Prioritize CLI parameters over file/env if not set explicitly

[vbrandl]

I want to prioritize CLI parameters over parameters from a config file or the environment but only if the CLI parameters are explicitly set (e.g. not derived via the clap default value).

If I understand correctly, merging with clap parameters only works if every CLI parameter has a default value.

I have the following config struct:

#[derive(Serialize, Deserialize, Debug, Parser)]
#[command(version, about, long_about = None)]
pub struct Config {
    #[arg(long, default_value = "0.0.0.0")]
    pub host: String,
    #[arg(long, default_value = "1337")]
    pub port: u16,
}

And 2 functions to parse the configuration, with a different order for the config sources:

impl Config {
    pub fn load_env_first() -> Result<Self> {
        Ok(Figment::new()
            .merge(Env::prefixed("APP_"))
            .merge(Serialized::defaults(Config::parse()))
            .extract()?)
    }

    pub fn load_cli_first() -> Result<Self> {
        Ok(Figment::new()
            .merge(Serialized::defaults(Config::parse()))
            .merge(Env::prefixed("APP_"))
            .extract()?)
    }
}

My general expectation for CLI tools is, that explicitly passed command line parameters overwrite everything else (e.g. parameters from env or config files). If I want this, i have to use the load_env_first function. But since for this to work, every clap parameter needs a default value, those default values overwrite everything from env and config files, if I place the serialized defaults last.

$ APP_HOST=127.0.0.1 ./app
env first: Config {
    host: "0.0.0.0",
    port: 1337,
}
cli first: Config {
    host: "127.0.0.1",
    port: 1337,
}

Is it possible to achieve the following behavior using figment, clap and configuration files/environment variables?

# default values
$ ./app
=> host = 0.0.0.0
# value from env (and NOT the clap default)
$ APP_HOST=127.0.0.1 ./app
=> host = 127.0.0.1
# value from cli
$ ./app --host 127.0.0.1
=> host = 127.0.0.1
# if both env and cli params are set, the CLI param should be prioritized
$ APP_HOST=127.0.0.1 ./app --host 127.0.0.2
=> host = 127.0.0.2