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Ownership and Borrowing

Level: intro (score: 1)

🎯 In Python 🐍, strings are immutable and passed by reference under the hood. In Rust 🦀, you explicitly choose between borrowing(&str) and owning (String). Returning data can either borrow from an input (or something it outlives), or allocate a new owned value.

In this exercise, you'll implement three small functions, each demonstrating a key Rust concept:

✅ Ownership transfer
✅ Shared borrowing
✅ Mutable borrowing

✅ Your task

Implement the following functions in src/main.rs:

transfer_ownership: takes ownership of a String, uses it, and returns a new owned value
longest_word_length: takes two string references and returns the length of the longer one, without taking ownership
to_uppercase: takes a mutable reference to a String and modifies it in place to be uppercase

🔄 Concepts Covered

Moving ownership (String → String)
Shared borrowing (&String)
Mutable borrowing (&mut String)

No lifetimes required. Use this exercise to start building muscle memory for Rust’s ownership model and borrowing rules.

✅ Example tests

assert_eq!(transfer_ownership(String::from("hello")), String::from("hello world"));
assert_eq!(longest_word_length(&String::from("hello"), &String::from("world!")), 6);
let mut s = String::from("hello");
to_uppercase(&mut s);
assert_eq!(s, "HELLO");

ownership borrowing

Code

// --- Ownership: move in ---
pub fn transfer_ownership(name: String) -> String {
    // TODO: Return a greeting like "Welcome, <name>!"
    todo!()
}

// --- Borrowing: shared ---
pub fn longest_word_length(word1: &String, word2: &String) -> usize {
    // TODO: Return the length of the longer word
    todo!()
}

// --- Borrowing: mutable ---
pub fn to_uppercase(text: &mut String) {
    // TODO: Convert the input string to uppercase in-place
    todo!()
}

#[cfg(test)]
mod tests {
    use super::*;

#[test]
    fn test_transfer_ownership() {
        let username = String::from("Alice");
        let result = transfer_ownership(username);
        assert_eq!(result, "Welcome, Alice!");
    }

#[test]
    fn test_longest_word_length() {
        let text1 = String::from("borrow");
        let text2 = String::from("rust");
        let result = longest_word_length(&text1, &text2);
        assert_eq!(result, 6);
    }

#[test]
    fn test_to_uppercase() {
        let mut word = String::from("rust");
        to_uppercase(&mut word);
        assert_eq!(word, "RUST");
    }
}