Rust Developer - Lesson 2

Borrowing means passing a value by reference instead of transferring ownership. When a function takes &Tas a parameter, it borrows the value — it can read it but does not own it. After the function returns, the original variable is still valid because ownership was never moved. Borrowing avoids unnecessary cloning and allows functions to work with data without taking it over. This lesson shows how to write functions that borrow their arguments.

fn show(val: &i32) {
    println!("{}", val);
}
fn main() {
    let x = 5;
    show(&x);
}

• fn show(val: &i32) takes a reference to an i32.
• The function borrows x when called with show(&x).
• Inside the function, val is a reference that can be read.
• After show returns, x is still valid.
• The output is 5.

fn show(msg: &String) {
    println!("{}", msg);
}
fn main() {
    let s = String::from("hi");
    show(&s);
    println!("{}", s);
}

• fn show(msg: &String) borrows a String reference.
• After calling show(&s), s is still usable because ownership was not moved.
• Without borrowing, passing s directly would move ownership and make s invalid.
• Both calls print hi.
• Borrowing preserves the original variable's validity.

fn add(a: &i32, b: &i32) -> i32 {
    *a + *b
}
fn main() {
    let x = 3;
    let y = 7;
    println!("{}", add(&x, &y));
}

• fn add(a: &i32, b: &i32) -> i32 borrows two values and returns a new one.
• *a + *b dereferences both references to compute the sum.
• The original x and y are not moved.
• The returned i32 is owned by the caller.
• The output is 10.

fn length(s: &String) -> usize {
    s.len()
}
fn main() {
    let word = String::from("Rust");
    println!("{}", length(&word));
}

• s.len() works directly on a &String because method calls auto-dereference.
• The function borrows word, so word remains valid after the call.
• length can read the string but cannot modify it.
• The return type is usize, the length of the string.
• The output is 4.

fn first(s: &String) -> &str {
    &s[0..1]
}
fn main() {
    let word = String::from("Hi");
    println!("{}", first(&word));
}

• Borrowing can return a slice (&str) of the borrowed data.
• &s[0..1] creates a string slice of the first character.
• The returned slice is valid as long as the borrowed String is alive.
• The function only reads the string, it does not modify it.
• The output is H.

fn print_both(a: &i32, b: &i32) {
    println!("{}", a);
    println!("{}", b);
}
fn main() {
    let x = 1;
    let y = 2;
    print_both(&x, &y);
}

• A function can borrow multiple values at once.
• Both a and b are references passed to print_both.
• The function reads them but does not take ownership.
• After the call, x and y are still valid.
• The output is 1 and 2 on separate lines.