Macro register_fields 

macro_rules! register_fields {
    (
        $(
            $(#[$outer:meta])*
            enum $name:ident {
                $($field:ident = $offset:tt : $len:tt,)*
            }
        )*
    ) => { ... };
    (
        $(
            $(#[$outer:meta])*
            pub enum $name:ident {
                $($field:ident = $offset:tt : $len:tt,)*
            }
        )*
    ) => { ... };
}