Added newtype struct support for flattening

serde/src/private/de.rs

@@ -2143,10 +2143,21 @@ impl<'a, 'de, E> Deserializer<'de> for FlatMapDeserializer<'a, 'de, E>
         visitor.visit_map(FlatMapAccess::new(self.0.iter_mut(), Some(fields)))
     }
 
+    fn deserialize_newtype_struct<V>(
+        self,
+        _name: &str,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>,
+    {
+        visitor.visit_newtype_struct(self)
+    }
+
     forward_to_deserialize_any! {
         bool i8 i16 i32 i64 u8 u16 u32 u64 f32 f64 char str string bytes
-        byte_buf option unit unit_struct newtype_struct seq tuple
-        tuple_struct identifier ignored_any
+        byte_buf option unit unit_struct seq tuple tuple_struct identifier
+        ignored_any
     }
 }
 

serde/src/private/ser.rs

@@ -64,7 +64,6 @@ enum Unsupported {
     Optional,
     Unit,
     UnitStruct,
-    NewtypeStruct,
     Sequence,
     Tuple,
     TupleStruct,
@@ -83,7 +82,6 @@ impl Display for Unsupported {
             Unsupported::Optional => formatter.write_str("an optional"),
             Unsupported::Unit => formatter.write_str("unit"),
             Unsupported::UnitStruct => formatter.write_str("unit struct"),
-            Unsupported::NewtypeStruct => formatter.write_str("newtype struct"),
             Unsupported::Sequence => formatter.write_str("a sequence"),
             Unsupported::Tuple => formatter.write_str("a tuple"),
             Unsupported::TupleStruct => formatter.write_str("a tuple struct"),
@@ -1153,13 +1151,12 @@ impl<'a, M> Serializer for FlatMapSerializer<'a, M>
     fn serialize_newtype_struct<T: ?Sized>(
         self,
         _: &'static str,
-        _value: &T,
+        value: &T,
     ) -> Result<Self::Ok, Self::Error>
     where
         T: Serialize,
     {
-        // TODO: can we do better here?
-        Err(self.bad_type(Unsupported::NewtypeStruct))
+        value.serialize(self)
     }
 
     fn serialize_newtype_variant<T: ?Sized>(

test_suite/tests/test_annotations.rs

@@ -127,9 +127,12 @@ enum FlattenStructEnum {
 struct FlattenStructTagContentEnumWrapper {
     outer: u32,
     #[serde(flatten)]
-    data: FlattenStructTagContentEnum,
+    data: FlattenStructTagContentEnumNewtype,
 }
 
+#[derive(Debug, PartialEq, Serialize, Deserialize)]
+struct FlattenStructTagContentEnumNewtype(pub FlattenStructTagContentEnum);
+
 #[derive(Debug, PartialEq, Serialize, Deserialize)]
 #[serde(rename_all = "snake_case", tag = "type", content = "value")]
 enum FlattenStructTagContentEnum {
@@ -1394,10 +1397,12 @@ fn test_flatten_struct_enum() {
 fn test_flatten_struct_tag_content_enum() {
     let change_request = FlattenStructTagContentEnumWrapper {
         outer: 42,
-        data: FlattenStructTagContentEnum::InsertInteger {
-            index: 0,
-            value: 42
-        },
+        data: FlattenStructTagContentEnumNewtype(
+            FlattenStructTagContentEnum::InsertInteger {
+                index: 0,
+                value: 42
+            }
+        ),
     };
     assert_de_tokens(
         &change_request,