go/src/cmd/go/internal/modload/stat_unix.go

// 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 { … }