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    use crate::cmp;
    use crate::fmt::{self, Debug};
    use crate::iter::{DoubleEndedIterator, ExactSizeIterator, FusedIterator, Iterator};
    use crate::iter::{InPlaceIterable, SourceIter, TrustedLen};

    /// An iterator that iterates two other iterators simultaneously.
    ///
    /// This `struct` is created by [`Iterator::zip`]. See its documentation
    /// for more.
    #[derive(Clone)]
    #[must_use = "iterators are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Zip<A, B> {
        a: A,
        b: B,
        // index and len are only used by the specialized version of zip
        index: usize,
        len: usize,
    }
    impl<A: Iterator, B: Iterator> Zip<A, B> {
        pub(in crate::iter) fn new(a: A, b: B) -> Zip<A, B> {
            ZipImpl::new(a, b)
        }
        fn super_nth(&mut self, mut n: usize) -> Option<(A::Item, B::Item)> {
            while let Some(x) = Iterator::next(self) {
                if n == 0 {
                    return Some(x);
                }
                n -= 1;
            }
            None
        }
    }

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

        #[inline]
        fn next(&mut self) -> Option<Self::Item> {
            ZipImpl::next(self)
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            ZipImpl::size_hint(self)
        }

        #[inline]
        fn nth(&mut self, n: usize) -> Option<Self::Item> {
            ZipImpl::nth(self, n)
        }

        #[inline]
        unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> Self::Item
        where
            Self: TrustedRandomAccess,
        {
            // SAFETY: `ZipImpl::__iterator_get_unchecked` has same safety
            // requirements as `Iterator::__iterator_get_unchecked`.
            unsafe { ZipImpl::get_unchecked(self, idx) }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A, B> DoubleEndedIterator for Zip<A, B>
    where
        A: DoubleEndedIterator + ExactSizeIterator,
        B: DoubleEndedIterator + ExactSizeIterator,
    {
        #[inline]
        fn next_back(&mut self) -> Option<(A::Item, B::Item)> {
            ZipImpl::next_back(self)
        }
    }

    // Zip specialization trait
    #[doc(hidden)]
    trait ZipImpl<A, B> {
        type Item;
        fn new(a: A, b: B) -> Self;
        fn next(&mut self) -> Option<Self::Item>;
        fn size_hint(&self) -> (usize, Option<usize>);
        fn nth(&mut self, n: usize) -> Option<Self::Item>;
        fn next_back(&mut self) -> Option<Self::Item>
        where
            A: DoubleEndedIterator + ExactSizeIterator,
            B: DoubleEndedIterator + ExactSizeIterator;
        // This has the same safety requirements as `Iterator::__iterator_get_unchecked`
        unsafe fn get_unchecked(&mut self, idx: usize) -> <Self as Iterator>::Item
        where
            Self: Iterator + TrustedRandomAccess;
    }

    // General Zip impl
    #[doc(hidden)]
    impl<A, B> ZipImpl<A, B> for Zip<A, B>
    where
        A: Iterator,
        B: Iterator,
    {
        type Item = (A::Item, B::Item);
        default fn new(a: A, b: B) -> Self {
            Zip {
                a,
                b,
                index: 0, // unused
                len: 0,   // unused
            }
        }

        #[inline]
        default fn next(&mut self) -> Option<(A::Item, B::Item)> {
            let x = self.a.next()?;
            let y = self.b.next()?;
            Some((x, y))
        }

        #[inline]
        default fn nth(&mut self, n: usize) -> Option<Self::Item> {
            self.super_nth(n)
        }

        #[inline]
        default fn next_back(&mut self) -> Option<(A::Item, B::Item)>
        where
            A: DoubleEndedIterator + ExactSizeIterator,
            B: DoubleEndedIterator + ExactSizeIterator,
        {
            let a_sz = self.a.len();
            let b_sz = self.b.len();
            if a_sz != b_sz {
                // Adjust a, b to equal length
                if a_sz > b_sz {
                    for _ in 0..a_sz - b_sz {
                        self.a.next_back();
                    }
                } else {
                    for _ in 0..b_sz - a_sz {
                        self.b.next_back();
                    }
                }
            }
            match (self.a.next_back(), self.b.next_back()) {
                (Some(x), Some(y)) => Some((x, y)),
                (None, None) => None,
                _ => unreachable!(),
            }
        }

        #[inline]
        default fn size_hint(&self) -> (usize, Option<usize>) {
            let (a_lower, a_upper) = self.a.size_hint();
            let (b_lower, b_upper) = self.b.size_hint();

            let lower = cmp::min(a_lower, b_lower);

            let upper = match (a_upper, b_upper) {
                (Some(x), Some(y)) => Some(cmp::min(x, y)),
                (Some(x), None) => Some(x),
                (None, Some(y)) => Some(y),
                (None, None) => None,
            };

            (lower, upper)
        }

        default unsafe fn get_unchecked(&mut self, _idx: usize) -> <Self as Iterator>::Item
        where
            Self: TrustedRandomAccess,
        {
            unreachable!("Always specialized");
        }
    }

    #[doc(hidden)]
    impl<A, B> ZipImpl<A, B> for Zip<A, B>
    where
        A: TrustedRandomAccess + Iterator,
        B: TrustedRandomAccess + Iterator,
    {
        fn new(a: A, b: B) -> Self {
            let len = cmp::min(a.size(), b.size());
            Zip { a, b, index: 0, len }
        }

        #[inline]
        fn next(&mut self) -> Option<(A::Item, B::Item)> {
            if self.index < self.len {
                let i = self.index;
                self.index += 1;
                // SAFETY: `i` is smaller than `self.len`, thus smaller than `self.a.len()` and `self.b.len()`
                unsafe {
                    Some((self.a.__iterator_get_unchecked(i), self.b.__iterator_get_unchecked(i)))
                }
            } else if A::may_have_side_effect() && self.index < self.a.size() {
                // match the base implementation's potential side effects
                // SAFETY: we just checked that `self.index` < `self.a.len()`
                unsafe {
                    self.a.__iterator_get_unchecked(self.index);
                }
                self.index += 1;
                None
            } else {
                None
            }
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let len = self.len - self.index;
            (len, Some(len))
        }

        #[inline]
        fn nth(&mut self, n: usize) -> Option<Self::Item> {
            let delta = cmp::min(n, self.len - self.index);
            let end = self.index + delta;
            while self.index < end {
                let i = self.index;
                self.index += 1;
                if A::may_have_side_effect() {
                    // SAFETY: the usage of `cmp::min` to calculate `delta`
                    // ensures that `end` is smaller than or equal to `self.len`,
                    // so `i` is also smaller than `self.len`.
                    unsafe {
                        self.a.__iterator_get_unchecked(i);
                    }
                }
                if B::may_have_side_effect() {
                    // SAFETY: same as above.
                    unsafe {
                        self.b.__iterator_get_unchecked(i);
                    }
                }
            }

            self.super_nth(n - delta)
        }

        #[inline]
        fn next_back(&mut self) -> Option<(A::Item, B::Item)>
        where
            A: DoubleEndedIterator + ExactSizeIterator,
            B: DoubleEndedIterator + ExactSizeIterator,
        {
            let a_side_effect = A::may_have_side_effect();
            let b_side_effect = B::may_have_side_effect();
            if a_side_effect || b_side_effect {
                let sz_a = self.a.size();
                let sz_b = self.b.size();
                // Adjust a, b to equal length, make sure that only the first call
                // of `next_back` does this, otherwise we will break the restriction
                // on calls to `self.next_back()` after calling `get_unchecked()`.
                if sz_a != sz_b {
                    let sz_a = self.a.size();
                    if a_side_effect && sz_a > self.len {
                        for _ in 0..sz_a - cmp::max(self.len, self.index) {
                            self.a.next_back();
                        }
                    }
                    let sz_b = self.b.size();
                    if b_side_effect && sz_b > self.len {
                        for _ in 0..sz_b - self.len {
                            self.b.next_back();
                        }
                    }
                }
            }
            if self.index < self.len {
                self.len -= 1;
                let i = self.len;
                // SAFETY: `i` is smaller than the previous value of `self.len`,
                // which is also smaller than or equal to `self.a.len()` and `self.b.len()`
                unsafe {
                    Some((self.a.__iterator_get_unchecked(i), self.b.__iterator_get_unchecked(i)))
                }
            } else {
                None
            }
        }

        #[inline]
        unsafe fn get_unchecked(&mut self, idx: usize) -> <Self as Iterator>::Item {
            // SAFETY: the caller must uphold the contract for
            // `Iterator::__iterator_get_unchecked`.
            unsafe { (self.a.__iterator_get_unchecked(idx), self.b.__iterator_get_unchecked(idx)) }
        }
    }

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

    #[doc(hidden)]
    #[unstable(feature = "trusted_random_access", issue = "none")]
    unsafe impl<A, B> TrustedRandomAccess for Zip<A, B>
    where
        A: TrustedRandomAccess,
        B: TrustedRandomAccess,
    {
        fn may_have_side_effect() -> bool {
            A::may_have_side_effect() || B::may_have_side_effect()
        }
    }

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

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<A, B> TrustedLen for Zip<A, B>
    where
        A: TrustedLen,
        B: TrustedLen,
    {
    }

    // Arbitrarily selects the left side of the zip iteration as extractable "source"
    // it would require negative trait bounds to be able to try both
    #[unstable(issue = "none", feature = "inplace_iteration")]
    unsafe impl<S, A, B> SourceIter for Zip<A, B>
    where
        A: SourceIter<Source = S>,
        B: Iterator,
        S: Iterator,
    {
        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.a) }
        }
    }

    #[unstable(issue = "none", feature = "inplace_iteration")]
    // Limited to Item: Copy since interaction between Zip's use of TrustedRandomAccess
    // and Drop implementation of the source is unclear.
    //
    // An additional method returning the number of times the source has been logically advanced
    // (without calling next()) would be needed to properly drop the remainder of the source.
    unsafe impl<A: InPlaceIterable, B: Iterator> InPlaceIterable for Zip<A, B> where A::Item: Copy {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A: Debug, B: Debug> Debug for Zip<A, B> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            ZipFmt::fmt(self, f)
        }
    }

    trait ZipFmt<A, B> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result;
    }

    impl<A: Debug, B: Debug> ZipFmt<A, B> for Zip<A, B> {
        default fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Zip").field("a", &self.a).field("b", &self.b).finish()
        }
    }

    impl<A: Debug + TrustedRandomAccess, B: Debug + TrustedRandomAccess> ZipFmt<A, B> for Zip<A, B> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            // It's *not safe* to call fmt on the contained iterators, since once
            // we start iterating they're in strange, potentially unsafe, states.
            f.debug_struct("Zip").finish()
        }
    }

    /// An iterator whose items are random-accessible efficiently
    ///
    /// # Safety
    ///
    /// The iterator's `size_hint` must be exact and cheap to call.
    ///
    /// `size` may not be overridden.
    ///
    /// `<Self as Iterator>::__iterator_get_unchecked` must be safe to call
    /// provided the following conditions are met.
    ///
    /// 1. `0 <= idx` and `idx < self.size()`.
    /// 2. If `self: !Clone`, then `get_unchecked` is never called with the same
    ///    index on `self` more than once.
    /// 3. After `self.get_unchecked(idx)` has been called then `next_back` will
    ///    only be called at most `self.size() - idx - 1` times.
    /// 4. After `get_unchecked` is called, then only the following methods will be
    ///    called on `self`:
    ///     * `std::clone::Clone::clone`
    ///     * `std::iter::Iterator::size_hint()`
    ///     * `std::iter::Iterator::next_back()`
    ///     * `std::iter::Iterator::__iterator_get_unchecked()`
    ///     * `std::iter::TrustedRandomAccess::size()`
    ///
    /// Further, given that these conditions are met, it must guarantee that:
    ///
    /// * It does not change the value returned from `size_hint`
    /// * It must be safe to call the methods listed above on `self` after calling
    ///   `get_unchecked`, assuming that the required traits are implemented.
    /// * It must also be safe to drop `self` after calling `get_unchecked`.
    #[doc(hidden)]
    #[unstable(feature = "trusted_random_access", issue = "none")]
    #[rustc_specialization_trait]
    pub unsafe trait TrustedRandomAccess: Sized {
        // Convenience method.
        fn size(&self) -> usize
        where
            Self: Iterator,
        {
            self.size_hint().0
        }
        /// Returns `true` if getting an iterator element may have
        /// side effects. Remember to take inner iterators into account.
        fn may_have_side_effect() -> bool;
    }

    /// Like `Iterator::__iterator_get_unchecked`, but doesn't require the compiler to
    /// know that `U: TrustedRandomAccess`.
    ///
    /// ## Safety
    ///
    /// Same requirements calling `get_unchecked` directly.
    #[doc(hidden)]
    pub(in crate::iter::adapters) unsafe fn try_get_unchecked<I>(it: &mut I, idx: usize) -> I::Item
    where
        I: Iterator,
    {
        // SAFETY: the caller must uphold the contract for
        // `Iterator::__iterator_get_unchecked`.
        unsafe { it.try_get_unchecked(idx) }
    }

    unsafe trait SpecTrustedRandomAccess: Iterator {
        /// If `Self: TrustedRandomAccess`, it must be safe to call a
        /// `Iterator::__iterator_get_unchecked(self, index)`.
        unsafe fn try_get_unchecked(&mut self, index: usize) -> Self::Item;
    }

    unsafe impl<I: Iterator> SpecTrustedRandomAccess for I {
        default unsafe fn try_get_unchecked(&mut self, _: usize) -> Self::Item {
            panic!("Should only be called on TrustedRandomAccess iterators");
        }
    }

    unsafe impl<I: Iterator + TrustedRandomAccess> SpecTrustedRandomAccess for I {
        unsafe fn try_get_unchecked(&mut self, index: usize) -> Self::Item {
            // SAFETY: the caller must uphold the contract for
            // `Iterator::__iterator_get_unchecked`.
            unsafe { self.__iterator_get_unchecked(index) }
        }
    }