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diff --git a/library/core/src/iter/adapters/map.rs b/library/core/src/iter/adapters/map.rs
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+++ b/library/core/src/iter/adapters/map.rs
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+use crate::fmt;
+use crate::iter::adapters::{
+    zip::try_get_unchecked, SourceIter, TrustedRandomAccess, TrustedRandomAccessNoCoerce,
+};
+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> = [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 ['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 ['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> {
+    // Used for `SplitWhitespace` and `SplitAsciiWhitespace` `as_str` methods
+    pub(crate) 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<Output = 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))
+    }
+
+    #[inline]
+    unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> B
+    where
+        Self: TrustedRandomAccessNoCoerce,
+    {
+        // 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<Output = 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 {}
+
+#[doc(hidden)]
+#[unstable(feature = "trusted_random_access", issue = "none")]
+unsafe impl<I, F> TrustedRandomAccessNoCoerce for Map<I, F>
+where
+    I: TrustedRandomAccessNoCoerce,
+{
+    const MAY_HAVE_SIDE_EFFECT: bool = true;
+}
+
+#[unstable(issue = "none", feature = "inplace_iteration")]
+unsafe impl<I, F> SourceIter for Map<I, F>
+where
+    I: SourceIter,
+{
+    type Source = I::Source;
+
+    #[inline]
+    unsafe fn as_inner(&mut self) -> &mut I::Source {
+        // 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 {}