How is returning a variable "let" inside a function allowed in rust?

Question

This example compiles and returns the "expected" output. But is this not a dangling pointer scenario? If so, how come the rust compiler allows this?

use serde_json::{Value, json};
use std::io::Result;

fn main(){
    println!("{:#?}", test_json_lifetime());
}

fn test_json_lifetime() -> Result<(Value)> {                            
    let j = json!({ "name" : "test" }); 
    Ok(j)
}

Answer 1

It sounds like you are thinking that j is allocated on the stack frame of test_json_lifetime() (that memory gets deallocated at the end of the function when the stack unwinds), and we return a reference to j (which would result in a dangling pointer).

In this case, you are correct that j gets allocated on the stack, however when we return Ok(j) we do not return a reference to j, rather we copy j to the space for the Result<(Value)> that was allocated on main()'s stack frame before the function call to test_json_lifetime().

Answer 2

I found the answer in this section.

fn main() {
let s1 = gives_ownership();         // gives_ownership moves its return
                                    // value into s1

let s2 = String::from("hello");     // s2 comes into scope

let s3 = takes_and_gives_back(s2);  // s2 is moved into
                                    // takes_and_gives_back, which also
                                    // moves its return value into s3
} 

// Here, s3 goes out of scope and is dropped. s2 goes out of scope but was
// moved, so nothing happens. s1 goes out of scope and is dropped.

fn gives_ownership() -> String {             // gives_ownership will move its
                                             // return value into the function
                                             // that calls it

    let some_string = String::from("hello"); // some_string comes into scope

    some_string                              // some_string is returned and
                                             // moves out to the calling
                                             // function
}