Crate konst

source ·
Expand description

Const equivalents of std functions and const parsing.


This crate provides:

  • Const fn equivalents of standard library functions and methods.

  • Compile-time parsing through the Parser type, and parser_method macro.


Parsing an enum

This example demonstrates how you can parse a simple enum from an environment variable, at compile-time.

use konst::{

#[derive(Debug, PartialEq)]
enum Direction {

impl Direction {
    const fn try_parse(input: &str) -> Result<Self, ParseDirectionError> {
        // As of Rust 1.65.0, string patterns don't work in const contexts
        match () {
            _ if eq_str(input, "forward") => Ok(Direction::Forward),
            _ if eq_str(input, "backward") => Ok(Direction::Backward),
            _ if eq_str(input, "left") => Ok(Direction::Left),
            _ if eq_str(input, "right") => Ok(Direction::Right),
            _ => Err(ParseDirectionError),

const CHOICE: &str = option::unwrap_or!(option_env!("chosen-direction"), "forward");

const DIRECTION: Direction = unwrap_ctx!(Direction::try_parse(CHOICE));

fn main() {
    match DIRECTION {
        Direction::Forward => assert_eq!(CHOICE, "forward"),
        Direction::Backward => assert_eq!(CHOICE, "backward"),
        Direction::Left => assert_eq!(CHOICE, "left"),
        Direction::Right => assert_eq!(CHOICE, "right"),

#[derive(Debug, PartialEq)]
pub struct ParseDirectionError;

use std::fmt::{self, Display};

impl Display for ParseDirectionError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str("Failed to parse a Direction")

impl ParseDirectionError {
    const fn panic(&self) -> ! {
        panic!("failed to parse a Direction")

Parsing CSV

This example demonstrates how CSV can be parsed into integers.

This example requires the "parsing" and "iter" features (both are enabled by default).

use konst::{
    iter, string,

const CSV: &str = "3, 8, 13, 21, 34";

static PARSED: [u64; 5] = iter::collect_const!(u64 =>
    string::split(CSV, ","),
        map(|s| unwrap_ctx!(parse_u64(s))),

assert_eq!(PARSED, [3, 8, 13, 21, 34]);

Parsing a struct

This example demonstrates how a key-value pair format can be parsed into a struct.

This requires the "parsing_proc" feature (enabled by default).

use konst::{
    parsing::{Parser, ParseValueResult},
    for_range, parser_method, try_, unwrap_ctx,

const PARSED: Struct = {
    // You can also parse strings from environment variables, or from an `include_str!(....)`
    let input = "\
        colors = red, blue, green, blue
        amount = 1000
        repeating = circle
        name = bob smith

fn main(){
            name: "bob smith",
            amount: 1000,
            repeating: Shape::Circle,
            colors: [Color::Red, Color::Blue, Color::Green, Color::Blue],

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Struct<'a> {
    pub name: &'a str,
    pub amount: usize,
    pub repeating: Shape,
    pub colors: [Color; 4],

#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Shape {

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

pub const fn parse_struct(mut parser: Parser<'_>) -> ParseValueResult<'_, Struct<'_>> {
    let mut name = "<none>";
    let mut amount = 0;
    let mut repeating = Shape::Circle;
    let mut colors = [Color::Red; 4];
    parser = parser.trim_end();
    if !parser.is_empty() {
        loop {
            let mut prev_parser = parser.trim_start();

            parser = try_!(parser.find_skip('='));

            parser_method!{prev_parser, strip_prefix;
                "name" => (name, parser) = try_!(parser.trim_start().split_keep('\n')),
                "amount" => (amount, parser) = try_!(parser.trim_start().parse_usize()),
                "repeating" => (repeating, parser) = try_!(parse_shape(parser.trim_start())),
                "colors" => (colors, parser) = try_!(parse_colors(parser.trim_start())),
                _ => {
                    let err = &"could not parse Struct field name";
                    return Err(prev_parser.into_other_error(err));

            if parser.is_empty() {
            parser = try_!(parser.strip_prefix("\n"));

    Ok((Struct{name, amount, repeating, colors}, parser))

pub const fn parse_shape(mut parser: Parser<'_>) -> ParseValueResult<'_, Shape> {
    let shape = parser_method!{parser, strip_prefix;
        "circle" => Shape::Circle,
        "square" => Shape::Square,
        "line" => Shape::Line,
        _ => return Err(parser.into_other_error(&"could not parse Shape"))
    Ok((shape, parser))

pub const fn parse_colors<const LEN: usize>(
    mut parser: Parser<'_>,
) -> ParseValueResult<'_, [Color; LEN]> {
    let mut colors = [Color::Red; LEN];

    for_range!{i in 0..LEN =>
        (colors[i], parser) = try_!(parse_color(parser.trim_start()));
        match parser.strip_prefix(",") {
            Ok(next) => parser = next,
            Err(_) if i == LEN - 1 => {}
            Err(e) => return Err(e),

    Ok((colors, parser))

pub const fn parse_color(mut parser: Parser<'_>) -> ParseValueResult<'_, Color> {
    let color = parser_method!{parser, strip_prefix;
        "red" => Color::Red,
        "blue" => Color::Blue,
        "green" => Color::Green,
        _ => return Err(parser.into_other_error(&"could not parse Color"))
    Ok((color, parser))

Cargo features

These are the features of these crates:

  • "iter"(enabled by default): Enables all iteration items, including macros/functions that take/return iterators,

  • "cmp"(enabled by default): Enables all comparison functions and macros, the string equality and ordering comparison functions don’t require this feature.

  • "parsing_proc"(enabled by default): Enables the "parsing" feature, compiles the konst_proc_macros dependency, and enables the parser_method macro. You can use this feature instead of "parsing" if the slightly longer compile times aren’t a problem.

  • "parsing"(enabled by default): Enables the parsing module (for parsing from &str and &[u8]), the primitive::parse_* functions, try_rebind, and rebind_if_ok macros.

  • "alloc": Enables items that use types from the alloc crate, including Vec and String.

None of thse features are enabled by default.

  • "rust_latest_stable": enables the latest "rust_1_*" feature(there’s currently none). Only recommendable if you can update the Rust compiler every stable release.

  • "mut_refs"(disabled by default): Enables const functions that take mutable references. Use this whenever mutable references in const contexts are stabilized. Also enables the "rust_latest_stable" feature.

  • "nightly_mut_refs"(disabled by default): Enables the "mut_refs" feature. Requires Rust nightly.

No-std support

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

Minimum Supported Rust Version

konst requires Rust 1.65.0.

Features that require newer versions of Rust, or the nightly compiler, need to be explicitly enabled with crate features.



  • Generic constants for types from the alloc crate, including String and Vec.
  • Const equivalents of array functions.
  • Const equivalents of char functions.
  • cmpcmp
    Comparisong-related items.
  • const equivalents of core::ffi functions
  • iteriter
    Const equivalent of iterators with a specific next function signature.
  • Const fn equivalents of ManuallyDrop<T> methods.
  • Const fn equivalents of MaybeUninit<T> methods.
  • const fn equivalents of NonZero* methods.
  • const equivalents of Option methods.
  • const fn equivalents of methods from miscelaneous standard library types.
  • parsingparsing
    Parsing using const fn methods.
  • Miscelaneous items used for emulating polymorphism without trait methods.
  • const fn equivalents of primitive type methods.
  • Const equivalents of raw pointer and NonNull methods.
  • const fn equivalents of range methods.
  • const equivalents of Result methods.
  • const fn equivalents of slice methods.
  • const fn equivalents of str methods.


  • For asserting that two values are equal.
  • For asserting that two values are unequal.
  • Coerces reference to a type that has a const_eq or const_cmp method.
  • Compares two values for ordering.
  • Compares two standard library types for ordering, that can’t be compared with const_cmp.
  • Compares two values for equality.
  • Compares two standard library types for equality, that can’t be compared with const_eq.
  • For loop over a range
  • For implementing const comparison semi-manually.
  • Emulates the inline const feature, eg: const{ foo() },
  • parse_withparsing
    Parses a type that impls HasParser with the passed in Parser.
  • parser_methodparsing_proc
    Calls a Parser method with many alternative string literals.
  • Like an if let Ok, but also reassigns variables with the value in the Ok variant.
  • ?-like macro, which allows optionally mapping errors.
  • Evaluates to $ord if it is Ordering::Equal, otherwise returns it from the enclosing function.
  • ?-like macro for Options.
  • try_rebindparsing
    Like the ? operator, but also reassigns variables with the value in the Ok variant.