feat(macros): arg_enum! and simple_enum! provide a Vec<&str> of varia…

…nt names

You can now use  ArgName::variants() to get a Vec<&'static str> of
vairant names (Nice for Arg::possible_values()). It's not as good as
implementing Iterator for the enum, but that can't be done without being
able to concat AST tokens in order to make an "EnumIter" type which
implements iterator and keeps track of when to yield None. But the
overall intent of this issue was to be able to iterate variant names,
which is now possible.

Closes #119

src/macros.rs

@@ -275,7 +275,8 @@ macro_rules! value_t_or_exit {
 }
 
 /// Convenience macro generated a simple enum with variants to be used as a type when parsing
-/// arguments.
+/// arguments. This enum also provides a `variants()` function which can be used to retrieve a
+/// `Vec<&'static str>` of the variant names.
 ///
 /// **NOTE:** This macro automaically implements std::str::FromStr and std::fmt::Display
 ///
@@ -332,11 +333,20 @@ macro_rules! simple_enum {
                 }
             }
         }
+
+        impl $e {
+            pub fn variants() -> Vec<&'static str> {
+                vec![
+                    $(stringify!($v),)+
+                ]
+            }
+        }
     };
 }
 
 /// Convenience macro to generate more complete enums with variants to be used as a type when
-/// parsing arguments.
+/// parsing arguments. This enum also provides a `variants()` function which can be used to retrieve a
+/// `Vec<&'static str>` of the variant names.
 ///
 /// **NOTE:** Case insensitivity is supported for ASCII characters
 ///
@@ -405,6 +415,14 @@ macro_rules! arg_enum {
                 }
             }
         }
+
+        impl $e {
+            fn variants() -> Vec<&'static str> {
+                vec![
+                    $(stringify!($v),)+
+                ]
+            }
+        }
     };
     (pub enum $e:ident { $($v:ident),+ } ) => {
         pub enum $e {
@@ -439,6 +457,14 @@ macro_rules! arg_enum {
                 }
             }
         }
+
+        impl $e {
+            pub fn variants() -> Vec<&'static str> {
+                vec![
+                    $(stringify!($v),)+
+                ]
+            }
+        }
     };
     (#[derive($($d:ident),+)] enum $e:ident { $($v:ident),+ } ) => {
         #[derive($($d,)+)]
@@ -474,6 +500,14 @@ macro_rules! arg_enum {
                 }
             }
         }
+
+        impl $e {
+            pub fn variants() -> Vec<&'static str> {
+                vec![
+                    $(stringify!($v),)+
+                ]
+            }
+        }
     };
     (#[derive($($d:ident),+)] pub enum $e:ident { $($v:ident),+ } ) => {
         #[derive($($d,)+)]
@@ -509,6 +543,14 @@ macro_rules! arg_enum {
                 }
             }
         }
+
+        impl $e {
+            pub fn variants() -> Vec<&'static str> {
+                vec![
+                    $(stringify!($v),)+
+                ]
+            }
+        }
     };
 }