Added automatic type stub generation with pyo3-introspection

[varchasgopalaswamy]

This PR adds a basic mechanism for type stub generation with pyo3-introspection.

Perhaps there is a better way to do it, but the most obvious way to get the stubs into the wheel seemed to be by

1. Generating the stubs right after compile.
2. Writing the stubs out in a folder right next to where the .so/.dll file ends up
3. Stuffing the paths to the stubs into BuildArtifact
4. Having binding_generator append the stubs into the additional_files field that it carries around to put python files into the wheel

I've currently just implemented the last step for pyo3_binding but I guess there's no reason why it couldnt be applied elsewhere. I suppose the stubs could also be generated during the binding generation rather than during compile, but I think it's nice to be able to see the generated stubs and be able to check that they look good before they get stuffed into a wheel.

[Tpt]

[for context I am the main person working on introspection in Pyo3]

Thank you for this! I am not sure pyo3-introspection in its released version is ready to be part of a build pipeline, there are still to many issues. I am scared it will move people out of the feature even if it's quite promising. A good first step might a new maturin generate-stubs command or something like this that would allow people to have a look at the generated output and maybe fix it and commit it?

For context, I also have a draft commit adding a --introspect-stubs option to the build pipeline, it hooks into maturin in the generate_binding function: 92b90be. Not sure which approach is best. But anyway, I would prefer to make crystal clear the current state of the stub introspection code is not ready for use at the moment.

[varchasgopalaswamy]

Yes, I think feature gating it, maybe behind a CLI arg might be a good idea to let people know it's experimental and let them opt in. But my two cents is that exposing it even just as a preview feature through maturin would make it a lot easier for folks to give feedback! Since pyo3-introspection is lib only, it requires a bit of work to get it to start generating stubs.

That said, it's working pretty well so far!

[Tpt]

would make it a lot easier for folks to give feedback!

Yes! But I am afraid the missing features will be frustrating to the people trying and I guess most of the feedback will be elements still on the TODO list or already fixed in the unreleased version. Waiting a few month for at least the next pyo3 release seems a bit better to me.

[varchasgopalaswamy]

would make it a lot easier for folks to give feedback!

Yes! But I am afraid the missing features will be frustrating to the people trying and I guess most of the feedback will be elements still on the TODO list or already fixed in the unreleased version. Waiting a few month for at least the next pyo3 release seems a bit better to me.

Fair point. I guess we can leave this PR open so people can test-drive the introspection changes, and re-visit this once more of the TODOs are dealt with!