title: Why are len() and is_empty() not defined in a trait?
body: <p>Most patterns in Rust are captured by traits (<code>Iterator</code>, <code>From</code>, <code>Borrow</code>, etc.).</p>
  
  <p>How come a pattern as pervasive as <code>len</code>/<code>is_empty</code> has no associated trait in the standard library? Would that cause problems which I do not foresee? Was it deemed useless? Or is it only that nobody thought of it (which seems unlikely)? </p>