Stacked borrow implementation of miri may mistreat statics since they aren't places anymore

[oli-obk]

cc @spastorino @RalfJung

The way we represent statics and promoteds in MIR changed from them being an explicit place to them just being a constant that is a reference to the static (rust-lang/rust#67000). Since this was supported before anyway, we remove the special case of statics being referencable both via a Place and via a constant.

This change may have broken how stacked borrows process references to statics. We should run miri on a few examples and see whether the borrows assigned change in a significant way.

related discussion on zulip: https://rust-lang.zulipchat.com/#narrow/stream/146212-t-compiler.2Fconst-eval/topic/promoteds.20are.20not.20places

[RalfJung]

Looks like Miri's test suite still works fine...

I think what'll happen is that the embedded const pointers end up with their tag determined by static_base_ptr, which is exactly what we want: they count as "root pointers", and those (for statics) have SharedReadWrite permission.

[RalfJung]

Wait, rust-lang/rust#67000 didn't land yet, so what is the current status?

[oli-obk]

rust-lang/rust#67000 is for promoteds. statics have already landed

[RalfJung]

Oh I see. Do you have a PR reference for that?

[oli-obk]

rust-lang/rust#66587

[RalfJung]

How can that have landed without touching interpret at all...? There was a case somewhere there for Static places, right?

Oh I guess that still exists for promoteds?

[RalfJung]

Hm no, there's still this:

https://github.com/rust-lang/rust/blob/3982d3514efbb65b3efac6bb006b3fa496d16663/src/librustc_mir/interpret/place.rs#L625

[oli-obk]

Yea all of these are dead code to be removed by rust-lang/rust#67000

[RalfJung]

Well let's get back to this issue then once rust-lang/rust#67000 lands. Clearly right now everything is a mess.^^

[RalfJung]

Also I think it might be better to move this to the Miri repo. This is specifically about the Stacked Borrows implementation, which we track there. So I'll move it if I can.

[RalfJung]

rust-lang/rust#67000 has landed, and from all I can see Stacked Borrows for globals behaves exactly as it should. The miri engine uses tag_global_base_ptr to determine the tag when directly accessing a global; if there were untagged accesses to such globals we would notice (as in rust-lang/rust#70479).

So, closing this as everything seems just fine.