Crate const_init_build 

Generate Rust constant variables from the data in your configuration file (currently only JSON) at build time.

The goal is to improve performance by initializing your types with build-time values and benefit from compiler optimizations. This is meant to be used with const_init_macros that helps do constant build-time initialization with your custom types.

Workflow

Starts from JSON configuration file, for example settings.json:

{
  "foo": true,
  "bar": 1,
}

Generate Rust constants at build-time in build.rs:

use std::path::PathBuf;
use const_init_build::generate_constants_from_json;
fn main() {
    let manifest_path = std::env::var("CARGO_MANIFEST_DIR").unwrap();
    // We read the settings from "settings.json" file
    let json_input: PathBuf = [&manifest_path, "settings.json"].iter().collect();
    // We output "settings.rs" containing the variables of "settings.json" as constants
    let rust_output: PathBuf = [&manifest_path, "examples", "generated", "settings.rs"]
        .iter()
        .collect();

    generate_constants_from_json(&json_input, &rust_output);
}

Obtain Rust constant variables at examples/generated/settings.rs:

pub const FOO: bool = true;
pub const BAR: isize = 1;

Limitations

File format

Currently only JSON is supported but there are no difficulties to support other formats such as TOML.

Json to Rust

Certain JSON types do not translate perfectly into Rust types.

• JSON integers which are not float are all turned into Rust isize
• JSON arrays containing different types are not handled
• JSON null is unsupported
• JSON Nan is unsupported

Functions

generate_constants_from_json

This function is used in build scripts to generate a rust file from a json file. The output file contains all the variables from input file as rust constants.