4.1 Variables and state

Explanation

A let binding gives a name to a value. In Rust, a binding is immutable by default. Add mut only when the binding needs to change.

fn main() {
    let x = 2;
    let mut y = 3;
    y += x;
    println!("{y}");
}

Values also have owners. For simple numbers this is usually invisible, but the same rule matters later for arrays, strings, and larger data. Rust makes mutation explicit, so a reader can see which names may change.

For scientific code, avoid hidden global mutable state. Put state in values that are passed to functions, returned from functions, or stored in clearly named objects.

Things to look up

let binding
mut
Ownership
Global state

Exercise

Before running the program above, predict the value printed by println!. Then change let mut y = 3; to let y = 3; and ask an AI agent why mut is required for y += x.

Notes for the exercise

Separate a name binding from mutation of the object or value behind the name.
Track which line changes a value.
Explain why making mutation visible helps review generated code.