[Rusky]

You don't get two owners that way. You get something slightly less powerful than owners, which still has all its preconditions satisfied: you can check that it is in bounds, and if so you will find an element of the expected type. You cannot corrupt the underlying allocator's data structures, you cannot resize the array when there are outstanding pointers to its elements, and you cannot violate the type system.

Rust's goal was never to exclude all shared mutability. (Otherwise why support things like locks?) Rather, it excludes only the kinds of shared mutability that open the door to undefined behavior. The point of all these sorts of "workarounds" is that, because there is no longer a single unrestricted kind of shared mutability, you now get to pick which version of restricted mutability you want: reference counting vs bounds checking vs ghostcell's higher-rank lifetimes vs whatever else.