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    use std::future::Future;
    use std::pin::Pin;
    use std::task::{Context, Poll};

    /// Future for the [`ok`](ok()) function.
    ///
    /// `pub` in order to use the future as an associated type in a sealed trait.
    #[derive(Debug)]
    // Used as an associated type in a "sealed" trait.
    #[allow(unreachable_pub)]
    pub struct Ready<T>(Option<T>);

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

    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().unwrap())
        }
    }

    /// Creates a future that is immediately ready with a success value.
    pub(crate) fn ok<T, E>(t: T) -> Ready<Result<T, E>> {
        Ready(Some(Ok(t)))
    }