1 #![stable(feature = "futures_api", since = "1.36.0")]
3 //! Asynchronous basic functionality.
5 //! Please see the fundamental [`async`] and [`await`] keywords and the [async book]
6 //! for more information on asynchronous programming in Rust.
8 //! [`async`]: ../../std/keyword.async.html
9 //! [`await`]: ../../std/keyword.await.html
10 //! [async book]: https://rust-lang.github.io/async-book/
13 ops::{Generator, GeneratorState},
16 task::{Context, Poll},
25 #[stable(feature = "futures_api", since = "1.36.0")]
26 pub use self::future::Future;
28 #[unstable(feature = "into_future", issue = "67644")]
29 pub use into_future::IntoFuture;
31 #[stable(feature = "future_readiness_fns", since = "1.48.0")]
32 pub use pending::{pending, Pending};
33 #[stable(feature = "future_readiness_fns", since = "1.48.0")]
34 pub use ready::{ready, Ready};
36 #[unstable(feature = "future_poll_fn", issue = "72302")]
37 pub use poll_fn::{poll_fn, PollFn};
39 /// This type is needed because:
41 /// a) Generators cannot implement `for<'a, 'b> Generator<&'a mut Context<'b>>`, so we need to pass
42 /// a raw pointer (see <https://github.com/rust-lang/rust/issues/68923>).
43 /// b) Raw pointers and `NonNull` aren't `Send` or `Sync`, so that would make every single future
44 /// non-Send/Sync as well, and we don't want that.
46 /// It also simplifies the HIR lowering of `.await`.
48 #[unstable(feature = "gen_future", issue = "50547")]
49 #[derive(Debug, Copy, Clone)]
50 pub struct ResumeTy(NonNull<Context<'static>>);
52 #[unstable(feature = "gen_future", issue = "50547")]
53 unsafe impl Send for ResumeTy {}
55 #[unstable(feature = "gen_future", issue = "50547")]
56 unsafe impl Sync for ResumeTy {}
58 /// Wrap a generator in a future.
60 /// This function returns a `GenFuture` underneath, but hides it in `impl Trait` to give
61 /// better error messages (`impl Future` rather than `GenFuture<[closure.....]>`).
62 // This is `const` to avoid extra errors after we recover from `const async fn`
63 #[lang = "from_generator"]
65 #[unstable(feature = "gen_future", issue = "50547")]
66 #[rustc_const_unstable(feature = "gen_future", issue = "50547")]
68 pub const fn from_generator<T>(gen: T) -> impl Future<Output = T::Return>
70 T: Generator<ResumeTy, Yield = ()>,
72 #[rustc_diagnostic_item = "gen_future"]
73 struct GenFuture<T: Generator<ResumeTy, Yield = ()>>(T);
75 // We rely on the fact that async/await futures are immovable in order to create
76 // self-referential borrows in the underlying generator.
77 impl<T: Generator<ResumeTy, Yield = ()>> !Unpin for GenFuture<T> {}
79 impl<T: Generator<ResumeTy, Yield = ()>> Future for GenFuture<T> {
80 type Output = T::Return;
81 fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
82 // SAFETY: Safe because we're !Unpin + !Drop, and this is just a field projection.
83 let gen = unsafe { Pin::map_unchecked_mut(self, |s| &mut s.0) };
85 // Resume the generator, turning the `&mut Context` into a `NonNull` raw pointer. The
86 // `.await` lowering will safely cast that back to a `&mut Context`.
87 match gen.resume(ResumeTy(NonNull::from(cx).cast::<Context<'static>>())) {
88 GeneratorState::Yielded(()) => Poll::Pending,
89 GeneratorState::Complete(x) => Poll::Ready(x),
97 #[lang = "get_context"]
99 #[unstable(feature = "gen_future", issue = "50547")]
102 pub unsafe fn get_context<'a, 'b>(cx: ResumeTy) -> &'a mut Context<'b> {
103 // SAFETY: the caller must guarantee that `cx.0` is a valid pointer
104 // that fulfills all the requirements for a mutable reference.
105 unsafe { &mut *cx.0.as_ptr().cast() }