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

    /// An iterator that filters the elements of `iter` with `predicate`.
    ///
    /// This `struct` is created by the [`filter`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`filter`]: Iterator::filter
    /// [`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 Filter<I, P> {
        iter: I,
        predicate: P,
    }
    impl<I, P> Filter<I, P> {
        pub(in crate::iter) fn new(iter: I, predicate: P) -> Filter<I, P> {
            Filter { iter, predicate }
        }
    }

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

    fn filter_fold<T, Acc>(
        mut predicate: impl FnMut(&T) -> bool,
        mut fold: impl FnMut(Acc, T) -> Acc,
    ) -> impl FnMut(Acc, T) -> Acc {
        move |acc, item| if predicate(&item) { fold(acc, item) } else { acc }
    }

    fn filter_try_fold<'a, T, Acc, R: Try<Ok = Acc>>(
        predicate: &'a mut impl FnMut(&T) -> bool,
        mut fold: impl FnMut(Acc, T) -> R + 'a,
    ) -> impl FnMut(Acc, T) -> R + 'a {
        move |acc, item| if predicate(&item) { fold(acc, item) } else { try { acc } }
    }

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

        #[inline]
        fn next(&mut self) -> Option<I::Item> {
            self.iter.find(&mut self.predicate)
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let (_, upper) = self.iter.size_hint();
            (0, upper) // can't know a lower bound, due to the predicate
        }

        // this special case allows the compiler to make `.filter(_).count()`
        // branchless. Barring perfect branch prediction (which is unattainable in
        // the general case), this will be much faster in >90% of cases (containing
        // virtually all real workloads) and only a tiny bit slower in the rest.
        //
        // Having this specialization thus allows us to write `.filter(p).count()`
        // where we would otherwise write `.map(|x| p(x) as usize).sum()`, which is
        // less readable and also less backwards-compatible to Rust before 1.10.
        //
        // Using the branchless version will also simplify the LLVM byte code, thus
        // leaving more budget for LLVM optimizations.
        #[inline]
        fn count(self) -> usize {
            #[inline]
            fn to_usize<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut(T) -> usize {
                move |x| predicate(&x) as usize
            }

            self.iter.map(to_usize(self.predicate)).sum()
        }

        #[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, filter_try_fold(&mut self.predicate, fold))
        }

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

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: DoubleEndedIterator, P> DoubleEndedIterator for Filter<I, P>
    where
        P: FnMut(&I::Item) -> bool,
    {
        #[inline]
        fn next_back(&mut self) -> Option<I::Item> {
            self.iter.rfind(&mut self.predicate)
        }

        #[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, filter_try_fold(&mut self.predicate, fold))
        }

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

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

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