Async result warn

[graingert]

• Target the master branch for bug fixes, documentation updates and trivial changes.
• Target the features branch for new features and removals/deprecations.
• Include documentation when adding new features.
• Include new tests or update existing tests when applicable.

Unless your change is trivial or a small documentation fix (e.g., a typo or reword of a small section) please:

• Create a new changelog file in the changelog folder, with a name like <ISSUE NUMBER>.<TYPE>.rst. See changelog/README.rst for details.
• Add yourself to AUTHORS in alphabetical order;

[nicoddemus]

Follow up to #5734 I assume? Can you please rebase given that #5734 was merged already? Thanks

[codecov]

Codecov Report

Merging #5742 into master will increase coverage by 0.8%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #5742     +/-   ##
========================================
+ Coverage    93.1%   93.9%   +0.8%     
========================================
  Files         117     117             
  Lines       25884   25888      +4     
  Branches     2498    2499      +1     
========================================
+ Hits        24100   24311    +211     
+ Misses       1444    1249    -195     
+ Partials      340     328     -12

Impacted Files	Coverage Δ
testing/acceptance_test.py	96.49% <ø> (+0.77%)	⬆️
src/_pytest/python.py	92.79% <100%> (+0.15%)	⬆️
testing/test_terminal.py	98.43% <0%> (+0.31%)	⬆️
testing/test_config.py	99.5% <0%> (+0.33%)	⬆️
testing/python/fixtures.py	99.09% <0%> (+0.56%)	⬆️
src/_pytest/pathlib.py	90.67% <0%> (+1.03%)	⬆️
src/_pytest/assertion/util.py	92.36% <0%> (+1.14%)	⬆️
src/_pytest/capture.py	93.01% <0%> (+1.31%)	⬆️
... and 10 more

---

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[nicoddemus]

Do we really need this? I mean, Python itself already issues a warning which is clear enough IMHO:

async def foo():
    pass

def test_foo():
    return foo()

====================== warnings summary ======================
test_async.py::test_foo
  c:\users\bruno.esss.co\pytest\src\_pytest\python.py:165: RuntimeWarning: coroutine 'foo' was never awaited
    testfunction(**testargs)

-- Docs: https://docs.pytest.org/en/latest/warnings.html

[nicoddemus]

Oh for the future: new warnings should target features, because they might break test suites. No need to change this now because we are about to release 5.1.0. 👍

[graingert]

I'm experiencing the problem with twisted Deferreds, however. Where they are awaitable but no error is thrown if you do not await them and there is no way to determine if a function will return one statically like inspect.iscoro...

[nicoddemus]

Oh I see, thanks!