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

If you would like to access elements of the Vec with index checking, you can use the Vec as a slice and then use its get method. For example, consider the following code.

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

This outputs:

Some(3)
None