Function check_redundant_vis_repetition 

fn check_redundant_vis_repetition(
    err: &mut Diag<'_>,
    sess: &Session,
    seq: &SequenceRepetition,
    span: &DelimSpan,
)

Checks if a vis nonterminal fragment is unnecessarily wrapped in an optional repetition.

When a vis fragment (which can already be empty) is wrapped in $(...)?, this suggests removing the redundant repetition syntax since it provides no additional benefit.