Emit a warning when a async def function is not handled by a plugin

Fix pytest-dev#2224

changelog/2224.feature.rst

@@ -0,0 +1,2 @@
+A warning is emitted when a ``async`` test function is encountered and is not handled by any
+of the existing async frameworks (such as ``pytest-asyncio`` or ``pytest-trio``).

src/_pytest/python.py

@@ -156,6 +156,14 @@ def pytest_configure(config):
 @hookimpl(trylast=True)
 def pytest_pyfunc_call(pyfuncitem):
     testfunction = pyfuncitem.obj
+    if inspect.iscoroutinefunction(testfunction):
+        msg = "test function {} is a coroutine and not natively supported.\n"
+        msg += "you need to install a suitable plugin for your async framework, for example:\n"
+        msg += "  - pytest-asyncio\n"
+        msg += "  - pytest-trio\n"
+        msg += "  - pytest-tornasync"
+        warnings.warn(PytestWarning(msg.format(pyfuncitem.nodeid)))
+        return None
     funcargs = pyfuncitem.funcargs
     testargs = {arg: funcargs[arg] for arg in pyfuncitem._fixtureinfo.argnames}
     testfunction(**testargs)

testing/acceptance_test.py

@@ -1179,3 +1179,23 @@ def test_fixture_mock_integration(testdir):
 def test_usage_error_code(testdir):
     result = testdir.runpytest("-unknown-option-")
     assert result.ret == EXIT_USAGEERROR
+
+
+@pytest.mark.skipif(
+    sys.version_info[:2] < (3, 5), reason="async def syntax python 3.5+ only"
+)
+@pytest.mark.filterwarnings("default")
+def test_warn_on_async_function(testdir):
+    testdir.makepyfile(
+        test_async="""
+        async def test_1():
+            pass
+    """
+    )
+    result = testdir.runpytest()
+    result.stdout.fnmatch_lines(
+        [
+            "*test function test_async.py::test_1 is a coroutine*",
+            "*1 passed, 1 warnings in*",
+        ]
+    )