57 lines
1.7 KiB
Rust
57 lines
1.7 KiB
Rust
#![expect(unsafe_code, reason = "Futures require unsafe code.")]
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//! Utilities for working with [`Future`]s.
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use core::{
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future::Future,
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pin::Pin,
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task::{Context, Poll, RawWaker, RawWakerVTable, Waker},
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};
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/// Consumes a future, polls it once, and immediately returns the output
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/// or returns `None` if it wasn't ready yet.
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///
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/// This will cancel the future if it's not ready.
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pub fn now_or_never<F: Future>(mut future: F) -> Option<F::Output> {
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let noop_waker = noop_waker();
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let mut cx = Context::from_waker(&noop_waker);
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// SAFETY: `future` is not moved and the original value is shadowed
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let future = unsafe { Pin::new_unchecked(&mut future) };
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match future.poll(&mut cx) {
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Poll::Ready(x) => Some(x),
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_ => None,
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}
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}
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/// Polls a future once, and returns the output if ready
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/// or returns `None` if it wasn't ready yet.
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pub fn check_ready<F: Future + Unpin>(future: &mut F) -> Option<F::Output> {
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let noop_waker = noop_waker();
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let mut cx = Context::from_waker(&noop_waker);
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let future = Pin::new(future);
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match future.poll(&mut cx) {
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Poll::Ready(x) => Some(x),
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_ => None,
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}
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}
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fn noop_clone(_data: *const ()) -> RawWaker {
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noop_raw_waker()
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}
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fn noop(_data: *const ()) {}
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const NOOP_WAKER_VTABLE: RawWakerVTable = RawWakerVTable::new(noop_clone, noop, noop, noop);
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fn noop_raw_waker() -> RawWaker {
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RawWaker::new(core::ptr::null(), &NOOP_WAKER_VTABLE)
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}
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fn noop_waker() -> Waker {
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// SAFETY: the `RawWakerVTable` is just a big noop and doesn't violate any of the rules in `RawWakerVTable`s documentation
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// (which talks about retaining and releasing any "resources", of which there are none in this case)
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unsafe { Waker::from_raw(noop_raw_waker()) }
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}
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