Safety of scheme for downcasting data with lifetime

[LegionMammal978]

@quinedot

It has some of the concerns as static mut -- i.e. the mere existence of two aliasing &mut is technically UB. If I follow your safety guidelines for &'_ mut i32 for example, your code has that situation for a brief period when transforming into/out-of DynBox .

I suppose you're referring to the simultaneous ManuallyDrop<&mut i32> and returned &mut i32 in transmute. But as I understand it, creating the second reference only makes the reference in the ManuallyDrop an invalid value. Since we destruct the ManuallyDrop without touching it again, we never have to assert the first reference's validity; otherwise, methods like ManuallyDrop::drop() would be wildly unsafe. I've depended on this property myself for dropping a ManuallyDrop<Box<_>> within a Drop impl, and Ralf Jung seemed to tentatively agree with the reasoning.