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    use core::fmt;
    use core::iter::FusedIterator;
    use core::marker::PhantomData;

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

    /// A mutable iterator over the elements of a `VecDeque`.
    ///
    /// This `struct` is created by the [`iter_mut`] method on [`super::VecDeque`]. See its
    /// documentation for more.
    ///
    /// [`iter_mut`]: super::VecDeque::iter_mut
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct IterMut<'a, T: 'a> {
        // Internal safety invariant: the entire slice is dereferencable.
        pub(crate) ring: *mut [T],
        pub(crate) tail: usize,
        pub(crate) head: usize,
        pub(crate) phantom: PhantomData<&'a mut [T]>,
    }

    // SAFETY: we do nothing thread-local and there is no interior mutability,
    // so the usual structural `Send`/`Sync` apply.
    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<T: Send> Send for IterMut<'_, T> {}
    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<T: Sync> Sync for IterMut<'_, T> {}

    #[stable(feature = "collection_debug", since = "1.17.0")]
    impl<T: fmt::Debug> fmt::Debug for IterMut<'_, T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            let (front, back) = RingSlices::ring_slices(self.ring, self.head, self.tail);
            // SAFETY: these are the elements we have not handed out yet, so aliasing is fine.
            // The `IterMut` invariant also ensures everything is dereferencable.
            let (front, back) = unsafe { (&*front, &*back) };
            f.debug_tuple("IterMut").field(&front).field(&back).finish()
        }
    }

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

        #[inline]
        fn next(&mut self) -> Option<&'a mut 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 {
                let elem = self.ring.get_unchecked_mut(tail);
                Some(&mut *elem)
            }
        }

        #[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);
            // SAFETY: these are the elements we have not handed out yet, so aliasing is fine.
            // The `IterMut` invariant also ensures everything is dereferencable.
            let (front, back) = unsafe { (&mut *front, &mut *back) };
            accum = front.iter_mut().fold(accum, &mut f);
            back.iter_mut().fold(accum, &mut f)
        }

        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 mut T> {
            self.next_back()
        }
    }

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

            unsafe {
                let elem = self.ring.get_unchecked_mut(self.head);
                Some(&mut *elem)
            }
        }

        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);
            // SAFETY: these are the elements we have not handed out yet, so aliasing is fine.
            // The `IterMut` invariant also ensures everything is dereferencable.
            let (front, back) = unsafe { (&mut *front, &mut *back) };
            accum = back.iter_mut().rfold(accum, &mut f);
            front.iter_mut().rfold(accum, &mut f)
        }
    }

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

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