Tracking Issue for macro fragment fields

This is an experimental tracking issue for the RFC "Macro Fragment Fields" (rust-lang/rfcs#3714) (not yet accepted).
The feature gates for the issue are #![feature(macro_fragment_fields)] and #![feature(macro_fragments_more)].

Lang team experiment request: https://github.com/rust-lang/lang-team/issues/352

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.
Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

• Implement the RFC (@joshtriplett: in progress)
• Adjust documentation (see instructions on rustc-dev-guide)
• Style updates for any new syntax (nightly-style-procedure)
  • Style team decision on new formatting
  • Formatting for new syntax has been added to the Style Guide
  • (non-blocking) Formatting has been implemented in rustfmt
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

Implementation history

Start with RFC 3714 and the linked lang-team experiment request to understand the proposed macro fragment fields and current implementation status. Review the tracking checklist and the rustc-dev-guide links; this work is done only when implementation, documentation, style updates, and stabilization requirements are addressed.