Load config from file and use it in rust code everywhere

Question

I'm new to rust and want to understand how to load config from file to use it in code.

In main.rs and other files i want to use config loaded from file:

mod config;
use crate::config::config;

fn main() {
  println!("{:?}", config);
}

In config.rs file i want to read backend.conf file once at runtime, check it and store it as immutable to use it everywhere. My attempts so far got me only errors:

use hocon::HoconLoader;
use serde::Deserialize;

#[derive(Deserialize, Debug)]
pub struct Config {
    pub host: String,
    pub port: String,
}

pub const config: Config = get_config(); //err: calls in constants are limited to constant functions, tuple structs and tuple variants

fn get_config() -> Config {
    let config: Config = HoconLoader::new() // err: could not evaluate constant pattern
        .load_file("./backend.conf")
        .expect("Config load err")
        .resolve()
        .expect("Config deserialize err");

    config
}

What i can't wrap my head around, is to how you should do it in rust?

As Netwave suggested this is how it works:

main.rs:

 #[macro_use]
   extern crate lazy_static;

    mod config;
    use crate::config::CONFIG;

    fn main() {
      println!("{:?}", CONFIG.host);
    }

config.rs:

use hocon::HoconLoader;
use serde::Deserialize;

#[derive(Deserialize, Debug)]
pub struct Config {
    pub host: String,
    pub port: String,
}

lazy_static! {
    pub static ref CONFIG: Config = get_config();
}

fn get_config() -> Config {
    let configs: Config = HoconLoader::new()
        .load_file("./backend.conf")
        .expect("Config load err")
        .resolve()
        .expect("Config deserialize err");

    configs
}

backend.config:

{
    host: "127.0.0.1"
    port: "3001"
}

You could use the [`lazy_static` crate](https://crates.io/crates/lazy_static). However, that might make error handling a bit more troublesome. Instead I'd load the config in main, and pass a reference to the things that need it. — vallentin, Feb 07 '21 at 16:28
Does this answer your question? [Is it possible to use global variables in Rust?](https://stackoverflow.com/questions/19605132/is-it-possible-to-use-global-variables-in-rust) — pretzelhammer, Feb 07 '21 at 16:28

Netwave · Accepted Answer · 2021-02-07T17:58:33.980

5

Use lazy_static:

lazy_static!{
    pub static ref CONFIG: Config = get_config(); 
}

Alternatively, you may load it in the program entry point and attach it in some kind of context and pass it around where you need it.

edited Feb 07 '21 at 17:58

answered Feb 07 '21 at 16:28

Netwave

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Load config from file and use it in rust code everywhere

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