Plugins that run a fixture inside another fixture can't declare that dependency #15124

What's the problem this feature will solve?

pytest async plugins (like anyio and pytest-asyncio) run async fixtures that users write with plain @pytest.fixture.

Via pytest_fixture_setup, they run each fixture on an event loop or backend that is itself a fixture (which is itself often parametrized). The only way to access that fixture is request.getfixturevalue(), so pytest doesn't see the dependency so parametrization and caching can break.

# conftest.py
import pytest

current_backend = None  # stands in for the running event loop

@pytest.fixture(scope="session", params=["a", "b"])
def backend(request):
    return request.param

@pytest.hookimpl(wrapper=True)
def pytest_fixture_setup(fixturedef, request):
    # mimic a plugin that runs `resource` inside `backend`
    global current_backend
    if fixturedef.argname == "resource":
        current_backend = request.getfixturevalue("backend")
    return (yield)

@pytest.fixture(scope="session")
def resource():  # user-written
    return current_backend

def test_both(backend, resource):
    assert resource == backend

def test_resource(resource):
    pass

$ pytest -v --tb=no
test_mre.py::test_both[a] PASSED
test_mre.py::test_both[b] FAILED
test_mre.py::test_resource PASSED
FAILED test_mre.py::test_both[b] - AssertionError: assert 'a' == 'b'

test_resource isn't parametrized, and test_both[b] gets the resource built on backend[a].

Adding an unused backend parameter to resource fixes it. anyio's docs tell users to do that for higher-scoped fixtures, and forgetting it silently runs tests against a fixture built on asyncio via trio.

Describe the solution you'd like

A way at collection time for a plugin that runs a fixture to name fixtures it needs without passing them as arguments, so pytest treats them like parameters.

One idea that might work is a new hook:

def pytest_fixture_dependencies(
    fixturedef: FixtureDef,
    config: Config,
) -> Iterable[str] | None: ...

Alternative Solutions

Fabricating __signature__ or using register_fixture only helps with fixtures the plugin defines itself (e.g., pytest_asyncio.fixture but not pytest.fixture).

Writing to FixtureDef.argnames is private and has to happen before collection.

Runtime tracking of getfixturevalue (#14103, #14104) can't affect parametrization, which is decided at collection.

Additional context

Relates to #14991, which is about reading rather than adding to the fixture graph.