Replace proc-macro-error with proc-macro-error2

[Sympatron]

proc-macro-error depends on syn 1.0 and is no longer maintained. This PR replaces it with the drop-in replacement proc-macro-error2.

[jannic]

I don't have any experience with proc-macro-error2, but it indeed looks like a maintained fork of proc-macro-error.

However, it's only 22 days old, so it's unclear for how long it will stay maintained.

Not saying that this change is bad - just not sure if it is a long-term solution. Any opinions?

[Sympatron]

I thought about that too, but replacing it with more established crates like manyhow would require changes to the code base I could not do, because of my limited knowledge of proc macros.

The main reason I would want to update is to get rid of syn 1.0. That goal would be achieved independent of future maintenance. So If it becomes unmaintained in the future you could still switch to a different crate.

I primarily considered it to be ok because defmt did the same: knurling-rs/defmt@3a1c58a

[jannic]

I primarily considered it to be ok because defmt did the same: knurling-rs/defmt@3a1c58a

That's an interesting data point. And yes, completing the switch from syn 1 to syn 2 would be nice.

I'll still not merge this right now to give others a chance to comment, but that doesn't mean I don't like the change.

[jannic]

I'll still not merge this right now to give others a chance to comment, but that doesn't mean I don't like the change.

No further comments in about 2 weeks. With proc-macro-error2 quickly gaining popularity according to crates.io, I don't think there's a reason to wait any further. (And in case there are any objections, we can easily revert the commit.)