Macro sum_type 

macro_rules! sum_type {
    (
        $( #[$outer:meta] )*
        pub enum $name:ident {
            $(
                $( #[$inner:meta] )*
                $var_name:ident($var_ty:ty),
                )*
        }) => { ... };
    (
        $( #[$outer:meta] )*
        enum $name:ident {
            $(
                $( #[$inner:meta] )*
                $var_name:ident($var_ty:ty),
                )*
        }) => { ... };
    (
        $( #[$outer:meta] )*
        pub enum $name:ident {
            $(
                $( #[$inner:meta] )*
                $var_name:ident,
                )*
        }) => { ... };
    (
        $( #[$outer:meta] )*
        enum $name:ident {
            $(
                $( #[$inner:meta] )*
                $var_name:ident($var_ty:ty),
                )*
        }) => { ... };
}

The entire point.