Macro loose_enum 

macro_rules! loose_enum {
    (
        $(#[$outer:meta])*
        $vis:vis enum $name:ident: String {
            $($body:tt)*
        }
    ) => { ... };
    (
        $(#[$outer:meta])*
        $vis:vis enum $name:ident: $ty:ident {
            $($body:tt)*
        }
    ) => { ... };
    (
        $(#[$outer:meta])*
        $vis:vis enum $name:ident<$ty:ident $( : $first_bound:tt $(+ $other_bounds:tt)* )?>
        {
            $($body:tt)*
        }
    ) => { ... };
}

Defines a repr enum that supports any value. If a value does not match any case, it will be parsed as Undefined.