Remove the [mut] and it compiles, what is the difference?

lifetime_shortened_by_mutability.rs

fn main() {
  let mut string = String::from("Hello World!");
  println!("{}", get_back_the_same_string(string.as_mut_str()));
}

fn get_back_the_same_string<'a>(string: &'a mut str) -> &'a str {
    let nt = NewType(string);
    return nt.get_string()
}

struct NewType<'b>(&'b mut str);
//                     ^^^ if this was immutable, it compiles and below self.0 can be returned from the method. if it is mutable, it can't

impl <'b> NewType<'b> {
  fn get_string(&self) -> &'b str {
    return self.0
  }
}