Add derivation of Copy trait for each proxy

[ubnt-intrepid]

No description provided.

[ubnt-intrepid]

It is impossible to derive Copy trait for proxy because Rc doesn't have a implementation for Copy (in current implementation).

An alternative is using raw pointer instead of smart pointer. But it is bad situation because it may cause unexpected behaviours related to "unsafe" implementation.

[leoyvens]

They need to be Rc because indexes may change. What if the API gave never changing indexes to the variables (which are Copy) and then mapped those to the real indexes (Rc and may change)?

[ubnt-intrepid]

It sounds a good idea, but their immediate implementation is difficult because the change of internal structure of Model is required.

I'm preparing for next release, so do you mind this matter be a little deferred?

[ubnt-intrepid]

draft: https://gist.github.com/ys-nuem/beb69d252993e97b31e0a7cc8f5cddd6

[leoyvens]

Thanks for taking this into consideration, I don't mind at all, you're doing an amazing work with this lib!

[ubnt-intrepid]

Another alternative is that all methods of Model make "immutable" (receives &self instead of &mut self). It means that the instance of Model will be treated as an "immutable" object (on the surface). All instances of Proxy can borrow their own indices from the model without violating the Rust's borrowing rule.

[ubnt-intrepid]

Above scheme is failure because RefCell<T> throws a panic when illegal borrowing is occurred. The avoidance is to use UnsafeCell<T>, but it means that the implementation requires more "unsafe" code.

I think the benefit obtained by implementing Copy trait is less than the implementation cost to avoid Rust's borrowing rule. So I decide not to implement this feature at the present stage.

[ubnt-intrepid]

I'd misconceptions about the behavior of RefCell ... I'll try again this issue :)