[Rust] non-redundent-digit numbers demo

use std::io;
use std::io::Write;
use std::process;
use std::collections::HashSet;

fn main() {
    // Prompt for user input
    print!("Input a number: ");
    let _ = io::stdout().flush();

    // Receive user input
    let mut input = String::new();
    io::stdin()
        .read_line(&mut input)
        .expect("failed to read from stdin");

    // Parse the number
    let n = match input.trim().parse::<u32>() {
        Ok(i) => i,
        Err(_) => {
            println!("Invalid integer");

            // Exit the program with abnormal status code
            process::exit(1);
        }
    };

    let x: i32 = 10;
    for i in 1..(x.pow(n)) {
        /* Convert number to an iterator of char.
           Then, insert char to set. */
        let num_string = i.to_string();
        let mut chars = num_string.chars();
        let mut set = HashSet::new();
        let mut c = chars.next();
        while c != None {
            set.insert(c);
            c = chars.next();
        }

        // Get the digit number of i
        let mut digit = 0;
        let mut j = i;
        while j > 0 {
            j = j / 10;
            digit += 1;
        }

        // Check whether i fits our criteria
        if set.len() as i32 >= digit {
            // Show i in console
            print!("{} ", i);
        }
    }

    // Print tailing newline
    println!("");
}