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) {

func (q *query) matchesPath(path string) bool {
	if q.matchWildcard != nil && !gover.IsToolchain(path) {
		return q.matchWildcard(path)
	}
	return path == q.pattern
}

// canMatchInModule reports whether the given module path can potentially
// contain q.pattern.
func (q *query) canMatchInModule(mPath string) bool {
	if gover.IsToolchain(mPath) {
		return false
	}
	if q.canMatchWildcardInModule != nil {

func goModSummary(m module.Version) (*modFileSummary, error) {
	if m.Version == "" && !inWorkspaceMode() && MainModules.Contains(m.Path) {
		panic("internal error: goModSummary called on a main module")
	}
	if gover.IsToolchain(m.Path) {
		return rawGoModSummary(m)
	}

	if cfg.BuildMod == "vendor" {
		summary := &modFileSummary{
			module: module.Version{Path: m.Path},
		}

		readVendorList(VendorDir())

		info.GoVersion = v.(string)
	}

	// completeFromModCache fills in the extra fields in m using the module cache.
	completeFromModCache := func(m *modinfo.ModulePublic) {
		if gover.IsToolchain(m.Path) {
			return
		}

		checksumOk := func(suffix string) bool {
			return rs == nil || m.Version == "" || !mustHaveSums() ||
				modfetch.HaveSum(module.Version{Path: m.Path, Version: m.Version + suffix})

// AllowedFunc of qm.
func (qm *queryMatcher) allowsVersion(ctx context.Context, v string) bool {
	if qm.prefix != "" && !strings.HasPrefix(v, qm.prefix) {
		if gover.IsToolchain(qm.path) && strings.TrimSuffix(qm.prefix, ".") == v {
			// Allow 1.21 to match "1.21." prefix.
		} else {
			return false
		}
	}
	if qm.filter != nil && !qm.filter(v) {
		return false
	}

		}
	}

	if !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) {