How to define mutually competing flags

[tailhook]

There is a common pattern with flags which is like:

 --debug / --no-debug    enable or disable debugging

or similarly -v and -q could stand as verbose and quiet.

The only way I can find to implement it is using two different flags, but there are two problems with that:

1. The last option should win. With two options I have to define which one has a higher priority over the other.
2. Docs are a bit long (especially if you have many such flags)

Is there a way to implement this now? Will such a feature be accepted?

[CreepySkeleton]

Is there a way to implement this now?

AFAIK, no, sorry. Also, related #126

Will such a feature be accepted?

This depends a good deal on the precise design. If you're willing to try implementing this variant, which both @TeXitoi and I agree with, it will be accepted. If you have any other thoughts/improvements - we may need to discuss them first.

[tailhook]

Well this one looks promising but I'm not sure it's good enough:

#[derive(StructOpt)]
struct DebugOpt {
    #[structopt(long = "debug", parse(from_occurences)]
    on: u8,

    #[structopt(long = "no-debug", parse(from_occurences)]
    off: u8,
}

Then the following:

command --no-debug --no-debug --debug

Would disable debug instead of enabling. I.e. I'm not sure how to implement last argument wins semantics with collapse. (It's also a lot more verbose than I'd like for such common functionality, and can't be abstracted away, as far as I understand, because the inner structure have exact argument names).

[tailhook]

Oh well, actually:

    #[structopt(long, help="enabled debug (disable with --no-debug)")]
    pub debug: bool,
    #[structopt(long, overrides_with="debug", hidden=true)]
    pub _no_debug: bool,

Works pretty well (the key here is overrides_with, and _no_debug field can be ignored).

The --no-debug is hidden because otherwise it is put in a different place because of sorting.

I would still probably want better help, but this is not that critical and should probably be done after clap 3.0.

[CreepySkeleton]

Yeah, I was mostly answering to -q -v question. I believe the --debug/--no-debug thing must be done in clap, there's an issue for that already.