[nine_k]

Said promise should also be checked for sanity. E.g.

  for i in range(N) {
    for i in range(M) {
      # Typo; wanted j.
      # The compiler should complain.
    }
  }

[Maxatar]

The Rust compiler would complain in this case that the initial i variable is unused. Unused variables should be named with an underscore, _.