let mut

Goal of This Episode

Understand that Rust variables are immutable by default, and that you need mut to change their values.

Main Text

Today let’s talk about one of Rust’s most distinctive design decisions — variables are immutable by default.

First, See What Happens

fn main() {
    let x = 5;
    x = 10;
    println!("{}", x);
}

Do you think it prints 10? It doesn’t. You get a compile error. Rust is telling you: “x is immutable — you can’t give it a new value.”

Wait, Why Not?

In many programming languages, variables can be changed freely. But Rust’s attitude is: if you don’t intend to change it, don’t let it be changeable.

Why? Because if you know a value never changes, you don’t have to worry about it being modified behind your back while reading the code. This matters a lot in large programs.

To Change It, Add mut

If you really do need to change the value, add mut (short for “mutable”):

fn main() {
    let mut x = 5;
    println!("x was originally {}", x);
    x = 10;
    println!("x is now {}", x);
}

This time it works! By writing let mut, you’ve told Rust: “I’m going to change this variable later.”

Quick Summary

fn main() {
    let x = 5;     // Immutable; can't be changed later
    let mut x = 5; // Mutable; can be changed later
}

Rust isn’t forbidding you from changing variables — it just wants you to say so explicitly. This is part of Rust’s design philosophy: make choices consciously.

Recap

• Rust variables are immutable by default; they can’t be reassigned.
• To make a variable changeable, add mut when declaring it: let mut x = 5;.
• To modify a mutable variable’s value, just write x = new_value; (no let needed again).
• This is Rust’s design philosophy: make you choose explicitly, rather than silently allowing modification.