| // Licensed to the Apache Software Foundation (ASF) under one |
| // or more contributor license agreements. See the NOTICE file |
| // distributed with this work for additional information |
| // regarding copyright ownership. The ASF licenses this file |
| // to you under the Apache License, Version 2.0 (the |
| // "License"); you may not use this file except in compliance |
| // with the License. You may obtain a copy of the License at |
| // |
| // http://www.apache.org/licenses/LICENSE-2.0 |
| // |
| // Unless required by applicable law or agreed to in writing, |
| // software distributed under the License is distributed on an |
| // "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY |
| // KIND, either express or implied. See the License for the |
| // specific language governing permissions and limitations |
| // under the License.. |
| |
| //! Asynchronous values. |
| |
| #[cfg(bootstrap)] |
| use core::{ |
| cell::Cell, |
| marker::Unpin, |
| ops::{Drop, Generator, GeneratorState}, |
| pin::Pin, |
| ptr::NonNull, |
| task::{Context, Poll}, |
| }; |
| |
| #[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.....]>`). |
| // This is `const` to avoid extra errors after we recover from `const async fn` |
| #[cfg(bootstrap)] |
| #[doc(hidden)] |
| pub const 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. |
| #[cfg(bootstrap)] |
| #[doc(hidden)] |
| #[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. |
| #[cfg(bootstrap)] |
| impl<T: Generator<Yield = ()>> !Unpin for GenFuture<T> {} |
| |
| #[cfg(bootstrap)] |
| #[doc(hidden)] |
| 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) }; |
| let _guard = unsafe { set_task_context(cx) }; |
| match gen.resume(()) { |
| GeneratorState::Yielded(()) => Poll::Pending, |
| GeneratorState::Complete(x) => Poll::Ready(x), |
| } |
| } |
| } |
| |
| #[cfg(bootstrap)] |
| thread_local! { |
| static TLS_CX: Cell<Option<NonNull<Context<'static>>>> = Cell::new(None); |
| } |
| |
| #[cfg(bootstrap)] |
| struct SetOnDrop(Option<NonNull<Context<'static>>>); |
| |
| #[cfg(bootstrap)] |
| impl Drop for SetOnDrop { |
| fn drop(&mut self) { |
| TLS_CX.with(|tls_cx| { |
| tls_cx.set(self.0.take()); |
| }); |
| } |
| } |
| |
| // Safety: the returned guard must drop before `cx` is dropped and before |
| // any previous guard is dropped. |
| #[cfg(bootstrap)] |
| unsafe fn set_task_context(cx: &mut Context<'_>) -> SetOnDrop { |
| // transmute the context's lifetime to 'static so we can store it. |
| let cx = core::mem::transmute::<&mut Context<'_>, &mut Context<'static>>(cx); |
| let old_cx = TLS_CX.with(|tls_cx| tls_cx.replace(Some(NonNull::from(cx)))); |
| SetOnDrop(old_cx) |
| } |
| |
| #[cfg(bootstrap)] |
| #[doc(hidden)] |
| /// 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, |
| { |
| 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::poll(f, cx_ptr.as_mut()) } |
| } |