constants - Rust By Example
Description: A description

# constants

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 `mut`able variable with `'static` lifetime.

One special case is the `"string"` literal. It can be assigned directly to a `static` variable without modification because its type signature: `&'static str` has the required lifetime of `'static`. All other reference types must be specifically annotated so that they fulfill the `'static` lifetime. This may seem minor though because the required explicit annotation hides the distinction.

    // Globals are declared outside all other scopes.
    static LANGUAGE: &'static str = "Rust";
    const  THRESHOLD: i32 = 10;

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

    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