pyo3, rust-numpy, ndarray, and num-complex MSRV issue

[mtreinish]

🐛 Bug Reports

There is an MSRV conflict between ndarray, rust-numpy, and pyo3. Specifically the msrv of ndarray was increased to 1.42 in ndarray 0.14.0 and to rust 1.49 in 0.15 (see https://github.com/rust-ndarray/ndarray/blob/master/RELEASES.md ) so to use ndarray with numpy we need ndarray 0.13.x which still supports building with rust 1.41. However, if we need to use num-complex with all of the above for a complex number we have to use the version pinned in the ndarray 0.13.1 package which is 0.2:

https://github.com/rust-ndarray/ndarray/blob/0.13.1/Cargo.toml#L33

but this has an interop issue with pyo3's num-complex optional dependency which requires 0.4:

https://github.com/PyO3/pyo3/blob/main/Cargo.toml#L24

which causes a version mismatch if there is an object taken from a numpy array converted to a readonly ndarray that gets converted to a pyobject (which I hit).

🌍 Environment

• Your operating system and version: linux
• Your python version: 3.9
• How did you install python (e.g. apt or pyenv)? Did you use a virtualenv?: arch linux package
• Your Rust version (rustc --version): 1.41.1
• Your PyO3 version: 0.14.2
• Have you tried using latest PyO3 main (replace version = "0.x.y" with git = "https://github.com/PyO3/pyo3")?: no (but it has the same issue)

💥 Reproducing

[dependencies]
numpy = "0.14.1"
num-complex = "0.2"
ndarray = "0.13.1"

[dependencies.pyo3]
version = "0.14.2"
features = ["extension-module", "num-complex"]

use ndarray::prelude::*;
use pyo3::prelude::*;
use pyo3::Python;
use numpy::PyReadonlyArray2;
use num_complex::Complex64;

#[pyfunction]
fn reproduce<'p>(py: Python<'p>, matrix: PyReadonlyArray2<'p, Complex64>) -> Complex64 {
    let array = matrix.as_array();
    array[[0, 0]]
}

The only way I can get this to work with all 4 libs is by using the latest rust version and the latest ndarray and num-complex. Then everything builds as expected and is using num-complex 0.4. I really can't wait for cargo to support msrv package metadata so this kind of situation can be automatically resolved.