Tracking Issue for item-level `const` blocks

[GrigorenkoPV]

There has been no RFC, afaict. However, t-lang have expressed enthusiasm about seeing this added.
The feature gate for the issue is #![feature(const_block_items)].

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

• Implementation
• 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

• RFC needed?
• it has been decided that module-level const { ... } item is equivalent to const _: () =, but what about statement-items (i.e. inside function body) or assoc items?

Implementation history

• Module-level const blocks: const blocks as a mod item #149174