Macro required 

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

Helper for parsing a required parameter using a TokenStream

This will try to execute the parse (which returns an Option). If the parse failed, it will mark an error on the TokenStream.

Note that this does not change the control flow. The caller should still use the ? operator for early return if needed. This is intentional to make the early return more obvious.