Rollup merge of #117243 - chfogelman:replace-not-swap-comment, r=thomcc

Explain implementation of mem::replace

This adds a comment to explain why `mem::replace` is not implemented in terms of `mem::swap` to prevent [naïfs like me](https://github.com/rust-lang/rust/pull/117189) from trying to "fix" it.

library/core/src/mem/mod.rs

@@ -909,6 +909,10 @@ pub fn take<T: Default>(dest: &mut T) -> T {
 #[rustc_const_unstable(feature = "const_replace", issue = "83164")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "mem_replace")]
 pub const fn replace<T>(dest: &mut T, src: T) -> T {
+    // It may be tempting to use `swap` to avoid `unsafe` here. Don't!
+    // The compiler optimizes the implementation below to two `memcpy`s
+    // while `swap` would require at least three. See PR#83022 for details.
+
     // SAFETY: We read from `dest` but directly write `src` into it afterwards,
     // such that the old value is not duplicated. Nothing is dropped and
     // nothing here can panic.