Why does the Rust compiler allow index out of bounds?

Question

Can someone explain why this compiles:

fn main() {
    let a = vec![1, 2, 3];
    println!(\"{:?}\", a[4]);
}

When running it, I got:

Answer 1

Maybe what you mean is :

fn main() {
    let a = vec![1, 2, 3];
    println!("{:?}", a[4]);
}

This returns an Option so it will return Some or None. Compare this to:

fn main() {
    let a = vec![1, 2, 3];
    println!("{:?}", &a[4]);
}

This accesses by reference so it directly accesses the address and causes the panic in your program.