Function break_query_cycles 

pub fn break_query_cycles<'tcx>(query_map: QueryMap<'tcx>, registry: &Registry)

Detects query cycles by using depth first search over all active query jobs. If a query cycle is found it will break the cycle by finding an edge which uses a query latch and then resuming that waiter. There may be multiple cycles involved in a deadlock, so this searches all active queries for cycles before finally resuming all the waiters at once.