Safe trasnsmute for slices; e.g. &[u64] -> &[u32], particularly SIMD types

[briansmith]

@comex

I'd say that ship has sailed. The "underspecified language semantics" section of that RFC does list "the legal aliasing rules between unsafe pointers", but the sort of unsafe casting found in the OP is useful often enough that I would be surprised if there wasn't a significant amount of real code depending on it by this point. As an example, bindgen currently generates it for unions.

I agree. TBAA would be a backward incompatible change and thus seems unlikely.