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

    use fmt;
    use marker::Unpin;
    use ptr::NonNull;

    /// A `Waker` is a handle for waking up a task by notifying its executor that it
    /// is ready to be run.
    ///
    /// This handle contains a trait object pointing to an instance of the `UnsafeWake`
    /// trait, allowing notifications to get routed through it.
    #[repr(transparent)]
    pub struct Waker {
        inner: NonNull<dyn UnsafeWake>,
    }

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

    impl Waker {
        /// Constructs a new `Waker` directly.
        ///
        /// Note that most code will not need to call this. Implementers of the
        /// `UnsafeWake` trait will typically provide a wrapper that calls this
        /// but you otherwise shouldn't call it directly.
        ///
        /// If you're working with the standard library then it's recommended to
        /// use the `Waker::from` function instead which works with the safe
        /// `Arc` type and the safe `Wake` trait.
        #[inline]
        pub unsafe fn new(inner: NonNull<dyn UnsafeWake>) -> Self {
            Waker { inner }
        }

        /// Wake up the task associated with this `Waker`.
        #[inline]
        pub fn wake(&self) {
            unsafe { self.inner.as_ref().wake() }
        }

        /// Returns whether or not this `Waker` and `other` awaken 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.inner == other.inner
        }

        /// Returns whether or not this `Waker` and `other` `LocalWaker` awaken
        /// 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_local(&self, other: &LocalWaker) -> bool {
            self.will_wake(&other.0)
        }
    }

    impl Clone for Waker {
        #[inline]
        fn clone(&self) -> Self {
            unsafe {
                self.inner.as_ref().clone_raw()
            }
        }
    }

    impl fmt::Debug for Waker {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("Waker")
                .finish()
        }
    }

    impl Drop for Waker {
        #[inline]
        fn drop(&mut self) {
            unsafe {
                self.inner.as_ref().drop_raw()
            }
        }
    }

    /// A `LocalWaker` is a handle for waking up a task by notifying its executor that it
    /// is ready to be run.
    ///
    /// This is similar to the `Waker` type, but cannot be sent across threads.
    /// Task executors can use this type to implement more optimized single-threaded wakeup
    /// behavior.
    #[repr(transparent)]
    #[derive(Clone)]
    pub struct LocalWaker(Waker);

    impl Unpin for LocalWaker {}
    impl !Send for LocalWaker {}
    impl !Sync for LocalWaker {}

    impl LocalWaker {
        /// Constructs a new `LocalWaker` directly.
        ///
        /// Note that most code will not need to call this. Implementers of the
        /// `UnsafeWake` trait will typically provide a wrapper that calls this
        /// but you otherwise shouldn't call it directly.
        ///
        /// If you're working with the standard library then it's recommended to
        /// use the `local_waker_from_nonlocal` or `local_waker` to convert a `Waker`
        /// into a `LocalWaker`.
        ///
        /// For this function to be used safely, it must be sound to call `inner.wake_local()`
        /// on the current thread.
        #[inline]
        pub unsafe fn new(inner: NonNull<dyn UnsafeWake>) -> Self {
            LocalWaker(Waker::new(inner))
        }

        /// Borrows this `LocalWaker` as a `Waker`.
        ///
        /// `Waker` is nearly identical to `LocalWaker`, but is threadsafe
        /// (implements `Send` and `Sync`).
        #[inline]
        pub fn as_waker(&self) -> &Waker {
            &self.0
        }

        /// Converts this `LocalWaker` into a `Waker`.
        ///
        /// `Waker` is nearly identical to `LocalWaker`, but is threadsafe
        /// (implements `Send` and `Sync`).
        #[inline]
        pub fn into_waker(self) -> Waker {
            self.0
        }

        /// Wake up the task associated with this `LocalWaker`.
        #[inline]
        pub fn wake(&self) {
            unsafe { self.0.inner.as_ref().wake_local() }
        }

        /// Returns whether or not this `LocalWaker` and `other` `LocalWaker` awaken the same task.
        ///
        /// This function works on a best-effort basis, and may return false even
        /// when the `LocalWaker`s would awaken the same task. However, if this function
        /// returns true, it is guaranteed that the `LocalWaker`s will awaken the same
        /// task.
        ///
        /// This function is primarily used for optimization purposes.
        #[inline]
        pub fn will_wake(&self, other: &LocalWaker) -> bool {
            self.0.will_wake(&other.0)
        }

        /// Returns whether or not this `LocalWaker` and `other` `Waker` awaken 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 `LocalWaker`s will awaken the same
        /// task.
        ///
        /// This function is primarily used for optimization purposes.
        #[inline]
        pub fn will_wake_nonlocal(&self, other: &Waker) -> bool {
            self.0.will_wake(other)
        }
    }

    impl From<LocalWaker> for Waker {
        /// Converts a `LocalWaker` into a `Waker`.
        ///
        /// This conversion turns a `!Sync` `LocalWaker` into a `Sync` `Waker`, allowing a wakeup
        /// object to be sent to another thread, but giving up its ability to do specialized
        /// thread-local wakeup behavior.
        #[inline]
        fn from(local_waker: LocalWaker) -> Self {
            local_waker.0
        }
    }

    impl fmt::Debug for LocalWaker {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("LocalWaker")
                .finish()
        }
    }

    /// An unsafe trait for implementing custom memory management for a `Waker` or `LocalWaker`.
    ///
    /// A `Waker` conceptually is a cloneable trait object for `Wake`, and is
    /// most often essentially just `Arc<dyn Wake>`. However, in some contexts
    /// (particularly `no_std`), it's desirable to avoid `Arc` in favor of some
    /// custom memory management strategy. This trait is designed to allow for such
    /// customization.
    ///
    /// When using `std`, a default implementation of the `UnsafeWake` trait is provided for
    /// `Arc<T>` where `T: Wake`.
    pub unsafe trait UnsafeWake: Send + Sync {
        /// Creates a clone of this `UnsafeWake` and stores it behind a `Waker`.
        ///
        /// This function will create a new uniquely owned handle that under the
        /// hood references the same notification instance. In other words calls
        /// to `wake` on the returned handle should be equivalent to calls to
        /// `wake` on this handle.
        ///
        /// # Unsafety
        ///
        /// This function is unsafe to call because it's asserting the `UnsafeWake`
        /// value is in a consistent state, i.e., hasn't been dropped.
        unsafe fn clone_raw(&self) -> Waker;

        /// Drops this instance of `UnsafeWake`, deallocating resources
        /// associated with it.
        ///
        /// FIXME(cramertj)
        /// This method is intended to have a signature such as:
        ///
        /// ```ignore (not-a-doctest)
        /// fn drop_raw(self: *mut Self);
        /// ```
        ///
        /// Unfortunately in Rust today that signature is not object safe.
        /// Nevertheless it's recommended to implement this function *as if* that
        /// were its signature. As such it is not safe to call on an invalid
        /// pointer, nor is the validity of the pointer guaranteed after this
        /// function returns.
        ///
        /// # Unsafety
        ///
        /// This function is unsafe to call because it's asserting the `UnsafeWake`
        /// value is in a consistent state, i.e., hasn't been dropped.
        unsafe fn drop_raw(&self);

        /// Indicates that the associated task is ready to make progress and should
        /// be `poll`ed.
        ///
        /// Executors generally maintain a queue of "ready" tasks; `wake` should place
        /// the associated task onto this queue.
        ///
        /// # Panics
        ///
        /// Implementations should avoid panicking, but clients should also be prepared
        /// for panics.
        ///
        /// # Unsafety
        ///
        /// This function is unsafe to call because it's asserting the `UnsafeWake`
        /// value is in a consistent state, i.e., hasn't been dropped.
        unsafe fn wake(&self);

        /// Indicates that the associated task is ready to make progress and should
        /// be `poll`ed. This function is the same as `wake`, but can only be called
        /// from the thread that this `UnsafeWake` is "local" to. This allows for
        /// implementors to provide specialized wakeup behavior specific to the current
        /// thread. This function is called by `LocalWaker::wake`.
        ///
        /// Executors generally maintain a queue of "ready" tasks; `wake_local` should place
        /// the associated task onto this queue.
        ///
        /// # Panics
        ///
        /// Implementations should avoid panicking, but clients should also be prepared
        /// for panics.
        ///
        /// # Unsafety
        ///
        /// This function is unsafe to call because it's asserting the `UnsafeWake`
        /// value is in a consistent state, i.e., hasn't been dropped, and that the
        /// `UnsafeWake` hasn't moved from the thread on which it was created.
        unsafe fn wake_local(&self) {
            self.wake()
        }
    }

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