Borrow checker in rust

main.rs

fn main() {
    let mut s1 = String::from("Hello");
    
    let s2 = &mut s1;
    
    s2.push('!');
    
    // change order of next tow lines and program will not compile
    println!("s2 :{}", s2);
    println!("s1 :{}", s1);
    
    s2.push('.'); // comment this line to to make program compile
}