The article says that this code doesn't compile in 2023. Assuming the intention was to print "fox", I don't see how it is incorrect:

  let mut animal = "fox".to_string();
  let mut capture_animal = || {
      animal.push_str("es");
  };
  //ERROR:cannot borrow `animal` as immutable because it is also borrowed as mutable
  println!("Outside closure: {animal}");
  capture_animal();