use itertools::Itertools;
use rand::{distributions::Uniform, Rng}; // 0.6.5

fn main() {
    let mut rng = rand::thread_rng();
    let range = Uniform::new(0, 20);

    // we stop at 1000 try, use std::iter::repeat(()) if you want infinite
    let vals: Vec<u64> = (0..1000)
        .map(|_| rng.sample(&range))
        .unique()
        .take(100)
        .collect();

    println!("{:?}", vals);
}