Macro list 

macro_rules! list {
    ( $ts:ident, $list:ident, $inner:expr ) => { ... };
}

helper for parsing a list of parameters using a TokenStream

Optional list would be

let mut v = vec![];
list!(ts, v, $inner)

Required (at least one) would be

let mut v = vec![required!(ts, $inner)?];
list!(ts, v, $inner)