pub fn secure_substring(string: &str, start: usize, end: usize) -> String {
assert!(end >= start);
string.chars().take(end).skip(start).collect()
}
#[cfg(test)]
mod test {
use super::*;
#[test]
fn should_get_secure_substring() {
assert_eq!(secure_substring("christian", 2, 5).as_str(), "ris");
assert_eq!(secure_substring("россия", 3, 5).as_str(), "си");
}
}