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

    /// An iterator with a `peek()` that returns an optional reference to the next
    /// element.
    ///
    /// This `struct` is created by the [`peekable`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`peekable`]: Iterator::peekable
    /// [`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 Peekable<I: Iterator> {
        iter: I,
        /// Remember a peeked value, even if it was None.
        peeked: Option<Option<I::Item>>,
    }

    impl<I: Iterator> Peekable<I> {
        pub(in crate::iter) fn new(iter: I) -> Peekable<I> {
            Peekable { iter, peeked: None }
        }
    }

    // Peekable must remember if a None has been seen in the `.peek()` method.
    // It ensures that `.peek(); .peek();` or `.peek(); .next();` only advances the
    // underlying iterator at most once. This does not by itself make the iterator
    // fused.
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: Iterator> Iterator for Peekable<I> {
        type Item = I::Item;

        #[inline]
        fn next(&mut self) -> Option<I::Item> {
            match self.peeked.take() {
                Some(v) => v,
                None => self.iter.next(),
            }
        }

        #[inline]
        #[rustc_inherit_overflow_checks]
        fn count(mut self) -> usize {
            match self.peeked.take() {
                Some(None) => 0,
                Some(Some(_)) => 1 + self.iter.count(),
                None => self.iter.count(),
            }
        }

        #[inline]
        fn nth(&mut self, n: usize) -> Option<I::Item> {
            match self.peeked.take() {
                Some(None) => None,
                Some(v @ Some(_)) if n == 0 => v,
                Some(Some(_)) => self.iter.nth(n - 1),
                None => self.iter.nth(n),
            }
        }

        #[inline]
        fn last(mut self) -> Option<I::Item> {
            let peek_opt = match self.peeked.take() {
                Some(None) => return None,
                Some(v) => v,
                None => None,
            };
            self.iter.last().or(peek_opt)
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            let peek_len = match self.peeked {
                Some(None) => return (0, Some(0)),
                Some(Some(_)) => 1,
                None => 0,
            };
            let (lo, hi) = self.iter.size_hint();
            let lo = lo.saturating_add(peek_len);
            let hi = match hi {
                Some(x) => x.checked_add(peek_len),
                None => None,
            };
            (lo, hi)
        }

        #[inline]
        fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
        where
            Self: Sized,
            F: FnMut(B, Self::Item) -> R,
            R: Try<Ok = B>,
        {
            let acc = match self.peeked.take() {
                Some(None) => return try { init },
                Some(Some(v)) => f(init, v)?,
                None => init,
            };
            self.iter.try_fold(acc, f)
        }

        #[inline]
        fn fold<Acc, Fold>(self, init: Acc, mut fold: Fold) -> Acc
        where
            Fold: FnMut(Acc, Self::Item) -> Acc,
        {
            let acc = match self.peeked {
                Some(None) => return init,
                Some(Some(v)) => fold(init, v),
                None => init,
            };
            self.iter.fold(acc, fold)
        }
    }

    #[stable(feature = "double_ended_peek_iterator", since = "1.38.0")]
    impl<I> DoubleEndedIterator for Peekable<I>
    where
        I: DoubleEndedIterator,
    {
        #[inline]
        fn next_back(&mut self) -> Option<Self::Item> {
            match self.peeked.as_mut() {
                Some(v @ Some(_)) => self.iter.next_back().or_else(|| v.take()),
                Some(None) => None,
                None => self.iter.next_back(),
            }
        }

        #[inline]
        fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R
        where
            Self: Sized,
            F: FnMut(B, Self::Item) -> R,
            R: Try<Ok = B>,
        {
            match self.peeked.take() {
                Some(None) => try { init },
                Some(Some(v)) => match self.iter.try_rfold(init, &mut f).into_result() {
                    Ok(acc) => f(acc, v),
                    Err(e) => {
                        self.peeked = Some(Some(v));
                        Try::from_error(e)
                    }
                },
                None => self.iter.try_rfold(init, f),
            }
        }

        #[inline]
        fn rfold<Acc, Fold>(self, init: Acc, mut fold: Fold) -> Acc
        where
            Fold: FnMut(Acc, Self::Item) -> Acc,
        {
            match self.peeked {
                Some(None) => init,
                Some(Some(v)) => {
                    let acc = self.iter.rfold(init, &mut fold);
                    fold(acc, v)
                }
                None => self.iter.rfold(init, fold),
            }
        }
    }

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

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

    impl<I: Iterator> Peekable<I> {
        /// Returns a reference to the next() value without advancing the iterator.
        ///
        /// Like [`next`], if there is a value, it is wrapped in a `Some(T)`.
        /// But if the iteration is over, `None` is returned.
        ///
        /// [`next`]: Iterator::next
        ///
        /// Because `peek()` returns a reference, and many iterators iterate over
        /// references, there can be a possibly confusing situation where the
        /// return value is a double reference. You can see this effect in the
        /// examples below.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let xs = [1, 2, 3];
        ///
        /// let mut iter = xs.iter().peekable();
        ///
        /// // peek() lets us see into the future
        /// assert_eq!(iter.peek(), Some(&&1));
        /// assert_eq!(iter.next(), Some(&1));
        ///
        /// assert_eq!(iter.next(), Some(&2));
        ///
        /// // The iterator does not advance even if we `peek` multiple times
        /// assert_eq!(iter.peek(), Some(&&3));
        /// assert_eq!(iter.peek(), Some(&&3));
        ///
        /// assert_eq!(iter.next(), Some(&3));
        ///
        /// // After the iterator is finished, so is `peek()`
        /// assert_eq!(iter.peek(), None);
        /// assert_eq!(iter.next(), None);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn peek(&mut self) -> Option<&I::Item> {
            let iter = &mut self.iter;
            self.peeked.get_or_insert_with(|| iter.next()).as_ref()
        }

        /// Returns a mutable reference to the next() value without advancing the iterator.
        ///
        /// Like [`next`], if there is a value, it is wrapped in a `Some(T)`.
        /// But if the iteration is over, `None` is returned.
        ///
        /// Because `peek_mut()` returns a reference, and many iterators iterate over
        /// references, there can be a possibly confusing situation where the
        /// return value is a double reference. You can see this effect in the examples
        /// below.
        ///
        /// [`next`]: Iterator::next
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// #![feature(peekable_peek_mut)]
        /// let mut iter = [1, 2, 3].iter().peekable();
        ///
        /// // Like with `peek()`, we can see into the future without advancing the iterator.
        /// assert_eq!(iter.peek_mut(), Some(&mut &1));
        /// assert_eq!(iter.peek_mut(), Some(&mut &1));
        /// assert_eq!(iter.next(), Some(&1));
        ///
        /// // Peek into the iterator and set the value behind the mutable reference.
        /// if let Some(p) = iter.peek_mut() {
        ///     assert_eq!(*p, &2);
        ///     *p = &5;
        /// }
        ///
        /// // The value we put in reappears as the iterator continues.
        /// assert_eq!(iter.collect::<Vec<_>>(), vec![&5, &3]);
        /// ```
        #[inline]
        #[unstable(feature = "peekable_peek_mut", issue = "78302")]
        pub fn peek_mut(&mut self) -> Option<&mut I::Item> {
            let iter = &mut self.iter;
            self.peeked.get_or_insert_with(|| iter.next()).as_mut()
        }

        /// Consume and return the next value of this iterator if a condition is true.
        ///
        /// If `func` returns `true` for the next value of this iterator, consume and return it.
        /// Otherwise, return `None`.
        ///
        /// # Examples
        /// Consume a number if it's equal to 0.
        /// ```
        /// let mut iter = (0..5).peekable();
        /// // The first item of the iterator is 0; consume it.
        /// assert_eq!(iter.next_if(|&x| x == 0), Some(0));
        /// // The next item returned is now 1, so `consume` will return `false`.
        /// assert_eq!(iter.next_if(|&x| x == 0), None);
        /// // `next_if` saves the value of the next item if it was not equal to `expected`.
        /// assert_eq!(iter.next(), Some(1));
        /// ```
        ///
        /// Consume any number less than 10.
        /// ```
        /// let mut iter = (1..20).peekable();
        /// // Consume all numbers less than 10
        /// while iter.next_if(|&x| x < 10).is_some() {}
        /// // The next value returned will be 10
        /// assert_eq!(iter.next(), Some(10));
        /// ```
        #[stable(feature = "peekable_next_if", since = "1.51.0")]
        pub fn next_if(&mut self, func: impl FnOnce(&I::Item) -> bool) -> Option<I::Item> {
            match self.next() {
                Some(matched) if func(&matched) => Some(matched),
                other => {
                    // Since we called `self.next()`, we consumed `self.peeked`.
                    assert!(self.peeked.is_none());
                    self.peeked = Some(other);
                    None
                }
            }
        }

        /// Consume and return the next item if it is equal to `expected`.
        ///
        /// # Example
        /// Consume a number if it's equal to 0.
        /// ```
        /// let mut iter = (0..5).peekable();
        /// // The first item of the iterator is 0; consume it.
        /// assert_eq!(iter.next_if_eq(&0), Some(0));
        /// // The next item returned is now 1, so `consume` will return `false`.
        /// assert_eq!(iter.next_if_eq(&0), None);
        /// // `next_if_eq` saves the value of the next item if it was not equal to `expected`.
        /// assert_eq!(iter.next(), Some(1));
        /// ```
        #[stable(feature = "peekable_next_if", since = "1.51.0")]
        pub fn next_if_eq<T>(&mut self, expected: &T) -> Option<I::Item>
        where
            T: ?Sized,
            I::Item: PartialEq<T>,
        {
            self.next_if(|next| next == expected)
        }
    }

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

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