1 #![stable(feature = "futures_api", since = "1.36.0")]
3 //! Asynchronous values.
6 ops::{Generator, GeneratorState},
18 #[stable(feature = "futures_api", since = "1.36.0")]
19 pub use self::future::Future;
21 #[unstable(feature = "into_future", issue = "67644")]
22 pub use into_future::IntoFuture;
24 #[unstable(feature = "future_readiness_fns", issue = "70921")]
25 pub use pending::{pending, Pending};
26 #[unstable(feature = "future_readiness_fns", issue = "70921")]
27 pub use ready::{ready, Ready};
29 #[unstable(feature = "future_poll_fn", issue = "72302")]
30 pub use poll_fn::{poll_fn, PollFn};
32 /// This type is needed because:
34 /// a) Generators cannot implement `for<'a, 'b> Generator<&'a mut Context<'b>>`, so we need to pass
35 /// a raw pointer (see https://github.com/rust-lang/rust/issues/68923).
36 /// b) Raw pointers and `NonNull` aren't `Send` or `Sync`, so that would make every single future
37 /// non-Send/Sync as well, and we don't want that.
39 /// It also simplifies the HIR lowering of `.await`.
41 #[unstable(feature = "gen_future", issue = "50547")]
42 #[derive(Debug, Copy, Clone)]
43 pub struct ResumeTy(NonNull<Context<'static>>);
45 #[unstable(feature = "gen_future", issue = "50547")]
46 unsafe impl Send for ResumeTy {}
48 #[unstable(feature = "gen_future", issue = "50547")]
49 unsafe impl Sync for ResumeTy {}
51 /// Wrap a generator in a future.
53 /// This function returns a `GenFuture` underneath, but hides it in `impl Trait` to give
54 /// better error messages (`impl Future` rather than `GenFuture<[closure.....]>`).
55 // This is `const` to avoid extra errors after we recover from `const async fn`
56 #[lang = "from_generator"]
58 #[unstable(feature = "gen_future", issue = "50547")]
60 pub const fn from_generator<T>(gen: T) -> impl Future<Output = T::Return>
62 T: Generator<ResumeTy, Yield = ()>,
64 #[rustc_diagnostic_item = "gen_future"]
65 struct GenFuture<T: Generator<ResumeTy, Yield = ()>>(T);
67 // We rely on the fact that async/await futures are immovable in order to create
68 // self-referential borrows in the underlying generator.
69 impl<T: Generator<ResumeTy, Yield = ()>> !Unpin for GenFuture<T> {}
71 impl<T: Generator<ResumeTy, Yield = ()>> Future for GenFuture<T> {
72 type Output = T::Return;
73 fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
74 // Safety: Safe because we're !Unpin + !Drop, and this is just a field projection.
75 let gen = unsafe { Pin::map_unchecked_mut(self, |s| &mut s.0) };
77 // Resume the generator, turning the `&mut Context` into a `NonNull` raw pointer. The
78 // `.await` lowering will safely cast that back to a `&mut Context`.
79 match gen.resume(ResumeTy(NonNull::from(cx).cast::<Context<'static>>())) {
80 GeneratorState::Yielded(()) => Poll::Pending,
81 GeneratorState::Complete(x) => Poll::Ready(x),
89 #[lang = "get_context"]
91 #[unstable(feature = "gen_future", issue = "50547")]
93 pub unsafe fn get_context<'a, 'b>(cx: ResumeTy) -> &'a mut Context<'b> {
94 // SAFETY: the caller must guarantee that `cx.0` is a valid pointer
95 // that fulfills all the requirements for a mutable reference.
96 unsafe { &mut *cx.0.as_ptr().cast() }