Adding upstream version 1.16.10.upstream/1.16.10upstream

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

1 files changed, 2594 insertions, 0 deletions

diff --git a/src/cmd/go/internal/load/pkg.go b/src/cmd/go/internal/load/pkg.go
new file mode 100644
index 0000000..61fde89
--- /dev/null
+++ b/src/cmd/go/internal/load/pkg.go
@@ -0,0 +1,2594 @@
+// 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"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"go/build"
+	"go/scanner"
+	"go/token"
+	"io/fs"
+	"os"
+	"path"
+	pathpkg "path"
+	"path/filepath"
+	"runtime"
+	"sort"
+	"strconv"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"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/internal/sys"
+
+	"golang.org/x/mod/module"
+)
+
+var IgnoreImports bool // control whether we ignore imports in packages
+
+// 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.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?
+
+	// 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
+	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
+
+	// 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,
+		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]"
+	}
+	return p.ImportPath
+}
+
+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)
+	RawImports        []string             // this package's original imports as they appear in the text of the program
+	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
+	CoverMode         string               // preprocess Go source files with the coverage tool in this mode
+	CoverVars         map[string]*CoverVar // variables created by coverage analysis
+	OmitDebug         bool                 // tell linker not to write debug information
+	GobinSubdir       bool                 // install target would be subdir of GOBIN
+	BuildInfo         string               // add this info to package main
+	TestmainGo        *[]byte              // content for _testmain.go
+	Embed             map[string][]string  // //go:embed comment mapping
+
+	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,
+	}
+
+	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(pp *build.Package) {
+	p.Internal.Build = pp
+
+	if pp.PkgTargetRoot != "" && cfg.BuildPkgdir != "" {
+		old := pp.PkgTargetRoot
+		pp.PkgRoot = cfg.BuildPkgdir
+		pp.PkgTargetRoot = cfg.BuildPkgdir
+		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.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
+	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 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
+}
+
+// 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 = (*modload.ImportMissingError)(nil)
+	_ ImportPathError = (*modload.ImportMissingSumError)(nil)
+)
+
+type importError struct {
+	importPath string
+	err        error // created with fmt.Errorf
+}
+
+func ImportErrorf(path, format string, args ...interface{}) 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 interface{}) bool {
+		return shouldDelete[key.(importSpec).path]
+	})
+	packageDataCache.DeleteIf(func(key interface{}) bool {
+		return shouldDelete[key.(string)]
+	})
+}
+
+// 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 interface{}) bool {
+			return key.(importSpec).path == p.ImportPath
+		})
+		packageDataCache.Delete(p.ImportPath)
+	}
+	return LoadImport(context.TODO(), arg, base.Cwd, nil, 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
+)
+
+// 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.
+func LoadImport(ctx context.Context, path, srcDir string, parent *Package, stk *ImportStack, importPos []token.Position, mode int) *Package {
+	return loadImport(ctx, nil, path, srcDir, parent, stk, importPos, mode)
+}
+
+func loadImport(ctx context.Context, pre *preload, path, srcDir string, parent *Package, stk *ImportStack, importPos []token.Position, mode int) *Package {
+	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(path, parentPath, srcDir, parentRoot, parentIsStd, mode)
+	if loaded && pre != nil && !IgnoreImports {
+		pre.preloadImports(bp.Imports, bp)
+	}
+	if bp == nil {
+		if importErr, ok := err.(ImportPathError); !ok || importErr.ImportPath() != path {
+			// Only add path to the error's import stack if it's not already present on the error.
+			stk.Push(path)
+			defer stk.Pop()
+		}
+		return &Package{
+			PackagePublic: PackagePublic{
+				ImportPath: path,
+				Error: &PackageError{
+					ImportStack: stk.Copy(),
+					Err:         err,
+				},
+			},
+		}
+	}
+
+	importPath := bp.ImportPath
+	p := packageCache[importPath]
+	if p != nil {
+		stk.Push(path)
+		p = reusePackage(p, stk)
+		stk.Pop()
+	} else {
+		p = new(Package)
+		p.Internal.Local = build.IsLocalImport(path)
+		p.ImportPath = importPath
+		packageCache[importPath] = p
+
+		// Load package.
+		// loadPackageData may return bp != nil even if an error occurs,
+		// in order to return partial information.
+		p.load(ctx, 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(srcDir, parent, parentPath, p, stk); perr != p {
+		perr.Error.setPos(importPos)
+		return perr
+	}
+	if mode&ResolveImport != 0 {
+		if perr := disallowVendor(srcDir, path, parentPath, p, stk); perr != p {
+			perr.Error.setPos(importPos)
+			return perr
+		}
+	}
+
+	if p.Name == "main" && parent != nil && parent.Dir != p.Dir {
+		perr := *p
+		perr.Error = &PackageError{
+			ImportStack: stk.Copy(),
+			Err:         ImportErrorf(path, "import %q is a program, not an importable package", path),
+		}
+		perr.Error.setPos(importPos)
+		return &perr
+	}
+
+	if p.Internal.Local && parent != nil && !parent.Internal.Local {
+		perr := *p
+		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.Error = &PackageError{
+			ImportStack: stk.Copy(),
+			Err:         err,
+		}
+		perr.Error.setPos(importPos)
+		return &perr
+	}
+
+	return p
+}
+
+// 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(path, parentPath, parentDir, parentRoot string, parentIsStd bool, mode int) (bp *build.Package, loaded bool, err error) {
+	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() interface{} {
+		var r resolvedImport
+		if build.IsLocalImport(path) {
+			r.dir = filepath.Join(parentDir, path)
+			r.path = dirToImportPath(r.dir)
+		} else if cfg.ModulesEnabled {
+			r.dir, r.path, r.err = modload.Lookup(parentPath, parentIsStd, path)
+		} 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
+	}).(resolvedImport)
+	// 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.
+	data := packageDataCache.Do(r.path, func() interface{} {
+		loaded = true
+		var data packageData
+		if r.dir != "" {
+			var buildMode build.ImportMode
+			if !cfg.ModulesEnabled {
+				buildMode = build.ImportComment
+			}
+			data.p, data.err = cfg.BuildContext.ImportDir(r.dir, buildMode)
+			if data.p.Root == "" && cfg.ModulesEnabled {
+				if info := modload.PackageModuleInfo(path); info != nil {
+					data.p.Root = info.Dir
+				}
+			}
+		} 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
+	}).(packageData)
+
+	return data.p, loaded, data.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
+}
+
+// packageData holds information loaded from a package. It is the value type
+// in packageDataCache.
+type packageData struct {
+	p   *build.Package
+	err error
+}
+
+// resolvedImportCache maps import strings (importSpec) to canonical package names
+// (resolvedImport).
+var resolvedImportCache par.Cache
+
+// packageDataCache maps canonical package names (string) to package metadata
+// (packageData).
+var packageDataCache par.Cache
+
+// 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(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(pkg, "", base.Cwd, "", false, mode)
+					<-pre.sema
+					if bp != nil && loaded && err == nil && !IgnoreImports {
+						pre.preloadImports(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(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(path, parent.ImportPath, parent.Dir, parent.Root, parentIsStd, ResolveImport)
+				<-pre.sema
+				if bp != nil && loaded && err == nil && !IgnoreImports {
+					pre.preloadImports(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
+
+func isDir(path string) bool {
+	return isDirCache.Do(path, func() interface{} {
+		fi, err := fsys.Stat(path)
+		return err == nil && fi.IsDir()
+	}).(bool)
+}
+
+// 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 {
+		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
+)
+
+// goModPath returns the module path in the go.mod in dir, if any.
+func goModPath(dir string) (path string) {
+	return goModPathCache.Do(dir, func() interface{} {
+		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
+	}).(string)
+}
+
+// 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(srcDir string, importer *Package, importerPath string, p *Package, stk *ImportStack) *Package {
+	// 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 p
+	}
+
+	// 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 p
+	}
+
+	// We can't check standard packages with gccgo.
+	if cfg.BuildContext.Compiler == "gccgo" && p.Standard {
+		return p
+	}
+
+	// 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 p
+	}
+
+	// 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 p
+	}
+
+	// Check for "internal" element: three cases depending on begin of string and/or end of string.
+	i, ok := findInternal(p.ImportPath)
+	if !ok {
+		return p
+	}
+
+	// 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 p
+		}
+
+		// Look for symlinks before reporting error.
+		srcDir = expandPath(srcDir)
+		parent = expandPath(parent)
+		if str.HasFilePathPrefix(filepath.Clean(srcDir), filepath.Clean(parent)) {
+			return p
+		}
+	} 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 module, this will resolve to ".",
+			// which is not a prefix of any valid module.
+			importerPath = modload.DirImportPath(importer.Dir)
+		}
+		parentOfInternal := p.ImportPath[:i]
+		if str.HasPathPrefix(importerPath, parentOfInternal) {
+			return p
+		}
+	}
+
+	// Internal is present, and srcDir is outside parent's tree. Not allowed.
+	perr := *p
+	perr.Error = &PackageError{
+		alwaysPrintStack: true,
+		ImportStack:      stk.Copy(),
+		Err:              ImportErrorf(p.ImportPath, "use of internal package "+p.ImportPath+" not allowed"),
+	}
+	perr.Incomplete = true
+	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 new package containing just an appropriate error.
+func disallowVendor(srcDir string, path string, importerPath string, p *Package, stk *ImportStack) *Package {
+	// 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 p
+	}
+
+	if perr := disallowVendorVisibility(srcDir, p, importerPath, stk); perr != p {
+		return perr
+	}
+
+	// Paths like x/vendor/y must be imported as y, never as x/vendor/y.
+	if i, ok := FindVendor(path); ok {
+		perr := *p
+		perr.Error = &PackageError{
+			ImportStack: stk.Copy(),
+			Err:         ImportErrorf(path, "%s must be imported as %s", path, path[i+len("vendor/"):]),
+		}
+		perr.Incomplete = true
+		return &perr
+	}
+
+	return p
+}
+
+// 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) *Package {
+	// 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 p
+	}
+
+	// Check for "vendor" element.
+	i, ok := FindVendor(p.ImportPath)
+	if !ok {
+		return p
+	}
+
+	// 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 p
+	}
+	parent := p.Dir[:truncateTo]
+	if str.HasFilePathPrefix(filepath.Clean(srcDir), filepath.Clean(parent)) {
+		return p
+	}
+
+	// Look for symlinks before reporting error.
+	srcDir = expandPath(srcDir)
+	parent = expandPath(parent)
+	if str.HasFilePathPrefix(filepath.Clean(srcDir), filepath.Clean(parent)) {
+		return p
+	}
+
+	// Vendor is present, and srcDir is outside parent's tree. Not allowed.
+	perr := *p
+	perr.Error = &PackageError{
+		ImportStack: stk.Copy(),
+		Err:         errors.New("use of vendored package not allowed"),
+	}
+	perr.Incomplete = true
+	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,
+}
+
+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, path string, stk *ImportStack, importPos []token.Position, bp *build.Package, err error) {
+	p.copyBuild(bp)
+
+	// The localPrefix is the path we interpret ./ imports relative to.
+	// Synthesized main packages sometimes override this.
+	if p.Internal.Local {
+		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,
+			}
+
+			// 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()
+		full := cfg.BuildContext.GOOS + "_" + cfg.BuildContext.GOARCH + "/" + elem
+		if cfg.BuildContext.GOOS != base.ToolGOOS || cfg.BuildContext.GOARCH != base.ToolGOARCH {
+			// 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, "/") && 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(base.ToolDir, elem)
+			} else {
+				p.Target = filepath.Join(cfg.GOROOTpkg, "tool", full)
+			}
+		}
+		if p.Target != "" && cfg.BuildContext.GOOS == "windows" {
+			p.Target += ".exe"
+		}
+	} else if p.Internal.Local {
+		// Local import turned into absolute path.
+		// No permanent install target.
+		p.Target = ""
+	} else {
+		p.Target = p.Internal.Build.PkgObj
+		if cfg.BuildLinkshared && p.Target != "" {
+			// TODO(bcmills): The reliance on p.Target implies that -linkshared does
+			// not work for any package that lacks a Target — such as a non-main
+			// package in module mode. We should probably fix that.
+			shlibnamefile := p.Target[:len(p.Target)-2] + ".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)
+		}
+	}
+
+	// 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()
+
+	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
+	}
+
+	// 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 := LoadImport(ctx, path, p.Dir, p, stk, p.Internal.Build.ImportPos[path], ResolveImport)
+
+		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
+	p.collectDeps()
+
+	// unsafe is a fake package.
+	if p.Standard && (p.ImportPath == "unsafe" || cfg.BuildContext.Compiler == "gccgo") {
+		p.Target = ""
+	}
+
+	// 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
+	}
+
+	if cfg.ModulesEnabled && p.Error == nil {
+		mainPath := p.ImportPath
+		if p.Internal.CmdlineFiles {
+			mainPath = "command-line-arguments"
+		}
+		p.Module = modload.PackageModuleInfo(mainPath)
+		if p.Name == "main" && len(p.DepsErrors) == 0 {
+			p.Internal.BuildInfo = modload.PackageBuildInfo(mainPath, p.Deps)
+		}
+	}
+}
+
+// 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++
+
+		// Check pattern is valid for //go:embed.
+		if _, err := path.Match(pattern, ""); err != nil || !validEmbedPattern(pattern) {
+			return nil, nil, fmt.Errorf("invalid pattern syntax")
+		}
+
+		// Glob to find matches.
+		match, err := fsys.Glob(pkgdir + string(filepath.Separator) + filepath.FromSlash(pattern))
+		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 {
+			rel := filepath.ToSlash(file[len(pkgdir)+1:]) // file, relative to p.Dir
+
+			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(path[len(pkgdir)+1:])
+					name := info.Name()
+					if path != file && (isBadEmbedName(name) || name[0] == '.' || name[0] == '_') {
+						// 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
+}
+
+// collectDeps populates p.Deps and p.DepsErrors by iterating over
+// p.Internal.Imports.
+//
+// TODO(jayconrod): collectDeps iterates over transitive imports for every
+// package. We should only need to visit direct imports.
+func (p *Package) collectDeps() {
+	deps := make(map[string]*Package)
+	var q []*Package
+	q = append(q, p.Internal.Imports...)
+	for i := 0; i < len(q); i++ {
+		p1 := q[i]
+		path := p1.ImportPath
+		// The same import path could produce an error or not,
+		// depending on what tries to import it.
+		// Prefer to record entries with errors, so we can report them.
+		p0 := deps[path]
+		if p0 == nil || p1.Error != nil && (p0.Error == nil || len(p0.Error.ImportStack) > len(p1.Error.ImportStack)) {
+			deps[path] = p1
+			for _, p2 := range p1.Internal.Imports {
+				if deps[p2.ImportPath] != p2 {
+					q = append(q, p2)
+				}
+			}
+		}
+	}
+
+	p.Deps = make([]string, 0, len(deps))
+	for dep := range deps {
+		p.Deps = append(p.Deps, dep)
+	}
+	sort.Strings(p.Deps)
+	for _, dep := range p.Deps {
+		p1 := deps[dep]
+		if p1 == nil {
+			panic("impossible: missing entry in package cache for " + dep + " imported by " + p.ImportPath)
+		}
+		if p1.Error != nil {
+			p.DepsErrors = append(p.DepsErrors, p1.Error)
+		}
+	}
+}
+
+// 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")
+	}
+
+	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.
+	switch cfg.BuildContext.GOOS {
+	case "android":
+		if cfg.BuildContext.GOARCH != "arm64" {
+			return true
+		}
+	case "ios":
+		return true
+	}
+
+	// Currently build modes c-shared, pie (on systems that do not
+	// support PIE with internal linking mode (currently all
+	// systems: issue #18968)), plugin, and -linkshared force
+	// external linking mode, as of course does
+	// -ldflags=-linkmode=external. External linking mode forces
+	// an import of runtime/cgo.
+	// If there are multiple -linkmode options, the last one wins.
+	pieCgo := cfg.BuildBuildmode == "pie" && !sys.InternalLinkPIESupported(cfg.BuildContext.GOOS, cfg.BuildContext.GOARCH)
+	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 cfg.BuildBuildmode == "c-shared" || cfg.BuildBuildmode == "plugin" || pieCgo || cfg.BuildLinkshared || 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)
+}
+
+// InternalGoFiles 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, 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 := LoadImport(ctx, path, root.Dir, root, &stk, root.Internal.Build.TestImportPos[path], ResolveImport)
+		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 {
+	p := LoadImport(context.TODO(), path, srcDir, parent, stk, importPos, mode)
+	setToolFlags(p)
+	return p
+}
+
+// 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.
+//
+// TODO(#40775): Change this from a global variable to an explicit function
+// argument where needed.
+var ModResolveTests 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, 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() {
+				return []*Package{GoFilesPackage(ctx, patterns)}
+			}
+		}
+	}
+
+	var matches []*search.Match
+	if modload.Init(); cfg.ModulesEnabled {
+		loadOpts := modload.PackageOpts{
+			ResolveMissingImports: true,
+			LoadTests:             ModResolveTests,
+			SilenceErrors:         true,
+		}
+		matches, _ = modload.LoadPackages(ctx, loadOpts, patterns...)
+	} else {
+		matches = search.ImportPaths(patterns)
+	}
+
+	var (
+		pkgs    []*Package
+		stk     ImportStack
+		seenPkg = make(map[*Package]bool)
+	)
+
+	pre := newPreload()
+	defer pre.flush()
+	pre.preloadMatches(matches)
+
+	for _, m := range matches {
+		for _, pkg := range m.Pkgs {
+			if pkg == "" {
+				panic(fmt.Sprintf("ImportPaths returned empty package for pattern %s", m.Pattern()))
+			}
+			p := loadImport(ctx, pre, pkg, base.Cwd, nil, &stk, nil, 0)
+			p.Match = append(p.Match, m.Pattern())
+			p.Internal.CmdlinePkg = true
+			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.
+				p.Internal.CmdlinePkgLiteral = true
+			}
+			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)
+		}
+	}
+
+	// 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...)
+
+	return pkgs
+}
+
+// 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) {
+	printed := map[*PackageError]bool{}
+	for _, pkg := range pkgs {
+		if pkg.Error != nil {
+			base.Errorf("%v", pkg.Error)
+			printed[pkg.Error] = true
+		}
+		for _, err := range pkg.DepsErrors {
+			// Since these are errors in dependencies,
+			// the same error might show up multiple times,
+			// once in each package that depends on it.
+			// Only print each once.
+			if !printed[err] {
+				printed[err] = true
+				base.Errorf("%v", err)
+			}
+		}
+	}
+	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) {
+		if seen[pkg.ImportPath] && !reported[pkg.ImportPath] {
+			reported[pkg.ImportPath] = true
+			base.Errorf("internal error: duplicate loads of %s", pkg.ImportPath)
+		}
+		seen[pkg.ImportPath] = true
+	}
+	base.ExitIfErrors()
+}
+
+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, 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),
+			}
+			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.Split(file)
+		if dir1 == "" {
+			dir1 = "./"
+		}
+		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, "command-line-arguments", &stk, nil, bp, err)
+	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)
+		}
+	}
+
+	setToolFlags(pkg)
+
+	return pkg
+}