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    //! Definition of the `JoinAll` combinator, waiting for all of a list of futures
    //! to finish.

    use std::prelude::v1::*;

    use std::fmt;
    use std::mem;

    use {Future, IntoFuture, Poll, Async};

    #[derive(Debug)]
    enum ElemState<T> where T: Future {
        Pending(T),
        Done(T::Item),
    }

    /// A future which takes a list of futures and resolves with a vector of the
    /// completed values.
    ///
    /// This future is created with the `join_all` method.
    #[must_use = "futures do nothing unless polled"]
    pub struct JoinAll<I>
        where I: IntoIterator,
              I::Item: IntoFuture,
    {
        elems: Vec<ElemState<<I::Item as IntoFuture>::Future>>,
    }

    impl<I> fmt::Debug for JoinAll<I>
        where I: IntoIterator,
              I::Item: IntoFuture,
              <<I as IntoIterator>::Item as IntoFuture>::Future: fmt::Debug,
              <<I as IntoIterator>::Item as IntoFuture>::Item: fmt::Debug,
    {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.debug_struct("JoinAll")
                .field("elems", &self.elems)
                .finish()
        }
    }

    /// Creates a future which represents a collection of the results of the futures
    /// given.
    ///
    /// The returned future will drive execution for all of its underlying futures,
    /// collecting the results into a destination `Vec<T>` in the same order as they
    /// were provided. If any future returns an error then all other futures will be
    /// canceled and an error will be returned immediately. If all futures complete
    /// successfully, however, then the returned future will succeed with a `Vec` of
    /// all the successful results.
    ///
    /// # Examples
    ///
    /// ```
    /// use futures::future::*;
    ///
    /// let f = join_all(vec![
    ///     ok::<u32, u32>(1),
    ///     ok::<u32, u32>(2),
    ///     ok::<u32, u32>(3),
    /// ]);
    /// let f = f.map(|x| {
    ///     assert_eq!(x, [1, 2, 3]);
    /// });
    ///
    /// let f = join_all(vec![
    ///     Box::new(ok::<u32, u32>(1)),
    ///     Box::new(err::<u32, u32>(2)),
    ///     Box::new(ok::<u32, u32>(3)),
    /// ]);
    /// let f = f.then(|x| {
    ///     assert_eq!(x, Err(2));
    ///     x
    /// });
    /// ```
    pub fn join_all<I>(i: I) -> JoinAll<I>
        where I: IntoIterator,
              I::Item: IntoFuture,
    {
        let elems = i.into_iter().map(|f| {
            ElemState::Pending(f.into_future())
        }).collect();
        JoinAll { elems: elems }
    }

    impl<I> Future for JoinAll<I>
        where I: IntoIterator,
              I::Item: IntoFuture,
    {
        type Item = Vec<<I::Item as IntoFuture>::Item>;
        type Error = <I::Item as IntoFuture>::Error;

        fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
            let mut all_done = true;

            for idx in 0 .. self.elems.len() {
                let done_val = match self.elems[idx] {
                    ElemState::Pending(ref mut t) => {
                        match t.poll() {
                            Ok(Async::Ready(v)) => Ok(v),
                            Ok(Async::NotReady) => {
                                all_done = false;
                                continue
                            }
                            Err(e) => Err(e),
                        }
                    }
                    ElemState::Done(ref mut _v) => continue,
                };

                match done_val {
                    Ok(v) => self.elems[idx] = ElemState::Done(v),
                    Err(e) => {
                        // On completion drop all our associated resources
                        // ASAP.
                        self.elems = Vec::new();
                        return Err(e)
                    }
                }
            }

            if all_done {
                let elems = mem::replace(&mut self.elems, Vec::new());
                let result = elems.into_iter().map(|e| {
                    match e {
                        ElemState::Done(t) => t,
                        _ => unreachable!(),
                    }
                }).collect();
                Ok(Async::Ready(result))
            } else {
                Ok(Async::NotReady)
            }
        }
    }

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