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    #![stable(feature = "futures_api", since = "1.36.0")]

    use crate::fmt;
    use crate::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`.
    ///
    /// [`Waker`]: struct.Waker.html
    #[derive(PartialEq, Debug)]
    #[stable(feature = "futures_api", since = "1.36.0")]
    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]
        #[stable(feature = "futures_api", since = "1.36.0")]
        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.
    ///
    /// [`RawWaker`]: struct.RawWaker.html
    #[stable(feature = "futures_api", since = "1.36.0")]
    #[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`].
        ///
        /// [`Waker`]: struct.Waker.html
        /// [`RawWaker`]: struct.RawWaker.html
        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 implementation of this function must make sure to release any
        /// resources that are associated with this instance of a [`RawWaker`] and
        /// associated task.
        ///
        /// [`Waker`]: struct.Waker.html
        /// [`RawWaker`]: struct.RawWaker.html
        wake: unsafe fn(*const ()),

        /// This function will be called when `wake_by_ref` is called on the [`Waker`].
        /// It must wake up the task associated with this [`RawWaker`].
        ///
        /// This function is similar to `wake`, but must not consume the provided data
        /// pointer.
        ///
        /// [`Waker`]: struct.Waker.html
        /// [`RawWaker`]: struct.RawWaker.html
        wake_by_ref: 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.
        ///
        /// [`RawWaker`]: struct.RawWaker.html
        drop: unsafe fn(*const ()),
    }

    impl RawWakerVTable {
        /// Creates a new `RawWakerVTable` from the provided `clone`, `wake`,
        /// `wake_by_ref`, 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 implementation of this function must make sure to release any
        /// resources that are associated with this instance of a [`RawWaker`] and
        /// associated task.
        ///
        /// # `wake_by_ref`
        ///
        /// This function will be called when `wake_by_ref` is called on the [`Waker`].
        /// It must wake up the task associated with this [`RawWaker`].
        ///
        /// This function is similar to `wake`, but 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.
        ///
        /// [`Waker`]: struct.Waker.html
        /// [`RawWaker`]: struct.RawWaker.html
        #[rustc_promotable]
        #[cfg_attr(stage0, unstable(feature = "futures_api_const_fn_ptr", issue = "50547"))]
        #[cfg_attr(not(stage0), stable(feature = "futures_api", since = "1.36.0"))]
        // `rustc_allow_const_fn_ptr` is a hack that should not be used anywhere else
        // without first consulting with T-Lang.
        //
        // FIXME: remove whenever we have a stable way to accept fn pointers from const fn
        // (see https://github.com/rust-rfcs/const-eval/issues/19#issuecomment-472799062)
        #[cfg_attr(not(stage0), rustc_allow_const_fn_ptr)]
        pub const fn new(
            clone: unsafe fn(*const ()) -> RawWaker,
            wake: unsafe fn(*const ()),
            wake_by_ref: unsafe fn(*const ()),
            drop: unsafe fn(*const ()),
        ) -> Self {
            Self {
                clone,
                wake,
                wake_by_ref,
                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.
    #[stable(feature = "futures_api", since = "1.36.0")]
    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`.
        #[stable(feature = "futures_api", since = "1.36.0")]
        #[inline]
        pub fn from_waker(waker: &'a Waker) -> Self {
            Context {
                waker,
                _marker: PhantomData,
            }
        }

        /// Returns a reference to the `Waker` for the current task.
        #[stable(feature = "futures_api", since = "1.36.0")]
        #[inline]
        pub fn waker(&self) -> &'a Waker {
            &self.waker
        }
    }

    #[stable(feature = "futures_api", since = "1.36.0")]
    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`].
    ///
    /// [`RawWaker`]: struct.RawWaker.html
    #[repr(transparent)]
    #[stable(feature = "futures_api", since = "1.36.0")]
    pub struct Waker {
        waker: RawWaker,
    }

    #[stable(feature = "futures_api", since = "1.36.0")]
    impl Unpin for Waker {}
    #[stable(feature = "futures_api", since = "1.36.0")]
    unsafe impl Send for Waker {}
    #[stable(feature = "futures_api", since = "1.36.0")]
    unsafe impl Sync for Waker {}

    impl Waker {
        /// Wake up the task associated with this `Waker`.
        #[inline]
        #[stable(feature = "futures_api", since = "1.36.0")]
        pub fn wake(self) {
            // The actual wakeup call is delegated through a virtual function call
            // to the implementation which is defined by the executor.
            let wake = self.waker.vtable.wake;
            let data = self.waker.data;

            // Don't call `drop` -- the waker will be consumed by `wake`.
            crate::mem::forget(self);

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

        /// Wake up the task associated with this `Waker` without consuming the `Waker`.
        ///
        /// This is similar to `wake`, but may be slightly less efficient in the case
        /// where an owned `Waker` is available. This method should be preferred to
        /// calling `waker.clone().wake()`.
        #[inline]
        #[stable(feature = "futures_api", since = "1.36.0")]
        pub fn wake_by_ref(&self) {
            // The actual wakeup call is delegated through a virtual function call
            // to the implementation which is defined by the executor.

            // SAFETY: see `wake`
            unsafe { (self.waker.vtable.wake_by_ref)(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]
        #[stable(feature = "futures_api", since = "1.36.0")]
        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.
        ///
        /// [`RawWaker`]: struct.RawWaker.html
        /// [`RawWakerVTable`]: struct.RawWakerVTable.html
        #[inline]
        #[stable(feature = "futures_api", since = "1.36.0")]
        pub unsafe fn from_raw(waker: RawWaker) -> Waker {
            Waker {
                waker,
            }
        }
    }

    #[stable(feature = "futures_api", since = "1.36.0")]
    impl Clone for Waker {
        #[inline]
        fn clone(&self) -> Self {
            Waker {
                // SAFETY: This is safe because `Waker::from_raw` 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) },
            }
        }
    }

    #[stable(feature = "futures_api", since = "1.36.0")]
    impl Drop for Waker {
        #[inline]
        fn drop(&mut self) {
            // SAFETY: This is safe because `Waker::from_raw` 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) }
        }
    }

    #[stable(feature = "futures_api", since = "1.36.0")]
    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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