PyO3 does not respect struct memory alignment

[onsmith]

Bug Description

PyO3 does not respect memory alignment rules when defining Python classes using the #[pyclass] and #[new] annotations.

PyO3 Python classes with Rust-defined memory alignment requirements (for example, defined with #[repr(align())]) cause runtime panics or segfaults.

Steps to Reproduce

I created a complete minimal example to reproduce the issue. Key snippet:

#[derive(Clone)]
#[repr(align(64))]
struct Aligned64<T>(T);

#[pyclass]
pub(crate) struct StructWithAlignedFields {
    aligned_field: Aligned64<u64>,
}

#[pymethods]
impl StructWithAlignedFields {
    #[new]
    pub(crate) fn new() -> PyResult<Self> {
        Ok(StructWithAlignedFields {
            aligned_field: Aligned64(0),
        })
    }
}

#[pymodule]
fn foopy(_py: Python<'_>, m: &PyModule) -> PyResult<()> {
    m.add_class::<StructWithAlignedFields>()?;
    Ok(())
}

This code panics when foopy.StructWithAlignedFields() is executed in Python 3:

misaligned pointer dereference: address must be a multiple of 0x40 but is 0x7f8a02ef9f30

Backtrace

No response

Your operating system and version

Amazon Linux 2

Your Python version (python --version)

Python 3.9.18

Your Rust version (rustc --version)

rustc 1.73.0 (cc66ad468 2023-10-03)

Your PyO3 version

0.20.0

How did you install python? Did you use a virtualenv?

make install from a source tarball from python.org

Additional Info

My guess is that the macro which copies the field attributes is not correctly grabbing the alignment or is unable to.