Confusion about temporary values - rust

Question

I am writing a small function that reads /proc/bus/input/devices line by line and looks for a pattern in each block.
let mut handlers = Vec::<&str>::new();
let entry = RefCell::new(String::new());
let re = Regex::new(r"(?m)(event\d+)").unwrap();
for line in lines {
let l = line.unwrap();
if !l.is_empty() {
entry.borrow_mut().push_str(&l);
continue
}
if entry.borrow().contains("EV=120013") {
if let Some(captures) = re.captures(entry.borrow().as_str().clone()) {
if let Some(m) = captures.get(0) {
&handlers.push(m.as_str());
}
}
}
entry.borrow_mut().clear();
}
However, the build fails with the following error:
error[E0716]: temporary value dropped while borrowed
--> src/linux/keylogger.rs:50:53
|
50 | if let Some(captures) = re.captures(entry.borrow().as_str().clone()) {
| ^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
51 | if let Some(m) = captures.get(0) {
52 | &handlers.push(m.as_str());
| ------------------------- borrow later used here
...
55 | }
| - temporary value is freed at the end of this statement
|
= note: consider using a `let` binding to create a longer lived value
I tried to create such binding, but I couldn't get it to work ...

Answer 1

entry.borrow() returns a so called guard. You have to keep this guard alive for the entire time you access the content.
As you are only using it inline, it goes out of scope right away and the borrowed value gets returned.
You need to store it in a local variable that stays alive for as long as you need the borrowed content.
That's only the first problem, though. The second one is that you can't you can't store &str that aren't &'static str in a vector. &str do not keep the content of the string alive, they are only references. To store them somewhere, you need the owning version, String.
use std::cell::RefCell;
use regex::Regex;
fn main() {
let lines = [Some("aaa"), Some("bbb")];
let mut handlers = Vec::<String>::new();
let entry = RefCell::new(String::new());
let re = Regex::new(r"(?m)(event\d+)").unwrap();
for line in lines {
let l = line.unwrap();
if !l.is_empty() {
entry.borrow_mut().push_str(&l);
continue;
}
if entry.borrow().contains("EV=120013") {
let entry_guard = entry.borrow();
if let Some(captures) = re.captures(&entry_guard) {
if let Some(m) = captures.get(0) {
handlers.push(m.as_str().to_string());
}
}
}
entry.borrow_mut().clear();
}
}