Listing all primes up to a given limit
primes.rs:11:13: 11:25 warning: value assigned to `is_prime` is never read, #[warn(unused_assignments)] on by default
primes.rs:11         let mut is_prime = 0;
                         ^~~~~~~~~~~~


You don't have to initialize a variable when you declare it. That is, you can write

let mut is_prime;


You just have to initialize it before reading from it; if the compiler cannot guarantee that the variable will be initialized at that point, it will raise an error.



primes.rs:14:17: 14:25 warning: value assigned to `is_prime` is never read, #[warn(unused_assignments)] on by default
primes.rs:14                 is_prime = 1;
                             ^~~~~~~~


That warning comes from this block:

if n &lt; 4 {
    is_prime = 1;
}


This block is useless, because you are unconditionally setting is_prime to 1 after a couple of other if blocks, and you don't read back the variable between these assignments.



is_prime should be a boolean instead of an integer. Rust has a built-in boolean type, bool, and the keywords true and false, which you can use instead of 1 and 0.



Instead of having a mutable is_prime variable, I'd factor out the code that decides if a number is prime in a separate function. We can use early returns to avoid defining a mutable variable.

use std::io;

fn is_prime(n: u64) -&gt; bool {
    if n &lt; 4 {
        // Skip all the following checks, we know it's a prime!
        return true;
    }

    // We don't need to check if n is 2 or 3 anymore,
    // since if n is one of these values, we won't reach here.
    if n % 2 == 0 {
        return false;
    }

    if n % 3 == 0 {
        return false;
    }

    let mut m = 5;
    while m &lt;= (n as f64).sqrt() as u64 {
        if n % m == 0 {
            return false;
        }

        if n % (m + 2) == 0 {
            return false;
        }

        m += 6;
    }

    // Implicit return
    true
}

fn main() {
    println!("Enter a number: ");
    let mut limit = String::new();
    io::stdin().read_line(&amp;mut limit).expect("failed to read line");
    let limit: u64 = limit.trim().parse().expect("Please enter a number.");
    if limit &lt;= 0 {
        println!("Number must be greater than or equal to 0.");
    } else {
        for n in 2..limit + 1 {
            if is_prime(n) {
                println!("{}", n);
            }
        }
    }
}