Map requires types that implement `Copy` about concache

[mmstick]

At least you could provide an additional set of get methods that takes a closure, such as get_with, so the option is available for larger types.

[jonhoo]

Ideally we wouldn't require Copy or Clone, but sadly they're required to provide the get interface we currently have (i.e., fn get(&self, key) -> V). Because of the nature of the concurrent map, we can't easily provide get(..) -> &V like traditional maps. The likely path forward here is to have get take a closure that is given &V, but that requires some more work on our part.

The reason we currently require Copy over Clone is to discourage users from using values that are expensive to clone in the map (since their get would become pretty slow). That might not be a good way to do so, but I think I'd rather see a move to get with a closure rather than moving to Clone. @andrewtshen @saligrama what do you think?

[jonhoo]

I completely agree! The codebase belongs to @andrewtshen and @saligrama though — I'm just mentoring them — so I'll let them make the final call.

[andrewtshen]

Yea, that definitely seems like a good addition, I'll try to add that if I have time