Tracking Issue for item-level `const` blocks

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)].

Currently, this feature allows you to write const blocks as items in modules. So, it makes the following code possible:

#![feature(const_block_items)]

const {
    let a = 2 + 2;
    assert!(a != 5)
}

fn main() { }

Which is equivalent to the current stable's:

const _: () = {
    let a = 2 + 2;
    assert!(a != 5)
};
fn main() { }

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.

See also

• https://github.com/rust-lang/lang-team/issues/251
• Zulip: #t-lang > item-level const blocks
• Zulip: #t-lang > inline static expressions

Steps

• Implementation (see at the very bottom)
• 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?
• Making const { ... } sometimes an item and sometimes an expression complicates our story about that.
  • Other possibilities would be things like const _ = ...; to clarify that it's an item, but shorten the syntax (like one can omit -> () on functions) nobody was too enthusiastic about this approach: see https://github.com/rust-lang/rust/pull/149738
• The diagnostics for module-level const block items are subpar, and could benefit from
  • mentioning that the return type of the block is expected to be ()
  • actually having the phrase "const block item" in messages instead of "malformed const"
  • not leaking the dummy _ identifiers that are inserted during AST->HIR lowering of const { ... } to const _: () = const { ... };
  • rejecting visibility qualifiers on module-level const block items, since those have no effect (see also: #147136)
• module-level const block items currently reject inner attributes during parsing. Is this the desired behavior?
• maybe use static { ... } instead of const { ... }?

Implementation history

• Module-level const blocks: #149174
• Ability to omit : () for const items: #149738

Start by reading the const_block_items feature gate, implementation history in #149174 and #149738, and the unresolved questions listed here. Completion requires resolving the design questions, implementing the feature, updating documentation and syntax style, and completing stabilization work.