Macro arg_count

macro_rules! arg_count {
    (
        $arg:ident $(,$tail:ident)* $(,)?
    ) => { ... };
    (
        [$($t:tt)*]
        $arg:ident $($tail:ident)*
    ) => { ... };
    () => { ... };
    (
        [$($t:tt)*]
    ) => { ... };
}