From 94ecabfc4eea1c58dfd412b00e4a518f0dc87431 Mon Sep 17 00:00:00 2001
From: Carter Hunt Fogelman <chfogelman@gmail.com>
Date: Thu, 26 Oct 2023 11:10:04 -0700
Subject: [PATCH] Add comment to mem::replace to explain why it's not
 implemented via mem::swap

---
 library/core/src/mem/mod.rs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/library/core/src/mem/mod.rs b/library/core/src/mem/mod.rs
index a79a204e2c6a0..7ef84b0f5b540 100644
--- a/library/core/src/mem/mod.rs
+++ b/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.