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    //! Asynchronous values.

    use core::cell::Cell;
    use core::marker::Unpin;
    use core::pin::Pin;
    use core::option::Option;
    use core::ptr::NonNull;
    use core::task::{Waker, Poll};
    use core::ops::{Drop, Generator, GeneratorState};

    #[doc(inline)]
    pub use core::future::*;

    /// Wrap a generator in a future.
    ///
    /// This function returns a `GenFuture` underneath, but hides it in `impl Trait` to give
    /// better error messages (`impl Future` rather than `GenFuture<[closure.....]>`).
    #[unstable(feature = "gen_future", issue = "50547")]
    pub fn from_generator<T: Generator<Yield = ()>>(x: T) -> impl Future<Output = T::Return> {
        GenFuture(x)
    }

    /// A wrapper around generators used to implement `Future` for `async`/`await` code.
    #[unstable(feature = "gen_future", issue = "50547")]
    #[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)]
    struct GenFuture<T: Generator<Yield = ()>>(T);

    // We rely on the fact that async/await futures are immovable in order to create
    // self-referential borrows in the underlying generator.
    impl<T: Generator<Yield = ()>> !Unpin for GenFuture<T> {}

    #[unstable(feature = "gen_future", issue = "50547")]
    impl<T: Generator<Yield = ()>> Future for GenFuture<T> {
        type Output = T::Return;
        fn poll(self: Pin<&mut Self>, waker: &Waker) -> Poll<Self::Output> {
            // Safe because we're !Unpin + !Drop mapping to a ?Unpin value
            let gen = unsafe { Pin::map_unchecked_mut(self, |s| &mut s.0) };
            set_task_waker(waker, || match gen.resume() {
                GeneratorState::Yielded(()) => Poll::Pending,
                GeneratorState::Complete(x) => Poll::Ready(x),
            })
        }
    }

    thread_local! {
        static TLS_WAKER: Cell<Option<NonNull<Waker>>> = Cell::new(None);
    }

    struct SetOnDrop(Option<NonNull<Waker>>);

    impl Drop for SetOnDrop {
        fn drop(&mut self) {
            TLS_WAKER.with(|tls_waker| {
                tls_waker.set(self.0.take());
            });
        }
    }

    #[unstable(feature = "gen_future", issue = "50547")]
    /// Sets the thread-local task context used by async/await futures.
    pub fn set_task_waker<F, R>(waker: &Waker, f: F) -> R
    where
        F: FnOnce() -> R
    {
        let old_waker = TLS_WAKER.with(|tls_waker| {
            tls_waker.replace(Some(NonNull::from(waker)))
        });
        let _reset_waker = SetOnDrop(old_waker);
        f()
    }

    #[unstable(feature = "gen_future", issue = "50547")]
    /// Retrieves the thread-local task waker used by async/await futures.
    ///
    /// This function acquires exclusive access to the task waker.
    ///
    /// Panics if no waker has been set or if the waker has already been
    /// retrieved by a surrounding call to get_task_waker.
    pub fn get_task_waker<F, R>(f: F) -> R
    where
        F: FnOnce(&Waker) -> R
    {
        let waker_ptr = TLS_WAKER.with(|tls_waker| {
            // Clear the entry so that nested `get_task_waker` calls
            // will fail or set their own value.
            tls_waker.replace(None)
        });
        let _reset_waker = SetOnDrop(waker_ptr);

        let waker_ptr = waker_ptr.expect(
            "TLS Waker not set. This is a rustc bug. \
            Please file an issue on https://github.com/rust-lang/rust.");
        unsafe { f(waker_ptr.as_ref()) }
    }

    #[unstable(feature = "gen_future", issue = "50547")]
    /// Polls a future in the current thread-local task waker.
    pub fn poll_with_tls_waker<F>(f: Pin<&mut F>) -> Poll<F::Output>
    where
        F: Future
    {
        get_task_waker(|waker| F::poll(f, waker))
    }

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