[vitalyd]

> Concurrency is not special here I beg to differ. Concurrency comes with its own bag of hazards, as I mentioned in my reply to burntsushi. Comparing its invariants with Vec's length invariant/HashMap's RH invariant, and any other single threaded/internal invariants misses the point.

Unsafe discussions that I've seen rarely talk about fences - they tend to focus on raw pointer ops, ffi, transmutes, unbound lifetimes, and in general, are single thread focused.

[Manishearth]

Right, but I can make the same point about other invariants. Each comes with its own bag of hazards. You can write pages about the robin hood invariants.

Concurrency is particularly complex, perhaps. I think one of the reasons you don't see that much discussion of this is that in general folks in Rust don't write that many internally-unsafe concurrent abstractions. There are a bunch of great safe building blocks out there (stdlib ones, rayon, crossbeam) which folks use for concurrency; it's very rare to build your own. So that might be it.

At least with the stuff I work on like 50% of the unsafe Rust discussions have been around thread safety and ordering and fences, but we're in that relatively rare situation where we need to build those abstractions, so perhaps it's just me who sees these discussions happening.

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It's also probably just that discussions introducing unsafe will deal with problems people are used to -- and memory safety is a far more "normal" problem than thread safety.