Default in derive API from Default::default()

[Shatur]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

Clap Version

3.1.8

Describe your use case

I have a struct with non-optional values but with defaults. Currently I define it like this:

#[derive(Args, Clone)]
#[cfg_attr(test, derive(PartialEq, Debug))]
pub(crate) struct ServerSettings {
    /// Server name that will be visible to other players.
    #[clap(short, long, default_value_t = ServerSettings::default().server_name)]
    pub(crate) server_name: String,

    /// IP address to bind.
    #[clap(short, long, default_value_t = ServerSettings::default().ip)]
    pub(crate) ip: String,

    /// Port to use.
    #[clap(short, long, default_value_t = ServerSettings::default().port)]
    pub(crate) port: u16,

    /// Game mode.
    #[clap(short, long, default_value_t = ServerSettings::default().game_mode)]
    pub(crate) game_mode: GameMode,

    /// Game map.
    #[clap(short, long, default_value_t = ServerSettings::default().map)]
    pub(crate) map: Map,

    /// Choose heroes randomly.
    #[clap(short, long)]
    pub(crate) random_heroes: bool,
}

Because default_value_t uses Default::default for the underlying type, not from the struct. It's very unergonomic and error-prone.

Describe the solution you'd like

It would be nice to have an attribute to use default value from the struct's Default trait.

Alternatives, if applicable

No response

Additional Context

No response

[epage]

Looks like we have #3116 for this, closing in favor if that issue.