Guessing Game example from The Rust Programming Language book.

guessing_game.rs

extern crate rand;

use std::io;
use std::cmp::Ordering;
use rand::Rng;

fn main() {
    println!("Guess the number!");

    let secret_number = rand::thread_rng().gen_range(1, 101);

    println!("The secret number is: {}", secret_number);
    
    loop {

        println!("Please input your guess.");

        let mut guess = String::new();

        io::stdin().read_line(&mut guess)
            .expect("Failed to read line");

        let guess: u32 = match trim_and_validate(guess) {
            Some(num) => num,
            None => continue,
        };

        println!("You guessed: {}", guess);

        match guess.cmp(&secret_number) {
            Ordering::Less => println!("Too small!"),
            Ordering::Greater => println!("Too big!"),
            Ordering::Equal => {
                println!("You win!");
                break;
            }
        }
    }
}

fn trim_and_validate(input: String) -> Option<u32> {
    match input.trim().parse() {
        Ok(num) => return Some(num),
        Err(_) => return None,
    };
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_trim_and_validate() {
        let input = String::from("23");

        let output: u32 = match trim_and_validate(input) {
            Some(num) => num,
            None => panic!("Something went wrong"),
        };

        assert_eq!(output, 23); 
    }
}