// It returns -1 if rune is not present in s.
// If r is [utf8.RuneError], it returns the first instance of any
// invalid UTF-8 byte sequence.
func IndexRune(s []byte, r rune) int {
	const haveFastIndex = bytealg.MaxBruteForce > 0
	switch {
	case 0 <= r && r < utf8.RuneSelf:
		return IndexByte(s, byte(r))
	case r == utf8.RuneError:
		for i := 0; i < len(s); {
			r1, n := utf8.DecodeRune(s[i:])
			if r1 == utf8.RuneError {

	// test fallback to IndexRune
	{"oxoxoxoxoxoxoxoxoxoxox☺", "☺", 22},
	// invalid UTF-8 byte sequence (must be longer than bytealg.MaxBruteForce to
	// test that we don't use IndexRune)
	{"xx0123456789012345678901234567890123456789012345678901234567890120123456789012345678901234567890123456xxx\xed\x9f\xc0", "\xed\x9f\xc0", 105},
}