Macro join_impl::parse_n_or_empty_unit_fn_signature

macro_rules! parse_n_or_empty_unit_fn_signature {
    (0, true, $fn_name: ident) => { ... };
    ($unit_count: tt, false, $fn_name: ident) => { ... };
    ($unit_count: tt, true, $fn_name: ident) => { ... };
}