feat: add Runner.dummy

src/core/runner.ml

@@ -43,5 +43,14 @@ module For_runner_implementors = struct
   let k_cur_runner : t option ref TLS.key = Types_.k_cur_runner
 end
 
+let dummy : t =
+  For_runner_implementors.create
+    ~size:(fun () -> 0)
+    ~num_tasks:(fun () -> 0)
+    ~shutdown:(fun ~wait:_ () -> ())
+    ~run_async:(fun ~ls:_ _ ->
+      failwith "Runner.dummy: cannot actually run tasks")
+    ()
+
 let get_current_runner = get_current_runner
 let get_current_storage = get_current_storage

src/core/runner.mli

@@ -51,6 +51,10 @@ val run_wait_block : ?ls:Task_local_storage.t -> t -> (unit -> 'a) -> 'a
       about the required discipline to avoid deadlocks).
     @raise Shutdown if the runner was already shut down *)
 
+val dummy : t
+(** Runner that fails when scheduling tasks on it.
+    Calling {!run_async} on it will raise Failure. *)
+
 (** {2 Implementing runners} *)
 
 (** This module is specifically intended for users who implement their

src/fib/fiber.ml

@@ -65,7 +65,7 @@ let create_done_ ~res () : _ t =
       @@ Alive { children = FM.empty; on_cancel = Int_map.empty; cancel_id = 0 };
     id;
     res;
-    runner = Immediate_runner.runner;
+    runner = Runner.dummy;
     ls = Task_local_storage.dummy;
   }