title: Getting size of an array passed in as an argument
body: <p>I can't seem to make this work. I keep getting an error saying 'len' doesn't exist on type '&amp;[String]'.</p>
  
  <pre><code>fn testLength(arr: &amp;[String]) {
      if arr.len &gt;= 10 {
          // Do stuff
      }
  }
  </code></pre>
  
  <p>I'm still pretty new to Rust, and I understand this is a pointer to a raw string somewhere. Why can't I get the length of the underlying string at runtime? Googling things like "length of string in rust" and "length of array in rust" lead me absolutely no where.</p>