Rollup merge of rust-lang#122601 - joboet:ptr_replace, r=workingjubilee

Optimize `ptr::replace`

rust-lang#83022 optimized `mem::replace` to reduce the number of `memcpy`s. `ptr::replace`, which is [documented to behave just like `mem::replace`](https://doc.rust-lang.org/nightly/std/ptr/fn.replace.html), was not optimized however, leading to [worse code](https://godbolt.org/z/T3hdEEdfe) and missed optimizations. This PR simply forwards `ptr::replace` to `mem::replace` to take advantage of the better implementation.

Author: fmease

core/src/ptr/mod.rs

@@ -1114,7 +1114,7 @@ const unsafe fn swap_nonoverlapping_simple_untyped<T>(x: *mut T, y: *mut T, coun
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_unstable(feature = "const_replace", issue = "83164")]
 #[rustc_diagnostic_item = "ptr_replace"]
-pub const unsafe fn replace<T>(dst: *mut T, mut src: T) -> T {
+pub const unsafe fn replace<T>(dst: *mut T, src: T) -> T {
     // SAFETY: the caller must guarantee that `dst` is valid to be
     // cast to a mutable reference (valid for writes, aligned, initialized),
     // and cannot overlap `src` since `dst` must point to a distinct
@@ -1128,9 +1128,8 @@ pub const unsafe fn replace<T>(dst: *mut T, mut src: T) -> T {
                 align: usize = align_of::<T>(),
             ) => is_aligned_and_not_null(addr, align)
         );
-        mem::swap(&mut *dst, &mut src); // cannot overlap
+        mem::replace(&mut *dst, src)
     }
-    src
 }
 
 /// Reads the value from `src` without moving it. This leaves the