rust / expert
Snippet
Implementing a Custom Manual Future Poller
This snippet demonstrates how to manually implement and execute a Future without using async/await syntax. It constructs a dummy Waker using a custom virtual table (RawWakerVTable) and polls a stateful countdown future within a manual execution loop.
snippet.rs
rust
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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 => {}}}}
Breakdown
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static VTABLE: RawWakerVTable = ...
Defines a no-op virtual table for the custom raw waker to fulfill the task interface requirement.
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cx.waker().wake_by_ref();
Instructs the executor that the task is ready to be polled again immediately by waking its ref.
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let waker = unsafe { Waker::from_raw(raw_waker) };
Reconstitutes a safe Waker instance from our raw waker pointer structure.
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loop { match pinned.as_mut().poll(&mut cx) { ... } }
Drives the future to completion by calling poll inside a standard control flow loop.