Const Functions (`const fn`)

const fn marks a function as eligible for compile-time evaluation.

This wiki page is an overview. The canonical specification is const functions.

Example#

const fn add (a: int, b: int) -> int {
  return a + b;
}

const answer: int = add(20, 22);

fn main () -> int {
  return answer;
}

Const functions are intended for const initializer evaluation.
Const functions are compile-time-only: they are not callable from runtime code and are not emitted as runtime/linkable symbols.
The Supported forms targets scalar-only const evaluation; string/aggregate return values and higher-order const evaluation are planned.
In the Supported forms, const fn bodies must not allocate (new) or use regions/with, must not contain panic, and may only call other const fn declarations.