tokio: add load and compare_exchange_weak to loom StaticAtomicU64 (#5356)

Author: dtolnay

tokio/src/loom/std/atomic_u64_static_once_cell.rs

@@ -23,13 +23,34 @@ impl StaticAtomicU64 {
         }
     }
 
+    pub(crate) fn load(&self, order: Ordering) -> u64 {
+        *self.inner().lock()
+    }
+
     pub(crate) fn fetch_add(&self, val: u64, order: Ordering) -> u64 {
         let mut lock = self.inner().lock();
         let prev = *lock;
         *lock = prev + val;
         prev
     }
 
+    pub(crate) fn compare_exchange_weak(
+        &self,
+        current: u64,
+        new: u64,
+        _success: Ordering,
+        _failure: Ordering,
+    ) -> Result<u64, u64> {
+        let mut lock = self.inner().lock();
+
+        if *lock == current {
+            *lock = new;
+            Ok(current)
+        } else {
+            Err(*lock)
+        }
+    }
+
     fn inner(&self) -> &Mutex<u64> {
         self.cell.get(|| Mutex::new(self.init))
     }