toml_migrate

A crate that lets you read versioned config files and easily migrate them to the latest version. Inspired by the magic_migrate library.

Why?

Many applications need their configuration files to evolve over time as features get added and changed. This aims to simplify the process of taking a old config file and transforming it to the latest version.

How does it work?

For every version of your library, define a configuration struct that can be deserialized. This struct should not include the version field:

#[derive(Deserialize)]
struct ConfigV1 {
    name: String,
    timeout: u32,
}

#[derive(Deserialize)]
struct ConfigV2 {
    name: String,
    timeout: u32,
    // new!
    retries: u8,
}

Then, implement From<PreviousConfig> for all of your config types:

impl From<ConfigV1> for ConfigV2 {
    fn from(prev: ConfigV1) -> Self {
        Self {
            name: prev.name,
            timeout: prev.timeout,
            retries: 4,
        }
    }
}

Finally, use the build_migration_chain! macro to automatically implement the Migrate trait for all of your structs:

build_migration_chain!(ConfigV1 = 1, ConfigV2 = 2);

From there, you can use the ConfigMigrator to easily migrate your config file from a string:

fn read_config() -> ConfigV2 {
    let config_str = r#"
        version = 1
        name = "MyApp"
        timeout = 60
    "#;

    let migrator = ConfigMigrator::new("version");
    let (config, _) = migrator.migrate_config(config_str).unwrap();

    config
}