Add inverse for required_unless

[brmmm3]

clap provides required_if and required_unless. The first argument requires a value for comparison. The second one has exactly the opposite behavior what I need. I would like to have something like:

#[derive(Debug, StructOpt)]
pub struct Options {
    #[structopt(short, long, parse(from_os_str), help = "Foo")]
    pub foo: Option<PathBuf>,

    #[structopt(short, long, parse(from_os_str, !required_unless(foo)), help = "Bar")]
    pub bar: Option<PathBuf>,

Please note the "!" before the required_unless.
Currently I've solved the missing feature with some lines of code in Rust, but I think it would be helpful for several projects to have the above extension.

[pksunkara]

Can you explain a bit more on what exactly you are trying to achieve with !required_unless(foo)?

Would you say that bar needs to be present if foo is not present?

[brmmm3]

I would like to have the following behavior:
bar needs to be present if foo is present.

[pksunkara]

You can do requires("bar") on foo with https://docs.rs/clap/3.0.0-beta.2/clap/struct.Arg.html#method.requires

[brmmm3]

Oh thanks. That would solve one of my issues. But what about the following usecase:

#[derive(Debug, StructOpt)]
pub struct Options {
    #[structopt(short, long, parse(from_os_str), help = "Foo")]
    pub foo: Option<PathBuf>,

    #[structopt(short, long, parse(from_os_str, !required_unless(foo)), help = "Bar1")]
    pub bar1: Option<PathBuf>,

    #[structopt(short, long, parse(from_os_str, !required_unless(foo)), help = "Bar2")]
    pub bar2: Option<PathBuf>,
}

foo is only required if either bar1 and/or bar2 is present. Your solution would not work here, because requires("bar1", "bar2") would make both, bar1 and bar2, mandatory.

As an example:
I've got an optional option which enables a grep feature. 2 further optional options, before and after, are only available if the grep feature is enabled, which in other word means before and after make grep mandatory.

[pksunkara]

You can put bar1 and bar2 in an ArgGroup called bar and then do requires("bar") on foo.

Please go through the examples and documentation. Almost every argument relationship is supported.