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    // Copyright 2018 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! Asynchronous values.

    use core::cell::Cell;
    use core::marker::Unpin;
    use core::mem::PinMut;
    use core::option::Option;
    use core::ptr::NonNull;
    use core::task::{self, Poll};
    use core::ops::{Drop, Generator, GeneratorState};

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

    /// Wrap a future in a generator.
    ///
    /// 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: PinMut<Self>, cx: &mut task::Context) -> Poll<Self::Output> {
            set_task_cx(cx, || match unsafe { PinMut::get_mut_unchecked(self).0.resume() } {
                GeneratorState::Yielded(()) => Poll::Pending,
                GeneratorState::Complete(x) => Poll::Ready(x),
            })
        }
    }

    thread_local! {
        static TLS_CX: Cell<Option<NonNull<task::Context<'static>>>> = Cell::new(None);
    }

    struct SetOnDrop(Option<NonNull<task::Context<'static>>>);

    impl Drop for SetOnDrop {
        fn drop(&mut self) {
            TLS_CX.with(|tls_cx| {
                tls_cx.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_cx<F, R>(cx: &mut task::Context, f: F) -> R
    where
        F: FnOnce() -> R
    {
        let old_cx = TLS_CX.with(|tls_cx| {
            tls_cx.replace(NonNull::new(
                cx
                    as *mut task::Context
                    as *mut ()
                    as *mut task::Context<'static>
            ))
        });
        let _reset_cx = SetOnDrop(old_cx);
        f()
    }

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

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

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

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