Allow --flag, --flag=true and --flag=false with any default.

[iago-lito]

I wish my boolean setting to be input either as:

• --flag=true or --flag : meaning true.
• --flag=false : meaning false.
• (nothing) : meaning <default value> (either true or false).

What I have come up with is:

    #[arg(
        long,
        num_args(0..=1),
        default_value_t = true, // (or false)
        default_missing_value("true"),
    )]
    flag: std::primitive::bool,

It works the way I expect, but it looks very complicated. Is there a better way?
Or should I need to ask for some new magic like:

#[arg(long, default_value_t = true, explicit_value_allowed)]
flag: bool,

or something?

[epage]

Another way of writing this is

    #[arg(
        long,
        num_args(0..=1),
        action = clap::builder::ArgAction::Set,
        default_value = "false",
        default_missing_value = "true",
    )]
    flag: bool,

At the time, there is not a better way. I would not want to provide a way of doing this without an explicit num_args so people are more likely to notice all of the cautions and decide whether they want to add require_equals = true.

As an alternative, I think if we relax the debug_assert on SetTrue from only allowing num_args(0) to anything with a cap of 1, then most of this could probably simplified down to

    #[arg(
        long,
        num_args(0..=1),
    )]
    flag: bool,