title: struct definition with var instead of const in zig language
body: <p>I am now learning <code>zig</code> language. I have seen definitions of structs with <code>const</code> keyword like</p>
  <pre><code>const X = struct {
      n: i32,
  };
  </code></pre>
  <p>My understanding is that <code>const</code> is a kind of complementary to <code>var</code>, the latter allows change, the former does not. But what would mean defining struct with <code>var</code>?</p>
  <pre><code>var Y = struct {
      n: i32,
  };
  </code></pre>
  <p>Is this legal? I compiles, so yes, it is. But what is the meaning and use of this?</p>