I was confused that the string slice retrieved from the struct was not allowed to outlive it. It was because I had put the same lifetime bound on the borrow of self as on the contained string

lifetime_shortened_by_newtype.rs

fn main() {
  println!("{}", get_back_the_same_string("Hello World!"));
}

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

struct NewType<'b>(&'b str);

impl <'b> NewType<'b> {
  fn get_string(&'b self) -> &'b str {
    //           ^^ this lifetime caused the problem, I guess it prevents 'b from outliving self
    return &self.0
  }
}