RefCell<T> where T: Copy and T in current crate

[pythonesque]

In this situation you can always replace it with finer-grained Cells, you'd only turn it off if you have some specific reason not to do that. In my experience this is usually a bug.

[lambda-fairy]

What's the reason for checking that T is in the current crate?

[pythonesque]

@lfairy If it's not in the current crate, it's possible that you can't refactor a type that is Copy, but large, by making smaller elements Copy instead. If you can't do that, it's possible (albeit unlikely) that the overhead of copying the entire type would make RefCell attractive. Maybe this lint could be refined so that it warns unless the type is "large" and not in the current crate, but I'm not sure how much of that information is actually available at lint time.