[fixes #3605] Remove outer type reference when marking class as static

The class now implements Serializable to trigger generation of this$0
fields in the inner class. Since Java 18 the compiler only generates the
reference to the outer class if it is in use or the class is
Serializable.

src/core/lombok/javac/handlers/HandleUtilityClass.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2015-2021 The Project Lombok Authors.
+ * Copyright (C) 2015-2024 The Project Lombok Authors.
  * 
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to deal
@@ -107,7 +107,7 @@ public class HandleUtilityClass extends JavacAnnotationHandler<UtilityClass> {
 			if ((typeDecl.mods.flags & (Flags.INTERFACE | Flags.ANNOTATION)) != 0) markStatic = false;
 		}
 		
-		if (markStatic) classDecl.mods.flags |= Flags.STATIC;
+		if (markStatic) markClassAsStatic(classDecl);
 		
 		for (JavacNode element : typeNode.down()) {
 			if (element.getKind() == Kind.FIELD) {
@@ -125,21 +125,23 @@ public class HandleUtilityClass extends JavacAnnotationHandler<UtilityClass> {
 				
 				methodDecl.mods.flags |= Flags.STATIC;
 			} else if (element.getKind() == Kind.TYPE) {
-				JCClassDecl innerClassDecl = (JCClassDecl) element.get();
-				innerClassDecl.mods.flags |= Flags.STATIC;
-				ClassSymbol innerClassSymbol = innerClassDecl.sym;
-				if (innerClassSymbol != null) {
-					if (innerClassSymbol.type instanceof ClassType) {
-						((ClassType) innerClassSymbol.type).setEnclosingType(Type.noType);
-					}
-					innerClassSymbol.erasure_field = null;
-				}
+				markClassAsStatic((JCClassDecl) element.get());
 			}
 		}
 		
 		if (makeConstructor) createPrivateDefaultConstructor(typeNode);
 	}
 	
+	private void markClassAsStatic(JCClassDecl classDecl) {
+		classDecl.mods.flags |= Flags.STATIC; // maybe Flags.NOOUTERTHIS too?
+		ClassSymbol innerClassSymbol = classDecl.sym;
+		if (innerClassSymbol != null && innerClassSymbol.type instanceof ClassType) {
+			ClassType classType = (ClassType) innerClassSymbol.type;
+			classType.setEnclosingType(Type.noType);
+			innerClassSymbol.erasure_field = null;
+		}
+	}
+	
 	private void createPrivateDefaultConstructor(JavacNode typeNode) {
 		JavacTreeMaker maker = typeNode.getTreeMaker();
 		JCModifiers mods = maker.Modifiers(Flags.PRIVATE, List.<JCAnnotation>nil());

test/transform/resource/after-delombok/UtilityClassInner.java

@@ -1,5 +1,6 @@
+@java.lang.SuppressWarnings("serial")
 class UtilityClassInner {
-	static final class UtilClass {
+	static final class UtilClass implements java.io.Serializable {
 		@java.lang.SuppressWarnings("all")
 		@lombok.Generated
 		private UtilClass() {

test/transform/resource/after-ecj/UtilityClassInner.java

@@ -1,5 +1,5 @@
-class UtilityClassInner {
-  static final @lombok.experimental.UtilityClass class UtilClass {
+@java.lang.SuppressWarnings("serial") class UtilityClassInner {
+  static final @lombok.experimental.UtilityClass class UtilClass implements java.io.Serializable {
     private @java.lang.SuppressWarnings("all") @lombok.Generated UtilClass() {
       super();
       throw new java.lang.UnsupportedOperationException("This is a utility class and cannot be instantiated");

test/transform/resource/before/UtilityClassInner.java

@@ -1,4 +1,5 @@
+@java.lang.SuppressWarnings("serial")
 class UtilityClassInner {
 	@lombok.experimental.UtilityClass
-	class UtilClass {}
+	class UtilClass implements java.io.Serializable {}
 }