🦀 Tip of the day 07: const – Values the Compiler Knows Before Your Program Even Starts
You already know from tip 5 that let variables are immutable by default. But const is a different beast entirely.
#![allow(unused)]
fn main() {
let max_players = 8; // Decided at runtime, when this line runs
const MAX_PLAYERS: u32 = 8; // Decided at compile time, baked into the binary
}The compiler does not reserve memory for const. It replaces every use of MAX_PLAYERS with the literal 8 before your program even runs.
Rules that come with it:
Since the value must be known at compile time, you are limited to literals, math on other constants, and a few built-in operations. You cannot call non-const functions to initialize a const (more on that later).
const TIMEOUT: u64 = 30; // Type annotation is required, no inference allowed
const PI: f64 = 3.14159; // Can live at global scope, unlike `let` which can only exist inside functions
const ROWS: usize = 8;
const COLS: usize = 8;
const TOTAL: usize = ROWS * COLS; // Math on other consts works
const NAME: String = String::from("hello"); // ERROR: String::from is not a const function
fn main() {
const SIZE: usize = 10;
let numbers = [0; SIZE]; // Array sizes must be compile-time, so const works here but let does not
}Naming convention: ALL_CAPS_SNAKE_CASE. The compiler will not enforce it, but every Rustacean will judge you.
How other languages handle it:
Go and Kotlin also have true compile-time constants. Go’s is interesting: constants are untyped by default and pick up their type at the point of use. Kotlin’s const val is stricter, only primitives and String, and it must live at the top level or inside a companion object. C++ has constexpr/consteval, which are the closest cousins to Rust’s const and the original inspiration for compile-time evaluation.
Then there are the pretenders. JavaScript’s const just means “can’t reassign”, the value is still computed at runtime, and you can freely mutate the object it points to. Java’s static final looks like a constant but the language makes no compile-time guarantee. Python does not have constants at all. ALL_CAPS is just a polite suggestion that everyone ignores.
Think of const as a fact about your program that is true before it even starts, like the number of cards in a deck, or the value of pi. let is for things you figure out while running. const is for things you already know.
🔇 JavaScript really named their “you can’t reassign this but go ahead and mutate whatever is inside” feature const. Brave naming choice.