diff --git a/src/cmd/go/internal/str/str.go b/src/cmd/go/internal/str/str.go
index 975869d760..af7c699972 100644
--- a/src/cmd/go/internal/str/str.go
+++ b/src/cmd/go/internal/str/str.go
@@ -6,8 +6,8 @@
 package str
 
 import (
-	"bytes"
 	"fmt"
+	"strings"
 	"unicode"
 	"unicode/utf8"
 )
@@ -49,7 +49,7 @@ func ToFold(s string) string {
 	return s
 
 Slow:
-	var buf bytes.Buffer
+	var b strings.Builder
 	for _, r := range s {
 		// SimpleFold(x) cycles to the next equivalent rune > x
 		// or wraps around to smaller values. Iterate until it wraps,
@@ -65,9 +65,9 @@ Slow:
 		if 'A' <= r && r <= 'Z' {
 			r += 'a' - 'A'
 		}
-		buf.WriteRune(r)
+		b.WriteRune(r)
 	}
-	return buf.String()
+	return b.String()
 }
 
 // FoldDup reports a pair of strings from the list that are
diff --git a/src/go/internal/gccgoimporter/parser.go b/src/go/internal/gccgoimporter/parser.go
index 536083ae08..de9df0bbfb 100644
--- a/src/go/internal/gccgoimporter/parser.go
+++ b/src/go/internal/gccgoimporter/parser.go
@@ -5,7 +5,6 @@
 package gccgoimporter
 
 import (
-	"bytes"
 	"errors"
 	"fmt"
 	"go/constant"
@@ -129,16 +128,16 @@ func (p *parser) parseUnquotedString() string {
 	if p.tok == scanner.EOF {
 		p.error("unexpected EOF")
 	}
-	var buf bytes.Buffer
-	buf.WriteString(p.scanner.TokenText())
+	var b strings.Builder
+	b.WriteString(p.scanner.TokenText())
 	// This loop needs to examine each character before deciding whether to consume it. If we see a semicolon,
 	// we need to let it be consumed by p.next().
 	for ch := p.scanner.Peek(); ch != '\n' && ch != ';' && ch != scanner.EOF && p.scanner.Whitespace&(1<<uint(ch)) == 0; ch = p.scanner.Peek() {
-		buf.WriteRune(ch)
+		b.WriteRune(ch)
 		p.scanner.Next()
 	}
 	p.next()
-	return buf.String()
+	return b.String()
 }
 
 func (p *parser) next() {
diff --git a/src/net/mail/message.go b/src/net/mail/message.go
index c91aa3af12..5de47eb45b 100644
--- a/src/net/mail/message.go
+++ b/src/net/mail/message.go
@@ -829,18 +829,18 @@ func isQtext(r rune) bool {
 
 // quoteString renders a string as an RFC 5322 quoted-string.
 func quoteString(s string) string {
-	var buf strings.Builder
-	buf.WriteByte('"')
+	var b strings.Builder
+	b.WriteByte('"')
 	for _, r := range s {
 		if isQtext(r) || isWSP(r) {
-			buf.WriteRune(r)
+			b.WriteRune(r)
 		} else if isVchar(r) {
-			buf.WriteByte('\\')
-			buf.WriteRune(r)
+			b.WriteByte('\\')
+			b.WriteRune(r)
 		}
 	}
-	buf.WriteByte('"')
-	return buf.String()
+	b.WriteByte('"')
+	return b.String()
 }
 
 // isVchar reports whether r is an RFC 5322 VCHAR character.