Fix size on empty oneOf marshalling

planetscale#130 did not correctly
account for the key size impacting the size. This fixes the original PR,
and adds another test case to cover the issue

conformance/internal/conformance/oneof_test.go

@@ -1,16 +1,25 @@
 package conformance
 
 import (
-	"testing"
-
+	"github.com/planetscale/vtprotobuf/types/known/structpb"
 	"github.com/stretchr/testify/require"
 	"google.golang.org/protobuf/proto"
+	upstreamstructpb "google.golang.org/protobuf/types/known/structpb"
+	"testing"
 )
 
-func TestEmptyOneoff(t *testing.T) {
+func TestEmptyOneof(t *testing.T) {
 	// Regression test for https://github.com/planetscale/vtprotobuf/issues/61
-	msg := &TestAllTypesProto3{OneofField: &TestAllTypesProto3_OneofNestedMessage{}}
-	upstream, _ := proto.Marshal(msg)
-	vt, _ := msg.MarshalVTStrict()
-	require.Equal(t, upstream, vt)
+	t.Run("all proto", func(t *testing.T) {
+		msg := &TestAllTypesProto3{OneofField: &TestAllTypesProto3_OneofNestedMessage{}}
+		upstream, _ := proto.Marshal(msg)
+		vt, _ := msg.MarshalVTStrict()
+		require.Equal(t, upstream, vt)
+	})
+	t.Run("structpb", func(t *testing.T) {
+		msg := &structpb.Value{Kind: &upstreamstructpb.Value_StructValue{}}
+		upstream, _ := proto.Marshal((*upstreamstructpb.Value)(msg))
+		vt, _ := msg.MarshalVTStrict()
+		require.Equal(t, upstream, vt)
+	})
 }

features/size/size.go

@@ -267,9 +267,9 @@ func (p *size) field(oneof bool, field *protogen.Field, sizeName string) {
 	}
 	// Empty protobufs should emit a message or compatibility with Golang protobuf;
 	// See https://github.com/planetscale/vtprotobuf/issues/61
-	// Size is always 3 so just hardcode that here
+	// Size is always keysize + 1 so just hardcode that here
 	if oneof && field.Desc.Kind() == protoreflect.MessageKind && !field.Desc.IsMap() && !field.Desc.IsList() {
-		p.P("} else { n += 3 }")
+		p.P("} else { n += ", strconv.Itoa(key + 1), " }")
 	} else if repeated || nullable {
 		p.P(`}`)
 	}