> 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.

Only because I've had several people ask me how Rust handles memory ordering after they've learned of Sync. Sync documentation focuses very narrowly on there being no data races against the type that implements it. There's also obviously a difference between a struct consisting of a single AtomicUInt (for example) and a struct consisting of a plain uint. The former can be published unsafely because it internally already provides the appropriate fences by virtue of the atomic. In the latter, it requires the publishing to provide the necessary fences. Again, I don't mean to say that doesn't make sense or is unexpected (after one thinks about it), but I'd urge a bit more focus on that part as well.

Given there's no official memory model, similar to say Java Memory Model, there's not much to go by (correct me if I'm wrong). The JMM, for instance, spells out what needs to happen for happens-before edges to be created. It also talks about racy publication of values. Granted there's no close analog to Rust's unsafe in Java. But saying "T: Sync means it's free of data races in safe code", while correct, is a slightly vacuous statement since that T interacts with other components. And yes, those components will likely involve unsafe code, and unsafe code has its own caveats, but still, I don't think it hurts to make this more pronounced. Concurrency is a special beast, rife with its own hard-to-debug hazards. Being a bit more verbose and possibly repetitive about the hazards won't hurt :).