If you have more time

If you happen to have more time, you can use the clap crate to configure the file to read, the maximum number of threads, and the number of times you will run the benchmarks. Also, you might want to print a summary of the most frequent characters used, with a configurable number of characters printed:

$ cargo run -- --help
Usage: char-counter [OPTIONS] <FILE>

  <FILE>  The file to count characters from

  -m, --max <MAX>      The maximum level of parallelism to achieve [default: 8]
  -t, --times <TIMES>  The number of times to run each test [default: 100]
  -s, --stats <rank>   Display statistics
  -h, --help           Print help

Exercise 4.a: Do that, and make sure you send a mail to the faculty telling them you did it so that you may earn extra points (and put the code in your git repository so that they can check).