| //! 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::{Context, 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>, cx: &mut Context<'_>) -> 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_context(cx, || match gen.resume() { |
| GeneratorState::Yielded(()) => Poll::Pending, |
| GeneratorState::Complete(x) => Poll::Ready(x), |
| }) |
| } |
| } |
| |
| thread_local! { |
| static TLS_CX: Cell<Option<NonNull<Context<'static>>>> = Cell::new(None); |
| } |
| |
| struct SetOnDrop(Option<NonNull<Context<'static>>>); |
| |
| impl Drop for SetOnDrop { |
| fn drop(&mut self) { |
| TLS_CX.with(|tls_cx| { |
| tls_cx.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_context<F, R>(cx: &mut Context<'_>, f: F) -> R |
| where |
| F: FnOnce() -> R |
| { |
| // transmute the context's lifetime to 'static so we can store it. |
| let cx = unsafe { |
| core::mem::transmute::<&mut Context<'_>, &mut Context<'static>>(cx) |
| }; |
| let old_cx = TLS_CX.with(|tls_cx| { |
| tls_cx.replace(Some(NonNull::from(cx))) |
| }); |
| let _reset = SetOnDrop(old_cx); |
| f() |
| } |
| |
| #[unstable(feature = "gen_future", issue = "50547")] |
| /// Retrieves the thread-local task context used by async/await futures. |
| /// |
| /// This function acquires exclusive access to the task context. |
| /// |
| /// Panics if no context has been set or if the context has already been |
| /// retrieved by a surrounding call to get_task_context. |
| pub fn get_task_context<F, R>(f: F) -> R |
| where |
| F: FnOnce(&mut Context<'_>) -> R |
| { |
| let cx_ptr = TLS_CX.with(|tls_cx| { |
| // Clear the entry so that nested `get_task_waker` calls |
| // will fail or set their own value. |
| tls_cx.replace(None) |
| }); |
| let _reset = SetOnDrop(cx_ptr); |
| |
| let mut cx_ptr = cx_ptr.expect( |
| "TLS Context not set. This is a rustc bug. \ |
| Please file an issue on https://github.com/rust-lang/rust."); |
| |
| // Safety: we've ensured exclusive access to the context by |
| // removing the pointer from TLS, only to be replaced once |
| // we're done with it. |
| // |
| // The pointer that was inserted came from an `&mut Context<'_>`, |
| // so it is safe to treat as mutable. |
| unsafe { f(cx_ptr.as_mut()) } |
| } |
| |
| #[unstable(feature = "gen_future", issue = "50547")] |
| /// Polls a future in the current thread-local task waker. |
| pub fn poll_with_tls_context<F>(f: Pin<&mut F>) -> Poll<F::Output> |
| where |
| F: Future |
| { |
| get_task_context(|cx| F::poll(f, cx)) |
| } |