}
		if bytes.Contains(slurp, funcBenchmark) {
			return true
		}
	}
	return false
}

// makeGOROOTUnwritable makes all $GOROOT files & directories non-writable to
// check that no tests accidentally write to $GOROOT.
func (t *tester) makeGOROOTUnwritable() (undo func()) {
	dir := os.Getenv("GOROOT")
	if dir == "" {
		panic("GOROOT not set")
	}

	type pathMode struct {