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

    /// An iterator that skips over `n` elements of `iter`.
    ///
    /// This `struct` is created by the [`skip`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`skip`]: Iterator::skip
    /// [`Iterator`]: trait.Iterator.html
    #[derive(Clone, Debug)]
    #[must_use = "iterators are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Skip<I> {
        iter: I,
        n: usize,
    }

    impl<I> Skip<I> {
        pub(in crate::iter) fn new(iter: I, n: usize) -> Skip<I> {
            Skip { iter, n }
        }
    }

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

        #[inline]
        fn next(&mut self) -> Option<I::Item> {
            if unlikely(self.n > 0) {
                self.iter.nth(crate::mem::take(&mut self.n) - 1);
            }
            self.iter.next()
        }

        #[inline]
        fn nth(&mut self, n: usize) -> Option<I::Item> {
            // Can't just add n + self.n due to overflow.
            if self.n > 0 {
                let to_skip = self.n;
                self.n = 0;
                // nth(n) skips n+1
                self.iter.nth(to_skip - 1)?;
            }
            self.iter.nth(n)
        }

        #[inline]
        fn count(mut self) -> usize {
            if self.n > 0 {
                // nth(n) skips n+1
                if self.iter.nth(self.n - 1).is_none() {
                    return 0;
                }
            }
            self.iter.count()
        }

        #[inline]
        fn last(mut self) -> Option<I::Item> {
            if self.n > 0 {
                // nth(n) skips n+1
                self.iter.nth(self.n - 1)?;
            }
            self.iter.last()
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let (lower, upper) = self.iter.size_hint();

            let lower = lower.saturating_sub(self.n);
            let upper = match upper {
                Some(x) => Some(x.saturating_sub(self.n)),
                None => None,
            };

            (lower, upper)
        }

        #[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>,
        {
            let n = self.n;
            self.n = 0;
            if n > 0 {
                // nth(n) skips n+1
                if self.iter.nth(n - 1).is_none() {
                    return try { init };
                }
            }
            self.iter.try_fold(init, fold)
        }

        #[inline]
        fn fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
        where
            Fold: FnMut(Acc, Self::Item) -> Acc,
        {
            if self.n > 0 {
                // nth(n) skips n+1
                if self.iter.nth(self.n - 1).is_none() {
                    return init;
                }
            }
            self.iter.fold(init, fold)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I> ExactSizeIterator for Skip<I> where I: ExactSizeIterator {}

    #[stable(feature = "double_ended_skip_iterator", since = "1.9.0")]
    impl<I> DoubleEndedIterator for Skip<I>
    where
        I: DoubleEndedIterator + ExactSizeIterator,
    {
        fn next_back(&mut self) -> Option<Self::Item> {
            if self.len() > 0 { self.iter.next_back() } else { None }
        }

        #[inline]
        fn nth_back(&mut self, n: usize) -> Option<I::Item> {
            let len = self.len();
            if n < len {
                self.iter.nth_back(n)
            } else {
                if len > 0 {
                    // consume the original iterator
                    self.iter.nth_back(len - 1);
                }
                None
            }
        }

        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>,
        {
            fn check<T, Acc, R: Try<Ok = Acc>>(
                mut n: usize,
                mut fold: impl FnMut(Acc, T) -> R,
            ) -> impl FnMut(Acc, T) -> ControlFlow<R, Acc> {
                move |acc, x| {
                    n -= 1;
                    let r = fold(acc, x);
                    if n == 0 { ControlFlow::Break(r) } else { ControlFlow::from_try(r) }
                }
            }

            let n = self.len();
            if n == 0 { try { init } } else { self.iter.try_rfold(init, check(n, fold)).into_try() }
        }

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

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

    #[stable(feature = "fused", since = "1.26.0")]
    impl<I> FusedIterator for Skip<I> where I: FusedIterator {}

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