return nil, errors.New("go.mod changed during editing; not overwriting")
		}
		return out, nil
	})
	if err != nil {
		base.Fatal(err)
	}
}

// parsePathVersion parses -flag=arg expecting arg to be path@version.
func parsePathVersion(flag, arg string) (path, version string) {
	before, after, found := strings.Cut(arg, "@")
	if !found {
		base.Fatalf("go: -%s=%s: need path@version", flag, arg)
	}