Temporary value is freed at the end of this statem

问题:

This question already has answers here:

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Closed last year.

I'm trying to scrape a webpage using the Select crate:

let document = Document::from_read(response).unwrap();

for node in document.find(Class("lia-list-row")) {
    let title = node.find(Class("page-link")).next().unwrap();
    let title_text = title.text().trim();

    println!("{}\n", title_text);
}

Which results in following error:

let title_text = title.text().trim();
                 ^^^^^^^^^^^^       - temporary value is freed at the end of this statement
                 |
                 creates a temporary which is freed while still in use

println!("{} - {}\n", i, title_text);
                         ---------- borrow used here, in later iteration of loop

I solved it by separating the .text() and .trim()

let title_text = title.text();
let trim_text = title_text.trim();

What is the difference? Why did the first attempt fail?

回答1:

This one seems convoluted at first, but remember that String and &str are different beasts.

String can live and be used on its own, but &str is just a reference to part of String. So, &str can live as long as referenced String lives. Lets see how it should work on return signatures.

let title_text = title   .text()   .trim();
//               ^       ^         ^
//               Node    String <- &str

1. Here, title is a select::Node.

2. Node::text returns a String, but nothing binds it to context.

3. String::trim, in turn, returns a &str which is a reference to part of String itself.

In the end, the borrow checker just doesn't understand how it should process a reference to String that will not live long enough in context, as it is a temporary value (non-bound).