1 //! Asynchronous values.
4 use core::marker::Unpin;
6 use core::option::Option;
7 use core::ptr::NonNull;
8 use core::task::{Context, Poll};
9 use core::ops::{Drop, Generator, GeneratorState};
12 #[stable(feature = "futures_api", since = "1.36.0")]
13 pub use core::future::*;
15 /// Wrap a generator in a future.
17 /// This function returns a `GenFuture` underneath, but hides it in `impl Trait` to give
18 /// better error messages (`impl Future` rather than `GenFuture<[closure.....]>`).
20 #[unstable(feature = "gen_future", issue = "50547")]
21 pub fn from_generator<T: Generator<Yield = ()>>(x: T) -> impl Future<Output = T::Return> {
25 /// A wrapper around generators used to implement `Future` for `async`/`await` code.
27 #[unstable(feature = "gen_future", issue = "50547")]
28 #[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)]
29 #[cfg_attr(not(test), rustc_diagnostic_item = "gen_future")]
30 struct GenFuture<T: Generator<Yield = ()>>(T);
32 // We rely on the fact that async/await futures are immovable in order to create
33 // self-referential borrows in the underlying generator.
34 impl<T: Generator<Yield = ()>> !Unpin for GenFuture<T> {}
37 #[unstable(feature = "gen_future", issue = "50547")]
38 impl<T: Generator<Yield = ()>> Future for GenFuture<T> {
39 type Output = T::Return;
40 fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
41 // Safe because we're !Unpin + !Drop mapping to a ?Unpin value
42 let gen = unsafe { Pin::map_unchecked_mut(self, |s| &mut s.0) };
43 let _guard = unsafe { set_task_context(cx) };
45 GeneratorState::Yielded(()) => Poll::Pending,
46 GeneratorState::Complete(x) => Poll::Ready(x),
52 static TLS_CX: Cell<Option<NonNull<Context<'static>>>> = Cell::new(None);
55 struct SetOnDrop(Option<NonNull<Context<'static>>>);
57 impl Drop for SetOnDrop {
59 TLS_CX.with(|tls_cx| {
60 tls_cx.set(self.0.take());
65 // Safety: the returned guard must drop before `cx` is dropped and before
66 // any previous guard is dropped.
67 unsafe fn set_task_context(cx: &mut Context<'_>) -> SetOnDrop {
68 // transmute the context's lifetime to 'static so we can store it.
69 let cx = core::mem::transmute::<&mut Context<'_>, &mut Context<'static>>(cx);
70 let old_cx = TLS_CX.with(|tls_cx| {
71 tls_cx.replace(Some(NonNull::from(cx)))
77 #[unstable(feature = "gen_future", issue = "50547")]
78 /// Polls a future in the current thread-local task waker.
79 pub fn poll_with_tls_context<F>(f: Pin<&mut F>) -> Poll<F::Output>
83 let cx_ptr = TLS_CX.with(|tls_cx| {
84 // Clear the entry so that nested `get_task_waker` calls
85 // will fail or set their own value.
88 let _reset = SetOnDrop(cx_ptr);
90 let mut cx_ptr = cx_ptr.expect(
91 "TLS Context not set. This is a rustc bug. \
92 Please file an issue on https://github.com/rust-lang/rust.");
94 // Safety: we've ensured exclusive access to the context by
95 // removing the pointer from TLS, only to be replaced once
96 // we're done with it.
98 // The pointer that was inserted came from an `&mut Context<'_>`,
99 // so it is safe to treat as mutable.
100 unsafe { F::poll(f, cx_ptr.as_mut()) }