Naive prime number generation in Rust

prime-naive.rs

//! Conforms to the input data format of http://www.spoj.com/problems/PRIME1/ but not usable as a
//! solution due to the slowness of the algorithm

use std::io::{stdin, BufRead};
use std::error::Error;

fn get_line<R: BufRead>(r: &mut R) -> Result<String, Box<Error>> {
    let mut line = String::new();
    try!(r.read_line(&mut line));
    Ok(line)
}

fn is_prime(num: i32) -> bool {
    if num < 2 { return false; }

    let root = (num as f64).sqrt() as i32;

    for i in 2..root+1 {
        if num % i == 0 { return false; }
    }

    true
}

fn check_range(a: i32, b: i32) {
    for i in a..b+1 {
        if is_prime(i) {
            println!("{}", i);
        }
    }
}

fn go() -> Result<(), Box<Error>> {
    let stdin = stdin();
    let mut stdin = stdin.lock();

    let cnt = try!(try!(get_line(&mut stdin)).trim().parse::<i32>());

    for _ in 0..cnt {
        let line = try!(get_line(&mut stdin));

        let mut it = line.split_whitespace();

        match (it.next(), it.next()) {
            (Some(a), Some(b)) => check_range(try!(a.parse::<i32>()), try!(b.parse::<i32>())),
            _ => panic!("Invalid format"),
        }

        println!("");
    }

    Ok(())
}

fn main() {
    go().unwrap();
}