This function seems pretty niche, not actually that hard to implement as unsafe code goes, and it seems doubtful it is going to have a lot of users, while it will add more noise to every single instance of the slice method documentation which is transcluded into... many other types. Is it really appropriate for std?

And if it was to make it easier for performance wins, it seems an unfounded assumption based on the idea that unsafe code is inherently faster (as opposed to potentially a blocker for optimization). The attempt to use it in std to implement swap was itself reverted.