シーザー暗号 in Rust

caesar.rs

static SHIFT:u8=3;

fn main() {
    use std::env;
    let args: Vec<_> = env::args().collect();
    if args.len() > 2 {
        if args[1] == "encrypt" {
            println!("encrypt!");
            for s in &args[2..] {
                println!("{}", encrypt(&s, true));
            }
        }else if args[1] == "decrypt" {
            println!("decrypt!");
            for s in &args[2..] {
                println!("{}", encrypt(&s, false));
            }
        }
    }
}

fn encrypt(s: &str, is_encrypt: bool) -> String{
    let mut result = Vec::new();
    if is_encrypt {
        for c in s.bytes(){
            result.push(c+SHIFT);
        }
    }else{
        for c in s.bytes(){
            result.push(c-SHIFT);
        }
    }
    return String::from_utf8(result).unwrap();
}