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

    /// An iterator that only accepts elements while `predicate` returns `Some(_)`.
    ///
    /// This `struct` is created by the [`map_while`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`map_while`]: Iterator::map_while
    /// [`Iterator`]: trait.Iterator.html
    #[must_use = "iterators are lazy and do nothing unless consumed"]
    #[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
    #[derive(Clone)]
    pub struct MapWhile<I, P> {
        iter: I,
        predicate: P,
    }

    impl<I, P> MapWhile<I, P> {
        pub(in crate::iter) fn new(iter: I, predicate: P) -> MapWhile<I, P> {
            MapWhile { iter, predicate }
        }
    }

    #[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
    impl<I: fmt::Debug, P> fmt::Debug for MapWhile<I, P> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("MapWhile").field("iter", &self.iter).finish()
        }
    }

    #[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
    impl<B, I: Iterator, P> Iterator for MapWhile<I, P>
    where
        P: FnMut(I::Item) -> Option<B>,
    {
        type Item = B;

        #[inline]
        fn next(&mut self) -> Option<B> {
            let x = self.iter.next()?;
            (self.predicate)(x)
        }

        #[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
        }

        #[inline]
        fn try_fold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R
        where
            Self: Sized,
            Fold: FnMut(Acc, Self::Item) -> R,
            R: Try<Ok = Acc>,
        {
            let Self { iter, predicate } = self;
            iter.try_fold(init, |acc, x| match predicate(x) {
                Some(item) => ControlFlow::from_try(fold(acc, item)),
                None => ControlFlow::Break(try { acc }),
            })
            .into_try()
        }

        #[inline]
        fn fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
        where
            Self: Sized,
            Fold: FnMut(Acc, Self::Item) -> Acc,
        {
            #[inline]
            fn ok<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> Result<B, !> {
                move |acc, x| Ok(f(acc, x))
            }

            self.try_fold(init, ok(fold)).unwrap()
        }
    }

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