Allow other Result types in #[pyfunction]

[marioortizmanero]

Thank you for contributing to pyo3!

Please consider adding the following to your pull request:

• an entry in CHANGELOG.md
• docs to all new functions and / or detail in the guide
• tests for all new or changed functions

Be aware the CI pipeline will check your pull request for the following:

• Rust tests (Just cargo test or make test if you need to test examples)
• Rust lints (make clippy)
• Rust formatting (cargo fmt)
• Python formatting (black . --check. You can install black with pip install black)
• Compatibility with all supported Python versions for all examples. This uses tox; you can do run it using make test_py.

You can run a similar set of checks as the CI pipeline using make test.

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I've implemented #1106, see that issue for a description. It was quite simple but I haven't touched this codebase before so please let me know if anything can be improved.

I included a couple tests, one where it works correctly, and another where it fails to compile because from From<MyErr> for PyErr isn't implemented. What I would like to know before this is merged:

• Is it possible to improve the error message that appears when it fails to compile? See the tests/ui/invalid_result_conversion.stderr for the current error message.
• I still have to include this in the documentation. Any pointers would be appreciated for that, since I don't know where this should go in the book.

[kngwyu]

Thanks!
I have some minor concerns but it looks that it's nicely fixed 👍

[davidhewitt]

Thanks! Looks like this was even easier than expected 😄

As for the guide: maybe in this section for now? https://pyo3.rs/v0.11.1/conversions.html#returning-rust-values-to-python

I plan to make a bunch of PRs to improve the guide soon anyway, so as long as this functionality is mentioned in the guide I'll probably move it around if necessary ;)

[marioortizmanero]

Great, I've made all the suggested fixes now, including mentioning this in the docs.

[davidhewitt]

Thanks, LGTM! Regarding the UI test - I don't think we'll easily improve the error message (that's pretty much part of the compiler), so I'm just going to merge for now and maybe we can improve this later.

[marioortizmanero]

Thanks! I'm glad I could contribute to PyO3. Cheers :)