src/libstd/future.rs

   1 //! Asynchronous values.
   2
   3 use core::cell::Cell;
   4 use core::marker::Unpin;
   5 use core::ops::{Drop, Generator, GeneratorState};
   6 use core::option::Option;
   7 use core::pin::Pin;
   8 use core::ptr::NonNull;
   9 use core::task::{Context, Poll};
  10
  11 #[doc(inline)]
  12 #[stable(feature = "futures_api", since = "1.36.0")]
  13 pub use core::future::*;
  14
  15 /// Wrap a generator in a future.
  16 ///
  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.....]>`).
  19 // This is `const` to avoid extra errors after we recover from `const async fn`
  20 #[doc(hidden)]
  21 #[unstable(feature = "gen_future", issue = "50547")]
  22 pub const fn from_generator<T: Generator<Yield = ()>>(x: T) -> impl Future<Output = T::Return> {
  23     GenFuture(x)
  24 }
  25
  26 /// A wrapper around generators used to implement `Future` for `async`/`await` code.
  27 #[doc(hidden)]
  28 #[unstable(feature = "gen_future", issue = "50547")]
  29 #[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)]
  30 struct GenFuture<T: Generator<Yield = ()>>(T);
  31
  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> {}
  35
  36 #[doc(hidden)]
  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) };
  44         match gen.resume(
  45             #[cfg(not(bootstrap))]
  46             (),
  47         ) {
  48             GeneratorState::Yielded(()) => Poll::Pending,
  49             GeneratorState::Complete(x) => Poll::Ready(x),
  50         }
  51     }
  52 }
  53
  54 thread_local! {
  55     static TLS_CX: Cell<Option<NonNull<Context<'static>>>> = Cell::new(None);
  56 }
  57
  58 struct SetOnDrop(Option<NonNull<Context<'static>>>);
  59
  60 impl Drop for SetOnDrop {
  61     fn drop(&mut self) {
  62         TLS_CX.with(|tls_cx| {
  63             tls_cx.set(self.0.take());
  64         });
  65     }
  66 }
  67
  68 // Safety: the returned guard must drop before `cx` is dropped and before
  69 // any previous guard is dropped.
  70 unsafe fn set_task_context(cx: &mut Context<'_>) -> SetOnDrop {
  71     // transmute the context's lifetime to 'static so we can store it.
  72     let cx = core::mem::transmute::<&mut Context<'_>, &mut Context<'static>>(cx);
  73     let old_cx = TLS_CX.with(|tls_cx| tls_cx.replace(Some(NonNull::from(cx))));
  74     SetOnDrop(old_cx)
  75 }
  76
  77 #[doc(hidden)]
  78 #[unstable(feature = "gen_future", issue = "50547")]
  79 /// Polls a future in the current thread-local task waker.
  80 pub fn poll_with_tls_context<F>(f: Pin<&mut F>) -> Poll<F::Output>
  81 where
  82     F: Future,
  83 {
  84     let cx_ptr = TLS_CX.with(|tls_cx| {
  85         // Clear the entry so that nested `get_task_waker` calls
  86         // will fail or set their own value.
  87         tls_cx.replace(None)
  88     });
  89     let _reset = SetOnDrop(cx_ptr);
  90
  91     let mut cx_ptr = cx_ptr.expect(
  92         "TLS Context not set. This is a rustc bug. \
  93         Please file an issue on https://github.com/rust-lang/rust.",
  94     );
  95
  96     // Safety: we've ensured exclusive access to the context by
  97     // removing the pointer from TLS, only to be replaced once
  98     // we're done with it.
  99     //
 100     // The pointer that was inserted came from an `&mut Context<'_>`,
 101     // so it is safe to treat as mutable.
 102     unsafe { F::poll(f, cx_ptr.as_mut()) }
 103 }