encoding/json: call reflect.TypeOf with nil pointers rather than allocating

Updates #26775

Change-Id: I83c9eeda59769d2f35e0cc98f3a8579861d5978b
Reviewed-on: https://go-review.googlesource.com/119715Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/json/decode.go

@@ -611,7 +611,7 @@ func (d *decodeState) array(v reflect.Value) error {
 }
 
 var nullLiteral = []byte("null")
-var textUnmarshalerType = reflect.TypeOf(new(encoding.TextUnmarshaler)).Elem()
+var textUnmarshalerType = reflect.TypeOf((*encoding.TextUnmarshaler)(nil)).Elem()
 
 // object consumes an object from d.data[d.off-1:], decoding into v.
 // The first byte ('{') of the object has been read already.

src/encoding/json/encode.go

@@ -381,8 +381,8 @@ func typeEncoder(t reflect.Type) encoderFunc {
 }
 
 var (
-	marshalerType     = reflect.TypeOf(new(Marshaler)).Elem()
-	textMarshalerType = reflect.TypeOf(new(encoding.TextMarshaler)).Elem()
+	marshalerType     = reflect.TypeOf((*Marshaler)(nil)).Elem()
+	textMarshalerType = reflect.TypeOf((*encoding.TextMarshaler)(nil)).Elem()
 )
 
 // newTypeEncoder constructs an encoderFunc for a type.