#no-std #const-fn #parsing

no-std konst

Compile-time comparison, parsing, and other const functionality

7 releases

0.2.4 Mar 31, 2021
0.2.3 Mar 30, 2021
0.1.1 Mar 22, 2021
0.0.0 Mar 15, 2021

#40 in Parser tooling

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Zlib license

3.5K SLoC

Rust crates-io api-docs

Compile-time comparison, parsing, and const equivalents of std methods.


This crate provides:


Parsing an enum

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

use konst::eq_str;
use konst::{unwrap_opt_or, unwrap_ctx};

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

impl Direction {
    const fn try_parse(input: &str) -> Result<Self, ParseDirectionError> {
        // As of Rust 1.51.0, 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 = unwrap_opt_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"),

Parsing integers

You can parse integers using the parse_* functions in primitive, returning a None if the string as a whole isn't a valid integer.

use konst::primitive::parse_i128;

const N_100: Option<i128> = parse_i128("100");
assert_eq!(N_100, Some(100));

const N_N3: Option<i128> = parse_i128("-3");
assert_eq!(N_N3, Some(-3));

const NONE: Option<i128> = parse_i128("-");
assert_eq!(NONE, None);

const PAIR: Option<i128> = parse_i128("1,2");
assert_eq!(PAIR, None);

For parsing an integer inside a larger string, you can use Parser::parse_u128 method and the other parse_* methods

use konst::{Parser, unwrap_ctx};

const PAIR: (i64, u128) = {;
    let parser = Parser::from_str("1365;6789");

    // Parsing "1365"
    let (l, parser) = unwrap_ctx!(parser.parse_i64());

    // Skipping the ";"
    let parser = unwrap_ctx!(parser.strip_prefix(";"));

    // Parsing "6789"
    let (r, parser) = unwrap_ctx!(parser.parse_u128());
    (l, r)
assert_eq!(PAIR.0, 1365);
assert_eq!(PAIR.1, 6789);

Parsing a struct

This example demonstrates how you can use Parser to parse a struct at compile-time.

use konst::{
    parsing::{Parser, ParseValueResult},
    for_range, parse_any, try_rebind, unwrap_ctx,

const PARSED: Struct = {
    // You can also parse strings from environment variables, or from an `include_str!(....)`
    let input = "\
        red, blue, green, blue,

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

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Struct {
    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> {
    try_rebind!{(let amount, parser) = parser.trim_start().parse_usize()}
    try_rebind!{parser = parser.strip_prefix(",")}

    try_rebind!{(let repeating, parser) = parse_shape(parser.trim_start())}
    try_rebind!{parser = parser.strip_prefix(",")}

    try_rebind!{(let colors, parser) = parse_colors(parser.trim_start())}

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

pub const fn parse_shape(mut parser: Parser<'_>) -> ParseValueResult<'_, Shape> {
    let shape = parse_any!{parser, strip_prefix;
        "circle" => Shape::Circle,
        "square" => Shape::Square,
        "line" => Shape::Line,
        _ => return Err(parser.into_other_error())
    Ok((shape, parser))

pub const fn parse_colors(mut parser: Parser<'_>) -> ParseValueResult<'_, [Color; 4]> {
    let mut colors = [Color::Red; 4];

    for_range!{i in 0..4 =>
        try_rebind!{(colors[i], parser) = parse_color(parser.trim_start())}
        try_rebind!{parser = parser.strip_prefix(",")}

    Ok((colors, parser))

pub const fn parse_color(mut parser: Parser<'_>) -> ParseValueResult<'_, Color> {
    let color = parse_any!{parser, strip_prefix;
        "red" => Color::Red,
        "blue" => Color::Blue,
        "green" => Color::Green,
        _ => return Err(parser.into_other_error())
    Ok((color, parser))

Cargo features

These are the features of these crates:

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

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

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

  • "constant_time_slice"(disabled by default):
    Requires Rust nightly. Improves the performance of slice functions that split slices, from taking linear time to taking constant time.
    Note that only functions which mention this feature in their documentation are affected.

  • "const_generics" (disabled by default): Requires Rust 1.51.0. Enables items that require const generics, and impls for arrays to use const generics instead of only supporting small arrays.

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

  • "deref_raw_in_fn" (disabled by default): Requires Rust nightly. Enables const fns that need to dereference raw pointers.

No-std support

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

Minimum Supported Rust Version

konst requires Rust 1.46.0, because it uses looping an branching in const contexts.

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