Move Semantics

Moves invalidate the old name

fn main() {
    let s1 = String::from("AAPL");
    let s2 = s1; // s1 moved into s2
    // println!("{s1}"); // error: borrow of moved value
    println!("s2={s2}");
}

s2=AAPL

Copy types skip moves

i32, f64, bool, char, and tuples/arrays of Copy types implement Copy:

let x = 5;
let y = x; // both valid

Functions take ownership

fn eat(s: String) {
    println!("{s}");
}

fn main() {
    let t = String::from("X");
    eat(t);
    // t is gone — cannot use t here
}

X

How quants usually fix this

1. Borrow (&str / &String) if you only read
2. Clone if you truly need two owners (costs allocation)
3. Return ownership back if the function should give it back

fn tag(mut s: String) -> String {
    s.push_str("-G1");
    s
}

Exercise

Observe a move, then fix with clone and with returning ownership.