在此语句结束时释放临时值 [英] Temporary value is freed at the end of this statement

问题描述

我正在尝试使用 Select crate 抓取网页:

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);
}

导致以下错误:

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

我通过分离 .text() 和 .trim()

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

有什么区别?为什么第一次尝试失败?

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

推荐答案

这个乍一看似乎很复杂，但请记住 String 和 &str 是不同的野兽.

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

String 可以单独存在和使用，但 &str 只是对 String 的一部分的引用.因此，&str 的生命周期与引用的 String 生命周期一样长.让我们看看它应该如何处理返回签名.

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. 这里，title 是一个 select::Node.

节点::text 返回一个 String，但没有将其绑定到上下文.

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

String::trim 反过来返回一个 &str，它是对 String 本身的一部分的引用.

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

最后，借用检查器只是不明白它应该如何处理对 String 的引用，因为它是一个临时值(非绑定).

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).

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