Never type impls

[Person-93]

With the never type stabilized in rust 1.100 we can add trait impls for it and use the never-say-never trick to make it work in older rust versions.

[Person-93]

@davidhewitt do you know what might be causing the compiler errors about conflicting implementations from the pyclass macro? https://github.com/PyO3/pyo3/actions/runs/35962300436/job/107513513485?pr=6451#step:14:261

[mejrs]

We already have impls for Infallible, which is an alias for !.

[Tpt]

+1 to @mejrs. In 1.100 Infallible is an alias for ! so I guess we can just implement traits for Infallible and get something that works for ! in 1.100+

[mejrs]

The major difference is that Infallible doesn't spontaneously coerce into anything (prior to it being aliased with rust 1.100) because empty enums do not have the same coercion behavior as !. This means you (and users) might have to insert a bunch of match never {} blocks here and there.

I think I would prefer that over the conditional compilation and the never say never hack

[Person-93]

I think I would prefer that over the conditional compilation and the never say never hack

Prior to never type stabilization I'd have agreed with you. It relied on a compiler bug which might be fixed in future versions. But with the conditional compilation, we are not relying on it in future versions.

[davidhewitt]

It certainly seems like the current implementation has issues on MSRV; perhaps let's just implement only for 1.100+ once that releases?