#[document_codes] attribute macro

[zkat]

So it turns out that derive macros can't expand anything in-place: they can only generate new code in a completely different area.

We need a #[document_codes] attribute macro that you can slap at the toplevel of anything using the Diagnostic derive, which goes through and adds #[doc(alias = "my::code::here")] to the error struct or enum, in the appropriate places.

This might be tricky because you're injecting stuff in the middle of a tokenstream, but maybe it's easier than it sounds?

[zkat]

Update: This isn't possible on stable Rust yet (booo).

Tracking issue for stabilization: rust-lang/rust#81119

[zkat]

You know what? I'm just gonna scrap this idea for the foreseeable future, seeing as we have url(...) (and particulaly url(docsrs) now. Wee