Is there any way to use field in macro attribute declaration?

[hzqd]

I code this:

#[derive(Parser)]
pub struct Args {
    /// Specify the input file name
    #[clap(short)]
    pub input: String,

    /// Specify the output file name
    #[clap(short, default_value = input + ".xx")]
    pub output: String,
}

The compiler told me that the input field cannot be found in the #[clap(...)] scope.

Is there any way to use field in macro attribute declaration?

If I can't do this, how should I achieve the functionality?

[epage]

No, this won't work. What the macro expands out to is a CLI definition followed by a parse phase. One reason we can't make syntax like this work is that default_value is used for help text and we might not be able to instantiate Args to access input when help is shown. Even if we instead make this about referencing the intermediate value as we build up Args, there are still cases where input won't exist (e.g. $ cmd --help).

In #3008, we are looking at exploring custom runtime default logic that also allows a user to provide custom help hints which would enable something like this.