Soundness conflicts

[RalfJung]

Sometimes it happens that two unsafe-using libraries are sound in isolation, but unsound when combined. Each time that happens, Rust has to decide which side to consider sound. Ultimately this boils down to precisely specifying the safety invariants of all our types -- but doing that requires something like RustBelt, so it is hard to have that discussion in full generality. (Also that would involve T-types at least as much as WG-UCG / T-opsem.)

But meanwhile, it would be good to collect the cases of these conflicts here that we find out there in the wild.

The most famous case of this is of course leakpocalypse: Rc vs pre-Rust-1.0-scoped-threads, which famously got decided in favor of Rc (and mem::forget). Another case is that without union and ManuallyDrop, josephine would be sound. Again the resolution for the ecosystem is clearly in favor of unions and ManuallyDrop.

The point of this thread is not to discuss any of these conflicts and figure out which side we want to bless. It is solely to collect the known cases in a central location.

• take_mut / replace_with vs partial-borrow: see here for details and discussion.
• mk_static vs Stack Tokens: see reddit for details.
• Pin is seriously under-specified and probably has quite a few of these conflicts, here is one.
• "Partial move out of Drop type" vs a pattern that assumes that exactly that does not happen (coming up in this RFC).
• TLS vs stackful coroutines.

[RalfJung]

I am pretty sure I saw another case of this recently, but forgot where...

[RalfJung]

This thread is not meant to discuss concrete soundness conflicts, only to collect them. Discussing them all in the same thread would be a big mess.^^

I've opened an issue for the TLS discussion: #484.
Please re-post there; sadly github doesn't allow moving comments. I'm going to lock this issue as otherwise it seems people will inevitably start discussing here, no matter how much one has bold text in the issue description saying not to do that. (No hard feelings, that comment box Github offers is very tempting.) If you have something to add to the list, please mention it on Zulip.