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    use crate::fmt;
    use crate::iter::adapters::{zip::try_get_unchecked, SourceIter, TrustedRandomAccess};
    use crate::iter::{FusedIterator, InPlaceIterable, TrustedLen};
    use crate::ops::Try;

    /// An iterator that maps the values of `iter` with `f`.
    ///
    /// This `struct` is created by the [`map`] method on [`Iterator`]. See its
    /// documentation for more.
    ///
    /// [`map`]: Iterator::map
    /// [`Iterator`]: trait.Iterator.html
    ///
    /// # Notes about side effects
    ///
    /// The [`map`] iterator implements [`DoubleEndedIterator`], meaning that
    /// you can also [`map`] backwards:
    ///
    /// ```rust
    /// let v: Vec<i32> = vec![1, 2, 3].into_iter().map(|x| x + 1).rev().collect();
    ///
    /// assert_eq!(v, [4, 3, 2]);
    /// ```
    ///
    /// [`DoubleEndedIterator`]: trait.DoubleEndedIterator.html
    ///
    /// But if your closure has state, iterating backwards may act in a way you do
    /// not expect. Let's go through an example. First, in the forward direction:
    ///
    /// ```rust
    /// let mut c = 0;
    ///
    /// for pair in vec!['a', 'b', 'c'].into_iter()
    ///                                .map(|letter| { c += 1; (letter, c) }) {
    ///     println!("{:?}", pair);
    /// }
    /// ```
    ///
    /// This will print "('a', 1), ('b', 2), ('c', 3)".
    ///
    /// Now consider this twist where we add a call to `rev`. This version will
    /// print `('c', 1), ('b', 2), ('a', 3)`. Note that the letters are reversed,
    /// but the values of the counter still go in order. This is because `map()` is
    /// still being called lazily on each item, but we are popping items off the
    /// back of the vector now, instead of shifting them from the front.
    ///
    /// ```rust
    /// let mut c = 0;
    ///
    /// for pair in vec!['a', 'b', 'c'].into_iter()
    ///                                .map(|letter| { c += 1; (letter, c) })
    ///                                .rev() {
    ///     println!("{:?}", pair);
    /// }
    /// ```
    #[must_use = "iterators are lazy and do nothing unless consumed"]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Clone)]
    pub struct Map<I, F> {
        iter: I,
        f: F,
    }
    impl<I, F> Map<I, F> {
        pub(in crate::iter) fn new(iter: I, f: F) -> Map<I, F> {
            Map { iter, f }
        }
    }

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

    fn map_fold<T, B, Acc>(
        mut f: impl FnMut(T) -> B,
        mut g: impl FnMut(Acc, B) -> Acc,
    ) -> impl FnMut(Acc, T) -> Acc {
        move |acc, elt| g(acc, f(elt))
    }

    fn map_try_fold<'a, T, B, Acc, R>(
        f: &'a mut impl FnMut(T) -> B,
        mut g: impl FnMut(Acc, B) -> R + 'a,
    ) -> impl FnMut(Acc, T) -> R + 'a {
        move |acc, elt| g(acc, f(elt))
    }

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

        #[inline]
        fn next(&mut self) -> Option<B> {
            self.iter.next().map(&mut self.f)
        }

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

        fn try_fold<Acc, G, R>(&mut self, init: Acc, g: G) -> R
        where
            Self: Sized,
            G: FnMut(Acc, Self::Item) -> R,
            R: Try<Ok = Acc>,
        {
            self.iter.try_fold(init, map_try_fold(&mut self.f, g))
        }

        fn fold<Acc, G>(self, init: Acc, g: G) -> Acc
        where
            G: FnMut(Acc, Self::Item) -> Acc,
        {
            self.iter.fold(init, map_fold(self.f, g))
        }

        unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> B
        where
            Self: TrustedRandomAccess,
        {
            // SAFETY: the caller must uphold the contract for
            // `Iterator::__iterator_get_unchecked`.
            unsafe { (self.f)(try_get_unchecked(&mut self.iter, idx)) }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<B, I: DoubleEndedIterator, F> DoubleEndedIterator for Map<I, F>
    where
        F: FnMut(I::Item) -> B,
    {
        #[inline]
        fn next_back(&mut self) -> Option<B> {
            self.iter.next_back().map(&mut self.f)
        }

        fn try_rfold<Acc, G, R>(&mut self, init: Acc, g: G) -> R
        where
            Self: Sized,
            G: FnMut(Acc, Self::Item) -> R,
            R: Try<Ok = Acc>,
        {
            self.iter.try_rfold(init, map_try_fold(&mut self.f, g))
        }

        fn rfold<Acc, G>(self, init: Acc, g: G) -> Acc
        where
            G: FnMut(Acc, Self::Item) -> Acc,
        {
            self.iter.rfold(init, map_fold(self.f, g))
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<B, I: ExactSizeIterator, F> ExactSizeIterator for Map<I, F>
    where
        F: FnMut(I::Item) -> B,
    {
        fn len(&self) -> usize {
            self.iter.len()
        }

        fn is_empty(&self) -> bool {
            self.iter.is_empty()
        }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl<B, I: FusedIterator, F> FusedIterator for Map<I, F> where F: FnMut(I::Item) -> B {}

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<B, I, F> TrustedLen for Map<I, F>
    where
        I: TrustedLen,
        F: FnMut(I::Item) -> B,
    {
    }

    #[doc(hidden)]
    #[unstable(feature = "trusted_random_access", issue = "none")]
    unsafe impl<I, F> TrustedRandomAccess for Map<I, F>
    where
        I: TrustedRandomAccess,
    {
        #[inline]
        fn may_have_side_effect() -> bool {
            true
        }
    }

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