Sub string for rust

sub_string.rs

pub fn sub_string(model: &String, begin: usize, len: usize) -> Option<String>
{
    let mut to_return = String::with_capacity(len);
    let mut end = begin + len;

    if begin > model.len(){
        return None;
    }
    if begin + len > model.len(){
        end = model.len() - begin;
    }
    Some(String::from_str(&(model[begin..end])) )
}

#[test]
fn test_sub_string()
{
    let test1 = sub_string(&"aaabbbaaa".to_string(), 3, 3).unwrap();
    println!("test1: {}", test1);
    assert_eq!(test1.as_slice(), "bbb");

    let test2 = sub_string(&"bbb".to_string(), 0, 3).unwrap();
    println!("test2: {}", test2);
    assert_eq!(test2.as_slice(), "bbb");

    let test3 = sub_string(&"b".to_string(), 0, 3).unwrap();
    println!("test3: {}", test3);
    assert_eq!(test3.as_slice(), "b");

    let test4 = sub_string(&"b".to_string(), 5, 3);
    assert_eq!(test4, None);
}