all: use strings.Builder instead of bytes.Buffer where appropriate

I grepped for "bytes.Buffer" and "buf.String" and mostly ignored test
files. I skipped a few on purpose and probably missed a few others,
but otherwise I think this should be most of them.

Updates #18990

Change-Id: I5a6ae4296b87b416d8da02d7bfaf981d8cc14774
Reviewed-on: https://go-review.googlesource.com/102479
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/archive/tar/writer.go

@@ -5,7 +5,6 @@
 package tar
 
 import (
-	"bytes"
 	"fmt"
 	"io"
 	"path"
@@ -176,7 +175,7 @@ func (tw *Writer) writePAXHeader(hdr *Header, paxHdrs map[string]string) error {
 		sort.Strings(keys)
 
 		// Write each record to a buffer.
-		var buf bytes.Buffer
+		var buf strings.Builder
 		for _, k := range keys {
 			rec, err := formatPAXRecord(k, paxHdrs[k])
 			if err != nil {

src/cmd/cover/html.go

@@ -15,6 +15,7 @@ import (
 	"math"
 	"os"
 	"path/filepath"
+	"strings"
 )
 
 // htmlOutput reads the profile data from profile and generates an HTML
@@ -41,7 +42,7 @@ func htmlOutput(profile, outfile string) error {
 		if err != nil {
 			return fmt.Errorf("can't read %q: %v", fn, err)
 		}
-		var buf bytes.Buffer
+		var buf strings.Builder
 		err = htmlGen(&buf, src, profile.Boundaries(src))
 		if err != nil {
 			return err

src/expvar/expvar.go

@@ -22,7 +22,6 @@
 package expvar
 
 import (
-	"bytes"
 	"encoding/json"
 	"fmt"
 	"log"
@@ -32,6 +31,7 @@ import (
 	"runtime"
 	"sort"
 	"strconv"
+	"strings"
 	"sync"
 	"sync/atomic"
 )
@@ -111,7 +111,7 @@ type KeyValue struct {
 }
 
 func (v *Map) String() string {
-	var b bytes.Buffer
+	var b strings.Builder
 	fmt.Fprintf(&b, "{")
 	first := true
 	v.Do(func(kv KeyValue) {

src/go/types/methodset.go

@@ -7,9 +7,9 @@
 package types
 
 import (
-	"bytes"
 	"fmt"
 	"sort"
+	"strings"
 )
 
 // A MethodSet is an ordered set of concrete or abstract (interface) methods;
@@ -24,7 +24,7 @@ func (s *MethodSet) String() string {
 		return "MethodSet {}"
 	}
 
-	var buf bytes.Buffer
+	var buf strings.Builder
 	fmt.Fprintln(&buf, "MethodSet {")
 	for _, f := range s.list {
 		fmt.Fprintf(&buf, "\t%s\n", f)

src/mime/encodedword.go

@@ -11,7 +11,6 @@ import (
 	"fmt"
 	"io"
 	"strings"
-	"sync"
 	"unicode"
 	"unicode/utf8"
 )
@@ -51,16 +50,15 @@ func needsEncoding(s string) bool {
 
 // encodeWord encodes a string into an encoded-word.
 func (e WordEncoder) encodeWord(charset, s string) string {
-	buf := getBuffer()
-	defer putBuffer(buf)
+	var buf strings.Builder
 
-	e.openWord(buf, charset)
+	e.openWord(&buf, charset)
 	if e == BEncoding {
-		e.bEncode(buf, charset, s)
+		e.bEncode(&buf, charset, s)
 	} else {
-		e.qEncode(buf, charset, s)
+		e.qEncode(&buf, charset, s)
 	}
-	closeWord(buf)
+	closeWord(&buf)
 
 	return buf.String()
 }
@@ -77,7 +75,7 @@ const (
 var maxBase64Len = base64.StdEncoding.DecodedLen(maxContentLen)
 
 // bEncode encodes s using base64 encoding and writes it to buf.
-func (e WordEncoder) bEncode(buf *bytes.Buffer, charset, s string) {
+func (e WordEncoder) bEncode(buf *strings.Builder, charset, s string) {
 	w := base64.NewEncoder(base64.StdEncoding, buf)
 	// If the charset is not UTF-8 or if the content is short, do not bother
 	// splitting the encoded-word.
@@ -109,7 +107,7 @@ func (e WordEncoder) bEncode(buf *bytes.Buffer, charset, s string) {
 
 // qEncode encodes s using Q encoding and writes it to buf. It splits the
 // encoded-words when necessary.
-func (e WordEncoder) qEncode(buf *bytes.Buffer, charset, s string) {
+func (e WordEncoder) qEncode(buf *strings.Builder, charset, s string) {
 	// We only split encoded-words when the charset is UTF-8.
 	if !isUTF8(charset) {
 		writeQString(buf, s)
@@ -139,7 +137,7 @@ func (e WordEncoder) qEncode(buf *bytes.Buffer, charset, s string) {
 }
 
 // writeQString encodes s using Q encoding and writes it to buf.
-func writeQString(buf *bytes.Buffer, s string) {
+func writeQString(buf *strings.Builder, s string) {
 	for i := 0; i < len(s); i++ {
 		switch b := s[i]; {
 		case b == ' ':
@@ -155,7 +153,7 @@ func writeQString(buf *bytes.Buffer, s string) {
 }
 
 // openWord writes the beginning of an encoded-word into buf.
-func (e WordEncoder) openWord(buf *bytes.Buffer, charset string) {
+func (e WordEncoder) openWord(buf *strings.Builder, charset string) {
 	buf.WriteString("=?")
 	buf.WriteString(charset)
 	buf.WriteByte('?')
@@ -164,12 +162,12 @@ func (e WordEncoder) openWord(buf *bytes.Buffer, charset string) {
 }
 
 // closeWord writes the end of an encoded-word into buf.
-func closeWord(buf *bytes.Buffer) {
+func closeWord(buf *strings.Builder) {
 	buf.WriteString("?=")
 }
 
 // splitWord closes the current encoded-word and opens a new one.
-func (e WordEncoder) splitWord(buf *bytes.Buffer, charset string) {
+func (e WordEncoder) splitWord(buf *strings.Builder, charset string) {
 	closeWord(buf)
 	buf.WriteByte(' ')
 	e.openWord(buf, charset)
@@ -224,10 +222,9 @@ func (d *WordDecoder) Decode(word string) (string, error) {
 		return "", err
 	}
 
-	buf := getBuffer()
-	defer putBuffer(buf)
+	var buf strings.Builder
 
-	if err := d.convert(buf, charset, content); err != nil {
+	if err := d.convert(&buf, charset, content); err != nil {
 		return "", err
 	}
 
@@ -243,8 +240,7 @@ func (d *WordDecoder) DecodeHeader(header string) (string, error) {
 		return header, nil
 	}
 
-	buf := getBuffer()
-	defer putBuffer(buf)
+	var buf strings.Builder
 
 	buf.WriteString(header[:i])
 	header = header[i:]
@@ -296,7 +292,7 @@ func (d *WordDecoder) DecodeHeader(header string) (string, error) {
 			buf.WriteString(header[:start])
 		}
 
-		if err := d.convert(buf, charset, content); err != nil {
+		if err := d.convert(&buf, charset, content); err != nil {
 			return "", err
 		}
 
@@ -322,7 +318,7 @@ func decode(encoding byte, text string) ([]byte, error) {
 	}
 }
 
-func (d *WordDecoder) convert(buf *bytes.Buffer, charset string, content []byte) error {
+func (d *WordDecoder) convert(buf *strings.Builder, charset string, content []byte) error {
 	switch {
 	case strings.EqualFold("utf-8", charset):
 		buf.Write(content)
@@ -346,7 +342,7 @@ func (d *WordDecoder) convert(buf *bytes.Buffer, charset string, content []byte)
 		if err != nil {
 			return err
 		}
-		if _, err = buf.ReadFrom(r); err != nil {
+		if _, err = io.Copy(buf, r); err != nil {
 			return err
 		}
 	}
@@ -422,21 +418,3 @@ func fromHex(b byte) (byte, error) {
 	}
 	return 0, fmt.Errorf("mime: invalid hex byte %#02x", b)
 }
-
-var bufPool = sync.Pool{
-	New: func() interface{} {
-		return new(bytes.Buffer)
-	},
-}
-
-func getBuffer() *bytes.Buffer {
-	return bufPool.Get().(*bytes.Buffer)
-}
-
-func putBuffer(buf *bytes.Buffer) {
-	if buf.Len() > 1024 {
-		return
-	}
-	buf.Reset()
-	bufPool.Put(buf)
-}

src/mime/mediatype.go

@@ -5,7 +5,6 @@
 package mime
 
 import (
-	"bytes"
 	"errors"
 	"fmt"
 	"sort"
@@ -19,7 +18,7 @@ import (
 // When any of the arguments result in a standard violation then
 // FormatMediaType returns the empty string.
 func FormatMediaType(t string, param map[string]string) string {
-	var b bytes.Buffer
+	var b strings.Builder
 	if slash := strings.Index(t, "/"); slash == -1 {
 		if !isToken(t) {
 			return ""
@@ -167,7 +166,7 @@ func ParseMediaType(v string) (mediatype string, params map[string]string, err e
 
 	// Stitch together any continuations or things with stars
 	// (i.e. RFC 2231 things with stars: "foo*0" or "foo*")
-	var buf bytes.Buffer
+	var buf strings.Builder
 	for key, pieceMap := range continuation {
 		singlePartKey := key + "*"
 		if v, ok := pieceMap[singlePartKey]; ok {
@@ -265,7 +264,7 @@ func consumeValue(v string) (value, rest string) {
 	}
 
 	// parse a quoted-string
-	buffer := new(bytes.Buffer)
+	buffer := new(strings.Builder)
 	for i := 1; i < len(v); i++ {
 		r := v[i]
 		if r == '"' {

src/net/http/cookie.go

@@ -5,7 +5,6 @@
 package http
 
 import (
-	"bytes"
 	"log"
 	"net"
 	"strconv"
@@ -143,7 +142,7 @@ func (c *Cookie) String() string {
 	if c == nil || !isCookieNameValid(c.Name) {
 		return ""
 	}
-	var b bytes.Buffer
+	var b strings.Builder
 	b.WriteString(sanitizeCookieName(c.Name))
 	b.WriteRune('=')
 	b.WriteString(sanitizeCookieValue(c.Value))
@@ -168,17 +167,14 @@ func (c *Cookie) String() string {
 			log.Printf("net/http: invalid Cookie.Domain %q; dropping domain attribute", c.Domain)
 		}
 	}
+	var buf [len(TimeFormat)]byte
 	if validCookieExpires(c.Expires) {
 		b.WriteString("; Expires=")
-		b2 := b.Bytes()
-		b.Reset()
-		b.Write(c.Expires.UTC().AppendFormat(b2, TimeFormat))
+		b.Write(c.Expires.UTC().AppendFormat(buf[:0], TimeFormat))
 	}
 	if c.MaxAge > 0 {
 		b.WriteString("; Max-Age=")
-		b2 := b.Bytes()
-		b.Reset()
-		b.Write(strconv.AppendInt(b2, int64(c.MaxAge), 10))
+		b.Write(strconv.AppendInt(buf[:0], int64(c.MaxAge), 10))
 	} else if c.MaxAge < 0 {
 		b.WriteString("; Max-Age=0")
 	}

src/net/http/httptest/server.go

@@ -7,7 +7,6 @@
 package httptest
 
 import (
-	"bytes"
 	"crypto/tls"
 	"crypto/x509"
 	"flag"
@@ -17,6 +16,7 @@ import (
 	"net/http"
 	"net/http/internal"
 	"os"
+	"strings"
 	"sync"
 	"time"
 )
@@ -224,7 +224,7 @@ func (s *Server) Close() {
 func (s *Server) logCloseHangDebugInfo() {
 	s.mu.Lock()
 	defer s.mu.Unlock()
-	var buf bytes.Buffer
+	var buf strings.Builder
 	buf.WriteString("httptest.Server blocked in Close after 5 seconds, waiting for connections:\n")
 	for c, st := range s.conns {
 		fmt.Fprintf(&buf, "  %T %p %v in state %v\n", c, c, c.RemoteAddr(), st)

src/net/mail/message.go

@@ -19,7 +19,6 @@ package mail
 
 import (
 	"bufio"
-	"bytes"
 	"errors"
 	"fmt"
 	"io"
@@ -735,7 +734,7 @@ func isQtext(r rune) bool {
 
 // quoteString renders a string as an RFC 5322 quoted-string.
 func quoteString(s string) string {
-	var buf bytes.Buffer
+	var buf strings.Builder
 	buf.WriteByte('"')
 	for _, r := range s {
 		if isQtext(r) || isWSP(r) {

src/net/url/url.go

@@ -11,7 +11,6 @@ package url
 // contain references to issue numbers with details.
 
 import (
-	"bytes"
 	"errors"
 	"fmt"
 	"sort"
@@ -737,7 +736,7 @@ func validOptionalPort(port string) bool {
 //	- if u.RawQuery is empty, ?query is omitted.
 //	- if u.Fragment is empty, #fragment is omitted.
 func (u *URL) String() string {
-	var buf bytes.Buffer
+	var buf strings.Builder
 	if u.Scheme != "" {
 		buf.WriteString(u.Scheme)
 		buf.WriteByte(':')
@@ -878,7 +877,7 @@ func (v Values) Encode() string {
 	if v == nil {
 		return ""
 	}
-	var buf bytes.Buffer
+	var buf strings.Builder
 	keys := make([]string, 0, len(v))
 	for k := range v {
 		keys = append(keys, k)
@@ -886,12 +885,13 @@ func (v Values) Encode() string {
 	sort.Strings(keys)
 	for _, k := range keys {
 		vs := v[k]
-		prefix := QueryEscape(k) + "="
+		keyEscaped := QueryEscape(k)
 		for _, v := range vs {
 			if buf.Len() > 0 {
 				buf.WriteByte('&')
 			}
-			buf.WriteString(prefix)
+			buf.WriteString(keyEscaped)
+			buf.WriteByte('=')
 			buf.WriteString(QueryEscape(v))
 		}
 	}

src/regexp/onepass.go

@@ -5,9 +5,9 @@
 package regexp
 
 import (
-	"bytes"
 	"regexp/syntax"
 	"sort"
+	"strings"
 	"unicode"
 )
 
@@ -54,7 +54,7 @@ func onePassPrefix(p *syntax.Prog) (prefix string, complete bool, pc uint32) {
 	}
 
 	// Have prefix; gather characters.
-	var buf bytes.Buffer
+	var buf strings.Builder
 	for iop(i) == syntax.InstRune && len(i.Rune) == 1 && syntax.Flags(i.Arg)&syntax.FoldCase == 0 {
 		buf.WriteRune(i.Rune[0])
 		pc, i = i.Out, &p.Inst[i.Out]

src/regexp/syntax/parse_test.go

@@ -5,8 +5,8 @@
 package syntax
 
 import (
-	"bytes"
 	"fmt"
+	"strings"
 	"testing"
 	"unicode"
 )
@@ -282,7 +282,7 @@ func testParseDump(t *testing.T, tests []parseTest, flags Flags) {
 // dump prints a string representation of the regexp showing
 // the structure explicitly.
 func dump(re *Regexp) string {
-	var b bytes.Buffer
+	var b strings.Builder
 	dumpRegexp(&b, re)
 	return b.String()
 }
@@ -312,7 +312,7 @@ var opNames = []string{
 // dumpRegexp writes an encoding of the syntax tree for the regexp re to b.
 // It is used during testing to distinguish between parses that might print
 // the same using re's String method.
-func dumpRegexp(b *bytes.Buffer, re *Regexp) {
+func dumpRegexp(b *strings.Builder, re *Regexp) {
 	if int(re.Op) >= len(opNames) || opNames[re.Op] == "" {
 		fmt.Fprintf(b, "op%d", re.Op)
 	} else {

src/regexp/syntax/prog.go

@@ -5,8 +5,8 @@
 package syntax
 
 import (
-	"bytes"
 	"strconv"
+	"strings"
 	"unicode"
 )
 
@@ -117,7 +117,7 @@ type Inst struct {
 }
 
 func (p *Prog) String() string {
-	var b bytes.Buffer
+	var b strings.Builder
 	dumpProg(&b, p)
 	return b.String()
 }
@@ -153,7 +153,7 @@ func (p *Prog) Prefix() (prefix string, complete bool) {
 	}
 
 	// Have prefix; gather characters.
-	var buf bytes.Buffer
+	var buf strings.Builder
 	for i.op() == InstRune && len(i.Rune) == 1 && Flags(i.Arg)&FoldCase == 0 {
 		buf.WriteRune(i.Rune[0])
 		i = p.skipNop(i.Out)
@@ -267,18 +267,18 @@ func (i *Inst) MatchEmptyWidth(before rune, after rune) bool {
 }
 
 func (i *Inst) String() string {
-	var b bytes.Buffer
+	var b strings.Builder
 	dumpInst(&b, i)
 	return b.String()
 }
 
-func bw(b *bytes.Buffer, args ...string) {
+func bw(b *strings.Builder, args ...string) {
 	for _, s := range args {
 		b.WriteString(s)
 	}
 }
 
-func dumpProg(b *bytes.Buffer, p *Prog) {
+func dumpProg(b *strings.Builder, p *Prog) {
 	for j := range p.Inst {
 		i := &p.Inst[j]
 		pc := strconv.Itoa(j)
@@ -298,7 +298,7 @@ func u32(i uint32) string {
 	return strconv.FormatUint(uint64(i), 10)
 }
 
-func dumpInst(b *bytes.Buffer, i *Inst) {
+func dumpInst(b *strings.Builder, i *Inst) {
 	switch i.Op {
 	case InstAlt:
 		bw(b, "alt -> ", u32(i.Out), ", ", u32(i.Arg))

src/regexp/syntax/regexp.go

@@ -8,7 +8,6 @@ package syntax
 // In this package, re is always a *Regexp and r is always a rune.
 
 import (
-	"bytes"
 	"strconv"
 	"strings"
 	"unicode"
@@ -114,7 +113,7 @@ func (x *Regexp) Equal(y *Regexp) bool {
 }
 
 // writeRegexp writes the Perl syntax for the regular expression re to b.
-func writeRegexp(b *bytes.Buffer, re *Regexp) {
+func writeRegexp(b *strings.Builder, re *Regexp) {
 	switch re.Op {
 	default:
 		b.WriteString("<invalid op" + strconv.Itoa(int(re.Op)) + ">")
@@ -245,14 +244,14 @@ func writeRegexp(b *bytes.Buffer, re *Regexp) {
 }
 
 func (re *Regexp) String() string {
-	var b bytes.Buffer
+	var b strings.Builder
 	writeRegexp(&b, re)
 	return b.String()
 }
 
 const meta = `\.+*?()|[]{}^$`
 
-func escape(b *bytes.Buffer, r rune, force bool) {
+func escape(b *strings.Builder, r rune, force bool) {
 	if unicode.IsPrint(r) {
 		if strings.ContainsRune(meta, r) || force {
 			b.WriteRune('\\')

src/testing/example.go

@@ -5,7 +5,6 @@
 package testing
 
 import (
-	"bytes"
 	"fmt"
 	"io"
 	"os"
@@ -72,7 +71,7 @@ func runExample(eg InternalExample) (ok bool) {
 	os.Stdout = w
 	outC := make(chan string)
 	go func() {
-		var buf bytes.Buffer
+		var buf strings.Builder
 		_, err := io.Copy(&buf, r)
 		r.Close()
 		if err != nil {

src/testing/testing.go

@@ -376,7 +376,7 @@ func (c *common) decorate(s string) string {
 		file = "???"
 		line = 1
 	}
-	buf := new(bytes.Buffer)
+	buf := new(strings.Builder)
 	// Every line is indented at least one tab.
 	buf.WriteByte('\t')
 	fmt.Fprintf(buf, "%s:%d: ", file, line)