Decorate absent external translation units

core/org.eclipse.cdt.ui/plugin.properties

@@ -655,6 +655,8 @@ ShiftLeftAction.label= S&hift Left
 # Decorators
 indexedFilesDecorator.label = C/C++ Indexed Files
 indexedFilesDecorator.description = Decorates files indexed by C/C++ Indexer.
+translationUnitDecorator.label = C/C++ Translation Units
+translationUnitDecorator.description = Decorates C/C++ translation unit source and header files.
 excludedFile.name = C/C++ Files and Folders Excluded from Build
 excludedFile.description = Decorates source files and folders excluded from C/C++ build.
 includeFolderDecorator.name = C/C++ Missing Include Folders

core/org.eclipse.cdt.ui/plugin.xml

@@ -4428,6 +4428,17 @@
             </or>
          </enablement>
       </decorator>
+      <decorator
+            class="org.eclipse.cdt.internal.ui.viewsupport.TranslationUnitDecorator"
+            id="org.eclipse.cdt.ui.translationUnit"
+            label="%translationUnitDecorator.label"
+            lightweight="true"
+            state="true">
+         <description>%translationUnitDecorator.description</description>
+         <enablement>
+            <objectClass name="org.eclipse.cdt.core.model.ITranslationUnit"/>
+         </enablement>
+      </decorator>
       <decorator
             adaptable="true"
             class="org.eclipse.cdt.internal.ui.viewsupport.ExcludedFileDecorator"

core/org.eclipse.cdt.ui/src/org/eclipse/cdt/internal/ui/viewsupport/TranslationUnitDecorator.java

@@ -0,0 +1,44 @@
+/*******************************************************************************
+ * Copyright (c) 2023 John Dallaway and others.
+ *
+ * This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * which accompanies this distribution, and is available at
+ * https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ *     John Dallaway - initial implementation (#563)
+ *******************************************************************************/
+package org.eclipse.cdt.internal.ui.viewsupport;
+
+import org.eclipse.cdt.core.model.ITranslationUnit;
+import org.eclipse.jface.preference.JFacePreferences;
+import org.eclipse.jface.resource.JFaceResources;
+import org.eclipse.jface.viewers.BaseLabelProvider;
+import org.eclipse.jface.viewers.IDecoration;
+import org.eclipse.jface.viewers.ILightweightLabelDecorator;
+import org.eclipse.swt.graphics.Color;
+
+/**
+ * Decorates translation unit source and header files
+ */
+public final class TranslationUnitDecorator extends BaseLabelProvider implements ILightweightLabelDecorator {
+
+	@Override
+	public boolean isLabelProperty(Object element, String property) {
+		return false;
+	}
+
+	@Override
+	public void decorate(Object element, IDecoration decoration) {
+		// if an external translation unit source file that is not present locally
+		if (element instanceof ITranslationUnit tu && (null == tu.getResource()) && !tu.exists()) {
+			// decorate label to indicate file is absent
+			Color color = JFaceResources.getColorRegistry().get(JFacePreferences.QUALIFIER_COLOR);
+			decoration.setForegroundColor(color);
+		}
+	}
+
+}