2 files changed, 79 insertions, 3 deletions

diff --git a/src/bufio/bufio.go b/src/bufio/bufio.go
index 7483946fc0a..bcc273c78b2 100644
--- a/src/bufio/bufio.go
+++ b/src/bufio/bufio.go
@@ -731,13 +731,28 @@ func (b *Writer) WriteRune(r rune) (size int, err error) {
 // If the count is less than len(s), it also returns an error explaining
 // why the write is short.
 func (b *Writer) WriteString(s string) (int, error) {
+	var sw io.StringWriter
+	tryStringWriter := true
+
 	nn := 0
 	for len(s) > b.Available() && b.err == nil {
-		n := copy(b.buf[b.n:], s)
-		b.n += n
+		var n int
+		if b.Buffered() == 0 && sw == nil && tryStringWriter {
+			// Check at most once whether b.wr is a StringWriter.
+			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:]
-		b.Flush()
 	}
 	if b.err != nil {
 		return nn, b.err
diff --git a/src/bufio/bufio_test.go b/src/bufio/bufio_test.go
index ff3396e9469..b3456d23411 100644
--- a/src/bufio/bufio_test.go
+++ b/src/bufio/bufio_test.go
@@ -762,6 +762,67 @@ func TestWriteString(t *testing.T) {
 	}
 }
 
+func TestWriteStringStringWriter(t *testing.T) {
+	const BufSize = 8
+	{
+		tw := &teststringwriter{}
+		b := NewWriterSize(tw, BufSize)
+		b.WriteString("1234")
+		tw.check(t, "", "")
+		b.WriteString("56789012")   // longer than BufSize
+		tw.check(t, "12345678", "") // but not enough (after filling the partially-filled buffer)
+		b.Flush()
+		tw.check(t, "123456789012", "")
+	}
+	{
+		tw := &teststringwriter{}
+		b := NewWriterSize(tw, BufSize)
+		b.WriteString("123456789")   // long string, empty buffer:
+		tw.check(t, "", "123456789") // use WriteString
+	}
+	{
+		tw := &teststringwriter{}
+		b := NewWriterSize(tw, BufSize)
+		b.WriteString("abc")
+		tw.check(t, "", "")
+		b.WriteString("123456789012345")      // long string, non-empty buffer
+		tw.check(t, "abc12345", "6789012345") // use Write and then WriteString since the remaining part is still longer than BufSize
+	}
+	{
+		tw := &teststringwriter{}
+		b := NewWriterSize(tw, BufSize)
+		b.Write([]byte("abc")) // same as above, but use Write instead of WriteString
+		tw.check(t, "", "")
+		b.WriteString("123456789012345")
+		tw.check(t, "abc12345", "6789012345") // same as above
+	}
+}
+
+type teststringwriter struct {
+	write       string
+	writeString string
+}
+
+func (w *teststringwriter) Write(b []byte) (int, error) {
+	w.write += string(b)
+	return len(b), nil
+}
+
+func (w *teststringwriter) WriteString(s string) (int, error) {
+	w.writeString += s
+	return len(s), nil
+}
+
+func (w *teststringwriter) check(t *testing.T, write, writeString string) {
+	t.Helper()
+	if w.write != write {
+		t.Errorf("write: expected %q, got %q", write, w.write)
+	}
+	if w.writeString != writeString {
+		t.Errorf("writeString: expected %q, got %q", writeString, w.writeString)
+	}
+}
+
 func TestBufferFull(t *testing.T) {
 	const longString = "And now, hello, world! It is the time for all good men to come to the aid of their party"
 	buf := NewReaderSize(strings.NewReader(longString), minReadBufferSize)