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

    /// An iterator that only accepts elements while `predicate` returns `true`.
    ///
    /// This `struct` is created by the [`take_while`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`take_while`]: Iterator::take_while
    /// [`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 TakeWhile<I, P> {
        iter: I,
        flag: bool,
        predicate: P,
    }

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

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

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

        #[inline]
        fn next(&mut self) -> Option<I::Item> {
            if self.flag {
                None
            } else {
                let x = self.iter.next()?;
                if (self.predicate)(&x) {
                    Some(x)
                } else {
                    self.flag = true;
                    None
                }
            }
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            if self.flag {
                (0, Some(0))
            } else {
                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, fold: Fold) -> R
        where
            Self: Sized,
            Fold: FnMut(Acc, Self::Item) -> R,
            R: Try<Ok = Acc>,
        {
            fn check<'a, T, Acc, R: Try<Ok = Acc>>(
                flag: &'a mut bool,
                p: &'a mut impl FnMut(&T) -> bool,
                mut fold: impl FnMut(Acc, T) -> R + 'a,
            ) -> impl FnMut(Acc, T) -> ControlFlow<R, Acc> + 'a {
                move |acc, x| {
                    if p(&x) {
                        ControlFlow::from_try(fold(acc, x))
                    } else {
                        *flag = true;
                        ControlFlow::Break(try { acc })
                    }
                }
            }

            if self.flag {
                try { init }
            } else {
                let flag = &mut self.flag;
                let p = &mut self.predicate;
                self.iter.try_fold(init, check(flag, p, 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()
        }
    }

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

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