clap::arg_enum!

macro_rules! arg_enum {
    (enum $e:ident { $($v:ident),+ } ) => {
        enum $e {
            $($v),+
        }

        impl ::std::str::FromStr for $e {
            type Err = String;

            fn from_str(s: &str) -> Result<Self,Self::Err> {
                use ::std::ascii::AsciiExt;
                match s {
                    $(stringify!($v) |
                    _ if s.eq_ignore_ascii_case(stringify!($v)) => Ok($e::$v),)+
                    _                => Err({
                                            let v = vec![
                                                $(stringify!($v),)+
                                            ];
                                            format!("valid values:{}",
                                                v.iter().fold(String::new(), |a, i| {
                                                    a + &format!(" {}", i)[..]
                                                }))
                                        })
                }
            }
        }

        impl ::std::fmt::Display for $e {
            fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
                match *self {
                    $($e::$v => write!(f, stringify!($v)),)+
                }
            }
        }

        impl $e {
            #[allow(dead_code)]
            fn variants() -> Vec<&'static str> {
                vec![
                    $(stringify!($v),)+
                ]
            }
        }
    };
    (pub enum $e:ident { $($v:ident),+ } ) => {
        pub enum $e {
            $($v),+
        }

        impl ::std::str::FromStr for $e {
            type Err = String;

            fn from_str(s: &str) -> Result<Self,Self::Err> {
                use ::std::ascii::AsciiExt;
                match s {
                    $(stringify!($v) |
                    _ if s.eq_ignore_ascii_case(stringify!($v)) => Ok($e::$v),)+
                    _                => Err({
                                            let v = vec![
                                                $(stringify!($v),)+
                                            ];
                                            format!("valid values:{}",
                                                v.iter().fold(String::new(), |a, i| {
                                                    a + &format!(" {}", i)[..]
                                                }))
                                        })
                }
            }
        }

        impl ::std::fmt::Display for $e {
            fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
                match *self {
                    $($e::$v => write!(f, stringify!($v)),)+
                }
            }
        }

        impl $e {
            #[allow(dead_code)]
            pub fn variants() -> Vec<&'static str> {
                vec![
                    $(stringify!($v),)+
                ]
            }
        }
    };
    (#[derive($($d:ident),+)] enum $e:ident { $($v:ident),+ } ) => {
        #[derive($($d,)+)]
        enum $e {
            $($v),+
        }

        impl ::std::str::FromStr for $e {
            type Err = String;

            fn from_str(s: &str) -> Result<Self,Self::Err> {
                use ::std::ascii::AsciiExt;
                match s {
                    $(stringify!($v) |
                    _ if s.eq_ignore_ascii_case(stringify!($v)) => Ok($e::$v),)+
                    _                => Err({
                                            let v = vec![
                                                $(stringify!($v),)+
                                            ];
                                            format!("valid values:{}",
                                                v.iter().fold(String::new(), |a, i| {
                                                    a + &format!(" {}", i)[..]
                                                }))
                                        })
                }
            }
        }

        impl ::std::fmt::Display for $e {
            fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
                match *self {
                    $($e::$v => write!(f, stringify!($v)),)+
                }
            }
        }

        impl $e {
            #[allow(dead_code)]
            pub fn variants() -> Vec<&'static str> {
                vec![
                    $(stringify!($v),)+
                ]
            }
        }
    };
    (#[derive($($d:ident),+)] pub enum $e:ident { $($v:ident),+ } ) => {
        #[derive($($d,)+)]
        pub enum $e {
            $($v),+
        }

        impl ::std::str::FromStr for $e {
            type Err = String;

            fn from_str(s: &str) -> Result<Self,Self::Err> {
                use ::std::ascii::AsciiExt;
                match s {
                    $(stringify!($v) |
                    _ if s.eq_ignore_ascii_case(stringify!($v)) => Ok($e::$v),)+
                    _                => Err({
                                            let v = vec![
                                                $(stringify!($v),)+
                                            ];
                                            format!("valid values:{}",
                                                v.iter().fold(String::new(), |a, i| {
                                                    a + &format!(" {}", i)[..]
                                                }))
                                        })
                }
            }
        }

        impl ::std::fmt::Display for $e {
            fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
                match *self {
                    $($e::$v => write!(f, stringify!($v)),)+
                }
            }
        }

        impl $e {
            #[allow(dead_code)]
            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. 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

NOTE: This macro automaically implements std::str::FromStr and std::fmt::Display

These enums support pub (or not) and use of the #[derive()] traits

Example

arg_enum!{
    #[derive(Debug)]
    pub enum Foo {
        Bar,
        Baz,
        Qux
    }
}
// Foo enum can now be used via Foo::Bar, or Foo::Baz, etc
// and implements std::str::FromStr to use with the value_t! macros
fn main() {
    let m = App::new("app")
                .arg_from_usage("<foo> 'the foo'")
                .get_matches();
    let f = value_t_or_exit!(m.value_of("foo"), Foo);

    // Use f like any other Foo variant...
}