Declarative macro repetition counts

[nikomatsakis]

Summary

Add new syntax to allow declarative macro authors to easily access the count or index of declarative macro repetitions.

Info

• Lead: @markbt
• Lang-team liaison: @Lokathor
• Charter
• Draft RFC
• Zulip stream #t-lang/macro-repetition-count on the rust-lang Zulip

What is this issue?

This issue represents an active project group. It is meant to be used for
the group to post updates to the lang team (and others) in a lightweight
fashion. Please do not use the comments here for discussion, that should be kept
in the Zulip stream (discussion comments here will be marked as off-topic).

[nikomatsakis]

No update this week.

[markbt]

Project update

I've started looking at expanding more on the idea of general-purpose metavariable expressions (${ ... }) for use in extending the macro language, where macro repetition counts are the first user. It's coming together pretty nicely.

For a real-world example, this line in one of my crates would become:

let size = ${count(value)};

There is a prototype implementation on the markbt/rust:metavariable_expressions branch. If you'd like to try it out, any feedback would be welcome.

I've started drafting an RFC on the markbt/lang-team:metavar_expr_rfc branch (Rendered). It's still in its early stages, but feedback and contributions are always welcome.

Cc: @Lokathor @rust-lang/lang

[nikomatsakis]

Project update

• RFC: Declarative macro metavariable expressions rfcs#3086 is posted!

[nikomatsakis]

Closing this in favor of rust-lang/rust#83527