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    use super::assert_future;
    use core::pin::Pin;
    use futures_core::future::{FusedFuture, Future};
    use futures_core::task::{Context, Poll};

    /// Future for the [`ready`](ready()) function.
    #[derive(Debug, Clone)]
    #[must_use = "futures do nothing unless you `.await` or poll them"]
    pub struct Ready<T>(Option<T>);

    impl<T> Ready<T> {
        /// Unwraps the value from this immediately ready future.
        #[inline]
        pub fn into_inner(mut self) -> T {
            self.0.take().unwrap()
        }
    }

    impl<T> Unpin for Ready<T> {}

    impl<T> FusedFuture for Ready<T> {
        fn is_terminated(&self) -> bool {
            self.0.is_none()
        }
    }

    impl<T> Future for Ready<T> {
        type Output = T;

        #[inline]
        fn poll(mut self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<T> {
            Poll::Ready(self.0.take().expect("Ready polled after completion"))
        }
    }

    /// Creates a future that is immediately ready with a value.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures::executor::block_on(async {
    /// use futures::future;
    ///
    /// let a = future::ready(1);
    /// assert_eq!(a.await, 1);
    /// # });
    /// ```
    pub fn ready<T>(t: T) -> Ready<T> {
        assert_future::<T, _>(Ready(Some(t)))
    }

    /// Create a future that is immediately ready with a success value.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures::executor::block_on(async {
    /// use futures::future;
    ///
    /// let a = future::ok::<i32, i32>(1);
    /// assert_eq!(a.await, Ok(1));
    /// # });
    /// ```
    pub fn ok<T, E>(t: T) -> Ready<Result<T, E>> {
        Ready(Some(Ok(t)))
    }

    /// Create a future that is immediately ready with an error value.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures::executor::block_on(async {
    /// use futures::future;
    ///
    /// let a = future::err::<i32, i32>(1);
    /// assert_eq!(a.await, Err(1));
    /// # });
    /// ```
    pub fn err<T, E>(err: E) -> Ready<Result<T, E>> {
        Ready(Some(Err(err)))
    }