Notify the user when the index is out of date

ocaml-lsp-server/src/ocaml_lsp_server.ml

@@ -427,17 +427,30 @@ let selection_range (state : State.t)
     in
     List.filter_opt ranges
 
-let references (state : State.t)
+let references rpc (state : State.t)
     { ReferenceParams.textDocument = { uri }; position; _ } =
   let doc = Document_store.get state.store uri in
   match Document.kind doc with
   | `Other -> Fiber.return None
   | `Merlin doc ->
-    let+ locs, _desync =
+    let* locs, desync =
       Document.Merlin.dispatch_exn
         doc
         (Occurrences (`Ident_at (Position.logical position), `Project))
     in
+    let+ () =
+      if not desync then Fiber.return () else
+      let msg =
+        let message =
+          "The index might be out-of-sync and only local results are shown. \
+           If you use Dune you can build the target `@ocaml-index` to refresh \
+           the index."
+        in
+        ShowMessageParams.create ~message ~type_:Warning
+      in
+      task_if_running state.detached ~f:(fun () ->
+          Server.notification rpc (ShowMessage msg))
+    in
     Some
       (List.map locs ~f:(fun loc ->
            let range = Range.of_loc loc in
@@ -607,7 +620,7 @@ let on_request :
       | Some _ | None -> Hover_req.Default
     in
     later (fun (_ : State.t) () -> Hover_req.handle rpc req mode) ()
-  | TextDocumentReferences req -> later references req
+  | TextDocumentReferences req -> later (references rpc) req
   | TextDocumentCodeLensResolve codeLens -> now codeLens
   | TextDocumentCodeLens req -> (
     match state.configuration.data.codelens with