m := utf8.RuneLen(needle)
		for o := m; o > 0; {
			_, sz := utf8.DecodeLastRune(buf)
			copy(buf[len(buf)-sz:], "\x00\x00\x00\x00")
			buf = buf[:len(buf)-sz]
			o -= sz
		}
		buf = utf8.AppendRune(buf[:n-m], needle)

		n -= m // adjust for rune len
		for i := 0; i < b.N; i++ {
			j := IndexRune(buf, needle)
			if j != n {
				b.Fatal("bad index", j)
			}
		}
		for i := range buf {