const fn

Initially added:

Expanded in many releases, see each aspect below for more details.

A const fn allows you to execute code in a "const context." For example:

#![allow(unused)]
fn main() {
const fn five() -> i32 {
    5
}

const FIVE: i32 = five();
}

You cannot execute arbitrary code; the reasons why boil down to "you can destroy the type system." The details are a bit too much to put here, but the core idea is that const fn started off allowing the absolutely minimal subset of the language, and has slowly added more abilities over time. Therefore, while you can create a const fn in Rust 1.31, you cannot do much with it. This is why we didn't add const fn to the Rust 2018 section; it truly didn't become useful until after the release of the 2018 edition. This means that if you read this document top to bottom, the earlier versions may describe restrictions that are relaxed in later versions.

Additionally, this has allowed more and more of the standard library to be made const, we won't put all of those changes here, but you should know that it is becoming more const over time.

Arithmetic and comparison operators on integers

You can do arithmetic on integer literals:

#![allow(unused)]
fn main() {
const fn foo() -> i32 {
    5 + 6
}
}

Many boolean operators

You can use boolean operators other than && and ||, because they short-circut evaluation:

#![allow(unused)]
fn main() {
const fn mask(val: u8) -> u8 {
    let mask = 0x0f;

    mask & val
}
}

Constructing arrays, structs, enums, and tuples

You can create arrays, structs, enums, and tuples:

#![allow(unused)]
fn main() {
struct Point {
    x: i32,
    y: i32,
}

enum Error {
    Incorrect,
    FileNotFound,
}

const fn foo() {
    let array = [1, 2, 3];

    let point = Point {
        x: 5,
        y: 10,
    };

    let error = Error::FileNotFound;

    let tuple = (1, 2, 3);
}
}

Calls to other const fns

You can call const fn from a const fn:

#![allow(unused)]
fn main() {
const fn foo() -> i32 {
    5
}

const fn bar() -> i32 {
    foo()
}
}

Index expressions on arrays and slices

You can index into an array or slice:

#![allow(unused)]
fn main() {
const fn foo() -> i32 {
    let array = [1, 2, 3];

    array[1]
}
}

Field accesses on structs and tuples

You can access parts of a struct or tuple:

#![allow(unused)]
fn main() {
struct Point {
    x: i32,
    y: i32,
}

const fn foo() {
    let point = Point {
        x: 5,
        y: 10,
    };

    let tuple = (1, 2, 3);

    point.x;
    tuple.0;
}
}

Reading from constants

You can read from a constant:

#![allow(unused)]
fn main() {
const FOO: i32 = 5;

const fn foo() -> i32 {
    FOO
}
}

Note that this is only const, not static.

& and * of references

You can create and de-reference references:

#![allow(unused)]
fn main() {
const fn foo(r: &i32) {
    *r;

    &5;
}
}

Casts, except for raw pointer to integer casts

You may cast things, except for raw pointers may not be casted to an integer:

#![allow(unused)]
fn main() {
const fn foo() {
    let x: usize = 5;

    x as i32;
}
}

Irrefutable destructuring patterns

You can use irrefutable patterns that destructure values. For example:

#![allow(unused)]
fn main() {
const fn foo((x, y): (u8, u8)) {
    // ...
}
}

Here, foo destructures the tuple into x and y. if let is another place that uses irrefutable patterns.

let bindings

You can use both mutable and immutable let bindings:

#![allow(unused)]
fn main() {
const fn foo() {
    let x = 5;
    let mut y = 10;
}
}

Assignment

You can use assignment and assignment operators:

#![allow(unused)]
fn main() {
const fn foo() {
    let mut x = 5;
    x = 10;
}
}

Calling unsafe fn

You can call an unsafe fn inside a const fn:

#![allow(unused)]
fn main() {
const unsafe fn foo() -> i32 { 5 }

const fn bar() -> i32 {
    unsafe { foo() }
}
}