pytest-anyio should raise error if sync test relies on async fixture

[jakkdl]

Things to check first

• I have searched the existing issues and didn't find my feature already requested there

Feature description

Sync test function attempting to use async fixture fails silently. pytest-trio will raise a RuntimeError in this instance.

The one possible downside is that this will break users who have async autouse fixtures in the same file where they have sync tests. But this restriction probably isn't too onerous, and may catch bugs as well.

Use case

Catch bugs and align behavior with pytest-trio

import pytest

pytestmark = pytest.mark.anyio

@pytest.fixture(autouse=True)
async def async_fixture():
    assert False
    yield


def test_1(async_fixture):
    assert True

def test_2():
    assert True

will pass, but probably shouldn't

[euri10]

big +1 on this

[jakkdl]

It appears this works if the sync test explicitly requests the anyio_backend fixture

import pytest

@pytest.fixture
async def fix():
    return 1

def test_foo(fix, anyio_backend):
    assert fix == 1

but if using @pytest.mark.anyio, pytestmark = pytest.mark.anyio, or forgetting to add anything, you get an unawaited coroutine object as the value.

EDIT:
ah, it's about the fixture requesting (directly, or indirectly from the test requesting it) anyio_backend, so this also works:

import pytest

@pytest.fixture
async def fix(anyio_backend):
    return 1

def test_foo(fix):
    assert fix == 1