Add compile-time validation for effectively always cancelling listeners (#112)

Suggests changing code that always returns true inside a possibly cancelling listener into a void return always cancelling listener, as the latter allows EventBus to know at listener registration time and can perform additional optimisations based on that knowledge.

Author: PaintNinja

eventbus-test/src/test/java/net/minecraftforge/eventbus/test/compiletime/SubscribeEventValidatorTests.java

@@ -74,6 +74,13 @@ public void testSubscribeEventReturnTypeCancellable() {
         """);
         assertThat(compilation).hadWarningContaining("consider using a void return type");
 
+        compilation = compile("""
+            @SubscribeEvent
+            boolean alwaysCancellingEvent(CancelableEvent event) { return true; }
+        """);
+        assertThat(compilation).hadWarningContaining("consider using a void return type");
+        assertThat(compilation).hadWarningContaining("alwaysCancelling = true");
+
         compilation = compile("""
             @SubscribeEvent(alwaysCancelling = true)
             void alwaysCancellingEvent(CancelableEvent event) {}

eventbus-validator/src/main/java/net/minecraftforge/eventbus/validator/SubscribeEventValidator.java

@@ -104,12 +104,20 @@ private void validate(ExecutableElement method) {
             var scanner = new ReturnScanner();
             scanner.scan(methodPath, null);
 
-            if (scanner.sawReturn && !scanner.sawNonLiteralReturn && scanner.sawLiteralFalse && !scanner.sawLiteralTrue) {
-                // if we get here, all returns in this method are literal `return false;` statements
-                processingEnv.getMessager().printWarning(
-                        "Listener always returns false, consider using a void return type instead",
-                        method
-                );
+            if (scanner.sawReturn && !scanner.sawNonLiteralReturn) {
+                if (scanner.sawLiteralFalse && !scanner.sawLiteralTrue) {
+                    // if we get here, all returns in this method are literal `return false;` statements
+                    processingEnv.getMessager().printWarning(
+                            "Listener always returns false, consider using a void return type instead",
+                            method
+                    );
+                } else if (scanner.sawLiteralTrue && !scanner.sawLiteralFalse) {
+                    // if we get here, all returns in this method are literal `return true;` statements
+                    processingEnv.getMessager().printWarning(
+                            "Listener always returns true, consider using a void return type combined with @SubscribeEvent(alwaysCancelling = true) or CancellableEventBus#addListener(true, ...) instead",
+                            method
+                    );
+                }
             }
         }
     }