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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.

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

    use fmt;
    use future::Future;
    use marker::{PhantomData, Unpin};
    use pin::PinMut;
    use task::{Context, Poll};

    /// A custom trait object for polling futures, roughly akin to
    /// `Box<dyn Future<Output = T> + 'a>`.
    ///
    /// This custom trait object was introduced for two reasons:
    /// - Currently it is not possible to take `dyn Trait` by value and
    ///   `Box<dyn Trait>` is not available in no_std contexts.
    /// - The `Future` trait is currently not object safe: The `Future::poll`
    ///   method makes uses the arbitrary self types feature and traits in which
    ///   this feature is used are currently not object safe due to current compiler
    ///   limitations. (See tracking issue for arbitrary self types for more
    ///   information #44874)
    pub struct LocalFutureObj<'a, T> {
        ptr: *mut (),
        poll_fn: unsafe fn(*mut (), &mut Context) -> Poll<T>,
        drop_fn: unsafe fn(*mut ()),
        _marker: PhantomData<&'a ()>,
    }

    impl<'a, T> Unpin for LocalFutureObj<'a, T> {}

    impl<'a, T> LocalFutureObj<'a, T> {
        /// Create a `LocalFutureObj` from a custom trait object representation.
        #[inline]
        pub fn new<F: UnsafeFutureObj<'a, T> + 'a>(f: F) -> LocalFutureObj<'a, T> {
            LocalFutureObj {
                ptr: f.into_raw(),
                poll_fn: F::poll,
                drop_fn: F::drop,
                _marker: PhantomData,
            }
        }

        /// Converts the `LocalFutureObj` into a `FutureObj`
        /// To make this operation safe one has to ensure that the `UnsafeFutureObj`
        /// instance from which this `LocalFutureObj` was created actually
        /// implements `Send`.
        #[inline]
        pub unsafe fn into_future_obj(self) -> FutureObj<'a, T> {
            FutureObj(self)
        }
    }

    impl<'a, T> fmt::Debug for LocalFutureObj<'a, T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("LocalFutureObj")
                .finish()
        }
    }

    impl<'a, T> From<FutureObj<'a, T>> for LocalFutureObj<'a, T> {
        #[inline]
        fn from(f: FutureObj<'a, T>) -> LocalFutureObj<'a, T> {
            f.0
        }
    }

    impl<'a, T> Future for LocalFutureObj<'a, T> {
        type Output = T;

        #[inline]
        fn poll(self: PinMut<Self>, cx: &mut Context) -> Poll<T> {
            unsafe {
                (self.poll_fn)(self.ptr, cx)
            }
        }
    }

    impl<'a, T> Drop for LocalFutureObj<'a, T> {
        fn drop(&mut self) {
            unsafe {
                (self.drop_fn)(self.ptr)
            }
        }
    }

    /// A custom trait object for polling futures, roughly akin to
    /// `Box<dyn Future<Output = T> + Send + 'a>`.
    ///
    /// This custom trait object was introduced for two reasons:
    /// - Currently it is not possible to take `dyn Trait` by value and
    ///   `Box<dyn Trait>` is not available in no_std contexts.
    /// - The `Future` trait is currently not object safe: The `Future::poll`
    ///   method makes uses the arbitrary self types feature and traits in which
    ///   this feature is used are currently not object safe due to current compiler
    ///   limitations. (See tracking issue for arbitrary self types for more
    ///   information #44874)
    pub struct FutureObj<'a, T>(LocalFutureObj<'a, T>);

    impl<'a, T> Unpin for FutureObj<'a, T> {}
    unsafe impl<'a, T> Send for FutureObj<'a, T> {}

    impl<'a, T> FutureObj<'a, T> {
        /// Create a `FutureObj` from a custom trait object representation.
        #[inline]
        pub fn new<F: UnsafeFutureObj<'a, T> + Send>(f: F) -> FutureObj<'a, T> {
            FutureObj(LocalFutureObj::new(f))
        }
    }

    impl<'a, T> fmt::Debug for FutureObj<'a, T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("FutureObj")
                .finish()
        }
    }

    impl<'a, T> Future for FutureObj<'a, T> {
        type Output = T;

        #[inline]
        fn poll(self: PinMut<Self>, cx: &mut Context) -> Poll<T> {
            let pinned_field = unsafe { PinMut::map_unchecked(self, |x| &mut x.0) };
            pinned_field.poll(cx)
        }
    }

    /// A custom implementation of a future trait object for `FutureObj`, providing
    /// a hand-rolled vtable.
    ///
    /// This custom representation is typically used only in `no_std` contexts,
    /// where the default `Box`-based implementation is not available.
    ///
    /// The implementor must guarantee that it is safe to call `poll` repeatedly (in
    /// a non-concurrent fashion) with the result of `into_raw` until `drop` is
    /// called.
    pub unsafe trait UnsafeFutureObj<'a, T>: 'a {
        /// Convert an owned instance into a (conceptually owned) void pointer.
        fn into_raw(self) -> *mut ();

        /// Poll the future represented by the given void pointer.
        ///
        /// # Safety
        ///
        /// The trait implementor must guarantee that it is safe to repeatedly call
        /// `poll` with the result of `into_raw` until `drop` is called; such calls
        /// are not, however, allowed to race with each other or with calls to
        /// `drop`.
        unsafe fn poll(ptr: *mut (), cx: &mut Context) -> Poll<T>;

        /// Drops the future represented by the given void pointer.
        ///
        /// # Safety
        ///
        /// The trait implementor must guarantee that it is safe to call this
        /// function once per `into_raw` invocation; that call cannot race with
        /// other calls to `drop` or `poll`.
        unsafe fn drop(ptr: *mut ());
    }

    unsafe impl<'a, T, F> UnsafeFutureObj<'a, T> for &'a mut F
        where F: Future<Output = T> + Unpin + 'a
    {
        fn into_raw(self) -> *mut () {
            self as *mut F as *mut ()
        }

        unsafe fn poll(ptr: *mut (), cx: &mut Context) -> Poll<T> {
            PinMut::new_unchecked(&mut *(ptr as *mut F)).poll(cx)
        }

        unsafe fn drop(_ptr: *mut ()) {}
    }

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