title: Leave associated type unspecified in trait implementation
body: <p>I want to leave an associated type specification in a trait implementation for later, as it requires some work. Is there any way I can tell the compiler to ignore the missing associated type?</p>
  <pre><code>impl A for B {
      type C = todo()!;
  }
  </code></pre>
  <p>It throws this error</p>
  <pre><code>error: non-type macro in type position: ::core::panic
  </code></pre>