Just my 2cs, but the complexity of this code is here to remind you there are a lot of cases you have to take into account :

    let path = Path::new(&app_path);
    if let Some(ostr) = path.file_name() {
       if let Some(str) = ostr.to_str() {
           println!("file name : {}", str);
       } else {
          println!("WARNING : file name is not a valid unicode sequence ! (file name : {:?}", ostr);
       }
    } else {
      println!("Path is either a root directory or a dot entry, it has no filename");
    }