// if it becomes too large, WriteRune will panic with [ErrTooLarge].
func (b *Buffer) WriteRune(r rune) (n int, err error) {
	// Compare as uint32 to correctly handle negative runes.
	if uint32(r) < utf8.RuneSelf {
		b.WriteByte(byte(r))
		return 1, nil
	}
	b.lastRead = opInvalid
	m, ok := b.tryGrowByReslice(utf8.UTFMax)
	if !ok {
		m = b.grow(utf8.UTFMax)
	}