Rust — Constants

Values that never change

A constant stores a value that is fixed for the entire run of the program — things like the number of minutes in an hour.

const BIRTHYEAR: i32 = 1980;
const MINUTES_PER_HOUR: i32 = 60;

fn main() {
    println!("Born in {}", BIRTHYEAR);
}

Constants must have a type

Unlike regular variables, you must write the type when declaring a constant. The compiler won't infer it for you:

const BIRTHYEAR: i32 = 1980; // Ok
const BIRTHYEAR = 1980;      // Error: missing type

Naming convention

Write constants in UPPER_SNAKE_CASE. Not required by the compiler, but it's how every Rust codebase signals "this value never changes":

• MAX_SPEED
• PI
• MINUTES_PER_HOUR

Constants vs variables

	const	let
Can change later?	No	Yes, with mut
Type required?	Yes	Optional (inferred)
Usable at compile time?	Yes	No

When to reach for a constant

Use one whenever a value has meaning and shouldn't drift. Compare:

if speed > 100 { ... }                    // what is 100?
const MAX_SPEED: i32 = 100;
if speed > MAX_SPEED { ... }              // self-documenting

Note: don't confuse const with mut. A let mut variable can change; a constant never can — and Rust will stop compilation if your code tries.