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    use core::fmt;
    use core::iter::FusedIterator;
    use core::ops::Try;

    use super::{count, wrap_index, RingSlices};

    /// An iterator over the elements of a `VecDeque`.
    ///
    /// This `struct` is created by the [`iter`] method on [`super::VecDeque`]. See its
    /// documentation for more.
    ///
    /// [`iter`]: super::VecDeque::iter
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Iter<'a, T: 'a> {
        pub(crate) ring: &'a [T],
        pub(crate) tail: usize,
        pub(crate) head: usize,
    }

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl<T: fmt::Debug> fmt::Debug for Iter<'_, T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            let (front, back) = RingSlices::ring_slices(self.ring, self.head, self.tail);
            f.debug_tuple("Iter").field(&front).field(&back).finish()
        }
    }

    // FIXME(#26925) Remove in favor of `#[derive(Clone)]`
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> Clone for Iter<'_, T> {
        fn clone(&self) -> Self {
            Iter { ring: self.ring, tail: self.tail, head: self.head }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> Iterator for Iter<'a, T> {
        type Item = &'a T;

        #[inline]
        fn next(&mut self) -> Option<&'a T> {
            if self.tail == self.head {
                return None;
            }
            let tail = self.tail;
            self.tail = wrap_index(self.tail.wrapping_add(1), self.ring.len());
            unsafe { Some(self.ring.get_unchecked(tail)) }
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let len = count(self.tail, self.head, self.ring.len());
            (len, Some(len))
        }

        fn fold<Acc, F>(self, mut accum: Acc, mut f: F) -> Acc
        where
            F: FnMut(Acc, Self::Item) -> Acc,
        {
            let (front, back) = RingSlices::ring_slices(self.ring, self.head, self.tail);
            accum = front.iter().fold(accum, &mut f);
            back.iter().fold(accum, &mut f)
        }

        fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
        where
            Self: Sized,
            F: FnMut(B, Self::Item) -> R,
            R: Try<Ok = B>,
        {
            let (mut iter, final_res);
            if self.tail <= self.head {
                // single slice self.ring[self.tail..self.head]
                iter = self.ring[self.tail..self.head].iter();
                final_res = iter.try_fold(init, &mut f);
            } else {
                // two slices: self.ring[self.tail..], self.ring[..self.head]
                let (front, back) = self.ring.split_at(self.tail);
                let mut back_iter = back.iter();
                let res = back_iter.try_fold(init, &mut f);
                let len = self.ring.len();
                self.tail = (self.ring.len() - back_iter.len()) & (len - 1);
                iter = front[..self.head].iter();
                final_res = iter.try_fold(res?, &mut f);
            }
            self.tail = self.head - iter.len();
            final_res
        }

        fn nth(&mut self, n: usize) -> Option<Self::Item> {
            if n >= count(self.tail, self.head, self.ring.len()) {
                self.tail = self.head;
                None
            } else {
                self.tail = wrap_index(self.tail.wrapping_add(n), self.ring.len());
                self.next()
            }
        }

        #[inline]
        fn last(mut self) -> Option<&'a T> {
            self.next_back()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
        #[inline]
        fn next_back(&mut self) -> Option<&'a T> {
            if self.tail == self.head {
                return None;
            }
            self.head = wrap_index(self.head.wrapping_sub(1), self.ring.len());
            unsafe { Some(self.ring.get_unchecked(self.head)) }
        }

        fn rfold<Acc, F>(self, mut accum: Acc, mut f: F) -> Acc
        where
            F: FnMut(Acc, Self::Item) -> Acc,
        {
            let (front, back) = RingSlices::ring_slices(self.ring, self.head, self.tail);
            accum = back.iter().rfold(accum, &mut f);
            front.iter().rfold(accum, &mut f)
        }

        fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R
        where
            Self: Sized,
            F: FnMut(B, Self::Item) -> R,
            R: Try<Ok = B>,
        {
            let (mut iter, final_res);
            if self.tail <= self.head {
                // single slice self.ring[self.tail..self.head]
                iter = self.ring[self.tail..self.head].iter();
                final_res = iter.try_rfold(init, &mut f);
            } else {
                // two slices: self.ring[self.tail..], self.ring[..self.head]
                let (front, back) = self.ring.split_at(self.tail);
                let mut front_iter = front[..self.head].iter();
                let res = front_iter.try_rfold(init, &mut f);
                self.head = front_iter.len();
                iter = back.iter();
                final_res = iter.try_rfold(res?, &mut f);
            }
            self.head = self.tail + iter.len();
            final_res
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> ExactSizeIterator for Iter<'_, T> {
        fn is_empty(&self) -> bool {
            self.head == self.tail
        }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl<T> FusedIterator for Iter<'_, T> {}