This is a tracking issue for taking references to values of types that (may) contain UnsafeCell within const contexts.
The feature gate for the issue is #![feature(const_refs_to_cell)].

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.

Steps

• Implement the RFC (Allow references to interior mutable data behind a feature gate #80418)
• Adjust documentation: const_eval: update for const_mut_refs and const_refs_to_cell stabilization reference#1590
• Stabilization PR: Stabilize &mut (and *mut) as well as &Cell (and *const Cell) in const #129195

Unresolved questions

• The feature gate permits const BAR: AtomicUsize = ..; static FOO: &AtomicUsize = &BAR; and only prevents const FOO: &AtomicUsize = &BAR;. This decision was made because the implementation is simpler this way and we would already allow the static FOO case if BAR were a static itself.
  • This could be problematic, because LLVM ends up deduplicating some unnamed allocations like the one in static FOO
• The soundness of this feature depends on the fact that if a StorageDead exists for a local, then for every assignment to the local, on each pass from that assignment to any return, there will be a StorageDead for this local.
• Once we get heap allocations in constants, it would be possible for users to write a const fn foo() -> &'static Cell<i32>, which could then be used to initialize a const FOO: &'static Cell<i32> which is unsound (and only caught by dynamic checks). The tracking issue for const heap is Tracking Issue for const_heap #79597

Implementation history

• Allow references to interior mutable data behind a feature gate #80418