cw_multi_test: accept closures in new()

[dakom]

The current implementation makes two restrictive decisions:

1. The only way to create a wrapper is to pass function pointers (can't pass closures or any ol impl Fn)
2. All the fields are private

The internal new() creator is able to create a closure from the passed-in function pointer, but the current setup disallows the caller to do this too. e.g. it may want to wrap functions before passing them in.

Suggestion: change the new() functions to accept any impl Fn

edit: the Contract trait is pub so making the fields on ContractWrapper wouldn't add much. The real blocker is using the simple new() methods with closures being passed in

[hashedone]

The problem is that implementation if ContractWrapper is not trivial, it would require it to be generic over passed handlers types (right now it is generic over magic internals). I think it is doable, I will take a look at it next week, but personally I would suggest just implementing your custom type implementing Contract instead of using ContractWrapper.

[dakom]

Yup! totally agree @hashedone - looks like it would require adding more generics everywhere, which would be painful on the library side (but would make the API nicer for users)

Also agree, implementing our own custom type is totally workable, the issue here should be thought of more as improving developer experience rather than actually blocking anything important.

[ethanfrey]

I would put a deep refactor of multi-test before this should be approached (or fix this as part of a refactor). We may be able to simplify the traits and generics if some big brains spend several days focused on it.