@SuppressWarnings({"unused", "nullness"})
    Future<?> possiblyIgnoredError = serializer.submitAsync(firstCallable, directExecutor());
    TestCallable secondCallable = new TestCallable(Futures.<Void>immediateFuture(null));
    @SuppressWarnings({"unused", "nullness"})
    Future<?> possiblyIgnoredError1 = serializer.submitAsync(secondCallable, directExecutor());
    assertThat(firstCallable.called).isTrue();

   *
   * @throws RejectedExecutionException if the task cannot be scheduled for execution
   * @since 23.0
   */
  public static <O extends @Nullable Object> ListenableFuture<O> submitAsync(
      AsyncCallable<O> callable, Executor executor) {
    TrustedListenableFutureTask<O> task = TrustedListenableFutureTask.create(callable);
    executor.execute(task);
    return task;
  }

  /**