title: Why is the size of a tuple or struct not the sum of the members?
body: <pre><code>assert_eq!(12, mem::size_of::&lt;(i32, f64)&gt;()); // failed
  assert_eq!(16, mem::size_of::&lt;(i32, f64)&gt;()); // succeed
  assert_eq!(16, mem::size_of::&lt;(i32, f64, i32)&gt;()); // succeed
  </code></pre>
  <p>Why is it not 12 (4 + 8)?
  Does Rust have special treatment for tuples?</p>