| //! 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::{LocalWaker, Poll}; |
| use core::ops::{Drop, Generator, GeneratorState}; |
| |
| #[doc(inline)] |
| pub use core::future::*; |
| |
| /// Wrap a future in a generator. |
| /// |
| /// 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>, lw: &LocalWaker) -> Poll<Self::Output> { |
| set_task_waker(lw, || match unsafe { Pin::get_unchecked_mut(self).0.resume() } { |
| GeneratorState::Yielded(()) => Poll::Pending, |
| GeneratorState::Complete(x) => Poll::Ready(x), |
| }) |
| } |
| } |
| |
| thread_local! { |
| static TLS_WAKER: Cell<Option<NonNull<LocalWaker>>> = Cell::new(None); |
| } |
| |
| struct SetOnDrop(Option<NonNull<LocalWaker>>); |
| |
| 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>(lw: &LocalWaker, f: F) -> R |
| where |
| F: FnOnce() -> R |
| { |
| let old_waker = TLS_WAKER.with(|tls_waker| { |
| tls_waker.replace(Some(NonNull::from(lw))) |
| }); |
| 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(&LocalWaker) -> 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 LocalWaker 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(|lw| F::poll(f, lw)) |
| } |