if MainModules.Len() != 1 || MainModules.ModRoot(MainModules.Versions()[0]) == "" {
		// We aren't in a module, so we don't have anywhere to write a go.mod file.
		return nil, nil, nil, errNoChange
	}
	mainModule := MainModules.mustGetSingleMainModule()
	modFile = MainModules.ModFile(mainModule)
	if modFile == nil {

			if inWorkspaceMode() {
				for _, m := range MainModules.Versions() {
					reqs, _ := rootsFromModFile(m, MainModules.ModFile(m), omitToolchainRoot)
					mg.g.Require(m, append(reqs, vendorMod))
				}
				mg.g.Require(vendorMod, vendorList)

			} else {
				mainModule := MainModules.mustGetSingleMainModule()
				mg.g.Require(mainModule, append(rs.rootModules, vendorMod))
				mg.g.Require(vendorMod, vendorList)

	// It's not strictly necessary but helpful to keep the checks.
	var pkgNotFoundErr error
	pkgNotFoundLongestPrefix := ""
	for _, mainModule := range MainModules.Versions() {
		modRoot := MainModules.ModRoot(mainModule)
		if modRoot != "" && str.HasFilePathPrefix(absDir, modRoot) && !strings.Contains(absDir[len(modRoot):], "@") {
			suffix := filepath.ToSlash(str.TrimFilePathPrefix(absDir, modRoot))

			// restricted to matching packages in the main module.
			pkgPattern, mainModule := modload.MainModules.DirImportPath(ctx, q.pattern)
			if pkgPattern == "." {
				modload.MustHaveModRoot()
				var modRoots []string
				for _, m := range modload.MainModules.Versions() {
					modRoots = append(modRoots, modload.MainModules.ModRoot(m))
				}
				var plural string
				if len(modRoots) != 1 {
					plural = "s"

		// users) by writing the failing input to the package's testdata directory.
		// (See https://golang.org/issue/48495 and test_fuzz_modcache.txt.)
		mainMods := modload.MainModules
		if m := pkgs[0].Module; m != nil && m.Path != "" {
			if !mainMods.Contains(m.Path) {
				base.Fatalf("cannot use -fuzz flag on package outside the main module")
			}