[bilkow]

> It's complexity for its own sake.

Hey, let's talk about the complexity here, I think there's reason for it! They wanted to use static dispatch so that the return type has more information on the logic being returned and is able to optimize better while also avoiding allocations at all. Reduced the original example (that doesn't compile) here: https://play.rust-lang.org/?version=stable&mode=debug&editio...

If you want to do what other languages do, you can just use dyn Trait (trait objects, dynamic dispatch) instead of impl Trait (concrete objects, static dispatch). It's less efficient and requires allocations, which is what other languages usually do, but all you have to do is wrap it in a Box. Example: https://play.rust-lang.org/?version=stable&mode=debug&editio...

If you want to use static dispatch, then you have to look a bit more into it, there's a lot of discussion in the comments already but for this specific case Option's into_iter + flatten works fine, while Either is a more generic solution or you could even implement your own iterator for this use-case if you wanted to.

I'd say the complexity here comes from the explicitness and being forced to make a choice: Don't care about performance? Ok, just wrap it in a Box and call it a day. Wants to take the most out of it? Then you'll have to look deeper into it.