/// Worker shutdown controller.
usetokio::sync::oneshot;/// Dropping the dropper will cause runtime to shutdown.
#[derive(Debug)]pubstructDropper{pub(crate)close:Option<oneshot::Sender<()>>,
}implDrop forDropper{fndrop(&mutself){// Send a signal to say i am dropping.
self.close.take().map(|v|v.send(()));}}