References & Borrowing: The Borrow Checker

Immutable borrows (&T), mutable borrows (&mut T), and data race prevention

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Instead of taking full ownership, functions can "borrow" access to values using References (&). The Borrow Checker enforces data race prevention at compile-time.

The Two Rules of Borrowing

At any given time, you can have EITHER:

1Any number of immutable references (&T) to a resource, OR...

2Exactly ONE mutable reference (&mut T) to a resource.

You can NEVER have a mutable reference simultaneously with an immutable reference. This rule prevents Data Races at compile-time!

Borrowing with References in Rust

Try It Yourself »

fn calculate_length(s: &String) -> usize {
    s.len() // Borrowed reference; does NOT take ownership!
}

fn append_suffix(s: &mut String) {
    s.push_str(" Academy"); // Mutating through mutable reference
}

fn main() {
    let mut title = String::from("Codexia");
    
    let len = calculate_length(&title); // Immutable borrow
    println!("Length: {}", len);

    append_suffix(&mut title); // Mutable borrow
    println!("Updated: {}", title);
}

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Test Yourself With Exercise

+15 XP

Under Rust borrowing rules, can you have an active mutable reference (`&mut T`) while one or more immutable references (`&T`) exist for the same value?

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