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

Fix #2224

changelog/2224.feature.rst

@@ -0,0 +1,4 @@
+``async`` test functions are skipped and a warning is emitted when a suitable
+async plugin is not installed (such as ``pytest-asyncio`` or ``pytest-trio``).
+
+Previously ``async`` functions would not execute at all but still be marked as "passed".

src/_pytest/python.py

@@ -43,6 +43,7 @@ from _pytest.mark import MARK_GEN
 from _pytest.mark.structures import get_unpacked_marks
 from _pytest.mark.structures import normalize_mark_list
 from _pytest.outcomes import fail
+from _pytest.outcomes import skip
 from _pytest.pathlib import parts
 from _pytest.warning_types import PytestWarning
 
@@ -156,6 +157,15 @@ def pytest_configure(config):
 @hookimpl(trylast=True)
 def pytest_pyfunc_call(pyfuncitem):
     testfunction = pyfuncitem.obj
+    iscoroutinefunction = getattr(inspect, "iscoroutinefunction", None)
+    if iscoroutinefunction is not None and iscoroutinefunction(testfunction):
+        msg = "Coroutine functions are not natively supported and have been skipped.\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)))
+        skip(msg="coroutine function and no async plugin installed (see warnings)")
     funcargs = pyfuncitem.funcargs
     testargs = {arg: funcargs[arg] for arg in pyfuncitem._fixtureinfo.argnames}
     testfunction(**testargs)

testing/acceptance_test.py

@@ -1179,3 +1179,31 @@ 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
+        async def test_2():
+            pass
+    """
+    )
+    result = testdir.runpytest()
+    result.stdout.fnmatch_lines(
+        [
+            "test_async.py::test_1",
+            "test_async.py::test_2",
+            "*Coroutine functions are not natively supported*",
+            "*2 skipped, 2 warnings in*",
+        ]
+    )
+    # ensure our warning message appears only once
+    assert (
+        result.stdout.str().count("Coroutine functions are not natively supported") == 1
+    )