reflect/prototype: avoid unescaping string defaults

According to google.protobuf.FieldDescriptorProto.default_value documentation:
>>>
For strings, contains the default text contents (not escaped in any way).
For bytes, contains the C escaped value.  All bytes >= 128 are escaped.
<<<

Thus, use the default value ad-verbatim if the kind is a string.
For the bytes kind, we use the text format hack to unescape the string,
since the text format uses the same unescape syntax as C due to its heritage.

Change-Id: I79e6d4d7e8f0d1ebedccecb9424a49384485589d
Reviewed-on: https://go-review.googlesource.com/138515
Reviewed-by: Damien Neil <dneil@google.com>

reflect/prototype/protofile_desc.go

@@ -404,16 +404,16 @@ func parseDefault(s string, k protoreflect.Kind) (protoreflect.Value, error) {
 			}
 			return protoreflect.ValueOf(float64(v)), nil
 		}
-	case protoreflect.StringKind, protoreflect.BytesKind:
-		// String values use the same escaping as the text format,
+	case protoreflect.StringKind:
+		// String values are already unescaped and can be used as is.
+		return protoreflect.ValueOf(s), nil
+	case protoreflect.BytesKind:
+		// Bytes values use the same escaping as the text format,
 		// however they lack the surrounding double quotes.
 		// TODO: Export unmarshalString in the text package to avoid this hack.
 		v, err := text.Unmarshal([]byte(`["` + s + `"]:0`))
 		if err == nil && len(v.Message()) == 1 {
 			s := v.Message()[0][0].String()
-			if k == protoreflect.StringKind {
-				return protoreflect.ValueOf(s), nil
-			}
 			return protoreflect.ValueOf([]byte(s)), nil
 		}
 	}

reflect/prototype/type_test.go

@@ -131,7 +131,7 @@ func TestFile(t *testing.T) {
 				Number:      1,
 				Cardinality: pref.Optional,
 				Kind:        pref.StringKind,
-				Default:     pref.ValueOf("hello"),
+				Default:     pref.ValueOf("hello, \"world!\"\n"),
 				OneofName:   "O1",
 			}, {
 				Name:        "field_two", // "test.B.field_two"
@@ -248,7 +248,7 @@ func TestFile(t *testing.T) {
 				Number:       protoV1.Int32(1),
 				Label:        descriptorV1.FieldDescriptorProto_Label(pref.Optional).Enum(),
 				Type:         descriptorV1.FieldDescriptorProto_Type(pref.StringKind).Enum(),
-				DefaultValue: protoV1.String("hello"),
+				DefaultValue: protoV1.String("hello, \"world!\"\n"),
 				OneofIndex:   protoV1.Int32(0),
 			}, {
 				Name:         protoV1.String("field_two"),
@@ -452,7 +452,7 @@ func testFileAccessors(t *testing.T, fd pref.FileDescriptor) {
 						"Name":      pref.Name("field_one"),
 						"Index":     0,
 						"JSONName":  "fieldOne",
-						"Default":   "hello",
+						"Default":   "hello, \"world!\"\n",
 						"OneofType": M{"Name": pref.Name("O1"), "IsPlaceholder": false},
 					},
 					"ByJSONName:fieldTwo": nil,
@@ -702,7 +702,7 @@ func testFileFormat(t *testing.T, fd pref.FileDescriptor) {
 			Kind:        string
 			JSONName:    "fieldOne"
 			HasDefault:  true
-			Default:     "hello"
+			Default:     "hello, \"world!\"\n"
 			OneofType:   O1
 		}, {
 			Name:        field_two