Custom marks don't play nice with `pytest_plugins = ('my_plugin',)` in test file

I've got a pytest plugin that registers a custom mark in the pytest_configure hook using config.addinivalue_line. When I load my plugin by putting pytest_plugins = ("my_plugin",) in my test file (as described here) and add the mark to one of my tests, I still get a warning PytestUnknownMarkWarning: Unknown pytest.mark.my_mark - is this a typo?

When I load my plugin by putting pytest_plugins = ("my_plugin",) in the conftest.py file, I do not get the warning.

Here's my example plugin:

import pytest

# This fixture is just so I know the plugin is working.
# If it's not installed correctly, I'll get an error when the test tries to use the foo fixture
@pytest.fixture
def foo():
  return "foo"

def pytest_configure(config):
    config.addinivalue_line("markers", "my_mark")

Here's the test file that triggers the warning:

import pytest


@pytest.mark.my_mark
def test_01(foo):
  assert foo == "foo"

Here's my environment

Package       Version
------------- -------
attrs         21.4.0
iniconfig     1.1.1
my-plugin     1.0.0
packaging     21.3
pip           20.3.4
pkg-resources 0.0.0
pluggy        1.0.0
py            1.11.0
pyparsing     3.0.7
pytest        7.0.1
setuptools    44.1.1
tomli         2.0.1

I'm aware that I can use a setuptools entrypoint to install my plugin, but that isn't an option in the more complicated environment where I encountered the problem. Likewise, the use of conftest.py to load the plugin is a bit frowned upon - we're trying declare the plugins we're using in the test files themselves so they're more stand-alone readable (among other reasons).

I suspect what's going wrong here is the warning is triggered during assertion re-writing in the test module. If I drop a breakpoint here I see the following near the end of the backtrace

. . .
.../_pytest/assertion/rewrite.py(170)exec_module()
-> exec(co, module.__dict__)
.../my_test.py(4)<module>()
-> @pytest.mark.my_mark
.../_pytest/mark/structures.py(520)__getattr__()
-> warnings.warn(

That happens before my pytest_configure hook is run, so it has no idea that my_mark is a custom mark yet.

Unfortunately, this is about the limit of my understanding so I can't be more helpful by trying stuff like "suppress this warning during assertion re-writing" to see if that fixes the problem. It's a bummer that this mark triggers a warning.

Reproduce the example with pytest_plugins in a test file and compare it with loading the plugin from conftest.py. Start at _pytest/assertion/rewrite.py and _pytest/mark/structures.py, then trace when pytest_configure runs relative to mark access. Done means the custom mark is recognized without a warning when the plugin is declared in the test file.