[1.21] Mixins get ClassNotFoundException when referencing the mod's Kotlin classes

[filloax]

Hi, I'm using KFF 5.6.0 in my 1.21 neoforge mod, but when it loads mixins I get this error when the mixin code tries to use a Kotlin class (even just a static reference):

Exception in thread "main" java.lang.RuntimeException: org.spongepowered.asm.mixin.transformer.throwables.MixinTransformerError: An unexpected critical error was encountered
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:32)
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:53)
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:71)
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.Launcher.run(Launcher.java:103)
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.Launcher.main(Launcher.java:74)
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.BootstrapLaunchConsumer.accept(BootstrapLaunchConsumer.java:26)
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.BootstrapLaunchConsumer.accept(BootstrapLaunchConsumer.java:23)
	at [email protected]/cpw.mods.bootstraplauncher.BootstrapLauncher.run(BootstrapLauncher.java:210)
	at [email protected]/cpw.mods.bootstraplauncher.BootstrapLauncher.main(BootstrapLauncher.java:69)
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
	at [email protected]/net.neoforged.devlaunch.Main.main(Main.java:55)
Caused by: org.spongepowered.asm.mixin.transformer.throwables.MixinTransformerError: An unexpected critical error was encountered
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:392)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClass(MixinTransformer.java:250)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.service.modlauncher.MixinTransformationHandler.processClass(MixinTransformationHandler.java:131)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.launch.MixinLaunchPluginLegacy.processClass(MixinLaunchPluginLegacy.java:131)
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.serviceapi.ILaunchPluginService.processClassWithFlags(ILaunchPluginService.java:156)
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.LaunchPluginHandler.offerClassNodeToPlugins(LaunchPluginHandler.java:94)
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.ClassTransformer.transform(ClassTransformer.java:120)
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.TransformingClassLoader.maybeTransformClassBytes(TransformingClassLoader.java:57)
	at cpw.mods.securejarhandler/cpw.mods.cl.ModuleClassLoader.readerToClass(ModuleClassLoader.java:190)
	at cpw.mods.securejarhandler/cpw.mods.cl.ModuleClassLoader.lambda$findClass$20(ModuleClassLoader.java:298)
	at cpw.mods.securejarhandler/cpw.mods.cl.ModuleClassLoader.loadFromModule(ModuleClassLoader.java:311)
	at cpw.mods.securejarhandler/cpw.mods.cl.ModuleClassLoader.findClass(ModuleClassLoader.java:298)
	at cpw.mods.securejarhandler/cpw.mods.cl.ModuleClassLoader.loadClass(ModuleClassLoader.java:214)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:526)
	at java.base/java.lang.Class.getDeclaredMethods0(Native Method)
	at java.base/java.lang.Class.privateGetDeclaredMethods(Class.java:3578)
	at java.base/java.lang.Class.privateGetPublicMethods(Class.java:3603)
	at java.base/java.lang.Class.getMethods(Class.java:2185)
	at MC-BOOTSTRAP/[email protected]/net.neoforged.fml.earlydisplay.DisplayWindow.updateModuleReads(DisplayWindow.java:605)
	at MC-BOOTSTRAP/[email protected]/net.neoforged.fml.loading.ImmediateWindowHandler.acceptGameLayer(ImmediateWindowHandler.java:94)
	at MC-BOOTSTRAP/[email protected]/net.neoforged.fml.loading.FMLLoader.beforeStart(FMLLoader.java:160)
	at MC-BOOTSTRAP/[email protected]/net.neoforged.fml.loading.targets.CommonLaunchHandler.launchService(CommonLaunchHandler.java:115)
	at MC-BOOTSTRAP/[email protected]/cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:30)
	... 11 more
Caused by: org.spongepowered.asm.mixin.transformer.throwables.MixinPreProcessorException: Attach error for growsseth.mixins.json:client.RuinsMapIcon_GuiGraphicsMixin from mod growsseth during activity: [Transform -> Method onRenderItemDecorations(Lnet/minecraft/client/gui/Font;Lnet/minecraft/world/item/ItemStack;IILjava/lang/String;Lorg/spongepowered/asm/mixin/injection/callback/CallbackInfo;)V -> GETSTATIC -> com/ruslan/growsseth/item/GrowssethItems::RUINS_MAP:Lnet/minecraft/world/item/MapItem;]
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.MixinPreProcessorStandard.attach(MixinPreProcessorStandard.java:313)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.MixinPreProcessorStandard.createContextFor(MixinPreProcessorStandard.java:277)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.MixinInfo.createContextFor(MixinInfo.java:1289)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.MixinApplicatorStandard.apply(MixinApplicatorStandard.java:187)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.TargetClassContext.apply(TargetClassContext.java:437)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.TargetClassContext.applyMixins(TargetClassContext.java:418)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:363)
	... 33 more
Caused by: java.lang.RuntimeException: java.lang.ClassNotFoundException: com.ruslan.growsseth.item.GrowssethItems
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.MixinPreProcessorStandard.transformMemberReference(MixinPreProcessorStandard.java:791)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.MixinPreProcessorStandard.transformField(MixinPreProcessorStandard.java:782)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.MixinPreProcessorStandard.transform(MixinPreProcessorStandard.java:745)
	at MC-BOOTSTRAP/org.spongepowered.mixin/org.spongepowered.asm.mixin.transformer.MixinPreProcessorStandard.attach(MixinPreProcessorStandard.java:307)
	... 39 more
Caused by: java.lang.ClassNotFoundException: com.ruslan.growsseth.item.GrowssethItems

This doesn't happen in the Fabric version of the mod, where the mixins can use the Kt classes with no issue - am I missing something, or is this a bug?

[thedarkcolour]

Please send the entire log.

[thedarkcolour]

Does this issue happen on 5.5.0?

[filloax]

It does happen in 5.5.0, and here's the log:
latest.log

[filloax]

After grueling investigation, I can confirm that this only seems to happen with the atomicfu plugin enabled in Gradle (regardless of using any of its classes). Changing to another plugin for now, but to note that this doesn't happen on Fabric with Fabric Language Kotlin, so it's likely a conflict of some sort.