}

	// We know it's an error: we have width==1 and implicitly r==utf8.RuneError.
	// Is the error because there wasn't a full rune to be decoded?
	// FullRune distinguishes correctly between erroneous and incomplete encodings.
	if !atEOF && !utf8.FullRune(data) {
		// Incomplete; get more bytes.
		return 0, nil, nil
	}

	// We have a real UTF-8 encoding error. Return a properly encoded error rune

// and returns unicode.ReplacementChar (U+FFFD) with a size of 1.
func (b *Reader) ReadRune() (r rune, size int, err error) {
	for b.r+utf8.UTFMax > b.w && !utf8.FullRune(b.buf[b.r:b.w]) && b.err == nil && b.w-b.r < len(b.buf) {
		b.fill() // b.w-b.r < len(buf) => buffer is not full
	}
	b.lastRuneSize = -1
	if b.r == b.w {
		return 0, 0, b.readErr()
	}

pkg unicode/utf8, func DecodeRune([]uint8) (int32, int)
pkg unicode/utf8, func DecodeRuneInString(string) (int32, int)
pkg unicode/utf8, func EncodeRune([]uint8, int32) int
pkg unicode/utf8, func FullRune([]uint8) bool
pkg unicode/utf8, func FullRuneInString(string) bool
pkg unicode/utf8, func RuneCount([]uint8) int
pkg unicode/utf8, func RuneCountInString(string) int
pkg unicode/utf8, func RuneLen(int32) int