Flex TOML Configs in Rust

A simple example on how to make extendable configurations from TOML files.

Base Config

Example of a common config file you can have for your project.

[base]
param1 = value1
param2 = value2
# optional_param_with_default
# optional_param_no_default

Extended Config

Another example that extends the base config from the previous example, and includes a new plugin section.

[base]
param1 = value1
param2 = value2
# optional_param_with_default = value3
# optional_param_no_default = value4

[plugin]
param1 = value1
param2 = value2
# optional_param_with_default = value3
# optional_param_no_default = value4

Example

TOML configurations are loaded into Rust structs. And these structs are composed of Config and Block structs:

Base Config

Here, BaseConfig defines a block with fields we want:

#[derive(Default, Debug, Deserialize, PartialEq)]
pub struct BaseConfig {
    pub name: String,
    pub version: String,
    #[serde(default = "default_host")]
    pub host: String,
    #[serde(default = "default_port")]
    pub port: u16,
}

We provide default values for optional fields:

fn default_host() -> String {
    "localhost".to_string()
}

fn default_port() -> u16 {
    8080
}

Finally we define AppConfig as our config, which uses the BaseConfig as a section.

#[derive(Default, Debug, Deserialize, Serialize, PartialEq)]
pub struct AppConfig {
    pub base: BaseConfig,
}

Reading TOMLs

We can use load_config to read a TOML file and return our AppConfig struct. However, first we need AppConfig to implement Config which is the generic type accepted by the load_config function.

# the Config trait
pub trait Config: Default + Debug + Serialize + for<'de> Deserialize<'de> {}

# Implementing the trait for other configs
impl Config for AppConfig {}

Thenm, you can call load_config, using type annotation to specify the config type.

let config: plugin::AppConfig = load_config("path/to/config.toml").unwrap();

Saving configs to TOML file

We can call to_file from a parsed config, providing a name, to save it as a TOML file.

config.to_file("test_toml.toml")