Note that you can do Swift's protocol oriented programming / portocol extensions in Rust, too:

  trait Test {
      fn value(&self) -> i32;
  }

  impl dyn Test {
      fn multiplied(&self) -> i32 {
          self.value() * 2
      }
  }

What does `impl dyn` mean, exactly? That this only works for trait objects?

That it works for any type implementing the trait, via vtables. Otherwise, it’s a method specific to the particular type implementing the trait (a concrete implementation but distinct to the specific impl).