Unsoundness mapping/downgrading in RwLock

[Kestrer]

Version: Tokio 1.0.0

In the code:

use std::cell::Cell;
use tokio::sync::{RwLock, RwLockWriteGuard};

#[tokio::main]
async fn main() {
    let lock = RwLock::new(Cell::new(10_u32));
    let guard = lock.write().await;
    let guard = RwLockWriteGuard::map(guard, Cell::get_mut);
    let guard = RwLockWriteGuard::downgrade(guard);
    let value_ref: &u32 = &*guard;
    lock.read().await.set(11);
        
    drop(value_ref);
}

We obtain a reference to the Cell as well as a reference to the inner value - this can be seen in the error running Miri on the playground.

This same unsoundness occurred in parking_lot, and they had to remove the downgrade API (on mapped guards) as a result. However, this isn't really an option for Tokio now that it's stable.

(sorry for the edits, I accidentally posted this before I'd finished writing it)

[Darksonn]

Well that's problematic. It was added in #2733.

[MikailBag]

AFAIK fixing soundness bug is valid reason to break backwards compatibility.
For example Rust RFC 1122 allows it.