.0.0/go.mod h1:TrBl/3xTPFJ2gmMIYz53h2gkNtg0dokszEMuyS1QEb0=
+example.com/ambiguous/a/b v0.0.0-empty/go.mod h1:MajJq5jPEBnnXP+NTWIeXX7kwaPS1sbVEJdooTmsePQ=
+-- go.sum.a-only --
+example.com/ambiguous/a v1.0.0 h1:pGZhTXy6+titE2rNfwHwJykSjXDR4plO52PfZrBM0T8=
+example.com/ambiguous/a v1.0.0/go.mod h1:TrBl/3xTPFJ2gmMIYz53h2gkNtg0dokszEMuyS1QEb0=
+example.com/ambiguous/a/b v0.0.0-empty/go.mod h1:MajJq5jPEBnnXP+NTWIeXX7kwaPS1sbVEJdooTmsePQ=
+-- go.sum.b-only --
+example.com/ambiguous/a v1.0.0/go.mod h1:TrBl/3xTPFJ2gmMIYz53h2gkNtg0dokszEMuyS1QEb0=
+example.com/ambiguous/a/b v0.0.0-empty h1:xS29ReXXuhjT7jc79mo91h/PevaZ2oS9PciF1DucXtg=
+example.com/ambiguous/a/b v0.0.0-empty/go.mod h1:MajJq5jPEBnnXP+NTWIeXX7kwaPS1sbVEJdooTmsePQ=
+-- use.go --
+package use
+
+import _ "example.com/ambiguous/a/b"
diff --git a/src/cmd/go/testdata/script/mod_sum_issue56222.txt b/src/cmd/go/testdata/script/mod_sum_issue56222.txt
new file mode 100644
index 0000000..12848c9
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_sum_issue56222.txt
@@ -0,0 +1,91 @@
+# Regression test for #56222: 'go get -t' and 'go mod tidy'
+# should save enough checksums to run 'go test' on the named
+# packages or any package in "all" respectively.
+
+# 'go mod tidy' in a module at go 1.21 or higher should preserve
+# checksums needed to run 'go test all'.
+cd m1
+go mod tidy
+
+go list -f '{{if eq .ImportPath "example.com/generics"}}{{.Module.GoVersion}}{{end}}' -deps -test example.com/m2/q
+stdout 1.18
+[!short] go test -o $devnull -c all
+
+cat go.sum
+replace 'example.com/generics v1.0.0/go.mod' 'example.com/notgenerics v1.0.0/go.mod' go.sum
+
+! go list -f '{{if eq .ImportPath "example.com/generics"}}{{.Module.GoVersion}}{{end}}' -deps -test example.com/m2/q
+stderr '^go: can''t load test package: \.\.'${/}m2${/}q${/}'q_test.go:3:8: example\.com/generics@v1\.0\.0: missing go.sum entry for go.mod file; to add it:\n\tgo mod download example\.com/generics$'
+
+go mod download -json example.com/generics
+stdout '"GoModSum":'
+go list -f '{{if eq .ImportPath "example.com/generics"}}{{.Module.GoVersion}}{{end}}' -deps -test example.com/m2/q
+stdout 1.18
+
+
+# At go 1.20 or earlier, 'go mod tidy' should preserve the historical go.sum
+# contents, but 'go test' should flag the missing checksums (instead of trying
+# to build the test dependency with the wrong language version).
+
+go mod tidy -go=1.20
+! go test -o $devnull -c all
+stderr '^# example.com/m2/q\n'..${/}m2${/}q${/}'q_test.go:3:8: example.com/generics@v1.0.0: missing go.sum entry for go.mod file; to add it:\n\tgo mod download example.com/generics$'
+
+go mod download -json example.com/generics
+go list -f '{{if eq .ImportPath "example.com/generics"}}{{.Module.GoVersion}}{{end}}' -deps -test example.com/m2/q
+stdout 1.18
+
+
+# Even at go 1.20 or earlier, 'go mod tidy' shouldn't need go.mod files or
+# checksums that it won't record.
+
+go mod tidy -go=1.20
+go clean -modcache  # Remove checksums from the module cache, so that only go.sum is used.
+
+# Issue 60667: 'go list' without -mod=mod shouldn't report the checksums as
+# dirty either.
+go list -m -u all
+
+env OLDSUMDB=$GOSUMDB
+env GOSUMDB=bad
+go mod tidy
+
+env GOSUMDB=$OLDSUMDB
+
+
+# Regardless of the go version in go.mod, 'go get -t' should fetch
+# enough checksums to run 'go test' on the named package.
+
+rm p
+go mod tidy -go=1.20
+go list -m all
+! stdout example.com/generics
+go get -t example.com/m2/q@v1.0.0
+go list -f '{{if eq .ImportPath "example.com/generics"}}{{.Module.GoVersion}}{{end}}' -deps -test example.com/m2/q
+stdout 1.18
+[!short] go test -o $devnull -c example.com/m2/q
+
+
+-- m1/go.mod --
+module example.com/m1
+
+go 1.21
+
+require example.com/m2 v1.0.0
+replace example.com/m2 => ../m2
+-- m1/p/p.go --
+package p
+
+import _ "example.com/m2/q"
+-- m2/go.mod --
+module example.com/m2
+
+go 1.19
+
+require example.com/generics v1.0.0
+-- m2/q/q.go --
+package q
+-- m2/q/q_test.go --
+package q
+
+import _ "example.com/generics"
diff --git a/src/cmd/go/testdata/script/mod_sum_lookup.txt b/src/cmd/go/testdata/script/mod_sum_lookup.txt
new file mode 100644
index 0000000..7513f7f
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_sum_lookup.txt
@@ -0,0 +1,34 @@
+# When we attempt to resolve an import that doesn't exist, we should not save
+# hashes for downloaded modules.
+# Verifies golang.org/issue/36260.
+# TODO(golang.org/issue/26603): use 'go mod tidy -e' when implemented.
+go list -e -mod=mod -tags=ignore ./noexist
+! exists go.sum
+
+# When an import is resolved successfully, we should only save hashes for
+# the module that provides the package, not for other modules looked up.
+# Verifies golang.org/issue/31580.
+go get ./exist
+grep '^example.com/join v1.1.0 h1:' go.sum
+! grep '^example.com/join/subpkg' go.sum
+cp go.sum go.list.sum
+go mod tidy
+cmp go.sum go.list.sum
+
+-- go.mod --
+module m
+
+go 1.15
+
+-- noexist/use.go --
+// ignore tags prevents errors in 'go mod tidy'
+// +build ignore
+
+package use
+
+import _ "example.com/join/subpkg/noexist"
+
+-- exist/use.go --
+package use
+
+import _ "example.com/join/subpkg"
diff --git a/src/cmd/go/testdata/script/mod_sum_readonly.txt b/src/cmd/go/testdata/script/mod_sum_readonly.txt
new file mode 100644
index 0000000..c426073
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_sum_readonly.txt
@@ -0,0 +1,87 @@
+# Test that go.sum does not get updated when -mod=readonly flag is set
+env GO111MODULE=on
+
+# When a sum is needed to load the build list, we get an error for the
+# specific module. The .mod file is not downloaded, and go.sum is not written.
+! go list -m all
+stderr '^go: rsc.io/quote@v1.5.2: missing go.sum entry for go.mod file; to add it:\n\tgo mod download rsc.io/quote$'
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.mod
+! exists go.sum
+
+# If go.sum exists but contains hashes from an algorithm we don't know about,
+# we should see the same error.
+cp go.sum.h2only go.sum
+! go list -m all
+stderr '^go: rsc.io/quote@v1.5.2: missing go.sum entry for go.mod file; to add it:\n\tgo mod download rsc.io/quote$'
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.mod
+cmp go.sum go.sum.h2only
+rm go.sum
+
+# If we replace a module, we should see a missing sum error for the replacement.
+cp go.mod go.mod.orig
+go mod edit -replace rsc.io/quote@v1.5.2=rsc.io/quote@v1.5.1
+! go list -m all
+stderr '^go: rsc.io/quote@v1.5.2 \(replaced by rsc.io/quote@v1.5.1\): missing go.sum entry for go.mod file; to add it:\n\tgo mod download rsc.io/quote$'
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.1.mod
+! exists go.sum
+cp go.mod.orig go.mod
+
+# Control: when sums are present, loading the build list downloads .mod files.
+cp go.sum.buildlistonly go.sum
+go list -m all
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.mod
+
+
+# When a sum is needed to load a .mod file for a package outside the build list,
+# we get a generic missing import error.
+! go list example.com/doesnotexist
+stderr '^no required module provides package example.com/doesnotexist; to add it:\n\tgo get example.com/doesnotexist$'
+
+# When a sum is needed to load a .zip file, we get a more specific error.
+# The .zip file is not downloaded.
+! go list rsc.io/quote
+stderr '^missing go.sum entry for module providing package rsc.io/quote; to add:\n\tgo mod download rsc.io/quote$'
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.zip
+
+# The error is attached to the package from the missing module. We can load
+# a package that imports it without that error.
+go list -e -deps -f '{{.ImportPath}}{{with .Error}} {{.Err}}{{end}}' .
+stdout '^m$'
+stdout '^rsc.io/quote missing go.sum entry for module providing package rsc.io/quote \(imported by m\); to add:\n\tgo get m$'
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.zip
+
+# go.sum should not have been written.
+cmp go.sum go.sum.buildlistonly
+
+# Control: when sums are present, 'go list' downloads .zip files.
+cp go.sum.tidy go.sum
+go list .
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.zip
+
+-- go.mod --
+module m
+
+go 1.15
+
+require rsc.io/quote v1.5.2
+-- use.go --
+package use
+
+import _ "rsc.io/quote"
+-- go.sum.h2only --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h2:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.5.2/go.mod h2:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0/go.mod h2:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+-- go.sum.buildlistonly --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+-- go.sum.tidy --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c h1:pvCbr/wm8HzDD3fVywevekufpn6tCGPY3spdHeZJEsw=
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.5.2 h1:3fEykkD9k7lYzXqCYrwGAf7iNhbk4yCjHmKBN9td4L0=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0 h1:HLGR/BgEtI3r0uymSP/nl2uPLsUnNJX8toRyhfpBTII=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+rsc.io/testonly v1.0.0 h1:K/VWHdO+Jv7woUXG0GzVNx1czBXUt3Ib1deaMn+xk64=
+rsc.io/testonly v1.0.0/go.mod h1:OqmGbIFOcF+XrFReLOGZ6BhMM7uMBiQwZsyNmh74SzY=
diff --git a/src/cmd/go/testdata/script/mod_sum_replaced.txt b/src/cmd/go/testdata/script/mod_sum_replaced.txt
new file mode 100644
index 0000000..6c322a0
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_sum_replaced.txt
@@ -0,0 +1,28 @@
+env GO111MODULE=on
+
+# After 'go get', the go.sum file should contain the sum for the module.
+go get rsc.io/quote@v1.5.0
+grep 'rsc.io/quote v1.5.0' go.sum
+
+# If we replace the module and run 'go mod tidy', we should get a sum for the replacement.
+go mod edit -replace rsc.io/quote@v1.5.0=rsc.io/quote@v1.5.1
+go mod tidy
+grep 'rsc.io/quote v1.5.1' go.sum
+cp go.sum go.sum.tidy
+
+# 'go mod vendor' should preserve that sum, and should not need to add any new entries.
+go mod vendor
+grep 'rsc.io/quote v1.5.1' go.sum
+cmp go.sum go.sum.tidy
+
+-- go.mod --
+module golang.org/issue/27868
+
+require rsc.io/quote v1.5.0
+
+-- main.go --
+package main
+
+import _ "rsc.io/quote"
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/mod_sumdb.txt b/src/cmd/go/testdata/script/mod_sumdb.txt
new file mode 100644
index 0000000..d06db4a
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_sumdb.txt
@@ -0,0 +1,45 @@
+env GO111MODULE=on
+env sumdb=$GOSUMDB
+env proxy=$GOPROXY
+env GOPROXY GONOPROXY GOSUMDB GONOSUMDB
+env dbname=localhost.localdev/sumdb
+
+# disagreeing with the sumdb produces security errors
+# (this also populates tiles on the sumdb server).
+cp go.mod.orig go.mod
+env GOSUMDB=$sumdb' '$proxy/sumdb-wrong
+! go get rsc.io/quote
+stderr 'go: rsc.io/quote@v1.5.2: verifying module: checksum mismatch'
+stderr 'downloaded: h1:3fEy'
+stderr 'localhost.localdev/sumdb: h1:wrong'
+stderr 'SECURITY ERROR\nThis download does NOT match the one reported by the checksum server.'
+! go get rsc.io/sampler
+! go get golang.org/x/text
+
+go mod edit -require rsc.io/quote@v1.5.2
+! go mod tidy
+stderr 'go: rsc.io/quote@v1.5.2: verifying go.mod: checksum mismatch'
+stderr 'SECURITY ERROR\n'
+
+rm go.sum
+
+# switching to truthful sumdb detects timeline inconsistency
+cp go.mod.orig go.mod
+env GOSUMDB=$sumdb
+! go get rsc.io/fortune
+stderr 'SECURITY ERROR\ngo.sum database server misbehavior detected!'
+stderr 'proof of misbehavior:'
+
+# removing the cached wrong tree head and cached tiles clears the bad data
+rm $GOPATH/pkg/sumdb/$dbname/latest
+go clean -modcache
+go get rsc.io/fortune
+
+-- go.mod.orig --
+module m
+
+go 1.16
+-- m.go --
+package m
+
+import _ "rsc.io/quote"
diff --git a/src/cmd/go/testdata/script/mod_sumdb_cache.txt b/src/cmd/go/testdata/script/mod_sumdb_cache.txt
new file mode 100644
index 0000000..063fd20
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_sumdb_cache.txt
@@ -0,0 +1,47 @@
+env GO111MODULE=on
+env sumdb=$GOSUMDB
+env proxy=$GOPROXY
+env GOPROXY GONOPROXY GOSUMDB GONOSUMDB
+
+# rejected proxy fails verification
+cp go.mod.orig go.mod
+rm go.sum
+env GOPROXY=$proxy/sumdb-503
+! go get rsc.io/quote
+stderr 503
+
+# fetch through working proxy is OK
+cp go.mod.orig go.mod
+rm go.sum
+env GOPROXY=$proxy
+go get rsc.io/quote
+
+# repeated fetch works entirely from cache, does not consult sumdb
+cp go.mod.orig go.mod
+rm go.sum
+env GOPROXY=$proxy/sumdb-503
+go get rsc.io/quote
+rm go.sum
+
+# fetch specific module can work without proxy, using cache or go.sum
+cp go.mod.orig go.mod
+rm go.sum
+env GOPROXY=off
+go get rsc.io/quote@v1.5.2 # using cache
+rm $GOPATH/pkg/mod/cache/download/sumdb/localhost.localdev/sumdb/lookup/rsc.io/quote@v1.5.2
+go get rsc.io/quote@v1.5.2 # using go.sum
+
+# fetch fails once we lose access to both cache and go.sum
+rm go.sum
+env GOPROXY=$proxy/sumdb-504
+! go get rsc.io/quote@v1.5.2
+stderr 504
+
+# GOINSECURE does not bypass checksum lookup
+env GOINSECURE=rsc.io
+env GOPROXY=$proxy/sumdb-504
+! go get rsc.io/quote@v1.5.2
+stderr 504
+
+-- go.mod.orig --
+module m
diff --git a/src/cmd/go/testdata/script/mod_sumdb_file_path.txt b/src/cmd/go/testdata/script/mod_sumdb_file_path.txt
new file mode 100644
index 0000000..2f42cb5
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_sumdb_file_path.txt
@@ -0,0 +1,58 @@
+[!net:proxy.golang.org] skip
+[!net:sum.golang.org] skip
+
+env GO111MODULE=on
+[go-builder] env GOSUMDB=
+[!go-builder] env GOSUMDB=sum.golang.org  # Set explicitly in case GOROOT/go.env is modified.
+env GOPATH=$WORK/gopath1
+
+# With a file-based proxy with an empty checksum directory,
+# downloading a new module should fail, even if a subsequent
+# proxy contains a more complete mirror of the sum database.
+#
+# TODO(bcmills): The error message here is a bit redundant.
+# It comes from the sumweb package, which isn't yet producing structured errors.
+[GOOS:windows] env GOPROXY=file:///$WORK/sumproxy,https://proxy.golang.org
+[!GOOS:windows] env GOPROXY=file://$WORK/sumproxy,https://proxy.golang.org
+! go get golang.org/x/text@v0.3.2
+stderr '^go: golang.org/x/text@v0.3.2: verifying module: golang.org/x/text@v0.3.2: reading file://.*/sumdb/sum.golang.org/lookup/golang.org/x/text@v0.3.2: (no such file or directory|.*cannot find the path specified.*)'
+
+# If the proxy does not claim to support the database,
+# checksum verification should fall through to the next proxy,
+# and downloading should succeed.
+[GOOS:windows] env GOPROXY=file:///$WORK/emptyproxy,https://proxy.golang.org
+[!GOOS:windows] env GOPROXY=file://$WORK/emptyproxy,https://proxy.golang.org
+go get golang.org/x/text@v0.3.2
+
+# After a successful sumdb lookup, the lookup can be repeated
+# using the download cache as a proxy.
+cp supported $GOPATH/pkg/mod/cache/download/sumdb/sum.golang.org/supported
+[GOOS:windows] env GOPROXY=file:///$WORK/gopath1/pkg/mod/cache/download,file:///$WORK/sumproxy
+[!GOOS:windows] env GOPROXY=file://$WORK/gopath1/pkg/mod/cache/download,file://$WORK/sumproxy
+env GOPATH=$WORK/gopath2
+rm go.sum
+go get -x -v golang.org/x/text@v0.3.2
+
+# Once the checksum is present in the go.sum file,
+# an empty file-based sumdb can be used in conjunction with
+# a fallback module mirror.
+grep golang.org/x/text go.sum
+env GOPATH=$WORK/gopath3
+[GOOS:windows] env GOPROXY=file:///$WORK/sumproxy
+[!GOOS:windows] env GOPROXY=file://$WORK/sumproxy
+! go get golang.org/x/text@v0.3.2
+[GOOS:windows] env GOPROXY=file:///$WORK/sumproxy,https://proxy.golang.org
+[!GOOS:windows] env GOPROXY=file://$WORK/sumproxy,https://proxy.golang.org
+go get golang.org/x/text@v0.3.2
+
+-- supported --
+
+-- go.mod --
+module example.com
+go 1.13
+-- $WORK/emptyproxy/README.md --
+This proxy contains no modules.
+-- $WORK/sumproxy/README.md --
+This proxy contains no modules.
+-- $WORK/sumproxy/sumdb/sum.golang.org/supported --
+This proxy blocks checksum downloads from sum.golang.org.
diff --git a/src/cmd/go/testdata/script/mod_sumdb_golang.txt b/src/cmd/go/testdata/script/mod_sumdb_golang.txt
new file mode 100644
index 0000000..067e2e3
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_sumdb_golang.txt
@@ -0,0 +1,83 @@
+# Test default GOPROXY and GOSUMDB
+[go-builder] env GOPROXY=
+[go-builder] env GOSUMDB=
+[go-builder] go env GOPROXY
+[go-builder] stdout '^https://proxy.golang.org,direct$'
+[go-builder] go env GOSUMDB
+[go-builder] stdout '^sum.golang.org$'
+[go-builder] env GOPROXY=https://proxy.golang.org
+[go-builder] go env GOSUMDB
+[go-builder] stdout '^sum.golang.org$'
+
+# Download direct from github.
+
+[!net:proxy.golang.org] skip
+[!net:sum.golang.org] skip
+[!git] skip
+env GOSUMDB=sum.golang.org
+env GOPROXY=direct
+
+go get rsc.io/quote@v1.5.2
+cp go.sum saved.sum
+
+
+# Download from proxy.golang.org with go.sum entry already.
+# Use 'go list' instead of 'go get' since the latter may download extra go.mod
+# files not listed in go.sum.
+
+go clean -modcache
+env GOSUMDB=sum.golang.org
+env GOPROXY=https://proxy.golang.org,direct
+
+go list -x -m all  # Download go.mod files.
+! stderr github
+stderr proxy.golang.org/rsc.io/quote
+! stderr sum.golang.org/tile
+! stderr sum.golang.org/lookup/rsc.io/quote
+
+go list -x -deps rsc.io/quote  # Download module source.
+! stderr github
+stderr proxy.golang.org/rsc.io/quote
+! stderr sum.golang.org/tile
+! stderr sum.golang.org/lookup/rsc.io/quote
+
+cmp go.sum saved.sum
+
+
+# Download again.
+# Should use the checksum database to validate new go.sum lines,
+# but not need to fetch any new data from the proxy.
+
+rm go.sum
+
+go list -mod=mod -x -m all  # Add checksums for go.mod files.
+stderr sum.golang.org/tile
+! stderr github
+! stderr proxy.golang.org/rsc.io/quote
+stderr sum.golang.org/lookup/rsc.io/quote
+
+go list -mod=mod -x rsc.io/quote  # Add checksums for module source.
+! stderr .  # Adds checksums, but for entities already in the module cache.
+
+cmp go.sum saved.sum
+
+
+# test fallback to direct
+
+env TESTGOPROXY404=1
+go clean -modcache
+rm go.sum
+
+go list -mod=mod -x -m all  # Download go.mod files
+stderr 'proxy.golang.org.*404 testing'
+stderr github.com/rsc
+
+go list -mod=mod -x rsc.io/quote  # Download module source.
+stderr 'proxy.golang.org.*404 testing'
+stderr github.com/rsc
+
+cmp go.sum saved.sum
+
+
+-- go.mod --
+module m
diff --git a/src/cmd/go/testdata/script/mod_sumdb_proxy.txt b/src/cmd/go/testdata/script/mod_sumdb_proxy.txt
new file mode 100644
index 0000000..194c0c9
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_sumdb_proxy.txt
@@ -0,0 +1,65 @@
+env GO111MODULE=on
+env sumdb=$GOSUMDB
+env proxy=$GOPROXY
+env GOPROXY GONOPROXY GOSUMDB GONOSUMDB
+
+# basic fetch (through proxy) works
+cp go.mod.orig go.mod
+go get rsc.io/fortune@v1.0.0 # note: must use test proxy, does not exist in real world
+rm $GOPATH/pkg/mod/cache/download/sumdb # rm sumdb cache but NOT package download cache
+rm go.sum
+
+# can fetch by explicit URL
+cp go.mod.orig go.mod
+env GOSUMDB=$sumdb' '$proxy/sumdb-direct
+go get rsc.io/fortune@v1.0.0
+rm $GOPATH/pkg/mod/cache/download/sumdb
+rm go.sum
+
+# direct access fails (because localhost.localdev does not exist)
+# web.get is providing the error message - there's no actual network access.
+cp go.mod.orig go.mod
+env GOSUMDB=$sumdb
+env GOPROXY=direct
+! go get rsc.io/fortune@v1.0.0
+stderr 'verifying module: rsc.io/fortune@v1.0.0: .*: no such host localhost.localdev'
+rm $GOPATH/pkg/mod/cache/download/sumdb
+rm go.sum
+
+# proxy 404 falls back to direct access (which fails)
+cp go.mod.orig go.mod
+env GOSUMDB=$sumdb
+env GOPROXY=$proxy/sumdb-404
+! go get rsc.io/fortune@v1.0.0
+stderr 'verifying.*localhost.localdev'
+rm $GOPATH/pkg/mod/cache/download/sumdb
+rm go.sum
+
+# proxy non-200/404/410 stops direct access
+cp go.mod.orig go.mod
+env GOSUMDB=$sumdb
+env GOPROXY=$proxy/sumdb-503
+! go get rsc.io/fortune@v1.0.0
+stderr '503 Service Unavailable'
+rm $GOPATH/pkg/mod/cache/download/sumdb
+rm go.sum
+
+# the error from the last attempted proxy should be returned.
+cp go.mod.orig go.mod
+env GOSUMDB=$sumdb
+env GOPROXY=$proxy/sumdb-404,$proxy/sumdb-503
+! go get rsc.io/fortune@v1.0.0
+stderr '503 Service Unavailable'
+rm $GOPATH/pkg/mod/cache/download/sumdb
+rm go.sum
+
+# if proxies are separated with '|', fallback is allowed on any error.
+cp go.mod.orig go.mod
+env GOSUMDB=$sumdb
+env GOPROXY=$proxy/sumdb-503|https://0.0.0.0|$proxy
+go get rsc.io/fortune@v1.0.0
+rm $GOPATH/pkg/mod/cache/download/sumdb
+rm go.sum
+
+-- go.mod.orig --
+module m
diff --git a/src/cmd/go/testdata/script/mod_symlink.txt b/src/cmd/go/testdata/script/mod_symlink.txt
new file mode 100644
index 0000000..0604e1a
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_symlink.txt
@@ -0,0 +1,45 @@
+env GO111MODULE=on
+[!symlink] skip
+
+# 'go get' should resolve modules of imported packages.
+go get
+go list -deps -f '{{.Module}}' .
+stdout golang.org/x/text
+
+go get ./subpkg
+go list -deps -f '{{.Module}}' ./subpkg
+stdout golang.org/x/text
+
+# Create a copy of the module using symlinks in src/links.
+mkdir links
+symlink links/go.mod -> $GOPATH/src/go.mod
+symlink links/go.sum -> $GOPATH/src/go.sum
+symlink links/issue.go -> $GOPATH/src/issue.go
+mkdir links/subpkg
+symlink links/subpkg/issue.go -> $GOPATH/src/subpkg/issue.go
+
+# We should see the copy as a valid module root.
+cd links
+go env GOMOD
+stdout links[/\\]go.mod
+go list -m
+stdout golang.org/issue/28107
+
+# The symlink-based copy should contain the same packages
+# and have the same dependencies as the original.
+go list -deps -f '{{.Module}}' .
+stdout golang.org/x/text
+go list -deps -f '{{.Module}}' ./subpkg
+stdout golang.org/x/text
+
+-- go.mod --
+module golang.org/issue/28107
+
+-- issue.go --
+package issue
+
+import _ "golang.org/x/text/language"
+-- subpkg/issue.go --
+package issue
+
+import _ "golang.org/x/text/language"
diff --git a/src/cmd/go/testdata/script/mod_symlink_dotgo.txt b/src/cmd/go/testdata/script/mod_symlink_dotgo.txt
new file mode 100644
index 0000000..d4cc143
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_symlink_dotgo.txt
@@ -0,0 +1,17 @@
+env GO111MODULE=on
+[!symlink] skip
+
+symlink dir.go -> dir
+
+# Issue #39841: symlinks to directories should be ignored, not treated as source files.
+go list -f '{{range .GoFiles}}{{.}}{{"\n"}}{{end}}' .
+stdout 'p\.go$'
+! stdout 'dir\.go$'
+
+-- go.mod --
+module example.com
+go 1.15
+-- p.go --
+package p
+-- dir/README.txt --
+This file exists to ensure that dir is a directory.
diff --git a/src/cmd/go/testdata/script/mod_tagged_import_cycle.txt b/src/cmd/go/testdata/script/mod_tagged_import_cycle.txt
new file mode 100644
index 0000000..0491acb
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tagged_import_cycle.txt
@@ -0,0 +1,106 @@
+# Because 'go mod' subcommands ignore build constraints, they can encounter
+# package-import cycles that are not possible in an ordinary build. This test
+# verifies that such cycles are handled even when they cross module boundaries.
+
+# First, verify that the import graph depends on build tags as expected.
+go list -deps example.com/left
+stdout '^example.com/right$'
+go list -deps example.com/right
+! stdout left
+
+env GOFLAGS=-tags=mirror
+go list -deps example.com/left
+! stdout right
+go list -deps example.com/right
+stdout '^example.com/left$'
+env GOFLAGS=''
+
+# 'go mod why' should be agnostic to build tags.
+go mod why example.com/left
+stdout '^example.com/chiral$\n^example.com/left$'
+go mod why example.com/right
+stdout '^example.com/chiral$\n^example.com/right$'
+
+env GOFLAGS='-tags=mirror'
+go mod why example.com/left
+stdout '^example.com/chiral$\n^example.com/left$'
+go mod why example.com/right
+stdout '^example.com/chiral$\n^example.com/right$'
+env GOFLAGS=''
+
+# 'go mod tidy' should successfully handle the cycle.
+env GOFLAGS=-mod=readonly
+go mod tidy
+
+# 'go mod vendor' should copy in both packages without crashing.
+go mod vendor
+exists vendor/example.com/left/default.go
+exists vendor/example.com/left/mirror.go
+exists vendor/example.com/right/default.go
+exists vendor/example.com/right/mirror.go
+
+-- go.mod --
+module example.com/chiral
+
+go 1.14
+
+require (
+	example.com/left v0.1.0
+	example.com/right v0.1.0
+)
+
+replace (
+	example.com/left => ./left
+	example.com/right => ./right
+)
+-- chiral.go --
+// Package chiral imports packages in an order that depends on build tags.
+package chiral
+-- default.go --
+// +build !mirror
+
+package chiral
+
+import _ "example.com/left"
+-- mirror.go --
+// +build mirror
+
+package chiral
+
+import _ "example.com/right"
+-- left/go.mod --
+module example.com/left
+
+go 1.14
+
+require example.com/right v0.1.0
+
+replace example.com/right v0.1.0 => ../right
+-- left/default.go --
+// +build !mirror
+
+package left
+
+import _ "example.com/right"
+-- left/mirror.go --
+// +build mirror
+
+package left
+-- right/go.mod --
+module example.com/right
+
+go 1.14
+
+require example.com/left v0.1.0
+
+replace example.com/left v0.1.0 => ../left
+-- right/default.go --
+// +build !mirror
+
+package right
+-- right/mirror.go --
+// +build mirror
+
+package right
+
+import _ "example.com/left"
diff --git a/src/cmd/go/testdata/script/mod_test.txt b/src/cmd/go/testdata/script/mod_test.txt
new file mode 100644
index 0000000..76f1d7a
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_test.txt
@@ -0,0 +1,130 @@
+env GO111MODULE=on
+env GOFLAGS=-mod=mod
+[short] skip
+
+# TODO(bcmills): Convert the 'go test' calls below to 'go list -test' once 'go
+# list' is more sensitive to package loading errors.
+
+# A test in the module's root package should work.
+cd a/
+cp go.mod.empty go.mod
+go list -test
+! stderr error
+
+cp go.mod.empty go.mod
+go list -deps
+! stdout ^testing$
+
+# list all should include test dependencies, like testing
+cp go.mod.empty go.mod
+go list all
+stdout ^testing$
+stdout ^rsc.io/quote$
+stdout ^rsc.io/testonly$
+
+# list -deps -tests should also include testing
+# but not deps of tests of deps (rsc.io/testonly).
+go list -deps -test
+stdout ^testing$
+stdout ^rsc.io/quote$
+! stdout ^rsc.io/testonly$
+
+# list -test all should succeed
+cp go.mod.empty go.mod
+go list -test all
+stdout '^testing'
+
+cp go.mod.empty go.mod
+go list -test
+! stderr error
+
+# A test with the "_test" suffix in the module root should also work.
+cd ../b/
+go list -test
+! stderr error
+
+# A test with the "_test" suffix of a *package* with a "_test" suffix should
+# even work (not that you should ever do that).
+cd ../c_test
+go list -test
+! stderr error
+
+cd ../d_test
+go list -test
+! stderr error
+
+cd ../e
+go list -test
+! stderr error
+
+-- a/go.mod.empty --
+module example.com/user/a
+
+go 1.11
+
+-- a/a.go --
+package a
+
+-- a/a_test.go --
+package a
+
+import "testing"
+import _ "rsc.io/quote"
+
+func Test(t *testing.T) {}
+
+-- b/go.mod --
+module example.com/user/b
+
+-- b/b.go --
+package b
+
+-- b/b_test.go --
+package b_test
+
+import "testing"
+
+func Test(t *testing.T) {}
+
+-- c_test/go.mod --
+module example.com/c_test
+
+-- c_test/umm.go --
+// Package c_test is the non-test package for its import path!
+package c_test
+
+-- c_test/c_test_test.go --
+package c_test_test
+
+import "testing"
+
+func Test(t *testing.T) {}
+
+-- d_test/go.mod --
+// Package d is an ordinary package in a deceptively-named directory.
+module example.com/d
+
+-- d_test/d.go --
+package d
+
+-- d_test/d_test.go --
+package d_test
+
+import "testing"
+
+func Test(t *testing.T) {}
+
+-- e/go.mod --
+module example.com/e_test
+
+-- e/wat.go --
+// Package e_test is the non-test package for its import path,
+// in a deceptively-named directory!
+package e_test
+
+-- e/e_test.go --
+package e_test_test
+
+import "testing"
+
+func Test(t *testing.T) {}
diff --git a/src/cmd/go/testdata/script/mod_test_cached.txt b/src/cmd/go/testdata/script/mod_test_cached.txt
new file mode 100644
index 0000000..3da4358
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_test_cached.txt
@@ -0,0 +1,76 @@
+[short] skip
+
+env GO111MODULE=on
+env GOCACHE=$WORK/gocache
+env GODEBUG=gocachetest=1
+
+# The first run of a test should not be cached.
+# The second run should be.
+go test -run=WriteTmp .
+! stdout '(cached)'
+go test -run=WriteTmp .
+stdout '(cached)'
+
+# 'go test' without arguments should never be cached.
+go test -run=WriteTmp
+! stdout '(cached)'
+go test -run=WriteTmp
+! stdout '(cached)'
+
+# We should never cache a test run from command-line files.
+go test -run=WriteTmp ./foo_test.go
+! stdout '(cached)'
+go test -run=WriteTmp ./foo_test.go
+! stdout '(cached)'
+
+[!exec:sleep] stop
+# The go command refuses to cache access to files younger than 2s, so sleep that long.
+exec sleep 2
+
+# Touching a file that the test reads from within its testdata should invalidate the cache.
+go test -run=ReadTestdata .
+! stdout '(cached)'
+go test -run=ReadTestdata .
+stdout '(cached)'
+cp testdata/bar.txt testdata/foo.txt
+go test -run=ReadTestdata .
+! stdout '(cached)'
+
+-- go.mod --
+module golang.org/issue/29111/foo
+
+-- foo.go --
+package foo
+
+-- testdata/foo.txt --
+foo
+-- testdata/bar.txt --
+bar
+
+-- foo_test.go --
+package foo_test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestWriteTmp(t *testing.T) {
+	dir, err := os.MkdirTemp("", "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+	err = os.WriteFile(filepath.Join(dir, "x"), nil, 0666)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestReadTestdata(t *testing.T) {
+	_, err := os.ReadFile("testdata/foo.txt")
+	if err != nil {
+		t.Fatal(err)
+	}
+}
diff --git a/src/cmd/go/testdata/script/mod_test_files.txt b/src/cmd/go/testdata/script/mod_test_files.txt
new file mode 100644
index 0000000..6f520c7
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_test_files.txt
@@ -0,0 +1,50 @@
+env GO111MODULE=on
+
+cd foo
+
+# Testing an explicit source file should use the same import visibility as the
+# package in the same directory.
+go list -test -deps
+go list -test -deps foo_test.go
+
+# If the file is inside the main module's vendor directory, it should have
+# visibility based on the vendor-relative import path.
+mkdir vendor/example.com/foo
+cp foo_test.go vendor/example.com/foo
+go list -test -deps vendor/example.com/foo/foo_test.go
+
+# If the file is outside the main module entirely, it should be treated as outside.
+cp foo_test.go ../foo_test.go
+! go list -test -deps ../foo_test.go
+stderr 'use of internal package'
+
+-- foo/go.mod --
+module example.com/foo
+go 1.12
+require example.com/internal v0.0.0
+replace example.com/internal => ../internal
+
+-- foo/internal.go --
+package foo
+import _ "example.com/internal"
+
+-- foo/foo_test.go --
+package foo_test
+
+import (
+	"testing"
+	"example.com/internal"
+)
+
+func TestHacksEnabled(t *testing.T) {
+	if !internal.Hacks {
+		t.Fatal("hacks not enabled")
+	}
+}
+
+-- internal/go.mod --
+module example.com/internal
+
+-- internal/internal.go --
+package internal
+const Hacks = true
diff --git a/src/cmd/go/testdata/script/mod_tidy.txt b/src/cmd/go/testdata/script/mod_tidy.txt
new file mode 100644
index 0000000..b1d9371
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy.txt
@@ -0,0 +1,72 @@
+env GO111MODULE=on
+
+# tidy removes unused y, but everything else is used
+go mod tidy -v
+stderr '^unused y.1'
+! stderr '^unused [^y]'
+
+grep 'go 1.10' go.mod
+
+go list -m all
+! stdout '^y'
+stdout '^w.1 v1.2.0'
+stdout '^z.1 v1.2.0'
+
+# empty tidy should not crash
+cd triv
+! grep 'go ' go.mod
+go mod tidy
+
+# tidy should add missing go line
+grep 'go ' go.mod
+
+-- go.mod --
+module m
+
+go 1.10
+
+require (
+	x.1 v1.0.0
+	y.1 v1.0.0
+	w.1 v1.2.0
+)
+
+replace x.1 v1.0.0 => ./x
+replace y.1 v1.0.0 => ./y
+replace z.1 v1.1.0 => ./z
+replace z.1 v1.2.0 => ./z
+replace w.1 => ./w
+
+-- m.go --
+package m
+
+import _ "x.1"
+import _ "z.1/sub"
+
+-- w/go.mod --
+module w
+
+-- w/w.go --
+package w
+
+-- x/go.mod --
+module x
+require w.1 v1.1.0
+require z.1 v1.1.0
+
+-- x/x.go --
+package x
+import _ "w.1"
+
+-- y/go.mod --
+module y
+require z.1 v1.2.0
+
+-- z/go.mod --
+module z
+
+-- z/sub/sub.go --
+package sub
+
+-- triv/go.mod --
+module triv
diff --git a/src/cmd/go/testdata/script/mod_tidy_compat.txt b/src/cmd/go/testdata/script/mod_tidy_compat.txt
new file mode 100644
index 0000000..724c83e
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_compat.txt
@@ -0,0 +1,95 @@
+# https://golang.org/issue/46141: 'go mod tidy' for a Go 1.17 module should by
+# default preserve enough checksums for the module to be used by Go 1.16.
+#
+# We don't have a copy of Go 1.16 handy, but we can simulate it by editing the
+# 'go' version in the go.mod file to 1.16, without actually updating the
+# requirements to match.
+
+[short] skip
+
+env MODFMT='{{with .Module}}{{.Path}} {{.Version}}{{end}}'
+
+
+# This module has the same module dependency graph in Go 1.16 as in Go 1.17,
+# but in 1.16 requires (checksums for) additional (irrelevant) go.mod files.
+#
+# The module graph under both versions looks like:
+#
+# m ---- example.com/version v1.1.0
+# |
+# + ---- example.net/lazy v0.1.0 ---- example.com/version v1.0.1
+#
+# Go 1.17 avoids loading the go.mod file for example.com/version v1.0.1
+# (because it is lower than the version explicitly required by m,
+# and the module that requires it — m — specifies 'go 1.17').
+#
+# That go.mod file happens not to affect the final 1.16 module graph anyway,
+# so the pruned graph is equivalent to the unpruned one.
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod go.mod.orig
+
+go list -m all
+cmp stdout m_all.txt
+
+go mod edit -go=1.16
+go list -m all
+cmp stdout m_all.txt
+
+
+# If we explicitly drop compatibility with 1.16, we retain fewer checksums,
+# which gives a cleaner go.sum file but causes 1.16 to fail in readonly mode.
+
+cp go.mod.orig go.mod
+go mod tidy -compat=1.17
+cmp go.mod go.mod.orig
+
+go list -m all
+cmp stdout m_all.txt
+
+go mod edit -go=1.16
+! go list -m all
+stderr '^go: example.net/lazy@v0.1.0 requires\n\texample.com/version@v1.0.1: missing go.sum entry for go.mod file; to add it:\n\tgo mod download example.com/version$'
+
+
+-- go.mod --
+// Module m happens to have the exact same build list as what would be
+// selected under Go 1.16, but computes that build list without looking at
+// as many go.mod files.
+module example.com/m
+
+go 1.17
+
+replace example.net/lazy v0.1.0 => ./lazy
+
+require (
+	example.com/version v1.1.0
+	example.net/lazy v0.1.0
+)
+-- m_all.txt --
+example.com/m
+example.com/version v1.1.0
+example.net/lazy v0.1.0 => ./lazy
+-- compatible.go --
+package compatible
+
+import (
+	_ "example.com/version"
+	_ "example.net/lazy"
+)
+-- lazy/go.mod --
+// Module lazy requires example.com/version v1.0.1.
+//
+// However, since this module is lazy, its dependents
+// should not need checksums for that version of the module
+// unless they actually import packages from it.
+module example.net/lazy
+
+go 1.17
+
+require example.com/version v1.0.1
+-- lazy/lazy.go --
+package lazy
+
+import _ "example.com/version"
diff --git a/src/cmd/go/testdata/script/mod_tidy_compat_added.txt b/src/cmd/go/testdata/script/mod_tidy_compat_added.txt
new file mode 100644
index 0000000..b3f75ad
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_compat_added.txt
@@ -0,0 +1,105 @@
+# https://golang.org/issue/46141: 'go mod tidy' for a Go 1.17 module should by
+# default preserve enough checksums for the module to be used by Go 1.16.
+#
+# We don't have a copy of Go 1.16 handy, but we can simulate it by editing the
+# 'go' version in the go.mod file to 1.16, without actually updating the
+# requirements to match.
+
+[short] skip
+
+env MODFMT='{{with .Module}}{{.Path}} {{.Version}}{{end}}'
+
+
+# For this module, Go 1.17 produces an error for one module, and Go 1.16
+# produces a different error for a different module.
+
+cp go.mod go.mod.orig
+
+! go mod tidy
+
+stderr '^go: example\.com/m imports\n\texample\.net/added: module example\.net/added@latest found \(v0\.3\.0, replaced by \./a1\), but does not contain package example\.net/added$'
+
+cmp go.mod go.mod.orig
+
+
+# When we run 'go mod tidy -e', we should proceed past the first error and follow
+# it with a second error describing the version discrepancy.
+#
+# We should not provide advice on how to push past the version discrepancy,
+# because the '-e' flag should already do that, writing out an otherwise-tidied
+# go.mod file.
+
+go mod tidy -e
+
+stderr '^go: example\.com/m imports\n\texample\.net/added: module example\.net/added@latest found \(v0\.3\.0, replaced by \./a1\), but does not contain package example\.net/added\ngo: example\.net/added failed to load from any module,\n\tbut go 1\.16 would load it from example\.net/added@v0\.2\.0$'
+
+! stderr '\n\tgo mod tidy'
+
+cmp go.mod go.mod.tidy
+
+
+-- go.mod --
+module example.com/m
+
+go 1.17
+
+replace (
+	example.net/added v0.1.0 => ./a1
+	example.net/added v0.2.0 => ./a2
+	example.net/added v0.3.0 => ./a1
+	example.net/lazy v0.1.0 => ./lazy
+	example.net/pruned v0.1.0 => ./pruned
+)
+
+require (
+	example.net/added v0.1.0
+	example.net/lazy v0.1.0
+)
+-- go.mod.tidy --
+module example.com/m
+
+go 1.17
+
+replace (
+	example.net/added v0.1.0 => ./a1
+	example.net/added v0.2.0 => ./a2
+	example.net/added v0.3.0 => ./a1
+	example.net/lazy v0.1.0 => ./lazy
+	example.net/pruned v0.1.0 => ./pruned
+)
+
+require example.net/lazy v0.1.0
+-- m.go --
+package m
+
+import (
+	_ "example.net/added"
+	_ "example.net/lazy"
+)
+
+-- a1/go.mod --
+module example.net/added
+
+go 1.17
+-- a2/go.mod --
+module example.net/added
+
+go 1.17
+-- a2/added.go --
+package added
+
+-- lazy/go.mod --
+module example.net/lazy
+
+go 1.17
+
+require example.net/pruned v0.1.0
+-- lazy/lazy.go --
+package lazy
+
+-- pruned/go.mod --
+module example.net/pruned
+
+go 1.17
+
+require example.net/added v0.2.0
diff --git a/src/cmd/go/testdata/script/mod_tidy_compat_ambiguous.txt b/src/cmd/go/testdata/script/mod_tidy_compat_ambiguous.txt
new file mode 100644
index 0000000..5316220
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_compat_ambiguous.txt
@@ -0,0 +1,97 @@
+# https://golang.org/issue/46141: 'go mod tidy' for a Go 1.17 module should by
+# default preserve enough checksums for the module to be used by Go 1.16.
+#
+# We don't have a copy of Go 1.16 handy, but we can simulate it by editing the
+# 'go' version in the go.mod file to 1.16, without actually updating the
+# requirements to match.
+
+[short] skip
+
+env MODFMT='{{with .Module}}{{.Path}} {{.Version}}{{end}}'
+
+# For this module, the dependency providing package
+# example.net/ambiguous/nested/pkg is unambiguous in Go 1.17 (because only one
+# root of the module graph contains the package), whereas it is ambiguous in
+# Go 1.16 (because two different modules contain plausible packages and Go 1.16
+# does not privilege roots above other dependencies).
+#
+# However, the overall build list is identical for both versions.
+
+cp go.mod go.mod.orig
+
+! go mod tidy
+
+stderr '^go: example\.com/m imports\n\texample\.net/indirect imports\n\texample\.net/ambiguous/nested/pkg loaded from example\.net/ambiguous/nested@v0\.1\.0,\n\tbut go 1.16 would fail to locate it:\n\tambiguous import: found package example\.net/ambiguous/nested/pkg in multiple modules:\n\texample\.net/ambiguous v0.1.0 \(.*\)\n\texample\.net/ambiguous/nested v0.1.0 \(.*\)\n\n'
+
+stderr '\n\nTo proceed despite packages unresolved in go 1\.16:\n\tgo mod tidy -e\nIf reproducibility with go 1.16 is not needed:\n\tgo mod tidy -compat=1\.17\nFor other options, see:\n\thttps://golang\.org/doc/modules/pruning\n'
+
+cmp go.mod go.mod.orig
+
+
+# If we run 'go mod tidy -e', we should still save enough checksums to run
+# 'go list -m all' reproducibly with go 1.16, even though we can't list
+# the specific package.
+
+go mod tidy -e
+! stderr '\n\tgo mod tidy'
+cmp go.mod go.mod.orig
+
+go list -m all
+cmp stdout all-m.txt
+
+go list -f $MODFMT example.net/ambiguous/nested/pkg
+stdout '^example.net/ambiguous/nested v0\.1\.0$'
+! stderr .
+
+go mod edit -go=1.16
+go list -m all
+cmp stdout all-m.txt
+
+! go list -f $MODFMT example.net/ambiguous/nested/pkg
+stderr '^ambiguous import: found package example\.net/ambiguous/nested/pkg in multiple modules:\n\texample\.net/ambiguous v0\.1\.0 \(.*\)\n\texample\.net/ambiguous/nested v0\.1\.0 \(.*\)\n'
+
+
+# On the other hand, if we use -compat=1.17, 1.16 can't even load
+# the build list (due to missing checksums).
+
+cp go.mod.orig go.mod
+go mod tidy -compat=1.17
+! stderr .
+go list -m all
+cmp stdout all-m.txt
+
+go mod edit -go=1.16
+! go list -m all
+stderr '^go: example\.net/indirect@v0\.1\.0 requires\n\texample\.net/ambiguous@v0\.1\.0: missing go\.sum entry for go\.mod file; to add it:\n\tgo mod download example\.net/ambiguous\n'
+
+
+-- go.mod --
+module example.com/m
+
+go 1.17
+
+replace example.net/indirect v0.1.0 => ./indirect
+
+require example.net/indirect v0.1.0
+
+require example.net/ambiguous/nested v0.1.0 // indirect
+-- all-m.txt --
+example.com/m
+example.net/ambiguous v0.1.0
+example.net/ambiguous/nested v0.1.0
+example.net/indirect v0.1.0 => ./indirect
+-- m.go --
+package m
+
+import _ "example.net/indirect"
+
+-- indirect/go.mod --
+module example.net/indirect
+
+go 1.17
+
+require example.net/ambiguous v0.1.0
+-- indirect/indirect.go --
+package indirect
+
+import _ "example.net/ambiguous/nested/pkg"
diff --git a/src/cmd/go/testdata/script/mod_tidy_compat_deleted.txt b/src/cmd/go/testdata/script/mod_tidy_compat_deleted.txt
new file mode 100644
index 0000000..b148fc1
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_compat_deleted.txt
@@ -0,0 +1,128 @@
+# https://golang.org/issue/46141: 'go mod tidy' for a Go 1.17 module should by
+# default preserve enough checksums for the module to be used by Go 1.16.
+#
+# We don't have a copy of Go 1.16 handy, but we can simulate it by editing the
+# 'go' version in the go.mod file to 1.16, without actually updating the
+# requirements to match.
+
+[short] skip
+
+env MODFMT='{{with .Module}}{{.Path}} {{.Version}}{{end}}'
+
+
+# For this module, the "deleted" dependency contains an imported package, but
+# Go 1.16 selects a higher version (in which that package has been deleted).
+
+cp go.mod go.mod.orig
+
+! go mod tidy
+
+stderr '^go: example\.com/m imports\n\texample\.net/deleted loaded from example\.net/deleted@v0\.1\.0,\n\tbut go 1\.16 would fail to locate it in example\.net/deleted@v0\.2\.0\n\n'
+
+stderr '\n\nTo upgrade to the versions selected by go 1.16, leaving some packages unresolved:\n\tgo mod tidy -e -go=1\.16 && go mod tidy -e -go=1\.17\nIf reproducibility with go 1.16 is not needed:\n\tgo mod tidy -compat=1\.17\nFor other options, see:\n\thttps://golang\.org/doc/modules/pruning\n'
+
+
+# The suggested 'go mod tidy -e' command should proceed anyway.
+
+go mod tidy -e
+cmp go.mod go.mod.tidy
+
+
+# In 'go 1.16' mode we should error out in the way we claimed.
+
+cd 116-outside
+! go list -deps -f $MODFMT example.com/m
+stderr '^\.\.[/\\]m\.go:4:2: no required module provides package example\.net/deleted; to add it:\n\tgo get example\.net/deleted$'
+cd ..
+
+go mod edit -go=1.16
+! go list -deps -f $MODFMT example.com/m
+stderr '^go: updates to go\.mod needed; to update it:\n\tgo mod tidy$'
+
+! go mod tidy
+stderr '^go: example\.com/m imports\n\texample\.net/deleted: module example\.net/deleted@latest found \(v0\.2\.0, replaced by \./d2\), but does not contain package example\.net/deleted$'
+
+
+-- go.mod --
+module example.com/m
+
+go 1.17
+
+replace (
+	example.net/deleted v0.1.0 => ./d1
+	example.net/deleted v0.2.0 => ./d2
+	example.net/lazy v0.1.0 => ./lazy
+	example.net/pruned v0.1.0 => ./pruned
+)
+
+require (
+	example.net/deleted v0.1.0
+	example.net/deleted v0.1.0 // redundant
+	example.net/lazy v0.1.0
+)
+-- go.mod.tidy --
+module example.com/m
+
+go 1.17
+
+replace (
+	example.net/deleted v0.1.0 => ./d1
+	example.net/deleted v0.2.0 => ./d2
+	example.net/lazy v0.1.0 => ./lazy
+	example.net/pruned v0.1.0 => ./pruned
+)
+
+require (
+	example.net/deleted v0.1.0
+	example.net/lazy v0.1.0
+)
+-- 116-outside/go.mod --
+module outside
+
+go 1.16
+
+replace (
+	example.com/m => ../
+	example.net/deleted v0.1.0 => ../d1
+	example.net/deleted v0.2.0 => ../d2
+	example.net/lazy v0.1.0 => ../lazy
+	example.net/pruned v0.1.0 => ../pruned
+)
+
+require example.com/m v0.1.0
+-- m.go --
+package m
+
+import (
+	_ "example.net/deleted"
+	_ "example.net/lazy"
+)
+
+-- d1/go.mod --
+module example.net/deleted
+
+go 1.17
+-- d1/deleted.go --
+package deleted
+-- d2/go.mod --
+module example.net/deleted
+
+go 1.17
+-- d2/README --
+There is no longer a Go package here.
+
+-- lazy/go.mod --
+module example.net/lazy
+
+go 1.17
+
+require example.net/pruned v0.1.0
+-- lazy/lazy.go --
+package lazy
+
+-- pruned/go.mod --
+module example.net/pruned
+
+go 1.17
+
+require example.net/deleted v0.2.0
diff --git a/src/cmd/go/testdata/script/mod_tidy_compat_implicit.txt b/src/cmd/go/testdata/script/mod_tidy_compat_implicit.txt
new file mode 100644
index 0000000..26e4749
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_compat_implicit.txt
@@ -0,0 +1,128 @@
+# https://golang.org/issue/46141: 'go mod tidy' for a Go 1.17 module should by
+# default preserve enough checksums for the module to be used by Go 1.16.
+#
+# We don't have a copy of Go 1.16 handy, but we can simulate it by editing the
+# 'go' version in the go.mod file to 1.16, without actually updating the
+# requirements to match.
+
+[short] skip
+
+env MODFMT='{{with .Module}}{{.Path}} {{.Version}}{{end}}'
+
+
+# For this module, Go 1.16 selects the same versions of all explicit dependencies
+# as Go 1.17 does. However, Go 1.16 selects a higher version of an *implicit*
+# dependency, imported by a test of one of the (external) imported packages.
+# As a result, Go 1.16 also needs checksums for the module sources for that higher
+# version.
+#
+# The Go 1.16 module graph looks like:
+#
+# m ---- lazy v0.1.0 ---- incompatible v1.0.0
+#         |
+#         + ------------- requireincompatible v0.1.0 ---- incompatible v2.0.0+incompatible
+#
+# The Go 1.17 module graph is the same except that the dependencies of
+# requireincompatible are pruned out (because the module that requires
+# it — lazy v0.1.0 — specifies 'go 1.17', and it is not otherwise relevant to
+# the main module).
+
+# 'go mod tidy' should by default diagnose the difference in dependencies as an
+# error, with useful suggestions about how to resolve it.
+
+cp go.mod go.mod.orig
+! go mod tidy
+stderr '^go: example\.com/m imports\n\texample\.net/lazy tested by\n\texample\.net/lazy.test imports\n\texample\.com/retract/incompatible loaded from example\.com/retract/incompatible@v1\.0\.0,\n\tbut go 1\.16 would select v2\.0\.0\+incompatible\n\n'
+stderr '\n\nTo upgrade to the versions selected by go 1\.16:\n\tgo mod tidy -go=1\.16 && go mod tidy -go=1\.17\nIf reproducibility with go 1.16 is not needed:\n\tgo mod tidy -compat=1.17\nFor other options, see:\n\thttps://golang\.org/doc/modules/pruning\n'
+
+cmp go.mod go.mod.orig
+
+# The suggested '-compat' flag to ignore differences should silence the error
+# and leave go.mod unchanged, resulting in checksum errors when Go 1.16 tries
+# to load a module pruned out by Go 1.17.
+
+go mod tidy -compat=1.17
+! stderr .
+cmp go.mod go.mod.orig
+
+go list -deps -test -f $MODFMT ./...
+stdout '^example.net/lazy v0.1.0$'
+
+go mod edit -go=1.16
+! go list -deps -test -f $MODFMT ./...
+
+stderr -count=1 '^go: example\.net/lazy@v0\.1\.0 requires\n\texample\.com/retract/incompatible@v1\.0\.0: missing go\.sum entry for go\.mod file; to add it:\n\tgo mod download example\.com/retract/incompatible$'
+
+
+# If we combine a Go 1.16 go.sum file...
+go mod tidy -go=1.16
+
+# ...with a Go 1.17 go.mod file...
+cp go.mod.orig go.mod
+
+# ...then Go 1.17 no longer works. 😞
+! go list -deps -test -f $MODFMT all
+stderr -count=1 '^go: can''t load test package: lazy[/\\]lazy_test.go:3:8: missing go\.sum entry for module providing package example\.com/retract/incompatible \(imported by example\.net/lazy\); to add:\n\tgo get -t example.net/lazy@v0\.1\.0$'
+
+
+# However, if we take the union of the go.sum files...
+go list -mod=mod -deps -test all
+cmp go.mod go.mod.orig
+
+# ...then Go 1.17 continues to work...
+go list -deps -test -f $MODFMT all
+stdout '^example\.com/retract/incompatible v1\.0\.0$'
+
+# ...and 1.16 also works(‽), but selects a different version for the
+# external-test dependency.
+go mod edit -go=1.16
+go list -deps -test -f $MODFMT all
+stdout '^example\.com/retract/incompatible v2\.0\.0\+incompatible$'
+
+
+-- go.mod --
+// Module m imports packages from the same versions under Go 1.17
+// as under Go 1.16, but under 1.16 its (implicit) external test dependencies
+// are higher.
+module example.com/m
+
+go 1.17
+
+replace (
+	example.net/lazy v0.1.0 => ./lazy
+	example.net/requireincompatible v0.1.0 => ./requireincompatible
+)
+
+require example.net/lazy v0.1.0
+-- implicit.go --
+package implicit
+
+import _ "example.net/lazy"
+-- lazy/go.mod --
+// Module lazy requires example.com/retract/incompatible v1.0.0.
+//
+// When viewed from the outside it also has a transitive dependency
+// on v2.0.0+incompatible, but in lazy mode that transitive dependency
+// is pruned out.
+module example.net/lazy
+
+go 1.17
+
+exclude example.com/retract/incompatible v2.0.0+incompatible
+
+require (
+	example.com/retract/incompatible v1.0.0
+	example.net/requireincompatible v0.1.0
+)
+-- lazy/lazy.go --
+package lazy
+-- lazy/lazy_test.go --
+package lazy_test
+
+import _ "example.com/retract/incompatible"
+-- requireincompatible/go.mod --
+module example.net/requireincompatible
+
+go 1.15
+
+require example.com/retract/incompatible v2.0.0+incompatible
diff --git a/src/cmd/go/testdata/script/mod_tidy_compat_incompatible.txt b/src/cmd/go/testdata/script/mod_tidy_compat_incompatible.txt
new file mode 100644
index 0000000..9e2a9ee
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_compat_incompatible.txt
@@ -0,0 +1,133 @@
+# https://golang.org/issue/46141: 'go mod tidy' for a Go 1.17 module should by
+# default preserve enough checksums for the module to be used by Go 1.16.
+#
+# We don't have a copy of Go 1.16 handy, but we can simulate it by editing the
+# 'go' version in the go.mod file to 1.16, without actually updating the
+# requirements to match.
+
+[short] skip
+
+env MODFMT='{{with .Module}}{{.Path}} {{.Version}}{{end}}'
+
+
+# For this module, Go 1.17 prunes out a (transitive and otherwise-irrelevant)
+# requirement on a retracted higher version of a dependency.
+# However, when Go 1.16 reads the same requirements from the go.mod file,
+# it does not prune out that requirement, and selects the retracted version.
+#
+# The Go 1.16 module graph looks like:
+#
+# m ---- lazy v0.1.0 ---- requireincompatible v0.1.0 ---- incompatible v2.0.0+incompatible
+# |        |
+# + -------+------------- incompatible v1.0.0
+#
+# The Go 1.17 module graph is the same except that the dependencies of
+# requireincompatible are pruned out (because the module that requires
+# it — lazy v0.1.0 — specifies 'go 1.17', and it is not otherwise relevant to
+# the main module).
+
+
+# 'go mod tidy' should by default diagnose the difference in dependencies as an
+# error, with useful suggestions about how to resolve it.
+
+cp go.mod go.mod.orig
+! go mod tidy
+stderr '^go: example\.com/m imports\n\texample\.net/lazy imports\n\texample\.com/retract/incompatible loaded from example\.com/retract/incompatible@v1\.0\.0,\n\tbut go 1\.16 would select v2\.0\.0\+incompatible\n\n'
+stderr '\n\nTo upgrade to the versions selected by go 1\.16:\n\tgo mod tidy -go=1\.16 && go mod tidy -go=1\.17\nIf reproducibility with go 1\.16 is not needed:\n\tgo mod tidy -compat=1.17\nFor other options, see:\n\thttps://golang\.org/doc/modules/pruning\n'
+
+cmp go.mod go.mod.orig
+
+
+# The suggested '-compat' flag to ignore differences should silence the error
+# and leave go.mod unchanged, resulting in checksum errors when Go 1.16 tries
+# to load a module pruned out by Go 1.17.
+
+go mod tidy -compat=1.17
+! stderr .
+cmp go.mod go.mod.orig
+
+go mod edit -go=1.16
+! go list -f $MODFMT -deps ./...
+stderr -count=1 '^go: example\.net/lazy@v0\.1\.0 requires\n\texample\.net/requireincompatible@v0\.1\.0 requires\n\texample\.com/retract/incompatible@v2\.0\.0\+incompatible: missing go.sum entry for go.mod file; to add it:\n\tgo mod download example.com/retract/incompatible$'
+
+
+# There are two ways for the module author to bring the two into alignment.
+# One is to *explicitly* 'exclude' the version that is already *implicitly*
+# pruned out under 1.17.
+
+go mod edit -exclude=example.com/retract/incompatible@v2.0.0+incompatible
+go list -f $MODFMT -deps ./...
+stdout '^example.com/retract/incompatible v1\.0\.0$'
+! stdout 'v2\.0\.0'
+
+
+# The other is to explicitly upgrade the version required under Go 1.17
+# to match the version selected by Go 1.16. The commands suggested by
+# 'go mod tidy' should do exactly that.
+
+cp go.mod.orig go.mod
+
+go mod tidy -go=1.16
+go list -f $MODFMT -deps ./...
+stdout '^example.com/retract/incompatible v2\.0\.0\+incompatible$'
+! stdout 'v1\.0\.0'
+
+go mod tidy -go=1.17
+go list -f $MODFMT -deps ./...
+stdout '^example.com/retract/incompatible v2\.0\.0\+incompatible$'
+! stdout 'v1\.0\.0'
+
+go mod edit -go=1.16
+go list -f $MODFMT -deps ./...
+stdout '^example.com/retract/incompatible v2\.0\.0\+incompatible$'
+! stdout 'v1\.0\.0'
+
+
+-- go.mod --
+// Module m indirectly imports a package from
+// example.com/retract/incompatible. Its selected version of
+// that module is lower under Go 1.17 semantics than under Go 1.16.
+module example.com/m
+
+go 1.17
+
+replace (
+	example.net/lazy v0.1.0 => ./lazy
+	example.net/requireincompatible v0.1.0 => ./requireincompatible
+)
+
+require example.net/lazy v0.1.0
+
+require example.com/retract/incompatible v1.0.0 // indirect
+-- incompatible.go --
+package incompatible
+
+import _ "example.net/lazy"
+
+-- lazy/go.mod --
+// Module lazy requires example.com/retract/incompatible v1.0.0.
+//
+// When viewed from the outside it also has a transitive dependency
+// on v2.0.0+incompatible, but in lazy mode that transitive dependency
+// is pruned out.
+module example.net/lazy
+
+go 1.17
+
+exclude example.com/retract/incompatible v2.0.0+incompatible
+
+require (
+	example.com/retract/incompatible v1.0.0
+	example.net/requireincompatible v0.1.0
+)
+-- lazy/lazy.go --
+package lazy
+
+import _ "example.com/retract/incompatible"
+
+-- requireincompatible/go.mod --
+module example.net/requireincompatible
+
+go 1.15
+
+require example.com/retract/incompatible v2.0.0+incompatible
diff --git a/src/cmd/go/testdata/script/mod_tidy_compat_irrelevant.txt b/src/cmd/go/testdata/script/mod_tidy_compat_irrelevant.txt
new file mode 100644
index 0000000..e4eaea0
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_compat_irrelevant.txt
@@ -0,0 +1,98 @@
+# https://golang.org/issue/46141: 'go mod tidy' for a Go 1.17 module should by
+# default preserve enough checksums for the module to be used by Go 1.16.
+#
+# We don't have a copy of Go 1.16 handy, but we can simulate it by editing the
+# 'go' version in the go.mod file to 1.16, without actually updating the
+# requirements to match.
+
+[short] skip
+
+env MODFMT='{{with .Module}}{{.Path}} {{.Version}}{{end}}'
+
+
+# This module selects the same versions in Go 1.16 and 1.17 for all modules
+# that provide packages (or test dependencies of packages) imported by the
+# main module. However, in Go 1.16 it selects a higher version of a
+# transitive module dependency that is not otherwise relevant to the main module.
+# As a result, Go 1.16 needs an additional checksum for the go.mod file of
+# that irrelevant dependency.
+#
+# The Go 1.16 module graph looks like:
+#
+# m ---- lazy v0.1.0 ---- incompatible v1.0.0
+#         |
+#         + ------------- requireincompatible v0.1.0 ---- incompatible v2.0.0+incompatible
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod go.mod.orig
+
+go list -deps -test -f $MODFMT all
+cp stdout out-117.txt
+
+go mod edit -go=1.16
+go list -deps -test -f $MODFMT all
+cmp stdout out-117.txt
+
+
+# If we explicitly drop compatibility with 1.16, we retain fewer checksums,
+# which gives a cleaner go.sum file but causes 1.16 to fail in readonly mode.
+
+cp go.mod.orig go.mod
+go mod tidy -compat=1.17
+cmp go.mod go.mod.orig
+
+go list -deps -test -f $MODFMT all
+cmp stdout out-117.txt
+
+go mod edit -go=1.16
+! go list -deps -test -f $MODFMT all
+stderr -count=1 '^go: example.net/lazy@v0.1.0 requires\n\texample.com/retract/incompatible@v1.0.0: missing go.sum entry for go.mod file; to add it:\n\tgo mod download example.com/retract/incompatible$'
+
+
+-- go.mod --
+// Module m imports packages from the same versions under Go 1.17
+// as under Go 1.16, but under 1.16 its (implicit) external test dependencies
+// are higher.
+module example.com/m
+
+go 1.17
+
+replace (
+	example.net/lazy v0.1.0 => ./lazy
+	example.net/requireincompatible v0.1.0 => ./requireincompatible
+)
+
+require example.net/lazy v0.1.0
+-- m.go --
+package m
+
+import _ "example.net/lazy"
+-- lazy/go.mod --
+// Module lazy requires example.com/retract/incompatible v1.0.0.
+//
+// When viewed from the outside it also has a transitive dependency
+// on v2.0.0+incompatible, but in lazy mode that transitive dependency
+// is pruned out.
+module example.net/lazy
+
+go 1.17
+
+exclude example.com/retract/incompatible v2.0.0+incompatible
+
+require (
+	example.com/retract/incompatible v1.0.0
+	example.net/requireincompatible v0.1.0
+)
+-- lazy/lazy.go --
+package lazy
+-- lazy/unimported/unimported.go --
+package unimported
+
+import _ "example.com/retract/incompatible"
+-- requireincompatible/go.mod --
+module example.net/requireincompatible
+
+go 1.15
+
+require example.com/retract/incompatible v2.0.0+incompatible
diff --git a/src/cmd/go/testdata/script/mod_tidy_convergence.txt b/src/cmd/go/testdata/script/mod_tidy_convergence.txt
new file mode 100644
index 0000000..45f74b4
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_convergence.txt
@@ -0,0 +1,202 @@
+# This test demonstrates a simple case in which 'go mod tidy' may resolve a
+# missing package, only to remove that package when resolving its dependencies.
+#
+# If we naively iterate 'go mod tidy' until the dependency graph converges, this
+# scenario may fail to converge.
+
+# The import graph used in this test looks like:
+#
+# m --- x
+#       |
+#       x_test --- y
+#
+# The module dependency graph of m is initially empty.
+# Modules x and y look like:
+#
+# x.1 (provides package x that imports y, but does not depend on module y)
+#
+# x.2-pre (no dependencies, but does not provide package x)
+#
+# y.1 (no dependencies, but provides package y)
+#
+# y.2 --- x.2-pre (provides package y)
+#
+#
+# When we resolve the missing import of y in x_test, we add y@latest — which is
+# y.2, not y.1 — as a new dependency. That upgrades to x to x.2-pre, which
+# removes package x (and also the need for module y). We can then safely remove
+# the dependency on module y, because nothing imports package y any more!
+#
+# We might be tempted to remove the dependency on module x for the same reason:
+# it no longer provides any imported package. However, that would cause 'go mod
+# tidy -e' to become unstable: with x.2-pre out of the way, we could once again
+# resolve the missing import of package x by re-adding x.1.
+
+cp go.mod go.mod.orig
+
+# 'go mod tidy' without -e should fail without modifying go.mod,
+# because it cannot resolve x and y simultaneously.
+! go mod tidy
+
+cmp go.mod go.mod.orig
+
+stderr '^go: found example\.net/y in example\.net/y v0.2.0$'
+stderr '^go: finding module for package example\.net/x$'
+
+	# TODO: This error message should be clearer — it doesn't indicate why v0.2.0-pre is required.
+stderr '^go: example\.net/m imports\n\texample\.net/x: package example\.net/x provided by example\.net/x at latest version v0\.1\.0 but not at required version v0\.2\.0-pre$'
+
+
+# 'go mod tidy -e' should follow upgrades to try to resolve the modules that it
+# can, and then stop. When we resolve example.net/y, we upgrade to example.net/x
+# to v0.2.0-pre. At that version, package x no longer exists and no longer
+# imports package y, so the import of x should be left unsatisfied and the
+# existing dependency on example.net/x removed.
+#
+# TODO(bcmills): It would be ever better if we could keep the original
+# dependency on example.net/x v0.1.0, but I don't see a way to do that without
+# making the algorithm way too complicated. (We would have to detect that the
+# new dependency on example.net/y interferes with the package that caused us to
+# to add that dependency in the first place, and back out that part of the change
+# without also backing out any other needed changes.)
+
+go mod tidy -e
+cmp go.mod go.mod.tidye
+stderr '^go: found example\.net/y in example\.net/y v0.2.0$'
+
+	# TODO: This error message should be clearer — it doesn't indicate why v0.2.0-pre is required.
+stderr '^go: example\.net/m imports\n\texample\.net/x: package example\.net/x provided by example\.net/x at latest version v0\.1\.0 but not at required version v0\.2\.0-pre$'
+
+
+# Since we attempt to resolve the dependencies of package x whenever we add x itself,
+# this end state is stable.
+
+go mod tidy -e
+cmp go.mod go.mod.tidye
+
+
+# An explicit 'go get' with the correct versions should allow 'go mod tidy' to
+# succeed and remain stable. y.1 does not upgrade x, and can therefore be used
+# with it.
+
+go get example.net/x@v0.1.0 example.net/y@v0.1.0
+go mod tidy
+cmp go.mod go.mod.postget
+
+
+# The 'tidy' logic for a lazy main module is somewhat different from that for an
+# eager main module, but the overall behavior is the same.
+
+cp go.mod.orig go.mod
+go mod edit -go=1.17 go.mod
+go mod edit -go=1.17 go.mod.tidye
+
+go mod tidy -e
+cmp go.mod go.mod.tidye
+stderr '^go: found example\.net/y in example\.net/y v0.2.0$'
+stderr '^go: example\.net/m imports\n\texample\.net/x: package example\.net/x provided by example\.net/x at latest version v0\.1\.0 but not at required version v0\.2\.0-pre$'
+
+go get example.net/x@v0.1.0 example.net/y@v0.1.0
+go mod tidy
+cmp go.mod go.mod.postget-117
+
+
+-- go.mod --
+module example.net/m
+
+go 1.16
+
+replace (
+	example.net/x v0.1.0 => ./x1
+	example.net/x v0.2.0-pre => ./x2-pre
+	example.net/y v0.1.0 => ./y1
+	example.net/y v0.2.0 => ./y2
+)
+
+require (
+	example.net/x v0.1.0
+)
+-- go.mod.tidye --
+module example.net/m
+
+go 1.16
+
+replace (
+	example.net/x v0.1.0 => ./x1
+	example.net/x v0.2.0-pre => ./x2-pre
+	example.net/y v0.1.0 => ./y1
+	example.net/y v0.2.0 => ./y2
+)
+-- go.mod.postget --
+module example.net/m
+
+go 1.16
+
+replace (
+	example.net/x v0.1.0 => ./x1
+	example.net/x v0.2.0-pre => ./x2-pre
+	example.net/y v0.1.0 => ./y1
+	example.net/y v0.2.0 => ./y2
+)
+
+require (
+	example.net/x v0.1.0
+	example.net/y v0.1.0 // indirect
+)
+-- go.mod.postget-117 --
+module example.net/m
+
+go 1.17
+
+replace (
+	example.net/x v0.1.0 => ./x1
+	example.net/x v0.2.0-pre => ./x2-pre
+	example.net/y v0.1.0 => ./y1
+	example.net/y v0.2.0 => ./y2
+)
+
+require example.net/x v0.1.0
+
+require example.net/y v0.1.0 // indirect
+-- m.go --
+package m
+
+import _ "example.net/x"
+
+-- x1/go.mod --
+module example.net/x
+
+go 1.16
+-- x1/x.go --
+package x
+-- x1/x_test.go --
+package x
+
+import _ "example.net/y"
+
+-- x2-pre/go.mod --
+module example.net/x
+
+go 1.16
+-- x2-pre/README.txt --
+There is no package x here. Use example.com/x/subpkg instead.
+-- x2-pre/subpkg/subpkg.go --
+package subpkg  // import "example.net/x/subpkg"
+
+-- y1/go.mod --
+module example.net/y
+
+go 1.16
+-- y1/y.go --
+package y
+
+-- y2/go.mod --
+module example.net/y
+
+go 1.16
+
+require example.net/x v0.2.0-pre
+-- y2/y.go --
+package y
+
+import _ "example.net/x/subpkg"
diff --git a/src/cmd/go/testdata/script/mod_tidy_convergence_loop.txt b/src/cmd/go/testdata/script/mod_tidy_convergence_loop.txt
new file mode 100644
index 0000000..ec58159
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_convergence_loop.txt
@@ -0,0 +1,329 @@
+# This test demonstrates a simple case in which 'go mod tidy' may resolve a
+# missing package, only to remove that package when resolving its dependencies.
+#
+# If we naively iterate 'go mod tidy' until the dependency graph converges, this
+# scenario may fail to converge.
+
+# The import graph used in this test looks like:
+#
+# m --- w
+# |
+# + --- x
+# |
+# + --- y
+# |
+# + --- z
+#
+# The module dependency graph of m initially contains w.1 (and, by extension,
+# y.2-pre and z.2-pre). This is an arbitrary point in the cycle of possible
+# configurations.
+#
+# w.1 requires y.2-pre and z.2-pre
+# x.1 requires z.2-pre and w.2-pre
+# y.1 requires w.2-pre and x.2-pre
+# z.1 requires x.2-pre and y.2-pre
+#
+# At each point, exactly one missing package can be resolved by adding a
+# dependency on the .1 release of the module that provides that package.
+# However, adding that dependency causes the module providing another package to
+# roll over from its .1 release to its .2-pre release, which removes the
+# package. Once the package is removed, 'go mod tidy -e' no longer sees the
+# module as relevant to the main module, and will happily remove the existing
+# dependency on it.
+#
+# The cycle is of length 4 so that at every step only one package can be
+# resolved. This is important because it prevents the iteration from ever
+# reaching a state in which every package is simultaneously over-upgraded — such
+# a state is stable and does not exhibit failure to converge.
+
+cp go.mod go.mod.orig
+
+# 'go mod tidy' without -e should fail without modifying go.mod,
+# because it cannot resolve x, y, and z simultaneously.
+! go mod tidy
+
+cmp go.mod go.mod.orig
+
+stderr '^go: finding module for package example\.net/w$'
+stderr '^go: finding module for package example\.net/x$'
+stderr -count=2 '^go: finding module for package example\.net/y$'
+stderr -count=2 '^go: finding module for package example\.net/z$'
+stderr '^go: found example\.net/x in example\.net/x v0.1.0$'
+
+	# TODO: These error messages should be clearer — it doesn't indicate why v0.2.0-pre is required.
+stderr '^go: example\.net/m imports\n\texample\.net/w: package example\.net/w provided by example\.net/w at latest version v0\.1\.0 but not at required version v0\.2\.0-pre$'
+stderr '^go: example\.net/m imports\n\texample\.net/y: package example\.net/y provided by example\.net/y at latest version v0\.1\.0 but not at required version v0\.2\.0-pre$'
+stderr '^go: example\.net/m imports\n\texample\.net/z: package example\.net/z provided by example\.net/z at latest version v0\.1\.0 but not at required version v0\.2\.0-pre$'
+
+
+# 'go mod tidy -e' should preserve all of the upgrades to modules that could
+# provide the missing packages but don't. That would at least explain why they
+# are missing, and why no individual module can be upgraded in order to satisfy
+# a missing import.
+#
+# TODO(bcmills): Today, it doesn't preserve those upgrades, and instead advances
+# the state by one through the cycle of semi-tidy states.
+
+go mod tidy -e
+
+cmp go.mod go.mod.tidye1
+
+stderr '^go: finding module for package example\.net/w$'
+stderr '^go: finding module for package example\.net/x$'
+stderr -count=2 '^go: finding module for package example\.net/y$'
+stderr -count=2 '^go: finding module for package example\.net/z$'
+stderr '^go: found example\.net/x in example\.net/x v0.1.0$'
+
+stderr '^go: example\.net/m imports\n\texample\.net/w: package example\.net/w provided by example\.net/w at latest version v0\.1\.0 but not at required version v0\.2\.0-pre$'
+stderr '^go: example\.net/m imports\n\texample\.net/y: package example\.net/y provided by example\.net/y at latest version v0\.1\.0 but not at required version v0\.2\.0-pre$'
+stderr '^go: example\.net/m imports\n\texample\.net/z: package example\.net/z provided by example\.net/z at latest version v0\.1\.0 but not at required version v0\.2\.0-pre$'
+
+
+go mod tidy -e
+cmp go.mod go.mod.tidye2
+
+go mod tidy -e
+cmp go.mod go.mod.tidye3
+
+go mod tidy -e
+cmp go.mod go.mod.orig
+
+
+# If we upgrade away all of the packages simultaneously, the resulting tidy
+# state converges at "no dependencies", because simultaneously adding all of the
+# packages simultaneously over-upgrades all of the dependencies, and 'go mod
+# tidy' treats "no package can be added" as a terminal state.
+
+go get example.net/w@v0.2.0-pre example.net/x@v0.2.0-pre example.net/y@v0.2.0-pre example.net/z@v0.2.0-pre
+go mod tidy -e
+cmp go.mod go.mod.postget
+go mod tidy -e
+cmp go.mod go.mod.postget
+
+
+# The 'tidy' logic for a lazy main module requires more iterations to converge,
+# because it is willing to drop dependencies on non-root modules that do not
+# otherwise provide imported packages.
+#
+# On the first iteration, it adds x.1 as a root, which upgrades z and w,
+# dropping w.1's requirement on y. w.1 was initially a root, so the upgraded
+# w.2-pre is retained as a root.
+#
+# On the second iteration, it adds y.1 as a root, which upgrades w and x,
+# dropping x.1's requirement on z. x.1 was added as a root in the previous step,
+# so the upgraded x.2-pre is retained as a root.
+#
+# On the third iteration, it adds z.1 as a root, which upgrades x and y.
+# x and y were already roots (from the previous steps), so their upgraded versions
+# are retained (not dropped) and the iteration stops.
+#
+# At that point, we have z.1 as a root providing package z,
+# and w, x, and y have all been upgraded to no longer provide any packages.
+# So only z is retained as a new root.
+#
+# (From the above, we can see that in a lazy module we still cycle through the
+# same possible root states, but in a different order from the eager case.)
+#
+# TODO(bcmills): if we retained the upgrades on w, x, and y (since they are
+# lexical prefixes for unresolved packages w, x, and y, respectively), then 'go
+# mod tidy -e' itself would become stable and no longer cycle through states.
+
+cp go.mod.orig go.mod
+go mod edit -go=1.17 go.mod
+cp go.mod go.mod.117
+go mod edit -go=1.17 go.mod.tidye1
+go mod edit -go=1.17 go.mod.tidye2
+go mod edit -go=1.17 go.mod.tidye3
+go mod edit -go=1.17 go.mod.postget
+
+go list -m all
+
+go mod tidy -e
+cmp go.mod go.mod.tidye3
+
+go mod tidy -e
+cmp go.mod go.mod.tidye2
+
+go mod tidy -e
+cmp go.mod go.mod.tidye1
+
+go mod tidy -e
+cmp go.mod go.mod.117
+
+
+# As in the eager case, for the lazy module the fully-upgraded dependency graph
+# becomes empty, and the empty graph is stable.
+
+go get example.net/w@v0.2.0-pre example.net/x@v0.2.0-pre example.net/y@v0.2.0-pre example.net/z@v0.2.0-pre
+go mod tidy -e
+cmp go.mod go.mod.postget
+go mod tidy -e
+cmp go.mod go.mod.postget
+
+
+-- m.go --
+package m
+
+import (
+	_ "example.net/w"
+	_ "example.net/x"
+	_ "example.net/y"
+	_ "example.net/z"
+)
+
+-- go.mod --
+module example.net/m
+
+go 1.16
+
+replace (
+	example.net/w v0.1.0 => ./w1
+	example.net/w v0.2.0-pre => ./w2-pre
+	example.net/x v0.1.0 => ./x1
+	example.net/x v0.2.0-pre => ./x2-pre
+	example.net/y v0.1.0 => ./y1
+	example.net/y v0.2.0-pre => ./y2-pre
+	example.net/z v0.1.0 => ./z1
+	example.net/z v0.2.0-pre => ./z2-pre
+)
+
+require example.net/w v0.1.0
+-- go.mod.tidye1 --
+module example.net/m
+
+go 1.16
+
+replace (
+	example.net/w v0.1.0 => ./w1
+	example.net/w v0.2.0-pre => ./w2-pre
+	example.net/x v0.1.0 => ./x1
+	example.net/x v0.2.0-pre => ./x2-pre
+	example.net/y v0.1.0 => ./y1
+	example.net/y v0.2.0-pre => ./y2-pre
+	example.net/z v0.1.0 => ./z1
+	example.net/z v0.2.0-pre => ./z2-pre
+)
+
+require example.net/x v0.1.0
+-- go.mod.tidye2 --
+module example.net/m
+
+go 1.16
+
+replace (
+	example.net/w v0.1.0 => ./w1
+	example.net/w v0.2.0-pre => ./w2-pre
+	example.net/x v0.1.0 => ./x1
+	example.net/x v0.2.0-pre => ./x2-pre
+	example.net/y v0.1.0 => ./y1
+	example.net/y v0.2.0-pre => ./y2-pre
+	example.net/z v0.1.0 => ./z1
+	example.net/z v0.2.0-pre => ./z2-pre
+)
+
+require example.net/y v0.1.0
+-- go.mod.tidye3 --
+module example.net/m
+
+go 1.16
+
+replace (
+	example.net/w v0.1.0 => ./w1
+	example.net/w v0.2.0-pre => ./w2-pre
+	example.net/x v0.1.0 => ./x1
+	example.net/x v0.2.0-pre => ./x2-pre
+	example.net/y v0.1.0 => ./y1
+	example.net/y v0.2.0-pre => ./y2-pre
+	example.net/z v0.1.0 => ./z1
+	example.net/z v0.2.0-pre => ./z2-pre
+)
+
+require example.net/z v0.1.0
+-- go.mod.postget --
+module example.net/m
+
+go 1.16
+
+replace (
+	example.net/w v0.1.0 => ./w1
+	example.net/w v0.2.0-pre => ./w2-pre
+	example.net/x v0.1.0 => ./x1
+	example.net/x v0.2.0-pre => ./x2-pre
+	example.net/y v0.1.0 => ./y1
+	example.net/y v0.2.0-pre => ./y2-pre
+	example.net/z v0.1.0 => ./z1
+	example.net/z v0.2.0-pre => ./z2-pre
+)
+-- w1/go.mod --
+module example.net/w
+
+go 1.16
+
+require (
+	example.net/y v0.2.0-pre
+	example.net/z v0.2.0-pre
+)
+-- w1/w.go --
+package w
+-- w2-pre/go.mod --
+module example.net/w
+
+go 1.16
+-- w2-pre/README.txt --
+Package w has been removed.
+
+-- x1/go.mod --
+module example.net/x
+
+go 1.16
+
+require (
+	example.net/z v0.2.0-pre
+	example.net/w v0.2.0-pre
+)
+-- x1/x.go --
+package x
+-- x2-pre/go.mod --
+module example.net/x
+
+go 1.16
+-- x2-pre/README.txt --
+Package x has been removed.
+
+-- y1/go.mod --
+module example.net/y
+
+go 1.16
+
+require (
+	example.net/w v0.2.0-pre
+	example.net/x v0.2.0-pre
+)
+-- y1/y.go --
+package y
+
+-- y2-pre/go.mod --
+module example.net/y
+
+go 1.16
+-- y2-pre/README.txt --
+Package y has been removed.
+
+-- z1/go.mod --
+module example.net/z
+
+go 1.16
+
+require (
+	example.net/x v0.2.0-pre
+	example.net/y v0.2.0-pre
+)
+-- z1/z.go --
+package z
+
+-- z2-pre/go.mod --
+module example.net/z
+
+go 1.16
+-- z2-pre/README.txt --
+Package z has been removed.
diff --git a/src/cmd/go/testdata/script/mod_tidy_cycle.txt b/src/cmd/go/testdata/script/mod_tidy_cycle.txt
new file mode 100644
index 0000000..e46f37d
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_cycle.txt
@@ -0,0 +1,75 @@
+# Regression test for https://golang.org/issue/34086:
+# 'go mod tidy' produced different go.mod file from other
+# subcommands when certain kinds of cycles were present
+# in the build graph.
+
+env GO111MODULE=on
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod go.mod.orig
+
+# If the go.mod file is already tidy, 'go mod graph' should not modify it.
+go mod graph
+cmp go.mod go.mod.orig
+
+-- go.mod --
+module root
+
+go 1.13
+
+replace (
+	a v0.1.0 => ./a1
+	b v0.1.0 => ./b1
+	b v0.2.0 => ./b2
+	c v0.1.0 => ./c1
+	c v0.2.0 => ./c2
+)
+
+require (
+	a v0.1.0
+	b v0.2.0 // indirect
+)
+-- main.go --
+package main
+
+import _ "a"
+
+func main() {}
+
+-- a1/go.mod --
+module a
+
+go 1.13
+
+require b v0.1.0
+-- a1/a.go --
+package a
+
+import _ "c"
+-- b1/go.mod --
+module b
+
+go 1.13
+
+require c v0.1.0
+-- b2/go.mod --
+module b
+
+go 1.13
+
+require c v0.2.0
+-- c1/go.mod --
+module c
+
+go 1.13
+-- c2/c.go --
+package c
+-- c2/go.mod --
+module c
+
+go 1.13
+
+require b v0.2.0
+-- c2/c.go --
+package c
diff --git a/src/cmd/go/testdata/script/mod_tidy_downgrade_ambiguous.txt b/src/cmd/go/testdata/script/mod_tidy_downgrade_ambiguous.txt
new file mode 100644
index 0000000..8b508c7
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_downgrade_ambiguous.txt
@@ -0,0 +1,58 @@
+# Verifies golang.org/issue/47738.
+
+# In this test, the user has rewritten their imports to use rsc.io/quote/v3,
+# but their go.mod still requires rsc.io/quote@v1.5.2, and they indirectly
+# require rsc.io/quote@v1.5.1 but don't import anything from it.
+go list -m -f '{{.Path}}@{{.Version}}{{if .Indirect}} indirect{{end}}' all
+stdout '^rsc.io/quote@v1.5.2$'
+! stdout 'rsc.io/quote/v3'
+go list -e all
+! stdout '^rsc.io/quote$'
+
+# 'go mod tidy' should preserve the requirement on rsc.io/quote but mark it
+# indirect. This prevents a downgrade to v1.5.1, which could introduce
+# an ambiguity.
+go mod tidy
+go list -m -f '{{.Path}}@{{.Version}}{{if .Indirect}} indirect{{end}}' all
+stdout '^rsc.io/quote@v1.5.2 indirect$'
+stdout '^rsc.io/quote/v3@v3.0.0$'
+
+-- go.mod --
+module use
+
+go 1.16
+
+require (
+	old-indirect v0.0.0
+	rsc.io/quote v1.5.2
+)
+
+replace old-indirect v0.0.0 => ./old-indirect
+-- go.sum --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c h1:pvCbr/wm8HzDD3fVywevekufpn6tCGPY3spdHeZJEsw=
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.5.1/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/quote v1.5.2 h1:3fEykkD9k7lYzXqCYrwGAf7iNhbk4yCjHmKBN9td4L0=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0 h1:HLGR/BgEtI3r0uymSP/nl2uPLsUnNJX8toRyhfpBTII=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+-- use.go --
+package use
+
+import (
+	_ "old-indirect/empty"
+
+	_ "rsc.io/quote/v3"
+)
+-- old-indirect/empty/empty.go --
+package empty
+-- old-indirect/go.mod --
+module old-indirect
+
+go 1.16
+
+require rsc.io/quote v1.5.1
+-- old-indirect/go.sum --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
diff --git a/src/cmd/go/testdata/script/mod_tidy_duplicates.txt b/src/cmd/go/testdata/script/mod_tidy_duplicates.txt
new file mode 100644
index 0000000..d454c8d
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_duplicates.txt
@@ -0,0 +1,38 @@
+env GO111MODULE=on
+
+# Regression test for golang.org/issue/28456:
+# 'go mod tidy' should not leave duplicate lines when re-writing the file.
+
+go mod tidy
+cmp go.sum golden.sum
+
+-- go.mod --
+module use
+
+go 1.16
+
+require rsc.io/quote v1.5.2
+
+-- go.sum --
+rsc.io/quote v1.5.2 h1:3fEykkD9k7lYzXqCYrwGAf7iNhbk4yCjHmKBN9td4L0=
+rsc.io/quote v1.5.2 h1:3fEykkD9k7lYzXqCYrwGAf7iNhbk4yCjHmKBN9td4L0=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0 h1:HLGR/BgEtI3r0uymSP/nl2uPLsUnNJX8toRyhfpBTII=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+rsc.io/testonly v1.0.0 h1:K/VWHdO+Jv7woUXG0GzVNx1czBXUt3Ib1deaMn+xk64=
+rsc.io/testonly v1.0.0/go.mod h1:OqmGbIFOcF+XrFReLOGZ6BhMM7uMBiQwZsyNmh74SzY=
+-- golden.sum --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c h1:pvCbr/wm8HzDD3fVywevekufpn6tCGPY3spdHeZJEsw=
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.5.2 h1:3fEykkD9k7lYzXqCYrwGAf7iNhbk4yCjHmKBN9td4L0=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0 h1:HLGR/BgEtI3r0uymSP/nl2uPLsUnNJX8toRyhfpBTII=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+rsc.io/testonly v1.0.0 h1:K/VWHdO+Jv7woUXG0GzVNx1czBXUt3Ib1deaMn+xk64=
+rsc.io/testonly v1.0.0/go.mod h1:OqmGbIFOcF+XrFReLOGZ6BhMM7uMBiQwZsyNmh74SzY=
+-- main.go --
+package use
+
+import _ "rsc.io/quote"
diff --git a/src/cmd/go/testdata/script/mod_tidy_error.txt b/src/cmd/go/testdata/script/mod_tidy_error.txt
new file mode 100644
index 0000000..fc6cd78
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_error.txt
@@ -0,0 +1,39 @@
+env GO111MODULE=on
+
+# Regression test for golang.org/issue/27063:
+# 'go mod tidy' and 'go mod vendor' should not hide loading errors.
+
+! go mod tidy
+! stderr 'package nonexist is not in std'
+stderr '^go: issue27063 imports\n\tnonexist.example.com: cannot find module providing package nonexist.example.com'
+stderr '^go: issue27063 imports\n\tissue27063/other imports\n\tother.example.com/nonexist: cannot find module providing package other.example.com/nonexist'
+
+! go mod vendor
+! stderr 'package nonexist is not in std'
+stderr '^go: issue27063 imports\n\tnonexist.example.com: no required module provides package nonexist.example.com; to add it:\n\tgo get nonexist.example.com$'
+stderr '^go: issue27063 imports\n\tissue27063/other imports\n\tother.example.com/nonexist: no required module provides package other.example.com/nonexist; to add it:\n\tgo get other.example.com/nonexist$'
+
+-- go.mod --
+module issue27063
+
+go 1.13
+
+require issue27063/other v0.0.0
+replace issue27063/other => ./other
+-- x.go --
+package main
+
+import (
+	"nonexist"
+
+	"nonexist.example.com"
+	"issue27063/other"
+)
+
+func main() {}
+-- other/go.mod --
+module issue27063/other
+-- other/other.go --
+package other
+
+import "other.example.com/nonexist"
diff --git a/src/cmd/go/testdata/script/mod_tidy_indirect.txt b/src/cmd/go/testdata/script/mod_tidy_indirect.txt
new file mode 100644
index 0000000..1f092b2
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_indirect.txt
@@ -0,0 +1,67 @@
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod go.mod.orig
+
+-- go.mod --
+module example.com/tidy
+
+go 1.16
+
+require (
+	example.net/incomplete v0.1.0
+	example.net/indirect v0.2.0 // indirect
+	example.net/toolow v0.1.0
+)
+
+replace (
+	example.net/incomplete v0.1.0 => ./incomplete
+	example.net/indirect v0.1.0 => ./indirect.1
+	example.net/indirect v0.2.0 => ./indirect.2
+	example.net/toolow v0.1.0 => ./toolow
+)
+-- tidy.go --
+package tidy
+
+import (
+	_ "example.net/incomplete"
+	_ "example.net/toolow"
+)
+
+-- incomplete/go.mod --
+module example.net/incomplete
+
+go 1.16
+
+// This module omits a needed requirement on example.net/indirect.
+-- incomplete/incomplete.go --
+package incomplete
+
+import _ "example.net/indirect/newpkg"
+
+-- toolow/go.mod --
+module example.net/toolow
+
+go 1.16
+
+require example.net/indirect v0.1.0
+-- toolow/toolow.go --
+package toolow
+
+import _ "example.net/indirect/oldpkg"
+
+-- indirect.1/go.mod --
+module example.net/indirect
+
+go 1.16
+-- indirect.1/oldpkg/oldpkg.go --
+package oldpkg
+
+
+-- indirect.2/go.mod --
+module example.net/indirect
+
+go 1.16
+-- indirect.2/oldpkg/oldpkg.go --
+package oldpkg
+-- indirect.2/newpkg/newpkg.go --
+package newpkg
diff --git a/src/cmd/go/testdata/script/mod_tidy_issue60313.txt b/src/cmd/go/testdata/script/mod_tidy_issue60313.txt
new file mode 100644
index 0000000..6963994
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_issue60313.txt
@@ -0,0 +1,69 @@
+# Regression test for https://go.dev/issue/60313: 'go mod tidy' did not preserve
+# dependencies needed to prevent 'ambiguous import' errors in external test
+# dependencies.
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod go.mod.orig
+
+-- go.mod --
+module example
+
+go 1.21
+
+require (
+	example.net/a v0.1.0
+	example.net/b v0.1.0
+)
+
+require example.net/outer/inner v0.1.0 // indirect
+
+replace (
+	example.net/a v0.1.0 => ./a
+	example.net/b v0.1.0 => ./b
+	example.net/outer v0.1.0 => ./outer
+	example.net/outer/inner v0.1.0 => ./inner
+)
+-- example.go --
+package example
+
+import (
+	_ "example.net/a"
+	_ "example.net/b"
+)
+-- a/go.mod --
+module example.net/a
+
+go 1.21
+
+require example.net/outer/inner v0.1.0
+-- a/a.go --
+package a
+-- a/a_test.go --
+package a_test
+
+import _ "example.net/outer/inner"
+-- b/go.mod --
+module example.net/b
+
+go 1.21
+
+require example.net/outer v0.1.0
+-- b/b.go --
+package b
+-- b/b_test.go --
+package b_test
+
+import _ "example.net/outer/inner"
+-- inner/go.mod --
+module example.net/outer/inner
+
+go 1.21
+-- inner/inner.go --
+package inner
+-- outer/go.mod --
+module example.net/outer
+
+go 1.21
+-- outer/inner/inner.go --
+package inner
diff --git a/src/cmd/go/testdata/script/mod_tidy_lazy_self.txt b/src/cmd/go/testdata/script/mod_tidy_lazy_self.txt
new file mode 100644
index 0000000..9abbabd
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_lazy_self.txt
@@ -0,0 +1,66 @@
+# Regression test for https://golang.org/issue/46078:
+# 'go mod tidy' should not panic if the main module initially
+# requires an older version of itself.
+
+# A module may require an older version of itself without error. This is
+# inconsistent (the required version is never selected), but we still get
+# a reproducible build list.
+go list -m all
+stdout '^golang.org/issue/46078$'
+
+# 'go mod tidy' should fix this (and not crash).
+go mod tidy
+
+
+# We prune out redundant roots very early on in module loading, and at that
+# point the indirect requirement on example.net/x v0.1.0 appears to be
+# irrelevant. It should be pruned out; when the import of "example.net/x" is
+# later resolved, it should resolve at the latest version (v0.2.0), not the
+# version implied by the (former) misleading requirement on the older version of
+# the main module.
+
+cmp go.mod go.mod.tidy
+
+
+-- go.mod --
+module golang.org/issue/46078
+
+go 1.17
+
+replace (
+	example.net/x v0.1.0 => ./x
+	example.net/x v0.2.0 => ./x
+	golang.org/issue/46078 v0.1.0 => ./old
+)
+
+require golang.org/issue/46078 v0.1.0
+-- go.mod.tidy --
+module golang.org/issue/46078
+
+go 1.17
+
+replace (
+	example.net/x v0.1.0 => ./x
+	example.net/x v0.2.0 => ./x
+	golang.org/issue/46078 v0.1.0 => ./old
+)
+
+require example.net/x v0.2.0
+-- issue46078/issue.go --
+package issue46078
+
+import _ "example.net/x"
+
+-- old/go.mod --
+module golang.org/issue/46078
+
+go 1.17
+
+require example.net/x v0.1.0
+
+-- x/go.mod --
+module example.net/x
+
+go 1.17
+-- x/x.go --
+package x
diff --git a/src/cmd/go/testdata/script/mod_tidy_newroot.txt b/src/cmd/go/testdata/script/mod_tidy_newroot.txt
new file mode 100644
index 0000000..3abd5ef
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_newroot.txt
@@ -0,0 +1,82 @@
+# https://golang.org/issue/45952: 'go mod tidy' in an eager module failed due
+# to an erroneous check on root completeness.
+#
+# Per the issue report:
+# > It may have to do with:
+# >
+# > package A imports package B in go.mod, which imports package C in its own go.mod
+# > package A drops direct dependency on package B …
+#
+# We infer from that that package C is still needed by some other indirect
+# dependency, and must be at a higher version than what is required by that
+# dependency (or else no new root would be needed). An additional package D
+# in its own module satisfies that condition, reproducing the bug.
+
+go mod tidy
+cmp go.mod go.mod.tidy
+
+-- go.mod --
+module example.net/a
+
+go 1.16
+
+require (
+	example.net/b v0.1.0
+	example.net/d v0.1.0
+)
+
+replace (
+	example.net/b v0.1.0 => ./b
+	example.net/c v0.1.0 => ./c
+	example.net/c v0.2.0 => ./c
+	example.net/d v0.1.0 => ./d
+)
+-- go.mod.tidy --
+module example.net/a
+
+go 1.16
+
+require (
+	example.net/c v0.2.0 // indirect
+	example.net/d v0.1.0
+)
+
+replace (
+	example.net/b v0.1.0 => ./b
+	example.net/c v0.1.0 => ./c
+	example.net/c v0.2.0 => ./c
+	example.net/d v0.1.0 => ./d
+)
+-- a.go --
+package a
+
+import _ "example.net/d"
+
+-- b/go.mod --
+module example.net/b
+
+go 1.16
+
+require example.net/c v0.2.0
+-- b/b.go --
+package b
+
+import _ "example.net/c"
+
+-- c/go.mod --
+module example.net/c
+
+go 1.16
+-- c/c.go --
+package c
+
+-- d/go.mod --
+module example.net/d
+
+go 1.16
+
+require example.net/c v0.1.0
+-- d/d.go --
+package d
+
+import _ "example.net/c"
diff --git a/src/cmd/go/testdata/script/mod_tidy_old.txt b/src/cmd/go/testdata/script/mod_tidy_old.txt
new file mode 100644
index 0000000..7428f0c
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_old.txt
@@ -0,0 +1,46 @@
+# 'go mod tidy' should remove content sums for module versions that aren't
+# in the build list. It should preserve go.mod sums for module versions that
+# are in the module graph though.
+# Verifies golang.org/issue/33008.
+go mod tidy
+! grep '^rsc.io/quote v1.5.0 h1:' go.sum
+grep '^rsc.io/quote v1.5.0/go.mod h1:' go.sum
+
+-- go.mod --
+module m
+
+go 1.15
+
+require (
+	rsc.io/quote v1.5.2
+	example.com/r v0.0.0
+)
+
+replace example.com/r => ./r
+
+-- go.sum --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c h1:pvCbr/wm8HzDD3fVywevekufpn6tCGPY3spdHeZJEsw=
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.5.0 h1:6fJa6E+wGadANKkUMlZ0DhXFpoKlslOQDCo259XtdIE=
+rsc.io/quote v1.5.0/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/quote v1.5.2 h1:3fEykkD9k7lYzXqCYrwGAf7iNhbk4yCjHmKBN9td4L0=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0 h1:HLGR/BgEtI3r0uymSP/nl2uPLsUnNJX8toRyhfpBTII=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+rsc.io/testonly v1.0.0 h1:K/VWHdO+Jv7woUXG0GzVNx1czBXUt3Ib1deaMn+xk64=
+rsc.io/testonly v1.0.0/go.mod h1:OqmGbIFOcF+XrFReLOGZ6BhMM7uMBiQwZsyNmh74SzY=
+
+-- r/go.mod --
+module example.com/r
+
+require rsc.io/quote v1.5.0
+
+-- use.go --
+package use
+
+import _ "example.com/r"
+
+-- r/use.go --
+package use
+
+import _ "rsc.io/quote"
diff --git a/src/cmd/go/testdata/script/mod_tidy_oldgo.txt b/src/cmd/go/testdata/script/mod_tidy_oldgo.txt
new file mode 100644
index 0000000..0e88b06
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_oldgo.txt
@@ -0,0 +1,21 @@
+# Modules were introduced in Go 1.11, but for various reasons users may
+# decide to declare a (much!) older go version in their go.mod file.
+# Modules with very old versions should not be rejected, and should have
+# the same module-graph semantics as in Go 1.11.
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod go.mod.orig
+
+-- go.mod --
+module example.com/legacy/go1
+
+go 1.0
+
+require golang.org/x/text v0.3.0
+-- main.go --
+package main
+
+import _ "golang.org/x/text/language"
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/mod_tidy_quote.txt b/src/cmd/go/testdata/script/mod_tidy_quote.txt
new file mode 100644
index 0000000..423c7c2
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_quote.txt
@@ -0,0 +1,26 @@
+# Check that mod tidy does not introduce repeated
+# require statements when input go.mod has quoted requirements.
+env GO111MODULE=on
+
+go mod tidy
+grep -count=1 rsc.io/quote go.mod
+
+cp go.mod2 go.mod
+go mod tidy
+grep -count=1 rsc.io/quote go.mod
+
+
+-- go.mod --
+module x
+
+-- x.go --
+package x
+import "rsc.io/quote"
+func main() { _ = quote.Hello }
+
+-- go.mod2 --
+module x
+require (
+	"rsc.io/sampler" v1.3.0
+	"rsc.io/quote" v1.5.2
+)
diff --git a/src/cmd/go/testdata/script/mod_tidy_replace.txt b/src/cmd/go/testdata/script/mod_tidy_replace.txt
new file mode 100644
index 0000000..274f3bf
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_replace.txt
@@ -0,0 +1,145 @@
+env GO111MODULE=on
+env GOFLAGS=-mod=mod
+[short] skip
+
+# golang.org/issue/30166: 'go mod tidy' should not crash if a replaced module is
+# involved in a cycle.
+cd cycle
+env GOTRACEBACK=off
+go mod tidy
+cd ..
+
+# From inside the module, 'go list -m all' should NOT include transitive
+# requirements of modules that have been replaced.
+go list -m all
+stdout 'rsc.io/quote/v3 v3.0.0'
+! stdout 'rsc.io/sampler'
+! stdout 'golang.org/x/text'
+
+# From outside the module, 'go list -m all' should include them.
+cd outside
+go list -m all
+stdout 'rsc.io/quote/v3 v3.0.0'
+stdout 'rsc.io/sampler v1.3.0'
+stdout 'golang.org/x/text'
+cd ..
+
+# 'go list all' should add indirect requirements to satisfy the packages
+# imported from replacement modules.
+! grep 'rsc.io/sampler' go.mod
+! grep 'golang.org/x/text' go.mod
+go list all
+grep 'rsc.io/sampler' go.mod
+grep 'golang.org/x/text' go.mod
+
+# 'go get' and 'go mod tidy' should follow the requirements of the replacements,
+# not the originals, even if that results in a set of versions that are
+# misleading or redundant without those replacements.
+go get rsc.io/sampler@v1.2.0
+go mod tidy
+go list -m all
+stdout 'rsc.io/quote/v3 v3.0.0'
+stdout 'rsc.io/sampler v1.2.0'
+stdout 'golang.org/x/text'
+
+# The requirements seen from outside may be higher (or lower)
+# than those seen from within the module.
+grep 'rsc.io/sampler v1.2.0' go.mod
+cd outside
+go list -m all
+stdout 'rsc.io/sampler v1.3.0'
+cd ..
+
+# The same module can't be used as two different paths.
+cd multiple-paths
+! go mod tidy
+stderr 'rsc.io/quote/v3@v3.0.0 used for two different module paths \(not-rsc.io/quote/v3 and rsc.io/quote/v3\)'
+
+-- go.mod --
+module example.com/tidy
+
+require rsc.io/quote/v3 v3.0.0
+replace rsc.io/quote/v3 => ./not-rsc.io/quote/v3
+
+-- imports.go --
+package tidy
+
+import _ "rsc.io/quote/v3"
+
+-- outside/go.mod --
+module example.com/tidy/outside
+
+require example.com/tidy v0.0.0
+replace example.com/tidy => ./..
+
+-- not-rsc.io/quote/v3/go.mod --
+module not-rsc.io/quote/v3
+
+// No requirements specified!
+
+-- not-rsc.io/quote/v3/quote.go --
+package quote
+
+import (
+	_ "rsc.io/sampler"
+	_ "golang.org/x/text/language"
+)
+
+-- cycle/go.mod --
+module golang.org/issue/30166
+
+require (
+	golang.org/issue/30166/a v0.0.0
+	golang.org/issue/30166/b v0.0.0
+)
+
+replace (
+	golang.org/issue/30166/a => ./a
+	golang.org/issue/30166/b => ./b
+)
+-- cycle/cycle.go --
+package cycle
+
+import (
+	_ "golang.org/issue/30166/a"
+	_ "golang.org/issue/30166/b"
+)
+-- cycle/a/a.go --
+package a
+-- cycle/a/go.mod --
+module golang.org/issue/30166/a
+
+require golang.org/issue/30166/b v0.0.0
+-- cycle/b/b.go --
+package b
+-- cycle/b/go.mod --
+module golang.org/issue/30166/b
+
+require golang.org/issue/30166/a v0.0.0
+-- multiple-paths/main.go --
+package main
+
+import (
+	"fmt"
+	"rsc.io/quote/v3"
+)
+
+func main() {
+	fmt.Println(quote.GoV3())
+}
+-- multiple-paths/go.mod --
+module quoter
+
+require (
+	rsc.io/quote/v3 v3.0.0
+	not-rsc.io/quote/v3 v3.0.0
+)
+
+replace not-rsc.io/quote/v3 => rsc.io/quote/v3 v3.0.0
+-- multiple-paths/use.go --
+package quoter
+
+import (
+	_ "not-rsc.io/quote/v3"
+	_ "rsc.io/quote/v3"
+)
diff --git a/src/cmd/go/testdata/script/mod_tidy_replace_old.txt b/src/cmd/go/testdata/script/mod_tidy_replace_old.txt
new file mode 100644
index 0000000..18605b1
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_replace_old.txt
@@ -0,0 +1,34 @@
+# Regression test for https://golang.org/issue/46659.
+#
+# If a 'replace' directive specifies an older-than-selected version of a module,
+# 'go mod tidy' shouldn't try to add that version to the build list to resolve a
+# missing package: it won't be selected, and would cause the module loader to
+# loop indefinitely trying to resolve the package.
+
+cp go.mod go.mod.orig
+
+! go mod tidy
+! stderr panic
+stderr '^go: golang\.org/issue46659 imports\n\texample\.com/missingpkg/deprecated: package example\.com/missingpkg/deprecated provided by example\.com/missingpkg at latest version v1\.0\.0 but not at required version v1\.0\.1-beta$'
+
+go mod tidy -e
+
+cmp go.mod go.mod.orig
+
+-- go.mod --
+module golang.org/issue46659
+
+go 1.17
+
+replace example.com/missingpkg v1.0.1-alpha => example.com/missingpkg v1.0.0
+
+require example.com/usemissingpre v1.0.0
+
+require example.com/missingpkg v1.0.1-beta // indirect
+-- m.go --
+package m
+
+import (
+	_ "example.com/missingpkg/deprecated"
+	_ "example.com/usemissingpre"
+)
diff --git a/src/cmd/go/testdata/script/mod_tidy_sum.txt b/src/cmd/go/testdata/script/mod_tidy_sum.txt
new file mode 100644
index 0000000..5a15818
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_sum.txt
@@ -0,0 +1,33 @@
+env GO111MODULE=on
+
+# go.sum should list directly used modules and dependencies
+go get rsc.io/quote@v1.5.2
+go mod tidy
+grep rsc.io/sampler go.sum
+
+# go.sum should not normally lose old entries
+go get rsc.io/quote@v1.0.0
+grep 'rsc.io/quote v1.0.0' go.sum
+grep 'rsc.io/quote v1.5.2' go.sum
+grep rsc.io/sampler go.sum
+
+# go mod tidy should clear dead entries from go.sum
+go mod tidy
+grep 'rsc.io/quote v1.0.0' go.sum
+! grep 'rsc.io/quote v1.5.2' go.sum
+! grep rsc.io/sampler go.sum
+
+# go.sum with no entries is OK to keep
+# (better for version control not to delete and recreate.)
+cp x.go.noimports x.go
+go mod tidy
+exists go.sum
+! grep . go.sum
+
+-- go.mod --
+module x
+-- x.go --
+package x
+import _ "rsc.io/quote"
+-- x.go.noimports --
+package x
diff --git a/src/cmd/go/testdata/script/mod_tidy_support_buildx.txt b/src/cmd/go/testdata/script/mod_tidy_support_buildx.txt
new file mode 100644
index 0000000..d2135e1
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_support_buildx.txt
@@ -0,0 +1,18 @@
+# This test checks that "go mod tidy -x" print
+# commands tidy executes.
+# Verifies golang.org/issue/35849
+
+rm $GOPATH/pkg/mod/cache/download/rsc.io/quote
+go mod tidy
+! stderr 'get '$GOPROXY
+
+rm $GOPATH/pkg/mod/cache/download/rsc.io/quote
+go mod tidy -x
+stderr 'get '$GOPROXY
+
+-- go.mod --
+module example.com/mod
+
+-- a.go --
+package mod
+import _ "rsc.io/quote"
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/mod_tidy_symlink_issue35941.txt b/src/cmd/go/testdata/script/mod_tidy_symlink_issue35941.txt
new file mode 100644
index 0000000..d4658c6
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_symlink_issue35941.txt
@@ -0,0 +1,36 @@
+env GO111MODULE=on
+[!symlink] skip
+
+cd m
+symlink symlink -> ../outside
+
+cp go.mod go.mod.orig
+
+# Issue 35941: suppress symlink warnings when running 'go mod tidy'.
+# 'go mod tidy' should not scan packages in symlinked subdirectories.
+go mod tidy
+! stderr 'warning: ignoring symlink'
+cmp go.mod go.mod.orig
+
+! go build ./symlink
+stderr '^symlink[\\/]symlink.go:3:8: module example.net/unresolved provides package example.net/unresolved and is replaced but not required; to add it:\n\tgo get example.net/unresolved@v0.1.0$'
+
+-- m/go.mod --
+module example.net/m
+
+go 1.16
+
+replace example.net/unresolved v0.1.0 => ../unresolved
+-- m/a.go --
+package a
+-- outside/symlink.go --
+package symlink
+
+import _ "example.net/unresolved"
+-- unresolved/go.mod --
+module example.net/unresolved
+
+go 1.16
+-- unresolved/unresolved.go --
+// Package unresolved exists, but 'go mod tidy' won't add it.
+package unresolved
diff --git a/src/cmd/go/testdata/script/mod_tidy_temp.txt b/src/cmd/go/testdata/script/mod_tidy_temp.txt
new file mode 100644
index 0000000..8a2fa4e
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_temp.txt
@@ -0,0 +1,26 @@
+# Regression test for https://go.dev/issue/51992
+
+# 'go mod tidy' should error instead of throwing panic in the situation below.
+# 1. /tmp/go.mod exists
+# 2. run 'go mod tidy' in /tmp or in the child directory not having go.mod.
+
+[GOOS:plan9] stop  # Plan 9 has no $TMPDIR variable to set.
+
+env GOROOT=$TESTGO_GOROOT
+env TMP=$WORK
+env TMPDIR=$WORK
+mkdir $WORK/child
+
+! go mod tidy
+! stdout .
+stderr 'go: go.mod file not found in current directory or any parent directory'
+
+cd $WORK/child
+! go mod tidy
+! stdout .
+stderr 'go: go.mod file not found in current directory or any parent directory'
+
+-- $WORK/go.mod --
+module issue51992
+
+go 1.18
diff --git a/src/cmd/go/testdata/script/mod_tidy_version.txt b/src/cmd/go/testdata/script/mod_tidy_version.txt
new file mode 100644
index 0000000..e3f2561
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_version.txt
@@ -0,0 +1,394 @@
+# https://golang.org/issue/45094: 'go mod tidy' now accepts a '-go' flag
+# to change the language version in use.
+#
+# The package import graph used in this test looks like:
+#
+# m --- a --- b
+#             |
+#             b_test --- c
+#                        |
+#                        c_test --- d
+#
+# The module diagram looks like:
+#
+# m --- a --- b
+# |
+# + --- c
+# |
+# + --- d
+#
+# Module b omits its dependency on c, and module c omits its dependency on d.
+#
+# In go 1.15, the tidy main module must require a (because it is direct),
+# c (because it is a missing test dependency of an imported package),
+# and d (because it is a missing transitive test dependency).
+#
+# In go 1.16, the tidy main module can omit d because it is no longer
+# included in "all".
+#
+# In go 1.17, the main module must explicitly require b
+# (because it is transitively imported by the main module).
+
+cp go.mod go.mod.orig
+
+	# Pretend we're a release version so that we can theoretically
+	# write our version in toolchain lines.
+env goversion=1.99.0
+env TESTGO_VERSION=go${goversion}
+
+# An invalid argument should be rejected.
+
+! go mod tidy -go=bananas
+stderr '^invalid value "bananas" for flag -go: expecting a Go version like "'$goversion'"$'
+cmp go.mod go.mod.orig
+
+! go mod tidy -go=0.9
+stderr '^invalid value "0.9" for flag -go: expecting a Go version like "'$goversion'"$'
+
+! go mod tidy -go=2000.0
+stderr '^invalid value "2000.0" for flag -go: maximum supported Go version is '$goversion'$'
+
+
+# Supported versions should change the go.mod file to be tidy according to the
+# indicated version.
+
+go mod tidy -go=1.15
+cmp go.mod go.mod.115
+
+go mod tidy
+cmp go.mod go.mod.115
+
+
+go mod tidy -go=1.16
+cmp go.mod go.mod.116
+
+go mod tidy
+cmp go.mod go.mod.116
+
+
+go mod tidy -go=1.17
+cmp go.mod go.mod.117
+
+go mod tidy
+cmp go.mod go.mod.117
+
+
+# If we downgrade back to 1.15, we should re-resolve d to v0.2.0 instead
+# of the original v0.1.0 (because the original requirement is lost).
+
+go mod tidy -go=1.15
+cmp go.mod go.mod.115-2
+
+
+# -go= (with an empty argument) maintains the existing version or adds the
+#  default version (just like omitting the flag).
+
+go mod tidy -go=''
+cmp go.mod go.mod.115-2
+
+cp go.mod.orig go.mod
+go mod tidy -go=''
+cmpenv go.mod go.mod.latest
+
+# Repeat with go get go@ instead of mod tidy.
+
+# Go 1.16 -> 1.17 should be a no-op.
+cp go.mod.116 go.mod
+go get go@1.16
+cmp go.mod go.mod.116
+
+# Go 1.17 -> 1.16 should leave b (go get is not tidy).
+cp go.mod.117 go.mod
+go get go@1.16
+cmp go.mod go.mod.116from117
+
+# Go 1.15 -> 1.16 should leave d (go get is not tidy).
+cp go.mod.115 go.mod
+go get go@1.16
+cmp go.mod go.mod.116from115
+
+# Go 1.16 -> 1.17 should add b.
+cp go.mod.116 go.mod
+go get go@1.17
+stderr '^\tnote: expanded dependencies to upgrade to go 1.17 or higher; run ''go mod tidy'' to clean up'
+cmp go.mod go.mod.117
+
+# Go 1.16 -> 1.15 should add d,
+# but 'go get' doesn't load enough packages to know that.
+# (This leaves the module untidy, but the user can fix it by running 'go mod tidy'.)
+cp go.mod.116 go.mod
+go get go@1.15 toolchain@none
+cmp go.mod go.mod.115from116
+go mod tidy
+cmp go.mod go.mod.115-2
+
+# Updating the go line to 1.21 or higher also updates the toolchain line,
+# only if the toolchain is higher than what would be implied by the go line.
+
+cp go.mod.117 go.mod
+go mod tidy -go=$goversion
+cmpenv go.mod go.mod.latest
+
+cp go.mod.117 go.mod
+go mod tidy -go=1.21.0  # lower than $goversion
+cmpenv go.mod go.mod.121toolchain
+
+
+-- go.mod --
+module example.com/m
+
+require example.net/a v0.1.0
+
+require (
+	example.net/c v0.1.0 // indirect
+	example.net/d v0.1.0 // indirect
+)
+
+replace (
+	example.net/a v0.1.0 => ./a
+	example.net/a v0.2.0 => ./a
+	example.net/b v0.1.0 => ./b
+	example.net/b v0.2.0 => ./b
+	example.net/c v0.1.0 => ./c
+	example.net/c v0.2.0 => ./c
+	example.net/d v0.1.0 => ./d
+	example.net/d v0.2.0 => ./d
+)
+-- m.go --
+package m
+
+import _ "example.net/a"
+
+-- go.mod.115 --
+module example.com/m
+
+go 1.15
+
+require example.net/a v0.1.0
+
+require (
+	example.net/c v0.1.0 // indirect
+	example.net/d v0.1.0 // indirect
+)
+
+replace (
+	example.net/a v0.1.0 => ./a
+	example.net/a v0.2.0 => ./a
+	example.net/b v0.1.0 => ./b
+	example.net/b v0.2.0 => ./b
+	example.net/c v0.1.0 => ./c
+	example.net/c v0.2.0 => ./c
+	example.net/d v0.1.0 => ./d
+	example.net/d v0.2.0 => ./d
+)
+-- go.mod.115from116 --
+module example.com/m
+
+go 1.15
+
+require example.net/a v0.1.0
+
+require example.net/c v0.1.0 // indirect
+
+replace (
+	example.net/a v0.1.0 => ./a
+	example.net/a v0.2.0 => ./a
+	example.net/b v0.1.0 => ./b
+	example.net/b v0.2.0 => ./b
+	example.net/c v0.1.0 => ./c
+	example.net/c v0.2.0 => ./c
+	example.net/d v0.1.0 => ./d
+	example.net/d v0.2.0 => ./d
+)
+-- go.mod.116from115 --
+module example.com/m
+
+go 1.16
+
+require example.net/a v0.1.0
+
+require (
+	example.net/c v0.1.0 // indirect
+	example.net/d v0.1.0 // indirect
+)
+
+replace (
+	example.net/a v0.1.0 => ./a
+	example.net/a v0.2.0 => ./a
+	example.net/b v0.1.0 => ./b
+	example.net/b v0.2.0 => ./b
+	example.net/c v0.1.0 => ./c
+	example.net/c v0.2.0 => ./c
+	example.net/d v0.1.0 => ./d
+	example.net/d v0.2.0 => ./d
+)
+-- go.mod.115-2 --
+module example.com/m
+
+go 1.15
+
+require example.net/a v0.1.0
+
+require (
+	example.net/c v0.1.0 // indirect
+	example.net/d v0.2.0 // indirect
+)
+
+replace (
+	example.net/a v0.1.0 => ./a
+	example.net/a v0.2.0 => ./a
+	example.net/b v0.1.0 => ./b
+	example.net/b v0.2.0 => ./b
+	example.net/c v0.1.0 => ./c
+	example.net/c v0.2.0 => ./c
+	example.net/d v0.1.0 => ./d
+	example.net/d v0.2.0 => ./d
+)
+-- go.mod.116 --
+module example.com/m
+
+go 1.16
+
+require example.net/a v0.1.0
+
+require example.net/c v0.1.0 // indirect
+
+replace (
+	example.net/a v0.1.0 => ./a
+	example.net/a v0.2.0 => ./a
+	example.net/b v0.1.0 => ./b
+	example.net/b v0.2.0 => ./b
+	example.net/c v0.1.0 => ./c
+	example.net/c v0.2.0 => ./c
+	example.net/d v0.1.0 => ./d
+	example.net/d v0.2.0 => ./d
+)
+-- go.mod.117 --
+module example.com/m
+
+go 1.17
+
+require example.net/a v0.1.0
+
+require (
+	example.net/b v0.1.0 // indirect
+	example.net/c v0.1.0 // indirect
+)
+
+replace (
+	example.net/a v0.1.0 => ./a
+	example.net/a v0.2.0 => ./a
+	example.net/b v0.1.0 => ./b
+	example.net/b v0.2.0 => ./b
+	example.net/c v0.1.0 => ./c
+	example.net/c v0.2.0 => ./c
+	example.net/d v0.1.0 => ./d
+	example.net/d v0.2.0 => ./d
+)
+-- go.mod.116from117 --
+module example.com/m
+
+go 1.16
+
+require example.net/a v0.1.0
+
+require (
+	example.net/b v0.1.0 // indirect
+	example.net/c v0.1.0 // indirect
+)
+
+replace (
+	example.net/a v0.1.0 => ./a
+	example.net/a v0.2.0 => ./a
+	example.net/b v0.1.0 => ./b
+	example.net/b v0.2.0 => ./b
+	example.net/c v0.1.0 => ./c
+	example.net/c v0.2.0 => ./c
+	example.net/d v0.1.0 => ./d
+	example.net/d v0.2.0 => ./d
+)
+-- go.mod.latest --
+module example.com/m
+
+go $goversion
+
+require example.net/a v0.1.0
+
+require (
+	example.net/b v0.1.0 // indirect
+	example.net/c v0.1.0 // indirect
+)
+
+replace (
+	example.net/a v0.1.0 => ./a
+	example.net/a v0.2.0 => ./a
+	example.net/b v0.1.0 => ./b
+	example.net/b v0.2.0 => ./b
+	example.net/c v0.1.0 => ./c
+	example.net/c v0.2.0 => ./c
+	example.net/d v0.1.0 => ./d
+	example.net/d v0.2.0 => ./d
+)
+-- go.mod.121toolchain --
+module example.com/m
+
+go 1.21.0
+
+toolchain $TESTGO_VERSION
+
+require example.net/a v0.1.0
+
+require (
+	example.net/b v0.1.0 // indirect
+	example.net/c v0.1.0 // indirect
+)
+
+replace (
+	example.net/a v0.1.0 => ./a
+	example.net/a v0.2.0 => ./a
+	example.net/b v0.1.0 => ./b
+	example.net/b v0.2.0 => ./b
+	example.net/c v0.1.0 => ./c
+	example.net/c v0.2.0 => ./c
+	example.net/d v0.1.0 => ./d
+	example.net/d v0.2.0 => ./d
+)
+-- a/go.mod --
+module example.net/a
+
+go 1.15
+
+require example.net/b v0.1.0
+-- a/a.go --
+package a
+
+import _ "example.net/b"
+
+-- b/go.mod --
+module example.net/b
+
+go 1.15
+-- b/b.go --
+package b
+-- b/b_test.go --
+package b_test
+
+import _ "example.net/c"
+
+-- c/go.mod --
+module example.net/c
+
+go 1.15
+-- c/c.go --
+package c
+-- c/c_test.go --
+package c_test
+
+import _ "example.net/d"
+
+-- d/go.mod --
+module example.net/d
+
+go 1.15
+-- d/d.go --
+package d
diff --git a/src/cmd/go/testdata/script/mod_tidy_version_tooold.txt b/src/cmd/go/testdata/script/mod_tidy_version_tooold.txt
new file mode 100644
index 0000000..713ef1a
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_tidy_version_tooold.txt
@@ -0,0 +1,23 @@
+env TESTGO_VERSION=go1.22.0
+
+! go mod tidy -go=1.21
+stderr '^go: example.net/a@v0.1.0 requires go@1.22, but 1.21 is requested$'
+
+-- go.mod --
+module example
+
+go 1.22
+
+require example.net/a v0.1.0
+
+replace example.net/a v0.1.0 => ./a
+-- example.go --
+package example
+
+import "example.net/a"
+-- a/go.mod --
+module example.net/a
+
+go 1.22
+-- a/a.go --
+package a
diff --git a/src/cmd/go/testdata/script/mod_toolchain.txt b/src/cmd/go/testdata/script/mod_toolchain.txt
new file mode 100644
index 0000000..c771cae
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_toolchain.txt
@@ -0,0 +1,76 @@
+env TESTGO_VERSION=go1.100.0
+env TESTGO_VERSION_SWITCH=switch
+
+go get toolchain@go1.22.1
+stderr '^go: added toolchain go1.22.1$'
+! stderr '(added|removed|upgraded|downgraded) go'
+grep 'toolchain go1.22.1' go.mod
+
+go get toolchain@none
+stderr '^go: removed toolchain go1.22.1$'
+! stderr '(added|removed|upgraded|downgraded) go'
+! grep toolchain go.mod
+
+go get toolchain@go1.22.1
+stderr '^go: added toolchain go1.22.1$'
+! stderr '(added|removed|upgraded|downgraded) go'
+grep 'toolchain go1.22.1' go.mod
+
+go get go@1.22.3
+stderr '^go: upgraded go 1.10 => 1.22.3$'
+stderr '^go: upgraded toolchain go1.22.1 => go1.100.0$'
+grep 'go 1.22.3' go.mod
+
+go get go@1.22.3 toolchain@1.22.3
+stderr '^go: removed toolchain go1.100.0$'
+! grep toolchain go.mod
+
+go get go@1.22.1 toolchain@go1.22.3
+stderr '^go: downgraded go 1.22.3 => 1.22.1$'
+stderr '^go: added toolchain go1.22.3$'
+grep 'go 1.22.1' go.mod
+grep 'toolchain go1.22.3' go.mod
+
+go get go@1.22.3 toolchain@1.22.3
+stderr '^go: upgraded go 1.22.1 => 1.22.3$'
+stderr '^go: removed toolchain go1.22.3$'
+grep 'go 1.22.3' go.mod
+! grep toolchain go.mod
+
+go get toolchain@1.22.1
+stderr '^go: downgraded go 1.22.3 => 1.22.1$'
+! stderr toolchain # already gone, was not added
+grep 'go 1.22.1' go.mod
+! grep toolchain go.mod
+
+env TESTGO_VERSION=go1.22.1
+env GOTOOLCHAIN=local
+! go get go@1.22.3
+stderr 'go: updating go.mod requires go >= 1.22.3 \(running go 1.22.1; GOTOOLCHAIN=local\)$'
+
+env TESTGO_VERSION=go1.30
+go get toolchain@1.22.3
+grep 'toolchain go1.22.3' go.mod
+
+go get go@1.22.1
+grep 'go 1.22.1' go.mod
+go get m2@v1.0.0
+stderr '^go: upgraded go 1.22.1 => 1.23$'
+stderr '^go: added m2 v1.0.0$'
+grep 'go 1.23$' go.mod
+
+go get toolchain@go1.23.9 go@1.23.5
+go get toolchain@none
+stderr '^go: removed toolchain go1.23.9'
+! stderr ' go 1'
+grep 'go 1.23.5' go.mod
+
+-- go.mod --
+module m
+go 1.10
+
+replace m2 v1.0.0 => ./m2
+
+-- m2/go.mod --
+module m2
+go 1.23
diff --git a/src/cmd/go/testdata/script/mod_toolchain_slash.txt b/src/cmd/go/testdata/script/mod_toolchain_slash.txt
new file mode 100644
index 0000000..bb1f770
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_toolchain_slash.txt
@@ -0,0 +1,32 @@
+[!exec:/bin/sh] skip
+
+chmod 0777 go1.999999-/run.sh
+chmod 0777 run.sh
+
+! go list all
+! stdout 'RAN SCRIPT'
+
+cd subdir
+! go list all
+! stdout 'RAN SCRIPT'
+
+-- go.mod --
+module exploit
+
+go 1.21
+toolchain go1.999999-/run.sh
+-- go1.999999-/run.sh --
+#!/bin/sh
+printf 'RAN SCRIPT\n'
+exit 1
+-- run.sh --
+#!/bin/sh
+printf 'RAN SCRIPT\n'
+exit 1
+-- subdir/go.mod --
+module exploit
+
+go 1.21
+toolchain go1.999999-/../../run.sh
+-- subdir/go1.999999-/README.txt --
+heh heh heh
diff --git a/src/cmd/go/testdata/script/mod_update_sum_readonly.txt b/src/cmd/go/testdata/script/mod_update_sum_readonly.txt
new file mode 100644
index 0000000..41f12e4
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_update_sum_readonly.txt
@@ -0,0 +1,34 @@
+# When finding the latest version of a module, we should not download version
+# contents. Previously, we downloaded .zip files to determine whether a real
+# .mod file was present in order to decide whether +incompatible versions
+# could be "latest".
+#
+# Verifies #47377.
+
+# rsc.io/breaker has two versions, neither of which has a .mod file.
+go list -m -versions rsc.io/breaker
+stdout '^rsc.io/breaker v1.0.0 v2.0.0\+incompatible$'
+go mod download rsc.io/breaker@v1.0.0
+! grep '^go' $GOPATH/pkg/mod/cache/download/rsc.io/breaker/@v/v1.0.0.mod
+go mod download rsc.io/breaker@v2.0.0+incompatible
+! grep '^go' $GOPATH/pkg/mod/cache/download/rsc.io/breaker/@v/v2.0.0+incompatible.mod
+
+# Delete downloaded .zip files.
+go clean -modcache
+
+# Check for updates.
+go list -m -u rsc.io/breaker
+stdout '^rsc.io/breaker v1.0.0 \[v2.0.0\+incompatible\]$'
+
+# We should not have downloaded zips.
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/breaker/@v/v1.0.0.zip
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/breaker/@v/v2.0.0+incompatible.zip
+
+-- go.mod --
+module m
+
+go 1.16
+
+require rsc.io/breaker v1.0.0
+-- go.sum --
+rsc.io/breaker v1.0.0/go.mod h1:s5yxDXvD88U1/ESC23I2FK3Lkv4YIKaB1ij/Hbm805g=
diff --git a/src/cmd/go/testdata/script/mod_upgrade_patch.txt b/src/cmd/go/testdata/script/mod_upgrade_patch.txt
new file mode 100644
index 0000000..79a9808
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_upgrade_patch.txt
@@ -0,0 +1,110 @@
+env GO111MODULE=on
+[short] skip
+
+# Initially, we are at v1.0.0 for all dependencies.
+go get
+cp go.mod go.mod.orig
+go list -m all
+stdout '^patch.example.com/direct v1.0.0'
+stdout '^patch.example.com/indirect v1.0.0'
+! stdout '^patch.example.com/depofdirectpatch'
+
+# @patch should be rejected for modules not already in the build list.
+! go get patch.example.com/depofdirectpatch@patch
+stderr '^go: can''t query version "patch" of module patch.example.com/depofdirectpatch: no existing version is required$'
+cmp go.mod.orig go.mod
+
+# get -u=patch, with no arguments, should patch-update all dependencies
+# of the package in the current directory, pulling in transitive dependencies
+# and also patching those.
+cp go.mod.orig go.mod
+go get -u=patch
+go list -m all
+stdout '^patch.example.com/direct v1.0.1'
+stdout '^patch.example.com/indirect v1.0.1'
+stdout '^patch.example.com/depofdirectpatch v1.0.0'
+
+# 'get all@patch' should patch the modules that provide packages in 'all'.
+cp go.mod.orig go.mod
+go get all@patch
+go list -m all
+stdout '^patch.example.com/direct v1.0.1'
+stdout '^patch.example.com/indirect v1.0.1'
+stdout '^patch.example.com/depofdirectpatch v1.0.0'
+
+# ...but 'all@patch' should fail if any of the affected modules do not already
+# have a selected version.
+cp go.mod.orig go.mod
+go mod edit -droprequire=patch.example.com/direct
+cp go.mod go.mod.dropped
+! go get all@patch
+stderr '^go: all@patch: can''t query version "patch" of module patch.example.com/direct: no existing version is required$'
+cmp go.mod.dropped go.mod
+
+# Requesting the direct dependency with -u=patch but without an explicit version
+# should patch-update it and its dependencies.
+cp go.mod.orig go.mod
+go get -u=patch patch.example.com/direct
+go list -m all
+stdout '^patch.example.com/direct v1.0.1'
+stdout '^patch.example.com/indirect v1.0.1'
+stdout '^patch.example.com/depofdirectpatch v1.0.0'
+
+# Requesting only the indirect dependency should not update the direct one.
+cp go.mod.orig go.mod
+go get -u=patch patch.example.com/indirect
+go list -m all
+stdout '^patch.example.com/direct v1.0.0'
+stdout '^patch.example.com/indirect v1.0.1'
+! stdout '^patch.example.com/depofdirectpatch'
+
+# @patch should apply only to the specific module,
+# but the result must reflect its upgraded requirements.
+cp go.mod.orig go.mod
+go get patch.example.com/direct@patch
+go list -m all
+stdout '^patch.example.com/direct v1.0.1'
+stdout '^patch.example.com/indirect v1.0.0'
+stdout '^patch.example.com/depofdirectpatch v1.0.0'
+
+# An explicit @patch should override a general -u.
+cp go.mod.orig go.mod
+go get -u patch.example.com/direct@patch
+go list -m all
+stdout '^patch.example.com/direct v1.0.1'
+stdout '^patch.example.com/indirect v1.1.0'
+stdout '^patch.example.com/depofdirectpatch v1.0.0'
+
+# An explicit @latest should override a general -u=patch.
+cp go.mod.orig go.mod
+go get -u=patch patch.example.com/direct@latest
+go list -m all
+stdout '^patch.example.com/direct v1.1.0'
+stdout '^patch.example.com/indirect v1.0.1'
+! stdout '^patch.example.com/depofdirectpatch'
+
+# Standard library packages cannot be upgraded explicitly.
+cp go.mod.orig go.mod
+! go get cmd/vet@patch
+stderr 'go: can''t request explicit version "patch" of standard library package cmd/vet$'
+
+# However, standard-library packages without explicit versions are fine.
+go get -u=patch cmd/go
+
+# We can upgrade to a new version of a module with no root package.
+go get example.com/noroot@v1.0.0
+go list -m all
+stdout '^example.com/noroot v1.0.0$'
+go get example.com/noroot@patch
+go list -m all
+stdout '^example.com/noroot v1.0.1$'
+
+
+-- go.mod --
+module x
+
+require patch.example.com/direct v1.0.0
+
+-- main.go --
+package x
+import _ "patch.example.com/direct"
diff --git a/src/cmd/go/testdata/script/mod_vcs_missing.txt b/src/cmd/go/testdata/script/mod_vcs_missing.txt
new file mode 100644
index 0000000..7f63e9d
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_vcs_missing.txt
@@ -0,0 +1,28 @@
+[exec:bzr] skip 'tests NOT having bzr'
+[!net:launchpad.net] skip
+
+env GO111MODULE=on
+env GOPROXY=direct
+
+cd empty
+! go get launchpad.net/gocheck
+stderr '"bzr": executable file not found'
+cd ..
+
+# 1.11 used to give the cryptic error "cannot find module for path" here, but
+# only for a main package.
+cd main
+! go build -mod=mod
+stderr '"bzr": executable file not found'
+cd ..
+
+-- empty/go.mod --
+module m
+-- main/go.mod --
+module m
+-- main/main.go --
+package main
+
+import _ "launchpad.net/gocheck"
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/mod_vendor.txt b/src/cmd/go/testdata/script/mod_vendor.txt
new file mode 100644
index 0000000..b02341d
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_vendor.txt
@@ -0,0 +1,354 @@
+env GO111MODULE=on
+
+# Without vendoring, a build should succeed unless -mod=vendor is set.
+[!short] go build
+[!short] ! go build -mod=vendor
+
+# Without vendoring, 'go list' should report the replacement directory for
+# a package in a replaced module.
+go list -f {{.Dir}} x
+stdout 'src[\\/]x'
+
+# 'go mod vendor' should copy all replaced modules to the vendor directory.
+go mod vendor -v
+stderr '^# x v1.0.0 => ./x'
+stderr '^x'
+stderr '^# y v1.0.0 => ./y'
+stderr '^y'
+stderr '^# z v1.0.0 => ./z'
+stderr '^z'
+! stderr '^w'
+grep 'a/foo/bar/b\na/foo/bar/c' vendor/modules.txt # must be sorted
+
+# An explicit '-mod=mod' should ignore the vendor directory.
+go list -mod=mod -f {{.Dir}} x
+stdout 'src[\\/]x'
+
+go list -mod=mod -f {{.Dir}} -m x
+stdout 'src[\\/]x'
+
+# An explicit '-mod=vendor' should report package directories within
+# the vendor directory.
+go list -mod=vendor -f {{.Dir}} x
+stdout 'src[\\/]vendor[\\/]x'
+
+# 'go list -mod=vendor -m' should successfully list vendored modules,
+# but should not provide a module directory because no directory contains
+# the complete module.
+go list -mod=vendor -f '{{.Version}} {{.Dir}}' -m x
+stdout '^v1.0.0 $'
+
+# -mod=vendor should cause 'go list' flags that look up versions to fail.
+! go list -mod=vendor -versions -m x
+stderr '^go: can''t determine available versions using the vendor directory\n\t\(Use -mod=mod or -mod=readonly to bypass.\)$'
+! go list -mod=vendor -u -m x
+stderr '^go: can''t determine available upgrades using the vendor directory\n\t\(Use -mod=mod or -mod=readonly to bypass.\)$'
+
+# 'go list -mod=vendor -m' on a transitive dependency that does not
+# provide vendored packages should give a helpful error rather than
+# 'not a known dependency'.
+! go list -mod=vendor -f '{{.Version}} {{.Dir}}' -m diamondright
+stderr 'go: module diamondright: can''t resolve module using the vendor directory\n\t\(Use -mod=mod or -mod=readonly to bypass.\)'
+
+# 'go list -mod=mod' should report packages outside the import graph,
+# but 'go list -mod=vendor' should error out for them.
+go list -mod=mod -f {{.Dir}} w
+stdout 'src[\\/]w'
+! go list -mod=vendor -f {{.Dir}} w
+stderr 'package w is not in std'
+
+go list -mod=mod -f {{.Dir}} diamondright
+stdout 'src[\\/]diamondright'
+
+# Test dependencies should not be copied.
+! exists vendor/x/testdata
+! exists vendor/a/foo/bar/b/ignored.go
+! exists vendor/a/foo/bar/b/main_test.go
+
+# Licenses and other metadata for each module should be copied
+# if any package within their module is copied.
+exists vendor/a/foo/AUTHORS.txt
+exists vendor/a/foo/CONTRIBUTORS
+exists vendor/a/foo/LICENSE
+exists vendor/a/foo/PATENTS
+exists vendor/a/foo/COPYING
+exists vendor/a/foo/COPYLEFT
+exists vendor/x/NOTICE!
+exists vendor/mysite/myname/mypkg/LICENSE.txt
+
+! exists vendor/a/foo/licensed-to-kill
+! exists vendor/w
+! exists vendor/w/LICENSE
+! exists vendor/x/x2
+! exists vendor/x/x2/LICENSE
+
+# 'go mod vendor' should work with an alternative vendor directory if the -o flag is provided.
+go mod vendor -v -o alternative-vendor-dir
+exists alternative-vendor-dir/modules.txt
+exists alternative-vendor-dir/a/foo/LICENSE
+
+# 'go mod vendor' should interpret paths relative to the current working directory when the -o flag is provided.
+mkdir dir1
+mkdir dir2
+
+cd dir1
+go mod vendor -v -o relative-vendor-dir
+
+go mod vendor -v -o ../dir2/relative-vendor-dir
+
+cd ..
+exists dir1/relative-vendor-dir/modules.txt
+exists dir1/relative-vendor-dir/a/foo/LICENSE
+exists dir2/relative-vendor-dir/modules.txt
+exists dir2/relative-vendor-dir/a/foo/LICENSE
+
+# 'go mod vendor' should fall back to the default 'vendor' directory when an empty argument is passed to the -o flag
+# the same behavior should be exhibited both on the module root directory, as well as nested subdirectories
+
+go mod vendor -v -o ''
+exists vendor/modules.txt
+
+env GOFLAGS=-o=foo
+go mod vendor -v -o ''
+exists vendor/modules.txt
+env GOFLAGS=''
+
+mkdir -p nested/dir
+cd nested/dir
+go mod vendor -v -o ''
+! exists vendor/
+exists ../../vendor/modules.txt
+cd ../..
+
+# 'go mod vendor' should work with absolute paths as well
+go mod vendor -v -o $WORK/tmp/absolute-vendor-dir
+exists $WORK/tmp/absolute-vendor-dir/modules.txt
+
+[short] stop
+
+# 'go build' and 'go test' using vendored packages should succeed.
+go build -mod=mod
+go build -mod=vendor
+go test -mod=vendor . ./subdir
+go test -mod=vendor ./...
+go fmt -mod=vendor ./...
+
+-- go.mod --
+module m
+
+go 1.13
+
+require (
+	a v1.0.0
+	diamondroot v0.0.0
+	mysite/myname/mypkg v1.0.0
+	w v1.0.0 // indirect
+	x v1.0.0
+	y v1.0.0
+	z v1.0.0
+)
+
+replace (
+	a v1.0.0 => ./a
+	diamondleft => ./diamondleft
+	diamondpoint => ./diamondpoint
+	diamondright => ./diamondright
+	diamondroot => ./diamondroot
+	mysite/myname/mypkg v1.0.0 => ./mypkg
+	w v1.0.0 => ./w
+	x v1.0.0 => ./x
+	y v1.0.0 => ./y
+	z v1.0.0 => ./z
+)
+
+-- a/foo/AUTHORS.txt --
+-- a/foo/CONTRIBUTORS --
+-- a/foo/LICENSE --
+-- a/foo/PATENTS --
+-- a/foo/COPYING --
+-- a/foo/COPYLEFT --
+-- a/foo/licensed-to-kill --
+-- w/LICENSE --
+-- x/NOTICE! --
+-- x/x2/LICENSE --
+-- mypkg/LICENSE.txt --
+
+-- a/foo/bar/b/main.go --
+package b
+-- a/foo/bar/b/ignored.go --
+// This file is intended for use with "go run"; it isn't really part of the package.
+
+// +build ignore
+
+package main
+
+func main() {}
+-- a/foo/bar/b/main_test.go --
+package b
+
+import (
+	"os"
+	"testing"
+)
+
+func TestDir(t *testing.T) {
+	if _, err := os.Stat("../testdata/1"); err != nil {
+		t.Fatalf("testdata: %v", err)
+	}
+}
+-- a/foo/bar/c/main.go --
+package c
+import _ "a/foo/bar/b"
+-- a/foo/bar/c/main_test.go --
+package c
+
+import (
+	"os"
+	"testing"
+)
+
+func TestDir(t *testing.T) {
+	if _, err := os.Stat("../../../testdata/1"); err != nil {
+		t.Fatalf("testdata: %v", err)
+	}
+	if _, err := os.Stat("./testdata/1"); err != nil {
+		t.Fatalf("testdata: %v", err)
+	}
+}
+-- a/foo/bar/c/testdata/1 --
+-- a/foo/bar/testdata/1 --
+-- a/go.mod --
+module a
+-- a/main.go --
+package a
+-- a/main_test.go --
+package a
+
+import (
+	"os"
+	"testing"
+)
+
+func TestDir(t *testing.T) {
+	if _, err := os.Stat("./testdata/1"); err != nil {
+		t.Fatalf("testdata: %v", err)
+	}
+}
+-- a/testdata/1 --
+-- appengine.go --
+// +build appengine
+
+package m
+
+import _ "appengine"
+import _ "appengine/datastore"
+-- mypkg/go.mod --
+module me
+-- mypkg/mydir/d.go --
+package mydir
+-- subdir/v1_test.go --
+package m
+
+import _ "mysite/myname/mypkg/mydir"
+-- testdata1.go --
+package m
+
+import _ "a"
+-- testdata2.go --
+package m
+
+import _ "a/foo/bar/c"
+-- v1.go --
+package m
+
+import _ "x"
+-- v2.go --
+// +build abc
+
+package mMmMmMm
+
+import _ "y"
+-- v3.go --
+// +build !abc
+
+package m
+
+import _ "z"
+-- v4.go --
+// +build notmytag
+
+package m
+
+import _ "x/x1"
+-- importdiamond.go --
+package m
+
+import _ "diamondroot"
+-- w/go.mod --
+module w
+-- w/w.go --
+package w
+-- x/go.mod --
+module x
+-- x/testdata/x.txt --
+placeholder - want directory with no go files
+-- x/x.go --
+package x
+-- x/x1/x1.go --
+// +build notmytag
+
+package x1
+-- x/x2/dummy.txt --
+dummy
+-- x/x_test.go --
+package x
+
+import _ "w"
+-- y/go.mod --
+module y
+-- y/y.go --
+package y
+-- z/go.mod --
+module z
+-- z/z.go --
+package z
+
+-- diamondroot/go.mod --
+module diamondroot
+
+require (
+	diamondleft v0.0.0
+	diamondright v0.0.0
+)
+-- diamondroot/x.go --
+package diamondroot
+
+import _ "diamondleft"
+-- diamondroot/unused/unused.go --
+package unused
+
+import _ "diamondright"
+-- diamondleft/go.mod --
+module diamondleft
+
+require (
+	diamondpoint v0.0.0
+)
+-- diamondleft/x.go --
+package diamondleft
+
+import _ "diamondpoint"
+-- diamondright/go.mod --
+module diamondright
+
+require (
+	diamondpoint v0.0.0
+)
+-- diamondright/x.go --
+package diamondright
+
+import _ "diamondpoint"
+-- diamondpoint/go.mod --
+module diamondpoint
+-- diamondpoint/x.go --
+package diamondpoint
diff --git a/src/cmd/go/testdata/script/mod_vendor_auto.txt b/src/cmd/go/testdata/script/mod_vendor_auto.txt
new file mode 100644
index 0000000..2cafcda
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_vendor_auto.txt
@@ -0,0 +1,259 @@
+# Integration test for golang.org/issue/33848: automatically check and use vendored packages.
+
+env GO111MODULE=on
+
+[short] skip
+
+cd $WORK/auto
+cp go.mod go.mod.orig
+cp $WORK/modules-1.13.txt $WORK/auto/modules.txt
+
+# An explicit -mod=vendor should force use of the vendor directory.
+env GOFLAGS=-mod=vendor
+
+# Pass -e to permit an error: tools.go imports a main package
+# "example.com/printversion".
+# TODO(#59186): investigate why it didn't fail without -e.
+go list -f {{.Dir}} -tags tools -e all
+stdout '^'$WORK'[/\\]auto$'
+stdout '^'$WORK'[/\\]auto[/\\]vendor[/\\]example.com[/\\]printversion$'
+stdout '^'$WORK'[/\\]auto[/\\]vendor[/\\]example.com[/\\]version$'
+
+! go list -m all
+stderr 'go: can''t compute ''all'' using the vendor directory\n\t\(Use -mod=mod or -mod=readonly to bypass.\)'
+
+! go list -m -f '{{.Dir}}' all
+stderr 'go: can''t compute ''all'' using the vendor directory\n\t\(Use -mod=mod or -mod=readonly to bypass.\)'
+
+# An explicit -mod=mod should force the vendor directory to be ignored.
+env GOFLAGS=-mod=mod
+
+go list -f {{.Dir}} -tags tools -e all
+stdout '^'$WORK'[/\\]auto$'
+stdout '^'$GOPATH'[/\\]pkg[/\\]mod[/\\]example.com[/\\]printversion@v1.0.0$'
+stdout '^'$WORK'[/\\]auto[/\\]replacement-version$'
+
+go list -m all
+stdout '^example.com/auto$'
+stdout 'example.com/printversion v1.0.0'
+stdout 'example.com/version v1.0.0'
+
+go list -m -f '{{.Dir}}' all
+stdout '^'$WORK'[/\\]auto$'
+stdout '^'$GOPATH'[/\\]pkg[/\\]mod[/\\]example.com[/\\]printversion@v1.0.0$'
+stdout '^'$WORK'[/\\]auto[/\\]replacement-version$'
+
+# If the main module's "go" directive says 1.13, we should default to -mod=mod.
+env GOFLAGS=
+go mod edit -go=1.13
+
+go list -f {{.Dir}} -tags tools -e all
+stdout '^'$WORK'[/\\]auto$'
+stdout '^'$GOPATH'[/\\]pkg[/\\]mod[/\\]example.com[/\\]printversion@v1.0.0$'
+stdout '^'$WORK'[/\\]auto[/\\]replacement-version$'
+
+go list -m -f '{{.Dir}}' all
+stdout '^'$WORK'[/\\]auto$'
+stdout '^'$GOPATH'[/\\]pkg[/\\]mod[/\\]example.com[/\\]printversion@v1.0.0$'
+stdout '^'$WORK'[/\\]auto[/\\]replacement-version$'
+
+# A 'go 1.14' directive in the main module's go.mod file should enable
+# -mod=vendor by default, along with stronger checks for consistency
+# between the go.mod file and vendor/modules.txt.
+# A 'go 1.13' vendor/modules.txt file is not usually sufficient
+# to pass those checks.
+go mod edit -go=1.14
+
+! go list -f {{.Dir}} -tags tools all
+stderr '^go: inconsistent vendoring in '$WORK[/\\]auto':$'
+stderr '^\texample.com/printversion@v1.0.0: is explicitly required in go.mod, but not marked as explicit in vendor/modules.txt'
+stderr '^\texample.com/unused: is replaced in go.mod, but not marked as replaced in vendor/modules.txt'
+stderr '^\texample.com/version@v1.2.0: is replaced in go.mod, but not marked as replaced in vendor/modules.txt'
+stderr '^\tTo ignore the vendor directory, use -mod=readonly or -mod=mod.\n\tTo sync the vendor directory, run:\n\t\tgo mod vendor$'
+
+# Module-specific subcommands should continue to load the full module graph.
+go mod graph
+stdout '^example.com/printversion@v1.0.0 example.com/version@v1.0.0$'
+
+# An explicit -mod=mod should still force the vendor directory to be ignored.
+env GOFLAGS=-mod=mod
+
+go list -f {{.Dir}} -tags tools -e all
+stdout '^'$WORK'[/\\]auto$'
+stdout '^'$GOPATH'[/\\]pkg[/\\]mod[/\\]example.com[/\\]printversion@v1.0.0$'
+stdout '^'$WORK'[/\\]auto[/\\]replacement-version$'
+
+go list -m all
+stdout '^example.com/auto$'
+stdout 'example.com/printversion v1.0.0'
+stdout 'example.com/version v1.0.0'
+
+go list -m -f '{{.Dir}}' all
+stdout '^'$WORK'[/\\]auto$'
+stdout '^'$GOPATH'[/\\]pkg[/\\]mod[/\\]example.com[/\\]printversion@v1.0.0$'
+stdout '^'$WORK'[/\\]auto[/\\]replacement-version$'
+
+# 'go mod vendor' should repair vendor/modules.txt so that the implicit
+# -mod=vendor works again.
+env GOFLAGS=
+
+go mod edit -go=1.14
+go mod vendor
+
+go list -f {{.Dir}} -tags tools -e all
+stdout '^'$WORK'[/\\]auto$'
+stdout '^'$WORK'[/\\]auto[/\\]vendor[/\\]example.com[/\\]printversion$'
+stdout '^'$WORK'[/\\]auto[/\\]vendor[/\\]example.com[/\\]version$'
+
+# ...but 'go list -m' should continue to fail, this time without
+# referring to a -mod default that the user didn't set.
+! go list -m all
+stderr 'go: can''t compute ''all'' using the vendor directory\n\t\(Use -mod=mod or -mod=readonly to bypass.\)'
+
+! go list -m -f '{{.Dir}}' all
+stderr 'go: can''t compute ''all'' using the vendor directory\n\t\(Use -mod=mod or -mod=readonly to bypass.\)'
+
+
+# 'go mod init' should work if there is already a GOPATH-mode vendor directory
+# present. If there are no module dependencies, -mod=vendor should be used by
+# default and should not fail the consistency check even though no module
+# information is present.
+
+rm go.mod
+rm vendor/modules.txt
+
+go mod init example.com/auto
+go list -f {{.Dir}} -tags tools -e all
+stdout '^'$WORK'[/\\]auto$'
+stdout '^'$WORK'[/\\]auto[/\\]vendor[/\\]example.com[/\\]printversion$'
+stdout '^'$WORK'[/\\]auto[/\\]vendor[/\\]example.com[/\\]version$'
+
+# If information about dependencies is added to a 1.14 go.mod file, subsequent
+# list commands should error out if vendor/modules.txt is missing or incomplete.
+
+cp go.mod.orig go.mod
+go mod edit -go=1.14
+! go list -f {{.Dir}} -tags tools -e all
+stderr '^go: inconsistent vendoring in '$WORK[/\\]auto':$'
+stderr '^\texample.com/printversion@v1.0.0: is explicitly required in go.mod, but not marked as explicit in vendor/modules.txt'
+stderr '^\texample.com/unused: is replaced in go.mod, but not marked as replaced in vendor/modules.txt'
+stderr '^\texample.com/version@v1.2.0: is replaced in go.mod, but not marked as replaced in vendor/modules.txt'
+stderr '^\tTo ignore the vendor directory, use -mod=readonly or -mod=mod.\n\tTo sync the vendor directory, run:\n\t\tgo mod vendor$'
+
+# If -mod=vendor is set, limited consistency checks should apply even when
+# the go version is 1.13 or earlier.
+# An incomplete or missing vendor/modules.txt should resolve the vendored packages...
+go mod edit -go=1.13
+go list -mod=vendor -f {{.Dir}} -tags tools -e all
+stdout '^'$WORK'[/\\]auto$'
+stdout '^'$WORK'[/\\]auto[/\\]vendor[/\\]example.com[/\\]printversion$'
+stdout '^'$WORK'[/\\]auto[/\\]vendor[/\\]example.com[/\\]version$'
+
+# ...but a version mismatch for an explicit dependency should be noticed.
+cp $WORK/modules-bad-1.13.txt vendor/modules.txt
+! go list -mod=vendor -f {{.Dir}} -tags tools all
+stderr '^go: inconsistent vendoring in '$WORK[/\\]auto':$'
+stderr '^\texample.com/printversion@v1.0.0: is explicitly required in go.mod, but vendor/modules.txt indicates example.com/printversion@v1.1.0$'
+stderr '^\tTo ignore the vendor directory, use -mod=readonly or -mod=mod.\n\tTo sync the vendor directory, run:\n\t\tgo mod vendor$'
+
+# If the go version is still 1.13, 'go mod vendor' should write a
+# matching vendor/modules.txt containing the corrected 1.13 data.
+go mod vendor
+cmp $WORK/modules-1.13.txt vendor/modules.txt
+
+go list -mod=vendor -f {{.Dir}} -tags tools -e all
+stdout '^'$WORK'[/\\]auto$'
+stdout '^'$WORK'[/\\]auto[/\\]vendor[/\\]example.com[/\\]printversion$'
+stdout '^'$WORK'[/\\]auto[/\\]vendor[/\\]example.com[/\\]version$'
+
+# When the version is upgraded to 1.14, 'go mod vendor' should write a
+# vendor/modules.txt with the updated 1.14 annotations.
+go mod edit -go=1.14
+go mod vendor
+cmp $WORK/modules-1.14.txt vendor/modules.txt
+
+# Then, -mod=vendor should kick in automatically and succeed.
+go list -f {{.Dir}} -tags tools -e all
+stdout '^'$WORK'[/\\]auto$'
+stdout '^'$WORK'[/\\]auto[/\\]vendor[/\\]example.com[/\\]printversion$'
+stdout '^'$WORK'[/\\]auto[/\\]vendor[/\\]example.com[/\\]version$'
+
+# 'go get' should update from the network or module cache,
+# even if a vendor directory is present.
+go get example.com/version@v1.1.0
+! go list -f {{.Dir}} -tags tools all
+stderr '^go: inconsistent vendoring'
+
+-- $WORK/auto/go.mod --
+module example.com/auto
+
+go 1.13
+
+require example.com/printversion v1.0.0
+
+replace (
+	example.com/unused => nonexistent.example.com/unused v1.0.0-whatever
+	example.com/version v1.0.0 => ./replacement-version
+	example.com/version v1.2.0 => nonexistent.example.com/version v1.2.0
+)
+-- $WORK/auto/tools.go --
+// +build tools
+
+package auto
+
+import _ "example.com/printversion"
+-- $WORK/auto/auto.go --
+package auto
+-- $WORK/auto/replacement-version/go.mod --
+module example.com/version
+-- $WORK/auto/replacement-version/version.go --
+package version
+
+const V = "v1.0.0-replaced"
+-- $WORK/modules-1.14.txt --
+# example.com/printversion v1.0.0
+## explicit
+example.com/printversion
+# example.com/version v1.0.0 => ./replacement-version
+example.com/version
+# example.com/unused => nonexistent.example.com/unused v1.0.0-whatever
+# example.com/version v1.2.0 => nonexistent.example.com/version v1.2.0
+-- $WORK/modules-1.13.txt --
+# example.com/printversion v1.0.0
+example.com/printversion
+# example.com/version v1.0.0 => ./replacement-version
+example.com/version
+-- $WORK/modules-bad-1.13.txt --
+# example.com/printversion v1.1.0
+example.com/printversion
+# example.com/version v1.1.0
+example.com/version
+-- $WORK/auto/vendor/example.com/printversion/go.mod --
+module example.com/printversion
+
+require example.com/version v1.0.0
+replace example.com/version v1.0.0 => ../oops v0.0.0
+exclude example.com/version v1.0.1
+-- $WORK/auto/vendor/example.com/printversion/printversion.go --
+package main
+
+import (
+	"fmt"
+	"os"
+	"runtime/debug"
+
+	_ "example.com/version"
+)
+
+func main() {
+	info, _ := debug.ReadBuildInfo()
+	fmt.Fprintf(os.Stdout, "path is %s\n", info.Path)
+	fmt.Fprintf(os.Stdout, "main is %s %s\n", info.Main.Path, info.Main.Version)
+	for _, m := range info.Deps {
+		fmt.Fprintf(os.Stdout, "using %s %s\n", m.Path, m.Version)
+	}
+}
+-- $WORK/auto/vendor/example.com/version/version.go --
+package version
+
+const V = "v1.0.0-replaced"
diff --git a/src/cmd/go/testdata/script/mod_vendor_build.txt b/src/cmd/go/testdata/script/mod_vendor_build.txt
new file mode 100644
index 0000000..4efda55
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_vendor_build.txt
@@ -0,0 +1,41 @@
+env GO111MODULE=on
+[short] skip
+
+# Populate go.mod and go.sum.
+go mod tidy
+
+# initial conditions: using sampler v1.3.0, not listed in go.mod.
+go list -deps
+stdout rsc.io/sampler
+! grep 'rsc.io/sampler v1.3.0' go.mod
+
+# update to v1.3.1, now indirect in go.mod.
+go get rsc.io/sampler@v1.3.1
+grep 'rsc.io/sampler v1.3.1 // indirect' go.mod
+cp go.mod go.mod.good
+
+# vendoring can but should not need to make changes.
+go mod vendor
+cmp go.mod go.mod.good
+
+# go list -mod=vendor (or go build -mod=vendor) must not modify go.mod.
+# golang.org/issue/26704
+go list -mod=vendor
+cmp go.mod go.mod.good
+
+# With a clean (and empty) module cache, 'go list -mod=vendor' should not download modules.
+go clean -modcache
+env GOPROXY=off
+! go list ...
+go list -mod=vendor ...
+
+# However, it should still list packages in the main module.
+go list -mod=vendor m/...
+stdout m
+
+-- go.mod --
+module m
+go 1.12
+-- x.go --
+package x
+import _ "rsc.io/quote"
diff --git a/src/cmd/go/testdata/script/mod_vendor_embed.txt b/src/cmd/go/testdata/script/mod_vendor_embed.txt
new file mode 100644
index 0000000..b14fd99
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_vendor_embed.txt
@@ -0,0 +1,179 @@
+go mod vendor
+cmp vendor/example.com/a/samedir_embed.txt a/samedir_embed.txt
+cmp vendor/example.com/a/subdir/embed.txt a/subdir/embed.txt
+cmp vendor/example.com/a/subdir/test/embed.txt a/subdir/test/embed.txt
+cmp vendor/example.com/a/subdir/test/xtest/embed.txt a/subdir/test/xtest/embed.txt
+
+cd broken_no_matching_files
+! go mod vendor
+stderr 'go: pattern foo.txt: no matching files found'
+
+cd ../broken_bad_pattern
+! go mod vendor
+stderr 'go: pattern ../foo.txt: invalid pattern syntax'
+
+# matchPotentialSourceFile prunes out tests and unbuilt code.
+# Make sure that they are vendored if they are embedded files.
+cd ../embed_unbuilt
+go mod vendor
+cmp vendor/example.com/dep/unbuilt.go dep/unbuilt.go
+cmp vendor/example.com/dep/dep_test.go dep/dep_test.go
+! exists vendor/example.com/dep/not_embedded_unbuilt.go
+! exists vendor/example.com/dep/not_embedded_dep_test.go
+-- go.mod --
+module example.com/foo
+go 1.16
+
+require (
+	example.com/a v0.1.0
+)
+
+replace (
+	example.com/a v0.1.0 => ./a
+)
+-- foo.go --
+package main
+
+import (
+	"fmt"
+
+	"example.com/a"
+)
+
+func main() {
+    fmt.Println(a.Str())
+}
+-- a/go.mod --
+module example.com/a
+-- a/a.go --
+package a
+
+import _ "embed"
+
+//go:embed samedir_embed.txt
+var sameDir string
+
+//go:embed subdir/embed.txt
+var subDir string
+
+func Str() string {
+	return sameDir + subDir
+}
+-- a/a_test.go --
+package a
+
+import _ "embed"
+
+//go:embed subdir/test/embed.txt
+var subderTest string
+-- a/a_x_test.go --
+package a_test
+
+import _ "embed"
+
+//go:embed subdir/test/xtest/embed.txt
+var subdirXtest string
+-- a/samedir_embed.txt --
+embedded file in same directory as package
+-- a/subdir/embed.txt --
+embedded file in subdirectory of package
+-- a/subdir/test/embed.txt --
+embedded file of test in subdirectory of package
+-- a/subdir/test/xtest/embed.txt --
+embedded file of xtest in subdirectory of package
+-- broken_no_matching_files/go.mod --
+module example.com/broken
+go 1.16
+
+require (
+	example.com/brokendep v0.1.0
+)
+
+replace (
+	example.com/brokendep v0.1.0 => ./brokendep
+)
+-- broken_no_matching_files/f.go --
+package broken
+
+import _ "example.com/brokendep"
+
+func F() {}
+-- broken_no_matching_files/brokendep/go.mod --
+module example.com/brokendep
+go 1.16
+-- broken_no_matching_files/brokendep/f.go --
+package brokendep
+
+import _ "embed"
+
+//go:embed foo.txt
+var foo string
+-- broken_bad_pattern/go.mod --
+module example.com/broken
+go 1.16
+
+require (
+	example.com/brokendep v0.1.0
+)
+
+replace (
+	example.com/brokendep v0.1.0 => ./brokendep
+)
+-- broken_bad_pattern/f.go --
+package broken
+
+import _ "example.com/brokendep"
+
+func F() {}
+-- broken_bad_pattern/brokendep/go.mod --
+module example.com/brokendep
+go 1.16
+-- broken_bad_pattern/brokendep/f.go --
+package brokendep
+
+import _ "embed"
+
+//go:embed ../foo.txt
+var foo string
+-- embed_unbuilt/go.mod --
+module example.com/foo
+go 1.16
+
+require (
+	example.com/dep v0.1.0
+)
+
+replace (
+	example.com/dep v0.1.0 => ./dep
+)
+-- embed_unbuilt/foo.go --
+package a
+
+import _ "example.com/dep"
+
+func F() {}
+-- embed_unbuilt/dep/go.mod --
+module example.com/dep
+go 1.16
+-- embed_unbuilt/dep/dep.go --
+package dep
+
+import _ "embed"
+
+//go:embed unbuilt.go
+var unbuilt string
+
+//go:embed dep_test.go
+var depTest string
+-- embed_unbuilt/dep/unbuilt.go --
+// +build ignore
+
+package dep
+-- embed_unbuilt/dep/not_embedded_unbuilt.go --
+// +build ignore
+
+package dep
+-- embed_unbuilt/dep/dep_test.go --
+package dep
+-- embed_unbuilt/dep/not_embedded_dep_test.go --
+package dep
diff --git a/src/cmd/go/testdata/script/mod_vendor_gomod.txt b/src/cmd/go/testdata/script/mod_vendor_gomod.txt
new file mode 100644
index 0000000..3f6ea35
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_vendor_gomod.txt
@@ -0,0 +1,38 @@
+# https://golang.org/issue/42970: As of Go 1.17, go.mod and go.sum files should
+# be stripped from vendored dependencies.
+
+go mod vendor
+cd vendor/example.net/x
+go list all
+! stdout '^example.net/m'
+stdout '^example.net/x$'
+exists ./go.sum
+
+cd ../../..
+go mod edit -go=1.17
+go mod vendor
+cd vendor/example.net/x
+go list all
+stdout '^example.net/m$'
+stdout '^example.net/x$'
+! exists ./go.sum
+
+-- go.mod --
+module example.net/m
+
+go 1.16
+
+require example.net/x v0.1.0
+
+replace example.net/x v0.1.0 => ./x
+-- m.go --
+package m
+
+import _ "example.net/x"
+-- x/go.mod --
+module example.net/x
+
+go 1.16
+-- x/go.sum --
+-- x/x.go --
+package x
diff --git a/src/cmd/go/testdata/script/mod_vendor_goversion.txt b/src/cmd/go/testdata/script/mod_vendor_goversion.txt
new file mode 100644
index 0000000..838c557
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_vendor_goversion.txt
@@ -0,0 +1,102 @@
+# https://golang.org/issue/36876: As of Go 1.17, vendor/modules.txt should
+# indicate the language version used by each dependency.
+
+[short] skip
+
+# Control case: without a vendor directory, need117 builds and bad114 doesn't.
+
+go build example.net/need117
+! go build example.net/bad114
+stderr '^bad114[/\\]bad114.go:15:2: duplicate method Y( .*)?$'
+
+
+# With a vendor/modules.txt lacking language versions, the world is topsy-turvy,
+# because we have to guess a uniform version for everything.
+#
+# We always guess Go 1.16, because that was the last version for which
+# 'go mod vendor' failed to record dependency versions, and it has most of
+# the language features added since modules were introduced in Go 1.11.
+#
+# Even so, modules that declare 'go 1.17' and use 1.17 features spuriously fail
+# to build, and modules that declare an older version and use features from a
+# newer one spuriously build (instead of failing as they ought to).
+
+go mod vendor
+
+! grep 1.17 vendor/modules.txt
+! go build example.net/need117
+stderr '^vendor[/\\]example\.net[/\\]need117[/\\]need117.go:5:1[89]:'
+stderr 'conversion of slices to array pointers requires go1\.17 or later'
+
+! grep 1.13 vendor/modules.txt
+go build example.net/bad114
+
+
+# Upgrading the main module to 1.17 adds version annotations.
+# Then everything is once again consistent with the non-vendored world.
+
+go mod edit -go=1.17
+go mod vendor
+
+grep '^## explicit; go 1.17$' vendor/modules.txt
+go build example.net/need117
+
+grep '^## explicit; go 1.13$' vendor/modules.txt
+! go build example.net/bad114
+stderr '^vendor[/\\]example\.net[/\\]bad114[/\\]bad114.go:15:2: duplicate method Y( .+)?$'
+
+-- go.mod --
+module example.net/m
+
+go 1.16
+
+require (
+	example.net/bad114 v0.1.0
+	example.net/need117 v0.1.0
+)
+
+replace (
+	example.net/bad114 v0.1.0 => ./bad114
+	example.net/need117 v0.1.0 => ./need117
+)
+-- m.go --
+package m
+
+import _ "example.net/bad114"
+import _ "example.net/need117"
+
+-- bad114/go.mod --
+// Module bad114 requires Go 1.14 or higher, but declares Go 1.13.
+module example.net/bad114
+
+go 1.13
+-- bad114/bad114.go --
+package bad114
+
+type XY interface {
+	X()
+	Y()
+}
+
+type YZ interface {
+	Y()
+	Z()
+}
+
+type XYZ interface {
+	XY
+	YZ
+}
+
+-- need117/go.mod --
+// Module need117 requires Go 1.17 or higher.
+module example.net/need117
+
+go 1.17
+-- need117/need117.go --
+package need117
+
+func init() {
+		 s := make([]byte, 4)
+		 _ = (*[4]byte)(s)
+}
diff --git a/src/cmd/go/testdata/script/mod_vendor_issue46867.txt b/src/cmd/go/testdata/script/mod_vendor_issue46867.txt
new file mode 100644
index 0000000..38ae87b
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_vendor_issue46867.txt
@@ -0,0 +1,31 @@
+# Regression test for golang.org/issue/46867:
+# 'go mod vendor' on Windows attempted to open and copy
+# files from directories outside of the module.
+
+cd subdir
+go mod vendor
+! exists vendor/example.net/NOTICE
+exists vendor/example.net/m/NOTICE
+
+-- subdir/go.mod --
+module golang.org/issue46867
+
+go 1.17
+
+replace example.net/m v0.1.0 => ./m
+
+require example.net/m v0.1.0
+-- subdir/issue.go --
+package issue
+
+import _ "example.net/m/n"
+-- subdir/m/go.mod --
+module example.net/m
+
+go 1.17
+-- subdir/m/n/n.go --
+package n
+-- subdir/m/NOTICE --
+This notice is in module m and SHOULD be vendored.
+-- subdir/NOTICE --
+This notice is outside of module m and SHOULD NOT be vendored.
diff --git a/src/cmd/go/testdata/script/mod_vendor_nodeps.txt b/src/cmd/go/testdata/script/mod_vendor_nodeps.txt
new file mode 100644
index 0000000..e9a84ca
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_vendor_nodeps.txt
@@ -0,0 +1,9 @@
+env GO111MODULE=on
+
+go mod vendor
+stderr '^go: no dependencies to vendor'
+
+-- go.mod --
+module x
+-- x.go --
+package x
diff --git a/src/cmd/go/testdata/script/mod_vendor_redundant_requirement.txt b/src/cmd/go/testdata/script/mod_vendor_redundant_requirement.txt
new file mode 100644
index 0000000..3f6f5c5
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_vendor_redundant_requirement.txt
@@ -0,0 +1,29 @@
+# 'go list -mod=vendor' should succeed even when go.mod contains redundant
+# requirements. Verifies #47565.
+go list -mod=vendor
+
+-- go.mod --
+module m
+
+go 1.17
+
+require example.com/m v0.0.0
+require example.com/m v0.0.0
+
+replace example.com/m v0.0.0 => ./m
+-- m/go.mod --
+module example.com/m
+
+go 1.17
+-- m/m.go --
+package m
+-- use.go --
+package use
+
+import _ "example.com/m"
+-- vendor/example.com/m/m.go --
+package m
+-- vendor/modules.txt --
+# example.com/m v0.0.0 => ./m
+## explicit; go 1.17
+example.com/m
diff --git a/src/cmd/go/testdata/script/mod_vendor_replace.txt b/src/cmd/go/testdata/script/mod_vendor_replace.txt
new file mode 100644
index 0000000..c492999
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_vendor_replace.txt
@@ -0,0 +1,89 @@
+env GO111MODULE=on
+
+# Replacement should not use a vendor directory as the target.
+! go mod vendor
+stderr 'replacement path ./vendor/not-rsc.io/quote/v3 inside vendor directory'
+
+cp go.mod1 go.mod
+rm -r vendor
+
+# Before vendoring, we expect to see the original directory.
+go list -f '{{with .Module}}{{.Version}}{{end}} {{.Dir}}' rsc.io/quote/v3
+stdout 'v3.0.0'
+stdout '.*[/\\]not-rsc.io[/\\]quote[/\\]v3'
+
+# Since all dependencies are replaced, 'go mod vendor' should not
+# have to download anything from the network.
+go mod vendor
+! stderr 'downloading'
+! stderr 'finding'
+
+# After vendoring, we expect to see the replacement in the vendor directory,
+# without attempting to look up the non-replaced version.
+cmp vendor/rsc.io/quote/v3/quote.go local/not-rsc.io/quote/v3/quote.go
+
+go list -mod=vendor -f '{{with .Module}}{{.Version}}{{end}} {{.Dir}}' rsc.io/quote/v3
+stdout 'v3.0.0'
+stdout '.*[/\\]vendor[/\\]rsc.io[/\\]quote[/\\]v3'
+! stderr 'finding'
+! stderr 'lookup disabled'
+
+# 'go list' should provide the original replacement directory as the module's
+# replacement path.
+go list -mod=vendor -f '{{with .Module}}{{with .Replace}}{{.Path}}{{end}}{{end}}' rsc.io/quote/v3
+stdout '.*[/\\]not-rsc.io[/\\]quote[/\\]v3'
+
+# The same module can't be used as two different paths.
+cd multiple-paths
+! go mod vendor
+stderr 'rsc.io/quote/v3@v3.0.0 used for two different module paths \(not-rsc.io/quote/v3 and rsc.io/quote/v3\)'
+
+-- go.mod --
+module example.com/replace
+
+require rsc.io/quote/v3 v3.0.0
+replace rsc.io/quote/v3 => ./vendor/not-rsc.io/quote/v3
+
+-- go.mod1 --
+module example.com/replace
+
+require rsc.io/quote/v3 v3.0.0
+replace rsc.io/quote/v3 => ./local/not-rsc.io/quote/v3
+
+-- imports.go --
+package replace
+
+import _ "rsc.io/quote/v3"
+
+-- local/not-rsc.io/quote/v3/go.mod --
+module not-rsc.io/quote/v3
+
+-- local/not-rsc.io/quote/v3/quote.go --
+package quote
+
+-- multiple-paths/main.go --
+package main
+import (
+	"fmt"
+	"rsc.io/quote/v3"
+)
+func main() {
+	fmt.Println(quote.GoV3())
+}
+-- multiple-paths/go.mod --
+module quoter
+require (
+	rsc.io/quote/v3 v3.0.0
+	not-rsc.io/quote/v3 v3.0.0
+)
+replace not-rsc.io/quote/v3 => rsc.io/quote/v3 v3.0.0
+-- multiple-paths/go.sum --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote/v3 v3.0.0/go.mod h1:yEA65RcK8LyAZtP9Kv3t0HmxON59tX3rD+tICJqUlj0=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+
+-- vendor/not-rsc.io/quote/v3/go.mod --
+module not-rsc.io/quote/v3
+
+-- vendor/not-rsc.io/quote/v3/quote.go --
+package quote
diff --git a/src/cmd/go/testdata/script/mod_vendor_trimpath.txt b/src/cmd/go/testdata/script/mod_vendor_trimpath.txt
new file mode 100644
index 0000000..d9d9d98
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_vendor_trimpath.txt
@@ -0,0 +1,49 @@
+# Check that when -trimpath and -mod=vendor are used together,
+# paths in vendored packages are properly trimmed.
+# Verifies golang.org/issue/36566.
+
+[short] skip
+
+# Only the main module has a root directory in vendor mode.
+go mod vendor
+go list -f {{.Module.Dir}} example.com/main
+stdout $PWD
+go list -f {{.Module.Dir}} example.com/stack
+! stdout .
+
+# The program prints a file name from a vendored package.
+# Without -trimpath, the name should include the vendor directory.
+go run main.go
+stdout vendor
+
+# With -trimpath, everything before the package path should be trimmed.
+# As with -mod=mod, the version should appear as part of the module path.
+go run -mod=vendor -trimpath main.go
+stdout '^example.com/stack@v1.0.0/stack.go$'
+
+# With pristinely vendored source code, a trimmed binary built from vendored
+# code should have the same behavior as one build from the module cache.
+go run -mod=mod -trimpath main.go
+stdout '^example.com/stack@v1.0.0/stack.go$'
+
+-- go.mod --
+module example.com/main
+
+go 1.17
+
+require example.com/stack v1.0.0
+-- go.sum --
+example.com/stack v1.0.0 h1:IEDLeew5NytZ8vrgCF/QVem3H3SR3QMttdu9HfJvk9I=
+example.com/stack v1.0.0/go.mod h1:7wFEbaV5e5O7wJ8aBdqQOR//UXppm/pwnwziMKViuI4=
+-- main.go --
+package main
+
+import (
+  "fmt"
+
+  "example.com/stack"
+)
+
+func main() {
+  fmt.Println(stack.TopFile())
+}
diff --git a/src/cmd/go/testdata/script/mod_vendor_unused.txt b/src/cmd/go/testdata/script/mod_vendor_unused.txt
new file mode 100644
index 0000000..96251bb
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_vendor_unused.txt
@@ -0,0 +1,67 @@
+# Auxiliary test for inclusion of otherwise-unused replacements in
+# vendor/modules.txt for golang.org/issue/33848.
+# We need metadata about replacements in order to verify that modules.txt
+# remains in sync with the main module's go.mod file.
+
+env GO111MODULE=on
+
+go mod vendor
+cmp go1.14-modules.txt vendor/modules.txt
+
+-- go.mod --
+module example.com/foo
+go 1.14
+
+require (
+	example.com/a v0.1.0
+)
+
+replace (
+	example.com/a v0.1.0 => ./a
+	example.com/b v0.1.0 => ./b1
+	example.com/b v0.2.0-unused => ./b2
+	example.com/c => ./c
+	example.com/d v0.1.0 => ./d1
+	example.com/d v0.2.0 => ./d2
+	example.com/e => example.com/e v0.1.0-unused
+)
+-- foo.go --
+package foo
+import _ "example.com/a"
+-- a/go.mod --
+module example.com/a
+require (
+	example.com/b v0.1.0 // indirect
+	example.com/c v0.1.0 // indirect
+)
+-- a/a.go --
+package a
+import _ "example.com/d"
+-- b1/go.mod --
+module example.com/b
+require example.com/d v0.1.0
+-- b2/go.mod --
+module example.com/b
+require example.com/c v0.2.0
+-- c/go.mod --
+module example.com/c
+require example.com/d v0.2.0
+-- d1/go.mod --
+module example.com/d
+-- d1/d1.go --
+package d
+-- d2/go.mod --
+module example.com/d
+-- d2/d2.go --
+package d
+-- go1.14-modules.txt --
+# example.com/a v0.1.0 => ./a
+## explicit
+example.com/a
+# example.com/d v0.2.0 => ./d2
+example.com/d
+# example.com/b v0.1.0 => ./b1
+# example.com/b v0.2.0-unused => ./b2
+# example.com/c => ./c
+# example.com/d v0.1.0 => ./d1
+# example.com/e => example.com/e v0.1.0-unused
diff --git a/src/cmd/go/testdata/script/mod_vendor_unused_only.txt b/src/cmd/go/testdata/script/mod_vendor_unused_only.txt
new file mode 100644
index 0000000..accd9f3
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_vendor_unused_only.txt
@@ -0,0 +1,19 @@
+# Ensure that we generate a vendor/modules.txt file even when the only
+# requirements in go.mod are unused.  Regression test for
+# golang.org/issue/36580
+
+env GO111MODULE=on
+
+go mod vendor
+cmp go1.14-modules.txt vendor/modules.txt
+
+-- go.mod --
+module example.com/m
+go 1.14
+
+require example.com v1.0.0 // indirect
+-- go.sum --
+example.com v1.0.0/go.mod h1:WRiieAqDBb1hVdDXLLdxNtCDWNfehn7FWyPC5Oz2vB4=
+-- go1.14-modules.txt --
+# example.com v1.0.0
+## explicit
diff --git a/src/cmd/go/testdata/script/mod_verify.txt b/src/cmd/go/testdata/script/mod_verify.txt
new file mode 100644
index 0000000..018709e
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_verify.txt
@@ -0,0 +1,82 @@
+env GO111MODULE=on
+[short] skip
+
+# With good go.sum, verify succeeds by avoiding download.
+cp go.sum.good go.sum
+go mod verify
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.1.0.zip
+
+# With bad go.sum, verify succeeds by avoiding download.
+cp go.sum.bad go.sum
+go mod verify
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.1.0.zip
+
+# With bad go.sum, sync (which must download) fails.
+rm go.sum
+cp go.sum.bad go.sum
+! go mod tidy
+stderr 'checksum mismatch'
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.1.0.zip
+
+# With good go.sum, sync works.
+rm go.sum
+cp go.sum.good go.sum
+go mod tidy
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.1.0.zip
+exists $GOPATH/pkg/mod/rsc.io/quote@v1.1.0/quote.go
+
+# go.sum should have the new checksum for go.mod
+grep '^rsc.io/quote v1.1.0/go.mod ' go.sum
+
+# verify should work
+go mod verify
+
+# basic loading of module graph should detect incorrect go.mod files.
+go mod graph
+cp go.sum.bad2 go.sum
+! go mod graph
+stderr 'go.mod: checksum mismatch'
+
+# go.sum should be created and updated automatically.
+rm go.sum
+go mod tidy
+grep '^rsc.io/quote v1.1.0/go.mod ' go.sum
+grep '^rsc.io/quote v1.1.0 ' go.sum
+
+# verify should fail on a missing ziphash. tidy should restore it.
+rm $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.1.0.ziphash
+! go mod verify
+stderr '^rsc.io/quote v1.1.0: missing ziphash: open '$GOPATH'[/\\]pkg[/\\]mod[/\\]cache[/\\]download[/\\]rsc.io[/\\]quote[/\\]@v[/\\]v1.1.0.ziphash'
+go mod tidy
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.1.0.ziphash
+go mod verify
+
+# Packages below module root should not be mentioned in go.sum.
+rm go.sum
+go mod edit -droprequire rsc.io/quote
+go get rsc.io/quote/buggy
+grep '^rsc.io/quote v1.5.2/go.mod ' go.sum
+! grep buggy go.sum
+
+# non-existent packages below module root should not be mentioned in go.sum
+go mod edit -droprequire rsc.io/quote
+! go list rsc.io/quote/morebuggy
+grep '^rsc.io/quote v1.5.2/go.mod ' go.sum
+! grep buggy go.sum
+
+-- go.mod --
+module x
+require rsc.io/quote v1.1.0
+
+-- x.go --
+package x
+import _ "rsc.io/quote"
+
+-- go.sum.good --
+rsc.io/quote v1.1.0 h1:a3YaZoizPtXyv6ZsJ74oo2L4/bwOSTKMY7MAyo4O/0c=
+
+-- go.sum.bad --
+rsc.io/quote v1.1.0 h1:a3YaZoizPtXyv6ZsJ74oo2L4/bwOSTKMY7MAyo4O/1c=
+
+-- go.sum.bad2 --
+rsc.io/quote v1.1.0/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl1=
diff --git a/src/cmd/go/testdata/script/mod_verify_work.txt b/src/cmd/go/testdata/script/mod_verify_work.txt
new file mode 100644
index 0000000..d9f5a54
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_verify_work.txt
@@ -0,0 +1,24 @@
+# Regression test for Issue #62663: we would filter out the toolchain and
+# main modules from the build list incorrectly, leading to the workspace
+# modules being checked for correct sums. Specifically this would happen when
+# the module name sorted after the virtual 'go' version module name because
+# it could not get chopped off when we removed the MainModules.Len() modules
+# at the beginning of the build list and we would remove the go module instead.
+
+go mod verify
+
+-- go.work --
+go 1.21
+
+use (
+    ./a
+    ./b
+)
+-- a/go.mod --
+module hexample.com/a // important for test that module name sorts after 'go'
+
+go 1.21
+-- b/go.mod --
+module hexample.com/b // important for test that module name sorts after 'go'
+
+go 1.21
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/mod_versions.txt b/src/cmd/go/testdata/script/mod_versions.txt
new file mode 100644
index 0000000..9e6322b
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_versions.txt
@@ -0,0 +1,14 @@
+# Test rejection of pkg@version in GOPATH mode.
+env GO111MODULE=off
+! go get rsc.io/quote@v1.5.1
+stderr '^go: can only use path@version syntax with ''go get'' and ''go install'' in module-aware mode$'
+! go build rsc.io/quote@v1.5.1
+stderr '^package rsc.io/quote@v1.5.1: can only use path@version syntax with ''go get'' and ''go install'' in module-aware mode$'
+
+env GO111MODULE=on
+cd x
+! go build rsc.io/quote@v1.5.1
+stderr '^package rsc.io/quote@v1.5.1: can only use path@version syntax with ''go get'' and ''go install'' in module-aware mode$'
+
+-- x/go.mod --
+module x
diff --git a/src/cmd/go/testdata/script/mod_why.txt b/src/cmd/go/testdata/script/mod_why.txt
new file mode 100644
index 0000000..b3036fa
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_why.txt
@@ -0,0 +1,130 @@
+env GO111MODULE=on
+[short] skip
+
+# Populate go.sum.
+go mod tidy
+cp go.mod go.mod.orig
+
+go list -test all
+stdout rsc.io/quote
+stdout golang.org/x/text/language
+
+# why a package?
+go mod why golang.org/x/text/language
+cmp stdout why-language.txt
+
+# why a module?
+go mod why -m golang.org...
+cmp stdout why-text-module.txt
+
+# why a package used only in tests?
+go mod why rsc.io/testonly
+cmp stdout why-testonly.txt
+
+# why a module used only in a test of a dependency?
+go mod why -m rsc.io/testonly
+cmp stdout why-testonly.txt
+
+# test package not needed
+go mod why golang.org/x/text/unused
+cmp stdout why-unused.txt
+
+# vendor doesn't use packages used only in tests.
+go mod why -vendor rsc.io/testonly
+cmp stdout why-vendor.txt
+
+# vendor doesn't use modules used only in tests.
+go mod why -vendor -m rsc.io/testonly
+cmp stdout why-vendor-module.txt
+
+# test multiple packages
+go mod why golang.org/x/text/language golang.org/x/text/unused
+cmp stdout why-both.txt
+
+# test multiple modules
+go mod why -m rsc.io/quote rsc.io/sampler
+cmp stdout why-both-module.txt
+
+# package in a module that isn't even in the module graph
+# (https://golang.org/issue/26977)
+go mod why rsc.io/fortune
+cmp stdout why-missing.txt
+
+# None of these command should have changed the go.mod file.
+cmp go.mod go.mod.orig
+
+-- go.mod --
+module mymodule
+require rsc.io/quote v1.5.2
+
+-- x/x.go --
+package x
+import _ "mymodule/z"
+
+-- y/y.go --
+package y
+
+-- y/y_test.go --
+package y
+import _ "rsc.io/quote"
+
+-- z/z.go --
+package z
+import _ "mymodule/y"
+
+
+-- why-language.txt --
+# golang.org/x/text/language
+mymodule/y
+mymodule/y.test
+rsc.io/quote
+rsc.io/sampler
+golang.org/x/text/language
+-- why-unused.txt --
+# golang.org/x/text/unused
+(main module does not need package golang.org/x/text/unused)
+-- why-text-module.txt --
+# golang.org/x/text
+mymodule/y
+mymodule/y.test
+rsc.io/quote
+rsc.io/sampler
+golang.org/x/text/language
+-- why-testonly.txt --
+# rsc.io/testonly
+mymodule/y
+mymodule/y.test
+rsc.io/quote
+rsc.io/sampler
+rsc.io/sampler.test
+rsc.io/testonly
+-- why-vendor.txt --
+# rsc.io/testonly
+(main module does not need to vendor package rsc.io/testonly)
+-- why-vendor-module.txt --
+# rsc.io/testonly
+(main module does not need to vendor module rsc.io/testonly)
+-- why-both.txt --
+# golang.org/x/text/language
+mymodule/y
+mymodule/y.test
+rsc.io/quote
+rsc.io/sampler
+golang.org/x/text/language
+
+# golang.org/x/text/unused
+(main module does not need package golang.org/x/text/unused)
+-- why-both-module.txt --
+# rsc.io/quote
+mymodule/y
+mymodule/y.test
+rsc.io/quote
+
+# rsc.io/sampler
+mymodule/y
+mymodule/y.test
+rsc.io/quote
+rsc.io/sampler
+-- why-missing.txt --
+# rsc.io/fortune
+(main module does not need package rsc.io/fortune)
diff --git a/src/cmd/go/testdata/script/modfile_flag.txt b/src/cmd/go/testdata/script/modfile_flag.txt
new file mode 100644
index 0000000..6d28759
--- /dev/null
+++ b/src/cmd/go/testdata/script/modfile_flag.txt
@@ -0,0 +1,98 @@
+# Tests the behavior of the -modfile flag in commands that support it.
+# The go.mod file exists but should not be read or written.
+# Same with go.sum.
+
+env GOFLAGS=-modfile=go.alt.mod
+cp go.mod go.mod.orig
+cp go.sum go.sum.orig
+
+
+# go mod init should create a new file, even though go.mod already exists.
+go mod init example.com/m
+grep example.com/m go.alt.mod
+
+# 'go env GOMOD' should print the path to the real file.
+# 'go env' does not recognize the '-modfile' flag.
+go env GOMOD
+stdout '^'$WORK${/}gopath${/}src${/}'go\.mod$'
+
+# 'go list -m' should print the effective go.mod file as GoMod though.
+go list -m -f '{{.GoMod}}'
+stdout '^go.alt.mod$'
+
+# go mod edit should operate on the alternate file
+go mod edit -require rsc.io/quote@v1.5.2
+grep rsc.io/quote go.alt.mod
+
+# 'go list -m' should add sums to the alternate go.sum.
+go list -m -mod=mod all
+grep '^rsc.io/quote v1.5.2/go.mod ' go.alt.sum
+! grep '^rsc.io/quote v1.5.2 ' go.alt.sum
+
+# other 'go mod' commands should work. 'go mod vendor' is tested later.
+go mod download rsc.io/quote
+go mod graph
+stdout rsc.io/quote
+go mod tidy
+grep rsc.io/quote go.alt.sum
+go mod verify
+go mod why rsc.io/quote
+
+
+# 'go list' and other commands with build flags should work.
+# They should update the alternate go.mod when a dependency is missing.
+go mod edit -droprequire rsc.io/quote
+go list -mod=mod .
+grep rsc.io/quote go.alt.mod
+go build -n -mod=mod .
+go test -n -mod=mod .
+go get rsc.io/quote
+
+
+# 'go mod vendor' should work.
+go mod vendor
+exists vendor
+
+# Automatic vendoring should be broken by editing an explicit requirement
+# in the alternate go.mod file.
+go mod edit -require rsc.io/quote@v1.5.1
+! go list .
+go list -mod=mod
+rm vendor
+
+
+# 'go generate' should use the alternate file when resolving packages.
+# Recursive go commands started with 'go generate' should not get an explicitly
+# passed -modfile, but they should see arguments from GOFLAGS.
+cp go.alt.mod go.gen.mod
+env OLD_GOFLAGS=$GOFLAGS
+env GOFLAGS=-modfile=go.gen.mod
+go generate -modfile=go.alt.mod .
+env GOFLAGS=$OLD_GOFLAGS
+grep example.com/exclude go.gen.mod
+! grep example.com/exclude go.alt.mod
+
+
+# The original files should not have been modified.
+cmp go.mod go.mod.orig
+cmp go.sum go.sum.orig
+
+
+# If the alternate mod file does not have a ".mod" suffix, an error
+# should be reported.
+cp go.alt.mod goaltmod
+! go mod tidy -modfile=goaltmod
+stderr '-modfile=goaltmod: file does not have .mod extension'
+
+-- go.mod --
+ʕ◔ϖ◔ʔ
+-- go.sum --
+ʕ◔ϖ◔ʔ
+-- use.go --
+package main
+
+import _ "rsc.io/quote"
+-- gen.go --
+//go:generate go mod edit -exclude example.com/exclude@v1.0.0
+
+package main
diff --git a/src/cmd/go/testdata/script/noncanonical_import.txt b/src/cmd/go/testdata/script/noncanonical_import.txt
new file mode 100644
index 0000000..018cb01
--- /dev/null
+++ b/src/cmd/go/testdata/script/noncanonical_import.txt
@@ -0,0 +1,19 @@
+env GO111MODULE=off
+
+! go build canonical/d
+stderr '^canonical[/\\]b[/\\]b.go:3:8: non-canonical import path "canonical/a/": should be "canonical/a"$'
+
+-- canonical/a/a.go --
+package a
+
+import _ "c"
+-- canonical/b/b.go --
+package b
+
+import _ "canonical/a/"
+-- canonical/a/vendor/c/c.go --
+package c
+-- canonical/d/d.go --
+package d
+
+import _ "canonical/b"
diff --git a/src/cmd/go/testdata/script/old_tidy_toolchain.txt b/src/cmd/go/testdata/script/old_tidy_toolchain.txt
new file mode 100644
index 0000000..d4b5af2
--- /dev/null
+++ b/src/cmd/go/testdata/script/old_tidy_toolchain.txt
@@ -0,0 +1,28 @@
+# Commands in an old module with no go line and no toolchain line,
+# or with only a go line, should succeed.
+# (They should not fail due to the go.mod not being tidy.)
+
+# No go line, no toolchain line.
+go list
+
+# Old go line, no toolchain line.
+go mod edit -go=1.16
+go list
+
+go mod edit -go=1.20
+go list
+
+# New go line, no toolchain line, using same toolchain.
+env TESTGO_VERSION=1.21
+go mod edit -go=1.21
+go list
+
+# New go line, no toolchain line, using newer Go version.
+# (Until we need to update the go line, no toolchain addition.)
+env TESTGO_VERSION=1.21.0
+go list
+
+-- go.mod --
+module m
+-- p.go --
+package p
diff --git a/src/cmd/go/testdata/script/pattern_syntax_error.txt b/src/cmd/go/testdata/script/pattern_syntax_error.txt
new file mode 100644
index 0000000..9a1f5e5
--- /dev/null
+++ b/src/cmd/go/testdata/script/pattern_syntax_error.txt
@@ -0,0 +1,12 @@
+env GO111MODULE=off
+
+# patterns match directories with syntax errors
+! go list ./...
+! go build ./...
+! go install ./...
+
+-- mypkg/x.go --
+package mypkg
+
+-- mypkg/y.go --
+pkg mypackage
diff --git a/src/cmd/go/testdata/script/prevent_sys_unix_import.txt b/src/cmd/go/testdata/script/prevent_sys_unix_import.txt
new file mode 100644
index 0000000..ea1ad78
--- /dev/null
+++ b/src/cmd/go/testdata/script/prevent_sys_unix_import.txt
@@ -0,0 +1,6 @@
+# Policy decision: we shouldn't vendor golang.org/x/sys/unix in std
+# See https://golang.org/issue/32102
+
+env GO111MODULE=on
+go list std
+! stdout vendor/golang.org/x/sys/unix
diff --git a/src/cmd/go/testdata/script/repro_build.txt b/src/cmd/go/testdata/script/repro_build.txt
new file mode 100644
index 0000000..7c6e317
--- /dev/null
+++ b/src/cmd/go/testdata/script/repro_build.txt
@@ -0,0 +1,22 @@
+# Check that goroutine scheduling does not affect compiler output.
+# If it does, reproducible builds will not work very well.
+[short] skip
+[GOOS:aix] env CGO_ENABLED=0  # go.dev/issue/56896
+env GOMAXPROCS=16
+go build -a -o http16.o net/http
+env GOMAXPROCS=17
+go build -a -o http17.o net/http
+cmp -q http16.o http17.o
+env GOMAXPROCS=18
+go build -a -o http18.o net/http
+cmp -q http16.o http18.o
+
+# Check that goroutine scheduling does not affect linker output.
+env GOMAXPROCS=16
+go build -a -o gofmt16.exe cmd/gofmt
+env GOMAXPROCS=17
+go build -a -o gofmt17.exe cmd/gofmt
+cmp -q gofmt16.exe gofmt17.exe
+env GOMAXPROCS=18
+go build -a -o gofmt18.exe cmd/gofmt
+cmp -q gofmt16.exe gofmt18.exe
diff --git a/src/cmd/go/testdata/script/reuse_git.txt b/src/cmd/go/testdata/script/reuse_git.txt
new file mode 100644
index 0000000..0357d67
--- /dev/null
+++ b/src/cmd/go/testdata/script/reuse_git.txt
@@ -0,0 +1,370 @@
+[short] skip
+[!git] skip
+
+env GO111MODULE=on
+env GOPROXY=direct
+env GOSUMDB=off
+
+# go mod download with the pseudo-version should invoke git but not have a TagSum or Ref.
+go mod download -x -json vcs-test.golang.org/git/hello.git@v0.0.0-20170922010558-fc3a09f3dc5c
+stderr 'git fetch'
+cp stdout hellopseudo.json
+! stdout '"(Query|TagPrefix|TagSum|Ref)"'
+stdout '"Version": "v0.0.0-20170922010558-fc3a09f3dc5c"'
+stdout '"VCS": "git"'
+stdout '"URL": ".*/git/hello"'
+stdout '"Hash": "fc3a09f3dc5cfe0d7a743ea18f1f5226e68b3777"'
+go clean -modcache
+
+# go mod download vcstest/hello should invoke git, print origin info
+go mod download -x -json vcs-test.golang.org/git/hello.git@latest
+stderr 'git fetch'
+cp stdout hello.json
+stdout '"Version": "v0.0.0-20170922010558-fc3a09f3dc5c"'
+stdout '"VCS": "git"'
+stdout '"URL": ".*/git/hello"'
+stdout '"Query": "latest"'
+! stdout '"TagPrefix"'
+stdout '"TagSum": "t1:47DEQpj8HBSa[+]/TImW[+]5JCeuQeRkm5NMpJWZG3hSuFU="'
+stdout '"Ref": "HEAD"'
+stdout '"Hash": "fc3a09f3dc5cfe0d7a743ea18f1f5226e68b3777"'
+
+# pseudo-version again should not invoke git fetch (it has the version from the @latest query)
+# but still be careful not to include a TagSum or a Ref, especially not Ref set to HEAD,
+# which is easy to do when reusing the cached version from the @latest query.
+go mod download -x -json vcs-test.golang.org/git/hello.git@v0.0.0-20170922010558-fc3a09f3dc5c
+! stderr 'git fetch'
+cp stdout hellopseudo2.json
+cmpenv hellopseudo.json hellopseudo2.json
+
+# go mod download vcstest/hello@hash needs to check TagSum to find pseudoversion base.
+go mod download -x -json vcs-test.golang.org/git/hello.git@fc3a09f3dc5c
+! stderr 'git fetch'
+cp stdout hellohash.json
+stdout '"Version": "v0.0.0-20170922010558-fc3a09f3dc5c"'
+stdout '"Query": "fc3a09f3dc5c"'
+stdout '"VCS": "git"'
+stdout '"URL": ".*/git/hello"'
+stdout '"TagSum": "t1:47DEQpj8HBSa[+]/TImW[+]5JCeuQeRkm5NMpJWZG3hSuFU="'
+stdout '"Hash": "fc3a09f3dc5cfe0d7a743ea18f1f5226e68b3777"'
+
+# go mod download vcstest/hello/v9 should fail, still print origin info
+! go mod download -x -json vcs-test.golang.org/git/hello.git/v9@latest
+cp stdout hellov9.json
+stdout '"Version": "latest"'
+stdout '"Error":.*no matching versions'
+! stdout '"TagPrefix"'
+stdout '"TagSum": "t1:47DEQpj8HBSa[+]/TImW[+]5JCeuQeRkm5NMpJWZG3hSuFU="'
+! stdout '"(Ref|Hash|RepoSum)":'
+
+# go mod download vcstest/hello/sub/v9 should also fail, print origin info with TagPrefix
+! go mod download -x -json vcs-test.golang.org/git/hello.git/sub/v9@latest
+cp stdout hellosubv9.json
+stdout '"Version": "latest"'
+stdout '"Error":.*no matching versions'
+stdout '"TagPrefix": "sub/"'
+stdout '"TagSum": "t1:47DEQpj8HBSa[+]/TImW[+]5JCeuQeRkm5NMpJWZG3hSuFU="'
+! stdout '"(Ref|Hash|RepoSum)":'
+
+# go mod download vcstest/hello@nonexist should fail, still print origin info
+! go mod download -x -json vcs-test.golang.org/git/hello.git@nonexist
+cp stdout hellononexist.json
+stdout '"Version": "nonexist"'
+stdout '"Error":.*unknown revision nonexist'
+stdout '"RepoSum": "r1:c0/9JCZ25lxoBiK3[+]3BhACU4giH49flcJmBynJ[+]Jvmc="'
+! stdout '"(TagPrefix|TagSum|Ref|Hash)"'
+
+# go mod download vcstest/hello@1234567890123456789012345678901234567890 should fail, still print origin info
+# (40 hex digits is assumed to be a full hash and is a slightly different code path from @nonexist)
+! go mod download -x -json vcs-test.golang.org/git/hello.git@1234567890123456789012345678901234567890
+cp stdout hellononhash.json
+stdout '"Version": "1234567890123456789012345678901234567890"'
+stdout '"Error":.*unknown revision 1234567890123456789012345678901234567890'
+stdout '"RepoSum": "r1:c0/9JCZ25lxoBiK3[+]3BhACU4giH49flcJmBynJ[+]Jvmc="'
+! stdout '"(TagPrefix|TagSum|Ref|Hash)"'
+
+# go mod download vcstest/hello@v0.0.0-20220101120101-123456789abc should fail, still print origin info
+# (non-existent pseudoversion)
+! go mod download -x -json vcs-test.golang.org/git/hello.git@v0.0.0-20220101120101-123456789abc
+cp stdout hellononpseudo.json
+stdout '"Version": "v0.0.0-20220101120101-123456789abc"'
+stdout '"Error":.*unknown revision 123456789abc'
+stdout '"RepoSum": "r1:c0/9JCZ25lxoBiK3[+]3BhACU4giH49flcJmBynJ[+]Jvmc="'
+! stdout '"(TagPrefix|TagSum|Ref|Hash)"'
+
+# go mod download vcstest/tagtests should invoke git, print origin info
+go mod download -x -json vcs-test.golang.org/git/tagtests.git@latest
+stderr 'git fetch'
+cp stdout tagtests.json
+stdout '"Version": "v0.2.2"'
+stdout '"Query": "latest"'
+stdout '"VCS": "git"'
+stdout '"URL": ".*/git/tagtests"'
+! stdout '"TagPrefix"'
+stdout '"TagSum": "t1:Dp7yRKDuE8WjG0429PN9hYWjqhy2te7P9Oki/sMEOGo="'
+stdout '"Ref": "refs/tags/v0.2.2"'
+stdout '"Hash": "59356c8cd18c5fe9a598167d98a6843e52d57952"'
+
+# go mod download vcstest/tagtests@v0.2.2 should print origin info, no TagSum needed
+go mod download -x -json vcs-test.golang.org/git/tagtests.git@v0.2.2
+cp stdout tagtestsv022.json
+stdout '"Version": "v0.2.2"'
+! stdout '"Query":'
+stdout '"VCS": "git"'
+stdout '"URL": ".*/git/tagtests"'
+! stdout '"TagPrefix"'
+! stdout '"TagSum"'
+stdout '"Ref": "refs/tags/v0.2.2"'
+stdout '"Hash": "59356c8cd18c5fe9a598167d98a6843e52d57952"'
+
+# go mod download vcstest/tagtests@master needs a TagSum again
+go mod download -x -json vcs-test.golang.org/git/tagtests.git@master
+cp stdout tagtestsmaster.json
+stdout '"Version": "v0.2.3-0.20190509225625-c7818c24fa2f"'
+stdout '"Query": "master"'
+stdout '"VCS": "git"'
+stdout '"URL": ".*/git/tagtests"'
+! stdout '"TagPrefix"'
+stdout '"TagSum": "t1:Dp7yRKDuE8WjG0429PN9hYWjqhy2te7P9Oki/sMEOGo="'
+stdout '"Ref": "refs/heads/master"'
+stdout '"Hash": "c7818c24fa2f3f714c67d0a6d3e411c85a518d1f"'
+
+# go mod download vcstest/prefixtagtests should invoke git, print origin info
+go mod download -x -json vcs-test.golang.org/git/prefixtagtests.git/sub@latest
+stderr 'git fetch'
+cp stdout prefixtagtests.json
+stdout '"Version": "v0.0.10"'
+stdout '"Query": "latest"'
+stdout '"VCS": "git"'
+stdout '"URL": ".*/git/prefixtagtests"'
+stdout '"Subdir": "sub"'
+stdout '"TagPrefix": "sub/"'
+stdout '"TagSum": "t1:YGSbWkJ8dn9ORAr[+]BlKHFK/2ZhXLb9hVuYfTZ9D8C7g="'
+stdout '"Ref": "refs/tags/sub/v0.0.10"'
+stdout '"Hash": "2b7c4692e12c109263cab51b416fcc835ddd7eae"'
+
+# go mod download of a bunch of these should fail (some are invalid) but write good JSON for later
+! go mod download -json vcs-test.golang.org/git/hello.git@latest vcs-test.golang.org/git/hello.git/v9@latest vcs-test.golang.org/git/hello.git/sub/v9@latest vcs-test.golang.org/git/tagtests.git@latest vcs-test.golang.org/git/tagtests.git@v0.2.2 vcs-test.golang.org/git/tagtests.git@master
+cp stdout all.json
+
+# clean the module cache, make sure that makes go mod download re-run git fetch, clean again
+go clean -modcache
+go mod download -x -json vcs-test.golang.org/git/hello.git@latest
+stderr 'git fetch'
+go clean -modcache
+
+# reuse go mod download vcstest/hello result
+go mod download -reuse=hello.json -x -json vcs-test.golang.org/git/hello.git@latest
+! stderr 'git fetch'
+stdout '"Reuse": true'
+stdout '"Version": "v0.0.0-20170922010558-fc3a09f3dc5c"'
+stdout '"VCS": "git"'
+stdout '"URL": ".*/git/hello"'
+! stdout '"TagPrefix"'
+stdout '"TagSum": "t1:47DEQpj8HBSa[+]/TImW[+]5JCeuQeRkm5NMpJWZG3hSuFU="'
+stdout '"Ref": "HEAD"'
+stdout '"Hash": "fc3a09f3dc5cfe0d7a743ea18f1f5226e68b3777"'
+! stdout '"Dir"'
+! stdout '"Info"'
+! stdout '"GoMod"'
+! stdout '"Zip"'
+
+# reuse go mod download vcstest/hello pseudoversion result
+go mod download -reuse=hellopseudo.json -x -json vcs-test.golang.org/git/hello.git@v0.0.0-20170922010558-fc3a09f3dc5c
+! stderr 'git fetch'
+stdout '"Reuse": true'
+stdout '"Version": "v0.0.0-20170922010558-fc3a09f3dc5c"'
+stdout '"VCS": "git"'
+stdout '"URL": ".*/git/hello"'
+! stdout '"(Query|TagPrefix|TagSum|Ref)"'
+stdout '"Hash": "fc3a09f3dc5cfe0d7a743ea18f1f5226e68b3777"'
+! stdout '"(Dir|Info|GoMod|Zip)"'
+
+# reuse go mod download vcstest/hello@hash
+go mod download -reuse=hellohash.json -x -json vcs-test.golang.org/git/hello.git@fc3a09f3dc5c
+! stderr 'git fetch'
+stdout '"Reuse": true'
+stdout '"Query": "fc3a09f3dc5c"'
+stdout '"Version": "v0.0.0-20170922010558-fc3a09f3dc5c"'
+stdout '"VCS": "git"'
+stdout '"URL": ".*/git/hello"'
+! stdout '"(TagPrefix|Ref)"'
+stdout '"TagSum": "t1:47DEQpj8HBSa[+]/TImW[+]5JCeuQeRkm5NMpJWZG3hSuFU="'
+stdout '"Hash": "fc3a09f3dc5cfe0d7a743ea18f1f5226e68b3777"'
+! stdout '"(Dir|Info|GoMod|Zip)"'
+
+# reuse go mod download vcstest/hello/v9 error result
+! go mod download -reuse=hellov9.json -x -json vcs-test.golang.org/git/hello.git/v9@latest
+! stderr 'git fetch'
+stdout '"Reuse": true'
+stdout '"Error":.*no matching versions'
+! stdout '"TagPrefix"'
+stdout '"TagSum": "t1:47DEQpj8HBSa[+]/TImW[+]5JCeuQeRkm5NMpJWZG3hSuFU="'
+! stdout '"(Ref|Hash)":'
+! stdout '"(Dir|Info|GoMod|Zip)"'
+
+# reuse go mod download vcstest/hello/sub/v9 error result
+! go mod download -reuse=hellosubv9.json -x -json vcs-test.golang.org/git/hello.git/sub/v9@latest
+! stderr 'git fetch'
+stdout '"Reuse": true'
+stdout '"Error":.*no matching versions'
+stdout '"TagPrefix": "sub/"'
+stdout '"TagSum": "t1:47DEQpj8HBSa[+]/TImW[+]5JCeuQeRkm5NMpJWZG3hSuFU="'
+! stdout '"(Ref|Hash)":'
+! stdout '"(Dir|Info|GoMod|Zip)"'
+
+# reuse go mod download vcstest/hello@nonexist
+! go mod download -reuse=hellononexist.json -x -json vcs-test.golang.org/git/hello.git@nonexist
+! stderr 'git fetch'
+stdout '"Reuse": true'
+stdout '"Version": "nonexist"'
+stdout '"Error":.*unknown revision nonexist'
+stdout '"RepoSum": "r1:c0/9JCZ25lxoBiK3[+]3BhACU4giH49flcJmBynJ[+]Jvmc="'
+! stdout '"(TagPrefix|TagSum|Ref|Hash)"'
+! stdout '"(Dir|Info|GoMod|Zip)"'
+
+# reuse go mod download vcstest/hello@1234567890123456789012345678901234567890
+! go mod download -reuse=hellononhash.json -x -json vcs-test.golang.org/git/hello.git@1234567890123456789012345678901234567890
+! stderr 'git fetch'
+stdout '"Reuse": true'
+stdout '"Version": "1234567890123456789012345678901234567890"'
+stdout '"Error":.*unknown revision 1234567890123456789012345678901234567890'
+stdout '"RepoSum": "r1:c0/9JCZ25lxoBiK3[+]3BhACU4giH49flcJmBynJ[+]Jvmc="'
+! stdout '"(TagPrefix|TagSum|Ref|Hash)"'
+! stdout '"(Dir|Info|GoMod|Zip)"'
+
+# reuse go mod download vcstest/hello@v0.0.0-20220101120101-123456789abc
+! go mod download -reuse=hellononpseudo.json -x -json vcs-test.golang.org/git/hello.git@v0.0.0-20220101120101-123456789abc
+! stderr 'git fetch'
+stdout '"Reuse": true'
+stdout '"Version": "v0.0.0-20220101120101-123456789abc"'
+stdout '"Error":.*unknown revision 123456789abc'
+stdout '"RepoSum": "r1:c0/9JCZ25lxoBiK3[+]3BhACU4giH49flcJmBynJ[+]Jvmc="'
+! stdout '"(TagPrefix|TagSum|Ref|Hash)"'
+! stdout '"(Dir|Info|GoMod|Zip)"'
+
+# reuse go mod download vcstest/tagtests result
+go mod download -reuse=tagtests.json -x -json vcs-test.golang.org/git/tagtests.git@latest
+! stderr 'git fetch'
+stdout '"Reuse": true'
+stdout '"Version": "v0.2.2"'
+stdout '"Query": "latest"'
+stdout '"VCS": "git"'
+stdout '"URL": ".*/git/tagtests"'
+! stdout '"TagPrefix"'
+stdout '"TagSum": "t1:Dp7yRKDuE8WjG0429PN9hYWjqhy2te7P9Oki/sMEOGo="'
+stdout '"Ref": "refs/tags/v0.2.2"'
+stdout '"Hash": "59356c8cd18c5fe9a598167d98a6843e52d57952"'
+! stdout '"(Dir|Info|GoMod|Zip)"'
+
+# reuse go mod download vcstest/tagtests@v0.2.2 result
+go mod download -reuse=tagtestsv022.json -x -json vcs-test.golang.org/git/tagtests.git@v0.2.2
+! stderr 'git fetch'
+stdout '"Reuse": true'
+stdout '"Version": "v0.2.2"'
+! stdout '"Query":'
+stdout '"VCS": "git"'
+stdout '"URL": ".*/git/tagtests"'
+! stdout '"TagPrefix"'
+! stdout '"TagSum"'
+stdout '"Ref": "refs/tags/v0.2.2"'
+stdout '"Hash": "59356c8cd18c5fe9a598167d98a6843e52d57952"'
+! stdout '"(Dir|Info|GoMod|Zip)"'
+
+# reuse go mod download vcstest/tagtests@master result
+go mod download -reuse=tagtestsmaster.json -x -json vcs-test.golang.org/git/tagtests.git@master
+! stderr 'git fetch'
+stdout '"Reuse": true'
+stdout '"Version": "v0.2.3-0.20190509225625-c7818c24fa2f"'
+stdout '"Query": "master"'
+stdout '"VCS": "git"'
+stdout '"URL": ".*/git/tagtests"'
+! stdout '"TagPrefix"'
+stdout '"TagSum": "t1:Dp7yRKDuE8WjG0429PN9hYWjqhy2te7P9Oki/sMEOGo="'
+stdout '"Ref": "refs/heads/master"'
+stdout '"Hash": "c7818c24fa2f3f714c67d0a6d3e411c85a518d1f"'
+! stdout '"(Dir|Info|GoMod|Zip)"'
+
+# reuse go mod download vcstest/tagtests@master result again with all.json
+go mod download -reuse=all.json -x -json vcs-test.golang.org/git/tagtests.git@master
+! stderr 'git fetch'
+stdout '"Reuse": true'
+stdout '"Version": "v0.2.3-0.20190509225625-c7818c24fa2f"'
+stdout '"Query": "master"'
+stdout '"VCS": "git"'
+stdout '"URL": ".*/git/tagtests"'
+! stdout '"TagPrefix"'
+stdout '"TagSum": "t1:Dp7yRKDuE8WjG0429PN9hYWjqhy2te7P9Oki/sMEOGo="'
+stdout '"Ref": "refs/heads/master"'
+stdout '"Hash": "c7818c24fa2f3f714c67d0a6d3e411c85a518d1f"'
+! stdout '"(Dir|Info|GoMod|Zip)"'
+
+# go mod download vcstest/prefixtagtests result with json
+go mod download -reuse=prefixtagtests.json -x -json vcs-test.golang.org/git/prefixtagtests.git/sub@latest
+! stderr 'git fetch'
+stdout '"Version": "v0.0.10"'
+stdout '"Query": "latest"'
+stdout '"VCS": "git"'
+stdout '"URL": ".*/git/prefixtagtests"'
+stdout '"Subdir": "sub"'
+stdout '"TagPrefix": "sub/"'
+stdout '"TagSum": "t1:YGSbWkJ8dn9ORAr[+]BlKHFK/2ZhXLb9hVuYfTZ9D8C7g="'
+stdout '"Ref": "refs/tags/sub/v0.0.10"'
+stdout '"Hash": "2b7c4692e12c109263cab51b416fcc835ddd7eae"'
+! stdout '"(Dir|Info|GoMod|Zip)"'
+
+# reuse the bulk results with all.json
+! go mod download -reuse=all.json -json vcs-test.golang.org/git/hello.git@latest vcs-test.golang.org/git/hello.git/v9@latest vcs-test.golang.org/git/hello.git/sub/v9@latest vcs-test.golang.org/git/tagtests.git@latest vcs-test.golang.org/git/tagtests.git@v0.2.2 vcs-test.golang.org/git/tagtests.git@master
+! stderr 'git fetch'
+stdout '"Reuse": true'
+! stdout '"(Dir|Info|GoMod|Zip)"'
+
+# reuse attempt with stale hash should reinvoke git, not report reuse
+cp tagtestsv022.json tagtestsv022badhash.json
+replace '57952' '56952XXX' tagtestsv022badhash.json
+go mod download -reuse=tagtestsv022badhash.json -x -json vcs-test.golang.org/git/tagtests.git@v0.2.2
+stderr 'git fetch'
+! stdout '"Reuse": true'
+stdout '"Version": "v0.2.2"'
+! stdout '"Query"'
+stdout '"VCS": "git"'
+stdout '"URL": ".*/git/tagtests"'
+! stdout '"(TagPrefix|TagSum)"'
+stdout '"Ref": "refs/tags/v0.2.2"'
+stdout '"Hash": "59356c8cd18c5fe9a598167d98a6843e52d57952"'
+stdout '"Dir"'
+stdout '"Info"'
+stdout '"GoMod"'
+stdout '"Zip"'
+
+# reuse with stale repo URL
+cp tagtestsv022.json tagtestsv022badurl.json
+replace 'git/tagtests\"' 'git/tagtestsXXX\"' tagtestsv022badurl.json
+go mod download -reuse=tagtestsv022badurl.json -x -json vcs-test.golang.org/git/tagtests.git@v0.2.2
+! stdout '"Reuse": true'
+stdout '"URL": ".*/git/tagtests"'
+stdout '"Dir"'
+stdout '"Info"'
+stdout '"GoMod"'
+stdout '"Zip"'
+
+# reuse with stale VCS
+cp tagtestsv022.json tagtestsv022badvcs.json
+replace '\"git\"' '\"gitXXX\"' tagtestsv022badvcs.json
+go mod download -reuse=tagtestsv022badvcs.json -x -json vcs-test.golang.org/git/tagtests.git@v0.2.2
+! stdout '"Reuse": true'
+stdout '"URL": ".*/git/tagtests"'
+
+# reuse with stale Dir
+cp tagtestsv022.json tagtestsv022baddir.json
+replace '\t\t\"Ref\":' '\t\t\"Subdir\": \"subdir\",\n\t\t\"Ref\":' tagtestsv022baddir.json
+go mod download -reuse=tagtestsv022baddir.json -x -json vcs-test.golang.org/git/tagtests.git@v0.2.2
+! stdout '"Reuse": true'
+stdout '"URL": ".*/git/tagtests"'
+
+# reuse with stale TagSum
+cp tagtests.json tagtestsbadtagsum.json
+replace 'sMEOGo=' 'sMEoGo=XXX' tagtestsbadtagsum.json
+go mod download -reuse=tagtestsbadtagsum.json -x -json vcs-test.golang.org/git/tagtests.git@latest
+! stdout '"Reuse": true'
+stdout '"TagSum": "t1:Dp7yRKDuE8WjG0429PN9hYWjqhy2te7P9Oki/sMEOGo="'
diff --git a/src/cmd/go/testdata/script/run_dirs.txt b/src/cmd/go/testdata/script/run_dirs.txt
new file mode 100644
index 0000000..bd5cfbe
--- /dev/null
+++ b/src/cmd/go/testdata/script/run_dirs.txt
@@ -0,0 +1,21 @@
+cd rundir
+
+! go run x.go sub/sub.go
+stderr 'named files must all be in one directory; have . and sub'
+! go run sub/sub.go x.go
+stderr 'named files must all be in one directory; have sub and .'
+
+cd ../
+go run rundir/foo.go ./rundir/bar.go
+stderr 'hello world'
+
+-- rundir/sub/sub.go --
+package main
+-- rundir/x.go --
+package main
+-- rundir/foo.go --
+package main
+func main() { println(msg) }
+-- rundir/bar.go --
+package main
+const msg = "hello world"
diff --git a/src/cmd/go/testdata/script/run_hello.txt b/src/cmd/go/testdata/script/run_hello.txt
new file mode 100644
index 0000000..939b661
--- /dev/null
+++ b/src/cmd/go/testdata/script/run_hello.txt
@@ -0,0 +1,9 @@
+env GO111MODULE=off
+
+# hello world
+go run hello.go
+stderr 'hello world'
+
+-- hello.go --
+package main
+func main() { println("hello world") }
diff --git a/src/cmd/go/testdata/script/run_hello_pkg.txt b/src/cmd/go/testdata/script/run_hello_pkg.txt
new file mode 100644
index 0000000..ea2b4d7
--- /dev/null
+++ b/src/cmd/go/testdata/script/run_hello_pkg.txt
@@ -0,0 +1,17 @@
+go run m/hello
+stderr 'hello, world'
+
+cd hello
+go run .
+stderr 'hello, world'
+
+-- go.mod --
+module m
+
+go 1.16
+-- hello/hello.go --
+package main
+
+func main() {
+	println("hello, world")
+}
diff --git a/src/cmd/go/testdata/script/run_internal.txt b/src/cmd/go/testdata/script/run_internal.txt
new file mode 100644
index 0000000..d021850
--- /dev/null
+++ b/src/cmd/go/testdata/script/run_internal.txt
@@ -0,0 +1,64 @@
+env GO111MODULE=off
+
+go list -e -f '{{.Incomplete}}' m/runbad1.go
+stdout true
+! go run m/runbad1.go
+stderr 'use of internal package m/x/internal not allowed'
+
+go list -e -f '{{.Incomplete}}' m/runbad2.go
+stdout true
+! go run m/runbad2.go
+stderr 'use of internal package m/x/internal/y not allowed'
+
+go list -e -f '{{.Incomplete}}' m/runok.go
+stdout false
+go run m/runok.go
+
+cd m
+env GO111MODULE=on
+
+go list -e -f '{{.Incomplete}}' runbad1.go
+stdout true
+! go run runbad1.go
+stderr 'use of internal package m/x/internal not allowed'
+
+go list -e -f '{{.Incomplete}}' runbad2.go
+stdout true
+! go run runbad2.go
+stderr 'use of internal package m/x/internal/y not allowed'
+
+go list -e -f '{{.Incomplete}}' runok.go
+stdout false
+go run runok.go
+
+
+-- m/go.mod --
+module m
+
+-- m/x/internal/internal.go --
+package internal
+
+-- m/x/internal/y/y.go --
+package y
+
+-- m/internal/internal.go --
+package internal
+
+-- m/internal/z/z.go --
+package z
+
+-- m/runbad1.go --
+package main
+import _ "m/x/internal"
+func main() {}
+
+-- m/runbad2.go --
+package main
+import _ "m/x/internal/y"
+func main() {}
+
+-- m/runok.go --
+package main
+import _ "m/internal"
+import _ "m/internal/z"
+func main() {}
diff --git a/src/cmd/go/testdata/script/run_issue11709.txt b/src/cmd/go/testdata/script/run_issue11709.txt
new file mode 100644
index 0000000..c8ba998
--- /dev/null
+++ b/src/cmd/go/testdata/script/run_issue11709.txt
@@ -0,0 +1,15 @@
+# 'go run' should not pass extraneous environment variables to the subprocess.
+go run run.go
+! stdout .
+! stderr .
+
+-- run.go --
+package main
+
+import "os"
+
+func main() {
+	if os.Getenv("TERM") != "" {
+		os.Exit(1)
+	}
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/run_issue51125.txt b/src/cmd/go/testdata/script/run_issue51125.txt
new file mode 100644
index 0000000..8fa4486
--- /dev/null
+++ b/src/cmd/go/testdata/script/run_issue51125.txt
@@ -0,0 +1,54 @@
+# Regression test for https://go.dev/issue/51125:
+# Relative import paths (a holdover from GOPATH) were accidentally allowed in module mode.
+
+cd $WORK
+
+# Relative imports should not be allowed with a go.mod file.
+
+! go run driver.go
+stderr '^driver.go:3:8: "./mypkg" is relative, but relative import paths are not supported in module mode$'
+
+go list -e -f '{{with .Error}}{{.}}{{end}}' -deps driver.go
+stdout '^driver.go:3:8: "./mypkg" is relative, but relative import paths are not supported in module mode$'
+! stderr .
+
+
+# Relative imports should not be allowed in module mode even without a go.mod file.
+rm go.mod
+
+! go run driver.go
+stderr '^driver.go:3:8: "./mypkg" is relative, but relative import paths are not supported in module mode$'
+
+go list -e -f '{{with .Error}}{{.}}{{end}}' -deps driver.go
+stdout '^driver.go:3:8: "./mypkg" is relative, but relative import paths are not supported in module mode$'
+! stderr .
+
+
+# In GOPATH mode, they're still allowed (but only outside of GOPATH/src).
+env GO111MODULE=off
+
+[!short] go run driver.go
+
+go list -deps driver.go
+
+
+-- $WORK/go.mod --
+module example
+
+go 1.17
+-- $WORK/driver.go --
+package main
+
+import "./mypkg"
+
+func main() {
+	mypkg.MyFunc()
+}
+-- $WORK/mypkg/code.go --
+package mypkg
+
+import "fmt"
+
+func MyFunc() {
+	fmt.Println("Hello, world!")
+}
diff --git a/src/cmd/go/testdata/script/run_set_executable_name.txt b/src/cmd/go/testdata/script/run_set_executable_name.txt
new file mode 100644
index 0000000..54ddee9
--- /dev/null
+++ b/src/cmd/go/testdata/script/run_set_executable_name.txt
@@ -0,0 +1,50 @@
+env GO111MODULE=on
+[short] skip
+
+# Check for correct naming of temporary executable
+
+#Test for single file specified
+cd x/y/z
+go run foo.go
+stderr 'foo'
+
+#Test for current directory
+go run .
+stderr 'z'
+
+#Test for set path
+go run m/x/y/z/
+stderr 'z'
+
+-- m/x/y/z/foo.go --
+package main
+import(
+	"os"
+	"path/filepath"
+)
+func main() {
+	println(filepath.Base(os.Args[0]))
+}
+
+-- x/y/z/foo.go --
+package main
+import(
+	"os"
+	"path/filepath"
+)
+func main() {
+	println(filepath.Base(os.Args[0]))
+}
+
+-- x/y/z/foo.go --
+package main
+import(
+	"os"
+	"path/filepath"
+)
+func main() {
+	println(filepath.Base(os.Args[0]))
+}
+
+-- go.mod --
+module m
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/run_vendor.txt b/src/cmd/go/testdata/script/run_vendor.txt
new file mode 100644
index 0000000..46cac06
--- /dev/null
+++ b/src/cmd/go/testdata/script/run_vendor.txt
@@ -0,0 +1,34 @@
+# Run
+env GO111MODULE=off
+cd vend/hello
+go run hello.go
+stdout 'hello, world'
+
+-- vend/hello/hello.go --
+package main
+
+import (
+	"fmt"
+	"strings" // really ../vendor/strings
+)
+
+func main() {
+	fmt.Printf("%s\n", strings.Msg)
+}
+-- vend/hello/hello_test.go --
+package main
+
+import (
+	"strings" // really ../vendor/strings
+	"testing"
+)
+
+func TestMsgInternal(t *testing.T) {
+	if strings.Msg != "hello, world" {
+		t.Fatalf("unexpected msg: %v", strings.Msg)
+	}
+}
+-- vend/vendor/strings/msg.go --
+package strings
+
+var Msg = "hello, world"
diff --git a/src/cmd/go/testdata/script/run_wildcard.txt b/src/cmd/go/testdata/script/run_wildcard.txt
new file mode 100644
index 0000000..3e7e7b7
--- /dev/null
+++ b/src/cmd/go/testdata/script/run_wildcard.txt
@@ -0,0 +1,7 @@
+env GO111MODULE=off
+
+# Fix for https://github.com/golang/go/issues/28696:
+# go run x/... should not panic when directory x doesn't exist.
+
+! go run nonexistent/...
+stderr '^go: no packages loaded from nonexistent/...$'
diff --git a/src/cmd/go/testdata/script/run_work_versioned.txt b/src/cmd/go/testdata/script/run_work_versioned.txt
new file mode 100644
index 0000000..eb0f22d
--- /dev/null
+++ b/src/cmd/go/testdata/script/run_work_versioned.txt
@@ -0,0 +1,16 @@
+[short] skip
+go run example.com/printversion@v0.1.0
+stdout '^main is example.com/printversion v0.1.0$'
+
+-- go.work --
+go 1.18
+
+use (
+	.
+)
+-- go.mod --
+module example
+
+go 1.18
+
+require example.com/printversion v1.0.0
diff --git a/src/cmd/go/testdata/script/script_help.txt b/src/cmd/go/testdata/script/script_help.txt
new file mode 100644
index 0000000..dd7f203
--- /dev/null
+++ b/src/cmd/go/testdata/script/script_help.txt
@@ -0,0 +1,7 @@
+help -v
+
+# To see help for a specific command or condition, run 'help' for it here.
+# For example:
+help wait
+help [verbose]
+help [exec:bash]
diff --git a/src/cmd/go/testdata/script/script_wait.txt b/src/cmd/go/testdata/script/script_wait.txt
new file mode 100644
index 0000000..f253060
--- /dev/null
+++ b/src/cmd/go/testdata/script/script_wait.txt
@@ -0,0 +1,28 @@
+env GO111MODULE=off
+
+[!exec:echo] skip
+[!exec:false] skip
+
+exec echo foo
+stdout foo
+
+exec echo foo &
+exec echo bar &
+! exec false &
+
+# Starting a background process should clear previous output.
+! stdout foo
+
+# Wait should set the output to the concatenated outputs of the background
+# programs, in the order in which they were started.
+wait
+stdout 'foo\nbar'
+
+# The end of the test should interrupt or kill any remaining background
+# programs, but that should not cause the test to fail if it does not
+# care about the exit status of those programs.
+[exec:sleep] ? exec sleep 86400 &
+
+# It should also cancel any backgrounded builtins that respond to Context
+# cancellation.
+? sleep 86400s &
diff --git a/src/cmd/go/testdata/script/slashpath.txt b/src/cmd/go/testdata/script/slashpath.txt
new file mode 100644
index 0000000..22b3e9d
--- /dev/null
+++ b/src/cmd/go/testdata/script/slashpath.txt
@@ -0,0 +1,18 @@
+# .a files should use slash-separated paths even on windows
+# This is important for reproducing native builds with cross-compiled builds.
+go build -o x.a text/template
+! grep 'GOROOT\\' x.a
+! grep 'text\\template' x.a
+! grep 'c:\\' x.a
+
+# executables should use slash-separated paths even on windows
+# This is important for reproducing native builds with cross-compiled builds.
+go build -o hello.exe hello.go
+! grep 'GOROOT\\' hello.exe
+! grep '\\runtime' hello.exe
+! grep 'runtime\\' hello.exe
+! grep 'gofile..[A-Za-z]:\\' hello.exe
+
+-- hello.go --
+package main
+func main() { println("hello") }
diff --git a/src/cmd/go/testdata/script/src_file.txt b/src/cmd/go/testdata/script/src_file.txt
new file mode 100644
index 0000000..8d5c20b
--- /dev/null
+++ b/src/cmd/go/testdata/script/src_file.txt
@@ -0,0 +1,9 @@
+# Files in src should not be treated as packages
+
+exists $GOROOT/src/regexp/testdata/README
+go list -f '{{.Dir}}' regexp/testdata/README
+
+-- go.mod --
+module regexp/testdata/README
+-- p.go --
+package p
diff --git a/src/cmd/go/testdata/script/std_vendor.txt b/src/cmd/go/testdata/script/std_vendor.txt
new file mode 100644
index 0000000..731ee9e
--- /dev/null
+++ b/src/cmd/go/testdata/script/std_vendor.txt
@@ -0,0 +1,43 @@
+env GO111MODULE=off
+
+[!compiler:gc] skip
+
+# 'go list' should report imports from _test.go in the TestImports field.
+go list -f '{{.TestImports}}'
+stdout net/http # from .TestImports
+
+# 'go list' should report standard-vendored packages by path.
+go list -f '{{.Dir}}' vendor/golang.org/x/net/http2/hpack
+stdout $GOROOT[/\\]src[/\\]vendor
+
+# 'go list -test' should report vendored transitive dependencies of _test.go
+# imports in the Deps field, with a 'vendor' prefix on their import paths.
+go list -test -f '{{.Deps}}'
+stdout golang.org/x/crypto # dep of .TestImports
+
+# Packages outside the standard library should not use its copy of vendored packages.
+cd broken
+! go build
+stderr 'cannot find package'
+
+-- go.mod --
+module m
+
+-- x.go --
+package x
+
+-- x_test.go --
+package x
+import "testing"
+import _ "net/http"
+func Test(t *testing.T) {}
+
+-- broken/go.mod --
+module broken
+-- broken/http.go --
+package broken
+
+import (
+	_ "net/http"
+	_ "golang.org/x/net/http/httpproxy"
+)
diff --git a/src/cmd/go/testdata/script/test2json_interrupt.txt b/src/cmd/go/testdata/script/test2json_interrupt.txt
new file mode 100644
index 0000000..763c336
--- /dev/null
+++ b/src/cmd/go/testdata/script/test2json_interrupt.txt
@@ -0,0 +1,58 @@
+[short] skip 'links and runs a test binary'
+[!fuzz] skip 'tests SIGINT behavior for interrupting fuzz tests'
+[GOOS:windows] skip 'windows does not support os.Interrupt'
+
+? go test -json -fuzz FuzzInterrupt -run '^$' -parallel 1
+stdout -count=1 '"Action":"pass","Package":"example","Test":"FuzzInterrupt"'
+stdout -count=1 '"Action":"pass","Package":"example","Elapsed":'
+
+mkdir $WORK/fuzzcache
+go test -c . -fuzz=. -o example_test.exe
+? go tool test2json -p example -t ./example_test.exe -test.v -test.paniconexit0 -test.fuzzcachedir $WORK/fuzzcache -test.fuzz FuzzInterrupt -test.run '^$' -test.parallel 1
+stdout -count=1 '"Action":"pass","Package":"example","Test":"FuzzInterrupt"'
+stdout -count=1 '"Action":"pass","Package":"example","Elapsed":'
+
+-- go.mod --
+module example
+go 1.20
+-- example_test.go --
+package example_test
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+	"testing"
+	"strings"
+	"time"
+)
+
+func FuzzInterrupt(f *testing.F) {
+	pids := os.Getenv("GO_TEST_INTERRUPT_PIDS")
+	if pids == "" {
+		// This is the main test process.
+		// Set the environment variable for fuzz workers.
+		pid := os.Getpid()
+		ppid := os.Getppid()
+		os.Setenv("GO_TEST_INTERRUPT_PIDS", fmt.Sprintf("%d,%d", ppid, pid))
+	}
+
+	sentInterrupt := false
+	f.Fuzz(func(t *testing.T, orig string) {
+		if !sentInterrupt {
+			// Simulate a ctrl-C on the keyboard by sending SIGINT
+			// to the main test process and its parent.
+			for _, pid := range strings.Split(pids, ",") {
+				i, err := strconv.Atoi(pid)
+				if err != nil {
+					t.Fatal(err)
+				}
+				if p, err := os.FindProcess(i); err == nil {
+					p.Signal(os.Interrupt)
+					sentInterrupt = true // Only send interrupts once.
+				}
+			}
+		}
+		time.Sleep(1 * time.Millisecond)  // Delay the fuzzer a bit to avoid wasting CPU.
+	})
+}
diff --git a/src/cmd/go/testdata/script/test_bad_example.txt b/src/cmd/go/testdata/script/test_bad_example.txt
new file mode 100644
index 0000000..1d147b6
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_bad_example.txt
@@ -0,0 +1,13 @@
+# Tests that invalid examples are ignored.
+# Verifies golang.org/issue/35284
+go test x_test.go
+
+-- x_test.go --
+package  x
+
+import "fmt"
+
+func ExampleThisShouldNotHaveAParameter(thisShouldntExist int) {
+	fmt.Println("X")
+	// Output:
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/test_badtest.txt b/src/cmd/go/testdata/script/test_badtest.txt
new file mode 100644
index 0000000..e79fc51
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_badtest.txt
@@ -0,0 +1,49 @@
+env GO111MODULE=off
+
+! go test badtest/badexec
+! stdout ^ok
+stdout ^FAIL\tbadtest/badexec
+
+! go test badtest/badsyntax
+! stdout ^ok
+stdout ^FAIL\tbadtest/badsyntax
+
+! go test badtest/badvar
+! stdout ^ok
+stdout ^FAIL\tbadtest/badvar
+
+! go test notest
+! stdout ^ok
+stderr '^notest.hello.go:6:1: syntax error: non-declaration statement outside function body' # Exercise issue #7108
+
+-- badtest/badexec/x_test.go --
+package badexec
+
+func init() {
+	panic("badexec")
+}
+
+-- badtest/badsyntax/x.go --
+package badsyntax
+
+-- badtest/badsyntax/x_test.go --
+package badsyntax
+
+func func func func func!
+
+-- badtest/badvar/x.go --
+package badvar
+
+-- badtest/badvar/x_test.go --
+package badvar_test
+
+func f() {
+	_ = notdefined
+}
+-- notest/hello.go --
+package notest
+
+func hello() {
+	println("hello world")
+}
+Hello world
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/test_benchmark_1x.txt b/src/cmd/go/testdata/script/test_benchmark_1x.txt
new file mode 100644
index 0000000..b1d4c39
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_benchmark_1x.txt
@@ -0,0 +1,37 @@
+# Test that -benchtime 1x only runs a total of 1 loop iteration.
+# See golang.org/issue/32051.
+
+go test -run ^$ -bench . -benchtime 1x
+
+-- go.mod --
+module bench
+
+go 1.16
+-- x_test.go --
+package bench
+
+import (
+	"fmt"
+	"os"
+	"testing"
+)
+
+var called = false
+
+func TestMain(m *testing.M) {
+	m.Run()
+	if !called {
+		fmt.Println("benchmark never called")
+		os.Exit(1)
+	}
+}
+
+func Benchmark(b *testing.B) {
+	if b.N > 1 {
+		b.Fatalf("called with b.N=%d; want b.N=1 only", b.N)
+	}
+	if called {
+		b.Fatal("called twice")
+	}
+	called = true
+}
diff --git a/src/cmd/go/testdata/script/test_benchmark_chatty_fail.txt b/src/cmd/go/testdata/script/test_benchmark_chatty_fail.txt
new file mode 100644
index 0000000..6031ead
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_benchmark_chatty_fail.txt
@@ -0,0 +1,32 @@
+# Run chatty tests. Assert on CONT lines.
+! go test chatty_test.go -v -bench . chatty_bench
+
+# Sanity check that output occurs.
+stdout -count=2 'this is sub-0'
+stdout -count=2 'this is sub-1'
+stdout -count=2 'this is sub-2'
+stdout -count=1 'error from sub-0'
+stdout -count=1 'error from sub-1'
+stdout -count=1 'error from sub-2'
+
+# Benchmarks should not print CONT.
+! stdout CONT
+
+-- chatty_test.go --
+package chatty_bench
+
+import (
+	"testing"
+	"fmt"
+)
+
+func BenchmarkChatty(b *testing.B) {
+    for i := 0; i < 3; i++ {
+        b.Run(fmt.Sprintf("sub-%d", i), func(b *testing.B) {
+            for j := 0; j < 2; j++ {
+                b.Logf("this is sub-%d", i)
+            }
+            b.Errorf("error from sub-%d", i)
+        })
+    }
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/test_benchmark_chatty_success.txt b/src/cmd/go/testdata/script/test_benchmark_chatty_success.txt
new file mode 100644
index 0000000..a1c0d65
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_benchmark_chatty_success.txt
@@ -0,0 +1,29 @@
+# Run chatty tests. Assert on CONT lines.
+go test chatty_test.go -v -bench . chatty_bench
+
+# Sanity check that output happens. We don't provide -count because the amount
+# of output is variable.
+stdout 'this is sub-0'
+stdout 'this is sub-1'
+stdout 'this is sub-2'
+
+# Benchmarks should not print CONT.
+! stdout CONT
+
+-- chatty_test.go --
+package chatty_bench
+
+import (
+	"testing"
+	"fmt"
+)
+
+func BenchmarkChatty(b *testing.B) {
+    for i := 0; i < 3; i++ {
+        b.Run(fmt.Sprintf("sub-%d", i), func(b *testing.B) {
+            for j := 0; j < 2; j++ {
+                b.Logf("this is sub-%d", i)
+            }
+        })
+    }
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/test_benchmark_fatal.txt b/src/cmd/go/testdata/script/test_benchmark_fatal.txt
new file mode 100644
index 0000000..e281379
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_benchmark_fatal.txt
@@ -0,0 +1,19 @@
+# Test that calling t.Fatal in a benchmark causes a non-zero exit status.
+
+! go test -run '^$' -bench . benchfatal
+! stdout ^ok
+! stderr ^ok
+stdout FAIL.*benchfatal
+
+-- go.mod --
+module benchfatal
+
+go 1.16
+-- x_test.go --
+package benchfatal
+
+import "testing"
+
+func BenchmarkThatCallsFatal(b *testing.B) {
+	b.Fatal("called by benchmark")
+}
diff --git a/src/cmd/go/testdata/script/test_benchmark_labels.txt b/src/cmd/go/testdata/script/test_benchmark_labels.txt
new file mode 100644
index 0000000..6b424c1
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_benchmark_labels.txt
@@ -0,0 +1,23 @@
+# Tests that go test -bench prints out goos, goarch, and pkg.
+
+# Check for goos, goarch, and pkg.
+go test -run ^$ -bench . bench
+stdout '^goos: '$GOOS
+stdout '^goarch: '$GOARCH
+stdout '^pkg: bench'
+
+# Check go test does not print pkg multiple times
+! stdout 'pkg:.*pkg: '
+! stderr 'pkg:.*pkg:'
+
+-- go.mod --
+module bench
+
+go 1.16
+-- x_test.go --
+package bench
+
+import "testing"
+
+func Benchmark(b *testing.B) {
+}
diff --git a/src/cmd/go/testdata/script/test_benchmark_timeout.txt b/src/cmd/go/testdata/script/test_benchmark_timeout.txt
new file mode 100644
index 0000000..4bae7e7
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_benchmark_timeout.txt
@@ -0,0 +1,18 @@
+# Tests issue #18845
+[short] skip
+
+go test -bench . -timeout=750ms timeoutbench_test.go
+stdout ok
+stdout PASS
+
+-- timeoutbench_test.go --
+package timeoutbench_test
+
+import (
+	"testing"
+	"time"
+)
+
+func BenchmarkSleep1s(b *testing.B) {
+	time.Sleep(1 * time.Second)
+}
diff --git a/src/cmd/go/testdata/script/test_build_failure.txt b/src/cmd/go/testdata/script/test_build_failure.txt
new file mode 100644
index 0000000..e8c984f
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_build_failure.txt
@@ -0,0 +1,31 @@
+[short] skip
+
+! go test -x coverbad
+! stderr '[\\/]coverbad\.test( |$)' # 'go test' should not claim to have run the test.
+stderr 'undefined: g'
+[cgo] stderr 'undefined: j'
+
+-- go.mod --
+module coverbad
+
+go 1.16
+-- p.go --
+package p
+
+func f() {
+	g()
+}
+-- p1.go --
+package p
+
+import "C"
+
+func h() {
+	j()
+}
+-- p_test.go --
+package p
+
+import "testing"
+
+func Test(t *testing.T) {}
diff --git a/src/cmd/go/testdata/script/test_buildvcs.txt b/src/cmd/go/testdata/script/test_buildvcs.txt
new file mode 100644
index 0000000..db844f8
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_buildvcs.txt
@@ -0,0 +1,104 @@
+# https://go.dev/issue/51723: 'go test' should not stamp VCS metadata
+# in the build settings. (It isn't worth the latency hit, given that
+# test binaries are almost never distributed to users.)
+
+[short] skip
+[!git] skip
+
+exec git init
+
+# The test binaries should not have VCS settings stamped by default.
+# (The test itself verifies that.)
+go test . ./testonly
+
+# However, setting -buildvcs explicitly should override that and
+# stamp anyway (https://go.dev/issue/52648).
+go test -buildvcs -c -o ./testonly.exe ./testonly
+! exec ./testonly.exe
+stdout 'unexpected VCS setting: vcs\.modified=true'
+
+
+# Remove 'git' from $PATH. The test should still build.
+# This ensures that we aren't loading VCS metadata that
+# we subsequently throw away.
+env PATH=''
+env path=''
+
+# Compiling the test should not require the VCS tool.
+go test -c -o $devnull .
+
+
+# When listing a main package, in general we need its VCS metadata to determine
+# the .Stale and .StaleReason fields.
+! go list -buildvcs=true .
+stderr '^go: missing Git command\. See https://golang\.org/s/gogetcmd\nerror obtaining VCS status: .*\n\tUse -buildvcs=false to disable VCS stamping.'
+
+# Adding the -test flag should be strictly additive — it should not suppress the error.
+! go list -buildvcs=true -test .
+stderr '^go: missing Git command\. See https://golang\.org/s/gogetcmd\nerror obtaining VCS status: .*\n\tUse -buildvcs=false to disable VCS stamping.'
+
+# Adding the suggested flag should suppress the error.
+go list -test -buildvcs=false .
+! stderr .
+
+
+# Since the ./testonly package doesn't itself produce an actual binary, we shouldn't
+# invoke a VCS tool to compute a build stamp by default when listing it.
+go list ./testonly
+! stderr .
+go list -test ./testonly
+! stderr .
+
+# Again, setting -buildvcs explicitly should force the use of the VCS tool.
+! go list -buildvcs ./testonly
+stderr '^go: missing Git command\. See https://golang\.org/s/gogetcmd\nerror obtaining VCS status: .*\n\tUse -buildvcs=false to disable VCS stamping.'
+! go list -buildvcs -test ./testonly
+stderr '^go: missing Git command\. See https://golang\.org/s/gogetcmd\nerror obtaining VCS status: .*\n\tUse -buildvcs=false to disable VCS stamping.'
+
+
+-- go.mod --
+module example
+
+go 1.18
+-- example.go --
+package main
+-- example_test.go --
+package main
+
+import (
+	"runtime/debug"
+	"strings"
+	"testing"
+)
+
+func TestDetail(t *testing.T) {
+	bi, ok := debug.ReadBuildInfo()
+	if !ok {
+		t.Fatal("BuildInfo not present")
+	}
+	for _, s := range bi.Settings {
+		if strings.HasPrefix(s.Key, "vcs.") {
+			t.Fatalf("unexpected VCS setting: %s=%s", s.Key, s.Value)
+		}
+	}
+}
+-- testonly/main_test.go --
+package main
+
+import (
+	"runtime/debug"
+	"strings"
+	"testing"
+)
+
+func TestDetail(t *testing.T) {
+	bi, ok := debug.ReadBuildInfo()
+	if !ok {
+		t.Fatal("BuildInfo not present")
+	}
+	for _, s := range bi.Settings {
+		if strings.HasPrefix(s.Key, "vcs.") {
+			t.Fatalf("unexpected VCS setting: %s=%s", s.Key, s.Value)
+		}
+	}
+}
diff --git a/src/cmd/go/testdata/script/test_cache_inputs.txt b/src/cmd/go/testdata/script/test_cache_inputs.txt
new file mode 100644
index 0000000..3705c70
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_cache_inputs.txt
@@ -0,0 +1,270 @@
+env GO111MODULE=off
+
+# Test that cached test results are invalidated in response to
+# changes to the external inputs to the test.
+
+[short] skip
+[GODEBUG:gocacheverify=1] skip
+
+# We're testing cache behavior, so start with a clean GOCACHE.
+env GOCACHE=$WORK/cache
+
+# Build a helper binary to invoke os.Chtimes.
+go build -o mkold$GOEXE mkold.go
+
+# Make test input files appear to be a minute old.
+exec ./mkold$GOEXE 1m testcache/file.txt
+exec ./mkold$GOEXE 1m testcache/script.sh
+
+# If the test reads an environment variable, changes to that variable
+# should invalidate cached test results.
+env TESTKEY=x
+go test testcache -run=TestLookupEnv
+go test testcache -run=TestLookupEnv
+stdout '\(cached\)'
+
+env TESTKEY=y
+go test testcache -run=TestLookupEnv
+! stdout '\(cached\)'
+go test testcache -run=TestLookupEnv
+stdout '\(cached\)'
+
+# Changes in arguments forwarded to the test should invalidate cached test
+# results.
+go test testcache -run=TestOSArgs -v hello
+! stdout '\(cached\)'
+stdout 'hello'
+go test testcache -run=TestOSArgs -v goodbye
+! stdout '\(cached\)'
+stdout 'goodbye'
+
+# golang.org/issue/36134: that includes the `-timeout` argument.
+go test testcache -run=TestOSArgs -timeout=20m -v
+! stdout '\(cached\)'
+stdout '-test\.timeout[= ]20m'
+go test testcache -run=TestOSArgs -timeout=5s -v
+! stdout '\(cached\)'
+stdout '-test\.timeout[= ]5s'
+
+# If the test stats a file, changes to the file should invalidate the cache.
+go test testcache -run=FileSize
+go test testcache -run=FileSize
+stdout '\(cached\)'
+
+cp 4x.txt testcache/file.txt
+go test testcache -run=FileSize
+! stdout '\(cached\)'
+go test testcache -run=FileSize
+stdout '\(cached\)'
+
+# Files should be tracked even if the test changes its working directory.
+go test testcache -run=Chdir
+go test testcache -run=Chdir
+stdout '\(cached\)'
+cp 6x.txt testcache/file.txt
+go test testcache -run=Chdir
+! stdout '\(cached\)'
+go test testcache -run=Chdir
+stdout '\(cached\)'
+
+# The content of files should affect caching, provided that the mtime also changes.
+exec ./mkold$GOEXE 1m testcache/file.txt
+go test testcache -run=FileContent
+go test testcache -run=FileContent
+stdout '\(cached\)'
+cp 2y.txt testcache/file.txt
+exec ./mkold$GOEXE 50s testcache/file.txt
+go test testcache -run=FileContent
+! stdout '\(cached\)'
+go test testcache -run=FileContent
+stdout '\(cached\)'
+
+# Directory contents read via os.ReadDirNames should affect caching.
+go test testcache -run=DirList
+go test testcache -run=DirList
+stdout '\(cached\)'
+rm testcache/file.txt
+go test testcache -run=DirList
+! stdout '\(cached\)'
+go test testcache -run=DirList
+stdout '\(cached\)'
+
+# Files outside GOROOT and GOPATH should not affect caching.
+env TEST_EXTERNAL_FILE=$WORK/external.txt
+go test testcache -run=ExternalFile
+go test testcache -run=ExternalFile
+stdout '\(cached\)'
+
+rm $WORK/external.txt
+go test testcache -run=ExternalFile
+stdout '\(cached\)'
+
+# The -benchtime flag without -bench should not affect caching.
+go test testcache -run=Benchtime -benchtime=1x
+go test testcache -run=Benchtime -benchtime=1x
+stdout '\(cached\)'
+
+go test testcache -run=Benchtime -bench=Benchtime -benchtime=1x
+go test testcache -run=Benchtime -bench=Benchtime -benchtime=1x
+! stdout '\(cached\)'
+
+# golang.org/issue/47355: that includes the `-failfast` argument.
+go test testcache -run=TestOSArgs -failfast
+! stdout '\(cached\)'
+go test testcache -run=TestOSArgs -failfast
+stdout '\(cached\)'
+
+# Executables within GOROOT and GOPATH should affect caching,
+# even if the test does not stat them explicitly.
+
+[!exec:/bin/sh] skip
+chmod 0755 ./testcache/script.sh
+
+exec ./mkold$GOEXEC 1m testcache/script.sh
+go test testcache -run=Exec
+go test testcache -run=Exec
+stdout '\(cached\)'
+
+exec ./mkold$GOEXE 50s testcache/script.sh
+go test testcache -run=Exec
+! stdout '\(cached\)'
+go test testcache -run=Exec
+stdout '\(cached\)'
+
+-- testcache/file.txt --
+xx
+-- 4x.txt --
+xxxx
+-- 6x.txt --
+xxxxxx
+-- 2y.txt --
+yy
+-- $WORK/external.txt --
+This file is outside of GOPATH.
+-- testcache/script.sh --
+#!/bin/sh
+exit 0
+-- testcache/testcache_test.go --
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testcache
+
+import (
+	"io"
+	"os"
+	"testing"
+)
+
+func TestChdir(t *testing.T) {
+	os.Chdir("..")
+	defer os.Chdir("testcache")
+	info, err := os.Stat("testcache/file.txt")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if info.Size()%2 != 1 {
+		t.Fatal("even file")
+	}
+}
+
+func TestOddFileContent(t *testing.T) {
+	f, err := os.Open("file.txt")
+	if err != nil {
+		t.Fatal(err)
+	}
+	data, err := io.ReadAll(f)
+	f.Close()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(data)%2 != 1 {
+		t.Fatal("even file")
+	}
+}
+
+func TestOddFileSize(t *testing.T) {
+	info, err := os.Stat("file.txt")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if info.Size()%2 != 1 {
+		t.Fatal("even file")
+	}
+}
+
+func TestOddGetenv(t *testing.T) {
+	val := os.Getenv("TESTKEY")
+	if len(val)%2 != 1 {
+		t.Fatal("even env value")
+	}
+}
+
+func TestLookupEnv(t *testing.T) {
+	_, ok := os.LookupEnv("TESTKEY")
+	if !ok {
+		t.Fatal("env missing")
+	}
+}
+
+func TestDirList(t *testing.T) {
+	f, err := os.Open(".")
+	if err != nil {
+		t.Fatal(err)
+	}
+	f.Readdirnames(-1)
+	f.Close()
+}
+
+func TestExec(t *testing.T) {
+	// Note: not using os/exec to make sure there is no unexpected stat.
+	p, err := os.StartProcess("./script.sh", []string{"script"}, new(os.ProcAttr))
+	if err != nil {
+		t.Fatal(err)
+	}
+	ps, err := p.Wait()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !ps.Success() {
+		t.Fatalf("script failed: %v", err)
+	}
+}
+
+func TestExternalFile(t *testing.T) {
+	os.Open(os.Getenv("TEST_EXTERNAL_FILE"))
+	_, err := os.Stat(os.Getenv("TEST_EXTERNAL_FILE"))
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestOSArgs(t *testing.T) {
+	t.Log(os.Args)
+}
+
+func TestBenchtime(t *testing.T) {
+}
+
+-- mkold.go --
+package main
+
+import (
+	"log"
+	"os"
+	"time"
+)
+
+func main() {
+	d, err := time.ParseDuration(os.Args[1])
+	if err != nil {
+		log.Fatal(err)
+	}
+	path := os.Args[2]
+	old := time.Now().Add(-d)
+	err = os.Chtimes(path, old, old)
+	if err != nil {
+		log.Fatal(err)
+	}
+}
diff --git a/src/cmd/go/testdata/script/test_chatty_fail.txt b/src/cmd/go/testdata/script/test_chatty_fail.txt
new file mode 100644
index 0000000..a5ef559
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_chatty_fail.txt
@@ -0,0 +1,32 @@
+# Run chatty tests. Assert on CONT lines.
+! go test chatty_test.go -v
+
+# Sanity check that output occurs.
+stdout -count=2 'this is sub-0'
+stdout -count=2 'this is sub-1'
+stdout -count=2 'this is sub-2'
+stdout -count=1 'error from sub-0'
+stdout -count=1 'error from sub-1'
+stdout -count=1 'error from sub-2'
+
+# Non-parallel tests should not print CONT.
+! stdout CONT
+
+-- chatty_test.go --
+package chatty_test
+
+import (
+	"testing"
+	"fmt"
+)
+
+func TestChatty(t *testing.T) {
+    for i := 0; i < 3; i++ {
+        t.Run(fmt.Sprintf("sub-%d", i), func(t *testing.T) {
+            for j := 0; j < 2; j++ {
+                t.Logf("this is sub-%d", i)
+            }
+            t.Errorf("error from sub-%d", i)
+        })
+    }
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/test_chatty_parallel_fail.txt b/src/cmd/go/testdata/script/test_chatty_parallel_fail.txt
new file mode 100644
index 0000000..62f97db
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_chatty_parallel_fail.txt
@@ -0,0 +1,62 @@
+# Run parallel chatty tests.
+# Check that multiple parallel outputs continue running.
+! go test -parallel 3 chatty_parallel_test.go -v
+
+stdout -count=1 '^=== CONT  TestChattyParallel/sub-0'
+stdout -count=1 '^=== CONT  TestChattyParallel/sub-1'
+stdout -count=1 '^=== CONT  TestChattyParallel/sub-2'
+
+stdout -count=1 '^=== (CONT|NAME)  TestChattyParallel/sub-0\n    chatty_parallel_test.go:38: error from sub-0$'
+stdout -count=1 '^=== (CONT|NAME)  TestChattyParallel/sub-1\n    chatty_parallel_test.go:38: error from sub-1$'
+stdout -count=1 '^=== (CONT|NAME)  TestChattyParallel/sub-2\n    chatty_parallel_test.go:38: error from sub-2$'
+
+# Run parallel chatty tests with -json.
+# Check that each output is attributed to the right test.
+! go test -json -parallel 3 chatty_parallel_test.go -v
+stdout -count=1 '"Test":"TestChattyParallel/sub-0","Output":"    chatty_parallel_test.go:38: error from sub-0\\n"'
+stdout -count=1 '"Test":"TestChattyParallel/sub-1","Output":"    chatty_parallel_test.go:38: error from sub-1\\n"'
+stdout -count=1 '"Test":"TestChattyParallel/sub-2","Output":"    chatty_parallel_test.go:38: error from sub-2\\n"'
+
+-- chatty_parallel_test.go --
+package chatty_parallel_test
+
+import (
+	"testing"
+	"fmt"
+	"flag"
+)
+
+// This test ensures the order of CONT lines in parallel chatty tests.
+func TestChattyParallel(t *testing.T) {
+	t.Parallel()
+
+	// The number of concurrent tests running. This is closely tied to the
+	// -parallel test flag, so we grab it from the flag rather than setting it
+	// to some constant.
+	parallel := flag.Lookup("test.parallel").Value.(flag.Getter).Get().(int)
+
+	// ready is a synchronization mechanism that causes subtests to execute
+	// round robin.
+	ready := make([]chan bool, parallel)
+	for i := range ready {
+		ready[i] = make(chan bool, 1)
+	}
+	ready[0] <- true
+
+	for i := range ready {
+		i := i
+		t.Run(fmt.Sprintf("sub-%d", i), func(t *testing.T) {
+			t.Parallel()
+
+			// Some basic log output to precede the failures.
+			<-ready[i]
+			t.Logf("this is sub-%d", i)
+			ready[(i+1)%len(ready)] <- true
+
+			// The actual failure messages we care about.
+			<-ready[i]
+			t.Errorf("error from sub-%d", i)
+			ready[(i+1)%len(ready)] <- true
+		})
+	}
+}
diff --git a/src/cmd/go/testdata/script/test_chatty_parallel_success.txt b/src/cmd/go/testdata/script/test_chatty_parallel_success.txt
new file mode 100644
index 0000000..01653ac
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_chatty_parallel_success.txt
@@ -0,0 +1,52 @@
+# Run parallel chatty tests. Assert on CONT or NAME lines. This test makes sure that
+# multiple parallel outputs have the appropriate test name lines between them.
+go test -parallel 3 chatty_parallel_test.go -v
+stdout -count=2 '^=== (CONT|NAME)  TestChattyParallel/sub-0\n    chatty_parallel_test.go:32: this is sub-0$'
+stdout -count=2 '^=== (CONT|NAME)  TestChattyParallel/sub-1\n    chatty_parallel_test.go:32: this is sub-1$'
+stdout -count=2 '^=== (CONT|NAME)  TestChattyParallel/sub-2\n    chatty_parallel_test.go:32: this is sub-2$'
+
+# Run parallel chatty tests with -json.
+# Assert test2json has properly attributed output.
+go test -json -parallel 3 chatty_parallel_test.go -v
+stdout -count=2 '"Test":"TestChattyParallel/sub-0","Output":"    chatty_parallel_test.go:32: this is sub-0\\n"'
+stdout -count=2 '"Test":"TestChattyParallel/sub-1","Output":"    chatty_parallel_test.go:32: this is sub-1\\n"'
+stdout -count=2 '"Test":"TestChattyParallel/sub-2","Output":"    chatty_parallel_test.go:32: this is sub-2\\n"'
+
+-- chatty_parallel_test.go --
+package chatty_parallel_test
+
+import (
+	"testing"
+	"fmt"
+	"flag"
+)
+
+// This test ensures the order of CONT lines in parallel chatty tests.
+func TestChattyParallel(t *testing.T) {
+	t.Parallel()
+
+	// The number of concurrent tests running. This is closely tied to the
+	// -parallel test flag, so we grab it from the flag rather than setting it
+	// to some constant.
+	parallel := flag.Lookup("test.parallel").Value.(flag.Getter).Get().(int)
+
+	// ready is a synchronization mechanism that causes subtests to execute
+	// round robin.
+	ready := make([]chan bool, parallel)
+	for i := range ready {
+		ready[i] = make(chan bool, 1)
+	}
+	ready[0] <- true
+
+	for i := range ready {
+		i := i
+		t.Run(fmt.Sprintf("sub-%d", i), func(t *testing.T) {
+			t.Parallel()
+			for j := 0; j < 2; j++ {
+				<-ready[i]
+				t.Logf("this is sub-%d", i)
+				ready[(i+1)%len(ready)] <- true
+			}
+		})
+	}
+}
diff --git a/src/cmd/go/testdata/script/test_chatty_parallel_success_run.txt b/src/cmd/go/testdata/script/test_chatty_parallel_success_run.txt
new file mode 100644
index 0000000..1d4b6d6
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_chatty_parallel_success_run.txt
@@ -0,0 +1,41 @@
+# Run parallel chatty tests. Assert on CONT or NAME lines. This test makes sure that
+# multiple parallel outputs have the appropriate CONT lines between them.
+go test -parallel 3 chatty_parallel -v
+
+stdout '=== RUN   TestInterruptor/interruption\n=== (CONT|NAME)  TestLog\n    chatty_parallel_test.go:28: this is the second TestLog log\n--- PASS: Test(Log|Interruptor) \([0-9.]{4}s\)'
+
+-- go.mod --
+module chatty_parallel
+
+go 1.18
+-- chatty_parallel_test.go --
+package chatty_parallel_test
+
+import (
+	"testing"
+)
+
+var (
+	afterFirstLog = make(chan struct{})
+	afterSubTest  = make(chan struct{})
+	afterSecondLog = make(chan struct{})
+)
+
+func TestInterruptor(t *testing.T) {
+	t.Parallel()
+
+	<-afterFirstLog
+	t.Run("interruption", func (t *testing.T) {})
+	close(afterSubTest)
+	<-afterSecondLog // Delay the "PASS: TestInterruptor" line until after "CONT  TestLog".
+}
+
+func TestLog(t *testing.T) {
+	t.Parallel()
+
+	t.Logf("this is the first TestLog log")
+	close(afterFirstLog)
+	<-afterSubTest
+	t.Logf("this is the second TestLog log")
+	close(afterSecondLog)
+}
diff --git a/src/cmd/go/testdata/script/test_chatty_success.txt b/src/cmd/go/testdata/script/test_chatty_success.txt
new file mode 100644
index 0000000..8bfa569
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_chatty_success.txt
@@ -0,0 +1,27 @@
+# Run chatty tests. Assert on CONT lines.
+go test chatty_test.go -v
+
+# Non-parallel tests should not print CONT.
+! stdout CONT
+
+# The assertion is condensed into one line so that it precisely matches output,
+# rather than skipping lines and allow rogue CONT lines.
+stdout '=== RUN   TestChatty\n=== RUN   TestChatty/sub-0\n    chatty_test.go:12: this is sub-0\n    chatty_test.go:12: this is sub-0\n=== RUN   TestChatty/sub-1\n    chatty_test.go:12: this is sub-1\n    chatty_test.go:12: this is sub-1\n=== RUN   TestChatty/sub-2\n    chatty_test.go:12: this is sub-2\n    chatty_test.go:12: this is sub-2\n--- PASS: TestChatty'
+
+-- chatty_test.go --
+package chatty_test
+
+import (
+	"testing"
+	"fmt"
+)
+
+func TestChatty(t *testing.T) {
+    for i := 0; i < 3; i++ {
+        t.Run(fmt.Sprintf("sub-%d", i), func(t *testing.T) {
+            for j := 0; j < 2; j++ {
+                t.Logf("this is sub-%d", i)
+            }
+        })
+    }
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/test_cleanup_failnow.txt b/src/cmd/go/testdata/script/test_cleanup_failnow.txt
new file mode 100644
index 0000000..0aba8c7
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_cleanup_failnow.txt
@@ -0,0 +1,46 @@
+# For issue 41355
+[short] skip
+
+# This test could fail if the testing package does not wait until
+# a panicking test does the panic. Turn off multithreading and GC
+# to increase the probability of such a failure.
+env GOMAXPROCS=1
+env GOGC=off
+
+# If the test exits with 'no tests to run', it means the testing package
+# implementation is incorrect and does not wait until a test panic.
+# If the test exits with '(?s)panic: die.*panic: die', it means
+# the testing package did an extra panic for a panicking test.
+
+! go test -v cleanup_failnow/panic_nocleanup_test.go
+! stdout 'no tests to run'
+stdout '(?s)panic: die \[recovered\].*panic: die'
+! stdout '(?s)panic: die \[recovered\].*panic: die.*panic: die'
+
+! go test -v cleanup_failnow/panic_withcleanup_test.go
+! stdout 'no tests to run'
+stdout '(?s)panic: die \[recovered\].*panic: die'
+! stdout '(?s)panic: die \[recovered\].*panic: die.*panic: die'
+
+-- cleanup_failnow/panic_nocleanup_test.go --
+package panic_nocleanup_test
+import "testing"
+func TestX(t *testing.T) {
+	t.Run("x", func(t *testing.T) {
+		panic("die")
+	})
+}
+
+-- cleanup_failnow/panic_withcleanup_test.go --
+package panic_withcleanup_test
+import "testing"
+func TestCleanupWithFailNow(t *testing.T) {
+	t.Cleanup(func() {
+		t.FailNow()
+	})
+	t.Run("x", func(t *testing.T) {
+		t.Run("y", func(t *testing.T) {
+			panic("die")
+		})
+	})
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/test_compile_binary.txt b/src/cmd/go/testdata/script/test_compile_binary.txt
new file mode 100644
index 0000000..63bb8ec
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_compile_binary.txt
@@ -0,0 +1,8 @@
+env GO111MODULE=off
+
+! go test -c compile_binary/...
+stderr 'build comment'
+
+-- compile_binary/foo_test.go --
+// +build foo
+package foo
diff --git a/src/cmd/go/testdata/script/test_compile_multi_pkg.txt b/src/cmd/go/testdata/script/test_compile_multi_pkg.txt
new file mode 100644
index 0000000..921ef5c
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_compile_multi_pkg.txt
@@ -0,0 +1,50 @@
+[short] skip 'links test binaries'
+
+# Verify test -c can output multiple executables to a directory.
+
+# This test also serves as a regression test for https://go.dev/issue/62221:
+# prior to the fix for that issue, it occasionally failed with ETXTBSY when
+# run on Unix platforms.
+
+go test -c -o $WORK/some/nonexisting/directory/ ./pkg/...
+exists -exec $WORK/some/nonexisting/directory/pkg1.test$GOEXE
+exists -exec $WORK/some/nonexisting/directory/pkg2.test$GOEXE
+
+go test -c ./pkg/...
+exists -exec pkg1.test$GOEXE
+exists -exec pkg2.test$GOEXE
+
+! go test -c -o $WORK/bin/test/bin.test.exe ./pkg/...
+stderr '^with multiple packages, -o must refer to a directory or '$devnull
+
+! go test -c ./...
+stderr '^cannot write test binary pkg1.test for multiple packages:\nexample/anotherpkg/pkg1\nexample/pkg/pkg1'
+
+! go test -c -o $WORK/bin/test/ ./...
+stderr '^cannot write test binary pkg1.test for multiple packages:\nexample/anotherpkg/pkg1\nexample/pkg/pkg1'
+
+! go test -o $WORK/bin/filename.exe ./pkg/...
+stderr '^with multiple packages, -o must refer to a directory or '$devnull
+
+! go test -o $WORK/bin/ ./...
+stderr '^cannot write test binary pkg1.test for multiple packages:\nexample/anotherpkg/pkg1\nexample/pkg/pkg1'
+
+go test -c -o $devnull ./...
+
+rm pkg1.test$GOEXE
+rm pkg2.test$GOEXE
+go test -o . ./pkg/...
+exists -exec pkg1.test$GOEXE
+exists -exec pkg2.test$GOEXE
+
+-- go.mod --
+module example
+
+-- pkg/pkg1/pkg1_test.go --
+package pkg1
+
+-- pkg/pkg2/pkg2_test.go --
+package pkg2
+
+-- anotherpkg/pkg1/pkg1_test.go --
+package pkg1
diff --git a/src/cmd/go/testdata/script/test_compile_tempfile.txt b/src/cmd/go/testdata/script/test_compile_tempfile.txt
new file mode 100644
index 0000000..05f721a
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_compile_tempfile.txt
@@ -0,0 +1,11 @@
+[short] skip
+
+# Ensure that the target of 'go build -o' can be an existing, empty file so that
+# its name can be reserved using os.CreateTemp or the 'mktemp` command.
+
+go build -o empty-file$GOEXE main.go
+
+-- main.go --
+package main
+func main() {}
+-- empty-file$GOEXE --
diff --git a/src/cmd/go/testdata/script/test_deadline.txt b/src/cmd/go/testdata/script/test_deadline.txt
new file mode 100644
index 0000000..06ae16f
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_deadline.txt
@@ -0,0 +1,54 @@
+[short] skip
+
+go test -timeout=0 -run=TestNoDeadline
+go test -timeout=1m -run=TestDeadlineWithinMinute
+go test -timeout=1m -run=TestSubtestDeadlineWithinMinute
+
+-- go.mod --
+module m
+
+go 1.16
+-- deadline_test.go --
+package testing_test
+
+import (
+	"testing"
+	"time"
+)
+
+func TestNoDeadline(t *testing.T) {
+	d, ok := t.Deadline()
+	if ok || !d.IsZero() {
+		t.Fatalf("t.Deadline() = %v, %v; want 0, false", d, ok)
+	}
+}
+
+func TestDeadlineWithinMinute(t *testing.T) {
+	now := time.Now()
+	d, ok := t.Deadline()
+	if !ok || d.IsZero() {
+		t.Fatalf("t.Deadline() = %v, %v; want nonzero deadline", d, ok)
+	}
+	if !d.After(now) {
+		t.Fatalf("t.Deadline() = %v; want after start of test (%v)", d, now)
+	}
+	if d.Sub(now) > time.Minute {
+		t.Fatalf("t.Deadline() = %v; want within one minute of start of test (%v)", d, now)
+	}
+}
+
+func TestSubtestDeadlineWithinMinute(t *testing.T) {
+	t.Run("sub", func(t *testing.T) {
+		now := time.Now()
+		d, ok := t.Deadline()
+		if !ok || d.IsZero() {
+			t.Fatalf("t.Deadline() = %v, %v; want nonzero deadline", d, ok)
+		}
+		if !d.After(now) {
+			t.Fatalf("t.Deadline() = %v; want after start of test (%v)", d, now)
+		}
+		if d.Sub(now) > time.Minute {
+			t.Fatalf("t.Deadline() = %v; want within one minute of start of test (%v)", d, now)
+		}
+	})
+}
diff --git a/src/cmd/go/testdata/script/test_empty.txt b/src/cmd/go/testdata/script/test_empty.txt
new file mode 100644
index 0000000..5ebbecd
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_empty.txt
@@ -0,0 +1,53 @@
+[!race] skip
+
+cd $GOPATH/src/empty/pkg
+go test -cover -coverpkg=. -race
+
+[short] stop # Only run first case in short mode
+
+cd $GOPATH/src/empty/test
+go test -cover -coverpkg=. -race
+
+cd $GOPATH/src/empty/xtest
+go test -cover -coverpkg=. -race
+
+cd $GOPATH/src/empty/pkgtest
+go test -cover -coverpkg=. -race
+
+cd $GOPATH/src/empty/pkgxtest
+go test -cover -coverpkg=. -race
+
+cd $GOPATH/src/empty/pkgtestxtest
+go test -cover -coverpkg=. -race
+
+cd $GOPATH/src/empty/testxtest
+go test -cover -coverpkg=. -race
+
+-- empty/go.mod --
+module empty
+
+go 1.16
+-- empty/pkg/pkg.go --
+package p
+-- empty/pkgtest/pkg.go --
+package p
+-- empty/pkgtest/test_test.go --
+package p
+-- empty/pkgtestxtest/pkg.go --
+package p
+-- empty/pkgtestxtest/test_test.go --
+package p
+-- empty/pkgtestxtest/xtest_test.go --
+package p_test
+-- empty/pkgxtest/pkg.go --
+package p
+-- empty/pkgxtest/xtest_test.go --
+package p_test
+-- empty/test/test_test.go --
+package p
+-- empty/testxtest/test_test.go --
+package p
+-- empty/testxtest/xtest_test.go --
+package p_test
+-- empty/xtest/xtest_test.go --
+package p_test
diff --git a/src/cmd/go/testdata/script/test_env_term.txt b/src/cmd/go/testdata/script/test_env_term.txt
new file mode 100644
index 0000000..8a5f79a
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_env_term.txt
@@ -0,0 +1,15 @@
+# Tests golang.org/issue/12096
+
+env TERM=''
+go test test_test.go
+! stdout '^ok.*\[no tests to run\]'
+stdout '^ok'
+
+-- test_test.go --
+package main
+import ("os"; "testing")
+func TestEnv(t *testing.T) {
+	if os.Getenv("TERM") != "" {
+		t.Fatal("TERM is set")
+	}
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/test_example_goexit.txt b/src/cmd/go/testdata/script/test_example_goexit.txt
new file mode 100644
index 0000000..984f434
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_example_goexit.txt
@@ -0,0 +1,29 @@
+# For issue golang.org/issue/41084
+[short] skip
+
+! go test -v examplegoexit
+stdout '(?s)--- PASS.*--- FAIL.*'
+stdout 'panic: test executed panic\(nil\) or runtime\.Goexit'
+
+-- go.mod --
+module examplegoexit
+
+go 1.16
+-- example_test.go --
+package main
+
+import (
+	"fmt"
+	"runtime"
+)
+
+func ExamplePass() {
+	fmt.Println("pass")
+	// Output:
+	// pass
+}
+
+func ExampleGoexit() {
+	runtime.Goexit()
+	// Output:
+}
diff --git a/src/cmd/go/testdata/script/test_exit.txt b/src/cmd/go/testdata/script/test_exit.txt
new file mode 100644
index 0000000..3703ba5
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_exit.txt
@@ -0,0 +1,131 @@
+# Builds and runs test binaries, so skip in short mode.
+[short] skip
+
+env GO111MODULE=on
+
+# If a test invoked by 'go test' exits with a zero status code,
+# it will panic.
+! go test ./zero
+! stdout ^ok
+! stdout 'exit status'
+stdout 'panic'
+stdout ^FAIL
+
+# If a test exits with a non-zero status code, 'go test' fails normally.
+! go test ./one
+! stdout ^ok
+stdout 'exit status'
+! stdout 'panic'
+stdout ^FAIL
+
+# Ensure that other flags still do the right thing.
+go test -list=. ./zero
+stdout ExitZero
+
+! go test -bench=. ./zero
+stdout 'panic'
+
+# 'go test' with no args streams output without buffering. Ensure that it still
+# catches a zero exit with missing output.
+cd zero
+! go test
+stdout 'panic'
+cd ../normal
+go test
+stdout ^ok
+cd ..
+
+# If a TestMain exits with a zero status code, 'go test' shouldn't
+# complain about that. It's a common way to skip testing a package
+# entirely.
+go test ./main_zero
+! stdout 'skipping all tests'
+stdout ^ok
+
+# With -v, we'll see the warning from TestMain.
+go test -v ./main_zero
+stdout 'skipping all tests'
+stdout ^ok
+
+# Listing all tests won't actually give a result if TestMain exits. That's okay,
+# because this is how TestMain works. If we decide to support -list even when
+# TestMain is used to skip entire packages, we can change this test case.
+go test -list=. ./main_zero
+stdout 'skipping all tests'
+! stdout TestNotListed
+
+# Running the test directly still fails, if we pass the flag.
+go test -c -o ./zero.exe ./zero
+! exec ./zero.exe -test.paniconexit0
+
+# Using -json doesn't affect the exit status.
+! go test -json ./zero
+! stdout '"Output":"ok'
+! stdout 'exit status'
+stdout 'panic'
+stdout '"Output":"FAIL'
+
+# Running the test via test2json also fails.
+! go tool test2json ./zero.exe -test.v -test.paniconexit0
+! stdout '"Output":"ok'
+! stdout 'exit status'
+stdout 'panic'
+
+-- go.mod --
+module m
+
+-- ./normal/normal.go --
+package normal
+-- ./normal/normal_test.go --
+package normal
+
+import "testing"
+
+func TestExitZero(t *testing.T) {
+}
+
+-- ./zero/zero.go --
+package zero
+-- ./zero/zero_test.go --
+package zero
+
+import (
+	"os"
+	"testing"
+)
+
+func TestExitZero(t *testing.T) {
+	os.Exit(0)
+}
+
+-- ./one/one.go --
+package one
+-- ./one/one_test.go --
+package one
+
+import (
+	"os"
+	"testing"
+)
+
+func TestExitOne(t *testing.T) {
+	os.Exit(1)
+}
+
+-- ./main_zero/zero.go --
+package zero
+-- ./main_zero/zero_test.go --
+package zero
+
+import (
+	"fmt"
+	"os"
+	"testing"
+)
+
+func TestMain(m *testing.M) {
+	fmt.Println("skipping all tests")
+	os.Exit(0)
+}
+
+func TestNotListed(t *testing.T) {}
diff --git a/src/cmd/go/testdata/script/test_fail_fast.txt b/src/cmd/go/testdata/script/test_fail_fast.txt
new file mode 100644
index 0000000..132ea70
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fail_fast.txt
@@ -0,0 +1,113 @@
+[short] skip
+
+# test fail fast
+! go test ./failfast_test.go -run='TestFailingA' -failfast=true
+stdout -count=1 'FAIL - '
+! go test ./failfast_test.go -run='TestFailing[AB]' -failfast=true
+stdout -count=1 'FAIL - '
+! go test ./failfast_test.go -run='TestFailing[AB]' -failfast=false
+stdout -count=2 'FAIL - '
+
+# mix with non-failing tests
+! go test ./failfast_test.go -run='TestA|TestFailing[AB]' -failfast=true
+stdout -count=1 'FAIL - '
+! go test ./failfast_test.go -run='TestA|TestFailing[AB]' -failfast=false
+stdout -count=2 'FAIL - '
+
+# mix with parallel tests
+! go test ./failfast_test.go -run='TestFailingB|TestParallelFailingA' -failfast=true
+stdout -count=2 'FAIL - '
+! go test ./failfast_test.go -run='TestFailingB|TestParallelFailingA' -failfast=false
+stdout -count=2 'FAIL - '
+! go test ./failfast_test.go -run='TestFailingB|TestParallelFailing[AB]' -failfast=true
+stdout -count=3 'FAIL - '
+! go test ./failfast_test.go -run='TestFailingB|TestParallelFailing[AB]' -failfast=false
+stdout -count=3 'FAIL - '
+
+# mix with parallel sub-tests
+! go test ./failfast_test.go -run='TestFailingB|TestParallelFailing[AB]|TestParallelFailingSubtestsA' -failfast=true
+stdout -count=3 'FAIL - '
+! go test ./failfast_test.go -run='TestFailingB|TestParallelFailing[AB]|TestParallelFailingSubtestsA' -failfast=false
+stdout -count=5 'FAIL - '
+! go test ./failfast_test.go -run='TestParallelFailingSubtestsA' -failfast=true
+stdout -count=1 'FAIL - '
+
+# only parallels
+! go test ./failfast_test.go -run='TestParallelFailing[AB]' -failfast=false
+stdout -count=2 'FAIL - '
+
+# non-parallel subtests
+! go test ./failfast_test.go -run='TestFailingSubtestsA' -failfast=true
+stdout -count=1 'FAIL - '
+! go test ./failfast_test.go -run='TestFailingSubtestsA' -failfast=false
+stdout -count=2 'FAIL - '
+
+# fatal test
+! go test ./failfast_test.go -run='TestFatal[CD]' -failfast=true
+stdout -count=1 'FAIL - '
+! go test ./failfast_test.go -run='TestFatal[CD]' -failfast=false
+stdout -count=2 'FAIL - '
+
+-- failfast_test.go --
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package failfast
+
+import "testing"
+
+func TestA(t *testing.T) {
+	// Edge-case testing, mixing unparallel tests too
+	t.Logf("LOG: %s", t.Name())
+}
+
+func TestFailingA(t *testing.T) {
+	t.Errorf("FAIL - %s", t.Name())
+}
+
+func TestB(t *testing.T) {
+	// Edge-case testing, mixing unparallel tests too
+	t.Logf("LOG: %s", t.Name())
+}
+
+func TestParallelFailingA(t *testing.T) {
+	t.Parallel()
+	t.Errorf("FAIL - %s", t.Name())
+}
+
+func TestParallelFailingB(t *testing.T) {
+	t.Parallel()
+	t.Errorf("FAIL - %s", t.Name())
+}
+
+func TestParallelFailingSubtestsA(t *testing.T) {
+	t.Parallel()
+	t.Run("TestFailingSubtestsA1", func(t *testing.T) {
+		t.Errorf("FAIL - %s", t.Name())
+	})
+	t.Run("TestFailingSubtestsA2", func(t *testing.T) {
+		t.Errorf("FAIL - %s", t.Name())
+	})
+}
+
+func TestFailingSubtestsA(t *testing.T) {
+	t.Run("TestFailingSubtestsA1", func(t *testing.T) {
+		t.Errorf("FAIL - %s", t.Name())
+	})
+	t.Run("TestFailingSubtestsA2", func(t *testing.T) {
+		t.Errorf("FAIL - %s", t.Name())
+	})
+}
+
+func TestFailingB(t *testing.T) {
+	t.Errorf("FAIL - %s", t.Name())
+}
+
+func TestFatalC(t *testing.T) {
+	t.Fatalf("FAIL - %s", t.Name())
+}
+
+func TestFatalD(t *testing.T) {
+	t.Fatalf("FAIL - %s", t.Name())
+}
diff --git a/src/cmd/go/testdata/script/test_fail_newline.txt b/src/cmd/go/testdata/script/test_fail_newline.txt
new file mode 100644
index 0000000..43cee56
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fail_newline.txt
@@ -0,0 +1,65 @@
+[short] skip
+
+# In package list mode, output is buffered.
+# Check that a newline is printed after the buffer's contents.
+cd fail
+! go test .
+! stderr .
+stdout '^exitcode=1\n'
+stdout '^FAIL\s+example/fail'
+
+# In local directory mode output is streamed, so we don't know
+# whether the test printed anything at all, so we print the exit code
+# (just in case it failed without emitting any output at all),
+# and that happens to add the needed newline as well.
+! go test
+! stderr .
+stdout '^exitcode=1exit status 1\n'
+stdout '^FAIL\s+example/fail'
+
+# In package list mode, if the test passes the 'ok' message appears
+# on its own line.
+cd ../skip
+go test -v .
+! stderr .
+stdout '^skipping\n'
+stdout '^ok\s+example/skip'
+
+# If the output is streamed and the test passes, we can't tell whether it ended
+# in a partial line, and don't want to emit any extra output in the
+# overwhelmingly common case that it did not.
+# (In theory we could hook the 'os' package to report whether output
+# was emitted and whether it ended in a newline, but that seems too invasive.)
+go test
+! stderr .
+stdout '^skippingok\s+example/skip'
+
+
+-- go.mod --
+module example
+
+go 1.18
+-- fail/fail_test.go --
+package fail
+
+import (
+	"os"
+	"testing"
+)
+
+func TestMain(m *testing.M) {
+	os.Stderr.WriteString("exitcode=1")
+	os.Exit(1)
+}
+-- skip/skip_test.go --
+package skip
+
+import (
+	"os"
+	"testing"
+)
+
+func TestMain(m *testing.M) {
+	os.Stderr.WriteString("skipping")
+	os.Exit(0)
+}
diff --git a/src/cmd/go/testdata/script/test_finished_subtest_goroutines.txt b/src/cmd/go/testdata/script/test_finished_subtest_goroutines.txt
new file mode 100644
index 0000000..8db821e
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_finished_subtest_goroutines.txt
@@ -0,0 +1,52 @@
+# Regression test for https://golang.org/issue/45127:
+# Goroutines for completed parallel subtests should exit immediately,
+# not block until earlier subtests have finished.
+
+[short] skip
+
+! go test .
+stdout 'panic: slow failure'
+! stdout '\[chan send'
+
+-- go.mod --
+module golang.org/issue45127
+
+go 1.16
+-- issue45127_test.go --
+package main
+
+import (
+	"fmt"
+	"runtime"
+	"runtime/debug"
+	"sync"
+	"testing"
+)
+
+func TestTestingGoroutineLeak(t *testing.T) {
+	debug.SetTraceback("all")
+
+	var wg sync.WaitGroup
+	const nFast = 10
+
+	t.Run("slow", func(t *testing.T) {
+		t.Parallel()
+		wg.Wait()
+		for i := 0; i < nFast; i++ {
+			// If the subtest goroutines are going to park on the channel
+			// send, allow them to park now. If they're not going to park,
+			// make sure they have had a chance to run to completion so
+			// that they aren't spuriously parked when we panic.
+			runtime.Gosched()
+		}
+		panic("slow failure")
+	})
+
+	wg.Add(nFast)
+	for i := 0; i < nFast; i++ {
+		t.Run(fmt.Sprintf("leaky%d", i), func(t *testing.T) {
+			t.Parallel()
+			wg.Done()
+		})
+	}
+}
diff --git a/src/cmd/go/testdata/script/test_flag.txt b/src/cmd/go/testdata/script/test_flag.txt
new file mode 100644
index 0000000..6ef4529
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_flag.txt
@@ -0,0 +1,38 @@
+[short] skip
+
+go test flag_test.go -v -args -v=7 # Two distinct -v flags
+go test -v flag_test.go -args -v=7 # Two distinct -v flags
+
+# Using a custom flag mixed with regular 'go test' flags should be OK.
+go test -count=1 -custom -args -v=7
+
+# However, it should be an error to use custom flags when -c is used,
+# since we know for sure that no test binary will run at all.
+! go test -c -custom
+stderr '^go: unknown flag -custom cannot be used with -c$'
+
+# The same should apply even if -c comes after a custom flag.
+! go test -custom -c
+stderr '^go: unknown flag -custom cannot be used with -c$'
+
+-- go.mod --
+module m
+-- flag_test.go --
+package flag_test
+
+import (
+	"flag"
+	"log"
+	"testing"
+)
+
+var v = flag.Int("v", 0, "v flag")
+
+var custom = flag.Bool("custom", false, "")
+
+// Run this as go test pkg -v=7
+func TestVFlagIsSet(t *testing.T) {
+	if *v != 7 {
+		log.Fatal("v flag not set")
+	}
+}
diff --git a/src/cmd/go/testdata/script/test_flags.txt b/src/cmd/go/testdata/script/test_flags.txt
new file mode 100644
index 0000000..3f7964b
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_flags.txt
@@ -0,0 +1,131 @@
+env GO111MODULE=on
+
+[short] skip
+
+# Arguments after the flag terminator should be ignored.
+# If we pass '-- -test.v', we should not get verbose output
+# *and* output from the test should not be echoed.
+go test ./x -- -test.v
+stdout '\Aok\s+example.com/x\s+[0-9.s]+\n\z'
+! stderr .
+
+# For backward-compatibility with previous releases of the 'go' command,
+# arguments that appear after unrecognized flags should not be treated
+# as packages, even if they are unambiguously not arguments to flags.
+# Even though ./x looks like a package path, the real package should be
+# the implicit '.'.
+! go test --answer=42 ./x
+stderr '^no Go files in '$PWD'$'
+
+# However, *flags* that appear after unrecognized flags should still be
+# interpreted as flags, under the (possibly-erroneous) assumption that
+# unrecognized flags are non-boolean.
+
+go test -v -x ./x -timeout 24h -boolflag=true foo -timeout 25h
+stdout 'args: foo -timeout 25h'
+stdout 'timeout: 24h0m0s$'  # -timeout is unambiguously not a flag, so the real flag wins.
+
+go test -v -x ./x -timeout 24h -boolflag foo -timeout 25h
+stdout 'args: foo -test\.timeout=25h0m0s'  # For legacy reasons, '-timeout ' is erroneously rewritten to -test.timeout; see https://golang.org/issue/40763.
+stdout 'timeout: 24h0m0s$'  # Actual flag wins.
+
+go test -v -x ./x -timeout 24h -stringflag foo -timeout 25h
+stdout 'args: $'
+stdout 'timeout: 25h0m0s$'  # Later flag wins.
+
+# An explicit '-outputdir=' argument should set test.outputdir
+# to the 'go' command's working directory, not zero it out
+# for the test binary.
+go test -x -coverprofile=cover.out '-outputdir=' ./x
+stderr '-test.outputdir=[^ ]'
+exists ./cover.out
+! exists ./x/cover.out
+
+# Test flags from GOFLAGS should be forwarded to the test binary,
+# with the 'test.' prefix in the GOFLAGS entry...
+env GOFLAGS='-test.timeout=24h0m0s -count=1'
+go test -v -x ./x
+stdout 'timeout: 24h0m0s$'
+stderr '-test.count=1'
+
+# ...or without.
+env GOFLAGS='-timeout=24h0m0s -count=1'
+go test -v -x ./x
+stdout 'timeout: 24h0m0s$'
+stderr '-test.count=1'
+
+# Arguments from the command line should override GOFLAGS...
+go test -v -x -timeout=25h0m0s ./x
+stdout 'timeout: 25h0m0s$'
+stderr '-test.count=1'
+
+# ...even if they use a different flag name.
+go test -v -x -test.timeout=26h0m0s ./x
+stdout 'timeout: 26h0m0s$'
+stderr '-test\.timeout=26h0m0s'
+! stderr 'timeout=24h0m0s'
+stderr '-test.count=1'
+
+# Invalid flags should be reported exactly once.
+! go test -covermode=walrus ./x
+stderr -count=1 'invalid value "walrus" for flag -covermode: valid modes are .*$'
+stderr '^usage: go test .*$'
+stderr '^Run ''go help test'' and ''go help testflag'' for details.$'
+
+# Passing -help to the test binary should show flag help.
+go test ./x -args -help
+stdout 'usage_message'
+
+# -covermode, -coverpkg, and -coverprofile should imply -cover
+go test -covermode=set ./x
+stdout '\s+coverage:\s+'
+
+go test -coverpkg=encoding/binary ./x
+stdout '\s+coverage:\s+'
+
+go test -coverprofile=cover.out ./x
+stdout '\s+coverage:\s+'
+exists ./cover.out
+rm ./cover.out
+
+# -*profile and -trace flags should force output to the current working directory
+# or -outputdir, not the directory containing the test.
+
+go test -memprofile=mem.out ./x
+exists ./mem.out
+rm ./mem.out
+
+go test -trace=trace.out ./x
+exists ./trace.out
+rm ./trace.out
+
+# Relative paths with -outputdir should be relative to the go command's working
+# directory, not the directory containing the test.
+mkdir profiles
+go test -memprofile=mem.out -outputdir=./profiles ./x
+exists ./profiles/mem.out
+rm profiles
+
+-- go.mod --
+module example.com
+go 1.14
+-- x/x_test.go --
+package x
+
+import (
+	"flag"
+	"strings"
+	"testing"
+)
+
+var _ = flag.String("usage_message", "", "dummy flag to check usage message")
+var boolflag = flag.Bool("boolflag", false, "ignored boolean flag")
+var stringflag = flag.String("stringflag", "", "ignored string flag")
+
+func TestLogTimeout(t *testing.T) {
+	t.Logf("timeout: %v", flag.Lookup("test.timeout").Value)
+}
+
+func TestLogArgs(t *testing.T) {
+	t.Logf("args: %s", strings.Join(flag.Args(), " "))
+}
diff --git a/src/cmd/go/testdata/script/test_fullpath.txt b/src/cmd/go/testdata/script/test_fullpath.txt
new file mode 100644
index 0000000..8e01552
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fullpath.txt
@@ -0,0 +1,21 @@
+[short] skip
+
+# test with -fullpath
+! go test ./x/... -fullpath
+stdout '^ +.+/gopath/src/x/fullpath/fullpath_test.go:8: test failed'
+# test without -fullpath
+! go test ./x/...
+stdout '^ +fullpath_test.go:8: test failed'
+
+-- go.mod --
+module example
+-- x/fullpath/fullpath_test.go --
+package fullpath_test
+
+import (
+	"testing"
+)
+
+func TestFullPath(t *testing.T) {
+	t.Error("test failed")
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz.txt b/src/cmd/go/testdata/script/test_fuzz.txt
new file mode 100644
index 0000000..37170bf
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz.txt
@@ -0,0 +1,500 @@
+[!fuzz] skip
+[short] skip
+env GOCACHE=$WORK/cache
+
+# Test that running a fuzz target that returns without failing or calling
+# f.Fuzz fails and causes a non-zero exit status.
+! go test noop_fuzz_test.go
+! stdout ^ok
+stdout FAIL
+
+# Test that fuzzing a fuzz target that returns without failing or calling
+# f.Fuzz fails and causes a non-zero exit status.
+! go test -fuzz=Fuzz -fuzztime=1x noop_fuzz_test.go
+! stdout ^ok
+stdout FAIL
+
+# Test that calling f.Error in a fuzz target causes a non-zero exit status.
+! go test -fuzz=Fuzz -fuzztime=1x error_fuzz_test.go
+! stdout ^ok
+stdout FAIL
+
+# Test that calling f.Fatal in a fuzz target causes a non-zero exit status.
+! go test fatal_fuzz_test.go
+! stdout ^ok
+stdout FAIL
+
+# Test that successful test exits cleanly.
+go test success_fuzz_test.go
+stdout ^ok
+! stdout FAIL
+
+# Test that successful fuzzing exits cleanly.
+go test -fuzz=Fuzz -fuzztime=1x success_fuzz_test.go
+stdout ok
+! stdout FAIL
+
+# Test that calling f.Fatal while fuzzing causes a non-zero exit status.
+! go test -fuzz=Fuzz -fuzztime=1x fatal_fuzz_test.go
+! stdout ^ok
+stdout FAIL
+
+# Test error with seed corpus in f.Fuzz
+! go test -run FuzzError fuzz_add_test.go
+! stdout ^ok
+stdout FAIL
+stdout 'error here'
+
+[short] stop
+
+# Test that calling panic(nil) in a fuzz target causes a non-zero exit status.
+! go test panic_fuzz_test.go
+! stdout ^ok
+stdout FAIL
+
+# Test that skipped test exits cleanly.
+go test skipped_fuzz_test.go
+stdout ok
+! stdout FAIL
+
+# Test that f.Fatal within f.Fuzz panics
+! go test fatal_fuzz_fn_fuzz_test.go
+! stdout ^ok
+! stdout 'fatal here'
+stdout FAIL
+stdout 'fuzz target'
+
+# Test that f.Error within f.Fuzz panics
+! go test error_fuzz_fn_fuzz_test.go
+! stdout ^ok
+! stdout 'error here'
+stdout FAIL
+stdout 'fuzz target'
+
+# Test that f.Fail within f.Fuzz panics
+! go test fail_fuzz_fn_fuzz_test.go
+! stdout ^ok
+stdout FAIL
+stdout 'fuzz target'
+
+# Test that f.Skip within f.Fuzz panics
+! go test skip_fuzz_fn_fuzz_test.go
+! stdout ^ok
+! stdout 'skip here'
+stdout FAIL
+stdout 'fuzz target'
+
+# Test that f.Skipped within f.Fuzz panics
+! go test skipped_fuzz_fn_fuzz_test.go
+! stdout ^ok
+! stdout 'f.Skipped is'
+stdout FAIL
+stdout 'fuzz target'
+stdout 't.Skipped is false'
+
+# Test that runtime.Goexit within the fuzz function is an error.
+! go test goexit_fuzz_fn_fuzz_test.go
+! stdout ^ok
+stdout FAIL
+
+# Test that a call to f.Fatal after the Fuzz func is executed.
+! go test fatal_after_fuzz_func_fuzz_test.go
+! stdout ok
+stdout FAIL
+
+# Test that missing *T in f.Fuzz causes a non-zero exit status.
+! go test incomplete_fuzz_call_fuzz_test.go
+! stdout ^ok
+stdout FAIL
+
+# Test that a panic in the Cleanup func is executed.
+! go test cleanup_fuzz_test.go
+! stdout ^ok
+stdout FAIL
+stdout 'failed some precondition'
+
+# Test success with seed corpus in f.Fuzz
+go test -run FuzzPass fuzz_add_test.go
+stdout ok
+! stdout FAIL
+! stdout 'off by one error'
+
+# Test fatal with seed corpus in f.Fuzz
+! go test -run FuzzFatal fuzz_add_test.go
+! stdout ^ok
+stdout FAIL
+stdout 'fatal here'
+
+# Test panic with seed corpus in f.Fuzz
+! go test -run FuzzPanic fuzz_add_test.go
+! stdout ^ok
+stdout FAIL
+stdout 'off by one error'
+
+# Test panic(nil) with seed corpus in f.Fuzz
+! go test -run FuzzNilPanic fuzz_add_test.go
+! stdout ^ok
+stdout FAIL
+
+# Test panic with unsupported seed corpus
+! go test -run FuzzUnsupported fuzz_add_test.go
+! stdout ^ok
+stdout FAIL
+
+# Test panic with different number of args to f.Add
+! go test -run FuzzAddDifferentNumber fuzz_add_test.go
+! stdout ^ok
+stdout FAIL
+
+# Test panic with different type of args to f.Add
+! go test -run FuzzAddDifferentType fuzz_add_test.go
+! stdout ^ok
+stdout FAIL
+
+# Test that the wrong type given with f.Add will fail.
+! go test -run FuzzWrongType fuzz_add_test.go
+! stdout ^ok
+stdout '\[string int\], want \[\[\]uint8 int8\]'
+stdout FAIL
+
+# Test fatal with testdata seed corpus
+! go test -run FuzzFail corpustesting/fuzz_testdata_corpus_test.go
+! stdout ^ok
+stdout FAIL
+stdout 'fatal here'
+
+# Test pass with testdata seed corpus
+go test -run FuzzPass corpustesting/fuzz_testdata_corpus_test.go
+stdout ok
+! stdout FAIL
+! stdout 'fatal here'
+
+# Test pass with testdata and f.Add seed corpus
+go test -run FuzzPassString corpustesting/fuzz_testdata_corpus_test.go
+stdout ok
+! stdout FAIL
+
+# Fuzzing pass with testdata and f.Add seed corpus (skip running tests first)
+go test -run=None -fuzz=FuzzPassString corpustesting/fuzz_testdata_corpus_test.go -fuzztime=10x
+stdout ok
+! stdout FAIL
+
+# Fuzzing pass with testdata and f.Add seed corpus
+go test -run=FuzzPassString -fuzz=FuzzPassString corpustesting/fuzz_testdata_corpus_test.go -fuzztime=10x
+stdout ok
+! stdout FAIL
+
+# Test panic with malformed seed corpus
+! go test -run FuzzFail corpustesting/fuzz_testdata_corpus_test.go
+! stdout ^ok
+stdout FAIL
+
+# Test pass with file in other nested testdata directory
+go test -run FuzzInNestedDir corpustesting/fuzz_testdata_corpus_test.go
+stdout ok
+! stdout FAIL
+! stdout 'fatal here'
+
+# Test fails with file containing wrong type
+! go test -run FuzzWrongType corpustesting/fuzz_testdata_corpus_test.go
+! stdout ^ok
+stdout FAIL
+
+-- noop_fuzz_test.go --
+package noop_fuzz
+
+import "testing"
+
+func Fuzz(f *testing.F) {}
+
+-- error_fuzz_test.go --
+package error_fuzz
+
+import "testing"
+
+func Fuzz(f *testing.F) {
+    f.Error("error in target")
+}
+
+-- fatal_fuzz_test.go --
+package fatal_fuzz
+
+import "testing"
+
+func Fuzz(f *testing.F) {
+    f.Fatal("fatal in target")
+}
+
+-- panic_fuzz_test.go --
+package panic_fuzz
+
+import "testing"
+
+func FuzzPanic(f *testing.F) {
+    panic(nil)
+}
+
+-- success_fuzz_test.go --
+package success_fuzz
+
+import "testing"
+
+func Fuzz(f *testing.F) {
+    f.Fuzz(func (*testing.T, []byte) {})
+}
+
+-- skipped_fuzz_test.go --
+package skipped_fuzz
+
+import "testing"
+
+func Fuzz(f *testing.F) {
+    f.Skip()
+}
+
+-- fatal_fuzz_fn_fuzz_test.go --
+package fatal_fuzz_fn_fuzz
+
+import "testing"
+
+func Fuzz(f *testing.F) {
+    f.Add([]byte("aa"))
+    f.Fuzz(func(t *testing.T, b []byte) {
+        f.Fatal("fatal here")
+    })
+}
+
+-- error_fuzz_fn_fuzz_test.go --
+package error_fuzz_fn_fuzz
+
+import "testing"
+
+func Fuzz(f *testing.F) {
+    f.Add([]byte("aa"))
+    f.Fuzz(func(t *testing.T, b []byte) {
+        f.Error("error here")
+    })
+}
+
+-- fail_fuzz_fn_fuzz_test.go --
+package skip_fuzz_fn_fuzz
+
+import "testing"
+
+func Fuzz(f *testing.F) {
+    f.Add([]byte("aa"))
+    f.Fuzz(func(t *testing.T, b []byte) {
+        f.Fail()
+    })
+}
+
+-- skip_fuzz_fn_fuzz_test.go --
+package skip_fuzz_fn_fuzz
+
+import "testing"
+
+func Fuzz(f *testing.F) {
+    f.Add([]byte("aa"))
+    f.Fuzz(func(t *testing.T, b []byte) {
+        f.Skip("skip here")
+    })
+}
+
+-- skipped_fuzz_fn_fuzz_test.go --
+package skipped_fuzz_fn_fuzz
+
+import "testing"
+
+func Fuzz(f *testing.F) {
+    f.Add([]byte("aa"))
+    f.Fuzz(func(t *testing.T, b []byte) {
+        t.Logf("t.Skipped is %t\n", t.Skipped())
+        t.Logf("f.Skipped is %t\n", f.Skipped())
+    })
+}
+
+-- goexit_fuzz_fn_fuzz_test.go --
+package goexit_fuzz_fn_fuzz
+
+import "testing"
+
+func Fuzz(f *testing.F) {
+    f.Add([]byte("aa"))
+    f.Fuzz(func(t *testing.T, b []byte) {
+        runtime.Goexit()
+    })
+}
+
+-- fatal_after_fuzz_func_fuzz_test.go --
+package fatal_after_fuzz_func_fuzz
+
+import "testing"
+
+func Fuzz(f *testing.F) {
+    f.Fuzz(func(t *testing.T, b []byte) {
+        // no-op
+    })
+    f.Fatal("this shouldn't be called")
+}
+
+-- incomplete_fuzz_call_fuzz_test.go --
+package incomplete_fuzz_call_fuzz
+
+import "testing"
+
+func Fuzz(f *testing.F) {
+    f.Fuzz(func(b []byte) {
+        // this is missing *testing.T as the first param, so should panic
+    })
+}
+
+-- cleanup_fuzz_test.go --
+package cleanup_fuzz_test
+
+import "testing"
+
+func Fuzz(f *testing.F) {
+    f.Cleanup(func() {
+        panic("failed some precondition")
+    })
+    f.Fuzz(func(t *testing.T, b []byte) {
+        // no-op
+    })
+}
+
+-- fuzz_add_test.go --
+package fuzz_add
+
+import "testing"
+
+func add(f *testing.F) {
+    f.Helper()
+    f.Add([]byte("123"))
+    f.Add([]byte("12345"))
+    f.Add([]byte(""))
+}
+
+func FuzzPass(f *testing.F) {
+    add(f)
+    f.Fuzz(func(t *testing.T, b []byte) {
+        if len(b) == -1 {
+            t.Fatal("fatal here") // will not be executed
+        }
+    })
+}
+
+func FuzzError(f *testing.F) {
+    add(f)
+    f.Fuzz(func(t *testing.T, b []byte) {
+        if len(b) == 3 {
+            t.Error("error here")
+        }
+    })
+}
+
+func FuzzFatal(f *testing.F) {
+    add(f)
+    f.Fuzz(func(t *testing.T, b []byte) {
+        if len(b) == 0 {
+            t.Fatal("fatal here")
+        }
+    })
+}
+
+func FuzzPanic(f *testing.F) {
+    add(f)
+    f.Fuzz(func(t *testing.T, b []byte) {
+        if len(b) == 5 {
+            panic("off by one error")
+        }
+    })
+}
+
+func FuzzNilPanic(f *testing.F) {
+    add(f)
+    f.Fuzz(func(t *testing.T, b []byte) {
+        if len(b) == 3 {
+            panic(nil)
+        }
+    })
+}
+
+func FuzzUnsupported(f *testing.F) {
+    m := make(map[string]bool)
+    f.Add(m)
+    f.Fuzz(func(*testing.T, []byte) {})
+}
+
+func FuzzAddDifferentNumber(f *testing.F) {
+    f.Add([]byte("a"))
+    f.Add([]byte("a"), []byte("b"))
+    f.Fuzz(func(*testing.T, []byte) {})
+}
+
+func FuzzAddDifferentType(f *testing.F) {
+    f.Add(false)
+    f.Add(1234)
+    f.Fuzz(func(*testing.T, []byte) {})
+}
+
+func FuzzWrongType(f *testing.F) {
+    f.Add("hello", 50)
+    f.Fuzz(func(*testing.T, []byte, int8) {})
+}
+
+-- corpustesting/fuzz_testdata_corpus_test.go --
+package fuzz_testdata_corpus
+
+import "testing"
+
+func fuzzFn(f *testing.F) {
+    f.Helper()
+    f.Fuzz(func(t *testing.T, b []byte) {
+        if string(b) == "12345" {
+            t.Fatal("fatal here")
+        }
+    })
+}
+
+func FuzzFail(f *testing.F) {
+    fuzzFn(f)
+}
+
+func FuzzPass(f *testing.F) {
+    fuzzFn(f)
+}
+
+func FuzzPassString(f *testing.F) {
+    f.Add("some seed corpus")
+    f.Fuzz(func(*testing.T, string) {})
+}
+
+func FuzzPanic(f *testing.F) {
+    f.Fuzz(func(t *testing.T, b []byte) {})
+}
+
+func FuzzInNestedDir(f *testing.F) {
+    f.Fuzz(func(t *testing.T, b []byte) {})
+}
+
+func FuzzWrongType(f *testing.F) {
+    f.Fuzz(func(t *testing.T, b []byte) {})
+}
+
+-- corpustesting/testdata/fuzz/FuzzFail/1 --
+go test fuzz v1
+[]byte("12345")
+-- corpustesting/testdata/fuzz/FuzzPass/1 --
+go test fuzz v1
+[]byte("00000")
+-- corpustesting/testdata/fuzz/FuzzPassString/1 --
+go test fuzz v1
+string("hello")
+-- corpustesting/testdata/fuzz/FuzzPanic/1 --
+malformed
+-- corpustesting/testdata/fuzz/FuzzInNestedDir/anotherdir/1 --
+go test fuzz v1
+[]byte("12345")
+-- corpustesting/testdata/fuzz/FuzzWrongType/1 --
+go test fuzz v1
+int("00000")
diff --git a/src/cmd/go/testdata/script/test_fuzz_cache.txt b/src/cmd/go/testdata/script/test_fuzz_cache.txt
new file mode 100644
index 0000000..752ab3a
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_cache.txt
@@ -0,0 +1,97 @@
+[!fuzz-instrumented] skip
+
+[short] skip
+env GOCACHE=$WORK/cache
+
+# Fuzz cache should not exist after a regular test run.
+go test .
+exists $GOCACHE
+! exists $GOCACHE/fuzz
+
+# Fuzzing should write interesting values to the cache.
+go test -fuzz=FuzzY -fuzztime=100x .
+go run ./contains_files $GOCACHE/fuzz/example.com/y/FuzzY
+
+# 'go clean -cache' should not delete the fuzz cache.
+go clean -cache
+exists $GOCACHE/fuzz
+
+# 'go clean -fuzzcache' should delete the fuzz cache but not the build cache.
+go build -x ./empty
+stderr '(compile|gccgo)( |\.exe).*empty.go'
+go clean -fuzzcache
+! exists $GOCACHE/fuzz
+go build -x ./empty
+! stderr '(compile|gccgo)( |\.exe).*empty.go'
+
+# Fuzzing indicates that one new interesting value was found with an empty
+# corpus, and the total size of the cache is now 1.
+go clean -fuzzcache
+go test -fuzz=FuzzEmpty -fuzztime=10000x .
+stdout 'new interesting: 1'
+stdout 'total: 1'
+
+# Fuzzing again with a small fuzztime does not find any other interesting
+# values but still indicates that the cache size is 1.
+go test -fuzz=FuzzEmpty -fuzztime=2x .
+stdout 'new interesting: 0'
+stdout 'total: 1'
+
+! go clean -fuzzcache example.com/y
+stderr 'go: clean -fuzzcache cannot be used with package arguments'
+
+-- go.mod --
+module example.com/y
+
+go 1.16
+-- y_test.go --
+package y
+
+import (
+	"io"
+	"testing"
+)
+
+func FuzzEmpty(f *testing.F) {
+    f.Fuzz(func (*testing.T, []byte) {})
+}
+
+func FuzzY(f *testing.F) {
+	f.Add([]byte("y"))
+	f.Fuzz(func(t *testing.T, b []byte) { Y(io.Discard, b) })
+}
+-- y.go --
+package y
+
+import (
+	"bytes"
+	"io"
+)
+
+func Y(w io.Writer, b []byte) {
+	if !bytes.Equal(b, []byte("y")) {
+		w.Write([]byte("not equal"))
+	}
+}
+-- empty/empty.go --
+package empty
+-- contains_files/contains_files.go --
+package main
+
+import (
+	"fmt"
+	"path/filepath"
+	"io/ioutil"
+	"os"
+)
+
+func main() {
+	infos, err := ioutil.ReadDir(filepath.Clean(os.Args[1]))
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	if len(infos) == 0 {
+		os.Exit(1)
+	}
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_chatty.txt b/src/cmd/go/testdata/script/test_fuzz_chatty.txt
new file mode 100644
index 0000000..01a68cb
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_chatty.txt
@@ -0,0 +1,103 @@
+[!fuzz] skip
+[short] skip
+env GOCACHE=$WORK/cache
+
+# Run chatty fuzz targets with an error.
+! go test -v chatty_error_fuzz_test.go
+! stdout '^ok'
+stdout 'FAIL'
+stdout 'error in target'
+
+# Run chatty fuzz targets with a fatal.
+! go test -v chatty_fatal_fuzz_test.go
+! stdout '^ok'
+stdout 'FAIL'
+stdout 'fatal in target'
+
+# Run chatty fuzz target with a panic
+! go test -v chatty_panic_fuzz_test.go
+! stdout ^ok
+stdout FAIL
+stdout 'this is bad'
+
+# Run skipped chatty fuzz targets.
+go test -v chatty_skipped_fuzz_test.go
+stdout ok
+stdout SKIP
+! stdout FAIL
+
+# Run successful chatty fuzz targets.
+go test -v chatty_fuzz_test.go
+stdout ok
+stdout PASS
+stdout 'all good here'
+! stdout FAIL
+
+# Fuzz successful chatty fuzz target that includes a separate unit test.
+go test -v chatty_with_test_fuzz_test.go -fuzz=Fuzz -fuzztime=1x
+stdout ok
+stdout PASS
+! stdout FAIL
+stdout -count=1 'all good here'
+# Verify that the unit test is only run once.
+stdout -count=1 'logged foo'
+
+-- chatty_error_fuzz_test.go --
+package chatty_error_fuzz
+
+import "testing"
+
+func Fuzz(f *testing.F) {
+    f.Error("error in target")
+}
+
+-- chatty_fatal_fuzz_test.go --
+package chatty_fatal_fuzz
+
+import "testing"
+
+func Fuzz(f *testing.F) {
+    f.Fatal("fatal in target")
+}
+
+-- chatty_panic_fuzz_test.go --
+package chatty_panic_fuzz
+
+import "testing"
+
+func Fuzz(f *testing.F) {
+    panic("this is bad")
+}
+
+-- chatty_skipped_fuzz_test.go --
+package chatty_skipped_fuzz
+
+import "testing"
+
+func Fuzz(f *testing.F) {
+    f.Skip()
+}
+
+-- chatty_fuzz_test.go --
+package chatty_fuzz
+
+import "testing"
+
+func Fuzz(f *testing.F) {
+    f.Log("all good here")
+    f.Fuzz(func(*testing.T, []byte) {})
+}
+
+-- chatty_with_test_fuzz_test.go --
+package chatty_with_test_fuzz
+
+import "testing"
+
+func TestFoo(t *testing.T) {
+    t.Log("logged foo")
+}
+
+func Fuzz(f *testing.F) {
+    f.Log("all good here")
+    f.Fuzz(func(*testing.T, []byte) {})
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_cleanup.txt b/src/cmd/go/testdata/script/test_fuzz_cleanup.txt
new file mode 100644
index 0000000..5f86498
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_cleanup.txt
@@ -0,0 +1,67 @@
+[!fuzz] skip
+[short] skip
+env GOCACHE=$WORK/cache
+
+# Cleanup should run after F.Skip.
+go test -run=FuzzTargetSkip
+stdout cleanup
+
+# Cleanup should run after F.Fatal.
+! go test -run=FuzzTargetFatal
+stdout cleanup
+
+# Cleanup should run after an unexpected runtime.Goexit.
+! go test -run=FuzzTargetGoexit
+stdout cleanup
+
+# Cleanup should run after panic.
+! go test -run=FuzzTargetPanic
+stdout cleanup
+
+# Cleanup should run in fuzz function on seed corpus.
+go test -v -run=FuzzFunction
+stdout '(?s)inner.*outer'
+
+# TODO(jayconrod): test cleanup while fuzzing. For now, the worker process's
+# stdout and stderr is connected to the coordinator's, but it should eventually
+# be connected to os.DevNull, so we wouldn't see t.Log output.
+
+-- go.mod --
+module cleanup
+
+go 1.15
+-- cleanup_test.go --
+package cleanup
+
+import (
+	"runtime"
+	"testing"
+)
+
+func FuzzTargetSkip(f *testing.F) {
+	f.Cleanup(func() { f.Log("cleanup") })
+	f.Skip()
+}
+
+func FuzzTargetFatal(f *testing.F) {
+	f.Cleanup(func() { f.Log("cleanup") })
+	f.Fatal()
+}
+
+func FuzzTargetGoexit(f *testing.F) {
+	f.Cleanup(func() { f.Log("cleanup") })
+	runtime.Goexit()
+}
+
+func FuzzTargetPanic(f *testing.F) {
+	f.Cleanup(func() { f.Log("cleanup") })
+	panic("oh no")
+}
+
+func FuzzFunction(f *testing.F) {
+	f.Add([]byte{0})
+	f.Cleanup(func() { f.Log("outer") })
+	f.Fuzz(func(t *testing.T, b []byte) {
+		t.Cleanup(func() { t.Logf("inner") })
+	})
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_cov.txt b/src/cmd/go/testdata/script/test_fuzz_cov.txt
new file mode 100644
index 0000000..c0844a3
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_cov.txt
@@ -0,0 +1,36 @@
+# Test that coverage instrumentation is working. Without the instrumentation
+# it is _extremely_ unlikely that the fuzzer would produce this particular
+# input in any reasonable amount of time.
+
+[short] skip
+[!fuzz-instrumented] skip
+env GOCACHE=$WORK/cache
+
+# TODO(#51484): enabled debugging info to help diagnose a deadlock in the fuzzer
+env GODEBUG=fuzzdebug=1
+! go test -fuzz=FuzzCov -v
+! stderr 'cov instrumentation working'
+
+-- go.mod --
+module test
+
+-- cov_test.go --
+package cov
+
+import "testing"
+
+func FuzzCov(f *testing.F) {
+	f.Fuzz(func(t *testing.T, b []byte) {
+		if len(b) == 8 &&
+			b[0] == 'h' &&
+			b[1] == 'e' &&
+			b[2] == 'l' &&
+			b[3] == 'l' &&
+			b[4] == 'o' &&
+			b[5] == ' ' &&
+			b[6] == ':' &&
+			b[7] == ')' {
+			panic("cov instrumentation working")
+		}
+	})
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_deadline.txt b/src/cmd/go/testdata/script/test_fuzz_deadline.txt
new file mode 100644
index 0000000..46d3521
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_deadline.txt
@@ -0,0 +1,36 @@
+[!fuzz] skip
+[short] skip
+env GOCACHE=$WORK/cache
+
+# The fuzz function should be able to detect whether -timeout
+# was set with T.Deadline. Note there is no F.Deadline, and
+# there is no timeout while fuzzing, even if -fuzztime is set.
+go test -run=FuzzDeadline -wantdeadline=true # -timeout defaults to 10m
+go test -run=FuzzDeadline -timeout=0 -wantdeadline=false
+! go test -run=FuzzDeadline -timeout=1s -wantdeadline=false
+go test -run=FuzzDeadline -timeout=1s -wantdeadline=true
+go test -fuzz=FuzzDeadline -timeout=0 -fuzztime=1s -wantdeadline=false
+go test -fuzz=FuzzDeadline -timeout=0 -fuzztime=100x -wantdeadline=false
+
+-- go.mod --
+module fuzz
+
+go 1.16
+-- fuzz_deadline_test.go --
+package fuzz_test
+
+import (
+	"flag"
+	"testing"
+)
+
+var wantDeadline = flag.Bool("wantdeadline", false, "whether the test should have a deadline")
+
+func FuzzDeadline(f *testing.F) {
+	f.Add("run once")
+	f.Fuzz(func (t *testing.T, _ string) {
+		if _, hasDeadline := t.Deadline(); hasDeadline != *wantDeadline {
+			t.Fatalf("function got %v; want %v", hasDeadline, *wantDeadline)
+		}
+	})
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_dup_cache.txt b/src/cmd/go/testdata/script/test_fuzz_dup_cache.txt
new file mode 100644
index 0000000..f54a77c
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_dup_cache.txt
@@ -0,0 +1,53 @@
+[!fuzz] skip
+[short] skip
+env GOCACHE=$WORK/cache
+
+# This test checks that cached corpus loading properly handles duplicate entries (this can
+# happen when a f.Add value has a duplicate entry in the cached corpus.) Duplicate entries
+# should be discarded, and the rest of the cache should be loaded as normal.
+
+env GOCACHE=$WORK/cache
+env GODEBUG=fuzzdebug=1
+
+mkdir -p $GOCACHE/fuzz/fuzztest/FuzzTarget
+go run ./populate $GOCACHE/fuzz/fuzztest/FuzzTarget
+
+go test -fuzz=FuzzTarget -fuzztime=10x .
+stdout 'entries: 5'
+
+-- go.mod --
+module fuzztest
+
+go 1.17
+
+-- fuzz_test.go --
+package fuzz
+
+import "testing"
+
+func FuzzTarget(f *testing.F) {
+    f.Add(int(0))
+    f.Fuzz(func(t *testing.T, _ int) {})
+}
+
+-- populate/main.go --
+package main
+
+import (
+    "path/filepath"
+	"fmt"
+	"os"
+)
+
+func main() {
+	for i := 0; i < 10; i++ {
+		b := byte(0)
+		if i > 5 {
+			b = byte(i)
+		}
+        tmpl := "go test fuzz v1\nint(%d)\n"
+		if err := os.WriteFile(filepath.Join(os.Args[1], fmt.Sprint(i)), []byte(fmt.Sprintf(tmpl, b)), 0777); err != nil {
+			panic(err)
+		}
+	}
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/test_fuzz_err_deadlock.txt b/src/cmd/go/testdata/script/test_fuzz_err_deadlock.txt
new file mode 100644
index 0000000..4feb41a
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_err_deadlock.txt
@@ -0,0 +1,50 @@
+[short] skip
+[!fuzz-instrumented] skip
+
+env GOCACHE=$WORK/cache
+! go test -fuzz=FuzzDead -v
+# This is a somewhat inexact check, but since we don't prefix the error with anything
+# and as the error suffix is platform dependent, this is the best we can do. In the
+# deadlock failure case, the test will just deadlock and timeout anyway, so it should
+# be clear that that failure mode is different.
+stdout 'open'
+
+-- go.mod --
+module test
+
+-- cov_test.go --
+package dead
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+func FuzzDead(f *testing.F) {
+	go func() {
+		c := filepath.Join(os.Getenv("GOCACHE"), "fuzz", "test", "FuzzDead")
+		t := time.NewTicker(time.Second)
+		for range t.C {
+			files, _ := os.ReadDir(c)
+			if len(files) > 0 {
+				os.RemoveAll(c)
+			}
+		}
+	}()
+
+	f.Fuzz(func(t *testing.T, b []byte) {
+		if len(b) == 8 &&
+			b[0] == 'h' &&
+			b[1] == 'e' &&
+			b[2] == 'l' &&
+			b[3] == 'l' &&
+			b[4] == 'o' &&
+			b[5] == ' ' &&
+			b[6] == ':' &&
+			b[7] == ')' {
+			return
+		}
+	})
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_fuzztime.txt b/src/cmd/go/testdata/script/test_fuzz_fuzztime.txt
new file mode 100644
index 0000000..28ef3bf
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_fuzztime.txt
@@ -0,0 +1,120 @@
+[!fuzz] skip
+[short] skip
+env GOCACHE=$WORK/cache
+
+# There are no seed values, so 'go test' should finish quickly.
+go test
+
+# Fuzzing should exit 0 after fuzztime, even if timeout is short.
+go test -timeout=3s -fuzz=FuzzFast -fuzztime=5s
+
+# We should see the same behavior when invoking the test binary directly.
+go test -c
+exec ./fuzz.test$GOEXE -test.timeout=3s -test.fuzz=FuzzFast -test.fuzztime=5s -test.parallel=1 -test.fuzzcachedir=$WORK/cache
+
+# Timeout should not cause inputs to be written as crashers.
+! exists testdata/fuzz
+
+# When we use fuzztime with an "x" suffix, it runs a specific number of times.
+# This fuzz function creates a file with a unique name ($pid.$count) on each
+# run. We count the files to find the number of runs.
+mkdir count
+go test -fuzz=FuzzTestCount -fuzztime=1000x -fuzzminimizetime=1x
+go run check_file_count.go count 1000
+
+# When we use fuzzminimizetime with an "x" suffix, it runs a specific number of
+# times while minimizing. This fuzz function creates a file with a unique name
+# ($pid.$count) on each run once the first crash has been found. That means that
+# there should be one file for each execution of the fuzz function during
+# minimization, so we count these to determine how many times minimization was
+# run.
+mkdir minimizecount
+! go test -fuzz=FuzzMinimizeCount -fuzzminimizetime=3x -parallel=1
+go run check_file_count.go minimizecount 3
+
+-- go.mod --
+module fuzz
+
+go 1.16
+-- fuzz_fast_test.go --
+package fuzz_test
+
+import "testing"
+
+func FuzzFast(f *testing.F) {
+	f.Fuzz(func (*testing.T, []byte) {})
+}
+-- fuzz_count_test.go --
+package fuzz
+
+import (
+	"fmt"
+	"os"
+	"testing"
+)
+
+func FuzzTestCount(f *testing.F) {
+	pid := os.Getpid()
+	n := 0
+	f.Fuzz(func(t *testing.T, _ []byte) {
+		name := fmt.Sprintf("count/%v.%d", pid, n)
+		if err := os.WriteFile(name, nil, 0666); err != nil {
+			t.Fatal(err)
+		}
+		n++
+	})
+}
+-- fuzz_minimize_count_test.go --
+package fuzz
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"testing"
+)
+
+func FuzzMinimizeCount(f *testing.F) {
+	pid := os.Getpid()
+	n := 0
+	seed := bytes.Repeat([]byte("a"), 357)
+	f.Add(seed)
+	crashFound := false
+	f.Fuzz(func(t *testing.T, b []byte) {
+		if crashFound {
+			name := fmt.Sprintf("minimizecount/%v.%d", pid, n)
+			if err := os.WriteFile(name, nil, 0666); err != nil {
+				t.Fatal(err)
+			}
+			n++
+		}
+		if !bytes.Equal(b, seed) {  // this should happen right away
+			crashFound = true
+			t.Error("minimize this!")
+		}
+	})
+}
+-- check_file_count.go --
+// +build ignore
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+)
+
+func main() {
+	dir, err := os.ReadDir(os.Args[1])
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	got := len(dir)
+	want, _ := strconv.Atoi(os.Args[2])
+	if got != want {
+		fmt.Fprintf(os.Stderr, "got %d files; want %d\n", got, want)
+		os.Exit(1)
+	}
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_io_error.txt b/src/cmd/go/testdata/script/test_fuzz_io_error.txt
new file mode 100644
index 0000000..01b4da6
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_io_error.txt
@@ -0,0 +1,102 @@
+# Test that when the coordinator experiences an I/O error communicating
+# with a worker, the coordinator stops the worker and reports the error.
+# The coordinator should not record a crasher.
+#
+# We simulate an I/O error in the test by writing garbage to fuzz_out.
+# This is unlikely, but possible. It's difficult to simulate interruptions
+# due to ^C and EOF errors which are more common. We don't report those.
+[short] skip
+[!fuzz] skip
+env GOCACHE=$WORK/cache
+
+# If the I/O error occurs before F.Fuzz is called, the coordinator should
+# stop the worker and say that.
+! go test -fuzz=FuzzClosePipeBefore -parallel=1
+stdout '\s*fuzzing process terminated without fuzzing:'
+! stdout 'communicating with fuzzing process'
+! exists testdata
+
+# If the I/O error occurs after F.Fuzz is called (unlikely), just exit.
+# It's hard to distinguish this case from the worker being interrupted by ^C
+# or exiting with status 0 (which it should do when interrupted by ^C).
+! go test -fuzz=FuzzClosePipeAfter -parallel=1
+stdout '^\s*communicating with fuzzing process: invalid character ''!'' looking for beginning of value$'
+! exists testdata
+
+-- go.mod --
+module test
+
+go 1.17
+-- io_error_test.go --
+package io_error
+
+import (
+	"flag"
+	"testing"
+	"time"
+)
+
+func isWorker() bool {
+	f := flag.Lookup("test.fuzzworker")
+	if f == nil {
+		return false
+	}
+	get, ok := f.Value.(flag.Getter)
+	if !ok {
+		return false
+	}
+	return get.Get() == interface{}(true)
+}
+
+func FuzzClosePipeBefore(f *testing.F) {
+	if isWorker() {
+		sendGarbageToCoordinator(f)
+		time.Sleep(3600 * time.Second) // pause until coordinator terminates the process
+	}
+	f.Fuzz(func(*testing.T, []byte) {})
+}
+
+func FuzzClosePipeAfter(f *testing.F) {
+	f.Fuzz(func(t *testing.T, _ []byte) {
+		if isWorker() {
+			sendGarbageToCoordinator(t)
+			time.Sleep(3600 * time.Second) // pause until coordinator terminates the process
+		}
+	})
+}
+-- io_error_windows_test.go --
+package io_error
+
+import (
+	"fmt"
+	"os"
+	"testing"
+)
+
+func sendGarbageToCoordinator(tb testing.TB) {
+	v := os.Getenv("GO_TEST_FUZZ_WORKER_HANDLES")
+	var fuzzInFD, fuzzOutFD uintptr
+	if _, err := fmt.Sscanf(v, "%x,%x", &fuzzInFD, &fuzzOutFD); err != nil {
+		tb.Fatalf("parsing GO_TEST_FUZZ_WORKER_HANDLES: %v", err)
+	}
+	f := os.NewFile(fuzzOutFD, "fuzz_out")
+	if _, err := f.Write([]byte("!!")); err != nil {
+		tb.Fatalf("writing fuzz_out: %v", err)
+	}
+}
+-- io_error_notwindows_test.go --
+// +build !windows
+
+package io_error
+
+import (
+	"os"
+	"testing"
+)
+
+func sendGarbageToCoordinator(tb testing.TB) {
+	f := os.NewFile(4, "fuzz_out")
+	if _, err := f.Write([]byte("!!")); err != nil {
+		tb.Fatalf("writing fuzz_out: %v", err)
+	}
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_limit_dup_entry.txt b/src/cmd/go/testdata/script/test_fuzz_limit_dup_entry.txt
new file mode 100644
index 0000000..01217ae
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_limit_dup_entry.txt
@@ -0,0 +1,37 @@
+[!fuzz] skip
+[short] skip
+env GOCACHE=$WORK/cache
+
+# FuzzA attempts to cause the mutator to create duplicate inputs that generate
+# new coverage. Previously this would trigger a corner case when the fuzzer
+# had a execution limit, causing it to deadlock and sit in the coordinator
+# loop indefinitely, failing to exit once the limit had been exhausted.
+
+go test -fuzz=FuzzA -fuzztime=100x -parallel=1
+
+-- go.mod --
+module m
+
+go 1.16
+-- fuzz_test.go --
+package fuzz_test
+
+import (
+	"fmt"
+	"testing"
+)
+
+func FuzzA(f *testing.F) {
+	f.Add([]byte("seed"))
+	i := 0
+	f.Fuzz(func(t *testing.T, b []byte) {
+		i++
+		if string(b) == "seed" {
+			if i == 0 {
+				fmt.Println("a")
+			} else if i > 1 {
+				fmt.Println("b")
+			}
+		}
+	})
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_match.txt b/src/cmd/go/testdata/script/test_fuzz_match.txt
new file mode 100644
index 0000000..d149586
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_match.txt
@@ -0,0 +1,40 @@
+[!fuzz] skip
+[short] skip
+env GOCACHE=$WORK/cache
+
+# Matches only fuzz targets to test.
+go test standalone_fuzz_test.go
+! stdout '^ok.*\[no tests to run\]'
+stdout '^ok'
+
+# Matches only for fuzzing.
+go test -fuzz Fuzz -fuzztime 1x standalone_fuzz_test.go
+! stdout '^ok.*\[no tests to run\]'
+stdout '^ok'
+
+# Matches none for fuzzing but will run the fuzz target as a test.
+go test -fuzz ThisWillNotMatch -fuzztime 1x standalone_fuzz_test.go
+! stdout '^ok.*no tests to run'
+stdout '^ok'
+stdout 'no fuzz tests to fuzz'
+
+[short] stop
+
+# Matches only fuzz targets to test with -run.
+go test -run Fuzz standalone_fuzz_test.go
+! stdout '^ok.*\[no tests to run\]'
+stdout '^ok'
+
+# Matches no fuzz targets.
+go test -run ThisWillNotMatch standalone_fuzz_test.go
+stdout '^ok.*no tests to run'
+! stdout 'no fuzz tests to fuzz'
+
+-- standalone_fuzz_test.go --
+package standalone_fuzz
+
+import "testing"
+
+func Fuzz(f *testing.F) {
+	f.Fuzz(func (*testing.T, []byte) {})
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_minimize.txt b/src/cmd/go/testdata/script/test_fuzz_minimize.txt
new file mode 100644
index 0000000..a6dc3f1
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_minimize.txt
@@ -0,0 +1,203 @@
+[!fuzz] skip
+[short] skip
+
+# We clean the fuzz cache during this test. Don't clean the user's cache.
+env GOCACHE=$WORK/gocache
+
+# Test that fuzzminimizetime cannot be negative seconds
+! go test -fuzz=FuzzMinimizerRecoverable -run=FuzzMinimizerRecoverable -fuzztime=10000x -fuzzminimizetime=-1ms .
+! stdout '^ok'
+! stdout 'contains a non-zero byte'
+stdout 'invalid duration'
+stdout FAIL
+
+# Test that fuzzminimizetime cannot be negative times
+! go test -fuzz=FuzzMinimizerRecoverable -run=FuzzMinimizerRecoverable -fuzztime=10000x -fuzzminimizetime=-1x .
+! stdout '^ok'
+! stdout 'contains a non-zero byte'
+stdout 'invalid count'
+stdout FAIL
+
+# Test that fuzzminimizetime can be zero seconds, and minimization is disabled
+! go test -fuzz=FuzzMinimizeZeroDurationSet -run=FuzzMinimizeZeroDurationSet -fuzztime=10000x -fuzzminimizetime=0s .
+! stdout '^ok'
+! stdout 'minimizing'
+stdout 'there was an Error'
+stdout FAIL
+
+# Test that fuzzminimizetime can be zero times, and minimization is disabled
+! go test -fuzz=FuzzMinimizeZeroLimitSet -run=FuzzMinimizeZeroLimitSet -fuzztime=10000x -fuzzminimizetime=0x .
+! stdout '^ok'
+! stdout 'minimizing'
+stdout -count=1 'there was an Error'
+stdout FAIL
+
+# Test that minimization is working for recoverable errors.
+! go test -fuzz=FuzzMinimizerRecoverable -run=FuzzMinimizerRecoverable -fuzztime=10000x .
+! stdout '^ok'
+stdout 'got the minimum size!'
+# The error message that was printed should be for the one written to testdata.
+stdout 'contains a non-zero byte of length 50'
+stdout FAIL
+
+# Check that the bytes written to testdata are of length 50 (the minimum size)
+go run ./check_testdata FuzzMinimizerRecoverable 50
+
+# Test that re-running the minimized value causes a crash.
+! go test -run=FuzzMinimizerRecoverable .
+rm testdata
+
+# Test that minimization is working for recoverable errors. Run it with -v this
+# time to ensure the command line output still looks right.
+! go test -v -fuzz=FuzzMinimizerRecoverable -run=FuzzMinimizerRecoverable -fuzztime=10000x .
+! stdout '^ok'
+stdout 'got the minimum size!'
+# The error message that was printed should be for the one written to testdata.
+stdout 'contains a non-zero byte of length 50'
+stdout FAIL
+
+# Check that the bytes written to testdata are of length 50 (the minimum size)
+go run ./check_testdata FuzzMinimizerRecoverable 50
+
+# Test that re-running the minimized value causes a crash.
+! go test -run=FuzzMinimizerRecoverable .
+rm testdata
+
+# Test that minimization doesn't run for non-recoverable errors.
+! go test -fuzz=FuzzMinimizerNonrecoverable -run=FuzzMinimizerNonrecoverable -fuzztime=10000x .
+! stdout '^ok'
+! stdout 'minimizing'
+stdout -count=1 '^\s+fuzzing process hung or terminated unexpectedly: exit status 99'
+stdout FAIL
+
+# Check that re-running the value causes a crash.
+! go test -run=FuzzMinimizerNonrecoverable .
+rm testdata
+
+# Clear the fuzzing cache. There may already be minimized inputs that would
+# interfere with the next stage of the test.
+go clean -fuzzcache
+
+# Test that minimization can be cancelled by fuzzminimizetime and the latest
+# crash will still be logged and written to testdata.
+! go test -fuzz=FuzzMinimizerRecoverable -run=FuzzMinimizerRecoverable -fuzztime=100x -fuzzminimizetime=1x .
+! stdout '^ok'
+stdout 'testdata[/\\]fuzz[/\\]FuzzMinimizerRecoverable[/\\]'
+! stdout 'got the minimum size!'  # it shouldn't have had enough time to minimize it
+stdout FAIL
+
+# Test that re-running the unminimized value causes a crash.
+! go test -run=FuzzMinimizerRecoverable .
+
+# TODO(jayconrod,katiehockman): add a test which verifies that the right bytes
+# are written to testdata in the case of an interrupt during minimization.
+
+-- go.mod --
+module example.com/y
+
+go 1.16
+-- y_test.go --
+package y
+
+import (
+	"os"
+	"testing"
+)
+
+func FuzzMinimizeZeroDurationSet(f *testing.F) {
+	f.Fuzz(func(t *testing.T, b []byte) {
+		if len(b) > 5 {
+			t.Errorf("there was an Error")
+		}
+	})
+}
+
+func FuzzMinimizeZeroLimitSet(f *testing.F) {
+	f.Fuzz(func(t *testing.T, b []byte) {
+		if len(b) > 5 {
+			t.Errorf("there was an Error")
+		}
+	})
+}
+
+func FuzzMinimizerRecoverable(f *testing.F) {
+	f.Add(make([]byte, 100))
+	f.Fuzz(func(t *testing.T, b []byte) {
+		if len(b) < 50 {
+			// Make sure that b is large enough that it can be minimized
+			return
+		}
+		// Given the randomness of the mutations, this should allow the
+		// minimizer to trim down the value a bit.
+		for _, n := range b {
+			if n != 0 {
+				if len(b) == 50 {
+					t.Log("got the minimum size!")
+				}
+				t.Fatalf("contains a non-zero byte of length %d", len(b))
+			}
+		}
+	})
+}
+
+func FuzzMinimizerNonrecoverable(f *testing.F) {
+	f.Fuzz(func(t *testing.T, b []byte) {
+		os.Exit(99)
+	})
+}
+-- empty/empty.go --
+package empty
+-- check_testdata/check_testdata.go --
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strconv"
+)
+
+func main() {
+	target := os.Args[1]
+	numBytes, err := strconv.Atoi(os.Args[2])
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+
+	// Open the file in testdata (there should only be one)
+	dir := fmt.Sprintf("testdata/fuzz/%s", target)
+	files, err := ioutil.ReadDir(dir)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	if len(files) != 1 {
+		fmt.Fprintf(os.Stderr, "expected one file, got %d", len(files))
+		os.Exit(1)
+	}
+	got, err := ioutil.ReadFile(filepath.Join(dir, files[0].Name()))
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+
+	// Trim the newline at the end of the file
+	got = bytes.TrimSpace(got)
+
+	// Make sure that there were exactly 100 bytes written to the corpus entry
+	prefix := []byte("[]byte(")
+	i := bytes.Index(got, prefix)
+	gotBytes := got[i+len(prefix) : len(got)-1]
+	s, err := strconv.Unquote(string(gotBytes))
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	if want, got := numBytes, len(s); want != got {
+		fmt.Fprintf(os.Stderr, "want %d bytes, got %d\n", want, got)
+		os.Exit(1)
+	}
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_minimize_dirty_cov.txt b/src/cmd/go/testdata/script/test_fuzz_minimize_dirty_cov.txt
new file mode 100644
index 0000000..1279f6e
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_minimize_dirty_cov.txt
@@ -0,0 +1,85 @@
+# Test that minimization doesn't use dirty coverage snapshots when it
+# is unable to actually minimize the input. We do this by checking that
+# a expected value appears in the cache. If a dirty coverage map is used
+# (i.e. the coverage map generated during the last minimization step,
+# rather than the map provided with the initial input) then this value
+# is unlikely to appear in the cache, since the map generated during
+# the last minimization step should not increase the coverage.
+
+[short] skip
+[!fuzz-instrumented] skip
+
+env GOCACHE=$WORK/gocache
+go test -fuzz=FuzzCovMin -fuzztime=500000x -test.fuzzcachedir=$GOCACHE/fuzz
+go run check_file/main.go $GOCACHE/fuzz/FuzzCovMin ab
+
+-- go.mod --
+module test
+
+-- covmin_test.go --
+package covmin
+
+import "testing"
+
+func FuzzCovMin(f *testing.F) {
+	f.Add([]byte("aa"))
+	f.Fuzz(func(t *testing.T, data []byte) {
+		if len(data) == 2 && data[0] == 'a' && data[1] == 'b' {
+			return
+		}
+	})
+}
+
+-- check_file/main.go --
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strconv"
+)
+
+func checkFile(name, expected string) (bool, error) {
+	data, err := os.ReadFile(name)
+	if err != nil {
+		return false, err
+	}
+	for _, line := range bytes.Split(data, []byte("\n")) {
+		m := valRe.FindSubmatch(line)
+		if m == nil {
+			continue
+		}
+		fmt.Println(strconv.Unquote(string(m[1])))
+		if s, err := strconv.Unquote(string(m[1])); err != nil {
+			return false, err
+		} else if s == expected {
+			return true, nil
+		}
+	}
+	return false, nil
+}
+
+var valRe = regexp.MustCompile(`^\[\]byte\(([^)]+)\)$`)
+
+func main() {
+	dir, expected := os.Args[1], os.Args[2]
+	ents, err := os.ReadDir(dir)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	for _, ent := range ents {
+		name := filepath.Join(dir, ent.Name())
+		if good, err := checkFile(name, expected); err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(1)
+		} else if good {
+			os.Exit(0)
+		}
+	}
+	fmt.Fprintln(os.Stderr, "input over minimized")
+	os.Exit(1)
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_minimize_interesting.txt b/src/cmd/go/testdata/script/test_fuzz_minimize_interesting.txt
new file mode 100644
index 0000000..e61c4f9
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_minimize_interesting.txt
@@ -0,0 +1,230 @@
+[short] skip
+[!fuzz-instrumented] skip
+
+# Test that when an interesting value is discovered (one that expands coverage),
+# the fuzzing engine minimizes it before writing it to the cache.
+#
+# The program below starts with a seed value of length 100, but more coverage
+# will be found for any value other than the seed. We should end with a value
+# in the cache of length 1 (the minimizer currently does not produce empty
+# strings). check_cache.go confirms that.
+#
+# We would like to verify that ALL values in the cache were minimized to a
+# length of 1, but this isn't always possible when new coverage is found in
+# functions called by testing or internal/fuzz in the background.
+
+go test -c -fuzz=.  # Build using shared build cache for speed.
+env GOCACHE=$WORK/gocache
+exec ./fuzz.test$GOEXE -test.fuzzcachedir=$GOCACHE/fuzz -test.fuzz=FuzzMinCache -test.fuzztime=1000x
+go run check_cache/check_cache.go $GOCACHE/fuzz/FuzzMinCache
+
+# Test that minimization occurs for a crash that appears while minimizing a
+# newly found interesting input. There must be only one worker for this test to
+# be flaky like we want.
+! exec ./fuzz.test$GOEXE -test.fuzzcachedir=$GOCACHE/fuzz -test.fuzz=FuzzMinimizerCrashInMinimization -test.run=XXX -test.fuzztime=10000x -test.parallel=1
+! stdout '^ok'
+stdout -count=1 'got the minimum size!'
+stdout -count=1 'bad input'
+stdout FAIL
+# Check that the input written to testdata will reproduce the error, and is the
+# smallest possible.
+go run check_testdata/check_testdata.go FuzzMinimizerCrashInMinimization 1
+
+# Test that a nonrecoverable error that occurs while minimizing an interesting
+# input is reported correctly.
+! exec ./fuzz.test$GOEXE -test.fuzzcachedir=$GOCACHE/fuzz -test.fuzz=FuzzMinimizerNonrecoverableCrashInMinimization -test.run=XXX -test.fuzztime=10000x -test.parallel=1
+! stdout '^ok'
+stdout -count=1 'fuzzing process hung or terminated unexpectedly while minimizing'
+stdout -count=1 'EOF'
+stdout FAIL
+# Check that the input written to testdata will reproduce the error.
+go run check_testdata/check_testdata.go FuzzMinimizerNonrecoverableCrashInMinimization 1
+
+-- go.mod --
+module fuzz
+
+go 1.17
+-- y.go --
+package fuzz
+
+import (
+	"bytes"
+	"io"
+)
+
+func Y(w io.Writer, s string) {
+	if !bytes.Equal([]byte(s), []byte("y")) {
+		w.Write([]byte("not equal"))
+	}
+}
+-- fuzz_test.go --
+package fuzz
+
+import (
+	"bytes"
+	"os"
+	"testing"
+)
+
+func FuzzMinimizerCrashInMinimization(f *testing.F) {
+	seed := bytes.Repeat([]byte{255}, 100)
+	f.Add(seed)
+	f.Fuzz(func(t *testing.T, b []byte) {
+		if bytes.Equal(seed, b) {
+			return
+		}
+		t.Error("bad input")
+		if len(b) == 1 {
+			t.Error("got the minimum size!")
+		}
+	})
+}
+
+var fuzzing bool
+
+func FuzzMinimizerNonrecoverableCrashInMinimization(f *testing.F) {
+	seed := bytes.Repeat([]byte{255}, 100)
+	f.Add(seed)
+	f.Fuzz(func(t *testing.T, b []byte) {
+		if bytes.Equal(seed, b) {
+			return
+		} else if len(b) == 1 {
+			os.Exit(1)
+		}
+	})
+}
+
+func FuzzMinCache(f *testing.F) {
+	seed := bytes.Repeat([]byte("a"), 20)
+	f.Add(seed)
+	f.Fuzz(func(t *testing.T, buf []byte) {
+		if bytes.Equal(buf, seed) {
+			return
+		}
+	})
+}
+-- check_testdata/check_testdata.go --
+//go:build ignore
+// +build ignore
+
+// check_testdata.go checks that the string written
+// is not longer than the provided length.
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strconv"
+)
+
+func main() {
+	wantLen, err := strconv.Atoi(os.Args[2])
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	testName := os.Args[1]
+	dir := filepath.Join("testdata/fuzz", testName)
+
+	files, err := ioutil.ReadDir(dir)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+
+	if len(files) == 0 {
+		fmt.Fprintf(os.Stderr, "expect at least one failure to be written to testdata\n")
+		os.Exit(1)
+	}
+
+	for _, f := range files {
+		data, err := ioutil.ReadFile(filepath.Join(dir, f.Name()))
+		if err != nil {
+			panic(err)
+		}
+		var containsVal bool
+		for _, line := range bytes.Split(data, []byte("\n")) {
+			m := valRe.FindSubmatch(line)
+			if m == nil {
+				continue
+			}
+			containsVal = true
+			s, err := strconv.Unquote(string(m[1]))
+			if err != nil {
+				panic(err)
+			}
+			if len(s) != wantLen {
+				fmt.Fprintf(os.Stderr, "expect length %d, got %d (%q)\n", wantLen, len(s), line)
+				os.Exit(1)
+			}
+		}
+		if !containsVal {
+			fmt.Fprintln(os.Stderr, "corpus file contained no values")
+			os.Exit(1)
+		}
+	}
+}
+
+var valRe = regexp.MustCompile(`^\[\]byte\(([^)]+)\)$`)
+
+-- check_cache/check_cache.go --
+//go:build ignore
+// +build ignore
+
+// check_cache.go checks that each file in the cached corpus has a []byte
+// of length at most 1. This verifies that at least one cached input is minimized.
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strconv"
+)
+
+func main() {
+	dir := os.Args[1]
+	ents, err := os.ReadDir(dir)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	for _, ent := range ents {
+		name := filepath.Join(dir, ent.Name())
+		if good, err := checkCacheFile(name); err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(1)
+		} else if good {
+			os.Exit(0)
+		}
+	}
+	fmt.Fprintln(os.Stderr, "no cached inputs were minimized")
+	os.Exit(1)
+}
+
+func checkCacheFile(name string) (good bool, err error) {
+	data, err := os.ReadFile(name)
+	if err != nil {
+		return false, err
+	}
+	for _, line := range bytes.Split(data, []byte("\n")) {
+		m := valRe.FindSubmatch(line)
+		if m == nil {
+			continue
+		}
+		if s, err := strconv.Unquote(string(m[1])); err != nil {
+			return false, err
+		} else if len(s) <= 1 {
+			return true, nil
+		}
+	}
+	return false, nil
+}
+
+var valRe = regexp.MustCompile(`^\[\]byte\(([^)]+)\)$`)
diff --git a/src/cmd/go/testdata/script/test_fuzz_modcache.txt b/src/cmd/go/testdata/script/test_fuzz_modcache.txt
new file mode 100644
index 0000000..c0f18ea
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_modcache.txt
@@ -0,0 +1,58 @@
+# This test demonstrates the fuzz corpus behavior for packages outside of the main module.
+# (See https://golang.org/issue/48495.)
+
+[short] skip
+
+# Set -modcacherw so that the test behaves the same regardless of whether the
+# module cache is writable. (For example, on some platforms it can always be
+# written if the user is running as root.) At one point, a failing fuzz test
+# in a writable module cache would corrupt module checksums in the cache.
+env GOFLAGS=-modcacherw
+
+
+# When the upstream module has no test corpus, running 'go test' should succeed,
+# but 'go test -fuzz=.' should error out before running the test.
+# (It should NOT corrupt the module cache by writing out new fuzz inputs,
+# even if the cache is writable.)
+
+go get -t example.com/fuzzfail@v0.1.0
+go test example.com/fuzzfail
+
+! go test -fuzz=. example.com/fuzzfail
+! stdout .
+stderr '^cannot use -fuzz flag on package outside the main module$'
+
+go mod verify
+
+
+# If the module does include a test corpus, 'go test' (without '-fuzz') should
+# load that corpus and run the fuzz tests against it, but 'go test -fuzz=.'
+# should continue to be rejected.
+
+go get -t example.com/fuzzfail@v0.2.0
+
+! go test example.com/fuzzfail
+stdout '^\s*fuzzfail_test\.go:7: oops:'
+
+! go test -fuzz=. example.com/fuzzfail
+! stdout .
+stderr '^cannot use -fuzz flag on package outside the main module$'
+
+go mod verify
+
+
+# Packages in 'std' cannot be fuzzed when the corresponding GOROOT module is not
+# the main module — either the failures would not be recorded or the behavior of
+# the 'std' tests would change globally.
+
+! go test -fuzz . encoding/json
+stderr '^cannot use -fuzz flag on package outside the main module$'
+
+! go test -fuzz . cmd/buildid
+stderr '^cannot use -fuzz flag on package outside the main module$'
+
+
+-- go.mod --
+module example.com/m
+
+go 1.18
diff --git a/src/cmd/go/testdata/script/test_fuzz_multiple.txt b/src/cmd/go/testdata/script/test_fuzz_multiple.txt
new file mode 100644
index 0000000..c96112f
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_multiple.txt
@@ -0,0 +1,50 @@
+# This test checks that 'go test' prints a reasonable error when fuzzing is
+# enabled, and multiple package or multiple fuzz targets match.
+# TODO(#46312): support fuzzing multiple targets in multiple packages.
+
+[!fuzz] skip
+[short] skip
+env GOCACHE=$WORK/cache
+
+# With fuzzing disabled, multiple targets can be tested.
+go test ./...
+
+# With fuzzing enabled, at most one package may be tested,
+# even if only one package contains fuzz targets.
+! go test -fuzz=. ./...
+stderr '^cannot use -fuzz flag with multiple packages$'
+! go test -fuzz=. ./zero ./one
+stderr '^cannot use -fuzz flag with multiple packages$'
+go test -fuzz=. -fuzztime=1x ./one
+
+# With fuzzing enabled, at most one target in the same package may match.
+! go test -fuzz=. ./two
+stdout '^testing: will not fuzz, -fuzz matches more than one fuzz test: \[FuzzOne FuzzTwo\]$'
+go test -fuzz=FuzzTwo -fuzztime=1x ./two
+
+-- go.mod --
+module fuzz
+
+go 1.18
+-- zero/zero.go --
+package zero
+-- one/one_test.go --
+package one
+
+import "testing"
+
+func FuzzOne(f *testing.F) {
+  f.Fuzz(func(*testing.T, []byte) {})
+}
+-- two/two_test.go --
+package two
+
+import "testing"
+
+func FuzzOne(f *testing.F) {
+  f.Fuzz(func(*testing.T, []byte) {})
+}
+
+func FuzzTwo(f *testing.F) {
+  f.Fuzz(func(*testing.T, []byte) {})
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_mutate_crash.txt b/src/cmd/go/testdata/script/test_fuzz_mutate_crash.txt
new file mode 100644
index 0000000..4b9b36d
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_mutate_crash.txt
@@ -0,0 +1,323 @@
+[!fuzz] skip
+
+# Tests that a crash caused by a mutator-discovered input writes the bad input
+# to testdata, and fails+reports correctly. This tests the end-to-end behavior
+# of the mutator finding a crash while fuzzing, adding it as a regression test
+# to the seed corpus in testdata, and failing the next time the test is run.
+
+[short] skip
+env GOCACHE=$WORK/cache
+
+# Running the seed corpus for all of the targets should pass the first
+# time, since nothing in the seed corpus will cause a crash.
+go test
+
+# Running the fuzzer should find a crashing input quickly.
+! go test -fuzz=FuzzWithBug -fuzztime=100x -fuzzminimizetime=1000x
+stdout 'testdata[/\\]fuzz[/\\]FuzzWithBug[/\\]'
+stdout 'this input caused a crash!'
+go run check_testdata.go FuzzWithBug
+
+# Now, the failing bytes should have been added to the seed corpus for
+# the target, and should fail when run without fuzzing.
+! go test
+stdout 'FuzzWithBug/[a-f0-9]{16}'
+stdout 'this input caused a crash!'
+
+! go test -run=FuzzWithNilPanic -fuzz=FuzzWithNilPanic -fuzztime=100x -fuzzminimizetime=1000x
+stdout 'testdata[/\\]fuzz[/\\]FuzzWithNilPanic[/\\]'
+stdout 'panic called with nil argument|test executed panic.nil. or runtime.Goexit'
+go run check_testdata.go FuzzWithNilPanic
+
+! go test -run=FuzzWithGoexit -fuzz=FuzzWithGoexit -fuzztime=100x -fuzzminimizetime=1000x
+stdout 'testdata[/\\]fuzz[/\\]FuzzWithGoexit[/\\]'
+stdout 'runtime.Goexit'
+go run check_testdata.go FuzzWithGoexit
+
+! go test -run=FuzzWithFail -fuzz=FuzzWithFail -fuzztime=100x -fuzzminimizetime=1000x
+stdout 'testdata[/\\]fuzz[/\\]FuzzWithFail[/\\]'
+go run check_testdata.go FuzzWithFail
+
+! go test -run=FuzzWithLogFail -fuzz=FuzzWithLogFail -fuzztime=100x -fuzzminimizetime=1000x
+stdout 'testdata[/\\]fuzz[/\\]FuzzWithLogFail[/\\]'
+stdout 'logged something'
+go run check_testdata.go FuzzWithLogFail
+
+! go test -run=FuzzWithErrorf -fuzz=FuzzWithErrorf -fuzztime=100x -fuzzminimizetime=1000x
+stdout 'testdata[/\\]fuzz[/\\]FuzzWithErrorf[/\\]'
+stdout 'errorf was called here'
+go run check_testdata.go FuzzWithErrorf
+
+! go test -run=FuzzWithFatalf -fuzz=FuzzWithFatalf -fuzztime=100x -fuzzminimizetime=1000x
+stdout 'testdata[/\\]fuzz[/\\]FuzzWithFatalf[/\\]'
+stdout 'fatalf was called here'
+go run check_testdata.go FuzzWithFatalf
+
+! go test -run=FuzzWithBadExit -fuzz=FuzzWithBadExit -fuzztime=100x -fuzzminimizetime=1000x
+stdout 'testdata[/\\]fuzz[/\\]FuzzWithBadExit[/\\]'
+stdout '^\s+fuzzing process hung or terminated unexpectedly: exit status'
+go run check_testdata.go FuzzWithBadExit
+
+! go test -run=FuzzDeadlock -fuzz=FuzzDeadlock -fuzztime=100x -fuzzminimizetime=0x
+stdout 'testdata[/\\]fuzz[/\\]FuzzDeadlock[/\\]'
+stdout '^\s+fuzzing process hung or terminated unexpectedly: exit status'
+go run check_testdata.go FuzzDeadlock
+
+# Running the fuzzer should find a crashing input quickly for fuzzing two types.
+! go test -run=FuzzWithTwoTypes -fuzz=FuzzWithTwoTypes -fuzztime=100x -fuzzminimizetime=1000x
+stdout 'testdata[/\\]fuzz[/\\]FuzzWithTwoTypes[/\\]'
+stdout 'these inputs caused a crash!'
+go run check_testdata.go FuzzWithTwoTypes
+
+# Running the fuzzer should find a crashing input quickly for an integer.
+! go test -run=FuzzInt -fuzz=FuzzInt -fuzztime=100x -fuzzminimizetime=1000x
+stdout 'testdata[/\\]fuzz[/\\]FuzzInt[/\\]'
+stdout 'this input caused a crash!'
+go run check_testdata.go FuzzInt
+
+! go test -run=FuzzUint -fuzz=FuzzUint -fuzztime=100x -fuzzminimizetime=1000x
+stdout 'testdata[/\\]fuzz[/\\]FuzzUint[/\\]'
+stdout 'this input caused a crash!'
+go run check_testdata.go FuzzUint
+
+# Running the fuzzer should find a crashing input quickly for a bool.
+! go test -run=FuzzBool -fuzz=FuzzBool -fuzztime=100x -fuzzminimizetime=1000x
+stdout 'testdata[/\\]fuzz[/\\]FuzzBool[/\\]'
+stdout 'this input caused a crash!'
+go run check_testdata.go FuzzBool
+
+# Running the fuzzer should find a crashing input quickly for a float.
+! go test -run=FuzzFloat -fuzz=FuzzFloat -fuzztime=100x -fuzzminimizetime=1000x
+stdout 'testdata[/\\]fuzz[/\\]FuzzFloat[/\\]'
+stdout 'this input caused a crash!'
+go run check_testdata.go FuzzFloat
+
+# Running the fuzzer should find a crashing input quickly for a byte.
+! go test -run=FuzzByte -fuzz=FuzzByte -fuzztime=100x -fuzzminimizetime=1000x
+stdout 'testdata[/\\]fuzz[/\\]FuzzByte[/\\]'
+stdout 'this input caused a crash!'
+go run check_testdata.go FuzzByte
+
+# Running the fuzzer should find a crashing input quickly for a rune.
+! go test -run=FuzzRune -fuzz=FuzzRune -fuzztime=100x -fuzzminimizetime=1000x
+stdout 'testdata[/\\]fuzz[/\\]FuzzRune[/\\]'
+stdout 'this input caused a crash!'
+go run check_testdata.go FuzzRune
+
+# Running the fuzzer should find a crashing input quickly for a string.
+! go test -run=FuzzString -fuzz=FuzzString -fuzztime=100x -fuzzminimizetime=1000x
+stdout 'testdata[/\\]fuzz[/\\]FuzzString[/\\]'
+stdout 'this input caused a crash!'
+go run check_testdata.go FuzzString
+
+-- go.mod --
+module m
+
+go 1.16
+-- fuzz_crash_test.go --
+package fuzz_crash
+
+import (
+	"os"
+	"runtime"
+	"testing"
+)
+
+func FuzzWithBug(f *testing.F) {
+	f.Add([]byte("aa"))
+	f.Fuzz(func(t *testing.T, b []byte) {
+		if string(b) != "aa" {
+			panic("this input caused a crash!")
+		}
+	})
+}
+
+func FuzzWithNilPanic(f *testing.F) {
+	f.Add([]byte("aa"))
+	f.Fuzz(func(t *testing.T, b []byte) {
+		if string(b) != "aa" {
+			panic(nil)
+		}
+	})
+}
+
+func FuzzWithGoexit(f *testing.F) {
+	f.Add([]byte("aa"))
+	f.Fuzz(func(t *testing.T, b []byte) {
+		if string(b) != "aa" {
+			runtime.Goexit()
+		}
+	})
+}
+
+func FuzzWithFail(f *testing.F) {
+	f.Add([]byte("aa"))
+	f.Fuzz(func(t *testing.T, b []byte) {
+		if string(b) != "aa" {
+			t.Fail()
+		}
+	})
+}
+
+func FuzzWithLogFail(f *testing.F) {
+	f.Add([]byte("aa"))
+	f.Fuzz(func(t *testing.T, b []byte) {
+		if string(b) != "aa" {
+			t.Log("logged something")
+			t.Fail()
+		}
+	})
+}
+
+func FuzzWithErrorf(f *testing.F) {
+	f.Add([]byte("aa"))
+	f.Fuzz(func(t *testing.T, b []byte) {
+		if string(b) != "aa" {
+			t.Errorf("errorf was called here")
+		}
+	})
+}
+
+func FuzzWithFatalf(f *testing.F) {
+	f.Add([]byte("aa"))
+	f.Fuzz(func(t *testing.T, b []byte) {
+		if string(b) != "aa" {
+			t.Fatalf("fatalf was called here")
+		}
+	})
+}
+
+func FuzzWithBadExit(f *testing.F) {
+	f.Add([]byte("aa"))
+	f.Fuzz(func(t *testing.T, b []byte) {
+		if string(b) != "aa" {
+			os.Exit(1)
+		}
+	})
+}
+
+func FuzzDeadlock(f *testing.F) {
+	f.Add(int(0))
+	f.Fuzz(func(t *testing.T, n int) {
+		if n != 0 {
+			select {}
+		}
+	})
+}
+
+func FuzzWithTwoTypes(f *testing.F) {
+	f.Fuzz(func(t *testing.T, a, b []byte) {
+		if len(a) > 0 && len(b) > 0 {
+			panic("these inputs caused a crash!")
+		}
+	})
+}
+
+func FuzzInt(f *testing.F) {
+	f.Add(0)
+	f.Fuzz(func(t *testing.T, a int) {
+		if a != 0 {
+			panic("this input caused a crash!")
+		}
+	})
+}
+
+func FuzzUint(f *testing.F) {
+	f.Add(uint(0))
+	f.Fuzz(func(t *testing.T, a uint) {
+		if a != 0 {
+			panic("this input caused a crash!")
+		}
+	})
+}
+
+func FuzzBool(f *testing.F) {
+	f.Add(false)
+	f.Fuzz(func(t *testing.T, a bool) {
+		if a {
+			panic("this input caused a crash!")
+		}
+	})
+}
+
+func FuzzFloat(f *testing.F) {
+	f.Fuzz(func(t *testing.T, a float64) {
+		if a != 0 {
+			panic("this input caused a crash!")
+		}
+	})
+}
+
+func FuzzByte(f *testing.F) {
+	f.Add(byte(0))
+	f.Fuzz(func(t *testing.T, a byte) {
+		if a != 0 {
+			panic("this input caused a crash!")
+		}
+	})
+}
+
+func FuzzRune(f *testing.F) {
+	f.Add(rune(0))
+	f.Fuzz(func(t *testing.T, a rune) {
+		if a != 0 {
+			panic("this input caused a crash!")
+		}
+	})
+}
+
+func FuzzString(f *testing.F) {
+	f.Add("")
+	f.Fuzz(func(t *testing.T, a string) {
+		if a != "" {
+			panic("this input caused a crash!")
+		}
+	})
+}
+
+-- check_testdata.go --
+// +build ignore
+
+package main
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+)
+
+func main() {
+	target := os.Args[1]
+	dir := filepath.Join("testdata/fuzz", target)
+
+	files, err := ioutil.ReadDir(dir)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+
+	if len(files) == 0 {
+		fmt.Fprintf(os.Stderr, "expect at least one new mutation to be written to testdata\n")
+		os.Exit(1)
+	}
+
+	fname := files[0].Name()
+	contents, err := ioutil.ReadFile(filepath.Join(dir, fname))
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	if bytes.Equal(contents, []byte("aa")) {
+		fmt.Fprintf(os.Stderr, "newly written testdata entry was not mutated\n")
+		os.Exit(1)
+	}
+	// The hash of the bytes in the file should match the filename.
+	h := []byte(fmt.Sprintf("%x", sha256.Sum256(contents)))
+	if !bytes.HasPrefix(h, []byte(fname)) {
+		fmt.Fprintf(os.Stderr, "hash of bytes %q does not match filename %q\n", h, fname)
+		os.Exit(1)
+	}
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_mutate_fail.txt b/src/cmd/go/testdata/script/test_fuzz_mutate_fail.txt
new file mode 100644
index 0000000..213b73a
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_mutate_fail.txt
@@ -0,0 +1,103 @@
+[!fuzz] skip
+
+# Check that if a worker does not call F.Fuzz or calls F.Fail first,
+# 'go test' exits non-zero and no crasher is recorded.
+
+[short] skip
+env GOCACHE=$WORK/cache
+
+! go test -fuzz=FuzzReturn
+! exists testdata
+
+! go test -fuzz=FuzzSkip
+! exists testdata
+
+! go test -fuzz=FuzzFail
+! exists testdata
+
+! go test -fuzz=FuzzPanic
+! exists testdata
+
+! go test -fuzz=FuzzNilPanic
+! exists testdata
+
+! go test -fuzz=FuzzGoexit
+! exists testdata
+
+! go test -fuzz=FuzzExit
+! exists testdata
+
+-- go.mod --
+module m
+
+go 1.17
+-- fuzz_fail_test.go --
+package fuzz_fail
+
+import (
+	"flag"
+	"os"
+	"runtime"
+	"testing"
+)
+
+func isWorker() bool {
+	f := flag.Lookup("test.fuzzworker")
+	if f == nil {
+		return false
+	}
+	get, ok := f.Value.(flag.Getter)
+	if !ok {
+		return false
+	}
+	return get.Get() == interface{}(true)
+}
+
+func FuzzReturn(f *testing.F) {
+	if isWorker() {
+		return
+	}
+	f.Fuzz(func(*testing.T, []byte) {})
+}
+
+func FuzzSkip(f *testing.F) {
+	if isWorker() {
+		f.Skip()
+	}
+	f.Fuzz(func(*testing.T, []byte) {})
+}
+
+func FuzzFail(f *testing.F) {
+	if isWorker() {
+		f.Fail()
+	}
+	f.Fuzz(func(*testing.T, []byte) {})
+}
+
+func FuzzPanic(f *testing.F) {
+	if isWorker() {
+		panic("nope")
+	}
+	f.Fuzz(func(*testing.T, []byte) {})
+}
+
+func FuzzNilPanic(f *testing.F) {
+	if isWorker() {
+		panic(nil)
+	}
+	f.Fuzz(func(*testing.T, []byte) {})
+}
+
+func FuzzGoexit(f *testing.F) {
+	if isWorker() {
+		runtime.Goexit()
+	}
+	f.Fuzz(func(*testing.T, []byte) {})
+}
+
+func FuzzExit(f *testing.F) {
+	if isWorker() {
+		os.Exit(99)
+	}
+	f.Fuzz(func(*testing.T, []byte) {})
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_mutator.txt b/src/cmd/go/testdata/script/test_fuzz_mutator.txt
new file mode 100644
index 0000000..cc1f989
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_mutator.txt
@@ -0,0 +1,166 @@
+[!fuzz] skip
+
+# Test basic fuzzing mutator behavior.
+#
+# fuzz_test.go has two fuzz targets (FuzzA, FuzzB) which both add a seed value.
+# Each fuzz function writes the input to a log file. The coordinator and worker
+# use separate log files. check_logs.go verifies that the coordinator only
+# tests seed values and the worker tests mutated values on the fuzz target.
+
+[short] skip
+env GOCACHE=$WORK/cache
+
+go test -fuzz=FuzzA -fuzztime=100x -parallel=1 -log=fuzz
+go run check_logs.go fuzz fuzz.worker
+
+# TODO(b/181800488): remove -parallel=1, here and below. For now, when a
+# crash is found, all workers keep running, wasting resources and reducing
+# the number of executions available to the minimizer, increasing flakiness.
+
+# Test that the mutator is good enough to find several unique mutations.
+! go test -fuzz=FuzzMutator -parallel=1 -fuzztime=100x mutator_test.go
+! stdout '^ok'
+stdout FAIL
+stdout 'mutator found enough unique mutations'
+
+-- go.mod --
+module m
+
+go 1.16
+-- fuzz_test.go --
+package fuzz_test
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"testing"
+)
+
+var (
+	logPath = flag.String("log", "", "path to log file")
+	logFile *os.File
+)
+
+func TestMain(m *testing.M) {
+	flag.Parse()
+	var err error
+	logFile, err = os.OpenFile(*logPath, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0666)
+	if os.IsExist(err) {
+		*logPath += ".worker"
+		logFile, err = os.OpenFile(*logPath, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0666)
+	}
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	os.Exit(m.Run())
+}
+
+func FuzzA(f *testing.F) {
+	f.Add([]byte("seed"))
+	f.Fuzz(func(t *testing.T, b []byte) {
+		fmt.Fprintf(logFile, "FuzzA %q\n", b)
+	})
+}
+
+func FuzzB(f *testing.F) {
+	f.Add([]byte("seed"))
+	f.Fuzz(func(t *testing.T, b []byte) {
+		fmt.Fprintf(logFile, "FuzzB %q\n", b)
+	})
+}
+
+-- check_logs.go --
+// +build ignore
+
+package main
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+)
+
+func main() {
+	coordPath, workerPath := os.Args[1], os.Args[2]
+
+	coordLog, err := os.Open(coordPath)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	defer coordLog.Close()
+	if err := checkCoordLog(coordLog); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+
+	workerLog, err := os.Open(workerPath)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	defer workerLog.Close()
+	if err := checkWorkerLog(workerLog); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}
+
+func checkCoordLog(r io.Reader) error {
+	b, err := io.ReadAll(r)
+	if err != nil {
+		return err
+	}
+	if string(bytes.TrimSpace(b)) != `FuzzB "seed"` {
+		return fmt.Errorf("coordinator: did not test FuzzB seed")
+	}
+	return nil
+}
+
+func checkWorkerLog(r io.Reader) error {
+	scan := bufio.NewScanner(r)
+	var sawAMutant bool
+	for scan.Scan() {
+		line := scan.Text()
+		if !strings.HasPrefix(line, "FuzzA ") {
+			return fmt.Errorf("worker: tested something other than target: %s", line)
+		}
+		if strings.TrimPrefix(line, "FuzzA ") != `"seed"` {
+			sawAMutant = true
+		}
+	}
+	if err := scan.Err(); err != nil && err != bufio.ErrTooLong {
+		return err
+	}
+	if !sawAMutant {
+		return fmt.Errorf("worker: did not test any mutants")
+	}
+	return nil
+}
+-- mutator_test.go --
+package fuzz_test
+
+import (
+	"testing"
+)
+
+// TODO(katiehockman): re-work this test once we have a better fuzzing engine
+// (ie. more mutations, and compiler instrumentation)
+func FuzzMutator(f *testing.F) {
+	// TODO(katiehockman): simplify this once we can dedupe crashes (e.g.
+	// replace map with calls to panic, and simply count the number of crashes
+	// that were added to testdata)
+	crashes := make(map[string]bool)
+	// No seed corpus initiated
+	f.Fuzz(func(t *testing.T, b []byte) {
+		crashes[string(b)] = true
+		if len(crashes) >= 10 {
+			panic("mutator found enough unique mutations")
+		}
+	})
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_mutator_repeat.txt b/src/cmd/go/testdata/script/test_fuzz_mutator_repeat.txt
new file mode 100644
index 0000000..3b005c9
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_mutator_repeat.txt
@@ -0,0 +1,75 @@
+# TODO(jayconrod): support shared memory on more platforms.
+[!GOOS:darwin] [!GOOS:linux] [!GOOS:windows] skip
+
+# Verify that the fuzzing engine records the actual crashing input, even when
+# a worker process terminates without communicating the crashing input back
+# to the coordinator.
+
+[short] skip
+env GOCACHE=$WORK/cache
+
+# Start fuzzing. The worker crashes after 100 iterations.
+# The fuzz function writes the crashing input to "want" before exiting.
+# The fuzzing engine reconstructs the crashing input and saves it to testdata.
+! exists want
+! go test -fuzz=. -parallel=1 -fuzztime=110x -fuzzminimizetime=10x -v
+stdout '^\s+fuzzing process hung or terminated unexpectedly: exit status'
+stdout 'Failing input written to testdata'
+
+# Run the fuzz target without fuzzing. The fuzz function is called with the
+# crashing input in testdata. The test passes if that input is identical to
+# the one saved in "want".
+exists want
+go test -want=want
+
+-- go.mod --
+module fuzz
+
+go 1.17
+-- fuzz_test.go --
+package fuzz
+
+import (
+	"bytes"
+	"flag"
+	"os"
+	"testing"
+)
+
+var wantFlag = flag.String("want", "", "file containing previous crashing input")
+
+func FuzzRepeat(f *testing.F) {
+	i := 0
+	f.Fuzz(func(t *testing.T, b []byte) {
+		i++
+		if i == 100 {
+			f, err := os.OpenFile("want", os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0666)
+			if err != nil {
+				// Couldn't create the file. Return without crashing, and try
+				// again.
+				i--
+				t.Skip(err)
+			}
+			if _, err := f.Write(b); err != nil {
+				// We already created the file, so if we failed to write it
+				// there's not much we can do. The test will fail anyway, but
+				// at least make sure the error is logged to stdout.
+				t.Fatal(err)
+			}
+			if err := f.Close(); err != nil {
+				t.Fatal(err)
+			}
+			os.Exit(1) // crash without communicating
+		}
+
+		if *wantFlag != "" {
+			want, err := os.ReadFile(*wantFlag)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if !bytes.Equal(want, b) {
+				t.Fatalf("inputs are not equal!\n got: %q\nwant:%q", b, want)
+			}
+		}
+	})
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_non_crash_signal.txt b/src/cmd/go/testdata/script/test_fuzz_non_crash_signal.txt
new file mode 100644
index 0000000..94a0421
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_non_crash_signal.txt
@@ -0,0 +1,76 @@
+# NOTE: this test is skipped on Windows, since there's no concept of signals.
+# When a process terminates another process, it provides an exit code.
+[GOOS:windows] skip
+[!fuzz] skip
+[short] skip
+env GOCACHE=$WORK/cache
+
+# FuzzNonCrash sends itself a signal that does not appear to be a crash.
+# We should not save a crasher.
+! go test -fuzz=FuzzNonCrash
+! exists testdata
+! stdout unreachable
+! stderr unreachable
+stdout 'fuzzing process terminated by unexpected signal; no crash will be recorded: signal: terminated'
+
+# FuzzKill sends itself a signal that cannot be caught by the worker process
+# and does not appear to be a crash.
+# We should not save a crasher.
+! go test -fuzz=FuzzKill
+! exists testdata
+! stdout unreachable
+! stderr unreachable
+stdout 'fuzzing process terminated by unexpected signal; no crash will be recorded: signal: killed'
+
+# FuzzCrash sends itself a signal that looks like a crash.
+# We should save a crasher.
+! go test -fuzz=FuzzCrash
+exists testdata/fuzz/FuzzCrash
+stdout '^\s+fuzzing process hung or terminated unexpectedly: exit status'
+
+-- go.mod --
+module test
+
+go 1.17
+-- fuzz_posix_test.go --
+// +build darwin freebsd linux
+
+package fuzz
+
+import (
+	"syscall"
+	"testing"
+)
+
+func FuzzNonCrash(f *testing.F) {
+	f.Fuzz(func(*testing.T, bool) {
+		pid := syscall.Getpid()
+		if err := syscall.Kill(pid, syscall.SIGTERM); err != nil {
+			panic(err)
+		}
+		// signal may not be received immediately. Wait for it.
+		select{}
+	})
+}
+
+func FuzzKill(f *testing.F) {
+	f.Fuzz(func(*testing.T, bool) {
+		pid := syscall.Getpid()
+		if err := syscall.Kill(pid, syscall.SIGKILL); err != nil {
+			panic(err)
+		}
+		// signal may not be received immediately. Wait for it.
+		select{}
+	})
+}
+
+func FuzzCrash(f *testing.F) {
+	f.Fuzz(func(*testing.T, bool) {
+		pid := syscall.Getpid()
+		if err := syscall.Kill(pid, syscall.SIGILL); err != nil {
+			panic(err)
+		}
+		// signal may not be received immediately. Wait for it.
+		select{}
+	})
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_parallel.txt b/src/cmd/go/testdata/script/test_fuzz_parallel.txt
new file mode 100644
index 0000000..8ff965a
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_parallel.txt
@@ -0,0 +1,67 @@
+[!fuzz] skip
+[short] skip
+env GOCACHE=$WORK/cache
+
+# When running seed inputs, T.Parallel should let multiple inputs run in
+# parallel.
+go test -run=FuzzSeed
+
+# When fuzzing, T.Parallel should be safe to call, but it should have no effect.
+# We just check that it doesn't hang, which would be the most obvious
+# failure mode.
+# TODO(jayconrod): check for the string "after T.Parallel". It's not printed
+# by 'go test', so we can't distinguish that crasher from some other panic.
+! go test -run=FuzzMutate -fuzz=FuzzMutate
+exists testdata/fuzz/FuzzMutate
+
+# Testdata should now contain a corpus entry which will fail FuzzMutate.
+# Run the test without fuzzing, setting -parallel to different values to make
+# sure it fails, and doesn't hang.
+! go test -run=FuzzMutate -parallel=1
+! go test -run=FuzzMutate -parallel=2
+! go test -run=FuzzMutate -parallel=4
+
+-- go.mod --
+module fuzz_parallel
+
+go 1.17
+-- fuzz_parallel_test.go --
+package fuzz_parallel
+
+import (
+	"sort"
+	"sync"
+	"testing"
+)
+
+func FuzzSeed(f *testing.F) {
+	for _, v := range [][]byte{{'a'}, {'b'}, {'c'}} {
+		f.Add(v)
+	}
+
+	var mu sync.Mutex
+	var before, after []byte
+	f.Cleanup(func() {
+		sort.Slice(after, func(i, j int) bool { return after[i] < after[j] })
+		got := string(before) + string(after)
+		want := "abcabc"
+		if got != want {
+			f.Fatalf("got %q; want %q", got, want)
+		}
+	})
+
+	f.Fuzz(func(t *testing.T, b []byte) {
+		before = append(before, b...)
+		t.Parallel()
+		mu.Lock()
+		after = append(after, b...)
+		mu.Unlock()
+	})
+}
+
+func FuzzMutate(f *testing.F) {
+	f.Fuzz(func(t *testing.T, _ []byte) {
+		t.Parallel()
+		t.Error("after T.Parallel")
+	})
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_profile_flags.txt b/src/cmd/go/testdata/script/test_fuzz_profile_flags.txt
new file mode 100644
index 0000000..5434c72
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_profile_flags.txt
@@ -0,0 +1,38 @@
+[!fuzz] skip
+
+! go test -fuzz=FuzzTrivial -coverprofile=prof
+! stdout .
+stderr '^cannot use -coverprofile flag with -fuzz flag$'
+
+! go test -fuzz=FuzzTrivial -blockprofile=prof
+! stdout .
+stderr '^cannot use -blockprofile flag with -fuzz flag$'
+
+! go test -fuzz=FuzzTrivial -cpuprofile=prof
+! stdout .
+stderr '^cannot use -cpuprofile flag with -fuzz flag$'
+
+! go test -fuzz=FuzzTrivial -memprofile=prof
+! stdout .
+stderr '^cannot use -memprofile flag with -fuzz flag$'
+
+! go test -fuzz=FuzzTrivial -mutexprofile=prof
+! stdout .
+stderr '^cannot use -mutexprofile flag with -fuzz flag$'
+
+! go test -fuzz=FuzzTrivial -trace=prof
+! stdout .
+stderr '^cannot use -trace flag with -fuzz flag$'
+
+-- go.mod --
+module example
+
+go 1.18
+-- fuzz_test.go --
+package example
+
+import "testing"
+
+func FuzzTrivial(f *testing.F) {
+	f.Fuzz(func(t *testing.T, _ []byte) {})
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_return.txt b/src/cmd/go/testdata/script/test_fuzz_return.txt
new file mode 100644
index 0000000..63275aa
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_return.txt
@@ -0,0 +1,19 @@
+[short] skip
+
+! go test .
+stdout '^panic: testing: fuzz target must not return a value \[recovered\]$'
+
+-- go.mod --
+module test
+go 1.18
+-- x_test.go --
+package test
+
+import "testing"
+
+func FuzzReturnErr(f *testing.F) {
+	f.Add("hello, validation!")
+	f.Fuzz(func(t *testing.T, in string) string {
+		return in
+	})
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_run.txt b/src/cmd/go/testdata/script/test_fuzz_run.txt
new file mode 100644
index 0000000..99a4413
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_run.txt
@@ -0,0 +1,143 @@
+[!fuzz] skip
+[short] skip
+env GOCACHE=$WORK/cache
+
+# Tests which verify the behavior and command line output when
+# running a fuzz target as a unit test.
+
+# Tests without -run.
+
+! go test
+stdout FAIL
+stdout 'error here'
+
+! go test -v
+stdout FAIL
+stdout 'error here'
+stdout '=== RUN   FuzzFoo/thisfails'
+stdout '--- FAIL: FuzzFoo/thisfails'
+stdout '=== RUN   FuzzFoo/thispasses'
+stdout '--- PASS: FuzzFoo/thispasses'
+
+# Tests where -run matches all seed corpora.
+
+! go test -run FuzzFoo/this
+stdout FAIL
+stdout 'error here'
+! stdout 'no tests to run'
+
+! go test -run /this
+stdout FAIL
+stdout 'error here'
+! stdout 'no tests to run'
+
+! go test -v -run FuzzFoo/this
+stdout FAIL
+stdout 'error here'
+stdout '=== RUN   FuzzFoo/thisfails'
+stdout '--- FAIL: FuzzFoo/thisfails'
+stdout '=== RUN   FuzzFoo/thispasses'
+stdout '--- PASS: FuzzFoo/thispasses'
+! stdout 'no tests to run'
+
+! go test -v -run /this
+stdout FAIL
+stdout 'error here'
+stdout '=== RUN   FuzzFoo/thisfails'
+stdout '--- FAIL: FuzzFoo/thisfails'
+stdout '=== RUN   FuzzFoo/thispasses'
+stdout '--- PASS: FuzzFoo/thispasses'
+! stdout 'no tests to run'
+
+# Tests where -run only matches one seed corpus which passes.
+
+go test -run FuzzFoo/thispasses
+stdout ok
+! stdout 'no tests to run'
+
+go test -run /thispasses
+stdout ok
+! stdout 'no tests to run'
+
+# Same tests in verbose mode
+go test -v -run FuzzFoo/thispasses
+stdout '=== RUN   FuzzFoo/thispasses'
+stdout '--- PASS: FuzzFoo/thispasses'
+! stdout '=== RUN   FuzzFoo/thisfails'
+! stdout 'no tests to run'
+
+go test -v -run /thispasses
+stdout '=== RUN   FuzzFoo/thispasses'
+stdout '--- PASS: FuzzFoo/thispasses'
+! stdout '=== RUN   FuzzFoo/thisfails'
+! stdout 'no tests to run'
+
+# Tests where -run only matches one seed corpus which fails.
+
+! go test -run FuzzFoo/thisfails
+stdout FAIL
+stdout 'error here'
+! stdout 'no tests to run'
+
+! go test -run /thisfails
+stdout FAIL
+stdout 'error here'
+! stdout 'no tests to run'
+
+! go test -v -run FuzzFoo/thisfails
+stdout 'error here'
+stdout '=== RUN   FuzzFoo/thisfails'
+stdout '--- FAIL: FuzzFoo/thisfails'
+! stdout '=== RUN   FuzzFoo/thispasses'
+! stdout 'no tests to run'
+
+! go test -v -run /thisfails
+stdout 'error here'
+stdout '=== RUN   FuzzFoo/thisfails'
+stdout '--- FAIL: FuzzFoo/thisfails'
+! stdout '=== RUN   FuzzFoo/thispasses'
+! stdout 'no tests to run'
+
+# Tests where -run doesn't match any seed corpora.
+
+go test -run FuzzFoo/nomatch
+stdout ok
+
+go test -run /nomatch
+stdout ok
+
+go test -v -run FuzzFoo/nomatch
+stdout '=== RUN   FuzzFoo'
+stdout '--- PASS: FuzzFoo'
+stdout ok
+! stdout 'no tests to run'
+
+go test -v -run /nomatch
+stdout '=== RUN   FuzzFoo'
+stdout '--- PASS: FuzzFoo'
+stdout ok
+! stdout 'no tests to run'
+
+-- go.mod --
+module example.com/x
+
+go 1.16
+-- x_test.go --
+package x
+
+import "testing"
+
+func FuzzFoo(f *testing.F) {
+    f.Add("this is fine")
+    f.Fuzz(func(t *testing.T, s string) {
+        if s == "fails" {
+            t.Error("error here")
+        }
+    })
+}
+-- testdata/fuzz/FuzzFoo/thisfails --
+go test fuzz v1
+string("fails")
+-- testdata/fuzz/FuzzFoo/thispasses --
+go test fuzz v1
+string("passes")
diff --git a/src/cmd/go/testdata/script/test_fuzz_seed_corpus.txt b/src/cmd/go/testdata/script/test_fuzz_seed_corpus.txt
new file mode 100644
index 0000000..3e3fbad
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_seed_corpus.txt
@@ -0,0 +1,203 @@
+[!fuzz-instrumented] skip
+[short] skip
+env GOCACHE=$WORK/cache
+
+# Test that fuzzing a target with a failure in f.Add prints the crash
+# and doesn't write anything to testdata/fuzz
+! go test -fuzz=FuzzWithAdd -run=FuzzWithAdd -fuzztime=1x
+! stdout ^ok
+! stdout 'Failing input written to testdata[/\\]fuzz[/\\]FuzzWithAdd[/\\]'
+stdout FAIL
+
+# Test that fuzzing a target with a success in f.Add and a fuzztime of only
+# 1 does not produce a crash.
+go test -fuzz=FuzzWithGoodAdd -run=FuzzWithGoodAdd -fuzztime=1x
+stdout ok
+! stdout FAIL
+
+# Test that fuzzing a target with a failure in testdata/fuzz prints the crash
+# and doesn't write anything to testdata/fuzz
+! go test -fuzz=FuzzWithTestdata -run=FuzzWithTestdata -fuzztime=1x
+! stdout ^ok
+! stdout 'Failing input written to testdata[/\\]fuzz[/\\]FuzzWithTestdata[/\\]'
+stdout 'failure while testing seed corpus entry: FuzzWithTestdata/1'
+stdout FAIL
+
+# Test that fuzzing a target with no seed corpus or cache finds a crash, prints
+# it, and write it to testdata
+! go test -fuzz=FuzzWithNoCache -run=FuzzWithNoCache -fuzztime=1x
+! stdout ^ok
+stdout 'Failing input written to testdata[/\\]fuzz[/\\]FuzzWithNoCache[/\\]'
+stdout FAIL
+
+# Write a crashing input to the cache
+mkdir $GOCACHE/fuzz/example.com/x/FuzzWithCache
+cp cache-file $GOCACHE/fuzz/example.com/x/FuzzWithCache/1
+
+# Test that fuzzing a target with a failure in the cache prints the crash
+# and writes this as a "new" crash to testdata/fuzz
+! go test -fuzz=FuzzWithCache -run=FuzzWithCache -fuzztime=1x
+! stdout ^ok
+stdout 'Failing input written to testdata[/\\]fuzz[/\\]FuzzWithCache[/\\]'
+stdout FAIL
+
+# Write a crashing input to the cache
+mkdir $GOCACHE/fuzz/example.com/x/FuzzWithMinimizableCache
+cp cache-file-bytes $GOCACHE/fuzz/example.com/x/FuzzWithMinimizableCache/1
+
+# Test that fuzzing a target with a failure in the cache minimizes it and writes
+# the new crash to testdata/fuzz
+! go test -fuzz=FuzzWithMinimizableCache -run=FuzzWithMinimizableCache -fuzztime=10000x
+! stdout ^ok
+stdout 'gathering baseline coverage'
+stdout 'got the minimum size!'
+stdout 'contains a non-zero byte of length 10'
+stdout 'Failing input written to testdata[/\\]fuzz[/\\]FuzzWithMinimizableCache[/\\]'
+stdout FAIL
+# Make sure this crash didn't come from fuzzing
+# (the log line that states fuzzing began shouldn't have printed)
+! stdout 'execs'
+
+# Clear the fuzz cache and make sure it's gone
+go clean -fuzzcache
+! exists $GOCACHE/fuzz
+
+# The tests below should operate the exact same as the previous tests. If -fuzz
+# is enabled, then whatever target is going to be fuzzed shouldn't be run by
+# anything other than the workers.
+
+# Test that fuzzing a target (with -run=None set) with a failure in f.Add prints
+# the crash and doesn't write anything to testdata/fuzz -fuzztime=1x
+! go test -fuzz=FuzzWithAdd -run=None
+! stdout ^ok
+! stdout 'Failing input written to testdata[/\\]fuzz[/\\]FuzzWithAdd[/\\]'
+stdout FAIL
+
+# Test that fuzzing a target (with -run=None set) with a success in f.Add and a
+# fuzztime of only 1 does not produce a crash.
+go test -fuzz=FuzzWithGoodAdd -run=None -fuzztime=1x
+stdout ok
+! stdout FAIL
+
+# Test that fuzzing a target (with -run=None set) with a failure in
+# testdata/fuzz prints the crash and doesn't write anything to testdata/fuzz
+! go test -fuzz=FuzzWithTestdata -run=None -fuzztime=1x
+! stdout ^ok
+! stdout 'Failing input written to testdata[/\\]fuzz[/\\]FuzzWithTestdata[/\\]'
+stdout FAIL
+
+# Write a crashing input to the cache
+mkdir $GOCACHE/fuzz/example.com/x/FuzzRunNoneWithCache
+cp cache-file $GOCACHE/fuzz/example.com/x/FuzzRunNoneWithCache/1
+
+# Test that fuzzing a target (with -run=None set) with a failure in the cache
+# prints the crash and writes this as a "new" crash to testdata/fuzz
+! go test -fuzz=FuzzRunNoneWithCache -run=None -fuzztime=1x
+! stdout ^ok
+stdout 'Failing input written to testdata[/\\]fuzz[/\\]FuzzRunNoneWithCache[/\\]'
+stdout FAIL
+
+# Clear the fuzz cache and make sure it's gone
+go clean -fuzzcache
+! exists $GOCACHE/fuzz
+
+# The tests below should operate the exact same way for the previous tests with
+# a seed corpus (namely, they should still fail). However, the binary is built
+# without instrumentation, so this should be a "testing only" run which executes
+# the seed corpus before attempting to fuzz.
+
+go test -c
+! exec ./x.test$GOEXE -test.fuzz=FuzzWithAdd -test.run=FuzzWithAdd -test.fuzztime=1x -test.fuzzcachedir=$WORK/cache
+! stdout ^ok
+! stdout 'Failing input written to testdata[/\\]fuzz[/\\]FuzzWithAdd[/\\]'
+stdout FAIL
+stderr warning
+
+go test -c
+! exec ./x.test$GOEXE -test.fuzz=FuzzWithTestdata -test.run=FuzzWithTestdata -test.fuzztime=1x -test.fuzzcachedir=$WORK/cache
+! stdout ^ok
+! stdout 'Failing input written to testdata[/\\]fuzz[/\\]FuzzWithTestdata[/\\]'
+stdout FAIL
+stderr warning
+
+-- go.mod --
+module example.com/x
+
+go 1.16
+-- x_test.go --
+package x
+
+import "testing"
+
+func FuzzWithAdd(f *testing.F) {
+    f.Add(10)
+    f.Fuzz(func(t *testing.T, i int) {
+        if i == 10 {
+            t.Error("bad thing here")
+        }
+    })
+}
+
+func FuzzWithGoodAdd(f *testing.F) {
+    f.Add(10)
+    f.Fuzz(func(t *testing.T, i int) {
+        if i != 10 {
+            t.Error("bad thing here")
+        }
+    })
+}
+
+func FuzzWithTestdata(f *testing.F) {
+    f.Fuzz(func(t *testing.T, i int) {
+        if i == 10 {
+            t.Error("bad thing here")
+        }
+    })
+}
+
+func FuzzWithNoCache(f *testing.F) {
+    f.Fuzz(func(t *testing.T, i int) {
+        t.Error("bad thing here")
+    })
+}
+
+func FuzzWithCache(f *testing.F) {
+    f.Fuzz(func(t *testing.T, i int) {
+        if i == 10 {
+            t.Error("bad thing here")
+        }
+    })
+}
+
+func FuzzWithMinimizableCache(f *testing.F) {
+    f.Fuzz(func(t *testing.T, b []byte) {
+		if len(b) < 10 {
+			return
+		}
+		for _, n := range b {
+			if n != 0 {
+				if len(b) == 10 {
+					t.Log("got the minimum size!")
+				}
+				t.Fatalf("contains a non-zero byte of length %d", len(b))
+			}
+		}
+    })
+}
+
+func FuzzRunNoneWithCache(f *testing.F) {
+    f.Fuzz(func(t *testing.T, i int) {
+        if i == 10 {
+            t.Error("bad thing here")
+        }
+    })
+}
+-- testdata/fuzz/FuzzWithTestdata/1 --
+go test fuzz v1
+int(10)
+-- cache-file --
+go test fuzz v1
+int(10)
+-- cache-file-bytes --
+go test fuzz v1
+[]byte("11111111111111111111")
diff --git a/src/cmd/go/testdata/script/test_fuzz_setenv.txt b/src/cmd/go/testdata/script/test_fuzz_setenv.txt
new file mode 100644
index 0000000..1370cd8
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_setenv.txt
@@ -0,0 +1,46 @@
+[!fuzz] skip
+[short] skip
+env GOCACHE=$WORK/cache
+
+go test -fuzz=FuzzA -fuzztime=100x fuzz_setenv_test.go
+
+-- fuzz_setenv_test.go --
+package fuzz
+
+import (
+  "flag"
+  "os"
+  "testing"
+)
+
+func FuzzA(f *testing.F) {
+  if s := os.Getenv("TEST_FUZZ_SETENV_A"); isWorker() && s == "" {
+    f.Fatal("environment variable not set")
+  } else if !isWorker() && s != "" {
+    f.Fatal("environment variable already set")
+  }
+  f.Setenv("TEST_FUZZ_SETENV_A", "A")
+  if os.Getenv("TEST_FUZZ_SETENV_A") == "" {
+    f.Fatal("Setenv did not set environment variable")
+  }
+  f.Fuzz(func(*testing.T, []byte) {})
+}
+
+func FuzzB(f *testing.F) {
+  if os.Getenv("TEST_FUZZ_SETENV_A") != "" {
+    f.Fatal("environment variable not cleared after FuzzA")
+  }
+  f.Skip()
+}
+
+func isWorker() bool {
+	f := flag.Lookup("test.fuzzworker")
+	if f == nil {
+		return false
+	}
+	get, ok := f.Value.(flag.Getter)
+	if !ok {
+		return false
+	}
+	return get.Get() == interface{}(true)
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_test_race.txt b/src/cmd/go/testdata/script/test_fuzz_test_race.txt
new file mode 100644
index 0000000..1bed47d
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_test_race.txt
@@ -0,0 +1,40 @@
+# Test that when both race detection and coverage instrumentation are enabled,
+# and seed values are being executed, the race detector isn't mistakenly
+# triggered.
+
+[short] skip
+[!fuzz] skip
+[!race] skip
+env GOCACHE=$WORK/cache
+
+# Test with coverage instrumentation enabled (-fuzz) and race instrumentation
+# but without actually fuzzing the target (by using a non-matching pattern)
+go test -fuzz=xxx -race -v
+! stderr 'race detected during execution of test'
+
+# Test with just race instrumentation enabled
+go test -race -v
+! stderr 'race detected during execution of test'
+
+# Test with coverage and race instrumentation enabled, and a matching fuzz
+# pattern
+go test -fuzz=FuzzRace -race -v -fuzztime=200x
+! stderr 'race detected during execution of test'
+
+-- go.mod --
+module test
+
+-- race_test.go --
+package race
+
+import "testing"
+
+func FuzzRace(f *testing.F) {
+	for i := 0; i < 100; i++ {
+		f.Add(i)
+	}
+
+	f.Fuzz(func(t *testing.T, i int) {
+		t.Parallel()
+	})
+}
diff --git a/src/cmd/go/testdata/script/test_fuzz_unsupported.txt b/src/cmd/go/testdata/script/test_fuzz_unsupported.txt
new file mode 100644
index 0000000..1ed0b8a
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_fuzz_unsupported.txt
@@ -0,0 +1,18 @@
+[fuzz] skip
+
+! go test -fuzz=. -fuzztime=1x
+! stdout .
+stderr '^-fuzz flag is not supported on '$GOOS'/'$GOARCH'$'
+
+-- go.mod --
+module example
+
+go 1.18
+-- fuzz_test.go --
+package example
+
+import "testing"
+
+func FuzzTrivial(f *testing.F) {
+	f.Fuzz(func(t *testing.T, _ []byte) {})
+}
diff --git a/src/cmd/go/testdata/script/test_generated_main.txt b/src/cmd/go/testdata/script/test_generated_main.txt
new file mode 100644
index 0000000..2e991a5
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_generated_main.txt
@@ -0,0 +1,34 @@
+# Tests that the generated test main file has a generated code comment.
+# This is needed by analyzers that access source files through 'go list'.
+# Verifies golang.org/issue/31971.
+# TODO(jayconrod): This test is brittle. We should write _testmain.go as
+# a build action instead of with an ad-hoc WriteFile call
+# in internal/test/test.go. Then we could just grep 'go get -n'.
+go test x_test.go
+
+-- x_test.go --
+package x
+
+import (
+	"os"
+	"path/filepath"
+	"regexp"
+	"testing"
+)
+
+func Test(t *testing.T) {
+	exePath, err := os.Executable()
+	if err != nil {
+		t.Fatal(err)
+	}
+	testmainPath := filepath.Join(filepath.Dir(exePath), "_testmain.go")
+	source, err := os.ReadFile(testmainPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if matched, err := regexp.Match(`(?m)^// Code generated .* DO NOT EDIT\.$`, source); err != nil {
+		t.Fatal(err)
+	} else if !matched {
+		t.Error("_testmain.go does not have generated code comment")
+	}
+}
diff --git a/src/cmd/go/testdata/script/test_go111module_cache.txt b/src/cmd/go/testdata/script/test_go111module_cache.txt
new file mode 100644
index 0000000..ca1de43
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_go111module_cache.txt
@@ -0,0 +1,15 @@
+env GO111MODULE=on
+go mod init foo
+go test
+stdout ^ok\s+foo
+env GO111MODULE=off
+go test
+stdout ^ok\s+
+! stdout ^ok\s+(cache)$
+
+-- main_test.go --
+package main
+
+import "testing"
+
+func TestF(t *testing.T) {}
diff --git a/src/cmd/go/testdata/script/test_goroot_PATH.txt b/src/cmd/go/testdata/script/test_goroot_PATH.txt
new file mode 100644
index 0000000..a6278ca
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_goroot_PATH.txt
@@ -0,0 +1,41 @@
+# https://go.dev/issue/51473: to avoid the need for tests to rely on
+# runtime.GOROOT, 'go test' should run the test with its own GOROOT/bin
+# at the beginning of $PATH.
+
+[short] skip
+
+[!GOOS:plan9] env PATH=
+[GOOS:plan9] env path=
+go test .
+
+[!GOOS:plan9] env PATH=$WORK${/}bin
+[GOOS:plan9] env path=$WORK${/}bin
+go test .
+
+-- go.mod --
+module example
+
+go 1.19
+-- example_test.go --
+package example
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"testing"
+)
+
+func TestGoCommandExists(t *testing.T) {
+	got, err := exec.LookPath("go")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	want := filepath.Join(os.Getenv("GOROOT"), "bin", "go" + os.Getenv("GOEXE"))
+	if got != want {
+		t.Fatalf(`exec.LookPath("go") = %q; want %q`, got, want)
+	}
+}
+-- $WORK/bin/README.txt --
+This directory contains no executables.
diff --git a/src/cmd/go/testdata/script/test_import_error_stack.txt b/src/cmd/go/testdata/script/test_import_error_stack.txt
new file mode 100644
index 0000000..6c60f3d
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_import_error_stack.txt
@@ -0,0 +1,31 @@
+env GO111MODULE=off
+! go test testdep/p1
+stderr 'package testdep/p1 \(test\)\n\timports testdep/p2\n\timports testdep/p3: build constraints exclude all Go files ' # check for full import stack
+! go vet testdep/p1
+stderr 'package testdep/p1 \(test\)\n\timports testdep/p2\n\timports testdep/p3: build constraints exclude all Go files ' # check for full import stack
+
+env GO111MODULE=on
+cd testdep
+! go test testdep/p1
+stderr 'package testdep/p1 \(test\)\n\timports testdep/p2\n\timports testdep/p3: build constraints exclude all Go files ' # check for full import stack
+! go vet testdep/p1
+stderr 'package testdep/p1 \(test\)\n\timports testdep/p2\n\timports testdep/p3: build constraints exclude all Go files ' # check for full import stack
+
+-- testdep/go.mod --
+module testdep
+
+go 1.16
+-- testdep/p1/p1.go --
+package p1
+-- testdep/p1/p1_test.go --
+package p1
+
+import _ "testdep/p2"
+-- testdep/p2/p2.go --
+package p2
+
+import _ "testdep/p3"
+-- testdep/p3/p3.go --
+// +build ignore
+
+package ignored
diff --git a/src/cmd/go/testdata/script/test_issue45477.txt b/src/cmd/go/testdata/script/test_issue45477.txt
new file mode 100644
index 0000000..f435b6a
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_issue45477.txt
@@ -0,0 +1,12 @@
+[short] skip  # links and runs a test binary
+
+go test -v .
+
+-- go.mod --
+module example.com/pkg_test
+
+-- pkg.go --
+package pkg_test
+
+-- pkg_test.go --
+package pkg_test
diff --git a/src/cmd/go/testdata/script/test_json.txt b/src/cmd/go/testdata/script/test_json.txt
new file mode 100644
index 0000000..6207c2e
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_json.txt
@@ -0,0 +1,80 @@
+[compiler:gccgo] skip # gccgo does not have standard packages
+[short] skip
+
+env GOCACHE=$WORK/tmp
+
+# Run go test -json on errors m/empty/pkg and m/skipper
+# It would be nice to test that the output is interlaced
+# but it seems to be impossible to do that in a short test
+# that isn't also flaky. Just check that we get JSON output.
+go test -json -short -v errors m/empty/pkg m/skipper
+
+# Check errors for run action
+stdout '"Package":"errors"'
+stdout '"Action":"start","Package":"errors"'
+stdout '"Action":"run","Package":"errors"'
+
+# Check m/empty/pkg for output and skip actions
+stdout '"Action":"start","Package":"m/empty/pkg"'
+stdout '"Action":"output","Package":"m/empty/pkg","Output":".*no test files'
+stdout '"Action":"skip","Package":"m/empty/pkg"'
+
+# Check skipper for output and skip actions
+stdout '"Action":"start","Package":"m/skipper"'
+stdout '"Action":"output","Package":"m/skipper","Test":"Test","Output":"--- SKIP:'
+stdout '"Action":"skip","Package":"m/skipper","Test":"Test"'
+
+# Check that starts were ordered properly.
+stdout '(?s)"Action":"start","Package":"errors".*"Action":"start","Package":"m/empty/pkg".*"Action":"start","Package":"m/skipper"'
+
+# Run go test -json on errors and check it's cached
+go test -json -short -v errors
+stdout '"Action":"output","Package":"errors","Output":".*\(cached\)'
+
+go test -json -bench=NONE -short -v errors
+stdout '"Package":"errors"'
+stdout '"Action":"run"'
+
+# Test running test2json
+go test -o $WORK/tmp/errors.test$GOEXE -c errors
+go tool test2json -p errors $WORK/tmp/errors.test$GOEXE -test.v -test.short
+stdout '"Package":"errors"'
+stdout '"Action":"run"'
+stdout '\{"Action":"pass","Package":"errors"\}'
+
+-- go.mod --
+module m
+
+go 1.16
+-- skipper/skip_test.go --
+package skipper
+
+import "testing"
+
+func Test(t *testing.T) {
+	t.Skip("skipping")
+}
+-- empty/pkg/pkg.go --
+package p
+-- empty/pkgtest/pkg.go --
+package p
+-- empty/pkgtest/test_test.go --
+package p
+-- empty/pkgtestxtest/pkg.go --
+package p
+-- empty/pkgtestxtest/test_test.go --
+package p
+-- empty/pkgtestxtest/xtest_test.go --
+package p_test
+-- empty/pkgxtest/pkg.go --
+package p
+-- empty/pkgxtest/xtest_test.go --
+package p_test
+-- empty/test/test_test.go --
+package p
+-- empty/testxtest/test_test.go --
+package p
+-- empty/testxtest/xtest_test.go --
+package p_test
+-- empty/xtest/xtest_test.go --
+package p_test
diff --git a/src/cmd/go/testdata/script/test_json_exit.txt b/src/cmd/go/testdata/script/test_json_exit.txt
new file mode 100644
index 0000000..dc7ffb0
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_json_exit.txt
@@ -0,0 +1,102 @@
+[short] skip
+
+go test -c -o mainpanic.exe ./mainpanic &
+go test -c -o mainexit0.exe ./mainexit0 &
+go test -c -o testpanic.exe ./testpanic &
+go test -c -o testbgpanic.exe ./testbgpanic &
+wait
+
+# Test binaries that panic in TestMain should be marked as failing.
+
+! go test -json ./mainpanic
+stdout '"Action":"fail"'
+! stdout '"Action":"pass"'
+
+! go tool test2json ./mainpanic.exe
+stdout '"Action":"fail"'
+! stdout '"Action":"pass"'
+
+# Test binaries that exit with status 0 should be marked as passing.
+
+go test -json ./mainexit0
+stdout '"Action":"pass"'
+! stdout '"Action":"fail"'
+
+go tool test2json ./mainexit0.exe
+stdout '"Action":"pass"'
+! stdout '"Action":"fail"'
+
+# Test functions that panic should never be marked as passing
+# (https://golang.org/issue/40132).
+
+! go test -json ./testpanic
+stdout '"Action":"fail"'
+! stdout '"Action":"pass"'
+
+! go tool test2json ./testpanic.exe -test.v
+stdout '"Action":"fail"'
+! stdout '"Action":"pass"'
+
+! go tool test2json ./testpanic.exe
+stdout '"Action":"fail"'
+! stdout '"Action":"pass"'
+
+# Tests that panic in a background goroutine should be marked as failing.
+
+! go test -json ./testbgpanic
+stdout '"Action":"fail"'
+! stdout '"Action":"pass"'
+
+! go tool test2json ./testbgpanic.exe -test.v
+stdout '"Action":"fail"'
+! stdout '"Action":"pass"'
+
+! go tool test2json ./testbgpanic.exe
+stdout '"Action":"fail"'
+! stdout '"Action":"pass"'
+
+-- go.mod --
+module m
+go 1.14
+-- mainpanic/mainpanic_test.go --
+package mainpanic_test
+
+import "testing"
+
+func TestMain(m *testing.M) {
+	panic("haha no")
+}
+-- mainexit0/mainexit0_test.go --
+package mainexit0_test
+
+import (
+	"fmt"
+	"os"
+	"testing"
+)
+
+func TestMain(m *testing.M) {
+	fmt.Println("nothing to do")
+	os.Exit(0)
+}
+-- testpanic/testpanic_test.go --
+package testpanic_test
+
+import "testing"
+
+func TestPanic(*testing.T) {
+	panic("haha no")
+}
+-- testbgpanic/testbgpanic_test.go --
+package testbgpanic_test
+
+import "testing"
+
+func TestPanicInBackground(*testing.T) {
+	c := make(chan struct{})
+	go func() {
+		panic("haha no")
+		close(c)
+	}()
+	<-c
+}
diff --git a/src/cmd/go/testdata/script/test_json_interleaved.txt b/src/cmd/go/testdata/script/test_json_interleaved.txt
new file mode 100644
index 0000000..e2d349e
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_json_interleaved.txt
@@ -0,0 +1,27 @@
+# Regression test for https://golang.org/issue/40657: output from the main test
+# function should be attributed correctly even if interleaved with the PAUSE
+# line for a new parallel subtest.
+
+[short] skip
+
+go test -json
+stdout '"Test":"TestWeirdTiming","Output":"[^"]* logging to outer again\\n"'
+
+-- go.mod --
+module example.com
+go 1.15
+-- main_test.go --
+package main
+
+import (
+	"testing"
+)
+
+func TestWeirdTiming(outer *testing.T) {
+	outer.Run("pauser", func(pauser *testing.T) {
+		outer.Logf("logging to outer")
+		pauser.Parallel()
+	})
+
+	outer.Logf("logging to outer again")
+}
diff --git a/src/cmd/go/testdata/script/test_json_panic_exit.txt b/src/cmd/go/testdata/script/test_json_panic_exit.txt
new file mode 100644
index 0000000..5f1d033
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_json_panic_exit.txt
@@ -0,0 +1,69 @@
+# Verifies golang.org/issue/37555.
+
+[short] skip
+
+# 'go test -json' should say a test passes if it says it passes.
+go test -json ./pass
+stdout '"Action":"pass","Package":"[^"]*","Elapsed":[^"]*}\n\z'
+! stdout '"Test":.*\n\z'
+
+# 'go test -json' should say a test passes if it exits 0 and prints nothing.
+# TODO(golang.org/issue/29062): this should fail in the future.
+go test -json ./exit0main
+stdout '"Action":"pass".*\n\z'
+! stdout '"Test":.*\n\z'
+
+# 'go test -json' should say a test fails if it exits 1 and prints nothing.
+! go test -json ./exit1main
+stdout '"Action":"fail".*\n\z'
+! stdout '"Test":.*\n\z'
+
+# 'go test -json' should say a test fails if it panics.
+! go test -json ./panic
+stdout '"Action":"fail".*\n\z'
+! stdout '"Test":.*\n\z'
+
+-- go.mod --
+module example.com/test
+
+go 1.14
+
+-- pass/pass_test.go --
+package pass_test
+
+import "testing"
+
+func TestPass(t *testing.T) {}
+
+-- exit0main/exit0main_test.go --
+package exit0_test
+
+import (
+	"os"
+	"testing"
+)
+
+func TestMain(m *testing.M) {
+	os.Exit(0)
+}
+
+-- exit1main/exit1main_test.go --
+package exit1_test
+
+import (
+	"os"
+	"testing"
+)
+
+func TestMain(m *testing.M) {
+	os.Exit(1)
+}
+
+-- panic/panic_test.go --
+package panic_test
+
+import "testing"
+
+func TestPanic(t *testing.T) {
+	panic("oh no")
+}
diff --git a/src/cmd/go/testdata/script/test_json_prints.txt b/src/cmd/go/testdata/script/test_json_prints.txt
new file mode 100644
index 0000000..f979998
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_json_prints.txt
@@ -0,0 +1,48 @@
+go test -json
+
+stdout '"Action":"output","Package":"p","Output":"M1"}'
+stdout '"Action":"output","Package":"p","Test":"Test","Output":"=== RUN   Test\\n"}'
+stdout '"Action":"output","Package":"p","Test":"Test","Output":"T1"}'
+stdout '"Action":"output","Package":"p","Test":"Test/Sub1","Output":"=== RUN   Test/Sub1\\n"}'
+stdout '"Action":"output","Package":"p","Test":"Test/Sub1","Output":"Sub1    x_test.go:19: SubLog1\\n"}'
+stdout '"Action":"output","Package":"p","Test":"Test/Sub1","Output":"Sub2"}'
+stdout '"Action":"output","Package":"p","Test":"Test/Sub1","Output":"--- PASS: Test/Sub1 \([\d.]+s\)\\n"}'
+stdout '"Action":"pass","Package":"p","Test":"Test/Sub1","Elapsed"'
+stdout '"Action":"output","Package":"p","Test":"Test/Sub3","Output":"foo bar"}'
+stdout '"Action":"output","Package":"p","Test":"Test/Sub3","Output":"baz\\n"}'
+stdout '"Action":"output","Package":"p","Test":"Test","Output":"T2"}'
+stdout '"Action":"output","Package":"p","Test":"Test","Output":"--- PASS: Test \([\d.]+s\)\\n"}'
+stdout '"Action":"pass","Package":"p","Test":"Test","Elapsed"'
+stdout '"Action":"output","Package":"p","Output":"M2ok  \\tp\\t[\d.]+s\\n"}'
+stdout '"Action":"pass","Package":"p","Elapsed"'
+
+-- go.mod --
+module p
+
+-- x_test.go --
+package p
+
+import (
+	"os"
+	"testing"
+)
+
+func TestMain(m *testing.M) {
+	print("M1")
+	code := m.Run()
+	print("M2")
+	os.Exit(code)
+}
+
+func Test(t *testing.T) {
+	print("T1")
+	t.Run("Sub1", func(t *testing.T) {
+		print("Sub1")
+		t.Log("SubLog1")
+		print("Sub2")
+	})
+	t.Run("Sub3", func(t *testing.T) {
+		print("\x16foo bar\x16baz\n")
+	})
+	print("T2")
+}
diff --git a/src/cmd/go/testdata/script/test_json_timeout.txt b/src/cmd/go/testdata/script/test_json_timeout.txt
new file mode 100644
index 0000000..0a2329b
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_json_timeout.txt
@@ -0,0 +1,19 @@
+! go test -json -timeout=1ms
+
+stdout '"Action":"output","Package":"p","Output":"FAIL\\tp\\t'
+stdout '"Action":"fail","Package":"p","Elapsed":'
+
+-- go.mod --
+module p
+
+-- x_test.go --
+package p
+
+import (
+	"testing"
+	"time"
+)
+
+func Test(t *testing.T) {
+	time.Sleep(1*time.Hour)
+}
diff --git a/src/cmd/go/testdata/script/test_main.txt b/src/cmd/go/testdata/script/test_main.txt
new file mode 100644
index 0000000..25d02e4
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_main.txt
@@ -0,0 +1,92 @@
+# Test TestMain
+go test standalone_main_normal_test.go
+! stdout '^ok.*\[no tests to run\]'
+! stderr '^ok.*\[no tests to run\]'
+stdout '^ok'
+
+# Test TestMain sees testing flags
+go test standalone_testmain_flag_test.go
+stdout '^ok.*\[no tests to run\]'
+
+# Test TestMain with wrong signature (Issue #22388)
+! go test standalone_main_wrong_test.go
+stderr 'wrong signature for TestMain, must be: func TestMain\(m \*testing.M\)'
+
+# Test TestMain does not call os.Exit (Issue #34129)
+! go test standalone_testmain_not_call_os_exit_test.go
+! stdout '^ok'
+
+-- standalone_main_normal_test.go --
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package standalone_main_normal_test
+
+import "testing"
+
+func TestMain(t *testing.T) {
+}
+-- standalone_main_wrong_test.go --
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package standalone_main_wrong_test
+
+import "testing"
+
+func TestMain(m *testing.Main) {
+}
+-- standalone_testmain_flag_test.go --
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package standalone_testmain_flag_test
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"testing"
+)
+
+func TestMain(m *testing.M) {
+	// A TestMain should be able to access testing flags if it calls
+	// flag.Parse without needing to use testing.Init.
+	flag.Parse()
+	found := false
+	flag.VisitAll(func(f *flag.Flag) {
+		if f.Name == "test.count" {
+			found = true
+		}
+	})
+	if !found {
+		fmt.Println("testing flags not registered")
+		os.Exit(1)
+	}
+	os.Exit(m.Run())
+}
+-- standalone_testmain_not_call_os_exit_test.go --
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package standalone_testmain_not_call_os_exit_test
+
+import (
+	"testing"
+)
+
+func TestWillFail(t *testing.T) {
+	t.Error("this test will fail.")
+}
+
+func TestMain(m *testing.M) {
+	defer func() {
+		recover()
+	}()
+	exit := m.Run()
+	panic(exit)
+}
diff --git a/src/cmd/go/testdata/script/test_main_archive.txt b/src/cmd/go/testdata/script/test_main_archive.txt
new file mode 100644
index 0000000..410d923
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_main_archive.txt
@@ -0,0 +1,32 @@
+env GO111MODULE=off
+
+# Test that a main_test of 'package main' imports the package,
+# not the installed binary.
+
+[short] skip
+
+env GOBIN=$WORK/bin
+go test main_test
+go install main_test
+
+go list -f '{{.Stale}}' main_test
+stdout false
+
+go test main_test
+
+-- main_test/m.go --
+package main
+
+func F()    {}
+func main() {}
+-- main_test/m_test.go --
+package main_test
+
+import (
+	. "main_test"
+	"testing"
+)
+
+func Test1(t *testing.T) {
+	F()
+}
diff --git a/src/cmd/go/testdata/script/test_main_panic.txt b/src/cmd/go/testdata/script/test_main_panic.txt
new file mode 100644
index 0000000..45887c5
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_main_panic.txt
@@ -0,0 +1,30 @@
+[short] skip
+[!race] skip
+
+! go test -v -race main_panic/testmain_parallel_sub_panic_test.go
+! stdout 'DATA RACE'
+-- main_panic/testmain_parallel_sub_panic_test.go --
+package testmain_parallel_sub_panic_test
+
+import "testing"
+
+func setup()    { println("setup()") }
+func teardown() { println("teardown()") }
+func TestA(t *testing.T) {
+	t.Run("1", func(t *testing.T) {
+		t.Run("1", func(t *testing.T) {
+			t.Parallel()
+			panic("A/1/1 panics")
+		})
+		t.Run("2", func(t *testing.T) {
+			t.Parallel()
+			println("A/1/2 is ok")
+		})
+	})
+}
+
+func TestMain(m *testing.M) {
+	setup()
+	defer teardown()
+	m.Run()
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/test_main_twice.txt b/src/cmd/go/testdata/script/test_main_twice.txt
new file mode 100644
index 0000000..f32d4fc
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_main_twice.txt
@@ -0,0 +1,27 @@
+[short] skip
+
+env GOCACHE=$WORK/tmp
+go test -v multimain
+stdout -count=2 notwithstanding # check tests ran twice
+
+-- go.mod --
+module multimain
+
+go 1.16
+-- multimain_test.go --
+package multimain_test
+
+import "testing"
+
+func TestMain(m *testing.M) {
+	// Some users run m.Run multiple times, changing
+	// some kind of global state between runs.
+	// This used to work so I guess now it has to keep working.
+	// See golang.org/issue/23129.
+	m.Run()
+	m.Run()
+}
+
+func Test(t *testing.T) {
+	t.Log("notwithstanding")
+}
diff --git a/src/cmd/go/testdata/script/test_match_benchmark_labels.txt b/src/cmd/go/testdata/script/test_match_benchmark_labels.txt
new file mode 100644
index 0000000..13c4007
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_match_benchmark_labels.txt
@@ -0,0 +1,18 @@
+# Benchmark labels, file outside gopath
+# TODO(matloob): This test was called TestBenchmarkLabelsOutsideGOPATH
+# why "OutsideGOPATH"? Does the go command need to be run outside GOPATH?
+# Do the files need to exist outside GOPATH?
+cp $GOPATH/src/standalone_benchmark_test.go $WORK/tmp/standalone_benchmark_test.go
+go test -run '^$' -bench . $WORK/tmp/standalone_benchmark_test.go
+stdout '^goos: '$GOOS
+stdout '^goarch: '$GOARCH
+! stdout '^pkg:'
+! stderr '^pkg:'
+
+-- standalone_benchmark_test.go --
+package standalone_benchmark
+
+import "testing"
+
+func Benchmark(b *testing.B) {
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/test_match_no_benchmarks.txt b/src/cmd/go/testdata/script/test_match_no_benchmarks.txt
new file mode 100644
index 0000000..30f4be8
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_match_no_benchmarks.txt
@@ -0,0 +1,13 @@
+# Matches no benchmarks
+go test -run '^$' -bench ThisWillNotMatch standalone_benchmark_test.go
+! stdout '^ok.*\[no tests to run\]'
+! stderr '^ok.*\[no tests to run\]'
+stdout '^ok'
+
+-- standalone_benchmark_test.go --
+package standalone_benchmark
+
+import "testing"
+
+func Benchmark(b *testing.B) {
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/test_match_no_subtests.txt b/src/cmd/go/testdata/script/test_match_no_subtests.txt
new file mode 100644
index 0000000..7abb1eb
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_match_no_subtests.txt
@@ -0,0 +1,12 @@
+# The subtests don't match
+go test -run Test/ThisWillNotMatch standalone_sub_test.go
+stdout '^ok.*\[no tests to run\]'
+
+-- standalone_sub_test.go --
+package standalone_sub_test
+
+import "testing"
+
+func Test(t *testing.T) {
+	t.Run("Sub", func(t *testing.T) {})
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/test_match_no_subtests_failure.txt b/src/cmd/go/testdata/script/test_match_no_subtests_failure.txt
new file mode 100644
index 0000000..b3c5b92
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_match_no_subtests_failure.txt
@@ -0,0 +1,15 @@
+# Matches no subtests, but parent test still fails
+! go test -run TestThatFails/ThisWillNotMatch standalone_fail_sub_test.go
+! stdout '^ok.*\[no tests to run\]'
+! stderr '^ok.*\[no tests to run\]'
+stdout 'FAIL'
+
+-- standalone_fail_sub_test.go --
+package standalone_fail_sub_test
+
+import "testing"
+
+func TestThatFails(t *testing.T) {
+	t.Run("Sub", func(t *testing.T) {})
+	t.Fail()
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/test_match_no_subtests_parallel.txt b/src/cmd/go/testdata/script/test_match_no_subtests_parallel.txt
new file mode 100644
index 0000000..11c734c
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_match_no_subtests_parallel.txt
@@ -0,0 +1,19 @@
+# Matches no subtests, parallel
+go test -run Test/Sub/ThisWillNotMatch standalone_parallel_sub_test.go
+stdout '^ok.*\[no tests to run\]'
+
+-- standalone_parallel_sub_test.go --
+package standalone_parallel_sub_test
+
+import "testing"
+
+func Test(t *testing.T) {
+	ch := make(chan bool, 1)
+	t.Run("Sub", func(t *testing.T) {
+		t.Parallel()
+		<-ch
+		t.Run("Nested", func(t *testing.T) {})
+	})
+	// Ensures that Sub will finish after its t.Run call already returned.
+	ch <- true
+}
diff --git a/src/cmd/go/testdata/script/test_match_no_tests.txt b/src/cmd/go/testdata/script/test_match_no_tests.txt
new file mode 100644
index 0000000..1ad2097
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_match_no_tests.txt
@@ -0,0 +1,11 @@
+# Matches no tests
+go test -run ThisWillNotMatch standalone_test.go
+stdout '^ok.*\[no tests to run\]'
+
+-- standalone_test.go --
+package standalone_test
+
+import "testing"
+
+func Test(t *testing.T) {
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/test_match_no_tests_build_failure.txt b/src/cmd/go/testdata/script/test_match_no_tests_build_failure.txt
new file mode 100644
index 0000000..e1c9643
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_match_no_tests_build_failure.txt
@@ -0,0 +1,19 @@
+# Test that when there's a build failure and a -run flag that doesn't match,
+# that the error for not matching tests does not override the error for
+# the build failure.
+
+! go test -run ThisWillNotMatch syntaxerror
+! stderr '(?m)^ok.*\[no tests to run\]'
+stdout 'FAIL'
+
+-- go.mod --
+module syntaxerror
+
+go 1.16
+-- x.go --
+package p
+-- x_test.go --
+package p
+
+func f() (x.y, z int) {
+}
diff --git a/src/cmd/go/testdata/script/test_match_no_tests_with_subtests.txt b/src/cmd/go/testdata/script/test_match_no_tests_with_subtests.txt
new file mode 100644
index 0000000..0d94918
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_match_no_tests_with_subtests.txt
@@ -0,0 +1,12 @@
+# Matches no tests with subtests
+go test -run ThisWillNotMatch standalone_sub_test.go
+stdout '^ok.*\[no tests to run\]'
+
+-- standalone_sub_test.go --
+package standalone_sub_test
+
+import "testing"
+
+func Test(t *testing.T) {
+	t.Run("Sub", func(t *testing.T) {})
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/test_match_only_benchmarks.txt b/src/cmd/go/testdata/script/test_match_only_benchmarks.txt
new file mode 100644
index 0000000..5dfb96e
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_match_only_benchmarks.txt
@@ -0,0 +1,13 @@
+# Matches only benchmarks
+go test -run '^$' -bench . standalone_benchmark_test.go
+! stdout '^ok.*\[no tests to run\]'
+! stderr '^ok.*\[no tests to run\]'
+stdout '^ok'
+
+-- standalone_benchmark_test.go --
+package standalone_benchmark
+
+import "testing"
+
+func Benchmark(b *testing.B) {
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/test_match_only_example.txt b/src/cmd/go/testdata/script/test_match_only_example.txt
new file mode 100644
index 0000000..515ccb3
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_match_only_example.txt
@@ -0,0 +1,31 @@
+[short] skip
+
+# Check that it's okay for test pattern to match only examples.
+go test -run Example example1_test.go
+! stderr '^ok.*\[no tests to run\]'
+stdout '^ok'
+
+-- example1_test.go --
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Make sure that go test runs Example_Z before Example_A, preserving source order.
+
+package p
+
+import "fmt"
+
+var n int
+
+func Example_Z() {
+	n++
+	fmt.Println(n)
+	// Output: 1
+}
+
+func Example_A() {
+	n++
+	fmt.Println(n)
+	// Output: 2
+}
diff --git a/src/cmd/go/testdata/script/test_match_only_subtests.txt b/src/cmd/go/testdata/script/test_match_only_subtests.txt
new file mode 100644
index 0000000..beea895
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_match_only_subtests.txt
@@ -0,0 +1,14 @@
+# Matches only subtests
+go test -run Test/Sub standalone_sub_test.go
+! stdout '^ok.*\[no tests to run\]'
+! stderr '^ok.*\[no tests to run\]'
+stdout '^ok'
+
+-- standalone_sub_test.go --
+package standalone_sub_test
+
+import "testing"
+
+func Test(t *testing.T) {
+	t.Run("Sub", func(t *testing.T) {})
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/test_match_only_subtests_parallel.txt b/src/cmd/go/testdata/script/test_match_only_subtests_parallel.txt
new file mode 100644
index 0000000..11872c2
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_match_only_subtests_parallel.txt
@@ -0,0 +1,21 @@
+# Matches only subtests, parallel
+go test -run Test/Sub/Nested standalone_parallel_sub_test.go
+! stdout '^ok.*\[no tests to run\]'
+! stderr '^ok.*\[no tests to run\]'
+stdout '^ok'
+
+-- standalone_parallel_sub_test.go --
+package standalone_parallel_sub_test
+
+import "testing"
+
+func Test(t *testing.T) {
+	ch := make(chan bool, 1)
+	t.Run("Sub", func(t *testing.T) {
+		t.Parallel()
+		<-ch
+		t.Run("Nested", func(t *testing.T) {})
+	})
+	// Ensures that Sub will finish after its t.Run call already returned.
+	ch <- true
+}
diff --git a/src/cmd/go/testdata/script/test_match_only_tests.txt b/src/cmd/go/testdata/script/test_match_only_tests.txt
new file mode 100644
index 0000000..9185793
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_match_only_tests.txt
@@ -0,0 +1,13 @@
+# Matches only tests
+go test -run Test standalone_test.go
+! stdout '^ok.*\[no tests to run\]'
+! stderr '^ok.*\[no tests to run\]'
+stdout '^ok'
+
+-- standalone_test.go --
+package standalone_test
+
+import "testing"
+
+func Test(t *testing.T) {
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/test_minus_n.txt b/src/cmd/go/testdata/script/test_minus_n.txt
new file mode 100644
index 0000000..9900dbc
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_minus_n.txt
@@ -0,0 +1,14 @@
+# The intent here is to verify that 'go test -n' works without crashing.
+# Any test will do.
+
+go test -n x_test.go
+
+-- x_test.go --
+package x_test
+
+import (
+	"testing"
+)
+
+func TestEmpty(t *testing.T) {
+}
diff --git a/src/cmd/go/testdata/script/test_no_run_example.txt b/src/cmd/go/testdata/script/test_no_run_example.txt
new file mode 100644
index 0000000..53ac755
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_no_run_example.txt
@@ -0,0 +1,30 @@
+go test -v norunexample
+stdout 'File with non-runnable example was built.'
+
+-- go.mod --
+module norunexample
+
+go 1.16
+-- example_test.go --
+package pkg_test
+
+import "os"
+
+func init() {
+	os.Stdout.Write([]byte("File with non-runnable example was built.\n"))
+}
+
+func Example_test() {
+	// This test will not be run, it has no "Output:" comment.
+}
+-- test_test.go --
+package pkg
+
+import (
+	"os"
+	"testing"
+)
+
+func TestBuilt(t *testing.T) {
+	os.Stdout.Write([]byte("A normal test was executed.\n"))
+}
diff --git a/src/cmd/go/testdata/script/test_no_tests.txt b/src/cmd/go/testdata/script/test_no_tests.txt
new file mode 100644
index 0000000..2d624d1
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_no_tests.txt
@@ -0,0 +1,15 @@
+# Tests issue #26242
+
+go test testnorun
+stdout 'testnorun\t\[no test files\]'
+
+-- go.mod --
+module testnorun
+
+go 1.16
+-- p.go --
+package p
+
+func init() {
+	panic("go test must not link and run test binaries without tests")
+}
diff --git a/src/cmd/go/testdata/script/test_overlay.txt b/src/cmd/go/testdata/script/test_overlay.txt
new file mode 100644
index 0000000..b6bdc11
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_overlay.txt
@@ -0,0 +1,24 @@
+[short] skip
+
+cd $WORK/gopath/src/foo
+go test -list=. -overlay=overlay.json .
+stdout 'TestBar'
+
+-- go.mod --
+module test.pkg
+-- foo/foo_test.go --
+package foo
+
+import "testing"
+
+func TestFoo(t *testing.T) { }
+-- tmp/bar_test.go --
+package foo
+
+import "testing"
+
+func TestBar(t *testing.T) {
+	t.Fatal("dummy failure")
+}
+-- foo/overlay.json --
+{"Replace": {"foo_test.go": "../tmp/bar_test.go"}}
diff --git a/src/cmd/go/testdata/script/test_parallel_number.txt b/src/cmd/go/testdata/script/test_parallel_number.txt
new file mode 100644
index 0000000..4eb9794
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_parallel_number.txt
@@ -0,0 +1,25 @@
+[short] skip
+
+# go test -parallel -1 shouldn't work
+! go test -parallel -1 standalone_parallel_sub_test.go
+stdout '-parallel can only be given'
+
+# go test -parallel 0 shouldn't work
+! go test -parallel 0 standalone_parallel_sub_test.go
+stdout '-parallel can only be given'
+
+-- standalone_parallel_sub_test.go --
+package standalone_parallel_sub_test
+
+import "testing"
+
+func Test(t *testing.T) {
+	ch := make(chan bool, 1)
+	t.Run("Sub", func(t *testing.T) {
+		t.Parallel()
+		<-ch
+		t.Run("Nested", func(t *testing.T) {})
+	})
+	// Ensures that Sub will finish after its t.Run call already returned.
+	ch <- true
+}
diff --git a/src/cmd/go/testdata/script/test_ppc64_linker_funcs.txt b/src/cmd/go/testdata/script/test_ppc64_linker_funcs.txt
new file mode 100644
index 0000000..735b5dc
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_ppc64_linker_funcs.txt
@@ -0,0 +1,49 @@
+# Tests that the linker implements the PPC64 ELFv2 ABI
+# register save and restore functions as defined in
+# section 2.3.3.1 of the PPC64 ELFv2 ABI when linking
+# external objects most likely compiled with gcc's
+# -Os option.
+#
+# Verifies golang.org/issue/52366 for linux/ppc64le
+[!GOOS:linux] skip
+[!compiler:gc] skip
+[!cgo] skip
+[!GOARCH:ppc64le] skip
+
+go build -ldflags='-linkmode=internal'
+exec ./abitest
+stdout success
+
+-- go.mod --
+module abitest
+
+-- abitest.go --
+package main
+
+/*
+#cgo CFLAGS: -Os
+
+int foo_fpr() {
+        asm volatile("":::"fr31","fr30","fr29","fr28");
+}
+int foo_gpr0() {
+        asm volatile("":::"r30","r29","r28");
+}
+int foo_gpr1() {
+        asm volatile("":::"fr31", "fr30","fr29","fr28","r30","r29","r28");
+}
+int foo_vr() {
+        asm volatile("":::"v31","v30","v29","v28");
+}
+*/
+import "C"
+
+import "fmt"
+
+func main() {
+	C.foo_fpr()
+	C.foo_gpr0()
+	C.foo_gpr1()
+	C.foo_vr()
+	fmt.Println("success")
+}
diff --git a/src/cmd/go/testdata/script/test_ppc64le_cgo_inline_plt.txt b/src/cmd/go/testdata/script/test_ppc64le_cgo_inline_plt.txt
new file mode 100644
index 0000000..ef9ff03
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_ppc64le_cgo_inline_plt.txt
@@ -0,0 +1,38 @@
+# Verify the linker will correctly resolve
+# ppc64le objects compiled with gcc's -fno-plt
+# option. This inlines PLT calls, and generates
+# additional reloc types which the internal linker
+# should handle.
+#
+# Verifies golang.org/issue/53345
+#
+# Note, older gcc/clang may accept this option, but
+# ignore it if binutils does not support the relocs.
+[!compiler:gc] skip
+[!cgo] skip
+[!GOARCH:ppc64le] skip
+
+env CGO_CFLAGS='-fno-plt -O2 -g'
+
+go build -ldflags='-linkmode=internal'
+exec ./noplttest
+stdout helloworld
+
+-- go.mod --
+module noplttest
+
+-- noplttest.go --
+package main
+
+/*
+#include <stdio.h>
+void helloworld(void) {
+   printf("helloworld\n");
+   fflush(stdout);
+}
+*/
+import "C"
+
+func main() {
+	C.helloworld()
+}
diff --git a/src/cmd/go/testdata/script/test_profile.txt b/src/cmd/go/testdata/script/test_profile.txt
new file mode 100644
index 0000000..9110706
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_profile.txt
@@ -0,0 +1,19 @@
+[compiler:gccgo] skip 'gccgo has no standard packages'
+[short] skip
+
+# Check go test -cpuprofile creates errors.test
+go test -cpuprofile errors.prof errors
+exists -exec errors.test$GOEXE
+
+# Check go test -cpuprofile -o myerrors.test creates errors.test
+go test -cpuprofile errors.prof -o myerrors.test$GOEXE errors
+exists -exec myerrors.test$GOEXE
+
+# Check go test -mutexprofile creates errors.test
+go test -mutexprofile errors.prof errors
+exists -exec errors.test$GOEXE
+
+# Check go test -mutexprofile -o myerrors.test creates errors.test
+go test -mutexprofile errors.prof -o myerrors.test$GOEXE errors
+exists -exec myerrors.test$GOEXE
+
diff --git a/src/cmd/go/testdata/script/test_race.txt b/src/cmd/go/testdata/script/test_race.txt
new file mode 100644
index 0000000..2ffea46
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_race.txt
@@ -0,0 +1,51 @@
+[short] skip
+[!race] skip
+
+go test testrace
+
+! go test -race testrace
+stdout 'FAIL: TestRace'
+! stdout 'PASS'
+! stderr 'PASS'
+
+! go test -race testrace -run XXX -bench .
+stdout 'FAIL: BenchmarkRace'
+! stdout 'PASS'
+! stderr 'PASS'
+
+-- go.mod --
+module testrace
+
+go 1.16
+-- race_test.go --
+package testrace
+
+import "testing"
+
+func TestRace(t *testing.T) {
+	for i := 0; i < 10; i++ {
+		c := make(chan int)
+		x := 1
+		go func() {
+			x = 2
+			c <- 1
+		}()
+		x = 3
+		<-c
+		_ = x
+	}
+}
+
+func BenchmarkRace(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		c := make(chan int)
+		x := 1
+		go func() {
+			x = 2
+			c <- 1
+		}()
+		x = 3
+		<-c
+		_ = x
+	}
+}
diff --git a/src/cmd/go/testdata/script/test_race_cover_mode_issue20435.txt b/src/cmd/go/testdata/script/test_race_cover_mode_issue20435.txt
new file mode 100644
index 0000000..eacc882
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_race_cover_mode_issue20435.txt
@@ -0,0 +1,48 @@
+[short] skip
+[!race] skip
+
+# Make sure test is functional.
+go test testrace
+
+# Now, check that -race -covermode=set is not allowed.
+! go test -race -covermode=set testrace
+stderr '-covermode must be "atomic", not "set", when -race is enabled'
+! stdout PASS
+! stderr PASS
+
+-- go.mod --
+module testrace
+
+go 1.16
+-- race_test.go --
+package testrace
+
+import "testing"
+
+func TestRace(t *testing.T) {
+	for i := 0; i < 10; i++ {
+		c := make(chan int)
+		x := 1
+		go func() {
+			x = 2
+			c <- 1
+		}()
+		x = 3
+		<-c
+		_ = x
+	}
+}
+
+func BenchmarkRace(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		c := make(chan int)
+		x := 1
+		go func() {
+			x = 2
+			c <- 1
+		}()
+		x = 3
+		<-c
+		_ = x
+	}
+}
diff --git a/src/cmd/go/testdata/script/test_race_install.txt b/src/cmd/go/testdata/script/test_race_install.txt
new file mode 100644
index 0000000..918d7e9
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_race_install.txt
@@ -0,0 +1,12 @@
+[!race] skip
+[short] skip
+
+mkdir $WORKDIR/tmp/pkg
+go install -race -pkgdir=$WORKDIR/tmp/pkg std
+
+-- go.mod --
+module empty
+
+go 1.16
+-- pkg/pkg.go --
+package p
diff --git a/src/cmd/go/testdata/script/test_race_install_cgo.txt b/src/cmd/go/testdata/script/test_race_install_cgo.txt
new file mode 100644
index 0000000..e1fe4f2
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_race_install_cgo.txt
@@ -0,0 +1,91 @@
+# Tests Issue #10500
+
+[!race] skip
+
+env GOBIN=$WORK/bin
+go install m/mtime m/sametime
+
+go tool -n cgo
+cp stdout cgopath.txt
+exec $GOBIN/mtime cgopath.txt # get the mtime of the file whose name is in cgopath.txt
+cp stdout cgotime_before.txt
+
+ # For this test, we don't actually care whether 'go test -race -i' succeeds.
+ # It may fail if GOROOT is read-only (perhaps it was installed as root).
+ # We only care that it does not overwrite cmd/cgo regardless.
+? go test -race -i runtime/race
+
+exec $GOBIN/mtime cgopath.txt # get the mtime of the file whose name is in cgopath.txt
+cp stdout cgotime_after.txt
+exec $GOBIN/sametime cgotime_before.txt cgotime_after.txt
+
+-- go.mod --
+module m
+
+go 1.16
+-- mtime/mtime.go --
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"strings"
+)
+
+func main() {
+	b, err := os.ReadFile(os.Args[1])
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	filename := strings.TrimSpace(string(b))
+	info, err := os.Stat(filename)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	if err := json.NewEncoder(os.Stdout).Encode(info.ModTime()); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}
+-- sametime/sametime.go --
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"time"
+)
+
+
+func main() {
+	var t1 time.Time
+	b1, err := os.ReadFile(os.Args[1])
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	if err := json.Unmarshal(b1, &t1); err != nil  {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+
+	var t2 time.Time
+	b2, err := os.ReadFile(os.Args[2])
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	if err := json.Unmarshal(b2, &t2); err != nil  {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+
+	if !t1.Equal(t2) {
+		fmt.Fprintf(os.Stderr, "time in %v (%v) is not the same as time in %v (%v)", os.Args[1], t1, os.Args[2], t2)
+		os.Exit(1)
+	}
+}
diff --git a/src/cmd/go/testdata/script/test_race_tag.txt b/src/cmd/go/testdata/script/test_race_tag.txt
new file mode 100644
index 0000000..4b18ebc
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_race_tag.txt
@@ -0,0 +1,29 @@
+# Tests Issue #54468
+
+[short] skip 'links a test binary'
+[!race] skip
+
+go mod tidy
+go test -c -o=$devnull -race .
+
+! stderr 'cannot find package'
+
+-- go.mod --
+module testrace
+
+go 1.18
+
+require rsc.io/sampler v1.0.0
+-- race_test.go --
+//go:build race
+
+package testrace
+
+import (
+        "testing"
+
+        _ "rsc.io/sampler"
+)
+
+func TestRaceTag(t *testing.T) {
+}
diff --git a/src/cmd/go/testdata/script/test_rebuildall.txt b/src/cmd/go/testdata/script/test_rebuildall.txt
new file mode 100644
index 0000000..38233c1
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_rebuildall.txt
@@ -0,0 +1,14 @@
+env GO111MODULE=off
+
+# Regression test for golang.org/issue/6844:
+# 'go test -a' should force dependencies in the standard library to be rebuilt.
+
+[short] skip
+
+go test -x -a -c testdata/dep_test.go
+stderr '^.*[/\\]compile'$GOEXE'["]? (.* )?regexp .*[/\\]regexp\.go'
+
+-- testdata/dep_test.go --
+package deps
+
+import _ "testing"
diff --git a/src/cmd/go/testdata/script/test_regexps.txt b/src/cmd/go/testdata/script/test_regexps.txt
new file mode 100644
index 0000000..2f33080
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_regexps.txt
@@ -0,0 +1,79 @@
+go test -cpu=1 -run=X/Y -bench=X/Y -count=2 -v testregexp
+
+# Test the following:
+
+# TestX is run, twice
+stdout -count=2 '^=== RUN   TestX$'
+stdout -count=2 '^    x_test.go:6: LOG: X running$'
+
+# TestX/Y is run, twice
+stdout -count=2 '^=== RUN   TestX/Y$'
+stdout -count=2 '^    x_test.go:8: LOG: Y running$'
+
+# TestXX is run, twice
+stdout -count=2 '^=== RUN   TestXX$'
+stdout -count=2 '^    z_test.go:10: LOG: XX running'
+
+# TestZ is not run
+! stdout '^=== RUN   TestZ$'
+
+# BenchmarkX is run with N=1 once, only to discover what sub-benchmarks it has,
+# and should not print a final summary line.
+stdout -count=1 '^    x_test.go:13: LOG: X running N=1$'
+! stdout '^\s+BenchmarkX: x_test.go:13: LOG: X running N=\d\d+'
+! stdout 'BenchmarkX\s+\d+'
+
+# Same for BenchmarkXX.
+stdout -count=1 '^    z_test.go:18: LOG: XX running N=1$'
+! stdout  '^    z_test.go:18: LOG: XX running N=\d\d+'
+! stdout 'BenchmarkXX\s+\d+'
+
+# BenchmarkX/Y is run in full twice due to -count=2.
+# "Run in full" means that it runs for approximately the default benchtime,
+# but may cap out at N=1e9.
+# We don't actually care what the final iteration count is, but it should be
+# a large number, and the last iteration count prints right before the results.
+stdout -count=2 '^    x_test.go:15: LOG: Y running N=[1-9]\d{4,}\nBenchmarkX/Y\s+\d+'
+
+-- go.mod --
+module testregexp
+
+go 1.16
+-- x_test.go --
+package x
+
+import "testing"
+
+func TestX(t *testing.T) {
+	t.Logf("LOG: X running")
+	t.Run("Y", func(t *testing.T) {
+		t.Logf("LOG: Y running")
+	})
+}
+
+func BenchmarkX(b *testing.B) {
+	b.Logf("LOG: X running N=%d", b.N)
+	b.Run("Y", func(b *testing.B) {
+		b.Logf("LOG: Y running N=%d", b.N)
+	})
+}
+-- z_test.go --
+package x
+
+import "testing"
+
+func TestZ(t *testing.T) {
+	t.Logf("LOG: Z running")
+}
+
+func TestXX(t *testing.T) {
+	t.Logf("LOG: XX running")
+}
+
+func BenchmarkZ(b *testing.B) {
+	b.Logf("LOG: Z running N=%d", b.N)
+}
+
+func BenchmarkXX(b *testing.B) {
+	b.Logf("LOG: XX running N=%d", b.N)
+}
diff --git a/src/cmd/go/testdata/script/test_relative_cmdline.txt b/src/cmd/go/testdata/script/test_relative_cmdline.txt
new file mode 100644
index 0000000..96f7b87
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_relative_cmdline.txt
@@ -0,0 +1,52 @@
+# Relative imports in command line package
+
+env GO111MODULE=off
+
+# Run tests outside GOPATH.
+env GOPATH=$WORK/tmp
+
+go test ./testimport/p.go ./testimport/p_test.go ./testimport/x_test.go
+stdout '^ok'
+
+-- testimport/p.go --
+package p
+
+func F() int { return 1 }
+-- testimport/p1/p1.go --
+package p1
+
+func F() int { return 1 }
+-- testimport/p2/p2.go --
+package p2
+
+func F() int { return 1 }
+-- testimport/p_test.go --
+package p
+
+import (
+	"./p1"
+
+	"testing"
+)
+
+func TestF(t *testing.T) {
+	if F() != p1.F() {
+		t.Fatal(F())
+	}
+}
+-- testimport/x_test.go --
+package p_test
+
+import (
+	. "../testimport"
+
+	"./p2"
+
+	"testing"
+)
+
+func TestF1(t *testing.T) {
+	if F() != p2.F() {
+		t.Fatal(F())
+	}
+}
diff --git a/src/cmd/go/testdata/script/test_relative_import.txt b/src/cmd/go/testdata/script/test_relative_import.txt
new file mode 100644
index 0000000..938a875
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_relative_import.txt
@@ -0,0 +1,31 @@
+# Relative imports in go test
+env GO111MODULE=off # relative import not supported in module mode
+
+# Run tests outside GOPATH.
+env GOPATH=$WORK/tmp
+
+go test ./testimport
+stdout '^ok'
+
+-- testimport/p.go --
+package p
+
+func F() int { return 1 }
+-- testimport/p1/p1.go --
+package p1
+
+func F() int { return 1 }
+-- testimport/p_test.go --
+package p
+
+import (
+	"./p1"
+
+	"testing"
+)
+
+func TestF(t *testing.T) {
+	if F() != p1.F() {
+		t.Fatal(F())
+	}
+}
diff --git a/src/cmd/go/testdata/script/test_script_cmdcd.txt b/src/cmd/go/testdata/script/test_script_cmdcd.txt
new file mode 100644
index 0000000..6e6f67e
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_script_cmdcd.txt
@@ -0,0 +1,13 @@
+# Tests that after a cd command, where usually the UNIX path separator is used,
+# a match against $PWD does not fail on Windows.
+
+cd $WORK/a/b/c/pkg
+
+go list -find -f {{.Root}}
+stdout $PWD
+
+-- $WORK/a/b/c/pkg/go.mod --
+module pkg
+
+-- $WORK/a/b/c/pkg/pkg.go --
+package pkg
diff --git a/src/cmd/go/testdata/script/test_shuffle.txt b/src/cmd/go/testdata/script/test_shuffle.txt
new file mode 100644
index 0000000..98029f5
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_shuffle.txt
@@ -0,0 +1,134 @@
+# Shuffle order of tests and benchmarks
+
+[short] skip 'builds and repeatedly runs a test binary'
+
+# Run tests
+go test -v foo_test.go
+! stdout '-test.shuffle '
+stdout '(?s)TestOne(.*)TestTwo(.*)TestThree'
+
+go test -v -shuffle=off foo_test.go
+! stdout '-test.shuffle '
+stdout '(?s)TestOne(.*)TestTwo(.*)TestThree'
+
+go test -v -shuffle=42 foo_test.go
+stdout '^-test.shuffle 42'
+stdout '(?s)TestThree(.*)TestOne(.*)TestTwo'
+
+go test -v -shuffle=0 foo_test.go
+stdout '^-test.shuffle 0'
+stdout '(?s)TestTwo(.*)TestOne(.*)TestThree'
+
+go test -v -shuffle -1 foo_test.go
+stdout '^-test.shuffle -1'
+stdout '(?s)TestThree(.*)TestOne(.*)TestTwo'
+
+go test -v -shuffle=on foo_test.go
+stdout '^-test.shuffle '
+stdout '(?s)=== RUN   TestOne(.*)--- PASS: TestOne'
+stdout '(?s)=== RUN   TestTwo(.*)--- PASS: TestTwo'
+stdout '(?s)=== RUN   TestThree(.*)--- PASS: TestThree'
+
+
+# Run tests and benchmarks
+go test -v -bench=. foo_test.go
+! stdout '-test.shuffle '
+stdout '(?s)TestOne(.*)TestTwo(.*)TestThree(.*)BenchmarkOne(.*)BenchmarkTwo(.*)BenchmarkThree'
+
+go test -v -bench=. -shuffle=off foo_test.go
+! stdout '-test.shuffle '
+stdout '(?s)TestOne(.*)TestTwo(.*)TestThree(.*)BenchmarkOne(.*)BenchmarkTwo(.*)BenchmarkThree'
+
+go test -v -bench=. -shuffle=42 foo_test.go
+stdout '^-test.shuffle 42'
+stdout '(?s)TestThree(.*)TestOne(.*)TestTwo(.*)BenchmarkThree(.*)BenchmarkOne(.*)BenchmarkTwo'
+
+go test -v -bench=. -shuffle=0 foo_test.go
+stdout '^-test.shuffle 0'
+stdout '(?s)TestTwo(.*)TestOne(.*)TestThree(.*)BenchmarkThree(.*)BenchmarkOne(.*)BenchmarkTwo'
+
+go test -v -bench=. -shuffle -1 foo_test.go
+stdout '^-test.shuffle -1'
+stdout '(?s)TestThree(.*)TestOne(.*)TestTwo(.*)BenchmarkOne(.*)BenchmarkThree(.*)BenchmarkTwo'
+
+go test -v -bench=. -shuffle=on foo_test.go
+stdout '^-test.shuffle '
+stdout '(?s)=== RUN   TestOne(.*)--- PASS: TestOne'
+stdout '(?s)=== RUN   TestTwo(.*)--- PASS: TestTwo'
+stdout '(?s)=== RUN   TestThree(.*)--- PASS: TestThree'
+stdout -count=2 'BenchmarkOne'
+stdout -count=2 'BenchmarkTwo'
+stdout -count=2 'BenchmarkThree'
+
+
+# When running go test -count=N, each of the N runs distinct runs should maintain the same
+# shuffled order of these tests.
+go test -v -shuffle=43 -count=4 foo_test.go
+stdout '^-test.shuffle 43'
+stdout '(?s)TestThree(.*)TestTwo(.*)TestOne(.*)TestThree(.*)TestTwo(.*)TestOne(.*)TestThree(.*)TestTwo(.*)TestOne(.*)TestThree(.*)TestTwo(.*)TestOne'
+
+go test -v -bench=. -shuffle=44 -count=2 foo_test.go
+stdout '^-test.shuffle 44'
+stdout '(?s)TestOne(.*)TestThree(.*)TestTwo(.*)TestOne(.*)TestThree(.*)TestTwo(.*)BenchmarkTwo(.*)BenchmarkOne(.*)BenchmarkThree(.*)'
+
+
+# The feature should work with test binaries as well
+go test -c
+exec ./m.test -test.shuffle=off
+! stdout '^-test.shuffle '
+
+exec ./m.test -test.shuffle=on
+stdout '^-test.shuffle '
+
+exec ./m.test -test.v -test.bench=. -test.shuffle=0 foo_test.go
+stdout '^-test.shuffle 0'
+stdout '(?s)TestTwo(.*)TestOne(.*)TestThree(.*)BenchmarkThree(.*)BenchmarkOne(.*)BenchmarkTwo'
+
+exec ./m.test -test.v -test.bench=. -test.shuffle=123 foo_test.go
+stdout '^-test.shuffle 123'
+stdout '(?s)TestThree(.*)TestOne(.*)TestTwo(.*)BenchmarkThree(.*)BenchmarkTwo(.*)BenchmarkOne'
+
+exec ./m.test -test.v -test.bench=. -test.shuffle=-1 foo_test.go
+stdout '^-test.shuffle -1'
+stdout '(?s)TestThree(.*)TestOne(.*)TestTwo(.*)BenchmarkOne(.*)BenchmarkThree(.*)BenchmarkTwo'
+
+exec ./m.test -test.v -test.bench=. -test.shuffle=44 -test.count=2 foo_test.go
+stdout '^-test.shuffle 44'
+stdout '(?s)TestOne(.*)TestThree(.*)TestTwo(.*)TestOne(.*)TestThree(.*)TestTwo(.*)BenchmarkTwo(.*)BenchmarkOne(.*)BenchmarkThree(.*)'
+
+
+# Negative testcases for invalid input
+! go test -shuffle -count=2
+stderr 'invalid value "-count=2" for flag -shuffle: -shuffle argument must be "on", "off", or an int64: strconv.ParseInt: parsing "-count=2": invalid syntax'
+
+! go test -shuffle=
+stderr '(?s)invalid value "" for flag -shuffle: -shuffle argument must be "on", "off", or an int64: strconv.ParseInt: parsing "": invalid syntax'
+
+! go test -shuffle=' '
+stderr '(?s)invalid value " " for flag -shuffle: -shuffle argument must be "on", "off", or an int64: strconv.ParseInt: parsing " ": invalid syntax'
+
+! go test -shuffle=true
+stderr 'invalid value "true" for flag -shuffle: -shuffle argument must be "on", "off", or an int64: strconv.ParseInt: parsing "true": invalid syntax'
+
+! go test -shuffle='abc'
+stderr 'invalid value "abc" for flag -shuffle: -shuffle argument must be "on", "off", or an int64: strconv.ParseInt: parsing "abc": invalid syntax'
+
+-- go.mod --
+module m
+
+go 1.16
+-- foo_test.go --
+package foo
+
+import "testing"
+
+func TestOne(t *testing.T)   {}
+func TestTwo(t *testing.T)   {}
+func TestThree(t *testing.T) {}
+
+func BenchmarkOne(b *testing.B)   {}
+func BenchmarkTwo(b *testing.B)   {}
+func BenchmarkThree(b *testing.B) {}
+
+-- foo.go --
+package foo
diff --git a/src/cmd/go/testdata/script/test_skip.txt b/src/cmd/go/testdata/script/test_skip.txt
new file mode 100644
index 0000000..94d20b9
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_skip.txt
@@ -0,0 +1,34 @@
+go test -v -run Test -skip T skip_test.go
+! stdout RUN
+stdout '^ok.*\[no tests to run\]'
+
+go test -v -skip T skip_test.go
+! stdout RUN
+
+go test -v -skip 1 skip_test.go
+! stdout Test1
+stdout RUN.*Test2
+stdout RUN.*Test2/3
+
+go test -v -skip 2/3 skip_test.go
+stdout RUN.*Test1
+stdout RUN.*Test2
+! stdout Test2/3
+
+go test -v -skip 2/4 skip_test.go
+stdout RUN.*Test1
+stdout RUN.*Test2
+stdout RUN.*Test2/3
+
+
+-- skip_test.go --
+package skip_test
+
+import "testing"
+
+func Test1(t *testing.T) {
+}
+
+func Test2(t *testing.T) {
+	t.Run("3", func(t *testing.T) {})
+}
diff --git a/src/cmd/go/testdata/script/test_source_order.txt b/src/cmd/go/testdata/script/test_source_order.txt
new file mode 100644
index 0000000..2865276
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_source_order.txt
@@ -0,0 +1,54 @@
+[short] skip
+
+# Control
+! go test example2_test.go example1_test.go
+
+# This test only passes if the source order is preserved
+go test example1_test.go example2_test.go
+
+-- example1_test.go --
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Make sure that go test runs Example_Z before Example_A, preserving source order.
+
+package p
+
+import "fmt"
+
+var n int
+
+func Example_Z() {
+	n++
+	fmt.Println(n)
+	// Output: 1
+}
+
+func Example_A() {
+	n++
+	fmt.Println(n)
+	// Output: 2
+}
+-- example2_test.go --
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Make sure that go test runs Example_Y before Example_B, preserving source order.
+
+package p
+
+import "fmt"
+
+func Example_Y() {
+	n++
+	fmt.Println(n)
+	// Output: 3
+}
+
+func Example_B() {
+	n++
+	fmt.Println(n)
+	// Output: 4
+}
diff --git a/src/cmd/go/testdata/script/test_status.txt b/src/cmd/go/testdata/script/test_status.txt
new file mode 100644
index 0000000..aa6ad3c
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_status.txt
@@ -0,0 +1,18 @@
+env GO111MODULE=off
+
+! go test x y
+stdout ^FAIL\s+x
+stdout ^ok\s+y
+stdout (?-m)FAIL\n$
+
+-- x/x_test.go --
+package x
+
+import "testing"
+
+func TestNothingJustFail(t *testing.T) {
+    t.Fail()
+}
+
+-- y/y_test.go --
+package y
diff --git a/src/cmd/go/testdata/script/test_syntax_error_says_fail.txt b/src/cmd/go/testdata/script/test_syntax_error_says_fail.txt
new file mode 100644
index 0000000..44ff6e2
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_syntax_error_says_fail.txt
@@ -0,0 +1,25 @@
+# Test that the error message for a syntax error in a test go file
+# says FAIL.
+
+env GO111MODULE=off
+! go test syntaxerror
+stderr 'x_test.go:' # check that the error is diagnosed
+stdout 'FAIL' # check that go test says FAIL
+
+env GO111MODULE=on
+cd syntaxerror
+! go test syntaxerror
+stderr 'x_test.go:' # check that the error is diagnosed
+stdout 'FAIL' # check that go test says FAIL
+
+-- syntaxerror/go.mod --
+module syntaxerror
+
+go 1.16
+-- syntaxerror/x.go --
+package p
+-- syntaxerror/x_test.go --
+package p
+
+func f() (x.y, z int) {
+}
diff --git a/src/cmd/go/testdata/script/test_timeout.txt b/src/cmd/go/testdata/script/test_timeout.txt
new file mode 100644
index 0000000..4de4df4
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_timeout.txt
@@ -0,0 +1,23 @@
+[short] skip
+env GO111MODULE=off
+cd a
+
+# If no timeout is set explicitly, 'go test' should set
+# -test.timeout to its internal deadline.
+go test -v . --
+stdout '10m0s'
+
+# An explicit -timeout argument should be propagated to -test.timeout.
+go test -v -timeout 30m . --
+stdout '30m0s'
+
+-- a/timeout_test.go --
+package t
+import (
+	"flag"
+	"fmt"
+	"testing"
+)
+func TestTimeout(t *testing.T) {
+	fmt.Println(flag.Lookup("test.timeout").Value.String())
+}
diff --git a/src/cmd/go/testdata/script/test_timeout_stdin.txt b/src/cmd/go/testdata/script/test_timeout_stdin.txt
new file mode 100644
index 0000000..f2de0a6
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_timeout_stdin.txt
@@ -0,0 +1,88 @@
+# Regression test for https://go.dev/issue/24050:
+# a test that exits with an I/O stream held open
+# should fail after a reasonable delay, not wait forever.
+# (As of the time of writing, that delay is 10% of the timeout,
+# but this test does not depend on its specific value.)
+
+[short] skip 'runs a test that hangs until its WaitDelay expires'
+
+! go test -v -timeout=1m .
+
+	# After the test process itself prints PASS and exits,
+	# the kernel closes its stdin pipe to to the orphaned subprocess.
+	# At that point, we expect the subprocess to print 'stdin closed'
+	# and periodically log to stderr until the WaitDelay expires.
+	#
+	# Once the WaitDelay expires, the copying goroutine for 'go test' stops and
+	# closes the read side of the stderr pipe, and the subprocess will eventually
+	# exit due to a failed write to that pipe.
+
+stdout '^--- PASS: TestOrphanCmd .*\nPASS\nstdin closed'
+stdout '^\*\*\* Test I/O incomplete \d+.* after exiting\.\nexec: WaitDelay expired before I/O complete\nFAIL\s+example\s+\d+(\.\d+)?s'
+
+-- go.mod --
+module example
+
+go 1.20
+-- main_test.go --
+package main
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"os/exec"
+	"testing"
+	"time"
+)
+
+func TestMain(m *testing.M) {
+	if os.Getenv("TEST_TIMEOUT_HANG") == "1" {
+		io.Copy(io.Discard, os.Stdin)
+		if _, err := os.Stderr.WriteString("stdin closed\n"); err != nil {
+			os.Exit(1)
+		}
+
+		ticker := time.NewTicker(100 * time.Millisecond)
+		for t := range ticker.C {
+			_, err := fmt.Fprintf(os.Stderr, "still alive at %v\n", t)
+			if err != nil {
+				os.Exit(1)
+			}
+		}
+	}
+
+	m.Run()
+}
+
+func TestOrphanCmd(t *testing.T) {
+	exe, err := os.Executable()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cmd := exec.Command(exe)
+	cmd.Env = append(cmd.Environ(), "TEST_TIMEOUT_HANG=1")
+
+	// Hold stdin open until this (parent) process exits.
+	if _, err := cmd.StdinPipe(); err != nil {
+		t.Fatal(err)
+	}
+
+	// Forward stderr to the subprocess so that it can hold the stream open.
+	cmd.Stderr = os.Stderr
+
+	if err := cmd.Start(); err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("started %v", cmd)
+
+	// Intentionally leak cmd when the test completes.
+	// This will allow the test process itself to exit, but (at least on Unix
+	// platforms) will keep the parent process's stderr stream open.
+	go func() {
+		if err := cmd.Wait(); err != nil {
+			os.Exit(3)
+		}
+	}()
+}
diff --git a/src/cmd/go/testdata/script/test_trimpath.txt b/src/cmd/go/testdata/script/test_trimpath.txt
new file mode 100644
index 0000000..065f9ce
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_trimpath.txt
@@ -0,0 +1,51 @@
+[short] skip
+
+go test -trimpath -v .
+! stdout '[/\\]pkg_test[/\\]'
+stdout -count=3 '[/\\]pkg[/\\]'
+
+-- go.mod --
+module example.com/pkg
+
+go 1.17
+
+-- pkg.go --
+package pkg
+
+import "runtime"
+
+func PrintFile() {
+	_, file, _, _ := runtime.Caller(0)
+	println(file)
+}
+
+-- pkg_test.go --
+package pkg
+
+import "runtime"
+
+func PrintFileForTest() {
+	_, file, _, _ := runtime.Caller(0)
+	println(file)
+}
+
+-- pkg_x_test.go --
+package pkg_test
+
+import (
+	"runtime"
+	"testing"
+
+	"example.com/pkg"
+)
+
+func TestMain(m *testing.M) {
+	pkg.PrintFile()
+	pkg.PrintFileForTest()
+	PrintFileInXTest()
+}
+
+func PrintFileInXTest() {
+	_, file, _, _ := runtime.Caller(0)
+	println(file)
+}
diff --git a/src/cmd/go/testdata/script/test_trimpath_main.txt b/src/cmd/go/testdata/script/test_trimpath_main.txt
new file mode 100644
index 0000000..c076212
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_trimpath_main.txt
@@ -0,0 +1,38 @@
+[short] skip
+
+go test -trimpath -v .
+! stdout '[/\\]pkg_test[/\\]'
+stdout -count=2 '[/\\]pkg[/\\]'
+
+-- go.mod --
+module example.com/pkg
+
+go 1.17
+
+-- main.go --
+package main
+
+import "runtime"
+
+func PrintFile() {
+	_, file, _, _ := runtime.Caller(0)
+	println(file)
+}
+
+-- main_test.go --
+package main
+
+import (
+	"runtime"
+	"testing"
+)
+
+func PrintFileForTest() {
+	_, file, _, _ := runtime.Caller(0)
+	println(file)
+}
+
+func TestMain(m *testing.M) {
+	PrintFile()
+	PrintFileForTest()
+}
diff --git a/src/cmd/go/testdata/script/test_trimpath_test_suffix.txt b/src/cmd/go/testdata/script/test_trimpath_test_suffix.txt
new file mode 100644
index 0000000..6cbad83
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_trimpath_test_suffix.txt
@@ -0,0 +1,40 @@
+[short] skip
+
+go test -trimpath -v .
+! stdout '[/\\]pkg_test_test[/\\]'
+stdout -count=2 '[/\\]pkg_test[/\\]'
+
+-- go.mod --
+module example.com/pkg_test
+
+go 1.17
+
+-- pkg.go --
+package pkg_test
+
+import "runtime"
+
+func PrintFile() {
+	_, file, _, _ := runtime.Caller(0)
+	println(file)
+}
+
+-- pkg_x_test.go --
+package pkg_test_test
+
+import (
+	"runtime"
+	"testing"
+
+	"example.com/pkg_test"
+)
+
+func PrintFileForTest() {
+	_, file, _, _ := runtime.Caller(0)
+	println(file)
+}
+
+func TestMain(m *testing.M) {
+	pkg_test.PrintFile()
+	PrintFileForTest()
+}
diff --git a/src/cmd/go/testdata/script/test_vendor.txt b/src/cmd/go/testdata/script/test_vendor.txt
new file mode 100644
index 0000000..c6a88b6
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_vendor.txt
@@ -0,0 +1,57 @@
+# In GOPATH mode, vendored packages can replace std packages.
+env GO111MODULE=off
+cd vend/hello
+go test -v
+stdout TestMsgInternal
+stdout TestMsgExternal
+
+# In module mode, they cannot.
+env GO111MODULE=on
+! go test -mod=vendor
+stderr 'undefined: strings.Msg'
+
+-- vend/hello/go.mod --
+module vend/hello
+
+go 1.16
+-- vend/hello/hello.go --
+package main
+
+import (
+	"fmt"
+	"strings" // really ../vendor/strings
+)
+
+func main() {
+	fmt.Printf("%s\n", strings.Msg)
+}
+-- vend/hello/hello_test.go --
+package main
+
+import (
+	"strings" // really ../vendor/strings
+	"testing"
+)
+
+func TestMsgInternal(t *testing.T) {
+	if strings.Msg != "hello, world" {
+		t.Fatalf("unexpected msg: %v", strings.Msg)
+	}
+}
+-- vend/hello/hellox_test.go --
+package main_test
+
+import (
+	"strings" // really ../vendor/strings
+	"testing"
+)
+
+func TestMsgExternal(t *testing.T) {
+	if strings.Msg != "hello, world" {
+		t.Fatalf("unexpected msg: %v", strings.Msg)
+	}
+}
+-- vend/vendor/strings/msg.go --
+package strings
+
+var Msg = "hello, world"
diff --git a/src/cmd/go/testdata/script/test_vet.txt b/src/cmd/go/testdata/script/test_vet.txt
new file mode 100644
index 0000000..6151f91
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_vet.txt
@@ -0,0 +1,123 @@
+[short] skip
+
+# Test file
+! go test p1_test.go
+stderr 'Logf format %d'
+go test -vet=off
+stdout '^ok'
+
+# Non-test file
+! go test p1.go
+stderr 'Printf format %d'
+go test -x -vet=shift p1.go
+stderr '[\\/]vet.*-shift'
+stdout '\[no test files\]'
+go test -vet=off p1.go
+! stderr '[\\/]vet.*-shift'
+stdout '\[no test files\]'
+
+# ensure all runs non-default vet
+! go test -vet=all ./vetall/...
+stderr 'using resp before checking for errors'
+
+# Test issue #47309
+! go test -vet=bools,xyz ./vetall/...
+stderr '-vet argument must be a supported analyzer'
+
+# Test with a single analyzer
+! go test -vet=httpresponse ./vetall/...
+stderr 'using resp before checking for errors'
+
+# Test with a list of analyzers
+go test -vet=atomic,bools,nilfunc ./vetall/...
+stdout 'm/vetall.*\[no tests to run\]'
+
+# Test issue #22890
+go test m/vetcycle
+stdout 'm/vetcycle.*\[no test files\]'
+
+# Test with ...
+! go test ./vetfail/...
+stderr 'Printf format %d'
+stdout 'ok\s+m/vetfail/p2'
+
+# Check there's no diagnosis of a bad build constraint in vetxonly mode.
+# Use -a so that we need to recompute the vet-specific export data for
+# vetfail/p1.
+go test -a m/vetfail/p2
+! stderr 'invalid.*constraint'
+
+-- go.mod --
+module m
+
+go 1.16
+-- p1_test.go --
+package p
+
+import "testing"
+
+func Test(t *testing.T) {
+	t.Logf("%d") // oops
+}
+-- p1.go --
+package p
+
+import "fmt"
+
+func F() {
+	fmt.Printf("%d") // oops
+}
+-- vetall/p.go --
+package p
+
+import "net/http"
+
+func F() {
+	resp, err := http.Head("example.com")
+	defer resp.Body.Close()
+	if err != nil {
+		panic(err)
+	}
+	// (defer statement belongs here)
+}
+-- vetall/p_test.go --
+package p
+-- vetcycle/p.go --
+package p
+
+type (
+	_  interface{ m(B1) }
+	A1 interface{ a(D1) }
+	B1 interface{ A1 }
+	C1 interface {
+		B1 /* ERROR issue #18395 */
+	}
+	D1 interface{ C1 }
+)
+
+var _ A1 = C1 /* ERROR cannot use C1 */ (nil)
+-- vetfail/p1/p1.go --
+// +build !foo-bar
+
+package p1
+
+import "fmt"
+
+func F() {
+	fmt.Printf("%d", "hello") // causes vet error
+}
+-- vetfail/p2/p2.go --
+package p2
+
+import _ "m/vetfail/p1"
+
+func F() {
+}
+-- vetfail/p2/p2_test.go --
+package p2
+
+import "testing"
+
+func TestF(t *testing.T) {
+	F()
+}
diff --git a/src/cmd/go/testdata/script/test_write_profiles_on_timeout.txt b/src/cmd/go/testdata/script/test_write_profiles_on_timeout.txt
new file mode 100644
index 0000000..0db183f
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_write_profiles_on_timeout.txt
@@ -0,0 +1,26 @@
+# Tests issue 19394
+
+[short] skip
+
+! go test -cpuprofile cpu.pprof -memprofile mem.pprof -timeout 1ms
+stdout '^panic: test timed out'
+grep . cpu.pprof
+grep . mem.pprof
+
+-- go.mod --
+module profiling
+
+go 1.16
+-- timeout_test.go --
+package timeouttest_test
+
+import (
+	"testing"
+	"time"
+)
+
+func TestSleep(t *testing.T) {
+	for {
+		time.Sleep(1 * time.Second)
+	}
+}
diff --git a/src/cmd/go/testdata/script/test_xtestonly_works.txt b/src/cmd/go/testdata/script/test_xtestonly_works.txt
new file mode 100644
index 0000000..8e150db
--- /dev/null
+++ b/src/cmd/go/testdata/script/test_xtestonly_works.txt
@@ -0,0 +1,27 @@
+[short] skip
+
+go test xtestonly
+! stdout '^ok.*\[no tests to run\]'
+stdout '^ok'
+
+-- go.mod --
+module xtestonly
+
+go 1.16
+-- f.go --
+package xtestonly
+
+func F() int { return 42 }
+-- f_test.go --
+package xtestonly_test
+
+import (
+	"testing"
+	"xtestonly"
+)
+
+func TestF(t *testing.T) {
+	if x := xtestonly.F(); x != 42 {
+		t.Errorf("f.F() = %d, want 42", x)
+	}
+}
diff --git a/src/cmd/go/testdata/script/testing_coverage.txt b/src/cmd/go/testdata/script/testing_coverage.txt
new file mode 100644
index 0000000..6cf6adb
--- /dev/null
+++ b/src/cmd/go/testdata/script/testing_coverage.txt
@@ -0,0 +1,57 @@
+
+# Rudimentary test of testing.Coverage().
+
+[short] skip
+[!GOEXPERIMENT:coverageredesign] skip
+
+# Simple test.
+go test -v -cover -count=1
+
+# Make sure test still passes when test executable is built and
+# run outside the go command.
+go test -c -o t.exe -cover
+exec ./t.exe
+
+-- go.mod --
+module hello
+
+go 1.20
+-- hello.go --
+package hello
+
+func Hello() {
+	println("hello")
+}
+
+// contents not especially interesting, just need some code
+func foo(n int) int {
+	t := 0
+	for i := 0; i < n; i++ {
+		for j := 0; j < i; j++ {
+			t += i ^ j
+			if t == 1010101 {
+				break
+			}
+		}
+	}
+	return t
+}
+
+-- hello_test.go --
+package hello
+
+import "testing"
+
+func TestTestCoverage(t *testing.T) {
+	Hello()
+	C1 := testing.Coverage()
+	foo(29)
+	C2 := testing.Coverage()
+	if C1 == 0.0 || C2 == 0.0 {
+		t.Errorf("unexpected zero values C1=%f C2=%f", C1, C2)
+	}
+	if C1 >= C2 {
+		t.Errorf("testing.Coverage() not monotonically increasing C1=%f C2=%f", C1, C2)
+	}
+}
+
diff --git a/src/cmd/go/testdata/script/testing_issue40908.txt b/src/cmd/go/testdata/script/testing_issue40908.txt
new file mode 100644
index 0000000..839320e
--- /dev/null
+++ b/src/cmd/go/testdata/script/testing_issue40908.txt
@@ -0,0 +1,25 @@
+[short] skip
+[!race] skip
+
+go test -race testrace
+
+-- go.mod --
+module testrace
+
+go 1.16
+-- race_test.go --
+package testrace
+
+import "testing"
+
+func TestRace(t *testing.T) {
+	helperDone := make(chan struct{})
+	go func() {
+		t.Logf("Something happened before cleanup.")
+		close(helperDone)
+	}()
+
+	t.Cleanup(func() {
+		<-helperDone
+	})
+}
diff --git a/src/cmd/go/testdata/script/toolexec.txt b/src/cmd/go/testdata/script/toolexec.txt
new file mode 100644
index 0000000..20a5968
--- /dev/null
+++ b/src/cmd/go/testdata/script/toolexec.txt
@@ -0,0 +1,128 @@
+[short] skip
+
+# Build our simple toolexec program.
+go build ./cmd/mytool
+
+# Use an ephemeral build cache so that our toolexec output is not cached
+# for any stale standard-library dependencies.
+#
+# TODO(#27628): This should not be necessary.
+env GOCACHE=$WORK/gocache
+
+# Build the main package with our toolexec program. For each action, it will
+# print the tool's name and the TOOLEXEC_IMPORTPATH value. We expect to compile
+# each package once, and link the main package once.
+# Don't check the entire output at once, because the order in which the tools
+# are run is irrelevant here.
+# Finally, note that asm and cgo are run twice.
+
+go build -toolexec=$PWD/mytool
+[GOARCH:amd64] stderr -count=2 '^asm'${GOEXE}' TOOLEXEC_IMPORTPATH="test/main/withasm"$'
+stderr -count=1 '^compile'${GOEXE}' TOOLEXEC_IMPORTPATH="test/main/withasm"$'
+[cgo] stderr -count=2 '^cgo'${GOEXE}' TOOLEXEC_IMPORTPATH="test/main/withcgo"$'
+[cgo] stderr -count=1 '^compile'${GOEXE}' TOOLEXEC_IMPORTPATH="test/main/withcgo"$'
+stderr -count=1 '^compile'${GOEXE}' TOOLEXEC_IMPORTPATH="test/main"$'
+stderr -count=1 '^link'${GOEXE}' TOOLEXEC_IMPORTPATH="test/main"$'
+
+# Test packages are a little bit trickier.
+# We have four variants of test/main, as reported by 'go list -test':
+#
+#    test/main                        - the regular non-test package
+#    test/main.test                   - the generated test program
+#    test/main [test/main.test]       - the test package for foo_test.go
+#    test/main_test [test/main.test]  - the test package for foo_separate_test.go
+#
+# As such, TOOLEXEC_IMPORTPATH must see the same strings, to be able to uniquely
+# identify each package being built as reported by 'go list -f {{.ImportPath}}'.
+# Note that these are not really "import paths" anymore, but that naming is
+# consistent with 'go list -json' at least.
+
+go test -toolexec=$PWD/mytool
+
+stderr -count=2 '^# test/main\.test$'
+stderr -count=1 '^compile'${GOEXE}' TOOLEXEC_IMPORTPATH="test/main\.test"$'
+stderr -count=1 '^link'${GOEXE}' TOOLEXEC_IMPORTPATH="test/main\.test"$'
+
+stderr -count=1 '^# test/main \[test/main\.test\]$'
+stderr -count=1 '^compile'${GOEXE}' TOOLEXEC_IMPORTPATH="test/main \[test/main\.test\]"$'
+
+stderr -count=1 '^# test/main_test \[test/main\.test\]$'
+stderr -count=1 '^compile'${GOEXE}' TOOLEXEC_IMPORTPATH="test/main_test \[test/main\.test\]"$'
+
+-- go.mod --
+module test/main
+-- foo.go --
+// Simple package so we can test a program build with -toolexec.
+// With a dummy import, to test different TOOLEXEC_IMPORTPATH values.
+// Includes dummy uses of cgo and asm, to cover those tools as well.
+package main
+
+import (
+	_ "test/main/withasm"
+	_ "test/main/withcgo"
+)
+
+func main() {}
+-- foo_test.go --
+package main
+
+import "testing"
+
+func TestFoo(t *testing.T) {}
+-- foo_separate_test.go --
+package main_test
+
+import "testing"
+
+func TestSeparateFoo(t *testing.T) {}
+-- withcgo/withcgo.go --
+package withcgo
+
+// int fortytwo()
+// {
+//     return 42;
+// }
+import "C"
+-- withcgo/stub.go --
+package withcgo
+
+// Stub file to ensure we build without cgo too.
+-- withasm/withasm.go --
+package withasm
+
+// Note that we don't need to declare the Add func at all.
+-- withasm/withasm_amd64.s --
+TEXT ·Add(SB),$0-24
+	MOVQ a+0(FP), AX
+	ADDQ b+8(FP), AX
+	MOVQ AX, ret+16(FP)
+	RET
+-- cmd/mytool/main.go --
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+)
+
+func main() {
+	tool, args := os.Args[1], os.Args[2:]
+	toolName := filepath.Base(tool)
+	if len(args) > 0 && args[0] == "-V=full" {
+		// We can't alter the version output.
+	} else {
+		// Print which tool we're running, and on what package.
+		fmt.Fprintf(os.Stdout, "%s TOOLEXEC_IMPORTPATH=%q\n", toolName, os.Getenv("TOOLEXEC_IMPORTPATH"))
+	}
+
+	// Simply run the tool.
+	cmd := exec.Command(tool, args...)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	if err := cmd.Run(); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}
diff --git a/src/cmd/go/testdata/script/tooltags.txt b/src/cmd/go/testdata/script/tooltags.txt
new file mode 100644
index 0000000..27068ee
--- /dev/null
+++ b/src/cmd/go/testdata/script/tooltags.txt
@@ -0,0 +1,57 @@
+env GOOS=linux
+
+env GOARCH=amd64
+env GOAMD64=v3
+go list -f '{{context.ToolTags}}'
+stdout 'amd64.v1 amd64.v2 amd64.v3'
+
+env GOARCH=arm
+env GOARM=6
+go list -f '{{context.ToolTags}}'
+stdout 'arm.5 arm.6'
+
+env GOARCH=mips
+env GOMIPS=hardfloat
+go list -f '{{context.ToolTags}}'
+stdout 'mips.hardfloat'
+
+env GOARCH=mips64
+env GOMIPS=hardfloat
+go list -f '{{context.ToolTags}}'
+stdout 'mips64.hardfloat'
+
+env GOARCH=ppc64
+env GOPPC64=power9
+go list -f '{{context.ToolTags}}'
+stdout 'ppc64.power8 ppc64.power9'
+
+env GOARCH=ppc64
+env GOPPC64=power10
+go list -f '{{context.ToolTags}}'
+stdout 'ppc64.power8 ppc64.power9 ppc64.power10'
+
+env GOARCH=ppc64le
+env GOPPC64=power9
+go list -f '{{context.ToolTags}}'
+stdout 'ppc64le.power8 ppc64le.power9'
+
+env GOARCH=ppc64le
+env GOPPC64=power10
+go list -f '{{context.ToolTags}}'
+stdout 'ppc64le.power8 ppc64le.power9 ppc64le.power10'
+
+env GOARCH=386
+env GO386=sse2
+go list -f '{{context.ToolTags}}'
+stdout '386.sse2'
+
+env GOARCH=wasm
+env GOWASM=satconv
+go list -f '{{context.ToolTags}}'
+stdout 'wasm.satconv'
+
+-- go.mod --
+module m
+
+-- p.go --
+package p
diff --git a/src/cmd/go/testdata/script/trampoline_reuse_test.txt b/src/cmd/go/testdata/script/trampoline_reuse_test.txt
new file mode 100644
index 0000000..41e86e4
--- /dev/null
+++ b/src/cmd/go/testdata/script/trampoline_reuse_test.txt
@@ -0,0 +1,100 @@
+# Verify PPC64 does not reuse a trampoline which is too far away.
+# This tests an edge case where the direct call relocation addend should
+# be ignored when computing the distance from the direct call to the
+# already placed trampoline
+[short] skip
+[!GOARCH:ppc64] [!GOARCH:ppc64le] skip
+[GOOS:aix] skip
+
+# Note, this program does not run. Presumably, 'DWORD $0' is simpler to
+# assembly 2^26 or so times.
+#
+# We build something which should be laid out as such:
+#
+# bar.Bar
+# main.Func1
+# bar.Bar+400-tramp0
+# main.BigAsm
+# main.Func2
+# bar.Bar+400-tramp1
+#
+# bar.Bar needs to be placed far enough away to generate relocations
+# from main package calls. and main.Func1 and main.Func2 are placed
+# a bit more than the direct call limit apart, but not more than 0x400
+# bytes beyond it (to verify the reloc calc).
+
+go build
+
+-- go.mod --
+
+module foo
+
+go 1.19
+
+-- main.go --
+
+package main
+
+import "foo/bar"
+
+func Func1()
+
+func main() {
+        Func1()
+        bar.Bar2()
+}
+
+-- foo.s --
+
+TEXT main·Func1(SB),0,$0-0
+        CALL bar·Bar+0x400(SB)
+        CALL main·BigAsm(SB)
+// A trampoline will be placed here to bar.Bar
+
+// This creates a gap sufficiently large to prevent trampoline reuse
+#define NOP64 DWORD $0; DWORD $0; DWORD $0; DWORD $0; DWORD $0; DWORD $0; DWORD $0; DWORD $0;
+#define NOP256 NOP64 NOP64 NOP64 NOP64
+#define NOP2S10 NOP256 NOP256 NOP256 NOP256
+#define NOP2S12 NOP2S10 NOP2S10 NOP2S10 NOP2S10
+#define NOP2S14 NOP2S12 NOP2S12 NOP2S12 NOP2S12
+#define NOP2S16 NOP2S14 NOP2S14 NOP2S14 NOP2S14
+#define NOP2S18 NOP2S16 NOP2S16 NOP2S16 NOP2S16
+#define NOP2S20 NOP2S18 NOP2S18 NOP2S18 NOP2S18
+#define NOP2S22 NOP2S20 NOP2S20 NOP2S20 NOP2S20
+#define NOP2S24 NOP2S22 NOP2S22 NOP2S22 NOP2S22
+#define BIGNOP NOP2S24 NOP2S24
+TEXT main·BigAsm(SB),0,$0-0
+        // Fill to the direct call limit so Func2 must generate a new trampoline.
+        // As the implicit trampoline above is just barely unreachable.
+        BIGNOP
+        MOVD $main·Func2(SB), R3
+
+TEXT main·Func2(SB),0,$0-0
+        CALL bar·Bar+0x400(SB)
+// Another trampoline should be placed here.
+
+-- bar/bar.s --
+
+#define NOP64 DWORD $0; DWORD $0; DWORD $0; DWORD $0; DWORD $0; DWORD $0; DWORD $0; DWORD $0;
+#define NOP256 NOP64 NOP64 NOP64 NOP64
+#define NOP2S10 NOP256 NOP256 NOP256 NOP256
+#define NOP2S12 NOP2S10 NOP2S10 NOP2S10 NOP2S10
+#define NOP2S14 NOP2S12 NOP2S12 NOP2S12 NOP2S12
+#define NOP2S16 NOP2S14 NOP2S14 NOP2S14 NOP2S14
+#define NOP2S18 NOP2S16 NOP2S16 NOP2S16 NOP2S16
+#define NOP2S20 NOP2S18 NOP2S18 NOP2S18 NOP2S18
+#define NOP2S22 NOP2S20 NOP2S20 NOP2S20 NOP2S20
+#define NOP2S24 NOP2S22 NOP2S22 NOP2S22 NOP2S22
+#define BIGNOP NOP2S24 NOP2S24 NOP2S10
+// A very big not very interesting function.
+TEXT bar·Bar(SB),0,$0-0
+        BIGNOP
+
+-- bar/bar.go --
+
+package bar
+
+func Bar()
+
+func Bar2() {
+}
diff --git a/src/cmd/go/testdata/script/vendor_complex.txt b/src/cmd/go/testdata/script/vendor_complex.txt
new file mode 100644
index 0000000..290efdb
--- /dev/null
+++ b/src/cmd/go/testdata/script/vendor_complex.txt
@@ -0,0 +1,75 @@
+env GO111MODULE=off
+
+# smoke test for complex build configuration
+go build -o complex.exe complex
+[!cross] [exec:gccgo] go build -compiler=gccgo -o complex.exe complex
+
+-- complex/main.go --
+package main
+
+import (
+	_ "complex/nest/sub/test12"
+	_ "complex/nest/sub/test23"
+	"complex/w"
+	"v"
+)
+
+func main() {
+	println(v.Hello + " " + w.World)
+}
+
+-- complex/nest/sub/test12/p.go --
+package test12
+
+// Check that vendor/v1 is used but vendor/v2 is NOT used (sub/vendor/v2 wins).
+
+import (
+	"v1"
+	"v2"
+)
+
+const x = v1.ComplexNestVendorV1
+const y = v2.ComplexNestSubVendorV2
+
+-- complex/nest/sub/test23/p.go --
+package test23
+
+// Check that vendor/v3 is used but vendor/v2 is NOT used (sub/vendor/v2 wins).
+
+import (
+	"v2"
+	"v3"
+)
+
+const x = v3.ComplexNestVendorV3
+const y = v2.ComplexNestSubVendorV2
+
+-- complex/nest/sub/vendor/v2/v2.go --
+package v2
+
+const ComplexNestSubVendorV2 = true
+
+-- complex/nest/vendor/v1/v1.go --
+package v1
+
+const ComplexNestVendorV1 = true
+
+-- complex/nest/vendor/v2/v2.go --
+package v2
+
+const ComplexNestVendorV2 = true
+
+-- complex/nest/vendor/v3/v3.go --
+package v3
+
+const ComplexNestVendorV3 = true
+
+-- complex/vendor/v/v.go --
+package v
+
+const Hello = "hello"
+
+-- complex/w/w.go --
+package w
+
+const World = "world"
diff --git a/src/cmd/go/testdata/script/vendor_gopath_issue11409.txt b/src/cmd/go/testdata/script/vendor_gopath_issue11409.txt
new file mode 100644
index 0000000..c85f674
--- /dev/null
+++ b/src/cmd/go/testdata/script/vendor_gopath_issue11409.txt
@@ -0,0 +1,52 @@
+[!GOOS:windows] [short] stop 'this test only applies to Windows'
+env GO111MODULE=off
+
+go build run_go.go
+exec ./run_go$GOEXE $GOPATH $GOPATH/src/vend/hello
+stdout 'hello, world'
+
+-- run_go.go --
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+)
+
+func changeVolume(s string, f func(s string) string) string {
+	vol := filepath.VolumeName(s)
+	return f(vol) + s[len(vol):]
+}
+
+func main() {
+	gopath := changeVolume(os.Args[1], strings.ToLower)
+	dir := changeVolume(os.Args[2], strings.ToUpper)
+	cmd := exec.Command("go", "run", "hello.go")
+	cmd.Dir = dir
+	cmd.Env = append(os.Environ(), "GOPATH="+gopath)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	if err := cmd.Run(); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}
+
+-- vend/hello/hello.go --
+package main
+
+import (
+	"fmt"
+	"strings" // really ../vendor/strings
+)
+
+func main() {
+	fmt.Printf("%s\n", strings.Msg)
+}
+-- vend/vendor/strings/msg.go --
+package strings
+
+var Msg = "hello, world"
diff --git a/src/cmd/go/testdata/script/vendor_import.txt b/src/cmd/go/testdata/script/vendor_import.txt
new file mode 100644
index 0000000..c64af2c
--- /dev/null
+++ b/src/cmd/go/testdata/script/vendor_import.txt
@@ -0,0 +1,106 @@
+# Imports
+env GO111MODULE=off
+
+# Pass -e to permit errors (e.g. bad.go, invalid.go)
+go list -f  '{{.ImportPath}} {{.Imports}}' -e 'vend/...' 'vend/vendor/...' 'vend/x/vendor/...'
+cmp stdout want_vendor_imports.txt
+
+-- want_vendor_imports.txt --
+vend [vend/vendor/p r]
+vend/dir1 []
+vend/hello [fmt vend/vendor/strings]
+vend/subdir [vend/vendor/p r]
+vend/x [vend/x/vendor/p vend/vendor/q vend/x/vendor/r vend/dir1 vend/vendor/vend/dir1/dir2]
+vend/x/invalid [vend/x/invalid/vendor/foo]
+vend/vendor/p []
+vend/vendor/q []
+vend/vendor/strings []
+vend/vendor/vend/dir1/dir2 []
+vend/x/vendor/p []
+vend/x/vendor/p/p [notfound]
+vend/x/vendor/r []
+-- vend/bad.go --
+package vend
+
+import _ "r"
+-- vend/dir1/dir1.go --
+package dir1
+-- vend/good.go --
+package vend
+
+import _ "p"
+-- vend/hello/hello.go --
+package main
+
+import (
+	"fmt"
+	"strings" // really ../vendor/strings
+)
+
+func main() {
+	fmt.Printf("%s\n", strings.Msg)
+}
+-- vend/hello/hello_test.go --
+package main
+
+import (
+	"strings" // really ../vendor/strings
+	"testing"
+)
+
+func TestMsgInternal(t *testing.T) {
+	if strings.Msg != "hello, world" {
+		t.Fatalf("unexpected msg: %v", strings.Msg)
+	}
+}
+-- vend/hello/hellox_test.go --
+package main_test
+
+import (
+	"strings" // really ../vendor/strings
+	"testing"
+)
+
+func TestMsgExternal(t *testing.T) {
+	if strings.Msg != "hello, world" {
+		t.Fatalf("unexpected msg: %v", strings.Msg)
+	}
+}
+-- vend/subdir/bad.go --
+package subdir
+
+import _ "r"
+-- vend/subdir/good.go --
+package subdir
+
+import _ "p"
+-- vend/vendor/p/p.go --
+package p
+-- vend/vendor/q/q.go --
+package q
+-- vend/vendor/strings/msg.go --
+package strings
+
+var Msg = "hello, world"
+-- vend/vendor/vend/dir1/dir2/dir2.go --
+package dir2
+-- vend/x/invalid/invalid.go --
+package invalid
+
+import "vend/x/invalid/vendor/foo"
+-- vend/x/vendor/p/p/p.go --
+package p
+
+import _ "notfound"
+-- vend/x/vendor/p/p.go --
+package p
+-- vend/x/vendor/r/r.go --
+package r
+-- vend/x/x.go --
+package x
+
+import _ "p"
+import _ "q"
+import _ "r"
+import _ "vend/dir1"      // not vendored
+import _ "vend/dir1/dir2" // vendored
diff --git a/src/cmd/go/testdata/script/vendor_import_missing.txt b/src/cmd/go/testdata/script/vendor_import_missing.txt
new file mode 100644
index 0000000..8e50dfe
--- /dev/null
+++ b/src/cmd/go/testdata/script/vendor_import_missing.txt
@@ -0,0 +1,7 @@
+# Missing package error message
+! go build vend/x/vendor/p/p
+
+-- vend/x/vendor/p/p/p.go --
+package p
+
+import _ "notfound"
diff --git a/src/cmd/go/testdata/script/vendor_import_wrong.txt b/src/cmd/go/testdata/script/vendor_import_wrong.txt
new file mode 100644
index 0000000..73bf595
--- /dev/null
+++ b/src/cmd/go/testdata/script/vendor_import_wrong.txt
@@ -0,0 +1,20 @@
+# Wrong import path
+env GO111MODULE=off
+! go build vend/x/invalid
+stderr 'must be imported as foo'
+
+env GO111MODULE=
+cd vend/x/invalid
+! go build vend/x/invalid
+stderr 'must be imported as foo'
+
+-- vend/x/invalid/go.mod --
+module vend/x/invalid
+
+go 1.16
+
+-- vend/x/invalid/invalid.go --
+package invalid
+
+import "vend/x/invalid/vendor/foo"
+
diff --git a/src/cmd/go/testdata/script/vendor_internal.txt b/src/cmd/go/testdata/script/vendor_internal.txt
new file mode 100644
index 0000000..4c0f1fa
--- /dev/null
+++ b/src/cmd/go/testdata/script/vendor_internal.txt
@@ -0,0 +1,16 @@
+go build ./vendor/foo.com/internal/bar/a
+
+-- go.mod --
+module example.com/x
+go 1.19
+
+require "foo.com/internal/bar" v1.0.0
+-- vendor/modules.txt --
+# foo.com/internal/bar v1.0.0
+## explicit
+foo.com/internal/bar/a
+-- vendor/foo.com/internal/bar/a/a.go --
+package a
+import _ "foo.com/internal/bar/b"
+-- vendor/foo.com/internal/bar/b/b.go --
+package b
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/vendor_issue12156.txt b/src/cmd/go/testdata/script/vendor_issue12156.txt
new file mode 100644
index 0000000..ac95c6d
--- /dev/null
+++ b/src/cmd/go/testdata/script/vendor_issue12156.txt
@@ -0,0 +1,16 @@
+# Tests issue #12156, a former index out of range panic.
+
+env GO111MODULE=off
+env GOPATH=$WORK/gopath/src/testvendor2 # vendor/x is directly in $GOPATH, not in $GOPATH/src
+cd $WORK/gopath/src/testvendor2/src/p
+
+! go build p.go
+! stderr panic # Make sure it doesn't panic
+stderr 'cannot find package "x"'
+
+-- testvendor2/src/p/p.go --
+package p
+
+import "x"
+-- testvendor2/vendor/x/x.go --
+package x
diff --git a/src/cmd/go/testdata/script/vendor_list_issue11977.txt b/src/cmd/go/testdata/script/vendor_list_issue11977.txt
new file mode 100644
index 0000000..f519175
--- /dev/null
+++ b/src/cmd/go/testdata/script/vendor_list_issue11977.txt
@@ -0,0 +1,17 @@
+[!net:github.com] skip
+[!git] skip
+env GO111MODULE=off
+
+go get github.com/rsc/go-get-issue-11864
+
+go list -f '{{join .TestImports "\n"}}' github.com/rsc/go-get-issue-11864/t
+stdout 'go-get-issue-11864/vendor/vendor.org/p'
+
+go list -f '{{join .XTestImports "\n"}}' github.com/rsc/go-get-issue-11864/tx
+stdout 'go-get-issue-11864/vendor/vendor.org/p'
+
+go list -f '{{join .XTestImports "\n"}}' github.com/rsc/go-get-issue-11864/vendor/vendor.org/tx2
+stdout 'go-get-issue-11864/vendor/vendor.org/tx2'
+
+go list -f '{{join .XTestImports "\n"}}' github.com/rsc/go-get-issue-11864/vendor/vendor.org/tx3
+stdout 'go-get-issue-11864/vendor/vendor.org/tx3'
diff --git a/src/cmd/go/testdata/script/vendor_outside_module.txt b/src/cmd/go/testdata/script/vendor_outside_module.txt
new file mode 100644
index 0000000..3ad4579
--- /dev/null
+++ b/src/cmd/go/testdata/script/vendor_outside_module.txt
@@ -0,0 +1,36 @@
+# baz.go (importing just fmt) works with -mod=mod,  -mod=vendor.
+go build -x -mod=mod my-module/vendor/example.com/another-module/foo/bar/baz.go
+go build -x -mod=readonly my-module/vendor/example.com/another-module/foo/bar/baz.go
+go build -x -mod=vendor my-module/vendor/example.com/another-module/foo/bar/baz.go
+
+# baz_with_outside_dep.go (with a non-std dependency) works with -mod=mod
+# but not with -mod=readonly and -mod=vendor.
+go build -x -mod=mod my-module/vendor/example.com/another-module/foo/bar/baz_with_outside_dep.go
+! go build -x -mod=readonly my-module/vendor/example.com/another-module/foo/bar/baz_with_outside_dep.go
+stderr 'no required module provides package rsc.io/quote'
+! go build -x -mod=vendor my-module/vendor/example.com/another-module/foo/bar/baz_with_outside_dep.go
+stderr 'no required module provides package rsc.io/quote'
+
+-- my-module/go.mod --
+module example.com/my-module
+
+go 1.20
+-- my-module/vendor/example.com/another-module/foo/bar/baz.go --
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("hello, world.")
+}
+-- my-module/vendor/example.com/another-module/foo/bar/baz_with_outside_dep.go --
+package main
+
+import (
+    "fmt"
+    "rsc.io/quote"
+)
+
+func main() {
+	fmt.Println(quote.Hello())
+}
diff --git a/src/cmd/go/testdata/script/vendor_resolve.txt b/src/cmd/go/testdata/script/vendor_resolve.txt
new file mode 100644
index 0000000..bc8cf0a
--- /dev/null
+++ b/src/cmd/go/testdata/script/vendor_resolve.txt
@@ -0,0 +1,21 @@
+env GO111MODULE=off
+! go build p
+stderr 'must be imported as x'
+
+-- p/p.go --
+package p
+
+import (
+	_ "q/y"
+	_ "q/z"
+)
+-- q/vendor/x/x.go --
+package x
+-- q/y/y.go --
+package y
+
+import _ "x"
+-- q/z/z.go --
+package z
+
+import _ "q/vendor/x"
diff --git a/src/cmd/go/testdata/script/vendor_test_issue11864.txt b/src/cmd/go/testdata/script/vendor_test_issue11864.txt
new file mode 100644
index 0000000..9e34811
--- /dev/null
+++ b/src/cmd/go/testdata/script/vendor_test_issue11864.txt
@@ -0,0 +1,12 @@
+[!net:github.com] skip
+[!git] skip
+env GO111MODULE=off
+
+go get github.com/rsc/go-get-issue-11864
+
+# test should work too
+go test github.com/rsc/go-get-issue-11864
+go test github.com/rsc/go-get-issue-11864/t
+
+# external tests should observe internal test exports (golang.org/issue/11977)
+go test github.com/rsc/go-get-issue-11864/vendor/vendor.org/tx2
diff --git a/src/cmd/go/testdata/script/vendor_test_issue14613.txt b/src/cmd/go/testdata/script/vendor_test_issue14613.txt
new file mode 100644
index 0000000..9535fc1
--- /dev/null
+++ b/src/cmd/go/testdata/script/vendor_test_issue14613.txt
@@ -0,0 +1,18 @@
+[!net:github.com] skip
+[!git] skip
+env GO111MODULE=off
+
+cd $GOPATH
+
+go get github.com/clsung/go-vendor-issue-14613
+
+# test folder should work
+go test github.com/clsung/go-vendor-issue-14613
+
+# test with specified _test.go should work too
+cd $GOPATH/src
+go test github.com/clsung/go-vendor-issue-14613/vendor_test.go
+
+# test with imported and not used
+! go test github.com/clsung/go-vendor-issue-14613/vendor/mylibtesttest/myapp/myapp_test.go
+stderr 'imported and not used'
diff --git a/src/cmd/go/testdata/script/version.txt b/src/cmd/go/testdata/script/version.txt
new file mode 100644
index 0000000..0a2ac1e
--- /dev/null
+++ b/src/cmd/go/testdata/script/version.txt
@@ -0,0 +1,92 @@
+# Without arguments, we just print Go's own version.
+go version
+stdout '^go version'
+
+# Flags without files, or paths to missing files, should error.
+! go version missing.exe
+! go version -m
+stderr 'with arguments'
+! go version -v
+stderr 'with arguments'
+
+# Check that 'go version' succeed even when it does not contain Go build info.
+# It should print an error if the file has a known Go binary extension.
+#
+go version empty.txt
+! stdout .
+! stderr .
+go version empty.exe
+stderr 'could not read Go build info'
+go version empty.so
+stderr 'could not read Go build info'
+go version empty.dll
+stderr 'could not read Go build info'
+
+# Neither of the two flags above should be an issue via GOFLAGS.
+env GOFLAGS='-m -v'
+go version
+stdout '^go version'
+env GOFLAGS=
+
+env GO111MODULE=on
+
+# Check that very basic version lookup succeeds.
+go build empty.go
+go version empty$GOEXE
+[cgo] go build -ldflags=-linkmode=external empty.go
+[cgo] go version empty$GOEXE
+
+# Skip the remaining builds if we are running in short mode.
+[short] skip
+
+# Check that 'go version' and 'go version -m' work on a binary built in module mode.
+go get rsc.io/fortune
+go build -o fortune.exe rsc.io/fortune
+go version fortune.exe
+stdout '^fortune.exe: .+'
+go version -m fortune.exe
+stdout -buildmode=exe
+stdout '^\tpath\trsc.io/fortune'
+stdout '^\tmod\trsc.io/fortune\tv1.0.0'
+
+# Check the build info of a binary built from $GOROOT/src/cmd
+go build -o test2json.exe cmd/test2json
+go version -m test2json.exe
+stdout -buildmode=exe
+stdout '^test2json.exe: .+'
+stdout '^\tpath\tcmd/test2json$'
+! stdout 'mod[^e]'
+
+# Repeat the test with -buildmode=pie.
+[!buildmode:pie] stop
+go build -buildmode=pie -o external.exe rsc.io/fortune
+go version external.exe
+stdout '^external.exe: .+'
+go version -m external.exe
+stdout -buildmode=pie
+stdout '^\tpath\trsc.io/fortune'
+stdout '^\tmod\trsc.io/fortune\tv1.0.0'
+
+# Also test PIE with internal linking.
+# currently only supported on linux/amd64, linux/arm64 and windows/amd64.
+[!GOOS:linux] [!GOOS:windows] stop
+[!GOARCH:amd64] [!GOARCH:arm64] stop
+go build -buildmode=pie -ldflags=-linkmode=internal -o internal.exe rsc.io/fortune
+go version internal.exe
+stdout '^internal.exe: .+'
+go version -m internal.exe
+stdout -buildmode=pie
+stdout '^\tpath\trsc.io/fortune'
+stdout '^\tmod\trsc.io/fortune\tv1.0.0'
+
+-- go.mod --
+module m
+
+-- empty.go --
+package main
+func main(){}
+
+-- empty.txt --
+-- empty.exe --
+-- empty.so --
+-- empty.dll --
diff --git a/src/cmd/go/testdata/script/version_build_settings.txt b/src/cmd/go/testdata/script/version_build_settings.txt
new file mode 100644
index 0000000..cd3f5cf
--- /dev/null
+++ b/src/cmd/go/testdata/script/version_build_settings.txt
@@ -0,0 +1,89 @@
+[short] skip
+
+# Compiler name is always added.
+go build
+go version -m m$GOEXE
+stdout '^\tbuild\t-compiler=gc$'
+stdout '^\tbuild\tGOOS='
+stdout '^\tbuild\tGOARCH='
+[GOARCH:amd64] stdout '^\tbuild\tGOAMD64='
+! stdout asmflags|gcflags|ldflags|gccgoflags
+
+# Toolchain flags are added if present.
+# The raw flags are included, with package patterns if specified.
+go build -asmflags=example.com/m=-D=FOO=bar
+go version -m m$GOEXE
+stdout '^\tbuild\t-asmflags=example\.com/m=-D=FOO=bar$'
+
+go build -gcflags=example.com/m=-N
+go version -m m$GOEXE
+stdout '^\tbuild\t-gcflags=example\.com/m=-N$'
+
+go build -ldflags=example.com/m=-w
+go version -m m$GOEXE
+stdout '^\tbuild\t-ldflags=example\.com/m=-w$'
+
+go build -trimpath
+go version -m m$GOEXE
+stdout '\tbuild\t-trimpath=true$'
+
+# gccgoflags are not added when gc is used, and vice versa.
+# TODO: test gccgo.
+go build -gccgoflags=all=UNUSED
+go version -m m$GOEXE
+! stdout gccgoflags
+
+# Build and tool tags are added but not release tags.
+# "race" is included with build tags but not "cgo".
+go build -tags=a,b
+go version -m m$GOEXE
+stdout '^\tbuild\t-tags=a,b$'
+[race] go build -race
+[race] go version -m m$GOEXE
+[race] ! stdout '^\tbuild\t-tags='
+[race] stdout '^\tbuild\t-race=true$'
+
+# CGO flags are separate settings.
+# CGO_ENABLED is always present.
+# Other flags are added if CGO_ENABLED is true.
+env CGO_ENABLED=0
+go build
+go version -m m$GOEXE
+stdout '^\tbuild\tCGO_ENABLED=0$'
+! stdout CGO_CPPFLAGS|CGO_CFLAGS|CGO_CXXFLAGS|CGO_LDFLAGS
+
+[cgo] env CGO_ENABLED=1
+[cgo] env CGO_CPPFLAGS=-DFROM_CPPFLAGS=1
+[cgo] env CGO_CFLAGS=-DFROM_CFLAGS=1
+[cgo] env CGO_CXXFLAGS=-DFROM_CXXFLAGS=1
+[cgo] env CGO_LDFLAGS=-L/extra/dir/does/not/exist
+[cgo] go build '-ldflags=all=-linkmode=external -extldflags=-L/bonus/dir/does/not/exist'
+[cgo] go version -m m$GOEXE
+[cgo] stdout '^\tbuild\t-ldflags="all=-linkmode=external -extldflags=-L/bonus/dir/does/not/exist"$'
+[cgo] stdout '^\tbuild\tCGO_ENABLED=1$'
+[cgo] stdout '^\tbuild\tCGO_CPPFLAGS=-DFROM_CPPFLAGS=1$'
+[cgo] stdout '^\tbuild\tCGO_CFLAGS=-DFROM_CFLAGS=1$'
+[cgo] stdout '^\tbuild\tCGO_CXXFLAGS=-DFROM_CXXFLAGS=1$'
+[cgo] stdout '^\tbuild\tCGO_LDFLAGS=-L/extra/dir/does/not/exist$'
+
+# https://go.dev/issue/52372: a cgo-enabled binary should not be stamped with
+# CGO_ flags that contain paths.
+[cgo] env CGO_ENABLED=1
+[cgo] env CGO_CPPFLAGS=-DFROM_CPPFLAGS=1
+[cgo] env CGO_CFLAGS=-DFROM_CFLAGS=1
+[cgo] env CGO_CXXFLAGS=-DFROM_CXXFLAGS=1
+[cgo] env CGO_LDFLAGS=-L/extra/dir/does/not/exist
+[cgo] go build -trimpath '-ldflags=all=-linkmode=external -extldflags=-L/bonus/dir/does/not/exist'
+[cgo] go version -m m$GOEXE
+[cgo] ! stdout '/extra/dir/does/not/exist'
+[cgo] ! stdout '/bonus/dir/does/not/exist'
+[cgo] stdout '^\tbuild\tCGO_ENABLED=1$'
+
+-- go.mod --
+module example.com/m
+
+go 1.18
+-- m.go --
+package main
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/version_buildvcs_bzr.txt b/src/cmd/go/testdata/script/version_buildvcs_bzr.txt
new file mode 100644
index 0000000..85db9ba
--- /dev/null
+++ b/src/cmd/go/testdata/script/version_buildvcs_bzr.txt
@@ -0,0 +1,107 @@
+# This test checks that VCS information is stamped into Go binaries by default,
+# controlled with -buildvcs. This test focuses on Bazaar specifics.
+# The Git test covers common functionality.
+
+[!exec:bzr] skip
+[short] skip
+env GOBIN=$WORK/gopath/bin
+env oldpath=$PATH
+env HOME=$WORK
+cd repo/a
+exec bzr whoami 'J.R. Gopher <gopher@golang.org>'
+
+# If there's no local repository, there's no VCS info.
+go install
+go version -m $GOBIN/a$GOEXE
+! stdout bzrrevision
+rm $GOBIN/a$GOEXE
+
+# If there is a repository, but it can't be used for some reason,
+# there should be an error. It should hint about -buildvcs=false.
+cd ..
+mkdir .bzr
+env PATH=$WORK${/}fakebin${:}$oldpath
+chmod 0755 $WORK/fakebin/bzr
+! exec bzr help
+cd a
+! go install
+stderr '^error obtaining VCS status: exit status 1\n\tUse -buildvcs=false to disable VCS stamping.$'
+rm $GOBIN/a$GOEXE
+cd ..
+env PATH=$oldpath
+rm .bzr
+
+# If there is an empty repository in a parent directory, only "modified" is tagged.
+exec bzr init
+cd a
+go install
+go version -m $GOBIN/a$GOEXE
+stdout '^\tbuild\tvcs=bzr$'
+! stdout vcs.revision
+! stdout vcs.time
+stdout '^\tbuild\tvcs.modified=true$'
+cd ..
+
+# Revision and commit time are tagged for repositories with commits.
+exec bzr add a README
+exec bzr commit -m 'initial commit'
+cd a
+go install
+go version -m $GOBIN/a$GOEXE
+stdout '^\tbuild\tvcs=bzr$'
+stdout '^\tbuild\tvcs.revision='
+stdout '^\tbuild\tvcs.time='
+stdout '^\tbuild\tvcs.modified=false$'
+rm $GOBIN/a$GOEXE
+
+# Building an earlier commit should still build clean.
+cp ../../outside/empty.txt ../NEWS
+exec bzr add ../NEWS
+exec bzr commit -m 'add NEWS'
+exec bzr update -r1
+go install
+go version -m $GOBIN/a$GOEXE
+stdout '^\tbuild\tvcs=bzr$'
+stdout '^\tbuild\tvcs.revision='
+stdout '^\tbuild\tvcs.time='
+stdout '^\tbuild\tvcs.modified=false$'
+
+# Building with -buildvcs=false suppresses the info.
+go install -buildvcs=false
+go version -m $GOBIN/a$GOEXE
+! stdout vcs.revision
+rm $GOBIN/a$GOEXE
+
+# An untracked file is shown as modified, even if it isn't part of the build.
+cp ../../outside/empty.txt .
+go install
+go version -m $GOBIN/a$GOEXE
+stdout '^\tbuild\tvcs.modified=true$'
+rm empty.txt
+rm $GOBIN/a$GOEXE
+
+# An edited file is shown as modified, even if it isn't part of the build.
+cp ../../outside/empty.txt ../README
+go install
+go version -m $GOBIN/a$GOEXE
+stdout '^\tbuild\tvcs.modified=true$'
+exec bzr revert ../README
+rm $GOBIN/a$GOEXE
+
+-- $WORK/fakebin/bzr --
+#!/bin/sh
+exit 1
+-- $WORK/fakebin/bzr.bat --
+exit 1
+-- repo/README --
+Far out in the uncharted backwaters of the unfashionable end of the western
+spiral arm of the Galaxy lies a small, unregarded yellow sun.
+-- repo/a/go.mod --
+module example.com/a
+
+go 1.18
+-- repo/a/a.go --
+package main
+
+func main() {}
+-- outside/empty.txt --
diff --git a/src/cmd/go/testdata/script/version_buildvcs_fossil.txt b/src/cmd/go/testdata/script/version_buildvcs_fossil.txt
new file mode 100644
index 0000000..bd6b89d
--- /dev/null
+++ b/src/cmd/go/testdata/script/version_buildvcs_fossil.txt
@@ -0,0 +1,94 @@
+# This test checks that VCS information is stamped into Go binaries by default,
+# controlled with -buildvcs. This test focuses on Fossil specifics.
+# The Git test covers common functionality.
+
+# "fossil" is the Fossil file server on Plan 9.
+[GOOS:plan9] skip
+[!exec:fossil] skip
+[short] skip
+env GOBIN=$WORK/gopath/bin
+env oldpath=$PATH
+env HOME=$WORK
+env USER=gopher
+[!GOOS:windows] env fslckout=.fslckout
+[GOOS:windows] env fslckout=_FOSSIL_
+exec pwd
+exec fossil init repo.fossil
+cd repo/a
+
+# If there's no local repository, there's no VCS info.
+go install
+go version -m $GOBIN/a$GOEXE
+! stdout vcs.revision
+rm $GOBIN/a$GOEXE
+
+# If there is a repository, but it can't be used for some reason,
+# there should be an error. It should hint about -buildvcs=false.
+cd ..
+mv fslckout $fslckout
+env PATH=$WORK${/}fakebin${:}$oldpath
+chmod 0755 $WORK/fakebin/fossil
+! exec fossil help
+cd a
+! go install
+stderr '^error obtaining VCS status: exit status 1\n\tUse -buildvcs=false to disable VCS stamping.$'
+rm $GOBIN/a$GOEXE
+cd ..
+env PATH=$oldpath
+rm $fslckout
+
+# Revision and commit time are tagged for repositories with commits.
+exec fossil open ../repo.fossil -f
+exec fossil add a README
+exec fossil commit -m 'initial commit'
+cd a
+go install
+go version -m $GOBIN/a$GOEXE
+stdout '^\tbuild\tvcs=fossil\n'
+stdout '^\tbuild\tvcs.revision='
+stdout '^\tbuild\tvcs.time='
+stdout '^\tbuild\tvcs.modified=false$'
+rm $GOBIN/a$GOEXE
+
+# Building with -buildvcs=false suppresses the info.
+go install -buildvcs=false
+go version -m $GOBIN/a$GOEXE
+! stdout vcs.revision
+rm $GOBIN/a$GOEXE
+
+# An untracked file is shown as modified, even if it isn't part of the build.
+cp ../../outside/empty.txt .
+go install
+go version -m $GOBIN/a$GOEXE
+stdout '^\tbuild\tvcs=fossil\n'
+stdout '^\tbuild\tvcs.modified=true$'
+rm empty.txt
+rm $GOBIN/a$GOEXE
+
+# An edited file is shown as modified, even if it isn't part of the build.
+cp ../../outside/empty.txt ../README
+go install
+go version -m $GOBIN/a$GOEXE
+stdout '^\tbuild\tvcs=fossil\n'
+stdout '^\tbuild\tvcs.modified=true$'
+exec fossil revert ../README
+rm $GOBIN/a$GOEXE
+
+-- $WORK/fakebin/fossil --
+#!/bin/sh
+exit 1
+-- $WORK/fakebin/fossil.bat --
+exit 1
+-- repo/README --
+Far out in the uncharted backwaters of the unfashionable end of the western
+spiral arm of the Galaxy lies a small, unregarded yellow sun.
+-- repo/fslckout --
+-- repo/a/go.mod --
+module example.com/a
+
+go 1.18
+-- repo/a/a.go --
+package main
+
+func main() {}
+-- outside/empty.txt --
diff --git a/src/cmd/go/testdata/script/version_buildvcs_git.txt b/src/cmd/go/testdata/script/version_buildvcs_git.txt
new file mode 100644
index 0000000..680e492
--- /dev/null
+++ b/src/cmd/go/testdata/script/version_buildvcs_git.txt
@@ -0,0 +1,182 @@
+# This test checks that VCS information is stamped into Go binaries by default,
+# controlled with -buildvcs. This test focuses on Git. Other tests focus on
+# other VCS tools but may not cover common functionality.
+
+[!git] skip
+[short] skip
+env GOBIN=$WORK/gopath/bin
+env oldpath=$PATH
+cd repo/a
+
+# If there's no local repository, there's no VCS info.
+go install
+go version -m $GOBIN/a$GOEXE
+! stdout vcs.revision
+rm $GOBIN/a$GOEXE
+
+# If there's an orphan .git file left by a git submodule, it's not a git
+# repository, and there's no VCS info.
+cd ../gitsubmodule
+go install
+go version -m $GOBIN/gitsubmodule$GOEXE
+! stdout vcs.revision
+rm $GOBIN/gitsubmodule$GOEXE
+
+# If there is a repository, but it can't be used for some reason,
+# there should be an error. It should hint about -buildvcs=false.
+# Also ensure that multiple errors are collected by "go list -e".
+cd ..
+mkdir .git
+env PATH=$WORK${/}fakebin${:}$oldpath
+chmod 0755 $WORK/fakebin/git
+! exec git help
+cd a
+! go install
+stderr '^error obtaining VCS status: exit status 1\n\tUse -buildvcs=false to disable VCS stamping.$'
+go list -e -f '{{.ImportPath}}: {{.Error}}' ./...
+stdout -count=1 '^example\.com/a: error obtaining VCS status'
+stdout -count=1 '^example\.com/a/library: <nil>'
+stdout -count=1 '^example\.com/a/othermain: error obtaining VCS status'
+cd ..
+env PATH=$oldpath
+rm .git
+
+# If there is an empty repository in a parent directory, only "uncommitted" is tagged.
+exec git init
+exec git config user.email gopher@golang.org
+exec git config user.name 'J.R. Gopher'
+cd a
+go install
+go version -m $GOBIN/a$GOEXE
+stdout '^\tbuild\tvcs=git$'
+stdout '^\tbuild\tvcs.modified=true$'
+! stdout vcs.revision
+! stdout vcs.time
+rm $GOBIN/a$GOEXE
+
+# Revision and commit time are tagged for repositories with commits.
+exec git add -A
+exec git commit -m 'initial commit'
+go install
+go version -m $GOBIN/a$GOEXE
+stdout '^\tbuild\tvcs.revision='
+stdout '^\tbuild\tvcs.time='
+stdout '^\tbuild\tvcs.modified=false$'
+rm $GOBIN/a$GOEXE
+
+# Building with -buildvcs=false suppresses the info.
+go install -buildvcs=false
+go version -m $GOBIN/a$GOEXE
+! stdout vcs.revision
+rm $GOBIN/a$GOEXE
+
+# An untracked file is shown as uncommitted, even if it isn't part of the build.
+cp ../../outside/empty.txt .
+go install
+go version -m $GOBIN/a$GOEXE
+stdout '^\tbuild\tvcs.modified=true$'
+rm empty.txt
+rm $GOBIN/a$GOEXE
+
+# An edited file is shown as uncommitted, even if it isn't part of the build.
+cp ../../outside/empty.txt ../README
+go install
+go version -m $GOBIN/a$GOEXE
+stdout '^\tbuild\tvcs.modified=true$'
+exec git checkout ../README
+rm $GOBIN/a$GOEXE
+
+# If the build doesn't include any packages from the repository,
+# there should be no VCS info.
+go install example.com/cmd/a@v1.0.0
+go version -m $GOBIN/a$GOEXE
+! stdout vcs.revision
+rm $GOBIN/a$GOEXE
+
+go mod edit -require=example.com/c@v0.0.0
+go mod edit -replace=example.com/c@v0.0.0=../../outside/c
+go install example.com/c
+go version -m $GOBIN/c$GOEXE
+! stdout vcs.revision
+rm $GOBIN/c$GOEXE
+exec git checkout go.mod
+
+# If the build depends on a package in the repository, but it's not in the
+# main module, there should be no VCS info.
+go mod edit -require=example.com/b@v0.0.0
+go mod edit -replace=example.com/b@v0.0.0=../b
+go mod edit -require=example.com/d@v0.0.0
+go mod edit -replace=example.com/d@v0.0.0=../../outside/d
+go install example.com/d
+go version -m $GOBIN/d$GOEXE
+! stdout vcs.revision
+exec git checkout go.mod
+rm $GOBIN/d$GOEXE
+
+# If we're loading multiple main packages,
+# but they share the same VCS repository,
+# we only need to execute VCS status commands once.
+go list -x ./...
+stdout -count=3 '^example.com'
+stderr -count=1 '^git status'
+stderr -count=1 '^git -c log.showsignature=false show'
+
+-- $WORK/fakebin/git --
+#!/bin/sh
+exit 1
+-- $WORK/fakebin/git.bat --
+exit 1
+-- repo/README --
+Far out in the uncharted backwaters of the unfashionable end of the western
+spiral arm of the Galaxy lies a small, unregarded yellow sun.
+-- repo/a/go.mod --
+module example.com/a
+
+go 1.18
+-- repo/a/a.go --
+package main
+
+func main() {}
+-- repo/a/library/f.go --
+package library
+-- repo/a/othermain/f.go --
+package main
+
+func main() {}
+-- repo/b/go.mod --
+module example.com/b
+
+go 1.18
+-- repo/b/b.go --
+package b
+-- repo/gitsubmodule/.git --
+gitdir: ../.git/modules/gitsubmodule
+-- repo/gitsubmodule/go.mod --
+module example.com/gitsubmodule
+
+go 1.18
+-- repo/gitsubmodule/main.go --
+package main
+
+func main() {}
+-- outside/empty.txt --
+-- outside/c/go.mod --
+module example.com/c
+
+go 1.18
+-- outside/c/main.go --
+package main
+
+func main() {}
+-- outside/d/go.mod --
+module example.com/d
+
+go 1.18
+
+require example.com/b v0.0.0
+-- outside/d/main.go --
+package main
+
+import _ "example.com/b"
+
+func main() {}
diff --git a/src/cmd/go/testdata/script/version_buildvcs_hg.txt b/src/cmd/go/testdata/script/version_buildvcs_hg.txt
new file mode 100644
index 0000000..fbbd886
--- /dev/null
+++ b/src/cmd/go/testdata/script/version_buildvcs_hg.txt
@@ -0,0 +1,91 @@
+# This test checks that VCS information is stamped into Go binaries by default,
+# controlled with -buildvcs. This test focuses on Mercurial specifics.
+# The Git test covers common functionality.
+
+[!exec:hg] skip
+[short] skip
+env GOBIN=$WORK/gopath/bin
+env oldpath=$PATH
+cd repo/a
+
+# If there's no local repository, there's no VCS info.
+go install
+go version -m $GOBIN/a$GOEXE
+! stdout hgrevision
+rm $GOBIN/a$GOEXE
+
+# If there is a repository, but it can't be used for some reason,
+# there should be an error. It should hint about -buildvcs=false.
+cd ..
+mkdir .hg
+env PATH=$WORK${/}fakebin${:}$oldpath
+chmod 0755 $WORK/fakebin/hg
+! exec hg help
+cd a
+! go install
+stderr '^error obtaining VCS status: exit status 1\n\tUse -buildvcs=false to disable VCS stamping.$'
+rm $GOBIN/a$GOEXE
+cd ..
+env PATH=$oldpath
+rm .hg
+
+# If there is an empty repository in a parent directory, only "uncommitted" is tagged.
+exec hg init
+cd a
+go install
+go version -m $GOBIN/a$GOEXE
+! stdout vcs.revision
+! stdout vcs.time
+stdout '^\tbuild\tvcs.modified=true$'
+cd ..
+
+# Revision and commit time are tagged for repositories with commits.
+exec hg add a README
+exec hg commit -m 'initial commit'
+cd a
+go install
+go version -m $GOBIN/a$GOEXE
+stdout '^\tbuild\tvcs.revision='
+stdout '^\tbuild\tvcs.time='
+stdout '^\tbuild\tvcs.modified=false$'
+rm $GOBIN/a$GOEXE
+
+# Building with -buildvcs=false suppresses the info.
+go install -buildvcs=false
+go version -m $GOBIN/a$GOEXE
+! stdout hgrevision
+rm $GOBIN/a$GOEXE
+
+# An untracked file is shown as uncommitted, even if it isn't part of the build.
+cp ../../outside/empty.txt .
+go install
+go version -m $GOBIN/a$GOEXE
+stdout '^\tbuild\tvcs.modified=true$'
+rm empty.txt
+rm $GOBIN/a$GOEXE
+
+# An edited file is shown as uncommitted, even if it isn't part of the build.
+cp ../../outside/empty.txt ../README
+go install
+go version -m $GOBIN/a$GOEXE
+stdout '^\tbuild\tvcs.modified=true$'
+exec hg revert ../README
+rm $GOBIN/a$GOEXE
+
+-- $WORK/fakebin/hg --
+#!/bin/sh
+exit 1
+-- $WORK/fakebin/hg.bat --
+exit 1
+-- repo/README --
+Far out in the uncharted backwaters of the unfashionable end of the western
+spiral arm of the Galaxy lies a small, unregarded yellow sun.
+-- repo/a/go.mod --
+module example.com/a
+
+go 1.18
+-- repo/a/a.go --
+package main
+
+func main() {}
+-- outside/empty.txt --
diff --git a/src/cmd/go/testdata/script/version_buildvcs_nested.txt b/src/cmd/go/testdata/script/version_buildvcs_nested.txt
new file mode 100644
index 0000000..6dab847
--- /dev/null
+++ b/src/cmd/go/testdata/script/version_buildvcs_nested.txt
@@ -0,0 +1,51 @@
+[!git] skip
+[!exec:hg] skip
+[short] skip
+env GOFLAGS='-n -buildvcs'
+
+# Create a root module in a root Git repository.
+mkdir root
+cd root
+go mod init example.com/root
+exec git init
+
+# Nesting repositories in parent directories are ignored, as the current
+# directory main package, and containing main module are in the same repository.
+# This is an error in GOPATH mode (to prevent VCS injection), but for modules,
+# we assume users have control over repositories they've checked out.
+mkdir hgsub
+cd hgsub
+exec hg init
+cp ../../main.go main.go
+! go build
+stderr '^error obtaining VCS status: main module is in repository ".*root" but current directory is in repository ".*hgsub"$'
+stderr '^\tUse -buildvcs=false to disable VCS stamping.$'
+go build -buildvcs=false
+go mod init example.com/root/hgsub
+go build
+cd ..
+
+# It's an error to build a package from a nested Git repository if the package
+# is in a separate repository from the current directory or from the module
+# root directory.
+mkdir gitsub
+cd gitsub
+exec git init
+exec git config user.name 'J.R.Gopher'
+exec git config user.email 'gopher@golang.org'
+cp ../../main.go main.go
+! go build
+stderr '^error obtaining VCS status: main module is in repository ".*root" but current directory is in repository ".*gitsub"$'
+go build -buildvcs=false
+go mod init example.com/root/gitsub
+exec git commit --allow-empty -m empty # status commands fail without this
+go build
+rm go.mod
+cd ..
+! go build ./gitsub
+stderr '^error obtaining VCS status: main package is in repository ".*gitsub" but current directory is in repository ".*root"$'
+go build -buildvcs=false -o=gitsub${/} ./gitsub
+
+-- main.go --
+package main
+func main() {}
diff --git a/src/cmd/go/testdata/script/version_cshared.txt b/src/cmd/go/testdata/script/version_cshared.txt
new file mode 100644
index 0000000..29e21fc
--- /dev/null
+++ b/src/cmd/go/testdata/script/version_cshared.txt
@@ -0,0 +1,19 @@
+[short] skip
+[!buildmode:c-shared] stop
+
+env GO111MODULE=on
+
+go get rsc.io/fortune
+go build -buildmode=c-shared -o external.so rsc.io/fortune
+go version external.so
+stdout '^external.so: .+'
+go version -m external.so
+stdout '^\tpath\trsc.io/fortune'
+stdout '^\tmod\trsc.io/fortune\tv1.0.0'
+
+-- go.mod --
+module m
+
+-- empty.go --
+package main
+func main(){}
diff --git a/src/cmd/go/testdata/script/version_gc_sections.txt b/src/cmd/go/testdata/script/version_gc_sections.txt
new file mode 100644
index 0000000..4bda23b
--- /dev/null
+++ b/src/cmd/go/testdata/script/version_gc_sections.txt
@@ -0,0 +1,24 @@
+# This test checks that external linking with --gc-sections does not strip version information.
+
+[short] skip
+[!cgo] skip
+[GOOS:aix] skip  # no --gc-sections
+[GOOS:darwin] skip  # no --gc-sections
+
+go build -ldflags='-linkmode=external -extldflags=-Wl,--gc-sections'
+go version hello$GOEXE
+! stdout 'not a Go executable'
+! stderr 'not a Go executable'
+
+-- go.mod --
+module hello
+-- hello.go --
+package main
+
+/*
+*/
+import "C"
+
+func main() {
+	println("hello")
+}
diff --git a/src/cmd/go/testdata/script/version_goexperiment.txt b/src/cmd/go/testdata/script/version_goexperiment.txt
new file mode 100644
index 0000000..4b165eb
--- /dev/null
+++ b/src/cmd/go/testdata/script/version_goexperiment.txt
@@ -0,0 +1,16 @@
+# Test that experiments appear in "go version <binary>"
+
+# This test requires rebuilding the runtime, which takes a while.
+[short] skip
+
+env GOEXPERIMENT=fieldtrack
+go build -o main$GOEXE version.go
+go version main$GOEXE
+stdout 'X:fieldtrack$'
+exec ./main$GOEXE
+stderr 'X:fieldtrack$'
+
+-- version.go --
+package main
+import "runtime"
+func main() { println(runtime.Version()) }
diff --git a/src/cmd/go/testdata/script/version_replace.txt b/src/cmd/go/testdata/script/version_replace.txt
new file mode 100644
index 0000000..82b8504
--- /dev/null
+++ b/src/cmd/go/testdata/script/version_replace.txt
@@ -0,0 +1,33 @@
+[short] skip
+
+go mod download example.com/printversion@v0.1.0 example.com/printversion@v1.0.0
+go get example.com/printversion@v0.1.0
+go install example.com/printversion
+
+go run example.com/printversion
+cmp stdout out.txt
+
+go version -m $GOPATH/bin/printversion$GOEXE
+stdout '^.*[/\\]bin[/\\]printversion'$GOEXE': .*$'
+stdout '^	path	example.com/printversion$'
+stdout '^	mod	example.com/printversion	v0.1.0$'
+stdout '^	=>	example.com/printversion	v1.0.0	h1:.*$'
+stdout '^	dep	example.com/version	v1.0.0$'
+stdout '^	=>	example.com/version	v1.0.1	h1:.*$'
+
+-- go.mod --
+module golang.org/issue/37392
+go 1.14
+require (
+	example.com/printversion v0.1.0
+)
+replace (
+	example.com/printversion => example.com/printversion v1.0.0
+	example.com/version v1.0.0 => example.com/version v1.0.1
+)
+-- out.txt --
+path is example.com/printversion
+main is example.com/printversion v0.1.0
+	(replaced by example.com/printversion v1.0.0)
+using example.com/version v1.0.0
+	(replaced by example.com/version v1.0.1)
diff --git a/src/cmd/go/testdata/script/vet.txt b/src/cmd/go/testdata/script/vet.txt
new file mode 100644
index 0000000..6573ae3
--- /dev/null
+++ b/src/cmd/go/testdata/script/vet.txt
@@ -0,0 +1,62 @@
+# Package with external tests
+! go vet m/vetpkg
+stderr 'Printf'
+
+# With tags
+! go vet -tags tagtest m/vetpkg
+stderr 'c\.go.*Printf'
+
+# With flags on
+! go vet -printf m/vetpkg
+stderr 'Printf'
+
+# With flags off
+go vet -printf=false m/vetpkg
+! stderr .
+
+# With only test files (tests issue #23395)
+go vet m/onlytest
+! stderr .
+
+# With only cgo files (tests issue #24193)
+[!cgo] skip
+[short] skip
+go vet m/onlycgo
+! stderr .
+
+-- go.mod --
+module m
+
+go 1.16
+-- vetpkg/a_test.go --
+package p_test
+-- vetpkg/b.go --
+package p
+
+import "fmt"
+
+func f() {
+	fmt.Printf("%d")
+}
+-- vetpkg/c.go --
+// +build tagtest
+
+package p
+
+import "fmt"
+
+func g() {
+	fmt.Printf("%d", 3, 4)
+}
+-- onlytest/p_test.go --
+package p
+
+import "testing"
+
+func TestMe(*testing.T) {}
+-- onlycgo/p.go --
+package p
+
+import "C"
+
+func F() {}
diff --git a/src/cmd/go/testdata/script/vet_asm.txt b/src/cmd/go/testdata/script/vet_asm.txt
new file mode 100644
index 0000000..ae2db97
--- /dev/null
+++ b/src/cmd/go/testdata/script/vet_asm.txt
@@ -0,0 +1,32 @@
+env GO111MODULE=off
+
+# Issue 27665. Verify that "go vet" analyzes non-Go files.
+
+[!GOARCH:amd64] skip
+! go vet -asmdecl a
+stderr 'f: invalid MOVW of x'
+
+# -c flag shows context
+! go vet -c=2 -asmdecl a
+stderr '...invalid MOVW...'
+stderr '1	.*TEXT'
+stderr '2		MOVW'
+stderr '3		RET'
+stderr '4'
+
+# -json causes success, even with diagnostics and errors.
+go vet -json -asmdecl a
+stderr '"a": {'
+stderr   '"asmdecl":'
+stderr     '"posn": ".*asm.s:2:1",'
+stderr     '"message": ".*invalid MOVW.*"'
+
+-- a/a.go --
+package a
+
+func f(x int8)
+
+-- a/asm.s --
+TEXT ·f(SB),0,$0-1
+	MOVW	x+0(FP), AX
+	RET
diff --git a/src/cmd/go/testdata/script/vet_deps.txt b/src/cmd/go/testdata/script/vet_deps.txt
new file mode 100644
index 0000000..b2a8f16
--- /dev/null
+++ b/src/cmd/go/testdata/script/vet_deps.txt
@@ -0,0 +1,34 @@
+env GO111MODULE=off
+
+# Issue 30296. Verify that "go vet" uses only immediate dependencies.
+
+# First run fills the cache.
+go vet a
+
+go vet -x a
+! stderr 'transitive'
+
+-- a/a.go --
+package a
+
+import "b"
+
+func F() {
+	b.F()
+}
+
+-- b/b.go --
+package b
+
+import "transitive"
+
+func F() {
+	transitive.F()
+}
+
+-- transitive/c.go --
+package transitive
+
+func F() {
+}
+
diff --git a/src/cmd/go/testdata/script/vet_flags.txt b/src/cmd/go/testdata/script/vet_flags.txt
new file mode 100644
index 0000000..73f4e41
--- /dev/null
+++ b/src/cmd/go/testdata/script/vet_flags.txt
@@ -0,0 +1,91 @@
+env GO111MODULE=on
+
+# Issue 35837: "go vet -<analyzer> <std package>" should use the requested
+# analyzers, not the default analyzers for 'go test'.
+go vet -n -buildtags=false runtime
+stderr '-buildtags=false'
+! stderr '-unsafeptr=false'
+
+# Issue 37030: "go vet <std package>" without other flags should disable the
+# unsafeptr check by default.
+go vet -n runtime
+stderr '-unsafeptr=false'
+! stderr '-unreachable=false'
+
+# However, it should be enabled if requested explicitly.
+go vet -n -unsafeptr runtime
+stderr '-unsafeptr'
+! stderr '-unsafeptr=false'
+
+# -unreachable is disabled during test but on during plain vet.
+go test -n runtime
+stderr '-unreachable=false'
+
+# A flag terminator should be allowed before the package list.
+go vet -n -- .
+
+[short] stop
+
+# Analyzer flags should be included from GOFLAGS, and should override
+# the defaults.
+go vet .
+env GOFLAGS='-tags=buggy'
+! go vet .
+stderr 'possible Printf formatting directive'
+
+# Enabling one analyzer in GOFLAGS should disable the rest implicitly...
+env GOFLAGS='-tags=buggy -unsafeptr'
+go vet .
+
+# ...but enabling one on the command line should not disable the analyzers
+# enabled via GOFLAGS.
+env GOFLAGS='-tags=buggy -printf'
+! go vet -unsafeptr
+stderr 'possible Printf formatting directive'
+
+# Analyzer flags don't exist unless we're running 'go vet',
+# and we shouldn't run the vet tool to discover them otherwise.
+# (Maybe someday we'll hard-code the analyzer flags for the default vet
+# tool to make this work, but not right now.)
+env GOFLAGS='-unsafeptr'
+! go list .
+stderr 'go: parsing \$GOFLAGS: unknown flag -unsafeptr'
+env GOFLAGS=
+
+# "go test" on a user package should by default enable an explicit list of analyzers.
+go test -n -run=none .
+stderr '[/\\]vet'$GOEXE'["]? .* -errorsas .* ["]?\$WORK[/\\][^ ]*[/\\]vet\.cfg'
+
+# An explicitly-empty -vet argument should imply the default analyzers.
+go test -n -vet= -run=none .
+stderr '[/\\]vet'$GOEXE'["]? .* -errorsas .* ["]?\$WORK[/\\][^ ]*[/\\]vet\.cfg'
+
+# "go test" on a standard package should by default disable an explicit list.
+go test -n -run=none encoding/binary
+stderr '[/\\]vet'$GOEXE'["]? -unsafeptr=false -unreachable=false ["]?\$WORK[/\\][^ ]*[/\\]vet\.cfg'
+
+go test -n -vet= -run=none encoding/binary
+stderr '[/\\]vet'$GOEXE'["]? -unsafeptr=false -unreachable=false ["]?\$WORK[/\\][^ ]*[/\\]vet\.cfg'
+
+# Both should allow users to override via the -vet flag.
+go test -n -vet=unreachable -run=none .
+stderr '[/\\]vet'$GOEXE'["]? -unreachable ["]?\$WORK[/\\][^ ]*[/\\]vet\.cfg'
+go test -n -vet=unreachable -run=none encoding/binary
+stderr '[/\\]vet'$GOEXE'["]? -unreachable ["]?\$WORK[/\\][^ ]*[/\\]vet\.cfg'
+
+-- go.mod --
+module example.com/x
+-- x.go --
+package x
+-- x_test.go --
+package x
+-- x_tagged.go --
+// +build buggy
+
+package x
+
+import "fmt"
+
+func init() {
+	fmt.Sprint("%s") // oops!
+}
diff --git a/src/cmd/go/testdata/script/vet_internal.txt b/src/cmd/go/testdata/script/vet_internal.txt
new file mode 100644
index 0000000..85f7093
--- /dev/null
+++ b/src/cmd/go/testdata/script/vet_internal.txt
@@ -0,0 +1,71 @@
+env GO111MODULE=off
+
+# Issue 36173. Verify that "go vet" prints line numbers on load errors.
+
+! go vet a/a.go
+stderr '^package command-line-arguments\n\ta[/\\]a.go:5:3: use of internal package'
+
+! go vet a/a_test.go
+stderr '^package command-line-arguments \(test\)\n\ta[/\\]a_test.go:4:3: use of internal package'
+
+! go vet a
+stderr '^package a\n\ta[/\\]a.go:5:3: use of internal package'
+
+go vet b/b.go
+! stderr 'use of internal package'
+
+! go vet b/b_test.go
+stderr '^package command-line-arguments \(test\)\n\tb[/\\]b_test.go:4:3: use of internal package'
+
+! go vet depends-on-a/depends-on-a.go
+stderr '^package command-line-arguments\n\timports a\n\ta[/\\]a.go:5:3: use of internal package'
+
+! go vet depends-on-a/depends-on-a_test.go
+stderr '^package command-line-arguments \(test\)\n\timports a\n\ta[/\\]a.go:5:3: use of internal package a/x/internal/y not allowed'
+
+! go vet depends-on-a
+stderr '^package depends-on-a\n\timports a\n\ta[/\\]a.go:5:3: use of internal package'
+
+-- a/a.go --
+// A package with bad imports in both src and test
+package a
+
+import (
+  _ "a/x/internal/y"
+)
+
+-- a/a_test.go --
+package a
+
+import (
+  _ "a/x/internal/y"
+)
+
+-- b/b.go --
+// A package with a bad import in test only
+package b
+
+-- b/b_test.go --
+package b
+
+import (
+  _ "a/x/internal/y"
+)
+
+-- depends-on-a/depends-on-a.go --
+// A package that depends on a package with a bad import
+package depends
+
+import (
+  _ "a"
+)
+
+-- depends-on-a/depends-on-a_test.go --
+package depends
+
+import (
+  _ "a"
+)
+
+-- a/x/internal/y/y.go --
+package y
diff --git a/src/cmd/go/testdata/script/work.txt b/src/cmd/go/testdata/script/work.txt
new file mode 100644
index 0000000..e229ab6
--- /dev/null
+++ b/src/cmd/go/testdata/script/work.txt
@@ -0,0 +1,154 @@
+! go work init doesnotexist
+stderr 'go: directory doesnotexist does not exist'
+go env GOWORK
+! stdout .
+
+go work init ./a ./b
+cmpenv go.work go.work.want
+go env GOWORK
+stdout '^'$WORK'(\\|/)gopath(\\|/)src(\\|/)go.work$'
+
+! go run  example.com/b
+stderr 'a(\\|/)a.go:4:8: no required module provides package rsc.io/quote; to add it:\n\tcd '$WORK(\\|/)gopath(\\|/)src(\\|/)a'\n\tgo get rsc.io/quote'
+cd a
+go get rsc.io/quote
+cat go.mod
+go env GOMOD # go env GOMOD reports the module in a single module context
+stdout $GOPATH(\\|/)src(\\|/)a(\\|/)go.mod
+cd ..
+go run example.com/b
+stdout 'Hello, world.'
+
+# And try from a different directory
+cd c
+go run  example.com/b
+stdout 'Hello, world.'
+cd $GOPATH/src
+
+go list all # all includes both modules
+stdout 'example.com/a'
+stdout 'example.com/b'
+
+# -mod can only be set to readonly in workspace mode
+go list -mod=readonly all
+! go list -mod=mod all
+stderr '^go: -mod may only be set to readonly when in workspace mode'
+env GOWORK=off
+go list -mod=mod all
+env GOWORK=
+
+# Test that duplicates in the use list return an error
+cp go.work go.work.backup
+cp go.work.dup go.work
+! go run example.com/b
+stderr 'reading go.work: path .* appears multiple times in workspace'
+cp go.work.backup go.work
+
+cp go.work.d go.work
+go work use # update go version
+go run example.com/d
+
+# Test that we don't run into "newRequirements called with unsorted roots"
+# panic with unsorted main modules.
+cp go.work.backwards go.work
+go work use # update go version
+go run example.com/d
+
+# Test that command-line-arguments work inside and outside modules.
+# This exercises the code that determines which module command-line-arguments
+# belongs to.
+go list ./b/main.go
+env GOWORK=off
+go build -n -o foo foo.go
+env GOWORK=
+go build -n -o foo foo.go
+
+-- go.work.dup --
+go 1.18
+
+use (
+	a
+	b
+	../src/a
+)
+-- go.work.want --
+go $goversion
+
+use (
+	./a
+	./b
+)
+-- go.work.d --
+go 1.18
+
+use (
+	a
+	b
+	d
+)
+-- a/go.mod --
+
+module example.com/a
+
+-- a/a.go --
+package a
+
+import "fmt"
+import "rsc.io/quote"
+
+func HelloFromA() {
+	fmt.Println(quote.Hello())
+}
+
+-- b/go.mod --
+
+module example.com/b
+
+-- b/main.go --
+package main
+
+import "example.com/a"
+
+func main() {
+	a.HelloFromA()
+}
+-- b/lib/hello.go --
+package lib
+
+import "example.com/a"
+
+func Hello() {
+	a.HelloFromA()
+}
+
+-- c/README --
+Create this directory so we can cd to
+it and make sure paths are interpreted
+relative to the go.work, not the cwd.
+-- d/go.mod --
+module example.com/d
+
+-- d/main.go --
+package main
+
+import "example.com/b/lib"
+
+func main() {
+	lib.Hello()
+}
+
+-- go.work.backwards --
+go 1.18
+
+use (
+	d
+	b
+	a
+)
+
+-- foo.go --
+package main
+import "fmt"
+func main() {
+	fmt.Println("Hello, World")
+}
diff --git a/src/cmd/go/testdata/script/work_build_no_modules.txt b/src/cmd/go/testdata/script/work_build_no_modules.txt
new file mode 100644
index 0000000..c9859b4
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_build_no_modules.txt
@@ -0,0 +1,13 @@
+! go build .
+stderr 'go: no modules were found in the current workspace; see ''go help work'''
+
+-- go.work --
+go 1.18
+-- go.mod --
+go 1.18
+
+module foo
+-- foo.go --
+package main
+
+func main() {}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/work_disablevendor.txt b/src/cmd/go/testdata/script/work_disablevendor.txt
new file mode 100644
index 0000000..c4c580b
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_disablevendor.txt
@@ -0,0 +1,56 @@
+# Test that mod=vendor is disabled in workspace mode, even
+# with a single workspace module.
+
+cd workspace
+
+# Base case: ensure the module would default to mod=vendor
+# outside of workspace mode.
+env GOWORK=off
+go list -f '{{.Dir}}' example.com/dep
+stdout $GOPATH[\\/]src[\\/]workspace[\\/]vendor[\\/]example.com[\\/]dep
+
+# Test case: endure the module does not enter mod=vendor outside
+# worspace mode.
+env GOWORK=''
+go list -f '{{.Dir}}' example.com/dep
+stdout $GOPATH[\\/]src[\\/]dep
+
+-- workspace/go.work --
+use .
+replace example.com/dep => ../dep
+-- workspace/main.go --
+package main
+
+import "example.com/dep"
+
+func main() {
+	dep.Dep()
+}
+-- workspace/go.mod --
+module example.com/mod
+
+go 1.20
+
+require example.com/dep v1.0.0
+-- workspace/vendor/example.com/dep/dep.go --
+package dep
+
+import "fmt"
+
+func Dep() {
+	fmt.Println("the vendored dep")
+}
+-- workspace/vendor/modules.txt --
+# example.com/dep v1.0.0
+## explicit
+example.com/dep
+-- dep/go.mod --
+module example.com/dep
+-- dep/dep.go --
+package dep
+
+import "fmt"
+
+func Dep () {
+    fmt.Println("the real dep")
+}
diff --git a/src/cmd/go/testdata/script/work_edit.txt b/src/cmd/go/testdata/script/work_edit.txt
new file mode 100644
index 0000000..c67696d
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_edit.txt
@@ -0,0 +1,166 @@
+# Test editing go.work files.
+
+go work init m
+cmpenv go.work go.work.want_initial
+
+go work edit -use n
+cmpenv go.work go.work.want_use_n
+
+grep go go.work
+go work edit -go none
+! grep go go.work
+
+go work edit -go 1.18
+cmp go.work go.work.want_go_118
+
+go work edit -dropuse m
+cmp go.work go.work.want_dropuse_m
+
+go work edit -replace=x.1@v1.3.0=y.1@v1.4.0 -replace='x.1@v1.4.0 = ../z'
+cmp go.work go.work.want_add_replaces
+
+go work edit -use n -use ../a -use /b -use c -use c
+cmp go.work go.work.want_multiuse
+
+go work edit -dropuse /b -dropuse n
+cmp go.work go.work.want_multidropuse
+
+go work edit -dropreplace='x.1@v1.4.0'
+cmp go.work go.work.want_dropreplace
+
+go work edit -print -go 1.19 -use b -dropuse c -replace 'x.1@v1.4.0 = ../z' -dropreplace x.1 -dropreplace x.1@v1.3.0
+cmp stdout go.work.want_print
+
+go work edit -json -go 1.19 -use b -dropuse c -replace 'x.1@v1.4.0 = ../z' -dropreplace x.1 -dropreplace x.1@v1.3.0
+cmp stdout go.work.want_json
+
+env GOWORK=$GOPATH/src/unformatted
+go work edit -print -fmt
+cmp stdout formatted
+
+-- m/go.mod --
+module m
+
+go 1.18
+-- go.work.want_initial --
+go $goversion
+
+use ./m
+-- go.work.want_use_n --
+go $goversion
+
+use (
+	./m
+	./n
+)
+-- go.work.want_go_118 --
+go 1.18
+
+use (
+	./m
+	./n
+)
+-- go.work.want_dropuse_m --
+go 1.18
+
+use ./n
+-- go.work.want_add_replaces --
+go 1.18
+
+use ./n
+
+replace (
+	x.1 v1.3.0 => y.1 v1.4.0
+	x.1 v1.4.0 => ../z
+)
+-- go.work.want_multiuse --
+go 1.18
+
+use (
+	../a
+	./c
+	./n
+	/b
+)
+
+replace (
+	x.1 v1.3.0 => y.1 v1.4.0
+	x.1 v1.4.0 => ../z
+)
+-- go.work.want_multidropuse --
+go 1.18
+
+use (
+	../a
+	./c
+)
+
+replace (
+	x.1 v1.3.0 => y.1 v1.4.0
+	x.1 v1.4.0 => ../z
+)
+-- go.work.want_dropreplace --
+go 1.18
+
+use (
+	../a
+	./c
+)
+
+replace x.1 v1.3.0 => y.1 v1.4.0
+-- go.work.want_print --
+go 1.19
+
+use (
+	../a
+	./b
+)
+
+replace x.1 v1.4.0 => ../z
+-- go.work.want_json --
+{
+	"Go": "1.19",
+	"Use": [
+		{
+			"DiskPath": "../a"
+		},
+		{
+			"DiskPath": "./b"
+		}
+	],
+	"Replace": [
+		{
+			"Old": {
+				"Path": "x.1",
+				"Version": "v1.4.0"
+			},
+			"New": {
+				"Path": "../z"
+			}
+		}
+	]
+}
+-- unformatted --
+go 1.18
+ use (
+ a
+  b
+  c
+  )
+  replace (
+  x.1 v1.3.0 => y.1 v1.4.0
+                            x.1 v1.4.0 => ../z
+                            )
+-- formatted --
+go 1.18
+
+use (
+	a
+	b
+	c
+)
+
+replace (
+	x.1 v1.3.0 => y.1 v1.4.0
+	x.1 v1.4.0 => ../z
+)
diff --git a/src/cmd/go/testdata/script/work_edit_toolchain.txt b/src/cmd/go/testdata/script/work_edit_toolchain.txt
new file mode 100644
index 0000000..b4e260d
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_edit_toolchain.txt
@@ -0,0 +1,20 @@
+# Test support for go work edit -toolchain to set toolchain to use
+
+env GOTOOLCHAIN=local
+env GO111MODULE=on
+
+! grep toolchain go.work
+go work edit -toolchain=go1.9
+grep 'toolchain go1.9' go.work
+
+go work edit -toolchain=default
+grep 'toolchain default' go.work
+
+go work edit -toolchain=none
+! grep toolchain go.work
+
+-- go.work --
+go 1.8
+use .
+-- go.mod --
+module m
diff --git a/src/cmd/go/testdata/script/work_empty_panic_GOPATH.txt b/src/cmd/go/testdata/script/work_empty_panic_GOPATH.txt
new file mode 100644
index 0000000..43ebf11
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_empty_panic_GOPATH.txt
@@ -0,0 +1,13 @@
+# Regression test for https://go.dev/issue/58767:
+# with an empty go.work file in GOPATH mode, calls to load.defaultGODEBUG for a
+# package named "main" panicked in modload.MainModules.GoVersion.
+
+env GO111MODULE=off
+cd example
+go list example/m
+
+-- example/go.work --
+go 1.21
+-- example/m/main.go --
+package main
+func main() {}
diff --git a/src/cmd/go/testdata/script/work_env.txt b/src/cmd/go/testdata/script/work_env.txt
new file mode 100644
index 0000000..8b1779e
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_env.txt
@@ -0,0 +1,28 @@
+go env GOWORK
+stdout '^'$GOPATH'[\\/]src[\\/]go.work$'
+go env
+stdout '^(set )?GOWORK=''?'$GOPATH'[\\/]src[\\/]go.work''?$'
+
+cd ..
+go env GOWORK
+! stdout .
+go env
+stdout 'GOWORK=("")?'
+
+cd src
+go env GOWORK
+stdout 'go.work'
+
+env GOWORK='off'
+go env GOWORK
+stdout 'off'
+
+! go env -w GOWORK=off
+stderr '^go: GOWORK cannot be modified$'
+
+-- go.work --
+go 1.18
+
+use a
+-- a/go.mod --
+module example.com/a
diff --git a/src/cmd/go/testdata/script/work_get_toolchain.txt b/src/cmd/go/testdata/script/work_get_toolchain.txt
new file mode 100644
index 0000000..5a851bb
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_get_toolchain.txt
@@ -0,0 +1,24 @@
+# go get should update the go and toolchain lines in go.work
+env TESTGO_VERSION=go1.21
+env TESTGO_VERSION_SWITCH=switch
+env GOTOOLCHAIN=auto
+cp go.mod.new go.mod
+cp go.work.new go.work
+go get rsc.io/needgo121 rsc.io/needgo122 rsc.io/needgo123 rsc.io/needall
+stderr '^go: rsc.io/needall@v0.0.1 requires go >= 1.23; switching to go1.23.9$'
+stderr '^go: added rsc.io/needall v0.0.1'
+grep 'go 1.23$' go.mod
+grep 'go 1.23$' go.work
+grep 'toolchain go1.23.9' go.mod
+grep 'toolchain go1.23.9' go.work
+
+-- go.mod.new --
+module m
+go 1.1
+
+-- p.go --
+package p
+
+-- go.work.new --
+go 1.18
+use .
diff --git a/src/cmd/go/testdata/script/work_goline_order.txt b/src/cmd/go/testdata/script/work_goline_order.txt
new file mode 100644
index 0000000..fe1cddb
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_goline_order.txt
@@ -0,0 +1,34 @@
+# Check that go line in go.work is always >= go line of used modules.
+
+# Using an old Go version, fails during module loading, but we rewrite the error to the
+# same one a switching version would use, without the auto-switch.
+# This is a misconfigured system that should not arise in practice.
+env TESTGO_VERSION=go1.21.1
+env TESTGO_VERSION_SWITCH=switch
+cp go.work go.work.orig
+! go list
+stderr '^go: module . listed in go.work file requires go >= 1.21.2, but go.work lists go 1.21.1; to update it:\n\tgo work use$'
+go work use
+go list
+
+# Using a new enough Go version, fails later and can suggest 'go work use'.
+env TESTGO_VERSION=go1.21.2
+env TESTGO_VERSION_SWITCH=switch
+cp go.work.orig go.work
+! go list
+stderr '^go: module . listed in go.work file requires go >= 1.21.2, but go.work lists go 1.21.1; to update it:\n\tgo work use$'
+
+# go work use fixes the problem.
+go work use
+go list
+
+-- go.work --
+go 1.21.1
+use .
+
+-- go.mod --
+module m
+go 1.21.2
+
+-- p.go --
+package p
diff --git a/src/cmd/go/testdata/script/work_goproxy_off.txt b/src/cmd/go/testdata/script/work_goproxy_off.txt
new file mode 100644
index 0000000..0a602e3
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_goproxy_off.txt
@@ -0,0 +1,59 @@
+go work init
+go work use . ./sub
+
+# Verify that the go.mod files for both modules in the workspace are tidy,
+# and add missing go.sum entries as needed.
+
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod go.mod.orig
+
+cd sub
+cp go.mod go.mod.orig
+go mod tidy
+cmp go.mod go.mod.orig
+cd ..
+
+go list -m all
+stdout '^rsc\.io/quote v1\.5\.1$'
+stdout '^rsc\.io/sampler v1\.3\.1$'
+
+# Now remove the module dependencies from the module cache.
+# Because one module upgrades a transitive dependency needed by another,
+# listing the modules in the workspace should error out.
+
+go clean -modcache
+env GOPROXY=off
+! go list -m all
+stderr '^go: rsc.io/sampler@v1.3.0: module lookup disabled by GOPROXY=off$'
+
+-- example.go --
+package example
+
+import _ "rsc.io/sampler"
+-- go.mod --
+module example
+
+go 1.19
+
+require rsc.io/sampler v1.3.0
+
+require (
+	golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c // indirect
+	rsc.io/testonly v1.0.0 // indirect
+)
+-- sub/go.mod --
+module example/sub
+
+go 1.19
+
+require rsc.io/quote v1.5.1
+
+require (
+	golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c // indirect
+	rsc.io/sampler v1.3.1 // indirect
+)
+-- sub/sub.go --
+package example
+
+import _ "rsc.io/quote"
diff --git a/src/cmd/go/testdata/script/work_gowork.txt b/src/cmd/go/testdata/script/work_gowork.txt
new file mode 100644
index 0000000..1cfbf0c
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_gowork.txt
@@ -0,0 +1,24 @@
+env GOWORK=stop.work
+! go list a # require absolute path
+! stderr panic
+env GOWORK=doesnotexist
+! go list a
+! stderr panic
+
+env GOWORK=$GOPATH/src/stop.work
+go list -n a
+go build -n a
+go test -n a
+
+-- stop.work --
+go 1.18
+
+use ./a
+-- a/a.go --
+package a
+-- a/a_test.go --
+package a
+-- a/go.mod --
+module a
+
+go 1.18
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/work_init_gowork.txt b/src/cmd/go/testdata/script/work_init_gowork.txt
new file mode 100644
index 0000000..55ac99b
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_init_gowork.txt
@@ -0,0 +1,19 @@
+# Test that the GOWORK environment variable flag is used by go work init.
+
+! exists go.work
+go work init
+exists go.work
+
+env GOWORK=$GOPATH/src/foo/foo.work
+! exists foo/foo.work
+go work init
+exists foo/foo.work
+
+env GOWORK=
+cd foo/bar
+! go work init
+stderr 'already exists'
+
+# Create directories to make go.work files in.
+-- foo/dummy.txt --
+-- foo/bar/dummy.txt --
diff --git a/src/cmd/go/testdata/script/work_init_path.txt b/src/cmd/go/testdata/script/work_init_path.txt
new file mode 100644
index 0000000..e397788
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_init_path.txt
@@ -0,0 +1,17 @@
+# Regression test for https://go.dev/issue/51448.
+# 'go work init . foo/bar' should produce a go.work file
+# with the same paths as 'go work init; go work use -r .'.
+
+go work init . foo/bar
+mv go.work go.work.init
+
+go work init
+go work use -r .
+cmp go.work go.work.init
+
+-- go.mod --
+module example
+go 1.18
+-- foo/bar/go.mod --
+module example
+go 1.18
diff --git a/src/cmd/go/testdata/script/work_init_toolchain.txt b/src/cmd/go/testdata/script/work_init_toolchain.txt
new file mode 100644
index 0000000..900ea2c
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_init_toolchain.txt
@@ -0,0 +1,35 @@
+
+# Create basic modules and work space.
+# Note that toolchain lines in modules should be completely ignored.
+env TESTGO_VERSION=go1.50
+mkdir m1_22_0
+go mod init -C m1_22_0
+go mod edit -C m1_22_0 -go=1.22.0 -toolchain=go1.99.0
+
+# work init writes the current Go version to the go line
+go work init
+grep '^go 1.50$' go.work
+! grep toolchain go.work
+
+# work init with older modules should leave go 1.50 in the go.work.
+rm go.work
+go work init ./m1_22_0
+grep '^go 1.50$' go.work
+! grep toolchain go.work
+
+# work init with newer modules should bump go,
+# including updating to a newer toolchain as needed.
+# Because work init writes the current toolchain as the go version,
+# it writes the bumped go version, not the max of the used modules.
+env TESTGO_VERSION=go1.21
+env TESTGO_VERSION_SWITCH=switch
+rm go.work
+env GOTOOLCHAIN=local
+! go work init ./m1_22_0
+stderr '^go: m1_22_0'${/}'go.mod requires go >= 1.22.0 \(running go 1.21; GOTOOLCHAIN=local\)$'
+env GOTOOLCHAIN=auto
+go work init ./m1_22_0
+stderr '^go: m1_22_0'${/}'go.mod requires go >= 1.22.0; switching to go1.22.9$'
+cat go.work
+grep '^go 1.22.9$' go.work
+! grep toolchain go.work
diff --git a/src/cmd/go/testdata/script/work_install_submodule.txt b/src/cmd/go/testdata/script/work_install_submodule.txt
new file mode 100644
index 0000000..3d11717
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_install_submodule.txt
@@ -0,0 +1,36 @@
+# This is a regression test for golang.org/issue/50036
+# Don't check sums for other modules in the workspace.
+
+cd m/sub
+go install -n
+
+-- go.work --
+go 1.18
+
+use (
+    ./m
+    ./m/sub
+)
+-- m/go.mod --
+module example.com/m
+
+go 1.18
+
+-- m/m.go --
+package m
+
+func M() {}
+-- m/sub/go.mod --
+module example.com/m/sub
+
+go 1.18
+
+require example.com/m v1.0.0
+-- m/sub/main.go --
+package main
+
+import "example.com/m"
+
+func main() {
+    m.M()
+}
diff --git a/src/cmd/go/testdata/script/work_issue51204.txt b/src/cmd/go/testdata/script/work_issue51204.txt
new file mode 100644
index 0000000..d483002
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_issue51204.txt
@@ -0,0 +1,57 @@
+go work sync
+
+go list -f '{{.Dir}}' example.com/test
+stdout '^'$PWD${/}test'$'
+
+-- go.work --
+go 1.18
+
+use (
+	./test2
+	./test2/sub
+)
+-- test/go.mod --
+module example.com/test
+
+go 1.18
+-- test/file.go --
+package test
+
+func DoSomething() {
+}
+-- test2/go.mod --
+module example.com/test2
+
+go 1.18
+
+replace example.com/test => ../test
+
+require example.com/test v0.0.0-00010101000000-000000000000
+-- test2/file.go --
+package test2
+
+import (
+	"example.com/test"
+)
+
+func DoSomething() {
+	test.DoSomething()
+}
+-- test2/sub/go.mod --
+module example.com/test2/sub
+
+go 1.18
+
+replace example.com/test => ../../test
+
+require example.com/test v0.0.0
+-- test2/sub/file.go --
+package test2
+
+import (
+	"example.com/test"
+)
+
+func DoSomething() {
+	test.DoSomething()
+}
diff --git a/src/cmd/go/testdata/script/work_issue54048.txt b/src/cmd/go/testdata/script/work_issue54048.txt
new file mode 100644
index 0000000..ced3d90
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_issue54048.txt
@@ -0,0 +1,19 @@
+! go list -m -json all
+stderr 'go: module example.com/foo appears multiple times in workspace'
+
+-- go.work --
+go 1.18
+
+use (
+        ./a
+        ./b
+)
+-- a/go.mod --
+module example.com/foo
+
+go 1.18
+
+-- b/go.mod --
+module example.com/foo
+
+go 1.18
diff --git a/src/cmd/go/testdata/script/work_issue54372.txt b/src/cmd/go/testdata/script/work_issue54372.txt
new file mode 100644
index 0000000..bd3108a
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_issue54372.txt
@@ -0,0 +1,37 @@
+# go mod verify should not try to verify the workspace modules.
+# This is a test for #54372.
+
+go mod verify
+stdout 'all modules verified'
+! stderr .
+
+-- go.work --
+go 1.21
+
+use (
+    ./a
+    ./b
+    ./c
+    ./d
+)
+-- a/go.mod --
+module example.com/a
+
+go 1.21
+
+require rsc.io/quote v1.1.0
+-- a/a.go --
+package a
+import _ "rsc.io/quote"
+-- b/go.mod --
+module example.com/b
+
+go 1.21
+-- c/go.mod --
+module example.com/c
+
+go 1.21
+-- d/go.mod --
+module example.com/d
+
+go 1.21
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/work_module_not_in_go_work.txt b/src/cmd/go/testdata/script/work_module_not_in_go_work.txt
new file mode 100644
index 0000000..074bac5
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_module_not_in_go_work.txt
@@ -0,0 +1,40 @@
+# This is a regression test for issue #49632.
+# The Go command should mention go.work if the user
+# tries to load a local package that's in a module
+# that's not in go.work and can't be resolved.
+
+! go list ./...
+stderr 'pattern ./...: directory prefix . does not contain modules listed in go.work or their selected dependencies'
+
+! go list ./a/c
+stderr 'directory a[\\/]c is contained in a module that is not one of the workspace modules listed in go.work. You can add the module to the workspace using:\n\tgo work use a'
+
+! go install ./a/c
+stderr 'directory a[\\/]c is contained in a module that is not one of the workspace modules listed in go.work. You can add the module to the workspace using:\n\tgo work use a'
+
+cd a/c
+! go run .
+stderr 'current directory is contained in a module that is not one of the workspace modules listed in go.work. You can add the module to the workspace using:\n\tgo work use \.\.'
+
+cd ../..
+! go run .
+stderr 'current directory outside modules listed in go.work or their selected dependencies'
+
+-- go.work --
+go 1.18
+
+use ./b
+-- a/go.mod --
+module example.com/a
+
+go 1.18
+-- a/a.go --
+package a
+-- a/c/c.go --
+package main
+-- b/go.mod --
+module example.com/b
+
+go 1.18
+-- foo.go --
+package foo
diff --git a/src/cmd/go/testdata/script/work_nowork.txt b/src/cmd/go/testdata/script/work_nowork.txt
new file mode 100644
index 0000000..b4c9b1d
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_nowork.txt
@@ -0,0 +1,20 @@
+! go work use
+stderr '^go: no go\.work file found\n\t\(run ''go work init'' first or specify path using GOWORK environment variable\)$'
+
+! go work use .
+stderr '^go: no go\.work file found\n\t\(run ''go work init'' first or specify path using GOWORK environment variable\)$'
+
+! go work edit
+stderr '^go: no go\.work file found\n\t\(run ''go work init'' first or specify path using GOWORK environment variable\)$'
+
+! go work edit -go=1.18
+stderr '^go: no go\.work file found\n\t\(run ''go work init'' first or specify path using GOWORK environment variable\)$'
+
+! go work sync
+stderr '^go: no go\.work file found\n\t\(run ''go work init'' first or specify path using GOWORK environment variable\)$'
+
+-- go.mod --
+module example
+go 1.18
+-- README.txt --
+There is no go.work file here.
diff --git a/src/cmd/go/testdata/script/work_prune.txt b/src/cmd/go/testdata/script/work_prune.txt
new file mode 100644
index 0000000..b1f569e
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_prune.txt
@@ -0,0 +1,103 @@
+# This test makes sure workspace mode's handling of the module graph
+# is compatible with module pruning. The graph we load from either of
+# the workspace modules should be the same, even if their graphs
+# don't overlap.
+#
+# This is the module graph in the test:
+#
+#  example.com/a -> example.com/b v1.0.0 -> example.com/q v1.1.0
+#  example.com/p -> example.com/q v1.0.0
+#
+# If we didn't load the whole graph and didn't load the dependencies of b
+# when loading p, we would end up loading q v1.0.0, rather than v1.1.0,
+# which is selected by MVS.
+
+go list -m -f '{{.Version}}' example.com/q
+stdout '^v1.1.0$'
+
+-- go.work --
+go 1.18
+
+use (
+	./a
+	./p
+)
+-- a/go.mod --
+module example.com/a
+
+go 1.18
+
+require example.com/b v1.0.0
+
+replace example.com/b v1.0.0 => ../b
+-- a/foo.go --
+package main
+
+import "example.com/b"
+
+func main() {
+	b.B()
+}
+-- b/go.mod --
+module example.com/b
+
+go 1.18
+
+require example.com/q v1.1.0
+
+replace example.com/q v1.0.0 => ../q1_0_0
+replace example.com/q v1.1.0 => ../q1_1_0
+-- b/b.go --
+package b
+
+func B() {
+}
+-- b/b_test.go --
+package b
+
+import "example.com/q"
+
+func TestB() {
+	q.PrintVersion()
+}
+-- p/go.mod --
+module example.com/p
+
+go 1.18
+
+require example.com/q v1.0.0
+
+replace example.com/q v1.0.0 => ../q1_0_0
+replace example.com/q v1.1.0 => ../q1_1_0
+-- p/main.go --
+package main
+
+import "example.com/q"
+
+func main() {
+	q.PrintVersion()
+}
+-- q1_0_0/go.mod --
+module example.com/q
+
+go 1.18
+-- q1_0_0/q.go --
+package q
+
+import "fmt"
+
+func PrintVersion() {
+	fmt.Println("version 1.0.0")
+}
+-- q1_1_0/go.mod --
+module example.com/q
+
+go 1.18
+-- q1_1_0/q.go --
+package q
+
+import "fmt"
+
+func PrintVersion() {
+	fmt.Println("version 1.1.0")
+}
diff --git a/src/cmd/go/testdata/script/work_prune_all.txt b/src/cmd/go/testdata/script/work_prune_all.txt
new file mode 100644
index 0000000..a7ad9c0
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_prune_all.txt
@@ -0,0 +1,176 @@
+# This test makes sure workspace mode's handling of the module graph
+# is compatible with module pruning. The graph we load from either of
+# the workspace modules should be the same, even if their graphs
+# don't overlap.
+#
+# This is the module graph in the test:
+#
+#  example.com/p -> example.com/q v1.0.0
+#  example.com/a -> example.com/b v1.0.0 -> example.com/q v1.1.0 -> example.com/w v1.0.0 -> example.com/x v1.0.0 -> example.com/y v1.0.0
+#                |-> example.com/z v1.0.0                        |-> example.com/z v1.1.0
+#                            |-> example.com/q v1.0.5 -> example.com/r v1.0.0
+# If we didn't load the whole graph and didn't load the dependencies of b
+# when loading p, we would end up loading q v1.0.0, rather than v1.1.0,
+# which is selected by MVS.
+
+go list -m all
+stdout 'example.com/w v1.0.0'
+stdout 'example.com/q v1.1.0'
+stdout 'example.com/z v1.1.0'
+stdout 'example.com/x v1.0.0'
+! stdout 'example.com/r'
+! stdout 'example.com/y'
+
+-- go.work --
+go 1.18
+
+use (
+	./a
+	./p
+)
+
+replace example.com/b v1.0.0 => ./b
+replace example.com/q v1.0.0 => ./q1_0_0
+replace example.com/q v1.0.5 => ./q1_0_5
+replace example.com/q v1.1.0 => ./q1_1_0
+replace example.com/r v1.0.0 => ./r
+replace example.com/w v1.0.0 => ./w
+replace example.com/x v1.0.0 => ./x
+replace example.com/y v1.0.0 => ./y
+replace example.com/z v1.0.0 => ./z1_0_0
+replace example.com/z v1.1.0 => ./z1_1_0
+
+-- a/go.mod --
+module example.com/a
+
+go 1.18
+
+require example.com/b v1.0.0
+require example.com/z v1.0.0
+-- a/foo.go --
+package main
+
+import "example.com/b"
+
+func main() {
+	b.B()
+}
+-- b/go.mod --
+module example.com/b
+
+go 1.18
+
+require example.com/q v1.1.0
+-- b/b.go --
+package b
+
+func B() {
+}
+-- p/go.mod --
+module example.com/p
+
+go 1.18
+
+require example.com/q v1.0.0
+
+replace example.com/q v1.0.0 => ../q1_0_0
+replace example.com/q v1.1.0 => ../q1_1_0
+-- p/main.go --
+package main
+
+import "example.com/q"
+
+func main() {
+	q.PrintVersion()
+}
+-- q1_0_0/go.mod --
+module example.com/q
+
+go 1.18
+-- q1_0_0/q.go --
+package q
+
+import "fmt"
+
+func PrintVersion() {
+	fmt.Println("version 1.0.0")
+}
+-- q1_0_5/go.mod --
+module example.com/q
+
+go 1.18
+
+require example.com/r v1.0.0
+-- q1_0_5/q.go --
+package q
+
+import _ "example.com/r"
+-- q1_1_0/go.mod --
+module example.com/q
+
+require example.com/w v1.0.0
+require example.com/z v1.1.0
+
+go 1.18
+-- q1_1_0/q.go --
+package q
+
+import _ "example.com/w"
+import _ "example.com/z"
+
+import "fmt"
+
+func PrintVersion() {
+	fmt.Println("version 1.1.0")
+}
+-- r/go.mod --
+module example.com/r
+
+go 1.18
+
+require example.com/r v1.0.0
+-- r/r.go --
+package r
+-- w/go.mod --
+module example.com/w
+
+go 1.18
+
+require example.com/x v1.0.0
+-- w/w.go --
+package w
+-- w/w_test.go --
+package w
+
+import _ "example.com/x"
+-- x/go.mod --
+module example.com/x
+
+go 1.18
+-- x/x.go --
+package x
+-- x/x_test.go --
+package x
+import _ "example.com/y"
+-- y/go.mod --
+module example.com/y
+
+go 1.18
+-- y/y.go --
+package y
+-- z1_0_0/go.mod --
+module example.com/z
+
+go 1.18
+
+require example.com/q v1.0.5
+-- z1_0_0/z.go --
+package z
+
+import _ "example.com/q"
+-- z1_1_0/go.mod --
+module example.com/z
+
+go 1.18
+-- z1_1_0/z.go --
+package z
diff --git a/src/cmd/go/testdata/script/work_regression_hang.txt b/src/cmd/go/testdata/script/work_regression_hang.txt
new file mode 100644
index 0000000..a7661b6
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_regression_hang.txt
@@ -0,0 +1,71 @@
+# This test makes checks against a regression of a bug in the Go command
+# where the module loader hung forever because all main module dependencies
+# kept workspace pruning instead of adopting the pruning in their go.mod
+# files, and the loader kept adding dependencies on the queue until they
+# were either pruned or unpruned, never breaking a module dependency cycle.
+#
+# This is the module graph in the test:
+#
+#                               /-------------------------\
+#                              |                          |
+#                              V                          |
+#  example.com/a -> example.com/b v1.0.0 -> example.com/c v1.1.0
+
+go list -m -f '{{.Version}}' example.com/c
+
+-- go.work --
+go 1.16
+
+use (
+	./a
+)
+-- a/go.mod --
+module example.com/a
+
+go 1.18
+
+require example.com/b v1.0.0
+
+replace example.com/b v1.0.0 => ../b
+replace example.com/c v1.0.0 => ../c
+-- a/foo.go --
+package main
+
+import "example.com/b"
+
+func main() {
+	b.B()
+}
+-- b/go.mod --
+module example.com/b
+
+go 1.18
+
+require example.com/c v1.0.0
+-- b/b.go --
+package b
+
+func B() {
+}
+-- b/cmd/main.go --
+package main
+
+import "example.com/c"
+
+func main() {
+	c.C()
+}
+-- c/go.mod --
+module example.com/c
+
+go 1.18
+
+require example.com/b v1.0.0
+-- c/c.go --
+package c
+
+import "example.com/b"
+
+func C() {
+	b.B()
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/work_reject_modfile.txt b/src/cmd/go/testdata/script/work_reject_modfile.txt
new file mode 100644
index 0000000..f0cfa3b
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_reject_modfile.txt
@@ -0,0 +1,34 @@
+# Test that -modfile=path/to/go.mod is rejected in workspace mode.
+
+! go list -m -modfile=./a/go.alt.mod
+stderr 'go: -modfile cannot be used in workspace mode'
+
+env GOFLAGS=-modfile=./a/go.alt.mod
+! go list -m
+stderr 'go: -modfile cannot be used in workspace mode'
+
+-- go.work --
+go 1.20
+
+use (
+    ./a
+)
+
+-- a/go.mod --
+module example.com/foo
+
+go 1.20
+
+-- a/go.alt.mod --
+module example.com/foo
+
+go 1.20
+
+-- a/main.go --
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("Hello world!")
+}
diff --git a/src/cmd/go/testdata/script/work_replace.txt b/src/cmd/go/testdata/script/work_replace.txt
new file mode 100644
index 0000000..81268e5
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_replace.txt
@@ -0,0 +1,55 @@
+# Support replace statement in go.work file
+
+# Replacement in go.work file, and none in go.mod file.
+go list -m example.com/dep
+stdout 'example.com/dep v1.0.0 => ./dep'
+
+# Wildcard replacement in go.work file overrides version replacement in go.mod
+# file.
+go list -m example.com/other
+stdout 'example.com/other v1.0.0 => ./other2'
+
+-- go.work --
+use m
+
+replace example.com/dep => ./dep
+replace example.com/other => ./other2
+
+-- m/go.mod --
+module example.com/m
+
+require example.com/dep v1.0.0
+require example.com/other v1.0.0
+
+replace example.com/other v1.0.0 => ./other
+-- m/m.go --
+package m
+
+import "example.com/dep"
+import "example.com/other"
+
+func F() {
+	dep.G()
+	other.H()
+}
+-- dep/go.mod --
+module example.com/dep
+-- dep/dep.go --
+package dep
+
+func G() {
+}
+-- other/go.mod --
+module example.com/other
+-- other/dep.go --
+package other
+
+func G() {
+}
+-- other2/go.mod --
+module example.com/other
+-- other2/dep.go --
+package other
+
+func G() {
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/work_replace_conflict.txt b/src/cmd/go/testdata/script/work_replace_conflict.txt
new file mode 100644
index 0000000..7b71b0f
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_replace_conflict.txt
@@ -0,0 +1,53 @@
+# Conflicting replaces in workspace modules returns error that suggests
+# overriding it in the go.work file.
+
+! go list -m example.com/dep
+stderr 'go: conflicting replacements for example.com/dep@v1.0.0:\n\t'$PWD${/}'dep1\n\t'$PWD${/}'dep2\nuse "go work edit -replace example.com/dep@v1.0.0=\[override\]" to resolve'
+go work edit -replace example.com/dep@v1.0.0=./dep1
+go list -m example.com/dep
+stdout 'example.com/dep v1.0.0 => ./dep1'
+
+-- foo --
+-- go.work --
+use m
+use n
+-- m/go.mod --
+module example.com/m
+
+require example.com/dep v1.0.0
+replace example.com/dep v1.0.0 => ../dep1
+-- m/m.go --
+package m
+
+import "example.com/dep"
+
+func F() {
+	dep.G()
+}
+-- n/go.mod --
+module example.com/n
+
+require example.com/dep v1.0.0
+replace example.com/dep v1.0.0 => ../dep2
+-- n/n.go --
+package n
+
+import "example.com/dep"
+
+func F() {
+	dep.G()
+}
+-- dep1/go.mod --
+module example.com/dep
+-- dep1/dep.go --
+package dep
+
+func G() {
+}
+-- dep2/go.mod --
+module example.com/dep
+-- dep2/dep.go --
+package dep
+
+func G() {
+}
diff --git a/src/cmd/go/testdata/script/work_replace_conflict_override.txt b/src/cmd/go/testdata/script/work_replace_conflict_override.txt
new file mode 100644
index 0000000..c62084b
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_replace_conflict_override.txt
@@ -0,0 +1,57 @@
+# Conflicting workspace module replaces can be overridden by a replace in the
+# go.work file.
+
+go list -m example.com/dep
+stdout 'example.com/dep v1.0.0 => ./dep3'
+
+-- go.work --
+use m
+use n
+replace example.com/dep => ./dep3
+-- m/go.mod --
+module example.com/m
+
+require example.com/dep v1.0.0
+replace example.com/dep => ./dep1
+-- m/m.go --
+package m
+
+import "example.com/dep"
+
+func F() {
+	dep.G()
+}
+-- n/go.mod --
+module example.com/n
+
+require example.com/dep v1.0.0
+replace example.com/dep => ./dep2
+-- n/n.go --
+package n
+
+import "example.com/dep"
+
+func F() {
+	dep.G()
+}
+-- dep1/go.mod --
+module example.com/dep
+-- dep1/dep.go --
+package dep
+
+func G() {
+}
+-- dep2/go.mod --
+module example.com/dep
+-- dep2/dep.go --
+package dep
+
+func G() {
+}
+-- dep3/go.mod --
+module example.com/dep
+-- dep3/dep.go --
+package dep
+
+func G() {
+}
diff --git a/src/cmd/go/testdata/script/work_replace_main_module.txt b/src/cmd/go/testdata/script/work_replace_main_module.txt
new file mode 100644
index 0000000..b213764
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_replace_main_module.txt
@@ -0,0 +1,45 @@
+# Ensure that replaces of the main module in workspace modules
+# are ignored, and replaces in the go.work file are disallowed.
+# This tests against an issue where requirements of the
+# main module were being ignored because the main module
+# was replaced in a transitive dependency with another
+# version.
+
+go list example.com/dep
+
+cp replace_main_module.go.work go.work
+! go list example.com/dep
+stderr 'go: workspace module example.com/mainmoda is replaced at all versions in the go.work file. To fix, remove the replacement from the go.work file or specify the version at which to replace the module.'
+
+-- replace_main_module.go.work --
+go 1.18
+use (
+    ./mainmoda
+    ./mainmodb
+)
+replace example.com/mainmoda => ../mainmodareplacement
+-- go.work --
+go 1.18
+use (
+    ./mainmoda
+    ./mainmodb
+)
+-- mainmoda/go.mod --
+module example.com/mainmoda
+
+go 1.18
+
+require example.com/dep v1.0.0
+replace example.com/dep => ../dep
+
+-- dep/go.mod --
+module example.com/dep
+-- dep/dep.go --
+package dep
+-- mainmodb/go.mod --
+module example.com/mainmodb
+go 1.18
+replace example.com/mainmoda => ../mainmodareplacement
+-- mainmodareplacement/go.mod --
+module example.com/mainmoda
+go 1.18
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/work_sum.txt b/src/cmd/go/testdata/script/work_sum.txt
new file mode 100644
index 0000000..19dbb90
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_sum.txt
@@ -0,0 +1,34 @@
+# Test adding sums to go.work.sum when sum isn't in go.mod.
+
+go run .
+cmp go.work.sum want.sum
+
+-- want.sum --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c h1:pvCbr/wm8HzDD3fVywevekufpn6tCGPY3spdHeZJEsw=
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.5.2 h1:3fEykkD9k7lYzXqCYrwGAf7iNhbk4yCjHmKBN9td4L0=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+-- go.work --
+go 1.18
+
+use .
+-- go.mod --
+go 1.18
+
+module example.com/hi
+
+require "rsc.io/quote" v1.5.2
+-- go.sum --
+rsc.io/sampler v1.3.0 h1:HLGR/BgEtI3r0uymSP/nl2uPLsUnNJX8toRyhfpBTII=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+-- main.go --
+package main
+
+import (
+	"fmt"
+	"rsc.io/quote"
+)
+
+func main() {
+	fmt.Println(quote.Hello())
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/work_sum_mismatch.txt b/src/cmd/go/testdata/script/work_sum_mismatch.txt
new file mode 100644
index 0000000..ca5d71d
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_sum_mismatch.txt
@@ -0,0 +1,61 @@
+# Test mismatched sums in go.sum files
+
+! go run ./a
+cmpenv stderr want-error
+
+-- want-error --
+verifying rsc.io/sampler@v1.3.0/go.mod: checksum mismatch
+	downloaded: h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+	$WORK${/}gopath${/}src${/}a${/}go.sum:     h1:U1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+
+SECURITY ERROR
+This download does NOT match an earlier download recorded in go.sum.
+The bits may have been replaced on the origin server, or an attacker may
+have intercepted the download attempt.
+
+For more information, see 'go help module-auth'.
+-- go.work --
+go 1.18
+
+use ./a
+use ./b
+-- a/go.mod --
+go 1.18
+
+module example.com/hi
+
+require "rsc.io/quote" v1.5.2
+-- a/go.sum --
+rsc.io/sampler v1.3.0 h1:HLGR/BgEtI3r0uymSP/nl2uPLsUnNJX8toRyhfpBTII=
+rsc.io/sampler v1.3.0/go.mod h1:U1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+-- a/main.go --
+package main
+
+import (
+	"fmt"
+	"rsc.io/quote"
+)
+
+func main() {
+	fmt.Println(quote.Hello())
+}
+-- b/go.mod --
+go 1.18
+
+module example.com/hi2
+
+require "rsc.io/quote" v1.5.2
+-- b/go.sum --
+rsc.io/sampler v1.3.0 h1:HLGR/BgEtI3r0uymSP/nl2uPLsUnNJX8toRyhfpBTII=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+-- b/main.go --
+package main
+
+import (
+	"fmt"
+	"rsc.io/quote"
+)
+
+func main() {
+	fmt.Println(quote.Hello())
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/work_sync.txt b/src/cmd/go/testdata/script/work_sync.txt
new file mode 100644
index 0000000..69167d4
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_sync.txt
@@ -0,0 +1,119 @@
+go work sync
+cmp a/go.mod a/want_go.mod
+cmp b/go.mod b/want_go.mod
+
+-- go.work --
+go 1.18
+
+use (
+	./a
+	./b
+)
+
+-- a/go.mod --
+go 1.18
+
+module example.com/a
+
+require (
+	example.com/p v1.0.0
+	example.com/q v1.1.0
+	example.com/r v1.0.0
+)
+
+replace (
+	example.com/p => ../p
+	example.com/q => ../q
+	example.com/r => ../r
+)
+-- a/want_go.mod --
+go 1.18
+
+module example.com/a
+
+require (
+	example.com/p v1.1.0
+	example.com/q v1.1.0
+)
+
+replace (
+	example.com/p => ../p
+	example.com/q => ../q
+	example.com/r => ../r
+)
+-- a/a.go --
+package a
+
+import (
+	"example.com/p"
+	"example.com/q"
+)
+
+func Foo() {
+	p.P()
+	q.Q()
+}
+-- b/go.mod --
+go 1.18
+
+module example.com/b
+
+require (
+	example.com/p v1.1.0
+	example.com/q v1.0.0
+)
+
+replace (
+	example.com/p => ../p
+	example.com/q => ../q
+)
+-- b/want_go.mod --
+go 1.18
+
+module example.com/b
+
+require (
+	example.com/p v1.1.0
+	example.com/q v1.1.0
+)
+
+replace (
+	example.com/p => ../p
+	example.com/q => ../q
+)
+-- b/b.go --
+package b
+
+import (
+	"example.com/p"
+	"example.com/q"
+)
+
+func Foo() {
+	p.P()
+	q.Q()
+}
+-- p/go.mod --
+go 1.18
+
+module example.com/p
+-- p/p.go --
+package p
+
+func P() {}
+-- q/go.mod --
+go 1.18
+
+module example.com/q
+-- q/q.go --
+package q
+
+func Q() {}
+-- r/go.mod --
+go 1.18
+
+module example.com/r
+-- r/q.go --
+package r
+
+func R() {}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/work_sync_irrelevant_dependency.txt b/src/cmd/go/testdata/script/work_sync_irrelevant_dependency.txt
new file mode 100644
index 0000000..072323d
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_sync_irrelevant_dependency.txt
@@ -0,0 +1,119 @@
+# Test of go work sync in a workspace in which some dependency needed by `a`
+# appears at a lower version in the build list of `b`, but is not needed at all
+# by `b` (so it should not be upgraded within b).
+#
+# a -> p 1.1
+# b -> q 1.0 -(through a test dependency)-> p 1.0
+go work sync
+cmp a/go.mod a/want_go.mod
+cmp b/go.mod b/want_go.mod
+
+-- go.work --
+go 1.18
+
+use (
+	./a
+	./b
+)
+
+-- a/go.mod --
+go 1.18
+
+module example.com/a
+
+require (
+	example.com/p v1.1.0
+)
+
+replace (
+	example.com/p => ../p
+)
+-- a/want_go.mod --
+go 1.18
+
+module example.com/a
+
+require (
+	example.com/p v1.1.0
+)
+
+replace (
+	example.com/p => ../p
+)
+-- a/a.go --
+package a
+
+import (
+	"example.com/p"
+)
+
+func Foo() {
+	p.P()
+}
+-- b/go.mod --
+go 1.18
+
+module example.com/b
+
+require (
+	example.com/q v1.0.0
+)
+
+replace (
+	example.com/q => ../q
+)
+-- b/want_go.mod --
+go 1.18
+
+module example.com/b
+
+require (
+	example.com/q v1.0.0
+)
+
+replace (
+	example.com/q => ../q
+)
+-- b/b.go --
+package b
+
+import (
+	"example.com/q"
+)
+
+func Foo() {
+	q.Q()
+}
+-- p/go.mod --
+go 1.18
+
+module example.com/p
+-- p/p.go --
+package p
+
+func P() {}
+-- q/go.mod --
+go 1.18
+
+module example.com/q
+
+require (
+	example.com/p v1.0.0
+)
+
+replace (
+	example.com/p => ../p
+)
+-- q/q.go --
+package q
+
+func Q() {
+}
+-- q/q_test.go --
+package q
+
+import example.com/p
+
+func TestQ(t *testing.T) {
+	p.P()
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/work_sync_missing_module.txt b/src/cmd/go/testdata/script/work_sync_missing_module.txt
new file mode 100644
index 0000000..0018c73
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_sync_missing_module.txt
@@ -0,0 +1,12 @@
+# Ensure go work sync works without any modules in go.work.
+go work sync
+
+# Ensure go work sync works even without a go.mod file.
+rm go.mod
+go work sync
+
+-- go.work --
+go 1.18
+-- go.mod --
+go 1.18
+module foo
diff --git a/src/cmd/go/testdata/script/work_sync_relevant_dependency.txt b/src/cmd/go/testdata/script/work_sync_relevant_dependency.txt
new file mode 100644
index 0000000..d799702
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_sync_relevant_dependency.txt
@@ -0,0 +1,106 @@
+# Test of go work sync in a workspace in which some dependency in the build
+# list of 'b' (but not otherwise needed by `b`, so not seen when lazy loading
+# occurs) actually is relevant to `a`.
+#
+# a -> p 1.0
+# b -> q 1.1 -> p 1.1
+go work sync
+cmp a/go.mod a/want_go.mod
+cmp b/go.mod b/want_go.mod
+
+-- go.work --
+go 1.18
+
+use (
+	./a
+	./b
+)
+
+-- a/go.mod --
+go 1.18
+
+module example.com/a
+
+require (
+	example.com/p v1.0.0
+)
+
+replace (
+	example.com/p => ../p
+)
+-- a/want_go.mod --
+go 1.18
+
+module example.com/a
+
+require example.com/p v1.1.0
+
+replace example.com/p => ../p
+-- a/a.go --
+package a
+
+import (
+	"example.com/p"
+)
+
+func Foo() {
+	p.P()
+}
+-- b/go.mod --
+go 1.18
+
+module example.com/b
+
+require (
+	example.com/q v1.1.0
+)
+
+replace (
+	example.com/q => ../q
+)
+-- b/want_go.mod --
+go 1.18
+
+module example.com/b
+
+require (
+	example.com/q v1.1.0
+)
+
+replace (
+	example.com/q => ../q
+)
+-- b/b.go --
+package b
+
+import (
+	"example.com/q"
+)
+
+func Foo() {
+	q.Q()
+}
+-- p/go.mod --
+go 1.18
+
+module example.com/p
+-- p/p.go --
+package p
+
+func P() {}
+-- q/go.mod --
+go 1.18
+
+module example.com/q
+
+require example.com/p v1.1.0
+
+replace example.com/p => ../p
+-- q/q.go --
+package q
+
+import example.com/p
+
+func Q() {
+	p.P()
+}
diff --git a/src/cmd/go/testdata/script/work_sync_sum.txt b/src/cmd/go/testdata/script/work_sync_sum.txt
new file mode 100644
index 0000000..656fd31
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_sync_sum.txt
@@ -0,0 +1,40 @@
+# Test that the sum file data state is properly reset between modules in
+# go work sync so that the sum file that's written is correct.
+# Exercises the fix to #50038.
+
+cp b/go.sum b/go.sum.want
+
+# As a sanity check, verify b/go.sum is tidy.
+cd b
+go mod tidy
+cd ..
+cmp b/go.sum b/go.sum.want
+
+# Run go work sync and verify it doesn't change b/go.sum.
+go work sync
+cmp b/go.sum b/go.sum.want
+
+-- b/go.sum --
+rsc.io/quote v1.0.0 h1:kQ3IZQzPTiDJxSZI98YaWgxFEhlNdYASHvh+MplbViw=
+rsc.io/quote v1.0.0/go.mod h1:v83Ri/njykPcgJltBc/gEkJTmjTsNgtO1Y7vyIK1CQA=
+-- go.work --
+go 1.18
+use (
+    ./a
+    ./b
+)
+replace example.com/c => ./c
+-- a/go.mod --
+module example.com/a
+go 1.18
+require rsc.io/fortune v1.0.0
+-- a/a.go --
+package a
+import "rsc.io/fortune"
+-- b/go.mod --
+module example.com/b
+go 1.18
+require rsc.io/quote v1.0.0
+-- b/b.go --
+package b
+import _ "rsc.io/quote"
diff --git a/src/cmd/go/testdata/script/work_sync_toolchain.txt b/src/cmd/go/testdata/script/work_sync_toolchain.txt
new file mode 100644
index 0000000..b752462
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_sync_toolchain.txt
@@ -0,0 +1,45 @@
+# Create basic modules and work space.
+env TESTGO_VERSION=go1.50
+mkdir m1_22_0
+go mod init -C m1_22_0
+go mod edit -C m1_22_0 -go=1.22.0 -toolchain=go1.99.0
+mkdir m1_22_1
+go mod init -C m1_22_1
+go mod edit -C m1_22_1 -go=1.22.1 -toolchain=go1.99.1
+mkdir m1_24_rc0
+go mod init -C m1_24_rc0
+go mod edit -C m1_24_rc0 -go=1.24rc0 -toolchain=go1.99.2
+
+go work init ./m1_22_0 ./m1_22_1
+grep '^go 1.50$' go.work
+! grep toolchain go.work
+
+# work sync with older modules should leave go 1.50 in the go.work.
+go work sync
+cat go.work
+grep '^go 1.50$' go.work
+! grep toolchain go.work
+
+# work sync with newer modules should update go 1.21 -> 1.22.1 and toolchain -> go1.22.9 in go.work
+env TESTGO_VERSION=go1.21
+env TESTGO_VERSION_SWITCH=switch
+go work edit -go=1.21
+grep '^go 1.21$' go.work
+! grep toolchain go.work
+env GOTOOLCHAIN=local
+! go work sync
+stderr '^go: cannot load module m1_22_0 listed in go.work file: m1_22_0'${/}'go.mod requires go >= 1.22.0 \(running go 1.21; GOTOOLCHAIN=local\)$'
+stderr '^go: cannot load module m1_22_1 listed in go.work file: m1_22_1'${/}'go.mod requires go >= 1.22.1 \(running go 1.21; GOTOOLCHAIN=local\)$'
+env GOTOOLCHAIN=auto
+go work sync
+stderr '^go: m1_22_1'${/}'go.mod requires go >= 1.22.1; switching to go1.22.9$'
+grep '^go 1.22.1$' go.work
+grep '^toolchain go1.22.9$' go.work
+
+# work sync with newer modules should update go 1.22.1 -> 1.24rc1 and drop toolchain
+go work edit -use=./m1_24_rc0
+go work sync
+stderr '^go: m1_24_rc0'${/}'go.mod requires go >= 1.24rc0; switching to go1.24rc1$'
+cat go.work
+grep '^go 1.24rc0$' go.work
+grep '^toolchain go1.24rc1$' go.work
diff --git a/src/cmd/go/testdata/script/work_use.txt b/src/cmd/go/testdata/script/work_use.txt
new file mode 100644
index 0000000..7470899
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_use.txt
@@ -0,0 +1,36 @@
+go work use -r foo
+cmp go.work go.want_work_r
+
+! go work use other
+stderr '^go: error reading other'${/}'go.mod: missing module declaration'
+
+go mod edit -C other -module=other
+go work use other
+cmp go.work go.want_work_other
+-- go.work --
+go 1.18
+
+use (
+	foo
+	foo/bar // doesn't exist
+)
+-- go.want_work_r --
+go 1.18
+
+use (
+	./foo
+	./foo/bar/baz
+)
+-- go.want_work_other --
+go 1.18
+
+use (
+	./foo
+	./foo/bar/baz
+	./other
+)
+-- foo/go.mod --
+module foo
+-- foo/bar/baz/go.mod --
+module baz
+-- other/go.mod --
diff --git a/src/cmd/go/testdata/script/work_use_deleted.txt b/src/cmd/go/testdata/script/work_use_deleted.txt
new file mode 100644
index 0000000..b379cbc
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_use_deleted.txt
@@ -0,0 +1,22 @@
+go work use -r .
+cmp go.work go.work.want
+
+-- go.work --
+go 1.18
+
+use (
+	.
+	./sub
+	./sub/dir/deleted
+)
+-- go.work.want --
+go 1.18
+
+use ./sub/dir
+-- sub/README.txt --
+A go.mod file has been deleted from this directory.
+In addition, the entire subdirectory sub/dir/deleted
+has been deleted, along with sub/dir/deleted/go.mod.
+-- sub/dir/go.mod --
+module example/sub/dir
+go 1.18
diff --git a/src/cmd/go/testdata/script/work_use_dot.txt b/src/cmd/go/testdata/script/work_use_dot.txt
new file mode 100644
index 0000000..8f21042
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_use_dot.txt
@@ -0,0 +1,49 @@
+cp go.work go.work.orig
+
+# If the current directory contains a go.mod file,
+# 'go work use .' should add an entry for it.
+cd bar/baz
+go work use .
+cmp ../../go.work ../../go.work.rel
+
+# If the current directory lacks a go.mod file, 'go work use .'
+# should remove its entry.
+mv go.mod go.mod.bak
+go work use .
+cmp ../../go.work ../../go.work.orig
+
+# If the path is absolute, it should remain absolute.
+mv go.mod.bak go.mod
+go work use $PWD
+grep -count=1 '^use ' ../../go.work
+grep '^use ["]?'$PWD'["]?$' ../../go.work
+
+# An absolute path should replace an entry for the corresponding relative path
+# and vice-versa.
+go work use .
+cmp ../../go.work ../../go.work.rel
+go work use $PWD
+grep -count=1 '^use ' ../../go.work
+grep '^use ["]?'$PWD'["]?$' ../../go.work
+
+# If both the absolute and relative paths are named, 'go work use' should error
+# out: we don't know which one to use, and shouldn't add both because the
+# resulting workspace would contain a duplicate module.
+cp ../../go.work.orig ../../go.work
+! go work use $PWD .
+stderr '^go: already added "\./bar/baz" as "'$PWD'"$'
+cmp ../../go.work ../../go.work.orig
+
+
+-- go.mod --
+module example
+go 1.18
+-- go.work --
+go 1.18
+-- go.work.rel --
+go 1.18
+
+use ./bar/baz
+-- bar/baz/go.mod --
+module example/bar/baz
+go 1.18
diff --git a/src/cmd/go/testdata/script/work_use_issue50958.txt b/src/cmd/go/testdata/script/work_use_issue50958.txt
new file mode 100644
index 0000000..7a25531
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_use_issue50958.txt
@@ -0,0 +1,17 @@
+go work use -r .
+cmp go.work go.work.want
+
+-- go.mod --
+module example
+go 1.18
+-- go.work --
+go 1.18
+
+use sub
+-- go.work.want --
+go 1.18
+
+use .
+-- sub/README.txt --
+This directory no longer contains a go.mod file.
+
diff --git a/src/cmd/go/testdata/script/work_use_issue55952.txt b/src/cmd/go/testdata/script/work_use_issue55952.txt
new file mode 100644
index 0000000..befec67
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_use_issue55952.txt
@@ -0,0 +1,11 @@
+! go list .
+stderr '^go: cannot load module y listed in go\.work file: open y'${/}'go\.mod:'
+
+-- go.work --
+use ./y
+-- x/go.mod --
+module x
+
+go 1.19
+-- x/m.go --
+package m
diff --git a/src/cmd/go/testdata/script/work_use_only_dirs.txt b/src/cmd/go/testdata/script/work_use_only_dirs.txt
new file mode 100644
index 0000000..5d0fcdd
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_use_only_dirs.txt
@@ -0,0 +1,17 @@
+! go work use foo bar baz
+
+stderr '^go: foo is not a directory'
+stderr '^go: directory baz does not exist'
+cmp go.work go.work_want
+
+! go work use -r qux
+stderr '^go: qux is not a directory'
+
+-- go.work --
+go 1.18
+-- go.work_want --
+go 1.18
+-- foo --
+-- qux --
+-- bar/go.mod --
+module bar
diff --git a/src/cmd/go/testdata/script/work_use_toolchain.txt b/src/cmd/go/testdata/script/work_use_toolchain.txt
new file mode 100644
index 0000000..bb3db9c
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_use_toolchain.txt
@@ -0,0 +1,51 @@
+# Create basic modules and work space.
+env TESTGO_VERSION=go1.50
+mkdir m1_22_0
+go mod init -C m1_22_0
+go mod edit -C m1_22_0 -go=1.22.0 -toolchain=go1.99.0
+mkdir m1_22_1
+go mod init -C m1_22_1
+go mod edit -C m1_22_1 -go=1.22.1 -toolchain=go1.99.1
+mkdir m1_24_rc0
+go mod init -C m1_24_rc0
+go mod edit -C m1_24_rc0 -go=1.24rc0 -toolchain=go1.99.2
+
+go work init
+grep '^go 1.50$' go.work
+! grep toolchain go.work
+
+# work use with older modules should leave go 1.50 in the go.work.
+go work use ./m1_22_0
+grep '^go 1.50$' go.work
+! grep toolchain go.work
+
+# work use with newer modules should bump go and toolchain,
+# including updating to a newer toolchain as needed.
+env TESTGO_VERSION=go1.21
+env TESTGO_VERSION_SWITCH=switch
+rm go.work
+go work init
+env GOTOOLCHAIN=local
+! go work use ./m1_22_0
+stderr '^go: m1_22_0'${/}'go.mod requires go >= 1.22.0 \(running go 1.21; GOTOOLCHAIN=local\)$'
+env GOTOOLCHAIN=auto
+go work use ./m1_22_0
+stderr '^go: m1_22_0'${/}'go.mod requires go >= 1.22.0; switching to go1.22.9$'
+grep '^go 1.22.0$' go.work
+grep '^toolchain go1.22.9$' go.work
+
+# work use with an even newer module should bump go again.
+go work use ./m1_22_1
+! stderr switching
+grep '^go 1.22.1$' go.work
+grep '^toolchain go1.22.9$' go.work  # unchanged
+
+# work use with an even newer module should bump go and toolchain again.
+env GOTOOLCHAIN=go1.22.9
+! go work use ./m1_24_rc0
+stderr '^go: m1_24_rc0'${/}'go.mod requires go >= 1.24rc0 \(running go 1.22.9; GOTOOLCHAIN=go1.22.9\)$'
+env GOTOOLCHAIN=auto
+go work use ./m1_24_rc0
+stderr '^go: m1_24_rc0'${/}'go.mod requires go >= 1.24rc0; switching to go1.24rc1$'
+grep '^go 1.24rc0$' go.work
+grep '^toolchain go1.24rc1$' go.work
diff --git a/src/cmd/go/testdata/script/work_vet.txt b/src/cmd/go/testdata/script/work_vet.txt
new file mode 100644
index 0000000..f95cadd
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_vet.txt
@@ -0,0 +1,19 @@
+! go vet ./a
+stderr 'fmt.Println call has possible Printf formatting directive'
+
+-- go.work --
+go 1.18
+
+use ./a
+-- a/go.mod --
+module example.com/a
+
+go 1.18
+-- a/a.go --
+package a
+
+import "fmt"
+
+func A() {
+    fmt.Println("%s")
+}
\ No newline at end of file
diff --git a/src/cmd/go/testdata/script/work_why_download_graph.txt b/src/cmd/go/testdata/script/work_why_download_graph.txt
new file mode 100644
index 0000000..8f1aedd
--- /dev/null
+++ b/src/cmd/go/testdata/script/work_why_download_graph.txt
@@ -0,0 +1,65 @@
+# Test go mod download, why, and graph work in workspace mode.
+# TODO(bcmills): clarify the interaction with #44435
+
+go mod download rsc.io/quote
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.info
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.mod
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.zip
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.info
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.mod
+grep '^rsc\.io/quote v1\.5\.2/go\.mod h1:' go.work.sum
+grep '^rsc\.io/quote v1\.5\.2 h1:' go.work.sum
+
+go clean -modcache
+rm go.work.sum
+go mod download
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.info
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.mod
+exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.zip
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.info
+! exists $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.0.mod
+grep '^rsc\.io/quote v1\.5\.2/go\.mod h1:' go.work.sum
+grep '^rsc\.io/quote v1\.5\.2 h1:' go.work.sum
+
+go mod why rsc.io/quote
+stdout '# rsc.io/quote\nexample.com/a\nrsc.io/quote'
+
+go mod graph
+stdout 'example.com/a rsc.io/quote@v1.5.2\nexample.com/b example.com/c@v1.0.0\nrsc.io/quote@v1.5.2 rsc.io/sampler@v1.3.0\nrsc.io/sampler@v1.3.0 golang.org/x/text@v0.0.0-20170915032832-14c0d48ead0c'
+
+-- go.work --
+go 1.18
+
+use (
+	./a
+	./b
+)
+-- a/go.mod --
+go 1.18
+
+module example.com/a
+
+require "rsc.io/quote" v1.5.2
+-- a/main.go --
+package main
+
+import (
+	"fmt"
+	"rsc.io/quote"
+)
+
+func main() {
+	fmt.Println(quote.Hello())
+}
+-- b/go.mod --
+go 1.18
+
+module example.com/b
+
+require example.com/c v1.0.0
+replace example.com/c => ../c
+-- c/go.mod --
+go 1.18
+
+module example.com/c
+
diff --git a/src/cmd/go/testdata/vcstest/README b/src/cmd/go/testdata/vcstest/README
new file mode 100644
index 0000000..f3a0e15
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/README
@@ -0,0 +1,14 @@
+The scripts in this directory set up version-control repos for use in
+tests of cmd/go and its subpackages.
+
+They are written in a dialect of the same script language as in
+cmd/go/testdata/script, and the outputs are hosted by the server in
+cmd/go/internal/vcweb.
+
+To see the conditions and commands available for these scripts, run:
+
+	go test cmd/go/internal/vcweb -v --run=TestHelp
+
+To host these scripts in a standalone server, run:
+
+	go test cmd/go/internal/vcweb/vcstest -v --port=0
diff --git a/src/cmd/go/testdata/vcstest/auth/or401.txt b/src/cmd/go/testdata/vcstest/auth/or401.txt
new file mode 100644
index 0000000..10da48d
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/auth/or401.txt
@@ -0,0 +1,29 @@
+handle auth
+
+modzip vcs-test.golang.org/auth/or401/@v/v0.0.0-20190405155051-52df474c8a8b.zip vcs-test.golang.org/auth/or401@v0.0.0-20190405155051-52df474c8a8b .moddir
+
+-- .access --
+{
+	"Username": "aladdin",
+	"Password": "opensesame",
+	"StatusCode": 401,
+	"Message": "ACCESS DENIED, buddy"
+}
+-- index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/auth/or401 mod https://vcs-test.golang.org/auth/or401">
+-- vcs-test.golang.org/auth/or401/@v/list --
+v0.0.0-20190405155051-52df474c8a8b
+-- vcs-test.golang.org/auth/or401/@v/v0.0.0-20190405155051-52df474c8a8b.info --
+{"Version":"v0.0.0-20190405155051-52df474c8a8b","Time":"2019-04-05T15:50:51Z"}
+-- vcs-test.golang.org/auth/or401/@v/v0.0.0-20190405155051-52df474c8a8b.mod --
+module vcs-test.golang.org/auth/or401
+
+go 1.13
+-- .moddir/go.mod --
+module vcs-test.golang.org/auth/or401
+
+go 1.13
+-- .moddir/or401.go --
+package or401
diff --git a/src/cmd/go/testdata/vcstest/auth/or404.txt b/src/cmd/go/testdata/vcstest/auth/or404.txt
new file mode 100644
index 0000000..9e393c7
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/auth/or404.txt
@@ -0,0 +1,30 @@
+handle auth
+
+modzip vcs-test.golang.org/auth/or404/@v/v0.0.0-20190405155004-2234c475880e.zip vcs-test.golang.org/auth/or404@v0.0.0-20190405155004-2234c475880e .moddir
+
+-- .access --
+{
+	"Username": "aladdin",
+	"Password": "opensesame",
+	"StatusCode": 404,
+	"Message": "File? What file?"
+}
+-- index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/auth/or404 mod https://vcs-test.golang.org/auth/or404">
+-- vcs-test.golang.org/auth/or404/@v/list --
+v0.0.0-20190405155004-2234c475880e
+-- vcs-test.golang.org/auth/or404/@v/v0.0.0-20190405155004-2234c475880e.info --
+{"Version":"v0.0.0-20190405155004-2234c475880e","Time":"2019-04-05T15:50:04Z"}
+-- vcs-test.golang.org/auth/or404/@v/v0.0.0-20190405155004-2234c475880e.mod --
+module vcs-test.golang.org/auth/or404
+
+go 1.13
+-- .moddir/go.mod --
+module vcs-test.golang.org/auth/or404
+
+go 1.13
+-- .moddir/or404.go --
+package or404
+-- vcs-test.golang.org/go/modauth404/@v/list --
diff --git a/src/cmd/go/testdata/vcstest/auth/ormanylines.txt b/src/cmd/go/testdata/vcstest/auth/ormanylines.txt
new file mode 100644
index 0000000..41cf5fe
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/auth/ormanylines.txt
@@ -0,0 +1,9 @@
+handle auth
+
+-- .access --
+{
+	"Username": "aladdin",
+	"Password": "opensesame",
+	"StatusCode": 404,
+	"Message": "line 1\nline 2\nline 3\nline 4\nline 5\nline 6\nline 7\nline 8\nline 9\nline 10\nline 11\nline 12\nline 13\nline 14\nline 15\nline 16"
+}
diff --git a/src/cmd/go/testdata/vcstest/auth/oronelongline.txt b/src/cmd/go/testdata/vcstest/auth/oronelongline.txt
new file mode 100644
index 0000000..d27653a
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/auth/oronelongline.txt
@@ -0,0 +1,9 @@
+handle auth
+
+-- .access --
+{
+	"Username": "aladdin",
+	"Password": "opensesame",
+	"StatusCode": 404,
+	"Message": "blahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblahblah"
+}
diff --git a/src/cmd/go/testdata/vcstest/bzr/hello.txt b/src/cmd/go/testdata/vcstest/bzr/hello.txt
new file mode 100644
index 0000000..7d06503
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/bzr/hello.txt
@@ -0,0 +1,32 @@
+handle bzr
+
+env BZR_EMAIL='Russ Cox <rsc@google.com>'
+
+bzr init-repo .
+
+bzr init b
+cd b
+cp ../hello.go .
+bzr add hello.go
+bzr commit --commit-time='2017-09-21 21:20:12 -0400' -m 'hello world'
+bzr push ..
+cd ..
+rm b
+
+bzr log
+cmp stdout .bzr-log
+
+-- .bzr-log --
+------------------------------------------------------------
+revno: 1
+committer: Russ Cox <rsc@google.com>
+branch nick: b
+timestamp: Thu 2017-09-21 21:20:12 -0400
+message:
+  hello world
+-- hello.go --
+package main
+
+func main() {
+	println("hello, world")
+}
diff --git a/src/cmd/go/testdata/vcstest/fossil/hello.txt b/src/cmd/go/testdata/vcstest/fossil/hello.txt
new file mode 100644
index 0000000..48fb774
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/fossil/hello.txt
@@ -0,0 +1,22 @@
+handle fossil
+
+env USER=rsc
+fossil init --date-override 2017-09-22T01:15:36Z hello.fossil
+fossil open --keep hello.fossil
+
+fossil add hello.go
+fossil commit --no-prompt --nosign --date-override 2017-09-22T01:19:07Z --comment 'hello world'
+
+fossil timeline --oneline
+cmp stdout .fossil-timeline
+
+-- .fossil-timeline --
+d4c7dcdc29 hello world
+58da0d15e9 initial empty check-in
++++ no more data (2) +++
+-- hello.go --
+package main
+
+func main() {
+	println("hello, world")
+}
diff --git a/src/cmd/go/testdata/vcstest/git/commit-after-tag.txt b/src/cmd/go/testdata/vcstest/git/commit-after-tag.txt
new file mode 100644
index 0000000..eb13a63
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/git/commit-after-tag.txt
@@ -0,0 +1,39 @@
+handle git
+
+env GIT_AUTHOR_NAME='Bryan C. Mills'
+env GIT_AUTHOR_EMAIL='bcmills@google.com'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+git init
+
+at 2019-07-15T17:16:47-04:00
+git add go.mod main.go
+git commit -m 'all: add go.mod and main.go'
+git branch -m master
+git tag v1.0.0
+
+at 2019-07-15T17:17:27-04:00
+cp _next/main.go main.go
+git add main.go
+git commit -m 'add init function'
+
+git log --oneline --decorate=short
+cmp stdout .git-log
+
+-- .git-log --
+b325d82 (HEAD -> master) add init function
+8da67e0 (tag: v1.0.0) all: add go.mod and main.go
+-- go.mod --
+module vcs-test.golang.org/git/commit-after-tag.git
+
+go 1.13
+-- main.go --
+package main
+
+func main() {}
+-- _next/main.go --
+package main
+
+func main() {}
+func init() {}
diff --git a/src/cmd/go/testdata/vcstest/git/empty-v2-without-v1.txt b/src/cmd/go/testdata/vcstest/git/empty-v2-without-v1.txt
new file mode 100644
index 0000000..afe407e
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/git/empty-v2-without-v1.txt
@@ -0,0 +1,24 @@
+handle git
+
+env GIT_AUTHOR_NAME='Bryan C. Mills'
+env GIT_AUTHOR_EMAIL='bcmills@google.com'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+git init
+
+at 2019-10-07T14:15:32-04:00
+git add go.mod
+git commit -m 'add go.mod file without go source files'
+git branch -m master
+git tag v2.0.0
+
+git log --oneline --decorate=short
+cmp stdout .git-log
+
+-- .git-log --
+122733c (HEAD -> master, tag: v2.0.0) add go.mod file without go source files
+-- go.mod --
+module vcs-test.golang.org/git/empty-v2-without-v1.git/v2
+
+go 1.14
diff --git a/src/cmd/go/testdata/vcstest/git/emptytest.txt b/src/cmd/go/testdata/vcstest/git/emptytest.txt
new file mode 100644
index 0000000..4526202
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/git/emptytest.txt
@@ -0,0 +1,21 @@
+handle git
+
+env GIT_AUTHOR_NAME='Russ Cox'
+env GIT_AUTHOR_EMAIL='rsc@golang.org'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+git init
+
+at 2018-07-03T22:35:49-04:00
+git add go.mod
+git commit -m 'initial'
+git branch -m master
+
+git log --oneline
+cmp stdout .git-log
+
+-- .git-log --
+7bb9146 initial
+-- go.mod --
+module vcs-test.golang.org/git/emptytest.git
diff --git a/src/cmd/go/testdata/vcstest/git/gitrepo1.txt b/src/cmd/go/testdata/vcstest/git/gitrepo1.txt
new file mode 100644
index 0000000..7919089
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/git/gitrepo1.txt
@@ -0,0 +1,68 @@
+handle git
+
+env GIT_AUTHOR_NAME='Russ Cox'
+env GIT_AUTHOR_EMAIL='rsc@golang.org'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+git init
+
+at 2018-04-17T15:43:22-04:00
+unquote ''
+cp stdout README
+git add README
+git commit -a -m 'empty README'
+git branch -m master
+git tag v1.2.3
+
+at 2018-04-17T15:45:48-04:00
+git branch v2
+git checkout v2
+echo 'v2'
+cp stdout v2
+git add v2
+git commit -a -m 'v2'
+git tag v2.3
+git tag v2.0.1
+git branch v2.3.4
+
+at 2018-04-17T16:00:19-04:00
+echo 'intermediate'
+cp stdout foo.txt
+git add foo.txt
+git commit -a -m 'intermediate'
+
+at 2018-04-17T16:00:32-04:00
+echo 'another'
+cp stdout another.txt
+git add another.txt
+git commit -a -m 'another'
+git tag v2.0.2
+
+at 2018-04-17T16:16:52-04:00
+git checkout master
+git branch v3
+git checkout v3
+mkdir v3/sub/dir
+echo 'v3/sub/dir/file'
+cp stdout v3/sub/dir/file.txt
+git add v3
+git commit -a -m 'add v3/sub/dir/file.txt'
+
+at 2018-04-17T22:23:00-04:00
+git checkout master
+git tag -a v1.2.4-annotated -m 'v1.2.4-annotated'
+
+git show-ref --tags --heads
+cmp stdout .git-refs
+
+-- .git-refs --
+ede458df7cd0fdca520df19a33158086a8a68e81 refs/heads/master
+9d02800338b8a55be062c838d1f02e0c5780b9eb refs/heads/v2
+76a00fb249b7f93091bc2c89a789dab1fc1bc26f refs/heads/v2.3.4
+a8205f853c297ad2c3c502ba9a355b35b7dd3ca5 refs/heads/v3
+ede458df7cd0fdca520df19a33158086a8a68e81 refs/tags/v1.2.3
+b004e48a345a86ed7a2fb7debfa7e0b2f9b0dd91 refs/tags/v1.2.4-annotated
+76a00fb249b7f93091bc2c89a789dab1fc1bc26f refs/tags/v2.0.1
+9d02800338b8a55be062c838d1f02e0c5780b9eb refs/tags/v2.0.2
+76a00fb249b7f93091bc2c89a789dab1fc1bc26f refs/tags/v2.3
diff --git a/src/cmd/go/testdata/vcstest/git/hello.txt b/src/cmd/go/testdata/vcstest/git/hello.txt
new file mode 100644
index 0000000..17ba09c
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/git/hello.txt
@@ -0,0 +1,25 @@
+handle git
+
+env GIT_AUTHOR_NAME=bwk
+env GIT_AUTHOR_EMAIL=bwk
+env GIT_COMMITTER_NAME='Russ Cox'
+env GIT_COMMITTER_EMAIL='rsc@golang.org'
+
+git init
+
+at 2017-09-21T21:05:58-04:00
+git add hello.go
+git commit -a -m 'hello'
+git branch -m master
+
+git log --oneline --decorate=short
+cmp stdout .git-log
+
+-- .git-log --
+fc3a09f (HEAD -> master) hello
+-- hello.go --
+package main
+
+func main() {
+	println("hello, world")
+}
diff --git a/src/cmd/go/testdata/vcstest/git/insecurerepo.txt b/src/cmd/go/testdata/vcstest/git/insecurerepo.txt
new file mode 100644
index 0000000..e0ea62c
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/git/insecurerepo.txt
@@ -0,0 +1,32 @@
+handle git
+
+env GIT_AUTHOR_NAME='Bryan C. Mills'
+env GIT_AUTHOR_EMAIL='bcmills@google.com'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+git init
+
+at 2019-04-03T13:30:35-04:00
+git add go.mod
+git commit -m 'all: initialize module'
+git branch -m master
+
+at 2019-09-04T14:39:48-04:00
+git add main.go
+git commit -m 'main: add Go source file'
+
+git log --oneline --decorate=short
+cmp stdout .git-log
+
+-- .git-log --
+6fecd21 (HEAD -> master) main: add Go source file
+d1a15cd all: initialize module
+-- go.mod --
+module vcs-test.golang.org/insecure/go/insecure
+
+go 1.13
+-- main.go --
+package main
+
+func main() {}
diff --git a/src/cmd/go/testdata/vcstest/git/mainonly.txt b/src/cmd/go/testdata/vcstest/git/mainonly.txt
new file mode 100644
index 0000000..d294e34
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/git/mainonly.txt
@@ -0,0 +1,23 @@
+handle git
+
+env GIT_AUTHOR_NAME='Bryan C. Mills'
+env GIT_AUTHOR_EMAIL='bcmills@google.com'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+git init
+
+at 2019-09-05T14:07:43-04:00
+git add main.go
+git commit -a -m 'add main.go'
+git branch -m master
+
+git log --oneline --decorate=short
+cmp stdout .git-log
+
+-- .git-log --
+8a27e8b (HEAD -> master) add main.go
+-- main.go --
+package main
+
+func main() {}
diff --git a/src/cmd/go/testdata/vcstest/git/missingrepo.txt b/src/cmd/go/testdata/vcstest/git/missingrepo.txt
new file mode 100644
index 0000000..b947d8c
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/git/missingrepo.txt
@@ -0,0 +1,10 @@
+handle dir
+
+-- missingrepo-git/index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/go/missingrepo/missingrepo-git git https://vcs-test.golang.org/git/missingrepo">
+-- missingrepo-git/notmissing/index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/go/missingrepo/missingrepo-git/notmissing git https://vcs-test.golang.org/git/mainonly">
diff --git a/src/cmd/go/testdata/vcstest/git/modlegacy1-new.txt b/src/cmd/go/testdata/vcstest/git/modlegacy1-new.txt
new file mode 100644
index 0000000..ee14454
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/git/modlegacy1-new.txt
@@ -0,0 +1,33 @@
+handle git
+
+env GIT_AUTHOR_NAME='Russ Cox'
+env GIT_AUTHOR_EMAIL='rsc@golang.org'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+git init
+
+at 2018-04-25T11:00:57-04:00
+git add go.mod new.go p1 p2
+git commit -m 'initial commit'
+git branch -m master
+
+git log --oneline --decorate=short
+cmp stdout .git-log
+
+-- .git-log --
+36cc50a (HEAD -> master) initial commit
+-- go.mod --
+module "vcs-test.golang.org/git/modlegacy1-new.git/v2"
+-- new.go --
+package new
+
+import _ "vcs-test.golang.org/git/modlegacy1-new.git/v2/p2"
+-- p1/p1.go --
+package p1
+
+import _ "vcs-test.golang.org/git/modlegacy1-old.git/p2"
+import _ "vcs-test.golang.org/git/modlegacy1-new.git"
+import _ "vcs-test.golang.org/git/modlegacy1-new.git/p2"
+-- p2/p2.go --
+package p2
diff --git a/src/cmd/go/testdata/vcstest/git/modlegacy1-old.txt b/src/cmd/go/testdata/vcstest/git/modlegacy1-old.txt
new file mode 100644
index 0000000..bca8f06
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/git/modlegacy1-old.txt
@@ -0,0 +1,27 @@
+handle git
+
+env GIT_AUTHOR_NAME='Russ Cox'
+env GIT_AUTHOR_EMAIL='rsc@golang.org'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+git init
+
+at 2018-04-25T10:59:24-04:00
+git add p1 p2
+git commit -m 'initial commit'
+git branch -m master
+
+git log --oneline --decorate=short
+cmp stdout .git-log
+
+-- .git-log --
+6b4ba8b (HEAD -> master) initial commit
+-- p1/p1.go --
+package p1
+
+import _ "vcs-test.golang.org/git/modlegacy1-old.git/p2"
+import _ "vcs-test.golang.org/git/modlegacy1-new.git/p1"
+import _ "vcs-test.golang.org/git/modlegacy1-new.git"
+-- p2/p2.go --
+package p2
diff --git a/src/cmd/go/testdata/vcstest/git/no-tags.txt b/src/cmd/go/testdata/vcstest/git/no-tags.txt
new file mode 100644
index 0000000..5ff0091
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/git/no-tags.txt
@@ -0,0 +1,27 @@
+handle git
+
+env GIT_AUTHOR_NAME='Bryan C. Mills'
+env GIT_AUTHOR_EMAIL='bcmills@google.com'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+git init
+
+at 2019-07-15T17:20:47-04:00
+git add go.mod main.go
+git commit -m 'all: add go.mod and main.go'
+git branch -m master
+
+git log --oneline --decorate=short
+cmp stdout .git-log
+
+-- .git-log --
+e706ba1 (HEAD -> master) all: add go.mod and main.go
+-- go.mod --
+module vcs-test.golang.org/git/no-tags.git
+
+go 1.13
+-- main.go --
+package main
+
+func main() {}
diff --git a/src/cmd/go/testdata/vcstest/git/odd-tags.txt b/src/cmd/go/testdata/vcstest/git/odd-tags.txt
new file mode 100644
index 0000000..8e24867
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/git/odd-tags.txt
@@ -0,0 +1,49 @@
+handle git
+
+env GIT_AUTHOR_NAME='Bryan C. Mills'
+env GIT_AUTHOR_EMAIL='bcmills@google.com'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+git init
+
+at 2022-02-23T13:48:02-05:00
+git add README.txt
+git commit -m 'initial state'
+git branch -m main
+git tag 'v2.0.0+incompatible'
+
+at 2022-02-23T13:48:35-05:00
+git rm -r README.txt
+git add go.mod
+git commit -m 'migrate to Go modules'
+git tag 'v0.1.0+build-metadata'
+
+at 2022-02-23T14:41:55-05:00
+git branch v3-dev
+git checkout v3-dev
+cp v3/go.mod go.mod
+git commit go.mod -m 'update to /v3'
+git tag 'v3.0.0-20220223184802-12d19af20458'
+
+git checkout main
+
+git show-ref --tags --heads
+cmp stdout .git-refs
+
+-- .git-refs --
+9d863d525bbfcc8eda09364738c4032393711a56 refs/heads/main
+cce3d0f5d2ec85678cca3c45ac4a87f3be5efaca refs/heads/v3-dev
+9d863d525bbfcc8eda09364738c4032393711a56 refs/tags/v0.1.0+build-metadata
+12d19af204585b0db3d2a876ceddf5b9323f5a4a refs/tags/v2.0.0+incompatible
+cce3d0f5d2ec85678cca3c45ac4a87f3be5efaca refs/tags/v3.0.0-20220223184802-12d19af20458
+-- README.txt --
+This module lacks a go.mod file.
+-- go.mod --
+module vcs-test.golang.org/git/odd-tags.git
+
+go 1.18
+-- v3/go.mod --
+module vcs-test.golang.org/git/odd-tags.git/v3
+
+go 1.18
diff --git a/src/cmd/go/testdata/vcstest/git/prefixtagtests.txt b/src/cmd/go/testdata/vcstest/git/prefixtagtests.txt
new file mode 100644
index 0000000..6c89c85
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/git/prefixtagtests.txt
@@ -0,0 +1,53 @@
+handle git
+
+env GIT_AUTHOR_NAME='Jay Conrod'
+env GIT_AUTHOR_EMAIL='jayconrod@google.com'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+at 2019-05-09T18:35:00-04:00
+
+git init
+
+git add sub
+git commit -m 'create module sub'
+git branch -m master
+
+echo 'v0.1.0'
+cp stdout status
+git add status
+git commit -a -m 'v0.1.0'
+git tag 'v0.1.0'
+
+echo 'sub/v0.0.9'
+cp stdout status
+git commit -a -m 'sub/v0.0.9'
+git tag 'sub/v0.0.9'
+
+echo 'sub/v0.0.10'
+cp stdout status
+git commit -a -m 'sub/v0.0.10'
+git tag 'sub/v0.0.10'
+
+echo 'v0.2.0'
+cp stdout status
+git commit -a -m 'v0.2.0'
+git tag 'v0.2.0'
+
+echo 'after last tag'
+cp stdout status
+git commit -a -m 'after last tag'
+
+git show-ref --tags --heads
+cmp stdout .git-refs
+
+-- .git-refs --
+c3ee5d0dfbb9bf3c4d8bb2bce24cd8d14d2d4238 refs/heads/master
+2b7c4692e12c109263cab51b416fcc835ddd7eae refs/tags/sub/v0.0.10
+883885166298d79a0561d571a3044ec5db2e7c28 refs/tags/sub/v0.0.9
+db89fc573cfb939faf0aa0660671eb4cf8b8b673 refs/tags/v0.1.0
+1abe41965749e50828dd41de8d12c6ebc8e4e131 refs/tags/v0.2.0
+-- sub/go.mod --
+module vcs-test.golang.org/git/prefixtagtests.git/sub
+-- sub/sub.go --
+package sub
diff --git a/src/cmd/go/testdata/vcstest/git/querytest.txt b/src/cmd/go/testdata/vcstest/git/querytest.txt
new file mode 100644
index 0000000..b0f708a
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/git/querytest.txt
@@ -0,0 +1,273 @@
+handle git
+
+env GIT_AUTHOR_NAME='Russ Cox'
+env GIT_AUTHOR_EMAIL='rsc@golang.org'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+git init
+
+at 2018-07-03T22:31:01-04:00
+git add go.mod
+git commit -a -m 'v1'
+git branch -m master
+git tag start
+
+git branch v2
+
+at 2018-07-03T22:33:47-04:00
+echo 'before v0.0.0-pre1'
+cp stdout status
+git add status
+git commit -a -m 'before v0.0.0-pre1'
+
+echo 'at v0.0.0-pre1'
+cp stdout status
+git commit -a -m 'at v0.0.0-pre1'
+git tag 'v0.0.0-pre1'
+
+echo 'before v0.0.0'
+cp stdout status
+git commit -a -m 'before v0.0.0'
+
+echo 'at v0.0.0'
+cp stdout status
+git commit -a -m 'at v0.0.0'
+git tag 'v0.0.0'
+
+echo 'before v0.0.1'
+cp stdout status
+git commit -a -m 'before v0.0.1'
+
+echo 'at v0.0.1'
+cp stdout status
+git commit -a -m 'at v0.0.1'
+git tag 'v0.0.1'
+
+echo 'before v0.0.2'
+cp stdout status
+git commit -a -m 'before v0.0.2'
+
+echo 'at v0.0.2'
+cp stdout status
+git commit -a -m 'at v0.0.2'
+git tag 'v0.0.2'
+
+echo 'before v0.0.3'
+cp stdout status
+git commit -a -m 'before v0.0.3'
+
+echo 'at v0.0.3'
+cp stdout status
+git commit -a -m 'at v0.0.3'
+git tag 'v0.0.3'
+git tag favorite
+
+echo 'before v0.1.0'
+cp stdout status
+git commit -a -m 'before v0.1.0'
+
+echo 'at v0.1.0'
+cp stdout status
+git commit -a -m 'at v0.1.0'
+git tag v0.1.0
+
+echo 'before v0.1.1'
+cp stdout status
+git commit -a -m 'before v0.1.1'
+
+echo 'at v0.1.1'
+cp stdout status
+git commit -a -m 'at v0.1.1'
+git tag 'v0.1.1'
+
+echo 'before v0.1.2'
+cp stdout status
+git commit -a -m 'before v0.1.2'
+
+echo 'at v0.1.2'
+cp stdout status
+git commit -a -m 'at v0.1.2'
+git tag 'v0.1.2'
+
+echo 'before v0.3.0'
+cp stdout status
+git commit -a -m 'before v0.3.0'
+
+echo 'at v0.3.0'
+cp stdout status
+git commit -a -m 'at v0.3.0'
+git tag 'v0.3.0'
+
+echo 'before v1.0.0'
+cp stdout status
+git commit -a -m 'before v1.0.0'
+
+echo 'at v1.0.0'
+cp stdout status
+git commit -a -m 'at v1.0.0'
+git tag 'v1.0.0'
+
+echo 'before v1.1.0'
+cp stdout status
+git commit -a -m 'before v1.1.0'
+
+echo 'at v1.1.0'
+cp stdout status
+git commit -a -m 'at v1.1.0'
+git tag 'v1.1.0'
+
+echo 'before v1.9.0'
+cp stdout status
+git commit -a -m 'before v1.9.0'
+
+echo 'at v1.9.0'
+cp stdout status
+git commit -a -m 'at v1.9.0'
+git tag 'v1.9.0'
+
+echo 'before v1.9.9'
+cp stdout status
+git commit -a -m 'before v1.9.9'
+
+echo 'at v1.9.9'
+cp stdout status
+git commit -a -m 'at v1.9.9'
+git tag 'v1.9.9'
+
+at 2018-07-03T22:45:01-04:00
+echo 'before v1.9.10-pre1'
+cp stdout status
+git commit -a -m 'before v1.9.10-pre1'
+
+echo 'at v1.9.10-pre1'
+cp stdout status
+git commit -a -m 'at v1.9.10-pre1'
+git tag 'v1.9.10-pre1'
+
+at 2018-07-03T22:50:24-04:00
+git checkout v2
+cp v2/go.mod go.mod
+git add go.mod
+git commit -a -m 'v2'
+
+at 2018-07-03T22:51:14-04:00
+echo 'before v2.0.0'
+cp stdout status
+git add status
+git commit -a -m 'before v2.0.0'
+
+at 2018-07-03T22:51:14-04:00
+echo 'at v2.0.0'
+cp stdout status
+git commit -a -m 'at v2.0.0'
+git tag 'v2.0.0'
+
+at 2018-07-03T22:51:14-04:00
+echo 'before v2.1.0'
+cp stdout status
+git commit -a -m 'before v2.1.0'
+
+at 2018-07-03T22:51:14-04:00
+echo 'at v2.1.0'
+cp stdout status
+git commit -a -m 'at v2.1.0'
+git tag 'v2.1.0'
+
+at 2018-07-03T22:51:14-04:00
+echo 'before v2.2.0'
+cp stdout status
+git commit -a -m 'before v2.2.0'
+
+at 2018-07-03T22:51:14-04:00
+echo 'at v2.2.0'
+cp stdout status
+git commit -a -m 'at v2.2.0'
+git tag 'v2.2.0'
+
+at 2018-07-03T22:51:14-04:00
+echo 'before v2.5.5'
+cp stdout status
+git commit -a -m 'before v2.5.5'
+
+at 2018-07-03T22:51:14-04:00
+echo 'at v2.5.5'
+cp stdout status
+git commit -a -m 'at v2.5.5'
+git tag 'v2.5.5'
+
+at 2018-07-03T23:35:18-04:00
+echo 'after v2.5.5'
+cp stdout status
+git commit -a -m 'after v2.5.5'
+
+
+env GIT_AUTHOR_NAME='Bryan C. Mills'
+env GIT_AUTHOR_EMAIL=bcmills@google.com
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+git checkout v2.5.5
+
+at 2019-05-13T17:13:56-04:00
+echo 'before v2.6.0-pre1'
+cp stdout status
+git commit -a -m 'before v2.6.0-pre1'
+
+at 2019-05-13T17:13:56-04:00
+echo 'at v2.6.0-pre1'
+cp stdout status
+git commit -a -m 'at v2.6.0-pre1'
+git tag 'v2.6.0-pre1'
+
+git checkout master
+
+at 2019-05-13T16:11:25-04:00
+echo 'before v1.9.10-pre2+metadata'
+cp stdout status
+git commit -a -m 'before v1.9.10-pre2+metadata'
+
+at 2019-05-13T16:11:26-04:00
+echo 'at v1.9.10-pre2+metadata'
+cp stdout status
+git commit -a -m 'at v1.9.10-pre2+metadata'
+git tag 'v1.9.10-pre2+metadata'
+
+at 2019-12-20T08:46:14-05:00
+echo 'after v1.9.10-pre2+metadata'
+cp stdout status
+git commit -a -m 'after v1.9.10-pre2+metadata'
+
+
+git show-ref --tags --heads
+cmp stdout .git-refs
+
+-- .git-refs --
+ed5ffdaa1f5e7e0be6f5ba2d63097026506224f2 refs/heads/master
+feed8f518cf4a7215a3b2a8268b8b0746dcbb12d refs/heads/v2
+f6abd4e3ed7f2297bc8fd2888bd6d5412e255fcc refs/tags/favorite
+5e9e31667ddfe16e9350f4bd00acc933c8cd5e56 refs/tags/start
+0de900e0063bcc310ea0621bfbc227a9b4e3b020 refs/tags/v0.0.0
+e5ec98b1c15df29e3bd346d538d73b6e8c3b500c refs/tags/v0.0.0-pre1
+179bc86b1be3f6d4553f77ebe68a8b6d750ceff8 refs/tags/v0.0.1
+81da2346e009fa1072fe4de3a9a223398ea8ec39 refs/tags/v0.0.2
+f6abd4e3ed7f2297bc8fd2888bd6d5412e255fcc refs/tags/v0.0.3
+7a1b6bf60ae5bb2b2bd49d152e0bbad806056122 refs/tags/v0.1.0
+daedca9abee3171fe45e0344098a993675ac799e refs/tags/v0.1.1
+ce829e0f1c45a2eca0f1ad16d7c1aca7cddb433b refs/tags/v0.1.2
+44aadfee25d86acb32d6f352afd1d602b0e3a651 refs/tags/v0.3.0
+20756d3a393908b2edb5db0f0bb954e962860168 refs/tags/v1.0.0
+b0bf267f64b7d5b5cabe22fbcad22f3f1642b7e5 refs/tags/v1.1.0
+609dca58c03f0ddf1d8ebe46c1f74fc6a99f3e73 refs/tags/v1.9.0
+e0cf3de987e660c21b6950e85b317ce5f7fbb9d9 refs/tags/v1.9.10-pre1
+42abcb6df8eee6983aeca9a307c28ea40530aceb refs/tags/v1.9.10-pre2+metadata
+5ba9a4ea62136ae86213feba68bc73858f55b7e1 refs/tags/v1.9.9
+9763aa065ae27c6cacec5ca8b6dfa43a1b31dea0 refs/tags/v2.0.0
+23c28cb696ff40a2839ce406f2c173aa6c3cdda6 refs/tags/v2.1.0
+1828ee9f8074075675013e4d488d5d49ddc1b502 refs/tags/v2.2.0
+d7352560158175e3b6aa11e22efb06d9e87e6eea refs/tags/v2.5.5
+fb9e35b393eb0cccc37e13e243ce60b4ff8c7eea refs/tags/v2.6.0-pre1
+-- go.mod --
+module vcs-test.golang.org/git/querytest.git
+-- v2/go.mod --
+module vcs-test.golang.org/git/querytest.git/v2
diff --git a/src/cmd/go/testdata/vcstest/git/retract-pseudo.txt b/src/cmd/go/testdata/vcstest/git/retract-pseudo.txt
new file mode 100644
index 0000000..e189484
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/git/retract-pseudo.txt
@@ -0,0 +1,33 @@
+handle git
+
+env GIT_AUTHOR_NAME='Jay Conrod'
+env GIT_AUTHOR_EMAIL='jayconrod@google.com'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+at 2020-10-09T13:37:47-04:00
+
+git init
+
+git add go.mod p.go
+git commit -m 'create module retract-pseudo'
+git branch -m main
+git tag v1.0.0
+
+git mv p.go q.go
+git commit -m 'trivial change'
+
+git show-ref --tags --heads
+cmp stdout .git-refs
+
+-- .git-refs --
+713affd19d7b9b6dc876b603017f3dcaab8ba674 refs/heads/main
+64c061ed4371ef372b6bbfd58ee32015d6bfc3e5 refs/tags/v1.0.0
+-- go.mod --
+module vcs-test.golang.org/git/retract-pseudo.git
+
+go 1.16
+
+retract v1.0.0
+-- p.go --
+package p
diff --git a/src/cmd/go/testdata/vcstest/git/semver-branch.txt b/src/cmd/go/testdata/vcstest/git/semver-branch.txt
new file mode 100644
index 0000000..69e1762
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/git/semver-branch.txt
@@ -0,0 +1,53 @@
+handle git
+
+env GIT_AUTHOR_NAME='Bryan C. Mills'
+env GIT_AUTHOR_EMAIL='bcmills@google.com'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+git init
+
+at 2022-02-02T14:15:21-05:00
+git add pkg go.mod
+git commit -a -m 'pkg: add empty package'
+git branch -m main
+git tag 'v0.1.0'
+
+at 2022-02-02T14:19:44-05:00
+git branch 'v1.0.0'
+git branch 'v2.0.0'
+git checkout 'v1.0.0'
+cp v1/pkg/pkg.go pkg/pkg.go
+git commit -a -m 'pkg: start developing toward v1.0.0'
+
+at 2022-02-03T10:53:13-05:00
+git branch 'v3.0.0-devel'
+git checkout 'v3.0.0-devel'
+git checkout v0.1.0 pkg/pkg.go
+git commit -a -m 'pkg: remove panic'
+git tag v4.0.0-beta.1
+
+git checkout main
+
+git show-ref --tags --heads
+cmp stdout .git-refs
+
+-- .git-refs --
+33ea7ee36f3e3f44f528664b3712c9fa0cef7502 refs/heads/main
+09c4d8f6938c7b5eeae46858a72712b8700fa46a refs/heads/v1.0.0
+33ea7ee36f3e3f44f528664b3712c9fa0cef7502 refs/heads/v2.0.0
+d59622f6e4d77f008819083582fde71ea1921b0c refs/heads/v3.0.0-devel
+33ea7ee36f3e3f44f528664b3712c9fa0cef7502 refs/tags/v0.1.0
+d59622f6e4d77f008819083582fde71ea1921b0c refs/tags/v4.0.0-beta.1
+-- go.mod --
+module vcs-test.golang.org/git/semver-branch.git
+
+go 1.16
+-- pkg/pkg.go --
+package pkg
+-- v1/pkg/pkg.go --
+package pkg
+
+func init() {
+	panic("TODO")
+}
diff --git a/src/cmd/go/testdata/vcstest/git/tagtests.txt b/src/cmd/go/testdata/vcstest/git/tagtests.txt
new file mode 100644
index 0000000..92e79cd
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/git/tagtests.txt
@@ -0,0 +1,44 @@
+handle git
+
+env GIT_AUTHOR_NAME='Jay Conrod'
+env GIT_AUTHOR_EMAIL='jayconrod@google.com'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+at 2019-05-09T18:56:25-04:00
+
+git init
+
+git add go.mod tagtests.go
+git commit -m 'create module tagtests'
+git branch -m master
+git branch b
+
+git add v0.2.1
+git commit -m 'v0.2.1'
+git tag 'v0.2.1'
+
+git checkout b
+git add 'v0.2.2'
+git commit -m 'v0.2.2'
+git tag 'v0.2.2'
+
+git checkout master
+git merge b -m 'merge'
+
+git show-ref --tags --heads
+cmp stdout .git-refs
+
+-- .git-refs --
+59356c8cd18c5fe9a598167d98a6843e52d57952 refs/heads/b
+c7818c24fa2f3f714c67d0a6d3e411c85a518d1f refs/heads/master
+101c49f5af1b2466332158058cf5f03c8cca6429 refs/tags/v0.2.1
+59356c8cd18c5fe9a598167d98a6843e52d57952 refs/tags/v0.2.2
+-- go.mod --
+module vcs-test.golang.org/git/tagtests.git
+-- tagtests.go --
+package tagtests
+-- v0.2.1 --
+v0.2.1
+-- v0.2.2 --
+v0.2.2
diff --git a/src/cmd/go/testdata/vcstest/git/v2repo.txt b/src/cmd/go/testdata/vcstest/git/v2repo.txt
new file mode 100644
index 0000000..6cbe924
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/git/v2repo.txt
@@ -0,0 +1,26 @@
+handle git
+
+env GIT_AUTHOR_NAME='Bryan C. Mills'
+env GIT_AUTHOR_EMAIL='bcmills@google.com'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+git init
+
+at 2019-04-03T11:52:15-04:00
+env GIT_AUTHOR_DATE=2019-04-03T11:44:11-04:00
+git add go.mod
+git commit -m 'all: add go.mod'
+git branch -m master
+git tag 'v2.0.0'
+
+git show-ref --tags --heads
+cmp stdout .git-refs
+
+-- .git-refs --
+203b91c896acd173aa719e4cdcb7d463c4b090fa refs/heads/master
+203b91c896acd173aa719e4cdcb7d463c4b090fa refs/tags/v2.0.0
+-- go.mod --
+module vcs-test.golang.org/go/v2module/v2
+
+go 1.12
diff --git a/src/cmd/go/testdata/vcstest/git/v2sub.txt b/src/cmd/go/testdata/vcstest/git/v2sub.txt
new file mode 100644
index 0000000..5d4ab58
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/git/v2sub.txt
@@ -0,0 +1,35 @@
+handle git
+
+env GIT_AUTHOR_NAME='Bryan C. Mills'
+env GIT_AUTHOR_EMAIL='bcmills@google.com'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+git init
+
+at 2022-02-22T15:53:33-05:00
+git add v2sub.go v2
+git commit -m 'all: add package v2sub and v2sub/v2'
+git branch -m main
+git tag v2.0.0
+
+at 2022-02-22T15:55:07-05:00
+git add README.txt
+git commit -m 'v2sub: add README.txt'
+
+git show-ref --tags --heads
+cmp stdout .git-refs
+
+-- .git-refs --
+80beb17a16036f17a5aedd1bb5bd6d407b3c6dc5 refs/heads/main
+5fcd3eaeeb391d399f562fd45a50dac9fc34ae8b refs/tags/v2.0.0
+-- v2/go.mod --
+module vcs-test.golang.org/git/v2sub.git/v2
+
+go 1.16
+-- v2/v2sub.go --
+package v2sub
+-- v2sub.go --
+package v2sub
+-- README.txt --
+This root module lacks a go.mod file.
diff --git a/src/cmd/go/testdata/vcstest/git/v3pkg.txt b/src/cmd/go/testdata/vcstest/git/v3pkg.txt
new file mode 100644
index 0000000..af18e01
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/git/v3pkg.txt
@@ -0,0 +1,28 @@
+handle git
+
+env GIT_AUTHOR_NAME='Bryan C. Mills'
+env GIT_AUTHOR_EMAIL='bcmills@google.com'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+git init
+
+at 2019-07-15T14:01:24-04:00
+env GIT_AUTHOR_DATE=2019-07-15T13:59:34-04:00
+git add go.mod v3pkg.go
+git commit -a -m 'all: add go.mod with v3 path'
+git branch -m master
+git tag 'v3.0.0'
+
+git show-ref --tags --heads
+cmp stdout .git-refs
+
+-- .git-refs --
+a3eab1261b8e3164bcbde9171c23d5fd36e32a85 refs/heads/master
+a3eab1261b8e3164bcbde9171c23d5fd36e32a85 refs/tags/v3.0.0
+-- go.mod --
+module vcs-test.golang.org/git/v3pkg.git/v3
+
+go 1.13
+-- v3pkg.go --
+package v3pkg
diff --git a/src/cmd/go/testdata/vcstest/git/vgotest1.txt b/src/cmd/go/testdata/vcstest/git/vgotest1.txt
new file mode 100644
index 0000000..d2fc741
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/git/vgotest1.txt
@@ -0,0 +1,257 @@
+handle git
+
+env GIT_AUTHOR_NAME='Russ Cox'
+env GIT_AUTHOR_EMAIL='rsc@golang.org'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+git init
+
+at 2018-02-19T17:21:09-05:00
+git add LICENSE README.md
+git commit -m 'initial commit'
+git branch -m master
+
+git checkout --detach HEAD
+
+at 2018-02-19T18:10:06-05:00
+mkdir pkg
+echo 'package p // pkg/p.go'
+cp stdout pkg/p.go
+git add pkg/p.go
+git commit -m 'add pkg/p.go'
+git tag v0.0.0
+git tag v1.0.0
+git tag mytag
+
+git checkout --detach HEAD
+
+at 2018-02-19T18:14:23-05:00
+mkdir v2
+echo 'module "github.com/rsc/vgotest1/v2" // root go.mod'
+cp stdout go.mod
+git add go.mod
+git commit -m 'go.mod v2'
+git tag v2.0.1
+
+at 2018-02-19T18:15:11-05:00
+mkdir submod/pkg
+echo 'package p // submod/pkg/p.go'
+cp stdout submod/pkg/p.go
+git add submod/pkg/p.go
+git commit -m 'submod/pkg/p.go'
+git tag v2.0.2
+
+at 2018-02-19T18:16:04-05:00
+echo 'module "github.com/rsc/vgotest" // v2/go.mod'
+cp stdout v2/go.mod
+git add v2/go.mod
+git commit -m 'v2/go.mod: bad go.mod (no version)'
+git tag v2.0.3
+
+at 2018-02-19T19:03:38-05:00
+env GIT_AUTHOR_DATE=2018-02-19T18:16:38-05:00
+echo 'module "github.com/rsc/vgotest1/v2" // v2/go.mod'
+cp stdout v2/go.mod
+git add v2/go.mod
+git commit -m 'v2/go.mod: fix'
+git tag v2.0.4
+
+at 2018-02-19T19:03:59-05:00
+env GIT_AUTHOR_DATE=2018-02-19T18:17:02-05:00
+echo 'module "github.com/rsc/vgotest1" // root go.mod'
+cp stdout go.mod
+git add go.mod
+git commit -m 'go.mod: drop v2'
+git tag v2.0.5
+
+git checkout --detach mytag
+
+at 2018-02-19T18:10:28-05:00
+echo 'module "github.com/rsc/vgotest1" // root go.mod'
+cp stdout go.mod
+git add go.mod
+git commit -m 'go.mod'
+git tag v0.0.1
+git tag v1.0.1
+
+at 2018-02-19T18:11:28-05:00
+mkdir submod/pkg
+echo 'package pkg // submod/pkg/p.go'
+cp stdout submod/pkg/p.go
+git add submod
+git commit -m 'submod/pkg/p.go'
+git tag v1.0.2
+
+at 2018-02-19T18:12:07-05:00
+echo 'module "github.com/vgotest1/submod" // submod/go.mod'
+cp stdout submod/go.mod
+git add submod/go.mod
+git commit -m 'submod/go.mod'
+git tag v1.0.3
+git tag submod/v1.0.4
+
+at 2018-02-19T18:12:59-05:00
+git apply 0001-submod-go.mod-add-require-vgotest1-v1.1.0.patch
+git commit -a -m 'submod/go.mod: add require vgotest1 v1.1.0'
+git tag submod/v1.0.5
+
+at 2018-02-19T18:13:36-05:00
+git apply 0002-go.mod-add-require-submod-v1.0.5.patch
+git commit -a -m 'go.mod: add require submod v1.0.5'
+git tag v1.1.0
+
+git checkout master
+
+at 2018-02-19T17:23:01-05:00
+mkdir pkg
+echo 'package pkg'
+cp stdout pkg/p.go
+git add pkg/p.go
+git commit -m 'pkg: add'
+
+at 2018-02-19T17:30:23-05:00
+env GIT_AUTHOR_DATE=2018-02-19T17:24:48-05:00
+echo 'module "github.com/vgotest1/v2"'
+cp stdout go.mod
+git add go.mod
+git commit -m 'add go.mod'
+
+at 2018-02-19T17:30:45-05:00
+echo 'module "github.com/vgotest1"'
+cp stdout go.mod
+git add go.mod
+git commit -m 'bad mod path'
+
+at 2018-02-19T17:31:34-05:00
+mkdir v2
+echo 'module "github.com/vgotest1/v2"'
+cp stdout v2/go.mod
+git add v2/go.mod
+git commit -m 'add v2/go.mod'
+
+at 2018-02-19T17:32:37-05:00
+echo 'module "github.com/vgotest1/v2"'
+cp stdout go.mod
+git add go.mod
+git commit -m 'say v2 in root go.mod'
+
+git checkout --detach HEAD
+at 2018-02-19T17:51:24-05:00
+	# README.md at this commit lacked a trailing newline, so 'git apply' can't
+	# seem to apply it correctly as a patch. Instead, we use 'echo -e' to write
+	# the exact contents.
+unquote 'This is a test repo for versioned go.\nThere''s nothing useful here.\n\n	v0.0.0 - has pkg/p.go\n	v0.0.1 - has go.mod\n	\n	v1.0.0 - has pkg/p.go\n	v1.0.1 - has go.mod\n	v1.0.2 - has submod/pkg/p.go\n	v1.0.3 - has submod/go.mod\n	submod/v1.0.4 - same\n	submod/v1.0.5 - add requirement on v1.1.0\n	v1.1.0 - add requirement on submod/v1.0.5\n	\n	v2.0.0 - has pkg/p.go\n	v2.0.1 - has go.mod with v2 module path\n	v2.0.2 - has go.mod with v1 (no version) module path\n	v2.0.3 - has v2/go.mod with v2 module path\n	v2.0.5 - has go.mod AND v2/go.mod with v2 module path\n	'
+cp stdout README.md
+mkdir v2/pkg
+echo 'package q'
+cp stdout v2/pkg/q.go
+git add README.md v2/pkg/q.go
+git commit -m 'add q'
+git tag v2.0.6
+
+git checkout --detach mytag~1
+at 2018-07-18T21:21:27-04:00
+env GIT_AUTHOR_DATE=2018-02-19T18:10:06-05:00
+mkdir pkg
+echo 'package p // pkg/p.go'
+cp stdout pkg/p.go
+git add pkg/p.go
+unquote 'add pkg/p.go\n\nv2\n'
+cp stdout COMMIT_MSG
+git commit -F COMMIT_MSG
+git tag v2.0.0
+
+git checkout master
+
+git show-ref --tags --heads
+cmp stdout .git-refs
+
+-- .git-refs --
+a08abb797a6764035a9314ed5f1d757e0224f3bf refs/heads/master
+80d85c5d4d17598a0e9055e7c175a32b415d6128 refs/tags/mytag
+8afe2b2efed96e0880ecd2a69b98a53b8c2738b6 refs/tags/submod/v1.0.4
+70fd92eaa4dacf82548d0c6099f5b853ae2c1fc8 refs/tags/submod/v1.0.5
+80d85c5d4d17598a0e9055e7c175a32b415d6128 refs/tags/v0.0.0
+5a115c66393dd8c4a5cc3215653850d7f5640d0e refs/tags/v0.0.1
+80d85c5d4d17598a0e9055e7c175a32b415d6128 refs/tags/v1.0.0
+5a115c66393dd8c4a5cc3215653850d7f5640d0e refs/tags/v1.0.1
+2e38a1a347ba4d9e9946ec0ce480710ff445c919 refs/tags/v1.0.2
+8afe2b2efed96e0880ecd2a69b98a53b8c2738b6 refs/tags/v1.0.3
+b769f2de407a4db81af9c5de0a06016d60d2ea09 refs/tags/v1.1.0
+45f53230a74ad275c7127e117ac46914c8126160 refs/tags/v2.0.0
+ea65f87c8f52c15ea68f3bdd9925ef17e20d91e9 refs/tags/v2.0.1
+f7b23352af1cd750b11e4673b20b24c2d239430a refs/tags/v2.0.2
+f18795870fb14388a21ef3ebc1d75911c8694f31 refs/tags/v2.0.3
+1f863feb76bc7029b78b21c5375644838962f88d refs/tags/v2.0.4
+2f615117ce481c8efef46e0cc0b4b4dccfac8fea refs/tags/v2.0.5
+a01a0aef06cbd571294fc5451788cd4eadbfd651 refs/tags/v2.0.6
+-- LICENSE --
+Copyright (c) 2009 The Go Authors. All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+   * Redistributions of source code must retain the above copyright
+notice, this list of conditions and the following disclaimer.
+   * Redistributions in binary form must reproduce the above
+copyright notice, this list of conditions and the following disclaimer
+in the documentation and/or other materials provided with the
+distribution.
+   * Neither the name of Google Inc. nor the names of its
+contributors may be used to endorse or promote products derived from
+this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+-- README.md --
+This is a test repo for versioned go.
+There's nothing useful here.
+-- 0001-submod-go.mod-add-require-vgotest1-v1.1.0.patch --
+From 70fd92eaa4dacf82548d0c6099f5b853ae2c1fc8 Mon Sep 17 00:00:00 2001
+From: Russ Cox <rsc@golang.org>
+Date: Mon, 19 Feb 2018 18:12:59 -0500
+Subject: [PATCH] submod/go.mod: add require vgotest1 v1.1.0
+
+---
+ submod/go.mod | 1 +
+ 1 file changed, 1 insertion(+)
+
+diff --git a/submod/go.mod b/submod/go.mod
+index 7b18d93..c88de0f 100644
+--- a/submod/go.mod
++++ b/submod/go.mod
+@@ -1 +1,2 @@
+ module "github.com/vgotest1/submod" // submod/go.mod
++require "github.com/vgotest1" v1.1.0
+--
+2.36.1.838.g23b219f8e3
+-- 0002-go.mod-add-require-submod-v1.0.5.patch --
+From b769f2de407a4db81af9c5de0a06016d60d2ea09 Mon Sep 17 00:00:00 2001
+From: Russ Cox <rsc@golang.org>
+Date: Mon, 19 Feb 2018 18:13:36 -0500
+Subject: [PATCH] go.mod: add require submod v1.0.5
+
+---
+ go.mod | 1 +
+ 1 file changed, 1 insertion(+)
+
+diff --git a/go.mod b/go.mod
+index ac7a6d7..6118671 100644
+--- a/go.mod
++++ b/go.mod
+@@ -1 +1,2 @@
+ module "github.com/rsc/vgotest1" // root go.mod
++require "github.com/rsc/vgotest1/submod" v1.0.5
+--
+2.36.1.838.g23b219f8e3
diff --git a/src/cmd/go/testdata/vcstest/go/custom-hg-hello.txt b/src/cmd/go/testdata/vcstest/go/custom-hg-hello.txt
new file mode 100644
index 0000000..40b1ef6
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/go/custom-hg-hello.txt
@@ -0,0 +1,4 @@
+handle dir
+
+-- index.html --
+<meta name="go-import" content="vcs-test.golang.org/go/custom-hg-hello hg https://vcs-test.golang.org/hg/custom-hg-hello">
diff --git a/src/cmd/go/testdata/vcstest/go/insecure.txt b/src/cmd/go/testdata/vcstest/go/insecure.txt
new file mode 100644
index 0000000..6eb83c3
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/go/insecure.txt
@@ -0,0 +1,6 @@
+handle dir
+
+-- index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/insecure/go/insecure git https://vcs-test.golang.org/git/insecurerepo">
diff --git a/src/cmd/go/testdata/vcstest/go/missingrepo.txt b/src/cmd/go/testdata/vcstest/go/missingrepo.txt
new file mode 100644
index 0000000..9db6c14
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/go/missingrepo.txt
@@ -0,0 +1,18 @@
+handle dir
+
+-- missingrepo-git/index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/go/missingrepo/missingrepo-git git https://vcs-test.golang.org/git/missingrepo">
+-- missingrepo-git/notmissing/index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/go/missingrepo/missingrepo-git/notmissing git https://vcs-test.golang.org/git/mainonly">
+-- missingrepo-git-ssh/index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/go/missingrepo/missingrepo-git-ssh git ssh://nonexistent.vcs-test.golang.org/git/missingrepo">
+-- missingrepo-git-ssh/notmissing/index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/go/missingrepo/missingrepo-git-ssh/notmissing git https://vcs-test.golang.org/git/mainonly">
diff --git a/src/cmd/go/testdata/vcstest/go/mod/gitrepo1.txt b/src/cmd/go/testdata/vcstest/go/mod/gitrepo1.txt
new file mode 100644
index 0000000..0e727d3
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/go/mod/gitrepo1.txt
@@ -0,0 +1,6 @@
+handle dir
+
+-- index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/go/mod/gitrepo1 git https://vcs-test.golang.org/git/gitrepo1">
diff --git a/src/cmd/go/testdata/vcstest/go/modauth404.txt b/src/cmd/go/testdata/vcstest/go/modauth404.txt
new file mode 100644
index 0000000..51f25a9
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/go/modauth404.txt
@@ -0,0 +1,6 @@
+handle dir
+
+-- index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/go/modauth404 mod https://vcs-test.golang.org/auth/or404">
diff --git a/src/cmd/go/testdata/vcstest/go/test1-svn-git.txt b/src/cmd/go/testdata/vcstest/go/test1-svn-git.txt
new file mode 100644
index 0000000..42dc949
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/go/test1-svn-git.txt
@@ -0,0 +1,30 @@
+handle dir
+
+-- aaa/index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/go/test1-svn-git svn https://vcs-test.golang.org/svn/test1-svn-git">
+-- git-README-only/index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/go/test1-svn-git/git-README-only git https://vcs-test.golang.org/git/README-only">
+-- git-README-only/other/index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/go/test1-svn-git/git-README-only git https://vcs-test.golang.org/git/README-only">
+-- git-README-only/pkg/index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/go/test1-svn-git/git-README-only git https://vcs-test.golang.org/git/README-only">
+-- index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/go/test1-svn-git svn https://vcs-test.golang.org/svn/test1-svn-git">
+-- other/index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/go/test1-svn-git svn https://vcs-test.golang.org/svn/test1-svn-git">
+-- tiny/index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/go/test1-svn-git svn https://vcs-test.golang.org/svn/test1-svn-git">
diff --git a/src/cmd/go/testdata/vcstest/go/test2-svn-git.txt b/src/cmd/go/testdata/vcstest/go/test2-svn-git.txt
new file mode 100644
index 0000000..8aae5c8
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/go/test2-svn-git.txt
@@ -0,0 +1,26 @@
+handle dir
+
+-- test2main/index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/go/test2-svn-git/test2main git https://vcs-test.golang.org/git/test2main">
+-- test2pkg/index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/go/test2-svn-git/test2pkg git https://vcs-test.golang.org/git/README-only">
+-- test2pkg/pkg/index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/go/test2-svn-git/test2pkg git https://vcs-test.golang.org/git/README-only">
+-- test2PKG/index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/go/test2-svn-git/test2PKG svn https://vcs-test.golang.org/svn/test2-svn-git">
+-- test2PKG/p1/index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/go/test2-svn-git/test2PKG svn https://vcs-test.golang.org/svn/test2-svn-git">
+-- test2PKG/pkg/index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/go/test2-svn-git/test2PKG svn https://vcs-test.golang.org/svn/test2-svn-git">
diff --git a/src/cmd/go/testdata/vcstest/go/v2module.txt b/src/cmd/go/testdata/vcstest/go/v2module.txt
new file mode 100644
index 0000000..abcf2fd
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/go/v2module.txt
@@ -0,0 +1,6 @@
+handle dir
+
+-- v2/index.html --
+<!DOCTYPE html>
+<html>
+<meta name="go-import" content="vcs-test.golang.org/go/v2module/v2 git https://vcs-test.golang.org/git/v2repo">
diff --git a/src/cmd/go/testdata/vcstest/hg/custom-hg-hello.txt b/src/cmd/go/testdata/vcstest/hg/custom-hg-hello.txt
new file mode 100644
index 0000000..572cbdf
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/hg/custom-hg-hello.txt
@@ -0,0 +1,17 @@
+handle hg
+hg init
+
+hg add hello.go
+hg commit --user 'Russ Cox <rsc@golang.org>' --date '2017-10-10T19:39:36-04:00' --message 'hello'
+
+hg log -r ':' --template '{node|short} {desc|strip|firstline}\n'
+cmp stdout .hg-log
+
+-- .hg-log --
+a8c8e7a40da9 hello
+-- hello.go --
+package main // import "vcs-test.golang.org/go/custom-hg-hello"
+
+func main() {
+	println("hello")
+}
diff --git a/src/cmd/go/testdata/vcstest/hg/hello.txt b/src/cmd/go/testdata/vcstest/hg/hello.txt
new file mode 100644
index 0000000..10f114e
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/hg/hello.txt
@@ -0,0 +1,17 @@
+handle hg
+hg init
+
+hg add hello.go
+hg commit --user 'bwk' --date '2017-09-21T21:14:14-04:00' --message 'hello world'
+
+hg log -r ':' --template '{node|short} {desc|strip|firstline}\n'
+cmp stdout .hg-log
+
+-- .hg-log --
+e483a7d9f8c9 hello world
+-- hello.go --
+package main
+
+func main() {
+	println("hello, world")
+}
diff --git a/src/cmd/go/testdata/vcstest/hg/hgrepo1.txt b/src/cmd/go/testdata/vcstest/hg/hgrepo1.txt
new file mode 100644
index 0000000..1e4b83a
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/hg/hgrepo1.txt
@@ -0,0 +1,153 @@
+handle hg
+
+mkdir git
+cd git
+
+env GIT_AUTHOR_NAME='Russ Cox'
+env GIT_AUTHOR_EMAIL='rsc@golang.org'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+git init
+
+at 2018-04-17T15:43:22-04:00
+unquote ''
+cp stdout README
+git add README
+git commit -a -m 'empty README'
+git branch -m master
+git tag v1.2.3
+
+at 2018-04-17T15:45:48-04:00
+git branch v2
+git checkout v2
+echo 'v2'
+cp stdout v2
+git add v2
+git commit -a -m 'v2'
+git tag v2.3
+git tag v2.0.1
+git branch v2.3.4
+git tag branch-v2.3.4
+
+at 2018-04-17T16:00:19-04:00
+echo 'intermediate'
+cp stdout foo.txt
+git add foo.txt
+git commit -a -m 'intermediate'
+
+at 2018-04-17T16:00:32-04:00
+echo 'another'
+cp stdout another.txt
+git add another.txt
+git commit -a -m 'another'
+git tag v2.0.2
+git tag branch-v2
+
+at 2018-04-17T16:16:52-04:00
+git checkout master
+git branch v3
+git checkout v3
+mkdir v3/sub/dir
+echo 'v3/sub/dir/file'
+cp stdout v3/sub/dir/file.txt
+git add v3
+git commit -a -m 'add v3/sub/dir/file.txt'
+git tag branch-v3
+
+at 2018-04-17T22:23:00-04:00
+git checkout master
+git tag -a v1.2.4-annotated -m 'v1.2.4-annotated'
+
+cd ..
+
+hg init
+hg convert --datesort ./git .
+rm ./git
+
+hg update -C v2
+hg branch v2
+unquote ''
+cp stdout dummy
+hg add dummy
+hg commit --user 'Russ Cox <rsc@golang.org>' --date '2018-06-27T12:15:24-04:00' -m 'dummy'
+
+# 'hg convert' blindly stamps a tag-update commit at the end of whatever branch
+# happened to contain the last converted commit — in this case, v3. However, the
+# original vcs-test.golang.org copy of this repo had this commit on the v3
+# branch as a descendent of 'add v3/sub/dir/file.txt', so that's where we put it
+# here. That leaves the convert-repo 'update tags' commit only reachable as the
+# head of the default branch.
+hg update -r 4
+
+hg branch v3
+unquote ''
+cp stdout dummy
+hg add dummy
+hg commit --user 'Russ Cox <rsc@golang.org>' --date '2018-06-27T12:15:45-04:00' -m 'dummy'
+
+hg update v2.3.4
+hg branch v2.3.4
+unquote ''
+cp stdout dummy
+hg add dummy
+hg commit --user 'Russ Cox <rsc@golang.org>' --date '2018-06-27T12:16:10-04:00' -m 'dummy'
+
+hg tag --user 'Russ Cox <rsc@golang.org>' --date '2018-06-27T12:16:30-04:00' -m 'Removed tag branch-v2, branch-v3, branch-v2.3.4' --remove branch-v2 branch-v3 branch-v2.3.4
+
+# Adding commits to the above branches updates both the branch heads and the
+# corresponding bookmarks.
+# But apparently at some point it did not do so? The original copy of this repo
+# had bookmarks pointing to the base of each branch instead of the tip. 🤔
+# Either way, force the bookmarks we care about to match the original copy of
+# the repo.
+hg book v2 -r 3 --force
+hg book v2.3.4 -r 1 --force
+hg book v3 -r 5 --force
+
+hg log -G --debug
+
+hg tags
+cmp stdout .hg-tags
+
+	# 'hg convert' leaves an 'update tags' commit on the default branch, and that
+	# commit always uses the current date (so is not reproducible). Fortunately,
+	# that commit lands on the 'default' branch and is not tagged as 'tip', so it
+	# seems to be mostly harmless. However, because it is nondeterministic we
+	# should avoid listing it here.
+	#
+	# Unfortunately, some of our builders are still running Debian 9 “Stretch”,
+	# which shipped with a version of 'hg' that does not support 'hg branch -r'
+	# to list branches for specific versions. Although Stretch is past its
+	# end-of-life date, we need to keep the builders happy until they can be
+	# turned down (https://go.dev/issue/56414).
+hg branches
+? cmp stdout .hg-branches
+stdout 'v2\s+6:9a4f43d231ec'
+stdout 'v2.3.4\s+9:18518c07eb8e'
+stdout 'v3\s+7:a2cad8a2b1bb'
+stdout 'default\s+5:'
+
+# Likewise, bookmark v3 ends up on the nondeterministic commit.
+hg bookmarks
+? cmp stdout .hg-bookmarks
+stdout 'master\s+0:41964ddce118'
+stdout 'v2\s+3:8f49ee7a6ddc'
+stdout 'v2.3.4\s+1:88fde824ec8b'
+stdout 'v3\s+5:.*'
+
+-- .hg-branches --
+v2.3.4                         9:18518c07eb8e
+v3                             7:a2cad8a2b1bb
+v2                             6:9a4f43d231ec
+-- .hg-tags --
+tip                                9:18518c07eb8e
+v2.0.2                             3:8f49ee7a6ddc
+v2.3                               1:88fde824ec8b
+v2.0.1                             1:88fde824ec8b
+v1.2.4-annotated                   0:41964ddce118
+v1.2.3                             0:41964ddce118
+-- .hg-bookmarks --
+   master                    0:41964ddce118
+   v2                        3:8f49ee7a6ddc
+   v2.3.4                    1:88fde824ec8b
diff --git a/src/cmd/go/testdata/vcstest/hg/vgotest1.txt b/src/cmd/go/testdata/vcstest/hg/vgotest1.txt
new file mode 100644
index 0000000..e53c5e0
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/hg/vgotest1.txt
@@ -0,0 +1,322 @@
+handle hg
+
+cd git
+
+env GIT_AUTHOR_NAME='Russ Cox'
+env GIT_AUTHOR_EMAIL='rsc@golang.org'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+git init
+
+# 0
+at 2018-02-19T17:21:09-05:00
+git add LICENSE README.md
+git commit -m 'initial commit'
+git branch -m master
+
+# 1
+git branch mybranch
+git checkout mybranch
+
+at 2018-02-19T18:10:06-05:00
+mkdir pkg
+echo 'package p // pkg/p.go'
+cp stdout pkg/p.go
+git add pkg/p.go
+git commit -m 'add pkg/p.go'
+git tag v0.0.0
+git tag v1.0.0
+git tag v2.0.0
+git tag mytag
+
+git branch v1
+git branch v2
+git checkout v2
+
+# 2
+at 2018-02-19T18:14:23-05:00
+mkdir v2
+echo 'module "github.com/rsc/vgotest1/v2" // root go.mod'
+cp stdout go.mod
+git add go.mod
+git commit -m 'go.mod v2'
+git tag v2.0.1
+
+# 3
+at 2018-02-19T18:15:11-05:00
+mkdir submod/pkg
+echo 'package p // submod/pkg/p.go'
+cp stdout submod/pkg/p.go
+git add submod/pkg/p.go
+git commit -m 'submod/pkg/p.go'
+git tag v2.0.2
+
+# 4
+at 2018-02-19T18:16:04-05:00
+echo 'module "github.com/rsc/vgotest" // v2/go.mod'
+cp stdout v2/go.mod
+git add v2/go.mod
+git commit -m 'v2/go.mod: bad go.mod (no version)'
+git tag v2.0.3
+
+# 5
+at 2018-02-19T19:03:38-05:00
+env GIT_AUTHOR_DATE=2018-02-19T18:16:38-05:00
+echo 'module "github.com/rsc/vgotest1/v2" // v2/go.mod'
+cp stdout v2/go.mod
+git add v2/go.mod
+git commit -m 'v2/go.mod: fix'
+git tag v2.0.4
+
+# 6
+at 2018-02-19T19:03:59-05:00
+env GIT_AUTHOR_DATE=2018-02-19T18:17:02-05:00
+echo 'module "github.com/rsc/vgotest1" // root go.mod'
+cp stdout go.mod
+git add go.mod
+git commit -m 'go.mod: drop v2'
+git tag v2.0.5
+
+git checkout v1
+
+# 7
+at 2018-02-19T18:10:28-05:00
+echo 'module "github.com/rsc/vgotest1" // root go.mod'
+cp stdout go.mod
+git add go.mod
+git commit -m 'go.mod'
+git tag v0.0.1
+git tag v1.0.1
+
+# 8
+at 2018-02-19T18:11:28-05:00
+mkdir submod/pkg
+echo 'package pkg // submod/pkg/p.go'
+cp stdout submod/pkg/p.go
+git add submod
+git commit -m 'submod/pkg/p.go'
+git tag v1.0.2
+
+# 9
+at 2018-02-19T18:12:07-05:00
+echo 'module "github.com/vgotest1/submod" // submod/go.mod'
+cp stdout submod/go.mod
+git add submod/go.mod
+git commit -m 'submod/go.mod'
+git tag v1.0.3
+git tag submod/v1.0.4
+
+# 10
+at 2018-02-19T18:12:59-05:00
+git apply ../0001-submod-go.mod-add-require-vgotest1-v1.1.0.patch
+git commit -a -m 'submod/go.mod: add require vgotest1 v1.1.0'
+git tag submod/v1.0.5
+
+# 11
+at 2018-02-19T18:13:36-05:00
+git apply ../0002-go.mod-add-require-submod-v1.0.5.patch
+git commit -a -m 'go.mod: add require submod v1.0.5'
+git tag v1.1.0
+
+git checkout master
+
+# 12
+at 2018-02-19T17:23:01-05:00
+mkdir pkg
+echo 'package pkg'
+cp stdout pkg/p.go
+git add pkg/p.go
+git commit -m 'pkg: add'
+
+# 13
+at 2018-02-19T17:30:23-05:00
+env GIT_AUTHOR_DATE=2018-02-19T17:24:48-05:00
+echo 'module "github.com/vgotest1/v2"'
+cp stdout go.mod
+git add go.mod
+git commit -m 'add go.mod'
+
+# 14
+at 2018-02-19T17:30:45-05:00
+echo 'module "github.com/vgotest1"'
+cp stdout go.mod
+git add go.mod
+git commit -m 'bad mod path'
+
+# 15
+at 2018-02-19T17:31:34-05:00
+mkdir v2
+echo 'module "github.com/vgotest1/v2"'
+cp stdout v2/go.mod
+git add v2/go.mod
+git commit -m 'add v2/go.mod'
+
+# 16
+at 2018-02-19T17:32:37-05:00
+echo 'module "github.com/vgotest1/v2"'
+cp stdout go.mod
+git add go.mod
+git commit -m 'say v2 in root go.mod'
+
+# 17
+at 2018-02-19T17:51:24-05:00
+	# README.md at this commit lacked a trailing newline, so 'git apply' can't
+	# seem to apply it correctly as a patch. Instead, we use 'unquote' to write
+	# the exact contents.
+unquote 'This is a test repo for versioned go.\nThere''s nothing useful here.\n\n	v0.0.0 - has pkg/p.go\n	v0.0.1 - has go.mod\n	\n	v1.0.0 - has pkg/p.go\n	v1.0.1 - has go.mod\n	v1.0.2 - has submod/pkg/p.go\n	v1.0.3 - has submod/go.mod\n	submod/v1.0.4 - same\n	submod/v1.0.5 - add requirement on v1.1.0\n	v1.1.0 - add requirement on submod/v1.0.5\n	\n	v2.0.0 - has pkg/p.go\n	v2.0.1 - has go.mod with v2 module path\n	v2.0.2 - has go.mod with v1 (no version) module path\n	v2.0.3 - has v2/go.mod with v2 module path\n	v2.0.5 - has go.mod AND v2/go.mod with v2 module path\n	'
+cp stdout README.md
+mkdir v2/pkg
+echo 'package q'
+cp stdout v2/pkg/q.go
+git add README.md v2/pkg/q.go
+git commit -m 'add q'
+git tag v2.0.6
+
+cd ..
+
+hg init
+hg convert ./git .
+rm ./git
+
+# Note: commit #18 is an 'update tags' commit automatically generated by 'hg
+# convert'. We have no control over its timestamp, so it and its descendent
+# commit #19 both end up with unpredictable commit hashes.
+#
+# Fortunately, these commits don't seem to matter for the purpose of reproducing
+# the final branches and heads from the original copy of this repo.
+
+# 19
+hg update -C -r 18
+hg tag --user 'Russ Cox <rsc@golang.org>' --date '2018-07-18T21:24:45-04:00' -m 'Removed tag v2.0.0' --remove v2.0.0
+
+# 20
+hg branch default
+hg update -C -r 1
+echo 'v2'
+cp stdout v2
+hg add v2
+hg commit --user 'Russ Cox <rsc@golang.org>' --date '2018-07-18T21:25:08-04:00' -m 'v2.0.0'
+
+# 21
+hg tag --user 'Russ Cox <rsc@golang.org>' --date '2018-07-18T21:25:13-04:00' -r f0ababb31f75 -m 'Added tag v2.0.0 for changeset f0ababb31f75' v2.0.0
+
+# 22
+hg tag --user 'Russ Cox <rsc@golang.org>' --date '2018-07-18T21:26:02-04:00' -m 'Removed tag v2.0.0' --remove v2.0.0
+
+# 23
+hg update -C -r 1
+echo 'v2'
+cp stdout v2
+hg add v2
+hg commit --user 'Russ Cox <rsc@golang.org>' --date '2018-07-19T01:21:27+00:00' -m 'v2'
+
+# 24
+hg tag --user 'Russ Cox <rsc@golang.org>' --date '2018-07-18T21:26:33-04:00' -m 'Added tag v2.0.0 for changeset 814fce58e83a' -r 814fce58e83a v2.0.0
+
+hg book --delete v1
+hg book --delete v2
+hg book --force -r 16 master
+
+hg log -G --debug
+
+hg tags
+cmp stdout .hg-tags
+hg branches
+cmp stdout .hg-branches
+hg bookmarks
+cmp stdout .hg-bookmarks
+
+-- .hg-tags --
+tip                               24:645b06ca536d
+v2.0.0                            23:814fce58e83a
+v2.0.6                            17:3d4b89a2d059
+v1.1.0                            11:92c7eb888b4f
+submod/v1.0.5                     10:f3f560a6065c
+v1.0.3                             9:4e58084d459a
+submod/v1.0.4                      9:4e58084d459a
+v1.0.2                             8:3ccdce3897f9
+v1.0.1                             7:7890ea771ced
+v0.0.1                             7:7890ea771ced
+v2.0.5                             6:879ea98f7743
+v2.0.4                             5:bf6388016230
+v2.0.3                             4:a9ad6d1d14eb
+v2.0.2                             3:de3663002f0f
+v2.0.1                             2:f1fc0f22021b
+v1.0.0                             1:e125018e286a
+v0.0.0                             1:e125018e286a
+mytag                              1:e125018e286a
+-- .hg-branches --
+default                       24:645b06ca536d
+-- .hg-bookmarks --
+   master                    16:577bde103b24
+   mybranch                  1:e125018e286a
+-- git/LICENSE --
+Copyright (c) 2009 The Go Authors. All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+   * Redistributions of source code must retain the above copyright
+notice, this list of conditions and the following disclaimer.
+   * Redistributions in binary form must reproduce the above
+copyright notice, this list of conditions and the following disclaimer
+in the documentation and/or other materials provided with the
+distribution.
+   * Neither the name of Google Inc. nor the names of its
+contributors may be used to endorse or promote products derived from
+this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+-- git/README.md --
+This is a test repo for versioned go.
+There's nothing useful here.
+-- 0001-submod-go.mod-add-require-vgotest1-v1.1.0.patch --
+From 70fd92eaa4dacf82548d0c6099f5b853ae2c1fc8 Mon Sep 17 00:00:00 2001
+From: Russ Cox <rsc@golang.org>
+Date: Mon, 19 Feb 2018 18:12:59 -0500
+Subject: [PATCH] submod/go.mod: add require vgotest1 v1.1.0
+
+---
+ submod/go.mod | 1 +
+ 1 file changed, 1 insertion(+)
+
+diff --git a/submod/go.mod b/submod/go.mod
+index 7b18d93..c88de0f 100644
+--- a/submod/go.mod
++++ b/submod/go.mod
+@@ -1 +1,2 @@
+ module "github.com/vgotest1/submod" // submod/go.mod
++require "github.com/vgotest1" v1.1.0
+-- 
+2.36.1.838.g23b219f8e3
+-- 0002-go.mod-add-require-submod-v1.0.5.patch --
+From b769f2de407a4db81af9c5de0a06016d60d2ea09 Mon Sep 17 00:00:00 2001
+From: Russ Cox <rsc@golang.org>
+Date: Mon, 19 Feb 2018 18:13:36 -0500
+Subject: [PATCH] go.mod: add require submod v1.0.5
+
+---
+ go.mod | 1 +
+ 1 file changed, 1 insertion(+)
+
+diff --git a/go.mod b/go.mod
+index ac7a6d7..6118671 100644
+--- a/go.mod
++++ b/go.mod
+@@ -1 +1,2 @@
+ module "github.com/rsc/vgotest1" // root go.mod
++require "github.com/rsc/vgotest1/submod" v1.0.5
+-- 
+2.36.1.838.g23b219f8e3
diff --git a/src/cmd/go/testdata/vcstest/insecure.txt b/src/cmd/go/testdata/vcstest/insecure.txt
new file mode 100644
index 0000000..cbfb1b9
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/insecure.txt
@@ -0,0 +1 @@
+handle insecure
diff --git a/src/cmd/go/testdata/vcstest/svn/hello.txt b/src/cmd/go/testdata/vcstest/svn/hello.txt
new file mode 100644
index 0000000..c6ebd8d
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/svn/hello.txt
@@ -0,0 +1,87 @@
+handle svn
+
+mkdir db/transactions
+mkdir db/txn-protorevs
+chmod 0755 hooks/pre-revprop-change
+
+env ROOT=$PWD
+cd .checkout
+[GOOS:windows] svn checkout file:///$ROOT .
+[!GOOS:windows] svn checkout file://$ROOT .
+
+svn add hello.go
+svn commit --file MSG
+svn propset svn:author 'rsc' --revprop -r1
+svn propset svn:date '2017-09-22T01:12:45.861368Z' --revprop -r1
+
+svn update
+svn log --xml
+
+[GOOS:windows] replace '\n' '\r\n' .svn-log
+cmp stdout .svn-log
+
+-- .checkout/MSG --
+hello world
+
+-- .checkout/hello.go --
+package main
+
+func main() {
+	println("hello, world")
+}
+-- .checkout/.svn-log --
+<?xml version="1.0" encoding="UTF-8"?>
+<log>
+<logentry
+   revision="1">
+<author>rsc</author>
+<date>2017-09-22T01:12:45.861368Z</date>
+<msg>hello world
+
+</msg>
+</logentry>
+</log>
+-- conf/authz --
+-- conf/passwd --
+-- conf/svnserve.conf --
+-- db/current --
+0
+-- db/format --
+6
+layout sharded 1000
+-- db/fs-type --
+fsfs
+-- db/fsfs.conf --
+-- db/min-unpacked-rev --
+0
+-- db/revprops/0/0 --
+K 8
+svn:date
+V 27
+2017-09-22T01:11:53.895835Z
+END
+-- db/revs/0/0 --
+PLAIN
+END
+ENDREP
+id: 0.0.r0/17
+type: dir
+count: 0
+text: 0 0 4 4 2d2977d1c96f487abe4a1e202dd03b4e
+cpath: /
+
+
+17 107
+-- db/txn-current --
+0
+-- db/txn-current-lock --
+-- db/uuid --
+53cccb44-0fca-40a2-b0c5-acaf9e75039a
+-- db/write-lock --
+-- format --
+5
+-- hooks/pre-revprop-change --
+#!/bin/sh
+
+-- hooks/pre-revprop-change.bat --
+@exit
diff --git a/src/cmd/go/testdata/vcstest/svn/nonexistent.txt b/src/cmd/go/testdata/vcstest/svn/nonexistent.txt
new file mode 100644
index 0000000..a71ecf1
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/svn/nonexistent.txt
@@ -0,0 +1,5 @@
+handle svn
+
+# For this path, we turn on the svn handler but don't actually create the repo.
+# svnserve should use the svn protocol to tell the client that the repo doesn't
+# actually exist.
diff --git a/src/cmd/go/testdata/vcstest/svn/test1-svn-git.txt b/src/cmd/go/testdata/vcstest/svn/test1-svn-git.txt
new file mode 100644
index 0000000..2b94201
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/svn/test1-svn-git.txt
@@ -0,0 +1,188 @@
+handle svn
+
+# Note: this repo script does not produce a byte-for-byte copy of the original.
+#
+# The 'git init' operation in the nested Git repo creates some sample files
+# whose contents depend on the exact Git version in use, and the steps we take
+# to construct a fake 'git clone' status don't produce some log files that
+# a real 'git clone' leaves behind.
+#
+# However, the repo is probably accurate enough for the tests that need it.
+
+env GIT_AUTHOR_NAME='Russ Cox'
+env GIT_AUTHOR_EMAIL='rsc@golang.org'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+mkdir db/transactions
+mkdir db/txn-protorevs
+chmod 0755 hooks/pre-revprop-change
+
+env ROOT=$PWD
+cd .checkout
+[GOOS:windows] svn checkout file:///$ROOT .
+[!GOOS:windows] svn checkout file://$ROOT .
+
+cd git-README-only
+git init
+git config --add core.ignorecase true
+git config --add core.precomposeunicode true
+
+git add README
+at 2017-09-22T11:39:03-04:00
+git commit -a -m 'README'
+git branch -m master
+
+git rev-parse HEAD
+stdout '^7f800d2ac276dd7042ea0e8d7438527d236fd098$'
+
+	# Fake a clone from an origin repo at this commit.
+git remote add origin https://vcs-test.swtch.com/git/README-only
+mkdir .git/refs/remotes/origin
+echo 'ref: refs/remotes/origin/master'
+cp stdout .git/refs/remotes/origin/HEAD
+unquote '# pack-refs with: peeled fully-peeled \n7f800d2ac276dd7042ea0e8d7438527d236fd098 refs/remotes/origin/master\n'
+cp stdout .git/packed-refs
+git branch --set-upstream-to=origin/master
+
+git add pkg/pkg.go
+at 2017-09-22T11:41:28-04:00
+git commit -a -m 'add pkg'
+
+git log --oneline --decorate=short
+cmp stdout ../.git-log
+
+cd ..
+svn add git-README-only
+svn commit -m 'add modified git-README-only'
+svn propset svn:author rsc --revprop -r1
+svn propset svn:date 2017-09-22T15:41:54.145716Z --revprop -r1
+
+svn add pkg.go
+svn commit -m 'use git-README-only/pkg'
+svn propset svn:author rsc --revprop -r2
+svn propset svn:date 2017-09-22T15:49:11.130406Z --revprop -r2
+
+svn add other
+svn commit -m 'add other'
+svn propset svn:author rsc --revprop -r3
+svn propset svn:date 2017-09-22T16:56:16.665173Z --revprop -r3
+
+svn add tiny
+svn commit -m 'add tiny'
+svn propset svn:author rsc --revprop -r4
+svn propset svn:date 2017-09-27T17:48:18.350817Z --revprop -r4
+
+cd git-README-only
+git remote set-url origin https://vcs-test.golang.org/git/README-only
+cd ..
+replace 'vcs-test.swtch.com' 'vcs-test.golang.org' other/pkg.go
+replace 'vcs-test.swtch.com' 'vcs-test.golang.org' pkg.go
+svn commit -m 'move from vcs-test.swtch.com to vcs-test.golang.org'
+svn propset svn:author rsc --revprop -r5
+svn propset svn:date 2017-10-04T15:08:26.291877Z --revprop -r5
+
+svn update
+svn log --xml
+
+[GOOS:windows] replace '\n' '\r\n' .svn-log
+cmp stdout .svn-log
+
+-- .checkout/git-README-only/pkg/pkg.go --
+package pkg
+const Message = "code not in git-README-only"
+-- .checkout/git-README-only/README --
+README
+-- .checkout/.git-log --
+ab9f66b (HEAD -> master) add pkg
+7f800d2 (origin/master, origin/HEAD) README
+-- .checkout/pkg.go --
+package p
+
+import "vcs-test.swtch.com/go/test1-svn-git/git-README-only/pkg"
+
+const _ = pkg.Message
+-- .checkout/other/pkg.go --
+package other
+
+import _ "vcs-test.swtch.com/go/test1-svn-git/git-README-only/other"
+-- .checkout/tiny/tiny.go --
+package tiny
+-- .checkout/.svn-log --
+<?xml version="1.0" encoding="UTF-8"?>
+<log>
+<logentry
+   revision="5">
+<author>rsc</author>
+<date>2017-10-04T15:08:26.291877Z</date>
+<msg>move from vcs-test.swtch.com to vcs-test.golang.org</msg>
+</logentry>
+<logentry
+   revision="4">
+<author>rsc</author>
+<date>2017-09-27T17:48:18.350817Z</date>
+<msg>add tiny</msg>
+</logentry>
+<logentry
+   revision="3">
+<author>rsc</author>
+<date>2017-09-22T16:56:16.665173Z</date>
+<msg>add other</msg>
+</logentry>
+<logentry
+   revision="2">
+<author>rsc</author>
+<date>2017-09-22T15:49:11.130406Z</date>
+<msg>use git-README-only/pkg</msg>
+</logentry>
+<logentry
+   revision="1">
+<author>rsc</author>
+<date>2017-09-22T15:41:54.145716Z</date>
+<msg>add modified git-README-only</msg>
+</logentry>
+</log>
+-- conf/authz --
+-- conf/passwd --
+-- conf/svnserve.conf --
+-- db/current --
+0
+-- db/format --
+6
+layout sharded 1000
+-- db/fs-type --
+fsfs
+-- db/fsfs.conf --
+-- db/min-unpacked-rev --
+0
+-- db/revprops/0/0 --
+K 8
+svn:date
+V 27
+2017-09-22T01:11:53.895835Z
+END
+-- db/revs/0/0 --
+PLAIN
+END
+ENDREP
+id: 0.0.r0/17
+type: dir
+count: 0
+text: 0 0 4 4 2d2977d1c96f487abe4a1e202dd03b4e
+cpath: /
+
+
+17 107
+-- db/txn-current --
+0
+-- db/txn-current-lock --
+-- db/uuid --
+53cccb44-0fca-40a2-b0c5-acaf9e75039a
+-- db/write-lock --
+-- format --
+5
+-- hooks/pre-revprop-change --
+#!/bin/sh
+
+-- hooks/pre-revprop-change.bat --
+@exit
diff --git a/src/cmd/go/testdata/vcstest/svn/test2-svn-git.txt b/src/cmd/go/testdata/vcstest/svn/test2-svn-git.txt
new file mode 100644
index 0000000..bf82797
--- /dev/null
+++ b/src/cmd/go/testdata/vcstest/svn/test2-svn-git.txt
@@ -0,0 +1,154 @@
+handle svn
+
+# Note: this repo script does not produce a byte-for-byte copy of the original.
+#
+# The 'git init' operation in the nested Git repo creates some sample files
+# whose contents depend on the exact Git version in use, and the steps we take
+# to construct a fake 'git clone' status don't produce some log files that
+# a real 'git clone' leaves behind.
+#
+# However, the repo is probably accurate enough for the tests that need it.
+
+env GIT_AUTHOR_NAME='Russ Cox'
+env GIT_AUTHOR_EMAIL='rsc@golang.org'
+env GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
+env GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
+
+mkdir db/transactions
+mkdir db/txn-protorevs
+chmod 0755 hooks/pre-revprop-change
+
+env ROOT=$PWD
+cd .checkout
+[GOOS:windows] svn checkout file:///$ROOT .
+[!GOOS:windows] svn checkout file://$ROOT .
+
+git init
+git config --add core.ignorecase true
+git config --add core.precomposeunicode true
+
+git add README
+at 2017-09-22T11:39:03-04:00
+git commit -a -m 'README'
+git branch -m master
+
+git rev-parse HEAD
+stdout '^7f800d2ac276dd7042ea0e8d7438527d236fd098$'
+
+	# Fake a clone from an origin repo at this commit.
+git remote add origin https://vcs-test.swtch.com/git/README-only
+mkdir .git/refs/remotes/origin
+echo 'ref: refs/remotes/origin/master'
+cp stdout .git/refs/remotes/origin/HEAD
+unquote '# pack-refs with: peeled fully-peeled \n7f800d2ac276dd7042ea0e8d7438527d236fd098 refs/remotes/origin/master\n'
+cp stdout .git/packed-refs
+git branch --set-upstream-to=origin/master
+
+git add pkg/pkg.go
+at 2017-09-22T11:41:28-04:00
+git commit -a -m 'add pkg'
+
+git log --oneline --decorate=short
+cmp stdout .git-log
+
+rm README
+
+svn add .git pkg
+svn commit -m 'git'
+svn propset svn:author rsc --revprop -r1
+svn propset svn:date 2017-09-27T18:00:52.201719Z --revprop -r1
+
+svn add p1
+svn commit -m 'add p1'
+svn propset svn:author rsc --revprop -r2
+svn propset svn:date 2017-09-27T18:16:14.650893Z --revprop -r2
+
+git remote set-url origin https://vcs-test.golang.org/git/README-only
+svn commit -m 'move from vcs-test.swtch.com to vcs-test.golang.org'
+svn propset svn:author rsc --revprop -r3
+svn propset svn:date 2017-10-04T15:09:35.963034Z --revprop -r3
+
+svn update
+svn log --xml
+
+[GOOS:windows] replace '\n' '\r\n' .svn-log
+cmp stdout .svn-log
+
+-- .checkout/.git-log --
+ab9f66b (HEAD -> master) add pkg
+7f800d2 (origin/master, origin/HEAD) README
+-- .checkout/p1/p1.go --
+package p1
+-- .checkout/pkg/pkg.go --
+package pkg
+const Message = "code not in git-README-only"
+-- .checkout/README --
+README
+-- .checkout/p1/p1.go --
+package p1
+-- .checkout/.svn-log --
+<?xml version="1.0" encoding="UTF-8"?>
+<log>
+<logentry
+   revision="3">
+<author>rsc</author>
+<date>2017-10-04T15:09:35.963034Z</date>
+<msg>move from vcs-test.swtch.com to vcs-test.golang.org</msg>
+</logentry>
+<logentry
+   revision="2">
+<author>rsc</author>
+<date>2017-09-27T18:16:14.650893Z</date>
+<msg>add p1</msg>
+</logentry>
+<logentry
+   revision="1">
+<author>rsc</author>
+<date>2017-09-27T18:00:52.201719Z</date>
+<msg>git</msg>
+</logentry>
+</log>
+-- conf/authz --
+-- conf/passwd --
+-- conf/svnserve.conf --
+-- db/current --
+0
+-- db/format --
+6
+layout sharded 1000
+-- db/fs-type --
+fsfs
+-- db/fsfs.conf --
+-- db/min-unpacked-rev --
+0
+-- db/revprops/0/0 --
+K 8
+svn:date
+V 27
+2017-09-22T01:11:53.895835Z
+END
+-- db/revs/0/0 --
+PLAIN
+END
+ENDREP
+id: 0.0.r0/17
+type: dir
+count: 0
+text: 0 0 4 4 2d2977d1c96f487abe4a1e202dd03b4e
+cpath: /
+
+
+17 107
+-- db/txn-current --
+0
+-- db/txn-current-lock --
+-- db/uuid --
+53cccb44-0fca-40a2-b0c5-acaf9e75039a
+-- db/write-lock --
+-- format --
+5
+-- hooks/pre-revprop-change --
+#!/bin/sh
+
+-- hooks/pre-revprop-change.bat --
+@exit
diff --git a/src/cmd/go/testdata/vendormod.txt b/src/cmd/go/testdata/vendormod.txt
new file mode 100644
index 0000000..1bdaf2a
--- /dev/null
+++ b/src/cmd/go/testdata/vendormod.txt
@@ -0,0 +1,160 @@
+generated by: go run savedir.go vendormod
+
+-- a/foo/AUTHORS.txt --
+-- a/foo/CONTRIBUTORS --
+-- a/foo/LICENSE --
+-- a/foo/PATENTS --
+-- a/foo/COPYING --
+-- a/foo/COPYLEFT --
+-- a/foo/licensed-to-kill --
+-- w/LICENSE --
+-- x/NOTICE! --
+-- x/x2/LICENSE --
+-- mypkg/LICENSE.txt --
+-- a/foo/bar/b/main.go --
+package b
+-- a/foo/bar/b/main_test.go --
+package b
+
+import (
+	"os"
+	"testing"
+)
+
+func TestDir(t *testing.T) {
+	if _, err := os.Stat("../testdata/1"); err != nil {
+		t.Fatalf("testdata: %v", err)
+	}
+}
+-- a/foo/bar/c/main.go --
+package c
+-- a/foo/bar/c/main_test.go --
+package c
+
+import (
+	"os"
+	"testing"
+)
+
+func TestDir(t *testing.T) {
+	if _, err := os.Stat("../../../testdata/1"); err != nil {
+		t.Fatalf("testdata: %v", err)
+	}
+	if _, err := os.Stat("./testdata/1"); err != nil {
+		t.Fatalf("testdata: %v", err)
+	}
+}
+-- a/foo/bar/c/testdata/1 --
+-- a/foo/bar/testdata/1 --
+-- a/go.mod --
+module a
+-- a/main.go --
+package a
+-- a/main_test.go --
+package a
+
+import (
+	"os"
+	"testing"
+)
+
+func TestDir(t *testing.T) {
+	if _, err := os.Stat("./testdata/1"); err != nil {
+		t.Fatalf("testdata: %v", err)
+	}
+}
+-- a/testdata/1 --
+-- appengine.go --
+// +build appengine
+
+package m
+
+import _ "appengine"
+import _ "appengine/datastore"
+-- go.mod --
+module m
+
+require (
+	a v1.0.0
+	mysite/myname/mypkg v1.0.0
+	w v1.0.0 // indirect
+	x v1.0.0
+	y v1.0.0
+	z v1.0.0
+)
+
+replace (
+	a v1.0.0 => ./a
+	mysite/myname/mypkg v1.0.0 => ./mypkg
+	w v1.0.0 => ./w
+	x v1.0.0 => ./x
+	y v1.0.0 => ./y
+	z v1.0.0 => ./z
+)
+-- mypkg/go.mod --
+module me
+-- mypkg/mydir/d.go --
+package mydir
+-- subdir/v1_test.go --
+package m
+
+import _ "mysite/myname/mypkg/mydir"
+-- testdata1.go --
+package m
+
+import _ "a"
+-- testdata2.go --
+package m
+
+import _ "a/foo/bar/b"
+import _ "a/foo/bar/c"
+-- v1.go --
+package m
+
+import _ "x"
+-- v2.go --
+// +build abc
+
+package mMmMmMm
+
+import _ "y"
+-- v3.go --
+// +build !abc
+
+package m
+
+import _ "z"
+-- v4.go --
+// +build notmytag
+
+package m
+
+import _ "x/x1"
+-- w/go.mod --
+module w
+-- w/w.go --
+package w
+-- x/go.mod --
+module x
+-- x/testdata/x.txt --
+placeholder - want directory with no go files
+-- x/x.go --
+package x
+-- x/x1/x1.go --
+// +build notmytag
+
+package x1
+-- x/x2/dummy.txt --
+dummy
+-- x/x_test.go --
+package x
+
+import _ "w"
+-- y/go.mod --
+module y
+-- y/y.go --
+package y
+-- z/go.mod --
+module z
+-- z/z.go --
+package z