Fix panic in ConcurrencyLimiter when unwinding

Fixes #1275

src/concurrency_limiter.rs

@@ -10,6 +10,7 @@ pub(super) struct ConcurrencyLimiter {
     helper_thread: Option<HelperThread>,
     state: Arc<Mutex<state::ConcurrencyLimiterState>>,
     available_token_condvar: Arc<Condvar>,
+    finished: bool,
 }
 
 impl ConcurrencyLimiter {
@@ -32,6 +33,7 @@ pub(super) fn new(sess: &Session, pending_jobs: usize) -> Self {
             helper_thread: Some(helper_thread),
             state,
             available_token_condvar: Arc::new(Condvar::new()),
+            finished: false,
         }
     }
 
@@ -56,16 +58,23 @@ pub(super) fn job_already_done(&mut self) {
         let mut state = self.state.lock().unwrap();
         state.job_already_done();
     }
-}
 
-impl Drop for ConcurrencyLimiter {
-    fn drop(&mut self) {
-        //
+    pub(crate) fn finished(mut self) {
         self.helper_thread.take();
 
         // Assert that all jobs have finished
         let state = Mutex::get_mut(Arc::get_mut(&mut self.state).unwrap()).unwrap();
         state.assert_done();
+
+        self.finished = true;
+    }
+}
+
+impl Drop for ConcurrencyLimiter {
+    fn drop(&mut self) {
+        if !self.finished && !std::thread::panicking() {
+            panic!("Forgot to call finished() on ConcurrencyLimiter");
+        }
     }
 }
 

src/driver/aot.rs

@@ -106,7 +106,7 @@ pub(crate) fn join(
             }
         }
 
-        drop(self.concurrency_limiter);
+        self.concurrency_limiter.finished();
 
         (
             CodegenResults {