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    //! Definition of the Lazy combinator, deferring execution of a function until
    //! the future is polled.

    use core::mem;

    use {Future, IntoFuture, Poll};

    /// A future which defers creation of the actual future until a callback is
    /// scheduled.
    ///
    /// This is created by the `lazy` function.
    #[derive(Debug)]
    #[must_use = "futures do nothing unless polled"]
    pub struct Lazy<F, R: IntoFuture> {
        inner: _Lazy<F, R::Future>,
    }

    #[derive(Debug)]
    enum _Lazy<F, R> {
        First(F),
        Second(R),
        Moved,
    }

    /// Creates a new future which will eventually be the same as the one created
    /// by the closure provided.
    ///
    /// The provided closure is only run once the future has a callback scheduled
    /// on it, otherwise the callback never runs. Once run, however, this future is
    /// the same as the one the closure creates.
    ///
    /// # Examples
    ///
    /// ```
    /// use futures::future::*;
    ///
    /// let a = lazy(|| ok::<u32, u32>(1));
    ///
    /// let b = lazy(|| -> FutureResult<u32, u32> {
    ///     panic!("oh no!")
    /// });
    /// drop(b); // closure is never run
    /// ```
    pub fn lazy<F, R>(f: F) -> Lazy<F, R>
        where F: FnOnce() -> R,
              R: IntoFuture
    {
        Lazy {
            inner: _Lazy::First(f),
        }
    }

    impl<F, R> Lazy<F, R>
        where F: FnOnce() -> R,
              R: IntoFuture,
    {
        fn get(&mut self) -> &mut R::Future {
            match self.inner {
                _Lazy::First(_) => {}
                _Lazy::Second(ref mut f) => return f,
                _Lazy::Moved => panic!(), // can only happen if `f()` panics
            }
            match mem::replace(&mut self.inner, _Lazy::Moved) {
                _Lazy::First(f) => self.inner = _Lazy::Second(f().into_future()),
                _ => panic!(), // we already found First
            }
            match self.inner {
                _Lazy::Second(ref mut f) => f,
                _ => panic!(), // we just stored Second
            }
        }
    }

    impl<F, R> Future for Lazy<F, R>
        where F: FnOnce() -> R,
              R: IntoFuture,
    {
        type Item = R::Item;
        type Error = R::Error;

        fn poll(&mut self) -> Poll<R::Item, R::Error> {
            self.get().poll()
        }
    }

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