scalapb-json4s: add support for ScalaPB generated code compiled with Scala 3

[bjaglin]

Betting on Scala 2.13/3 compatibility, I tried to use scalapb-json4s_2.13 0.11.0 against classes compiled with Scala 3 from code generated by ScalaPB 0.11.3, and got the following errors

NoSuchFieldException

Caused by: java.lang.NoSuchFieldException: MODULE$
    at java.lang.Class.getField(Class.java:1999)
    at scalapb.json4s.FormatRegistry.companionForClass(JsonFormat.scala:52)
    at scalapb.json4s.FormatRegistry.registerMessageFormatter(JsonFormat.scala:63)
    at scalapb.json4s.FormatRegistry.registerWriter(JsonFormat.scala:86)
    at scalapb.json4s.JsonFormat$.<clinit>(JsonFormat.scala:711)

Looking at the classpath with sbt, it's unclear which binary version of scalapb-runtime is used (dependencyTree shows both but fullClasspath shows only 2.13?), but I assume the problem comes from the WKT classes included in scalapb-runtime_3, as we can see that the returned type inferred for companion() is different:

--- Timestamp_2_13.class.javap
+++ Timestamp_3.class.javap
-  public com.google.protobuf.timestamp.Timestamp$ companion();
+  public scalapb.GeneratedMessageCompanion<?> companion();

IncompatibleClassChangeError

java.lang.IncompatibleClassChangeError: Class scalapb.descriptors.PEnum$ does not implement the requested interface scala.Function1
    at scala.Option.fold(Option.scala:263)
    at scalapb.json4s.Parser.parseSingleValue(JsonFormat.scala:684)
    at scalapb.json4s.Parser.$anonfun$fromJsonToPMessage$2(JsonFormat.scala:580)
    at scala.collection.immutable.List.map(List.scala:246)
    at scalapb.json4s.Parser.parseValue$1(JsonFormat.scala:580)
    at scalapb.json4s.Parser.$anonfun$fromJsonToPMessage$3(JsonFormat.scala:602)
    at scala.collection.immutable.List.foreach(List.scala:333)
    at scalapb.json4s.Parser.fromJsonToPMessage(JsonFormat.scala:598)
    at scalapb.json4s.Parser.fromJson(JsonFormat.scala:530)
    at scalapb.json4s.Parser.fromJson(JsonFormat.scala:523)
    at scalapb.json4s.Parser.fromJsonString(JsonFormat.scala:517)
    at scalapb.json4s.JsonFormat$.fromJsonString(JsonFormat.scala:843)

Synthetic bytecode emitted for case classes has changed. For example, for case class Foo(a: Int)

--- Foo$_2_13.class.javap
+++ Foo$_3.class.javap
-public final class Foo$ extends scala.runtime.AbstractFunction1<java.lang.Object, Foo> implements java.io.Serializable {
+public final class Foo$ implements scala.deriving.Mirror$Product,java.io.Serializable {
   public static final Foo$ MODULE$;
   public static {};
-  public final java.lang.String toString();
   public Foo apply(int);
-  public scala.Option<java.lang.Object> unapply(Foo);
-  public java.lang.Object apply(java.lang.Object);
+  public Foo unapply(Foo);
+  public java.lang.String toString();
+  public Foo fromProduct(scala.Product);
+  public java.lang.Object fromProduct(scala.Product);
 }

[thesamet]

@bjaglin Is there additional work needed to resolve this issue?

[bjaglin]

@bjaglin Is there additional work needed to resolve this issue?

Not to my knowledge. Thanks for the ping, I just spent some time finishing up scalapb/scalapb-validate#123 which was the original driver for this, and it's green 👍

[bjaglin]

(I know it's not ideal and it would better be tested in ScalaPB itself, but at least if something regress we'll see it upon bumping https://github.com/scalapb/scalapb-validate)