Macro define_builder

macro_rules! define_builder {
    (@constructors $rust_type:ident, $alias:literal, $builder_type:ident $(($($req_ident:ident: $req_ty:ty,)*))?) => { ... };
    (
        $rust_type:ident, $alias:literal, $builder_type:ident {
            $($field_id:ident: $field_ty:ty $(= $field_expr:expr)?,)*
        }
        $(requires $required_tt:tt)?
    ) => { ... };
}