rust - How do I pass modified string parameters?

Question

I'm on chapter 12 of The Rust Programming Language, where a case insensitive line search is implemented. It doesn't make sense to me to implement the same logic twice, so I figured if I just called the case sensitive search function with the parameters converted to lower case, that might work. It did not.

This is my non working code:

fn main() {
    let a = search("Waldo", "where in
the world
is Waldo?");
    let b = search("waldo", "where in
the world
is Waldo?");
    let c = search_case_insensitive("waldo", "where in
the world
is Waldo?");

    println!("{:?}", a);
    println!("{:?}", b);
    println!("{:?}", c);
}

pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
    let mut results = Vec::new();

    for line in contents.lines() {
        if line.contains(query) {
            results.push(line);
        }
    }

    results
}

pub fn search_case_insensitive<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
    let query = query.to_lowercase();
    let contents2: &str = &contents.to_lowercase();

    search(&query, contents2)
}

The error in most versions I've come up with is inevitably something very much like:

error[E0597]: borrowed value does not live long enough
  --> src/main.rs:25:28
   |
25 |     let contents2: &str = &contents.to_lowercase();
   |                            ^^^^^^^^^^^^^^^^^^^^^^^ temporary value does not live long enough
...
28 | }
   | - temporary value only lives until here
   |
note: borrowed value must be valid for the lifetime 'a as defined on the function body at 23:1...
  --> src/main.rs:23:1
   |
23 | pub fn search_case_insensitive<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Answer 1

You have introduced an impossible constraint on the lifetime of variable contents2; by writing &'a you are attempting to assign to it the same lifetime as to the contents argument, but it is created and destroyed within the scope of search_case_insensitive and thus is outlived by contents.

In order for contents2 to outlive the body of search_case_insensitive you would need to either return it as a String and assign to some variable outside of it or pass it to search_case_insensitive by reference as long as it already exists as a String elsewhere.

Citing The Book:

It's important to understand that lifetime annotations are descriptive, not prescriptive. This means that how long a reference is valid is determined by the code, not by the annotations.