The tracking issue for this feature is: #24111

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

#![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);
}