Generate a random u8 from 0 to 255 inclusively

Use the gen method instead. This method will generate a random value from the whole set of possible values for the specified type.

extern crate rand;

use rand::Rng;

fn main() {
    println!("Guess the number!");
    let random_number: u8 = rand::thread_rng().gen();

    println!("Your random number is {}", random_number);
}