Add an as_mut method to MutableHandle

Signed-off-by: Greg Morenz <[email protected]>

Author: gmorenz

mozjs/src/gc/root.rs

@@ -216,6 +216,11 @@ impl<'a, T> MutableHandle<'a, T> {
         unsafe { *self.ptr = v }
     }
 
+    /// Safety: GC must not run during the lifetime of the returned reference.
+    pub unsafe fn as_mut<'b: 'a>(&mut self) -> &'b mut T {
+        &mut *(self.ptr)
+    }
+
     /// Creates a copy of this object, with a shorter lifetime, that holds a
     /// mutable borrow on the original object. When you write code that wants
     /// to use a `MutableHandle` more than once, you will typically need to