rust - Getting "temporary value dropped while borrowed" when trying to update an Option<&str> in a loop

Question

I'm trying to implement a commonly used pattern - using the result of a previous loop iteration in the next loop iteration. For example, to implement pagination where you need to give the id of the last value on the previous page.

struct Result {
    str: String,
}    

fn main() {
    let times = 10;
    let mut last: Option<&str> = None;

    for i in 0..times {
        let current = do_something(last);
        last = match current {
            Some(r) => Some(&r.str.to_owned()),
            None => None,
        };
    }
}

fn do_something(o: Option<&str>) -> Option<Result> {
    Some(Result {
        str: "whatever string".to_string(),
    })
}

However, I'm not sure how to actually get the value out of the loop. Currently, the compiler error is temporary value dropped while borrowed (at &r.str.to_owned()), though I made many other attempts, but to no avail.

The only way I found to actually get it working is to create some sort of local tmp_str variable and do a hack like this:

match current {
    Some(r) => {
        tmp_str.clone_from(&r.str);
        last = Some(&tmp_str);
    }
    None => {
        last = None;
    }
}

But that doesn't feel like it's the way it's supposed to be done.

Answer 1

In your code, it remains unclear who the owner of the String referenced in last: Option<&str> is supposed to be. You could introduce an extra mutable local variable that owns the string. But then you would have two variables: the owner and the reference, which seems redundant. It would be much simpler to just make last the owner:

struct MyRes {
    str: String,
}

fn main() {
    let times = 10;
    let mut last: Option<String> = None;

    for _i in 0..times {
        last = do_something(&last).map(|r| r.str);
    }
}

fn do_something(_o: &Option<String>) -> Option<MyRes> {
    Some(MyRes {
        str: "whatever string".to_string(),
    })
}

In do_something, you can just pass the whole argument by reference, this seems more likely to be what you wanted. Also note that naming your own struct Result is a bad idea, because Result is such a pervasive trait built deeply into the compiler (?-operator etc).

---

Follow-up question: Option<&str> or Option<String>?

Both Option<&str> and Option<String> have different trade-offs. One is better for passing string literals, other is better for passing owned Strings. I'd actually propose to use neither, and instead make the function generic over type S that implements AsRef<str>. Here is a comparison of various methods:

fn do_something(o: &Option<String>) {
    let _a: Option<&str> = o.as_ref().map(|r| &**r);
    let _b: Option<String> = o.clone();
}
fn do_something2(o: &Option<&str>) {
    let _a: Option<&str> = o.clone(); // do you need it?
    let _b: Option<String> = o.map(|r| r.to_string());
}
fn do_something3<S: AsRef<str>>(o: &Option<S>) {
    let _a: Option<&str> = o.as_ref().map(|s| s.as_ref());
    let _b: Option<String> = o.as_ref().map(|r| r.as_ref().to_string());
}

fn main() {
    let x: Option<String> = None;
    let y: Option<&str> = None;

    do_something(&x);                           // nice
    do_something(&y.map(|r| r.to_string()));    // awkward & expensive

    do_something2(&x.as_ref().map(|x| &**x));   // cheap but awkward
    do_something2(&y);                          // nice

    do_something3(&x);                          // nice
    do_something3(&y);                          // nice, in both cases
}

Note that not all of the above combinations are very idiomatic, some are added just for completeness (e.g. asking for AsRef<str> and then building an owned String out of seems a bit strange).