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    use crate::iter::adapters::{zip::try_get_unchecked, SourceIter, TrustedRandomAccess};
    use crate::iter::{FusedIterator, InPlaceIterable, TrustedLen};
    use crate::ops::{Add, AddAssign, Try};

    /// An iterator that yields the current count and the element during iteration.
    ///
    /// This `struct` is created by the [`enumerate`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`enumerate`]: Iterator::enumerate
    /// [`Iterator`]: trait.Iterator.html
    #[derive(Clone, Debug)]
    #[must_use = "iterators are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Enumerate<I> {
        iter: I,
        count: usize,
    }
    impl<I> Enumerate<I> {
        pub(in crate::iter) fn new(iter: I) -> Enumerate<I> {
            Enumerate { iter, count: 0 }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I> Iterator for Enumerate<I>
    where
        I: Iterator,
    {
        type Item = (usize, <I as Iterator>::Item);

        /// # Overflow Behavior
        ///
        /// The method does no guarding against overflows, so enumerating more than
        /// `usize::MAX` elements either produces the wrong result or panics. If
        /// debug assertions are enabled, a panic is guaranteed.
        ///
        /// # Panics
        ///
        /// Might panic if the index of the element overflows a `usize`.
        #[inline]
        fn next(&mut self) -> Option<(usize, <I as Iterator>::Item)> {
            let a = self.iter.next()?;
            let i = self.count;
            // Possible undefined overflow.
            AddAssign::add_assign(&mut self.count, 1);
            Some((i, a))
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }

        #[inline]
        fn nth(&mut self, n: usize) -> Option<(usize, I::Item)> {
            let a = self.iter.nth(n)?;
            // Possible undefined overflow.
            let i = Add::add(self.count, n);
            self.count = Add::add(i, 1);
            Some((i, a))
        }

        #[inline]
        fn count(self) -> usize {
            self.iter.count()
        }

        #[inline]
        fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
        where
            Self: Sized,
            Fold: FnMut(Acc, Self::Item) -> R,
            R: Try<Ok = Acc>,
        {
            #[inline]
            fn enumerate<'a, T, Acc, R>(
                count: &'a mut usize,
                mut fold: impl FnMut(Acc, (usize, T)) -> R + 'a,
            ) -> impl FnMut(Acc, T) -> R + 'a {
                move |acc, item| {
                    let acc = fold(acc, (*count, item));
                    // Possible undefined overflow.
                    AddAssign::add_assign(count, 1);
                    acc
                }
            }

            self.iter.try_fold(init, enumerate(&mut self.count, fold))
        }

        #[inline]
        fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
        where
            Fold: FnMut(Acc, Self::Item) -> Acc,
        {
            #[inline]
            fn enumerate<T, Acc>(
                mut count: usize,
                mut fold: impl FnMut(Acc, (usize, T)) -> Acc,
            ) -> impl FnMut(Acc, T) -> Acc {
                move |acc, item| {
                    let acc = fold(acc, (count, item));
                    // Possible undefined overflow.
                    AddAssign::add_assign(&mut count, 1);
                    acc
                }
            }

            self.iter.fold(init, enumerate(self.count, fold))
        }

        unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> <Self as Iterator>::Item
        where
            Self: TrustedRandomAccess,
        {
            // SAFETY: the caller must uphold the contract for
            // `Iterator::__iterator_get_unchecked`.
            let value = unsafe { try_get_unchecked(&mut self.iter, idx) };
            (Add::add(self.count, idx), value)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I> DoubleEndedIterator for Enumerate<I>
    where
        I: ExactSizeIterator + DoubleEndedIterator,
    {
        #[inline]
        fn next_back(&mut self) -> Option<(usize, <I as Iterator>::Item)> {
            let a = self.iter.next_back()?;
            let len = self.iter.len();
            // Can safely add, `ExactSizeIterator` promises that the number of
            // elements fits into a `usize`.
            Some((self.count + len, a))
        }

        #[inline]
        fn nth_back(&mut self, n: usize) -> Option<(usize, <I as Iterator>::Item)> {
            let a = self.iter.nth_back(n)?;
            let len = self.iter.len();
            // Can safely add, `ExactSizeIterator` promises that the number of
            // elements fits into a `usize`.
            Some((self.count + len, a))
        }

        #[inline]
        fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
        where
            Self: Sized,
            Fold: FnMut(Acc, Self::Item) -> R,
            R: Try<Ok = Acc>,
        {
            // Can safely add and subtract the count, as `ExactSizeIterator` promises
            // that the number of elements fits into a `usize`.
            fn enumerate<T, Acc, R>(
                mut count: usize,
                mut fold: impl FnMut(Acc, (usize, T)) -> R,
            ) -> impl FnMut(Acc, T) -> R {
                move |acc, item| {
                    count -= 1;
                    fold(acc, (count, item))
                }
            }

            let count = self.count + self.iter.len();
            self.iter.try_rfold(init, enumerate(count, fold))
        }

        #[inline]
        fn rfold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
        where
            Fold: FnMut(Acc, Self::Item) -> Acc,
        {
            // Can safely add and subtract the count, as `ExactSizeIterator` promises
            // that the number of elements fits into a `usize`.
            fn enumerate<T, Acc>(
                mut count: usize,
                mut fold: impl FnMut(Acc, (usize, T)) -> Acc,
            ) -> impl FnMut(Acc, T) -> Acc {
                move |acc, item| {
                    count -= 1;
                    fold(acc, (count, item))
                }
            }

            let count = self.count + self.iter.len();
            self.iter.rfold(init, enumerate(count, fold))
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I> ExactSizeIterator for Enumerate<I>
    where
        I: ExactSizeIterator,
    {
        fn len(&self) -> usize {
            self.iter.len()
        }

        fn is_empty(&self) -> bool {
            self.iter.is_empty()
        }
    }

    #[doc(hidden)]
    #[unstable(feature = "trusted_random_access", issue = "none")]
    unsafe impl<I> TrustedRandomAccess for Enumerate<I>
    where
        I: TrustedRandomAccess,
    {
        fn may_have_side_effect() -> bool {
            I::may_have_side_effect()
        }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl<I> FusedIterator for Enumerate<I> where I: FusedIterator {}

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<I> TrustedLen for Enumerate<I> where I: TrustedLen {}

    #[unstable(issue = "none", feature = "inplace_iteration")]
    unsafe impl<S: Iterator, I: Iterator> SourceIter for Enumerate<I>
    where
        I: SourceIter<Source = S>,
    {
        type Source = S;

        #[inline]
        unsafe fn as_inner(&mut self) -> &mut S {
            // SAFETY: unsafe function forwarding to unsafe function with the same requirements
            unsafe { SourceIter::as_inner(&mut self.iter) }
        }
    }

    #[unstable(issue = "none", feature = "inplace_iteration")]
    unsafe impl<I: InPlaceIterable> InPlaceIterable for Enumerate<I> {}