This doesn't demonstrate either named or default arguments. You can expand on it to implement the equivalent of default arguments, sure, but it doesn't solve the ergonomic reason to have default arguments, and it doesn't touch on named arguments at all.

You're missing the point. Here's a good article that talks about the problems of having neither overloading nor default parameters, but in Go.[1] The problem is made significantly worse in Rust when all struct fields need to be initialized and there are no varargs. I really see no solution to the "stable API with more options" problem in Rust, aside from a builder pattern, which I really hate.

1: https://dave.cheney.net/2014/10/17/functional-options-for-fr...

Why do you have the builder pattern (other than the verbosity of declaring them)?

I normally just leverage a couple of features: Derive(default) to have an "empty" initialized value and the struct field filling syntax

  let x = Foo { field_i_care_about, ..Default::default() };

In Go, you can use an interface to implement private struct fields. Then you can make a "New" function to implement default values as needed. I made a post about this last month:

https://stackoverflow.com/a/63552596