Cross compilation ignores PYO3_CROSS_PYTHON_VERSION

[messense]

Please provide the following information:

• Your python version (python -V): host Python 3.9, cross target Python 3.6
• Your pip version (pip -V): -
  • If this isn't the latest pip version, have you checked using the latest pip version?
• The version of the bindings you're using, if any (e.g. pyo3, rust-cpython or cffi): pyo3
• Does cargo build work? yes
• If on windows, have you checked that you aren't accidentally using unix path (those with the forward slash /)? -

Please list the exact steps required to reproduce your error with all command output and if possible with a repository:

https://github.com/messense/manylinux2014-cross-arm/runs/1975989661?check_suite_focus=true

SetPYO3_CROSS_PYTHON_VERSION =3.6 and PYO3_CROSS_LIB_DIR according to https://pyo3.rs/v0.13.2/building_and_distribution.html#cross-compiling , maturin builds fine but produces cp39-cp39 wheels instead of cp36-cp36m.

[messense]

It'd be nice if maturin can detect cross target Python version like pyo3 does https://github.com/PyO3/pyo3/blob/c02b71fc09017078034394c1f7d7011bddb5fd03/build.rs#L316

[messense]

I guess this is essentially the same issue as #375 because of its poor cross compilation support.

[konstin]

Unless we require passing extra data, this will require copying (or extracting in a shared crate) of https://github.com/PyO3/pyo3/blob/master/build.rs#L311-L510

For future reference, that part could be used for parsing the version:

    if let Some(cross_python_version) = env::var_os("PYO3_CROSS_PYTHON_VERSION") {
        if bridge == BridgeModel::Bin {
            bail!("You can't use PYO3_CROSS_PYTHON_VERSION with bin bindings");
        }

        let expr = Regex::new(r"(\d+).(\d+)").unwrap();
        let capture = expr
            .captures(&cross_python_version.to_string_lossy())
            .context(format!(
                "Invalid PYO3_CROSS_PYTHON_VERSION value (should be a version like 3.10): '{}'",
                cross_python_version.to_string_lossy()
            ));
        let context = "Expected a digit";
        let major = capture
            .get(1)
            .unwrap()
            .as_str()
            .parse::<usize>()
            .context(context)?;
        let minor = capture
            .get(2)
            .unwrap()
            .as_str()
            .parse::<usize>()
            .context(context)?;
    }

[messense]

I have found that you can workaround it by using the same major & minor version of Python for build and target. Eg: rust-cross/manylinux-cross@ef575f9