Update to loom v0.3 (#381)

src/loom.rs

@@ -2,8 +2,29 @@
 pub(crate) mod sync {
     pub(crate) mod atomic {
         pub(crate) use core::sync::atomic::{fence, AtomicPtr, AtomicUsize, Ordering};
+
+        pub(crate) trait AtomicMut<T> {
+            fn with_mut<F, R>(&mut self, f: F) -> R
+            where
+                F: FnOnce(&mut *mut T) -> R;
+        }
+
+        impl<T> AtomicMut<T> for AtomicPtr<T> {
+            fn with_mut<F, R>(&mut self, f: F) -> R
+            where
+                F: FnOnce(&mut *mut T) -> R,
+            {
+                f(self.get_mut())
+            }
+        }
     }
 }
 
 #[cfg(all(test, loom))]
-pub(crate) use ::loom::sync;
+pub(crate) mod sync {
+    pub(crate) mod atomic {
+        pub(crate) use loom::sync::atomic::{fence, AtomicPtr, AtomicUsize, Ordering};
+
+        pub(crate) trait AtomicMut<T> {}
+    }
+}