Interesting: the careful tests fail in exactly the same way as the tests fail on the opaque pyobject build. Maybe a useful hint that there's some unsafety happening somewhere...

the careful tests fail in exactly the same way as the tests fail on the opaque pyobject build.

It looks like all the tests on Python 3.12 and newer fail the abi3,full,multiple-pymethods tests. So it looks like the issue is using a PyVarObject in the limited API. Before this PR the base type is still a static PyObject.

As @davidhewitt pointed out when I chatted with him last Friday - it's worth checking to see if doing that introduces performance regressions. If it does, we need to make sure we set things up so a PyVarObject base type is only used on opaque pyobject builds.

codspeed uses the free-threaded build so it's not testing this configuration at all. I need to run benchmarks locally to see what the performance impact is. We also need a benchmark for creating PyClass instances - there is a benchmark for type initialization but not for instance initialization as far as I can see.

I opened #5816 with a benchmark for creating an instance of a PyClass. I also had to hack in abi3 support for the benchmark crate, see #5818.

Running the bench_pyclass_create benchmark from #5816, I see the following results on Python 3.12.12:

cargo bench --manifest-path=pyo3-benches/Cargo.toml --features=abi3 bench_pyclass_create

With opaque PyObject:
bench_pyclass_create    time:   [42.311 ns 42.374 ns 42.441 ns]

Without:
bench_pyclass_create    time:   [40.705 ns 40.790 ns 40.882 ns]

I ran the benchmarks on my personal development machine so there's some noise in the results. I'm showing the best of four runs of this benchmark for each commit. I see similar results on 3.15a6.

Very roughly, there's a 5% performance cost. That's on the bubble for being significant but probably is too steep to force extensions to pay that cost on the 3.12 stable ABI.

I think I'll go ahead and adjust this PR to only use PyVarObject as a base when it's absolutely needed when _Py_OPAQUE_PYOBJECT is defined.