borrow_read_lock.rs

let mut vec1 = vec![1, 2, 3];
{
    let vec2 = &vec1;
    let vec3 = &vec1;
    println!("{} {}", vec2.len(), vec3.len());

    // Error: cannot obtain mutable reference once
    // other references are in place
    let vec4 = &mut vec1;
}
println!("{}", vec1.len());