warning: lifetime name `'a` shadows a label name that is already in scope
  --> $DIR/loops-reject-lifetime-shadowing-label.rs:21:55
   |
LL |     'a: loop {
   |     -- first declared here
LL |         let b = Box::new(|x: &i8| *x) as Box<dyn for <'a> Fn(&'a i8) -> i8>;
   |                                                       ^^ lifetime 'a already in scope