Sorry, you are correct. I was thinking of situations like this:

  fn assign(a: &mut Bar, b: &Bar) {
    a.foo = b.foo;
  }
    
    assign(&mut a[0], &a[1]);

Bit contrived - next time Rust stops me doing something simple I will note it down!

Again to be clear, I'm not saying that there's anything wrong with Rust here.