n := copy(b.buf[b.n:], s)
	b.n += n
	nn += n
	return nn, nil
}

// ReadFrom implements [io.ReaderFrom]. If the underlying writer
// supports the ReadFrom method, this calls the underlying ReadFrom.
// If there is buffered data and an underlying ReadFrom, this fills
// the buffer and writes it before calling ReadFrom.
func (b *Writer) ReadFrom(r io.Reader) (n int64, err error) {
	if b.err != nil {
		return 0, b.err
	}