Support argument aliases that set multiple arguments

[darnir]

Problem Description

In my projects, I often have a scenario where there is a single shortcut switch which sets values for multiple switches. For example, in my current project, I have (taken directly from my code):

        .arg(
            Arg::with_name("UTILIZATION")
                .long("utilization")
                .short("u")
                .takes_value(true)
                .overrides_with_all(&["UBEGIN", "USTEP", "UEND"])
                .help("Utilization of generated taskset"),
        )
        .arg(
            Arg::with_name("UBEGIN")
                .long("utilization-begin")
                .takes_value(true)
                .default_value("0.025")
                .help("Begin Utilization Range"),
        )
        .arg(
            Arg::with_name("USTEP")
                .long("utilization-step")
                .default_value("0.1")
                .help("Step for Utilization Range"),
        )
        .arg(
            Arg::with_name("UEND")
                .long("utilization-end")
                .default_value("1.0")
                .help("End of Utilization Range"),
        )
        .group(
            ArgGroup::with_name("UTILIZATION_RANGE")
                .conflicts_with("UTILIZATION")
                .multiple(true),
        )

Now what I would like it for the argument UTILIZATION to explicitly set values for UBEGIN, USTEP and UEND.

As you can see, I currently have a overrides and conflicts rule and then later in the application code, UTILIZATION is tested for and the other values are set.

Another way to implement this would be by allowing the user to manipulate values in the ArgMatches struct. Then one could simply set the values there before returning to the main client for consumption.

This is not a very obscure feature but is often used in many programs. For a more real-world usage of such a feature, please look at GNU Wget, quoting from its manual here (Emphasis mine):

  --mirror
       Turn on options suitable for mirroring.  This option turns on recursion and time-stamping, sets infinite recursion depth and keeps FTP
       directory listings.  It is currently equivalent to **-r -N -l inf --no-remove-listing**.

[kbknapp]

Using Arg::default_value_if will probably get you about 90% of what you're looking for. Otherwise, you'll have to go with defining these aliases in user code. I'm not against adding something like this feature in the future, but it's not something I can work on right now as there are about 1,000 other things I need to finish first 😜

[epage]

Between default_value_if and replace (#2836), I think we've got this covered. If there is feedback on `replace, please put it there.