Design meeting: Declarative (`macro_rules!`) attributes and derives

[tomassedovic]

Summary

@joshtriplett wants to get a consensus on moving the declarative macro_rules! for attribute and derive macros forward:

• Declarative macro_rules! attribute macros rfcs#3697
• Declarative macro_rules! derive macros rfcs#3698

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.

[tomassedovic]

We've scheduled this in the planning call yesterday for 2025-07-09.

If both RFCs get approved ahead of time, we'll cancel the meeting.

[PoignardAzur]

I wonder if this would be a good point to bring up variadic generics as well.

Hopefully I should have put out more concrete material by then.

[traviscross]

This went into FCP so we don't need the meeting.