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    use crate::fmt;
    use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable};
    use crate::ops::Try;

    /// An iterator that calls a function with a reference to each element before
    /// yielding it.
    ///
    /// This `struct` is created by the [`inspect`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`inspect`]: Iterator::inspect
    /// [`Iterator`]: trait.Iterator.html
    #[must_use = "iterators are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Clone)]
    pub struct Inspect<I, F> {
        iter: I,
        f: F,
    }
    impl<I, F> Inspect<I, F> {
        pub(in crate::iter) fn new(iter: I, f: F) -> Inspect<I, F> {
            Inspect { iter, f }
        }
    }

    #[stable(feature = "core_impl_debug", since = "1.9.0")]
    impl<I: fmt::Debug, F> fmt::Debug for Inspect<I, F> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Inspect").field("iter", &self.iter).finish()
        }
    }

    impl<I: Iterator, F> Inspect<I, F>
    where
        F: FnMut(&I::Item),
    {
        #[inline]
        fn do_inspect(&mut self, elt: Option<I::Item>) -> Option<I::Item> {
            if let Some(ref a) = elt {
                (self.f)(a);
            }

            elt
        }
    }

    fn inspect_fold<T, Acc>(
        mut f: impl FnMut(&T),
        mut fold: impl FnMut(Acc, T) -> Acc,
    ) -> impl FnMut(Acc, T) -> Acc {
        move |acc, item| {
            f(&item);
            fold(acc, item)
        }
    }

    fn inspect_try_fold<'a, T, Acc, R>(
        f: &'a mut impl FnMut(&T),
        mut fold: impl FnMut(Acc, T) -> R + 'a,
    ) -> impl FnMut(Acc, T) -> R + 'a {
        move |acc, item| {
            f(&item);
            fold(acc, item)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: Iterator, F> Iterator for Inspect<I, F>
    where
        F: FnMut(&I::Item),
    {
        type Item = I::Item;

        #[inline]
        fn next(&mut self) -> Option<I::Item> {
            let next = self.iter.next();
            self.do_inspect(next)
        }

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

        #[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>,
        {
            self.iter.try_fold(init, inspect_try_fold(&mut self.f, fold))
        }

        #[inline]
        fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
        where
            Fold: FnMut(Acc, Self::Item) -> Acc,
        {
            self.iter.fold(init, inspect_fold(self.f, fold))
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: DoubleEndedIterator, F> DoubleEndedIterator for Inspect<I, F>
    where
        F: FnMut(&I::Item),
    {
        #[inline]
        fn next_back(&mut self) -> Option<I::Item> {
            let next = self.iter.next_back();
            self.do_inspect(next)
        }

        #[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>,
        {
            self.iter.try_rfold(init, inspect_try_fold(&mut self.f, fold))
        }

        #[inline]
        fn rfold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
        where
            Fold: FnMut(Acc, Self::Item) -> Acc,
        {
            self.iter.rfold(init, inspect_fold(self.f, fold))
        }
    }

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

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

    #[stable(feature = "fused", since = "1.26.0")]
    impl<I: FusedIterator, F> FusedIterator for Inspect<I, F> where F: FnMut(&I::Item) {}

    #[unstable(issue = "none", feature = "inplace_iteration")]
    unsafe impl<S: Iterator, I: Iterator, F> SourceIter for Inspect<I, F>
    where
        F: FnMut(&I::Item),
        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, F> InPlaceIterable for Inspect<I, F> where F: FnMut(&I::Item) {}