MSVC 5274 warning suppression for _Py_ALIGN_AS should be local

[encukou]

Bug report

In #133966 (comment) we added a Python.h-wide suppression for MSVC warning 5274 “Application of _Alignas on a structured type in C”.

I think it should be instead applied locally, after a bit of thought.

AFAICS, 5274 warns that structs compiled with MSVC versions older than 17.9 might end up with a different layout (ABI) than ones compiled with 17.9+. That can cause trouble for extensions.

In PyASCIIObject, _Py_ALIGN_AS is applied to an anonymous struct. I think we can assume that no one is pulling state's type out with something like typeof(my_PyASCIIObject.state) – and if they do, getting an 4B-aligned struct is very unlikely to cause trouble in practice.
But, that is the kind of analysis one should do when a 5274 warning pops up.

Linked PRs

• gh-135183: Suppress MSVC warning 5274 locally #135184