Pass function to default (for string)

[Liam Dawson]

I'm trying to use a function called default_config() to provide the default value for an option:

pub fn default_config() -> String {
    option_env!("BIN_DEFAULT_CONFIG_PATH")
        .unwrap_or("/etc/default/path")
        .to_owned()
}

#[derive(Options)]
pub struct FetchOpts {
    #[options(help = "Path to the config file", default = "default_config()")]
    pub config: String,
}

However, that is interpreted as the literal value "default_config()". Passing default = default_config() causes a syntax error panic instead.

Is there a sensible way to set that default value, taking advantage of the help text auto-generation? (If not, I can fall back to the unwrap method, which I was previously using)

[Murarth]

I've just pushed a commit that implements this feature, with a new attribute default_expr. The expression must be contained in a string (e.g. default_expr = "default_config()") because of how the Rust compiler parses item attributes.

[Liam Dawson]

Perfect, thank you! Will test it out soon 🙂

[Liam Dawson]

Can confirm, works as expected. Is the value meant to show up in the help string too, or am I confusing it with the clap version?

[Murarth]

A parseable default value string does show up in the usage, but the default_expr does not. Only the Rust expression is available at compile time (the usage string is a &'static str that is generated when derive(Options) is expanded) and displaying that would not provide any meaningful information to the user.