Restore supplier-disposal link in ClassRequestor without jakarta

PR eclipse-platform#2649 decoupled the E4 injector from jakarta annotations by replacing
the ClassRequestor pseudo-dependency descriptor with an empty array.
That
pseudo-link is what notifies an injected object when its supplier is
disposed. With an empty descriptor the link is no longer established, so
@PreDestroy of objects that only use constructor/@PostConstruct
injection
(no @Inject links) is no longer guaranteed to run before the supplying
context is disposed.

This broke disposal ordering and surfaced as a failure of
PartRenderingEngineTests.testBut336225 ("The shell should not have been
disposed first"), reported in eclipse-platform#2700.

Restore a non-empty, optional descriptor without re-introducing jakarta:
use a private marker interface InjectionLink that is never available
from
any supplier together with e4's own @optional annotation. This keeps the
injector free of specific jakarta annotation classes (the goal of eclipse-platform#2649)
while re-establishing the supplier-disposal link.

Fixes eclipse-platform#2700

Author: akurtakov

runtime/bundles/org.eclipse.e4.core.di/src/org/eclipse/e4/core/internal/di/ClassRequestor.java

@@ -13,8 +13,10 @@
  *******************************************************************************/
 package org.eclipse.e4.core.internal.di;
 
+import java.lang.reflect.Field;
 import org.eclipse.e4.core.di.IInjector;
 import org.eclipse.e4.core.di.InjectionException;
+import org.eclipse.e4.core.di.annotations.Optional;
 import org.eclipse.e4.core.di.suppliers.IObjectDescriptor;
 import org.eclipse.e4.core.di.suppliers.PrimaryObjectSupplier;
 
@@ -26,7 +28,30 @@
  */
 public class ClassRequestor extends Requestor<Class<?>> {
 
-	private static IObjectDescriptor[] pseudoVariableDescriptor = {};
+	/**
+	 * Marker type for the pseudo dependency. It is intentionally never available
+	 * from any supplier, so the (optional) dependency never resolves to a value.
+	 * Its sole purpose is to provide a non-empty, optional descriptor that
+	 * establishes the link between the supplier and the injected object, so that
+	 * the injected object is notified on the supplier's disposal.
+	 */
+	private interface InjectionLink {
+		// marker interface
+	}
+
+	@Optional
+	private static final InjectionLink PSEUDO_VARIABLE = null;
+
+	private static final IObjectDescriptor[] pseudoVariableDescriptor = createDescriptor();
+
+	private static IObjectDescriptor[] createDescriptor() {
+		try {
+			Field field = ClassRequestor.class.getDeclaredField("PSEUDO_VARIABLE"); //$NON-NLS-1$
+			return new IObjectDescriptor[] { new ObjectDescriptor(field.getGenericType(), field.getAnnotations()) };
+		} catch (NoSuchFieldException e) {
+			throw new IllegalStateException(e); // the field is declared right above, this cannot happen
+		}
+	}
 
 	public ClassRequestor(Class<?> clazz, IInjector injector, PrimaryObjectSupplier primarySupplier, PrimaryObjectSupplier tempSupplier, Object requestingObject, boolean track) {
 		super(clazz, injector, primarySupplier, tempSupplier, requestingObject, track);