encoding/xml: remove unnecessary memory allocation in Unmarshal

benchmark              old ns/op     new ns/op     delta
BenchmarkUnmarshal     75256         72626         -3.49%

benchmark              old allocs     new allocs     delta
BenchmarkUnmarshal     259            219            -15.44%

Change-Id: I7fd30739b045e35b95e6ef6a8ef2f15b0dd6839c
Reviewed-on: https://go-review.googlesource.com/2758Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/xml/marshal_test.go

@@ -1148,12 +1148,14 @@ func TestMarshalFlush(t *testing.T) {
 }
 
 func BenchmarkMarshal(b *testing.B) {
+	b.ReportAllocs()
 	for i := 0; i < b.N; i++ {
 		Marshal(atomValue)
 	}
 }
 
 func BenchmarkUnmarshal(b *testing.B) {
+	b.ReportAllocs()
 	xml := []byte(atomXml)
 	for i := 0; i < b.N; i++ {
 		Unmarshal(xml, &Feed{})

src/encoding/xml/xml.go

@@ -1119,12 +1119,12 @@ func (d *Decoder) name() (s string, ok bool) {
 	}
 
 	// Now we check the characters.
-	s = d.buf.String()
-	if !isName([]byte(s)) {
-		d.err = d.syntaxError("invalid XML name: " + s)
+	b := d.buf.Bytes()
+	if !isName(b) {
+		d.err = d.syntaxError("invalid XML name: " + string(b))
 		return "", false
 	}
-	return s, true
+	return string(b), true
 }
 
 // Read a name and append its bytes to d.buf.