Function remove_cycle 

fn remove_cycle<'tcx>(
    query_map: &QueryMap<'tcx>,
    jobs: &mut Vec<QueryJobId>,
    wakelist: &mut Vec<Arc<QueryWaiter<'tcx>>>,
) -> bool

Looks for query cycles starting from the last query in jobs. If a cycle is found, all queries in the cycle is removed from jobs and the function return true. If a cycle was not found, the starting query is removed from jobs and the function returns false.