mime: use strings.Builder

Change-Id: I2756fab046154a2df49435ba1978a32176de830a
Reviewed-on: https://go-review.googlesource.com/c/go/+/428265
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: xie cui <523516579@qq.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>

src/mime/multipart/formdata_test.go

@@ -5,7 +5,6 @@
 package multipart
 
 import (
-	"bytes"
 	"io"
 	"math"
 	"os"
@@ -92,7 +91,7 @@ func testFile(t *testing.T, fh *FileHeader, efn, econtent string) File {
 	if err != nil {
 		t.Fatal("opening file:", err)
 	}
-	b := new(bytes.Buffer)
+	b := new(strings.Builder)
 	_, err = io.Copy(b, f)
 	if err != nil {
 		t.Fatal("copying contents:", err)

src/mime/multipart/multipart_test.go

@@ -126,7 +126,7 @@ func TestMultipartSlowInput(t *testing.T) {
 func testMultipart(t *testing.T, r io.Reader, onlyNewlines bool) {
 	t.Parallel()
 	reader := NewReader(r, "MyBoundary")
-	buf := new(bytes.Buffer)
+	buf := new(strings.Builder)
 
 	// Part1
 	part, err := reader.NextPart()
@@ -416,7 +416,7 @@ func TestLineContinuation(t *testing.T) {
 		if err != nil {
 			t.Fatalf("didn't get a part")
 		}
-		var buf bytes.Buffer
+		var buf strings.Builder
 		n, err := io.Copy(&buf, part)
 		if err != nil {
 			t.Errorf("error reading part: %v\nread so far: %q", err, buf.String())
@@ -446,7 +446,7 @@ func testQuotedPrintableEncoding(t *testing.T, cte string) {
 	if te, ok := part.Header["Content-Transfer-Encoding"]; ok {
 		t.Errorf("unexpected Content-Transfer-Encoding of %q", te)
 	}
-	var buf bytes.Buffer
+	var buf strings.Builder
 	_, err = io.Copy(&buf, part)
 	if err != nil {
 		t.Error(err)
@@ -484,7 +484,7 @@ Content-Transfer-Encoding: quoted-printable
 	if _, ok := part.Header["Content-Transfer-Encoding"]; !ok {
 		t.Errorf("missing Content-Transfer-Encoding")
 	}
-	var buf bytes.Buffer
+	var buf strings.Builder
 	_, err = io.Copy(&buf, part)
 	if err != nil {
 		t.Error(err)
@@ -993,7 +993,7 @@ func roundTripParseTest() parseTest {
 			formData("foo", "bar"),
 		},
 	}
-	var buf bytes.Buffer
+	var buf strings.Builder
 	w := NewWriter(&buf)
 	for _, p := range t.want {
 		pw, err := w.CreatePart(p.header)

src/mime/multipart/writer_test.go

@@ -98,7 +98,7 @@ func TestWriterSetBoundary(t *testing.T) {
 		{"(boundary)", true},
 	}
 	for i, tt := range tests {
-		var b bytes.Buffer
+		var b strings.Builder
 		w := NewWriter(&b)
 		err := w.SetBoundary(tt.b)
 		got := err == nil
@@ -145,7 +145,7 @@ func TestWriterBoundaryGoroutines(t *testing.T) {
 }
 
 func TestSortedHeader(t *testing.T) {
-	var buf bytes.Buffer
+	var buf strings.Builder
 	w := NewWriter(&buf)
 	if err := w.SetBoundary("MIMEBOUNDARY"); err != nil {
 		t.Fatalf("Error setting mime boundary: %v", err)

src/mime/quotedprintable/reader_test.go

@@ -6,7 +6,6 @@ package quotedprintable
 
 import (
 	"bufio"
-	"bytes"
 	"errors"
 	"flag"
 	"fmt"
@@ -69,7 +68,7 @@ func TestReader(t *testing.T) {
 			want: "accept UTF-8 right quotation mark: ’"},
 	}
 	for _, tt := range tests {
-		var buf bytes.Buffer
+		var buf strings.Builder
 		_, err := io.Copy(&buf, NewReader(strings.NewReader(tt.in)))
 		if got := buf.String(); got != tt.want {
 			t.Errorf("for %q, got %q; want %q", tt.in, got, tt.want)
@@ -114,7 +113,7 @@ func TestExhaustive(t *testing.T) {
 		}
 	}
 
-	var buf bytes.Buffer
+	var buf strings.Builder
 	res := make(map[string]int)
 	n := 6
 	if testing.Short() {

src/mime/quotedprintable/writer_test.go

@@ -91,7 +91,7 @@ func testWriter(t *testing.T, binary bool) {
 	}
 
 	for _, tt := range tests {
-		buf := new(bytes.Buffer)
+		buf := new(strings.Builder)
 		w := NewWriter(buf)
 
 		want := tt.want