gh-107249: Implement Py_UNUSED() for MSVC

[vstinner]

Fix warnings C4100 in Py_UNUSED() when Python is built with "cl /W4".

Example with this function included by Python.h:

static inline unsigned int
PyUnicode_IS_READY(PyObject* Py_UNUSED(op))
{ return 1; }

Without this change, building a C program with "cl /W4" which just includes Python.h emits the warning:

Include\cpython/unicodeobject.h(199):
warning C4100: '_unused_op': unreferenced formal parameter

This change fix this warning.

• Issue: Implement Py_UNUSED() for Windows MSVC compiler #107249

[vstinner]

@zooba @serhiy-storchaka @methane: Do you want to trade a review? I consider that it's a feature change, so it requires to be approved to be merged, following new Steering Council guidelines :-)

[vstinner]

Do you think that it would too late to backport it to Python 3.12? We are close to Python 3.12.0 beta 4 (Tuesday, 2023-07-11). I'm not sure if it's a bugfix or a feature :-/ But I would prefer to not backport it to Python 3.11 since I'm not 100% sure that my solution works in all cases on all MSVC versions.

[methane]

But I would prefer to not backport it to Python 3.11 since I'm not 100% sure that my solution works in all cases on all MSVC versions.

+1.

[methane]

(void)name; in function name might be the simplest solution.
But existing Py_UNUSED() macro is used in function signature.
So I don't know better approach than this PR.

[vstinner]

Thanks for the review @methane. I merged my PR.

(void)name; in function name might be the simplest solution

Is it possible to put ; in the argument list? int test(int arg) { return 0; } would become int test(int (void)arg;) { return 0; }?

Well, Python 3.12 is already at beta stage. I prefer to not cause any regression. Let's wait until Python 3.13 is shipped. I don't think that this change is so important that it deserves to be backported.

[serhiy-storchaka]

C++17 supports

int test([[maybe_unused]] int arg)

I am not sure, but maybe it can be also written as

int test(int [[maybe_unused]] arg)

or

int test(int arg [[maybe_unused]])

If it is so, it can be utilized by Py_UNUSED() in extensions written in C++17.

[miss-islington-app]

Thanks @vstinner for the PR 🌮🎉.. I'm working now to backport this PR to: 3.12.
🐍🍒⛏🤖

[bedevere-app]

GH-127907 is a backport of this pull request to the 3.12 branch.

[vstinner]

GH-127907 is a backport of this pull request to the 3.12 branch.

I need this to backport test_cext to 3.12: issue gh-127906.