How to convert Option<Option<String>> to Option<Option<&str>> in Rust?

December 6, 2023

I am trying to make this conversion (Option<Option<String>> to Option<Option<&str>>) possible, but I still failed after trying many methods including using .map.

I know that the conversion is possible, however, if there’s no nested Option (i.e. Option<String> to Option<&str>): just using .as_deref().
Using .map(|inner| inner.as_deref()) violates the ownership.

>Solution :

The reason using a plain map violates ownership rules is because it takes self. You’re consuming the outer Option and then trying to borrow something from inside it, which doesn’t work.

However, there’s an easy approach here, using as_ref and then as_deref inside the map. as_ref turns &Option<T> into Option<&T>, where T is Option<String>, so when you call map, you get an &Option<String>, which you can then safely pass to as_deref.

fn main() {
    let foo = Some(Some(String::from("abc")));
    let bar: Option<Option<&str>> = foo.as_ref().map(|r| r.as_deref());
    eprintln!("{:?}", bar);
}