pan class='kw-2'>&amp;</span><span class='lifetime'>&#39;a</span> <span class='ident'>s</span>;
    }
}</pre>

<p>This signature of <code>as_str</code> takes a reference to a u32 with <em>some</em> lifetime, and
promises that it can produce a reference to a str that can live <em>just as long</em>.
Already we can see why this signature might be trouble. That basically implies
that we&#39;re going to find a str somewhere in the scope the reference
to the u32 originated in, or somewhere <em>even earlier</em>. 