Support paths relative to the current file in query_file!()

[abonander]

This would require proc_macro::SourceFile to be stabilized (or a similar API): rust-lang/rust#54725

Probably would require fixing rust-lang/rust#62892

We could make it work on nightly using --cfg procmacro2_semver_exempt but that's a half-measure.

[de-vri-es]

You can also make it work on nightly by using proc_macro2::Span::unwrap to get a real proc_macro::Span and then use the feature gated API. I did something similar here.

That still can't support stable, but atleast it doesn't require all transitive reverse dependencies to build with --cfg procmacro2_semver_exempt.

It doesn't seem nice to introduce behavioural differences in the same macro between stable and nightly though. If it's important enough, it might be better to add the functionality to a separate macro only available on nightly, possibly with a feature.

[mehcode]

The current paths are at least cargo workspace relative.. for me, that's good enough to not want to do anything until we can do it properly.

[mehcode]

It looks like we can support this soon once rust-lang/rust#69041 lands

[abonander]

That doesn't give us a stable way to get the path still.

[mehcode]

This doesn't seem like its going to happen in Rust in the near future so I'm going to mark this postponed and close. We just need to make sure our work-arounds are well documented and we're fine I think.