Vendor things

third-party/vendor/loom/tests/rwlock_regression1.rs

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+use loom::{
+    sync::{
+        atomic::{AtomicUsize, Ordering},
+        Arc, RwLock,
+    },
+    thread,
+};
+
+#[test]
+fn rwlock_two_writers() {
+    loom::model(|| {
+        let lock = Arc::new(RwLock::new(1));
+        let c_lock = lock.clone();
+        let c_lock2 = lock;
+
+        let atomic = Arc::new(AtomicUsize::new(0));
+        let c_atomic = atomic.clone();
+        let c_atomic2 = atomic;
+
+        thread::spawn(move || {
+            let mut w = c_lock.write().unwrap();
+            *w += 1;
+            c_atomic.fetch_add(1, Ordering::Relaxed);
+        });
+
+        thread::spawn(move || {
+            let mut w = c_lock2.write().unwrap();
+            *w += 1;
+            c_atomic2.fetch_add(1, Ordering::Relaxed);
+        });
+    });
+}