Struct str attribute must be reference? [duplicate] - string

Question

This question already has answers here:
How to solve "returns a value referencing data owned by the current function" error in Rust? [duplicate]
(1 answer)
Return local String as a slice (&str)
(7 answers)
Why can't I return an &str value generated from a String?
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Is there any way to return a reference to a variable created in a function?
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Closed 3 years ago.
For the following code:
fn get_lines() -> String {
String::from("Hello\nWorld")
}
fn get_first_line(s: &String) -> &str {
s.lines().next().unwrap()
}
struct World<'a> {
a_str: &'a str,
}
fn work<'a>() -> World<'a> {
let s1 = get_lines();
let s2 = get_first_line(&s1);
World { a_str: s2 }
}
fn main() {
let w = work();
}
I got the following error:
error[E0515]: cannot return value referencing local variable `s1`
--> src/main.rs:17:5
|
15 | let s2 = get_first_line(&s1);
| --- `s1` is borrowed here
16 |
17 | World { a_str: s2 }
| ^^^^^^^^^^^^^^^^^^^ returns a value referencing data owned by the current function
How to build a struct instance using s2? Is it a conceptual error of the World struct?

Answer 1

World refers to a str slice which must be owned by something else. Your function work allocates a new String (through get_lines), and makes a reference into it (through get_first_line). When it returns, the String goes out of scope and will be dropped, so you cannot keep a reference to it, since the thing it refers to is no longer there.
If you want a World object that does not depend on a String owned by something else, it will need to own the data itself: Contain a String instead of a &'a str.
See also 'dangling references' in the Rust Book.