From 4064a6bed2ee5675fb40f719b165b297efba33fb Mon Sep 17 00:00:00 2001
From: Yotam Ofek <yotam.ofek@gmail.com>
Date: Sun, 16 Feb 2025 10:14:53 +0000
Subject: [PATCH 1/2] Specialize `Iterator::{eq|cmp|partial_cmp}_by` for
 `TrustedLen` iterators

---
 library/core/src/iter/traits/iterator.rs | 42 ++++++++++++++++++++++--
 1 file changed, 39 insertions(+), 3 deletions(-)

diff --git a/library/core/src/iter/traits/iterator.rs b/library/core/src/iter/traits/iterator.rs
index 42886e90f997d..41670392a4e68 100644
--- a/library/core/src/iter/traits/iterator.rs
+++ b/library/core/src/iter/traits/iterator.rs
@@ -4,6 +4,7 @@ use super::super::{
     Product, Rev, Scan, Skip, SkipWhile, StepBy, Sum, Take, TakeWhile, TrustedRandomAccessNoCoerce,
     Zip, try_process,
 };
+use super::TrustedLen;
 use crate::array;
 use crate::cmp::{self, Ordering};
 use crate::num::NonZero;
@@ -3615,7 +3616,7 @@ pub trait Iterator {
             }
         }
 
-        match iter_compare(self, other.into_iter(), compare(cmp)) {
+        match SpecIterCompare::spec_iter_compare(self, other.into_iter(), compare(cmp)) {
             ControlFlow::Continue(ord) => ord,
             ControlFlow::Break(ord) => ord,
         }
@@ -3707,7 +3708,7 @@ pub trait Iterator {
             }
         }
 
-        match iter_compare(self, other.into_iter(), compare(partial_cmp)) {
+        match SpecIterCompare::spec_iter_compare(self, other.into_iter(), compare(partial_cmp)) {
             ControlFlow::Continue(ord) => Some(ord),
             ControlFlow::Break(ord) => ord,
         }
@@ -3762,7 +3763,7 @@ pub trait Iterator {
             }
         }
 
-        match iter_compare(self, other.into_iter(), compare(eq)) {
+        match SpecIterCompare::spec_iter_compare(self, other.into_iter(), compare(eq)) {
             ControlFlow::Continue(ord) => ord == Ordering::Equal,
             ControlFlow::Break(()) => false,
         }
@@ -3984,6 +3985,41 @@ pub trait Iterator {
     }
 }
 
+trait SpecIterCompare<B: Iterator>: Iterator {
+    fn spec_iter_compare<F, T>(self, b: B, f: F) -> ControlFlow<T, Ordering>
+    where
+        F: FnMut(Self::Item, B::Item) -> ControlFlow<T>;
+}
+
+impl<A: Iterator, B: Iterator> SpecIterCompare<B> for A {
+    #[inline]
+    default fn spec_iter_compare<F, T>(self, b: B, f: F) -> ControlFlow<T, Ordering>
+    where
+        F: FnMut(A::Item, <B as Iterator>::Item) -> ControlFlow<T>,
+    {
+        iter_compare(self, b, f)
+    }
+}
+
+impl<A: Iterator + TrustedLen, B: Iterator + TrustedLen> SpecIterCompare<B> for A {
+    #[inline]
+    fn spec_iter_compare<F, T>(self, b: B, f: F) -> ControlFlow<T, Ordering>
+    where
+        F: FnMut(Self::Item, <B as Iterator>::Item) -> ControlFlow<T>,
+    {
+        if let (_, Some(a)) = self.size_hint()
+            && let (_, Some(b)) = b.size_hint()
+        {
+            let ord = a.cmp(&b);
+            if ord != Ordering::Equal {
+                return ControlFlow::Continue(ord);
+            }
+        }
+
+        iter_compare(self, b, f)
+    }
+}
+
 /// Compares two iterators element-wise using the given function.
 ///
 /// If `ControlFlow::Continue(())` is returned from the function, the comparison moves on to the next

From d9f58d45d67bc8bdb3283f1691263d7dc0269065 Mon Sep 17 00:00:00 2001
From: Yotam Ofek <yotam.ofek@gmail.com>
Date: Sun, 16 Feb 2025 11:50:38 +0000
Subject: [PATCH 2/2] This optimization can't be applied to
 `{partial_}cmp{_by}`

---
 library/core/src/iter/traits/iterator.rs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/library/core/src/iter/traits/iterator.rs b/library/core/src/iter/traits/iterator.rs
index 41670392a4e68..3f0cf04648bbc 100644
--- a/library/core/src/iter/traits/iterator.rs
+++ b/library/core/src/iter/traits/iterator.rs
@@ -3616,7 +3616,7 @@ pub trait Iterator {
             }
         }
 
-        match SpecIterCompare::spec_iter_compare(self, other.into_iter(), compare(cmp)) {
+        match iter_compare(self, other.into_iter(), compare(cmp)) {
             ControlFlow::Continue(ord) => ord,
             ControlFlow::Break(ord) => ord,
         }
@@ -3708,7 +3708,7 @@ pub trait Iterator {
             }
         }
 
-        match SpecIterCompare::spec_iter_compare(self, other.into_iter(), compare(partial_cmp)) {
+        match iter_compare(self, other.into_iter(), compare(partial_cmp)) {
             ControlFlow::Continue(ord) => Some(ord),
             ControlFlow::Break(ord) => ord,
         }