src/libcore/future/mod.rs

   1 #![stable(feature = "futures_api", since = "1.36.0")]
   2
   3 //! Asynchronous values.
   4
   5 use crate::{
   6     ops::{Generator, GeneratorState},
   7     pin::Pin,
   8     ptr::NonNull,
   9     task::{Context, Poll},
  10 };
  11
  12 mod future;
  13 mod into_future;
  14 mod pending;
  15 mod poll_fn;
  16 mod ready;
  17
  18 #[stable(feature = "futures_api", since = "1.36.0")]
  19 pub use self::future::Future;
  20
  21 #[unstable(feature = "into_future", issue = "67644")]
  22 pub use into_future::IntoFuture;
  23
  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};
  28
  29 #[unstable(feature = "future_poll_fn", issue = "72302")]
  30 pub use poll_fn::{poll_fn, PollFn};
  31
  32 /// This type is needed because:
  33 ///
  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.
  38 ///
  39 /// It also simplifies the HIR lowering of `.await`.
  40 #[doc(hidden)]
  41 #[unstable(feature = "gen_future", issue = "50547")]
  42 #[derive(Debug, Copy, Clone)]
  43 pub struct ResumeTy(NonNull<Context<'static>>);
  44
  45 #[unstable(feature = "gen_future", issue = "50547")]
  46 unsafe impl Send for ResumeTy {}
  47
  48 #[unstable(feature = "gen_future", issue = "50547")]
  49 unsafe impl Sync for ResumeTy {}
  50
  51 /// Wrap a generator in a future.
  52 ///
  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 #[doc(hidden)]
  57 #[unstable(feature = "gen_future", issue = "50547")]
  58 #[inline]
  59 pub const fn from_generator<T>(gen: T) -> impl Future<Output = T::Return>
  60 where
  61     T: Generator<ResumeTy, Yield = ()>,
  62 {
  63     #[rustc_diagnostic_item = "gen_future"]
  64     struct GenFuture<T: Generator<ResumeTy, Yield = ()>>(T);
  65
  66     // We rely on the fact that async/await futures are immovable in order to create
  67     // self-referential borrows in the underlying generator.
  68     impl<T: Generator<ResumeTy, Yield = ()>> !Unpin for GenFuture<T> {}
  69
  70     impl<T: Generator<ResumeTy, Yield = ()>> Future for GenFuture<T> {
  71         type Output = T::Return;
  72         fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
  73             // Safety: Safe because we're !Unpin + !Drop, and this is just a field projection.
  74             let gen = unsafe { Pin::map_unchecked_mut(self, |s| &mut s.0) };
  75
  76             // Resume the generator, turning the `&mut Context` into a `NonNull` raw pointer. The
  77             // `.await` lowering will safely cast that back to a `&mut Context`.
  78             match gen.resume(ResumeTy(NonNull::from(cx).cast::<Context<'static>>())) {
  79                 GeneratorState::Yielded(()) => Poll::Pending,
  80                 GeneratorState::Complete(x) => Poll::Ready(x),
  81             }
  82         }
  83     }
  84
  85     GenFuture(gen)
  86 }
  87
  88 #[doc(hidden)]
  89 #[unstable(feature = "gen_future", issue = "50547")]
  90 #[inline]
  91 pub unsafe fn get_context<'a, 'b>(cx: ResumeTy) -> &'a mut Context<'b> {
  92     // SAFETY: the caller must guarantee that `cx.0` is a valid pointer
  93     // that fulfills all the requirements for a mutable reference.
  94     unsafe { &mut *cx.0.as_ptr().cast() }
  95 }