Pass unmodifiable lists to DisplayNameGenerator

Author: marcphilipp

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/descriptor/DisplayNameUtils.java

@@ -10,6 +10,8 @@
 
 package org.junit.jupiter.engine.descriptor;
 
+import static java.util.Collections.emptyList;
+import static java.util.Collections.unmodifiableList;
 import static org.junit.platform.commons.support.AnnotationSupport.findAnnotation;
 
 import java.lang.reflect.AnnotatedElement;
@@ -121,14 +123,18 @@ private static Supplier<String> createDisplayNameSupplier(Supplier<List<Class<?>
 			Class<?> testClass, JupiterConfiguration configuration,
 			BiFunction<DisplayNameGenerator, List<Class<?>>, String> generatorFunction) {
 		return () -> {
-			List<Class<?>> enclosingInstanceTypes = enclosingInstanceTypesSupplier.get();
+			List<Class<?>> enclosingInstanceTypes = makeUnmodifiable(enclosingInstanceTypesSupplier.get());
 			return findDisplayNameGenerator(enclosingInstanceTypes, testClass) //
 					.map(it -> generatorFunction.apply(it, enclosingInstanceTypes)) //
 					.orElseGet(() -> generatorFunction.apply(configuration.getDefaultDisplayNameGenerator(),
 						enclosingInstanceTypes));
 		};
 	}
 
+	private static <T> List<T> makeUnmodifiable(List<T> list) {
+		return list.isEmpty() ? emptyList() : unmodifiableList(list);
+	}
+
 	private static Optional<DisplayNameGenerator> findDisplayNameGenerator(List<Class<?>> enclosingInstanceTypes,
 			Class<?> testClass) {
 		Preconditions.notNull(testClass, "Test class must not be null");