Rust Platform

Mutable Borrowing: The One Exclusive Borrower

Level: intro (score: 1)

Rust enforces strict borrowing rules to prevent data races and ensure memory safety.

A variable can have multiple immutable references (&T), or one mutable reference (&mut T)—but never both at the same time.

Why This Matters

In Python, variables are mutable by default, so you can modify a value even when multiple references exist:

def append_exclamation(text):
    text += "!"
    return text

word = "Hello"
new_word = append_exclamation(word)
print(word)      # Still "Hello" (strings are immutable)
print(new_word)  # "Hello!"

However, Rust enforces stricter rules:

• If you borrow mutably (&mut), no other references (mutable or immutable) can exist at the same time.

• If you borrow immutably (&), multiple references are allowed, but you cannot modify the data.

Task

Implement the function: to_uppercase(text: &mut String)

• The function should take a mutable reference to a String.

• It should modify the string in place, converting all characters to uppercase.

• The function does not return anything—it should mutate the original value.

Hints

• Use .make_ascii_uppercase() to modify the string in place.

• Note that Rust does not allow an immutable borrow to exist while a mutable borrow is active. Try to make a new project cargo new dummy and test this behavior by putting this code in src/main.rs:

fn main() {
    let mut word = String::from("hello");

    let ref1 = &word; // Immutable borrow
    let ref2 = &word; // Another immutable borrow

    word.push_str("!"); // ❌ ERROR: Mutable borrow while immutable borrows exist

    println!("ref1: {}", ref1);
    println!("ref2: {}", ref2);

    // if removing the word.push_str("!") line, this code will compile
    word.make_ascii_uppercase(); // ✅ Mutable borrow after immutable borrows are dropped
}

Mutability rules in Rust help eliminate data races and undefined behavior at compile time.

You’ll never have to deal with "race conditions" like in multi-threaded Python code! 💡

Happy coding! 🚀