Leak checker misses pointers cast to integer

[RalfJung]

The leak checker considers allocations live when they can still be reached from a static. However, this scan can only consider pointers that are still stored as pointer values (with provenance), not pointers cast to an integer (where provenance is lost).

This causes false positives, at least, for

• Windows Mutex (Enable leak checker on Windows #1302)
• Any user of AtomicPtr
  • crossbeam (Memory leak from crossbeam_epoch::deferred::Deferred::new crossbeam-rs/crossbeam#464)
  • Leak checker doesn't see through AtomicPtr #1350

[RalfJung]

For the Windows Mutex, one option on the Miri side is to keep provenance on ptr-to-int casts, and only delete provenance later when the integer is actually used for an integer operation. I am not extremely happy about this but it is also not entirely awful. This is blocked on #841.

---

For crossbeam, it turns out the pointer is not just cast to an integer, it also has its last bits repurposed to store some flags. So, even if we do the above, we are still going to lose the provenance.

I would say trying to preserve provenance across these bitwise operations is conceptually just not the kind of semantics that Miri should give to Rust. It's also a game of whack-a-mole to get ever more effective at preserving provenance -- a game that the proper support for ptr-int-casts that we have now is supposed to just end, so I do not have a big interest in re-opening that game.

Another option might be to expect libraries that pull such tricks (storing the pointers that keep an allocation live without provenance) to give us some extra hints, via some Miri-specific instrumentation/intrinsics or so. (I recall @oli-obk proposed such operations a while ago for a different purpose and I found the idea quite disgusting. ;) However, the code doing the casting might not even be aware that it is working on data stored in a global static, so it is unclear how such an API could even look like.

Cc @jonhoo

[jonhoo]

I think if there were annotations libraries like crossbeam could give to miri to get tracking to work in these situations, they would be happy to add those :) At least I would be!