use std::task::{Context, Poll};
#[cfg(feature = "client")]
use std::task::{RawWaker, RawWakerVTable, Waker};
/// A function to help "yield" a future, such that it is re-scheduled immediately.
///
/// Useful for spin counts, so a future doesn't hog too much time.
pub(crate) fn yield_now(cx: &mut Context<'_>) -> Poll<std::convert::Infallible> {
cx.waker().wake_by_ref();
Poll::Pending
}
// TODO: replace with `std::task::Waker::noop()` once MSRV >= 1.85
#[cfg(feature = "client")]
fn noop_waker() -> Waker {
const NOOP_RAW_WAKER: RawWaker = RawWaker::new(std::ptr::null(), &NOOP_VTABLE);
const NOOP_VTABLE: RawWakerVTable = RawWakerVTable::new(
// `clone` returns the same noop waker again
|_: *const ()| NOOP_RAW_WAKER,
// `wake`, `wake_by_ref`, and `drop` do nothing
|_: *const ()| {},
|_: *const ()| {},
|_: *const ()| {},
);
// SAFETY: all functions in the vtable are safe to call, and Waker's safety does not require
// them to actually do anything.
unsafe { Waker::from_raw(NOOP_RAW_WAKER) }
}
/// Poll the future once and return `Some` if it is ready, else `None`.
///
/// If the future wasn't ready, it future likely can't be driven to completion any more: the polling
/// uses a no-op waker, so knowledge of what the pending future was waiting for is lost.
#[cfg(feature = "client")]
pub(crate) fn now_or_never<F: std::future::Future>(fut: F) -> Option<F::Output> {
let waker = noop_waker();
let mut cx = Context::from_waker(&waker);
// TODO: replace with std::pin::pin! and drop pin-utils once MSRV >= 1.68
pin_utils::pin_mut!(fut);
match fut.poll(&mut cx) {
Poll::Ready(res) => Some(res),
Poll::Pending => None,
}
}
