Simplify code for ProtoAdapters of types with no fields

[Egorand]

Example:

companion object {
    @JvmField
    val ADAPTER: ProtoAdapter<NodeEvent> = object : ProtoAdapter<NodeEvent>(
        FieldEncoding.LENGTH_DELIMITED,
        NodeEvent::class.java
    ) {
        override fun decode(reader: ProtoReader): NodeEvent {
            val unknownFields = reader.forEachTag { tag ->
                when (tag) {
                    else -> TagHandler.UNKNOWN_TAG
                }
            }
   ...
}

The when block is unnecessary, since NodeEvent doesn't have any fields. This can be simplified to:

val unknownFields = reader.forEachTag { TagHandler.UNKNOWN_TAG }

[swankjesse]

Does this ever happen?

[Egorand]

Yes, this is a real example from the real life

[JakeWharton]

Why isn't the type pruned completely instead?

[oldergod]

It cannot be pruned if it's an enclosing type

[JakeWharton]

Bytecode wise this source-based optimization should have no effect. I'll check though.

Enclosing types should be generated as objects which don't extend Message.

[oldergod]

Also, type with no fields exist as well right? We have a few message Request {}. If we were to generate an object type, how do we deal with unknown fields?
Or is it only if pruned, that an enclosing type should be an not extending Message object?

[JakeWharton]

I'm just saying that enclosing types are separate from messages with no fields. This case only occurs in the latter.

[JakeWharton]

Yep kotlinc doesn't even both with emitting anything for an empty when (except a warning).

$ cat Test.kt
fun test(tag: Int) {
  when (tag) {
    else -> println("Hi")
  }
}

$ kotlinc Test.kt
OpenJDK 64-Bit Server VM warning: Options -Xverify:none and -noverify were deprecated in JDK 13 and will likely be removed in a future release.
Test.kt:2:9: warning: the expression is unused
  when (tag) {
        ^

$ javap -c TestKt.class
Compiled from "Test.kt"
public final class TestKt {
  public static final void test(int);
    Code:
       0: iload_0
       1: istore_1
       2: ldc           #8                  // String Hi
       4: astore_2
       5: iconst_0
       6: istore_3
       7: getstatic     #14                 // Field java/lang/System.out:Ljava/io/PrintStream;
      10: aload_2
      11: invokevirtual #20                 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
      14: return
}