Improve enum shape validation (#178)

* Apply shape validation to enum with no variants. Previously, this would not have produced an error.
* If the receiver doesn't support any enum shapes, surface one diagnostic instead of one per variant

core/src/options/shape.rs

@@ -72,18 +72,28 @@ impl ToTokens for Shape {
         } else {
             let en = &self.enum_values;
             let st = &self.struct_values;
+
+            let enum_validation = if en.supports_none() {
+                let ty = en.prefix.trim_end_matches('_');
+                quote!(return ::darling::export::Err(::darling::Error::unsupported_shape(#ty));)
+            } else {
+                quote! {
+                    fn validate_variant(data: &::syn::Fields) -> ::darling::Result<()> {
+                        #en
+                    }
+
+                    for variant in &data.variants {
+                        validate_variant(&variant.fields)?;
+                    }
+
+                    Ok(())
+                }
+            };
+
             quote! {
                 match *__body {
                     ::syn::Data::Enum(ref data) => {
-                        fn validate_variant(data: &::syn::Fields) -> ::darling::Result<()> {
-                            #en
-                        }
-
-                        for variant in &data.variants {
-                            validate_variant(&variant.fields)?;
-                        }
-
-                        Ok(())
+                        #enum_validation
                     }
                     ::syn::Data::Struct(ref struct_data) => {
                         let data = &struct_data.fields;

tests/supports.rs

@@ -46,6 +46,12 @@ mod source {
         }
     }
 
+    pub fn empty_enum() -> DeriveInput {
+        parse_quote! {
+            enum Hello {}
+        }
+    }
+
     pub fn named_struct() -> DeriveInput {
         parse_quote! {
             struct Hello {
@@ -80,4 +86,5 @@ fn struct_named() {
     StructContainer::from_derive_input(&source::tuple_struct()).unwrap_err();
     StructContainer::from_derive_input(&source::named_field_enum()).unwrap_err();
     StructContainer::from_derive_input(&source::newtype_enum()).unwrap_err();
+    StructContainer::from_derive_input(&source::empty_enum()).unwrap_err();
 }