Show the unified diff for a file that is gone from the workspace

team/bundles/org.eclipse.compare/compare/org/eclipse/compare/internal/CompareUIPlugin.java

@@ -88,6 +88,7 @@
 import org.eclipse.jface.operation.IRunnableContext;
 import org.eclipse.jface.preference.IPreferenceStore;
 import org.eclipse.jface.resource.ImageDescriptor;
+import org.eclipse.jface.text.IDocument;
 import org.eclipse.jface.util.IPropertyChangeListener;
 import org.eclipse.jface.viewers.Viewer;
 import org.eclipse.osgi.service.debug.DebugOptions;
@@ -720,6 +721,12 @@ private boolean openUnifiedDiff(UnifiedDiffSource source, CompareEditorInput inp
 				closeIfOpenedHere(wpage, openedHere);
 				return false;
 			}
+			// A side that is not a workspace file, the version shown for a deleted file
+			// for example, is only worth overlaying when its content really loaded.
+			if (!(source.editorInput() instanceof IFileEditorInput) && !hasContent(textEditor)) {
+				closeIfOpenedHere(wpage, openedHere);
+				return false;
+			}
 			Action openTwoWayCompare = createOpenTwoWayCompareAction(input, page, editor, activate, textEditor);
 			IStatus status = UnifiedDiff.create(textEditor, source.diffSource(), source.mode())
 					.additionalActions(Arrays.asList(openTwoWayCompare))
@@ -744,6 +751,11 @@ private boolean openUnifiedDiff(UnifiedDiffSource source, CompareEditorInput inp
 		return false;
 	}
 
+	private static boolean hasContent(ITextEditor editor) {
+		IDocument document = editor.getDocumentProvider().getDocument(editor.getEditorInput());
+		return document != null && document.getLength() > 0;
+	}
+
 	private static void closeIfOpenedHere(IWorkbenchPage page, IEditorPart editor) {
 		if (editor != null) {
 			// Nothing here writes to the document, so there is nothing to save.
@@ -897,17 +909,31 @@ private static UnifiedDiffCandidate unifiedDiffCandidateOf(CompareEditorInput in
 		// plain left-vs-right overlay.
 		ITypedElement left = compareInput.getLeft();
 		ITypedElement right = compareInput.getRight();
-		// Only the side shown in the editor has to be a workspace file; an editor
-		// opened on anything else, a revision for example, comes up empty. The other
-		// side merely supplies the diff source and may be missing entirely, as for a
-		// newly added file.
-		if (documentKeyOf(left) instanceof IFileEditorInput leftEditorInput) {
+		IEditorInput leftEditorInput = documentKeyOf(left);
+		IEditorInput rightEditorInput = documentKeyOf(right);
+		// The side shown in the editor is preferably a workspace file; an editor opened
+		// on anything else, a revision for example, comes up empty. The other side
+		// merely supplies the diff source and may be missing entirely, as for a newly
+		// added file.
+		if (leftEditorInput instanceof IFileEditorInput) {
 			return new UnifiedDiffCandidate(compareInput, leftEditorInput, left, UnifiedDiffMode.REVERT_MODE, right);
 		}
-		if (documentKeyOf(right) instanceof IFileEditorInput rightEditorInput) {
+		if (rightEditorInput instanceof IFileEditorInput) {
+			return new UnifiedDiffCandidate(compareInput, rightEditorInput, right,
+					UnifiedDiffMode.OVERLAY_READ_ONLY_MODE, left);
+		}
+		// A deleted file leaves no workspace file to overlay. The version that still
+		// exists is then shown read-only, with its whole content marked as removed.
+		// Two sides that both exist are left to the classic compare editor, because
+		// neither of them is the state on disk.
+		if (rightEditorInput != null && isAbsent(left)) {
 			return new UnifiedDiffCandidate(compareInput, rightEditorInput, right,
 					UnifiedDiffMode.OVERLAY_READ_ONLY_MODE, left);
 		}
+		if (leftEditorInput != null && isAbsent(right)) {
+			return new UnifiedDiffCandidate(compareInput, leftEditorInput, left,
+					UnifiedDiffMode.OVERLAY_READ_ONLY_MODE, right);
+		}
 		return null;
 	}
 
@@ -953,6 +979,21 @@ private static record UnifiedDiffCandidate(ICompareInput compareInput, IEditorIn
 			ITypedElement element, UnifiedDiffMode mode, ITypedElement diffSource) {
 	}
 
+	/**
+	 * Returns whether the given side does not exist, as the workspace file of a
+	 * deleted resource or the previous version of an added file. Answered from the
+	 * element alone, so that picking a candidate stays free of content reads.
+	 */
+	private static boolean isAbsent(ITypedElement element) {
+		// null, or an element that only names the file
+		if (!(element instanceof IStreamContentAccessor)) {
+			return true;
+		}
+		IResource resource = element instanceof IResourceProvider provider ? provider.getResource()
+				: Adapters.adapt(element, IResource.class);
+		return resource != null && !resource.exists();
+	}
+
 	private static IEditorInput documentKeyOf(ITypedElement element) {
 		if (element == null) {
 			return null;

team/bundles/org.eclipse.compare/compare/org/eclipse/compare/unifieddiff/internal/HideAllDiffsRunnable.java

@@ -42,7 +42,7 @@ public HideAllDiffsRunnable(ITextViewer tv, IAnnotationModel model) {
 
 	public HideAllDiffsRunnable(ITextEditor textEditor) {
 		this.tv = textEditor.getAdapter(ITextViewer.class);
-		this.model = textEditor.getDocumentProvider().getAnnotationModel(textEditor.getEditorInput());
+		this.model = UnifiedDiffManager.annotationModelOf(textEditor);
 	}
 
 	public String getLabel() {

team/bundles/org.eclipse.compare/compare/org/eclipse/compare/unifieddiff/internal/UnifiedDiffManager.java

@@ -63,11 +63,13 @@
 import org.eclipse.jface.text.Position;
 import org.eclipse.jface.text.codemining.ICodeMining;
 import org.eclipse.jface.text.source.Annotation;
+import org.eclipse.jface.text.source.AnnotationModel;
 import org.eclipse.jface.text.source.AnnotationModelEvent;
 import org.eclipse.jface.text.source.IAnnotationModel;
 import org.eclipse.jface.text.source.IAnnotationModelExtension;
 import org.eclipse.jface.text.source.IAnnotationModelListener;
 import org.eclipse.jface.text.source.IAnnotationModelListenerExtension;
+import org.eclipse.jface.text.source.ISourceViewer;
 import org.eclipse.jface.text.source.ISourceViewerExtension5;
 import org.eclipse.jface.text.source.inlined.AbstractInlinedAnnotation;
 import org.eclipse.jface.text.source.projection.ProjectionViewer;
@@ -130,20 +132,49 @@ public static List<UnifiedDiff> get(ITextViewer viewer) {
 		return diffsByViewer.get(viewer);
 	}
 
+	/**
+	 * Returns the annotation model the diffs of the given editor live in, or
+	 * <code>null</code> when it has none. The document provider of an editor on
+	 * something other than a workspace file, a revision for example, supplies no
+	 * model, so the one installed on the viewer is used.
+	 */
+	public static IAnnotationModel annotationModelOf(ITextEditor editor) {
+		IAnnotationModel model = editor.getDocumentProvider().getAnnotationModel(editor.getEditorInput());
+		if (model != null) {
+			return model;
+		}
+		return editor.getAdapter(ITextViewer.class) instanceof ISourceViewer sourceViewer
+				? sourceViewer.getAnnotationModel()
+				: null;
+	}
+
+	/**
+	 * Installs an annotation model on the viewer of an editor whose document
+	 * provider has none, so that the diffs have somewhere to be shown.
+	 */
+	private static IAnnotationModel installAnnotationModel(ITextViewer viewer, IDocument document) {
+		if (document == null || !(viewer instanceof ISourceViewer sourceViewer)) {
+			return null;
+		}
+		sourceViewer.setDocument(document, new AnnotationModel());
+		return sourceViewer.getAnnotationModel();
+	}
+
 	public static IStatus open(ITextEditor editor, String source, UnifiedDiffMode mode, List<Action> additionalActions,
 			TokenComparatorFactory tokenComparatorFactory,
 			IgnoreWhitespaceContributorFactory ignoreWhitespaceContributorFactory, boolean ignoreWhiteSpace) {
 		ITextViewer viewer = editor.getAdapter(ITextViewer.class);
 		if (viewer instanceof ProjectionViewer pv) {
 			pv.doOperation(ProjectionViewer.EXPAND_ALL);
 		}
-		IAnnotationModel model = editor.getDocumentProvider().getAnnotationModel(editor.getEditorInput());
+		IDocument leftDocument = editor.getDocumentProvider().getDocument(editor.getEditorInput());
+		IAnnotationModel editorModel = annotationModelOf(editor);
+		IAnnotationModel model = editorModel != null ? editorModel : installAnnotationModel(viewer, leftDocument);
 		if (model == null) {
 			return Status.CANCEL_STATUS;
 		}
 		clearAll(viewer, model);
 
-		IDocument leftDocument = editor.getDocumentProvider().getDocument(editor.getEditorInput());
 		IDocument rightDocument = new Document(source);
 
 		List<UnifiedDiff> unifiedDiffs;