Added helpful error message if an async function is passed to trio.to_thread_run_sync

newsfragments/1573.bugfix.rst

@@ -0,0 +1 @@
+Added a helpful error message if an async function is passed to `trio.to_thread.run_sync`.

trio/_threads.py

@@ -179,7 +179,17 @@ def do_release_then_return_result():
     def worker_fn():
         TOKEN_LOCAL.token = current_trio_token
         try:
-            return sync_fn(*args)
+            ret = sync_fn(*args)
+
+            if inspect.iscoroutine(ret):
+                # Manually close coroutine to avoid RuntimeWarnings
+                ret.close()
+                raise TypeError(
+                    "Trio expected a sync function, but {!r} appears to be "
+                    "asynchronous".format(getattr(sync_fn, "__qualname__", sync_fn))
+                )
+
+            return ret
         finally:
             del TOKEN_LOCAL.token
 

trio/tests/test_threads.py

@@ -457,6 +457,15 @@ def thread_fn():
     assert callee_token == caller_token
 
 
+async def test_trio_to_thread_run_sync_expected_error():
+    # Test correct error when passed async function
+    async def async_fn():  # pragma: no cover
+        pass
+
+    with pytest.raises(TypeError, match="expected a sync function"):
+        await to_thread_run_sync(async_fn)
+
+
 async def test_trio_from_thread_run_sync():
     # Test that to_thread_run_sync correctly "hands off" the trio token to
     # trio.from_thread.run_sync()