if err != nil {
		return "", err
	}
	return filepath.Join(cfg.GOMODCACHE, "cache/download", enc, "/@v"), nil
}

func CachePath(ctx context.Context, m module.Version, suffix string) (string, error) {
	if gover.IsToolchain(m.Path) {
		return "", ErrToolchain
	}
	dir, err := cacheDir(ctx, m.Path)
	if err != nil {
		return "", err
	}
	if !gover.ModIsValid(m.Path, m.Version) {

		if gover.IsToolchain(path) {
			continue
		}
		old := r.initialVersion[path]
		new := reason.version
		if old != new && (old != "" || new != "none") {
			changes[path] = change{path, old, new}
		}
	}

	// Collect changes to explicit requirements in go.mod.
	for _, req := range oldReqs {
		if gover.IsToolchain(req.Path) {
			continue
		}

		}
	}

	if !strings.Contains(pkgArg, "@") || build.IsLocalImport(pkgArg) || filepath.IsAbs(pkgArg) {
		return false
	}
	path, version, _ := strings.Cut(pkgArg, "@")
	if path == "" || version == "" || gover.IsToolchain(path) {
		return false
	}

	if !modcacherwSeen && base.InGOFLAGS("-modcacherw") {
		fs := flag.NewFlagSet("goInstallVersion", flag.ExitOnError)
		fs.Var(modcacherwVal, "modcacherw", modcacherwFlag.Usage)

	// requirements in mg.
	for {
		var sumErrMods, altMods []module.Version
		for prefix := path; prefix != "."; prefix = pathpkg.Dir(prefix) {
			if gover.IsToolchain(prefix) {
				// Do not use the synthetic "go" module for "go/ast".
				continue
			}
			var (
				v  string
				ok bool
			)
			if mg == nil {
				v, ok = rs.rootSelected(prefix)
			} else {

// local download cache and returns the name of the directory
// corresponding to the root of the module's file tree.
func Download(ctx context.Context, mod module.Version) (dir string, err error) {
	if gover.IsToolchain(mod.Path) {
		return "", ErrToolchain
	}
	if err := checkCacheDir(ctx); err != nil {
		base.Fatal(err)
	}

	// The par.Cache here avoids duplicate work.
	return downloadCache.Do(mod, func() (string, error) {

// if dir is in the module cache copy of a module in our build list.
func pathInModuleCache(ctx context.Context, dir string, rs *Requirements) string {
	tryMod := func(m module.Version) (string, bool) {
		if gover.IsToolchain(m.Path) {
			return "", false
		}
		var root string
		var err error
		if repl := Replacement(m); repl.Path != "" && repl.Version == "" {
			root = repl.Path
			if !filepath.IsAbs(root) {