Adding upstream version 1.16.10.upstream/1.16.10upstream

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

1 files changed, 1986 insertions, 0 deletions

diff --git a/src/go/build/build.go b/src/go/build/build.go
new file mode 100644
index 0000000..217fadf
--- /dev/null
+++ b/src/go/build/build.go
@@ -0,0 +1,1986 @@
+// 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 build
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"go/ast"
+	"go/doc"
+	"go/token"
+	exec "internal/execabs"
+	"internal/goroot"
+	"internal/goversion"
+	"io"
+	"io/fs"
+	"io/ioutil"
+	"os"
+	pathpkg "path"
+	"path/filepath"
+	"runtime"
+	"sort"
+	"strconv"
+	"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 path
+
+	// 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 +build lines, file names
+	Compiler    string // compiler to assume when computing target paths
+
+	// The build 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 ReleaseTags,
+	// which 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 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
+	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 os.Stat.
+func (ctxt *Context) isDir(path string) bool {
+	if f := ctxt.IsDir; f != nil {
+		return f(path)
+	}
+	fi, err := os.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) {
+	const sep = string(filepath.Separator)
+	root = filepath.Clean(root)
+	if !strings.HasSuffix(root, sep) {
+		root += sep
+	}
+	dir = filepath.Clean(dir)
+	if !strings.HasPrefix(dir, root) {
+		return "", false
+	}
+	return filepath.ToSlash(dir[len(root):]), true
+}
+
+// readDir calls ctxt.ReadDir (if not nil) or else ioutil.ReadDir.
+func (ctxt *Context) readDir(path string) ([]fs.FileInfo, error) {
+	if f := ctxt.ReadDir; f != nil {
+		return f(path)
+	}
+	return ioutil.ReadDir(path)
+}
+
+// openFile calls ctxt.OpenFile (if not nil) or else os.Open.
+func (ctxt *Context) openFile(path string) (io.ReadCloser, error) {
+	if fn := ctxt.OpenFile; fn != nil {
+		return fn(path)
+	}
+
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, err // nil interface
+	}
+	return f, nil
+}
+
+// isFile determines whether path is a file by trying to open it.
+// It reuses openFile instead of adding another function to the
+// list in Context.
+func (ctxt *Context) isFile(path string) bool {
+	f, err := ctxt.openFile(path)
+	if err != nil {
+		return false
+	}
+	f.Close()
+	return 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
+}
+
+// SrcDirs returns a list of package source root directories.
+// It draws from the current Go root and Go path but omits directories
+// that do not exist.
+func (ctxt *Context) SrcDirs() []string {
+	var all []string
+	if ctxt.GOROOT != "" && ctxt.Compiler != "gccgo" {
+		dir := ctxt.joinPath(ctxt.GOROOT, "src")
+		if ctxt.isDir(dir) {
+			all = append(all, dir)
+		}
+	}
+	for _, p := range ctxt.gopath() {
+		dir := ctxt.joinPath(p, "src")
+		if ctxt.isDir(dir) {
+			all = append(all, dir)
+		}
+	}
+	return all
+}
+
+// Default is the default Context for builds.
+// It uses the GOARCH, GOOS, GOROOT, and GOPATH environment variables
+// if set, or else the compiled code's GOARCH, GOOS, and GOROOT.
+var Default Context = defaultContext()
+
+func defaultGOPATH() string {
+	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()) {
+			// Don't set the default GOPATH to GOROOT,
+			// as that will trigger warnings from the go tool.
+			return ""
+		}
+		return def
+	}
+	return ""
+}
+
+var defaultReleaseTags []string
+
+func defaultContext() Context {
+	var c Context
+
+	c.GOARCH = envOr("GOARCH", runtime.GOARCH)
+	c.GOOS = envOr("GOOS", runtime.GOOS)
+	c.GOROOT = pathpkg.Clean(runtime.GOROOT())
+	c.GOPATH = envOr("GOPATH", defaultGOPATH())
+	c.Compiler = runtime.Compiler
+
+	// Each major Go release in the Go 1.x series adds a new
+	// "go1.x" release tag. That is, the go1.x tag is present in
+	// all releases >= Go 1.x. Code that requires Go 1.x or later
+	// should say "+build go1.x", and code that should only be
+	// built before Go 1.x (perhaps it is the stub to use in that
+	// case) should say "+build !go1.x".
+	// The last element in ReleaseTags is the current release.
+	for i := 1; i <= goversion.Version; i++ {
+		c.ReleaseTags = append(c.ReleaseTags, "go1."+strconv.Itoa(i))
+	}
+
+	defaultReleaseTags = append([]string{}, c.ReleaseTags...) // our own private copy
+
+	env := os.Getenv("CGO_ENABLED")
+	if env == "" {
+		env = defaultCGO_ENABLED
+	}
+	switch env {
+	case "1":
+		c.CgoEnabled = true
+	case "0":
+		c.CgoEnabled = false
+	default:
+		// cgo must be explicitly enabled for cross compilation builds
+		if runtime.GOARCH == c.GOARCH && runtime.GOOS == c.GOOS {
+			c.CgoEnabled = cgoEnabled[c.GOOS+"/"+c.GOARCH]
+			break
+		}
+		c.CgoEnabled = false
+	}
+
+	return c
+}
+
+func envOr(name, def string) string {
+	s := os.Getenv(name)
+	if s == "" {
+		return def
+	}
+	return s
+}
+
+// An ImportMode controls the behavior of the Import method.
+type ImportMode uint
+
+const (
+	// If FindOnly is set, Import stops after locating the directory
+	// that should contain the sources for a package. It does not
+	// read any files in the directory.
+	FindOnly ImportMode = 1 << iota
+
+	// If AllowBinary is set, Import can be satisfied by a compiled
+	// package object without corresponding sources.
+	//
+	// Deprecated:
+	// The supported way to create a compiled-only package is to
+	// write source code containing a //go:binary-only-package comment at
+	// the top of the file. Such a package will be recognized
+	// regardless of this flag setting (because it has source code)
+	// and will have BinaryOnly set to true in the returned Package.
+	AllowBinary
+
+	// If ImportComment is set, parse import comments on package statements.
+	// Import returns an error if it finds a comment it cannot understand
+	// or finds conflicting comments in multiple source files.
+	// See golang.org/s/go14customimport for more information.
+	ImportComment
+
+	// By default, Import searches vendor directories
+	// that apply in the given source directory before searching
+	// the GOROOT and GOPATH roots.
+	// If an Import finds and returns a package using a vendor
+	// directory, the resulting ImportPath is the complete path
+	// to the package, including the path elements leading up
+	// to and including "vendor".
+	// For example, if Import("y", "x/subdir", 0) finds
+	// "x/vendor/y", the returned package's ImportPath is "x/vendor/y",
+	// not plain "y".
+	// See golang.org/s/go15vendor for more information.
+	//
+	// Setting IgnoreVendor ignores vendor directories.
+	//
+	// In contrast to the package's ImportPath,
+	// the returned package's Imports, TestImports, and XTestImports
+	// are always the exact import paths from the source files:
+	// Import makes no attempt to resolve or check those paths.
+	IgnoreVendor
+)
+
+// A Package describes the Go package found in a directory.
+type Package struct {
+	Dir           string   // directory containing package sources
+	Name          string   // package name
+	ImportComment string   // path in import comment on package statement
+	Doc           string   // documentation synopsis
+	ImportPath    string   // import path of package ("" if unknown)
+	Root          string   // root of Go tree where this package lives
+	SrcRoot       string   // package source root directory ("" if unknown)
+	PkgRoot       string   // package install root directory ("" if unknown)
+	PkgTargetRoot string   // architecture dependent install root directory ("" if unknown)
+	BinDir        string   // command install directory ("" if unknown)
+	Goroot        bool     // package found in Go root
+	PkgObj        string   // installed .a file
+	AllTags       []string // tags that can influence file selection in this directory
+	ConflictDir   string   // this directory shadows Dir in $GOPATH
+	BinaryOnly    bool     // cannot be rebuilt from source (has //go:binary-only-package comment)
+
+	// Source files
+	GoFiles           []string // .go source files (excluding CgoFiles, TestGoFiles, XTestGoFiles)
+	CgoFiles          []string // .go source files that import "C"
+	IgnoredGoFiles    []string // .go source files ignored for this build (including ignored _test.go files)
+	InvalidGoFiles    []string // .go source files with detected problems (parse error, wrong package name, and so on)
+	IgnoredOtherFiles []string // non-.go source files ignored for this build
+	CFiles            []string // .c source files
+	CXXFiles          []string // .cc, .cpp and .cxx source files
+	MFiles            []string // .m (Objective-C) 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 system object files to add to archive
+
+	// Cgo directives
+	CgoCFLAGS    []string // Cgo CFLAGS directives
+	CgoCPPFLAGS  []string // Cgo CPPFLAGS directives
+	CgoCXXFLAGS  []string // Cgo CXXFLAGS directives
+	CgoFFLAGS    []string // Cgo FFLAGS directives
+	CgoLDFLAGS   []string // Cgo LDFLAGS directives
+	CgoPkgConfig []string // Cgo pkg-config directives
+
+	// Test information
+	TestGoFiles  []string // _test.go files in package
+	XTestGoFiles []string // _test.go files outside package
+
+	// Dependency information
+	Imports        []string                    // import paths from GoFiles, CgoFiles
+	ImportPos      map[string][]token.Position // line information for Imports
+	TestImports    []string                    // import paths from TestGoFiles
+	TestImportPos  map[string][]token.Position // line information for TestImports
+	XTestImports   []string                    // import paths from XTestGoFiles
+	XTestImportPos map[string][]token.Position // line information for XTestImports
+
+	// //go:embed patterns found in Go source files
+	// For example, if a source file says
+	//	//go:embed a* b.c
+	// then the list will contain those two strings as separate entries.
+	// (See package embed for more details about //go:embed.)
+	EmbedPatterns        []string                    // patterns from GoFiles, CgoFiles
+	EmbedPatternPos      map[string][]token.Position // line information for EmbedPatterns
+	TestEmbedPatterns    []string                    // patterns from TestGoFiles
+	TestEmbedPatternPos  map[string][]token.Position // line information for TestEmbedPatterns
+	XTestEmbedPatterns   []string                    // patterns from XTestGoFiles
+	XTestEmbedPatternPos map[string][]token.Position // line information for XTestEmbedPatternPos
+}
+
+// IsCommand reports whether the package is considered a
+// command to be installed (not just a library).
+// Packages named "main" are treated as commands.
+func (p *Package) IsCommand() bool {
+	return p.Name == "main"
+}
+
+// ImportDir is like Import but processes the Go package found in
+// the named directory.
+func (ctxt *Context) ImportDir(dir string, mode ImportMode) (*Package, error) {
+	return ctxt.Import(".", dir, mode)
+}
+
+// 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:]
+}
+
+// Import returns details about the Go package named by the import path,
+// interpreting local import paths relative to the srcDir directory.
+// If the path is a local import path naming a package that can be imported
+// using a standard import path, the returned package will set p.ImportPath
+// to that path.
+//
+// In the directory containing the package, .go, .c, .h, and .s files are
+// considered part of the package except for:
+//
+//	- .go files in package documentation
+//	- files starting with _ or . (likely editor temporary files)
+//	- files with build constraints not satisfied by the context
+//
+// If an error occurs, Import returns a non-nil error and a non-nil
+// *Package containing partial information.
+//
+func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Package, error) {
+	p := &Package{
+		ImportPath: path,
+	}
+	if path == "" {
+		return p, fmt.Errorf("import %q: invalid import path", path)
+	}
+
+	var pkgtargetroot string
+	var pkga string
+	var pkgerr error
+	suffix := ""
+	if ctxt.InstallSuffix != "" {
+		suffix = "_" + ctxt.InstallSuffix
+	}
+	switch ctxt.Compiler {
+	case "gccgo":
+		pkgtargetroot = "pkg/gccgo_" + ctxt.GOOS + "_" + ctxt.GOARCH + suffix
+	case "gc":
+		pkgtargetroot = "pkg/" + ctxt.GOOS + "_" + ctxt.GOARCH + suffix
+	default:
+		// Save error for end of function.
+		pkgerr = fmt.Errorf("import %q: unknown compiler %q", path, ctxt.Compiler)
+	}
+	setPkga := func() {
+		switch ctxt.Compiler {
+		case "gccgo":
+			dir, elem := pathpkg.Split(p.ImportPath)
+			pkga = pkgtargetroot + "/" + dir + "lib" + elem + ".a"
+		case "gc":
+			pkga = pkgtargetroot + "/" + p.ImportPath + ".a"
+		}
+	}
+	setPkga()
+
+	binaryOnly := false
+	if IsLocalImport(path) {
+		pkga = "" // local imports have no installed path
+		if srcDir == "" {
+			return p, fmt.Errorf("import %q: import relative to unknown directory", path)
+		}
+		if !ctxt.isAbsPath(path) {
+			p.Dir = ctxt.joinPath(srcDir, path)
+		}
+		// p.Dir directory may or may not exist. Gather partial information first, check if it exists later.
+		// Determine canonical import path, if any.
+		// Exclude results where the import path would include /testdata/.
+		inTestdata := func(sub string) bool {
+			return strings.Contains(sub, "/testdata/") || strings.HasSuffix(sub, "/testdata") || strings.HasPrefix(sub, "testdata/") || sub == "testdata"
+		}
+		if ctxt.GOROOT != "" {
+			root := ctxt.joinPath(ctxt.GOROOT, "src")
+			if sub, ok := ctxt.hasSubdir(root, p.Dir); ok && !inTestdata(sub) {
+				p.Goroot = true
+				p.ImportPath = sub
+				p.Root = ctxt.GOROOT
+				setPkga() // p.ImportPath changed
+				goto Found
+			}
+		}
+		all := ctxt.gopath()
+		for i, root := range all {
+			rootsrc := ctxt.joinPath(root, "src")
+			if sub, ok := ctxt.hasSubdir(rootsrc, p.Dir); ok && !inTestdata(sub) {
+				// We found a potential import path for dir,
+				// but check that using it wouldn't find something
+				// else first.
+				if ctxt.GOROOT != "" && ctxt.Compiler != "gccgo" {
+					if dir := ctxt.joinPath(ctxt.GOROOT, "src", sub); ctxt.isDir(dir) {
+						p.ConflictDir = dir
+						goto Found
+					}
+				}
+				for _, earlyRoot := range all[:i] {
+					if dir := ctxt.joinPath(earlyRoot, "src", sub); ctxt.isDir(dir) {
+						p.ConflictDir = dir
+						goto Found
+					}
+				}
+
+				// sub would not name some other directory instead of this one.
+				// Record it.
+				p.ImportPath = sub
+				p.Root = root
+				setPkga() // p.ImportPath changed
+				goto Found
+			}
+		}
+		// It's okay that we didn't find a root containing dir.
+		// Keep going with the information we have.
+	} else {
+		if strings.HasPrefix(path, "/") {
+			return p, fmt.Errorf("import %q: cannot import absolute path", path)
+		}
+
+		if err := ctxt.importGo(p, path, srcDir, mode); err == nil {
+			goto Found
+		} else if err != errNoModules {
+			return p, err
+		}
+
+		gopath := ctxt.gopath() // needed twice below; avoid computing many times
+
+		// tried records the location of unsuccessful package lookups
+		var tried struct {
+			vendor []string
+			goroot string
+			gopath []string
+		}
+
+		// Vendor directories get first chance to satisfy import.
+		if mode&IgnoreVendor == 0 && srcDir != "" {
+			searchVendor := func(root string, isGoroot bool) bool {
+				sub, ok := ctxt.hasSubdir(root, srcDir)
+				if !ok || !strings.HasPrefix(sub, "src/") || strings.Contains(sub, "/testdata/") {
+					return false
+				}
+				for {
+					vendor := ctxt.joinPath(root, sub, "vendor")
+					if ctxt.isDir(vendor) {
+						dir := ctxt.joinPath(vendor, path)
+						if ctxt.isDir(dir) && hasGoFiles(ctxt, dir) {
+							p.Dir = dir
+							p.ImportPath = strings.TrimPrefix(pathpkg.Join(sub, "vendor", path), "src/")
+							p.Goroot = isGoroot
+							p.Root = root
+							setPkga() // p.ImportPath changed
+							return true
+						}
+						tried.vendor = append(tried.vendor, dir)
+					}
+					i := strings.LastIndex(sub, "/")
+					if i < 0 {
+						break
+					}
+					sub = sub[:i]
+				}
+				return false
+			}
+			if ctxt.Compiler != "gccgo" && searchVendor(ctxt.GOROOT, true) {
+				goto Found
+			}
+			for _, root := range gopath {
+				if searchVendor(root, false) {
+					goto Found
+				}
+			}
+		}
+
+		// Determine directory from import path.
+		if ctxt.GOROOT != "" {
+			// If the package path starts with "vendor/", only search GOROOT before
+			// GOPATH if the importer is also within GOROOT. That way, if the user has
+			// vendored in a package that is subsequently included in the standard
+			// distribution, they'll continue to pick up their own vendored copy.
+			gorootFirst := srcDir == "" || !strings.HasPrefix(path, "vendor/")
+			if !gorootFirst {
+				_, gorootFirst = ctxt.hasSubdir(ctxt.GOROOT, srcDir)
+			}
+			if gorootFirst {
+				dir := ctxt.joinPath(ctxt.GOROOT, "src", path)
+				if ctxt.Compiler != "gccgo" {
+					isDir := ctxt.isDir(dir)
+					binaryOnly = !isDir && mode&AllowBinary != 0 && pkga != "" && ctxt.isFile(ctxt.joinPath(ctxt.GOROOT, pkga))
+					if isDir || binaryOnly {
+						p.Dir = dir
+						p.Goroot = true
+						p.Root = ctxt.GOROOT
+						goto Found
+					}
+				}
+				tried.goroot = dir
+			}
+		}
+		if ctxt.Compiler == "gccgo" && goroot.IsStandardPackage(ctxt.GOROOT, ctxt.Compiler, path) {
+			p.Dir = ctxt.joinPath(ctxt.GOROOT, "src", path)
+			p.Goroot = true
+			p.Root = ctxt.GOROOT
+			goto Found
+		}
+		for _, root := range gopath {
+			dir := ctxt.joinPath(root, "src", path)
+			isDir := ctxt.isDir(dir)
+			binaryOnly = !isDir && mode&AllowBinary != 0 && pkga != "" && ctxt.isFile(ctxt.joinPath(root, pkga))
+			if isDir || binaryOnly {
+				p.Dir = dir
+				p.Root = root
+				goto Found
+			}
+			tried.gopath = append(tried.gopath, dir)
+		}
+
+		// If we tried GOPATH first due to a "vendor/" prefix, fall back to GOPATH.
+		// That way, the user can still get useful results from 'go list' for
+		// standard-vendored paths passed on the command line.
+		if ctxt.GOROOT != "" && tried.goroot == "" {
+			dir := ctxt.joinPath(ctxt.GOROOT, "src", path)
+			if ctxt.Compiler != "gccgo" {
+				isDir := ctxt.isDir(dir)
+				binaryOnly = !isDir && mode&AllowBinary != 0 && pkga != "" && ctxt.isFile(ctxt.joinPath(ctxt.GOROOT, pkga))
+				if isDir || binaryOnly {
+					p.Dir = dir
+					p.Goroot = true
+					p.Root = ctxt.GOROOT
+					goto Found
+				}
+			}
+			tried.goroot = dir
+		}
+
+		// package was not found
+		var paths []string
+		format := "\t%s (vendor tree)"
+		for _, dir := range tried.vendor {
+			paths = append(paths, fmt.Sprintf(format, dir))
+			format = "\t%s"
+		}
+		if tried.goroot != "" {
+			paths = append(paths, fmt.Sprintf("\t%s (from $GOROOT)", tried.goroot))
+		} else {
+			paths = append(paths, "\t($GOROOT not set)")
+		}
+		format = "\t%s (from $GOPATH)"
+		for _, dir := range tried.gopath {
+			paths = append(paths, fmt.Sprintf(format, dir))
+			format = "\t%s"
+		}
+		if len(tried.gopath) == 0 {
+			paths = append(paths, "\t($GOPATH not set. For more details see: 'go help gopath')")
+		}
+		return p, fmt.Errorf("cannot find package %q in any of:\n%s", path, strings.Join(paths, "\n"))
+	}
+
+Found:
+	if p.Root != "" {
+		p.SrcRoot = ctxt.joinPath(p.Root, "src")
+		p.PkgRoot = ctxt.joinPath(p.Root, "pkg")
+		p.BinDir = ctxt.joinPath(p.Root, "bin")
+		if pkga != "" {
+			p.PkgTargetRoot = ctxt.joinPath(p.Root, pkgtargetroot)
+			p.PkgObj = ctxt.joinPath(p.Root, pkga)
+		}
+	}
+
+	// If it's a local import path, by the time we get here, 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 *Package returned when an error occurs.
+	// We need to do this before we return early on FindOnly flag.
+	if IsLocalImport(path) && !ctxt.isDir(p.Dir) {
+		if ctxt.Compiler == "gccgo" && p.Goroot {
+			// gccgo has no sources for GOROOT packages.
+			return p, nil
+		}
+
+		// package was not found
+		return p, fmt.Errorf("cannot find package %q in:\n\t%s", path, p.Dir)
+	}
+
+	if mode&FindOnly != 0 {
+		return p, pkgerr
+	}
+	if binaryOnly && (mode&AllowBinary) != 0 {
+		return p, pkgerr
+	}
+
+	if ctxt.Compiler == "gccgo" && p.Goroot {
+		// gccgo has no sources for GOROOT packages.
+		return p, nil
+	}
+
+	dirs, err := ctxt.readDir(p.Dir)
+	if err != nil {
+		return p, err
+	}
+
+	var badGoError error
+	var Sfiles []string // files with ".S"(capital S)/.sx(capital s equivalent for case insensitive filesystems)
+	var firstFile, firstCommentFile 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)
+	fset := token.NewFileSet()
+	for _, d := range dirs {
+		if d.IsDir() {
+			continue
+		}
+		if d.Mode()&fs.ModeSymlink != 0 {
+			if ctxt.isDir(ctxt.joinPath(p.Dir, d.Name())) {
+				// Symlinks to directories are not source files.
+				continue
+			}
+		}
+
+		name := d.Name()
+		ext := nameExt(name)
+
+		badFile := func(err error) {
+			if badGoError == nil {
+				badGoError = err
+			}
+			p.InvalidGoFiles = append(p.InvalidGoFiles, name)
+		}
+
+		info, err := ctxt.matchFile(p.Dir, name, allTags, &p.BinaryOnly, fset)
+		if err != nil {
+			badFile(err)
+			continue
+		}
+		if info == nil {
+			if strings.HasPrefix(name, "_") || strings.HasPrefix(name, ".") {
+				// not due to build constraints - don't report
+			} else if ext == ".go" {
+				p.IgnoredGoFiles = append(p.IgnoredGoFiles, name)
+			} else if fileListForExt(p, ext) != nil {
+				p.IgnoredOtherFiles = append(p.IgnoredOtherFiles, name)
+			}
+			continue
+		}
+		data, filename := info.header, info.name
+
+		// 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
+		}
+
+		if info.parseErr != nil {
+			badFile(info.parseErr)
+			continue
+		}
+		pf := info.parsed
+
+		pkg := pf.Name.Name
+		if pkg == "documentation" {
+			p.IgnoredGoFiles = append(p.IgnoredGoFiles, name)
+			continue
+		}
+
+		isTest := strings.HasSuffix(name, "_test.go")
+		isXTest := false
+		if isTest && strings.HasSuffix(pkg, "_test") {
+			isXTest = true
+			pkg = pkg[:len(pkg)-len("_test")]
+		}
+
+		if p.Name == "" {
+			p.Name = pkg
+			firstFile = name
+		} else if pkg != p.Name {
+			badFile(&MultiplePackageError{
+				Dir:      p.Dir,
+				Packages: []string{p.Name, pkg},
+				Files:    []string{firstFile, name},
+			})
+			p.InvalidGoFiles = append(p.InvalidGoFiles, name)
+		}
+		// Grab the first package comment as docs, provided it is not from a test file.
+		if pf.Doc != nil && p.Doc == "" && !isTest && !isXTest {
+			p.Doc = doc.Synopsis(pf.Doc.Text())
+		}
+
+		if mode&ImportComment != 0 {
+			qcom, line := findImportComment(data)
+			if line != 0 {
+				com, err := strconv.Unquote(qcom)
+				if err != nil {
+					badFile(fmt.Errorf("%s:%d: cannot parse import comment", filename, line))
+				} else if p.ImportComment == "" {
+					p.ImportComment = com
+					firstCommentFile = name
+				} else if p.ImportComment != com {
+					badFile(fmt.Errorf("found import comments %q (%s) and %q (%s) in %s", p.ImportComment, firstCommentFile, com, name, p.Dir))
+				}
+			}
+		}
+
+		// Record imports and information about cgo.
+		isCgo := false
+		for _, imp := range info.imports {
+			if imp.path == "C" {
+				if isTest {
+					badFile(fmt.Errorf("use of cgo in test %s not supported", filename))
+					continue
+				}
+				isCgo = true
+				if imp.doc != nil {
+					if err := ctxt.saveCgo(filename, p, imp.doc); err != nil {
+						badFile(err)
+					}
+				}
+			}
+		}
+
+		var fileList *[]string
+		var importMap, embedMap map[string][]token.Position
+		switch {
+		case isCgo:
+			allTags["cgo"] = true
+			if ctxt.CgoEnabled {
+				fileList = &p.CgoFiles
+				importMap = importPos
+				embedMap = embedPos
+			} else {
+				// Ignore imports and embeds from cgo files if cgo is disabled.
+				fileList = &p.IgnoredGoFiles
+			}
+		case isXTest:
+			fileList = &p.XTestGoFiles
+			importMap = xTestImportPos
+			embedMap = xTestEmbedPos
+		case isTest:
+			fileList = &p.TestGoFiles
+			importMap = testImportPos
+			embedMap = testEmbedPos
+		default:
+			fileList = &p.GoFiles
+			importMap = importPos
+			embedMap = embedPos
+		}
+		*fileList = append(*fileList, name)
+		if importMap != nil {
+			for _, imp := range info.imports {
+				importMap[imp.path] = append(importMap[imp.path], fset.Position(imp.pos))
+			}
+		}
+		if embedMap != nil {
+			for _, emb := range info.embeds {
+				embedMap[emb.pattern] = append(embedMap[emb.pattern], emb.pos)
+			}
+		}
+	}
+
+	for tag := range allTags {
+		p.AllTags = append(p.AllTags, tag)
+	}
+	sort.Strings(p.AllTags)
+
+	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)
+
+	// add the .S/.sx files only if we are using cgo
+	// (which means gcc will compile them).
+	// The standard assemblers expect .s files.
+	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, &NoGoError{p.Dir}
+	}
+	return p, pkgerr
+}
+
+func fileListForExt(p *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
+}
+
+func uniq(list []string) []string {
+	if list == nil {
+		return nil
+	}
+	out := make([]string, len(list))
+	copy(out, list)
+	sort.Strings(out)
+	uniq := out[:0]
+	for _, x := range out {
+		if len(uniq) == 0 || uniq[len(uniq)-1] != x {
+			uniq = append(uniq, x)
+		}
+	}
+	return uniq
+}
+
+var errNoModules = errors.New("not using modules")
+
+// importGo checks whether it can use the go command to find the directory for path.
+// If using the go command is not appropriate, importGo returns errNoModules.
+// Otherwise, importGo tries using the go command and reports whether that succeeded.
+// Using the go command lets build.Import and build.Context.Import find code
+// in Go modules. In the long term we want tools to use go/packages (currently golang.org/x/tools/go/packages),
+// which will also use the go command.
+// Invoking the go command here is not very efficient in that it computes information
+// about the requested package and all dependencies and then only reports about the requested package.
+// Then we reinvoke it for every dependency. But this is still better than not working at all.
+// See golang.org/issue/26504.
+func (ctxt *Context) importGo(p *Package, path, srcDir string, mode ImportMode) error {
+	// To invoke the go command,
+	// we must not being doing special things like AllowBinary or IgnoreVendor,
+	// and all the file system callbacks must be nil (we're meant to use the local file system).
+	if mode&AllowBinary != 0 || mode&IgnoreVendor != 0 ||
+		ctxt.JoinPath != nil || ctxt.SplitPathList != nil || ctxt.IsAbsPath != nil || ctxt.IsDir != nil || ctxt.HasSubdir != nil || ctxt.ReadDir != nil || ctxt.OpenFile != nil || !equal(ctxt.ReleaseTags, defaultReleaseTags) {
+		return errNoModules
+	}
+
+	// Predict whether module aware mode is enabled by checking the value of
+	// GO111MODULE and looking for a go.mod file in the source directory or
+	// one of its parents. Running 'go env GOMOD' in the source directory would
+	// give a canonical answer, but we'd prefer not to execute another command.
+	go111Module := os.Getenv("GO111MODULE")
+	switch go111Module {
+	case "off":
+		return errNoModules
+	default: // "", "on", "auto", anything else
+		// Maybe use modules.
+	}
+
+	if srcDir != "" {
+		var absSrcDir string
+		if filepath.IsAbs(srcDir) {
+			absSrcDir = srcDir
+		} else if ctxt.Dir != "" {
+			return fmt.Errorf("go/build: Dir is non-empty, so relative srcDir is not allowed: %v", srcDir)
+		} else {
+			// Find the absolute source directory. hasSubdir does not handle
+			// relative paths (and can't because the callbacks don't support this).
+			var err error
+			absSrcDir, err = filepath.Abs(srcDir)
+			if err != nil {
+				return errNoModules
+			}
+		}
+
+		// If the source directory is in GOROOT, then the in-process code works fine
+		// and we should keep using it. Moreover, the 'go list' approach below doesn't
+		// take standard-library vendoring into account and will fail.
+		if _, ok := ctxt.hasSubdir(filepath.Join(ctxt.GOROOT, "src"), absSrcDir); ok {
+			return errNoModules
+		}
+	}
+
+	// For efficiency, if path is a standard library package, let the usual lookup code handle it.
+	if ctxt.GOROOT != "" {
+		dir := ctxt.joinPath(ctxt.GOROOT, "src", path)
+		if ctxt.isDir(dir) {
+			return errNoModules
+		}
+	}
+
+	// If GO111MODULE=auto, look to see if there is a go.mod.
+	// Since go1.13, it doesn't matter if we're inside GOPATH.
+	if go111Module == "auto" {
+		var (
+			parent string
+			err    error
+		)
+		if ctxt.Dir == "" {
+			parent, err = os.Getwd()
+			if err != nil {
+				// A nonexistent working directory can't be in a module.
+				return errNoModules
+			}
+		} else {
+			parent, err = filepath.Abs(ctxt.Dir)
+			if err != nil {
+				// If the caller passed a bogus Dir explicitly, that's materially
+				// different from not having modules enabled.
+				return err
+			}
+		}
+		for {
+			if f, err := ctxt.openFile(ctxt.joinPath(parent, "go.mod")); err == nil {
+				buf := make([]byte, 100)
+				_, err := f.Read(buf)
+				f.Close()
+				if err == nil || err == io.EOF {
+					// go.mod exists and is readable (is a file, not a directory).
+					break
+				}
+			}
+			d := filepath.Dir(parent)
+			if len(d) >= len(parent) {
+				return errNoModules // reached top of file system, no go.mod
+			}
+			parent = d
+		}
+	}
+
+	cmd := exec.Command("go", "list", "-e", "-compiler="+ctxt.Compiler, "-tags="+strings.Join(ctxt.BuildTags, ","), "-installsuffix="+ctxt.InstallSuffix, "-f={{.Dir}}\n{{.ImportPath}}\n{{.Root}}\n{{.Goroot}}\n{{if .Error}}{{.Error}}{{end}}\n", "--", path)
+
+	if ctxt.Dir != "" {
+		cmd.Dir = ctxt.Dir
+	}
+
+	var stdout, stderr strings.Builder
+	cmd.Stdout = &stdout
+	cmd.Stderr = &stderr
+
+	cgo := "0"
+	if ctxt.CgoEnabled {
+		cgo = "1"
+	}
+	cmd.Env = append(os.Environ(),
+		"GOOS="+ctxt.GOOS,
+		"GOARCH="+ctxt.GOARCH,
+		"GOROOT="+ctxt.GOROOT,
+		"GOPATH="+ctxt.GOPATH,
+		"CGO_ENABLED="+cgo,
+	)
+
+	if err := cmd.Run(); err != nil {
+		return fmt.Errorf("go/build: go list %s: %v\n%s\n", path, err, stderr.String())
+	}
+
+	f := strings.SplitN(stdout.String(), "\n", 5)
+	if len(f) != 5 {
+		return fmt.Errorf("go/build: importGo %s: unexpected output:\n%s\n", path, stdout.String())
+	}
+	dir := f[0]
+	errStr := strings.TrimSpace(f[4])
+	if errStr != "" && dir == "" {
+		// If 'go list' could not locate the package (dir is empty),
+		// return the same error that 'go list' reported.
+		return errors.New(errStr)
+	}
+
+	// If 'go list' did locate the package, ignore the error.
+	// It was probably related to loading source files, and we'll
+	// encounter it ourselves shortly if the FindOnly flag isn't set.
+	p.Dir = dir
+	p.ImportPath = f[1]
+	p.Root = f[2]
+	p.Goroot = f[3] == "true"
+	return nil
+}
+
+func equal(x, y []string) bool {
+	if len(x) != len(y) {
+		return false
+	}
+	for i, xi := range x {
+		if xi != y[i] {
+			return false
+		}
+	}
+	return true
+}
+
+// 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(ctxt *Context, dir string) bool {
+	ents, _ := ctxt.readDir(dir)
+	for _, ent := range ents {
+		if !ent.IsDir() && strings.HasSuffix(ent.Name(), ".go") {
+			return true
+		}
+	}
+	return false
+}
+
+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):
+		i := bytes.Index(data, newline)
+		if i < 0 {
+			i = len(data)
+		}
+		comment = data[2:i]
+	case bytes.HasPrefix(data, slashStar):
+		data = data[2:]
+		i := bytes.Index(data, starSlash)
+		if i < 0 {
+			// malformed comment
+			return "", 0
+		}
+		comment = data[:i]
+		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
+}
+
+// MatchFile reports whether the file with the given name in the given directory
+// matches the context and would be included in a Package created by ImportDir
+// of that directory.
+//
+// MatchFile considers the name of the file and may use ctxt.OpenFile to
+// read some or all of the file's content.
+func (ctxt *Context) MatchFile(dir, name string) (match bool, err error) {
+	info, err := ctxt.matchFile(dir, name, nil, nil, nil)
+	return info != nil, err
+}
+
+var dummyPkg 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
+	embedErr error
+}
+
+type fileImport struct {
+	path string
+	pos  token.Pos
+	doc  *ast.CommentGroup
+}
+
+type fileEmbed struct {
+	pattern string
+	pos     token.Position
+}
+
+// matchFile determines whether the file with the given name in the given directory
+// should be included in the package being constructed.
+// If the file should be included, matchFile returns a non-nil *fileInfo (and a nil error).
+// Non-nil errors are reserved for unexpected problems.
+//
+// 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.
+//
+// If allTags is non-nil, matchFile records any encountered build tag
+// by setting allTags[tag] = true.
+func (ctxt *Context) matchFile(dir, name string, allTags map[string]bool, binaryOnly *bool, 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 !ctxt.goodOSArchFile(name, allTags) && !ctxt.UseAllFiles {
+		return nil, nil
+	}
+
+	if ext != ".go" && fileListForExt(&dummyPkg, ext) == nil {
+		// skip
+		return nil, nil
+	}
+
+	info := &fileInfo{name: ctxt.joinPath(dir, name), fset: fset}
+	if ext == ".syso" {
+		// binary, no reading
+		return info, nil
+	}
+
+	f, err := ctxt.openFile(info.name)
+	if err != nil {
+		return nil, err
+	}
+
+	if strings.HasSuffix(name, ".go") {
+		err = readGoInfo(f, info)
+		if strings.HasSuffix(name, "_test.go") {
+			binaryOnly = nil // ignore //go:binary-only-package comments in _test.go files
+		}
+	} else {
+		binaryOnly = nil // ignore //go:binary-only-package comments in non-Go sources
+		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.
+	ok, sawBinaryOnly, err := ctxt.shouldBuild(info.header, allTags)
+	if err != nil {
+		return nil, err
+	}
+	if !ok && !ctxt.UseAllFiles {
+		return nil, nil
+	}
+
+	if binaryOnly != nil && sawBinaryOnly {
+		*binaryOnly = true
+	}
+
+	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
+}
+
+// Import is shorthand for Default.Import.
+func Import(path, srcDir string, mode ImportMode) (*Package, error) {
+	return Default.Import(path, srcDir, mode)
+}
+
+// ImportDir is shorthand for Default.ImportDir.
+func ImportDir(dir string, mode ImportMode) (*Package, error) {
+	return Default.ImportDir(dir, mode)
+}
+
+var (
+	bSlashSlash = []byte(slashSlash)
+	bStarSlash  = []byte(starSlash)
+	bSlashStar  = []byte(slashStar)
+
+	goBuildComment = []byte("//go:build")
+
+	errGoBuildWithoutBuild = errors.New("//go:build comment without // +build comment")
+	errMultipleGoBuild     = errors.New("multiple //go:build comments") // unused in Go 1.(N-1)
+)
+
+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")
+
+// 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.
+//
+// For each build tag it consults, shouldBuild sets allTags[tag] = true.
+//
+// shouldBuild reports whether the file should be built
+// and whether a //go:binary-only-package comment was found.
+func (ctxt *Context) shouldBuild(content []byte, allTags map[string]bool) (shouldBuild, binaryOnly bool, err error) {
+
+	// Pass 1. Identify leading run of // comments and blank lines,
+	// which must be followed by a blank line.
+	// Also identify any //go:build comments.
+	content, goBuild, sawBinaryOnly, err := parseFileHeader(content)
+	if err != nil {
+		return false, false, err
+	}
+
+	// Pass 2.  Process each +build line in the run.
+	p := content
+	shouldBuild = true
+	sawBuild := false
+	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) {
+			continue
+		}
+		line = bytes.TrimSpace(line[len(bSlashSlash):])
+		if len(line) > 0 && line[0] == '+' {
+			// Looks like a comment +line.
+			f := strings.Fields(string(line))
+			if f[0] == "+build" {
+				sawBuild = true
+				ok := false
+				for _, tok := range f[1:] {
+					if ctxt.match(tok, allTags) {
+						ok = true
+					}
+				}
+				if !ok {
+					shouldBuild = false
+				}
+			}
+		}
+	}
+
+	if goBuild != nil && !sawBuild {
+		return false, false, errGoBuildWithoutBuild
+	}
+
+	return shouldBuild, 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 false && goBuild != nil { // enabled in Go 1.N
+				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 *Package, cg *ast.CommentGroup) error {
+	text := cg.Text()
+	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 = strings.TrimSpace(line[4:])
+		i := strings.Index(line, ":")
+		if i < 0 {
+			return fmt.Errorf("%s: invalid #cgo line: %s", filename, orig)
+		}
+		line, argstr := line[:i], line[i+1:]
+
+		// 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.match(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)
+		}
+		var ok bool
+		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
+}
+
+// match reports whether the name is one of:
+//
+//	$GOOS
+//	$GOARCH
+//	cgo (if cgo is enabled)
+//	!cgo (if cgo is disabled)
+//	ctxt.Compiler
+//	!ctxt.Compiler
+//	tag (if tag is listed in ctxt.BuildTags or ctxt.ReleaseTags)
+//	!tag (if tag is not listed in ctxt.BuildTags or ctxt.ReleaseTags)
+//	a comma-separated list of any of these
+//
+func (ctxt *Context) match(name string, allTags map[string]bool) bool {
+	if name == "" {
+		if allTags != nil {
+			allTags[name] = true
+		}
+		return false
+	}
+	if i := strings.Index(name, ","); i >= 0 {
+		// comma-separated list
+		ok1 := ctxt.match(name[:i], allTags)
+		ok2 := ctxt.match(name[i+1:], allTags)
+		return ok1 && ok2
+	}
+	if strings.HasPrefix(name, "!!") { // bad syntax, reject always
+		return false
+	}
+	if strings.HasPrefix(name, "!") { // negation
+		return len(name) > 1 && !ctxt.match(name[1:], allTags)
+	}
+
+	if allTags != nil {
+		allTags[name] = true
+	}
+
+	// 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
+		}
+	}
+
+	// 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
+	}
+
+	// other tags
+	for _, tag := range ctxt.BuildTags {
+		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 {
+	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 ctxt.match(l[n-1], allTags) && ctxt.match(l[n-2], allTags)
+	}
+	if n >= 1 && (knownOS[l[n-1]] || knownArch[l[n-1]]) {
+		return ctxt.match(l[n-1], allTags)
+	}
+	return true
+}
+
+var knownOS = make(map[string]bool)
+var knownArch = make(map[string]bool)
+
+func init() {
+	for _, v := range strings.Fields(goosList) {
+		knownOS[v] = true
+	}
+	for _, v := range strings.Fields(goarchList) {
+		knownArch[v] = true
+	}
+}
+
+// ToolDir is the directory containing build tools.
+var ToolDir = getToolDir()
+
+// IsLocalImport reports whether the import path is
+// a local import path, like ".", "..", "./foo", or "../foo".
+func IsLocalImport(path string) bool {
+	return path == "." || path == ".." ||
+		strings.HasPrefix(path, "./") || strings.HasPrefix(path, "../")
+}
+
+// ArchChar returns "?" and an error.
+// In earlier versions of Go, the returned string was used to derive
+// the compiler and linker tool names, the default object file suffix,
+// and the default linker output name. As of Go 1.5, those strings
+// no longer vary by architecture; they are compile, link, .o, and a.out, respectively.
+func ArchChar(goarch string) (string, error) {
+	return "?", errors.New("architecture letter no longer used")
+}