Borrowing: Lending Values

Last time, giving a value away meant you didn’t have it anymore. But what if a friend just wants to look at it, and hand it right back? That’s called borrowing, and Rust has a friendly way to do it. 🤝

The Big Idea A reference (written with an &) lets you borrow a value without taking ownership. It's like lending a library book — someone reads it, then it still belongs to you.

The little `&` means “borrow”

When you put an & in front of a value, you’re saying “here, take a peek — but I still own it.” A function can take &String to look at the words without becoming the owner.

Think of it like this… Lending a book is different from giving it away. With &, a friend borrows it, reads for a bit, and you get it back good as new!

fn how_long(text: &String) {
    println!("That word has {} letters.", text.len());
}

fn main() {
    let name = String::from("Ferris");
    how_long(&name); // we LEND it
    println!("I still own: {}", name);
}

Because we only borrowed name, main still owns it afterward. We can keep using it! 🎉

Borrowing to change: `&mut`

Sometimes a borrower doesn’t just look — they actually edit what they borrowed. For that, Rust uses &mut, which means “borrow it and you may change it.”

fn add_sparkles(text: &mut String) {
    text.push_str(" ✨");
}

fn main() {
    let mut name = String::from("Ferris");
    add_sparkles(&mut name);
    println!("{}", name);
}

Rust has one simple rule to keep everything tidy: many readers OR one writer at a time.

Lots of people can look at the value together (many &). 👀
But only one person can change it at once (one &mut).

This stops two people from editing the same value at the same time and making a mess.

Ferris says: Many can look, but only one can change at a time. That keeps your data — and your code — safe! 🦀

Try this! In the sparkles box, change " ✨" to " 🌟" and press ▶ Run. The borrowed value comes back with a new sparkle!

Quick quiz

What does an & reference let you do?

Yes! & borrows — you peek at the value, and the owner still keeps it. 🤝

You learned… A reference (&) borrows a value, &mut lets you change what you borrowed, and the rule is many readers OR one writer. Next up: borrowing just a piece of something — A Slice of the Pie! 🍕

The little & means “borrow”

Borrowing to change: &mut

The sharing rule

Quick quiz

The little `&` means “borrow”

Borrowing to change: `&mut`