const

Goal of This Episode

Declare a never-changing constant with const, and understand how it differs from let.

Main Text

In Chapter 1 we learned to declare variables with let. Today let’s meet its good friend — const.

const means “constant,” as in: this value never changes, and it’s already determined at compile time.

Here’s the syntax:

fn main() {
    const MAX_SCORE: i32 = 100;
    println!("The highest score is: {}", MAX_SCORE);
}

Looks a lot like let, right? But there are several important differences:

Difference 1: const Must Have a Type Annotation

fn main() {
    const MAX_SCORE: i32 = 100; // ✅ The : i32 is required
    let max_score = 100;        // ✅ let can omit it; the compiler infers it
}

With const, you can’t lazily skip the type — the compiler will complain.

Difference 2: The Naming Convention Is ALL CAPS with Underscores

fn main() {
    const MAX_SCORE: i32 = 100;    // ✅ All caps, separated by underscores
    const PI_VALUE: f64 = 3.14159; // ✅ Like this
    const maxScore: i32 = 100;     // ⚠️ Compiles, but the compiler will warn you
}

This is the Rust community convention: constants use SCREAMING_SNAKE_CASE. Your program still runs if you don’t follow it, but the compiler will grumble.

Difference 3: const Can’t Take mut

#![allow(unused)]

fn main() {
    const mut MAX: i32 = 100; // ❌ No such thing exists
    let mut x = 5;            // ✅ This is fine
}

A constant is a constant — unchangeable means unchangeable. There’s no such contradiction as a “mutable constant.”

Difference 4: const Can Live Outside fn

const MAX_PLAYERS: i32 = 10;

fn main() {
    println!("At most {} players", MAX_PLAYERS);
}

const can be declared at the outermost level of a program; let cannot.

When to Use const?

When you have a value that’s fixed and unchanging, and you already know what it is while writing the program, use const. For example:

const TAX_RATE: f64 = 0.05;
const MAX_RETRY: i32 = 3;

fn main() {}

Recap

• const declares a compile-time constant whose value never changes.
• The type annotation is mandatory (can’t be omitted).
• The naming convention is all caps with underscores, like MAX_SCORE.
• No mut allowed.
• It can be placed outside fn so other parts of the program can use it.