Add error messages when compiling for ABI3 with unsupported macro features

[davidhewitt]

Building from #3835 (comment) and https://pyo3.rs/v0.20.3/building_and_distribution#missing-features

We have some #[pyclass] options which do not work on older ABI3 versions:

• #[pyo3(text_signature = "...")] does not work on classes until Python 3.10 or greater.
• #[pyclass(dict)] and #[pyclass(weakref)] options on classes are not supported until Python 3.9 or greater.

At the moment I think these conditions fail silently (they compile but the runtime behaviour is missing). The proc macros should instead detect these cases and emit a helpful warning.

[cojmeister]

Hi!
I can do this! But I'll need some guidance! Where should I start?

[davidhewitt]

Thanks for volunteering! To focus on #[pyclass(weakref)] as an initial example, try looking in pyo3-macros-backend/src/pyclass.rs for the part of the proc-macro code which handles the weakref option. pyo3-macros-backend/src/utils.rs has a method for checking if it's an ABI3 build, so you can use that method (and other information from pyo3_build_config) to work out if weakref has been set for an unsupported build.

Once that's figured out, look for e.g. the bail_spanned! / ensure_spanned! macros which is how most of PyO3's macro code emits errors.

[davidhewitt]

Once the error has been implemented, it would be worth adding a test for this as part of test_compile_error.

[cojmeister]

Hey @davidhewitt, I've opened PR #3993
Can you let me know if I'm going on the right direction?
Thanks!

[codeguru42]

I created a follow up PR #4194 that superceded #3993. This doesn't include any error message for text_signature which may no longer be necessary or can be a separate PR.