extern crate unicode_segmentation; // 1.2.1
extern crate unicode_width; // 0.1.5

pub fn substr<'a>(s: &'a str, from_index: usize, to_index: usize) -> Option<&'a str>{
    use unicode_segmentation::UnicodeSegmentation;
    use unicode_width::UnicodeWidthStr;

    assert!(!(from_index > to_index));

    //println!("{}", UnicodeWidthStr::width(s));

    let begin = match s.grapheme_indices(true).nth(from_index).map(|(i, _)| i){
        Some(i) => i,
        None => return None,
    };
    if to_index == UnicodeWidthStr::width(s){
        return Some(&s[begin..]);
    }
    else{
        match s.char_indices().nth(to_index).map(|(i, _)| i){
            Some(i) => Some(&s[begin..i]),
            None => return None,
        }
    }
}


fn main() {
    substr("😀😁 😂 🤣 😃 😄 😅 😆 😉 😊 😋 😎 😍 😘 😗 😙 😚😀 😁 😂 🤣 😃 😄 😅 😆 😉 😊 😋 😎 😍 😘😗 😙 😚", 89, 98).expect("Cant split");
}