test for forgetting locked mutex

Author: RalfJung

tests/pass/concurrency/sync_singlethread.rs

@@ -1,9 +1,11 @@
 use std::hint;
+use std::mem;
 use std::sync::atomic;
 use std::sync::{Mutex, TryLockError};
 
 fn main() {
     test_mutex_stdlib();
+    leak_mutex_guard();
     test_rwlock_stdlib();
     test_spin_loop_hint();
     test_thread_yield_now();
@@ -19,6 +21,12 @@ fn test_mutex_stdlib() {
     drop(m);
 }
 
+fn leak_mutex_guard() {
+    // Test for https://github.com/rust-lang/rust/issues/85434
+    let m = Mutex::new(5i32);
+    mem::forget(m.lock());
+}
+
 fn test_rwlock_stdlib() {
     use std::sync::RwLock;
     let rw = RwLock::new(0);