`PyMODINIT_FUNC` does not apply `__declspec(dllexport)` on Windows

[encukou]

The macro is documented as:

Declare an extension module initialization function. This macro:

• specifies the PyObject* return type,
• adds any special linkage declarations required by the platform, and
• for C++, declares the function as extern "C".

Specifically, the special linkage declarations should be __attribute__ ((visibility ("default"))) on GCC/clang, and __declspec(dllexport) on Windows.
However, the definition assumes we're building CPython itself, and only adds a linkage declaration with Py_ENABLE_SHARED -- that is, when Python itself is built with a shared library.

According to MSVC docs:

There are four methods for exporting a definition, listed in recommended order of use:

1. __declspec(dllexport) in the source code
2. An EXPORTS statement in a .def file
3. An /EXPORT specification in a LINK command
4. A comment directive in the source code, of the form #pragma comment(linker, "/export: definition ").

Setuptools, and historically distutils, does (2) -- and so does, presumably, any other build system for native extensions on Windows.
However, this breaks for modules that define the PEP-793 PyModExport hook.

It would be great to switch to the preferred (1), and then tell Setuptools to avoid /EXPORT for 3.15+.

Linked PRs

• gh-141671: PyMODINIT_FUNC: apply __declspec(dllexport) on Windows #141672

[encukou]

I filed a setuptools issue: pypa/setuptools#5118