Function visit_waiters 

fn visit_waiters<'tcx>(
    job_map: &QueryJobMap<'tcx>,
    query: QueryJobId,
    visit: impl FnMut(Span, QueryJobId) -> ControlFlow<Option<(QueryJobId, usize)>>,
) -> ControlFlow<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 and a span indicating the reason the query waited on query. If visit returns Break, this function also returns Break, and if all visit calls returns Continue it also returns Continue. For visits of non-resumable waiters it returns the return value of visit. For visits of resumable waiters it returns information required to resume that waiter.