borrow_mut_race.rs

let mut vec1 = vec![1, 2, 3];
{
    let vec2 = &mut vec1;
    // We can mutate s1's data via a mutable reference!
    vec2.push(4);
    // Error: cannot access data when a mutable 
    // reference is in place
    println!("{}", vec1.len());
}
println!("{}", vec1.len());