Macro config_sources 

macro_rules! config_sources {
    ($builder:ident {$($source:expr),* $(,)?}) => { ... };
    (@impl $builder:ident::<[$($source:expr),* $(,)?]>) => { ... };
    (@file file($wd:expr).required($req:expr)) => { ... };
}

create a macro to streamline the process of adding sources

use scsys_config::{config, config_sources};

use config::builder::{ConfigBuilder, DefaultState};

let mut builder: ConfigBuilder<DefaultState> = ConfigBuilder::default();

builder = config_sources! {
    builder {
        config::File::with_name("config.toml").required(false),
    }
};