features = ["abi3", "num-bigint"] silently ignore "num-bigint"

[youknowone]

Bug Description

It happens because num_bigint.rs has #![cfg(all(feature = "num-bigint", not(any(Py_LIMITED_API))))] on the top, right?

If the features are actually not compatible, could there be any error or warning?

Steps to Reproduce

1. Add num-bigint features and use the conversion features.
2. Add abi3
3. Then conversion code build fails.

Backtrace

No response

Your operating system and version

MacOS / 13.3.1

Your Python version (python --version)

Python 3.11.1

Your Rust version (rustc --version)

rustc 1.69.0 (84c898d65 2023-04-16)

Your PyO3 version

0.19.0

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

pyenv install 3.11.1 && pyenv virtualenv 3.11.1 pyo3 && pyenv local pyo3

Additional Info

Another suggestion is implementing num-bigint for Py_LIMITED_API. Since it is already using ffi::_PyLong_FromByteArray, using a python call through int.from_bytes and int.to_bytes will makes sense (with additional overhead).
I am sorry if this is a silly idea. Is this expected to be implemented on user side?

[davidhewitt]

In #3185 we are in the process of fixing a different abi3 case where support for num-complex diverged. Similarly here I would happily accept a PR where we used a lower performance fallback to add support here. (Most likely using int.from_bytes and int.to_bytes as you propose.)

[youknowone]

Thank you! I'll take a shot.