#build #config #parsing #generator


Create struct definitions from config files at build time

5 releases (breaking)

0.5.0 Jun 18, 2020
0.4.0 Apr 3, 2020
0.3.0 Oct 21, 2019
0.2.0 Jul 31, 2018
0.1.0 Dec 20, 2017

#86 in Configuration

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This is a library for converting config files into matching source files at build time.

Build Status Crates.io Docs.rs


This library is intended to be used in a build.rs file, so it needs to be added to [build-dependencies].

version = "~0.4.0"
features = ["toml-parsing"]

By default, config_struct is markup-language-agnostic, so include the relevant feature for whatever language your config file is written in. Choices are:

  1. json-parsing
  2. ron-parsing
  3. toml-parsing
  4. yaml-parsing


Now in your build.rs file, add code like the following:

use config_struct::{Error, StructOptions};

fn main() -> Result<(), Error> {

This will take the following config.toml file:

name = "Config name"

... and generate a config.rs file like the following:

// ...
use std::borrow::Cow;

#[derive(Debug, Clone)]
pub struct Config {
    pub name: Cow<'static, str>,

pub const CONFIG: Config = Config {
    name: Cow::Borrowed("Config name"),

Strings and arrays are represented by Cow types, which allows the entire Config struct to be either heap allocated at runtime, or a compile time constant, as shown above.

Support for serde

Unless you are specifically avoiding the serde family of crates at runtime, it's recommended to use the following options:

StructOptions {
    serde_support: SerdeSupport::Yes,
    generate_load_fns: true,
    .. my_other_options

This will derive the Serialize and Deserialize traits for your struct, as well as providing a handy load() method to read and parse the file at runtime.

If these are the only options you want to set beyond the defaults, you can use StructOptions::serde_default() as a shorthand.


There are a few different ways to access the config at runtime.

  1. Call the generated load function, e.g. let config = Config::load();
    • Note that this requires the generate_load_fns option described above.
  2. Access the CONFIG const directly, e.g. let x = CONFIG.name;
  3. Deserialize the config file manually, e.g. let config: Config = toml::from_str(file_contents)?
    • Note that this either requires the serde_support option above, or requires you to manually add serde::Serialize and serde::Deserialize to the derived_traits option.

The first method is recommended, as it will return the const value in release mode, but load from the filesystem in debug mode. This gives you flexibility during development and immutability in release.


Enum generation works in a similar way to structs, but for the keys of a map.

// build.rs
use config_struct::{Error, EnumOptions};

fn main() -> Result<(), Error> {

The above build script will take the following items.yaml file and generate a (not-formatted) items.rs like the following:

# items.yaml
    - data
    - more
    - data
// items.rs
// ...
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub enum Key {
impl Key {
    pub const ALL: &'static [Key] = &[Key::ItemOne, Key::ItemTwo];
impl Default for Key {
    fn default() -> Self {
impl std::fmt::Display for Key {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        write!(f, "{:?}", self)
impl std::str::FromStr for Key {
    type Err = ();
    fn from_str(s: &str) -> Result<Self, Self::Err> {
        const STRINGS: &'static [&'static str] = &["ItemOne", "ItemTwo"];
        for (index, &key) in STRINGS.iter().enumerate() {
            if key == s {
                return Ok(Key::ALL[index]);

As you can see, this provides more functionality out-of-the-box - most of which could be disabled in the EnumOptions. The intended purpose of this is to have a small efficient type to use as a key into the data stored in the initial config file.


~22K SLoC