You can have it all though - at least, you can in Scala, Rust ought to support something similar. Something like (hybrid syntax/pseudocode):

    (for {
      fname <- args
      f = fs::File::open(fname).orElse(
        e => panic!("input file {} could not be opened: {}", fname, e))
      r = io::BufReader::new(f)
      w <- f.lines().flatMap {
          line => line.unwrap().split_whitespace()
        } .distinct
    } yield Map(fname -> Vec(w.toString))).sum

Just as concise as yours (more concise even), but easier to reason about and safer to refactor. Possibly you might need to do the mutable version for performance if this was a hot loop, but you don't get to even ask that question until you've profiled it and found out that it was the bottleneck; 99.9% of the time it isn't.