Comment by vitalyd

I think it's fair to question a type being auto-derived to be Sync if only individual fields are Sync. It may lead to improper sharing of a reference to this value where threadsafety across fields is needed. That would be a bug, yes, but compiler never made the author pause to consider putting Sync there explicitly (and presumably thinking this through). So that "linting" aspect that's applied to, e.g., *mut T is not present.