I have a Python/Rust project that uses PyO3 to build a Python extension, written in Rust.

I have it set up with maturin and it works fine locally – it will build a wheel (.whl) and within that is my Python code and the Rust extension shared object, just as I’d expect.

I need to cross-compile this with Yocto (I’m stuck with Langdale, unfortunately), to put it in my embedded root filesystem, and I’ve quickly come to the realisation that I will need to ditch maturin for setuptools-rust, as there is no maturin support in Yocto Langdale at all.

So now my Rust/Python/PyO3 project is set up very much like the setuptools-rust example and I can build it locally, and the generated wheel contains what I’d expect.

Directory layout:

.
├── Cargo.lock
├── Cargo.toml
├── pyproject.toml
├── python
│   └── my_project
│       └── my_module.py
└── src
    └── lib.rs

project.toml (partial):

[build-system]
requires = ["setuptools>=61.0.0", "setuptools_scm>=8", "setuptools-rust"]
build-backend = "setuptools.build_meta"

[tool.setuptools.packages]
find = { where = ["python"] }

[[tool.setuptools-rust.ext-modules]]
target = "my_project._lib"
path = "Cargo.toml"
binding = "PyO3"

Cargo.toml (partial):

[lib]
name = "_lib"
crate-type = ["cdylib"]
path = "src/lib.rs"

[dependencies]
pyo3 = "0.22.0"

But it doesn’t build in Yocto with a recipe similar to:

inherit externalsrc
EXTERNALSRC = "${TOPDIR}/../../my_project"

inherit python_setuptools_build_meta

The error is:

|   File "build/tmp/work/cortexa72-cortexa53-xilinx-linux/my_package/1.0-r0/recipe-sysroot-native/usr/lib/python3.10/site-packages/picobuild/__init__.py", line 93, in __init__
|     pyproject = tomllib.load(f)
| AttributeError: module 'tomli' has no attribute 'load'

Not sure what to make about that. It looks something that tomli drop-in issue between Python <3.11 and Python 3.11 one sees sometimes.

After working out that I needed to inherit from python_setuptools3_rust instead of python_setuptools_build_meta, my bitbake recipe now looks something like this:

inherit externalsrc
EXTERNALSRC = "${TOPDIR}/../../my_project"

inherit python_setuptools3_rust

What this seems to do is successfully build and package the Python parts of my project, but the Rust extension (I expected to see a .so or .pyd file) is completely absent.

I’ve been through the Yocto logs and I can see references to python setup.py bdist_wheel (the old way to build, I guess), but nothing Rust-specific. I’m starting to think that python_setuptools3_rust is too old to handle pyproject.toml, but the bbclass python_setuptools_build_meta doesn’t seem to be able to build it either (due to that tomli error).

At this point I’ve run out of documentation or examples to guide me. I’m at a bit of a loss as to how to proceed.

It’s a real shame I’m stuck with Yocto Langdale because the latest LTS, Scarthgap, does seem to have maturin support.