Macro env_struct 

macro_rules! env_struct {
    (
        $(#[$outer:meta])*
        $vis:vis struct $struct_name:ident {
            $(
                $(#[$outer_field:meta])*
                $vis_ident:vis $field:ident = $fieldDef:expr,
            )*
        }
    ) => { ... };
    (
        $(#[$outer:meta])*
        $vis:vis struct $struct_name:ident {
            $(
                $(#[$outer_field:meta])*
                $vis_ident:vis $field:ident,
            )*
        }
    ) => { ... };
}

Macro for writing a env_struct