For generate a random string with fixed size in Rust, we can use Alphanumeric trait from rand crate:

use rand::{distributions::Alphanumeric, Rng}; // 0.8
 
fn main() {
  let s: String = rand::thread_rng()
    .sample_iter(&Alphanumeric)
    .take(7)
    .map(char::from)
    .collect();
  println!("{}", s);
}

ref: https://stackoverflow.com/questions/54275459/how-do-i-create-a-random-string-by-sampling-from-alphanumeric-characters