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    #![unstable(feature = "futures_api",
                reason = "futures in libcore are unstable",
                issue = "50547")]

    use fmt;
    use marker::{PhantomData, Unpin};

    /// A `RawWaker` allows the implementor of a task executor to create a [`Waker`]
    /// which provides customized wakeup behavior.
    ///
    /// [vtable]: https://en.wikipedia.org/wiki/Virtual_method_table
    ///
    /// It consists of a data pointer and a [virtual function pointer table (vtable)][vtable] that
    /// customizes the behavior of the `RawWaker`.
    #[derive(PartialEq, Debug)]
    pub struct RawWaker {
        /// A data pointer, which can be used to store arbitrary data as required
        /// by the executor. This could be e.g. a type-erased pointer to an `Arc`
        /// that is associated with the task.
        /// The value of this field gets passed to all functions that are part of
        /// the vtable as the first parameter.
        data: *const (),
        /// Virtual function pointer table that customizes the behavior of this waker.
        vtable: &'static RawWakerVTable,
    }

    impl RawWaker {
        /// Creates a new `RawWaker` from the provided `data` pointer and `vtable`.
        ///
        /// The `data` pointer can be used to store arbitrary data as required
        /// by the executor. This could be e.g. a type-erased pointer to an `Arc`
        /// that is associated with the task.
        /// The value of this poiner will get passed to all functions that are part
        /// of the `vtable` as the first parameter.
        ///
        /// The `vtable` customizes the behavior of a `Waker` which gets created
        /// from a `RawWaker`. For each operation on the `Waker`, the associated
        /// function in the `vtable` of the underlying `RawWaker` will be called.
        #[rustc_promotable]
        #[unstable(feature = "futures_api",
                reason = "futures in libcore are unstable",
                issue = "50547")]
        pub const fn new(data: *const (), vtable: &'static RawWakerVTable) -> RawWaker {
            RawWaker {
                data,
                vtable,
            }
        }
    }

    /// A virtual function pointer table (vtable) that specifies the behavior
    /// of a [`RawWaker`].
    ///
    /// The pointer passed to all functions inside the vtable is the `data` pointer
    /// from the enclosing [`RawWaker`] object.
    ///
    /// The functions inside this struct are only intended be called on the `data`
    /// pointer of a properly constructed [`RawWaker`] object from inside the
    /// [`RawWaker`] implementation. Calling one of the contained functions using
    /// any other `data` pointer will cause undefined behavior.
    #[derive(PartialEq, Copy, Clone, Debug)]
    pub struct RawWakerVTable {
        /// This function will be called when the [`RawWaker`] gets cloned, e.g. when
        /// the [`Waker`] in which the [`RawWaker`] is stored gets cloned.
        ///
        /// The implementation of this function must retain all resources that are
        /// required for this additional instance of a [`RawWaker`] and associated
        /// task. Calling `wake` on the resulting [`RawWaker`] should result in a wakeup
        /// of the same task that would have been awoken by the original [`RawWaker`].
        clone: unsafe fn(*const ()) -> RawWaker,

        /// This function will be called when `wake` is called on the [`Waker`].
        /// It must wake up the task associated with this [`RawWaker`].
        ///
        /// The implemention of this function must not consume the provided data
        /// pointer.
        wake: unsafe fn(*const ()),

        /// This function gets called when a [`RawWaker`] gets dropped.
        ///
        /// The implementation of this function must make sure to release any
        /// resources that are associated with this instance of a [`RawWaker`] and
        /// associated task.
        drop: unsafe fn(*const ()),
    }

    impl RawWakerVTable {
        /// Creates a new `RawWakerVTable` from the provided `clone`, `wake`, and
        /// `drop` functions.
        ///
        /// # `clone`
        ///
        /// This function will be called when the [`RawWaker`] gets cloned, e.g. when
        /// the [`Waker`] in which the [`RawWaker`] is stored gets cloned.
        ///
        /// The implementation of this function must retain all resources that are
        /// required for this additional instance of a [`RawWaker`] and associated
        /// task. Calling `wake` on the resulting [`RawWaker`] should result in a wakeup
        /// of the same task that would have been awoken by the original [`RawWaker`].
        ///
        /// # `wake`
        ///
        /// This function will be called when `wake` is called on the [`Waker`].
        /// It must wake up the task associated with this [`RawWaker`].
        ///
        /// The implemention of this function must not consume the provided data
        /// pointer.
        ///
        /// # `drop`
        ///
        /// This function gets called when a [`RawWaker`] gets dropped.
        ///
        /// The implementation of this function must make sure to release any
        /// resources that are associated with this instance of a [`RawWaker`] and
        /// associated task.
        #[rustc_promotable]
        #[unstable(feature = "futures_api",
                reason = "futures in libcore are unstable",
                issue = "50547")]
        pub const fn new(
            clone: unsafe fn(*const ()) -> RawWaker,
            wake: unsafe fn(*const ()),
            drop: unsafe fn(*const ()),
        ) -> Self {
            Self {
                clone,
                wake,
                drop,
            }
        }
    }

    /// The `Context` of an asynchronous task.
    ///
    /// Currently, `Context` only serves to provide access to a `&Waker`
    /// which can be used to wake the current task.
    pub struct Context<'a> {
        waker: &'a Waker,
        // Ensure we future-proof against variance changes by forcing
        // the lifetime to be invariant (argument-position lifetimes
        // are contravariant while return-position lifetimes are
        // covariant).
        _marker: PhantomData<fn(&'a ()) -> &'a ()>,
    }

    impl<'a> Context<'a> {
        /// Create a new `Context` from a `&Waker`.
        #[inline]
        pub fn from_waker(waker: &'a Waker) -> Self {
            Context {
                waker,
                _marker: PhantomData,
            }
        }

        /// Returns a reference to the `Waker` for the current task.
        #[inline]
        pub fn waker(&self) -> &'a Waker {
            &self.waker
        }
    }

    impl fmt::Debug for Context<'_> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("Context")
                .field("waker", &self.waker)
                .finish()
        }
    }

    /// A `Waker` is a handle for waking up a task by notifying its executor that it
    /// is ready to be run.
    ///
    /// This handle encapsulates a [`RawWaker`] instance, which defines the
    /// executor-specific wakeup behavior.
    ///
    /// Implements [`Clone`], [`Send`], and [`Sync`].
    #[repr(transparent)]
    pub struct Waker {
        waker: RawWaker,
    }

    impl Unpin for Waker {}
    unsafe impl Send for Waker {}
    unsafe impl Sync for Waker {}

    impl Waker {
        /// Wake up the task associated with this `Waker`.
        #[inline]
        pub fn wake(&self) {
            // The actual wakeup call is delegated through a virtual function call
            // to the implementation which is defined by the executor.

            // SAFETY: This is safe because `Waker::new_unchecked` is the only way
            // to initialize `wake` and `data` requiring the user to acknowledge
            // that the contract of `RawWaker` is upheld.
            unsafe { (self.waker.vtable.wake)(self.waker.data) }
        }

        /// Returns `true` if this `Waker` and another `Waker` have awoken the same task.
        ///
        /// This function works on a best-effort basis, and may return false even
        /// when the `Waker`s would awaken the same task. However, if this function
        /// returns `true`, it is guaranteed that the `Waker`s will awaken the same task.
        ///
        /// This function is primarily used for optimization purposes.
        #[inline]
        pub fn will_wake(&self, other: &Waker) -> bool {
            self.waker == other.waker
        }

        /// Creates a new `Waker` from [`RawWaker`].
        ///
        /// The behavior of the returned `Waker` is undefined if the contract defined
        /// in [`RawWaker`]'s and [`RawWakerVTable`]'s documentation is not upheld.
        /// Therefore this method is unsafe.
        #[inline]
        pub unsafe fn new_unchecked(waker: RawWaker) -> Waker {
            Waker {
                waker,
            }
        }
    }

    impl Clone for Waker {
        #[inline]
        fn clone(&self) -> Self {
            Waker {
                // SAFETY: This is safe because `Waker::new_unchecked` is the only way
                // to initialize `clone` and `data` requiring the user to acknowledge
                // that the contract of [`RawWaker`] is upheld.
                waker: unsafe { (self.waker.vtable.clone)(self.waker.data) },
            }
        }
    }

    impl Drop for Waker {
        #[inline]
        fn drop(&mut self) {
            // SAFETY: This is safe because `Waker::new_unchecked` is the only way
            // to initialize `drop` and `data` requiring the user to acknowledge
            // that the contract of `RawWaker` is upheld.
            unsafe { (self.waker.vtable.drop)(self.waker.data) }
        }
    }

    impl fmt::Debug for Waker {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            let vtable_ptr = self.waker.vtable as *const RawWakerVTable;
            f.debug_struct("Waker")
                .field("data", &self.waker.data)
                .field("vtable", &vtable_ptr)
                .finish()
        }
    }

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