Function visit_waiters

fn visit_waiters<F>(
    query_map: &QueryMap,
    query: QueryJobId,
    visit: F,
) -> Option<Option<(QueryJobId, usize)>>
where
    F: FnMut(Span, QueryJobId) -> Option<Option<(QueryJobId, usize)>>,

Visits all the non-resumable and resumable waiters of a query. Only waiters in a query are visited. visit is called for every waiter and is passed a query waiting on query_ref and a span indicating the reason the query waited on query_ref. If visit returns Some, this function returns. For visits of non-resumable waiters it returns the return value of visit. For visits of resumable waiters it returns Some(Some(Waiter)) which has the required information to resume the waiter. If all visit calls returns None, this function also returns None.