Serialize and deserialize a tagged newtype variant over unit () as if it was a unit variant.

serde/src/private/de.rs

@@ -1262,6 +1262,17 @@ mod content {
         {
             match self.content {
                 Content::Unit => visitor.visit_unit(),
+
+                // As a special case, allow deserializing newtype variant containing unit. E.G:
+                // #[derive(Deserialize)]
+                // #[serde(tag = "result")]
+                // enum Response<T> {
+                //     Success(T),
+                // }
+                //
+                // We want {"result": "Success"} to deserialize into `Response<T>`.
+                Content::Map(ref v) if v.is_empty() => visitor.visit_unit(),
+                Content::Seq(ref v) if v.is_empty() => visitor.visit_unit(),
                 _ => Err(self.invalid_type(&visitor)),
             }
         }

serde/src/private/ser.rs

@@ -51,7 +51,6 @@ enum Unsupported {
     String,
     ByteArray,
     Optional,
-    Unit,
     #[cfg(any(feature = "std", feature = "alloc"))]
     UnitStruct,
     Sequence,
@@ -70,7 +69,6 @@ impl Display for Unsupported {
             Unsupported::String => formatter.write_str("a string"),
             Unsupported::ByteArray => formatter.write_str("a byte array"),
             Unsupported::Optional => formatter.write_str("an optional"),
-            Unsupported::Unit => formatter.write_str("unit"),
             #[cfg(any(feature = "std", feature = "alloc"))]
             Unsupported::UnitStruct => formatter.write_str("unit struct"),
             Unsupported::Sequence => formatter.write_str("a sequence"),
@@ -184,7 +182,9 @@ where
     }
 
     fn serialize_unit(self) -> Result<Self::Ok, Self::Error> {
-        Err(self.bad_type(Unsupported::Unit))
+        let mut map = try!(self.delegate.serialize_map(Some(1)));
+        try!(map.serialize_entry(self.tag, self.variant_name));
+        map.end()
     }
 
     fn serialize_unit_struct(self, _: &'static str) -> Result<Self::Ok, Self::Error> {

test_suite/tests/test_annotations.rs

@@ -2302,6 +2302,34 @@ fn test_internally_tagged_enum_containing_flatten() {
     );
 }
 
+#[test]
+fn test_internally_tagged_enum_new_type_with_unit() {
+    #[derive(Serialize, Deserialize, PartialEq, Debug)]
+    #[serde(tag = "t")]
+    enum Data {
+        A(()),
+    }
+
+    assert_tokens(
+        &Data::A(()),
+        &[
+            Token::Map { len: Some(1) },
+            Token::Str("t"),
+            Token::Str("A"),
+            Token::MapEnd,
+        ],
+    );
+    assert_ser_tokens(
+        &Data::A(()),
+        &[
+            Token::Map { len: Some(1) },
+            Token::Str("t"),
+            Token::Str("A"),
+            Token::MapEnd,
+        ],
+    );
+}
+
 #[test]
 fn test_adjacently_tagged_enum_containing_flatten() {
     #[derive(Serialize, Deserialize, PartialEq, Debug)]