Implement Default in the Enum! and Command! macros

For the transition to structopt, we have to be able to easily construct
enum variants once we have added fields to them.  Therefore we implement
the Default trait in the generated macros by choosing the first variant
as the default.

src/arg_util.rs

@@ -76,6 +76,8 @@ macro_rules! Command {
  }
  }
  }
+
+ impl_default!($name => $( $name::$var(::std::default::Default::default()) , )*);
  };
  ( $name:ident, [ $( $var:ident => ($str:expr, $exec:expr), ) *] ) => {
  #[derive(Debug, PartialEq)]
@@ -128,6 +130,8 @@ macro_rules! Command {
  }
  }
  }
+
+ impl_default!($name => $( $name::$var , )*);
  };
 }
 
@@ -204,6 +208,19 @@ macro_rules! enum_int {
  }
  }
  }
+
+ impl_default!($name => $( $name::$var , )*);
+
+ };
+}
+
+macro_rules! impl_default {
+ ( $name:ident => $def:expr , $( $other:expr , ) *) => {
+ impl ::std::default::Default for $name {
+ fn default() -> Self {
+ $def
+ }
+ }
  };
 }