Auto merge of rust-lang#111803 - scottmcm:simple-swap-alternative, r=Mark-Simulacrum

Tweak the threshold for chunked swapping

Thanks to `@AngelicosPhosphoros` for the tests here, which I copied from rust-lang#98892.

This is an experiment as a simple alternative to that PR that just tweaks the existing threshold, since that PR showed that 3×Align (like `String`) currently doesn't work as well as it could.

Author: bors

core/src/mem/mod.rs

@@ -736,7 +736,7 @@ pub const fn swap<T>(x: &mut T, y: &mut T) {
         // tends to copy the whole thing to stack rather than doing it one part
         // at a time, so instead treat them as one-element slices and piggy-back
         // the slice optimizations that will split up the swaps.
-        if size_of::<T>() / align_of::<T>() > 4 {
+        if const { size_of::<T>() / align_of::<T>() > 2 } {
             // SAFETY: exclusive references always point to one non-overlapping
             // element and are non-null and properly aligned.
             return unsafe { ptr::swap_nonoverlapping(x, y, 1) };