Shared via Rust Playground

playground.rs

fn main() {
    println!("{:?}", substr("abcdef", 2, 4));
}

fn substr(string: &str, start_index: u32, length: u32) -> String {
    let mut result: String = String::new();
    let index: usize = start_index as usize;
    let mut lusize: usize = length as usize;
    lusize = string.len() - lusize;
    
    result.push_str(&string[index..lusize]);
    
    return result;
}