13 files changed, 215 insertions, 198 deletions

diff --git a/library/core/src/iter/adapters/array_chunks.rs b/library/core/src/iter/adapters/array_chunks.rs
index 9b479a9f8..d4fb88610 100644
--- a/library/core/src/iter/adapters/array_chunks.rs
+++ b/library/core/src/iter/adapters/array_chunks.rs
@@ -1,6 +1,6 @@
 use crate::array;
 use crate::iter::{ByRefSized, FusedIterator, Iterator};
-use crate::ops::{ControlFlow, NeverShortCircuit, Try};
+use crate::ops::{ControlFlow, Try};
 
 /// An iterator over `N` elements of the iterator at a time.
 ///
@@ -82,13 +82,7 @@ where
         }
     }
 
-    fn fold<B, F>(mut self, init: B, f: F) -> B
-    where
-        Self: Sized,
-        F: FnMut(B, Self::Item) -> B,
-    {
-        self.try_fold(init, NeverShortCircuit::wrap_mut_2(f)).0
-    }
+    impl_fold_via_try_fold! { fold -> try_fold }
 }
 
 #[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
@@ -126,13 +120,7 @@ where
         try { acc }
     }
 
-    fn rfold<B, F>(mut self, init: B, f: F) -> B
-    where
-        Self: Sized,
-        F: FnMut(B, Self::Item) -> B,
-    {
-        self.try_rfold(init, NeverShortCircuit::wrap_mut_2(f)).0
-    }
+    impl_fold_via_try_fold! { rfold -> try_rfold }
 }
 
 impl<I, const N: usize> ArrayChunks<I, N>
diff --git a/library/core/src/iter/adapters/by_ref_sized.rs b/library/core/src/iter/adapters/by_ref_sized.rs
index 477e7117c..1945e402f 100644
--- a/library/core/src/iter/adapters/by_ref_sized.rs
+++ b/library/core/src/iter/adapters/by_ref_sized.rs
@@ -1,4 +1,7 @@
-use crate::ops::{NeverShortCircuit, Try};
+use crate::{
+    const_closure::ConstFnMutClosure,
+    ops::{NeverShortCircuit, Try},
+};
 
 /// Like `Iterator::by_ref`, but requiring `Sized` so it can forward generics.
 ///
@@ -36,12 +39,13 @@ impl<I: Iterator> Iterator for ByRefSized<'_, I> {
     }
 
     #[inline]
-    fn fold<B, F>(self, init: B, f: F) -> B
+    fn fold<B, F>(self, init: B, mut f: F) -> B
     where
         F: FnMut(B, Self::Item) -> B,
     {
         // `fold` needs ownership, so this can't forward directly.
-        I::try_fold(self.0, init, NeverShortCircuit::wrap_mut_2(f)).0
+        I::try_fold(self.0, init, ConstFnMutClosure::new(&mut f, NeverShortCircuit::wrap_mut_2_imp))
+            .0
     }
 
     #[inline]
@@ -72,12 +76,17 @@ impl<I: DoubleEndedIterator> DoubleEndedIterator for ByRefSized<'_, I> {
     }
 
     #[inline]
-    fn rfold<B, F>(self, init: B, f: F) -> B
+    fn rfold<B, F>(self, init: B, mut f: F) -> B
     where
         F: FnMut(B, Self::Item) -> B,
     {
         // `rfold` needs ownership, so this can't forward directly.
-        I::try_rfold(self.0, init, NeverShortCircuit::wrap_mut_2(f)).0
+        I::try_rfold(
+            self.0,
+            init,
+            ConstFnMutClosure::new(&mut f, NeverShortCircuit::wrap_mut_2_imp),
+        )
+        .0
     }
 
     #[inline]
diff --git a/library/core/src/iter/adapters/copied.rs b/library/core/src/iter/adapters/copied.rs
index f9bfd77d7..62d3afb81 100644
--- a/library/core/src/iter/adapters/copied.rs
+++ b/library/core/src/iter/adapters/copied.rs
@@ -2,7 +2,10 @@ use crate::iter::adapters::{
     zip::try_get_unchecked, TrustedRandomAccess, TrustedRandomAccessNoCoerce,
 };
 use crate::iter::{FusedIterator, TrustedLen};
+use crate::mem::MaybeUninit;
+use crate::mem::SizedTypeProperties;
 use crate::ops::Try;
+use crate::{array, ptr};
 
 /// An iterator that copies the elements of an underlying iterator.
 ///
@@ -44,6 +47,15 @@ where
         self.it.next().copied()
     }
 
+    fn next_chunk<const N: usize>(
+        &mut self,
+    ) -> Result<[Self::Item; N], array::IntoIter<Self::Item, N>>
+    where
+        Self: Sized,
+    {
+        <I as SpecNextChunk<'_, N, T>>::spec_next_chunk(&mut self.it)
+    }
+
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.it.size_hint()
     }
@@ -166,3 +178,65 @@ where
     T: Copy,
 {
 }
+
+trait SpecNextChunk<'a, const N: usize, T: 'a>: Iterator<Item = &'a T>
+where
+    T: Copy,
+{
+    fn spec_next_chunk(&mut self) -> Result<[T; N], array::IntoIter<T, N>>;
+}
+
+impl<'a, const N: usize, I, T: 'a> SpecNextChunk<'a, N, T> for I
+where
+    I: Iterator<Item = &'a T>,
+    T: Copy,
+{
+    default fn spec_next_chunk(&mut self) -> Result<[T; N], array::IntoIter<T, N>> {
+        array::iter_next_chunk(&mut self.map(|e| *e))
+    }
+}
+
+impl<'a, const N: usize, T: 'a> SpecNextChunk<'a, N, T> for crate::slice::Iter<'a, T>
+where
+    T: Copy,
+{
+    fn spec_next_chunk(&mut self) -> Result<[T; N], array::IntoIter<T, N>> {
+        let mut raw_array = MaybeUninit::uninit_array();
+
+        let len = self.len();
+
+        if T::IS_ZST {
+            if len < N {
+                let _ = self.advance_by(len);
+                // SAFETY: ZSTs can be conjured ex nihilo; only the amount has to be correct
+                return Err(unsafe { array::IntoIter::new_unchecked(raw_array, 0..len) });
+            }
+
+            let _ = self.advance_by(N);
+            // SAFETY: ditto
+            return Ok(unsafe { MaybeUninit::array_assume_init(raw_array) });
+        }
+
+        if len < N {
+            // SAFETY: `len` indicates that this many elements are available and we just checked that
+            // it fits into the array.
+            unsafe {
+                ptr::copy_nonoverlapping(
+                    self.as_ref().as_ptr(),
+                    raw_array.as_mut_ptr() as *mut T,
+                    len,
+                );
+                let _ = self.advance_by(len);
+                return Err(array::IntoIter::new_unchecked(raw_array, 0..len));
+            }
+        }
+
+        // SAFETY: `len` is larger than the array size. Copy a fixed amount here to fully initialize
+        // the array.
+        unsafe {
+            ptr::copy_nonoverlapping(self.as_ref().as_ptr(), raw_array.as_mut_ptr() as *mut T, N);
+            let _ = self.advance_by(N);
+            Ok(MaybeUninit::array_assume_init(raw_array))
+        }
+    }
+}
diff --git a/library/core/src/iter/adapters/map_while.rs b/library/core/src/iter/adapters/map_while.rs
index 1e8d6bf3e..fbdeca4d4 100644
--- a/library/core/src/iter/adapters/map_while.rs
+++ b/library/core/src/iter/adapters/map_while.rs
@@ -64,19 +64,7 @@ where
         .into_try()
     }
 
-    #[inline]
-    fn fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
-    where
-        Self: Sized,
-        Fold: FnMut(Acc, Self::Item) -> Acc,
-    {
-        #[inline]
-        fn ok<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> Result<B, !> {
-            move |acc, x| Ok(f(acc, x))
-        }
-
-        self.try_fold(init, ok(fold)).unwrap()
-    }
+    impl_fold_via_try_fold! { fold -> try_fold }
 }
 
 #[unstable(issue = "none", feature = "inplace_iteration")]
diff --git a/library/core/src/iter/adapters/mod.rs b/library/core/src/iter/adapters/mod.rs
index bf4fabad3..8cc2b7cec 100644
--- a/library/core/src/iter/adapters/mod.rs
+++ b/library/core/src/iter/adapters/mod.rs
@@ -1,5 +1,5 @@
 use crate::iter::{InPlaceIterable, Iterator};
-use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, NeverShortCircuit, Residual, Try};
+use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, Residual, Try};
 
 mod array_chunks;
 mod by_ref_sized;
@@ -203,13 +203,7 @@ where
             .into_try()
     }
 
-    fn fold<B, F>(mut self, init: B, fold: F) -> B
-    where
-        Self: Sized,
-        F: FnMut(B, Self::Item) -> B,
-    {
-        self.try_fold(init, NeverShortCircuit::wrap_mut_2(fold)).0
-    }
+    impl_fold_via_try_fold! { fold -> try_fold }
 }
 
 #[unstable(issue = "none", feature = "inplace_iteration")]
diff --git a/library/core/src/iter/adapters/scan.rs b/library/core/src/iter/adapters/scan.rs
index 80bfd2231..62470512c 100644
--- a/library/core/src/iter/adapters/scan.rs
+++ b/library/core/src/iter/adapters/scan.rs
@@ -74,19 +74,7 @@ where
         self.iter.try_fold(init, scan(state, f, fold)).into_try()
     }
 
-    #[inline]
-    fn fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
-    where
-        Self: Sized,
-        Fold: FnMut(Acc, Self::Item) -> Acc,
-    {
-        #[inline]
-        fn ok<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> Result<B, !> {
-            move |acc, x| Ok(f(acc, x))
-        }
-
-        self.try_fold(init, ok(fold)).unwrap()
-    }
+    impl_fold_via_try_fold! { fold -> try_fold }
 }
 
 #[unstable(issue = "none", feature = "inplace_iteration")]
diff --git a/library/core/src/iter/adapters/skip.rs b/library/core/src/iter/adapters/skip.rs
index dbf0ae9ec..c6334880d 100644
--- a/library/core/src/iter/adapters/skip.rs
+++ b/library/core/src/iter/adapters/skip.rs
@@ -206,17 +206,7 @@ where
         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()
-    }
+    impl_fold_via_try_fold! { rfold -> try_rfold }
 
     #[inline]
     fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
diff --git a/library/core/src/iter/adapters/take.rs b/library/core/src/iter/adapters/take.rs
index 2962e0104..58a0b9d7b 100644
--- a/library/core/src/iter/adapters/take.rs
+++ b/library/core/src/iter/adapters/take.rs
@@ -98,19 +98,7 @@ where
         }
     }
 
-    #[inline]
-    fn fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
-    where
-        Self: Sized,
-        Fold: FnMut(Acc, Self::Item) -> Acc,
-    {
-        #[inline]
-        fn ok<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> Result<B, !> {
-            move |acc, x| Ok(f(acc, x))
-        }
-
-        self.try_fold(init, ok(fold)).unwrap()
-    }
+    impl_fold_via_try_fold! { fold -> try_fold }
 
     #[inline]
     #[rustc_inherit_overflow_checks]
diff --git a/library/core/src/iter/adapters/take_while.rs b/library/core/src/iter/adapters/take_while.rs
index ded216da9..ec66dc3ae 100644
--- a/library/core/src/iter/adapters/take_while.rs
+++ b/library/core/src/iter/adapters/take_while.rs
@@ -94,19 +94,7 @@ where
         }
     }
 
-    #[inline]
-    fn fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
-    where
-        Self: Sized,
-        Fold: FnMut(Acc, Self::Item) -> Acc,
-    {
-        #[inline]
-        fn ok<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> Result<B, !> {
-            move |acc, x| Ok(f(acc, x))
-        }
-
-        self.try_fold(init, ok(fold)).unwrap()
-    }
+    impl_fold_via_try_fold! { fold -> try_fold }
 }
 
 #[stable(feature = "fused", since = "1.26.0")]
diff --git a/library/core/src/iter/mod.rs b/library/core/src/iter/mod.rs
index 9514466bd..ef0f39782 100644
--- a/library/core/src/iter/mod.rs
+++ b/library/core/src/iter/mod.rs
@@ -352,6 +352,29 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
+// This needs to be up here in order to be usable in the child modules
+macro_rules! impl_fold_via_try_fold {
+    (fold -> try_fold) => {
+        impl_fold_via_try_fold! { @internal fold -> try_fold }
+    };
+    (rfold -> try_rfold) => {
+        impl_fold_via_try_fold! { @internal rfold -> try_rfold }
+    };
+    (@internal $fold:ident -> $try_fold:ident) => {
+        #[inline]
+        fn $fold<AAA, FFF>(mut self, init: AAA, mut fold: FFF) -> AAA
+        where
+            FFF: FnMut(AAA, Self::Item) -> AAA,
+        {
+            use crate::const_closure::ConstFnMutClosure;
+            use crate::ops::NeverShortCircuit;
+
+            let fold = ConstFnMutClosure::new(&mut fold, NeverShortCircuit::wrap_mut_2_imp);
+            self.$try_fold(init, fold).0
+        }
+    };
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::traits::Iterator;
 
diff --git a/library/core/src/iter/range.rs b/library/core/src/iter/range.rs
index f7aeee8c9..ac7b389b1 100644
--- a/library/core/src/iter/range.rs
+++ b/library/core/src/iter/range.rs
@@ -1150,19 +1150,7 @@ impl<A: Step> Iterator for ops::RangeInclusive<A> {
         self.spec_try_fold(init, f)
     }
 
-    #[inline]
-    fn fold<B, F>(mut self, init: B, f: F) -> B
-    where
-        Self: Sized,
-        F: FnMut(B, Self::Item) -> B,
-    {
-        #[inline]
-        fn ok<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> Result<B, !> {
-            move |acc, x| Ok(f(acc, x))
-        }
-
-        self.try_fold(init, ok(f)).unwrap()
-    }
+    impl_fold_via_try_fold! { fold -> try_fold }
 
     #[inline]
     fn last(mut self) -> Option<A> {
@@ -1230,19 +1218,7 @@ impl<A: Step> DoubleEndedIterator for ops::RangeInclusive<A> {
         self.spec_try_rfold(init, f)
     }
 
-    #[inline]
-    fn rfold<B, F>(mut self, init: B, f: F) -> B
-    where
-        Self: Sized,
-        F: FnMut(B, Self::Item) -> B,
-    {
-        #[inline]
-        fn ok<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> Result<B, !> {
-            move |acc, x| Ok(f(acc, x))
-        }
-
-        self.try_rfold(init, ok(f)).unwrap()
-    }
+    impl_fold_via_try_fold! { rfold -> try_rfold }
 }
 
 // Safety: See above implementation for `ops::Range<A>`
diff --git a/library/core/src/iter/traits/collect.rs b/library/core/src/iter/traits/collect.rs
index 12ca508be..e099700e3 100644
--- a/library/core/src/iter/traits/collect.rs
+++ b/library/core/src/iter/traits/collect.rs
@@ -228,6 +228,7 @@ pub trait FromIterator<A>: Sized {
 #[rustc_diagnostic_item = "IntoIterator"]
 #[rustc_skip_array_during_method_dispatch]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[const_trait]
 pub trait IntoIterator {
     /// The type of the elements being iterated over.
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -263,7 +264,7 @@ pub trait IntoIterator {
 
 #[rustc_const_unstable(feature = "const_intoiterator_identity", issue = "90603")]
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<I: ~const Iterator> const IntoIterator for I {
+impl<I: Iterator> const IntoIterator for I {
     type Item = I::Item;
     type IntoIter = I;
 
diff --git a/library/core/src/iter/traits/iterator.rs b/library/core/src/iter/traits/iterator.rs
index b2d08f4b0..789a87968 100644
--- a/library/core/src/iter/traits/iterator.rs
+++ b/library/core/src/iter/traits/iterator.rs
@@ -692,7 +692,7 @@ pub trait Iterator {
     /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
     /// assert_eq!(it.next(), Some(NotClone(1)));  // The next element from `v`.
     /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
-    /// assert_eq!(it.next(), Some(NotClone(2)));  // The last element from from `v`.
+    /// assert_eq!(it.next(), Some(NotClone(2)));  // The last element from `v`.
     /// assert_eq!(it.next(), None);               // The iterator is finished.
     /// ```
     ///
@@ -2431,22 +2431,13 @@ pub trait Iterator {
     ///
     /// # Example
     ///
-    /// Find the maximum value:
-    ///
     /// ```
-    /// fn find_max<I>(iter: I) -> Option<I::Item>
-    ///     where I: Iterator,
-    ///           I::Item: Ord,
-    /// {
-    ///     iter.reduce(|accum, item| {
-    ///         if accum >= item { accum } else { item }
-    ///     })
-    /// }
-    /// let a = [10, 20, 5, -23, 0];
-    /// let b: [u32; 0] = [];
+    /// let reduced: i32 = (1..10).reduce(|acc, e| acc + e).unwrap();
+    /// assert_eq!(reduced, 45);
     ///
-    /// assert_eq!(find_max(a.iter()), Some(&20));
-    /// assert_eq!(find_max(b.iter()), None);
+    /// // Which is equivalent to doing it with `fold`:
+    /// let folded: i32 = (1..10).fold(0, |acc, e| acc + e);
+    /// assert_eq!(reduced, folded);
     /// ```
     #[inline]
     #[stable(feature = "iterator_fold_self", since = "1.51.0")]
@@ -2906,14 +2897,14 @@ pub trait Iterator {
     /// Stopping at the first `true`:
     ///
     /// ```
-    /// let a = [1, 2, 3];
+    /// let a = [-1, 2, 3, 4];
     ///
     /// let mut iter = a.iter();
     ///
-    /// assert_eq!(iter.rposition(|&x| x == 2), Some(1));
+    /// assert_eq!(iter.rposition(|&x| x >= 2), Some(3));
     ///
     /// // we can still use `iter`, as there are more elements.
-    /// assert_eq!(iter.next(), Some(&1));
+    /// assert_eq!(iter.next(), Some(&-1));
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -3461,36 +3452,27 @@ pub trait Iterator {
     /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (2 * x).cmp(&y)), Ordering::Greater);
     /// ```
     #[unstable(feature = "iter_order_by", issue = "64295")]
-    fn cmp_by<I, F>(mut self, other: I, mut cmp: F) -> Ordering
+    fn cmp_by<I, F>(self, other: I, cmp: F) -> Ordering
     where
         Self: Sized,
         I: IntoIterator,
         F: FnMut(Self::Item, I::Item) -> Ordering,
     {
-        let mut other = other.into_iter();
-
-        loop {
-            let x = match self.next() {
-                None => {
-                    if other.next().is_none() {
-                        return Ordering::Equal;
-                    } else {
-                        return Ordering::Less;
-                    }
-                }
-                Some(val) => val,
-            };
-
-            let y = match other.next() {
-                None => return Ordering::Greater,
-                Some(val) => val,
-            };
-
-            match cmp(x, y) {
-                Ordering::Equal => (),
-                non_eq => return non_eq,
+        #[inline]
+        fn compare<X, Y, F>(mut cmp: F) -> impl FnMut(X, Y) -> ControlFlow<Ordering>
+        where
+            F: FnMut(X, Y) -> Ordering,
+        {
+            move |x, y| match cmp(x, y) {
+                Ordering::Equal => ControlFlow::CONTINUE,
+                non_eq => ControlFlow::Break(non_eq),
             }
         }
+
+        match iter_compare(self, other.into_iter(), compare(cmp)) {
+            ControlFlow::Continue(ord) => ord,
+            ControlFlow::Break(ord) => ord,
+        }
     }
 
     /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
@@ -3546,36 +3528,27 @@ pub trait Iterator {
     /// );
     /// ```
     #[unstable(feature = "iter_order_by", issue = "64295")]
-    fn partial_cmp_by<I, F>(mut self, other: I, mut partial_cmp: F) -> Option<Ordering>
+    fn partial_cmp_by<I, F>(self, other: I, partial_cmp: F) -> Option<Ordering>
     where
         Self: Sized,
         I: IntoIterator,
         F: FnMut(Self::Item, I::Item) -> Option<Ordering>,
     {
-        let mut other = other.into_iter();
-
-        loop {
-            let x = match self.next() {
-                None => {
-                    if other.next().is_none() {
-                        return Some(Ordering::Equal);
-                    } else {
-                        return Some(Ordering::Less);
-                    }
-                }
-                Some(val) => val,
-            };
-
-            let y = match other.next() {
-                None => return Some(Ordering::Greater),
-                Some(val) => val,
-            };
-
-            match partial_cmp(x, y) {
-                Some(Ordering::Equal) => (),
-                non_eq => return non_eq,
+        #[inline]
+        fn compare<X, Y, F>(mut partial_cmp: F) -> impl FnMut(X, Y) -> ControlFlow<Option<Ordering>>
+        where
+            F: FnMut(X, Y) -> Option<Ordering>,
+        {
+            move |x, y| match partial_cmp(x, y) {
+                Some(Ordering::Equal) => ControlFlow::CONTINUE,
+                non_eq => ControlFlow::Break(non_eq),
             }
         }
+
+        match iter_compare(self, other.into_iter(), compare(partial_cmp)) {
+            ControlFlow::Continue(ord) => Some(ord),
+            ControlFlow::Break(ord) => ord,
+        }
     }
 
     /// Determines if the elements of this [`Iterator`] are equal to those of
@@ -3613,29 +3586,26 @@ pub trait Iterator {
     /// assert!(xs.iter().eq_by(&ys, |&x, &y| x * x == y));
     /// ```
     #[unstable(feature = "iter_order_by", issue = "64295")]
-    fn eq_by<I, F>(mut self, other: I, mut eq: F) -> bool
+    fn eq_by<I, F>(self, other: I, eq: F) -> bool
     where
         Self: Sized,
         I: IntoIterator,
         F: FnMut(Self::Item, I::Item) -> bool,
     {
-        let mut other = other.into_iter();
-
-        loop {
-            let x = match self.next() {
-                None => return other.next().is_none(),
-                Some(val) => val,
-            };
-
-            let y = match other.next() {
-                None => return false,
-                Some(val) => val,
-            };
-
-            if !eq(x, y) {
-                return false;
+        #[inline]
+        fn compare<X, Y, F>(mut eq: F) -> impl FnMut(X, Y) -> ControlFlow<()>
+        where
+            F: FnMut(X, Y) -> bool,
+        {
+            move |x, y| {
+                if eq(x, y) { ControlFlow::CONTINUE } else { ControlFlow::BREAK }
             }
         }
+
+        match iter_compare(self, other.into_iter(), compare(eq)) {
+            ControlFlow::Continue(ord) => ord == Ordering::Equal,
+            ControlFlow::Break(()) => false,
+        }
     }
 
     /// Determines if the elements of this [`Iterator`] are unequal to those of
@@ -3860,6 +3830,46 @@ pub trait Iterator {
     }
 }
 
+/// Compares two iterators element-wise using the given function.
+///
+/// If `ControlFlow::CONTINUE` is returned from the function, the comparison moves on to the next
+/// elements of both iterators. Returning `ControlFlow::Break(x)` short-circuits the iteration and
+/// returns `ControlFlow::Break(x)`. If one of the iterators runs out of elements,
+/// `ControlFlow::Continue(ord)` is returned where `ord` is the result of comparing the lengths of
+/// the iterators.
+///
+/// Isolates the logic shared by ['cmp_by'](Iterator::cmp_by),
+/// ['partial_cmp_by'](Iterator::partial_cmp_by), and ['eq_by'](Iterator::eq_by).
+#[inline]
+fn iter_compare<A, B, F, T>(mut a: A, mut b: B, f: F) -> ControlFlow<T, Ordering>
+where
+    A: Iterator,
+    B: Iterator,
+    F: FnMut(A::Item, B::Item) -> ControlFlow<T>,
+{
+    #[inline]
+    fn compare<'a, B, X, T>(
+        b: &'a mut B,
+        mut f: impl FnMut(X, B::Item) -> ControlFlow<T> + 'a,
+    ) -> impl FnMut(X) -> ControlFlow<ControlFlow<T, Ordering>> + 'a
+    where
+        B: Iterator,
+    {
+        move |x| match b.next() {
+            None => ControlFlow::Break(ControlFlow::Continue(Ordering::Greater)),
+            Some(y) => f(x, y).map_break(ControlFlow::Break),
+        }
+    }
+
+    match a.try_for_each(compare(&mut b, f)) {
+        ControlFlow::Continue(()) => ControlFlow::Continue(match b.next() {
+            None => Ordering::Equal,
+            Some(_) => Ordering::Less,
+        }),
+        ControlFlow::Break(x) => x,
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<I: Iterator + ?Sized> Iterator for &mut I {
     type Item = I::Item;