show warning when Stacked Borrows skips a reborrow due to 'extern type'

[RalfJung]

When this happens, we can't actually be sure to catch all bugs -- LLVM will still get a noalias but Miri can't do reborrowing. That's not good.

Unfortunately it seems like the formatting machinery in std relies on extern type. :( So this can't land in its current shape.

[bors]

:umbrella: The latest upstream changes (presumably #3706) made this pull request unmergeable. Please resolve the merge conflicts.

[RalfJung]

With https://github.com/rust-lang/rust/pull/127859, this doesn't show any warnings for standard library code any more. :)

@rust-lang/miri what do you think?

[saethlin]

I think you want to update the PR description a bit. I support having a warning for this, and won't complain much if you prefer your current wording :p

[RalfJung]

All right, let's go with this then. :)

@bors r=saethlin

[bors]

:pushpin: Commit 9aa0893e0f371b962388fc2b1fc5351ebe5baf79 has been approved by saethlin

It is now in the queue for this repository.

[bors]

:hourglass: Testing commit 9aa0893e0f371b962388fc2b1fc5351ebe5baf79 with merge b10b02fc1e0b134b8aa9acdb10f170f426c97df9...

[bors]

:sunny: Test successful - checks-actions Approved by: saethlin Pushing b10b02fc1e0b134b8aa9acdb10f170f426c97df9 to master...