Mastering Rust Lifetime Elision

Rust's borrow checker is a powerful tool for memory safety, and understanding lifetimes is crucial for effective Rust programming. This challenge focuses on applying Rust's lifetime elision rules, a feature that simplifies code by allowing the compiler to infer lifetimes in many common scenarios. By completing this challenge, you'll gain a deeper appreciation for how Rust's compiler helps manage memory and make your code cleaner and more concise.

Problem Description

Your task is to implement a Rust function find_longest_string that takes two string slices (&str) as input and returns a string slice that refers to the longer of the two input strings. The key requirement is to implement this function without explicitly annotating lifetimes, relying entirely on Rust's lifetime elision rules.

Key Requirements:

The function signature must be fn find_longest_string(s1: &str, s2: &str) -> &str.
The function should return a reference to the input string slice that has the greater length.
If the two string slices have equal length, the function can return either one.
Crucially, no explicit lifetime annotations (like 'a, 'b, etc.) should be present in the function signature or its implementation.

Expected Behavior:

The function should correctly identify and return a reference to the longer string slice. The returned reference must have a lifetime that is valid for at least as long as the shorter of the two input lifetimes.

Edge Cases:

One or both input strings are empty.
Both input strings have the same length.

Examples

Example 1:

Input:
let string1 = String::from("abcd");
let string2 = "xyz";

let result = find_longest_string(string1.as_str(), string2);
// result should be a reference to "abcd"

Output: &str referring to "abcd" Explanation: "abcd" has a length of 4, while "xyz" has a length of 3. The function correctly identifies "abcd" as the longer string.

Example 2:

Input:
let s1 = "long string";
let s2 = "short";

let result = find_longest_string(s1, s2);
// result should be a reference to "long string"

Output: &str referring to "long string" Explanation: "long string" has a length of 11, and "short" has a length of 5. The function returns the reference to "long string".

Example 3:

Input:
let a = "equal";
let b = "equal";

let result = find_longest_string(a, b);
// result could be a reference to either "equal"

Output: &str referring to "equal" (either a or b) Explanation: Both input strings have the same length. The function can return a reference to either a or b.

Constraints

Input s1 and s2 will always be valid string slices (&str).
The function should not allocate new strings or modify the input strings.
The solution must adhere to Rust's borrowing rules.

Notes

Remember Rust's three lifetime elision rules:

Input Lifetimes: Each parameter that is a reference gets its own lifetime parameter. So if you have two parameters that are references, you have two lifetimes.
Output Lifetimes: If there is exactly one input lifetime parameter, that lifetime is assigned to all output lifetime parameters.
Output Lifetimes (Multiple Inputs): If there are multiple input lifetime parameters, but one of them is &self or &mut self (a method argument), the lifetime of self is assigned to all output lifetime parameters.

In this problem, you have two input reference parameters and one output reference parameter. Consider which of these rules will apply. The goal is to write the function signature so that the compiler can infer the correct lifetimes without explicit annotations.