Constants in Rust

Use const and static for compile‑time and global values. Both require explicit types.

const (inline constants)

Evaluated at compile time; usable in any scope and in const contexts.

const PI: f64 = 3.14159;
const MAX_RETRIES: u32 = 3;
fn area(r: f64) -> f64 { PI * r * r }

static (single address)

Has a fixed memory address for the program’s lifetime.

static APP_NAME: &str = "GistDev";

static mut (unsafe)

Mutable global state is unsafe and requires unsafe to read/write; avoid where possible.

static mut COUNTER: u32 = 0;
unsafe { COUNTER += 1; }

Prefer interior mutability (Mutex, Atomic*, or OnceLock) for safe shared state.

use std::sync::OnceLock;
static CONFIG: OnceLock<String> = OnceLock::new();

const fn

Define functions evaluable at compile time to produce consts.

const fn square(n: i32) -> i32 { n * n }
const NINE: i32 = square(3);

Types and literals

Constants require types; numeric literals can use suffixes (42u64) or context for inference.

Naming

Use SCREAMING_SNAKE_CASE for const and static.

Summary

• const = compile‑time values; static = one address for program lifetime
• Avoid static mut; use safe abstractions for global mutable state
• Use const fn to compute constants at compile time