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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 ops::Try;
    use result::Result;

    /// Indicates whether a value is available or if the current task has been
    /// scheduled to receive a wakeup instead.
    #[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)]
    pub enum Poll<T> {
        /// Represents that a value is immediately ready.
        Ready(T),

        /// Represents that a value is not ready yet.
        ///
        /// When a function returns `Pending`, the function *must* also
        /// ensure that the current task is scheduled to be awoken when
        /// progress can be made.
        Pending,
    }

    impl<T> Poll<T> {
        /// Change the ready value of this `Poll` with the closure provided
        pub fn map<U, F>(self, f: F) -> Poll<U>
            where F: FnOnce(T) -> U
        {
            match self {
                Poll::Ready(t) => Poll::Ready(f(t)),
                Poll::Pending => Poll::Pending,
            }
        }

        /// Returns whether this is `Poll::Ready`
        #[inline]
        pub fn is_ready(&self) -> bool {
            match *self {
                Poll::Ready(_) => true,
                Poll::Pending => false,
            }
        }

        /// Returns whether this is `Poll::Pending`
        #[inline]
        pub fn is_pending(&self) -> bool {
            !self.is_ready()
        }
    }

    impl<T, E> Poll<Result<T, E>> {
        /// Change the success value of this `Poll` with the closure provided
        pub fn map_ok<U, F>(self, f: F) -> Poll<Result<U, E>>
            where F: FnOnce(T) -> U
        {
            match self {
                Poll::Ready(Ok(t)) => Poll::Ready(Ok(f(t))),
                Poll::Ready(Err(e)) => Poll::Ready(Err(e)),
                Poll::Pending => Poll::Pending,
            }
        }

        /// Change the error value of this `Poll` with the closure provided
        pub fn map_err<U, F>(self, f: F) -> Poll<Result<T, U>>
            where F: FnOnce(E) -> U
        {
            match self {
                Poll::Ready(Ok(t)) => Poll::Ready(Ok(t)),
                Poll::Ready(Err(e)) => Poll::Ready(Err(f(e))),
                Poll::Pending => Poll::Pending,
            }
        }
    }

    impl<T> From<T> for Poll<T> {
        fn from(t: T) -> Poll<T> {
            Poll::Ready(t)
        }
    }

    impl<T, E> Try for Poll<Result<T, E>> {
        type Ok = Poll<T>;
        type Error = E;

        #[inline]
        fn into_result(self) -> Result<Self::Ok, Self::Error> {
            match self {
                Poll::Ready(Ok(x)) => Ok(Poll::Ready(x)),
                Poll::Ready(Err(e)) => Err(e),
                Poll::Pending => Ok(Poll::Pending),
            }
        }

        #[inline]
        fn from_error(e: Self::Error) -> Self {
            Poll::Ready(Err(e))
        }

        #[inline]
        fn from_ok(x: Self::Ok) -> Self {
            x.map(Ok)
        }
    }

    impl<T, E> Try for Poll<Option<Result<T, E>>> {
        type Ok = Poll<Option<T>>;
        type Error = E;

        #[inline]
        fn into_result(self) -> Result<Self::Ok, Self::Error> {
            match self {
                Poll::Ready(Some(Ok(x))) => Ok(Poll::Ready(Some(x))),
                Poll::Ready(Some(Err(e))) => Err(e),
                Poll::Ready(None) => Ok(Poll::Ready(None)),
                Poll::Pending => Ok(Poll::Pending),
            }
        }

        #[inline]
        fn from_error(e: Self::Error) -> Self {
            Poll::Ready(Some(Err(e)))
        }

        #[inline]
        fn from_ok(x: Self::Ok) -> Self {
            x.map(|x| x.map(Ok))
        }
    }

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