Code shared from the Rust Playground

playground.rs

fn main() -> Result<(), &'static str> {
    let mut a = [1,2,3,4];
    println!("{:?}", a);
    {
        let b = &mut a[0..2];
        // You can't access a at this point because it has been mutably borrowed
        // from. The following line won't compile, with the error message:
        // `cannot borrow `a` as immutable because it is also borrowed as mutable`:
        // println!("a: {:?}", a);
        println!("b: {:?}", b); // b: [1, 2]
        b[0] = 42;
        println!("b: {:?}", b); // b: [42, 2]
    }
    // a is accessible again:
    println!("a: {:?}", a); // a: [42, 2, 3, 4]
    Ok(())
}