title: Convert String to Vec<char> at compile time for pattern matching
body: <p>I'm writing a parser in Rust and I'm creating tokens from a <code>Vec&lt;char&gt;</code>. Currently, my code looks like</p>
  <pre class="lang-rust prettyprint-override"><code>match &amp;source[..] {
      ['l', 'e', 't', ..] =&gt; ...,
      ['t', 'r', 'u', 'e', ..] =&gt; ...,
      _ =&gt; ...
  }
  </code></pre>
  <p>Obviously this is a lot more verbose than I'd like, and not easy to read. Is there any way I can convert <code>&quot;let&quot;</code> to <code>['l', 'e', 't']</code> at compile time (with a macro or const function) in order to pattern match on it like this?</p>