Function collect_special_tys 

fn collect_special_tys<'tcx>(
    cx: &RustcPatCtxt<'_, 'tcx>,
    pat: &WitnessPat<'_, 'tcx>,
    special_tys: &mut FxIndexSet<RevealedTy<'tcx>>,
)

Collect types that require specific explanations when they show up in witnesses.