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

    /// An iterator to maintain state while iterating another iterator.
    ///
    /// This `struct` is created by the [`scan`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`scan`]: Iterator::scan
    /// [`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 Scan<I, St, F> {
        iter: I,
        f: F,
        state: St,
    }

    impl<I, St, F> Scan<I, St, F> {
        pub(in crate::iter) fn new(iter: I, state: St, f: F) -> Scan<I, St, F> {
            Scan { iter, state, f }
        }
    }

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

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

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

        #[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 scan function
        }

        #[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>,
        {
            fn scan<'a, T, St, B, Acc, R: Try<Ok = Acc>>(
                state: &'a mut St,
                f: &'a mut impl FnMut(&mut St, T) -> Option<B>,
                mut fold: impl FnMut(Acc, B) -> R + 'a,
            ) -> impl FnMut(Acc, T) -> ControlFlow<R, Acc> + 'a {
                move |acc, x| match f(state, x) {
                    None => ControlFlow::Break(try { acc }),
                    Some(x) => ControlFlow::from_try(fold(acc, x)),
                }
            }

            let state = &mut self.state;
            let f = &mut self.f;
            self.iter.try_fold(init, scan(state, f, fold)).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<St, F, B, S: Iterator, I: Iterator> SourceIter for Scan<I, St, F>
    where
        I: SourceIter<Source = S>,
        F: FnMut(&mut St, I::Item) -> Option<B>,
    {
        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<St, F, B, I: InPlaceIterable> InPlaceIterable for Scan<I, St, F> where
        F: FnMut(&mut St, I::Item) -> Option<B>
    {
    }