b.buf[b.r] = byte(b.lastByte)
	b.lastByte = -1
	b.lastRuneSize = -1
	return nil
}

// ReadRune reads a single UTF-8 encoded Unicode character and returns the
// rune and its size in bytes. If the encoded rune is invalid, it consumes one byte
// and returns unicode.ReplacementChar (U+FFFD) with a size of 1.
func (b *Reader) ReadRune() (r rune, size int, err error) {