References

Borrow a value with &. The original owner keeps ownership.

main.rs

fn calculate_length(s: &String) -> usize {
    s.len()
}

let s1 = String::from("hello");
let len = calculate_length(&s1);  // borrow s1
println!("{} has length {}", s1, len);  // s1 still valid!

📖

Borrow the book, return it later