Implement optional semantics for proto3

This commit adds support for the proto3 optional keyword.

Proto3 optional fields will have the label "optional" added,
to match the behaviour of proto2 optional fields.

Fields in proto3 that are not marked as optional are unaffected
and will continue to have no label (unless they are repeated, in
which case they will continue to have the "repeated" label).

plugin.go

@@ -21,6 +21,8 @@ type PluginOptions struct {
 	ExcludePatterns []*regexp.Regexp
 }
 
+var SupportedFeatures = uint64(plugin_go.CodeGeneratorResponse_FEATURE_PROTO3_OPTIONAL)
+
 // Plugin describes a protoc code generate plugin. It's an implementation of Plugin from github.com/pseudomuto/protokit
 type Plugin struct{}
 
@@ -56,6 +58,7 @@ func (p *Plugin) Generate(r *plugin_go.CodeGeneratorRequest) (*plugin_go.CodeGen
 		Name:    proto.String(options.OutputFile),
 		Content: proto.String(string(output)),
 	})
+	resp.SupportedFeatures = proto.Uint64(SupportedFeatures)
 
 	return resp, nil
 }

template.go

@@ -422,7 +422,7 @@ func parseFileExtension(pe *protokit.ExtensionDescriptor) *FileExtension {
 		LongName:           pe.GetLongName(),
 		FullName:           pe.GetFullName(),
 		Description:        description(pe.GetComments().String()),
-		Label:              labelName(pe.GetLabel(), pe.IsProto3()),
+		Label:              labelName(pe.GetLabel(), pe.IsProto3(), pe.GetProto3Optional()),
 		Type:               t,
 		LongType:           lt,
 		FullType:           ft,
@@ -473,7 +473,7 @@ func parseMessageField(pf *protokit.FieldDescriptor) *MessageField {
 	m := &MessageField{
 		Name:         pf.GetName(),
 		Description:  description(pf.GetComments().String()),
-		Label:        labelName(pf.GetLabel(), pf.IsProto3()),
+		Label:        labelName(pf.GetLabel(), pf.IsProto3(), pf.GetProto3Optional()),
 		Type:         t,
 		LongType:     lt,
 		FullType:     ft,
@@ -532,8 +532,8 @@ func baseName(name string) string {
 	return parts[len(parts)-1]
 }
 
-func labelName(lbl descriptor.FieldDescriptorProto_Label, proto3 bool) string {
-	if proto3 && lbl != descriptor.FieldDescriptorProto_LABEL_REPEATED {
+func labelName(lbl descriptor.FieldDescriptorProto_Label, proto3 bool, proto3Opt bool) string {
+	if proto3 && !proto3Opt && lbl != descriptor.FieldDescriptorProto_LABEL_REPEATED {
 		return ""
 	}
 

template_test.go

@@ -340,8 +340,7 @@ func TestFieldPropertiesProto3Optional(t *testing.T) {
 	field = findField("name", msg)
 	require.Equal(t, "name", field.Name)
 	require.Equal(t, "The name of the cookie.", field.Description)
-	//TODO(ezimanyi): This is a proto3 optional field, and should have label "optional"
-	require.Equal(t, "", field.Label)
+	require.Equal(t, "optional", field.Label)
 	require.Equal(t, "string", field.Type)
 	require.Equal(t, "string", field.LongType)
 	require.Equal(t, "string", field.FullType)