我错在哪里😥?。

fn main_FnOnce() {
    fn fn_immut<F>(f: F) where F: FnOnce() -> String {
        println!("run {}", f());
    }
    fn fn_immutb<F>(f: F) where F: for<'a> Fn() -> &'a String {
        println!("run {}", f());
    }
    let a = "Fn".to_string();
    let b = &"Fn".to_string();
    let f = || a;
    let fb = || b;
    fn_immut(f);
    fn_immutb(fb);
}

error[E0261]: use of undeclared lifetime name `'a`
  --> src\main.rs:56:21
   |
56 |         F: Fn() -> &'a String,
   |                     ^^ undeclared lifetime
   |
   = note: for more information on higher-ranked polymorphism, visit https://doc.rust-lang.org/nomicon/hrtb.html
help: consider introducing lifetime `'a` here
   |
54 |     fn fn_immutb<'a, F>(f: F)
   |                  ^^^
help: consider making the bound lifetime-generic with a new `'a` lifetime
   |
56 |         F: for<'a> Fn() -> &'a String,
   |            ^^^^^^^

error: aborting due to previous error; 1 warning emitted