title: Specifying generic parameter to belong to a small set of types
body: <p>Is it possible with to constrain a generic parameter to be one of the select few types without figuring out what traits precisely define those type?  e.g.</p>
  
  <pre><code>impl&lt;T&gt; Data&lt;T&gt; where T == u32 || T == u64 
  </code></pre>
  
  <p>Sometimes it's tedious to figure out what all traits to add to <code>where</code> to get the types you want, and sometimes one wouldn't want to allow a type even when it makes syntactic sense because of semantics. </p>