sorting the wheat from the chaff

Lifetimes and strings in Rust

Today I've finally cleared a couple of basic concepts about &str (string slice) and String (string) in Rust. Let's very briefly recap.

Let's see the following code snippet:

fn get_str<'a>() -> &'a str {
    let s = String::from("ok");

fn main() {
    println!("{}", get_str());

This won't compile:

error[E0515]: cannot return reference to temporary value
 --> src/main.rs:3:5
3 |     &s.to_owned()
  |     ^------------
  |     ||
  |     |temporary value created here
  |     returns a reference to data owned by the current function

and here is why. When we run the application we need to consider how the stack memory works:

  1. we are in the main body
  2. we invoke get_str()
  3. The function get_str() is allocated on the stack
  4. When we return from get_str() the stack will be freed and everything allocated inside will be destroyed

Rust will block you and here's why it won't even compile! It's warning you ("a reference to data owned by the current function") that it won't allow you to return a pointer (&str) to a deallocated memory location.

Inside get_str() we are allocating a new chunk of memory on the stack. This memory location cannot be referenced outside of get_str(), once we return.

The only solution to this is to fix as follows:

fn get_str() -> String {
    let s = String::from("ok");

fn main() {
    println!("{}", get_str());

So, as a rule of thumb it's always better to use &str when passing strings around, if the content is meant to be read-only, it's equivalent to doing:

char *ptr = malloc(10);
ptr[0] = '\0';

But be careful when you're returning a &str allocated inside a function. That won't be allowed.

Unless you bind that &str to a parameter lifetime: you explicit the fact that the &str the function receives, will outlive the function:

fn get_str<'a>(mut s: &'a str) -> &'a str {
    println!("{}", s);
    s = "new value";

fn main() {
    println!("{}", get_str("old value"));

This will print:

$ ./target/debug/test
old value
new value

The &str is allocated outside the function and even though it's modified inside it, we're allowed to return the pointer; or simply use the String object (like said before).

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