Scanner.java

// thank HalfOfTwo for this monstrosity
public class Scanner implements PreLaunchEntrypoint {
	private static final Field ACTIVE_TRANSFORMER, MIXIN_TRANSFORMER;

	static {
		Transformer.init();

		System.out.println("I am become Mass ASM, destroyer of LexManos");

		try {
			ACTIVE_TRANSFORMER = MixinEnvironment.class.getDeclaredField("transformer");
			ACTIVE_TRANSFORMER.setAccessible(true);
			MIXIN_TRANSFORMER = Class.forName("net.fabricmc.loader.launch.knot.KnotClassDelegate").getDeclaredField("mixinTransformer");
			MIXIN_TRANSFORMER.setAccessible(true);
		} catch (ReflectiveOperationException arg) {
			throw new RuntimeException(arg);
		}
	}

	public class FabricMixinTransformerProxyProxy extends FabricMixinTransformerProxy {
		private final FabricMixinTransformerProxy proxy;

		public FabricMixinTransformerProxyProxy(FabricMixinTransformerProxy proxy) {this.proxy = proxy;}

		@Override
		public byte[] transformClassBytes(String name, String transformedName, byte[] basicClass) {
			if (basicClass == null) {
				return this.proxy.transformClassBytes(name, transformedName, basicClass);
			}

			byte[] transformed = this.proxy.transformClassBytes(name, transformedName, basicClass);

			return Transformer.transform(transformedName, transformed);
		}
	}

	@Override
	public void onPreLaunch() {
		try {
			// first we need knot's delegate
			Object delegate = this.getDelegate();
			// then we get the current mixin transformer
			FabricMixinTransformerProxy proxy = (FabricMixinTransformerProxy) MIXIN_TRANSFORMER.get(delegate);

			Field active = FabricMixinTransformerProxy.class.getDeclaredField("transformer");
			active.setAccessible(true);

			// we need to delete mixin's current mixin transformer to get around it's constructor check
			Object currentTransformer = active.get(proxy);

			// we remove the active transformer
			ACTIVE_TRANSFORMER.set(null, null);
			// this action overrides the current transformer
			FabricMixinTransformerProxyProxy proxyProxy = new FabricMixinTransformerProxyProxy(proxy);
			// so we return the old one
			ACTIVE_TRANSFORMER.set(null, currentTransformer);

			MIXIN_TRANSFORMER.set(delegate, proxyProxy);
		} catch (Throwable e) {
			throw new RuntimeException(e);
		}
	}

	public Object getDelegate() throws ReflectiveOperationException {
		ClassLoader loader = Scanner.class.getClassLoader();
		Class<?> knotInterface = Class.forName("net.fabricmc.loader.launch.knot.KnotClassLoaderInterface");
		while (loader != null && !knotInterface.isInstance(loader)) {
			loader = loader.getParent();
		}

		Method method = knotInterface.getMethod("getDelegate");
		method.setAccessible(true);
		return method.invoke(loader);
	}
}