Why does the Rust compiler allow index out of bounds?

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));
}

Rust Playground

This outputs:

Some(3)
None