fix(derive): Define multiple policy for Special Types

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, whenver I've used the old `Arg::multiple`, I thought I was
getting `Arg::multiple_occurrences` only.  `Arg::multiple_values`,
without any bounds or delimeter requirement, can lead to a confusing
user experience but isn't a good default for these.  On top of that, if
someone does have an unbounded or a delimeter 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.

`Option<Vec<_>>` ends up matching `Vec<_>` which an raise the question
of why have it.  Some users might prefer the type.  Otherwise, this is
so users can no when 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 option would have been to not infer a setting if someone sets a
handful of settings manually, which would have avoided the confusion in
Issue clap-rs#2599 but I see that being confusing (for someone who knows the
default, they will be expecting it to be additive; which flags?) 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 clap-rs#2924
- `(T1, T2)`, `Vec<(T1, T2)>`, etc, see clap-rs#1717

Fixes clap-rs#1772
Fixes clap-rs#2599
See also clap-rs#2195

clap_derive/src/derives/args.rs

@@ -289,8 +289,7 @@ pub fn gen_augment(
                     Ty::OptionVec => quote_spanned! { ty.span()=>
                         .takes_value(true)
                         .value_name(#value_name)
-                        .multiple_values(true)
-                        .min_values(0)
+                        .multiple_occurrences(true)
                         #possible_values
                         #validator
                         #allow_invalid_utf8
@@ -300,7 +299,7 @@ pub fn gen_augment(
                         quote_spanned! { ty.span()=>
                             .takes_value(true)
                             .value_name(#value_name)
-                            .multiple_values(true)
+                            .multiple_occurrences(true)
                             #possible_values
                             #validator
                             #allow_invalid_utf8
@@ -313,7 +312,6 @@ pub fn gen_augment(
 
                     Ty::Other if flag => quote_spanned! { ty.span()=>
                         .takes_value(false)
-                        .multiple_values(false)
                     },
 
                     Ty::Other => {

clap_derive/tests/arg_enum.rs

@@ -419,7 +419,7 @@ fn vec_type() {
         Opt {
             arg: vec![ArgChoice::Foo, ArgChoice::Bar]
         },
-        Opt::try_parse_from(&["", "-a", "foo", "bar"]).unwrap()
+        Opt::try_parse_from(&["", "-a", "foo", "-a", "bar"]).unwrap()
     );
     assert!(Opt::try_parse_from(&["", "-a", "fOo"]).is_err());
 }
@@ -439,10 +439,6 @@ fn option_vec_type() {
     }
 
     assert_eq!(Opt { arg: None }, Opt::try_parse_from(&[""]).unwrap());
-    assert_eq!(
-        Opt { arg: Some(vec![]) },
-        Opt::try_parse_from(&["", "-a"]).unwrap()
-    );
     assert_eq!(
         Opt {
             arg: Some(vec![ArgChoice::Foo])
@@ -453,7 +449,7 @@ fn option_vec_type() {
         Opt {
             arg: Some(vec![ArgChoice::Foo, ArgChoice::Bar])
         },
-        Opt::try_parse_from(&["", "-a", "foo", "bar"]).unwrap()
+        Opt::try_parse_from(&["", "-a", "foo", "-a", "bar"]).unwrap()
     );
     assert!(Opt::try_parse_from(&["", "-a", "fOo"]).is_err());
 }

clap_derive/tests/options.rs

@@ -259,7 +259,7 @@ fn two_option_option_types() {
 }
 
 #[test]
-fn vec_type_is_multiple_values() {
+fn vec_type_is_multiple_occurrences() {
     #[derive(Parser, PartialEq, Debug)]
     struct Opt {
         #[clap(short, long)]
@@ -270,6 +270,42 @@ fn vec_type_is_multiple_values() {
         Opt::try_parse_from(&["test", "-a24"]).unwrap()
     );
     assert_eq!(Opt { arg: vec![] }, Opt::try_parse_from(&["test"]).unwrap());
+    assert_eq!(
+        Opt { arg: vec![24, 42] },
+        Opt::try_parse_from(&["test", "-a", "24", "-a", "42"]).unwrap()
+    );
+}
+
+#[test]
+fn vec_type_with_required() {
+    #[derive(Parser, PartialEq, Debug)]
+    struct Opt {
+        #[clap(short, long, required = true)]
+        arg: Vec<i32>,
+    }
+    assert_eq!(
+        Opt { arg: vec![24] },
+        Opt::try_parse_from(&["test", "-a24"]).unwrap()
+    );
+    assert!(Opt::try_parse_from(&["test"]).is_err());
+    assert_eq!(
+        Opt { arg: vec![24, 42] },
+        Opt::try_parse_from(&["test", "-a", "24", "-a", "42"]).unwrap()
+    );
+}
+
+#[test]
+fn vec_type_with_multiple_values_only() {
+    #[derive(Parser, PartialEq, Debug)]
+    struct Opt {
+        #[clap(short, long, multiple_values(true), multiple_occurrences(false))]
+        arg: Vec<i32>,
+    }
+    assert_eq!(
+        Opt { arg: vec![24] },
+        Opt::try_parse_from(&["test", "-a24"]).unwrap()
+    );
+    assert_eq!(Opt { arg: vec![] }, Opt::try_parse_from(&["test"]).unwrap());
     assert_eq!(
         Opt { arg: vec![24, 42] },
         Opt::try_parse_from(&["test", "-a", "24", "42"]).unwrap()
@@ -308,6 +344,28 @@ fn option_vec_type() {
         Opt::try_parse_from(&["test", "-a", "1"]).unwrap()
     );
 
+    assert_eq!(
+        Opt {
+            arg: Some(vec![1, 2])
+        },
+        Opt::try_parse_from(&["test", "-a", "1", "-a", "2"]).unwrap()
+    );
+
+    assert_eq!(Opt { arg: None }, Opt::try_parse_from(&["test"]).unwrap());
+}
+
+#[test]
+fn option_vec_type_structopt_behavior() {
+    #[derive(Parser, PartialEq, Debug)]
+    struct Opt {
+        #[clap(short, long, multiple_values(true), min_values(0))]
+        arg: Option<Vec<i32>>,
+    }
+    assert_eq!(
+        Opt { arg: Some(vec![1]) },
+        Opt::try_parse_from(&["test", "-a", "1"]).unwrap()
+    );
+
     assert_eq!(
         Opt {
             arg: Some(vec![1, 2])
@@ -337,9 +395,9 @@ fn two_option_vec_types() {
     assert_eq!(
         Opt {
             arg: Some(vec![1]),
-            b: Some(vec![])
+            b: None,
         },
-        Opt::try_parse_from(&["test", "-a", "1", "-b"]).unwrap()
+        Opt::try_parse_from(&["test", "-a", "1"]).unwrap()
     );
 
     assert_eq!(
@@ -355,7 +413,7 @@ fn two_option_vec_types() {
             arg: Some(vec![1, 2]),
             b: Some(vec![1, 2])
         },
-        Opt::try_parse_from(&["test", "-a", "1", "2", "-b", "1", "2"]).unwrap()
+        Opt::try_parse_from(&["test", "-a", "1", "-a", "2", "-b", "1", "-b", "2"]).unwrap()
     );
 
     assert_eq!(