build(rust): vendor rnix parser crate

third_party/crates/rowan-0.16.1/src/cow_mut.rs

@@ -0,0 +1,30 @@
+#[derive(Debug)]
+pub(crate) enum CowMut<'a, T> {
+    Owned(T),
+    Borrowed(&'a mut T),
+}
+
+impl<T> std::ops::Deref for CowMut<'_, T> {
+    type Target = T;
+    fn deref(&self) -> &T {
+        match self {
+            CowMut::Owned(it) => it,
+            CowMut::Borrowed(it) => *it,
+        }
+    }
+}
+
+impl<T> std::ops::DerefMut for CowMut<'_, T> {
+    fn deref_mut(&mut self) -> &mut T {
+        match self {
+            CowMut::Owned(it) => it,
+            CowMut::Borrowed(it) => *it,
+        }
+    }
+}
+
+impl<T: Default> Default for CowMut<'_, T> {
+    fn default() -> Self {
+        CowMut::Owned(T::default())
+    }
+}