Macro target_spec_enum 

macro_rules! target_spec_enum {
    (
        $( #[$attr:meta] )*
        pub enum $Name:ident {
            $(
                $( #[$variant_attr:meta] )*
                $Variant:ident = $string:literal,
            )*
        }
        parse_error_type = $parse_error_type:literal;
    ) => { ... };
    (
        $( #[$attr:meta] )*
        pub enum $Name:ident {
            $(
                $( #[$variant_attr:meta] )*
                $Variant:ident = $string:literal,
            )*
        }
        $( #[$other_variant_attr:meta] )*
        other_variant = $OtherVariant:ident;
    ) => { ... };
    (@common_impls $Name:ident) => { ... };
}