borrowed data escapes outside of closure - rust

Question

I created two structs, Car and CarModifier. Both Car and CarModifier are instantiated in main.rs. CarModifier modifies one of the fields of Car using data from curl request. Here is the code:
main.rs
use sample::{car::Car, car_modifier::CarModifier};
fn main() {
println!("Hello, world!");
let mut car = Car::new();
let mut car_modifier = CarModifier::new();
car_modifier.update_car(&mut car);
println!("{:?}", car.driver);
}
Car.rs
#[derive(Debug)]
pub struct Car<'a> {
pub driver: &'a [u8]
}
impl<'a> Car<'a> {
pub fn new() -> Self {
Self {
driver: &[26]
}
}
pub fn update_driver(&mut self, new_driver: &'a [u8]) {
self.driver = new_driver;
}
}
CarModifier.rs
use curl::easy::Easy;
use crate::{car::Car};
pub struct CarModifier;
impl CarModifier {
pub fn new() -> Self {
Self {
}
}
pub fn update_car(&mut self, car: &mut Car) {
let mut easy = Easy::new();
easy.url("https://www.rust-lang.org/").unwrap();
let mut transfer = easy.transfer();
transfer.write_function(|data: &[u8]| {
car.update_driver(data);
Ok(data.len())
}).unwrap();
transfer.perform().unwrap();
}
}
This is the error which I get when trying to run
error[E0521]: borrowed data escapes outside of closure
--> src/car_modifier.rs:19:13
|
13 | pub fn update_car(&mut self, car: &mut Car) {
| --- `car` declared here, outside of the closure body
...
18 | transfer.write_function(|data: &[u8]| {
| ---- `data` is a reference that is only valid in the closure body
19 | car.update_driver(data);
| ^^^^^^^^^^^^^^^^^^^^^^^ `data` escapes the closure body here
For more information about this error, try `rustc --explain E0521`.
error: could not compile `sample` due to previous error
I seem to understand from the error that data lives only in the closure body and the code tries to refer that in car.driver which will outlive the closure body, hence the error. Is this the right understanding?
I am also aware that instead of using &[u8], I could use Vec<u8> to resolve the error. But how can I keep using &[u8]?

Answer 1

Basically data belongs to the curl library. It may be a slice of some library internal buffer pool, and may be re-used for some other curl api calls later on, or freed after this call. You need to copy data into the Car structure instead of just having a pointer to the underlying buffer. Rust compiler helps you to catch that problem. In C++ the compiler won't even tell you this error and the C++ code will have runtime problems.