At a high and general level, yeah, it's all "unsafe". But, most conversations about unsafe don't talk about this aspect. So while what you say is true, I'm merely pointing out that this concurrency aspect doesn't seem to be mentioned much. And while it's implied at a high level, I think it's worth mentioning.
Basically, there's no issue - I just think this should be called out more when concurrency is discussed.
Concurrency is not special here. There are all kinds of invariants unsafe code might be required to uphold. So yeah, we could mention concurrency, but then we could also mention UTF8, noalias, initialization, the vector length invariant, the HashMap robin hood invariant, various BTreeMap invariants, etc etc. "Make sure you have fences" is just another semi-specific invariant.
I disagree that "most conversations about unsafe don't talk about this aspect", compartmentalizing unsafe invariants is a major part of these discussions (it's like the first chapter of the nomicon, even)
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.
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.
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 :).
It is vacuous at a big picture level where you're trying to understand the complete thread safety story, but that was never what that statement was trying to convey.
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I think documentation here can be improved, though. When I get more time one of the things I want to do is do a major revamp of the concurrency docs, including paragraphs on how the memory ordering stuff works. I'll include filling out the Sync docs in this, thanks for the feedback!
Sequential consistency is irrelevant to the example I gave of a single valued struct. I wasn't talking about any ordering with respect other loads/stores.
In any case, the only reason to think about publishing is how loads and stores are ordered, so it isn't irrelevant. The single valued struct can still have the partial initialisation problem, it just appears as the single field being completely uninitialised.
I know one can look at this as being unimportant because only unsafe code that works with Sync types carries the burden of upholding the invariants. But, I still contend that memory ordering ought to be discussed, somewhere, while there's no official memory model documentation (not even sure that's in the plans, although I feel like I may have come across a github issue for that).
Anyway, I don't think I'm saying anything new in this reply over my others. If you feel existing docs are adequate in this regard, that's fine - we can agree to disagree :).