Since the proc_macro implementation explicity declares an extern num_traits crate, any crate that wants to use it must also depend on num-traits.

This can be a problem in case a crate reexport a num-derive proc macros because it forces the user to also depend on num-traits otherwise it produces: error[E0463]: can't find crate for _num_traits`.

Would it be possible to add a feature flag that disable this behavior? Specifically it would be the user's responsability to make sure that the num-traits crate is in scope.