fix(derive): Define multiple policy for Special Types

[epage]

Before:

• bool: a flag
• Option<_>: not required
• Option<Option<_>> is not required and when it is present, the value
  is not required
• Vec<_>: multiple values, optional
• Option<Vec<_>>: multiple values, min values of 0, optional

After:

• bool: a flag
• Option<_>: not required
• Option<Option<_>> is not required and when it is present, the value
  is not required
• Vec<_>: multiple occurrences, optional
  • optional: Vec implies 0 or more, so should not imply required
• Option<Vec<_>>: multiple occurrences, optional
  • optional: Use over Vec to detect when no option being present when
    using multiple values

Motivations:

My priorities were:

1. Are we getting in the users way?
2. Does the API make sense?
3. Does the API encourage best practices?

I was originally concerned about the lack of composability with
Option<Option<_>> and Option<Vec<_>> (and eventually Vec<Vec<_>>).
It prescribes special meaning to each type depending on where it shows
up, rather than providing a single meaning for a type generally. You
then can't do things like have Option<_> mean "required argument with
optional value" without hand constructing it. However, in practice the
outer type correlates with the argument occurrence and the inner type with the
value. It is rare to want the value behavior without also the
occurrence behavior. So I figure it is probably fine as long as people
can set the flags to manually get the behavior they want.

Vec<_> implies multiple occurrences, rather than multiple values.
Anecdotally, whenever I've used the old Arg::multiple, I thought I was
getting Arg::multiple_occurrences only. Arg::multiple_values,
without any bounds or delimiter requirement, can lead to a confusing
user experience and isn't a good default for these. On top of that, if
someone does have an unbounded or a delimiter multiple values, they are
probably also using multiple occurrences.

Vec<_> is optional because a Vec implies 0 or more, so we stick to
the meaning of the rust type. At least for me, I also rarely need a
required with multiple occurrences argument but more often need optional
with multiple occurrences.

Option<Vec<_>> ends up matching Vec<_> which can raise the question
of why have it. Some users might prefer the type. Otherwise, this is
so users can detect whether the argument is present or not when using
min_values(0). Rather than defining an entire policy around this and
having users customize it, or setting min_values(0) without the rest
of a default policy, this gives people a blank slate to work from.

Another design option would have been to not infer any special-type
settings if someone sets a handful of settings manually, which would
have avoided the confusion in Issue #2599 but I see that being confusing
(for someone who knows the default, they will be expecting it to be
additive; which flags disable inferred settings?) and brittle (as flags
are added or changed, how do we ensure we keep this up?).

Tests were added to ensure we support people customizing the behavior to
match their needs.

This is not solving:

• Vec<Vec<_>>, see Stablize ArgMatches::grouped_value_of Tracking Issue #2924
• (T1, T2), Vec<(T1, T2)>, etc, see clap_derive: support tuples as shortcut for number_of_values = N, different validators #1717
• Vec<Option<_>> and many other potential combinations

Fixes #1772
Fixes #2599
See also #2195

[kbknapp]

I'm working on a write-up to put out my thoughts on our best guess for a "spec" (for the most loose possible value of "spec") so that we can ensure we're covering all aspects of this, and also properly communicating how clap goes about parsing these types of values since it's decently ambiguous and there isn't always a "correct" or "incorrect" assumption about clap should have proceeded. I hope to have this posted here by tomorrow evening.

[pksunkara]

I'm working on a write-up to put out my thoughts

Any update on that? No worries if not.

[pksunkara]

Rather than defining an entire policy around this and
having users customize it, or setting min_values(0) without the rest
of a default policy, this gives people a blank slate to work from.

My only question for this PR is, why not start with min_values(0) and let the user remove it if needed? Otherwise, what's the reason for choosing Option<Vec<_>> over Vec<_>?

[pksunkara]

Also keeping min_values(0) means less churn for people porting from structopt which is really hard since it is macros when compared to normal API churn.