Tracking Issue for RFC 3086: macro metavariable expressions

[nikomatsakis]

This is a tracking issue for the RFC "3086" (rust-lang/rfcs#3086).
The feature gate for the issue is #![feature(macro_metavar_expr)].

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Implement the RFC (cc @markbt -- has there been work done here already?)
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved questions and bugs

• Figure out problems around hygiene

Implementation history

• 2022-02-25, [1/2] Implement macro meta-variable expressions
• 2022-03-11, [2/2] Implement macro meta-variable expressions
• 2022-03-12, Fix remaining meta-variable expression TODOs
• 2019-03-21, [macro-metavar-expr] Fix generated tokens hygiene
• 2022-04-07, Kickstart the inner usage of macro_metavar_expr
• 2022-04-07, [macro_metavar_expr] Add tests to ensure the feature requirement
• 2023-05-24, [RFC-3086] Restrict the parsing of count
• 2023-10-22, [RFC 3086] Attempt to try to resolve blocking concerns

[markbt]

There is a working prototype on the markbt/rust/metavariable_expressions branch. This needs feature gating, and there are a couple of TODOs to resolve, but it's otherwise in reasonable shape. I'm planning to work on it over the coming weeks.

[markbt]

Update (2021-04-07)

I've not yet started work on productionizing the prototype on the markbt/rust/metavariable_expressions branch. I plan to start later this month, free time permitting.

[markbt]

Update (2021-05-09)

I still haven't started on this yet as some stuff came up last month that prevented from having the time to work on it. It's still in my plan to work on it, and hopefully I'll have some time soon.

[markbt]

Update (2021-06-29)

Still no progress, as I haven't had any spare time to work on this project. I'm still planning to work on it, and hopefully will get some time soon.

[joshtriplett]

@markbt If you don't expect to find the bandwidth in the near future, would you potentially be interested in seeking help in the form of another owner for this initiative? If you're still interested in driving this, that's fine, but if you'd like us to switch gears from pinging you to broadcasting that the project could use help, we'd be happy to do that.

[markbt]

I'd be happy with any help if there's someone available. I still plan to work on it, but personal stuff is getting in the way at the moment. Sorry about that.

To recap: I have a working prototype on my branch at https://github.com/markbt/rust/tree/metavariable_expressions . The next steps are to rebase that onto the latest master, and then polish it up so that it's ready for inclusion. Then there's also the doc work to make sure the new feature is documented well. Help with any of this would be appreciated.