Design meeting: `macro_metavar_expr`

[traviscross]

Summary

See:

• Decide what we want about macro_metavar_expr rust#137581

We'd need a volunteer to write the doc for this.

cc @jhpratt

About this issue

This issue corresponds to a lang-team design meeting proposal. It corresponds to a possible topic of discussion that may be scheduled for deeper discussion during one of our design meetings.

[jhpratt]

What would this document be, exactly?

[traviscross]

(Will reply and discuss elsewhere.)