sw, tryStringWriter = b.wr.(io.StringWriter)
		}
		if b.Buffered() == 0 && tryStringWriter {
			// Large write, empty buffer, and the underlying writer supports
			// WriteString: forward the write to the underlying StringWriter.
			// This avoids an extra copy.
			n, b.err = sw.WriteString(s)
		} else {
			n = copy(b.buf[b.n:], s)
			b.n += n
			b.Flush()
		}
		nn += n
		s = s[n:]