Add a protector test that demonstrates the base tag diagnostic

[saethlin]

Per #2519 (comment), this demonstrates this case for protector diagnostics:

help: <3131> was created here, as a base tag for alloc1623
  --> tests/fail/stacked_borrows/invalidate_against_protector3.rs:10:19
   |
10 |         let ptr = std::alloc::alloc(std::alloc::Layout::for_value(&0i32)) as *mut i32;
   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This diagnostic is inspired by what Miri used to do with rust-lang/rust#60076 (comment)

[RalfJung]

The diagnostic cannot just happen for protectors, right? At least that function that computes it seems to be more generic than just protectors.

[saethlin]

The diagnostic cannot just happen for protectors, right?

I think actually it can only happen for protectors. When the problem is that the used pointer has been invalidated, we print the creation span of the invalidated pointer, not the invalidating pointer. And since you can't invalidate the base tag, I don't think we'll ever print its span.

We also can't print the base tag's creation span for a use-outside-provenance issue, because the base tag is always valid for the whole allocation. And the base tag always has write permission.

[RalfJung]

I think actually it can only happen for protectors.

Ah, makes sense, thanks.

[RalfJung]

@bors r+

[bors]

📌 Commit 3cfb991 has been approved by RalfJung

It is now in the queue for this repository.

[bors]

⌛ Testing commit 3cfb991 with merge 2d50443...

[bors]

☀️ Test successful - checks-actions
Approved by: RalfJung
Pushing 2d50443 to master...