Add useful panic messages if queries fail to start

Author: rsmyth-ecmi

compiler/rustc_query_system/src/query/plumbing.rs

@@ -44,6 +44,18 @@ enum QueryResult {
     Poisoned,
 }
 
+impl QueryResult {
+    /// Unwraps the query job expecting that it has started.
+    fn expect_job(self) -> QueryJob {
+        match self {
+            Self::Started(job) => job,
+            Self::Poisoned => {
+                panic!("job for query failed to start and was poisoned")
+            }
+        }
+    }
+}
+
 impl<K> QueryState<K>
 where
     K: Eq + Hash + Copy + Debug,
@@ -169,10 +181,7 @@ where
 
         let job = {
             let mut lock = state.active.lock_shard_by_value(&key);
-            match lock.remove(&key).unwrap() {
-                QueryResult::Started(job) => job,
-                QueryResult::Poisoned => panic!(),
-            }
+            lock.remove(&key).unwrap().expect_job()
         };
 
         job.signal_complete();
@@ -190,10 +199,8 @@ where
         let state = self.state;
         let job = {
             let mut shard = state.active.lock_shard_by_value(&self.key);
-            let job = match shard.remove(&self.key).unwrap() {
-                QueryResult::Started(job) => job,
-                QueryResult::Poisoned => panic!(),
-            };
+            let job = shard.remove(&self.key).unwrap().expect_job();
+
             shard.insert(self.key, QueryResult::Poisoned);
             job
         };
@@ -277,11 +284,14 @@ where
                     // We didn't find the query result in the query cache. Check if it was
                     // poisoned due to a panic instead.
                     let lock = query.query_state(qcx).active.get_shard_by_value(&key).lock();
+
                     match lock.get(&key) {
-                        // The query we waited on panicked. Continue unwinding here.
-                        Some(QueryResult::Poisoned) => FatalError.raise(),
+                        Some(QueryResult::Poisoned) => {
+                            panic!("query '{}' not cached due to poisoning", query.name())
+                        }
                         _ => panic!(
-                            "query result must in the cache or the query must be poisoned after a wait"
+                            "query '{}' result must be in the cache or the query must be poisoned after a wait",
+                            query.name()
                         ),
                     }
                 })