Merge #10

10: Update to serde 0.9.0 and various improvements

1 files changed, 275 insertions, 256 deletions

diff --git a/src/ser/mod.rs b/src/ser/mod.rs
index f04c956..50355fd 100644
--- a/src/ser/mod.rs
+++ b/src/ser/mod.rs
@@ -2,7 +2,9 @@
 
 mod key;
 mod pair;
+mod part;
 mod value;
+mod void;
 
 use serde::ser;
 use std::borrow::Cow;
@@ -27,12 +29,9 @@ use url::form_urlencoded::Target as UrlEncodedTarget;
 ///     Ok("bread=baguette&cheese=comt%C3%A9&meat=ham&fat=butter".to_owned()));
 /// ```
 pub fn to_string<T: ser::Serialize>(input: &T) -> Result<String, Error> {
-    let mut output = String::new();
-    {
-        let mut urlencoder = UrlEncodedSerializer::new(&mut output);
-        try!(input.serialize(&mut Serializer::new(&mut urlencoder)));
-    }
-    Ok(output)
+    let mut urlencoder = UrlEncodedSerializer::new("".to_owned());
+    input.serialize(Serializer::new(&mut urlencoder))?;
+    Ok(urlencoder.finish())
 }
 
 /// A serializer for the `application/x-www-form-urlencoded` format.
@@ -44,13 +43,13 @@ pub fn to_string<T: ser::Serialize>(input: &T) -> Result<String, Error> {
 ///   unit structs and unit variants.
 ///
 /// * Newtype structs defer to their inner values.
-pub struct Serializer<'output, T: 'output + UrlEncodedTarget> {
-    urlencoder: &'output mut UrlEncodedSerializer<T>
+pub struct Serializer<'output, Target: 'output + UrlEncodedTarget> {
+    urlencoder: &'output mut UrlEncodedSerializer<Target>,
 }
 
-impl<'output, T: 'output + UrlEncodedTarget> Serializer<'output, T> {
+impl<'output, Target: 'output + UrlEncodedTarget> Serializer<'output, Target> {
     /// Returns a new `Serializer`.
-    pub fn new(urlencoder: &'output mut UrlEncodedSerializer<T>) -> Self {
+    pub fn new(urlencoder: &'output mut UrlEncodedSerializer<Target>) -> Self {
         Serializer { urlencoder: urlencoder }
     }
 }
@@ -59,15 +58,13 @@ impl<'output, T: 'output + UrlEncodedTarget> Serializer<'output, T> {
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub enum Error {
     Custom(Cow<'static, str>),
-    InvalidValue(Cow<'static, str>),
     Utf8(str::Utf8Error),
 }
 
 impl fmt::Display for Error {
-    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match *self {
             Error::Custom(ref msg) => msg.fmt(f),
-            Error::InvalidValue(ref msg) => write!(f, "invalid value: {}", msg),
             Error::Utf8(ref err) => write!(f, "invalid UTF-8: {}", err),
         }
     }
@@ -77,7 +74,6 @@ impl error::Error for Error {
     fn description(&self) -> &str {
         match *self {
             Error::Custom(ref msg) => msg,
-            Error::InvalidValue(ref msg) => msg,
             Error::Utf8(ref err) => error::Error::description(err),
         }
     }
@@ -85,409 +81,432 @@ impl error::Error for Error {
     /// The lower-level cause of this error, in the case of a `Utf8` error.
     fn cause(&self) -> Option<&error::Error> {
         match *self {
-            Error::Custom(_) | Error::InvalidValue(_) => None,
+            Error::Custom(_) => None,
             Error::Utf8(ref err) => Some(err),
         }
     }
 }
 
 impl ser::Error for Error {
-    fn custom<T: Into<String>>(msg: T) -> Self {
-        Error::Custom(msg.into().into())
-    }
-
-    fn invalid_value(msg: &str) -> Self {
-        Error::InvalidValue(String::from(msg).into())
+    fn custom<T: fmt::Display>(msg: T) -> Self {
+        Error::Custom(format!("{}", msg).into())
     }
 }
 
-/// State used when serializing sequences.
-pub struct SeqState {
-    _state: (),
+/// Sequence serializer.
+pub struct SeqSerializer<'output, Target: 'output + UrlEncodedTarget> {
+    urlencoder: &'output mut UrlEncodedSerializer<Target>,
 }
 
-/// State used when serializing tuples.
-pub struct TupleState {
-    _state: (),
+/// Tuple serializer.
+///
+/// Never instantiated, tuples are not supported at top-level.
+pub struct TupleSerializer<'output, T: 'output + UrlEncodedTarget> {
+    inner: void::VoidSerializer<&'output mut UrlEncodedSerializer<T>>,
 }
 
-/// State used when serializing tuple structs.
-pub struct TupleStructState {
-    _state: (),
+/// Tuple struct serializer.
+///
+/// Never instantiated, tuple structs are not supported.
+pub struct TupleStructSerializer<'output, T: 'output + UrlEncodedTarget> {
+    inner: void::VoidSerializer<&'output mut UrlEncodedSerializer<T>>,
 }
 
-/// State used when serializing tuple variants.
-pub struct TupleVariantState {
-    _state: (),
+/// Tuple variant serializer.
+///
+/// Never instantiated, tuple variants are not supported.
+pub struct TupleVariantSerializer<'output, T: 'output + UrlEncodedTarget> {
+    inner: void::VoidSerializer<&'output mut UrlEncodedSerializer<T>>,
 }
 
-/// State used when serializing maps.
-pub struct MapState {
-    key: Option<Cow<'static, str>>
+/// Map serializer.
+pub struct MapSerializer<'output, Target: 'output + UrlEncodedTarget> {
+    urlencoder: &'output mut UrlEncodedSerializer<Target>,
+    key: Option<Cow<'static, str>>,
 }
 
-/// State used when serializing structs.
-pub struct StructState {
-    _state: (),
+/// Struct serializer.
+pub struct StructSerializer<'output, Target: 'output + UrlEncodedTarget> {
+    urlencoder: &'output mut UrlEncodedSerializer<Target>,
 }
 
-/// State used when serializing struct variants.
-pub struct StructVariantState {
-    _state: (),
+/// Struct variant serializer.
+///
+/// Never instantiated, struct variants are not supported.
+pub struct StructVariantSerializer<'output, T: 'output + UrlEncodedTarget> {
+    inner: void::VoidSerializer<&'output mut UrlEncodedSerializer<T>>,
 }
 
 impl<'output, Target> ser::Serializer for Serializer<'output, Target>
-    where Target: 'output + UrlEncodedTarget
+    where Target: 'output + UrlEncodedTarget,
 {
+    type Ok = &'output mut UrlEncodedSerializer<Target>;
     type Error = Error;
-
-    /// State used when serializing sequences.
-    type SeqState = SeqState;
-
-    /// State used when serializing tuples.
-    type TupleState = TupleState;
-
-    /// State used when serializing tuple structs.
-    type TupleStructState = TupleStructState;
-
-    /// State used when serializing tuple variants.
-    type TupleVariantState = TupleVariantState;
-
-    /// State used when serializing maps.
-    type MapState = MapState;
-
-    /// State used when serializing structs.
-    type StructState = StructState;
-
-    /// State used when serializing struct variants.
-    type StructVariantState = StructVariantState;
-
-    /// Returns an error.
-    fn serialize_bool(&mut self, _v: bool) -> Result<(), Error> {
-        Err(Error::top_level())
-    }
-
-    /// Returns an error.
-    fn serialize_isize(&mut self, _v: isize) -> Result<(), Error> {
-        Err(Error::top_level())
-    }
+    type SerializeSeq = SeqSerializer<'output, Target>;
+    type SerializeTuple = TupleSerializer<'output, Target>;
+    type SerializeTupleStruct = TupleStructSerializer<'output, Target>;
+    type SerializeTupleVariant = TupleVariantSerializer<'output, Target>;
+    type SerializeMap = MapSerializer<'output, Target>;
+    type SerializeStruct = StructSerializer<'output, Target>;
+    type SerializeStructVariant = StructVariantSerializer<'output, Target>;
 
     /// Returns an error.
-    fn serialize_i8(&mut self, _v: i8) -> Result<(), Error> {
+    fn serialize_bool(self, _v: bool) -> Result<Self::Ok, Error> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_i16(&mut self, _v: i16) -> Result<(), Error> {
+    fn serialize_i8(self, _v: i8) -> Result<Self::Ok, Error> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_i32(&mut self, _v: i32) -> Result<(), Error> {
+    fn serialize_i16(self, _v: i16) -> Result<Self::Ok, Error> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_i64(&mut self, _v: i64) -> Result<(), Error> {
+    fn serialize_i32(self, _v: i32) -> Result<Self::Ok, Error> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_usize(&mut self, _v: usize) -> Result<(), Error> {
+    fn serialize_i64(self, _v: i64) -> Result<Self::Ok, Error> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_u8(&mut self, _v: u8) -> Result<(), Error> {
+    fn serialize_u8(self, _v: u8) -> Result<Self::Ok, Error> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_u16(&mut self, _v: u16) -> Result<(), Error> {
+    fn serialize_u16(self, _v: u16) -> Result<Self::Ok, Error> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_u32(&mut self, _v: u32) -> Result<(), Error> {
+    fn serialize_u32(self, _v: u32) -> Result<Self::Ok, Error> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_u64(&mut self, _v: u64) -> Result<(), Error> {
+    fn serialize_u64(self, _v: u64) -> Result<Self::Ok, Error> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_f32(&mut self, _v: f32) -> Result<(), Error> {
+    fn serialize_f32(self, _v: f32) -> Result<Self::Ok, Error> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_f64(&mut self, _v: f64) -> Result<(), Error> {
+    fn serialize_f64(self, _v: f64) -> Result<Self::Ok, Error> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_char(&mut self, _v: char) -> Result<(), Error> {
+    fn serialize_char(self, _v: char) -> Result<Self::Ok, Error> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_str(&mut self, _value: &str) -> Result<(), Error> {
+    fn serialize_str(self, _value: &str) -> Result<Self::Ok, Error> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_bytes(&mut self, _value: &[u8]) -> Result<(), Error> {
+    fn serialize_bytes(self, _value: &[u8]) -> Result<Self::Ok, Error> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_unit(&mut self) -> Result<(), Error> {
+    fn serialize_unit(self) -> Result<Self::Ok, Error> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_unit_struct(
-            &mut self, _name: &'static str)
-            -> Result<(), Error> {
+    fn serialize_unit_struct(self,
+                             _name: &'static str)
+                             -> Result<Self::Ok, Error> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_unit_variant(
-            &mut self,
-            _name: &'static str,
-            _variant_index: usize,
-            _variant: &'static str)
-            -> Result<(), Error> {
+    fn serialize_unit_variant(self,
+                              _name: &'static str,
+                              _variant_index: usize,
+                              _variant: &'static str)
+                              -> Result<Self::Ok, Error> {
         Err(Error::top_level())
     }
 
     /// Serializes the inner value, ignoring the newtype name.
-    fn serialize_newtype_struct<T>(
-            &mut self, _name: &'static str, value: T)
-            -> Result<(), Error>
-        where T: ser::Serialize
-    {
+    fn serialize_newtype_struct<T: ?Sized + ser::Serialize>
+        (self,
+         _name: &'static str,
+         value: &T)
+         -> Result<Self::Ok, Error> {
         value.serialize(self)
     }
 
     /// Returns an error.
-    fn serialize_newtype_variant<T>(
-            &mut self,                                    
-            _name: &'static str,                                    
-            _variant_index: usize,                                    
-            _variant: &'static str,                                    
-            _value: T)                                    
-            -> Result<(), Error>
-        where T: ser::Serialize
-    {
+    fn serialize_newtype_variant<T: ?Sized + ser::Serialize>
+        (self,
+         _name: &'static str,
+         _variant_index: usize,
+         _variant: &'static str,
+         _value: &T)
+         -> Result<Self::Ok, Error> {
         Err(Error::top_level())
     }
 
-    /// Returns an error.
-    fn serialize_none(&mut self) -> Result<(), Error> {
-        Ok(())
+    /// Returns `Ok`.
+    fn serialize_none(self) -> Result<Self::Ok, Error> {
+        Ok(self.urlencoder)
     }
 
-    /// Returns an error.
-    fn serialize_some<T>(&mut self, value: T) -> Result<(), Error>
-        where T: ser::Serialize
-    {
+    /// Serializes the given value.
+    fn serialize_some<T: ?Sized + ser::Serialize>
+        (self,
+         value: &T)
+         -> Result<Self::Ok, Error> {
         value.serialize(self)
     }
 
-    /// Begins to serialize a sequence, given length (if any) is ignored.
-    fn serialize_seq(
-            &mut self, _len: Option<usize>)
-            -> Result<SeqState, Error> {
-        Ok(SeqState { _state: () })
+    /// Serialize a sequence, given length (if any) is ignored.
+    fn serialize_seq(self,
+                     _len: Option<usize>)
+                     -> Result<Self::SerializeSeq, Error> {
+        Ok(SeqSerializer { urlencoder: self.urlencoder })
     }
 
-    /// Serializes a sequence element.
-    fn serialize_seq_elt<T>(
-            &mut self, _state: &mut SeqState, value: T)
-            -> Result<(), Error>
-        where T: ser::Serialize
-    {
-        value.serialize(&mut pair::PairSerializer::new(self.urlencoder))
-    }
-
-    /// Finishes serializing a sequence.
-    fn serialize_seq_end(&mut self, _state: SeqState) -> Result<(), Error> {
-        Ok(())
-    }
-
-    /// Begins to serialize a sequence, given length is ignored.
-    fn serialize_seq_fixed_size(
-            &mut self, _length: usize)
-            -> Result<SeqState, Error> {
-        Ok(SeqState { _state: () })
+    /// Serializes a sequence, given length is ignored.
+    fn serialize_seq_fixed_size(self,
+                                _len: usize)
+                                -> Result<Self::SerializeSeq, Error> {
+        Ok(SeqSerializer { urlencoder: self.urlencoder })
     }
 
     /// Returns an error.
-    fn serialize_tuple(&mut self, _len: usize) -> Result<TupleState, Error> {
+    fn serialize_tuple(self,
+                       _len: usize)
+                       -> Result<Self::SerializeTuple, Error> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_tuple_elt<T>(
-            &mut self, _state: &mut TupleState, _value: T)
-            -> Result<(), Error>
-        where T: ser::Serialize
-    {
+    fn serialize_tuple_struct(self,
+                              _name: &'static str,
+                              _len: usize)
+                              -> Result<Self::SerializeTupleStruct, Error> {
         Err(Error::top_level())
     }
 
     /// Returns an error.
-    fn serialize_tuple_end(&mut self, _state: TupleState) -> Result<(), Error> {
+    fn serialize_tuple_variant
+        (self,
+         _name: &'static str,
+         _variant_index: usize,
+         _variant: &'static str,
+         _len: usize)
+         -> Result<Self::SerializeTupleVariant, Error> {
         Err(Error::top_level())
     }
 
-    /// Returns an error.
-    fn serialize_tuple_struct(
-            &mut self, _name: &'static str, _len: usize)
-            -> Result<TupleStructState, Error> {
-        Err(Error::top_level())
+    /// Serializes a map, given length is ignored.
+    fn serialize_map(self,
+                     _len: Option<usize>)
+                     -> Result<Self::SerializeMap, Error> {
+        Ok(MapSerializer {
+            urlencoder: self.urlencoder,
+            key: None,
+        })
     }
 
-    /// Returns an error.
-    fn serialize_tuple_struct_elt<T>(
-            &mut self, _state: &mut TupleStructState, _value: T)
-            -> Result<(), Error>
-        where T: ser::Serialize
-    {
-        Err(Error::top_level())
+    /// Serializes a struct, given length is ignored.
+    fn serialize_struct(self,
+                        _name: &'static str,
+                        _len: usize)
+                        -> Result<Self::SerializeStruct, Error> {
+        Ok(StructSerializer { urlencoder: self.urlencoder })
     }
 
     /// Returns an error.
-    fn serialize_tuple_struct_end(
-            &mut self, _state: TupleStructState)
-            -> Result<(), Error>
-    {
+    fn serialize_struct_variant
+        (self,
+         _name: &'static str,
+         _variant_index: usize,
+         _variant: &'static str,
+         _len: usize)
+         -> Result<Self::SerializeStructVariant, Error> {
         Err(Error::top_level())
     }
+}
 
-    /// Returns an error.
-    fn serialize_tuple_variant(
-            &mut self,
-            _name: &'static str,
-            _variant_index: usize,
-            _variant: &'static str,
-            _len: usize)
-            -> Result<TupleVariantState, Error> {
-        Err(Error::top_level())
+impl<'output, Target> ser::SerializeSeq for SeqSerializer<'output, Target>
+    where Target: 'output + UrlEncodedTarget,
+{
+    type Ok = &'output mut UrlEncodedSerializer<Target>;
+    type Error = Error;
+
+    fn serialize_element<T: ?Sized + ser::Serialize>(&mut self,
+                                                     value: &T)
+                                                     -> Result<(), Error> {
+        value.serialize(pair::PairSerializer::new(self.urlencoder))
     }
 
-    /// Returns an error.
-    fn serialize_tuple_variant_elt<T>(
-            &mut self, _state: &mut TupleVariantState, _value: T)
-            -> Result<(), Error>
-        where T: ser::Serialize
-    {
-        Err(Error::top_level())
+    fn end(self) -> Result<Self::Ok, Error> {
+        Ok(self.urlencoder)
     }
+}
 
-    /// Returns an error.
-    fn serialize_tuple_variant_end(
-            &mut self, _state: TupleVariantState)
-            -> Result<(), Error> {
-        Err(Error::top_level())
+impl<'output, Target> ser::SerializeTuple for TupleSerializer<'output, Target>
+    where Target: 'output + UrlEncodedTarget,
+{
+    type Ok = &'output mut UrlEncodedSerializer<Target>;
+    type Error = Error;
+
+    fn serialize_element<T: ?Sized + ser::Serialize>(&mut self,
+                                                     value: &T)
+                                                     -> Result<(), Error> {
+        self.inner.serialize_element(value)
     }
 
-    /// Begins to serialize a map, given length (if any) is ignored.
-    fn serialize_map(
-            &mut self, _len: Option<usize>)
-            -> Result<MapState, Error> {
-        Ok(MapState { key: None })
+    fn end(self) -> Result<Self::Ok, Error> {
+        self.inner.end()
     }
+}
 
-    /// Serializes a map key.
-    fn serialize_map_key<T>(
-            &mut self, state: &mut MapState, key: T)
-            -> Result<(), Error>
-        where T: ser::Serialize
-    {
-        key.serialize(&mut key::MapKeySerializer::new(&mut state.key))
+impl<'output, Target> ser::SerializeTupleStruct
+    for TupleStructSerializer<'output, Target>
+    where Target: 'output + UrlEncodedTarget,
+{
+    type Ok = &'output mut UrlEncodedSerializer<Target>;
+    type Error = Error;
+
+    fn serialize_field<T: ?Sized + ser::Serialize>(&mut self,
+                                                   value: &T)
+                                                   -> Result<(), Error> {
+        self.inner.serialize_field(value)
     }
 
-    /// Serializes a map value.
-    fn serialize_map_value<T>(
-            &mut self, state: &mut MapState, value: T)
-            -> Result<(), Error>
-        where T: ser::Serialize
-    {
-        let mut value_serializer =
-            try!(value::ValueSerializer::new(&mut state.key, self.urlencoder));
-        value.serialize(&mut value_serializer)
+    fn end(self) -> Result<Self::Ok, Error> {
+        self.inner.end()
     }
+}
 
-    /// Finishes serializing a map.
-    fn serialize_map_end(&mut self, _state: MapState) -> Result<(), Error> {
-        Ok(())
+impl<'output, Target> ser::SerializeTupleVariant
+    for TupleVariantSerializer<'output, Target>
+    where Target: 'output + UrlEncodedTarget,
+{
+    type Ok = &'output mut UrlEncodedSerializer<Target>;
+    type Error = Error;
+
+    fn serialize_field<T: ?Sized + ser::Serialize>(&mut self,
+                                                   value: &T)
+                                                   -> Result<(), Error> {
+        self.inner.serialize_field(value)
     }
 
-    /// Begins to serialize a struct, given length is ignored.
-    fn serialize_struct(
-            &mut self, _name: &'static str, _len: usize)
-            -> Result<StructState, Error> {
-        Ok(StructState { _state: () })
+    fn end(self) -> Result<Self::Ok, Error> {
+        self.inner.end()
     }
+}
 
-    /// Serializes a struct element.
-    fn serialize_struct_elt<T>(
-            &mut self,
-            _state: &mut StructState,
-            key: &'static str,
-            value: T)
-            -> Result<(), Error>
-        where T: ser::Serialize
-    {
-        let mut key = Some(key.into());
-        let mut value_serializer =
-            value::ValueSerializer::new(&mut key, self.urlencoder).unwrap();
-        value.serialize(&mut value_serializer)
+impl<'output, Target> ser::SerializeMap for MapSerializer<'output, Target>
+    where Target: 'output + UrlEncodedTarget,
+{
+    type Ok = &'output mut UrlEncodedSerializer<Target>;
+    type Error = Error;
+
+    fn serialize_entry<K: ?Sized + ser::Serialize, V: ?Sized + ser::Serialize>
+        (&mut self,
+         key: &K,
+         value: &V)
+         -> Result<(), Error> {
+        let key_sink = key::KeySink::new(|key| {
+            let value_sink = value::ValueSink::new(self.urlencoder, &key);
+            value.serialize(part::PartSerializer::new(value_sink))?;
+            self.key = None;
+            Ok(())
+        });
+        let entry_serializer = part::PartSerializer::new(key_sink);
+        key.serialize(entry_serializer)
+    }
+
+    fn serialize_key<T: ?Sized + ser::Serialize>(&mut self,
+                                                 key: &T)
+                                                 -> Result<(), Error> {
+        let key_sink = key::KeySink::new(|key| Ok(key.into()));
+        let key_serializer = part::PartSerializer::new(key_sink);
+        self.key = Some(key.serialize(key_serializer)?);
+        Ok(())
     }
 
-    /// Finishes serializing a struct.
-    fn serialize_struct_end(&mut self, _state: StructState)
-                            -> Result<(), Error> {
+    fn serialize_value<T: ?Sized + ser::Serialize>(&mut self,
+                                                   value: &T)
+                                                   -> Result<(), Error> {
+        {
+            let key = self.key.as_ref().ok_or_else(|| Error::no_key())?;
+            let value_sink = value::ValueSink::new(self.urlencoder, &key);
+            value.serialize(part::PartSerializer::new(value_sink))?;
+        }
+        self.key = None;
         Ok(())
     }
 
-    /// Returns an error.
-    fn serialize_struct_variant(
-            &mut self,
-            _name: &'static str,
-            _variant_index: usize,
-            _variant: &'static str,
-            _len: usize)
-            -> Result<StructVariantState, Error> {
-        Err(Error::top_level())
+    fn end(self) -> Result<Self::Ok, Error> {
+        Ok(self.urlencoder)
     }
+}
 
-    /// Returns an error.
-    fn serialize_struct_variant_elt<T>(
-            &mut self,
-            _state: &mut StructVariantState,
-            _key: &'static str,
-            _value: T)
-            -> Result<(), Error> {
-        Err(Error::top_level())
+impl<'output, Target> ser::SerializeStruct for StructSerializer<'output, Target>
+    where Target: 'output + UrlEncodedTarget,
+{
+    type Ok = &'output mut UrlEncodedSerializer<Target>;
+    type Error = Error;
+
+    fn serialize_field<T: ?Sized + ser::Serialize>(&mut self,
+                                                   key: &'static str,
+                                                   value: &T)
+                                                   -> Result<(), Error> {
+        let value_sink = value::ValueSink::new(self.urlencoder, key);
+        value.serialize(part::PartSerializer::new(value_sink))
     }
 
-    /// Returns an error.
-    fn serialize_struct_variant_end(
-            &mut self, _state: StructVariantState)
-            -> Result<(), Error> {
-        Err(Error::top_level())
+    fn end(self) -> Result<Self::Ok, Error> {
+        Ok(self.urlencoder)
+    }
+}
+
+impl<'output, Target> ser::SerializeStructVariant
+    for StructVariantSerializer<'output, Target>
+    where Target: 'output + UrlEncodedTarget,
+{
+    type Ok = &'output mut UrlEncodedSerializer<Target>;
+    type Error = Error;
+
+    fn serialize_field<T: ?Sized + ser::Serialize>(&mut self,
+                                                   key: &'static str,
+                                                   value: &T)
+                                                   -> Result<(), Error> {
+        self.inner.serialize_field(key, value)
+    }
+
+    fn end(self) -> Result<Self::Ok, Error> {
+        self.inner.end()
     }
 }
 
 impl Error {
     fn top_level() -> Self {
-        Error::Custom(
-            "top-level serializer supports only maps and structs".into())
+        let msg = "top-level serializer supports only maps and structs";
+        Error::Custom(msg.into())
+    }
+
+    fn no_key() -> Self {
+        let msg = "tried to serialize a value before serializing key";
+        Error::Custom(msg.into())
     }
 }