DefaultLauncher no longer prunes TestDescriptor trees

Closes #732.

documentation/src/docs/asciidoc/release-notes-5.0.0-M5.adoc

@@ -65,6 +65,10 @@ is placed on the Java 9 module path.
   - `junit-platform-commons`: `ReflectionUtils.findAllClassesInClasspathRoot(Path, Predicate, Predicate)`
 * The `isLeaf()` method of the `org.junit.platform.engine.support.hierarchical.Node`
   interface has been removed.
+* The default methods `prune()` and `pruneTree()` have been removed from `TestDescriptor`.
+* The `DefaultLauncher` no longer prunes trees of `TestDescriptors` it receives from test
+  engines. It's the responsibility of test engines to only return `TestDescriptors` they
+  wish to execute.
 
 ===== New Features and Improvements
 

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/discovery/DiscoverySelectorResolver.java

@@ -64,7 +64,16 @@ public void resolveSelectors(EngineDiscoveryRequest request, TestDescriptor engi
 		request.getSelectorsByType(UniqueIdSelector.class).forEach(selector -> {
 			javaElementsResolver.resolveUniqueId(selector.getUniqueId());
 		});
-		engineDescriptor.pruneTree();
+		pruneTree(engineDescriptor);
+	}
+
+	private void pruneTree(TestDescriptor engineDescriptor) {
+		engineDescriptor.accept(testDescriptor -> {
+			if (testDescriptor.isRoot() || testDescriptor.hasTests()) {
+				return;
+			}
+			testDescriptor.removeFromHierarchy();
+		});
 	}
 
 	private JavaElementsResolver createJavaElementsResolver(TestDescriptor engineDescriptor) {

junit-platform-engine/src/main/java/org/junit/platform/engine/TestDescriptor.java

@@ -195,37 +195,6 @@ default boolean hasTests() {
 		return isTest() || getChildren().stream().anyMatch(TestDescriptor::hasTests);
 	}
 
-	/**
-	 * Remove this descriptor from the hierarchy unless it is a root or has tests.
-	 *
-	 * <p>A concrete {@link TestEngine} may override this method in order to implement
-	 * a different algorithm or to skip pruning altogether.
-	 *
-	 * @see #isRoot()
-	 * @see #hasTests()
-	 * @see #removeFromHierarchy()
-	 */
-	default void prune() {
-		if (isRoot() || hasTests()) {
-			return;
-		}
-		removeFromHierarchy();
-	}
-
-	/**
-	 * Remove this descriptor and its descendants from the hierarchy.
-	 *
-	 * <p>The default implementation supplies the {@link #prune()} method as a
-	 * {@link Visitor} to this descriptor, thereby effectively removing this
-	 * descriptor and all of its descendants.
-	 *
-	 * @see #accept(Visitor)
-	 * @see #prune()
-	 */
-	default void pruneTree() {
-		accept(TestDescriptor::prune);
-	}
-
 	/**
 	 * Find the descriptor with the supplied unique ID.
 	 *

junit-platform-launcher/src/main/java/org/junit/platform/launcher/core/DefaultLauncher.java

@@ -118,7 +118,6 @@ private Root discoverRoot(LauncherDiscoveryRequest discoveryRequest, String phas
 			engineRoot.ifPresent(rootDescriptor -> root.add(testEngine, rootDescriptor));
 		}
 		root.applyPostDiscoveryFilters(discoveryRequest);
-		root.prune();
 		return root;
 	}
 

junit-platform-launcher/src/main/java/org/junit/platform/launcher/core/Root.java

@@ -60,17 +60,6 @@ void applyPostDiscoveryFilters(LauncherDiscoveryRequest discoveryRequest) {
 		acceptInAllTestEngines(removeExcludedTestDescriptors);
 	}
 
-	/**
-	 * Prune all branches in the tree of {@link TestDescriptor TestDescriptors}
-	 * that do not have executable tests.
-	 *
-	 * <p>If a {@link TestEngine} ends up with no {@code TestDescriptors} after
-	 * pruning, it will <strong>not</strong> be removed.
-	 */
-	void prune() {
-		acceptInAllTestEngines(TestDescriptor::prune);
-	}
-
 	private boolean isExcluded(TestDescriptor descriptor, Filter<TestDescriptor> postDiscoveryFilter) {
 		return descriptor.getChildren().isEmpty() && postDiscoveryFilter.apply(descriptor).excluded();
 	}