Stable support

[Hoverbear]

I (@lambda-fairy) am adopting the top comment to summarize the unstable features used in Maud. The original text can be found at the bottom of this comment.

Summary of unstable features:

• Mandatory
  • proc_macro_hygiene (Tracking issue for procedural macros and "hygiene 2.0" rust-lang/rust#54727)
• Can be removed, but will make the library less nice to use
  • proc_macro_diagnostic (Tracking Issue: Procedural Macro Diagnostics (RFC 1566) rust-lang/rust#54140)
  • proc_macro_span (Tracking issue for proc_macro::Span inspection APIs rust-lang/rust#54725)
• Can be removed, but will make the library slower
  • specialization (Tracking issue for specialization (RFC 1210) rust-lang/rust#31844)
• Can be removed, but will make the implementation more complex
  • proc_macro_def_site (Tracking issue for Span::def_site() rust-lang/rust#54724)
  • proc_macro_quote (Tracking issue for the quote! macro in proc_macro rust-lang/rust#54722)

---

Original comment by Hoverbear:

Hey! You might be able to get this to work on stable with Syntex.

[lambda-fairy]

Good idea.

I'm busy with other things at the moment, but I'll be happy to take a PR if it's not too intrusive.

[TheNeikos]

This has been rendered not possible for now due to a feature usage: https://github.com/lfairy/maud/blob/master/maud/src/lib.rs#L1

[Hoverbear]

Specialize is expected to land eventually so this should be possible again within the next 18 weeks. :)

[lambda-fairy]

@Hoverbear do you have a source for that? According to rust-lang/rust#31844 there's still a soundness hole in the implementation, and I don't think they'd stabilize before that gets fixed.

[Hoverbear]

Ah yes you're right! Darn. :(

[lambda-fairy]

We probably want to skip straight to rust-lang/rust#38356 when that's ready.

The library only uses specialization for optimizations, so it's harmless to remove that feature if need be. That might be useful if somehow procedural macros stabilize before specialization.

[dmwp37]

proc-macro2 may help to remove the #![feature(proc_macro)] feature dependency

[lambda-fairy]

Macros 1.2 will stabilize function-like macros, but "only invoked in module contexts, not as expressions or inside functions."

This rules out Maud for the time being, but is a step in the right direction nonetheless.