I would quite like to see some syntax sugar around this. If we could elide the type and made `...` equivalent to `..Default::default()` it would be a lot closer to kwargs without having to introduce any new "real" features. So we could write:

     f(arg1, arg2, _ {name: "Hello", address, ...})

instead of

     f(arg1, arg2, MyKwargs {name: "Hello", address, ..Default::default()}

Using a struct for this feels natural as you get forwarding and non-exhaustiveness for adding additional fields for free.