Macro neli::impl_trait

macro_rules! impl_trait {
    ( $(#[$outer:meta])* $trait_name:ident, $to_from_ty:ty ) => { ... };
    ( $trait_name:ident, $to_from_ty:ty ) => { ... };
}

For generating a marker trait that flags a new enum as usable in a field that accepts a generic type. This way, the type can be constrained when the impl is provided to only accept enums that implement the marker trait that corresponds to the given marker trait. The current convention is to use impl_trait to create the trait with the name of the field that is the generic type and then use impl_var_trait to flag the new enum as usable in this field. See the examples below for more details.