Hierarchy validation/transformation does not consider superinterfaces

[zml2008]

Given a Mixin class with the signature:

@Mixin(Level.class)
public abstract class LevelMixin_Tracker implements LevelBridge, LevelAccessorMixin_Tracker {}

where LevelBridge is an ordinary interface, and LevelAccessorMixin_Tracker is an interface mixin targeting LevelAccessor (an interface which Level implements), I would expect Mixin to handle the implementation of LevelAccessorMixin_Tracker in the same way that it would handle a superclass that is a Mixin claass.

However, currently Mixin directly applies both interfaces to the target class, resulting in a raw reference to a Mixin class appearing in a real class. This causes classloading to fail with a cryptic message:

[23:24:10] [main/INFO] [mixin]: Mixing minecraft.world.level.LevelMixin_API from mixins.sponge.api.json into net.minecraft.world.level.Level
[23:24:10] [main/INFO] [mixin]: Mixing world.level.LevelMixin from mixins.sponge.core.json into net.minecraft.world.level.Level
[23:24:10] [main/INFO] [mixin]: Mixing world.level.LevelMixin_Timings from mixins.sponge.core.json into net.minecraft.world.level.Level
[23:24:10] [main/INFO] [mixin]: Mixing world.level.LevelMixin_Tracker from mixins.sponge.tracker.json into net.minecraft.world.level.Level
[23:24:10] [main/WARN] [mixin]: Write access to @Mutable @Final field dimensionType:Lnet/minecraft/world/level/dimension/DimensionType; in mixins.sponge.core.json:world.level.LevelMixin::bridge$adjustDimensionLogic
Exception in thread "main" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
	at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:39)
	at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54)
	at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72)
	at cpw.mods.modlauncher.Launcher.run(Launcher.java:82)
	at cpw.mods.modlauncher.Launcher.main(Launcher.java:66)
	at org.spongepowered.vanilla.applaunch.Main.run(Main.java:95)
	at org.spongepowered.vanilla.applaunch.Main.main(Main.java:65)
	at org.spongepowered.vanilla.applaunch.Main.main(Main.java:59)
Caused by: java.lang.reflect.InvocationTargetException
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
	at org.spongepowered.vanilla.applaunch.handler.dev.ClientDevLaunchHandler.launchService0(ClientDevLaunchHandler.java:45)
	at org.spongepowered.vanilla.applaunch.handler.AbstractVanillaLaunchHandler.lambda$2(AbstractVanillaLaunchHandler.java:197)
	at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37)
	... 7 more
Caused by: java.lang.NoClassDefFoundError: org.spongepowered.common.mixin.tracker.world.level.LevelAccessorMixin_Tracker is invalid
	at org.spongepowered.asm.service.modlauncher.ModLauncherClassTracker.handlesClass(ModLauncherClassTracker.java:91)
	at org.spongepowered.asm.launch.MixinLaunchPluginLegacy.handlesClass(MixinLaunchPluginLegacy.java:109)
	at cpw.mods.modlauncher.LaunchPluginHandler.computeLaunchPluginTransformerSet(LaunchPluginHandler.java:66)
	at cpw.mods.modlauncher.ClassTransformer.transform(ClassTransformer.java:64)
	at cpw.mods.modlauncher.TransformingClassLoader$DelegatedClassLoader.findClass(TransformingClassLoader.java:265)
	at cpw.mods.modlauncher.TransformingClassLoader.loadClass(TransformingClassLoader.java:136)
	at cpw.mods.modlauncher.TransformingClassLoader.loadClass(TransformingClassLoader.java:98)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:520)
	at java.base/java.lang.ClassLoader.defineClass1(Native Method)
	at java.base/java.lang.ClassLoader.defineClass(ClassLoader.java:1012)
	at cpw.mods.modlauncher.TransformingClassLoader.loadClass(TransformingClassLoader.java:138)
	at cpw.mods.modlauncher.TransformingClassLoader.loadClass(TransformingClassLoader.java:98)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:520)
	at net.minecraft.client.main.Main.main(Main.java:136)
	at org.spongepowered.vanilla.launch.ClientLaunch.lambda$0(ClientLaunch.java:51)
	at org.spongepowered.vanilla.launch.VanillaBootstrap.perform(VanillaBootstrap.java:65)
	at org.spongepowered.vanilla.launch.ClientLaunch.performBootstrap(ClientLaunch.java:51)
	at org.spongepowered.vanilla.launch.VanillaLaunch.launchPlatform(VanillaLaunch.java:119)
	at org.spongepowered.vanilla.launch.ClientLaunch.launch(ClientLaunch.java:41)
	... 14 more

Ideally superinterfaces that are Mixins would have the same handling as superclasses that are Mixins, but an acceptable alternative would be providing a more clear & specific error message indicating that this inheritance layout is not supported.

[Mumfrey]

Ideally superinterfaces that are Mixins would have the same handling as superclasses that are Mixins, but an acceptable alternative would be providing a more clear & specific error message indicating that this inheritance layout is not supported.

There are so many cases like this because of the way interfaces were originally designed to be handled not taking into account the Java 8 changes, it's why I'm looking at reimplementing the whole thing before allowing stuff like injectors in interfaces, the way mixin handles interfaces is just too outdated compared with how the use-cases available want to employ them.

This isn't an isolated issue and this is basically one small issue which goes away after tackling the larger problem which is "interfaces are second-class citizens in mixin". It's not even its own class of problem, it's related to several others.

Hence I'm not even really classing this as a bug because this is just a case that mixin was never designed to handle in the first place, and while it could admittedly handle this illegal state more gracefully, the real solution is all of the interface handling needs to be improved. Hence I'm labelling this as enhancement because it's really part of a feature which is "support interface mixins properly".