title: How can I append a formatted string to an existing String?
body: <p>Using <a href="http://doc.rust-lang.org/std/macro.format!.html" rel="noreferrer"><code>format!</code></a>, I can create a <code>String</code> from a format string, but what if I already have a <code>String</code> that I'd like to append to? I would like to avoid allocating the second string just to copy it and throw away the allocation.</p>
  
  <pre><code>let s = "hello ".to_string();
  append!(s, "{}", 5); // Doesn't exist
  </code></pre>
  
  <p>A close equivalent in C/C++ would be <a href="http://www.cplusplus.com/reference/cstdio/snprintf/" rel="noreferrer"><code>snprintf</code></a>. </p>