Because in branchless quicksort, no matter how long it takes to figure out where the element has to go relative to the pivot it doesn't obstruct the CPU's ability in this algorithm to start working on the next element the next cycle.

So a complement to the likely/unlikely annotations?

It probably depends on a core language accommodation because bitwise-swapping objects, or even primitive elements of objects, with constructors is UB.

Arthur O'Dwyer's P1144 https://wg21.link/p1144 explores that territory too.

Gcc has weird limits on generating CMOV instructions to implement this sort of optimization.

But running industrially important generic algorithms twice as fast as C++, automatically and without heroics, would be a Feather in Rust's Cap, and a Black Eye for C++. ("!!!!", as it were.) Or vice versa. Best would be if both got it, of course.

At present, both languages need heroics. I don't know of any other language where there is a reasonable prospect of getting such an optimization implemented generically, but that doesn't mean there aren't any.