Make RwLockReadGuard covariant

1 files changed, 13 insertions, 1 deletions

diff --git a/library/std/src/sync/rwlock/tests.rs b/library/std/src/sync/rwlock/tests.rs
index 53aa2b1e38a..1ffa13f8321 100644
--- a/library/std/src/sync/rwlock/tests.rs
+++ b/library/std/src/sync/rwlock/tests.rs
@@ -1,6 +1,6 @@
 use crate::sync::atomic::{AtomicUsize, Ordering};
 use crate::sync::mpsc::channel;
-use crate::sync::{Arc, RwLock, TryLockError};
+use crate::sync::{Arc, RwLock, TryLockError, RwLockReadGuard};
 use crate::thread;
 use rand::{self, Rng};
 
@@ -245,3 +245,15 @@ fn test_get_mut_poison() {
         Ok(x) => panic!("get_mut of poisoned RwLock is Ok: {x:?}"),
     }
 }
+
+#[test]
+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);
+}