extern crate rand;

use std::io;
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);

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

        let mut guess = String::new();

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

        let guess: u32 = guess.trim().parse()
            .ok()
            .expect("Please type a number!");

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

extern crate rand;

use std::io;
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);

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

        let mut guess = String::new();

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

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

--- 
+++ 
@@ -18,5 +18,9 @@
             .ok()
             .expect("Failed to read line");
 
+        let guess: u32 = guess.trim().parse()
+            .ok()
+            .expect("Please type a number!");
+
         println!("You guessed: {}", guess);
 }
Convert `guess` from `String` into `u32`
