Merge pull request #1179 from mitsuhiko/feature/flatten

Support for Flattening

serde/src/private/de.rs

@@ -11,13 +11,13 @@ use lib::*;
 use de::{Deserialize, DeserializeSeed, Deserializer, Error, IntoDeserializer, Visitor};
 
 #[cfg(any(feature = "std", feature = "alloc"))]
-use de::Unexpected;
+use de::{Unexpected, MapAccess};
 
 #[cfg(any(feature = "std", feature = "alloc"))]
 pub use self::content::{Content, ContentDeserializer, ContentRefDeserializer,
                         InternallyTaggedUnitVisitor, TagContentOtherField,
                         TagContentOtherFieldVisitor, TagOrContentField, TagOrContentFieldVisitor,
-                        TaggedContentVisitor, UntaggedUnitVisitor};
+                        TaggedContentVisitor, UntaggedUnitVisitor, EnumDeserializer};
 
 /// If the missing field is of type `Option<T>` then treat is as `None`,
 /// otherwise it is an error.
@@ -269,6 +269,14 @@ mod content {
     }
 
     impl<'de> Content<'de> {
+        pub fn as_str(&self) -> Option<&str> {
+            match *self {
+                Content::Str(x) => Some(x),
+                Content::String(ref x) => Some(x),
+                _ => None,
+            }
+        }
+
         fn unexpected(&self) -> Unexpected {
             match *self {
                 Content::Bool(b) => Unexpected::Bool(b),
@@ -1118,11 +1126,7 @@ mod content {
                 }
             };
 
-            visitor.visit_enum(EnumDeserializer {
-                variant: variant,
-                value: value,
-                err: PhantomData,
-            })
+            visitor.visit_enum(EnumDeserializer::new(variant, value))
         }
 
         fn deserialize_unit_struct<V>(
@@ -1170,7 +1174,7 @@ mod content {
         }
     }
 
-    struct EnumDeserializer<'de, E>
+    pub struct EnumDeserializer<'de, E>
     where
         E: de::Error,
     {
@@ -1179,6 +1183,18 @@ mod content {
         err: PhantomData<E>,
     }
 
+    impl<'de, E> EnumDeserializer<'de, E>
+        where E: de::Error
+    {
+        pub fn new(variant: Content<'de>, value: Option<Content<'de>>) -> EnumDeserializer<'de, E> {
+            EnumDeserializer {
+                variant: variant,
+                value: value,
+                err: PhantomData,
+            }
+        }
+    }
+
     impl<'de, E> de::EnumAccess<'de> for EnumDeserializer<'de, E>
     where
         E: de::Error,
@@ -1199,7 +1215,7 @@ mod content {
         }
     }
 
-    struct VariantDeserializer<'de, E>
+    pub struct VariantDeserializer<'de, E>
     where
         E: de::Error,
     {
@@ -2062,3 +2078,162 @@ where
         T::deserialize_in_place(deserializer, self.0)
     }
 }
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+pub struct FlatMapDeserializer<'a, 'de: 'a, E>(
+    pub &'a mut Vec<Option<(Content<'de>, Content<'de>)>>,
+    pub PhantomData<E>
+);
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, 'de, E> Deserializer<'de> for FlatMapDeserializer<'a, 'de, E>
+    where E: Error
+{
+    type Error = E;
+
+    fn deserialize_any<V>(self, _: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>,
+    {
+        Err(Error::custom("can only flatten structs and maps"))
+    }
+
+    fn deserialize_enum<V>(
+        self,
+        name: &'static str,
+        variants: &'static [&'static str],
+        visitor: V
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>,
+    {
+        for item in self.0.iter_mut() {
+            // items in the vector are nulled out when used.  So we can only use
+            // an item if it's still filled in and if the field is one we care
+            // about.
+            let use_item = match *item {
+                None => false,
+                Some((ref c, _)) => c.as_str().map_or(false, |x| variants.contains(&x))
+            };
+
+            if use_item {
+                let (key, value) = item.take().unwrap();
+                return visitor.visit_enum(EnumDeserializer::new(
+                    key,
+                    Some(value)
+                ));
+            }
+        }
+
+        Err(Error::custom(format_args!(
+            "no variant of enum {} not found in flattened data",
+            name
+        )))
+    }
+
+    fn deserialize_map<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>,
+    {
+        visitor.visit_map(FlatMapAccess::new(self.0.iter_mut(), None))
+    }
+
+    fn deserialize_struct<V>(
+        self,
+        _: &'static str,
+        fields: &'static [&'static str],
+        visitor: V
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>,
+    {
+        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 seq tuple tuple_struct identifier
+        ignored_any
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+pub struct FlatMapAccess<'a, 'de: 'a, E> {
+    iter: slice::IterMut<'a, Option<(Content<'de>, Content<'de>)>>,
+    pending_content: Option<Content<'de>>,
+    fields: Option<&'static [&'static str]>,
+    _marker: PhantomData<E>,
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, 'de, E> FlatMapAccess<'a, 'de, E> {
+    fn new(
+        iter: slice::IterMut<'a, Option<(Content<'de>, Content<'de>)>>,
+        fields: Option<&'static [&'static str]>
+    ) -> FlatMapAccess<'a, 'de, E> {
+        FlatMapAccess {
+            iter: iter,
+            pending_content: None,
+            fields: fields,
+            _marker: PhantomData,
+        }
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, 'de, E> MapAccess<'de> for FlatMapAccess<'a, 'de, E>
+    where E: Error
+{
+    type Error = E;
+
+    fn next_key_seed<T>(&mut self, seed: T) -> Result<Option<T::Value>, Self::Error>
+    where
+        T: DeserializeSeed<'de>,
+    {
+        while let Some(item) = self.iter.next() {
+            // items in the vector are nulled out when used.  So we can only use
+            // an item if it's still filled in and if the field is one we care
+            // about.  In case we do not know which fields we want, we take them all.
+            let use_item = match *item {
+                None => false,
+                Some((ref c, _)) => {
+                    c.as_str().map_or(self.fields.is_none(), |key| {
+                        match self.fields {
+                            None => true,
+                            Some(fields) if fields.contains(&key) => true,
+                            _ => false
+                        }
+                    })
+                }
+            };
+
+            if use_item {
+                let (key, content) = item.take().unwrap();
+                self.pending_content = Some(content);
+                return seed.deserialize(ContentDeserializer::new(key)).map(Some);
+            }
+        }
+        Ok(None)
+    }
+
+    fn next_value_seed<T>(&mut self, seed: T) -> Result<T::Value, Self::Error>
+    where
+        T: DeserializeSeed<'de>,
+    {
+        match self.pending_content.take() {
+            Some(value) => seed.deserialize(ContentDeserializer::new(value)),
+            None => Err(Error::custom("value is missing")),
+        }
+    }
+}

serde/src/private/ser.rs

@@ -11,7 +11,12 @@ use lib::*;
 use ser::{self, Impossible, Serialize, SerializeMap, SerializeStruct, Serializer};
 
 #[cfg(any(feature = "std", feature = "alloc"))]
-use self::content::{SerializeStructVariantAsMapValue, SerializeTupleVariantAsMapValue};
+use self::content::{
+    SerializeStructVariantAsMapValue,
+    SerializeTupleVariantAsMapValue,
+    ContentSerializer,
+    Content,
+};
 
 /// Used to check that serde(getter) attributes return the expected type.
 /// Not public API.
@@ -58,10 +63,11 @@ enum Unsupported {
     ByteArray,
     Optional,
     Unit,
+    #[cfg(any(feature = "std", feature = "alloc"))]
+    UnitStruct,
     Sequence,
     Tuple,
     TupleStruct,
-    #[cfg(not(any(feature = "std", feature = "alloc")))]
     Enum,
 }
 
@@ -76,10 +82,11 @@ 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"),
+            #[cfg(any(feature = "std", feature = "alloc"))]
+            Unsupported::UnitStruct => formatter.write_str("unit struct"),
             Unsupported::Sequence => formatter.write_str("a sequence"),
             Unsupported::Tuple => formatter.write_str("a tuple"),
             Unsupported::TupleStruct => formatter.write_str("a tuple struct"),
-            #[cfg(not(any(feature = "std", feature = "alloc")))]
             Unsupported::Enum => formatter.write_str("an enum"),
         }
     }
@@ -459,7 +466,7 @@ mod content {
     }
 
     #[derive(Debug)]
-    enum Content {
+    pub enum Content {
         Bool(bool),
 
         U8(u8),
@@ -584,12 +591,12 @@ mod content {
         }
     }
 
-    struct ContentSerializer<E> {
+    pub struct ContentSerializer<E> {
         error: PhantomData<E>,
     }
 
     impl<E> ContentSerializer<E> {
-        fn new() -> Self {
+        pub fn new() -> Self {
             ContentSerializer { error: PhantomData }
         }
     }
@@ -804,7 +811,7 @@ mod content {
         }
     }
 
-    struct SerializeSeq<E> {
+    pub struct SerializeSeq<E> {
         elements: Vec<Content>,
         error: PhantomData<E>,
     }
@@ -830,7 +837,7 @@ mod content {
         }
     }
 
-    struct SerializeTuple<E> {
+    pub struct SerializeTuple<E> {
         elements: Vec<Content>,
         error: PhantomData<E>,
     }
@@ -856,7 +863,7 @@ mod content {
         }
     }
 
-    struct SerializeTupleStruct<E> {
+    pub struct SerializeTupleStruct<E> {
         name: &'static str,
         fields: Vec<Content>,
         error: PhantomData<E>,
@@ -883,7 +890,7 @@ mod content {
         }
     }
 
-    struct SerializeTupleVariant<E> {
+    pub struct SerializeTupleVariant<E> {
         name: &'static str,
         variant_index: u32,
         variant: &'static str,
@@ -917,7 +924,7 @@ mod content {
         }
     }
 
-    struct SerializeMap<E> {
+    pub struct SerializeMap<E> {
         entries: Vec<(Content, Content)>,
         key: Option<Content>,
         error: PhantomData<E>,
@@ -967,7 +974,7 @@ mod content {
         }
     }
 
-    struct SerializeStruct<E> {
+    pub struct SerializeStruct<E> {
         name: &'static str,
         fields: Vec<(&'static str, Content)>,
         error: PhantomData<E>,
@@ -994,7 +1001,7 @@ mod content {
         }
     }
 
-    struct SerializeStructVariant<E> {
+    pub struct SerializeStructVariant<E> {
         name: &'static str,
         variant_index: u32,
         variant: &'static str,
@@ -1028,3 +1035,283 @@ mod content {
         }
     }
 }
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+pub struct FlatMapSerializer<'a, M: 'a>(pub &'a mut M);
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, M> FlatMapSerializer<'a, M>
+where
+    M: SerializeMap + 'a
+{
+    fn bad_type(self, what: Unsupported) -> M::Error {
+        ser::Error::custom(format_args!("can only flatten structs and maps (got {})", what))
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, M> Serializer for FlatMapSerializer<'a, M>
+    where M: SerializeMap + 'a
+{
+    type Ok = ();
+    type Error = M::Error;
+
+    type SerializeSeq = Impossible<Self::Ok, M::Error>;
+    type SerializeTuple = Impossible<Self::Ok, M::Error>;
+    type SerializeTupleStruct = Impossible<Self::Ok, M::Error>;
+    type SerializeMap = FlatMapSerializeMap<'a, M>;
+    type SerializeStruct = FlatMapSerializeStruct<'a, M>;
+    type SerializeTupleVariant = Impossible<Self::Ok, M::Error>;
+    type SerializeStructVariant = FlatMapSerializeStructVariantAsMapValue<'a, M>;
+
+    fn serialize_bool(self, _: bool) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Boolean))
+    }
+
+    fn serialize_i8(self, _: i8) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_i16(self, _: i16) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_i32(self, _: i32) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_i64(self, _: i64) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_u8(self, _: u8) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_u16(self, _: u16) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_u32(self, _: u32) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_u64(self, _: u64) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Integer))
+    }
+
+    fn serialize_f32(self, _: f32) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Float))
+    }
+
+    fn serialize_f64(self, _: f64) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Float))
+    }
+
+    fn serialize_char(self, _: char) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Char))
+    }
+
+    fn serialize_str(self, _: &str) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::String))
+    }
+
+    fn serialize_bytes(self, _: &[u8]) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::ByteArray))
+    }
+
+    fn serialize_none(self) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Optional))
+    }
+
+    fn serialize_some<T: ?Sized>(self, _: &T) -> Result<Self::Ok, Self::Error>
+    where
+        T: Serialize,
+    {
+        Err(self.bad_type(Unsupported::Optional))
+    }
+
+    fn serialize_unit(self) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Unit))
+    }
+
+    fn serialize_unit_struct(self, _: &'static str) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::UnitStruct))
+    }
+
+    fn serialize_unit_variant(
+        self,
+        _: &'static str,
+        _: u32,
+        _: &'static str,
+    ) -> Result<Self::Ok, Self::Error> {
+        Err(self.bad_type(Unsupported::Enum))
+    }
+
+    fn serialize_newtype_struct<T: ?Sized>(
+        self,
+        _: &'static str,
+        value: &T,
+    ) -> Result<Self::Ok, Self::Error>
+    where
+        T: Serialize,
+    {
+        value.serialize(self)
+    }
+
+    fn serialize_newtype_variant<T: ?Sized>(
+        self,
+        _: &'static str,
+        _: u32,
+        variant: &'static str,
+        value: &T,
+    ) -> Result<Self::Ok, Self::Error>
+    where
+        T: Serialize,
+    {
+        try!(self.0.serialize_key(variant));
+        self.0.serialize_value(value)
+    }
+
+    fn serialize_seq(self, _: Option<usize>) -> Result<Self::SerializeSeq, Self::Error> {
+        Err(self.bad_type(Unsupported::Sequence))
+    }
+
+    fn serialize_tuple(self, _: usize) -> Result<Self::SerializeTuple, Self::Error> {
+        Err(self.bad_type(Unsupported::Tuple))
+    }
+
+    fn serialize_tuple_struct(
+        self,
+        _: &'static str,
+        _: usize,
+    ) -> Result<Self::SerializeTupleStruct, Self::Error> {
+        Err(self.bad_type(Unsupported::TupleStruct))
+    }
+
+    fn serialize_tuple_variant(
+        self,
+        _: &'static str,
+        _: u32,
+        _: &'static str,
+        _: usize,
+    ) -> Result<Self::SerializeTupleVariant, Self::Error> {
+        Err(self.bad_type(Unsupported::Enum))
+    }
+
+    fn serialize_map(self, _: Option<usize>) -> Result<Self::SerializeMap, Self::Error> {
+        Ok(FlatMapSerializeMap(self.0))
+    }
+
+    fn serialize_struct(
+        self,
+        _: &'static str,
+        _: usize,
+    ) -> Result<Self::SerializeStruct, Self::Error> {
+        Ok(FlatMapSerializeStruct(self.0))
+    }
+
+    fn serialize_struct_variant(
+        self,
+        _: &'static str,
+        _: u32,
+        inner_variant: &'static str,
+        _: usize,
+    ) -> Result<Self::SerializeStructVariant, Self::Error> {
+        try!(self.0.serialize_key(inner_variant));
+        Ok(FlatMapSerializeStructVariantAsMapValue::new(self.0, inner_variant))
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+pub struct FlatMapSerializeMap<'a, M: 'a>(&'a mut M);
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, M> ser::SerializeMap for FlatMapSerializeMap<'a, M>
+    where M: SerializeMap + 'a
+{
+    type Ok = ();
+    type Error = M::Error;
+
+    fn serialize_key<T: ?Sized>(&mut self, key: &T) -> Result<(), Self::Error>
+    where
+        T: Serialize,
+    {
+        self.0.serialize_key(key)
+    }
+
+    fn serialize_value<T: ?Sized>(&mut self, value: &T) -> Result<(), Self::Error>
+    where
+        T: Serialize,
+    {
+        self.0.serialize_value(value)
+    }
+
+    fn end(self) -> Result<(), Self::Error> {
+        Ok(())
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+pub struct FlatMapSerializeStruct<'a, M: 'a>(&'a mut M);
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, M> ser::SerializeStruct for FlatMapSerializeStruct<'a, M>
+    where M: SerializeMap + 'a
+{
+    type Ok = ();
+    type Error = M::Error;
+
+    fn serialize_field<T: ?Sized>(&mut self, key: &'static str, value: &T) -> Result<(), Self::Error>
+    where
+        T: Serialize,
+    {
+        self.0.serialize_entry(key, value)
+    }
+
+    fn end(self) -> Result<(), Self::Error> {
+        Ok(())
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+pub struct FlatMapSerializeStructVariantAsMapValue<'a, M: 'a> {
+    map: &'a mut M,
+    name: &'static str,
+    fields: Vec<(&'static str, Content)>,
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, M> FlatMapSerializeStructVariantAsMapValue<'a, M>
+    where M: SerializeMap + 'a
+{
+    fn new(map: &'a mut M, name: &'static str) -> FlatMapSerializeStructVariantAsMapValue<'a, M> {
+        FlatMapSerializeStructVariantAsMapValue {
+            map: map,
+            name: name,
+            fields: Vec::new(),
+        }
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, M> ser::SerializeStructVariant for FlatMapSerializeStructVariantAsMapValue<'a, M>
+    where M: SerializeMap + 'a
+{
+    type Ok = ();
+    type Error = M::Error;
+
+    fn serialize_field<T: ?Sized>(&mut self, key: &'static str, value: &T) -> Result<(), Self::Error>
+    where
+        T: Serialize,
+    {
+        let value = try!(value.serialize(ContentSerializer::<M::Error>::new()));
+        self.fields.push((key, value));
+        Ok(())
+    }
+
+    fn end(self) -> Result<(), Self::Error> {
+        try!(self.map.serialize_value(&Content::Struct(self.name, self.fields)));
+        Ok(())
+    }
+}

serde_derive/src/de.rs

@@ -268,7 +268,11 @@ fn deserialize_in_place_body(cont: &Container, params: &Parameters) -> Option<St
 
     let code = match cont.data {
         Data::Struct(Style::Struct, ref fields) => {
-            deserialize_struct_in_place(None, params, fields, &cont.attrs, None)
+            if let Some(code) = deserialize_struct_in_place(None, params, fields, &cont.attrs, None) {
+                code
+            } else {
+                return None;
+            }
         }
         Data::Struct(Style::Tuple, ref fields) | Data::Struct(Style::Newtype, ref fields) => {
             deserialize_tuple_in_place(None, params, fields, &cont.attrs, None)
@@ -345,6 +349,8 @@ fn deserialize_tuple(
         split_with_de_lifetime(params);
     let delife = params.borrowed.de_lifetime();
 
+    debug_assert!(!cattrs.has_flatten());
+
     // If there are getters (implying private fields), construct the local type
     // and use an `Into` conversion to get the remote type. If there are no
     // getters then construct the target type directly.
@@ -440,6 +446,8 @@ fn deserialize_tuple_in_place(
         split_with_de_lifetime(params);
     let delife = params.borrowed.de_lifetime();
 
+    debug_assert!(!cattrs.has_flatten());
+
     let is_enum = variant_ident.is_some();
     let expecting = match variant_ident {
         Some(variant_ident) => format!("tuple variant {}::{}", params.type_name(), variant_ident),
@@ -522,6 +530,8 @@ fn deserialize_seq(
 ) -> Fragment {
     let vars = (0..fields.len()).map(field_i as fn(_) -> _);
 
+    // XXX: do we need an error for flattening here?
+
     let deserialized_count = fields
         .iter()
         .filter(|field| !field.attrs.skip_deserializing())
@@ -613,6 +623,8 @@ fn deserialize_seq_in_place(
 ) -> Fragment {
     let vars = (0..fields.len()).map(field_i as fn(_) -> _);
 
+    // XXX: do we need an error for flattening here?
+
     let deserialized_count = fields
         .iter()
         .filter(|field| !field.attrs.skip_deserializing())
@@ -793,10 +805,13 @@ fn deserialize_struct(
 
     let visit_seq = Stmts(deserialize_seq(&type_path, params, fields, true, cattrs));
 
-    let (field_visitor, fields_stmt, visit_map) =
-        deserialize_struct_visitor(&type_path, params, fields, cattrs);
+    let (field_visitor, fields_stmt, visit_map) = if cattrs.has_flatten() {
+        deserialize_struct_as_map_visitor(&type_path, params, fields, cattrs)
+    } else {
+        deserialize_struct_as_struct_visitor(&type_path, params, fields, cattrs)
+    };
     let field_visitor = Stmts(field_visitor);
-    let fields_stmt = Stmts(fields_stmt);
+    let fields_stmt = fields_stmt.map(Stmts);
     let visit_map = Stmts(visit_map);
 
     let visitor_expr = quote! {
@@ -813,6 +828,10 @@ fn deserialize_struct(
         quote! {
             _serde::de::VariantAccess::struct_variant(__variant, FIELDS, #visitor_expr)
         }
+    } else if cattrs.has_flatten() {
+        quote! {
+            _serde::Deserializer::deserialize_map(__deserializer, #visitor_expr)
+        }
     } else {
         let type_name = cattrs.name().deserialize_name();
         quote! {
@@ -827,10 +846,10 @@ fn deserialize_struct(
         quote!(mut __seq)
     };
 
-    // untagged struct variants do not get a visit_seq method
+    // untagged struct variants do not get a visit_seq method.  The same applies to structs that
+    // only have a map representation.
     let visit_seq = match *untagged {
-        Untagged::Yes => None,
-        Untagged::No => Some(quote! {
+        Untagged::No if !cattrs.has_flatten() => Some(quote! {
             #[inline]
             fn visit_seq<__A>(self, #visitor_var: __A) -> _serde::export::Result<Self::Value, __A::Error>
                 where __A: _serde::de::SeqAccess<#delife>
@@ -838,6 +857,7 @@ fn deserialize_struct(
                 #visit_seq
             }
         }),
+        _ => None,
     };
 
     quote_block! {
@@ -878,9 +898,15 @@ fn deserialize_struct_in_place(
     fields: &[Field],
     cattrs: &attr::Container,
     deserializer: Option<Tokens>,
-) -> Fragment {
+) -> Option<Fragment> {
     let is_enum = variant_ident.is_some();
 
+    // for now we do not support in_place deserialization for structs that
+    // are represented as map.
+    if cattrs.has_flatten() {
+        return None;
+    }
+
     let this = &params.this;
     let (de_impl_generics, de_ty_generics, ty_generics, where_clause) =
         split_with_de_lifetime(params);
@@ -893,10 +919,11 @@ fn deserialize_struct_in_place(
 
     let visit_seq = Stmts(deserialize_seq_in_place(params, fields, cattrs));
 
-    let (field_visitor, fields_stmt, visit_map) =
-        deserialize_struct_in_place_visitor(params, fields, cattrs);
+    let (field_visitor, fields_stmt, visit_map) = deserialize_struct_as_struct_in_place_visitor(
+        params, fields, cattrs);
+
     let field_visitor = Stmts(field_visitor);
-    let fields_stmt = Stmts(fields_stmt);
+    let fields_stmt = fields_stmt.map(Stmts);
     let visit_map = Stmts(visit_map);
 
     let visitor_expr = quote! {
@@ -940,7 +967,7 @@ fn deserialize_struct_in_place(
     let in_place_ty_generics = de_ty_generics.in_place();
     let place_life = place_lifetime();
 
-    quote_block! {
+    Some(quote_block! {
         #field_visitor
 
         struct __Visitor #in_place_impl_generics #where_clause {
@@ -968,7 +995,7 @@ fn deserialize_struct_in_place(
         #fields_stmt
 
         #dispatch
-    }
+    })
 }
 
 fn deserialize_enum(
@@ -1688,12 +1715,16 @@ fn deserialize_untagged_newtype_variant(
 fn deserialize_generated_identifier(
     fields: &[(String, Ident)],
     cattrs: &attr::Container,
-    is_variant: bool,
+    is_variant: bool
 ) -> Fragment {
     let this = quote!(__Field);
     let field_idents: &Vec<_> = &fields.iter().map(|&(_, ref ident)| ident).collect();
 
-    let (ignore_variant, fallthrough) = if is_variant || cattrs.deny_unknown_fields() {
+    let (ignore_variant, fallthrough) = if cattrs.has_flatten() {
+        let ignore_variant = quote!(__other(_serde::private::de::Content<'de>),);
+        let fallthrough = quote!(_serde::export::Ok(__Field::__other(__value)));
+        (Some(ignore_variant), Some(fallthrough))
+    } else if is_variant || cattrs.deny_unknown_fields() {
         (None, None)
     } else {
         let ignore_variant = quote!(__ignore,);
@@ -1706,11 +1737,18 @@ fn deserialize_generated_identifier(
         fields,
         is_variant,
         fallthrough,
+        cattrs.has_flatten(),
     ));
 
+    let lifetime = if cattrs.has_flatten() {
+        Some(quote!(<'de>))
+    } else {
+        None
+    };
+
     quote_block! {
         #[allow(non_camel_case_types)]
-        enum __Field {
+        enum __Field #lifetime {
             #(#field_idents,)*
             #ignore_variant
         }
@@ -1718,12 +1756,12 @@ fn deserialize_generated_identifier(
         struct __FieldVisitor;
 
         impl<'de> _serde::de::Visitor<'de> for __FieldVisitor {
-            type Value = __Field;
+            type Value = __Field #lifetime;
 
             #visitor_impl
         }
 
-        impl<'de> _serde::Deserialize<'de> for __Field {
+        impl<'de> _serde::Deserialize<'de> for __Field #lifetime {
             #[inline]
             fn deserialize<__D>(__deserializer: __D) -> _serde::export::Result<Self, __D::Error>
                 where __D: _serde::Deserializer<'de>
@@ -1804,6 +1842,7 @@ fn deserialize_custom_identifier(
         &names_idents,
         is_variant,
         fallthrough,
+        false,
     ));
 
     quote_block! {
@@ -1833,9 +1872,12 @@ fn deserialize_identifier(
     fields: &[(String, Ident)],
     is_variant: bool,
     fallthrough: Option<Tokens>,
+    collect_other_fields: bool
 ) -> Fragment {
     let field_strs = fields.iter().map(|&(ref name, _)| name);
+    let field_borrowed_strs = fields.iter().map(|&(ref name, _)| name);
     let field_bytes = fields.iter().map(|&(ref name, _)| Literal::byte_string(name.as_bytes()));
+    let field_borrowed_bytes = fields.iter().map(|&(ref name, _)| Literal::byte_string(name.as_bytes()));
 
     let constructors: &Vec<_> = &fields
         .iter()
@@ -1852,28 +1894,129 @@ fn deserialize_identifier(
 
     let variant_indices = 0u64..;
     let fallthrough_msg = format!("{} index 0 <= i < {}", index_expecting, fields.len());
-    let visit_index = quote! {
-        fn visit_u64<__E>(self, __value: u64) -> _serde::export::Result<Self::Value, __E>
-            where __E: _serde::de::Error
-        {
-            match __value {
-                #(
-                    #variant_indices => _serde::export::Ok(#constructors),
-                )*
-                _ => _serde::export::Err(_serde::de::Error::invalid_value(
-                            _serde::de::Unexpected::Unsigned(__value),
-                            &#fallthrough_msg))
+    let visit_other = if collect_other_fields {
+        Some(quote! {
+            fn visit_bool<__E>(self, __value: bool) -> Result<Self::Value, __E>
+                where __E: _serde::de::Error
+            {
+                Ok(__Field::__other(_serde::private::de::Content::Bool(__value)))
             }
-        }
+
+            fn visit_i8<__E>(self, __value: i8) -> Result<Self::Value, __E>
+                where __E: _serde::de::Error
+            {
+                Ok(__Field::__other(_serde::private::de::Content::I8(__value)))
+            }
+
+            fn visit_i16<__E>(self, __value: i16) -> Result<Self::Value, __E>
+                where __E: _serde::de::Error
+            {
+                Ok(__Field::__other(_serde::private::de::Content::I16(__value)))
+            }
+
+            fn visit_i32<__E>(self, __value: i32) -> Result<Self::Value, __E>
+                where __E: _serde::de::Error
+            {
+                Ok(__Field::__other(_serde::private::de::Content::I32(__value)))
+            }
+
+            fn visit_i64<__E>(self, __value: i64) -> Result<Self::Value, __E>
+                where __E: _serde::de::Error
+            {
+                Ok(__Field::__other(_serde::private::de::Content::I64(__value)))
+            }
+
+            fn visit_u8<__E>(self, __value: u8) -> Result<Self::Value, __E>
+                where __E: _serde::de::Error
+            {
+                Ok(__Field::__other(_serde::private::de::Content::U8(__value)))
+            }
+
+            fn visit_u16<__E>(self, __value: u16) -> Result<Self::Value, __E>
+                where __E: _serde::de::Error
+            {
+                Ok(__Field::__other(_serde::private::de::Content::U16(__value)))
+            }
+
+            fn visit_u32<__E>(self, __value: u32) -> Result<Self::Value, __E>
+                where __E: _serde::de::Error
+            {
+                Ok(__Field::__other(_serde::private::de::Content::U32(__value)))
+            }
+
+            fn visit_u64<__E>(self, __value: u64) -> Result<Self::Value, __E>
+                where __E: _serde::de::Error
+            {
+                Ok(__Field::__other(_serde::private::de::Content::U64(__value)))
+            }
+
+            fn visit_f32<__E>(self, __value: f32) -> Result<Self::Value, __E>
+                where __E: _serde::de::Error
+            {
+                Ok(__Field::__other(_serde::private::de::Content::F32(__value)))
+            }
+
+            fn visit_f64<__E>(self, __value: f64) -> Result<Self::Value, __E>
+                where __E: _serde::de::Error
+            {
+                Ok(__Field::__other(_serde::private::de::Content::F64(__value)))
+            }
+
+            fn visit_char<__E>(self, __value: char) -> Result<Self::Value, __E>
+                where __E: _serde::de::Error
+            {
+                Ok(__Field::__other(_serde::private::de::Content::Char(__value)))
+            }
+
+            fn visit_unit<__E>(self) -> Result<Self::Value, __E>
+                where __E: _serde::de::Error
+            {
+                Ok(__Field::__other(_serde::private::de::Content::Unit))
+            }
+        })
+    } else {
+        Some(quote! {
+            fn visit_u64<__E>(self, __value: u64) -> _serde::export::Result<Self::Value, __E>
+                where __E: _serde::de::Error
+            {
+                match __value {
+                    #(
+                        #variant_indices => _serde::export::Ok(#constructors),
+                    )*
+                    _ => _serde::export::Err(_serde::de::Error::invalid_value(
+                                _serde::de::Unexpected::Unsigned(__value),
+                                &#fallthrough_msg))
+                }
+            }
+        })
     };
 
-    let bytes_to_str = if fallthrough.is_some() {
+    let bytes_to_str = if fallthrough.is_some() || collect_other_fields {
         None
     } else {
-        let conversion = quote! {
+        Some(quote! {
             let __value = &_serde::export::from_utf8_lossy(__value);
-        };
-        Some(conversion)
+        })
+    };
+
+    let (value_as_str_content, value_as_borrowed_str_content,
+         value_as_bytes_content, value_as_borrowed_bytes_content) = if !collect_other_fields {
+        (None, None, None, None)
+    } else {
+        (
+            Some(quote! {
+                let __value = _serde::private::de::Content::String(__value.to_string());
+            }),
+            Some(quote! {
+                let __value = _serde::private::de::Content::Str(__value);
+            }),
+            Some(quote! {
+                let __value = _serde::private::de::Content::ByteBuf(__value.to_vec());
+            }),
+            Some(quote! {
+                let __value = _serde::private::de::Content::Bytes(__value);
+            })
+        )
     };
 
     let fallthrough_arm = if let Some(fallthrough) = fallthrough {
@@ -1893,7 +2036,7 @@ fn deserialize_identifier(
             _serde::export::Formatter::write_str(formatter, #expecting)
         }
 
-        #visit_index
+        #visit_other
 
         fn visit_str<__E>(self, __value: &str) -> _serde::export::Result<Self::Value, __E>
             where __E: _serde::de::Error
@@ -1902,7 +2045,39 @@ fn deserialize_identifier(
                 #(
                     #field_strs => _serde::export::Ok(#constructors),
                 )*
-                _ => #fallthrough_arm
+                _ => {
+                    #value_as_str_content
+                    #fallthrough_arm
+                }
+            }
+        }
+
+        fn visit_borrowed_str<__E>(self, __value: &'de str) -> _serde::export::Result<Self::Value, __E>
+            where __E: _serde::de::Error
+        {
+            match __value {
+                #(
+                    #field_borrowed_strs => _serde::export::Ok(#constructors),
+                )*
+                _ => {
+                    #value_as_borrowed_str_content
+                    #fallthrough_arm
+                }
+            }
+        }
+
+        fn visit_borrowed_bytes<__E>(self, __value: &'de [u8]) -> _serde::export::Result<Self::Value, __E>
+            where __E: _serde::de::Error
+        {
+            match __value {
+                #(
+                    #field_borrowed_bytes => _serde::export::Ok(#constructors),
+                )*
+                _ => {
+                    #bytes_to_str
+                    #value_as_borrowed_bytes_content
+                    #fallthrough_arm
+                }
             }
         }
 
@@ -1915,6 +2090,7 @@ fn deserialize_identifier(
                 )*
                 _ => {
                     #bytes_to_str
+                    #value_as_bytes_content
                     #fallthrough_arm
                 }
             }
@@ -1922,12 +2098,14 @@ fn deserialize_identifier(
     }
 }
 
-fn deserialize_struct_visitor(
+fn deserialize_struct_as_struct_visitor(
     struct_path: &Tokens,
     params: &Parameters,
     fields: &[Field],
     cattrs: &attr::Container,
-) -> (Fragment, Fragment, Fragment) {
+) -> (Fragment, Option<Fragment>, Fragment) {
+    debug_assert!(!cattrs.has_flatten());
+
     let field_names_idents: Vec<_> = fields
         .iter()
         .enumerate()
@@ -1946,7 +2124,27 @@ fn deserialize_struct_visitor(
 
     let visit_map = deserialize_map(struct_path, params, fields, cattrs);
 
-    (field_visitor, fields_stmt, visit_map)
+    (field_visitor, Some(fields_stmt), visit_map)
+}
+
+fn deserialize_struct_as_map_visitor(
+    struct_path: &Tokens,
+    params: &Parameters,
+    fields: &[Field],
+    cattrs: &attr::Container,
+) -> (Fragment, Option<Fragment>, Fragment) {
+    let field_names_idents: Vec<_> = fields
+        .iter()
+        .enumerate()
+        .filter(|&(_, field)| !field.attrs.skip_deserializing() && !field.attrs.flatten())
+        .map(|(i, field)| (field.attrs.name().deserialize_name(), field_i(i)))
+        .collect();
+
+    let field_visitor = deserialize_generated_identifier(&field_names_idents, cattrs, false);
+
+    let visit_map = deserialize_map(struct_path, params, fields, cattrs);
+
+    (field_visitor, None, visit_map)
 }
 
 fn deserialize_map(
@@ -1965,7 +2163,7 @@ fn deserialize_map(
     // Declare each field that will be deserialized.
     let let_values = fields_names
         .iter()
-        .filter(|&&(field, _)| !field.attrs.skip_deserializing())
+        .filter(|&&(field, _)| !field.attrs.skip_deserializing() && !field.attrs.flatten())
         .map(|&(field, ref name)| {
             let field_ty = &field.ty;
             quote! {
@@ -1973,10 +2171,22 @@ fn deserialize_map(
             }
         });
 
+    // Collect contents for flatten fields into a buffer
+    let let_collect = if cattrs.has_flatten() {
+        Some(quote! {
+            let mut __collect = Vec::<Option<(
+                _serde::private::de::Content,
+                _serde::private::de::Content
+            )>>::new();
+        })
+    } else {
+        None
+    };
+
     // Match arms to extract a value for a field.
     let value_arms = fields_names
         .iter()
-        .filter(|&&(field, _)| !field.attrs.skip_deserializing())
+        .filter(|&&(field, _)| !field.attrs.skip_deserializing() && !field.attrs.flatten())
         .map(|&(field, ref name)| {
             let deser_name = field.attrs.name().deserialize_name();
 
@@ -2008,7 +2218,15 @@ fn deserialize_map(
         });
 
     // Visit ignored values to consume them
-    let ignored_arm = if cattrs.deny_unknown_fields() {
+    let ignored_arm = if cattrs.has_flatten() {
+        Some(quote! {
+            __Field::__other(__name) => {
+                __collect.push(Some((
+                    __name,
+                    try!(_serde::de::MapAccess::next_value(&mut __map)))));
+            }
+        })
+    } else if cattrs.deny_unknown_fields() {
         None
     } else {
         Some(quote! {
@@ -2038,7 +2256,7 @@ fn deserialize_map(
 
     let extract_values = fields_names
         .iter()
-        .filter(|&&(field, _)| !field.attrs.skip_deserializing())
+        .filter(|&&(field, _)| !field.attrs.skip_deserializing() && !field.attrs.flatten())
         .map(|&(field, ref name)| {
             let missing_expr = Match(expr_is_missing(field, cattrs));
 
@@ -2050,6 +2268,35 @@ fn deserialize_map(
             }
         });
 
+    let extract_collected = fields_names
+        .iter()
+        .filter(|&&(field, _)| field.attrs.flatten())
+        .map(|&(field, ref name)| {
+            let field_ty = field.ty;
+            quote! {
+                let #name: #field_ty = try!(_serde::de::Deserialize::deserialize(
+                    _serde::private::de::FlatMapDeserializer(
+                        &mut __collect,
+                        _serde::export::PhantomData)));
+            }
+        });
+
+    let collected_deny_unknown_fields = if cattrs.has_flatten() && cattrs.deny_unknown_fields() {
+        Some(quote! {
+            if let Some(Some((__key, _))) = __collect.into_iter().filter(|x| x.is_some()).next() {
+                if let Some(__key) = __key.as_str() {
+                    return _serde::export::Err(
+                        _serde::de::Error::custom(format_args!("unknown field `{}`", &__key)));
+                } else {
+                    return _serde::export::Err(
+                        _serde::de::Error::custom(format_args!("unexpected map key")));
+                }
+            }
+        })
+    } else {
+        None
+    };
+
     let result = fields_names.iter().map(|&(field, ref name)| {
         let ident = field.ident.expect("struct contains unnamed fields");
         if field.attrs.skip_deserializing() {
@@ -2085,22 +2332,30 @@ fn deserialize_map(
     quote_block! {
         #(#let_values)*
 
+        #let_collect
+
         #match_keys
 
         #let_default
 
         #(#extract_values)*
 
+        #(#extract_collected)*
+
+        #collected_deny_unknown_fields
+
         _serde::export::Ok(#result)
     }
 }
 
 #[cfg(feature = "deserialize_in_place")]
-fn deserialize_struct_in_place_visitor(
+fn deserialize_struct_as_struct_in_place_visitor(
     params: &Parameters,
     fields: &[Field],
     cattrs: &attr::Container,
-) -> (Fragment, Fragment, Fragment) {
+) -> (Fragment, Option<Fragment>, Fragment) {
+    debug_assert!(!cattrs.has_flatten());
+
     let field_names_idents: Vec<_> = fields
         .iter()
         .enumerate()
@@ -2119,7 +2374,7 @@ fn deserialize_struct_in_place_visitor(
 
     let visit_map = deserialize_map_in_place(params, fields, cattrs);
 
-    (field_visitor, fields_stmt, visit_map)
+    (field_visitor, Some(fields_stmt), visit_map)
 }
 
 #[cfg(feature = "deserialize_in_place")]
@@ -2128,6 +2383,8 @@ fn deserialize_map_in_place(
     fields: &[Field],
     cattrs: &attr::Container,
 ) -> Fragment {
+    debug_assert!(!cattrs.has_flatten());
+
     // Create the field names for the fields.
     let fields_names: Vec<_> = fields
         .iter()
@@ -2145,6 +2402,18 @@ fn deserialize_map_in_place(
             }
         });
 
+    // Collect contents for flatten fields into a buffer
+    let let_collect = if cattrs.has_flatten() {
+        Some(quote! {
+            let mut __collect = Vec::<Option<(
+                _serde::private::de::Content,
+                _serde::private::de::Content
+            )>>::new();
+        })
+    } else {
+        None
+    };
+
     // Match arms to extract a value for a field.
     let value_arms_from = fields_names
         .iter()
@@ -2179,7 +2448,13 @@ fn deserialize_map_in_place(
         });
 
     // Visit ignored values to consume them
-    let ignored_arm = if cattrs.deny_unknown_fields() {
+    let ignored_arm = if cattrs.has_flatten() {
+        Some(quote! {
+            __Field::__other(__name) => {
+                __collect.push(Some((__name, try!(_serde::de::MapAccess::next_value(&mut __map)))));
+            }
+        })
+    } else if cattrs.deny_unknown_fields() {
         None
     } else {
         Some(quote! {
@@ -2187,7 +2462,7 @@ fn deserialize_map_in_place(
         })
     };
 
-    let all_skipped = fields.iter().all(|field| field.attrs.skip_deserializing());
+    let all_skipped = !cattrs.has_flatten() && fields.iter().all(|field| field.attrs.skip_deserializing());
 
     let match_keys = if cattrs.deny_unknown_fields() && all_skipped {
         quote! {
@@ -2251,15 +2526,47 @@ fn deserialize_map_in_place(
         }
     };
 
+    let extract_collected = fields_names
+        .iter()
+        .filter(|&&(field, _)| field.attrs.flatten())
+        .map(|&(field, ref name)| {
+            let field_ty = field.ty;
+            quote! {
+                let #name: #field_ty = try!(_serde::de::Deserialize::deserialize(
+                    _serde::private::de::FlatMapDeserializer(
+                        &mut __collect,
+                        _serde::export::PhantomData)));
+            }
+        });
+
+    let collected_deny_unknown_fields = if cattrs.has_flatten() && cattrs.deny_unknown_fields() {
+        Some(quote! {
+            if let _serde::export::Some(_serde::export::Some((__key, _))) =
+                __collect.into_iter().filter(|x| x.is_some()).next()
+            {
+                return _serde::export::Err(
+                    _serde::de::Error::custom(format_args!("unknown field `{}`", &__key)));
+            }
+        })
+    } else {
+        None
+    };
+
     quote_block! {
         #(#let_flags)*
 
+        #let_collect
+
         #match_keys
 
         #let_default
 
         #(#check_flags)*
 
+        #(#extract_collected)*
+
+        #collected_deny_unknown_fields
+
         _serde::export::Ok(())
     }
 }

serde_derive/src/lib.rs

@@ -26,7 +26,7 @@
 #![cfg_attr(feature = "cargo-clippy", allow(enum_variant_names, redundant_field_names,
                                             too_many_arguments, used_underscore_binding))]
 // The `quote!` macro requires deep recursion.
-#![recursion_limit = "256"]
+#![recursion_limit = "512"]
 
 #[macro_use]
 extern crate quote;

serde_derive/src/ser.rs

@@ -245,6 +245,14 @@ fn serialize_tuple_struct(
 fn serialize_struct(params: &Parameters, fields: &[Field], cattrs: &attr::Container) -> Fragment {
     assert!(fields.len() as u64 <= u64::from(u32::MAX));
 
+    if cattrs.has_flatten() {
+        serialize_struct_as_map(params, fields, cattrs)
+    } else {
+        serialize_struct_as_struct(params, fields, cattrs)
+    }
+}
+
+fn serialize_struct_as_struct(params: &Parameters, fields: &[Field], cattrs: &attr::Container) -> Fragment {
     let serialize_fields = serialize_struct_visitor(
         fields,
         params,
@@ -279,6 +287,44 @@ fn serialize_struct(params: &Parameters, fields: &[Field], cattrs: &attr::Contai
     }
 }
 
+fn serialize_struct_as_map(params: &Parameters, fields: &[Field], cattrs: &attr::Container) -> Fragment {
+    let serialize_fields = serialize_struct_visitor(
+        fields,
+        params,
+        false,
+        &StructTrait::SerializeMap,
+    );
+
+    let mut serialized_fields = fields
+        .iter()
+        .filter(|&field| !field.attrs.skip_serializing())
+        .peekable();
+
+    let let_mut = mut_if(serialized_fields.peek().is_some());
+
+    let len = if cattrs.has_flatten() {
+        quote!(_serde::export::None)
+    } else {
+        let len = serialized_fields
+            .map(|field| match field.attrs.skip_serializing_if() {
+                None => quote!(1),
+                Some(path) => {
+                    let ident = field.ident.expect("struct has unnamed fields");
+                    let field_expr = get_member(params, field, &Member::Named(ident));
+                    quote!(if #path(#field_expr) { 0 } else { 1 })
+                }
+            })
+            .fold(quote!(0), |sum, expr| quote!(#sum + #expr));
+        quote!(_serde::export::Some(#len))
+    };
+
+    quote_block! {
+        let #let_mut __serde_state = try!(_serde::Serializer::serialize_map(__serializer, #len));
+        #(#serialize_fields)*
+        _serde::ser::SerializeMap::end(__serde_state)
+    }
+}
+
 fn serialize_enum(params: &Parameters, variants: &[Variant], cattrs: &attr::Container) -> Fragment {
     assert!(variants.len() as u64 <= u64::from(u32::MAX));
 
@@ -859,6 +905,7 @@ fn serialize_struct_visitor(
         .filter(|&field| !field.attrs.skip_serializing())
         .map(|field| {
             let field_ident = field.ident.expect("struct has unnamed field");
+
             let mut field_expr = if is_enum {
                 quote!(#field_ident)
             } else {
@@ -877,20 +924,33 @@ fn serialize_struct_visitor(
             }
 
             let span = Span::def_site().located_at(field.original.span());
-            let func = struct_trait.serialize_field(span);
-            let ser = quote! {
-                try!(#func(&mut __serde_state, #key_expr, #field_expr));
+            let ser = if field.attrs.flatten() {
+                quote! {
+                    try!(_serde::Serialize::serialize(&#field_expr, _serde::private::ser::FlatMapSerializer(&mut __serde_state)));
+                }
+            } else {
+                let func = struct_trait.serialize_field(span);
+                quote! {
+                    try!(#func(&mut __serde_state, #key_expr, #field_expr));
+                }
             };
 
             match skip {
                 None => ser,
                 Some(skip) => {
-                    let skip_func = struct_trait.skip_field(span);
-                    quote! {
-                        if !#skip {
-                            #ser
-                        } else {
-                            try!(#skip_func(&mut __serde_state, #key_expr));
+                    if let Some(skip_func) = struct_trait.skip_field(span) {
+                        quote! {
+                            if !#skip {
+                                #ser
+                            } else {
+                                try!(#skip_func(&mut __serde_state, #key_expr));
+                            }
+                        }
+                    } else {
+                        quote! {
+                            if !#skip {
+                                #ser
+                            }
                         }
                     }
                 }
@@ -1011,6 +1071,7 @@ fn get_member(params: &Parameters, field: &Field, member: &Member) -> Tokens {
 }
 
 enum StructTrait {
+    SerializeMap,
     SerializeStruct,
     SerializeStructVariant,
 }
@@ -1018,6 +1079,9 @@ enum StructTrait {
 impl StructTrait {
     fn serialize_field(&self, span: Span) -> Tokens {
         match *self {
+            StructTrait::SerializeMap => {
+                quote_spanned!(span=> _serde::ser::SerializeMap::serialize_entry)
+            }
             StructTrait::SerializeStruct => {
                 quote_spanned!(span=> _serde::ser::SerializeStruct::serialize_field)
             }
@@ -1027,14 +1091,15 @@ impl StructTrait {
         }
     }
 
-    fn skip_field(&self, span: Span) -> Tokens {
+    fn skip_field(&self, span: Span) -> Option<Tokens> {
         match *self {
-            StructTrait::SerializeStruct => {
+            StructTrait::SerializeMap => None,
+            StructTrait::SerializeStruct => Some({
                 quote_spanned!(span=> _serde::ser::SerializeStruct::skip_field)
-            }
-            StructTrait::SerializeStructVariant => {
+            }),
+            StructTrait::SerializeStructVariant => Some({
                 quote_spanned!(span=> _serde::ser::SerializeStructVariant::skip_field)
-            }
+            })
         }
     }
 }

serde_derive_internals/src/ast.rs

@@ -48,7 +48,7 @@ pub enum Style {
 
 impl<'a> Container<'a> {
     pub fn from_ast(cx: &Ctxt, item: &'a syn::DeriveInput) -> Container<'a> {
-        let attrs = attr::Container::from_ast(cx, item);
+        let mut attrs = attr::Container::from_ast(cx, item);
 
         let mut data = match item.data {
             syn::Data::Enum(ref data) => {
@@ -63,6 +63,7 @@ impl<'a> Container<'a> {
             }
         };
 
+        let mut has_flatten = false;
         match data {
             Data::Enum(ref mut variants) => for variant in variants {
                 variant.attrs.rename_by_rule(attrs.rename_all());
@@ -71,10 +72,17 @@ impl<'a> Container<'a> {
                 }
             },
             Data::Struct(_, ref mut fields) => for field in fields {
+                if field.attrs.flatten() {
+                    has_flatten = true;
+                }
                 field.attrs.rename_by_rule(attrs.rename_all());
             },
         }
 
+        if has_flatten {
+            attrs.mark_has_flatten();
+        }
+
         let item = Container {
             ident: item.ident,
             attrs: attrs,

serde_derive_internals/src/attr.rs

@@ -27,6 +27,7 @@ use proc_macro2::{Span, TokenStream, TokenNode, TokenTree};
 
 pub use case::RenameRule;
 
+#[derive(Copy, Clone)]
 struct Attr<'c, T> {
     cx: &'c Ctxt,
     name: &'static str,
@@ -114,6 +115,7 @@ pub struct Container {
     type_into: Option<syn::Type>,
     remote: Option<syn::Path>,
     identifier: Identifier,
+    has_flatten: bool,
 }
 
 /// Styles of representing an enum.
@@ -373,6 +375,7 @@ impl Container {
             type_into: type_into.get(),
             remote: remote.get(),
             identifier: decide_identifier(cx, item, &field_identifier, &variant_identifier),
+            has_flatten: false,
         }
     }
 
@@ -419,6 +422,14 @@ impl Container {
     pub fn identifier(&self) -> Identifier {
         self.identifier
     }
+
+    pub fn has_flatten(&self) -> bool {
+        self.has_flatten
+    }
+
+    pub fn mark_has_flatten(&mut self) {
+        self.has_flatten = true;
+    }
 }
 
 fn decide_tag(
@@ -699,6 +710,7 @@ pub struct Field {
     de_bound: Option<Vec<syn::WherePredicate>>,
     borrowed_lifetimes: BTreeSet<syn::Lifetime>,
     getter: Option<syn::ExprPath>,
+    flatten: bool,
 }
 
 /// Represents the default to use for a field when deserializing.
@@ -732,6 +744,7 @@ impl Field {
         let mut de_bound = Attr::none(cx, "bound");
         let mut borrowed_lifetimes = Attr::none(cx, "borrow");
         let mut getter = Attr::none(cx, "getter");
+        let mut flatten = BoolAttr::none(cx, "flatten");
 
         let ident = match field.ident {
             Some(ref ident) => ident.to_string(),
@@ -878,6 +891,11 @@ impl Field {
                         }
                     }
 
+                    // Parse `#[serde(flatten)]`
+                    Meta(Word(word)) if word == "flatten" => {
+                        flatten.set_true();
+                    }
+
                     Meta(ref meta_item) => {
                         cx.error(format!(
                             "unknown serde field attribute `{}`",
@@ -970,6 +988,7 @@ impl Field {
             de_bound: de_bound.get(),
             borrowed_lifetimes: borrowed_lifetimes,
             getter: getter.get(),
+            flatten: flatten.get(),
         }
     }
 
@@ -1025,6 +1044,10 @@ impl Field {
     pub fn getter(&self) -> Option<&syn::ExprPath> {
         self.getter.as_ref()
     }
+
+    pub fn flatten(&self) -> bool {
+        self.flatten
+    }
 }
 
 type SerAndDe<T> = (Option<T>, Option<T>);

serde_derive_internals/src/check.rs

@@ -14,6 +14,7 @@ use Ctxt;
 /// object. Simpler checks should happen when parsing and building the attrs.
 pub fn check(cx: &Ctxt, cont: &Container) {
     check_getter(cx, cont);
+    check_flatten(cx, cont);
     check_identifier(cx, cont);
     check_variant_skip_attrs(cx, cont);
     check_internal_tag_field_name_conflict(cx, cont);
@@ -40,6 +41,38 @@ 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(_) => {
+            debug_assert!(!cont.attrs.has_flatten());
+        }
+        Data::Struct(_, _) => {
+            for field in cont.data.all_fields() {
+                if !field.attrs.flatten() {
+                    continue;
+                }
+                if field.attrs.skip_serializing() {
+                    cx.error(
+                        "#[serde(flatten] can not be combined with \
+                         #[serde(skip_serializing)]"
+                    );
+                } else if field.attrs.skip_serializing_if().is_some() {
+                    cx.error(
+                        "#[serde(flatten] can not be combined with \
+                         #[serde(skip_serializing_if = \"...\")]"
+                    );
+                } else if field.attrs.skip_deserializing() {
+                    cx.error(
+                        "#[serde(flatten] can not be combined with \
+                         #[serde(skip_deserializing)]"
+                    );
+                }
+            }
+        }
+    }
+}
+
 /// The `other` attribute must be used at most once and it must be the last
 /// variant of an enum that has the `field_identifier` attribute.
 ///

test_suite/tests/compile-fail/conflict/flatten-skip-deserializing.rs

@@ -0,0 +1,24 @@
+// Copyright 2018 Serde Developers
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Deserialize)] //~ ERROR: proc-macro derive panicked
+//~^ HELP: #[serde(flatten] can not be combined with #[serde(skip_deserializing)]
+struct Foo {
+    #[serde(flatten, skip_deserializing)]
+    other: Other,
+}
+
+#[derive(Deserialize)]
+struct Other {
+    x: u32
+}
+
+fn main() {}

test_suite/tests/compile-fail/conflict/flatten-skip-serializing-if.rs

@@ -0,0 +1,24 @@
+// Copyright 2018 Serde Developers
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)] //~ ERROR: proc-macro derive panicked
+//~^ HELP: #[serde(flatten] can not be combined with #[serde(skip_serializing_if = "...")]
+struct Foo {
+    #[serde(flatten, skip_serializing_if="Option::is_none")]
+    other: Option<Other>,
+}
+
+#[derive(Serialize)]
+struct Other {
+    x: u32
+}
+
+fn main() {}

test_suite/tests/compile-fail/conflict/flatten-skip-serializing.rs

@@ -0,0 +1,24 @@
+// Copyright 2018 Serde Developers
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)] //~ ERROR: proc-macro derive panicked
+//~^ HELP: #[serde(flatten] can not be combined with #[serde(skip_serializing)]
+struct Foo {
+    #[serde(flatten, skip_serializing)]
+    other: Other,
+}
+
+#[derive(Serialize)]
+struct Other {
+    x: u32
+}
+
+fn main() {}

test_suite/tests/compile-fail/enum-representation/flatten-enum.rs

@@ -0,0 +1,21 @@
+// Copyright 2018 Serde Developers
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)] //~ ERROR: proc-macro derive panicked
+//~^ HELP: unknown serde variant attribute `flatten`
+enum Foo {
+    #[serde(flatten)]
+    Foo {
+        x: u32,
+    }
+}
+
+fn main() {}

test_suite/tests/test_annotations.rs

@@ -12,12 +12,13 @@
 extern crate serde_derive;
 
 extern crate serde;
+use std::collections::HashMap;
 use self::serde::{Deserialize, Deserializer, Serialize, Serializer};
 use self::serde::de::{self, Unexpected};
 
 extern crate serde_test;
-use self::serde_test::{assert_de_tokens, assert_de_tokens_error, assert_ser_tokens, assert_tokens,
-                       Token};
+use self::serde_test::{assert_de_tokens, assert_de_tokens_error, assert_ser_tokens,
+                       assert_ser_tokens_error, assert_tokens, Token};
 
 trait MyDefault: Sized {
     fn my_default() -> Self;
@@ -94,6 +95,56 @@ where
     a5: E,
 }
 
+#[derive(Debug, PartialEq, Serialize, Deserialize)]
+struct CollectOther {
+    a: u32,
+    b: u32,
+    #[serde(flatten)]
+    extra: HashMap<String, u32>,
+}
+
+#[derive(Debug, PartialEq, Serialize, Deserialize)]
+struct FlattenStructEnumWrapper {
+    #[serde(flatten)]
+    data: FlattenStructEnum,
+    #[serde(flatten)]
+    extra: HashMap<String, String>,
+}
+
+#[derive(Debug, PartialEq, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+enum FlattenStructEnum {
+    InsertInteger {
+        index: u32,
+        value: u32
+    },
+}
+
+#[derive(Debug, PartialEq, Serialize, Deserialize)]
+struct FlattenStructTagContentEnumWrapper {
+    outer: u32,
+    #[serde(flatten)]
+    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 {
+    InsertInteger {
+        index: u32,
+        value: u32
+    },
+    NewtypeVariant(FlattenStructTagContentEnumNewtypeVariant),
+}
+
+#[derive(Debug, PartialEq, Serialize, Deserialize)]
+struct FlattenStructTagContentEnumNewtypeVariant {
+    value: u32,
+}
+
 #[test]
 fn test_default_struct() {
     assert_de_tokens(
@@ -1267,3 +1318,482 @@ fn test_from_into_traits() {
     assert_ser_tokens::<StructFromEnum>(&StructFromEnum(None), &[Token::None]);
     assert_de_tokens::<StructFromEnum>(&StructFromEnum(Some(2)), &[Token::Some, Token::U32(2)]);
 }
+
+#[test]
+fn test_collect_other() {
+    let mut extra = HashMap::new();
+    extra.insert("c".into(), 3);
+    assert_tokens(
+        &CollectOther { a: 1, b: 2, extra },
+        &[
+            Token::Map { len: None },
+            Token::Str("a"),
+            Token::U32(1),
+            Token::Str("b"),
+            Token::U32(2),
+            Token::Str("c"),
+            Token::U32(3),
+            Token::MapEnd,
+        ],
+    );
+}
+
+#[test]
+fn test_flatten_struct_enum() {
+    let mut extra = HashMap::new();
+    extra.insert("extra_key".into(), "extra value".into());
+    let change_request = FlattenStructEnumWrapper {
+        data: FlattenStructEnum::InsertInteger {
+            index: 0,
+            value: 42
+        },
+        extra,
+    };
+    assert_de_tokens(
+        &change_request,
+        &[
+            Token::Map { len: None },
+            Token::Str("insert_integer"),
+            Token::Map { len: None },
+            Token::Str("index"),
+            Token::U32(0),
+            Token::Str("value"),
+            Token::U32(42),
+            Token::MapEnd,
+            Token::Str("extra_key"),
+            Token::Str("extra value"),
+            Token::MapEnd
+        ],
+    );
+    assert_ser_tokens(
+        &change_request,
+        &[
+            Token::Map { len: None },
+            Token::Str("insert_integer"),
+            Token::Struct { len: 2, name: "insert_integer" },
+            Token::Str("index"),
+            Token::U32(0),
+            Token::Str("value"),
+            Token::U32(42),
+            Token::StructEnd,
+            Token::Str("extra_key"),
+            Token::Str("extra value"),
+            Token::MapEnd
+        ],
+    );
+}
+
+#[test]
+fn test_flatten_struct_tag_content_enum() {
+    let change_request = FlattenStructTagContentEnumWrapper {
+        outer: 42,
+        data: FlattenStructTagContentEnumNewtype(
+            FlattenStructTagContentEnum::InsertInteger {
+                index: 0,
+                value: 42
+            }
+        ),
+    };
+    assert_de_tokens(
+        &change_request,
+        &[
+            Token::Map { len: None },
+            Token::Str("outer"),
+            Token::U32(42),
+            Token::Str("type"),
+            Token::Str("insert_integer"),
+            Token::Str("value"),
+            Token::Map { len: None },
+            Token::Str("index"),
+            Token::U32(0),
+            Token::Str("value"),
+            Token::U32(42),
+            Token::MapEnd,
+            Token::MapEnd,
+        ],
+    );
+    assert_ser_tokens(
+        &change_request,
+        &[
+            Token::Map { len: None },
+            Token::Str("outer"),
+            Token::U32(42),
+            Token::Str("type"),
+            Token::Str("insert_integer"),
+            Token::Str("value"),
+            Token::Struct { len: 2, name: "insert_integer" },
+            Token::Str("index"),
+            Token::U32(0),
+            Token::Str("value"),
+            Token::U32(42),
+            Token::StructEnd,
+            Token::MapEnd,
+        ],
+    );
+}
+
+#[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,
+        ],
+    );
+}
+
+#[test]
+fn test_unknown_field_in_flatten() {
+    #[derive(Debug, PartialEq, Serialize, Deserialize)]
+    #[serde(deny_unknown_fields)]
+    struct Outer {
+        dummy: String,
+        #[serde(flatten)]
+        inner: Inner,
+    }
+
+    #[derive(Debug, PartialEq, Serialize, Deserialize)]
+    struct Inner {
+        foo: HashMap<String, u32>,
+    }
+
+    assert_de_tokens_error::<Outer>(
+        &[
+            Token::Struct {
+                name: "Outer",
+                len: 1,
+            },
+            Token::Str("dummy"),
+            Token::Str("23"),
+            Token::Str("foo"),
+            Token::Map { len: None },
+            Token::Str("a"),
+            Token::U32(1),
+            Token::Str("b"),
+            Token::U32(2),
+            Token::MapEnd,
+            Token::Str("bar"),
+            Token::U32(23),
+            Token::StructEnd,
+        ],
+        "unknown field `bar`",
+    );
+}
+
+#[test]
+fn test_complex_flatten() {
+    #[derive(Debug, PartialEq, Serialize, Deserialize)]
+    struct Outer {
+        y: u32,
+        #[serde(flatten)]
+        first: First,
+        #[serde(flatten)]
+        second: Second,
+        z: u32
+    }
+
+    #[derive(Debug, PartialEq, Serialize, Deserialize)]
+    struct First {
+        a: u32,
+        b: bool,
+        c: Vec<String>,
+        d: String,
+        e: Option<u64>,
+    }
+
+    #[derive(Debug, PartialEq, Serialize, Deserialize)]
+    struct Second {
+        f: u32,
+    }
+
+    assert_de_tokens(
+        &Outer {
+            y: 0,
+            first: First {
+                a: 1,
+                b: true,
+                c: vec!["a".into(), "b".into()],
+                d: "c".into(),
+                e: Some(2),
+            },
+            second: Second {
+                f: 3
+            },
+            z: 4
+        },
+        &[
+            Token::Map { len: None },
+            Token::Str("y"),
+            Token::U32(0),
+            Token::Str("a"),
+            Token::U32(1),
+            Token::Str("b"),
+            Token::Bool(true),
+            Token::Str("c"),
+            Token::Seq { len: Some(2) },
+            Token::Str("a"),
+            Token::Str("b"),
+            Token::SeqEnd,
+            Token::Str("d"),
+            Token::Str("c"),
+            Token::Str("e"),
+            Token::U64(2),
+            Token::Str("f"),
+            Token::U32(3),
+            Token::Str("z"),
+            Token::U32(4),
+            Token::MapEnd,
+        ],
+    );
+
+    assert_ser_tokens(
+        &Outer {
+            y: 0,
+            first: First {
+                a: 1,
+                b: true,
+                c: vec!["a".into(), "b".into()],
+                d: "c".into(),
+                e: Some(2),
+            },
+            second: Second {
+                f: 3
+            },
+            z: 4
+        },
+        &[
+            Token::Map { len: None },
+            Token::Str("y"),
+            Token::U32(0),
+            Token::Str("a"),
+            Token::U32(1),
+            Token::Str("b"),
+            Token::Bool(true),
+            Token::Str("c"),
+            Token::Seq { len: Some(2) },
+            Token::Str("a"),
+            Token::Str("b"),
+            Token::SeqEnd,
+            Token::Str("d"),
+            Token::Str("c"),
+            Token::Str("e"),
+            Token::Some,
+            Token::U64(2),
+            Token::Str("f"),
+            Token::U32(3),
+            Token::Str("z"),
+            Token::U32(4),
+            Token::MapEnd,
+        ],
+    );
+}
+
+#[test]
+fn test_flatten_unsupported_type() {
+    #[derive(Debug, PartialEq, Serialize, Deserialize)]
+    struct Outer {
+        outer: String,
+        #[serde(flatten)]
+        inner: String,
+    }
+
+    assert_ser_tokens_error(
+        &Outer {
+            outer: "foo".into(),
+            inner: "bar".into(),
+        },
+        &[
+            Token::Map { len: None },
+            Token::Str("outer"),
+            Token::Str("foo"),
+        ],
+        "can only flatten structs and maps (got a string)",
+    );
+    assert_de_tokens_error::<Outer>(
+        &[
+            Token::Map { len: None },
+            Token::Str("outer"),
+            Token::Str("foo"),
+            Token::Str("a"),
+            Token::Str("b"),
+            Token::MapEnd
+        ],
+        "can only flatten structs and maps",
+    );
+}
+
+#[test]
+fn test_non_string_keys() {
+    #[derive(Debug, PartialEq, Serialize, Deserialize)]
+    struct TestStruct {
+        name: String,
+        age: u32,
+        #[serde(flatten)]
+        mapping: HashMap<u32, u32>,
+    }
+
+    let mut mapping = HashMap::new();
+    mapping.insert(0, 42);
+    assert_tokens(
+        &TestStruct { name: "peter".into(), age: 3, mapping },
+        &[
+            Token::Map { len: None },
+            Token::Str("name"),
+            Token::Str("peter"),
+            Token::Str("age"),
+            Token::U32(3),
+            Token::U32(0),
+            Token::U32(42),
+            Token::MapEnd,
+        ],
+    );
+}
+
+#[test]
+fn test_lifetime_propagation_for_flatten() {
+    #[derive(Deserialize, Serialize, Debug, PartialEq)]
+    struct A<T> {
+        #[serde(flatten)]
+        t: T,
+    }
+
+    #[derive(Deserialize, Serialize, Debug, PartialEq)]
+    struct B<'a> {
+        #[serde(flatten, borrow)]
+        t: HashMap<&'a str, u32>,
+    }
+
+    #[derive(Deserialize, Serialize, Debug, PartialEq)]
+    struct C<'a> {
+        #[serde(flatten, borrow)]
+        t: HashMap<&'a [u8], u32>,
+    }
+
+    let mut owned_map = HashMap::new();
+    owned_map.insert("x".to_string(), 42u32);
+    assert_tokens(
+        &A { t: owned_map },
+        &[
+            Token::Map { len: None },
+            Token::Str("x"),
+            Token::U32(42),
+            Token::MapEnd,
+        ],
+    );
+
+    let mut borrowed_map = HashMap::new();
+    borrowed_map.insert("x", 42u32);
+    assert_ser_tokens(
+        &B { t: borrowed_map.clone() },
+        &[
+            Token::Map { len: None },
+            Token::BorrowedStr("x"),
+            Token::U32(42),
+            Token::MapEnd,
+        ],
+    );
+
+    assert_de_tokens(
+        &B { t: borrowed_map },
+        &[
+            Token::Map { len: None },
+            Token::BorrowedStr("x"),
+            Token::U32(42),
+            Token::MapEnd,
+        ],
+    );
+
+    let mut borrowed_map = HashMap::new();
+    borrowed_map.insert(&b"x"[..], 42u32);
+    assert_ser_tokens(
+        &C { t: borrowed_map.clone() },
+        &[
+            Token::Map { len: None },
+            Token::Seq { len: Some(1) },
+            Token::U8(120),
+            Token::SeqEnd,
+            Token::U32(42),
+            Token::MapEnd,
+        ],
+    );
+
+    assert_de_tokens(
+        &C { t: borrowed_map },
+        &[
+            Token::Map { len: None },
+            Token::BorrowedBytes(b"x"),
+            Token::U32(42),
+            Token::MapEnd,
+        ],
+    );
+}
+
+#[test]
+fn test_flatten_enum_newtype() {
+    #[derive(Serialize, Deserialize, PartialEq, Debug)]
+    struct S {
+        #[serde(flatten)]
+        flat: E,
+    }
+
+    #[derive(Serialize, Deserialize, PartialEq, Debug)]
+    enum E {
+        Q(HashMap<String, String>),
+    }
+
+    let e = E::Q({
+        let mut map = HashMap::new();
+        map.insert("k".to_owned(), "v".to_owned());
+        map
+    });
+    let s = S { flat: e };
+
+    assert_tokens(
+        &s,
+        &[
+            Token::Map { len: None },
+            Token::Str("Q"),
+            Token::Map { len: Some(1) },
+            Token::Str("k"),
+            Token::Str("v"),
+            Token::MapEnd,
+            Token::MapEnd,
+        ],
+    );
+}