Allow internally tagged newtype variant containing unit struct

serde/src/private/de.rs

@@ -1114,10 +1114,38 @@ mod content {
             )
         }
 
+        fn deserialize_unit_struct<V>(
+            self,
+            _name: &'static str,
+            visitor: V
+        ) -> Result<V::Value, Self::Error>
+        where
+            V: Visitor<'de>,
+        {
+            match self.content {
+                // As a special case, allow deserializing untagged newtype
+                // variant containing unit struct.
+                //
+                //     #[derive(Deserialize)]
+                //     struct Info;
+                //
+                //     #[derive(Deserialize)]
+                //     #[serde(tag = "topic")]
+                //     enum Message {
+                //         Info(Info),
+                //     }
+                //
+                // We want {"topic":"Info"} to deserialize even though
+                // ordinarily unit structs do not deserialize from empty map.
+                Content::Map(ref v) if v.is_empty() => visitor.visit_unit(),
+                _ => self.deserialize_any(visitor),
+            }
+        }
+
         forward_to_deserialize_any! {
             bool i8 i16 i32 i64 u8 u16 u32 u64 f32 f64 char str string bytes
-            byte_buf unit unit_struct seq tuple tuple_struct map struct
-            identifier ignored_any
+            byte_buf unit seq tuple tuple_struct map struct identifier
+            ignored_any
         }
     }
 

serde/src/private/ser.rs

@@ -60,7 +60,6 @@ enum Unsupported {
     ByteArray,
     Optional,
     Unit,
-    UnitStruct,
     Sequence,
     Tuple,
     TupleStruct,
@@ -79,7 +78,6 @@ impl Display for Unsupported {
             Unsupported::ByteArray => formatter.write_str("a byte array"),
             Unsupported::Optional => formatter.write_str("an optional"),
             Unsupported::Unit => formatter.write_str("unit"),
-            Unsupported::UnitStruct => formatter.write_str("a unit struct"),
             Unsupported::Sequence => formatter.write_str("a sequence"),
             Unsupported::Tuple => formatter.write_str("a tuple"),
             Unsupported::TupleStruct => formatter.write_str("a tuple struct"),
@@ -199,7 +197,9 @@ where
     }
 
     fn serialize_unit_struct(self, _: &'static str) -> Result<Self::Ok, Self::Error> {
-        Err(self.bad_type(Unsupported::UnitStruct))
+        let mut map = try!(self.delegate.serialize_map(Some(1)));
+        try!(map.serialize_entry(self.tag, self.variant_name));
+        map.end()
     }
 
     fn serialize_unit_variant(

test_suite/tests/test_macros.rs

@@ -1267,3 +1267,48 @@ fn test_rename_all() {
         ]
     );
 }
+
+#[test]
+fn test_untagged_newtype_variant_containing_unit_struct_not_map() {
+    #[derive(Debug, PartialEq, Serialize, Deserialize)]
+    struct Unit;
+
+    #[derive(Debug, PartialEq, Serialize, Deserialize)]
+    #[serde(untagged)]
+    enum Message {
+        Unit(Unit),
+        Map(BTreeMap<String, String>),
+    }
+
+    assert_tokens(
+        &Message::Map(BTreeMap::new()),
+        &[
+            Token::Map { len: Some(0) },
+            Token::MapEnd,
+        ],
+    );
+}
+
+#[test]
+fn test_internally_tagged_newtype_variant_containing_unit_struct() {
+    #[derive(Debug, PartialEq, Serialize, Deserialize)]
+    struct Info;
+
+    #[derive(Debug, PartialEq, Serialize, Deserialize)]
+    #[serde(tag = "topic")]
+    enum Message {
+        Info(Info),
+    }
+
+    assert_tokens(
+        &Message::Info(Info),
+        &[
+            Token::Map { len: Some(1) },
+
+            Token::Str("topic"),
+            Token::Str("Info"),
+
+            Token::MapEnd,
+        ],
+    );
+}