// [Reader.ReadBytes] or ReadString instead.
// ReadSlice returns err != nil if and only if line does not end in delim.
func (b *Reader) ReadSlice(delim byte) (line []byte, err error) {
	s := 0 // search start index
	for {
		// Search buffer.
		if i := bytes.IndexByte(b.buf[b.r+s:b.w], delim); i >= 0 {
			i += s
			line = b.buf[b.r : b.r+i+1]
			b.r += i + 1
			break
		}

		// Pending error?
		if b.err != nil {