[Manishearth]

I don't think it's vacuous. It's very important for compartmentalizing unsafe. It interacts with other components, and that statement tells you the responsibilities of both T implementing Sync, and abstractions accepting Sync types.

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!