Provide slightly better notes when tracking a pointer tag

[saethlin]

I slapped this in as a sort of advanced println-based debugging when trying to figure out a track-raw-pointers finding in smallvec. Perhaps this looks like a good idea to you all?

EDIT: User scenario

Run MIRIFLAGS=-Ztag-raw-pointers cargo miri test, get a diagnostic that looks like

error: Undefined Behavior: trying to reborrow for SharedReadOnly at alloc99465+0x9, but parent tag <265507> does not have an appropriate item in the borrow stack

So now run MIRIFLAGS=-Ztag-raw-pointers -Zmiri-track-pointer-tag=265507 cargo miri test
Old:

note: tracking was triggered
   --> src/lib.rs:822:36
    |
822 |                 vec: NonNull::from(self),
    |                                    ^^^^ popped tracked tag for item [SharedReadOnly for <265507>]

New:

note: tracking was triggered
   --> src/lib.rs:822:36
    |
822 |                 vec: NonNull::from(self),
    |                                    ^^^^ popped tracked tag for item [SharedReadOnly for <265507>] due to Write access for <265356>

So that if a user is now beginning to question their sanity because they don't really understand SB yet, they can then track the tag which caused the parent tag to be removed from the stack to be sure what's going on here:

   --> src/lib.rs:792:5
    |
792 | /     pub fn drain<R>(&mut self, range: R) -> Drain<'_, A>
793 | |     where
794 | |         R: RangeBounds<usize>,
795 | |     {
...   |
824 | |         }
825 | |     }
    | |_____^ created tag 265356

The existing diagnostic can tell you where the tag you'd need was invalidated, but it cannot tell you what and why that tag was invalidated.

[RalfJung]

What does the new output look like, compared to the old one? An example would be nice, to make it easier to evaluate the change.

[saethlin]

I've cleaned up the nomenclature a bit and elaborated on the user experience that made me want this in the opening comment.

[RalfJung]

I've cleaned up the nomenclature a bit and elaborated on the user experience that made me want this in the opening comment.

Ah I see, this is a case where it's also not clear from the code whether this is a read or write access. (NonNull::from(&*self) would possibly avoid invalidating the tag, if I guessed the involved types correctly.)

[saethlin]

Ah I see, this is a case where it's also not clear from the code whether this is a read or write access. (NonNull::from(&*self) would possibly avoid invalidating the tag, if I guessed the involved types correctly.)

Yup! I'm now understanding my own thinking a bit more: A lot of my use of Miri has been squinting at a diagnostic and trying to understand what exactly is causing there to be no borrow in the stack for a tag. The only way I could confirm my guess is by altering the code, and if Miri then passes, I just assume that my understanding was correct.

[RalfJung]

Looks good, thanks. :)
@bors r+

[bors]

📌 Commit cd69219 has been approved by RalfJung

[bors]

⌛ Testing commit cd69219 with merge 6f3061b...

[bors]

☀️ Test successful - checks-actions
Approved by: RalfJung
Pushing 6f3061b to master...