Declarative (macro_rules!) macro improvements #629

Summary

I propose to continue improving the state of declarative macros (e.g.
macro_rules! macros) in Rust, with the eventual aim of making them just as
capable as proc macros while being easier to write and faster to compile.

I'll also prototype extensions to the declarative macro system to make macros
easier to write. I intend to collaborate with the "Reflection and Comptime"
goal,
and work with experimental implementations of comptime when available.

The initial focus of this goal is the stabilization of declarative attribute
and derive macros, along with enough functionality to enable the adoption of
such macros by a reasonable subset of crates without substantive degradation of
functionality or usability.

This issue is intended for status updates only.

For general questions or comments, please contact a goal owner or post in the Zulip topic.