Whoa, very cool! Can't wait for 1.75.

-> impl Trait in public traits

That's a bummer. This works:

trait Base {
  fn op(&self);
}
trait Child : Base {
  fn other_op(&self);
}

trait A {
  fn some_fn(val: impl Base) {
    val.op();
  }
}

fn some_fn(val: impl Child) {
  val.op();
  val.other_op();
}

So it seems like returning an impl Child in a trait that binds impl Base should also work.

Hopefully this change means we'll see more of this kind of thing in the upcoming releases.