title: What does `impl ... for` mean?
body: <p>I see the following pattern coming up a lot in Rust codebases, but I can't find an explanation why this is used.</p>
  <p>For what end does one use the <code>impl ... for</code> construction?</p>
  <p>Pseudocode:</p>
  <pre><code>impl Handler {
      pub fn method1() -&gt; () {
      }
  }
  </code></pre>
  <pre><code>impl Dummy for Handler {
      pub fn method2() -&gt; () {
      }
  }
  </code></pre>