code.rs

let mut formatted_bytes = Vec::new();
{
    struct UnimplementedCliOptions;
    impl rustfmt_nightly::CliOptions for UnimplementedCliOptions {
        fn apply_to(self, _config: &mut Config) { unimplemented!() }
        fn config_path(&self) -> Option<&std::path::Path> { unimplemented!() }
    }

    let (mut config, _) = load_config::<UnimplementedCliOptions>(None, None).unwrap();
    config.set().emit_mode(EmitMode::Files);
    config.set().edition(Edition::Edition2018);

    let mut session = Session::new(config, Some(&mut formatted_bytes));
    session.format(Input::Text(code)).unwrap();
}

String::from_utf8_lossy(&formatted_bytes).to_string()