Avoid branch in Display match statement for empty enums (#196)

This removes a match arm that will never be triggered for non empty
enums.
So now only if their are no arms in the enum will the code for empty
enums be generated.

This changes no functionally and is a bug fix.

src/display.rs

@@ -68,9 +68,8 @@ pub fn expand(input: &syn::DeriveInput, trait_name: &str) -> Result<TokenStream>
             fn fmt(&self, _derive_more_display_formatter: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
                 #helper_struct
 
-                match self {
+                match *self {
                     #arms
-                    _ => Ok(()) // This is needed for empty enums
                 }
             }
         }
@@ -200,7 +199,7 @@ impl<'a, 'b> State<'a, 'b> {
                 let fields: TokenStream = (0..fields.unnamed.len())
                     .map(|n| {
                         let i = Ident::new(&format!("_{}", n), Span::call_site());
-                        quote!(#i,)
+                        quote!(ref #i,)
                     })
                     .collect();
                 quote!((#fields))
@@ -211,7 +210,7 @@ impl<'a, 'b> State<'a, 'b> {
                     .iter()
                     .map(|f| {
                         let i = f.ident.as_ref().unwrap();
-                        quote!(#i,)
+                        quote!(ref #i,)
                     })
                     .collect();
                 quote!({#fields})

tests/display.rs

@@ -160,6 +160,13 @@ fn check_display() {
     assert_eq!(DebugStructAsDisplay.to_string(), "DebugStructAsDisplay");
 }
 
+#[test]
+fn empty_enum_impls_display() {
+    trait S: std::fmt::Display {}
+
+    impl S for EmptyEnum {}
+}
+
 mod generic {
     #[derive(Display)]
     #[display(fmt = "Generic {}", field)]