Masking with # except last 4 string

     "64607935616" -->      "#######5616"
               "1" -->                "1"
                "" -->                 ""

 

 

 

fn maskify(cc: &str) -> String {
    let mask_length = cc.len().saturating_sub(4);
    "#".repeat(mask_length) + &cc[mask_length..]
}