* callable} or a callable that has begun to execute, but if the output future is cancelled before
   * {@link AsyncCallable#call()} is invoked, {@link AsyncCallable#call()} will not be invoked.
   */
  public <T extends @Nullable Object> ListenableFuture<T> submitAsync(
      AsyncCallable<T> callable, Executor executor) {
    checkNotNull(callable);
    checkNotNull(executor);

   * callable} or a callable that has begun to execute, but if the output future is cancelled before
   * {@link AsyncCallable#call()} is invoked, {@link AsyncCallable#call()} will not be invoked.
   */
  public <T extends @Nullable Object> ListenableFuture<T> submitAsync(
      AsyncCallable<T> callable, Executor executor) {
    checkNotNull(callable);
    checkNotNull(executor);

    WeakReference<Object> ref = new WeakReference<>(toBeGCed);
    SettableFuture<@Nullable Void> settableFuture = SettableFuture.create();
    ListenableFuture<?> ignored =
        serializer.submitAsync(
            new AsyncCallable<@Nullable Void>() {
              @Override
              public ListenableFuture<@Nullable Void> call() {
                return settableFuture;
              }
            },
            directExecutor());