Repository: rust
Issue: Tracking Issue for `UniqueRc`/`UniqueArc`
Labels: T-libs-api,C-tracking-issue

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Feature gate: `#![feature(unique_rc_arc)]`

This is a tracking issue for `UniqueRc` and `UniqueArc`, as discussed in https://github.com/rust-lang/libs-team/issues/90.

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This feature supports creating cyclic data structures by creating an RC that is guaranteed to be uniquely owned. The uniquely owned RC can then be converted to a regular RC. While uniquely owned, we can freely modify the contents of the `UniqueRc`/`UniqueArc` (i.e. they implement `DerefMut`). Weak pointers to the object can be made, but upgrading these will fail until it has been converted into a regular RC.

### Public API

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```rust
let rc = UniqueRc::new(42);
let weak = UniqueRc::downgrade(&rc);
assert!(weak.upgrade().is_none());

let _rc = UniqueRc::into_rc(rc);
assert_eq!(*weak.upgrade().unwrap(), 42);
```

### Steps / History

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- [x] Implementation:
  - [x] `UniqueRc`: #111849
  - [ ] `UniqueArc`
- [ ] Final comment period (FCP)[^1]
- [ ] Stabilization PR

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### Unresolved Questions

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- Do we also want `EmptyRc`/`EmptyArc`?
- Are we concerned about confusion around the existing `is_unique` methods? Could we add this functionality to `Rc`/`Arc`/ instead? ([comment](https://github.com/rust-lang/rust/pull/111849#discussion_r1225322362))
- Do we want consistency with [Linux Kernel `UniqueRc`](https://github.com/Rust-for-Linux/linux/blob/rust/rust/kernel/sync/arc.rs#L502-L578)? ([comment](https://github.com/rust-lang/rust/pull/111849#issuecomment-1592034589))

[^1]: https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html