const_fn - The Rust Unstable Book

# `const_fn`

The tracking issue for this feature is: #57563

The `const_fn` feature allows marking free functions and inherent methods as `const`, enabling them to be called in constants contexts, with constant arguments.

## Examples

    #![feature(const_fn)]

    const fn double(x: i32) -> i32 {
        x * 2
    }

    const FIVE: i32 = 5;
    const TEN: i32 = double(FIVE);

    fn main() {
        assert_eq!(5, FIVE);
        assert_eq!(10, TEN);
    }