#transmute #casting #const-fn

no-std constmuck

const-equivalents of bytemuck functions

5 releases (2 stable)

1.1.0 Jan 4, 2024
1.0.0 Sep 2, 2023
0.3.0 Oct 26, 2021
0.2.0 Oct 9, 2021
0.0.0 Sep 10, 2021

#147 in No standard library

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Rust crates-io api-docs

Const equivalents of many bytemuck functions.

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

Because the *_alt functions are const fns, they can't inspect the address of the reference parameter. This differs from their bytemuck equivalents, which use the address to determine alignment.

Examples

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.

Contiguous

This example demonstrates constructing an enum from its representation.


use constmuck::Contiguous;

use konst::{array, try_opt};

fn main() {
    const COLORS: Option<[Color; 5]> = Color::from_array([3, 4, 1, 0, 2]);
    assert_eq!(
        COLORS,
        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);
}

#[repr(u8)]
#[derive(Debug, PartialEq, Eq, Contiguous, Copy, Clone)]
pub enum Color {
    Red = 0,
    Blue,
    Green,
    White,
    Black,
}

impl Color {
    pub const fn from_int(n: u8) -> Option<Self> {
        constmuck::contiguous::from_integer(n)
    }
    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))))
    }
}


Wrapper

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


use constmuck::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)));
}

#[repr(transparent)]
#[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)
    }
}

impl SliceWrapper<u32> {
    pub const fn sum(&self) -> u64 {
        konst::iter::eval!(&self.0,copied(),fold(0, |l, r| l + r as u64))
    }
    pub const fn find_first_even(&self) -> Option<(usize, u32)> {
        konst::iter::eval!(&self.0,copied(),enumerate(),find(|(i, n)| *n % 2 == 0))
    }
}


Additional checks

The "debug_checks" crate feature (which is disabled by default) enables additional assertions in constmuck functions, these assertions panic in some cases where unsound impls of bytemuck traits would have caused Undefined Behavior.

Features

These are the features of this crate:

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

  • "debug_checks"(disabled by default): Enables additional safety checks for detecting some Undefined Behavior.

  • "rust_1_75" (disabled by default): allows constmuck::zeroed to construct types of any size.

  • "rust_latest_stable" (disabled by default): enables all "rust_1_*" features.

No-std support

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

Minimum Supported Rust Version

constmuck requires Rust 1.65.0.

You can use the "rust_latest_stable" crate feature to get all items and functionality that requires stable Rust versions after 1.65.0.

Dependencies

~755KB
~13K SLoC