When does schedule called? about async-task HOT 4 CLOSED

use async_task::Runnable;
use send_wrapper::SendWrapper;
use std::{cell::RefCell, collections::VecDeque, future::poll_fn, rc::Rc, task::Poll};

fn main() {
    let runnables = Rc::new(RefCell::new(VecDeque::<Runnable>::new()));

    let schedule = {
        let runnables = SendWrapper::new(runnables.clone());
        move |r| runnables.borrow_mut().push_back(r)
    };

    let (runnable, task) = async_task::spawn_local(
        poll_fn(|cx| {
            cx.waker().wake_by_ref();
            Poll::<()>::Pending
        }),
        schedule,
    );
    let t = std::thread::spawn(move || smol::block_on(task));
    runnable.schedule();
    loop {
        let next_task = runnables.borrow_mut().pop_front();
        if let Some(r) = next_task {
            r.run();
        } else {
            break;
        }
    }
    t.join().unwrap();
}