Rust has two different types of constants which can be declared in any scope including global. Both require explicit type annotation:

  • const: An unchangeable value (the common case).
  • static: A possibly mutable variable with 'static lifetime. The static lifetime is inferred and does not have to be specified. Accessing or modifying a mutable static variable is unsafe.
// Globals are declared outside all other scopes.
static LANGUAGE: &str = "Rust";
const THRESHOLD: i32 = 10;

fn is_big(n: i32) -> bool {
    // Access constant in some function

fn main() {
    let n = 16;

    // Access constant in the main thread
    println!("This is {}", LANGUAGE);
    println!("The threshold is {}", THRESHOLD);
    println!("{} is {}", n, if is_big(n) { "big" } else { "small" });

    // Error! Cannot modify a `const`.
    THRESHOLD = 5;
    // FIXME ^ Comment out this line

See also:

The const/static RFC, 'static lifetime