Fix AttachCapabilitiesEvent dispatch being very slow

EventBus strikes again...

Author: embeddedt

src/main/java/org/embeddedt/modernfix/common/mixin/perf/forge_cap_retrieval/AttachCapabilitiesEventMixin.java

@@ -0,0 +1,26 @@
+package org.embeddedt.modernfix.common.mixin.perf.forge_cap_retrieval;
+
+import net.minecraftforge.event.AttachCapabilitiesEvent;
+import net.minecraftforge.eventbus.api.Event;
+import org.spongepowered.asm.mixin.Mixin;
+
+@Mixin(AttachCapabilitiesEvent.class)
+public abstract class AttachCapabilitiesEventMixin extends Event {
+    /**
+     * @author embeddedt
+     * @reason EventSubclassTransformer is supposed to inject an override returning a constant on the class to avoid the
+     * {@link net.minecraftforge.eventbus.api.EventListenerHelper#isCancelable(Class)} slow path.
+     * However, the false case is only done for direct subclasses of Event (the true case is done for
+     * any cancelable event). This works for normal events because they must subclass Event directly, or be a subclass
+     * of an event that does. However, AttachCapabilitiesEvent subclasses GenericEvent, which does not pass through
+     * the EventSubclassTransformer as it comes from the EventBus library (where transformers are not run) rather than
+     * Forge which is on the GAME layer. The transformer on AttachCapabilitiesEvent then does not add the override as
+     * it expects it to be present on GenericEvent already.
+     * <p>
+     * The simplest workaround to that whole mess is to just inject the override ourselves.
+     */
+    @Override
+    public boolean isCancelable() {
+        return false;
+    }
+}