Add internal API to CEditor to allow CDT-LSP to contribute a "try it" banner

Part of https://github.com/eclipse-cdt/cdt/issues/968

core/org.eclipse.cdt.ui/META-INF/MANIFEST.MF

@@ -22,7 +22,11 @@ Export-Package: org.eclipse.cdt.internal.corext;x-internal:=true,
  org.eclipse.cdt.internal.ui.dialogs;x-internal:=true,
  org.eclipse.cdt.internal.ui.dialogs.cpaths;x-internal:=true,
  org.eclipse.cdt.internal.ui.dnd;x-internal:=true,
- org.eclipse.cdt.internal.ui.editor;x-friends:="org.eclipse.cdt.codan.ui,org.eclipse.cdt.codan.ui.cxx,org.eclipse.cdt.dsf.ui",
+ org.eclipse.cdt.internal.ui.editor;
+  x-friends:="org.eclipse.cdt.codan.ui,
+   org.eclipse.cdt.codan.ui.cxx,
+   org.eclipse.cdt.dsf.ui,
+   org.eclipse.cdt.lsp",
  org.eclipse.cdt.internal.ui.editor.asm;x-internal:=true,
  org.eclipse.cdt.internal.ui.filters;x-internal:=true,
  org.eclipse.cdt.internal.ui.help;x-internal:=true,
@@ -51,6 +55,7 @@ Export-Package: org.eclipse.cdt.internal.corext;x-internal:=true,
  org.eclipse.cdt.internal.ui.resources,
  org.eclipse.cdt.internal.ui.search;x-internal:=true,
  org.eclipse.cdt.internal.ui.search.actions;x-internal:=true,
+ org.eclipse.cdt.internal.ui.switchtolsp;x-friends:="org.eclipse.cdt.lsp",
  org.eclipse.cdt.internal.ui.text;x-friends:="org.eclipse.cdt.codan.ui.cxx,org.eclipse.cdt.dsf.ui",
  org.eclipse.cdt.internal.ui.text.asm;x-internal:=true,
  org.eclipse.cdt.internal.ui.text.c.hover;x-internal:=true,

core/org.eclipse.cdt.ui/src/org/eclipse/cdt/internal/ui/editor/CEditor.java

@@ -74,6 +74,7 @@ import org.eclipse.cdt.internal.ui.search.IOccurrencesFinder;
 import org.eclipse.cdt.internal.ui.search.IOccurrencesFinder.OccurrenceLocation;
 import org.eclipse.cdt.internal.ui.search.OccurrencesFinder;
 import org.eclipse.cdt.internal.ui.search.actions.SelectionSearchGroup;
+import org.eclipse.cdt.internal.ui.switchtolsp.ISwitchToLsp;
 import org.eclipse.cdt.internal.ui.text.CHeuristicScanner;
 import org.eclipse.cdt.internal.ui.text.CPairMatcher;
 import org.eclipse.cdt.internal.ui.text.CSourceViewerScalableConfiguration;
@@ -111,6 +112,7 @@ import org.eclipse.core.runtime.IStatus;
 import org.eclipse.core.runtime.ListenerList;
 import org.eclipse.core.runtime.NullProgressMonitor;
 import org.eclipse.core.runtime.Platform;
+import org.eclipse.core.runtime.SafeRunner;
 import org.eclipse.core.runtime.Status;
 import org.eclipse.core.runtime.content.IContentType;
 import org.eclipse.core.runtime.jobs.Job;
@@ -2786,8 +2788,10 @@ public class CEditor extends TextEditor
 
 	@Override
 	protected ISourceViewer createSourceViewer(Composite parent, IVerticalRuler ruler, int styles) {
+		Composite editorComposite = createTryLspBanner(parent);
+
 		IPreferenceStore store = getPreferenceStore();
-		AdaptedSourceViewer cSourceViewer = new AdaptedSourceViewer(parent, ruler, getOverviewRuler(),
+		AdaptedSourceViewer cSourceViewer = new AdaptedSourceViewer(editorComposite, ruler, getOverviewRuler(),
 				isOverviewRulerVisible(), styles, store);
 
 		/*
@@ -2818,6 +2822,29 @@ public class CEditor extends TextEditor
 		return cSourceViewer;
 	}
 
+	/**
+	 * Wraps {@link ISwitchToLsp#createTryLspEditor(org.eclipse.ui.texteditor.ITextEditor, Composite)}
+	 * with the needed service access + fallback checks.
+	 *
+	 * If the {@link ISwitchToLsp} service doesn't exist, or fails, this method
+	 * is a no-op that simply returns its input.
+	 *
+	 * @see ISwitchToLsp#createTryLspEditor(org.eclipse.ui.texteditor.ITextEditor, Composite)
+	 */
+	private Composite createTryLspBanner(Composite parent) {
+		Composite editorComposite = SafeRunner.run(() -> {
+			ISwitchToLsp switchToLsp = PlatformUI.getWorkbench().getService(ISwitchToLsp.class);
+			if (switchToLsp != null) {
+				return switchToLsp.createTryLspEditor(CEditor.this, parent);
+			}
+			return null;
+		});
+		if (editorComposite == null) {
+			editorComposite = parent;
+		}
+		return editorComposite;
+	}
+
 	/** Outliner context menu Id */
 	protected String fOutlinerContextMenuId;
 

core/org.eclipse.cdt.ui/src/org/eclipse/cdt/internal/ui/switchtolsp/ISwitchToLsp.java

@@ -0,0 +1,39 @@
+/*******************************************************************************
+ * Copyright (c) 2025 Kichwa Coders Canada Inc. 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
+ *******************************************************************************/
+
+package org.eclipse.cdt.internal.ui.switchtolsp;
+
+import org.eclipse.cdt.internal.ui.editor.CEditor;
+import org.eclipse.swt.widgets.Composite;
+import org.eclipse.ui.texteditor.ITextEditor;
+
+/**
+ * This interface can be implemented by CDT-LSP to display a banner in the {@link CEditor}
+ * to encourage users to try the new C/C++ editing experience based on CDT/LSP
+ *
+ * See org.eclipse.cdt.lsp/OSGI-INF/org.eclipse.cdt.lsp.internal.switchtolsp.SwitchToLsp.xml
+ * for the use.
+ *
+ * This interface is not public API, it is provided as a hook for CDT LSP and may
+ * be changed at any point.
+ */
+public interface ISwitchToLsp {
+
+	/**
+	 * Create the banner controls for the "try new experience"
+	 *
+	 * @param part the editor part that the banner is added on
+	 * @param parent the parent control
+	 * @return the new parent control the editor should use
+	 */
+	Composite createTryLspEditor(ITextEditor part, Composite parent);
+
+}