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

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Feature gate: `#![feature(rwlock_downgrade)]`

This is a tracking issue for a `downgrade` method for `RwLock` as discussed in https://github.com/rust-lang/libs-team/issues/392.

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The `downgrade` method on `RwLockWriteGuard` will transform a write-locked `RwLock` into a read-locked one.

### Public API

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```rust
impl<'a, T: ?Sized> RwLockWriteGuard<'a, T> {
    pub fn downgrade(s: Self) -> RwLockReadGuard<'a, T> {}
}
```

### Steps / History

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- [X] ACP: https://github.com/rust-lang/libs-team/issues/392
- [x] Implementation: #128219 
    - [x] `downgrade` implementation for [`futex.rs`](https://github.com/rust-lang/rust/blob/master/library/std/src/sys/sync/rwlock/futex.rs)
    - [x] `downgrade` implementation for [`no_threads.rs`](https://github.com/rust-lang/rust/blob/master/library/std/src/sys/sync/rwlock/no_threads.rs)
    - [x] `downgrade` implementation for [`queue.rs`](https://github.com/rust-lang/rust/blob/master/library/std/src/sys/sync/rwlock/queue.rs)
    - [x] `downgrade` implementation for [`solid.rs`](https://github.com/rust-lang/rust/blob/master/library/std/src/sys/sync/rwlock/solid.rs)
    - [x] `downgrade` implementation for [`teeos.rs`](https://github.com/rust-lang/rust/blob/master/library/std/src/sys/sync/rwlock/teeos.rs)
- [ ] Final comment period (FCP)[^1]
- [ ] Stabilization PR

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### Unresolved Questions

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- It is likely that the reader protocol for the futex implementation will need to change. See below.
- ~~How to go about implementing `downgrade` for the `queue.rs` implementation?~~
- ~~Does the `solid_asp3` platform already have a `downgrade` method on their `RwLock` implementation?~~
- ~~Does lock poisoning play into this at all?~~

[^1]: https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html