Auto merge of #96820 - r-raymond:master, r=cuviper

Make RwLockReadGuard covariant

Hi, first time contributor here, if anything is not as expected, please let me know.

`RwLockReadGoard`'s type constructor is invariant. Since it behaves like a smart pointer to an immutable reference, there is no reason that it should not be covariant. Take e.g.

```
fn test_read_guard_covariance() {
    fn do_stuff<'a>(_: RwLockReadGuard<'_, &'a i32>, _: &'a i32) {}
    let j: i32 = 5;
    let lock = RwLock::new(&j);
    {
        let i = 6;
        do_stuff(lock.read().unwrap(), &i);
    }
    drop(lock);
}
```
where the compiler complains that &i doesn't live long enough. If `RwLockReadGuard` is covariant, then the above code is accepted because the lifetime can be shorter than `'a`.

In order for `RwLockReadGuard` to be covariant, it can't contain a full reference to the `RwLock`, which can never be covariant (because it exposes a mutable reference to the underlying data structure). By reducing the data structure to the required pieces of `RwLock`, the rest falls in place.

If there is a better way to do a test that tests successful compilation, please let me know.

Fixes #80392

0 files changed, 0 insertions, 0 deletions