cmd/protoc-gen-go: don't generate default values for [default=""].

The previous generator considers a default value of "" (valid only for
string and bytes fields) to be unset, and does not generate a const or
var to hold the default value.

e.g.,

  message M {
    optional F string = 1 [default=""];
  }

does not generate this constant, even though the field has a default
value:

  const Default_M_F string = ""

Maintain consistent output.

Change-Id: Ib172b02d59c15c05e19a7056d05ce1c619a2fa40
Reviewed-on: https://go-review.googlesource.com/138518
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/cmd/protoc-gen-go/internal_gengo/main.go b/cmd/protoc-gen-go/internal_gengo/main.go
index 00fe773..1272ebb 100644
--- a/cmd/protoc-gen-go/internal_gengo/main.go
+++ b/cmd/protoc-gen-go/internal_gengo/main.go
@@ -456,7 +456,7 @@
 
 	// Constants and vars holding the default values of fields.
 	for _, field := range message.Fields {
-		if !field.Desc.HasDefault() {
+		if !fieldHasDefault(field) {
 			continue
 		}
 		defVarName := "Default_" + message.GoIdent.GoName + "_" + field.GoName
@@ -676,7 +676,7 @@
 	if field.Desc.Cardinality() == protoreflect.Repeated {
 		return "nil"
 	}
-	if field.Desc.HasDefault() {
+	if fieldHasDefault(field) {
 		defVarName := "Default_" + message.GoIdent.GoName + "_" + field.GoName
 		if field.Desc.Kind() == protoreflect.BytesKind {
 			return "append([]byte(nil), " + defVarName + "...)"
@@ -697,6 +697,26 @@
 	}
 }
 
+// fieldHasDefault returns true if we consider a field to have a default value.
+//
+// For consistency with the previous generator, it returns false for fields with
+// [default=""], preventing the generation of a default const or var for these
+// fields.
+//
+// TODO: Drop this special case.
+func fieldHasDefault(field *protogen.Field) bool {
+	if !field.Desc.HasDefault() {
+		return false
+	}
+	switch field.Desc.Kind() {
+	case protoreflect.StringKind:
+		return field.Desc.Default().String() != ""
+	case protoreflect.BytesKind:
+		return len(field.Desc.Default().Bytes()) > 0
+	}
+	return true
+}
+
 var wireTypes = map[protoreflect.Kind]string{
 	protoreflect.BoolKind:     "varint",
 	protoreflect.EnumKind:     "varint",
diff --git a/cmd/protoc-gen-go/testdata/proto2/fields.pb.go b/cmd/protoc-gen-go/testdata/proto2/fields.pb.go
index 04a7007..9527dc8 100644
--- a/cmd/protoc-gen-go/testdata/proto2/fields.pb.go
+++ b/cmd/protoc-gen-go/testdata/proto2/fields.pb.go
@@ -131,6 +131,8 @@
 	DefaultDouble       *float64                             `protobuf:"fixed64,314,opt,name=default_double,json=defaultDouble,def=3.1415" json:"default_double,omitempty"`
 	DefaultString       *string                              `protobuf:"bytes,315,opt,name=default_string,json=defaultString,def=x,y" json:"default_string,omitempty"`
 	DefaultBytes        []byte                               `protobuf:"bytes,316,opt,name=default_bytes,json=defaultBytes,def=x,y" json:"default_bytes,omitempty"`
+	DefaultZeroString   *string                              `protobuf:"bytes,350,opt,name=default_zero_string,json=defaultZeroString,def=" json:"default_zero_string,omitempty"`
+	DefaultZeroBytes    []byte                               `protobuf:"bytes,351,opt,name=default_zero_bytes,json=defaultZeroBytes,def=" json:"default_zero_bytes,omitempty"`
 	DefaultFloatNeginf  *float32                             `protobuf:"fixed32,400,opt,name=default_float_neginf,json=defaultFloatNeginf,def=-inf" json:"default_float_neginf,omitempty"`
 	DefaultFloatPosinf  *float32                             `protobuf:"fixed32,401,opt,name=default_float_posinf,json=defaultFloatPosinf,def=inf" json:"default_float_posinf,omitempty"`
 	DefaultFloatNan     *float32                             `protobuf:"fixed32,402,opt,name=default_float_nan,json=defaultFloatNan,def=nan" json:"default_float_nan,omitempty"`
@@ -708,6 +710,20 @@
 	return append([]byte(nil), Default_FieldTestMessage_DefaultBytes...)
 }
 
+func (m *FieldTestMessage) GetDefaultZeroString() string {
+	if m != nil && m.DefaultZeroString != nil {
+		return *m.DefaultZeroString
+	}
+	return ""
+}
+
+func (m *FieldTestMessage) GetDefaultZeroBytes() []byte {
+	if m != nil {
+		return m.DefaultZeroBytes
+	}
+	return nil
+}
+
 func (m *FieldTestMessage) GetDefaultFloatNeginf() float32 {
 	if m != nil && m.DefaultFloatNeginf != nil {
 		return *m.DefaultFloatNeginf
@@ -1600,120 +1616,123 @@
 func init() { proto.RegisterFile("proto2/fields.proto", fileDescriptor_fd8a9d72b841fd68) }
 
 var fileDescriptor_fd8a9d72b841fd68 = []byte{
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+	0x35, 0xbf, 0x7f, 0x27, 0x9f, 0x1a, 0xec, 0x57, 0x22, 0x4c, 0x55, 0xc3, 0xdb, 0xa8, 0xd4, 0xe8,
+	0xfe, 0x3d, 0xa8, 0x5a, 0x46, 0xa5, 0xe6, 0xb6, 0xed, 0x27, 0x42, 0xf6, 0x3a, 0x88, 0xaa, 0xb6,
+	0x9f, 0x08, 0x5d, 0x3f, 0x11, 0xb2, 0x47, 0x20, 0xa2, 0x8e, 0x9f, 0xad, 0x52, 0xb3, 0xfa, 0x0f,
+	0xa0, 0xaa, 0x39, 0x7e, 0xf2, 0x7a, 0xb1, 0xa1, 0x6a, 0xe9, 0xc1, 0xfb, 0x47, 0xd0, 0xb5, 0xf7,
+	0x2b, 0x51, 0x0b, 0xab, 0x0d, 0xcc, 0x55, 0x08, 0x03, 0x66, 0x42, 0xff, 0x09, 0x84, 0xab, 0xfb,
+	0x95, 0x08, 0x7d, 0xf4, 0x78, 0x36, 0x1d, 0xe0, 0x70, 0xfe, 0x33, 0xa8, 0x7c, 0xd3, 0x01, 0x8e,
+	0xe5, 0x92, 0xab, 0x08, 0xd9, 0x5f, 0x40, 0xd6, 0x29, 0xb9, 0x8a, 0xb0, 0xe4, 0x2a, 0x42, 0xf6,
+	0x57, 0x10, 0xae, 0xb8, 0xae, 0x76, 0xb7, 0x6a, 0x14, 0xff, 0x0d, 0x64, 0x9e, 0xe9, 0x56, 0x8d,
+	0xe0, 0x62, 0xe7, 0x70, 0x46, 0xfe, 0x1d, 0x54, 0x8d, 0x62, 0xe7, 0x70, 0x3a, 0x9a, 0x0e, 0x70,
+	0x2c, 0xfe, 0x03, 0x44, 0x4d, 0xd3, 0x01, 0x0e, 0xc4, 0x0f, 0xeb, 0x75, 0xe9, 0x7b, 0xd1, 0x3f,
+	0x6b, 0x97, 0x7a, 0x1c, 0x60, 0x1a, 0xd1, 0x28, 0xdd, 0x57, 0x07, 0x18, 0xef, 0x44, 0xff, 0xaa,
+	0xc9, 0x67, 0x01, 0xbd, 0x73, 0x3f, 0x0b, 0x80, 0x54, 0x79, 0x21, 0x32, 0xeb, 0xc5, 0xfb, 0xd0,
+	0x1b, 0x75, 0x4f, 0xf9, 0xc3, 0xec, 0x70, 0x87, 0xbd, 0x5c, 0x97, 0xe7, 0xd8, 0x53, 0x47, 0xfd,
+	0xd9, 0x87, 0xd9, 0x8e, 0x2b, 0xe9, 0xb1, 0xef, 0xd6, 0xe5, 0xd1, 0xb3, 0x24, 0xbd, 0xab, 0x82,
+	0xb4, 0x9c, 0xa7, 0x0d, 0xce, 0xe3, 0x13, 0x5c, 0xf1, 0x96, 0x7c, 0xcc, 0x62, 0x9e, 0x42, 0x48,
+	0x19, 0xe4, 0x39, 0x3f, 0x99, 0x9c, 0xdf, 0x80, 0x98, 0xf7, 0x50, 0xfe, 0x56, 0x34, 0x3f, 0xa5,
+	0x30, 0xef, 0x29, 0xc8, 0xb3, 0x6e, 0x79, 0xce, 0xcd, 0x15, 0xf3, 0x5e, 0xc3, 0x2b, 0xae, 0xb9,
+	0xfe, 0x61, 0xe2, 0xbb, 0x08, 0x5d, 0x9e, 0x91, 0xb4, 0x43, 0xaa, 0x2f, 0x24, 0xa7, 0xf2, 0xa9,
+	0x63, 0x3d, 0x82, 0x7f, 0xe9, 0x63, 0xa4, 0xfe, 0x20, 0x1e, 0x2f, 0x12, 0xf9, 0x90, 0xb1, 0x1a,
+	0xe1, 0x8b, 0xbe, 0xff, 0xb4, 0x77, 0x35, 0x27, 0x57, 0xce, 0x9c, 0x7e, 0x76, 0x91, 0x06, 0x16,
+	0xd9, 0xb3, 0x8b, 0x5c, 0xe2, 0x6a, 0x6c, 0xb9, 0x66, 0x64, 0xeb, 0x8c, 0x39, 0x68, 0x7b, 0xae,
+	0xa0, 0xe7, 0xae, 0xed, 0x79, 0xd1, 0x5f, 0x8d, 0x96, 0xe1, 0x33, 0x84, 0x14, 0x67, 0x46, 0x3e,
+	0xb7, 0x93, 0x47, 0x40, 0xee, 0xed, 0xa1, 0x7c, 0xd0, 0xcb, 0xfe, 0x2d, 0x21, 0x89, 0xda, 0x99,
+	0xd1, 0xc9, 0xaa, 0x57, 0x1b, 0x64, 0x55, 0x55, 0xbd, 0xf6, 0x06, 0x52, 0x93, 0xd3, 0x72, 0x8d,
+	0xd4, 0x0e, 0xf6, 0xa2, 0x7b, 0x9d, 0x0a, 0x5d, 0x25, 0x70, 0x25, 0xee, 0x78, 0xbb, 0x2d, 0x33,
+	0x0e, 0x20, 0x69, 0x77, 0x9d, 0x34, 0xcc, 0x21, 0xdb, 0x7d, 0xf7, 0xc1, 0x3b, 0x87, 0xa3, 0xfc,
+	0xf9, 0xc5, 0xd1, 0xf6, 0x71, 0x36, 0xe9, 0x0e, 0xb3, 0x71, 0x9c, 0x0e, 0xbb, 0x72, 0xe5, 0x47,
+	0x8b, 0x41, 0xf7, 0x41, 0xaf, 0x7b, 0x3c, 0x39, 0xc1, 0xd7, 0xc7, 0xb7, 0x86, 0x49, 0x7a, 0x6b,
+	0x98, 0x75, 0xf3, 0x64, 0x9e, 0x9f, 0xc4, 0x79, 0x8c, 0xe1, 0xde, 0xff, 0x03, 0x00, 0x00, 0xff,
+	0xff, 0xd7, 0xb7, 0x5b, 0x69, 0x94, 0x16, 0x00, 0x00,
 }
diff --git a/cmd/protoc-gen-go/testdata/proto2/fields.proto b/cmd/protoc-gen-go/testdata/proto2/fields.proto
index 2779992..c2e9be0 100644
--- a/cmd/protoc-gen-go/testdata/proto2/fields.proto
+++ b/cmd/protoc-gen-go/testdata/proto2/fields.proto
@@ -89,6 +89,9 @@
   optional string   default_string   = 315 [default="x,y"];
   optional bytes    default_bytes    = 316 [default="x,y"];
 
+  optional string default_zero_string = 350 [default=""];
+  optional bytes  default_zero_bytes  = 351 [default=""];
+
   optional float  default_float_neginf  = 400 [default=-inf];
   optional float  default_float_posinf  = 401 [default=inf];
   optional float  default_float_nan     = 402 [default=nan];