warning: the loop variable `i` is only used to index `b`
 --> src/util.rs:8:14
  |
8 |     for i in 0..n {
  |              ^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_range_loop
  = note: `#[warn(clippy::needless_range_loop)]` on by default
help: consider using an iterator
  |
8 |     for <item> in b.iter_mut().take(n) {
  |         ~~~~~~    ~~~~~~~~~~~~~~~~~~~~

warning: function `rand_str` is never used
 --> src/util.rs:5:8
  |
5 | pub fn rand_str(n: usize) -> String {
  |        ^^^^^^^^

warning: constant `LETTERS` is never used
 --> src/util.rs:3:7
  |
3 | const LETTERS: &[u8] = b"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
  |       ^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default