trio_test_case: throw an exception if we forget to await a coroutine

Forgetting to await a coroutine may make a test do essentially
nothing.  We want to make sure that doesn't happen.

Probably raise_unraisables should be done automatically by
unittest.TestCase.

Ideally, forgetting an await would be an error at the Python level,
see also python-trio/pytest-trio#86

python/rsyscall/tests/trio_test_case.py

@@ -1,11 +1,25 @@
 "A trio-enabled variant of unittest.TestCase"
 import trio
 import unittest
+import contextlib
 import functools
+import sys
 import types
+import warnings
 from trio._core._run import Nursery
 from rsyscall import local_process, Process
 
+@contextlib.contextmanager
+def raise_unraisables():
+    unraisables = []
+    try:
+        orig_unraisablehook, sys.unraisablehook = sys.unraisablehook, unraisables.append
+        yield
+    finally:
+        sys.unraisablehook = orig_unraisablehook
+        if unraisables:
+            raise trio.MultiError([unr.exc_value for unr in unraisables])
+
 class TrioTestCase(unittest.TestCase):
     "A trio-enabled variant of unittest.TestCase"
     nursery: Nursery
@@ -43,7 +57,21 @@ async def test_with_setup() -> None:
                 nursery.cancel_scope.cancel()
         @functools.wraps(test_with_setup)
         def sync_test_with_setup(self) -> None:
-            trio.run(test_with_setup)
+            # Throw an exception if there were any "coroutine was never awaited" warnings, to fail the test.
+            # See https://github.com/python-trio/pytest-trio/issues/86
+            # We also need raise_unraisables, otherwise the exception is suppressed, since it's in __del__
+            with raise_unraisables():
+                # Restore the old warning filter after the test.
+                with warnings.catch_warnings():
+                    warnings.filterwarnings('error', message='.*was never awaited', category=RuntimeWarning)
+                    trio.run(test_with_setup)
         setattr(self, methodName, types.MethodType(sync_test_with_setup, self))
         super().__init__(methodName)
 
+class Test(unittest.TestCase):
+    def test_coro_warning(self) -> None:
+        class Test(TrioTestCase):
+            async def test(self):
+                trio.sleep(0)
+        with self.assertRaises(RuntimeWarning):
+            Test('test').test()