Auto merge of rust-lang#137122 - yotamofek:pr/std/iter-eq-exact-size, r=<try>

Specialize `Iterator::eq{_by}` for `TrustedLen` iterators

I'm sure I got some stuff wrong here, but opening this to get feedback and make sure it's a viable idea at all.

### Motivation
I had a piece of code that open-coded `Iterator::eq`, something like:
```rust
if current.len() != other.len()
    || current.iter().zip(other.iter()).any(|(a, b)| a != b) { ... }
```
... where both `current` and `other` are slices of the same type.
Changing the code to use `current.iter().eq(other)` made it a lot slower, since it wasn't checking the length of the two slices beforehand anymore, which in this instance made a big difference in perf. So I thought I'd see if I can improve `Iterator::eq`.

### Questions
1. I can't specialize for `ExactSizeIterator`, I think it's a limitation of `min_specialization` but not sure exactly why. Is specializing for `TrustedLen` good enough?
2. Should I make a codegen test for this? If so, then how? (I manually checked the assembly to make sure it works as expected)
3. Where should I put `SpecIterCompare`?
4. Can I get a perf run for this, please? I think the compiler uses this in a few places, so it might have an affect.

Author: bors

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;
@@ -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