title: Why does unwrapping a cloned Rc cause a panic?
body: <p>Why does this code panic on line 7? Shouldn't foo_unwrapped be Some(3) on line 5 instead of None?</p>
  <pre><code>use std::rc::Rc;
  fn main()
  {
      let foo: Rc&lt;i32&gt; = Rc::new(3);
      let mut foo_cloned = Rc::clone(&amp;foo);
      let mut foo_unwrapped = Rc::get_mut(&amp;mut foo_cloned).unwrap();  
      foo_unwrapped = &amp;mut 42;
  }
  </code></pre>