Add PEP 660 support

[messense]

Closes #624

[davidhewitt]

So I tried this out locally with the maturin_starter example from the PyO3 repo. I changed the pyproject.toml file to:

[build-system]
requires = ["maturin @ git+https://github.com/messense/maturin@pep660"]
build-backend = "maturin"

... and then I installed with pip install -e ..

The editable install completed successfully, which is great! I then changed the __init__.py file, started a new Python interpreter and imported maturin_starter, and expected to see the new additions I made to the __init__.py file.

That didn't seem to be the case; I had to run pip install -e . again to pick up the changes.

I thought that with editable installs the Python source files can be freely updated without needing to reinstall the package, though I could be misunderstanding something?

[messense]

I thought that with editable installs the Python source files can be freely updated without needing to reinstall the package, though I could be misunderstanding something?

You are right, it should allow you freely update Python source files. What version of pip are you using? It requires pip 21.3.

And it'll install a ${pkg_name}.pth file in site-packages, could you paste the content here? It should contain the parent directory of Python source directory which will be added to sys.path on interpreter startup. Maybe the order of sys.path matters then we may need to avoid putting Python source files in editable wheels I guess.

[messense]

I think maybe we can also make maturin develop work just like editable wheels by placing dist-info and a .pth file in site-packages? cc @konstin What do you think?

[messense]

@davidhewitt Can you give this another try? I've tested the pyo3-mixed example and it works for me.

[konstin]

I think maybe we can also make maturin develop work just like editable wheels by placing dist-info and a .pth file in site-packages? cc @konstin What do you think?

Yes please! The current develop behavior is mostly a hack and .pth would be much better

[davidhewitt]

👍 it's now working nicely for me too, awesome work!

[messense]

I think maybe we can also make maturin develop work just like editable wheels by placing dist-info and a .pth file in site-packages? cc @konstin What do you think?

Yes please! The current develop behavior is mostly a hack and .pth would be much better

Great, I'll do it later in follow-up PRs.