gh-106320: Remove private _PyLong_IsCompact() function

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Move the private _PyLong_IsCompact() and _PyLong_CompactValue() functions to the internal C API (pycore_long.h). Public PyUnstable_Long_IsCompact() and PyUnstable_Long_CompactValue() functions can be used instead.

Remove "const" qualifier from PyUnstable_Long_IsCompact() and PyUnstable_Long_CompactValue() parameter type.

• Issue: C API: Remove private C API functions (move them to the internal C API) #106320

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📚 Documentation preview 📚: https://cpython-previews--108742.org.readthedocs.build/

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With this change, PyUnstable_Long_IsCompact() and PyUnstable_Long_CompactValue() become regular opaque function calls. They are no longer implemented as static inline functions. That's needed to be able remove the 2 associated static inline functions from the public C API.

Remove "const" qualifier from PyUnstable_Long_IsCompact() and PyUnstable_Long_CompactValue() parameter type.

When I tried to use the const qualified in the public C API in the past, I had issues :-( I prefer to avoid it. For the internal C API with static inline functions, that's fine.

See:

• Const correctness capi-workgroup/problems#38
• [C API] Avoid "const PyObject*" type #91768
• gh-91768: C API no longer use "const PyObject*" type #91769

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@gvanrossum @markshannon: Are the private _PyLong_IsCompact() and _PyLong_CompactValue() functions exposed by <Python.h> on purpose? Or are you are ok to move them to the internal C API?

I don't know the impact on performance. I'm more in favor of hiding implementation details.

If these functions are moved to the internal C API, are you ok to also make this change in Python 3.12?

Remove "const" qualifier from PyUnstable_Long_IsCompact() and PyUnstable_Long_CompactValue() parameter type.

I don't have a strong opinion on that, I'm also fine with keeping const if you prefer :-) It's just that in the past, I tried to use const in the public C API and "I got issues".

[gvanrossum]

Please don’t touch these. They are meant to be exported.

[gvanrossum]

Also it’s too late for 3.12.

[vstinner]

Please don’t touch these. They are meant to be exported.

Ok, let's keep them.