MixinLoader breaks with Mixin 0.8.3+, as MixinProcessor became package-private, leading to
java.lang.IllegalAccessError: tried to access class org.spongepowered.asm.mixin.transformer.MixinProcessor from class net.minecraftforge.fml.common.Loader
at net.minecraftforge.fml.common.Loader.handler$zza000$beforeConstructingMods(Loader.java:1141)
MixinBooter fixed this here, perhaps you could implement something similar.
MixinLoader
breaks with Mixin 0.8.3+, asMixinProcessor
became package-private, leading toMixinBooter fixed this here, perhaps you could implement something similar.