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00ce47209d
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 |
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Cargo.toml |