add support for abi3t

[Icxolu]

This adds abi3t support by

• switching to opaque structs
• exposing data access helpers from slots 369-378
• redirect all field accesses through the access helpers

Todo:

• changelog entry
• CI testing for 3.15/3.15t

See numpy/numpy#31091
Closes #555

[ngoldbaum]

Ping @kumaraditya303 - we were just talking about this today.

[neutrinoceros]

I confirmed that this resolved the runtime error I reported in #555
example logs (remaining failures are unrelated, they're just due to numpy not having cp315 nightlies for windows yet)

[kumaraditya303]

I think we should also add a check that when abi3t is used, the minimum numpy version should be 2.5.

[Icxolu]

Looks like 32bit windows still has some issues... not sure why though
There was also https://github.com/PyO3/rust-numpy/actions/runs/27866655836/job/82471861978 earlier

[neutrinoceros]

My guess is that'd be because there are no wheels for cp315*-*win32
xref numpy/numpy-release#53

[Icxolu]

To me it looks like there are cp315-win32 wheels here and they installed fine. The version specific run had no issues as well. It's just the abi3t run that failed, so I think this is a genuine failure, but I'm not sure which fault it could be: ours, pyo3, numpy or python

[kumaraditya303]

I think it could alignment issue on 32 bit MSVC, I am not very familiar with rust but it seems flags is defined as npy_uint64 which rust gives 8 bytes alignment whereas in C I think it is 4 bytes which could be an issue.

[kumaraditya303]

@Icxolu Can you try forcing a 4 byte alignment layout using #[cfg_attr(all(Py_LIMITED_API, Py_GIL_DISABLED), repr(C, packed(4)))]?

[ngoldbaum]

If you all don't sort this out in the meantime I'll try to look on Monday. While win32 isn't so important per se, I do find it useful to detect and sort out CPU architecture-sensitive issues. It's possible this issue is in any one of CPython, NumPy, or rust-numpy.

Glad to see everything mostly works otherwise!

[Icxolu]

I did a bit of digging with compiler explorer: https://godbolt.org/z/qYe3Ghz7a
Both rustc and msvc agree on both architectures on the structure size and that flags is at offset 16 (and thus 8 byte aligned as it should), so I think there is no issue with our definitions here. I did however also find this: rust-lang/rust#112480, which does not really paint a great picture about win32... perhaps we ran into that...