Try It Yourself - SyntaxStudy Editor

HTML / CSS / JS

fn main() {
    // --- Immutable references ---
    let s = String::from("hello");
    let r1 = &s;
    let r2 = &s;    // multiple immutable refs are fine
    println!("r1={r1}  r2={r2}  s={s}");

// --- Mutable reference ---
    let mut data = String::from("foo");
    append_bar(&mut data);
    println!("data = {data}");

// --- Borrow checker prevents simultaneous mut + immut ---
    let mut v = vec![1, 2, 3];
    let first = &v[0];      // immutable borrow of v
    println!("first = {first}");
    // v.push(4);           // compile error: cannot borrow `v` as mutable
                            // because it is also borrowed as immutable
    // The immutable borrow ends after the last use of `first`
    v.push(4);              // now ok — first's lifetime has ended
    println!("v = {:?}", v);

// --- Only one mutable reference at a time ---
    let mut x = 5;
    let m1 = &mut x;
    *m1 += 10;
    // let m2 = &mut x;     // compile error: second mutable borrow
    println!("x = {x}");
}

fn append_bar(s: &mut String) {
    s.push_str("bar");
}