Stacked Borrows cannot properly handle extern type

[RalfJung]

While working on rust-lang/miri#1725, I realized that Stacked Borrows does not really work properly when extern type is used: on a reborrow, the newly created reference is only valid for the memory range that Miri can actually know belongs to this reference. So a reference with an extern type tail would be reborrowed in a way that that tail becomes inaccessible. This is incompatible with e.g. how the standard library format string machinery uses extern type as its implementation for Opaque.

In a sense this is a variant of #134, but it felt worth pointing out separately.

[CAD97]

#256 seems also worth noting as related. (It seems a more dynamic concept of how much memory pointers have provenance over would improve a lot of use cases...)

[RalfJung]

Yeah, #256 is essentially a duplicate of #134.

(It seems a more dynamic concept of how much memory pointers have provenance over would improve a lot of use cases...)

Very much agreed!