Rust: Lifetimes and “Temporary Value Dropped While Borrowed”

Using lifetimes to resolve the E0716 error: “temporary value dropped while borrowed.”


Rust: Lifetimes and “Temporary Value Dropped While Borrowed”

Easing myself back into Rust after taking a break from it for over a year. I revisited a Stack Overflow question that I had also looked at on 28/02/2024.

❶ Rust version 1.86.0 (05f9846f8 2025-03-31) still produces the same compilation error as described in the original post, which is:

error[E0716]: temporary value dropped while borrowed
  --> ./src/std_019_dropped_borrowed.rs:10:19
   |
9  | fn fun1(s1: &String, v1: &mut Vec1) {
   |                      -- has type `&mut Vec<&'1 String>`
10 |     v1.insert(0, &s1.clone());
   |     --------------^^^^^^^^^^-- temporary value is freed at the end of this statement
   |     |             |
   |     |             creates a temporary value which is freed while still in use
   |     argument requires that borrow lasts for `'1`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0716`.

And the original code is:

type Vec1<'a> = Vec::<&'a String>;

fn fun1(s1: &String, v1: &mut Vec1) {
    v1.insert(0, &s1.clone());
}

fn main() {
    let mut vec1 = Vec::new();
    let str1 = String::new();
    fun1(&str1, &mut vec1);
}

❷ My First Attempt at Getting the Code to Compile:

type Vec1<'a> = Vec::<&'a String>;

fn fun1<'a>(s1: &'a String, v1: &'a mut Vec1) {
    v1.insert(0, s1);
}

fn main() {
    let mut vec1 = Vec1::new();

    let str1 = String::from("abcdd");
    fun1(&str1, &mut vec1);

    println!("{:?}", vec1);
}

The following changes were made:

Added the lifetime 'a to fun1(...) as fn fun1<'a>(s1: &'a String, v1: &'a mut Vec1). The lifetime ensures that the string reference s1 remains valid as long as the reference to the vector vec1 is alive.
Updated the function body to use v1.insert(0, s1);. Since the entries in the vector vec1 are string references, and s1 is a string reference, it can be directly inserted into the vector.
For testing purposes, assigned an actual value to str1 in fn main(): let str1 = String::from("abcdd");.

This refactoring results in a new error:

error[E0621]: explicit lifetime required in the type of `v1`
  --> ./src/std_019_dropped_borrowed.rs:24:5
   |
23 | fn fun1<'a>(s1: &'a String, v1: &'a mut Vec1) {
   |                                 ------------ help: add explicit lifetime `'a` to the type of `v1`: `&'a mut Vec<&'a String>`
24 |     v1.insert(0, s1);
   |     ^^^^^^^^^^^^^^^^ lifetime `'a` required

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0621`.
behai@HP-Pavilion-15:~/rust/std$

The error is also shown in its original format in the screenshot below:

❸ The lifetime annotation for fun1(...) should be written as fn fun1<'a>(s1: &'a String, v1: &mut Vec1<'a>) (instead of fn fun1<'a>(s1: &'a String, v1: &'a mut Vec1)). The new, working version is listed in full below:

type Vec1<'a> = Vec::<&'a String>;

fn fun1<'a>(s1: &'a String, v1: &mut Vec1<'a>) {
    v1.insert(0, s1);
}

fn main() {
    let mut vec1 = Vec1::new();

    let str1 = String::from("abcdd");
    fun1(&str1, &mut vec1);

    println!("{:?}", vec1);
}

I couldn’t find this solution in the original Stack Overflow question, so I thought it would be worth documenting. While we don’t know exactly why the original author wanted to store string references in a vector, this solution aligns with that intention.

❹ I find Rust lifetimes a bit difficult to follow. Troubleshooting isolated problems like this one, however, feels insightful and helps enhance our understanding.

Thank you for reading. I hope you find the information in this post useful. Stay safe, as always.

✿✿✿

Feature image sources: