Regression: Crash when compiling trivial program using mypyc

[JukkaL]

When I try to compile a simple program using mypyc, I experience a crash (at least on macOS):

$ python -m mypyc t.py
Traceback (most recent call last):
  File "/Users/jukka/src/mypy/build/setup.py", line 4, in <module>
    setup(name='mypyc_output',
    ~~~~~^^^^^^^^^^^^^^^^^^^^^
          ext_modules=mypycify(['t/t.py'], opt_level="3", debug_level="1", strict_dunder_typing=False),
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/Users/jukka/venv/mypy/lib/python3.13/site-packages/setuptools/__init__.py", line 117, in setup
    return distutils.core.setup(**attrs)
           ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/Users/jukka/venv/mypy/lib/python3.13/site-packages/setuptools/_distutils/core.py", line 157, in setup
    dist.parse_config_files()
    ~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/Users/jukka/venv/mypy/lib/python3.13/site-packages/setuptools/dist.py", line 608, in parse_config_files
    pyprojecttoml.apply_configuration(self, filename, ignore_option_errors)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jukka/venv/mypy/lib/python3.13/site-packages/setuptools/config/pyprojecttoml.py", line 71, in apply_configuration
    config = read_configuration(filepath, True, ignore_option_errors, dist)
  File "/Users/jukka/venv/mypy/lib/python3.13/site-packages/setuptools/config/pyprojecttoml.py", line 143, in read_configuration
    return expand_configuration(asdict, root_dir, ignore_option_errors, dist)
  File "/Users/jukka/venv/mypy/lib/python3.13/site-packages/setuptools/config/pyprojecttoml.py", line 168, in expand_configuration
    return _ConfigExpander(config, root_dir, ignore_option_errors, dist).expand()
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/Users/jukka/venv/mypy/lib/python3.13/site-packages/setuptools/config/pyprojecttoml.py", line 216, in expand
    self._expand_all_dynamic(dist, package_dir)
    ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
  File "/Users/jukka/venv/mypy/lib/python3.13/site-packages/setuptools/config/pyprojecttoml.py", line 262, in _expand_all_dynamic
    readme=self._obtain_readme(dist),
           ~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/Users/jukka/venv/mypy/lib/python3.13/site-packages/setuptools/config/pyprojecttoml.py", line 328, in _obtain_readme
    self._ensure_previously_set(dist, "readme")
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^
  File "/Users/jukka/venv/mypy/lib/python3.13/site-packages/setuptools/config/pyprojecttoml.py", line 280, in _ensure_previously_set
    raise InvalidConfigError(msg)
setuptools.errors.InvalidConfigError: No configuration found for dynamic 'readme'.
Some dynamic fields need to be specified via `tool.setuptools.dynamic`

The program t.py that I compiled is trivial:

print('hello')

This appears to be a regression that bisects to #18146. cc @cdce8p

[brianschubert]

This appears to only happen when running mypyc from the root directory with mypy's pyproject.toml. Running mypyc from a different directory (without pyproject.toml) seems to work as expected.

I think what's happening is that 1) setuptools looks at the pyproject.toml file in the working directory, 2) mypy's pyproject.toml declares dynamic = ["readme"], meaning that setup.py needs to define long_description, 3) the setup.py generated by mypyc doesn't define long_description, hence setuptools complains.

[cdce8p]

Will take a look later today. I do have an idea in mind, just want to confirm it's working before opening the PR.