TestCallable secondCallable = new TestCallable(immediateVoidFuture());
    ListenableFuture<@Nullable Void> secondFuture =
        serializer.submitAsync(secondCallable, directExecutor());
    TestCallable thirdCallable = new TestCallable(immediateVoidFuture());
    @SuppressWarnings({"unused", "nullness"})

    // oldFuture completes.
    Runnable listener =
        () -> {
          if (taskFuture.isDone()) {
            // Since the value of oldFuture can only ever be immediateVoidFuture() or setFuture of a
            // future that eventually came from immediateVoidFuture(), this doesn't leak throwables
            // or completion values.
            newFuture.setFuture(oldFuture);