Allow flatten attribute in enums

serde_derive/src/internals/check.rs

@@ -6,7 +6,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use internals::ast::{Container, Data, Style};
+use internals::ast::{Container, Data, Field, Style};
 use internals::attr::{EnumTag, Identifier};
 use internals::Ctxt;
 
@@ -44,15 +44,24 @@ fn check_getter(cx: &Ctxt, cont: &Container) {
 /// Flattening has some restrictions we can test.
 fn check_flatten(cx: &Ctxt, cont: &Container) {
     match cont.data {
-        Data::Enum(_) => {
-            if cont.attrs.has_flatten() {
-                cx.error("#[serde(flatten)] cannot be used within enums");
+        Data::Enum(ref variants) => {
+            for variant in variants {
+                for field in &variant.fields {
+                    check_flatten_field(cx, variant.style, field);
                 }
             }
-        Data::Struct(style, _) => {
-            for field in cont.data.all_fields() {
+        }
+        Data::Struct(style, ref fields) => {
+            for field in fields {
+                check_flatten_field(cx, style, field);
+            }
+        }
+    }
+}
+
+fn check_flatten_field(cx: &Ctxt, style: Style, field: &Field) {
     if !field.attrs.flatten() {
-                    continue;
+        return;
     }
     match style {
         Style::Tuple => {
@@ -79,9 +88,6 @@ fn check_flatten(cx: &Ctxt, cont: &Container) {
              #[serde(skip_deserializing)]",
         );
     }
-            }
-        }
-    }
 }
 
 /// The `other` attribute must be used at most once and it must be the last