Let's say std::iter::from_fn didn't exist. Then you'd need to make something that ends the iterator when the function returns none, you could do it like this:

Let's say std::iter::from_fn didn't exist. Then you'd need to make something that ends the iterator when the function returns none, you could do it like this: std::iter::repeat_with(f).take_while(|elt| elt.is_some()).filter_map(|elt| elt) Where `.filter_map()` removes the Option layer and `repeat_with` creates an initially infinite iterator from your function `f`. Long time ago, `repeat_with` might not have existed. Maybe you'd do something like this instead for that in Rust 1.0.0: `std::iter::repeat(()).map(|_| f())...` You prefer to use std::iter::from_fn now that it exists, of course! To be honest, making a simple iterator adaptor is quite easy too, so you could have a knack at making one you are missing - if you can't find it in std or in the crate itertools already.