18 files changed, 6637 insertions, 0 deletions

diff --git a/src/cmd/go/internal/modload/build.go b/src/cmd/go/internal/modload/build.go
new file mode 100644
index 0000000..8ad5f83
--- /dev/null
+++ b/src/cmd/go/internal/modload/build.go
@@ -0,0 +1,361 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"bytes"
+	"context"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"internal/goroot"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/modinfo"
+	"cmd/go/internal/search"
+
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
+)
+
+var (
+	infoStart, _ = hex.DecodeString("3077af0c9274080241e1c107e6d618e6")
+	infoEnd, _   = hex.DecodeString("f932433186182072008242104116d8f2")
+)
+
+func isStandardImportPath(path string) bool {
+	return findStandardImportPath(path) != ""
+}
+
+func findStandardImportPath(path string) string {
+	if path == "" {
+		panic("findStandardImportPath called with empty path")
+	}
+	if search.IsStandardImportPath(path) {
+		if goroot.IsStandardPackage(cfg.GOROOT, cfg.BuildContext.Compiler, path) {
+			return filepath.Join(cfg.GOROOT, "src", path)
+		}
+	}
+	return ""
+}
+
+// PackageModuleInfo returns information about the module that provides
+// a given package. If modules are not enabled or if the package is in the
+// standard library or if the package was not successfully loaded with
+// LoadPackages or ImportFromFiles, nil is returned.
+func PackageModuleInfo(pkgpath string) *modinfo.ModulePublic {
+	if isStandardImportPath(pkgpath) || !Enabled() {
+		return nil
+	}
+	m, ok := findModule(pkgpath)
+	if !ok {
+		return nil
+	}
+	fromBuildList := true
+	listRetracted := false
+	return moduleInfo(context.TODO(), m, fromBuildList, listRetracted)
+}
+
+func ModuleInfo(ctx context.Context, path string) *modinfo.ModulePublic {
+	if !Enabled() {
+		return nil
+	}
+
+	listRetracted := false
+	if i := strings.Index(path, "@"); i >= 0 {
+		m := module.Version{Path: path[:i], Version: path[i+1:]}
+		fromBuildList := false
+		return moduleInfo(ctx, m, fromBuildList, listRetracted)
+	}
+
+	for _, m := range buildList {
+		if m.Path == path {
+			fromBuildList := true
+			return moduleInfo(ctx, m, fromBuildList, listRetracted)
+		}
+	}
+
+	return &modinfo.ModulePublic{
+		Path: path,
+		Error: &modinfo.ModuleError{
+			Err: "module not in current build",
+		},
+	}
+}
+
+// addUpdate fills in m.Update if an updated version is available.
+func addUpdate(ctx context.Context, m *modinfo.ModulePublic) {
+	if m.Version == "" {
+		return
+	}
+
+	if info, err := Query(ctx, m.Path, "upgrade", m.Version, CheckAllowed); err == nil && semver.Compare(info.Version, m.Version) > 0 {
+		m.Update = &modinfo.ModulePublic{
+			Path:    m.Path,
+			Version: info.Version,
+			Time:    &info.Time,
+		}
+	}
+}
+
+// addVersions fills in m.Versions with the list of known versions.
+// Excluded versions will be omitted. If listRetracted is false, retracted
+// versions will also be omitted.
+func addVersions(ctx context.Context, m *modinfo.ModulePublic, listRetracted bool) {
+	allowed := CheckAllowed
+	if listRetracted {
+		allowed = CheckExclusions
+	}
+	m.Versions, _ = versions(ctx, m.Path, allowed)
+}
+
+// addRetraction fills in m.Retracted if the module was retracted by its author.
+// m.Error is set if there's an error loading retraction information.
+func addRetraction(ctx context.Context, m *modinfo.ModulePublic) {
+	if m.Version == "" {
+		return
+	}
+
+	err := CheckRetractions(ctx, module.Version{Path: m.Path, Version: m.Version})
+	var rerr *ModuleRetractedError
+	if errors.As(err, &rerr) {
+		if len(rerr.Rationale) == 0 {
+			m.Retracted = []string{"retracted by module author"}
+		} else {
+			m.Retracted = rerr.Rationale
+		}
+	} else if err != nil && m.Error == nil {
+		m.Error = &modinfo.ModuleError{Err: err.Error()}
+	}
+}
+
+func moduleInfo(ctx context.Context, m module.Version, fromBuildList, listRetracted bool) *modinfo.ModulePublic {
+	if m == Target {
+		info := &modinfo.ModulePublic{
+			Path:    m.Path,
+			Version: m.Version,
+			Main:    true,
+		}
+		if HasModRoot() {
+			info.Dir = ModRoot()
+			info.GoMod = ModFilePath()
+			if modFile.Go != nil {
+				info.GoVersion = modFile.Go.Version
+			}
+		}
+		return info
+	}
+
+	info := &modinfo.ModulePublic{
+		Path:     m.Path,
+		Version:  m.Version,
+		Indirect: fromBuildList && loaded != nil && !loaded.direct[m.Path],
+	}
+	if v, ok := rawGoVersion.Load(m); ok {
+		info.GoVersion = v.(string)
+	}
+
+	// completeFromModCache fills in the extra fields in m using the module cache.
+	completeFromModCache := func(m *modinfo.ModulePublic) {
+		mod := module.Version{Path: m.Path, Version: m.Version}
+
+		if m.Version != "" {
+			if q, err := Query(ctx, m.Path, m.Version, "", nil); err != nil {
+				m.Error = &modinfo.ModuleError{Err: err.Error()}
+			} else {
+				m.Version = q.Version
+				m.Time = &q.Time
+			}
+
+			gomod, err := modfetch.CachePath(mod, "mod")
+			if err == nil {
+				if info, err := os.Stat(gomod); err == nil && info.Mode().IsRegular() {
+					m.GoMod = gomod
+				}
+			}
+			dir, err := modfetch.DownloadDir(mod)
+			if err == nil {
+				m.Dir = dir
+			}
+
+			if listRetracted {
+				addRetraction(ctx, m)
+			}
+		}
+
+		if m.GoVersion == "" {
+			if summary, err := rawGoModSummary(mod); err == nil && summary.goVersionV != "" {
+				m.GoVersion = summary.goVersionV[1:]
+			}
+		}
+	}
+
+	if !fromBuildList {
+		// If this was an explicitly-versioned argument to 'go mod download' or
+		// 'go list -m', report the actual requested version, not its replacement.
+		completeFromModCache(info) // Will set m.Error in vendor mode.
+		return info
+	}
+
+	r := Replacement(m)
+	if r.Path == "" {
+		if cfg.BuildMod == "vendor" {
+			// It's tempting to fill in the "Dir" field to point within the vendor
+			// directory, but that would be misleading: the vendor directory contains
+			// a flattened package tree, not complete modules, and it can even
+			// interleave packages from different modules if one module path is a
+			// prefix of the other.
+		} else {
+			completeFromModCache(info)
+		}
+		return info
+	}
+
+	// Don't hit the network to fill in extra data for replaced modules.
+	// The original resolved Version and Time don't matter enough to be
+	// worth the cost, and we're going to overwrite the GoMod and Dir from the
+	// replacement anyway. See https://golang.org/issue/27859.
+	info.Replace = &modinfo.ModulePublic{
+		Path:    r.Path,
+		Version: r.Version,
+	}
+	if v, ok := rawGoVersion.Load(m); ok {
+		info.Replace.GoVersion = v.(string)
+	}
+	if r.Version == "" {
+		if filepath.IsAbs(r.Path) {
+			info.Replace.Dir = r.Path
+		} else {
+			info.Replace.Dir = filepath.Join(ModRoot(), r.Path)
+		}
+		info.Replace.GoMod = filepath.Join(info.Replace.Dir, "go.mod")
+	}
+	if cfg.BuildMod != "vendor" {
+		completeFromModCache(info.Replace)
+		info.Dir = info.Replace.Dir
+		info.GoMod = info.Replace.GoMod
+		info.Retracted = info.Replace.Retracted
+	}
+	info.GoVersion = info.Replace.GoVersion
+	return info
+}
+
+// PackageBuildInfo returns a string containing module version information
+// for modules providing packages named by path and deps. path and deps must
+// name packages that were resolved successfully with LoadPackages.
+func PackageBuildInfo(path string, deps []string) string {
+	if isStandardImportPath(path) || !Enabled() {
+		return ""
+	}
+
+	target := mustFindModule(path, path)
+	mdeps := make(map[module.Version]bool)
+	for _, dep := range deps {
+		if !isStandardImportPath(dep) {
+			mdeps[mustFindModule(path, dep)] = true
+		}
+	}
+	var mods []module.Version
+	delete(mdeps, target)
+	for mod := range mdeps {
+		mods = append(mods, mod)
+	}
+	module.Sort(mods)
+
+	var buf bytes.Buffer
+	fmt.Fprintf(&buf, "path\t%s\n", path)
+
+	writeEntry := func(token string, m module.Version) {
+		mv := m.Version
+		if mv == "" {
+			mv = "(devel)"
+		}
+		fmt.Fprintf(&buf, "%s\t%s\t%s", token, m.Path, mv)
+		if r := Replacement(m); r.Path == "" {
+			fmt.Fprintf(&buf, "\t%s\n", modfetch.Sum(m))
+		} else {
+			fmt.Fprintf(&buf, "\n=>\t%s\t%s\t%s\n", r.Path, r.Version, modfetch.Sum(r))
+		}
+	}
+
+	writeEntry("mod", target)
+	for _, mod := range mods {
+		writeEntry("dep", mod)
+	}
+
+	return buf.String()
+}
+
+// mustFindModule is like findModule, but it calls base.Fatalf if the
+// module can't be found.
+//
+// TODO(jayconrod): remove this. Callers should use findModule and return
+// errors instead of relying on base.Fatalf.
+func mustFindModule(target, path string) module.Version {
+	pkg, ok := loaded.pkgCache.Get(path).(*loadPkg)
+	if ok {
+		if pkg.err != nil {
+			base.Fatalf("build %v: cannot load %v: %v", target, path, pkg.err)
+		}
+		return pkg.mod
+	}
+
+	if path == "command-line-arguments" {
+		return Target
+	}
+
+	base.Fatalf("build %v: cannot find module for path %v", target, path)
+	panic("unreachable")
+}
+
+// findModule searches for the module that contains the package at path.
+// If the package was loaded, its containing module and true are returned.
+// Otherwise, module.Version{} and false are returend.
+func findModule(path string) (module.Version, bool) {
+	if pkg, ok := loaded.pkgCache.Get(path).(*loadPkg); ok {
+		return pkg.mod, pkg.mod != module.Version{}
+	}
+	if path == "command-line-arguments" {
+		return Target, true
+	}
+	return module.Version{}, false
+}
+
+func ModInfoProg(info string, isgccgo bool) []byte {
+	// Inject a variable with the debug information as runtime.modinfo,
+	// but compile it in package main so that it is specific to the binary.
+	// The variable must be a literal so that it will have the correct value
+	// before the initializer for package main runs.
+	//
+	// The runtime startup code refers to the variable, which keeps it live
+	// in all binaries.
+	//
+	// Note: we use an alternate recipe below for gccgo (based on an
+	// init function) due to the fact that gccgo does not support
+	// applying a "//go:linkname" directive to a variable. This has
+	// drawbacks in that other packages may want to look at the module
+	// info in their init functions (see issue 29628), which won't
+	// work for gccgo. See also issue 30344.
+
+	if !isgccgo {
+		return []byte(fmt.Sprintf(`package main
+import _ "unsafe"
+//go:linkname __debug_modinfo__ runtime.modinfo
+var __debug_modinfo__ = %q
+`, string(infoStart)+info+string(infoEnd)))
+	} else {
+		return []byte(fmt.Sprintf(`package main
+import _ "unsafe"
+//go:linkname __set_debug_modinfo__ runtime.setmodinfo
+func __set_debug_modinfo__(string)
+func init() { __set_debug_modinfo__(%q) }
+`, string(infoStart)+info+string(infoEnd)))
+	}
+}
diff --git a/src/cmd/go/internal/modload/buildlist.go b/src/cmd/go/internal/modload/buildlist.go
new file mode 100644
index 0000000..0ed9853
--- /dev/null
+++ b/src/cmd/go/internal/modload/buildlist.go
@@ -0,0 +1,308 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/imports"
+	"cmd/go/internal/mvs"
+	"context"
+	"fmt"
+	"go/build"
+	"os"
+	"strings"
+
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
+)
+
+// buildList is the list of modules to use for building packages.
+// It is initialized by calling LoadPackages or ImportFromFiles,
+// each of which uses loaded.load.
+//
+// Ideally, exactly ONE of those functions would be called,
+// and exactly once. Most of the time, that's true.
+// During "go get" it may not be. TODO(rsc): Figure out if
+// that restriction can be established, or else document why not.
+//
+var buildList []module.Version
+
+// additionalExplicitRequirements is a list of modules paths for which
+// WriteGoMod should record explicit requirements, even if they would be
+// selected without those requirements. Each path must also appear in buildList.
+var additionalExplicitRequirements []string
+
+// capVersionSlice returns s with its cap reduced to its length.
+func capVersionSlice(s []module.Version) []module.Version {
+	return s[:len(s):len(s)]
+}
+
+// LoadAllModules loads and returns the list of modules matching the "all"
+// module pattern, starting with the Target module and in a deterministic
+// (stable) order, without loading any packages.
+//
+// Modules are loaded automatically (and lazily) in LoadPackages:
+// LoadAllModules need only be called if LoadPackages is not,
+// typically in commands that care about modules but no particular package.
+//
+// The caller must not modify the returned list, but may append to it.
+func LoadAllModules(ctx context.Context) []module.Version {
+	LoadModFile(ctx)
+	ReloadBuildList()
+	WriteGoMod()
+	return capVersionSlice(buildList)
+}
+
+// Selected returns the selected version of the module with the given path, or
+// the empty string if the given module has no selected version
+// (either because it is not required or because it is the Target module).
+func Selected(path string) (version string) {
+	if path == Target.Path {
+		return ""
+	}
+	for _, m := range buildList {
+		if m.Path == path {
+			return m.Version
+		}
+	}
+	return ""
+}
+
+// EditBuildList edits the global build list by first adding every module in add
+// to the existing build list, then adjusting versions (and adding or removing
+// requirements as needed) until every module in mustSelect is selected at the
+// given version.
+//
+// (Note that the newly-added modules might not be selected in the resulting
+// build list: they could be lower than existing requirements or conflict with
+// versions in mustSelect.)
+//
+// If the versions listed in mustSelect are mutually incompatible (due to one of
+// the listed modules requiring a higher version of another), EditBuildList
+// returns a *ConstraintError and leaves the build list in its previous state.
+func EditBuildList(ctx context.Context, add, mustSelect []module.Version) error {
+	var upgraded = capVersionSlice(buildList)
+	if len(add) > 0 {
+		// First, upgrade the build list with any additions.
+		// In theory we could just append the additions to the build list and let
+		// mvs.Downgrade take care of resolving the upgrades too, but the
+		// diagnostics from Upgrade are currently much better in case of errors.
+		var err error
+		upgraded, err = mvs.Upgrade(Target, &mvsReqs{buildList: upgraded}, add...)
+		if err != nil {
+			return err
+		}
+	}
+
+	downgraded, err := mvs.Downgrade(Target, &mvsReqs{buildList: append(upgraded, mustSelect...)}, mustSelect...)
+	if err != nil {
+		return err
+	}
+
+	final, err := mvs.Upgrade(Target, &mvsReqs{buildList: downgraded}, mustSelect...)
+	if err != nil {
+		return err
+	}
+
+	selected := make(map[string]module.Version, len(final))
+	for _, m := range final {
+		selected[m.Path] = m
+	}
+	inconsistent := false
+	for _, m := range mustSelect {
+		s, ok := selected[m.Path]
+		if !ok {
+			if m.Version != "none" {
+				panic(fmt.Sprintf("internal error: mvs.BuildList lost %v", m))
+			}
+			continue
+		}
+		if s.Version != m.Version {
+			inconsistent = true
+			break
+		}
+	}
+
+	if !inconsistent {
+		buildList = final
+		additionalExplicitRequirements = make([]string, 0, len(mustSelect))
+		for _, m := range mustSelect {
+			if m.Version != "none" {
+				additionalExplicitRequirements = append(additionalExplicitRequirements, m.Path)
+			}
+		}
+		return nil
+	}
+
+	// We overshot one or more of the modules in mustSelected, which means that
+	// Downgrade removed something in mustSelect because it conflicted with
+	// something else in mustSelect.
+	//
+	// Walk the requirement graph to find the conflict.
+	//
+	// TODO(bcmills): Ideally, mvs.Downgrade (or a replacement for it) would do
+	// this directly.
+
+	reqs := &mvsReqs{buildList: final}
+	reason := map[module.Version]module.Version{}
+	for _, m := range mustSelect {
+		reason[m] = m
+	}
+	queue := mustSelect[:len(mustSelect):len(mustSelect)]
+	for len(queue) > 0 {
+		var m module.Version
+		m, queue = queue[0], queue[1:]
+		required, err := reqs.Required(m)
+		if err != nil {
+			return err
+		}
+		for _, r := range required {
+			if _, ok := reason[r]; !ok {
+				reason[r] = reason[m]
+				queue = append(queue, r)
+			}
+		}
+	}
+
+	var conflicts []Conflict
+	for _, m := range mustSelect {
+		s, ok := selected[m.Path]
+		if !ok {
+			if m.Version != "none" {
+				panic(fmt.Sprintf("internal error: mvs.BuildList lost %v", m))
+			}
+			continue
+		}
+		if s.Version != m.Version {
+			conflicts = append(conflicts, Conflict{
+				Source:     reason[s],
+				Dep:        s,
+				Constraint: m,
+			})
+		}
+	}
+
+	return &ConstraintError{
+		Conflicts: conflicts,
+	}
+}
+
+// A ConstraintError describes inconsistent constraints in EditBuildList
+type ConstraintError struct {
+	// Conflict lists the source of the conflict for each version in mustSelect
+	// that could not be selected due to the requirements of some other version in
+	// mustSelect.
+	Conflicts []Conflict
+}
+
+func (e *ConstraintError) Error() string {
+	b := new(strings.Builder)
+	b.WriteString("version constraints conflict:")
+	for _, c := range e.Conflicts {
+		fmt.Fprintf(b, "\n\t%v requires %v, but %v is requested", c.Source, c.Dep, c.Constraint)
+	}
+	return b.String()
+}
+
+// A Conflict documents that Source requires Dep, which conflicts with Constraint.
+// (That is, Dep has the same module path as Constraint but a higher version.)
+type Conflict struct {
+	Source     module.Version
+	Dep        module.Version
+	Constraint module.Version
+}
+
+// ReloadBuildList resets the state of loaded packages, then loads and returns
+// the build list set by EditBuildList.
+func ReloadBuildList() []module.Version {
+	loaded = loadFromRoots(loaderParams{
+		PackageOpts: PackageOpts{
+			Tags: imports.Tags(),
+		},
+		listRoots:          func() []string { return nil },
+		allClosesOverTests: index.allPatternClosesOverTests(), // but doesn't matter because the root list is empty.
+	})
+	return capVersionSlice(buildList)
+}
+
+// CheckTidyVersion reports an error to stderr if the Go version indicated by
+// the go.mod file is not supported by this version of the 'go' command.
+//
+// If allowError is false, such an error terminates the program.
+func CheckTidyVersion(ctx context.Context, allowError bool) {
+	LoadModFile(ctx)
+	if index.goVersionV == "" {
+		return
+	}
+
+	tags := build.Default.ReleaseTags
+	maxGo := tags[len(tags)-1]
+	if !strings.HasPrefix(maxGo, "go") || !modfile.GoVersionRE.MatchString(maxGo[2:]) {
+		base.Fatalf("go: unrecognized go version %q", maxGo)
+	}
+	max := maxGo[2:]
+
+	if semver.Compare(index.goVersionV, "v"+max) > 0 {
+		have := index.goVersionV[1:]
+		if allowError {
+			fmt.Fprintf(os.Stderr, "go mod tidy: go.mod file indicates go %s, but maximum supported version is %s\n", have, max)
+		} else {
+			base.Fatalf("go mod tidy: go.mod file indicates go %s, but maximum supported version is %s\n", have, max)
+		}
+	}
+}
+
+// TidyBuildList trims the build list to the minimal requirements needed to
+// retain the same versions of all packages from the preceding call to
+// LoadPackages.
+func TidyBuildList() {
+	used := map[module.Version]bool{Target: true}
+	for _, pkg := range loaded.pkgs {
+		used[pkg.mod] = true
+	}
+
+	keep := []module.Version{Target}
+	var direct []string
+	for _, m := range buildList[1:] {
+		if used[m] {
+			keep = append(keep, m)
+			if loaded.direct[m.Path] {
+				direct = append(direct, m.Path)
+			}
+		} else if cfg.BuildV {
+			if _, ok := index.require[m]; ok {
+				fmt.Fprintf(os.Stderr, "unused %s\n", m.Path)
+			}
+		}
+	}
+
+	min, err := mvs.Req(Target, direct, &mvsReqs{buildList: keep})
+	if err != nil {
+		base.Fatalf("go: %v", err)
+	}
+	buildList = append([]module.Version{Target}, min...)
+}
+
+// checkMultiplePaths verifies that a given module path is used as itself
+// or as a replacement for another module, but not both at the same time.
+//
+// (See https://golang.org/issue/26607 and https://golang.org/issue/34650.)
+func checkMultiplePaths() {
+	firstPath := make(map[module.Version]string, len(buildList))
+	for _, mod := range buildList {
+		src := mod
+		if rep := Replacement(mod); rep.Path != "" {
+			src = rep
+		}
+		if prev, ok := firstPath[src]; !ok {
+			firstPath[src] = mod.Path
+		} else if prev != mod.Path {
+			base.Errorf("go: %s@%s used for two different module paths (%s and %s)", src.Path, src.Version, prev, mod.Path)
+		}
+	}
+	base.ExitIfErrors()
+}
diff --git a/src/cmd/go/internal/modload/help.go b/src/cmd/go/internal/modload/help.go
new file mode 100644
index 0000000..fd39ddd
--- /dev/null
+++ b/src/cmd/go/internal/modload/help.go
@@ -0,0 +1,64 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import "cmd/go/internal/base"
+
+var HelpModules = &base.Command{
+	UsageLine: "modules",
+	Short:     "modules, module versions, and more",
+	Long: `
+Modules are how Go manages dependencies.
+
+A module is a collection of packages that are released, versioned, and
+distributed together. Modules may be downloaded directly from version control
+repositories or from module proxy servers.
+
+For a series of tutorials on modules, see
+https://golang.org/doc/tutorial/create-module.
+
+For a detailed reference on modules, see https://golang.org/ref/mod.
+
+By default, the go command may download modules from https://proxy.golang.org.
+It may authenticate modules using the checksum database at
+https://sum.golang.org. Both services are operated by the Go team at Google.
+The privacy policies for these services are available at
+https://proxy.golang.org/privacy and https://sum.golang.org/privacy,
+respectively.
+
+The go command's download behavior may be configured using GOPROXY, GOSUMDB,
+GOPRIVATE, and other environment variables. See 'go help environment'
+and https://golang.org/ref/mod#private-module-privacy for more information.
+	`,
+}
+
+var HelpGoMod = &base.Command{
+	UsageLine: "go.mod",
+	Short:     "the go.mod file",
+	Long: `
+A module version is defined by a tree of source files, with a go.mod
+file in its root. When the go command is run, it looks in the current
+directory and then successive parent directories to find the go.mod
+marking the root of the main (current) module.
+
+The go.mod file format is described in detail at
+https://golang.org/ref/mod#go-mod-file.
+
+To create a new go.mod file, use 'go help init'. For details see
+'go help mod init' or https://golang.org/ref/mod#go-mod-init.
+
+To add missing module requirements or remove unneeded requirements,
+use 'go mod tidy'. For details, see 'go help mod tidy' or
+https://golang.org/ref/mod#go-mod-tidy.
+
+To add, upgrade, downgrade, or remove a specific module requirement, use
+'go get'. For details, see 'go help module-get' or
+https://golang.org/ref/mod#go-get.
+
+To make other changes or to parse go.mod as JSON for use by other tools,
+use 'go mod edit'. See 'go help mod edit' or
+https://golang.org/ref/mod#go-mod-edit.
+	`,
+}
diff --git a/src/cmd/go/internal/modload/import.go b/src/cmd/go/internal/modload/import.go
new file mode 100644
index 0000000..eda0568
--- /dev/null
+++ b/src/cmd/go/internal/modload/import.go
@@ -0,0 +1,600 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"go/build"
+	"internal/goroot"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/par"
+	"cmd/go/internal/search"
+
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
+)
+
+type ImportMissingError struct {
+	Path     string
+	Module   module.Version
+	QueryErr error
+
+	// isStd indicates whether we would expect to find the package in the standard
+	// library. This is normally true for all dotless import paths, but replace
+	// directives can cause us to treat the replaced paths as also being in
+	// modules.
+	isStd bool
+
+	// replaced the highest replaced version of the module where the replacement
+	// contains the package. replaced is only set if the replacement is unused.
+	replaced module.Version
+
+	// newMissingVersion is set to a newer version of Module if one is present
+	// in the build list. When set, we can't automatically upgrade.
+	newMissingVersion string
+}
+
+func (e *ImportMissingError) Error() string {
+	if e.Module.Path == "" {
+		if e.isStd {
+			return fmt.Sprintf("package %s is not in GOROOT (%s)", e.Path, filepath.Join(cfg.GOROOT, "src", e.Path))
+		}
+		if e.QueryErr != nil && e.QueryErr != ErrNoModRoot {
+			return fmt.Sprintf("cannot find module providing package %s: %v", e.Path, e.QueryErr)
+		}
+		if cfg.BuildMod == "mod" || (cfg.BuildMod == "readonly" && allowMissingModuleImports) {
+			return "cannot find module providing package " + e.Path
+		}
+
+		if e.replaced.Path != "" {
+			suggestArg := e.replaced.Path
+			if !modfetch.IsZeroPseudoVersion(e.replaced.Version) {
+				suggestArg = e.replaced.String()
+			}
+			return fmt.Sprintf("module %s provides package %s and is replaced but not required; to add it:\n\tgo get %s", e.replaced.Path, e.Path, suggestArg)
+		}
+
+		message := fmt.Sprintf("no required module provides package %s", e.Path)
+		if e.QueryErr != nil {
+			return fmt.Sprintf("%s: %v", message, e.QueryErr)
+		}
+		return fmt.Sprintf("%s; to add it:\n\tgo get %s", message, e.Path)
+	}
+
+	if e.newMissingVersion != "" {
+		return fmt.Sprintf("package %s provided by %s at latest version %s but not at required version %s", e.Path, e.Module.Path, e.Module.Version, e.newMissingVersion)
+	}
+
+	return fmt.Sprintf("missing module for import: %s@%s provides %s", e.Module.Path, e.Module.Version, e.Path)
+}
+
+func (e *ImportMissingError) Unwrap() error {
+	return e.QueryErr
+}
+
+func (e *ImportMissingError) ImportPath() string {
+	return e.Path
+}
+
+// An AmbiguousImportError indicates an import of a package found in multiple
+// modules in the build list, or found in both the main module and its vendor
+// directory.
+type AmbiguousImportError struct {
+	importPath string
+	Dirs       []string
+	Modules    []module.Version // Either empty or 1:1 with Dirs.
+}
+
+func (e *AmbiguousImportError) ImportPath() string {
+	return e.importPath
+}
+
+func (e *AmbiguousImportError) Error() string {
+	locType := "modules"
+	if len(e.Modules) == 0 {
+		locType = "directories"
+	}
+
+	var buf strings.Builder
+	fmt.Fprintf(&buf, "ambiguous import: found package %s in multiple %s:", e.importPath, locType)
+
+	for i, dir := range e.Dirs {
+		buf.WriteString("\n\t")
+		if i < len(e.Modules) {
+			m := e.Modules[i]
+			buf.WriteString(m.Path)
+			if m.Version != "" {
+				fmt.Fprintf(&buf, " %s", m.Version)
+			}
+			fmt.Fprintf(&buf, " (%s)", dir)
+		} else {
+			buf.WriteString(dir)
+		}
+	}
+
+	return buf.String()
+}
+
+// ImportMissingSumError is reported in readonly mode when we need to check
+// if a module contains a package, but we don't have a sum for its .zip file.
+// We might need sums for multiple modules to verify the package is unique.
+//
+// TODO(#43653): consolidate multiple errors of this type into a single error
+// that suggests a 'go get' command for root packages that transtively import
+// packages from modules with missing sums. load.CheckPackageErrors would be
+// a good place to consolidate errors, but we'll need to attach the import
+// stack here.
+type ImportMissingSumError struct {
+	importPath                string
+	found                     bool
+	mods                      []module.Version
+	importer, importerVersion string // optional, but used for additional context
+	importerIsTest            bool
+}
+
+func (e *ImportMissingSumError) Error() string {
+	var importParen string
+	if e.importer != "" {
+		importParen = fmt.Sprintf(" (imported by %s)", e.importer)
+	}
+	var message string
+	if e.found {
+		message = fmt.Sprintf("missing go.sum entry needed to verify package %s%s is provided by exactly one module", e.importPath, importParen)
+	} else {
+		message = fmt.Sprintf("missing go.sum entry for module providing package %s%s", e.importPath, importParen)
+	}
+	var hint string
+	if e.importer == "" {
+		// Importing package is unknown, or the missing package was named on the
+		// command line. Recommend 'go mod download' for the modules that could
+		// provide the package, since that shouldn't change go.mod.
+		if len(e.mods) > 0 {
+			args := make([]string, len(e.mods))
+			for i, mod := range e.mods {
+				args[i] = mod.Path
+			}
+			hint = fmt.Sprintf("; to add:\n\tgo mod download %s", strings.Join(args, " "))
+		}
+	} else {
+		// Importing package is known (common case). Recommend 'go get' on the
+		// current version of the importing package.
+		tFlag := ""
+		if e.importerIsTest {
+			tFlag = " -t"
+		}
+		version := ""
+		if e.importerVersion != "" {
+			version = "@" + e.importerVersion
+		}
+		hint = fmt.Sprintf("; to add:\n\tgo get%s %s%s", tFlag, e.importer, version)
+	}
+	return message + hint
+}
+
+func (e *ImportMissingSumError) ImportPath() string {
+	return e.importPath
+}
+
+type invalidImportError struct {
+	importPath string
+	err        error
+}
+
+func (e *invalidImportError) ImportPath() string {
+	return e.importPath
+}
+
+func (e *invalidImportError) Error() string {
+	return e.err.Error()
+}
+
+func (e *invalidImportError) Unwrap() error {
+	return e.err
+}
+
+// importFromBuildList finds the module and directory in the build list
+// containing the package with the given import path. The answer must be unique:
+// importFromBuildList returns an error if multiple modules attempt to provide
+// the same package.
+//
+// importFromBuildList can return a module with an empty m.Path, for packages in
+// the standard library.
+//
+// importFromBuildList can return an empty directory string, for fake packages
+// like "C" and "unsafe".
+//
+// If the package cannot be found in buildList,
+// importFromBuildList returns an *ImportMissingError.
+func importFromBuildList(ctx context.Context, path string, buildList []module.Version) (m module.Version, dir string, err error) {
+	if strings.Contains(path, "@") {
+		return module.Version{}, "", fmt.Errorf("import path should not have @version")
+	}
+	if build.IsLocalImport(path) {
+		return module.Version{}, "", fmt.Errorf("relative import not supported")
+	}
+	if path == "C" || path == "unsafe" {
+		// There's no directory for import "C" or import "unsafe".
+		return module.Version{}, "", nil
+	}
+	// Before any further lookup, check that the path is valid.
+	if err := module.CheckImportPath(path); err != nil {
+		return module.Version{}, "", &invalidImportError{importPath: path, err: err}
+	}
+
+	// Is the package in the standard library?
+	pathIsStd := search.IsStandardImportPath(path)
+	if pathIsStd && goroot.IsStandardPackage(cfg.GOROOT, cfg.BuildContext.Compiler, path) {
+		if targetInGorootSrc {
+			if dir, ok, err := dirInModule(path, targetPrefix, ModRoot(), true); err != nil {
+				return module.Version{}, dir, err
+			} else if ok {
+				return Target, dir, nil
+			}
+		}
+		dir := filepath.Join(cfg.GOROOT, "src", path)
+		return module.Version{}, dir, nil
+	}
+
+	// -mod=vendor is special.
+	// Everything must be in the main module or the main module's vendor directory.
+	if cfg.BuildMod == "vendor" {
+		mainDir, mainOK, mainErr := dirInModule(path, targetPrefix, ModRoot(), true)
+		vendorDir, vendorOK, _ := dirInModule(path, "", filepath.Join(ModRoot(), "vendor"), false)
+		if mainOK && vendorOK {
+			return module.Version{}, "", &AmbiguousImportError{importPath: path, Dirs: []string{mainDir, vendorDir}}
+		}
+		// Prefer to return main directory if there is one,
+		// Note that we're not checking that the package exists.
+		// We'll leave that for load.
+		if !vendorOK && mainDir != "" {
+			return Target, mainDir, nil
+		}
+		if mainErr != nil {
+			return module.Version{}, "", mainErr
+		}
+		readVendorList()
+		return vendorPkgModule[path], vendorDir, nil
+	}
+
+	// Check each module on the build list.
+	var dirs []string
+	var mods []module.Version
+	var sumErrMods []module.Version
+	for _, m := range buildList {
+		if !maybeInModule(path, m.Path) {
+			// Avoid possibly downloading irrelevant modules.
+			continue
+		}
+		needSum := true
+		root, isLocal, err := fetch(ctx, m, needSum)
+		if err != nil {
+			if sumErr := (*sumMissingError)(nil); errors.As(err, &sumErr) {
+				// We are missing a sum needed to fetch a module in the build list.
+				// We can't verify that the package is unique, and we may not find
+				// the package at all. Keep checking other modules to decide which
+				// error to report. Multiple sums may be missing if we need to look in
+				// multiple nested modules to resolve the import.
+				sumErrMods = append(sumErrMods, m)
+				continue
+			}
+			// Report fetch error.
+			// Note that we don't know for sure this module is necessary,
+			// but it certainly _could_ provide the package, and even if we
+			// continue the loop and find the package in some other module,
+			// we need to look at this module to make sure the import is
+			// not ambiguous.
+			return module.Version{}, "", err
+		}
+		if dir, ok, err := dirInModule(path, m.Path, root, isLocal); err != nil {
+			return module.Version{}, "", err
+		} else if ok {
+			mods = append(mods, m)
+			dirs = append(dirs, dir)
+		}
+	}
+	if len(mods) > 1 {
+		return module.Version{}, "", &AmbiguousImportError{importPath: path, Dirs: dirs, Modules: mods}
+	}
+	if len(sumErrMods) > 0 {
+		return module.Version{}, "", &ImportMissingSumError{
+			importPath: path,
+			mods:       sumErrMods,
+			found:      len(mods) > 0,
+		}
+	}
+	if len(mods) == 1 {
+		return mods[0], dirs[0], nil
+	}
+
+	var queryErr error
+	if !HasModRoot() {
+		queryErr = ErrNoModRoot
+	}
+	return module.Version{}, "", &ImportMissingError{Path: path, QueryErr: queryErr, isStd: pathIsStd}
+}
+
+// queryImport attempts to locate a module that can be added to the current
+// build list to provide the package with the given import path.
+//
+// Unlike QueryPattern, queryImport prefers to add a replaced version of a
+// module *before* checking the proxies for a version to add.
+func queryImport(ctx context.Context, path string) (module.Version, error) {
+	// To avoid spurious remote fetches, try the latest replacement for each
+	// module (golang.org/issue/26241).
+	if index != nil {
+		var mods []module.Version
+		for mp, mv := range index.highestReplaced {
+			if !maybeInModule(path, mp) {
+				continue
+			}
+			if mv == "" {
+				// The only replacement is a wildcard that doesn't specify a version, so
+				// synthesize a pseudo-version with an appropriate major version and a
+				// timestamp below any real timestamp. That way, if the main module is
+				// used from within some other module, the user will be able to upgrade
+				// the requirement to any real version they choose.
+				if _, pathMajor, ok := module.SplitPathVersion(mp); ok && len(pathMajor) > 0 {
+					mv = modfetch.ZeroPseudoVersion(pathMajor[1:])
+				} else {
+					mv = modfetch.ZeroPseudoVersion("v0")
+				}
+			}
+			mods = append(mods, module.Version{Path: mp, Version: mv})
+		}
+
+		// Every module path in mods is a prefix of the import path.
+		// As in QueryPattern, prefer the longest prefix that satisfies the import.
+		sort.Slice(mods, func(i, j int) bool {
+			return len(mods[i].Path) > len(mods[j].Path)
+		})
+		for _, m := range mods {
+			needSum := true
+			root, isLocal, err := fetch(ctx, m, needSum)
+			if err != nil {
+				if sumErr := (*sumMissingError)(nil); errors.As(err, &sumErr) {
+					return module.Version{}, &ImportMissingSumError{importPath: path}
+				}
+				return module.Version{}, err
+			}
+			if _, ok, err := dirInModule(path, m.Path, root, isLocal); err != nil {
+				return m, err
+			} else if ok {
+				if cfg.BuildMod == "readonly" {
+					return module.Version{}, &ImportMissingError{Path: path, replaced: m}
+				}
+				return m, nil
+			}
+		}
+		if len(mods) > 0 && module.CheckPath(path) != nil {
+			// The package path is not valid to fetch remotely,
+			// so it can only exist in a replaced module,
+			// and we know from the above loop that it is not.
+			return module.Version{}, &PackageNotInModuleError{
+				Mod:         mods[0],
+				Query:       "latest",
+				Pattern:     path,
+				Replacement: Replacement(mods[0]),
+			}
+		}
+	}
+
+	if search.IsStandardImportPath(path) {
+		// This package isn't in the standard library, isn't in any module already
+		// in the build list, and isn't in any other module that the user has
+		// shimmed in via a "replace" directive.
+		// Moreover, the import path is reserved for the standard library, so
+		// QueryPattern cannot possibly find a module containing this package.
+		//
+		// Instead of trying QueryPattern, report an ImportMissingError immediately.
+		return module.Version{}, &ImportMissingError{Path: path, isStd: true}
+	}
+
+	if cfg.BuildMod == "readonly" && !allowMissingModuleImports {
+		// In readonly mode, we can't write go.mod, so we shouldn't try to look up
+		// the module. If readonly mode was enabled explicitly, include that in
+		// the error message.
+		var queryErr error
+		if cfg.BuildModExplicit {
+			queryErr = fmt.Errorf("import lookup disabled by -mod=%s", cfg.BuildMod)
+		} else if cfg.BuildModReason != "" {
+			queryErr = fmt.Errorf("import lookup disabled by -mod=%s\n\t(%s)", cfg.BuildMod, cfg.BuildModReason)
+		}
+		return module.Version{}, &ImportMissingError{Path: path, QueryErr: queryErr}
+	}
+
+	// Look up module containing the package, for addition to the build list.
+	// Goal is to determine the module, download it to dir,
+	// and return m, dir, ImpportMissingError.
+	fmt.Fprintf(os.Stderr, "go: finding module for package %s\n", path)
+
+	candidates, err := QueryPackages(ctx, path, "latest", Selected, CheckAllowed)
+	if err != nil {
+		if errors.Is(err, fs.ErrNotExist) {
+			// Return "cannot find module providing package […]" instead of whatever
+			// low-level error QueryPattern produced.
+			return module.Version{}, &ImportMissingError{Path: path, QueryErr: err}
+		} else {
+			return module.Version{}, err
+		}
+	}
+
+	candidate0MissingVersion := ""
+	for i, c := range candidates {
+		cm := c.Mod
+		canAdd := true
+		for _, bm := range buildList {
+			if bm.Path == cm.Path && semver.Compare(bm.Version, cm.Version) > 0 {
+				// QueryPattern proposed that we add module cm to provide the package,
+				// but we already depend on a newer version of that module (and we don't
+				// have the package).
+				//
+				// This typically happens when a package is present at the "@latest"
+				// version (e.g., v1.0.0) of a module, but we have a newer version
+				// of the same module in the build list (e.g., v1.0.1-beta), and
+				// the package is not present there.
+				canAdd = false
+				if i == 0 {
+					candidate0MissingVersion = bm.Version
+				}
+				break
+			}
+		}
+		if canAdd {
+			return cm, nil
+		}
+	}
+	return module.Version{}, &ImportMissingError{
+		Path:              path,
+		Module:            candidates[0].Mod,
+		newMissingVersion: candidate0MissingVersion,
+	}
+}
+
+// maybeInModule reports whether, syntactically,
+// a package with the given import path could be supplied
+// by a module with the given module path (mpath).
+func maybeInModule(path, mpath string) bool {
+	return mpath == path ||
+		len(path) > len(mpath) && path[len(mpath)] == '/' && path[:len(mpath)] == mpath
+}
+
+var (
+	haveGoModCache   par.Cache // dir → bool
+	haveGoFilesCache par.Cache // dir → goFilesEntry
+)
+
+type goFilesEntry struct {
+	haveGoFiles bool
+	err         error
+}
+
+// dirInModule locates the directory that would hold the package named by the given path,
+// if it were in the module with module path mpath and root mdir.
+// If path is syntactically not within mpath,
+// or if mdir is a local file tree (isLocal == true) and the directory
+// that would hold path is in a sub-module (covered by a go.mod below mdir),
+// dirInModule returns "", false, nil.
+//
+// Otherwise, dirInModule returns the name of the directory where
+// Go source files would be expected, along with a boolean indicating
+// whether there are in fact Go source files in that directory.
+// A non-nil error indicates that the existence of the directory and/or
+// source files could not be determined, for example due to a permission error.
+func dirInModule(path, mpath, mdir string, isLocal bool) (dir string, haveGoFiles bool, err error) {
+	// Determine where to expect the package.
+	if path == mpath {
+		dir = mdir
+	} else if mpath == "" { // vendor directory
+		dir = filepath.Join(mdir, path)
+	} else if len(path) > len(mpath) && path[len(mpath)] == '/' && path[:len(mpath)] == mpath {
+		dir = filepath.Join(mdir, path[len(mpath)+1:])
+	} else {
+		return "", false, nil
+	}
+
+	// Check that there aren't other modules in the way.
+	// This check is unnecessary inside the module cache
+	// and important to skip in the vendor directory,
+	// where all the module trees have been overlaid.
+	// So we only check local module trees
+	// (the main module, and any directory trees pointed at by replace directives).
+	if isLocal {
+		for d := dir; d != mdir && len(d) > len(mdir); {
+			haveGoMod := haveGoModCache.Do(d, func() interface{} {
+				fi, err := fsys.Stat(filepath.Join(d, "go.mod"))
+				return err == nil && !fi.IsDir()
+			}).(bool)
+
+			if haveGoMod {
+				return "", false, nil
+			}
+			parent := filepath.Dir(d)
+			if parent == d {
+				// Break the loop, as otherwise we'd loop
+				// forever if d=="." and mdir=="".
+				break
+			}
+			d = parent
+		}
+	}
+
+	// Now committed to returning dir (not "").
+
+	// Are there Go source files in the directory?
+	// We don't care about build tags, not even "+build ignore".
+	// We're just looking for a plausible directory.
+	res := haveGoFilesCache.Do(dir, func() interface{} {
+		ok, err := fsys.IsDirWithGoFiles(dir)
+		return goFilesEntry{haveGoFiles: ok, err: err}
+	}).(goFilesEntry)
+
+	return dir, res.haveGoFiles, res.err
+}
+
+// fetch downloads the given module (or its replacement)
+// and returns its location.
+//
+// needSum indicates whether the module may be downloaded in readonly mode
+// without a go.sum entry. It should only be false for modules fetched
+// speculatively (for example, for incompatible version filtering). The sum
+// will still be verified normally.
+//
+// The isLocal return value reports whether the replacement,
+// if any, is local to the filesystem.
+func fetch(ctx context.Context, mod module.Version, needSum bool) (dir string, isLocal bool, err error) {
+	if mod == Target {
+		return ModRoot(), true, nil
+	}
+	if r := Replacement(mod); r.Path != "" {
+		if r.Version == "" {
+			dir = r.Path
+			if !filepath.IsAbs(dir) {
+				dir = filepath.Join(ModRoot(), dir)
+			}
+			// Ensure that the replacement directory actually exists:
+			// dirInModule does not report errors for missing modules,
+			// so if we don't report the error now, later failures will be
+			// very mysterious.
+			if _, err := fsys.Stat(dir); err != nil {
+				if os.IsNotExist(err) {
+					// Semantically the module version itself “exists” — we just don't
+					// have its source code. Remove the equivalence to os.ErrNotExist,
+					// and make the message more concise while we're at it.
+					err = fmt.Errorf("replacement directory %s does not exist", r.Path)
+				} else {
+					err = fmt.Errorf("replacement directory %s: %w", r.Path, err)
+				}
+				return dir, true, module.VersionError(mod, err)
+			}
+			return dir, true, nil
+		}
+		mod = r
+	}
+
+	if HasModRoot() && cfg.BuildMod == "readonly" && needSum && !modfetch.HaveSum(mod) {
+		return "", false, module.VersionError(mod, &sumMissingError{})
+	}
+
+	dir, err = modfetch.Download(ctx, mod)
+	return dir, false, err
+}
+
+type sumMissingError struct {
+	suggestion string
+}
+
+func (e *sumMissingError) Error() string {
+	return "missing go.sum entry" + e.suggestion
+}
diff --git a/src/cmd/go/internal/modload/import_test.go b/src/cmd/go/internal/modload/import_test.go
new file mode 100644
index 0000000..9420dc5
--- /dev/null
+++ b/src/cmd/go/internal/modload/import_test.go
@@ -0,0 +1,96 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"context"
+	"internal/testenv"
+	"regexp"
+	"strings"
+	"testing"
+
+	"golang.org/x/mod/module"
+)
+
+var importTests = []struct {
+	path string
+	m    module.Version
+	err  string
+}{
+	{
+		path: "golang.org/x/net/context",
+		m: module.Version{
+			Path: "golang.org/x/net",
+		},
+	},
+	{
+		path: "golang.org/x/net",
+		err:  `module golang.org/x/net@.* found \(v0.0.0-.*\), but does not contain package golang.org/x/net`,
+	},
+	{
+		path: "golang.org/x/text",
+		m: module.Version{
+			Path: "golang.org/x/text",
+		},
+	},
+	{
+		path: "github.com/rsc/quote/buggy",
+		m: module.Version{
+			Path:    "github.com/rsc/quote",
+			Version: "v1.5.2",
+		},
+	},
+	{
+		path: "github.com/rsc/quote",
+		m: module.Version{
+			Path:    "github.com/rsc/quote",
+			Version: "v1.5.2",
+		},
+	},
+	{
+		path: "golang.org/x/foo/bar",
+		err:  "cannot find module providing package golang.org/x/foo/bar",
+	},
+}
+
+func TestQueryImport(t *testing.T) {
+	testenv.MustHaveExternalNetwork(t)
+	testenv.MustHaveExecPath(t, "git")
+
+	oldAllowMissingModuleImports := allowMissingModuleImports
+	oldRootMode := RootMode
+	defer func() {
+		allowMissingModuleImports = oldAllowMissingModuleImports
+		RootMode = oldRootMode
+	}()
+	allowMissingModuleImports = true
+	RootMode = NoRoot
+
+	ctx := context.Background()
+
+	for _, tt := range importTests {
+		t.Run(strings.ReplaceAll(tt.path, "/", "_"), func(t *testing.T) {
+			// Note that there is no build list, so Import should always fail.
+			m, err := queryImport(ctx, tt.path)
+
+			if tt.err == "" {
+				if err != nil {
+					t.Fatalf("queryImport(_, %q): %v", tt.path, err)
+				}
+			} else {
+				if err == nil {
+					t.Fatalf("queryImport(_, %q) = %v, nil; expected error", tt.path, m)
+				}
+				if !regexp.MustCompile(tt.err).MatchString(err.Error()) {
+					t.Fatalf("queryImport(_, %q): error %q, want error matching %#q", tt.path, err, tt.err)
+				}
+			}
+
+			if m.Path != tt.m.Path || (tt.m.Version != "" && m.Version != tt.m.Version) {
+				t.Errorf("queryImport(_, %q) = %v, _; want %v", tt.path, m, tt.m)
+			}
+		})
+	}
+}
diff --git a/src/cmd/go/internal/modload/init.go b/src/cmd/go/internal/modload/init.go
new file mode 100644
index 0000000..8ec1c86
--- /dev/null
+++ b/src/cmd/go/internal/modload/init.go
@@ -0,0 +1,1072 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"go/build"
+	"internal/lazyregexp"
+	"os"
+	"path"
+	"path/filepath"
+	"sort"
+	"strconv"
+	"strings"
+	"sync"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/lockedfile"
+	"cmd/go/internal/modconv"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/mvs"
+	"cmd/go/internal/search"
+	"cmd/go/internal/str"
+
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
+)
+
+var (
+	initialized bool
+
+	modRoot string
+	Target  module.Version
+
+	// targetPrefix is the path prefix for packages in Target, without a trailing
+	// slash. For most modules, targetPrefix is just Target.Path, but the
+	// standard-library module "std" has an empty prefix.
+	targetPrefix string
+
+	// targetInGorootSrc caches whether modRoot is within GOROOT/src.
+	// The "std" module is special within GOROOT/src, but not otherwise.
+	targetInGorootSrc bool
+
+	gopath string
+
+	// RootMode determines whether a module root is needed.
+	RootMode Root
+
+	// ForceUseModules may be set to force modules to be enabled when
+	// GO111MODULE=auto or to report an error when GO111MODULE=off.
+	ForceUseModules bool
+
+	allowMissingModuleImports bool
+)
+
+type Root int
+
+const (
+	// AutoRoot is the default for most commands. modload.Init will look for
+	// a go.mod file in the current directory or any parent. If none is found,
+	// modules may be disabled (GO111MODULE=on) or commands may run in a
+	// limited module mode.
+	AutoRoot Root = iota
+
+	// NoRoot is used for commands that run in module mode and ignore any go.mod
+	// file the current directory or in parent directories.
+	NoRoot
+
+	// NeedRoot is used for commands that must run in module mode and don't
+	// make sense without a main module.
+	NeedRoot
+)
+
+// ModFile returns the parsed go.mod file.
+//
+// Note that after calling LoadPackages or LoadAllModules,
+// the require statements in the modfile.File are no longer
+// the source of truth and will be ignored: edits made directly
+// will be lost at the next call to WriteGoMod.
+// To make permanent changes to the require statements
+// in go.mod, edit it before loading.
+func ModFile() *modfile.File {
+	Init()
+	if modFile == nil {
+		die()
+	}
+	return modFile
+}
+
+func BinDir() string {
+	Init()
+	return filepath.Join(gopath, "bin")
+}
+
+// Init determines whether module mode is enabled, locates the root of the
+// current module (if any), sets environment variables for Git subprocesses, and
+// configures the cfg, codehost, load, modfetch, and search packages for use
+// with modules.
+func Init() {
+	if initialized {
+		return
+	}
+	initialized = true
+
+	// Keep in sync with WillBeEnabled. We perform extra validation here, and
+	// there are lots of diagnostics and side effects, so we can't use
+	// WillBeEnabled directly.
+	var mustUseModules bool
+	env := cfg.Getenv("GO111MODULE")
+	switch env {
+	default:
+		base.Fatalf("go: unknown environment setting GO111MODULE=%s", env)
+	case "auto":
+		mustUseModules = ForceUseModules
+	case "on", "":
+		mustUseModules = true
+	case "off":
+		if ForceUseModules {
+			base.Fatalf("go: modules disabled by GO111MODULE=off; see 'go help modules'")
+		}
+		mustUseModules = false
+		return
+	}
+
+	if err := fsys.Init(base.Cwd); err != nil {
+		base.Fatalf("go: %v", err)
+	}
+
+	// Disable any prompting for passwords by Git.
+	// Only has an effect for 2.3.0 or later, but avoiding
+	// the prompt in earlier versions is just too hard.
+	// If user has explicitly set GIT_TERMINAL_PROMPT=1, keep
+	// prompting.
+	// See golang.org/issue/9341 and golang.org/issue/12706.
+	if os.Getenv("GIT_TERMINAL_PROMPT") == "" {
+		os.Setenv("GIT_TERMINAL_PROMPT", "0")
+	}
+
+	// Disable any ssh connection pooling by Git.
+	// If a Git subprocess forks a child into the background to cache a new connection,
+	// that child keeps stdout/stderr open. After the Git subprocess exits,
+	// os /exec expects to be able to read from the stdout/stderr pipe
+	// until EOF to get all the data that the Git subprocess wrote before exiting.
+	// The EOF doesn't come until the child exits too, because the child
+	// is holding the write end of the pipe.
+	// This is unfortunate, but it has come up at least twice
+	// (see golang.org/issue/13453 and golang.org/issue/16104)
+	// and confuses users when it does.
+	// If the user has explicitly set GIT_SSH or GIT_SSH_COMMAND,
+	// assume they know what they are doing and don't step on it.
+	// But default to turning off ControlMaster.
+	if os.Getenv("GIT_SSH") == "" && os.Getenv("GIT_SSH_COMMAND") == "" {
+		os.Setenv("GIT_SSH_COMMAND", "ssh -o ControlMaster=no")
+	}
+
+	if modRoot != "" {
+		// modRoot set before Init was called ("go mod init" does this).
+		// No need to search for go.mod.
+	} else if RootMode == NoRoot {
+		if cfg.ModFile != "" && !base.InGOFLAGS("-modfile") {
+			base.Fatalf("go: -modfile cannot be used with commands that ignore the current module")
+		}
+		modRoot = ""
+	} else {
+		modRoot = findModuleRoot(base.Cwd)
+		if modRoot == "" {
+			if cfg.ModFile != "" {
+				base.Fatalf("go: cannot find main module, but -modfile was set.\n\t-modfile cannot be used to set the module root directory.")
+			}
+			if RootMode == NeedRoot {
+				base.Fatalf("go: %v", ErrNoModRoot)
+			}
+			if !mustUseModules {
+				// GO111MODULE is 'auto', and we can't find a module root.
+				// Stay in GOPATH mode.
+				return
+			}
+		} else if search.InDir(modRoot, os.TempDir()) == "." {
+			// If you create /tmp/go.mod for experimenting,
+			// then any tests that create work directories under /tmp
+			// will find it and get modules when they're not expecting them.
+			// It's a bit of a peculiar thing to disallow but quite mysterious
+			// when it happens. See golang.org/issue/26708.
+			modRoot = ""
+			fmt.Fprintf(os.Stderr, "go: warning: ignoring go.mod in system temp root %v\n", os.TempDir())
+			if !mustUseModules {
+				return
+			}
+		}
+	}
+	if cfg.ModFile != "" && !strings.HasSuffix(cfg.ModFile, ".mod") {
+		base.Fatalf("go: -modfile=%s: file does not have .mod extension", cfg.ModFile)
+	}
+
+	// We're in module mode. Set any global variables that need to be set.
+	cfg.ModulesEnabled = true
+	setDefaultBuildMod()
+	list := filepath.SplitList(cfg.BuildContext.GOPATH)
+	if len(list) == 0 || list[0] == "" {
+		base.Fatalf("missing $GOPATH")
+	}
+	gopath = list[0]
+	if _, err := fsys.Stat(filepath.Join(gopath, "go.mod")); err == nil {
+		base.Fatalf("$GOPATH/go.mod exists but should not")
+	}
+
+	if modRoot == "" {
+		// We're in module mode, but not inside a module.
+		//
+		// Commands like 'go build', 'go run', 'go list' have no go.mod file to
+		// read or write. They would need to find and download the latest versions
+		// of a potentially large number of modules with no way to save version
+		// information. We can succeed slowly (but not reproducibly), but that's
+		// not usually a good experience.
+		//
+		// Instead, we forbid resolving import paths to modules other than std and
+		// cmd. Users may still build packages specified with .go files on the
+		// command line, but they'll see an error if those files import anything
+		// outside std.
+		//
+		// This can be overridden by calling AllowMissingModuleImports.
+		// For example, 'go get' does this, since it is expected to resolve paths.
+		//
+		// See golang.org/issue/32027.
+	} else {
+		modfetch.GoSumFile = strings.TrimSuffix(ModFilePath(), ".mod") + ".sum"
+		search.SetModRoot(modRoot)
+	}
+}
+
+// WillBeEnabled checks whether modules should be enabled but does not
+// initialize modules by installing hooks. If Init has already been called,
+// WillBeEnabled returns the same result as Enabled.
+//
+// This function is needed to break a cycle. The main package needs to know
+// whether modules are enabled in order to install the module or GOPATH version
+// of 'go get', but Init reads the -modfile flag in 'go get', so it shouldn't
+// be called until the command is installed and flags are parsed. Instead of
+// calling Init and Enabled, the main package can call this function.
+func WillBeEnabled() bool {
+	if modRoot != "" || cfg.ModulesEnabled {
+		// Already enabled.
+		return true
+	}
+	if initialized {
+		// Initialized, not enabled.
+		return false
+	}
+
+	// Keep in sync with Init. Init does extra validation and prints warnings or
+	// exits, so it can't call this function directly.
+	env := cfg.Getenv("GO111MODULE")
+	switch env {
+	case "on", "":
+		return true
+	case "auto":
+		break
+	default:
+		return false
+	}
+
+	if modRoot := findModuleRoot(base.Cwd); modRoot == "" {
+		// GO111MODULE is 'auto', and we can't find a module root.
+		// Stay in GOPATH mode.
+		return false
+	} else if search.InDir(modRoot, os.TempDir()) == "." {
+		// If you create /tmp/go.mod for experimenting,
+		// then any tests that create work directories under /tmp
+		// will find it and get modules when they're not expecting them.
+		// It's a bit of a peculiar thing to disallow but quite mysterious
+		// when it happens. See golang.org/issue/26708.
+		return false
+	}
+	return true
+}
+
+// Enabled reports whether modules are (or must be) enabled.
+// If modules are enabled but there is no main module, Enabled returns true
+// and then the first use of module information will call die
+// (usually through MustModRoot).
+func Enabled() bool {
+	Init()
+	return modRoot != "" || cfg.ModulesEnabled
+}
+
+// ModRoot returns the root of the main module.
+// It calls base.Fatalf if there is no main module.
+func ModRoot() string {
+	if !HasModRoot() {
+		die()
+	}
+	return modRoot
+}
+
+// HasModRoot reports whether a main module is present.
+// HasModRoot may return false even if Enabled returns true: for example, 'get'
+// does not require a main module.
+func HasModRoot() bool {
+	Init()
+	return modRoot != ""
+}
+
+// ModFilePath returns the effective path of the go.mod file. Normally, this
+// "go.mod" in the directory returned by ModRoot, but the -modfile flag may
+// change its location. ModFilePath calls base.Fatalf if there is no main
+// module, even if -modfile is set.
+func ModFilePath() string {
+	if !HasModRoot() {
+		die()
+	}
+	if cfg.ModFile != "" {
+		return cfg.ModFile
+	}
+	return filepath.Join(modRoot, "go.mod")
+}
+
+func die() {
+	if cfg.Getenv("GO111MODULE") == "off" {
+		base.Fatalf("go: modules disabled by GO111MODULE=off; see 'go help modules'")
+	}
+	if dir, name := findAltConfig(base.Cwd); dir != "" {
+		rel, err := filepath.Rel(base.Cwd, dir)
+		if err != nil {
+			rel = dir
+		}
+		cdCmd := ""
+		if rel != "." {
+			cdCmd = fmt.Sprintf("cd %s && ", rel)
+		}
+		base.Fatalf("go: cannot find main module, but found %s in %s\n\tto create a module there, run:\n\t%sgo mod init", name, dir, cdCmd)
+	}
+	base.Fatalf("go: %v", ErrNoModRoot)
+}
+
+var ErrNoModRoot = errors.New("go.mod file not found in current directory or any parent directory; see 'go help modules'")
+
+// LoadModFile sets Target and, if there is a main module, parses the initial
+// build list from its go.mod file.
+//
+// LoadModFile may make changes in memory, like adding a go directive and
+// ensuring requirements are consistent. WriteGoMod should be called later to
+// write changes out to disk or report errors in readonly mode.
+//
+// As a side-effect, LoadModFile may change cfg.BuildMod to "vendor" if
+// -mod wasn't set explicitly and automatic vendoring should be enabled.
+func LoadModFile(ctx context.Context) {
+	if len(buildList) > 0 {
+		return
+	}
+
+	Init()
+	if modRoot == "" {
+		Target = module.Version{Path: "command-line-arguments"}
+		targetPrefix = "command-line-arguments"
+		buildList = []module.Version{Target}
+		return
+	}
+
+	gomod := ModFilePath()
+	data, err := lockedfile.Read(gomod)
+	if err != nil {
+		base.Fatalf("go: %v", err)
+	}
+
+	var fixed bool
+	f, err := modfile.Parse(gomod, data, fixVersion(ctx, &fixed))
+	if err != nil {
+		// Errors returned by modfile.Parse begin with file:line.
+		base.Fatalf("go: errors parsing go.mod:\n%s\n", err)
+	}
+	modFile = f
+	index = indexModFile(data, f, fixed)
+
+	if f.Module == nil {
+		// No module declaration. Must add module path.
+		base.Fatalf("go: no module declaration in go.mod. To specify the module path:\n\tgo mod edit -module=example.com/mod")
+	}
+
+	if err := checkModulePathLax(f.Module.Mod.Path); err != nil {
+		base.Fatalf("go: %v", err)
+	}
+
+	setDefaultBuildMod() // possibly enable automatic vendoring
+	modFileToBuildList()
+	if cfg.BuildMod == "vendor" {
+		readVendorList()
+		checkVendorConsistency()
+	}
+}
+
+// CreateModFile initializes a new module by creating a go.mod file.
+//
+// If modPath is empty, CreateModFile will attempt to infer the path from the
+// directory location within GOPATH.
+//
+// If a vendoring configuration file is present, CreateModFile will attempt to
+// translate it to go.mod directives. The resulting build list may not be
+// exactly the same as in the legacy configuration (for example, we can't get
+// packages at multiple versions from the same module).
+func CreateModFile(ctx context.Context, modPath string) {
+	modRoot = base.Cwd
+	Init()
+	modFilePath := ModFilePath()
+	if _, err := fsys.Stat(modFilePath); err == nil {
+		base.Fatalf("go: %s already exists", modFilePath)
+	}
+
+	if modPath == "" {
+		var err error
+		modPath, err = findModulePath(modRoot)
+		if err != nil {
+			base.Fatalf("go: %v", err)
+		}
+	} else if err := checkModulePathLax(modPath); err != nil {
+		base.Fatalf("go: %v", err)
+	}
+
+	fmt.Fprintf(os.Stderr, "go: creating new go.mod: module %s\n", modPath)
+	modFile = new(modfile.File)
+	modFile.AddModuleStmt(modPath)
+	addGoStmt() // Add the go directive before converted module requirements.
+
+	convertedFrom, err := convertLegacyConfig(modPath)
+	if convertedFrom != "" {
+		fmt.Fprintf(os.Stderr, "go: copying requirements from %s\n", base.ShortPath(convertedFrom))
+	}
+	if err != nil {
+		base.Fatalf("go: %v", err)
+	}
+
+	modFileToBuildList()
+	WriteGoMod()
+
+	// Suggest running 'go mod tidy' unless the project is empty. Even if we
+	// imported all the correct requirements above, we're probably missing
+	// some sums, so the next build command in -mod=readonly will likely fail.
+	//
+	// We look for non-hidden .go files or subdirectories to determine whether
+	// this is an existing project. Walking the tree for packages would be more
+	// accurate, but could take much longer.
+	empty := true
+	files, _ := os.ReadDir(modRoot)
+	for _, f := range files {
+		name := f.Name()
+		if strings.HasPrefix(name, ".") || strings.HasPrefix(name, "_") {
+			continue
+		}
+		if strings.HasSuffix(name, ".go") || f.IsDir() {
+			empty = false
+			break
+		}
+	}
+	if !empty {
+		fmt.Fprintf(os.Stderr, "go: to add module requirements and sums:\n\tgo mod tidy\n")
+	}
+}
+
+// checkModulePathLax checks that the path meets some minimum requirements
+// to avoid confusing users or the module cache. The requirements are weaker
+// than those of module.CheckPath to allow room for weakening module path
+// requirements in the future, but strong enough to help users avoid significant
+// problems.
+func checkModulePathLax(p string) error {
+	// TODO(matloob): Replace calls of this function in this CL with calls
+	// to module.CheckImportPath once it's been laxened, if it becomes laxened.
+	// See golang.org/issue/29101 for a discussion about whether to make CheckImportPath
+	// more lax or more strict.
+
+	errorf := func(format string, args ...interface{}) error {
+		return fmt.Errorf("invalid module path %q: %s", p, fmt.Sprintf(format, args...))
+	}
+
+	// Disallow shell characters " ' * < > ? ` | to avoid triggering bugs
+	// with file systems and subcommands. Disallow file path separators : and \
+	// because path separators other than / will confuse the module cache.
+	// See fileNameOK in golang.org/x/mod/module/module.go.
+	shellChars := "`" + `\"'*<>?|`
+	fsChars := `\:`
+	if i := strings.IndexAny(p, shellChars); i >= 0 {
+		return errorf("contains disallowed shell character %q", p[i])
+	}
+	if i := strings.IndexAny(p, fsChars); i >= 0 {
+		return errorf("contains disallowed path separator character %q", p[i])
+	}
+
+	// Ensure path.IsAbs and build.IsLocalImport are false, and that the path is
+	// invariant under path.Clean, also to avoid confusing the module cache.
+	if path.IsAbs(p) {
+		return errorf("is an absolute path")
+	}
+	if build.IsLocalImport(p) {
+		return errorf("is a local import path")
+	}
+	if path.Clean(p) != p {
+		return errorf("is not clean")
+	}
+
+	return nil
+}
+
+// fixVersion returns a modfile.VersionFixer implemented using the Query function.
+//
+// It resolves commit hashes and branch names to versions,
+// canonicalizes versions that appeared in early vgo drafts,
+// and does nothing for versions that already appear to be canonical.
+//
+// The VersionFixer sets 'fixed' if it ever returns a non-canonical version.
+func fixVersion(ctx context.Context, fixed *bool) modfile.VersionFixer {
+	return func(path, vers string) (resolved string, err error) {
+		defer func() {
+			if err == nil && resolved != vers {
+				*fixed = true
+			}
+		}()
+
+		// Special case: remove the old -gopkgin- hack.
+		if strings.HasPrefix(path, "gopkg.in/") && strings.Contains(vers, "-gopkgin-") {
+			vers = vers[strings.Index(vers, "-gopkgin-")+len("-gopkgin-"):]
+		}
+
+		// fixVersion is called speculatively on every
+		// module, version pair from every go.mod file.
+		// Avoid the query if it looks OK.
+		_, pathMajor, ok := module.SplitPathVersion(path)
+		if !ok {
+			return "", &module.ModuleError{
+				Path: path,
+				Err: &module.InvalidVersionError{
+					Version: vers,
+					Err:     fmt.Errorf("malformed module path %q", path),
+				},
+			}
+		}
+		if vers != "" && module.CanonicalVersion(vers) == vers {
+			if err := module.CheckPathMajor(vers, pathMajor); err != nil {
+				return "", module.VersionError(module.Version{Path: path, Version: vers}, err)
+			}
+			return vers, nil
+		}
+
+		info, err := Query(ctx, path, vers, "", nil)
+		if err != nil {
+			return "", err
+		}
+		return info.Version, nil
+	}
+}
+
+// AllowMissingModuleImports allows import paths to be resolved to modules
+// when there is no module root. Normally, this is forbidden because it's slow
+// and there's no way to make the result reproducible, but some commands
+// like 'go get' are expected to do this.
+func AllowMissingModuleImports() {
+	allowMissingModuleImports = true
+}
+
+// modFileToBuildList initializes buildList from the modFile.
+func modFileToBuildList() {
+	Target = modFile.Module.Mod
+	targetPrefix = Target.Path
+	if rel := search.InDir(base.Cwd, cfg.GOROOTsrc); rel != "" {
+		targetInGorootSrc = true
+		if Target.Path == "std" {
+			targetPrefix = ""
+		}
+	}
+
+	list := []module.Version{Target}
+	for _, r := range modFile.Require {
+		if index != nil && index.exclude[r.Mod] {
+			if cfg.BuildMod == "mod" {
+				fmt.Fprintf(os.Stderr, "go: dropping requirement on excluded version %s %s\n", r.Mod.Path, r.Mod.Version)
+			} else {
+				fmt.Fprintf(os.Stderr, "go: ignoring requirement on excluded version %s %s\n", r.Mod.Path, r.Mod.Version)
+			}
+		} else {
+			list = append(list, r.Mod)
+		}
+	}
+	buildList = list
+}
+
+// setDefaultBuildMod sets a default value for cfg.BuildMod if the -mod flag
+// wasn't provided. setDefaultBuildMod may be called multiple times.
+func setDefaultBuildMod() {
+	if cfg.BuildModExplicit {
+		// Don't override an explicit '-mod=' argument.
+		return
+	}
+
+	if cfg.CmdName == "get" || strings.HasPrefix(cfg.CmdName, "mod ") {
+		// 'get' and 'go mod' commands may update go.mod automatically.
+		// TODO(jayconrod): should this narrower? Should 'go mod download' or
+		// 'go mod graph' update go.mod by default?
+		cfg.BuildMod = "mod"
+		return
+	}
+	if modRoot == "" {
+		cfg.BuildMod = "readonly"
+		return
+	}
+
+	if fi, err := fsys.Stat(filepath.Join(modRoot, "vendor")); err == nil && fi.IsDir() {
+		modGo := "unspecified"
+		if index != nil && index.goVersionV != "" {
+			if semver.Compare(index.goVersionV, "v1.14") >= 0 {
+				// The Go version is at least 1.14, and a vendor directory exists.
+				// Set -mod=vendor by default.
+				cfg.BuildMod = "vendor"
+				cfg.BuildModReason = "Go version in go.mod is at least 1.14 and vendor directory exists."
+				return
+			} else {
+				modGo = index.goVersionV[1:]
+			}
+		}
+
+		// Since a vendor directory exists, we should record why we didn't use it.
+		// This message won't normally be shown, but it may appear with import errors.
+		cfg.BuildModReason = fmt.Sprintf("Go version in go.mod is %s, so vendor directory was not used.", modGo)
+	}
+
+	cfg.BuildMod = "readonly"
+}
+
+// convertLegacyConfig imports module requirements from a legacy vendoring
+// configuration file, if one is present.
+func convertLegacyConfig(modPath string) (from string, err error) {
+	for _, name := range altConfigs {
+		cfg := filepath.Join(modRoot, name)
+		data, err := os.ReadFile(cfg)
+		if err == nil {
+			convert := modconv.Converters[name]
+			if convert == nil {
+				return "", nil
+			}
+			cfg = filepath.ToSlash(cfg)
+			err := modconv.ConvertLegacyConfig(modFile, cfg, data)
+			return name, err
+		}
+	}
+	return "", nil
+}
+
+// addGoStmt adds a go directive to the go.mod file if it does not already include one.
+// The 'go' version added, if any, is the latest version supported by this toolchain.
+func addGoStmt() {
+	if modFile.Go != nil && modFile.Go.Version != "" {
+		return
+	}
+	tags := build.Default.ReleaseTags
+	version := tags[len(tags)-1]
+	if !strings.HasPrefix(version, "go") || !modfile.GoVersionRE.MatchString(version[2:]) {
+		base.Fatalf("go: unrecognized default version %q", version)
+	}
+	if err := modFile.AddGoStmt(version[2:]); err != nil {
+		base.Fatalf("go: internal error: %v", err)
+	}
+}
+
+var altConfigs = []string{
+	"Gopkg.lock",
+
+	"GLOCKFILE",
+	"Godeps/Godeps.json",
+	"dependencies.tsv",
+	"glide.lock",
+	"vendor.conf",
+	"vendor.yml",
+	"vendor/manifest",
+	"vendor/vendor.json",
+
+	".git/config",
+}
+
+func findModuleRoot(dir string) (root string) {
+	if dir == "" {
+		panic("dir not set")
+	}
+	dir = filepath.Clean(dir)
+
+	// Look for enclosing go.mod.
+	for {
+		if fi, err := fsys.Stat(filepath.Join(dir, "go.mod")); err == nil && !fi.IsDir() {
+			return dir
+		}
+		d := filepath.Dir(dir)
+		if d == dir {
+			break
+		}
+		dir = d
+	}
+	return ""
+}
+
+func findAltConfig(dir string) (root, name string) {
+	if dir == "" {
+		panic("dir not set")
+	}
+	dir = filepath.Clean(dir)
+	if rel := search.InDir(dir, cfg.BuildContext.GOROOT); rel != "" {
+		// Don't suggest creating a module from $GOROOT/.git/config
+		// or a config file found in any parent of $GOROOT (see #34191).
+		return "", ""
+	}
+	for {
+		for _, name := range altConfigs {
+			if fi, err := fsys.Stat(filepath.Join(dir, name)); err == nil && !fi.IsDir() {
+				return dir, name
+			}
+		}
+		d := filepath.Dir(dir)
+		if d == dir {
+			break
+		}
+		dir = d
+	}
+	return "", ""
+}
+
+func findModulePath(dir string) (string, error) {
+	// TODO(bcmills): once we have located a plausible module path, we should
+	// query version control (if available) to verify that it matches the major
+	// version of the most recent tag.
+	// See https://golang.org/issue/29433, https://golang.org/issue/27009, and
+	// https://golang.org/issue/31549.
+
+	// Cast about for import comments,
+	// first in top-level directory, then in subdirectories.
+	list, _ := os.ReadDir(dir)
+	for _, info := range list {
+		if info.Type().IsRegular() && strings.HasSuffix(info.Name(), ".go") {
+			if com := findImportComment(filepath.Join(dir, info.Name())); com != "" {
+				return com, nil
+			}
+		}
+	}
+	for _, info1 := range list {
+		if info1.IsDir() {
+			files, _ := os.ReadDir(filepath.Join(dir, info1.Name()))
+			for _, info2 := range files {
+				if info2.Type().IsRegular() && strings.HasSuffix(info2.Name(), ".go") {
+					if com := findImportComment(filepath.Join(dir, info1.Name(), info2.Name())); com != "" {
+						return path.Dir(com), nil
+					}
+				}
+			}
+		}
+	}
+
+	// Look for Godeps.json declaring import path.
+	data, _ := os.ReadFile(filepath.Join(dir, "Godeps/Godeps.json"))
+	var cfg1 struct{ ImportPath string }
+	json.Unmarshal(data, &cfg1)
+	if cfg1.ImportPath != "" {
+		return cfg1.ImportPath, nil
+	}
+
+	// Look for vendor.json declaring import path.
+	data, _ = os.ReadFile(filepath.Join(dir, "vendor/vendor.json"))
+	var cfg2 struct{ RootPath string }
+	json.Unmarshal(data, &cfg2)
+	if cfg2.RootPath != "" {
+		return cfg2.RootPath, nil
+	}
+
+	// Look for path in GOPATH.
+	var badPathErr error
+	for _, gpdir := range filepath.SplitList(cfg.BuildContext.GOPATH) {
+		if gpdir == "" {
+			continue
+		}
+		if rel := search.InDir(dir, filepath.Join(gpdir, "src")); rel != "" && rel != "." {
+			path := filepath.ToSlash(rel)
+			// TODO(matloob): replace this with module.CheckImportPath
+			// once it's been laxened.
+			// Only checkModulePathLax here. There are some unpublishable
+			// module names that are compatible with checkModulePathLax
+			// but they already work in GOPATH so don't break users
+			// trying to do a build with modules. gorelease will alert users
+			// publishing their modules to fix their paths.
+			if err := checkModulePathLax(path); err != nil {
+				badPathErr = err
+				break
+			}
+			return path, nil
+		}
+	}
+
+	reason := "outside GOPATH, module path must be specified"
+	if badPathErr != nil {
+		// return a different error message if the module was in GOPATH, but
+		// the module path determined above would be an invalid path.
+		reason = fmt.Sprintf("bad module path inferred from directory in GOPATH: %v", badPathErr)
+	}
+	msg := `cannot determine module path for source directory %s (%s)
+
+Example usage:
+	'go mod init example.com/m' to initialize a v0 or v1 module
+	'go mod init example.com/m/v2' to initialize a v2 module
+
+Run 'go help mod init' for more information.
+`
+	return "", fmt.Errorf(msg, dir, reason)
+}
+
+var (
+	importCommentRE = lazyregexp.New(`(?m)^package[ \t]+[^ \t\r\n/]+[ \t]+//[ \t]+import[ \t]+(\"[^"]+\")[ \t]*\r?\n`)
+)
+
+func findImportComment(file string) string {
+	data, err := os.ReadFile(file)
+	if err != nil {
+		return ""
+	}
+	m := importCommentRE.FindSubmatch(data)
+	if m == nil {
+		return ""
+	}
+	path, err := strconv.Unquote(string(m[1]))
+	if err != nil {
+		return ""
+	}
+	return path
+}
+
+var allowWriteGoMod = true
+
+// DisallowWriteGoMod causes future calls to WriteGoMod to do nothing at all.
+func DisallowWriteGoMod() {
+	allowWriteGoMod = false
+}
+
+// AllowWriteGoMod undoes the effect of DisallowWriteGoMod:
+// future calls to WriteGoMod will update go.mod if needed.
+// Note that any past calls have been discarded, so typically
+// a call to AlowWriteGoMod should be followed by a call to WriteGoMod.
+func AllowWriteGoMod() {
+	allowWriteGoMod = true
+}
+
+// MinReqs returns a Reqs with minimal additional dependencies of Target,
+// as will be written to go.mod.
+func MinReqs() mvs.Reqs {
+	retain := append([]string{}, additionalExplicitRequirements...)
+	for _, m := range buildList[1:] {
+		_, explicit := index.require[m]
+		if explicit || loaded.direct[m.Path] {
+			retain = append(retain, m.Path)
+		}
+	}
+	sort.Strings(retain)
+	str.Uniq(&retain)
+	min, err := mvs.Req(Target, retain, &mvsReqs{buildList: buildList})
+	if err != nil {
+		base.Fatalf("go: %v", err)
+	}
+	return &mvsReqs{buildList: append([]module.Version{Target}, min...)}
+}
+
+// WriteGoMod writes the current build list back to go.mod.
+func WriteGoMod() {
+	// If we're using -mod=vendor we basically ignored
+	// go.mod, so definitely don't try to write back our
+	// incomplete view of the world.
+	if !allowWriteGoMod || cfg.BuildMod == "vendor" {
+		return
+	}
+
+	// If we aren't in a module, we don't have anywhere to write a go.mod file.
+	if modRoot == "" {
+		return
+	}
+
+	if cfg.BuildMod != "readonly" {
+		addGoStmt()
+	}
+
+	if loaded != nil {
+		reqs := MinReqs()
+		min, err := reqs.Required(Target)
+		if err != nil {
+			base.Fatalf("go: %v", err)
+		}
+		var list []*modfile.Require
+		for _, m := range min {
+			list = append(list, &modfile.Require{
+				Mod:      m,
+				Indirect: !loaded.direct[m.Path],
+			})
+		}
+		modFile.SetRequire(list)
+	}
+	modFile.Cleanup()
+
+	dirty := index.modFileIsDirty(modFile)
+	if dirty && cfg.BuildMod == "readonly" {
+		// If we're about to fail due to -mod=readonly,
+		// prefer to report a dirty go.mod over a dirty go.sum
+		if cfg.BuildModExplicit {
+			base.Fatalf("go: updates to go.mod needed, disabled by -mod=readonly")
+		} else if cfg.BuildModReason != "" {
+			base.Fatalf("go: updates to go.mod needed, disabled by -mod=readonly\n\t(%s)", cfg.BuildModReason)
+		} else {
+			base.Fatalf("go: updates to go.mod needed; to update it:\n\tgo mod tidy")
+		}
+	}
+
+	if !dirty && cfg.CmdName != "mod tidy" {
+		// The go.mod file has the same semantic content that it had before
+		// (but not necessarily the same exact bytes).
+		// Don't write go.mod, but write go.sum in case we added or trimmed sums.
+		// 'go mod init' shouldn't write go.sum, since it will be incomplete.
+		if cfg.CmdName != "mod init" {
+			modfetch.WriteGoSum(keepSums(true))
+		}
+		return
+	}
+
+	new, err := modFile.Format()
+	if err != nil {
+		base.Fatalf("go: %v", err)
+	}
+	defer func() {
+		// At this point we have determined to make the go.mod file on disk equal to new.
+		index = indexModFile(new, modFile, false)
+
+		// Update go.sum after releasing the side lock and refreshing the index.
+		// 'go mod init' shouldn't write go.sum, since it will be incomplete.
+		if cfg.CmdName != "mod init" {
+			modfetch.WriteGoSum(keepSums(true))
+		}
+	}()
+
+	// Make a best-effort attempt to acquire the side lock, only to exclude
+	// previous versions of the 'go' command from making simultaneous edits.
+	if unlock, err := modfetch.SideLock(); err == nil {
+		defer unlock()
+	}
+
+	errNoChange := errors.New("no update needed")
+
+	err = lockedfile.Transform(ModFilePath(), func(old []byte) ([]byte, error) {
+		if bytes.Equal(old, new) {
+			// The go.mod file is already equal to new, possibly as the result of some
+			// other process.
+			return nil, errNoChange
+		}
+
+		if index != nil && !bytes.Equal(old, index.data) {
+			// The contents of the go.mod file have changed. In theory we could add all
+			// of the new modules to the build list, recompute, and check whether any
+			// module in *our* build list got bumped to a different version, but that's
+			// a lot of work for marginal benefit. Instead, fail the command: if users
+			// want to run concurrent commands, they need to start with a complete,
+			// consistent module definition.
+			return nil, fmt.Errorf("existing contents have changed since last read")
+		}
+
+		return new, nil
+	})
+
+	if err != nil && err != errNoChange {
+		base.Fatalf("go: updating go.mod: %v", err)
+	}
+}
+
+// keepSums returns a set of module sums to preserve in go.sum. The set
+// includes entries for all modules used to load packages (according to
+// the last load function such as LoadPackages or ImportFromFiles).
+// It also contains entries for go.mod files needed for MVS (the version
+// of these entries ends with "/go.mod").
+//
+// If keepBuildListZips is true, the set also includes sums for zip files for
+// all modules in the build list with replacements applied. 'go get' and
+// 'go mod download' may add sums to this set when adding a requirement on a
+// module without a root package or when downloading a direct or indirect
+// dependency.
+func keepSums(keepBuildListZips bool) map[module.Version]bool {
+	// Re-derive the build list using the current list of direct requirements.
+	// Keep the sum for the go.mod of each visited module version (or its
+	// replacement).
+	modkey := func(m module.Version) module.Version {
+		return module.Version{Path: m.Path, Version: m.Version + "/go.mod"}
+	}
+	keep := make(map[module.Version]bool)
+	var mu sync.Mutex
+	reqs := &keepSumReqs{
+		Reqs: &mvsReqs{buildList: buildList},
+		visit: func(m module.Version) {
+			// If we build using a replacement module, keep the sum for the replacement,
+			// since that's the code we'll actually use during a build.
+			mu.Lock()
+			r := Replacement(m)
+			if r.Path == "" {
+				keep[modkey(m)] = true
+			} else {
+				keep[modkey(r)] = true
+			}
+			mu.Unlock()
+		},
+	}
+	buildList, err := mvs.BuildList(Target, reqs)
+	if err != nil {
+		panic(fmt.Sprintf("unexpected error reloading build list: %v", err))
+	}
+
+	actualMods := make(map[string]module.Version)
+	for _, m := range buildList[1:] {
+		if r := Replacement(m); r.Path != "" {
+			actualMods[m.Path] = r
+		} else {
+			actualMods[m.Path] = m
+		}
+	}
+
+	// Add entries for modules in the build list with paths that are prefixes of
+	// paths of loaded packages. We need to retain sums for modules needed to
+	// report ambiguous import errors. We use our re-derived build list,
+	// since the global build list may have been tidied.
+	if loaded != nil {
+		for _, pkg := range loaded.pkgs {
+			if pkg.testOf != nil || pkg.inStd || module.CheckImportPath(pkg.path) != nil {
+				continue
+			}
+			for prefix := pkg.path; prefix != "."; prefix = path.Dir(prefix) {
+				if m, ok := actualMods[prefix]; ok {
+					keep[m] = true
+				}
+			}
+		}
+	}
+
+	// Add entries for the zip of each module in the build list.
+	// We might not need all of these (tidy does not add them), but they may be
+	// added by a specific 'go get' or 'go mod download' command to resolve
+	// missing import sum errors.
+	if keepBuildListZips {
+		for _, m := range actualMods {
+			keep[m] = true
+		}
+	}
+
+	return keep
+}
+
+// keepSumReqs embeds another Reqs implementation. The Required method
+// calls visit for each version in the module graph.
+type keepSumReqs struct {
+	mvs.Reqs
+	visit func(module.Version)
+}
+
+func (r *keepSumReqs) Required(m module.Version) ([]module.Version, error) {
+	r.visit(m)
+	return r.Reqs.Required(m)
+}
+
+func TrimGoSum() {
+	// Don't retain sums for the zip file of every module in the build list.
+	// We may not need them all to build the main module's packages.
+	keepBuildListZips := false
+	modfetch.TrimGoSum(keepSums(keepBuildListZips))
+}
diff --git a/src/cmd/go/internal/modload/list.go b/src/cmd/go/internal/modload/list.go
new file mode 100644
index 0000000..7b1aa7f
--- /dev/null
+++ b/src/cmd/go/internal/modload/list.go
@@ -0,0 +1,191 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"runtime"
+	"strings"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/modinfo"
+	"cmd/go/internal/search"
+
+	"golang.org/x/mod/module"
+)
+
+func ListModules(ctx context.Context, args []string, listU, listVersions, listRetracted bool) []*modinfo.ModulePublic {
+	mods := listModules(ctx, args, listVersions, listRetracted)
+
+	type token struct{}
+	sem := make(chan token, runtime.GOMAXPROCS(0))
+	if listU || listVersions || listRetracted {
+		for _, m := range mods {
+			add := func(m *modinfo.ModulePublic) {
+				sem <- token{}
+				go func() {
+					if listU {
+						addUpdate(ctx, m)
+					}
+					if listVersions {
+						addVersions(ctx, m, listRetracted)
+					}
+					if listRetracted || listU {
+						addRetraction(ctx, m)
+					}
+					<-sem
+				}()
+			}
+
+			add(m)
+			if m.Replace != nil {
+				add(m.Replace)
+			}
+		}
+	}
+	// Fill semaphore channel to wait for all tasks to finish.
+	for n := cap(sem); n > 0; n-- {
+		sem <- token{}
+	}
+
+	return mods
+}
+
+func listModules(ctx context.Context, args []string, listVersions, listRetracted bool) []*modinfo.ModulePublic {
+	LoadAllModules(ctx)
+	if len(args) == 0 {
+		return []*modinfo.ModulePublic{moduleInfo(ctx, buildList[0], true, listRetracted)}
+	}
+
+	var mods []*modinfo.ModulePublic
+	matchedBuildList := make([]bool, len(buildList))
+	for _, arg := range args {
+		if strings.Contains(arg, `\`) {
+			base.Fatalf("go: module paths never use backslash")
+		}
+		if search.IsRelativePath(arg) {
+			base.Fatalf("go: cannot use relative path %s to specify module", arg)
+		}
+		if !HasModRoot() && (arg == "all" || strings.Contains(arg, "...")) {
+			base.Fatalf("go: cannot match %q: %v", arg, ErrNoModRoot)
+		}
+		if i := strings.Index(arg, "@"); i >= 0 {
+			path := arg[:i]
+			vers := arg[i+1:]
+			var current string
+			for _, m := range buildList {
+				if m.Path == path {
+					current = m.Version
+					break
+				}
+			}
+
+			allowed := CheckAllowed
+			if IsRevisionQuery(vers) || listRetracted {
+				// Allow excluded and retracted versions if the user asked for a
+				// specific revision or used 'go list -retracted'.
+				allowed = nil
+			}
+			info, err := Query(ctx, path, vers, current, allowed)
+			if err != nil {
+				mods = append(mods, &modinfo.ModulePublic{
+					Path:    path,
+					Version: vers,
+					Error:   modinfoError(path, vers, err),
+				})
+				continue
+			}
+			mod := moduleInfo(ctx, module.Version{Path: path, Version: info.Version}, false, listRetracted)
+			mods = append(mods, mod)
+			continue
+		}
+
+		// Module path or pattern.
+		var match func(string) bool
+		var literal bool
+		if arg == "all" {
+			match = func(string) bool { return true }
+		} else if strings.Contains(arg, "...") {
+			match = search.MatchPattern(arg)
+		} else {
+			match = func(p string) bool { return arg == p }
+			literal = true
+		}
+		matched := false
+		for i, m := range buildList {
+			if i == 0 && !HasModRoot() {
+				// The root module doesn't actually exist: omit it.
+				continue
+			}
+			if match(m.Path) {
+				matched = true
+				if !matchedBuildList[i] {
+					matchedBuildList[i] = true
+					mods = append(mods, moduleInfo(ctx, m, true, listRetracted))
+				}
+			}
+		}
+		if !matched {
+			if literal {
+				if listVersions {
+					// Don't make the user provide an explicit '@latest' when they're
+					// explicitly asking what the available versions are.
+					// Instead, resolve the module, even if it isn't an existing dependency.
+					info, err := Query(ctx, arg, "latest", "", nil)
+					if err == nil {
+						mod := moduleInfo(ctx, module.Version{Path: arg, Version: info.Version}, false, listRetracted)
+						mods = append(mods, mod)
+					} else {
+						mods = append(mods, &modinfo.ModulePublic{
+							Path:  arg,
+							Error: modinfoError(arg, "", err),
+						})
+					}
+					continue
+				}
+				if cfg.BuildMod == "vendor" {
+					// In vendor mode, we can't determine whether a missing module is “a
+					// known dependency” because the module graph is incomplete.
+					// Give a more explicit error message.
+					mods = append(mods, &modinfo.ModulePublic{
+						Path:  arg,
+						Error: modinfoError(arg, "", errors.New("can't resolve module using the vendor directory\n\t(Use -mod=mod or -mod=readonly to bypass.)")),
+					})
+				} else {
+					mods = append(mods, &modinfo.ModulePublic{
+						Path:  arg,
+						Error: modinfoError(arg, "", errors.New("not a known dependency")),
+					})
+				}
+			} else {
+				fmt.Fprintf(os.Stderr, "warning: pattern %q matched no module dependencies\n", arg)
+			}
+		}
+	}
+
+	return mods
+}
+
+// modinfoError wraps an error to create an error message in
+// modinfo.ModuleError with minimal redundancy.
+func modinfoError(path, vers string, err error) *modinfo.ModuleError {
+	var nerr *NoMatchingVersionError
+	var merr *module.ModuleError
+	if errors.As(err, &nerr) {
+		// NoMatchingVersionError contains the query, so we don't mention the
+		// query again in ModuleError.
+		err = &module.ModuleError{Path: path, Err: err}
+	} else if !errors.As(err, &merr) {
+		// If the error does not contain path and version, wrap it in a
+		// module.ModuleError.
+		err = &module.ModuleError{Path: path, Version: vers, Err: err}
+	}
+
+	return &modinfo.ModuleError{Err: err.Error()}
+}
diff --git a/src/cmd/go/internal/modload/load.go b/src/cmd/go/internal/modload/load.go
new file mode 100644
index 0000000..154fc3c
--- /dev/null
+++ b/src/cmd/go/internal/modload/load.go
@@ -0,0 +1,1364 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+// This file contains the module-mode package loader, as well as some accessory
+// functions pertaining to the package import graph.
+//
+// There are two exported entry points into package loading — LoadPackages and
+// ImportFromFiles — both implemented in terms of loadFromRoots, which itself
+// manipulates an instance of the loader struct.
+//
+// Although most of the loading state is maintained in the loader struct,
+// one key piece - the build list - is a global, so that it can be modified
+// separate from the loading operation, such as during "go get"
+// upgrades/downgrades or in "go mod" operations.
+// TODO(#40775): It might be nice to make the loader take and return
+// a buildList rather than hard-coding use of the global.
+//
+// Loading is an iterative process. On each iteration, we try to load the
+// requested packages and their transitive imports, then try to resolve modules
+// for any imported packages that are still missing.
+//
+// The first step of each iteration identifies a set of “root” packages.
+// Normally the root packages are exactly those matching the named pattern
+// arguments. However, for the "all" meta-pattern, the final set of packages is
+// computed from the package import graph, and therefore cannot be an initial
+// input to loading that graph. Instead, the root packages for the "all" pattern
+// are those contained in the main module, and allPatternIsRoot parameter to the
+// loader instructs it to dynamically expand those roots to the full "all"
+// pattern as loading progresses.
+//
+// The pkgInAll flag on each loadPkg instance tracks whether that
+// package is known to match the "all" meta-pattern.
+// A package matches the "all" pattern if:
+// 	- it is in the main module, or
+// 	- it is imported by any test in the main module, or
+// 	- it is imported by another package in "all", or
+// 	- the main module specifies a go version ≤ 1.15, and the package is imported
+// 	  by a *test of* another package in "all".
+//
+// When we implement lazy loading, we will record the modules providing packages
+// in "all" even when we are only loading individual packages, so we set the
+// pkgInAll flag regardless of the whether the "all" pattern is a root.
+// (This is necessary to maintain the “import invariant” described in
+// https://golang.org/design/36460-lazy-module-loading.)
+//
+// Because "go mod vendor" prunes out the tests of vendored packages, the
+// behavior of the "all" pattern with -mod=vendor in Go 1.11–1.15 is the same
+// as the "all" pattern (regardless of the -mod flag) in 1.16+.
+// The allClosesOverTests parameter to the loader indicates whether the "all"
+// pattern should close over tests (as in Go 1.11–1.15) or stop at only those
+// packages transitively imported by the packages and tests in the main module
+// ("all" in Go 1.16+ and "go mod vendor" in Go 1.11+).
+//
+// Note that it is possible for a loaded package NOT to be in "all" even when we
+// are loading the "all" pattern. For example, packages that are transitive
+// dependencies of other roots named on the command line must be loaded, but are
+// not in "all". (The mod_notall test illustrates this behavior.)
+// Similarly, if the LoadTests flag is set but the "all" pattern does not close
+// over test dependencies, then when we load the test of a package that is in
+// "all" but outside the main module, the dependencies of that test will not
+// necessarily themselves be in "all". (That configuration does not arise in Go
+// 1.11–1.15, but it will be possible in Go 1.16+.)
+//
+// Loading proceeds from the roots, using a parallel work-queue with a limit on
+// the amount of active work (to avoid saturating disks, CPU cores, and/or
+// network connections). Each package is added to the queue the first time it is
+// imported by another package. When we have finished identifying the imports of
+// a package, we add the test for that package if it is needed. A test may be
+// needed if:
+// 	- the package matches a root pattern and tests of the roots were requested, or
+// 	- the package is in the main module and the "all" pattern is requested
+// 	  (because the "all" pattern includes the dependencies of tests in the main
+// 	  module), or
+// 	- the package is in "all" and the definition of "all" we are using includes
+// 	  dependencies of tests (as is the case in Go ≤1.15).
+//
+// After all available packages have been loaded, we examine the results to
+// identify any requested or imported packages that are still missing, and if
+// so, which modules we could add to the module graph in order to make the
+// missing packages available. We add those to the module graph and iterate,
+// until either all packages resolve successfully or we cannot identify any
+// module that would resolve any remaining missing package.
+//
+// If the main module is “tidy” (that is, if "go mod tidy" is a no-op for it)
+// and all requested packages are in "all", then loading completes in a single
+// iteration.
+// TODO(bcmills): We should also be able to load in a single iteration if the
+// requested packages all come from modules that are themselves tidy, regardless
+// of whether those packages are in "all". Today, that requires two iterations
+// if those packages are not found in existing dependencies of the main module.
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"go/build"
+	"io/fs"
+	"os"
+	"path"
+	pathpkg "path"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"sort"
+	"strings"
+	"sync"
+	"sync/atomic"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/imports"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/mvs"
+	"cmd/go/internal/par"
+	"cmd/go/internal/search"
+	"cmd/go/internal/str"
+
+	"golang.org/x/mod/module"
+)
+
+// loaded is the most recently-used package loader.
+// It holds details about individual packages.
+var loaded *loader
+
+// PackageOpts control the behavior of the LoadPackages function.
+type PackageOpts struct {
+	// Tags are the build tags in effect (as interpreted by the
+	// cmd/go/internal/imports package).
+	// If nil, treated as equivalent to imports.Tags().
+	Tags map[string]bool
+
+	// ResolveMissingImports indicates that we should attempt to add module
+	// dependencies as needed to resolve imports of packages that are not found.
+	//
+	// For commands that support the -mod flag, resolving imports may still fail
+	// if the flag is set to "readonly" (the default) or "vendor".
+	ResolveMissingImports bool
+
+	// AllowPackage, if non-nil, is called after identifying the module providing
+	// each package. If AllowPackage returns a non-nil error, that error is set
+	// for the package, and the imports and test of that package will not be
+	// loaded.
+	//
+	// AllowPackage may be invoked concurrently by multiple goroutines,
+	// and may be invoked multiple times for a given package path.
+	AllowPackage func(ctx context.Context, path string, mod module.Version) error
+
+	// LoadTests loads the test dependencies of each package matching a requested
+	// pattern. If ResolveMissingImports is also true, test dependencies will be
+	// resolved if missing.
+	LoadTests bool
+
+	// UseVendorAll causes the "all" package pattern to be interpreted as if
+	// running "go mod vendor" (or building with "-mod=vendor").
+	//
+	// This is a no-op for modules that declare 'go 1.16' or higher, for which this
+	// is the default (and only) interpretation of the "all" pattern in module mode.
+	UseVendorAll bool
+
+	// AllowErrors indicates that LoadPackages should not terminate the process if
+	// an error occurs.
+	AllowErrors bool
+
+	// SilenceErrors indicates that LoadPackages should not print errors
+	// that occur while loading packages. SilenceErrors implies AllowErrors.
+	SilenceErrors bool
+
+	// SilenceMissingStdImports indicates that LoadPackages should not print
+	// errors or terminate the process if an imported package is missing, and the
+	// import path looks like it might be in the standard library (perhaps in a
+	// future version).
+	SilenceMissingStdImports bool
+
+	// SilenceUnmatchedWarnings suppresses the warnings normally emitted for
+	// patterns that did not match any packages.
+	SilenceUnmatchedWarnings bool
+}
+
+// LoadPackages identifies the set of packages matching the given patterns and
+// loads the packages in the import graph rooted at that set.
+func LoadPackages(ctx context.Context, opts PackageOpts, patterns ...string) (matches []*search.Match, loadedPackages []string) {
+	LoadModFile(ctx)
+	if opts.Tags == nil {
+		opts.Tags = imports.Tags()
+	}
+
+	patterns = search.CleanPatterns(patterns)
+	matches = make([]*search.Match, 0, len(patterns))
+	allPatternIsRoot := false
+	for _, pattern := range patterns {
+		matches = append(matches, search.NewMatch(pattern))
+		if pattern == "all" {
+			allPatternIsRoot = true
+		}
+	}
+
+	updateMatches := func(ld *loader) {
+		for _, m := range matches {
+			switch {
+			case m.IsLocal():
+				// Evaluate list of file system directories on first iteration.
+				if m.Dirs == nil {
+					matchLocalDirs(m)
+				}
+
+				// Make a copy of the directory list and translate to import paths.
+				// Note that whether a directory corresponds to an import path
+				// changes as the build list is updated, and a directory can change
+				// from not being in the build list to being in it and back as
+				// the exact version of a particular module increases during
+				// the loader iterations.
+				m.Pkgs = m.Pkgs[:0]
+				for _, dir := range m.Dirs {
+					pkg, err := resolveLocalPackage(dir)
+					if err != nil {
+						if !m.IsLiteral() && (err == errPkgIsBuiltin || err == errPkgIsGorootSrc) {
+							continue // Don't include "builtin" or GOROOT/src in wildcard patterns.
+						}
+
+						// If we're outside of a module, ensure that the failure mode
+						// indicates that.
+						ModRoot()
+
+						if ld != nil {
+							m.AddError(err)
+						}
+						continue
+					}
+					m.Pkgs = append(m.Pkgs, pkg)
+				}
+
+			case m.IsLiteral():
+				m.Pkgs = []string{m.Pattern()}
+
+			case strings.Contains(m.Pattern(), "..."):
+				m.Errs = m.Errs[:0]
+				matchPackages(ctx, m, opts.Tags, includeStd, buildList)
+
+			case m.Pattern() == "all":
+				if ld == nil {
+					// The initial roots are the packages in the main module.
+					// loadFromRoots will expand that to "all".
+					m.Errs = m.Errs[:0]
+					matchPackages(ctx, m, opts.Tags, omitStd, []module.Version{Target})
+				} else {
+					// Starting with the packages in the main module,
+					// enumerate the full list of "all".
+					m.Pkgs = ld.computePatternAll()
+				}
+
+			case m.Pattern() == "std" || m.Pattern() == "cmd":
+				if m.Pkgs == nil {
+					m.MatchPackages() // Locate the packages within GOROOT/src.
+				}
+
+			default:
+				panic(fmt.Sprintf("internal error: modload missing case for pattern %s", m.Pattern()))
+			}
+		}
+	}
+
+	loaded = loadFromRoots(loaderParams{
+		PackageOpts: opts,
+
+		allClosesOverTests: index.allPatternClosesOverTests() && !opts.UseVendorAll,
+		allPatternIsRoot:   allPatternIsRoot,
+
+		listRoots: func() (roots []string) {
+			updateMatches(nil)
+			for _, m := range matches {
+				roots = append(roots, m.Pkgs...)
+			}
+			return roots
+		},
+	})
+
+	// One last pass to finalize wildcards.
+	updateMatches(loaded)
+
+	// Report errors, if any.
+	checkMultiplePaths()
+	for _, pkg := range loaded.pkgs {
+		if pkg.err != nil {
+			if sumErr := (*ImportMissingSumError)(nil); errors.As(pkg.err, &sumErr) {
+				if importer := pkg.stack; importer != nil {
+					sumErr.importer = importer.path
+					sumErr.importerVersion = importer.mod.Version
+					sumErr.importerIsTest = importer.testOf != nil
+				}
+			}
+			silence := opts.SilenceErrors
+			if stdErr := (*ImportMissingError)(nil); errors.As(pkg.err, &stdErr) &&
+				stdErr.isStd && opts.SilenceMissingStdImports {
+				silence = true
+			}
+
+			if !silence {
+				if opts.AllowErrors {
+					fmt.Fprintf(os.Stderr, "%s: %v\n", pkg.stackText(), pkg.err)
+				} else {
+					base.Errorf("%s: %v", pkg.stackText(), pkg.err)
+				}
+			}
+		}
+		if !pkg.isTest() {
+			loadedPackages = append(loadedPackages, pkg.path)
+		}
+	}
+	if !opts.SilenceErrors {
+		// Also list errors in matching patterns (such as directory permission
+		// errors for wildcard patterns).
+		for _, match := range matches {
+			for _, err := range match.Errs {
+				if opts.AllowErrors {
+					fmt.Fprintf(os.Stderr, "%v\n", err)
+				} else {
+					base.Errorf("%v", err)
+				}
+			}
+		}
+	}
+	base.ExitIfErrors()
+
+	if !opts.SilenceUnmatchedWarnings {
+		search.WarnUnmatched(matches)
+	}
+
+	// Success! Update go.mod (if needed) and return the results.
+	WriteGoMod()
+	sort.Strings(loadedPackages)
+	return matches, loadedPackages
+}
+
+// matchLocalDirs is like m.MatchDirs, but tries to avoid scanning directories
+// outside of the standard library and active modules.
+func matchLocalDirs(m *search.Match) {
+	if !m.IsLocal() {
+		panic(fmt.Sprintf("internal error: resolveLocalDirs on non-local pattern %s", m.Pattern()))
+	}
+
+	if i := strings.Index(m.Pattern(), "..."); i >= 0 {
+		// The pattern is local, but it is a wildcard. Its packages will
+		// only resolve to paths if they are inside of the standard
+		// library, the main module, or some dependency of the main
+		// module. Verify that before we walk the filesystem: a filesystem
+		// walk in a directory like /var or /etc can be very expensive!
+		dir := filepath.Dir(filepath.Clean(m.Pattern()[:i+3]))
+		absDir := dir
+		if !filepath.IsAbs(dir) {
+			absDir = filepath.Join(base.Cwd, dir)
+		}
+		if search.InDir(absDir, cfg.GOROOTsrc) == "" && search.InDir(absDir, ModRoot()) == "" && pathInModuleCache(absDir) == "" {
+			m.Dirs = []string{}
+			m.AddError(fmt.Errorf("directory prefix %s outside available modules", base.ShortPath(absDir)))
+			return
+		}
+	}
+
+	m.MatchDirs()
+}
+
+// resolveLocalPackage resolves a filesystem path to a package path.
+func resolveLocalPackage(dir string) (string, error) {
+	var absDir string
+	if filepath.IsAbs(dir) {
+		absDir = filepath.Clean(dir)
+	} else {
+		absDir = filepath.Join(base.Cwd, dir)
+	}
+
+	bp, err := cfg.BuildContext.ImportDir(absDir, 0)
+	if err != nil && (bp == nil || len(bp.IgnoredGoFiles) == 0) {
+		// golang.org/issue/32917: We should resolve a relative path to a
+		// package path only if the relative path actually contains the code
+		// for that package.
+		//
+		// If the named directory does not exist or contains no Go files,
+		// the package does not exist.
+		// Other errors may affect package loading, but not resolution.
+		if _, err := fsys.Stat(absDir); err != nil {
+			if os.IsNotExist(err) {
+				// Canonicalize OS-specific errors to errDirectoryNotFound so that error
+				// messages will be easier for users to search for.
+				return "", &fs.PathError{Op: "stat", Path: absDir, Err: errDirectoryNotFound}
+			}
+			return "", err
+		}
+		if _, noGo := err.(*build.NoGoError); noGo {
+			// A directory that does not contain any Go source files — even ignored
+			// ones! — is not a Go package, and we can't resolve it to a package
+			// path because that path could plausibly be provided by some other
+			// module.
+			//
+			// Any other error indicates that the package “exists” (at least in the
+			// sense that it cannot exist in any other module), but has some other
+			// problem (such as a syntax error).
+			return "", err
+		}
+	}
+
+	if modRoot != "" && absDir == modRoot {
+		if absDir == cfg.GOROOTsrc {
+			return "", errPkgIsGorootSrc
+		}
+		return targetPrefix, nil
+	}
+
+	// Note: The checks for @ here are just to avoid misinterpreting
+	// the module cache directories (formerly GOPATH/src/mod/foo@v1.5.2/bar).
+	// It's not strictly necessary but helpful to keep the checks.
+	if modRoot != "" && strings.HasPrefix(absDir, modRoot+string(filepath.Separator)) && !strings.Contains(absDir[len(modRoot):], "@") {
+		suffix := filepath.ToSlash(absDir[len(modRoot):])
+		if strings.HasPrefix(suffix, "/vendor/") {
+			if cfg.BuildMod != "vendor" {
+				return "", fmt.Errorf("without -mod=vendor, directory %s has no package path", absDir)
+			}
+
+			readVendorList()
+			pkg := strings.TrimPrefix(suffix, "/vendor/")
+			if _, ok := vendorPkgModule[pkg]; !ok {
+				return "", fmt.Errorf("directory %s is not a package listed in vendor/modules.txt", absDir)
+			}
+			return pkg, nil
+		}
+
+		if targetPrefix == "" {
+			pkg := strings.TrimPrefix(suffix, "/")
+			if pkg == "builtin" {
+				// "builtin" is a pseudo-package with a real source file.
+				// It's not included in "std", so it shouldn't resolve from "."
+				// within module "std" either.
+				return "", errPkgIsBuiltin
+			}
+			return pkg, nil
+		}
+
+		pkg := targetPrefix + suffix
+		if _, ok, err := dirInModule(pkg, targetPrefix, modRoot, true); err != nil {
+			return "", err
+		} else if !ok {
+			return "", &PackageNotInModuleError{Mod: Target, Pattern: pkg}
+		}
+		return pkg, nil
+	}
+
+	if sub := search.InDir(absDir, cfg.GOROOTsrc); sub != "" && sub != "." && !strings.Contains(sub, "@") {
+		pkg := filepath.ToSlash(sub)
+		if pkg == "builtin" {
+			return "", errPkgIsBuiltin
+		}
+		return pkg, nil
+	}
+
+	pkg := pathInModuleCache(absDir)
+	if pkg == "" {
+		return "", fmt.Errorf("directory %s outside available modules", base.ShortPath(absDir))
+	}
+	return pkg, nil
+}
+
+var (
+	errDirectoryNotFound = errors.New("directory not found")
+	errPkgIsGorootSrc    = errors.New("GOROOT/src is not an importable package")
+	errPkgIsBuiltin      = errors.New(`"builtin" is a pseudo-package, not an importable package`)
+)
+
+// pathInModuleCache returns the import path of the directory dir,
+// if dir is in the module cache copy of a module in our build list.
+func pathInModuleCache(dir string) string {
+	tryMod := func(m module.Version) (string, bool) {
+		var root string
+		var err error
+		if repl := Replacement(m); repl.Path != "" && repl.Version == "" {
+			root = repl.Path
+			if !filepath.IsAbs(root) {
+				root = filepath.Join(ModRoot(), root)
+			}
+		} else if repl.Path != "" {
+			root, err = modfetch.DownloadDir(repl)
+		} else {
+			root, err = modfetch.DownloadDir(m)
+		}
+		if err != nil {
+			return "", false
+		}
+
+		sub := search.InDir(dir, root)
+		if sub == "" {
+			return "", false
+		}
+		sub = filepath.ToSlash(sub)
+		if strings.Contains(sub, "/vendor/") || strings.HasPrefix(sub, "vendor/") || strings.Contains(sub, "@") {
+			return "", false
+		}
+
+		return path.Join(m.Path, filepath.ToSlash(sub)), true
+	}
+
+	for _, m := range buildList[1:] {
+		if importPath, ok := tryMod(m); ok {
+			// checkMultiplePaths ensures that a module can be used for at most one
+			// requirement, so this must be it.
+			return importPath
+		}
+	}
+	return ""
+}
+
+// ImportFromFiles adds modules to the build list as needed
+// to satisfy the imports in the named Go source files.
+func ImportFromFiles(ctx context.Context, gofiles []string) {
+	LoadModFile(ctx)
+
+	tags := imports.Tags()
+	imports, testImports, err := imports.ScanFiles(gofiles, tags)
+	if err != nil {
+		base.Fatalf("go: %v", err)
+	}
+
+	loaded = loadFromRoots(loaderParams{
+		PackageOpts: PackageOpts{
+			Tags:                  tags,
+			ResolveMissingImports: true,
+		},
+		allClosesOverTests: index.allPatternClosesOverTests(),
+		listRoots: func() (roots []string) {
+			roots = append(roots, imports...)
+			roots = append(roots, testImports...)
+			return roots
+		},
+	})
+	WriteGoMod()
+}
+
+// DirImportPath returns the effective import path for dir,
+// provided it is within the main module, or else returns ".".
+func DirImportPath(dir string) string {
+	if !HasModRoot() {
+		return "."
+	}
+	LoadModFile(context.TODO())
+
+	if !filepath.IsAbs(dir) {
+		dir = filepath.Join(base.Cwd, dir)
+	} else {
+		dir = filepath.Clean(dir)
+	}
+
+	if dir == modRoot {
+		return targetPrefix
+	}
+	if strings.HasPrefix(dir, modRoot+string(filepath.Separator)) {
+		suffix := filepath.ToSlash(dir[len(modRoot):])
+		if strings.HasPrefix(suffix, "/vendor/") {
+			return strings.TrimPrefix(suffix, "/vendor/")
+		}
+		return targetPrefix + suffix
+	}
+	return "."
+}
+
+// TargetPackages returns the list of packages in the target (top-level) module
+// matching pattern, which may be relative to the working directory, under all
+// build tag settings.
+func TargetPackages(ctx context.Context, pattern string) *search.Match {
+	// TargetPackages is relative to the main module, so ensure that the main
+	// module is a thing that can contain packages.
+	LoadModFile(ctx)
+	ModRoot()
+
+	m := search.NewMatch(pattern)
+	matchPackages(ctx, m, imports.AnyTags(), omitStd, []module.Version{Target})
+	return m
+}
+
+// ImportMap returns the actual package import path
+// for an import path found in source code.
+// If the given import path does not appear in the source code
+// for the packages that have been loaded, ImportMap returns the empty string.
+func ImportMap(path string) string {
+	pkg, ok := loaded.pkgCache.Get(path).(*loadPkg)
+	if !ok {
+		return ""
+	}
+	return pkg.path
+}
+
+// PackageDir returns the directory containing the source code
+// for the package named by the import path.
+func PackageDir(path string) string {
+	pkg, ok := loaded.pkgCache.Get(path).(*loadPkg)
+	if !ok {
+		return ""
+	}
+	return pkg.dir
+}
+
+// PackageModule returns the module providing the package named by the import path.
+func PackageModule(path string) module.Version {
+	pkg, ok := loaded.pkgCache.Get(path).(*loadPkg)
+	if !ok {
+		return module.Version{}
+	}
+	return pkg.mod
+}
+
+// PackageImports returns the imports for the package named by the import path.
+// Test imports will be returned as well if tests were loaded for the package
+// (i.e., if "all" was loaded or if LoadTests was set and the path was matched
+// by a command line argument). PackageImports will return nil for
+// unknown package paths.
+func PackageImports(path string) (imports, testImports []string) {
+	pkg, ok := loaded.pkgCache.Get(path).(*loadPkg)
+	if !ok {
+		return nil, nil
+	}
+	imports = make([]string, len(pkg.imports))
+	for i, p := range pkg.imports {
+		imports[i] = p.path
+	}
+	if pkg.test != nil {
+		testImports = make([]string, len(pkg.test.imports))
+		for i, p := range pkg.test.imports {
+			testImports[i] = p.path
+		}
+	}
+	return imports, testImports
+}
+
+// Lookup returns the source directory, import path, and any loading error for
+// the package at path as imported from the package in parentDir.
+// Lookup requires that one of the Load functions in this package has already
+// been called.
+func Lookup(parentPath string, parentIsStd bool, path string) (dir, realPath string, err error) {
+	if path == "" {
+		panic("Lookup called with empty package path")
+	}
+
+	if parentIsStd {
+		path = loaded.stdVendor(parentPath, path)
+	}
+	pkg, ok := loaded.pkgCache.Get(path).(*loadPkg)
+	if !ok {
+		// The loader should have found all the relevant paths.
+		// There are a few exceptions, though:
+		//	- during go list without -test, the p.Resolve calls to process p.TestImports and p.XTestImports
+		//	  end up here to canonicalize the import paths.
+		//	- during any load, non-loaded packages like "unsafe" end up here.
+		//	- during any load, build-injected dependencies like "runtime/cgo" end up here.
+		//	- because we ignore appengine/* in the module loader,
+		//	  the dependencies of any actual appengine/* library end up here.
+		dir := findStandardImportPath(path)
+		if dir != "" {
+			return dir, path, nil
+		}
+		return "", "", errMissing
+	}
+	return pkg.dir, pkg.path, pkg.err
+}
+
+// A loader manages the process of loading information about
+// the required packages for a particular build,
+// checking that the packages are available in the module set,
+// and updating the module set if needed.
+type loader struct {
+	loaderParams
+
+	work *par.Queue
+
+	// reset on each iteration
+	roots    []*loadPkg
+	pkgCache *par.Cache // package path (string) → *loadPkg
+	pkgs     []*loadPkg // transitive closure of loaded packages and tests; populated in buildStacks
+
+	// computed at end of iterations
+	direct map[string]bool // imported directly by main module
+}
+
+// loaderParams configure the packages loaded by, and the properties reported
+// by, a loader instance.
+type loaderParams struct {
+	PackageOpts
+
+	allClosesOverTests bool // Does the "all" pattern include the transitive closure of tests of packages in "all"?
+	allPatternIsRoot   bool // Is the "all" pattern an additional root?
+
+	listRoots func() []string
+}
+
+func (ld *loader) reset() {
+	select {
+	case <-ld.work.Idle():
+	default:
+		panic("loader.reset when not idle")
+	}
+
+	ld.roots = nil
+	ld.pkgCache = new(par.Cache)
+	ld.pkgs = nil
+}
+
+// A loadPkg records information about a single loaded package.
+type loadPkg struct {
+	// Populated at construction time:
+	path   string // import path
+	testOf *loadPkg
+
+	// Populated at construction time and updated by (*loader).applyPkgFlags:
+	flags atomicLoadPkgFlags
+
+	// Populated by (*loader).load:
+	mod         module.Version // module providing package
+	dir         string         // directory containing source code
+	err         error          // error loading package
+	imports     []*loadPkg     // packages imported by this one
+	testImports []string       // test-only imports, saved for use by pkg.test.
+	inStd       bool
+
+	// Populated by (*loader).pkgTest:
+	testOnce sync.Once
+	test     *loadPkg
+
+	// Populated by postprocessing in (*loader).buildStacks:
+	stack *loadPkg // package importing this one in minimal import stack for this pkg
+}
+
+// loadPkgFlags is a set of flags tracking metadata about a package.
+type loadPkgFlags int8
+
+const (
+	// pkgInAll indicates that the package is in the "all" package pattern,
+	// regardless of whether we are loading the "all" package pattern.
+	//
+	// When the pkgInAll flag and pkgImportsLoaded flags are both set, the caller
+	// who set the last of those flags must propagate the pkgInAll marking to all
+	// of the imports of the marked package.
+	//
+	// A test is marked with pkgInAll if that test would promote the packages it
+	// imports to be in "all" (such as when the test is itself within the main
+	// module, or when ld.allClosesOverTests is true).
+	pkgInAll loadPkgFlags = 1 << iota
+
+	// pkgIsRoot indicates that the package matches one of the root package
+	// patterns requested by the caller.
+	//
+	// If LoadTests is set, then when pkgIsRoot and pkgImportsLoaded are both set,
+	// the caller who set the last of those flags must populate a test for the
+	// package (in the pkg.test field).
+	//
+	// If the "all" pattern is included as a root, then non-test packages in "all"
+	// are also roots (and must be marked pkgIsRoot).
+	pkgIsRoot
+
+	// pkgImportsLoaded indicates that the imports and testImports fields of a
+	// loadPkg have been populated.
+	pkgImportsLoaded
+)
+
+// has reports whether all of the flags in cond are set in f.
+func (f loadPkgFlags) has(cond loadPkgFlags) bool {
+	return f&cond == cond
+}
+
+// An atomicLoadPkgFlags stores a loadPkgFlags for which individual flags can be
+// added atomically.
+type atomicLoadPkgFlags struct {
+	bits int32
+}
+
+// update sets the given flags in af (in addition to any flags already set).
+//
+// update returns the previous flag state so that the caller may determine which
+// flags were newly-set.
+func (af *atomicLoadPkgFlags) update(flags loadPkgFlags) (old loadPkgFlags) {
+	for {
+		old := atomic.LoadInt32(&af.bits)
+		new := old | int32(flags)
+		if new == old || atomic.CompareAndSwapInt32(&af.bits, old, new) {
+			return loadPkgFlags(old)
+		}
+	}
+}
+
+// has reports whether all of the flags in cond are set in af.
+func (af *atomicLoadPkgFlags) has(cond loadPkgFlags) bool {
+	return loadPkgFlags(atomic.LoadInt32(&af.bits))&cond == cond
+}
+
+// isTest reports whether pkg is a test of another package.
+func (pkg *loadPkg) isTest() bool {
+	return pkg.testOf != nil
+}
+
+var errMissing = errors.New("cannot find package")
+
+// loadFromRoots attempts to load the build graph needed to process a set of
+// root packages and their dependencies.
+//
+// The set of root packages is returned by the params.listRoots function, and
+// expanded to the full set of packages by tracing imports (and possibly tests)
+// as needed.
+func loadFromRoots(params loaderParams) *loader {
+	ld := &loader{
+		loaderParams: params,
+		work:         par.NewQueue(runtime.GOMAXPROCS(0)),
+	}
+
+	var err error
+	reqs := &mvsReqs{buildList: buildList}
+	buildList, err = mvs.BuildList(Target, reqs)
+	if err != nil {
+		base.Fatalf("go: %v", err)
+	}
+
+	addedModuleFor := make(map[string]bool)
+	for {
+		ld.reset()
+
+		// Load the root packages and their imports.
+		// Note: the returned roots can change on each iteration,
+		// since the expansion of package patterns depends on the
+		// build list we're using.
+		inRoots := map[*loadPkg]bool{}
+		for _, path := range ld.listRoots() {
+			root := ld.pkg(path, pkgIsRoot)
+			if !inRoots[root] {
+				ld.roots = append(ld.roots, root)
+				inRoots[root] = true
+			}
+		}
+
+		// ld.pkg adds imported packages to the work queue and calls applyPkgFlags,
+		// which adds tests (and test dependencies) as needed.
+		//
+		// When all of the work in the queue has completed, we'll know that the
+		// transitive closure of dependencies has been loaded.
+		<-ld.work.Idle()
+
+		ld.buildStacks()
+
+		if !ld.ResolveMissingImports || (!HasModRoot() && !allowMissingModuleImports) {
+			// We've loaded as much as we can without resolving missing imports.
+			break
+		}
+		modAddedBy := ld.resolveMissingImports(addedModuleFor)
+		if len(modAddedBy) == 0 {
+			break
+		}
+
+		// Recompute buildList with all our additions.
+		reqs = &mvsReqs{buildList: buildList}
+		buildList, err = mvs.BuildList(Target, reqs)
+		if err != nil {
+			// If an error was found in a newly added module, report the package
+			// import stack instead of the module requirement stack. Packages
+			// are more descriptive.
+			if err, ok := err.(*mvs.BuildListError); ok {
+				if pkg := modAddedBy[err.Module()]; pkg != nil {
+					base.Fatalf("go: %s: %v", pkg.stackText(), err.Err)
+				}
+			}
+			base.Fatalf("go: %v", err)
+		}
+	}
+	base.ExitIfErrors()
+
+	// Compute directly referenced dependency modules.
+	ld.direct = make(map[string]bool)
+	for _, pkg := range ld.pkgs {
+		if pkg.mod == Target {
+			for _, dep := range pkg.imports {
+				if dep.mod.Path != "" && dep.mod.Path != Target.Path && index != nil {
+					_, explicit := index.require[dep.mod]
+					if allowWriteGoMod && cfg.BuildMod == "readonly" && !explicit {
+						// TODO(#40775): attach error to package instead of using
+						// base.Errorf. Ideally, 'go list' should not fail because of this,
+						// but today, LoadPackages calls WriteGoMod unconditionally, which
+						// would fail with a less clear message.
+						base.Errorf("go: %[1]s: package %[2]s imported from implicitly required module; to add missing requirements, run:\n\tgo get %[2]s@%[3]s", pkg.path, dep.path, dep.mod.Version)
+					}
+					ld.direct[dep.mod.Path] = true
+				}
+			}
+		}
+	}
+	base.ExitIfErrors()
+
+	// If we didn't scan all of the imports from the main module, or didn't use
+	// imports.AnyTags, then we didn't necessarily load every package that
+	// contributes “direct” imports — so we can't safely mark existing
+	// dependencies as indirect-only.
+	// Conservatively mark those dependencies as direct.
+	if modFile != nil && (!ld.allPatternIsRoot || !reflect.DeepEqual(ld.Tags, imports.AnyTags())) {
+		for _, r := range modFile.Require {
+			if !r.Indirect {
+				ld.direct[r.Mod.Path] = true
+			}
+		}
+	}
+
+	return ld
+}
+
+// resolveMissingImports adds module dependencies to the global build list
+// in order to resolve missing packages from pkgs.
+//
+// The newly-resolved packages are added to the addedModuleFor map, and
+// resolveMissingImports returns a map from each newly-added module version to
+// the first package for which that module was added.
+func (ld *loader) resolveMissingImports(addedModuleFor map[string]bool) (modAddedBy map[module.Version]*loadPkg) {
+	var needPkgs []*loadPkg
+	for _, pkg := range ld.pkgs {
+		if pkg.err == nil {
+			continue
+		}
+		if pkg.isTest() {
+			// If we are missing a test, we are also missing its non-test version, and
+			// we should only add the missing import once.
+			continue
+		}
+		if !errors.As(pkg.err, new(*ImportMissingError)) {
+			// Leave other errors for Import or load.Packages to report.
+			continue
+		}
+
+		needPkgs = append(needPkgs, pkg)
+
+		pkg := pkg
+		ld.work.Add(func() {
+			pkg.mod, pkg.err = queryImport(context.TODO(), pkg.path)
+		})
+	}
+	<-ld.work.Idle()
+
+	modAddedBy = map[module.Version]*loadPkg{}
+	for _, pkg := range needPkgs {
+		if pkg.err != nil {
+			continue
+		}
+
+		fmt.Fprintf(os.Stderr, "go: found %s in %s %s\n", pkg.path, pkg.mod.Path, pkg.mod.Version)
+		if addedModuleFor[pkg.path] {
+			// TODO(bcmills): This should only be an error if pkg.mod is the same
+			// version we already tried to add previously.
+			base.Fatalf("go: %s: looping trying to add package", pkg.stackText())
+		}
+		if modAddedBy[pkg.mod] == nil {
+			modAddedBy[pkg.mod] = pkg
+			buildList = append(buildList, pkg.mod)
+		}
+	}
+
+	return modAddedBy
+}
+
+// pkg locates the *loadPkg for path, creating and queuing it for loading if
+// needed, and updates its state to reflect the given flags.
+//
+// The imports of the returned *loadPkg will be loaded asynchronously in the
+// ld.work queue, and its test (if requested) will also be populated once
+// imports have been resolved. When ld.work goes idle, all transitive imports of
+// the requested package (and its test, if requested) will have been loaded.
+func (ld *loader) pkg(path string, flags loadPkgFlags) *loadPkg {
+	if flags.has(pkgImportsLoaded) {
+		panic("internal error: (*loader).pkg called with pkgImportsLoaded flag set")
+	}
+
+	pkg := ld.pkgCache.Do(path, func() interface{} {
+		pkg := &loadPkg{
+			path: path,
+		}
+		ld.applyPkgFlags(pkg, flags)
+
+		ld.work.Add(func() { ld.load(pkg) })
+		return pkg
+	}).(*loadPkg)
+
+	ld.applyPkgFlags(pkg, flags)
+	return pkg
+}
+
+// applyPkgFlags updates pkg.flags to set the given flags and propagate the
+// (transitive) effects of those flags, possibly loading or enqueueing further
+// packages as a result.
+func (ld *loader) applyPkgFlags(pkg *loadPkg, flags loadPkgFlags) {
+	if flags == 0 {
+		return
+	}
+
+	if flags.has(pkgInAll) && ld.allPatternIsRoot && !pkg.isTest() {
+		// This package matches a root pattern by virtue of being in "all".
+		flags |= pkgIsRoot
+	}
+
+	old := pkg.flags.update(flags)
+	new := old | flags
+	if new == old || !new.has(pkgImportsLoaded) {
+		// We either didn't change the state of pkg, or we don't know anything about
+		// its dependencies yet. Either way, we can't usefully load its test or
+		// update its dependencies.
+		return
+	}
+
+	if !pkg.isTest() {
+		// Check whether we should add (or update the flags for) a test for pkg.
+		// ld.pkgTest is idempotent and extra invocations are inexpensive,
+		// so it's ok if we call it more than is strictly necessary.
+		wantTest := false
+		switch {
+		case ld.allPatternIsRoot && pkg.mod == Target:
+			// We are loading the "all" pattern, which includes packages imported by
+			// tests in the main module. This package is in the main module, so we
+			// need to identify the imports of its test even if LoadTests is not set.
+			//
+			// (We will filter out the extra tests explicitly in computePatternAll.)
+			wantTest = true
+
+		case ld.allPatternIsRoot && ld.allClosesOverTests && new.has(pkgInAll):
+			// This variant of the "all" pattern includes imports of tests of every
+			// package that is itself in "all", and pkg is in "all", so its test is
+			// also in "all" (as above).
+			wantTest = true
+
+		case ld.LoadTests && new.has(pkgIsRoot):
+			// LoadTest explicitly requests tests of “the root packages”.
+			wantTest = true
+		}
+
+		if wantTest {
+			var testFlags loadPkgFlags
+			if pkg.mod == Target || (ld.allClosesOverTests && new.has(pkgInAll)) {
+				// Tests of packages in the main module are in "all", in the sense that
+				// they cause the packages they import to also be in "all". So are tests
+				// of packages in "all" if "all" closes over test dependencies.
+				testFlags |= pkgInAll
+			}
+			ld.pkgTest(pkg, testFlags)
+		}
+	}
+
+	if new.has(pkgInAll) && !old.has(pkgInAll|pkgImportsLoaded) {
+		// We have just marked pkg with pkgInAll, or we have just loaded its
+		// imports, or both. Now is the time to propagate pkgInAll to the imports.
+		for _, dep := range pkg.imports {
+			ld.applyPkgFlags(dep, pkgInAll)
+		}
+	}
+}
+
+// load loads an individual package.
+func (ld *loader) load(pkg *loadPkg) {
+	if strings.Contains(pkg.path, "@") {
+		// Leave for error during load.
+		return
+	}
+	if build.IsLocalImport(pkg.path) || filepath.IsAbs(pkg.path) {
+		// Leave for error during load.
+		// (Module mode does not allow local imports.)
+		return
+	}
+
+	if search.IsMetaPackage(pkg.path) {
+		pkg.err = &invalidImportError{
+			importPath: pkg.path,
+			err:        fmt.Errorf("%q is not an importable package; see 'go help packages'", pkg.path),
+		}
+		return
+	}
+
+	pkg.mod, pkg.dir, pkg.err = importFromBuildList(context.TODO(), pkg.path, buildList)
+	if pkg.dir == "" {
+		return
+	}
+	if pkg.mod == Target {
+		// Go ahead and mark pkg as in "all". This provides the invariant that a
+		// package that is *only* imported by other packages in "all" is always
+		// marked as such before loading its imports.
+		//
+		// We don't actually rely on that invariant at the moment, but it may
+		// improve efficiency somewhat and makes the behavior a bit easier to reason
+		// about (by reducing churn on the flag bits of dependencies), and costs
+		// essentially nothing (these atomic flag ops are essentially free compared
+		// to scanning source code for imports).
+		ld.applyPkgFlags(pkg, pkgInAll)
+	}
+	if ld.AllowPackage != nil {
+		if err := ld.AllowPackage(context.TODO(), pkg.path, pkg.mod); err != nil {
+			pkg.err = err
+		}
+	}
+
+	pkg.inStd = (search.IsStandardImportPath(pkg.path) && search.InDir(pkg.dir, cfg.GOROOTsrc) != "")
+
+	var imports, testImports []string
+
+	if cfg.BuildContext.Compiler == "gccgo" && pkg.inStd {
+		// We can't scan standard packages for gccgo.
+	} else {
+		var err error
+		imports, testImports, err = scanDir(pkg.dir, ld.Tags)
+		if err != nil {
+			pkg.err = err
+			return
+		}
+	}
+
+	pkg.imports = make([]*loadPkg, 0, len(imports))
+	var importFlags loadPkgFlags
+	if pkg.flags.has(pkgInAll) {
+		importFlags = pkgInAll
+	}
+	for _, path := range imports {
+		if pkg.inStd {
+			// Imports from packages in "std" and "cmd" should resolve using
+			// GOROOT/src/vendor even when "std" is not the main module.
+			path = ld.stdVendor(pkg.path, path)
+		}
+		pkg.imports = append(pkg.imports, ld.pkg(path, importFlags))
+	}
+	pkg.testImports = testImports
+
+	ld.applyPkgFlags(pkg, pkgImportsLoaded)
+}
+
+// pkgTest locates the test of pkg, creating it if needed, and updates its state
+// to reflect the given flags.
+//
+// pkgTest requires that the imports of pkg have already been loaded (flagged
+// with pkgImportsLoaded).
+func (ld *loader) pkgTest(pkg *loadPkg, testFlags loadPkgFlags) *loadPkg {
+	if pkg.isTest() {
+		panic("pkgTest called on a test package")
+	}
+
+	createdTest := false
+	pkg.testOnce.Do(func() {
+		pkg.test = &loadPkg{
+			path:   pkg.path,
+			testOf: pkg,
+			mod:    pkg.mod,
+			dir:    pkg.dir,
+			err:    pkg.err,
+			inStd:  pkg.inStd,
+		}
+		ld.applyPkgFlags(pkg.test, testFlags)
+		createdTest = true
+	})
+
+	test := pkg.test
+	if createdTest {
+		test.imports = make([]*loadPkg, 0, len(pkg.testImports))
+		var importFlags loadPkgFlags
+		if test.flags.has(pkgInAll) {
+			importFlags = pkgInAll
+		}
+		for _, path := range pkg.testImports {
+			if pkg.inStd {
+				path = ld.stdVendor(test.path, path)
+			}
+			test.imports = append(test.imports, ld.pkg(path, importFlags))
+		}
+		pkg.testImports = nil
+		ld.applyPkgFlags(test, pkgImportsLoaded)
+	} else {
+		ld.applyPkgFlags(test, testFlags)
+	}
+
+	return test
+}
+
+// stdVendor returns the canonical import path for the package with the given
+// path when imported from the standard-library package at parentPath.
+func (ld *loader) stdVendor(parentPath, path string) string {
+	if search.IsStandardImportPath(path) {
+		return path
+	}
+
+	if str.HasPathPrefix(parentPath, "cmd") {
+		if Target.Path != "cmd" {
+			vendorPath := pathpkg.Join("cmd", "vendor", path)
+			if _, err := os.Stat(filepath.Join(cfg.GOROOTsrc, filepath.FromSlash(vendorPath))); err == nil {
+				return vendorPath
+			}
+		}
+	} else if Target.Path != "std" || str.HasPathPrefix(parentPath, "vendor") {
+		// If we are outside of the 'std' module, resolve imports from within 'std'
+		// to the vendor directory.
+		//
+		// Do the same for importers beginning with the prefix 'vendor/' even if we
+		// are *inside* of the 'std' module: the 'vendor/' packages that resolve
+		// globally from GOROOT/src/vendor (and are listed as part of 'go list std')
+		// are distinct from the real module dependencies, and cannot import
+		// internal packages from the real module.
+		//
+		// (Note that although the 'vendor/' packages match the 'std' *package*
+		// pattern, they are not part of the std *module*, and do not affect
+		// 'go mod tidy' and similar module commands when working within std.)
+		vendorPath := pathpkg.Join("vendor", path)
+		if _, err := os.Stat(filepath.Join(cfg.GOROOTsrc, filepath.FromSlash(vendorPath))); err == nil {
+			return vendorPath
+		}
+	}
+
+	// Not vendored: resolve from modules.
+	return path
+}
+
+// computePatternAll returns the list of packages matching pattern "all",
+// starting with a list of the import paths for the packages in the main module.
+func (ld *loader) computePatternAll() (all []string) {
+	for _, pkg := range ld.pkgs {
+		if pkg.flags.has(pkgInAll) && !pkg.isTest() {
+			all = append(all, pkg.path)
+		}
+	}
+	sort.Strings(all)
+	return all
+}
+
+// scanDir is like imports.ScanDir but elides known magic imports from the list,
+// so that we do not go looking for packages that don't really exist.
+//
+// The standard magic import is "C", for cgo.
+//
+// The only other known magic imports are appengine and appengine/*.
+// These are so old that they predate "go get" and did not use URL-like paths.
+// Most code today now uses google.golang.org/appengine instead,
+// but not all code has been so updated. When we mostly ignore build tags
+// during "go vendor", we look into "// +build appengine" files and
+// may see these legacy imports. We drop them so that the module
+// search does not look for modules to try to satisfy them.
+func scanDir(dir string, tags map[string]bool) (imports_, testImports []string, err error) {
+	imports_, testImports, err = imports.ScanDir(dir, tags)
+
+	filter := func(x []string) []string {
+		w := 0
+		for _, pkg := range x {
+			if pkg != "C" && pkg != "appengine" && !strings.HasPrefix(pkg, "appengine/") &&
+				pkg != "appengine_internal" && !strings.HasPrefix(pkg, "appengine_internal/") {
+				x[w] = pkg
+				w++
+			}
+		}
+		return x[:w]
+	}
+
+	return filter(imports_), filter(testImports), err
+}
+
+// buildStacks computes minimal import stacks for each package,
+// for use in error messages. When it completes, packages that
+// are part of the original root set have pkg.stack == nil,
+// and other packages have pkg.stack pointing at the next
+// package up the import stack in their minimal chain.
+// As a side effect, buildStacks also constructs ld.pkgs,
+// the list of all packages loaded.
+func (ld *loader) buildStacks() {
+	if len(ld.pkgs) > 0 {
+		panic("buildStacks")
+	}
+	for _, pkg := range ld.roots {
+		pkg.stack = pkg // sentinel to avoid processing in next loop
+		ld.pkgs = append(ld.pkgs, pkg)
+	}
+	for i := 0; i < len(ld.pkgs); i++ { // not range: appending to ld.pkgs in loop
+		pkg := ld.pkgs[i]
+		for _, next := range pkg.imports {
+			if next.stack == nil {
+				next.stack = pkg
+				ld.pkgs = append(ld.pkgs, next)
+			}
+		}
+		if next := pkg.test; next != nil && next.stack == nil {
+			next.stack = pkg
+			ld.pkgs = append(ld.pkgs, next)
+		}
+	}
+	for _, pkg := range ld.roots {
+		pkg.stack = nil
+	}
+}
+
+// stackText builds the import stack text to use when
+// reporting an error in pkg. It has the general form
+//
+//	root imports
+//		other imports
+//		other2 tested by
+//		other2.test imports
+//		pkg
+//
+func (pkg *loadPkg) stackText() string {
+	var stack []*loadPkg
+	for p := pkg; p != nil; p = p.stack {
+		stack = append(stack, p)
+	}
+
+	var buf bytes.Buffer
+	for i := len(stack) - 1; i >= 0; i-- {
+		p := stack[i]
+		fmt.Fprint(&buf, p.path)
+		if p.testOf != nil {
+			fmt.Fprint(&buf, ".test")
+		}
+		if i > 0 {
+			if stack[i-1].testOf == p {
+				fmt.Fprint(&buf, " tested by\n\t")
+			} else {
+				fmt.Fprint(&buf, " imports\n\t")
+			}
+		}
+	}
+	return buf.String()
+}
+
+// why returns the text to use in "go mod why" output about the given package.
+// It is less ornate than the stackText but contains the same information.
+func (pkg *loadPkg) why() string {
+	var buf strings.Builder
+	var stack []*loadPkg
+	for p := pkg; p != nil; p = p.stack {
+		stack = append(stack, p)
+	}
+
+	for i := len(stack) - 1; i >= 0; i-- {
+		p := stack[i]
+		if p.testOf != nil {
+			fmt.Fprintf(&buf, "%s.test\n", p.testOf.path)
+		} else {
+			fmt.Fprintf(&buf, "%s\n", p.path)
+		}
+	}
+	return buf.String()
+}
+
+// Why returns the "go mod why" output stanza for the given package,
+// without the leading # comment.
+// The package graph must have been loaded already, usually by LoadPackages.
+// If there is no reason for the package to be in the current build,
+// Why returns an empty string.
+func Why(path string) string {
+	pkg, ok := loaded.pkgCache.Get(path).(*loadPkg)
+	if !ok {
+		return ""
+	}
+	return pkg.why()
+}
+
+// WhyDepth returns the number of steps in the Why listing.
+// If there is no reason for the package to be in the current build,
+// WhyDepth returns 0.
+func WhyDepth(path string) int {
+	n := 0
+	pkg, _ := loaded.pkgCache.Get(path).(*loadPkg)
+	for p := pkg; p != nil; p = p.stack {
+		n++
+	}
+	return n
+}
diff --git a/src/cmd/go/internal/modload/modfile.go b/src/cmd/go/internal/modload/modfile.go
new file mode 100644
index 0000000..c6667d0
--- /dev/null
+++ b/src/cmd/go/internal/modload/modfile.go
@@ -0,0 +1,591 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"path/filepath"
+	"strings"
+	"sync"
+	"unicode"
+
+	"cmd/go/internal/base"
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/lockedfile"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/par"
+	"cmd/go/internal/trace"
+
+	"golang.org/x/mod/modfile"
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
+)
+
+// narrowAllVersionV is the Go version (plus leading "v") at which the
+// module-module "all" pattern no longer closes over the dependencies of
+// tests outside of the main module.
+const narrowAllVersionV = "v1.16"
+const go116EnableNarrowAll = true
+
+var modFile *modfile.File
+
+// A modFileIndex is an index of data corresponding to a modFile
+// at a specific point in time.
+type modFileIndex struct {
+	data            []byte
+	dataNeedsFix    bool // true if fixVersion applied a change while parsing data
+	module          module.Version
+	goVersionV      string // GoVersion with "v" prefix
+	require         map[module.Version]requireMeta
+	replace         map[module.Version]module.Version
+	highestReplaced map[string]string // highest replaced version of each module path; empty string for wildcard-only replacements
+	exclude         map[module.Version]bool
+}
+
+// index is the index of the go.mod file as of when it was last read or written.
+var index *modFileIndex
+
+type requireMeta struct {
+	indirect bool
+}
+
+// CheckAllowed returns an error equivalent to ErrDisallowed if m is excluded by
+// the main module's go.mod or retracted by its author. Most version queries use
+// this to filter out versions that should not be used.
+func CheckAllowed(ctx context.Context, m module.Version) error {
+	if err := CheckExclusions(ctx, m); err != nil {
+		return err
+	}
+	if err := CheckRetractions(ctx, m); err != nil {
+		return err
+	}
+	return nil
+}
+
+// ErrDisallowed is returned by version predicates passed to Query and similar
+// functions to indicate that a version should not be considered.
+var ErrDisallowed = errors.New("disallowed module version")
+
+// CheckExclusions returns an error equivalent to ErrDisallowed if module m is
+// excluded by the main module's go.mod file.
+func CheckExclusions(ctx context.Context, m module.Version) error {
+	if index != nil && index.exclude[m] {
+		return module.VersionError(m, errExcluded)
+	}
+	return nil
+}
+
+var errExcluded = &excludedError{}
+
+type excludedError struct{}
+
+func (e *excludedError) Error() string     { return "excluded by go.mod" }
+func (e *excludedError) Is(err error) bool { return err == ErrDisallowed }
+
+// CheckRetractions returns an error if module m has been retracted by
+// its author.
+func CheckRetractions(ctx context.Context, m module.Version) error {
+	if m.Version == "" {
+		// Main module, standard library, or file replacement module.
+		// Cannot be retracted.
+		return nil
+	}
+
+	// Look up retraction information from the latest available version of
+	// the module. Cache retraction information so we don't parse the go.mod
+	// file repeatedly.
+	type entry struct {
+		retract []retraction
+		err     error
+	}
+	path := m.Path
+	e := retractCache.Do(path, func() (v interface{}) {
+		ctx, span := trace.StartSpan(ctx, "checkRetractions "+path)
+		defer span.Done()
+
+		if repl := Replacement(module.Version{Path: m.Path}); repl.Path != "" {
+			// All versions of the module were replaced with a local directory.
+			// Don't load retractions.
+			return &entry{nil, nil}
+		}
+
+		// Find the latest version of the module.
+		// Ignore exclusions from the main module's go.mod.
+		const ignoreSelected = ""
+		var allowAll AllowedFunc
+		rev, err := Query(ctx, path, "latest", ignoreSelected, allowAll)
+		if err != nil {
+			return &entry{nil, err}
+		}
+
+		// Load go.mod for that version.
+		// If the version is replaced, we'll load retractions from the replacement.
+		//
+		// If there's an error loading the go.mod, we'll return it here.
+		// These errors should generally be ignored by callers of checkRetractions,
+		// since they happen frequently when we're offline. These errors are not
+		// equivalent to ErrDisallowed, so they may be distinguished from
+		// retraction errors.
+		//
+		// We load the raw file here: the go.mod file may have a different module
+		// path that we expect if the module or its repository was renamed.
+		// We still want to apply retractions to other aliases of the module.
+		rm := module.Version{Path: path, Version: rev.Version}
+		if repl := Replacement(rm); repl.Path != "" {
+			rm = repl
+		}
+		summary, err := rawGoModSummary(rm)
+		if err != nil {
+			return &entry{nil, err}
+		}
+		return &entry{summary.retract, nil}
+	}).(*entry)
+
+	if err := e.err; err != nil {
+		// Attribute the error to the version being checked, not the version from
+		// which the retractions were to be loaded.
+		var mErr *module.ModuleError
+		if errors.As(err, &mErr) {
+			err = mErr.Err
+		}
+		return &retractionLoadingError{m: m, err: err}
+	}
+
+	var rationale []string
+	isRetracted := false
+	for _, r := range e.retract {
+		if semver.Compare(r.Low, m.Version) <= 0 && semver.Compare(m.Version, r.High) <= 0 {
+			isRetracted = true
+			if r.Rationale != "" {
+				rationale = append(rationale, r.Rationale)
+			}
+		}
+	}
+	if isRetracted {
+		return module.VersionError(m, &ModuleRetractedError{Rationale: rationale})
+	}
+	return nil
+}
+
+var retractCache par.Cache
+
+type ModuleRetractedError struct {
+	Rationale []string
+}
+
+func (e *ModuleRetractedError) Error() string {
+	msg := "retracted by module author"
+	if len(e.Rationale) > 0 {
+		// This is meant to be a short error printed on a terminal, so just
+		// print the first rationale.
+		msg += ": " + ShortRetractionRationale(e.Rationale[0])
+	}
+	return msg
+}
+
+func (e *ModuleRetractedError) Is(err error) bool {
+	return err == ErrDisallowed
+}
+
+type retractionLoadingError struct {
+	m   module.Version
+	err error
+}
+
+func (e *retractionLoadingError) Error() string {
+	return fmt.Sprintf("loading module retractions for %v: %v", e.m, e.err)
+}
+
+func (e *retractionLoadingError) Unwrap() error {
+	return e.err
+}
+
+// ShortRetractionRationale returns a retraction rationale string that is safe
+// to print in a terminal. It returns hard-coded strings if the rationale
+// is empty, too long, or contains non-printable characters.
+func ShortRetractionRationale(rationale string) string {
+	const maxRationaleBytes = 500
+	if i := strings.Index(rationale, "\n"); i >= 0 {
+		rationale = rationale[:i]
+	}
+	rationale = strings.TrimSpace(rationale)
+	if rationale == "" {
+		return "retracted by module author"
+	}
+	if len(rationale) > maxRationaleBytes {
+		return "(rationale omitted: too long)"
+	}
+	for _, r := range rationale {
+		if !unicode.IsGraphic(r) && !unicode.IsSpace(r) {
+			return "(rationale omitted: contains non-printable characters)"
+		}
+	}
+	// NOTE: the go.mod parser rejects invalid UTF-8, so we don't check that here.
+	return rationale
+}
+
+// Replacement returns the replacement for mod, if any, from go.mod.
+// If there is no replacement for mod, Replacement returns
+// a module.Version with Path == "".
+func Replacement(mod module.Version) module.Version {
+	if index != nil {
+		if r, ok := index.replace[mod]; ok {
+			return r
+		}
+		if r, ok := index.replace[module.Version{Path: mod.Path}]; ok {
+			return r
+		}
+	}
+	return module.Version{}
+}
+
+// indexModFile rebuilds the index of modFile.
+// If modFile has been changed since it was first read,
+// modFile.Cleanup must be called before indexModFile.
+func indexModFile(data []byte, modFile *modfile.File, needsFix bool) *modFileIndex {
+	i := new(modFileIndex)
+	i.data = data
+	i.dataNeedsFix = needsFix
+
+	i.module = module.Version{}
+	if modFile.Module != nil {
+		i.module = modFile.Module.Mod
+	}
+
+	i.goVersionV = ""
+	if modFile.Go != nil {
+		// We're going to use the semver package to compare Go versions, so go ahead
+		// and add the "v" prefix it expects once instead of every time.
+		i.goVersionV = "v" + modFile.Go.Version
+	}
+
+	i.require = make(map[module.Version]requireMeta, len(modFile.Require))
+	for _, r := range modFile.Require {
+		i.require[r.Mod] = requireMeta{indirect: r.Indirect}
+	}
+
+	i.replace = make(map[module.Version]module.Version, len(modFile.Replace))
+	for _, r := range modFile.Replace {
+		if prev, dup := i.replace[r.Old]; dup && prev != r.New {
+			base.Fatalf("go: conflicting replacements for %v:\n\t%v\n\t%v", r.Old, prev, r.New)
+		}
+		i.replace[r.Old] = r.New
+	}
+
+	i.highestReplaced = make(map[string]string)
+	for _, r := range modFile.Replace {
+		v, ok := i.highestReplaced[r.Old.Path]
+		if !ok || semver.Compare(r.Old.Version, v) > 0 {
+			i.highestReplaced[r.Old.Path] = r.Old.Version
+		}
+	}
+
+	i.exclude = make(map[module.Version]bool, len(modFile.Exclude))
+	for _, x := range modFile.Exclude {
+		i.exclude[x.Mod] = true
+	}
+
+	return i
+}
+
+// allPatternClosesOverTests reports whether the "all" pattern includes
+// dependencies of tests outside the main module (as in Go 1.11–1.15).
+// (Otherwise — as in Go 1.16+ — the "all" pattern includes only the packages
+// transitively *imported by* the packages and tests in the main module.)
+func (i *modFileIndex) allPatternClosesOverTests() bool {
+	if !go116EnableNarrowAll {
+		return true
+	}
+	if i != nil && semver.Compare(i.goVersionV, narrowAllVersionV) < 0 {
+		// The module explicitly predates the change in "all" for lazy loading, so
+		// continue to use the older interpretation. (If i == nil, we not in any
+		// module at all and should use the latest semantics.)
+		return true
+	}
+	return false
+}
+
+// modFileIsDirty reports whether the go.mod file differs meaningfully
+// from what was indexed.
+// If modFile has been changed (even cosmetically) since it was first read,
+// modFile.Cleanup must be called before modFileIsDirty.
+func (i *modFileIndex) modFileIsDirty(modFile *modfile.File) bool {
+	if i == nil {
+		return modFile != nil
+	}
+
+	if i.dataNeedsFix {
+		return true
+	}
+
+	if modFile.Module == nil {
+		if i.module != (module.Version{}) {
+			return true
+		}
+	} else if modFile.Module.Mod != i.module {
+		return true
+	}
+
+	if modFile.Go == nil {
+		if i.goVersionV != "" {
+			return true
+		}
+	} else if "v"+modFile.Go.Version != i.goVersionV {
+		if i.goVersionV == "" && cfg.BuildMod == "readonly" {
+			// go.mod files did not always require a 'go' version, so do not error out
+			// if one is missing — we may be inside an older module in the module
+			// cache, and should bias toward providing useful behavior.
+		} else {
+			return true
+		}
+	}
+
+	if len(modFile.Require) != len(i.require) ||
+		len(modFile.Replace) != len(i.replace) ||
+		len(modFile.Exclude) != len(i.exclude) {
+		return true
+	}
+
+	for _, r := range modFile.Require {
+		if meta, ok := i.require[r.Mod]; !ok {
+			return true
+		} else if r.Indirect != meta.indirect {
+			if cfg.BuildMod == "readonly" {
+				// The module's requirements are consistent; only the "// indirect"
+				// comments that are wrong. But those are only guaranteed to be accurate
+				// after a "go mod tidy" — it's a good idea to run those before
+				// committing a change, but it's certainly not mandatory.
+			} else {
+				return true
+			}
+		}
+	}
+
+	for _, r := range modFile.Replace {
+		if r.New != i.replace[r.Old] {
+			return true
+		}
+	}
+
+	for _, x := range modFile.Exclude {
+		if !i.exclude[x.Mod] {
+			return true
+		}
+	}
+
+	return false
+}
+
+// rawGoVersion records the Go version parsed from each module's go.mod file.
+//
+// If a module is replaced, the version of the replacement is keyed by the
+// replacement module.Version, not the version being replaced.
+var rawGoVersion sync.Map // map[module.Version]string
+
+// A modFileSummary is a summary of a go.mod file for which we do not need to
+// retain complete information — for example, the go.mod file of a dependency
+// module.
+type modFileSummary struct {
+	module     module.Version
+	goVersionV string // GoVersion with "v" prefix
+	require    []module.Version
+	retract    []retraction
+}
+
+// A retraction consists of a retracted version interval and rationale.
+// retraction is like modfile.Retract, but it doesn't point to the syntax tree.
+type retraction struct {
+	modfile.VersionInterval
+	Rationale string
+}
+
+// goModSummary returns a summary of the go.mod file for module m,
+// taking into account any replacements for m, exclusions of its dependencies,
+// and/or vendoring.
+//
+// m must be a version in the module graph, reachable from the Target module.
+// In readonly mode, the go.sum file must contain an entry for m's go.mod file
+// (or its replacement). goModSummary must not be called for the Target module
+// itself, as its requirements may change. Use rawGoModSummary for other
+// module versions.
+//
+// The caller must not modify the returned summary.
+func goModSummary(m module.Version) (*modFileSummary, error) {
+	if m == Target {
+		panic("internal error: goModSummary called on the Target module")
+	}
+
+	if cfg.BuildMod == "vendor" {
+		summary := &modFileSummary{
+			module: module.Version{Path: m.Path},
+		}
+		if vendorVersion[m.Path] != m.Version {
+			// This module is not vendored, so packages cannot be loaded from it and
+			// it cannot be relevant to the build.
+			return summary, nil
+		}
+
+		// For every module other than the target,
+		// return the full list of modules from modules.txt.
+		readVendorList()
+
+		// TODO(#36876): Load the "go" version from vendor/modules.txt and store it
+		// in rawGoVersion with the appropriate key.
+
+		// We don't know what versions the vendored module actually relies on,
+		// so assume that it requires everything.
+		summary.require = vendorList
+		return summary, nil
+	}
+
+	actual := Replacement(m)
+	if actual.Path == "" {
+		actual = m
+	}
+	if HasModRoot() && cfg.BuildMod == "readonly" && actual.Version != "" {
+		key := module.Version{Path: actual.Path, Version: actual.Version + "/go.mod"}
+		if !modfetch.HaveSum(key) {
+			suggestion := fmt.Sprintf("; to add it:\n\tgo mod download %s", m.Path)
+			return nil, module.VersionError(actual, &sumMissingError{suggestion: suggestion})
+		}
+	}
+	summary, err := rawGoModSummary(actual)
+	if err != nil {
+		return nil, err
+	}
+
+	if actual.Version == "" {
+		// The actual module is a filesystem-local replacement, for which we have
+		// unfortunately not enforced any sort of invariants about module lines or
+		// matching module paths. Anything goes.
+		//
+		// TODO(bcmills): Remove this special-case, update tests, and add a
+		// release note.
+	} else {
+		if summary.module.Path == "" {
+			return nil, module.VersionError(actual, errors.New("parsing go.mod: missing module line"))
+		}
+
+		// In theory we should only allow mpath to be unequal to m.Path here if the
+		// version that we fetched lacks an explicit go.mod file: if the go.mod file
+		// is explicit, then it should match exactly (to ensure that imports of other
+		// packages within the module are interpreted correctly). Unfortunately, we
+		// can't determine that information from the module proxy protocol: we'll have
+		// to leave that validation for when we load actual packages from within the
+		// module.
+		if mpath := summary.module.Path; mpath != m.Path && mpath != actual.Path {
+			return nil, module.VersionError(actual, fmt.Errorf(`parsing go.mod:
+	module declares its path as: %s
+	        but was required as: %s`, mpath, m.Path))
+		}
+	}
+
+	if index != nil && len(index.exclude) > 0 {
+		// Drop any requirements on excluded versions.
+		// Don't modify the cached summary though, since we might need the raw
+		// summary separately.
+		haveExcludedReqs := false
+		for _, r := range summary.require {
+			if index.exclude[r] {
+				haveExcludedReqs = true
+				break
+			}
+		}
+		if haveExcludedReqs {
+			s := new(modFileSummary)
+			*s = *summary
+			s.require = make([]module.Version, 0, len(summary.require))
+			for _, r := range summary.require {
+				if !index.exclude[r] {
+					s.require = append(s.require, r)
+				}
+			}
+			summary = s
+		}
+	}
+	return summary, nil
+}
+
+// rawGoModSummary returns a new summary of the go.mod file for module m,
+// ignoring all replacements that may apply to m and excludes that may apply to
+// its dependencies.
+//
+// rawGoModSummary cannot be used on the Target module.
+func rawGoModSummary(m module.Version) (*modFileSummary, error) {
+	if m == Target {
+		panic("internal error: rawGoModSummary called on the Target module")
+	}
+
+	type cached struct {
+		summary *modFileSummary
+		err     error
+	}
+	c := rawGoModSummaryCache.Do(m, func() interface{} {
+		summary := new(modFileSummary)
+		var f *modfile.File
+		if m.Version == "" {
+			// m is a replacement module with only a file path.
+			dir := m.Path
+			if !filepath.IsAbs(dir) {
+				dir = filepath.Join(ModRoot(), dir)
+			}
+			gomod := filepath.Join(dir, "go.mod")
+
+			data, err := lockedfile.Read(gomod)
+			if err != nil {
+				return cached{nil, module.VersionError(m, fmt.Errorf("reading %s: %v", base.ShortPath(gomod), err))}
+			}
+			f, err = modfile.ParseLax(gomod, data, nil)
+			if err != nil {
+				return cached{nil, module.VersionError(m, fmt.Errorf("parsing %s: %v", base.ShortPath(gomod), err))}
+			}
+		} else {
+			if !semver.IsValid(m.Version) {
+				// Disallow the broader queries supported by fetch.Lookup.
+				base.Fatalf("go: internal error: %s@%s: unexpected invalid semantic version", m.Path, m.Version)
+			}
+
+			data, err := modfetch.GoMod(m.Path, m.Version)
+			if err != nil {
+				return cached{nil, err}
+			}
+			f, err = modfile.ParseLax("go.mod", data, nil)
+			if err != nil {
+				return cached{nil, module.VersionError(m, fmt.Errorf("parsing go.mod: %v", err))}
+			}
+		}
+
+		if f.Module != nil {
+			summary.module = f.Module.Mod
+		}
+		if f.Go != nil && f.Go.Version != "" {
+			rawGoVersion.LoadOrStore(m, f.Go.Version)
+			summary.goVersionV = "v" + f.Go.Version
+		}
+		if len(f.Require) > 0 {
+			summary.require = make([]module.Version, 0, len(f.Require))
+			for _, req := range f.Require {
+				summary.require = append(summary.require, req.Mod)
+			}
+		}
+		if len(f.Retract) > 0 {
+			summary.retract = make([]retraction, 0, len(f.Retract))
+			for _, ret := range f.Retract {
+				summary.retract = append(summary.retract, retraction{
+					VersionInterval: ret.VersionInterval,
+					Rationale:       ret.Rationale,
+				})
+			}
+		}
+
+		return cached{summary, nil}
+	}).(cached)
+
+	return c.summary, c.err
+}
+
+var rawGoModSummaryCache par.Cache // module.Version → rawGoModSummary result
diff --git a/src/cmd/go/internal/modload/mvs.go b/src/cmd/go/internal/modload/mvs.go
new file mode 100644
index 0000000..3101519
--- /dev/null
+++ b/src/cmd/go/internal/modload/mvs.go
@@ -0,0 +1,113 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"context"
+	"errors"
+	"os"
+	"sort"
+
+	"cmd/go/internal/modfetch"
+
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
+)
+
+// mvsReqs implements mvs.Reqs for module semantic versions,
+// with any exclusions or replacements applied internally.
+type mvsReqs struct {
+	buildList []module.Version
+}
+
+func (r *mvsReqs) Required(mod module.Version) ([]module.Version, error) {
+	if mod == Target {
+		// Use the build list as it existed when r was constructed, not the current
+		// global build list.
+		return r.buildList[1:], nil
+	}
+
+	if mod.Version == "none" {
+		return nil, nil
+	}
+
+	summary, err := goModSummary(mod)
+	if err != nil {
+		return nil, err
+	}
+	return summary.require, nil
+}
+
+// Max returns the maximum of v1 and v2 according to semver.Compare.
+//
+// As a special case, the version "" is considered higher than all other
+// versions. The main module (also known as the target) has no version and must
+// be chosen over other versions of the same module in the module dependency
+// graph.
+func (*mvsReqs) Max(v1, v2 string) string {
+	if v1 != "" && (v2 == "" || semver.Compare(v1, v2) == -1) {
+		return v2
+	}
+	return v1
+}
+
+// Upgrade is a no-op, here to implement mvs.Reqs.
+// The upgrade logic for go get -u is in ../modget/get.go.
+func (*mvsReqs) Upgrade(m module.Version) (module.Version, error) {
+	return m, nil
+}
+
+func versions(ctx context.Context, path string, allowed AllowedFunc) ([]string, error) {
+	// Note: modfetch.Lookup and repo.Versions are cached,
+	// so there's no need for us to add extra caching here.
+	var versions []string
+	err := modfetch.TryProxies(func(proxy string) error {
+		repo, err := lookupRepo(proxy, path)
+		if err != nil {
+			return err
+		}
+		allVersions, err := repo.Versions("")
+		if err != nil {
+			return err
+		}
+		allowedVersions := make([]string, 0, len(allVersions))
+		for _, v := range allVersions {
+			if err := allowed(ctx, module.Version{Path: path, Version: v}); err == nil {
+				allowedVersions = append(allowedVersions, v)
+			} else if !errors.Is(err, ErrDisallowed) {
+				return err
+			}
+		}
+		versions = allowedVersions
+		return nil
+	})
+	return versions, err
+}
+
+// Previous returns the tagged version of m.Path immediately prior to
+// m.Version, or version "none" if no prior version is tagged.
+//
+// Since the version of Target is not found in the version list,
+// it has no previous version.
+func (*mvsReqs) Previous(m module.Version) (module.Version, error) {
+	// TODO(golang.org/issue/38714): thread tracing context through MVS.
+
+	if m == Target {
+		return module.Version{Path: m.Path, Version: "none"}, nil
+	}
+
+	list, err := versions(context.TODO(), m.Path, CheckAllowed)
+	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return module.Version{Path: m.Path, Version: "none"}, nil
+		}
+		return module.Version{}, err
+	}
+	i := sort.Search(len(list), func(i int) bool { return semver.Compare(list[i], m.Version) >= 0 })
+	if i > 0 {
+		return module.Version{Path: m.Path, Version: list[i-1]}, nil
+	}
+	return module.Version{Path: m.Path, Version: "none"}, nil
+}
diff --git a/src/cmd/go/internal/modload/mvs_test.go b/src/cmd/go/internal/modload/mvs_test.go
new file mode 100644
index 0000000..50e93c3
--- /dev/null
+++ b/src/cmd/go/internal/modload/mvs_test.go
@@ -0,0 +1,31 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"testing"
+)
+
+func TestReqsMax(t *testing.T) {
+	type testCase struct {
+		a, b, want string
+	}
+	reqs := new(mvsReqs)
+	for _, tc := range []testCase{
+		{a: "v0.1.0", b: "v0.2.0", want: "v0.2.0"},
+		{a: "v0.2.0", b: "v0.1.0", want: "v0.2.0"},
+		{a: "", b: "v0.1.0", want: ""}, // "" is Target.Version
+		{a: "v0.1.0", b: "", want: ""},
+		{a: "none", b: "v0.1.0", want: "v0.1.0"},
+		{a: "v0.1.0", b: "none", want: "v0.1.0"},
+		{a: "none", b: "", want: ""},
+		{a: "", b: "none", want: ""},
+	} {
+		max := reqs.Max(tc.a, tc.b)
+		if max != tc.want {
+			t.Errorf("(%T).Max(%q, %q) = %q; want %q", reqs, tc.a, tc.b, max, tc.want)
+		}
+	}
+}
diff --git a/src/cmd/go/internal/modload/query.go b/src/cmd/go/internal/modload/query.go
new file mode 100644
index 0000000..8affd17
--- /dev/null
+++ b/src/cmd/go/internal/modload/query.go
@@ -0,0 +1,1111 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	pathpkg "path"
+	"path/filepath"
+	"sort"
+	"strings"
+	"sync"
+	"time"
+
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/imports"
+	"cmd/go/internal/modfetch"
+	"cmd/go/internal/search"
+	"cmd/go/internal/str"
+	"cmd/go/internal/trace"
+
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
+)
+
+// Query looks up a revision of a given module given a version query string.
+// The module must be a complete module path.
+// The version must take one of the following forms:
+//
+// - the literal string "latest", denoting the latest available, allowed
+//   tagged version, with non-prereleases preferred over prereleases.
+//   If there are no tagged versions in the repo, latest returns the most
+//   recent commit.
+// - the literal string "upgrade", equivalent to "latest" except that if
+//   current is a newer version, current will be returned (see below).
+// - the literal string "patch", denoting the latest available tagged version
+//   with the same major and minor number as current (see below).
+// - v1, denoting the latest available tagged version v1.x.x.
+// - v1.2, denoting the latest available tagged version v1.2.x.
+// - v1.2.3, a semantic version string denoting that tagged version.
+// - <v1.2.3, <=v1.2.3, >v1.2.3, >=v1.2.3,
+//   denoting the version closest to the target and satisfying the given operator,
+//   with non-prereleases preferred over prereleases.
+// - a repository commit identifier or tag, denoting that commit.
+//
+// current denotes the currently-selected version of the module; it may be
+// "none" if no version is currently selected, or "" if the currently-selected
+// version is unknown or should not be considered. If query is
+// "upgrade" or "patch", current will be returned if it is a newer
+// semantic version or a chronologically later pseudo-version than the
+// version that would otherwise be chosen. This prevents accidental downgrades
+// from newer pre-release or development versions.
+//
+// The allowed function (which may be nil) is used to filter out unsuitable
+// versions (see AllowedFunc documentation for details). If the query refers to
+// a specific revision (for example, "master"; see IsRevisionQuery), and the
+// revision is disallowed by allowed, Query returns the error. If the query
+// does not refer to a specific revision (for example, "latest"), Query
+// acts as if versions disallowed by allowed do not exist.
+//
+// If path is the path of the main module and the query is "latest",
+// Query returns Target.Version as the version.
+func Query(ctx context.Context, path, query, current string, allowed AllowedFunc) (*modfetch.RevInfo, error) {
+	var info *modfetch.RevInfo
+	err := modfetch.TryProxies(func(proxy string) (err error) {
+		info, err = queryProxy(ctx, proxy, path, query, current, allowed)
+		return err
+	})
+	return info, err
+}
+
+// AllowedFunc is used by Query and other functions to filter out unsuitable
+// versions, for example, those listed in exclude directives in the main
+// module's go.mod file.
+//
+// An AllowedFunc returns an error equivalent to ErrDisallowed for an unsuitable
+// version. Any other error indicates the function was unable to determine
+// whether the version should be allowed, for example, the function was unable
+// to fetch or parse a go.mod file containing retractions. Typically, errors
+// other than ErrDisallowd may be ignored.
+type AllowedFunc func(context.Context, module.Version) error
+
+var errQueryDisabled error = queryDisabledError{}
+
+type queryDisabledError struct{}
+
+func (queryDisabledError) Error() string {
+	if cfg.BuildModReason == "" {
+		return fmt.Sprintf("cannot query module due to -mod=%s", cfg.BuildMod)
+	}
+	return fmt.Sprintf("cannot query module due to -mod=%s\n\t(%s)", cfg.BuildMod, cfg.BuildModReason)
+}
+
+func queryProxy(ctx context.Context, proxy, path, query, current string, allowed AllowedFunc) (*modfetch.RevInfo, error) {
+	ctx, span := trace.StartSpan(ctx, "modload.queryProxy "+path+" "+query)
+	defer span.Done()
+
+	if current != "" && current != "none" && !semver.IsValid(current) {
+		return nil, fmt.Errorf("invalid previous version %q", current)
+	}
+	if cfg.BuildMod == "vendor" {
+		return nil, errQueryDisabled
+	}
+	if allowed == nil {
+		allowed = func(context.Context, module.Version) error { return nil }
+	}
+
+	if path == Target.Path && (query == "upgrade" || query == "patch") {
+		if err := allowed(ctx, Target); err != nil {
+			return nil, fmt.Errorf("internal error: main module version is not allowed: %w", err)
+		}
+		return &modfetch.RevInfo{Version: Target.Version}, nil
+	}
+
+	if path == "std" || path == "cmd" {
+		return nil, fmt.Errorf("can't query specific version (%q) of standard-library module %q", query, path)
+	}
+
+	repo, err := lookupRepo(proxy, path)
+	if err != nil {
+		return nil, err
+	}
+
+	// Parse query to detect parse errors (and possibly handle query)
+	// before any network I/O.
+	qm, err := newQueryMatcher(path, query, current, allowed)
+	if (err == nil && qm.canStat) || err == errRevQuery {
+		// Direct lookup of a commit identifier or complete (non-prefix) semantic
+		// version.
+
+		// If the identifier is not a canonical semver tag — including if it's a
+		// semver tag with a +metadata suffix — then modfetch.Stat will populate
+		// info.Version with a suitable pseudo-version.
+		info, err := repo.Stat(query)
+		if err != nil {
+			queryErr := err
+			// The full query doesn't correspond to a tag. If it is a semantic version
+			// with a +metadata suffix, see if there is a tag without that suffix:
+			// semantic versioning defines them to be equivalent.
+			canonicalQuery := module.CanonicalVersion(query)
+			if canonicalQuery != "" && query != canonicalQuery {
+				info, err = repo.Stat(canonicalQuery)
+				if err != nil && !errors.Is(err, fs.ErrNotExist) {
+					return info, err
+				}
+			}
+			if err != nil {
+				return nil, queryErr
+			}
+		}
+		if err := allowed(ctx, module.Version{Path: path, Version: info.Version}); errors.Is(err, ErrDisallowed) {
+			return nil, err
+		}
+		return info, nil
+	} else if err != nil {
+		return nil, err
+	}
+
+	// Load versions and execute query.
+	versions, err := repo.Versions(qm.prefix)
+	if err != nil {
+		return nil, err
+	}
+	releases, prereleases, err := qm.filterVersions(ctx, versions)
+	if err != nil {
+		return nil, err
+	}
+
+	lookup := func(v string) (*modfetch.RevInfo, error) {
+		rev, err := repo.Stat(v)
+		if err != nil {
+			return nil, err
+		}
+
+		if (query == "upgrade" || query == "patch") && modfetch.IsPseudoVersion(current) && !rev.Time.IsZero() {
+			// Don't allow "upgrade" or "patch" to move from a pseudo-version
+			// to a chronologically older version or pseudo-version.
+			//
+			// If the current version is a pseudo-version from an untagged branch, it
+			// may be semantically lower than the "latest" release or the latest
+			// pseudo-version on the main branch. A user on such a version is unlikely
+			// to intend to “upgrade” to a version that already existed at that point
+			// in time.
+			//
+			// We do this only if the current version is a pseudo-version: if the
+			// version is tagged, the author of the dependency module has given us
+			// explicit information about their intended precedence of this version
+			// relative to other versions, and we shouldn't contradict that
+			// information. (For example, v1.0.1 might be a backport of a fix already
+			// incorporated into v1.1.0, in which case v1.0.1 would be chronologically
+			// newer but v1.1.0 is still an “upgrade”; or v1.0.2 might be a revert of
+			// an unsuccessful fix in v1.0.1, in which case the v1.0.2 commit may be
+			// older than the v1.0.1 commit despite the tag itself being newer.)
+			currentTime, err := modfetch.PseudoVersionTime(current)
+			if err == nil && rev.Time.Before(currentTime) {
+				if err := allowed(ctx, module.Version{Path: path, Version: current}); errors.Is(err, ErrDisallowed) {
+					return nil, err
+				}
+				return repo.Stat(current)
+			}
+		}
+
+		return rev, nil
+	}
+
+	if qm.preferLower {
+		if len(releases) > 0 {
+			return lookup(releases[0])
+		}
+		if len(prereleases) > 0 {
+			return lookup(prereleases[0])
+		}
+	} else {
+		if len(releases) > 0 {
+			return lookup(releases[len(releases)-1])
+		}
+		if len(prereleases) > 0 {
+			return lookup(prereleases[len(prereleases)-1])
+		}
+	}
+
+	if qm.mayUseLatest {
+		latest, err := repo.Latest()
+		if err == nil {
+			if qm.allowsVersion(ctx, latest.Version) {
+				return lookup(latest.Version)
+			}
+		} else if !errors.Is(err, fs.ErrNotExist) {
+			return nil, err
+		}
+	}
+
+	if (query == "upgrade" || query == "patch") && current != "" && current != "none" {
+		// "upgrade" and "patch" may stay on the current version if allowed.
+		if err := allowed(ctx, module.Version{Path: path, Version: current}); errors.Is(err, ErrDisallowed) {
+			return nil, err
+		}
+		return lookup(current)
+	}
+
+	return nil, &NoMatchingVersionError{query: query, current: current}
+}
+
+// IsRevisionQuery returns true if vers is a version query that may refer to
+// a particular version or revision in a repository like "v1.0.0", "master",
+// or "0123abcd". IsRevisionQuery returns false if vers is a query that
+// chooses from among available versions like "latest" or ">v1.0.0".
+func IsRevisionQuery(vers string) bool {
+	if vers == "latest" ||
+		vers == "upgrade" ||
+		vers == "patch" ||
+		strings.HasPrefix(vers, "<") ||
+		strings.HasPrefix(vers, ">") ||
+		(semver.IsValid(vers) && isSemverPrefix(vers)) {
+		return false
+	}
+	return true
+}
+
+// isSemverPrefix reports whether v is a semantic version prefix: v1 or v1.2 (not v1.2.3).
+// The caller is assumed to have checked that semver.IsValid(v) is true.
+func isSemverPrefix(v string) bool {
+	dots := 0
+	for i := 0; i < len(v); i++ {
+		switch v[i] {
+		case '-', '+':
+			return false
+		case '.':
+			dots++
+			if dots >= 2 {
+				return false
+			}
+		}
+	}
+	return true
+}
+
+type queryMatcher struct {
+	path               string
+	prefix             string
+	filter             func(version string) bool
+	allowed            AllowedFunc
+	canStat            bool // if true, the query can be resolved by repo.Stat
+	preferLower        bool // if true, choose the lowest matching version
+	mayUseLatest       bool
+	preferIncompatible bool
+}
+
+var errRevQuery = errors.New("query refers to a non-semver revision")
+
+// newQueryMatcher returns a new queryMatcher that matches the versions
+// specified by the given query on the module with the given path.
+//
+// If the query can only be resolved by statting a non-SemVer revision,
+// newQueryMatcher returns errRevQuery.
+func newQueryMatcher(path string, query, current string, allowed AllowedFunc) (*queryMatcher, error) {
+	badVersion := func(v string) (*queryMatcher, error) {
+		return nil, fmt.Errorf("invalid semantic version %q in range %q", v, query)
+	}
+
+	matchesMajor := func(v string) bool {
+		_, pathMajor, ok := module.SplitPathVersion(path)
+		if !ok {
+			return false
+		}
+		return module.CheckPathMajor(v, pathMajor) == nil
+	}
+
+	qm := &queryMatcher{
+		path:               path,
+		allowed:            allowed,
+		preferIncompatible: strings.HasSuffix(current, "+incompatible"),
+	}
+
+	switch {
+	case query == "latest":
+		qm.mayUseLatest = true
+
+	case query == "upgrade":
+		if current == "" || current == "none" {
+			qm.mayUseLatest = true
+		} else {
+			qm.mayUseLatest = modfetch.IsPseudoVersion(current)
+			qm.filter = func(mv string) bool { return semver.Compare(mv, current) >= 0 }
+		}
+
+	case query == "patch":
+		if current == "none" {
+			return nil, &NoPatchBaseError{path}
+		}
+		if current == "" {
+			qm.mayUseLatest = true
+		} else {
+			qm.mayUseLatest = modfetch.IsPseudoVersion(current)
+			qm.prefix = semver.MajorMinor(current) + "."
+			qm.filter = func(mv string) bool { return semver.Compare(mv, current) >= 0 }
+		}
+
+	case strings.HasPrefix(query, "<="):
+		v := query[len("<="):]
+		if !semver.IsValid(v) {
+			return badVersion(v)
+		}
+		if isSemverPrefix(v) {
+			// Refuse to say whether <=v1.2 allows v1.2.3 (remember, @v1.2 might mean v1.2.3).
+			return nil, fmt.Errorf("ambiguous semantic version %q in range %q", v, query)
+		}
+		qm.filter = func(mv string) bool { return semver.Compare(mv, v) <= 0 }
+		if !matchesMajor(v) {
+			qm.preferIncompatible = true
+		}
+
+	case strings.HasPrefix(query, "<"):
+		v := query[len("<"):]
+		if !semver.IsValid(v) {
+			return badVersion(v)
+		}
+		qm.filter = func(mv string) bool { return semver.Compare(mv, v) < 0 }
+		if !matchesMajor(v) {
+			qm.preferIncompatible = true
+		}
+
+	case strings.HasPrefix(query, ">="):
+		v := query[len(">="):]
+		if !semver.IsValid(v) {
+			return badVersion(v)
+		}
+		qm.filter = func(mv string) bool { return semver.Compare(mv, v) >= 0 }
+		qm.preferLower = true
+		if !matchesMajor(v) {
+			qm.preferIncompatible = true
+		}
+
+	case strings.HasPrefix(query, ">"):
+		v := query[len(">"):]
+		if !semver.IsValid(v) {
+			return badVersion(v)
+		}
+		if isSemverPrefix(v) {
+			// Refuse to say whether >v1.2 allows v1.2.3 (remember, @v1.2 might mean v1.2.3).
+			return nil, fmt.Errorf("ambiguous semantic version %q in range %q", v, query)
+		}
+		qm.filter = func(mv string) bool { return semver.Compare(mv, v) > 0 }
+		qm.preferLower = true
+		if !matchesMajor(v) {
+			qm.preferIncompatible = true
+		}
+
+	case semver.IsValid(query):
+		if isSemverPrefix(query) {
+			qm.prefix = query + "."
+			// Do not allow the query "v1.2" to match versions lower than "v1.2.0",
+			// such as prereleases for that version. (https://golang.org/issue/31972)
+			qm.filter = func(mv string) bool { return semver.Compare(mv, query) >= 0 }
+		} else {
+			qm.canStat = true
+			qm.filter = func(mv string) bool { return semver.Compare(mv, query) == 0 }
+			qm.prefix = semver.Canonical(query)
+		}
+		if !matchesMajor(query) {
+			qm.preferIncompatible = true
+		}
+
+	default:
+		return nil, errRevQuery
+	}
+
+	return qm, nil
+}
+
+// allowsVersion reports whether version v is allowed by the prefix, filter, and
+// AllowedFunc of qm.
+func (qm *queryMatcher) allowsVersion(ctx context.Context, v string) bool {
+	if qm.prefix != "" && !strings.HasPrefix(v, qm.prefix) {
+		return false
+	}
+	if qm.filter != nil && !qm.filter(v) {
+		return false
+	}
+	if qm.allowed != nil {
+		if err := qm.allowed(ctx, module.Version{Path: qm.path, Version: v}); errors.Is(err, ErrDisallowed) {
+			return false
+		}
+	}
+	return true
+}
+
+// filterVersions classifies versions into releases and pre-releases, filtering
+// out:
+// 	1. versions that do not satisfy the 'allowed' predicate, and
+// 	2. "+incompatible" versions, if a compatible one satisfies the predicate
+// 	   and the incompatible version is not preferred.
+//
+// If the allowed predicate returns an error not equivalent to ErrDisallowed,
+// filterVersions returns that error.
+func (qm *queryMatcher) filterVersions(ctx context.Context, versions []string) (releases, prereleases []string, err error) {
+	needIncompatible := qm.preferIncompatible
+
+	var lastCompatible string
+	for _, v := range versions {
+		if !qm.allowsVersion(ctx, v) {
+			continue
+		}
+
+		if !needIncompatible {
+			// We're not yet sure whether we need to include +incomptaible versions.
+			// Keep track of the last compatible version we've seen, and use the
+			// presence (or absence) of a go.mod file in that version to decide: a
+			// go.mod file implies that the module author is supporting modules at a
+			// compatible version (and we should ignore +incompatible versions unless
+			// requested explicitly), while a lack of go.mod file implies the
+			// potential for legacy (pre-modules) versioning without semantic import
+			// paths (and thus *with* +incompatible versions).
+			//
+			// This isn't strictly accurate if the latest compatible version has been
+			// replaced by a local file path, because we do not allow file-path
+			// replacements without a go.mod file: the user would have needed to add
+			// one. However, replacing the last compatible version while
+			// simultaneously expecting to upgrade implicitly to a +incompatible
+			// version seems like an extreme enough corner case to ignore for now.
+
+			if !strings.HasSuffix(v, "+incompatible") {
+				lastCompatible = v
+			} else if lastCompatible != "" {
+				// If the latest compatible version is allowed and has a go.mod file,
+				// ignore any version with a higher (+incompatible) major version. (See
+				// https://golang.org/issue/34165.) Note that we even prefer a
+				// compatible pre-release over an incompatible release.
+				ok, err := versionHasGoMod(ctx, module.Version{Path: qm.path, Version: lastCompatible})
+				if err != nil {
+					return nil, nil, err
+				}
+				if ok {
+					// The last compatible version has a go.mod file, so that's the
+					// highest version we're willing to consider. Don't bother even
+					// looking at higher versions, because they're all +incompatible from
+					// here onward.
+					break
+				}
+
+				// No acceptable compatible release has a go.mod file, so the versioning
+				// for the module might not be module-aware, and we should respect
+				// legacy major-version tags.
+				needIncompatible = true
+			}
+		}
+
+		if semver.Prerelease(v) != "" {
+			prereleases = append(prereleases, v)
+		} else {
+			releases = append(releases, v)
+		}
+	}
+
+	return releases, prereleases, nil
+}
+
+type QueryResult struct {
+	Mod      module.Version
+	Rev      *modfetch.RevInfo
+	Packages []string
+}
+
+// QueryPackages is like QueryPattern, but requires that the pattern match at
+// least one package and omits the non-package result (if any).
+func QueryPackages(ctx context.Context, pattern, query string, current func(string) string, allowed AllowedFunc) ([]QueryResult, error) {
+	pkgMods, modOnly, err := QueryPattern(ctx, pattern, query, current, allowed)
+
+	if len(pkgMods) == 0 && err == nil {
+		return nil, &PackageNotInModuleError{
+			Mod:         modOnly.Mod,
+			Replacement: Replacement(modOnly.Mod),
+			Query:       query,
+			Pattern:     pattern,
+		}
+	}
+
+	return pkgMods, err
+}
+
+// QueryPattern looks up the module(s) containing at least one package matching
+// the given pattern at the given version. The results are sorted by module path
+// length in descending order. If any proxy provides a non-empty set of candidate
+// modules, no further proxies are tried.
+//
+// For wildcard patterns, QueryPattern looks in modules with package paths up to
+// the first "..." in the pattern. For the pattern "example.com/a/b.../c",
+// QueryPattern would consider prefixes of "example.com/a".
+//
+// If any matching package is in the main module, QueryPattern considers only
+// the main module and only the version "latest", without checking for other
+// possible modules.
+//
+// QueryPattern always returns at least one QueryResult (which may be only
+// modOnly) or a non-nil error.
+func QueryPattern(ctx context.Context, pattern, query string, current func(string) string, allowed AllowedFunc) (pkgMods []QueryResult, modOnly *QueryResult, err error) {
+	ctx, span := trace.StartSpan(ctx, "modload.QueryPattern "+pattern+" "+query)
+	defer span.Done()
+
+	base := pattern
+
+	firstError := func(m *search.Match) error {
+		if len(m.Errs) == 0 {
+			return nil
+		}
+		return m.Errs[0]
+	}
+
+	var match func(mod module.Version, root string, isLocal bool) *search.Match
+	matchPattern := search.MatchPattern(pattern)
+
+	if i := strings.Index(pattern, "..."); i >= 0 {
+		base = pathpkg.Dir(pattern[:i+3])
+		if base == "." {
+			return nil, nil, &WildcardInFirstElementError{Pattern: pattern, Query: query}
+		}
+		match = func(mod module.Version, root string, isLocal bool) *search.Match {
+			m := search.NewMatch(pattern)
+			matchPackages(ctx, m, imports.AnyTags(), omitStd, []module.Version{mod})
+			return m
+		}
+	} else {
+		match = func(mod module.Version, root string, isLocal bool) *search.Match {
+			m := search.NewMatch(pattern)
+			prefix := mod.Path
+			if mod == Target {
+				prefix = targetPrefix
+			}
+			if _, ok, err := dirInModule(pattern, prefix, root, isLocal); err != nil {
+				m.AddError(err)
+			} else if ok {
+				m.Pkgs = []string{pattern}
+			}
+			return m
+		}
+	}
+
+	var queryMatchesMainModule bool
+	if HasModRoot() {
+		m := match(Target, modRoot, true)
+		if len(m.Pkgs) > 0 {
+			if query != "upgrade" && query != "patch" {
+				return nil, nil, &QueryMatchesPackagesInMainModuleError{
+					Pattern:  pattern,
+					Query:    query,
+					Packages: m.Pkgs,
+				}
+			}
+			if err := allowed(ctx, Target); err != nil {
+				return nil, nil, fmt.Errorf("internal error: package %s is in the main module (%s), but version is not allowed: %w", pattern, Target.Path, err)
+			}
+			return []QueryResult{{
+				Mod:      Target,
+				Rev:      &modfetch.RevInfo{Version: Target.Version},
+				Packages: m.Pkgs,
+			}}, nil, nil
+		}
+		if err := firstError(m); err != nil {
+			return nil, nil, err
+		}
+
+		if matchPattern(Target.Path) {
+			queryMatchesMainModule = true
+		}
+
+		if (query == "upgrade" || query == "patch") && queryMatchesMainModule {
+			if err := allowed(ctx, Target); err == nil {
+				modOnly = &QueryResult{
+					Mod: Target,
+					Rev: &modfetch.RevInfo{Version: Target.Version},
+				}
+			}
+		}
+	}
+
+	var (
+		results          []QueryResult
+		candidateModules = modulePrefixesExcludingTarget(base)
+	)
+	if len(candidateModules) == 0 {
+		if modOnly != nil {
+			return nil, modOnly, nil
+		} else if queryMatchesMainModule {
+			return nil, nil, &QueryMatchesMainModuleError{
+				Pattern: pattern,
+				Query:   query,
+			}
+		} else {
+			return nil, nil, &PackageNotInModuleError{
+				Mod:     Target,
+				Query:   query,
+				Pattern: pattern,
+			}
+		}
+	}
+
+	err = modfetch.TryProxies(func(proxy string) error {
+		queryModule := func(ctx context.Context, path string) (r QueryResult, err error) {
+			ctx, span := trace.StartSpan(ctx, "modload.QueryPattern.queryModule ["+proxy+"] "+path)
+			defer span.Done()
+
+			pathCurrent := current(path)
+			r.Mod.Path = path
+			r.Rev, err = queryProxy(ctx, proxy, path, query, pathCurrent, allowed)
+			if err != nil {
+				return r, err
+			}
+			r.Mod.Version = r.Rev.Version
+			needSum := true
+			root, isLocal, err := fetch(ctx, r.Mod, needSum)
+			if err != nil {
+				return r, err
+			}
+			m := match(r.Mod, root, isLocal)
+			r.Packages = m.Pkgs
+			if len(r.Packages) == 0 && !matchPattern(path) {
+				if err := firstError(m); err != nil {
+					return r, err
+				}
+				return r, &PackageNotInModuleError{
+					Mod:         r.Mod,
+					Replacement: Replacement(r.Mod),
+					Query:       query,
+					Pattern:     pattern,
+				}
+			}
+			return r, nil
+		}
+
+		allResults, err := queryPrefixModules(ctx, candidateModules, queryModule)
+		results = allResults[:0]
+		for _, r := range allResults {
+			if len(r.Packages) == 0 {
+				modOnly = &r
+			} else {
+				results = append(results, r)
+			}
+		}
+		return err
+	})
+
+	if queryMatchesMainModule && len(results) == 0 && modOnly == nil && errors.Is(err, fs.ErrNotExist) {
+		return nil, nil, &QueryMatchesMainModuleError{
+			Pattern: pattern,
+			Query:   query,
+		}
+	}
+	return results[:len(results):len(results)], modOnly, err
+}
+
+// modulePrefixesExcludingTarget returns all prefixes of path that may plausibly
+// exist as a module, excluding targetPrefix but otherwise including path
+// itself, sorted by descending length.
+func modulePrefixesExcludingTarget(path string) []string {
+	prefixes := make([]string, 0, strings.Count(path, "/")+1)
+
+	for {
+		if path != targetPrefix {
+			if _, _, ok := module.SplitPathVersion(path); ok {
+				prefixes = append(prefixes, path)
+			}
+		}
+
+		j := strings.LastIndexByte(path, '/')
+		if j < 0 {
+			break
+		}
+		path = path[:j]
+	}
+
+	return prefixes
+}
+
+func queryPrefixModules(ctx context.Context, candidateModules []string, queryModule func(ctx context.Context, path string) (QueryResult, error)) (found []QueryResult, err error) {
+	ctx, span := trace.StartSpan(ctx, "modload.queryPrefixModules")
+	defer span.Done()
+
+	// If the path we're attempting is not in the module cache and we don't have a
+	// fetch result cached either, we'll end up making a (potentially slow)
+	// request to the proxy or (often even slower) the origin server.
+	// To minimize latency, execute all of those requests in parallel.
+	type result struct {
+		QueryResult
+		err error
+	}
+	results := make([]result, len(candidateModules))
+	var wg sync.WaitGroup
+	wg.Add(len(candidateModules))
+	for i, p := range candidateModules {
+		ctx := trace.StartGoroutine(ctx)
+		go func(p string, r *result) {
+			r.QueryResult, r.err = queryModule(ctx, p)
+			wg.Done()
+		}(p, &results[i])
+	}
+	wg.Wait()
+
+	// Classify the results. In case of failure, identify the error that the user
+	// is most likely to find helpful: the most useful class of error at the
+	// longest matching path.
+	var (
+		noPackage   *PackageNotInModuleError
+		noVersion   *NoMatchingVersionError
+		noPatchBase *NoPatchBaseError
+		notExistErr error
+	)
+	for _, r := range results {
+		switch rErr := r.err.(type) {
+		case nil:
+			found = append(found, r.QueryResult)
+		case *PackageNotInModuleError:
+			// Given the option, prefer to attribute “package not in module”
+			// to modules other than the main one.
+			if noPackage == nil || noPackage.Mod == Target {
+				noPackage = rErr
+			}
+		case *NoMatchingVersionError:
+			if noVersion == nil {
+				noVersion = rErr
+			}
+		case *NoPatchBaseError:
+			if noPatchBase == nil {
+				noPatchBase = rErr
+			}
+		default:
+			if errors.Is(rErr, fs.ErrNotExist) {
+				if notExistErr == nil {
+					notExistErr = rErr
+				}
+			} else if err == nil {
+				if len(found) > 0 || noPackage != nil {
+					// golang.org/issue/34094: If we have already found a module that
+					// could potentially contain the target package, ignore unclassified
+					// errors for modules with shorter paths.
+
+					// golang.org/issue/34383 is a special case of this: if we have
+					// already found example.com/foo/v2@v2.0.0 with a matching go.mod
+					// file, ignore the error from example.com/foo@v2.0.0.
+				} else {
+					err = r.err
+				}
+			}
+		}
+	}
+
+	// TODO(#26232): If len(found) == 0 and some of the errors are 4xx HTTP
+	// codes, have the auth package recheck the failed paths.
+	// If we obtain new credentials for any of them, re-run the above loop.
+
+	if len(found) == 0 && err == nil {
+		switch {
+		case noPackage != nil:
+			err = noPackage
+		case noVersion != nil:
+			err = noVersion
+		case noPatchBase != nil:
+			err = noPatchBase
+		case notExistErr != nil:
+			err = notExistErr
+		default:
+			panic("queryPrefixModules: no modules found, but no error detected")
+		}
+	}
+
+	return found, err
+}
+
+// A NoMatchingVersionError indicates that Query found a module at the requested
+// path, but not at any versions satisfying the query string and allow-function.
+//
+// NOTE: NoMatchingVersionError MUST NOT implement Is(fs.ErrNotExist).
+//
+// If the module came from a proxy, that proxy had to return a successful status
+// code for the versions it knows about, and thus did not have the opportunity
+// to return a non-400 status code to suppress fallback.
+type NoMatchingVersionError struct {
+	query, current string
+}
+
+func (e *NoMatchingVersionError) Error() string {
+	currentSuffix := ""
+	if (e.query == "upgrade" || e.query == "patch") && e.current != "" && e.current != "none" {
+		currentSuffix = fmt.Sprintf(" (current version is %s)", e.current)
+	}
+	return fmt.Sprintf("no matching versions for query %q", e.query) + currentSuffix
+}
+
+// A NoPatchBaseError indicates that Query was called with the query "patch"
+// but with a current version of "" or "none".
+type NoPatchBaseError struct {
+	path string
+}
+
+func (e *NoPatchBaseError) Error() string {
+	return fmt.Sprintf(`can't query version "patch" of module %s: no existing version is required`, e.path)
+}
+
+// A WildcardInFirstElementError indicates that a pattern passed to QueryPattern
+// had a wildcard in its first path element, and therefore had no pattern-prefix
+// modules to search in.
+type WildcardInFirstElementError struct {
+	Pattern string
+	Query   string
+}
+
+func (e *WildcardInFirstElementError) Error() string {
+	return fmt.Sprintf("no modules to query for %s@%s because first path element contains a wildcard", e.Pattern, e.Query)
+}
+
+// A PackageNotInModuleError indicates that QueryPattern found a candidate
+// module at the requested version, but that module did not contain any packages
+// matching the requested pattern.
+//
+// NOTE: PackageNotInModuleError MUST NOT implement Is(fs.ErrNotExist).
+//
+// If the module came from a proxy, that proxy had to return a successful status
+// code for the versions it knows about, and thus did not have the opportunity
+// to return a non-400 status code to suppress fallback.
+type PackageNotInModuleError struct {
+	Mod         module.Version
+	Replacement module.Version
+	Query       string
+	Pattern     string
+}
+
+func (e *PackageNotInModuleError) Error() string {
+	if e.Mod == Target {
+		if strings.Contains(e.Pattern, "...") {
+			return fmt.Sprintf("main module (%s) does not contain packages matching %s", Target.Path, e.Pattern)
+		}
+		return fmt.Sprintf("main module (%s) does not contain package %s", Target.Path, e.Pattern)
+	}
+
+	found := ""
+	if r := e.Replacement; r.Path != "" {
+		replacement := r.Path
+		if r.Version != "" {
+			replacement = fmt.Sprintf("%s@%s", r.Path, r.Version)
+		}
+		if e.Query == e.Mod.Version {
+			found = fmt.Sprintf(" (replaced by %s)", replacement)
+		} else {
+			found = fmt.Sprintf(" (%s, replaced by %s)", e.Mod.Version, replacement)
+		}
+	} else if e.Query != e.Mod.Version {
+		found = fmt.Sprintf(" (%s)", e.Mod.Version)
+	}
+
+	if strings.Contains(e.Pattern, "...") {
+		return fmt.Sprintf("module %s@%s found%s, but does not contain packages matching %s", e.Mod.Path, e.Query, found, e.Pattern)
+	}
+	return fmt.Sprintf("module %s@%s found%s, but does not contain package %s", e.Mod.Path, e.Query, found, e.Pattern)
+}
+
+func (e *PackageNotInModuleError) ImportPath() string {
+	if !strings.Contains(e.Pattern, "...") {
+		return e.Pattern
+	}
+	return ""
+}
+
+// ModuleHasRootPackage returns whether module m contains a package m.Path.
+func ModuleHasRootPackage(ctx context.Context, m module.Version) (bool, error) {
+	needSum := false
+	root, isLocal, err := fetch(ctx, m, needSum)
+	if err != nil {
+		return false, err
+	}
+	_, ok, err := dirInModule(m.Path, m.Path, root, isLocal)
+	return ok, err
+}
+
+func versionHasGoMod(ctx context.Context, m module.Version) (bool, error) {
+	needSum := false
+	root, _, err := fetch(ctx, m, needSum)
+	if err != nil {
+		return false, err
+	}
+	fi, err := os.Stat(filepath.Join(root, "go.mod"))
+	return err == nil && !fi.IsDir(), nil
+}
+
+// A versionRepo is a subset of modfetch.Repo that can report information about
+// available versions, but cannot fetch specific source files.
+type versionRepo interface {
+	ModulePath() string
+	Versions(prefix string) ([]string, error)
+	Stat(rev string) (*modfetch.RevInfo, error)
+	Latest() (*modfetch.RevInfo, error)
+}
+
+var _ versionRepo = modfetch.Repo(nil)
+
+func lookupRepo(proxy, path string) (repo versionRepo, err error) {
+	err = module.CheckPath(path)
+	if err == nil {
+		repo = modfetch.Lookup(proxy, path)
+	} else {
+		repo = emptyRepo{path: path, err: err}
+	}
+
+	if index == nil {
+		return repo, err
+	}
+	if _, ok := index.highestReplaced[path]; !ok {
+		return repo, err
+	}
+
+	return &replacementRepo{repo: repo}, nil
+}
+
+// An emptyRepo is a versionRepo that contains no versions.
+type emptyRepo struct {
+	path string
+	err  error
+}
+
+var _ versionRepo = emptyRepo{}
+
+func (er emptyRepo) ModulePath() string                         { return er.path }
+func (er emptyRepo) Versions(prefix string) ([]string, error)   { return nil, nil }
+func (er emptyRepo) Stat(rev string) (*modfetch.RevInfo, error) { return nil, er.err }
+func (er emptyRepo) Latest() (*modfetch.RevInfo, error)         { return nil, er.err }
+
+// A replacementRepo augments a versionRepo to include the replacement versions
+// (if any) found in the main module's go.mod file.
+//
+// A replacementRepo suppresses "not found" errors for otherwise-nonexistent
+// modules, so a replacementRepo should only be constructed for a module that
+// actually has one or more valid replacements.
+type replacementRepo struct {
+	repo versionRepo
+}
+
+var _ versionRepo = (*replacementRepo)(nil)
+
+func (rr *replacementRepo) ModulePath() string { return rr.repo.ModulePath() }
+
+// Versions returns the versions from rr.repo augmented with any matching
+// replacement versions.
+func (rr *replacementRepo) Versions(prefix string) ([]string, error) {
+	repoVersions, err := rr.repo.Versions(prefix)
+	if err != nil && !errors.Is(err, os.ErrNotExist) {
+		return nil, err
+	}
+
+	versions := repoVersions
+	if index != nil && len(index.replace) > 0 {
+		path := rr.ModulePath()
+		for m, _ := range index.replace {
+			if m.Path == path && strings.HasPrefix(m.Version, prefix) && m.Version != "" && !modfetch.IsPseudoVersion(m.Version) {
+				versions = append(versions, m.Version)
+			}
+		}
+	}
+
+	if len(versions) == len(repoVersions) { // No replacement versions added.
+		return versions, nil
+	}
+
+	sort.Slice(versions, func(i, j int) bool {
+		return semver.Compare(versions[i], versions[j]) < 0
+	})
+	str.Uniq(&versions)
+	return versions, nil
+}
+
+func (rr *replacementRepo) Stat(rev string) (*modfetch.RevInfo, error) {
+	info, err := rr.repo.Stat(rev)
+	if err == nil || index == nil || len(index.replace) == 0 {
+		return info, err
+	}
+
+	v := module.CanonicalVersion(rev)
+	if v != rev {
+		// The replacements in the go.mod file list only canonical semantic versions,
+		// so a non-canonical version can't possibly have a replacement.
+		return info, err
+	}
+
+	path := rr.ModulePath()
+	_, pathMajor, ok := module.SplitPathVersion(path)
+	if ok && pathMajor == "" {
+		if err := module.CheckPathMajor(v, pathMajor); err != nil && semver.Build(v) == "" {
+			v += "+incompatible"
+		}
+	}
+
+	if r := Replacement(module.Version{Path: path, Version: v}); r.Path == "" {
+		return info, err
+	}
+	return rr.replacementStat(v)
+}
+
+func (rr *replacementRepo) Latest() (*modfetch.RevInfo, error) {
+	info, err := rr.repo.Latest()
+
+	if index != nil {
+		path := rr.ModulePath()
+		if v, ok := index.highestReplaced[path]; ok {
+			if v == "" {
+				// The only replacement is a wildcard that doesn't specify a version, so
+				// synthesize a pseudo-version with an appropriate major version and a
+				// timestamp below any real timestamp. That way, if the main module is
+				// used from within some other module, the user will be able to upgrade
+				// the requirement to any real version they choose.
+				if _, pathMajor, ok := module.SplitPathVersion(path); ok && len(pathMajor) > 0 {
+					v = modfetch.PseudoVersion(pathMajor[1:], "", time.Time{}, "000000000000")
+				} else {
+					v = modfetch.PseudoVersion("v0", "", time.Time{}, "000000000000")
+				}
+			}
+
+			if err != nil || semver.Compare(v, info.Version) > 0 {
+				return rr.replacementStat(v)
+			}
+		}
+	}
+
+	return info, err
+}
+
+func (rr *replacementRepo) replacementStat(v string) (*modfetch.RevInfo, error) {
+	rev := &modfetch.RevInfo{Version: v}
+	if modfetch.IsPseudoVersion(v) {
+		rev.Time, _ = modfetch.PseudoVersionTime(v)
+		rev.Short, _ = modfetch.PseudoVersionRev(v)
+	}
+	return rev, nil
+}
+
+// A QueryMatchesMainModuleError indicates that a query requests
+// a version of the main module that cannot be satisfied.
+// (The main module's version cannot be changed.)
+type QueryMatchesMainModuleError struct {
+	Pattern string
+	Query   string
+}
+
+func (e *QueryMatchesMainModuleError) Error() string {
+	if e.Pattern == Target.Path {
+		return fmt.Sprintf("can't request version %q of the main module (%s)", e.Query, e.Pattern)
+	}
+
+	return fmt.Sprintf("can't request version %q of pattern %q that includes the main module (%s)", e.Query, e.Pattern, Target.Path)
+}
+
+// A QueryMatchesPackagesInMainModuleError indicates that a query cannot be
+// satisfied because it matches one or more packages found in the main module.
+type QueryMatchesPackagesInMainModuleError struct {
+	Pattern  string
+	Query    string
+	Packages []string
+}
+
+func (e *QueryMatchesPackagesInMainModuleError) Error() string {
+	if len(e.Packages) > 1 {
+		return fmt.Sprintf("pattern %s matches %d packages in the main module, so can't request version %s", e.Pattern, len(e.Packages), e.Query)
+	}
+
+	if search.IsMetaPackage(e.Pattern) || strings.Contains(e.Pattern, "...") {
+		return fmt.Sprintf("pattern %s matches package %s in the main module, so can't request version %s", e.Pattern, e.Packages[0], e.Query)
+	}
+
+	return fmt.Sprintf("package %s is in the main module, so can't request version %s", e.Packages[0], e.Query)
+}
diff --git a/src/cmd/go/internal/modload/query_test.go b/src/cmd/go/internal/modload/query_test.go
new file mode 100644
index 0000000..e225a0e
--- /dev/null
+++ b/src/cmd/go/internal/modload/query_test.go
@@ -0,0 +1,216 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"context"
+	"internal/testenv"
+	"log"
+	"os"
+	"path"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"cmd/go/internal/cfg"
+
+	"golang.org/x/mod/module"
+)
+
+func TestMain(m *testing.M) {
+	os.Exit(testMain(m))
+}
+
+func testMain(m *testing.M) int {
+	cfg.GOPROXY = "direct"
+
+	dir, err := os.MkdirTemp("", "modload-test-")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+
+	os.Setenv("GOPATH", dir)
+	cfg.BuildContext.GOPATH = dir
+	cfg.GOMODCACHE = filepath.Join(dir, "pkg/mod")
+	return m.Run()
+}
+
+var (
+	queryRepo   = "vcs-test.golang.org/git/querytest.git"
+	queryRepoV2 = queryRepo + "/v2"
+	queryRepoV3 = queryRepo + "/v3"
+
+	// Empty version list (no semver tags), not actually empty.
+	emptyRepoPath = "vcs-test.golang.org/git/emptytest.git"
+)
+
+var queryTests = []struct {
+	path    string
+	query   string
+	current string
+	allow   string
+	vers    string
+	err     string
+}{
+	/*
+		git init
+		echo module vcs-test.golang.org/git/querytest.git >go.mod
+		git add go.mod
+		git commit -m v1 go.mod
+		git tag start
+		for i in v0.0.0-pre1 v0.0.0 v0.0.1 v0.0.2 v0.0.3 v0.1.0 v0.1.1 v0.1.2 v0.3.0 v1.0.0 v1.1.0 v1.9.0 v1.9.9 v1.9.10-pre1 v1.9.10-pre2+metadata unversioned; do
+			echo before $i >status
+			git add status
+			git commit -m "before $i" status
+			echo at $i >status
+			git commit -m "at $i" status
+			git tag $i
+		done
+		git tag favorite v0.0.3
+
+		git branch v2 start
+		git checkout v2
+		echo module vcs-test.golang.org/git/querytest.git/v2 >go.mod
+		git commit -m v2 go.mod
+		for i in v2.0.0 v2.1.0 v2.2.0 v2.5.5 v2.6.0-pre1; do
+			echo before $i >status
+			git add status
+			git commit -m "before $i" status
+			echo at $i >status
+			git commit -m "at $i" status
+			git tag $i
+		done
+		git checkout v2.5.5
+		echo after v2.5.5 >status
+		git commit -m 'after v2.5.5' status
+		git checkout master
+		zip -r ../querytest.zip
+		gsutil cp ../querytest.zip gs://vcs-test/git/querytest.zip
+		curl 'https://vcs-test.golang.org/git/querytest?go-get=1'
+	*/
+	{path: queryRepo, query: "<v0.0.0", vers: "v0.0.0-pre1"},
+	{path: queryRepo, query: "<v0.0.0-pre1", err: `no matching versions for query "<v0.0.0-pre1"`},
+	{path: queryRepo, query: "<=v0.0.0", vers: "v0.0.0"},
+	{path: queryRepo, query: ">v0.0.0", vers: "v0.0.1"},
+	{path: queryRepo, query: ">=v0.0.0", vers: "v0.0.0"},
+	{path: queryRepo, query: "v0.0.1", vers: "v0.0.1"},
+	{path: queryRepo, query: "v0.0.1+foo", vers: "v0.0.1"},
+	{path: queryRepo, query: "v0.0.99", err: `vcs-test.golang.org/git/querytest.git@v0.0.99: invalid version: unknown revision v0.0.99`},
+	{path: queryRepo, query: "v0", vers: "v0.3.0"},
+	{path: queryRepo, query: "v0.1", vers: "v0.1.2"},
+	{path: queryRepo, query: "v0.2", err: `no matching versions for query "v0.2"`},
+	{path: queryRepo, query: "v0.0", vers: "v0.0.3"},
+	{path: queryRepo, query: "v1.9.10-pre2+metadata", vers: "v1.9.10-pre2.0.20190513201126-42abcb6df8ee"},
+	{path: queryRepo, query: "ed5ffdaa", vers: "v1.9.10-pre2.0.20191220134614-ed5ffdaa1f5e"},
+
+	// golang.org/issue/29262: The major version for for a module without a suffix
+	// should be based on the most recent tag (v1 as appropriate, not v0
+	// unconditionally).
+	{path: queryRepo, query: "42abcb6df8ee", vers: "v1.9.10-pre2.0.20190513201126-42abcb6df8ee"},
+
+	{path: queryRepo, query: "v1.9.10-pre2+wrongmetadata", err: `vcs-test.golang.org/git/querytest.git@v1.9.10-pre2+wrongmetadata: invalid version: unknown revision v1.9.10-pre2+wrongmetadata`},
+	{path: queryRepo, query: "v1.9.10-pre2", err: `vcs-test.golang.org/git/querytest.git@v1.9.10-pre2: invalid version: unknown revision v1.9.10-pre2`},
+	{path: queryRepo, query: "latest", vers: "v1.9.9"},
+	{path: queryRepo, query: "latest", current: "v1.9.10-pre1", vers: "v1.9.9"},
+	{path: queryRepo, query: "upgrade", vers: "v1.9.9"},
+	{path: queryRepo, query: "upgrade", current: "v1.9.10-pre1", vers: "v1.9.10-pre1"},
+	{path: queryRepo, query: "upgrade", current: "v1.9.10-pre2+metadata", vers: "v1.9.10-pre2.0.20190513201126-42abcb6df8ee"},
+	{path: queryRepo, query: "upgrade", current: "v0.0.0-20190513201126-42abcb6df8ee", vers: "v0.0.0-20190513201126-42abcb6df8ee"},
+	{path: queryRepo, query: "upgrade", allow: "NOMATCH", err: `no matching versions for query "upgrade"`},
+	{path: queryRepo, query: "upgrade", current: "v1.9.9", allow: "NOMATCH", err: `vcs-test.golang.org/git/querytest.git@v1.9.9: disallowed module version`},
+	{path: queryRepo, query: "upgrade", current: "v1.99.99", err: `vcs-test.golang.org/git/querytest.git@v1.99.99: invalid version: unknown revision v1.99.99`},
+	{path: queryRepo, query: "patch", current: "", vers: "v1.9.9"},
+	{path: queryRepo, query: "patch", current: "v0.1.0", vers: "v0.1.2"},
+	{path: queryRepo, query: "patch", current: "v1.9.0", vers: "v1.9.9"},
+	{path: queryRepo, query: "patch", current: "v1.9.10-pre1", vers: "v1.9.10-pre1"},
+	{path: queryRepo, query: "patch", current: "v1.9.10-pre2+metadata", vers: "v1.9.10-pre2.0.20190513201126-42abcb6df8ee"},
+	{path: queryRepo, query: "patch", current: "v1.99.99", err: `vcs-test.golang.org/git/querytest.git@v1.99.99: invalid version: unknown revision v1.99.99`},
+	{path: queryRepo, query: ">v1.9.9", vers: "v1.9.10-pre1"},
+	{path: queryRepo, query: ">v1.10.0", err: `no matching versions for query ">v1.10.0"`},
+	{path: queryRepo, query: ">=v1.10.0", err: `no matching versions for query ">=v1.10.0"`},
+	{path: queryRepo, query: "6cf84eb", vers: "v0.0.2-0.20180704023347-6cf84ebaea54"},
+
+	// golang.org/issue/27173: A pseudo-version may be based on the highest tag on
+	// any parent commit, or any existing semantically-lower tag: a given commit
+	// could have been a pre-release for a backport tag at any point.
+	{path: queryRepo, query: "3ef0cec634e0", vers: "v0.1.2-0.20180704023347-3ef0cec634e0"},
+	{path: queryRepo, query: "v0.1.2-0.20180704023347-3ef0cec634e0", vers: "v0.1.2-0.20180704023347-3ef0cec634e0"},
+	{path: queryRepo, query: "v0.1.1-0.20180704023347-3ef0cec634e0", vers: "v0.1.1-0.20180704023347-3ef0cec634e0"},
+	{path: queryRepo, query: "v0.0.4-0.20180704023347-3ef0cec634e0", vers: "v0.0.4-0.20180704023347-3ef0cec634e0"},
+
+	// Invalid tags are tested in cmd/go/testdata/script/mod_pseudo_invalid.txt.
+
+	{path: queryRepo, query: "start", vers: "v0.0.0-20180704023101-5e9e31667ddf"},
+	{path: queryRepo, query: "5e9e31667ddf", vers: "v0.0.0-20180704023101-5e9e31667ddf"},
+	{path: queryRepo, query: "v0.0.0-20180704023101-5e9e31667ddf", vers: "v0.0.0-20180704023101-5e9e31667ddf"},
+
+	{path: queryRepo, query: "7a1b6bf", vers: "v0.1.0"},
+
+	{path: queryRepoV2, query: "<v0.0.0", err: `no matching versions for query "<v0.0.0"`},
+	{path: queryRepoV2, query: "<=v0.0.0", err: `no matching versions for query "<=v0.0.0"`},
+	{path: queryRepoV2, query: ">v0.0.0", vers: "v2.0.0"},
+	{path: queryRepoV2, query: ">=v0.0.0", vers: "v2.0.0"},
+
+	{path: queryRepoV2, query: "v2", vers: "v2.5.5"},
+	{path: queryRepoV2, query: "v2.5", vers: "v2.5.5"},
+	{path: queryRepoV2, query: "v2.6", err: `no matching versions for query "v2.6"`},
+	{path: queryRepoV2, query: "v2.6.0-pre1", vers: "v2.6.0-pre1"},
+	{path: queryRepoV2, query: "latest", vers: "v2.5.5"},
+
+	// Commit e0cf3de987e6 is actually v1.19.10-pre1, not anything resembling v3,
+	// and it has a go.mod file with a non-v3 module path. Attempting to query it
+	// as the v3 module should fail.
+	{path: queryRepoV3, query: "e0cf3de987e6", err: `vcs-test.golang.org/git/querytest.git/v3@v3.0.0-20180704024501-e0cf3de987e6: invalid version: go.mod has non-.../v3 module path "vcs-test.golang.org/git/querytest.git" (and .../v3/go.mod does not exist) at revision e0cf3de987e6`},
+
+	// The querytest repo does not have any commits tagged with major version 3,
+	// and the latest commit in the repo has a go.mod file specifying a non-v3 path.
+	// That should prevent us from resolving any version for the /v3 path.
+	{path: queryRepoV3, query: "latest", err: `no matching versions for query "latest"`},
+
+	{path: emptyRepoPath, query: "latest", vers: "v0.0.0-20180704023549-7bb914627242"},
+	{path: emptyRepoPath, query: ">v0.0.0", err: `no matching versions for query ">v0.0.0"`},
+	{path: emptyRepoPath, query: "<v10.0.0", err: `no matching versions for query "<v10.0.0"`},
+}
+
+func TestQuery(t *testing.T) {
+	testenv.MustHaveExternalNetwork(t)
+	testenv.MustHaveExecPath(t, "git")
+
+	ctx := context.Background()
+
+	for _, tt := range queryTests {
+		allow := tt.allow
+		if allow == "" {
+			allow = "*"
+		}
+		allowed := func(ctx context.Context, m module.Version) error {
+			if ok, _ := path.Match(allow, m.Version); !ok {
+				return module.VersionError(m, ErrDisallowed)
+			}
+			return nil
+		}
+		tt := tt
+		t.Run(strings.ReplaceAll(tt.path, "/", "_")+"/"+tt.query+"/"+tt.current+"/"+allow, func(t *testing.T) {
+			t.Parallel()
+
+			info, err := Query(ctx, tt.path, tt.query, tt.current, allowed)
+			if tt.err != "" {
+				if err == nil {
+					t.Errorf("Query(_, %q, %q, %q, %v) = %v, want error %q", tt.path, tt.query, tt.current, allow, info.Version, tt.err)
+				} else if err.Error() != tt.err {
+					t.Errorf("Query(_, %q, %q, %q, %v): %v\nwant error %q", tt.path, tt.query, tt.current, allow, err, tt.err)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("Query(_, %q, %q, %q, %v): %v\nwant %v", tt.path, tt.query, tt.current, allow, err, tt.vers)
+			}
+			if info.Version != tt.vers {
+				t.Errorf("Query(_, %q, %q, %q, %v) = %v, want %v", tt.path, tt.query, tt.current, allow, info.Version, tt.vers)
+			}
+		})
+	}
+}
diff --git a/src/cmd/go/internal/modload/search.go b/src/cmd/go/internal/modload/search.go
new file mode 100644
index 0000000..1fe742d
--- /dev/null
+++ b/src/cmd/go/internal/modload/search.go
@@ -0,0 +1,219 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"context"
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
+	"cmd/go/internal/imports"
+	"cmd/go/internal/search"
+
+	"golang.org/x/mod/module"
+)
+
+type stdFilter int8
+
+const (
+	omitStd = stdFilter(iota)
+	includeStd
+)
+
+// matchPackages is like m.MatchPackages, but uses a local variable (rather than
+// a global) for tags, can include or exclude packages in the standard library,
+// and is restricted to the given list of modules.
+func matchPackages(ctx context.Context, m *search.Match, tags map[string]bool, filter stdFilter, modules []module.Version) {
+	m.Pkgs = []string{}
+
+	isMatch := func(string) bool { return true }
+	treeCanMatch := func(string) bool { return true }
+	if !m.IsMeta() {
+		isMatch = search.MatchPattern(m.Pattern())
+		treeCanMatch = search.TreeCanMatchPattern(m.Pattern())
+	}
+
+	have := map[string]bool{
+		"builtin": true, // ignore pseudo-package that exists only for documentation
+	}
+	if !cfg.BuildContext.CgoEnabled {
+		have["runtime/cgo"] = true // ignore during walk
+	}
+
+	type pruning int8
+	const (
+		pruneVendor = pruning(1 << iota)
+		pruneGoMod
+	)
+
+	walkPkgs := func(root, importPathRoot string, prune pruning) {
+		root = filepath.Clean(root)
+		err := fsys.Walk(root, func(path string, fi fs.FileInfo, err error) error {
+			if err != nil {
+				m.AddError(err)
+				return nil
+			}
+
+			want := true
+			elem := ""
+
+			// Don't use GOROOT/src but do walk down into it.
+			if path == root {
+				if importPathRoot == "" {
+					return nil
+				}
+			} else {
+				// Avoid .foo, _foo, and testdata subdirectory trees.
+				_, elem = filepath.Split(path)
+				if strings.HasPrefix(elem, ".") || strings.HasPrefix(elem, "_") || elem == "testdata" {
+					want = false
+				}
+			}
+
+			name := importPathRoot + filepath.ToSlash(path[len(root):])
+			if importPathRoot == "" {
+				name = name[1:] // cut leading slash
+			}
+			if !treeCanMatch(name) {
+				want = false
+			}
+
+			if !fi.IsDir() {
+				if fi.Mode()&fs.ModeSymlink != 0 && want {
+					if target, err := fsys.Stat(path); err == nil && target.IsDir() {
+						fmt.Fprintf(os.Stderr, "warning: ignoring symlink %s\n", path)
+					}
+				}
+				return nil
+			}
+
+			if !want {
+				return filepath.SkipDir
+			}
+			// Stop at module boundaries.
+			if (prune&pruneGoMod != 0) && path != root {
+				if fi, err := os.Stat(filepath.Join(path, "go.mod")); err == nil && !fi.IsDir() {
+					return filepath.SkipDir
+				}
+			}
+
+			if !have[name] {
+				have[name] = true
+				if isMatch(name) {
+					if _, _, err := scanDir(path, tags); err != imports.ErrNoGo {
+						m.Pkgs = append(m.Pkgs, name)
+					}
+				}
+			}
+
+			if elem == "vendor" && (prune&pruneVendor != 0) {
+				return filepath.SkipDir
+			}
+			return nil
+		})
+		if err != nil {
+			m.AddError(err)
+		}
+	}
+
+	if filter == includeStd {
+		walkPkgs(cfg.GOROOTsrc, "", pruneGoMod)
+		if treeCanMatch("cmd") {
+			walkPkgs(filepath.Join(cfg.GOROOTsrc, "cmd"), "cmd", pruneGoMod)
+		}
+	}
+
+	if cfg.BuildMod == "vendor" {
+		if HasModRoot() {
+			walkPkgs(ModRoot(), targetPrefix, pruneGoMod|pruneVendor)
+			walkPkgs(filepath.Join(ModRoot(), "vendor"), "", pruneVendor)
+		}
+		return
+	}
+
+	for _, mod := range modules {
+		if !treeCanMatch(mod.Path) {
+			continue
+		}
+
+		var (
+			root, modPrefix string
+			isLocal         bool
+		)
+		if mod == Target {
+			if !HasModRoot() {
+				continue // If there is no main module, we can't search in it.
+			}
+			root = ModRoot()
+			modPrefix = targetPrefix
+			isLocal = true
+		} else {
+			var err error
+			const needSum = true
+			root, isLocal, err = fetch(ctx, mod, needSum)
+			if err != nil {
+				m.AddError(err)
+				continue
+			}
+			modPrefix = mod.Path
+		}
+
+		prune := pruneVendor
+		if isLocal {
+			prune |= pruneGoMod
+		}
+		walkPkgs(root, modPrefix, prune)
+	}
+
+	return
+}
+
+// MatchInModule identifies the packages matching the given pattern within the
+// given module version, which does not need to be in the build list or module
+// requirement graph.
+//
+// If m is the zero module.Version, MatchInModule matches the pattern
+// against the standard library (std and cmd) in GOROOT/src.
+func MatchInModule(ctx context.Context, pattern string, m module.Version, tags map[string]bool) *search.Match {
+	match := search.NewMatch(pattern)
+	if m == (module.Version{}) {
+		matchPackages(ctx, match, tags, includeStd, nil)
+	}
+
+	LoadModFile(ctx)
+
+	if !match.IsLiteral() {
+		matchPackages(ctx, match, tags, omitStd, []module.Version{m})
+		return match
+	}
+
+	const needSum = true
+	root, isLocal, err := fetch(ctx, m, needSum)
+	if err != nil {
+		match.Errs = []error{err}
+		return match
+	}
+
+	dir, haveGoFiles, err := dirInModule(pattern, m.Path, root, isLocal)
+	if err != nil {
+		match.Errs = []error{err}
+		return match
+	}
+	if haveGoFiles {
+		if _, _, err := scanDir(dir, tags); err != imports.ErrNoGo {
+			// ErrNoGo indicates that the directory is not actually a Go package,
+			// perhaps due to the tags in use. Any other non-nil error indicates a
+			// problem with one or more of the Go source files, but such an error does
+			// not stop the package from existing, so it has no impact on matching.
+			match.Pkgs = []string{pattern}
+		}
+	}
+	return match
+}
diff --git a/src/cmd/go/internal/modload/stat_openfile.go b/src/cmd/go/internal/modload/stat_openfile.go
new file mode 100644
index 0000000..5842b85
--- /dev/null
+++ b/src/cmd/go/internal/modload/stat_openfile.go
@@ -0,0 +1,28 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build js,wasm plan9
+
+// On plan9, per http://9p.io/magic/man2html/2/access: “Since file permissions
+// are checked by the server and group information is not known to the client,
+// access must open the file to check permissions.”
+//
+// aix and js,wasm are similar, in that they do not define syscall.Access.
+
+package modload
+
+import (
+	"io/fs"
+	"os"
+)
+
+// hasWritePerm reports whether the current user has permission to write to the
+// file with the given info.
+func hasWritePerm(path string, _ fs.FileInfo) bool {
+	if f, err := os.OpenFile(path, os.O_WRONLY, 0); err == nil {
+		f.Close()
+		return true
+	}
+	return false
+}
diff --git a/src/cmd/go/internal/modload/stat_unix.go b/src/cmd/go/internal/modload/stat_unix.go
new file mode 100644
index 0000000..f49278e
--- /dev/null
+++ b/src/cmd/go/internal/modload/stat_unix.go
@@ -0,0 +1,32 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package modload
+
+import (
+	"io/fs"
+	"os"
+	"syscall"
+)
+
+// hasWritePerm reports whether the current user has permission to write to the
+// file with the given info.
+//
+// Although the root user on most Unix systems can write to files even without
+// permission, hasWritePerm reports false if no appropriate permission bit is
+// set even if the current user is root.
+func hasWritePerm(path string, fi fs.FileInfo) bool {
+	if os.Getuid() == 0 {
+		// The root user can access any file, but we still want to default to
+		// read-only mode if the go.mod file is marked as globally non-writable.
+		// (If the user really intends not to be in readonly mode, they can
+		// pass -mod=mod explicitly.)
+		return fi.Mode()&0222 != 0
+	}
+
+	const W_OK = 0x2
+	return syscall.Access(path, W_OK) == nil
+}
diff --git a/src/cmd/go/internal/modload/stat_windows.go b/src/cmd/go/internal/modload/stat_windows.go
new file mode 100644
index 0000000..0ac2391
--- /dev/null
+++ b/src/cmd/go/internal/modload/stat_windows.go
@@ -0,0 +1,21 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build windows
+
+package modload
+
+import "io/fs"
+
+// hasWritePerm reports whether the current user has permission to write to the
+// file with the given info.
+func hasWritePerm(_ string, fi fs.FileInfo) bool {
+	// Windows has a read-only attribute independent of ACLs, so use that to
+	// determine whether the file is intended to be overwritten.
+	//
+	// Per https://golang.org/pkg/os/#Chmod:
+	// “On Windows, only the 0200 bit (owner writable) of mode is used; it
+	// controls whether the file's read-only attribute is set or cleared.”
+	return fi.Mode()&0200 != 0
+}
diff --git a/src/cmd/go/internal/modload/vendor.go b/src/cmd/go/internal/modload/vendor.go
new file mode 100644
index 0000000..d8fd91f
--- /dev/null
+++ b/src/cmd/go/internal/modload/vendor.go
@@ -0,0 +1,219 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package modload
+
+import (
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+
+	"cmd/go/internal/base"
+
+	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
+)
+
+var (
+	vendorOnce      sync.Once
+	vendorList      []module.Version          // modules that contribute packages to the build, in order of appearance
+	vendorReplaced  []module.Version          // all replaced modules; may or may not also contribute packages
+	vendorVersion   map[string]string         // module path → selected version (if known)
+	vendorPkgModule map[string]module.Version // package → containing module
+	vendorMeta      map[module.Version]vendorMetadata
+)
+
+type vendorMetadata struct {
+	Explicit    bool
+	Replacement module.Version
+}
+
+// readVendorList reads the list of vendored modules from vendor/modules.txt.
+func readVendorList() {
+	vendorOnce.Do(func() {
+		vendorList = nil
+		vendorPkgModule = make(map[string]module.Version)
+		vendorVersion = make(map[string]string)
+		vendorMeta = make(map[module.Version]vendorMetadata)
+		data, err := os.ReadFile(filepath.Join(ModRoot(), "vendor/modules.txt"))
+		if err != nil {
+			if !errors.Is(err, fs.ErrNotExist) {
+				base.Fatalf("go: %s", err)
+			}
+			return
+		}
+
+		var mod module.Version
+		for _, line := range strings.Split(string(data), "\n") {
+			if strings.HasPrefix(line, "# ") {
+				f := strings.Fields(line)
+
+				if len(f) < 3 {
+					continue
+				}
+				if semver.IsValid(f[2]) {
+					// A module, but we don't yet know whether it is in the build list or
+					// only included to indicate a replacement.
+					mod = module.Version{Path: f[1], Version: f[2]}
+					f = f[3:]
+				} else if f[2] == "=>" {
+					// A wildcard replacement found in the main module's go.mod file.
+					mod = module.Version{Path: f[1]}
+					f = f[2:]
+				} else {
+					// Not a version or a wildcard replacement.
+					// We don't know how to interpret this module line, so ignore it.
+					mod = module.Version{}
+					continue
+				}
+
+				if len(f) >= 2 && f[0] == "=>" {
+					meta := vendorMeta[mod]
+					if len(f) == 2 {
+						// File replacement.
+						meta.Replacement = module.Version{Path: f[1]}
+						vendorReplaced = append(vendorReplaced, mod)
+					} else if len(f) == 3 && semver.IsValid(f[2]) {
+						// Path and version replacement.
+						meta.Replacement = module.Version{Path: f[1], Version: f[2]}
+						vendorReplaced = append(vendorReplaced, mod)
+					} else {
+						// We don't understand this replacement. Ignore it.
+					}
+					vendorMeta[mod] = meta
+				}
+				continue
+			}
+
+			// Not a module line. Must be a package within a module or a metadata
+			// directive, either of which requires a preceding module line.
+			if mod.Path == "" {
+				continue
+			}
+
+			if strings.HasPrefix(line, "## ") {
+				// Metadata. Take the union of annotations across multiple lines, if present.
+				meta := vendorMeta[mod]
+				for _, entry := range strings.Split(strings.TrimPrefix(line, "## "), ";") {
+					entry = strings.TrimSpace(entry)
+					if entry == "explicit" {
+						meta.Explicit = true
+					}
+					// All other tokens are reserved for future use.
+				}
+				vendorMeta[mod] = meta
+				continue
+			}
+
+			if f := strings.Fields(line); len(f) == 1 && module.CheckImportPath(f[0]) == nil {
+				// A package within the current module.
+				vendorPkgModule[f[0]] = mod
+
+				// Since this module provides a package for the build, we know that it
+				// is in the build list and is the selected version of its path.
+				// If this information is new, record it.
+				if v, ok := vendorVersion[mod.Path]; !ok || semver.Compare(v, mod.Version) < 0 {
+					vendorList = append(vendorList, mod)
+					vendorVersion[mod.Path] = mod.Version
+				}
+			}
+		}
+	})
+}
+
+// checkVendorConsistency verifies that the vendor/modules.txt file matches (if
+// go 1.14) or at least does not contradict (go 1.13 or earlier) the
+// requirements and replacements listed in the main module's go.mod file.
+func checkVendorConsistency() {
+	readVendorList()
+
+	pre114 := false
+	if semver.Compare(index.goVersionV, "v1.14") < 0 {
+		// Go versions before 1.14 did not include enough information in
+		// vendor/modules.txt to check for consistency.
+		// If we know that we're on an earlier version, relax the consistency check.
+		pre114 = true
+	}
+
+	vendErrors := new(strings.Builder)
+	vendErrorf := func(mod module.Version, format string, args ...interface{}) {
+		detail := fmt.Sprintf(format, args...)
+		if mod.Version == "" {
+			fmt.Fprintf(vendErrors, "\n\t%s: %s", mod.Path, detail)
+		} else {
+			fmt.Fprintf(vendErrors, "\n\t%s@%s: %s", mod.Path, mod.Version, detail)
+		}
+	}
+
+	// Iterate over the Require directives in their original (not indexed) order
+	// so that the errors match the original file.
+	for _, r := range modFile.Require {
+		if !vendorMeta[r.Mod].Explicit {
+			if pre114 {
+				// Before 1.14, modules.txt did not indicate whether modules were listed
+				// explicitly in the main module's go.mod file.
+				// However, we can at least detect a version mismatch if packages were
+				// vendored from a non-matching version.
+				if vv, ok := vendorVersion[r.Mod.Path]; ok && vv != r.Mod.Version {
+					vendErrorf(r.Mod, fmt.Sprintf("is explicitly required in go.mod, but vendor/modules.txt indicates %s@%s", r.Mod.Path, vv))
+				}
+			} else {
+				vendErrorf(r.Mod, "is explicitly required in go.mod, but not marked as explicit in vendor/modules.txt")
+			}
+		}
+	}
+
+	describe := func(m module.Version) string {
+		if m.Version == "" {
+			return m.Path
+		}
+		return m.Path + "@" + m.Version
+	}
+
+	// We need to verify *all* replacements that occur in modfile: even if they
+	// don't directly apply to any module in the vendor list, the replacement
+	// go.mod file can affect the selected versions of other (transitive)
+	// dependencies
+	for _, r := range modFile.Replace {
+		vr := vendorMeta[r.Old].Replacement
+		if vr == (module.Version{}) {
+			if pre114 && (r.Old.Version == "" || vendorVersion[r.Old.Path] != r.Old.Version) {
+				// Before 1.14, modules.txt omitted wildcard replacements and
+				// replacements for modules that did not have any packages to vendor.
+			} else {
+				vendErrorf(r.Old, "is replaced in go.mod, but not marked as replaced in vendor/modules.txt")
+			}
+		} else if vr != r.New {
+			vendErrorf(r.Old, "is replaced by %s in go.mod, but marked as replaced by %s in vendor/modules.txt", describe(r.New), describe(vr))
+		}
+	}
+
+	for _, mod := range vendorList {
+		meta := vendorMeta[mod]
+		if meta.Explicit {
+			if _, inGoMod := index.require[mod]; !inGoMod {
+				vendErrorf(mod, "is marked as explicit in vendor/modules.txt, but not explicitly required in go.mod")
+			}
+		}
+	}
+
+	for _, mod := range vendorReplaced {
+		r := Replacement(mod)
+		if r == (module.Version{}) {
+			vendErrorf(mod, "is marked as replaced in vendor/modules.txt, but not replaced in go.mod")
+			continue
+		}
+		if meta := vendorMeta[mod]; r != meta.Replacement {
+			vendErrorf(mod, "is marked as replaced by %s in vendor/modules.txt, but replaced by %s in go.mod", describe(meta.Replacement), describe(r))
+		}
+	}
+
+	if vendErrors.Len() > 0 {
+		base.Fatalf("go: inconsistent vendoring in %s:%s\n\n\tTo ignore the vendor directory, use -mod=readonly or -mod=mod.\n\tTo sync the vendor directory, run:\n\t\tgo mod vendor", modRoot, vendErrors)
+	}
+}