Macro one_way_slot_map::define_key_type

macro_rules! define_key_type {
    ($visability : vis $key_type : ident < $pointer_type : ty >
 $(: $derive_1 : ident $(+ $more_derives : ident) *) ?) => { ... };
}

Macro for creating a simple Key type for one-way slot maps. Key types can be created from scratch, but for most cases, this will produce what you want