Added support for newtype variant serialization

serde/src/private/ser.rs

@@ -1164,12 +1164,12 @@ impl<'a, M> Serializer for FlatMapSerializer<'a, M>
         _: &'static str,
         _: u32,
         _: &'static str,
-        _: &T,
+        value: &T,
     ) -> Result<Self::Ok, Self::Error>
     where
         T: Serialize,
     {
-        Err(self.bad_type(Unsupported::Enum))
+        value.serialize(self)
     }
 
     fn serialize_seq(self, _: Option<usize>) -> Result<Self::SerializeSeq, Self::Error> {

test_suite/tests/test_annotations.rs

@@ -140,6 +140,12 @@ enum FlattenStructTagContentEnum {
         index: u32,
         value: u32
     },
+    NewtypeVariant(FlattenStructTagContentEnumNewtypeVariant),
+}
+
+#[derive(Debug, PartialEq, Serialize, Deserialize)]
+struct FlattenStructTagContentEnumNewtypeVariant {
+    value: u32,
 }
 
 #[test]
@@ -1441,3 +1447,49 @@ fn test_flatten_struct_tag_content_enum() {
         ],
     );
 }
+
+#[test]
+fn test_flatten_struct_tag_content_enum_newtype() {
+    let change_request = FlattenStructTagContentEnumWrapper {
+        outer: 42,
+        data: FlattenStructTagContentEnumNewtype(
+            FlattenStructTagContentEnum::NewtypeVariant(
+                FlattenStructTagContentEnumNewtypeVariant {
+                    value: 23
+                }
+            )
+        ),
+    };
+    assert_de_tokens(
+        &change_request,
+        &[
+            Token::Map { len: None },
+            Token::Str("outer"),
+            Token::U32(42),
+            Token::Str("type"),
+            Token::Str("newtype_variant"),
+            Token::Str("value"),
+            Token::Map { len: None },
+            Token::Str("value"),
+            Token::U32(23),
+            Token::MapEnd,
+            Token::MapEnd,
+        ],
+    );
+    assert_ser_tokens(
+        &change_request,
+        &[
+            Token::Map { len: None },
+            Token::Str("outer"),
+            Token::U32(42),
+            Token::Str("type"),
+            Token::Str("newtype_variant"),
+            Token::Str("value"),
+            Token::Struct { len: 1, name: "FlattenStructTagContentEnumNewtypeVariant" },
+            Token::Str("value"),
+            Token::U32(23),
+            Token::StructEnd,
+            Token::MapEnd,
+        ],
+    );
+}