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//! Asynchronous values.
use core::cell::Cell;
use core::marker::Unpin;
use core::pin::Pin;
use core::option::Option;
use core::ptr::NonNull;
use core::task::{Waker, Poll};
use core::ops::{Drop, Generator, GeneratorState};
#[doc(inline)]
pub use core::future::*;
/// Wrap a generator in a future.
///
/// This function returns a `GenFuture` underneath, but hides it in `impl Trait` to give
/// better error messages (`impl Future` rather than `GenFuture<[closure.....]>`).
#[unstable(feature = "gen_future", issue = "50547")]
pub fn from_generator<T: Generator<Yield = ()>>(x: T) -> impl Future<Output = T::Return> {
GenFuture(x)
}
/// A wrapper around generators used to implement `Future` for `async`/`await` code.
#[unstable(feature = "gen_future", issue = "50547")]
#[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)]
struct GenFuture<T: Generator<Yield = ()>>(T);
// We rely on the fact that async/await futures are immovable in order to create
// self-referential borrows in the underlying generator.
impl<T: Generator<Yield = ()>> !Unpin for GenFuture<T> {}
#[unstable(feature = "gen_future", issue = "50547")]
impl<T: Generator<Yield = ()>> Future for GenFuture<T> {
type Output = T::Return;
fn poll(self: Pin<&mut Self>, waker: &Waker) -> Poll<Self::Output> {
// Safe because we're !Unpin + !Drop mapping to a ?Unpin value
let gen = unsafe { Pin::map_unchecked_mut(self, |s| &mut s.0) };
set_task_waker(waker, || match gen.resume() {
GeneratorState::Yielded(()) => Poll::Pending,
GeneratorState::Complete(x) => Poll::Ready(x),
})
}
}
thread_local! {
static TLS_WAKER: Cell<Option<NonNull<Waker>>> = Cell::new(None);
}
struct SetOnDrop(Option<NonNull<Waker>>);
impl Drop for SetOnDrop {
fn drop(&mut self) {
TLS_WAKER.with(|tls_waker| {
tls_waker.set(self.0.take());
});
}
}
#[unstable(feature = "gen_future", issue = "50547")]
/// Sets the thread-local task context used by async/await futures.
pub fn set_task_waker<F, R>(waker: &Waker, f: F) -> R
where
F: FnOnce() -> R
{
let old_waker = TLS_WAKER.with(|tls_waker| {
tls_waker.replace(Some(NonNull::from(waker)))
});
let _reset_waker = SetOnDrop(old_waker);
f()
}
#[unstable(feature = "gen_future", issue = "50547")]
/// Retrieves the thread-local task waker used by async/await futures.
///
/// This function acquires exclusive access to the task waker.
///
/// Panics if no waker has been set or if the waker has already been
/// retrieved by a surrounding call to get_task_waker.
pub fn get_task_waker<F, R>(f: F) -> R
where
F: FnOnce(&Waker) -> R
{
let waker_ptr = TLS_WAKER.with(|tls_waker| {
// Clear the entry so that nested `get_task_waker` calls
// will fail or set their own value.
tls_waker.replace(None)
});
let _reset_waker = SetOnDrop(waker_ptr);
let waker_ptr = waker_ptr.expect(
"TLS Waker not set. This is a rustc bug. \
Please file an issue on https://github.com/rust-lang/rust.");
unsafe { f(waker_ptr.as_ref()) }
}
#[unstable(feature = "gen_future", issue = "50547")]
/// Polls a future in the current thread-local task waker.
pub fn poll_with_tls_waker<F>(f: Pin<&mut F>) -> Poll<F::Output>
where
F: Future
{
get_task_waker(|waker| F::poll(f, waker))
}
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