test.rs

struct Foo {
  bar: ~Bar
}

struct Bar {
  age: int
}

impl IndexMut<int,Bar> for Foo {
  fn index_mut<'a>(&'a mut self, element: &int) -> &'a mut Bar {
    &mut *self.bar
  }
}

fn main() {
  let foo = &mut Foo{ bar: ~Bar{ age: 10 } };
  let bar: &mut Bar = &mut foo[10];
}