amend RFC 769 (sound generic drop) re: drop via `&mut T` overwrite

[pnkfelix]

spawned off of: #769 (comment) :

@glaebhoerl notes:

The impl Drop for T can also be invoked through an &mut T, e.g. assignment through it invokes the destructor on the previous value. This possibility doesn't appear to be covered by "the type of v owns data of type T".

@pnkfelix replies:

any destructor that actually writes to a contained &'a mut T or &'a RefCell<T> within a value v will need to either:

• be attached to a type that is parametric in 'a (which will force 'a to strictly outlive v, and thus I think will force T also to strictly outlive v), or
• be hidden behind a generic type with some method that does the write to &'a mut T or &'a RefCell<T> -- but again, I think we will again here see the 'a leak up and end up being forced to strictly outlive v?

[Centril]

Triage: @pnkfelix what's the status of this?

[pnkfelix]

nothings been done. At this point I guess the real place to file such an issue would be the nomicon.