go/ast: use strings.Builder

Change-Id: I9c4c3ada3a8f5d8d198cc42a4afc06972ee00c61
GitHub-Last-Rev: 4ed80119e3
GitHub-Pull-Request: golang/go#54916
Reviewed-on: https://go-review.googlesource.com/c/go/+/428921
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/go/ast/commentmap_test.go

@@ -7,12 +7,12 @@
 package ast_test
 
 import (
-	"bytes"
 	"fmt"
 	. "go/ast"
 	"go/parser"
 	"go/token"
 	"sort"
+	"strings"
 	"testing"
 )
 
@@ -94,7 +94,7 @@ var res = map[string]string{
 }
 
 func ctext(list []*CommentGroup) string {
-	var buf bytes.Buffer
+	var buf strings.Builder
 	for _, g := range list {
 		buf.WriteString(g.Text())
 	}

src/go/ast/example_test.go

@@ -5,12 +5,12 @@
 package ast_test
 
 import (
-	"bytes"
 	"fmt"
 	"go/ast"
 	"go/format"
 	"go/parser"
 	"go/token"
+	"strings"
 )
 
 // This example demonstrates how to inspect the AST of a Go program.
@@ -186,11 +186,11 @@ func main() {
 	f.Comments = cmap.Filter(f).Comments()
 
 	// Print the modified AST.
-	var buf bytes.Buffer
+	var buf strings.Builder
 	if err := format.Node(&buf, fset, f); err != nil {
 		panic(err)
 	}
-	fmt.Printf("%s", buf.Bytes())
+	fmt.Printf("%s", buf.String())
 
 	// Output:
 	// // This is the package comment.

src/go/ast/filter_test.go

@@ -7,11 +7,11 @@
 package ast_test
 
 import (
-	"bytes"
 	"go/ast"
 	"go/format"
 	"go/parser"
 	"go/token"
+	"strings"
 	"testing"
 )
 
@@ -73,7 +73,7 @@ func TestFilterDuplicates(t *testing.T) {
 	merged := ast.MergePackageFiles(pkg, ast.FilterFuncDuplicates)
 
 	// pretty-print
-	var buf bytes.Buffer
+	var buf strings.Builder
 	if err := format.Node(&buf, fset, merged); err != nil {
 		t.Fatal(err)
 	}

src/go/ast/print_test.go

@@ -5,7 +5,6 @@
 package ast
 
 import (
-	"bytes"
 	"strings"
 	"testing"
 )
@@ -84,7 +83,7 @@ func trim(s string) string {
 }
 
 func TestPrint(t *testing.T) {
-	var buf bytes.Buffer
+	var buf strings.Builder
 	for _, test := range tests {
 		buf.Reset()
 		if err := Fprint(&buf, nil, test.x, nil); err != nil {