Also seems like you could benefit from the entry() api (available on both BTreeMap and HashMap):

http://doc.rust-lang.org/std/collections/struct.BTreeMap.htm...

I think the example used in the docs is your exact use case.

This bit:

  let found = match map.get_mut(..) {
    ..
  }
  if !found {
    ..
  }

can be replaced with

  match map.entry(word) {
      Occupied(mut view) => { *view.get_mut() += 1; }
      Vacant(view) => { view.insert(1); }
  }