Implementing a Custom Manual Future Poller

use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll, RawWaker, RawWakerVTable, Waker};
 
static VTABLE: RawWakerVTable = RawWakerVTable::new(
    |_| RawWaker::new(std::ptr::null(), &VTABLE),
    |_| {},
    |_| {},
    |_| {},
);
 
struct CountdownFuture {
    count: usize,
}
 
impl Future for CountdownFuture {
    type Output = String;
    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        if self.count == 0 {
            Poll::Ready("Finished!".to_string())
        } else {
            self.count -= 1;
            cx.waker().wake_by_ref();
            Poll::Pending
        }
    }
}
 
fn execute_future() -> String {
    let mut fut = CountdownFuture { count: 3 };
    let mut pinned = Pin::new(&mut fut);
    let raw_waker = RawWaker::new(std::ptr::null(), &VTABLE);
    let waker = unsafe { Waker::from_raw(raw_waker) };
    let mut cx = Context::from_waker(&waker);
    loop {
        match pinned.as_mut().poll(&mut cx) {
            Poll::Ready(result) => return result,
            Poll::Pending => {}
        }
    }
}