Crate constmuck[][src]

Expand description

Const equivalents of many bytemuck functions, and additional functionality.

constmuck uses bytemuck’s traits, any type that implements those traits can be used with the relevant functions from this crate.

The *_alt functions aren’t exactly equivalent to the bytemuck ones, each one describes how it’s different.

This crate avoids requiring (unstable as of 2021) trait bounds in const fns by using marker types to require that a trait is implemented.


These examples use bytemuck’s derives to show how users don’t need to write unsafe to use this crate, and use the konst crate to make writing the const functions easier.


This example demonstrates constructing an enum from its representation.

use constmuck::{Contiguous, infer};

use konst::{array, try_opt};

fn main() {
    const COLORS: Option<[Color; 5]> = Color::from_array([3, 4, 1, 0, 2]);
        Some([Color::White, Color::Black, Color::Blue, Color::Red, Color::Green]),

    const NONE_COLORS: Option<[Color; 4]> = Color::from_array([1, 2, 3, 5]);
    assert_eq!(NONE_COLORS, None);

#[derive(Debug, PartialEq, Eq, Contiguous, Copy, Clone)]
pub enum Color {
    Red = 0,

impl Color {
    pub const fn from_int(n: u8) -> Option<Self> {
        constmuck::contiguous::from_u8(n, infer!())
    pub const fn from_array<const N: usize>(input: [u8; N]) -> Option<[Self; N]> {
        // `try_opt` returns from `from_array` on `None`,
        // because `konst::array::map` allows the passed-in expression
        // to return from the surrounding named function.
        Some(array::map!(input, |n| try_opt!(Self::from_int(n))))


This example demonstrates a type that wraps a [T], constructed by reference.

use constmuck::{IsTW, TransparentWrapper};

fn main() {
    const SLICE: &[u32] = &[3, 5, 8, 13, 21];
    const WRAPPER: &SliceWrapper<u32> = SliceWrapper::new(SLICE);

    const SUM: u64 = WRAPPER.sum();
    assert_eq!(SUM, 50);

    const FIRST_EVEN: Option<(usize, u32)> = WRAPPER.find_first_even();
    assert_eq!(FIRST_EVEN, Some((2, 8)));

#[derive(Debug, PartialEq, Eq, TransparentWrapper)]
pub struct SliceWrapper<T>(pub [T]);

impl<T> SliceWrapper<T> {
    // Using `constmuck` allows safely defining this function as a `const fn`
    pub const fn new(reff: &[T]) -> &Self {
        constmuck::wrapper::wrap_ref!(reff, IsTW!())

impl SliceWrapper<u32> {
    pub const fn sum(&self) -> u64 {
        let mut sum = 0;
        konst::for_range!{i in 0..self.0.len() =>
            sum += self.0[i] as u64;
    pub const fn find_first_even(&self) -> Option<(usize, u32)> {
        konst::for_range!{i in 0..self.0.len() =>
            if self.0[i] % 2 == 0 {
                return Some((i, self.0[i]));

Additional checks

Additional checks are enabled in debug builds, all of which cause panics when it’d have otherwise been Undefined Behavior (caused by unsound unsafe impls or calling unsafe constructor functions), which means that there is a bug in some unsafe code somewhere.

The precise checks are left unspecified so that they can change at any time.

These checks are disabled by default in release builds, to enable them you can use this in your Cargo.toml:

debug-assertions = true


These are the features of this crate:

  • "derive"(disabled by default): Enables bytemuck’s "derive" feature and reexports its derives.

  • "rust_latest_stable"(disabled by default): Enables all items and functionality that requires stable Rust versions after 1.56.0. Currently doesn’t enable any other feature.

  • "rust_1_57"(disabled by default, requires Rust 1.57.0): Causes this crate to use the const_panic dependency, to improve the quality of panic messages.

No-std support

constmuck is #![no_std], it can be used anywhere Rust can be used.

Minimum Supported Rust Version

constmuck requires Rust 1.56.0, because it uses transmute inside const fns.

You can use the "rust_latest_stable" crate feature to get all items and functionality that requires stable Rust versions after 1.56.0.


pub use bytemuck;
pub use bytemuck::Contiguous;
pub use bytemuck::Pod;
pub use bytemuck::PodCastError;
pub use bytemuck::TransparentWrapper;
pub use bytemuck::Zeroable;


Functions for converting types that implement Contiguous into and from their integer representation.

For copying values in generic contexts.

Functions for wrapping/peeling types that implement TransparentWrapper.


Constructs an IsContiguous<$T, $IntRepr>, requires $T:Contiguous<Int = $IntRepr>.

Constructs an IsCopy<$T>, requires $T:Pod for reasons explained in the bound section.

Constructs an IsPod<$T>, requires $T:Pod.

Constructs an IsTransparentWrapper<$Outer, $Inner>, requiring that $Outer implements TransparentWrapper<$Inner>.

Constructs an IsZeroable<$T>, requires $T:Zeroable.

For constructing types that implement Infer, this includes Is* types.

Maps the bound field of a TypeSize.


Encodes a T:Contiguous<Int = IntRepr> bound as a value.

Encodes that a T is trivially copyable.

Encodes a T:Pod bound as a value.

Encodes a Outer:TransparentWrapper<Inner> bound as a value.

Encodes a T:Zeroable bound as a value.

For passing a type along with its size and additional bounds.


For constructing Is* types (values that represent trait bounds), and tuples of them.


Casts &T to &[u8; SIZE]

Casts T into U

Cast a &T to &U

Casts &[T] to &[U]

Tries to cast T into U

Tries to cast &T to &U

Tries to cast &[T] to &[U]

Constructs a zero-initialized T, equivalent to std::mem::zeroed::<T>().

Constructs a zero-initialized [T; N], equivalent to std::mem::zeroed::<[T; N]>().